424Understanding IPv6, Second Edition

4.Which extension headers are fragmentable and why? Which extension headers are not fragmentable and why?

Fragmentable:

Authentication header—Needed only by final destination ESP header and trailer—Needed only by final destination

Destination Options header (for final destination)—Needed only by final destination

Not fragmentable:

Hop-by-Hop Options header—Needed by every intermediate router

Destination Options header (for intermediate destinations)—Might be needed by intermediate destinations

Routing header—Might be needed by intermediate destinations Fragment header—Not present prior to fragmentation

5.Describe a situation that results in an IPv6 packet that contains a Fragment header in which the Fragment Offset field is set to 0 and the More Fragments flag is not set to 1.

IPv6 packets sent to IPv4 destinations that undergo IPv6-to-IPv4 header translation might receive a path maximum transmission unit (PMTU) update of less than 1280 bytes. In this case, the sending host sends IPv6 packets with a Fragment header and a smaller payload size of 1272 bytes. In the Fragment header, the Fragment Offset field is set to 0 and the More Fragments flag is not set to 1. The Fragment header is included so that the IPv6-to-IPv4 translator can use the Identification field in the Fragment header to perform IPv4 fragmentation to reach the IPv4 destination.

6.Describe how the new upper-layer checksum calculation affects transport layer protocols such as TCP and UDP.

TCP and UDP implementations must be updated to perform the checksum calculation that includes the new IPv6 pseudo-header when sending or receiving data over IPv6.

7.If the minimum MTU for IPv6 packets is 1280 bytes, how are 1280-byte packets sent on a link that supports only 512-byte frames?

The link layer must provide a fragmentation and reassembly scheme that is independent of and transparent to IPv6.

Chapter 5: ICMPv6

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

The value of the Type field for error messages is in the range 0 to 127. (The high-order bit is set to 0.) The value of the Type field for informational messages is in the range 128 to 255. (The high-order bit is set to 1.)

426 Understanding IPv6, Second Edition

Neighbor unreachability detection; ability to advertise changes in link-layer addresses and the node’s role on the network; ability to advertise configuration parameters, address prefixes, and routes.

3.List the five different ND messages and the options that can be included with them.

Router Solicitation: Source Link-Layer Address option

Router Advertisement: Source Link-Layer Address, Prefix Information, MTU, Advertisement Interval, Home Agent Information, and Route Information options

Neighbor Solicitation: Source Link-Layer Address option

Neighbor Advertisement: Target Link-Layer Address option

Redirect: Redirected Header and Target Link-Layer Address options

4.Describe the interpretation of the Length field in ND options.

The Length field is the number of 8-byte blocks in the entire Neighbor Discovery option.

5.What is the value of the Length field for a maximum-sized Redirected Header option (assuming no IPv6 extension headers are present)?

[1280 – 40 (IPv6 header) – 40 (ICMPv6 Redirect message header)]/8 = 150

6.Describe how you would use the MTU option to provide seamless connectivity between Ethernet nodes and Asynchronous Transfer Mode (ATM) nodes on a transparently bridged link.

Set the MTU option on the router to advertise a 1500-byte link maximum transmission unit (MTU) so that the ATM nodes do not send 9180-byte IPv6 packets.

7.Why is the Source Link-Layer Address option not included in the Neighbor Solicitation message sent during duplicate address detection?

It is not included because the reply must be multicast to all nodes on the link, rather than unicast to the sender of the Neighbor Solicitation message.

8.Describe the configuration parameters and their corresponding fields sent in the Router Advertisement message (not including options). Describe the configuration parameters and their corresponding fields sent in the Prefix Information option.

Router Advertisement message:

Default value of the Hop Limit field: Current Hop Limit

Whether to use a stateful address configuration protocol to obtain addresses or other configuration information: Managed Address Configuration (M) flag, Other Stateful Configuration (O) flag

Whether the advertising router is capable of acting as a home agent: Home Agent flag

The default router preference level of the advertising router: Default Router Preference

Whether the advertising router is a default router, and for how long: Router Lifetime

The value of the reachable time for neighbor unreachability detection: Reachable Time

The time interval between successive Neighbor Solicitation messages: Retransmission Timer

Prefix Information option:

The prefix: Prefix Length, Prefix

Whether the advertised prefix is on-link: On-link flag

Whether to create a stateless address based on the prefix: Autonomous flag

Whether the Prefix field contains the address of the Home Agent: Router Address flag

Whether to update the site prefix table with a site prefix: Site Prefix flag, Site Prefix Length

The valid lifetime of the stateless address: Valid Lifetime

The preferred lifetime of the stateless address: Preferred Lifetime

9.Under what circumstances is an unsolicited Neighbor Advertisement message sent?

