Sunday, May 31, 2009

Logic Validation

Looking for some input on folks familiar with low-level TCP. I've got some general questions about proper values of Sequence # and Ack # over the course of a TCP session.

ACKing Packets

A segment on the retransmission queue is fully acknowledged if the sum of its sequence number and length is less or equal than the acknowledgment value in the incoming segment.

A packet is received with sequence number "seqn" and TCP payload with "len" octets of data. The receiving party should compare "seqn" to the variable outlined in the RFC, "rcv.nxt". If they match, "" is incremented by "len".

The ACK number of outgoing packets is always sent to ''-1.

What is done with packets received out-of-order, but that are entirely valid? Buffer them, then check each one's SEQ number when rcv.nxt is incremented?

Delayed ACK response
This also raises the question of how to properly ACK a received message, when packets aren't already being set. In some instances, it may be necessary to send an empty packet with the ACK flag set. How to determine when this needs to be done?


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