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MPLS                                                        H. Song, Ed.
Internet-Draft                                                     Z. Li
Intended status: Informational                                   T. Zhou
Expires: August 19, 2019                                          Huawei
                                                            L. Andersson
                                                Bronze Dragon Consulting
                                                       February 15, 2019


              Options for MPLS Extension Header Indicator
                    draft-song-mpls-eh-indicator-00

Abstract

   This document describes the schemes that indicates the presence of
   the MPLS extension header(s) following the MPLS label stack.  After a
   thorough evaluation of these options by comparing their pros and
   cons, one should be chosen as the final scheme for MPLS extension
   header indicator.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119][RFC8174] when, and only when, they appear in all
   capitals, as shown here.

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
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on August 19, 2019.







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Copyright Notice

   Copyright (c) 2019 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
   (https://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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Dedicated Extension Header Label  . . . . . . . . . . . . . .   3
     2.1.  Special Purpose Label . . . . . . . . . . . . . . . . . .   3
     2.2.  Extension Label plus an Extended Special Purpose Label  .   3
   3.  Generic Associated Channel Extension  . . . . . . . . . . . .   4
     3.1.  GAL and Associated Channel Header . . . . . . . . . . . .   4
     3.2.  GAL and a Different Nibble Value  . . . . . . . . . . . .   4
   4.  Configured FEC Labels . . . . . . . . . . . . . . . . . . . .   5
   5.  Summary . . . . . . . . . . . . . . . . . . . . . . . . . . .   6
   6.  Considerations of EHI . . . . . . . . . . . . . . . . . . . .   7
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   9.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .   8
   10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   8
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     11.1.  Normative References . . . . . . . . . . . . . . . . . .   8
     11.2.  Informative References . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   The document [I-D.song-mpls-extension-header] presents the
   motivation, specification, and use cases of MPLS Extension Header
   (EH).  However, multiple options are possible to indicate the
   presence of the extension header(s).

   We propound three categories of methods which can be further
   partitioned into five unique schemes.  Four of them use explicit data
   plane encoding to indicate the EH and the last one implies the EH
   through control plane configuration.  This document details and




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   compares these schemes, in order to foster further discussions until
   a final decision is made.

2.  Dedicated Extension Header Label

   A straightforward method is to directly encode an Extension Header
   Label (EHL) in the MPLS label stack.  Two derived schemes are as
   follows.

2.1.  Special Purpose Label

   A new special purpose label, EHL, can be used to indicate EHs.  As
   specified in [RFC7274], so far eight special purpose label values are
   still left unsigned by IANA (which are 4 to 6 and 8 to 12).  This
   single label scheme is elegant but arguably demands a scarce
   resource.  We cannot rule out the possibility of requiring more than
   one label value to differentiate EH classes (e.g., Hop-by-Hop, End-
   to-End, or both).  If this happens, it can only aggravate the
   situation.

   Another benefit of this scheme is that an EHL can potentially be
   located anywhere in an MPLS label stack.  It is easier and quicker
   for a router to figure out the existence of extension header(s) if
   the EHL is close to or at the top of the label stack.  However, if
   there are legacy devices which can reach the EHL but do not recognize
   it in a network, then for backward compatibility, the EHL must be
   located at the bottom of the stack (i.e., only the MPLS tunnel ends
   and EHL-aware nodes will look up and process it).

   The format of an EHL is the same as an MPLS label.  The first 20-bit
   label value will be assigned by IANA.  The BoS bit is used to
   indicate the location of the label.  The other fields, CoS and TTL,
   currently have no use in the context of EHL.  However, these two
   fields can potentially be used to encode other information, which is
   beyond the scope of this document.

2.2.  Extension Label plus an Extended Special Purpose Label

   [RFC7274] specifies the Extension Label (XL) with the value of 15.
   An extended special purpose label (ESPL) following XL can be used as
   EHL.  A large number of ESPL values are available for allocation.
   The XL+EHL scheme eases the concern on the reserved label space at
   the cost of one more label in the label stack.

   Except for the fact that one more label is needed, The XL+EHL scheme
   shares the same property as the single special purpose EHL scheme.





