Network Layer Flashcards
What is the functionality of the network layer?
Delivery transport layer segment from sending to receiving host
What is routing?
Determine route taken by packets from source to destination
What is forwarding?
Moving packets from routers input link to appropriate output link
What is the data plane?
Local, per router function
Determines how datagrams arriving at input are forwarded to output
What is the control plane?
Network wide logic
Determines how datagram is routed among routers along end to end path from source to destination host
What is longest prefix matching?
When looking for forwarding table entry for given destination address, use longest address prefix that matches destination address
What is input port queuing?
If switch fabric is slower than input ports combined, queueing occurs at input queues
What is head of line blocking?
Queued datagram at front stops others moving forward
How does buffer management work?
Drop certain packets when buffer is full
Tail drop = drop arriving packet
Priority drop = drop on priority basis
What is weighted fair queueing?
Generalised round robin
Minimum bandwidth guarantee per class = wi / sum i(wj)
i = class, j = weight
What is IP addressing?
32 bit identifier
Connection between host router and physical link
What are the 3 types of packet scheduling?
FCFS = transmitted in order or arrival to output port Priority based = arriving traffic queued by class and send packet from highest priority queue with buffered packets Round robin = arriving traffic is queued by class and cyclically send complete packet from each class in turn
What are subnets?
Device interfaces that can physically reach each other without passing through router
How does DCHP work?
Host broadcasts discover message
DHCP server responds with DHCP offer message
Host requests IP address with DHCP request message
DHCP server sends back IP with DHCP ack message
What is NAT (network address translation)?
All devices in local network have address in private IP space that can only be used in local network
When do route oscillations occur?
When link costs depend on traffic volume
How does the distance vector algorithm work?
- Each node waits for change in local link cost or message from neighbour
- Recompute dv estimates using dv received from neighbour
- If dv to any destination has changed, notify neighbours
How do distance vector messages work?
- x receives new dv message from neighbour
- Updates own dv
- Estimated Dx(y) converges to actual least cost Dx(y)
What is the complexity of the distance vector algorithm?
Message: varies
Speed of convergence: varies
What happens to errors in the distance vector algorithm?
Propagate through net work
What does Dijkstras link state routing algorithm do?
Computes least cost nodes from one node to all the others
What is ICMP?
Internet control message protocol
Used by hosts and routers to communicate network level info
How are ICMP messages sent?
Carried in IP datagrams
Type, code, and 1st 8 bytes of IP datagram causing error
How does ICMP work?
- Source sends UDP segment to datagram
- Datagram in nth set arrives to nth router
- Router discards datagram and sends source ICMP message
- Record RTTs when message arrives at source
- Stops when UDP segment arrives at destination host
What is interconnected AS?
Routing among different AS
Gateways perform inter-domain routing
Intra-AS determine entries within AS
Intra and inter determine external entries
What is an example of inter-AS?
BGP routing
How does BGP routing work?
Allows AS to eBGP and iBGP
- 2 BGP routers exchange BGP messages over TCP
- When AS3 gateway 3a advertises path AS3,X to AS2 gateway 2a
- AS3 promises to forward datagrams to x
What is eBGP?
Obtain subnet reachability info from neighbour AS
What is iBGP?
Propagate reachability info to AS-internal routers
What is intra-connected AS?
Routing within same AS
What is an example of intra-AS?
OSPF routing
How does OSPF routing work?
- Each router floors OSPF link-state advertisements to all routers in AS
- Each router uses dijkstras to compute forwarding table
- 2 level hierarchy to flood only local area or backbone
What are the differences between inter and intra AS for policy?
Inter - admin wants control of who/how routed
Intra - single admin so lesser concern
What are the differences between inter and Intra AS for scale?
Hierarchical routing reduces table size
Reduces update traffic
What are the differences between inter and Intra AS for performance?
Inter - policy over performance
Intra - performance
