Roughly, FIN means "I have no more data to send" and SYN means "this is the
first packet of a new connection". It's probably valid to have SYN and FIN
both set if you only have less than one MSS-worth of data to transmit, but
this is a pretty unlikely circumstance to occur. I think transaction TCP
works this way but I'm not sure.
>This does remind me of one other question, though: is FIN+RST (w/ or w/o
>ACK set) ever a valid combination? I tend to think not (by pretty much the
No. RST combined with SYN or FIN isn't a sensible thing to send. A RST
causes the connection to be immediately torn down and this is incompatible
with either gracefully closing it or initiating a new one. :-)
RST+ACK is valid though.
p.
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