Network Working Group J. Rosenberg
Request for Comments: 3219 dynamicsoft
Category: Standards Track H. Salama
Cisco Systems
M. Squire
Hatteras Networks
January 2002
Telephony Routing over IP (TRIP)
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
This document presents the Telephony Routing over IP (TRIP). TRIP is
a policy driven inter-administrative domain protocol for advertising
the reachability of telephony destinations between location servers,
and for advertising attributes of the routes to those destinations.
TRIP's operation is independent of any signaling protocol, hence TRIP
can serve as the telephony routing protocol for any signaling
protocol.
The Border Gateway Protocol (BGP-4) is used to distribute routing
information between administrative domains. TRIP is used to
distribute telephony routing information between telephony
administrative domains. The similarity between the two protocols is
obvious, and hence TRIP is modeled after BGP-4.
Table of Contents
1 Terminology and Definitions .............................. 3
2 Introduction ............................................. 4
3 Summary of Operation ..................................... 53.1 Peering Session Establishment and Maintenance ............ 53.2 Database Exchanges ....................................... 63.3 Internal Versus External Synchronization ................. 63.4 Advertising TRIP Routes .................................. 6
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RFC 3219 Telephony Routing over IP (TRIP) January 2002
3.5 Telephony Routing Information Bases ...................... 73.6 Routes in TRIP ........................................... 93.7 Aggregation .............................................. 9
4 Message Formats .......................................... 104.1 Message Header Format .................................... 104.2 OPEN Message Format ...................................... 114.3 UPDATE Message Format .................................... 154.4 KEEPALIVE Message Format ................................ 224.5 NOTIFICATION Message Format ............................. 23
5 TRIP Attributes ......................................... 245.1 WithdrawnRoutes .......................................... 245.2 ReachableRoutes .......................................... 285.3 NextHopServer ........................................... 295.4 AdvertisementPath ....................................... 315.5 RoutedPath ............................................... 355.6 AtomicAggregate ......................................... 365.7 LocalPreference ......................................... 375.8 MultiExitDisc ............................................ 385.9 Communities .............................................. 395.10 ITAD Topology .......................................... 415.11 ConvertedRoute ........................................... 435.12 Considerations for Defining New TRIP Attributes ......... 44
6 TRIP Error Detection and Handling ....................... 446.1 Message Header Error Detection and Handling ............. 456.2 OPEN Message Error Detection and Handling ............... 456.3 UPDATE Message Error Detection and Handling ............. 466.4 NOTIFICATION Message Error Detection and Handling ....... 486.5 Hold Timer Expired Error Handling ....................... 486.6 Finite State Machine Error Handling ..................... 486.7 Cease ................................................... 486.8 Connection Collision Detection .......................... 48
7 TRIP Version Negotiation ................................ 49
8 TRIP Capability Negotiation ............................. 50
9 TRIP Finite State Machine ............................... 50
10 UPDATE Message Handling ................................. 5510.1 Flooding Process ........................................ 5610.2 Decision Process ........................................ 5810.3 Update-Send Process ..................................... 6210.4 Route Selection Criteria ................................ 6710.5 Originating TRIP Routes ................................. 67
11 TRIP Transport .......................................... 68
12 ITAD Topology ........................................... 68
13 IANA Considerations ...................................... 6813.1 TRIP Capabilities ....................................... 6813.2 TRIP Attributes ........................................ 6913.3 Destination Address Families ............................ 6913.4 TRIP Application Protocols .............................. 6913.5 ITAD Numbers ............................................ 70
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RFC 3219 Telephony Routing over IP (TRIP) January 2002
14 Security Considerations ................................. 70
A1 Appendix 1: TRIP FSM State Transitions and Actions ...... 71
A2 Appendix 2: Implementation Recommendations .............. 73
Acknowledgments ................................................ 75
References ..................................................... 75
Intellectual Property Notice ................................... 77
Authors' Addresses ............................................. 78
Full Copyright Statement ....................................... 79
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [1].
A framework for Telephony Routing over IP (TRIP) is described in [2].
We assume the reader is familiar with the framework and terminology
of [2]. We define and use the following terms in addition to those
defined in [2].
Telephony Routing Information Base (TRIB): The database of reachable
telephony destinations built and maintained at an LS as a result of
its participation in TRIP.
IP Telephony Administrative Domain (ITAD): The set of resources
(gateways, location servers, etc.) under the control of a single
administrative authority. End users are customers of an ITAD.
Less/More Specific Route: A route X is said to be less specific than
a route Y if every destination in Y is also a destination in X, and X
and Y are not equal. In this case, Y is also said to be more
specific than X.
Aggregation: Aggregation is the process by which multiple routes are
combined into a single less specific route that covers the same set
of destinations. Aggregation is used to reduce the size of the TRIB
being synchronized with peer LSs by reducing the number of exported
TRIP routes.
Peers: Two LSs that share a logical association (a transport
connection). If the LSs are in the same ITAD, they are internal
peers. Otherwise, they are external peers. The logical association
between two peer LSs is called a peering session.
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RFC 3219 Telephony Routing over IP (TRIP) January 2002
Telephony Routing Information Protocol (TRIP): The protocol defined
in this specification. The function of TRIP is to advertise the
reachability of telephony destinations, attributes associated with
the destinations, as well as the attributes of the path towards those
destinations.
TRIP destination: TRIP can be used to manage routing tables for
multiple protocols (SIP, H323, etc.). In TRIP, a destination is the
combination of (a) a set of addresses (given by an address family and
address prefix), and (b) an application protocol (SIP, H323, etc).
The gateway location and routing problem has been introduced in [2].
It is considered one of the more difficult problems in IP telephony.
The selection of an egress gateway for a telephony call, traversing
an IP network towards an ultimate destination in the PSTN, is driven
in large part by the policies of the various parties along the path,
and by the relationships established between these parties. As such,
a global directory of egress gateways in which users look up
destination phone numbers is not a feasible solution. Rather,
information about the availability of egress gateways is exchanged
between providers, and subject to policy, made available locally and
then propagated to other providers in other ITADs, thus creating
routes towards these egress gateways. This would allow each provider
to create its own database of reachable phone numbers and the
associated routes - such a database could be very different for each
provider depending on policy.
TRIP is an inter-domain (i.e., inter-ITAD) gateway location and
routing protocol. The primary function of a TRIP speaker, called a
location server (LS), is to exchange information with other LSs.
This information includes the reachability of telephony destinations,
the routes towards these destinations, and information about gateways
towards those telephony destinations residing in the PSTN. The TRIP
requirements are set forth in [2].
LSs exchange sufficient routing information to construct a graph of
ITAD connectivity so that routing loops may be prevented. In
addition, TRIP can be used to exchange attributes necessary to
enforce policies and to select routes based on path or gateway
characteristics. This specification defines TRIP's transport and
synchronization mechanisms, its finite state machine, and the TRIP
data. This specification defines the basic attributes of TRIP. The
TRIP attribute set is extendible, so additional attributes may be
defined in future documents.
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RFC 3219 Telephony Routing over IP (TRIP) January 2002
TRIP is modeled after the Border Gateway Protocol 4 (BGP-4) [3] and
enhanced with some link state features, as in the Open Shortest Path
First (OSPF) protocol [4], IS-IS [5], and the Server Cache
Synchronization Protocol (SCSP) [6]. TRIP uses BGP's inter-domain
transport mechanism, BGP's peer communication, BGP's finite state
machine, and similar formats and attributes as BGP. Unlike BGP
however, TRIP permits generic intra-domain LS topologies, which
simplifies configuration and increases scalability in contrast to
BGP's full mesh requirement of internal BGP speakers. TRIP uses an
intra-domain flooding mechanism similar to that used in OSPF [4],
IS-IS [5], and SCSP [6].
TRIP permits aggregation of routes as they are advertised through the
network. TRIP does not define a specific route selection algorithm.
TRIP runs over a reliable transport protocol. This eliminates the
need to implement explicit fragmentation, retransmission,
acknowledgment, and sequencing. The error notification mechanism
used in TRIP assumes that the transport protocol supports a graceful
close, i.e., that all outstanding data will be delivered before the
connection is closed.
TRIP's operation is independent of any particular telephony signaling
protocol. Therefore, TRIP can be used as the routing protocol for
any of these protocols, e.g., H.323 [7] and SIP [8].
The LS peering topology is independent of the physical topology of
the network. In addition, the boundaries of an ITAD are independent
of the boundaries of the layer 3 routing autonomous systems. Neither
internal nor external TRIP peers need to be physically adjacent.
Two peer LSs form a transport protocol connection between one
another. They exchange messages to open and confirm the connection
parameters, and to negotiate the capabilities of each LS as well as
the type of information to be advertised over this connection.
KeepAlive messages are sent periodically to ensure adjacent peers are
operational. Notification messages are sent in response to errors or
special conditions. If a connection encounters an error condition, a
Notification message is sent and the connection is closed.
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Once the peer connection has been established, the initial data flow
is a dump of all routes relevant to the new peer (In the case of an
external peer, all routes in the LS's Adj-TRIB-Out for that external
peer. In the case of an internal peer, all routes in the Ext-TRIB
and all Adj-TRIBs-In). Note that the different TRIBs are defined in
Section 3.5.
Incremental updates are sent as the TRIP routing tables (TRIBs)
change. TRIP does not require periodic refresh of the routes.
Therefore, an LS must retain the current version of all routing
entries.
If a particular ITAD has multiple LSs and is providing transit
service for other ITADs, then care must be taken to ensure a
consistent view of routing within the ITAD. When synchronized the
TRIP routing tables, i.e., the Loc-TRIBs, of all internal peers are
identical.
As with BGP, TRIP distinguishes between internal and external peers.
Within an ITAD, internal TRIP uses link-state mechanisms to flood
database updates over an arbitrary topology. Externally, TRIP uses
point-to-point peering relationships to exchange database
information.
To achieve internal synchronization, internal peer connections are
configured between LSs of the same ITAD such that the resulting
intra-domain LS topology is connected and sufficiently redundant.
This is different from BGP's approach that requires all internal
peers to be connected in a full mesh topology, which may result in
scaling problems. When an update is received from an internal peer,
the routes in the update are checked to determine if they are newer
than the version already in the database. Newer routes are then
flooded to all other peers in the same domain.
In TRIP, a route is defined as the combination of (a) a set of
destination addresses (given by an address family indicator and an
address prefix), and (b) an application protocol (e.g. SIP, H323,
etc.). Generally, there are additional attributes associated with
each route (for example, the next-hop server).
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RFC 3219 Telephony Routing over IP (TRIP) January 2002
TRIP routes are advertised between a pair of LSs in UPDATE messages.
The destination addresses are included in the ReachableRoutes
attribute of the UPDATE, while other attributes describe things like
the path or egress gateway.
If an LS chooses to advertise a TRIP route, it may add to or modify
the attributes of the route before advertising it to a peer. TRIP
provides mechanisms by which an LS can inform its peer that a
previously advertised route is no longer available for use. There
are three methods by which a given LS can indicate that a route has
been withdrawn from service:
- Include the route in the WithdrawnRoutes Attribute in an UPDATE
message, thus marking the associated destinations as being no
longer available for use.
- Advertise a replacement route with the same set of destinations
in the ReachableRoutes Attribute.
- For external peers where flooding is not in use, the LS-to-LS
peer connection can be closed, which implicitly removes from
service all routes which the pair of LSs had advertised to each
other over that peer session. Note that terminating an
internal peering session does not necessarily remove the routes
advertised by the peer LS as the same routes may have been
received from multiple internal peers because of flooding. If
an LS determines that another internal LS is no longer active
(from the ITAD Topology attributes of the UPDATE messages from
other internal peers), then it MUST remove all routes
originated into the LS by that LS and rerun its decision
process.
A TRIP LS processes three types of routes:
- External routes: An external route is a route received from an
external peer LS
- Internal routes: An internal route is a route received from an
internal LS in the same ITAD.
- Local routes: A local route is a route locally injected into
TRIP, e.g. by configuration or by route redistribution from
another routing protocol.
The Telephony Routing Information Base (TRIB) within an LS consists
of four distinct parts:
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RFC 3219 Telephony Routing over IP (TRIP) January 2002
- Adj-TRIBs-In: The Adj-TRIBs-In store routing information that
has been learned from inbound UPDATE messages. Their contents
represent TRIP routes that are available as an input to the
Decision Process. These are the "unprocessed" routes received.
The routes from each external peer LS and each internal LS are
maintained in this database independently, so that updates from
one peer do not affect the routes received from another LS.
Note that there is an Adj-TRIB-In for every LS within the
domain, even those with which the LS is not directly peered.
- Ext-TRIB: There is only one Ext-TRIB database per LS. The LS
runs the route selection algorithm on all external routes
(stored in the Adj-TRIBs-In of the external peers) and local
routes (may be stored in an Adj-TRIB-In representing the local
LS) and selects the best route for a given destination and
stores it in the Ext-TRIB. The use of Ext-TRIB will be
explained further in Section 10.3.1
- Loc-TRIB: The Loc-TRIB contains the local TRIP routing
information that the LS has selected by applying its local
policies to the routing information contained in its Adj-
TRIBs-In of internal LSs and the Ext-TRIB.
- Adj-TRIBs-Out: The Adj-TRIBs-Out store the information that
the local LS has selected for advertisement to its external
peers. The routing information stored in the Adj-TRIBs-Out
will be carried in the local LS's UPDATE messages and
advertised to its peers.
Figure 1 illustrates the relationship between the four parts of the
routing information base.
Loc-TRIB
^
|
Decision Process
^ ^ |
| | |
Adj-TRIBs-In | V
(Internal LSs) | Adj-TRIBs-Out
|
|
|
Ext-TRIB
^ ^
| |
Adj-TRIB-In Local Routes
(External Peers)
Figure 1: TRIB Relationships
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RFC 3219 Telephony Routing over IP (TRIP) January 2002
Although the conceptual model distinguishes between Adj-TRIBs-In,
Ext-TRIB, Loc-TRIB, and Adj-TRIBs-Out, this neither implies nor
requires that an implementation must maintain four separate copies of
the routing information. The choice of implementation (for example,
4 copies of the information vs. 1 copy with pointers) is not
constrained by the protocol.
A route in TRIP specifies a range of numbers by being a prefix of
those numbers (the exact definition & syntax of route are in 5.1.1).
Arbitrary ranges of numbers are not atomically representable by a
route in TRIP. A prefix range is the only type of range supported
atomically. An arbitrary range can be accomplished by using multiple
prefixes in a ReachableRoutes attribute (see Section 5.1 & 5.2). For
example, 222-xxxx thru 999-xxxx could be represented by including the
prefixes 222, 223, 224,...,23,24,...,3,4,...,9 in a ReachableRoutes
attribute.
Aggregation is a scaling enhancement used by an LS to reduce the
number of routing entries that it has to synchronize with its peers.
