Adding Explicit Congestion Notification (ECN) to TCP control packets and TCP retransmissions
draft-bagnulo-tcpm-generalized-ecn-01
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft whose latest revision state is "Replaced".
|
|
|---|---|---|---|
| Authors | Marcelo Bagnulo , Bob Briscoe | ||
| Last updated | 2017-04-13 | ||
| Replaces | draft-bagnulo-tsvwg-generalized-ecn | ||
| Replaced by | draft-ietf-tcpm-generalized-ecn | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Formats | |||
| Additional resources | |||
| Stream | WG state | (None) | |
| Document shepherd | (None) | ||
| IESG | IESG state | I-D Exists | |
| Consensus boilerplate | Unknown | ||
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | (None) |
draft-bagnulo-tcpm-generalized-ecn-01
Network Working Group M. Bagnulo
Internet-Draft UC3M
Intended status: Experimental B. Briscoe
Expires: October 15, 2017 Simula Research Lab
April 13, 2017
Adding Explicit Congestion Notification (ECN) to TCP control packets and
TCP retransmissions
draft-bagnulo-tcpm-generalized-ecn-01
Abstract
This document describes an experimental modification to ECN when used
with TCP. It allows the use of ECN on the following TCP packets:
SYNs, Pure ACKs, Window probes, FINs, RSTs and retransmissions.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on October 15, 2017.
Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Bagnulo & Briscoe Expires October 15, 2017 [Page 1]
Internet-Draft ECN and TCP control packets April 2017
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Motivation . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Experiment goals . . . . . . . . . . . . . . . . . . . . 4
1.3. Document structure . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Specification . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1. Network behaviour . . . . . . . . . . . . . . . . . . . . 6
3.2. Endpoint behaviour . . . . . . . . . . . . . . . . . . . 6
3.2.1. SYN . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.2.2. SYN-ACK . . . . . . . . . . . . . . . . . . . . . . . 10
3.2.3. Pure ACK . . . . . . . . . . . . . . . . . . . . . . 10
3.2.4. Window Probe . . . . . . . . . . . . . . . . . . . . 11
3.2.5. FIN . . . . . . . . . . . . . . . . . . . . . . . . . 12
3.2.6. RST . . . . . . . . . . . . . . . . . . . . . . . . . 12
3.2.7. Retransmissions . . . . . . . . . . . . . . . . . . . 13
4. Interaction with popular variants or derivatives of TCP . . . 14
4.1. SCTP . . . . . . . . . . . . . . . . . . . . . . . . . . 14
4.2. TFO . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
4.3. IW10 . . . . . . . . . . . . . . . . . . . . . . . . . . 15
5. Discussion of the arguments in RFC 3168 . . . . . . . . . . . 16
5.1. The reliability argument . . . . . . . . . . . . . . . . 16
5.2. SYNs . . . . . . . . . . . . . . . . . . . . . . . . . . 16
5.2.1. Argument 1a: Loss of congestion notification on the
SYN . . . . . . . . . . . . . . . . . . . . . . . . . 17
5.2.2. Argument 1b: Unknown Handling of Unexpected ECN . . . 18
5.2.3. Argument 2: DoS attacks. . . . . . . . . . . . . . . 19
5.3. Pure ACKs. . . . . . . . . . . . . . . . . . . . . . . . 20
5.4. Window probes . . . . . . . . . . . . . . . . . . . . . . 22
5.5. Retransmitted packets. . . . . . . . . . . . . . . . . . 22
6. Security considerations . . . . . . . . . . . . . . . . . . . 23
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 23
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 23
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 23
9.1. Normative References . . . . . . . . . . . . . . . . . . 24
9.2. Informative References . . . . . . . . . . . . . . . . . 24
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 26
1. Introduction
RFC 3168 [RFC3168] specifies support of Explicit Congestion
Notification (ECN) in IP (v4 and v6). By using the ECN capability,
switches performing Active Queue Management (AQM) can use ECN marks
instead of packet drops to signal congestion to the endpoints of a
communication. This results in lower packet loss and increased
performance. RFC 3168 also specifies support for ECN in TCP, but
solely on data packets. For various reasons it precludes the use of
Bagnulo & Briscoe Expires October 15, 2017 [Page 2]
Internet-Draft ECN and TCP control packets April 2017
ECN on TCP control packets (TCP SYN, TCP SYN-ACK, pure ACKs, Window
probes) and on retransmitted packets. RFC 3168 is silent about the
use of ECN on RST and FIN packets. RFC 5562 [RFC5562] is an
experimental modification to ECN that enables ECN support for TCP
SYN-ACK packets.
This document defines an experimental modification to ECN [RFC3168]
that enables ECN support on all the aforementioned types of TCP
packet. [I-D.ietf-tsvwg-ecn-experimentation] is a standards track
procedural device that updates RFC 3168 to allow the present
experiment, which RFC 3168 would otherwise prohibit.
1.1. Motivation
The absence of ECN support on TCP control packets and retransmissions
has a potential harmful effect. In any ECN deployment, non-ECN-
capable packets suffer a penalty when they traverse a congested
bottleneck. For instance, with a drop probability of 1%, 1% of
connection attempts suffer a timeout of about 1 second before the SYN
is retransmitted, which is highly detrimental to the performance of
short flows. TCP control packets, such as TCP SYNs and pure ACKs,
are important for performance, so dropping them is best avoided.
Non-ECN control packets particularly harm performance in environments
where the ECN marking level is high. For example, [judd-nsdi] shows
that in a data centre (DC) environment where ECN is used (in
conjunction with DCTCP), the probability of being able to establish a
new connection using a non-ECN SYN packet drops to close to zero even
when there are only 16 ongoing TCP flows transmitting at full speed.
In this data centre context, the issue is that DCTCP's aggressive
response to packet marking leads to a high marking probability for
ECN-capable packets, and in turn a high drop probability for non-ECN
packets. Therefore non-ECN SYNs are dropped aggressively, rendering
it nearly impossible to establish a new connection in the presence of
even mild traffic load.
Finally, there are ongoing experimental efforts to promote the
adoption of a slightly modified variant of DCTCP (and similar
congestion controls) over the Internet to achieve low latency, low
loss and scalable throughput (L4S) for all communications
[I-D.briscoe-tsvwg-l4s-arch]. In such an approach, L4S packets
identify themselves using an ECN codepoint. Preventing TCP control
packets from obtaining the benefits of ECN would not only expose them
to the prevailing level of congestion loss, but it would also stop
them from being classified into the low latency (L4S) queue, which
would greatly degrade L4S performance.
