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.
This document specifies the architecture for Multiprotocol Label
Switching (MPLS).
Note that the use of MPLS for multicast is left for further study.
Rosen, et al. Standards Track [Page 3]
RFC 3031 MPLS Architecture January 2001
As a packet of a connectionless network layer protocol travels from
one router to the next, each router makes an independent forwarding
decision for that packet. That is, each router analyzes the packet's
header, and each router runs a network layer routing algorithm. Each
router independently chooses a next hop for the packet, based on its
analysis of the packet's header and the results of running the
routing algorithm.
Packet headers contain considerably more information than is needed
simply to choose the next hop. Choosing the next hop can therefore
be thought of as the composition of two functions. The first
function partitions the entire set of possible packets into a set of
"Forwarding Equivalence Classes (FECs)". The second maps each FEC to
a next hop. Insofar as the forwarding decision is concerned,
different packets which get mapped into the same FEC are
indistinguishable. All packets which belong to a particular FEC and
which travel from a particular node will follow the same path (or if
certain kinds of multi-path routing are in use, they will all follow
one of a set of paths associated with the FEC).
In conventional IP forwarding, a particular router will typically
consider two packets to be in the same FEC if there is some address
prefix X in that router's routing tables such that X is the "longest
match" for each packet's destination address. As the packet
traverses the network, each hop in turn reexamines the packet and
assigns it to a FEC.
In MPLS, the assignment of a particular packet to a particular FEC is
done just once, as the packet enters the network. The FEC to which
the packet is assigned is encoded as a short fixed length value known
as a "label". When a packet is forwarded to its next hop, the label
is sent along with it; that is, the packets are "labeled" before they
are forwarded.
At subsequent hops, there is no further analysis of the packet's
network layer header. Rather, the label is used as an index into a
table which specifies the next hop, and a new label. The old label
is replaced with the new label, and the packet is forwarded to its
next hop.
In the MPLS forwarding paradigm, once a packet is assigned to a FEC,
no further header analysis is done by subsequent routers; all
forwarding is driven by the labels. This has a number of advantages
over conventional network layer forwarding.
Rosen, et al. Standards Track [Page 4]
RFC 3031 MPLS Architecture January 2001
- MPLS forwarding can be done by switches which are capable of
doing label lookup and replacement, but are either not capable
of analyzing the network layer headers, or are not capable of
analyzing the network layer headers at adequate speed.
- Since a packet is assigned to a FEC when it enters the network,
the ingress router may use, in determining the assignment, any
information it has about the packet, even if that information
cannot be gleaned from the network layer header. For example,
packets arriving on different ports may be assigned to
different FECs. Conventional forwarding, on the other hand,
can only consider information which travels with the packet in
the packet header.
- A packet that enters the network at a particular router can be
labeled differently than the same packet entering the network
at a different router, and as a result forwarding decisions
that depend on the ingress router can be easily made. This
cannot be done with conventional forwarding, since the identity
of a packet's ingress router does not travel with the packet.
- The considerations that determine how a packet is assigned to a
FEC can become ever more and more complicated, without any
impact at all on the routers that merely forward labeled
packets.
- Sometimes it is desirable to force a packet to follow a
particular route which is explicitly chosen at or before the
time the packet enters the network, rather than being chosen by
the normal dynamic routing algorithm as the packet travels
through the network. This may be done as a matter of policy,
or to support traffic engineering. In conventional forwarding,
this requires the packet to carry an encoding of its route
along with it ("source routing"). In MPLS, a label can be used
to represent the route, so that the identity of the explicit
route need not be carried with the packet.
Some routers analyze a packet's network layer header not merely to
choose the packet's next hop, but also to determine a packet's
"precedence" or "class of service". They may then apply different
discard thresholds or scheduling disciplines to different packets.
MPLS allows (but does not require) the precedence or class of service
to be fully or partially inferred from the label. In this case, one
may say that the label represents the combination of a FEC and a
precedence or class of service.
Rosen, et al. Standards Track [Page 5]
RFC 3031 MPLS Architecture January 2001
MPLS stands for "Multiprotocol" Label Switching, multiprotocol
because its techniques are applicable to ANY network layer protocol.
In this document, however, we focus on the use of IP as the network
layer protocol.
A router which supports MPLS is known as a "Label Switching Router",
or LSR.
This section gives a general conceptual overview of the terms used in
this document. Some of these terms are more precisely defined in
later sections of the document.
DLCI a label used in Frame Relay networks to
identify frame relay circuits
forwarding equivalence class a group of IP packets which are
forwarded in the same manner (e.g.,
over the same path, with the same
forwarding treatment)
frame merge label merging, when it is applied to
operation over frame based media, so
that the potential problem of cell
interleave is not an issue.
label a short fixed length physically
contiguous identifier which is used to
identify a FEC, usually of local
significance.
label merging the replacement of multiple incoming
labels for a particular FEC with a
single outgoing label
label swap the basic forwarding operation
consisting of looking up an incoming
label to determine the outgoing label,
encapsulation, port, and other data
handling information.
label swapping a forwarding paradigm allowing
streamlined forwarding of data by using
labels to identify classes of data
packets which are treated
indistinguishably when forwarding.
Rosen, et al. Standards Track [Page 6]
RFC 3031 MPLS Architecture January 2001
label switched hop the hop between two MPLS nodes, on which
forwarding is done using labels.
label switched path The path through one or more LSRs at one
level of the hierarchy followed by a
packets in a particular FEC.
label switching router an MPLS node which is capable of
forwarding native L3 packets
layer 2 the protocol layer under layer 3 (which
therefore offers the services used by
layer 3). Forwarding, when done by the
swapping of short fixed length labels,
occurs at layer 2 regardless of whether
the label being examined is an ATM
VPI/VCI, a frame relay DLCI, or an MPLS
label.
layer 3 the protocol layer at which IP and its
associated routing protocols operate
link layer synonymous with layer 2
loop detection a method of dealing with loops in which
loops are allowed to be set up, and data
may be transmitted over the loop, but
the loop is later detected
loop prevention a method of dealing with loops in which
data is never transmitted over a loop
label stack an ordered set of labels
merge point a node at which label merging is done
MPLS domain a contiguous set of nodes which operate
MPLS routing and forwarding and which
are also in one Routing or
Administrative Domain
MPLS edge node an MPLS node that connects an MPLS
domain with a node which is outside of
the domain, either because it does not
run MPLS, and/or because it is in a
different domain. Note that if an LSR
has a neighboring host which is not
running MPLS, that that LSR is an MPLS
edge node.
Rosen, et al. Standards Track [Page 7]
RFC 3031 MPLS Architecture January 2001
MPLS egress node an MPLS edge node in its role in
handling traffic as it leaves an MPLS
domain
MPLS ingress node an MPLS edge node in its role in
handling traffic as it enters an MPLS
domain
MPLS label a label which is carried in a packet
header, and which represents the
packet's FEC
MPLS node a node which is running MPLS. An MPLS
node will be aware of MPLS control
protocols, will operate one or more L3
routing protocols, and will be capable
of forwarding packets based on labels.
An MPLS node may optionally be also
capable of forwarding native L3 packets.
MultiProtocol Label Switching an IETF working group and the
effort associated with the working
group
network layer synonymous with layer 3
stack synonymous with label stack
switched path synonymous with label switched path
virtual circuit a circuit used by a connection-oriented
layer 2 technology such as ATM or Frame
Relay, requiring the maintenance of
state information in layer 2 switches.
VC merge label merging where the MPLS label is
carried in the ATM VCI field (or
combined VPI/VCI field), so as to allow
multiple VCs to merge into one single VC
VP merge label merging where the MPLS label is
carried din the ATM VPI field, so as to
allow multiple VPs to be merged into one
single VP. In this case two cells would
have the same VCI value only if they
originated from the same node. This
allows cells from different sources to
be distinguished via the VCI.
Rosen, et al. Standards Track [Page 8]
RFC 3031 MPLS Architecture January 2001
VPI/VCI a label used in ATM networks to identify
circuits
The ideas and text in this document have been collected from a number
of sources and comments received. We would like to thank Rick
Boivie, Paul Doolan, Nancy Feldman, Yakov Rekhter, Vijay Srinivasan,
and George Swallow for their inputs and ideas.
A label is a short, fixed length, locally significant identifier
which is used to identify a FEC. The label which is put on a
particular packet represents the Forwarding Equivalence Class to
which that packet is assigned.
Rosen, et al. Standards Track [Page 9]
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Most commonly, a packet is assigned to a FEC based (completely or
partially) on its network layer destination address. However, the
label is never an encoding of that address.
If Ru and Rd are LSRs, they may agree that when Ru transmits a packet
to Rd, Ru will label with packet with label value L if and only if
the packet is a member of a particular FEC F. That is, they can
agree to a "binding" between label L and FEC F for packets moving
from Ru to Rd. As a result of such an agreement, L becomes Ru's
"outgoing label" representing FEC F, and L becomes Rd's "incoming
label" representing FEC F.
Note that L does not necessarily represent FEC F for any packets
other than those which are being sent from Ru to Rd. L is an
arbitrary value whose binding to F is local to Ru and Rd.
When we speak above of packets "being sent" from Ru to Rd, we do not
imply either that the packet originated at Ru or that its destination
is Rd. Rather, we mean to include packets which are "transit
packets" at one or both of the LSRs.
Sometimes it may be difficult or even impossible for Rd to tell, of
an arriving packet carrying label L, that the label L was placed in
the packet by Ru, rather than by some other LSR. (This will
typically be the case when Ru and Rd are not direct neighbors.) In
such cases, Rd must make sure that the binding from label to FEC is
one-to-one. That is, Rd MUST NOT agree with Ru1 to bind L to FEC F1,
while also agreeing with some other LSR Ru2 to bind L to a different
FEC F2, UNLESS Rd can always tell, when it receives a packet with
incoming label L, whether the label was put on the packet by Ru1 or
whether it was put on by Ru2.
It is the responsibility of each LSR to ensure that it can uniquely
interpret its incoming labels.
Suppose Ru and Rd have agreed to bind label L to FEC F, for packets
sent from Ru to Rd. Then with respect to this binding, Ru is the
"upstream LSR", and Rd is the "downstream LSR".
To say that one node is upstream and one is downstream with respect
to a given binding means only that a particular label represents a
particular FEC in packets travelling from the upstream node to the
downstream node. This is NOT meant to imply that packets in that FEC
would actually be routed from the upstream node to the downstream
node.
Rosen, et al. Standards Track [Page 10]
RFC 3031 MPLS Architecture January 2001
A "labeled packet" is a packet into which a label has been encoded.
In some cases, the label resides in an encapsulation header which
exists specifically for this purpose. In other cases, the label may
reside in an existing data link or network layer header, as long as
there is a field which is available for that purpose. The particular
encoding technique to be used must be agreed to by both the entity
which encodes the label and the entity which decodes the label.
In the MPLS architecture, the decision to bind a particular label L
to a particular FEC F is made by the LSR which is DOWNSTREAM with
respect to that binding. The downstream LSR then informs the
upstream LSR of the binding. Thus labels are "downstream-assigned",
and label bindings are distributed in the "downstream to upstream"
direction.
If an LSR has been designed so that it can only look up labels that
fall into a certain numeric range, then it merely needs to ensure
that it only binds labels that are in that range.
A particular binding of label L to FEC F, distributed by Rd to Ru,
may have associated "attributes". If Ru, acting as a downstream LSR,
also distributes a binding of a label to FEC F, then under certain
conditions, it may be required to also distribute the corresponding
attribute that it received from Rd.
A label distribution protocol is a set of procedures by which one LSR
informs another of the label/FEC bindings it has made. Two LSRs
which use a label distribution protocol to exchange label/FEC binding
information are known as "label distribution peers" with respect to
the binding information they exchange. If two LSRs are label
distribution peers, we will speak of there being a "label
distribution adjacency" between them.
(N.B.: two LSRs may be label distribution peers with respect to some
set of bindings, but not with respect to some other set of bindings.)
The label distribution protocol also encompasses any negotiations in
which two label distribution peers need to engage in order to learn
of each other's MPLS capabilities.
