Routing Flashcards
What are the two main types of data transfer?
Packet switching
Circuit switching
What are the advantages and disadvantages of circuit switching?
Pro: dedicated connection between machines, guaranteed performance
Con: Expensive
What are the pros and cons of packet switching?
Pros: good utilisation of available bandwidth
Cons: may have poor performance at busy times, packets may have to queue
What are the main considerations of routing?
Minimising packet delays
Minimising hop count
Maximising available bandwidth
minimising financial cost
What are the two main approaches to routing?
Distance vector protocols
Link state protocols
Describe distance vector protocols
Neighbours exchange lists of distances to known destinations
Best next hop is determined at each node
Describe link state protocols
Link state information is flooded to all routers in the network
Routers have complete topology information
Route is calculated based on desired criteria
Name 5 alternative routing approaches
Flooding
Deflection routing ('hot potato')
Source routing
Gradient descent
RPLDescribe flooding and state an advantage and disadvantage
Packets are flooded to all nodes
Pro: good for survivability of the packet
Con: exponential growth of traffic
Describe deflection routing and name an advantage and disadvantage
Switches maintain multiple paths to the destination, if the preferred path is congested an alternative can be used
Pro: switches have reduced buffer size
Con: time to delivery and packet reordering have greater variability
Describe source routing and state an advantage and disadvantage
Sender specifies a complete route to the destination. Nodes strip off their identifier on receipt and forward to the next node in the path
Pros: less load on intermediate nodes, can avoid particular hosts
Cons: source must know the route to all hosts, link failures cause disruption
Describe gradient descent and state an advantage
Gradient is built around a single sink node, nodes can only pass messages to lower tiers
Pros: adds path diversity, allows energy management using elevation
What does MANET stand for?
Mobile Ad-Hoc Networks
What are the advantages of a MANET?
Ease of deployment
Speed of deployment
Decreased infrastructure dependence
What are the variants of MANET?
Fully symmetrical
Asymmetric
Describe dynamic source routing (DSR)
The network is flooded with request messages
When the destination receives the request it sends a route reply (RREP)
The RREP is sent back on the reversed route the request was sent on
When the source node receives the RREP it caches the route and the data is transmitted
What are the disadvantages of route caching? (DSR)
Stale caches can negatively affect performance
Host mobility may become stale
Several routes may be tried before a good route is found
What are the advantages of DSR?
Routes maintained only between communicating nodes
Route caching may reduce discovery overheads
What are the disadvantages of DSR?
Packets grow with route length
Flood requests may not reach all nodes
What is the difference between ad-hoc on demand distance vector (AODV) routing and dynamic source routing (DSR)?
DSR stores the route in the packet header
AODV maintains routing tables at the nodes
How does AODV work?
The route request is forwarded
On forwarding the node sets up a temporary reverse route to the source (AODV assumes bi-directional links)
When the destination node receives the request, it sends an RREP back along the temporary reverse path set up by the request
Describe route replies (AODV)
Intermediate nodes may reply with routes
The likelihood of a node issuing a RREP is not as high as in DSR
Describe timeouts (AODV)
Routing tables maintaining reverse paths are wiped after a timeout period
Forward paths are purged if not used for a set timeout interval
How are link failures reported in AODV?
When a next-hop link is broken all active neighbours are informed
Link failures are propagated using route error (RERR) messages that also update destination sequence numbers
How are route errors handled in AODV?
If a node is unable to forward a packet it generates an RERR message
When the source node receives the RERR it initiates a new route discovery
How are link failures detected in AODV?
Neighbouring nodes periodically send each other short messages to indicate active neighbour presence
Absence of this message is an indicator of link failure