An unsolicited Neighbor Advertisement message is sent in response to a duplicate address detection Neighbor Solicitation and when either the link-layer address or the role of the node changes.

10.What are the differences in address resolution and duplicate address detection node behavior for anycast addresses?

In Neighbor Advertisement messages, the Override flag is always set to 0. Duplicate address detection is not performed for anycast addresses.

11.Why is the response to a duplicate address detection sent as multicast? Who sends the response, the offending or defending node?

The response is multicast because the sender of the Neighbor Solicitation message cannot receive unicast packets at the duplicated IPv6 address. The defending node always sends the response.

12.Why is the value of the Hop Limit field set to 255 for all ND messages?

The value is set to 255 to prevent ND-based attacks from being launched from off-link nodes. The Hop Limit field for all traffic of an off-link node is always less than 255.

428Understanding IPv6, Second Edition

13.Describe the purpose of each of the host data structures described in RFC 2461.

Destination cache: Maps a destination address to a next-hop address, and stores the PMTU to the destination

Neighbor cache: Maps a next-hop address to a link-layer address, and stores the state of the entry for neighbor unreachability detection

Prefix list: Stores all the on-link prefixes

Default router list: Stores all the routers that advertised themselves as default routers

14.What field in the Redirect message contains the next-hop address of the better router to use for packets addressed to a specific destination? Describe how the contents of that field are used to update the conceptual host data structures for subsequent data sent to the destination.

The Target Address field. The Target Address field updates the Next-Hop Address field of the destination cache entry corresponding to the Destination Address field on the host that receives the Redirect message.

15.Under what circumstances does a router send a Router Advertisement?

Pseudo-periodically—the interval between unsolicited advertisements is randomized to reduce synchronization issues when there are multiple advertising routers on a link— and in response to a Router Solicitation message.

16.For Host A and Host B on the same link, why is the exchange of a Neighbor Solicitation message (sent by Host A to Host B) and a Neighbor Advertisement message (sent by Host B to Host A) not considered by Host B as proof that Host A is reachable?

Host B receives no confirmation that Host A received and processed the Neighbor Advertisement sent by Host B.

Chapter 7: Multicast Listener Discovery and MLD

Version 2

1.Why is the IPv6 Router Alert option used in the Hop-by-Hop Options header for Multicast Listener Discovery (MLD) and MLDv2 messages?

The IPv6 Router Alert option is used to ensure that routers process MLD and MLDv2 messages that are sent to multicast addresses on which the router is not listening.

2.Which addresses are used as the source address in MLD and MLDv2 messages?

The Source Address field is set to the link-local address of the interface on which the message is being sent. If a Multicast Listener Report message is for a solicited-node multicast address corresponding to a unicast address for which duplicate address detection has not been completed, the source address is set to the unspecified address (::).

3.Which addresses are used as the IPv6 destination address in MLD and MLDv2 messages?

MLD Multicast Listener Query: For the general query, the Destination Address field is set to the link-local scope all-nodes multicast address (FF02::1). For the multicast-address-specific query, the Destination Address field is set to the specific multicast address being queried.

MLD Multicast Listener Report: The specific multicast address being reported.

MLD Multicast Listener Done: The link-local scope all-routers multicast address (FF02::2).

MLDv2 Multicast Listener Query: For the general query, the Destination Address field is set to the link-local scope all-nodes multicast address (FF02::1). For the multicast-address-specific and multicast-address-and-source-specific queries, the Destination Address field is set to the specific multicast address being queried.

MLDv2 Multicast Listener Report: The All MLDv2-Capable Routers address (FF02::16).

4.How do you distinguish a general query from a multicast-address-specific query in the Multicast Listener Query message?

In the general query, the Destination Address field in the IPv6 header is set to the linklocal scope all-nodes multicast address (FF02::1) and the Multicast Address field in the message is set to the unspecified address (::). In the multicast-address-specific query, the Destination Address field in the IPv6 header and the Multicast Address field in the message are set to the specific address being queried.

5.How do you distinguish a multicast-address-specific query from a multicast-address- and-source-specific query in the MLDv2 Multicast Listener Query message?

The multicast-address-and-source-specific query contains a list of multicast sources and the multicast-address-specific query does not.

6.For which multicast addresses are Multicast Listener Report messages never sent?

The only multicast addresses that are not reported are the link-local scope all-nodes multicast address (FF02::1), and all multicast addresses with a scope of 0 (reserved) or 1 (interface-local).

7.In which MLD message is the value of the Maximum Response Delay field significant? MLD Multicast Listener Query (both general and multicast-address-specific)

8.Describe the use of the Multicast Address field for each MLD and MLDv2 message.

MLD or MLDv2 Multicast Listener Query: Requests reporting for all multicast addresses (except excluded ones) or for a specified multicast address