path: root/doc/rfc/rfc6389.txt

Internet Engineering Task Force (IETF)                       R. Aggarwal
Request for Comments: 6389                              Juniper Networks
Category: Standards Track                                    JL. Le Roux
ISSN: 2070-1721                                                   Orange
                                                           November 2011


                 MPLS Upstream Label Assignment for LDP

Abstract

   This document describes procedures for distributing upstream-assigned
   labels for the Label Distribution Protocol (LDP).  It also describes
   how these procedures can be used for avoiding branch Label Switching
   Router (LSR) traffic replication on a LAN for LDP point-to-multipoint
   (P2MP) Label Switched Paths (LSPs).

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6389.

Copyright Notice

   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.






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   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.

Table of Contents

   1. Introduction ....................................................3
   2. Specification of Requirements ...................................3
   3. LDP Upstream Label Assignment Capability ........................3
   4. Distributing Upstream-Assigned Labels in LDP ....................4
      4.1. Procedures .................................................4
   5. LDP Tunnel Identifier Exchange ..................................5
   6. LDP Point-to-Multipoint LSPs on a LAN ...........................9
   7. IANA Considerations ............................................11
      7.1. LDP TLVs ..................................................11
      7.2. Interface Type Identifiers ................................11
   8. Security Considerations ........................................12
   9. Acknowledgements ...............................................12
   10. References ....................................................12
      10.1. Normative References .....................................12
      10.2. Informative References ...................................13






















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1.  Introduction

   This document describes procedures for distributing upstream-assigned
   labels [RFC5331] for Label Distribution Protocol (LDP) [RFC5036].
   These procedures follow the architecture for MPLS upstream label
   assignment described in [RFC5331].

   This document describes extensions to LDP that a Label Switching
   Router (LSR) can use to advertise whether the LSR supports upstream
   label assignment to its neighboring LSRs.

   This document also describes extensions to LDP to distribute
   upstream-assigned labels.

   The usage of MPLS upstream label assignment using LDP to avoid branch
   LSR traffic replication on a LAN for LDP point-to-multipoint (P2MP)
   Label Switched Paths (LSPs) [RFC6388] is also described.

2.  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 [RFC2119].

3.  LDP Upstream Label Assignment Capability

   According to [RFC5331], upstream-assigned label bindings MUST NOT be
   used unless it is known that a downstream LSR supports them.  This
   implies that there MUST be a mechanism to enable an LSR to advertise
   to its LDP neighbor LSR(s) its support of upstream-assigned labels.

   A new Capability Parameter, the LDP Upstream Label Assignment
   Capability, is introduced to allow an LDP peer to exchange with its
   peers, its support of upstream label assignment.  This parameter
   follows the format and procedures for exchanging Capability
   Parameters defined in [RFC5561].

   Following is the format of the LDP Upstream Label Assignment
   Capability Parameter:

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |1|0| Upstrm Lbl Ass Cap(0x0507)|      Length (= 1)             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |1| Reserved    |
      +-+-+-+-+-+-+-+-+




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   If an LSR includes the Upstream Label Assignment Capability in LDP
   Initialization messages, it implies that the LSR is capable of both
   distributing upstream-assigned label bindings and receiving upstream-
   assigned label bindings.  The reserved bits MUST be set to zero on
   transmission and ignored on receipt.  The Upstream Label Assignment
   Capability Parameter MUST be carried only in LDP Initialization
   messages and MUST be ignored if received in LDP Capability messages.

4.  Distributing Upstream-Assigned Labels in LDP

   An optional LDP TLV, Upstream-Assigned Label Request TLV, is
   introduced.  To request an upstream-assigned label, an LDP peer MUST
   include this TLV in a Label Request message.

       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|Upstrm-Ass Lbl Req (0x0205)|      Length                   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                       Reserved                                |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   An optional LDP TLV, Upstream-Assigned Label TLV, is introduced to
   signal an upstream-assigned label.  Upstream-Assigned Label TLVs are
   carried by the messages used to advertise, release, and withdraw
   upstream-assigned label mappings.

       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| Upstrm-Ass Label (0x0204) |      Length                   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                         Reserved                              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                   Label                                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The Label field is a 20-bit label value as specified in [RFC3032],
   represented as a 20-bit number in a 4-octet field as specified in
   Section 3.4.2.1 of RFC 5036 [RFC5036].

