Re: [AVTCORE] WGLC on draft-ietf-avtcore-ecn-for-rtp-04
"Piers O'Hanlon" <p.ohanlon@gmail.com> Fri, 14 October 2011 14:55 UTC
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From: Piers O'Hanlon <p.ohanlon@gmail.com>
Date: Fri, 14 Oct 2011 15:54:44 +0100
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To: Qin Wu <bill.wu@huawei.com>
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Cc: Magnus Westerlund <magnus.westerlund@ericsson.com>, avt@ietf.org
Subject: Re: [AVTCORE] WGLC on draft-ietf-avtcore-ecn-for-rtp-04
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Dear Qin, Thanks for your feedback. Comments inline [omitting Thomas's clarifications] On 11 October 2011 03:46, Qin Wu <bill.wu@huawei.com> wrote: > Hi, > I have reviewed this draft and support it moving towards publication. > Here is my minor editional comments to this 04 version draft. > 1. Section 1 > [Qin]:Is it better to move the first paragraph next to the third paragraph. > > 2. Section 3, 1st paragraph > " > ECN has been specified for use with TCP [RFC3168], SCTP [RFC4960], > and DCCP [RFC4340] transports. These are all unicast protocols which > negotiate the use of ECN during the initial connection establishment > handshake (supporting incremental deployment, and checking if ECN > marked packets pass all middleboxes on the path). > " > [Qin]: I think "these" in the second sentence can't be "ECN" > mentioned in the first sentence. Is it better to replace "these" > the first word in this sentence with "these transport"? > The 'these' in the second sentance refers to the transport protocols (not ECN). I think the existing wording is probably ok. > " > ECN-CE marks are > immediately echoed back to the sender by the receiving end-point > using an additional bit in feedback messages, and the sender then > interprets the mark as equivalent to a packet loss for congestion > control purposes. > " > > [Qin]: For this sentence, it is not clear to me what is the feedback > message? > Is feedback message a new RTCP Feedback message defined in this > document or TCP message or SCTP message? If it is latter, I would like > to change it as these transport protocols header > This paragraph is aiming to describe the existing ECN enabled transport protocols so it does not refer to RTCP/AVPF messages. We have attempted to describe the feedback process in a generic way as they all have their own way of feeding back the ECN state information - so there is inevitably some lack of specificity. > 4. Section 3.1 > > [Qin]: Any requirements that should be met by the middlebox or intermediate > node? > These are just a list of requirements on the protocol - and correspondingly on any equipment that implements this protocol (which could include a middlebox, intermediate node, MCU, router etc). So such support is implied for any of these boxes. > > 5. Section 3.2,3rd paragraph > " > Topo-Point-to-Point: This is a standard unicast flow. > .... > " > > [Qin]: Topo-point-to-point is not flow but a topology. Should this sentence > be > rephrased as "This is for a standard unicast flow". > This may benefit from some rewording. > " > Accordingly, we allow ECN to be used by an RTP sender using multicast UDP > provided the sender has verified that the paths to all its known receivers > support ECN, > and irrespective of whether the paths from other senders to their receivers > support ECN > ("all its known receivers" are all the SSRCs that the RTP sender has > received RTP or RTCP > from the last five reporting intervals, i.e., they have not timed out). > " > [Qin]: Five is default value of the timeout multiplier, do you think other > value is not allowed? > That issue is down to compliance with the RTP spec: http://tools.ietf.org/html/rfc3550#section-6.3.5 Where 5 is specified as the default value, so in principle it's possible to choose something else but that could lead to problems. > " > Since only a subset of the RTP packets generated by a sender are forwarded > to the > receivers, a video switching MCU can break ECN negotiation (the success of > the > ECN negotiation may depend on the voice activity of the participant at the > instant the > negotiation takes place - shout if you want ECN). > " > > [Qin]: Is "shout if you want ECN" appropirate language here? > Well it serves the purpose - given that this not a recommended topology and is shall not be used with ECN. > > 6. Section 3.2, 4th paragraph > " > This is a great benefit as the rate of an > RTP session can be