IPv6 Headers Fields
This post will provides a more detailed overview of IPv6 packet headers. In the first part, the basic IPv6 header will be described, and in the following sections, extension headers will be presented. (Extension headers are additional headers that can be present in the IPv6 packet.)
IPV6 Header
IPv6 Headers Fields :
Version
The 4-bit Version field contains the number 6. This field is the same size as the IPv4 version field that contains the number 4. Nevertheless, the use of this field is limited because IPv4 and IPv6 packets are not distinguished on the basis of the value contained in it, but as a function of a different protocol type present in the layer 2 envelope (for example, Ethernet or PPP).
Priority or Traffic Class
The 4-bit Priority field in the IPv6 header can assume 16 different values.
It enables the source node to differentiate packets it generates by associating different delivery priorities to them. These 16 possible values are further divided into two groups: from 0 through 7 and from 8 through 15. Values 0 through 7 are used to specify the priority of traffic for which the source is providing traffic control. Values 8 through 15 are used to specify the priority of traffic that does not back off in response to congestion. Priority 8 is associated with those packets that the network will discard first under conditions of congestion (for example, high-fidelity
video traffic), and priority 15 is associated with those packets that the sender will discard at the end, only if absolutely necessary (for example, low-quality telephone audio traffic).
Flow Label
The 24-bit Flow Label field in the IPv6 header can be used by a source to label a set of packets belonging to the same flow. A flow is uniquely identified by the combination of the source address and of a nonzero Flow Label. Multiple active flows may exist from a source to a destination (with the same source address but with nonzero and different Flow Labels) as well as traffic that is not associated with any flow (carrying a Flow Label of zero). Packets belonging to the same flow must be coherently handled by IPv6 routers. The way to handle packets belonging to a given flow can be specified by information within the packets themselves or conveyed by a control protocol such as RSVP (Resource reSerVation Protocol). The Flow Label assigned to a flow is a numeric value randomly chosen by the source node from the range 1 to FFFFFF (hexadecimal). This value must be different from Flow Labels in use on the source node or used in the recent past. All packets belonging to the same flow must be sent with the same source address, destination address, priority, and Flow Label. Moreover, if any Hop-by-Hop or Routing extension headers are present, they must be the same in all packets belonging to the same flow.
Payload Length
The 16-bit Payload Length field contains the payload length—that is, the length of the data field following the IPv6 header, in octets. Because it is a 16-bit field, the maximum length of an IPv6 packet payload is 64 Kbytes. If a wider data field is needed, a Jumbo Payload extension header can be used. The presence of a Jumbo Payload is indicated by the value zero in the Payload Length field.
Next Header
The 8-bit Next Header field identifies the type of header immediately following the IPv6 header and located at the beginning of the data field (payload) of the IPv6 packet. The two most common kinds of Next Headers are clearly TCP and UDP , but many other headers are possible. The format adopted for this field is the one proposed for IPv4 by RFC 17003;. The Next Header field is generally the same as the IPv4 Protocol field.
Hop Limit
The 8-bit Hop Limit field is decremented by one by each node (typically a router) that forwards a packet. If the Hop Limit field is decremented to zero, the packet is discarded. The main function of this field is to identify and to discard packets that are looping because of erroneous routing information. Clearly, between two IPv6 nodes, we cannot have more than 255 hops (links), which means no more than 254 routers.
Source Address
The 128-bit Source Address field contains the IPv6 address of the node originating the packet.
Destination Address
The 128-bit Destination Address field contains the IPv6 address of the node recipient of the packet. If a Routing header is present, this address is not that of the ultimate recipient.
Extension Header used in IPv6
The IPv4 header has space for some optional fields requiring a particular processing of packets. These optional fields are not used often, and they can deteriorate router performance remarkably because their presence must be checked for each packet. IPv6 replaces optional fields with extension headers.
Extension headers are based on the principle that most of the packets need a very simple processing, and therefore basic fields of the IPv6 header are sufficient. Each extension header has a length equal to a multiple of 8 octets (64 bits).
A full implementation of IPv6 must include support for the following extension headers:
1. IPv6 header
2. Hop-by-Hop Options header
3. Destination Options header (see note 1)
4. Routing header
5. Fragment header
6. Authentication header
7. Encapsulating Security Payload header
8. Destination Options header (see note 2)
9. Upper layer header
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