Problem Max. Points Act. Points Grader

Transcription

1 dvanced Networking ourse: International University remen ate: r. Jürgen Schönwälder eadline: Midterm xamination The IU ode of cademic Integrity applies to this examination. Please fill in your name (please write readable) and sign below. Name: Signature: This exam is open book in the sense that you are allowed to use course slides and your personal notes during the exam. You are not allowed to use books nor are you allowed to use any electronic equipment such as computers or cell phones. Please answer the questions on the problem sheets. If you need more space, feel free to write on the back of the pages. Please keep the papers stapled. Problem Max. Points ct. Points Grader M.1 10 M.2 30 M.3 20 M.4 20 M.5 10 M.6 10 Total 100

2 Problem M.1: multicast basics ( =10 points) Indicate which of the following statements are correct or incorrect by marking the appropriate boxes. For every correctly marked box, you will earn two points. For every incorrectly marked box, you will loose one point. Statements which are not marked or which are marked as true and false will be ignored. The minimum number of points you can achieve is zero. true false n open multicast groups allows new members to join a group while a closed multicast group has a fixed set of members. single I multicast M address may be used by multiple IP multicast addresses. To receive an IPv4 source-specific multicast stream, a node has to subscribe to the multicast group G by using the setsockopt() system call and the ip mreq data structure. The IGMP protocol handles the registration of multicast receivers at the leafs of multicast distribution trees. ridges use IGMP snooping to restrict the forwarding of multicast traffic in a bridged LN to those segments which either have multicast receivers or which connect segments in the spanning tree which have multicast receivers. true false n open multicast groups allows new members to join a group while a closed multicast group has a fixed set of members. single I multicast M address may be used by multiple IP multicast addresses. To receive an IPv4 source-specific multicast stream, a node has to subscribe to the multicast group G by using the setsockopt() system call and the ip mreq data structure. The IGMP protocol handles the registration of multicast receivers at the leafs of multicast distribution trees. ridges use IGMP snooping to restrict the forwarding of multicast traffic in a bridged LN to those segments which either have multicast receivers or which connect segments in the spanning tree which have multicast receivers.

3 Problem M.2: reverse path forwarding ( =30 points) onsider the following network topology: a) For every router, compute the shortest unicast paths to all the other routers (minimize the total link costs). b) ompute the reverse path forwarding (RPF) multicast trees for the sources, and. c) ssume, and start a video conference with three channels (one channel for each source). What is the utilization of the various links in the network caused by this video conference? What would be the link utilization if multiple unicast streams would be used instead of multicasting? (The bandwidth required by a channel originating from a source S is given by (S)). a) Shortest unicast paths: est. Next ost est. Next ost est. Next ost / 3 est. Next ost 2 / est. Next ost b) RPF tree for source (dotted links may be pruned): RPF tree for source (dotted links may be pruned):

4 RPF tree for source (dotted links may be pruned): c) Link load: Link RPF Load RPF Load (pruned) Unicast - () + () + () () + () + () () + 2() + () - () + () + () () + f) + () () + () + 2() - () + () () + () () () 0 0

5 Problem M.3: traffic shaping (10+10=20 points) a) host on a network is regulated by a token bucket. The token bucket is filled at a rate of 1M tokens/sec. It is initially filled to capacity with 2M tokens. It takes one token for a data packet to be released into the network. Now suppose that data comes in three 4M-packet bursts (i.e., each burst contains 4M packets). ursts occur at the beginning of the first, second, and third second, and each lasts for 800ms. How long does it take the host to release all the packets onto the network? (ssume that the first burst starts at t = 0.) b) The transmission schedule for a given flow lists for each second the number of packets sent between that time and the following second. Time [s] Packets Sent The flow must stay within the bounds of a token bucket filter. ssume the bucket is initially full. What bucket depth does the flow need for the token rates r = 2packets/sec and r = 4packets/sec? a) t the end of the first second (t = 1), 4M packets have come in. We started with 2M tokens and gained an additional 1M tokens in the second, so in the end, 1M packets are still left needing tokens (buffered). t t = 2, 4M packets have come in for a total of 5M packets. 1M of these packets are consumed by the generated 1M tokens, so 4M packets are buffered. t t = 3, 4M packets come in for a total of 8M packets. 1M are consumed by the generated 1M tokens, so 7M packets are left. t this point, every second 1M packets are consumed, so it takes t = = 10 seconds. b) r = 2packets/sec: uring the interval (0, 2) the flow sends 10 packets but receives 4 tokens. Thus the bucket size must be at least 6. Then during (2, 4) the flow sends one packet but receives 4 tokens. Thus, at time 4 the flow has saved 3 tokens in the bucket. uring interval (4, 5) the flow needs to send 6 packets; however, it has 3 tokens and will receive another 2, which only allows 5 packets to be sent. Thus, it needs another token, which means that the bucket size must start with at least 7 tokens. r = 4packets/sec: In a similar way, we see that a bucket of size 2 is sufficient.

