6 - Network Layer

Question 1

Network Layer Responsibility (3)

Answer 1

To get packet from source to destination

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

Question 2

N.L Design Objectives (3)

Answer 2

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

Question 3

Fixed Packet Size Adv (3)

Answer 3

• 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

Question 4

N.L Contents (6)

Answer 4

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

Question 5

Connection-based Service (Virtual Circuit) Analogy

Answer 5

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

Question 6

Conectionless (Datagram) Scheme Analogy

Answer 6

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

Question 7

Routing Algorithms for VC and Datagram services

Answer 7

• VC: route is chosen once per session

- Datagram: route is chosen once for each packet

Question 8

Routing Alg Desirable Properties (6)

Answer 8

• correctness
• simplicity
• robustness
• stability
• fairness
• optimality

Question 9

Robustness is significant (2)

Answer 9

• difficult to change routing all software

- Network topologies often change hosts, IMPs and lines fail

Question 10

Types of Routing Algorithm (2)

Answer 10

• Non-adaptive (route is computed in advance)

- Adaptive (dynamic routing)

Question 11

Flooding Algorithm Solutions (5)

Answer 11

• 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

Question 12

Distance Vector Routing

Answer 12

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

Question 13

Count to Infinity Issue

Answer 13

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

Question 14

Link State Routing (5)

Answer 14

• 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

Question 15

Congestion Control

Answer 15

Concerned with ensuring that subnet can carry the offered traffic

Question 16

Flow Control

Answer 16

Concerned with end-to-end control

Question 17

Congestion Metrics (4)

Answer 17

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

Question 18

Open Loop Control VC (2)

Answer 18

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

Question 19

Closed Loop Control VC (3)

Answer 19

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

Question 20

Load Shedding

Answer 20

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

Question 21

Load Shedding Constraint (3)

Answer 21

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

Question 22

Discarding Packet Strategies

Answer 22

• 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