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Network Layer 2

Sam Wells Networks Degree Inglés 42 fichas

Fichas del conjunto (42)

  1. 1What are router queues?A queue in which packets are placed into to be processed
  2. 2What is congestion and when does it occur?Too many packets are in transit through a particular section of a subnet
    Occurs if the load on the network is greater than the capacity of the network
  3. 3What are the effects of congestion?- Long delays
    - Packet loss
  4. 4What layer does congestion occur in?Network Layer
  5. 5Name same methods of avoiding congestion- Traffic Shaping
    - Open-loop Congestion Control
    - Closed-loop Congestion Control
  6. 6What is traffic shaping and what are its benefits?A mechanism to control the rate of traffic in the network
    + Performance Guarantee
    + Latency Improvement
    + Inc. available bandwidth for certain packets (whilst delaying others)
  7. 7What are the two main types of traffic shaping algorithms?- Leaky Bucket
    - Token Bucket
  8. 8Leaky Bucket- Bucket with hole leaks at constant rate
    - Rate of filling from tap can vary although the leak remains constant
    i.e only take packet if not full, discard otherwise
  9. 9Token BucketTokens added/removed as space becomes available
    - Non-conformant packets: if packet is longer(bytes) than number of tokens discard, enqueue or send anyway (low priority)
  10. 10Packet scheduling with reference to large packet issue- Aggressive sender can hog capacity
    - Weighted fair queuing gives priority to certain types for data
  11. 11Types of open-loop congestion control- Re-transmission: packet re-trans. if sender thinks packet has been lost
    - Window: Selective repeat or go back to N
    - Acknowledgement: Sender waits for ACK from receiver, determines if sender needs to slow
    - Discarding: Used for media trans. discard less sensitive packets then congestion is likely
    - Admission: QoS mechanism in VC networks, deny est. connection due to congestion
  12. 12Types of closed-loop congestion control- Implicit Signalling: No comm. between congested node and source (e.g. no ACK)
    - Explicit Signalling: Congested node sends explicit congestion message (not choke)
    - Backward Signalling: Add a bit warning of congestion to packet travelling in opposite direction of congestion
    - Forward Signalling: Add a bit warning of congestion to packet travelling in same direction of the congestion
  13. 13Types of closed-loop congestion- Backpressure: Congested router stops receiving datagrams from immediate nodes
    - Choke Packet: Sent to source to notify of congestion
  14. 14What is load shedding?When a router becomes congested with packets, it discards them all with no consideration.
    Alternatively packets discarded based upon importance
  15. 15What type of routing do Virtual Circuits (VC) use?Session Routing
  16. 16What is the difference between adaptive and non-adaptive routing?- Adaptive: Routing tables are updated dynamically
    - Non-Adaptive: Hard wired routing tables (AKA Static Routing)
  17. 17What is the difference between direct and indirect delivery?- Direct: 1 packet/datagram from 1:1
    both hosts connected to each other via a physical network and same underlying physical system
    - Indirect: Opposite of direct, router used
  18. 18What are the main properties associated with routing?- Stability
    - Simplicity
    - Robustness
    - Fairness
    - Correctness
    - Efficiency
  19. 19What does the optimality principle state?“If router j is on an optimal path from router i to k, then the optimal path from j to k also falls along the same route”
  20. 20What is a sink tree?A tree with no loops that is also not unique
  21. 21Shortest path routing: What do we mean by shortest?- Number of hops
    - Geographical distance
    - Transmission delay
    - Bandwidth
  22. 22Name an example of a shortest path routing algorithmDijkstra's shortest-path
    SEE LECTURE 11 SLIDE 33
  23. 23What is flooding and how does it work?A static algorithm
    Every incoming packet is sent out to every machine except the one it was sent from
  24. 24What is selective flooding?Sending packets out on lines in the right direction
  25. 25Distance-Vector RoutingDynamic routing algorithm
    Least cost rout between any 2 nodes is route w/ minimum distance
    Each node has a vector (table) of shortest distance to each node
    Nodes share tables in order to calculate shortest distance
  26. 26What is the Count-to-Infinity problem?A informs B that it has a valid path, B cannot tell if it is on that path
    All routers will eventually end up on infinity paths
  27. 27What is Link-State Routing and how does it work?Algorithm to replace distance-vector routing
    Each node periodically sends info on link states by flooding network w/ Link State Packets (LSP)
    Nodes receive LSPs and use Dijkstra's algorithm to build and maintain a routing table
  28. 285 Steps of Link-Stage Routing1. Discover neighbouring routers and network addresses
    2. Measure line cost/delay to neighbouring routers
    3. Build link state packet with new info (LSP's)
    4. Distribute and receive LSPs
    5. Compute new routes to every router
  29. 29Link-Stage Routing: Step 1- Send HELLO packet to each router
    - Receiving routers respond w/ their name
  30. 30Link-Stage Routing: Step 2- Cost inversely prop. to bandwidth of link between 2 routers
    - Geog. spread may introduce requirement for delay to be added to cost
  31. 31Link-Stage Routing: Step 3- Each router build a packet containing data available about link costs
  32. 32Link-Stage Routing: Step 4- Distribute packets using flooding
    - Each packet has a seq. No.
    - Routers check numbers against what they have already seen
    - Age field of LSP decremented every second
  33. 33Link-Stage Routing: Step 5- Each router uses Dijkstra's algorithm to compute new table
    - Each router now has a network graph representing every link
  34. 34A single network is known as an ________________________Autonomous System (AS)
  35. 35A routing algorithm that routes within a single AS is known as a _______________________________Interior Gateway Protocol (intradomain routing)
  36. 36A protocol routing between many autonomous systems is an _______________________________Exterior Gateway Protocol (Interdomain Routing)
  37. 37Issues with Intradomain Protocols- Distance vector subject to instability
    - Link state uses large quantities of resources and can create traffic
  38. 38Name a inter domain routing solutionPath Vector
  39. 39What is Path-Vector Routing?- Similar to distance-vector routing
    - Considers routes a packet takes based upon where source wants to avoid
    - Best route determined by source and its policy
    - Used to route packets between ISPs
  40. 40What does RIP stand for and what is it?Routing Information Protocol
    Implementation of distance-vector routing using UDP
  41. 41What does OSPF stand for and what is it?Open Shortest Path First
    Interior Gateway Routing Protocol
    Based on link-state algorithm
    Supports 3 types of networks:
    1. point 2 point
    2. Multiaccess w/ broadcasting (LAN)
    3. Multiaccess w/o broadcasting (LAN)
  42. 42What does BGP stand for and what is it?Border Gateway Protocol
    - Based on path-vector algorithm
    - each note holds a cost to each dest. and previous used routes
    - Paths periodically updated or when routing changes occur

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Descripción
Lecture 11
Creado por
Sam Wells · may 2016
Actualizado
9 may 2016
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