use Dijkstra's shortest-route set of rules to compute the shortest route from E to all community nodes. Show your operating in a table: every column indicating a vacation spot node, every row indicating an generation of the set of rules. [10 marks] (b) In 2008 an ISP changed into suggested to have hijacked visitors for YouTube causing site visitors for YouTube to be diverted into the ISP's community.

Despite YouTube the usage of handiest three addresses, every browser's YouTube URL requests are in the long run routed to the nearest of over a dozen facts-centres Google operates global-wide. (i) Describe concepts from the route that make this viable. [2 marks] (ii) State the smallest marketed netblock that could pick out all YouTube addresses. [1 mark] (iii) In an try to solve the problem, YouTube advertised the netblock 208.Sixty five.153.0/24, however this was the same netblock as advertised through the rogue ISP. Why could this not remedy the problem? [2 marks] (iv) YouTube advertised two smaller netblocks, each one 1/2 of 208.65.153.Zero/24. Why should this now paintings? [2 marks] (v) BGP networks may additionally optionally filter internet that during (b)(iii). Estimate the size of the netblock clear out. [1 mark] (vi) Why does BGP put in force such filtering? [2 marks] 7 (TURN OVER) CST.2015.Five.Eight 6 Computer Networking (a) An older domestic-community router has an upload bandwidth of 1Mbit/s to the Internet, and a 100kbyte first-in first-out (FIFO) buffer for packets looking forward to transmission. Packets have a maximum transmission unit (MTU) of 1500 bytes. (i) If the buffer is completely full, how lengthy does it take the router to transmit all of the bytes in the buffer? [2 marks] (ii) Suppose the router supports two FIFO queues, one excessive-priority for interactive programs (like Voice over IP) and the alternative lower-priority for all last site visitors. If a VoIP packet arrives while the queue for interactive applications is empty, what is the most time before the router starts offevolved transmitting the VoIP packet? (Assume that the router does now not preempt any ongoing packet transmission.)

) imply an average range of values in a 1Gbit/s neighborhood place community with hyperlink-period less than 500m. State your assumptions during. [4 3 marks] (c) A lecturer feedback that "centralised multiplexing" offers capability profits in efficiency over non-centralised multiplexing. Give two motives why this could be so. In every, nation sincerely what characteristic ought to be centralised to obtain these profits. [4 marks] eight CST.2015.5.Nine 7 Concurrent and Distributed Systems (a) (i) Name the four essential conditions for impasse. [2 marks] (ii) Which of these situations is often precluded in running-device kernel designs if you want to prevent deadlocks and why? [2 marks] (b) Deadlocks are not limited to locks; cycles of waiting on circumstance variables also can lead to the "lethal embrace". (i) Explain why it might be more difficult to debug deadlocks regarding condition variables than those truly concerning locks. [2 marks] (ii) Briefly describe a circumstance-variable API change that might allow this problem to be solved in a few cases; explain why it can't always help. [2 marks] (c) FreeBSD's WITNESS function checks statically described and dynamically determined lock orders. Each time a lock is received, any formerly undiscovered graph edges concerning lock kinds presently held by the thread and the newly received lock type can be added to the graph. Cycle detection is achieved, and debug statistics is outlined if a previously unreported cycle is observed. (i) Describe a commonplace code shape wherein programmers are possibly in order to outline a static order between lock types. [2 marks] (ii) Describe a commonplace case in which programmers are possibly to rely as an alternative on dynamic discovery of an order among lock kinds. [2 marks] (iii) Unlike the impasse-detection set of rules presented in lecture, the WITNESS set of rules does now not dispose of edges whilst locks are launched. Explain why WITNESS's behaviour might be extra useful in exercise. [2 marks] (iv) WITNESS is situation to fake positives: warnings may be emitted due to legitimate cycles despite the fact that, with the aid of layout, the cycle could by no means cause an real impasse. Describe a scenario in which this could get up, and explain why deadlock may want to by no means occur. [3 marks] (v) WITNESS, as written, is supposed to be used with mutexes and other lock sorts presenting mutual exclusion. A developer might navely expand WITNESS to aid reader-creator locks (rwlock) by means of introducing graph edges for both study and write acquires because it does for mutex acquires. Explain why this could no longer usually lead to the favored end result. [3 marks] 9 (TURN OVER) CST.2015.Five.10 8 Concurrent and Distributed Systems (a) (i) Define the time period functionality. [2 marks] (ii) What fields should RBAC-based totally ACL entries constantly include? [2 marks] (b) Network-Attached Secure Disks (NASD) utilise report managers and block servers. File-manager RPCs change an authorised consumer ID, password, and object ID for a keyed cryptographic capability granting block get entry to: f(okay, ObjID, rights). (i) Describe the effects of a user learning the cost of key ok. [2 marks] (ii) Alice obtains a functionality for object Oi . Bob then issues an RPC to the report manager revoking Alice's access to Oi . Describe what happens while Alice performs her next block-server read on Oi . [2 marks] (iii) Explain why it is probably acceptable, from a protection angle, to add a timeout subject t, protected by means of the keyed hash, to the capability. [2 marks] (iv) Developers make bigger NASD to assist Quorum-replicated block servers. What new failure mode may stand up at some point of a Quorum block write, relative to unmodified NASD talents, in adding functionality timeouts? [2 marks] (c) The Andrew File System (AFS) is authenticated and encrypted using Kerberos; ACLs expressing positive and terrible rights for users and corporations. Multiuser AFS clients (e.G., UNIX servers) construct a comfortable RPC connection for each neighborhood person, authenticated with their Kerberos price tag, and trouble RPCs (e.G., report read) on their behalf simplest thru their own connection. If no suitable Kerberos price tag is to be had (e.G., the ticket has expired, the consumer has destroyed their price ticket, or a activity is strolling unattended), then an insecure connection is used rather. (i) The group machine:anyuser holds the union of unauthenticated (anonymous) customers and all authenticated customers. Explain why an ACL granting examine get right of entry to to device:anyuser through a advantageous access, but denying read get entry to to user rnw via an overriding negative entry, may prove elaborate. [2 marks] (ii) Describe the effects to AFS authentication and authorisation of a malicious nearby person gaining root get right of entry to on a multiuser purchaser. [2 marks] (iii) An AFS consumer makes use of the unauthenticated Network Time Protocol (NTP) to synchronise its clock with the AFS server. Attacker Mallory is capable of investigate, drop, and insert packets between the AFS consumer and server (e.G., by means of controlling a network switch). Describe an assault that permits Mallory to inject malicious content material into the patron's AFS cache, however that does not allow Mallory to jot down content at once to the AFS server.

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