One document matched: draft-ietf-dccp-tfrc-rtt-option-02.txt
Differences from draft-ietf-dccp-tfrc-rtt-option-01.txt
DCCP Working Group G. Renker
Internet-Draft G. Fairhurst
Updates: 4342, 5622 University of Aberdeen
(if approved) January 31, 2011
Intended status: Standards Track
Expires: August 4, 2011
Sender RTT Estimate Option for DCCP
draft-ietf-dccp-tfrc-rtt-option-02.txt
Abstract
This document describes an update to CCID-3/4 that addresses
parameter-estimation problems occurring with TFRC-based DCCP
congestion control.
The fix uses a recommendation made in the original TFRC
specification. It avoids the inherent problems of receiver-based RTT
sampling, by utilising higher-accuracy RTT samples already available
at the sender. It is integrated into the feature set of DCCP as an
end-to-end negotiable extension.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on August 4, 2011.
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
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
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publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Problems caused by sampling the RTT at the receiver . . . . . 4
2.1. List of problems encountered with a real implementation . 4
2.2. Other areas affected by the RTT sampling problems . . . . 5
2.2.1. Measured Receive Rate X_recv . . . . . . . . . . . . . 6
2.2.2. Disambiguation and Accuracy of Loss Intervals . . . . 6
2.2.3. Determining Quiescence . . . . . . . . . . . . . . . . 6
2.2.4. Practical Considerations . . . . . . . . . . . . . . . 6
3. Specification . . . . . . . . . . . . . . . . . . . . . . . . 8
3.1. Conventions . . . . . . . . . . . . . . . . . . . . . . . 8
3.2. Options and Features . . . . . . . . . . . . . . . . . . . 8
3.2.1. RTT Estimate Option . . . . . . . . . . . . . . . . . 8
3.2.2. Send RTT Estimate Feature . . . . . . . . . . . . . . 10
3.3. Basic Usage . . . . . . . . . . . . . . . . . . . . . . . 11
3.4. Receiver Robustness Measures . . . . . . . . . . . . . . . 12
4. Security Considerations . . . . . . . . . . . . . . . . . . . 13
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14
5.1. Option Types . . . . . . . . . . . . . . . . . . . . . . . 14
5.2. Feature Numbers . . . . . . . . . . . . . . . . . . . . . 15
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 17
6.1. Normative References . . . . . . . . . . . . . . . . . . . 17
6.2. Informative References . . . . . . . . . . . . . . . . . . 17
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 22
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1. Introduction
This document defines a Standards Track update to both a sender and
receiver that implement DCCP CCID-3 [RFC4342] or CCID-4 [RFC5622],
addressing RTT estimation problems that were observed when using a
real implementation.
To fix these problems, this document presents a solution based on a
concept first recommended in [RFC5348], 3.2.1; i.e. to measure the
RTT at the sender. This results in a higher reliability and
frequency of samples, and avoids the inherent problems of receiver-
based RTT sampling discussed below.
We begin by analysing the encountered problems in the next section.
The update is presented in Section 3. We then discuss security
considerations in Section 4, and list the resulting IANA
considerations in Section 5.
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2. Problems caused by sampling the RTT at the receiver
There are at least six areas that make a TFRC receiver vulnerable to
inaccuracies or absence of (receiver-based) RTT samples:
o the measured sending rate, X_recv ([RFC5348], 6.2);
o synthesis of the first loss interval ([RFC5348], 6.3.1);
o disambiguation of loss events ([RFC4342], 10.2);
o validation of loss intervals ([RFC4342], 6.1);
o ensuring that at least one feedback packet is sent per RTT
([RFC4342], 10.3);
o determining quiescence periods ([RFC4342], 6.4).
2.1. List of problems encountered with a real implementation
This section summarizes several years of experience using the Linux
implementation of CCID-3 and CCID-4. It lists the problems
encountered with receiver-based RTT sampling over real networks, in a
variety of wired and wireless environments and under different link-
layer conditions.
