FINAL EXAM - SLOT 2 TCP/IP NETWORKING Duration: 90 min. With Solutions

Transcription

1 First name: Family name: FINAL EXAM - SLOT 2 TCP/IP NETWORKING Duration: 90 min. With Solutions Jean-Yves Le Boudec, Patrick Thiran 2011 January 15 INSTRUCTIONS 1. The exam is in two time slots. Slot 1 must be completed entirely (sheets returned to us) before starting slot 2. Its duration is indicated on the top of this document. Slot 1 covers part 2 of the course and is mandatory for everyone. Slot 2 starts once slot 1 is over and covers part 1 of the course. If you had the maximum grade at the mid term, you should skip slot 2. If you are finished with slot 1 ahead of time, you may leave the room and come back at the beginning of slot 2. Reminder: Your final theory grade is T = max(m 1, F 1 ) + F 2 2 where F 1, F 2 are your grades at this exam, and M 1 is your grade at the mid term. Slot 1 gives you grade F 2, slot 2 gives grade F Write your solution into this document and return it to us. You may use additional sheets if needed. Do not forget to put your name on this document and all additional sheets of your solution. 3. If you need to make assumptions in order to solve some questions, please write them down explicitly. 4. Figures are on a separate sheet, for your convenience. 5. No documents, no electronic equipments are allowed. 6. Give short explanations for your answers. 1

2 PROBLEM 1 Consider the network in Figure 1. H1 to H3 are hosts, B1 is a bridge, R0, R1 and R2 are routers. All routers run RIP and the entire network is a single domain. IPv4 is used. The links are all Ethernet and the cost of each link is equal to 1; there is no other link than indicated on the figure. The IP addresses are shown on the figure. The MAC addresses of hosts are H1, H2,..; those of the routers are R11, R12, etc, as shown. The circled numbers represent observation points (where we put packet sniffers). Unless otherwise specified, all machines are configured to set TTL = 64 in the IP packets for which they are the source. Assume that the ARP caches and the DNS caches are populated with the correct values. The default gateway at H1 is set to and at H2 to Proxy ARP is not used anywhere. 1. Give possible (and correct) values for the subnet masks at H1, H2 and H3. Possible values for the netmasks are: H1: H2: H3: Give the routing table at R2 after RIP has converged (do not give the value of the interface field). At R2 Destination Network Next-Hop Distance /25 on-link /25 on-link / / H1 now sends some traffic to H2. What is the path followed by the resulting IP packets? The packets go first to R1 since the destination is not on-link with H1. R forwards the packet to R2, so the traffic goes twice through B1. Optional answer: Probably, R1 will notice that it is sending the packet on the same interface as it received it and will therefore generate an ICMP redirect to H1. H1, if it accepts it, will write a host route into its routing table, with destination= and next hop = We observe the packets flowing from H1 to H2 at observation points 1, 2 and 3. Give the IP and MAC source and destination addresses and TTL that are observed in the packets. Make sure to describe all assumptions you are making. MAC IPv4 at point source addr dest address source address destination address TTL 1 H1 R R11 R R22 H

3 4. In this question only, we assume that H1 is configured by mistake with netmask = Assume H1 sends a packet to H2 and one packet to H3. Explain the sequence of actions that may happen. H1 to H2: H1 wrongly believes that it is on-link with the destination. It therefore broadcasts an ARP-request in order to learn the MAC address of the corresponding interface of H2. Nobody has the requested IP address and no ARP reply is sent. H1 does not send the data packet. H1 to H3: The first 16 bits of the IP addresses of H1 and H3 are obviously not the same. Therefore H1 correctly believes that H3 is in an other separate subnetwork. H1 sends the data packet to R1, as it is its default gateway. The packet will eventually arrive at H3, the wrong netmask has no influence in this case. 5. At some time t 1 the link between R1 and R0 fails. Explain the sequence of actions for RIP. Both R0 and R1 notice the failure (by the keep-alive mechanism or by a local management system). R0 sets the cost to infinity (16) for all the destination networks that have R1 as next hop. That is, for /25 and /25. In the same way, R1 sets the cost to infinity for /8. It then sends a message to its RIP neighbors (that is, only R2 here) with its new distance vector. Since R2 was using R1 as next hop for /8, it updates the cost for this destination network, and sets it to infinity. It then notifies its neighbors respecting the split-horizon rule. As its only neighbor is R1 and the update came from R1, it does not send anything, RIP has converged. 6. Assume at time t 2 > t 1 RIP has converged again. What is the path followed by traffic from H1 to H3? and from H3 to H1? As the network is now disconnected, H1 and H3 cannot reach each other. The packets sent by H1 follow the path H1 B1 R1 and are dropped at R1. The packets sent by H3 follow the path H3 R0 and are dropped at R0. 7. At some time t 0 < t 1 H1 successfully opens a TCP connection to H3 and starts sending some data. What happens to this connection when the failure occurs at time t 1? When the connection breaks, H1 and H3 cannot communicate anymore. The packets sent by H1 are lost at R1, and obviously no ack are received by H1 for these packets. The TCP connection will thus eventually timeout. Alternative answer: if implemented, R1 may send an ICMP Destination Unreachable message (more precisely, an ICMP Network unreachable error message) to H1, upon receiving a packet to H3. In this case, the ICMP RFC states that such a message must be reported to the transport layer of H1, hence breaking the TCP connection. PROBLEM 2 Consider the network in Figure 2. All boxes are BGP routers. All physical links are shown with thick lines. There are no other routers than shown on the figure. All routers run BGP, unless otherwise specified. All routers inside AS B run RIP with all link costs equal to 1. We never re-distribute BGP into an interior routing protocol. Routers do not perform aggregation, unless otherwise specified. 3

