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Independent Submission S. Floyd
Request for Comments: 5690 ICIR
Category: Informational A. Arcia
ISSN: 2070-1721 D. Ros
TELECOM Bretagne
J. Iyengar
Franklin & Marshall College
February 2010
Adding Acknowledgement Congestion Control to TCP
Abstract
This document describes a possible congestion control mechanism for
acknowledgement (ACKs) traffic in TCP. The document specifies an
end-to-end acknowledgement congestion control mechanism for TCP that
uses participation from both TCP hosts: the TCP data sender and the
TCP data receiver. The TCP data sender detects lost or Explicit
Congestion Notification (ECN)-marked ACK packets, and tells the TCP
data receiver the ACK Ratio R to use to respond to the congestion on
the reverse path from the data receiver to the data sender. The TCP
data receiver sends roughly one ACK packet for every R data packets
received. This mechanism is based on the acknowledgement congestion
control in the Datagram Congestion Control Protocol's (DCCP's)
Congestion Control Identifier (CCID) 2. This acknowledgement
congestion control mechanism is being specified for further
evaluation by the network community.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This is a contribution to the RFC Series, independently of any other
RFC stream. The RFC Editor has chosen to publish this document at
its discretion and makes no statement about its value for
implementation or deployment. Documents approved for publication by
the RFC Editor are not a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc5690.
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IESG Note
The content of this RFC was at one time considered by the IETF, and
therefore it may resemble a current IETF work in progress or a
published IETF work.
Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document.
Table of Contents
1. Introduction ....................................................3
2. Conventions and Terminology .....................................4
3. Overview ........................................................4
4. Acknowledgement Congestion Control ..............................6
4.1. The ACK Congestion Control Permitted Option ................6
4.2. The TCP ACK Ratio Option ...................................7
4.3. The Receiver: Implementing the ACK Ratio ...................7
4.4. The Sender: Determining Lost or Marked ACK Packets .........8
4.4.1. The Sender: Detecting Lost ACK Packets
after a Congestion Event ...........................10
4.5. The Sender: Adjusting the ACK Ratio .......................10
4.5.1. Possible Addition: Decreasing the ACK Ratio
after a Congestion Window Decrease .................12
4.6. The Receiver: Sending ACKs for Out-of-Order Data
Segments ..................................................12
4.7. The Sender: Response to ACK Packets .......................13
4.8. Possible Addition: Receiver Bounds on the ACK Ratio .......15
5. Possible Complications .........................................15
5.1. Possible Complication: Delayed Acknowledgements ...........15
5.2. Possible Complication: Duplicate Acknowledgements .........15
5.3. Possible Complication: Two-Way Traffic ....................16
5.4. Possible Complication: Reordering of ACK Packets ..........16
5.5. Possible Complication: Abrupt Changes in the ACK Path .....17
5.6. Possible Complication: Corruption .........................17
5.7. Possible Complication: ACKs That Don't Contribute
to Congestion .............................................17
5.8. Possible Complication: TCP Implementations that
Skip ACK Packets ..........................................20
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5.9. Possible Complication: Router or Middlebox-Based
ACK Mechanisms ............................................21
5.10. Possible Complication: Data-Limited Senders ..............21
5.11. Other Issues .............................................22
6. Evaluating ACK Congestion Control ..............................22
6.1. Contention in Wireless Links or in Non-Switched Ethernet ..22
6.2. Keep-Alive and Other Special ACK Packets ..................22
7. Measurements of ACK Traffic and Congestion .....................23
8. Acknowledgement Congestion Control in DCCP's CCID 2 ............23
9. Security Considerations ........................................24
10. IANA Considerations ...........................................25
11. Conclusions ...................................................26
12. Acknowledgements ..............................................26
Appendix A. Related Work ..........................................27
A.1. ECN-Only Mechanisms .......................................28
A.2. Receiver-Only Mechanisms ..................................28
A.3. Middlebox-Based Mechanisms ................................29
Appendix B. Design Considerations .................................29
B.1. The TCP ACK Ratio Option, or an AckNow Bit in
Data Packets? .............................................29
Normative References ..............................................30
Informative References ............................................30
1. Introduction
This document describes a congestion control mechanism for
acknowledgements (ACKs) to TCP. This mechanism is based on the
acknowledgement congestion control in DCCP's CCID 2 ([RFC4340],
[RFC4341]), which is a successor to the TCP acknowledgement
congestion control mechanism proposed by Balakrishnan, et al. in
[BPK97].
In this document we use the terminology of senders and receivers,
with the sender sending data traffic and the receiver sending
acknowledgement traffic in response. In CCID 2's acknowledgement
congestion control, specified in Section 6.1 of [RFC4341], the
receiver uses an ACK Ratio R reported to it by the sender, sending
roughly one ACK packet for every R data packets received. The CCID 2
sender keeps the acknowledgement rate roughly TCP-friendly by
monitoring the acknowledgement stream for lost and marked ACK packets
and modifying the ACK Ratio accordingly. For every round-trip time
(RTT) containing an ACK congestion event (that is, a lost or marked
ACK packet), the sender halves the acknowledgement rate by doubling
the ACK Ratio; for every RTT containing no ACK congestion event, the
sender additively increases the acknowledgement rate through gradual
decreases in the ACK Ratio.
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The goal of this document is to explore a similar congestion control
mechanism for acknowledgement traffic for TCP. The assumption is
that in some environments with congestion on the reverse path,
reducing the sending rate for ACK traffic traversing the congested
path can help to reduce the congestion itself. For those
environments where the reverse path is congested but where TCP ACK
traffic does not appreciably contribute to that aggregate congestion,
the goal is for TCP's ACK congestion control to have a minimal
negative effect on the performance of the TCP connection.
Adding acknowledgement congestion control as an option in TCP would
require the following:
* An agreement from the TCP hosts on the use of ACK congestion
control. For the mechanism specified in this document, the TCP
hosts would use a new TCP option, the ACK Congestion Control
Permitted option.
* A mechanism for the TCP sender to detect lost and ECN-marked pure
acknowledgement packets.
* A mechanism for adjusting the ACK Ratio. The TCP sender would
adjust the ACK Ratio as specified in Section 6.1.2 of [RFC4341].
* A method for the TCP sender to inform the TCP receiver of a new
value for the ACK Ratio. For the mechanism specified in this
document, the TCP sender would use a new TCP option, the ACK Ratio
option.
2. Conventions and Terminology
MSS refers to the Maximum Segment Size.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. Overview
This section gives an overview of acknowledgement congestion control
for TCP.
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---------------------------------------------------------------
TCP Host A Router TCP Host B
(data sender) (data receiver)
---------- ------ ----------
<--- SYN with AckCC Permitted.
SYN/ACK with AckCC Permitted --->
. . .
Data packets --->
<--- one ACK packet
for every two data packets
. . .
Sender detects a lost ACK packet.
Data packet with an ACK Ratio option of 4 --->
<--- one ACK packet
for at most every four data packets
. . .
Sender detects a period with no lost ACK packets.
Data packet with an ACK Ratio option of 3 --->
<--- one ACK packet
for at most every three data packets
---------------------------------------------------------------
Figure 1: Acknowledgement Congestion Control,
Host B as the Connection Initiator, for a Connection without ECN
Figure 1 gives an example of acknowledgement congestion control
(AckCC) with TCP Host B as the connection initiator.
During connection initiation, TCP host B sends an ACK Congestion
Control Permitted option on its SYN or SYN/ACK packet. This allows
TCP host A (now called the sender) to send instructions to TCP host B
(now called the receiver) about the ACK Ratio to use in responding to
data packets.
Also during connection initiation, TCP host A sends an ACK Congestion
Control Permitted option on its SYN or SYN/ACK packet. In
combination with TCP host B's sending of an ACK Congestion Control
Permitted option, and with the negotiation of ECN-Capability as
specified in [RFC3168], this would allow TCP host B to send its ACK
packets as ECN-Capable.
