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IPv6 Destination Option 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 7837.
Authors Suresh Krishnan , Mirja Kühlewind , Bob Briscoe , Carlos Ucendo
Last updated 2016-05-13 (Latest revision 2016-01-17)
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 Dirk Kutscher
Shepherd write-up Show Last changed 2015-05-08
IESG IESG state Became RFC 7837 (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
ConEx Working Group                                          S. Krishnan
Internet-Draft                                                  Ericsson
Intended status: Experimental                              M. Kuehlewind
Expires: July 21, 2016                                        ETH Zurich
                                                              B. Briscoe
                                              Simula Research Laboratory
                                                                C. Ralli
                                                        January 18, 2016

        IPv6 Destination Option for Congestion Exposure (ConEx)


   Congestion Exposure (ConEx) is a mechanism by which senders inform
   the network about the congestion encountered by packets earlier in
   the same flow.  This document specifies an IPv6 destination option
   that is capable of carrying ConEx markings in IPv6 datagrams.

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 July 21, 2016.

Copyright Notice

   Copyright (c) 2016 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

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   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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions used in this document . . . . . . . . . . . . . .   3
   3.  Requirements for the coding of ConEx in IPv6  . . . . . . . .   3
   4.  ConEx Destination Option (CDO)  . . . . . . . . . . . . . . .   4
   5.  Implementation in the fast path of ConEx-aware routers  . . .   7
   6.  Tunnel Processing . . . . . . . . . . . . . . . . . . . . . .   8
   7.  Compatibility with use of IPsec . . . . . . . . . . . . . . .   8
   8.  Mitigating flooding attacks by using preferential drop  . . .   9
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  10
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  11
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  11
     12.2.  Informative References . . . . . . . . . . . . . . . . .  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction

   Congestion Exposure (ConEx) [I-D.ietf-conex-abstract-mech] is a
   mechanism by which senders inform the network about the congestion
   encountered by packets earlier in the same flow.  This document
   specifies an IPv6 destination option [RFC2460] that can be used for
   performing ConEx markings in IPv6 datagrams.

   This document specifies the ConEx wire protocol in IPv6.  The ConEx
   information can be used by any network element on the path to e.g. do
   traffic management or egress policing.  Additionally this information
   will potentially be used by an audit function that checks the
   integrity of the sender's signaling.  Further each transport
   protocol, that supports ConEx signaling, will need to specify
   precisely when the transport sets ConEx markings (e.g. the behavior
   for TCP is specified in [I-D.ietf-conex-tcp-modifications]).

   This document specifies ConEx for IPv6 only.  Due to space
   limitations in the IPv4 header and the risk of options that might be
   stripped by middlebox in IPv4 the primary goal of the working goal
   was to specify ConEx in IPv6 for experimentation.

   This specification is experimental to allow the IETF to assess
   whether the decision to implement the ConEx signal as a destination
   option fulfills the requirements stated in this document, as well as

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   to evaluate the proposed encoding of the ConEx signals as described
   in [I-D.ietf-conex-abstract-mech].

   The duration of this experiment is expected to be no less than two
   years from publication of this document as infrastructure is needed
   to be set up to determine the outcome of this experiment.
   Experimenting with Conex requires IPv6 traffic.  Even though the
   amount of IPv6 traffic is growing, the traffic mix carried over IPv6
   is still very different as over IPv4.  Therefore, it might taker
   longer to find a suitable test scenario where only IPv6 traffic is
   managed using ConEx.

2.  Conventions used in this document

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

3.  Requirements for the coding of ConEx in IPv6

   A set of requirement for an ideal concrete ConEx wire protocol is
   given in [I-D.ietf-conex-abstract-mech].  In the ConEx working group
   is was recognized that it will be difficult to find an encoding in
   IPv6 that satisfies all requirements.  The choice in this document to
   implement the ConEx information in a destination option aims to
   satisfy those requirements that constrain the placement of ConEx

   R-1: The marking mechanism needs to be visible to all ConEx-capable
   nodes on the path.

   R-2: The mechanism needs to be able to traverse nodes that do not
   understand the markings.  This is required to ensure that ConEx can
   be incrementally deployed over the Internet.

