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Versions: (draft-raggarwa-mpls-upstream-label) 00 01 02 03 04 05 06 07 RFC 5331

Network Working Group                                        R. Aggarwal
Internet Draft                                          Juniper Networks
Category: Standards Track
Expiration Date: October 2008                                 Y. Rekhter
                                                        Juniper Networks

                                                                E. Rosen
                                                     Cisco Systems, Inc.

                                                          April 30, 2008


    MPLS Upstream Label Assignment and Context-Specific Label Space


                 draft-ietf-mpls-upstream-label-05.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

 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  ......   5
 5          Assigning Upstream-Assigned Labels  ....................   5
 6          Distributing Upstream-Assigned Labels  .................   6
 7          Upstream Neighbor Label Space  .........................   6
 8          Context Label on LANs  .................................   9
 9          Usage of Upstream-Assigned Labels  .....................  10
10          IANA Considerations  ...................................  10
11          Security Considerations  ...............................  11
12          Acknowledgements  ......................................  11
13          References  ............................................  11
13.1        Normative References  ..................................  11
13.2        Informative References  ................................  11
14          Author's Address  ......................................  12
15          Intellectual Property Statement  .......................  12
16          Copyright Notice  ......................................  13






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 Forwarding Equivalence Class (FEC) F is made by the
   Label Switching Router (LSR) which is DOWNSTREAM with respect to that
   binding. The downstream LSR then informs the upstream LSR of the



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   binding. Thus labels are "downstream-assigned", and label bindings
   are distributed in the "downstream to upstream" direction."

   Upstream assignment of MPLS labels has been discussed and mentioned
   before [RFC3353, MVPN]. However the architecture for upstream
   assignment of MPLS labels 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
   assignment of MPLS labels 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 is 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.

   Per-interface label space [RFC3031] is an example of a context-
   specific label space used for downstream-assigned labels. Context-
   specific label spaces can also be used for upstream-assigned labels,
   as described below.

   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 Label Switched Path (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.




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   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, after tunnel decapsulation, is looked up in a label space
   that is specific to the root of the tunnel. This does imply that Lr
   be able to determine the tunnel over which the packet was received.
   Therefore, if the tunnel is a MPLS tunnel, penultimate-hop-popping
   (PHP) MUST be disabled for the tunnel.

   Another example of such a context is the neighbor from which MPLS
   packets on LSP1 may be received. In this case the top label of the
   MPLS packet, transmitted by the neighbor on LSP1, is looked up in a
   "Neighbor Specific Label Space".

   The above two examples are further described in section 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. A "context label" is one which identifies a
   label table in which the label immediately below the context label
   should be looked up. A context label carried as an outermost label
   over a particular multi-access subnet/tunnel  MUST be unique within
   the scope of that subnet/tunnel.


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
   "downstream LSR" [RFC3031]. Consider two LSRs, Ru and Rd that have
   agreed to bind Label L to a FEC, F, for packets 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 about upstream-assigned labels is the
   following. When an upstream-assigned label L is at the top of the
   label stack, it must be looked up by an LSR which is not the LSR that
   assigned and distributed the label binding for L. Therefore an
   upstream-assigned label must always be looked up in a context-
   specific label space, as described in section 7.







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4.1. Upstream-Assigned and Downstream-Assigned Labels

   It is possible that some LSRs on a LSP for FEC F, distribute
   downstream-assigned label bindings for FEC F, while other LSRs
   distribute upstream-assigned label bindings. It is possible for a LSR
   to distribute 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. When two LSRs Ru
   and Rd are adjacent on an LSP for FEC F (with Ru being the upstream
   neighbor and Rd the downstream neighbor), either Ru distributes an
   upstream-assigned label binding for F to Rd, or else Rd distributes a
   downstream-assigned label binding to Ru, but NOT both.  Whether
   upstream-assigned or downstream-assigned labels are to be used for a
   particular FEC depends on the application using the LSP.  Any
   application which requires the use of upstream-assigned labels MUST
   specify that explicitly, or else it is to be assumed that downstream-
   assigned labels are used. An application on an LSR uses a label
   distribution protocol to indicate to its peer LSRs whether a
   particular label binding distributed by the LSR uses upstream-
   assigned or downstream-assigned label. Details of such procedures are
   outside the scope of this document. In some cases, the decision as to
   which is used for a particular application may be made by a
   configuration option.


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.  An upstream LSR which is the head end of multiple tunnels SHOULD
   by default assign the upstream-assigned labels, for all the LSPs
   carried over these tunnels, from a single label space, which is
   common to all those tunnels.  Further an upstream LSR which is the
   head of multiple tunnels SHOULD use the same IP address as the head
   identifier of these tunnels, provided that the head identifier of
   these tunnels includes an IP address. The LSR could assign 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
   outgoing 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.



