quartz/content/notes/20-network-layer-control-plane.md

title	aliases	tags	sr-due	sr-interval	sr-ease
20-network-layer-control-plane		cosc203 lecture	2022-10-21	3	250

per-router control vs SDN

per router

• each router runs a routing algorithm
• router communicate with each other
• and creates a forwarding table

SDN software defined networking

a remote controller computes, and install fowarding tables in routers

• much faster: less work for each router
• separate control and data plane

routing algorithms

• goal is to determine "good" paths from sending to recieving host through network of routers
• path: sequence of routers
• "good": least "cost", "fastest", "least congested"

graph abstraction

• nodes: set of routers
• edges: set of links

c_{a,b} cost the link directly connecting a and b. if there is no link the cost is ♾

algorithm classification

global: all routers have complete toplogy, link cost into

• link state algotihms

decentralized: interative process of computation, exchange of into with neighbors

• routers only know link costs to attacked neighbors
• "distance vecotr algorithms

static: routes change very slowly over time

dynamic: routes change more quickly

• periodic updates or in response to link cost changes

notations

link state routing

centralized: network topology, link costs are known to all nodes

• each node gathers informatin on each link to its neighbors
• build link state packets and flood to all other nodes

computes least costs paths from one node to all other nodes

• gives a fowarding table for that node

iterative: after k interations, know least cost path to k destinations

Dijkstra's algorithm

foward search algorithm

///initialization
N` = {u}
for all nodes v
	if v adjacent to u
		then D(v) = Cuv
	else D(v) = infinity
	
//loop
loop until all nodes in N`
	find W not in N` such that D(w) is a min
	add w to N`
	update D(v) for all v adjacent to w and not in N`:
		D(v) = min(D(v), D(w)+Cwv)
	//new least-path-cost to v is either old least-cost-path to v or known least-path-cost to w plus direct-cost from w to v

algorithm complexity:

• n nodes
• for each of n interation: need to check all nodes, w, not in N
• n(n+1)/2 comparisons: O(n²)
• more efficient implementations possible: O(nlogn)

message complexity

• each router must broadcast its link state information to other n routers
• efficient broadcast algorithms: O(n) link crossings to dissenimate a bradcast message from one source
• each router's message crosses O(n) links: overall message compexity O(n²)

Distance vector routing

bellman-ford equation

backward search algorithm D_{x}(y) = min_{v} \{c_{x, v} + D_{v}(y)\}

distance vector algorithm

each node:

• wait for change in local link cost or msg from neighbor

• recompute DV estimate using DV recieved from neighbor

• if DV to any destination had changed, notify neighbors

• iterative, asynchronous

• distributed, self-stopping

  • no notifications recieved, no action taken

"good news (decrease on link cost) travels quickly"

"bad news (increase on link cost) travels slowly" - count-to-infinity problem

• if a link is broken other routers become aware slowly
• routers unaware of broken link can "advertise" incorrect costs and create a routing loop
• loop is broken if a hop count threshold is passed

link state (LS) vs Distance vector (DV)

message complexity

• LS: n routers O(n²) messages sent
• DV: exhange between neighbors; convergence time varies

speed of convergence:

• LS: O(n²) algorithm, O(n²) messages
• DV: varies
  • may have routing loops
  • count-to-infinity problem

robustness

• LS:
  • router can advertise incorrect link cost
  • each router computes only its own table
• DV:
  • router can advertise incorrect path cost: black holing
  • each routers table is used by others: errors propagate through network

Routing in internet

• autonomous system (AS): a group of networks and routers controled by a single administrative authority
• Intra-AS routing:
  • routing information protocol (RIP): distance vector routing
  • open shortest path first (OSPF): link state routing
• Inter-AS routing:
  • border gateway protocol (BGP)L path-vector routing

ICMP

• used by hosts and routers to communicate netowrk level information
  • error reporting:
  • echo requeset/reply
• network layer "above" IP
  • ICMP message carried in IP datagrams
• ICMP message: type, code plus first 8 bytes of IP datagram causing error

traceroute

• sends a set of UDP segments to destination
• datagram in nth set arrives to nth router
  • router discards datagram and sends source ICMP message (type 11 code 0)
  • ICMP message possibly incldes name of router and IP
• when ICMP message arrives at source: record RTTs

stopping:

• when a packet arrives that the destination, it returns ICMP "port unreachable message" (type 3 code 3)
• source stops