   R-3: The presence of the marking mechanism should not significantly
   alter the processing of the packet.  This is required to ensure that
   ConEx marked packets do not face any undue delays or drops due to a
   badly chosen mechanism.

   R-4: The markings should be immutable once set by the sender.  At the
   very least, any tampering should be detectable.

   Based on these requirements four solutions to implement the ConEx
   information in the IPv6 header have been investigated: hop-by-hop
   options, destination options, using IPv6 header bits (from the flow
   label), and new extension headers.  After evaluating the different

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   solutions, the ConEx working group concluded that the use of a
   destination option would best address these requirements.

   Hop-by-hop options would have been the best solution for carrying
   ConEx markings if requirement R-3 would have been met.  There is
   currently some work ongoing in the 6man wgto address this very issue
   [I-D.ietf-6man-hbh-header-handling].  This new behavior would address
   R-3 and would make hop-by-hop options the preferred solution for
   carrying ConEx markings.

   Choosing to use a destination option does not necessarily satisfy the
   requirement for on-path visibility, because it can be encapsulated by
   additional IP header(s).  Therefore, ConEx-aware network devices,
   including policy or audit devices, might have to follow the chaining
   (extension-)headers into inner IP headers to find ConEx information.
   This choice was a compromise between fast-path performance of Conex-
   aware network nodes and visibility, as discussed in
   Section Section 5.

   Please note that the IPv6 specification [RFC2460] does not require or
   expect intermediate nodes to inspect destination options such as the
   CDO.  This implies that ConEx-aware intermediate nodes following this
   specification need updated extension header processing code to be
   able read the destination options.

4.  ConEx Destination Option (CDO)

   The ConEx Destination Option (CDO) is a destination option that can
   be included in IPv6 datagrams that are sent by ConEx-aware senders in
   order to inform ConEx-aware nodes on the path about the congestion
   encountered by packets earlier in the same flow or the expected risk
   of encountering congestion in the future.  The CDO has an alignment
   requirement of (none).

    0                   1                   2
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
   |  Option Type  | Option Length |X|L|E|C|  res  |

                 Figure 1: ConEx Destination Option Layout

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     Option Type

        8-bit identifier of the type of option. Set to the value
        30 (0x1E) allocated for experimental work.

     Option Length

        8-bit unsigned integer.  The length of the option in octets
        (excluding the Option Type and Option Length fields). Set to
        the value 1.

     X Bit

        When this bit is set, the transport sender is using ConEx with
        this packet. If it is not set, the sender is not using ConEx
        with this packet.

     L Bit

        When this bit is set, the transport sender has experienced a

     E Bit

        When this bit is set, the transport sender has experienced
        congestion signaled using Explicite Congestion Notification
        (ECN) [RFC3168].

     C Bit

        When this bit is set, the transport sender is building up
        congestion credit in the audit function.

     Reserved (res)

        These four bits are not used in the current specification.
        They are set to zero on the sender and are ignored on the

                               Option Layout

   All packets sent over a ConEx-capable TCP connection or belonging to
   the same ConEx-capable flow MUST carry the CDO.  The chg bit (the
   third-highest-order bit) in the CDO Option Type field is set to zero,
   meaning that the CDO option is immutable.  Network devices with
   ConEx-aware functions read the flags, but all network devices MUST
   forward the CDO unaltered.

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   The CDO SHOULD be placed as the first option in the destination
   option header before the AH and/or ESP (if present).  IPsec
   Authentication Header (AH) MAY be used to verify that the CDO has not
   been modified.

   If the X bit is zero all other three bits are undefined and thus MUST
   be ignored and forwarded unchanged by network nodes.  The X bit set
   to zero means that the connection is ConEx-capable but this packet
   MUST NOT be counted when determining ConEx information in an audit
   function.  This can be the case if no congestion feedback is
   (currently) available e.g. in TCP if one endpoint has been receiving
   data but sending nothing but pure ACKs (no user data) for some time.
   This is because pure ACKs do not advance the sequence number, so the
   TCP endpoint receiving them cannot reliably tell whether any have
   been lost due to congestion.  Pure TCP ACKs cannot be ECN-marked
   either [RFC3168].