4.1.  Procedures

   Procedures for Label Mapping, Label Request, Label Abort, Label
   Withdraw, and Label Release follow [RFC5036] other than the
   modifications pointed out in this section.





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   An LDP LSR MUST NOT distribute the Upstream-Assigned Label TLV to a
   neighboring LSR if the neighboring LSR has not previously advertised
   the Upstream Label Assignment Capability in its LDP Initialization
   messages.  An LDP LSR MUST NOT send the Upstream-Assigned Label
   Request TLV to a neighboring LSR if the neighboring LSR has not
   previously advertised the Upstream Label Assignment Capability in its
   LDP Initialization messages.

   As described in [RFC5331], the distribution of upstream-assigned
   labels is similar to either ordered LSP control or independent LSP
   control of the downstream-assigned labels.

   When the label distributed in a Label Mapping message is an upstream-
   assigned label, the Upstream-Assigned Label TLV MUST be included in
   the Label Mapping message.  When an LSR receives a Label Mapping
   message with an Upstream-Assigned Label TLV and it does not recognize
   the TLV, it MUST generate a Notification message with a status code
   of "Unknown TLV" [RFC5036].  If it does recognize the TLV but is
   unable to process the upstream label, it MUST generate a Notification
   message with a status code of "No Label Resources".  If the Label
   Mapping message was generated in response to a Label Request message,
   the Label Request message MUST contain an Upstream-Assigned Label
   Request TLV.  An LSR that generates an upstream-assigned label
   request to a neighbor LSR, for a given FEC, MUST NOT send a
   downstream label mapping to the neighbor LSR for that FEC unless it
   withdraws the upstream-assigned label binding.  Similarly, if an LSR
   generates a downstream-assigned label request to a neighbor LSR, for
   a given FEC, it MUST NOT send an upstream label mapping to that LSR
   for that FEC, unless it aborts the downstream-assigned label request.

   The Upstream-Assigned Label TLV may be optionally included in Label
   Withdraw and Label Release messages that withdraw/release a
   particular upstream-assigned label binding.

5.  LDP Tunnel Identifier Exchange

   As described in [RFC5331], a specific upstream LSR (Ru) MAY transmit
   an MPLS packet, the top label of which (L) is upstream assigned, to
   its downstream neighbor LSR (Rd).  In this case, the fact that L is
   upstream assigned is determined by Rd by the tunnel on which the
   packet is received.  There must be a mechanism for Ru to inform Rd
   that a particular tunnel from Ru to Rd will be used by Ru for
   transmitting MPLS packets with upstream-assigned MPLS labels.

   When LDP is used for upstream label assignment, the Interface ID TLV
   [RFC3472] is used for signaling the Tunnel Identifier.  If Ru uses an
   IP or MPLS tunnel to transmit MPLS packets with upstream assigned
   labels to Rd, Ru MUST include the Interface ID TLV in the Label



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   Mapping messages along with the Upstream-Assigned Label TLV.  The
   IPv4/IPv6 Next/Previous Hop Address and the Logical Interface ID
   fields in the Interface ID TLV SHOULD be set to 0 by the sender and
   ignored by the receiver.  The Length field indicates the total length
   of the TLV, i.e., 4 + the length of the Value field in octets.  A
   Value field whose length is not a multiple of four MUST be zero-
   padded so that the TLV is four-octet aligned.

   Hence the IPv4 Interface ID TLV has 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|     Type (0x082d)         |             Length            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                 IPv4 Next/Previous Hop Address (0)            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     Logical Interface ID (0)                  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                           Sub-TLVs                            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The IPv6 Interface ID TLV has 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|     Type (0x082e)         |             Length            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                 IPv6 Next/Previous Hop Address (0)            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     Logical Interface ID (0)                  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                           Sub-TLVs                            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   As shown in the above figures, the Interface ID TLV carries sub-TLVs.
   Four new Interface ID sub-TLVs are introduced to support RSVP -
   Traffic Engineering (RSVP-TE) P2MP LSPs, LDP P2MP LSPs, IP Multicast
   Tunnels, and context labels.  The sub-TLV value in the sub-TLV acts
   as the tunnel identifier.










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   The following sub-TLVs are introduced:

   1. RSVP-TE P2MP LSP TLV (Type = 28)

   The value of the TLV is the RSVP-TE P2MP LSP SESSION Object
   [RFC4875].

