Network Working Group K. Kompella
Request for Comments: 4206 Y. Rekhter
Category: Standards Track Juniper Networks
October 2005 Label Switched Paths (LSP) Hierarchy with
Generalized Multi-Protocol Label Switching (GMPLS)
Traffic Engineering (TE)
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 (2005).
To improve scalability of Generalized Multi-Protocol Label Switching
(GMPLS) it may be useful to aggregate Label Switched Paths (LSPs) by
creating a hierarchy of such LSPs. A way to create such a hierarchy
is by (a) a Label Switching Router (LSR) creating a Traffic
Engineering Label Switched Path (TE LSP), (b) the LSR forming a
forwarding adjacency (FA) out of that LSP (by advertising this LSP as
a Traffic Engineering (TE) link into the same instance of ISIS/OSPF
as the one that was used to create the LSP), (c) allowing other LSRs
to use FAs for their path computation, and (d) nesting of LSPs
originated by other LSRs into that LSP (by using the label stack
This document describes the mechanisms to accomplish this.
Table of Contents
1. Overview ........................................................22. Specification of Requirements ...................................33. Routing Aspects .................................................43.1. Traffic Engineering Parameters .............................43.1.1. Link Type (OSPF Only) ...............................53.1.2. Link ID (OSPF Only) .................................53.1.3. Local and Remote Interface IP Address ...............53.1.4. Local and Remote Link Identifiers ...................53.1.5. Traffic Engineering Metric ..........................53.1.6. Maximum Bandwidth ...................................53.1.7. Unreserved Bandwidth ................................53.1.8. Resource Class/Color ................................53.1.9. Interface Switching Capability ......................63.1.10. SRLG Information ...................................64. Other Considerations ............................................65. Controlling FA-LSPs Boundaries ..................................75.1. LSP Regions ................................................76. Signalling Aspects ..............................................86.1. Common Procedures ..........................................86.1.1. RSVP-TE .............................................86.1.2. CR-LDP ..............................................96.2. Specific Procedures .......................................106.3. FA-LSP Holding Priority ...................................117. Security Considerations ........................................118. Acknowledgements ...............................................129. Normative References ...........................................1210. Informative References ........................................131. Overview
An LSR uses Generalized MPLS (GMPLS) TE procedures to create and
maintain an LSP. The LSR then may (under local configuration
control) announce this LSP as a Traffic Engineering (TE) link into
the same instance of the GMPLS control plane (or, more precisely, its
ISIS/OSPF component) as the one that was used to create the LSP. We
call such a link a "forwarding adjacency" (FA). We refer to the LSP
as the "forwarding adjacency LSP", or just FA-LSP. Note that an FA-
LSP is both created and used as a TE link by exactly the same
instance of the GMPLS control plane. Thus, the concept of an FA is
applicable only when an LSP is both created and used as a TE link by
exactly the same instance of the GMPLS control plane. Note also that
an FA is a TE link between two GMPLS nodes whose path transits zero
or more (G)MPLS nodes in the same instance of the GMPLS control
The nodes connected by a 'basic' TE link may have a routing
adjacency; however, the nodes connected by an FA would not usually
have a routing adjacency. A TE link of any kind (either 'basic' or
FA) would have to have a signaling adjacency in order for it to be
used to establish an LSP across it.
In general, the creation/termination of an FA and its FA-LSP could be
driven either by mechanisms outside of GMPLS (e.g., via configuration
control on the LSR at the head-end of the adjacency), or by
mechanisms within GMPLS (e.g., as a result of the LSR at the head-end
of the adjacency receiving LSP setup requests originated by some
ISIS/OSPF floods the information about FAs just as it floods the
information about any other links. As a result of this flooding, an
LSR has in its TE link state database the information about not just
basic TE links, but FAs as well.
