[dccp] Re: RFC: some tweaking in ccid3_hc_tx_send_packet

[Ian McDonald <imcdnzl at gmail.com>]

>         Ah, quick comment without further too much thinking about the full
> implications: isn't initializing t_ipi to TFRC_INITIAL_TIMEOUT (2 seconds)
> a bit excessive?
> 
Yes. The spec for CCID3 says:
    RFC 3448 (Section 4) specifies an initial sending rate of one packet
    per RTT (Round-Trip Time) as follows: The sender initializes the
    allowed sending rate to one packet per second.  As soon as a
    feedback packet is received from the receiver, the sender has a
    measurement of the round-trip time, and then sets the initial
    allowed sending rate to one packet per RTT.  However, while the
    initial TCP window used to be one segment, RFC 2581 allows an
    initial TCP window of two segments, and RFC 3390 allows an initial
    TCP window of three or four segments (up to 4380 bytes).  RFC 3390
    gives an upper bound on the initial window of
               min(4*MSS, max(2*MSS, 4380 bytes)).
    Translating this to the packet-based congestion control of CCID 3,
    the initial CCID 3 sending rate is allowed to be at least two
    packets per RTT, and at most four packets per RTT, depending on the
    packet size.  The initial rate is only allowed to be three or four
    packets per RTT when, in terms of segment size, that translates to
    at most 4380 bytes per RTT.

so I think that if we go for four packets per RTT as initial maximum
then we can set first t_ipi as 250 milliseconds. We shouldn't alter
the constant, but create a new one, as that constant is used for other
things.

If anybody from dccp IETF working group wishes to comment on whether I
have this right/wrong please advise....

Regards,

Ian