Hosts or local area networks. 3 Links. E Routers

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Moses Ellis
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What is the RIP?
Whose packets are re - ordered at the receiving host?

1 Albert Ludwigs University Freiburg Department of Computer Science Prof. Dr. Stefan Leue and Corina Apachite Distributed Systems - WS 2001/2002 Assignment 2 - Solutions Question 2.1 Explain how it is possible for a sequence of packets transmitted through a wide area network to arrive their destination in an order that diers from that in which theywere sent. Why can't this happen in a local network? Can it happen in an ATM network? (2 points) Answer 2.1 The route for transmitting a packet in a wide area network is determined dinamically for eachpacket. So, there is no guarantee that all the packets addressed to the same destination will follow the same route or at least will need the same amount of time to reach the destination. Connection-oriented protocols (TCP) readdress this by adding sequence numbers to the packets and re-ordering them at the receiving host.(1 point) It cannot happen in LANs because there is only a single communication medium, the messages are transmitted in stric sequence.(0.5 points) The data transmission in ATM networks is connection oriented, the communication is always through virtual circuits and these assure the order preservation. (0.5 points) Question 2.2 Show the sequence of changes to the routing tables listed bellow that would occur (according to the RIP algorithm given in Figure 2) after the link labeled 3 in Figure 1 is broken. The routings table are: D 1 2 1

2 A 1 B Hosts or local area networks 3 Links C D 6 E Routers Figure 1: Routing in a wide area network A 3 1 B 3 2 (4 points) Answer 2.2 After the link labelled 3 is broken, the routing tables at A and Darechanging (1 point): D 1 2 2

3 Send: Each t seconds or when Tl changes, send Tl on each non-faulty outgoing link. Receive: Whenever a routing table Tr is received on link n: for all rows Rr in Tr{ if (Rr.link!= n) { Rr.cost=Rr.cost+1 Rr.link=n if (Rr.destination is not in Tl) add Rr to Tl //add new destination to Tl else for all rows Rl in Tl { if (Rr.destination = Rl.destination and (Rr.cost < Rl.cost or Rl.link=n)) Rl=Rr //Rr.cost<Rl.cost: remote node has better route //Rl.link=n: remote node is more authoritative } } } Figure 2: Pseudo-code for RIP algorithm A 3 1 B 3 1 The routers in A and D send the new routing tables on every outgoing link. The changes that occur (1 point): D 1 1 3

4 A 3 1 B 3 1 After the second exchange of routing tables (1 point): D 4 2 A 6 3 B 6 2 After the third exchange of routing tables (1 point): 4

5 D 1 3 D 4 2 A 6 3 B 6 2 Question 2.3 (4 points) Discuss the problems that could occur in MobileIP Protocol. Answer 2.3 v6): Some of the problems that we can have with MobileIp (v4, or Network access: how the mobile host authenticates itself on the foreign network? (in MobileIPv6 there is no authentication, therefore, we have no authorization, no access control, no accounting) Where does the mobile host get his new IP-address? (stateless address conguration: mobile host creates the CoA with via IPv6 mechanism vulnerable against Denial of service Attacks or stateful address autoconguration: more messages need to be exchanged but provides better security features not the preferred solution because of high latency of rgistration process) Privacy The requierements on privacy are not clear today: 1. What is the identity that shoul be protected? (CoA, Home Address..) 2. Against whom do we protect our identity? (someone on the wireless link in the visited network, anyone between the mobile node and the correspondent node, against the correspondent node proling the mobile node..) 3. Should we also protect the home-realm information of the mobile user? Binding update How can we keep consistently the binding information? (problem that appears when a clientmoves relativly often from one foreign network to another foreign network) 5

6 Do we need to replicate the home agent? Is enough to have only one? 6

WAN Data Link Protocols

WAN Data Link Protocols In addition to Physical layer devices, WANs require Data Link layer protocols to establish the link across the communication line from the sending to the receiving device. 1 Data

Hosts or local area networks. 3 Links. E Routers

What is the RIP?

Whose packets are re - ordered at the receiving host?