Transparent Interconnection of Lots of Links (TRILL): MTU Negotiation

Internet Engineering Task Force (IETF)                          M. Zhang
Request for Comments: 8249                                      X. Zhang
Updates: 6325, 7177, 7780                                D. Eastlake 3rd
Category: Standards Track                                         Huawei
ISSN: 2070-1721                                               R. Perlman
                                                                Dell EMC
                                                           S. Chatterjee
                                                                   Cisco
                                                          September 2017


         Transparent Interconnection of Lots of Links (TRILL):
                            MTU Negotiation

Abstract

   The base IETF TRILL (Transparent Interconnection of Lots of Links)
   protocol has a TRILL campus-wide MTU feature, specified in RFCs 6325
   and 7177, that assures that link-state changes can be successfully
   flooded throughout the campus while being able to take advantage of a
   campus-wide capability to support jumbo packets.  This document
   specifies recommended updates to that MTU feature to take advantage,
   for appropriate link-local packets, of link-local MTUs that exceed
   the TRILL campus MTU.  In addition, it specifies an efficient
   algorithm for local MTU testing.  This document updates RFCs 6325,
   7177, and 7780.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8249.

Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   described in the Simplified BSD License.

Table of Contents

   1. Introduction ....................................................3
      1.1. Conventions Used in This Document ..........................3
   2. Link-Wide TRILL MTU Size ........................................4
      2.1. Operations .................................................5
   3. Testing Link MTU Size ...........................................6
   4. Refreshing Sz ...................................................8
   5. Relationship between Port MTU, Lz, and Sz .......................9
   6. LSP Synchronization ............................................10
   7. Recommendations for Traffic Link Testing of MTU Size ...........10
   8. Backward Compatibility .........................................11
   9. Security Considerations ........................................11
   10. Additions to Configuration ....................................12
      10.1. Per-RBridge Configuration ................................12
      10.2. Per-RBridge Port Configuration ...........................12
   11. IANA Considerations ...........................................12
   12. References ....................................................12
      12.1. Normative References .....................................12
      12.2. Informative References ...................................14
   Acknowledgements ..................................................14
   Authors' Addresses ................................................14

1.  Introduction

   [RFC6325] describes the way RBridges agree on the campus-wide minimum
   acceptable inter-RBridge MTU (Maximum Transmission Unit) size (called
   "Sz") to ensure that link-state flooding operates properly and all
   RBridges converge to the same link state.  For the proper operation
   of TRILL (Transparent Interconnection of Lots of Links) IS-IS, all
   RBridges format their Link State Protocol Data Units (LSPs) to fit
   in Sz.

   [RFC7177] diagrams the state transitions of an adjacency.  If MTU
   testing is enabled, "Link MTU size is successfully tested" is part of
   an event (event A6) causing the transition from the "2-Way" state
   [RFC7177] to the "Report" state for an adjacency.  This means that
   the link MTU testing of size x succeeds, and x is greater than or
   equal to Sz [RFC6325].  If this link cannot support an MTU of Sz, it
   will not be reported as part of the campus topology.

   In this document, a new RECOMMENDED link-wide minimum inter-RBridge
   MTU size, "Lz", is specified.  As further discussed in Section 2, by
   calculating and using Lz as specified herein, link-scoped Protocol
   Data Units (PDUs) can be formatted greater than Sz, up to the
   link-wide minimum acceptable inter-RBridge MTU size, potentially
   improving the efficiency of link utilization and speeding link-state
   convergence.

   An optional TRILL MTU size-testing algorithm is specified in
   Section 3 as an efficient method to update the old MTU testing method
   described in Section 4.3.2 of [RFC6325] and in [RFC7177].  The new
   MTU size-testing method specified in this document is backward
   compatible with the old one.  Multicasting the MTU-probes is
   recommended when there are multiple RBridges on a link responding to
   the probing with an MTU-ack [RFC7177].  The testing method and rules
   of this document are devised in a way that minimizes the number of
   MTU-probes for testing, therefore reducing the number of multicast
   packets for MTU testing.

   This document updates RFCs 6325, 7177, and 7780.  The update to
   [RFC6325] and [RFC7177] is specified in Section 3.  The update to
   [RFC7780] is specified in Section 4.

1.1.  Conventions Used in This Document

   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.

2.  Link-Wide TRILL MTU Size

   This document specifies a new value "Lz" for the minimum acceptable
   inter-RBridge link MTU size on a local link.  Link-wide Lz is the
   minimum Lz supported and agreed upon amongst all RBridges on a
   specific link.  If the link is usable, Lz will be greater than or
   equal to Sz.