Rosen, et al. Standards Track [Page 11]
RFC 3031 MPLS Architecture January 2001
THE ARCHITECTURE DOES NOT ASSUME THAT THERE IS ONLY A SINGLE LABEL
DISTRIBUTION PROTOCOL. In fact, a number of different label
distribution protocols are being standardized. Existing protocols
have been extended so that label distribution can be piggybacked on
them (see, e.g., [MPLS-BGP], [MPLS-RSVP-TUNNELS]). New protocols
have also been defined for the explicit purpose of distributing
labels (see, e.g., [MPLS-LDP], [MPLS-CR-LDP].
In this document, we try to use the acronym "LDP" to refer
specifically to the protocol defined in [MPLS-LDP]; when speaking of
label distribution protocols in general, we try to avoid the acronym.
The MPLS architecture allows an LSR to explicitly request, from its
next hop for a particular FEC, a label binding for that FEC. This is
known as "downstream-on-demand" label distribution.
The MPLS architecture also allows an LSR to distribute bindings to
LSRs that have not explicitly requested them. This is known as
"unsolicited downstream" label distribution.
It is expected that some MPLS implementations will provide only
downstream-on-demand label distribution, and some will provide only
unsolicited downstream label distribution, and some will provide
both. Which is provided may depend on the characteristics of the
interfaces which are supported by a particular implementation.
However, both of these label distribution techniques may be used in
the same network at the same time. On any given label distribution
adjacency, the upstream LSR and the downstream LSR must agree on
which technique is to be used.
An LSR Ru may receive (or have received) a label binding for a
particular FEC from an LSR Rd, even though Rd is not Ru's next hop
(or is no longer Ru's next hop) for that FEC.
Ru then has the choice of whether to keep track of such bindings, or
whether to discard such bindings. If Ru keeps track of such
bindings, then it may immediately begin using the binding again if Rd
eventually becomes its next hop for the FEC in question. If Ru
discards such bindings, then if Rd later becomes the next hop, the
binding will have to be reacquired.
Rosen, et al. Standards Track [Page 12]
RFC 3031 MPLS Architecture January 2001
If an LSR supports "Liberal Label Retention Mode", it maintains the
bindings between a label and a FEC which are received from LSRs which
are not its next hop for that FEC. If an LSR supports "Conservative
Label Retention Mode", it discards such bindings.
Liberal label retention mode allows for quicker adaptation to routing
changes, but conservative label retention mode though requires an LSR
to maintain many fewer labels.
So far, we have spoken as if a labeled packet carries only a single
label. As we shall see, it is useful to have a more general model in
which a labeled packet carries a number of labels, organized as a
last-in, first-out stack. We refer to this as a "label stack".
Although, as we shall see, MPLS supports a hierarchy, the processing
of a labeled packet is completely independent of the level of
hierarchy. The processing is always based on the top label, without
regard for the possibility that some number of other labels may have
been "above it" in the past, or that some number of other labels may
be below it at present.
An unlabeled packet can be thought of as a packet whose label stack
is empty (i.e., whose label stack has depth 0).
If a packet's label stack is of depth m, we refer to the label at the
bottom of the stack as the level 1 label, to the label above it (if
such exists) as the level 2 label, and to the label at the top of the
stack as the level m label.
The utility of the label stack will become clear when we introduce
the notion of LSP Tunnel and the MPLS Hierarchy (section 3.27).
The "Next Hop Label Forwarding Entry" (NHLFE) is used when forwarding
a labeled packet. It contains the following information:
1. the packet's next hop
2. the operation to perform on the packet's label stack; this is one
of the following operations:
a) replace the label at the top of the label stack with a
specified new label
b) pop the label stack
Rosen, et al. Standards Track [Page 13]
RFC 3031 MPLS Architecture January 2001
c) replace the label at the top of the label stack with a
specified new label, and then push one or more specified new
labels onto the label stack.
It may also contain:
d) the data link encapsulation to use when transmitting the packet
e) the way to encode the label stack when transmitting the packet
f) any other information needed in order to properly dispose of
the packet.
Note that at a given LSR, the packet's "next hop" might be that LSR
itself. In this case, the LSR would need to pop the top level label,
and then "forward" the resulting packet to itself. It would then
make another forwarding decision, based on what remains after the
label stacked is popped. This may still be a labeled packet, or it
may be the native IP packet.
This implies that in some cases the LSR may need to operate on the IP
header in order to forward the packet.
If the packet's "next hop" is the current LSR, then the label stack
operation MUST be to "pop the stack".
The "Incoming Label Map" (ILM) maps each incoming label to a set of
NHLFEs. It is used when forwarding packets that arrive as labeled
packets.
If the ILM maps a particular label to a set of NHLFEs that contains
more than one element, exactly one element of the set must be chosen
before the packet is forwarded. The procedures for choosing an
element from the set are beyond the scope of this document. Having
the ILM map a label to a set containing more than one NHLFE may be
useful if, e.g., it is desired to do load balancing over multiple
equal-cost paths.
The "FEC-to-NHLFE" (FTN) maps each FEC to a set of NHLFEs. It is
used when forwarding packets that arrive unlabeled, but which are to
be labeled before being forwarded.
Rosen, et al. Standards Track [Page 14]
RFC 3031 MPLS Architecture January 2001
If the FTN maps a particular label to a set of NHLFEs that contains
more than one element, exactly one element of the set must be chosen
before the packet is forwarded. The procedures for choosing an
element from the set are beyond the scope of this document. Having
the FTN map a label to a set containing more than one NHLFE may be
useful if, e.g., it is desired to do load balancing over multiple
equal-cost paths.
Label swapping is the use of the following procedures to forward a
packet.
In order to forward a labeled packet, a LSR examines the label at the
top of the label stack. It uses the ILM to map this label to an
NHLFE. Using the information in the NHLFE, it determines where to
forward the packet, and performs an operation on the packet's label
stack. It then encodes the new label stack into the packet, and
forwards the result.
In order to forward an unlabeled packet, a LSR analyzes the network
layer header, to determine the packet's FEC. It then uses the FTN to
map this to an NHLFE. Using the information in the NHLFE, it
determines where to forward the packet, and performs an operation on
the packet's label stack. (Popping the label stack would, of course,
be illegal in this case.) It then encodes the new label stack into
the packet, and forwards the result.
IT IS IMPORTANT TO NOTE THAT WHEN LABEL SWAPPING IS IN USE, THE NEXT
HOP IS ALWAYS TAKEN FROM THE NHLFE; THIS MAY IN SOME CASES BE
DIFFERENT FROM WHAT THE NEXT HOP WOULD BE IF MPLS WERE NOT IN USE.
A given LSR Rd may bind label L1 to FEC F, and distribute that
binding to label distribution peer Ru1. Rd may also bind label L2 to
FEC F, and distribute that binding to label distribution peer Ru2.
Whether or not L1 == L2 is not determined by the architecture; this
is a local matter.
A given LSR Rd may bind label L to FEC F1, and distribute that
binding to label distribution peer Ru1. Rd may also bind label L to
FEC F2, and distribute that binding to label distribution peer Ru2.
IF (AND ONLY IF) RD CAN TELL, WHEN IT RECEIVES A PACKET WHOSE TOP
LABEL IS L, WHETHER THE LABEL WAS PUT THERE BY RU1 OR BY RU2, THEN
THE ARCHITECTURE DOES NOT REQUIRE THAT F1 == F2. In such cases, we
may say that Rd is using a different "label space" for the labels it
distributes to Ru1 than for the labels it distributes to Ru2.
Rosen, et al. Standards Track [Page 15]
RFC 3031 MPLS Architecture January 2001
In general, Rd can only tell whether it was Ru1 or Ru2 that put the
particular label value L at the top of the label stack if the
following conditions hold:
- Ru1 and Ru2 are the only label distribution peers to which Rd
distributed a binding of label value L, and
- Ru1 and Ru2 are each directly connected to Rd via a point-to-
point interface.
When these conditions hold, an LSR may use labels that have "per
interface" scope, i.e., which are only unique per interface. We may
say that the LSR is using a "per-interface label space". When these
conditions do not hold, the labels must be unique over the LSR which
has assigned them, and we may say that the LSR is using a "per-
platform label space."
If a particular LSR Rd is attached to a particular LSR Ru over two
point-to-point interfaces, then Rd may distribute to Ru a binding of
label L to FEC F1, as well as a binding of label L to FEC F2, F1 !=
F2, if and only if each binding is valid only for packets which Ru
sends to Rd over a particular one of the interfaces. In all other
cases, Rd MUST NOT distribute to Ru bindings of the same label value
to two different FECs.
This prohibition holds even if the bindings are regarded as being at
different "levels of hierarchy". In MPLS, there is no notion of
having a different label space for different levels of the hierarchy;
when interpreting a label, the level of the label is irrelevant.
The question arises as to whether it is possible for an LSR to use
multiple per-platform label spaces, or to use multiple per-interface
label spaces for the same interface. This is not prohibited by the
architecture. However, in such cases the LSR must have some means,
not specified by the architecture, of determining, for a particular
incoming label, which label space that label belongs to. For
example, [MPLS-SHIM] specifies that a different label space is used
for unicast packets than for multicast packets, and uses a data link
layer codepoint to distinguish the two label spaces.
A "Label Switched Path (LSP) of level m" for a particular packet P is
a sequence of routers,
<R1, ..., Rn>
with the following properties:
Rosen, et al. Standards Track [Page 16]
RFC 3031 MPLS Architecture January 2001
1. R1, the "LSP Ingress", is an LSR which pushes a label onto P's
label stack, resulting in a label stack of depth m;
2. For all i, 1<i<n, P has a label stack of depth m when received
by LSR Ri;
3. At no time during P's transit from R1 to R[n-1] does its label
stack ever have a depth of less than m;
4. For all i, 1<i<n: Ri transmits P to R[i+1] by means of MPLS,
i.e., by using the label at the top of the label stack (the
level m label) as an index into an ILM;
5. For all i, 1<i<n: if a system S receives and forwards P after P
is transmitted by Ri but before P is received by R[i+1] (e.g.,
Ri and R[i+1] might be connected via a switched data link
subnetwork, and S might be one of the data link switches), then
S's forwarding decision is not based on the level m label, or
on the network layer header. This may be because:
a) the decision is not based on the label stack or the network
layer header at all;
b) the decision is based on a label stack on which additional
labels have been pushed (i.e., on a level m+k label, where
k>0).
In other words, we can speak of the level m LSP for Packet P as the
sequence of routers:
1. which begins with an LSR (an "LSP Ingress") that pushes on a
level m label,
2. all of whose intermediate LSRs make their forwarding decision
by label Switching on a level m label,
3. which ends (at an "LSP Egress") when a forwarding decision is
made by label Switching on a level m-k label, where k>0, or
when a forwarding decision is made by "ordinary", non-MPLS
forwarding procedures.
A consequence (or perhaps a presupposition) of this is that whenever
an LSR pushes a label onto an already labeled packet, it needs to
make sure that the new label corresponds to a FEC whose LSP Egress is
the LSR that assigned the label which is now second in the stack.
Rosen, et al. Standards Track [Page 17]
RFC 3031 MPLS Architecture January 2001
We will call a sequence of LSRs the "LSP for a particular FEC F" if
it is an LSP of level m for a particular packet P when P's level m
label is a label corresponding to FEC F.
Consider the set of nodes which may be LSP ingress nodes for FEC F.
Then there is an LSP for FEC F which begins with each of those nodes.
If a number of those LSPs have the same LSP egress, then one can
consider the set of such LSPs to be a tree, whose root is the LSP
egress. (Since data travels along this tree towards the root, this
may be called a multipoint-to-point tree.) We can thus speak of the
"LSP tree" for a particular FEC F.
Note that according to the definitions of section 3.15, if <R1, ...,
Rn> is a level m LSP for packet P, P may be transmitted from R[n-1]
to Rn with a label stack of depth m-1. That is, the label stack may
be popped at the penultimate LSR of the LSP, rather than at the LSP
Egress.
From an architectural perspective, this is perfectly appropriate.
The purpose of the level m label is to get the packet to Rn. Once
R[n-1] has decided to send the packet to Rn, the label no longer has
any function, and need no longer be carried.
There is also a practical advantage to doing penultimate hop popping.