An LSR, when performing path computation, uses not just basic TE
links, but FAs as well. Once a path is computed, the LSR uses
RSVP/CR-LDP [RSVP-TE, CR-LDP] for establishing label binding along
In this document we define mechanisms/procedures to accomplish the
above. These mechanisms/procedures cover both the routing
(ISIS/OSPF) and the signalling (RSVP/CR-LDP) aspects.
Note that an LSP may be advertised as a point-to-point link into ISIS
or OSPF, to be used in normal SPF by nodes other than the head-end.
While this is similar in spirit to an FA, this is beyond the scope of
Scenarios where an LSP is created (and maintained) by one instance of
the GMPLS control plane, and is used as a (TE) link by another
instance of the GMPLS control plane, are outside the scope of this
2. Specification of Requirements
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in BCP 14, RFC 2119
3. Routing Aspects
In this section we describe procedures for constructing FAs out of
LSPs, and handling of FAs by ISIS/OSPF. Specifically, this section
describes how to construct the information needed to advertise LSPs
as links into ISIS/OSPF. Procedures for creation/termination of such
LSPs are defined in Section 5, "Controlling FA-LSPs boundaries".
FAs may be represented as either unnumbered or numbered links. If
FAs are numbered with IPv4 addresses, the local and remote IPv4
addresses come out of a /31 that is allocated by the LSR that
originates the FA-LSP; the head-end address of the FA-LSP is the one
specified as the IPv4 tunnel sender address; the remote (tail-end)
address can then be inferred. If the LSP is bidirectional, the
tail-end can thus know the addresses to assign to the reverse FA.
If there are multiple LSPs that all originate on one LSR and all
terminate on another LSR, then at one end of the spectrum all these
LSPs could be merged (under control of the head-end LSR) into a
single FA using the concept of Link Bundling (see [BUNDLE]); while at
the other end of the spectrum each such LSP could be advertised as
its own adjacency.
When an FA is created under administrative control (static
provisioning), the attributes of the FA-LSP have to be provided via
configuration. Specifically, the following attributes may be
configured for the FA-LSP: the head-end address (if left
unconfigured, this defaults to the head-end LSR's Router ID); the
tail-end address; bandwidth and resource colors constraints. The
path taken by the FA-LSP may be either computed by the LSR at the
head-end of the FA-LSP, or specified by explicit configuration; this
choice is determined by configuration.
When an FA is created dynamically, the attributes of its FA-LSP are
inherited from the LSP that induced its creation. Note that the
bandwidth of the FA-LSP must be at least as big as the LSP that
induced it, but may be bigger if only discrete bandwidths are
available for the FA-LSP. In general, for dynamically provisioned
FAs, a policy-based mechanism may be needed to associate attributes
to the FA-LSPs.
3.1. Traffic Engineering Parameters
In this section, the Traffic Engineering parameters (see [OSPF-TE]
and [ISIS-TE]) for FAs are described.
3.1.1. Link Type (OSPF Only)
The Link Type of an FA is set to "point-to-point".
3.1.2. Link ID (OSPF Only)
The Link ID is set to the Router ID of the tail-end of FA-LSP.
3.1.3. Local and Remote Interface IP Address
If the FA is to be numbered, the local interface IP address (OSPF) or
IPv4 interface address (ISIS) is set to the head-end address of the
FA-LSP. The remote interface IP address (OSPF) or IPv4 neighbor
address (ISIS) is set to the tail-end address of the FA-LSP.
3.1.4. Local and Remote Link Identifiers
For an unnumbered FA, the assignment and handling of the local and
remote link identifiers is specified in [UNNUM-RSVP], [UNNUM-CRLDP].
3.1.5. Traffic Engineering Metric
By default the TE metric on the FA is set to max(1, (the TE metric of
the FA-LSP path) - 1) so that it attracts traffic in preference to
setting up a new LSP. This may be overridden via configuration at
the head-end of the FA.
