1. CS4470 Computer Networking Protocols
8/1/2018
CS Computer Networking Protocols
12. Routing protocols
Huiping Guo
Department of Computer Science
California State University, Los Angeles

2. Outline Hierarchical routing Internet routing ICMP Intro-AS Routing
RIP
OSPF
Inter-AS Routing
BGP
ICMP
12. Routing protocols CS4470

3. Hierarchical Routing scale: with 200 million routers:
Our routing study thus far – idealization
We’ve viewed the network simply as a collection of interconnected routers
All routers execute the same routing algorithm to compute routing paths through the entire network
Not true in practice
scale: with 200 million routers:
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
12. Routing protocols CS4470

4. Hierarchical Routing
Aggregate routers into regions, “autonomous systems” (AS)
Each AS contains a group of routers under the same administrative control
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
12. Routing protocols CS4470

5. Interconnected ASes 3c 3a 2c 3b 2a AS3 2b 1c
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
12. Routing protocols CS4470

6. Routing a datagram Source and destination are in the same AS
Each router knows the optimal path and where to forward
Source and destination are in different ASes and the source AS only has one gateway router
Forward the packet to the gateway router
Source and destination are in different ASes and the source AS has multiple gateway routers
Which gateway router to forward the datagram to?
12. Routing protocols CS4470

7. 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? 3b 1d 3a 1c 2a
AS3
AS1
AS2 1a 2c 2b 1b 3c
12. Routing protocols CS4470

8. Example: Setting forwarding table in router 1d
Suppose AS1 learns from the inter-AS protocol that subnet x is reachable from AS3 (gateway 1c) but not from 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.
Puts in forwarding table entry (x, I).
12. Routing protocols CS4470

9. 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.
This is also the job of inter-AS routing protocol!
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
12. Routing protocols CS4470

10. Intra-AS Routing Also known as Interior Gateway Protocols (IGP)
8/1/2018
Intra-AS Routing
Also known as Interior Gateway Protocols (IGP)
Most common Intra-AS routing protocols:
RIP: Routing Information Protocol
OSPF: Open Shortest Path First
IGRP: Interior Gateway Routing Protocol (Cisco proprietary)
12. Routing protocols CS4470

11. RIP ( Routing Information Protocol)
Distance vector algorithm
Cost metric: # of hops
# of hops: the number of subnets traversed along the shortest path from source router to destination subnet
Each link has a cost of 1
The maximum cost of a path is limited to 15
Can only use RIP in ASes that are fewer than 15 hops diameter
12. Routing protocols CS4470

12. RIP ( Routing Information Protocol)D C B A u v w x y z
destination hops u v w x y z
From router A to subsets:
12. Routing protocols CS4470

13. Review: Distance Vector algorithm
Neighboring routers exchange distance vectors with each other
The distance vector for any one router contains the current estimate of the shortest path distances from that router to other subnets
12. Routing protocols CS4470

14. RIP advertisements
Routing tables are exchanged among neighbors every 30 sec using a RIP Response Message (also called RIP advertisement)
The response message contains a list of up to 15 destination subnets within the AS, as well as the sender’s distance to each of those subnets
12. Routing protocols CS4470

15. RIP: Example z w x y A D B C y B 2 z B 7 x -- 1
Destination Network Next Router Num. of hops to dest.
w A 2
y B 2
z B 7 x
…. …
Routing table in D
12. Routing protocols CS4470

16. RIP: Example Advertisement from A to D w x y z A C D B
Dest Next hops w x
z C 4
…. …
Advertisement
from A to D w x y z A C D B
Destination Network Next Router Num. of hops to dest.
w A 2
y B 2
z B A 7 5 x
…. …
Routing table in D
12. Routing protocols CS4470

17. OSPF (Open Shortest Path First)
“open”: specification publicly available
As opposed to Cisco’s EIGRP protocol
Uses Link State algorithm
Uses flooding of link-state information
Construct a complete topological map of the entire AS
Route computation using Dijkstra’s algorithm
Usually deployed in upper-tier Internet Service Providers (ISPs)
RIP is deployed in lower-tier ISPs and enterprise networks
12. Routing protocols CS4470

18. OSPF (Open Shortest Path First)
A router broadcasts routing information all other routers in the AS system
whenever there is a change in a link’s state
And periodically (at least once every 30 minutes) even if the link’s state has not changed
OSPF advertisements carried in OSPF messages directly over IP
OSPF must itself implement functionality such as reliable message transfer
12. Routing protocols CS4470

19. OSPF “advanced” features (not in RIP)
Security: all OSPF messages authenticated (to prevent malicious intrusion)
Multiple same-cost paths allowed (only one path in RIP)
Load balancing: if 2 routes to the same destination have the same cost, the traffic to the destination will be distributed to the two routes.
Multiple cost metrics
It’s up to the network administrator to choose the cost metrics
Support for Hierarchy within a single routing domain
The most significant advance in OSPF is the ability to structure an AS hierarchically
12. Routing protocols CS4470

20. Hierarchical OSPF An OSPF AS can be configured into areas
8/1/2018
Hierarchical OSPF
An OSPF AS can be configured into areas
Each area runs its own OSPF link state routing algorithm
Each router in an area broadcasts its link state to all other routers in the area
The internal details of an area are invisible to all routers outside the area
Intra-area routing involves only those routers within the same area
12. Routing protocols CS4470