   Some TRILL IS-IS PDUs are exchanged only between neighbors instead of
   throughout the whole campus.  They are confined by the link-wide Lz
   instead of Sz.  Complete Sequence Number PDUs (CSNPs) and Partial
   Sequence Number PDUs (PSNPs) are examples of such PDUs.  These PDUs
   are exchanged only on the local link.  (While TRILL IS-IS Hellos are
   also link local, they are always limited to 1470 bytes for
   robustness.)

   [RFC7356] defines the PDUs that support flooding scopes in addition
   to area-wide scopes and domain-wide scopes.  As specified in
   [RFC8139], RBridges support the Extended L1 Circuit Scope (E-L1CS)
   Flooding Scope LSP (FS-LSP) [RFC7780].  The originatingSNPBufferSize
   for a port is the minimum of the following two quantities but
   not less than 1470 bytes: (1) the MTU of the port and (2) the maximum
   LSP size that the TRILL IS-IS implementation can handle.  They use
   that flooding to exchange their maximum supported value of "Lz".  The
   smallest value of the Lz advertised by the RBridges on a link, but
   not less than Sz, is the link-wide Lz.  An RBridge on a local link
   will be able to tell which other RBridges on that link support E-L1CS
   FS-LSPs because, as required by [RFC7780], all RBridges include the
   Scope Flooding Support TLV [RFC7356] in their TRILL Hellos.

   The maximum size for a level-1 link-local PDU (such as a PSNP or
   CSNP) that may be generated by a system is controlled by the value of
   the management parameter originatingL1SNPBufferSize.  This value
   determines Lz.  The TRILL APPsub-TLV shown in Figure 1 SHOULD be
   included in a TRILL GENINFO TLV [RFC7357] in an E-L1CS FS-LSP
   fragment zero.  If it is missing from an E-L1CS FS-LSP fragment zero
   or there is no E-L1CS FS-LSP fragment zero, it is assumed that its
   originating IS is implicitly advertising its originatingSNPBufferSize
   value as Sz octets.

   E-L1CS FS-LSPs are link local and can also be sent up to a size of Lz
   but, for robustness, E-L1CS FS-LSP fragment zero MUST NOT exceed
   1470 bytes.

              +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
              | Type = 21                     |   (2 bytes)
              +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
              | Length = 2                    |   (2 bytes)
              +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
              | originatingSNPBufferSize      |   (2 bytes)
              +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

             Figure 1: The originatingSNPBufferSize APPsub-TLV

   Type: Set to the originatingSNPBufferSize APPsub-TLV (TRILL
      APPsub-TLV type 21).  Two bytes, because this APPsub-TLV appears
      in an extended TLV [RFC7356].

   Length: Set to 2.

   originatingSNPBufferSize: The local value of
      originatingL1SNPBufferSize as an unsigned integer, limited to the
      range from 1470 to 65,535 bytes.  (A value less than 1470 will be
      ignored.)

2.1.  Operations

   Lz MAY be reported using an originatingSNPBufferSize APPsub-TLV that
   occurs in fragment zero of the RBridge's E-L1CS FS-LSP.  An
   originatingSNPBufferSize APPsub-TLV occurring in any other fragment
   is ignored.  If more than one originatingSNPBufferSize APPsub-TLV
   occurs in fragment zero, the one advertising the smallest value for
   originatingSNPBufferSize, but not less than 1470 bytes, is used.

   Even if all RBridges on a specific link have reached consensus on the
   value of link-wide Lz based on advertised originatingSNPBufferSize,
   it does not mean that these RBridges can safely exchange PDUs between
   each other.  Figure 2 shows such a corner case.  RB1, RB2, and RB3
   are three RBridges on the same link and their Lz is 1800, so the
   link-wide Lz of this link is 1800.  There is an intermediate bridge
   (say B1) between RB2 and RB3 whose port MTU size is 1700.  If RB2
   sends PDUs formatted in chunks of size 1800, those PDUs will be
   discarded by B1.

                         Lz:1800               Lz:1800
                          +---+         |         +---+
                          |RB1|(2000)---|---(2000)|RB2|
                          +---+         |         +---+
                                        |
                  Lz:1800               |
                   +---+               +--+
                   |RB3|(2000)---(1700)|B1|
                   +---+               +--+
                                        |

       Figure 2: Link-Wide Lz = 1800 vs. Tested Link MTU Size = 1700

   Therefore, the link MTU size SHOULD be tested.  After the link MTU
   size of an adjacency is successfully tested, those link-local PDUs,
   such as CSNPs, PSNPs, and E-L1CS FS-LSPs, will be formatted
   no greater than the tested link MTU size and will be safely
   transmitted on this link.

