What are IPv6 Extension Headers and how do routers handle fragmentation in IPv6?

Extension headers carry optional Layer 3 info (fragmentation, security, mobility) between the IPv6 header and payload; IPv6 routers never fragment — the source host fragments based on Path MTU and only the destination reassembles.

IPv6 Extension Headers (EH):

• Provide optional network layer information
• Are optional - not required in every packet
• Placed between the IPv6 header and the payload

Uses for Extension Headers:

• Fragmentation information
• Security (IPsec)
• Mobility support
• Routing options

Critical difference from IPv4:

Routers do NOT fragment IPv6 packets.

In IPv6, fragmentation is handled by:

1. The source host must discover the Path MTU
2. The source fragments before sending if needed
3. Only the destination reassembles fragments

Context: Moving fragmentation responsibility to endpoints simplifies router processing and improves forwarding performance.

Go deeper:

• RFC 8200 §4 — Extension Headers — how the Next-Header chain encodes fragmentation, routing, and options.
• Wikipedia — IPv6 packet (extension headers) — the extension-header types and ordering.