Network Working Group Eric C. Rosen
Internet Draft Cisco Systems, Inc.
Expiration Date: September 1998
Arun Viswanathan
Lucent Technologies
Ross Callon
IronBridge Networks, Inc.
March 1998
Multiprotocol Label Switching Architecture
draft-ietf-mpls-arch-01.txt
Status of this Memo
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Abstract
This internet draft specifies the architecture for multiprotocol
label switching (MPLS). The architecture is based on other label
switching approaches [2-11] as well as on the MPLS Framework document
[1].
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Table of Contents
1 Introduction to MPLS ............................... 4
1.1 Overview ........................................... 4
1.2 Terminology ........................................ 6
1.3 Acronyms and Abbreviations ......................... 9
1.4 Acknowledgments .................................... 10
2 Outline of Approach ................................ 11
2.1 Labels ............................................. 11
2.2 Upstream and Downstream LSRs ....................... 12
2.3 Labeled Packet ..................................... 12
2.4 Label Assignment and Distribution; Attributes ...... 12
2.5 Label Distribution Protocol (LDP) .................. 13
2.6 The Label Stack .................................... 13
2.7 The Next Hop Label Forwarding Entry (NHLFE) ........ 14
2.8 Incoming Label Map (ILM) ........................... 14
2.9 Stream-to-NHLFE Map (STN) .......................... 15
2.10 Label Swapping ..................................... 15
2.11 Scope and Uniqueness of Labels ..................... 15
2.12 Label Switched Path (LSP), LSP Ingress, LSP Egress . 16
2.13 Penultimate Hop Popping ............................ 18
2.14 LSP Next Hop ....................................... 19
2.15 Route Selection .................................... 20
2.16 Time-to-Live (TTL) ................................. 21
2.17 Loop Control ....................................... 22
2.17.1 Loop Prevention .................................... 23
2.17.2 Interworking of Loop Control Options ............... 25
2.18 Merging and Non-Merging LSRs ....................... 26
2.18.1 Stream Merge ....................................... 27
2.18.2 Non-merging LSRs ................................... 27
2.18.3 Labels for Merging and Non-Merging LSRs ............ 28
2.18.4 Merge over ATM ..................................... 29
2.18.4.1 Methods of Eliminating Cell Interleave ............. 29
2.18.4.2 Interoperation: VC Merge, VP Merge, and Non-Merge .. 29
2.19 LSP Control: Egress versus Local ................... 30
2.20 Granularity ........................................ 32
2.21 Tunnels and Hierarchy .............................. 33
2.21.1 Hop-by-Hop Routed Tunnel ........................... 33
2.21.2 Explicitly Routed Tunnel ........................... 33
2.21.3 LSP Tunnels ........................................ 33
2.21.4 Hierarchy: LSP Tunnels within LSPs ................. 34
2.21.5 LDP Peering and Hierarchy .......................... 34
2.22 LDP Transport ...................................... 36
2.23 Label Encodings .................................... 36
2.23.1 MPLS-specific Hardware and/or Software ............. 36
2.23.2 ATM Switches as LSRs ............................... 37
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2.23.3 Interoperability among Encoding Techniques ......... 38
2.24 Multicast .......................................... 39
3 Some Applications of MPLS .......................... 39
3.1 MPLS and Hop by Hop Routed Traffic ................. 39
3.1.1 Labels for Address Prefixes ........................ 39
3.1.2 Distributing Labels for Address Prefixes ........... 39
3.1.2.1 LDP Peers for a Particular Address Prefix .......... 39
3.1.2.2 Distributing Labels ................................ 40
3.1.3 Using the Hop by Hop path as the LSP ............... 41
3.1.4 LSP Egress and LSP Proxy Egress .................... 41
3.1.5 The POP Label ...................................... 42
3.1.6 Option: Egress-Targeted Label Assignment ........... 43
3.2 MPLS and Explicitly Routed LSPs .................... 44
3.2.1 Explicitly Routed LSP Tunnels: Traffic Engineering . 44
3.3 Label Stacks and Implicit Peering .................. 45
3.4 MPLS and Multi-Path Routing ........................ 46
3.5 LSP Trees as Multipoint-to-Point Entities .......... 46
3.6 LSP Tunneling between BGP Border Routers ........... 47
3.7 Other Uses of Hop-by-Hop Routed LSP Tunnels ........ 49
3.8 MPLS and Multicast ................................. 49
4 LDP Procedures for Hop-by-Hop Routed Traffic ....... 50
4.1 The Procedures for Advertising and Using labels .... 50
4.1.1 Downstream LSR: Distribution Procedure ............. 50
4.1.1.1 PushUnconditional .................................. 51
4.1.1.2 PushConditional .................................... 51
4.1.1.3 PulledUnconditional ................................ 52
4.1.1.4 PulledConditional .................................. 52
4.1.2 Upstream LSR: Request Procedure .................... 53
4.1.2.1 RequestNever ....................................... 53
4.1.2.2 RequestWhenNeeded .................................. 53
4.1.2.3 RequestOnRequest ................................... 53
4.1.3 Upstream LSR: NotAvailable Procedure ............... 54
4.1.3.1 RequestRetry ....................................... 54
4.1.3.2 RequestNoRetry ..................................... 54
4.1.4 Upstream LSR: Release Procedure .................... 54
4.1.4.1 ReleaseOnChange .................................... 54
4.1.4.2 NoReleaseOnChange .................................. 54
4.1.5 Upstream LSR: labelUse Procedure ................... 55
4.1.5.1 UseImmediate ....................................... 55
4.1.5.2 UseIfLoopFree ...................................... 55
4.1.5.3 UseIfLoopNotDetected ............................... 55
4.1.6 Downstream LSR: Withdraw Procedure ................. 56
4.2 MPLS Schemes: Supported Combinations of Procedures . 56
4.2.1 TTL-capable LSP Segments ........................... 57
4.2.2 Using ATM Switches as LSRs ......................... 57
4.2.2.1 Without Multipoint-to-point Capability ............. 58
4.2.2.2 With Multipoint-To-Point Capability ................ 58
4.2.3 Interoperability Considerations .................... 59
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4.2.4 How to do Loop Prevention .......................... 60
4.2.5 How to do Loop Detection ........................... 60
4.2.6 Security Considerations ............................ 60
5 Authors' Addresses ................................. 60
6 References ......................................... 61
1. Introduction to MPLS
1.1. Overview
In connectionless network layer protocols, as a packet travels from
one router hop to the next, an independent forwarding decision is
made at each hop. Each router runs a network layer routing
algorithm. As a packet travels through the network, each router
analyzes the packet header. The choice of next hop for a packet is
based on the header analysis and the result 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. Such
a set of packets may be called a "stream".
In conventional IP forwarding, a particular router will typically
consider two packets to be in the same stream 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 stream.
In MPLS, the assignment of a particular packet to a particular stream
is done just once, as the packet enters the network. The stream to
which the packet is assigned is encoded with 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".
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
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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. If assignment to a stream is based on a "longest match",
this eliminates the need to perform a longest match computation for
each packet at each hop; the computation can be performed just once.
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", in order to apply different
discard thresholds or scheduling disciplines to different packets.
MPLS allows the precedence or class of service to be inferred from
the label, so that no further header analysis is needed; in some
cases MPLS provides a way to explicitly encode a class of service in
the "label header".
