Network Working Group E. Mannie, Ed.
Request for Comments: 3945 October 2004
Category: Standards Track
Generalized Multi-Protocol Label Switching (GMPLS) Architecture
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright (C) The Internet Society (2004).
Future data and transmission networks will consist of elements such
as routers, switches, Dense Wavelength Division Multiplexing (DWDM)
systems, Add-Drop Multiplexors (ADMs), photonic cross-connects
(PXCs), optical cross-connects (OXCs), etc. that will use Generalized
Multi-Protocol Label Switching (GMPLS) to dynamically provision
resources and to provide network survivability using protection and
This document describes the architecture of GMPLS. GMPLS extends
MPLS to encompass time-division (e.g., SONET/SDH, PDH, G.709),
wavelength (lambdas), and spatial switching (e.g., incoming port or
fiber to outgoing port or fiber). The focus of GMPLS is on the
control plane of these various layers since each of them can use
physically diverse data or forwarding planes. The intention is to
cover both the signaling and the routing part of that control plane.
The architecture described in this document covers the main building
blocks needed to build a consistent control plane for multiple
switching layers. It does not restrict the way that these layers
work together. Different models can be applied, e.g., overlay,
augmented or integrated. Moreover, each pair of contiguous layers
may collaborate in different ways, resulting in a number of possible
combinations, at the discretion of manufacturers and operators.
This architecture clearly separates the control plane and the
forwarding plane. In addition, it also clearly separates the control
plane in two parts, the signaling plane containing the signaling
protocols and the routing plane containing the routing protocols.
This document is a generalization of the Multi-Protocol Label
Switching (MPLS) architecture [RFC3031], and in some cases may differ
slightly from that architecture since non packet-based forwarding
planes are now considered. It is not the intention of this document
to describe concepts already described in the current MPLS
architecture. The goal is to describe specific concepts of
Generalized MPLS (GMPLS).
However, some of the concepts explained hereafter are not part of the
current MPLS architecture and are applicable to both MPLS and GMPLS
(i.e., link bundling, unnumbered links, and LSP hierarchy). Since
these concepts were introduced together with GMPLS and since they are
of paramount importance for an operational GMPLS network, they will
be discussed here.
The organization of the remainder of this document is as follows. We
begin with an introduction of GMPLS. We then present the specific
GMPLS building blocks and explain how they can be combined together
to build an operational GMPLS network. Specific details of the
separate building blocks can be found in the corresponding documents.
1.1. Acronyms & Abbreviations
AS Autonomous System
BGP Border Gateway Protocol
CR-LDP Constraint-based Routing LDP
CSPF Constraint-based Shortest Path First
DWDM Dense Wavelength Division Multiplexing
FA Forwarding Adjacency
GMPLS Generalized Multi-Protocol Label Switching
IGP Interior Gateway Protocol
LDP Label Distribution Protocol
LMP Link Management Protocol
LSA Link State Advertisement
LSR Label Switching Router
LSP Label Switched Path
MIB Management Information Base
MPLS Multi-Protocol Label Switching
NMS Network Management System
OXC Optical Cross-Connect
PXC Photonic Cross-Connect
RSVP ReSource reserVation Protocol
SDH Synchronous Digital Hierarchy
SONET Synchronous Optical Networks
STM(-N) Synchronous Transport Module (-N)
STS(-N) Synchronous Transport Signal-Level N (SONET)
TDM Time Division Multiplexing
TE Traffic Engineering
1.2. Multiple Types of Switching and Forwarding Hierarchies
Generalized MPLS (GMPLS) differs from traditional MPLS in that it
supports multiple types of switching, i.e., the addition of support
for TDM, lambda, and fiber (port) switching. The support for the
additional types of switching has driven GMPLS to extend certain base
functions of traditional MPLS and, in some cases, to add
functionality. These changes and additions impact basic LSP
properties: how labels are requested and communicated, the
unidirectional nature of LSPs, how errors are propagated, and
information provided for synchronizing the ingress and egress LSRs.
The MPLS architecture [RFC3031] was defined to support the forwarding
of data based on a label. In this architecture, Label Switching
Routers (LSRs) were assumed to have a forwarding plane that is
capable of (a) recognizing either packet or cell boundaries, and (b)
being able to process either packet headers (for LSRs capable of
recognizing packet boundaries) or cell headers (for LSRs capable of
recognizing cell boundaries).