varied in a number of ways, > " > [Qin]: Should "rate" in this sentence be data rate or bitrate? > It is intentionally left open as one could alter the bit/byte rate or the packet rate. > 7. Section 3.2, 5th paragraph > " > The standard RTP/ AVP profile [RFC3551] does not allow any early or > immediate > transmission of RTCP feedback, and has a minimal RTCP interval whose default > value > (5 seconds) is many times the normal RTT between sender and receiver. > " > [Qin]: What's the purpose of last sentence of this paragraph? > Are you saying RFC3551 following Regular RTCP timing rule? > or something else? > Yes - That sentence is just reiterating the RFC3551 timing rules so that the read may understand the limitations of using that approach (as opposed to employing AVPF). > Another question is according to RFC3551, the negotiation of > parameters or membership control may be not support. in other wonder, > there may be no way to negotiate ECN with RTP using SDP signaling, > I am wondering can we say ECN with RTP can be used with the standard RTP/ > AVP profile [RFC3551]. > We have explicitly mentioned that it is possible to use RFC3551 but performance may be limited. RFC3551 doesn't have any impact upon the SDP negotiations. > 8. Section 3.3,3rd paragraph > " > For interoperability we mandate the implementation of the RTP/AVPF > profile, with both RTCP extensions and the necessary signalling to > support a common operations mode. This specification recommends the > use of RTP/AVPF in all cases as negotiation of the common > interoperability point requires RTP/AVPF, mixed negotiation of RTP/ > AVP and RTP/AVPF depending on other SDP attributes in the same media > block is difficult, > " > [Qin] How mixed negotiation is avoided when you don't put all the SDP > attribute > into the same media block? I don't really understand the rationale. Would > you like to clarify a little bit? > > 9. Section 4, 1st paragraph > " > 2. Initiation and initial verification of ECN capable transport > " > [Qin]: It looks the bullet 2 has two steps, > a. Initiation procedure > b. inital verification procedure, If there is no STUN exchange in between, > if the step (a) initiation procedure > is required? > This list is meant to describe the overall pieces of the solution - not necessarily every single step. > 10. Section 4, 2nd paragraph > " > This document defines the information that needs to be negotiated, and > provides a mapping to SDP > for use in both declarative and offer/answer contexts. > " > [Qin]: Would it be better to add an anchor pointing to the section 6.1.1 and > 6.1.2? > I'm not sure if that is necessary. > 11. Section 4, 3rd paragraph > " > o The sender may generate a (small) subset of its RTP data packets > with the ECN field set to ECT(0) or ECT(1). > " > > [Qin] It is not clear to me where the ECN field is located. > Is ECN field in the IP header or UDP header or RTP header? > > 12. Section 5, 1st paragraph > " > This memo defines two new RTCP extensions: one RTP/AVPF [RFC4585] > transport layer feedback format for urgent ECN information, > " > [Qin]: Should the "urgent ECN information" be "urgent ECN information > reporting" > > 13.Section 5.1,1st paragraph > " > This RTP/AVPF transport layer feedback format is intended for use in > RTP/AVPF early or immediate feedback modes when information needs to > urgently reach the sender. Thus its main use is to report reception > of an ECN-CE marked RTP packet so that the sender may perform > congestion control, or to speed up the initiation procedures by > rapidly reporting that the path can support ECN-marked traffic. > " > > [Qin]: are you saying slow initiation procedure may result in network > congestion? > In my understanding, when ECN-CE marked packet is received, the only choice > for sender > is to do congestion control since you are not sure if initiation procedure > lead to such > congestion. If it is not intiation procedure that lead to congestion, you > get risk to worse > congestion condition? > > 14.Section 6.1, 1st paragraph > " > One new SDP attribute, "a=ecn-capable-rtp", is defined. This is a > media level attribute, and MUST NOT be used at the session level. It > is not subject to the character set chosen. > " > > [Qin] Would it better to put reference to the section 6 of RFC4566 for > character set? > Since it is independent of the character set it is probably necessary to add the reference. > 15. Sectioin 6.1, 2nd paragraph > " > ect: This parameter makes it