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TRILL: ECN (Explicit Congestion Notification) Support

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This is an older version of an Internet-Draft whose latest revision state is "Active".
Authors Donald E. Eastlake 3rd , Bob Briscoe
Last updated 2016-10-21 (Latest revision 2016-10-19)
Replaces draft-eastlake-trill-ecn-support
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Send notices to "Susan Hares" <>
TRILL Working Group                                      Donald Eastlake
INTERNET-DRAFT                                                    Huawei
Intended status: Proposed Standard                           Bob Briscoe
                                                     Simula Research Lab
Expires: April 18, 2017                                 October 19, 2016

         TRILL: ECN (Explicit Congestion Notification) Support


   Explicit congestion notification (ECN) allows a forwarding element to
   notify downstream devices, including the destination, of the onset of
   congestion without having to drop packets. This document extends this
   capability to TRILL switches, including integration with IP ECN.

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 <>.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-

   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."

   The list of current Internet-Drafts can be accessed at The list of Internet-Draft
   Shadow Directories can be accessed at

D. Eastlake & B.Briscoe                                         [Page 1]
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Table of Contents

      1. Introduction............................................3
      1.1 Conventions used in this document......................4

      2. The ECN Specific Extended Header Flags..................5

      3. ECN Support.............................................6
      3.1 Ingress ECN Support....................................6
      3.2 Transit ECN Support....................................6
      3.3 Egress ECN Support.....................................7

      4. TRILL Support for ECN Variants..........................9
      4.1 Pre-Congestion Notification (PCN)......................9
      4.2 Low Latency, Low Loss, Scalable Throughput (L4S)......10

      5. IANA Considerations....................................11
      6. Security Considerations................................12
      7. Acknowledgements.......................................12

      Normative References......................................13
      Informative References....................................13

      Appendix A. TRILL Transit RBridge Behavior to Support L4S.15

      Authors' Addresses........................................17

D. Eastlake & B.Briscoe                                         [Page 2]
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1. Introduction

   Explicit congestion notification (ECN [RFC3168]) allows a forwarding
   element, such as a router, to notify downstream devices, including
   the destination, of the onset of congestion without having to drop
   packets.  Instead, the forwarding element can explicitly mark a
   proportion of packets in an ECN field. For example, a two-bit field
   is available for ECN marking in IP headers.

                     .                           .
                 +---------+                     .
    +------+     | Ingress |                     .
    |Source|  +->| RBridge |                     .   +----------+
    +---+--+  |  |   RB1   |                     .   |Forwarding|
        |     |  +------+--+  +----------+       .   | Element  |
        v     |      .  |     | Transit  |       .   |    Y     |
      +-------+--+   .  +---->| RBridges |       .   +--------+-+
      |Forwarding|   .        |   RBn    |       .      ^     |
      | Element  |   .        +-------+--+  +---------+ |     v
      |    X     |   .                |     | Egress  | |  +-----------+
      +----------+   .                +---->| RBridge +-+  |Destination|
                     .                      |   RB9   |    +-----------+
                     .  TRILL               +---------+
                     .  campus                   .

                  Figure 1. Example Path Forwarding Nodes

   In [RFC3168] it was recognized that tunnels and lower layer protocols
   would need to support ECN, and ECN markings would need to be
   propagated, as headers were encapsulated and decapsulated.
   [ECNencapGuide] gives guidelines on the addition of ECN to protocols
   like TRILL that often encapsulate IP packets, including propagation
   of ECN from and to IP.

   In the figure above, assuming IP traffic, RB1 is an encapsulator and
   RB9 a decapsulator. Traffic from Source to RB1 might or might not get
   marked as having experienced congestion in forwarding elements, such
   as X, before being encapsulated at ingress RB1. Any such ECN marking
   is encapsulated with a TRILL Header.

   This specification provides for any ECN marking in the traffic at the
   ingress to be copied into the TRILL Extension Header Flags Word. It
   also enables congestion marking by a congested RBridge such as RBn or
   RB1 above in the TRILL Header Extension Flags Word [RFC7179].

   At RB9, the TRILL egress, it specifies how any ECN markings in the
   TRILL Header Flags Word and in the encapsulated traffic are combined

D. Eastlake & B.Briscoe                                         [Page 3]
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   so that subsequent forwarding elements, such as Y and the
   Destination, can see if congestion was experienced at any previous
   point in the path from Source.

