Internet DRAFT - draft-ietf-tcpm-accurate-ecn
draft-ietf-tcpm-accurate-ecn
TCP Maintenance & Minor Extensions (tcpm) B. Briscoe
Internet-Draft Independent
Updates: 3168 (if approved) M. Kühlewind
Intended status: Standards Track Ericsson
Expires: 20 May 2024 R. Scheffenegger
NetApp
17 November 2023
More Accurate Explicit Congestion Notification (ECN) Feedback in TCP
draft-ietf-tcpm-accurate-ecn-28
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 endpoints. Receivers with an ECN-capable
transport protocol feed back this information to the sender. ECN was
originally specified for TCP in such a way that only one feedback
signal can be transmitted per Round-Trip Time (RTT). Recent new TCP
mechanisms like Congestion Exposure (ConEx), Data Center TCP (DCTCP)
or Low Latency, Low Loss, and Scalable Throughput (L4S) need more
accurate ECN feedback information whenever more than one marking is
received in one RTT. This document updates the original ECN
specification in RFC 3168 to specify a scheme that provides more than
one feedback signal per RTT in the TCP header. Given TCP header
space is scarce, it allocates a reserved header bit previously
assigned to the ECN-Nonce. It also overloads the two existing ECN
flags in the TCP header. The resulting extra space is exploited to
feed back the IP-ECN field received during the 3-way handshake as
well. Supplementary feedback information can optionally be provided
in two new TCP option alternatives, which are never used on the TCP
SYN. The document also specifies the treatment of this updated TCP
wire protocol by middleboxes.
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 https://datatracker.ietf.org/drafts/current/.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Document Roadmap . . . . . . . . . . . . . . . . . . . . 6
1.2. Goals . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.3. Terminology . . . . . . . . . . . . . . . . . . . . . . . 6
1.4. Recap of Existing ECN feedback in IP/TCP . . . . . . . . 7
2. AccECN Protocol Overview and Rationale . . . . . . . . . . . 9
2.1. Capability Negotiation . . . . . . . . . . . . . . . . . 10
2.2. Feedback Mechanism . . . . . . . . . . . . . . . . . . . 10
2.3. Delayed ACKs and Resilience Against ACK Loss . . . . . . 11
2.4. Feedback Metrics . . . . . . . . . . . . . . . . . . . . 12
2.5. Generic (Mechanistic) Reflector . . . . . . . . . . . . . 12
3. AccECN Protocol Specification . . . . . . . . . . . . . . . . 13
3.1. Negotiating to use AccECN . . . . . . . . . . . . . . . . 13
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3.1.1. Negotiation during the TCP handshake . . . . . . . . 13
3.1.2. Backward Compatibility . . . . . . . . . . . . . . . 14
3.1.3. Forward Compatibility . . . . . . . . . . . . . . . . 17
3.1.4. Multiple SYNs or SYN/ACKs . . . . . . . . . . . . . . 17
3.1.4.1. Retransmitted SYNs . . . . . . . . . . . . . . . 17
3.1.4.2. Retransmitted SYN/ACKs . . . . . . . . . . . . . 19
3.1.5. Implications of AccECN Mode . . . . . . . . . . . . . 20
3.2. AccECN Feedback . . . . . . . . . . . . . . . . . . . . . 24
3.2.1. Initialization of Feedback Counters . . . . . . . . . 25
3.2.2. The ACE Field . . . . . . . . . . . . . . . . . . . . 25
3.2.2.1. ACE Field on the ACK of the SYN/ACK . . . . . . . 26
3.2.2.2. Encoding and Decoding Feedback in the ACE
Field . . . . . . . . . . . . . . . . . . . . . . . 29
3.2.2.3. Testing for Mangling of the IP/ECN Field . . . . 31
3.2.2.4. Testing for Zeroing of the ACE Field . . . . . . 33
3.2.2.5. Safety against Ambiguity of the ACE Field . . . . 34
3.2.3. The AccECN Option . . . . . . . . . . . . . . . . . . 36
3.2.3.1. Encoding and Decoding Feedback in the AccECN Option
Fields . . . . . . . . . . . . . . . . . . . . . . 39
3.2.3.2. Path Traversal of the AccECN Option . . . . . . . 39
3.2.3.3. Usage of the AccECN TCP Option . . . . . . . . . 43
3.3. AccECN Compliance Requirements for TCP Proxies, Offload
Engines and other Middleboxes . . . . . . . . . . . . . . 46
3.3.1. Requirements for TCP Proxies . . . . . . . . . . . . 46
3.3.2. Requirements for Transparent Middleboxes and TCP
Normalizers . . . . . . . . . . . . . . . . . . . . . 46
3.3.3. Requirements for TCP ACK Filtering . . . . . . . . . 47
3.3.4. Requirements for TCP Segmentation Offload . . . . . . 48
4. Updates to RFC 3168 . . . . . . . . . . . . . . . . . . . . . 49
5. Interaction with TCP Variants . . . . . . . . . . . . . . . . 50
5.1. Compatibility with SYN Cookies . . . . . . . . . . . . . 50
5.2. Compatibility with TCP Experiments and Common TCP
Options . . . . . . . . . . . . . . . . . . . . . . . . . 51
5.3. Compatibility with Feedback Integrity Mechanisms . . . . 51
6. Summary: Protocol Properties . . . . . . . . . . . . . . . . 52
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 55
8. Security Considerations . . . . . . . . . . . . . . . . . . . 56
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 57
9.1. Normative References . . . . . . . . . . . . . . . . . . 57
9.2. Informative References . . . . . . . . . . . . . . . . . 58
Appendix A. Example Algorithms . . . . . . . . . . . . . . . . . 60
A.1. Example Algorithm to Encode/Decode the AccECN Option . . 60
A.2. Example Algorithm for Safety Against Long Sequences of ACK
Loss . . . . . . . . . . . . . . . . . . . . . . . . . . 62
A.2.1. Safety Algorithm without the AccECN Option . . . . . 62
A.2.2. Safety Algorithm with the AccECN Option . . . . . . . 64
A.3. Example Algorithm to Estimate Marked Bytes from Marked
Packets . . . . . . . . . . . . . . . . . . . . . . . . . 66
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A.4. Example Algorithm to Count Not-ECT Bytes . . . . . . . . 67
Appendix B. Rationale for Usage of TCP Header Flags . . . . . . 67
B.1. Three TCP Header Flags in the SYN-SYN/ACK Handshake . . . 67
B.2. Four Codepoints in the SYN/ACK . . . . . . . . . . . . . 68
B.3. Space for Future Evolution . . . . . . . . . . . . . . . 69
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 70
Comments Solicited . . . . . . . . . . . . . . . . . . . . . . . 71
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 71
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 endpoints. Receivers with an
ECN-capable transport protocol feed back this information to the
sender. In RFC 3168, ECN was specified for TCP in such a way that
only one feedback signal could be transmitted per Round-Trip Time
(RTT). Recently, proposed mechanisms like Congestion Exposure (ConEx
[RFC7713]), DCTCP [RFC8257] or L4S [RFC9330] need to know when more
than one marking is received in one RTT, which is information that
cannot be provided by the feedback scheme as specified in [RFC3168].
This document specifies an update to the ECN feedback scheme of RFC
3168 that provides more accurate information and could be used by
these and potentially other future TCP extensions, while still also
supporting the pre-existing TCP congestion controllers that use just
one feedback signal per round. A fuller treatment of the motivation
for this specification is given in the associated requirements
document [RFC7560].
This document specifies a standards track 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. This document updates RFC 3168 with respect to negotiation
and use of the feedback scheme for TCP. All aspects of RFC 3168
other than the TCP feedback scheme and its negotiation remain
unchanged by this specification. In particular the definition of ECN
at the IP layer is unaffected. Section 4 gives a more detailed
specification of exactly which aspects of RFC 3168 this document
updates.
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This document uses the term Classic ECN feedback when it needs to
distinguish the TCP/ECN feedback scheme defined in [RFC3168] from the
AccECN TCP feedback scheme. AccECN is intended to offer a complete
replacement for Classic TCP/ECN feedback, not a fork in the design of
TCP. AccECN feedback complements TCP's loss feedback and it can
coexist alongside hosts using Classic TCP/ECN feedback. So its
applicability is intended to include all public and private IP
networks (and even any non-IP networks over which TCP is used),
whether or not any nodes on the path support ECN, of whatever
flavour.
AccECN feedback overloads the two existing ECN flags in the TCP
header and allocates the currently reserved flag (previously called
NS) in the TCP header, to be used as one three-bit counter field for
feeding back the number of packets marked as congestion experienced
(CE). Given the new definitions of these three bits, both ends have
to support the new wire protocol before it can be used. Therefore,
during the TCP handshake, the two ends use these three bits in the
TCP header to negotiate the most advanced feedback protocol that they
can both support, in a way that is backward compatible with
[RFC3168].
AccECN is solely a change to the TCP wire protocol; it covers 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, but
ultimately the motivation for accurate ECN feedback. Like Classic
ECN feedback, AccECN can be used by standard Reno or CUBIC congestion
control [RFC5681] [RFC9438] to respond to the existence of at least
one congestion notification within a round trip. Or, unlike Reno or
CUBIC, AccECN can be used to respond to the extent of congestion
notification over a round trip, as for example DCTCP does in
controlled environments [RFC8257]. For congestion response, this
specification refers to [RFC3168], as updated to allow ECN
experiments like those referred to in [RFC8311], namely: a TCP-based
Low Latency Low Loss Scalable (L4S) congestion control [RFC9330]; or
Alternative Backoff with ECN (ABE) [RFC8511].
Section 5.2 explains how AccECN is compatible with current commonly
used TCP options, and a number of current experimental modifications
to TCP, as well as SYN cookies.
It is RECOMMENDED that the AccECN protocol is implemented alongside
SACK [RFC2018]. If SACK is implemented with AccECN, DSACK [RFC2883]
MUST also be implemented.
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1.1. Document Roadmap
The following introductory section outlines the goals of AccECN
(Section 1.2). Then, terminology is defined (Section 1.3) and a
recap of existing prerequisite technology is given (Section 1.4).
Section 2 gives an informative overview of the AccECN protocol. Then
Section 3 gives the normative protocol specification, and Section 3.3
collects together requirements for proxies, offload engines and other
middleboxes. Section 4 clarifies which aspects of RFC 3168 are
updated by AccECN. Section 5 assesses the interaction of AccECN with
commonly used variants of TCP, whether standardized or not. Then
Section 6 summarizes the features and properties of AccECN.
Section 7 summarizes the protocol fields and numbers that IANA will
need to assign and Section 8 points to the aspects of the protocol
that will be of interest to the security community.
Appendix A gives pseudocode examples for the various algorithms that
AccECN uses and Appendix B explains why AccECN uses flags in the main
TCP header and quantifies the space left for future use.
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 recognizes that a perfect scheme that fully satisfies all the
requirements is unlikely and trade-offs between requirements are
likely. Section 6 presents the properties of AccECN against these
requirements and discusses the trade-offs made.
The requirements document recognizes that a protocol as ubiquitous as
TCP needs to be able to serve as-yet-unspecified requirements.
Therefore an AccECN receiver acts as a generic (mechanistic)
reflector of congestion information with the aim that in future new
sender behaviours can be deployed unilaterally (see Section 2.5).
1.3. 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
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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 (ACK=1).
Pure ACK: A TCP acknowledgement without a data payload.
Acceptable packet / segment: A packet or segment that passes the
acceptability tests in [RFC9293] and [RFC5961], or that has passed
other tests with equivalent protection.
TCP Client: The TCP stack that originates a connection (the
initiator).
TCP Server: The TCP stack that responds to a connection request (the
listener).
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.
In a mild abuse of terminology, this document sometimes refers to
'TCP packets' instead of 'TCP segments'.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
1.4. Recap of Existing ECN feedback in IP/TCP
ECN [RFC3168] uses a two bit field in the IP header. Once ECN has
been negotiated for a transport layer connection, the Data Sender for
either half-connection can set two possible codepoints (ECT(0) or
ECT(1)) in the IP header of a data packet to indicate an ECN-capable
transport (ECT). If the ECN codepoint is 0b00, 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 the ECN codepoint to 0b11, which is
termed 'Congestion Experienced' (CE), or loosely a 'congestion mark'.
Table 1 summarises these codepoints.
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+==================+================+===========================+
| IP-ECN codepoint | Codepoint name | Description |
+==================+================+===========================+
| 0b00 | Not-ECT | Not ECN-Capable Transport |
+------------------+----------------+---------------------------+
| 0b01 | ECT(1) | ECN-Capable Transport (1) |
+------------------+----------------+---------------------------+
| 0b10 | ECT(0) | ECN-Capable Transport (0) |
+------------------+----------------+---------------------------+
| 0b11 | CE | Congestion Experienced |
+------------------+----------------+---------------------------+
Table 1: The ECN Field in the IP Header
In the TCP header the first two bits in byte 14 (the TCP header flags
at bit offsets 8 and 9 labelled CWR and ECE in Figure 1) are defined
as flags for the use of Classic ECN [RFC3168]. A TCP Client
indicates that it supports Classic ECN feedback by setting (CWR,ECE)
= (1,1) in the SYN, and an ECN-enabled TCP Server confirms Classic
ECN support by setting (CWR,ECE) = (0,1) in the SYN/ACK. On
reception of a CE-marked packet at the IP layer, the Data Receiver
for that half-connection starts to set the Echo Congestion
Experienced (ECE) flag continuously in the TCP header of ACKs, which
gives the signal resilience to loss or reordering of ACKs. The Data
Sender for the same half-connection confirms that it has received at
least one ECE signal by responding with the congestion window reduced
(CWR) flag, which allows the Data Receiver to stop repeating the ECN-
Echo flag. This always leads to a full RTT of ACKs with ECE set.
Thus Classic ECN cannot feed back any additional CE markings arriving
within this RTT.