possible to express the preferred ECT > marking. This is either "random", "0", or "1", with "0" being > implied if not specified. The "ect" parameter describes a > receiver preference, and is useful in the case where the receiver > knows it is behind a link using IP header compression, the > efficiency of which would be seriously disrupted if it were to > receive packets with randomly chosen ECT marks. It is RECOMMENDED > that ECT(0) marking be used. > " > > [Qin]: It looks ect is not well specified here. > e.g.,what it means if ect is set to "random" or "1" from above paragraph. > what does "randomly chosen" stand for? > I think when ect is set 0, it means ECT(0) is chosen, when ect is set 1, > it means ECT(1) is chosen, when ect is set random, it means either ECT(0) or > ECT(1) is chosen. I think it should be clear in the text. > As you explain that is what is meant in the paragraph. It seems fairly clear as we have put it - Though the mapping between "0" and ECT(0) is not explicitly mentioned till the end so we may need to address this. > Also I think the text like ""0" being implied if not specified" in this > paragraph is not correct. > This is correct - it just another way of saying this the default value but the current wording makes sense > 16. Section 6.1.3, 2nd paragraph > " > If timely feedback is not required it is still RECOMMENDED > to used RTP/AVPF. > " > > [Qin]: Should "used" in this sentence be "use"? > Yes that needs changing. > 17.Section 7.1,2nd paragraph > " > The "rtcp-fb" attribute with the "nack" parameter "ecn" MUST be used > whenever the initialization method or a congestion control algorithm > requiring timely sender side knowledge of received CE markings. > " > [Qin]: Should "requiring" in this sentence be "requires"? > Yes that sentence may need some modification. > 18.Section 7.2, 2nd paragraph > " > At the start of the RTP session, when the first packets with ECT are > sent, it is important to verify that IP packets with ECN field values > of ECT or ECN-CE will reach their destination(s). > " > [Qin]: Should "first packets" be "first packet" and "IP packets" be "IP > packet"? > No - as there are more than of these packets. > " > If the path between the sender and the receivers exhibits either of these > behaviours one > " > [Qin]: s/the receivers/receiver > I'm not sure that is necessary. > " > If the path between the sender and the receivers exhibits either of these > behaviours one > needs to stop using ECN immediately to protect both the network and > the application. > " > [Qin]: How does sender or receiver detects ECN is misused? Who is the > "one" in this sentence? If it is middlebox, the middlebox should also > tell the sender ECN is not used. > As mentioned earlier in the section the "behaviours" will be detected by the end-systems. The 'one' is the parties involved in the connection setup - which could in certain cases be a 'middlebox' - but that doesn't need to be explicitly covered here. > 19.Section 7.2.1, 1st paragraph > " > Then any resulting lack of congestion response is likely to have little > damaging affect on others. > " > > [Qin] s/affect/effect > I think you're correct there. > " > Note: The above optimization supports peer to peer unicast > transport with several SSRCs multiplexed onto the same flow > (e.g., a single participant with two video cameras, or SSRC > multiplexed RTP retransmission [RFC4588]). It is desirable to > " > [Qin]: In SSRC multiplexed RTP Retransmission case, original stream and > associated retransmission > stream possible have different > transport characteristics, however the original stream may be > multicast stream, which contradict to > what you said about "The above optimization supports peer to peer > unicast > transport", am I wrong? > " In the case of multicast the optimisation wouldn't apply. > be able to rapidly negotiate ECN support for such a session, > but the optimisation above can fail if there are > implementations that use the same CNAME for different parts of > a distributed implementation that have different transport > characteristics (e.g., if a single logical endpoint is split > across multiple hosts). > " > [Qin]: I don't get this example clearly. are you saying when two streams > with the same CNAME > is sent from the same RTP sender, such above optimization applies.However > when two streams > with the same CNAME are sent from two differnt RTP entities,such above > optimization can not apply. > For the first part you