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   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.


7. Upstream Neighbor Label Space

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

   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 a MPLS label which is upstream-
   assigned by Ru. This label may be the top label on the label stack of
   a packet received from Ru or it may be exposed as the top label on
   the label stack, as a result of Rd popping one or more labels off the
   label stack, from such a packet.




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   This implies that Rd MUST be able to determine whether the top label
   of a MPLS packet being processed is upstream-assigned and if yes, the
   "context" 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.

   If Ru transmits this packet by encapsulating it in an IP or MPLS
   tunnel, then the fact that L is upstream-assigned is determined by Rd
   by the tunnel on which the packet is received. Whether a given tunnel
   can be used for transmitting MPLS packets with either downstream-
   assigned or upstream assigned MPLS labels, or both, depends on the
   tunnel type and is described in [MPLS-MCAST-ENCAPS]. 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. 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. Such a mechanism
   will be provided by the label distribution protocol between Ru and Rd
   and will likely require extensions to existing label distribution
   protocols. The description of such a mechanism is outside the scope
   of this document.

   Rd maintains an "Upstream Neighbor Label Space" for upstream assigned
   labels, assigned by Ru. When Ru transmits MPLS packets the top label
   of which is upstream assigned over IP or MPLS tunnels, then Rd MUST
   be able to determine the root of these IP/MPLS tunnels.  Rd MUST then
   use a separate label space for each unique root.

   The root is identified by the head-end IP address of the Tunnel. If
   the same upstream router, Ru, uses different head-end IP addresses
   for different tunnels then the downstream router, Rd, MUST maintain a
   different Upstream Neighbor Label Space for each such head-end IP
   address.

   Consider the following conditions:

      1) Ru is the "root" of two tunnels, call them A and B.

      2) IP address X is an IP address of Ru.

      3) The signaling protocol used to set up tunnel A identified  A's
         root node as IP address X.

      4) The signaling protocol used to set up tunnel B identified B's
         root node as IP address X.






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      5) Packets sent through tunnels A  and B may be carrying upstream-
         assigned labels.

      6) Ru is the LSR that assigned the upstream-assigned labels
         mentioned in condition 5.

   Under these conditions, Ru  MUST use the same label space when
   assigning the upstream-assigned labels.

   Suppose that Rd is a node that belongs to tunnels A and B, but is not
   the root node of either tunnel. Then Rd may assume that the same
   upstream-assigned label space is used on both tunnels IF AND ONLY IF
   the signaling protocol used to set up tunnel A identified the root
   node as IP address X and the signaling protocol used to set up tunnel
   B identified the root node as the same IP address X.

   In addition, the protocol that is used for distributing the upstream-
   assigned label to be used over a particular tunnel MUST identify the
   "assigner" using the same IP address that is used, by the protocol
   that sets up the tunnel, to identify the root node of the tunnel.
   Implementors must take note of this, even if the tunnel setup
   protocol is different from the protocol that is used for distributing
   the upstream-assigned label to be used over the tunnel.

   The precise set of procedures for identifying the IP address of the
   root of the tunnel depend, of course, on the protocol used to set up
   the tunnel. For P2P tunnels, the intention is that the headend of the
   tunnel is the "root". For P2MP or MP2MP tunnels, one can always
   identify one node as being the "root" of the tunnel.

   Some tunnels may be set up by configuration, rather than by
   signaling. In these cases, the IP address of the root of the tunnel
   must be configured.

   Some tunnels may not even require configuration, e.g., a GRE tunnel
   can be "created" just by encapsulating packets and transmitting them.
   In such a case the IP address of the root is considered to be the IP
   source address of the encapsulated packets.

   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



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   in the IP header.

   When Ru and Rd are adjacent to each other on a multi-access data link
   media, if Ru would transmit the packet, with top label L, by
   encapsulating it in a data link frame, then whether L is upstream-
   assigned or downstream assigned can be determined by Rd as described
   in [MPLS-MCAST-ENCAPS].  This is possible because if L is upstream-
   assigned then [MPLS-MCAST-ENCAPS] uses a different ether type in the
   data link frame. However this is not sufficient for Rd to determine
   the context of this packet. In order for Rd to determine the context
   of this packet, Ru encapsulates the packet, in a one hop MPLS tunnel.
   This tunnel uses an MPLS context label that is assigned by Ru.
   Section 8 describes how the context label is assigned.  Rd maintains
   a separate "Upstream Neighbor Label Space" for Ru. The "context" of
   this packet, i.e. Ru's upstream neighbor label space, in which L was
   reserved, is determined by Rd from the top context label and the
   interface on which the packet is received. The ether type in the data
   link frame is set to indicate that the top label is upstream-
   assigned.  The second label in the stack is L.


