that the sender of this segment is currently willing to receive. (See and .) Checksum (16 bits)The 16-bit checksum field is used for error-checking of the TCP header, the payload and an IP pseudo-header. The pseudo-header consists of the source IP address, the destination IP address, the protocol number for the TCP protocol (6) and the length of the TCP headers and payload (in bytes). Urgent pointer (16 bits)If the URG flag is set, then this 16-bit field is an offset from the sequence number indicating the last urgent data byte. Options (Variable 0–320 bits, in units of 32 bits)The length of this field is determined by the data offset field. Options have up to three fields: Option-Kind (1 byte), Option-Length (1 byte), Option-Data (variable). The Option-Kind field indicates the type of option and is the only field that is not optional. Depending on Option-Kind value, the next two fields may be set. Option-Length indicates the total length of the option, and Option-Data contains data associated with the option, if applicable. For example, an Option-Kind byte of 1 indicates that this is a no operation option used only for padding, and does not have an Option-Length or Option-Data fields following it. An Option-Kind byte of 0 marks the end of options, and is also only one byte. An Option-Kind byte of 2 is used to indicate Maximum Segment Size option, and will be followed by an Option-Length byte specifying the length of the MSS field. Option-Length is the total length of the given options field, including Option-Kind and Option-Length fields. So while the MSS value is typically expressed in two bytes, Option-Length will be 4. As an example, an MSS option field with a value of 0x05B4 is coded as (0x02 0x04 0x05B4) in the TCP options section. Some options may only be sent when SYN is set; they are indicated below as [SYN]. Option-Kind and standard lengths given as (Option-Kind, Option-Length). {| class="wikitable" |- ! Option-Kind ! Option-Length ! Option-Data ! Purpose ! Notes |- |0 | | |End of options list | |- |1 | | |No operation |This may be used to align option fields on 32-bit boundaries for better performance. |- |2 |4 |SS |Maximum segment size |See [SYN] |- |3 |3 |S |Window scale |See for details [SYN] |- |4 |2 | |Selective Acknowledgement permitted |See for details[SYN] |- |5 |N (10, 18, 26, or