Previous

Content

Next 

5.0.- Sequence Number Validity  

DCCP endpoints should ignore packets with invalid sequence numbers. This should be done using what is called a LossWindow. Each sender gives the receiver a loss window W. This number reflects how many packets the sender expects to have in flight. Only the sender can anticipate this number. A good guideline is to set W to 3 or 4 times the maximum number of packets the sender expects to send in any round-trip time. W defaults to 1000.
The receiver sets up a loss window of W consecutive sequence number containing the Greatest Sequence Number (GSN) it has received on any valid packet from the sender. The loss window may be centered on GSN or arranged asymetically. Then, sequence numbers outside this loss window are invalid. Packets with invalid sequence numbers are themselve invalid, unless both next conditions hold:

  1. No valid packet has been received within at least one RTT.

  2. The packet includes a correct Identification or Challenge option, and a valid ACK number (the ACK number is within the loss window).

  

The receiver should igonre invalid packets. However, the receiver may send a DCCP-ack packet to the sender acknowledging the last received valid sequence number and containing a Challenge option.
Previous

Content

Next