School of Computing Science

1

• School of Computing Science
• Simon Fraser University
• CMPT 880 Internet Architectures and Protocols
• Introduction II
• Instructor Dr. Mohamed Hefeeda

2
Review of Basic Networking Concepts

• Internet structure
• Protocol layering and encapsulation
• Internet services and socket programming
• Network Layer
• Network types Circuit switching, Packet
  switching
• Addressing, Forwarding, Routing
• Transport layer
• Reliability and congestion control
• TCP, UDP
• Link Layer
• Multiple Access Protocols
• Ethernet

3
Network Layer in the Internet

• Recall the big picture

Transport layer TCP, UDP
Network layer
Link layer
physical layer
4
Graph Abstraction

• Graph G (N,E)
• N set of routers u, v, w, x, y, z
• E set of links (u,v), (u,x), (v,x), (v,w),
  (x,w), (x,y), (w,y), (w,z), (y,z)
• cost of link (x1, x2)
• Metric value, e.g., c(w,z) 5
• could be
• 1 (typical), or
• inversely related to bandwidth, or
• inversely related to congestion

Routing algorithm find the least-cost path
5
Classification of Routing Algorithms

• Global or local information?
• Global
• all routers have complete topology, link cost
  info
• link state algorithms
• Local
• each router knows physically-connected neighbors,
  link costs to neighbors
• iterative process of computation, exchange of
  info with neighbors
• distance vector algorithms

6
A Link-State Routing Algorithm

• Dijkstras algorithm
• net topology, link costs known to all nodes
• accomplished via link state broadcast
• all nodes have same info
• computes least cost paths from one node (source)
  to all other nodes
• gives forwarding table for that node
• iterative after k iterations, know least cost
  path to k destinations

7
A Link-State Routing Algorithm

• Notation
• c(x,y) link cost from node x to y
• c(x,y) 8 if not direct neighbors
• D(v) current value of cost of path from source
  to dest. v
• p(v) predecessor node along path from source to
  v
• N' set of nodes whose least cost path
  definitively known

8
Dijsktras Algorithm
1 Initialization 2 N' u 3 for all
nodes v 4 if v adjacent to u 5
then D(v) c(u,v) 6 else D(v) 8 7 8
Loop 9 find w not in N' such that D(w) is a
minimum 10 add w to N' 11 update D(v) for
all v adjacent to w and not in N' 12
D(v) min D(v), D(w) c(w,v) 13 / new
cost to v is either old cost to v or known 14
shortest path cost to w plus cost from w to v /
15 until all nodes in N'
9
Dijkstras algorithm example
D(v),p(v) 2,u 2,u 2,u
D(x),p(x) 1,u
Step 0 1 2 3 4 5
D(w),p(w) 5,u 4,x 3,y 3,y
D(y),p(y) 8 2,x
N' u ux uxy uxyv uxyvw uxyvwz
D(z),p(z) 8 8 4,y 4,y 4,y
10
Dijkstras algorithm example (2)
Resulting shortest-path tree from u
Resulting forwarding table in u
11
Dijkstras algorithm, discussion

• What is the time complexity of Dijkstras
  algorithm?
• Input n nodes (other than source)
• each iteration need to check all nodes not in N
• 1st iteration n comparisons
• 2nd n -1
• 3rd n-2
• nth 1
• Total n(n1)/2 comparisons ? complexity O(n2)
• more efficient implementations possible O(nlogn)
• Using heap data structure

12
Distance Vector Algorithm

• Bellman-Ford Equation (dynamic programming)
• Define
• dx(y) cost of least-cost path from x to y
• Then
• dx(y) min c(x,v) dv(y)
• where min is taken over all neighbors v of x

v
13
Bellman-Ford example
Determine du(z)
Clearly, dv(z) 5, dx(z) 3, dw(z) 3
B-F equation says
du(z) min c(u,v) dv(z),
c(u,x) dx(z), c(u,w)
dw(z) min 2 5,
1 3, 5 3 4
How would you use BF equation to construct
shortest paths?
14
Distance Vector Algorithm

• Define
• Dx(y) estimate of least cost from x to y
• Distance vector Dx Dx(y) y ? N
• Node x knows cost to each neighbor v c(x,v)
• Node x maintains Dx Dx(y) y ? N
• Node x also maintains its neighbors distance
  vectors
• For each neighbor v, x maintains Dv Dv(y) y ?
  N