The last bit in byte 13 of the TCP header (the TCP header flag at bit
offset 7 in Figure 1) was defined as the Nonce Sum (NS) for the ECN
Nonce [RFC3540]. In the absence of widespread deployment RFC 3540
has been reclassified as historic [RFC8311] and the respective flag
has been marked as "reserved", making this TCP flag available for use
by AccECN 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: TCP header flags as defined before the Nonce Sum flag
reverted to Reserved
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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 general TCP approach, the Data Receiver of each TCP half-
connection sends AccECN feedback to the Data Sender on TCP
acknowledgements, reusing data packets of the other half-connection
whenever possible.
The AccECN protocol has had to be designed in two parts:
* an essential part that re-uses the TCP-ECN header bits for the
Data Receiver to feed back the number of packets arriving with CE
in the IP-ECN field. This provides more accuracy than Classic ECN
feedback, but limited resilience against ACK loss;
* a supplementary part using one of two new alternative AccECN TCP
Options that provide additional feedback on the number of bytes
that arrive marked with each of the three ECN codepoints in the
IP-ECN field (not just CE marks). This provides greater
resilience against ACK loss than the essential feedback, but it is
currently 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 might 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 Classic
ECN. This design choice deliberately allows AccECN peers to replace
the Classic ECN feedback protocol, rather than leaving Classic ECN
feedback intact and adding more accurate feedback separately because:
* this efficiently reuses scarce TCP header space, given TCP option
space is approaching saturation;
* a single upgrade path for the TCP protocol is preferable to a fork
in the design;
* otherwise Classic and accurate ECN feedback could give conflicting
feedback about the same segment, which could open up new security
concerns and make implementations unnecessarily complex;
* middleboxes are more likely to faithfully forward the TCP ECN
flags than newly defined areas of the TCP header.
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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
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
TCP 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 TCP
Client falls back to the same variant of ECN feedback.
An AccECN TCP Client does not send an AccECN Option on the SYN as SYN
option space is limited. The TCP Server sends an AccECN Option on
the SYN/ACK and the TCP Client sends one on the first ACK to test
whether the network path forwards these options correctly.
2.2. Feedback Mechanism
A Data Receiver maintains four counters initialized at the start of
the half-connection. Three count the number of arriving payload
bytes marked CE, ECT(1) and ECT(0) in the IP-ECN field. 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 3 later). The 24
LSBs of some or all of the byte counters can be optionally carried in
an AccECN Option. For efficient use of limited option space, two
alternative forms of AccECN Option are specified with the fields in
opposite order to each other.
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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. There is no need to acknowledge these continually repeated
counters, so the congestion window reduced (CWR) mechanism of
[RFC3168] is no longer used. Even if some ACKs are lost, the Data
Sender ought to 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 not allowed to delay sending an ACK to such an extent
that the ACE field would cycle. However ACKs received at the Data
Sender could still cycle because a whole sequence of ACKs carrying
intervening values of the field might all be lost or delayed in
transit.
The fields in an 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, provided that an AccECN Option is
available, it can be treated as a dependable feedback channel.
If an 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 [RFC8311]. Because the
3-bit ACE field is so small, when it is the only field available, the
Data Sender has to interpret it assuming the most likely wrap, but
with a degree of conservatism.
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 options are reaching the
sender and 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.
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2.4. Feedback Metrics
The CE packet counter in the ACE field and the CE byte counter in
AccECN Options 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 an AccECN Option will
provide the more accurate information needed for modern congestion
control and policing schemes, such as L4S, DCTCP or ConEx. If AccECN
Options are stripped, a simple algorithm to estimate the number of
marked bytes from the ACE field is given in Appendix A.3.
The AccECN design has been generalized so that it ought to be able to
support possible future uses of the experimental ECT(1) codepoint
other than the L4S experiment [RFC9330], such as a lower severity or
a more instant congestion signal than CE.
Feedback in bytes is provided to protect against the receiver or a
middlebox 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 (Mechanistic) Reflector
The ACE field provides feedback about CE markings in the IP-ECN field
of both data and control packets. According to [RFC3168] the Data
Sender is meant to set the IP-ECN field of control packets to Not-
ECT. However, mechanisms in certain private networks (e.g. data
centres) set control packets to be ECN 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 Section 3.2.2.3, Section 3.2.2.4 and Section 3.2.3.2 of
the present document and paragraph 2 of Section 20.2 of [RFC3168]).
The initial SYN and SYN/ACK are the most critical control packets, so
AccECN feeds back their IP-ECN fields. Although RFC 3168 prohibits
ECN-capable SYNs and SYN/ACKs, providing feedback of ECN marking on
the SYN and SYN/ACK supports future scenarios in which SYNs might be
ECN-enabled (without prejudging whether they ought to be). For
instance, [RFC8311] updates this aspect of RFC 3168 to allow
experimentation with ECN-capable TCP control packets.
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Even if the TCP Client (or Server) has set the SYN (or SYN/ACK) to
not-ECT in compliance with RFC 3168, feedback on the state of the IP-
ECN field when it arrives at the receiver could still be useful,
because middleboxes have been known to overwrite the IP-ECN field as
if it is still part of the old Type of Service (ToS) field
[Mandalari18]. For example, if a TCP Client has set the SYN to Not-
ECT, but receives feedback that the IP-ECN field on the SYN arrived
with a different codepoint, it can detect such middlebox
interference. Previously, neither end knew what IP-ECN field the
other had sent. So, if a TCP Server received ECT or CE on a SYN, it
could not know whether it was invalid because only the TCP Client
knew whether it originally marked the SYN as Not-ECT (or ECT).
Therefore, prior to AccECN, the Server's only safe course of action
in this example was to disable ECN for the connection. Instead, the
AccECN protocol allows the Server and Client to feed back the ECN
field received on the SYN and SYN/ACK to their peer, 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] has been reclassified as historic
[RFC8311], the TCP flag that was previously called NS (Nonce Sum) is
renamed as the AE (Accurate ECN) flag (the TCP header flag at bit
offset 7 in Figure 2). See the IANA Considerations in Section 7.
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
| | | A | C | E | U | A | P | R | S | F |
| Header Length | Reserved | E | W | C | R | C | S | S | Y | I |
| | | | R | E | G | K | H | T | N | N |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
Figure 2: The new definition of the TCP header flags during the
TCP handshake
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,CWR,ECE) = (1,1,1) in the initial SYN segment.
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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 AE, CWR and ECE TCP flags on the
SYN/ACK to the combination in the top block of Table 2 that feeds
back the IP-ECN field that arrived on the SYN. This applies whether
or not the Server itself supports setting the IP-ECN field on a SYN
or SYN/ACK (see Section 2.5 for rationale).
When the TCP Server returns any of the 4 combinations in the top
block of Table 2, it confirms that it supports AccECN. The TCP
Server MUST NOT set one of these 4 combination of flags on the SYN/
ACK unless the preceding SYN requested support for AccECN as above.
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 TCP Client
MUST NOT enter AccECN mode (or any feedback mode) before it has
received the first SYN/ACK.
Once in AccECN mode, a TCP Client or Server has the rights and
obligations to participate in the ECN protocol defined in
Section 3.1.5.
The procedures to follow for retransmission of SYNs or SYN/ACKs are
given in Section 3.1.4.
3.1.2. Backward Compatibility
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: Supports more Accurate ECN Feedback (the present
specification)
Nonce: Supports ECN Nonce feedback [RFC3540]
ECN: Supports 'Classic' ECN feedback [RFC3168]
No ECN: Not ECN-capable. Implicit congestion notification using
packet drop.
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+========+========+============+============+======================+
| A | B | SYN | SYN/ACK | Feedback Mode of A |
| | | A->B | B->A | |
| | | AE CWR ECE | AE CWR ECE | |
+========+========+============+============+======================+
| AccECN | AccECN | 1 1 1 | 0 1 0 | AccECN (Not-ECT SYN) |
| AccECN | AccECN | 1 1 1 | 0 1 1 | AccECN (ECT1 on SYN) |
| AccECN | AccECN | 1 1 1 | 1 0 0 | AccECN (ECT0 on SYN) |
| AccECN | AccECN | 1 1 1 | 1 1 0 | AccECN (CE on SYN) |
+--------+--------+------------+------------+----------------------+
+--------+--------+------------+------------+----------------------+
| AccECN | Nonce | 1 1 1 | 1 0 1 | (Reserved) |
| 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 |
+--------+--------+------------+------------+----------------------+
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 in Section 3.1
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. If the TCP Client has set
itself into Classic ECN feedback mode it MUST then comply with
[RFC3168].
An AccECN implementation has no need to recognize or support the
Server response labelled 'Nonce' or ECN Nonce feedback more
generally [RFC3540], which has been reclassified as historic
[RFC8311]. AccECN is compatible with alternative ECN feedback
integrity approaches to the nonce (see Section 5.3). The SYN/ACK
labelled 'Nonce' with (AE,CWR,ECE) = (1,0,1) is reserved for
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future use. A TCP Client (A) that receives such a SYN/ACK
follows the procedure for forward compatibility given in
Section 3.1.3.
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.
When an AccECN-enabled TCP Server (B) receives a SYN with
(AE,CWR,ECE) = (0,1,1) it MUST do one of the following:
* set both its half connections into the Classic ECN feedback
mode and return a SYN/ACK with (AE,CWR,ECE) = (0,0,1) as
shown. Then it MUST comply with [RFC3168].
* set both its half-connections into Not ECN mode and return a
SYN/ACK with (AE,CWR,ECE) = (0,0,0), then continue with ECN
disabled. This latter case is unlikely to be desirable, but
it is allowed as a possibility, e.g. for minimal TCP
implementations.
When an AccECN-enabled TCP Server (B) receives a SYN with
(AE,CWR,ECE) = (0,0,0) it MUST set both its half connections into
the Not ECN feedback mode, return a SYN/ACK with (AE,CWR,ECE) =
(0,0,0) as shown and continue with ECN disabled.
4. The fourth block displays a combination labelled `Broken'. Some
older TCP Server implementations incorrectly set the TCP-ECN
flags in the SYN/ACK by reflecting those in the SYN. Such broken
TCP Servers (B) cannot support ECN, so as soon as an AccECN-
capable TCP Client (A) receives such a broken SYN/ACK it MUST
fall back to Not ECN mode for both its half connections and
continue with ECN disabled.
The following additional rules do not fit the structure of the table,
but they complement it:
Simultaneous Open: An originating AccECN Host (A), having sent a SYN
with (AE,CWR,ECE) = (1,1,1), 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.
In-window SYN during TIME-WAIT: Many TCP implementations create a
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new TCP connection if they receive an in-window SYN packet during
TIME-WAIT state. When a TCP host enters TIME-WAIT or CLOSED
state, it ought to ignore any previous state about the negotiation
of AccECN for that connection and renegotiate the feedback mode
according to Table 2.
3.1.3. Forward Compatibility
If a TCP Server that implements AccECN receives a SYN with the three
TCP header flags (AE,CWR,ECE) set to any combination other than
(0,0,0), (0,1,1) or (1,1,1) and it does not have logic specific to
such a combination, the Server MUST negotiate the use of AccECN as if
the three flags had been set to (1,1,1). However, an AccECN Client
implementation MUST NOT send a SYN with any combination other than
the three listed.
If a TCP Client has sent a SYN requesting AccECN feedback with
(AE,CWR,ECE) = (1,1,1) then receives a SYN/ACK with the currently
reserved combination (AE,CWR,ECE) = (1,0,1) but it does not have
logic specific to such a combination, the Client MUST enable AccECN
mode as if the SYN/ACK confirmed that the Server supported AccECN and
as if it fed back that the IP-ECN field on the SYN had arrived
unchanged. However, an AccECN Server implementation MUST NOT send a
SYN/ACK with this combination (AE,CWR,ECE) = (1,0,1).
| For the avoidance of doubt, the behaviour described in the
| present specification applies whether or not the three
| remaining reserved TCP header flags are zero.
All these requirements ensure that future uses of all the Reserved
combinations on a SYN or SYN/ACK can rely on consistent behaviour
from the installed base of AccECN implementations. See Appendix B.3
for related discussion.
3.1.4. Multiple SYNs or SYN/ACKs
3.1.4.1. Retransmitted SYNs
If the sender of an AccECN SYN (the TCP Client) times out before
receiving the SYN/ACK, it SHOULD attempt to negotiate the use of
AccECN at least one more time by continuing to set all three TCP ECN
flags (AE,CWR,ECE) = (1,1,1) on the first retransmitted SYN (using
the usual retransmission time-outs). If this first retransmission
also fails to be acknowledged, in deployment scenarios where AccECN
path traversal might be problematic, the TCP Client SHOULD send
subsequent retransmissions of the SYN with the three TCP-ECN flags
cleared (AE,CWR,ECE) = (0,0,0). Such a retransmitted SYN MUST use
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the same ISN as the original SYN.
Retrying once before fall-back adds delay in the case where a
middlebox drops an AccECN (or ECN) SYN deliberately. However, recent
measurements [Mandalari18] imply that a drop is less likely to be due
to middlebox interference than other intermittent causes of loss,
e.g. congestion, wireless transmission loss, etc.
Implementers MAY use other fall-back strategies if they are found to
be more effective (e.g. attempting to negotiate AccECN on the SYN
only once or more than twice (most appropriate during high levels of
congestion).
Further it might make sense to also remove any other new or
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. For instance, even
if taking part in an [RFC8311] experiment that allows ECT on a SYN,
it would be advisable to try it without.
Whichever fall-back strategy is used, the TCP initiator SHOULD cache
failed connection attempts. If it does, 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 needs to be arranged to expire
so that the initiator will infrequently attempt to check whether the
problem has been resolved.
