TRILL: Header Extension

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Document Type Active Internet-Draft (trill WG)
Authors Donald Eastlake  , Anoop Ghanwani  , Vishwas Manral  , Li Yizhou  , Caitlin Bestler 
Last updated 2012-06-07 (latest revision 2012-05-02)
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TRILL Working Group                                      Donald Eastlake
INTERNET-DRAFT                                                    Huawei
Intended status: Proposed Standard                        Anoop Ghanwani
                                                          Vishwas Manral
                                                               Yizhou Li
                                                         Caitlin Bestler
Expires: November 1, 2012                                    May 2, 2012

                        TRILL: Header Extension


   The IETF TRILL (Transparent Interconnection of Lots of Links) base
   protocol specifies minimal hooks to safely support TRILL Header
   extensions. This document specifies an initial extension providing
   additional flag bits and specifies two of those bits.

Status of This Memo

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

   Distribution of this document is unlimited. Comments should be sent
   to the TRILL working group mailing list <>.

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Table of Contents

      1. Introduction............................................3
      1.1 Conventions used in this document......................3

      2. TRILL Header Extensions.................................4
      2.1 RBridge Extended Flag Handling Requirements............5
      2.2 No Critical Surprises..................................5
      2.3 Extended Header Flags..................................6
      2.3.1 Critical Summary Bits................................7
      2.4 Conflict of Extensions.................................8

      3. Specific Extended Header Flags..........................9
      3.1 The RBridge Channel Alert Extended Flags...............9

      4. Additions to IS-IS.....................................10

      5. IANA Considerations....................................10
      6. Security Considerations................................11
      7. Acknowledgements.......................................11
      8. Normative References...................................12
      9. Informative References.................................12

      Change History............................................13

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1. Introduction

   The base IETF TRILL protocol [RFC6325] [RFC6326bis] provides a TRILL
   Header extension feature and describes minimal hooks to safely
   support header extension. (This feature is called "options" in
   Section 3.8 of [RFC6325].) But, except for the first two bits, the
   base TRILL protocol does not specify the structure of extensions to
   the TRILL Header nor the details of any particular extension. This
   document specifies an initial extension providing additional flag
   bits and specifies one of those bits. Additional extensions,
   including TLV (Type, Length, Value) encoded options, may be specified
   in later documents.

   Section 2 below describes some general principles of TRILL header
   extensions and an initial extension.  Section 3 specifies pair of
   flags in this initial extension.

1.1 Conventions used in this document

   The terminology and acronyms defined in [RFC6325] are used herein
   with the same meaning.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

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2. TRILL Header Extensions

   The base TRILL Protocol includes a feature for extension of the TRILL
   Header (see [RFC6325] Sections 3.5 and 3.8).  The 5-bit Op-Length
   header field gives the length of the extensions to the TRILL Header
   in units of 4 octets, which allows up to 124 octets of header
   extension. If Op-Length is zero there are no header extensions
   present; else, the extension area follows immediately after the
   Ingress RBridge Nickname field of the TRILL Header. The first 32-bit
   word of the optional extensions area consists of an extended flags
   area and critical summary bits as specified in this document.

   As described below, provision is made for

      o  hop-by-hop flags, which might affect any RBridge that receives
         a TRILL Data frame with such a flag set,

      o  ingress-to-egress flags, which would only necessarily affect
         the RBridge(s) where a TRILL frame is decapsulated,

      o  flag affecting an as yet unspecified class of RBridges, for
         example border RBridges in a TRILL campus extended to support
         multi-level IS-IS, and

      o  both "critical" and "non-critical" flags.

   Any RBridge receiving a frame with a critical hop-by-hop extension
   that it does not implement MUST discard the frame because it is
   unsafe to process the frame without understanding such a critical

   Any egress RBridge receiving a frame with a critical ingress-to-
   egress extension it does not implement MUST drop the frame if it is a
   known unicast frame; if it is a multi-destination TRILL Data frame,
   then it MUST NOT be egressed at that RBridge but the egress RBridge
   still forwards such a frame on the distribution tree.

   Non-critical extensions can be safely ignored.

   Any extended flag indicating a significant change in the structure or
   interpretation of later parts of the frame which, if the extended
   flag were ignored, could cause a failure of service or violation of
   security policy MUST be a critical extension. If such an extended
   flag affects any fields that transit RBridges will examine, it MUST
   be a hop-by-hop critical extended flag.