If one does not do this, then when the LSP egress receives a packet,
it first looks up the top label, and determines as a result of that
lookup that it is indeed the LSP egress. Then it must pop the stack,
and examine what remains of the packet. If there is another label on
the stack, the egress will look this up and forward the packet based
on this lookup. (In this case, the egress for the packet's level m
LSP is also an intermediate node for its level m-1 LSP.) If there is
no other label on the stack, then the packet is forwarded according
to its network layer destination address. Note that this would
require the egress to do TWO lookups, either two label lookups or a
label lookup followed by an address lookup.
If, on the other hand, penultimate hop popping is used, then when the
penultimate hop looks up the label, it determines:
- that it is the penultimate hop, and
- who the next hop is.
The penultimate node then pops the stack, and forwards the packet
based on the information gained by looking up the label that was
previously at the top of the stack. When the LSP egress receives the
Rosen, et al. Standards Track [Page 18]
RFC 3031 MPLS Architecture January 2001
packet, the label which is now at the top of the stack will be the
label which it needs to look up in order to make its own forwarding
decision. Or, if the packet was only carrying a single label, the
LSP egress will simply see the network layer packet, which is just
what it needs to see in order to make its forwarding decision.
This technique allows the egress to do a single lookup, and also
requires only a single lookup by the penultimate node.
The creation of the forwarding "fastpath" in a label switching
product may be greatly aided if it is known that only a single lookup
is ever required:
- the code may be simplified if it can assume that only a single
lookup is ever needed
- the code can be based on a "time budget" that assumes that only
a single lookup is ever needed.
In fact, when penultimate hop popping is done, the LSP Egress need
not even be an LSR.
However, some hardware switching engines may not be able to pop the
label stack, so this cannot be universally required. There may also
be some situations in which penultimate hop popping is not desirable.
Therefore the penultimate node pops the label stack only if this is
specifically requested by the egress node, OR if the next node in the
LSP does not support MPLS. (If the next node in the LSP does support
MPLS, but does not make such a request, the penultimate node has no
way of knowing that it in fact is the penultimate node.)
An LSR which is capable of popping the label stack at all MUST do
penultimate hop popping when so requested by its downstream label
distribution peer.
Initial label distribution protocol negotiations MUST allow each LSR
to determine whether its neighboring LSRS are capable of popping the
label stack. A LSR MUST NOT request a label distribution peer to pop
the label stack unless it is capable of doing so.
It may be asked whether the egress node can always interpret the top
label of a received packet properly if penultimate hop popping is
used. As long as the uniqueness and scoping rules of section 3.14
are obeyed, it is always possible to interpret the top label of a
received packet unambiguously.
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The LSP Next Hop for a particular labeled packet in a particular LSR
is the LSR which is the next hop, as selected by the NHLFE entry used
for forwarding that packet.
The LSP Next Hop for a particular FEC is the next hop as selected by
the NHLFE entry indexed by a label which corresponds to that FEC.
Note that the LSP Next Hop may differ from the next hop which would
be chosen by the network layer routing algorithm. We will use the
term "L3 next hop" when we refer to the latter.
What should an LSR do if it receives a labeled packet with a
particular incoming label, but has no binding for that label? It is
tempting to think that the labels can just be removed, and the packet
forwarded as an unlabeled IP packet. However, in some cases, doing
so could cause a loop. If the upstream LSR thinks the label is bound
to an explicit route, and the downstream LSR doesn't think the label
is bound to anything, and if the hop by hop routing of the unlabeled
IP packet brings the packet back to the upstream LSR, then a loop is
formed.
It is also possible that the label was intended to represent a route
which cannot be inferred from the IP header.
Therefore, when a labeled packet is received with an invalid incoming
label, it MUST be discarded, UNLESS it is determined by some means
(not within the scope of the current document) that forwarding it
unlabeled cannot cause any harm.
Some FECs correspond to address prefixes which are distributed via a
dynamic routing algorithm. The setup of the LSPs for these FECs can
be done in one of two ways: Independent LSP Control or Ordered LSP
Control.
In Independent LSP Control, each LSR, upon noting that it recognizes
a particular FEC, makes an independent decision to bind a label to
that FEC and to distribute that binding to its label distribution
peers. This corresponds to the way that conventional IP datagram
routing works; each node makes an independent decision as to how to
treat each packet, and relies on the routing algorithm to converge
rapidly so as to ensure that each datagram is correctly delivered.
Rosen, et al. Standards Track [Page 20]
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In Ordered LSP Control, an LSR only binds a label to a particular FEC
if it is the egress LSR for that FEC, or if it has already received a
label binding for that FEC from its next hop for that FEC.
If one wants to ensure that traffic in a particular FEC follows a
path with some specified set of properties (e.g., that the traffic
does not traverse any node twice, that a specified amount of
resources are available to the traffic, that the traffic follows an
explicitly specified path, etc.) ordered control must be used. With
independent control, some LSRs may begin label switching a traffic in
the FEC before the LSP is completely set up, and thus some traffic in
the FEC may follow a path which does not have the specified set of
properties. Ordered control also needs to be used if the recognition
of the FEC is a consequence of the setting up of the corresponding
LSP.
Ordered LSP setup may be initiated either by the ingress or the
egress.
Ordered control and independent control are fully interoperable.
However, unless all LSRs in an LSP are using ordered control, the
overall effect on network behavior is largely that of independent
control, since one cannot be sure that an LSP is not used until it is
fully set up.
This architecture allows the choice between independent control and
ordered control to be a local matter. Since the two methods
interwork, a given LSR need support only one or the other. Generally
speaking, the choice of independent versus ordered control does not
appear to have any effect on the label distribution mechanisms which
need to be defined.
One way of partitioning traffic into FECs is to create a separate FEC
for each address prefix which appears in the routing table. However,
within a particular MPLS domain, this may result in a set of FECs
such that all traffic in all those FECs follows the same route. For
example, a set of distinct address prefixes might all have the same
egress node, and label swapping might be used only to get the the
traffic to the egress node. In this case, within the MPLS domain,
the union of those FECs is itself a FEC. This creates a choice:
should a distinct label be bound to each component FEC, or should a
single label be bound to the union, and that label applied to all
traffic in the union?
The procedure of binding a single label to a union of FECs which is
itself a FEC (within some domain), and of applying that label to all
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traffic in the union, is known as "aggregation". The MPLS
architecture allows aggregation. Aggregation may reduce the number
of labels which are needed to handle a particular set of packets, and
may also reduce the amount of label distribution control traffic
needed.
Given a set of FECs which are "aggregatable" into a single FEC, it is
possible to (a) aggregate them into a single FEC, (b) aggregate them
into a set of FECs, or (c) not aggregate them at all. Thus we can
speak of the "granularity" of aggregation, with (a) being the
"coarsest granularity", and (c) being the "finest granularity".
When order control is used, each LSR should adopt, for a given set of
FECs, the granularity used by its next hop for those FECs.
When independent control is used, it is possible that there will be
two adjacent LSRs, Ru and Rd, which aggregate some set of FECs
differently.
If Ru has finer granularity than Rd, this does not cause a problem.
Ru distributes more labels for that set of FECs than Rd does. This
means that when Ru needs to forward labeled packets in those FECs to
Rd, it may need to map n labels into m labels, where n > m. As an
option, Ru may withdraw the set of n labels that it has distributed,
and then distribute a set of m labels, corresponding to Rd's level of
granularity. This is not necessary to ensure correct operation, but
it does result in a reduction of the number of labels distributed by
Ru, and Ru is not gaining any particular advantage by distributing
the larger number of labels. The decision whether to do this or not
is a local matter.
If Ru has coarser granularity than Rd (i.e., Rd has distributed n
labels for the set of FECs, while Ru has distributed m, where n > m),
it has two choices:
- It may adopt Rd's finer level of granularity. This would
require it to withdraw the m labels it has distributed, and
distribute n labels. This is the preferred option.
- It may simply map its m labels into a subset of Rd's n labels,
if it can determine that this will produce the same routing.
For example, suppose that Ru applies a single label to all
traffic that needs to pass through a certain egress LSR,
whereas Rd binds a number of different labels to such traffic,
depending on the individual destination addresses of the
packets. If Ru knows the address of the egress router, and if
Rd has bound a label to the FEC which is identified by that
address, then Ru can simply apply that label.
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In any event, every LSR needs to know (by configuration) what
granularity to use for labels that it assigns. Where ordered control
is used, this requires each node to know the granularity only for
FECs which leave the MPLS network at that node. For independent
control, best results may be obtained by ensuring that all LSRs are
consistently configured to know the granularity for each FEC.
However, in many cases this may be done by using a single level of
granularity which applies to all FECs (such as "one label per IP
prefix in the forwarding table", or "one label per egress node").
Route selection refers to the method used for selecting the LSP for a
particular FEC. The proposed MPLS protocol architecture supports two
options for Route Selection: (1) hop by hop routing, and (2) explicit
routing.
Hop by hop routing allows each node to independently choose the next
hop for each FEC. This is the usual mode today in existing IP
networks. A "hop by hop routed LSP" is an LSP whose route is
selected using hop by hop routing.
In an explicitly routed LSP, each LSR does not independently choose
the next hop; rather, a single LSR, generally the LSP ingress or the
LSP egress, specifies several (or all) of the LSRs in the LSP. If a
single LSR specifies the entire LSP, the LSP is "strictly" explicitly
routed. If a single LSR specifies only some of the LSP, the LSP is
"loosely" explicitly routed.
The sequence of LSRs followed by an explicitly routed LSP may be
chosen by configuration, or may be selected dynamically by a single
node (for example, the egress node may make use of the topological
information learned from a link state database in order to compute
the entire path for the tree ending at that egress node).
Explicit routing may be useful for a number of purposes, such as
policy routing or traffic engineering. In MPLS, the explicit route
needs to be specified at the time that labels are assigned, but the
explicit route does not have to be specified with each IP packet.
This makes MPLS explicit routing much more efficient than the
alternative of IP source routing.
The procedures for making use of explicit routes, either strict or
loose, are beyond the scope of this document.
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When a labeled packet is traveling along an LSP, it may occasionally
happen that it reaches an LSR at which the ILM does not map the
packet's incoming label into an NHLFE, even though the incoming label
is itself valid. This can happen due to transient conditions, or due
to an error at the LSR which should be the packet's next hop.
It is tempting in such cases to strip off the label stack and attempt
to forward the packet further via conventional forwarding, based on
its network layer header. However, in general this is not a safe
procedure:
- If the packet has been following an explicitly routed LSP, this
could result in a loop.
- The packet's network header may not contain enough information
to enable this particular LSR to forward it correctly.
Unless it can be determined (through some means outside the scope of
this document) that neither of these situations obtains, the only
safe procedure is to discard the packet.
In conventional IP forwarding, each packet carries a "Time To Live"
(TTL) value in its header. Whenever a packet passes through a
router, its TTL gets decremented by 1; if the TTL reaches 0 before
the packet has reached its destination, the packet gets discarded.
This provides some level of protection against forwarding loops that
may exist due to misconfigurations, or due to failure or slow
convergence of the routing algorithm. TTL is sometimes used for
other functions as well, such as multicast scoping, and supporting
the "traceroute" command. This implies that there are two TTL-
related issues that MPLS needs to deal with: (i) TTL as a way to
suppress loops; (ii) TTL as a way to accomplish other functions, such
as limiting the scope of a packet.
When a packet travels along an LSP, it SHOULD emerge with the same
TTL value that it would have had if it had traversed the same
sequence of routers without having been label switched. If the
packet travels along a hierarchy of LSPs, the total number of LSR-
hops traversed SHOULD be reflected in its TTL value when it emerges
from the hierarchy of LSPs.
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The way that TTL is handled may vary depending upon whether the MPLS
label values are carried in an MPLS-specific "shim" header [MPLS-
SHIM], or if the MPLS labels are carried in an L2 header, such as an
ATM header [MPLS-ATM] or a frame relay header [MPLS-FRMRLY].
If the label values are encoded in a "shim" that sits between the
data link and network layer headers, then this shim MUST have a TTL
field that SHOULD be initially loaded from the network layer header
TTL field, SHOULD be decremented at each LSR-hop, and SHOULD be
copied into the network layer header TTL field when the packet
emerges from its LSP.
