Download Internet Topology

1
LINK STATE ROUTING
PROTOCOLS
Dr. Rocky K. C. Chang
22 November 2010
Link state approach
2


In the DV approach, each router discovers the
network topology through the distance vectors
received from its neighbors.
In the LS approach, each router discovers the
network topology through link state advertisements
(LSAs).
 Each
router is responsible for discovering its neighbors
and the states of the links between them, and informs
other routers.
 Upon receiving all the link states, a router can form a
local view of the entire network.
An example [from [3])
3
Routers connected by point-to-point links
 Networks
connected to the routers are omitted.
 Link states are generally asymmetric.
B
4
2
3
A
1
E
5
C
1

3
D
After exchanging LSAs with neighbors (from [1])
4

For router A
B
Link
A
4
4
A
Link
B
4
C
3
E
1
3
1
E
Link
A
1
C
Link
A
3
After receiving all LSAs (from [1])
5
After the protocol converges, the routers’ link state
databases are identical.
B
Link
A
4
D
2
4
A
Link
B
4
C
3
E
1
2
C
Link
A
3
D
5
E
1
3
1
E
Link
A
1
C
1
D
3
1

5
3
D
Link
B
2
C
5
E
3
Link state routing protocols
6

Therefore, there are two main parts in a link state
routing protocol.
 (Topology
dissemination) Construct and maintain a
consistent link state database in all routers.
 (Route computation) Each router computes minimum-cost
paths to all destinations in the network.
Topology dissemination
7

Neighbor discovery
 Use
Hello protocols to discover neighbors and to elect a
designated router on a shared medium.

Bringing up adjacencies in a point-to-point network
A
set of protocols (database description, link state
request/update) to synchronize the link state databases
of the two routers

On a shared medium, a designated router
 forms
adjacency with other routers, and
 responsible for advertising the link states.
Topology dissemination
8
A
C
B
D
A
B
C
D
N(N-1)/2
adjacencies
Shared medium
A
B
A
B
LAN
C
D
(N-1)
adjacencies
C
D
Link
advertisements
Link state advertisements
9

A link-state router generates a link state
advertisement (LSA) when discovering
a
new neighbor router, or
 the cost of the link to an existing neighbor changes
(including link failure).

The LSA describes the latest changes in the network
topology and must be transmitted to all other
routers as soon as possible.
 Dissemination
cannot be based on the routing
information. Why not?
An example
10

If B fails, A needs to update the link state for A->B.
 But
the spanning tree computed based on the original
topology does not reach all nodes.
A
B
Reliable flooding protocol
11

The advertising router floods an LSA to all other
interfaces.


Other routers forward the LSA to all interfaces except the
one that the LSA is received.
To ensure reliability, all LSAs are positiveacknowledged.
A checksum is used to detect data corruption of an LSA,
which covers the entire LSA except for some mutable fields.
 The checksum is used both when the LSA is flooded and kept
in the router’s database.

Packet storm prevention
12

To prevent packet storms from occurring, use some
information to differentiate new LSAs from old LSAs.
 Each
router must keep the latest LSAs.
 LSAs that are not newer are discarded.

Differentiating between new and old LSAs
 Timestamps
with local and global meanings (stamped
by the advertising router).
 A combination of sequence number (SN) and age.
LSAs’ sequence numbers and ages
13

A new LSA is indicated by a larger SN (the
advertising router increments the SN for every LSA
it sends).
 The
age and other information may also be used when
the SNs are identical.

Design of the sequence numbering
 Circular
SN space (SN wraparounds)
 Linear SN space (no SN wraparounds).
Linear sequence number space
14

The SN starts from some value, say 0 (this initial
value may be negative).
 The
SN space should be large enough, say 32-bit SN
space.

When attempting to increment the SN beyond the
largest SN,
 Prematurely
age the LSA and reflooding it.
 After receiving ACKs from all neighbor routers, send the
LSA with the initial SN.
LSA messages
15

To recap, an LSA message must minimally include the
following information
 Some
identification information of the advertising router
 Link state type and its value
 Age, sequence number, and checksum
Route computation
16

Given the network topology, each router computes
minimum-cost paths from itself to other destinations (a
spanning tree).




Use any efficient single-source, shortest-path algorithms,
such as Dijskstra’s algorithm.
The link state can be based on any type of service
(TOS), such as delay, reliability, etc.
Compute routes for different TOS, and equal-cost paths
for load balancing.
The computed routes can be used in source routing or
hop-by-hop routing.
An example
17
B
4
2
3
5
1
1
C
D
3
B
E
4
2
3
A
1
E
5
C
1
A
3
D
The optimality principle
18

If router E is on the optimal path from router A to
router D, then
 the
optimal path from router E to router D also falls
along the same path.