   A large part of the guidelines for adding ECN to lower layer
   protocols [ECNencapGuide] concerns safe propagation of congestion
   notifications in scenarios where some of the nodes do not support or
   understand ECN. Whichever RBridges do not support ECN, this
   specification ensures congestion notification will propagate safely
   to Destination or the packet will be dropped if congestion
   notification cannot be propagaed.

1.1 Conventions used in this document

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

   In this documents, "IP" refers to both IPv4 and IPv6.

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


      AQM - Active Queue Management

      CCE - Critical Congestion Experienced

      CE - Congestion Experienced

      CItE - Critical Ingress-to-Egress

      ECN - Explicit Congestion Notification

      ECT - ECN Capable Transport

      L4S - Low Latency, Low Loss, Scalable throughput

      NCHbH - Non-Critical Hop-by-Hop

      NCCE - Non-Critical Congestion Experienced

      Not-ECT - Not ECN-Capable Transport

      PCN - Pre-Congestion Notification

D. Eastlake & B.Briscoe                                         [Page 4]
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2. The ECN Specific Extended Header Flags

   The extension header fields for explicit congestion notification
   (ECN) in TRILL are defined as a two-bit TRILL-ECN field and a one-bit
   Critical Congestion Experienced (CCE) field in the TRILL Header
   Extension Flags Word [RFC7780].

   These fields are show in Figure 2 as "ECN" and "CCE". The TRILL-ECN
   field consists of bits 12 and 13, which are in the range reserved for
   non-critical hop-by-hop (NCHbH) bits. The CCE field consists of bit
   26, which is in the range reserved for Critical Ingress-to-Egress
   (CItE) bits. See [RFC7780] and [RFC7179] for the meaning of the other

       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
      |Crit.|  CHbH   |   NCHbH   |CRSV | NCRSV |   CItE    |  NCItE  |
      |C|C|C|       |C|N|     |   |     |       |         | |   |     |
      |R|R|R|       |R|C|     |ECN| Ext |       |         |C|Ext|     |
      |H|I|R|       |C|C|     |   | Hop |       |         |C|Clr|     |
      |b|t|s|       |A|A|     |   | Cnt |       |         |E|   |     |
      |H|E|v|       |F|F|     |   |     |       |         | |   |     |

     Figure 2 The ECN and CCE TRILL Header Extension Flags Word Fields

   Table 1 shows the meaning of the codepoints in the TRILL-ECN field.
   Note that the first three have the same meaning as the corresponding
   ECN field codepoints in the IPv4 or IPv6 header as defined in
   [RFC3168]. However codepoint 11 is called Non-Critical Congestion
   Experienced (NCCE) to distinguish it from Congestion Experienced in

          Binary  Name     Meaning
          ------  -------  -----------------------------------
            00     Not-ECT Not ECN-Capable Transport
            01     ECT(1)  ECN-Capable Transport (1)
            10     ECT(0)  ECN-Capable Transport (0)
            11     NCCE    Non-Critical Congestion Experienced

                    Table 1. TRILL-ECN Field Codepoints

D. Eastlake & B.Briscoe                                         [Page 5]
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3. ECN Support

   The subsections below describe the required behavior to support ECN
   at TRILL ingress, transit, and egress. The ingress behavior logically
   occurs as a native frame is encapsulated with a TRILL Header to
   produce a TRILL Data packet. The transit behavior logically occurs in
   all RBridges where TRILL Data packets are queued, usually at the
   output port.  The egress behavior logically occurs as a TRILL Data
   packet is decapsulated and output as a native frame through an
   RBridge port.

   An RBridge that supports ECN MUST behave as described in the relevant
   subsections below, which correspond to the recommended provisions of
   [ECNencapGuide]. Nonetheless, the scheme is designed to safely
   propagate some form of congestion notification even if some RBridges
   in the path followed by a TRILL Data packet support ECN and others do

3.1 Ingress ECN Support

   The ingress behavior is as follows:

   o  When encapsulating an IP frame, the ingress RBridge MUST:

      +  set the F flag in the main TRILL header [RFC7780];
      +  create a Flags Word as part of the TRILL Header;
      +  copy the two ECN bits from the IP header into the TRILL-ECN
         field (Flags Word bits 12 and 13)
      +  ensure the CCE flag is cleared to zero (Flags Word bit 26).

   o  When encapsulating a frame for a non-IP protocol, where that
      protocol has a means of indicating ECN that is understood by the
      ingress RBridge, it MUST follow the guidelines in [ECNencapGuide]
      to add a Flags Word to the TRILL Header. For a non-IP protocol
      with a similar ECN field to IP, this would be achieved by copying
      into the TRILL-ECN field from the encapsulated native frame.