The fact that a packet is assigned to a stream just once, rather than
at every hop, allows the use of sophisticated forwarding paradigms.
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 point ("policy routing") can be easily made. In fact,
the policy used to assign a packet to a stream need not have only the
network layer header as input; it may use arbitrary information about
the packet, and/or arbitrary policy information as input. Since this
decouples forwarding from routing, it allows one to use MPLS to
support a large variety of routing policies that are difficult or
impossible to support with just conventional network layer
forwarding.
Similarly, MPLS facilitates the use of explicit routing, without
requiring that each IP packet carry the explicit route. Explicit
routes may be useful to support policy routing and traffic
engineering.
MPLS makes use of a routing approach whereby the normal mode of
operation is that L3 routing (e.g., existing IP routing protocols
and/or new IP routing protocols) is used by all nodes to determine
the routed path.
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.
A general discussion of issues related to MPLS is presented in "A
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Framework for Multiprotocol Label Switching" [1].
1.2. Terminology
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.
aggregate stream synonym of "stream"
DLCI a label used in Frame Relay networks to
identify frame relay circuits
flow a single instance of an application to
application flow of data (as in the RSVP
and IFMP use of the term "flow")
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 stream merge, 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 stream, usually of local
significance.
label information base the database of information containing
label bindings
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 streams of data to be
forwarded.
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label switched hop the hop between two MPLS nodes, on which
forwarding is done using labels.
label switched path the path created by the concatenation of
one or more label switched hops, allowing
a packet to be forwarded by swapping
labels from an MPLS node to another MPLS
node.
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 and closed
loop prevention a method of dealing with loops in which
data is never transmitted over a loop
label stack an ordered set of labels
loop survival a method of dealing with loops in which
data may be transmitted over a loop, but
means are employed to limit the amount of
network resources which may be consumed
by the looping data
label switched path The path through one or more LSRs at one
level of the hierarchy followed by a
stream.
label switching router an MPLS node which is capable of
forwarding native L3 packets
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merge point the node at which multiple streams and
switched paths are combined into a single
stream sent over a single path.
Mlabel abbreviation for MPLS label
MPLS core standards the standards which describe the core
MPLS technology
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.
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 placed in a short MPLS shim
header used to identify streams
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
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stream an aggregate of one or more flows,
treated as one aggregate for the purpose
of forwarding in L2 and/or L3 nodes
(e.g., may be described using a single
label). In many cases a stream may be the
aggregate of a very large number of
flows. Synonymous with "aggregate
stream".
stream merge the merging of several smaller streams
into a larger stream, such that for some
or all of the path the larger stream can
be referred to using a single label.
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 stream merge when it is specifically
applied to VCs, specifically so as to
allow multiple VCs to merge into one
single VC
VP merge stream merge when it is applied to VPs,
specifically so as to allow multiple VPs
to merge into one single VP. In this case
the VCIs need to be unique. This allows
cells from different sources to be
distinguished via the VCI.
VPI/VCI a label used in ATM networks to identify
circuits
1.3. Acronyms and Abbreviations
ATM Asynchronous Transfer Mode
BGP Border Gateway Protocol
DLCI Data Link Circuit Identifier
FEC Forwarding Equivalence Class
STN stream to NHLFE Map
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IGP Interior Gateway Protocol
ILM Incoming Label Map
IP Internet Protocol
LIB Label Information Base
LDP Label Distribution Protocol
L2 Layer 2
L3 Layer 3
LSP Label Switched Path
LSR Label Switching Router
MPLS MultiProtocol Label Switching
MPT Multipoint to Point Tree
NHLFE Next Hop Label Forwarding Entry
SVC Switched Virtual Circuit
SVP Switched Virtual Path
TTL Time-To-Live
VC Virtual Circuit
VCI Virtual Circuit Identifier
VP Virtual Path
VPI Virtual Path Identifier
1.4. Acknowledgments
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.
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2. Outline of Approach
In this section, we introduce some of the basic concepts of MPLS and
describe the general approach to be used.
2.1. Labels
A label is a short, fixed length, locally significant identifier
which is used to identify a stream. The label is based on the stream
or Forwarding Equivalence Class that a packet is assigned to. The
label does not directly encode the network layer address. The choice
of label depends on the network layer address only to the extent that
the Forwarding Equivalence Class depends on that address.
If Ru and Rd are LSRs, and Ru transmits a packet to Rd, they may
agree to use label L to represent stream S for packets which are sent
from Ru to Rd. That is, they can agree to a "mapping" between label
L and stream S for packets moving from Ru to Rd. As a result of such
an agreement, L becomes Ru's "outgoing label" corresponding to stream
S for such packets; L becomes Rd's "incoming label" corresponding to
stream S for such packets.
Note that L does not necessarily correspond to stream S for any
packets other than those which are being sent from Ru to Rd. Also, L
is not an inherently meaningful value and does not have any network-
wide value; the particular value assigned to L gets its meaning
solely from the agreement between Ru and Rd.
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 mapping from label to FEC is
one-to-one. That is, in such cases, Rd must not agree with Ru1 to
use L for one purpose, while also agreeing with some other LSR Ru2 to
use L for a different purpose.
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2.2. Upstream and Downstream LSRs
Suppose Ru and Rd have agreed to map label L to stream S, for packets
sent from Ru to Rd. Then with respect to this mapping, Ru is the
"upstream LSR", and Rd is the "downstream LSR".
The notion of upstream and downstream relate to agreements between
nodes of the label values to be assigned for packets belonging to a
particular stream that might be traveling from an upstream node to a
downstream node. This is independent of whether the routing protocol
actually will cause any packets to be transmitted in that particular
direction. Thus, Rd is the downstream LSR for a particular mapping
for label L if it recognizes L-labeled packets from Ru as being in
stream S. This may be true even if routing does not actually forward
packets for stream S between nodes Rd and Ru, or if routing has made
Ru downstream of Rd along the path which is actually used for packets
in stream S.
2.3. Labeled Packet
A "labeled packet" is a packet into which a label has been encoded.
The encoding can be done by means of an encapsulation which exists
specifically for this purpose, or by placing the label in an
available location in either of the data link or network layer
headers. Of course, the encoding technique must be agreed to by the
entity which encodes the label and the entity which decodes the
label.
2.4. Label Assignment and Distribution; Attributes
For unicast traffic in the MPLS architecture, the decision to bind a
particular label L to a particular stream S is made by the LSR which
is downstream with respect to that mapping. The downstream LSR then
informs the upstream LSR of the mapping. Thus labels are
"downstream-assigned", and are "distributed upstream".
A particular mapping of label L to stream S, distributed by Rd to Ru,
may have associated "attributes". If Ru, acting as a downstream LSR,
also distributes a mapping of a label to stream S, then under certain
conditions, it may be required to also distribute the corresponding
attribute that it received from Rd.
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2.5. Label Distribution Protocol (LDP)
A Label Distribution Protocol (LDP) is a set of procedures by which
one LSR informs another of the label/Stream mappings it has made.
Two LSRs which use an LDP to exchange label/Stream mapping
information are known as "LDP Peers" with respect to the mapping
information they exchange; we will speak of there being an "LDP
Adjacency" between them.