Aggregation may be performed by an LS when there is a set of routes
{R1, R2, ...} in its TRIB such that there exists a less specific
route R where every valid destination in R is also a valid
destination in {R1, R2, ...} and vice-versa. Section 5 includes a
description of how to combine each attribute (by type) on the {R1,
R2, ...} routes into an attribute for R.
Note that there is no mechanism within TRIP to communicate that a
particular address prefix is not used or valid within a particular
address family, and thus that these addresses could be skipped during
aggregation. LSs may use methods outside of TRIP to learn of invalid
prefixes that may be ignored during aggregation.
An LS is not required to perform aggregation, however it is
recommended whenever maintaining a smaller TRIB is important. An LS
decides based on its local policy whether or not to aggregate a set
of routes into a single aggregate route.
Whenever an LS aggregates multiple routes where the NextHopServer is
not identical in all aggregated routes, the NextHopServer attribute
of the aggregate route must be set to a signalling server in the
aggregating LS's domain.
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RFC 3219 Telephony Routing over IP (TRIP) January 2002
When an LS resets the NextHopServer of any route, and this may be
performed because of aggregation or other reasons, it has the effect
of adding another signalling server along the signalling path to
these destinations. The end result is that the signalling path
between two destinations may consist of multiple signalling servers
across multiple domains.
This section describes message formats used by TRIP. Messages are
sent over a reliable transport protocol connection. A message MUST
be processed only after it is entirely received. The maximum message
size is 4096 octets. All implementations MUST support this maximum
message size. The smallest message that MAY be sent consists of a
TRIP header without a data portion, or 3 octets.
Each message has a fixed-size header. There may or may not be a data
portion following the header, depending on the message type. The
layout of the header fields is shown in Figure 2.
0 1 2
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
+--------------+----------------+---------------+
| Length | Type |
+--------------+----------------+---------------+
Figure 2: TRIP Header
Length: This 2-octet unsigned integer indicates the total length of
the message, including the header, in octets. Thus, it allows one to
locate, in the transport-level stream, the beginning of the next
message. The value of the Length field must always be at least 3 and
no greater than 4096, and may be further constrained depending on the
message type. No padding of extra data after the message is allowed,
so the Length field must have the smallest value possible given the
rest of the message.
Type: This 1-octet unsigned integer indicates the type code of the
message. The following type codes are defined:
1 - OPEN
2 - UPDATE
3 - NOTIFICATION
4 - KEEPALIVE
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RFC 3219 Telephony Routing over IP (TRIP) January 2002
After a transport protocol connection is established, the first
message sent by each side is an OPEN message. If the OPEN message is
acceptable, a KEEPALIVE message confirming the OPEN is sent back.
Once the OPEN is confirmed, UPDATE, KEEPALIVE, and NOTIFICATION
messages may be exchanged.
The minimum length of the OPEN message is 17 octets (including
message header). OPEN messages not meeting this minimum requirement
are handled as defined in Section 6.2.
In addition to the fixed-size TRIP header, the OPEN message contains
the following fields:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+--------------+----------------+
| Version | Reserved | Hold Time |
+---------------+---------------+--------------+----------------+
| My ITAD |
+---------------+---------------+--------------+----------------+
| TRIP Identifier |
+---------------+---------------+--------------+----------------+
| Optional Parameters Len |Optional Parameters (variable)...
+---------------+---------------+--------------+----------------+
Figure 3: TRIP OPEN Header
Version:
This 1-octet unsigned integer indicates the protocol version of the
message. The current TRIP version number is 1.
Hold Time:
This 2-octet unsigned integer indicates the number of seconds that
the sender proposes for the value of the Hold Timer. Upon receipt of
an OPEN message, an LS MUST calculate the value of the Hold Timer by
using the smaller of its configured Hold Time and the Hold Time
received in the OPEN message. The Hold Time MUST be either zero or
at least three seconds. An implementation MAY reject connections on
the basis of the Hold Time. The calculated value indicates the
maximum number of seconds that may elapse between the receipt of
successive KEEPALIVE and/or UPDATE messages by the sender.
This 4-octet unsigned integer indicates the ITAD number of the
sender. The ITAD number must be unique for this domain within this
confederation of cooperating LSs.
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RFC 3219 Telephony Routing over IP (TRIP) January 2002
ITAD numbers are assigned by IANA as specified in Section 13. This
document reserves ITAD number 0. ITAD numbers from 1 to 255 are
designated for private use.
TRIP Identifier:
This 4-octet unsigned integer indicates the TRIP Identifier of the
sender. The TRIP Identifier MUST uniquely identify this LS within
its ITAD. A given LS MAY set the value of its TRIP Identifier to an
IPv4 address assigned to that LS. The value of the TRIP Identifier
is determined on startup and MUST be the same for all peer
connections. When comparing two TRIP identifiers, the TRIP
Identifier is interpreted as a numerical 4-octet unsigned integer.
Optional Parameters Length:
This 2-octet unsigned integer indicates the total length of the
Optional Parameters field in octets. If the value of this field is
zero, no Optional Parameters are present.
Optional Parameters:
This field may contain a list of optional parameters, where each
parameter is encoded as a <Parameter Type, Parameter Length,
Parameter Value> triplet.
0 1 2
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+--------------+----------------+
| Parameter Type | Parameter Length |
+---------------+---------------+--------------+----------------+
| Parameter Value (variable)...
+---------------+---------------+--------------+----------------+
Figure 4: Optional Parameter Encoding
Parameter Type:
This is a 2-octet field that unambiguously identifies individual
parameters.
Parameter Length:
This is a 2-octet field that contains the length of the Parameter
Value field in octets.
Parameter Value:
This is a variable length field that is interpreted according to the
value of the Parameter Type field.
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RFC 3219 Telephony Routing over IP (TRIP) January 2002
Capability Information uses Optional Parameter type 1. This is an
optional parameter used by an LS to convey to its peer the list of
capabilities supported by the LS. This permits an LS to learn of the
capabilities of its peer LSs. Capability negotiation is defined in
Section 8.
The parameter contains one or more triples <Capability Code,
Capability Length, Capability Value>, where each triple is encoded as
shown below:
0 1 2
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+--------------+----------------+
| Capability Code | Capability Length |
+---------------+---------------+--------------+----------------+
| Capability Value (variable)...
+---------------+---------------+--------------+----------------+
Figure 5: Capability Optional Parameter
Capability Code:
Capability Code is a 2-octet field that unambiguously identifies
individual capabilities.
Capability Length:
Capability Length is a 2-octet field that contains the length of the
Capability Value field in octets.
Capability Value:
Capability Value is a variable length field that is interpreted
according to the value of the Capability Code field.
Any particular capability, as identified by its Capability Code, may
appear more than once within the Optional Parameter.
This document reserves Capability Codes 32768-65535 for vendor-
specific applications (these are the codes with the first bit of the
code value equal to 1). This document reserves value 0. Capability
Codes (other than those reserved for vendor specific use) are
controlled by IANA. See Section 13 for IANA considerations.
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RFC 3219 Telephony Routing over IP (TRIP) January 2002
The following Capability Codes are defined by this specification:
Code Capability
1 Route Types Supported
2 Send Receive Capability
The Route Types Supported Capability Code lists the route types
supported in this peering session by the transmitting LS. An LS MUST
NOT use route types that are not supported by the peer LS in any
particular peering session. If the route types supported by a peer
are not satisfactory, an LS SHOULD terminate the peering session.
The format for a Route Type is:
0 1 2
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+--------------+----------------+
| Address Family | Application Protocol |
+---------------+---------------+--------------+----------------+
Figure 6: Route Types Supported Capability
The Address Family and Application Protocol are as defined in Section
5.1.1. Address Family gives the address family being routed (within
the ReachableRoutes attribute). The application protocol lists the
application for which the routes apply. As an example, a route type
for TRIP could be <E.164, SIP>, indicating a set of E.164
destinations for the SIP protocol.
The Route Types Supported Capability MAY contain multiple route types
in the capability. The number of route types within the capability
is the maximum number that can fit given the capability length. The
Capability Code is 1 and the length is variable.
This capability specifies the mode in which the LS will operate with
this particular peer. The possible modes are: Send Only mode,
Receive Only mode, or Send Receive mode. The default mode is Send
Receive mode.
In Send Only mode, an LS transmits UPDATE messages to its peer, but
the peer MUST NOT transmit UPDATE messages to that LS. If an LS in
Send Only mode receives an UPDATE message from its peer, it MUST
discard that message, but no further action should be taken.
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The UPDATE messages sent by an LS in Send Only mode to its intra-
domain peer MUST include the ITAD Topology attribute whenever the
topology changes. A useful application of an LS in Send Only mode
with an external peer is to enable gateway registration services.
If a service provider terminates calls to a set of gateways it owns,
but never initiates calls, it can set its LSs to operate in Send Only
mode, since they only ever need to generate UPDATE messages, not
receive them. If an LS in Send Receive mode has a peering session
with a peer in Send Only mode, that LS MUST set its route
dissemination policy such that it does not send any UPDATE messages
to its peer.
In Receive Only mode, the LS acts as a passive TRIP listener. It
receives and processes UPDATE messages from its peer, but it MUST NOT
transmit any UPDATE messages to its peer. This is useful for
management stations that wish to collect topology information for
display purposes.
The behavior of an LS in Send Receive mode is the default TRIP
operation specified throughout this document.
The Send Receive capability is a 4-octet unsigned numeric value. It
can only take one of the following three values:
1 - Send Receive mode
2 - Send only mode
3 - Receive Only mode
A peering session MUST NOT be established between two LSs if both of
them are in Send Only mode or if both of them are in Receive Only
mode. If a peer LS detects such a capability mismatch when
processing an OPEN message, it MUST respond with a NOTIFICATION
message and close the peer session. The error code in the
NOTIFICATION message must be set to "Capability Mismatch."
An LS MUST be configured in the same Send Receive mode for all peers.
UPDATE messages are used to transfer routing information between LSs.
The information in the UPDATE packet can be used to construct a graph
describing the relationships between the various ITADs. By applying
rules to be discussed, routing information loops and some other
anomalies can be prevented.
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RFC 3219 Telephony Routing over IP (TRIP) January 2002
An UPDATE message is used to both advertise and withdraw routes from
service. An UPDATE message may simultaneously advertise and withdraw
TRIP routes.
In addition to the TRIP header, the TRIP UPDATE contains a list of
routing attributes as shown in Figure 7. There is no padding between
routing attributes.
+------------------------------------------------+--...
| First Route Attribute | Second Route Attribute | ...
+------------------------------------------------+--...
Figure 7: TRIP UPDATE Format
The minimum length of an UPDATE message is 3 octets (there are no
mandatory attributes in TRIP).
A variable length sequence of routing attributes is present in every
UPDATE message. Each attribute is a triple <attribute type,
attribute length, attribute value> of variable length.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+--------------+----------------+
| Attr. Flags |Attr. Type Code| Attr. Length |
+---------------+---------------+--------------+----------------+
| Attribute Value (variable) |
+---------------+---------------+--------------+----------------+
Figure 8: Routing Attribute Format
Attribute Type is a two-octet field that consists of the Attribute
Flags octet followed by the Attribute Type Code octet.
The Attribute Type Code defines the type of attribute. The basic
TRIP-defined Attribute Type Codes are discussed later in this
section. Attributes MUST appear in the UPDATE message in numerical
order of the Attribute Type Code. An attribute MUST NOT be included
more than once in the same UPDATE message. Attribute Flags are used
to control attribute processing when the attribute type is unknown.
Attribute Flags are further defined in Section 4.3.2.
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This document reserves Attribute Type Codes 224-255 for vendor-
specific applications (these are the codes with the first three bits
of the code equal to 1). This document reserves value 0. Attribute
Type Codes (other than those reserved for vendor specific use) are
controlled by IANA. See Section 13 for IANA considerations.
The third and the fourth octets of the route attribute contain the
length of the attribute value field in octets.
The remaining octets of the attribute represent the Attribute Value
and are interpreted according to the Attribute Flags and the
Attribute Type Code. The basic supported attribute types, their
values, and their uses are defined in this specification. These are
the attributes necessary for proper loop free operation of TRIP, both
inter-domain and intra-domain. Additional attributes may be defined
in future documents.
It is clear that the set of attributes for TRIP will evolve over
time. Hence it is essential that mechanisms be provided to handle
attributes with unrecognized types. The handling of unrecognized
attributes is controlled via the flags field of the attribute.
Recognized attributes should be processed according to their specific
definition.
The following are the attribute flags defined by this specification:
Bit Flag
0 Well-Known Flag
1 Transitive Flag
2 Dependent Flag
3 Partial Flag
4 Link-state Encapsulated Flag
The high-order bit (bit 0) of the Attribute Flags octet is the Well-
Known Bit. It defines whether the attribute is not well-known (if
set to 1) or well-known (if set to 0). Implementations are not
required to support not well-known attributes, but MUST support
well-known attributes.
The second high-order bit (bit 1) of the Attribute Flags octet is the
Transitive bit. It defines whether a not well-known attribute is
transitive (if set to 1) or non-transitive (if set to 0). For well-
known attributes, the Transitive bit MUST be zero on transmit and
MUST be ignored on receipt.
The third high-order bit (bit 2) of the Attribute Flags octet is the
Dependent bit. It defines whether a transitive attribute is
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dependent (if set to 1) or independent (if set to 0). For well-known
attributes and for non-transitive attributes, the Dependent bit is
irrelevant, and MUST be set to zero on transmit and MUST be ignored
on receipt.
The fourth high-order bit (bit 3) of the Attribute Flags octet is the
Partial bit. It defines whether the information contained in the not
well-known transitive attribute is partial (if set to 1) or complete
(if set to 0). For well-known attributes and for non-transitive
attributes the Partial bit MUST be set to 0 on transmit and MUST be
ignored on receipt.
The fifth high-order bit (bit 4) of the Attribute Flags octet is the
Link-state Encapsulation bit. This bit is only applicable to certain
attributes (ReachableRoutes and WithdrawnRoutes) and determines the
encapsulation of the routes within those attributes. If this bit is
set, link-state encapsulation is used within the attribute.
Otherwise, standard encapsulation is used within the attribute. The
Link-state Encapsulation technique is described in Section 4.3.2.4.
This flag is only valid on the ReachableRoutes and WithdrawnRoutes
attributes. It MUST be cleared on transmit and MUST be ignored on
receipt for all other attributes.
The other bits of the Attribute Flags octet are unused. They MUST be
zeroed on transmit and ignored on receipt.
TRIP provides for two variations of transitivity due to the fact that
intermediate LSs need not modify the NextHopServer when propagating
routes. Attributes may be non-transitive, dependent transitive, or
independent transitive. An attribute cannot be both dependent
transitive and independent transitive.
Unrecognized independent transitive attributes may be propagated by
any intermediate LS. Unrecognized dependent transitive attributes
MAY only be propagated if the LS is NOT changing the next-hop server.
The transitivity variations permit some unrecognized attributes to be
carried end-to-end (independent transitive), some to be carried
between adjacent next-hop servers (dependent transitive), and other
to be restricted to peer LSs (non-transitive).
