This document discusses transport issues that arise within protocols
for Authentication, Authorization and Accounting (AAA). It also
provides recommendations on the use of transport by AAA protocols.
This includes usage of standards-track RFCs as well as experimental
proposals.
In this document, the key words "MAY", "MUST, "MUST NOT", "optional",
"recommended", "SHOULD", and "SHOULD NOT", are to be interpreted as
described in [RFC2119].
Accounting
The act of collecting information on resource usage for the
purpose of trend analysis, auditing, billing, or cost
allocation.
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Administrative Domain
An internet, or a collection of networks, computers, and
databases under a common administration.
Agent A AAA agent is an intermediary that communicates with AAA
clients and servers. Several types of AAA agents exist,
including Relays, Re-directs, and Proxies.
Application-driven transport
Transport behavior is said to be "application-driven" when
the rate at which messages are sent is limited by the rate
at which the application generates data, rather than by the
size of the congestion window. In the most extreme case,
the time between transactions exceeds the round-trip time
between sender and receiver, implying that the application
operates with an effective congestion window of one. AAA
transport is typically application driven.
Attribute Value Pair (AVP)
The variable length concatenation of a unique Attribute
(represented by an integer) and a Value containing the
actual value identified by the attribute.
Authentication
The act of verifying a claimed identity, in the form of a
pre-existing label from a mutually known name space, as the
originator of a message (message authentication) or as the
end-point of a channel (entity authentication).
Authorization
The act of determining if a particular right, such as
access to some resource, can be granted to the presenter of
a particular credential.
Billing The act of preparing an invoice.
Network Access Identifier
The Network Access Identifier (NAI) is the userID submitted
by the host during network access authentication. In
roaming, the purpose of the NAI is to identify the user as
well as to assist in the routing of the authentication
request. The NAI may not necessarily be the same as the
user's e-mail address or the user-ID submitted in an
application layer authentication.
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Network Access Server (NAS)
A Network Access Server (NAS) is a device that hosts
connect to in order to get access to the network.
Proxy In addition to forwarding requests and responses, proxies
enforce policies relating to resource usage and
provisioning. This is typically accomplished by tracking
the state of NAS devices. While proxies typically do not
respond to client Requests prior to receiving a Response
from the server, they may originate Reject messages in
cases where policies are violated. As a result, proxies
need to understand the semantics of the messages passing
through them, and may not support all extensions.
Local Proxy
A Local Proxy is a proxy that exists within the same
administrative domain as the network device (e.g. NAS) that
issued the AAA request. Typically a local proxy is used to
multiplex AAA messages to and from a large number of
network devices, and may implement policy.
Store and forward proxy
Store and forward proxies distinguish themselves from other
proxy species by sending a reply to the NAS prior to
proxying the request to the server. As a result, store and
forward proxies need to implement AAA client and server
functionality for the messages that they handle. Store and
Forward proxies also typically keep state on conversations
in progress in order to assure delivery of proxied Requests
and Responses. While store and forward proxies are most
frequently deployed for accounting, they also can be used
to implement authentication/authorization policy.
Network-driven transport
Transport behavior is said to be "network driven" when the
rate at which messages are sent is limited by the
congestion window, not by the rate at which the application
can generate data. File transfer is an example of an
application where transport is network driven.
Re-direct Rather than forwarding Requests and Responses between
clients and servers, Re-directs refer clients to servers
and allow them to communicate directly. Since Re-directs
do not sit in the forwarding path, they do not alter any
AVPs transitting between client and server. Re-directs do
not originate messages and are capable of handling any
message type. A Re-direct may be configured only to re-
direct messages of certain types, while acting as a Relay
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or Proxy for other types. As with Relays, re-directs do
not keep state with respect to conversations or NAS
resources.
Relay Relays forward requests and responses based on routing-
related AVPs and domain forwarding table entries. Since
relays do not enforce policies, they do not examine or
alter non-routing AVPs. As a result, relays never
originate messages, do not need to understand the semantics
of messages or non-routing AVPs, and are capable of
handling any extension or message type. Since relays make
decisions based on information in routing AVPs and domain
forwarding tables they do not keep state on NAS resource
usage or conversations in progress.
Issues that arise in AAA transport usage include:
Application-driven versus network-driven
Slow failover
Use of Nagle Algorithm
Multiple connections
Duplicate detection
Invalidation of transport parameter estimates
Inability to use fast re-transmit
Congestion avoidance
Delayed acknowledgments
Premature Failover
Head of line blocking
Connection load balancing
We discuss each of these issues in turn.
AAA transport behavior is typically application rather than network
driven. This means that the rate at which messages are sent is
typically limited by how quickly they are generated by the
application, rather than by the size of the congestion window.
For example, let us assume a 48-port NAS with an average session time
of 20 minutes. This device will, on average, send only 144
authentication/authorization requests/hour, and an equivalent number
of accounting requests. This represents an average inter-packet
spacing of 25 seconds, which is much larger than the Round Trip Time
(RTT) in most networks.
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Even on much larger NAS devices, the inter-packet spacing is often
larger than the RTT. For example, consider a 2048-port NAS with an
average session time of 10 minutes. It will on average send 3.4
authentication/authorization requests/second, and an equivalent
number of accounting requests. This translates to an average inter-
packet spacing of 293 ms.
However, even where transport behavior is largely application-driven,
periods of network-driven behavior can occur. For example, after a
NAS reboot, previously stored accounting records may be sent to the
accounting server in rapid succession. Similarly, after recovery
from a power failure, users may respond with a large number of
simultaneous logins. In both cases, AAA messages may be generated
more quickly than the network will allow them to be sent, and a queue
will build up.
Network congestion can occur when transport behavior is network-
driven or application-driven. For example, while a single NAS may
not send substantial AAA traffic, many NASes may communicate with a
single AAA proxy or server. As a result, routers close to a heavily
loaded proxy or server may experience congestion, even though traffic
from each individual NAS is light. Such "convergent congestion" can
result in dropped packets in routers near the AAA server, or even
within the AAA server itself.
Let us consider what happens when 10,000 48-ports NASes, each with an
average session time of 20 minutes, are configured with the same AAA
agent or server. The unfortunate proxy or server would receive 400
authentication/authorization requests/second and an equivalent number
of accounting requests. For 1000 octet requests, this would generate
6.4 Mbps of incoming traffic at the AAA agent or server.
While this transaction load is within the capabilities of the fastest
AAA agents and servers, implementations exist that cannot handle such
a high load. Thus high queuing delays and/or dropped packets may be
experienced at the agent or server, even if routers on the path are
not congested. Thus, a well designed AAA protocol needs to be able
to handle congestion occurring at the AAA server, as well as
congestion experienced within the network.
Where TCP [RFC793] is used as the transport, AAA implementations will
experience very slow fail over times if they wait until a TCP
connection times out before resending on another connection. This is
not an issue for SCTP [RFC2960], which supports endpoint and path
failure detection. As described in section 8 of [RFC2960], when the
number of retransmissions exceeds the maximum
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("Association.Max.Retrans"), the peer endpoint is considered
unreachable, the association enters the CLOSED state, and the failure
is reported to the application. This enables more rapid failure
detection.
AAA protocol messages are often smaller than the maximum segment size
(MSS). While exceptions occur when certificate-based authentication
messages are issued or where a low path MTU is found, typically AAA
protocol messages are less than 1000 octets. Therefore, when using
TCP [RFC793], the total packet count and associated network overhead
can be reduced by combining multiple AAA messages within a single
packet.
Where AAA runs over TCP and transport behavior is network-driven,
such as after a reboot when many users login simultaneously, or many
stored accounting records need to be sent, the Nagle algorithm will
result in "transport layer batching" of AAA messages. While this
does not reduce the work required by the application in parsing
packets and responding to the messages, it does reduce the number of
packets processed by routers along the path. The Nagle algorithm is
not used with SCTP.
