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8.15: Epilog and Exercises

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  • IPv4 has run out of large address blocks, as of 2011. IPv6 has reached a mature level of development. Most common operating systems provide excellent IPv6 support.

    Yet conversion has been slow. Many ISPs still provide limited (to nonexistent) support, and inexpensive IPv6 firewalls to replace the ubiquitous consumer-grade NAT routers are just beginning to appear. Time will tell how all this evolves. However, while IPv6 has now been around for twenty years, top-level IPv4 address blocks disappeared much more recently. It is quite possible that this will prove to be just the catalyst IPv6 needs.

    8.16 Exercises

    Exercises are given fractional (floating point) numbers, to allow for interpolation of new exercises.

    1.0. Each IPv6 address is associated with a specific solicited-node multicast address.

    (a). Explain why, on a typical Ethernet, if the original IPv6 host address was obtained via SLAAC then the LAN multicast group corresponding to the host’s solicited-node multicast addresses is likely to be small, in many cases consisting of one host only. (Packet delivery to small LAN multicast groups can be much more efficient than delivery to large multicast groups.)

    (b). What steps might a DHCPv6 server take to ensure that, for the IPv6 addresses it hands out, the LAN multicast groups corresponding to the host addresses’ solicited-node multicast addresses will be small?

    2.0. If an attacker sends a large number of probe packets via IPv4, you can block them by blocking the attacker’s IP address. Now suppose the attacker uses IPv6 to launch the probes; for each probe, the attacker changes the low-order 64 bits of the address. Can these probes be blocked efficiently? If so, what do you have to block? Might you also be blocking other users?

    3.0. Suppose someone tried to implement ping6 so that, if the address was a link-local address and no interface was specified, the ICMPv6 Echo Request was sent out all non-loopback interfaces. Could the end result be different than conventional ping6 with the correct interface supplied? If so, how likely is this?

    4.0. Create an IPv6 ssh connection as in 8.12   IPv6 Examples Without a Router. Examine the connection’s packets using WireShark or the equivalent. Does the TCP handshake (12.3   TCP Connection Establishment) look any different over IPv6?

    5.0. Create an IPv6 ssh connection using manually configured addresses as in 8.12.3   Manual address configuration. Again use WireShark or the equivalent to monitor the connection. Is DAD (8.7.1   Duplicate Address Detection) used?

    6.0. An IPv6 fixed-header is 40 bytes. Taking this as the minimum packet size, how long will it take to send 1015 hosts (one quadrillion) probe packets to a site, if the bandwidth is 1 Gbps?

    7.0. Suppose host A gets its IPv6 traffic through tunnel provider H, as in 8.13   IPv6 Connectivity via Tunneling. To improve security, A blocks all packets that are not part of connections it has initiated, and makes no exception for ICMPv6 traffic. H is correctly configured to know the MTU of the A–H link. For (a) and (b), this MTU is 1280, the minimum allowed for IPv6. Much of the Internet, however, allows larger MTU values.

    A ─── H ─── Internet ─── B

    (a). If A attempts to send a larger-than-1280-byte IPv6 packet to remote host B, will A be informed of the resultant failure? Why or why not?

    (b). Suppose B attempts to send a larger-than-1280-byte IPv6 packet to A. Will B receive an ICMPv6 Packet Too Big message? Why or why not?

    (c). Now suppose the MTU of the A–H link is raised to 1400 bytes. Outline a scenario in which A sends a packet of size greater than 1280 bytes to remote host B, the packet is too big to make it all the way to B, and yet A receives no notification of this.