If the label values are encoded in a data link layer header (e.g.,
the VPI/VCI field in ATM's AAL5 header), and the labeled packets are
forwarded by an L2 switch (e.g., an ATM switch), and the data link
layer (like ATM) does not itself have a TTL field, then it will not
be possible to decrement a packet's TTL at each LSR-hop. An LSP
segment which consists of a sequence of LSRs that cannot decrement a
packet's TTL will be called a "non-TTL LSP segment".
When a packet emerges from a non-TTL LSP segment, it SHOULD however
be given a TTL that reflects the number of LSR-hops it traversed. In
the unicast case, this can be achieved by propagating a meaningful
LSP length to ingress nodes, enabling the ingress to decrement the
TTL value before forwarding packets into a non-TTL LSP segment.
Sometimes it can be determined, upon ingress to a non-TTL LSP
segment, that a particular packet's TTL will expire before the packet
reaches the egress of that non-TTL LSP segment. In this case, the
LSR at the ingress to the non-TTL LSP segment must not label switch
the packet. This means that special procedures must be developed to
support traceroute functionality, for example, traceroute packets may
be forwarded using conventional hop by hop forwarding.
On a non-TTL LSP segment, by definition, TTL cannot be used to
protect against forwarding loops. The importance of loop control may
depend on the particular hardware being used to provide the LSR
functions along the non-TTL LSP segment.
Suppose, for instance, that ATM switching hardware is being used to
provide MPLS switching functions, with the label being carried in the
VPI/VCI field. Since ATM switching hardware cannot decrement TTL,
there is no protection against loops. If the ATM hardware is capable
of providing fair access to the buffer pool for incoming cells
carrying different VPI/VCI values, this looping may not have any
deleterious effect on other traffic. If the ATM hardware cannot
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provide fair buffer access of this sort, however, then even transient
loops may cause severe degradation of the LSR's total performance.
Even if fair buffer access can be provided, it is still worthwhile to
have some means of detecting loops that last "longer than possible".
In addition, even where TTL and/or per-VC fair queuing provides a
means for surviving loops, it still may be desirable where practical
to avoid setting up LSPs which loop. All LSRs that may attach to
non-TTL LSP segments will therefore be required to support a common
technique for loop detection; however, use of the loop detection
technique is optional. The loop detection technique is specified in
[MPLS-ATM] and [MPLS-LDP].
In order to transmit a label stack along with the packet whose label
stack it is, it is necessary to define a concrete encoding of the
label stack. The architecture supports several different encoding
techniques; the choice of encoding technique depends on the
particular kind of device being used to forward labeled packets.
If one is using MPLS-specific hardware and/or software to forward
labeled packets, the most obvious way to encode the label stack is to
define a new protocol to be used as a "shim" between the data link
layer and network layer headers. This shim would really be just an
encapsulation of the network layer packet; it would be "protocol-
independent" such that it could be used to encapsulate any network
layer. Hence we will refer to it as the "generic MPLS
encapsulation".
The generic MPLS encapsulation would in turn be encapsulated in a
data link layer protocol.
The MPLS generic encapsulation is specified in [MPLS-SHIM].
It will be noted that MPLS forwarding procedures are similar to those
of legacy "label swapping" switches such as ATM switches. ATM
switches use the input port and the incoming VPI/VCI value as the
index into a "cross-connect" table, from which they obtain an output
port and an outgoing VPI/VCI value. Therefore if one or more labels
can be encoded directly into the fields which are accessed by these
legacy switches, then the legacy switches can, with suitable software
upgrades, be used as LSRs. We will refer to such devices as "ATM-
LSRs".
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There are three obvious ways to encode labels in the ATM cell header
(presuming the use of AAL5):
1. SVC Encoding
Use the VPI/VCI field to encode the label which is at the top
of the label stack. This technique can be used in any network.
With this encoding technique, each LSP is realized as an ATM
SVC, and the label distribution protocol becomes the ATM
"signaling" protocol. With this encoding technique, the ATM-
LSRs cannot perform "push" or "pop" operations on the label
stack.
2. SVP Encoding
Use the VPI field to encode the label which is at the top of
the label stack, and the VCI field to encode the second label
on the stack, if one is present. This technique some
advantages over the previous one, in that it permits the use of
ATM "VP-switching". That is, the LSPs are realized as ATM
SVPs, with the label distribution protocol serving as the ATM
signaling protocol.
However, this technique cannot always be used. If the network
includes an ATM Virtual Path through a non-MPLS ATM network,
then the VPI field is not necessarily available for use by
MPLS.
When this encoding technique is used, the ATM-LSR at the egress
of the VP effectively does a "pop" operation.
3. SVP Multipoint Encoding
Use the VPI field to encode the label which is at the top of
the label stack, use part of the VCI field to encode the second
label on the stack, if one is present, and use the remainder of
the VCI field to identify the LSP ingress. If this technique
is used, conventional ATM VP-switching capabilities can be used
to provide multipoint-to-point VPs. Cells from different
packets will then carry different VCI values. As we shall see
in section 3.26, this enables us to do label merging, without
running into any cell interleaving problems, on ATM switches
which can provide multipoint-to-point VPs, but which do not
have the VC merge capability.
This technique depends on the existence of a capability for
assigning 16-bit VCI values to each ATM switch such that no
single VCI value is assigned to two different switches. (If an
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adequate number of such values could be assigned to each
switch, it would be possible to also treat the VCI value as the
second label in the stack.)
If there are more labels on the stack than can be encoded in the ATM
header, the ATM encodings must be combined with the generic
encapsulation.
If <R1, R2, R3> is a segment of a LSP, it is possible that R1 will
use one encoding of the label stack when transmitting packet P to R2,
but R2 will use a different encoding when transmitting a packet P to
R3. In general, the MPLS architecture supports LSPs with different
label stack encodings used on different hops. Therefore, when we
discuss the procedures for processing a labeled packet, we speak in
abstract terms of operating on the packet's label stack. When a
labeled packet is received, the LSR must decode it to determine the
current value of the label stack, then must operate on the label
stack to determine the new value of the stack, and then encode the
new value appropriately before transmitting the labeled packet to its
next hop.
Unfortunately, ATM switches have no capability for translating from
one encoding technique to another. The MPLS architecture therefore
requires that whenever it is possible for two ATM switches to be
successive LSRs along a level m LSP for some packet, that those two
ATM switches use the same encoding technique.
Naturally there will be MPLS networks which contain a combination of
ATM switches operating as LSRs, and other LSRs which operate using an
MPLS shim header. In such networks there may be some LSRs which have
ATM interfaces as well as "MPLS Shim" interfaces. This is one
example of an LSR with different label stack encodings on different
hops. Such an LSR may swap off an ATM encoded label stack on an
incoming interface and replace it with an MPLS shim header encoded
label stack on the outgoing interface.
Suppose that an LSR has bound multiple incoming labels to a
particular FEC. When forwarding packets in that FEC, one would like
to have a single outgoing label which is applied to all such packets.
The fact that two different packets in the FEC arrived with different
incoming labels is irrelevant; one would like to forward them with
the same outgoing label. The capability to do so is known as "label
merging".
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Let us say that an LSR is capable of label merging if it can receive
two packets from different incoming interfaces, and/or with different
labels, and send both packets out the same outgoing interface with
the same label. Once the packets are transmitted, the information
that they arrived from different interfaces and/or with different
incoming labels is lost.
Let us say that an LSR is not capable of label merging if, for any
two packets which arrive from different interfaces, or with different
labels, the packets must either be transmitted out different
interfaces, or must have different labels. ATM-LSRs using the SVC or
SVP Encodings cannot perform label merging. This is discussed in
more detail in the next section.
If a particular LSR cannot perform label merging, then if two packets
in the same FEC arrive with different incoming labels, they must be
forwarded with different outgoing labels. With label merging, the
number of outgoing labels per FEC need only be 1; without label
merging, the number of outgoing labels per FEC could be as large as
the number of nodes in the network.
With label merging, the number of incoming labels per FEC that a
particular LSR needs is never be larger than the number of label
distribution adjacencies. Without label merging, the number of
incoming labels per FEC that a particular LSR needs is as large as
the number of upstream nodes which forward traffic in the FEC to the
LSR in question. In fact, it is difficult for an LSR to even
determine how many such incoming labels it must support for a
particular FEC.
The MPLS architecture accommodates both merging and non-merging LSRs,
but allows for the fact that there may be LSRs which do not support
label merging. This leads to the issue of ensuring correct
interoperation between merging LSRs and non-merging LSRs. The issue
is somewhat different in the case of datagram media versus the case
of ATM. The different media types will therefore be discussed
separately.
The MPLS forwarding procedures is very similar to the forwarding
procedures used by such technologies as ATM and Frame Relay. That
is, a unit of data arrives, a label (VPI/VCI or DLCI) is looked up in
a "cross-connect table", on the basis of that lookup an output port
is chosen, and the label value is rewritten. In fact, it is possible
to use such technologies for MPLS forwarding; a label distribution
protocol can be used as the "signalling protocol" for setting up the
cross-connect tables.
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Unfortunately, these technologies do not necessarily support the
label merging capability. In ATM, if one attempts to perform label
merging, the result may be the interleaving of cells from various
packets. If cells from different packets get interleaved, it is
impossible to reassemble the packets. Some Frame Relay switches use
cell switching on their backplanes. These switches may also be
incapable of supporting label merging, for the same reason -- cells
of different packets may get interleaved, and there is then no way to
reassemble the packets.
We propose to support two solutions to this problem. First, MPLS
will contain procedures which allow the use of non-merging LSRs.
Second, MPLS will support procedures which allow certain ATM switches
to function as merging LSRs.
Since MPLS supports both merging and non-merging LSRs, MPLS also
contains procedures to ensure correct interoperation between them.
An upstream LSR which supports label merging needs to be sent only
one label per FEC. An upstream neighbor which does not support label
merging needs to be sent multiple labels per FEC. However, there is
no way of knowing a priori how many labels it needs. This will
depend on how many LSRs are upstream of it with respect to the FEC in
question.
In the MPLS architecture, if a particular upstream neighbor does not
support label merging, it is not sent any labels for a particular FEC
unless it explicitly asks for a label for that FEC. The upstream
neighbor may make multiple such requests, and is given a new label
each time. When a downstream neighbor receives such a request from
upstream, and the downstream neighbor does not itself support label
merging, then it must in turn ask its downstream neighbor for another
label for the FEC in question.
It is possible that there may be some nodes which support label
merging, but can only merge a limited number of incoming labels into
a single outgoing label. Suppose for example that due to some
hardware limitation a node is capable of merging four incoming labels
into a single outgoing label. Suppose however, that this particular
node has six incoming labels arriving at it for a particular FEC. In
this case, this node may merge these into two outgoing labels.
Whether label merging is applicable to explicitly routed LSPs is for
further study.
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There are several methods that can be used to eliminate the cell
interleaving problem in ATM, thereby allowing ATM switches to support
stream merge:
1. VP merge, using the SVP Multipoint Encoding
When VP merge is used, multiple virtual paths are merged into a
virtual path, but packets from different sources are
distinguished by using different VCIs within the VP.
2. VC merge
When VC merge is used, switches are required to buffer cells
from one packet until the entire packet is received (this may
be determined by looking for the AAL5 end of frame indicator).
VP merge has the advantage that it is compatible with a higher
percentage of existing ATM switch implementations. This makes it
more likely that VP merge can be used in existing networks. Unlike
VC merge, VP merge does not incur any delays at the merge points and
also does not impose any buffer requirements. However, it has the
disadvantage that it requires coordination of the VCI space within
each VP. There are a number of ways that this can be accomplished.
Selection of one or more methods is for further study.
This tradeoff between compatibility with existing equipment versus
protocol complexity and scalability implies that it is desirable for
the MPLS protocol to support both VP merge and VC merge. In order to
do so each ATM switch participating in MPLS needs to know whether its
immediate ATM neighbors perform VP merge, VC merge, or no merge.
The interoperation of the various forms of merging over ATM is most
easily described by first describing the interoperation of VC merge
with non-merge.
