TCP modifications for Congestion Exposure
draft-ietf-conex-tcp-modifications-04

Congestion Exposure (ConEx)                           M. Kuehlewind, Ed.
Internet-Draft                                   University of Stuttgart
Intended status: Experimental                           R. Scheffenegger
Expires: January 16, 2014                                   NetApp, Inc.
                                                           July 15, 2013

               TCP modifications for Congestion Exposure
                 draft-ietf-conex-tcp-modifications-04

Abstract

   Congestion Exposure (ConEx) is a mechanism by which senders inform
   the network about the congestion encountered by previous packets on
   the same flow.  This document describes the necessary modifications
   to use ConEx with the Transmission Control Protocol (TCP).

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on January 16, 2014.

Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   described in the Simplified BSD License.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  Sender-side Modifications . . . . . . . . . . . . . . . . . .   3
   3.  Accounting congestion . . . . . . . . . . . . . . . . . . . .   4
     3.1.  ECN . . . . . . . . . . . . . . . . . . . . . . . . . . .   5
       3.1.1.  Accurate ECN feedback . . . . . . . . . . . . . . . .   6
       3.1.2.  Classic ECN support . . . . . . . . . . . . . . . . .   6
     3.2.  Loss Detection with/without SACK  . . . . . . . . . . . .   8
   4.  Setting the ConEx Bits  . . . . . . . . . . . . . . . . . . .   9
     4.1.  Setting the E and the L Bit . . . . . . . . . . . . . . .   9
     4.2.  Credit Bits . . . . . . . . . . . . . . . . . . . . . . .   9
     4.3.  Loss of ConEx information . . . . . . . . . . . . . . . .  10
   5.  Timeliness of the ConEx Signals . . . . . . . . . . . . . . .  11
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  11
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  11
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  12
   Appendix A.  Revision history . . . . . . . . . . . . . . . . . .  13
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  14

1.  Introduction

   Congestion Exposure (ConEx) is a mechanism by which senders inform
   the network about the congestion encountered by previous packets on
   the same flow.  ConEx concepts and use cases are further explained in
   [RFC6789].  The abstract ConEx mechanism is explained in
   [draft-ietf-conex-abstract-mech].  This document describes the
   necessary modifications to use ConEx with the Transmission Control
   Protocol (TCP).

   ConEx is defined as a destination option for IPv6
   [draft-ietf-conex-destopt].  The use of four bits have been defined,
   namely the X (ConEx-capable), the L (loss experienced), the E (ECN
   experienced) and C (credit) bit.

   The ConEx signal is based on loss or Explicit Congestion Notification
   (ECN) marks [RFC3168] as a congestion indication.  This congestion
   information is retrieved by the sender based on existing feedback
   mechanisms from the receiver to the sender in TCP.

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   This document describes mechanisms for both TCP with and without the
   Selective Acknowledgment (SACK) extension [RFC2018].  However, ConEx
   benefits from more accurate information about the number of packets
   dropped in the network.  We therefore recommend using the SACK
   extension when using TCP with ConEx.

   While loss-based congestion feedback should be minimized, ECN could
   actually provide more fine-grained feedback information.  ConEx-based
   traffic measurement or management mechanism would benefit from this.
   Unfortunately the current ECN does not reflect multiple congestion
   markings which occur within the same Round-Trip Time (RTT).  A more
   accurate feedback extension to ECN is defined in a separate document
   [draft-kuehlewind-tcpm-accurate-ecn], as this is also useful for
   other mechanisms.

1.1.  Requirements Language

   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].

2.  Sender-side Modifications

   A ConEx sender MUST negotiate for both SACK and ECN or the more
   accurate ECN feedback in the TCP handshake if these TCP extension are
   available at the sender.  Thus a ConEx SHOULD also implement SACK and
   ECN.  Depending on the capability of the receiver, the following
   operation modes exist:

   o  SACK-accECN-ConEx (SACK and accurate ECN feedback)

   o  accECN-ConEx (no SACK but accurate ECN feedback)

   o  ECN-ConEx (no SACK and no accurate ECN feedback but 'classic' ECN)

   o  SACK-ECN-ConEx (SACK and 'classic' instead of accurate ECN)

   o  SACK-ConEx (SACK but no ECN at all)

   o  Basic-ConEx (neither SACK nor ECN)

   A ConEx sender MUST expose all congestion information to the network
   according to the congestion information received by ECN or based on
   loss information provided by the TCP feedback loop.  A TCP sender
   SHOULD account congestion byte-wise (and not packet-wise).  A sender
   MUST mark subsequent packets (after the congestion notification) with
   the respective ConEx bit in the IP header.