The Linux DCCP/TFRC implementation is based on the RTT-sampling
algorithm specified in [RFC4342], 8.1. This algorithm relies on a
coarse-grained window-counter (units of RTT/4), and uses packet
inter-arrival times to estimate the current RTT of the network.
The algorithm is effective only for packets with modulo-16 CCVal
differences less than 5, due to limitations noted in sections 8.1 and
10.3 of [RFC4342]. A CCVal difference less than 4 means sampling at
sub-RTT scale; [RFC4342], 8.1 thus suggests differences between 2 and
4, the latter being preferable (equivalent to a full RTT). The same
section limits the maximum CCVal difference between data-carrying
packets to 5, in order to avoid wrap-around. As a consequence, the
timing interval for adjacent packets with a CCVal difference greater
than 4 can not be determined: such samples have to be discarded.
A second problem arises when there are holes in the sequence space.
Because the 4-bit CCVal counter may cycle around multiple times, it
is not possible to determine window-counter wrap-around whenever
sequence numbers of subsequent packets are not immediately adjacent.
This problem occurs when packets are delayed, reordered, or lost in
the network.
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As a consequence, RTT sampling has to be paused during times of loss.
This however aggravates the problem, since the sender now requires
new feedback from the receiver, but the receiver is unable to provide
accurate and up-to-date information: the receiver is unable to sample
the RTT, accordingly also not able to estimate X_recv correctly,
which then in turn affects X_Bps at the sender.
The third limitation arises from using inter-arrival times as
representatives of network inter-packet gaps. It is well known that
the inter-packet gap of packets is not constant along a network path.
Furthermore, modern network interface cards do not necessarily
deliver each packet at the time it is received, but rather in a
bunch, to avoid overly frequent interrupts [MR97]. As a result,
inter-packet arrival times may converge to zero, when subsequent
packets are being delivered at virtually the same time.
The fourth problem is that of under-sampling and thus related to the
first limitation. If loss occurs while the receiver has not yet had
a chance to sample the RTT, it needs to fall back to some fixed RTT
constant to plug into the equation of [RFC5348], 6.3.1. (The sender,
for example, uses a fixed value of 1 second when it can not obtain an
initial RTT sample, see [RFC5348], 4.2).
In particular, if the loss is caused by a transient condition, this
fourth problem causes a subsequent deterioration of the connection
(rate reduction), further aggravated by the fact that TFRC takes
longer than common window-based protocols to recover from a reduction
of its allowed sending rate.
Trying to smooth over these effects by imposing heavy filtering on
the RTT samples did not substantially improve the situation, nor does
it solve the problem of under-sampling.
The TFRC sender, on the other hand, is much better equipped to
estimate the RTT and can do this more accurately. This is in
particular due to the use of timestamps and elapsed time information
([RFC5348], 3.2.2), which are mandatory in CCID-3 (sections 6 and 8.2
of [RFC4342]).
2.2. Other areas affected by the RTT sampling problems
We here analyse the impact that unreliability of receiver-based RTT
sampling has on the areas listed at the begin of this section.
In addition, benefits of sender-based RTT sampling have already been
pointed out in [RFC5348], and in the specification of CCID-3
[RFC4342], at the end of section 10.2.
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2.2.1. Measured Receive Rate X_recv
A key problem is that the reliability of X_recv [RFC4342] depends
directly upon the reliability and accuracy of RTT samples. This
means that failures propagate from one parameter to another.
Errata IDs 610 and 611 update [RFC4342] to use the definition of the
receive rate as specified in [RFC5348].
Having an explicit (rather than a coarse-grained) RTT estimate allows
measurement of X_recv with greater accuracy, and isolates failure.
An explicit RTT estimate also enables the receiver to more accurately
perform the test in step (2) of [RFC4342], 6.2, i.e. to check whether
less or more than one RTT has passed since the last feedback.
2.2.2. Disambiguation and Accuracy of Loss Intervals
Since a loss event is defined as one or more lost (ECN-marked) data
packets in one RTT ([RFC5348], 5.2), the receiver needs accurate RTT
estimates to validate and accurately separate loss events. Moreover,
[RFC5348], 5.2 expressly points out the sender RTT estimate as
RECOMMENDED for this purpose.
