Assured Forwarding PHB Group

The AF PHB group provides delivery of IP packets in four independently forwarded AF classes. Within each AF class, an IP packet can be assigned one of three different levels of drop precedence. A DS node does not reorder IP packets of the same microflow if they belong to the same AF class.

It is desirable that a site may exceed the subscribed profile with the understanding that the excess traffic is not delivered with as high probability as the traffic that is within the profile. It is also important that the network does not reorder packets that belong to the same microflow, as defined in, no matter if they are in or out of the profile. Assured Forwarding (AF) PHB group is a means for a provider DS domain to offer different levels of forwarding assurances for IP packets received from a customer DS domain. Four AF classes are defined, where each AF class is in each DS node allocated a certain amount of forwarding resources (buffer space and bandwidth). IP packets that wish to use the services provided by the AF PHB group are assigned by the customer or the provider DS domain into one or more of these AF classes according to the services that the customer has subscribed to. Within each AF class IP packets are marked (again by the customer or the provider DS domain) with one of three possible drop precedence values. In case of congestion, the drop precedence of a packet determines the relative importance of the packet within the AF class. A congested DS node tries to protect packets with a lower drop precedence value from being lost by preferably discarding packets with a higher drop precedence value.

In a DS node, the level of forwarding assurance of an IP packet thus depends on
(1) how much forwarding resources has been allocated to the AF class that the packet belongs to,
(2) what is the current load of the AF class, and, in case of congestion within the class,
(3) what is the drop precedence of the packet.

An otherwise DS-compliant node is not required to implement this PHB group in order to be considered DS-compliant, but when a DS-compliant node is said to implement an AF PHB group, it must conform to the specification in this document.

An Expedited Forwarding PHB

We show the generality of this PHB by noting that it can be produced by more than one mechanism and give an example of its use to produce at least one service, a Virtual Leased Line. Network nodes that implement the differentiated services enhancements to IP use a codepoint in the IP header to select a per-hop behavior (PHB) as the specific forwarding treatment for that packet. The EF PHB can be used to build a low loss, low latency, low jitter, assured bandwidth, end-to-end service through DS domains. Such a service appears to the endpoints like a point-to-point connection or a "virtual leased line".

Loss, latency and jitter are all due to the queues traffic experiences while transiting the network. Therefore providing low loss, latency and jitter for some traffic aggregate means ensuring that the aggregate sees no (or very small) queues. Queues arise when (short-term) traffic arrival rate exceeds departure rate at some node. Thus a service that ensures no queues for some aggregate is equivalent to bounding rates such that, at every transit node, the aggregate's maximum arrival rate is less than that aggregate's minimum departure rate. Creating such a service has two parts:
1) Configuring nodes so that the aggregate has a well-defined minimum departure rate. ("Well-defined" means independent of the dynamic state of the node. In particular, independent of the intensity of other traffic at the node.)
2) Conditioning the aggregate (via policing and shaping) so that its arrival rate at any node is always less than that node's configured minimum departure rate.

The EF PHB provides the first part of the service. The network boundary traffic conditioners described in provide the second part.

The EF PHB is not a mandatory part of the Differentiated Services architecture, i.e., a node is not required to implement the EF PHB in order to be considered DS-compliant. However, when a DS-compliant node claims to implement the EF PHB, the implementation must conform to the specification given in this document.

DiffServ PHB Aggregations

在DiffServ的架構中，提供需求不同的資料不同的服務是利用在IP header 上的codepoint來選擇不同的傳送方式（Per-Hop Behavior），其中針對有時間敏感度的訊務， IETF的Network Working Group提出了Assured Forwarding將資料分成四個不同的class來傳送與一個特別的PHB稱為Expedited Forwarding（EF）。

　　除了提供不同等級的服務，AF PHB的每一個class中的封包可以被指派三種不同的drop precedence，沒有使用超過預定資源的資料有較高的priority，而超過的部份則會有較低的priority。當網路壅塞時，而DiffServ網域上的節點會傾向忽略被指派較高drop precedence的封包來保護有較低drop precedence的資料。　　在DiffServ網域，不同等級的傳遞品質保證是受到配置給其所屬AF class的資源、此AF class的負荷與同一class中封包的drop precedence所影響。

　　EF PHB可以用來建立通過DiffServ網域，低latency、低loss、低jitter，保証頻寬的高品質服務，不論是loss rate, latency或jitter 都是由於資料傳送時在網域某節點的queue上等待的所造成的。所以要達成上述的目標，在網域的節點設定上，不論網域與其他aggregation忙碌與否，EF aggregation的資料要有一個最小的離開速率；此外，必須對進入網域EF aggregation的資料嚴格控管，使得不論在網域任何一個節點，訊務離開之速率要比進入得快。

　　DiffServ的精神在於提供per-aggregation 品質保證，對每一個aggregation裡的資料提供一致性的服務，在沒有指定路徑的情況下，對於有時間敏感度或高 priority的訊務，並沒有辦法提供per-flow的保證。在資源配置上，AF PHB中每個aggregation都必須有基本的資源配置，部份網段的壅塞或許不會對高priority 的訊務產生影響，但會使得大量的低priority訊務遺失，造成其他網段的資源浪費，甚至惡化網路壅塞的情況 EF PHB則以控制訊務進出節點的流速來提供高品質的服務，當網域某些地方訊務繁忙時，為了不降低服務品質，只好減少進入網域或通過節點的訊務，造成其他網段資源的浪費，整體資源的使用率與系統業者的收入也相形降低。