In TCP recovery state, problem of computing the pipe(amount of data in flight).

[hiren panchasara]

On 02/29/16 at 05:40P, Yongmin Cho wrote:
> Thanks very much for the quick reply.
> 
> I'm sorry, I didn't make patch file.
> First, I wanted to discuss my opinion is right.
> 
> Let me give you example, please check this.
> 
> <- ACK (cumack = 1,  sack[3-4], sack[6-7], sack[9-10])
> * here segment/byte 2, 5, and 8 are missing.
> <- ACK (cumack = 1, sack[12-13], sack[15-16], sack[18-19])
> * here segment/bytes 11, 14 and 17 are also reported missing.
> <- ACK (cumack = 1, sack[18-19], sack[21-24], sack[27, 28])
> * here segment/bytes 20 and 25, 26 are missing. (triggered fast
> retransmission)
> <- ACK.....(cumack = 1, sack[21-24], sack[27, 28], sack[32, 33])
> <- ACK.....(cumack = 1, sack[34-35], sack[37, 40], sack[42, 44])
> <- ACK.....(cumack = 1, sack[34-35], sack[37, 40], sack[42, 45])
> <- ACKs.....(many duplication acks, and new sacked blocks)
> 
> In the fast recovery phase, the pipe is caculated like below, If the
> net.inet.tcp.rfc6675_pipe is turned on.
> pipe = snd_max - snd_una + sack_bytes_rexmit(1 MSS size) -
> sacked_bytes(10 = 34-35, 37-40, 42-45 tcp_sack.c:390)
> One segment is sended(sack_bytes_rexmit), when triggered fast
> retransmission.
> Because the snd_cwnd was set 1 mss size. (tcp_input.c:2609)
> In the fast recovery phase, The sender can send data,
> If this condition is right(awnd < tp->snd_ssthresh tcp_input.c:2568).
> When in the network, It still has many in flight packets,
> snd_max and snd_una will not be changed, and sack_bytes_rexmit is one MSS 
> size, and sacked_bytes is caculated by last ACK that has three SACK
> blocks(34-35, 37-40, 42-45).
> So, sometimes(In my test environment) the awnd(pipe) value can't go
> down less than the snd_ssthresh, while receiving each ACKs
> in fast recovery phase.
> You know, If the awnd value can't go down less than the snd_ssthresh,
> The sender can't send data that is included snd_holes.
> 
> So, I think, the sacked_bytes should be caculated by all of sacked
> blocks that is greater than snd_una, like below.
> pipe = snd_max - snd_una + sack_bytes_rexmit - sacked_bytes(3-4, 6-7,
> 9-10, 12-13, 15-16, 18-19, 21-24, 27-28, ...).
> 
> But on current implementation, the sacked_bytes is caculated by three(or four)
> sacked blocks that is in last ACK, like below.
> pipe = snd_max - snd_una + sack_bytes_rexmit - sacked_bytes(34-35,
> 37-40, 42-45 -> sacked blocks of last ACK).
> 
> My opinion may not be right. Just I want to check implementation of
> computing pipe.

Your opinion seems correct to me. If you can create a patch based on
this, that'd be great. As I cannot spend time on this until next week.

Cheers,
Hiren
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