network protocols handbook.pdf

110Protocols GuideSecurity and VPN - Secured Routing ProtocolsSecured Routing ProtocolsProtocol NameDiffServ: Differentiated ServiceArchitectureProtocol DescriptionDiifrentiated Service (DiffServ) defines an architecture for implementingscalable service differentiation in the Internet. A “Service”defines some significant characteristics of packet transmissionin one direction across a set of one or more paths withina network. These characteristics may be specified in quantitativeor statistical terms of throughput, delay, jitter, and/or loss, ormay otherwise be specified in terms of some relative priority ofaccess to network resources. Service differentiation is desiredto accommodate heterogeneous application requirements anduser expectations, and to permit differentiated pricing of Internetservice.DiffServ architecture is composed of a number of functional elementsimplemented in network nodes, including a small set ofper-hop forwarding behaviors, packet classification functions,and traffic conditioning functions including metering, marking,shaping, and policing. This architecture achieves scalability byimplementing complex classification and conditioning functionsonly at network boundary nodes, and by applying per-hop behaviorsto aggregates of traffic which have been appropriatelymarked using the DS field in the IPv4 or IPv6 headers [DS-FIELD]. Per-hop behaviors are defined to permit a reasonablygranular means of allocating buffer and bandwidth resourcesat each node among competing traffic streams. Per-applicationflow or per-customer forwarding state need not be maintainedwithin the core of the network.The differentiated services architecture is based on a simplemodel where traffic entering a network is classified and possiblyconditioned at the boundaries of the network, and assignedto different behavior aggregates. Each behavior aggregate isidentified by a single DS codepoint. Within the core of the network,packets are forwarded according to the per-hop behaviorassociated with the DS codepoint. In this section, we discussthe key components within a differentiated services region, trafficclassification and conditioning functions, and how differentiatedservices are achieved through the combination of trafficconditioning and PHB-based forwarding.Protocol StructureIn DiffServ, a replacement header field, called the DS field, is defined,which is intended to supersede the existing definitions ofthe IPv4 TOS octet and the IPv6 Traffic Class octet. The formatof the header as follows:DSCP• DSCP - differentiated services codepoint to select thePHB a packet experiences at each node• CU - currently unusedRelated protocolsIP, IPv6Sponsor SourceDiffServ is defined by IETF (http://www.ietf.org) in RFC 2474and 2475.Referencehttp://www.javvin.com/protocol/rfc2475.pdfAn Architecture for Differentiated Serviceshttp://www.javvin.com/protocol/rfc2475.pdfDifferentiated Services FieldCU6 8bit