3.1.6. Maximum Bandwidth
By default, the Maximum Reservable Bandwidth and the initial Maximum
LSP Bandwidth for all priorities of the FA is set to the bandwidth of
the FA-LSP. These may be overridden via configuration at the head-
end of the FA (note that the Maximum LSP Bandwidth at any one
priority should be no more than the bandwidth of the FA-LSP).
3.1.7. Unreserved Bandwidth
The initial unreserved bandwidth for all priority levels of the FA is
set to the bandwidth of the FA-LSP.
3.1.8. Resource Class/Color
By default, an FA does not have resource colors (administrative
groups). This may be overridden by configuration at the head-end of
3.1.9. Interface Switching Capability
The (near-end) Interface Switching Capability associated with the FA
is the (near end) Interface Switching Capability of the first link in
When the (near-end) Interface Switching Capability field is PSC-1,
PSC-2, PSC-3, or PSC-4, the specific information includes Interface
MTU and Minimum LSP Bandwidth. The Interface MTU is the minimum MTU
along the path of the FA-LSP; the Minimum LSP Bandwidth is the
bandwidth of the LSP.
3.1.10. SRLG Information
An FA advertisement could contain the information about the Shared
Risk Link Groups (SRLG) for the path taken by the FA-LSP associated
with that FA. This information may be used for path calculation by
other LSRs. The information carried is the union of the SRLGs of the
underlying TE links that make up the FA-LSP path; it is carried in
the SRLG TLV in IS-IS or the SRLG sub-TLV of the TE Link TLV in OSPF.
See [GMPLS-ISIS, GMPLS-OSPF] for details on the format of this
It is possible that the underlying path information might change over
time, via configuration updates or dynamic route modifications,
resulting in the change of the SRLG TLV.
If FAs are bundled (via link bundling), and if the resulting bundled
link carries an SRLG TLV, it MUST be the case that the list of SRLGs
in the underlying path, followed by each of the FA-LSPs that form the
component links, is the same (note that the exact paths need not be
4. Other Considerations
It is expected that FAs will not be used for establishing ISIS/OSPF
peering relation between the routers at the ends of the adjacency.
It may be desired in some cases to use FAs only in Traffic
Engineering path computations. In IS-IS, this can be accomplished by
setting the default metric of the extended IS reachability TLV for
the FA to the maximum link metric (2^24 - 1). In OSPF, this can be
accomplished by not advertising the link as a regular LSA, but only
as a TE opaque LSA.
5. Controlling FA-LSPs Boundaries
To facilitate controlling the boundaries of FA-LSPs, this document
introduces two new mechanisms: Interface Switching Capability (see
[GMPLS-ISIS, GMPLS-OSPF], and "LSP region" (or just "region").
5.1. LSP Regions
The information carried in the Interface Switching Capabilities is
used to construct LSP regions and to determine regions' boundaries as
Define an ordering among interface switching capabilities as follows:
PSC-1 < PSC-2 < PSC-3 < PSC-4 < TDM < LSC < FSC. Given two
interfaces if-1 and if-2 with interface switching capabilities isc-1
and isc-2 respectively, say that if-1 < if-2 iff isc-1 < isc-2 or
isc-1 == isc-2 == TDM, and if-1's max LSP bandwidth is less than if-
2's max LSP bandwidth.
Suppose an LSP's path is as follows: node-0, link-1, node-1, link-2,
node-2, ..., link-n, node-n. Moreover, for link-i denote by [link-i,
node-(i-1)] the interface that connects link-i to node-(i-1), and by
[link-i, node-i] the interface that connects link-i to node-i.
If [link-(i+1), node-i)] < [link-(i+1), node-(i+1)], we say that the
LSP has crossed a region boundary at node-i; with respect to that LSP
path, the LSR at node-i is an edge LSR. The 'other edge' of the
region with respect to the LSP path is node-k, where k is the
smallest number greater than i such that [link-(i+1), node-(i+1)]
equal [link-k, node-(k-1)], and [link-k, node-(k-1)] > [link-k,
Path computation may take region boundaries into account when
computing a path for an LSP. For example, path computation may
restrict the path taken by an LSP to only the links whose Interface
Switching Capability is PSC-1.
