Network Working Group K. Kompella

Network Working Group K. Kompella
Request for Comments: 4201 Y. Rekhter
Updates: 3471, 3472, 3473 Juniper Networks
Category: Standards Track L. Berger
Movaz Networks
October 2005

Link Bundling in MPLS 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 Notice

Copyright (C) The Internet Society (2005).

Abstract

For the purpose of Generalized Multi-Protocol Label Switching (GMPLS)
signaling, in certain cases a combination of <link identifier, label>
is not sufficient to unambiguously identify the appropriate resource
used by a Label Switched Path (LSP). Such cases are handled by using
the link bundling construct, which is described in this document.
This document updates the interface identification TLVs, which are
defined in the GMPLS Signaling Functional Description.

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Table of Contents

1. Introduction ................................................. 2
1.1. Specification of Requirements .......................... 2
2. Link Bundling ................................................ 3
2.1. Restrictions on Bundling ............................... 4
2.2. Routing Considerations ................................. 4
2.3. Signaling Considerations ............................... 5
2.3.1. Interface Identification TLV Format ............ 6
2.3.2. Errored Component Identification ............... 7
3. Traffic Engineering Parameters for Bundled Links ............. 7
3.1. OSPF Link Type ......................................... 7
3.2. OSPF Link ID ........................................... 7
3.3. Local and Remote Interface IP Address .................. 7
3.4. Local and Remote Identifiers ........................... 8
3.5. Traffic Engineering Metric ............................. 8
3.6. Maximum Bandwidth ...................................... 8
3.7. Maximum Reservable Bandwidth ........................... 8
3.8. Unreserved Bandwidth ................................... 8
3.9. Resource Classes (Administrative Groups) ............... 8
3.10. Maximum LSP Bandwidth ................................. 8
4. Bandwidth Accounting ......................................... 9
5. Security Considerations ...................................... 9
6. IANA Considerations .......................................... 9
7. References ................................................... 10
7.1. Normative References ................................... 10
7.2. Informative References ................................. 11

1. Introduction

For the purpose of Generalized Multi-Protocol Label Switching (GMPLS)
signaling, in certain cases a combination of <link identifier, label>
is not sufficient to unambiguously identify the appropriate resource
used by a Label Switched Path (LSP). Such cases are handled by using
the link bundling construct, which is described in this document.
This document updates the interface identification TLVs, which are
defined in the GMPLS Signaling Functional Description.

1.1. 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 RFC 2119 [RFC2119].

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2. Link Bundling

As defined in [GMPLS-ROUTING], a traffic engineering (TE) link is a
logical construct that represents a way to group/map information
about certain physical resources (and their properties) that
interconnect LSRs with information that is used by Constrained SPF
(for the purpose of path computation) and by GMPLS signaling.

As stated in [GMPLS-ROUTING], depending on the nature of resources
that form a particular TE link for the purpose of GMPLS signaling, in
some cases a combination of <TE link identifier, label> is sufficient
to unambiguously identify the appropriate resource used by an LSP.
In other cases, a combination of <TE link identifier, label> is not
sufficient. Consider, for example, a TE link between a pair of
SONET/SDH cross-connects, where this TE link is composed of several
fibers. In this case the label is a TDM time slot, and moreover,
this time slot is significant only within a particular fiber. Thus,
when signaling an LSP over such a TE link, one needs to specify not
just the identity of the link, but also the identity of a particular
fiber within that TE link, as well as a particular label (time slot)
within that fiber. Such cases are handled by using the link bundling
construct, which is described in this document.

Consider a TE link such that, for the purpose of GMPLS signaling, a
combination of <TE link identifier, label> is not sufficient to
unambiguously identify the appropriate resources used by an LSP. In
this situation, the link bundling construct assumes that the set of
resources that form the TE link could be partitioned into disjoint
subsets, such that (a) the partition is minimal, and (b) within each
subset, a label is sufficient to unambiguously identify the
appropriate resources used by an LSP. We refer to such subsets as
"component links", and to the whole TE link as a "bundled link".
Furthermore, we restrict the identifiers that can be used to identify
component links such that they are unique for a given node. On a
bundled link, a combination of <component link identifier, label> is
sufficient to unambiguously identify the appropriate resources used
by an LSP.

The partition of resources that form a bundled link into component
links has to be done consistently at both ends of the bundled link.
Both ends of the bundled link also have to understand the other end's
component link identifiers.

The purpose of link bundling is to improve routing scalability by
reducing the amount of information that has to be handled by OSPF
and/or IS-IS. This reduction is accomplished by performing
information aggregation/abstraction. As with any other information
aggregation/abstraction, this results in losing some of the

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information. To limit the amount of losses, one needs to restrict
the type of information that can be aggregated/abstracted.