   As for Sz, RBridges continue to propagate their
   originatingL1LSPBufferSize across the campus through the
   advertisement of LSPs as defined in Section 4.3.2 of [RFC6325].  The
   smallest value of Sz advertised by any RBridge, but not less than
   1470, will be deemed as Sz.  Each RBridge formats their "campus-wide"
   PDUs -- for example, LSPs -- no greater than what they determine
   as Sz.

3.  Testing Link MTU Size

   [RFC7177] defines event A6 as indicating that the MTU test was
   successful if MTU testing is enabled.  As described in Section 4.3.2
   of [RFC6325], this is a combination of the following event and
   condition:

   o  Event: The link MTU size has been tested.

   o  Condition: The link can support Sz.

   This condition can be efficiently tested by the following "binary
   search algorithm" and rules.  This updates [RFC6325] and [RFC7177].

   x, lowerBound, and upperBound are local integer variables.  The
   MTU-probe and MTU-ack PDUs are specified in Section 3 of [RFC7176].
   It is RECOMMENDED that one Round-Trip Time (RTT) between the two
   adjacent RBridges be used as the minimum interval between two
   successive probes.  Note that RTT estimation is out of scope for this
   document.  If operators cannot estimate the RTT, the default value of
   5 milliseconds should be assumed.

   Step 0: RB1 sends an MTU-probe padded to the size of link-wide Lz.

      1) If RB1 successfully receives the MTU-ack from RB2 to the probe
         of the value of link-wide Lz within k tries (where k is a
         configurable parameter whose default is 3), the link MTU size
         is set to the size of link-wide Lz.  Stop.

      2) RB1 tries to send an MTU-probe padded to 1470 bytes.

         a) If RB1 fails to receive an MTU-ack from RB2 after k tries
            (an MTU-ack should be considered to have failed two RTTs
            after the probe is sent out), RB1 sets the "failed minimum
            MTU test" flag for RB2 in RB1's Hello.  Stop.

         b) The link MTU size is set to 1470; lowerBound is set to 1470;
            upperBound is set to the link-wide Lz; x is set to
            [(lowerBound + upperBound) / 2], rounded down to the nearest
            integer.

   Step 1: RB1 tries to send an MTU-probe padded to the size x.

      1) If RB1 fails to receive an MTU-ack from RB2 after k tries:

         upperBound is set to x - 1; x is set to
         [(lowerBound + upperBound) / 2], rounded down to the nearest
         integer.

      2) If RB1 receives an MTU-ack to a probe of size x from RB2:

         The link MTU size is set to x; lowerBound is set to x; x is set
         to [(lowerBound + upperBound) / 2], rounded down to the nearest
         integer.  If lowerBound equals upperBound - 1, then x is set to
         upperBound.

      3) If lowerBound >= upperBound or Step 1 has been repeated n times
         (where n is a configurable parameter whose default value is 5),
         stop.

      4) Repeat Step 1.

   After the testing, the two connected RBridges agree on the value of
   the link MTU size.  MTU testing is only done in the Designated VLAN
   [RFC7177].  Since the execution of the above algorithm can be
   resource consuming, it is RECOMMENDED that the Designated RBridge
   (DRB) [RFC7177] take the responsibility to do the testing.  Multicast
   MTU-probes are used instead of unicast when multiple RBridges are
   desired to respond with an MTU-ack on the link.  The binary search
   algorithm given here is a way to minimize the probing attempts; it
   reduces the number of multicast packets for MTU-probing.

   The following rules are designed to determine whether the
   aforementioned "Condition" holds.

   RBridges have figured out the upper bound and lower bound of the link
   MTU size from the execution of the above algorithm.  If Sz is smaller
   than the lower bound or greater than the upper bound, RBridges can
   directly judge whether the link supports Sz without MTU-probing.

   (a) If lowerBound >= Sz, this link can support Sz.

   (b) Else if upperBound <= Sz, this link cannot support Sz.

   Otherwise, RBridges SHOULD test whether the link can support Sz as in
   item (c) below.  If they do not, the only safe assumption will be
   that the link cannot support Sz.  This assumption, without testing,
   might rule out the use of a link that can, in fact, handle packets up
   to Sz.  In the worst case, this might result in unnecessary network
   partition.