Given that the link state databases are identical in
all routers, each local forwarding decision gives a
globally optimal path.
If the link state databases are not identical, can the
routers forward packets on an optimal path?
Area routing
19



To make the link state routing more scalable, some
routers and their networks may be grouped into the
same area.
Intra-area routing is essentially a separate copy of
the link state routing protocol.
Inter-area routing relies on area border routers that
have routing info in more than one area.
 E.g.,
Router D “summarizes” the routing information in
area 2 and floods it to the area 1, and vice versa.
Area routing example (from [1])
20
area 1
A
C
B
area 2
E
D
F
Link and router failures
21


When a link fails, the routers on both side send
updated LSAs for this link to age the LSAs in other
routers.
To cater for router failures, each LSA is associated
with an age (determined by the advertising router).
 Each
router periodically sends out its LSAs even though
they are not changed, e.g., every half-an-hour.
 An aged LSA will also be sent out to other routers to
age the LSAs in other routers.
Initial sequence number
22


If a router boots up (after failure), it needs to know
the latest SNs for its LSAs.
One solution is to use a distinct initial sequence
number to start with. Let NO be the oldest SN (2**31 + 1 in OSPF).
 When
the router boots up, its sends LSAs with SN = NO.
 When other routers received LSAs with SN = NO, they
inform this router of the latest SNs.
Network partitions
23





Link/router failures could partition a network.
After the partition is healed, the link state databases in the
two previously partitioned networks may not be consistent.
After the network partition, router B is unaware of the failure
of link E-G.
After the link C-F is restored, router B may forward packets to
router E via router G.
Whenever a failed link is restored, bringing up adjacency is
performed.
Network partition example (from [1])
24
A
E
3
2
1
B
C
D
F
G
H
Open shortest path first protocol
25


OSPF is an implementation of the link state
approach.
OSPF calculates separate routes for each TOS.
 When
several equal-cost routes to a destination exist,
traffic is distributed equally among them.
 The cost of a route is described by a single dimensionless
metric.

OSPF allows sets of networks to be grouped
together, i.e., areas.
 The
topology of an area is hidden from outside.
Open shortest path first protocol
26

OSPF enables the flexible configuration of IP subnets.
 Each
route distributed by OSPF has a destination and
mask.
 Two different subnets of the same IP network number
may have different sizes.


All OSPF protocol exchanges are authenticated.
Externally derived routing data is passed
transparently throughout the OSPF network.
An OSPF network example
27


A cost is associated with the output side of each router
interface.
Three kinds of nodes:
Router (transit)
 Network (transit), e.g., N8 and N9
 Stub network (nontransit), e.g., N1 and N2




