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Differentiated services

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DiffServ or differentiated services is a method of trying to guarantee quality of service on large networks such as the Internet.

DiffServ deals with bulk flows of data rather than single flows and single reservations. This means that a single negotiation will be made for all of the packets from, for example, a single ISP, or a single university. The contracts resulting from these negotiations are called "service level agreements", and will inevitably involve money changing hands. These service level agreements will specify what classes of traffic will be provided, what guarantees are needed for each class, and how much data will be sent for each class.

A "DiffServ cloud" is a collection of DiffServ routers. When packets enter a DiffServ cloud they are first classified by the sender. The sender sets the "type of service" field (which hence is also called DiffServ Code Point - DSCP), in the IP header according to the class of the data, so that the better classes get higher numbers.

As the packets enter the DiffServ cloud they are policed by the receiver. If there is so much traffic that it breaches the service level agreement, then the sender may be liable for fines, according to the details of the contract. Within the DiffServ cloud, all the individual routers need to do is to give highest priority to the packets with the highest value in the type of service field, which is simple to implement. There may also be a discard policy on the frequencies with which each type of packet is discarded if the router runs out of buffer space.

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There are many ways to split up traffic into classes. For example, the traffic may be split into Gold, Silver, and Bronze classes. In each router, Gold traffic takes precedence over Silver traffic, which takes precedence over Bronze.

Special handling may be done in at least two different ways:

  • preferential forwarding, where more recent higher precedence packets are allowed to jump the queue over old lower precedence packets
  • preferential discarding, where buffer space for higher-preference packets is allowed to grow at the expense of lower precedence packets which are discarded

There are also many other schemes involving hybrids of these and other Quality of Service strategies.

Example: Prioritizing specific data on communications networks.

  • Usually it is done by the router which connects a local network to the Internet. The router then decides for example, to put interactive traffic like remote shells[?] or online games to maximum priority in order to reduce ping time. Other traffic like HTTP or SMTP then get some lower priority while usual downloads like FTP or peer to peer networks are getting the lowest priority.
  • The decision about which traffic should get high priority usually depends on the intended usage of the network connection. Another approach for deciding which traffic is important is the TOS/DiffServ field in the IP header.

Advantages of DiffServ

One advantage of DiffServ, is that all the policing and classifying is done at the boundaries between DiffServ clouds. This means that in the core of the Internet, routers can get on with doing the job of routing, and not care about the complexities of collecting payment or enforcing agreements.

Disadvantages of DiffServ

End-to-end and peering problems

One disadvantage is that the details of how individual routers deal with the type of service field is somewhat arbitrary, and it is difficult to predict end-to-end behaviour. This is complicated further if a packet crosses two or more DiffServ clouds before reaching its destination.

From a commercial viewpoint, this is a major flaw, as it means that it is impossible to sell different classes of end-to-end connectivity to end users, as one provider's Gold packet may be another's Bronze. Internet operators could fix this, by enforcing standardised policies across networks, but are not keen on adding new levels of complexity to their already complex peering agreements. One of the reasons for this is set out below.

DiffServ vs. more capacity

The greatest disadavantage of DiffServ is that at the very highest level, it can be regarded as a technical solution for a technical problem which does not exist.

Since DiffServ is simply a mechanism for deciding which packets to delay or drop at the expense of others in a situation where there is not enough network capacity, consider that when DiffServ is working by dropping packets selectively, traffic on the link in question must already be very close to saturation. Any further increase in traffic will result in Bronze services being taken out altogether. Since Internet traffic is highly bursty, this is almost certain to happen on a regular basis if traffic on a link is near the limit at which DiffServ becomes needed.

For this reason, many people think that DiffServ will always be inferior to adding sufficient network capacity to avoid packet loss on all classes of traffic.

As of 2003, there is a glut of fibre capacity in most parts of the telecoms market, with it being far easier and cheaper to add more capacity than to employ elaborate DiffServ policies as a way of increasing customer satisfaction. In fact, this is what is generally done in the core of the Internet, which is generally fast and dumb with "fat pipes" connecting its routers.

DiffServ as rationing

Hence, DiffServ is for most ISPs only a way of rationing[?] customer network utilisation to allow greater overbooking[?] of their existing capacity. A good example of this is the use of DiffServ tools to suppress peer-to-peer traffic, because of its ability to saturate customer links indefinitely, disrupting the ISP's business model which relies on 1%-10% link utilization for most customers.

See also: IntServ, quality of service, traffic shaping, traffic engineering, Paris Metro pricing[?]

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