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Inter-domain routing

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Exercises

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Consider the interdomain topology shown in the figure below.

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In this network, what are the paths :

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from `AS1` to `AS4`

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from `AS4` to `AS2`

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from `AS4` to `AS1`

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Consider the interdomain topology shown in the figure below. Assuming, that `AS1` advertises prefix ``2001:db8:1::/48``,  `AS2` prefix ``2001:db8:2::/48``, ... compute the routing tables of the different ASes.

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Are all ASes capable of reaching all the other ASes in this simple Internet ?

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In this internet, some ASes cannot reach all other ASes. Can you fix the problem by adding one shared-cost peering link or one customer-provider peering link ?

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Consider the network below in which a stub domain, `AS456`, is connected to two providers `AS123` and `AS789`. `AS456` advertises its prefix to both its providers. On the other hand, `AS123` advertises ``2001:db8:dead::/48`` while `AS789` advertises ``2001:db8:beef::/48`` and ``2001:db8:dead:cafe::/63``. Via which provider will the packets destined to ``2001:db8:dead:cafe::1`` will be received by `AS456` ?

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Should `AS123` change its configuration ?

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Consider that the AS stub (`AS456`) shown in the figure below decides to advertise two ``/48`` prefixes instead of its allocated ``/47`` prefix.

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Via which provider does `AS456` receive the packets destined to ``2001:db8:caff::bb`` and ``2001:db8:cafe::aa`` ?

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How is the reachability of these addresses affected when link `R1`-`R3` fails ?

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Propose a configuration on R1 that achieves the same objective as the one shown in the figure but also preserves the reachability of all IP addresses inside `AS456` if one of `AS456`'s interdomain links fails.

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Consider the network shown below. In this network, the metric of each link is set to `1` except link `A-B` whose metric is set to `4` in both directions. In this network, there are two paths with the same cost between `D` and `C`. Old routers would randomly select one of these equal cost paths and install it in their forwarding table. Recent routers are able to use up to `N` equal cost paths towards the same destination.

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A simple network

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On recent routers, a lookup in the forwarding table for a destination address returns a set of outgoing interfaces. How would you design an algorithm that selects the outgoing interface used for each packet, knowing that to avoid reordering, all segments of a given TCP connection should follow the same path ?

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A ``traceroute6`` towards ``ipv6.google.com`` provides the following output :