TCP Maintenance & Minor Extensions (tcpm) B. Briscoe
Internet-Draft Simula Research Laboratory
Intended status: Experimental M. Kuehlewind
Expires: December 1, 2017 ETH Zurich
R. Scheffenegger
May 30, 2017
More Accurate ECN Feedback in TCP
draft-ietf-tcpm-accurate-ecn-03
Abstract
Explicit Congestion Notification (ECN) is a mechanism where network
nodes can mark IP packets instead of dropping them to indicate
incipient congestion to the end-points. Receivers with an ECN-
capable transport protocol feed back this information to the sender.
ECN is specified for TCP in such a way that only one feedback signal
can be transmitted per Round-Trip Time (RTT). Recently, new TCP
mechanisms like Congestion Exposure (ConEx) or Data Center TCP
(DCTCP) need more accurate ECN feedback information whenever more
than one marking is received in one RTT. This document specifies an
experimental scheme to provide more than one feedback signal per RTT
in the TCP header. Given TCP header space is scarce, it overloads
the three existing ECN-related flags in the TCP header and provides
additional information in a new TCP option.
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
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This Internet-Draft will expire on December 1, 2017.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Document Roadmap . . . . . . . . . . . . . . . . . . . . 4
1.2. Goals . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.3. Experiment Goals . . . . . . . . . . . . . . . . . . . . 5
1.4. Terminology . . . . . . . . . . . . . . . . . . . . . . . 6
1.5. Recap of Existing ECN feedback in IP/TCP . . . . . . . . 6
2. AccECN Protocol Overview and Rationale . . . . . . . . . . . 7
2.1. Capability Negotiation . . . . . . . . . . . . . . . . . 8
2.2. Feedback Mechanism . . . . . . . . . . . . . . . . . . . 9
2.3. Delayed ACKs and Resilience Against ACK Loss . . . . . . 9
2.4. Feedback Metrics . . . . . . . . . . . . . . . . . . . . 10
2.5. Generic (Dumb) Reflector . . . . . . . . . . . . . . . . 10
3. AccECN Protocol Specification . . . . . . . . . . . . . . . . 11
3.1. Negotiating to use AccECN . . . . . . . . . . . . . . . . 11
3.1.1. Negotiation during the TCP handshake . . . . . . . . 11
3.1.2. Retransmission of the SYN . . . . . . . . . . . . . . 14
3.2. AccECN Feedback . . . . . . . . . . . . . . . . . . . . . 15
3.2.1. The ACE Field . . . . . . . . . . . . . . . . . . . . 15
3.2.2. Testing for Zeroing of the ACE Field . . . . . . . . 16
3.2.3. Safety against Ambiguity of the ACE Field . . . . . . 17
3.2.4. The AccECN Option . . . . . . . . . . . . . . . . . . 17
3.2.5. Path Traversal of the AccECN Option . . . . . . . . . 19
3.2.6. Usage of the AccECN TCP Option . . . . . . . . . . . 22
3.3. AccECN Compliance by TCP Proxies, Offload Engines and
other Middleboxes . . . . . . . . . . . . . . . . . . . . 23
4. Interaction with Other TCP Variants . . . . . . . . . . . . . 24
4.1. Compatibility with SYN Cookies . . . . . . . . . . . . . 24
4.2. Compatibility with Other TCP Options and Experiments . . 25
4.3. Compatibility with Feedback Integrity Mechanisms . . . . 25
5. Protocol Properties . . . . . . . . . . . . . . . . . . . . . 26
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 28
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7. Security Considerations . . . . . . . . . . . . . . . . . . . 29
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 29
9. Comments Solicited . . . . . . . . . . . . . . . . . . . . . 30
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 30
10.1. Normative References . . . . . . . . . . . . . . . . . . 30
10.2. Informative References . . . . . . . . . . . . . . . . . 30
Appendix A. Example Algorithms . . . . . . . . . . . . . . . . . 33
A.1. Example Algorithm to Encode/Decode the AccECN Option . . 33
A.2. Example Algorithm for Safety Against Long Sequences of
ACK Loss . . . . . . . . . . . . . . . . . . . . . . . . 34
A.2.1. Safety Algorithm without the AccECN Option . . . . . 34
A.2.2. Safety Algorithm with the AccECN Option . . . . . . . 36
A.3. Example Algorithm to Estimate Marked Bytes from Marked
Packets . . . . . . . . . . . . . . . . . . . . . . . . . 37
A.4. Example Algorithm to Beacon AccECN Options . . . . . . . 38
A.5. Example Algorithm to Count Not-ECT Bytes . . . . . . . . 39
Appendix B. Alternative Design Choices (To Be Removed Before
Publication) . . . . . . . . . . . . . . . . . . . . 39
Appendix C. Open Protocol Design Issues (To Be Removed Before
Publication) . . . . . . . . . . . . . . . . . . . . 40
Appendix D. Changes in This Version (To Be Removed Before
Publication) . . . . . . . . . . . . . . . . . . . . 40
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 40
1. Introduction
Explicit Congestion Notification (ECN) [RFC3168] is a mechanism where
network nodes can mark IP packets instead of dropping them to
indicate incipient congestion to the end-points. Receivers with an
ECN-capable transport protocol feed back this information to the
sender. ECN is specified for TCP in such a way that only one
feedback signal can be transmitted per Round-Trip Time (RTT).
Recently, proposed mechanisms like Congestion Exposure (ConEx
[RFC7713]), DCTCP [I-D.ietf-tcpm-dctcp] or L4S
[I-D.ietf-tsvwg-l4s-arch] need more accurate ECN feedback information
whenever more than one marking is received in one RTT. A fuller
treatment of the motivation for this specification is given in the
associated requirements document [RFC7560].
This documents specifies an experimental scheme for ECN feedback in
the TCP header to provide more than one feedback signal per RTT. It
will be called the more accurate ECN feedback scheme, or AccECN for
short. If AccECN progresses from experimental to the standards
track, it is intended to be a complete replacement for classic ECN
feedback, not a fork in the design of TCP. Thus, the applicability
of AccECN is intended to include all public and private IP networks
(and even any non-IP networks over which TCP is used today). Until
the AccECN experiment succeeds, [RFC3168] will remain as the
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standards track specification for adding ECN to TCP. To avoid
confusion, in this document we use the term 'classic ECN' for the
pre-existing ECN specification [RFC3168].
AccECN feedback overloads flags and fields in the main TCP header
with new definitions, so both ends have to support the new wire
protocol before it can be used. Therefore during the TCP handshake
the two ends use the three ECN-related flags in the TCP header to
negotiate the most advanced feedback protocol that they can both
support.
AccECN is solely an (experimental) change to the TCP wire protocol;
it only specifies the negotiation and signaling of more accurate ECN
feedback from a TCP Data Receiver to a Data Sender. It is completely
independent of how TCP might respond to congestion feedback, which is
out of scope. For that we refer to [RFC3168] or any RFC that
specifies a different response to TCP ECN feedback, for example:
[I-D.ietf-tcpm-dctcp]; or the ECN experiments referred to in
[I-D.ietf-tsvwg-ecn-experimentation], namely: a TCP-based Low Latency
Low Loss Scalable (L4S) congestion control [I-D.ietf-tsvwg-l4s-arch];
ECN-capable TCP control packets [I-D.bagnulo-tcpm-generalized-ecn],
or Alternative Backoff with ECN (ABE)
[I-D.ietf-tcpm-alternativebackoff-ecn].
It is likely (but not required) that the AccECN protocol will be
implemented along with the following experimental additions to the
TCP-ECN protocol: ECN-capable TCP control packets and retransmissions
[I-D.bagnulo-tcpm-generalized-ecn], which includes the ECN-capable
SYN-ACK experiment [RFC5562]; and testing receiver non-compliance
[I-D.moncaster-tcpm-rcv-cheat].
1.1. Document Roadmap
The following introductory sections outline the goals of AccECN
(Section 1.2) and the goal of experiments with ECN (Section 1.3) so
that it is clear what success would look like. Then terminology is
defined (Section 1.4) and a recap of existing prerequisite technology
is given (Section 1.5).
Section 2 gives an informative overview of the AccECN protocol. Then
Section 3 gives the normative protocol specification. Section 4
assesses the interaction of AccECN with commonly used variants of
TCP, whether standardised or not. Section 5 summarises the features
and properties of AccECN.
Section 6 summarises the protocol fields and numbers that IANA will
need to assign and Section 7 points to the aspects of the protocol
that will be of interest to the security community.
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Appendix A gives pseudocode examples for the various algorithms that
AccECN uses.
1.2. Goals
[RFC7560] enumerates requirements that a candidate feedback scheme
will need to satisfy, under the headings: resilience, timeliness,
integrity, accuracy (including ordering and lack of bias),
complexity, overhead and compatibility (both backward and forward).
It recognises that a perfect scheme that fully satisfies all the
requirements is unlikely and trade-offs between requirements are
likely. Section 5 presents the properties of AccECN against these
requirements and discusses the trade-offs made.
The requirements document recognises that a protocol as ubiquitous as
TCP needs to be able to serve as-yet-unspecified requirements.
Therefore an AccECN receiver aims to act as a generic (dumb)
reflector of congestion information so that in future new sender
behaviours can be deployed unilaterally.
1.3. Experiment Goals
TCP is critical to the robust functioning of the Internet, therefore
any proposed modifications to TCP need to be thoroughly tested. The
present specification describes an experimental protocol that adds
more accurate ECN feedback to the TCP protocol. The intention is to
specify the protocol sufficiently so that more than one
implementation can be built in order to test its function, robustness
and interoperability (with itself and with previous version of ECN
and TCP).
