Re: [lwip-users] TCP Buffer Size Increment

[Gaurav Jain]

My scenario is like this:
I have a windows machine acting as a client and a Virtex 707 acting as
Server (implemented using Lwip). From the client side I had set the
limit to 512 bytes per send request. However, on logging the
activities on the server side, what I observed was that the first
packet received is of 512 bytes, as sent by the client. However, there
onwards the next packet received was 1460 bytes.

In addition I even observed packet queuing behaviour, i.e.

Now since the client is sending packets in chunks of 512 bytes and as
per my observation, the server waits for 1460 bytes before processing,
it implies that until it receives 3 packets of 512 bytes, it would not
push the data upward (provided a close is not requested). Now once it
does receive them, what it does, as I have observed, is that
512+512+512 > 1460, so it pushes the 1460 bytes first, then it pushes
the remaining 76 bytes as well. Now how do you explain this behavior
if the congestion window mechanism was not coming into picture? Or is
it something with my implementation?


On Wed, Feb 3, 2016 at 2:35 PM, Simon Goldschmidt <address@hidden> wrote:
> Gaurav Jain wrote:
>> TCP has a mechanism to increase the buffer after every successful
>> transaction. So after receiving how does it increase it to 1460 bytes(
>> the upper limit)?
>
> It's the sender that has this limit. As in your example, lwIP is the receiver,
> slow start doesn't help you.
>
> Also, what you are talking about is actually a limit to the window, not to
> the maximum segment size. Being like that, 1460 is NOT the upper limit.
>
> Again, without having the remote side under control, you can NOT ensure to
> get 512 byte sized segments. That's just not what TCP is like.
>
>
> Simon
>
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