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Versions: (draft-kuehlewind-conex-tcp-modifications) 00 01 02 03 04 05 06 07 08 09 10 RFC 7786

Congestion Exposure (ConEx)                           M. Kuehlewind, Ed.
Internet-Draft                                                ETH Zurich
Intended status: Experimental                           R. Scheffenegger
Expires: May 17, 2015                                       NetApp, Inc.
                                                       November 13, 2014


               TCP modifications for Congestion Exposure
                 draft-ietf-conex-tcp-modifications-06

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
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   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."

   This Internet-Draft will expire on May 17, 2015.

Copyright Notice

   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   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.  Loss Detection  . . . . . . . . . . . . . . . . . . . . .   5
       3.1.1.  Without SACK Support  . . . . . . . . . . . . . . . .   6
     3.2.  ECN . . . . . . . . . . . . . . . . . . . . . . . . . . .   6
       3.2.1.  Accurate ECN feedback . . . . . . . . . . . . . . . .   8
       3.2.2.  Classic ECN support . . . . . . . . . . . . . . . . .   8
   4.  Setting the ConEx Bits  . . . . . . . . . . . . . . . . . . .   9
     4.1.  Setting the E and the L Bit . . . . . . . . . . . . . . .   9
     4.2.  Credit Bits . . . . . . . . . . . . . . . . . . . . . . .   9
   5.  Loss of ConEx information . . . . . . . . . . . . . . . . . .  11
   6.  Timeliness of the ConEx Signals . . . . . . . . . . . . . . .  11
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  11
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  12
     10.2.  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).

   The needed markings to provide ConEx signaling are defined in the
   ConEx Destination Option (CDO) for IPv6 [draft-ietf-conex-destopt].
   Specifically, the use of four bits are defined: the X (ConEx-
   capable), the L (loss experienced), the E (ECN experienced) and C
   (credit) bit.

   ConEx signaling 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.  No changes are
   needed at the receiver to implement ConEx signaling.  Therefore no
   additional negotiation is needed to implement and use ConEx at the




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   sender.  This document specifies actions needed by sender to provide
   meaningful ConEx information to the network.

   Section 2 provides an overview of the needed modifications for TCP
   senders to implement ConEx.  First congestion information have to be
   extracted from loss or ECN feedback in TCP as described in section
   3".  Section 4 details how to set the CDO marking based on the
   accounted congestion information.  Section 6 finally discusses
   timeliness of the ConEx feedback signal as congestion is a temporary
   state.

   This document describes congestion accounting for both TCP with and
   without the Selective Acknowledgment (SACK) extension [RFC2018] in
   section 3.1.  However, ConEx benefits from more accurate information
   about the number of packets dropped in the network.  It is therefore
   recommend to use the SACK extension when using TCP with ConEx.  The
   detailed mechanism to respectively set the L bit in response to loss-
   based congestion feedback signal is given in section 4.1.

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

   The congestion accounting for both, with the classic ECN feedback as
   well as a more accurate ECN feedback are explained in detail in
   section 3.2 while the setting of the E bit in response to ECN-based
   congestion feedback is again detailed in section 4.1.

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

   This section gives an overview of actions that need to be taken by a
   TCP sender that would like to use ConEx signaling.

   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.  Therefore a ConEx sender SHOULD also




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   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.  Furthermore, a ConEx
   sender must send enough credit to cover all experienced congestion
   for the connection so far, as well as the risk of congestion for the
   current transmission (see Section 4.2).

   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 but not the 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.

   Detailed mechanisms for congestion accounting in each operation mode
   are described in the next section.  Further handling of the IPv6 bits
   itself if congestion was accounted is described in the subsequent
   section afterwards.

3.  Accounting congestion

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






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   The outstanding bytes for congestion indications based on loss are
   maintained in the loss exposure gauge (LEG) and the accounting is
   explained in Section 3.1.

   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 3.2.

   Furthermore, those counters will be reduced every time a ConEx
   capable packet with the E or L bit set is sent.  This is explained
   for 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 for
   simplification 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 all packets that were sent
   during the last RTT are known.  Otherwise if no additional
   information is available the worst case number of packets and thus
   header bytes 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.  Loss Detection

   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 contained by the retransmission,
   assuming equal sized segments such that the retransmitted packet will
   have the same number of header bytes as the original ones.

   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



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   [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.

3.1.1.  Without SACK Support

   If multiple losses occur within one RTT and SACK is not used, it may
   take several RTTs until all lost data is retransmitted.  With the
   scheme described above, the ConEx information will be delayed
   strongly but timeliness is important for ConEx.

   For ConEx it is not important to know which data got lost but only
   how much.  During the first RTT after the initial loss detection, the
   amount of received data and thus also the amount of lost data can be
   estimated based on the number of received ACKs.  Thus without SACK,
   the needed information for the ConEx feedback can be available with
   an additionally delay of one RTT by using the following estimation
   algorithm:

   If SACK information is not available, a ConEx sender should maintain
   an additional Loss Estimation Counter (LEC).  With the first
   retransmission of a congestion event LEC is set to:

   LEC = f - 3*SMSS

   where f the is current flight size in bytes.  At this point of time
   in the transmission, in the worst case, all packets in flight minus
   three that trigged the dupACks could have been lost.  For each
   retransmission that is sent, the LEG will still be increased but the
   LEC will also be decreased by the payload size of the retransmission.
   During the following RTT, LEC should be reduced by SMSS for each ACK
   that is received.  Thus after one RTT the LEC estimates the number of
   outstanding bytes that should be ConEx L marked.  To not further
   delay this information, now LEG should be increased by LEC.  From
   then on every following retransmission should only reduce the LEC and
   not increase the LEG until the LEC is zero, as those bytes were
   already accounted.

