18.10 TCP Options

The TCP header can contain options (Figure 17.2). The only options defined in the original TCP specification are the end of option list, no operation, and the maximum segment size option. We have seen the MSS option in almost every SYN segment in our examples.

Newer RFCs, specifically RFC 1323 [Jacobson, Braden, and Borman 1992], define additional TCP options, most of which are found only in the latest implementations . (We describe these new options in Chapter 24.) Figure 18.20 shows the format of the current TCP options ” those from RFC 793 and RFC 1323.

Figure 18.20. TCP options.

Every option begins with a 1-byte kind that specifies the type of option. The options with a kind of 0 and 1 occupy a single byte. The other options have a len byte that follows the kind byte. The length is the total length, including the kind and len bytes.

The reason for the no operation (NOP) option is to allow the sender to pad fields to a multiple of 4 bytes. If we initiate a TCP connection from a 4.4BSD system, the following TCP options are output by tcpdump on the initial SYN segment:

 <mss 512,nop,wscale 0,nop,nop,timestamp 146647 0>

The MSS option is set to 512, followed by a NOP, followed by the window scale option. The reason for the first NOP is to pad the 3-byte window scale option to a 4-byte boundary. Similarly, the 10-byte timestamp option is preceded by two NOPs, to occupy 12 bytes, placing the two 4-byte timestamps onto 4-byte boundaries.

Four other options have been proposed, with kinds of 4, 5, 6, and 7 called the selective-ACK and echo options. We don't show them in Figure 18.20 because the echo options have been replaced with the timestamp option, and selective ACKs, as currently defined, are still under discussion and were not included in RFC 1323. Also, the T/TCP proposal for TCP transactions (Section 24.7) specifies three options with kinds of 11,12, and 13.