15
Distance vector algorithm

• Basic idea
• Each node periodically sends its own distance
  vector estimate to neighbors
• When a node x receives new DV estimate from
  neighbor, it updates its own DV using B-F
  equation

Dx(y) ? minvc(x,v) Dv(y) for each node y ?
N

• Under minor, natural conditions, the estimate
  Dx(y) converge to the actual least cost dx(y)

16
Distance Vector Algorithm
Each node

• Iterative
• Continues until no more info is exchanged
• Each iteration caused by
• local link cost change
• DV update message from neighbor
• Asynchronous
• Nodes do not operate in lockstep
• Distributed
• Each node receives info only from its directly
  attached neighbors
• NO Global info

17
Dx(z) minc(x,y) Dy(z),
c(x,z) Dz(z) min21 , 70 3
Dx(y) minc(x,y) Dy(y), c(x,z) Dz(y)
min20 , 71 2
node x table
cost to
cost to
x y z
x y z
x
0 2 3
x
0 2 3
y
from
2 0 1
y
from
2 0 1
z
7 1 0
z
3 1 0
node y table
cost to
cost to
cost to
x y z
x y z
x y z
x
8
8
x
0 2 7
x
0 2 3
8 2 0 1
y
y
from
y
2 0 1
from
from
2 0 1
z
z
8
8
8
z
7 1 0
3 1 0
cost to
cost to
Example
node z table
cost to
x y z
x y z
x y z
x
0 2 3
x
0 2 7
x
8 8 8
y
y
2 0 1
from
from
y
2 0 1
from
8
8
8
z
z
z
3 1 0
3 1 0
7
1
0
time
18
Distance Vector link cost changes

• Link cost decreased
• node detects local link cost change
• updates routing info, recalculates distance
  vector
• if DV changes, notify neighbors

1
4
50
At time t0, y detects the link-cost change,
updates its DV, and informs its neighbors. At
time t1, z receives the update from y and updates
its table. It computes a new least cost to x
and sends its neighbors its DV. At time t2, y
receives zs update and updates its distance
table. ys least costs do not change and hence y
does not send any message to z.
good news travels fast
19
Distance Vector link cost changes

• Link cost increased
• t0 y detects change, updates its cost to x to
  be 6. Why?
• Because z previously told y that I can reach x
  with cost of 5.
• 6 min 600, 15
• Now we have a routing loop!
• Pkts destined to x from y go back and forth
  between y and z forever (or until loop is broken)
• t1 z gets the update from y. z updates its cost
  to x to be??
• 7 min 500, 16
• Algorithm will take 44 iterations to stabilize
• This is called count to infinity problem!
• Solutions?

1
4
50
Bad news travels slow
20
Distance Vector link cost changes

• Poisoned reverse
• If z routes through y to get to x
• Then z tells y that its (zs) distance to x is
  infinity (so y wont route to x via z)
• Will this completely solve count to infinity
  problem?
• No! Loops involving three or more nodes will not
  be detected

21
Comparison of LS and DV algorithms

• Message complexity
• LS with n nodes, E links, O(nE) msgs sent
• DV exchange between neighbors only
• But send entire table
• Speed of Convergence
• LS O(n2) algorithm requires O(nE) msgs
• may have oscillations
• DV convergence time varies
• may be routing loops
• count-to-infinity problem

• Robustness what happens if router malfunctions?
• LS node can advertise incorrect link cost
• each node computes only its own table ? some
  degree of robustness
• DV node can advertise incorrect path cost
• each nodes table used by others ?error
  propagates thru network
• In The Internet
• LS OSPF (recent, more features)
• DV RIP (old, small nets)

22
Hierarchical Routing

• Our routing study thus far - idealization
• all routers identical
• network flat not true in practice

• scale with 200 million destinations
• cant store all dests in routing tables!
• routing table exchange would swamp links!

• administrative autonomy
• internet network of networks
• each network admin may want to control routing in
  its own network

23
Hierarchical Routing

• aggregate routers into regions, autonomous
  systems (AS)
• routers in same AS run same routing protocol
• intra-AS routing protocol
• routers in different AS can run different
  intra-AS routing protocol
• Gateway router
• Direct link to router in another AS

24
Interconnected ASes

• Forwarding table is configured by both intra- and
  inter-AS routing algorithm
• Intra-AS sets entries for internal dests
• Inter-AS Intra-As sets entries for external
  dests

25
Inter-AS tasks

• AS1 needs
• to learn which dests are reachable through AS2
  and which through AS3
• to propagate this reachability info to all
  routers in AS1
• Job of inter-AS routing!

