RE: [lwip-devel] TCP ack timing

[Mountifield, Tony]

Kieran,

Many thanks for your interesting response:

> Sounds like there's something strange going on, but it's
> almost correct.  TCP delays acknowledging data in order
> to avoid sending packets that only contain ACKs and no
> payload.  As you're sending data in one direction only,
> it never gets a packet to send it with (and this is why
> sending data in the reverse direction solved the problem). 
> However, there are supposed to be limits on the delay in the 
> absence of traffic:
>  - if its been 200ms since the acknowledgement was requested,
> send the ACK anyway;
>  - if the amount of unacknowledged data is >2*TCP_MSS, send
> the ACK anyway.

OK, that sounds like an explaination of what I'm seeing...

> To do this relies on the TCP timers being called
> appropriately.  As this is an embedded system, are you sure
> this is the case?

Yes, the timers are ok.

> Can anyone provide a (link to a) document/previous post
> that describes what is required?  As you get ACKS sent
> after 0.2s (200ms) it sounds like it is working.
> 
> This suggests that the second limit is not being obeyed.
> Having had a look at the code, lwIP uses a slightly
> different algorithm: it sends the ACK if *two ACKs* have
> been requested, not if *two MSS of data need ACKing*.
> This could result in the behaviour you're seeing if 
> you have a large TCP_MSS (similar to the receive window),
> and send a single segment that closes the receive window
> entirely.  Then you can't send a second segment, and so
> it waits for the 200ms limit to be hit.

Yes, I think that is what is happening. In lwipopts.h I have
2048 as the TCP window size (which is the default in opt.h),
and 1500 as the MSS (default was 128, and I changed it to
what I thought was more appropriate for Ethernet without
understanding its interaction with the window size).

> This is possibly a bug, but I think you might see similar 
> problems even if lwIP did the standard 2*TCP_MSS check,
> as you'd still never reach 2*TCP_MSS if TCP_MSS is > TCP_WND/2.
> 
> Could you verify if this is the case?  If not, I'll give it some more
> thought.  

Yes, I think you've hit the nail on the head, and I'll try
playing with the values.

Many thanks again
Tony


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