Where AAA transport is application-driven, the NAS will typically
receive a reply from the home server prior to having another request
to send. This implies, for example, that accounting requests will
typically be sent individually rather than being batched by the
transport layer. As a result, within the application-driven regime,
the Nagle algorithm [RFC896] is ineffective.
Since the RADIUS [RFC2865] Identifier field is a single octet, a
maximum of 256 requests can be in progress between two endpoints
described by a 5-tuple: (Client IP address, Client port, UDP, Server
IP address, Server port). In order to get around this limitation,
RADIUS clients have utilized more than one sending port, sometimes
even going to the extreme of using a different UDP source port for
each NAS port.
Were this behavior to be extended to AAA protocols operating over
reliable transport, the result would be multiplication of the
effective slow-start ramp-up by the number of connections. For
example, if a AAA client had ten connections open to a AAA agent, and
used a per-connection initial window [RFC3390] of 2, then the
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effective initial window would be 20. This is inappropriate, since
it would permit the AAA client to send a large burst of packets into
the network.
Where a AAA client maintains connections to multiple AAA agents or
servers, and where failover/failback or connection load balancing is
supported, it is possible for multiple agents or servers to receive
duplicate copies of the same transaction. A transaction may be sent
on another connection before expiration of the "time wait" interval
necessary to guarantee that all packets sent on the original
connection have left the network. Therefore it is conceivable that
transactions sent on the alternate connection will arrive before
those sent on the failed connection. As a result, AAA agents and
servers MUST be prepared to handle duplicates, and MUST assume that
duplicates can arrive on any connection.
For example, in billing, it is necessary to be able to weed out
duplicate accounting records, based on the accounting session-id,
event-timestamp and NAS identification information. Where
authentication requests are always idempotent, the resultant
duplicate responses from multiple servers will presumably be
identical, so that little harm will result.
However, there are situations where the response to an authentication
request will depend on a previously established state, such as when
simultaneous usage restrictions are being enforced. In such cases,
authentication requests will not be idempotent. For example, while
an initial request might elicit an Accept response, a duplicate
request might elicit a Reject response from another server, if the
user were already presumed to be logged in, and only one simultaneous
session were permitted. In these situations, the AAA client might
receive both Accept and Reject responses to the same duplicate
request, and the outcome will depend on which response arrives first.
Congestion control principles [Congest],[RFC2914] require the ability
of a transport protocol to respond effectively to congestion, as
sensed via increasing delays, packet loss, or explicit congestion
notification.
With network-driven applications, it is possible to respond to
congestion on a timescale comparable to the round-trip time (RTT).
However, with AAA protocols, the time between sends may be longer
than the RTT, so that the network conditions can not be assumed to
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persist between sends. For example, the congestion window may grow
during a period in which congestion is being experienced because few
packets are sent, limiting the opportunity for feedback. Similarly,
after congestion is detected, the congestion window may remain small,
even though the network conditions that existed at the time of
congestion no longer apply by the time when the next packets are
sent. In addition, due to the low sampling interval, estimates of
RTT and RTO made via the procedure described in [RFC2988] may become
invalid.
When congestion window validation [RFC2861] is implemented, the
result is that AAA protocols operate much of the time in slow-start
with an initial congestion window set to 1 or 2, depending on the
implementation [RFC3390]. This implies that AAA protocols gain
little benefit from the windowing features of reliable transport.
Since the congestion window is so small, it is generally not possible
to receive enough duplicate ACKs (3) to trigger fast re-transmit. In
addition, since AAA traffic is two-way, ACKs including data will not
count as part of the duplicate ACKs necessary to trigger fast re-
transmit. As a result, dropped packets will require a retransmission
timeout (RTO).
The law of conservation of packets [Congest] suggests that a client
should not send another packet into the network until it can be
reasonably sure that a packet has exited the network on the same
path. In the case of a AAA client, the law suggests that it should
not retransmit to the same server or choose another server until it
can be reasonably sure that a packet has exited the network on the
same path. If the client advances the window as responses arrive,
then the client will "self clock", adjusting its transmission rate to
the available bandwidth.
While a AAA client using a reliable transport such as TCP [RFC793] or
SCTP [RFC2960] will self-clock when communicating directly with a
AAA-server, end-to-end self-clocking is not assured when AAA agents
are present.
As described in the Appendix, AAA agents include Relays, Proxies,
Re-directs, Store and Forward proxies, and Transport proxies. Of
these agents, only Transport proxies and Re-directs provide a direct
transport connection between the AAA client and server, allowing
end-to-end self-clocking to occur.
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With Relays, Proxies or Store and Forward proxies, two separate and
de-coupled transport connections are used. One connection operates
between the AAA client and agent, and another between the agent and
server. Since the two transport connections are de-coupled,
transport layer ACKs do not flow end-to-end, and self-clocking does
not occur.
For example, consider what happens when the bottleneck exists between
a AAA Relay and a AAA server. Self-clocking will occur between the
AAA client and AAA Relay, causing the AAA client to adjust its
sending rate to the rate at which transport ACKs flow back from the
AAA Relay. However, since this rate is higher than the bottleneck
bandwidth, the overall system will not self-clock.
Since there is no direct transport connection between the AAA client
and AAA server, the AAA client does not have the ability to estimate
end-to-end transport parameters and adjust its sending rate to the
bottleneck bandwidth between the Relay and server. As a result, the
incoming rate at the AAA Relay can be higher than the rate at which
packets can be sent to the AAA server.
In this case, the end-to-end performance will be determined by
details of the agent implementation. In general, the end-to-end
transport performance in the presence of Relays, Proxies or Store and
Forward proxies will always be worse in terms of delay and packet
loss than if the AAA client and server were communicating directly.
For example, if the agent operates with a large receive buffer, it is
possible that a large queue will develop on the receiving side, since
the AAA client is able to send packets to the AAA agent more rapidly
than the agent can send them to the AAA server. Eventually, the
buffer will overflow, causing wholesale packet loss as well as high
delay.
Methods to induce fine-grained coupling between the two transport
connections are difficult to implement. One possible solution is for
the AAA agent to operate with a receive buffer that is no larger than
its send buffer. If this is done, "back pressure" (closing of the
receive window) will cause the agent to reduce the AAA client sending
rate when the agent send buffer fills. However, unless multiple
connections exist between the AAA client and AAA agent, closing of
the receive window will affect all traffic sent by the AAA client,
even traffic destined to AAA servers where no bottleneck exists.
Since multiple connections between a AAA client and agent result in
multiplication of the effective slow-start ramp rate, this is not
recommended. As a result, use of "back pressure" cannot enable
individual AAA client-server conversations to self-clock, and this
technique appears impractical for use in AAA.
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As described in Appendix B, ACKs may comprise as much as half of the
traffic generated in a AAA exchange. This occurs because AAA
conversations are typically application-driven, and therefore there
is frequently not enough traffic to enable ACK piggybacking. As a
result, AAA protocols running over TCP or SCTP transport may
experience a doubling of traffic as compared with implementations
utilizing UDP transport.
It is typically not possible to address this issue via the sockets
API. ACK parameters (such as the value of the delayed ACK timer) are
typically fixed by TCP and SCTP implementations and are therefore not
tunable by the application.
RADIUS failover implementations are typically based on the concept of
primary and secondary servers, in which all traffic flows to the
primary server unless it is unavailable. However, the failover
algorithm was not specified in [RFC2865] or [RFC2866]. As a result,
RADIUS failover implementations vary in quality, with some failing
over prematurely, violating the law of "conservation of packets".