The experimental protocol will be considered successful if it
satisfies the requirements of [RFC7560] in the consensus opinion of
the IETF tcpm working group. In short, this requires that it
improves the accuracy and timeliness of TCP's ECN feedback, as
claimed in Section 5, while striking a balance between the
conflicting requirements of resilience, integrity and minimisation of
overhead. It also requires that it is not unduly complex, and that
it is compatible with prevalent equipment behaviours in the current
Internet, whether or not they comply with standards.
Testing will mostly focus on fall-back strategies in case of
middlebox interference. Current recommended strategies are specified
in Sections 3.1.2, 3.2.2 and 3.2.5. The effectiveness of these
strategies depends on the actual deployment situation of middleboxes.
Therefore experimental verification to confirm large-scale path
traversal in the Internet is needed to finalize this specification on
Standards Track.
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1.4. Terminology
AccECN: The more accurate ECN feedback scheme will be called AccECN
for short.
Classic ECN: the ECN protocol specified in [RFC3168].
Classic ECN feedback: the feedback aspect of the ECN protocol
specified in [RFC3168], including generation, encoding,
transmission and decoding of feedback, but not the Data Sender's
subsequent response to that feedback.
ACK: A TCP acknowledgement, with or without a data payload.
Pure ACK: A TCP acknowledgement without a data payload.
TCP client: The TCP stack that originates a connection.
TCP server: The TCP stack that responds to a connection request.
Data Receiver: The endpoint of a TCP half-connection that receives
data and sends AccECN feedback.
Data Sender: The endpoint of a TCP half-connection that sends data
and receives AccECN feedback.
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 RFC 2119 [RFC2119].
1.5. Recap of Existing ECN feedback in IP/TCP
ECN [RFC3168] uses two bits in the IP header. Once ECN has been
negotiated with the receiver at the transport layer, an ECN sender
can set two possible codepoints (ECT(0) or ECT(1)) in the IP header
to indicate an ECN-capable transport (ECT). If both ECN bits are
zero, the packet is considered to have been sent by a Not-ECN-capable
Transport (Not-ECT). When a network node experiences congestion, it
will occasionally either drop or mark a packet, with the choice
depending on the packet's ECN codepoint. If the codepoint is Not-
ECT, only drop is appropriate. If the codepoint is ECT(0) or ECT(1),
the node can mark the packet by setting both ECN bits, which is
termed 'Congestion Experienced' (CE), or loosely a 'congestion mark'.
Table 1 summarises these codepoints.
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+-----------------------+---------------+---------------------------+
| IP-ECN codepoint | Codepoint | Description |
| (binary) | name | |
+-----------------------+---------------+---------------------------+
| 00 | Not-ECT | Not ECN-Capable Transport |
| 01 | ECT(1) | ECN-Capable Transport (1) |
| 10 | ECT(0) | ECN-Capable Transport (0) |
| 11 | CE | Congestion Experienced |
+-----------------------+---------------+---------------------------+
Table 1: The ECN Field in the IP Header
In the TCP header the first two bits in byte 14 are defined as flags
for the use of ECN (CWR and ECE in Figure 1 [RFC3168]). A TCP client
indicates it supports ECN by setting ECE=CWR=1 in the SYN, and an
ECN-enabled server confirms ECN support by setting ECE=1 and CWR=0 in
the SYN/ACK. On reception of a CE-marked packet at the IP layer, the
Data Receiver starts to set the Echo Congestion Experienced (ECE)
flag continuously in the TCP header of ACKs, which ensures the signal
is received reliably even if ACKs are lost. The TCP sender confirms
that it has received at least one ECE signal by responding with the
congestion window reduced (CWR) flag, which allows the TCP receiver
to stop repeating the ECN-Echo flag. This always leads to a full RTT
of ACKs with ECE set. Thus any additional CE markings arriving
within this RTT cannot be fed back.
The last bit in byte 13 of the TCP header was defined as the Nonce
Sum (NS) for the ECN Nonce [RFC3540]. RFC 3540 was never deployed so
it is being reclassified as historic, making this TCP flag available
for use by the AccECN experiment instead.
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
| | | N | C | E | U | A | P | R | S | F |
| Header Length | Reserved | S | W | C | R | C | S | S | Y | I |
| | | | R | E | G | K | H | T | N | N |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
Figure 1: The (post-ECN Nonce) definition of the TCP header flags
2. AccECN Protocol Overview and Rationale
This section provides an informative overview of the AccECN protocol
that will be normatively specified in Section 3
Like the original TCP approach, the Data Receiver of each TCP half-
connection sends AccECN feedback to the Data Sender on TCP
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acknowledgements, reusing data packets of the other half-connection
whenever possible.
The AccECN protocol has had to be designed in two parts:
o an essential part that re-uses ECN TCP header bits to feed back
the number of arriving CE marked packets. This provides more
accuracy than classic ECN feedback, but limited resilience against
ACK loss;
o a supplementary part using a new AccECN TCP Option that provides
additional feedback on the number of bytes that arrive marked with
each of the three ECN codepoints (not just CE marks). This
provides greater resilience against ACK loss than the essential
feedback, but it is more likely to suffer from middlebox
interference.
The two part design was necessary, given limitations on the space
available for TCP options and given the possibility that certain
incorrectly designed middleboxes prevent TCP using any new options.
The essential part overloads the previous definition of the three
flags in the TCP header that had been assigned for use by ECN. This
design choice deliberately replaces the classic ECN feedback
protocol, rather than leaving classic ECN feedback intact and adding
more accurate feedback separately because:
o this efficiently reuses scarce TCP header space, given TCP option
space is approaching saturation;
o a single upgrade path for the TCP protocol is preferable to a fork
in the design;
o otherwise classic and accurate ECN feedback could give conflicting
feedback on the same segment, which could open up new security
concerns and make implementations unnecessarily complex;
o middleboxes are more likely to faithfully forward the TCP ECN
flags than newly defined areas of the TCP header.
AccECN is designed to work even if the supplementary part is removed
or zeroed out, as long as the essential part gets through.
2.1. Capability Negotiation
AccECN is a change to the wire protocol of the main TCP header,
therefore it can only be used if both endpoints have been upgraded to
understand it. The TCP client signals support for AccECN on the
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initial SYN of a connection and the TCP server signals whether it
supports AccECN on the SYN/ACK. The TCP flags on the SYN that the
client uses to signal AccECN support have been carefully chosen so
that a TCP server will interpret them as a request to support the
most recent variant of ECN feedback that it supports. Then the
client falls back to the same variant of ECN feedback.
An AccECN TCP client does not send the new AccECN Option on the SYN
as SYN option space is limited and successful negotiation using the
flags in the main header is taken as sufficient evidence that both
ends also support the AccECN Option. The TCP server sends the AccECN
Option on the SYN/ACK and the client sends it on the first ACK to
test whether the network path forwards the option correctly.
2.2. Feedback Mechanism
A Data Receiver maintains four counters initialised at the start of
the half-connection. Three count the number of arriving payload
bytes marked CE, ECT(1) and ECT(0) respectively. The fourth counts
the number of packets arriving marked with a CE codepoint (including
control packets without payload if they are CE-marked).
The Data Sender maintains four equivalent counters for the half
connection, and the AccECN protocol is designed to ensure they will
match the values in the Data Receiver's counters, albeit after a
little delay.
Each ACK carries the three least significant bits (LSBs) of the
packet-based CE counter using the ECN bits in the TCP header, now
renamed the Accurate ECN (ACE) field (see Figure 2 later). The LSBs
of each of the three byte counters are carried in the AccECN Option.
2.3. Delayed ACKs and Resilience Against ACK Loss
With both the ACE and the AccECN Option mechanisms, the Data Receiver
continually repeats the current LSBs of each of its respective
counters. Then, even if some ACKs are lost, the Data Sender should
be able to infer how much to increment its own counters, even if the
protocol field has wrapped.
The 3-bit ACE field can wrap fairly frequently. Therefore, even if
it appears to have incremented by one (say), the field might have
actually cycled completely then incremented by one. The Data
Receiver is required not to delay sending an ACK to such an extent
that the ACE field would cycle. However cyling is still a
possibility at the Data Sender because a whole sequence of ACKs
carrying intervening values of the field might all be lost or delayed
in transit.
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The fields in the AccECN Option are larger, but they will increment
in larger steps because they count bytes not packets. Nonetheless,
their size has been chosen such that a whole cycle of the field would
never occur between ACKs unless there had been an infeasibly long
sequence of ACK losses. Therefore, as long as the AccECN Option is
available, it can be treated as a dependable feedback channel.
If the AccECN Option is not available, e.g. it is being stripped by a
middlebox, the AccECN protocol will only feed back information on CE
markings (using the ACE field). Although not ideal, this will be
sufficient, because it is envisaged that neither ECT(0) nor ECT(1)
will ever indicate more severe congestion than CE, even though future
uses for ECT(0) or ECT(1) are still unclear
[I-D.ietf-tsvwg-ecn-experimentation]. Because the 3-bit ACE field is
so small, when it is the only field available the Data Sender has to
interpret it conservatively assuming the worst possible wrap.
Certain specified events trigger the Data Receiver to include an
AccECN Option on an ACK. The rules are designed to ensure that the
order in which different markings arrive at the receiver is
communicated to the sender (as long as there is no ACK loss).
Implementations are encouraged to send an AccECN Option more
frequently, but this is left up to the implementer.
2.4. Feedback Metrics
The CE packet counter in the ACE field and the CE byte counter in the
AccECN Option both provide feedback on received CE-marks. The CE
packet counter includes control packets that do not have payload
data, while the CE byte counter solely includes marked payload bytes.
If both are present, the byte counter in the option will provide the
more accurate information needed for modern congestion control and
policing schemes, such as DCTCP or ConEx. If the option is stripped,
a simple algorithm to estimate the number of marked bytes from the
ACE field is given in Appendix A.3.
Feedback in bytes is recommended in order to protect against the
receiver using attacks similar to 'ACK-Division' to artificially
inflate the congestion window, which is why [RFC5681] now recommends
that TCP counts acknowledged bytes not packets.