The original MPLS architecture is here being extended to include LSRs
whose forwarding plane recognizes neither packet, nor cell
boundaries, and therefore, cannot forward data based on the
information carried in either packet or cell headers. Specifically,
such LSRs include devices where the switching decision is based on
time slots, wavelengths, or physical ports. So, the new set of LSRs,
or more precisely interfaces on these LSRs, can be subdivided into
the following classes:
1. Packet Switch Capable (PSC) interfaces:
Interfaces that recognize packet boundaries and can forward data
based on the content of the packet header. Examples include
interfaces on routers that forward data based on the content of
the IP header and interfaces on routers that switch data based on
the content of the MPLS "shim" header.
2. Layer-2 Switch Capable (L2SC) interfaces:
Interfaces that recognize frame/cell boundaries and can switch
data based on the content of the frame/cell header. Examples
include interfaces on Ethernet bridges that switch data based on
the content of the MAC header and interfaces on ATM-LSRs that
forward data based on the ATM VPI/VCI.
3. Time-Division Multiplex Capable (TDM) interfaces:
Interfaces that switch data based on the data's time slot in a
repeating cycle. An example of such an interface is that of a
SONET/SDH Cross-Connect (XC), Terminal Multiplexer (TM), or Add-
Drop Multiplexer (ADM). Other examples include interfaces
providing G.709 TDM capabilities (the "digital wrapper") and PDH
4. Lambda Switch Capable (LSC) interfaces:
Interfaces that switch data based on the wavelength on which the
data is received. An example of such an interface is that of a
Photonic Cross-Connect (PXC) or Optical Cross-Connect (OXC) that
can operate at the level of an individual wavelength. Additional
examples include PXC interfaces that can operate at the level of a
group of wavelengths, i.e., a waveband and G.709 interfaces
providing optical capabilities.
5. Fiber-Switch Capable (FSC) interfaces:
Interfaces that switch data based on a position of the data in the
(real world) physical spaces. An example of such an interface is
that of a PXC or OXC that can operate at the level of a single or
A circuit can be established only between, or through, interfaces of
the same type. Depending on the particular technology being used for
each interface, different circuit names can be used, e.g., SDH
circuit, optical trail, light-path, etc. In the context of GMPLS,
all these circuits are referenced by a common name: Label Switched
The concept of nested LSP (LSP within LSP), already available in the
traditional MPLS, facilitates building a forwarding hierarchy, i.e.,
a hierarchy of LSPs. This hierarchy of LSPs can occur on the same
interface, or between different interfaces.
For example, a hierarchy can be built if an interface is capable of
multiplexing several LSPs from the same technology (layer), e.g., a
lower order SONET/SDH LSP (e.g., VT2/VC-12) nested in a higher order
SONET/SDH LSP (e.g., STS-3c/VC-4). Several levels of signal (LSP)
nesting are defined in the SONET/SDH multiplexing hierarchy.
The nesting can also occur between interface types. At the top of
the hierarchy are FSC interfaces, followed by LSC interfaces,
followed by TDM interfaces, followed by L2SC, and followed by PSC
interfaces. This way, an LSP that starts and ends on a PSC interface
can be nested (together with other LSPs) into an LSP that starts and
ends on a L2SC interface. This LSP, in turn, can be nested (together
with other LSPs) into an LSP that starts and ends on a TDM interface.
In turn, this LSP can be nested (together with other LSPs) into an
LSP that starts and ends on a LSC interface, which in turn can be
nested (together with other LSPs) into an LSP that starts and ends on
a FSC interface.
1.3. Extension of the MPLS Control Plane
The establishment of LSPs that span only Packet Switch Capable (PSC)
or Layer-2 Switch Capable (L2SC) interfaces is defined for the
original MPLS and/or MPLS-TE control planes. GMPLS extends these
control planes to support each of the five classes of interfaces
(i.e., layers) defined in the previous section.
Note that the GMPLS control plane supports an overlay model, an
augmented model, and a peer (integrated) model. In the near term,
GMPLS appears to be very suitable for controlling each layer
independently. This elegant approach will facilitate the future
deployment of other models.