are correct, but the the two stream case only talks about a 'logical endpoint' i.e. a single RTP endpoint which is split over multiple hosts, not separate RTP endpoints. > 20. Section 7.2.2,1st paragraph > " > This section describes an OPTIONAL method that can be used to avoid > media impact and also ensure an ECN capable path prior to media > " > [Qin]: What's the media impact? It looks like a new term that needs to be > explained somewhere. > It refers to the effect of the ECN path discovery mechanisms that need to be normally employed at session initiation. We could explain this a little better. > 21. Section 7.2.2,3rd paragraph > " > A STUN server thatdoesn't understand the extension, or is incapable of > reading the ECN values on incoming STUN packets, should follow the rule in > the STUN > specification for unknown comprehension-optional attributes, and ignore the > attribute, > resulting in the sender receiving a STUN response without the ECN Check > STUN attribute. > > " > [Qin]: Is the sender in this sentence "the media sender" or new participant? > The sender would be the entity that sent the STUN message in the first place. > 22. Section 7.2.2, last paragraph > " > If that is successfully completed the ECT properties of the session are > maintained for the other senders in the session. > " > [Qin]: Who is the other senders? are the other senders "new participant" > or other participants? It should be clear in the text. > The other senders are the other existing participants (i.e. Not the new participant). > 23. Section 7.3, 1st paragraph > " > Once ECN has been successfully initiated for an RTP sender, that > sender begins sending all RTP data packets as ECT-marked, and its > receivers send ECN feedback information via RTCP packets. This > section describes procedures for sending ECT-marked data, providing > ECN feedback information via RTCP, responding to ECN feedback > information, and detecting failures and misbehaving receivers. > " > [Qin]It looks failure detection is discussed in different section, i.e., > section 7.4. > Suggest to remove ", and detecting failures and misbehaving receivers" from > this paragraph. > Agreed. > 24. Section 7.3.1, 2nd paragraph > " > The sender SHALL NOT include ECT marks on outgoing RTCP packets, > " > [Qin]: Is it true, since RTP/AVPF Transport Layer ECN Feedback packet > include the satistics of ECT marks? > Yes - whilst the RTCP packet may contain information regarding the ECN markings, they are not themselves ECT marked. > 25. Section 7.4.1,4th paragraph > " > load balancing devices may in worst case result in that some packets take a > different network path then the others; > " > [Qin]: Should "then" in this sentence be "than" ? > No. > 26. Section 7.4.2, 4th paragraph > " > The rate of packet duplication is tracked, allowing one to take the > duplication into account. The value of the ECN field for duplicates > will also be counted and when comparing the figures one needs to take > " > [Qin]: What and where are the figures? > The 'figures' refers to the values of the various counters of ECN markings, as explained earlier in the paragraph. > 27.Section 8.2,8th paragraph > " > When rounding counter values in the translated RTCP packet, the > translator should try to ensure that they sum to the number of RTP > packets in the pre-translation sequence number space (numOrig). The > translator should also try to ensure that no non-zero counter is > rounded to a zero value, since that will lose information that a > particular type of event has occurred. It is recognised that it may > be impossible to satisfy both of these constraints; > " > > [Qin]: What's the difficulty to satisfy both constraints, why non-zero > counter can be rounded into zero? what kind of round method is used? > can you explain a little bit? > Because the translator actually alters the packet stream there is not always a clear mapping between the values taken from the original packets vs the translated packets. But if a particular event has been reported it is best to report that event (e.g. ECN-CE marking) has occurred in the translated packets - but they may not always add up correctly if one wants to indicate that a marking is now 'spread' over more than one packet in the translated stream. The rounding used means that it shouldn't round down to zero unless the pre-translated values are zero. > 28.Section 8.4, 2nd paragraph > " > An ECN-aware RTP mixer must generate RTCP ECN feedback packets and > RTCP