CS 6250- Computer Networks Exam Questions with Verified Solutions Latest Update

CS 6250- Computer Networks Exam Questions with Verified Solutions Latest Update DARPA Paper: Main Goal - Answers Multiplexed utilization of existing interconnected networks DARPA Paper: Secondary Goals - Answers 1. Survivability 2. Multiple types of service 3. Variety of networks 4. Distributed management 5. Cost effective 6. Ease of attachment 7. Accountability Fate Sharing (definition) - Answers It is acceptable to lose state information for an entity if that entity is lost. (i.e. If a router crashes, its routing tables can be lost) Fate Sharing (consequences) - Answers 1. Protects against any number of intermediate failures (vs. replication) 2. More trust placed in the host machine (vs. an architecture where the network ensures reliable delivery) DARPA Paper: Survivability Mechanisms - Answers Fate sharing, replication DARPA Paper: Datagram (usefulness) - Answers Eliminates the need for connection state within intermediate switching nodes. Provides a basic building block out of which a variety of types of service can be implemented. Represents the minimum network service assumption, which has permitted a wide variety of networks to be incorporated into various Internet realizations. DARPA Paper: Datagram (definition) - Answers The fundamental architectural feature of the Internet - the entity which is transported across the underlying networks. Important note: The role of the datagram is as a building block, and not as a service in itself. Network Service: Basic Assumptions - Answers The network can transport a packet or datagram of reasonable size, and it will be delivered with reasonable but not perfect reliability. The network must have some suitable form of addressing if it is more than a point-to-point link. Network Services: Explicitly Not Assumed - Answers 1. Reliable or sequenced delivery 2. Network level broadcast or multicast 3. Priority ranking of transmitted packets 4. Support for multiple types of service 5. Internal knowledge of failures, speeds, or delays Why Flow Control in TCP is Based on Byte Number (vs. packet number) - Answers 1. Permits the insertion of control information into the sequence space of the bytes. (later dropped) 2. Permits the TCP packet to be broken up into smaller packets if needed. (later moved to the IP layer) 3. Permits a number of small packets to be gathered together into one larger packet if retransmission of the data is necessary. (turned out to be critical) DARPA Paper: "Flow" / "Soft State" - Answers A building block to replace the datagram. It would identify a sequence of packets traveling from source to destination, without assuming any particular type of service. The type of service would be enforced by the end points,w hich would periodically send messages to ensure that the proper type of service was being associated with the flow. Narrow Waist - Answers Every internet device "speaks" IP - allows interconnection of existing networks while hiding underlying technology from applications. Internet Layers + Examples and Guarantees - Answers 1. Physical Layer 2. Link Layer (ethernet) Guarantees P2P connectivity. 3. Network Layer (IP) Guarantees end-to-end connectivity. 4. Transport Layer (TCP, UDP) Guarantees reliable transport and congestion control. 5. Application Layer (HTTP, SMTP) End to End Argument (definition) - Answers The intelligence required to implement an application should be placed at the endpoints. "Dumb network, intelligent endpoints" End to End Argument Violations - Answers NATs VPN tunnels TCP splitting - especially used when last hop is wireless Spam filters P2P systems Caches & in-network aggregation End to End Argument Paper: Reasoning - Answers Functions placed at low levels of a system may be redundant or of little value. Many functions can be completely and correctly implemented only with the knowledge and help of the application standing at the endpoints of the system. NAT (definition) - Answers Network Address Translation. Maps public IP addresses / port numbers to private IP addresses / port numbers. Why does NAT violate the end to end argument? - Answers Devices are not globally addressable, so other devices can't initiate inbound connections. ARP (definition) - Answers Address Reolution Protocol. ARP table maps IP addresses to MAC addresses ARP (process) - Answers 1. Host queries with IP address - broadcast. 2. Host with IP address responds with MAC address - unicast. 3. When host receives reply, adds to ARP table. 4. To send packets, host puts IP packet inside Ethernet frame using MAC address destination. Hubs vs. Switches - Answers Hub: Broadcast medium where all packets on the network are seen everywhere. Switch: Partitions LAN into separate broadcast domains / LAN segments. Frames bound for a host in the same segment aren't forwarded to other segments. Switch Table (definition) - Answers Maps destination MAC address to switch output ports Learning Switches (process) - Answers 1. Switch receives a packet with a destination address.