   Below is the RSVP-TE P2MP LSP TLV format when carried in the IPv4
   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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    Type (0x1c)                |  16                           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                       P2MP ID                                 |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  MUST be zero                 |      Tunnel ID                |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      Extended Tunnel ID                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Below is the RSVP-TE P2MP LSP TLV format when carried in the IPv6
   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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    Type (0x1c)                |  28                           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                       P2MP ID                                 |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  MUST be zero                 |      Tunnel ID                |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      Extended Tunnel ID                       |
      |                                                               |
      |                             .......                           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   This TLV identifies the RSVP-TE P2MP LSP.  It allows Ru to tunnel an
   "inner" LDP P2MP LSP, the label for which is upstream assigned, over
   an "outer" RSVP-TE P2MP LSP that has leaves <Rd1...Rdn>.  The RSVP-TE
   P2MP LSP IF_ID TLV allows Ru to signal to <Rd1...Rdn> the binding of
   the inner LDP P2MP LSP to the outer RSVP-TE P2MP LSP.  The control-
   plane signaling between Ru and <Rd1...Rdn> for the inner P2MP LSP
   uses targeted LDP signaling messages.





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   2. LDP P2MP LSP TLV (Type = 29)

   The value of the TLV is the LDP P2MP FEC as defined in [RFC6388] and
   has to be set as per the procedures in [RFC6388].  Here is the format
   of the LDP P2MP FEC as defined in [RFC6388]:

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |P2MP Type      |        Address Family         | Address Length|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ~                       Root Node Address                       ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    Opaque Length              |    Opaque Value ...           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
      ~                                                               ~
      |                                                               |
      |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The Address Family MUST be set to IPv4, the Address Length MUST be
   set to 4, and the Root Node Address MUST be set to an IPv4 address
   when the LDP P2MP LSP TLV is carried in the IPv4 Interface ID TLV.
   The Address Family MUST be set to IPv6, the Address Length MUST be
   set to 16, and the Root Node Address MUST be set to an IPv6 address
   when the LDP P2MP LSP TLV is carried in the IPv6 Interface ID TLV.

   The TLV value identifies the LDP P2MP LSP.  It allows Ru to tunnel an
   inner LDP P2MP LSP, the label for which is upstream assigned, over an
   outer LDP P2MP LSP that has leaves <Rd1...Rdn>.  The LDP P2MP LSP
   IF_ID TLV allows Ru to signal to <Rd1...Rdn> the binding of the inner
   LDP P2MP LSP to the outer LDP-P2MP LSP.  The control-plane signaling
   between Ru and <Rd1...Rdn> for the inner P2MP LSP uses targeted LDP
   signaling messages.

   3. IP Multicast Tunnel TLV (Type = 30)

   In this case, the TLV value is a <Source Address, Multicast Group
   Address> tuple.  Source Address is the IP address of the root of the
   tunnel (i.e., Ru), and Multicast Group Address is the Multicast Group
   Address used by the tunnel.  The addresses MUST be IPv4 addresses
   when the IP Multicast Tunnel TLV is included in the IPv4 Interface ID
   TLV.  The addresses MUST be IPv6 addresses when the IP Multicast
   Tunnel TLV is included in the IPv6 Interface ID TLV.






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   4. MPLS Context Label TLV (Type = 31)

   In this case, the TLV value is a <Source Address, MPLS Context Label>
   tuple.  The Source Address belongs to Ru and the MPLS Context Label
   is an upstream-assigned label, assigned by Ru.  The Source Address
   MUST be set to an IPv4 address when the MPLS Context Label TLV is
   carried in the IPv4 Interface ID TLV.  The Source Address MUST be set
   to an IPv6 address when the MPLS Context Label TLV is carried in the
   IPv6 Interface ID TLV.  This allows Ru to tunnel an inner LDP P2MP
   LSP, the label of which is upstream assigned, over an outer one-hop
   MPLS LSP, where the outer one-hop LSP has the following property:

      o  The label pushed by Ru for the outer MPLS LSP is an upstream-
         assigned context label, assigned by Ru.  When <Rd1...Rdn>
         perform an MPLS label lookup on this label, a combination of
         this label and the incoming interface MUST be sufficient for
         <Rd1...Rdn> to uniquely determine Ru's context-specific label
         space to look up the next label on the stack.  <Rd1...Rdn> MUST
         receive the data sent by Ru with the context-specific label
         assigned by Ru being the top label on the label stack.

   Currently, the usage of the Context Label TLV is limited only to LDP
   P2MP LSPs on a LAN as specified in the next section.  The Context
   Label TLV MUST NOT be used for any other purpose.