The TCP receiver starts with an ACK Ratio of two, generally sending
one ACK packet for every two data packets received.
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The TCP sender detects a lost or ECN-marked ACK packet from the TCP
receiver and sends an ACK Ratio option of four to the receiver. The
TCP receiver changes to an ACK Ratio of four, sending one ACK packet
for at most four data packets. The TCP sender uses Appropriate Byte
Counting and rate-based pacing in responding to these ACK packets.
The TCP sender detects a period with no lost ACK packets and sends an
ACK Ratio option of three to the TCP receiver. The TCP receiver
changes back to an ACK Ratio of three, sending one ACK packet for at
most three data packets.
4. Acknowledgement Congestion Control
The goal of the mechanism proposed in this document is to control
pure ACK traffic on the path from the TCP data receiver to the TCP
data sender. Note that the approach outlined here is an end-to-end
one (as is the approach followed by DCCP's CCID 2 [RFC4341]), but it
may also take advantage of explicit congestion information from the
network, conveyed by ECN [RFC3168], if available. The ECN
specification ([RFC3168], see Section 6.1.4) prohibits a TCP receiver
from setting the ECT(0) or ECT(1) codepoints in IP packets carrying
pure ACKs, but *only* as long as the receiver does *not* implement
any form of ACK congestion control. Unlike some of the related work
cited in the appendix, in this document we are proposing an end-to-
end ACK congestion control mechanism that controls congestion on the
reverse path (the path followed by the ACK traffic) by detecting and
responding to either marked or dropped ACK packets.
4.1. The ACK Congestion Control Permitted Option
The TCP end-points would negotiate the use of ACK congestion control
(AckCC) with a TCP option: the ACK Congestion Control Permitted
option. The option number would be allocated by IANA.
The ACK Congestion Control Permitted option can only be sent on
packets that have the SYN bit set. If TCP end-point A receives an
ACK Congestion Control Permitted option from TCP end-point B, then
the TCP end-points may use ACK congestion control on the pure
acknowledgements sent from B to A. This means that TCP end-point A
may send ACK Ratio values to TCP end-point B, for TCP end-point B to
use on pure acknowledgement packets. Equivalently, if TCP end-point
A *does not* receive an ACK Congestion Control Permitted option from
TCP end-point B, then TCP end-point A knows not to waste its time
detecting lost ACK packets and adjusting and sending the ACK Ratio
values.
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If TCP end-point B receives an ACK Congestion Control Permitted
option from TCP end-point A, then the TCP end-points may use ACK
congestion control on the pure acknowledgements sent from A to B.
If TCP end-point B receives an ACK Congestion Control Permitted
option from TCP end-point A and also sent an ACK Congestion Control
Permitted option to TCP end-point A, and if ECN-Capability has been
negotiated, then TCP end-point B can send its pure ACK packets as
ECN-Capable.
TCP ACK Congestion Control Permitted Option:
Kind: TBD1
+-----------+-----------+
| Kind=TBD1 | Length=2 |
+-----------+-----------+
When ACK congestion control is used, the default initial ACK Ratio is
two, with the receiver acknowledging at least every other data
packet.
4.2. The TCP ACK Ratio Option
The sender uses an ACK Ratio TCP option to communicate the ACK Ratio
value from the sender to the receiver.
TCP ACK Ratio Option:
Kind: TBD2
+-----------+-----------+-----------+
| Kind=TBD2 | Length=3 | ACK Ratio |
+-----------+-----------+-----------+
The ACK Ratio option is only sent on data packets. Because TCP uses
reliable delivery for data packets, the TCP sender can tell if the
TCP receiver has received an ACK Ratio option.
4.3. The Receiver: Implementing the ACK Ratio
With an ACK Ratio of R, the receiver should send one pure ACK for
every R newly received data packets unless the delayed ACK timer
expires first. A receiver could simply maintain a counter that
increments by one for each new data packet received, and send an ACK
packet when the counter reaches R. The receiver would reset the
counter to zero whenever a pure or piggybacked ACK is sent.
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If the receiver has buffer limitations, the receiver might have to
acknowledge K packets, for some K less than the current ACK Ratio R.
In this case, the sender could observe from the acknowledgements that
the receiver is acknowledging less than R packets.
It is possible for there to be lost or marked ACK packets when there
haven't yet been any lost or marked data packets. Thus, the sender
could increase the ACK Ratio R even during the initial slow-start.
[RFC5681] recommends that the receiver SHOULD acknowledge out-of-
order data packets immediately, sending an immediate duplicate ACK
when it receives a data segment above a gap in the sequence space,
and sending an immediate ACK when it receives a data segment that
fills in all or part of a gap in the sequence space.
When ACK congestion control is being used and the ACK Ratio is at
most two, the TCP receiver acknowledges each out-of-order data packet
immediately. For an ACK Ratio greater than two, Section 4.6
specifies in detail the receiver's behavior for sending ACKs for out-
of-order data packets.
4.4. The Sender: Determining Lost or Marked ACK Packets
The TCP data sender uses its knowledge of the ACK Ratio in use by the
receiver to infer when an ACK packet has been lost.
Because the TCP sender knows the ACK Ratio R in use by the receiver,
the TCP sender knows that in the absence of dropped or reordered
acknowledgement packets, each new acknowledgement received will
acknowledge at most R additional data packets. Thus, if the sender
receives an acknowledgement acknowledging more than R data packets,
and does not receive a subsequent acknowledgement acknowledging a
strict subset (with a smaller cumulative acknowledgement, or with the
same cumulative acknowledgement but a strict subset of data
acknowledged in selective acknowledgement (SACK) blocks), then the
sender can infer that an ACK packet has been dropped. The use of
SACK options in ACK packets would help the sender in detecting lost
ACK packets.
Similarly, the TCP sender knows that in the absence of dropped or
delayed data packets from the sender, and in the absence of delayed
acknowledgements due to a timer expiring at the receiver, each new
pure acknowledgement received will acknowledge at least R additional
data packets. In terms of ACK congestion control, the TCP sender
does not have to take any actions when it receives an acknowledgement
acknowledging less than R additional packets.
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Out-of-order data packets:
If the ACK Ratio is at most two, then the TCP receiver sends a
duplicate acknowledgement (DupACK) for every out-of-order data
packet. In this case, the TCP sender should be able to detect
lost DupACK packets by counting the number of DupACKs that arrive
between the beginning of the loss event and the arrival of the
first full or partial ACK, and comparing this number with the
number of DupACKs that should have arrived (based on the number of
packets being ACKed by the full or partial ACK). Simulations
and/or experiments will be needed to determine whether, in
practice, it works for the TCP sender to assess lost ACK packets
during loss events, for an ACK Ratio of at most two.
If the ACK Ratio is greater than two, the TCP receiver does not
send a DupACK for every out-of-order data packet, as specified in
Section 4.6. For simplicity, the TCP sender does not attempt to
detect lost ACK packets during loss events involving forward-path
data traffic. That is, as soon as the sender infers a packet loss
for a forward-path data packet, it stops detection of ACK loss on
the reverse path. The sender waits until a new cumulative
acknowledgement is received that covers the retransmitted data,
and then restarts detection of ACK loss for reverse-path traffic.
Detecting lost ACK packets after changes in the ACK Ratio:
In detecting lost ACK packets, the sender relies on its knowledge
of the ACK Ratio used by the receiver. But when the sender makes
a change in the ACK Ratio and then receives ACK packets, how does
the sender know whether the receiver was using the new or the old
ACK Ratio when it sent those ACK packets? As specified in the
next section, the sender can make only one of two possible changes
to the ACK Ratio within one round-trip time. The sender can
decrease the ACK Ratio by one, from R to R-1, or the sender can
double the ACK Ratio, increasing it from R to 2R. But, in
detecting lost ACK packets after an increase in the ACK Ratio, the
sender needs to know whether the receiver was using the old ACK
Ratio R or the new ACK Ratio 2R.