All fall-back strategies will need to follow all the normative rules
in Section 3.1.5, which concern behaviour when SYNs or SYN/ACKs
negotiating different types of feedback have been sent within the
same connection, including the possibility that they arrive out of
order. As examples, the following non-normative bullets call out
those rules from Section 3.1.5 that apply to the above fall-back
strategies:
* Once the TCP Client has sent SYNs with (AE,CWR,ECE) = (1,1,1) and
with (AE,CWR,ECE) = (0,0,0), it might eventually receive a SYN/ACK
from the Server in response to one, the other, or both and
possibly reordered;
* Such a TCP Client enters the feedback mode appropriate to the
first SYN/ACK it receives according to Table 2, and it does not
switch to a different mode, whatever other SYN/ACKs it might
receive or send;
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* If a TCP Client has entered AccECN mode but then subsequently
sends a SYN or receives a SYN/ACK with (AE,CWR,ECE) = (0,0,0), it
is still allowed to set ECT on packets for the rest of the
connection. Note that this rule is different to that of a Server
in an equivalent position (Section 3.1.5 explains).
* Having entered AccECN mode, in general a TCP Client commits to
respond to any incoming congestion feedback, whether or not it
sets ECT on outgoing packets (for rationale and some exceptions
see Section 3.2.2.3, Section 3.2.2.4);
* Having entered AccECN mode, a TCP Client commits to using AccECN
to feed back the IP-ECN field in incoming packets for the rest of
the connection, as specified in Section 3.2, even if it is not
itself setting ECT on outgoing packets.
3.1.4.2. Retransmitted SYN/ACKs
A TCP Server might send multiple SYN/ACKs indicating different
feedback modes. For instance, when falling back to sending a SYN/ACK
with (AE,CWR,ECE) = (0,0,0) after previous AccECN SYN/ACKs have timed
out (Section 3.2.3.2.2); or to acknowledge different retransmissions
of the SYN (Section 3.1.4.1).
All fall-back strategies will need to follow all the normative rules
in Section 3.1.5, which concern behaviour when SYNs or SYN/ACKs
negotiating different types of feedback are sent within the same
connection, including the possibility that they arrive out of order.
As examples, the following non-normative bullets call out those rules
from Section 3.1.5 that apply to the above fall-back strategies:
* An AccECN-capable TCP Server enters the feedback mode appropriate
to the first SYN it receives using Table 2, and it does not switch
to a different mode, whatever other SYNs it might receive and
whatever SYN/ACKs it might send;
* if a TCP Server in AccECN mode receives a SYN with (AE,CWR,ECE) =
(0,0,0), it preferably acknowledges it first using an AccECN SYN/
ACK, but it can retry using a SYN/ACK with (AE,CWR,ECE) = (0,0,0);
* If a TCP Server in AccECN mode sends multiple AccECN SYN/ACKs, it
uses the TCP-ECN flags in each SYN/ACK to feed back the IP-ECN
field on the latest SYN to have arrived;
* If a TCP Server enters AccECN mode then subsequently sends a SYN/
ACK or receives a SYN with (AE,CWR,ECE) = (0,0,0), it is
prohibited from setting ECT on any packet for the rest of the
connection;
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* Having entered AccECN mode, in general a TCP Server commits to
respond to any incoming congestion feedback, whether or not it
sets ECT on outgoing packets (for rationale and some exceptions
see Section 3.2.2.3, Section 3.2.2.4);
* Having entered AccECN mode, a TCP Server commits to using AccECN
to feed back the IP-ECN field in incoming packets for the rest of
the connection, as specified in Section 3.2, even if it is not
itself setting ECT on outgoing packets.
3.1.5. Implications of AccECN Mode
Section 3.1.1 describes the only ways that a host can enter AccECN
mode, whether as a Client or as a Server.
An implementation that supports AccECN has the rights and obligations
concerning the use of ECN defined below, which update those in
Section 6.1.1 of [RFC3168]. This section uses the following
definitions:
'During the handshake': The connection states prior to
synchronization;
'Valid SYN': A SYN that has the same port numbers and the same ISN
as the SYN that first caused the Server to open the connection.
An 'Acceptable' packet is defined in Section 1.3.
Handling SYNs or SYN/ACKs of multiple types (e.g. fall-back):
* Any implementation that supports AccECN:
- MUST NOT switch into a different feedback mode to the one it
first entered according to Table 2, no matter whether it
subsequently receives valid SYNs or Acceptable SYN/ACKs of
different types.
- SHOULD ignore the TCP-ECN flags in SYNs or SYN/ACKs that are
received after the implementation reaches the Established
state, in line with the general TCP approach [RFC9293];
Reason: Reaching established state implies that at least one
SYN and one SYN/ACK have successfully been delivered. And all
the rules for handshake fall-back are designed to work based on
those packets that successfully traverse the path, whatever
other handshake packets are lost or delayed.
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- MUST NOT send a 'Classic' ECN-setup SYN [RFC3168] with
(AE,CWR,ECE) = (0,1,1) and a SYN with (AE,CWR,ECE) = (1,1,1)
requesting AccECN feedback within the same connection;
- MUST NOT send a 'Classic' ECN-setup SYN/ACK [RFC3168] with
(AE,CWR,ECE) = (0,0,1) and a SYN/ACK agreeing to use AccECN
feedback within the same connection;
- MUST reset the connection with a RST packet, if it receives a
'Classic' ECN-setup SYN with (AE,CWR,ECE) = (0,1,1) and a SYN
requesting AccECN feedback during the same handshake;
- MUST reset the connection with a RST packet, if it receives
'Classic' ECN-setup SYN/ACK with (AE,CWR,ECE) = (0,0,1) and a
SYN/ACK agreeing to use AccECN feedback during the same
handshake;
The last four rules are necessary because, if one peer were to
negotiate the feedback mode in two different types of handshake,
it would not be possible for the other peer to know for certain
which handshake packet(s) the other end had eventually received or
in which order it received them. So, in the absence of these
rules, the two peers could end up using different ECN feedback
modes without knowing it.
* A host in AccECN mode that is feeding back the IP-ECN field on a
SYN or SYN/ACK:
- MUST feed back the IP-ECN field on the latest valid SYN or
acceptable SYN/ACK to arrive.
* A TCP Server already in AccECN mode:
- SHOULD acknowledge a valid SYN arriving with (AE,CWR,ECE) =
(0,0,0) by emitting an AccECN SYN/ACK (with the appropriate
combination of TCP-ECN flags to feed back the IP-ECN field of
this latest SYN);
- MAY acknowledge a valid SYN arriving with (AE,CWR,ECE) =
(0,0,0) by sending a SYN/ACK with (AE,CWR,ECE) = (0,0,0);
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Rationale: When a SYN arrives with (AE,CWR,ECE) = (0,0,0) at a TCP
Server that is already in AccECN mode, it implies that the TCP
Client had probably not received the previous AccECN SYN/ACK
emitted by the TCP Server. Therefore, the first bullet recommends
attempting at least one more AccECN SYN/ACK. Nonetheless, the
second bullet recognizes that the Server might eventually need to
fall back to a non-ECN SYN/ACK. In either case, the TCP Server
remains in AccECN feedback mode (according to the earlier
requirement not to switch modes).
* An AccECN-capable TCP Server already in Not ECN mode:
- SHOULD respond to any subsequent valid SYN using a SYN/ACK with
(AE,CWR,ECE) = (0,0,0), even if the SYN is offering to
negotiate Classic ECN or AccECN feedback mode;
Rationale: There would be no point in the Server offering any
type of ECN feedback, because the Client will not be using ECN.
However, there is no interoperability reason to make this rule
mandatory.
If for any reason a host is not willing to provide ECN feedback on a
particular TCP connection, it SHOULD clear the AE, CWR and ECE flags
in all SYN and/or SYN/ACK packets that it sends.
Sending ECT:
* Any implementation that supports AccECN:
- MUST NOT set ECT if it is in Not ECN feedback mode.
A Data Sender in AccECN mode:
- can set an ECT codepoint in the IP header of packets to
indicate to the network that the transport is capable and
willing to participate in ECN for this packet;
- does not have to set ECT on any packet (for instance if it has
reason to believe such a packet would be blocked);
A TCP Server in AccECN mode:
- MUST NOT set ECT on any packet for the rest of the connection,
if it has received or sent at least one valid SYN or Acceptable
SYN/ACK with (AE,CWR,ECE) = (0,0,0) during the handshake.
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This rule solely applies to a Server because, when a Server
enters AccECN mode it doesn't know for sure whether the Client
will end up in AccECN mode. But when a Client enters AccECN
mode, it can be certain that the Server is already in AccECN
feedback mode.
Congestion response:
* A host in AccECN mode:
- is obliged to respond appropriately to AccECN feedback that
indicates there were ECN marks on packets it had previously
sent, where 'appropriately' is defined in Section 6.1 of
[RFC3168] and updated by Sections 2.1 and 4.1 of [RFC8311];
- is still obliged to respond appropriately to congestion
feedback, even when it is solely sending non-ECN-capable
packets (for rationale, some examples and some exceptions see
Section 3.2.2.3, Section 3.2.2.4).
- is still obliged to respond appropriately to congestion
feedback, even if it has sent or received a SYN or SYN/ACK
packet with (AE,CWR,ECE) = (0,0,0) during the handshake;
- MUST NOT set CWR to indicate that it has received and responded
to indications of congestion.
For the avoidance of doubt, this is unlike an RFC 3168 data
sender and this does not preclude the Data Sender from setting
the bits of the ACE counter field, which includes an overloaded
use of the same bit.
Receiving ECT:
* A host in AccECN mode:
- MUST feed back the information in the IP-ECN field of incoming
packets using Accurate ECN feedback, as specified in
Section 3.2.
For the avoidance of doubt, this requirement stands even if the
AccECN host has also sent or received a SYN or SYN/ACK with
(AE,CWR,ECE) = (0,0,0). Reason: Such a SYN or SYN/ACK implies
some form of packet mangling might be present. Even if the
remote peer is not setting ECT, it could still be set
erroneously by packet mangling at the IP layer (see
Section 3.2.2.3). In such cases, the Data Sender is best
placed to decide whether ECN markings are valid, but it can
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only do that if the Data Receiver mechanistically feeds back
any ECN markings. This approach will not lead to TCP Options
being generated unnecessarily if the recommended simple scheme
in Section 3.2.3.3 is used, because no byte counters will
change if no packets are set to ECT.
- MUST NOT use reception of packets with ECT set in the IP-ECN
field as an implicit signal that the peer is ECN-capable.
Reason: ECT at the IP layer does not explicitly confirm the
peer has the correct ECN feedback logic, because the packets
could have been mangled at the IP layer.
3.2. AccECN Feedback
Each Data Receiver of each half connection maintains four counters,
r.cep, r.ceb, r.e0b and r.e1b:
* The Data Receiver MUST increment the CE packet counter (r.cep),
for every Acceptable packet that it receives with the CE code
point in the IP ECN field, including CE marked control packets and
retransmissions but excluding CE on SYN packets (SYN=1; ACK=0).
* A Data Receiver that supports sending of AccECN TCP Options MUST
increment the r.ceb, r.e0b or r.e1b byte counters by the number of
TCP payload octets in Acceptable packets marked with the CE,
ECT(0) and ECT(1) codepoint in their IP-ECN field, including any
payload octets on control packets and retransmissions, but not
including any payload octets on SYN packets (SYN=1; ACK=0).
Each Data Sender of each half connection maintains four counters,
s.cep, s.ceb, s.e0b and s.e1b intended to track the equivalent
counters at the Data Receiver.
A Data Receiver feeds back the CE packet counter using the Accurate
ECN (ACE) field, as explained in Section 3.2.2. And it optionally
feeds back all the byte counters using the AccECN TCP Option, as
specified in Section 3.2.3.
Whenever a Data Receiver feeds back the value of any counter, it MUST
report the most recent value, no matter whether it is in a pure ACK,
or an ACK piggybacked on a packet used by the other half-connection,
whether new payload data or a retransmission. Therefore the feedback
piggybacked on a retransmitted packet is unlikely to be the same as
the feedback on the original packet.
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3.2.1. Initialization of Feedback Counters
When a host first enters AccECN mode, in its role as a Data Receiver
it initializes its counters to r.cep = 5, r.e0b = r.e1b = 1 and r.ceb
= 0,
Non-zero initial values are used to support a stateless handshake
(see Section 5.1) and to be distinct from cases where the fields are
incorrectly zeroed (e.g. by middleboxes - see Section 3.2.3.2.4).
When a host enters AccECN mode, in its role as a Data Sender it
initializes its counters to s.cep = 5, s.e0b = s.e1b = 1 and s.ceb =
0.
3.2.2. 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 3.
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 3: Definition of the ACE field within bytes 13 and 14 of
the TCP Header (when AccECN has been negotiated and SYN=0).
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 unconditionally; it merely overloads them with
another name and definition once an AccECN connection has been
established.
With one exception (Section 3.2.2.1), a host with both of its half-
connections in AccECN mode MUST interpret the AE, CWR and ECE flags
as the 3-bit ACE counter on a segment with the SYN flag cleared
(SYN=0). On such a packet, a Data Receiver MUST encode the three
least significant bits of its r.cep counter into the ACE field that
it feeds back to the Data Sender. The least significant bit is at
bit offset 9 in Figure 3. 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.
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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).
3.2.2.1. ACE Field on the ACK of the SYN/ACK
A TCP Client (A) in AccECN mode MUST feed back which of the 4
possible values of the IP-ECN field was on the SYN/ACK by writing it
into the ACE field of a pure ACK with no SACK blocks using the binary
encoding in Table 3 (which is the same as that used on the SYN/ACK in
Table 2). This shall be called the handshake encoding of the ACE
field, and it is the only exception to the rule that the ACE field
carries the 3 least significant bits of the r.cep counter on packets
with SYN=0.