• Suppose router in AS1 receives datagram for which
  dest is outside of AS1
• Router should forward packet towards one of the
  gateway routers, but which one?

26
Example Choosing among multiple ASes

• Now suppose AS1 learns from the inter-AS protocol
  that subnet x is reachable from AS3 and from AS2
• To configure forwarding table, router 1d must
  determine towards which gateway it should forward
  packets for dest x
• Hot potato routing send packet towards closest
  of two routers

27
Internet inter-AS routing BGP

• BGP (Border Gateway Protocol) the de facto
  standard
• BGP provides each AS a means to
• Obtain subnet reachability information from
  neighboring ASes
• Propagate the reachability information to all
  routers internal to the AS
• Determine good routes to subnets based on
  reachability information and policy
• Allows a subnet to advertise its existence to
  rest of the Internet I am here

28
BGP basics

• Pairs of routers (BGP peers) exchange routing
  info over semi-permanent TCP connections BGP
  sessions
• Note BGP sessions do not correspond to physical
  links
• When AS2 advertises a prefix to AS1, AS2 is
  promising it will forward any datagrams destined
  to that prefix towards the prefix
• AS2 can aggregate prefixes in its advertisement

29
Distributing reachability info

• With eBGP session between 3a and 1c, AS3 sends
  prefix reachability info to AS1
• 1c can then use iBGP to distribute this new
  prefix reach info to all routers in AS1
• 1b can then re-advertise the new reachability
  info to AS2 over the 1b-to-2a eBGP session
• When router learns about a new prefix, it creates
  an entry for the prefix in its forwarding table.

30
Path attributes BGP routes

• When advertising a prefix, advert. includes BGP
  attributes
• prefix attributes route
• Two important attributes
• AS-PATH contains the ASes on the path to the
  prefix
• NEXT-HOP Indicates the specific internal-AS
  router to next-hop AS. (There may be multiple
  links from current AS to next-hop-AS.)
• When gateway router receives route advert., uses
  import policy to accept/decline

31
BGP messages

• BGP messages exchanged using TCP
• BGP messages
• OPEN opens TCP connection to peer and
  authenticates sender
• UPDATE advertises new path (or withdraws old)
• KEEPALIVE keeps connection alive in absence of
  UPDATES also ACKs OPEN request
• NOTIFICATION reports errors in previous msg
  also used to close connection

32
BGP route selection

• Router may learn about more than 1 route to some
  prefix. Router must select a route
• Elimination rules
• Local preference value attribute policy decision
• Shortest AS-PATH
• Closest NEXT-HOP router hot potato routing
• Additional criteria

33
BGP Routing Policy

• A,B,C are provider networks
• X,W,Y are customer (of provider networks)
• X is dual-homed attached to two provider
  networks
• X does not want to route traffic from B via X to
  C
• so X will not advertise to B a route to C

34
BGP Routing Policy (2)

• A advertises to B the path AW
• B advertises to X (its client) the path BAW
• Should B advertise to C the path BAW?
• No way! B gets no revenue for routing CBAW
  since neither W nor C are Bs customers
• Rule of thumb a provider wants to route only
  to/from its customers! (unless there is a mutual
  peering deal)

35
Why different Intra- and Inter-AS routing ?

• Policy
• Inter-AS admin wants control over how its
  traffic routed, who routes through its net.
• Intra-AS single admin, so no policy decisions
  needed
• Scale
• hierarchical routing saves table size, reduced
  update traffic
• Performance
• Intra-AS can focus on performance
• Inter-AS policy may dominate over performance

36
Unicast, multicast, broadcast

• Unicast one source, one destination
• E.g., web session
• Multicast one source, multiple destinations
• Subset of all possible destinations
• E.g., streaming a hockey game to interested fans
• Broadcast one source, all destinations
• E.g., broadcasting link state info to ALL routers
  in a domain in OSPF protocol
• Anycast multiple possible sources, one
  destination
• Sources have same (anycast) address
• Request is forwarded to appropriate source
• (Still in research phases)
• We will not cover these topics!