TCP modifications for Congestion Exposure
draft-ietf-conex-tcp-modifications-00
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 7786.
|
|
|---|---|---|---|
| Authors | Mirja Kühlewind , Richard Scheffenegger | ||
| Last updated | 2011-12-21 | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Formats | |||
| Reviews | |||
| Additional resources | Mailing list discussion | ||
| Stream | WG state | WG Document | |
| Document shepherd | (None) | ||
| IESG | IESG state | Became RFC 7786 (Experimental) | |
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| Send notices to | (None) |
draft-ietf-conex-tcp-modifications-00
Congestion Exposure (ConEx) M. Kuehlewind, Ed.
Internet-Draft University of Stuttgart
Intended status: Experimental R. Scheffenegger
Expires: June 23, 2012 NetApp, Inc.
December 21, 2011
TCP modifications for Congestion Exposure
draft-ietf-conex-tcp-modifications-00
Abstract
Congestion Exposure (ConEx) is a mechanism by which senders inform
the network about the congestion encountered by previous packets on
the same flow. This document describes the necessary modifications
to use ConEx with the Transmission Control Protocol (TCP).
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
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on June 23, 2012.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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described in the Simplified BSD License.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Sender-side Modifications . . . . . . . . . . . . . . . . . . 3
3. Accounting congestion . . . . . . . . . . . . . . . . . . . . 4
3.1. ECN . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.1.1. Accurate ECN feedback . . . . . . . . . . . . . . . . 5
3.1.2. Classic ECN support . . . . . . . . . . . . . . . . . 5
3.2. Loss Detection with/without SACK . . . . . . . . . . . . . 7
4. Setting the ConEx IPv6 Bits . . . . . . . . . . . . . . . . . 7
4.1. Setting the E and the L Bit . . . . . . . . . . . . . . . 8
4.2. Credit Bits . . . . . . . . . . . . . . . . . . . . . . . 8
5. Timeliness of the ConEx Signals . . . . . . . . . . . . . . . 9
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
8. Security Considerations . . . . . . . . . . . . . . . . . . . 10
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
9.1. Normative References . . . . . . . . . . . . . . . . . . . 10
9.2. Informative References . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11
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1. Introduction
Congestion Exposure (ConEx) is a mechanism by which senders inform
the network about the congestion encountered by previous packets on
the same flow. This document describes the necessary modifications
to use ConEx with the Transmission Control Protocol (TCP). The ConEx
signal is based on loss or ECN marks [RFC3168] as a congestion
indication.
With standard TCP without Selective Acknowledgments (SACK) [RFC2018]
the actual number of losses is hard to detect, thus we recommend to
enable SACK when using ConEx. However, we discuss both cases, with
and without SACK support, later on.
Explicit Congestion Notification (ECN) is defined in such a way that
only a single congestion signal is guaranteed to be delivered per
Round-trip Time (RTT). For ConEx a more accurate feedback signal
would be beneficial. Such an extension to ECN is defined in a
seperate document [draft-kuehlewind-conex-accurate-ecn], as it can
also be useful for other mechanisms, as e.g. [DCTCP] or whenever the
congestion control reaction should be proportional to the expirienced
congestion.
ConEx is currently/will be defined as an destination option for IPv6.
The use of four bits have been defined, namely the X (ConEx-capable),
the L (loss experienced), the E (ECN experienced) and C (credit) bit.
1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
2. Sender-side Modifications
A ConEx sender MUST negotitate for both SACK and ECN or the more
accurate ECN feedback in the TCP handshake if these TCP extension are
available at the sender. Depending on the capability of the
receiver, the following operation modes exist:
o Full-ConEx (SACK and accurate ECN feedback)
o accECN-ConEx (no SACK but accurate ECN feedback)
o ECN-ConEx (no SACK and no accurate ECN feedback but 'classic' ECN)
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o SACK-ECN-ConEx (SACK and 'classic' instead of accurate ECN)
o SACK-ConEx (SACK but no ECN at all)
o Basic-ConEx (neither SACK nor ECN)
A ConEx sender MUST expose congestion to the network according to the
congestion information received by ECN or based on loss provided by
the TCP feedback loop. A TCP sender MUST account congestion byte-
wise (and not packet-wise) and MUST mark the respective number of
payload bytes in subsequent packets (after the congestion
notification) with the respective ConEx bit in the IP header. The
congestion accounting based on different operation modes is described
in the next section and the handling of the IPv6 bits itself in the
subsequent section afterwards.
3. Accounting congestion
A TCP sender MUST account congestion byte-wise (and not packet-wise)
based the congestion information received by ECN or loss detection
provided by TCP. For this purpose a TCP sender will maintain two
different counters for number outstanding bytes that need to be ConEx
marked either with the E bit or the L Bit.
