Network Working Group Kireeti Kompella
Internet Draft Juniper Networks
Expiration Date: March 2001 Yakov Rekhter
Cisco Systems
Lou Berger
LabN Consulting, LLC
Link Bundling in MPLS Traffic Engineering
draft-kompella-mpls-bundle-03.txt
1. Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
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The list of current Internet-Drafts can be accessed at
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2. Abstract
In some cases a pair of Label Switching Routers (LSRs) may be
connected by several (parallel) links. From the MPLS Traffic
Engineering point of view for reasons of scalability it may be
desirable to advertise all these links as a single link into OSPF
and/or IS-IS. This document describes how to accomplish this. This
document also defines corresponding signaling (RSVP-TE) support.
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Internet Draft draft-kompella-mpls-bundle-03.txt September 2000
3. Link Bundling
When a pair of LSRs are connected by multiple links, then for the
purpose of MPLS Traffic Engineering it is possible to advertise
several (or all) of these links as a single link into OSPF and/or
IS-IS. We refer to this process as "link bundling", or just
"bundling". We refer to the link that is advertised into OSPF/IS-IS
as a "bundled link". We refer to the links associated with that
bundled link as "component links".
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
information. To limit the amount of losses one need to restrict the
type of the information that can be aggregated/abstracted.
3.1. Restrictions on Bundling
All component links in a bundle must have the same Link Type (if
any), the same Traffic Engineering metric, the same set of resource
classes, and the same Link Multiplex Capability (see [LSP-HIER]).
If the component links are all multi-access links, the set of IS-IS
or OSPF routers 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.
3.2. Numbering Considerations
Component links may be either numbered or unnumbered. If all the
component links within a bundled link are unnumbered, then the
bundled link is unnumbered by default. In all cases, the bundled
link's addresses may be overridden by configuration with IP addresses
assigned to some "virtual" interfaces on an LSR (it is assumed that
an LSR may have multiple virtual interfaces).
If a component link is numbered, this document assumes that the link
has a dedicated control channel (which may be the link itself) that
could be used to send RSVP/CR-LDP messages. If a component link is
unnumbered, then the link may, but doesn't have to have a dedicated
control channel. In this case, the bundled link must have a control
channel that is shared by all component links that do not have
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Internet Draft draft-kompella-mpls-bundle-03.txt September 2000
dedicated control channels.
If a bundled link is unnumbered, then it has to have its own
interface identifier, just like any other unnumbered link [UNNUM].
This document assumes that each unnumbered component link on a Label
Switching Router (LSR) is given a 16-bit identifier by this LSR. We
refer to this identifier as the component interface identifier. The
scope of this identifier is the bundled link that contains the
component link. Thus multiple component links on an LSR may share
the same component interface identifier, as long as these component
links belong to different bundled links. The component interface
identifiers are not visible to the IS-IS and/or OSPF, but are visible
to RSVP and/or CR-LDP.
3.3. Other Considerations
If several component links are bundled, IS-IS/OSPF flooding can be
restricted to just one of the component links [ZININ]. Similarly,
IS-IS/OSPF hellos can be restricted to just one component link;
however, it may be useful to send hellos on all links that do not
have a link layer mechanism (e.g., keep-alive) to ensure that a
failure of the link is detected.
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.
4. 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 [ISIS] and [OSPF];
we use the terminology from [OSPF].
4.1. Link Type
The Link Type of a bundled link is the (unique) Link Type of the
component links. (Note: this parameter is not present in IS-IS.)
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4.2. 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:
this parameter is not present in IS-IS.)
4.3. Local and Remote Interface IP Address
(Note: in IS-IS, these are known as IPv4 Interface Address and IPv4
Neighbor Address, respectively.)
If the bundled link is numbered (see section 3.1), 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.
If the bundled link is unnumbered, the local interface IP address
(OSPF) or IPv4 interface address (ISIS) is set to the Router ID of
advertising LSR. The first two octets of the remote interface IP
address (OSPF) or IPv4 neighbor address (ISIS) are set to zero; the
remaining two octets are set to the interface identifier assigned to
the bundled link.
4.4. Traffic Engineering Metric
The Traffic Engineering Metric for a bundled link is that of the
component links.