      Note: Most RBridges implementations are expected to be optimized
      for simple and common cases of frame forwarding and processing.
      Although the hard limit on the header extensions area length, the
      32-bit alignment of the extension area, and the presence of

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      critical extension summary bits, as described below, are intended
      to assist in the efficient hardware processing of frames with a
      TRILL header extensions area, nevertheless the inclusion of
      extensions may cause frame processing using a "slow path" with
      inferior performance to "fast path" processing. Limited slow path
      throughput of such frames could cause some of them to be

2.1 RBridge Extended Flag Handling Requirements

   All RBridges MUST check whether there are any critical flags set that
   are necessarily applicable to their processing of the frame. To
   assist in this task, critical summary bits are provided that cover
   not only the extended flags specified herein but will cover any
   further extensions that may be specified in future documents
   [Options]. If an RBridge does not implement all critical flags in a
   TRILL Data frame, it MUST discard the frame or, in some circumstances
   as described for certain multi-destination frames, continue to
   forward the frame but MUST NOT egress the frame. A transit or egress
   RBridge may assume that the critical summary bits are correct.

   In addition, a transit RBridge:

   o  MAY set or clear hop-by-hop flags as specified for such flags;
   o  MUST adjust the length of the extensions area, including changing
      Op-Length in the TRILL header, as appropriate if it adds or
      removes the word of extended header flags;
   o  MUST, if it adds the word of extended header flags or changes any
      critical flags, correctly set the critical summary bits in the
      extended header flags word;
   o  MUST NOT remove the extended header flags word unless it is all
      zero (either on arrival or after permitted modifications);
   o  MUST NOT set or clear ingress-to-egress or reserved extended
      header flags except as specifically permitted in the specification
      of such flags.

2.2 No Critical Surprises

   RBridges advertise the extended header flags they support in IS-IS
   PDUs [RFC6326bis]. Unless an RBridge advertises support for a
   critical extended header flag, it will not normally receive frames
   with that flag set. An RBridge is not required to support any

   An RBridge SHOULD NOT set a critical extended flag in a frame unless,

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   -  for a critical hop-by-hop extended header flag, it has determined
      that the next hop RBridge or RBridges that will accept the frame
      support that flag,
   -  for a critical ingress-to-egress extended header flag, it has
      determined that the RBridge or RBridges that will egress the frame
      support that flag, or
   -  for a critical reserved extended header flag, it may set such a
      flag only if it understands which RBridges it is applicable to and
      has determined that those RBridges that will accept the frame
      support that flag.

   "SHOULD NOT" is specified above since there may be cases where it is
   acceptable for those frames, particularly for the multi-destination
   case, to be discarded or not egressed by any RBridges that do not
   implement the extended flag.

2.3 Extended Header Flags

   If any extensions are present in a TRILL Header, as indicated by a
   non-zero Op-Length field, the first 32 bits of the extensions area
   consist of Extended Header Flags, as described below. The remainder
   of the extensions area, if any, after this initial 32 bits, may be
   specified in later documents [Options].

   Any RBridge adding an extensions area to a TRILL Header must set the
   first 32 bits to zero except when permitted or required to set one or
   more of those bits as specified. For TRILL Data frames with
   extensions present, any transit RBridge that does not discard the
   frame MUST transparently copy the extended flags word, except for
   modifictions permitted by an extension implemented by that RBridge.

   The word of Extended Header Flags is illustrated below and the
   meanings of these bits is further described in the list following the

                        1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 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
   |Crit.|  CHbH   |   NCHbH   |CRSV | NCRSV |   CItE    |  NCItE  |
   | ... additional optional 32-bit aligned words of extension     |
   |     possibly including TLV extensions ...

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   (The first two critical summary bits are as specified in [RFC6325].
   In this document an "S", for Summary, has been added at the end of
   their acronyms.)

   Bit(s)   Description

    0-3  Crit.: Critical summary bits.
         0 CHbHS: Critical Hop-by-Hop extension(s) are present.
         1 CItES: Critical Ingress-to-Egress extension(s) are present.
         2 CRSVS: Critical reserved extension(s) are present.

    3-7   CHbH: Critical Hob-by-Hop extended Flag bits.
    8-13  NCHbH: Non-critical Hop-by-Hop extended Flag bits.

    14-16 CRSV: Critical Reserved extended Flag bits.
    17-20 NCRSV: Non-critical Reserved extended Flag bits.