Having the sender RTT Estimate available further increases the
accuracy of the information reported by the receiver. The definition
of Loss Intervals in [RFC4342], 6.1 needs the RTT to separate the
lossy parts; in particular, lossy parts spanning a period of more
than one RTT are invalid.
A similar benefit arises in the computation of the loss event rate:
as discussed in section 9.2 of [RFC4342], it may happen that sender
and receiver compute different loss event rates, due to differences
in the available timing information. An explicit RTT estimate
increases the accuracy of information available at the receiver, thus
the sender may not need to recompute the (less reliable) loss event
rate reported by the receiver.
2.2.3. Determining Quiescence
The quiescence period is defined as max(2 * RTT, 0.2 sec) in section
6.4 of [RFC4342]. An explicit RTT estimate avoids under- and over-
estimating quiescence periods.
2.2.4. Practical Considerations
Using explicit RTT estimates contributes to greater robustness and
can also result in simpler implementation.
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First, it becomes easier to separate adjacent loss events. The 4-bit
counter value wraps relatively frequently, which requires additional
procedures to avoid aliasing effects.
Second, the receiver is better able to determine when to send
feedback packets. It can perform the test described in step (2) of
[RFC5348], 6.2 more accurately. Moreover, unnecessary expiration of
the nofeedback timer (as described in [RFC4342], 10.3) can be
avoided.
Lastly, a sender-based RTT estimate option can be used by middleboxes
to verify that a flow uses conforming end-to-end congestion control
([RFC4342], 10.2).
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3. Specification
3.1. Conventions
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].
This document uses the conventions of [RFC5348], [RFC4340],
[RFC4342], and [RFC5622].
All multi-byte field descriptions presented in this documented are in
network byte order (most significant byte first).
3.2. Options and Features
This document defines a single TFRC-specific option, RTT Estimate,
described in the next subsection.
Following the guidelines in [RFC4340], section 15, the use of the RTT
Estimate Option is governed by an associated feature, Send RTT
Estimate Feature. This feature is described in the second
subsection.
3.2.1. RTT Estimate Option
The sender communicates its current RTT estimate to the receiver
using a RTT Estimate Option.
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==> RFC Editor's Note:
Please replace 'XX' with IANA value when published and delete this
note.
+------+---------------+--------------+------------+
| Type | Option Length | Meaning | DCCP Data? |
+------+---------------+--------------+------------+
| XX | 3/4/5 | RTT Estimate | Y |
+------+---------------+--------------+------------+
Table 1: The RTT Estimate Option defined by this document
Column meanings are as per [RFC4340], section 5.8 (table 3). This
option is permitted in any DCCP packet, has option number XX and a
length of 3-5 bytes.
A Sender RTT Estimate Option is valid if it satisfies one of the
three following formats:
+--------+--------+--------+
|xxxxxxxx|00000011| RTT |
+--------+--------+--------+
Type=XX Length=3 Estimate
+--------+--------+--------+--------+
|xxxxxxxx|00000100| RTT |
+--------+--------+--------+--------+
Type=XX Length=4 Estimate
+--------+--------+--------+--------+--------+
|xxxxxxxx|00000101| RTT |
+--------+--------+--------+--------+--------+
Type=XX Length=5 Estimate
The 1..3 value bytes of the option data carry the current RTT
estimate of the sender, using a granularity of 1 microsecond. This
allows values up to 16.7 seconds (corresponding to 0xFFFFFE) to be
communicated.
The value 0xFFFFFF is reserved to indicate significant delay spikes,
larger than 16.7 seconds. This is qualitative rather than
quantitative information, to alert the receiver that there is a
network problem (for instance jamming on a wireless channel).
The use of the RTT Estimate Option on networks with RTTs larger than
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16.7 seconds is not specified by this document.