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An LS that passes an unrecognized transitive attribute to a peer MUST
set the Partial flag on that attribute. Any LS along a path MAY
insert a transitive attribute into a route. If any LS except the
originating LS inserts a new independent transitive attribute into a
route, then it MUST set the Partial flag on that attribute. If any
LS except an LS that modifies the NextHopServer inserts a new
dependent transitive attribute into a route, then it MUST set the
Partial flag on that attribute. The Partial flag indicates that not
every LS along the relevant path has processed and understood the
attribute. For independent transitive attributes, the "relevant
path" is the path given in the AdvertisementPath attribute. For
dependent transitive attributes, the relevant path consists only of
those domains thru which this object has passed since the
NextHopServer was last modified. The Partial flag in an independent
transitive attribute MUST NOT be unset by any other LS along the
path. The Partial flag in a dependent transitive attribute MUST be
reset whenever the NextHopServer is changed, but MUST NOT be unset by
any LS that is not changing the NextHopServer.
The rules governing the addition of new non-transitive attributes are
defined independently for each non-transitive attribute. Any
attribute MAY be updated by an LS in the path.
Each attribute defines how it is to be handled during route
aggregation.
The rules governing the handling of unknown attributes are guided by
the Attribute Flags. Unrecognized transitive attributes are dropped
during aggregation. There should be no unrecognized non-transitive
attributes during aggregation because non-transitive attributes must
be processed by the local LS in order to be propagated.
Normally attributes have the simple format as described in Section
4.3.1. If the Link-state Encapsulation Flag is set, then the two
additional fields are added to the attribute header as shown in
Figure 9.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+--------------+----------------+
| Attr. Flags |Attr. Type Code| Attr. Length |
+---------------+---------------+--------------+----------------+
| Originator TRIP Identifier |
+---------------+---------------+--------------+----------------+
| Sequence Number |
+---------------+---------------+--------------+----------------+
| Attribute Value (variable) |
+---------------+---------------+--------------+----------------+
Figure 9: Link State Encapsulation
The Originator TRIP ID and Sequence Number are used to control the
flooding of routing updates within a collection of servers. These
fields are used to detect duplicate and old routes so that they are
not further propagated to other LSs. The use of these fields is
defined in Section 10.1.
There are no Mandatory attributes in TRIP. However, there are
Conditional Mandatory attributes. A conditional mandatory attribute
is an attribute, which MUST be included in an UPDATE message if
another attribute is included in that message. For example, if an
UPDATE message includes a ReachableRoutes attribute, it MUST include
an AdvertisementPath attribute as well.
The three base attributes in TRIP are WithdrawnRoutes,
ReachableRoutes, and ITAD Topology. Their presence in an UPDATE
message is entirely optional and independent of any other attributes.
This section summarizes the attributes that may be carried in an
UPDATE message. Attributes MUST appear in the UPDATE message in
increasing order of the Attribute Type Code. Additional details are
provided in Section 5.
This attribute lists a set of routes that are being withdrawn from
service. The transmitting LS has determined that these routes should
no longer be advertised, and is propagating this information to its
peers.
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This attribute lists a set of routes that are being added to service.
These routes will have the potential to be inserted into the Adj-
TRIBs-In of the receiving LS and the route selection process will be
applied to them.
This attribute gives the identity of the entity to which messages
should be sent along this routed path. It specifies the identity of
the next hop server as either a host domain name or an IP address.
It MAY optionally specify the UDP/TCP port number for the next hop
signaling server. If not specified, then the default port SHOULD be
used. The NextHopServer is specific to the set of destinations and
application protocol defined in the ReachableRoutes attribute. Note
that this is NOT necessarily the address to which media (voice,
video, etc.) should be transmitted, it is only for the application
protocol as given in the ReachableRoutes attribute.
The AdvertisementPath is analogous to the AS_PATH in BGP4 [3]. The
attribute records the sequence of domains through which this
advertisement has passed. The attribute is used to detect when the
routing advertisement is looping. This attribute does NOT reflect
the path through which messages following this route would traverse.
Since the next-hop need not be modified by each LS, the actual path
to the destination might not have to traverse every domain in the
AdvertisementPath.
The RoutedPath attribute is analogous to the AdvertisementPath
attribute, except that it records the actual path (given by the list
of domains) *to* the destinations. Unlike AdvertisementPath, which
is modified each time the route is propagated, RoutedPath is only
modified when the NextHopServer attribute changes. Thus, it records
the subset of the AdvertisementPath which signaling messages
following this particular route would traverse.
The AtomicAggregate attribute indicates that a route may actually
include domains not listed in the RoutedPath. If an LS, when
presented with a set of overlapping routes from a peer LS, selects a
less specific route without selecting the more specific route, then
the LS MUST include the AtomicAggregate attribute with the route. An
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LS receiving a route with an AtomicAggregate attribute MUST NOT make
the set of destinations more specific when advertising it to other
LSs.
The LocalPreference attribute is an intra-domain attribute used to
inform other LSs of the local LS's preference for a given route. The
preference of a route is calculated at the ingress to a domain and
passed as an attribute with that route throughout the domain. Other
LSs within the same ITAD use this attribute in their route selection
process. This attribute has no significance between domains.
There may be more than one LS peering relationship between
neighboring domains. The MultiExitDisc attribute is used by an LS to
express a preference for one link between the domains over another
link between the domains. The use of the MultiExitDisc attribute is
controlled by local policy.
The Communities attribute is not a well-known attribute. It is used
to facilitate and simplify the control of routing information by
grouping destinations into communities.
TRIP does not use any transport-based keep-alive mechanism to
determine if peers are reachable. Instead, KEEPALIVE messages are
exchanged between peers often enough as not to cause the Hold Timer
to expire. A reasonable maximum time between KEEPALIVE messages
would be one third of the Hold Time interval. KEEPALIVE messages
MUST NOT be sent more than once every 3 seconds. An implementation
SHOULD adjust the rate at which it sends KEEPALIVE messages as a
function of the negotiated Hold Time interval.
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If the negotiated Hold Time interval is zero, then periodic KEEPALIVE
messages MUST NOT be sent.
The KEEPALIVE message consists of only a message header and has a
length of 3 octets.
A NOTIFICATION message is sent when an error condition is detected.
The TRIP transport connection is closed immediately after sending a
NOTIFICATION message.
In addition to the fixed-size TRIP header, the NOTIFICATION message
contains the following fields:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+--------------+----------------+
| Error Code | Error Subcode | Data... (variable)
+---------------+---------------+--------------+----------------+
Figure 10: TRIP NOTIFICATION Format
Error Code:
This 1-octet unsigned integer indicates the type of NOTIFICATION.
The following Error Codes have been defined:
Error Code Symbolic Name Reference
1 Message Header Error Section 6.1
2 OPEN Message Error Section 6.2
3 UPDATE Message Error Section 6.3
4 Hold Timer Expired Section 6.5
5 Finite State Machine Error Section 6.6
6 Cease Section 6.7
Error Subcode:
This 1-octet unsigned integer provides more specific information
about the nature of the reported error. Each Error Code may have one
or more Error Subcodes associated with it. If no appropriate Error
Subcode is defined, then a zero (Unspecific) value is used for the
Error Subcode field.
Message Header Error Subcodes:
1 - Bad Message Length.
2 - Bad Message Type.
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OPEN Message Error Subcodes:
1 - Unsupported Version Number.
2 - Bad Peer ITAD.
3 - Bad TRIP Identifier.
4 - Unsupported Optional Parameter.
5 - Unacceptable Hold Time.
6 - Unsupported Capability.
7 - Capability Mismatch.
UPDATE Message Error Subcodes:
1 - Malformed Attribute List.
2 - Unrecognized Well-known Attribute.
3 - Missing Well-known Mandatory Attribute.
4 - Attribute Flags Error.
5 - Attribute Length Error.
6 - Invalid Attribute.
Data:
This variable-length field is used to diagnose the reason for the
NOTIFICATION. The contents of the Data field depend upon the Error
Code and Error Subcode.
Note that the length of the data can be determined from the message
length field by the formula:
Data Length = Message Length - 5
The minimum length of the NOTIFICATION message is 5 octets (including
message header).
Conditional Mandatory: False.
Required Flags: Well-known.
Potential Flags: Link-State Encapsulation (when flooding).
TRIP Type Code: 1
The WithdrawnRoutes specifies a set of routes that are to be removed
from service by the receiving LS(s). The set of routes MAY be empty,
indicated by a length field of zero.
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The WithdrawnRoutes Attribute encodes a sequence of routes in its
value field. The format for individual routes is given in Section
5.1.1.1. The WithdrawnRoutes Attribute lists the individual routes
sequentially with no padding as shown in Figure 11. Each route
includes a length field so that the individual routes within the
attribute can be delineated.
+---------------------+---------------------+...
| WithdrawnRoute1... | WithdrawnRoute2... |...
+---------------------+---------------------+...
Figure 11: WithdrawnRoutes Format
The generic format for a TRIP route is given in Figure 12.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+--------------+----------------+
| Address Family | Application Protocol |
+---------------+---------------+--------------+----------------+
| Length | Address (variable) ...
+---------------+---------------+--------------+----------------+
Figure 12: Generic TRIP Route Format
Address Family:
The address family field gives the type of address for the route.
Three address families are defined in this Section:
Code Address Family
1 Decimal Routing Numbers
2 PentaDecimal Routing Numbers
3 E.164 Numbers
This document reserves address family code 0. This document reserves
address family codes 32768-65535 for vendor-specific applications
(these are the codes with the first bit of the code value equal to
1). Additional address families may be defined in the future.
Assignment of address family codes is controlled by IANA. See
Section 13 for IANA considerations.
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Application Protocol:
The application protocol gives the protocol for which this routing
table is maintained. The currently defined application protocols
are:
Code Protocol
1 SIP
2 H.323-H.225.0-Q.931
3 H.323-H.225.0-RAS
4 H.323-H.225.0-Annex-G
This document reserves application protocol code 0. This document
reserves application protocol codes 32768-65535 for vendor-specific
applications (these are the codes with the first bit of the code
value equal to 1). Additional application protocols may be defined
in the future. Assignment of application protocol codes is
controlled by IANA. See Section 13 for IANA considerations.
Length:
The length of the address field, in bytes.
Address:
This is an address (prefix) of the family type given by Address
Family. The octet length of the address is variable and is
determined by the length field of the route.
The Decimal Routing Numbers address family is a super set of all
E.164 numbers, national numbers, local numbers, and private numbers.
It can also be used to represent the decimal routing numbers used in
conjunction with Number Portability in some countries/regions. A set
of telephone numbers is specified by a Decimal Routing Number prefix.
Decimal Routing Number prefixes are represented by a string of
digits, each digit encoded by its ASCII character representation.
This routing object covers all phone numbers starting with this
prefix. The syntax for the Decimal Routing Number prefix is:
Decimal-routing-number = *decimal-digit
decimal-digit = DECIMAL-DIGIT
DECIMAL-DIGIT = "0"|"1"|"2"|"3"|"4"|"5"|"6"|"7"|"8"|"9"
This DECIMAL Routing Number prefix is not bound in length. This
format is similar to the format for a global telephone number as
defined in SIP [8] without visual separators and without the "+"
prefix for international numbers. This format facilitates efficient
comparison when using TRIP to route SIP or H323, both of which use
character based representations of phone numbers. The prefix length
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RFC 3219 Telephony Routing over IP (TRIP) January 2002
is determined from the length field of the route. The type of
Decimal Routing Number (private, local, national, or international)
can be deduced from the first few digits of the prefix.
This address family is used to represent PentaDecimal Routing Numbers
used in conjunction with Number Portability in some
countries/regions. PentaDecimal Routing Number prefixes are
represented by a string of digits, each digit encoded by its ASCII
character representation. This routing object covers all routing
numbers starting with this prefix. The syntax for the PentaDecimal
Routing Number prefix is:
PentaDecimal-routing-number = *pentadecimal-digit
pentadecimal-routing-digit = PENTADECIMAL-DIGIT
PENTADECIMAL-DIGIT = "0"|"1"|"2"|"3"|"4"|"5"|"6"|"7"|
"8"|"9"|"A"|"B"|"C"|"D"|"E"
Note the difference in alphabets between Decimal Routing Numbers and
PentaDecimal Routing Numbers. A PentaDecimal Routing Number prefix
is not bound in length.
Note that the address family, which suits the routing numbers of a
specific country/region depends on the alphabets used for routing
numbers in that country/region. For example, North American routing
numbers SHOULD use the Decimal Routing Numbers address family,
because their alphabet is limited to the digits "0" through "9".
Another example, in most European countries routing numbers use the
alphabet "0" through "9" and "A" through "E", and hence these
countries SHOULD use the PentaDecimal Routing Numbers address family.
The E.164 Numbers address family is dedicated to fully qualified
E.164 numbers. A set of telephone numbers is specified by a E.164
prefix. E.164 prefixes are represented by a string of digits, each
digit encoded by its ASCII character representation. This routing
object covers all phone numbers starting with this prefix. The
syntax for the E.164 prefix is:
E164-number = *e164-digit
E164-digit = E164-DIGIT
E164-DIGIT = "0"|"1"|"2"|"3"|"4"|"5"|"6"|"7"|"8"|"9"
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This format facilitates efficient comparison when using TRIP to route
SIP or H323, both of which use character based representations of
phone numbers. The prefix length is determined from the length field
of the route.
The E.164 Numbers address family and the Decimal Routing Numbers
address family have the same alphabet. The E.164 Numbers address
family SHOULD be used whenever possible. The Decimal Routing Numbers
address family can be used in case of private numbering plans or
applications that do not desire to advertise fully expanded, fully
qualified telephone numbers. If Decimal Routing Numbers are used to
advertise non-fully qualified prefixes, the prefixes may have to be
manipulated (e.g. expanded) at the boundary between ITADs. This adds
significant complexity to the ITAD-Border LS, because, it has to map
the prefixes from the format used in its own ITAD to the format used
in the peer ITAD.
Conditional Mandatory: False.
Required Flags: Well-known.
Potential Flags: Link-State Encapsulation (when flooding).
TRIP Type Code: 2
The ReachableRoutes attribute specifies a set of routes that are to
be added to service by the receiving LS(s). The set of routes MAY be
empty, as indicated by setting the length field to zero.
Routes are injected into TRIP by a method outside the scope of this
specification. Possible methods include a front-end protocol, an
intra-domain routing protocol, or static configuration.
To aggregate multiple routes, the set of ReachableRoutes to be
aggregated MUST combine to form a less specific set.
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There is no mechanism within TRIP to communicate that a particular
address prefix is not used and thus that these addresses could be
skipped during aggregation. LSs MAY use methods outside of TRIP to
learn of invalid prefixes that may be ignored during aggregation.
If an LS advertises an aggregated route, it MUST include the
AtomicAggregate attribute.