Where a Relay, Proxy or Store and Forward proxy is present, the AAA
client has no direct connection to a AAA server, and is unable to
estimate the end-to-end transport parameters. As a result, a AAA
client awaiting an application-layer response from the server has no
transport-based mechanism for determining an appropriate failover
timer.
For example, if the path between the AAA agent and server includes a
high delay link, or if the AAA server is very heavily loaded, it is
possible that the NAS will failover to another agent while packets
are still in flight. This violates the principle of "conservation of
packets", since the AAA client will inject additional packets into
the network before having evidence that a previously sent packet has
left the network. Such behavior can result in a worse situation on
an already congested link, resulting in congestive collapse
[Congest].
Head of line blocking occurs during periods of packet loss where the
time between sends is shorter than the re-transmission timeout value
(RTO). In such situations, packets back up in the send queue until
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the lost packet can be successfully re-transmitted. This can be an
issue for SCTP when using ordered delivery over a single stream, and
for TCP.
Head of line blocking is typically an issue only on larger NASes.
For example, a 48-port NAS with an average inter-packet spacing of 25
seconds is unlikely to have an RTO greater than this, unless severe
packet loss has been experienced. However, a 2048-port NAS with an
average inter-packet spacing of 293 ms may experience head-of-line
blocking since the inter-packet spacing is less than the minimum RTO
value of 1 second [RFC2988].
In order to lessen queuing delays and address head of line blocking,
a AAA implementation may wish to load balance between connections to
multiple destinations. While it is possible to employ dynamic load
balancing techniques, this level of sophistication may not be
required. In many situations, adequate reliability and load
balancing can be achieved via static load balancing, where traffic is
distributed between destinations based on static "weights".
In order to address AAA transport issues, it is recommended that AAA
protocols make use of standards track as well as experimental
techniques. More details are provided in the sections that follow.
AAA Servers MUST support TCP and SCTP. AAA clients SHOULD support
SCTP, but MUST support TCP if SCTP is not available. As support for
SCTP improves, it is possible that SCTP support will be required on
clients at some point in the future. AAA agents inherit all the
obligations of Servers with respect to transport support.
While AAA protocols typically operate in the application-driven
regime, there are circumstances in which they are network driven.
For example, where an NAS reboots, or where connectivity is restored
between an NAS and a AAA agent, it is possible that multiple packets
will be available for sending.
As a result, there are circumstances where the transport-layer
batching provided by the Nagle Algorithm (12) is useful, and as a
result, AAA implementations running over TCP MUST enable the Nagle
algorithm, [RFC896]. The Nagle algorithm is not used with SCTP.
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AAA protocols SHOULD use only a single persistent connection between
a AAA client and a AAA agent or server. They SHOULD provide for
pipelining of requests, so that more than one request can be in
progress at a time. In order to minimize use of inactive connections
in roaming situations, a AAA client or agent MAY bring down a
connection to a AAA agent or server if the connection has been
unutilized (discounting the watchdog) for a certain period of time,
which MUST NOT be less than BRINGDOWN_INTERVAL (5 minutes).
While a AAA client/agent SHOULD only use a single persistent
connection to a given AAA agent or server, it MAY have connections to
multiple AAA agents or servers. A AAA client/agent connected to
multiple agents/servers can treat them as primary/secondary or
balance load between them.
In order to enable AAA implementations to more quickly detect
transport and application-layer failures, AAA protocols MUST support
an application layer watchdog message.
The application layer watchdog message enables failover from a peer
that has failed, either because it is unreachable or because its
applications functions have failed. This is distinct from the
purpose of the SCTP heartbeat, which is to enable failover between
interfaces. The SCTP heartbeat may enable a failover to another path
to reach the same server, but does not address the situation where
the server system or the application service has failed. Therefore
both mechanisms MAY be used together.
The watchdog is used in order to enable a AAA client or agent to
determine when to resend on another connection. It operates on all
open connections and is used to suspend and eventually close
connections that are experiencing difficulties. The watchdog is also
used to re-open and validate connections that have returned to
health. The watchdog may be utilized either within primary/secondary
or load balancing configurations. However, it is not intended as a
cluster heartbeat mechanism.
The application layer watchdog is designed to detect failures of the
immediate peer, and not to be affected by failures of downstream
proxies or servers. This prevents instability in downstream AAA
components from propagating upstream. While the receipt of any AAA
Response from a peer is taken as evidence that the peer is up, lack
of a Response is insufficient to conclude that the peer is down.
Since the lack of Response may be the result of problems with a
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downstream proxy or server, only after failure to respond to the
watchdog message can it be determined that the peer is down.
Since the watchdog algorithm takes any AAA Response into account in
determining peer liveness, decreases in the watchdog timer interval
do not significantly increase the level of watchdog traffic on
heavily loaded networks. This is because watchdog messages do not
need to be sent where other AAA Response traffic serves as a constant
reminder of peer liveness. Watchdog traffic only increases when AAA
traffic is light, and therefore a AAA Response "signal" is not
present. Nevertheless, decreasing the timer interval TWINIT does
increase the probability of false failover significantly, and so this
decision should be made with care.
The watchdog behavior is controlled by an algorithm defined in this
section. This algorithm is appropriate for use either within
primary/secondary or load balancing configurations. Implementations
SHOULD implement this algorithm, which operates as follows:
[1] Watchdog behavior is controlled by a single timer (Tw). The
initial value of Tw, prior to jittering is Twinit. The default
value of Twinit is 30 seconds. This value was selected because
it minimizes the probability that failover will be initiated due
to a routing flap, as noted in [Paxson].
While Twinit MAY be set as low as 6 seconds (not including
jitter), it MUST NOT be set lower than this. Note that setting
such a low value for Twinit is likely to result in an increased
probability of duplicates, as well as an increase in spurious
failover and failback attempts.
In order to avoid synchronization behaviors that can occur with
fixed timers among distributed systems, each time the watchdog
interval is calculated with a jitter by using the Twinit value
and randomly adding a value drawn between -2 and 2 seconds.
Alternative calculations to create jitter MAY be used. These
MUST be pseudo-random, generated by a PRNG seeded as per
[RFC1750].
[2] When any AAA message is received, Tw is reset. This need not be
a response to a watchdog request. Receiving a watchdog response
from a peer constitutes activity, and Tw should be reset. If the
watchdog timer expires and no watchdog response is pending, then
a watchdog message is sent. On sending a watchdog request, Tw is
reset.
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Watchdog packets are not retransmitted by the AAA protocol, since
AAA protocols run over reliable transports that will handle all
retransmissions internally. As a result, a watchdog request is
only sent when there is no watchdog response pending.
[3] If the watchdog timer expires and a watchdog response is pending,
then failover is initiated. In order for a AAA client or agent
to perform failover procedures, it is necessary to maintain a
pending message queue for a given peer. When an answer message
is received, the corresponding request is removed from the queue.
The Hop-by-Hop Identifier field MAY be used to match the answer
with the queued request.
When failover is initiated, all messages in the queue are sent to
an alternate agent, if available. Multiple identical requests or
answers may be received as a result of a failover. The
combination of an end-to-end identifier and the origin host MUST
be used to identify duplicate messages.
Note that where traffic is heavy, the application layer watchdog
can take as long as 2Tw to determine that a peer has gone down.
For peers receiving a high volume of AAA Requests, AAA Responses
will continually reset the timer, so that after a failure it will
take Tw for the lack of traffic to be noticed, and for the
watchdog message to be sent. Another Tw will elapse before
failover is initiated.
On a lightly loaded network without much AAA Response traffic,
the watchdog timer will typically expire without being reset, so
that a watchdog response will be outstanding and failover will be
initiated after only a single timer interval has expired.
