Network Working Group                                       Naiming Shen
Internet Draft                                                 Enke Chen
                                                             Albert Tian
Expiration Date: January 2005                           Redback Networks

                 Discovering LDP Next-Nexthop Labels


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   This document specifies extensions to LDP in support of next-nexthop
   label discovery. The next-nexthop label information can be used to
   fast re-route LDP LSP traffic into an explicitly routed tunnel
   for nexthop node protection in the case of a link or node failure.

Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC-2119 [5].

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1. Introduction

   As currently specified in [1], the LDP protocol only needs to know
   label mapping for the adjacent peers and there is no way for an LSR
   to learn the adjacent peer's downstream label mapping. This document
   proposes an LDP extension that allows an LSR to discover the
   next-nexthop label mapping from its downstream peers.

   One application for learning the next-nexthop label mapping is
   for fast re-route. Similar to the facility based node-protection
   of LSP Fast ReRoute [2], the NFRR (Nexthop Fast ReRoute) [3] scheme
   allows an LSR to perform Fast ReRoute on any type of traffic,
   including LDP LSP traffic. When the Nexthop Fast ReRoute is used
   for node-protection of LDP LSP traffic, the next-nexthop labels are
   needed to tunnel the data traffic into the next-nexthop LSR in the
   case of a link or node failure.

   A new Status TLV code is specified for an LSR to indicate its
   interest in receiving the next-nexthop label mapping information.
   A new Next-Nexthop Label TLV is specified to pass the downstream
   label mapping to the upstream LSR in the Label Mapping Message.

   The extension specified in this document assumes the next-nexthop
   nodes use platform-wide label space for LDP. It is outside the
   scope of this document when the next-nexthop nodes use
   per-interface label space.

2. LDP Next-Nexthop Label Mapping Scheme

2.1 Example

   Confiser LSRs interconnected with LDP as the following:

                 |    lsp2  | (link-protection)
                 |          V
                 |    ||=>c[R3]
                 |    ||
       [R1]====>[R2]a=||                  X
                 |    ||                 /
                 |    ||===>b[R4]====>[R5]
                 |      (x2)     (x1)   ^
                 |                      |
                 |  (z1)        (z2)    |
                   lsp1 (node-protection)

       Figure 1: NFRR node-protection for LDP data traffic

   R2 is the PLR (Point of Local Repair) node, the lsp1 is the

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   NFRR [3] LSP for the purpose of protecting node R4 over R2's
   interface "a". The lsp2 is the NFRR LSP for link protection in
   the case of interface "a" or "c" is down. We will only be
   concerned with node-protection using lsp1 in this document.

   R5 advertises FEC X to R4 with label x1. R4 advertises the same
   FECs with label x2 to upstream peer R2. The RSVP signaled lsp1
   uses label z1 from R2 to R6 and z2 from R6 to R5, and z2 can
   also be an implicit null.

   When R2 detects either the interface "a" is down, or the nexthop
   "b" is unreachable, or LSR R4 is down, the forwarding engine on
   R2 will re-direct the LDP data traffic into the NFRR tunnel lsp1.
   This can be quickly done by pushing the label x1 onto the label
   stack and send the packet through the lsp1 for LDP data traffic
   going to FEC X. As long as the platform-wide label space is
   used on LSR R5, the R5 does not even know the difference. In
   this case, the next-nexthop label x1 is used by PLR node R2
   for fast re-route with node-protection. For this scheme to work,
   LSR R4 needs to advertise the next-nexthop label x1 to the
   upstream LSR R2 in addition to their own label mapping of x2 for
   the same FEC.

2.2 Next-Nexthop Label Request

   Take the same example as in section 2.1, a user can statically
   configure on LSR R4 that it needs to include downstream labels
   to all or some of the upstream peers while it advertises the
   label mappings. A better way is for LSR R2 to make a request
   to its peer R4 that it is interested in receiving the next-nexthop
   label mapping information, since R2 has already been configured
   to perform node-protection for LSR R4.

   When the LDP peer between R2 and R4 is up, and there is at least
   one NFRR lsp configured on R2 to perform node-protection of R4,
   R2 can optionally send a Notification Message with the Next-Nexthop
   Label Request bit set in the Status TLV. When the last NFRR LSP
   protecting node R4 is removed, R2 can optionally send the
   Notification Message to R4 with the Next-Nexthop Label Withdraw bit
   set in the Status TLV.