Bagnulo & Briscoe Expires October 15, 2017 [Page 3]
Internet-Draft ECN and TCP control packets April 2017
1.2. Experiment goals
The goal of the experimental modifications defined in this document
is to allow the use of ECN (both ECT and CE codepoints) on all TCP
packets. Experiments are expected in the public Internet as well as
in controlled environments to understand the following issues:
o How SYNs, Window probes, pure ACKs, FINs, RSTs and retransmissions
that carry the ECT(0), ECT(1) or CE codepoints are processed by
the TCP endpoints and the network (including routers, firewalls
and other middleboxes). In particular we would like to learn if
these packets are frequently blocked or if these packets are
usually forwarded and processed.
o The scale of deployment of the different flavours of ECN,
including [RFC3168], [RFC5562], [RFC3540] and
[I-D.ietf-tcpm-accurate-ecn].
o How much the performance of TCP communications is improved by
allowing ECN marking of each packet type.
o To identify any issues (including security issues) raised by
enabling ECN marking of these packets.
The data gathered through the experiments described in this document,
particularly under the first 2 bullets above, will help in the design
of the final mechanism (if any) for adding ECN support to the
different packet types considered in this document. Whenever data
input is needed to assist in a design choice, it is spelled out
throughout the document.
Success criteria: The experiment will be a success if we obtain
enough data to have a clearer view of the deployability and benefits
of ECN marking all TCP packets, as well as any issues. If the
results of the experiment show that it is feasible to deploy such
changes; that there are gains to be achieved though the changes
described in this specification; and that no other major issues may
interfere with the deployment of the proposed changes; then it would
be reasonable to adopt the proposed changes in a standards track
specification that would update RFC 3168.
1.3. Document structure
The remainder of this document is structured as follows. In
Section 2, we present the terminology used in the rest of the
document. In Section 3, we specify the modifications to provide ECN
support to TCP SYNs, pure ACKs, Window probes, FINs, RSTs and
retransmissions. We describe both the network behaviour and the
Bagnulo & Briscoe Expires October 15, 2017 [Page 4]
Internet-Draft ECN and TCP control packets April 2017
endpoint behaviour. Section 4 discusses variations of the
specification that will be necessary to interwork with a number of
popular variants or derivatives of TCP. RFC 3168 provides a number
of specific reasons why ECN support is not appropriate for each
packet type. In Section 5, we revisit each of these arguments and
explore the possibility of enabling the ECN capability for each
packet type in turn.
2. Terminology
The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
document, are to be interpreted as described in [RFC2119].
Pure ACK: A TCP segment with the ACK flag set and no data payload.
SYN: A TCP segment with the SYN (synchronize) flag set. It may carry
data if TCP Fast Open is used.
Window probe: Defined in [RFC1122], a window probe is a TCP segment
with only one byte of data sent to learn if the receive window is
still zero.
FIN: A TCP segment with the FIN (finish) flag set.
RST: A TCP segment with the RST (reset) flag set.
Retransmission: A TCP segment that has been retransmitted by the TCP
sender because it determined that the original segment was lost,
which may or may not be the case.
ECT: ECN-Capable Transport. One of the two codepoints ECT(0) or
ECT(1) in the ECN field [RFC3168] of the IP header (v4 or v6). An
ECN-capable sender sets one of these to indicate that both transport
end-points support ECN. When this specification says the sender sets
an ECT codepoint, by default it means ECT(0). Optionally, it could
mean ECT(1), which is in the process of being redefined for use by
L4S experiments [I-D.ietf-tsvwg-ecn-experimentation]
[I-D.briscoe-tsvwg-ecn-l4s-id].
Not-ECT: The ECN codepoint that indicates that the transport is not
ECN-capable.
CE: Congestion Experienced. The ECN codepoint that an intermediate
node sets to indicate congestion [RFC3168]. A node sets an
increasing proportion of ECT packets to CE as the level of congestion
increases.
Bagnulo & Briscoe Expires October 15, 2017 [Page 5]
Internet-Draft ECN and TCP control packets April 2017
3. Specification
3.1. Network behaviour
Previously the specification of ECN for TCP [RFC3168] required the
sender to set not-ECT on TCP control packets and retransmissions.
Some readers might have erroneously interpreted this as a requirement
for firewalls, intrusion detection systems, etc. to check and enforce
this behaviour. Now that the present experimental specification
allows TCP senders to set ECT on all TCP packets (control and data),
it needs to be clear that a firewall (or any network node) SHOULD NOT
treat any ECN-capable packet differently dependent on what type of
TCP packet it is.
The previous sentence says "SHOULD NOT" rather than "MUST NOT"
because one potential exception is envisaged. A security function
that has detected an ongoing attack MAY drop more ECT marked SYNs
than not-ECT marked SYNs. Such a policy MUST NOT be applied
routinely. It can only be applied if an attack is detected, and
preferably only if it is determined that the ECT capability is
intensifying the attack.
3.2. Endpoint behaviour
The changes to the specification of TCP over ECN [RFC3168] defined
here solely alter the behaviour of a sending host.
The feedback behaviour at the receiver depends on whether classic ECN
TCP feedback [RFC3168] or Accurate ECN (AccECN) TCP feedback
[I-D.ietf-tcpm-accurate-ecn] has been negotiated. Nonetheless,
neither receiver feedback behaviour is altered by the present
specification.
For each type of control packet or retransmission, the following
sections detail changes to the sender's behaviour in two respects: i)
whether it sets ECT; and ii) its response to congestion feedback.
Table 1 summarises these two behaviours for each type of packet, but
the relevant subsection below should be referred to for the detailed
behaviour. The subsection on the SYN is more complex than the
others, because it has to include fall-back behaviour if the ECT
packet appears not to have got through, and caching of the outcome to
detect persistent failures.
Bagnulo & Briscoe Expires October 15, 2017 [Page 6]
Internet-Draft ECN and TCP control packets April 2017
+----------+------------------+--------------------+----------------+
| TCP | ECN field if | ECN field if RFC | Congestion |
| packet | AccECN f/b | 3168 f/b | Response |
| type | negotiated* | negotiated* | |
+----------+------------------+--------------------+----------------+
| SYN | ECT | not-ECT | Reduce IW |
| | | | |
| SYN-ACK | ECT | ECT | Reduce IW as |
| | | | in [RFC5562] |
| | | | |
| Pure ACK | ECT | ECT | None or |
| | | | optionally |
| | | | [RFC5690] |
| | | | |
| W Probe | ECT | ECT | Usual response |
| | | | |
| FIN | ECT | ECT | None or |
| | | | optionally |
| | | | [RFC5690] |
| | | | |
| RST | ECT | ECT | N/A |
| | | | |
| Re-XMT | ECT | ECT | Usual response |
+----------+------------------+--------------------+----------------+
Window probe and retransmission are abbreviated to W Probe an Re-XMT.
* For a SYN, "negotiated" means "requested".