Normally, a TCP Client acknowledges a SYN/ACK with an ACK that
satisfies the above conditions anyway (SYN=0, no data, no SACK
blocks). If an AccECN TCP Client intends to acknowledge the SYN/ACK
with a packet that does not satisfy these conditions (e.g. it has
data to include on the ACK), it SHOULD first send a pure ACK that
does satisfy these conditions (see Section 5.2), so that it can feed
back which of the four values of the IP-ECN field arrived on the SYN/
ACK. A valid exception to this "SHOULD" would be where the
implementation will only be used in an environment where mangling of
the ECN field is unlikely.
The TCP Client MUST also use the handshake encoding for the pure ACK
of any retransmitted SYN/ACK that confirms that the TCP Server
supports AccECN. The procedure for the TCP Server to follow if the
final ACK of the handshake does not arrive before its retransmission
timer expires is given in Section 3.1.4.2.
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+==================+================+=====================+
| IP-ECN codepoint | ACE on pure | r.cep of TCP Client |
| on SYN/ACK | ACK of SYN/ACK | in AccECN mode |
+==================+================+=====================+
| Not-ECT | 0b010 | 5 |
+------------------+----------------+---------------------+
| ECT(1) | 0b011 | 5 |
+------------------+----------------+---------------------+
| ECT(0) | 0b100 | 5 |
+------------------+----------------+---------------------+
| CE | 0b110 | 6 |
+------------------+----------------+---------------------+
Table 3: The encoding of the ACE field in the ACK of
the SYN-ACK to reflect the SYN-ACK's IP-ECN field
When an AccECN Server in SYN-RCVD state receives a pure ACK with
SYN=0 and no SACK blocks, instead of treating the ACE field as a
counter, it MUST infer the meaning of each possible value of the ACE
field from Table 4, which also shows the value that an AccECN Server
MUST set s.cep to as a result.
Given this encoding of the ACE field on the ACK of a SYN/ACK is
exceptional, an AccECN Server using large receive offload (LRO) might
prefer to disable LRO until such an ACK has transitioned it out of
SYN-RCVD state.
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+============+==========================+=====================+
| ACE on ACK | IP-ECN codepoint on SYN/ | s.cep of TCP Server |
| of SYN/ACK | ACK inferred by Server | in AccECN mode |
+============+==========================+=====================+
| 0b000 | {Notes 1, 3} | Disable s.cep |
+------------+--------------------------+---------------------+
| 0b001 | {Notes 2, 3} | 5 |
+------------+--------------------------+---------------------+
| 0b010 | Not-ECT | 5 |
+------------+--------------------------+---------------------+
| 0b011 | ECT(1) | 5 |
+------------+--------------------------+---------------------+
| 0b100 | ECT(0) | 5 |
+------------+--------------------------+---------------------+
| 0b101 | Currently Unused {Note | 5 |
| | 2} | |
+------------+--------------------------+---------------------+
| 0b110 | CE | 6 |
+------------+--------------------------+---------------------+
| 0b111 | Currently Unused {Note | 5 |
| | 2} | |
+------------+--------------------------+---------------------+
Table 4: Meaning of the ACE field on the ACK of the SYN/ACK
{Note 1}: If the Server is in AccECN mode and in SYN-RCVD state, and
if it receives a value of zero on a pure ACK with SYN=0 and no SACK
blocks, for the rest of the connection the Server MUST NOT set ECT on
outgoing packets and MUST NOT respond to AccECN feedback.
Nonetheless, as a Data Receiver it MUST NOT disable AccECN feedback.
Any of the circumstances below could cause a value of zero but,
whatever the cause, the actions above would be the appropriate
response:
* The TCP Client has somehow entered No ECN feedback mode (most
likely if the Server received a SYN or sent a SYN/ACK with
(AE,CWR,ECE) = (0,0,0) after entering AccECN mode, but possible
even if it didn't);
* The TCP Client genuinely might be in AccECN mode, but its count of
received CE marks might have caused the ACE field to wrap to zero.
This is highly unlikely, but not impossible because the Server
might have already sent multiple packets while still in SYN-RCVD
state, e.g. using TFO (see Section 5.2) and some might have been
CE-marked. Then ACE on the first ACK seen by the Server might be
zero, due to previous ACKs experiencing an unfortunate pattern of
loss or delay.
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* Some form of non-compliance at the TCP Client or on the path (see
Section 3.2.2.4).
{Note 2}: If the Server is in AccECN mode, these values are Currently
Unused but the AccECN Server's behaviour is still defined for forward
compatibility. Then the designer of a future protocol can know for
certain what AccECN Servers will do with these codepoints.
{Note 3}: In the case where a Server that implements AccECN is also
using a stateless handshake (termed a SYN cookie) it will not
remember whether it entered AccECN mode. The values 0b000 or 0b001
will remind it that it did not enter AccECN mode, because AccECN does
not use them (see Section 5.1 for details). If a Server that uses a
stateless handshake and implements AccECN receives either of these
two values in the ACK, its action is implementation-dependent and
outside the scope of this spec. It will certainly not take the
action in the third column because, after it receives either of these
values, it is not in AccECN mode. I.e., it will not disable ECN (at
least not just because ACE is 0b000) and it will not set s.cep.
3.2.2.2. Encoding and Decoding Feedback in the ACE Field
Whenever the Data Receiver sends an ACK with SYN=0 (with or without
data), unless the handshake encoding in Section 3.2.2.1 applies, the
Data Receiver MUST encode the least significant 3 bits of its r.cep
counter into the ACE field (see Appendix A.2).
Whenever the Data Sender receives an ACK with SYN=0 (with or without
data), it first checks whether it has already been superseded
(defined in Appendix A.1) by another ACK in which case it ignores the
ECN feedback. If the ACK has not been superseded, and if the special
handshake encoding in Section 3.2.2.1 does not apply, the Data Sender
decodes the ACE field as follows (see Appendix A.2 for examples).
* It takes the least significant 3 bits of its local s.cep counter
and subtracts them from the incoming ACE counter to work out the
minimum positive increment it could apply to s.cep (assuming the
ACE field only wrapped at most once).
* It then follows the safety procedures in Section 3.2.2.5.2 to
calculate or estimate how many packets the ACK could have
acknowledged under the prevailing conditions to determine whether
the ACE field might have wrapped more than once.
The encode/decode procedures during the three-way handshake are
exceptions to the general rules given so far, so they are spelled out
step by step below for clarity:
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* If a TCP Server in AccECN mode receives a CE mark in the IP-ECN
field of a SYN (SYN=1, ACK=0), it MUST NOT increment r.cep (it
remains at its initial value of 5).
Reason: It would be redundant for the Server to include CE-marked
SYNs in its r.cep counter, because it already reliably delivers
feedback of any CE marking using the encoding in the top block of
Table 2 in the SYN/ACK. This also ensures that, when the Server
starts using the ACE field, it has not unnecessarily consumed more
than one initial value, given they can be used to negotiate
variants of the AccECN protocol (see Appendix B.3).
* If a TCP Client in AccECN mode receives CE feedback in the TCP
flags of a SYN/ACK, it MUST NOT increment s.cep (it remains at its
initial value of 5), so that it stays in step with r.cep on the
Server. Nonetheless, the TCP Client still triggers the congestion
control actions necessary to respond to the CE feedback.
* If a TCP Client in AccECN mode receives a CE mark in the IP-ECN
field of a SYN/ACK, it MUST increment r.cep, but no more than once
no matter how many CE-marked SYN/ACKs it receives
(i.e. incremented from 5 to 6, but no further).
Reason: Incrementing r.cep ensures the Client will eventually
deliver any CE marking to the Server reliably when it starts using
the ACE field. Even though the Client also feeds back any CE
marking on the ACK of the SYN/ACK using the encoding in Table 3,
this ACK is not delivered reliably, so it can be considered as a
timely notification that is redundant but unreliable. The Client
does not increment r.cep more than once, because the Server can
only increment s.cep once (see next bullet). Also, this limits
the unnecessarily consumed initial values of the ACE field to two.
* If a TCP Server in AccECN mode and in SYN-RCVD state receives CE
feedback in the TCP flags of a pure ACK with no SACK blocks, it
MUST increment s.cep (from 5 to 6). The TCP Server then triggers
the congestion control actions necessary to respond to the CE
feedback.
Reasoning: The TCP Server can only increment s.cep once, because
the first ACK it receives will cause it to transition out of SYN-
RCVD state. The Server's congestion response would be no
different even if it could receive feedback of more than one CE-
marked SYN/ACK.
Once the TCP Server transitions to ESTABLISHED state, it might
later receive other pure ACK(s) with the handshake encoding in the
ACE field. A Server MAY implement a test for such a case, but it
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is not required. Therefore, once in the ESTABLISHED state, it
will be sufficient for the Server to consider the ACE field to be
encoded as the normal ACE counter on all packets with SYN=0.
Reasoning: Such ACKs will be quite unusual, e.g. a SYN/ACK (or ACK
of the SYN/ACK) that is delayed for longer than the Server's
retransmission timeout; or packet duplication by the network. And
the impact of any error in the feedback on such ACKs will only be
temporary.
3.2.2.3. Testing for Mangling of the IP/ECN Field
The value of the TCP-ECN flags on the SYN/ACK indicates the value of
the IP-ECN field when the SYN arrived at the Server. The TCP Client
can compare this with how it originally set the IP-ECN field on the
SYN. If this comparison implies an invalid transition (defined
below) of the IP-ECN field, for the remainder of the half-connection
the Client is advised to send non-ECN-capable packets, but it still
ought to respond to any feedback of CE markings (explained below).
However, the TCP Client MUST remain in the AccECN feedback mode and
it MUST continue to feed back any ECN markings on arriving packets
(in its role as Data Receiver).
The value of the ACE field on the last ACK of the 3WHS indicates the
value of the IP-ECN field when the SYN/ACK arrived at the TCP Client.
The Server can compare this with how it originally set the IP-ECN
field on the SYN/ACK. If this comparison implies an invalid
transition of the IP-ECN field, for the remainder of the half-
connection the Server is advised to send non-ECN-capable packets, but
it still ought to respond to any feedback of CE markings (explained
below). However, the Server MUST remain in the AccECN feedback mode
and it MUST continue to feed back any ECN markings on arriving
packets (in its role as Data Receiver).
If a Data Sender in AccECN mode starts sending non-ECN-capable
packets because it has detected mangling, it is still advised to
respond to CE feedback. Reason: any CE-marking arriving at the Data
Receiver could be due to something early in the path mangling the
non-ECN-capable IP-ECN field into an ECN-capable codepoint and then,
later in the path, a network bottleneck might be applying CE-markings
to indicate genuine congestion. This argument applies whether the
handshake packet originally sent by the TCP Client or Server was non-
ECN-capable or ECN-capable because, in either case, an unsafe
transition could imply that non-ECN-capable packets later in the
connection might get mangled.
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Once a Data Sender has entered AccECN mode it is advised to check
whether it is receiving continuous feedback of CE. Specifying
exactly how to do this is beyond the scope of the present
specification, but the sender might check whether the feedback for
every packet it sends for the first three or four rounds indicates
CE-marking. If continuous CE-marking is detected, for the remainder
of the half-connection, the Data Sender ought to send non-ECN-capable
packets and it is advised not to respond to any feedback of CE
markings. The Data Sender might occasionally test whether it can
resume sending ECN-capable packets.
The above advice on switching to sending non-ECN-capable packets but
still responding to CE-markings unless they become continuous is not
stated normatively (in capitals), because the best strategy might
depend on experience of the most likely types of mangling, which can
only be known at the time of deployment.
As always, once a host has entered AccECN mode, it follows the
general mandatory requirements (Section 3.1.5) to remain in the same
feedback mode and to continue feeding back any ECN markings on
arriving packets using AccECN feedback. This follows the general
approach where an AccECN Data Receiver mechanistically reflects
whatever it receives (Section 2.5).
The ACK of the SYN/ACK is not reliably delivered (nonetheless, the
count of CE marks is still eventually delivered reliably). If this
ACK does not arrive, the Server is advised to continue to send ECN-
capable packets without having tested for mangling of the IP-ECN
field on the SYN/ACK.
All the fall-back behaviours in this section are necessary in case
mangling of the IP-ECN field is asymmetric, which is currently common
over some mobile networks [Mandalari18]. Then one end might see no
unsafe transition and continue sending ECN-capable packets, while the
other end sees an unsafe transition and stops sending ECN-capable
packets.
Invalid transitions of the IP-ECN field are defined in section 18 of
[RFC3168] and repeated here for convenience:
* the not-ECT codepoint changes;
* either ECT codepoint transitions to not-ECT;
* the CE codepoint changes.
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RFC 3168 says that a router that changes ECT to not-ECT is invalid
but safe. However, from a host's viewpoint, this transition is
unsafe because it could be the result of two transitions at different
routers on the path: ECT to CE (safe) then CE to not-ECT (unsafe).
This scenario could well happen where an ECN-enabled home router
congests its upstream mobile broadband bottleneck link, then the
ingress to the mobile network clears the ECN field [Mandalari18].
3.2.2.4. Testing for Zeroing of the ACE Field
Section 3.2.2 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 counter ought to be non-zero.
This section does not concern the case where the ACE field is zero
when the handshake encoding has been used on the ACK of the SYN/ACK
under the carefully worded conditions in Section 3.2.2.1.
If AccECN has been successfully negotiated, the Data Sender MAY check
the value of the ACE counter in the first feedback packet (with or
without data) that arrives after the 3-way handshake. If the value
of this ACE field is found to be zero (0b000), for the remainder of
the half-connection the Data Sender ought to send non-ECN-capable
packets and it is advised not to respond to any feedback of CE
markings. Reason: the symptoms imply any or all of the following: i)
the remote peer has somehow entered Not ECN feedback mode; ii) a
broken remote TCP implementation; iii) potential mangling of the ECN
fields in the TCP headers (although unlikely given they clearly
survived during the handshake). This advice is not stated
normatively (in capitals), because the best strategy might depend on
experience of the most likely scenarios, which can only be known at
the time of deployment.