3.2 Transit ECN Support

   The transit behavior, show below, is required at all RBridges where
   TRILL Data packets are queued, usually at the output port.

   o  An RBridge that supports ECN MUST implement some form of active
      queue management (AQM) according to the guidelines of [RFC7567].
      The RBridge detects congestion either by monitoring its own queue
      depth or by participating in a link-specific protocol.

D. Eastlake & B.Briscoe                                         [Page 6]
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   o  If the TRILL Header Flags Word is present, whenever the AQM
      algorithm decides to indicate congestion on a TRILL Data packet it
      MUST set the CCE flag (Flags Word bit 26).

   o  If the TRILL header Flags Word is not present, to indicate
      congestion the RBridge will either drop the packet or it MAY do
      all of the following instead:

      +  set the F flag in the main TRILL header;
      +  add a Flags Word to the TRILL Header;
      +  set the TRILL-ECN field to Not-ECT (00);
      +  and set the CCE flag and the Ingress-to-Egress critical summary
         bit (CRIbE).

   Note that a transit RBridge that supports ECN does not refer to the
   TRILL-ECN field before signalling CCE in a packet. It signals CCE
   irrespective of whether the packet indicates that the transport is
   ECN-capable. The egress/decapsulation behavior (described next)
   ensures that a CCE indication is converted to a drop if the transport
   is not ECN-capable.

3.3 Egress ECN Support

   If the egress RBridge does not support ECN, it will ignore bits 12
   and 13 of any Flags Word that is present, because it does not contain
   any special ECN logic. Nonetheless, if a transit RBridge has set the
   CCE flag, the egress will drop the packet. This is because drop is
   the default behavior for an RBridge decapsulating a Critical Ingress-
   to-Egress flag when it has no specific logic to understand it. Drop
   is the intended behavior for such a packet, as required by

   If an RBridge supports ECN, the egress behavior is as follows:

   o  When decapsulating an inner IP packet, the RBridge sets the ECN
      field of the outgoing native IP packet using Table 2. It MUST set
      the ECN field of the outgoing IP packet to the codepoint at the
      intersection of the row for the arriving encapsulated IP packet
      and the column for 3-bit ECN codepoint in the arriving outer TRILL
      Data packet TRILL Header. If no TRILL Header Extension Flags Word
      is present, the 3-bit ECN codepoint is assumed to be all zero
         The name of the TRILL 3-bit ECN codepoint is defined using the
      combination of the TRILL-ECN and CCE fields in Table 3.
      Specifically, the TRILL 3-bit ECN codepoint is called CE if either
      NCCE or CCE is set in the TRILL Header Extension Flags Word.
      Otherwise it has the same name as the 2-bit TRILL-ECN codepoint.
         In the case where the TRILL 3-bit ECN codepoint indicates

D. Eastlake & B.Briscoe                                         [Page 7]
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      congestion experienced (CE) but the encapsulated native IP frame
      indicates a not ECN-capable transport (Not-ECT), the RBridge MUST
      drop the packet.  Such packet dropping is necessary because a
      transport above the IP layer that is not ECN-capable will have no
      ECN logic, so it will only understand dropped packets as an
      indication of congestion.

   o  When decapsulating a non-IP protocol frame with a means of
      indicating ECN that is understood by the RBridge, it MUST follow
      the guideines in [ECNencapGuide] when setting the ECN information
      in the decapsulated native frame. For a non-IP protocol with a
      similar ECN field to IP, this would be achieved by combining the
      information in the TRILL Header Flags Word with the encapsulated
      non-IP native frame, as specified in Table 2.

        | Inner   |  Arriving TRILL 3-bit ECN Codepoint Name     |
        | Native  +---------+------------+------------+----------+
        | Header  | Not-ECT | ECT(0)     | ECT(1)     |     CE   |
        | Not-ECT | Not-ECT | Not-ECT(*) | Not-ECT(*) |  <drop>  |
        |  ECT(0) |  ECT(0) |  ECT(0)    |  ECT(1)    |     CE   |
        |  ECT(1) |  ECT(1) |  ECT(1)(*) |  ECT(1)    |     CE   |
        |    CE   |      CE |      CE    |      CE(*) |     CE   |

                       Table 2: Egress ECN Behavior

   An asterisk in the above table indicates a currently unused
   combination that SHOULD be logged. In contrast to [RFC6040], in TRILL
   the drop condition is the result of a valid combination of events and
   need not be logged.

