[lwip-devel] TCP implementation voilates RFC 793

[Elena Gurevich]

Hi dear all,

 

I’m following the implementation of tcp_receive() function and it looks like function logic does not follow RFC793 specification of segment_arrives event:

 

RFC specifies that:

 

--------------------- snipped from RFC 793 --------------------------

first check sequence number

 

      SYN-RECEIVED STATE

      ESTABLISHED STATE

      FIN-WAIT-1 STATE

      FIN-WAIT-2 STATE

      CLOSE-WAIT STATE

      CLOSING STATE

      LAST-ACK STATE

      TIME-WAIT STATE

 

        Segments are processed in sequence.  Initial tests on arrival

        are used to discard old duplicates, but further processing is

        done in SEG.SEQ order.  If a segment's contents straddle the

        boundary between old and new, only the new parts should be

        processed.

 

        There are four cases for the acceptability test for an incoming

        segment:

 

        Segment Receive  Test

        Length  Window

        ------- -------  -------------------------------------------

 

           0       0     SEG.SEQ = RCV.NXT

 

           0      >0     RCV.NXT =< SEG.SEQ < RCV.NXT+RCV.WND

 

          >0       0     not acceptable

 

          >0      >0     RCV.NXT =< SEG.SEQ < RCV.NXT+RCV.WND

                      or RCV.NXT =< SEG.SEQ+SEG.LEN-1 < RCV.NXT+RCV.WND

 

        If the RCV.WND is zero, no segments will be acceptable, but

        special allowance should be made to accept valid ACKs, URGs and

        RSTs.

 

        If an incoming segment is not acceptable, an acknowledgment

        should be sent in reply (unless the RST bit is set, if so drop

        the segment and return):

 

          <SEQ=SND.NXT><ACK=RCV.NXT><CTL=ACK>

 

        After sending the acknowledgment, drop the unacceptable segment

        and return.

 

        In the following it is assumed that the segment is the idealized

        segment that begins at RCV.NXT and does not exceed the window.

        One could tailor actual segments to fit this assumption by

        trimming off any portions that lie outside the window (including

        SYN and FIN), and only processing further if the segment then

        begins at RCV.NXT.  Segments with higher begining sequence

        numbers may be held for later processing.

 

--------------------- snipped from RFC 793 --------------------------

 

As I understand that all control information ( SYN, FIN, ACK, ack_no, window ) filed should be ignored and not processed in case of unacceptable segment arrival.

According tcp_receive() function from first of all it takes care of ack_no and window field for segment with ACK ? uses actual ack_no and windows values to update tcb

and takes into account segment seq_no only at data processing step.

 

This looks like strong violation of RFC defined behavior.

 

I am a new member of lwIP and intended to use the TCP implementation as a "golden" reference, but very confused now.

Please explain me if I felt into some misunderstanding !!!

 

with best regards,

Lena