IPv6 Headers Explained

Overview

IPv6 offers several improvements over IPv4, primarily in addressing capacity, but also features a more efficient header format. Despite the larger address size, the IPv6 header is only twice as large as IPv4's due to its streamlined structure.

Fixed Header

• Length: 40 bytes
• Includes essential information for routing packets.
• Fields:
  1. Version (4-bits): Indicates the protocol version, i.e., 0110 for IPv6.
  2. Traffic Class (8-bits):
     • 6 bits for type of service.
     • 2 bits for Explicit Congestion Notification (ECN).
  3. Flow Label (20-bits): Helps maintain packet sequence, used for streaming/real-time data.
  4. Payload Length (16-bits): Indicates the byte count of the payload, allows up to 65535 bytes.
  5. Next Header (8-bits): Specifies the type of Extension Header or the Upper Layer Protocol.
  6. Hop Limit (8-bits): Prevents infinite packet looping, similar to TTL in IPv4.
  7. Source Address (128-bits): Originator address of the packet.
  8. Destination Address (128-bits): Recipient address of the packet.

Extension Headers

• Placed between the Fixed Header and the Upper Layer Header.
• Allow IPv6 to include optional information not necessary for every packet.
• Identification:
  • Fixed Header's Next Header field directs to the first Extension Header.
  • Each Extension Header's Next-Header field points to subsequent headers, forming a linked list.
• No Further Headers: A value of 59 indicates there are no subsequent headers.
• Must be supported as per RFC 2460.

Sequence of Extension Headers

• Should be processed by first and subsequent destinations and by the final destination.
• Arranged in a linked list format, as depicted in relevant diagrams.

Additional Resources

• IPv6 - Overview
• IPv6 - Features
• IPv6 - Addressing Modes
• IPv6 - Communication

These headers and their structures are crucial for understanding the robustness and flexibility of IPv6 compared to IPv4.