ConEx Working Group S. Krishnan
Internet-Draft Ericsson
Intended status: Experimental M. Kuehlewind
Expires: April 21, 2016 ETH Zurich
C. Ralli
Telefonica
October 19, 2015
IPv6 Destination Option for Congestion Exposure (ConEx)
draft-ietf-conex-destopt-11
Abstract
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.
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This Internet-Draft will expire on April 21, 2016.
Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
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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. Auditing . . . . . . . . . . . . . . . . . . . . . . . . . . 7
7. Tunnel Processing . . . . . . . . . . . . . . . . . . . . . . 8
8. Compatibility with use of IPsec . . . . . . . . . . . . . . . 9
9. Mitigating flooding attacks by using preferential drop . . . 10
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 11
11. Security Considerations . . . . . . . . . . . . . . . . . . . 11
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
13.1. Normative References . . . . . . . . . . . . . . . . . . 12
13.2. Informative References . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13
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.
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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].
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",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
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
information:
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
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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.
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
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.
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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.
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.
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
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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
connection.
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.
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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
unchanged.
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 SHOULD place the CDO as the
first destination option in the destination options header.
6. Auditing
An audit element shall be a shadow device in the network, i.e. its
presence should not be detectable for well-behaving senders. The
objective of an audit is to verify that senders correctly signals
ConEx information and to penalize cheaters. For this, the audit
element has to maintain state for any active ConEx-enabled flow.
Flows must be audited independently, as there are no dependencies.
There are two aspects the audit element has to check for each flow:
o if the congestion reported using the ConEx mechanism (L- or
E-marked bytes) matches the congestion actually observed by the
receivers (loss or ECN-CE-marks) and
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o if sufficient credit marks have been sent to signal the congestion
risk in advance (number of C-marked bytes is larger than lost or
ECN-EC-marked bytes).
By the Re-Echo-Loss signal a sender exposes to the network that this
transport has experienced loss very recently. By the Re-Echo-ECN
signal a sender exposes to the network that this transport has
experienced an ECN-CE mark very recently. For the audit this means,
that if it detects a loss or an ECN-CE mark for a ConEx-enabled flow,
for a compliant sender the corresponding Re-Echo-Loss or Re-Echo-ECN
signals must be observed in the near future (e.g. within a 1-2 RTT
after the congestion has been observed).
The Credit signal represents potential for congestion. A ConEx-
enabled sender SHOULD signal sufficient credit in advance to any
congestion event. If a congestion event occurs, a corresponding
amount of credit is consumed. If the sender intends to take the same
risk again, it just must replace this consumed credit as non-consumed
credit does not expire.
The audit penalizes a flow if it fails either of these two criteria.
In order to not incentivize senders to simply start new flows when
detecting being penalized by an audit element, the penalty of a
misbehaving flow SHOULD be proportional to the misbehavior. This
document does not mandate a particular audit design. Implementation
considerations are further discussed in [I-D.wagner-conex-audit].
The CDO MUST only be used alongside protocols that provide a way to
audit loss in the network. Any protocol specification that uses the
CDO MUST define how an audit device can detect loss on the forward
path (e.g. sequence number monitoring) and the means for protecting
against likely attacks on such mechanisms.
7. 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
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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.
8. Compatibility with use of IPsec
If the endpoints are using the IPsec Authentication Header (AH)
[RFC4302] to detect alteration of IP headers along the path, AH will
also ensure the e2e integrity of the CDO header. A network-based
attacker could alter ConEx information to fool an audit function in a
downstream network into discarding packets. However, other existing
attacks from one network on another such a TTL expiry attacks are
more damaging (because ConEx audit discards silently) and less
traceable (because TTL is meant to change, whereas CDO is not).
Inside an IPv6 packet, a Destination Option header 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]. When 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 was used
with encryption, the CDO cannot be viewed and interpreted by ConEx-
aware intermediate nodes 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
ConEx.
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9. 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
above).
+----------------------+----------------+
| | 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
policer).
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].
10. Acknowledgements
The authors would like to thank Marcelo Bagnulo, Bob Briscoe, 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.
Special thanks to Bob Briscoe who contributed text and analysis work
on preferential dropping and to David Wagner for providing the text
on which the audting section is based.
11. 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.
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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.
12. 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
time.
13. References
13.1. Normative References
[I-D.ietf-conex-abstract-mech]
Mathis, M. and B. Briscoe, "Congestion Exposure (ConEx)
Concepts, Abstract Mechanism and Requirements", draft-
ietf-conex-abstract-mech-13 (work in progress), October
2014.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
[RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
of Explicit Congestion Notification (ECN) to IP", RFC
3168, September 2001.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.
[RFC4302] Kent, S., "IP Authentication Header", RFC 4302, December
2005.
[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)", RFC
4303, DOI 10.17487/RFC4303, December 2005,
<http://www.rfc-editor.org/info/rfc4303>.
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13.2. Informative References
[I-D.ietf-conex-tcp-modifications]
Kuehlewind, M. and R. Scheffenegger, "TCP modifications
for Congestion Exposure", draft-ietf-conex-tcp-
modifications-08 (work in progress), April 2015.
[I-D.wagner-conex-audit]
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
Ericsson
8400 Blvd Decarie
Town of Mount Royal, Quebec
Canada
Email: suresh.krishnan@ericsson.com
Mirja Kuehlewind
ETH Zurich
Email: mirja.kuehlewind@tik.ee.ethz.ch
Carlos Ralli Ucendo
Telefonica
Email: ralli@tid.es
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