The ReachableRoutes attribute is recomputed at each LS except where
flooding is being used (e.g., within a domain). It is therefore
possible for an LS to change the Application Protocol field of a
route before advertising that route to an external peer.
If an LS changes the Application Protocol of a route it advertises,
it MUST include the ConvertedRoute attribute in the UPDATE message.
and PentaDecimal Routing Numbers
An LS that has routes to all valid numbers in a specific prefix
SHOULD advertise that prefix as the ReachableRoutes, even if there
are more specific prefixes that do not actually exist on the PSTN.
Generally, it takes 10 Decimal Routing/E.164 prefixes, or 15
PentaDecimal Routing prefixes, of length n to aggregate into a prefix
of length n-1. However, if an LS is aware that a prefix is an
invalid Decimal Routing/E.164 prefix, or PentaDecimal Routing prefix,
then the LS MAY aggregate by skipping this prefix. For example, if
the Decimal Routing prefix 19191 is known not to exist, then an LS
can aggregate to 1919 without 19191. A prefix representing an
invalid set of PSTN destinations is sometimes referred to as a
"black-hole." The method by which an LS is aware of black-holes is
not within the scope of TRIP, but if an LS has such knowledge, it can
use the knowledge when aggregating.
Conditional Mandatory: True (if ReachableRoutes and/or
WithdrawnRoutes attribute is present).
Required Flags: Well-known.
Potential Flags: None.
TRIP Type Code: 3.
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Given a route with application protocol A and destinations D, the
NextHopServer indicates to the next-hop that messages of protocol A
destined for D should be sent to it. This may or may not represent
the ultimate destination of those messages.
For generality, the address of the next-hop server may be of various
types (domain name, IPv4, IPv6, etc). The NextHopServer attribute
includes the ITAD number of next-hop server, a length field, and a
next-hop name or address.
The syntax for the NextHopServer is given in Figure 13.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+--------------+----------------+
| Next Hop ITAD |
+---------------+---------------+--------------+----------------+
| Length | Server (variable) ...
+---------------+---------------+--------------+----------------+
Figure 13: NextHopServer Syntax
The Next-Hop ITAD indicates the domain of the next-hop. Length field
gives the number of octets in the Server field, and the Server field
contains the name or address of the next-hop server. The server
field is represented as a string of ASCII characters. It is defined
as follows:
Server = host [":" port ]
host = < A legal Internet host domain name
or an IPv4 address using the textual representation
defined in Section 2.1 of RFC 1123 [9]
or an IPv6 address using the textual representation
defined in Section 2.2 of RFC 2373 [10]. The IPv6
address MUST be enclosed in "[" and "]"
characters.>
port = *DIGIT
If the port is empty or not given, the default port is assumed (e.g.,
port 5060 if the application protocol is SIP).
When an LS originates a routing object into TRIP, it MUST include a
NextHopServer within its domain. The NextHopServer could be an
address of the egress gateway or of a signaling proxy.
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When aggregating multiple routing objects into a single routing
object, an LS MUST insert a new signaling server from within its
domain as the new NextHopServer unless all of the routes being
aggregated have the same next-hop.
When propagating routing objects to peers, an LS may choose to insert
a signaling proxy within its domain as the new next-hop, or it may
leave the next-hop unchanged. Inserting a new next-hop will cause
the signaling messages to be sent to that address, and will provide
finer control over the signaling path. Leaving the next-hop
unchanged will yield a more efficient signaling path (fewer hops).
It is a local policy decision of the LS to decide whether to
propagate or change the NextHopServer.
Conditional Mandatory: True (if ReachableRoutes and/or
WithdrawnRoutes attribute is present).
Required Flags: Well-known.
Potential Flags: None.
TRIP Type Code: 4.
This attribute identifies the ITADs through which routing information
carried in an advertisement has passed. The AdvertisementPath
attribute is analogous to the AS_PATH attribute in BGP. The
attributes differ in that BGP's AS_PATH also reflects the path to the
destination. In TRIP, not every domain need modify the next-hop, so
the AdvertisementPath may include many more hops than the actual path
to the destination. The RoutedPath attribute (Section 5.5) reflects
the actual signaling path to the destination.
AdvertisementPath is a variable length attribute that is composed of
a sequence of ITAD path segments. Each ITAD path segment is
represented by a type-length-value triple.
The path segment type is a 1-octet long field with the following
values defined:
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RFC 3219 Telephony Routing over IP (TRIP) January 2002
Value Segment Type
1 AP_SET: unordered set of ITADs a route in the
advertisement message has traversed
2 AP_SEQUENCE: ordered set of ITADs a route in
the advertisement message has traversed
The path segment length is a 1-octet long field containing the number
of ITADs in the path segment value field.
The path segment value field contains one or more ITAD numbers, each
encoded as a 4-octets long field. ITAD numbers uniquely identify an
Internet Telephony Administrative Domain, and must be obtained from
IANA. See Section 13 for procedures to obtain an ITAD number from
IANA.
When an LS originates a route then:
- The originating LS shall include its own ITAD number in the
AdvertisementPath attribute of all advertisements sent to LSs
located in neighboring ITADs. In this case, the ITAD number of
the originating LS's ITAD will be the only entry in the
AdvertisementPath attribute.
- The originating LS shall include an empty AdvertisementPath
attribute in all advertisements sent to LSs located in its own
ITAD. An empty AdvertisementPath attribute is one whose length
field contains the value zero.
The AdvertisementPath may be used for route selection. Possible
criteria to be used are the number of hops on the path and the
presence or absence of particular ITADs on the path.
As discussed in Section 10, the AdvertisementPath is used to prevent
routing information from looping. If an LS receives a route with its
own ITAD already in the AdvertisementPath, the route MUST be
discarded.
The rules for aggregating AdvertisementPath attributes are given in
the following sections, where the term "path" used in Section 5.4.4.1
and 5.4.4.2 is understood to mean AdvertisementPath.
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For the purpose of aggregating path attributes we model each ITAD
within the path as a pair <type, value>, where "type" identifies a
type of the path segment (AP_SEQUENCE or AP_SET), and "value" is the
ITAD number. Two ITADs are said to be the same if their
corresponding <type, value> are the same.
If the routes to be aggregated have different path attributes, then
the aggregated path attribute shall satisfy all of the following
conditions:
- All pairs of the type AP_SEQUENCE in the aggregated path MUST
appear in all of the paths of routes to be aggregated.
- All pairs of the type AP_SET in the aggregated path MUST appear
in at least one of the paths of the initial set (they may
appear as either AP_SET or AP_SEQUENCE types).
- For any pair X of the type AP_SEQUENCE that precedes pair Y in
the aggregated path, X precedes Y in each path of the initial
set that contains Y, regardless of the type of Y.
- No pair with the same value shall appear more than once in the
aggregated path, regardless of the pair's type.
An implementation may choose any algorithm that conforms to these
rules. At a minimum, a conformant implementation MUST be able to
perform the following algorithm that meets all of the above
conditions:
- Determine the longest leading sequence of tuples (as defined
above) common to all the paths of the routes to be aggregated.
Make this sequence the leading sequence of the aggregated path.
- Set the type of the rest of the tuples from the paths of the
routes to be aggregated to AP_SET, and append them to the
aggregated path.
- If the aggregated path has more than one tuple with the same
value (regardless of tuple's type), eliminate all but one such
tuple by deleting tuples of the type AP_SET from the aggregated
path.
An implementation that chooses to provide a path aggregation
algorithm that retains significant amounts of path information may
wish to use the procedure of Section 5.4.4.3.
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An example algorithm to aggregate two paths works as follows:
- Identify the ITADs (as defined in Section 5.4.1) within each
path attribute that are in the same relative order within both
path attributes. Two ITADs, X and Y, are said to be in the
same order if either X precedes Y in both paths, or if Y
precedes X in both paths.
- The aggregated path consists of ITADs identified in (a) in
exactly the same order as they appear in the paths to be
aggregated. If two consecutive ITADs identified in (a) do not
immediately follow each other in both of the paths to be
aggregated, then the intervening ITADs (ITADs that are between
the two consecutive ITADs that are the same) in both attributes
are combined into an AP_SET path segment that consists of the
intervening ITADs from both paths; this segment is then placed
in between the two consecutive ITADs identified in (a) of the
aggregated attribute. If two consecutive ITADs identified in
(a) immediately follow each other in one attribute, but do not
follow in another, then the intervening ITADs of the latter are
combined into an AP_SET path segment; this segment is then
placed in between the two consecutive ITADs identified in (a)
of the aggregated path.
If as a result of the above procedure a given ITAD number appears
more than once within the aggregated path, all but the last instance
(rightmost occurrence) of that ITAD number should be removed from the
aggregated path.
When an LS propagates a route which it has learned from another LS,
it shall modify the route's AdvertisementPath attribute based on the
location of the LS to which the route will be sent.
- When a LS advertises a route to another LS located in its own
ITAD, the advertising LS MUST NOT modify the AdvertisementPath
attribute associated with the route.
- When a LS advertises a route to an LS located in a neighboring
ITAD, then the advertising LS MUST update the AdvertisementPath
attribute as follows:
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* If the first path segment of the AdvertisementPath is of
type AP_SEQUENCE, the local system shall prepend its own
ITAD number as the last element of the sequence (put it in
the leftmost position).
* If the first path segment of the AdvertisementPath is of
type AP_SET, the local system shall prepend a new path
segment of type AP_SEQUENCE to the AdvertisementPath,
including its own ITAD number in that segment.
Conditional Mandatory: True
(if ReachableRoutes attribute is present).
Required Flags: Well-known.
Potential Flags: None.
TRIP Type Code: 5.
This attribute identifies the ITADs through which messages sent using
this route would pass. The ITADs in this path are a subset of those
in the AdvertisementPath.
When an LS originates a route it MUST include the RoutedPath
attribute.
- The originating LS shall include its own ITAD number in the
RoutedPath attribute of all advertisements sent to LSs located
in neighboring ITADs. In this case, the ITAD number of the
originating LS's ITAD will be the only entry in the RoutedPath
attribute.
- The originating LS shall include an empty RoutedPath attribute
in all advertisements sent to LSs located in its own ITAD. An
empty RoutedPath attribute is one whose length field contains
the value zero.
The RoutedPath MAY be used for route selection, and in most cases is
preferred over the AdvertisementPath for this role. Some possible
criteria to be used are the number of hops on the path and the
presence or absence of particular ITADs on the path.
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The rules for aggregating RoutedPath attributes are given in Section
5.4.4.1 and 5.4.4.2, where the term "path" used in Section 5.4.4.1
and 5.4.4.2 is understood to mean RoutedPath.
When an LS propagates a route that it learned from another LS, it
modifies the route's RoutedPath attribute based on the location of
the LS to which the route is sent.
- When an LS advertises a route to another LS located in its own
ITAD, the advertising LS MUST NOT modify the RoutedPath
attribute associated with the route.
- If the LS has not changed the NextHopServer attribute, then the
LS MUST NOT change the RoutedPath attribute.
- Otherwise, the LS changed the NextHopServer and is advertising
the route to an LS in another ITAD. The advertising LS MUST
update the RoutedPath attribute as follows:
* If the first path segment of the RoutedPath is of type
AP_SEQUENCE, the local system shall prepend its own ITAD
number as the last element of the sequence (put it in the
leftmost position).
* If the first path segment of the RoutedPath is of type
AP_SET, the local system shall prepend a new path segment of
type AP_SEQUENCE to the RoutedPath, including its own ITAD
number in that segment.
Conditional Mandatory: False.
Required Flags: Well-known.
Potential Flags: None.
TRIP Type Code: 6.
The AtomicAggregate attribute indicates that a route may traverse
domains not listed in the RoutedPath. If an LS, when presented with
a set of overlapping routes from a peer LS, selects the less specific
route without selecting the more specific route, then the LS includes
the AtomicAggregate attribute with the routing object.
This attribute has length zero (0); the value field is empty.
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If an LS, when presented with a set of overlapping routes from a peer
LS, selects the less specific route (see Section 0) without selecting
the more specific route, then the LS MUST include the AtomicAggregate
attribute with the routing object (if it is not already present).
An LS receiving a routing object with an AtomicAggregate attribute
MUST NOT make the set of destinations more specific when advertising
it to other LSs, and MUST NOT remove the attribute when propagating
this object to a peer LS.
Conditional Mandatory: False.
Required Flags: Well-known.
Potential Flags: None.
TRIP Type Code: 7.
The LocalPreference attribute is only used intra-domain, it indicates
the local LS's preference for the routing object to other LSs within
the same domain. This attribute MUST NOT be included when
communicating to an LS in another domain, and MUST be included over
intra-domain links.
The LocalPreference attribute is a 4-octet unsigned numeric value. A
higher value indicates a higher preference.
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Routes MUST NOT be originated with the LocalPreference attribute to
inter-domain peers. Routes to intra-domain peers MUST be originated
with the LocalPreference attribute.
The LocalPreference attribute allows one LS in a domain to calculate
a preference for a route, and to communicate this preference to other
LSs within the domain.
An LS MUST include the LocalPreference attribute when communicating
with peer LSs within its own domain. An LS MUST NOT include the
LocalPreference attribute when communicating with LSs in other
domains. LocalPreference attributes received from inter-domain peers
MUST be ignored.
Conditional Mandatory: False.
Required Flags: Well-known.
Potential Flags: None.
TRIP Type Code: 8.
When two ITADs are connected by more than one set of peers, the
MultiExitDisc attribute may be used to specify preferences for routes
received over one of those links versus routes received over other
links. The MultiExitDisc parameter is used only for route selection.
Routes originated to intra-domain peers MUST NOT be originated with
the MultiExitDisc attribute. When originating a route to an inter-
domain peer, the MultiExitDisc attribute may be included.
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The MultiExitDisc attribute is used to express a preference when
there are multiple links between two domains. If all other factors
are equal, then a route with a higher MultiExitDisc attribute is
preferred over a route with a lower MultiExitDisc attribute.
Routes with differing MultiExitDisc parameters MUST NOT be
aggregated. Routes with the same value in the MultiExitDisc
attribute MAY be aggregated and the same MultiExitDisc attribute
attached to the aggregated object.
If received from a peer LS in another domain, an LS MAY propagate the
MultiExitDisc to other LSs within its domain. The MultiExitDisc
attribute MUST NOT be propagated to LSs in other domains.
An LS may add the MultiExitDisc attribute when propagating routing
objects to an LS in another domain. The inclusion of the
MultiExitDisc attribute is a matter of policy, as is the value of the
attribute.
Conditional Mandatory: False.
Required Flags: Not Well-Known, Independent Transitive.
Potential Flags: None.
TRIP Type Code: 9.
A community is a group of destinations that share some common
property.
The Communities attribute is used to group destinations so that the
routing decision can be based on the identity of the group. Using
the Communities attribute should significantly simplify the
distribution of routing information by providing an administratively
defined aggregation unit.
Each ITAD administrator may define the communities to which a
particular route belongs. By default, all routes belong to the
general Internet Telephony community.