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   With SACK only the number of lost payload bytes is known, but not the
   number of packets carrying these bytes.  With classic ECN only an
   indication is given that a marking occurred which is not giving an
   exact number of payload bytes nor packets.  As network congestion is
   usually byte-congestion [draft-briscoe-tsvwg-byte-pkt-mark], the
   exact number of bytes should be taken into account if available to
   make the ConEx signal as exact as possible.

   The congestion accounting based on different operation modes is
   described in the next section and the handling of the IPv6 bits
   itself in the subsequent section afterwards.

3.  Accounting congestion

   A ConEx sender, thats accounts congestion byte-wise based on the
   congestion information received by ECN or loss detection provided by
   TCP, will maintain two different counters.  These counters hold the
   number outstanding bytes that need to be ConEx marked either with the
   E bit or the L bit.

   The outstanding bytes accounted based on ECN feedback information are
   maintained in the congestion exposure gauge (CEG).  The accounting of
   these bytes from the ECN feedback is explained in more detail next in
   Section 3.1.

   The outstanding bytes for congestion indications based on loss are
   maintained in the loss exposure gauge (LEG) and the accounting is
   explained in subsequent to the CEG accounting in Section 3.2.

   Furthermore, those counters will be reduced every time a ConEx
   capable packet with the E or L bit set is sent.  This is explained
   from both counters in Section 4.1.

   Usually all bytes of an IP packet must be accounted.  Therefore the
   sender SHOULD take the headers into account, too.  If equal sized
   packets, or at least equally distributed packet sizes can be assumed,
   the sender MAY only account the TCP payload bytes.  In this case
   there should be about the same number of ConEx marked packets as the
   original packets that were causing the congestion.  Thus both contain
   about the same number of header bytes.  This case is assumed in the
   following sections.

   Otherwise if this is not the case and a sender sends different sized
   packets (with unequally distributed packet sizes), the sender needs
   to memorize or estimate the number of ECN-marked or lost packets.  A
   sender might be able to reconstruct the number of packets and thus
   the header bytes if the packet sizes of the last RTT are known.
   Otherwise if no additional information is available the worst case

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   number of headers should be estimated in a conservative way based on
   a minimum packet size (of all packets sent in the last RTT).  If the
   number of ConEx marked packets is smaller (or larger) than the
   estimated number of ECN-marked or lost packets, the additional header
   bytes should the added to (or can be subtracted from) the respective
   counter.

3.1.  ECN

   ECN [RFC3168] is an IP/TCP mechanism that allows network nodes to
   mark packets with the Congestion Experienced (CE) mark instead of
   (early) dropping them when congestion occurs.  As soon as a CE mark
   is seen at the receiver, with classic ECN it will feed this
   information back to the sender by setting the Echo Congestion
   Experienced (ECE) bit in the TCP header of all subsequent ACKs until
   a packet with Congestion Window Reduced (CWR) bit in the TCP header
   is received to acknowledge the reception of the congestion
   notification.  The sender sets the CWR bit in the TCP header once
   when the first ECE of a congestion notification is received.

   A receiver can support 'classic' ECN, a more accurate ECN feedback
   scheme, or neither.  In the case ECN is not supported at all, of
   course, no ECN marks will occur, thus the E bit will never be set.
   Otherwise, a ConEx sender must maintain a counter, the congestion
   exposure gauge (CEG), for the number of outstanding bytes that have
   to be ConEx marked with the E bit.

   The CEG is increased when ECN information is received from an ECN-
   capable receiver supporting the 'classic' ECN scheme or the accurate
   ECN feedback scheme.  When the ConEx sender receives an ACK
   indicating one or more segments were received with a CE mark, CEG is
   increased by the appropriate number of bytes.

