Re: [lwip-users] tcp_listen_input: ACK in LISTEN, sending reset

[Johan Högberg]

Quoting Leon Woestenberg <address@hidden>:

> Good call, but maybe first check this with a good book on TCP, or even
> the RFC (if this describes this corner case unambiguously).

Sorry, I did read some in "TCP/IP Illustrated" before posting but I'm
a newbie on TCP. I've now checked with RFC793 and I believe it proves
my point.

Regards,
Johan

http://www.faqs.org/rfcs/rfc793.html
(page 36, Reset Generation)
  2.  If the connection is in any non-synchronized state (LISTEN,
  ....

  If the incoming segment has an ACK field, the reset takes its
  sequence number from the ACK field of the segment, otherwise the
  reset has sequence number zero and the ACK field is set to the sum
  of the sequence number and segment length of the incoming segment.
  The connection remains in the same state.

(page 65, SEGMENT ARRIVES)
 If the state is LISTEN then

    first check for an RST

      An incoming RST should be ignored.  Return.

    second check for an ACK

      Any acknowledgment is bad if it arrives on a connection still in
      the LISTEN state.  An acceptable reset segment should be formed
      for any arriving ACK-bearing segment.  The RST should be
      formatted as follows:

        <SEQ=SEG.ACK><CTL=RST>

      Return.