Subject: Re: [Ice] Continue discussion of global pacing based on feedback from "transport guys"

From: "Black, David" <[email-masked]>
Date: Wed, 28 Sep 2016 15:45:16 +0000

I'm apparently one of the "transport guys" referred to in the subject line ;-).

In https://www.ietf.org/mail-archive/web/ice/current/msg00378.html, Peter Thatcher
described the current proposal:

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1.  A maximum number of outstanding request packets, call it MaxPackets which SHOULD be 10.

2.  A timeout for request packets, call it handshake timeout or HTO which SHOULD be 2*RTT if the RTT is known and 500ms otherwise. 

3.  A global pacing rate of 5ms (like a global Ta).

If all packets are dropped and the RTT is not known, then 1+2 are effectively 20 packets per second,
well below what you would get with the bitrate limits we were asking for.
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There's been some further discussion with the "transport guys" about packet size and bit rate.   In essence, the rationale for 10 as the maximum number of outstanding request packets (item 1 above) is based on the typical TCP segment size of about 1500 bytes.  ICE request packets tend to be much smaller, 100 bytes or less, which suggests that a factor of 15 could be available if bit rate matters but packet count doesn't.    That may well be the case ...

RFC 7141 (2014 BCP on AQM configuration/implementation) supports that view by recommending that AQM implementations count octets (i.e., forwarding resources are bit-congestible) as opposed to counting packets (i.e., forwarding resources are not packet-congestible).  That RFC effectively asserts that the current Internet is bit-congestible only: https://tools.ietf.org/html/rfc7141

That would suggest a value of 150 for MaxPackets *when* all the packets are 100 octets or less, as should be case for ICE request packets.  A possible area of concern (based on ICE use by Web RTC) is residential gateway/NAT boxes, but the 5ms pacing (item 3 above) ought to be sufficient to avoid overrunning them.

What do people with more knowledge of and/or experience with residential gateway boxes think of this line of reasoning?

Thanks, --David