    21-26 CItE: Critical Ingress-to-Egress extended Flag bits.
    27-31 NCItE: Non-critical Ingress-to-Egress extended Flag bits.

2.3.1 Critical Summary Bits

   The top three bits of the Extended Header Flags area, bits 0, 1, and
   2 above, are called the critical summary bits. They summarize the
   presence of critical extensions as follows:

   CHbHS: If the CHbHS (Critical Hop by Hop Summary) bit is one, one or
      more critical hop-by-hop extensions are present. These might be
      critical hop-by-hop extended header flags or critical hop-by-hop
      extensions after the first word in the extensions area. Transit
      RBridges that do not support all of the critical hop-by-hop
      extensions present, for example an RBridge that supported no
      critical hop-by-hop extensions, MUST drop the frame. If the CHbHS
      bit is zero, the frame is safe, from the point of view of
      extensions processing, for a transit RBridge to forward,
      regardless of what extensions that RBridge does or does not

   CItES: If the CItES (Critical Ingress to Egress Summary) bit is a
      one, one or more critical ingress-to-egress extensions are
      present. These might be critical ingress-to-egress extended header
      flags or critical ingress-to-egress extensions after the first
      word in the extensions area.  If the CItES bit is zero, no such
      extensions are present.  If either CHbHS or CItES is non-zero,
      egress RBridges that do not support all critical extensions
      present, for example an RBridge that supports no critical
      extensions, MUST drop the frame.  If both CHbHS and CItES are

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      zero, the frame is safe, from the point of view of extensions, for
      an egress RBridge to process, regardless of what extensions that
      RBridge does or does not support.

   CRSVS: If the CRSVS (Critical Reserved Summary) bit is a one, one or
      more critical extensions are present that are reserved to apply to
      a class of RBridges to be specified in the future, for example
      border RBridges in a TRILL campus extended to support multi-level
      IS-IS. This class will be a subset of transit RBridges. RBridges
      in this class MUST drop frames with the CRSVS bit set unless they
      implement all critical hop-by-hop and all critical reserved
      extensions present in the frame.

   The critical summary bits enable simple and efficient processing of
   TRILL Data frames by egress RBridges that support no critical
   extensions, by transit RBridges that support no critical hop-by-hop
   extensions, and by RBridges in the reserved class that support no
   critical hop-by-hop or reserved extensions. Such RBridges need only
   check whether Op-Length is non-zero and, if it is, the top one, two,
   or three bits just after the fixed portion of the TRILL Header. Based
   on those three bits, such RBridges can decide whether to discard or
   forward / process the frame.

2.4 Conflict of Extensions

   Defining TRILL extensions including Extended Header Flags that
   conflict with each other would be undesirable. Should conflicting
   extensions appear in the same packet, the results would be
   unpredictable, if different implementations processed them in
   different orders. While rules could be defined to specify how to
   predictably process conflicting extensions, such rules would also
   limit implementation flexibility and could impose substantial
   processing burdens.

   Conflicting extensions SHOULD NOT be defined, but if they are,
   careful thought should be given as to whether and how to specify the
   handling of conflicting extensions.

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3. Specific Extended Header Flags

   The table below shows the state of TRILL Header extended flag
   assignments. See Section 6 for IANA Considerations.

       Bits    Purpose                              Section
        0-2    Critical Summary Bits                2.3.1
        3-6    available critical hop-by-hop flags
        7      Critical Channel Alert Flag          3.1
        8      Non-critical Channel Alert Flag      3.1
        9-13   available non-critical hop-by-hop flags
       14-16   available critical reserved flags
       17-20   available non-critical reserved flags
       21-26   available critical ingress-to-egress flags
       27-31   available non-critical ingress-to-egress flags

                    Table 1. Extended Header Flags Area

3.1 The RBridge Channel Alert Extended Flags

   The RBridge Channel Alert Extended Flags indicates that the frame is
   an RBridge Channel frame [Channel] that requests processing at each

   If the critical Channel Alert flag (bit 7) is a one and the RBridge
   does not implement the RBridge Channel feature or the particular
   RBridge Channel protocol involved [Channel] or the frame does not
   actually appear to be an RBridge Channel message, then the frame is
   discarded. This permits implementation, for example, of a Channel
   message requiring strict source routing or the like, with assurance
   that it will be discarded rather than deviate from the directed path.

