[e2e] tcp retransmission packet size.

[Dennis Rockwell]

On 26 Nov, Tianbo Kuang wrote:

> I noticed in a captured trace that a TCP sender actually retransmitted a
> bigger packet than the orignal one (after the ACK was dropped in the
> network). My question is,
> (1) Is this allowed by specification?

Yes, and encouraged.  You're not retransmitting the segment
(which is a transient entity), you're retransmitting
starting with the lost bytes.  Any bytes that have been
enqueued after the initial transmission will be included, up
to the maximum segment size.

> (2) If so, that means that a TCP receiver will always have to use the
> duplicated packet as the useful data as it contains more information. Is
> that true in the implementation?

Yes.  The reassembly code puts together a sequence of bytes
and trims away duplicates.  It doesn't matter which copy you
delete, they're identical.

Dennis