References

The following references were cited in this chapter:

■RFC 1191 — “Path MTU Discovery”

■RFC 1981 — “Path MTU Discovery for IP version 6”

■RFC 2460 — “Internet Protocol, Version 6 (IPv6) Specification”

■RFC 4443 — “Internet Control Message Protocol (ICMPv6) for the Internet Protocol Version 6 (IPv6)”

■RFC 4884 — “Extended ICMP to Support Multi-Part Messages”

You can obtain these RFCs from the \RFCs_and_Drafts folder on the companion CD-ROM or from http://www.ietf.org/rfc.html.

Testing for Understanding

To test your understanding of ICMPv6, answer the following questions. See Appendix D, “Testing for Understanding Answers,” to check your answers.

1.How do you distinguish ICMPv6 error messages from ICMPv6 informational messages?

2.Which fields of the Echo Request message are echoed in the Echo Reply message?

3.For a maximum-sized IPv6 packet with a Fragment extension header sent on an Ethernet link, how many bytes of the original payload are returned in an ICMPv6 Destination Unreachable message?

4.How can you tell whether a returned packet was discarded by a firewall that is enforcing network policy or a router that could not resolve the link-layer address of the destination?

5.Why is the MTU field in the ICMPv6 Packet Too Big message 4 bytes long when the Next Hop MTU field in the ICMPv4 Destination Unreachable-Fragmentation Needed and DF Set message is only 2 bytes long?

6.Why isn’t the ICMPv6 Parameter Problem-Unrecognized Option message sent when the two high-order bits of an option’s Option Type field are set to either 00 (binary) or 01 (binary)?

7.Based on the IPv6 design requirement to minimize processing at IPv6 routers, why is there no equivalent to the ICMPv4 Source Quench message in IPv6?

124 Understanding IPv6, Second Edition

IPv6 ND processes include the following:

■Router discovery During router discovery, a host discovers the local routers on an attached link. This process is equivalent to ICMPv4 router discovery. For more information, see the “Router Discovery” section in this chapter.

■Prefix discovery Prefix discovery is the process by which hosts discover the network prefixes for local link destinations. This is similar to the exchange of the ICMPv4 Address Mask Request and Address Mask Reply messages. For more information, see the “Router Discovery” section in this chapter.

■Parameter discovery The parameter discovery process enables hosts to discover additional operating parameters, including the link maximum transmission unit (MTU) and the default hop limit for outgoing packets. For more information, see the “Router Discovery” section in this chapter.

■Address autoconfiguration During address autoconfiguration, IP addresses are configured for interfaces in either the presence or absence of an address configuration server, such as a Dynamic Host Configuration Protocol for IPv6 (DHCPv6) server. For more information, see Chapter 8, “Address Autoconfiguration.”

■Address resolution Address resolution is the process by which nodes resolve a neighbor’s IPv6 address to its link-layer address. It is equivalent to ARP in IPv4. For more information, see the “Address Resolution” section in this chapter.

■Next-hop determination During next-hop determination, a node determines the IPv6 address of the neighbor to which a packet is being forwarded, based on the destination address. The next-hop address is either the destination address or the address of an onlink default router. For more information, see “Host Sending Algorithm” in this chapter.

■Neighbor unreachability detection The neighbor unreachability detection process is the means by which a node determines that the IPv6 layer of a neighbor is no longer receiving packets or that an IPv6 address has moved to a different physical interface. For more information, see the “Neighbor Unreachability Detection” section in this chapter.

■Duplicate address detection During duplicate address detection, a node determines that an address considered for use is not already in use by a neighboring node. This process is equivalent to using gratuitous ARP frames in IPv4. For more information, see the “Duplicate Address Detection” section in this chapter.

■Redirect function The redirect function is the process of informing a host of a better first-hop IPv6 address to reach a destination. It is equivalent to the use of the ICMPv4 Redirect message. For more information, see the “Redirect Function” section in this chapter.