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... motivated the design choices in this specification. Sections 3 and 4
describe the Type of Service octet in the IP header and the values
which the TOS field of that octet may contain. Section 5 describes
...
... Type of Service octet
in the IP datagram header. The Type of Service octet consists of
three fields:
...
... quality of service as close as
possible to the one requested in the TOS field in the IP header.
Long ago an attempt (RFC-795) was made to codify how this might be
...
...
At first glance goals (3) and (4) seem to be pretty much mutually
exclusive. The IP header currently has only three unused bits, so
at most three new type of service ...
... type of service bits could be defined without
resorting to the impractical step of changing the IP header
format. Since one of them would need to be allocated to meet goal
...
... meet goal (3) and goal (4) without having to use up all of the
remaining reserved bits in the IP header. The obvious way to do
that was to change the interpretation of TOS values so that they
...
... Requirements, but would reduce to one the
number of reserved bits in the IP header. Expanding the TOS field
to four bits ...
... bits would restrict future expansion to more modest levels
(11 new TOS values), but would leave an additional IP header bit
free. The IETF ...