New tagging technique boosts IP QoS
 
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Applications such as Web browsing and e-mail have been working well with the best-effort quality of service (QoS) provided by the Internet. However, with best-effort services, data can easily be lost or delayed.
An emerging array of high-bandwidth and delay-sensitive applications, such as voice over IP, video over IP and virtual private networks, have become a strong motivating force for the development of improved Internet QoS levels.
A new technology making its way through the Internet Engineering Task Force (IETF), known as Differentiated Services (Diff-Serv), could give users higher levels of Internet QoS.
Diff-Serv is a very simple technology that allows large corporate IP backbone users and ISPs to quickly deploy different QoS levels in the Internet backbone.
Previous attempts to improve Internet QoS included Resource Reservation Protocol (RSVP), also developed by the IETF. RSVP is based on a sophisticated per-connection signaling system that requires routers in the network to "agree" to a specific level of service. Unfortunately, it is widely acknowledged that RSVP is complex to deploy and does not scale well.
Diff-Serv does not specify a signaling system like RSVP, but rather a method to mark, or tag, packets, allowing routers to modify their forwarding behavior appropriately. Various types of traffic requiring different QoS have different tags applied. In place of complex dynamic signaling, ISPs could offer various service-level agreements (SLA) based on Diff-Serv packet markings.
Diff-Serv has a built-in aggregation mechanism: all traffic with the same tag is treated in the same way; each voice connection is not handled separately. This is an important reason why Diff-Serv can scale to support larger environments.
David Clark, senior research scientist at the MIT Laboratory for Computer Science, initially led the effort, producing a first Internet draft on differentiated services in the middle of last year.
The draft proposed partial redefinition of the type-of-service (ToS) byte in the IP packet header to implement Diff-Serv markings.
Reusing an existing IP header field was a significant step in creating a simple standard, since the basic IP protocol would not change.
Further, the draft proposed essentially three marking options: "none," "assured and in profile" and "assured and out of profile." "None" offered existing best-effort service. The definitions of "assured and in profile " and "assure and out of profile" would be described in an SLA between the customer and network service provider.
In late February, the IETF formed the Diff-Serv working group that recently produced an architecture draft (draft-ietf-diffserv-arch-00.txt).
According to this draft, a router's forwarding process modified by a Diff-Serv marking is known as a Per-Hop-Behavior (PHB). A PHB can be defined to just specify minimum bandwidth. A significant effort of the Diff-Serv working group was to determine to what initial standard PHBs should adhere.
Two PHBs are proposed: default and expedited.
Default PHB is defined as today's best-effort service. Expedited PHB is the other extreme: low absolute delay, low delay variation and low packet loss.
The draft further defines Diff-Serv implementation in two types of routers: traffic conditioners and DS-capable. Traffic conditioners perform sophisticated traffic classification, monitoring, shaping, scheduling and marking. They are most likely to be access routers.
DS-capable routers have scheduling capabilities and must modify their forwarding behavior based on the markings. They are most likely to be backbone routers.
This separation of function is another reason for Diff-Serv's simplicity: most of the complexity is in the traffic conditioner, which is at the edge of the network. At the same time, DS-capable routers in the core need only support modified forwarding operations.
A basic requirement for traffic conditioners is to allow an ISP, and potentially ISP customers, to mark packets based on an agreed SLA. The SLA may include limits the traffic conditioner must adhere to, or soft limits that when exceeded cause packets to be marked with a different PHB.
It is likely that ISPs will typically offer a limited number of service levels on their DS-capable backbones.
Traffic conditioners at the edge of the network that support richer classification and scheduling schemes, such as class-based queuing, will provide the flexibility to build an extensible set of SLAs on top of the basic Diff-Serv building blocks.
While it may take some time to determine the success of Diff-Serv, its simplicity, flexibility and initial wide acceptance in the user, vendor and ISP communities could finally make end-to-end Internet QoS a reality.
Greene is chief technology officer and vice president of software with Xedia Corp., an Internet access software developer in Littleton, Mass. He can be reached at (978) 952-6000.
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