TRILL (TRansparent Interconnection of Lots of Links): ECN (Explicit Congestion Notification) Support
draft-ietf-trill-ecn-support-07

TRILL Working Group                                      Donald Eastlake
INTERNET-DRAFT                                                    Huawei
Intended status: Proposed Standard                           Bob Briscoe
                                                               CableLabs
Expires: August 24, 2018                               February 25, 2018

         TRILL (TRansparent Interconnection of Lots of Links):
             ECN (Explicit Congestion Notification) Support
                 <draft-ietf-trill-ecn-support-07.txt>

Abstract

   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 can improve network
   efficiency through better congestion control without packet drops.
   This document extends ECN to TRILL (TRansparent Interconnection of
   Lots of Links) switches, including integration with IP ECN, and
   provides for ECN marking in the TRILL Header Extension Flags Word
   (see RFC 7179).

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 <trill@ietf.org>.

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

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   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."

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   http://www.ietf.org/1id-abstracts.html. The list of Internet-Draft
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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..................6

      3. ECN Support.............................................7
      3.1 Ingress ECN Support....................................7
      3.2 Transit ECN Support....................................7
      3.3 Egress ECN Support.....................................8
      3.3.1 Non-ECN Egress RBridges..............................8
      3.3.2 ECN Egress RBridges..................................8

      4. TRILL Support for ECN Variants.........................11
      4.1 Pre-Congestion Notification (PCN).....................11
      4.2 Low Latency, Low Loss, Scalable Throughput (L4S)......12

      5. IANA Considerations....................................13
      6. Security Considerations................................14

      7. Acknowledgements.......................................14

      Normative References......................................15
      Informative References....................................16

      Appendix A. TRILL Transit RBridge Behavior to Support L4S.17

      Authors' Addresses........................................19

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

   Explicit congestion notification (ECN [RFC3168] [RFC8311]) 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. This can improve network efficiency through better
   congestion control without packet drops. The forwarding element can
   explicitly mark a proportion of packets in an ECN field instead of
   dropping the packet. 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 (TRansparent Interconnection of Lots of Links) 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 [RFC6325].

   This document specifies how ECN marking in traffic at the ingress is
   copied into the TRILL Extension Header Flags Word and requires such
   copying for IP traffic. It also enables congestion marking by a
   congested RBridge such as RBn or RB1 above in the TRILL Header
   Extension Flags Word [RFC7179].

D. Eastlake & B. Briscoe                                        [Page 3]
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   At RB9, the TRILL egress, it specifies how any ECN markings in the
   TRILL Header Flags Word and in the encapsulated traffic are combined
   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. Such ECN ignorance is not a major problem with
   RBridges using this specification because the method specified
   assures that, if an egress RBridge is ECN ignorant (so it cannot
   further propagate ECN) and congestion has been encountered, the
   egress RBridge will at least drop the packet and this drop will
   itself indicate congestion to end stations.

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",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119] [RFC8174]
   when, and only when, they appear in all capitals, as shown here.

   Acronyms:

      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

D. Eastlake & B. Briscoe                                        [Page 4]
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      Not-ECT - Not ECN-Capable Transport

      PCN - Pre-Congestion Notification

D. Eastlake & B. Briscoe                                        [Page 5]
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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 32-bit TRILL
   Header Extension Flags Word [RFC7780].

   These fields are shown 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. The CRItE bit is the critical Ingress-to-Egress summary
   bit and will be one if and only if any of the bits in the CItE range
   (21-26) are one or there is a critical feature invoked in some
   further extension of the TRILL Header after the Extension Flags Word.
   The other bits and fields shown in Figure 2 are not relevant to ECN.
   See [RFC7780], [RFC7179], and [IANAthFlags] for the meaning of these
   other bits and fields.

       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 TRILL-ECN and CCE
                 TRILL Header Extension Flags Word Fields

   Table 1 shows the meaning of the codepoints in the TRILL-ECN field.
   The first three have the same meaning as the corresponding ECN field
   codepoints in the IP header as defined in [RFC3168]. However,
   codepoint 0b11 is called Non-Critical Congestion Experienced (NCCE)
   to distinguish it from Congestion Experienced in IP.

          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 6]
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3. ECN Support

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

   The subsections below describe the required behavior to support ECN
   at TRILL ingress, transit, and egress. The ingress behavior occurs as
   a native frame is encapsulated with a TRILL Header to produce a TRILL
   Data packet. The transit behavior occurs in all RBridges where TRILL
   Data packets are queued, usually at the output port.  The egress
   behavior occurs where 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 in
   Section 3 and Sections 5.1-5.4 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 not.

3.1 Ingress ECN Support

   The behavior at an ingress RBridge 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 set 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 Section 5.3 of
      [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, shown 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

D. Eastlake & B. Briscoe                                        [Page 7]
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      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.

   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 signaling 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

3.3.1 Non-ECN Egress RBridges

   If the egress RBridge does not support ECN, that RBridge 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
   Section 5.4 of [ECNencapGuide].

3.3.2 ECN Egress RBridges

   If an RBridge supports ECN, for the two cases of an IP and a non-IP
   inner packet, the egress behavior is as follows:

      Decapsulating an inner IP packet: The RBridge sets the ECN field

D. Eastlake & B. Briscoe                                        [Page 8]
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         of the outgoing native IP packet using Table 3. 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 bits.