Table 1: Summary of sender behaviour. In each case the relevant
section below should be referred to for the detailed behaviour
It can be seen that the sender can set ECT in all cases, except if it
is not requesting AccECN feedback on the SYN. Therefore it is
RECOMMENDED that the experimental AccECN specification
[I-D.ietf-tcpm-accurate-ecn] is implemented, because it is expected
that ECT on the SYN will give the most significant performance gain,
particularly for short flows. Nonetheless, this specification also
caters for the case where AccECN feedback is not implemented.
3.2.1. SYN
3.2.1.1. Setting ECT on the SYN
With classic [RFC3168] ECN feedback, the SYN was never expected to be
ECN-capable, so the flag provided to feed back congestion was put to
another use (it is used in combination with other flags to indicate
that the responder supports ECN). In contrast, Accurate ECN (AccECN)
feedback [I-D.ietf-tcpm-accurate-ecn] provides a codepoint in the
Bagnulo & Briscoe Expires October 15, 2017 [Page 7]
Internet-Draft ECN and TCP control packets April 2017
SYN-ACK for the responder to feed back that the SYN arrived marked
CE.
Therefore, a TCP initiator MUST NOT set ECT on a SYN unless it also
attempts to negotiate Accurate ECN feedback in the same SYN.
For the experiments proposed here, if the SYN is requesting AccECN
feedback, the TCP sender will also set ECT on the SYN. It can ignore
the prohibition in section 6.1.1 of RFC 3168 against setting ECT on
such a SYN.
The following subsections about the SYN solely apply to this case
where the initiator sent an ECT SYN.
3.2.1.2. Caching Failed Connection Attempts
Until AccECN servers become widely deployed, a TCP initiator that
implements AccECN and sets ECT on a SYN SHOULD also maintain a cache
per server to record any failure of the previous attempt. It SHOULD
record whether a server does not support AccECN and MAY record
whether the ECT SYN is persistently lost (see fall-back below). The
TCP initiator will not subsequently attempt any behaviour recorded as
persistently problematic. However, the cache should be arranged to
expire so that the initiator will infrequently attempt to check
whether each problem has been resolved.
There is no need to cache successful attempts, because the default
ECT SYN behaviour performs optimally on success.
Servers that do not support ECN as a whole can be recorded as non-
support of AccECN and do not need to be distinguished, because there
is no performance penalty in always attempting to negotiate classic
[RFC3168] ECN support.
3.2.1.3. SYN Congestion Response
Here, we use IW0 to denote the initial window of the TCP initiator
[RFC5681].
If the SYN-ACK returned to the TCP initiator confirms that the server
supports AccECN, it will also indicate whether or not the SYN was CE-
marked. If the SYN was CE-marked, the initiator MUST reduce its
Initial Window (IW) and SHOULD reduce it to 1 SMSS (sender maximum
segment size).
If the SYN-ACK shows that the server does not support AccECN, the TCP
initiator MUST conservatively reduce its Initial Window and SHOULD
reduce it to 1 SMSS. A reduction to greater than 1 SMSS MAY be
Bagnulo & Briscoe Expires October 15, 2017 [Page 8]
Internet-Draft ECN and TCP control packets April 2017
appropriate (see discussion below). Conservatism is necessary
because a non-AccECN SYN-ACK cannot show whether the SYN was CE-
marked.
If the TCP initiator (host A) receives a SYN from the remote end
(host B) after it has sent a SYN to B, it indicates the (unusual)
case of a simultaneous open. Host A will respond with a SYN-ACK.
Host A will probably then receive a SYN-ACK in response to its own
SYN, after which it can follow the appropriate one of the two
paragraphs above.
In all the above cases, the initiator does not have to back off its
retransmission timer as it would in response to a timeout following
no response to its SYN [RFC6298], because both the SYN and the SYN-
ACK have been successfully delivered through the network. Also, the
initiator does not need to exit slow start or reduce ssthresh, which
is not even required when a SYN is lost [RFC5681],
DISCUSSION: In the case where the server does not support AccECN,
because we impose a conservative reduction in initial window, we
are penalizing those that deploy AccECN with ECT SYNs, rather than
improving performance as intended. Nonetheless, if such cases are
cached, performance will only suffer on the first attempt to
access a non-AccECN server. Also, the data sent initially by a
TCP client is often a small request that usually fits within 1
SMSS anyway {ToDo: reference? (this information was given
informally by Yuchung Cheng)}.
See Section 4 for cases where TCP Fast Open (TFO [RFC7413]) or an
initial window of 10 (IW10 [RFC6928]) are also implemented.
3.2.1.4. Fall-back Following a Lost ECT SYN (or SYN-ACK))
An ECT SYN might be lost due to an over-zealous path element (or
server) blocking ECT packets that do not conform to RFC 3168.
However, loss is commonplace for numerous other reasons, e.g.
congestion loss at a non-ECN queue on the forward or reverse path,
transmission errors, etc. Alternatively, the cause of the blockage
might be the attempt to negotiate AccECN, or possibly other unrelated
options on the SYN.
To expedite connection set-up if, after sending an ECT SYN, the
retransmission timer expires, the TCP initiator SHOULD send a SYN
with the not-ECT codepoint in the IP header and not attempt to
negotiate AccECN. It would make sense to also remove any other
experimental fields or options on the SYN, but that will depend on
the specification of the other option(s). Other fall-back strategies
that are considered to improve performance MAY be adopted.
Bagnulo & Briscoe Expires October 15, 2017 [Page 9]
Internet-Draft ECN and TCP control packets April 2017
If the TCP initiator is caching failed connection attempts, it SHOULD
NOT give up using ECT on the first SYN of subsequent connection
attempts until it is clear that the blockage persistently and
specifically affects ECT on SYNs. This is because loss is so
commonplace for other reasons.
DISCUSSION: If initial experiments show that blocking of ECT on
SYNs is widespread, it MAY be necessary to cache successful
attempts as well as failures. Then, if there is no entry in the
cache for a particular server, the TCP initiator could send a not-
ECT SYN soon after the first ECT SYN. This would reduce the
performance penalty for those deploying ECT SYN support.
3.2.2. SYN-ACK
To comply with the present specification, the responder (server) part
of a TCP implementation MUST also comply with [RFC5562], which
defines the use of ECT on a SYN-ACK and the congestion response of
the TCP listener if a SYN-ACK is CE-marked.
Feedback by the initiator in response to a CE-marked SYN-ACK from the
responder depends on whether classic ECN feedback or AccECN feedback
[I-D.ietf-tcpm-accurate-ecn] has been negotiated. In either case no
change is required to RFC 5562 or the AccECN specification
respectively.
3.2.3. Pure ACK
For the experiments proposed here, the TCP implementation will set
ECT on Pure ACKs. It can ignore the requirement in section 6.1.4 of
RFC 3168 to set not-ECT on a Pure ACK.
