Question regarding TCP behavior

Question regarding TCP behavior

[Adayadil Thomas]

Greetings.

I have a question about how linux's TCP stack behaves.
I apologize if this is not the right place to ask this question and
please redirect me.

When a TCP end point (A) sends x bytes of data to the other end point (B),
does B immediately ACK the received bytes or will it do so only when the data
is passed to the upper layer ?


Thanks
Thomas

RE: Question regarding TCP behavior

[David Schwartz]

> Greetings.
>
> I have a question about how linux's TCP stack behaves.
> I apologize if this is not the right place to ask this question and
> please redirect me.
>
> When a TCP end point (A) sends x bytes of data to the other end point (B),
> does B immediately ACK the received bytes or will it do so only
> when the data
> is passed to the upper layer ?
>
>
> Thanks
> Thomas

If the TCP connection was idle and there is no unacknowledged data in either
direction, the answer is neither.

Acknowledging the data immediately is wasteful. There's a very good chance
another packet is right behind this one and delaying the acknowledgement
would save a packet.

Waiting for upper layers is disastrous, it could result in the other end
timing out and retransmitting and would limit the end of slow start based on
user-space speeds.

Google "delayed ACK".
http://www.freesoft.org/CIE/RFC/1122/110.htm

DS

RE: Question regarding TCP behavior

[Jan Engelhardt]

On Thursday 2008-05-29 18:56, David Schwartz wrote:
>> Greetings.
>>
>> I have a question about how linux's TCP stack behaves.
>> I apologize if this is not the right place to ask this question and
>> please redirect me.
>>
>> When a TCP end point (A) sends x bytes of data to the other end point (B),
>> does B immediately ACK the received bytes or will it do so only
>> when the data
>> is passed to the upper layer ?
>>
>>
>> Thanks
>> Thomas
>
>If the TCP connection was idle and there is no unacknowledged data in either
>direction, the answer is neither.
>
>Acknowledging the data immediately is wasteful.[...]

And for everything else, there is the PSH bit. (I assume?)