[AVTCORE] Comments on draft-ietf-avtcore-rtp-circuit-breakers-05
Magnus Westerlund <magnus.westerlund@ericsson.com> Thu, 27 February 2014 14:59 UTC
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Date: Thu, 27 Feb 2014 15:58:56 +0100
From: Magnus Westerlund <magnus.westerlund@ericsson.com>
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Subject: [AVTCORE] Comments on draft-ietf-avtcore-rtp-circuit-breakers-05
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Hi, (as individual) I have reviewed draft-ietf-avtcore-rtp-circuit-breakers-05 and have some comments: 1) Section 3: This modifies the RTCP timing rules to allow RTCP reports to be sent early, in some cases immediately, provided the average RTCP reporting interval remains unchanged. I would suggest changing "provided the average RTCP reporting interval remains unchanged." is changed to: provided the RTCP transmission keeps within its bandwidth allocation. The reason is that AVPF does not necessarily keep within the average reporting interval. As that is commonly under using the RTCP bandwidth. 2) Section 3: The use of the RTP/AVPF profile is dependent on signalling, but is otherwise generally backwards compatible with the RTP/AVP profile, as it keeps the same average RTCP reporting interval as the base RTP specification. I wouldn't claim that it is generally backwards compatible. In many configurations it would not, and even with the timeout clarification being proposed in draft-ietf-avtcore-rtp-multi-stream-03 there will still be cases where interoperability would be questionable. I suggest some other formulations, maybe saying that one can configure AVPF to be backwards compatible. 3) Section 4: TCP congestion control intentionally tries to fill the router queues, and uses the resulting packet loss as congestion feedback. I think a reference would be appropriate after "TCP congestion control". 4) Section 4.1: Accordingly, if a sender of RTP data packets receives two or more consecutive RTCP SR or RR packets from the same receiver, and those packets correspond to its transmission and have a non-increasing extended highest sequence number received field (i.e., the sender receivers at least three RTCP SR or RR packets that report the same value in the extended highest sequence number received field for an SSRC, but the sender has sent RTP data packets for that SSRC that would have caused an increase in the reported value of the extended highest sequence number received if they had reached the receiver), then that sender SHOULD cease transmission (see Section 4.5). It is a long sentence, and my comment relate to "has sent RTP data packets for that SSRC that would have caused an increase in the reported value of the extended highest sequence number" If the sender sends very few packets, this effects the probability for this CB to trigger. So if I send only 1 packet per reporting interval, and each packet is lost with a probability of 10%, then I have 1% (0.1^2) probability of this being triggered. If I send 10 packets per interval, then I have 0.1^20 = 1E-20. Thus, it might be worth making some lower threshold recommendation here. This example assumes equal distribution of loss also, but if we take into burst a short packet burst may be likely to be completely lost, while the same amount of packets may be likelier to have at least one arrive if distributed over the reporting interval. 5) Section 4.2: Accordingly, an RTP sender that has not received any RTCP SR or RTCP RR packets reporting on the SSRC it is using for three or more RTCP reporting intervals SHOULD cease transmission (see Section 4.5). Maybe one should clarify that the reporting intervals used in this calculation is the interval calculated for the SSRC sending the media. This is in comparision with 4.1 where the trigger is calculated on a per remote basis for the received reports from that particular remote SSRC. Is there an issue if one gets reports from multiple remote SSRCs on ones transmission. From a media timeout in a point to point case, these reports although from many SSRCs should not skew the result as they would be based on the same underlying data. But, using any two consecutive reports, does not ensure that one is measuring over an sufficient long interval. 6) Section 4.2: When calculating the timeout, the fixed minimum RTCP reporting interval SHOULD be used (based on the rationale in Section 6.2 of RFC 3550 [RFC3550]). I think this sentence provides an minor issue if one tries to match this with the RFC 3550 terminology. fixed minimum (interval) is used in section 6.2. I think the added "RTCP reporting" is confusing here, I would suggest to use another formulation here. I think you may have to define what fixed minimum RTCP reporting interval means, i.e. the RTCP reporting interval (T or Td?, i.e. with or without randomization) (and in case of randomization, two random draws or simple 2*T?) calculated with the current set of RTCP stack parameters and using the fixed minimum interval (SHOULD be 5 sec). 7) Section 5: As there is a difference between CB #1 and #3 and #2 that has to do with that the first (#1 and #3) uses incoming RTCP reports and that #2 uses the CB implementers estimate of what the regular reporting interval is. Thus, I wonder if there needs to exist include a reflection over t_rr_int as well as the RTCP SSRC timeout calculation that we recommend to use 5 seconds as fixed minimal value in those calculations (if 5 sec larger than t_rr_int). 8) Section 5: Reports of ECN-CE packets sent as reduced-size RTCP ECN feedback packets without an RTCP SR/RR packet MUST be ignored. Shouldn't then be accounted as loss event in CB #3? Or are you assuming that this will anyway be counted in the next compound packets ECN XR summary? 9) Section 9: This attack can be avoided if RTCP packets are authenticated, for example using the Secure RTP profile [RFC3711]. Maybe this should reference draft-ietf-rtp-security-options for other potential solutions? Cheers Magnus Westerlund ---------------------------------------------------------------------- Services, Media and Network features, Ericsson Research EAB/TXM ---------------------------------------------------------------------- Ericsson AB | Phone +46 10 7148287 Färögatan 6 | Mobile +46 73 0949079 SE-164 80 Stockholm, Sweden | mailto: magnus.westerlund@ericsson.com ----------------------------------------------------------------------
- [AVTCORE] Comments on draft-ietf-avtcore-rtp-circ… Magnus Westerlund
- Re: [AVTCORE] Comments on draft-ietf-avtcore-rtp-… Colin Perkins
- Re: [AVTCORE] Comments on draft-ietf-avtcore-rtp-… Colin Perkins