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IPv6 Destination Option for Congestion Exposure (ConEx)

The information below is for an old version of the document.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 7837.
Authors Suresh Krishnan , Mirja Kühlewind , Carlos Ucendo
Last updated 2015-10-14 (Latest revision 2015-08-05)
RFC stream Internet Engineering Task Force (IETF)
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 IESG Evaluation::Revised I-D Needed
Consensus boilerplate Yes
Telechat date (None)
Needs a YES.
Responsible AD Martin Stiemerling
Send notices to (None)
IANA IANA review state IANA OK - Actions Needed
ConEx Working Group                                          S. Krishnan
Internet-Draft                                                  Ericsson
Intended status: Experimental                              M. Kuehlewind
Expires: February 6, 2016                                     ETH Zurich
C. Ralli
August 5, 2015

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 February 6, 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
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 . . . . . . . . . . . . . . . . . . .  10
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 [ID.conex-tcp-modifications]).

This document specifies ConEx for IPv6 only.  Due to space limitation
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

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

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

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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.

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                   3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
|  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. The option identifier
for the ConEx destination option will be allocated by the IANA.

Option Length

8-bit unsigned integer.  The length of the option (excluding
the Option Type and Option Length fields). The sender MUST set
this field to 1 but ConEx-aware nodes MUST accept an option
length of 1 or more.

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 loss.

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 receiver.


All packets sent over a ConEx-capable TCP connection or belonging to
the same ConEx-capable flow MUST carry the CDO.  The CDO is
immutable.  Network devices with ConEx-aware functions read the
flags, but all network devices MUST forward the CDO unaltered.

CDO MUST 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.

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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.

A transport sends credits prior to the occurrence of congestion (loss
or ECN-CE marks) and the amount of credits should cover the
congestion risk.  This is further specified in
[I-D.ietf-conex-abstract-mech] and described in detail for the case
of TCP in [I-D.ietf-conex-tcp-modifications].  Note, the maximum
congestion risk is that all packets in flight get lost or ECN marked.

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

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 a network
node must be aware that this estimation can be quite wrong, if e.g.
different sized packed are sent and thus it is not reliable.

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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.

It might be possible to implement a proxy for a ConEx sender, as long
as it is located where receiver feedback is always visible.  A ConEx
proxy MUST NOT introduce a CDO header into a packet already carrying
one and it MUST NOT alter the information in any existing CDO header.
However, it can add a CDO header to any packets without one, taking
care not to disrupt any integrity or authentication mechanisms as
well as to not exceed the MTU.

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
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 MUST place the CDO as the first
destination option in the destination options header.

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6.  Tunnel Processing

As with any destination option, an ingress tunnel endpoint will not
natively 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 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.

7.  Compatibility with use of IPsec

If the transport network cannot be trusted, IPsec Authentication
should be used to ensure integrity of the ConEx information.  If an
attacker would be able to remove the ConEx marks, this could cause an
audit device to penalize the respective connection, while the sender
cannot easily detect that ConEx information is missing.

In IPv6 a Destination Option header can be placed in two possible
position in the order of possible headers, either before the Routing
header or after the Encapsulating Security Payload (ESP) header
[RFC2460].  As the CDO is placed in the destination option header
before the AH and/or ESP, it is not encrypted in transport mode
[RFC4301].  Otherwise, if the CDO were placed in the latter position
and an ESP header were used, the CDO would also be encrypted and
could not be interpreted by ConEx-aware devices.

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.

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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
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%

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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 Marcelo Bagnulo, Bob Briscoe, Ingemar
Johansson, Joel Halpern and John Leslie for the discussions that led
to this document.

Special thanks to Bob Briscoe who contributed text and analysis work
on preferential dropping.

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 chg bit in the CDO option type field is set to zero, meaning that
the CDO option is immutable.  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.

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11.  IANA Considerations

This document defines a new IPv6 ConEx destination option for
carrying ConEx markings.  IANA is requested to assign a new
destination option type in the Destination Options registry
maintained at <TBA1>
ConEx Destination Option [RFCXXXX] The act bits for this option need
to be 00.  The chg bit need to be 0.  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 ConEx.  Please also update the
describe of the Option Type in section 4 after assignment!

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,

[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,

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12.2.  Informative References

Kuehlewind, M. and R. Scheffenegger, "TCP modifications
for Congestion Exposure", draft-ietf-conex-tcp-
modifications-08 (work in progress), April 2015.

[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,

Authors' Addresses

Suresh Krishnan
8400 Blvd Decarie
Town of Mount Royal, Quebec


Mirja Kuehlewind
ETH Zurich


Carlos Ralli Ucendo


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