TCP does not normally detect or respond to loss of pure ACKs.
Therefore, any response to CE markings on Pure ACKs is not required
in order to comply with the present specification. Nonetheless, a
congestion response is not precluded either. It could be arranged
using any one of the following approaches.
TCP never acknowledges Pure ACKs. So classic [RFC3168] ECN provides
no mechanism to feed back a CE marking on a Pure ACK, unless the
feedback is added to the ACK of a later data packet (if one arises).
In contrast, an AccECN receiver [I-D.ietf-tcpm-accurate-ecn]
continually feeds back a count of the number of CE-marked packets
that it has received (and, if possible, a count of CE-marked bytes).
So a TCP sender that has negotiated AccECN and is setting ECT on pure
ACKs will receive congestion feedback if any Pure ACKs are CE-marked
in transit.
Bagnulo & Briscoe Expires October 15, 2017 [Page 10]
Internet-Draft ECN and TCP control packets April 2017
In either case (classic or AccECN feedback), if the TCP sender does
receive feedback about CE-markings on Pure ACKs, it will react in the
usual way by reducing its congestion window accordingly. This will
regulate the rate of any data packets it is sending amongst the Pure
ACKs. However, reducing the congestion window will have no effect on
the rate of Pure ACKs. So while it is only sending Pure ACKs the
sender will not be responding to congestion.
Any pair of TCP end-points can already choose to regulate the rate of
Pure ACKs by agreeing to regulate the delayed ACK ratio in response
to loss or CE-marking of Pure ACKs, using the Acknowledgement
Congestion Control (AckCC) techniques documented in [RFC5690]
(informational). However, AckCC is not required.
RFC 5690 proposed new TCP options to address the problems that TCP
had no mechanism to allow ECT to be set on Pure ACKs and no mechanism
to feed back loss or CE-marking of Pure ACKs. A combination of the
present specification and AccECN addresses both these problems, at
least for ECN marking. So it might now be possible to design an ECN-
specific ACK congestion control scheme without the extra TCP options
proposed in RFC 5690. However, such a mechanism is out of scope of
the present document.
3.2.4. Window Probe
For the experiments proposed here, the TCP sender will set ECT on
window probes. It can ignore the prohibition in section 6.1.6 of RFC
3168 against setting ECT on a window probe.
A window probe contains a single octet, so it is no different from a
regular TCP data segment. Therefore a TCP receiver will feed back
any CE marking on a window probe as normal (either using classic ECN
feedback or AccECN feedback). The sender of the probe will then
reduce its congestion window as normal.
A receive window of zero indicates that the application is not
consuming data fast enough and does not imply anything about network
congestion. Once the receive window opens, the congestion window
might become the limiting factor, so it is correct that CE-marked
probes reduce the congestion window. However, CE-marking on window
probes does not reduce the rate of the probes themselves. This is
unlikely to present a problem, given a window probe is sent only
every 2 minutes [RFC0793] as long as the receiver is advertising a
zero window.
Bagnulo & Briscoe Expires October 15, 2017 [Page 11]
Internet-Draft ECN and TCP control packets April 2017
3.2.5. FIN
A TCP implementation can set ECT on a FIN.
A congestion response to a CE-marking on a FIN is not required.
After sending a FIN, the endpoint will not send any more data in the
connection. Therefore, even if the FIN-ACK indicates that the FIN
was CE-marked (whether using classic or AccECN feedback), reducing
the congestion window will not affect anything.
After sending a FIN, a host might send one or more pure ACKs. If it
is using one of the techniques in Section 3.2.3 to regulate the
delayed ACK ratio for Pure ACKs, it could equally be applied after a
FIN. But this is not required.
3.2.6. RST
A TCP implementation can set ECT on a RST.
A congestion response to a CE-marking on a RST is not required (and
actually not possible).
The host generating the RST message does not have an open connection
after sending it (either because there was no such connection when
the packet that triggered the RST message was received or because the
packet that triggered the RST message also triggered the closure of
the connection).
Moreover, the receiver of a CE-marked RST message can either: i)
accept the RST message and close the connection; ii) emit a so-called
challenge ACK in response (with suitable throttling) [RFC5961] and
otherwise ignore the RST (e.g. because the sequence number is in-
window but not the precise number expected next); or iii) discard the
RST message (e.g. because the sequence number is out-of-window). In
the first two cases there is no point in echoing any CE mark received
because the sender closed its connection when it sent the RST. In
the third case it makes sense to discard the CE signal as well as the
RST. So, in all these cases it does not make sense to generate
feedback about a CE mark on a RST message.
The following factors have been considered before deciding whether
ECT ought to be allowed on a RST message:
o As explained above, a congestion response by the sender of a CE-
marked RST message is not possible;
Bagnulo & Briscoe Expires October 15, 2017 [Page 12]
Internet-Draft ECN and TCP control packets April 2017
o So the only reason for the sender setting ECT on a RST would be to
improve the reliability of the message's delivery;
o RST messages are used to both mount and mitigate attacks:
* Spoofed RST messages are used by attackers to terminate ongoing
connections, although the mitigations in RFC 5961 have
considerably raised the bar against off-path RST attacks;
* Legitimate RST messages allow endpoints to inform their peers
to eliminate existing state that correspond to non existing
connections, liberating resources e.g. in DoS attacks
scenarios;
o AQMs are advised to disable ECN marking during persistent
overload, so:
* it is harder for an attacker to exploit ECN to intensify an
attack;
* it is harder for a legitimate user to exploit ECN to more
reliably mitigate an attack
o Prohibiting ECT on a RST would deny the benefit of ECN to
legitimate RST messages, but not to attackers who can disregard
RFCs;
o If ECT were prohibited on RSTs, security middleboxes could discard
any RSTs that were exploiting ECN to intensify an attack;
o However, unlike a SYN flood, a RST flood is easier to distinguish
from legitimate traffic, so it is easier to ignore or eliminate
without harming legitimate traffic.
So, on balance, it has been decided that it is not necessary to
prohibit ECT on RSTs. However, there is always the possibility that
someone might demonstrate a new RST attack that proves this decision
to be unwise.
3.2.7. Retransmissions
For the experiments proposed here, the TCP sender will set ECT on
retransmitted segments. It can ignore the prohibition in section
6.1.5 of RFC 3168 against setting ECT on retransmissions.
Nonetheless, the requirement in RFC 3168 that "the TCP data receiver
SHOULD ignore the CE codepoint on out-of-window packets" still holds.
Bagnulo & Briscoe Expires October 15, 2017 [Page 13]
Internet-Draft ECN and TCP control packets April 2017
If the TCP sender receives feedback that a retransmitted packet was
CE-marked, it will react as it would to any feedback of CE-marking on
a data packet.