   If the frame is not discarded as above then the presence of either
   the Critical or Non-critical hop-by-hop Channel Alert flag alerts
   transit RBridges to the presence of an RBridge Channel message
   [Channel] that may require special handling. The non-critical alert
   flag supports, for example, an RBridge Channel protocol message
   including a "record route" function where not recording transit
   RBridges that do not support this function is acceptable.

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4. Additions to IS-IS

   RBridges use IS-IS LSP PDUs to inform other RBridges which Extended
   Header Flags they support. The IS-IS PDU(s), TLV(s), or sub-TLV(s)
   used to encode and advertise this information are specified in a
   separate document [RFC6326bis].

5. IANA Considerations

   IANA is requested to create a subregistry within the TRILL Parameters
   registry: The "TRILL Extended Header Flags" subregistry, that is
   initially populated as specified in Table 1 in Section 3. References
   in that table to sections of this document are to be replaced in the
   IANA subregistry by references to this document as an RFC.

   New TRILL Extended Header Flags are allocated by Standards Action
   [RFC5226] as modified by [RFC4020].

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6. Security Considerations

   For general TRILL protocol security considerations, see [RFC6325].

   For security considerations related to extended header flags, see the
   document where the flag is specified.

   It is important that the critical summary bits in the Extended Header
   Flags word be set properly. If set when critical extensions of the
   appropriate category are not present, frames may be unnecessarily
   discarded. If not set when critical extensions are present, frames
   may be mishandled or corrupted and intended security policies may be

   The RBridge Channel Alert extended flags have the following security
   considerations. Implementations should keep in mind that they might
   be erroneously set in a frame. If either RBridge Channel Alert flag
   is found set in a frame that is not an RBridge Channel message
   [Channel], the flag MAY be cleared and should have no effect except,
   possibly, delaying processing of the frame. If either Channel Alert
   flag is erroneously omitted from a frame, desired per hop processing
   for the frame may not occur.

7. Acknowledgements

   The following are thanked for their valuable contributions: Thomas

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8. Normative References

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

   [RFC4020] - Kompella, K. and A. Zinin, "Early IANA Allocation of
         Standards Track Code Points", BCP 100, RFC 4020, February 2005.

   [RFC5226] - Narten, T. and H. Alvestrand, "Guidelines for Writing an
         IANA Considerations Section in RFCs", BCP 26, RFC 5226, May

   [RFC6325] - Perlman, R., D. Eastlake, D. Dutt, S. Gai, and A.
         Ghanwani, "Routing Bridges (RBridges): Base Protocol
         Specification", July 2011.

   [Channel] - draft-ietf-trill-rbridge-channel, work in progress.

   [RFC6326bis] - draft-eastlake-isis-rfc6326bis, work in progress.

9. Informative References

   [Options] - draft-ietf-trill-rbridge-options, work in progress.
         - draft-eastlake-trill-rbridge-more-options, work in progress.

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Change History

   The sections below summarize changes between successive versions of
   this draft. RFC Editor: Please delete this section before

   Version 00 of this draft is an extract and simplification of draft-
   ietf-trill-rbridge-options-05.txt as discussed at the TRILL Working
   Group meeting at IETF 81 and on the TRILL WG mailing list.

From -00 to -01

   1. Update Author Addresses.

   2. Assorted editorial changes.

From -01 to -02

   1. Addition of the Critical RBridge Channel Alert Flag.

   2. Assorted editorial and author changes.

From -02 to -03

   1. Replacement of Section 2.4 to eliminate detailed constraints on
      the processing of conflicting extensions and to warn against the
      future specification of conflicting options.

   2. Minor editorial changes.

From -03 to -04

   Editorial changes including the deletion of Section 2.3.2 that was
   completely redundant with earlier parts of Section 2.3.

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Authors' Addresses

   Donald Eastlake
   Huawei R&D USA
   155 Beaver Street
   Milford, MA 01757 USA

   Phone: +1-508-333-2270

   Anoop Ghanwani
   350 Holger Way
   San Jose, CA 95134 USA

   Phone: +1-408-571-3500

   Vishwas Manral
   HP Networking
   19111 Pruneridge Avenue
   Cupertino, CA 95014 USA

   Phone: +1-408-477-0000

   Yizhou Li
   Huawei Technologies
   101 Software Avenue,
   Nanjing 210012, China

   Phone: +86-25-56622310

   Caitlin Bestler
   1650 Technology Drive , Suite 700
   San Jose, CA 95110 USA

   Phone: +1-408-944-4000

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