(N.B.: two LSRs may be LDP Peers with respect to some set of
mappings, but not with respect to some other set of mappings.)
The LDP also encompasses any negotiations in which two LDP Peers need
to engage in order to learn of each other's MPLS capabilities.
2.6. The Label Stack
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".
IN MPLS, EVERY FORWARDING DECISION IS BASED EXCLUSIVELY ON THE LABEL
AT THE TOP OF THE 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 (sections 2.21.3 and
2.21.4).
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2.7. The Next Hop Label Forwarding Entry (NHLFE)
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 data link encapsulation to use when transmitting the packet
3. the way to encode the label stack when transmitting the packet
4. 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
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.
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".
2.8. Incoming Label Map (ILM)
The "Incoming Label Map" (ILM) is a mapping from incoming labels to
NHLFEs. It is used when forwarding packets that arrive as labeled
packets.
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2.9. Stream-to-NHLFE Map (STN)
The "Stream-to-NHLFE" (STN) is a mapping from stream to NHLFEs. It is
used when forwarding packets that arrive unlabeled, but which are to
be labeled before being forwarded.
2.10. Label Swapping
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 stream. It then uses the STN
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.
2.11. Scope and Uniqueness of Labels
A given LSR Rd may map label L1 to stream S, and distribute that
mapping to LDP peer Ru1. Rd may also map label L2 to stream S, and
distribute that mapping to LDP peer Ru2. Whether or not L1 == L2 is
not determined by the architecture; this is a local matter.
A given LSR Rd may map label L to stream S1, and distribute that
mapping to LDP peer Ru1. Rd may also map label L to stream S2, and
distribute that mapping to LDP 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 S1 == S2. 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:
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- Ru1 and Ru2 are the only LDP peers to which Rd distributed a
mapping 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. When
these conditions do not hold, the labels must be unique over the LSR
which has assigned them.
If a particular LSR Rd is attached to a particular LSR Ru over two
point-to-point interfaces, then Rd may distribute to Rd a mapping of
label L to stream S1, as well as a mapping of label L to stream S2,
S1 != S2, if and only if each mapping 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 mappings of the same label value
to two different streams.
This prohibition holds even if the mappings 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.
2.12. Label Switched Path (LSP), LSP Ingress, LSP Egress
A "Label Switched Path (LSP) of level m" for a particular packet P is
a sequence of routers,
<R1, ..., Rn>
with the following properties:
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;
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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.
We will call a sequence of LSRs the "LSP for a particular stream S"
if it is an LSP of level m for a particular packet P when P's level m
label is a label corresponding to stream S.
Consider the set of nodes which may be LSP ingress nodes for stream
S. Then there is an LSP for stream S 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 stream S.
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2.13. Penultimate Hop Popping
Note that according to the definitions of section 2.11, 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 forward the packet
based on the information gained by looking up the label that was at
the top of the stack. When the LSP egress receives the packet, the
label 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
every required:
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- 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 LDP peer.
Initial LDP negotiations must allow each LSR to determine whether its
neighboring LSRS are capable of popping the label stack. A LSR will
not request an LDP 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 2.11
are obeyed, it is always possible to interpret the top label of a
received packet unambiguously.
2.14. LSP Next Hop
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 stream is the next hop as selected
by the NHLFE entry indexed by a label which corresponds to that
stream.
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.
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2.15. Route Selection
Route selection refers to the method used for selecting the LSP for a
particular stream. 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 the path for a stream. This is the normal mode today with
existing datagram IP networks. A hop by hop routed LSP refers to an
LSP whose route is selected using hop by hop routing.
An explicitly routed LSP is an LSP where, at a given LSR, the LSP
next hop is not chosen by each local node, but rather is chosen by a
single node (usually the ingress or egress node of the LSP). 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 allowing
policy routing and/or facilitating traffic engineering. With 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 implies that explicit routing with MPLS is
relatively efficient (when compared with the efficiency of explicit
routing for pure datagrams).
For any one LSP (at any one level of hierarchy), there are two
possible options: (i) The entire LSP may be hop by hop routed from
ingress to egress; (ii) The entire LSP may be explicit routed from
ingress to egress. Intermediate cases do not make sense: In general,
an LSP will be explicit routed specifically because there is a good
reason to use an alternative to the hop by hop routed path. This
implies that if some of the nodes along the path follow an explicit
route but some of the nodes make use of hop by hop routing, then
inconsistent routing will result and loops (or severely inefficient
paths) may form.
For this reason, it is important that if an explicit route is
specified for an LSP, then that route must be followed. Note that it
is relatively simple to *follow* an explicit route which is specified
in a LDP setup. We therefore propose that the LDP specification
require that all MPLS nodes implement the ability to follow an
explicit route if this is specified.
It is not necessary for a node to be able to create an explicit
route. However, in order to ensure interoperability it is necessary
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to ensure that either (i) Every node knows how to use hop by hop
routing; or (ii) Every node knows how to create and follow an
explicit route. We propose that due to the common use of hop by hop
routing in networks today, it is reasonable to make hop by hop
routing the default that all nodes need to be able to use.
2.16. Time-to-Live (TTL)
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.
The way that TTL is handled may vary depending upon whether the MPLS
label values are carried in an MPLS-specific "shim" header, or if the
MPLS labels are carried in an L2 header such as an ATM header or a
frame relay header.
If the label values are encoded in a "shim" that sits between the
data link and network layer headers, then this shim should have a TTL
field that is initially loaded from the network layer header TTL
field, is decremented at each LSR-hop, and is copied into the network
layer header TTL field when the packet emerges from its LSP.
If the label values are encoded in an L2 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). This implies that unless the data
link layer itself has a TTL field (unlike ATM), 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".
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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.
2.17. Loop Control
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
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.
The MPLS architecture will therefore provide a technique for ensuring
that looping LSP segments can be detected, and a technique for
ensuring that looping LSP segments are never created.
All LSRs will be required to support a common technique for loop
detection. Support for the loop prevention technique is optional,
though it is recommended in ATM-LSRs that have no other way to
protect themselves against the effects of looping data packets. Use
of the loop prevention technique, when supported, is optional.
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2.17.1. Loop Prevention
NOTE: The loop prevention technique described here is being
reconsidered, and may be changed.
LSR's maintain for each of their LSP's an LSR id list. This list is a
list of all the LSR's downstream from this LSR on a given LSP. The
LSR id list is used to prevent the formation of switched path loops.
The LSR ID list is propagated upstream from a node to its neighbor
nodes. The LSR ID list is used to prevent loops as follows:
When a node, R, detects a change in the next hop for a given stream,
it asks its new next hop for a label and the associated LSR ID list
for that stream.
The new next hop responds with a label for the stream and an
associated LSR id list.
R looks in the LSR id list. If R determines that it, R, is in the
list then we have a route loop. In this case, we do nothing and the
old LSP will continue to be used until the route protocols break the
loop. The means by which the old LSP is replaced by a new LSP after
the route protocols breathe loop is described below.
If R is not in the LSR id list, R will start a "diffusion"
computation [12]. The purpose of the diffusion computation is to
prune the tree upstream of R so that we remove all LSR's from the
tree that would be on a looping path if R were to switch over to the
new LSP. After those LSR's are removed from the tree, it is safe for
R to replace the old LSP with the new LSP (and the old LSP can be
released).
