Chapter 20

"Do I Know This Already?"

9.A, B, C, D
11.B and D
12.A and D


1.The benefits of IPv6 include
  • Larger address space, because of its 128-bit addresses

  • Simplified packet header, which can be processed more efficiently

  • IPv6 mobility built in

  • IPsec built in

  • Many ways to transition from IPv4

2.The fields in the IPv6 header are described as follows:
  • Version— A 4-bit field, set to the number six for IPv6.

  • Traffic Class— Also called priority. Similar to ToS field in IPv4, this 8-bit field describes relative priority and is used for QoS.

  • Flow Label— The 20-bit flow label allows traffic to be tagged so that it can be handled faster, on a per-flow basis; this field can also be used to associate flows with traffic classes.

  • Payload Length— This 16-bit field is the length of the data in the packet.

  • Next Header— Like the protocol field in the IPv4 header, this 8-bit field indicates how the fields after the IPv6 basic header should be interpreted. It could indicate that the following field is TCP or UDP transport layer information, or it could indicate that an extension header is present.

  • Hop Limit— Similar to the TTL field of IPv4, this 8-bit field is decremented by intermediate routers and, to prevent looping, the packet is discarded and a message is sent back to the source if this field reaches zero.

  • Source Address and Destination Address— These 128-bit fields are the IPv6 source and destination addresses of the communicating devices.

  • Extension Headers— Zero or more extension headers follow the basic IPv6 header, for example before the transport layer data. The next header field within an extension header points to the next header in the chain.

3.Extension headers may follow the IPv6 basic header; possible extension headers include Hop-by-Hop options, Destination options, Routing, Fragment, Authentication, and ESP.
4.The following fields are significantly different in the IPv6 header and in the IPv4 header:
  • Version: The value is set to six instead of four.

  • Flow Label: This new field in the IPv6 header allows tagging and simplifies forwarding.

  • Source Address and Destination Address: These fields contain 128-bit addresses instead of 32-bit addresses.

  • Extension Headers: Extension headers are new in IPv6 and provide a flexible way to support other features.

Another difference is that the IPv6 header does not include a checksum, whereas the IPv4 header does. Fragmentation is also handled differently: in IPv6, intermediate routers do not fragment packets. Fragmentation is performed by the source node and fragmentation information has been moved to an extension header.

6.Broadcast addresses do not exist in IPv6. Broadcasts can be thought of as a special case of multicasting, where every device is the intended recipient. In IPv4, sending a broadcast causes all devices to process the packet, even those that are not concerned with the contents. Using multicasts is much more efficient because the packets can be targeted to a subset of devices, such as to routers running OSPF.
7.The EUI-64 bit interface ID is derived from the MAC address by inserting the hexadecimal number FFFE between the OUI field (the upper three bytes) and the vendor code (the lower three bytes) of the MAC address. The seventh bit in the first byte of the resulting interface ID, corresponding to the U/L bit, is set to binary 1. Thus, the EUI-64 bit interface ID for this interface is 020C:0AFF:FE28:121C. This value could be used in stateless autoconfiguration; the EUI-64 format interface ID is appended to the prefix advertised by the local router.
8.An address may be manually assigned, or dynamically assigned through DHCP or stateless autoconfiguration.
9.An IPv6 anycast address is a global unicast address that is assigned to two or more devices. Other devices route to the closest active device with the anycast address; the routing protocol metric determines which is closest.

Anycast addresses are created by assigning the same unicast address to more than one device; there is no reserved address space for anycast. Nodes on which the address is assigned must be explicitly configured to use and know that the address is an anycast address.

10.All IPv6 multicast addresses start with the prefix FF00::/8. The next four bits are flags; the four bits after the flags indicate the scope of the address and limit how far the multicast may travel. IPv4 uses TTL as a crude way to accomplish this, but there are times when the distance allowed by TTL is too far in one direction and not far enough in another. The IPv6 multicast scope is flexible enough to limit the multicast to a link, to a site, or to an enterprise.
11.The following IPv6 addresses exist on an interface:
  • Any global unicast or anycast assigned (in the range 2000::/3)

  • Link-local address (FE80::/10, using autoconfiguration for the EUI-64 format interface ID)

  • Loopback (::1/128)

  • All-nodes multicast addresses (FF01::1 and FF02::1)

  • Solicited-nodes multicast addresses (appending the last 24 bits of the corresponding unicast or anycast address of the device to FF02::1:FF00:/104)

  • Any other multicast group addresses assigned (in the range FF00::/8)

12.Routers must respond to the following additional IPv6 addresses on each of their interfaces:
  • Subnet-router anycast (the unicast address of the subnet with the interface ID—the host bits—set to ::0)

  • All-routers multicast addresses (FF01::2, FF02::2, and FF05::2)

13.Stateless autoconfiguration can be used to renumber a network, by changing the network information only on the routers. Each router advertises network information (either periodically or at the request of a host), including the 64-bit prefix, on each of its links. By listening for this advertisement, end-systems create a unique address by concatenating the new prefix and the EUI-64 format interface ID.
14.An IPv6 mobile node has a home address on its home network and a care-of address on its current network. A node communicating with a mobile node is called a correspondent node of the mobile node. The association between a the home address and the care-of address of the mobile node is known as a binding. When a mobile node roams away from its home network, it sends a binding update to its home agent, a router on its home network.

A mobile node and a correspondent node can communicate either via the home agent or directly.