Connected: An Internet Encyclopedia
5.3.2 Type of Service (TOS)
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5.3.2 Type of Service (TOS)
5.3.2 Type of Service (TOS)
The Type-of-Service byte in the IP header is divided into three
sections: the Precedence field (high-order 3 bits), a field that
is customarily called Type of Service or "TOS (next 4 bits), and a
reserved bit (the low order bit). Rules governing the reserved
bit were described in Section [4.2.2.3]. The Precedence field
will be discussed in Section [5.3.3]. A more extensive discussion
of the TOS field and its use can be found in [ROUTE:11].
A router SHOULD consider the TOS field in a packet's IP header
when deciding how to forward it. The remainder of this section
describes the rules that apply to routers that conform to this
requirement.
A router MUST maintain a TOS value for each route in its routing
table. Routes learned through a routing protocol that does not
support TOS MUST be assigned a TOS of zero (the default TOS).
To choose a route to a destination, a router MUST use an algorithm
equivalent to the following:
- The router locates in its routing table all available routes
to the destination (see Section [5.2.4]).
- If there are none, the router drops the packet because the
destination is unreachable. See section [5.2.4].
- If one or more of those routes have a TOS that exactly matches
the TOS specified in the packet, the router chooses the route
with the best metric.
- Otherwise, the router repeats the above step, except looking
at routes whose TOS is zero.
- If no route was chosen above, the router drops the packet
because the destination is unreachable. The router returns
an ICMP Destination Unreachable error specifying the
appropriate code: either Network Unreachable with Type of
Service (code 11) or Host Unreachable with Type of Service
(code 12).
- DISCUSSION
-
Although TOS has been little used in the past, its use by hosts is
now mandated by the Requirements for Internet Hosts RFCs
([INTRO:2] and [INTRO:3]). Support for TOS in routers may become
a MUST in the future, but is a SHOULD for now until we get more
experience with it and can better judge both its benefits and its
costs.
Various people have proposed that TOS should affect other aspects
of the forwarding function. For example:
- A router could place packets that have the Low Delay bit set
ahead of other packets in its output queues.
- a router is forced to discard packets, it could try to avoid
discarding those which have the High Reliability bit set.
These ideas have been explored in more detail in [INTERNET:17] but
we don't yet have enough experience with such schemes to make
requirements in this area.
Next: 5.3.3 IP Precedence
Connected: An Internet Encyclopedia
5.3.2 Type of Service (TOS)