The sender sends ACK Ratio options only on data packets, and these
data packets are acknowledged by the receiver. One possibility
would be for the sender to save the sequence number of the last
data packet that contained an ACK Ratio option and to remember
whether that ACK Ratio option was for an increase or a decrease in
the ACK Ratio. Then, if the sender receives an ACK packet
acknowledging the saved sequence number, the sender knows that the
receiver has begun using the new ACK Ratio.
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It *might* be sufficient for the sender just to save the
information of whether the last change in the ACK Ratio was an
increase or a decrease, without saving the sequence number
associated with the last ACK Ratio option. In this way, if the
sender recently increased the ACK Ratio from R to 2R, the sender
could be more cautious in detecting lost ACK packets. Another
possibility would be that, after sending an ACK Ratio option, the
sender waits until that data has been ACKed, with the new ACK
Ratio in use by the receiver, before resuming the detection of
lost ACK packets. However, we do not explore either of these
approaches in more detail in this document.
4.4.1. The Sender: Detecting Lost ACK Packets after a Congestion Event
After a sender's retransmit timeout or fast retransmit, the sender
might retransmit a number of data packets dropped from a single
window of data. In particular, during a loss recovery period (from
the sender's detection of the congestion event up until the sender
receives an acknowledgement of all data packets transmitted before
the loss recovery period began), retransmitted data packets can fill
holes in the receiver's sequence space, resulting in irregular jumps
in the cumulative acknowledgement field in ACK packets from the
receiver. These jumps in the cumulative acknowledgement field make
it difficult for the sender to reliably detect lost ACK packets
during a loss recovery period.
Because of this uneven progress of the cumulative acknowledgement
field during a loss recovery period, the sender should not attempt to
detect lost ACK packets during a loss recovery period. As a
consequence, the sender will not increase the ACK Ratio in response
to ACK packets that are lost during a loss recovery period.
4.5. The Sender: Adjusting the ACK Ratio
The TCP sender will adjust the ACK Ratio as specified in Section
6.1.2 of [RFC4341], as follows.
The ACK Ratio always meets the following three constraints.
(1) The ACK Ratio is an integer.
(2) The minimum ACK sending rate: The ACK Ratio does not exceed
max(2, cwnd/(K*MSS)), rounded up, for K=2. As a result, the TCP
receiver generally sends at least two ACKs in response to a
window of at least four full-sized segments.
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(3) If the congestion window is at least as large as four full-sized
segments, then the ACK Ratio is at least two. In other words, an
ACK Ratio of one is only allowed when the congestion window is at
most three full-sized segments.
The sender changes the ACK Ratio within those constraints as follows.
For each congestion window of data with lost or marked ACK packets,
the ACK Ratio R is doubled; for each cwnd/(MSS*(R^2 - R)) consecutive
congestion windows of data with no lost or marked ACK packets, the
ACK Ratio is decreased by 1. (See Appendix A of RFC 4341 for the
derivation. Note that Appendix A of RFC 4341 assumes a congestion
window W in packets, while we use cwnd in bytes.) As stated in the
previous section, when the ACK Ratio is greater than two, the sender
does not attempt to detect lost ACK packets during loss events for
forward-path traffic.
For a constant congestion window, these modifications to the ACK
Ratio give an ACK sending rate that is roughly TCP-friendly. Of
course, cwnd usually varies over time; the dynamics will be rather
complex, but roughly TCP friendly. We recommend that the sender
determines when to decrease the ACK Ratio by one (i.e., by
calculating the number of in-order data packets to count) right after
an ACK loss event.
The frequency of ACK Ratio negotiations:
The sender need not keep the ACK Ratio completely up to date. For
instance, it may rate-limit ACK Ratio renegotiations to once every
four or five round-trip times, or to once every second or two.
The sender should not attempt to change the ACK Ratio more than
once per round-trip time. In particular, before sending a packet
with a new value for the ACK Ratio, the sender should verify that
the receiver has acknowledged a data packet containing an ACK
Ratio option for the old value of the ACK Ratio. Additionally,
the sender may enforce a minimum ACK Ratio of two, or it may set
the ACK Ratio to one for half-connections with persistent
congestion windows of 1 or 2 packets.
The minimum ACK sending rate:
From rule (2) above, the TCP receiver always sends at least K=2
ACKs for a window of data, even in the face of very heavy
congestion on the reverse path. We would note, however, that if
congestion is sufficiently heavy, all the ACK packets are dropped,
and then the sender falls back on an exponentially backed-off
timeout. Thus, if congestion is sufficiently heavy on the reverse
path, then the sender reduces its sending rate on the forward
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path, which reduces the rate on the reverse path as well. One
possibility would be to use a higher minimum ACK-sending rate,
adding a constant upper bound on the ACK Ratio. That is, if the
ACK Ratio also had an upper bound of J, independent of cwnd, then
the receiver would always send at least one ACK for every J data
packets, regardless of the level of congestion on the reverse
path.
4.5.1. Possible Addition: Decreasing the ACK Ratio after a Congestion
Window Decrease
After a lost or ECN-marked data packet, the data sender halves the
congestion window, thus halving the sending rate for data packets,
while making no change to the ACK Ratio R. As a result, after a
congestion event involving a data packet, the sending rate for ACK
packets on the return path is also halved. If the congestion event
was a lost or ECN-marked data packet, this was due to congestion on
the forward path, which may have been unrelated to conditions on the
reverse path. Thus, it has been suggested that the sender could
decrease the ACK Ratio R when it halves the congestion window; in
this case, the halving of the sending rate for data packets would not
be accompanied by a halving of the sending rate for ACK packets also.
However, there are a few cases where a congestion event involving
data packets could in fact have been caused by congestion on the
reverse path. As one example, the path could include a congested
multiaccess link where forward-path and reverse-path traffic can
interfere with each other. Thus, in this case it might be desirable
if a congestion event resulted in a reduction in the sending rate of
ACK packets as well as of data packets.
As a second example of a congestion event involving congestion of the
reverse path, a congestion event could be caused not by a dropped or
ECN-marked data packet, but by a window of dropped ACK packets,
resulting in a retransmit timeout at the data sender. After a
retransmit timeout, the TCP sender will slow-start, reducing the
congestion window to the initial window and setting the ACK Ratio to
at most two.
Until further investigation, the sender will not decrease the ACK
Ratio as a result of a congestion event involving a data packet.
4.6. The Receiver: Sending ACKs for Out-of-Order Data Segments
RFC 5681 says that "a TCP receiver SHOULD send an immediate duplicate
ACK when an out-of-order segment arrives". After three duplicate
ACKs are received, the TCP sender infers a packet loss and implements
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fast retransmit and fast recovery, retransmitting the missing packet.
When the ACK Ratio is at most two, the TCP receiver should still send
an immediate duplicate ACK when an out-of-order segment arrives.
In general, when the ACK Ratio is greater than two, the TCP receiver
still should send an immediate duplicate ACK for each of the first
three out-of-order segments that arrive in a reordering event. (We
define a reordering event at the receiver as beginning when an out-
of-order segment arrives, and ending when the receiver holds no more
out-of-order segments.) However, when the ACK Ratio is greater than
two, after the first three duplicate ACKs have been sent, the TCP
receiver should perform ACK congestion control on the remaining ACKs
to be sent during the current reordering event. That is, after the
first three duplicate ACKs have been sent, the TCP receiver should
return to sending an ACK for every R segments, instead of sending an
ACK for every out-of-order segment in that reordering event. (We
note that the fast recovery procedure of the TCP sender might have to
be modified to take this change into account.) In addition, a
receiver must not withhold an ACK for more than 500 ms.