   If the X bit is set, any of the other three bits (L, E, C) might be
   set.  Whenever one of these bits is set, the number of bytes carried
   by this IP packet (including the IP header that directly encapsulates
   the CDO and everything that IP header encapsulates) SHOULD be counted
   to determine congestion or credit information.  In IPv6 the number of
   bytes can easily be calculated by adding the number 40 (length of the
   IPv6 header in bytes) to the value present in the Payload Length
   field in the IPv6 header.

   The credit signal represents potential for congestion.  If a
   congestion event occurs, a corresponding amount of credit is consumed
   as outlined in [I-D.ietf-conex-abstract-mech].  A ConEx-enabled
   sender SHOULD, therefore, signal sufficient credit in advance to any
   congestion event to cover the (estimated maximum) amount of lost or
   CE-marked bytes that could occur in such a congestion event.  This
   estmation depends on the heuristics used and aggressiveness of the
   sender whening deciding about the apropriate sending rate (congestion
   control).  Note, the maximum congestion risk is that all packets in
   flight get lost or CE-marked, and therefore this would be the most
   conservative estimation for the congestion risk.  After a congestion
   event, if the sender intends to take the same risk again, it just
   needs to replace the consumed credit as non-consumed credit does not
   expire.  For the case of TCP, this is described in detail in

   If the L or E bit is set, a congestion signal in the form of a loss
   or, respectively, an ECN mark was previously experienced by the same

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   In principle all of these three bits (L, E, C) might be set in the
   same packet.  In this case the packet size MUST be counted more than
   once for each respective ConEx information counter.

   If a network node extracts the ConEx information from a connection,
   it is expected to hold this information in bytes, e.g. comparing the
   total number of bytes sent with the number of bytes sent with ConEx
   congestion marks (L, E) to determine the current whole path
   congestion level.  Therefore a ConEx-aware nodes, that processes the
   CDO, MUST use the Payload length field of the preceding IPv6 header
   for byte-based counting.  When a ratio is measured and equally sized
   packets can be assumed, counting the number of packets (instead of
   the number of bytes) should deliver the same result.  But an audit
   function must be aware that this estimation can be quite wrong, if
   e.g. different sized packed are sent and thus it is not reliable.

   All remaining bits in the CDO are reserved for future use (which are
   currently the last four bits of the eight bit option space).  A ConEx
   sender SHOULD set the reserved bits in the CDO to zero.  Other nodes
   MUST ignore these bits and ConEx-aware intermediate nodes MUST
   forward them unchanged, whatever their values.  They MAY log the
   presence of a non-zero reserved field.

   The CDO is only applicable on unicast or anycast packets (see
   [I-D.ietf-conex-abstract-mech] note regarding item J on multicast at
   the end of section 3.3 for reasoning).  A ConEx sender MUST NOT send
   a packet with the CDO to a multicast address.  ConEx-capable network
   nodes MUST treat a multicast packet with the X flag set the same as
   an equivalent packet without the CDO, and they SHOULD forward it

   As stated in [I-D.ietf-conex-abstract-mech] (see section 3.3 item N
   on network layer requirements) protocol specs should describe any
   warning or error messages relevant to the encoding.  There are no
   warnings or error messages associated with the CDO.

5.  Implementation in the fast path of ConEx-aware routers

   The ConEx information is being encoded into a destination option so
   that it does not impact forwarding performance in the non-ConEx-aware
   nodes on the path.  Since destination options are not usually
   processed by routers, the existence of the CDO does not affect the
   fast path processing of the datagram on non-ConEx-aware routers, i.e.
   they are not pushed into the slow path towards the control plane for
   exception processing.

   ConEx-aware nodes still need to process the CDO without severely
   affecting forwarding.  For this to be possible, the ConEx-aware

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   routers need to quickly ascertain the presence of the CDO and process
   the option if it is present.  To efficiently perform this, the CDO
   needs to be placed in a fairly deterministic location.  In order to
   facilitate forwarding on ConEx-aware routers, ConEx-aware senders
   that send IPv6 datagrams with the CDO SHOULD place the CDO as the
   first destination option in the destination options header.

6.  Tunnel Processing

   As with any destination option, an ingress tunnel endpoint will not
   normally copy the CDO when adding an encapsulating outer IP header.
   In general an ingress tunnel SHOULD NOT copy the CDO to the outer
   header as this would changed the number of bytes that would be
   counted.  However, it MAY copy the CDO to the outer header in order
   to facilitate visibility by subsequent on-path ConEx functions if the
   configuration of the tunnel ingress and the ConEx nodes is co-
   ordinated.  This trades off the performance of ConEx functions
   against that of tunnel processing.

