Extension Headers

Extension Headers:

The paradigm of a fixed base header followed by a set of optional extension headers was chosen as a compromise between generality and efficiency. IPV6 needs to Include mechanisms to support functions such as fragmentation, source routing, and authentication. However, choosing to allocate fixed fields in datagram header for all mechanisms is inefficient because most datagrams don't use all mechanisms; the large IPV6 address size exacerbates the inefficiency.

The IPV6 extension header paradigm works similar to IPV4 options a sender can choose which extension headers to include in a given datagram and which to omit. Thus, extension headers provide maximum flexibility.

The length of the base header is 40 bytes. However, in IPv6 header can be followed by up to 6 extension header. This is to give more functionality to IP datagram. Extension headers of variable size contain a Header Extension Length field and must use padding as needed to ensure that their size is a multiple of 8 bytes. Next Header field in the IPv6 header and zero or more extension headers form a chain of pointers. Each pointer indicates the type of header that comes after the immediate header until the upper layer protocol is ultimately identified. Extension headers must be processed strictly in the order they appear in the packet.

Types of Extension Headers:

1. Hop-by-Hop Options Header:

• Special options that require hop-by-hop processing

2. Destination Options Header:

• Used to carry optional information for destination node

3. Routing Options Header:

• Lists one or more IPv6 node to be “visited” on the way to a packet destination. Fragmentation Options Header: Fragmentation and reassembly
• Only source node can fragment a packet in IPv6

4. Authentication Options Header:

• Provide Integrity and authentication, security

5. Encapsulating Security Payload:

• Provides Encryption Security, Confidentiality

Fig: Next Header Code used in IPV6

References:

1. A.S. Tanenbaum, “Computer Networks”, 3rd Edition, Prentice Hall India, 1997.
2. W. Stallings, “Data and Computer Communication”, Macmillan Press, 1989.
3. Kurose Ross, “Computer Networking: A top-down approach”, 2nd Edition, Pearson Education
4. Larry L. Peterson, Bruce S. Davie, “Computer Networks: A Systems Approach”, 3rd Edition, Morgan Kaufmann Publishers