We note that in an environment with systematic reordering in the data
path (e.g., every set of K data packets arrives in inverted order,
for some value of K), the guideline above could result in the
receiver sending an ACK for every data packet, regardless of the ACK
Ratio. In such an environment with persistent reordering, the
receiver may decide not to send an immediate duplicate ACK for each
of the first three out-of-order segments that arrive in a reordering
event. We leave the investigation of mechanisms for effective ACK
congestion control in environments with systematic reordering for
future work.
4.7. The Sender: Response to ACK Packets
The use of a large ACK Ratio can generate line-rate data bursts at a
TCP sender. When the ACK Ratio is greater than two, the TCP sender
should use some form of burst mitigation or rate-based pacing for
sending data packets in response to a single acknowledgement. The
use of rate-based pacing will be limited by the timer granularity at
the TCP sender.
We note that the interaction of ACK congestion control and burst
mitigation schemes needs further study.
Byte counting at the sender:
In addition to the impact of a large ACK Ratio on the burstiness
of the TCP sender's sending rate, a large ACK Ratio can also
affect the data-sending rate by slowing down the increase of the
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congestion window cwnd. As specified in RFC 5681, in slow-start
the TCP sender increases cwnd by one full-sized segment for each
new ACK received (in this context, a "new ACK" is an ACK that
acknowledges new data). RFC 5681 also specifies that in
congestion avoidance, the TCP sender increases cwnd by roughly
1/cwnd full-sized segments for each ACK received, resulting in an
increase in cwnd of roughly one full-sized segment per round-trip
time. In this case, the use of a large ACK Ratio would slow down
the increase of the sender's congestion window.
RFC 5681 notes that during congestion avoidance, it is also
acceptable to count the number of bytes acknowledged by new ACKs
and to increase cwnd based on the number of bytes acknowledged,
rather than on the number of new ACKs received. Thus, the sender
should use this form of byte counting with acknowledgement
congestion control, so that the acknowledgement congestion control
doesn't slow down the window increases for the data traffic sent
by the sender. Because rate-based pacing should be used with
acknowledgement congestion control, as recommended earlier in this
section, the TCP sender may increase the congestion window by more
than two MSS for each ACK.
We note that for Appropriate Byte Counting (ABC) as specified in
[RFC3465], during slow-start the sender is allowed to increase the
congestion window by at most two MSS for each ACK. It has not yet
been determined whether, with acknowledgement congestion control,
the TCP sender could use ABC during slow-start. If ABC is used
with acknowledgement congestion control, then when the TCP sender
is in slow-start and the ACK Ratio is greater than two, the TCP
sender may increase the congestion window by more that two MSS in
response to a single ACK. Section 4.2 of [LL07] explores some of
the issues with the use of ABC for TCP connections with a fixed
ACK Ratio greater than two.
Inferring lost data packets:
As cited earlier, RFC 5681 infers that a packet has been lost
after it receives three duplicate acknowledgements. Because ACK
congestion control is only used when there is congestion on the
reverse path, after a packet loss, one or more of the three
duplicate ACKs sent by the receiver could be lost on the reverse
path, and the receiver might wait until it has received R more
out-of-order segments before sending the next duplicate ACK. All
this could slow down fast recovery and fast retransmit quite a
bit. The use of SACK can help reduce the potential delay in
detecting a lost packet. With SACK, a TCP sender can use the
information in the SACK option to detect when the receiver has
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received at least three out-of-order data packets and to initiate
fast retransmit and fast recovery in this case, even if the TCP
sender has not yet received three duplicate ACKs.
4.8. Possible Addition: Receiver Bounds on the ACK Ratio
It has been suggested that in some environments, the TCP receiver
might want to set lower bounds on the ACK Ratio. For example, the
TCP receiver might know from configuration or from past experience
that the bandwidth on the return path is limited, and might want to
set a lower bound (greater than two) on the ACK Ratio R. If this is
included, this would require a TCP option from the TCP receiver to
the TCP sender, reporting the lower bound on the ACK Ratio. Care
would also be needed so that the lower bound on the ACK Ratio was
only in effect when the TCP sender's congestion window was
sufficiently high.
5. Possible Complications
5.1. Possible Complication: Delayed Acknowledgements
The receiver could send a delayed acknowledgement acknowledging a
single packet, even when the ACK Ratio is two or more.
This should not cause false positives (when the TCP sender infers a
loss when no loss happened). The TCP sender only infers that a pure
ACK packet has been lost when no data packet has been lost and an ACK
packet arrives acknowledging more than R new packets.
Delayed acknowledgements could, however, cause false negatives, with
the TCP sender unable to detect the loss of an ACK packet sent as a
delayed acknowledgement. False negatives seem acceptable; this would
result in approximate ACK congestion control, which would be better
than no ACK congestion control at all. In particular, when this form
of false negative occurs, it is because the receiver is sending
acknowledgements at such a low rate that it is sending delayed
acknowledgements, rather than acknowledging at least R data packets
with each acknowledgement.
5.2. Possible Complication: Duplicate Acknowledgements
As discussed in Section 4.3, RFC 5681 states that "a TCP receiver
SHOULD send an immediate duplicate ACK when an out-of-order segment
arrives", and that "a TCP receiver SHOULD send an immediate ACK when
the incoming segment fills in all or part of a gap in the sequence
space" [RFC5681]. When ACK congestion control is used, the TCP
receiver instead uses the guidelines from Section 4.6 to govern the
sending of duplicate ACKs. More work would be useful to evaluate the
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advantages and disadvantages of this approach in terms of the
potential delay in triggering fast retransmit, and to explore
alternate possibilities.
5.3. Possible Complication: Two-Way Traffic
In a TCP connection with two-way traffic, the receiver could send
some pure ACK packets and some acknowledgements piggybacked on data
packets. The receiver would still follow the rule of only sending a
pure ACK packet when there is a need for a delayed ACK or when there
are R new data packets to acknowledge.
In a connection with two-way traffic, the TCP sender would not always
be able to infer when a pure ACK packet had been lost. For example,
the receiver could send a pure ACK packet acknowledging packet K and,
soon afterwards, the receiver could send a newly generated data
packet for the reverse-path flow also acknowledging packet K. The
pure ACK packet could be dropped in the network, and the sender would
not be able to detect this drop.
Fortunately, there are limitations to the potential problems caused
by undetected ACK losses in two-way traffic. The sender will only
fail to detect the loss of a pure ACK packet if the ACK packet was
followed by a data packet with the same acknowledgement number. If
the reverse-path traffic for the connection is dominated by data
traffic, then the congestion control for the data traffic is more
important than the congestion control for the pure ACK traffic. If
the reverse-path traffic is dominated by pure ACK traffic, then the
sender would detect any losses of pure ACK packets followed by other
pure ACK packets, and this would include most of the pure ACK packets
for that connection. Thus, the sender's failure to detect the loss
of a pure ACK packet followed by a data packet with the same
acknowledgement number would not disable acknowledgement congestion
control for a TCP connection with two-way traffic.
5.4. Possible Complication: Reordering of ACK Packets
It is possible for ACK packets to be reordered on the reverse path.
The TCP sender could either use a parallel mechanism to the DupACK
threshold to infer when an ACK packet has been lost, as with TCP, or,
more robustly, the TCP sender could wait an entire round-trip time
before inferring that an ACK packet has been lost [RFC4653].
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5.5. Possible Complication: Abrupt Changes in the ACK Path
What happens when there are abrupt changes in the reverse path, such
as from vertical handovers? Can there be any problems that would be
worse than those experienced by a TCP connection that is not using
ACK congestion control?
5.6. Possible Complication: Corruption
As with data packets, it is possible for ACK packets to be dropped in
the network due to corruption rather than congestion. The current
assumption of ACK congestion control is that all losses should be
taken as indications of congestion. If there is ever some better
mechanism for identifying and responding to corrupted TCP data
packets, the same solution hopefully would apply to corrupted ACK
packets as well.