2.5. Generic (Dumb) Reflector
The ACE field provides information about CE markings on both data and
control packets. According to [RFC3168] the Data Sender is meant to
set control packets to Not-ECT. However, mechanisms in certain
private networks (e.g. data centres) set control packets to be ECN
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capable because they are precisely the packets that performance
depends on most.
For this reason, AccECN is designed to be a generic reflector of
whatever ECN markings it sees, whether or not they are compliant with
a current standard. Then as standards evolve, Data Senders can
upgrade unilaterally without any need for receivers to upgrade too.
It is also useful to be able to rely on generic reflection behaviour
when senders need to test for unexpected interference with markings
(for instance [I-D.kuehlewind-tcpm-ecn-fallback] and
[I-D.moncaster-tcpm-rcv-cheat]).
The initial SYN is the most critical control packet, so AccECN
provides feedback on whether it is CE marked. Although RFC 3168
prohibits an ECN-capable SYN, providing feedback of CE marking on the
SYN supports future scenarios in which SYNs might be ECN-enabled
(without prejudging whether they ought to be). For instance,
[I-D.ietf-tsvwg-ecn-experimentation] updates this aspect of RFC 3168
to allow experimentation with ECN-capable TCP control packets.
Even if the TCP client has set the SYN to not-ECT in compliance with
RFC 3168, feedback on whether it has been CE-marked could still be
useful, because middleboxes have been known to overwrite the ECN IP
field as if it is still part of the old Type of Service (ToS) field.
If a TCP client has set the SYN to Not-ECT, but receives CE feedback,
it can detect such middlebox interference and send Not-ECT for the
rest of the connection (see [I-D.kuehlewind-tcpm-ecn-fallback]).
Today, if a TCP server receives CE on a SYN, it cannot know whether
it is invalid (or valid) because only the TCP client knows whether it
originally marked the SYN as Not-ECT (or ECT). Therefore, prior to
AccECN, the server's only safe course of action was to disable ECN
for the connection. Instead, the AccECN protocol allows the server
to feed back the CE marking to the client, which then has all the
information to decide whether the connection has to fall-back from
supporting ECN (or not).
3. AccECN Protocol Specification
3.1. Negotiating to use AccECN
3.1.1. Negotiation during the TCP handshake
Given the ECN Nonce [RFC3540] is being reclassified as historic, the
present specification renames the TCP flag at bit 7 of the TCP header
flags from NS (Nonce Sum) to AE (Accurate ECN) (see IANA
Considerations in Section 6).
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During the TCP handshake at the start of a connection, to request
more accurate ECN feedback the TCP client (host A) MUST set the TCP
flags AE=1, CWR=1 and ECE=1 in the initial SYN segment.
If a TCP server (B) that is AccECN-enabled receives a SYN with the
above three flags set, it MUST set both its half connections into
AccECN mode. Then it MUST set the TCP flags CWR=1 and ECE=0 on its
response in the SYN/ACK segment to confirm that it supports AccECN.
The TCP server MUST NOT set this combination of flags unless the
preceding SYN requested support for AccECN as above.
A TCP server in AccECN mode MUST additionally set the TCP flag AE=1
on the SYN/ACK if the IP/ECN field of the SYN was CE-marked (see
Section 2.5 for rationale). If the IP/ECN field of the received SYN
was Not-ECT, ECT(0) or ECT(1), it MUST clear the TCP AE flag (AE=0)
on the SYN/ACK.
Once a TCP client (A) has sent the above SYN to declare that it
supports AccECN, and once it has received the above SYN/ACK segment
that confirms that the TCP server supports AccECN, the TCP client
MUST set both its half connections into AccECN mode.
The procedure for the client to follow if a SYN/ACK does not arrive
before its retransmission timer expires is given in Section 3.1.2.
The three flags set to 1 to indicate AccECN support on the SYN have
been carefully chosen to enable natural fall-back to prior stages in
the evolution of ECN. Table 2 tabulates all the negotiation
possibilities for ECN-related capabilities that involve at least one
AccECN-capable host. The entries in the first two columns have been
abbreviated, as follows:
AccECN: More Accurate ECN Feedback (the present specification)
Nonce: ECN Nonce feedback [RFC3540]
ECN: 'Classic' ECN feedback [RFC3168]
No ECN: Not-ECN-capable. Implicit congestion notification using
packet drop.
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+--------+---------+------------+--------------+--------------------+
| A | B | SYN A->B | SYN/ACK B->A | Feedback Mode |
+--------+---------+------------+--------------+--------------------+
| | | AE CWR ECE | AE CWR ECE | |
| AccECN | AccECN | 1 1 1 | 0 1 0 | AccECN |
| AccECN | AccECN | 1 1 1 | 1 1 0 | AccECN (CE on SYN) |
| | | | | |
| AccECN | Nonce | 1 1 1 | 1 0 1 | classic ECN |
| AccECN | ECN | 1 1 1 | 0 0 1 | classic ECN |
| AccECN | No ECN | 1 1 1 | 0 0 0 | Not ECN |
| | | | | |
| Nonce | AccECN | 0 1 1 | 0 0 1 | classic ECN |
| ECN | AccECN | 0 1 1 | 0 0 1 | classic ECN |
| No ECN | AccECN | 0 0 0 | 0 0 0 | Not ECN |
| | | | | |
| AccECN | Broken | 1 1 1 | 1 1 1 | Not ECN |
| AccECN | AccECN+ | 1 1 1 | 0 1 1 | AccECN (CU) |
| AccECN | AccECN+ | 1 1 1 | 1 0 0 | AccECN (CU) |
+--------+---------+------------+--------------+--------------------+
Table 2: ECN capability negotiation between Client (A) and Server (B)
Table 2 is divided into blocks each separated by an empty row.
1. The top block shows the case already described where both
endpoints support AccECN and how the TCP server (B) indicates
congestion feedback.
2. The second block shows the cases where the TCP client (A)
supports AccECN but the TCP server (B) supports some earlier
variant of TCP feedback, indicated in its SYN/ACK. Therefore, as
soon as an AccECN-capable TCP client (A) receives the SYN/ACK
shown it MUST set both its half connections into the feedback
mode shown in the rightmost column.
3. The third block shows the cases where the TCP server (B) supports
AccECN but the TCP client (A) supports some earlier variant of
TCP feedback, indicated in its SYN. Therefore, as soon as an
AccECN-enabled TCP server (B) receives the SYN shown, it MUST set
both its half connections into the feedback mode shown in the
rightmost column.
4. The fourth block displays combinations that are not valid or
currently unused. The first case (labelled `Broken' is where all
bits set in the SYN are reflected by the receiver in the SYN/ACK,
which happens quite often if the TCP connection is proxied. In
this case, both ends MUST fall-back to Not ECN for both half
connections. The other two cases (labelled 'AccECN (CU)') are
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currently unassigned and available for an RFC to extend TCP in
future, tagged as 'AccECN+' (see Appendix B for possible uses).
For forward compatibility, as soon as an AccECN-capable TCP
client (A) receives either of these SYN/ACKs it MUST set both its
half connections into AccECN mode, as if the SYN/ACK had been
AE=0, CWR=1, ECE=0.
The following exceptional cases need some explanation:
ECN Nonce: An AccECN implementation, whether client or server,
sender or receiver, does not need to implement the ECN Nonce
feedback mode [RFC3540], which is being reclassified as historic
[I-D.ietf-tsvwg-ecn-experimentation]. AccECN is compatible with
an alternative ECN feedback integrity approach that does not use
up the ECT(1) codepoint and can be implemented solely at the
sender (see Section 4.3).
Simultaneous Open: An originating AccECN Host (A), having sent a SYN
with AE=1, CWR=1 and ECE=1, might receive another SYN from host B.
Host A MUST then enter the same feedback mode as it would have
entered had it been a responding host and received the same SYN.
Then host A MUST send the same SYN/ACK as it would have sent had
it been a responding host (see the third block above).
3.1.2. Retransmission of the SYN
If the sender of an AccECN SYN times out before receiving the SYN/
ACK, the sender SHOULD attempt to negotiate the use of AccECN at
least one more time by continuing to set all three TCP ECN flags on
the first retransmitted SYN (using the usual retransmission time-
outs). If this first retransmission also fails to be acknowledged,
the sender SHOULD send subsequent retransmissions of the SYN without
any ECN flags set. This adds delay, in the case where a middlebox
drops an AccECN (or ECN) SYN deliberately. However, current
measurements imply that a drop is less likely to be due to middlebox
interference than other intermittent causes of loss, e.g. congestion,
wireless interference, etc.
Implementers MAY use other fall-back strategies if they are found to
be more effective (e.g. attempting to retransmit an AccECN SYN only
once or more than twice (most appropriate during high levels of
congestion); or falling back to classic ECN feedback rather than non-
ECN). Further it may make sense to also remove any other
experimental fields or options on the SYN in case a middlebox might
be blocking them, although the required behaviour will depend on the
specification of the other option(s) and any attempt to co-ordinate
fall-back between different modules of the stack. In any case, the
TCP initiator SHOULD cache failed connection attempts. If it does,
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it SHOULD NOT give up attempting to negotiate AccECN on the SYN of
subsequent connection attempts until it is clear that the blockage is
persistently and specifically due to AccECN. The cache should be
arranged to expire so that the initiator will infrequently attempt to
check whether the problem has been resolved.
The fall-back procedure if the TCP server receives no ACK to
acknowledge a SYN/ACK that tried to negotiate AccECN is specified in
Section 3.2.5.