2.3 Next-Nexthop Label Advertisement

   When an LSR advertises the FEC-label bindings to its peer, if it
   has received the Next-Nexthop Label Request from that peer or the
   LSR is configured with this capability, it SHOULD include the
   next-nexthop label mapping information when applicable in the
   Label Mapping Message.

   An optional Next-Nexthop Label TLV is defined to be used in the
   Label Mapping Message. The Next-Nexthop Label contains a list

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   of (label, downstream router-id) tuples. More than one tuple
   can be used when there is an ECMP case to different downstream
   nodes for the same FECs. It is an implementation and local
   configuration issue whether to announce only one or multiple
   tuples in the ECMP case.

   If some FECs are not advertised with next-nexthop labels, then
   no node-protection can be performed on those FECs. But they
   can still be fast re-routed with NFRR link-protection scheme [3].
   If there is a NFRR LSP built from R2 to R4, then the LDP data
   traffic will be re-routed directly onto R4 itself. The
   node-protection is not meant for all the situations. Usually
   node-protection is used in the backbone portion of the network,
   and link-protection is used close to the edge of the network.

2.4 Next-Nexthop Label Update

   If an LSR advertises the Next-Nexthop Label TLV in the Label
   Mapping Messages, and when the next-nexthop label information
   changes, it MUST re-send the Label Mapping Message with updated
   next-nexthop label information. The LSR SHOULD implement a means
   to dampen the re-advertisement to avoid potentially excessive
   updating due to link flapping.

3. Next-Nexthop Label Packet Encoding

3.1 Next-Nexthop Label Bits in Status TLV

   The Next-Nexthop Label Request/Withdraw information is sent in the
   Notification Message. Two bits (to be allocated by IANA) are
   defined in this document, one for Request and one for Withdraw.
   Unlike most of the bits already defined in the Status TLV,
   the Next-Nexthop Label Bits are used by an LSR to dynamically
   announce a capability to its peers.

   The E bit and F bit MUST be set to zero if Next-Nexthop Label
   Request or Withdraw is the only status code set. The Next-Nexthop
   Label Bits SHOULD only be used in Notification Message, otherwise
   it MUST be quietly ignored upon receipt.

3.2 Next-Nexthop Label TLV in Label Mapping Message

   The Next-Nexthop Label TLV can be optionally carried in the Optional
   Parameters field of a Label Mapping Message. The TLV consists a
   list of (label, router-id) pairs with the following format:

    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| Next-Nexthop Label (IANA) |   Length (4 + N * 8 bytes)    |

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   |     NNhop-Label 1                                             |
   |     NNhop Router-ID 1                                         |
   //                                                              //
   //                                                              //
   |     NNhop-Label N                                             |
   |     NNhop Router-ID N                                         |

      Next-Nexthop Label. This is a 20-bit label value as specified
      in [4] represented as a 20-bit number in a 4 octet field.

   NNhop Router-ID
      Next-Nexthop router-ID which advertised that next-nexthop label.
      This is a 4 octet number.

4. Security Considerations

   This mechanism does not introduce any new security issue in LDP.

5. IANA Considerations

   Two new bits in Status TLV and a new LDP TLV Type is defined in
   section 3.  This LDP extension requires that IANA allocate those

6. Acknowledgments


7. References

   [1]  Andersson, L., Doolan, P., Feldman, N., Fredette, A. and B.
        Thomas, "LDP Specification", RFC 3036, January 2001.

   [2]  Pan, P., Gan, D., Swallow, G., Vasseur, J.Ph., Copper, D.,
        Atlas, A., Jork, M., "Fast Reroute Technique in RSVP-TE",
        Internet draft, draft-ietf-mpls-rsvp-lsp-fastreroute-06.txt,
        work in progress.

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   [3]  Shen, N., Pan, P., "Nexthop Fast ReRoute for IP and MPLS",
        Internet draft, draft-shen-nhop-fastreroute-01.txt, work in

   [4]  Rosen, E., Tappan, D., Federkow, G., Rekhter, Y., Farinacci, D.,
        Li, T. and A. Conta, "MPLS Label Stack Encoding", RFC 3032,
        January 2001.

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

8. Authors' Addresses

   Naiming Shen
   Redback Networks, Inc.
   300 Holger Way
   San Jose, CA 95134

   Enke Chen
   Redback Networks, Inc.
   300 Holger Way
   San Jose, CA 95134

   Albert Tian
   Redback Networks, Inc.
   300 Holger Way
   San Jose, CA 95134

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