9. NETWORK FEATURES

Quality of Service

In the fields of packet-switched networks and computer networking, the traffic engineering term Quality of Service (QoS) refers to the probability of the network meeting a given traffic contract, or in many cases is used informally to refer the probability of a packet passing between two points in the network. While most of the issues involving QoS relate to wide area networking, with todays high levels of traffic running over Local Area Networks QoS is becoming more of an issue within the LAN, and is often supported by Ethernet switches.

There is a school of thought which says why bother with a QOS mechanism. If the network is that busy that it needs to start dropping packets then its better to add more bandwidth, because eventually the priority packets will be discarded due to too much traffic.

Why do we require a Quality of Service?

When the Internet was first being created, there was no perceived need for a QoS application. In fact the entire internet ran on a “best effort” system. There were 4 “type of service” bits and three “precedence” bits provided in each message, but they were largely unused. There are many things that can happen to packets as they travel from origin to destination and they result in the following problems, as seen from the point of view of the sender and receiver:

• dropped packets – the routers might fail to deliver (drop) some packets if they arrive when their buffers are already full. Some, none, or all of the packets might be dropped, depending on the state of the network, and it is impossible to determine what happened in advance. The receiving application must ask for this information to be retransmitted, possibly causing severe delays in the overall transmission.
• delay – it might take a long time for a packet to reach its destination, because it gets held up in long queues, or takes a more indirect route to avoid congestion. Alternatively, it might follow a fast, direct route. The delay is very unpredictable.
• out-of-order delivery – when a collection of related packets are routed through the internet, different packets may take different routes, each resulting in a different delay. The result is that the packets arrive in a different order than the one with which they were sent. This problem necessitates special additional protocols responsible for rearranging out-of-order packets once they reach their destination.
• error – sometimes packets are misdirected, or combined together, or corrupted, while en route. The receiver has to detect this and, just as if the packet was dropped, ask the sender to repeat itself.

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Applications requiring QoS

A Quality of Service may be required for certain types of network traffic, for example:

• streaming multimedia may require guaranteed throughput
• IP telephony may require strict limits on jitter and delay
• dedicated link emulation requires both guaranteed throughput and imposes limits on maximum delay
• a safety-critical application, such as remote surgery may require a guaranteed level of availability (this is also called hard QoS).

These types of service are called inelastic, meaning that they require a certain level of bandwidth to function – if they get more than that they can’t use it, and if they get less, then they can’t function at all. By contrast, elastic applications can take advantage of however much or little bandwidth is available.

Obtaining QoS

There are essentially two ways to provide QoS guarantees. The first is simply to provide lots of resources, enough to meet the expected peak demand with a substantial safety margin. This is nice and simple, but some people believe it to be expensive in practice, and can’t cope if the peak demand increases faster than predicted: deploying the extra resources takes time.

The second one is to require the network to make reservations for certain traffic types.

Types of QOS

IntServ

In computer networking IntServ or integrated services is a system that attempts to guarantee quality of service (QoS) on networks. In other words, IntServ is designed to allow video and sound to reach the user without interruption.

It is a fine-grained system which is often contrasted with DiffServ’s coarse-grained system.

The idea of IntServ is that every router in the system implements IntServ, and every application that requires some kind of guarantee has to make an individual reservation. “Flow Specs” describe what the reservation is for, while “RSVP” is the underlying mechanism for making them.

Flow Specs

There are two parts to a flow spec:

• What does the traffic look like? Done in the Traffic SPECification or TSPEC part.
• What guarantees does it need? Done in the service Request SPECification or RSPEC part.

RSVP

The Resource ReSerVation Protocol (RSVP) is described in RFC 2205. All machines on the network capable of sending QoS data send a PATH message every 30 seconds, which spreads out through the network. Those who want to listen to them send a corresponding RESV (short for “Reserve”) message which then traces the path backwards to the sender. The RESV message contains the flow specs.

The routers between the sender and listener have to decide if they can support the reservation being requested, and if they cannot then send a reject message to let the listener know about it. Otherwise, once they accept the reservation they have to carry the traffic.

The routers then store the nature of the flow, and also police it. This is all done in soft state, so if nothing is heard for a certain length of time, then the reader will time out and the reservation will be cancelled. This solves the problem if either the sender or the receiver crash or are shut down incorrectly without first cancelling the reservation. The individual routers may, at their option, police the traffic to check that it conforms to the flow specs.

In summary, RSVP has the following attributes:

• RSVP makes resource reservations for both unicast and many-to-many multicast applications, adapting dynamically to changing group membership as well as to changing routes.
• RSVP is simplex, i.e., it makes reservations for unidirectional data flows.
• RSVP is receiver-oriented, i.e., the receiver of a data flow initiates and maintains the resource reservation used for that flow.
• RSVP maintains “soft” state in routers and hosts, providing graceful support for dynamic membership changes and automatic adaptation to routing changes.
• RSVP is not a routing protocol but depends upon present and future routing protocols.
• RSVP transports and maintains traffic control and policy control parameters that are opaque to RSVP.

Problems

The problem with IntServ is that many states must be stored in each router. As a result, IntServ works on a small-scale, but as you scale up to a system the size of the Internet, it is difficult to keep track of all of the reservations. As a result, IntServ is not very popular.

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DiffServe

DiffServ or differentiated services is a method of trying to guarantee quality of service on large networks such as the Internet, but it is increasingly being used within the LAN on higher end Ethernet switches.

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”. 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.

Example

There are many ways to split up traffic into classes. For example, the traffic may be split into first, second, and third classes. In each router, First class traffic takes precedence over second class traffic, which takes precedence over third class.

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.

• Usually it is done by the router which connects a local area 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

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 first class packet may be another’s third class packet. 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.

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MultiLayer Network Equipment

Network equipment, that supports DiffServ and perhaps IntServ, are called multilayer network equipment. A switch that supports DiffServ and perhaps IntServ is called a multilayer switch.

However, the market has not yet favoured QoS services. Some people believe that this is because a “dumb” network that offers sufficient bandwidth for most applications, most of the time, is already economically stable, with little incentive to deploy non-standard stateful QoS-based applications.

Internet peering arrangements are already complex, and there appears to be no enthusiasm among providers for supporting QoS across peering connections, or agreement about what policies should be supported in order to do so.

QoS sceptics further point out that if you are dropping many packets on elastic low-QoS connections, you are already dangerously close to the point of congestion collapse on your inelastic high-QoS applications, without any way of further dropping traffic without violating traffic contracts.

MPLS (Multiprotocol Label Switching)

Multiprotocol Label Switching (MPLS) is a data-carrying mechanism, operating at a layer below protocols such as IP. It was designed to provide a unified data-carrying service for both circuit-based clients and packet-switching clients which provide a datagram service model. It can be used to carry many different kinds of traffic, including both voice telephone traffic and IP packets.

With MPLS the edge routers assign a lable to the packet which defines its path through the network, in much the same way Frame Relay assigns a DLCI. This allows the various routers in the core of the network to pass the packets through without the need to refer to a routing table, thus eliminating the delays associated with making routing decisions at each stage.

Comparison of MPLS versus IP

Unlike IP, MPLS does not define a directly usable end-point protocol. It only defines a way of encapsulating other layer 2 and layer 3 protocols. In this regard, it is similar to a protocol like PPP. Also unlike IP, MPLS explicitly decouples routing from forwarding, although it can fall back to using IP-style routing if necessary.

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