The diffusion computation works as follows:
R adds its LSR id to the list and sends a query message to each of
its "upstream" neighbors (i.e. to each of its neighbors that is not
the new "downstream" next hop).
A node S that receives such a query will process the query as
follows:
- If node R is not node S's next hop for the given stream, node S
will respond to node R will an "OK" message meaning that as far
as node S is concerned it is safe for node R to switch over to
the new LSP.
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- If node R is node S's next hop for the stream, node S will check
to see if it, node S, is in the LSR id list that it received from
node R. If it is, we have a route loop and S will respond with a
"LOOP" message. R will unsplice the connection to S pruning S
from the tree. The mechanism by which S will get a new LSP for
the stream after the route protocols break the loop is described
below.
- If node S is not in the LSR id list, S will add its LSR id to the
LSR id list and send a new query message further upstream. The
diffusion computation will continue to propagate upstream along
each of the paths in the tree upstream of S until either a loop
is detected, in which case the node is pruned as described above
or we get to a point where a node gets a response ("OK" or
"LOOP") from each of its neighbors perhaps because none of those
neighbors considers the node in question to be its downstream
next hop. Once a node has received a response from each of its
upstream neighbors, it returns an "OK" message to its downstream
neighbor. When the original node, node R, gets a response from
each of its neighbors, it is safe to replace the old LSP with the
new one because all the paths that would loop have been pruned
from the tree.
There are a couple of details to discuss:
- First, we need to do something about nodes that for one reason or
another do not produce a timely response in response to a query
message. If a node Y does not respond to a query from node X
because of a failure of some kind, X will not be able to respond
to its downstream neighbors (if any) or switch over to a new LSP
if X is, like R above, the node that has detected the route
change. This problem is handled by timing out the query message.
If a node doesn't receive a response within a "reasonable" period
of time, it "unsplices" its VC to the upstream neighbor that is
not responding and proceeds as it would if it had received the
"LOOP" message.
- We also need to be concerned about multiple concurrent routing
updates. What happens, for example, when a node M receives a
request for an LSP from an upstream neighbor, N, when M is in the
middle of a diffusion computation i.e., it has sent a query
upstream but hasn't received all the responses. Since a
downstream node, node R is about to change from one LSP to
another, M needs to pass to N an LSR id list corresponding to the
union of the old and new LSP's if it is to avoid loops both
before and after the transition. This is easily accomplished
since M already has the LSR id list for the old LSP and it gets
the LSR id list for the new LSP in the query message. After R
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makes the switch from the old LSP to the new one, R sends a new
establish message upstream with the LSR id list of (just) the new
LSP. At this point, the nodes upstream of R know that R has
switched over to the new LSP and that they can return the id list
for (just) the new LSP in response to any new requests for LSP's.
They can also grow the tree to include additional nodes that
would not have been valid for the combined LSR id list.
- We also need to discuss how a node that doesn't have an LSP for a
given stream at the end of a diffusion computation (because it
would have been on a looping LSP) gets one after the routing
protocols break the loop. If node L has been pruned from the
tree and its local route protocol processing entity breaks the
loop by changing L's next hop, L will request a new LSP from its
new downstream neighbor which it will use once it executes the
diffusion computation as described above. If the loop is broken
by a route change at another point in the loop, i.e. at a point
"downstream" of L, L will get a new LSP as the new LSP tree grows
upstream from the point of the route change as discussed in the
previous paragraph.
- Note that when a node is pruned from the tree, the switched path
upstream of that node remains "connected". This is important
since it allows the switched path to get "reconnected" to a
downstream switched path after a route change with a minimal
amount of unsplicing and resplicing once the appropriate
diffusion computation(s) have taken place.
The LSR Id list can also be used to provide a "loop detection"
capability. To use it in this manner, an LSR which sees that it is
already in the LSR Id list for a particular stream will immediately
unsplice itself from the switched path for that stream, and will NOT
pass the LSR Id list further upstream. The LSR can rejoin a switched
path for the stream when it changes its next hop for that stream, or
when it receives a new LSR Id list from its current next hop, in
which it is not contained. The diffusion computation would be
omitted.
2.17.2. Interworking of Loop Control Options
The MPLS protocol architecture allows some nodes to be using loop
prevention, while some other nodes are not (i.e., the choice of
whether or not to use loop prevention may be a local decision). When
this mix is used, it is not possible for a loop to form which
includes only nodes which do loop prevention. However, it is possible
for loops to form which contain a combination of some nodes which do
loop prevention, and some nodes which do not.
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There are at least four identified cases in which it makes sense to
combine nodes which do loop prevention with nodes which do not: (i)
For transition, in intermediate states while transitioning from all
non-loop-prevention to all loop prevention, or vice versa; (ii) For
interoperability, where one vendor implements loop prevention but
another vendor does not; (iii) Where there is a mixed ATM and
datagram media network, and where loop prevention is desired over the
ATM portions of the network but not over the datagram portions; (iv)
where some of the ATM switches can do fair access to the buffer pool
on a per-VC basis, and some cannot, and loop prevention is desired
over the ATM portions of the network which cannot.
Note that interworking is straightforward. If an LSR is not doing
loop prevention, and it receives from a downstream LSR a label
mapping which contains loop prevention information, it (a) accepts
the label mapping, (b) does NOT pass the loop prevention information
upstream, and (c) informs the downstream neighbor that the path is
loop-free.
Similarly, if an LSR R which is doing loop prevention receives from a
downstream LSR a label mapping which does not contain any loop
prevention information, then R passes the label mapping upstream with
loop prevention information included as if R were the egress for the
specified stream.
Optionally, a node is permitted to implement the ability of either
doing or not doing loop prevention as options, and is permitted to
choose which to use for any one particular LSP based on the
information obtained from downstream nodes. When the label mapping
arrives from downstream, then the node may choose whether to use loop
prevention so as to continue to use the same approach as was used in
the information passed to it. Note that regardless of whether loop
prevention is used the egress nodes (for any particular LSP) always
initiates exchange of label mapping information without waiting for
other nodes to act.
2.18. Merging and Non-Merging LSRs
Merge allows multiple upstream LSPs to be merged into a single
downstream LSP. When implemented by multiple nodes, this results in
the traffic going to a particular egress nodes, based on one
particular stream, to follow a multipoint to point tree (MPT), with
the MPT rooted at the egress node and associated with the stream.
This can have a significant effect on reducing the number of labels
that need to be maintained by any one particular node.
If merge was not used at all it would be necessary for each node to
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provide the upstream neighbors with a label for each stream for each
upstream node which may be forwarding traffic over the link. This
implies that the number of labels needed might not in general be
known a priori. However, the use of merge allows a single label to be
used per stream, therefore allowing label assignment to be done in a
common way without regard for the number of upstream nodes which will
be using the downstream LSP.
The proposed MPLS protocol architecture supports LSP merge, while
allowing nodes which do not support LSP merge. This leads to the
issue of ensuring correct interoperation between nodes which
implement merge and those which do not. 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.
2.18.1. Stream Merge
Let us say that an LSR is capable of Stream Merge 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. This in effect takes two incoming streams and merges
them into one. 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 Stream Merge 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.
