Internet Draft
Network Working Group Kireeti Kompella
Internet Draft Juniper Networks
Expiration Date: March 2001 Yakov Rekhter
Cisco Systems
Alan Kullberg
NetPlane Systems
Signalling Unnumbered Links in CR-LDP
draft-kompella-mpls-crldp-unnum-00.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
Task Force (IETF), its areas, and its working groups. Note that
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and may be updated, replaced, or obsoleted by other documents at any
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The list of current Internet-Drafts can be accessed at
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2. Abstract
Current signalling used by MPLS TE doesn't provide support for
unnumbered links. This document defines procedures and extensions to
CR-LDP, one of the MPLS TE signalling protocols, that are needed in
order to support unnumbered links.
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3. Overview
Supporting MPLS TE over unnumbered links (i.e., links that do not
have IP addresses) involves two components: (a) the ability to carry
(TE) information about unnumbered links in IGP TE extensions (ISIS or
OSPF), and (b) the ability to specify unnumbered links in MPLS TE
signalling. The former is covered in [ISIS-TE, OSPF-TE]. The focus
of this document is on the latter.
Current signalling used by MPLS TE doesn't provide support for
unnumbered links because the current signalling doesn't provide a way
to indicate an unnumbered link in its Explicit Route Objects. This
document proposes simple procedures and extensions that allow CR-LDP
[CR-LDP] signalling to be used with unnumbered links.
4. Interface Identifiers
Since unnumbered links are not identified by an IP address, then for
the purpose of MPLS TE they need some other identifier. We assume
that each unnumbered link on a Label Switched Router (LSR) is given a
unique 16-bit identifier. The scope of this identifier is the LSR to
which the link belongs; moreover, the IS-IS and/or OSPF and CR-LDP
modules on an LSR must agree on interface identifiers.
Note that links are directed, i.e., a link l is from some LSR A to
some other LSR B. LSR A chooses the interface identifier for link l.
To be completely clear, we call this the "outgoing interface
identifier from LSR A's point of view". If there is a reverse link
from LSR B to LSR A (for example, a point-to-point SONET interface
connecting LSRs A and B would be represented as two links, one from A
to B, and another from B to A), B chooses the outgoing interface
identifier for the reverse link. There is no a priori relationship
between the two interface identifiers.
5. Unnumbered Forwarding Adjacencies
If an LSR that originates an LSP advertises this LSP as an unnumbered
Forwarding Adjacency in IS-IS or OSPF [LSP-HIER], the LSR MUST
allocate an interface ID to that Forwarding Adjacency. Moreover, the
Local CR-LSP ID in the LSPID TLV of the Request Message for the LSP
MUST be set to that interface ID, and the Ingress LSR Router ID in
the LSPID TLV of the LSP MUST be set to the Router ID of the LSR that
originates the LSP.
If the LSP is bidirectional, and the tail-end LSR (of the forward
LSP) advertises the reverse LSP as an unnumbered Forwarding
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Adjacency, the tail-end LSR MUST allocate an interface ID to the
reverse Forwarding Adjacency. Furthermore, it MUST set the "Reverse
Interface ID" field in the Reverse Interface ID TLV in the MAPPING
message to the reverse FA's interface ID. The Reverse Interface ID's
format is shown in Figure 1:
Figure 1: Reverse Interface ID TLV
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0|0| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MUST be zero | Reverse Interface ID (16 bits)|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Type (Reverse Interface ID) is to be determined by IETF consensus
and the Length is 4.
6. Signalling Unnumbered Links in EROs
A new subobject of the Explicit Route Object (ERO) is used to specify
unnumbered links. This subobject has the following format:
Figure 2: Unnumbered Interface ID Subobject
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0|0| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|L| MUST be zero | Interface ID (16 bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
This subobject MUST be strict (i.e., the L bit MUST be 0). The Type
is 0x0805 (Unnumbered Interface ID) and the Length is 4.
