18.10 TCP Options
Options may include a TCP header portion (FIG 17--2). Option is only defined in the original TCP specification is the end of the option list, no operation and maximum segment length. In our case, almost every SYN packet segment we have encountered MSS option.
The new RFC, mainly RFC 1323 [Jacobson, Braden and Borman 1992], defines a new TCP option, only the most recent implementations of TCP in order to see these options (we will introduce in the new Chapter 2 4 options). FIG 18--20 shows the current format of the TCP options, these options by the definition in RFC 793 and RFC 1323.
The start of each option is the 1-byte kind field, indicating the type of option. options with a kind 0 and 1 only one byte. Other options in the kind byte after byte len there. It is noted that the length of the total length, including the kind and len bytes.
Options provided no reason for padding fields allow the sender of a multiple of 4 bytes. If we use the system 4 4 BSD TCP connection is initialized, the tcpdump displayed on the initial SYN TCP following options: <mss 512, nop, wscale 0, nop, nop, timestamp 146647 0>the MSS option to 512, followed by the NOP, followed by the window scale option. The first option will be extended for a NOP padded to 4-byte boundary window. Similarly, 10-byte timestamp option in two after the NOP, accounting for 12 bytes, while the 4-byte timestamp satisfies two 4-byte boundary.
Four options other kind is called 4,5,6 and 7 of the ACK and echo selection options. Since the echo option has been substituted with the timestamp option, the option is selected ACK currently defined are still under discussion, not included in the RFC 1323, and therefore FIG 18--20 thereof is not listed. Further, as a TCP transaction (2 4. Section 7) of the T / TCP kind specified is also recommended that 11, 12 and 13 of the three options.