The outstanding bytes accounted based on ECN feedback information are
maintained in the congestion exposure gauge (CEG). The accounting of
these bytes from the ECN feedback is explained in more detail next.
The outstanding bytes for congestion indications based on loss are
maintained in the loss exposure gauge (LEG) and the accounting is
explained in subsequent to the CEG accounting.
The subtraction of bytes which have been ConEx marked from both
counters is explained in the next section.
Usually all byte of an IP packet must be accounted. If we assume
equal sized packets or at least equally distributed packet sizes the
sender MAY only account the TCP payload bytes, as the ConEx marked
packets as well as the original packets causing the congestion will
both contain about the same number of headers. Otherwise the sender
MUST take the headers into account. A sender which sends different
sized packets with unequally distributed packet sizes should know
about reason to do so and thus may be able to reconstruct the exact
number of headers based on this information. Otherwise if no
additional information is available the worse case number of headers
SHOULD be estimated in a conservative way based on a minimum packet
size (of all packets sent in the last RTT).
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3.1. ECN
A receiver can support the accurate ECN feedback scheme, the
'classic' ECN or neither. In the case ECN is not supported at all,
the transport is not ECN-capable and no ECN marks will occur, thus
the E bit will never be set. In the other cases a ConEx sender MUST
maintain a gauge for the number of outstanding bytes that has to be
ConEx marked with the E bit, the congestion exposure gauge (CEG).
The CEG is increased when ECN information is received from an ECN-
capable receiver supporting the 'classic' ECN scheme or the accurate
ECN feedback scheme. When the ConEx sender receives an ACK
indicating one or more segments were received with a CE mark, CEG is
increased by the appropriate number of bytes. The two cases,
depending on the receiver capability, are discussed in the following
sections.
3.1.1. Accurate ECN feedback
With an more accurate ECN feedback scheme either the number of marked
packets/received CE marks is know or the number of marked bytes
directly. In the later case the CEG can directly be increased by the
number of marked bytes. Otherwise when the accurate ECN feedback
scheme is supported by the receiver, the receiver will maintain an
echo congestion counter (ECC). The ECC will hold the number of CE
marks received. A sender that is understanding the accurate ECN
feedback will be able to reconstruct this ECC value on the sender
side by maintaining a counter ECC.r.
On the arrival of every ACK, the sender calculates the difference D
between the local ECC.r counter, and the signaled value of the
receiver side ECC counter. The value of ECC.r is increased by D, and
D is assumed to be the number of CE marked packets that arrived at
the receiver since it sent the previously received ACK.
Whenever the counter ECC.r is increased, the gauge CEG has to be
increased by the amount of bytes sent which were marked:
CEG += min( SMSS*D, acked_bytes )
3.1.2. Classic ECN support
A ConEx sender that communicates with a classic ECN receiver
(conforming to [RFC3168] or [RFC5562]) MAY run in one of these modes:
o Full compliance mode:
The ConEx sender fully conforms to all the semantics of the ECN
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signaling as defined by [RFC5562]. In this mode, only a single
congestion indication can be signaled by the receiver per RTT.
Whenever the ECE flag toggles from "0" to "1", the gauge CEG is
increased by the SMSS:
CEG += SMSS
Note that under severe congestion, a session adhering to these
semantics may not provide enough ConEx marks. This may cause
appropriate sanctions by an audit device in a ConEx enabled
network.
o Simple compatibility mode:
The sender will set the CWR permanently to force the receiver to
signal only one ECE per CE mark. Unfortunately, in a high
congestion situation where all packets are CE marled over a
certain period of time, the use of delayed ACKs, as it is usually
done today, will prevent a feedback of every CE mark. With an ACK
rate of m, about m-1/m CE indications will not be signaled back by
the receiver (e.g. 50% with M=2). Thus, in this mode the ConEx
sender MUST increase CEG by a count of M*SMSS for each received
ECE signal:
CEG += M*SMSS
In case of a congestion event with low congestion (that means when
only a very smaller number of packets get marked), the sender
might miss the whole congestion event. In average the sender will
sent sufficient ConEx marks due to the scheme proposed above but
these ConEx marks might be timely shifted. Regarding congestion
control it is not a general problem to miss a congestion event as
by chance a marking scheme in the network node might also miss a
certain flow. Even if then no other flow is reacting, the
congestion level will increase and it will get more likely that
the congestion feedback is delivered. But to provide a fair share
over time, a TCP sender could react more strong when receiving a
ECN feedback signal. This of course depends on the congestion
control used. A TCP sender using this scheme MUST take the impact
on congestion control into account.
o Advanced compatibility mode:
More sophisticated heuristics, such as a phase locked loop, to set
CWR only on those data segments, that will actually trigger an
(delayed) ACK, could extract congestion notifications more timely.