4. Interaction with popular variants or derivatives of TCP
The following subsections specify additional behaviour necessary when
setting ECT on all data and control packets while using the following
popular variants or derivatives of TCP: SCTP, TFO, IW10. The
subsection on IW10 discusses changes to specifications but does not
recommend any, because the specification as it stands is safe, and
there is only a corner-case where performance could be occasionally
improved.
TCP variants that have been assessed and found not to interact
adversely with ECT on TCP control packets are: SYN cookies (see
Appendix A of [RFC4987]) and L4S [I-D.briscoe-tsvwg-l4s-arch].
4.1. SCTP
Stream Control Transmission Protocol (SCTP [RFC4960]) is a standards
track protocol derived from TCP. SCTP currently does not include ECN
support, but a draft on the addition of ECN to SCTP has been produced
[I-D.stewart-tsvwg-sctpecn]. This draft avoids setting ECT on
control packets and retransmissions, closely following the arguments
in RFC 3168. When ECN is finally added to SCTP, experience from
experiments on adding ECN support to all TCP packets ought to be
directly transferable to SCTP.
4.2. TFO
TCP Fast Open (TFO [RFC7413]) is an experiment to remove the round
trip delay of TCP's 3-way hand-shake (3WHS). A TFO initiator caches
a cookie from a previous connection with a TFO-enabled server. Then,
for subsequent connections to the same server, any data included on
the SYN and any other data segments sent directly after the SYN (up
to the initial window limit) can be passed directly to the server
application, which can then return response data with the SYN-ACK
(again, up to the initial window limit).
If a TFO initiator has cached that the server supported ECN in the
previous connection, it would be safe to set ECT on any data segments
it sends before a SYN-ACK returns from the responder (server). Note
that there is no space in the SYN-ACK itself (whether classic or
AccECN feedback has been negotiated) to include feedback about any CE
on data packets. Nonetheless, it is safe to set ECT on data packets
within the handshake because any CE-marking on these data segments
can be fed back by the responder on the first data segment it sends
Bagnulo & Briscoe Expires October 15, 2017 [Page 14]
Internet-Draft ECN and TCP control packets April 2017
after the SYN-ACK (or on an additional Pure ACK if it has no more
data to send).
Note that the prohibition in Section 3.2.1.1 against setting ECT on
the SYN if the same SYN is not requesting AccECN feedback still
applies.
Strictly even a non-TFO TCP initiator can send up to an initial
window of data segments straight after the SYN. However, this is
rare because a non-TFO TCP server will not deliver them to the
application until the 3WHS completes. Therefore the question of ECT
on data segments within the handshake only becomes important with
TFO. A TFO initiator's first ever connection with a server never
uses a fast open, so the initiator always has a chance to cache
whether a server supports ECN before it uses a fast open.
4.3. IW10
IW10 is an experiment to determine whether it is safe for TCP to use
an initial window of 10 SMSS [RFC6928].
This subsection does not recommend any additions to the present
specification in order to interwork with IW10. The specifications as
they stand are safe, and there is only a corner-case where
performance could be occasionally improved, as explained below.
As specified in Section 3.2.1.1, a TCP initiator can only set ECT on
the SYN if it requests AccECN support. If, however, the SYN-ACK
tells the initiator that the responder does not support AccECN,
Section 3.2.1.1 advises the initiator to conservatively reduce its
initial window to 1 SMSS because, if the SYN was CE-marked, the SYN-
ACK has no way to feed that back.
If the initiator implements IW10, it seems rather over-conservative
to reduce IW to 1 in this scenario. Nonetheless, it will rarely hit
performance if we leave the advice at 1 SMSS, because:
o as long as the initiator is caching failures to negotiate AccECN,
subsequent attempts to access the same server will not use ECT on
the SYN anyway, so there will no longer be any need to
conservatively reduce IW;
o currently it is not common for a TCP initiator (client) to have
more than one segment to send {ToDo: evidence/reference?} - IW10
is primarily exploited by TCP servers.
Bagnulo & Briscoe Expires October 15, 2017 [Page 15]
Internet-Draft ECN and TCP control packets April 2017
5. Discussion of the arguments in RFC 3168
This section is informative, not normative. It presents counter-
arguments against the justifications in the RFC series for disabling
ECN marking on each type of packet. First it addresses over-arching
arguments used for most packet types, then it addresses the specific
arguments for each packet type in turn.
5.1. The reliability argument
Section 5.2 of RFC 3168 states:
"To ensure the reliable delivery of the congestion indication of
the CE codepoint, an ECT codepoint MUST NOT be set in a packet
unless the loss of that packet [at a subsequent node] in the
network would be detected by the end nodes and interpreted as an
indication of congestion."
We believe this argument is overly conservative. The principle to
determine whether a packet is ECN-capable ought to be "do no extra
harm", meaning that the reliability of a congestion signal's delivery
ought to be no worse with ECN than without. In particular, setting
the CE codepoint on the very same packet fulfills this criterion,
since either the packet is delivered and the CE signal is delivered
to the endpoint, or the packet is dropped and the original congestion
signal (packet loss) is delivered to the endpoint.
TCP does not deliver control packets reliably. So it is more
important to allow control packets to be ECN-capable, which greatly
improves reliable delivery of the control packets themselves. This
outweighs by far the concern that a CE marking applied to a control
packet by one node might subsequently be dropped by another node.
Particularly given that, without ECN, the transport does not attempt
to detect the drop of most control packets anyway.
5.2. SYNs
RFC 5562 presents two arguments against ECT marking of SYN packets
(quoted verbatim):
"First, when the TCP SYN packet is sent, there are no guarantees
that the other TCP endpoint (node B in Figure 2) is ECN-Capable,
or that it would be able to understand and react if the ECN CE
codepoint was set by a congested router.
Second, the ECN-Capable codepoint in TCP SYN packets could be
misused by malicious clients to "improve" the well-known TCP SYN
attack. By setting an ECN-Capable codepoint in TCP SYN packets, a
Bagnulo & Briscoe Expires October 15, 2017 [Page 16]
Internet-Draft ECN and TCP control packets April 2017
malicious host might be able to inject a large number of TCP SYN
packets through a potentially congested ECN-enabled router,
congesting it even further."
The first point actually describes two subtly different issues. So
below three arguments are countered in turn.
5.2.1. Argument 1a: Loss of congestion notification on the SYN
This argument certainly applied at the time RFC 5562 was written,
when no ECN responder mechanism had any logic to recognize or feed
back a CE marking on a SYN. The problem was that, during the 3WHS,
the flag in the TCP header for ECN feedback (called Echo Congestion
Experienced) had been overloaded to negotiate the use of ECN itself.