N3, 6, 8, 9 are broadcast networks.
RT5 and RT7 are AS border routers.
N12-N14 are external networks, and the routes to N12
are advertised by both RT5 and RT7.
28
+
| 3+---+
N12
N14
N1|--|RT1|\ 1
\ N13 /
| +---+ \
8\ |8/8
+
\ ____
\|/
/
\
1+---+8
8+---+6
* N3 *---|RT4|------|RT5|--------+
\____/
+---+
+---+
|
+
/
|
|7
|
| 3+---+ /
|
|
|
N2|--|RT2|/1
|1
|6
|
| +---+
+---+8
6+---+
|
+
|RT3|--------------|RT6|
|
+---+
+---+
|
|2
Ia|7
|
|
|
|
+---------+
|
|
N4
|
|
|
|
|
|
N11
|
|
+---------+
|
|
|
|
|
N12
|3
|
|6 2/
+---+
|
+---+/
|RT9|
|
|RT7|---N15
+---+
|
+---+ 9
|1
+
|
|1
_|__
|
Ib|5
__|_
/
\
1+----+2
| 3+----+1
/
\
* N9 *------|RT11|----|---|RT10|---* N6 *
\____/
+----+
|
+----+
\____/
|
|
|
|1
+
|1
+--+
10+----+
N8
+---+
|H1|-----|RT12|
|RT8|
+--+SLIP +----+
+---+
|2
|4
|
|
+---------+
+--------+
N10
N7
**FROM**
*
*
T
O
*
*
29
|RT|RT|RT|RT|RT|RT|RT|RT|RT|RT|RT|RT|
|1 |2 |3 |4 |5 |6 |7 |8 |9 |10|11|12|N3|N6|N8|N9|
----- --------------------------------------------RT1| | | | | | | | | | | | |0 | | | |
RT2| | | | | | | | | | | | |0 | | | |
RT3| | | | | |6 | | | | | | |0 | | | |
RT4| | | | |8 | | | | | | | |0 | | | |
RT5| | | |8 | |6 |6 | | | | | | | | | |
RT6| | |8 | |7 | | | | |5 | | | | | | |
RT7| | | | |6 | | | | | | | | |0 | | |
RT8| | | | | | | | | | | | | |0 | | |
RT9| | | | | | | | | | | | | | | |0 |
RT10| | | | | |7 | | | | | | | |0 |0 | |
RT11| | | | | | | | | | | | | | |0 |0 |
RT12| | | | | | | | | | | | | | | |0 |
N1|3 | | | | | | | | | | | | | | | |
N2| |3 | | | | | | | | | | | | | | |
N3|1 |1 |1 |1 | | | | | | | | | | | | |
N4| | |2 | | | | | | | | | | | | | |
N6| | | | | | |1 |1 | |1 | | | | | | |
N7| | | | | | | |4 | | | | | | | | |
N8| | | | | | | | | |3 |2 | | | | | |
N9| | | | | | | | |1 | |1 |1 | | | | |
N10| | | | | | | | | | | |2 | | | | |
N11| | | | | | | | |3 | | | | | | | |
N12| | | | |8 | |2 | | | | | | | | | |
N13| | | | |8 | | | | | | | | | | | |
N14| | | | |8 | | | | | | | | | | | |
N15| | | | | | |9 | | | | | | | | | |
H1| | | | | | | | | | | |10| | | | |
Link state representations: routers and LANs
30
**FROM**
*
*
T
O
*
*
|RT12|N9|N10|H1|
-------------------RT12|
| |
| |
N9|1
| |
| |
N10|2
| |
| |
H1|10 | |
| |
RT12's router links
advertisement
**FROM**
*
*
T
O
*
*
|RT9|RT11|RT12|N9|
---------------------RT9|
|
|
|0 |
RT11|
|
|
|0 |
RT12|
|
|
|0 |
N9|
|
|
| |
N9's network links
advertisement
Shortest-path tree rooted at RT6
31


Links without labels have costs of 0, e.g., N3 to R1,
2, 4.
RT6 has a link to address Ib (destination) and
another link to RT10 (transit).
 Numbered



vs unnumbered point-to-point links
RT10 uses RT7 to reach external network N12.
RT6 to address Ia (destination)?
Only the next hop to the destination is used in the IP
forwarding.
32
RT6(origin)
RT5 o------------o-----------o Ib
/|\
6
|\
7
8/8|8\
| \
/ | \
| \
o
|
o
6 |
\7
N12 o N14
|
\
N13
2 |
\
N4 o-----o RT3 \
/
\
5
1/
RT10 o-------o Ia
/
|\
RT4 o-----o N3
3| \1
/|
| \ N6
RT7
/ |
N8 o
o---------o
/ |
|
|
/|
RT2 o
o RT1
|
|
2/ |9
/
|
|
|RT8
/ |
/3
|3
RT11 o
o
o
o
/
|
|
|
N12 N15
N2 o
o N1
1|
|4
|
|
N9 o
o N7
/|
/ |
N11
RT9
/ |RT12
o--------o-------o
o--------o H1
3
|
10
|2
|
o N10
R6’s forwarding table for internal destinations
33
Destination
Next Hop
Distance
__________________________________
N1
RT3
10
N2
RT3
10
N3
RT3
7
N4
RT3
8
Ib
*
7
Ia
RT10
12
N6
RT10
8
N7
RT10
12
N8
RT10
10
N9
RT10
11
N10
RT10
13
N11
RT10
14
H1
RT10
21
R6’s forwarding table for external destinations
34
Destination
Next Hop
Distance
__________________________________
N12
RT10
10
N13
RT5
14
N14
RT5
14
N15
RT10
17
OSPF areas
35

OSPF uses the concept of areas to define the routing
environment.
An area is a logical collection of networks, identified by a
32-bit ID.
 All routers belonging to the same area disclose complete
information about all networks within that area.
 There is a special area that must be defined for all OSPF
domains---the backbone area (0.0.0.0).