3.2. AccECN Feedback
Each Data Receiver maintains four counters, r.cep, r.ceb, r.e0b and
r.e1b. The CE packet counter (r.cep), counts the number of packets
the host receives with the CE code point in the IP ECN field,
including CE marks on control packets without data. r.ceb, r.e0b and
r.e1b count the number of TCP payload bytes in packets marked
respectively with the CE, ECT(0) and ECT(1) codepoint in their IP-ECN
field. When a host first enters AccECN mode, it initialises its
counters to r.cep = 6, r.e0b = 1 and r.ceb = r.e1b.= 0 (see
Appendix A.5). Non-zero initial values are used to support a
stateless handshake (see Section 4.1) and to be distinct from cases
where the fields are incorrectly zeroed (e.g. by middleboxes - see
Section 3.2.5.4).
A host feeds back the CE packet counter using the Accurate ECN (ACE)
field, as explained in the next section. And it feeds back all the
byte counters using the AccECN TCP Option, as specified in
Section 3.2.4. Whenever a host feeds back the value of any counter,
it MUST report the most recent value, no matter whether it is in a
pure ACK, an ACK with new payload data or a retransmission.
3.2.1. The ACE Field
After AccECN has been negotiated on the SYN and SYN/ACK, both hosts
overload the three TCP flags (AE, CWR and ECE) in the main TCP header
as one 3-bit field. Then the field is given a new name, ACE, as
shown in Figure 2.
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
| | | | U | A | P | R | S | F |
| Header Length | Reserved | ACE | R | C | S | S | Y | I |
| | | | G | K | H | T | N | N |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
Figure 2: Definition of the ACE field within bytes 13 and 14 of the
TCP Header (when AccECN has been negotiated and SYN=0).
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The original definition of these three flags in the TCP header,
including the addition of support for the ECN Nonce, is shown for
comparison in Figure 1. This specification does not rename these
three TCP flags to ACE for always; it merely overloads them with
another name and definition once an AccECN connection has been
established.
A host MUST interpret the AE, CWR and ECE flags as the 3-bit ACE
counter on a segment with the SYN flag cleared (SYN=0) that it sends
or receives if both of its half-connections are set into AccECN mode
having successfully negotiated AccECN (see Section 3.1). A host MUST
NOT interpret the 3 flags as a 3-bit ACE field on any segment with
SYN=1 (whether ACK is 0 or 1), or if AccECN negotiation is incomplete
or has not succeeded.
Both parts of each of these conditions are equally important. For
instance, even if AccECN negotiation has been successful, the ACE
field is not defined on any segments with SYN=1 (e.g. a
retransmission of an unacknowledged SYN/ACK, or when both ends send
SYN/ACKs after AccECN support has been successfully negotiated during
a simultaneous open).
The ACE field encodes the three least significant bits of the r.cep
counter, therefore its initial value will be 0b110 (decimal 6). If
the SYN/ACK was CE marked, the client MUST increase its r.cep counter
before it sends its first ACK, therefore the initial value of the ACE
field will be 0b111 (decimal 7). To support a stateless handshake
(see Section 4.1), these values have been chosen deliberately so that
they are distinct from [RFC5562] behaviour, where the TCP client
would set ECE on the first ACK as feedback for a CE mark on the SYN/
ACK.
3.2.2. Testing for Zeroing of the ACE Field
Section 3.2.1 required the Data Receiver to initialize the r.cep
counter to a non-zero value. Therefore, in either direction the
initial value of the ACE field ought to be non-zero.
If AccECN has been successfully negotiated, the Data Sender SHOULD
check the initial value of the ACE field in the first arriving
segment with SYN=0. If the initial value of the ACE field is zero
(0b000), the Data Sender MUST disable sending ECN-capable packets for
the remainder of the half-connection by setting the IP/ECN field in
all subsequent packets to Not-ECT.
For example, the server checks the ACK of the SYN/ACK or the first
data segment from the client, while the client checks the first data
segment from the server. More precisely, the "first segment with
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SYN=0" is defined as: the segment with SYN=0 that i) acknowledges
sequence space at least covering the initial sequence number (ISN)
plus 1; and ii) arrives before any other segments with SYN=0 so it is
unlikely to be a retransmission. If no such segment arrives (e.g.
because it is lost and the ISN is first acknowledged by a subsequent
segment), no test for invalid initialization can be conducted, and
the half-connection will continue in AccECN mode.
Note that the Data Sender MUST NOT test whether the arriving counter
in the initial ACE field has been initialized to a specific valid
value - the above check solely tests whether the ACE fields have been
incorrectly zeroed. This allows hosts to use different initial
values as an additional signalling channel in future.
3.2.3. Safety against Ambiguity of the ACE Field
If too many CE-marked segments are acknowledged at once, or if a long
run of ACKs is lost, the 3-bit counter in the ACE field might have
cycled between two ACKs arriving at the Data Sender.
Therefore an AccECN Data Receiver SHOULD immediately send an ACK once
'n' CE marks have arrived since the previous ACK, where 'n' SHOULD be
2 and MUST be no greater than 6.
If the Data Sender has not received AccECN TCP Options to give it
more dependable information, and it detects that the ACE field could
have cycled under the prevailing conditions, it SHOULD conservatively
assume that the counter did cycle. It can detect if the counter
could have cycled by using the jump in the acknowledgement number
since the last ACK to calculate or estimate how many segments could
have been acknowledged. An example algorithm to implement this
policy is given in Appendix A.2. An implementer MAY develop an
alternative algorithm as long as it satisfies these requirements.
If missing acknowledgement numbers arrive later (reordering) and
prove that the counter did not cycle, the Data Sender MAY attempt to
neutralise the effect of any action it took based on a conservative
assumption that it later found to be incorrect.
3.2.4. The AccECN Option
The AccECN Option is defined as shown below in Figure 3. It consists
of three 24-bit fields that provide the 24 least significant bits of
the r.e0b, r.ceb and r.e1b counters, respectively. The initial 'E'
of each field name stands for 'Echo'.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Kind = TBD1 | Length = 11 | EE0B field |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| EE0B (cont'd) | ECEB field |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| EE1B field |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: The AccECN Option
The Data Receiver MUST set the Kind field to TBD1, which is
registered in Section 6 as a new TCP option Kind called AccECN. An
experimental TCP option with Kind=254 MAY be used for initial
experiments, with magic number 0xACCE.
Appendix A.1 gives an example algorithm for the Data Receiver to
encode its byte counters into the AccECN Option, and for the Data
Sender to decode the AccECN Option fields into its byte counters.
Note that there is no field to feedback Not-ECT bytes. Nonetheless
an algorithm for the Data Sender to calculate the number of payload
bytes received as Not-ECT is given in Appendix A.5.
Whenever a Data Receiver sends an AccECN Option, the rules in
Section 3.2.6 expect it to always send a full-length option. To cope
with option space limitations, it can omit unchanged fields from the
tail of the option, as long as it preserves the order of the
remaining fields and includes any field that has changed. The length
field MUST indicate which fields are present as follows:
Length=11: EE0B, ECEB, EE1B
Length=8: EE0B, ECEB
Length=5: EE0B
Length=2: (empty)
The empty option of Length=2 is provided to allow for a case where an
AccECN Option has to be sent (e.g. on the SYN/ACK to test the path),
but there is very limited space for the option. For initial
experiments, the Length field MUST be 2 greater to accommodate the
16-bit magic number.
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All implementations of a Data Sender MUST be able to read in AccECN
Options of any of the above lengths. They MUST ignore an AccECN
Option of any other length.
3.2.5. Path Traversal of the AccECN Option
3.2.5.1. Testing the AccECN Option during the Handshake
The TCP client MUST NOT include the AccECN TCP Option on the SYN.
Nonetheless, if the AccECN negotiation using the ECN flags in the
main TCP header (Section 3.1) is successful, it implicitly declares
that the endpoints also support the AccECN TCP Option. A fall-back
strategy for the loss of the SYN (possibly due to middlebox
interference) is specified in Section 3.1.2.
A TCP server that confirms its support for AccECN (in response to an
AccECN SYN from the client as described in Section 3.1) SHOULD also
include an AccECN TCP Option in the SYN/ACK.
A TCP client that has successfully negotiated AccECN SHOULD include
an AccECN Option in the first ACK at the end of the 3WHS. However,
this first ACK is not delivered reliably, so the TCP client SHOULD
also include an AccECN Option on the first data segment it sends (if
it ever sends one).
A host MAY NOT include an AccECN Option in any of these three cases
if it has cached knowledge that the packet would be likely to be
blocked on the path to the other host if it included an AccECN
Option.
3.2.5.2. Testing for Loss of Packets Carrying the AccECN Option
If after the normal TCP timeout the TCP server has not received an
ACK to acknowledge its SYN/ACK, the SYN/ACK might just have been
lost, e.g. due to congestion, or a middlebox might be blocking the
AccECN Option. To expedite connection setup, the TCP server SHOULD
retransmit the SYN/ACK with the same TCP flags (AE, CWR and ECE) but
with no AccECN Option. If this retransmission times out, to expedite
connection setup, the TCP server SHOULD disable AccECN and ECN for
this connection by retransmitting the SYN/ACK with AE=CWR=ECE=0 and
no AccECN Option. Implementers MAY use other fall-back strategies if
they are found to be more effective (e.g. falling back to classic
ECN feedback on the first retransmission; retrying the AccECN Option
for a second time before fall-back (most appropriate during high
levels of congestion); or falling back to classic ECN feedback rather
than non-ECN on the third retransmission).
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If the TCP client detects that the first data segment it sent with
the AccECN Option was lost, it SHOULD fall back to no AccECN Option
on the retransmission. Again, implementers MAY use other fall-back
strategies such as attempting to retransmit a second segment with the
AccECN Option before fall-back, and/or caching whether the AccECN
Option is blocked for subsequent connections.
Either host MAY include the AccECN Option in a subsequent segment to
retest whether the AccECN Option can traverse the path.
