6.2. QoS in IPv6 Protocols
The designers of IPv6 have focused not on requiring specific mechanisms for QoS, but on offering as much flexibility as possible to support different QoS mechanisms. This section describes the elements in the IPv6 header and the Extension headers that can be used for QoS services.
6.2.1. IPv6 Header
There are two fields in the IPv6 header that can be used for QoS: the Traffic Class and the Flow Label field.
188.8.131.52. Traffic Class
The use of the 1-byte Traffic Class field is specified in RFC 2474. As already mentioned, this RFC introduces the term "DS field" for the Traffic Class field. The goal of this specification is that DiffServ routers have a known set of DS routines, which are determined by the value in the DS field. These DSCP values are mapped to Per-Hop Behaviors (PHB) and can be either performance- or class-based. Figure 6-1 shows the DS field.
Figure 6-1. Format of the DS field
The DSCP field within the DS field (the six most significant bits of the DS field) is used for the codepoint, which specifies the PHB. With this field, 64 different codepoints can be specified. This codepoint pool has been divided into three parts to control the assignment of PHBs. Table 6-1 shows the division of the DSCP pools.
A pool of 32 recommended codepoints (pool 1) is assigned through formal standardization; a pool of 16 more codepoints (pool 2) is reserved for experimental or local use; the final pool of 16 codepoints (pool 3) is initially available for experimental or local use but should be used as an overflow pool if pool 1 is used up.
The PHBs specify how packets should be forwarded. A default PHB denominated by an all-zeros DS codepoint must be provided by any DS router. The default PHB describes the common, best-effort forwarding behavior available in existing routers. Such packets are forwarded without adhering to any priority policy; in other words, the network will deliver as many of these packets as possible as soon as possible, based on existing resources such as memory or processing capacity. Packets received with an undefined codepoint should also be forwarded as though they were marked for the default behavior.
The DS field does not specifiy PHBs; it specifies codepoints. The number of codepoints is limited to 64, whereas the number of PHBs is unlimited. There are recommended mappings of codepoints to PHBs. These mappings can be defined individually within administrative domains, which makes the number of possible PHBs unlimited. The coding rules for PHB IDs are specified in RFC 3140, "Per Hop Behavior Identification Codes." RFC 2597 defines a PHB group called Assured Forwarding (AF); RFC 3246 defines a PHB called Expedited Forwarding (EF).
Recommended codepoints and PHB IDs are assigned by IANA. The list of codepoints can be found at http://www.iana.org/assignments/dscp-registry, and the list of PHB IDs at http://www.iana.org/assignments/phbid-codes.
Figure 6-2 shows the DS field in a trace file.
Figure 6-2. The DS field in a trace file
This is a RIPng (RIP Next Generation) Response from our Cisco router. It is sent to the RIP Routers Multicast address of FF02::9. The DS field is set to 0xE0 (decimal notation 224, binary notation 1110 0000), which is decoded by Sniffer with Preferential Forwarding.
The remaining two bits of the DS field (see Figure 6-1) are not used according to RFC 2474, and are specified in RFC 3168, "The Addition of Explicit Congestion Notification (ECN) to IP." They provide four possible codepoints (00 to 11) that are used for Congestion Notification. Usually the overload of a router could only be determined based on packet loss. With the use of these Congestion Notification Codepoints, a router can signal overload before packet loss. This method is similar to Frame Relay's use of BECNs and FECNs (Backwards and Forwards Explicit Congestion Notification).
The two bits are used as follows:
184.108.40.206. Flow Label
The 20-bit Flow Label field in the IPv6 header may be used by a source to label packets for which it requests special handling by the IPv6 routers, such as nondefault QoS or real-time service. A flow label is assigned to a flow by the flow's source node. Between a sender and a receiver, there can be multiple flows active in parallel, along with the exchange of packets with no QoS requirements. New flow labels must be chosen randomly from the range 00001 to FFFFF. The purpose of the random allocation is to make any combination of bits within the Flow Label field suitable for use as a hash key by routers for looking up the state associated with the flow.
Hosts or routers that do not support the functions of the Flow Label field (most of today's applications, which will not be modified to use the Flow Label, or which do not need QoS handling) are required to set the field to all zeros when sending a packet, to pass the field on unchanged when forwarding a packet, and to ignore the field content when receiving a packet.
All packets belonging to the same flow must be sent with the same IP Source address, IP Destination address, identical source and destination ports, and a nonzero flow label. If any of these packets includes a Hop-By-Hop Options header, they all must be originated with the same Hop-By-Hop Options header contents (excluding the Next Header field of the Hop-By-Hop Options header, which is allowed to differ). If any packet includes a Routing Extension header, they all must be created with the same contents in all Extension headers up to and including the Routing Extension header (again excluding the Next Header field in the Routing Extension header). The routers or receivers are allowed to verify that these conditions are satisfied. If a violation of these consistency rules is detected, a corresponding error message is returned, indicating the exact location of the rule violation.
The handling of the flow label on routers is efficient, and when IPsec is used, it is always available because the IPv6 header is not encrypted by ESP or authenticated by AH (in transport mode). This implies that the integrity of the information in the DS field cannot be guaranteed by IPsec.
RFC 3697, "IPv6 Flow Label Specification," is a new specification of the Flow Label. A flow is defined as a sequence of packets from a sender to a specific unicast, anycast, or multicast address labeled as a flow by the sender. A flow is not necessarily associated with a transport connection. A host running multiple sessions with another host should be able to assign a different flow label to each session. Where the original specification defines a flow based on five criteria, the new specification definies a flow based on three criteria (Source and Destination address and flow label). The reason for this is that these three fields are always available for examination by routers, whereas the source and destination port number can be hidden by ESP.
6.2.2. IPv6 Extension Headers
As outlined earlier, two IPv6 Extension headers can be used to signal QoS requirements:
A detailed description of these headers can be found in Chapter 2. An updated list of router alert types can be found at http://www.iana.org/assignments/ipv6-routeralert-values.
6.2.3. IPv6 Label Switch Architecture (6LSA)
One new proposal using the Flow Label field is the IPv6 Label Switch Architecture (6LSA). A new usage of the Flow Label field is proposed, the 6LSA architecture is described, and a method of binding packets to Forwarding Equivalence Classes (FECs) is discussed.
The 6LSA architecture is similar in many respects to Multiprotocol Label Switching (MPLS). Labels are assigned to IPv6 packets, which are then used to provide QoS services across a 6LSA domain. 6LSA uses the Flow Label field instead of a shim header, however, to carry the label information, thus avoiding fragmentation and certain performance issues, as well as providing an end-to-end layer 3 tag for QoS.
The 6LSA architecture is still a very new concept and will need to go through peer review in the IETF before it is finalized. While its ultimate acceptance and deployment are unknown, its development is a sign that the QoS community for IPv6 continues to work on providing better models.