The backbone is responsible for distributing routing
information between areas.
Inter-area routing
36


Internal, area border, backbone, AS boundary
routers
Inter-area routing: from source to
 an
area border router to
 a backbone path between the source and destination
areas to
 another intra-area path to the destination

The correct area border to use for exiting the
source area is chosen exactly the same way as how
external routers are chosen (e.g., N12).
37
...........................
.
+
.
.
| 3+---+
.
N12
N14
. N1|--|RT1|\ 1
.
\ N13 /
.
| +---+ \
.
8\ |8/8
.
+
\ ____
.
\|/
.
/
\
1+---+8
8+---+6
.
* N3 *---|RT4|------|RT5|--------+
.
\____/
+---+
+---+
|
.
+
/
\
.
|7
|
.
| 3+---+ /
\ .
|
|
. N2|--|RT2|/1
1\ .
|6
|
.
| +---+
+---+8
6+---+
|
.
+
|RT3|------|RT6|
|
.
+---+
+---+
|
.
2/ .
Ia|7
|
.
/ .
|
|
.
+---------+ .
|
|
.Area 1
N4
.
|
|
...........................
|
|
..........................
|
|
.
N11
.
|
|
.
+---------+
.
|
|
.
|
.
|
|
N12
.
|3
.
Ib|5
|6 2/
.
+---+
.
+----+
+---+/
.
|RT9|
.
.........|RT10|.....|RT7|---N15.
.
+---+
.
.
+----+
+---+ 9
.
.
|1
.
.
+ /3
1\
|1
.
.
_|__
.
.
| /
\
__|_
.
.
/
\
1+----+2
|/
\ /
\
.
.
* N9 *------|RT11|----|
* N6 *
.
.
\____/
+----+
|
\____/
.
.
|
.
.
|
|
.
.
|1
.
.
+
|1
.
. +--+
10+----+
.
.
N8
+---+
.
. |H1|-----|RT12|
.
.
|RT8|
.
. +--+SLIP +----+
.
.
+---+
.
.
|2
.
.
|4
.
.
|
.
.
|
.
.
+---------+
.
.
+--------+
.
.
N10
.
.
N7
.
.
.
.Area 2
.
.Area 3
.
................................
..........................
Advertising link states into Area 1
38




Internal destinations: N1-N4
Area border routers RT3 and RT4 advertise into
Area 1 the distances to all destinations external to
the area.
RT3 and RT4 must also advertise into Area 1 the
locations of the AS boundary routers RT5 and RT7.
Finally, external advertisements from RT5 and RT7
are flooded throughout the entire AS, including
Area 1.
**FROM**
|RT|RT|RT|RT|RT|RT|
|1 |2 |3 |4 |5 |7 |N3|
----- ------------------RT1| | | | | | |0 |
RT2| | | | | | |0 |
RT3| | | | | | |0 |
*
RT4| | | | | | |0 |
*
RT5| | |14|8 | | | |
T
RT7| | |20|14| | | |
O
N1|3 | | | | | | |
*
N2| |3 | | | | | |
*
N3|1 |1 |1 |1 | | | |
N4| | |2 | | | | |
Ia,Ib| | |15|22| | | |
N6| | |16|15| | | |
N7| | |20|19| | | |
N8| | |18|18| | | |
N9-N11,H1| | |19|16| | | |
N12| | | | |8 |2 | |
N13| | | | |8 | | |
N14| | | | |8 | | |
N15| | | | | |9 | |
39
Figure 7: Area 1's Database.
**FROM**
|RT|RT|RT|RT|RT|RT|RT
|3 |4 |5 |6 |7 |10|11|
-----------------------RT3| | | |6 | | | |
RT4| | |8 | | | | |
RT5| |8 | |6 |6 | | |
RT6|8 | |7 | | |5 | |
RT7| | |6 | | | | |
* RT10| | | |7 | | |2 |
* RT11| | | | | |3 | |
T
N1|4 |4 | | | | | |
O
N2|4 |4 | | | | | |
*
N3|1 |1 | | | | | |
*
N4|2 |3 | | | | | |
Ia| | | | | |5 | |
Ib| | | |7 | | | |
N6| | | | |1 |1 |3 |
N7| | | | |5 |5 |7 |
N8| | | | |4 |3 |2 |
N9-N11,H1| | | | | | |1 |
N12| | |8 | |2 | | |
N13| | |8 | | | | |
N14| | |8 | | | | |
N15| | | | |9 | | |
Figure 8: The backbone's database.
Summary
40




LS protocols have been deployed for intra-domain
routing, such as IS-IS and OSPF.
LS protocols rely on reliable mechanisms to update
link states of the network in all routers.
Compared with the DV approach, LS protocols are
more complex to design and also more difficult to
manage.
While the LS approach scales better than the DV
approach, the LS still cannot be used in interdomain
routing.
References
41
1.
2.
3.
4.
5.
M. Steenstrup, Routing in Communications Networks,
Prentice Hall, 1995.
R. Perlman, Interconnection, Second Edition, Addison
Wesley, 2000.
S. Keshav, An Engineering Approach to Computer
Networking, Addison Wesley, 1997.
C. Huitema, Routing in the Internet, Prentice Hall
PTR, Second Edition, 1999.
OSPF version 2 (RFC 1583)