[4] The client MUST NOT close the primary connection until the
primary's watchdog timer has expired at least twice without a
response (note that the watchdog is not sent a second time,
however). Once this has occurred, the client SHOULD cause a
transport reset or close to be done on the connection.
Once the primary connection has failed, subsequent requests are
sent to the alternate server until the watchdog timer on the
primary connection is reset.
Suspension of the primary connection prevents flapping between
primary and alternate connections, and ensures that failover
behavior remains consistent. The application may not receive a
response to the watchdog request message due to a connectivity
problem, in which case a transport layer ACK will not have been
received, or the lack of response may be due to an application
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problem. Without transport layer visibility, the application is
unable to tell the difference, and must behave conservatively.
In situations where no transport layer ACK is received on the
primary connection after multiple re-transmissions, the RTO will
be exponentially backed off as described in [RFC2988]. Due to
Karn's algorithm as implemented in SCTP and TCP, the RTO
estimator will not be reset until another ACK is received in
response to a non-re-transmitted request. Thus, in cases where
the problem occurs at the transport layer, after the client fails
over to the alternate server, the RTO of the primary will remain
at a high value unless an ACK is received on the primary
connection.
In the case where the problem occurs at the transport layer,
subsequent requests sent on the primary connection will not
receive the same service as was originally provided. For
example, instead of failover occurring after 3 retransmissions,
failover might occur without even a single retransmission if RTO
has been sufficiently backed off. Of course, if the lack of a
watchdog response was due to an application layer problem, then
RTO will not have been backed off. However, without transport
layer visibility, there is no way for the application to know
this.
Suspending use of the primary connection until a response to a
watchdog message is received guarantees that the RTO timer will
have been reset before the primary connection is reused. If no
response is received after the second watchdog timer expiration,
then the primary connection is closed and the suspension becomes
permanent.
[5] While the connection is in the closed state, the AAA client MUST
NOT attempt to send further watchdog messages on the connection.
However, after the connection is closed, the AAA client continues
to periodically attempt to reopen the connection.
The AAA client SHOULD wait for the transport layer to report
connection failure before attempting again, but MAY choose to
bound this wait time by the watchdog interval, Tw. If the
connection is successfully opened, then the watchdog message is
sent. Once three watchdog messages have been sent and responded
to, the connection is returned to service, and transactions are
once again sent over it. Connection validation via receipt of
multiple watchdogs is not required when a connection is initially
brought up -- in this case, the connection can immediately be put
into service.
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[6] When using SCTP as a transport, it is not necessary to disable
SCTP's transport-layer heartbeats. However, if AAA
implementations have access to SCTP's heartbeat parameters, they
MAY chose to ensure that SCTP's heartbeat interval is longer than
the AAA watchdog interval, Tw. This will ensure that alternate
paths are still probed by SCTP, while the primary path has a
minimum of heartbeat redundancy.
The watchdog timer MAY be integrated with primary/secondary style
failover so as to provide improved reliability and basic load
balancing. In order to balance load among multiple AAA servers, each
AAA server is designated the primary for a portion of the clients,
and designated as secondaries of varying priority for the remainder.
In this way, load can be balanced among the AAA servers.
Within primary/secondary configurations, the watchdog timer operates
as follows:
[1] Assume that each client or agent is initially configured with a
single primary agent or server, and one or more secondary
connections.
[2] The watchdog mechanism is used to suspend and eventually close
primary connections that are experiencing difficulties. It is
also used to re-open and validate connections that have returned
to health.
[3] Once a secondary is promoted to primary status, either on a
temporary or permanent basis, the next server on the list of
secondaries is promoted to fill the open secondary slot.
[4] The client or agent periodically attempts to re-open closed
connections, so that it is possible that a previously closed
connection can be returned to service and become eligible for use
again. Implementations will typically retain a limit on the
number of connections open at a time, so that once a previously
closed connection is brought online again, the lowest priority
secondary connection will be closed. In order to prevent
periodic closing and re-opening of secondary connections, it is
recommended that functioning connections remain open for a
minimum of 5 minutes.
[5] In order to enable diagnosis of failover behavior, it is
recommended that a table of failover events be kept within the
MIB. These failover events SHOULD include appropriate
transaction identifiers so that client and server data can be
Aboba & Wood Standards Track [Page 17]
RFC 3539 AAA Transport Profile June 2003
compared, providing insight into the cause of the problem
(transport or application layer).
Primary/secondary failover is capable of providing improved
resilience and basic load balancing. However, it does not address
TCP head of line blocking, since only a single connection is in use
at a time.
A AAA client or agent maintaining connections to multiple agents or
servers MAY load balance between them. Establishing connections to
multiple agents or servers reduces, but does not eliminate, head of
line blocking issues experienced on TCP connections. This issue does
not exist with SCTP connections utilizing multiple streams.
In connection load balancing configurations, the application watchdog
operates as follows:
[1] Assume that each client or agent is initially configured with
connections to multiple AAA agents or servers, with one
connection between a given client/agent and an agent/server.
[2] In static load balancing, transactions are apportioned among the
connections based on the total number of connections and a
"weight" assigned to each connection. Pearson's hash [RFC3074]
applied to the NAI [RFC2486] can be used to determine which
connection will handle a given transaction. Hashing on the NAI
provides highly granular load balancing, while ensuring that all
traffic for a given conversation will be sent to the same agent
or server. In dynamic load balancing, the value of the "weight"
can vary based on conditions such as AAA server load. Such
techniques, while sophisticated, are beyond the scope of this
document.
[3] Transactions are distributed to connections based on the total
number of available connections and their weights. A change in
the number of available connections forces recomputation of the
hash table. In order not to cause conversations in progress to
be switched to new destinations, on recomputation, a transitional
period is required in which both old and new hash tables are
needed in order to permit aging out of conversations in progress.
Note that this requires a way to easily determine whether a
Request represents a new conversation or the continuation of an
existing conversation. As a result, removing and adding of
connections is an expensive operation, and it is recommended that
the hash table only be recomputed once a connection is closed or
returned to service.
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RFC 3539 AAA Transport Profile June 2003
Suspended connections, although they are not used, do not force
hash table reconfiguration until they are closed. Similarly,
re-opened connections not accumulating sufficient watchdog
responses do not force a reconfiguration until they are returned
to service.
While a connection is suspended, transactions that were to have
been assigned to it are instead assigned to the next available
server. While this results in a momentary imbalance, it is felt
that this is a relatively small price to pay in order to reduce
hash table thrashing.
[4] In order to enable diagnosis of load balancing behavior, it is
recommended that in addition to a table of failover events, a
table of statistics be kept on each client, indexed by a AAA
server. That way, the effectiveness of the load balancing
algorithm can be evaluated.
Multiple facilities are required to enable duplicate detection.
These include session identifiers as well as hop-by-hop and end-to-
end message identifiers. Hop-by-hop identifiers whose value may
change at each hop are not sufficient, since a AAA server may receive
the same message from multiple agents. For example, a AAA client can
send a request to Agent1, then failover and resend the request to
Agent2; both agents forward the request to the home AAA server, with
different hop-by-hop identifiers. A Session Identifier is
insufficient as it does not distinguish different messages for the
the same session.
Proper treatment of the end-to-end message identifier ensures that
AAA operations are idempotent. For example, without an end-to-end
identifier, a AAA server keeping track of simultaneous logins might
send an Accept in response to an initial Request, and then a Reject
in response to a duplicate Request (where the user was allowed only
one simultaneous login). Depending on which Response arrived first,
the user might be allowed access or not.
However, if the server were to store the end-to-end message
identifier along with the simultaneous login information, then the
duplicate Request (which utilizes the same end-to-end message
identifier) could be identified and the correct response could be
returned.