8. Context Label on LANs

   The procedure described below applies to LSRs using IPv4 and does not
   apply to LSRs only using IPv6. A solution for IPv6 LSRs is outside
   the scope of this document.

   For a labeled packet with an ether type of 'upstream label
   assignment' the top label is used as the context. The context label
   value is assigned by the upstream LSR and advertised to the
   downstream LSRs.  Mechanisms for advertising the context label will
   be provided by the label distribution protocol between the upstream
   and downstream LSRs.  The description of such a mechanism is outside
   the scope of this document.

   The context label assigned by a LSR on a LAN interface MUST be unique
   across all the context labels assigned by other LSRs on the same LAN.
   Each LAN interface is normally configured with a primary IPv4 address
   that is unique on that LAN. The host part of the IPv4 address,
   identified by the network mask, is unique. If the IPv4 network mask
   is greater then 12 bits, it is possible to map the remaining 20 bits
   into an unique context label value. This enables the LSRs on the LAN
   to assign an unique context label without the need for additional
   configuration. To avoid assigning context label values that fall into
   the reserved label space range [RFC3032], the value of the host part
   of the IPv4 address is offset with 0x10, if this value is not greater
   then 0xFFFEF. Values of the host part of the IPv4 address greater
   then 0xFFFEF are not allowed to be used as the context label.



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   Consider LSRs Rm (downstream) connected to Ru1 (upstream) on a LAN
   interface and to Ru2 (upstream) on a different LAN interface. Rm
   could receive a context label value derived from the LAN interface
   from Ru1 and from Ru2. It is possible that the context label values
   used by Ru1 and Ru2 are the same. This would occur if the LAN
   interfaces of both Ru1 and Ru2 are configured with a primary IPv4
   address where the lowest 20 bits are equal. To avoid these conflicts
   the context label MUST be looked up in the context of the LAN
   interface on which the packet is received. A receiving LSR that
   receives a packet with a context label of Lc over LAN interface
   identified by X, MUST use the label space specific to X to lookup Lc.
   This determines the context to lookup the label below Lc in the label
   stack.


9. Usage of Upstream-Assigned Labels

   A typical usage of upstream-assigned labels is when an upstream LSR
   Ru is adjacent to several 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 the case of a tunnel 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 tunnel.
   In the case of a multi-access media 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 the context label
   that identifies Ru, and the label immediately below is L, on the
   multi-access media. 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.


10. IANA Considerations

   This document has no actions for IANA.











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11. Security Considerations

   The security considerations that apply to upstream-assigned labels
   and context labels are no different in kind than those that apply to
   downstream-assigned labels.

   Note that procedures for distributing upstream-assigned labels and/or
   context labels are not within the scope of this document.  Therefore
   the security considerations that may apply to such procedures are not
   considered here.

   Section 8 of this document describes a procedure which enables an LSR
   to automatically generate a unique context label for a LAN.  This
   procedure assumes that the IP addresses of all the LSR interfaces on
   the LAN will be unique in their low-order 20 bits. If two LSRs whose
   IP addresses have the same low-order 20 bits are placed on the LAN,
   other LSRs are likely to misroute packets transmitted to the LAN by
   either of the two LSRs in question.


12. Acknowledgements

   Thanks to IJsbrand Wijnands's contribution, specifically for the text
   on which section 8 is based.


13. References

13.1. Normative References

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

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

   [MPLS-MCAST-ENCAPS] T. Eckert, E. Rosen, R. Aggarwal, Y. Rekhter,
   draft-ietf-mpls-multicast-encaps-08.txt


13.2. Informative References

   [MVPN] E. Rosen, R. Aggarwal [Editors], Multicast in BGP/MPLS VPNs",
   draft-ietf-l3vpn-2547bis-mcast-06.txt

   [RFC3353] D. Ooms, et. al., "Overview of IP Multicast in a Multi-
   Protocol Label Switching (MPLS) Environment.", August 2002.




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   [RFC3032] E. Rosen, et. al., "MPLS Label Stack Encoding", January
   2001.



14. Author's Address

   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



15. Intellectual Property Statement

   The IETF takes no position regarding the validity or scope of any
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   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
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   on the procedures with respect to rights in RFC documents can be
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   Copies of IPR disclosures made to the IETF Secretariat and any
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   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.


16. Copyright Notice

   Copyright (C) The IETF Trust (2008).

   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
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   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
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   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
































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