On 12/19/2014 2:24 PM, Brian Trammell wrote:
...
> I was trying to make a much less subtle point from the interface side:
> since SCTP actually _has_ a concept of application PDU, it can bundle
> them, while TCP has no such concept, so what it's bundling can't really
> be PDUs.
TCP has three notions of application interaction regarding user data
boundaries:
- a bytestream
at which point the entire stream is the user API PDU
(that's why CLOSE implies EOF; it's really
"end of stream")
- URG
a one-bit flag that is an offset into the bytestream
- PSH
Of these, only the first two provide user-level "PDU" information. The
last is essentially a flow-control signal.
However, because of how URG works, it's not useful as an intra-stream
marker (it is set for all segments between when activated and not).
So I wouldn't say that TCP has no concept of an application PDU. What it
lacks are internal PDU boundaries. The stream itself is the PDU.
Joe
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