An LSR which is capable of Stream Merge (a "Merging LSR") needs to
maintain only one outgoing label for each FEC. AN LSR which is not
capable of Stream Merge (a "Non-merging LSR") may need to maintain as
many as N outgoing labels per FEC, where N is the number of LSRs in
the network. Hence by supporting Stream Merge, an LSR can reduce its
number of outgoing labels by a factor of O(N). Since each label in
use requires the dedication of some amount of resources, this can be
a significant savings.
2.18.2. Non-merging LSRs
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
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use such technologies for MPLS forwarding; LDP can be used as the
"signalling protocol" for setting up the cross-connect tables.
Unfortunately, these technologies do not necessarily support the
Stream Merge capability. In ATM, if one attempts to perform Stream
Merge, 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 Stream Merge, 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.
2.18.3. Labels for Merging and Non-Merging LSRs
An upstream LSR which supports Stream Merge needs to be sent only one
label per FEC. An upstream neighbor which does not support Stream
Merge 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 Stream Merge, 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 Stream
Merge, 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 merge, but
have a limited number of upstream streams which may be merged into a
single downstream streams. Suppose for example that due to some
hardware limitation a node is capable of merging four upstream LSPs
into a single downstream LSP. Suppose however, that this particular
node has six upstream LSPs arriving at it for a particular stream. In
this case, this node may merge these into two downstream LSPs
(corresponding to two labels that need to be obtained from the
downstream neighbor). In this case, the normal operation of the LDP
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implies that the downstream neighbor will supply this node with a
single label for the stream. This node can then ask its downstream
neighbor for one additional label for the stream, implying that the
node will thereby obtain the required two labels.
The interaction between explicit routing and merge is FFS.
2.18.4. Merge over ATM
2.18.4.1. Methods of Eliminating Cell Interleave
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
When VP merge is used, multiple virtual paths are merged into a
virtual path, but packets from different sources are
distinguished by using different VCs 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 FFS.
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.
2.18.4.2. Interoperation: VC Merge, VP Merge, and Non-Merge
The interoperation of the various forms of merging over ATM is most
easily described by first describing the interoperation of VC merge
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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
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).
2.19. LSP Control: Egress versus Local
There is a choice to be made regarding whether the initial setup of
LSPs will be initiated by the egress node, or locally by each
individual node.
When LSP control is done locally, then each node may at any time pass
label bindings to its neighbors for each FEC recognized by that node.
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In the normal case that the neighboring nodes recognize the same
FECs, then nodes may map incoming labels to outgoing labels as part
of the normal label swapping forwarding method.
When LSP control is done by the egress, then initially only the
egress node passes label bindings to its neighbors corresponding to
any FECs which leave the MPLS network at that egress node. Other
nodes wait until they get a label from downstream for a particular
FEC before passing a corresponding label for the same FEC to upstream
nodes.
With local control, since each LSR is (at least initially)
independently assigning labels to FECs, it is possible that different
LSRs may make inconsistent decisions. For example, an upstream LSR
may make a coarse decision (map multiple IP address prefixes to a
single label) while its downstream neighbor makes a finer grain
decision (map each individual IP address prefix to a separate label).
With downstream label assignment this can be corrected by having LSRs
withdraw labels that it has assigned which are inconsistent with
downstream labels, and replace them with new consistent label
assignments.
Even with egress control it is possible that the choice of egress
node may change, or the egress may (based on a change in
configuration) change its mind in terms of the granularity which is
to be used. This implies the same mechanism will be necessary to
allow changes in granularity to bubble up to upstream nodes. The
choice of egress or local control may therefore effect the frequency
with which this mechanism is used, but will not effect the need for a
mechanism to achieve consistency of label granularity. Generally
speaking, the choice of local versus egress control does not appear
to have any effect on the LDP mechanisms which need to be defined.
Egress control and local control can interwork in a very
straightforward manner (although when both methods exist in the
network, the overall behavior of the network is largely that of local
control). With either approach, (assuming downstream label
assignment) the egress node will initially assign labels for
particular FECs and will pass these labels to its neighbors. With
either approach these label assignments will bubble upstream, with
the upstream nodes choosing labels that are consistent with the
labels that they receive from downstream. The difference between the
two approaches is therefore primarily an issue of what each node does
prior to obtaining a label assignment for a particular FEC from
downstream nodes: Does it wait, or does it assign a preliminary label
under the expectation that it will (probably) be correct?
Regardless of which method is used (local control or egress control)
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each node needs to know (possibly by configuration) what granularity
to use for labels that it assigns. Where egress control is used, this
requires each node to know the granularity only for streams which
leave the MPLS network at that node. For local control, in order to
avoid the need to withdraw inconsistent labels, each node in the
network would need to be configured consistently to know the
granularity for each stream. However, in many cases this may be done
by using a single level of granularity which applies to all streams
(such as "one label per IP prefix in the forwarding table").
This architecture allows the choice between local control and egress
control to be a local matter. Since the two methods interwork, a
given LSR need support only one or the other.
2.20. Granularity
When forwarding by label swapping, a stream of packets following a
stream arriving from upstream may be mapped into an equal or coarser
grain stream. However, a coarse grain stream (for example, containing
packets destined for a short IP address prefix covering many subnets)
cannot be mapped directly into a finer grain stream (for example,
containing packets destined for a longer IP address prefix covering a
single subnet). This implies that there needs to be some mechanism
for ensuring consistency between the granularity of LSPs in an MPLS
network.
The method used for ensuring compatibility of granularity may depend
upon the method used for LSP control.
When LSP control is local, it is possible that a node may pass a
coarse grain label to its upstream neighbor(s), and subsequently
receive a finer grain label from its downstream neighbor. In this
case the node has two options: (i) It may forward the corresponding
packets using normal IP datagram forwarding (i.e., by examination of
the IP header); (ii) It may withdraw the label mappings that it has
passed to its upstream neighbors, and replace these with finer grain
label mappings.
When LSP control is egress based, the label setup originates from the
egress node and passes upstream. It is therefore straightforward with
this approach to maintain equally-grained mappings along the route.
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2.21. Tunnels and Hierarchy
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".
2.21.1. Hop-by-Hop Routed Tunnel
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.
2.21.2. Explicitly Routed Tunnel
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.
2.21.3. LSP Tunnels
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 becomes
a stream, and each LSR in the tunnel must assign a label to that
stream (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.
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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.
2.21.4. Hierarchy: LSP Tunnels within LSPs
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>.
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.
2.21.5. LDP Peering and Hierarchy
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 LDP peer is
R21. From the perspective of the Level 1 LSP, R2's LDP peers are R1
and R3. One can have LDP peers at each layer of hierarchy. We will
see in sections 3.6 and 3.7 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 LDP
Peers". When two LSRs may be LDP peers, but are not IGP neighbors,
we will refer to them as "Remote LDP Peers". In the above example,
R2 and R21 are local LDP peers, but R2 and R3 are remote LDP peers.
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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 LDP Peers by opening
LDP connections to them. One can perform label Distribution with
one's Remote LDP Peers in one of two ways:
1. Explicit Peering
In explicit peering, one sets up LDP connections between Remote
LDP Peers, exactly as one would do for Local LDP Peers. This
technique is most useful when the number of Remote LDP Peers is
small, or the number of higher level label mappings is large,
or the Remote LDP 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 3.1.2.