So there was no space for feedback in a SYN-ACK.
The accurate ECN (AccECN) protocol [I-D.ietf-tcpm-accurate-ecn] has
since been designed to solve this problem, using a two-pronged
approach. First AccECN uses the 3 ECN bits in the TCP header as 8
codepoints, so there is space for the responder to feed back whether
there was CE on the SYN. Second a TCP initiator can always request
AccECN support on every SYN, and any responder reveals its level of
ECN support: AccECN, classic ECN, or no ECN. Therefore, if a
responder does indicate that it supports AccECN, the initiator can be
sure that, if there is no CE feedback on the SYN-ACK, then there
really was no CE on the SYN.
An initiator can combine AccECN with three possible strategies for
setting ECT on a SYN:
(S1): Pessimistic ECT with positive cache: The initiator always
requests AccECN in the SYN, but without setting ECT. Then it
records those servers that confirm that they support AccECN in
a cache. On a subsequent connection to any server that
supports AccECN, the initiator can then set ECT on the SYN.
(S2): Optimistic ECT: The initiator always sets ECT optimistically
on the initial SYN and it always requests AccECN support.
Then, if the server response shows it has no AccECN logic (so
it cannot feed back a CE mark), the initiator conservatively
behaves as if the SYN was CE-marked, by reducing its initial
window.
A. With no cache: The optimistic ECT strategy ought to work
pretty well without caching any responses.
B. With negative cache: The optimistic ECT strategy can be
improved by recording solely those servers that do not
Bagnulo & Briscoe Expires October 15, 2017 [Page 17]
Internet-Draft ECN and TCP control packets April 2017
support AccECN. On subsequent connections to these non-
AccECN servers, the initiator will still request AccECN
but not set ECT on the SYN. Then, the initiator can use
its full initial window (if it has enough request data to
need it). Longer term, as servers upgrade to AccECN, the
initiator will remove them from the cache and use ECT on
subsequent SYNs to that server.
(S3): ECT by configuration: In a controlled environment, the
administrator can make sure that servers support ECN-capable
SYN packets. Examples of controlled environments are single-
tenant DCs, and possibly multi-tenant DCs if we assume that
each tenant mostly communicates with its own VMs.
For unmanaged environments like the public Internet, the choice is
between strategies (S1) and (S2B):
o The "pessimistic ECT with positive cache" strategy (S1) suffers
from exposing the initial SYN to the prevailing loss level, even
if the server supports ECT on SYNs, but only on the first
connection to each AccECN server.
o The "optimistic ECT with negative cache" strategy (S2B) exploits a
server's support for ECT on SYNs from the very first attempt. But
if the server turns out not to support AccECN, the initiator has
to conservatively limit its initial window - usually
unnecessarily. Nonetheless, initiator request data (as opposed to
server response data) is rarely larger than 1 SMSS anyway (see
Section 4.3).
The normative specification for ECT on a SYN in Section 3.2.1 uses
the "optimistic ECT with negative cache" strategy on the assumption
that an initial window of 1 SMSS is usually sufficient for client
requests anyway. For clients that often initially send more than 1
SMSS of data, strategy (S1) could be used during initial deployment
and strategy (S2B) later (when the probability of servers supporting
AccECN and the likelihood of seeing some CE marking is higher).
Also, as deployment proceeds a positive cache (S1) starts off small
then grows, while a negative cache (S2B) becomes large at first, then
shrinks.
5.2.2. Argument 1b: Unknown Handling of Unexpected ECN
GIven ECT-marked SYN packets have previously been prohibited, it
cannot be assumed they will be accepted. According to a study using
2014 data [ecn-pam] from a limited range of vantage points, out of
the top 1M Alexa web sites, 4791 (0.82%) IPv4 sites and 104 (0.61%)
IPv6 sites failed to establish a connection when they received a TCP
Bagnulo & Briscoe Expires October 15, 2017 [Page 18]
Internet-Draft ECN and TCP control packets April 2017
SYN with any ECN codepoint set in the IP header and the appropriate
ECN flags in the TCP header. Of these, about 41% failed to establish
a connection due to the ECN flags in the TCP header even with a Not-
ECT ECN field in the IP header (i.e. despite full compliance with RFC
3168). Therefore adding the ECN-capability to SYNs was increasing
connection establishment failures by about 0.4%.
We will need to investigate which of numerous possible causes is
leading to these failures. RFC 3168 says "a host MUST NOT set ECT on
SYN [...] packets", but it does not say what the responder should do
if an ECN-capable SYN arrives. So perhaps some responder
implementations are checking that the SYN complies with RFC 3168,
then silently ignoring non-compliant SYNs (or perhaps returning a
RST). Also some middleboxes (e.g. firewalls) might be discarding
non-compliant SYNs themselves. For the future,
[I-D.ietf-tsvwg-ecn-experimentation] clarifies that middleboxes
"SHOULD NOT" do this, but that does not alter the past.
Whereas RSTs can be dealt with immediately, silent failures introduce
a retransmission timeout delay (default 1 second) at the initiator
before it attempts any fall back strategy. Ironically, making SYNs
ECN-capable is intended to avoid the timeout when a SYN is lost due
to congestion. Fortunately, where discard of ECN-capable SYNs is due
to policy it will occur predictably, not randomly like congestion.
So the initiator can avoid it by caching those sites that do not
support ECN-capable SYNs.
This further justifies the use of the "optimistic ECT with negative
cache" strategy in Section 3.2.1.
It might seem tempting to first send an ECT SYN and then a non-ECT
SYN (possibly with a small delay between them) and only accept the
non-ECT connection if it returned first. However, even a cache of a
dozen or so sites ought to avoid all ECN-related performance problems
with roughly the Alexa top thousand. So it is questionable whether
the level of failure of ECT on SYNs warrants always sending two SYNs,
particularly given failures at well-maintained sites could reduce if
ECT SYNs are standardized.
5.2.3. Argument 2: DoS attacks.
[RFC5562] says that ECT SYN packets could be misused by malicious
clients to augment "the well-known TCP SYN attack". It goes on to
say "a malicious host might be able to inject a large number of TCP
SYN packets through a potentially congested ECN-enabled router,
congesting it even further."
Bagnulo & Briscoe Expires October 15, 2017 [Page 19]
Internet-Draft ECN and TCP control packets April 2017
We assume this is a reference to the TCP SYN flood attack (see
https://en.wikipedia.org/wiki/SYN_flood), which is an attack against
a responder end point. We assume the idea of this attack is to use
ECT to get more packets through an ECN-enabled router in preference
to other non-ECN traffic so that they can go on to use the SYN
flooding attack to inflict more damage on the responder end point.
This argument could apply to flooding with any type of packet, but we
assume SYNs are singled out because their source address is easier to
spoof, whereas floods of other types of packets are easier to block.
