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Versions: 00                                                            
Network Working Group                                              X. Xu
Internet-Draft                                                   M. Chen
Intended status: Standards Track                                  Huawei
Expires: June 28, 2014                                 December 25, 2013


             Advertising Global Labels or SIDs Using IS-IS
                 draft-xu-isis-global-label-sid-adv-00

Abstract

   Segment Routing (SR) is a new MPLS paradigm in which each SR-capable
   router is required to advertise global MPLS labels or Segment IDs
   (SID ) for its attached prefixes by using link-state IGPs, e.g., IS-
   IS.  One major challenge associated with such global MPLS label or
   SID advertisement mechanism is how to avoid a given global MPLS label
   or SID from being allocated by different routers to different
   prefixes.  Although such global label or SID allocation collision
   problem can be addressed through manual allocation , it is error-
   prone and nonautomatic therefore may not be suitable in large-scale
   SR network environments.  This document proposes an alternative
   approach for allocating and advertising global MPLS labels or SIDs
   via IS-IS so as to eliminate the potential risk of label allocation
   collision.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on June 28, 2014.

Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.





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   This document is subject to BCP 78 and the IETF Trust's Legal
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Advertising Label Bindings for Prefixes using IS-IS . . . . .   3
   4.  Advertising SID Bindings for Prefixes using IS-IS . . . . . .   4
   5.  Requesting Label Bindings for Prefixes using IS-IS  . . . . .   5
   6.  Requesting SID Bindings for Prefixes using IS-IS  . . . . . .   5
   7.  Mapping Server Redundancy . . . . . . . . . . . . . . . . . .   6
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   7
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   10. Security Considerations . . . . . . . . . . . . . . . . . . .   7
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     11.1.  Normative References . . . . . . . . . . . . . . . . . .   7
     11.2.  Informative References . . . . . . . . . . . . . . . . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   Segment Routing (SR) [I-D.filsfils-rtgwg-segment-routing] is a new
   MPLS paradigm in which each SR-capable router is required to
   advertise global MPLS labels or Segment IDs (SID) for its attached
   prefixes by using link-state IGPs, e.g., IS-
   IS[I-D.previdi-isis-segment-routing-extensions] . One major challenge
   associated with such global MPLS label or SID advertisement mechanism
   is how to avoid a given global MPLS label or SID from being allocated
   by different routers to different prefixes.  Although such global
   label or SID allocation collision problem can be addressed through
   manual allocation , it is error-prone and nonautomatic therefore may
   not be suitable in large-scale SR network environments.

   This document proposes an alternative approach for allocating and
   advertising global MPLS labels or SIDs via IS-IS so as to eliminate
   the potential risk of label allocation collision.  The basic idea of
   this approach is to allow a particular IGP router to allocate global
   MPLS labels or SIDs for those prefixes attached to each SR-capable
   router and meanwhile advertise the corresponding label or SID



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   bindings in the IGP domain scope.  That particular IGP rouer is
   therefore refered to as a mapping server.  As for how the mapping
   server know which prefixes need to be allocated with global labels or
   SIDs, it can be achieved either by configuration on the mapping
   server or by advertisement from SR-capable routers.  In the multi-
   level scenario where route summarization between levels is enabled,
   the IP longest-match algorithm SHOULD be used by SR-capable routers
   when processing label or SID bindings advertised by the mapping
   server, just as the mechanism defined in [RFC5283] .

1.1.  Requirements Language

   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 RFC 2119 [RFC2119].

2.  Terminology

   This memo makes use of the terms defined in
   [I-D.filsfils-rtgwg-segment-routing] and [RFC4971].

