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TCP Modifications for Congestion Exposure (ConEx)

The information below is for an old version of the document that is already published as an RFC.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 7786.
Authors Mirja Kühlewind , Richard Scheffenegger
Last updated 2016-05-13 (Latest revision 2015-10-13)
RFC stream Internet Engineering Task Force (IETF)
Intended RFC status Experimental
Additional resources Mailing list discussion
Stream WG state Submitted to IESG for Publication
Document shepherd Suresh Krishnan
Shepherd write-up Show Last changed 2015-05-06
IESG IESG state Became RFC 7786 (Experimental)
Action Holders
Consensus boilerplate Yes
Telechat date (None)
Responsible AD Martin Stiemerling
Send notices to (None)
IANA IANA review state Version Changed - Review Needed
IANA action state No IANA Actions
Congestion Exposure (ConEx)                           M. Kuehlewind, Ed.
Internet-Draft                                                ETH Zurich
Intended status: Experimental                           R. Scheffenegger
Expires: April 15, 2016                                     NetApp, Inc.
                                                        October 13, 2015

               TCP modifications for Congestion Exposure


   Congestion Exposure (ConEx) is a mechanism by which senders inform
   the network about expected congestion based on congestion feedback
   from previous packets in 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

   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 April 15, 2016.

Copyright Notice

   Copyright (c) 2015 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
   ( 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

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  Sender-side Modifications . . . . . . . . . . . . . . . . . .   3
   3.  Counting Congestion . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Loss Detection  . . . . . . . . . . . . . . . . . . . . .   6
       3.1.1.  Without SACK Support  . . . . . . . . . . . . . . . .   7
     3.2.  ECN . . . . . . . . . . . . . . . . . . . . . . . . . . .   8
       3.2.1.  Accurate ECN Feedback . . . . . . . . . . . . . . . .  10
       3.2.2.  Classic ECN Support . . . . . . . . . . . . . . . . .  10
   4.  Setting the ConEx Flags . . . . . . . . . . . . . . . . . . .  11
     4.1.  Setting the E or the L Flag . . . . . . . . . . . . . . .  11
     4.2.  Setting the Credit Flag . . . . . . . . . . . . . . . . .  11
   5.  Loss of ConEx Information . . . . . . . . . . . . . . . . . .  14
   6.  Timeliness of the ConEx Signals . . . . . . . . . . . . . . .  14
   7.  Open Areas for Experimentation  . . . . . . . . . . . . . . .  15
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  17
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  17
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  17
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  18
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  18
     11.2.  Informative References . . . . . . . . . . . . . . . . .  19
   Appendix A.  Revision history . . . . . . . . . . . . . . . . . .  20
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  20

1.  Introduction

   Congestion Exposure (ConEx) is a mechanism by which senders inform
   the network about expected congestion based on congestion feedback
   from previous packets in 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 markings for ConEx signaling are defined in the ConEx Destination
   Option (CDO) for IPv6 [draft-ietf-conex-destopt].  Specifically, the
   use of four flags is defined: X (ConEx-capable), L (loss
   experienced), E (ECN experienced) and C (credit).

   ConEx signaling is based on loss or Explicit Congestion Notification
   (ECN) marks [RFC3168] as congestion indications.  The sender collects
   this congestion information based on existing TCP feedback mechanisms
   from the receiver to the sender.  No changes are needed at the

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   receiver to implement ConEx signaling.  Therefore no additional
   negotiation is needed to implement and use ConEx at the sender.  This
   document specifies the sender's actions that are needed to provide
   meaningful ConEx information to the network.

   Section 2 provides an overview of the modifications needed for TCP
   senders to implement ConEx.  First congestion information has to be
   extracted from TCP's loss or ECN feedback as described in section 3.
   Section 4 details how to set the CDO marking based on this congestion
   information.  Section 5 discusses loss of packets carrying ConEx
   information.  Section 6 discusses timeliness of the ConEx feedback
   signal, given congestion is a temporary state.

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

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

   Congestion accounting for both classic ECN feedback and AccECN
   feedback is explained in detail in section 3.2.  Setting the E flag
   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",
   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 modified to use ConEx signaling.