As an example, the Communities attribute could be used to define an
alliance between a group of Internet Telephony service providers for
a specific subset of routing information. In this case, members of
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that alliance would accept only routes for destinations in this group
that are advertised by other members of the alliance. Other
destinations would be more freely accepted. To achieve this, a
member would tag each route with a designated Community attribute
value before disseminating it. This relieves the members of such an
alliance, from the responsibility of keeping track of the identities
of all other members of that alliance.
Another example use of the Communities attribute is with aggregation.
It is often useful to advertise both the aggregate route and the
component more-specific routes that were used to form the aggregate.
These information components are only useful to the neighboring TRIP
peer, and perhaps the ITAD of the neighboring TRIP peer, so it is
desirable to filter out the component routes. This can be achieved
by specifying a Community attribute value that the neighboring peers
will match and filter on. That way it can be assured that the more
specific routes will not propagate beyond their desired scope.
The Communities attribute is of variable length. It consists of a
set of 8-octet values, each of which specifies a community. The
first 4 octets of the Community value are the Community ITAD Number
and the next 4 octets are the Community ID.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+--------------+----------------+
| Community ITAD Number 1 |
+---------------+---------------+--------------+----------------+
| Community ID 1 |
+---------------+---------------+--------------+----------------+
| . . . . . . . . .
+---------------+---------------+--------------+----------------+
Figure 14: Communities Syntax
For administrative assignment, the following assumptions may be made:
The Community attribute values starting with a Community ITAD
Number of 0x00000000 are hereby reserved.
The following communities have global significance and their
operation MUST be implemented in any Community attribute-aware TRIP
LS.
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- NO_EXPORT (Community ITAD Number = 0x00000000 and Community ID
= 0xFFFFFF01). Any received route with a community attribute
containing this value MUST NOT be advertised outside of the
receiving TRIP ITAD.
Other community values MUST be encoded using an ITAD number in the
four most significant octets. The semantics of the final four octets
(the Community ID octets) may be defined by the ITAD (e.g., ITAD 690
may define research, educational, and commercial community IDs that
may be used for policy routing as defined by the operators of that
ITAD).
The Communities attribute is not well-known. If a route has a
Communities attribute associated with it, the LS MUST include that
attribute in the advertisement it originates.
The Communities attribute may be used for route selection. A route
that is a member of a certain community may be preferred over another
route that is not a member of that community. Likewise, routes
without a certain community value may be excluded from consideration.
If a set of routes is to be aggregated and the resultant aggregate
does not carry an Atomic_Aggregate attribute, then the resulting
aggregate should have a Communities attribute that contains the union
of the Community attributes of the aggregated routes.
An LS may manipulate the Communities attribute before disseminating a
route to a peer. Community attribute manipulation may include adding
communities, removing communities, adding a Communities attribute (if
none exists), deleting the Communities attribute, etc.
Conditional Mandatory: False.
Required Flags: Well-known, Link-State encapsulated.
Potential Flags: None.
TRIP Type Code: 10.
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Within an ITAD, each LS must know the status of other LSs so that LS
failure can be detected. To do this, each LS advertises its internal
topology to other LSs within the domain. When an LS detects that
another LS is no longer active, the information sourced by that LS
can be deleted (the Adj-TRIB-In for that peer may be cleared). The
ITAD Topology attribute is used to communicate this information to
other LSs within the domain.
An LS MUST send a topology update each time it detects a change in
its internal peer set. The topology update may be sent in an UPDATE
message by itself or it may be piggybacked on an UPDATE message which
includes ReachableRoutes and/or WithdrawnRoutes information.
When an LS receives a topology update from an internal LS, it MUST
recalculate which LSs are active within the ITAD via a connectivity
algorithm on the topology.
The ITAD Topology attribute indicates the LSs with which the LS is
currently peering. The attribute consists of a list of the TRIP
Identifiers with which the LS is currently peering, the format is
given in Figure 15. This attribute MUST use the link-state
encapsulation as defined in Section 4.3.2.4.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+--------------+----------------+
| TRIP Identifier 1 |
+---------------+---------------+--------------+----------------+
| TRIP Identifier 2 ... |
+---------------+---------------+--------------+----------------+
Figure 15: ITAD Topology Syntax
The ITAD Topology attribute is independent of any routes in the
UPDATE. Whenever the set of internal peers of an LS changes, it MUST
create an UPDATE with the ITAD Topology Attribute included listing
the current set of internal peers. The LS MUST include this
attribute in the first UPDATE it sends to a peer after the peering
session is established.
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This attribute is independent of any routing information in the
UPDATE. When an LS receives an UPDATE with an ITAD Topology
attribute, it MUST compute the set of LSs currently active in the
domain by performing a connectivity test on the ITAD topology as
given by the set of originated ITAD Topology attributes. The LS MUST
locally purge the Adj-TRIB-In for any LS that is no longer active in
the domain. The LS MUST NOT propagate this purging information to
other LSs as they will make a similar decision.
Conditional Mandatory: False.
Required Flags: Well-known.
Potential Flags: None.
TRIP Type Code: 12.
The ConvertedRoute attribute indicates that an intermediate LS has
altered the route by changing the route's Application Protocol. For
example, if an LS receives a route with Application Protocol X and
changes the Application Protocol to Y before advertising the route to
an external peer, the LS MUST include the ConvertedRoute attribute.
The attribute is an indication that the advertised application
protocol will not be used end-to-end, i.e., the information
advertised about this route is not complete.
Routes are never originated with the ConvertedRoute attribute.
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If an LS changes the Application Protocol of a route before
advertising the route to an external peer, the LS MUST include the
ConvertedRoute attribute.
Any proposal for defining new TRIP attributes should specify the
following:
- the use of this attribute,
- the attribute's flags,
- the attribute's syntax,
- how the attribute works with route origination,
- how the attribute works with route aggregation, and
- how the attribute works with route dissemination and the
attribute's scope (e.g., intra-domain only like
LocalPreference)
IANA will manage the assignment of TRIP attribute type codes to new
attributes.
This section describes errors to be detected and the actions to be
taken while processing TRIP messages.
When any of the conditions described here are detected, a
NOTIFICATION message with the indicated Error Code, Error Subcode,
and Data fields MUST be sent, and the TRIP connection MUST be closed.
If no Error Subcode is specified, then a zero Subcode MUST be used.
The phrase "the TRIP connection is closed" means that the transport
protocol connection has been closed and that all resources for that
TRIP connection have been de-allocated. If the connection was
inter-domain, then routing table entries associated with the remote
peer MUST be marked as invalid. Routing table entries MUST NOT be
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RFC 3219 Telephony Routing over IP (TRIP) January 2002
marked as invalid if an internal peering session is terminated. The
fact that the routes have been marked as invalid is passed to other
TRIP peers before the routes are deleted from the system.
Unless specified explicitly, the Data field of the NOTIFICATION
message that is sent to indicate an error MUST be empty.
All errors detected while processing the Message Header are indicated
by sending the NOTIFICATION message with the Error Code Message
Header Error. The Error Subcode elaborates on the specific nature of
the error. The error checks in this section MUST be performed by
each LS upon receipt of every message.
If the Length field of the message header is less than 3 or greater
than 4096, or if the Length field of an OPEN message is less than the
minimum length of the OPEN message, or if the Length field of an
UPDATE message is less than the minimum length of the UPDATE message,
or if the Length field of a KEEPALIVE message is not equal to 3, or
if the Length field of a NOTIFICATION message is less than the
minimum length of the NOTIFICATION message, then the Error Subcode
MUST be set to Bad Message Length. The Data field contains the
erroneous Length field.
If the Type field of the message header is not recognized, then the
Error Subcode MUST be set to "Bad Message Type." The Data field
contains the erroneous Type field.
All errors detected while processing the OPEN message are indicated
by sending the NOTIFICATION message with the Error Code "OPEN Message
Error." The Error Subcode elaborates on the specific nature of the
error. The error checks in this section MUST be performed by each LS
upon receipt of every OPEN message.
If the version number contained in the Version field of the received
OPEN message is not supported, then the Error Subcode MUST be set to
"Unsupported Version Number." The Data field is a 1-octet unsigned
integer, which indicates the largest locally supported version
number, which is less than the version of the remote TRIP peer bid
(as indicated in the received OPEN message).
If the ITAD field of the OPEN message is unacceptable, then the Error
Subcode MUST be set to "Bad Peer ITAD." The determination of
acceptable ITAD numbers is outside the scope of this protocol.
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If the Hold Time field of the OPEN message is unacceptable, then the
Error Subcode MUST be set to "Unacceptable Hold Time." An
implementation MUST reject Hold Time values of one or two seconds.
An implementation MAY reject any proposed Hold Time. An
implementation that accepts a Hold Time MUST use the negotiated value
for the Hold Time.
If the TRIP Identifier field of the OPEN message is not valid, then
the Error Subcode MUST be set to "Bad TRIP Identifier." A TRIP
identifier is 4-octets in length and can take any value. An LS
considers the TRIP Identifier invalid if it already has an open
connection with another peer LS that has the same ITAD and TRIP
Identifier.
Any two LSs within the same ITAD MUST NOT have equal TRIP Identifier
values. This restriction does not apply to LSs in different ITADs
since the purpose is to uniquely identify an LS using its TRIP
Identifier and its ITAD number.
If one of the Optional Parameters in the OPEN message is not
recognized, then the Error Subcode MUST be set to "Unsupported
Optional Parameters."
If the Optional Parameters of the OPEN message include Capability
Information with an unsupported capability (unsupported in either
capability type or value), then the Error Subcode MUST be set to
"Unsupported Capability," and the entirety of the unsupported
capabilities MUST be listed in the Data field of the NOTIFICATION
message.
If the Optional Parameters of the OPEN message include Capability
Information which does not match the receiving LS's capabilities,
then the Error Subcode MUST be set to "Capability Mismatch," and the
entirety of the mismatched capabilities MUST be listed in the Data
field of the NOTIFICATION message.
All errors detected while processing the UPDATE message are indicated
by sending the NOTIFICATION message with the Error Code "UPDATE
Message Error." The Error Subcode elaborates on the specific nature
of the error. The error checks in this section MUST be performed by
each LS upon receipt of every UPDATE message. These error checks
MUST occur before flooding procedures are invoked with internal
peers.
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If any recognized attribute has Attribute Flags that conflict with
the Attribute Type Code, then the Error Subcode MUST be set to
"Attribute Flags Error." The Data field contains the erroneous
attribute (type, length and value).
If any recognized attribute has an Attribute Length that conflicts
with the expected length (based on the attribute type code), then the
Error Subcode MUST be set to "Attribute Length Error." The Data
field contains the erroneous attribute (type, length and value).
If any of the mandatory (i.e., conditional mandatory attribute and
the conditions for including it in the UPDATE message are fulfilled)
well-known attributes are not present, then the Error Subcode MUST be
set to "Missing Well-known Mandatory Attribute." The Data field
contains the Attribute Type Code of the missing well-known
conditional mandatory attributes.
If any of the well-known attributes are not recognized, then the
Error Subcode MUST be set to "Unrecognized Well-known Attribute."
The Data field contains the unrecognized attribute (type, length and
value).
If any attribute has a syntactically incorrect value, or an undefined
value, then the Error Subcode is set to "Invalid Attribute." The
Data field contains the incorrect attribute (type, length and value).
Such a NOTIFICATION message is sent, for example, when a
NextHopServer attribute is received with an invalid address.
The information carried by the AdvertisementPath attribute is checked
for ITAD loops. ITAD loop detection is done by scanning the full
AdvertisementPath, and checking that the ITAD number of the local
ITAD does not appear in the AdvertisementPath. If the local ITAD
number appears in the AdvertisementPath, then the route MAY be stored
in the Adj-TRIB-In. However unless the LS is configured to accept
routes with its own ITAD in the advertisement path, the route MUST
not be passed to the TRIP Decision Process. The operation of an LS
that is configured to accept routes with its own ITAD number in the
advertisement path are outside the scope of this document.
If the UPDATE message was received from an internal peer and either
the WithdrawnRoutes, ReachableRoutes, or ITAD Topology attribute does
not have the Link-State Encapsulation flag set, then the Error
Subcode is set to "Invalid Attribute" and the data field contains the
attribute. Likewise, the attribute is invalid if received from an
external peer and the Link-State Flag is set.
If any attribute appears more than once in the UPDATE message, then
the Error Subcode is set to "Malformed Attribute List."
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If a peer sends a NOTIFICATION message, and there is an error in that
message, there is unfortunately no means of reporting this error via
a subsequent NOTIFICATION message. Any such error, such as an
unrecognized Error Code or Error Subcode, should be noticed, logged
locally, and brought to the attention of the administration of the
peer. The means to do this, however, are outside the scope of this
document.
If a system does not receive successive messages within the period
specified by the negotiated Hold Time, then a NOTIFICATION message
with a "Hold Timer Expired" Error Code MUST be sent and the TRIP
connection MUST be closed.
An error detected by the TRIP Finite State Machine (e.g., receipt of
an unexpected event) MUST result in sending a NOTIFICATION message
with the Error Code "Finite State Machine Error" and the TRIP
connection MUST be closed.
In the absence of any fatal errors (that are indicated in this
section), a TRIP peer MAY choose at any given time to close its TRIP
connection by sending the NOTIFICATION message with the Error Code
"Cease." However, the Cease NOTIFICATION message MUST NOT be used
when a fatal error indicated by this section exists.
If a pair of LSs try simultaneously to establish a transport
connection to each other, then two parallel connections between this
pair of speakers might well be formed. We refer to this situation as
connection collision. Clearly, one of these connections must be
closed.
Based on the value of the TRIP Identifier, a convention is
established for detecting which TRIP connection is to be preserved
when a collision occurs. The convention is to compare the TRIP
Identifiers of the peers involved in the collision and to retain only
the connection initiated by the LS with the higher-valued TRIP
Identifier.
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Upon receipt of an OPEN message, the local LS MUST examine all of its
connections that are in the OpenConfirm state. An LS MAY also
examine connections in an OpenSent state if it knows the TRIP
Identifier of the peer by means outside of the protocol. If among
these connections there is a connection to a remote LS, whose TRIP
Identifier equals the one in the OPEN message, then the local LS MUST
perform the following collision resolution procedure:
The TRIP Identifier and ITAD of the local LS is compared to the TRIP
Identifier and ITAD of the remote LS (as specified in the OPEN
message). TRIP Identifiers are treated as 4-octet unsigned integers
for comparison.
If the value of the local TRIP Identifier is less than the remote
one, or if the two TRIP Identifiers are equal and the value of the
ITAD of the local LS is less than value of the ITAD of the remote LS,
then the local LS MUST close the TRIP connection that already exists
(the one that is already in the OpenConfirm state), and accept the
TRIP connection initiated by the remote LS:
1. Otherwise, the local LS closes the newly created TRIP
connection and continues to use the existing one (the one that
is already in the OpenConfirm state).