One problem with the interaction of packet corruption and congestion
control, for both data and ACK packets, is that it is not always
obvious when the packet corruption is related to congestion and when
the packet corruption is independent of the level of congestion on
the corrupting link. In environments where packet corruption exists
and is independent of the level of congestion on the corrupting link,
applying ACK congestion control would only make the connection more
sensitive to ACK packet corruption by reducing the number of ACKs
that are sent.
5.7. Possible Complication: ACKs that Don't Contribute to Congestion
It is possible for the ACK packets in a TCP connection to traverse a
congested path where ACK packets are dropped but where the ACK
packets themselves don't significantly contribute to the congestion
on the path. In scenarios where ACK packets are dropped but where
ACK traffic doesn't make a significant contribution of the congestion
on the path, the use of ACK congestion control would not contribute
to reducing the aggregate congestion on the path. In this case, one
goal is to minimize the negative impact of ACK congestion control on
the overall performance of the TCP connection.
J TCP conns. link L -> J TCP conns.
data -> |---| |---| <- ACKs
<-------------> | | | | <------------->
| | <-------------> | |
<-------------> | | | | <------------->
K TCP conns. |---| |---| K TCP conns.
ACKs -> <- link L1 <- data
Figure 2. A Scenario with J Forward and K Reverse TCP Connections
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To explore the relative contribution of ACK traffic on congestion, it
is useful to consider a simple scenario with a congested
unidirectional link L carrying data traffic from J TCP connections
(the forward TCP connections) and ACK traffic from K TCP connections
(the reverse TCP connections). We assume that all TCP connections
have the same round-trip time R and the same data packet size S of
1500 bytes. We further assume that all of the forward TCP
connections have the same data packet drop rate p and the same
congestion window W, and that all of the reverse TCP connections have
the same congestion window W1 and the same ACK packet drop rate p1.
(The packet drop rate for data packets is defined as the fraction of
arriving data packets that are dropped; similarly, the packet drop
rate for ACK packets is the fraction of arriving ACK packets that are
dropped.) The J TCP connections each use a bandwidth on link L of
1500*W/R bytes per second, and the K TCP connections, without ACK
congestion control, each use a bandwidth on link L of 40*(W1/2)/R
bytes per second. This gives a ratio of 75*(J/K)*(W/W1) for TCP data
bandwidth to TCP ACK bandwidth on link L. The ratio J/K is the ratio
between the number of forward and reverse TCP connections on link L,
and could have a wide range of values (e.g., large for an access link
from a web server, and small for an access link to a web server).
For this scenario, the ratio W/W1 is largely a function of the
different levels of congestion on the forward and reverse paths.
To explore the possibilities, we will consider some of the range of
congestion control mechanisms for the congested link. First, we
consider scenarios where the limitation on the congested path is in
the link bandwidth in bytes per second.
Cases (1), (2), (3), (5), and (7) below represent the best scenarios
for ACK congestion control, where the fraction of packet drops for
TCP ACK packets roughly matches the TCP ACK packets' contribution to
congestion. (In several of these cases this is, at best, a rough
match because the data packets are a factor in the bandwidth and in
the queue limitations, while the TCP ACK packets are only a factor in
the queue limitations.) Cases (4) and (8) below represent
problematic scenarios where the fraction of packet drops for TCP ACK
packets is much higher than the TCP ACK packets' contribution to
congestion (in terms of taking space in a congested queue, using
scarce CPU cycles at the congested router, or using scarce
bandwidth). Case (6) below represents scenarios where ACK congestion
control would not be effective because it would not be invoked. In
the scenarios in case (6), the fraction of packet drops for TCP ACK
packets would be much smaller than the TCP ACK packets' contribution
to congestion.
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(1) The Drop-Tail queue for link L is measured in packets. In this
case, the congested queue can accommodate N packets (regardless
of packet size), there is a limitation of both bandwidth in bytes
per second and also in queue space in packets, and large data
packets and small TCP ACK packets should see similar packet drop
rates. Although TCP ACK packets most likely aren't a major
factor in the bandwidth limitation, they can be a significant
contribution to the limitation of queue space. So, while the
packet drop rate for ACK packets could be high in times of
congestion, the ACK packets are contributing to that congestion
somewhat by using scarce buffer space.
(2) The Drop-Tail queue is measured in bytes. In this case, the
congested queue can accommodate M bytes of packets, and TCP ACK
packets don't make a significant contribution to either the
bandwidth limitation or to the limitation in queue space. It is
also the case that, in this scenario, even if there is heavy
congestion, the packet drop rate for TCP ACK packets should be
small (because small ACK packets can often find space on the
congested queue when large data packets can't find space). In
this case, ACK congestion control should not present any
problems; the TCP ACK packets aren't contributing significantly
to congestion and aren't experiencing significant packet drop
rates.
(3) The RED queue is in packet mode and is measured in packets. This
is similar to case (1) above. Because the queue is measured in
packets, small TCP ACK packets contribute to the limitation in
queue space but not to the limitation in link bandwidth. Because
the queue is in packet mode, large data packets and small TCP ACK
packets should see similar packet drop rates.
(4) The RED queue is in packet mode but is measured in bytes.
Because the queue is measured in bytes, small TCP ACK packets
don't contribute significantly to either the limitation in queue
space or to the limitation in link bandwidth. Because the queue
is in packet mode, large data packets and small TCP ACK packets
should see similar packet drop rates. If it existed, this case
would be problematic, because the TCP ACK packets would not be
contributing significantly to the congestion but they would see a
similar packet drop rate as the large data packets that are
contributing to congestion.
(5) The RED queue is in byte mode and is measured in bytes. This is
similar to case (2) above. Because the queue is measured in
bytes, small TCP ACK packets don't contribute significantly to
either the limitation in queue space or to the limitation in link
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bandwidth. At the same time, because the queue is in byte mode,
small TCP ACK packets see much smaller packet drop rates than
those of large data packets.
(6) The RED queue is in byte mode but is measured in packets.
Because the queue is measured in packets, small TCP ACK packets
contribute to the limitation in queue space but not to the
limitation in link bandwidth. Because the queue is in byte mode,
small TCP ACK packets see much smaller packet drop rates than
those of large data packets. If this case existed, TCP ACK
packets would contribute somewhat to congestion but would see a
much smaller packet drop rate than that of large data packets.
Next, we consider scenarios where the limitation on the congested
link is in CPU cycles at the router in packets per second, not in
bandwidth in bytes per second.
(7) The CPU load imposed by TCP ACK packets is similar to the load
imposed by other packets (e.g., TCP data packets). ACK
congestion control would be useful in this scenario, particularly
if TCP ACK packets saw the same packet drop rates as TCP data
packets.
(8) The CPU load imposed by TCP ACK packets is much less than the
load imposed by other packets (e.g., TCP data packets). If TCP
ACK packets saw a smaller packet drop rate than TCP data packets,
then the TCP ACK packet drop rate would roughly match the TCP ACK
packets' contribution to congestion, and this would be good. If
TCP ACK packets saw the same packet drop rate as TCP data
packets, this case would be problematic, because the TCP ACK
packets would not be contributing significantly to the
congestion, but they would see a similar packet drop rate as the
large data packets that are contributing to congestion.
5.8. Possible Complication: TCP Implementations that Skip ACK Packets
It has been reported in IETF meetings that current TCP
implementations do not always acknowledge at least every other data
packet, as required by the TCP specifications. In particular, it has
been reported that if a TCP receiver receives many data packets in a
burst, before it is able to send an acknowledgement, then it might
send a single acknowledgement for the burst of packets. We note that
such a behavior would cause complications for a TCP connection that
used ACK congestion control, as the sender would not be able to
determine when an ACK packet had been dropped in the network or when
the packet had been skipped by the receiver because it was processing
a burst of data packet arrivals.
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One possibility for addressing this problem would be for TCP
receivers using ACK congestion control to be required to send an
acknowledgement for each R packets, for ACK Ratio R. In this case,
if the receiver received a large burst of data packets back-to-back,
the receiver would be required to send a responding burst of ACK
packets, one for each set of R data packets.