   (c) lowerBound < Sz < upperBound.  RBridges probe the link with
       MTU-probe messages padded to Sz.  If an MTU-ack is received
       within k tries, this link can support Sz.  Otherwise, this link
       cannot support Sz.  Through this test, the lower bound and upper
       bound of the link MTU size can be updated accordingly.

4.  Refreshing Sz

   RBridges may join or leave the campus; this may change Sz.

   1) Joining

      a) When a new RBridge joins the campus and its
         originatingL1LSPBufferSize is smaller than the current Sz,
         reporting its originatingL1LSPBufferSize in its LSPs will cause
         other RBridges to decrease their Sz.  Then, any LSP greater
         than the reduced Sz MUST be split, and/or the LSP contents in
         the campus MUST be otherwise redistributed so that no LSP is
         greater than the new Sz.

      b) If the joining RBridge's originatingL1LSPBufferSize is greater
         than or equal to the current Sz, reporting its
         originatingL1LSPBufferSize will not change Sz.

   2) Leaving

      a) From the specification of the Joining process, we know that if
         an RBridge's originatingL1LSPBufferSize is smaller than Sz,
         this RBridge will not join this campus.

      b) When an RBridge leaves the campus and its
         originatingL1LSPBufferSize equals Sz, its LSPs are purged from
         the remainder of the campus after reaching MaxAge [IS-IS].  Sz
         MAY be recalculated and MAY increase.  In other words, while in
         most cases RB1 ignores link-state information for IS-IS
         unreachable RBridge RB2 [RFC7780], originatingL1LSPBufferSize
         is meaningful.  Its value, even from IS-IS unreachable
         RBridges, is used in determining Sz.  This updates [RFC7780].

      c) When an RBridge leaves the campus and its
         originatingL1LSPBufferSize is greater than Sz, Sz will not be
         updated, since Sz is determined by another RBridge with a
         smaller originatingL1LSPBufferSize.

   Frequent LSP "resizing" is harmful to the stability of the TRILL
   campus, so, to avoid this, upward resizing SHOULD be dampened.  When
   an upward resizing event is noticed by an RBridge, it is RECOMMENDED
   that a timer be set at that RBridge via a configurable parameter --
   LSPresizeTime -- whose default value is 300 seconds.  Before this
   timer expires, all subsequent upward resizing will be dampened
   (ignored).  Of course, in a well-configured campus with all RBridges
   configured to have the same originatingL1LSPBufferSize, no resizing
   will be necessary.  It does not matter if different RBridges have
   different dampening timers or if some RBridges resize upward more
   quickly than others.

   If the refreshed Sz is smaller than the lower bound or greater than
   the upper bound of the tested link MTU size, the issue of resource
   consumption from testing the link MTU size can be avoided according
   to rule (a) or (b) as specified in Section 3.  Otherwise, RBridges
   test the link MTU size according to rule (c).

5.  Relationship between Port MTU, Lz, and Sz

   When the port MTU of an RBridge is smaller than the local
   originatingL1SNPBufferSize of an RBridge (an inconsistent
   configuration), that port SHOULD be disabled, since, in any case, an
   adjacency cannot be formed through such a port.  On the other hand,
   when an RBridge receives an LSP or E-L1CS FS-LSP with size greater
   than the link-wide Lz or Sz but not greater than its port MTU size,
   this LSP is processed normally.  If the size of an LSP is greater
   than the MTU size of a port over which it is to be propagated, this
   LSP MUST NOT be sent over the port and an LSPTooLargeToPropagate
   alarm shall be generated [IS-IS].

6.  LSP Synchronization

   An RBridge participates in LSP synchronization on a link as soon as
   it has at least one adjacency on that link that has advanced to at
   least the 2-Way state [RFC7177].  On a LAN link, CSNPs and PSNPs are
   used for synchronization.  On a point-to-point link, only PSNPs are
   used.

   The CSNPs and PSNPs can be formatted in chunks of size (at most)
   link-wide Lz but are processed normally if received having a larger
   size.  Since the link MTU size may not have been tested in the 2-Way
   state, link-wide Lz may be greater than the supported link MTU size.
   In that case, a CSNP or PSNP may be discarded.  After the link MTU
   size is successfully tested, RBridges will begin to format these PDUs
   with a size no greater than that MTU; therefore, these PDUs will
   eventually get through.

