[Web10g-user] difference in curCWND and SlowStart

[Valdis.Kletnieks at vt.edu]

On Fri, 11 Jul 2014 11:50:39 -0400, mojgan ghasemi said:

> I have a connection that according to Web10G, never exists the SS, and its
> SlowStart variable is 235, meaning that the CWND has been increased a total
> of 235 times.
>
> However, when I look at the packet traces of the connection using tcpdump,
> I see more than 15K new ACKs being delieverd for this connection (even with
> byte counting approach), which according to many RFCs such as 2581, each of
> them must make the CWND to increase by one MSS. So why only increase CWND
> 235 times?

Is TCP Window Scaling in effect?  If so, you could be seeing "steps" in
the window.   For example, say you start with a (hypothetical) window of
12, and a scaling of 3, and packets coming in that each increase the window
by 3.

packet    internal window     on the wire
0           12                   1
1           15                   1
2           18                   2
3           21                   2
4           24                   3
5           27                   3
6           30                   3
7           33                   4

You get the idea.  15K ACKs and 235 updates works out to 63 acks per
update - making me take a wild guess that you have TCP scaling of 6
in effect?


-------------- next part --------------
A non-text attachment was scrubbed...
Name: not available
Type: application/pgp-signature
Size: 848 bytes
Desc: not available
Url : http://lists.psc.edu/pipermail/web10g-user/attachments/20140711/78f11fee/attachment.bin