If the TCP server receives a second SYN with a request for AccECN
support, it should resend the SYN/ACK, again confirming its support
for AccECN, but this time without the AccECN Option. This approach
rules out any interference by middleboxes that may drop packets with
unknown options, even though it is more likely that the SYN/ACK would
have been lost due to congestion. The TCP server MAY try to send
another packet with the AccECN Option at a later point during the
connection but should monitor if that packet got lost as well, in
which case it SHOULD disable the sending of the AccECN Option for
this half-connection.
Similarly, an AccECN end-point MAY separately memorize which data
packets carried an AccECN Option and disable the sending of AccECN
Options if the loss probability of those packets is significantly
higher than that of all other data packets in the same connection.
3.2.5.3. Testing for Stripping of the AccECN Option
If the TCP client has successfully negotiated AccECN but does not
receive an AccECN Option on the SYN/ACK, it switches into a mode that
assumes that the AccECN Option is not available for this half
connection.
Similarly, if the TCP server has successfully negotiated AccECN but
does not receive an AccECN Option on the first segment that
acknowledges sequence space at least covering the ISN, it switches
into a mode that assumes that the AccECN Option is not available for
this half connection.
While a host is in this mode that assumes incoming AccECN Options are
not available, it MUST adopt the conservative interpretation of the
ACE field discussed in Section 3.2.3. However, it cannot make any
assumption about support of outgoing AccECN Options on the other half
connection, so it SHOULD continue to send the AccECN Option itself
(unless it has established that sending the AccECN Option is causing
packets to be blocked as in Section 3.2.5.2).
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If a host is in the mode that assumes incoming AccECN Options are not
available, but it receives an AccECN Option at any later point during
the connection, this clearly indicates that the AccECN Option is not
blocked on the respective path, and the AccECN endpoint MAY switch
out of the mode that assumes the AccECN Option is not available for
this half connection.
3.2.5.4. Test for Zeroing of the AccECN Option
For a related test for invalid initialization of the ACE field, see
Section 3.2.2
Section 3.2 required the Data Receiver to initialize the r.e0b
counter to a non-zero value. Therefore, in either direction the
initial value of the EE0B field in the AccECN Option (if one exists)
ought to be non-zero. If AccECN has been negotiated:
o the TCP server MAY check the initial value of the EE0B field in
the first segment that acknowledges sequence space that at least
covers the ISN plus 1. If the initial value of the EE0B field is
zero, the server will switch into a mode that ignores the AccECN
Option for this half connection.
o the TCP client MAY check the initial value of the EE0B field on
the SYN/ACK. If the initial value of the EE0B field is zero, the
client will switch into a mode that ignores the AccECN Option for
this half connection.
While a host is in the mode that ignores the AccECN Option it MUST
adopt the conservative interpretation of the ACE field discussed in
Section 3.2.3.
Note that the Data Sender MUST NOT test whether the arriving byte
counters in the initial AccECN Option have been initialized to
specific valid values - the above checks solely test whether these
fields have been incorrectly zeroed. This allows hosts to use
different initial values as an additional signalling channel in
future. Also note that the initial value of either field might be
greater than its expected initial value, because the counters might
already have been incremented. Nonetheless, the initial values of
the counters have been chosen so that they cannot wrap to zero on
these initial segments.
3.2.5.5. Consistency between AccECN Feedback Fields
When the AccECN Option is available it supplements but does not
replace the ACE field. An endpoint using AccECN feedback MUST always
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consider the information provided in the ACE field whether or not the
AccECN Option is also available.
If the AccECN option is present, the s.cep counter might increase
while the s.ceb counter does not (e.g. due to a CE-marked control
packet). The sender's response to such a situation is out of scope,
and needs to be dealt with in a specification that uses ECN-capable
control packets. Theoretically, this situation could also occur if a
middlebox mangled the AccECN Option but not the ACE field. However,
the Data Sender has to assume that the integrity of the AccECN Option
is sound, based on the above test of the well-known initial values
and optionally other integrity tests (Section 4.3).
If either end-point detects that the s.ceb counter has increased but
the s.cep has not (and by testing ACK coverage it is certain how much
the ACE field has wrapped), this invalid protocol transition has to
be due to some form of feedback mangling. So, the Data Sender MUST
disable sending ECN-capable packets for the remainder of the half-
connection by setting the IP/ECN field in all subsequent packets to
Not-ECT.
3.2.6. Usage of the AccECN TCP Option
The following rules determine when a Data Receiver in AccECN mode
sends the AccECN TCP Option, and which fields to include:
Change-Triggered ACKs: If an arriving packet increments a different
byte counter to that incremented by the previous packet, the Data
Receiver SHOULD immediately send an ACK with an AccECN Option,
without waiting for the next delayed ACK (this is in addition to
the safety recommendation in Section 3.2.3 against ambiguity of
the ACE field). Certain offload hardware might not be able to
support change-triggered ACKs, but otherwise it is important to
keep exceptions to this rule to a minimum so that Data Senders can
generally rely on this behaviour;
Continual Repetition: Otherwise, if arriving packets continue to
increment the same byte counter, the Data Receiver can include an
AccECN Option on most or all (delayed) ACKs, but it does not have
to. If option space is limited on a particular ACK, the Data
Receiver MUST give precedence to SACK information about loss. It
SHOULD include an AccECN Option if the r.ceb counter has
incremented and it MAY include an AccECN Option if r.ec0b or
r.ec1b has incremented;
Full-Length Options Preferred: It SHOULD always use full-length
AccECN Options. It MAY use shorter AccECN Options if space is
limited, but it MUST include the counter(s) that have incremented
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since the previous AccECN Option and it MUST only truncate fields
from the right-hand tail of the option to preserve the order of
the remaining fields (see Section 3.2.4);
Beaconing Full-Length Options: Nonetheless, it MUST include a full-
length AccECN TCP Option on at least three ACKs per RTT, or on all
ACKs if there are less than three per RTT (see Appendix A.4 for an
example algorithm that satisfies this requirement).
The following example series of arriving IP/ECN fields illustrates
when a Data Receiver will emit an ACK if it is using a delayed ACK
factor of 2 segments and change-triggered ACKs: 01 -> ACK, 01, 01 ->
ACK, 10 -> ACK, 10, 01 -> ACK, 01, 11 -> ACK, 01 -> ACK.
For the avoidance of doubt, the change-triggered ACK mechanism is
deliberately worded to ignore the arrival of a control packet with no
payload, which therefore does not alter any byte counters, because it
is important that TCP does not acknowledge pure ACKs. The change-
triggered ACK approach will lead to some additional ACKs but it feeds
back the timing and the order in which ECN marks are received with
minimal additional complexity.
Implementation note: sending an AccECN Option each time a different
counter changes and including a full-length AccECN Option on every
delayed ACK will satisfy the requirements described above and might
be the easiest implementation, as long as sufficient space is
available in each ACK (in total and in the option space).
Appendix A.3 gives an example algorithm to estimate the number of
marked bytes from the ACE field alone, if the AccECN Option is not
available.
If a host has determined that segments with the AccECN Option always
seem to be discarded somewhere along the path, it is no longer
obliged to follow the above rules.
3.3. AccECN Compliance by TCP Proxies, Offload Engines and other
Middleboxes
A large class of middleboxes split TCP connections. Such a middlebox
would be compliant with the AccECN protocol if the TCP implementation
on each side complied with the present AccECN specification and each
side negotiated AccECN independently of the other side.
Another large class of middleboxes intervenes to some degree at the
transport layer, but attempts to be transparent (invisible) to the
end-to-end connection. A subset of this class of middleboxes
attempts to `normalise' the TCP wire protocol by checking that all
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values in header fields comply with a rather narrow interpretation of
the TCP specifications. To comply with the present AccECN
specification, such a middlebox MUST NOT change the ACE field or the
AccECN Option and it MUST attempt to preserve the timing of each ACK
(for example, if it coalesced ACKs it would not be AccECN-compliant).
A middlebox claiming to be transparent at the transport layer MUST
forward the AccECN TCP Option unaltered, whether or not the length
value matches one of those specified in Section 3.2.4, and whether or
not the initial values of the byte-counter fields are correct. This
is because blocking apparently invalid values does not improve
security (because AccECN hosts are required to ignore invalid values
anyway), while it prevents the standardised set of values being
extended in future (because outdated normalisers would block updated
hosts from using the extended AccECN standard).
Hardware to offload certain TCP processing represents another large
class of middleboxes, even though it is often a function of a host's
network interface and rarely in its own 'box'. Leeway has been
allowed in the present AccECN specification in the expectation that
offload hardware could comply and still serve its function.
Nonetheless, such hardware MUST attempt to preserve the timing of
each ACK (for example, if it coalesced ACKs it would not be AccECN-
compliant).
4. Interaction with Other TCP Variants
This section is informative, not normative.
4.1. Compatibility with SYN Cookies
A TCP server can use SYN Cookies (see Appendix A of [RFC4987]) to
protect itself from SYN flooding attacks. It places minimal commonly
used connection state in the SYN/ACK, and deliberately does not hold
any state while waiting for the subsequent ACK (e.g. it closes the
thread). Therefore it cannot record the fact that it entered AccECN
mode for both half-connections. Indeed, it cannot even remember
whether it negotiated the use of classic ECN [RFC3168].
Nonetheless, such a server can determine that it negotiated AccECN as
follows. If a TCP server using SYN Cookies supports AccECN and if
the first segment it receives that at least covers the ISN contains
an ACE field with the value 0b110 or 0b111, it can assume that:
o the TCP client must have requested AccECN support on the SYN
o it (the server) must have confirmed that it supported AccECN
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Therefore the server can switch itself into AccECN mode, and continue
as if it had never forgotten that it switched itself into AccECN mode
earlier. For other values of ACE field, heuristics to infer what
other type of ECN the client supports are out of scope.
4.2. Compatibility with Other TCP Options and Experiments
AccECN is compatible (at least on paper) with the most commonly used
TCP options: MSS, time-stamp, window scaling, SACK and TCP-AO. It is
also compatible with the recent promising experimental TCP options
TCP Fast Open (TFO [RFC7413]) and Multipath TCP (MPTCP [RFC6824]).