Examples of the use of explicit peering is found in sections
3.2.1 and 3.6.
2. Implicit Peering
In Implicit Peering, one does not have LDP connections to one's
remote LDP peers, but only to one's local LDP peers. To
distribute higher level labels to ones remote LDP peers, one
encodes the higher level labels as an attribute of the lower
level labels, and distributes the lower level label, along with
this attribute, to the local LDP peers. The local LDP peers
then propagate the information to their peers. This process
continues till the information reaches remote LDP peers. Note
that the intermediary nodes may also be remote LDP peers.
This technique is most useful when the number of Remote LDP
Peers is large. Implicit peering does not require a n-square
peering mesh to distribute labels to the remote LDP peers
because the information is piggybacked through the local LDP
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
3.3.
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2.22. LDP Transport
LDP is used between nodes in an MPLS network to establish and
maintain the label mappings. In order for LDP to operate correctly,
LDP information needs to be transmitted reliably, and the LDP
messages pertaining to a particular FEC need to be transmitted in
sequence. Flow control is also required, as is the capability to
carry multiple LDP messages in a single datagram.
These goals will be met by using TCP as the underlying transport for
LDP.
(The use of multicast techniques to distribute label mappings is
FFS.)
2.23. Label Encodings
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.
2.23.1. MPLS-specific Hardware and/or Software
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 generic MPLS encapsulation should contain the following fields:
1. the label stack,
2. a Time-to-Live (TTL) field
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3. a Class of Service (CoS) field
The TTL field permits MPLS to provide a TTL function similar to what
is provided by IP.
The CoS field permits LSRs to apply various scheduling packet
disciplines to labeled packets, without requiring separate labels for
separate disciplines.
2.23.2. ATM Switches as LSRs
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".
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 LDP 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
LDP 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.
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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, so multipoint-
to-point VPs can be provided without any cell interleaving
problems.
This technique depends on the existence of a capability for
assigning small unique values to each ATM switch.
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. This does presuppose that it be possible to tell,
when reassembling the ATM cells into packets, whether the generic
encapsulation is also present.
2.23.3. Interoperability among Encoding Techniques
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
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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.
2.24. Multicast
This section is for further study
3. Some Applications of MPLS
3.1. MPLS and Hop by Hop Routed Traffic
One use of MPLS is to simplify the process of forwarding packets
using hop by hop routing.
3.1.1. Labels for Address Prefixes
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 stream
are just those packets which match a given address prefix in R's
routing table. In this case, a stream can be identified with an
address prefix.
If packet P must traverse a sequence of routers, and at each router
in the sequence P matches the same address prefix, MPLS simplifies
the forwarding process by enabling all routers but the first to avoid
executing the best match algorithm; they need only look up the label.
3.1.2. Distributing Labels for Address Prefixes
3.1.2.1. LDP Peers for a Particular Address Prefix
LSRs R1 and R2 are considered to be LDP 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
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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
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 LDP peers for that
address prefix are the IGP neighbors. If the route to a particular
address prefix is distributed via BGP, the LDP peers for that address
prefix are the BGP peers. In other cases of LSP tunneling, the
tunnel endpoints are LDP peers.
3.1.2.2. Distributing Labels
In order to use MPLS for the forwarding of normally routed traffic,
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 an LDP to distribute the
mapping of a label to X to each of its LDP Peers for X.
There is also one circumstance in which an LSR must distribute a
label mapping 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 mapping
between T 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
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speakers.
These rules are intended to indicate which label mappings must be
distributed by a given LSR to which other LSRs, NOT to indicate the
conditions under which the distribution is to be made. That is
discussed in section 2.19.
3.1.3. Using the Hop by Hop path as the LSP
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
10.2.153/22, 10.2.154/22, 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 stream.
3.1.4. LSP Egress and LSP Proxy Egress
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. R1 has an address Y, such that X is the address prefix in R1'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, R2 is a "deaggregation point" for address prefix X.
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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 R1 and R2 are not LDP 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
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.
3.1.5. The POP Label
The POP 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 mapping of the POP label 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 mapping between the POP label and an address
prefix X to LSR Ru if and only if:
1. the rules of Section 3.1.2 indicate that Rd distributes to Ru a
label mapping for X, and
2. when the LDP connection between Ru and Rd was opened, Ru
indicated that it could support the POP label, 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 POP label mapping 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
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Egress, it acts just as if the LSP Egress had distributed the POP
label for X.
3.1.6. Option: Egress-Targeted Label Assignment
There are situations in which an LSP Ingress, Ri, knows that packets
of several different streams must all follow the same LSP,
terminating at, say, LSP Egress Re. In this case, proper routing can
be achieved by using a single label can be used for all such streams;
it is not necessary to have a distinct label for each stream. 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 streams
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 stream? There are a couple 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 stream must leave the routing
domain or area.
- It is possible to use LDP 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:
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- 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 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.
3.2. MPLS and Explicitly Routed LSPs
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
(i.e., to allow intentional management of the loading of the
bandwidth through the nodes and links in the network).
3.2.1. Explicitly Routed LSP Tunnels: Traffic Engineering
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 is known as Traffic Engineering.
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MPLS allows this to be easily done by means of Explicitly Routed LSP
Tunnels. All that is needed is:
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 LDP peers. The label mappings they need to exchange are of
no interest to the LSRs along the tunnel.
3.3. Label Stacks and Implicit Peering
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
mapping, which we call the "Stack Attribute". We impose the
following rules:
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- When LSR Ru initially labels an untagged packet, if the longest
match for the packet's destination address is X, and R's LSP next
hop for X is Rd, and Rd has distributed to R1 a mapping of label
L1 X, along with a stack attribute of L2, then
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 mappings for X to its LDP 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.
3.4. MPLS and Multi-Path Routing
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 mapping 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 mappings for a particular address prefix are
specified, they may have distinct attributes.
3.5. LSP Trees as Multipoint-to-Point Entities
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 mapping to R2. R2 binds label L2 to X, and distributes this
mapping 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
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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,
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 2.23) can be
used, the LSPs can be realized as multipoint-to-point SVPs.
3.6. LSP Tunneling between BGP Border Routers
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:
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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,
e) B2 has bound label L1 to X, and has distributed this
mapping 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 mapping 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 mappings for
address prefixes that are not even known to the IGP routing, the BGP
routers should become explicit LDP peers with each other.
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3.7. Other Uses of Hop-by-Hop Routed LSP Tunnels
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 LDP peers. The label mappings they need to exchange are of
no interest to the LSRs along the tunnel.
3.8. MPLS and Multicast
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 mapping 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 mapping 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.
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4. LDP Procedures for Hop-by-Hop Routed Traffic
4.1. The Procedures for Advertising and Using labels
In this section, we consider only label mappings 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.
There are a number of different procedures that may be used to
distribute label mappings. One such procedure is executed by the
downstream LSR, and the others 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 4.2, where the
interoperability between different combinations will also be
discussed.
4.1.1. Downstream LSR: Distribution Procedure
The Distribution Procedure is used by a downstream LSR to determine
when it should distribute a label mapping for a particular address
prefix to its LDP peers. The architecture supports four different
distribution procedures.