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   In the TCP handshake, a ConEx sender MUST negotiate for SACK and ECN
   preferably with AccECN feedback.  Therefore a ConEx sender MUST 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  SACK-ECN-ConEx (SACK and 'classic' instead of accurate ECN)

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

   o  ECN-ConEx (no SACK and no accurate ECN feedback but 'classic' 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 count congestion byte-wise (rather than packet-wise; see next
   paragraph).  After any congestion notification, a sender MUST mark
   subsequent packets with the appropriate ConEx flag 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 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 [RFC7141], the byte-size of a packet marked with a CDO
   flag is defined to represent that number of bytes of congestion
   signaling [draft-ietf-conex-destopt].  Therefore 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 counting in each operation mode
   are described in the next section.

3.  Counting Congestion

   A ConEx TCP sender maintains two counters: one that counts congestion
   based on the information retrieved by loss detection, and a second
   that accounts for ECN based congestion feedback.  These counters hold
   the number of outstanding bytes that should be ConEx marked with
   respectively the E flag or the L flag in subsequent packets.

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

   The outstanding bytes counted based on ECN feedback information are
   maintained in the congestion exposure gauge (CEG), as explained in
   Section 3.2.

   When the sender sends a ConEx capable packet with the E or L flag
   set, it reduces the respective counter by the byte-size of the
   packet.  This is explained for both counters in Section 4.1.

   Note that all bytes of an IP packet must be counted in the LEG or CEG
   to capture the right number of bytes that should be marked.
   Therefore the sender SHOULD take the payload and headers into
   account, up to and including the IP header.  However, in TCP the
   information regarding how large the headers of a lost or marked
   packet were is usually not available, as only payload data will be

   If equal-sized packets, or at least equally distributed packet sizes,
   can be assumed, the sender MAY only add and subtract 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
   so they will cancel out.  This case is assumed for simplicity in the
   following sections.

   Otherwise, if a sender sends different sized packets (with unequally
   distributed packet sizes), the sender needs to memorize or estimate
   the number of lost or ECN-marked packets.  If the sender has
   sufficient memory available, the most accurate way to reconstruct the
   number of lost or marked packets is to remember the sequence number
   of all sent but not acknowledged packets.  In this case a sender is
   able to reconstruct the number of packets and thus the header bytes
   that were sent during the last RTT.  Otherwise, if e.g. not enough
   memory is available, the sender should estimate the packet size, e.g.
   if the packet size distribution follows a certain known pattern, or
   by using the minimum packet size seen in the last RTT.

   If the number of newly sent-out packets with the ConEx L or E flag
   set is smaller (or larger) than this estimated number of lost/ECN-
   marked packets, the additional header bytes should be added to (or
   can be subtracted from) the respective gauge.

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3.1.  Loss Detection

   This section applies whether or not SACK support is available.  The
   following subsection (Section 3.1.1) handles the case when SACK is
   not available.

   A TCP sender detects losses and subsequently retransmits the lost
   data.  Therefore, ConEx sender can simply set the ConEx L flag on all
   retransmissions in order to at least cover the amount of bytes lost.
   If this approach is taken, no LEG is needed.

   However, any retransmission may be spurious.  In this case more bytes
   have been marked than necessary.  To compensate for this effect a
   ConEx sender can maintain a local signed counter, the (LEG), that
   indicates the number of outstanding bytes to be sent with the ConEx L
   flag and also can become negative.

   Using the LEG, when a TCP sender decides that a data segment needs to
   be retransmitted, it will increase LEG by the size of the TCP payload
   bytes in the retransmission (assuming equal sized segments such that
   the retransmitted packet will have the same number of header bytes as
   the original ones):

   For each retransmission:

   LEG += payload

   Note, how the LEG is reduced when the ConEx L marking are set is
   described in section Section 4.

   Further to accommodate spurious retransmissions, a ConEx sender
   SHOULD make use of heuristics to detect such spurious retransmissions
   (e.g.  F-RTO [RFC5682], DSACK [RFC3708], and Eifel [RFC3522],
   [RFC4015]) if already available in a given implementation.  If no
   mechanism for detecting spurious retransmissions is available, the
   ConEx sender MAY chose to implement one of the mechanism stated
   above.  However, given the inaccuracy that ConEx may have anyway and
   the timeliness of ConEx information, a ConEx MAY also chose to not
   compensate for spurious retransmission.  In this case if spurious
   retransmissions occur, the ConEx sender simple has sent too many
   ConEx signals which e.g. would decrease the congestion allowance in a
   ConEx policer unnecessarily.

