How does packet interaction with TCP Selective Acknowledgement work?

Question

Can anybody explain how the packet interaction with TCP Selective Acknowledgment works? I found the definition on Wikipedia, but I cannot get a clear picture what Selective Acknowledgment really does compared to Positive Acknowledgment and Negative Acknowledgment.

Answer 1

TCP breaks the information it sends into segments... essentially segments are chunks of data no larger than the current value of the TCP MSS (maximum segment size) received from the other end. Those chunks have incrementing sequence numbers (based on total data byte counts sent in the TCP session) that allow TCP to know when something got lost in-flight; the first TCP sequence number is chosen at random, and for security-purposes it should not be a pseudo-random number. Most of the time, the MTU of your local ethernet is smaller than the MSS, so they could send multiple segments to you before you can ACK.

It is helpful to think of these things in the time sequence they got standardized...

First came Positive Acknowledgement, which is the mechanism of telling the sender you got the data, and the sequence number you ACK with is the maximum byte-sequence received per TCP chunk (a.k.a segment) he sent.

I will demonstrate below, but in my examples you will see small TCP segment numbers like 1,2,3,4,5... in reality these byte-sequence numbers will be large, incrementing, and have gaps between them (but that's normal... TCP typically sends data in chunks at least 500 bytes long).

So, let's suppose the sender xmits segment numbers 1,2,3,4,5 before send your first ACK. If all goes well, you send an ACK for 1,2,3,4,5 and life is good. If 2 gets lost, everything is on hold till the sender realizes that 2 has never been ACK'd; he knows because you send duplicate ACKs for 1. Upon the proper timeout, the sender xmits 2,3,4,5 again.

Then Selective Acknowledgement was proposed as a way to make this more efficient. In the same example, you ACK 1, and SACK segments 3 through 5 along with it (if you use a sniffer, you'll see something like "ACK:1, SACK:3-5" for the ACK packets from you). This way, the sender knows it only has to retransmit TCP segment 2... so life is better. Also, note that the SACK defined the edges of the contiguous data you have received; however, multiple non-contiguous data segments can be SACK'd at the same time.

Negative Acknowledgement is the mechanism of telling the sender only about missing data. If you don't tell them something is missing, they keep sending the data 'till you cry uncle.

HTH, \m