A second possibility for addressing this problem would be to define a
TCP option or flag that the TCP receiver could use when sending an
ACK packet to inform the sender that the TCP receiver `skipped' some
ACK packets, so that the sender should not infer ACK loss if some
previous ACK packets seem to be missing.
Future work will explore the costs and benefits of these two
approaches.
5.9. Possible Complication: Router or Middlebox-Based ACK Mechanisms
One possible complication would be the interaction of ACK congestion
control with router-based or middlebox-based ACK mechanisms, such as
ACK filtering along the reverse path ([BPK97], [WWCM99], [BA03],
[KLS07]). We are not aware of the deployment of ACK filtering in the
Internet, but any testing of ACK congestion control would have to
look for interactions with any middlebox-based mechanisms regarding
ACK packets. In particular, we would consider interactions of ACK
congestion control with the possible deployment of ACK filtering on
satellite links, cable modems, or the like.
5.10. Possible Complication: Data-Limited Senders
The mechanism for adjusting the ACK Ratio is designed with the goal
of having the TCP receiver send at least two ACKs in response to each
window of at least four full-sized data packets. However, with ACK
congestion control in combination with a data-limited sender, it is
possible for the sender to send at least four full-sized data packets
in a round-trip time, with the receiver sending less than two ACKs in
response.
As an example, consider a connection where the sender's congestion
window W is greater than four and the ACK Ratio R is at its maximum
value of W/2. If the sender becomes data-limited and sends less than
W data packets in a round-trip time, then the receiver can send less
than two ACK packets in response. This behavior makes the connection
more sensitive to the loss of an occasional ACK packet.
Of course, there is still the safety mechanism of the receiver
sending an ACK packet when the delayed ACK timer expires. However,
more work would be useful to explore the conflicting goals of a
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congestion-controlled ACK flow and a timely ACK response to the
sender for the specific case of a connection with a data-limited
sender and a congested ACK path.
5.11. Other Issues
Are there any problems caused by the combination of two-way traffic
and reordering? Or other issues that have not yet been addressed?
6. Evaluating ACK Congestion Control
Evaluating ACK congestion control will have two components: (1)
evaluating the effects of ACK congestion control on an individual TCP
connection, and (2) evaluating the effects of ACK congestion control
on aggregate traffic (including the effects of ACK congestion control
on the aggregate congestion of the path).
The first part, evaluating ACK congestion control on the performance
of an individual TCP connection, will have to examine those scenarios
where ACK congestion control might help the performance of a TCP
connection and those scenarios where the use of ACK congestion
control might cause problems.
The second part, evaluating the effects of ACK congestion control on
aggregate traffic, should consider scenarios where the use of ACK
congestion control helps all of the connections sharing a path by
reducing the aggregate congestion on the path. This part should also
see if there are scenarios where ACK congestion control causes
problems by increasing the burstiness of aggregate traffic or by
otherwise changing traffic dynamics.
6.1. Contention in Wireless Links or in Non-Switched Ethernet
One possible benefit of ACK congestion control is that it could
reduce contention in wireless links, shared Ethernet, or other
environments with contention between forward-path and reverse-path
traffic ([AJ03], [KIA07]). At the same time, contention on the
shared medium won't necessarily result in dropped ACK packets, and
therefore wouldn't necessarily be detected by ACK congestion control.
6.2. Keep-Alive and Other Special ACK Packets
Some TCP hosts send keep-alive packets when no data or ACK packets
have been received over a long period of time [KEEP-ALIVE]. This
keep-alive mechanism is not addressed in TCP specifications.
However, such keep-alive packets, if used, should not interact with
ACK congestion control one way or another. For ACK congestion
control, the ACK Ratio is set small enough to allow the receiver to
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generally send at least two ACKs for a window of data. In addition,
the receiver uses a delayed ACK timer with the ACK Ratio, always
sending an acknowledgement if the delayed ACK timer expires. Thus,
ACK congestion control will never cause the receiver to delay
indefinitely in sending an acknowledgement for a received data
packet.
Some TCP implementations send pure ACK packets as window probes, to
solicit an ACK packet from the other end with current window
information. Such ACK packets will generally be orthogonal to the
ACK congestion control specified in this document.
TCP receivers also can send pure ACK packets as window update packets
announcing a new value for the receive window, even when the
acknowledgement number and SACK options in the ACK packet are not
new. The receiver may send window update packets even if the ACK
congestion control mechanism would say that it is not time yet to
send a pure ACK. The sender will not necessarily be able to detect
the loss of a window update ACK packet.
7. Measurements of ACK Traffic and Congestion
There are a number of studies about the traffic composition on
various links in the Internet, reporting the fraction of bandwidth
used by TCP data and by TCP ACK traffic [Studies].
Are there any studies that show the relative packet drop rates for
TCP data and ACK traffic, for particular links or for particular TCP
connections?
Are there any studies of congested links that show the fraction of
traffic on the congested link, or in the congested queue, that
consist of TCP ACK packets?
8. Acknowledgement Congestion Control in DCCP's CCID 2
In the transport protocol DCCP [RFC4340], the congestion control
mechanism for the CCID 2 profile is based on that of TCP. This
section briefly discusses some of the issues that have been addressed
in the acknowledgement congestion control already standardized in
CCID 2 [RFC4341].
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Rate-based pacing:
For CCID 2, RFC 4341 says that "senders MAY use a form of rate-
based pacing when sending multiple data packets liberated by a
single ACK packet, rather than sending all liberated data packets
in a single burst." However, rate-based pacing is not required in
CCID 2.
Increasing the congestion window:
For CCID 2, RFC 4341 says that "when cwnd < ssthresh, meaning that
the sender is in slow-start, the congestion window is increased by
one packet for every two newly acknowledged data packets with ACK
Vector State 0 (not ECN-marked), up to a maximum of ACK Ratio/2
packets per acknowledgement. This is a modified form of
Appropriate Byte Counting [RFC3465] that is consistent with TCP's
current standard (which does not include byte counting), but
allows CCID 2 to increase as aggressively as TCP when CCID 2's ACK
Ratio is greater than the default value of two. When cwnd >=
ssthresh, the congestion window is increased by one packet for
every window of data acknowledged without lost or marked packets."
9. Security Considerations
What are the sender's incentives to cheat on ACK congestion control?
What are the receiver's incentives to cheat? What are the avenues
open for cheating?
As long as ACK congestion control is optional, neither host can be
forced to use ACK congestion control if it doesn't want to. So ACK
congestion control will only be used if the sender or receiver have
some chance of receiving some benefit.
As long as ACK congestion control is optional for TCP, there is
little incentive for the TCP end nodes to cheat on non-ECN-based ACK
congestion control. There is nothing now that requires TCP hosts to
use congestion control in response to dropped ACK packets.
What avenues for cheating are opened by the use of ECN-Capable ACK
packets? If the end nodes can use ECN to have ACK packets marked
rather than dropped, and if the end nodes can then avoid the use of
ACK congestion control that goes along with the use of ECN on ACK
packets, then the end nodes could have an incentive to cheat.
Senders could cheat by not instructing the receiver to use a higher
ACK Ratio; the receiver would have a hard time detecting this
cheating. Receivers could cheat by not using the ACK Ratio they were
instructed to use, but senders could easily detect this cheating.
However, receivers could also cheat by not using ACK congestion
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control and still sending ACK packets as ECN-Capable, so ACK
congestion control is not a necessary component for receivers to
cheat about sending ECN-Capable ACK packets. One question would be
whether there is any way for receivers to cheat about sending ECN-
Capable ACK packets and not using appropriate ACK congestion control
without this cheating being easily detected by the sender.
