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TCP Message (Segment) Format
(Page 4 of 4)
TCP Options and Option Field Values
Table 157
shows the main options currently defined for TCP.
Table 157: TCP Options
Option-Kind
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Option-Length
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Option-Data
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Description
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0
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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.
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1
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No-Operation:
A spacer that can be included between options to align a
subsequent option on a 32-bit boundary if needed.
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2
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4
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Maximum Segment
Size Value
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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.
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3
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3
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Window
Size Shift Bits
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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.
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4
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2
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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.
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5
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Variable
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Blocks
Of Data Selectively Acknowledged
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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.
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14
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3
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Alternate
Checksum Algorithm
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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.
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15
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Variable
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Alternate
Checksum
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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.
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I have not shown every
TCP option in Table 157,
just the basic ones defined in RFC 793 and a few others that are interesting
and correspond to features described elsewhere in the Guide. Note that
most options are sent only in connection request (SYN) segments.
This includes the Maximum Segment Size, Window Scale,
Selective Acknowledgement Permitted and Alternate Checksum
Request options above. In contrast, Selective Acknowledgment
and Alternate Checksum options appear in regular data segments,
when used.
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Version 3.0 - Version Date: September 20, 2005
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