THE HONG KONG POLYTECHNIC UNIVERSITY. Department of Computing. This is an open-book examination.

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1 THE HONG KONG POLYTECHNIC UNIVERSITY Department of Computing This is an open-book examination. () Internetworking Protocols and Software 7 January hours [Answer all ten questions.]

2 2 Please answer ALL questions in this section. You should always attach a succinct explanation to your answer. 1. [10 MARKS] (Subnet mask) Figure 1 shows the content of the ARP cache of a host with IP address after issuing arp -a. Note that the first five and the seventh dynamic entries bind to the same MAC address. The static entries are for multicast and broadcast. Figure 1: The content of the ARP cache in the host with IP address (a) [2 MARKS] What is the subnet-directed broadcast address of the subnet that the host belongs to? (b) [6 MARKS] By comparing the IP addresses in the dynamic bindings except for those for c-07-ac-00 with the entry in (a), determine the subnet mask of the subnet that this host belongs to. We again assume that only contiguous subnet mask can be used (i.e., the 1 s in the subnet mask must not be separated by 0.). (c) [2 MARKS] Based on the ARP cache content, is variable-length subnet mask used in the class B network ?

3 3 2. [10 MARKS] (UDP traceroute with fragmentation) A student performed traceroute to using PingPlotter, and the traceroute configuration is shown in Figure 2. Note that the packet size includes the 20-byte IP header. The host that the traceroute was performed was connected to a network with an MTU of 1500 bytes. Figure 2: The packet setting for the traceroute using PingPlotter. Based on the result, the IP packet sent by PingPlotter was fragmented into two fragments. Moreover, the first-hop will return an ICMP Time Exceeded message only for the first fragment. (a) [2 MARKS] How many protocol headers (including the IP header) are in the first fragment, and what are they? Repeat for the second fragment. (b) [2 MARKS] What is the fragment offset s value in the second fragment? (c) [4 MARKS] Why is an ICMP Time Exceeded message returned only for the first fragment, but not for the second? (d) [2 MARKS] How many protocol headers (including the IP header) are in the packet containing the ICMP Time Exceeded message, and what are they?

4 4 3. [10 MARKS] (ICMP messages from within an IP tunnel) RFC 2003 states that After an encapsulated (tunneled) datagram has been sent, the encapsulator may receive an ICMP message from any intermediate router within the tunnel other than the tunnel exit point. The action taken by the encapsulator depends on the type of ICMP message received. When the received message contains enough information, the encapsulator MAY use the incoming message to create a similar ICMP message, to be sent to the originator of the original unencapsulated IP datagram (the original sender). This process will be referred to as "relaying" the ICMP message from the tunnel. Consider the following scenario for the ICMP relaying. A host sends an IP datagram containing a TCP packet to a destination. An encapsulator receives this packet and tunnels it to a decapsulator. But a router within the tunnel returns an ICMP error message to the encapsulator. (a) [3 MARKS] How many protocol headers (including the IP header) are in the packet that carries the ICMP error message? (b) [4 MARKS] Assuming that the tunnel IP header does not have option, would the encapsulator be able to relay a similar ICMP error message to the host? (c) [3 MARKS] Repeat part (b) if the tunnel IP header includes a 12-byte option.

5 5 4. [10 MARKS] (IP forwarding) Figure 3 shows two LAN segments connected by a router and a LAN switch (only with layer-two capability). The class B network is subnetted with subnet mask Assume that H1 has a proper forwarding table, and the router serves as the default router for both H1 and H2. Figure 3: Two LANs connected by a router and a LAN switch. (a) [2 MARKS] Write down the main entries in H1 s forwarding table. Repeat for the router. (The forwarding table contains the Network/Subnet, Mask, and Next-hop entries.) (b) [5 MARKS] Assume that the LAN switch has learnt the MAC addresses of H1, H2, and the router. If H1 sends a datagram to H2, will H2 receive a duplicate copy of the datagram? Also assume that H1 s ARP cache is empty. Explain your answer. (c) [3 MARKS] Repeat part (a) if the LAN switch has not learnt the MAC addresses ofh1, H2, and the router.

6 6 5. [10 MARKS] (A complete TCP connection) Figure 4 shows a Wireshark trace of a short web session: from a client with IP address of and a port of 4676 to the COMP web server with port 80. Assume that both sides use an initial sequence number (SN) of 0. Therefore, the SN and the acknowledgment number (AN) in the SYN-ACK packet are 0 and 1, respectively. The SN and AN in the third hand-shaking packet are both 1. Figure 4: A complete TCP connection captured in a short web session. Answer the following questions concerning Figure 4. (a) [2 MARKS] At time 0.073, the server sent two pure ACKs. The first (second) one acknowledges the first (second) data segment sent by the client. What are the ANs in the two pure ACKs? (b) [3 MARKS] What is the SN of the only data segment sent by the server? (c) [3 MARKS] What is the SN of the last ACK sent by the server? (d) [2 MARKS] Which side will enter into the TIME WAIT state?