What about the ability of routers or middleboxes to detect TCP
receivers that cheat by inappropriately sending ACK packets as ECN-
Capable? The router will only know if the receiver is authorized to
send ACK packets as ECN-Capable if the router can see traffic on both
the forward and reverse paths and monitored both the SYN and SYN/ACK
packets (and was able to read the TCP options in the packet headers).
If ACK congestion control has been negotiated, the router will only
know if ACK congestion control is being used correctly by the
receiver if it can monitor the ACK Ratio options sent from the sender
to the receiver. If ACK congestion control is being used, the router
will not necessarily be able to tell if ACK congestion control is
being used correctly by the sender, because drops of ACK packets
might be occurring after the ACK packets have left the router.
However, if the router sees the ACK Ratio options sent from the
sender, the router will be able to tell if the sender is correctly
accounting for those ACK packets that are dropped or ECN-marked on
the path from the receiver to the router.
10. IANA Considerations
No IANA action is needed at this time. If this document was advanced
as Experimental or Proposed Standard, then IANA would allocate the
option numbers for the two TCP options, the ACK Congestion Control
Permitted option, and the ACK Ratio option. In such a case, the
following two lines would be added to the TCP Option Numbers registry
(maintained by IANA -- http://www.iana.org):
Kind Length Meaning Reference
---- ------ --------------------------------- -----------
TBD1 2 ACK Congestion Control Permitted [RFCXXXX]
TBD2 3 ACK Ratio [RFCXXXX]
In the absence of TCP option numbers allocated by IANA, experimenters
may use the TCP Option Numbers set aside for Experimentation in RFC
4727 [RFC4727]. As stressed in Section 1 of RFC 3692 [RFC3692], the
TCP Option Numbers in the experimental range are intended for
experimentation and testing and not for wide or general deployments;
these option numbers could be in use by other experimentors for other
purposes.
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11. Conclusions
This document specifies a congestion control mechanism for
acknowledgement (ACKs) traffic for TCP and discusses the possible
complications. We are deferring a recommendation on the use of this
mechanism for TCP until it has been evaluated more fully.
12. Acknowledgements
Many thanks for feedback from Mark Allman, Armando Caro, Alfred
Hoenes, Ilpoo Jarvinen, Sara Landstrom, Anantha Ramaiah, and Michael
Welzl, and for contributed text from Michael Welzl.
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Appendix A. Related Work
There exist several papers dealing with controlling congestion in the
reverse path of a TCP connection, especially in the context of
networks with bandwidth asymmetry. Some of these proposals require
explicit support from routers or middleboxes, whereas others are
"pure" end-to-end schemes.
RFC 3449 [RFC3449] discusses TCP performance problems that arise in
TCP connections over asymmetric paths. Section 3 of RFC 3449
describes in detail how congestion on the ACK path can affect overall
TCP performance. RFC 3449 also outlines a number of proposed
mitigations, including ACK congestion control. The experimental ACK
congestion control mechanism discussed in that RFC relies on ECN,
with the TCP sender detecting congestion on the ACK path from ECN-
marked packets. RFC 3449 also discusses two receiver-based
mechanisms, the Window Prediction Mechanism (WPM) [CLP98] and
Acknowledgement based on Cwnd Estimation (ACE) [MJW00], for using a
dynamic ACK Ratio. RFC 3449 also considers link- and network-layer
techniques that address congestion on the upstream path. These
include header compression as well as bandwidth management techniques
for the upstream link, including ACK filtering and ACK
reconstruction.
RFC 3135 [RFC3135], "Performance Enhancing Proxies Intended to
Mitigate Link-Related Degradations", surveys a range of Performance
Enhancing Proxies used to improve TCP behavior, including proxies for
ACK filtering and reconstruction. RFC 2760 [RFC2760], "Ongoing TCP
Research Related to Satellites", discusses both ACK congestion
control and ACK filtering and reconstruction, with detailed
descriptions of the mechanisms proposed by Balakrishnan, et al. in
[BPK97].
Landstrom, et al. in [LL07] explore a mechanism where the receiver
sends only four acknowledgements per window of data, along with the
sender using a form of Appropriate Byte Counting. In addition, the
receiver reverts to a lower acknowledgement frequency after a
timeout, to avoid unnecessary retransmit timeouts. One conclusion of
the paper is that pacing at the sender introduces an additional delay
and might not be necessary. A key result of the paper is that, with
the use of some form of byte counting at the sender, it is possible
for TCP to use a lower acknowledgement frequency than that of delayed
acknowledgements.
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A.1. ECN-Only Mechanisms
Balakrishnan, et al. in [BPK97] describe the use of ECN to detect
congestion in the return path, in order to reduce the sending rate of
ACKs. The use of a RED queue in the reverse path allows for marking
of ACK packets. The sender echoes back ECN congestion marks to the
receiver. The receiver keeps an ACK Ratio d (called the "delayed-ACK
factor"), specifying the number of data segments that have to be
received before the receiver sends a new ACK. The ACK Ratio d is
managed using multiplicative-increase, additive-decrease; upon
reception of a congestion mark, the receiver doubles the value of d
(hence dividing the ACK sending rate by two). The ACK Ratio
decreases linearly for each RTT in which no ECN-marked ACKs are
received. Multiple congestion marks received in an RTT are treated
as a single congestion event, i.e., d can be doubled at most once per
RTT. The TCP timestamp option is used to keep track of the RTT
values.
A.2. Receiver-Only Mechanisms
In [MJW00], Tam Ming-Chit, et al. propose a receiver-based method for
calculating an "appropriate" number of ACKs per congestion window
(cwnd) of data, in order to alleviate congestion on the reverse path.
The sender's cwnd is estimated at the receiver by counting the number
of received packets per RTT (which also has to be estimated by the
receiver). From this estimate, a simple algorithm is used to compute
the number of ACKs to be sent per cwnd. The algorithm enforces a
lower bound on the number of ACKs per cwnd, aiming at minimizing the
probability of timeout at the sender due to ACK loss. Similarly, the
ACK Ratio is upper-bounded so as to avoid excessive ACK delay.
Blandford, et al. [BGG+07] propose an end-to-end, receiver-oriented
scheme called "smartacking". The algorithm is based upon the
receiver's monitoring the inter-segment arrival time for data packets
and adapting the ACK sending rate in response. When the bottleneck
link is underutilized, ACKs are sent frequently (up to one ACK per
received segment) to promote fast growth of the congestion window.
On the other hand, when the bottleneck is close to full utilization,
the algorithm tries to reduce control traffic overhead and slow
congestion window growth by generating ACKs at the minimum rate
needed to keep the data pipe full.
Reducing the number of ACKs (or, equivalently, increasing the amount
of bytes acknowledged by each ACK) can increase the burstiness of the
TCP sender. Hence, any mechanism as those cited above should be
coupled with a burst mitigation technique, such as rate-based pacing,
that paces the sending of data segments ([AB05], [ASA00], [BPK97]).
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A.3. Middlebox-Based Mechanisms
ACK filtering (AF) [BPK97] from Balakrishnan, et al. is a router-
based technique that tries to reduce the number of ACKs sent over the
congested return link. With AF, an arriving ACK may replace
preceding, older ACKs at the bottleneck queue. An aggressive
replacement policy might guarantee that at most one ACK per
connection is waiting in the queue, alleviating congestion. However,
as in other proposals, care must be taken to avoid sender timeouts in
case the (too few) ACKs resulting from the filtering get lost. The
idea of filtering ACKs has been extended in [YMH03] to deal with SACK
information.
Aweya, et al. [AOM02] present a middlebox-based approach for
mitigating data packet bursts and for controlling the uplink ACK
congestion. The main idea is to perform pacing on ACK segments on an
edge device close to the sender, so as to control the ACK arrival
rate at the sender.
Appendix B. Design Considerations
B.1. The TCP ACK Ratio Option or an AckNow Bit in Data Packets?
