Internet Routing Protocols: Key Concepts of Link-State and Distance Vector

Internet Routing Protocols:
Fundamental Concepts of Link-State
and Distance Vector Routing*

*From Christian Huitema’s Routing in the Internet
Vishal Sharma, Ph D
Tellabs Research Center
vsharma@trc tellabs com
November 20, 1998

Outline of the Talk

 What does a router do?
 A look at the forwarding process at a router
 How does it know where to forward packets?
 Distance vector routing
An illustration of its operation
Its drawbacks and how they affect performance

Some solutions to those drawbacks

 Link state routing
A look at its major components
An illustration of their operation

Its features and advantages

 OSPF: a link state based intra-domain routing protocol
 BGP: a distance vector based inter-domain routing
protocol
Copyright © 1998 Internet Routing Protocols: Fundamental Operation 2

Importance of Routing in the Internet

The structure that glues together the worldwide
Internet
 Without routing there would be no Internet!


Routing process at a router
Receive incoming pkt.
 Destination address (DA)
based forwarding DA=my_add or
Yes Deliver datagram to
protocol module
 Longest prefix matching DA= IP brdcst add.
?
(TCP/UDP) specified
in IP hdr.

No

Routing Table Yes Send pkt. to next-hop
RT entry =
complete DA? router or to directly
DA Next hop Network connected interface.
router Interface
Host entry 198.168.7.3 X 2 No

Host entry 198.168.7.4 X 3
RT entry = Yes Send pkt. to next-hop
Host entry 198.168.7.1 198.168.7.5 1 Destn. n/w id? router or to directly
connected interface.
Host entry 198.168.7.2 198.168.7.5 1
No
N/w entry 198.100.x.x 198.100.9.1 4
N/w entry 128.72.x.x 128.72.55.4 5 Yes
Send pkt. to
Default entry
exists? next-hop router.
Default x.x.x.x 128.84.73.1 6
No

Datagram undeliverable. (Use ICMP to inform source.)

Routing process at a router
Longest prefix match in DA Next hop N/w
Packet generated RT gives next hop router Int.
router as 198 100 9 1 Host entry 198.168.7.3 X 2
DA = 198 100 9 75
and outgoing interface Host entry 198.168.7.4 X 3
as 4
198.168.7.3 Host entry 198.168.7.1 198.168.7.5 1
198.168.7.4 Host entry 198.168.7.2 198.168.7.5 1
198.168.7.1

198.168.7.5 N/w entry 198.100.x.x 198.100.9.1 4
N/w entry 128.72.x.x 128.72.55.4 5
198.100.x.x Default x.x.x.x 128.84.73.1 6
2
1 3
Routing table (RT) at 198 168 7 6
198.168.7.2 4 198.100.9.1
198.168.7.6
5 198.100.9.75
6
128.84.x.x
128.72.x.x

128.72. 55.4
128.84. 73.1

 How do routers build their routing tables?
 By exchanging information with each other using routing protocols


Routing Protocols: Distance vector (DV)
Routing

Local dispersal of global information
 Each router has unique ID
 Each router knows cost of its outgoing links

 Router starts with distance vector “0” for itself, and “infinity”
for all other destinations
 Transmits DV to each neighbor -- periodically or upon
change
 Saves the most recently received DV from each neighbor

 Calculates new DV based on minimizing cost for each
destination
 Recalculations occur when:
DV with new values received from a neighbor
Link(s) fails


Operation of Distance Vector Routing (1)
From A Link Cost From B Link Cost

A local 0 B local 0

Letters represent
Node names
A B
1

A=0 2 From C Link Cost
C C local 0

3 4
Numbers on links represent
link identifiers (not cost)
5

6
From D Link Cost D E From E Link Cost

D local 0 E local 0



A local 0 B local 0
A 1 1
B=0, A=1
A B
1

2 From C Link Cost
C
C local 0

3 4
D=0, A=1

5

6

D local 0 E local 0
A 3 1



A local 0 B local 0
B 1 1 A 1 1
D 3 1 A=0, B=1, D=1

A B
1
C=0, B=1, A=2
2 From C Link Cost
C
C local 0
B 2 1
3 4 A 2 2

5

6

D local 0 E=0, B=1, A=2, E local 0
A 3 1 B 4 1
D=1
A 4 2
D 6 1



A local 0 B local 0
B 1 1 A 1 1
D 3 1 B=0, A=1, D=2, D 1 2
C=1, E=1 C 2 1
A B E 5 1
1