6 Problem M.4: fair queuing and weighted fair queuing (10+10=20 points) onsider a router with three incoming interfaces,, and a single outgoing interface. The queues of the router are initially empty. The router received the following packets: Time Interface Size t 0 5 bytes t 1 3 bytes t 1 2 bytes t 2 2 bytes t 3 3 bytes t 4 1 bytes a) In which order are the packets processed if the router applies fair queuing? What are the computed finish times? b) In which order are the packets processed if the router applies weighted fair queuing with the weights w = 1, w = 2, and w = 1 for the interfaces,, and? a) t t 0, the interface is empty and there is only a single packet. So the first packet p1 is put straight into the interface. The other packets are getting queued. omputation of the finish times using fair queuing: p2 -> p3 -> p4 -> p5 -> p6 -> 10 The computed fair queuing finish times are therefore p3(04), p2(06), p4(07), p6(10), p5(11) and the packets will be dequeued in the order p1, p3, p2, p4, p6, and p5. b) Like above, we ignore the first packet and look at the weighted fair queue. With the given weights, packets from interface can ship two bytes per cycle. Or alternatively, we simply consider the packets from the interfaces and to have double size. This leads to the following: p2 -> p3 -> p4 -> p5 -> p6 -> The computed fair queuing finish times are therefore p2(06), p3(10), p4(14), p5(15), p6(16) and the packets will be dequeued in the order p1, p2, p3, p4, p5, p6.

7 Problem M.5: multimedia transport and signaling (3+4+3=10 points) riefly answer the following questions: a) The RTP message format contains an SSR identifier and optionally a number of SR identifiers. xplain the purpose of these identifiers and in which situations SR identifiers will be used. b) What is the purpose of the all-i and Seq SIP header fields? c) The parties involved in a multimedia session must agree on the codecs they want to use. Outline how SIP deals with the negotiation of supported codecs. a) The synchronization source (SSR) identifier identifies the source of a multimedia stream which is authoritative for the synchronization of this particular stream. ontributing source identifiers are added when a mixer mixes several streams (each with a separate SSR) into a single stream. b) The all-i field contains a globally unique identifier for a call, generated by the combination of a random string and the softphone s host name or IP address. The Seq field contains a sequence number which is incremented for each new request within a dialog. c) SIP messages use SP to describe the details of the audio/video streams used in a multimedia session. The caller announces the locally supported codecs in the SP body of the SIP INVIT message while the callee announces his set of supported codecs in the SIP 200 response message. The details can be found in RF 3264.

8 Problem M.6: stream control transmission protocol ( =10 points) Indicate which of the following statements are correct or incorrect by marking the appropriate boxes. For every correctly marked box, you will earn two points. For every incorrectly marked box, you will loose one point. Statements which are not marked or which are marked as true and false will be ignored. The minimum number of points you can achieve is zero. true false STP support arly ongestion Notification (N), a mechanism in which a router sends N IMP messages towards the source of a data stream in case the router determines that it soon experiences congestion. STP uses a four-way handshake in order to protect against SYN flooding attacks. very stream in an STP association maintains its own sequence number space. STP does not have a TIM-WIT state since the verification tag protects STP endpoints from accepting chunks from old connections. SPT maintains congestion control parameters for each destination address associated with a stream. true false STP support arly ongestion Notification (N), a mechanism in which a router sends N IMP messages towards the source of a data stream in case the router determines that it soon experiences congestion. STP uses a four-way handshake in order to protect against SYN flooding attacks. very stream in an STP association maintains its own sequence number space. STP does not have a TIM-WIT state since the verification tag protects STP endpoints from accepting chunks from old connections. SPT maintains congestion control parameters for each destination address associated with a stream.