AccECN is friendly to all these protocols, because space for TCP
options is particularly scarce on the SYN, where AccECN consumes zero
additional header space.
When option space is under pressure from other options, Section 3.2.6
provides guidance on how important it is to send an AccECN Option and
whether it needs to be a full-length option.
4.3. Compatibility with Feedback Integrity Mechanisms
Three alternative mechanisms are available to assure the integrity of
ECN and/or loss signals. AccECN is compatible with any of these
approaches:
o The Data Sender can test the integrity of the receiver's ECN (or
loss) feedback by occasionally setting the IP-ECN field to a value
normally only set by the network (and/or deliberately leaving a
sequence number gap). Then it can test whether the Data
Receiver's feedback faithfully reports what it expects
[I-D.moncaster-tcpm-rcv-cheat]. Unlike the ECN Nonce [RFC3540],
this approach does not waste the ECT(1) codepoint in the IP
header, it does not require standardisation and it does not rely
on misbehaving receivers volunteering to reveal feedback
information that allows them to be detected. However, setting the
CE mark by the sender might conceal actual congestion feedback
from the network and should therefore only be done sparsely.
o Networks generate congestion signals when they are becoming
congested, so networks are more likely than Data Senders to be
concerned about the integrity of the receiver's feedback of these
signals. A network can enforce a congestion response to its ECN
markings (or packet losses) using congestion exposure (ConEx)
audit [RFC7713]. Whether the receiver or a downstream network is
suppressing congestion feedback or the sender is unresponsive to
the feedback, or both, ConEx audit can neutralise any advantage
that any of these three parties would otherwise gain.
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ConEx is a change to the Data Sender that is most useful when
combined with AccECN. Without AccECN, the ConEx behaviour of a
Data Sender would have to be more conservative than would be
necessary if it had the accurate feedback of AccECN.
o The TCP authentication option (TCP-AO [RFC5925]) can be used to
detect any tampering with AccECN feedback between the Data
Receiver and the Data Sender (whether malicious or accidental).
The AccECN fields are immutable end-to-end, so they are amenable
to TCP-AO protection, which covers TCP options by default.
However, TCP-AO is often too brittle to use on many end-to-end
paths, where middleboxes can make verification fail in their
attempts to improve performance or security, e.g. by
resegmentation or shifting the sequence space.
Originally the ECN Nonce [RFC3540] was proposed to ensure integrity
of congestion feedback. With minor changes AccECN could be optimised
for the possibility that the ECT(1) codepoint might be used as an ECN
Nonce . However, given RFC 3540 is being reclassified as historic,
the AccECN design has been generalised so that it ought to be able to
support other possible uses of the ECT(1) codepoint, such as a lower
severity or a more instant congestion signal than CE.
5. Protocol Properties
This section is informative not normative. It describes how well the
protocol satisfies the agreed requirements for a more accurate ECN
feedback protocol [RFC7560].
Accuracy: From each ACK, the Data Sender can infer the number of new
CE marked segments since the previous ACK. This provides better
accuracy on CE feedback than classic ECN. In addition if the
AccECN Option is present (not blocked by the network path) the
number of bytes marked with CE, ECT(1) and ECT(0) are provided.
Overhead: The AccECN scheme is divided into two parts. The
essential part reuses the 3 flags already assigned to ECN in the
IP header. The supplementary part adds an additional TCP option
consuming up to 11 bytes. However, no TCP option is consumed in
the SYN.
Ordering: The order in which marks arrive at the Data Receiver is
preserved in AccECN feedback, because the Data Receiver is
expected to send an ACK immediately whenever a different mark
arrives.
Timeliness: While the same ECN markings are arriving continually at
the Data Receiver, it can defer ACKs as TCP does normally, but it
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will immediately send an ACK as soon as a different ECN marking
arrives.
Timeliness vs Overhead: Change-Triggered ACKs are intended to enable
latency-sensitive uses of ECN feedback by capturing the timing of
transitions but not wasting resources while the state of the
signalling system is stable. The receiver can control how
frequently it sends the AccECN TCP Option and therefore it can
control the overhead induced by AccECN.
Resilience: All information is provided based on counters.
Therefore if ACKs are lost, the counters on the first ACK
following the losses allows the Data Sender to immediately recover
the number of the ECN markings that it missed.
Resilience against Bias: Because feedback is based on repetition of
counters, random losses do not remove any information, they only
delay it. Therefore, even though some ACKs are change-triggered,
random losses will not alter the proportions of the different ECN
markings in the feedback.
Resilience vs Overhead: If space is limited in some segments (e.g.
because more option are need on some segments, such as the SACK
option after loss), the Data Receiver can send AccECN Options less
frequently or truncate fields that have not changed, usually down
to as little as 5 bytes. However, it has to send a full-sized
AccECN Option at least three times per RTT, which the Data Sender
can rely on as a regular beacon or checkpoint.
Resilience vs Timeliness and Ordering: Ordering information and the
timing of transitions cannot be communicated in three cases: i)
during ACK loss; ii) if something on the path strips the AccECN
Option; or iii) if the Data Receiver is unable to support Change-
Triggered ACKs.
Complexity: An AccECN implementation solely involves simple counter
increments, some modulo arithmetic to communicate the least
significant bits and allow for wrap, and some heuristics for
safety against fields cycling due to prolonged periods of ACK
loss. Each host needs to maintain eight additional counters. The
hosts have to apply some additional tests to detect tampering by
middleboxes, but in general the protocol is simple to understand,
simple to implement and requires few cycles per packet to execute.
Integrity: AccECN is compatible with at least three approaches that
can assure the integrity of ECN feedback. If the AccECN Option is
stripped the resolution of the feedback is degraded, but the
integrity of this degraded feedback can still be assured.
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Backward Compatibility: If only one endpoint supports the AccECN
scheme, it will fall-back to the most advanced ECN feedback scheme
supported by the other end.
Backward Compatibility: If the AccECN Option is stripped by a
middlebox, AccECN still provides basic congestion feedback in the
ACE field. Further, AccECN can be used to detect mangling of the
IP ECN field; mangling of the TCP ECN flags; blocking of ECT-
marked segments; and blocking of segments carrying the AccECN
Option. It can detect these conditions during TCP's 3WHS so that
it can fall back to operation without ECN and/or operation without
the AccECN Option.
Forward Compatibility: The behaviour of endpoints and middleboxes is
carefully defined for all reserved or currently unused codepoints
in the scheme, to ensure that any blocking of anomalous values is
always at least under reversible policy control.
6. IANA Considerations
This document reassigns bit 7 of the TCP header flags to the AccECN
experiment. This bit was previously called the Nonce Sum (NS) flag
[RFC3540], but RFC 3540 is being reclassified as historic. The flag
will now be defined as:
+-----+-------------------+-----------+
| Bit | Name | Reference |
+-----+-------------------+-----------+
| 7 | AE (Accurate ECN) | RFC XXXX |
+-----+-------------------+-----------+
[TO BE REMOVED: This registration should take place at the following
location: https://www.iana.org/assignments/tcp-header-flags/tcp-
header-flags.xhtml#tcp-header-flags-1 ]
This document also defines a new TCP option for AccECN, assigned a
value of TBD1 (decimal) from the TCP option space. This value is
defined as:
+------+--------+-----------------------+-----------+
| Kind | Length | Meaning | Reference |
+------+--------+-----------------------+-----------+
| TBD1 | N | Accurate ECN (AccECN) | RFC XXXX |
+------+--------+-----------------------+-----------+
[TO BE REMOVED: This registration should take place at the following
location: http://www.iana.org/assignments/tcp-parameters/tcp-
parameters.xhtml#tcp-parameters-1 ]
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Early implementation before the IANA allocation MUST follow [RFC6994]
and use experimental option 254 and magic number 0xACCE (16 bits),
then migrate to the new option after the allocation.
7. Security Considerations
If ever the supplementary part of AccECN based on the new AccECN TCP
Option is unusable (due for example to middlebox interference) the
essential part of AccECN's congestion feedback offers only limited
resilience to long runs of ACK loss (see Section 3.2.3). These
problems are unlikely to be due to malicious intervention (because if
an attacker could strip a TCP option or discard a long run of ACKs it
could wreak other arbitrary havoc). However, it would be of concern
if AccECN's resilience could be indirectly compromised during a
flooding attack. AccECN is still considered safe though, because if
the option is not presented, the AccECN Data Sender is then required
to switch to more conservative assumptions about wrap of congestion
indication counters (see Section 3.2.3 and Appendix A.2).
Section 4.1 describes how a TCP server can negotiate AccECN and use
the SYN cookie method for mitigating SYN flooding attacks.
There is concern that ECN markings could be altered or suppressed,
particularly because a misbehaving Data Receiver could increase its
own throughput at the expense of others. AccECN is compatible with
the three schemes known to assure the integrity of ECN feedback (see
Section 4.3 for details). If the AccECN Option is stripped by an
incorrectly implemented middlebox, the resolution of the feedback
will be degraded, but the integrity of this degraded information can
still be assured.
The AccECN protocol is not believed to introduce any new privacy
concerns, because it merely counts and feeds back signals at the
transport layer that had already been visible at the IP layer.
8. Acknowledgements
We want to thank Koen De Schepper, Praveen Balasubramanian and
Michael Welzl for their input and discussion. The idea of using the
three ECN-related TCP flags as one field for more accurate TCP-ECN
feedback was first introduced in the re-ECN protocol that was the
ancestor of ConEx.
Bob Briscoe was part-funded by the European Community under its
Seventh Framework Programme through the Reducing Internet Transport
Latency (RITE) project (ICT-317700) and through the Trilogy 2 project
(ICT-317756). The views expressed here are solely those of the
authors.
