Computer Networks

1. Chapter 4
Network Layer
Computer Networking: A
Top Down Approach
4th edition.
Jim Kurose, Keith Ross
Addison-Wesley, July
2007.

2. Hierarchical Routing
 All routers run the same routing
Algorithm?
… not true in practice
Scale: With millions of
destinations:
 Can’t store all destinations
in routing tables
 Exchange of Routing
updates would swamp links!
Administrative Autonomy
 Internet = network of
networks
 Each network administrator
may want to control routing
in its own network

3. Hierarchical Routing
Gateway Routers
 One or more of the
into “Autonomous
routers in an AS will
Systems” (ASs).
have the added task of
 Routers in same
administrative domain
forwarding packets to
destinations outside
 Routers in same AS run
the AS.
same routing protocol
 Routers are organized


The routing algorithm
within an autonomous
system is called an
“intra-AS” routing
protocol
Routers in different AS
can run different intraAS routing protocol
1c
1a
1d
1b
AS1

4. Interconnected ASs
3c
3b
3a
2a
AS3
1c
1a
1d
2c
2b
AS2
1b
AS1
1b, 1c, 2a and 3a are Gateway Routers

5. Inter-AS tasks
AS1 must:
1. Learn which destinations are
reachable through AS2, which
through AS3
2. Propagate this reachability
information to all routers in
AS1.

Configure Forwarding table
Job of inter-AS routing protocol!
 Suppose router in AS1
receives datagram destined
outside of AS1:
 Router should forward
packet to gateway
router, but which one?
3c
3a
3b AS3
1a
2a
1c
1d
1b
Intra-AS
Routing
algorithm
AS1
Inter-AS
Routing
algorithm
Forwarding
table
2c
AS2
2b

6. Example: Setting forwarding table in router 1d
 Suppose AS1 learns (via inter-AS protocol) that
network x reachable via AS3 (gateway 1c) but not via
AS2.
 Inter-AS protocol propagates reachability info to all
internal routers.
 Router 1d determines from intra-AS routing info that
its interface I is on the least cost path to 1c.
 installs forwarding table entry (x,I)
x
3c
3a
3b
AS3
1a
2a
1c
1d
1b AS1
2c
2b
AS2

7. Example: Choosing among multiple ASs
 Now suppose AS1 learns from inter-AS protocol
that a network 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.
x
3c
3a
3b
AS3
1a
2a
1c
1d
1b
2c
AS2
AS1
2b

8. Example: Choosing among multiple ASs
Learn from inter-AS
protocol that network
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 the
interface I that leads
to least-cost gateway.
Enter (x,I) in
forwarding table
Steps in adding an outside AS destination in a routers forwarding Table

9. Routing Protocols

10. Path Vector Routing

11. Path Vector Routing
Loop Prevention
If a router receives a message, checks
to see if its autonomous system is in the
path list to the destination.
If it is, looping is involved and the
message is ignored.
Policy Routing
If one of the path in the path vector is
against the policy it can ignore that
path and does not update its routing
table.

12. Inter-AS Routing in Internet: BGP
 BGP (Border Gateway Protocol)
 BGP provides each AS a means to:
1. Obtain subnet reachability information from
neighboring ASs.
2. Propagate reachability information to all ASinternal routers.
3. Determine “good” routes to subnets based on
reachability information and policy.
 Allows subnet to advertise its existence to
rest of Internet: “I am here”

13. BGP Basics
 Pairs of routers (BGP peers) exchange routing info over TCP
connections using port 179.
 For each TCP connection, the two routers at the end of the
connection are called BGP Peers
 The TCP connection along with all the BGP messages sent over
the connection is called a BGP Session
 Internal (IBGP) neighbours
 A pair of BGP speakers within the same AS

External (EBGP) neighbours
 Two BGP speakers from two different AS
eBGP session
3c
3a
3b
AS3
1a
AS1
iBGP session
2a
1c
1d
1b
2c
AS2
2b

14. BGP Basics
 BGP allows each AS to learn which destinations
are reachable via its neighboring ASs.
 Destinations are not hosts but instead are
CIDRized prefixes.

E.g. AS1 has four subnets attached to it
It will aggregate the prefixes of these four and will
advertise the single prefix
 AS1 and AS2 send reachability information
trough their gateway routers 1b and 2a.
 When a gateway router receives e-BGP learned
prefixes, the gateway router uses iBGP sessions
to distribute the prefixes to other routers in
the AS.

15. Distributing Reachability info
 Using eBGP session between 3a and 1c, AS3 sends
prefix reachability info to AS1.
 1c can then use iBGP do distribute new prefix
info to all routers in AS1
 1b can then re-advertise new reachability info
to AS2 over 1b-to-2a eBGP session
 When router learns of new prefix, it creates entry
for prefix in its forwarding table.
eBGP session
3c
3a
3b
AS3
1a
AS1
iBGP session
2a
1c
1d
1b
2c
AS2
2b

16. Path Attributes & BGP Routes
 In BGP an AS is recognized by its
globally unique Autonomous System
Number (ASN).
 Assigned by ICANN Regional
Registries.
 Advertised prefix includes BGP
attributes.
 prefix + attributes = “route”
 Two important attributes:
 AS-PATH and NEXT-HOP

17. Path Attributes & BGP Routes
 AS-PATH
 Contains
ASs through which prefix
advertisement has passed.
 When a prefix is passed into an AS, the As
adds its ASN to the AS-PATH attribute.
E.g suppose that the prefix
138.16.64.0/24 is first advertised from
AS2 to AS1.
If AS1 then advertises the prefix to
AS3, AS-PATH would be AS2 AS1.
 Detect and prevent looping advertisements

18. Routing Loop Prevention
AS2
[ Net1, (AS3,AS4) ]
[ Net1,
(AS2,AS3,AS4)]
AS3
AS1
[Net1, (AS4)]
AS4
Net1
[ Net1,
(AS1,AS2,AS3,AS4)]
AS3 won’t forward this further