2.1. Restrictions on Bundling

All component links in a bundle have the same Link Type (i.e.,
point-to-point or multi-access), the same Traffic Engineering metric,
the same set of resource classes at each end of the links, and must
begin and end on the same pair of LSRs.

A Forwarding Adjacency may be a component link; in fact, a bundle can
consist of a mix of point-to-point links and FAs.

If the component links are all multi-access links, the set of IS-IS
or OSPF routers that are connected to each component link must be the
same, and the Designated Router for each component link must be the
same. If these conditions cannot be enforced, multi-access links
must not be bundled.

Component link identifiers MUST be unique across both TE and
component link identifiers on a particular node. This means that
unnumbered identifiers have a node-wide scope, and that numbered
identifiers have the same scope as IP addresses.

2.2. Routing Considerations

A component link may be either numbered or unnumbered. A bundled
link may itself be numbered or unnumbered, independent of whether the
component links of that bundled link are numbered.

Handling identifiers for unnumbered component links, including the
case in which a link is formed by a Forwarding Adjacency, follows the
same rules as those for an unnumbered TE link (see Section "Link
Identifiers" of [RFC3477]/[RFC3480]). Furthermore, link local
identifiers for all unnumbered links of a given LSR (whether
component links, Forwarding Adjacencies, or bundled links) MUST be
unique in the context of that LSR.

The "liveness" of the bundled link is determined by the liveness of
each of the component links within the bundled link; a bundled link
is alive when at least one of its component links is determined to be
alive. The liveness of a component link can be determined by any of
several means: IS-IS or OSPF hellos over the component link, RSVP
Hello, LMP hellos (see [LMP]), or from layer 1 or layer 2
indications.

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Once a bundled link is determined to be alive, it can be advertised
as a TE link and the TE information can be flooded. If IS-IS/OSPF
hellos are run over the component links, IS-IS/OSPF flooding can be
restricted to just one of the component links. Procedures for doing
this are outside the scope of this document.

In the future, as new Traffic Engineering parameters are added to
IS-IS and OSPF, they should be accompanied by descriptions as to how
they can be bundled, and possible restrictions on bundling.

2.3. Signaling Considerations

Because information about the bundled link is flooded, but
information about the component links is not, typically, an LSP's ERO
will identify the bundled link to be used for the LSP, but not the
component link. While Discovery of component link identities to be
used in an ERO is outside the scope of the document, it is envisioned
that such information may be provided via configuration or via future
RRO extensions. When the bundled link is identified in an ERO or is
dynamically identified, the choice of the component link for the LSP
is a local matter between the two LSRs at each end of the bundled
link.

Signaling must identify both the component link and label to use.
The choice of the component link to use is always made by the sender
of the Path/REQUEST message. If an LSP is bidirectional [RFC3471],
the sender chooses a component link in each direction. The handling
of labels is not modified by this document.

Component link identifiers are carried in RSVP messages, as described
in section 8 of [RFC3473]. Component link identifiers are carried in
CR-LDP messages, as described in section 8 of [RFC3473]. Additional
processing related to unnumbered links is described in the
"Processing the IF_ID RSVP_HOP object"/"Processing the IF_ID TLV",
and "Unnumbered Forwarding Adjacencies" sections of
[RFC3477]/[RFC3480].

[RFC3471] defines the Interface Identification type-length-value
(TLV) types. This document specifies that the TLV types 1, 2, and 3
SHOULD be used to indicate component links in IF_ID RSVP_HOP objects
and IF_ID TLVs.

Type 1 TLVs are used for IPv4 numbered component link identifiers.

Type 2 TLVs are used for IPv6 numbered component link identifiers.

Type 3 TLVs are used for unnumbered component link identifiers.

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The Component Interface TLVs, TLV types 4 and 5, SHOULD NOT be used.
Note, in Path and REQUEST messages, link identifiers MUST be
specified from the sender's perspective.

Except in the special case noted below, for a unidirectional LSP,
only a single TLV SHOULD be used in an IF_ID RSVP_HOP object or IF_ID
TLV. This TLV indicates the component link identifier of the
downstream data channel on which label allocation must be done.

Except in the special case noted below, for a bidirectional LSP, only
one or two TLVs SHOULD be used in an IF_ID RSVP_HOP object or IF_ID
TLV. The first TLV always indicates the component link identifier of
the downstream data channel on which label allocation must be done.
When present, the second TLV always indicates the component link
identifier of the upstream data channel on which label allocation
must be done. When only one TLV is present, it indicates the
component link identifier for both downstream and upstream data
channels.

