options (TCP)
The option field is a variable-length data field in the IP header that can be used to define service options. With the help of this field, the IP services can be adapted to the respective requirements of the higher protocols.
Therefore, the length of the option field depends on the type and number of options selected. Basically, there are two standard formats for the option field: the 8-bit long option field and the variable length option field.
The 8 bits of the option field define the option type. The first bit of this group is the copy flag( CF), which indicates whether the option was copied into each fragment during the fragmentation process. The following three bits indicate the option classes( CLASS), for example whether a check takes place, a debugging or whether the options are reserved. This is followed by four bits for the option type. These can be: no operation statement, security, quality of service, user group, listing of all routers traversed to the receiver, Internet timestamp, stream ID, route recording, etc.
An important option is used, for example, when establishing a connection with the specification of the maximum segment size.
The option field, if present, is variable in field length and depends on the option type, option length, and option data. The option field can have a maximum length of 40 bytes and is always padded by padding to a bit number that is a multiple of 32. The value of 40 bytes results from the data field for the header length of maximum 60 bytes and the fixed IP header of 20 bytes.