1. Introduction to Computer Networks
Routing
University of Ilam
By: Dr. Mozafar Bag-Mohammadi

2. Routing Process Primary solutions:
Question? How to populate the lookup table?
Primary solutions:
Build the lookup table Manually? Is it practical? The answer is no.
Flooding- Broadcast to all node except the one we have received the packet.
Waste the bandwidth
Does not scale well.

3. Overview Network as a Graph
Problem: Find lowest cost, or shortest, path between two nodes
The process is distributed and this makes it complicated, i.e, it may create loop.
Factors
static: topology
dynamic: load

4. Distance Vector Each node maintains a set of triples
(Destination, Cost, NextHop)
Exchange updates with directly connected neighbors
periodically ( on the order of several seconds)
whenever its table changes (called triggered update)
Each update is a list of pairs:
(Destination, Cost)
Update local table if receive a “better” route
smaller cost
came from next-hop
Refresh existing routes; delete if they time out

5. Example Destination Cost NextHop Distance of other nodes from Node B.
A A
C C
D C
E A
F A
G A
Distance of other nodes from Node B.
The cost between two nodes has been assumed 1.
All nodes keep a routing table from themselves.

6. The Bellman-Ford Algorithm
Bellman-Ford algorithm solve the distance Vector problem in general case.
1. Set: Xo = ( , , ,…, ).
2. Send updates of components of Xn to neighbors
3. Calculate: Xn+1 = F(Xn)
4. If Xn+1  Xn then go to (2)
5. Stop

7. Bellman-Ford Algorithm
Example:
Calculate from R8 R5 R3 R7 R8 R6 R4 R2 R1 1 4 2 3 R3 R2 R5 R7 R4 R6 R8 R1 1 4 2 3
2nd step

8. Bellman-Ford Algorithm1 4 2 3
3rd step
Result: R8 R6 R4 R2 R1 R3 R5 1 4 2 3 R7

9. Node Failure F detects that link to G has failed
F sets distance to G to infinity and sends update to A
A sets distance to G to infinity since it uses F to reach G
A receives periodic update from C with 2-hop path to G
A sets distance to G to 3 and sends update to F
F decides it can reach G in 4 hops via A

10. Routing Loops link from A to E fails
A advertises distance of infinity to E
B and C advertise a distance of 2 to E
B decides it can reach E in 3 hops; advertises this to A
A decides it can read E in 4 hops; advertises this to C
C decides that it can reach E in 5 hops…

11. The count-to-infinity problem

12. Loop-Breaking Heuristics
Set infinity to a reasonably small number. For instance, RIP sets to 16
Split horizon: Don’t announce the distance to the node the distance has been gotten from.
Split horizon with poison reverse: Instead of not announcing the distance put negative numbers.

13. Link State Strategy Link State Packet (LSP)
send to all nodes (not just neighbors) information about directly connected links (not entire routing table)
Link State Packet (LSP)
id of the node that created the LSP
cost of the link to each directly connected neighbor
sequence number (SEQNO)
time-to-live (TTL) for this packet

14. Link State (cont.) Reliable flooding
store most recent LSP from each node
forward LSP to all nodes but one that sent it
generate new LSP periodically
increment SEQNO
start SEQNO at 0 when reboot
decrement TTL of each stored LSP
discard when TTL=0

15. Route Calculation Dijkstra’s shortest path algorithm Let
N denotes set of nodes in the graph
l (i, j) denotes non-negative cost (weight) for edge (i, j)
s denotes this node
M denotes the set of nodes incorporated so far
C(n) denotes cost of the path from s to node n
M = {s}
for each n in N - {s}
C(n) = l(s, n)
while (N != M)
M = M union {w} such that C(w)
is the minimum for all w in (N - M)
for each n in (N - M)
C(n) = MIN(C(n), C (w) + l(w, n ))

16. Shortest Path Routing: Dijkstra Algorithm

17. Subnetting Add another level to address/routing hierarchy: subnet
Subnet masks define variable partition of host part
Subnets visible only within site
Network number
Host number
Class B address
Subnet mask ( )
Network number
Subnet ID
Host ID
Subnetted address

18. Subnet Example Subnet 111….1.0xxx….x Net host
Subnet mask:
Subnet number: H1 R1
Subnet mask:
Subnet number: R2 H2
Subnet mask:
Subnet number: H3
111….1.0xxx….x
Net
host
Forwarding table at router R1
Subnet # Subnet Mask Next Hop
interface 0
interface 1 R2

19. Supernetting (first_network_address, count)
Assign block of contiguous network numbers to nearby networks
Called CIDR: Classless Inter-Domain Routing
Represent blocks with a single pair
(first_network_address, count)
Restrict block sizes to powers of 2
Use a bit mask (CIDR mask) to identify block size
All routers must understand CIDR addressing

20. Route Propagation Know a smarter router Autonomous System (AS)
hosts know local router
local routers know site routers
site routers know core router
core routers know everything
Autonomous System (AS)
corresponds to an administrative domain
examples: University, company, backbone network
assign each AS a 16-bit number
Two-level route propagation hierarchy
interior gateway protocol (each AS selects its own)
exterior gateway protocol (Internet-wide standard)

