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TCP Checksum Calculation and the TCP "Pseudo Header"
(Page 2 of 3)
Increasing The Scope of Detected Errors: the TCP Pseudo Header
To this end, a change was made in
how the TCP checksum is computed. This special TCP checksum algorithm
was eventually also adopted
for use by the User Datagram Protocol (UDP).
Instead of computing the checksum
over only the actual data fields of the TCP segment, a 12-byte TCP pseudo
header is created prior to checksum calculation. This header contains
important information taken from fields in both the TCP header and the
IP
datagram into which the TCP segment will
be encapsulated. The TCP pseudo header has the format shown in Table 158
and Figure 217.
Table 158: TCP Pseudo Header For Checksum Calculation
Field
Name
|
Size (bytes)
|
Description
|
Source
Address
|
4
|
Source Address:
The 32-bit IP address of the originator of the datagram, taken from
the IP header.
|
Destination
Address
|
4
|
Destination
Address: The 32-bit IP address of the intended recipient of
the datagram, also from the IP header.
|
Reserved
|
1
|
Reserved: 8 bits
of zeroes.
|
Protocol
|
1
|
Protocol:
The Protocol field from the IP header. This indicates what higher-layer
protocol is carried in the IP datagram. Of course, we already know what
this protocol is, it's TCP! So, this field will normally have the value
6.
|
TCP Length
|
2
|
TCP Length: The
length of the TCP segment, including both header and data. Note that
this is not a specific field in the TCP header; it is computed.
|
Figure 217: TCP Pseudo Header For Checksum Calculation
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Once this 96-bit header
has been formed, it is placed in a buffer, following which the TCP segment
itself is placed. Then, the checksum is computed over the entire set
of data (pseudo header plus TCP segment). The value of the checksum
is placed into the Checksum field of the TCP header, and the
pseudo header is discardedit is not an actual part
of the TCP segment and is not transmitted. This process is illustrated
in Figure 218.
Note: The Checksum field is itself part of the TCP header and thus one of the fields over which the checksum is calculated, creating a chicken and egg situation of sorts. This field is assumed to be all zeroes during calculation of the checksum. |
Figure 218: TCP Header Checksum Calculation To calculate the TCP segment headers Checksum field, the TCP pseudo header is first constructed and placed, logically, before the TCP segment. The checksum is then calculated over both the pseudo header and the TCP segment. The pseudo header is then discarded.
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When the TCP segment arrives
at its destination, the receiving TCP software performs the same calculation.
It forms the pseudo header, prepends it to the actual TCP segment, and
then performs the checksum (setting the Checksum field to zero
for the calculation as before). If there is a mismatch between its calculation
and the value the source device put in the Checksum field, this
indicates that an error of some sort occurred and the segment is normally
discarded.
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Version 3.0 - Version Date: September 20, 2005
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