Option-Kind

Option-Length

Option-Data

Description

0

—

—

End Of Option List: A single byte option that marks the end of all options included in this segment. This only needs to be included when the end of the options doesn't coincide with the end of the TCP header.

1

—

—

No-Operation: A “spacer” that can be included between options to align a subsequent option on a 32-bit boundary if needed.

2

4

Maximum Segment Size Value

Maximum Segment Size: Conveys the size of the largest segment the sender of the segment wishes to receive. Used only in connection request (SYN) messages.

3

3

Window Size Shift Bits

Window Scale: Implements the optional window scale feature, which allows devices to specify much larger window sizes than would be possible with the normal Window field. The value in Option-Data specifies the power of two that the Window field should be multiplied by to get the true window size the sender of the option is using. For example, if the value of Option-Data is 3, this means values in the Window field should be multiplied by 8, assuming both devices agree to use this feature. This allows very large windows to be advertised when needed on high-performance links. See the topic on data transfer for more.

4

2

—

Selective Acknowledgment Permitted: Specifies that this device supports the selective acknowledgment (SACK) feature. This was implemented as a two-byte option with no Option-Data field, instead of a single-byte option like End Of Option List or No-Operation. This was necessary because it was defined after the original TCP specification, so an explicit option length had to be indicated for backwards compatibility.

5

Variable

Blocks Of Data Selectively Acknowledged

Selective Acknowledgment: Allows devices supporting the optional selective acknowledgment feature to specify non-contiguous blocks of data that have been received so they are not retransmitted if intervening segments do not show up and need to be retransmitted.

14

3

Alternate Checksum Algorithm

Alternate Checksum Request: Lets a device request that a checksum generation algorithm other than the standard TCP algorithm be used for this connection. Both devices must agree to the algorithm for it to be used.

15

Variable

Alternate Checksum

Alternate Checksum: If the checksum value needed to implement an alternate checksum is too large to fit in the standard 16-bit Checksum field, it is placed in this option.