   If a heuristic method is used to detect spurious retransmission and
   has determined that a certain number of packets were retransmitted
   erroneously, the ConEx sender subtracts the payload size of these TCP
   packets from LEG.

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   If a spurious retransmission is detected:

   LEG -= payload

   Note that LEG can become negative, if too many L marking have already
   been sent.  This case is further discussed in section Section 6.

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
   considerably, but timeliness is important for ConEx.  For ConEx, it
   is important to know how much data was lot; it is not important to
   know what data is lost.  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.

   Therefore a ConEx sender can use the following algorithm to estimated
   the number of lost bytes with an additional delay of one RTT using an
   additional Loss Estimation Counter (LEC):

      flight_bytes:      current flight size in bytes
      retransmit_bytes:  payload size of the retransmission

      At the first retransmission in a congestion event LEC is set:

         LEC = flight_bytes - 3*SMSS

         (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.)

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      Then during the first RTT of the congestion event:

         For each retransmission:
            LEG += retransmit_bytes
            LEC -= retransmit_bytes

         For each ACK:
            LEC -= SMSS

      After one RTT:

         LEG += LEC

         (The LEC now estimates the number of outstanding bytes
         that should be ConEx L marked.)

      After the first RTT for each following retransmissions:

         if (LEC > 0): LEC -= retransmit_bytes
         else if (LEC==0): LEG += retransmit_bytes

         if (LEC < 0): LEG += -LEC

         (The LEG is not increased for those bytes that were
         already counted.)

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
   dropping them when congestion occurs.

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

   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

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   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 that 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.  The formula depends on whether SACK is available: if SACK is
   not available SACK_diff is always zero, whereas is ACK information is
   available is_dup and is_after_dup are always zero.

   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_bytes), that has previously already been
   accumulated into DeliveredData based on SACK information.

   Otherwise without SACK, DeliveredData is increased by 1 SMSS on
   duplicate acknowledgements because duplicate acknowledgements do not
   acknowledge any new data (and acked_bytes will be zero).  For the
   subsequent partial or full ACK, acked_bytes cover all newly
   acknowledged bytes including those already accounted for with the
   receipt of any duplicate acknowledgement.  Therefore DeliveredData is
   reduced by one SMSS for each preceding duplicate ACK.  Consequently,
   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 (which usually
   is 3 or more).

   With classic ECN, one congestion marked packet causes continuous
   congestion feedback for a whole round trip, thus hiding the arrival
   of any further congestion marked packets during that round trip.  A
   more accurate ECN feedback scheme (AccECN) is needed to ensure that
   feedback properly reflects the extent of congestion marking.  The two
   cases, with and without a receiver capable of AccECN, are discussed
   in the following sections.

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3.2.1.  Accurate ECN Feedback

   With a more accurate ECN feedback scheme (AccECN) that is supported
   by the receiver, either the number of marked packets or the number of
   marked bytes will be fed back from the receiver to the sender and is
   therefore know at sender-side.  In the latter 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

   With classic ECN, as soon as a CE mark is seen at the receiver, it
   will feed this information back to the sender by setting the Echo
   Congestion Experienced (ECE) flag in the TCP header of subsequent
   ACKs.  Once the sender receives the first ECE of a congestion
   notification, it sets the CWR flag in the TCP header once.  When this
   packet with Congestion Window Reduced (CWR) flag in the TCP header
   arrives at the receiver, acknowledging its first ECE feedback, the
   receiver stops setting ECE.

   If the ConEx sender fully conforms to the semantics of ECN signaling
   as defined by [RFC3168], 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 many packets have
   actually been CE marked during this RTT, the most conservative
   assumption MAY 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 the following technique
   (that not conforms to [RFC3168]), called advanced compatibility mode,
   to considerably improve its estimate of the number of ECN-marked

   To extract more than one ECE indication per RTT, a ConEx sender could
   set the CWR flag continuously to force the receiver to signal only
   one ECE per CE mark.  Unfortunately, the use of delayed ACKs
   [RFC5681] (which is common) will prevent feedback of every CE mark;
   if a CWR confirmation is received before the ECE can be sent out on
   the next ACK, ECN feedback information could get lost (depending on
   the actual receiver implementation).  Thus a sender SHOULD set CWR
   only on those data segments that will presumably trigger a (delayed)

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   ACK.  The sender would need an additional control loop to estimate
   which data segments will trigger an ACK in order to extract more
   timely congestion notifications.  Still, the CEG SHOULD be increased
   by DeliveredData, as one or more CE marked packets could be
   acknowledged by one delayed ACK.