         The name of the TRILL 3-bit ECN codepoint is defined using the
         combination of the TRILL-ECN and CCE fields in Table 2.
         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
         congestion experienced (CE) but the encapsulated native IP
         frame indicates a not ECN-capable transport (Not-ECT), it can
         be seen that 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.

      Decapsulating a non-IP protocol frame: If the frame has a means of
         indicating ECN that is understood by the RBridge, it MUST
         follow the guidelines in Section 5.4 of [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 3.

                +------------+-----+---------------------+
                | 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 2. Mapping of TRILL-ECN and CCE Fields to
                    the TRILL 3-bit ECN Codepoint Name

D. Eastlake & B. Briscoe                                        [Page 9]
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        +---------+----------------------------------------------+
        | 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 3. Egress ECN Behavior

   An asterisk in the above table indicates a combination that is
   currently unused in all variants of ECN marking (see Section 4) and
   therefore SHOULD be logged.

   With one exception, the mappings in Table 3 are consistent with those
   for IP-in-IP tunnels [RFC6040], which ensures backward compatibility
   with all current and past variants of ECN marking (see Section 4). It
   also ensures forward compatibility with any future form of ECN
   marking that complies with the guidelines in [ECNencapGuide],
   including cases where ECT(1) represents a second level of marking
   severity below CE.

   The one exception is that the drop condition in Table 3 need not be
   logged because, with TRILL, it is the result of a valid combination
   of events.

D. Eastlake & B. Briscoe                                       [Page 10]
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4. TRILL Support for ECN Variants

   This section is informative, not normative; it discusses interworking
   between TRILL and variants of the standardized form of ECN in IP
   [RFC3168]. See also [RFC8311].

   The ECN wire protocol for TRILL (Section 2) and the ingress (Section
   3.1) and egress (Section 3.3) ECN behaviors have 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
   variants.

4.1 Pre-Congestion Notification (PCN)

   The PCN wire protocol [RFC6660] is recognized 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)
      codepoint

   o  01 ECT(1) as a lesser severity level, termed the Threshold-Marked
      (ThM) codepoint. (This difference between ECT(1) and ECT(0) only
      applies to PCN, not to the classic ECN support specified for TRILL
      in this document before Section 4.)

   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 11]
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4.2 Low Latency, Low Loss, Scalable Throughput (L4S)

   L4S is currently on 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 12]
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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
   following:

      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 13]
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6. Security Considerations

   TRILL support of ECN is a straightforward combination of previously
   specified ECN and TRILL with no significant new security
   considerations.

   For ECN tunneling security considerations, see [RFC6040].

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

7. Acknowledgements

   The helpful comments of Loa Andersson and Adam Roach are hereby
   acknowledged.

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

D. Eastlake & B. Briscoe                                       [Page 14]
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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, <http://www.rfc-editor.org/info/rfc2119>.

   [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-
         editor.org/info/rfc3168>.

   [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-
         editor.org/info/rfc4774>.

   [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,
         <http://www.rfc-editor.org/info/rfc6325>.

   [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, <http://www.rfc-editor.org/info/rfc7179>.

   [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-
         editor.org/info/rfc7567>.

   [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,
         <http://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, <http://www.rfc-editor.org/info/rfc8174>

   [RFC8311] - Black, D., "Relaxing Restrictions on Explicit Congestion
         Notification (ECN) Experimentation", RFC 8311, DOI
         10.17487/RFC8311, January 2018, <https://www.rfc-
         editor.org/info/rfc8311>.

   [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.

D. Eastlake & B. Briscoe                                       [Page 15]
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Informative References

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

   [IANAthFlags] - IANA TRILL Extended Header word flags:
         http://www.iana.org/assignments/trill-parameters/trill-
         parameters.xhtml#extended-header-flags

   [RFC6040] - Briscoe, B., "Tunnelling of Explicit Congestion
         Notification", RFC 6040, DOI 10.17487/RFC6040, November 2010,
         <http://www.rfc-editor.org/info/rfc6040>.

   [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-
         editor.org/info/rfc6660>.

D. Eastlake & B. Briscoe                                       [Page 16]
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Appendix A. TRILL Transit RBridge Behavior to Support L4S

   The specification of the Low Latency, Low Loss, Scalable throughput
   (L4S) wire protocol for IP is given in [ECNL4S]. L4S is one example
   of the ways TRILL ECN handling may evolve [RFC8311]. It is similar to
   the original ECN wire protocol 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.
       * bit13 is the least significant bit of the TRILL-ECN field
       */

      % On TRILL transit
      if (bit13 == 0 ) {
            % Classic Queue
            if (p > max(random(), random()) )
               mark(CCE)                         % likelihood: p^2

      } else {
            % L4S Queue
            if (p > random() ) {
               if (p > random() )
                  mark(CCE)                      % likelihood: p^2
               else
                  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 17]
INTERNET-DRAFT                                         TRILL ECN Support

   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
         inner.

            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 18]
INTERNET-DRAFT                                         TRILL ECN Support

Authors' Addresses

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

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

      Bob Briscoe
      CableLabs
      UK

      Email: ietf@bobbriscoe.net
      URI:   http://bobbriscoe.net/

D. Eastlake & B. Briscoe                                       [Page 19]
INTERNET-DRAFT                                         TRILL ECN Support

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D. Eastlake & B. Briscoe                                       [Page 20]