Figure 69-4: logical view of packet classification and traffic conditioning at the edge router in Java

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Figure 69-4: logical view of packet classification and traffic conditioning at the edge router
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693 Per-Hops Behavior
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So far, we have focused on the edge functions in the differentiated services architecture The second key component of the DS architecture involves the per hop behavior (ie, packet forwarding function) performed by DS-capable routers The per-hop behavior (PHB) is rather cryptically, but carefully, defined as "a description of the externally observable forwarding behavior of a DS node applied to a particular DS behavior aggregate" [RFC 2475] Digging a little deeper into this definition, we can see several important considerations embedded within:
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A PHB can result in different classes of traffic ( ie, traffic with different DS field values) receiving different performance (ie, different externally observable forwarding behavior) While a PHB defines differences in performance (behavior) among classes, it does not mandate any particular mechanism for achieving these behaviors As long as the externally observable performance criteria are met, any implementation mechanism and any buffer/bandwidth allocation policy can be used For example, a PHB would not require that a particular packet queueing discipline, eg, a priority queue versus a weighted-fair-queueing queue versus a firstcome-first-served queue, be used to achieve a particular behavior The PHB is the "end", to which resource allocation and implemention mechanisms are the "means" Differences in performance must be observable, and hence measurable
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An example of a simple PHB is one that guarantees that a given class of marked packets receive at least x % of the outgoing link bandwidth over some interval of time Another per-hop behavior might specify that one class of traffic will always receive strict priority over another class of traffic - ie, if a high priority packet and low priority are present in a router's queue at the same time, the high priority packet will always leave first Note that while a priority queueing discipline might be a natural choice for implementing this second PHB, any queueing discipline that implements the required observable behavior is acceptable Currently, two PHB's are under active discussion within the diffserv working group: an Expedited Forwarding (EF) PHB [Jacobson 1999] and an Assured Forwarding (AF) PHB [Heinanen 1999]:
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The Expedited Forwarding PHB specifies that the departure rate of a class of traffic from a router must equal or exceed a configured rate That is, during any interval of time, the class of traffic can be guaranteed to receive enough bandwidth so that the output rate of the traffic equals or exceeds this minimum configured rate Note that the EF per hop behavior implies some form of isolation among traffic classes, as this guarantee is made independently of the traffic intensity of any other classes that are arriving to a router Thus, even if the other classes of traffic are overwhelming router and link resources, enough of those resources must still be made available to the class to ensure that it receives its minimum rate guarantee EF thus provides a class with the simple abstraction of a link with a minumum guaranteed link bandwidth The Assured Forwarding PHB is more complex AF divides traffic into four classes, where each AF class is guaranteed to be provided with some minimum amount of bandwidth and
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buffering Within each class, packets are further partitioned into one of three "drop preference" categories When congestion occurs within an AF class, a router can then discard (drop) packets based on their drop preference values See [Heinanen 1999] for details By varying the amount of resources allocated to each class, an ISP can provide different levels of performance to the different AF traffic classes The AF PHB could be used as a building block to provide different levels of service to the end systems, eg, an Olympic-like gold, silver, and bronze classes of service But what would be required to do so If gold service is indeed going to be "better" (and presumably more expensive!) than silver service, then the ISP must ensure that gold packets receive lower delay and/or loss than silver packets Recall, however, that a minimum amount of bandwidth and buffering are to be allocated to eachclass What would happen if gold service was allocated x% of a link's bandwidth and silver service was allocated x/2 % of the link's bandwidth, but the traffic intensity of gold packets was 100 times higher than that of silver packets In this case, it is likely that silver packets would receive betterperformance than the gold packets! (An outcome that leaves the silver service buyers happy, but the high-spending gold service buyers extremely unhappy!) Clearly, when creating a service out of a PHB, more than just the PHB itself will come into play In this example, the dimensioning of resources - determining how much resources will be allocated to each class of service - must be done hand-in-hand with knowledge about the traffic demands of the various classes of traffic
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