Note that a host in AccECN mode MUST continue to provide accurate ECN
feedback to its peer, even if it is no longer sending ECT itself over
the other half connection.
If reordering occurs, "the first packet ... that arrives" will not
necessarily be the same as the first packet in sequence order. The
test has been specified loosely like this to simplify implementation,
and because it would not have been any more precise to have specified
the first packet in sequence order, which would not necessarily be
the first ACE counter that the Data Receiver fed back anyway, given
it might have been a retransmission.
The possibility of re-ordering means that there is a small chance
that the ACE field on the first packet to arrive is genuinely zero
(without middlebox interference). This would cause a host to
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unnecessarily disable ECN for a half connection. Therefore, in
environments where there is no evidence of the ACE field being
zeroed, implementations can skip this test.
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.
3.2.2.5. 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 or thinned out, the 3-bit counter in the ACE
field might have cycled between two ACKs arriving at the Data Sender.
The following safety procedures minimize this ambiguity.
3.2.2.5.1. Packet Receiver Safety Procedures
The following rules define when the receiver of a packet in AccECN
mode emits an ACK:
Change-Triggered ACKs: An AccECN Data Receiver SHOULD emit an ACK
whenever a data packet marked CE arrives after the previous packet
was not CE.
Even though this rule is stated as a "SHOULD", it is important for
a transition to trigger an ACK if at all possible, The only valid
exception to this rule is given below these bullets.
For the avoidance of doubt, this rule is deliberately worded to
apply solely when _data_ packets arrive, but the comparison with
the previous packet includes any packet, not just data packets.
Increment-Triggered ACKs: An AccECN receiver of a packet MUST emit
an ACK if 'n' CE marks have arrived since the previous ACK. If
there is unacknowledged data at the receiver, 'n' SHOULD be 2. If
there is no unacknowledged data at the receiver, 'n' SHOULD be 3
and MUST be no less than 3. In either case, 'n' MUST be no
greater than 7.
The above rules for when to send an ACK are designed to be
complemented by those in Section 3.2.3.3, which concern whether an
AccECN TCP Option ought to be included on ACKs.
If the arrivals of a number of data packets are all processed as one
event, e.g. using large receive offload (LRO) or generic receive
offload (GRO), both the above rules SHOULD be interpreted as
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requiring multiple ACKs to be emitted back-to-back (for each
transition and for each repetition by 'n' CE marks). If this is
problematic for high performance, either rule can be interpreted as
requiring just a single ACK at the end of the whole receive event.
Even if a number of data packets do not arrive as one event, the
'Change-Triggered ACKs' rule could sometimes cause the ACK rate to be
problematic for high performance (although high performance protocols
such as DCTCP already successfully use change-triggered ACKs). The
rationale for change-triggered ACKs is so that the Data Sender can
rely on them to detect queue growth as soon as possible, particularly
at the start of a flow. The approach can lead to some additional
ACKs but it feeds back the timing and the order in which ECN marks
are received with minimal additional complexity. If CE marks are
infrequent, as is the case for most AQMs at the time of writing, or
there are multiple marks in a row, the additional load will be low.
However, marking patterns with numerous non-contiguous CE marks could
increase the load significantly. One possible compromise would be
for the receiver to heuristically detect whether the sender is in
slow-start, then to implement change-triggered ACKs while the sender
is in slow-start, and offload otherwise.
In a scenario where both endpoints support AccECN, if host B has
chosen to use ECN-capable pure ACKs (as allowed in [RFC8311]
experiments) and enough of these ACKs become CE-marked, then the
'Increment-Triggered ACKs' rule ensures that its peer (host A) gives
B sufficient feedback about this congestion on the ACKs from B to A.
Normally, for instance in a unidirectional data scenario from host A
to B, the Data Sender (A) can piggyback that feedback on its data.
But if A stops sending data, the second part of the 'Increment-
Triggered ACKs' rule requires A to emit a pure ACK for at least every
third CE-marked incoming ACK over the subsequent round trip.
Although TCP normally only ACKs data segments, in this case the
increment-triggered ACK rule makes it mandatory for A to emit ACKs of
ACKs. This is justifiable because the ACKs in this case are ECN-
capable and so, even though the ACKs of these ACKs do not acknowledge
new data, they feed back new congestion state (useful in case B
starts sending). The minimum of 3 for 'n' in this case ensures that,
even if A also uses ECN-capable pure ACKs, and even if there is
pathological congestion in both directions, any resulting ping-pong
of ACKs will be rapidly damped.
In the above bidirectional scenario, incoming ACKs of ACKs could be
mistaken for duplicate ACKs. But ACKs of ACKs can be distinguished
from duplicate ACKs because they do not contain any SACK blocks even
when SACK has been negotiated. It is outside the scope of this
AccECN spec to normatively specify this additional test for DupACKs,
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because ACKs of ACKs can only arise if the original ACKs are ECN-
capable. Instead any spec that allows ECN-capable pure ACKs MUST
make sending ACKs of ACKs conditional on measures to distinguish ACKs
of ACKs from DupACKs (see for example
[I-D.ietf-tcpm-generalized-ecn]). All that is necessary here is to
require that these ACKs of ACKs MUST NOT contain any SACK blocks
(which would normally not happen anyway).
3.2.2.5.2. Data Sender Safety Procedures
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, it SHOULD deem whether it cycled by taking the safest
likely case under the prevailing conditions. 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 implementation MAY use an
alternative algorithm as long as it satisfies the requirements in
this subsection.
If missing acknowledgement numbers arrive later (reordering) and
prove that the counter did not cycle, the Data Sender MAY attempt to
neutralize the effect of any action it took based on a conservative
assumption that it later found to be incorrect.
The Data Sender can estimate how many packets (of any marking) an ACK
acknowledges. If the ACE counter on an ACK seems to imply that the
minimum number of newly CE-marked packets is greater than the number
of newly acknowledged packets, the Data Sender SHOULD consider the
ACE counter to be correct (and its count of control packets to be
incomplete), unless it can be sure that it is counting all control
packets correctly.
3.2.3. The AccECN Option
Two alternative AccECN Options are defined as shown in Figure 4. 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 = 172 | Length = 11 | EE0B field |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| EE0B (cont'd) | ECEB field |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| EE1B field | Order 0
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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 = 174 | Length = 11 | EE1B field |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| EE1B (cont'd) | ECEB field |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| EE0B field | Order 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: The Two Alternative AccECN TCP Options
Figure 4 shows two option field orders; order 0 and order 1. They
both consists of three 24-bit fields. Order 0 provides the 24 least
significant bits of the r.e0b, r.ceb and r.e1b counters,
respectively. Order 1 provides the same fields, but in the opposite
order. On each packet, the Data Receiver can use whichever order is
more efficient. In either case, the bytes within the fields are in
network byte order (big-endian).
When a Data Receiver sends an AccECN Option, it MUST set the Kind
field to 172 if using Order 0, or to 174 if using Order 1. These two
new TCP Option Kinds are registered in Section 7 and called
respectively AccECN0 and AccECN1.
Note that there is no field to feed back 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.4.
Whenever a Data Receiver sends an AccECN Option, the rules in
Section 3.2.3.3 allow it to omit unchanged fields from the tail of
the option, to help cope with option space limitations, 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:
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+========+==================+==================+
| Length | Order 0 | Order 1 |
+========+==================+==================+
| 11 | EE0B, ECEB, EE1B | EE1B, ECEB, EE0B |
+--------+------------------+------------------+
| 8 | EE0B, ECEB | EE1B, ECEB |
+--------+------------------+------------------+
| 5 | EE0B | EE1B |
+--------+------------------+------------------+
| 2 | (empty) | (empty) |
+--------+------------------+------------------+
Table 5: Fields included in AccECN TCP
Options of each length and order
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.
All implementations of a Data Sender that read any AccECN Option MUST
be able to read AccECN Options of any of the above lengths. For
forward compatibility, if the AccECN Option is of any other length,
implementations MUST use those whole 3-octet fields that fit within
the length and ignore the remainder of the option, treating it as
padding.
AccECN Options have to be optional to implement, because both sender
and receiver have to be able to cope without options anyway - in
cases where they do not traverse a network path. It is RECOMMENDED
to implement both sending and receiving of AccECN Options. Support
for AccECN Options is particularly valuable over paths that introduce
a high degree of ACK filtering, where the 3-bit ACE counter alone
might sometimes be insufficient, when it is ambiguous whether it has
wrapped. If sending of AccECN Options is implemented, the fall-backs
described in this document will need to be implemented as well
(unless solely for a controlled environment where path traversal is
not considered a problem). Even if a developer does not implement
logic to understand received AccECN Options, it is RECOMMENDED that
they implement logic to send AccECN Options. Otherwise, those remote
peers that implement the receiving logic will still be excluded from
congestion feedback that is robust against the increasingly
aggressive ACK filtering in the Internet. The logic to send AccECN
Options is the simpler to implement of the two sides.
If a Data Receiver intends to send an AccECN Option at any time
during the rest of the connection it is RECOMMENDED to also test path
traversal of the AccECN Option as specified in Section 3.2.3.2.
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3.2.3.1. Encoding and Decoding Feedback in the AccECN Option Fields
Whenever the Data Receiver includes any of the counter fields (ECEB,
EE0B, EE1B) in an AccECN Option, it MUST encode the 24 least
significant bits of the current value of the associated counter into
the field (respectively r.ceb, r.e0b, r.e1b).
Whenever the Data Sender receives an ACK carrying an AccECN Option,
it first checks whether the ACK has already been superseded by
another ACK in which case it ignores the ECN feedback. If the ACK
has not been superseded, the Data Sender normally decodes the fields
in the AccECN Option as follows. For each field, it takes the least
significant 24 bits of its associated local counter (s.ceb, s.e0b or
s.e1b) and subtracts them from the counter in the associated field of
the incoming AccECN Option (respectively ECEB, EE0B, EE1B), to work
out the minimum positive increment it could apply to s.ceb, s.e0b or
s.e1b (assuming the field in the option only wrapped at most once).
Appendix A.1 gives an example algorithm for the Data Receiver to
encode its byte counters into an AccECN Option, and for the Data
Sender to decode the AccECN Option fields into its byte counters.
Note that, as specified in Section 3.2, any data on the SYN (SYN=1,
ACK=0) is not included in any of the byte counters held locally for
each ECN marking nor in an AccECN Option on the wire.
3.2.3.2. Path Traversal of the AccECN Option
3.2.3.2.1. Testing the AccECN Option during the Handshake
The TCP Client MUST NOT include an AccECN TCP Option on the SYN. If
there is somehow an AccECN Option on a SYN, it MUST be ignored when
forwarded or received.
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
include an AccECN TCP Option on 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).
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A host MAY omit an AccECN Option in any of the above three cases due
to insufficient option space or 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.
3.2.3.2.2. Testing for Loss of Packets Carrying the AccECN Option
If the TCP Server has not received an ACK to acknowledge its SYN/ACK
after the normal TCP timeout or it receives a second SYN with a
request for AccECN support, then either the SYN/ACK might just have
been lost, e.g. due to congestion, or a middlebox might be blocking
AccECN Options. To expedite connection setup in deployment scenarios
where AccECN path traversal might be problematic, the TCP Server
SHOULD retransmit the SYN/ACK, but with no AccECN Option. If this
retransmission times out, to expedite connection setup, the TCP
Server SHOULD retransmit the SYN/ACK with (AE,CWR,ECE) = (0,0,0) and
no AccECN Option, but it remains in AccECN feedback mode (per
Section 3.1.5).
| Note that a retransmitted AccECN SYN/ACK will not necessarily
| have the same TCP-ECN flags as the original SYN/ACK, because it
| feeds back the IP-ECN field of the latest SYN to have arrived
| (by the rule in Section 3.1.5).
The above fall-back approach limits any interference by middleboxes
that might drop packets with unknown options, even though it is more
likely that SYN/ACK loss is due to congestion. The TCP Server MAY
try to send another packet with an AccECN Option at a later point
during the connection but it ought to monitor if that packet got lost
as well, in which case it SHOULD disable the sending of AccECN
Options for this half-connection.
Implementers MAY use other fall-back strategies if they are found to
be more effective (e.g. retrying an AccECN Option for a second time
before fall-back - most appropriate during high levels of
congestion). However, other fall-back strategies will need to follow
all the rules in Section 3.1.5, which concern behaviour when SYNs or
SYN/ACKs negotiating different types of feedback have been sent
within the same connection.
Further it might make sense to also remove any other new or
experimental fields or options on the SYN/ACK, although the required
behaviour will depend on the specification of the other option(s) and
on any attempt to co-ordinate fall-back between different modules of
the stack.
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If the TCP Client detects that the first data segment it sent with an
AccECN Option was lost, in deployment scenarios where AccECN path
traversal might be problematic, 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 an AccECN Option before fall-back, and/or caching
whether AccECN Options are blocked for subsequent connections.
[RFC9040] further discusses caching of TCP parameters and status
information.
If a middlebox is dropping packets with options it does not
recognize, a host that is sending little or no data but mostly pure
ACKs will not inherently detect such losses. Such a host MAY detect
loss of ACKs carrying the AccECN Option by detecting whether the
acknowledged data always reappears as a retransmission. In such
cases, the host SHOULD disable the sending of the AccECN Option for
this half-connection.
If a host falls back to not sending AccECN Options, it will continue
to process any incoming AccECN Options as normal.
Either host MAY include AccECN Options in a subsequent segment or
segments to retest whether AccECN Options can traverse the path.
Similarly, an AccECN endpoint 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.
3.2.3.2.3. Testing for Absence of the AccECN Option
If the TCP Client has successfully negotiated AccECN but does not
receive an AccECN Option on the SYN/ACK (e.g. because is has been
stripped by a middlebox or not sent by the Server), the Client
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.
