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Network Working Group                                        R. Aggarwal
Internet Draft                                          Juniper Networks
Expiration Date: July 2005
                                                              Y. Rekhter
                                                        Juniper Networks

                                                                E. Rosen
                                                     Cisco Systems, Inc.

                                                            January 2005


    MPLS Upstream Label Assignment and Context Specific Label Space


               draft-raggarwa-mpls-upstream-label-00.txt

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Abstract

   RFC 3031 limits the MPLS architecture to downstream assigned MPLS
   labels.  This document introduces the notion of upstream assigned
   MPLS labels. It describes the procedures for upstream MPLS label
   assignment and introduces the concept of a "Context-Specific Label
   Space".




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Table of Contents

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 1          Specification of requirements  .........................   2
 2          Introduction  ..........................................   2
 3          Context-Specific Label Space  ..........................   3
 4          Upstream Label Assignment  .............................   4
 4.1        Upstream Assigned and Downstream Assigned Labels  ......   4
 5          Assigning Upstream Assigned Labels  ....................   5
 6          Distributing Upstream Assigned Labels  .................   5
 7          Upstream Neighbor Label Space and Tunnel Label Space  ..   6
 8          Usage of Upstream Assigned Labels  .....................   7
 9          References  ............................................   7
 9.1        Normative References  ..................................   7
 9.2        Informative References  ................................   8
10          Author Information  ....................................   8
11          Intellectual Property Statement  .......................   8
12          Full Copyright Statement  ..............................   9




1. 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].


2. Introduction

   RFC 3031 [RFC3031] limits the MPLS architecture to downstream
   assigned MPLS labels. To quote from RFC 3031:

   "In the MPLS architecture, the decision to bind a particular label L
   to a particular FEC F is made by the LSR which is DOWNSTREAM with
   respect to that binding.  The downstream LSR then informs the
   upstream LSR of the binding.  Thus labels are "downstream-assigned",
   and label bindings are distributed in the "downstream to upstream"
   direction."

   MPLS upstream label assignment has been discussed and mentioned
   before [RFC3353, MVPN]. However the architecture for MPLS upstream
   label assignment and the associated procedures have not been
   described. This document introduces the notion of upstream assigned
   MPLS labels to the MPLS architecture. The procedures for upstream



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   MPLS label assignment are described.

   RFC 3031 describes per-platform and per-interface label space.  This
   document generalizes the latter to a "Context-Specific Label Space"
   and describes a "Neighbor Label Space" as an example of this.
   Upstream assigned labels are always looked up in a context-specific
   label space.


3. Context-Specific Label Space

   RFC 3031 describes per-platform and per-interface label spaces. This
   document introduces the more general concept of a "Context-Specific
   Label Space". A LSR may contain one or more context-specific label
   spaces. In  general, labels are looked  up in the  per-platform label
   space unless something about the context determines that a label be
   looked up in a particular context-specific label space.

   One example of  a context-specific label space is  the per-interface
   label space  discussed in  RFC 3031. When a MPLS packet is received
   over a particular interface the top label of the packet may need to
   be looked up in the receiving interface's per-interface label space.
   In this case the receiving interface determines the context of the
   packet. Whether MPLS packets  received over a particular  interface
   need  to  have  their  top  labels  looked  up  in  a per-interface
   label space depends on some characteristic or configuration of the
   interface.

   There may be more than one kind of context-specific label space.
   Context-specific label spaces can be used for downstream assigned
   labels or upstream assigned labels. Per-interface label space
   [RFC3031] is an example of a context-specific label space used for
   downstream assigned labels.

   When MPLS labels are upstream assigned the context of a MPLS label L
   is provided by the LSR that assigns the label and binds the label to
   a FEC F for a LSP LSP1. The LSR that assigns the label distributes
   the binding and context to a LSR Lr that then receives MPLS packets
   on LSP1 with label L. When Lr receives a MPLS packet on LSP1 it MUST
   be able to determine the context of this packet.

   An example of such a context is a Tunnel over which MPLS packets on
   LSP1 may be received and in this case the top label of the MPLS
   packet is looked up in a "Tunnel Specific Label Space". This does
   imply that Lr be able to determine the tunnel over which the packet
   was received. If the tunnel is a MPLS tunnel, penultimate-hop-popping
   (PHP) must be disabled for the tunnel. Another example of such a con-
   text is the neighbor from which MPLS packets on LSP1 may be received



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   and in this case the top label of the MPLS packet is looked up in a
   "Neighbor Specific Label Space". These are further described in sec-
   tion 7.

   There may be other sorts of contexts as well. For instance, we define
   the notion of a MPLS label being used to establish a context, i.e.
   identify a label space.