3.  Advertising Label Bindings for Prefixes using IS-IS

   A mapping server could uses one or more of the following TLVs to
   advertise global labels for those prefixes which need to be allocated
   with global labels.

   o  TLV-135 (IPv4) [RFC5305]

   o  TLV-235 (MT-IPv4) [RFC5120]

   o  TLV-236 (IPv6) [RFC5308]

   o  TLV-237 (MT-IPv6) [RFC5120]

   A Label Binding Sub-TLV (TBD) as shown below is associated with a
   prefix which is contained in one of the above TLVs:

      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=TBD    |    Length     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |P|  Reserved   |             MPLS Label (20 bit)               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Type: TBD




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      Length: 4

      P-Flag: if set, the penultimate hop router MUST perform PHP action
      on the allocated MPLS label.  For a given prefix, the P-Flag in
      the Label Binding Sub-TLV MUST be set to the same value as that of
      the P-Flag in the Label Request Sub-TLV if a label request message
      (See Section 5 of this document) for that prefix is received by
      the mapping server.

      MPLS Label: a global label for the prefix which is carried in the
      TLV containing this sub-TLV.

   Since the mapping server uses these TLVs for label binding
   advertisement purpose other than building the normal IP routing
   table, the Metric field MUST be set to a value larger than
   MAX_PATH_METRIC (i.e., 0xFE000000) according to the following
   specification as defined in [RFC5305] "...If a prefix is advertised
   with a metric larger then MAX_PATH_METRIC (0xFE000000, see paragraph
   3.0), this prefix MUST NOT be considered during the normal SPF
   computation.  This allows advertisement of a prefix for purposes
   other than building the normal IP routing table...".  In addition,
   when propagating those TLVs across levels, the Label Binding Sub-TLVs
   contained in them MUST be preserved.

4.  Advertising SID Bindings for Prefixes using IS-IS

   A mapping server could uses one or more of the Extended IP
   Reachability TLVs (i.e., TLV-135, TLV-235, TLV-236 and TLV-237) to
   advertise SIDs for those prefixes which need to be allocated with
   SIDs.

   A SID Binding Sub-TLV (TBD) as shown below is associated with a
   prefix which is contained in one of the above TLVs:

      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=TBD    |    Length     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                              SID                              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Type: TBD

      Length: 4

      SID: a SID for the prefix which is carried in the TLV containing
      this sub-TLV.



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   Since the mapping server uses these TLVs for label binding
   advertisement purpose other than building the normal IP routing
   table, the Metric field MUST be set to a value larger than
   MAX_PATH_METRIC (i.e., 0xFE000000).  In addition, when propagating
   those TLVs across levels, the SID Binding Sub-TLVs contained in them
   MUST be preserved.

5.  Requesting Label Bindings for Prefixes using IS-IS

   When advertising IP reachability information by using one of the
   Extended IP Reachability TLVs (i.e., TLV-135, TLV-235, TLV-236 and
   TLV-237), SR-capable IS-IS routers SHOULD mark those attached
   prefixes which need to be allocated with global labels by associating
   each of these prefixes with a Label Request sub-TLV (type code=TBD)
   as shown below.  In addition, when propagating those TLVs across
   levels, the Label Request Sub-TLVs contained in them MUST be
   preserved.

      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=TBD    |    Length     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |P|                         Reserved                            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Type: TBD

      Length: 4

      P-Flag: if set, the penultimate hop router MUST perform PHP action
      on the required label.

   In the multi-level scenario where route summarization between levels
   is required, separate Extended IP Reachability TLVs other than those
   for IP reachability advertisement purpose SHOULD be used for label
   binding request purpose.  Since these separate TLVs are not used for
   the purpose of building the normal IP routing table, the Metric field
   MUST be set to a value larger than MAX_PATH_METRIC (i.e.,
   0xFE000000).  In addition, when propagating those TLVs across levels,
   the Label Request Sub-TLVs contained in them MUST be preserved.