4.  Setting the ConEx Flags

   By setting the X flag, a packet is marked as ConEx-capable.  All
   packets carrying payload MUST be marked with the X flag set,
   including retransmissions.  Only if no congestion feedback
   information is (currently) available, the X flag SHOULD be zero (e.g.
   for control packets on a connection that not sent any user data for
   some time and therefore is sending only pure ACKs that are not
   carrying any payload).

4.1.  Setting the E or the L Flag

   As described in section Section 3.1, the sender needs to maintain a
   CEG counter and might maintain a LEG counter.  If no LEG is used, all
   retransmission will be marked with the L flag.

   Further, as long as the LEG or CEG counter is positive, the sender
   marks each ConEx-capable packet with L or E respectively, and
   decreases the LEG or CEG counter by the TCP payload bytes carried in
   the marked packet (assuming headers are not being counted because
   packet sizes are regular).  No matter how small the value of LEG or
   CEG, if the value is positive the sender MUST NOT defer packet
   marking; this ensure ConEx signals are timely.  Therefore the value
   of LEG and CEG will commonly be negative.

   If both LEG and CEG are positive, the sender MUST mark each ConEx-
   capable packet with both L and E.  If a credit signal is also pending
   (see next section), the C flag can be set as well.

4.2.  Setting the Credit Flag

   The ConEx abstract mechanism [draft-ietf-conex-abstract-mech]
   requires that sufficient credit MUST be signaled in advance to cover
   the expected congestion during the feedback delay of one RTT.

   To monitor the credit state at the audit, a ConEx sender needs to
   maintain a Credit State Counter (CSC) in bytes.  If congestion
   occurs, credits will be consumed and the CSC is 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 CSC becomes zero but cannot go negative.

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   To be sure that the credit state in the audit never reaches zero, the
   number of credits should always equal the number of bytes in flight
   as all packets could potentially get lost or congestion marked.  In
   this case a ConEx sender also monitors the number of bytes in flight
   F.  If F ever becomes larger than CSC, the ConEx sender sets the C
   flag on each ConEx-capable packet and increase CSC by the payload
   size of each marked packet until CSC is no less than F again.
   However, a ConEx sender might also be less conservative and send
   fewer credits, if it e.g. assumes based on previous experience that
   the congestion will be low on a certain path.

   Recall that CSC will be decreased whenever congestion occurs;
   therefore CSC will need to be replenished as soon as CSC drops below
   F.  Also recall that the sender can set the C flag on a ConEx-capable
   packet whether or not the E or L flags are also set.

   In TCP Slow Start, the congestion window might grow much larger than
   during the rest of the transmission.  Likely, a sender could consider
   sending fewer than F credits but risking being penalized by an audit
   function.  However, the credits should at least cover the increase in
   sending rate.  Given the exponential increase as implemented in the
   TCP Slow Start algorithm which means that the sending rate doubles
   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
   solely depend on the sender's actions.  In general, the behavior of
   the cross traffic, whether Active Queue Management (AQM) is used and
   how it is parameterized influence how many packets might be dropped
   or marked.  As long as any AQM encountered is not overly aggressive
   with ECN marking, sending half the flight size as credits should be
   sufficient whether congestion is signaled by loss or ECN.

   To maintain half of the packets in flight as credits, also half of
   the packet of the initial window must be C marked.  In Slow Start
   marking every fourth packet introduces the correct amount of credit
   as can be seen in Figure 1.

