IPv6 Essentials

6.1. QoS Basics

The current IP model treats all packets alike. They are all forwarded with best effort treatment according to the "first-come, first-served" principle. Which path a packet takes through the network depends on the available routers, routing tables, and general network load.

QoS protocols have the task of providing different data streams with priorities and guaranteeing qualities such as bandwidth and delay times. There are currently two main architectures: Integrated Services (IntServ) and Differentiated Services (DiffServ). Both architectures use traffic policies and can be combined to allow for QoS in the LAN as well as in the WAN.

Traffic policies can be used to make the transmission of data dependent on certain criteriafor example, whether there are enough resources available to forward the data according to its QoS requirements. Traffic policies can also monitor data streams and make adjustments or restrictions if necessary. Besides ensuring QoS requirements for delay-sensitive traffic, they can also be used for commercial reasons, such as controlling cost depending on different service levels.

6.1.1. Integrated Services

The Integrated Services Architecture (IntServ) is based on the paradigm that bandwidth and all related resources per flow are reserved on an end-to-end basis. This presupposes that routers store information about flows and analyze each packet to determine whether it belongs to a specific flow in order to forward the packet according to the criteria for that specific flow.

RSVP (Resource Reservation Protocol, RFC 2205) is part of the IntServ architecture. RFC 2210, "The Use of RSVP with IETF Integrated Services," describes the use of RSVP with IntServ. RSVP is a signaling protocol used to reserve bandwidth and other QoS resources across an IP network. IntServ combined with RSVP can be complex to implement and, because of its limited scalability, is inadequate to offer a general QoS solution for the global Internet.

If you are interested in further reading about RSVP and other QoS signaling protocols, refer to the informational RFC 4094," Analysis of Existing Quality-of-Service Signaling Protocols."

6.1.2. Differentiated Services

While IntServ offers the capability to allocate bandwidth to different flows, the Differentiated Services (DiffServ) architecture was designed to make a less granular differentiation of classes in order to increase its scalability and usability in large networks and in the Internet.

Differentiated Services is specified in RFCs 2474 and 2475. RFC 2474, "Definition of the Differentiated Services Field (DS Field) in the IPv4 and IPv6 Headers," specifies the DS field. This is implemented in the ToS field in the IPv4 header and the Traffic Class field in the IPv6 header. The DS field is used by DiffServ routers to determine the QoS forwarding requirements of packets. Communicating nodes can categorize their communication through a so-called Per-Hop Behavior (PHB). Based on the PHB, packets receive specific treatment on DiffServ routers.

A DiffServ (DS) domain is a contiguous group of DS routers that work with a common service policy implemented on all routers. A DS domain is defined by DS boundary routers. The boundary routers classify incoming data streams and ensure that all packets traversing the domain are labeled appropriately and use a Per-Hop Behavior from the set available for the domain. Routers within the domain choose the forwarding rules based on the DiffServ values in packets, which they map to the corresponding PHBs. The Differentiated Services Codepoint (DSCP; refer to Figure 6-1, shown later) value can use either the default mapping (DSCP=0) or an individually configured mapping for the domain. A DS domain usually consists of one network or a set of networks, which constitute an administrative unit.

A DS region is a set of contiguous DS domains. DS regions can ensure DS services for domain spanning paths. The single domains can use individual PHB definitions and PHB-codepoint mappings internally. Between the domains within a region, Traffic Conditioners are responsible for providing correct translation of the different PHBs and mappings. If the policies, PHB groups, and codepoint mappings are the same in all the domains within the region, no Traffic Conditioners are needed.

Packet Classifiers choose packets from a data stream based on information in the packet headers and according to predefined rules. There are two types of classifiers: the Behavior Aggregate Classifier (BA) classifies packets based on the DS field, and the Multi Field Classifier (MF) classifies packets based on either different header fields or a combination of header fields, such as Source or Destination address, DS field, protocol number, source or destination port, or information such as incoming interface.