   Note that when the outer P2MP LSP is signaled with RSVP-TE or MLDP,
   the above procedures assume that Ru has a priori knowledge of all the
   <Rd1, ... Rdn>.  In the scenario where the outer P2MP LSP is signaled
   using RSVP-TE, Ru can obtain this information from RSVP-TE.  However,
   in the scenario where the outer P2MP LSP is signaled using MLDP, MLDP
   does not provide this information to Ru.  In this scenario, the
   procedures by which Ru could acquire this information are outside the
   scope of this document.

6.  LDP Point-to-Multipoint LSPs on a LAN

   This section describes one application of upstream label assignment
   using LDP.  Further applications are to be described in separate
   documents.

   [RFC6388] describes how to setup P2MP LSPs using LDP.  On a LAN the
   solution relies on "ingress replication".  An LSR on a LAN, that is a
   branch LSR for a P2MP LSP (say Ru), sends a separate copy of a packet
   that it receives on the P2MP LSP to each of the downstream LSRs on
   the LAN (say <Rd1...Rdn>) that are adjacent to it in the P2MP LSP.

   It is desirable for Ru to send a single copy of the packet for the
   LDP P2MP LSP on the LAN, when there are multiple downstream routers



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   on the LAN that are adjacent to Ru in that LDP P2MP LSP.  This
   requires that each of <Rd1...Rdn> must be able to associate the label
   L, used by Ru to transmit packets for the P2MP LSP on the LAN, with
   that P2MP LSP.  It is possible to achieve this using LDP upstream-
   assigned labels with the following procedures.

   Consider an LSR Rd that receives the LDP P2MP FEC [RFC6388] from its
   downstream LDP peer.  Additionally, consider that the upstream
   interface to reach LSR Ru that is the next hop to the P2MP LSP root
   address (Pr) in the LDP P2MP FEC is a LAN interface (Li) and that Rd
   and Ru support upstream-assigned labels.  In this case, instead of
   sending a Label Mapping message as described in [RFC6388], Rd sends a
   Label Request message to Ru.  This Label Request message MUST contain
   an Upstream-Assigned Label Request TLV.

   On receiving this message, Ru sends back a Label Mapping message to
   Rd with an upstream-assigned label.  This message also contains an
   Interface ID TLV with an MPLS Context Label sub-TLV, as described in
   the previous section, with the value of the MPLS label set to a value
   assigned by Ru on interface Li as specified in [RFC5331].  Processing
   of the Label Request and Label Mapping messages for LDP upstream-
   assigned labels is as described in Section 4.1.  If Ru receives a
   Label Request for an upstream-assigned label for the same P2MP FEC
   from multiple downstream LSRs on the LAN, <Rd1...Rdn>, it MUST send
   the same upstream-assigned label to each of <Rd1...Rdn>.

   Ru transmits the MPLS packet using the procedures defined in
   [RFC5331] and [RFC5332].  The MPLS packet transmitted by Ru contains
   as the top label the context label assigned by Ru on the LAN
   interface, Li.  The bottom label is the upstream label assigned by Ru
   to the LDP P2MP LSP.  The top label is looked up in the context of
   the LAN interface (Li) by a downstream LSR on the LAN.  This lookup
   enables the downstream LSR to determine the context-specific label
   space in which to look up the inner label.

   Note that <Rd1...Rdn> may have more than one equal-cost next hop on
   the LAN to reach Pr.  It MAY be desirable for all of them to send the
   label request to the same upstream LSR and they MAY select one
   upstream LSR using the following procedure:

   1. The candidate upstream LSRs are numbered from lower to higher IP
      address.

   2. The following hash is performed: H = (Sum Opaque value) modulo N,
      where N is the number of candidate upstream LSRs.  The Opaque
      value is defined in [RFC6388] and comprises the P2MP LSP
      identifier.




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   3. The selected upstream LSR U is the LSR that has the number H.

   This allows for load balancing of a set of LSPs among a set of
   candidate upstream LSRs, while ensuring that on a LAN interface, a
   single upstream LSR is selected.  It is also to be noted that the
   procedures in this section can still be used by Rd and Ru if other
   LSRs on the LAN do not support upstream label assignment.  Ingress
   replication and downstream label assignment will continue to be used
   for LSRs that do not support upstream label assignment.

7.  IANA Considerations

7.1.  LDP TLVs

   IANA maintains a registry of LDP TLVs at the registry "Label
   Distribution Protocol" in the sub-registry called "TLV Type Name
   Space".

