Network Layer Flashcards
What are the network layer services?
- move packets from origin to destination
- avoid congestion
- internetworking
- lowest layer to deal with end to end issues
- must know topology to choose paths
What is congestion control?
- under heavy traffic performance will collapse
- all resources involved to make sure network can carry offered traffic
What is flow control?
Origin to destination point to point control
- fast senders shouldn’t overwhelm slow receivers
What is a virtual circuit?
connection oriented
routing decision only once
nodes know where to forward packets
What is a datagram?
connectionless service
no route in advance
datagrams follow different routes
more work but more robust and copes with congestion
What are the virtual circuit signalling protocols?
4 way handshake
1. initiate call
2. incoming call
3. accept call
4. call connected
What does a routing algorithm do?
determines end to end path through network
What does a forwarding table do?
determines local forwarding at particular router
What does the control plane do?
how datagram routed in end to end path from sender to destination
What does the data plane do?
how datagrams arriving to router’s input link is forwarded to routers output link
What is an example of a forwarding policy and how does it work?
weighted fair queueing
- generalised round robin
- each traffic class gets weighted amount of service in each cycle
How does datagram fragmentation work?
break down datagrams into smaller ones
reassemble at destination
What is the payload?
part actually transferred minus the header
What is MTU?
maximum transfer unit
- keep chopping datagram into pieces small enough to fit the MTU
What is CIDR?
classless interdomain routing
- subnet portion of address of arbitrary length
- format: a.b.c.d/x
- x is no of bits in subnet portion of address
How do subnets work in IP addressing?
all the things in the subnet need to know which network they’re connected to
What 3 things does host need to connect to internet in DHCP?
- IP address
- subnet mask
- address of nearby router
How does DHCP work?
each time user moves or relocated the 3 elements required must be reconfigured
What is required for DHCP?
- IP address
- address of first-hop router for client
- name and IP address of DNS server
- network mask
What is the difference between IPv4 and IPv6?
shorter header to avoid multiple fragmentation
What is tunneling?
Can have an IPv4 tunnel connecting IPv6 routers
What is a non-adaptive routing algorithm?
no consideration to traffic or topology changes
What are the 4 characteristics of an adaptive routing algorithm?
DRIC
1. Distributed: global and local info
2. Reacts to changes
3. Isolated: local info
4.Centralized: global info
What is the objective of a least-cost path algorithm?
find minimum length path from one origin to destination
What is a distance vector decentralised routing algorithm?
- calculates least cost in iterative, distributed manner
- each node begins with only knowledge of cost of attached links
- exchanges info with nieghbours
- gradually calculates least cost
What is the distance vector algorithm?
each node:
- wait for change in local link cost c(x,v) or message from neighbour
- update D_x(y) estimates
- if D_x(y) changes then notify neighbours
What is a link state global routing algorithm?
- calculate least cost path using complete, global knowledge about network
- mechanism to obtain info
- calculations performed in one site or replicated at multiple
What is Dijkstra’s algorithm an example of?
link-state algorithm
How does Dijkstra’s work?
initialise:
- N’ = {u}
- for all nodes v adjacent to u
- assign D(v) = c(u,v)
- otherwise D(v) = infinity
Do steps until all w inside N’:
1. find w not in N’ so D(w) is min
2. add w to N’
3. update D(v) for all v adjacent to w and not in N’
4. D(v) = min{D(v), D(w) + c(w,v)}
Compare link state and distance vector algorithms for message complexity
LS: n nodes, E links, O(nE) messages sent
DV: exchange between neighbours only
Compare link state and distance vector algorithms for speed of convergence
LS: O(n^2) algorithm required O(nE) messages
DV: convergence time varies
Compare link state and distance vector algorithms for robustness
LS:
- node advertise incorrect link cost
- node computes only own table
DV:
- node advertise path cost
- table used by others
- error propagates through network
What is an autonomous system?
- owned by one administration
- internal and external routing protocols
What are the 2 internet routing protocols?
interior gateway protocol
exterior gateway protocol
What is interior gateway protocol (IGP)?
relies on IP addresses to construct path
- routing info protocol (RIP)
- open shortest path first (OSPF)
What is exterior gateway protocol (EGP)?
relies on AS numbers to construct AS paths
- border gate protocol (BGPv4)
How does RIP work?
- RIP runs on top of UDP
- distance vector
- in RIP each router learns from neighbours distance to each dest
- metric for computation of path is no of hops
- router sends update message to neighbours every 30 secs
- mechanisms to reduce routing loops
How does OSPF work?
- runs over IP
- link state
- each router learns complete network topology
- routers monitor cost of link to each neighbour
- floods link state info to other routers on network
- each router builds identical topology
How does BGPv4 work?
- TCP connections
- path vector
- de-facto inter-domain routing protocol
- based on classless address prefixes and policy based routing
- mechanisms for address aggregation
- routers keep global view of internet in routing info bases (RIB)
- speaker exchange network reachability with other adjacent speakers (sequence of ASs that packets traverse to reach dest)
What is network management?
- deployment, integration and coordination of hardware, software and human elements
- monitor, test poll, configure, analyse, evaluate, control network and element resources
- to meet real time operational performance and QoS requirements
- at sustainable costs
What is the network management framework?
- managing server and operations personnel
- managed device equipment
- managed objects: pieces of hardware and configuration parameters
- management info base
- network management agent
- network management protocol needed between MS and MDs
What is the simple network management protocol (SNMP)?
- application layer protocol
- conveys management control and info between managing server and agent
- request response mode
- lies on top of UDP
What is the internet control message protocol (ICMP)?
- used by hosts and routers to communicate network layer info
- lies on top of IP
- ICMP messages carried in IP datagrams
- host de-multiplexes datagram to ICMP
What is Little’s theorem?
crowded systems are associates with long customer delays
Little’s theorem formula
N = lamda T
When is mean departure rate equal to mean arrival rate?
at equilibrium
if and only if traffic load is less than maximum service rate
What is Jackson’s theorem?
in a network of queues each node/link behaves as an independent M/M/1 or M/M/n with Poisson flows
How do we evaluate routing algorithms?
quantify traffic congestion in terms of stats of arrival process on network of queues
- measure congestion by average traffic carried or packet delay
What is the quasi-static assumption?
origin-destination traffic process is stationary over time
What is the fast settling assumption?
transients in the flow F_ij due to changes in routing are negligible
What is the synchronous update assumption?
all link rates F_ij are measured, received and updated simultaneously
What 3 assumptions are made for optimal routing problems?
- quasi-static
- fast settling
- synchronous update