2. If a connection collision occurs with an existing TRIP
connection that is in the Established state, then the LS MUST
unconditionally close off the newly created connection. Note
that a connection collision cannot be detected with connections
in Idle, Connect, or Active states.
3. To close the TRIP connection (that results from the collision
resolution procedure), an LS MUST send a NOTIFICATION message
with the Error Code "Cease" and the TRIP connection MUST be
closed.
Peer LSs may negotiate the version of the protocol by making multiple
attempts to open a TRIP connection, starting with the highest version
number each supports. If an open attempt fails with an Error Code
"OPEN Message Error" and an Error Subcode "Unsupported Version
Number," then the LS has available the version number it tried, the
version number its peer tried, the version number passed by its peer
in the NOTIFICATION message, and the version numbers that it
supports. If the two peers support one or more common versions, then
this will allow them to rapidly determine the highest common version.
In order to support TRIP version negotiation, future versions of TRIP
must retain the format of the OPEN and NOTIFICATION messages.
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An LS MAY include the Capabilities Option in its OPEN message to a
peer to indicate the capabilities supported by the LS. An LS
receiving an OPEN message MUST NOT use any capabilities that were not
included in the OPEN message of the peer when communicating with that
peer.
This section specifies TRIP operation in terms of a Finite State
Machine (FSM). Following is a brief summary and overview of TRIP
operations by state as determined by this FSM. A condensed version
of the TRIP FSM is found in Appendix 1. There is one TRIP FSM per
peer and these FSMs operate independently.
Idle state:
Initially TRIP is in the Idle state for each peer. In this state,
TRIP refuses all incoming connections. No resources are allocated to
the peer. In response to the Start event (initiated by either the
system or the operator), the local system initializes all TRIP
resources, starts the ConnectRetry timer, initiates a transport
connection to the peer, starts listening for a connection that may be
initiated by the remote TRIP peer, and changes its state to Connect.
The exact value of the ConnectRetry timer is a local matter, but
should be sufficiently large to allow TCP initialization.
If an LS detects an error, it closes the transport connection and
changes its state to Idle. Transitioning from the Idle state
requires generation of the Start event. If such an event is
generated automatically, then persistent TRIP errors may result in
persistent flapping of the LS. To avoid such a condition, Start
events MUST NOT be generated immediately for a peer that was
previously transitioned to Idle due to an error. For a peer that was
previously transitioned to Idle due to an error, the time between
consecutive Start events, if such events are generated automatically,
MUST exponentially increase. The value of the initial timer SHOULD
be 60 seconds, and the time SHOULD be at least doubled for each
consecutive retry up to some maximum value.
Any other event received in the Idle state is ignored.
Connect State:
In this state, an LS is waiting for a transport protocol connection
to be completed to the peer, and is listening for inbound transport
connections from the peer.
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If the transport protocol connection succeeds, the local LS clears
the ConnectRetry timer, completes initialization, sends an OPEN
message to its peer, sets its Hold Timer to a large value, and
changes its state to OpenSent. A Hold Timer value of 4 minutes is
suggested.
If the transport protocol connect fails (e.g., retransmission
timeout), the local system restarts the ConnectRetry timer, continues
to listen for a connection that may be initiated by the remote LS,
and changes its state to Active state.
In response to the ConnectRetry timer expired event, the local LS
cancels any outstanding transport connection to the peer, restarts
the ConnectRetry timer, initiates a transport connection to the
remote LS, continues to listen for a connection that may be initiated
by the remote LS, and stays in the Connect state.
If the local LS detects that a remote peer is trying to establish a
connection to it and the IP address of the peer is not an expected
one, then the local LS rejects the attempted connection and continues
to listen for a connection from its expected peers without changing
state.
If an inbound transport protocol connection succeeds, the local LS
clears the ConnectRetry timer, completes initialization, sends an
OPEN message to its peer, sets its Hold Timer to a large value, and
changes its state to OpenSent. A Hold Timer value of 4 minutes is
suggested.
The Start event is ignored in the Connect state.
In response to any other event (initiated by either the system or the
operator), the local system releases all TRIP resources associated
with this connection and changes its state to Idle.
Active state:
In this state, an LS is listening for an inbound connection from the
peer, but is not in the process of initiating a connection to the
peer.
If an inbound transport protocol connection succeeds, the local LS
clears the ConnectRetry timer, completes initialization, sends an
OPEN message to its peer, sets its Hold Timer to a large value, and
changes its state to OpenSent. A Hold Timer value of 4 minutes is
suggested.
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In response to the ConnectRetry timer expired event, the local system
restarts the ConnectRetry timer, initiates a transport connection to
the TRIP peer, continues to listen for a connection that may be
initiated by the remote TRIP peer, and changes its state to Connect.
If the local LS detects that a remote peer is trying to establish a
connection to it and the IP address of the peer is not an expected
one, then the local LS rejects the attempted connection and continues
to listen for a connection from its expected peers without changing
state.
Start event is ignored in the Active state.
In response to any other event (initiated by either the system or the
operator), the local system releases all TRIP resources associated
with this connection and changes its state to Idle.
OpenSent state:
In this state, an LS has sent an OPEN message to its peer and is
waiting for an OPEN message from its peer. When an OPEN message is
received, all fields are checked for correctness. If the TRIP
message header checking or OPEN message checking detects an error
(see Section 6.2) or a connection collision (see Section 6.8), the
local system sends a NOTIFICATION message and changes its state to
Idle.
If there are no errors in the OPEN message, TRIP sends a KEEPALIVE
message and sets a KeepAlive timer. The Hold Timer, which was
originally set to a large value (see above), is replaced with the
negotiated Hold Time value (see Section 4.2). If the negotiated Hold
Time value is zero, then the Hold Time timer and KeepAlive timers are
not started. If the value of the ITAD field is the same as the local
ITAD number, then the connection is an "internal" connection;
otherwise, it is "external" (this will affect UPDATE processing).
Finally, the state is changed to OpenConfirm.
If the local LS detects that a remote peer is trying to establish a
connection to it and the IP address of the peer is not an expected
one, then the local LS rejects the attempted connection and continues
to listen for a connection from its expected peers without changing
state.
If a disconnect notification is received from the underlying
transport protocol, the local LS closes the transport connection,
restarts the ConnectRetry timer, continues to listen for a connection
that may be initiated by the remote TRIP peer, and goes into the
Active state.
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If the Hold Timer expires, the local LS sends a NOTIFICATION message
with the Error Code "Hold Timer Expired" and changes its state to
Idle.
In response to the Stop event (initiated by either system or
operator) the local LS sends a NOTIFICATION message with the Error
Code "Cease" and changes its state to Idle.
The Start event is ignored in the OpenSent state.
In response to any other event the local LS sends a NOTIFICATION
message with the Error Code "Finite State Machine Error" and changes
its state to Idle.
Whenever TRIP changes its state from OpenSent to Idle, it closes the
transport connection and releases all resources associated with that
connection.
OpenConfirm state:
In this state, an LS has sent an OPEN to its peer, received an OPEN
from its peer, and sent a KEEPALIVE in response to the OPEN. The LS
is now waiting for a KEEPALIVE or NOTIFICATION message in response to
its OPEN.
If the local LS receives a KEEPALIVE message, it changes its state to
Established.
If the Hold Timer expires before a KEEPALIVE message is received, the
local LS sends NOTIFICATION message with the Error Code "Hold Timer
Expired" and changes its state to Idle.
If the local LS receives a NOTIFICATION message, it changes its state
to Idle.
If the KeepAlive timer expires, the local LS sends a KEEPALIVE
message and restarts its KeepAlive timer.
If a disconnect notification is received from the underlying
transport protocol, the local LS closes the transport connection,
restarts the ConnectRetry timer, continues to listen for a connection
that may be initiated by the remote TRIP peer, and goes into the
Active state.
In response to the Stop event (initiated by either the system or the
operator) the local LS sends NOTIFICATION message with the Error Code
"Cease" and changes its state to Idle.
The Start event is ignored in the OpenConfirm state.
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In response to any other event the local LS sends a NOTIFICATION
message with the Error Code "Finite State Machine Error" and changes
its state to Idle.
Whenever TRIP changes its state from OpenConfirm to Idle, it closes
the transport connection and releases all resources associated with
that connection.
Established state:
In the Established state, an LS can exchange UPDATE, NOTIFICATION,
and KEEPALIVE messages with its peer.
If the negotiated Hold Timer is zero, then no procedures are
necessary for keeping a peering session alive. If the negotiated
Hold Time value is non-zero, the procedures of this paragraph apply.
If the Hold Timer expires, the local LS sends a NOTIFICATION message
with the Error Code "Hold Timer Expired" and changes its state to
Idle. If the KeepAlive Timer expires, then the local LS sends a
KeepAlive message and restarts the KeepAlive Timer. If the local LS
receives an UPDATE or KEEPALIVE message, then it restarts its Hold
Timer. Each time the LS sends an UPDATE or KEEPALIVE message, it
restarts its KeepAlive Timer.
If the local LS receives a NOTIFICATION message, it changes its state
to Idle.
If the local LS receives an UPDATE message and the UPDATE message
error handling procedure (see Section6.3) detects an error, the local
LS sends a NOTIFICATION message and changes its state to Idle.
If a disconnect notification is received from the underlying
transport protocol, the local LS changes its state to Idle.
In response to the Stop event (initiated by either the system or the
operator), the local LS sends a NOTIFICATION message with the Error
Code "Cease" and changes its state to Idle.
The Start event is ignored in the Established state.
In response to any other event, the local LS sends a NOTIFICATION
message with Error Code "Finite State Machine Error" and changes its
state to Idle.
Whenever TRIP changes its state from Established to Idle, it closes
the transport connection and releases all resources associated with
that connection. Additionally, if the peer is an external peer, the
LS deletes all routes derived from that connection.
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An UPDATE message may be received only in the Established state.
When an UPDATE message is received, each field is checked for
validity as specified in Section 6.3. The rest of this section
presumes that the UPDATE message has passed the error-checking
procedures of Section 6.3.
If the UPDATE message was received from an internal peer, the
flooding procedures of Section 10.1 MUST be applied. The flooding
process synchronizes the Loc-TRIBs of all LSs within the domain.
Certain routes within the UPDATE may be marked as old or duplicates
by the flooding process and are ignored during the rest of the UPDATE
processing.
If the UPDATE message contains withdrawn routes, then the
corresponding previously advertised routes shall be removed from the
Adj-TRIB-In. This LS MUST rerun its Decision Process since the
previously advertised route is no longer available for use.
If the UPDATE message contains a route, then the route MUST be placed
in the appropriate Adj-TRIB-In, and the following additional actions
MUST be taken:
1. If its destinations are identical to those of a route currently
stored in the Adj-TRIB-In, then the new route MUST replace the
older route in the Adj-TRIB-In, thus implicitly withdrawing the
older route from service. The LS MUST rerun its Decision
Process since the older route is no longer available for use.
2. If the new route is more specific than an earlier route
contained in the Adj-TRIB-In and has identical attributes, then
no further actions are necessary.
3. If the new route is more specific than an earlier route
contained in the Adj-TRIB-In but does not have identical
attributes, then the LS MUST run its Decision Process since the
more specific route has implicitly made a portion of the less
specific route unavailable for use.
4. If the new route has destinations that are not present in any
of the routes currently stored in the Adj-TRIB-In, then the LS
MUST run its Decision Process.
5. If the new route is less specific than an earlier route
contained in the Adj-TRIB-In, the LS MUST run its Decision
Process on the set of destinations that are described only by
the less specific route.
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When an LS receives an UPDATE message from an internal peer, the LS
floods the new information from that message to all of its other
internal peers. Flooding is used to efficiently synchronize all of
the LSs within a domain without putting any constraints on the
domain's internal topology. The flooding mechanism is based on the
techniques used in OSPF [4] and SCSP [6]. One may argue that TRIP's
flooding process is in reality a controlled broadcast mechanism.
The LS MUST maintain the sequence number and originating TRIP
identifier for each link-state encapsulated attribute in an internal
Adj-TRIB-In. These values are included with the route in the
ReachableRoutes, WithdrawnRoutes, and ITAD Topology attributes. The
originating TRIP identifier gives the internal LS that originated
this route into the ITAD, the sequence number gives the version of
this route at the originating LS.
For each route in the ReachableRoutes or WithdrawnRoutes field, the
LS decides if the route is new or old. This is determined by
comparing the Sequence Number of the route in the UPDATE with the
Sequence Number of the route saved in the Adj-TRIB-In. The route is
new if either the route does not exist in the Adj-TRIB-In for the
originating LS, or if the route does exist in the Adj-TRIB-In but the
Sequence Number in the UPDATE is greater than the Sequence Number
saved in the Adj-TRIBs-In. Note that the newness test is
independently applied to each link-state encapsulated attribute in
the UPDATE (WithdrawnRoutes or ReachableRoutes or ITAD Topology).
Each route in the ReachableRoutes or WithdrawnRoutes field that is
determined to be old is ignored in further processing. If the route
is determined to be new then the following actions occur.
If the route is being withdrawn, then the LS MUST flood the withdrawn
route to all other internal peers, and MUST mark the route as
withdrawn. An LS MUST maintain routes marked as withdrawn in its
databases for MaxPurgeTime seconds.
If the route is being updated, then the LS MUST update the route in
the Adj-TRIB-In and MUST flood it to all other internal peers.
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If these procedures result in changes to the Adj-TRIB-In, then the
route is also made available for local route processing as described
early in Section 10.
To implement flooding, the following is recommended. All routes
received in a single UPDATE message that are determined to be new
should be forwarded to all other internal peers in a single UPDATE
message. Other variations of flooding are possible, but the local LS
MUST ensure that each new route (and any associated attributes)
received from an internal peer get forwarded to every other internal
peer.
The Sequence Number is used to determine when one version of a Route
is newer than another version of a route. A larger Sequence Number
indicates a newer version. The Sequence Number is assigned by the LS
originating the route into the local ITAD. The Sequence Number is an
unsigned 4-octet integer in the range of 1 thru 2^31-1 MinSequenceNum
thru MaxSequenceNum). The value 0 is reserved. When an LS first
originates a route (including when the LS restarts/reboots) into its
ITAD, it MUST originate it with a Sequence Number of MinSequenceNum.
Each time the route is updated within the ITAD by the originator, the
Sequence Number MUST be increased.
If it is ever the case that the sequence number is MaxSequenceNum-1
and it needs to be increased, then the TRIP module of the LS MUST be
disabled for a period of TripDisableTime so that all routes
originated by this LS with high sequence numbers can be removed.
To withdraw a route that it originated within the ITAD, an LS
includes the route in the WithdrawnRoutes field of an UPDATE message.
The Sequence Number MUST be greater than the last valid version of
the route. The LS MAY choose to use a sequence number of
MaxSequenceNum when withdrawing routes within its ITAD, but this is
not required.