                | TRILL-ECN  | CCE | Arriving TRILL 3-bit|
                |            |     | ECN codepoint name  |
                | Not-ECT 00 |  0  | Not-ECT             |
                | ECT(1)  01 |  0  | ECT(1)              |
                | ECT(0)  10 |  0  | ECT(0)              |
                | NCCE    11 |  0  | CE                  |
                | Not-ECT 00 |  1  | CE                  |
                | ECT(1)  01 |  1  | CE                  |
                | ECT(0)  10 |  1  | CE                  |
                | NCCE    11 |  1  | CE                  |

   Table 3: Mapping of TRILL-ECN and CCE Fields to TRILL 3-bit ECN
                              Codepoint Name

D. Eastlake & B.Briscoe                                         [Page 8]
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4. TRILL Support for ECN Variants

   This section is informative, not normative.

   Section 3 specifies interworking between TRILL and the original
   standardized form of ECN in IP [RFC3168].

   The ECN wire protocol for TRILL (Section 2) has been designed to
   support the other known variants of ECN, as detailed below. New
   variants of ECN will have to comply with the guidelines for defining
   alternative ECN semantics [RFC4774]. It is expected that the TRILL
   ECN wire protocol is generic enough to support such potential future

4.1 Pre-Congestion Notification (PCN)

   The PCN wire protocol [RFC6660] is recognised by the use of a PCN-
   compatible Diffserv codepoint in the IP header and a non-zero IP-ECN
   field. For TRILL or any lower layer protocol, equivalent traffic
   classification codepoints would have to be defined, but that is
   outside the scope of the current document.

   The PCN wire protocol is similar to ECN, except it indicates
   congestion with two levels of severity. It uses:

   o  11 (CE) as the most severe, termed the Excess-traffic-marked (ETM)

   o  01 ECT(1) as a lesser severity level, termed the Threshold-Marked
      (ThM) codepoint.

   To implement PCN on a transit RBridge would require a detailed
   specification. But in brief:

   o  the TRILL Critical Congestion Experienced (CCE) flag would be used
      for the Excess-Traffic-Marked (ETM) codepoint;

   o  ECT(1) in the TRILL-ECN field would be used for the Threshold-
      Marked codepoint.

   Then the ingress and egress behaviors defined in Section 3 would not
   need to be altered to ensure support for PCN as well as ECN.

D. Eastlake & B.Briscoe                                         [Page 9]
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4.2 Low Latency, Low Loss, Scalable Throughput (L4S)

   L4S is currently only a proposal being considered for adoption onto
   the IETF's experimental track. An outline of how a transit TRILL
   RBridge would support L4S [ECNL4S] is given in Appendix A.

D. Eastlake & B.Briscoe                                        [Page 10]
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5. IANA Considerations

   IANA is requested to update the TRILL Extended Header Flags registry
   by replacing the lines for bits 9-13 and for bits 21-26 with the

      Bits   Purpose                                       Reference
      -----  -------                                       ---------
       9-11  available non-critical hop-by-hop flags
      12-13  TRILL-ECN (Explicit Congestion Notification)  [this doc]

      21-25  available critical ingress-to-egress flags
         26  Critical Congestion Experienced (CCE)         [this doc]

D. Eastlake & B.Briscoe                                        [Page 11]
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6. Security Considerations

   TRILL support of ECN is a straight forward combination of previously
   specified ECN and TRILL with no significnat new security

   For ECN tunneling security considerations, see [RFC6040].

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

7. Acknowledgements

   This document was prepared with basic NROFF. All macros used were
   defined in the source file.

D. Eastlake & B.Briscoe                                        [Page 12]
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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, <>.

   [RFC3168] - Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
         of Explicit Congestion Notification (ECN) to IP", RFC 3168, DOI
         10.17487/RFC3168, September 2001, <http://www.rfc->.

   [RFC4774] - Floyd, S., "Specifying Alternate Semantics for the
         Explicit Congestion Notification (ECN) Field", BCP 124, RFC
         4774, DOI 10.17487/RFC4774, November 2006, <http://www.rfc->.

   [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,

   [RFC7179] - Eastlake 3rd, D., Ghanwani, A., Manral, V., Li, Y., and
         C. Bestler, "Transparent Interconnection of Lots of Links
         (TRILL): Header Extension", RFC 7179, DOI 10.17487/RFC7179, May
         2014, <>.

