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The IP protocol was originally designed for providing best-effort
services. Traffic is processed as quickly as possible but without
any guarantee of timeliness of actual delivery. There are no mechanisms
to differentiate some traffic in order to give it preferential treatment.
This is not optimal since different applications have varying requirements
for network charateristics such as bandwidth, packet loss, delay,
and delay variation (jitter). Voice-over-IP requires e.g. a small
but guaranteed bandwidth, low delay and low jitter. Other applications,
such as file transfers, require more bandwidth but are insentitive
to delay and jitter.
Diffserv is a simple method of providing differentiated classes
of service, also known as Quality of Service, for Internet traffic.
A small bit-pattern in each IP packet is used to mark a packet to
receive a particular forwarding treatment, or per-hop behavior (PHB),
at each network node. At the network border, service classes are
identified and packets are marked as belonging to a particular service.
Within the network, routers examine IP headers to determine which
forwarding treatment is appropriate for each packet.
A simple example of a PHB is to guarantee 30% of the bandwith on
a link to a particular traffic class. Per-Hop Behaviours are implemented
via scheduling and buffer management. In addition to the default
PHB, which is the normal best-effort behaviour, the IETF has standardized
two PHBs:
The Assured
Forwarding (AF) PHB divides traffic into four classes where
each AF class is guaranteed some minimum resources (capacity
and buffering). Within each class, packets are further partitioned
into one of three drop preference categories. Congested routers
then drop/mark based on their preference values The level of
forwarding assurance of an IP packet depends on the amount of
resources reserved for its AF class, the current load of its
AF class, and the drop precedence of the packet.
The Expedited Forwarding (EF) PHB defines a service with low
packet loss, low latency, low jitter and guaranteed bandwidth.
The idea is to always keep the total EF traffic passing through
any link in the network under a limit, which is set to be smaller
than the link bandwidth. A simple priority queue is then used
to schedule EF packets before packets from the other service
classes. Since the receiving rate of EF traffic is always smaller
than the sending rate at every router, the EF traffic is guaranteed
for minimized delay and assured bandwidth. |
Interpeak has implemented Diffserv as a built-in feature of the
IPNET dual-mode IPv4/IPv6 stack. It is available for a range of
operating systems including INTEGRITY, OSE, VxWorks, and Linux,
and supports the following IETF standards:
- RFC 2474 - Definition of the Differentiated Services Field (DS
Field) in IPv4 and IPv6 Headers.
- RFC 2475 - An Architecture for Differentiated Services
- RFC 2597 - Assured Forwarding PHB Group
- RFC 2598 - Expedited Forwarding PHB Group
- RFC 2697 - A Two Rate Three Color Marker
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