ABSTRACT zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA quality - PDF document

ABSTRACT zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA quality of service zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA QoS that they require. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA they receive the end-to-end zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA The growing use of multimedia communication applications with specific bandwidth and real-time delivery requirements has created the need for an integrated services Internet in which traditional best-effort datagram delivery can coexist with additional enhanced (QoS) delivery classes. Such classes provide data flows with QoS commitments with regard t o bandwidth, packet loss, and delay through the reservation of network resources along the data path, which can be done using the Resource Reservation Protocol (RSVP). This article is a tutorial on h o w RSVP can be used by end applications t o ensure that Paul P. White, University College London he current Internet consists of a multitude of networks the routers traversed by a data flow,l can provide the data built from various link-layer technologies and relies on flow with certain QoS commitments. In contrast, best-effort the Internet Protocol (IP) to interwork between them. IP traffic entering a router will receive no such service commit- ment and will have to make do with whatcvcr resources arc makes no assumptions about the underlying protocol stacks available. The level of QoS provided by these enhanced QoS and offers an unreliable, connectionless network-layer service that is subject to packet loss, reordering, and packet duplica- classes is programmable on a per-flow basis according to tion, all of which, together with queuing delay in router requests from the end applications. These requests can be buffers, will increase with network load. Because of the lack passed to the routers by network management procedures or, of any firm guarantees, the traditional IP delivery model is more commonly, using a reservation protocol such as RSVP, oftcn referred to as “best-cffort” with an additional highcr- which is dcscribed in thc third scction. The requcsts dictate layer end-to-end protocol such as the Transmission Control the level of resources (e.g., bandwidth, buffer space) that must Protocol (TCP) required to provide end-to-end reliability. be reserved along with the transmission scheduling behavior TCP does this through the use of such mechanisms as packet that must be installed in the routers to provide the desired retransmission, which further adds to the overall information end-to-end QoS commitment for the data flow. of zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA transfer delay. In determining the resuurce allocalions riccessary to satisfy For traditional non-real-time Internet traffic such as a request, the router needs to take account of the QoS sup- File Transfer Protocol (FTP) data, the best-effort delivery port provided by the link layer in the data forwarding path. model of IP has not been a problem. However, as we move Furthermore, in the case of a QoS-active link layer such as enhanced quality of service zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA asynchronous transfer mode (ATM) or certain types of local further into the age of multimedia communications, many real-time applications are being developed that are delay- area network (LAN), the router is responsible for negotia- sensitive to the point where the best-effort delivery model tions with the link layer to ensure that the link layer installs IP can be inadequate even under modest network loads. appropriate QoS support should the request be accepted. This Although the problem has been alleviated somewhat mapping to link-layer QoS is medium-dependent, and the through making certain applications adaptive to network mechanisms for doing so are currently being defined by the load where possible, there is still a firm need to provide Integrated Services over Specific Lower Layers (issll) Working 1 A data flow identij?es the set ofpackets to receive special &OS. It zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA many applications with additional service classes offering Group of the IETF [4]. In the case of a QoS-passive link layer with regard to band- such as a leased line, the mapping to the link-layer QoS is ( Q o S ) width, packet queuing delay, and loss. These additional trivial since transmission capacity is handled entirely by the enhanced QoS delivery classes would supplement the best- router’s packet scheduler. effort delivery service in what could be described as an Each router must apply admission control to requests to integrated services Internct [l]. i s F INTEGRATED SERVICES defined by a ‘‘session” comprising the IP address, transport-layer protocol 0163-6804/97/$10.00 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA n response to the growing demand for an integrated services type, and port number o the destination along with a list of specijic f Internet, the Internet Engineering Task Force (IETF) [2] set senders to that session that are entitled to receive the special QoS. Each up an Integrated Services (intserv) Working Group [3], which sznder- is identified by source address and port nurnber; while ils prvtocol has since defined several service classes that, if supported by type must be the same as for the session. IEEE Communications Magazine 0 May 1997 1997 IEEE 0 100