Irrespective of the particular procedure that is used, if a label
mapping for a particular address prefix has been distributed by a
downstream LSR Rd to an upstream LSR Ru, and if at any time the
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attributes (as defined above) of that mapping 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 maps multiple
labels to the address prefix (one per route), and hence distributes
multiple mappings.
4.1.1.1. PushUnconditional
Let Rd be an LSR. Suppose that:
1. X is an address prefix in Rd's routing table
2. Ru is an LDP Peer of Rd with respect to X
Whenever these conditions hold, Rd must map a label to X and
distribute that mapping to Ru. It is the responsibility of Rd to
keep track of the mappings which it has distributed to Ru, and to
make sure that Ru always has these mappings.
4.1.1.2. PushConditional
Let Rd be an LSR. Suppose that:
1. X is an address prefix in Rd's routing table
2. Ru is an LDP 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 mapping to Rd.
Then as soon as these conditions all hold, Rd should map a label to X
and distribute that mapping to Ru.
Whereas PushUnconditional causes the distribution of label mappings
for all address prefixes in the routing table, PushConditional causes
the distribution of label mappings only for those address prefixes
for which one has received label mappings from one's LSP next hop, or
for which one does not have an MPLS-capable L3 next hop.
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4.1.1.3. PulledUnconditional
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 map a label to X and
distribute the mapping to Ru
Then Rd should map a label to X and distribute that mapping to Ru.
Note that if X is not in Rd's routing table, or if Rd is not an LDP
peer of Ru with respect to X, then Rd must inform Ru that it cannot
provide a mapping at this time.
If Rd has already distributed a mapping for address prefix X to Ru,
and it receives a new request from Ru for a mapping for address
prefix X, it will map a second label, and distribute the new mapping
to Ru. The first label mapping remains in effect.
4.1.1.4. PulledConditional
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 map a label to X and
distribute the mapping 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 mapping to Rd,
or
Then as soon as these conditions all hold, Rd should map a label to X
and distribute that mapping 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 mapping
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 mapping from Rn.
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If Rd has distributed a label mapping for address prefix X to Ru, and
at some later time, any attribute of the label mapping changes, then
Rd must redistribute the label mapping to Ru, with the new attribute.
It must do this even though Ru does not issue a new Request.
In section 4.2, 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.
4.1.2. Upstream LSR: Request Procedure
The Request Procedure is used by the upstream LSR for an address
prefix to determine when to explicitly request that the downstream
LSR map a label to that prefix and distribute the mapping. There are
three possible procedures that can be used.
4.1.2.1. RequestNever
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.
4.1.2.2. RequestWhenNeeded
Make a request whenever the L3 next hop to the address prefix
changes, and one doesn't already have a label mapping from that next
hop for the given address prefix.
4.1.2.3. RequestOnRequest
Issue a request whenever a request is received, in addition to
issuing a request when needed (as described in section 4.1.2.2). 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, map it to X, and distribute that mapping. (Whether
Rd can distribute this mapping to Ru immediately or not depends on
the Distribution Procedure being used.)
This procedure is useful when the LSRs are implemented on
conventional ATM switching hardware.
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4.1.3. Upstream LSR: NotAvailable Procedure
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 mapping for X from Rd, but Rd replies that
it cannot provide a mapping at this time, then the NotAvailable
procedure determines how Ru responds. There are two possible
procedures governing Ru's behavior:
4.1.3.1. RequestRetry
Ru should issue the request again at a later time. That is, the
requester is responsible for trying again later to obtain the needed
mapping.
4.1.3.2. RequestNoRetry
Ru should never reissue the request, instead assuming that Rd will
provide the mapping automatically when it is available. This is
useful if Rd uses the PushUnconditional procedure or the
PushConditional procedure.
4.1.4. Upstream LSR: Release Procedure
Suppose that Rd is an LSR which has bound a label to address prefix
X, and has distributed that mapping 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 Rd 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:
4.1.4.1. ReleaseOnChange
Ru should release the mapping, and inform Rd that it has done so.
4.1.4.2. NoReleaseOnChange
Ru should maintain the mapping, so that it can use it again
immediately if Rd later becomes Ru's L3 next hop for X.
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4.1.5. Upstream LSR: labelUse Procedure
Suppose Ru is an LSR which has received label mapping 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 mapping if Rd is Ru's L3 next hop for X. If,
at the time the mapping is received by Ru, Rd is NOT Ru's L3 next hop
for X, Ru does not make any use of the mapping at that time. Ru may
however start using the mapping 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
mapping.
There are three procedures which Ru may use:
4.1.5.1. UseImmediate
Ru may put the mapping into use immediately. At any time when Ru has
a mapping 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.
4.1.5.2. UseIfLoopFree
Ru will use the mapping only if it determines that by doing so, it
will not cause a forwarding loop.
If Ru has a mapping for X from Rd, and Rd is (or becomes) Ru's L3
next hop for X, but Rd is NOT Ru's current LSP next hop for X, Ru
does NOT immediately make Rd its LSP next hop. Rather, it initiates
a loop prevention algorithm. If, upon the completion of this
algorithm, Rd is still the L3 next hop for X, Ru will make Rd the LSP
next hop for X, and use L as the outgoing label.
The loop prevention algorithm to be used is still under
consideration.
4.1.5.3. UseIfLoopNotDetected
This procedure is the same as UseImmediate, unless Ru has detected a
loop in the LSP. If a loop has been detected, Ru will discard
packets that would otherwise have been labeled with L and sent to Rd.
This will continue until the next hop for X changes, or until the
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loop is no longer detected.
4.1.6. Downstream LSR: Withdraw Procedure
In this case, there is only a single procedure.
When LSR Rd decides to break the mapping between label L and address
prefix X, then this unmapping must be distributed to all LSRs to
which the mapping was distributed.
It is desirable, though not required, that the unmapping of L from X
be distributed by Rd to a LSR Ru before Rd distributes to Ru any new
mapping of L to any other address prefix Y, where X != Y. If Ru
learns of the new mapping of L to Y before it learns of the unmapping
of L from X, and if packets matching both X and Y are forwarded by Ru
to Rd, then for a period of time, Ru will label both packets matching
X and packets matching Y with label L.
The distribution and withdrawal of label mappings is done via a label
distribution protocol, or LDP. LDP is a two-party protocol. If LSR R1
has received label mappings from LSR R2 via an instance of an LDP,
and that instance of that protocol is closed by either end (whether
as a result of failure or as a matter of normal operation), then all
mappings learned over that instance of the protocol must be
considered to have been withdrawn.
As long as the relevant LDP connection remains open, label mappings
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 map 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 mapping of that
label to the first address prefix, but to use that label for both
address prefixes.
4.2. MPLS Schemes: Supported Combinations of Procedures
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
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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.
4.2.1. TTL-capable LSP Segments
If Ru and Rd are MPLS peers, and both are capable of decrementing a
TTL field in the MPLS header, then the MPLS scheme in use between Ru
and Rd must be one of the following:
<PushUnconditional, RequestNever, N/A, NoReleaseOnChange,
UseImmediate>
<PushConditional, RequestWhenNeeded, RequestNoRetry, *, *>
The former, roughly speaking, is "local control with downstream label
assignment". The latter is an egress control scheme.
4.2.2. Using ATM Switches as LSRs
The procedures for using ATM switches as LSRs depends on whether the
ATM switches can realize LSP trees as multipoint-to-point VCs or VPs.
