Handbook of Local Area Networks, 1998 Edition:Advanced LAN Interconnectivity Issues and Solutions Click Here! Search the site:   ITLibrary ITKnowledge EXPERT SEARCH Programming Languages Databases Security Web Services Network Services Middleware Components Operating Systems User Interfaces Groupware & Collaboration Content Management Productivity Applications Hardware Fun & Games EarthWeb sites Crossnodes Datamation Developer.com DICE EarthWeb.com EarthWeb Direct ERP Hub Gamelan GoCertify.com HTMLGoodies Intranet Journal IT Knowledge IT Library JavaGoodies JARS JavaScripts.com open source IT RoadCoders Y2K Info Previous Table of Contents Next IPV6 EXTENSION HEADERS AND OPTIONS In IPv6, optional IP layer information is encoded in separate extension headers that are placed between the IPv6 basic header and the higher-layer protocol header. An IPv6 packet may carry zero, one, or more such extension headers, each identified by the next header field of the preceding header and each containing an even multiple of 64 bits (see Exhibit 4-1-4). A fully compliant implementation of IPv6 includes support for the following extension headers and corresponding options: Exhibit 4-1-4.  IPv6 Extension Header Examples •  The hop-by-hop options header. This header carries information that must be examined by every node along a packet’s path. Three options are included in this category. The pad1 option is used to insert a single octet of padding into the options area of a header for 64-bit alignment, whereas the padN option is used to insert two or more octets of padding. The jumbo payload option is used to indicate the length of the packet when the payload portion is longer than 65,535 octets. This option is employed when the payload length field is set to zero. •  The routing header. This header is used by an IPv6 source to list one or more intermediate nodes that must be visited as part of the packet’s path to the destination; this option is functionally similar to IPv4’s loose and strict source route options. This header contains a list of addresses and an indication of whether each address is strict or loose. If an address is marked strict, it means that this node must be a neighbor of the previously addressed node; if an address is marked loose, this node does not have to be a neighbor of the previous node. •  The fragment header. This header is used by an IPv6 source to send packets that are larger than the maximum transmission unit (MTU) on the path to the destination. This header contains a packet identifier, fragment offset, and final fragment indicator. Unlike IPv4, where fragmentation information is carried in every packet header, IPv6 only carries fragmentation/reassembly information in those packets that are fragmented. In another departure from IPv4, fragmentation in IPv6 is performed only by the source and not by the routers along a packet’s path. All IPv6 hosts and routers must support an MTU of 576 octets; it is recommended that path MTU discovery procedures (per RFC 1981) be invoked to discover, and take advantage of, those paths with a larger MTU. •  The destination options header. This header carries optional information that has to be examined only by a packet’s destination node(s). The only destination options defined so far are pad1 and padN, as described above. •  The IP authentication header (AH) and IP encapsulating security payload (ESP). These are IPv6 security mechanisms (a section on IPv6 security appears later in this chapter). With the exception of the hop-by-hop option, extension headers are only examined or processed by the intended destination nodes. The contents of each extension header determine whether or not to proceed to the next header and, therefore, extension headers must be processed in the order that they appear in the packet. IPV6 QUALITY-OF-SERVICE (QOS) PARAMETERS The priority and flow label fields in the IPv6 header are used by a source to identify packets needing special handling by network routers. The concept of a flow in IP is a major departure from IPv4 and most other connectionless protocols; flows are sometimes referred to as a form of connection-less virtual circuit because all packets with the same flow label are treated similarly and the network views them as associated entities. Special handling for nondefault quality of service is an important capability for supporting applications that require guaranteed throughput, end-to-end delay, and jitter, such as multimedia or real-time communication. These QOS parameters are an extension of IPv4’s type-of-service (TOS) capability. The priority field allows the source to identify the desired priority of a packet. Values 0 through 7 are used for congestion-controlled traffic, or traffic that backs off in response to network congestion, such as TCP segments. For this type of traffic, the following priority values are recommended: •  Zero is recommended for uncharacterized traffic. •  One is recommended for “filler” traffic (e.g., Netnews). •  Two is recommended for unattended data transfer (e.g., E-mail). •  Three is recommended for reserved traffic. •  Four is recommended for attended bulk transfer, such as FTP or hypertext transfer protocol (HTTP). •  Five is also recommended for reserved traffic. •  Six is recommended for interactive traffic (i.e., telnet). •  Seven is recommended for Internet control traffic (i.e., routing protocols and simple network management protocol). Values 8 through 15 are defined for noncongestion-controlled traffic, or traffic that does not back off in response to network congestion, such as real-time packets being sent at a constant rate. For this type of traffic, the lowest priority value (8) should be used for packets that the sender is most willing to have discarded under congestion conditions (e.g., high-fidelity video traffic) and the highest value (15) should be used for those packets that the sender is least willing to have discarded (e.g., low-fidelity audio traffic). The flow label is used by a source to identify packets needing nondefault QOS. The nature of the special handling might be conveyed to the network routers by a control protocol, such as the resource reservation protocol (RSVP), or by information within the flow packets themselves, such as a hop-by-hop option. There may be multiple active flows from a source to a destination, as well as traffic that is not associated with any flow (i.e., flow label = 0). A flow is uniquely identified by the combination of a source address and a nonzero flow label. This aspect of IPv6 is still in the experimental stage and future definition is expected. IPV6 SECURITY In the early days of TCP/IP, the ARPANET user community was small and close, and security mechanisms were not the primary concern. As the number of TCP/IP hosts grew, and the user community became one of strangers (some nefarious) rather than friends, security became more important. As critical and sensitive data travels on today’s Internet, security is of paramount concern. Although many of today’s TCP/IP applications have their own devices, security should be implemented at the lowest possible protocol layer. IPv4 has few, if any, security mechanisms, and authentication and privacy at lower protocol layers is largely absent. IPv6 builds two security schemes into the basic protocol. Previous Table of Contents Next Use of this site is subject certain Terms & Conditions. 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