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3.  Generic Associated Channel Extension

   The similar "header extension" requirement for MPLS has led to some
   proposals before.  A special Generic Associated Channel Label (GAL)
   [RFC5586] with the value of 13 has been assigned to support the
   identification of an Associated Channel Header (ACH).  We can extend
   this existing mechanism to encode the MPLS EH indicator.

3.1.  GAL and Associated Channel Header

   The ACH is located below the bottom label.  It has a 16-bit Channel
   Type field which provides abundant space to encode the MPLS EH
   indicator.  This scheme has the same header overhead as the XL+EHL
   scheme.  The format is depicted in Figure 1.

        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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                GAL (13)               | EXP |1|      TTL      |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |0 0 0 1|Version|   Reserved    |  Extension Header Indicator   |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       |                     HEH and EH(s)                             |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     Figure 1: Associated Channel Header as Extension Header Indicator

   GAL has several applications already yet its heritage also has
   several limitations.  The GAL must be located at the bottom of a
   label stack for its chief use cases such as MPLS-TP.  So a router
   needs to search the entire label stack for the BoS bit and check if
   the corresponding label is GAL.  This can impact the performance when
   the label stack is deep.  A more serious concern is that [RFC5586]
   states that GAL+ACH MUST NOT be used to transport user traffic and an
   ACH is supposed to be followed by a non-service payload.

   None of these is insurmountable but it does require an overhaul of
   the existing RFC in order to extend the usage of GAL.

3.2.  GAL and a Different Nibble Value

   To avoid changing the established semantics of ACH, a variation can
   be used.  ACH starts with a nibble value "0001".  A different nibble
   value may be used to redefine the remaining part of the word.  The
   idea has been exploited by [I-D.guichard-sfc-mpls-metadata] to define
   a Metadata Channel Header (MCH) with the leading nibble value "0000".



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   Similarly, we can use another nibble value (e.g., "0010") to define a
   new header, namely the MPLS Extension Header Indicator (EHI).

   The format of the GAL and EHI is depicted in Figure 2.

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                GAL (13)               | EXP |1|      TTL      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |0 0 1 0|Version|   Reserved    |  Extension Header Class       |<-EHI
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     |                     HEH and EH(s)                             |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 2: Extension Header Indicator Format

   The Extension Header Class field in EHI is used to differentiate the
   extension headers.  Potentially there are three classes: Hop-by-Hop
   (HbH), End-to-End (E2E), or both.  If finally we decide to not
   differentiate the extension headers, we have the opportunity to merge
   the HEH (see [I-D.song-mpls-extension-header] for details) into EHI,
   so we can reduce the header overhead by four bytes.  The header
   format is depicted in Figure 3.

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                GAL (13)               | EXP |1|      TTL      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |0 0 1 0|     EHCNT     |       EHTLEN          |      NH       |<-HEH
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     |                           EH(s)                               |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                        Figure 3: Merge HEH to EHI

4.  Configured FEC Labels

   It is also possible to use FEC labels to indicate the presence of
   extension headers.  An FEC label has the same forwarding semantics as
   the original label, but it also means that one or more extension
   headers exist below the label stack.




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   Although this approach avoids the need of new header encoding
   standards, it introduces a good deal of complexity into the control
   plane.  Since every label needs an FEC label to indicate EH, this
   scheme also significantly reduces the available label space.  Another
   issue is that this solution may not work for incremental deployment
   where some legacy routers cannot understand and apply the FEC labels
   for EH.  Moreover, this configuration-based solution certainly makes
   the cross-domain interoperability more difficult.  Hence, this is the
   least preferred option.  We only include it here for the completeness
   of the discussion.

5.  Summary

   Evidenced by the existing and emerging use cases, MPLS networks need
   a standard way to support extension headers.  In Figure 4, we
   summarize the potential schemes that allow MPLS packets to carry
   extension headers and list the main pros and cons for each scheme.


