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                                        in_flight  credits
                RTT1  |------XC------>|     1         1
                      |------X------->|     2         1
                      |------XC------>|     3         2
                      |               |
                RTT2  |------X------->|     3         2
                      |------X------->|     4         2
                      |------X------->|     4         2
                      |------XC------>|     5         3
                      |------X------->|     5         3
                      |------X------->|     6         3
                      |               |
                RTT3  |------X------->|     6         3
                      |------XC------>|     7         4
                      |------X------->|     7         4
                      |------X------->|     8         4
                      |------X------->|     8         4
                      |------XC------>|     9         5
                      |------X------->|     9         5
                      |------X------->|    10         5
                      |------X------->|    10         5
                      |------XC------>|    11         6
                      |------X------->|    11         6
                      |------X------->|    12         6
                      |      .        |
                      |      :        |

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

   It is possible that a TCP flow will encounter an audit function
   without relevant flow state, due to e.g. rerouting or memory
   limitations.  Therefore, the sender needs to detect this case and
   resend credits.  A ConEx sender might reset the credit counter CSC to
   zero if losses occur in subsequent RTTs (assuming that the sending
   rate was correctly reduced based on the received congestion signal
   and using a conservatively large RTT estimation).

   This section proposes a concrete algorithm for determining how much
   credit to signal (with a separate approach used for Slow Start).
   However, experimentation in credit setting algorithms is expected and
   encouraged.  The wider goal of ConEx is to reflect the 'cost' of the
   risk of causing congestion on those that contribute most to it.
   Thus, experimentation is encouraged to improve or maintain
   performance while reducing the risk of causing congestion, and
   therefore potentially reducing the need to signal so much credit.

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5.  Loss of ConEx Information

   Packets carrying ConEx signals could be discarded themselves.  This
   will be a second order problem (e.g. if the loss probability is 0.1%,
   the probability of losing a ConEx L signal will be 0.1% of 0.1% =
   0.01%).  Further, the penalty an audit induces should be proportional
   to the mismatch of expected ConEx marks and observed congestion,
   therefore the audit might only slightly increase the loss level of
   this flow.  Therefore, an implementer MAY choose to ignore this
   problem, accepting instead the risk that an audit function might
   wrongly penalize a flow.

   Nonetheless, a ConEx sender is responsible for always signalling
   sufficient congestion feedback and therefore SHOULD remember which
   packet was marked with either the L, the E or the C flag.  If one of
   these packets is detected as lost, the sender SHOULD increase the
   respective gauge(s), LEG or CEG, by the number of lost payload bytes
   in addition to increasing LEG for the loss.

6.  Timeliness of the ConEx Signals

   ConEx signals will only be useful to a network node within a time
   delay of about one RTT after the congestion occurred.  To avoid
   further delays, a ConEx sender SHOULD send the ConEx signaling on the
   next available packet.

   Any or all of the ConEx flags can be used in the same packet, which
   allows delay to be minimized when multiple signals are pending.  The
   need to set multiple ConEx flags at the same time can occur if e.g an
   ACK is received by the sender that simultaneously indicates that at
   least one ECN mark was received, and that one or more segments were
   lost.  This may happen during excessive congestion, if the queues
   overflow even though ECN was used and currently all forwarded packets
   are marked, while others have to be dropped.  Another case when this
   might happen is when ACKs are lost, so that a subsequent ACK carries
   summary information not previously available to the sender.

   If a flow becomes application-limited, there could be insufficient
   bytes to send to reduce the gauges to zero or below.  In such cases,
   the sender cannot help but delay ConEx signals.  Nonetheless, as long
   as the sender is marking all outgoing packets, an audit function is
   unlikely to penalize ConEx-marked packets.  Therefore, no matter how
   long a gauge has been positive, a sender MUST NOT reduce the gauge by
   more than the ConEx marked bytes it has sent.

   If the CEG or LEG counter is negative, the respective counter MAY be
   reset to zero within one RTT after it was decreased the last time or
   one RTT after recovery if no further congestion occurred.

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7.  Open Areas for Experimentation

   All proposed mechanisms in this document are experimental, and
   therefore further large-scale experimentation in the Internet is
   required to evaluate if the signaling provided by these mechanisms is
   accurate and timely enough to produce value for ConEx-based (traffic
   management or other) mechanisms.

   The current ConEx specifications assume that congestion is counted in
   number of bytes (including the IP header that directly encapsulates
   the CDO and everything that IP header encapsulates)
   [draft-ietf-conex-destopt].  This decision was taken because most
   network devices today experience byte-congestion where the memory is
   filled exactly with the number of bytes a packet carries [RFC7141].
   However, there are also devices that may allocate a certain amount of
   memory per packet, no matter how large a packet is.  These devices
   get congested based on the number of packets in their memory and
   therefore in this case congestion is determined by the number of
   packets that have been lost or marked.  Furthermore, a transport
   layer endpoint, such as a TCP sender or receiver, might not know the
   exact number of bytes that a lower layer was carrying.  Therefore a
   TCP endpoint may only be able to estimate the exact number of
   congested bytes (assuming that all lower layer header have the same
   length).  If this estimation is sufficient to work with, the ConEx
   signal needs to be further evaluated in tests in the Internet
   together with different auditor implementations.