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RFC 3539 AAA Transport Profile June 2003
In order to address invalidation of transport parameter estimates,
AAA protocol implementations MAY utilize Congestion Window Validation
[RFC2861] and RTO validation when using TCP. This specification also
recommends a procedure for RTO validation.
[RFC2581] and [RFC2861] both recommend that a connection go into
slow-start after a period where no traffic has been sent within the
RTO interval. [RFC2861] recommends only increasing the congestion
window if it was full when the ACK arrived. The congestion window is
reduced by half once every RTO interval if no traffic is received.
When Congestion Window Validation is used, the congestion window will
not build during application-driven periods, and instead will be
decayed. As a result, AAA applications operating within the
application-driven regime will typically run with a congestion window
equal to the initial window much of the time, operating in "perpetual
slowstart".
During periods in which AAA behavior is application-driven this will
have no effect. Since the time between packets will be larger than
RTT, AAA will operate with an effective congestion window equal to
the initial window. However, during network-driven periods, the
effect will be to space out sending of AAA packets. Thus instead of
being able to send a large burst of packets into the network, a
client will need to wait several RTTs as the congestion window builds
during slow-start.
For example, a client operating over TCP with an initial window of 2,
with 35 AAA requests to send would take approximately 6 RTTs to send
them, as the congestion window builds during slow start: 2, 3, 3, 6,
9, 12. After the backlog is cleared, the implementation will once
again be application-driven and the congestion window size will
decay. If the client were using SCTP, the number of RTTs needed to
transmit all requests would usually be less, and would depend on the
size of the requests, since SCTP tracks the progress for the opening
of the congestion window by bytes, not segments.
Note that [RFC2861] and [RFC2988] do not address the issue of RTO
validation. This is also a problem, particularly when the Congestion
Manager [RFC3124] is implemented. During periods of high packet
loss, the RTO may be repeatedly increased via exponential back-off,
and may attain a high value. Due to lack of timely feedback on RTT
and RTO during application-driven periods, the high RTO estimate may
persist long after the conditions that generated it have dissipated.
Aboba & Wood Standards Track [Page 20]
RFC 3539 AAA Transport Profile June 2003
RTO validation MAY be used to address this issue for TCP, via the
following procedure:
After the congestion window is decayed according to [RFC2861],
reset the estimated RTO to 3 seconds. After the next packet comes
in, re-calculate RTTavg, RTTdev, and RTO according to the method
described in [RFC2581].
To address this issue for SCTP, AAA implementations SHOULD use SCTP
heartbeats. [RFC2960] states that heartbeats should be enabled by
default, with an interval of 30 seconds. If this interval proves to
be too long to resolve this issue, AAA implementations MAY reduce the
heartbeat interval.
When Congestion Window Validation [RFC2861] is used, AAA
implementations will operate with a congestion window equal to the
initial window much of the time. As a result, the window size will
often not be large enough to enable use of fast re-transmit for TCP.
In addition, since AAA traffic is two-way, ACKs carrying data will
not count towards triggering fast re-transmit. SCTP is less likely
to encounter this issue, so the measures described below apply to
TCP.
To address this issue, AAA implementations SHOULD support selective
acknowledgement as described in [RFC2018] and [RFC2883]. AAA
implementations SHOULD also implement Limited Transmit for TCP, as
described in [RFC3042]. Rather than reducing the number of duplicate
ACKs required for triggering fast recovery, which would increase the
number of inappropriate re-transmissions, Limited Transmit enables
the window size be increased, thus enabling the sending of additional
packets which in turn may trigger fast re-transmit without a change
to the algorithm.
However, if congestion window validation [RFC2861] is implemented,
this proposal will only have an effect in situations where the time
between packets is less than the estimated retransmission timeout
(RTO). If the time between packets is greater than RTO, additional
packets will typically not be available for sending so as to take
advantage of the increased window size. As a result, AAA protocols
will typically operate with the lowest possible congestion window
size, resulting in a re-transmission timeout for every lost packet.
Aboba & Wood Standards Track [Page 21]
RFC 3539 AAA Transport Profile June 2003
TCP inherently does not provide a solution to the head-of-line
blocking problem, although its effects can be lessened by
implementation of Limited Transmit [RFC3042], and connection load
balancing.
Each AAA node SHOULD distribute its messages evenly across the range
of SCTP streams that it and its peer have agreed upon. (A lost
message in one stream will not cause any other streams to block.) A
trivial and effective implementation of this simply increments a
counter for the stream ID to send on. When the counter reaches the
maximum number of streams for the association, it resets to 0.
AAA peers MUST be able to accept messages on any stream. Note that
streams are used *solely* to prevent head-of-the-line blocking. All
identifying information is carried within the Diameter payload.
Messages distributed across multiple streams may not be received in
the order they are sent.
SCTP peers can allocate up to 65535 streams for an association. The
cost for idle streams may or may not be zero, depending on the
implementation, and the cost for non-idle streams is always greater
than 0. So administrators may wish to limit the number of possible
streams on their diameter nodes according to the resources (i.e.
memory, CPU power, etc.) of a particular node.
On a Diameter client, the number of streams may be determined by the
maximum number of peak users on the NAS. If a stream is available
per user, then this should be sufficient to prevent head-of-line
blocking. On a Diameter proxy, the number of streams may be
determined by the maximum number of peak sessions in progress from
that proxy to each downstream AAA server.
Stream IDs do not need to be preserved by relay agents. This
simplifies implementation, as agents can easily handle forwarding
between two associations with different numbers of streams. For
example, consider the following case, where a relay server DRL
forwards messages between a NAS and a home server, HMS. The NAS and
DRL have agreed upon 1000 streams for their association, and DRL and
HMS have agreed upon 2000 streams for their association. The
following figure shows the message flow from NAS to HMS via DRL, and
the stream ID assignments for each message:
Aboba & Wood Standards Track [Page 22]
RFC 3539 AAA Transport Profile June 2003
+------+ +------+ +------+
| | | | | |
| NAS | ---------> | DRL | ---------> | HMS |
| | | | | |
+------+ 1000 streams +------+ 2000 streams +------+
msg 1: str id 0 msg 1: str id 0
msg 2: str id 1 msg 2: str id 1
...
msg 1000: str id 999 msg 1000: str id 999
msg 1001: str id 0 msg 1001: str id 1000
DRL can forward messages 1 through 1000 to HMS using the same stream
ID that NAS used to send to DRL. However, since the NAS / DRL
association has only 1000 streams, NAS wraps around to stream ID 0
when sending message 1001. The DRL / HMS association, on the other
hand, has 2000 streams, so DRL can reassign message 1001 to stream ID
1000 when forwarding it on to HMS.
This distribution scheme acts like a hash table. It is possible, yet
unlikely, that two messages will end up in the same stream, and even
less likely that there will be message loss resulting in blocking
when this happens. If it does turn out to be a problem, local
administrators can increase the number of streams on their nodes to
improve performance.
In order to improve upon default timer estimates, AAA implementations
MAY implement the Congestion Manager (CM) [RFC3124]. CM is an end-
system module that:
(i) Enables an ensemble of multiple concurrent streams from a
sender destined to the same receiver and sharing the same
congestion properties to perform proper congestion avoidance
and control, and
(ii) Allows applications to easily adapt to network congestion.
The CM helps integrate congestion management across all applications
and transport protocols. The CM maintains congestion parameters
(available aggregate and per-stream bandwidth, per-receiver round-
trip times, etc.) and exports an API that enables applications to
learn about network characteristics, pass information to the CM,
share congestion information with each other, and schedule data
transmissions.