In the case where VC merge and non-merge nodes are interconnected the
forwarding of cells is based in all cases on a VC (i.e., the
concatenation of the VPI and VCI). For each node, if an upstream
neighbor is doing VC merge then that upstream neighbor requires only
a single VPI/VCI for a particular stream (this is analogous to the
requirement for a single label in the case of operation over frame
media). If the upstream neighbor is not doing merge, then the
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neighbor will require a single VPI/VCI per stream for itself, plus
enough VPI/VCIs to pass to its upstream neighbors. The number
required will be determined by allowing the upstream nodes to request
additional VPI/VCIs from their downstream neighbors (this is again
analogous to the method used with frame merge).
A similar method is possible to support nodes which perform VP merge.
In this case the VP merge node, rather than requesting a single
VPI/VCI or a number of VPI/VCIs from its downstream neighbor, instead
may request a single VP (identified by a VPI) but several VCIs within
the VP. Furthermore, suppose that a non-merge node is downstream
from two different VP merge nodes. This node may need to request one
VPI/VCI (for traffic originating from itself) plus two VPs (one for
each upstream node), each associated with a specified set of VCIs (as
requested from the upstream node).
In order to support all of VP merge, VC merge, and non-merge, it is
therefore necessary to allow upstream nodes to request a combination
of zero or more VC identifiers (consisting of a VPI/VCI), plus zero
or more VPs (identified by VPIs) each containing a specified number
of VCs (identified by a set of VCIs which are significant within a
VP). VP merge nodes would therefore request one VP, with a contained
VCI for traffic that it originates (if appropriate) plus a VCI for
each VC requested from above (regardless of whether or not the VC is
part of a containing VP). VC merge node would request only a single
VPI/VCI (since they can merge all upstream traffic into a single VC).
Non-merge nodes would pass on any requests that they get from above,
plus request a VPI/VCI for traffic that they originate (if
appropriate).
Sometimes a router Ru takes explicit action to cause a particular
packet to be delivered to another router Rd, even though Ru and Rd
are not consecutive routers on the Hop-by-hop path for that packet,
and Rd is not the packet's ultimate destination. For example, this
may be done by encapsulating the packet inside a network layer packet
whose destination address is the address of Rd itself. This creates
a "tunnel" from Ru to Rd. We refer to any packet so handled as a
"Tunneled Packet".
If a Tunneled Packet follows the Hop-by-hop path from Ru to Rd, we
say that it is in an "Hop-by-Hop Routed Tunnel" whose "transmit
endpoint" is Ru and whose "receive endpoint" is Rd.
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If a Tunneled Packet travels from Ru to Rd over a path other than the
Hop-by-hop path, we say that it is in an "Explicitly Routed Tunnel"
whose "transmit endpoint" is Ru and whose "receive endpoint" is Rd.
For example, we might send a packet through an Explicitly Routed
Tunnel by encapsulating it in a packet which is source routed.
It is possible to implement a tunnel as a LSP, and use label
switching rather than network layer encapsulation to cause the packet
to travel through the tunnel. The tunnel would be a LSP <R1, ...,
Rn>, where R1 is the transmit endpoint of the tunnel, and Rn is the
receive endpoint of the tunnel. This is called a "LSP Tunnel".
The set of packets which are to be sent though the LSP tunnel
constitutes a FEC, and each LSR in the tunnel must assign a label to
that FEC (i.e., must assign a label to the tunnel). The criteria for
assigning a particular packet to an LSP tunnel is a local matter at
the tunnel's transmit endpoint. To put a packet into an LSP tunnel,
the transmit endpoint pushes a label for the tunnel onto the label
stack and sends the labeled packet to the next hop in the tunnel.
If it is not necessary for the tunnel's receive endpoint to be able
to determine which packets it receives through the tunnel, as
discussed earlier, the label stack may be popped at the penultimate
LSR in the tunnel.
A "Hop-by-Hop Routed LSP Tunnel" is a Tunnel that is implemented as
an hop-by-hop routed LSP between the transmit endpoint and the
receive endpoint.
An "Explicitly Routed LSP Tunnel" is a LSP Tunnel that is also an
Explicitly Routed LSP.
Consider a LSP <R1, R2, R3, R4>. Let us suppose that R1 receives
unlabeled packet P, and pushes on its label stack the label to cause
it to follow this path, and that this is in fact the Hop-by-hop path.
However, let us further suppose that R2 and R3 are not directly
connected, but are "neighbors" by virtue of being the endpoints of an
LSP tunnel. So the actual sequence of LSRs traversed by P is <R1,
R2, R21, R22, R23, R3, R4>.
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When P travels from R1 to R2, it will have a label stack of depth 1.
R2, switching on the label, determines that P must enter the tunnel.
R2 first replaces the Incoming label with a label that is meaningful
to R3. Then it pushes on a new label. This level 2 label has a
value which is meaningful to R21. Switching is done on the level 2
label by R21, R22, R23. R23, which is the penultimate hop in the
R2-R3 tunnel, pops the label stack before forwarding the packet to
R3. When R3 sees packet P, P has only a level 1 label, having now
exited the tunnel. Since R3 is the penultimate hop in P's level 1
LSP, it pops the label stack, and R4 receives P unlabeled.
The label stack mechanism allows LSP tunneling to nest to any depth.
Suppose that packet P travels along a Level 1 LSP <R1, R2, R3, R4>,
and when going from R2 to R3 travels along a Level 2 LSP <R2, R21,
R22, R3>. From the perspective of the Level 2 LSP, R2's label
distribution peer is R21. From the perspective of the Level 1 LSP,
R2's label distribution peers are R1 and R3. One can have label
distribution peers at each layer of hierarchy. We will see in
sections 4.6 and 4.7 some ways to make use of this hierarchy. Note
that in this example, R2 and R21 must be IGP neighbors, but R2 and R3
need not be.
When two LSRs are IGP neighbors, we will refer to them as "local
label distribution peers". When two LSRs may be label distribution
peers, but are not IGP neighbors, we will refer to them as "remote
label distribution peers". In the above example, R2 and R21 are
local label distribution peers, but R2 and R3 are remote label
distribution peers.
The MPLS architecture supports two ways to distribute labels at
different layers of the hierarchy: Explicit Peering and Implicit
Peering.
One performs label distribution with one's local label distribution
peer by sending label distribution protocol messages which are
addressed to the peer. One can perform label distribution with one's
remote label distribution peers in one of two ways:
1. Explicit Peering
In explicit peering, one distributes labels to a peer by
sending label distribution protocol messages which are
addressed to the peer, exactly as one would do for local label
distribution peers. This technique is most useful when the
number of remote label distribution peers is small, or the
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number of higher level label bindings is large, or the remote
label distribution peers are in distinct routing areas or
domains. Of course, one needs to know which labels to
distribute to which peers; this is addressed in section 4.1.2.
Examples of the use of explicit peering is found in sections
4.2.1 and 4.6.
2. Implicit Peering
In Implicit Peering, one does not send label distribution
protocol messages which are addressed to one's peer. Rather,
to distribute higher level labels to ones remote label
distribution peers, one encodes a higher level label as an
attribute of a lower level label, and then distributes the
lower level label, along with this attribute, to one's local
label distribution peers. The local label distribution peers
then propagate the information to their local label
distribution peers. This process continues till the
information reaches the remote peer.
This technique is most useful when the number of remote label
distribution peers is large. Implicit peering does not require
an n-square peering mesh to distribute labels to the remote
label distribution peers because the information is piggybacked
through the local label distribution peering. However,
implicit peering requires the intermediate nodes to store
information that they might not be directly interested in.
An example of the use of implicit peering is found in section
4.3.
A label distribution protocol is used between nodes in an MPLS
network to establish and maintain the label bindings. In order for
MPLS to operate correctly, label distribution information needs to be
transmitted reliably, and the label distribution protocol messages
pertaining to a particular FEC need to be transmitted in sequence.
Flow control is also desirable, as is the capability to carry
multiple label messages in a single datagram.
One way to meet these goals is to use TCP as the underlying
transport, as is done in [MPLS-LDP] and [MPLS-BGP].
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This architecture does not establish hard and fast rules for choosing
which label distribution protocol to use in which circumstances.
However, it is possible to point out some of the considerations.
In many scenarios, it is desirable to bind labels to FECs which can
be identified with routes to address prefixes (see section 4.1). If
there is a standard, widely deployed routing algorithm which
distributes those routes, it can be argued that label distribution is
best achieved by piggybacking the label distribution on the
distribution of the routes themselves.
For example, BGP distributes such routes, and if a BGP speaker needs
to also distribute labels to its BGP peers, using BGP to do the label
distribution (see [MPLS-BGP]) has a number of advantages. In
particular, it permits BGP route reflectors to distribute labels,
thus providing a significant scalability advantage over using LDP to
distribute labels between BGP peers.
When RSVP is used to set up resource reservations for particular
flows, it can be desirable to label the packets in those flows, so
that the RSVP filterspec does not need to be applied at each hop. It
can be argued that having RSVP distribute the labels as part of its
path/reservation setup process is the most efficient method of
distributing labels for this purpose.
In some applications of MPLS, particularly those related to traffic
engineering, it is desirable to set up an explicitly routed path,
from ingress to egress. It is also desirable to apply resource
reservations along that path.
One can imagine two approaches to this:
- Start with an existing protocol that is used for setting up
resource reservations, and extend it to support explicit
routing and label distribution.
- Start with an existing protocol that is used for label
distribution, and extend it to support explicit routing and
resource reservations.
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The first approach has given rise to the protocol specified in
[MPLS-RSVP-TUNNELS], the second to the approach specified in [MPLS-
CR-LDP].
A number of uses of MPLS require that packets with a certain label be
forwarded along the same hop-by-hop routed path that would be used
for forwarding a packet with a specified address in its network layer
destination address field.
In general, router R determines the next hop for packet P by finding
the address prefix X in its routing table which is the longest match
for P's destination address. That is, the packets in a given FEC are
just those packets which match a given address prefix in R's routing
table. In this case, a FEC can be identified with an address prefix.
Note that a packet P may be assigned to FEC F, and FEC F may be
identified with address prefix X, even if P's destination address
does not match X.
LSRs R1 and R2 are considered to be label distribution peers for
address prefix X if and only if one of the following conditions
holds:
1. R1's route to X is a route which it learned about via a
particular instance of a particular IGP, and R2 is a neighbor
of R1 in that instance of that IGP
2. R1's route to X is a route which it learned about by some
instance of routing algorithm A1, and that route is
redistributed into an instance of routing algorithm A2, and R2
is a neighbor of R1 in that instance of A2
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3. R1 is the receive endpoint of an LSP Tunnel that is within
another LSP, and R2 is a transmit endpoint of that tunnel, and
R1 and R2 are participants in a common instance of an IGP, and
are in the same IGP area (if the IGP in question has areas),
and R1's route to X was learned via that IGP instance, or is
redistributed by R1 into that IGP instance
4. R1's route to X is a route which it learned about via BGP, and
R2 is a BGP peer of R1
In general, these rules ensure that if the route to a particular
address prefix is distributed via an IGP, the label distribution
peers for that address prefix are the IGP neighbors. If the route to
a particular address prefix is distributed via BGP, the label
distribution peers for that address prefix are the BGP peers. In
other cases of LSP tunneling, the tunnel endpoints are label
distribution peers.
In order to use MPLS for the forwarding of packets according to the
hop-by-hop route corresponding to any address prefix, each LSR MUST:
1. bind one or more labels to each address prefix that appears in
its routing table;
2. for each such address prefix X, use a label distribution
protocol to distribute the binding of a label to X to each of
its label distribution peers for X.
There is also one circumstance in which an LSR must distribute a
label binding for an address prefix, even if it is not the LSR which
bound that label to that address prefix:
3. If R1 uses BGP to distribute a route to X, naming some other
LSR R2 as the BGP Next Hop to X, and if R1 knows that R2 has
assigned label L to X, then R1 must distribute the binding
between L and X to any BGP peer to which it distributes that
route.
These rules ensure that labels corresponding to address prefixes
which correspond to BGP routes are distributed to IGP neighbors if
and only if the BGP routes are distributed into the IGP. Otherwise,
the labels bound to BGP routes are distributed only to the other BGP
speakers.
These rules are intended only to indicate which label bindings must
be distributed by a given LSR to which other LSRs.
