Why is an empty TCP segment at right edge of receive window not acceptable?

Question

The TCPv4 specification (RFC 793) classifies a received segment as unacceptable if it has zero length, a sequence number equal to RCV.NXT+RCV.WND while the receive window is not zero (second row in the table).

This essentially means that the segment will be discarded, other than possibly sending an ACK. No ACK processing or send window update will be done.

What is the justification of this?

Consider this scenario:

1. Host A sends all possible data segments to host B, just exhausting the receive window of B.
2. Host A shortly also sends an empty segment, e.g. a window update or acknowledgement of received data. This segment has sequence number equal to the right edge of the receive window of host B (RCV.NXT+RCV.WND), since it was set to the latest SND.NXT of host A.
3. The mentioned data packets are lost in the network or delayed, and host B receives the empty segment first.
4. Host B will classify the empty segment as not acceptable, and drop it, ignoring any acknowledgement or window update.

This seems to be particularly problematic, because the receiver will typically ignore seemingly older window updates based on the combination of sequence number and acknowledgement number. But just those window updates that won't get filtered out (highest sequence number) would be dropped even earlier due to segments not being considered acceptable.

Is there some part that I am not understanding correctly? Is this scenario really possible?

Answer 1

Segment in this context refers to PAYLOAD OF TCP. L=0 toss it. Segment outside of window toss it. S<RCV.NXT+RCV.WND but that is where they mess up in the spec, and it gets fixed in a later RFC.
Any S<RCV.NXT+RCV.WND will be accepted. Like a retransmission. ACKs, SACKS, and window updates are always processed. (The max is used to keep things straight in out-of-order situations)

Hope you graduated!

Answer 2

Okay, so here's that table:

    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

According to your scenario, for the packet of interest, SEG.SEQ = RCV.NXT+RCV.WND, SEG.LEN = 0, and RCV.WND > 0.

So yes, we need to look at the second row, and indeed, that test doesn't pass, which doesn't make a lot of sense.

Maybe report an erratum?

Answer 3

I do not see a situation where host A sends an empty segment in your scenario.

A is supposed to send packets up to the receiver window and then wait for ACKs [your item 1]. If nothing is received in response there are retransmissions, and retransmissions have sequence number < RCV.NXT+RCV.WND. A will not send an empty packet with sequence number outside of receiver window if it did not receive anything from B. So, your item 2 cannot happen.

If anything is received by A, that means that A has received an ACK (which updates SND.UNA) and with it new value of receiver window, so A either knows new receiver window or that receiver window is empty. This means next sequence number will be inside receiver window. Thus it will not result in missing check in line 2. Thus again, item 2 will not happen.

If A knows that receiver window is empty, it will indeed send a zero-length packet called zero-window probe. But this is conditioned on A receiving a zero-length receiver window from B, so again your situation cannot happen. Also, window probes are ACKed, so again both A and B know that the receiver window is zero, and A can select a sequence number SEG.SEQ = RCV.NXT.

So, to summarize, it is actually a sender error, if the sender sends a segment with a sequence number outside of receiver window.