   Unfortunately in case of duplicate acknowledgements the number of
   newly acknowledged bytes will be zero even though (CE marked) data
   has been received.  Therefore, we increase the CEG by DeliveredData,
   as defined below:

   DeliveredData covers the number of bytes which has been newly
   delivered to the receiver.  Therefore on each arrival of an ACK,
   DeliveredData will be calculated by the newly acknowledged bytes
   (acked_bytes) as indicated by the current ACK, relative to all past
   ACKs.  Moreover with SACK, DeliveredData is increased by the number
   of bytes provided by (new) SACK information (SACK_diff).  Note, if
   less unacknowledged bytes are announced in the new SACK information
   than in the previous ACK, SACK_diff can be negative.  In this case,
   data is newly acknowledged (in acked_byte), that has previously
   already been accounted to DeliveredData based on SACK information.

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   Without SACK, DeliveredData is estimated to be 1 SMSS on duplicate
   acknowledgements.  For the subsequent partial or full ACK,
   DeliveredData is estimated to be the the newly acknowledged bytes,
   minus one SMSS for each preceding duplicate ACK.

   DeliveredData = acked_bytes + SACK_diff + (is_dup)*1SMSS -
   (is_after_dup)*num_dup*1SMSS

   Thus is_dup is one if the current ACK is a duplicated ACK without
   SACK, and zero otherwise.  is_after_dup is only one for the next full
   or partial ACK after a number of duplicated ACKs without SACK and
   num_dup counts the number of duplicated ACKs in a row.

   The two cases, with and without more accurate ECN depending on the
   receiver capability, are discussed in the following sections.

3.1.1.  Accurate ECN feedback

   With a more accurate ECN feedback scheme either the number of marked
   packets/received CE marks or directly the number of marked bytes is
   known.  In the later case the CEG can directly be increased by the
   number of marked bytes.  Otherwise if D is assumed to be the number
   of marks, the gauge CEG has to be increased by the amount of bytes
   sent which were marked:

   CEG += min(SMSS*D, DeliveredData)

3.1.2.  Classic ECN support

   A ConEx sender that communicates with a classic ECN receiver
   (conforming to [RFC3168] or [RFC5562]) MAY run in one of these modes:

   o  Full compliance mode:

      The ConEx sender fully conforms to all the semantics of the ECN
      signaling as defined by [RFC5562].  In this mode, only a single
      congestion indication can be signaled by the receiver per RTT.
      Whenever the ECE flag toggles from "0" to "1", the gauge CEG is
      increased at maximum by the SMSS:

      CEG += min(SMSS, DeliveredData)

      Note that most often, a session adhering to these semantics may
      not provide enough ConEx marks as usually more than one CE mark
      will occur during one congestion event (within one RTT).  We
      assume that the credits build up during the Slow Start phase will
      cover the mismatch for short connections with only light
      congestion.  Otherwise this will cause appropriate sanctions by an

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      audit device in a ConEx enabled network.  To avoid this in any
      case, on whole RTT of packets need to be regarded as congestion
      marked.  Thus increasing the CEG by the number of DeliveredData
      for each ACK with the ECE bit set, would cover the worst case
      estimation.

   o  Simple compatibility mode:

      The sender will set the CWR permanently to force the receiver to
      signal only one ECE per CE mark.  Unfortunately, the use of
      delayed ACKs [RFC5681], as it is usually done today, will prevent
      a feedback of every CE mark.  An CWR confirmation will be received
      before the ECE can be sent out with the next ACK.  With an ACK
      rate of M, about M-1/M CE indications will not be signaled back by
      the receiver (e.g. 50% with M=2 for delayed ACKs).  Thus, in this
      mode the ConEx sender MUST increase CEG as if M congestion
      notification were received for each received ECE signal:

      CEG += min(M*SMSS, DeliveredData + (M-1)*SMSS)

      In case of a congestion event with low congestion (that means when
      only a very smaller number of packets get marked), the sender
      might miss the whole congestion event.  Even though the sender
      will send sufficient ConEx marks on average due to the scheme
      proposed above, these ConEx marks might be shifted in time and an
      audit might penalize this behavior.  Regarding congestion control,
      it is not a general problem to miss a congestion event as, by
      chance, a marking scheme in the network node might also miss a
      certain flow.  In the case where no other flow is reacting, the
      congestion level will increase and it will get more likely that
      the congestion feedback is delivered.  To provide a fair share
      over time, a TCP sender implementing this simple ECN compatibility
      mode could react more strongly when receiving an ECN feedback
      signal.  This of course depends on the congestion control used.