A ConEx sender MAY choose to implement such a heuristic. In
addition, further heuristics SHOULD be implemented, to determine
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the value of each ECE notification. E.g. for each consecutive ACK
received with the ECE flag set, CEG should be increased by min(
M*SSMS, acked_bytes). Else if the predecessor ACK was received
with the ECE flag cleared, CEG need only be increase by one SMSS:
if previous_marked: CEG += min( M*SSMS, acked_bytes)
else: CEG += SMSS
This heuristic is conservative during more serious congestion, and
more relaxed at low congestion levels.
3.2. Loss Detection with/without SACK
For all the data segments that are determined by a ConEx sender as
lost, an identical number of IP bytes MUST be be sent with the ConEx
L bit set. Loss detection typically happens by use of duplicate
ACKs, or the firing of the retransmission timer. A ConEx sender MUST
maintain a loss exposure gauge (LEG), indicating the number of
outstanding bytes that must be sent with the ConEx L bit. When a
data segment is retransmitted, LEG will be increased by the size of
the TCP payload packet containing the retransmission, assuming equal
sized segments such that the retransmitted packet will have the same
number of header as the original ones. When sending subsequent
segments (including TCP control segments), the ConEx L bit is set as
long as LEG is positive, and LEG is decreased by the size of the sent
TCP payload with the ConEx L bit set.
Any retransmission may be spurious. To accommodate that, a ConEx
sender SHOULD make use of heuristics to detect such spurious
retransmissions (e.g. F-RTO [RFC5682], DSACK [RFC3708], and Eifel
[RFC3522], [RFC4015]). When such a heuristic has determined, that a
certain number of packets were retransmitted erroneously, the ConEx
sender should subtract the payload size of these TCP packets from
LEG.
Note that the above heuristics delays the ConEx signal by one
segment, and also decouples them from the retransmissions themselves,
as some control packets (e.g. pure ACKs, window probes, or window
updates) may be sent in between data segment retransmissions. A
simpler approach would be to set the ConEx signal for each
retransmitted data segment. However, it is important to remember,
that a ConEx signal and TCP segments do not natively belong together.
4. Setting the ConEx IPv6 Bits
ConEx is currently/will be defined as an destination option for IPv6.
The use of four bits have been defined, namely the X (ConEx-capable),
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the L (loss experienced), the E (ECN experienced) and C (credit) bit.
By setting the X bit a packet is marked as ConEx-capable. All
packets carrying payload MUST be marked with the X bit set including
retransmissions. About control packets as pure ACKs which are not
carrying any payload no congestion feedback information is available
thus these packet should not be taken into account when determining
ConEx information. These packet MUST carry a ConEx Destination
Option with the X bit unset.
4.1. Setting the E and the L Bit
As long as the CEG/LEG is positive, ConEx-capable packets MUST be
marked with E or respective L and the CEG/LEG is decreased by the TCP
payload bytes carried in this packet. If the CEG/LEG is negative,
the CEG/LEG is drained by one byte with every packet sent out, as
ConEX information are only meaningful for a certain time:
if CEG > 0: CEG -= TCPpayload.length else: CEG--
if LEG > 0: LEG -= TCPpayload.length else: LEG--
4.2. Credit Bits
The ConEx abstract mechanism requires that the transport SHOULD
signal sufficient credit in advance to cover any reasonably expected
congestion during its feedback delay. To be very conservative the
number of credits would need to equal the number of packets in
flight, as every packet could get lost or congestion marked. With a
more moderate view, only an increase in the sending rate should cause
congestion.
For TCP sender using the [RFC5681] congestion control algorithm, we
recommend to only send credit in Slow Start, as in Congestion
Avoidance an increase of one segment per RTT should only cause a
minor amount of congestion marks (usually at max one). If a more
aggressive congestion control is used, a sufficient amount of credits
need to be set.
In TCP Slow Start the sending rate will increase exponentially and
that means double every RTT. Thus the number of credits should equal
half the number of packets in flight in every RTT. Under the
assumption that all marks will not get invalid for the whole Slow
Start phase, marks of a previous RTT have to be summed up. Thus the
marking of every fourth packet will allow sufficient credits in Slow
Start.