Aboba & Wood Standards Track [Page 23]
RFC 3539 AAA Transport Profile June 2003
The CM enables the AAA application to access transport parameters
(RTTavg, RTTdev) via callbacks. RTO estimates are currently not
available via the callback interface, though they probably should be.
Where available, transport parameters SHOULD be used to improve upon
default timer values.
Premature failover is prevented by the watchdog functionality
described above. If the next hop does not return a reply, the AAA
client will send a watchdog message to it to verify liveness. If a
watchdog reply is received, then the AAA client will know that the
next hop server is functioning at the application layer. As a
result, it is only necessary to provide terminal error messages, such
as the following:
"Busy": agent/Server too busy to handle additional requests, NAS
should failover all requests to another agent/server.
"Can't Locate": agent can't locate the AAA server for the
indicated realm; NAS should failover that request to another
proxy.
"Can't Forward": agent has tried both primary and secondary AAA
servers with no response; NAS should failover the request to
another agent.
Note that these messages differ in their scope. The "Busy" message
tells the NAS that the agent/server is too busy for ANY request. The
"Can't Locate" and "Can't Forward" messages indicate that the
ultimate destination cannot be reached or isn't responding, implying
per-request failover.
Since AAA clients, agents and servers serve as network access
gatekeepers, they are tempting targets for attackers. General
security considerations concerning TCP congestion control are
discussed in [RFC2581]. However, there are some additional
considerations that apply to this specification.
By enabling failover between AAA agents, this specification improves
the resilience of AAA applications. However, it may also open
avenues for denial of service attacks.
The failover algorithm is driven by lack of response to AAA requests
and watchdog packets. On a lightly loaded network where AAA
responses would not be received prior to expiration of the watchdog
Aboba & Wood Standards Track [Page 24]
RFC 3539 AAA Transport Profile June 2003
timer, an attacker can swamp the network, causing watchdog packets to
be dropped. This will cause the AAA client to switch to another AAA
agent, where the attack can be repeated. By causing the AAA client
to cycle between AAA agents, service can be denied to users desiring
network access.
Where TLS [RFC2246] is being used to provide AAA security, there will
be a vulnerability to spoofed reset packets, as well as other
transport layer denial of service attacks (e.g. SYN flooding). Since
SCTP offers improved denial of service resilience compared with TCP,
where AAA applications run over SCTP, this can be mitigated to some
extent.
Where IPsec [RFC2401] is used to provide security, it is important
that IPsec policy require IPsec on incoming packets. In order to
enable a AAA client to determine what security mechanisms are in use
on an agent or server without prior knowledge, it may be tempting to
initiate a connection in the clear, and then to have the AAA agent
respond with IKE [RFC2409]. While this approach minimizes required
client configuration, it increases the vulnerability to denial of
service attack, since a connection request can now not only tie up
transport resources, but also resources within the IKE
implementation.
[RFC793] Postel, J., "Transmission Control Protocol", STD 7, RFC
793, September 1981.
[RFC896] Nagle, J., "Congestion Control in IP/TCP internetworks",
RFC 896, January 1984.
[RFC1750] Eastlake, D., Crocker, S. and J. Schiller, "Randomness
Recommendations for Security", RFC 1750, December 1994.
[RFC2018] Mathis, M., Mahdavi, J., Floyd, S. and A. Romanow, "TCP
Selective Acknowledgment Options", RFC 2018, October 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
Aboba & Wood Standards Track [Page 25]
RFC 3539 AAA Transport Profile June 2003
[RFC2486] Aboba, B. and M. Beadles, "The Network Access Identifier",
RFC 2486, January 1999.
[RFC2581] Allman, M., Paxson, V. and W. Stevens, "TCP Congestion
Control", RFC 2581, April 1999.
[RFC2883] Floyd, S., Mahdavi, J., Mathis, M., Podolsky, M. and A.
Romanow, "An Extension to the Selective Acknowledgment
(SACK) Option for TCP", RFC 2883, July 2000.
[RFC2960] Stewart, R., Xie, Q., Morneault, K., Sharp, C.,
Schwarzbauer, H., Taylor, T., Rytina, I., Kalla, M., Zhang,
L. and V. Paxson, "Stream Control Transmission Protocol",
RFC 2960, October 2000.
[RFC2988] Paxson, V. and M. Allman, "Computing TCP's Retransmission
Timer", RFC 2988, November 2000.
[RFC3042] Allman, M., Balakrishnan H. and S. Floyd, "Enhancing TCP's
Loss Recovery Using Limited Transmit", RFC 3042, January
2001.
[RFC3074] Volz, B., Gonczi, S., Lemon, T. and R. Stevens, "DHC Load
Balancing Algorithm", RFC 3074, February 2001.
[RFC3124] Balakrishnan, H. and S. Seshan, "The Congestion Manager",
RFC 3124, June 2001.
[RFC2246] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
RFC 2246, January 1999.
[RFC2401] Atkinson, R. and S. Kent, "Security Architecture for the
Internet Protocol", RFC 2401, November 1998.
[RFC2409] Harkins, D. and D. Carrel, "The Internet Key Exchange
(IKE)", RFC 2409, November 1998.
[RFC2607] Aboba, B. and J. Vollbrecht, "Proxy Chaining and Policy
Implementation in Roaming", RFC 2607, June 1999.
[RFC2861] Handley, M., Padhye, J. and S. Floyd, "TCP Congestion
Window Validation", RFC 2861, June 2000.
[RFC2865] Rigney, C., Willens, S., Rubens, A. and W. Simpson, "Remote
Authentication Dial In User Service (RADIUS)", RFC 2865,
June 2000.
Aboba & Wood Standards Track [Page 26]
RFC 3539 AAA Transport Profile June 2003
[RFC2866] Rigney, C., "RADIUS Accounting", RFC 2866, June 2000.
[RFC2914] Floyd, S., "Congestion Control Principles", BCP 41, RFC
2914, September 2000.
[RFC2975] Aboba, B., Arkko, J. and D. Harrington, "Introduction to
Accounting Management", RFC 2975, June 2000.
[RFC3390] Allman, M., Floyd, S. and C. Partridge, "Increasing TCP's
Initial Window", RFC 3390, October 2002.
[Congest] Jacobson, V., "Congestion Avoidance and Control", Computer
Communication Review, vol. 18, no. 4, pp. 314-329, Aug.
1988. ftp://ftp.ee.lbl.gov/papers/congavoid.ps.Z
[Paxson] Paxson, V., "Measurement and Analysis of End-to-End
Internet Dynamics", Ph.D. Thesis, Computer Science
Division, University of California, Berkeley, April 1997.
Aboba & Wood Standards Track [Page 27]
RFC 3539 AAA Transport Profile June 2003
Appendix A - Detailed Watchdog Algorithm
In this Appendix, the memory control structure that contains all
information regarding a specific peer is referred to as a Peer
Control Block, or PCB. The PCB contains the following fields:
Status:
OKAY: The connection is up
SUSPECT: Failover has been initiated on the connection.
DOWN: Connection has been closed.
REOPEN: Attempting to reopen a closed connection
INITIAL: The initial state of the pcb when it is first created.
The pcb has never been opened.
Variables:
Pending: Set to TRUE if there is an outstanding unanswered
watchdog request
Tw: Watchdog timer value
NumDWA: Number of DWAs received during REOPEN
Tw is the watchdog timer, measured in seconds. Every second, Tw is
decremented. When it reaches 0, the OnTimerElapsed event (see below)
is invoked. Pseudo-code for the algorithm is included on the
following pages.
SetWatchdog()
{
/*
SetWatchdog() is called whenever it is necessary
to reset the watchdog timer Tw. The value of the
watchdog timer is calculated based on the default
initial value TWINIT and a jitter ranging from
-2 to 2 seconds. The default for TWINIT is 30 seconds,
and MUST NOT be set lower than 6 seconds.