   Further, the proposed marking schemes in this document are designed
   under the assumption that all TCP packets of a ConEx-capable flow are
   of equal size or that flows have a constant mean packet size over a
   rather small time frame, like one RTT or less.  In most
   implementations this assumption might be taken as well and probably
   is true for most of the traffic flows.  If this proposed scheme is
   used, it is necessary to evaluate how much accuracy degrades if this
   precondition is not met.  Evaluating with real traffic from different
   applications is especially important in making the decision regarding
   whether the proposed schemes are sufficient or whether a more complex
   scheme is needed.

   In this context the proposed scheme to set credit markings in Slow
   Start runs a risk to provide an insufficient number of markings which
   can cause an audit function to penalize this flow.  Both the proposed
   credit scheme for Slow Start as well as the scheme in Congestion
   Avoidance must be evaluated together with one or more specific
   implementations of an ConEx auditor to ensure that both algorithms,
   in the sender and in the auditor, work properly together with a low
   risk of false positives (which would lead to penalization of an
   honest sender).  However, if a sender is wrongly assumed to cheat,

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   the penalization of the audit should be adequate and should allow an
   honest sender using a congestion control scheme that is commonly used
   today to recover quickly.

   Another open issue is the accuracy of the ECN feedback signal.  At
   time of publication of this document there is no AccECN mechanism
   specified yet, and further AccECN will also take some time to be
   widely deployed.  This document proposes an advanced compatibility
   mode for Classic ECN.  The proposed mechanism can provide more
   accurate feedback by utilizing the way Classic ECN is specified but
   has a higher risk of losing information.  To figure out how high this
   risk is in a real deployment scenario, further experimental
   evaluation is needed.  The following argument is intended to prove
   that suppressing repetitions of ECE, however, is still safe against
   possible congestion collapse due to lost congestion feedback and
   should be further proven in experimentation:

   Repetition of ECE in classic ECN is intended to ensure reliable
   delivery of congestion feedback.  However, with advanced
   compatibility mode, it is possible to miss congestion notifications.
   This can happen in some implementations if delayed acknowledgements
   are used.  Further, an ACK containing ECE can simply get lost.  If
   only a few CE marks are received within one congestion event (e.g.,
   only one), the loss of one acknowledgements due to (heavy) congestion
   on the reverse path can prevent that any congestion notification is
   received by the sender.

   However, if loss of feedback exacerbates congestion on the forward
   path, more forward packets will be CE marked, increasing the
   likelihood that feedback from at least one CE will get through per
   RTT.  As long as one ECE reaches the sender per RTT, the sender's
   congestion response will be the same as if CWR were not continuous.
   The only way that heavy congestion on the forward path could be
   completely hidden would be if all ACKs on the reverse path were lost.
   If total ACK loss persisted, the sender would time out and do a
   congestion response anyway.  Therefore, the problem seems confined to
   potential suppression of a congestion response during light

   Furthermore, even if loss of all ECN feedback leads to no congestion
   response, the worst that could happen would be loss instead of ECN-
   signaled congestion on the forward path.  Given compatibility mode
   does not affect loss feedback, there would be no risk of congestion

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8.  Acknowledgements

   The authors would like to thank Bob Briscoe who contributed with this
   initial ideas [I-D.briscoe-conex-re-ecn-tcp] and valuable feedback.
   Moreover, thanks to Jana Iyengar who also provided valuable feedback.

9.  IANA Considerations

   This document does not have any requests to IANA.

10.  Security Considerations

   General ConEx security considerations are covered extensively in the
   ConEx abstract mechanism [draft-ietf-conex-abstract-mech].  This
   section covers TCP-specific concerns that may occur with the addition
   of ConEx to TCP (while not discussing general well-known attacks
   against TCP).  It is assumed that any altering of ConEx information
   can be detected by protection mechanisms in the IP layer and is
   therefore not discussed here but in [draft-ietf-conex-destopt].
   Further, [draft-ietf-conex-destopt] describes how to use ConEx to
   mitigate flooding attacks by using preferential drop where the use of
   ConEx can even increase security.