21. Architecture of Routing Protocols
Interior Gateway
Protocols (IGP) :
inside autonomous
systems
Exterior Gateway
Protocols (EGP) :
between autonomous
systems
AS 701
UUNet
OSPF, IS-IS,
RIP, EIGRP, ...
BGP
IGP
Metric Based
EGP
Policy Based
EGP
IGP
IGP
EGP
AT&T
Common Backbone
AT&T Research
AS 6431
AS 7018

22. Interior Gateway Protocols
RIP: Route Information Protocol
developed for XNS
distributed with Unix
distance-vector algorithm
based on hop-count
OSPF: Open Shortest Path First
recent Internet standard
uses link-state algorithm
supports load balancing
supports authentication

23. EGP: Exterior Gateway Protocol
Overview
designed for tree-structured Internet
concerned with reachability, not optimal routes
Protocol messages
neighbor acquisition: one router requests that another be its peer; peers exchange reachability information
neighbor reachability: one router periodically tests if the another is still reachable; exchange HELLO/ACK messages; uses a k-out-of-n rule
routing updates: peers periodically exchange their routing tables (distance-vector)

24. The Most Common Routing Protocols
BGP
RIP
Cisco proprietary
TCP
UDP
OSPF
IS-IS
EIGRP
IP (and ICMP)
This slide illustrates the Routing Protocol Encapsulation
BGP : TCP port number 179.
RIP : UDP port number 520.
OSPF : IP protocol number 89.
One reason why routing protocols are not more well-known is that they run on routers and do not provide services directly to end-user applications. These protocols are however VERY familiar to network administrators. They keep the network connected! LANS generally deal with OSPF or RIP. ISPs and backbone network providers deal with BGP. In fact, they devote considerable resources to the care and feeding of BGP.
Routing protocols exchange network
reachability information between routers.

25. BGP-4 BGP = Border Gateway Protocol Is a Policy-Based routing protocol
Is the de facto EGP of today’s global Internet
Relatively simple protocol, but configuration is complex and the entire world can see, and be impacted by, your mistakes.
1989 : BGP-1 [RFC 1105]
Replacement for EGP (1984, RFC 904)
1990 : BGP-2 [RFC 1163]
1991 : BGP-3 [RFC 1267]
1995 : BGP-4 [RFC 1771]
Support for Classless Interdomain Routing (CIDR)
Is complex like OSPF. But much more scary. If you screw up, the whole interet will know!
ISPs and NSP employ many network engineers provide care and
feeding of BGP.

26. BGP-4: Border Gateway Protocol
AS Types
stub AS: has a single connection to one other AS
carries local traffic only
multihomed AS: has connections to more than one AS
refuses to carry transit traffic
transit AS: has connections to more than one AS
carries both transit and local traffic
Each AS has:
one or more border routers
one BGP speaker that advertises:
local networks
other reachable networks (transit AS only)
gives path information

27. Policy-Based vs. Distance-Based Routing?
Host 1
Minimizing
“hop count” can
violate commercial
relationships that
constrain inter-
domain routing.
Cust1
YES NO
ISP1
ISP3
Host 2
ISP2
Cust3
Cust2

28. Why not minimize “AS hop count”?
YES
National
ISP1
National
ISP2 NO
Regional
ISP3
Regional
ISP2
Regional
ISP1
Cust3
Cust2
Cust3

29. BGP Operations Simplified
AS1
AS2
Establish Peering on
TCP port 179
BGP
Peers Exchange
All Routes
BGP speaking routers exchange information with their BGP peers.
Peers are configured by network engineer.
While connection
is ALIVE exchange
route UPDATE messages
Exchange Incremental
Updates

30. Two Types of BGP Neighbor Relationships
External Neighbor (eBGP) in a different Autonomous Systems
Internal Neighbor (iBGP) in the same Autonomous System
AS1
eBGP
eBGP presents a unified policy face to the outside world. eBGP peers must share the same physical medium.
iBGP is needed to ensure that all border routers synchronize and
present a consistent story to the outside world. They do not have to
share physical media, but do have to be fully meshed, so an IGP is
assumed.
iBGP
Physical Connection
AS2
Logical (TCP) Connection

31. Four Types of BGP Messages
Open : Establish a peering session.
Keep Alive : Handshake at regular intervals.
Notification : Shuts down a peering session.
Update : Announcing new routes or withdrawing previously announced routes.
announcement =
Network prefix + attributes

32. AS Path Attribute (cont.)
BGP at AS YYY will never accept a route whose AS Path contains YYY. This avoids interdomain routing loops.
AS702
UUnet
/16
AS Path =
Don’t Accept!

33. Local Preference Attribute
Used only in iBGP to prefer a point of exit
Frank’s Upstream Provider
AS 4
/16
AS Path = 4 1
Loc pref = 80
/16
AS Path = 3 1
Loc pref = 90
Frank’s
Internet Barn
Frank’s Local Competition
AS 3
/16
AS Path = 2 1
Loc pref = 100
Frank’s Customer
AS 2
Customer of Frank’s Customer
Higher Local
Preference Values
are more preferred
AS 1
/16

34. IP Version 6 Features Header 128-bit addresses (classless) multicast
real-time service
authentication and security
autoconfiguration
end-to-end fragmentation
protocol extensions
Header
40-byte “base” header
extension headers (fixed order, mostly fixed length)
fragmentation
source routing
other options

35. Tunneling

36. Routing for Mobile Hosts
1- finding location of the mobile host
2- hand-off
3- security

37. Routing for Mobile Hosts (2)
Packet routing for mobile users.