[c-nsp] TCP behavior under strict CAR rate-limiting

[bill fumerola]

On Thu, Jun 19, 2008 at 03:07:27PM -0700, Christopher Hunt wrote:
>    I am familiar with TCP's concept of Slow Start, but my understanding 
> is that it is the RWIN that is slow to start.  The packet does show the 
> first packet as 24 Byte payload, but even then the client RWIN is 5888 
> (scaled x7) (CentOS running 2.6.18 kernel).   The "server" is XP Pro 
> running an RWIN 65535 with scaling disabled.  As far as I can tell, TCP 
> slow start is not happenning.  What other signs of Slow Start should i 
> be looking for?

every second (+/- .1s) the drops occur and SACK kicks in:

23:05:46.809550 IP 192.168.10.2.33538 > 10.180.55.211.commplex-link: . 115865:117313(1448) ack 1 win 46 <nop,nop,timestamp 754051101 746671>
23:05:46.810977 IP 10.180.55.211.commplex-link > 192.168.10.2.33538: . ack 117313 win 65535 <nop,nop,timestamp 746671 754051101>
23:05:46.810997 IP 192.168.10.2.33538 > 10.180.55.211.commplex-link: . 117313:121657(4344) ack 1 win 46 <nop,nop,timestamp 754051103 746671>

ack 120209 begins, backlog to 126001 occurs:

23:05:46.812489 IP 10.180.55.211.commplex-link > 192.168.10.2.33538: . ack 120209 win 65535 <nop,nop,timestamp 746671 754051103>
23:05:46.812508 IP 192.168.10.2.33538 > 10.180.55.211.commplex-link: . 121657:124553(2896) ack 1 win 46 <nop,nop,timestamp 754051104 746671>

SACK kicks in:

23:05:46.813864 IP 10.180.55.211.commplex-link > 192.168.10.2.33538: . ack 120209 win 65535 <nop,nop,timestamp 746671 754051103,nop,nop,sack 1 {121657:123105}>
23:05:46.813883 IP 192.168.10.2.33538 > 10.180.55.211.commplex-link: . 124553:126001(1448) ack 1 win 46 <nop,nop,timestamp 754051105 746671>
23:05:46.814051 IP 10.180.55.211.commplex-link > 192.168.10.2.33538: . ack 120209 win 65535 <nop,nop,timestamp 746671 754051103,nop,nop,sack 1 {121657:124553}>
23:05:46.814070 IP 192.168.10.2.33538 > 10.180.55.211.commplex-link: . 126001:127449(1448) ack 1 win 46 <nop,nop,timestamp 754051106 746671>
23:05:46.815196 IP 10.180.55.211.commplex-link > 192.168.10.2.33538: . ack 120209 win 65535 <nop,nop,timestamp 746671 754051103,nop,nop,sack 1 {121657:126001}>

ack of last sack packet:

23:05:46.815214 IP 192.168.10.2.33538 > 10.180.55.211.commplex-link: . 120209:121657(1448) ack 1 win 46 <nop,nop,timestamp 754051107 746671>

.4 second delay, retransmit of ack of last sack packet:

23:05:47.237107 IP 192.168.10.2.33538 > 10.180.55.211.commplex-link: . 120209:121657(1448) ack 1 win 46 <nop,nop,timestamp 754051529 746671>

acks 120209-126001 finally occur, 10.180.55.211 moves on, .45 seconds later

23:05:47.238740 IP 10.180.55.211.commplex-link > 192.168.10.2.33538: . ack 126001 win 65535 <nop,nop,timestamp 746675 754051529>
23:05:47.238771 IP 192.168.10.2.33538 > 10.180.55.211.commplex-link: . 126001:127449(1448) ack 1 win 46 <nop,nop,timestamp 754051530 746675>

this pattern repeats frequently. sometimes with a retransmit, sometimes
without, always taking .3-.5 seconds or so. hence, the crap performance.


-- bill