7 7 6. [10 MARKS] (TCP congestion control I) In this question, we revisit the congestion control problem in assignment 3. Referring to Figure 5 for TCP packet transmissions in a TCP Reno connection, a number of full-sized TCP data segments were sent from a sender to a receiver, and the segments are numbered starting from 0. Each ACK acknowledges only one TCP data segment. The square symbol refers to a TCP data segment transmission, whereas a small dot refers to an ACK transmission. The TCP data segment and its ACK are drawn on the same line for easy reference (an example is shown for segment 10). TCP Reno implements the fast retransmission and fast recovery algorithms that we discussed in class. The figure shows that only segment 14 was lost, and it was fast retransmitted after receiving a third duplicate ACK. Note that there were a number of duplicate ACKs received. The send window was always given by the cwnd, and the sender always had data to send. Segment loss Retransmission 14 Duplicate ACKs ACK for segment 10 Figure 5: Packet transmission sequence of a TCP Reno connection. Answer the following questions about the trace in Figure 5. (a) [3 MARKS] Upon receiving the third duplicate ACK (dupack), the sender entered into the fast recovery phase. How many more dupacks were received in order for the sender to send the new segment 29? (b) [3 MARKS] Why were only six new segments sent during the fast recovery phase? (c) [4 MARKS] After the fast recovery phase was ended, each of the first six ACKs elicited a new data segment, but the seventh ACK elicited two. What is the reason for that?

8 8 7. [10 MARKS] (TCP congestion control II) In Figure 6, we show a TCP data-ack plot. You may ignore the receiver-side traces (i.e., Rcv Data and Rcv Ack). The two sender-side traces are defined as: Snd Data: TCP data segments sent by the TCP sender Snd Ack: TCP ACKs received by the TCP sender To simulate a packet reordering event, the packet that was supposed to be sent at around 38s (marked as + in the figure) is queued while the succeeding six packets were let through. Then the delayed packet was sent out (indicated by the arrow) with the next packet. There are no packet loss events in this set of traces. Figure 6: A TCP packet reordering scenario. (a) [3 MARKS] Around what time was the reordered packet recovered (from the sender pointof-view)? (b) [3 MARKS] How was the reordered packet recovered? (c) [4 MARKS] What happened to the sender s cwnd when the reordered packet was recovered?

9 9 8. [12 MARKS] Consider the IP network below. A new network segment, to which SUNBETA was attached, was added to improve the network resilience in terms of Internet access. Unfortunately, the network administrator simply duplicated the IP addresses for SUNBETA; as a result, two segments identified themselves with the same network address. All the networks used the same subnet mask of , and RIP-1 was used for routing. (a) [3 MARKS] If SUNALPHA s forwarding table contained a route to its directly attached segment and a default route to router D, how can it reach SUNGAMA? (b) [3 MARKS] If router D fails, can SUNALPHA connect to the Internet? (c) [2 MARKS] Can SUNALPHA and SUNBETA reach each other? (d) [4 MARKS] Can SUNGAMA, SUNDELTA, and SUNTHETA reach SUNALPHA and SUN- BETA? SUNGAMA router A I router B SUNDELTA SUNALPHA router C router D SUNTHETRA To Internet router E Originally, --SUNBETA this segment did not exist router F To Internet

10 10 9. [8 MARKS] (RIP with split horizon) Consider Figure 7 in which the three routers exchange their routes to an IP network denoted by Dest using RIP-1. Split horizon with poisonous reverse is used and a distance of 16 is interpreted as destination unreachable. H s default router is set tor 1. How does H send a packet to Dest for the following cases: (a) [2 MARKS] The distances announced by R 1, R 2, and R 3 are 5, 16, and 16, respectively. (b) [2 MARKS] The distances announced by R 1, R 2, and R 3 are 16, 16, and 5, respectively. (c) [2 MARKS] The distances announced by R 1, R 2, and R 3 are 16, 5, and 5, respectively. (d) [2 MARKS] The distances announced by R 1, R 2, and R 3 are 5, 5, and 16, respectively. Dest R1 R2 R3 H Figure 7: Three RIP-1 routers exchange routes for Dest on the LAN.

11 [10 MARKS] (Link-state routing) The topology of an OSPF network is given below. There are three routers RT1, RT2, and RT3, and three networks N1, N2, and N3. **FROM** RT RT RT N1 N2 N * RT * RT T RT3 0 0 O N1 1 * N * N3 2 3 (a) [4 MARKS] Draw the topology of the network. (b) [3 MARKS] Who is (are) the neighboring router(s) of RT1? (c) [3 MARKS] What is RT3 s next-hop router for reaching the hosts on N1? End of the Examination Paper