*/
Tw=TWINIT -2.0 + 4.0 * random() ;
SetTimer(Tw) ;
return ;
}
/*
OnReceive() is called whenever a message
is received from the peer. This message MAY
be a request or an answer, and can include
DWR and DWA messages. Pending is assumed to
be a global variable.
*/
OnReceive(pcb, msgType)
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RFC 3539 AAA Transport Profile June 2003
{
if (msgType == DWA) {
Pending = FALSE;
}
switch (pcb->Status){
case OKAY:
SetWatchdog();
break;
case SUSPECT:
pcb->Status = OKAY;
Failback(pcb);
SetWatchdog();
break;
case REOPEN:
if (msgType == DWA) {
NumDWA++;
if (NumDWA == 3) {
pcb->status = OKAY;
Failback();
}
} else {
Throwaway(received packet);
}
break;
case INITIAL:
case DOWN:
Throwaway(received packet);
break;
default:
Error("Shouldn't be here!");
break;
}
}
/*
OnTimerElapsed() is called whenever Tw reaches zero (0).
*/
OnTimerElapsed(pcb)
{
switch (pcb->status){
case OKAY:
if (!Pending) {
SendWatchdog(pcb);
SetWatchdog();
Pending = TRUE;
break;
}
pcb->status = SUSPECT;
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RFC 3539 AAA Transport Profile June 2003
FailOver(pcb);
SetWatchdog();
break ;
case SUSPECT:
pcb->status = DOWN;
CloseConnection(pcb);
SetWatchdog();
break;
case INITIAL:
case DOWN:
AttemptOpen(pcb);
SetWatchdog();
break;
case REOPEN:
if (!Pending) {
SendWatchdog(pbc);
SetWatchdog();
Pending = TRUE;
break;
}
if (NumDWA < 0) {
pcb->status = DOWN;
CloseConnection(pcb);
} else {
NumDWA = -1;
}
SetWatchdog();
break;
default:
error("Shouldn't be here!);
break;
}
}
/*
OnConnectionUp() is called whenever a connection comes up
*/
OnConnectionUp(pcb)
{
switch (pcb->status){
case INITIAL:
pcb->status = OKAY;
SetWatchdog();
break;
case DOWN:
pcb->status = REOPEN;
NumDWA = 0;
SendWatchdog(pcb);
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RFC 3539 AAA Transport Profile June 2003
SetWatchdog();
Pending = TRUE;
break;
default:
error("Shouldn't be here!);
break;
}
}
/*
OnConnectionDown() is called whenever a connection goes down
*/
OnConnectionDown(pcb)
{
pcb->status = DOWN;
CloseConnection();
switch (pcb->status){
case OKAY:
Failover(pcb);
SetWatchdog();
break;
case SUSPECT:
case REOPEN:
SetWatchdog();
break;
default:
error("Shouldn't be here!);
break;
}
}
/* Here is the state machine equivalent to the above code:
STATE Event Actions New State
===== ------ ------- ----------
OKAY Receive DWA Pending = FALSE
SetWatchdog() OKAY
OKAY Receive non-DWA SetWatchdog() OKAY
SUSPECT Receive DWA Pending = FALSE
Failback()
SetWatchdog() OKAY
SUSPECT Receive non-DWA Failback()
SetWatchdog() OKAY
REOPEN Receive DWA & Pending = FALSE
NumDWA == 2 NumDWA++
Failback() OKAY
REOPEN Receive DWA & Pending = FALSE
NumDWA < 2 NumDWA++ REOPEN
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RFC 3539 AAA Transport Profile June 2003
STATE Event Actions New State
===== ------ ------- ----------
REOPEN Receive non-DWA Throwaway() REOPEN
INITIAL Receive DWA Pending = FALSE
Throwaway() INITIAL
INITIAL Receive non-DWA Throwaway() INITIAL
DOWN Receive DWA Pending = FALSE
Throwaway() DOWN
DOWN Receive non-DWA Throwaway() DOWN
OKAY Timer expires & SendWatchdog()
!Pending SetWatchdog()
Pending = TRUE OKAY
OKAY Timer expires & Failover()
Pending SetWatchdog() SUSPECT
SUSPECT Timer expires CloseConnection()
SetWatchdog() DOWN
INITIAL Timer expires AttemptOpen()
SetWatchdog() INITIAL
DOWN Timer expires AttemptOpen()
SetWatchdog() DOWN
REOPEN Timer expires & SendWatchdog()
!Pending SetWatchdog()
Pending = TRUE REOPEN
REOPEN Timer expires & CloseConnection()
Pending & SetWatchdog()
NumDWA < 0 DOWN
REOPEN Timer expires & NumDWA = -1
Pending & SetWatchdog()
NumDWA >= 0 REOPEN
INITIAL Connection up SetWatchdog() OKAY
DOWN Connection up NumDWA = 0
SendWatchdog()
SetWatchdog()
Pending = TRUE REOPEN
OKAY Connection down CloseConnection()
Failover()
SetWatchdog() DOWN
SUSPECT Connection down CloseConnection()
SetWatchdog() DOWN
REOPEN Connection down CloseConnection()
SetWatchdog() DOWN
*/
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RFC 3539 AAA Transport Profile June 2003
Appendix B - AAA Agents
As described in [RFC2865] and [RFC2607], AAA agents have become
popular in order to support services such as roaming and shared use
networks. Such agents are used both for
authentication/authorization, as well as accounting [RFC2975].
AAA agents include:
Relays
Proxies
Re-directs
Store and Forward proxies
Transport layer proxies
The transport layer behavior of each of these agents is described
below.
While the application-layer behavior of relays and proxies are
different, at the transport layer the behavior is similar. In both
cases, two connections are established: one from the AAA client (NAS)
to the relay/proxy, and another from the relay/proxy to the AAA
server. The relay/proxy does not respond to a client request until
it receives a response from the server. Since the two connections
are de-coupled, the end-to-end conversation between the client and
server may not self clock.
Since AAA transport is typically application-driven, there is
frequently not enough traffic to enable ACK piggybacking. As a
result, the Nagle algorithm is rarely triggered, and delayed ACKs may
comprise nearly half the traffic. Thus AAA protocols running over
reliable transport will see packet traffic nearly double that
experienced with UDP transport. Since ACK parameters (such as the
value of the delayed ACK timer) are typically fixed by the TCP
implementation and are not tunable by the application, there is
little that can be done about this.
Aboba & Wood Standards Track [Page 33]
RFC 3539 AAA Transport Profile June 2003
A typical trace of a conversation between a NAS, proxy and server is
shown below:
Time NAS Relay/Proxy Server
------ --- ----------- ------
0 Request
------->
OTTnp + Tpr Request
------->
OTTnp + TdA Delayed ACK
<-------
OTTnp + OTTps + Reply/ACK
Tpr + Tsr <-------
OTTnp + OTTps +
Tpr + Tsr + Reply
OTTsp + TpR <-------
OTTnp + OTTps +
Tpr + Tsr + Delayed ACK
OTTsp + TdA ------->
OTTnp + OTTps +
OTTsp + OTTpn +
Tpr + Tsr + Delayed ACK
TpR + TdA ------->
Key
---
OTT = One-way Trip Time
OTTnp = One-way trip time (NAS to Relay/Proxy)
OTTpn = One-way trip time (Relay/Proxy to NAS)
OTTps = One-way trip time (Relay/Proxy to Server)
OTTsp = One-way trip time (Server to Relay/Proxy)
TdA = Delayed ACK timer
Tpr = Relay/Proxy request processing time
TpR = Relay/Proxy reply processing time
Tsr = Server request processing time
At time 0, the NAS sends a request to the relay/proxy. Ignoring the
serialization time, the request arrives at the relay/proxy at time
OTTnp, and the relay/proxy takes an additional Tpr in order to
forward the request toward the home server. At time TdA after
Aboba & Wood Standards Track [Page 34]
RFC 3539 AAA Transport Profile June 2003
receiving the request, the relay/proxy sends a delayed ACK. The
delayed ACK is sent, rather than being piggybacked on the reply, as
long as TdA < OTTps + OTTsp + Tpr + Tsr + TpR.