A value of 0 indicates the absence of a valid RTT sample. The sender
MUST set the value to 0 if it does not yet have an RTT estimate.
The sender SHOULD select the smallest format suitable to carry the
RTT estimate (i.e., less than 1 byte of leading zeroes).
3.2.2. Send RTT Estimate Feature
The Send RTT Estimate feature lets endpoints negotiate whether the
sender MUST provide RTT Estimate options on its data packets.
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==> RFC Editor's Note:
Please replace 'YY' with IANA value when published and delete this
note.
Send RTT Estimate has feature number YY and is server-priority. It
takes one-byte Boolean values; values greater than 1 are reserved.
+--------+-------------------+------------+---------------+-------+
| Number | Meaning | Rec'n Rule | Initial Value | Req'd |
+--------+-------------------+------------+---------------+-------+
| YY | Send RTT Estimate | SP | 0 | N |
+--------+-------------------+------------+---------------+-------+
Table 2: The Send RTT Estimate feature defined by this document
The column meanings are described in [RFC4340], section 6.4. In
particular, the feature is by default off (initial value of 0), and
the extension is not required to be understood by every DCCP
implementation (cf. [RFC4340], section 15).
DCCP B sends a "Mandatory Change R(Send RTT Estimate, 1)" to ask DCCP
A to send RTT Estimate options as part of its data traffic.
3.3. Basic Usage
When the Send RTT Estimate Feature is enabled, the sender MUST
provide an RTT Estimate Option on all of its Data, DataAck, Sync, and
SyncAck packets. It MAY in addition provide the RTT Estimate Option
on other packet types, such as DCCP-Ack.
The sender MUST implement and continue to update the CCVal window
counter as specified in [RFC4342], section 8.1, even when the Send
RTT Estimate Feature is on.
When the Send RTT Estimate Feature is enabled, the receiver MUST use
the value reported by the RTT Estimate Option in all places that
require a RTT (listed at the begin of Section 2). If the receiver
encounters an invalid RTT Estimate Option (Section 3.2.1), it MUST
reset the connection with Reset Code 5, "Option Error", where the
Data 1..3 fields are set to the first 3 bytes of the offending RTT
Estimate Option.
The receiver SHOULD track the long-term RTT estimate using a moving
average, such as the one specified in [RFC5348], 4.3. This long-term
estimate is referred to as "receiver_RTT" below.
When the Send RTT Estimate Feature is disabled, the receiver MUST
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estimate the RTT as previously specified in [RFC4340], [RFC4342], and
[RFC5622].
3.4. Receiver Robustness Measures
This subsection specifies robustness measures for the receiver when
the Send RTT Estimate Feature is on.
The 0-valued and 0xFFFFFF-valued RTT Estimate Options are both
referred to as "no-number RTT options". RTT Estimate Options with
values in the range of 1..0xFFFFFE are analogously called "numeric
RTT options".
Until the first numeric RTT option arrives, the receiver uses a value
of 0.5 seconds for receiver_RTT (to match the initial 2 second
timeout of the TFRC nofeedback timer, [RFC5348], 4.2).
If the path RTT is known, e.g. from a previous connection [RFC2140],
the receiver MAY reuse the previously known path RTT value to seed
its long-term RTT estimate.
The sender is permitted to occasionally send no-number RTT options,
covering for transient changes and spurious disruptions. During
these times, the receiver continues to use its long-term receiver_RTT
value.
To avoid that the long-term estimate at the receiver drifts in such a
way that it under-estimates the RTT, a simple back-off scheme is
employed: if the sender supplies no-number RTT options for longer
than receiver_RTT units of time, the receiver sets
receiver_RTT = MIN(2 * receiver_RTT, t_mbi)
where t_mbi = 64 seconds is the maximum backoff interval ([RFC5348],
Appendix A). For the next round of no-number RTT options, the
updated value of receiver_RTT applies.
This back-off mechanism ensures that short-term disruptions do not
have a lasting impact, whereas long-term problems will result in
asymptotically high receiver_RTT values.
