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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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