Typically Tpr < TdA, so that the delayed ACK is sent after the
relay/proxy forwards the request toward the server, but before the
relay/proxy receives the reply from the server. However, depending
on the TCP implementation on the relay/proxy and when the request is
received, it is also possible for the delayed ACK to be sent prior to
forwarding the request.
At time OTTnp + OTTps + Tpr, the server receives the request, and Tsr
later, it generates the reply. Where Tsr < TdA, the reply will
contain a piggybacked ACK. However, depending on the server
responsiveness and TCP implementation, the ACK and reply may be sent
separately. This can occur, for example, where a slow database or
storage system must be accessed prior to sending the reply.
At time OTTnp + OTTps + OTTsp + Tpr + Tsr the reply/ACK reaches the
relay/proxy, which then takes TpR additional time to forward the
reply to the NAS. At TdA after receiving the reply, the relay/proxy
generates a delayed ACK. Typically TpR < TdA so that the delayed ACK
is sent to the server after the relay/proxy forwards the reply to the
NAS. However, depending on the circumstances and the relay/proxy TCP
implementation, the delayed ACK may be sent first.
As with a delayed ACK sent in response to a request, which may be
piggybacked if the reply can be received quickly enough, piggybacking
of the ACK sent in response to a reply from the server is only
possible if additional request traffic is available. However, due to
the high inter-packet spacings in typical AAA scenarios, this is
unlikely unless the AAA protocol supports a reply ACK.
At time OTTnp + OTTps + OTTsp + OTTpn + Tpr + Tsr + TpR the NAS
receives the reply. TdA later, a delayed ACK is generated.
Re-directs operate by referring a NAS to the AAA server, enabling the
NAS to talk to the AAA server directly. Since a direct transport
connection is established, the end-to-end connection will self-clock.
With re-directs, delayed ACKs are less frequent than with
application-layer proxies since the Re-direct and Server will
typically piggyback replies with ACKs.
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RFC 3539 AAA Transport Profile June 2003
The sequence of events is as follows:
Time NAS Re-direct Server
------ --- --------- ------
0 Request
------->
OTTnp + Tpr Redirect/ACK
<-------
OTTnp + Tpr + Request
OTTpn + Tnr ------->
OTTnp + OTTpn +
Tpr + Tsr + Reply/ACK
OTTns <-------
OTTnp + OTTpn +
OTTns + OTTsn +
Tpr + Tsr + Delayed ACK
TdA ------->
Key
---
OTT = One-way Trip Time
OTTnp = One-way trip time (NAS to Re-direct)
OTTpn = One-way trip time (Re-direct to NAS)
OTTns = One-way trip time (NAS to Server)
OTTsn = One-way trip time (Server to NAS)
TdA = Delayed ACK timer
Tpr = Re-direct processing time
Tnr = NAS re-direct processing time
Tsr = Server request processing time
With a store and forward proxy, the proxy may send a reply to the NAS
prior to forwarding the request to the server. While store and
forward proxies are most frequently deployed for accounting
[RFC2975], they also can be used to implement
authentication/authorization policy, as described in [RFC2607].
As noted in [RFC2975], store and forward proxies can have a negative
effect on accounting reliability. By sending a reply to the NAS
without receiving one from the accounting server, store and forward
proxies fool the NAS into thinking that the accounting request had
been accepted by the accounting server when this is not the case. As
a result, the NAS can delete the accounting packet from non-volatile
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RFC 3539 AAA Transport Profile June 2003
storage before it has been accepted by the accounting server. That
leaves the proxy responsible for delivering accounting packets. If
the proxy involves moving parts (e.g. a disk drive) while the NAS
does not, overall system reliability can be reduced. As a result,
store and forward proxies SHOULD NOT be used.
The sequence of events is as follows:
Time NAS Proxy Server
------ --- ----- ------
0 Request
------->
OTTnp + TpR Reply/ACK
<-------
OTTnp + Tpr Request
------->
OTTnp + OTTph + Reply/ACK
Tpr + Tsr <-------
OTTnp + OTTph +
Tpr + Tsr + Reply
OTThp + TpR <-------
OTTnp + OTTph +
Tpr + Tsr + Delayed ACK
OTThp + TdA ------->
OTTnp + OTTph +
OTThp + OTTpn +
Tpr + Tsr + Delayed ACK
TpR + TdA ------->
Key
---
OTT = One-way Trip Time
OTTnp = One-way trip time (NAS to Proxy)
OTTpn = One-way trip time (Proxy to NAS)
OTTph = One-way trip time (Proxy to Home server)
OTThp = One-way trip time (Home Server to Proxy)
TdA = Delayed ACK timer
Tpr = Proxy request processing time
TpR = Proxy reply processing time
Tsr = Server request processing time
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RFC 3539 AAA Transport Profile June 2003
In addition to acting as proxies at the application layer, transport
layer proxies forward transport ACKs between the AAA client and
server. This splices together the client-proxy and proxy-server
connections into a single connection that behaves as though it
operates end-to-end, exhibiting self-clocking. However, since
transport proxies operate at the transport layer, they cannot be
implemented purely as applications and they are rarely deployed.
With a transport proxy, the sequence of events is as follows:
Time NAS Proxy Home Server
------ --- ----- -----------
0 Request
------->
OTTnp + Tpr Request
------->
OTTnp + OTTph + Reply/ACK
Tpr + Tsr <-------
OTTnp + OTTph +
Tpr + Tsr + Reply/ACK
OTThp + TpR <-------
OTTnp + OTTph +
OTThp + OTTpn +
Tpr + Tsr + Delayed ACK
TpR + TdA ------->
OTTnp + OTTph +
OTThp + OTTpn +
Tpr + Tsr + Delayed ACK
TpR + TpD ------->
Key
---
OTT = One-way Trip Time
OTTnp = One-way trip time (NAS to Proxy)
OTTpn = One-way trip time (Proxy to NAS)
OTTph = One-way trip time (Proxy to Home server)
OTThp = One-way trip time (Home Server to Proxy)
TdA = Delayed ACK timer
Tpr = Proxy request processing time
TpR = Proxy reply processing time
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RFC 3539 AAA Transport Profile June 2003
Tsr = Server request processing time
TpD = Proxy delayed ack processing time
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Acknowledgments
Thanks to Allison Mankin of AT&T, Barney Wolff of Databus, Steve Rich
of Cisco, Randy Bush of AT&T, Bo Landarv of IP Unplugged, Jari Arkko
of Ericsson, and Pat Calhoun of Blackstorm Networks for fruitful
discussions relating to AAA transport.
Aboba & Wood Standards Track [Page 39]
RFC 3539 AAA Transport Profile June 2003
Authors' Addresses
Bernard Aboba
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
Phone: +1 425 706 6605
Fax: +1 425 936 7329
EMail: bernarda@microsoft.com
Jonathan Wood
Sun Microsystems, Inc.
901 San Antonio Road
Palo Alto, CA 94303
EMail: jonwood@speakeasy.net
Aboba & Wood Standards Track [Page 40]
RFC 3539 AAA Transport Profile June 2003
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Aboba & Wood Standards Track [Page 41]