4. Upstream Label Assignment

   When two MPLS LSRs are adjacent in a MPLS label switched path (LSP)
   one of them can be termed an "upstream LSR" and the other a "down-
   stream LSR" [RFC3031]. Consider two LSRs, Ru and Rd that have agreed
   to bind Label L to a Forwarding Equivalence Class (FEC), F, for pack-
   ets sent from Ru to Rd.  Then with respect to this binding, Ru is the
   "upstream LSR", and Rd is the "downstream LSR"."

   When the label  binding for F is first made  by Rd and distributed by
   Rd to Ru,  the binding  is said  to be  "downstream assigned". When
   the label  binding for F is first made  by Ru and distributed  by Ru
   to Rd, the binding is said to be "upstream assigned".

   An important observation is that the downstream LSR Rd that receives
   MPLS packets with a top label L is not the LSR that assigns and dis-
   tributes label L. Rd must use a context-specific label space to
   lookup label L as described in section 7.



4.1. Upstream Assigned and Downstream Assigned Labels

   It is possible that some LSRs on a LSP for FEC F, distribute down-
   stream assigned label bindings for FEC F, while other LSRs distribute
   upstream assigned label bindings. It is possible for a LSR to dis-
   tribute a downstream assigned label binding for FEC F to its upstream
   adjacent LSR AND distribute an upstream assigned label binding for
   FEC F to its downstream adjacent LSR.  Two adjacent LSRs for a LSP
   that is bound to FEC F, MUST use either downstream assigned label
   distribution or upstream assigned label distribution, for FEC F, but
   NOT both. How these LSRs will determine which of the two is to be
   used is outside the scope of this document.









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5. Assigning Upstream Assigned Labels

   The only requirement on an upstream LSR assigning upstream assigned
   labels is that an upstream assigned label must be unambiguous in the
   context-specific label space in which the downstream LSR will look it
   up.  The LSR could ssign the same label value to both a downstream-
   assigned and an upstream-assigned label. The downstream LSR always
   looks up upstream assigned MPLS labels in a context-specific label
   space as described in section 7.

   An entry for the upstream assigned labels is not created in the
   Incoming Label Map (ILM) [RFC3031] at the upstream LSR as these
   labels are not incoming labels.  Instead an upstream label is an out-
   going label, with respect to the upstream LSR, for MPLS packets
   transmitted on the MPLS LSP in which the upstream LSR is adjacent to
   the downstream LSR. Hence an upstream label is part of a Next Hop
   Label Forwarding Entry (NHLFE) at the upstream LSR.

   When Ru advertises a binding of label L for FEC F to Rd, it creates a
   NHLFE entry corresponding to L. This NHLFE entry results in imposing
   the label L on the MPLS label stack of the packet forwarded using the
   NHLFE entry.  If Ru is a transit router on the LSP for FEC F, it
   binds the ILM for the LSP to this NHLFE. If Ru is an ingress router
   on the LSP for FEC F, it binds the FEC to the NHLFE entry.


6. Distributing Upstream Assigned Labels

   Upstream-assigned label bindings MUST NOT be used unless it is known
   that the downstream LSR supports them. How this is known is outside
   the scope of this document.

   MPLS upstream label assignment requires a label distribution protocol
   to distribute the binding from the upstream LSR to the downstream
   LSR.  Considerations that pertain to a label distribution protocol
   that are described in [RFC3031] apply.

   The distribution of the upstream-assigned labels is similar to either
   the ordered LSP control or independent LSP control of the downstream-
   assigned labels. In the former case a LSR distributes an upstream-
   assigned label binding for a FEC F if it is either (a) the ingress
   LSR for FEC F, or (b) if it has already received an upstream label
   binding for that FEC from its adjacent upstream LSR for FEC F, or (c)
   if it has received a request for a downstream label binding from its
   upstream adjacent LSR.  In the latter case each LSR, upon noting that
   it recognizes a particular FEC, makes an independent decision to bind
   an upstream-assigned label to that FEC and to distribute that binding
   to its label distribution peers.



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7. Upstream Neighbor Label Space and Tunnel Label Space

   If the top  label of an MPLS packet is  upstream assigned, when the
   packet  is  received  by  LSR  Rd  the  label  is  looked  up  in  a
   context-specific label space, not in a per-platform label space.

   One type of context-specific  label space is the "neighbor-specific"
   label  space.  Rd may maintain no  more than  one neighbor-specific
   label space for each of its neighbors.

   Rd uses a context-specific label space that it maintains for Ru to
   "reserve" MPLS labels assigned by Ru. Hence if Ru distributes an
   upstream assigned label binding L for FEC F to Rd, then Rd reserves L
   in the separate ILM for Ru's context-specific label space. This is
   the ILM that Rd uses to lookup MPLS packets received from Ru, the top
   label of which is upstream assigned by Ru.

   This implies that Rd MUST be able to determine whether the top label
   of a received MPLS packet is upstream assigned and if yes, the "con-
   text" of this packet. How this determination is made depends on the
   mechanism that is used by Ru to transmit the MPLS packet with an
   upstream assigned top label L, to Rd. Ru may transmit this packet to
   Rd by encapsulating it directly in a data link frame or by transmit-
   ting it in an IP or MPLS tunnel.

