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Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 1 ECSE-6660: Broadband Networks Exam 2: SOLUTIONS Time: 75 min (strictly enforced) Points: 50 YOUR.

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Presentation on theme: "Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 1 ECSE-6660: Broadband Networks Exam 2: SOLUTIONS Time: 75 min (strictly enforced) Points: 50 YOUR."— Presentation transcript:

1 Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 1 ECSE-6660: Broadband Networks Exam 2: SOLUTIONS Time: 75 min (strictly enforced) Points: 50 YOUR NAME: Be brief, but DO NOT omit necessary detail {Note: Simply copying text directly from the slides or notes will not earn (partial) credit. Brief, clear and consistent explanation will.}

2 Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 2 I. Quick True/False Questions: (8 points) Tick True or False (1 pt each). Add a 1 line explanation JUSTIFYING your choice (1 pt each) T F  x  PNNI is a distance-vector routing protocol used by each ATM cell PNNI is a link state protocol, and is used only in the control plane during signaling.  x  MPLS protocol is the result of the IP-over-ATM overlay mapping MPLS is a hybrid of the IP (control plane) and ATM (data-plane) technologies; an overlay would not violate the integrity of each layer.  x  IP forwarding in high-speed routers is done using hash-based address lookup IP forwarding uses the longest prefix match as its lookup method.  x  A signaled protocol (like ATM or MPLS) uses a global consistency condition (GCC) to ensure routing convergence ATM and MPLS utilize a VC-based connection-oriented signaled approach (I.e. by maping global path specifications to local labels) and hence do not need GCC.

3 Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 3 II. Below, you are given a true or false statement and asked a follow up question. 1. [4 pts] False statement: Virtual Tributary in SONET is the same as Concatenated mode Explain how multiplexing is done in SONET, featuring the differences between the concepts of “virtual tributaries” and concatenated mode Virtual Tributaries are used for multiplexing lower bandwidth pipes (T1, T3 etc) while the concatenated mode is for high-speed packet streams. In the concatenated mode packet stream cannot be demultiplexed into lower-speed streams. In case of VTs the payload is floating inside the STS-1 SPE and pointers are used to point to it.

4 Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 4 2. (14 pts) True statement: IP-over-ATM internetworking ran into complexities both in the control-plane and data-plane mappings. (6 pts) Discuss what were the primary IP-over-ATM and Ethernet-over-ATM internetworking approaches? What were the fundamental differences between them? (2 pts) What were the unresolved data-plane mapping issues? Explain briefly. (3 pts) What were the unresolved control-plane mapping issues? Explain briefly. (3 pts) How did IP switching and MPLS provide a solution and/or side-step these internetworking issues ? In order to define solutions for IP-over-ATM schemes were proposed to the following: Address Resolution  Classical IP over ATM( RFC 1577), NHRP and MPOA Formatting  RFC 1483 The primary Ethernet-over-ATM internetworking approach was the LAN Emulation (LANE). It differed from the IP-over-ATM approaches as the mapping to ATM address was done from the MAC (Ethernet) address rather than from the IP address. In the case of IP, because of issues with the control-plane mapping, the size of subnets had to be limited, and hence the notion of LIS and routing across LIS’es was introduced. Unresolved data-plane mapping issues: Complexity introduced by the address resolution methods (server: single point of failure, lack of cut-through forwarding across LIS’es), scalability issues due to speeds/infrastructure/packet formatting mismatches and overheads introduced as a result of mappings were some of the unresolved data-plane issues.

5 Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 5 Unresolved control-plane mapping issues: ATM being non-broadcast needs to have a full mesh structure (NBMA model) to support IP broadcast services and OSPF routing. The alternative point-to-multipoint model violated some of the IP subnet semantics (eg: direct reachability within the same subnet). Both these models added complexity. Similar issues arose in mapping BGP-4 to ATM. MPLS decouples the data and control planes allowing IP routing to work with ATM switching using labels. MPLS also allows flexibility in the choice of control plane algorithms (eg: constraint-based routing) and protocols (eg: RSVP or LDP).

6 Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 6 II. [12 pts] Traffic Engineering: Explain (4 pts) What are the differences between the goals of regular routing and traffic engineering? (4 pts) WHY does traffic engineering benefit from a signaled approach (eg: ATM or MPLS)? Why do current connectionless routing protocols fall short of providing sophisticated traffic engineering features. (4 pts) How does the BANANAS overcome some of these difficulties in a connectionless routing context? What are the features of MPLS it DOES NOT provide? Regular routing is concerned with finding a path to the destination (usually the shortest path or best-effort path available) whereas TE deals with the issue of performance optimization of operational IP networks. For example a TE concern would include load balancing: to map traffic in a balanced manner on links of the network. In a signaled approach the resources are reserved during the connection establishment phase and thus the paths are known explicitly making it easy to use TE. Connectionless routing protocols TODAY find only a SINGLE path. Therefore, if they want to re-map traffic, they will also need to incur routing dynamics and control traffic to re-route the paths (and achieve the TE goal in an INDIRECT manner)

7 Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 7 BANANAS implements TE by using Explicit Source Routing to discover and choose explicit paths to the destination within the context of connectionless routing. It does not provide resource reservation (QoS features) like MPLS. It also does not provide sophisticated multi-level tunneling mechanisms.

8 Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 8 III. [12 pts] High-Speed Routing: a) The key parts of high-speed routers are the lookup algorithm, switching fabric and buffering/scheduling (QoS support). Explain what are the central issues in each case, and how modern routers are able to meet the design requirements. (9 pts) b) How does design of telephone switch fabrics differ from that of packet switches ? (3 pts) Issues in Lookup Algorithm: The lookup speed is a major issue in case of lookup algorithms. Modern routers usually use high speed memories and combination of fast access data structures (tries, multi-way tries, etc.) for faster IP-prefix matching and address lookup. Issues in Switching Fabric: The main issues are connectivity, blocking and speed of switching. A combination of multi-stage switches, shared memory architectures and faster CPUs are used to address these issues. Issues in QoS Support: Large buffer requirements and complex scheduling are the major issues in QoS support. Input/Output queuing is used to overcome the problem of blocking and buffer misuse. Classification is also a complex problem (more complex than IP lookup). The telephone switch fabrics eliminate the need for lookups since the path is set at the time of connection establishment and the next-hop is decided purely from timing information (I.e. position of time-slot), and therefore there is no need to lookup in a table.

9 Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 9


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