Note that an interface may have multiple Interface Switching
Capabilities. In such a case, the test is whether if-i < if-j
depends on the Interface Switching Capabilities chosen for if-i and
if-j, which in turn determines whether or not there is a region
boundary at node-i.
6. Signalling Aspects
In this section we describe procedures that an LSR at the head-end of
an FA uses for handling LSP setup originated by other LSR.
As we mentioned before, establishment/termination of FA-LSPs may be
triggered either by mechanisms outside of GMPLS (e.g., via
administrative control), or by mechanisms within GMPLS (e.g., as a
result of the LSR at the edge of an aggregate LSP receiving LSP setup
requests originated by some other LSRs beyond LSP aggregate and its
edges). Procedures described in Section 6.1, "Common Procedures",
apply to both cases. Procedures described in Section 6.2, "Specific
Procedures", apply only to the latter case.
6.1. Common Procedures
For the purpose of processing the ERO in a Path/Request message of an
LSP that is to be tunneled over an FA, an LSR at the head-end of the
FA-LSP views the LSR at the tail of that FA-LSP as adjacent (one IP
How this is to be achieved for RSVP-TE and CR-LDP is described in the
In either case (RSVP-TE or CR-LDP), when an LSP is tunneled through
an FA-LSP, the LSR at the head-end of the FA-LSP subtracts the LSP's
bandwidth from the unreserved bandwidth of the FA.
In the presence of link bundling (when link bundling is applied to
FAs), when an LSP is tunneled through an FA-LSP, the LSR at the
head-end of the FA-LSP also needs to adjust Max LSP bandwidth of the
If one uses RSVP-TE to signal an LSP to be tunneled over an FA-LSP,
then the Path message MUST contain an IF_ID RSVP_HOP object
[GRSVP-TE, GSIG] instead of an RSVP_HOP object; and the data
interface identification MUST identify the FA-LSP.
The preferred method of sending the Path message is to set the
destination IP address of the Path message to the computed NHOP for
that Path message. This NHOP address must be a routable address; in
the case of separate control and data planes, this must be a control
Furthermore, the IP header for the Path message MUST NOT have the
Router Alert option. The Path message is intended to be IP-routed to
the tail-end of the FA-LSP without being intercepted and processed as
an RSVP message by any of the intermediate nodes.
Finally, the IP TTL vs. RSVP TTL check MUST NOT be made. In general,
if the IF_ID RSVP_HOP object is used, this check must be disabled, as
the number of hops over the control plane may be greater than one.
Instead, the following check is done by the receiver Y of the IF_ID
1. Make sure that the data interface identified in the IF_ID RSVP_HOP
object actually terminates on Y.
2. Find the "other end" of the above data interface, say X. Make
sure that the PHOP in the IF_ID RSVP_HOP object is a control
channel address that belongs to the same node as X.
How check #2 is carried out is beyond the scope of this document;
suffice it to say that it may require a Traffic Engineering Database,
or the use of LMP [LMP], or yet other means.
An alternative method is to encapsulate the Path message in an IP
tunnel (or, in the case that the Interface Switching Capability of
the FA-LSP is PSC[1-4], in the FA-LSP itself), and unicast the
message to the tail-end of the FA-LSP, without the Router Alert
option. This option may be needed if intermediate nodes process RSVP
messages regardless of whether the Router Alert option is present.
A PathErr sent in response to a Path message with an IF_ID RSVP_HOP
object SHOULD contain an IF_ID HOP object. (Note: a PathErr does not
normally carry an RSVP_HOP object, but in the case of separated
control and data, it is necessary to identify the data channel in the
The Resv message back to the head-end of the FA-LSP (PHOP) is IP-
routed to the PHOP in the Path message. If necessary, Resv Messages
MAY be encapsulated in another IP header whose destination IP address
is the PHOP of the received Path message.
