Packet Formats

IPV6 Header Format:

1.Version (4 bits)- 4 bits are used to indicate the version of IP and is set to 6

2. Traffic Class (8 bits)- Same function as the Type of Service field in the IPv4, distinguish different real-time delivery requirement

3. Flow Label (20 bits)

• Identifies a flow and it is intended to enable the router to identify packets that should be treated in a similar way without the need for deep lookups within those packets.
• Set by the source and should not be changed by routers along the path to the destination.

4. Payload Length (16 bits)– Only the length of the payload (Header length is fixed to 40 bytes)

5. Next Header (8 bits)- Indicates either the first extension header (if present) or the protocol in the upper layer PDU (such as TCP, UDP, or ICMPv6).

6. Hop Limit (8 bits)- IPv4 TTL was appropriately renamed Hop Limit because it is a variable that is decremented at each hop, and it does not have a temporal dimension.

7. Source Address (128 bits)- Stores the IPv6 address of the originating host.

8. Destination Address (128 bits)- Stores the IPv6 address of the current destination host.

Comparison to IPV4:

1. Removed (6): ID, flags, flag offset, TOS (types of services)/DS, header checksum
2. Changed (3): total lengths) payload length, protocol-) next header, TTL-) hop limit
3. Added (2): traffic class, flow label
4. Expanded: address 32 to 128 bits

Major Improvements:

1. No option field: replaced by extension header. Result in a fixed length, 40-byte IP header
2. No header checksum: result in fast processing
3. No fragmentation at intermediate nodes: result in fast IP forwarding

Optimization of writing of IPV6 address:

1. Leading zeroes of any group can be omitted

• Example: - 8000:0000:0000: 0123:4567:89AB: CDEF:2912 can be written as 8000:0000:0000: 123:4567:89AB:C0EF:2912.

2. 16-bit consecutive zeroes can be replaced with ::

• Example: - 8000:0000:0000: 123:4567:89AB: CDEF:2912 can be written as 8000::123:4567:89AB: CDEF:2912
• 0000:0000:1212: 2341:0000:0000: 1212:1212 can be written as
• Either: - ::1212:2341:0000: 0000:1212:1212
• Or: - ::0000:0000:1212:2341::1212:1212

[:: can be used only once)

202.51.77.123 IPV4 is equivalent to :: 202:51:77:123 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