DCCP Working Group G. Renker Internet-Draft G. Fairhurst Updates: 4342, 5622 University of Aberdeen (if approved) March 27, 2011 Intended status: Standards Track Expires: September 28, 2011 Sender RTT Estimate Option for DCCP draft-ietf-dccp-tfrc-rtt-option-05.txt Abstract This document specifies an update to the round trip time (RTT) estimation algorithm used for TFRC (TCP Friendly Rate Control) congestion control by the Datagram Congestion Control Protocol (DCCP). It updates specifications for the CCID-3 and CCID-4 Congestion Control IDs of DCCP. The update addresses parameter-estimation problems occurring with TFRC-based DCCP congestion control. It uses a recommendation made in the original TFRC specification to avoid 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 September 28, 2011. Copyright Notice Copyright (c) 2011 IETF Trust and the persons identified as the Renker & Fairhurst Expires September 28, 2011 [Page 1]
Internet-Draft Sender RTT Estimate Option for DCCP March 2011 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. 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 . . . . . . . . . . . . . . . . . . . . . . . . 23 Renker & Fairhurst Expires September 28, 2011 [Page 2]
Internet-Draft Sender RTT Estimate Option for DCCP March 2011 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. Renker & Fairhurst Expires September 28, 2011 [Page 3]
Internet-Draft Sender RTT Estimate Option for DCCP March 2011 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, it is not possible to determine the timing interval for adjacent packets with a CCVal difference greater than 4: 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. Renker & Fairhurst Expires September 28, 2011 [Page 4]
Internet-Draft Sender RTT Estimate Option for DCCP March 2011 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 is unable to 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. Renker & Fairhurst Expires September 28, 2011 [Page 5]
Internet-Draft Sender RTT Estimate Option for DCCP March 2011 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 (or 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. Renker & Fairhurst Expires September 28, 2011 [Page 6]
Internet-Draft Sender RTT Estimate Option for DCCP March 2011 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). Renker & Fairhurst Expires September 28, 2011 [Page 7]
Internet-Draft Sender RTT Estimate Option for DCCP March 2011 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. Renker & Fairhurst Expires September 28, 2011 [Page 8]
Internet-Draft Sender RTT Estimate Option for DCCP March 2011 ==> 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 MAY be placed 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: Renker & Fairhurst Expires September 28, 2011 [Page 9]
Internet-Draft Sender RTT Estimate Option for DCCP March 2011 ==> RFC Editor's Note: Please replace 'xxxxxxxx' below with the binary representation of the IANA 'Type' value (indicated in decimal as 'XX') when published and delete this note. +--------+--------+--------+ |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 16.7 seconds is not specified by this document (as per Section 3.3, the sender would then always report 0xFFFFFF). 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. Renker & Fairhurst Expires September 28, 2011 [Page 10]
Internet-Draft Sender RTT Estimate Option for DCCP March 2011 ==> 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. The Send RTT Estimate feature is OPTIONAL. An extension may implement it, but this specification does not require the feature to be understood by every DCCP implementation (see [RFC4340], section 15). The feature is off by default (initial value of 0). DCCP B sends a "Mandatory Change R(Send RTT Estimate, 1)" to require DCCP A to send RTT Estimate options as part of its data traffic (DCCP A will reset the connection if it does not understand this feature). 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. If the RTT is larger than the maximum representable value (0xFFFFFE), the sender MUST set the value of the RTT Estimate Option to 0xFFFFFF. 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. Renker & Fairhurst Expires September 28, 2011 [Page 11]
Internet-Draft Sender RTT Estimate Option for DCCP March 2011 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 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 MUST use 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 MAY occasionally send no-number RTT options, covering for transient changes and spurious disruptions. During these times, the receiver SHOULD continue to use its long-term receiver_RTT value. To avoid under-estimating the RTT in the absence of numeric options, the receiver MUST back off receiver_RTT in the following manner: 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. Renker & Fairhurst Expires September 28, 2011 [Page 12]
Internet-Draft Sender RTT Estimate Option for DCCP March 2011 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. Renker & Fairhurst Expires September 28, 2011 [Page 13]
Internet-Draft Sender RTT Estimate Option for DCCP March 2011 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. Renker & Fairhurst Expires September 28, 2011 [Page 14]
Internet-Draft Sender RTT Estimate Option for DCCP March 2011 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. Renker & Fairhurst Expires September 28, 2011 [Page 15]
Internet-Draft Sender RTT Estimate Option for DCCP March 2011 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.) Renker & Fairhurst Expires September 28, 2011 [Page 16]
Internet-Draft Sender RTT Estimate Option for DCCP March 2011 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. Renker & Fairhurst Expires September 28, 2011 [Page 17]
Internet-Draft Sender RTT Estimate Option for DCCP March 2011 ==> 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 samples, 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 Renker & Fairhurst Expires September 28, 2011 [Page 18]
Internet-Draft Sender RTT Estimate Option for DCCP March 2011 - 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 Renker & Fairhurst Expires September 28, 2011 [Page 19]
Internet-Draft Sender RTT Estimate Option for DCCP March 2011 (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 Renker & Fairhurst Expires September 28, 2011 [Page 20]
Internet-Draft Sender RTT Estimate Option for DCCP March 2011 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 Changes between revision 02 and 03 of this draft: o Section 3.2.2: - clarified the use of Mandatory Feature Negotiation, feedback from Eddie Changes between revision 03 and 04 of this draft: o Abstract: - clarified that the document updates the specifications of CCID-3 and CCID-4, AD review - changed wording so that all abbreviations are defined - Changelog: fixed typos, AD review o Section 2: - fixed grammatical nits in section 2.1, AD review - clarified meaning of lost vs ECN-marked packets in section 2.2.2, AD review o Section 3: - clarified normative applicability of option for packet types in section 3.2.1, AD review - added a note to the RFC editor to also update the binary representation of the Type value for the RTT Estimate Option in section 3.2.1 Renker & Fairhurst Expires September 28, 2011 [Page 21]
Internet-Draft Sender RTT Estimate Option for DCCP March 2011 - clarified optional nature of Send RTT Estimate feature in section 3.2.2, AD review - clarified normative use of initial value of receiver_RTT in section 3.4, AD review - clarified normative applicability of no-number RTT options in section 3.4, AD review - clarified normative applicability of receiver_RTT back-off scheme in section 3.4, AD review o Changelog: - fixed typos, AD review Changes between revision 04 and 05 of this draft: o Section 3.2.1: - added an explanation to indicate what happens in the pathological case of a network having an RTT larger than 16.7 seconds, Gen-ART review o Section 3.3: - clarified that the sender MUST use the no-number RTT Estimate Option of value 0xFFFFFF if the RTT is larger than the maximum representable value, Gen-ART review ====> END OF NOTE TO THE RFC EDITOR <==== Renker & Fairhurst Expires September 28, 2011 [Page 22]
Internet-Draft Sender RTT Estimate Option for DCCP March 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 September 28, 2011 [Page 23]