The GMPLS control plane is made of several building blocks as
described in more details in the following sections. These building
blocks are based on well-known signaling and routing protocols that
have been extended and/or modified to support GMPLS. They use IPv4
and/or IPv6 addresses. Only one new specialized protocol is required
to support the operations of GMPLS, a signaling protocol for link
GMPLS is indeed based on the Traffic Engineering (TE) extensions to
MPLS, a.k.a. MPLS-TE [RFC2702]. This, because most of the
technologies that can be used below the PSC level requires some
traffic engineering. The placement of LSPs at these levels needs in
general to consider several constraints (such as framing, bandwidth,
protection capability, etc) and to bypass the legacy Shortest-Path
First (SPF) algorithm. Note, however, that this is not mandatory and
that in some cases SPF routing can be applied.
In order to facilitate constrained-based SPF routing of LSPs, nodes
that perform LSP establishment need more information about the links
in the network than standard intra-domain routing protocols provide.
These TE attributes are distributed using the transport mechanisms
already available in IGPs (e.g., flooding) and taken into
consideration by the LSP routing algorithm. Optimization of the LSP
routes may also require some external simulations using heuristics
that serve as input for the actual path calculation and LSP
By definition, a TE link is a representation in the IS-IS/OSPF Link
State advertisements and in the link state database of certain
physical resources, and their properties, between two GMPLS nodes.
TE Links are used by the GMPLS control plane (routing and signaling)
for establishing LSPs.
Extensions to traditional routing protocols and algorithms are needed
to uniformly encode and carry TE link information, and explicit
routes (e.g., source routes) are required in the signaling. In
addition, the signaling must now be capable of transporting the
required circuit (LSP) parameters such as the bandwidth, the type of
signal, the desired protection and/or restoration, the position in a
particular multiplex, etc. Most of these extensions have already
been defined for PSC and L2SC traffic engineering with MPLS. GMPLS
primarily defines additional extensions for TDM, LSC, and FSC traffic
engineering. A very few elements are technology specific.
Thus, GMPLS extends the two signaling protocols defined for MPLS-TE
signaling, i.e., RSVP-TE [RFC3209] and CR-LDP [RFC3212]. However,
GMPLS does not specify which one of these two signaling protocols
must be used. It is the role of manufacturers and operators to
evaluate the two possible solutions for their own interest.
Since GMPLS signaling is based on RSVP-TE and CR-LDP, it mandates a
downstream-on-demand label allocation and distribution, with ingress
initiated ordered control. Liberal label retention is normally used,
but conservative label retention mode could also be used.
Furthermore, there is no restriction on the label allocation
strategy, it can be request/signaling driven (obvious for circuit
switching technologies), traffic/data driven, or even topology
driven. There is also no restriction on the route selection;
explicit routing is normally used (strict or loose) but hop-by-hop
routing could be used as well.
GMPLS also extends two traditional intra-domain link-state routing
protocols already extended for TE purposes, i.e., OSPF-TE [OSPF-TE]
and IS-IS-TE [ISIS-TE]. However, if explicit (source) routing is
used, the routing algorithms used by these protocols no longer need
to be standardized. Extensions for inter-domain routing (e.g., BGP)
are for further study.
The use of technologies like DWDM (Dense Wavelength Division
Multiplexing) implies that we can now have a very large number of
parallel links between two directly adjacent nodes (hundreds of
wavelengths, or even thousands of wavelengths if multiple fibers are
used). Such a large number of links was not originally considered
for an IP or MPLS control plane, although it could be done. Some
slight adaptations of that control plane are thus required if we want
to better reuse it in the GMPLS context.
For instance, the traditional IP routing model assumes the
establishment of a routing adjacency over each link connecting two
adjacent nodes. Having such a large number of adjacencies does not
scale well. Each node needs to maintain each of its adjacencies one
by one, and link state routing information must be flooded throughout
To solve this issue the concept of link bundling was introduced.
Moreover, the manual configuration and control of these links, even
if they are unnumbered, becomes impractical. The Link Management
Protocol (LMP) was specified to solve these issues.
LMP runs between data plane adjacent nodes and is used to manage TE
links. Specifically, LMP provides mechanisms to maintain control
channel connectivity (IP Control Channel Maintenance), verify the
physical connectivity of the data-bearing links (Link Verification),
correlate the link property information (Link Property Correlation),
and manage link failures (Fault Localization and Fault Notification).
A unique feature of LMP is that it is able to localize faults in both
opaque and transparent networks (i.e., independent of the encoding
scheme and bit rate used for the data).
LMP is defined in the context of GMPLS, but is specified
independently of the GMPLS signaling specification since it is a
local protocol running between data-plane adjacent nodes.