In the ACK congestion control mechanism specified in this document,
the sender uses the TCP ACK Ratio option to tell the receiver the ACK
Ratio to use. An alternate approach to the TCP ACK Ratio option
could be for the sender to use an AckNow bit in the TCP header of
data packets, telling the receiver to acknowledge this data packet.
In the discussion below, we call these two approaches the TCP ACK
Ratio option approach and the AckNow approach.
An advantage of an AckNow approach is that it would require less
state from the receiver; the receiver would not need to maintain a
variable for the current ACK Ratio and would not need to keep track
of the number of data packets un-ACKed to date.
However, a disadvantage of the AckNow approach is that the sender
does not know when packets will be reordered, delayed, or dropped on
the path to the receiver. In particular, the sender does not have
control over whether a data packet with the AckNow bit set is
reordered, delayed, or dropped in the network. For this reason, we
have chosen the approach of the sender determining the ACK Ratio that
should be used and sending the ACK Ratio to the receiver, rather than
the sender telling the receiver exactly which data packets to
acknowledge.
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An additional disadvantage of the AckNow approach is that it would
add complications and difficulties for the default cases of the
receiver using an ACK Ratio of one or two, as is done in the absence
of ACK congestion control.
For these reasons, we have specified that the sender determines the
ACK Ratio to use and tells the receiver, rather than the sender
setting an AckNow bit in the TCP Header of selected data packets.
Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3465] Allman, M., "TCP Congestion Control with Appropriate
Byte Counting (ABC)", RFC 3465, February 2003.
[RFC3692] Narten, T., "Assigning Experimental and Testing Numbers
Considered Useful", BCP 82, RFC 3692, January 2004.
[RFC4340] Kohler, E., Handley, M., and S. Floyd, "Datagram
Congestion Control Protocol (DCCP)", RFC 4340, March
2006.
[RFC4341] Floyd, S. and E. Kohler, "Profile for Datagram
Congestion Control Protocol (DCCP) Congestion Control ID
2: TCP-like Congestion Control", RFC 4341, March 2006.
[RFC4727] Fenner, B., "Experimental Values In IPv4, IPv6, ICMPv4,
ICMPv6, UDP, and TCP Headers", RFC 4727, November 2006.
[RFC5681] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
Control", RFC 5681, September 2009.
Informative References
[RFC2760] Allman, M., Ed., Dawkins, S., Glover, D., Griner, J.,
Tran, D., Henderson, T., Heidemann, J., Touch, J.,
Kruse, H., Ostermann, S., Scott, K., and J. Semke,
"Ongoing TCP Research Related to Satellites", RFC 2760,
February 2000.
[RFC3135] Border, J., Kojo, M., Griner, J., Montenegro, G., and Z.
Shelby, "Performance Enhancing Proxies Intended to
Mitigate Link-Related Degradations", RFC 3135, June
2001.
Floyd, et al. Informational [Page 30]
RFC 5690 TCPM - ACK Congestion Control February 2010
[RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
of Explicit Congestion Notification (ECN) to IP", RFC
3168, September 2001.
[RFC3449] Balakrishnan, H., Padmanabhan, V., Fairhurst, G., and M.
Sooriyabandara, "TCP Performance Implications of Network
Path Asymmetry", BCP 69, RFC 3449, December 2002.
[RFC4653] Bhandarkar, S., Reddy, A., Allman, M., and E. Blanton,
"Improving the Robustness of TCP to Non-Congestion
Events", RFC 4653, August 2006.
[ASA00] Aggarwal, A., Savage, S., and T. Anderson,
"Understanding the Performance of TCP Pacing", INFOCOM
(3), pp. 1157-1165, 2000.
[AB05] Allman, M., and E. Blanton, "Notes on Burst Mitigation
for Transport Protocols", SIGCOMM, Computer
Communications Review, 35(2):5360, 2005.
[AJ03] Altman, E., and T. Jimenez, "Novel Delayed ACK
Techniques for Improving TCP Performance in Multihop
Wireless Networks", Proc. of the Personal Wireless
Communications, 2003.
[AOM02] Aweya, J., Ouellette, M., and D. Y. Montuno, "A Self-
regulating TCP Acknowledgement (ack) Pacing Scheme",
International Journal of Network Management,
12(3):145163, 2002.
[BA03] Barakat, C., and E. Altman, "On ACK Filtering on a Slow
Reverse Channel", International Journal of Satellite
Communications and Networking, V.21 N.3, 2003.
[BPK97] Balakrishnan, H., Padmanabhan, V., and Katz, R., "The
Effects of Asymmetry on TCP Performance", Third ACM/IEEE
Mobicom Conference, September 1997.
[BGG+07] Blandford, D.K., Goldman, S.A., Gorinsky, S., Zhou, Y.,
and D.R. Dooly, "Smartacking: Improving TCP Performance
from the Receiving End", Journal of Internet
Engineering, 1(1), 2007.
[CLP98] Calveras, A., Linares, J., and J. Paradells, "Window
Prediction Mechanism for Improving TCP in Wireless
Asymmetric Links". Proc. IEEE Global Communications
Conference (GLOBECOM), Sydney Australia, pp. 533-538,
November 1998.
Floyd, et al. Informational [Page 31]
RFC 5690 TCPM - ACK Congestion Control February 2010
[KIA07] Keceli, F., Inan, I., and E. Ayanoglu, "TCP ACK
Congestion Control and Filtering for Fairness Provision
in the Uplink of IEEE 802.11 Infrastructure Basic
Service Set", Proc. IEEE ICC 2007, June 2007.
[KEEP-ALIVE] Busatto, F., "TCP Keepalive HOWTO", May 2007,
http://tldp.org/HOWTO/TCP-Keepalive-HOWTO/index.html.
[KLS07] Kim, H., Lee, H., and S. Shin, "On the Cross-Layer
Impact of TCP ACK Thinning on IEEE 802.11 Wireless MAC
Dynamics", IEICE Transactions on Communications, 2007.
[LL07] Landstrom, S., and Larzon, L.A., "Reducing the TCP
Acknowledgement Frequency", SIGCOMM, Computer
Communications Review, July 2007.
[MJW00] Ming-Chit, I.T., Jinsong, D., and W. Wang, "Improving
TCP Performance Over Asymmetric Networks", SIGCOMM,
Computer Communications Review (CCR), Vol.30, No.3,
2000.
[Studies] Floyd, S., "Measurement Studies of End-to-End Congestion
Control in the Internet",
http://www.icir.org/floyd/ccmeasure.html.
[WWCM99] Wu, H., Wu, J., Cheng, S., and J. Ma, "ACK Filtering on
Bandwidth Asymmetry Networks", IEEE Communications,
1999.
[YMH03] Yu, L., Minhua, Y., and Z. Huimin, "The Improvement of
TCP Performance in Bandwidth Asymmetric Network", IEEE
PIMRC, 1:482-486, September 2003.
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RFC 5690 TCPM - ACK Congestion Control February 2010
Authors' Addresses
Sally Floyd
ICSI Center for Internet Research
1947 Center Street, Suite 600
Berkeley, CA 94704
USA
EMail: floyd@icir.org
Andres Arcia
Networking, Security & Multimedia (RSM) Universidad de Los Andes
TELECOM Bretagne Facultad de Ingenieria
Rue de la Chataigneraie, CS 17607 Nucleo La Hechicera
35576 Cesson Sevigne Cedex Merida, Merida 5101
France Venezuela
EMail: ae.arcia@telecom-bretagne.eu EMail: amoret@ula.ve
URI: http://www.ula.ve
David Ros
Networking, Security & Multimedia (RSM) Dpt.
TELECOM Bretagne
Rue de la Chataigneraie, CS 17607
35576 Cesson Sevigne Cedex
France
EMail: David.Ros@telecom-bretagne.eu
Janardhan R. Iyengar
Math and Computer Science
Franklin & Marshall College
P. O. Box 3003
Lancaster, PA 17604-3003
USA
EMail: jiyengar@fandm.edu
Floyd, et al. Informational [Page 33]