Mandating Not-ECT in an RFC does not stop attackers using ECT for
flooding. Nonetheless, if a standard says SYNs are not meant to be
ECT it would make it legitimate for firewalls to discard them.
However this would negate the considerable benefit of ECT SYNs for
compliant transports and seems unnecessary because RFC 3168 already
provides the means to address this concern. In section 7, RFC 3168
says "During periods where ... the potential packet marking rate
would be high, our recommendation is that routers drop packets rather
then set the CE codepoint..." and this advice is repeated in
[RFC7567] (section 4.2.1). This makes it harder for flooding packets
to gain from ECT.
Further experiments are needed to test how much malicious hosts can
use ECT to augment flooding attacks without triggering AQMs to turn
off ECN support (flying "just under the radar"). If it is found that
ECT can only slightly augment flooding attacks, the risk of such
attacks will need to be weighed against the performance benefits of
ECT SYNs.
5.3. Pure ACKs.
RFC 3168 gives the following arguments for not allowing the ECT
marking of pure ACKs (ACKs not piggy-backed on data). In section 5.2
it reads:
"To ensure the reliable delivery of the congestion indication of
the CE codepoint, an ECT codepoint MUST NOT be set in a packet
unless the loss of that packet in the network would be detected by
the end nodes and interpreted as an indication of congestion.
Transport protocols such as TCP do not necessarily detect all
packet drops, such as the drop of a "pure" ACK packet; for
example, TCP does not reduce the arrival rate of subsequent ACK
packets in response to an earlier dropped ACK packet. Any
proposal for extending ECN- Capability to such packets would have
to address issues such as the case of an ACK packet that was
marked with the CE codepoint but was later dropped in the network.
We believe that this aspect is still the subject of research, so
Bagnulo & Briscoe Expires October 15, 2017 [Page 20]
Internet-Draft ECN and TCP control packets April 2017
this document specifies that at this time, "pure" ACK packets MUST
NOT indicate ECN-Capability."
Later on, in section 6.1.4 it reads:
"For the current generation of TCP congestion control algorithms,
pure acknowledgement packets (e.g., packets that do not contain
any accompanying data) MUST be sent with the not-ECT codepoint.
Current TCP receivers have no mechanisms for reducing traffic on
the ACK-path in response to congestion notification. Mechanisms
for responding to congestion on the ACK-path are areas for current
and future research. (One simple possibility would be for the
sender to reduce its congestion window when it receives a pure ACK
packet with the CE codepoint set). For current TCP
implementations, a single dropped ACK generally has only a very
small effect on the TCP's sending rate."
We next address each of the arguments presented above.
The first argument is a specific instance of the reliability argument
for the case of pure ACKs. This has already been addressed by
countering the general reliability argument in Section 5.1.
The second argument mentions that a sender does not reduce the load
of a stream of pure ACKs even if they are contributing to congestion.
Again, given that current TCP does not respond to pure ACK loss,
setting ECT on pure ACKs to allow them to carry congestion marks
would be no worse than not doing so (and not doing so would be
detrimental from a performance perspective).
The proposed AccECN modification to TCP feedback
[I-D.ietf-tcpm-accurate-ecn] involves a data receiver repeatedly
sending a count of received congestion marks. So AccECN could
include marks on pure ACKs in this count, even though it does not ACK
pure ACKs themselves. Then the sender of the pure ACKs will reduce
its congestion window, which will (correctly) reduce the rate at
which it sends any subsequent data. Nonetheless, even if the
original sender of the pure ACK does not respond to this feedback, or
if it is decided that AccECN will not provide this information, it
will still make sense to set ECT on pure ACKs, because the congestion
situation will be no worse than it is today with non-ECT pure ACKs.
In summary, allowing ECT (and CE) to be set on pure ACKs is no worse
than not doing so (and dropping the pure ACK). In contrast, not
setting ECT on pure ACKs is certainly detrimental to performance
because when a pure ACK is lost it can prevent the release of new
data.
Bagnulo & Briscoe Expires October 15, 2017 [Page 21]
Internet-Draft ECN and TCP control packets April 2017
5.4. Window probes
RFC 3168 presents only the reliability argument for preventing
setting the ECT codepoint in Window Probe packets. Specifically,
Section 6.1.6 states:
"If a window probe packet is dropped in the network, this loss is
not detected by the receiver. Therefore, the TCP data sender MUST
NOT set either an ECT codepoint or the CWR bit on window probe
packets.
However, because window probes use exact sequence numbers, they
cannot be easily spoofed in denial-of-service attacks. Therefore,
if a window probe arrives with the CE codepoint set, then the
receiver SHOULD respond to the ECN indications."
The reliability argument has already been addressed in Section 5.1.
Allowing ECT on window probes could considerably improve performance
because, if a window probe is lost in conditions when the Silly
Window Syndrome applies, the sender will stall until the next window
probe reaches the receiver (at least 2 minutes later).
On the bright side, RFC 3168 at least specifies the receiver
behaviour if a CE-marked window probe arrives, so changing the
behaviour ought to be less painful than for other packet types.
5.5. Retransmitted packets.
RFC 3168 says the sender "MUST NOT" set ECT on retransmitted packets.
The rationale for this consumes nearly 2 pages of RFC 3168, so the
reader is referred to section 6.1.5 of RFC 3168, rather than quoting
it all here. There are essentially three arguments namely,
reliability, DoS attacks and over-reaction to congestion. We address
them in order below.
The reliability argument has already been addressed in Section 5.1.
Protection against DoS attacks is not afforded by prohibiting ECT on
retransmitted packets. An attacker can set CE on spoofed
retransmissions whether or not it is prohibited by an RFC.
Protection against the DoS attack described in RFC 3168 is solely
afforded by the requirement that "the TCP data receiver SHOULD ignore
the CE codepoint on out-of-window packets". Therefore we propose to
allow ECT marking of retransmitted packets, in order to reduce the
chance of them being dropped.
Bagnulo & Briscoe Expires October 15, 2017 [Page 22]
Internet-Draft ECN and TCP control packets April 2017
Nonetheless, it is important to keep the RFC 3168 advice to ignore
the CE codepoint in out-of-window packets. This means that, for
those retransmitted packets that arrive at the receiver after the
original packet has been properly received, any CE marking will be
ignored. There is no problem with that because the delivery of the
original packet implies that the sender's original congestion
response (when it deemed the packet lost and retransmitted it) was
unnecessary. The data receiver is also advised to use the more
stringent input check for incoming segments in section 5.2 of
[RFC5961].
Finally, the third argument is about over-reacting to congestion.