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RTT1 |------XC------>|
|------X------->|
|------X------->| credit=1 in_flight=3
| |
RTT2 |------X------->|
|------XC------>|
|------X------->|
|------X------->|
|------X------->|
|------XC------>| credit=3 in_flight=6
| |
RTT3 |------X------->|
|------X------->|
|------X------->|
|------XC------>|
|------X------->|
|------X------->|
|------X------->|
|------XC------>|
|------X------->|
|------X------->|
|------X------->|
|------XC------>| credit=6 in_flight=12
| . |
| : |
Figure 1: Credits in Slow Start (with an initial window of 3)
If a ConEx sender detects an increasing number of losses even though
the sender reduced the sending rate, the sender SHOULD assume that
those losses are incorporated by an audit device and thus should send
further credits. Up to now its not clear if the credits say valid as
long as the connection is established or if an expiration of the
credits need to be assumed by the sender.
5. Timeliness of the ConEx Signals
ConEx signals will anyway be evaluated with a slight time delay of
about one RTT by a network node. Therefore, it would not be
absolutely necessary to immediately signal ConEx bits when they
become known (e.g. L and E bits), but a sender SHOULD sent the ConEx
signaling with the next available packet. If cases are available
where it is preferable to slight delay the ConEx signal, the sender
MUST NOT delay the ConEx signal more than one RTT.
Multiple ConEx bits may become available for signaling at the same
time, for example when an ACK is received by the sender, that
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indicates that at least one segment has been lost, and that one or
more ECN marks were received at the same time. This may happen
during excessive congestion, where buffer queues overflow and some
packets are marked, while others have to be dropped nevertheless.
Another possibility when this may happen are lost ACKs, so that a
subsequent ACK carries summary information not previously available
to the sender.
It is important to remember, that ConEx bits and TCP retransmissions
do not interact with each other. However, a retransmission should be
accompanied by one ConEx L bit in close proximity nevertheless. This
does not mean, that TCP retransmissions may never contain ConEx
marks. In a typical scenario using SACK, the first retransmission
would not carry a ConEx L bit, while subsequent retransmissions in
the same recovery episode, would be marked with the ConEx L bit.
Spreading the ConEx bits over a small number of segments increases
the likelihood that most devices along the path will see some ConEx
marks even during heavy congestion.
6. Acknowledgements
7. IANA Considerations
8. Security Considerations
9. References
9.1. Normative References
[RFC2018] Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP
Selective Acknowledgment Options", RFC 2018, October 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
of Explicit Congestion Notification (ECN) to IP",
RFC 3168, September 2001.
[RFC5562] Kuzmanovic, A., Mondal, A., Floyd, S., and K.
Ramakrishnan, "Adding Explicit Congestion Notification
(ECN) Capability to TCP's SYN/ACK Packets", RFC 5562,
June 2009.
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9.2. Informative References
[DCTCP] Alizadeh, M., Greenberg, A., Maltz, D., Padhye, J., Patel,
P., Prabhakar, B., Sengupta, S., and M. Sridharan, "DCTCP:
Efficient Packet Transport for the Commoditized Data
Center", Jan 2010.
[I-D.briscoe-tsvwg-re-ecn-tcp]
Briscoe, B., Jacquet, A., Moncaster, T., and A. Smith,
"Re-ECN: Adding Accountability for Causing Congestion to
TCP/IP", draft-briscoe-tsvwg-re-ecn-tcp-09 (work in
progress), October 2010.
[RFC3522] Ludwig, R. and M. Meyer, "The Eifel Detection Algorithm
for TCP", RFC 3522, April 2003.
[RFC3708] Blanton, E. and M. Allman, "Using TCP Duplicate Selective
Acknowledgement (DSACKs) and Stream Control Transmission
Protocol (SCTP) Duplicate Transmission Sequence Numbers
(TSNs) to Detect Spurious Retransmissions", RFC 3708,
February 2004.
[RFC4015] Ludwig, R. and A. Gurtov, "The Eifel Response Algorithm
for TCP", RFC 4015, February 2005.
[RFC5681] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
Control", RFC 5681, September 2009.
[RFC5682] Sarolahti, P., Kojo, M., Yamamoto, K., and M. Hata,
"Forward RTO-Recovery (F-RTO): An Algorithm for Detecting
Spurious Retransmission Timeouts with TCP", RFC 5682,
September 2009.
[draft-kuehlewind-conex-accurate-ecn]
Kuehlewind, M. and R. Scheffenegger, "Accurate ECN
Feedback in TCP", draft-kuehlewind-conex-accurate-ecn-00
(work in progress), Jun 2011.
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Authors' Addresses
Mirja Kuehlewind (editor)
University of Stuttgart
Pfaffenwaldring 47
Stuttgart 70569
Germany
Email: mirja.kuehlewind@ikr.uni-stuttgart.de
Richard Scheffenegger
NetApp, Inc.
Am Euro Platz 2
Vienna, 1120
Austria
Phone: +43 1 3676811 3146
Email: rs@netapp.com
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