Consequently, LMP can be used in other contexts with non-GMPLS
MPLS signaling and routing protocols require at least one bi-
directional control channel to communicate even if two adjacent nodes
are connected by unidirectional links. Several control channels can
be used. LMP can be used to establish, maintain and manage these
GMPLS does not specify how these control channels must be
implemented, but GMPLS requires IP to transport the signaling and
routing protocols over them. Control channels can be either in-band
or out-of-band, and several solutions can be used to carry IP. Note
also that one type of LMP message (the Test message) is used in-band
in the data plane and may not be transported over IP, but this is a
particular case, needed to verify connectivity in the data plane.
1.4. GMPLS Key Extensions to MPLS-TE
Some key extensions brought by GMPLS to MPLS-TE are highlighted in
the following. Some of them are key advantages of GMPLS to control
TDM, LSC and FSC layers.
- In MPLS-TE, links traversed by an LSP can include an intermix of
links with heterogeneous label encoding (e.g., links between
routers, links between routers and ATM-LSRs, and links between
ATM-LSRs. GMPLS extends this by including links where the label is
encoded as a time slot, or a wavelength, or a position in the
(real world) physical space.
- In MPLS-TE, an LSP that carries IP has to start and end on a
router. GMPLS extends this by requiring an LSP to start and end
on similar type of interfaces.
- The type of a payload that can be carried in GMPLS by an LSP is
extended to allow such payloads as SONET/SDH, G.709, 1Gb or 10Gb
- The use of Forwarding Adjacencies (FA) provides a mechanism that
can improve bandwidth utilization, when bandwidth allocation can
be performed only in discrete units. It offers also a mechanism
to aggregate forwarding state, thus allowing the number of
required labels to be reduced.
- GMPLS allows suggesting a label by an upstream node to reduce the
setup latency. This suggestion may be overridden by a downstream
node but in some cases, at the cost of higher LSP setup time.
- GMPLS extends on the notion of restricting the range of labels
that may be selected by a downstream node. In GMPLS, an upstream
node may restrict the labels for an LSP along either a single hop
or the entire LSP path. This feature is useful in photonic
networks where wavelength conversion may not be available.
- While traditional TE-based (and even LDP-based) LSPs are
unidirectional, GMPLS supports the establishment of bi-directional
- GMPLS supports the termination of an LSP on a specific egress
port, i.e., the port selection at the destination side.
- GMPLS with RSVP-TE supports an RSVP specific mechanism for rapid
Note also some other key differences between MPLS-TE and GMPLS:
- For TDM, LSC and FSC interfaces, bandwidth allocation for an LSP
can be performed only in discrete units.
- It is expected to have (much) fewer labels on TDM, LSC or FSC
links than on PSC or L2SC links, because the former are physical
labels instead of logical labels.
2. Routing and Addressing Model
GMPLS is based on the IP routing and addressing models. This assumes
that IPv4 and/or IPv6 addresses are used to identify interfaces but
also that traditional (distributed) IP routing protocols are reused.
Indeed, the discovery of the topology and the resource state of all
links in a routing domain is achieved via these routing protocols.
Since control and data planes are de-coupled in GMPLS, control-plane
neighbors (i.e., IGP-learnt neighbors) may not be data-plane
neighbors. Hence, mechanisms like LMP are needed to associate TE
links with neighboring nodes.
IP addresses are not used only to identify interfaces of IP hosts and
routers, but more generally to identify any PSC and non-PSC
interfaces. Similarly, IP routing protocols are used to find routes
for IP datagrams with a SPF algorithm; they are also used to find
routes for non-PSC circuits by using a CSPF algorithm.
However, some additional mechanisms are needed to increase the
scalability of these models and to deal with specific traffic
engineering requirements of non-PSC layers. These mechanisms will be
introduced in the following.
Re-using existing IP routing protocols allows for non-PSC layers
taking advantage of all the valuable developments that took place
since years for IP routing, in particular, in the context of intra-
domain routing (link-state routing) and inter-domain routing (policy
In an overlay model, each particular non-PSC layer can be seen as a
set of Autonomous Systems (ASs) interconnected in an arbitrary way.
Similarly to the traditional IP routing, each AS is managed by a
single administrative authority. For instance, an AS can be an
SONET/SDH network operated by a given carrier. The set of
interconnected ASs can be viewed as SONET/SDH internetworks.