   An egress tunnel endpoint SHOULD ignore any CDO in the outer header
   on decapsulation of an outer IP header.  The information in any inner
   CDO will always be considered correct, even if it differs from any
   outer CDO.  Therefore, the decapsulator can strip the outer CDO
   without comparison to the inner.  A decapsulator MAY compare the two,
   and MAY log any case where they differ.  However, the packet MUST be
   forwarded irrespective of any such anomaly, given an outer CDO is
   only a performance optimization.

   A network node that assesses ConEx information SHOULD search for
   encapsulated IP headers until a CDO is found.  At any specific
   network location, the maximum necessary depth of search is likely to
   be the same for all packets between a given set of tunnel endpoints.

7.  Compatibility with use of IPsec

   A network-based attacker could alter ConEx information to fool an
   audit function in a downstream network into discarding packets.  If
   the endpoints are using the IPsec Authentication Header (AH)
   [RFC2460] to detect alteration of IP headers along the path, AH will
   also detect alteration of the CDO header.  Nonetheless, AH protection
   will rarely need to be introduced for ConEx, because attacks by one
   network on another are rare if they are traceable.  Other known
   attacks from one network on another such a TTL expiry attacks are
   more damaging to the innocent network (because ConEx audit discards
   silently) and less traceable (because TTL is meant to change, whereas
   CDO is not).

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   Section 4 specifies that the CDO is placed in the destination option
   header before the AH and/or ESP headers so that ConEx information
   remains in the clear if ESP is being used to encrypt other
   transmitted information in transport mode [RFC4301].  In general, a
   Destination Option header inside an IPv6 packet can be placed in two
   possible positions, either before the Routing header or after the
   ESP/AH headers as described in Section 4.1 of [RFC2460].  If the CDO
   were placed in the latter position and an ESP header was used with
   encryption, ConEx-aware intermediate nodes would not be able to view
   and interpret the CDO, effectively rendering it useless.

   The IPv6 protocol architecture currently does not provide a mechanism
   for new headers to be copied to the outer IP header.  Therefore if
   IPsec encryption is used in tunnel mode, ConEx information cannot be
   accessed over the extent of the ESP tunnel.

   Also, the destination IP stack will not usually process the CDO,
   therefore the sender can send a CDO without checking if the receiver
   will understand it.  The CDO MUST still be forwarded to the
   destination IP stack, because the destination might check the
   integrity of the whole packet, irrespective of whether it understands

8.  Mitigating flooding attacks by using preferential drop

   This section is aspirational, and not critical to the use of ConEx
   for more general traffic management.  However, once CDO information
   is present, the CDO header could optionally also be used in the data
   plane of any IP-aware forwarding node to mitigate flooding attacks.

   Please note that ConEx is an experimental protocol and that any kind
   of mechanisms that reacts on information provided by the ConEx
   protocol needs to be evaluated in experimentation as well.  This is
   also true, or especially true, for the preferential drop mechanism
   described below.

   Dropping packets preferentially that are not ConEx-capable or do not
   carry a ConEx mark can be beneficial to migrate flooding attacks as
   ConEx-marked packets can be assumed to be already restricted by an
   ConEx ingress policer as further described in
   [I-D.ietf-conex-abstract-mech].  Therefore the following ConEx-based
   perferential dropping scheme is proposed:

   If a router queue experiences very high load so that it has to drop
   arriving packets, it MAY preferentially drop packets within the same
   DiffServ PHB using the preference order given in Table 1 (1 means
   drop first).  Additionally, if a router implements preferential drop
   based on ConEx it SHOULD also support ECN-marking.  Even though

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   preferential dropping can be difficult to implement on some hardware,
   if nowhere else, routers at the egress of a network SHOULD implement
   preferential drop based on ConEx markings (stronger than the MAY

                 |                      |   Preference   |
                 | Not-ConEx or no CDO  | 1 (drop first) |
                 | X (but not L,E or C) |       2        |
                 | X and L,E or C       |       3        |

                Table 1: Drop preference for ConEx packets

   A flooding attack is inherently about congestion of a resource.  As
   load focuses on a victim, upstream queues grow, requiring honest
   sources to pre-load packets with a higher fraction of ConEx-marks.