2 From C Link Cost
C C local 0
B 2 1
3 4 A 2 2
D=0, A=1, B=2
E=1
5

6

D local 0 E=0, B=1, A=2, E local 0
A 3 1 B 4 1
D=1, C=1
B 3 2 A 4 2
E 6 1 D 6 1
C 5 1



A local 0 B local 0
B 1 1 A 1 1
D 3 1 D 1 2
C 1 2 C 2 1
E 1 2 A B E 5 1
1

These do not alter 2 From C Link Cost
routing tables further C C local 0
Thus, no new 3 4
B 2 1
A 2 2
updates generated E 5 1
D 5 2
5

6

D local 0 E local 0
A 3 1 B 4 1
B 3 2 A 4 2
E 6 1 D 6 1
C 6 2 C 5 1

DV routing has now converged

Drawbacks of Distance-vector Routing

 Slow convergence after topology change
 “Counting to infinity” problem:
Loop exists
DVs do not converge till the link costs reach “infinity ”

 Problematic convergence with unequal link costs
 Bouncing effect:
Routing and link costs temporarily stabilize, but have a loop
Data packets circulate in the loop till time-to-live (TTL) expires

This changes only when network converges to a new,

coherent version of the routing tables


Drawbacks of Distance Vector Routing:
Counting to “Infinity” (1)
From A Link Cost
A’s stable routing table
A local 0
B 3 3 after link 1 fails
D 3 1
C 3 3
E 3 2 A B
1

A=0, B=3, D=1, 2
C=3, E=2 C
A transmits its last
3 4
DV before D does

Link 6 fails 5

6
From D Link Cost D E
D local 0
A 3 1 D’s routing table
B 6 inf immediately
E 6 inf
C 6 inf after link 6 fails


From A Link Cost

A local 0
B 3 3
D 3 1
C 3 3
E 3 2 A B
1

D transmits its 2
C
updated DV
D=0, A=1, B=4, 3 4
E=3, C=4

5

6
D local 0
A 3 1
B 3 4
E 3 3
C 3 4 D updates its routing table


From A Link Cost
Then A updates its routing table
A local 0
B 3 5
D 3 1
A=0, B=5, D=1,
C 3 5
C=5, E=4
E 3 4 A B
1

A transmits its 2
C
updated DV
3 4
We are in an
infinite loop! 5

6
D local 0
A 3 1
B 3 4 Infinite loop broken by some
E 3 3
C 3 4 convention on the representation
of “infinity ”

Bouncing Effect (1)
All links except 5 have unit
From A Link Cost cost, link 5 cost = 10 From B Link Cost

A local 0 B local 0
B 1 1 A 1 1
C 1 2 C 2 1
D 3 1 D 1 2
E 3 2 A B E 4 1
1

2
C
Routes towards C
3 4 From Link Cost
AC 1 2
BC 1 1
5 CC local 0
DC 3 3
6 EC 4 2

D local 0 E local 0
A 3 1 A 5 2
B 3 2 B 4 1
C 3 3 C 4 2
E 5 1 D 5 1


Bouncing Effect (2)
B’s routing table
A local 0 B local 0 immediately after
B 1 1 A 1 1
C 1 2 C 2 inf link 2 fails
D 3 1 D 1 2
E 3 2 A B E 4 1
1
Link 2 fails
A=0, B=1, C=2, 2
D=1,E=2 C
Routes towards C
A transmits its DV 3 4 From Link Cost
before B does AC 1 2
BC 1 inf
5 CC local 0
DC 3 3
6 EC 4 2