   The ConEx modifications to TCP provide no mechanism for a receiver to
   force a sender not to use ConEx.  A receiver can degrade the accuracy
   of ConEx by claiming that it does not support SACK, AccECN or ECN,
   but the sender will never have to turn ConEx off.  Further, the
   receiver cannot force the sender to have to mark ConEx more
   conservatively, in order to cover the risk of any inaccuracy.
   Instead it is always the sender's choice to either mark very
   conservatively which ensures that the audits always sees enough
   markings to not penalize the flow, or estimate the needed number of
   markings more tightly.  This second case lead to inaccurate marking
   and therefore increases the likelihood of loss at an audit function
   which will only harm the receiver itself.

   Assuming the sender is limited in some way by a congestion allowance
   or quota, a receiver could spoof more loss or ECN congestion feedback
   than it actually experiences, in an attempt to make the sender draw
   down its allowance faster than necessary.  However, over-declaring
   congestion simply makes the sender slow down.  If the receiver is
   interested in the content it will not want to harm its own

   However, if the receiver is solely interested in making the sender
   draw down its allowance, the net effect will depend on the sender's
   congestion control algorithm as permanently adding more and more
   additional congestion would cause the sender to more and more reduce

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   its sending rate.  Therefore a receiver can only maintain a certain
   congestion level that is corresponding to a certain sending rate.
   With New Reno [RFC5681], doubling congestion feedback causes the
   sender to reduce its sending rate such that it would only to consume
   sqrt(2) = 1.4 times more congestion allowance.  However, to improve
   scaling, congestion control algorithms are tending towards less
   responsive algorithms like Cubic or Compound TCP, and ultimately to
   linear algorithms like DCTCP [DCTCP] that aim to maintain the same
   congestion level independent of the current sending rate and always
   reduce its sending window if the signaled congestion feedback is
   higher.  In each case, if the receiver doubles congestion feedback,
   it causes the sender to respectively consume more allowance by a
   factor of 1.2, 1.15 or 1, where 1 implies the attack has become
   completely ineffective as no further congestion allowance is consumed
   but the flow will decrease its sending rate to a minimum instead.

11.  References

11.1.  Normative References

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

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

   [RFC2018]  Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP
              Selective Acknowledgment Options", RFC 2018,
              DOI 10.17487/RFC2018, October 1996,

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

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

   [RFC5681]  Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
              Control", RFC 5681, DOI 10.17487/RFC5681, September 2009,

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11.2.  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.

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

              Briscoe, B., Jacquet, A., Moncaster, T., and A. Smith,
              "Re-ECN: Adding Accountability for Causing Congestion to
              TCP/IP", draft-briscoe-conex-re-ecn-tcp-04 (work in
              progress), July 2014.

   [RFC3522]  Ludwig, R. and M. Meyer, "The Eifel Detection Algorithm
              for TCP", RFC 3522, DOI 10.17487/RFC3522, 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,
              DOI 10.17487/RFC3708, February 2004,

   [RFC4015]  Ludwig, R. and A. Gurtov, "The Eifel Response Algorithm
              for TCP", RFC 4015, DOI 10.17487/RFC4015, February 2005,

   [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,
              DOI 10.17487/RFC5682, September 2009,

   [RFC6789]  Briscoe, B., Ed., Woundy, R., Ed., and A. Cooper, Ed.,
              "Congestion Exposure (ConEx) Concepts and Use Cases",
              RFC 6789, DOI 10.17487/RFC6789, December 2012,

   [RFC7141]  Briscoe, B. and J. Manner, "Byte and Packet Congestion
              Notification", BCP 41, RFC 7141, DOI 10.17487/RFC7141,
              February 2014, <>.

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Appendix A.  Revision history

   RFC Editor: 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-

   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

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

   07 ... review feedback provided by Nandita

   08 ... based on Bob's feedback: Wording edits and structuring of a
   few paragraphs; change of SHOULD to MAY for resetting negative LEG/
   CEG; additional security considerations provided by Bob (thanks!).

   09 ... experimentation section added

   10 ... final review comments based on IETF last call

Authors' Addresses

   Mirja Kuehlewind (editor)
   ETH Zurich


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


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