   If Ru transmits this packet by encapsulating it in a data link frame,
   then whether L is upstream assigned or downstream assigned is deter-
   mined by Rd as described in [MPLS-MCAST-ENCAPS]. If L is upstream
   assigned then [MPLS-MCAST-ENCAPS] uses a different ether type in the
   data link frame.  Rd maintains a separate "Upstream Neighbor Label
   Space" for Ru. The "context" of this packet i.e. the upstream neigh-
   bor label space, in which L was reserved is determined by the data
   link header. For example if the data link header is ethernet, the
   source MAC address is used to determine the context.

   If Ru transmits this packet by encapsulating it in an IP or MPLS tun-
   nel, then the fact that L is upstream assigned is determined by Rd by
   the tunnel on which the packet is received. A given tunnel can be
   used for transmitting either downstream assigned MPLS packets or
   upstream assigned MPLS packets, or both. 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. The description of such a mechanism is outside the scope of
   this document. When Rd receives MPLS packets with a top label L on
   such a tunnel, it determines the "context" of this packet based on
   the tunnel that the packet is received on. In this case Rd may main-
   tain a separate "Tunnel Label Space" for the tunnel or may maintain a
   single "Upstream Neighbor Label Space" for Ru.



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   If the tunnel on which Rd receives MPLS packets with a top label L is
   a MPLS tunnel, then Rd determines a) That L is upstream assigned and
   b) The context for L, from the labels above L in the label stack.
   Note that one or more of these labels may also be upstream assigned
   labels.

   If the tunnel on which Rd receives MPLS packets with a top label L is
   an IP/GRE tunnel then Rd determines a) That L is upstream assigned
   [MPLS-MCAST-ENCAPS] and b) The context for L, from the source address
   in the IP header.



8. Usage of Upstream Assigned Labels

   A typical usage of upstream assigned labels is when an upstream LSR
   Ru is adjacent to more than downstream LSRs <Rd1...Rdn> in a LSP LSP1
   AND Ru is connected to <Rd1...Rdn> via a multi-access media or tunnel
   AND Ru wants to transmit a single copy of a MPLS packet on the LSP to
   <Rd1...Rdn>. In this case Ru can distribute an upstream assigned
   label L that is bound to the FEC for LSP1, to <Rd1..Rdn> and transmit
   a MPLS packet, the top label of which is L, on the multi-access media
   or tunnel. Each of <Rd1..Rdn> will then interpret this MPLS packet in
   the context of Ru and forward it appropriately.  This implies that
   <Rd1..Rdn> MUST all be able to support an Upstream Neighbor Label
   Space for Ru and Ru MUST be able to determine this. The mechanisms
   for determining this are specific to the application that is using
   upstream assigned labels and is outside the scope of this document.


9. References

9.1. Normative References

   [RFC3031] "MPLS Architecture", E. Rosen, A. Viswanathan, R. Callon,
   RFC 3031.

   [RFC2119] "Key words for use in RFCs to Indicate Requirement Lev-
   els.", Bradner, March 1997

   [MPLS-MCAST-ENCAPS] T. Eckert, E. Rosen, R. Aggarwal, Y. Rekhter,
   draft-rosen-mpls-codepoint-00.txt









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9.2. Informative References

   [MVPN] E. Rosen, R. Aggarwal [Editors], Multicast in BGP/MPLS VPNs"


10. Author Information

   Rahul Aggarwal
   Juniper Networks
   1194 North Mathilda Ave.
   Sunnyvale, CA 94089
   Email: rahul@juniper.net

   Yakov Rekhter
   Juniper Networks
   1194 North Mathilda Ave.
   Sunnyvale, CA 94089
   Email: yakov@juniper.net

   Eric C. Rosen
   Cisco Systems, Inc.
   1414 Massachusetts Avenue
   Boxborough, MA 01719
   Email: erosen@cisco.com



11. Intellectual Property Statement

   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; nor does it represent that it has
   made any independent effort to identify any such rights.  Information
   on the procedures with respect to rights in RFC documents can be
   found in BCP 78 and BCP 79.

   Copies of IPR disclosures made to the IETF Secretariat and any assur-
   ances of licenses to be made available, or the result of an attempt
   made to obtain a general license or permission for the use of such
   proprietary rights by implementers or users of this specification can
   be obtained from the IETF on-line IPR repository at
   http://www.ietf.org/ipr.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights that may cover technology that may be required to implement



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   this standard.  Please address the information to the IETF at ietf-
   ipr@ietf.org.



12. Full Copyright Statement

   Copyright (C) The Internet Society (2005).  This document is subject
   to the rights, licenses and restrictions contained in BCP 78 and
   except as set forth therein, the authors retain all their rights.

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
   INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFOR-
   MATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES
   OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

































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