Exchange of routing information between ASs can be done via an
inter-domain routing protocol like BGP-4. There is obviously a huge
value of re-using well-known policy routing facilities provided by
BGP in a non-PSC context. Extensions for BGP traffic engineering
(BGP-TE) in the context of non-PSC layers are left for further study.
Each AS can be sub-divided in different routing domains, and each can
run a different intra-domain routing protocol. In turn, each
routing-domain can be divided in areas.
A routing domain is made of GMPLS enabled nodes (i.e., a network
device including a GMPLS entity). These nodes can be either edge
nodes (i.e., hosts, ingress LSRs or egress LSRs), or internal LSRs.
An example of non-PSC host is an SONET/SDH Terminal Multiplexer (TM).
Another example is an SONET/SDH interface card within an IP router or
Note that traffic engineering in the intra-domain requires the use of
link-state routing protocols like OSPF or IS-IS.
GMPLS defines extensions to these protocols. These extensions are
needed to disseminate specific TDM, LSC and FSC static and dynamic
characteristics related to nodes and links. The current focus is on
intra-area traffic engineering. However, inter-area traffic
engineering is also under investigation.
2.1. Addressing of PSC and non-PSC Layers
The fact that IPv4 and/or IPv6 addresses are used does not imply at
all that they should be allocated in the same addressing space than
public IPv4 and/or IPv6 addresses used for the Internet. Private IP
addresses can be used if they do not require to be exchanged with any
other operator; public IP addresses are otherwise required. Of
course, if an integrated model is used, two layers could share the
same addressing space. Finally, TE links may be "unnumbered" i.e.,
not have any IP addresses, in case IP addresses are not available, or
the overhead of managing them is considered too high.
Note that there is a benefit of using public IPv4 and/or IPv6
Internet addresses for non-PSC layers if an integrated model with the
IP layer is foreseen.
If we consider the scalability enhancements proposed in the next
section, the IPv4 (32 bits) and the IPv6 (128 bits) addressing spaces
are both more than sufficient to accommodate any non-PSC layer. We
can reasonably expect to have much less non-PSC devices (e.g.,
SONET/SDH nodes) than we have today IP hosts and routers.
2.2. GMPLS Scalability Enhancements
TDM, LSC and FSC layers introduce new constraints on the IP
addressing and routing models since several hundreds of parallel
physical links (e.g., wavelengths) can now connect two nodes. Most
of the carriers already have today several tens of wavelengths per
fiber between two nodes. New generation of DWDM systems will allow
several hundreds of wavelengths per fiber.
It becomes rather impractical to associate an IP address with each
end of each physical link, to represent each link as a separate
routing adjacency, and to advertise and to maintain link states for
each of these links. For that purpose, GMPLS enhances the MPLS
routing and addressing models to increase their scalability.
Two optional mechanisms can be used to increase the scalability of
the addressing and the routing: unnumbered links and link bundling.
These two mechanisms can also be combined. They require extensions
to signaling (RSVP-TE and CR-LDP) and routing (OSPF-TE and IS-IS-TE)
2.3. TE Extensions to IP Routing Protocols
Traditionally, a TE link is advertised as an adjunct to a "regular"
OSPF or IS-IS link, i.e., an adjacency is brought up on the link.
When the link is up, both the regular IGP properties of the link
(basically, the SPF metric) and the TE properties of the link are
However, GMPLS challenges this notion in three ways:
- First, links that are non-PSC may yet have TE properties; however,
an OSPF adjacency could not be brought up directly on such links.
- Second, an LSP can be advertised as a point-to-point TE link in
the routing protocol, i.e., as a Forwarding Adjacency (FA); thus,
an advertised TE link need no longer be between two OSPF direct
neighbors. Forwarding Adjacencies (FA) are further described in
- Third, a number of links may be advertised as a single TE link
(e.g., for improved scalability), so again, there is no longer a
one-to-one association of a regular adjacency and a TE link.
Thus, we have a more general notion of a TE link. A TE link is a
logical link that has TE properties. Some of these properties may be
configured on the advertising LSR, others may be obtained from other
LSRs by means of some protocol, and yet others may be deduced from
the component(s) of the TE link.
An important TE property of a TE link is related to the bandwidth
accounting for that link. GMPLS will define different accounting
rules for different non-PSC layers. Generic bandwidth attributes are
however defined by the TE routing extensions and by GMPLS, such as
the unreserved bandwidth, the maximum reservable bandwidth and the
maximum LSP bandwidth.