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     +---+---------------------------+---------------------------------+
     |No.|    Description            |       Pros and Cons             |
     +---+---------------------------+---------------------------------+
     | 1 | Special purpose EHL       |+ Single label                   |
     |   |                           |+ Location freedom               |
     |   |                           |- Need standard extension        |
     |   |                           |- Use scarce resource            |
     +---+---------------------------+---------------------------------+
     | 2 | XL(15) + EHL              |+ Location freedom               |
     |   |                           |+ Established mechanism          |
     |   |                           |+ Abundant resource              |
     |   |                           |- One extra label than Optiona 1 |
     |   |                           |- Need standard extension        |
     +---+---------------------------+---------------------------------+
     | 3 | GAL + ACH with channel    |+ Reuse existing mechanism       |
     |   | type extension            |+ Abundant resource              |
     |   |                           |- Label location limitation      |
     |   |                           |- One more word than Option 1    |
     |   |                           |- Not for user traffic           |
     |   |                           |- Need standard extension/update |
     +---+---------------------------+---------------------------------+
     | 4 | GAL + another nibble value|+ No change to ACH semantics     |
     |   | to indicate EHs (e.g.,    |+ Potential overhead saving      |
     |   | "0010")                   |- Label location limitation      |
     |   |                           |- Hack scarce resource (nibble)  |
     |   |                           |- Need standard extension        |
     +---+---------------------------+---------------------------------+
     | 5 | FEC label as EH indicator |+ No need for header standard    |
     |   |                           |- Complex control plane          |
     |   |                           |- Cross-domain interoperability  |
     |   |                           |- Label space issue              |
     |   |                           |- Not for incremental deployment |
     +---+---------------------------+---------------------------------+


          Figure 4: Potential Schemes for MPLS Extension Headers

   Through comprehensive considerations on the pros and cons of each
   scheme, we expect a working group consensus can be reached to pick
   the final winner.

6.  Considerations of EHI

   The existence of Extension Headers will make the ECMP based on inner
   IP packet header impossible or harder.  If legacy routers need to
   conduct this kind of ECMP, the process either fails or generates
   unexpected results.  EH-aware routers can do this kind of ECMP but
   they need to skip all the EHs in order to access the inner packet



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   header which may not be efficient.  In this case, the Entropy Label
   (EL) is preferred for ECMP.  The Entropy Label Indicator (ELI) and EL
   should be put in front of the EHI to avoid confusing the legacy
   routers.

7.  Security Considerations

   TBD

8.  IANA Considerations

   If the EHL approach is adopted to indicate the presence of MPLS
   extension header(s), this document requests IANA to assign one or
   more new Special-Purpose MPLS Label Values from the Special-Purpose
   Multiprotocol Label Switching (MPLS) Label Values Registry of
   "Extension Header Label (EHL)".

9.  Contributors

   The other contributors of this document are listed as follows.

   o  James Guichard

   o  Stewart Bryant

10.  Acknowledgments

   TBD.

11.  References

11.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC5586]  Bocci, M., Ed., Vigoureux, M., Ed., and S. Bryant, Ed.,
              "MPLS Generic Associated Channel", RFC 5586,
              DOI 10.17487/RFC5586, June 2009,
              <https://www.rfc-editor.org/info/rfc5586>.

   [RFC7274]  Kompella, K., Andersson, L., and A. Farrel, "Allocating
              and Retiring Special-Purpose MPLS Labels", RFC 7274,
              DOI 10.17487/RFC7274, June 2014,
              <https://www.rfc-editor.org/info/rfc7274>.




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   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

11.2.  Informative References

   [I-D.guichard-sfc-mpls-metadata]
              Guichard, J., Pignataro, C., Spraggs, S., and S. Bryant,
              "Carrying Metadata in MPLS Networks", draft-guichard-sfc-
              mpls-metadata-00 (work in progress), September 2013.

   [I-D.song-mpls-extension-header]
              Song, H., Li, Z., Zhou, T., and L. Andersson, "MPLS
              Extension Header", draft-song-mpls-extension-header-02
              (work in progress), February 2019.

Authors' Addresses

   Haoyu Song (editor)
   Huawei
   2330 Central Expressway
   Santa Clara
   USA

   Email: haoyu.song@huawei.com


   Zhenbin Li
   Huawei
   156 Beiqing Road
   Beijing, 100095
   P.R. China

   Email: lizhenbin@huawei.com


   Tianran Zhou
   Huawei
   156 Beiqing Road
   Beijing, 100095
   P.R. China

   Email: zhoutianran@huawei.com








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   Loa Andersson
   Bronze Dragon Consulting
   Stockholm
   Sweden

   Email: loa@pi.nu













































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