This chapter describes how to use the Differentiated Services (DiffServ) feature so that a router can provide preferred service to appropriate IP data packets. Based on information in the IP header, the router classifies packets by matching them with predefined configurations in the policy database (created with the policy feature). See "Using the Policy Feature" for details. As a result, some packets may receive preferred service. This chapter consists of the following sections:
Most forwarding devices installed in an IP network today deliver standard best-effort service to data packets on a first-come, first-served basis. This delivery method is adequate for most traffic, but new applications are emerging that require faster and earlier transmission of certain packets.
The Differential Services (DiffServ) feature provides different levels of service to IP packets when a router processes them for transmission. DiffServ provides some packets with preferred service by reserving system resources (buffers) and link resources (bandwidth) for them. A DiffServ classifier function determines the type of service given to IP packets by examining various fields in the IP header, for example, ranges of IP source and destination addresses and port numbers, protocol type, and incoming TOS byte. To accomplish this in a scalable way, individual flows are aggregated into streams. Streams are the entities through which DiffServ manages access to buffers and bandwidth. Figure 38 shows how DiffServ processes the packets of a stream.
Figure 38. DiffServ Data Packet Path
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In addition to the traditional best-effort service, DiffServ provides the following types of service:
Local routers create and send control packets, so you must also leave enough resources free so that they receive adequate service.
DiffServ is currently implemented on PPP and Frame Relay links, and can be used by the RSVP subsystem. Figure 38 shows how packets of a stream are processed. When a router receives the first packet of a flow (assuming that it is designated for premium service), no indication of its service category exists in the fast path cache, so the packet is processed by the slow path. DiffServ invokes a search of the policy database to obtain the packet-handling criteria (policy). The policy-defined action is saved in the fast-path cache. When the router receives a subsequent packet of this flow, it finds that an entry in the fast-path cache for the flow already exists, so its policy-defined action is applied and the packet takes the fast path. Thus, subsequent packets from this flow receive premium service.
Figure 39 shows the relationship between the policer, buffer management, the queues, and the scheduler--some of the basic components that provide different quality of service levels.
Figure 39. Relationship of Buffers, Queues, and Scheduler
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The expedited forwarding (EF) and assured forwarding (AF) services have different characteristics, which are supported by three functions in the router: (1) The policer, (2) buffer management, and (3) the scheduler. These functions provide more sophisticated traffic control than is available in a traditional BE router device.
If traffic requires EF processing, any excess traffic must be dropped. A DiffServ policer function uses a token bucket to examine EF traffic and to determine whether a packet is in excess. If it is, the policer drops the packet.
If traffic is for AF, BE, or is EF traffic that the policer has allowed, the rate-based buffer management function processes it. This function allocates buffers from either a private pool for the interface, or from a common shared pool for all the interfaces. Use the Talk 6 set receive-buffers configuration command (see the set receive-buffers command in Access Integration Services Software User's Guide) to specify the total amount of physical buffer space available to an interface. Use the DiffServ Talk 6 set interface command to set the egress buffer size for the premium and assured queues. This is the buffer space that DiffServ manages. (DiffServ manages two separate pools--one for the premium (EF) queue and one for the assured forwarding (AF) queue. Ensure that the buffer space you specify reflects the actual amount of buffer space available in the system.) Buffer management determines whether buffers from its interface's private pool are available for the packet. If there are, it accepts and enqueues the packet. If they are not, it attempts to allocate buffer space from the shared pool and if it can, it enqueues the packet. If no shared buffer space is available, buffer management drops the packet.
The scheduler function examines the queues on a regular basis, dequeues enqueued packets, and sends them to the interface adapter for transmission. It is a self-clocked fair-queuing scheduler, which is a variation of weighted fair queuing. You may configure the scheduler weights and specify the frequency at which the scheduler examines the queues.
| Note: | It is possible to configure DiffServ options such that the resources are overcommitted or overbooked, that is, the traffic conditioner controls are configured as though there were more bandwidth or buffering than is actually available. DiffServ does not support overbooking. |
If a DiffServ stream becomes idle (no packets have been sent on the stream for some time), the system reclaims the resources so other streams can use them. If the stream reactivates, the resources are returned to it. If the resources are no longer available because of overbooking, then DiffServ attempts periodically to reallocate the resources.
After you have used the policy feature to configure appropriate policies, the first step in implementing DiffServ is to use the DiffServ set command to configure the options that define the system resources available to DiffServ. Then use the enable ds command to enable the DiffServ feature, and the enable interface command to enable the egress interface.
The following terms are used when discussing DiffServ:
The following procedures provide a high-level description of how to configure DiffServ to provide preferred service for selected packets. First, access the DiffServ feature:
* talk 6 Config>feature ds DS config>
DS config>enable ds DiffServ enabled
DS config>set interface
Enter Diffserv Interface number [0]? 2
Set Premium Queue Bandwidth (%) (1 - 99) [20]?
Assured Queue Bandwidth (%) = 80
Configure Advanced setting (y/n)? [No]: no
Accept input (y/n)? [Yes]:
| Note: | If you specify no to the Configure Advanced setting prompt, then default parameters for Premium Queue and Assured/BE queue will be used. |
Configure Advanced setting (y/n)? [No]: yes
Set Premium Queue Weight (%) (20 - 99) [90]?
Assured Queue Weight (%) = 10
EGRESS BufSize for Premium Queue (in bytes) (550 - 16500) [5500]?
Max EGRESS QoS Allocation for Premium Queue (%) (1 - 99) [95]?
EGRESS BufSize for Assured/BE Queue (in bytes) (5500 - 140800) [27500]?
Max EGRESS QoS Allocation for Assured/BE Queue (%) (1 - 99) [80]?
In this example, 20 percent of line bandwidth, and 90 percent of scheduler weight are given to the EF queue. The egress buffer size for the EF queue is 5500 (in bytes), out of which 95 percent is allocatable to QOS streams. The egress buffer size for the AF/BE queue is 27500 (in bytes), out of which 80 percent is allocatable to QOS streams.
After enabling DiffServ and setting interface parameters, you must restart or reload the device to activate DiffServ. For more details on specifying DiffServ commands, see "Configuring and Monitoring the Differentiated Services Feature".