6 - Network Layer Flashcards

1
Q

Network Layer Responsibility (3)

A

To get packet from source to destination

  • Source and dest functions
  • Store and forward functions
  • Network-wide Management functions
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2
Q

N.L Design Objectives (3)

A
  • independent of processor/communication topology
  • independent of the number, type and topology of the subnets
  • provide a uniform addr scheme for all hosts
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3
Q

Fixed Packet Size Adv (3)

A
  • Each node’s buffer size may be fixed
  • Simpler to prevent network congestion at dest node, since dest estimates the number of buffers to pre-locate
  • Simpler memory allocation schemes using ‘once-only’ allocation
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4
Q

N.L Contents (6)

A
  • Src and dest addr
  • Packet size
  • Message number
  • Control bits
  • Flow control info
  • Packet priority
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5
Q

Connection-based Service (Virtual Circuit) Analogy

A

Like a telephone conversation, where connection is opened and closed by both parties (session based service)

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6
Q

Conectionless (Datagram) Scheme Analogy

A

Like the postal service, with each packet carrying it src and dest addr

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7
Q

Routing Algorithms for VC and Datagram services

A
  • VC: route is chosen once per session

- Datagram: route is chosen once for each packet

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8
Q

Routing Alg Desirable Properties (6)

A
  • correctness
  • simplicity
  • robustness
  • stability
  • fairness
  • optimality
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9
Q

Robustness is significant (2)

A
  • difficult to change routing all software

- Network topologies often change hosts, IMPs and lines fail

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10
Q

Types of Routing Algorithm (2)

A
  • Non-adaptive (route is computed in advance)

- Adaptive (dynamic routing)

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11
Q

Flooding Algorithm Solutions (5)

A
  • Don’t retransmit packs on the link they arrived on
  • After hop count exceeds diameter of network, discard packet
  • ACKs only need to be transmitted on the link which the packet arrived on
  • Remember the link of minimum hop count
  • keep list of packets already seen and discard ones already seen
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12
Q

Distance Vector Routing

A

Keeps a table of best known distance to each destination, constantly updated

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13
Q

Count to Infinity Issue

A

If one router goes down, all the other routers record an infinite distance

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14
Q

Link State Routing (5)

A
  • Neighbour’s network addr
  • Measure delay to immediate neighbour
  • Construct link-state packets containing data learnt
  • Broadcast packet to all neighbours
  • Compute the shortest path to each router
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15
Q

Congestion Control

A

Concerned with ensuring that subnet can carry the offered traffic

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16
Q

Flow Control

A

Concerned with end-to-end control

17
Q

Congestion Metrics (4)

A
  • % of packets discarded for lack of buffer space
  • av router queue length
  • # of packets requiring retransmission
  • av packet delay
18
Q

Open Loop Control VC (2)

A
  • New VCs not created via an intermediate router low on memory
  • New VC may have to be created via another path
19
Q

Closed Loop Control VC (3)

A
  • Monitors local subnet for congestion
  • Paling info where corrective action may be taken
  • Adjustment of local op to correct problem
20
Q

Load Shedding

A

If not all demand can be met, some section is deliberately disadvantaged

21
Q

Load Shedding Constraint (3)

A
  • Must balance reasonable delay and user freedom
  • Maintain fair access to the network for all users
  • Respects rights of priority users
22
Q

Discarding Packet Strategies

A
  • consider priority
  • discard Data rather than ACKs
  • don’t discard packet that has travelled many hops
  • examine type of traffic being carried, e.g. Multimedia, file transfer