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IP Datagram General Format
(Page 3 of 3)
Type Of Service (TOS) Field
This one-byte field was originally
intended to provide certain quality
of service features for IP datagram delivery.
It allowed IP datagrams to be tagged with information indicating not
only their precedence, but the preferred manner in which they should
be delivered. It was divided into a number of subfields, as shown in
Table 57
(and Figure 86).
The lack of quality of service features
has been considered a weakness of IP for a long time. But as we can
see in Table 57,
these features were built into IP from the start. What's going on here?
The answer is that even though this field was defined in the standard
back in the early 1980s, it was not widely used by hardware and software.
For years, it was just passed around with all zeroes in the bits and
mostly ignored.
Table 57: Original Definition Of IPv4 Type Of Service (TOS) Field
Subfield
Name
|
Size (bytes)
|
Description
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Precedence
|
3/8
(3 bits)
|
|
D
|
1/8
(1 bit)
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Delay:
Set to 0 to request normal delay in delivery; set to 1 if
low delay delivery is requested.
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T
|
1/8
(1 bit)
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Throughput: Set
to 0 to request normal delivery throughput; set to 1 if
higher throughput delivery is requested.
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R
|
1/8
(1 bit)
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Reliability:
Set to 0 to request normal reliability in delivery; set
to 1 if higher reliability delivery is requested.
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Reserved
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2/8
(2 bits)
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Reserved: Not used.
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The IETF,
seeing the field unused, attempted to revive its use. In 1998, RFC 2474
redefines the first six bits of the TOS field to support a technique
called Differentiated Services (DS). Under DS, the values in
the TOS field are called codepoints and are associated
with different service levels. This starts to get rather complicated,
so refer to RFC 2474 if you want all the details.
Understanding the IP datagram format
is an important part of troubleshooting IP networks. Be sure to see
the following topic on options for more information on how IP options
are used in datagrams, and the topic
on fragmenting for some more context on
the use of fragmentation-related fields such as Identification,
Fragment Offset, and More Fragments.
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Version 3.0 - Version Date: September 20, 2005
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