D local 0 E local 0 Routes towards C
A 3 1 A 5 2
immediately after
B 3 2 B 4 1
C 3 3 C 4 2 B’s update of its
E 5 1 D 5 1 routing table


Bouncing Effect (3)
From A Link Cost cost, link 5 cost = 10 From B Link Cost However, B
updates its routing
A local 0 B local 0
B transmits its new DV A 1 1 table based on DV
B 1 1
C 1 2 B=0, A=1, C=3, C 1 3 from A (causing the
D 3 1 D=1,E=1 D 1 2
E 4 1
route towards C to
E 3 2 A B
1 change also)

2
C
Routes towards C
3 4 From Link Cost
AC 1 2
BC 1 3
A’s DV produces 5 CC local 0

no change at D DC 3 3
6 EC 4 2

D local 0 E local 0
A 3 1 A 5 2
B 3 2 B 4 1
C 3 3 C 4 2
E 5 1 D 5 1


Bouncing Effect (4)

A local 0 B local 0
B 1 1 A 1 1
C 1 4 C 1 3
D 3 1 D 1 2
E 3 2 A B E 4 1
1

Further DV exchanges 2
produce no change in C
Routes towards C
routing tables! Both 3 4 From Link Cost
routing and distances AC 1 4
Loop!
have (temporarily) BC 1 3
5 CC local 0
stabilized
DC 3 3
6 EC 4 4

D local 0 E local 0
A 3 1 A 5 2
B 3 2 B 4 1
C 3 3 C 4 4
E 5 1 D 5 1

Packets for C can now “bounce” between A and B

Some Solutions for Problems in Distance
Vector Routing

 Split Horizon: If A routes packets for X via B, it should
not announce to B that X is a short distance from A!
A B X

Simple algo : Omit from DV any info about destinations
routed on the link
 “Poisonous reverse” algo : Set distance of destination
routed on the link to infinity

 Triggered Updates: transmit updates as soon as routing
table changes, don’t wait for end of update period


Routing Protocols: Link State (LS)
Routing

Global dispersal of local information Each router:
 Identifies itself to all its neighbors

 Constructs a link state packet (LSP) with:
Names of each neighbor
Cost of link to each neighbor

 Floods its LSP in the network

 Stores most recent copy of LSP from every other router

 Using LSPs constructs a full map of network topology, and
computes shortest route(s) to each destination (using an
appropriate shortest path algorithm, such as Dijkstra’s)

Operation of Link State Routing (1)

 Meeting neighbors A Hello B
 Xmit special pkt over link Hello C
(if pt to pt ) or to a group
address (if broadcast n/w or
Hello
LAN)
D Hello E
Name of neighbor Cost of Link to
that neighbor
A
B/1 B
D/1 A/1
C/1 C
 Construct Link State Packet A B E/1 B/1
1 D/1
(LSP) LSP is sent when: Letters represent 2 C
 Neighbor changes Node names
4
 Link goes up/down
5
 Also, periodically E
Numbers on links are D
D 6 D/1
E/1 E C/1
link identifiers (not cost) A/1 B/1


Operation of Link State Routing (2)

 Disseminate LSP to all Receive message
from A, say
routers in the network
LSP distrn cannot use Record in
database
No - Add to database,
- Broadcast message.
routing database ⇒ use from A ?

Flooding algorithm
Yes

Each LSP has “msg #” and Record # in
Yes - Replace record by
database <
“age” to enable receiving Record # in
new value.
- Broadcast message.
msg.?
node to distinguish between
old and new info No

Record # in No Transmit database
database = value on incoming
 Advantage: Record # in
msg.?
interface.

