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Linux Kernel tcp flow tcp_validate_incoming and RFC793

antihtio
Posts: 1
RFC 793, page 69 state that:
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.
If I am not wrong the equivalent code can be found inside tcp_validate_incoming(...):
- /* Step 1: check sequence number */
- if (!tcp_sequence(tp, TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq)) {
- /* RFC793, page 37: "In all states except SYN-SENT, all reset
- * (RST) segments are validated by checking their SEQ-fields."
- * And page 69: "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)".
- */
- if (!th->rst) {
- if (th->syn)
- goto syn_challenge;
- if (!tcp_oow_rate_limited(sock_net(sk), skb,
- LINUX_MIB_TCPACKSKIPPEDSEQ,
- &tp->last_oow_ack_time))
- tcp_send_dupack(sk, skb);
- } else if (tcp_reset_check(sk, skb)) {
- tcp_reset(sk);
- }
- goto discard;
- }
However my question is with the following line in the RFC:
- If the RCV.WND is zero, no segments will be acceptable, but
- special allowance should be made to accept ***valid ACKs***, ***URGs*** and
- RSTs.
I can see that RSTs has been taken care of, in the if() statement, however I can't seems to figured out which part of the code process valid ACKs and URGs.
Any help is greatly appreciated !!
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