   This document defines a new LDP Upstream Label Assignment Capability
   TLV (Section 3).  IANA has assigned the value 0x0507 to this TLV.

   This document defines a new LDP Upstream-Assigned Label TLV (Section
   4).  IANA has assigned the type value of 0x204 to this TLV.

   This document defines a new LDP Upstream-Assigned Label Request TLV
   (Section 4).  IANA has assigned the type value of 0x205 to this TLV.

7.2.  Interface Type Identifiers

   [RFC3472] defines the LDP Interface ID IPv4 and IPv6 TLV.  These top-
   level TLVs can carry sub-TLVs dependent on the interface type.  These
   sub-TLVs are assigned "Interface ID Types".  IANA maintains a
   registry of Interface ID Types for use in GMPLS in the registry
   "Generalized Multi-Protocol Label Switching (GMPLS) Signaling
   Parameters" and sub-registry "Interface_ID Types".  IANA has made the
   corresponding allocations from this registry as follows:

      -  RSVP-TE P2MP LSP TLV (value 28)

      -  LDP P2MP LSP TLV (value 29)

      -  IP Multicast Tunnel TLV (value 30)

      -  MPLS Context Label TLV (value 31)







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8.  Security Considerations

   The security considerations discussed in [RFC5036], [RFC5331], and
   [RFC5332] apply to this document.

   More detailed discussion of security issues that are relevant in the
   context of MPLS and GMPLS, including security threats, related
   defensive techniques, and the mechanisms for detection and reporting,
   are discussed in "Security Framework for MPLS and GMPLS Networks"
   [RFC5920].

9.  Acknowledgements

   Thanks to Yakov Rekhter for his contribution.  Thanks to Ina Minei
   and Thomas Morin for their comments.  The hashing algorithm used on
   LAN interfaces is taken from [RFC6388].  Thanks to Loa Andersson,
   Adrian Farrel, and Eric Rosen for their comments and review.

10.  References

10.1.  Normative References

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

   [RFC5036] Andersson, L., Ed., Minei, I., Ed., and B. Thomas, Ed.,
             "LDP Specification", RFC 5036, October 2007.

   [RFC4875] Aggarwal, R., Ed., Papadimitriou, D., Ed., and S. Yasukawa,
             Ed., "Extensions to Resource Reservation Protocol - Traffic
             Engineering (RSVP-TE) for Point-to-Multipoint TE Label
             Switched Paths (LSPs)", RFC 4875, May 2007.

   [RFC5331] Aggarwal, R., Rekhter, Y., and E. Rosen, "MPLS Upstream
             Label Assignment and Context-Specific Label Space", RFC
             5331, August 2008.

   [RFC5332] Eckert, T., Rosen, E., Ed., Aggarwal, R., and Y. Rekhter,
             "MPLS Multicast Encapsulations", RFC 5332, August 2008.

   [RFC5561] Thomas, B., Raza, K., Aggarwal, S., Aggarwal, R., and JL.
             Le Roux, "LDP Capabilities", RFC 5561, July 2009.

   [RFC6388] Wijnands, IJ., Ed., Minei, I., Ed., Kompella, K., and B.
             Thomas, "Label Distribution Protocol Extensions for Point-
             to-Multipoint and Multipoint-to-Multipoint Label Switched
             Paths", RFC 6388, November 2011.




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RFC 6389         MPLS Upstream Label Assignment for LDP    November 2011


10.2.  Informative References

   [RFC3032] Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
             Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack
             Encoding", RFC 3032, January 2001.

   [RFC3472] Ashwood-Smith, P., Ed., and L. Berger, Ed., "Generalized
             Multi-Protocol Label Switching (GMPLS) Signaling
             Constraint-based Routed Label Distribution Protocol
             (CR-LDP) Extensions", RFC 3472, January 2003.

   [RFC5920] Fang, L., Ed., "Security Framework for MPLS and GMPLS
             Networks", RFC 5920, July 2010.

Author's Address

   Rahul Aggarwal
   Juniper Networks
   1194 North Mathilda Ave.
   Sunnyvale, CA 94089
   Phone: +1-408-936-2720
   EMail: raggarwa_1@yahoo.com


   Jean-Louis Le Roux
   France Telecom
   2, avenue Pierre-Marzin
   22307 Lannion Cedex
   France
   EMail: jeanlouis.leroux@orange-ftgroup.com





















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