4.5. Maximum Link Bandwidth
This TLV is not used. The maximum LSP Bandwidth (as described below)
replaces the maximum link bandwidth for bundled links. For backward
compatibility, one MAY advertise the Maximum LSP Bandwidth at
priority 7 of the bundle as the Maximum Link Bandwidth.
4.6. Maximum Reservable Bandwidth
We assume that for a given bundled link 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.
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4.7. 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.
4.8. Resource Classes (Administrative Groups)
The Resource Classes for a bundled link are the same as those of the
component links.
4.9. Maximum LSP Bandwidth
The Maximum LSP Bandwidth takes the place of the Maximum Link
Bandwidth. However, while Maximum Link Bandwidth is a single fixed
value (usually simply the link capacity), Maximum LSP Bandwidth is
carried per priority, and may vary as LSPs are set up and torn down.
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 each
component link.
If a component link is a simple (unbundled) link, define its Maximum
LSP Bandwidth at priority p to be the smaller of its unreserved
bandwidth at priority p and its maximum link bandwidth.
Since bundling may be applied recursively, a component link may
itself be a bundled link. In this case, its Maximum LSP Bandwidth as
a component link is the same as its Maximum LSP Bandwidth as a
bundled link.
In IS-IS, the Maximum LSP Bandwidth TLV is a sub-TLV of the Extended
IS Reachability TLV with type 21. In OSPF, this TLV is a sub-TLV of
the Link TLV within the Traffic Engineering LSA, with type 11. The
length of the Maximum LSP Bandwidth TLV is 32 octets. The value is a
list of eight 4 octet fields in IEEE floating point format of the
Maximum LSP Bandwidth of the bundle, from priority 0 to priority 7.
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5. Procedures
5.1. Bandwidth Accounting
The RSVP 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 maximum LSP
bandwidth >= b at priority p. If there are several such links, the
choice of which link is used for the LSP is up to the implementation.
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. This is done as
follows. If an LSP with bandwidth b and holding priority p is set up
through a component link, that component link's unreserved bandwidth
at priority p and lower is reduced by b. If an LSP with bandwidth b
and holding priority p that is currently set up through a component
link is torn down, the unreserved bandwidth at priority p and lower
for that component link is increased by b.
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.
5.2. Signaling
Signaling must identify both the component link to use and the label
to use. For unidirectional LSPs the sender of the Resv message
chooses the label (as before). The sender of the Path message
selects the component link to be used for the LSP and communicates
the choice downstream. For bidirectional LSPs [GMPLS-SIG], in
addition to the above, the sender of the Path message selects the
(upstream) label, and the component link to be used with the
(upstream) label, and communicates the choice downstream.
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Internet Draft draft-kompella-mpls-bundle-03.txt September 2000
There are two methods for communicating the selected component link.
The first method communicates the selected component link via the
COMPONENT_INTERFACE_ID object class defined below. Doing this assumes
that an LSR connected to a component link knows the component
interface identifier assigned to that link by the LSR at the other
end of the link. Exchanging the identity of a component link between
the LSRs connected by that link is accomplished either by
configuration or by means of a protocol such as [LMP], or in the case
where a component link is a Forwarding Adjacency by RSVP/CR-LDP
[LSP-HIER].
The second method is for the upstream node to indicate the selected
link by sending the Path message over the control channel
corresponding to the selected component link. This method may only
be used when LMP is not used or there are no configured component
link values, and when there is a one-to-one correspondence between
component links and control channels. When this method is used on
bidirectional LSPs, both upstream and downstream labels MUST be
assigned on the same component link.
When a component link is numbered, the first method is non-
applicable, and only the second method is used.
5.2.1. COMPONENT_INTERFACE_ID Object Class
A new object class, the COMPONENT_INTERFACE_ID object class, is
defined. The Length field is set to 8. The Class Num is TBD of form
0bbbbbbb. The DOWNSTREAM_COMPONENT_INTERFACE_ID object, which has a
C_Type of 1, is used to indicate the component interface to be used
for traffic flowing in the downstream direction. The
UPSTREAM_COMPONENT_INTERFACE_ID object, which has a C_Type of 2, is
used to indicate the component interface to be used for traffic
flowing in the upstream direction. Both objects have the same format
and carry a 16-bit Component Interface Identifier. The format of the
objects are:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length |Class Num (TBD)| C_Type (1|2) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Component Interface Identifier|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
We introduce a new error value for the error code "Routing problem",
namely "Unknown Component Interface ID" with error value 11.