XR report blocks for ECN summary information relating to the RTP > flows it terminates, in exactly the same way it would if it were an > RTP receiver. These reports form part of the congestion control loop > between the mixer and the media senders generating the streams it is > mixing. A separate control loop runs between each sender and the > mixer. > " > > [Qin]: what's the difference between congestion control loop and a separate > control loop > between media sender and mixer? are you saying congestion control loop is > unidirectional? > This may needs clarifying a little, though basically the 'control loop' refers to the overall exchange of messages - which are then used to drive 'congestion control loop'. The congestion control loop is not unidirectional. > 29. Section 12,1st paragraph > " > This section contain a few different examples of the signalling > mechanism defined in this specification in an SDP context. If there > are discrepancies between these examples and the specification text, > the specification text is definitive. > " > [Qin]: Why discrepacies is allowed? Should these example be consistent with > the sepcification text? > This a good point - we will take a look at them. > 30. Section 12.1 > " > This SDP offer offers a single media stream with 3 media payload > types. It proposes to use ECN with RTP, with the ICE based > initialization as being preferred over the RTP/RTCP one. Leap of > faith is not suggested to be used. The offerer is capable of both > setting and reading the ECN bits. In addition the RTCP ECN feedback > packet is configured and the RTCP XR ECN summary report. > " > [Qin]: Are you saying both ECN feedback and the XR ECN summary > report are allowed and configured or something else? > Yes that probably needs a small clarification. We're basically saying that they're both supported - though not necessarily configured as yet, Thanks, Piers. > Regards! > -Qin > > > ----- Original Message ----- > From: Roni Even > To: avt@ietf.org > Sent: Friday, September 30, 2011 8:29 PM > Subject: [AVTCORE] WGLC on draft-ietf-avtcore-ecn-for-rtp-04 > > Hi, > > I would like to start a WGLC on > http://tools.ietf.org/html/draft-ietf-avtcore-ecn-for-rtp-04 , Explicit > Congestion Notification (ECN) for RTP over UDP. > > > > The WGLC will end on October 21, 2011 > > > > Please review the draft and send comments to the list. > > > > > > Roni Even > > > > > > > > ________________________________ > > _______________________________________________ > Audio/Video Transport Core Maintenance > avt@ietf.org > https://www.ietf.org/mailman/listinfo/avt > > _______________________________________________ > Audio/Video Transport Core Maintenance > avt@ietf.org > https://www.ietf.org/mailman/listinfo/avt > >
- [AVTCORE] WGLC on draft-ietf-avtcore-ecn-for-rtp-… Roni Even
- Re: [AVTCORE] WGLC on draft-ietf-avtcore-ecn-for-… Belling, Thomas (NSN - DE/Munich)
- Re: [AVTCORE] WGLC on draft-ietf-avtcore-ecn-for-… Magnus Westerlund
- Re: [AVTCORE] WGLC on draft-ietf-avtcore-ecn-for-… Qin Wu
- Re: [AVTCORE] WGLC on draft-ietf-avtcore-ecn-for-… Belling, Thomas (NSN - DE/Munich)
- Re: [AVTCORE] WGLC on draft-ietf-avtcore-ecn-for-… Qin Wu
- Re: [AVTCORE] WGLC on draft-ietf-avtcore-ecn-for-… Bill Ver Steeg (versteb)
- Re: [AVTCORE] WGLC on draft-ietf-avtcore-ecn-for-… Dan Wing
- Re: [AVTCORE] WGLC on draft-ietf-avtcore-ecn-for-… Piers O'Hanlon
- Re: [AVTCORE] WGLC on draft-ietf-avtcore-ecn-for-… Qin Wu
- Re: [AVTCORE] WGLC on draft-ietf-avtcore-ecn-for-… Magnus Westerlund
- Re: [AVTCORE] WGLC on draft-ietf-avtcore-ecn-for-… Magnus Westerlund
- Re: [AVTCORE] WGLC on draft-ietf-avtcore-ecn-for-… Magnus Westerlund
- Re: [AVTCORE] WGLC on draft-ietf-avtcore-ecn-for-… Dan Wing
- Re: [AVTCORE] WGLC on draft-ietf-avtcore-ecn-for-… Piers O'Hanlon
- [AVTCORE] 答复: WGLC on draft-ietf-avtcore-ecn-for-… Qin Wu
- Re: [AVTCORE] WGLC on draft-ietf-avtcore-ecn-for-… Roni Even
- Re: [AVTCORE] WGLC on draft-ietf-avtcore-ecn-for-… Roni Even
- Re: [AVTCORE] WGLC on draft-ietf-avtcore-ecn-for-… Belling, Thomas (NSN - DE/Munich)
- Re: [AVTCORE] WGLC on draft-ietf-avtcore-ecn-for-… Magnus Westerlund
- Re: [AVTCORE] WGLC on draft-ietf-avtcore-ecn-for-… Roni Even
- Re: [AVTCORE] WGLC on draft-ietf-avtcore-ecn-for-… Bill Ver Steeg (versteb)
- Re: [AVTCORE] WGLC on draft-ietf-avtcore-ecn-for-… Piers O'Hanlon
- Re: [AVTCORE] WGLC on draft-ietf-avtcore-ecn-for-… Bill Ver Steeg (versteb)
- Re: [AVTCORE] WGLC on draft-ietf-avtcore-ecn-for-… Piers O'Hanlon