   o  Advanced compatibility mode:

      To avoid the loss of ECN feedback information in the proposed
      simple compatibility mode, a sender could set CWR only on those
      data segments, that will actually trigger a (delayed) ACK.  The
      sender would need an additional control loop to estimated which
      data segment will trigger an ACK.  Such a more sophisticated
      heuristics could extract congestion notifications more timely.  In
      addition, if this advanced compatibility mode is used, further
      heuristics SHOULD be implemented, to determine the value of each
      ECE notification.  E.g. for each consecutive ACK received with the
      ECE flag set, CEG should be increased by min( M*SSMS,
      DeliveredData).  Else if the predecessor ACK was received with the

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      ECE flag cleared, CEG need only be increase at maximum by one
      SMSS:

      if previous_marked: CEG += min( M*SSMS, DeliveredData)
      else: CEG += min(SMSS, DeliveredData)

      This heuristic is conservative during more serious congestion, and
      more relaxed at low congestion levels.

3.2.  Loss Detection with/without SACK

   For all the data segments that are determined by a ConEx sender as
   lost, (at least) the same number of TCP payload bytes MUST be be sent
   with the ConEx L bit set.  Loss detection typically happens by use of
   duplicate ACKs, or the firing of the retransmission timer.  A ConEx
   sender MUST maintain a loss exposure gauge (LEG), indicating the
   number of outstanding bytes that must be sent with the ConEx L bit.
   When a data segment is retransmitted, LEG will be increased by the
   size of the TCP payload bytes containing the retransmission, assuming
   equal sized segments such that the retransmitted packet will have the
   same number of header as the original ones.  When sending subsequent
   segments, the ConEx L bit is set as long as LEG is positive, and LEG
   is decreased by the size of the sent TCP payload bytes with the ConEx
   L bit set.

   Any retransmission may be spurious.  To accommodate that, a ConEx
   sender SHOULD make use of heuristics to detect such spurious
   retransmissions (e.g. F-RTO [RFC5682], DSACK [RFC3708], and Eifel
   [RFC3522], [RFC4015]).  When such a heuristic has determined, that a
   certain number of packets were retransmitted erroneously, the ConEx
   sender should subtract the payload size of these TCP packets from
   LEG.

   Note that the above heuristics delays the ConEx signal by one
   segment, and also decouples them from the retransmissions themselves,
   as some control packets (e.g. pure ACKs, window probes, or window
   updates) may be sent in between data segment retransmissions.  A
   simpler approach would be to set the ConEx signal for each
   retransmitted data segment.  However, it is important to remember,
   that a ConEx signal and TCP segments do not natively belong together.

   If SACK is not available or SACK information has been reset for any
   reason, spurious retransmission are more likely.  In this case it
   might be valuable to slightly delay the ConEx loss feedback until a
   spurious retransmission might be detected.  But the ConEx signal MUST
   NOT be delayed more than one RTT.

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4.  Setting the ConEx Bits

   ConEx is defined as a destination option for IPv6
   [draft-ietf-conex-destopt].  The use of four bits have been defined,
   namely the X (ConEx-capable), the L (loss experienced), the E (ECN
   experienced) and C (credit) bit.

   By setting the X bit a packet is marked as ConEx-capable.  All
   packets carrying payload MUST be marked with the X bit set including
   retransmissions.  No congestion feedback information are available
   about control packets as pure ACKs which are not carrying any
   payload.  Thus these packet should not be taken into account when
   determining ConEx information.  These packet MUST carry a ConEx
   Destination Option with the X bit unset.

4.1.  Setting the E and the L Bit

   As long as the CEG or LEG counter is positive, ConEx-capable packets
   SHOULD be marked with E or L respectively, and the CEG or LEG counter
   is decreased by the TCP payload bytes carried in this packet.  If the
   CEG or LEG counter is negative, the respective counter SHOULD be
   reset to zero within one RTT after it was decreased the last time or
   one RTT after recovery if no further congestion occurred.