If one uses CR-LDP to signal an LSP to be tunneled over an FA-LSP,
then the Request message MUST contain an IF_ID TLV [GCR-LDP] object,
and the data interface identification MUST identify the FA-LSP.
Furthermore, the head-end LSR must create a targeted LDP session with
the tail-end LSR. The Request (Mapping) message is unicast from the
head-end (tail-end) to the tail-end (head-end).
6.2. Specific Procedures
When an LSR receives a Path/Request message, the LSR determines
whether it is at the edge of a region with respect to the ERO carried
in the message. The LSR does this by looking up the interface
switching capabilities of the previous hop and the next hop in its
IGP database, and comparing them using the relation defined in this
section. If the LSR is not at the edge of a region, the procedures
in this section do not apply.
If the LSR is at the edge of a region, it must then determine the
other edge of the region with respect to the ERO, again using the IGP
database. The LSR then extracts (from the ERO) the subsequence of
hops from itself to the other end of the region.
The LSR then compares the subsequence of hops with all existing FA-
LSPs originated by the LSR. If a match is found, that FA-LSP has
enough unreserved bandwidth for the LSP being signaled, the L3PID of
the FA-LSP is compatible with the L3PID of the LSP being signaled,
and the LSR uses that FA-LSP as follows. The Path/Request message
for the original LSP is sent to the egress of the FA-LSP, not to the
next hop along the FA-LSP's path. The PHOP in the message is the
address of the LSR at the head-end of the FA-LSP. Before sending the
Path/Request message, the ERO in that message is adjusted by removing
the subsequence of the ERO that lies in the FA-LSP, and replacing it
with just the end point of the FA-LSP.
Otherwise (if no existing FA-LSP is found), the LSR sets up a new
FA-LSP. That is, it initiates a new LSP setup just for the FA-LSP.
Note that the new LSP may traverse either 'basic' TE links or FAs.
After the LSR establishes the new FA-LSP, the LSR announces this LSP
into IS-IS/OSPF as an FA.
The unreserved bandwidth of the FA is computed by subtracting the
bandwidth of sessions pending the establishment of the FA-LSP
associated from the bandwidth of the FA-LSP.
An FA-LSP could be torn down by the LSR at the head-end of the FA-LSP
as a matter of policy local to the LSR. It is expected that the FA-
LSP would be torn down once there are no more LSPs carried by the
FA-LSP. When the FA-LSP is torn down, the FA associated with the
FA-LSP is no longer advertised into IS-IS/OSPF.
6.3. FA-LSP Holding Priority
The value of the holding priority of an FA-LSP must be the minimum of
the configured holding priority of the FA-LSP and the holding
priorities of the LSPs tunneling through the FA-LSP (note that
smaller priority values denote higher priority). Thus, if an LSP of
higher priority than the FA-LSP tunnels through the FA-LSP, the FA-
LSP is itself promoted to the higher priority. However, if the
tunneled LSP is torn down, the FA-LSP need not drop its priority to
its old value right away; it may be advisable to apply hysteresis in
If the holding priority of an FA-LSP is configured, this document
restricts it to 0.
7. Security Considerations
From a security point of view, the primary change introduced in this
document is that the implicit assumption of a binding between data
interfaces and the interface over which a control message is sent is
no longer valid.
This means that the "sending interface" or "receiving interface" is
no longer well-defined, as the interface over which an RSVP message
is sent may change as routing changes. Therefore, mechanisms that
depend on these concepts (for example, the definition of a security
association) need a clearer definition.
[RFC2747] provides a solution: in Section 2.1, under "Key
Identifier", an IP address is a valid identifier for the sending (and
by analogy, receiving) interface. Since RSVP messages for a given
LSP are sent to an IP address that identifies the next/previous hop
for the LSP, one can replace all occurrences of 'sending [receiving]
interface' with 'receiver's [sender's] IP address' (respectively).
