6 - Network Layer Flashcards
Network Layer Responsibility (3)
To get packet from source to destination
- Source and dest functions
- Store and forward functions
- Network-wide Management functions
N.L Design Objectives (3)
- independent of processor/communication topology
- independent of the number, type and topology of the subnets
- provide a uniform addr scheme for all hosts
Fixed Packet Size Adv (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
N.L Contents (6)
- Src and dest addr
- Packet size
- Message number
- Control bits
- Flow control info
- Packet priority
Connection-based Service (Virtual Circuit) Analogy
Like a telephone conversation, where connection is opened and closed by both parties (session based service)
Conectionless (Datagram) Scheme Analogy
Like the postal service, with each packet carrying it src and dest addr
Routing Algorithms for VC and Datagram services
- VC: route is chosen once per session
- Datagram: route is chosen once for each packet
Routing Alg Desirable Properties (6)
- correctness
- simplicity
- robustness
- stability
- fairness
- optimality
Robustness is significant (2)
- difficult to change routing all software
- Network topologies often change hosts, IMPs and lines fail
Types of Routing Algorithm (2)
- Non-adaptive (route is computed in advance)
- Adaptive (dynamic routing)
Flooding Algorithm Solutions (5)
- 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
Distance Vector Routing
Keeps a table of best known distance to each destination, constantly updated
Count to Infinity Issue
If one router goes down, all the other routers record an infinite distance
Link State Routing (5)
- 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
Congestion Control
Concerned with ensuring that subnet can carry the offered traffic
Flow Control
Concerned with end-to-end control
Congestion Metrics (4)
- % of packets discarded for lack of buffer space
- av router queue length
- # of packets requiring retransmission
- av packet delay
Open Loop Control VC (2)
- New VCs not created via an intermediate router low on memory
- New VC may have to be created via another path
Closed Loop Control VC (3)
- Monitors local subnet for congestion
- Paling info where corrective action may be taken
- Adjustment of local op to correct problem
Load Shedding
If not all demand can be met, some section is deliberately disadvantaged
Load Shedding Constraint (3)
- Must balance reasonable delay and user freedom
- Maintain fair access to the network for all users
- Respects rights of priority users
Discarding Packet Strategies
- 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
