May 2006
Intermediate to advanced
480 pages
12h 20m
English
Content preview from Understanding TCP/IPBecome an O’Reilly member and get unlimited access to this title plus top books and audiobooks from O’Reilly and nearly 200 top publishers, thousands of courses curated by job role, 150+ live events each month,
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9.2 TCP Header Options
Mandatory TCP header items take up 20 B. These mandatory items are followed by optional items. An optional item is made up of the optional item type, optional item length, and value. The length of the TCP segment must be a multiple of four. If the header length is not a multiple of four, it is padded with NOP (no operation) options.
Since the header length field of the whole TCP segment is only four bits long, this field can only contain a maximum value of 11112=1510. The header length is determined in multiples of four, so the header can have a maximum length of 15x4=60 bytes. Mandatory items take up 20 bytes, so at the most only 40 bytes are left for optional items.
Figure 9.8 shows some TCP header options and their structure. ...
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