In the special case where the same label is to be valid across all
component links, two TLVs SHOULD be used in an IF_ID RSVP_HOP object
or IF_ID TLV. The first TLV indicates the TE link identifier of the
bundle on which label allocation must be done. The second TLV
indicates a bundle scope label. For TLV types 1 and 2, this is done
by using the special bit value of all ones (1) (e.g., 0xFFFFFFFF for
a type 1 TLV). Per [RFC3471], for TLV types 3, 4, and 5, this is
done by setting the Interface ID field to the special value
0xFFFFFFFF. Note that this special case applies to both
unidirectional and bidirectional LSPs.

Although it SHOULD NOT be used, when used, the type 5 TLV MUST NOT be
the first TLV in an IF_ID RSVP_HOP object or IF_ID TLV.

2.3.1. Interface Identification TLV Format

This section modifies section 9.1.1. of [RFC3471]. The definition of
the IP Address field of the TLV types 3, 4, and 5 is clarified.

For types 3, 4, and 5, the Value field has an identical format to
the contents of the C-Type 1 LSP_TUNNEL_INTERFACE_ID object
defined in [RFC3477]. Note that this results in the renaming of
the IP Address field defined in [RFC3471].

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2.3.2. Errored Component Identification

When Interface Identification TLVs are used, the TLVs are also used
to indicate the specific components associated with an error. For
RSVP, this means that any received TLVs SHOULD be copied into the
IF_ID ERROR_SPEC object (see Section 8.2 in [RFC3473]). The Error
Node Address field of the object SHOULD indicate the TE Link
associated with the error. For CR-LDP, this means that any received
TLVs SHOULD be copied into the IF_ID Status TLV (see Section 8.2 in
[RFC3472]). The HOP Address field of the TLV SHOULD indicate the TE
Link associated with the error.

3. Traffic Engineering Parameters for Bundled Links

In this section, we define the Traffic Engineering parameters to be
advertised for a bundled link, based on the configuration of the
component links and of the bundled link. The definition of these
parameters for component links was undertaken in [RFC3784] and
[RFC3630]; we use the terminology from [RFC3630].

3.1. OSPF Link Type

The Link Type of a bundled link is the (unique) Link Type of the
component links. Note that this parameter is not present in IS-IS.

3.2. OSPF Link ID

For point-to-point links, the Link ID of a bundled link is the
(unique) Router ID of the neighbor. For multi-access links, this is
the interface address of the (unique) Designated Router. Note that
this parameter is not present in IS-IS.

3.3. Local and Remote Interface IP Address

Note that in IS-IS, the Local Interface IP Address is known as the
IPv4 Interface Address and the Remote Interface IP Address is known
as the IPv4 Neighbor Address.

If the bundled link is numbered, the Local Interface IP Address is
the local address of the bundled link; similarly, the Remote
Interface IP Address is the remote address of the bundled link.

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3.4. Local and Remote Identifiers

If the bundled link is unnumbered, the link local identifier is set
to the identifier chosen for the bundle by the advertising LSR. The
link remote identifier is set to the identifier chosen by the
neighboring LSR for the reverse link corresponding to this bundle, if
known; otherwise, this is set to 0.

3.5. Traffic Engineering Metric

The Traffic Engineering Metric for a bundled link is that of the
component links.

3.6. Maximum Bandwidth

This parameter is not used. The maximum LSP Bandwidth (as described
below) replaces the Maximum Bandwidth for bundled links.

3.7. Maximum Reservable Bandwidth

For a given bundled link, we assume that either each of its component
links is configured with the Maximum Reservable Bandwidth, or the
bundled link is configured with the Maximum Reservable Bandwidth. In
the former case, the Maximum Reservable Bandwidth of the bundled link
is set to the sum of the Maximum Reservable Bandwidths of all
component links associated with the bundled link.

3.8. Unreserved Bandwidth

The unreserved bandwidth of a bundled link at priority p is the sum
of the unreserved bandwidths at priority p of all the component links
associated with the bundled link.

3.9. Resource Classes (Administrative Groups)

The Resource Classes for a bundled link are the same as those of the
component links.

3.10. Maximum LSP Bandwidth

The Maximum LSP Bandwidth takes the place of the Maximum Bandwidth.
For an unbundled link, the Maximum Bandwidth is defined in
[GMPLS-ROUTING]. The Maximum LSP Bandwidth of a bundled link at
priority p is defined to be the maximum of the Maximum LSP Bandwidth
at priority p of all of its component links.