4 Justify all answers in detail. 1. At time t 0, A1 sends to B1 the BGP announcements: 3.4.0/17, AS path =A, NEXT-HOP= /16, AS path =A, NEXT-HOP= Assume that, before t 0, B1 did not have any route in any of its RIB-INs for these two destinations. Will B1 accept these routes? To which routers will B1 announce a route to 3.4/16? to 3.4.0/17? B1 will accept both routes and add them to the Loc-RIB. B1 will announce these routes to all the I-BGP neighbors, namely, B0, B2 and B3. 2. Assume no other BGP announcement is sent from AS A to AS B and BGP has stabilized in all routers at time t 0. At time t 1 > t 0, A2 sends to B2 the BGP announcements /17, AS path =A, NEXT-HOP= /16, AS path =A, NEXT-HOP= At time t 1 BGP has stabilized again. Say which routes to 3.4/16 and to 3.4.0/17 the decision process at B2 has chosen. For prefix 3.4.0/17, B2 will choose the route announced by A1, as it is the only router that announced this route. For prefix 3.4/16, B2 will choose the route announced by A2 following the rule E-BGP > I-BGP. 3. At time t 2 > t 1 B0 has a packet to send to IP destination To which next hop does B0 send this packet? The address belongs to both networks 3.4.0/16 and /17. Prefix /17 is more specific (longest prefix match) and it was announced to B0 by B2. This means that the next hop for packet sent by B0 to destination is (via B2, recursive lookup). 4. At time t 3 > t 2, D1 sends to C1 the BGP announcement: /17, AS path =D, NEXT-HOP= Explain the sequence of events that follows. C1 already had a route to prefix /17 in its Loc-RIB. The route was sent by B3 and it had AS path = B A, NEXT-HOP= The route announced by D1 ( /17, AS path =D, NEXT-HOP= ) has shorter AS path, so C1 will replace the old route in its Loc-RIB with the new one. C1 will then announce the new route to B3. B3 will add it to its BGP database, but it won t promote it to its Loc-RIB, as it already has a route with shorter AS path ( /17, AS path =A, NEXT-HOP= ). When BGP stabilizes again (say at time t 3 ), which routes does C1 have to destination prefixes 3.4.0/17, /17 and 3.4/16? Same question for B3. Routes present in the Loc-RIB of C1: 3.4.0/17, AS path =B A, NEXT-HOP= (the only option) /17, AS path =D, NEXT-HOP= (shortest AS path) 3.4/16, AS path =B A, NEXT-HOP= (the only option) Routes present in the Loc-RIB of B3: 3.4.0/17, AS path =A, NEXT-HOP= (the only option) /17, AS path =A, NEXT-HOP= (shortest AS path) 3.4/16, AS path =A, NEXT-HOP= (shorter IGP distance) 4

5 To which next hop does C1 send a packet with destination address ? Same question for B3. In case of C1 next hop is , whereas in case of B3 next hop is In both cases the decision is made based on the AS path length. 5. At time t 4 > t 3, the link A2 B2 fails. Explain the sequence of events and messages exchanged by BGP routers. Router B2 will stop receiving KEEPALIVE messages from A2 and it will detect the failure. It will then announce to its I-BGP neighbors (B0, B1 and B3) that the routes with as next hop (i.e /17 and 3.4/16) are not available any more. Router B3 will promote the alternative route for the prefix /17 from its BGP database to the Loc-RIB ( /17, AS path =C D, NEXT-HOP= ). It will then announce the route to other routers inside AS B, which will add it to their Loc-RIBs. When BGP stabilizes again (say at time t 4 ), which routes does B1 have to destination prefixes 3.4.0/17, /17 and 3.4/16? These are the routes present in the Loc-RIB of B1 when BGP stabilizes: 3.4.0/17, AS path =A, NEXT-HOP= /17, AS path =C D, NEXT-HOP= /16, AS path =A, NEXT-HOP= To which next hop does C1 send a packet with destination address ? Same question for B0. Next hop for a packet with destination address , sent by C1 is After the failure of the link between A2 and B2, next hop for a a packet with destination address , sent by B0 is Assume we would like to avoid using BGP in B0 and replace the router B0 by a bridge. What would be an order of magnitude of the size of the MAC layer forwarding table at this bridge? Is it a viable solution? The MAC layer forwarding table would have a size limited to the maximum number of MAC addresses visible in the bridges network centered on this bridge. If the network is exactly as on the figure, this would mean 3 entries. This would be viable (but this bridge is a single point of failure, so probably such a topology is not recommended). 5

6 6

7 FINAL EXAM - SLOT 2 FIGURES Do not write your solution on this sheet, use only the main document. Do not return this sheet H1 1 B13 B B1 R1 R0 H3 R B12 R R21 R R22 H Figure 1: The network used in Problem 1 Figure 2: The network used in Problem 2 1