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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 3.2.2.5. However, it cannot make any
assumption about support of outgoing AccECN Options on the other half
connection, so it SHOULD continue to send AccECN Options itself
(unless it has established that sending AccECN Options is causing
packets to be blocked as in Section 3.2.3.2.2).
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 AccECN Options are no
longer blocked on the respective path, and the AccECN endpoint MAY
switch out of the mode that assumes AccECN Options are not available
for this half connection.
3.2.3.2.4. Test for Zeroing of the AccECN Option
For a related test for invalid initialization of the ACE field, see
Section 3.2.2.4
Section 3.2.1 required the Data Receiver to initialize the r.e0b and
r.e1b counters to a non-zero value. Therefore, in either direction
the initial value of the EE0B field or EE1B field in an AccECN Option
(if one exists) ought to be non-zero. If AccECN has been negotiated:
* the TCP Server MAY check that the initial value of the EE0B field
or the EE1B field is non-zero in the first segment that
acknowledges sequence space that at least covers the ISN plus 1.
If it runs a test and either initial value is zero, the Server
will switch into a mode that ignores AccECN Options for this half
connection.
* the TCP Client MAY check the initial value of the EE0B field or
the EE1B field is non-zero on the SYN/ACK. If it runs a test and
either initial value is zero, the Client will switch into a mode
that ignores AccECN Options for this half connection.
While a host is in the mode that ignores AccECN Options it MUST adopt
the conservative interpretation of the ACE field discussed in
Section 3.2.2.5.
Note that the Data Sender MUST NOT test whether the arriving byte
counters in an 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
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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.
3.2.3.2.5. Consistency between AccECN Feedback Fields
When AccECN Options are available they ought to provide more
unambiguous feedback. However, they supplement but do not replace
the ACE field. An endpoint using AccECN feedback MUST always
reconcile the information provided in the ACE field with that in any
AccECN Option, so that the state of the ACE-related packet counter
can be relied on if future feedback does not carry an AccECN Option.
If an AccECN Option is present, the s.cep counter might increase more
than expected from the increase of the s.ceb counter (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 an AccECN Option but not the
ACE field. However, the Data Sender has to assume that the integrity
of AccECN Options is sound, based on the above test of the well-known
initial values and optionally other integrity tests (Section 5.3).
If either endpoint 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), and if there is no explanation other than
an invalid protocol transition due to some form of feedback mangling,
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.
3.2.3.3. Usage of the AccECN TCP Option
If a Data Receiver in AccECN mode intends to use AccECN TCP Options
to provide feedback, the rules below determine when it includes an
AccECN TCP Option, and which fields to include, given other options
might be competing for limited option space:
Importance of Congestion Control: AccECN is for congestion control,
which implementations SHOULD generally prioritize over other TCP
options when there is insufficient space for all the options in
use.
If SACK has been negotiated [RFC2018], and the smallest
recommended AccECN Option would leave insufficient space for two
SACK blocks on a particular ACK, the Data Receiver MUST give
precedence to the SACK option (total 18 octets), because loss
feedback is more critical.
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Recommended Simple Scheme: The Data Receiver SHOULD include an
AccECN TCP Option on every scheduled ACK if any byte counter has
incremented since the last ACK. Whenever possible, it SHOULD
include a field for every byte counter that has changed at some
time during the connection (see examples later).
A scheduled ACK means an ACK that the Data Receiver would send by
its regular delayed ACK rules. Recall that Section 1.3 defines an
'ACK' as either with data payload or without. But the above rule
is worded so that, in the common case when most of the data is
from a Server to a Client, the Server only includes an AccECN TCP
Option while it is acknowledging data from the Client.
When available TCP option space is limited on particular packets, the
recommended scheme will need to include compromises. To guide the
implementer the rules below are ranked in order of importance, but
the final decision has to be implementation-dependent, because
tradeoffs will alter as new TCP options are defined and new use-cases
arise.
Necessary Option Length: The Data Receiver MUST only include an
AccECN TCP Option on a packet if it includes all the counter(s)
that have incremented since the previous AccECN Option. It MUST
only truncate unchanged fields from the right-hand tail of the
option to preserve the order of the remaining fields (see
Section 3.2.3);
Change-Triggered AccECN TCP Options: If an arriving packet
increments a different byte counter to that incremented by the
previous packet, the Data Receiver SHOULD feed it back in an
AccECN Option on the next scheduled ACK.
For the avoidance of doubt, this rule does not concern the arrival
of control packets with no payload, because they cannot alter any
byte counters.
Continual Repetition: Otherwise, if arriving packets continue to
increment the same byte counter:
* the Data Receiver SHOULD include a counter that has continued
to increment on the next scheduled ACK following a change-
triggered AccECN TCP Option;
* while the same counter continues to increment, it SHOULD
include the counter every n ACKs as consistently as possible,
where n can be chosen by the implementer;
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* It SHOULD always include an AccECN Option if the r.ceb counter
is incrementing and it MAY include an AccECN Option if r.ec0b
or r.ec1b is incrementing
* It SHOULD, include each counter at least once for every 2^22
bytes incremented to prevent overflow during continual
repetition.
The above rules complement those in Section 3.2.2.5, which determine
when to generate an ACK irrespective of whether an AccECN TCP Option
is to be included.
The recommended scheme is intended as a simple way to ensure that all
the relevant byte counters will be carried on any ACK that reaches
the Data Sender, no matter how many pure ACKs are filtered or
coalesced along the network path, and without consuming the space
available for payload data with counter field(s) that have never
changed.
As an example of the recommended scheme, if ECT(0) is the only
codepoint that has ever arrived in the IP-ECN field, the Data
Receiver will feed back an AccECN0 TCP Option with only the EE0B
field on every packet that acknowledges new data. However, as soon
as even one CE-marked packet arrives, on every packet that
acknowledges new data it will start to include an option with two
fields, EE0B and ECEB. As a second example, if the first packet to
arrive happens to be CE-marked, the Data Receiver will have to
arbitrarily choose whether to precede the ECEB field with an EE0B
field or an EE1B field. If it chooses, say, EEB0 but it turns out
never to receive ECT(0), it can start sending EE1B and ECEB instead -
it does not have to include the EE0B field if the r.e0b counter has
never changed during the connection.
With the recommended scheme, if the data sending direction switches
during a connection, there can be cases where the AccECN TCP Option
that is meant to feed back the counter values at the end of a volley
in one direction never reaches the other peer, due to packet loss.
ACE feedback ought to be sufficient to fill this gap, given accurate
feedback becomes moot after data transmission has paused.
Appendix A.3 gives an example algorithm to estimate the number of
marked bytes from the ACE field alone, if AccECN Options are not
available.
If a host has determined that segments with AccECN Options always
seem to be discarded somewhere along the path, it is no longer
obliged to follow any of the rules in this section.
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3.3. AccECN Compliance Requirements for TCP Proxies, Offload Engines
and other Middleboxes
Given AccECN alters the TCP protocol on the wire, this section
specifies new requirements on certain networking equipment that
forwards TCP and inspects TCP header information.
3.3.1. Requirements for TCP Proxies
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.
3.3.2. Requirements for Transparent Middleboxes and TCP Normalizers
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 `normalize' the TCP wire protocol by checking that all
values in header fields comply with a rather narrow interpretation of
the TCP specifications that is also not always up to date.
A middlebox that is not normalizing the TCP protocol and does not
itself act as a back-to-back pair of TCP endpoints (i.e. a middlebox
that intends to be transparent or invisible at the transport layer)
ought to forward AccECN TCP Options unaltered, whether or not the
length value matches one of those specified in Section 3.2.3, and
whether or not the initial values of the byte-counter fields match
those in Section 3.2.1. This is because blocking apparently invalid
values prevents the standardized set of values being extended in
future (given outdated normalizers would block updated hosts from
using the extended AccECN standard).
A TCP normalizer is likely to block or alter an AccECN TCP Option if
the length value or the initial values of its byte-counter fields do
not match one of those specified in Section 3.2.3 or Section 3.2.1.
However, to comply with the present AccECN specification, a middlebox
MUST NOT change the ACE field; or those fields of an AccECN Option
that are currently specified in Section 3.2.3; or any AccECN field
covered by integrity protection (e.g. [RFC5925]).
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3.3.3. Requirements for TCP ACK Filtering
Section 5.2.1 of BCP 69 [RFC3449] gives best current practice on
filtering (aka. thinning or coalescing) of pure TCP ACKs. It advises
that filtering ACKs carrying ECN feedback ought to preserve the
correct operation of ECN feedback. As the present specification
updates the operation of ECN feedback, this section discusses how an
ACK filter might preserve correct operation of AccECN feedback as
well.
The problem divides into two parts: determining if an ACK is part of
a connection that is using AccECN and then preserving the correct
operation of AccECN feedback:
* To determine whether a pure TCP ACK is part of an AccECN
connection without resorting to connection tracking and per-flow
state, a useful heuristic would be to check for a non-zero ECN
field at the IP layer (because the ECN++ experiment only allows
TCP pure ACKs to be ECN-capable if AccECN has been negotiated
[I-D.ietf-tcpm-generalized-ecn]). This heuristic is simple and
stateless. However, it might omit some AccECN ACKs, because
AccECN can be used without ECN++ and even if it is, ECN++ does not
have to make pure ACKs ECN-capable - only deployment experience
will tell. Also, TCP ACKs might be ECN-capable owing to some
scheme other than AccECN, e.g. [RFC5690] or some future standards
action. Again, only deployment experience will tell.
* The main concern with preserving correct AccECN operation involves
leaving enough ACKs for the Data Sender to work out whether the
3-bit ACE field has wrapped. In the worst case, in feedback about
a run of received packets that were all ECN-marked, the ACE field
will wrap every 8 acknowledged packets. ACE field wrap might be
of less concern if packets also carry AccECN TCP Options.
However, note that logic to read an AccECN TCP Option is optional
to implement (albeit recommended — see Section 3.2.3). So one end
writing an AccECN TCP Option into a packet does not necessarily
imply that the other end will read it.
Note that the present specification of AccECN in TCP does not presume
to rely on any of the above ACK filtering behaviour in the network,
because it has to be robust against pre-existing network nodes that
do not distinguish AccECN ACKs, and robust against ACK loss during
overload more generally.
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3.3.4. Requirements for TCP Segmentation Offload
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').
The ACE field changes with every received CE marking, so today's
receive offloading could lead to many interrupts in high congestion
situations. Although that would be useful (because congestion
information is received sooner), it could also significantly increase
processor load, particularly in scenarios such as DCTCP or L4S where
the marking rate is generally higher.
Current offload hardware ejects a segment from the coalescing process
whenever the TCP ECN flags change. In data centres it has been
fortunate for this offload hardware that DCTCP-style feedback changes
less often when there are long sequences of CE marks, which is more
common with a step marking threshold (but less likely the more short
flows are in the mix). The ACE counter approach has been designed so
that coalescing can continue over arbitrary patterns of marking and
only needs to stop when the counter wraps. Nonetheless, until the
particular offload hardware in use implements this more efficient
approach, it is likely to be more efficient for AccECN connections to
implement this counter-style logic using software segmentation
offload.
ECN encodes a varying signal in the ACK stream, so it is inevitable
that offload hardware will ultimately need to handle any form of ECN
feedback exceptionally. The ACE field has been designed as a counter
so that it is straightforward for offload hardware to pass on the
highest counter, and to push a segment from its cache before the
counter wraps. The purpose of working towards standardized TCP ECN
feedback is to reduce the risk for hardware developers, who would
otherwise have to guess which scheme is likely to become dominant.
The above process has been designed to enable a continuing
incremental deployment path - to more highly dynamic congestion
control. Once offload hardware supports AccECN, it will be able to
coalesce efficiently for any sequence of marks, instead of relying
for efficiency on the long marking sequences from step marking. In
the next stage, marking can evolve from a step to a ramp function.
That in turn will allow host congestion control algorithms to respond
faster to dynamics, while being backwards compatible with existing
host algorithms.
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4. Updates to RFC 3168
This section clarifies which parts of RFC3168 are updated and maps
them to the sections of the present AccECN specification that update
them:
* The whole of "6.1.1 TCP Initialization" of [RFC3168] is updated by
Section 3.1 of the present specification.
* In "6.1.2. The TCP Sender" of [RFC3168], all mentions of a
congestion response to an ECN-Echo (ECE) ACK packet are updated by
Section 3.2 of the present specification to mean an increment to
the sender's count of CE-marked packets, s.cep. And the
requirements to set the CWR flag no longer apply, as specified in
Section 3.1.5 of the present specification. Otherwise, the
remaining requirements in "6.1.2. The TCP Sender" still stand.
It will be noted that RFC 8311 already updates, or potentially
updates, a number of the requirements in "6.1.2. The TCP Sender".
Section 6.1.2 of RFC 3168 extended standard TCP congestion control
[RFC5681] to cover ECN marking as well as packet drop. Whereas,
RFC 8311 enables experimentation with alternative responses to ECN
marking, if specified for instance by an experimental RFC on the
IETF document stream. RFC 8311 also strengthened the statement
that "ECT(0) SHOULD be used" to a "MUST" (see [RFC8311] for the
details).
* The whole of "6.1.3. The TCP Receiver" of [RFC3168] is updated by
Section 3.2 of the present specification, with the exception of
the last paragraph (about congestion response to drop and ECN in
the same round trip), which still stands. Incidentally, this last
paragraph is in the wrong section, because it relates to "TCP
Sender" behaviour.
* The following text within "6.1.5. Retransmitted TCP packets":
"the TCP data receiver SHOULD ignore the ECN field on arriving
data packets that are outside of the receiver's current
window."
is updated by more stringent acceptability tests for any packet
(not just data packets) in the present specification.
Specifically, in the normative specification of AccECN (Section 3)
only 'Acceptable' packets contribute to the ECN counters at the
AccECN receiver and Section 1.3 defines an Acceptable packet as
one that passes acceptability tests equivalent in strength to
those in both [RFC9293] and [RFC5961].