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If the hop-by-hop path that packet P needs to follow is <R1, ...,
Rn>, then <R1, ..., Rn> can be an LSP as long as:
1. there is a single address prefix X, such that, for all i,
1<=i<n, X is the longest match in Ri's routing table for P's
destination address;
2. for all i, 1<i<n, Ri has assigned a label to X and distributed
that label to R[i-1].
Note that a packet's LSP can extend only until it encounters a router
whose forwarding tables have a longer best match address prefix for
the packet's destination address. At that point, the LSP must end
and the best match algorithm must be performed again.
Suppose, for example, that packet P, with destination address
10.2.153.178 needs to go from R1 to R2 to R3. Suppose also that R2
advertises address prefix 10.2/16 to R1, but R3 advertises
10.2.153/23, 10.2.154/23, and 10.2/16 to R2. That is, R2 is
advertising an "aggregated route" to R1. In this situation, packet P
can be label Switched until it reaches R2, but since R2 has performed
route aggregation, it must execute the best match algorithm to find
P's FEC.
An LSR R is considered to be an "LSP Egress" LSR for address prefix X
if and only if one of the following conditions holds:
1. R has an address Y, such that X is the address prefix in R's
routing table which is the longest match for Y, or
2. R contains in its routing tables one or more address prefixes Y
such that X is a proper initial substring of Y, but R's "LSP
previous hops" for X do not contain any such address prefixes
Y; that is, R is a "deaggregation point" for address prefix X.
An LSR R1 is considered to be an "LSP Proxy Egress" LSR for address
prefix X if and only if:
1. R1's next hop for X is R2, and R1 and R2 are not label
distribution peers with respect to X (perhaps because R2 does
not support MPLS), or
2. R1 has been configured to act as an LSP Proxy Egress for X
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The definition of LSP allows for the LSP Egress to be a node which
does not support MPLS; in this case the penultimate node in the LSP
is the Proxy Egress.
The Implicit NULL label is a label with special semantics which an
LSR can bind to an address prefix. If LSR Ru, by consulting its ILM,
sees that labeled packet P must be forwarded next to Rd, but that Rd
has distributed a binding of Implicit NULL to the corresponding
address prefix, then instead of replacing the value of the label on
top of the label stack, Ru pops the label stack, and then forwards
the resulting packet to Rd.
LSR Rd distributes a binding between Implicit NULL and an address
prefix X to LSR Ru if and only if:
1. the rules of Section 4.1.2 indicate that Rd distributes to Ru a
label binding for X, and
2. Rd knows that Ru can support the Implicit NULL label (i.e.,
that it can pop the label stack), and
3. Rd is an LSP Egress (not proxy egress) for X.
This causes the penultimate LSR on a LSP to pop the label stack.
This is quite appropriate; if the LSP Egress is an MPLS Egress for X,
then if the penultimate LSR does not pop the label stack, the LSP
Egress will need to look up the label, pop the label stack, and then
look up the next label (or look up the L3 address, if no more labels
are present). By having the penultimate LSR pop the label stack, the
LSP Egress is saved the work of having to look up two labels in order
to make its forwarding decision.
However, if the penultimate LSR is an ATM switch, it may not have the
capability to pop the label stack. Hence a binding of Implicit NULL
may be distributed only to LSRs which can support that function.
If the penultimate LSR in an LSP for address prefix X is an LSP Proxy
Egress, it acts just as if the LSP Egress had distributed a binding
of Implicit NULL for X.
There are situations in which an LSP Ingress, Ri, knows that packets
of several different FECs must all follow the same LSP, terminating
at, say, LSP Egress Re. In this case, proper routing can be achieved
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by using a single label for all such FECs; it is not necessary to
have a distinct label for each FEC. If (and only if) the following
conditions hold:
1. the address of LSR Re is itself in the routing table as a "host
route", and
2. there is some way for Ri to determine that Re is the LSP egress
for all packets in a particular set of FECs
Then Ri may bind a single label to all FECS in the set. This is
known as "Egress-Targeted Label Assignment."
How can LSR Ri determine that an LSR Re is the LSP Egress for all
packets in a particular FEC? There are a number of possible ways:
- If the network is running a link state routing algorithm, and
all nodes in the area support MPLS, then the routing algorithm
provides Ri with enough information to determine the routers
through which packets in that FEC must leave the routing domain
or area.
- If the network is running BGP, Ri may be able to determine that
the packets in a particular FEC must leave the network via some
particular router which is the "BGP Next Hop" for that FEC.
- It is possible to use the label distribution protocol to pass
information about which address prefixes are "attached" to
which egress LSRs. This method has the advantage of not
depending on the presence of link state routing.
If egress-targeted label assignment is used, the number of labels
that need to be supported throughout the network may be greatly
reduced. This may be significant if one is using legacy switching
hardware to do MPLS, and the switching hardware can support only a
limited number of labels.
One possible approach would be to configure the network to use
egress-targeted label assignment by default, but to configure
particular LSRs to NOT use egress-targeted label assignment for one
or more of the address prefixes for which it is an LSP egress. We
impose the following rule:
- If a particular LSR is NOT an LSP Egress for some set of
address prefixes, then it should assign labels to the address
prefixes in the same way as is done by its LSP next hop for
those address prefixes. That is, suppose Rd is Ru's LSP next
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hop for address prefixes X1 and X2. If Rd assigns the same
label to X1 and X2, Ru should as well. If Rd assigns different
labels to X1 and X2, then Ru should as well.
For example, suppose one wants to make egress-targeted label
assignment the default, but to assign distinct labels to those
address prefixes for which there are multiple possible LSP egresses
(i.e., for those address prefixes which are multi-homed.) One can
configure all LSRs to use egress-targeted label assignment, and then
configure a handful of LSRs to assign distinct labels to those
address prefixes which are multi-homed. For a particular multi-homed
address prefix X, one would only need to configure this in LSRs which
are either LSP Egresses or LSP Proxy Egresses for X.
It is important to note that if Ru and Rd are adjacent LSRs in an LSP
for X1 and X2, forwarding will still be done correctly if Ru assigns
distinct labels to X1 and X2 while Rd assigns just one label to the
both of them. This just means that R1 will map different incoming
labels to the same outgoing label, an ordinary occurrence.
Similarly, if Rd assigns distinct labels to X1 and X2, but Ru assigns
to them both the label corresponding to the address of their LSP
Egress or Proxy Egress, forwarding will still be done correctly. Ru
will just map the incoming label to the label which Rd has assigned
to the address of that LSP Egress.
There are a number of reasons why it may be desirable to use explicit
routing instead of hop by hop routing. For example, this allows
routes to be based on administrative policies, and allows the routes
that LSPs take to be carefully designed to allow traffic engineering
[MPLS-TRFENG].
In some situations, the network administrators may desire to forward
certain classes of traffic along certain pre-specified paths, where
these paths differ from the Hop-by-hop path that the traffic would
ordinarily follow. This can be done in support of policy routing, or
in support of traffic engineering. The explicit route may be a
configured one, or it may be determined dynamically by some means,
e.g., by constraint-based routing.
MPLS allows this to be easily done by means of Explicitly Routed LSP
Tunnels. All that is needed is:
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1. A means of selecting the packets that are to be sent into the
Explicitly Routed LSP Tunnel;
2. A means of setting up the Explicitly Routed LSP Tunnel;
3. A means of ensuring that packets sent into the Tunnel will not
loop from the receive endpoint back to the transmit endpoint.
If the transmit endpoint of the tunnel wishes to put a labeled packet
into the tunnel, it must first replace the label value at the top of
the stack with a label value that was distributed to it by the
tunnel's receive endpoint. Then it must push on the label which
corresponds to the tunnel itself, as distributed to it by the next
hop along the tunnel. To allow this, the tunnel endpoints should be
explicit label distribution peers. The label bindings they need to
exchange are of no interest to the LSRs along the tunnel.
Suppose a particular LSR Re is an LSP proxy egress for 10 address
prefixes, and it reaches each address prefix through a distinct
interface.
One could assign a single label to all 10 address prefixes. Then Re
is an LSP egress for all 10 address prefixes. This ensures that
packets for all 10 address prefixes get delivered to Re. However, Re
would then have to look up the network layer address of each such
packet in order to choose the proper interface to send the packet on.
Alternatively, one could assign a distinct label to each interface.
Then Re is an LSP proxy egress for the 10 address prefixes. This
eliminates the need for Re to look up the network layer addresses in
order to forward the packets. However, it can result in the use of a
large number of labels.
An alternative would be to bind all 10 address prefixes to the same
level 1 label (which is also bound to the address of the LSR itself),
and then to bind each address prefix to a distinct level 2 label.
The level 2 label would be treated as an attribute of the level 1
label binding, which we call the "Stack Attribute". We impose the
following rules:
- When LSR Ru initially labels a hitherto unlabeled packet, if
the longest match for the packet's destination address is X,
and Ru's LSP next hop for X is Rd, and Rd has distributed to Ru
a binding of label L1 to X, along with a stack attribute of L2,
then
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1. Ru must push L2 and then L1 onto the packet's label stack,
and then forward the packet to Rd;
2. When Ru distributes label bindings for X to its label
distribution peers, it must include L2 as the stack
attribute.
3. Whenever the stack attribute changes (possibly as a result
of a change in Ru's LSP next hop for X), Ru must distribute
the new stack attribute.
Note that although the label value bound to X may be different at
each hop along the LSP, the stack attribute value is passed
unchanged, and is set by the LSP proxy egress.
Thus the LSP proxy egress for X becomes an "implicit peer" with each
other LSR in the routing area or domain. In this case, explicit
peering would be too unwieldy, because the number of peers would
become too large.
If an LSR supports multiple routes for a particular stream, then it
may assign multiple labels to the stream, one for each route. Thus
the reception of a second label binding from a particular neighbor
for a particular address prefix should be taken as meaning that
either label can be used to represent that address prefix.
If multiple label bindings for a particular address prefix are
specified, they may have distinct attributes.
Consider the case of packets P1 and P2, each of which has a
destination address whose longest match, throughout a particular
routing domain, is address prefix X. Suppose that the Hop-by-hop
path for P1 is <R1, R2, R3>, and the Hop-by-hop path for P2 is <R4,
R2, R3>. Let's suppose that R3 binds label L3 to X, and distributes
this binding to R2. R2 binds label L2 to X, and distributes this
binding to both R1 and R4. When R2 receives packet P1, its incoming
label will be L2. R2 will overwrite L2 with L3, and send P1 to R3.
When R2 receives packet P2, its incoming label will also be L2. R2
again overwrites L2 with L3, and send P2 on to R3.
Note then that when P1 and P2 are traveling from R2 to R3, they carry
the same label, and as far as MPLS is concerned, they cannot be
distinguished. Thus instead of talking about two distinct LSPs, <R1,
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R2, R3> and <R4, R2, R3>, we might talk of a single "Multipoint-to-
Point LSP Tree", which we might denote as <{R1, R4}, R2, R3>.
This creates a difficulty when we attempt to use conventional ATM
switches as LSRs. Since conventional ATM switches do not support
multipoint-to-point connections, there must be procedures to ensure
that each LSP is realized as a point-to-point VC. However, if ATM
switches which do support multipoint-to-point VCs are in use, then
the LSPs can be most efficiently realized as multipoint-to-point VCs.
Alternatively, if the SVP Multipoint Encoding (section 3.25.2) can be
used, the LSPs can be realized as multipoint-to-point SVPs.
Consider the case of an Autonomous System, A, which carries transit
traffic between other Autonomous Systems. Autonomous System A will
have a number of BGP Border Routers, and a mesh of BGP connections
among them, over which BGP routes are distributed. In many such
cases, it is desirable to avoid distributing the BGP routes to
routers which are not BGP Border Routers. If this can be avoided,
the "route distribution load" on those routers is significantly
reduced. However, there must be some means of ensuring that the
transit traffic will be delivered from Border Router to Border Router
by the interior routers.
This can easily be done by means of LSP Tunnels. Suppose that BGP
routes are distributed only to BGP Border Routers, and not to the
interior routers that lie along the Hop-by-hop path from Border
Router to Border Router. LSP Tunnels can then be used as follows:
1. Each BGP Border Router distributes, to every other BGP Border
Router in the same Autonomous System, a label for each address
prefix that it distributes to that router via BGP.
