Re: [lwip-devel] pcb->lbb = ISS - 1?

[Kieran Mansley]

On Tue, 2012-04-10 at 12:49 +0800, narke wrote:
> I found the first segment (SYN) sent from a connection has a seqno of
> tcp_next_iss() - 1, and this is because the segment was created with a
> seqno equals with pcb-lbb, which was assigned with tcp_next_iss() in
> the tcp_connect() function.  Why is this '- 1' operation is
> necessary? 

The code in question is I think this, from tcp_connect():

  iss = tcp_next_iss();
  pcb->snd_nxt = iss;
  pcb->lastack = iss - 1;
  pcb->snd_lbb = iss - 1;

I'm not sure why it has been written that way.  It does strike me as
odd.  snd_lbb is the sequence number of next byte to be buffered, so it
would make sense for it to be equal to snd_nxt rather than lastack.

In tcp_alloc() we do this instead:

    iss = tcp_next_iss();
    pcb->snd_wl2 = iss;
    pcb->snd_nxt = iss;
    pcb->lastack = iss;
    pcb->snd_lbb = iss;

Kieran