   If ECN marking is supported by downstream queues, preferential
   dropping provides the most benefits because, if the queue is so
   congested that it drops traffic, it will be CE-marking 100% of any
   forwarded traffic.  Honest sources will therefore be sending 100%
   ConEx E-marked packets (and subject to rate-limiting at an ingress

   Senders under malicious control can either do the same as honest
   sources, and be rate-limited at ingress, or they can understate
   congestion and not set the E bit.

   If the preferential drop ranking is implemented on queues, these
   queues will preserve E/L-marked traffic until last.  So, the traffic
   from malicious sources will all be automatically dropped first.
   Either way, malicious sources cannot send more than honest sources.
   Therefore ConEx-based perferential drooping as describe above
   discriminates against attack traffic if done as part of the overall
   policing framework as described in [I-D.ietf-conex-abstract-mech].

9.  Acknowledgements

   The authors would like to thank David Wganer, Marcelo Bagnulo,
   Ingemar Johansson, Joel Halpern, John Leslie, Martin Stiemerling,
   Robert Sparks, Ron Bonica, Brian Haberman, Kathleen Moriarty, Bob
   Hinden, Ole Troan and Brian Carpenter for the discussions that made
   this document better.

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10.  Security Considerations

   [I-D.ietf-conex-abstract-mech] describes the overall audit framework
   for assuring that ConEx markings truly reflect actual path
   congestion.  This section focuses purely on the security of the
   encoding chosen for ConEx markings.

   The CDO Option Type is defined with a chg bit set to zero as
   described in Section 4.  If IPsec AH is used, a zero chg bit causes
   AH to cover the CDO option so that its end-to-end integrity can be
   verified, as explained in Section 4.

   This document specifies that the Reserved field in the CDO must be
   ignored and forwarded unchanged even if it does not contain all
   zeroes.  The Reserved field is also required to sit outside the
   Encapsulating Security Payload (ESP), at least in transport mode (see
   Section 7).  This allows the sender to use the Reserved field as a 4-
   bit-per-packet covert channel to send information to an on-path node
   outside the control of IPsec.  However, a covert channel is only a
   concern if it can circumvent IPsec in tunnel mode and, in the tunnel
   mode case, ESP would close the covert channel as outlined in
   Section 7.

11.  IANA Considerations

   The IPv6 ConEx destination option is used for carrying ConEx
   markings.  This document uses the experimental option type 0x1E with
   the act bits set to 00 and the chg bit set to 0 for realizing this
   option.  No further allocation action is required from IANA at this

12.  References

12.1.  Normative References

              Mathis, M. and B. Briscoe, "Congestion Exposure (ConEx)
              Concepts, Abstract Mechanism and Requirements", draft-
              ietf-conex-abstract-mech-13 (work in progress), October

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

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   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460,
              December 1998, <>.

   [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,

   [RFC4301]  Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", RFC 4301, DOI 10.17487/RFC4301,
              December 2005, <>.

   [RFC4302]  Kent, S., "IP Authentication Header", RFC 4302,
              DOI 10.17487/RFC4302, December 2005,

   [RFC4303]  Kent, S., "IP Encapsulating Security Payload (ESP)",
              RFC 4303, DOI 10.17487/RFC4303, December 2005,

12.2.  Informative References

              Baker, F., "IPv6 Hop-by-Hop Header Handling", draft-ietf-
              6man-hbh-header-handling-00 (work in progress), November

              Kuehlewind, M. and R. Scheffenegger, "TCP modifications
              for Congestion Exposure", draft-ietf-conex-tcp-
              modifications-10 (work in progress), October 2015.

              Wagner, D. and M. Kuehlewind, "Auditing of Congestion
              Exposure (ConEx) signals", draft-wagner-conex-audit-01
              (work in progress), February 2014.

Authors' Addresses

   Suresh Krishnan
   8400 Blvd Decarie
   Town of Mount Royal, Quebec


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   Mirja Kuehlewind
   ETH Zurich


   Bob Briscoe
   Simula Research Laboratory


   Carlos Ralli Ucendo


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