21.11 Repacketization

When TCP times out and retransmits, it does not have to retransmit the identical segment again. Instead, TCP is allowed to perform repacketization, sending a bigger segment, which can increase performance. (Naturally, this bigger segment cannot exceed the MSS announced by the other receiver.) This is allowed in the protocol because TCP identifies the data being sent and acknowledged by its byte number, not its segment number.

We can easily see this in action. We use our sock program to connect to the discard server and type one line. We then disconnect the Ethernet cable and type a second line. While this second line is being retransmitted, we type a third line. We expect the next retransmission to contain both the second and third lines.

 bsdi %  sock svr4 discard   hello there   first line gets sent OK   then we disconnect the Ethernet cable   line number 2   this line gets retransmitted   and 3   type this line before second line sent OK   then reconnect Ethernet cable

Figure 21.13 shows the tcpdump output. (We have removed the connection establishment, the connection termination, and all the window advertisements.)

Figure 21.13. Repacketization of data by TCP.

Lines 1 and 2 show the first line ("hello there") being sent and its acknowledgment. We then disconnect the Ethernet cable and type "line number 2" (14 bytes, including the newline). These bytes are transmitted on line 3, and then retransmitted on lines 4 and 5.

Before the retransmission on line 6 we type "and 3" (6 bytes, including the newline) and see this retransmission contain 20 bytes: both lines that we typed. When the acknowledgment arrives on line 9, it is for all 20 bytes.