Selective Repeat

See also ARQ Protocol for more on Sliding Window.

We essentially are doing RR requests, but now saying what is not there. You cannot RR7 until you RR5 (because it's cumulative), but you could say selective reject (SREJ) 6 and 7 so that you can eventually flush your buffer (ie: you just kill the packets that didn't make it).

On error sender Negative Acknowledges a frame
• Called a Selective Reject (e.g. SREJ 10)
• Sent by data receiver
• Specifies the sequence number of the missing frame