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The details of how Maximum LSP Bandwidth is carried in IS-IS is given
in [GMPLS-ISIS]. The details of how Maximum LSP Bandwidth is carried
in OSPF is given in [GMPLS-OSPF].

4. Bandwidth Accounting

The RSVP (or CR-LDP) Traffic Control module, or its equivalent, on an
LSR with bundled links must apply admission control on a per-
component link basis. An LSP with a bandwidth requirement b and
setup priority p fits in a bundled link if at least one component
link has a maximum LSP bandwidth >= b at priority p. If there are
several such links, the implementation will choose which link to use
for the LSP.

In order to know the maximum LSP bandwidth (per priority) of each
component link, the Traffic Control module must track the unreserved
bandwidth (per priority) for each component link.

A change in the unreserved bandwidth of a component link results in a
change in the unreserved bandwidth of the bundled link. It also
potentially results in a change in the maximum LSP bandwidth of the
bundle; thus, the maximum LSP bandwidth should be recomputed.

If one of the component links goes down, the associated bundled link
remains up and continues to be advertised, provided that at least one
component link associated with the bundled link is up. The
unreserved bandwidth of the component link that is down is set to
zero, and the unreserved bandwidth and maximum LSP bandwidth of the
bundle must be recomputed. If all the component links associated
with a given bundled link are down, the bundled link MUST not be
advertised into OSPF/IS-IS.

5. Security Considerations

This document defines ways of utilizing procedures defined in other
documents, referenced herein. Any security issues related to those
procedures are addressed in the referenced documents. Thus, this
document raises no new security issues for RSVP-TE [RFC3209] or CR-
LDP [RFC3212].

6. IANA Considerations

This document changes the recommended usage of two of the
Interface_ID Types defined in [RFC3471]. For this reason, the IANA
registry of GMPLS Signaling Parameters has been updated to read:

4 12 COMPONENT_IF_DOWNSTREAM - DEPRECATED
5 12 COMPONENT_IF_UPSTREAM - DEPRECATED

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7. References

7.1. Normative References

[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.

[GMPLS-ROUTING] Kompella, K., Ed. and Y. Rekhter, Ed., "Routing
Extensions in Support of Generalized Multi-Protocol
Label Switching (GMPLS)", RFC 4202, October 2005.

[RFC3471] Berger, L., "Generalized Multi-Protocol Label
Switching (GMPLS) Signaling Functional Description",
RFC 3471, January 2003.

[RFC3473] Berger, L., "Generalized Multi-Protocol Label
Switching (GMPLS) Signaling Resource ReserVation
Protocol-Traffic Engineering (RSVP-TE) Extensions",
RFC 3473, January 2003.

[RFC3472] 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.

[RFC3784] Smit, H. and T. Li, "Intermediate System to
Intermediate System (IS-IS) Extensions for Traffic
Engineering (TE)", RFC 3784, June 2004.

[RFC3630] Katz, D., Kompella, K., and D. Yeung, "Traffic
Engineering (TE) Extensions to OSPF Version 2", RFC
3630, September 2003.

[RFC3480] Kompella, K., Rekhter, Y., and A. Kullberg,
"Signalling Unnumbered Links in CR-LDP (Constraint-
Routing Label Distribution Protocol)", RFC 3480,
February 2003.

[RFC3477] Kompella, K. and Y. Rekhter, "Signalling Unnumbered
Links in Resource ReSerVation Protocol - Traffic
Engineering (RSVP-TE)", RFC 3477, January 2003.

Kompella, et al. Standards Track [Page 10]

RFC 4201 Link Bundling in MPLS-TE October 2005

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

[RFC3209] Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan,
V., and G. Swallow, "RSVP-TE: Extensions to RSVP for
LSP Tunnels", RFC 3209, December 2001.

[RFC3212] Jamoussi, B., Andersson, L., Callon, R., Dantu, R.,
Wu, L., Doolan, P., Worster, T., Feldman, N.,
Fredette, A., Girish, M., Gray, E., Heinanen, J.,
Kilty, T., and A. Malis, "Constraint-Based LSP Setup
using LDP", RFC 3212, January 2002.

7.2. Informative References

[LMP] Lang, J., Ed., "Link Management Protocol (LMP)", RFC
4204, October 2005.

Authors' Addresses

Kireeti Kompella
Juniper Networks, Inc.
1194 N. Mathilda Ave.
Sunnyvale, CA 94089

EMail: [email protected]

Yakov Rekhter
Juniper Networks, Inc.
1194 N. Mathilda Ave.
Sunnyvale, CA 94089

EMail: [email protected]

Lou Berger
Movaz Networks, Inc.

Phone: +1 703-847-1801
EMail: [email protected]

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