Home > Sample chapters

Understanding the IPv6 Header

IPv6 MTU

IPv6 requires that the link layer support a minimum MTU size of 1280 bytes. Link layers that do not support this MTU size must provide a link-layer fragmentation and reassembly scheme that is transparent to IPv6. For link layers that can support a configurable MTU size, RFC 2460 recommends that they be configured with an MTU size of at least 1500 bytes (the IPv6 MTU for Ethernet II encapsulation). An example of a configurable MTU is the Maximum Receive Unit (MRU) of a Point-to-Point Protocol (PPP) link.

Like IPv4, IPv6 provides a Path MTU Discovery process that uses the ICMPv6 Packet Too Big message described in the "Path MTU Discovery" section of Chapter 5. "ICMPv6." Path MTU Discovery allows the transmission of IPv6 packets that are larger than 1280 bytes.

IPv6 source hosts can fragment payloads of upper-layer protocols that are larger than the path MTU by using the process and Fragment header previously described. However, the use of IPv6 fragmentation is highly discouraged. An IPv6 node must be able to reassemble a fragmented packet that is at least 1500 bytes in size.

Table 4-5 lists commonly used local area network (LAN) and wide area network (WAN) technologies and their defined IPv6 MTUs.

Table 4-5. IPv6 MTUs for Common LAN and WAN Technologies

LAN or WAN Technology

IPv6 MTU

Ethernet (Ethernet II encapsulation)

1500, up to 9000 for jumbo frames

Ethernet (IEEE 802.3 SubNetwork Access Protocol [SNAP] encapsulation)

1492

IEEE 802.11

2312

Token Ring

Varies

Fiber Distributed Data Interface (FDDI)

4352

Attached Resource Computer Network (ARCNet)

9072

PPP

1500

X.25

1280

Frame Relay

1592

Asynchronous Transfer Mode (ATM) (Null or SNAP encapsulation)

9180.

For more information about LAN and WAN encapsulations for IPv6 packets, see Appendix E, "Link-Layer Support for IPv6."