To bail out from a hanging session, the receiver MAY close the
connection when receiver_RTT has reached the value MAX_RTT.
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4. Security Considerations
Security considerations for CCID-3 have been discussed in section 11
of [RFC4342]; for CCID-4 these have been discussed in section 13 of
[RFC5622], referring back to the same section of [RFC4342].
This document introduces an extension to communicate the current RTT
estimate of the sender to the receiver of a TFRC communication.
By altering the value of the RTT Estimate Option, it is possible to
interfere with the behaviour of a flow using TFRC. In particular,
since accuracy of the RTT estimate directly influences the accuracy
of the measured sending rate X_recv, it would be possible to obtain
either higher or lower sending rates than are warranted by the
current network conditions.
This is only possible if an attacker is on the same path as the DCCP
sender and receiver, and is able to guess valid sequence numbers.
Therefore the considerations of section 18 in [RFC4340] apply.
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5. IANA Considerations
This document requests identical allocation in the dccp-ccid3-
parameters and the dccp-ccid4-parameters registries.
5.1. Option Types
This document defines a single CCID-specific option for communicating
RTT estimates from the HC-sender to the HC-receiver. Following
[RFC4340], 10.3, this requires an option number for the RTT Estimate
Option in the range 128...191.
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Note to IANA and the RFC editor
When the IANA has allocated an option number for the `RTT Estimate'
option, please replace all occurrences of the placeholder `XX' in
this text with that number and delete this note. (Due to [RFC4340],
19.3 and [RFC4342], 12.2, the option number would be allocated in the
range 128...183/191.)
5.2. Feature Numbers
This document defines a single CCID-specific feature number for the
Send RTT Estimate feature which is located at the HC-sender.
Following [RFC4340], 10.3, a feature number in the range 128...191 is
required.
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Note to IANA and the RFC editor
When the IANA has allocated an option number for the `Send RTT
Estimate' feature, please replace all occurrences of the placeholder
`YY' in this text with that number and delete this note. (Due to
[RFC4340], 19.4 and [RFC4342], 12.3, the feature number would be
allocated in the range 128...183/191.)
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6. References
6.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4340] Kohler, E., Handley, M., and S. Floyd, "Datagram
Congestion Control Protocol (DCCP)", RFC 4340, March 2006.
[RFC4342] Floyd, S., Kohler, E., and J. Padhye, "Profile for
Datagram Congestion Control Protocol (DCCP) Congestion
Control ID 3: TCP-Friendly Rate Control (TFRC)", RFC 4342,
March 2006.
[RFC5348] Floyd, S., Handley, M., Padhye, J., and J. Widmer, "TCP
Friendly Rate Control (TFRC): Protocol Specification",
RFC 5348, September 2008.
6.2. Informative References
[MR97] Mogul, J. and K. Ramakrishnan, "Eliminating Receive
Livelock in an Interrupt-Driven Kernel", ACM Transactions
on Computer Systems (TOCS), 15(3):217-252, August 1997.
[RFC2140] Touch, J., "TCP Control Block Interdependence", RFC 2140,
April 1997.
[RFC5622] Floyd, S. and E. Kohler, "Profile for Datagram Congestion
Control Protocol (DCCP) Congestion ID 4: TCP-Friendly Rate
Control for Small Packets (TFRC-SP)", RFC 5622,
August 2009.
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==> NOTE TO THE RFC EDITOR: PLEASE REMOVE THIS LOG PRIOR TO PUBLICATION
The following changelog lists the changes since revision 01 of the
preceding individual submission draft-renker-dccp-tfrc-rtt-option.