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* Sections 5.2, 6.1.1, 6.1.4, 6.1.5 and 6.1.6 of [RFC3168] prohibit
use of ECN on TCP control packets and retransmissions. The
present specification does not update that aspect of RFC 3168, but
it does say what feedback an AccECN Data Receiver ought to provide
if it receives an ECN-capable control packet or retransmission.
This ensures AccECN is forward compatible with any future scheme
that allows ECN on these packets, as provided for in section 4.3
of [RFC8311] and as proposed in [I-D.ietf-tcpm-generalized-ecn].
5. Interaction with TCP Variants
This section is informative, not normative.
5.1. 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 it
receives a pure ACK that acknowledges an ISN that is a valid SYN
cookie, and if the ACK contains an ACE field with the value 0b010 to
0b111 (decimal 2 to 7), the Server can infer the first two stages of
the handshake:
* the TCP Client has to have requested AccECN support on the SYN;
* then, even though the Server kept no state, it has to have
confirmed that it supported AccECN.
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.
If the pure ACK that acknowledges a SYN cookie contains an ACE field
with the value 0b000 or 0b001, these values indicate that the TCP
Client did not request support for AccECN and therefore the Server
does not enter AccECN mode for this connection. Further, 0b001 on
the ACK implies that the Server sent an ECN-capable SYN/ACK, which
was marked CE in the network, and the non-AccECN TCP Client fed this
back by setting ECE on the ACK of the SYN/ACK.
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5.2. Compatibility with TCP Experiments and Common TCP Options
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 Multipath TCP (MPTCP [RFC8684]) and the
experimental TCP option TCP Fast Open (TFO [RFC7413]). 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 3.2.3.3 provides guidance on how important it is to send an
AccECN Option relative to other options, and which fields are more
important to include.
Implementers of TFO need to take careful note of the recommendation
in Section 3.2.2.1. That section recommends that, if the TCP Client
has successfully negotiated AccECN, when acknowledging the SYN/ACK,
even if it has data to send, it sends a pure ACK immediately before
the data. Then it can reflect the IP-ECN field of the SYN/ACK on
this pure ACK, which allows the Server to detect ECN mangling. Note
that, as specified in Section 3.2, any data on the SYN (SYN=1, ACK=0)
is not included in any of the byte counters held locally for each ECN
marking, nor in the AccECN Option on the wire.
AccECN feedback is compatible with the ECN++
[I-D.ietf-tcpm-generalized-ecn] experiment, which allows TCP control
packets and retransmissions to be ECN-capable ([RFC3168] was updated
by [RFC8311] to permit such experiments). AccECN is likely to
inherently support any experiment with ECN-capable packets, because
it feeds back the contents of the ECN field mechanistically, without
judging whether a packet ought to use the ECN capability or not
(Section 2.5). This specification does not discuss implementing
AccECN alongside [RFC5562], which was an earlier experimental
protocol with narrower scope than ECN++ and a 5-way handshake.
5.3. 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:
* 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 (similar to
paragraph 2 of Section 20.2 of [RFC3168]). Unlike the ECN Nonce
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[RFC3540], this approach does not waste the ECT(1) codepoint in
the IP header, it does not require standardization 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 therefore ought to only be done sparingly.
* 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 neutralize any advantage
that any of these three parties would otherwise gain.
ConEx is an experimental change to the Data Sender that would be
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.
* The standards track 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. Network
Address (and Port) Translation (NAT/NAPT), resegmentation or
shifting the sequence space.
6. Summary: 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 an
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
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essential part reuses the 3 flags already assigned to ECN in the
TCP header. The supplementary part adds an additional TCP option
consuming up to 11 bytes. However, no TCP option space 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
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. Within the constraints of the
change-triggered ACK rules, the receiver can control how
frequently it sends AccECN TCP Options and therefore to some
extent 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. And if data or
ACKs are reordered, stale congestion information can be identified
and ignored.
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 options are needed 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.
Resilience vs Timeliness and Ordering: Ordering information and the
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timing of transitions cannot be communicated in three cases: i)
during ACK loss; ii) if something on the path strips AccECN
Options; or iii) if the Data Receiver is unable to support Change-
Triggered ACKs. Following ACK reordering, the Data Sender can
reconstruct the order in which feedback was sent, but not until
all the missing feedback has arrived.
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 AccECN Options are
stripped the resolution of the feedback is degraded, but the
integrity of this degraded feedback can still be assured.
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.
If AccECN Options are 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 an 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 AccECN Options.
Forward Compatibility: The behaviour of endpoints and middleboxes is
carefully defined for all reserved or currently unused codepoints
in the scheme. Then, the designers of security devices can
understand which currently unused values might appear in future.
So, even if they choose to treat such values as anomalous while
they are not widely used, any blocking will at least be under
policy control not hard-coded. Then, if previously unused values
start to appear on the Internet (or in standards), such policies
could be quickly reversed.
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7. IANA Considerations
This document reassigns the TCP header flag at bit offset 7 to the
AccECN protocol. This bit was previously called the Nonce Sum (NS)
flag [RFC3540], but RFC 3540 has been reclassified as historic
[RFC8311]. The flag will now be defined as:
+=====+==============+===========+==============================+
| Bit | Name | Reference | Assignment Notes |
+=====+==============+===========+==============================+
| 7 | AE (Accurate | RFC XXXX | Previously used as NS (Nonce |
| | ECN) | | Sum) by [RFC3540], which is |
| | | | now historic [RFC8311] |
+-----+--------------+-----------+------------------------------+
Table 6: TCP header flag reassignment
[TO BE REMOVED: IANA is requested to update the existing entry in the
TCP Header Flags registry (https://www.iana.org/assignments/tcp-
parameters/tcp-parameters.xhtml#tcp-header-flags) for Bit 7 to "AE
(Accurate ECN)" and to change the reference to this RFC-to-be instead
of RFC8311. Also IANA is requested to change the assignment note to
"Previously used as NS (Nonce Sum) by [RFC3540], which is now
historic [RFC8311]."]
This document also defines two new TCP options for AccECN, assigned
values of 172 and 174 (decimal) from the TCP option space. These
values are defined as:
+======+========+================================+===========+
| Kind | Length | Meaning | Reference |
+======+========+================================+===========+
| 172 | N | Accurate ECN Order 0 (AccECN0) | RFC XXXX |
+------+--------+--------------------------------+-----------+
| 174 | N | Accurate ECN Order 1 (AccECN1) | RFC XXXX |
+------+--------+--------------------------------+-----------+
Table 7: New TCP Option assignments
[TO BE REMOVED: These registrations have taken place using the early
registration procedure, which may be temporary if this draft does not
proceed, at the following location: http://www.iana.org/assignments/
tcp-parameters/tcp-parameters.xhtml#tcp-parameters-1 ]
Early experimental implementations of the two AccECN Options used
experimental option 254 per [RFC6994] with the 16-bit magic numbers
0xACC0 and 0xACC1 respectively for Order 0 and 1, as allocated in the
IANA "TCP Experimental Option Experiment Identifiers (TCP ExIDs)"
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registry. Even earlier experimental implementations used the single
magic number 0xACCE (16 bits). Uses of these experimental options
SHOULD migrate to use the new option kinds (172 & 174).
[TO BE REMOVED: IANA is requested to replace the references for all
three of the above experimental options (0xACC0, 0xACC1 and 0xACCE)
with a reference to the present RFC XXXX.]
[TO BE REMOVED: If the early registrations, which may be temporary,
do not proceed, the three references to them in the TCP ExIDs
registry at the following location will also need to be edited out:
https://www.iana.org/assignments/tcp-parameters/tcp-
parameters.xhtml#tcp-exids ]
8. Security Considerations
If ever the supplementary part of AccECN based on one of the new
AccECN TCP Options 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 3.2.2.5). 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 AccECN Options are not present,
the AccECN Data Sender is then required to switch to more
conservative assumptions about wrap of congestion indication counters
(see Section 3.2.2.5 and Appendix A.2).
Section 5.1 describes how a TCP Server can negotiate AccECN and use
the SYN cookie method for mitigating SYN flooding attacks.
There is concern that ECN feedback 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 5.3 for details). If AccECN Options are 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. Assuring that Data Senders respond appropriately
to ECN feedback is possible, but the scope of the present document is
confined to the feedback protocol, and excludes the response to this
feedback.
In Section 3.2.3 a Data Sender is allowed to ignore an unrecognized
TCP AccECN Option length and read as many whole 3-octet fields from
it as possible up to a maximum of 3, treating the remainder as
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padding. This opens up a potential covert channel of up to 29B (40 -
(2+3*3)) B. However, it is really an overt channel (not hidden) and
it is no different to the use of unknown TCP options with unknown
option lengths in general. Therefore, where this is of concern, it
can already be adequately mitigated by regular TCP normalizer
technology (see Section 3.3.2).
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. A
covert channel can be used to compromise privacy. However, as
explained above, undefined TCP options in general open up such
channels and common techniques are available to close them off.
There is a potential concern that a Data Receiver could deliberately
omit AccECN Options pretending that they had been stripped by a
middlebox. No known way can yet be contrived for a receiver to take
advantage of this behaviour, which seems to always degrade its own
performance. However, the concern is mentioned here for
completeness.
9. References
9.1. Normative References
[RFC2018] Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP
Selective Acknowledgment Options", RFC 2018,
DOI 10.17487/RFC2018, October 1996,
<https://www.rfc-editor.org/info/rfc2018>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC2883] Floyd, S., Mahdavi, J., Mathis, M., and M. Podolsky, "An
Extension to the Selective Acknowledgement (SACK) Option
for TCP", RFC 2883, DOI 10.17487/RFC2883, July 2000,
<https://www.rfc-editor.org/info/rfc2883>.
[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,
<https://www.rfc-editor.org/info/rfc3168>.
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[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,
<https://www.rfc-editor.org/info/rfc5961>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC9293] Eddy, W., Ed., "Transmission Control Protocol (TCP)",
STD 7, RFC 9293, DOI 10.17487/RFC9293, August 2022,
<https://www.rfc-editor.org/info/rfc9293>.
9.2. Informative References
[I-D.ietf-tcpm-generalized-ecn]
Bagnulo, M. and B. Briscoe, "ECN++: Adding Explicit
Congestion Notification (ECN) to TCP Control Packets",
Work in Progress, Internet-Draft, draft-ietf-tcpm-
generalized-ecn-14, 6 November 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-tcpm-
generalized-ecn-14>.
[Mandalari18]
Mandalari, A., Lutu, A., Briscoe, B., Bagnulo, M., and Ö.
Alay, "Measuring ECN++: Good News for ++, Bad News for ECN
over Mobile", IEEE Communications Magazine , March 2018,
<http://www.it.uc3m.es/amandala/
ecn++/ecn_commag_2018.html>.
[RFC3449] Balakrishnan, H., Padmanabhan, V., Fairhurst, G., and M.
Sooriyabandara, "TCP Performance Implications of Network
Path Asymmetry", BCP 69, RFC 3449, DOI 10.17487/RFC3449,
December 2002, <https://www.rfc-editor.org/info/rfc3449>.
[RFC3540] Spring, N., Wetherall, D., and D. Ely, "Robust Explicit
Congestion Notification (ECN) Signaling with Nonces",
RFC 3540, DOI 10.17487/RFC3540, June 2003,
<https://www.rfc-editor.org/info/rfc3540>.
[RFC4987] Eddy, W., "TCP SYN Flooding Attacks and Common
Mitigations", RFC 4987, DOI 10.17487/RFC4987, August 2007,
<https://www.rfc-editor.org/info/rfc4987>.
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[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,
<https://www.rfc-editor.org/info/rfc5562>.
[RFC5681] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
Control", RFC 5681, DOI 10.17487/RFC5681, September 2009,
<https://www.rfc-editor.org/info/rfc5681>.
[RFC5690] Floyd, S., Arcia, A., Ros, D., and J. Iyengar, "Adding
Acknowledgement Congestion Control to TCP", RFC 5690,
DOI 10.17487/RFC5690, February 2010,
<https://www.rfc-editor.org/info/rfc5690>.
[RFC5925] Touch, J., Mankin, A., and R. Bonica, "The TCP
Authentication Option", RFC 5925, DOI 10.17487/RFC5925,
June 2010, <https://www.rfc-editor.org/info/rfc5925>.
[RFC6994] Touch, J., "Shared Use of Experimental TCP Options",
RFC 6994, DOI 10.17487/RFC6994, August 2013,
<https://www.rfc-editor.org/info/rfc6994>.
[RFC7323] Borman, D., Braden, B., Jacobson, V., and R.
Scheffenegger, Ed., "TCP Extensions for High Performance",
RFC 7323, DOI 10.17487/RFC7323, September 2014,
<https://www.rfc-editor.org/info/rfc7323>.
[RFC7413] Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP
Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014,
<https://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,
<https://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,
<https://www.rfc-editor.org/info/rfc7713>.
[RFC8257] Bensley, S., Thaler, D., Balasubramanian, P., Eggert, L.,
and G. Judd, "Data Center TCP (DCTCP): TCP Congestion
Control for Data Centers", RFC 8257, DOI 10.17487/RFC8257,
October 2017, <https://www.rfc-editor.org/info/rfc8257>.
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[RFC8311] Black, D., "Relaxing Restrictions on Explicit Congestion
Notification (ECN) Experimentation", RFC 8311,
DOI 10.17487/RFC8311, January 2018,
<https://www.rfc-editor.org/info/rfc8311>.
[RFC8511] Khademi, N., Welzl, M., Armitage, G., and G. Fairhurst,
"TCP Alternative Backoff with ECN (ABE)", RFC 8511,
DOI 10.17487/RFC8511, December 2018,
<https://www.rfc-editor.org/info/rfc8511>.
[RFC8684] Ford, A., Raiciu, C., Handley, M., Bonaventure, O., and C.
