Why "tp->write_seq = 1"?

[Xu�n Baldauf]

Hello,
during reading the tcp code of linux, I found an oddity:
In some (but not all) functions dealing with the "write_seq" field of a 
"struct tcp_sock", if this field ever was to be set as 0, it is actually 
set as 1.

Examples are:
   * http://lxr.linux.no/source/net/ipv4/tcp_ipv4.c#L593 :
                     if ((tp->write_seq =
                             tw->tw_snd_nxt + 65535 + 2) == 0)
                         tp->write_seq = 1;
   * http://lxr.linux.no/source/net/ipv4/tcp.c#L1821 :
         if ((tp->write_seq += tp->max_window + 2) == 0)
             tp->write_seq = 1;
But this kind of changing numbers changes the result of the calculation 
of "how much data is still unsent", as in

   * http://lxr.linux.no/source/net/ipv4/tcp.c#L452 :
         case SIOCOUTQ:
             if (sk->sk_state == TCP_LISTEN)
                 return -EINVAL;
             if ((1 << sk->sk_state) & (TCPF_SYN_SENT | TCPF_SYN_RECV))
                 answ = 0;
             else
                 answ = tp->write_seq - tp->snd_una;
             break;
   * http://lxr.linux.no/source/net/ipv4/tcp_diag.c#L155
         r->tcpdiag_wqueue = tp->write_seq - tp->snd_una;
So I ask: Is this a bug or a feature? Why is sometimes 
"tp->write_seq==1" allowed and sometimes not?

ciao,
Xu�n.
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