1. CMPT 880: Internet Architectures and Protocols
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
IP protocol
addressing conventions
datagram format
packet handling conventions
Routing protocols
path selection
RIP, OSPF, BGP
Network
layer
forwarding
table
ICMP protocol
error reporting
router “signaling”
Link layer
physical layer

4. Routing algorithm: find the least-cost path
Graph Abstraction u y x w v z 2 1 3 5
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
Dijkstra’s 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) = ∞ 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. Dijsktra’s Algorithm 1 Initialization: 2 N' = {u} 3 for all nodes v
if v adjacent to u
then D(v) = c(u,v)
else D(v) = ∞ 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' :
D(v) = min { D(v), D(w) + c(w,v) }
13 /* new cost to v is either old cost to v or known
shortest path cost to w plus cost from w to v */
15 until all nodes in N'

9. Dijkstra’s algorithm: example
Step 1 2 3 4 5 N' u ux
uxy
uxyv
uxyvw
uxyvwz
D(v),p(v)
2,u
D(w),p(w)
5,u
4,x
3,y
D(x),p(x)
1,u
D(y),p(y) ∞
2,x
D(z),p(z) ∞
4,y u y x w v z 2 1 3 5

10. Dijkstra’s algorithm: example (2)
Resulting shortest-path tree from u: u y x w v z
Resulting forwarding table in u: v x y w z
(u,v)
(u,x)
destination
link

11. Dijkstra’s algorithm, discussion
What is the time complexity of Dijkstra’s 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(n+1)/2 comparisons  complexity : O(n2)
more efficient implementations possible: O(nlogn)
Using heap data structure
Possible solutions for oscillation:
Routers do not run algorithm at the same time,
they randomize the time they send out link advertisement

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) How would you use BF equation tov z 2 1 3 5
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 є N ]
Node x knows cost to each neighbor v: c(x,v)
Node x maintains Dx = [Dx(y): y є N ]
Node x also maintains its neighbors’ distance vectors
For each neighbor v, x maintains Dv = [Dv(y): y є 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) ← minv{c(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
wait for (change in local link cost or msg from neighbor)
recompute estimates
if DV to any dest has changed, notify neighbors

17. z y x Dx(y) = min{c(x,y) + Dy(y), c(x,z) + Dz(y)} = min{2+0 , 7+1} = 2
Dx(z) = min{c(x,y) + Dy(z), c(x,z) + Dz(z)} = min{2+1 , 7+0} = 3
node x table
x y z x y z ∞
from
cost to
cost to
cost to
x y z
x y z x x
from y
from y z z
node y table
cost to
cost to
cost to x z 1 2 7 y
x y z
x y z
x y z x ∞ ∞ x ∞ x y
from y
from
from y z z ∞ ∞ ∞ z
node z table
cost to
cost to
Example
cost to
x y z
x y z
x y z x x x
∞ ∞ ∞
from y
from y
from y ∞ ∞ ∞ z z z 7 1
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 y 4 1 x z 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 z’s update and updates its distance table.
y’s 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 {60+0, 1+5}
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 {50+0, 1+6}
Algorithm will take 44 iterations to stabilize
This is called “count to infinity” problem!
Solutions? 60 y 4 1 x z 50
“Bad
news
travels
slow”

20. Distance Vector: link cost changesx z 1 4 50 y 60
Poisoned reverse:
If z routes through y to get to x:
Then z tells y that its (z’s) distance to x is infinity (so y won’t 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 node’s 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:
can’t store all dest’s 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 3c 3a 2c 3b 2a AS3 2b 1c 1a 1b AS1 1d
Intra-AS
Routing
algorithm
Inter-AS
Forwarding
table 3c
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:
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?
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! 3b 1d 3a 1c 2a
AS3
AS1
AS2 1a 2c 2b 1b 3c

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
Learn from inter-AS
protocol that subnet
x is reachable via
multiple gateways
Use routing info
from intra-AS
protocol to determine
costs of least-cost
paths to each
of the gateways
Hot potato routing:
Choose the gateway
that has the
smallest least cost
Determine from
forwarding table the
interface I that leads
to least-cost gateway.
Enter (x,I) in
forwarding table

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”
Why reachability? Why not exact route? Because exact route will impose too much overhead. Routers would
Have to handle a huge amount of information. With reachability (i.e., AS path), core routers have in their tables
Around 140,000 entries!
This is a tradeoff between scalability and route optimality. BGP
BGP is concerned with finding any route to destination, not necessarily the optimal route.

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 3b 1d 3a 1c 2a
AS3
AS1
AS2 1a 2c 2b 1b 3c
eBGP session
iBGP session

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. 3b 1d 3a 1c 2a
AS3
AS1
AS2 1a 2c 2b 1b 3c
eBGP session
iBGP session

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 B’s 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
Is routing in the Internet optimal?
NO, because:
- BGP policies may mandate longer paths
- BGP advertises only AS path. ASes have different sizes. Even if they have the same size, we do not know the cost of
paths within ASes.
- A route may (typically) crosses multiple ASes, each with its own intra-domain routing algorithm with different metric.
This makes it hard to even define an optimality criterion.

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!