3.2.  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



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   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 as described further
   below.

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

   DeliveredData covers the number of bytes which has been newly
   delivered to the receiver.  Therefore on each arrival of an ACK,
   DeliveredData will be increased 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.

   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 newly acknowledged bytes, minus
   one SMSS for each preceding duplicate ACK.  Therefore 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.




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

3.2.1.  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 will be conservatively increased by one SMSS
   for each marking or at max the number of newly acknowledged bytes:

   CEG += min(SMSS*D, DeliveredData)

3.2.2.  Classic ECN support

   If the ConEx sender fully conforms to the semantics of the ECN
   signaling as defined by [RFC5562], it will receive one full RTT of
   ACKs with the ECE flag set whenever at least one CE mark was received
   by the receiver.  As the sender cannot estimate how much packets have
   actually been CE marked during this RTT, the most conservative
   assumption should be taken, namely assuming that all packets were
   marked.  This can be achieved by increasing the CEG by DeliveredData
   for each ACK with the ECE flag:

   CEG += DeliveredData

   Optionally a ConEx sender could implement an Advanced Compatibility
   Mode:

   To extract more than one ECE indication per RTT, a ConEx sender could
   set the CWR flag opportunistically 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.  If an CWR confirmation will be received before the
   ECE can be sent out with the next ACK, ECN feedback information
   information could get lost.  Thus a sender should 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.  But such a more sophisticated
   heuristics could extract congestion notifications more timely.  Still
   the CEG need to be increased by DeliveredData, as one or more CE
   marked packets could be acknowledged by one delayed ACK.








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4.  Setting the ConEx Bits

   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 such as pure ACKs which are not carrying any
   payload.  Thus these packets 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.

   If SACK information is not available 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
   if as long as data packets are sent out.

4.2.  Credit Bits

   The ConEx abstract mechanism requires that sufficient credit must be
   signaled in advance to cover the expected congestion during the
   feedback delay of one RTT.  A ConEx sender should maintain a counter
   of the sent credits c in bytes.  If congestion occurs, credits will
   be consumed and the c counter should be reduced by the number of
   bytes that where lost or estimated to be ECN-marked.  If the risk of
   congestion was estimated wrongly and thus too few credits were sent,
   the c counter becomes zero but can not get negative.

   The number of credits sent should always equal the number of bytes in
   flight, as all packets could potentially get lost or congestion
   marked.  Thus a ConEx sender should monitor the number of bytes in
   flight f.  If f ever becomes larger than c, the ConEx sender SHOULD
   send new credits.  Remember that c will be decreased if congestion
   occurs.

   In TCP Slow Start, the congestion window might grow much larger than
   during the rest of the transmission.  Thus a sender could consider to
   sent fewer than f credits but risking potential penalization by an
   audit.  In any case the credits should at least cover the increase in
   sending rate.  As the sending rate increases exponentially in Slow



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   Start, thus double every RTT, a ConEx sender should at least cover
   half the number of packets in flight by credits.  Note, that the
   number of losses or markings within one RTT does not only depend
   actions taken by the sender.  In general, the behavior of the cross
   traffic, and if Active Queue Management (AQM) is used, the respective
   parameterization influence how many packets get dropped or marked.
   But if the used AQM is not overly aggressive with ECN marking,
   sending halve the flight size as credits should be sufficient for
   both, congestion signaled by loss or ECN.  Marking every fourth
   packet will allow the respective number of credits in Slow Start as
   it can be seen in Figure Figure 1.

             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)

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





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5.  Loss of ConEx information

   Of course also packets that carry a ConEx marking can get lost.  A
   ConEx sender must remember which packet was marked with either the L,
   the E or the C bit.  If one of these packets is detected to be lost,
   the should increase the respective gauge, LEG or CEG, by the number
   of lost payload bytes.

6.  Timeliness of the ConEx Signals

   ConEx signals can only be evaluated by a network nodewith a time
   delay of about one RTT after the congestion occured.  To avoiad
   further delays, a ConEx 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 at the same time that at least one segment has been lost,
   and that one or more ECN marks were received.  This may happen during
   excessive congestion, where the queues overflow even though ECN was
   used and currently all 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.

7.  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.

8.  IANA Considerations

   This document does not have any requests to IANA.

9.  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.



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10.  References

10.1.  Normative References

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

   [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.

10.2.  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.



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

   [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 5 and expanded discussion about ECN interaction.

   03 ... expanded the discussion around credit bits.

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

   05 ... changes on Loss Detection without SACK, support of classic ECN
   and credit handling.






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Authors' Addresses

   Mirja Kuehlewind (editor)
   ETH Zurich
   Switzerland

   Email: mirja.kuehlewind@tik.ee.ethz.ch


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

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


































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