   Note that the link MTU size is frequently greater than Sz.
   Link-local PDUs are limited in size by the link MTU size rather than
   Sz, which, when Lz is greater than Sz, promises a reduction in the
   number of PDUs and a faster LSP synchronization process.

7.  Recommendations for Traffic Link Testing of MTU Size

   Sz and link-wide Lz are used to limit the size of most TRILL IS-IS
   PDUs.  They are different from the MTU size restricting the size of
   TRILL Data packets.  The size of a TRILL Data packet is restricted by
   the physical MTU of the ports and links the packet traverses.  It is
   possible that a TRILL Data packet successfully gets through the
   campus but its size is greater than Sz or link-wide Lz values.

   The algorithm defined for testing the link MTU size can also be used
   in TRILL traffic MTU size testing; in that case, the link-wide Lz
   used in that algorithm is replaced by the port MTU of the RBridge
   sending MTU-probes.  The successfully tested size x MAY be advertised
   as an attribute of this link, using the MTU sub-TLV defined in
   [RFC7176].

   Unlike RBridges, end stations do not participate in the exchange of
   TRILL IS-IS PDUs; therefore, they cannot grasp the traffic link MTU
   size from a TRILL campus automatically.  An operator may collect
   these values using network management tools such as TRILL ping or
   TraceRoute.  Then, the path MTU can be set as the smallest tested
   link MTU on this path, and end stations should not generate frames
   that -- when encapsulated as TRILL Data packets -- exceed this
   path MTU.

8.  Backward Compatibility

   There can be a mixture of Lz-ignorant and Lz-aware RBridges on a
   link.  This configuration will behave properly, although it may
   not be as efficient as it would be if all RBridges on the link are
   Lz aware.

   For an Lz-ignorant RBridge, TRILL IS-IS PDUs are always formatted
   no greater than Sz.  Lz-aware RBridges as receivers can handle these
   PDUs, since they cannot be greater than the link-wide Lz.

   For an Lz-aware RBridge, in the case that link-wide Lz is greater
   than Sz, larger link-local TRILL IS-IS PDUs can be sent out to
   increase efficiency.  Lz-ignorant RBridges as receivers will have
   no problem handling them, since the originatingL1LSPBufferSize value
   of these RBridges had been tested and the link-wide Lz is not greater
   than that value.

   An Lz-ignorant RBridge might not support the link MTU size-testing
   algorithm defined in Section 3 but could be using some algorithm just
   to test for the Sz MTU on the link.  In any case, if an RBridge per
   [RFC6325] receives an MTU-probe, it MUST respond with an MTU-ack
   padded to the same size as the MTU-probe.

9.  Security Considerations

   This document raises no significant new security issues for TRILL.
   In TRILL, RBridges are generally considered to be trusted devices.
   Protection against forged TRILL IS-IS PDUs, including forged Hellos
   containing originatingSNPBufferSize APPsub-TLVs, can be obtained
   through IS-IS PDU cryptographic authentication [RFC5310].  The worst
   that an RBridge can do by reporting an erroneous
   originatingSNPBufferSize is reduce Lz to Sz and thus make unavailable
   the optimization of being able to use link MTUs that exceed the
   campus-wide MTU for link-local TRILL IS-IS PDUs.

   For general and adjacency-related TRILL security considerations, see
   [RFC6325] and [RFC7177].

10.  Additions to Configuration

   Implementation of the features specified in this document adds two
   RBridge configuration parameters, as follows:

10.1.  Per-RBridge Configuration

   Each RBridge implementing the RECOMMENDED LSP resizing damping
   strategy specified in Section 4 has an LSPresizeTime parameter that
   is an integer in the range of 0-65,535 and that defaults to 300.  It
   is the number of seconds for which an RBridge determines that Sz has
   increased before it will create any LSP or E-L1FS FS-LSP fragments.

10.2.  Per-RBridge Port Configuration

   Each RBridge port on which the calculation and use of Lz are
   implemented has an originatingL1SNPBufferSize parameter that is an
   integer in the range of 1470-65,535.  This parameter defaults to the
   minimum of the size that the port can accommodate and the link-local
   IS-IS PDU size that the TRILL implementation can accommodate.

11.  IANA Considerations

   IANA has assigned a new APPsub-TLV type for the TRILL
   originatingSNPBufferSize APPsub-TLV defined in Section 2 of this
   document.  This new type has been assigned from the range less than
   256 in the "TRILL APPsub-TLV Types under IS-IS TLV 251 Application
   Identifier 1" registry.  The entry is as follows:

      Type  Name                      Reference
      ----  ------------------------  ---------
      21    originatingSNPBufferSize  RFC 8249

12.  References

12.1.  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>.