After withdrawing a route, an LS MUST mark the route as "withdrawn"
in its database, and maintain the withdrawn route in its database for
MaxPurgeTime seconds. If the LS needs to re-originate a route that
had been purged but is still in its database, it can either re-
originate the route immediately using a Sequence Number that is
greater than that used in the withdraw, or the LS may wait until
MaxPurgeTime seconds have expired since the route was withdrawn.
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It is common for an LS to receive UPDATES for routes that it
originated within the ITAD via the flooding procedure. If the LS
receives an UPDATE for a route that it originated that is newer (has
a higher sequence number) than the LSs current version, then special
actions must be taken. This should be a relatively rare occurrence
and indicates that a route still exists within the ITAD since the LSs
last restart/reboot.
If an LS receives a self-originated route update that is newer than
the current version of the route at the LS, then the following
actions MUST be taken. If the LS still wishes to advertise the
information in the route, then the LS MUST increase the Sequence
Number of the route to a value greater than that received in the
UPDATE and re-originate the route. If the LS does not wish to
continue to advertise the route, then it MUST purge the route as
described in Section 10.1.5.
An LS SHOULD ensure that routes marked as withdrawn are removed from
the database in a timely fashion after the MaxPurgeTime has expired.
This could be done, for example, by periodically sweeping the
database, and deleting those entries that were withdrawn more than
MaxPurgeTime seconds ago.
The Decision Process selects routes for subsequent advertisement by
applying the policies in the local Policy Information Base (PIB) to
the routes stored in its Adj-TRIBs-In. The output of the Decision
process is the set of routes that will be advertised to all peers;
the selected routes will be stored in the local LS's Adj-TRIBs-Out.
The selection process is formalized by defining a function that takes
the attributes of a given route as an argument and returns a non-
negative integer denoting the degree of preference for the route.
The function that calculates the degree of preference for a given
route shall not use as its inputs any of the following: the
existence of other routes, the non-existence of other routes, or the
attributes of other routes. Route selection then consists of an
individual application of the degree of preference function to each
feasible route, followed by the choice of the one with the highest
degree of preference.
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All internal LSs in an ITAD MUST run the Decision Process and apply
the same decision criteria, otherwise it will not be possible to
synchronize their Loc-TRIBs.
The Decision Process operates on routes contained in each Adj-TRIBs-
In, and is responsible for:
- selection of routes to be advertised to internal peers
- selection of routes to be advertised to external peers
- route aggregation and route information reduction
The Decision Process takes place in three distinct phases, each
triggered by a different event:
- Phase 1 is responsible for calculating the degree of preference
for each route received from an external peer.
- Phase 2 is invoked on completion of phase 1. It is responsible
for choosing the best route out of all those available for each
distinct destination, and for installing each chosen route into
the Loc-TRIB.
- Phase 3 is invoked after the Loc-TRIB has been modified. It is
responsible for disseminating routes in the Loc-TRIB to each
external peer, according to the policies contained in the PIB.
Route aggregation and information reduction can optionally be
performed within this phase.
The Phase 1 decision function shall be invoked whenever the local LS
receives from a peer an UPDATE message that advertises a new route, a
replacement route, or a withdrawn route.
The Phase 1 decision function is a separate process that is completed
when it has no further work to do.
The Phase 1 decision function shall lock an Adj-TRIB-In prior to
operating on any route contained within it, and shall unlock it after
operating on all new or replacement routes contained within it.
The local LS MUST determine a degree of preference for each newly
received or replacement route. If the route is learned from an
internal peer, the value of the LocalPreference attribute MUST be
taken as the degree of preference. If the route is learned from an
external peer, then the degree of preference MUST be computed based
on pre-configured policy information and used as the LocalPreference
value in any intra-domain TRIP advertisement. The exact nature of
this policy information and the computation involved is a local
matter.
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The output of the degree of preference determination process is the
local preference of a route. The local LS computes the local
preference of routes learned from external peers or originated
internally at that LS. The local preference of a route learned from
an internal peer is included in the LocalPreference attribute
associated with that route.
The Phase 2 decision function shall be invoked on completion of Phase
1. The Phase 2 function is a separate process that completes when it
has no further work to do. Phase 2 consists of two sub-phases: 2a
and 2b. The same route selection function is applied in both sub-
phases, but the inputs to each phase are different. The Phase 2a
process MUST consider as inputs all external routes, that are present
in the Adj-TRIBs-In of external peers, and all local routes. The
output of Phase 2a is inserted into the Ext-TRIB. The Phase 2b
process shall be invoked upon completion of Phase 2a and it MUST
consider as inputs all routes in the Ext-TRIB and all routes that are
present in the Adj-TRIBs-In of internal LSs. The output of Phase 2b
is stored in the Loc-TRIB.
The Phase 2 decision function MUST be blocked from running while the
Phase 3 decision function is in process. The Phase 2 function MUST
lock all Adj-TRIBs-In and the Ext-TRIB prior to commencing its
function, and MUST unlock them on completion.
If the LS determines that the NextHopServer listed in a route is
unreachable, then the route MAY be excluded from the Phase 2 decision
function. The means by which such a determination is made is not
mandated here.
For each set of destinations for which one or more routes exist, the
local LS's route selection function MUST identify the route that has:
- the highest degree of preference, or
- is selected as a result of the tie breaking rules specified in
10.2.2.1.
Withdrawn routes MUST be removed from the Loc-TRIB, Ext-TRIB, and the
Adj-TRIBs-In.
Several routes to the same destination that have the same degree of
preference may be input to the Phase 2 route selection function. The
local LS can select only one of these routes for inclusion in the
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associated Ext-TRIB (Phase 2a) or Loc-TRIB (Phase 2b). The local LS
considers all routes with the same degrees of preference. The
following algorithm shall be used to break ties.
- If the local LS is configured to use the MultiExitDisc
attribute to break ties, and candidate routes received from the
same neighboring ITAD differ in the value of the MultiExitDisc
attribute, then select the route that has the larger value of
MultiExitDisc.
- If at least one of the routes was originated by an internal LS,
select the route route that was advertised by the internal LS
that has the lowest TRIP ID.
- Otherwise, select the route that was advertised by the neighbor
domain that has the lowest ITAD number.
The Phase 3 decision function MUST be invoked upon completion of
Phase 2 if Phase 2 results in changes to the Loc-TRIB or when a new
LS-to-LS peer session is established.
The Phase 3 function is a separate process that is completed when it
has no further work to do. The Phase 3 routing decision function
MUST be blocked from running while the Phase 2 decision function is
in process.
All routes in the Loc-TRIB shall be processed into a corresponding
entry in the associated Adj-TRIBs-Out. Route aggregation and
information reduction techniques (see 10.3.4) MAY optionally be
applied.
When the updating of the Adj-TRIBs-Out is complete, the local LS MUST
run the external update process of 10.3.2.
When overlapping routes are present in the same Adj-TRIB-In, the more
specific route shall take precedence, in order, from most specific to
least specific.
The set of destinations described by the overlap represents a portion
of the less specific route that is feasible, but is not currently in
use. If a more specific route is later withdrawn, the set of
destinations described by the more specific route will still be
reachable using the less specific route.
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If an LS receives overlapping routes, the Decision Process MUST take
into account the semantics of the overlapping routes. In particular,
if an LS accepts the less specific route while rejecting the more
specific route from the same peer, then the destinations represented
by the overlap may not forward along the domains listed in the
AdvertisementPath attribute of that route. Therefore, an LS has the
following choices:
1. Install both the less and the more specific routes
2. Install the more specific route only
3. Install the non-overlapping part of the less specific route
only (that implies disaggregation of the less-specific route)
4. Aggregate the two routes and install the aggregated route
5. Install the less specific route only
6. Install neither route
If an LS chooses 5), then it SHOULD add AtomicAggregate attribute to
the route. A route that carries AtomicAggregate attribute MUST NOT
be de-aggregated. That is, the route cannot be made more specific.
Forwarding along such a route does not guarantee that route traverses
only domains listed in the RoutedPath of the route. If an LS chooses
1), then it MUST NOT advertise the less specific route without the
more specific route.
The Update-Send process is responsible for advertising UPDATE
messages to all peers. For example, it distributes the routes chosen
by the Decision Process to other LSs that may be located in either
the same ITAD or a neighboring ITAD. Rules for information exchange
between peer LSs located in different ITADs are given in 10.3.2;
rules for information exchange between peer LSs located in the same
ITAD are given in 10.3.1.
Before forwarding routes to peers, an LS MUST determine which
attributes should be forwarded along with that route. If a not
well-known non-transitive attribute is unrecognized, it is quietly
ignored. If a not well-known dependent-transitive attribute is
unrecognized, and the NextHopServer attribute has been changed by the
LS, the unrecognized attribute is quietly ignored. If a not well-
known dependent-transitive attribute is unrecognized, and the
NextHopServer attribute has not been modified by the LS, the Partial
bit in the attribute flags octet is set to 1, and the attribute is
retained for propagation to other TRIP speakers. Similarly, if an
not well-known independent-transitive attribute is unrecognized, the
Partial bit in the attribute flags octet is set to 1, and the
attribute is retained for propagation to other TRIP speakers.
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If a not well-known attribute is recognized, and has a valid value,
then, depending on the type of the not well-known attribute, it is
updated, if necessary, for possible propagation to other TRIP
speakers.
The Internal update process is concerned with the distribution of
routing information to internal peers.
When an LS receives an UPDATE message from another TRIP LS located in
its own ITAD, it is flooded as described in Section 10.1.
When an LS receives a new route from an LS in a neighboring ITAD, or
if a local route is injected into TRIP, the LS determines the
preference of that route. If the new route has the highest degree of
preference for all external routes and local routes to a given
destination (or if the route was selected via a tie-breaking
procedure as specified in 10.3.1.1), the LS MUST insert that new
route into the Ext-TRIB database and the LS MUST advertise that route
to all other LSs in its ITAD by means of an UPDATE message. The LS
MUST advertise itself as the Originator of that route within the
ITAD.
When an LS receives an UPDATE message with a non-empty
WithdrawnRoutes attribute from an external peer, or if a local route
is withdrawn from TRIP, the LS MUST remove from its Adj-TRIB-In all
routes whose destinations were carried in this field. If the
withdrawn route was previously selected into the Ext-TRIB, the LS
MUST take the following additional steps:
- If a new route is selected for advertisement for those
destinations, then the LS MUST insert the replacement route
into Ext-TRIB to replace the withdrawn route and advertise it
to all internal LSs.
- If a replacement route is not available for advertisement, then
the LS MUST include the destinations of the route in the
WithdrawnRoutes attribute of an UPDATE message, and MUST send
this message to each internal peer. The LS MUST also remove
the withdrawn route from the Ext-TRIB.
If an LS has connections to several external peers, there will be
multiple Adj-TRIBs-In associated with these peers. These databases
might contain several equally preferable routes to the same
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destination, all of which were advertised by external peers. The
local LS shall select one of these routes according to the following
rules:
- If the LS is configured to use the MultiExitDisc attribute to
break ties, and the candidate routes differ in the value of the
MultiExitDisc attribute, then select the route that has the
lowest value of MultiExitDisc, else
- Select the route that was advertised by the external LS that
has the lowest TRIP Identifier.
The external update process is concerned with the distribution of
routing information to external peers. As part of the Phase 3 route
selection process, the LS has updated its Adj-TRIBs-Out. All newly
installed routes and all newly unfeasible routes for which there is
no replacement route MUST be advertised to external peers by means of
UPDATE messages.
Any routes in the Loc-TRIB marked as withdrawn MUST be removed.
Changes to the reachable destinations within its own ITAD SHALL also
be advertised in an UPDATE message.
The TRIP protocol constrains the amount of routing traffic (that is,
UPDATE messages) in order to limit both the link bandwidth needed to
advertise UPDATE messages and the processing power needed by the
Decision Process to digest the information contained in the UPDATE
messages.
The parameter MinRouteAdvertisementInterval determines the minimum
amount of time that must elapse between advertisements of routes to a
particular destination from a single LS. This rate limiting
procedure applies on a per-destination basis, although the value of
MinRouteAdvertisementInterval is set on a per LS peer basis.
Two UPDATE messages sent from a single LS that advertise feasible
routes to some common set of destinations received from external
peers MUST be separated by at least MinRouteAdvertisementInterval.
Clearly, this can only be achieved precisely by keeping a separate
timer for each common set of destinations. This would be unwarranted
overhead. Any technique which ensures that the interval between two
UPDATE messages sent from a single LS that advertise feasible routes
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to some common set of destinations received from external peers will
be at least MinRouteAdvertisementInterval, and will also ensure that
a constant upper bound on the interval is acceptable.
Two UPDATE messages, sent from a single LS to an external peer, that
advertise feasible routes to some common set of destinations received
from internal peers MUST be separated by at least
MinRouteAdvertisementInterval.
Since fast convergence is needed within an ITAD, this rate limiting
procedure does not apply to routes received from internal peers and
being broadcast to other internal peers. To avoid long-lived black
holes, the procedure does not apply to the explicit withdrawal of
routes (that is, routes whose destinations explicitly withdrawn by
UPDATE messages).
This procedure does not limit the rate of route selection, but only
the rate of route advertisement. If new routes are selected multiple
times while awaiting the expiration of MinRouteAdvertisementInterval,
the last route selected shall be advertised at the end of
MinRouteAdvertisementInterval.
The parameter MinITADOriginationInterval determines the minimum
amount of time that must elapse between successive advertisements of
UPDATE messages that report changes within the advertising LS's own
ITAD.
To minimize the likelihood that the distribution of TRIP messages by
a given LS will contain peaks, jitter should be applied to the timers
associated with MinITADOriginationInterval, KeepAlive, and
MinRouteAdvertisementInterval. A given LS shall apply the same
jitter to each of these quantities regardless of the destinations to
which the updates are being sent; that is, jitter will not be applied
on a "per peer" basis.
The amount of jitter to be introduced shall be determined by
multiplying the base value of the appropriate timer by a random
factor that is uniformly distributed in the range from 0.75 to 1.0.
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RFC 3219 Telephony Routing over IP (TRIP) January 2002
Having selected the routing information that it will advertise, a
TRIP speaker may use methods to organize this information in an
efficient manner. These methods are discussed in the following
sections.
Information reduction may imply a reduction in granularity of policy
control - after information has collapsed, the same policies will
apply to all destinations and paths in the equivalence class.
The Decision Process may optionally reduce the amount of information
that it will place in the Adj-TRIBs-Out by any of the following
methods:
- ReachableRoutes: A set of destinations can be usually
represented in compact form. For example, a set of E.164 phone
numbers can be represented in more compact form using E.164
prefixes.
- AdvertisementPath: AdvertisementPath information can be
represented as ordered AP_SEQUENCEs or unordered AP_SETs.
AP_SETs are used in the route aggregation algorithm described
in Section 5.4.4. They reduce the size of the AP_PATH
information by listing each ITAD number only once, regardless
of how many times it may have appeared in multiple
advertisement paths that were aggregated.