2. The IGP for the Autonomous System maintains a host route for
each BGP Border Router. Each interior router distributes its
labels for these host routes to each of its IGP neighbors.
3. Suppose that:
a) BGP Border Router B1 receives an unlabeled packet P,
b) address prefix X in B1's routing table is the longest match
for the destination address of P,
c) the route to X is a BGP route,
d) the BGP Next Hop for X is B2,
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e) B2 has bound label L1 to X, and has distributed this binding
to B1,
f) the IGP next hop for the address of B2 is I1,
g) the address of B2 is in B1's and I1's IGP routing tables as
a host route, and
h) I1 has bound label L2 to the address of B2, and distributed
this binding to B1.
Then before sending packet P to I1, B1 must create a label
stack for P, then push on label L1, and then push on label L2.
4. Suppose that BGP Border Router B1 receives a labeled Packet P,
where the label on the top of the label stack corresponds to an
address prefix, X, to which the route is a BGP route, and that
conditions 3b, 3c, 3d, and 3e all hold. Then before sending
packet P to I1, B1 must replace the label at the top of the
label stack with L1, and then push on label L2.
With these procedures, a given packet P follows a level 1 LSP all of
whose members are BGP Border Routers, and between each pair of BGP
Border Routers in the level 1 LSP, it follows a level 2 LSP.
These procedures effectively create a Hop-by-Hop Routed LSP Tunnel
between the BGP Border Routers.
Since the BGP border routers are exchanging label bindings for
address prefixes that are not even known to the IGP routing, the BGP
routers should become explicit label distribution peers with each
other.
It is sometimes possible to create Hop-by-Hop Routed LSP Tunnels
between two BGP Border Routers, even if they are not in the same
Autonomous System. Suppose, for example, that B1 and B2 are in AS 1.
Suppose that B3 is an EBGP neighbor of B2, and is in AS2. Finally,
suppose that B2 and B3 are on some network which is common to both
Autonomous Systems (a "Demilitarized Zone"). In this case, an LSP
tunnel can be set up directly between B1 and B3 as follows:
- B3 distributes routes to B2 (using EBGP), optionally assigning
labels to address prefixes;
- B2 redistributes those routes to B1 (using IBGP), indicating
that the BGP next hop for each such route is B3. If B3 has
assigned labels to address prefixes, B2 passes these labels
along, unchanged, to B1.
Rosen, et al. Standards Track [Page 46]
RFC 3031 MPLS Architecture January 2001
- The IGP of AS1 has a host route for B3.
The use of Hop-by-Hop Routed LSP Tunnels is not restricted to tunnels
between BGP Next Hops. Any situation in which one might otherwise
have used an encapsulation tunnel is one in which it is appropriate
to use a Hop-by-Hop Routed LSP Tunnel. Instead of encapsulating the
packet with a new header whose destination address is the address of
the tunnel's receive endpoint, the label corresponding to the address
prefix which is the longest match for the address of the tunnel's
receive endpoint is pushed on the packet's label stack. The packet
which is sent into the tunnel may or may not already be labeled.
If the transmit endpoint of the tunnel wishes to put a labeled packet
into the tunnel, it must first replace the label value at the top of
the stack with a label value that was distributed to it by the
tunnel's receive endpoint. Then it must push on the label which
corresponds to the tunnel itself, as distributed to it by the next
hop along the tunnel. To allow this, the tunnel endpoints should be
explicit label distribution peers. The label bindings they need to
exchange are of no interest to the LSRs along the tunnel.
Multicast routing proceeds by constructing multicast trees. The tree
along which a particular multicast packet must get forwarded depends
in general on the packet's source address and its destination
address. Whenever a particular LSR is a node in a particular
multicast tree, it binds a label to that tree. It then distributes
that binding to its parent on the multicast tree. (If the node in
question is on a LAN, and has siblings on that LAN, it must also
distribute the binding to its siblings. This allows the parent to
use a single label value when multicasting to all children on the
LAN.)
When a multicast labeled packet arrives, the NHLFE corresponding to
the label indicates the set of output interfaces for that packet, as
well as the outgoing label. If the same label encoding technique is
used on all the outgoing interfaces, the very same packet can be sent
to all the children.
In this section, we consider only label bindings that are used for
traffic to be label switched along its hop-by-hop routed path. In
these cases, the label in question will correspond to an address
prefix in the routing table.
Rosen, et al. Standards Track [Page 47]
RFC 3031 MPLS Architecture January 2001
There are a number of different procedures that may be used to
distribute label bindings. Some are executed by the downstream LSR,
and some by the upstream LSR.
The downstream LSR must perform:
- The Distribution Procedure, and
- the Withdrawal Procedure.
The upstream LSR must perform:
- The Request Procedure, and
- the NotAvailable Procedure, and
- the Release Procedure, and
- the labelUse Procedure.
The MPLS architecture supports several variants of each procedure.
However, the MPLS architecture does not support all possible
combinations of all possible variants. The set of supported
combinations will be described in section 5.2, where the
interoperability between different combinations will also be
discussed.
The Distribution Procedure is used by a downstream LSR to determine
when it should distribute a label binding for a particular address
prefix to its label distribution peers. The architecture supports
four different distribution procedures.
Irrespective of the particular procedure that is used, if a label
binding for a particular address prefix has been distributed by a
downstream LSR Rd to an upstream LSR Ru, and if at any time the
attributes (as defined above) of that binding change, then Rd must
inform Ru of the new attributes.
If an LSR is maintaining multiple routes to a particular address
prefix, it is a local matter as to whether that LSR binds multiple
labels to the address prefix (one per route), and hence distributes
multiple bindings.
Rosen, et al. Standards Track [Page 48]
RFC 3031 MPLS Architecture January 2001
Let Rd be an LSR. Suppose that:
1. X is an address prefix in Rd's routing table
2. Ru is a label distribution peer of Rd with respect to X
Whenever these conditions hold, Rd must bind a label to X and
distribute that binding to Ru. It is the responsibility of Rd to
keep track of the bindings which it has distributed to Ru, and to
make sure that Ru always has these bindings.
This procedure would be used by LSRs which are performing unsolicited
downstream label assignment in the Independent LSP Control Mode.
Let Rd be an LSR. Suppose that:
1. X is an address prefix in Rd's routing table
2. Ru is a label distribution peer of Rd with respect to X
3. Rd is either an LSP Egress or an LSP Proxy Egress for X, or
Rd's L3 next hop for X is Rn, where Rn is distinct from Ru, and
Rn has bound a label to X and distributed that binding to Rd.
Then as soon as these conditions all hold, Rd should bind a label to
X and distribute that binding to Ru.
Whereas PushUnconditional causes the distribution of label bindings
for all address prefixes in the routing table, PushConditional causes
the distribution of label bindings only for those address prefixes
for which one has received label bindings from one's LSP next hop, or
for which one does not have an MPLS-capable L3 next hop.
This procedure would be used by LSRs which are performing unsolicited
downstream label assignment in the Ordered LSP Control Mode.
Let Rd be an LSR. Suppose that:
1. X is an address prefix in Rd's routing table
2. Ru is a label distribution peer of Rd with respect to X
Rosen, et al. Standards Track [Page 49]
RFC 3031 MPLS Architecture January 2001
3. Ru has explicitly requested that Rd bind a label to X and
distribute the binding to Ru
Then Rd should bind a label to X and distribute that binding to Ru.
Note that if X is not in Rd's routing table, or if Rd is not a label
distribution peer of Ru with respect to X, then Rd must inform Ru
that it cannot provide a binding at this time.
If Rd has already distributed a binding for address prefix X to Ru,
and it receives a new request from Ru for a binding for address
prefix X, it will bind a second label, and distribute the new binding
to Ru. The first label binding remains in effect.
This procedure would be used by LSRs performing downstream-on-demand
label distribution using the Independent LSP Control Mode.
Let Rd be an LSR. Suppose that:
1. X is an address prefix in Rd's routing table
2. Ru is a label distribution peer of Rd with respect to X
3. Ru has explicitly requested that Rd bind a label to X and
distribute the binding to Ru
4. Rd is either an LSP Egress or an LSP Proxy Egress for X, or
Rd's L3 next hop for X is Rn, where Rn is distinct from Ru, and
Rn has bound a label to X and distributed that binding to Rd
Then as soon as these conditions all hold, Rd should bind a label to
X and distribute that binding to Ru. Note that if X is not in Rd's
routing table and a binding for X is not obtainable via Rd's next hop
for X, or if Rd is not a label distribution peer of Ru with respect
to X, then Rd must inform Ru that it cannot provide a binding at this
time.
However, if the only condition that fails to hold is that Rn has not
yet provided a label to Rd, then Rd must defer any response to Ru
until such time as it has receiving a binding from Rn.
If Rd has distributed a label binding for address prefix X to Ru, and
at some later time, any attribute of the label binding changes, then
Rd must redistribute the label binding to Ru, with the new attribute.
It must do this even though Ru does not issue a new Request.
Rosen, et al. Standards Track [Page 50]
RFC 3031 MPLS Architecture January 2001
This procedure would be used by LSRs that are performing downstream-
on-demand label allocation in the Ordered LSP Control Mode.
In section 5.2, we will discuss how to choose the particular
procedure to be used at any given time, and how to ensure
interoperability among LSRs that choose different procedures.
The Request Procedure is used by the upstream LSR for an address
prefix to determine when to explicitly request that the downstream
LSR bind a label to that prefix and distribute the binding. There
are three possible procedures that can be used.
Never make a request. This is useful if the downstream LSR uses the
PushConditional procedure or the PushUnconditional procedure, but is
not useful if the downstream LSR uses the PulledUnconditional
procedure or the the PulledConditional procedures.
This procedure would be used by an LSR when unsolicited downstream
label distribution and Liberal Label Retention Mode are being used.
Make a request whenever the L3 next hop to the address prefix
changes, or when a new address prefix is learned, and one doesn't
already have a label binding from that next hop for the given address
prefix.
This procedure would be used by an LSR whenever Conservative Label
Retention Mode is being used.
Issue a request whenever a request is received, in addition to
issuing a request when needed (as described in section 5.1.2.2). If
Ru is not capable of being an LSP ingress, it may issue a request
only when it receives a request from upstream.
If Rd receives such a request from Ru, for an address prefix for
which Rd has already distributed Ru a label, Rd shall assign a new
(distinct) label, bind it to X, and distribute that binding.
(Whether Rd can distribute this binding to Ru immediately or not
depends on the Distribution Procedure being used.)
Rosen, et al. Standards Track [Page 51]
RFC 3031 MPLS Architecture January 2001
This procedure would be used by an LSR which is doing downstream-on-
demand label distribution, but is not doing label merging, e.g., an
ATM-LSR which is not capable of VC merge.
If Ru and Rd are respectively upstream and downstream label
distribution peers for address prefix X, and Rd is Ru's L3 next hop
for X, and Ru requests a binding for X from Rd, but Rd replies that
it cannot provide a binding at this time, because it has no next hop
for X, then the NotAvailable procedure determines how Ru responds.
There are two possible procedures governing Ru's behavior:
Ru should issue the request again at a later time. That is, the
requester is responsible for trying again later to obtain the needed
binding. This procedure would be used when downstream-on-demand
label distribution is used.
Ru should never reissue the request, instead assuming that Rd will
provide the binding automatically when it is available. This is
useful if Rd uses the PushUnconditional procedure or the
PushConditional procedure, i.e., if unsolicited downstream label
distribution is used.
Note that if Rd replies that it cannot provide a binding to Ru,
because of some error condition, rather than because Rd has no next
hop, the behavior of Ru will be governed by the error recovery
conditions of the label distribution protocol, rather than by the
NotAvailable procedure.
Suppose that Rd is an LSR which has bound a label to address prefix
X, and has distributed that binding to LSR Ru. If Rd does not happen
to be Ru's L3 next hop for address prefix X, or has ceased to be Ru's
L3 next hop for address prefix X, then Ru will not be using the
label. The Release Procedure determines how Ru acts in this case.
There are two possible procedures governing Ru's behavior:
Ru should release the binding, and inform Rd that it has done so.
This procedure would be used to implement Conservative Label
Retention Mode.