   [RFC5310]  Bhatia, M., Manral, V., Li, T., Atkinson, R., White, R.,
              and M. Fanto, "IS-IS Generic Cryptographic
              Authentication", RFC 5310, DOI 10.17487/RFC5310,
              February 2009, <https://www.rfc-editor.org/info/rfc5310>.

   [RFC6325]  Perlman, R., Eastlake 3rd, D., Dutt, D., Gai, S., and A.
              Ghanwani, "Routing Bridges (RBridges): Base Protocol
              Specification", RFC 6325, DOI 10.17487/RFC6325, July 2011,
              <https://www.rfc-editor.org/info/rfc6325>.

   [RFC7176]  Eastlake 3rd, D., Senevirathne, T., Ghanwani, A., Dutt,
              D., and A. Banerjee, "Transparent Interconnection of Lots
              of Links (TRILL) Use of IS-IS", RFC 7176,
              DOI 10.17487/RFC7176, May 2014,
              <https://www.rfc-editor.org/info/rfc7176>.

   [RFC7177]  Eastlake 3rd, D., Perlman, R., Ghanwani, A., Yang, H., and
              V. Manral, "Transparent Interconnection of Lots of Links
              (TRILL): Adjacency", RFC 7177, DOI 10.17487/RFC7177,
              May 2014, <https://www.rfc-editor.org/info/rfc7177>.

   [RFC7356]  Ginsberg, L., Previdi, S., and Y. Yang, "IS-IS Flooding
              Scope Link State PDUs (LSPs)", RFC 7356,
              DOI 10.17487/RFC7356, September 2014,
              <https://www.rfc-editor.org/info/rfc7356>.

   [RFC7357]  Zhai, H., Hu, F., Perlman, R., Eastlake 3rd, D., and O.
              Stokes, "Transparent Interconnection of Lots of Links
              (TRILL): End Station Address Distribution Information
              (ESADI) Protocol", RFC 7357, DOI 10.17487/RFC7357,
              September 2014, <https://www.rfc-editor.org/info/rfc7357>.

   [RFC7780]  Eastlake 3rd, D., Zhang, M., Perlman, R., Banerjee, A.,
              Ghanwani, A., and S. Gupta, "Transparent Interconnection
              of Lots of Links (TRILL): Clarifications, Corrections, and
              Updates", RFC 7780, DOI 10.17487/RFC7780, February 2016,
              <https://www.rfc-editor.org/info/rfc7780>.

   [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>.

12.2.  Informative References

   [IS-IS]    International Organization for Standardization,
              "Information technology -- Telecommunications and
              information exchange between systems -- Intermediate
              System to Intermediate System intra-domain routeing
              information exchange protocol for use in conjunction with
              the protocol for providing the connectionless-mode network
              service (ISO 8473)", ISO/IEC 10589:2002, Second Edition,
              November 2002.

   [RFC8139]  Eastlake 3rd, D., Li, Y., Umair, M., Banerjee, A., and F.
              Hu, "Transparent Interconnection of Lots of Links (TRILL):
              Appointed Forwarders", RFC 8139, DOI 10.17487/RFC8139,
              June 2017, <https://www.rfc-editor.org/info/rfc8139>.

Acknowledgements

   The authors would like to thank Vishwas Manral for his comments and
   suggestions.

Authors' Addresses

   Mingui Zhang
   Huawei Technologies
   No. 156 Beiqing Rd. Haidian District
   Beijing  100095
   China

   Phone: +86-13810702575
   Email: zhangmingui@huawei.com


   Xudong Zhang
   Huawei Technologies
   No. 156 Beiqing Rd. Haidian District
   Beijing  100095
   China

   Email: zhangxudong@huawei.com
   Donald Eastlake 3rd
   Huawei Technologies
   155 Beaver Street
   Milford, MA  01757
   United States of America

   Phone: +1-508-333-2270
   Email: d3e3e3@gmail.com


   Radia Perlman
   Dell EMC
   505 1st Ave South
   Seattle, WA  98104
   United States of America

   Email: radia@alum.mit.edu


   Somnath Chatterjee
   Cisco Systems
   SEZ Unit, Cessna Business Park
   Outer Ring Road
   Bangalore  560087
   India

   Email: somnath.chatterjee01@gmail.com