Re: [dccp] Question on resetting nominal send time

[Gorry Fairhurst <gorry@erg.abdn.ac.uk>]

Ian,

I can see what you are saying, and indeed this is an interesting problem 
area, in that it impacts what we understand as correct behaviour, but 
also impinges on what we may accept as accceptable implementation cost.

I'll let others figure out their advice to you...

Gorry

Ian McDonald wrote:

> Folks,
> 
> While Gerrit and I have been refining the CCID3 implementation in
> Linux we have noticed some issues around packet scheduling. I would
> like some clarification around this please as I can't find the answers
> in the RFCs. It may well be that I have just missed something obvious.
> 
> Section 4.6 of RFC3448 talks about calculating the nominal sending
> time being the previous nominal sending time plus t_ipi (inter packet
> interval).
> 
> The aim of this is to allow an average packet rate per second and
> section 4.6 explicitly allows bursts of traffic.
> 
> This generally works well except for two scenarios that I can think of:
> 
> 1) The application sends at less than the permitted rate. This means
> that the nominal send time becomes further and further in the past for
> the current packet. This means we can basically transmit whenever we
> want until we catch up in time. I would guess that this is not what is
> intended, particularly as it means it will take time to respond to the
> beginning of increased loss.
> 
> 2) The sender becomes idle. However there is no talk of resetting the
> nominal sending time. So if we are idle for 10 seconds then when we
> become active again we can send 10 seconds worth of packets
> instantaneously. I am guessing that this was also not the intent of
> the RFC authors.
> 
> Can some clarification please be provided or pointing out what I have
> missed in the RFCs?
> 
> I'm guessing there should be some mechanism for resending the nominal 
> send time.
> 
> Regards,
> 
> Ian