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If the receiver doesn't recognize the COMPONENT_INTERFACE_ID object
class, per [RSVP], it SHOULD send an error message with an "Unknown
Object Class". If the class is recognize but the C-Type is not, per
[RSVP], the receiver SHOULD send an "Unknown Object C-Type" error. A
node that recognizes either COMPONENT_INTERFACE_ID objects, but that
is unable to support it (possibly because of a failure to allocate
labels) SHOULD send an error message with the error code "Routing
problem" and the error value "MPLS label allocation failure." If LMP
or some other link identification protocol is not running, or there
is no component link with the Component Interface Identifier in
either object, the receiver SHOULD send an error message with the
error code "Routing problem" and the error value "Unknown Component
Interface ID".
5.2.2. COMPONENT_INTERFACE_ID Object Class Usage
The COMPONENT_INTERFACE_ID objects are carried in RSVP messages as
part of the sender descriptor. They are optional with respect to the
protocol, and are only used when component links are being identified
using the COMPONENT_INTERFACE_ID objects. There are two formats for
the sender descriptor, one for traditional LSPs and one for
bidirectional LSPs.
The format of the sender descriptor for unidirectional LSPs is:
<sender descriptor> ::= <SENDER_TEMPLATE> <SENDER_TSPEC>
[ <ADSPEC> ]
[ <RECORD_ROUTE> ]
<DOWNSTREAM_COMPONENT_INTERFACE_ID>
[ <SUGGESTED_LABEL> ]
The format of the sender descriptor for bidirectional LSPs is:
<sender descriptor> ::= <SENDER_TEMPLATE> <SENDER_TSPEC>
[ <ADSPEC> ]
[ <RECORD_ROUTE> ]
<DOWNSTREAM_COMPONENT_INTERFACE_ID>
[ <SUGGESTED_LABEL> ]
<UPSTREAM_COMPONENT_INTERFACE_ID>
<UPSTREAM_LABEL>
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6. Security Considerations
This document raises no new security issues for IS-IS, OSPF or RSVP.
7. References
[GMPLS-SIG] Ashwood, P., et al., "Generalized MPLS - Signalling
Functional Description", draft-ashood-generalized-mpls-signalling-
00.txt
[ISIS] Smit, H., Li, T., "IS-IS extensions for Traffic Engineering",
draft-ietf-isis-traffic-01.txt (work in progress)
[LMP] Lang, J., Mitra, K., et al., "Link Management Protocol (LMP)",
draft-lang-mpls-lmp-00.txt (work in progress)
[OSPF] Katz, D., Yeung, D., "Traffic Engineering Extensions to OSPF",
draft-katz-yeung-ospf-traffic-01.txt (work in progress)
[RSVP] Braden, Ed., et. al., "Resource ReSerVation Protocol (RSVP) --
Version 1 Functional Specification", RFC2205, September 1997.
[RSVP-TE] Awduche, D., Berger, L., Gan, D., et al, "Extensions to
RSVP for LSP Tunnels", draft-ietf-mpls-rsvp-lsp-tunnel-05.txt (work
in progress)
[LSP-HIER] Kompella, K., Rekhter, Y., "LSP Hierarchy with MPLS TE",
draft-ietf-mpls-lsp-hierarchy-01.txt (work in progress)
[UNNUM] Kompella, K., Rekhter, Y., "Traffic Engineering with
Unnumbered Links", draft-kompella-mpls-unnum-01.txt (work in
progress)
[ZININ] Zinin, A., Shand, M., "Flooding optimizations in link-state
routing protocols", draft-zinin-flood-opt-00.txt (work in progress)
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8. Author Information
Kireeti Kompella
Juniper Networks, Inc.
1194 N. Mathilda Ave.
Sunnyvale, CA 94089
Email: kireeti@juniper.net
Yakov Rekhter
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 95134
Email: yakov@cisco.com
Lou Berger
LabN Consulting, LLC
Voice: +1 301 468 9228
Email: lberger@labn.net
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