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This work is partly supported by the European Commission under
Horizon 2020 grant agreement no. 688421 Measurement and Architecture
for a Middleboxed Internet (MAMI), and by the Swiss State Secretariat
for Education, Research, and Innovation under contract no. 15.0268.
This support does not imply endorsement.
9. Comments Solicited
Comments and questions are encouraged and very welcome. They can be
addressed to the IETF TCP maintenance and minor modifications working
group mailing list <tcpm@ietf.org>, and/or to the authors.
10. References
10.1. 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>.
[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>.
[RFC6994] Touch, J., "Shared Use of Experimental TCP Options",
RFC 6994, DOI 10.17487/RFC6994, August 2013,
<http://www.rfc-editor.org/info/rfc6994>.
10.2. Informative References
[I-D.bagnulo-tcpm-generalized-ecn]
Bagnulo, M. and B. Briscoe, "ECN++: Adding Explicit
Congestion Notification (ECN) to TCP Control Packets",
draft-bagnulo-tcpm-generalized-ecn-04 (work in progress),
May 2017.
[I-D.ietf-tcpm-alternativebackoff-ecn]
Khademi, N., Welzl, M., Armitage, G., and G. Fairhurst,
"TCP Alternative Backoff with ECN (ABE)", draft-ietf-tcpm-
alternativebackoff-ecn-01 (work in progress), May 2017.
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[I-D.ietf-tcpm-dctcp]
Bensley, S., Thaler, D., Balasubramanian, P., Eggert, L.,
and G. Judd, "Datacenter TCP (DCTCP): TCP Congestion
Control for Datacenters", draft-ietf-tcpm-dctcp-06 (work
in progress), May 2017.
[I-D.ietf-tsvwg-ecn-experimentation]
Black, D., "Explicit Congestion Notification (ECN)
Experimentation", draft-ietf-tsvwg-ecn-experimentation-02
(work in progress), April 2017.
[I-D.ietf-tsvwg-l4s-arch]
Briscoe, B., Schepper, K., and M. Bagnulo, "Low Latency,
Low Loss, Scalable Throughput (L4S) Internet Service:
Architecture", draft-ietf-tsvwg-l4s-arch-00 (work in
progress), May 2017.
[I-D.kuehlewind-tcpm-ecn-fallback]
Kuehlewind, M. and B. Trammell, "A Mechanism for ECN Path
Probing and Fallback", draft-kuehlewind-tcpm-ecn-
fallback-01 (work in progress), September 2013.
[I-D.moncaster-tcpm-rcv-cheat]
Moncaster, T., Briscoe, B., and A. Jacquet, "A TCP Test to
Allow Senders to Identify Receiver Non-Compliance", draft-
moncaster-tcpm-rcv-cheat-03 (work in progress), July 2014.
[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>.
[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>.
[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>.
[RFC5925] Touch, J., Mankin, A., and R. Bonica, "The TCP
Authentication Option", RFC 5925, DOI 10.17487/RFC5925,
June 2010, <http://www.rfc-editor.org/info/rfc5925>.
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[RFC6824] Ford, A., Raiciu, C., Handley, M., and O. Bonaventure,
"TCP Extensions for Multipath Operation with Multiple
Addresses", RFC 6824, DOI 10.17487/RFC6824, January 2013,
<http://www.rfc-editor.org/info/rfc6824>.
[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>.
[RFC7560] Kuehlewind, M., Ed., Scheffenegger, R., and B. Briscoe,
"Problem Statement and Requirements for Increased Accuracy
in Explicit Congestion Notification (ECN) Feedback",
RFC 7560, DOI 10.17487/RFC7560, August 2015,
<http://www.rfc-editor.org/info/rfc7560>.
[RFC7713] Mathis, M. and B. Briscoe, "Congestion Exposure (ConEx)
Concepts, Abstract Mechanism, and Requirements", RFC 7713,
DOI 10.17487/RFC7713, December 2015,
<http://www.rfc-editor.org/info/rfc7713>.
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Appendix A. Example Algorithms
This appendix is informative, not normative. It gives example
algorithms that would satisfy the normative requirements of the
AccECN protocol. However, implementers are free to choose other ways
to implement the requirements.
A.1. Example Algorithm to Encode/Decode the AccECN Option
The example algorithms below show how a Data Receiver in AccECN mode
could encode its CE byte counter r.ceb into the ECEB field within the
AccECN TCP Option, and how a Data Sender in AccECN mode could decode
the ECEB field into its byte counter s.ceb. The other counters for
bytes marked ECT(0) and ECT(1) in the AccECN Option would be
similarly encoded and decoded.
It is assumed that each local byte counter is an unsigned integer
greater than 24b (probably 32b), and that the following constant has
been assigned:
DIVOPT = 2^24
Every time a CE marked data segment arrives, the Data Receiver
increments its local value of r.ceb by the size of the TCP Data.
Whenever it sends an ACK with the AccECN Option, the value it writes
into the ECEB field is
ECEB = r.ceb % DIVOPT
where '%' is the modulo operator.
On the arrival of an AccECN Option, the Data Sender uses the TCP
acknowledgement number and any SACK options to calculate newlyAckedB,
the amount of new data that the ACK acknowledges in bytes. If
newlyAckedB is negative it means that a more up to date ACK has
already been processed, so this ACK has been superseded and the Data
Sender has to ignore the AccECN Option. Then the Data Sender
calculates the minimum difference d.ceb between the ECEB field and
its local s.ceb counter, using modulo arithmetic as follows:
if (newlyAckedB >= 0) {
d.ceb = (ECEB + DIVOPT - (s.ceb % DIVOPT)) % DIVOPT
s.ceb += d.ceb
}
For example, if s.ceb is 33,554,433 and ECEB is 1461 (both decimal),
then
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s.ceb % DIVOPT = 1
d.ceb = (1461 + 2^24 - 1) % 2^24
= 1460
s.ceb = 33,554,433 + 1460
= 33,555,893
A.2. Example Algorithm for Safety Against Long Sequences of ACK Loss
The example algorithms below show how a Data Receiver in AccECN mode
could encode its CE packet counter r.cep into the ACE field, and how
the Data Sender in AccECN mode could decode the ACE field into its
s.cep counter. The Data Sender's algorithm includes code to
heuristically detect a long enough unbroken string of ACK losses that
could have concealed a cycle of the congestion counter in the ACE
field of the next ACK to arrive.
Two variants of the algorithm are given: i) a more conservative
variant for a Data Sender to use if it detects that the AccECN Option
is not available (see Section 3.2.3 and Section 3.2.5); and ii) a
less conservative variant that is feasible when complementary
information is available from the AccECN Option.
A.2.1. Safety Algorithm without the AccECN Option
It is assumed that each local packet counter is a sufficiently sized
unsigned integer (probably 32b) and that the following constant has
been assigned:
DIVACE = 2^3
Every time a CE marked packet arrives, the Data Receiver increments
its local value of r.cep by 1. It repeats the same value of ACE in
every subsequent ACK until the next CE marking arrives, where
ACE = r.cep % DIVACE.
If the Data Sender received an earlier value of the counter that had
been delayed due to ACK reordering, it might incorrectly calculate
that the ACE field had wrapped. Therefore, on the arrival of every
ACK, the Data Sender uses the TCP acknowledgement number and any SACK
options to calculate newlyAckedB, the amount of new data that the ACK
acknowledges. If newlyAckedB is negative it means that a more up to
date ACK has already been processed, so this ACK has been superseded
and the Data Sender has to ignore the AccECN Option. If newlyAckedB
is zero, to break the tie the Data Sender could use timestamps (if
present) to work out newlyAckedT, the amount of new time that the ACK
acknowledges. Then the Data Sender calculates the minimum difference
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d.cep between the ACE field and its local s.cep counter, using modulo
arithmetic as follows:
if ((newlyAckedB > 0) || (newlyAckedB == 0 && newlyAckedT > 0))
d.cep = (ACE + DIVACE - (s.cep % DIVACE)) % DIVACE
Section 3.2.3 requires the Data Sender to assume that the ACE field
did cycle if it could have cycled under prevailing conditions. The
3-bit ACE field in an arriving ACK could have cycled and become
ambiguous to the Data Sender if a row of ACKs goes missing that
covers a stream of data long enough to contain 8 or more CE marks.
We use the word `missing' rather than `lost', because some or all the
missing ACKs might arrive eventually, but out of order. Even if some
of the lost ACKs are piggy-backed on data (i.e. not pure ACKs)
retransmissions will not repair the lost AccECN information, because
AccECN requires retransmissions to carry the latest AccECN counters,
not the original ones.
The phrase `under prevailing conditions' allows the Data Sender to
take account of the prevailing size of data segments and the
prevailing CE marking rate just before the sequence of ACK losses.
However, we shall start with the simplest algorithm, which assumes
segments are all full-sized and ultra-conservatively it assumes that
ECN marking was 100% on the forward path when ACKs on the reverse
path started to all be dropped. Specifically, if newlyAckedB is the
amount of data that an ACK acknowledges since the previous ACK, then
the Data Sender could assume that this acknowledges newlyAckedPkt
full-sized segments, where newlyAckedPkt = newlyAckedB/MSS. Then it
could assume that the ACE field incremented by
dSafer.cep = newlyAckedPkt - ((newlyAckedPkt - d.cep) % DIVACE),
For example, imagine an ACK acknowledges newlyAckedPkt=9 more full-
size segments than any previous ACK, and that ACE increments by a
minimum of 2 CE marks (d.cep=2). The above formula works out that it
would still be safe to assume 2 CE marks (because 9 - ((9-2) % 8) =
2). However, if ACE increases by a minimum of 2 but acknowledges 10
full-sized segments, then it would be necessary to assume that there
could have been 10 CE marks (because 10 - ((10-2) % 8) = 10).