21. Hierarchical OSPF
Exactly one OSPF area in the AS is configured to be the backbone area
Route traffic between the other areas in the AS
Contains all the areas border routers in the AS and may contain nonborder routers as well
Area border routers:
“summarize” distances to nets in own area
Route traffic outside the area
Belong to both an area and the backbone
12. Routing protocols CS4470

22. Hierarchical OSPF Backbone routers (non border routers)
run OSPF routing limited to backbone.
Perform routing within the backbone
Not the area border routers
Boundary routers
connect to other AS’s.
Internal routers
Belong to non-backbone areas
Perform only intra-area routing
12. Routing protocols CS4470

23. Hierarchical OSPF
12. Routing protocols CS4470

24. Intra-AS routing Summary
Two of the protocols
RIP
OSPF
The purpose of intra-AS routing algorithms
Decide the entries with dest. address in the same AS in the routing table
How to set the entries with dest. Addesses in different ASes?
Inter-AS routing: BGP
12. Routing protocols CS4470

25. Internet inter-AS routing: BGP
BGP (Border Gateway Protocol):
the de facto standard
Allows a subnet to advertise its existence to rest of the Internet: “I am here”
Make sure that all the ASes in the Internet know about the subnet and how to get there
BGP is a critical protocol for the Internet
It’s the protocol that glues the whole thing together
12. Routing protocols CS4470

26. Internet inter-AS routing: BGP
BGP provides each AS a means to:
Obtain subnet reachability information from neighboring ASs.
Propagate the reachability information to all routers internal to the AS.
Determine “good” routes to subnets based on reachability information and policy.
12. Routing protocols CS4470

27. BGP basics 3c 2c 3a 3b 2a AS3 2b 1c 1a 1b 1d AS1
Pairs of routers (BGP peers) exchange routing info over connections: BGP sessions
eBGP sessions: for gateway routers
iBGP sessions: for internal routers
Note that BGP sessions do not correspond to physical links.
Reachability information: advertised by the gateway routers
When AS2 advertises a prefix to AS1, AS2 is promising it will forward any datagram destined to that prefix 3b 1d 3a 1c 2a
AS3
AS1
AS2 1a 2c 2b 1b 3c
eBGP session
iBGP session
12. Routing protocols CS4470

28. Distributing reachability info
With eBGP session between 3a and 1c, AS3 sends prefix reachability info to AS1.
1c can then use iBGP do distribute this new prefix reach info to all routers in AS1
1b can then re-advertise the new reach 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
12. Routing protocols CS4470

29. 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 performanc
12. Routing protocols CS4470

30. IP Semantics IP is best-effort Datagrams can be Lost Delayed
Duplicated
Delivered out of order
Corrupted
12. Routing protocols CS4470

31. Error Detection IP is NOT careless, it attempts Errors detected
avoid errors
Report errors when they occur
Errors detected
Corrupted bits
Illegal addresses
Routing loops
Fragment loss
12. Routing protocols CS4470

32. Problems And Solutions
Corrupted header bits
Header checksum
Illegal destination address
Routing tables
Routing loop
Time-To-Live (TTL) field
Fragment loss
Timeout
12. Routing protocols CS4470

33. ICMP (Internet Control Management Protocol)
Separate protocol for
Errors
Information
Required part of IP
Sends error messages to original source
12. Routing protocols CS4470

34. Example ICMP Message Types
12. Routing protocols CS4470

35. Example ICMP Messages Time Exceeded Sent by router Sent by host
TTL on datagram reached zero
Not a request for retransmission
Sent by host
Reassembly timeout (some fragments lost)
12. Routing protocols CS4470

36. Example ICMP Messages Echo request and reply Not an error
Tests whether destination reachable
Request sent by ping program
Reply sent by ICMP on destination computer
12. Routing protocols CS4470

37. ICMP Message Transport
Messages carried in IP
Two levels of encapsulation
IP type field specifies ICMP
12. Routing protocols CS4470

38. Avoiding An Infinite Loop
Datagram D causes an ICMP error message, I1
Error message I1 causes another error, which generates ICMP message I2
Message I2 generates another error, I3
Error messages cascade
To avoid the problem
No error messages about ICMP error messages
12. Routing protocols CS4470

39. Traceroute
A network diagnostic program that finds the address of all routers between the user’s computer and a specific destination
Run the command
Windows
C:> tracert
C:>tracert
Unix
$traceroute
$traceroute
12. Routing protocols CS4470

40. Traceroute: how it works
The source sends three ICMP echo request messages to the destination with TTL set to 1
The first router sees the datagram, decreases TTL by 1 to 0, discards the datatgram and sends an ICMP time-exceeded message back to the source
The source sends three ICMP echo request messages to the destination with TTL set to 2
The first router sees the datagram, decreases TTL by 1 to 1
The second router sees the datagram, decreases TTL by 1 to 0, discards the datatgram and sends an ICMP time-exceeded message back to the source
3 probes
3 probe
12. Routing protocols CS4470

41. “Real” Internet delays and routes
C:>tracert
Three delay measurements
* means no response (probe lost, router not replying)
12. Routing protocols CS4470