   [RFC7567] - Baker, F., Ed., and G. Fairhurst, Ed., "IETF
         Recommendations Regarding Active Queue Management", BCP 197,
         RFC 7567, DOI 10.17487/RFC7567, July 2015, <http://www.rfc->.

   [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,

   [ECNencapGuide] - B. Briscoe, J. Kaippallimalil, P. Thaler,
         "Guidelines for Adding Congestion Notification to Protocols
         that Encapsulate IP", draft-ietf-tsvwg-ecn-encap-guidelines,
         work in progress.

Informative References

   [RFC6040] - Briscoe, B., "Tunnelling of Explicit Congestion
         Notification", RFC 6040, DOI 10.17487/RFC6040, November 2010,

D. Eastlake & B.Briscoe                                        [Page 13]
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   [RFC6660] - Briscoe, B., Moncaster, T., and M. Menth, "Encoding Three
         Pre-Congestion Notification (PCN) States in the IP Header Using
         a Single Diffserv Codepoint (DSCP)", RFC 6660, DOI
         10.17487/RFC6660, July 2012, <http://www.rfc->.

   [ECNL4S] - K. De Schepper, B. Briscoe, I. Tsang, "Identifying
         Modified Explicit Congestion Notification (ECN) Semantics for
         Ultra-Low Queueing Delay", draft-briscoe-tsvwg-ecn-l4s-id, work
         in progress.

D. Eastlake & B.Briscoe                                        [Page 14]
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Appendix A. TRILL Transit RBridge Behavior to Support L4S

   An initial specification of the Low Latency, Low Loss, Scalable
   throughput (L4S) wire protocol for IP is given in [ECNL4S]. It is
   similar to the original ECN wire protoocl for IP [RFC3168], except:

   o  An AQM that supports L4S classifies packets with ECT(1) or CE in
      the IP header into an L4S queue and a "Classic" queue otherwise.

   o  the meaning of CE markings applied by an L4S queue is not the same
      as the meaning of a drop by a "Classic" queue (contrary to the
      original requirement for ECN [RFC3168]). Instead the likelihood
      that the Classic queue drops packets is defined as the square of
      the likelihood that the L4S queue marks packets (e.g. when there
      is a drop probability of 0.0009 (0.09%) the L4S marking
      probability will be 0.03 (3%)).

   This seems to present a problem for the way that a transit TRILL
   RBridge defers the choice between marking and dropping to the egress.
   Nonetheless, the following pseudocode outlines how a transit TRILL
   RBridge can implement L4S marking in such a way that the egress
   behavior already described in Section 3.3 for Classic ECN [RFC3168]
   will produce the desired outcome.

      /* p is an internal variable calculated by any L4S AQM
       *  dependent on the delay being experienced in the Classic queue.
       * bit23 is the least significant bit of the TRILL-ECN field

      % On TRILL transit
      if (bit23 == 0 ) {
            % Classic Queue
            if (p > max(random(), random()) )
               mark(CCE)                         % likelihood: p^2

      } else {
            % L4S Queue
            if (p > max(random()) ) {
               if (p > max(random()) )
                  mark(CCE)                      % likelihood: p^2
                  mark(NCCE)                     % likelihood: p - p^2

   With the above transit behavior, an egress that supports ECN (Section
   3.3) will drop packets or propagate their ECN markings depending on
   whether the arriving inner header is from a non-ECN-capable or ECN-
   capable transport.

D. Eastlake & B.Briscoe                                        [Page 15]
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   Even if an egress has no L4S-specific logic of its own, it will drop
   packets with the square of the probability that an egress would if it
   did support ECN, for the following reasons:

   o Egress with ECN support:

      +  L4S: propagates both the Critical and Non-Critical CE marks
         (CCE & NCCE) as a CE mark.

            Likelihood: p^2 + p - p^2 = p

      +  Classic: Propagates CCE marks as CE or drop, depending on

            Likelihood: p^2

   o Egress without ECN support:

      +  L4S: does not propagate NCCE as a CE mark, but drops CCE marks.

            Likelihood: p^2

      +  Classic: drops CCE marks.

            Likelihood: p^2

D. Eastlake & B.Briscoe                                        [Page 16]
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Authors' Addresses

      Donald E. Eastlake, 3rd
      Huawei Technologies
      155 Beaver Street
      Milford, MA 01757 USA

      Tel: +1-508-333-2270

      Bob Briscoe (editor)
      Simula Research Lab


D. Eastlake & B.Briscoe                                        [Page 17]
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D. Eastlake & B.Briscoe                                        [Page 18]