Implementers could build in more heuristics to estimate prevailing
average segment size and prevailing ECN marking. For instance,
newlyAckedPkt in the above formula could be replaced with
newlyAckedPktHeur = newlyAckedPkt*p*MSS/s, where s is the prevailing
segment size and p is the prevailing ECN marking probability.
However, ultimately, if TCP's ECN feedback becomes inaccurate it
still has loss detection to fall back on. Therefore, it would seem
safe to implement a simple algorithm, rather than a perfect one.
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The simple algorithm for dSafer.cep above requires no monitoring of
prevailing conditions and it would still be safe if, for example,
segments were on average at least 5% of full-sized as long as ECN
marking was 5% or less. Assuming it was used, the Data Sender would
increment its packet counter as follows:
s.cep += dSafer.cep
If missing acknowledgement numbers arrive later (due to reordering),
Section 3.2.3 says "the Data Sender MAY attempt to neutralise the
effect of any action it took based on a conservative assumption that
it later found to be incorrect". To do this, the Data Sender would
have to store the values of all the relevant variables whenever it
made assumptions, so that it could re-evaluate them later. Given
this could become complex and it is not required, we do not attempt
to provide an example of how to do this.
A.2.2. Safety Algorithm with the AccECN Option
When the AccECN Option is available on the ACKs before and after the
possible sequence of ACK losses, if the Data Sender only needs CE-
marked bytes, it will have sufficient information in the AccECN
Option without needing to process the ACE field. However, if for
some reason it needs CE-marked packets, if dSafer.cep is different
from d.cep, it can calculate the average marked segment size that
each implies to determine whether d.cep is likely to be a safe enough
estimate. Specifically, it could use the following algorithm, where
d.ceb is the amount of newly CE-marked bytes (see Appendix A.1):
SAFETY_FACTOR = 2
if (dSafer.cep > d.cep) {
s = d.ceb/d.cep
if (s <= MSS) {
sSafer = d.ceb/dSafer.cep
if (sSafer < MSS/SAFETY_FACTOR)
dSafer.cep = d.cep % d.cep is a safe enough estimate
} % else
% No need for else; dSafer.cep is already correct,
% because d.cep must have been too small
}
The chart below shows when the above algorithm will consider d.cep
can replace dSafer.cep as a safe enough estimate of the number of CE-
marked packets:
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^
sSafer|
|
MSS+
|
| dSafer.cep
| is
MSS/2+--------------+ safest
| |
| d.cep is safe|
| enough |
+-------------------->
MSS s
The following examples give the reasoning behind the algorithm,
assuming MSS=1,460 [B]:
o if d.cep=0, dSafer.cep=8 and d.ceb=1,460, then s=infinity and
sSafer=182.5.
Therefore even though the average size of 8 data segments is
unlikely to have been as small as MSS/8, d.cep cannot have been
correct, because it would imply an average segment size greater
than the MSS.
o if d.cep=2, dSafer.cep=10 and d.ceb=1,460, then s=730 and
sSafer=146.
Therefore d.cep is safe enough, because the average size of 10
data segments is unlikely to have been as small as MSS/10.
o if d.cep=7, dSafer.cep=15 and d.ceb=10,200, then s=1,457 and
sSafer=680.
Therefore d.cep is safe enough, because the average data segment
size is more likely to have been just less than one MSS, rather
than below MSS/2.
If pure ACKs were allowed to be ECN-capable, missing ACKs would be
far less likely. However, because [RFC3168] currently precludes
this, the above algorithm assumes that pure ACKs are not ECN-capable.
A.3. Example Algorithm to Estimate Marked Bytes from Marked Packets
If the AccECN Option is not available, the Data Sender can only
decode CE-marking from the ACE field in packets. Every time an ACK
arrives, to convert this into an estimate of CE-marked bytes, it
needs an average of the segment size, s_ave. Then it can add or
subtract s_ave from the value of d.ceb as the value of d.cep
increments or decrements.
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To calculate s_ave, it could keep a record of the byte numbers of all
the boundaries between packets in flight (including control packets),
and recalculate s_ave on every ACK. However it would be simpler to
merely maintain a counter packets_in_flight for the number of packets
in flight (including control packets), which it could update once per
RTT. Either way, it would estimate s_ave as:
s_ave ~= flightsize / packets_in_flight,
where flightsize is the variable that TCP already maintains for the
number of bytes in flight. To avoid floating point arithmetic, it
could right-bit-shift by lg(packets_in_flight), where lg() means log
base 2.
An alternative would be to maintain an exponentially weighted moving
average (EWMA) of the segment size:
s_ave = a * s + (1-a) * s_ave,
where a is the decay constant for the EWMA. However, then it is
necessary to choose a good value for this constant, which ought to
depend on the number of packets in flight. Also the decay constant
needs to be power of two to avoid floating point arithmetic.
A.4. Example Algorithm to Beacon AccECN Options
Section 3.2.6 requires a Data Receiver to beacon a full-length AccECN
Option at least 3 times per RTT. This could be implemented by
maintaining a variable to store the number of ACKs (pure and data
ACKs) since a full AccECN Option was last sent and another for the
approximate number of ACKs sent in the last round trip time:
if (acks_since_full_last_sent > acks_in_round / BEACON_FREQ)
send_full_AccECN_Option()
For optimised integer arithmetic, BEACON_FREQ = 4 could be used,
rather than 3, so that the division could be implemented as an
integer right bit-shift by lg(BEACON_FREQ).
In certain operating systems, it might be too complex to maintain
acks_in_round. In others it might be possible by tagging each data
segment in the retransmit buffer with the number of ACKs sent at the
point that segment was sent. This would not work well if the Data
Receiver was not sending data itself, in which case it might be
necessary to beacon based on time instead, as follows:
if ( time_now > time_last_option_sent + (RTT / BEACON_FREQ) )
send_full_AccECN_Option()
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This time-based approach does not work well when all the ACKs are
sent early in each round trip, as is the case during slow-start. In
this case few options will be sent (evtl. even less than 3 per RTT).
However, when continuously sending data, data packets as well as ACKs
will spread out equally over the RTT and sufficient ACKs with the
AccECN option will be sent.
A.5. Example Algorithm to Count Not-ECT Bytes
A Data Sender in AccECN mode can infer the amount of TCP payload data
arriving at the receiver marked Not-ECT from the difference between
the amount of newly ACKed data and the sum of the bytes with the
other three markings, d.ceb, d.e0b and d.e1b. Note that, because
r.e0b is initialized to 1 and the other two counters are initialized
to 0, the initial sum will be 1, which matches the initial offset of
the TCP sequence number on completion of the 3WHS.
For this approach to be precise, it has to be assumed that spurious
(unnecessary) retransmissions do not lead to double counting. This
assumption is currently correct, given that RFC 3168 requires that
the Data Sender marks retransmitted segments as Not-ECT. However,
the converse is not true; necessary transmissions will result in
under-counting.
However, such precision is unlikely to be necessary. The only known
use of a count of Not-ECT marked bytes is to test whether equipment
on the path is clearing the ECN field (perhaps due to an out-dated
attempt to clear, or bleach, what used to be the ToS field). To
detect bleaching it will be sufficient to detect whether nearly all
bytes arrive marked as Not-ECT. Therefore there should be no need to
keep track of the details of retransmissions.
Appendix B. Alternative Design Choices (To Be Removed Before
Publication)
This appendix is informative, not normative. It records alternative
designs that the authors chose not to include in the normative
specification, but which the IETF might wish to consider for
inclusion:
Feedback all four ECN codepoints on the SYN/ACK: The last two
negotiation combinations in Table 2 could be used to indicate
AccECN support while also feeding back that the arriving SYN was
ECT(0) or ECT(1). This could be used to probe the client to
server path for incorrect forwarding of the ECN field
[I-D.kuehlewind-tcpm-ecn-fallback].
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Feedback all four ECN codepoints on the First ACK: To probe the
server to client path for incorrect ECN forwarding, it could be
useful to have four feedback states on the first ACK from the TCP
client. This could be achieved by assigning four combinations of
the ECN flags in the main TCP header, and only initializing the
ACE field on subsequent segments.
Appendix C. Open Protocol Design Issues (To Be Removed Before
Publication)
1. Currently it is specified that the receiver `SHOULD' use Change-
Triggered ACKs. It is controversial whether this ought to be a
`MUST' instead. A `SHOULD' would leave the Data Sender uncertain
whether it can rely on the timing and ordering information in
ACKs. If the sender guesses wrongly, it will probably introduce
at least 1 RTT of delay before it can use this timing
information. Ironically it will most likely be wanting this
information to reduce ramp-up delay. A `MUST' could make it hard
to implement AccECN in offload hardware. However, it is not
known whether AccECN would be hard to implement in such hardware
even with a `SHOULD' here. For instance, was it hard to offload
DCTCP to hardware because of change-triggered ACKs, or was this
just one of many reasons? The choice between MUST and SHOULD
here is critical. Before that choice is made, a clear use-case
for certainty of timing and ordering information is needed, plus
well-informed discussion about hardware offload constraints.
2. There is possibly a concern that a receiver could deliberately
omit the AccECN Option pretending that it had been stripped by a
middlebox. No known way can yet be contrived to take advantage
of this downgrade attack, but it is mentioned here in case
someone else can contrive one.
Appendix D. Changes in This Version (To Be Removed Before Publication)
The difference between any pair of versions can be displayed at
http://datatracker.ietf.org/doc/draft-kuehlewind-tcpm-accurate-ecn/
history/
Authors' Addresses
Bob Briscoe
Simula Research Laboratory
EMail: ietf@bobbriscoe.net
URI: http://bobbriscoe.net/
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Mirja Kuehlewind
ETH Zurich
Zurich
Switzerland
EMail: mirja.kuehlewind@tik.ee.ethz.ch
Richard Scheffenegger
Vienna
Austria
EMail: rscheff@gmx.at
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