4.2.  Credit Bits

   The ConEx abstract mechanism requires that the transport SHOULD
   signal sufficient credit in advance to cover any reasonably expected
   congestion during its feedback delay.  To be very conservative the
   number of credits would need to equal the number of packets in
   flight, as every packet could get lost or congestion marked.  With a
   more moderate view, only an increase in the sending rate should cause
   loss while the number of ECN markings within one RTT depends on
   parameterization of the used Active Queue management (AQM).  The
   average or maximum number of ECN marks per congestion event could
   potentially be estimated over time.  This case is not further
   expanded here.

   In TCP Slow Start the sending rate will increase exponentially and
   that means double every RTT.  Thus the number of credits should equal
   at least half the number of packets in flight in every RTT.  If the
   used AQM is not overly aggressive with ECN marking, maintaining the
   number of credit as half the number of packets in flight should be
   sufficient for both, congestion signaled by loss or ECN.  Under the
   assumption that all ConEx marks will not get invalid for the whole
   Slow Start phase, marks of a previous RTT have to be added up.  Thus
   the marking of every fourth packet will allow sufficient credits in
   Slow Start as it can be seen in Figure Figure 1.

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             RTT1  |------XC------>|
                   |------X------->|
                   |------X------->|   credit=1  in_flight=3
                   |               |
             RTT2  |------X------->|
                   |------XC------>|
                   |------X------->|
                   |------X------->|
                   |------X------->|
                   |------XC------>|   credit=3  in_flight=6
                   |               |
             RTT3  |------X------->|
                   |------X------->|
                   |------X------->|
                   |------XC------>|
                   |------X------->|
                   |------X------->|
                   |------X------->|
                   |------XC------>|
                   |------X------->|
                   |------X------->|
                   |------X------->|
                   |------XC------>|   credit=6  in_flight=12
                   |      .        |
                   |      :        |

       Figure 1: Credits in Slow Start (with an initial window of 3)

   Moreover, a ConEx sender should maintain a counter of the sent
   credits c. In Congestion Avoidance phase, the sender should to
   monitor the number of packets in flight f. If f every gets larger
   than c, the ConEx sender should send new credits.

   The audit might loose state due to e.g. rerouting or memory
   limitation.  Therefore, the sender needs to detect this case and
   resend credits.  Thus a ConEx sender should reset the credit count c
   if losses occur in two subsequent RTTs (assuming that the sending
   rate was correctly reduced based on the received congestion signal).

4.3.  Loss of ConEx information

   The audit can have wrong information if e.g. ConEx got lost on the
   channel (or a wrong number of ConEx marking has been estimated by the
   sender due to a lack of feedback information).  In this case the
   audit might penalize a sender wrongly.  The ConEx sender should
   detect this case and send further credits which should solve the
   situation (see Section 4.2).

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5.  Timeliness of the ConEx Signals

   ConEx signals will anyway be evaluated with a slight time delay of
   about one RTT by a network node.  Therefore, it is not absolutely
   necessary to immediately signal ConEx bits when they become known
   (e.g.  L and E bits), but a sender SHOULD sent the ConEx signaling
   with the next available packet.  In cases where it is preferable to
   slightly delay the ConEx signal, the sender MUST NOT delay the ConEx
   signal more than one RTT.

   Multiple ConEx bits may become available for signaling at the same
   time, for example when an ACK is received by the sender, that
   indicates that at least one segment has been lost, and that one or
   more ECN marks were received at the same time.  This may happen
   during excessive congestion, where buffer queues overflow and some
   packets are marked, while others have to be dropped nevertheless.
   Another possibility when this may happen are lost ACKs, so that a
   subsequent ACK carries summary information not previously available
   to the sender.  As ConEx-capable packet can carry different ConEx
   marks at the same time, these information do not need to be
   distributed over several packets and thus can be sent without further
   delay.

6.  Acknowledgements

   The authors would like to thank Bob Briscoe who contributed with this
   initial ideas and valuable feedback.  Moreover, thanks to Jana
   Iyengar who provided valuable feedback.

7.  IANA Considerations

   This document does not have any requests to IANA.

8.  Security Considerations

   With some of the advanced ECN compatibility modes it is possible to
   miss congestion notifications.  Thus a sender will not decrease its
   sending rate.  If the congestion is persistent, the likelihood to
   receive a congestion notification increases.  In the worst case the
   sender will still react correctly to loss.  This will prevent a
   congestion collapse.

