Re: [rtcweb] Ben Campbell's Yes on draft-ietf-rtcweb-fec-08: (with COMMENT)

Justin Uberti <juberti@google.com> Sat, 23 February 2019 01:17 UTC

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From: Justin Uberti <juberti@google.com>
Date: Fri, 22 Feb 2019 17:17:20 -0800
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To: Ben Campbell <ben@nostrum.com>
Cc: The IESG <iesg@ietf.org>, RTCWeb IETF <rtcweb@ietf.org>, rtcweb-chairs@ietf.org, draft-ietf-rtcweb-fec@ietf.org
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Subject: Re: [rtcweb] Ben Campbell's Yes on draft-ietf-rtcweb-fec-08: (with COMMENT)
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On Fri, Feb 22, 2019 at 4:57 PM Ben Campbell <ben@nostrum.com>; wrote:

> Thanks for the response. Comments below:
>
> On Feb 22, 2019, at 6:31 PM, Justin Uberti <juberti@google.com>; wrote:
>
> Thanks for your comments. See below:
>
> On Wed, Feb 20, 2019 at 9:56 AM Ben Campbell <ben@nostrum.com>; wrote:
>
>
> […]
>
> ----------------------------------------------------------------------
>>
>> Thanks for this effort. I am balloting "yes", but have some minor
>> comments:
>>
>> §8:
>> - "Given this, WebRTC implementations SHOULD consider using RTX or
>> flexfec retransmissions instead of FEC when RTT is low,"
>>
>> "consider" seems vague for a normative requirement. Can you describe
>> concrete
>> requirements? Otherwise I suggest descriptive language.
>>
>
> Perhaps "prefer" rather than "consider" would be less squishy.
>
>
> Prefer is better, thanks.
>
>
>> Can you give guidance on what RTT would be reasonable to consider as
>> "low"?
>>
>
> This is somewhat application-dependent, but ~100 ms would be a reasonable
> threshold.
>
>
> Would it make sense to say that the threshold for “low” is application
> dependent, but that would be a reasonable number for “typical” applications?
>
>
>
>> - "Note that when probing bandwidth, i.e., speculatively sending extra
>> data to determine if additional link capacity exists, FEC SHOULD be
>> used in all cases."
>>
>> I assume the point of this is the redundant FEC data should _be_ that
>> extra
>> data. But one could read this to mean that, if you are already sending
>> extra
>> data, you should also use FEC.
>>
>
> I see what you mean, agree this could be clarified.
>
>
> Thanks
>
>
>> §9, 2nd paragraph:
>>
>> I'm by no means an expert in this, and leave it to the crypto experts to
>> know
>> if this matters, but does the additional redundancy of FEC have any
>> impact on
>> SRTP encryption?
>>
>
> My understanding is that the fact that FEC is sent as its own stream with
> its own IV means that even if the data was identical to the primary
> payload, there would be no issue.
>
>
> Is that true for the case of in-band FEC at the codec level?
>

In the RFC 2198 case, SRTP is once again run over a new packet, again with
a new IV (from a new sequence number), and a new primary payload, so the
cipher state will be entirely different when the previous data is
re-encrypted. In the codec in-band case, you have a new bitstream, so there
are no repeated bytes.

>
> But given that no new mechanisms are being proposed in this document, it's
> probably covered elsewhere.
>
>
> I can accept that argument.
>
> Thanks!
>
> Ben.
>