LSP can be forwarded
Yes
without any shortest path
Do nothing.
computation, so faster

Operation of Link State Routing: The
Flooding Protocol at Work (1)
Link State A
B D’s Link State Database
Packet, LSP B/1 A/1 (amalgamation of LSPs from
D/1 C
A B C/1 remaining network nodes)
1 E/1 B/1 Node name LSP #
D/1
2 C A #1 B #1 C #1 D #1 E #1
B/1 A/1 B/1 E/1 D/1
3 4 D/1 C/1 D/1 A/1 C/1
E/1 B/1
5
E
D 6 D/1
D E/1 E
C/1
A/1 B/1

B #2
A/inf
A B C/1
1 E/4 A #1 B #1 C #1 D #1 E #1
A #2 2 C B/1 A/1 B/1 E/1 D/1
B/inf D/1 C/1 D/1 A/1 C/1
3 D/1 4 E/1 B/1

5
6
D E
Internet Routing Protocols: Fundamental Operation 24
Copyright © 1998

A B D’s Link State Database
1
2 C A #2 B #1 C #1 D #1 E #1
B #2 B/inf A/1 B/1 E/1 D/1
3 A/inf 4 D/1 C/1 D/1 A/1 C/1
C/1
E/1
E/1 B/1
5 B #2
A/inf
A #2 6 C/1
D E E/1
B/inf
D/1

A B
1
2 A #2 B #2 C #1 D #1 E #1
B #2 A #2 C
4 B/inf A/inf B/1 E/1 D/1
A/inf 3 B/inf
C/1 D/1
D/1 C/1 D/1 A/1 C/1
E/1 E/1 B/1
5
6
D E



A #2 D’s Link State Database
B/inf
A B
D/1
1
A #2 B #2 C #1 D #1 E #1
2 C B/inf A/inf B/1 E/1 D/1
D/1 C/1 D/1 A/1 C/1
3 4 E/1 B/1

5
6 No further changes take place
D E
in the LS database

D (and all other nodes) can now
calculate the shortest path to
every destination


Advantages of Link State Routing

 Fast, loopless convergence:
Rapid new-information transfer via flooding

Local computation of shortest path tree

 Supports precise and multiple metrics
 Full topology database ⇒ eliminates long convergence time

of DV protocols for unequal cost links
 One DV database per metric (!) vs small overhead per metric

in LSP
 Supports multiple paths to a destination
 Enables load splitting, which is known to be efficient (less

congestion)
 Gives smoother rerouting on failure of one path (reliability)

 Better representation of external routes
 Not limited by “infinity” of DV protocols

Computation for N routers is O(N logN) versus O(N^2) for DV

Open Shortest Path First (OSPF)

 Operates within an autonomous system (AS)

 Therefore, example of an interior gateway protocol (IGP)

 Based on the link-state routing algorithm


OSPF Features (1)

 Runs directly over IP (with protocol field = 189)
 Organized into “ASs” and “areas ”
 Area border routers summarize reachable destinations, so that
area routers can pick a more optimal exit point

Legend
External
world
AS Boundary routers
Area 1 Area 2
Area backbone routers

Area border routers

OSPF Autonomous
System (AS) Designated routers
Area 3

Area routers


OSPF Features (2)

 Reduces adjacencies over broadcast networks by electing a
designated router (DR) and backup designated router (BDR)


OSPF Features (3)

 Five link state advertisement (LSA) types:
 Router links: links that start from the advertising router (flooded
in an area)
 Network links:advertised by designated routers for transit
networks (broadcast and non-broadcast)
 Network summary links: networks reachable from outside the
area via area border router
 Boundary router summary links: path from generating area
backbone router to AS boundary routers
 External links: path from AS bdry router to destinations outside
AS


Border Gateway Protocol (BGP)

 Operates across autonomous systems (AS)

 Therefore, example of an exterior gateway protocol
(EGP)

 Based on the distance-vector routing algorithm, but
modified to eliminate looping (and its associated
problems)


BGP Features

 Runs over TCP, which ensures reliable communication
 Eliminates need for complex error recovery mechanisms
 Makes BGP message size independent of IP pkt size

 BGP routers transmit a sequence of AS #s along the path
to a destination
Intermediate router on detecting its AS # terminates the
path to prevent a loop

 Allows for policy routing:
 BGP router uses configuration info to rank routes
 Important when determining transit rules between ASs


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