Paasch, "TCP Extensions for Multipath Operation with
Multiple Addresses", RFC 8684, DOI 10.17487/RFC8684, March
2020, <https://www.rfc-editor.org/info/rfc8684>.
[RFC9040] Touch, J., Welzl, M., and S. Islam, "TCP Control Block
Interdependence", RFC 9040, DOI 10.17487/RFC9040, July
2021, <https://www.rfc-editor.org/info/rfc9040>.
[RFC9330] Briscoe, B., Ed., De Schepper, K., Bagnulo, M., and G.
White, "Low Latency, Low Loss, and Scalable Throughput
(L4S) Internet Service: Architecture", RFC 9330,
DOI 10.17487/RFC9330, January 2023,
<https://www.rfc-editor.org/info/rfc9330>.
[RFC9438] Xu, L., Ha, S., Rhee, I., Goel, V., and L. Eggert, Ed.,
"CUBIC for Fast and Long-Distance Networks", RFC 9438,
DOI 10.17487/RFC9438, August 2023,
<https://www.rfc-editor.org/info/rfc9438>.
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 an
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 an AccECN Option would be similarly
encoded and decoded.
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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 an AccECN Option, the value it writes
into the ECEB field is
ECEB = r.ceb % DIVOPT
where '%' is the remainder operator.
On the arrival of an AccECN Option, the Data Sender first makes sure
the ACK has not been superseded in order to avoid winding the s.ceb
counter backwards. It uses the TCP acknowledgement number and any
SACK options [RFC2018] to calculate newlyAckedB, the amount of new
data that the ACK acknowledges in bytes (newlyAckedB can be zero but
not negative). If newlyAckedB is zero, either the ACK has been
superseded or CE-marked packet(s) without data could have arrived.
To break the tie for the latter case, the Data Sender could use time-
stamps [RFC7323] (if present) to work out newlyAckedT, the amount of
new time that the ACK acknowledges. If the Data Sender determines
that the ACK has been superseded it ignores the AccECN Option.
Otherwise, the Data Sender calculates the minimum non-negative
difference d.ceb between the ECEB field and its local s.ceb counter,
using modulo arithmetic as follows:
if ((newlyAckedB > 0) || (newlyAckedT > 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
s.ceb % DIVOPT = 1
d.ceb = (1461 + 2^24 - 1) % 2^24
= 1460
s.ceb = 33,554,433 + 1460
= 33,555,893
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In practice an implementation might use heuristics to guess the
feedback in missing ACKs, then when it subsequently receives feedback
it might find that it needs to correct its earlier heuristics as part
of the decoding process. The above decoding process does not include
any such heuristics.
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 AccECN Options
are not available (see Section 3.2.2.5 and Section 3.2.3.2); and ii)
a less conservative variant that is feasible when complementary
information is available from AccECN Options.
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 an Acceptable CE marked packet arrives (Section 3.2.2.2),
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 ensures the ACK has not been superseded using
the TCP acknowledgement number, any SACK options and timestamps (if
available) to calculate newlyAckedB, as in Appendix A.1. If the ACK
has not been superseded, the Data Sender calculates the minimum
difference d.cep between the ACE field and its local s.cep counter,
using modulo arithmetic as follows:
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if ((newlyAckedB > 0) || (newlyAckedT > 0))
d.cep = (ACE + DIVACE - (s.cep % DIVACE)) % DIVACE
Section 3.2.2.5 expects the Data Sender to assume that the ACE field
cycled if it is the safest likely case under prevailing conditions.
The 3-bit ACE field in an arriving ACK could have cycled and become
ambiguous to the Data Sender if a sequence 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 missing ACKs were 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 for implementation-
dependent interpretation. A Data Sender might take account of the
prevailing size of data segments and the prevailing CE marking rate
just before the sequence of missing ACKs. 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).
Note that checks would need to be added to the above pseudocode for
(d.cep > newlyAckedPkt), which could occur if newlyAckedPkt had been
wrongly estimated using an inappropriate packet size.
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ACKs that acknowledge a large stretch of packets might be common in
data centres to achieve a high packet rate or might be due to ACK
thinning by a middlebox. In these cases, cycling of the ACE field
would often appear to have been possible, so the above algorithm
would be over-conservative, leading to a false high marking rate and
poor performance. Therefore it would be reasonable to only use
dSafer.cep rather than d.cep if the moving average of newlyAckedPkt
was well below 8.
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.
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 3.2.2.5 says "the Data Sender MAY attempt to neutralize 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 AccECN Options are 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 AccECN Options
without needing to process the ACE field. If for some reason it
needs CE-marked packets, if dSafer.cep is different from d.cep, it
can determine whether d.cep is likely to be a safe enough estimate by
checking whether the average marked segment size (s = d.ceb/d.cep) is
less than the MSS (where d.ceb is the amount of newly CE-marked bytes
- see Appendix A.1). Specifically, it could use the following
algorithm:
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SAFETY_FACTOR = 2
if (dSafer.cep > d.cep) {
if (d.ceb <= MSS * d.cep) { % Same as (s <= MSS), but no DBZ
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:
^
sSafer|
|
MSS+
|
| dSafer.cep
| is
MSS/SAFETY_FACTOR+--------------+ safest
| |
| d.cep is safe|
| enough |
+-------------------->
MSS s
The following examples give the reasoning behind the algorithm,
assuming MSS=1460 [B]:
* if d.cep=0, dSafer.cep=8 and d.ceb=1460, 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.
* if d.cep=2, dSafer.cep=10 and d.ceb=1460, 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.
* if d.cep=7, dSafer.cep=15 and d.ceb=10200, then s=1457 and
sSafer=680.
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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 AccECN Options are 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. Some possible ways to calculate s_ave are outlined
below. The precise details will depend on why an estimate of marked
bytes is needed.
The implementation 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 is reset 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 and '~=' means 'approximately equal to'.
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.
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A.4. 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.
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 retransmissions 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 IPv4 ToS byte or the
IPv6 Traffic Class field). To detect bleaching it will be sufficient
to detect whether nearly all bytes arrive marked as Not-ECT.
Therefore there ought to be no need to keep track of the details of
retransmissions.
Appendix B. Rationale for Usage of TCP Header Flags
B.1. Three TCP Header Flags in the SYN-SYN/ACK Handshake
AccECN uses a rather unorthodox approach to negotiate the highest
version TCP ECN feedback scheme that both ends support, as justified
below. It follows from the original TCP ECN capability negotiation
[RFC3168], in which the Client set the 2 least significant of the
original reserved flags in the TCP header, and fell back to no ECN
support if the Server responded with the 2 flags cleared, which had
previously been the default.
Classic ECN used header flags rather than a TCP option because it was
considered more efficient to use a header flag for 1 bit of feedback
per ACK, and this bit could be overloaded to indicate support for
Classic ECN during the handshake. During the development of ECN, 1
bit crept up to 2, in order to deliver the feedback reliably and to
work round some broken hosts that reflected the reserved flags during
the handshake.
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In order to be backward compatible with RFC 3168, AccECN continues
this approach, using the 3rd least significant TCP header flag that
had previously been allocated for the ECN nonce (now historic).
Then, whatever form of Server an AccECN Client encounters, the
connection can fall back to the highest version of feedback protocol
that both ends support, as explained in Section 3.1.
If AccECN capability negotiation had used the more orthodox approach
of a TCP option, it would still have had to set the two ECN flags in
the main TCP header, in order to be able to fall back to Classic RFC
3168 ECN, or to disable ECN support, without another round of
negotiation. Then AccECN would also have had to handle all the
different ways that Servers currently respond to settings of the ECN
flags in the main TCP header, including all the conflicting cases
where a Server might have said it supported one approach in the flags
and another approach in a new TCP option. And AccECN would have had
to deal with all the additional possibilities where a middlebox might
have mangled the ECN flags, or removed TCP options. Thus, usage of
the 3rd reserved TCP header flag simplified the protocol.
The third flag was used in a way that could be distinguished from the
ECN nonce, in case any nonce deployment was encountered. Previous
usage of this flag for the ECN nonce was integrated into the original
ECN negotiation. This further justified the 3rd flag's use for
AccECN, because a non-ECN usage of this flag would have had to use it
as a separate single bit, rather than in combination with the other 2
ECN flags.
Indeed, having overloaded the original uses of these three flags for
its handshake, AccECN overloads all three bits again as a 3-bit
counter.
B.2. Four Codepoints in the SYN/ACK
Of the 8 possible codepoints that the 3 TCP header flags can indicate
on the SYN/ACK, 4 already indicated earlier (or broken) versions of
ECN support, 1 now being historic. In the early design of AccECN, an
AccECN Server could use only 2 of the 4 remaining codepoints. They
both indicated AccECN support, but one fed back that the SYN had
arrived marked as CE. Even though ECN support on a SYN is not yet on
the standards track, the idea is for either end to act as a
mechanistic reflector, so that future capabilities can be
unilaterally deployed without requiring 2-ended deployment (justified
in Section 2.5).
During traversal testing it was discovered that the IP-ECN field in
the SYN was mangled on a non-negligible proportion of paths.
Therefore it was necessary to allow the SYN/ACK to feed all four IP-
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ECN codepoints that the SYN could arrive with back to the Client.
Without this, the Client could not know whether to disable ECN for
the connection due to mangling of the IP-ECN field (also explained in
Section 2.5). This development consumed the remaining 2 codepoints
on the SYN/ACK that had been reserved for future use by AccECN in
earlier versions.
B.3. Space for Future Evolution
Despite availability of usable TCP header space being extremely
scarce, the AccECN protocol has taken all possible steps to ensure
that there is space to negotiate possible future variants of the
protocol, either if a variant of AccECN is required, or if a
completely different ECN feedback approach is needed:
Future AccECN variants: When the AccECN capability is negotiated
during TCP's 3WHS, the rows in Table 2 tagged as 'Nonce' and
'Broken' in the column for the capability of node B are unused by
any current protocol in the RFC series. These could be used by
TCP Servers in future to indicate a variant of the AccECN
protocol. In recent measurement studies in which the response of
large numbers of Servers to an AccECN SYN has been tested,
e.g. [Mandalari18], a very small number of SYN/ACKs arrive with
the pattern tagged as 'Nonce', and a small but more significant
number arrive with the pattern tagged as 'Broken'. The 'Nonce'
pattern could be a sign that a few Servers have implemented the
ECN Nonce [RFC3540], which has now been reclassified as historic
[RFC8311], or it could be the random result of some unknown
middlebox behaviour. The greater prevalence of the 'Broken'
pattern suggests that some instances still exist of the broken
code that reflects the reserved flags on the SYN.
The requirement not to reject unexpected initial values of the ACE
counter (in the main TCP header) in the last paragraph of
Section 3.2.2.4 ensures that 3 unused codepoints on the ACK of the
SYN/ACK, 6 unused values on the first SYN=0 data packet from the
Client and 7 unused values on the first SYN=0 data packet from the
Server could be used to declare future variants of the AccECN
protocol. The word 'declare' is used rather than 'negotiate'
because, at this late stage in the 3WHS, it would be too late for
a negotiation between the endpoints to be completed. A similar
requirement not to reject unexpected initial values in AccECN TCP
Options (Section 3.2.3.2.4) is for the same purpose. If traversal
of AccECN TCP Options were reliable, this would have enabled a far
wider range of future variation of the whole AccECN protocol.
Nonetheless, it could be used to reliably negotiate a wide range
of variation in the semantics of the AccECN Option.
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Future non-AccECN variants: Five codepoints out of the 8 possible in
the 3 TCP header flags used by AccECN are unused on the initial
SYN (in the order (AE,CWR,ECE)): (0,0,1), (0,1,0), (1,0,0),
(1,0,1), (1,1,0). Section 3.1.3 ensures that the installed base
of AccECN Servers will all assume these are equivalent to AccECN
negotiation with (1,1,1) on the SYN. These codepoints would not
allow fall-back to Classic ECN support for a Server that did not
understand them, but this approach ensures they are available in
future, perhaps for uses other than ECN alongside the AccECN
scheme. All possible combinations of SYN/ACK could be used in
response except either (0,0,0) or reflection of the same values
sent on the SYN.
In order to extend AccECN or ECN in future, other ways could be
resorted to, although their traversal properties are likely to be
inferior. They include a new TCP option; using the remaining
reserved flags in the main TCP header (preferably extending the
3-bit combinations used by AccECN to 4-bit combinations, rather
than burning one bit for just one state); a non-zero urgent
pointer in combination with the URG flag cleared; or some other
unexpected combination of fields yet to be invented.
Acknowledgements
We want to thank Koen De Schepper, Praveen Balasubramanian, Michael
Welzl, Gorry Fairhurst, David Black, Spencer Dawkins, Michael Scharf,
Michael Tüxen, Yuchung Cheng, Kenjiro Cho, Olivier Tilmans, Ilpo
Järvinen, Neal Cardwell, Yoshifumi Nishida, Martin Duke, Jonathan
Morton, Vidhi Goel, Alex Burr, Markku Kojo, Grenville Armitage and
Wes Eddy 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.
The following contributed implementations of AccECN that validated
and helped to improve this specification:
Linux: Mirja Kühlewind, Ilpo Järvinen, Neal Cardwell and Chia-Yu
Chang;
FreeBSD: Richard Scheffenegger;
Apple OSs: Vidhi Goel.
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Bob Briscoe was part-funded by Apple Inc, the Comcast Innovation
Fund, 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), and the
Research Council of Norway through the TimeIn project. The views
expressed here are solely those of the authors.
Mirja Kühlewind was 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.
Comments Solicited
This section is to be removed before publishing as an RFC.
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.
Authors' Addresses
Bob Briscoe
Independent
United Kingdom
Email: ietf@bobbriscoe.net
URI: http://bobbriscoe.net/
Mirja Kühlewind
Ericsson
Germany
Email: ietf@kuehlewind.net
Richard Scheffenegger
NetApp
Vienna
Austria
Email: Richard.Scheffenegger@netapp.com
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