1. Introduction to Internet Routing
Lixin Gao
ECE Dept.
UMASS, Amherst
9/19/2018
ECE697F: Special Topics-Internet Routing

2. ECE697F: Special Topics-Internet Routing
Routing Protocols
Goal:
Determine path or route
Packet switching networks: determine routing table
Routing Table:
Used for forwarding each packet
Scalable, adaptive, stable
ECE697F: Special Topics-Internet Routing

3. Internet Protocol Stack
Application
Transport
Network
Link
Physical
ECE697F: Special Topics-Internet Routing

4. ECE697F: Special Topics-Internet Routing
Route Construction
Static
Listed Manually: change route slowly
not robust: reachability is independent of network condition
stable
Dynamic
Learn route via routing protocols
React to topology, traffic or configuration changes directly
Might not converge or oscillate
Might have loop
ECE697F: Special Topics-Internet Routing

5. Dynamic Routing Algorithms
Global or Link state algorithm
Use global knowledge about topology and cost
Decentralized or Distance Vector algorithm
Use only knowledge of attached links and neighbors
Distributed and iterative algorithm
ECE697F: Special Topics-Internet Routing

6. Global or Link State Algorithm
Dijkstra’s shortest path algorithm
Implementation:
Each node broadcast its connectivity and link costs to all nodes
ECE697F: Special Topics-Internet Routing

7. Link State Algorithm Issues
Scalability
Link cost is traffic carried: can cause route oscillation A A D C B
2+e
1+e 1 A A D C B
2+e e
1+e 1 1
1+e
2+e D B D B e 1 C
1+e C 1 1
…recompute
initially e
…recompute
… recompute
ECE697F: Special Topics-Internet Routing

8. Decentralized or Distance Vector Algorithm
each node communicates only with directly-attached neighbors
compute shortest path
continues until no nodes exchange information
nodes need not exchange info/iterate in lock step!
ECE697F: Special Topics-Internet Routing

9. Distance Vector Algorithm Issues
Robustness:
node can advertise incorrect path cost
each node’s table used by others
error propagate thru network
may have routing loops
Convergence Times:
count-to-infinity problem
ECE697F: Special Topics-Internet Routing

10. Evolution of the Internet
Regional
Backbone
Regional
Backbone
Campus
Network
NSFNET Backbone
Regional
Backbone
Regional
Backbone
ECE697F: Special Topics-Internet Routing

11. Internet Today NSFNET decommissioned in 1995: commercialization of
the Internet
(Inter)National Provider
NAP
Private
Peering
Regional
Provider
(Inter)National Provider
ECE697F: Special Topics-Internet Routing

12. Network Access Point (NAP)
MAE-East
MAE-West
Sprint NAP
AT&T
MCI
FDDI Ring
vBNS
ECE697F: Special Topics-Internet Routing

13. ECE697F: Special Topics-Internet Routing
Private Peering
Exchange Traffic
AT&T
MCI
ECE697F: Special Topics-Internet Routing

14. Internet Architecture
Segregated to Autonomous Systems (ASes) belong to
ISPs
Companies
Universities
One ISP might own several ASes
ISP Merger
ECE697F: Special Topics-Internet Routing

15. Inter-AS and Intra-AS Routing
Inter-AS routing
Intra-AS routing
ECE697F: Special Topics-Internet Routing

16. Why different Inter-AS and Intra-AS routing protocols?
Administrative: each AS runs different intra-AS routing protocol
Policy: AS has commercial agreement that determines routing policy
Scalability or hierarchy: hiding information within AS reduce the routing message size.
ECE697F: Special Topics-Internet Routing

17. ECE697F: Special Topics-Internet Routing
Forwarding Table
Inter-AS Routing
Routing Table
Intra-AS Routing
ECE697F: Special Topics-Internet Routing

18. Inter-AS Routing Protocols
Use EGP in NSFNET
Border Gateway Protocol (BGP)
BGP-4: de facto standard
Distance Vector Algorithm
ECE697F: Special Topics-Internet Routing

19. ECE697F: Special Topics-Internet Routing
Intra-AS Routing
Routing Information Protocol (RIP)
Distance Vector Algorithm
Open Shortest Path First (OSPF)
Link State Algorithm
IS-IS
ECE697F: Special Topics-Internet Routing

20. Border Gateway Protocol (BGP)
Exchange reachability information
Apply local policies for announcing and selecting route
Avoid Route Loops
Incremental Update
Use TCP
AS701
(1), (7018,1)
MCI
AS7018
AS1
(1), (701, 1) d
AT&T
BBN
ECE697F: Special Topics-Internet Routing

21. ECE697F: Special Topics-Internet Routing
OSPF
Hierarchy:
Divided into Areas
Load balancing:
Multiple same-cost paths
Security:
Authenticate update messeges
ECE697F: Special Topics-Internet Routing

22. Where do link costs and routing policies come from?
Someone’s head
Manually entered
Router Configuration
Cause failure
ECE697F: Special Topics-Internet Routing

23. Routing Protocol Issues
Stability
Policy Effect
Congestion Effect
Protocol Design Effect
Keep Alive Message
Redundant Incremental Update message ?
Reliability or Robustness
Alternate or Backup route
Human Error
ECE697F: Special Topics-Internet Routing

24. ECE697F: Special Topics-Internet Routing
More Issues
Convergence Speed
Security
Scalability
Hierarchy
Traffic Engineering
Load Balancing
Qos ?
ECE697F: Special Topics-Internet Routing