Most ATM switches existing today do NOT have a multipoint-to-point
VC-switching capability. Their cross-connect tables could easily be
programmed to move cells from multiple incoming VCs to a single
outgoing VC, but the result would be that cells from different
packets get interleaved.
Some ATM switches do support a multipoint-to-point VC-switching
capability. These switches will queue up all the incoming cells from
an incoming VC until a packet boundary is reached. Then they will
transmit the entire sequence of cells on the outgoing VC, without
allowing cells from any other packet to be interleaved.
Many ATM switches do support a multipoint-to-point VP-switching
capability, which can be used if the Multipoint SVP label encoding is
used.
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4.2.2.1. Without Multipoint-to-point Capability
Suppose that R1, R2, R3, and R4 are ATM switches which do not support
multipoint-to-point capability, 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), the MPLS scheme in use between R1 and R2
must be one of the following:
<PulledUnconditional, RequestOnRequest, RequestRetry,
ReleaseOnChange, UseImmediate>
<PulledConditional, RequestOnRequest, RequestNoRetry,
ReleaseOnChange, *>
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.
The first of these procedures is the "optimistic downstream-on-
demand" variant of local control. The second is the "conservative
downstream-on-demand" variant of local control.
An egress control scheme which works in the absence of multipoint-
to-point capability is for further study.
4.2.2.2. With Multipoint-To-Point Capability
If R1 and R2 are MPLS peers, and either of them is an LSR which is
implemented using ATM switching hardware with cell interleave
suppression, and neither is an LSR which is implemented using ATM
switching hardware that does not have cell interleave suppression,
then the MPLS scheme in use between R1 and R2 must be one of the
following;
<PushConditional, RequestWhenNeeded, RequestNoRetry, *, *>
<PushUnconditional, RequestNever, N/A, NoReleaseOnChange,
UseImmediate>
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<PulledConditional, RequestOnRequest, RequestNoRetry,
ReleaseOnChange, *>
The first of these is an egress control scheme. The second is is the
"downstream" variant of local control. The third is the
"conservative downstream-on-demand" variant of local control.
4.2.3. Interoperability Considerations
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
mappings 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 mappings.
- <*, RequestNever, *, *, ReleaseOnChange>
In these MPLS schemes, Rd releases mappings 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
mappings get properly distributed.
In this section, we specify rules to prevent a pair of LDP peers from
adopting procedures which lead to infeasible MPLS Schemes. These
rules require the exchange of information between LDP peers during
the initialization of the LDP connection between them.
1. Each must state whether it is an ATM switch, and if so, whether
it has cell interleave suppression.
2. If Rd is an ATM switch without cell interleave suppression, it
must state whether it intends to use the PulledUnconditional
procedure or the Pulledconditional procedure. If the former,
Ru MUST use the RequestRetry procedure; if the latter, Ru MUST
use the RequestNoRetry procedure.
3. If Ru is an ATM switch without cell interleave suppression, it
must state whether it intends to use the RequestRetry or the
RequestNoRetry procedure. If Rd is an ATM switch without cell
interleave suppression, Rd is not bound by this, and in fact Ru
MUST adopt Rd's preferences. However, if Rd is NOT an ATM
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switch without cell interleave suppression, then if Ru chooses
RequestRetry, Rd must use PulledUnconditional, and if Ru
chooses RequestNoRetry, Rd MUST use PulledConditional.
4. If Rd is an ATM switch with cell interleave suppression, it
must specify whether it prefers to use PushConditional,
PushUnconditional, or PulledConditional. If Ru is not an ATM
switch without cell interleave suppression, it must then use
RequestWhenNeeded and RequestNoRetry, or else RequestNever and
NoReleaseOnChange, respectively.
5. If Ru is an ATM switch with cell interleave suppression, it
must specify whether it prefers to use RequestWhenNeeded and
RequestNoRetry, or else RequestNever and NoReleaseOnChange. If
Rd is NOT an ATM switch with cell interleave suppression, it
must then use either PushConditional or PushUnconditional,
respectively.
4.2.4. How to do Loop Prevention
TBD
4.2.5. How to do Loop Detection
TBD.
4.2.6. Security Considerations
Security considerations are not discussed in this version of this
draft.
5. Authors' Addresses
Eric C. Rosen
Cisco Systems, Inc.
250 Apollo Drive
Chelmsford, MA, 01824
E-mail: erosen@cisco.com
Arun Viswanathan
Lucent Technologies
101 Crawford Corner Rd., #4D-537
Holmdel, NJ 07733
732-332-5163
Rosen, Viswanathan & Callon [Page 60]
Internet Draft draft-ietf-mpls-arch-01.txt March 1998
E-mail: arunv@dnrc.bell-labs.com
Ross Callon
IronBridge Networks
55 Hayden Avenue,
Lexington, MA 02173
+1-781-402-8017
E-mail: rcallon@ironbridgenetworks.com
6. References
[1] "A Framework for Multiprotocol Label Switching", R.Callon,
P.Doolan, N.Feldman, A.Fredette, G.Swallow, and A.Viswanathan, work
in progress, Internet Draft <draft-ietf-mpls-framework-02.txt>,
November 1997.
[2] "ARIS: Aggregate Route-Based IP Switching", A. Viswanathan, N.
Feldman, R. Boivie, R. Woundy, work in progress, Internet Draft
<draft-viswanathan-aris-overview-00.txt>, March 1997.
[3] "ARIS Specification", N. Feldman, A. Viswanathan, work in
progress, Internet Draft <draft-feldman-aris-spec-00.txt>, March
1997.
[4] "Tag Switching Architecture - Overview", Rekhter, Davie, Katz,
Rosen, Swallow, Farinacci, work in progress, Internet Draft <draft-
rekhter-tagswitch-arch-00.txt>, January, 1997.
[5] "Tag distribution Protocol", Doolan, Davie, Katz, Rekhter, Rosen,
work in progress, Internet Draft <draft-doolan-tdp-spec-01.txt>, May,
1997.
[6] "Use of Tag Switching with ATM", Davie, Doolan, Lawrence,
McGloghrie, Rekhter, Rosen, Swallow, work in progress, Internet Draft
<draft-davie-tag-switching-atm-01.txt>, January, 1997.
[7] "Label Switching: Label Stack Encodings", Rosen, Rekhter, Tappan,
Farinacci, Fedorkow, Li, Conta, work in progress, Internet Draft
<draft-ietf-mpls-label-encaps-01.txt>, February, 1998.
[8] "Partitioning Tag Space among Multicast Routers on a Common
Subnet", Farinacci, work in progress, internet draft <draft-
farinacci-multicast-tag-part-00.txt>, December, 1996.
[9] "Multicast Tag Binding and Distribution using PIM", Farinacci,
Rekhter, work in progress, internet draft <draft-farinacci-
multicast-tagsw-00.txt>, December, 1996.
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[10] "Toshiba's Router Architecture Extensions for ATM: Overview",
Katsube, Nagami, Esaki, RFC 2098, February, 1997.
[11] "Loop-Free Routing Using Diffusing Computations", J.J. Garcia-
Luna-Aceves, IEEE/ACM Transactions on Networking, Vol. 1, No. 1,
February 1993.
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