An AP_SET implies that the destinations advertised in the UPDATE
message can be reached through paths that traverse at least some of
the constituent ITADs. AP_SETs provide sufficient information to
avoid route looping; however their use may prune potentially feasible
paths, since such paths are no longer listed individually as in the
form of AP_SEQUENCEs. In practice this is not likely to be a
problem, since once a call arrives at the edge of a group of ITADs,
the LS at that point is likely to have more detailed path information
and can distinguish individual paths to destinations.
Aggregation is the process of combining the characteristics of
several different routes in such a way that a single route can be
advertised. Aggregation can occur as part of the decision process to
reduce the amount of routing information that is placed in the Adj-
TRIBs-Out.
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RFC 3219 Telephony Routing over IP (TRIP) January 2002
Aggregation reduces the amount of information an LS must store and
exchange with other LSs. Routes can be aggregated by applying the
following procedure separately to attributes of like type.
Routes that have the following attributes shall not be aggregated
unless the corresponding attributes of each route are identical:
MultiExitDisc, NextHopServer.
Attributes that have different type codes cannot be aggregated.
Attributes of the same type code may be aggregated. The rules for
aggregating each attribute MUST be provided together with attribute
definition. For example, aggregation rules for TRIP's basic
attributes, e.g., ReachableRoutes and AdvertisementPath, are given in
Section 5.
Generally speaking, additional rules for comparing routes among
several alternatives are outside the scope of this document. There
are two exceptions:
- If the local ITAD appears in the AdvertisementPath of the new
route being considered, then that new route cannot be viewed as
better than any other route. If such a route were ever used, a
routing loop could result (see Section 6.3).
- In order to achieve successful distributed operation, only
routes with a likelihood of stability can be chosen. Thus, an
ITAD must avoid using unstable routes, and it must not make
rapid spontaneous changes to its choice of route. Quantifying
the terms "unstable" and "rapid" in the previous sentence will
require experience, but the principle is clear.
An LS may originate local routes by injecting routing information
acquired by some other means (e.g. via an intra-domain routing
protocol or through manual configuration or some dynamic registration
mechanism/protocol) into TRIP. An LS that originates TRIP routes
shall assign the degree of preference to these routes by passing them
through the Decision Process (see Section 10.2). To TRIP, local
routes are identical to external routes and are subjected to the same
two phase route selection mechanism. A local route which is selected
into the Ext-TRIB MUST be advertised to all internal LSs. The
decision whether to distribute non-TRIP acquired routes within an
ITAD via TRIP or not depends on the environment within the ITAD (e.g.
type of intra-domain routing protocol) and should be controlled via
configuration.
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RFC 3219 Telephony Routing over IP (TRIP) January 2002
This specification defines the use of TCP as the transport layer for
TRIP. TRIP uses TCP port 6069. Running TRIP over other transport
protocols is for further study.
There are no restrictions on the intra-domain topology of TRIP LSs.
For example, LSs in an ITAD can be configured in a full mesh, star,
or any other connected topology. Similarly, there are no
restrictions on the topology of TRIP ITADs. For example, the ITADs
can be organized in a flat topology (mesh or ring) or in multi-level
hierarchy or any other topology.
The border between two TRIP ITADs may be located either on the link
between two TRIP LSs or it may coincide on a TRIP LS. In the latter
case, the same TRIP LS will be member in more than one ITAD, and it
appears to be an internal peer to LSs in each ITAD it is member of.
This document creates a new IANA registry for TRIP parameters. The
following TRIP parameters are included in the registry:
- TRIP Capabilities
- TRIP Attributes
- TRIP Address Families
- TRIP Application Protocols
- TRIP ITAD Numbers
Protocol parameters are frequently initialized/reset to 0. This
document reserves the value 0 of each of the above TRIP parameters in
order to clearly distinguish between an unset parameter and any other
registered values for that parameter.
The sub-registries for each of the above parameters are discussed in
the sections below.
Requests to add TRIP capabilities other than those defined in Section
4.2.1.1 must be submitted to iana@iana.org. Following the assigned
number policies outlined in [11], Capability Codes in the range
32768-65535 are reserved for Private Use (these are the codes with
the first bit of the code value equal to 1). This document reserves
value 0. Capability Codes 1 and 2 have been assigned in Section
4.2.1.1. Capability Codes in the range 2-32767 are controlled by
Rosenberg, et. al. Standards Track [Page 68]
RFC 3219 Telephony Routing over IP (TRIP) January 2002
IANA, and are allocated subject to the Specification Required (IETF
RFC or equivalent) condition. The specification MUST include a
description of the capability, the possible values it may take, and
what constitutes a capability mismatch.
This document reserves Attribute Type Codes 224-255 for Private Use
(these are the codes with the first three bits of the code equal to
1). This document reserves the value 0. Attribute Type Codes 1
through 11 have already been allocated by this document. Attribute
Type Codes 1 through 11 are defined in Sections 5.1 through 5.11.
Attribute Type Codes in the range 12-223 are controlled by IANA, and
require a Specification document (RFC or equivalent). The
specification MUST provide all information required in Section 5.12
of this document.
Attribute Type Code registration requests must be sent to
iana@iana.org. In addition to the specification requirement, the
request MUST include an indication of who has change control over the
attribute and contact information (postal and email address).
This document reserves address family 0. Requests to add TRIP address
families other than those defined in Section 5.1.1.1 ( address
families 1, 2, and 3), i.e., in the range 4-32767, must be submitted
to iana@iana.org. The request MUST include a brief description of
the address family, its alphabet, and special processing rules and
guidelines, such as guidelines for aggregation, if any. The requests
are subject to Expert Review. This document reserves the address
family codes 32768-65535 for vendor-specific applications.
This document creates a new IANA registry for TRIP application
protocols. This document reserves the application protocol code 0.
Requests to add TRIP application protocols other than those defined
in Section 5.1.1.1 (application protocols 1 through 4), i.e., in the
range 5-32767, must be submitted to iana@iana.org. The request MUST
include a brief background on the application protocol, and a
description of how TRIP can be used to advertise routes for that
protocol. The requests are subject to Expert Review. This document
reserves the application protocol codes 32768-65535 for vendor-
specific applications.
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RFC 3219 Telephony Routing over IP (TRIP) January 2002
This document reserves the ITAD number 0. ITAD numbers in the range
1-255 are designated for Private Use. ITAD numbers in the range from
256 to (2**32)-1 are allocated by IANA on a First-Come-First-Serve
basis. Requests for ITAD numbers must be submitted to iana@iana.org.
The requests MUST include the following:
- Information about the organization that will administer the
ITAD.
- Contact information (postal and email address).
This section covers security between peer TRIP LSs when TRIP runs
over TCP in an IP environment.
A security mechanism is clearly needed to prevent unauthorized
entities from using the protocol defined in this document for setting
up unauthorized peer sessions with other TRIP LSs or interfering with
authorized peer sessions. The security mechanism for the protocol,
when transported over TCP in an IP network, is IPsec [12]. IPsec
uses two protocols to provide traffic security: Authentication Header
(AH) [13] and Encapsulating Security Payload (ESP) [14].
The AH header affords data origin authentication, connectionless
integrity and optional anti-replay protection of messages passed
between the peer LSs. The ESP header provides origin authentication,
connectionless integrity, anti-replay protection, and confidentiality
of messages.
Implementations of the protocol defined in this document employing
the ESP header SHALL comply with section 5 of [14], which defines a
minimum set of algorithms for integrity checking and encryption.
Similarly, implementations employing the AH header SHALL comply with
section 5 of [13], which defines a minimum set of algorithms for
integrity checking using manual keys.
Implementations SHOULD use IKE [15] to permit more robust keying
options. Implementations employing IKE SHOULD support authentication
with RSA signatures and RSA public key encryption.
A Security Association (SA) [12] is a simplex "connection" that
affords security services to the traffic carried by it. Security
services are afforded to a SA by the use of AH, or ESP, but not both.
Two types of SAs are defined: transport mode and tunnel mode [12]. A
transport mode SA is a security association between two hosts, and is
appropriate for protecting the TRIP session between two peer LSs.
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RFC 3219 Telephony Routing over IP (TRIP) January 2002
This section presents some implementation recommendations.
A.2.1: Multiple Networks Per Message
The TRIP protocol allows for multiple address prefixes with the same
advertisement path and next-hop server to be specified in one
message. Making use of this capability is highly recommended. With
one address prefix per message there is a substantial increase in
overhead in the receiver. Not only does the system overhead increase
due to the reception of multiple messages, but the overhead of
scanning the routing table for updates to TRIP peers is incurred
multiple times as well. One method of building messages containing
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RFC 3219 Telephony Routing over IP (TRIP) January 2002
many address prefixes per advertisement path and next hop from a
routing table that is not organized per advertisement path is to
build many messages as the routing table is scanned. As each address
prefix is processed, a message for the associated advertisement path
and next hop is allocated, if it does not exist, and the new address
prefix is added to it. If such a message exists, the new address
prefix is just appended to it. If the message lacks the space to
hold the new address prefix, it is transmitted, a new message is
allocated, and the new address prefix is inserted into the new
message. When the entire routing table has been scanned, all
allocated messages are sent and their resources released. Maximum
compression is achieved when all the destinations covered by the
address prefixes share the same next hop server and common
attributes, making it possible to send many address prefixes in one
4096-byte message.
When peering with a TRIP implementation that does not compress
multiple address prefixes into one message, it may be necessary to
take steps to reduce the overhead from the flood of data received
when a peer is acquired or a significant network topology change
occurs. One method of doing this is to limit the rate of updates.
This will eliminate the redundant scanning of the routing table to
provide flash updates for TRIP peers. A disadvantage of this
approach is that it increases the propagation latency of routing
information. By choosing a minimum flash update interval that is not
much greater than the time it takes to process the multiple messages,
this latency should be minimized. A better method would be to read
all received messages before sending updates.
A.2.2: Processing Messages on a Stream Protocol
TRIP uses TCP as a transport mechanism. Due to the stream nature of
TCP, all the data of a received message does not necessarily arrive
at the same time. This can make it difficult to process the data as
messages, especially on systems where it is not possible to determine
how much data has been received but not yet processed.
One method that can be used in this situation is to first try to read
just the message header. For the KEEPALIVE message type, this is a
complete message; for other message types, the header should first be
verified, in particular the total length. If all checks are
successful, the specified length, minus the size of the message
header is the amount of data left to read. An implementation that
would "hang" the routing information process while trying to read
from a peer could set up a message buffer (4096 bytes) per peer and
fill it with data as available until a complete message has been
received.
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RFC 3219 Telephony Routing over IP (TRIP) January 2002
A.2.3: Reducing Route Flapping
To avoid excessive route flapping an LS which needs to withdraw a
destination and send an update about a more specific or less specific
route SHOULD combine them into the same UPDATE message.
A.2.4: TRIP Timers
TRIP employs seven timers: ConnectRetry, Hold Time, KeepAlive,
MaxPurgeTime, TripDisableTime, MinITADOriginationInterval, and
MinRouteAdvertisementInterval. The suggested value for the
ConnectRetry timer is 120 seconds. The suggested value for the Hold
Time is 90 seconds. The suggested value for the KeepAlive timer is
30 seconds. The suggested value for the MaxPurgeTime timer is 10
seconds. The suggested value for the TripDisableTime timer is 180
seconds. The suggested value for the MinITADOriginationInterval is
30 seconds. The suggested value for the
MinRouteAdvertisementInterval is 30 seconds.
An implementation of TRIP MUST allow these timers to be configurable.
A.2.5: AP_SET Sorting
Another useful optimization that can be done to simplify this
situation is to sort the ITAD numbers found in an AP_SET. This
optimization is entirely optional.
Acknowledgments
We wish to thank Dave Oran for his insightful comments and
suggestions.
References
[1] Bradner, S., "Keywords for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Rosenberg, J. and H. Schulzrinne, "A Framework for a Gateway
Location Protocol", RFC 2871, June 2000.
[3] Rekhter, Y. and T. Li, "Border Gateway Protocol 4 (BGP-4)," RFC
1771, March 1995.
[4] Moy, J., "Open Shortest Path First Version 2", STD 54, RFC
2328, April 1998.
Rosenberg, et. al. Standards Track [Page 75]
RFC 3219 Telephony Routing over IP (TRIP) January 2002
[5] "Intermediate System to Intermediate System Intra-Domain
Routing Exchange Protocol for use in Conjunction with the
Protocol for Providing the Connectionless-mode Network Service
(ISO 8473)," ISO DP 10589, February 1990.
[6] Luciani, J., Armitage, G., Halpern, J. and N. Doraswamy,
"Server Cache Synchronization Protocol (SCSP)", RFC 2334, April
1998.
[7] International Telecommunication Union, "Packet-Based Multimedia
Communication Systems," Recommendation H.323, Version 3
Telecommunication Standardization Sector of ITU, Geneva,
Switzerland, November 2000.
[8] Handley, H., Schulzrinne, H., Schooler, E. and J. Rosenberg,
"SIP: Session Initiation Protocol", RFC 2543, March 1999.
[9] Braden, R., "Requirements for Internet Hosts -- Application and
Support", STD 3, RFC 1123, October 1989.
[10] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 2373, July 1998.
[11] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", BCP 26, RFC 2434, October
1998.
[12] Kent, S. and R. Atkinson, "Security Architecture for the
Internet Protocol", RFC 2401, November 1998.
[13] Kent, S. and R. Atkinson, "IP Authentication Header", RFC 2402,
November 1998.
[14] Kent, S. and R. Atkinson, "IP Encapsulating Security Payload
(ESP)", RFC 2406, November 1998.
[15] Harkins, D. and D. Carrel, "The Internet Key Exchange (IKE)",
RFC 2409, November 1998.
Rosenberg, et. al. Standards Track [Page 76]
RFC 3219 Telephony Routing over IP (TRIP) January 2002
Intellectual Property Notice
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intellectual property or other rights that might be claimed to
pertain to the implementation or use of the technology described in
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The IETF has been notified of intellectual property rights claimed in
regard to some or all of the specification contained in this
document. For more information consult the online list of claimed
rights.
Rosenberg, et. al. Standards Track [Page 77]
RFC 3219 Telephony Routing over IP (TRIP) January 2002
Authors' Addresses
Jonathan Rosenberg
dynamicsoft
72 Eagle Rock Avenue
First Floor
East Hanover, NJ 07936
Phone: 973-952-5000
EMail: jdrosen@dynamicsoft.com
Hussein F. Salama
Cisco Systems
170 W. Tasman Drive
San Jose, CA 95134
Phone: 408-527-7147
EMail: hsalama@cisco.com
Matt Squire
Hatteras Networks
639 Davis Drive
Suite 200
Durham, NC 27713
EMail: mattsquire@acm.org
Rosenberg, et. al. Standards Track [Page 78]
RFC 3219 Telephony Routing over IP (TRIP) January 2002
Full Copyright Statement
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Acknowledgement
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Rosenberg, et. al. Standards Track [Page 79]