It is expected in a dynamic environment to have frequent changes of
bandwidth accounting information. A flexible policy for triggering
link state updates based on bandwidth thresholds and link-dampening
mechanism can be implemented.
TE properties associated with a link should also capture protection
and restoration related characteristics. For instance, shared
protection can be elegantly combined with bundling. Protection and
restoration are mainly generic mechanisms also applicable to MPLS. It
is expected that they will first be developed for MPLS and later on
generalized to GMPLS.
A TE link between a pair of LSRs does not imply the existence of an
IGP adjacency between these LSRs. A TE link must also have some
means by which the advertising LSR can know of its liveness (e.g., by
using LMP hellos). When an LSR knows that a TE link is up, and can
determine the TE link's TE properties, the LSR may then advertise
that link to its GMPLS enhanced OSPF or IS-IS neighbors using the TE
objects/TLVs. We call the interfaces over which GMPLS enhanced OSPF
or IS-IS adjacencies are established "control channels".
3. Unnumbered Links
Unnumbered links (or interfaces) are links (or interfaces) that do
not have IP addresses. Using such links involves two capabilities:
the ability to specify unnumbered links in MPLS TE signaling, and the
ability to carry (TE) information about unnumbered links in IGP TE
extensions of IS-IS-TE and OSPF-TE.
A. The ability to specify unnumbered links in MPLS TE signaling
requires extensions to RSVP-TE [RFC3477] and CR-LDP [RFC3480].
The MPLS-TE signaling does not provide support for unnumbered
links, because it does not provide a way to indicate an unnumbered
link in its Explicit Route Object/TLV and in its Record Route
Object (there is no such TLV for CR-LDP). GMPLS defines simple
extensions to indicate an unnumbered link in these two
Objects/TLVs, using a new Unnumbered Interface ID sub-object/sub-
Since unnumbered links are not identified by an IP address, then
for the purpose of MPLS TE each end need some other identifier,
local to the LSR to which the link belongs. LSRs at the two end-
points of an unnumbered link exchange with each other the
identifiers they assign to the link. Exchanging the identifiers
may be accomplished by configuration, by means of a protocol such
as LMP ([LMP]), by means of RSVP-TE/CR-LDP (especially in the case
where a link is a Forwarding Adjacency, see below), or by means of
IS-IS or OSPF extensions ([ISIS-TE-GMPLS], [OSPF-TE-GMPLS]).
Consider an (unnumbered) link between LSRs A and B. LSR A chooses
an identifier for that link. So does LSR B. From A's perspective
we refer to the identifier that A assigned to the link as the
"link local identifier" (or just "local identifier"), and to the
identifier that B assigned to the link as the "link remote
identifier" (or just "remote identifier"). Likewise, from B's
perspective the identifier that B assigned to the link is the
local identifier, and the identifier that A assigned to the link
is the remote identifier.
The new Unnumbered Interface ID sub-object/sub-TLV for the ER
Object/TLV contains the Router ID of the LSR at the upstream end
of the unnumbered link and the link local identifier with respect
to that upstream LSR.
The new Unnumbered Interface ID sub-object for the RR Object
contains the link local identifier with respect to the LSR that
adds it in the RR Object.
B. The ability to carry (TE) information about unnumbered links in
IGP TE extensions requires new sub-TLVs for the extended IS
reachability TLV defined in IS-IS-TE and for the TE LSA (which is
an opaque LSA) defined in OSPF-TE. A Link Local Identifier sub-
TLV and a Link Remote Identifier sub-TLV are defined.
3.1. Unnumbered Forwarding Adjacencies
If an LSR that originates an LSP advertises this LSP as an unnumbered
FA in IS-IS or OSPF, or the LSR uses this FA as an unnumbered
component link of a bundled link, the LSR must allocate an Interface
ID to that FA. If the LSP is bi-directional, the tail end does the
same and allocates an Interface ID to the reverse FA.
Signaling has been enhanced to carry the Interface ID of a FA in the
new LSP Tunnel Interface ID object/TLV. This object/TLV contains the
Router ID (of the LSR that generates it) and the Interface ID. It is
called the Forward Interface ID when it appears in a Path/REQUEST
message, and it is called the Reverse Interface ID when it appears in
the Resv/MAPPING message.