6.  Requesting SID Bindings for Prefixes using IS-IS

   When advertising IP reachability information by using one of the
   Extended IP Reachability TLVs (i.e., TLV-135, TLV-235, TLV-236 and
   TLV-237), SR-capable IS-IS routers SHOULD mark those attached
   prefixes which need to be allocated with SIDs by associating each of



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   these prefixes with a SID Request sub-TLV (Type Code=TBD and
   Length=0)

   In the multi-level scenario where route summarization between levels
   is required, separate Extended IP Reachability TLVs other than those
   for IP reachability advertisement purpose SHOULD be used for SID
   binding request purpose.  Since these separate TLVs are not used for
   the purpose of building the normal IP routing table, the Metric field
   MUST be set to a value larger than MAX_PATH_METRIC (i.e.,
   0xFE000000).  In addition, when propagating those TLVs across levels,
   the SID Request Sub-TLVs contained in them MUST be preserved.

7.  Mapping Server Redundancy

   For redundancy purpose, more than one router could be configured as
   candidates for mapping servers.  Each candidate for mapping servers
   SHOULD advertise its capability of being a mapping servers by using
   IS-IS Router Capability TLV.  The one with the highest priority
   SHOULD be elected as the primary mapping server which is eligible to
   allocate and advertise global labels or SIDs for prefixes on behalf
   of SR-capable routers.  The comparison of IS-IS System ID breaks the
   tie between two or more candidates with the same highest priority.
   Meanwhile, the one with the second highest priority SHOULD be elected
   as a backup mapping server.  This backup mapping server SHOULD
   advertise the same label bindings as those advertised by the primary
   mapping server.  In this way, the unnecessary changes to the data
   plane (i.e., MPLS forwarding table) of SR-capable routers can be
   avoided in the event of mapping server failover.

   Each candidate mapping server SHOULD advertise its capability of
   being a mapping server and the corresponding priority for mapping
   server election by attaching a Mapping Server Capability Sub-TLV
   (type code=TBD) shown as below to an IS-IS Router Capability TLV
   [RFC4971] with the S flag set (with domain-wide flooding scope).

      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=TBD    |    Length     |    Priority   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Type: TBD

      Length: 1

      Priority: the priority for mapping server election.





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8.  Acknowledgements

   The authors would like to thank .

9.  IANA Considerations

   TBD.

10.  Security Considerations

   This document does not introduce any new security considerations.

11.  References

11.1.  Normative References

   [I-D.previdi-isis-segment-routing-extensions]
              Previdi, S., Filsfils, C., Bashandy, A., Gredler, H., and
              S. Litkowski, "IS-IS Extensions for Segment Routing",
              draft-previdi-isis-segment-routing-extensions-04 (work in
              progress), October 2013.

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

   [RFC4971]  Vasseur, JP., Shen, N., and R. Aggarwal, "Intermediate
              System to Intermediate System (IS-IS) Extensions for
              Advertising Router Information", RFC 4971, July 2007.

   [RFC5120]  Przygienda, T., Shen, N., and N. Sheth, "M-ISIS: Multi
              Topology (MT) Routing in Intermediate System to
              Intermediate Systems (IS-ISs)", RFC 5120, February 2008.

   [RFC5283]  Decraene, B., Le Roux, JL., and I. Minei, "LDP Extension
              for Inter-Area Label Switched Paths (LSPs)", RFC 5283,
              July 2008.

   [RFC5305]  Li, T. and H. Smit, "IS-IS Extensions for Traffic
              Engineering", RFC 5305, October 2008.

   [RFC5308]  Hopps, C., "Routing IPv6 with IS-IS", RFC 5308, October
              2008.

11.2.  Informative References







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   [I-D.filsfils-rtgwg-segment-routing]
              Filsfils, C., Previdi, S., Bashandy, A., Decraene, B.,
              Litkowski, S., Horneffer, M., Milojevic, I., Shakir, R.,
              Ytti, S., Henderickx, W., Tantsura, J., and E. Crabbe,
              "Segment Routing Architecture", draft-filsfils-rtgwg-
              segment-routing-01 (work in progress), October 2013.

Authors' Addresses

   Xiaohu Xu
   Huawei

   Email: xuxiaohu@huawei.com


   Mach Chen
   Huawei

   Email: mach.chen@huawei.com
































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