Motivation. An Architecture for Differentiated Services. Background. DSCP Field. Network Working Group (RFC2475)

Transcription

1 Motivation An Architecture for Differentiated Services Network Working Group (RFC2475) S. Blake, D.Black, M. Carlson, E. Davies, Z.Wang, W. Weiss Presented by Tao Zhao To support various types of applications, and specific business requirements, there is clear a need for relatively simple and coarse methods of providing differentiated classes of service for internet traffic Differentiated services are intended to provide a framework and building blocks to enable deployment of scalable service discrimination in the internet Background In IPv4 header, the second byte is Type of Service (TOS) field. PRECEDENCE A replacement header field, called the DS field, is defined [RFC2474], which is intended to supersede the existing definitions of the IPv4 TOS octet [RFC791] and the IPv6 Traffic Class octet [IPv6]. DSCP D T R CU UNUSED DSCP: differentiated services codepoint CU: currently unused DSCP Field The DSCP field is six bits wide. DS- compliant nodes MUST select PHBs by treating the value of the field as a table index which is used to select a particular packet handling mechanism which has been implemented in that device. Implementations should support the recommended codepoint-to-phb mappings in their default configuration

2 Terminology Service defines some significant characteristics of customer s packet transmission including quantitative or statistical terms of throughput, delay, jitter, loss etc. Behavior Aggregate (BA) is a collection of packets with the same DS codepoint crossing a link in a particular direction. Per-Hop-Behavior (PHB) is the externally observable forwarding behavior applied at a node to packets. Traffic conditioning refers to control functions performed to enforce rules specified in agreement including metering, marking, shaping and policing. Architecture Overview The architecture is composed of a number of functional elements implemented: A small set of per-hop forwarding behaviors. Packet classification functions. Traffic conditioning functions. PHB s are defined to permit a reasonably granular mean of allocating buffer and bandwidth resources at each node among competing traffic streams. Service provisioning and traffic conditioning policies are sufficiently decoupled from the forwarding behaviors within the network interior. Requirements Should accommodate a wide variety of services and provisioning policies, extending end-to-end or within a particular network. Should allow decoupling of the service from the particular application in use. Should work with existing applications without the need for application programming interface changes or host software modification. Should require only a small set of forwarding behaviors whose implementation complexity does not dominate the cost of a network device. Related Approaches Relative priority marking model. Ipv4 precedence marking. Service marking model. Ipv4 type of service as an input to route selection. Label switching (virtual circuit). Frame relay, ATM and MPLS. Integrated services/rsvp model. Establish packet classification and forwarding state on each node along the path by exchanging signaling messages.

3 DS Architectural Model Traffic entering a network is classified and possibly conditioned at the boundaries of the network, and assigned to different BA. Each BA is identified by a single DS codepoint. Packets are forwarded according to the PHB associated with the DS codepoint. DS Domain A DS domain is a contiguous set of DS nodes which operate with a common service provisioning policy and set of PHB groups implemented on each node. A well-defined boundary of DS domain consisting of boundary nodes which classify and condition ingress traffic. Interior nodes within the domain select the forwarding behavior for packets based on their DS codepoint (codepoint -> PHB mapping). DS Region DS Region (Cont d) A DS region is a set of one or more contiguous DS domains. It is capable of supporting differentiated services along paths which span the domains within the region. To permit services which span across the domains, the peering DS domains must each establish agreements (SLA, TCA) which specifies how transit traffic from one domain to another. Domain Region TCA SLA Boundary Node TCA Interior Node Domain

4 Traffic Classification and Conditioning Traffic Classification and Conditioning Differentiated services are extended across a DS domain boundary by establishing a SLA between domains. TCA between domains is derived from SLA. The packet classification policy identifies the subset of traffic which may receive a DS within the domain. Traffic conditioning performs metering, shaping, policing and remarking to ensure that the traffic entering the domain conforms to the TCA. Packets Meter Marker Shaper/ Dropper Per-hop Behaviors Summary A description of the externally observable forwarding behavior of a DS node applied to a BA. The PHB is the means by which a node allocates resources to behavior aggregates. PHB is defined in terms of behavior characteristics relevant to service provisioning policies, and not in terms of particular implementation mechanisms. Differentiated services enhancements to the internet protocol are intended to enable scalable service discrimination in the internet. The architecture focuses on the general semantics of the behaviors rather than the specific mechanisms used to implement them. It achieves scalability by aggregating traffic classification state which is conveyed by packet marking using DS field.

5 Discussion How can the operating systems use this architecture to provide differentiated services to applications. Through API. Discussion What s the implications of this architecture brings to adaptive and distributed applications requiring differentiated services? Adaptive applications indicate some important features of the data in the packets. Distributed applications express preferences and police individual flows and sessions within its allocated bandwidth. Reference Nichols, K., Blake, S., Baker, F. and D. Black, Definition of the Differentiated Services Field (DS Field) in the IPv4 and IPv6 Headers, RFC 2474, December Y.Bernet, A Framework for Differentiated Services, Feb