9.  References

9.1.  Normative References

   [RFC2018]  Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP
              Selective Acknowledgment Options", RFC 2018, October 1996.

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

   [RFC3168]  Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
              of Explicit Congestion Notification (ECN) to IP", RFC
              3168, September 2001.

   [RFC5681]  Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
              Control", RFC 5681, September 2009.

   [draft-ietf-conex-abstract-mech]
              Mathis, M. and B. Briscoe, "Congestion Exposure (ConEx)
              Concepts and Abstract Mechanism", draft-ietf-conex-
              abstract-mech-06 (work in progress), October 2012.

   [draft-ietf-conex-destopt]
              Krishnan, S., Kuehlewind, M., and C. Ucendo, "IPv6
              Destination Option for ConEx", draft-ietf-conex-destopt-04
              (work in progress), March 2013.

9.2.  Informative References

   [DCTCP]    Alizadeh, M., Greenberg, A., Maltz, D., Padhye, J., Patel,
              P., Prabhakar, B., Sengupta, S., and M. Sridharan, "DCTCP:
              Efficient Packet Transport for the Commoditized Data
              Center", Jan 2010.

   [I-D.briscoe-tsvwg-re-ecn-tcp]
              Briscoe, B., Jacquet, A., Moncaster, T., and A. Smith,
              "Re-ECN: Adding Accountability for Causing Congestion to
              TCP/IP", draft-briscoe-tsvwg-re-ecn-tcp-09 (work in
              progress), October 2010.

   [RFC3522]  Ludwig, R. and M. Meyer, "The Eifel Detection Algorithm
              for TCP", RFC 3522, April 2003.

   [RFC3708]  Blanton, E. and M. Allman, "Using TCP Duplicate Selective
              Acknowledgement (DSACKs) and Stream Control Transmission
              Protocol (SCTP) Duplicate Transmission Sequence Numbers
              (TSNs) to Detect Spurious Retransmissions", RFC 3708,
              February 2004.

   [RFC4015]  Ludwig, R. and A. Gurtov, "The Eifel Response Algorithm
              for TCP", RFC 4015, February 2005.

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   [RFC5562]  Kuzmanovic, A., Mondal, A., Floyd, S., and K.
              Ramakrishnan, "Adding Explicit Congestion Notification
              (ECN) Capability to TCP's SYN/ACK Packets", RFC 5562, June
              2009.

   [RFC5682]  Sarolahti, P., Kojo, M., Yamamoto, K., and M. Hata,
              "Forward RTO-Recovery (F-RTO): An Algorithm for Detecting
              Spurious Retransmission Timeouts with TCP", RFC 5682,
              September 2009.

   [RFC6789]  Briscoe, B., Woundy, R., and A. Cooper, "Congestion
              Exposure (ConEx) Concepts and Use Cases", RFC 6789,
              December 2012.

   [draft-briscoe-tsvwg-byte-pkt-mark]
              Briscoe, B. and J. Manner, "Byte and Packet Congestion
              Notification", draft-briscoe-tsvwg-byte-pkt-mark-010 (work
              in progress), May 2013.

   [draft-kuehlewind-tcpm-accurate-ecn]
              Kuehlewind, M. and R. Scheffenegger, "More Accurate ECN
              Feedback in TCP", draft-kuehlewind-tcpm-accurate-ecn-02
              (work in progress), Jun 2013.

Appendix A.  Revision history

   RFC Editior: This section is to be removed before RFC publication.

   00 ... initial draft, early submission to meet deadline.

   01 ... refined draft, updated LEG "drain" from per-packet to RTT-
   based.

   02 ... added Section 4.3 and expanded discussion about ECN
   interaction.

   03 ... expanded the discussion around credit bits.

   04 ... review comments of Jana addressed.  (Change in full compliance
   mode.)

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Authors' Addresses

   Mirja Kuehlewind (editor)
   University of Stuttgart
   Pfaffenwaldring 47
   Stuttgart  70569
   Germany

   Email: mirja.kuehlewind@ikr.uni-stuttgart.de

   Richard Scheffenegger
   NetApp, Inc.
   Am Euro Platz 2
   Vienna  1120
   Austria

   Phone: +43 1 3676811 3146
   Email: rs@netapp.com

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