o General:
- added detailed changelog to track comments
- changed document name to reflect working group
- updated date to October
- made spelling of RTT Estimate Option (singular) consistent
- moved reference to RFC 5622 from normative to informative,
since document status is Experimental
o Section 2.1:
- clarified problematic cases of too small CCVal differences
and CCVal differences > 4, feedback by Pasi
o Section 3.1:
- clarified the byte ordering used by this document, feedback
by Pasi
o Section 3.2:
- corrected naming of Send RTT Estimate Feature, feedback by
Eddie
- removed superfluous remark regarding scaling to microsecond
granularity in 3.2.1, feedback by Pasi
- removed recommendation of preferring long-term RTT stamples,
since this can not be generalized (connection may be short or
path RTT may change, in both cases a long-term sample would not
be useful), feedback by Pasi
- made option variable-length (3/4/5 bytes), feedback by Eddie
- specified condition for syntactic option validity
- limited the maximum option size to 3 bytes and justified
decision why not to support RTTs greater than 16 seconds, in
reply to feedback by Eddie
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- clarified that the sender MUST use 0 to indicate absence of a
valid RTT estimate
- clarified the highest path RTT value supported by this
document (16.7 sec)
- reserved 0xFFFFFF as special value to communicate out-of-
bounds exceptions, network problems resulting in
disproportionately high delay spikes (> 16.7 seconds)
o Section 3.3:
- corrected naming of Send RTT Estimate Feature, feedback by
Eddie
- specified what happens if invalid RTT Estimate options are
received
- specified what happens if the sender persistently sends
0-valued RTT Estimate options, feedback by Eddie
- specified how the exceptional value 0xFFFFFF should be
handled
- added reference for reusing previously known path RTT value
Changes between revision 00 and 01 of this draft:
o General changes:
- incremented date and revision number
- various minor changes of syntax, typos, and paragraph
formatting
o Section 2.1:
- completely rewrote the description of the fifth problem in
order to more clearly/precisely identify problem causes,
following feedback from Michael
o Section 2.2.4:
- simplified sentence referring to aliasing effects (implicitly
referencing section 10.2 of RFC4342)
- clarified how middleboxes might use a sender-based RTT
estimate option to verify end-to-end congestion control
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(suggestion and quote taken from RFC4342, section 10.2)
o Section 3.3:
- clarified how the receiver must behave if the Send RTT
Estimate Feature is disabled, following feedback from Eddie
- removed the requirement that the receiver should additionally
track CCVal window counter values when the Send RTT Estimate
Feature is on
- removed suggestion that the receiver should take measures to
improve the quality of the connection, feedback by Michael
- moved all receiver robustness measures to the new section 3.4
- changed section title to reflect restructuring of content
o Section 3.4:
- new section, written from scratch, to address the
shortcomings of the previous scheme, which were identified by
Michael Welzl
- specifies what to do when the sender supplies no-number RTT
options for short and extended periods of time
Changes between revision 01 and 02 of this draft:
o General:
- incremented date and revision number
- removed compatibility clause in abstract
o Section 1:
- corrected placement of references, feedback from Eddie
o Section 2.1:
- removed description of a problem observed with CCID-3 over an
802.11 link (reduction of sending rate towards zero after a
short channel outage), since causes of the problem were not
conclusively understood
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o Section 3.2.2:
- clarified that values of 2 of the Send RTT Estimate feature
are reserved, feedback from Eddie
- fixed the issue of handling invalid values of the Send RTT
Estimate feature by using mandatory feature negotiation
([RFC4340], sec. 6.6.9), thanks to a suggestion by Eddie
o Section 3.3:
- clarified receiver behaviour when the Send RTT Estimate
feature is enabled, feedback from Eddie
====> END OF NOTE TO THE RFC EDITOR <====
Renker & Fairhurst Expires August 4, 2011 [Page 21]
Internet-Draft Sender RTT Estimate Option for DCCP January 2011
Authors' Addresses
Gerrit Renker
University of Aberdeen
School of Engineering
Fraser Noble Building
Aberdeen AB24 3UE
Scotland
Email: gerrit@erg.abdn.ac.uk
URI: http://www.erg.abdn.ac.uk
Godred Fairhurst
University of Aberdeen
School of Engineering
Fraser Noble Building
Aberdeen AB24 3UE
Scotland
Email: gorry@erg.abdn.ac.uk
URI: http://www.erg.abdn.ac.uk
Renker & Fairhurst Expires August 4, 2011 [Page 22]
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