Type of Service

The TOS byte in the IPv4 header has had various purposes over the years, and has been defined in different ways by five different RFCs. (RFC 791, RFC 1122, RFC 1349, RFC 2474, and RFC 3168.) The modern definition of the TOS byte is a six-bit Differentiated Services Code Point and a two-bit Explicit Congestion Notification field. For a full history of the TOS byte, see section 22 of RFC 3168.

Type of Service

8 bits in the IP header are reserved for the service type. They can be divided into 5 subfields:

The 3 precedence bits have a value from 0 to 2 and are used to indicate the importance of a datagram. Default is 0 (higher is better). Bits 3 4 5 represent the following:

* D: requests low delay
* T: requests high throughput
* R: requests high reliability

Support for ToS in routers may become a ‘MUST’ in the future, but for now it’s only a ‘SHOULD’. A router maintains a ToS value for each route in its routing table. Routes learned through a protocol that does not support ToS are assigned a ToS of zero. Routers use the ToS to choose a destination for the packet.

# The router locates in its routing table all available routes to the destination.
# If there are none, the router drops the packet because the destination is unreachable.
# 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).
#if no route was chosen above ,the router drops the packet.

References

* RFC 1349, Type of Service in the Internet Protocol Suite
* "Deploying IP and MPLS QoS for Multiservice Networks: Theory and Practice" by John Evans, Clarence Filsfils (Morgan Kaufmann, 2007, ISBN 0-370561-370548-5)

See also

* Class of Service
* DiffServ
* Quality of service