Rosen, et al. Standards Track [Page 52]
RFC 3031 MPLS Architecture January 2001
Ru should maintain the binding, so that it can use it again
immediately if Rd later becomes Ru's L3 next hop for X. This
procedure would be used to implement Liberal Label Retention Mode.
Suppose Ru is an LSR which has received label binding L for address
prefix X from LSR Rd, and Ru is upstream of Rd with respect to X, and
in fact Rd is Ru's L3 next hop for X.
Ru will make use of the binding if Rd is Ru's L3 next hop for X. If,
at the time the binding is received by Ru, Rd is NOT Ru's L3 next hop
for X, Ru does not make any use of the binding at that time. Ru may
however start using the binding at some later time, if Rd becomes
Ru's L3 next hop for X.
The labelUse Procedure determines just how Ru makes use of Rd's
binding.
There are two procedures which Ru may use:
Ru may put the binding into use immediately. At any time when Ru has
a binding for X from Rd, and Rd is Ru's L3 next hop for X, Rd will
also be Ru's LSP next hop for X. This procedure is used when loop
detection is not in use.
This procedure is the same as UseImmediate, unless Ru has detected a
loop in the LSP. If a loop has been detected, Ru will discontinue
the use of label L for forwarding packets to Rd.
This procedure is used when loop detection is in use.
This will continue until the next hop for X changes, or until the
loop is no longer detected.
In this case, there is only a single procedure.
When LSR Rd decides to break the binding between label L and address
prefix X, then this unbinding must be distributed to all LSRs to
which the binding was distributed.
Rosen, et al. Standards Track [Page 53]
RFC 3031 MPLS Architecture January 2001
It is required that the unbinding of L from X be distributed by Rd to
a LSR Ru before Rd distributes to Ru any new binding of L to any
other address prefix Y, where X != Y. If Ru were to learn of the new
binding of L to Y before it learned of the unbinding of L from X, and
if packets matching both X and Y were forwarded by Ru to Rd, then for
a period of time, Ru would label both packets matching X and packets
matching Y with label L.
The distribution and withdrawal of label bindings is done via a label
distribution protocol. All label distribution protocols require that
a label distribution adjacency be established between two label
distribution peers (except implicit peers). If LSR R1 has a label
distribution adjacency to LSR R2, and has received label bindings
from LSR R2 via that adjacency, then if adjacency is brought down by
either peer (whether as a result of failure or as a matter of normal
operation), all bindings received over that adjacency must be
considered to have been withdrawn.
As long as the relevant label distribution adjacency remains in
place, label bindings that are withdrawn must always be withdrawn
explicitly. If a second label is bound to an address prefix, the
result is not to implicitly withdraw the first label, but to bind
both labels; this is needed to support multi-path routing. If a
second address prefix is bound to a label, the result is not to
implicitly withdraw the binding of that label to the first address
prefix, but to use that label for both address prefixes.
Consider two LSRs, Ru and Rd, which are label distribution peers with
respect to some set of address prefixes, where Ru is the upstream
peer and Rd is the downstream peer.
The MPLS scheme which governs the interaction of Ru and Rd can be
described as a quintuple of procedures: <Distribution Procedure,
Request Procedure, NotAvailable Procedure, Release Procedure,
labelUse Procedure>. (Since there is only one Withdraw Procedure, it
need not be mentioned.) A "*" appearing in one of the positions is a
wild-card, meaning that any procedure in that category may be
present; an "N/A" appearing in a particular position indicates that
no procedure in that category is needed.
Only the MPLS schemes which are specified below are supported by the
MPLS Architecture. Other schemes may be added in the future, if a
need for them is shown.
Rosen, et al. Standards Track [Page 54]
RFC 3031 MPLS Architecture January 2001
If Ru and Rd are label distribution peers, and both support label
merging, one of the following schemes must be used:
1. <PushUnconditional, RequestNever, N/A, NoReleaseOnChange,
UseImmediate>
This is unsolicited downstream label distribution with
independent control, liberal label retention mode, and no loop
detection.
2. <PushUnconditional, RequestNever, N/A, NoReleaseOnChange,
UseIfLoopNotDetected>
This is unsolicited downstream label distribution with
independent control, liberal label retention, and loop
detection.
3. <PushConditional, RequestWhenNeeded, RequestNoRetry,
ReleaseOnChange, *>
This is unsolicited downstream label distribution with ordered
control (from the egress) and conservative label retention
mode. Loop detection is optional.
4. <PushConditional, RequestNever, N/A, NoReleaseOnChange, *>
This is unsolicited downstream label distribution with ordered
control (from the egress) and liberal label retention mode.
Loop detection is optional.
5. <PulledConditional, RequestWhenNeeded, RequestRetry,
ReleaseOnChange, *>
This is downstream-on-demand label distribution with ordered
control (initiated by the ingress), conservative label
retention mode, and optional loop detection.
6. <PulledUnconditional, RequestWhenNeeded, N/A, ReleaseOnChange,
UseImmediate>
This is downstream-on-demand label distribution with
independent control and conservative label retention mode,
without loop detection.
Rosen, et al. Standards Track [Page 55]
RFC 3031 MPLS Architecture January 2001
7. <PulledUnconditional, RequestWhenNeeded, N/A, ReleaseOnChange,
UseIfLoopNotDetected>
This is downstream-on-demand label distribution with
independent control and conservative label retention mode, with
loop detection.
Suppose that R1, R2, R3, and R4 are ATM switches which do not support
label merging, but are being used as LSRs. Suppose further that the
L3 hop-by-hop path for address prefix X is <R1, R2, R3, R4>, and that
packets destined for X can enter the network at any of these LSRs.
Since there is no multipoint-to-point capability, the LSPs must be
realized as point-to-point VCs, which means that there needs to be
three such VCs for address prefix X: <R1, R2, R3, R4>, <R2, R3, R4>,
and <R3, R4>.
Therefore, if R1 and R2 are MPLS peers, and either is an LSR which is
implemented using conventional ATM switching hardware (i.e., no cell
interleave suppression), or is otherwise incapable of performing
label merging, the MPLS scheme in use between R1 and R2 must be one
of the following:
1. <PulledConditional, RequestOnRequest, RequestRetry,
ReleaseOnChange, *>
This is downstream-on-demand label distribution with ordered
control (initiated by the ingress), conservative label
retention mode, and optional loop detection.
The use of the RequestOnRequest procedure will cause R4 to
distribute three labels for X to R3; R3 will distribute 2
labels for X to R2, and R2 will distribute one label for X to
R1.
2. <PulledUnconditional, RequestOnRequest, N/A, ReleaseOnChange,
UseImmediate>
This is downstream-on-demand label distribution with
independent control and conservative label retention mode,
without loop detection.
Rosen, et al. Standards Track [Page 56]
RFC 3031 MPLS Architecture January 2001
3. <PulledUnconditional, RequestOnRequest, N/A, ReleaseOnChange,
UseIfLoopNotDetected>
This is downstream-on-demand label distribution with
independent control and conservative label retention mode, with
loop detection.
It is easy to see that certain quintuples do NOT yield viable MPLS
schemes. For example:
- <PulledUnconditional, RequestNever, *, *, *>
<PulledConditional, RequestNever, *, *, *>
In these MPLS schemes, the downstream LSR Rd distributes label
bindings to upstream LSR Ru only upon request from Ru, but Ru
never makes any such requests. Obviously, these schemes are
not viable, since they will not result in the proper
distribution of label bindings.
- <*, RequestNever, *, *, ReleaseOnChange>
In these MPLS schemes, Rd releases bindings when it isn't using
them, but it never asks for them again, even if it later has a
need for them. These schemes thus do not ensure that label
bindings get properly distributed.
In this section, we specify rules to prevent a pair of label
distribution peers from adopting procedures which lead to infeasible
MPLS Schemes. These rules require either the exchange of information
between label distribution peers during the initialization of the
label distribution adjacency, or a priori knowledge of the
information (obtained through a means outside the scope of this
document).
1. Each must state whether it supports label merging.
2. If Rd does not support label merging, Rd must choose either the
PulledUnconditional procedure or the PulledConditional
procedure. If Rd chooses PulledConditional, Ru is forced to
use the RequestRetry procedure.
That is, if the downstream LSR does not support label merging,
its preferences take priority when the MPLS scheme is chosen.
Rosen, et al. Standards Track [Page 57]
RFC 3031 MPLS Architecture January 2001
3. If Ru does not support label merging, but Rd does, Ru must
choose either the RequestRetry or RequestNoRetry procedure.
This forces Rd to use the PulledConditional or
PulledUnConditional procedure respectively.
That is, if only one of the LSRs doesn't support label merging,
its preferences take priority when the MPLS scheme is chosen.
4. If both Ru and Rd both support label merging, then the choice
between liberal and conservative label retention mode belongs
to Ru. That is, Ru gets to choose either to use
RequestWhenNeeded/ReleaseOnChange (conservative) , or to use
RequestNever/NoReleaseOnChange (liberal). However, the choice
of "push" vs. "pull" and "conditional" vs. "unconditional"
belongs to Rd. If Ru chooses liberal label retention mode, Rd
can choose either PushUnconditional or PushConditional. If Ru
chooses conservative label retention mode, Rd can choose
PushConditional, PulledConditional, or PulledUnconditional.
These choices together determine the MPLS scheme in use.
Some routers may implement security procedures which depend on the
network layer header being in a fixed place relative to the data link
layer header. The MPLS generic encapsulation inserts a shim between
the data link layer header and the network layer header. This may
cause any such security procedures to fail.
An MPLS label has its meaning by virtue of an agreement between the
LSR that puts the label in the label stack (the "label writer"), and
the LSR that interprets that label (the "label reader"). If labeled
packets are accepted from untrusted sources, or if a particular
incoming label is accepted from an LSR to which that label has not
been distributed, then packets may be routed in an illegitimate
manner.
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.
Rosen, et al. Standards Track [Page 58]
RFC 3031 MPLS Architecture January 2001
Eric C. Rosen
Cisco Systems, Inc.
250 Apollo Drive
Chelmsford, MA, 01824
EMail: erosen@cisco.com
Arun Viswanathan
Force10 Networks, Inc.
1440 McCarthy Blvd.
Milpitas, CA 95035-7438
EMail: arun@force10networks.com
Ross Callon
Juniper Networks, Inc.
1194 North Mathilda Avenue
Sunnyvale, CA 94089 USA
EMail: rcallon@juniper.net
[MPLS-ATM] Davie, B., Lawrence, J., McCloghrie, K., Rekhter,
Y., Rosen, E., Swallow, G. and P. Doolan, "MPLS
using LDP and ATM VC Switching", RFC 3035,
January 2001.
[MPLS-BGP] "Carrying Label Information in BGP-4", Rekhter,
Rosen, Work in Progress.
[MPLS-CR-LDP] "Constraint-Based LSP Setup using LDP", Jamoussi,
Editor, Work in Progress.
[MPLS-FRMRLY] Conta, A., Doolan, P. and A. Malis, "Use of Label
Switching on Frame Relay Networks Specification",
RFC 3034, January 2001.
[MPLS-LDP] Andersson, L., Doolan, P., Feldman, N., Fredette,
A. and B. Thomas, "LDP Specification", RFC 3036,
January 2001.
Rosen, et al. Standards Track [Page 59]
RFC 3031 MPLS Architecture January 2001
[MPLS-RSVP-TUNNELS] "Extensions to RSVP for LSP Tunnels", Awduche,
Berger, Gan, Li, Swallow, Srinvasan, Work in
Progress.
[MPLS-SHIM] Rosen, E., Rekhter, Y., Tappan, D., Fedorkow, G.,
Farinacci, D. and A. Conta, "MPLS Label Stack
Encoding", RFC 3032, January 2001.
[MPLS-TRFENG] Awduche, D., Malcolm, J., Agogbua, J., O'Dell, M.
and J. McManus, "Requirements for Traffic
Engineering Over MPLS", RFC 2702, September 1999.
Rosen, et al. Standards Track [Page 60]
RFC 3031 MPLS Architecture January 2001
Copyright (C) The Internet Society (2001). All Rights Reserved.
This document and translations of it may be copied and furnished to
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HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
Rosen, et al. Standards Track [Page 61]