For example, in Section 4, third paragraph, instead of:
"Each sender SHOULD have distinct security associations (and keys)
per secured sending interface (or LIH). ... At the sender,
security association selection is based on the interface through
which the message is sent."
it should read:
"Each sender SHOULD have distinct security associations (and keys)
per secured receiver's IP address. ... At the sender, security
association selection is based on the IP address to which the
message is sent."
Note that CR-LDP does not have this issue, as CR-LDP messages are
sent over TCP sessions, and no assumption is made that these sessions
are to direct neighbors. The recommended mechanism for
authentication and integrity of LDP message exchange is to use the
TCP MD5 option [LDP].
Another consequence (relevant to RSVP) of the changes proposed in
this document is that IP destination address of Path messages be set
to the receiver's address, not to the session destination. Thus, the
objections raised in Section 1.2 of [RFC2747] should be revisited to
see if IPSec AH is now a viable means of securing RSVP-TE messages.
Many thanks to Alan Hannan, whose early discussions with Yakov
Rekhter contributed greatly to the notion of Forwarding Adjacencies.
We would also like to thank George Swallow, Quaizar Vohra and Ayan
9. Normative References
[GCR-LDP] Ashwood-Smith, P. and L. Berger, "Generalized Multi-
Protocol Label Switching (GMPLS) Signaling Constraint-
based Routed Label Distribution Protocol (CR-LDP)
Extensions", RFC 3472, January 2003.
[GMPLS-ISIS] Kompella, K., Ed., and Y. Rekhter, Ed., "Intermediate
System to Intermediate System (IS-IS) Extensions in
Support of Generalized Multi-Protocol Label Switching
(GMPLS)", RFC 4205, October 2005.
[GMPLS-OSPF] Kompella, K., Ed., and Y. Rekhter, Ed., "OSPF
Extensions in Support of Generalized Multi-Protocol
Label Switching (GMPLS)", RFC 4203, October 2005.
[GRSVP-TE] Berger, L., "Generalized Multi-Protocol Label Switching
(GMPLS) Signaling Resource ReserVation Protocol-Traffic
Engineering (RSVP-TE) Extensions", RFC 3473, January
[GSIG] Berger, L., "Generalized Multi-Protocol Label Switching
(GMPLS) Signaling Functional Description", RFC 3471,
[ISIS-TE] Smit, H. and T. Li, "Intermediate System to
Intermediate System (IS-IS) Extensions for Traffic
Engineering (TE)", RFC 3784, June 2004.
[LDP] Andersson, L., Doolan, P., Feldman, N., Fredette, A.,
and B. Thomas, "Label Distribution Protocol", RFC 3036,
[OSPF-TE] Katz, D., Kompella, K., and D. Yeung, "Traffic
Engineering (TE) Extensions to OSPF Version 2", RFC
3630, September 2003.
[UNNUM-CRLDP] Kompella, K., Rekhter, Y., and A. Kullberg, "Signalling
Unnumbered Links in CR-LDP (Constraint-Routing Label
Distribution Protocol)", RFC 3480, February 2003.
[UNNUM-RSVP] Kompella, K. and Y. Rekhter, "Signalling Unnumbered
Links in Resource ReSerVation Protocol - Traffic
Engineering (RSVP-TE)", RFC 3477, January 2003.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2747] Baker, F., Lindell, B., and M. Talwar, "RSVP
Cryptographic Authentication", RFC 2747, January 2000.
10. Informative References
[BUNDLE] Kompella, K., Rekhter, Y., and L. Berger, "Link
Bundling in MPLS Traffic Engineering (TE)", RFC 4201,
[LMP] Lang, L., Ed., "Link Management Protocol (LMP)", RFC
4204, October 2005.
Juniper Networks, Inc.
1194 N. Mathilda Ave
Sunnyvale, CA 94089
Juniper Networks, Inc.
1194 N. Mathilda Ave
Sunnyvale, CA 94089
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