An LSR sending a Request message that includes an Unnumbered
Interface ID subobject as the first subobject in the ERO MUST also
include a PHOP TLV, specifying the Router ID of the sending LSR.
This TLV is depicted in Figure 3.
The Type (PHOP TLV) is to be determined by IETF consensus and the
Length is 4.
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Figure 3: PHOP TLV
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0|0| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sending LSR's Router ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
6.1. Interpreting the Unnumbered Interface ID Subobject
The Interface ID is the outgoing interface identifier with respect to
the previous node in the path (i.e., the PHOP). If the Request
message contains an Unnumbered Interface ID subobject as the first
subobject in the ERO, then the PHOP object in the message must
contain the router ID of the previous node.
6.2. Processing the Unnumbered Interface ID Subobject
A node that receives a Request message with an Unnumbered Interface
ID as the first subobject in the ERO carried by the message MUST
check whether the tuple matches the tuple
of any of the LSPs for which the
node is a tail-end. If a match is found, the match identifies the
Forwarding Adjacency for which the node has to perform label
allocation.
Otherwise, the node MUST check whether the tuple
matches the tuple of
any of the bidirectional LSPs for which the node is the head-end. If
a match is found, the match identifies the Forwarding Adjacency for
which the node has to perform label allocation, namely, the reverse
Forwarding Adjacency for the LSP identified by the match.
Otherwise, it is assumed that the node has to perform label
allocation for the link over which the Request message was received.
In this case the receiving node MAY validate that it received the
Request Message correctly. To do so, the node must maintain a
database of Traffic Engineering information distributed by IS-IS
and/or OSPF.
To validate that it received the Request message correctly, the node
looks up in its Traffic Engineering database for the node
corresponding to the router ID of the sender of the Request message.
It then checks that there is a link from the previous node to itself
that carries the same Interface ID as the one in the ERO subobject.
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If this is not the case, the receiving node has received the message
in error and SHOULD return a "Bad Initial ER-Hop" error. Otherwise,
the receiving node removes the first subobject, and continues
processing the ERO.
6.3. Selecting the Next Hop
If, after processing and removing all initial subobjects in the ERO
that refer to itself, the receiving node finds a subobject of type
Unnumbered Interface ID, it determines the next hop as follows. The
Interface ID MUST refer to an outgoing interface identifier that this
node allocated; if not, the node SHOULD return a "Bad Strict Node"
error. The next hop is the node at the other end of the link that
the Interface ID refers to.
Furthermore, when sending a Request message to the next hop, the ERO
to be used is the current ERO (starting with the Unnumbered Interface
ID subobject).
7. Security Considerations
This document raises no new security concerns for CR-LDP.
8. Acknowledgments
Thanks to Rahul Aggarwal for his comments on the text.
9. References
[CR-LDP] Jamoussi, B., editor, "Constraint-Based LSP Setup using
LDP", draft-ietf-mpls-cr-ldp-04.txt (work in progress)
[ISIS-TE] Smit, H., and Li, T., "IS-IS extensions for Traffic
Engineering", draft-ietf-isis-traffic-02.txt (work in progress)
[LSP-HIER] Kompella, K., and Rekhter, Y., "LSP Hierarchy with MPLS
TE", draft-ietf-mpls-lsp-hierarchy-01.txt (work in progress)
[OSPF-TE] Katz, D., and Yeung, D., "Traffic Engineering Extensions to
OSPF", draft-katz-yeung-ospf-traffic-02.txt (work in progress)
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10. Author Information
Kireeti Kompella
Juniper Networks, Inc.
1194 N. Mathilda Ave.
Sunnyvale, CA 94089
e-mail: kireeti@juniper.net
Yakov Rekhter
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 95134
e-mail: yakov@cisco.com
Alan Kullberg
NetPlane Systems, Inc.
888 Washington St.
Dedham, MA 02026
e-mail: akullber@netplane.com
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