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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
   other groups may also distribute working documents as Internet-
   Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as ``work in progress.''

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt

   The list of Internet-Draft Shadow Directories 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 <PHOP, Interface ID> matches the tuple
   <Ingress LSR Router ID, CR-LSP ID> 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 <PHOP, Interface ID>
   matches the tuple <Ingress LSR Router ID, Reverse Interface ID> 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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