The argument goes that, if a retransmitted packet is dropped, the
sender will not detect it, so it will not react again to congestion
(it would have reduced its congestion window already when it
retransmitted the packet). Whereas, if retransmitted packets can be
CE tagged instead of dropped, senders could potentially react more
than once to congestion. However, we argue that it is legitimate to
respond again to congestion if it still persists in subsequent round
trip(s).
Therefore, in all three cases, it is not incorrect to set ECT on
retransmissions.
6. Security considerations
Section 3.2.6 considers the question of whether ECT on RSTs will
allow RST attacks to be intensified. There are several security
arguments presented in RFC 3168 for preventing the ECN marking of TCP
control packets and retransmitted segments. We believe all of them
have been properly addressed in Section 5, particularly Section 5.2.3
and Section 5.5 on DoS attacks using spoofed ECT-marked SYNs and
spoofed CE-marked retransmissions.
7. IANA Considerations
There are no IANA considerations in this memo.
8. Acknowledgments
Thanks to Mirja Kuehlewind and David Black for their useful reviews.
9. References
Bagnulo & Briscoe Expires October 15, 2017 [Page 23]
Internet-Draft ECN and TCP control packets April 2017
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[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,
<http://www.rfc-editor.org/info/rfc3168>.
[RFC5562] Kuzmanovic, A., Mondal, A., Floyd, S., and K.
Ramakrishnan, "Adding Explicit Congestion Notification
(ECN) Capability to TCP's SYN/ACK Packets", RFC 5562,
DOI 10.17487/RFC5562, June 2009,
<http://www.rfc-editor.org/info/rfc5562>.
[I-D.ietf-tcpm-accurate-ecn]
Briscoe, B., Kuehlewind, M., and R. Scheffenegger, "More
Accurate ECN Feedback in TCP", draft-ietf-tcpm-accurate-
ecn-02 (work in progress), October 2016.
[I-D.ietf-tsvwg-ecn-experimentation]
Black, D., "Explicit Congestion Notification (ECN)
Experimentation", draft-ietf-tsvwg-ecn-experimentation-01
(work in progress), March 2017.
9.2. Informative References
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, DOI 10.17487/RFC0793, September 1981,
<http://www.rfc-editor.org/info/rfc793>.
[RFC1122] Braden, R., Ed., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122,
DOI 10.17487/RFC1122, October 1989,
<http://www.rfc-editor.org/info/rfc1122>.
[RFC3540] Spring, N., Wetherall, D., and D. Ely, "Robust Explicit
Congestion Notification (ECN) Signaling with Nonces",
RFC 3540, DOI 10.17487/RFC3540, June 2003,
<http://www.rfc-editor.org/info/rfc3540>.
[RFC4960] Stewart, R., Ed., "Stream Control Transmission Protocol",
RFC 4960, DOI 10.17487/RFC4960, September 2007,
<http://www.rfc-editor.org/info/rfc4960>.
Bagnulo & Briscoe Expires October 15, 2017 [Page 24]
Internet-Draft ECN and TCP control packets April 2017
[RFC4987] Eddy, W., "TCP SYN Flooding Attacks and Common
Mitigations", RFC 4987, DOI 10.17487/RFC4987, August 2007,
<http://www.rfc-editor.org/info/rfc4987>.
[RFC5681] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
Control", RFC 5681, DOI 10.17487/RFC5681, September 2009,
<http://www.rfc-editor.org/info/rfc5681>.
[RFC5961] Ramaiah, A., Stewart, R., and M. Dalal, "Improving TCP's
Robustness to Blind In-Window Attacks", RFC 5961,
DOI 10.17487/RFC5961, August 2010,
<http://www.rfc-editor.org/info/rfc5961>.
[RFC5690] Floyd, S., Arcia, A., Ros, D., and J. Iyengar, "Adding
Acknowledgement Congestion Control to TCP", RFC 5690,
DOI 10.17487/RFC5690, February 2010,
<http://www.rfc-editor.org/info/rfc5690>.
[RFC6298] Paxson, V., Allman, M., Chu, J., and M. Sargent,
"Computing TCP's Retransmission Timer", RFC 6298,
DOI 10.17487/RFC6298, June 2011,
<http://www.rfc-editor.org/info/rfc6298>.
[RFC6928] Chu, J., Dukkipati, N., Cheng, Y., and M. Mathis,
"Increasing TCP's Initial Window", RFC 6928,
DOI 10.17487/RFC6928, April 2013,
<http://www.rfc-editor.org/info/rfc6928>.
[RFC7413] Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP
Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014,
<http://www.rfc-editor.org/info/rfc7413>.
[RFC7567] Baker, F., Ed. and G. Fairhurst, Ed., "IETF
Recommendations Regarding Active Queue Management",
BCP 197, RFC 7567, DOI 10.17487/RFC7567, July 2015,
<http://www.rfc-editor.org/info/rfc7567>.
[I-D.briscoe-tsvwg-ecn-l4s-id]
Schepper, K., Briscoe, B., and I. Tsang, "Identifying
Modified Explicit Congestion Notification (ECN) Semantics
for Ultra-Low Queuing Delay", draft-briscoe-tsvwg-ecn-l4s-
id-02 (work in progress), October 2016.
[I-D.briscoe-tsvwg-l4s-arch]
Briscoe, B., Schepper, K., and M. Bagnulo, "Low Latency,
Low Loss, Scalable Throughput (L4S) Internet Service:
Architecture", draft-briscoe-tsvwg-l4s-arch-02 (work in
progress), March 2017.
Bagnulo & Briscoe Expires October 15, 2017 [Page 25]
Internet-Draft ECN and TCP control packets April 2017
[I-D.stewart-tsvwg-sctpecn]
Stewart, R., Tuexen, M., and X. Dong, "ECN for Stream
Control Transmission Protocol (SCTP)", draft-stewart-
tsvwg-sctpecn-05 (work in progress), January 2014.
[judd-nsdi]
Judd, G., "Attaining the promise and avoiding the pitfalls
of TCP in the Datacenter", NSDI 2015, 2015.
[ecn-pam] Trammell, B., Kuehlewind, M., Boppart, D., Learmonth, I.,
Fairhurst, G., and R. Scheffenegger, "Enabling Internet-
Wide Deployment of Explicit Congestion Notification",
Int'l Conf. on on Passive and Active Network Measurement
(PAM'15) pp193-205, 2015.
Authors' Addresses
Marcelo Bagnulo
Universidad Carlos III de Madrid
Av. Universidad 30
Leganes, Madrid 28911
SPAIN
Phone: 34 91 6249500
Email: marcelo@it.uc3m.es
URI: http://www.it.uc3m.es
Bob Briscoe
Simula Research Lab
Email: ietf@bobbriscoe.net
URI: http://bobbriscoe.net/
Bagnulo & Briscoe Expires October 15, 2017 [Page 26]