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COMS/CSEE 4140
Networking Laboratory
Lecture 05
Salman Abdul Baset
Spring 2008
Announcements
Lab 4 (1-4) due next week before your lab slot
 Assignment 2 due next Monday
 Class participation




Help me update the router/linux commands
Glossary
Lab participation

TAs / myself will ask random questions
Midterm (March 10th, duration ~1.5 hours)
 Projects

2
Previous Lecture
CIDR multi-homing and IP forwarding
 The Internet





Routing protocols



IETF, IRTF, IESG, IRB
IANA, ICANN
IETF (eight areas, 119 WGs)
Distance vector vs. link state
Intra-domain vs. inter-domain (IGP vs. EGP)
Routing Information Protocol (RIP)
3
Previous Lecture: The Count-toInfinity Problem
1
A
A's Routing Table
to
C
via
(next hop)
C
B's Routing Table
cost
B
1
B
via
to
2
(next hop)
C
cost
C
1
-
oo
A
3
-
oo
now link B-C goes down
C
B
2
C
C
C
-
2
oo
C
C
C
B
C
oo
C
4
C
oo
3
C
4
C
oo
4
Agenda
Routing Information Protocol (RIPv2)
 Link state protocols
 Open Shortest Path First (OSPF)
 Autonomous Systems (AS)

5
The Gang of Four
Link State
IGP
EGP
OSPF
IS-IS
Vectoring
RIP
BGP
6
RIP - History

Late 1960s :

Mid-1970s:


1982
1988

1993

1997
1998

Distance Vector protocols were used in the
ARPANET
XNS (Xerox Network system) routing protocol is
the precursor of RIP in IP (and Novell’s IPX RIP
and Apple’s routing protocol)
Release of routed for BSD Unix
RIPv1 (RFC 1058)
- classful routing
RIPv2 (RFC 1388)
- adds subnet masks with each route entry
- allows classless routing
RIPng (IPv6)
Current version of RIPv2 (RFC 2453) and Internet
standard (STD 56) (IPv4)
7
Routing Information Protocol

RIPv2


Subnet masks, next hop addresses, authentication (plain text),
multicast (instead of broad cast)
Count-to-infinity solution

Split-horizon
1
A
B
1
C
A never advertises to B that its path to C goes through B

Hold-down timer
A
1
B
1
C
B ignores any updates for the link B-C for a hold-down time

Triggered updates
A
1
B
1
B immediately advertises that its link is down.
C
8
Routing Information Protocol

Looping solution (for RIP messages)


Link costs




UDP port 520 (msgs sent and rcvd on this port)
Complete or partial routing table?



per table: update (30s + /- 0 to 5) send complete routing table in
unsolicited response to every neighbor router.
per entry: each entry has a timeout timer (180s)
per entry: route-flush timer (120s)
Dedicated port


Always one or 16 (link-down)
RIP timers


Maximum number of hops is 16.
Complete (may spread over multiple fragments)
No reliable delivery
Multicast

224.0.0.9
9
RIPv1 Packet Format
IP header UDP header
RIP Message
1: RIPv1
2: for IP
0…0: request full routing table
Command Version
Set to 00...0
address family
Set to 00.00
32-bit address
Unused (Set to 00...0)
Address of destination
Cost (measured in hops)
One RIP message can have
up to 25 route entries
20x25=500 bytes + 8 (RIP hdr)
+ 8 (UDP) + 20 (IP)=536 bytes
Unused (Set to 00...0)
one route entry
(20 bytes)
1: request
2: response
metric (1-16)
Up to 24 more routes (each 20 bytes)
10
32 bits
RIPv2

RIPv2 is an extends RIPv1:





Subnet masks are carried in the route information
Authentication of routing messages
Route information carries next-hop address
Exploits IP multicasting
Extensions of RIPv2 are carried in unused fields
of RIPv1 messages
11
RIPv2 Packet Format
IP header UDP header
RIP Message
2: RIPv2
2: for IP
0…0: request full routing table
Command Version
Set to 00...0
address family
Set to 00.00
32-bit address
Unused (Set to 00...0)
Address of destination
Cost (measured in hops)
One RIP message can
have up to 25 route entries
Unused (Set to 00...0)
metric (1-16)
one route entry
(20 bytes)
1: request
2: response
Up to 24 more routes (each 20 bytes)
12
32 bits
RIPv2 Packet Format
Used to carry information
from other routing
protocols (e.g.,
autonomous system
number)
RIPv2 Message
Command Version
Set to 00.00
address family
route tag
IP address
Subnet mask for IP
address
Subnet Mask
Next-Hop IP address
Identifies a better next-hop
address on the same
subnet than the advertising
router, if one exists
(otherwise 0….0)
metric (1-16)
Up to 24 more routes (each 20 bytes)
2: RIPv2
one route entry
(20 bytes)
IP header UDP header
Any
problems?
13
32 bits
RIP Messages

Dedicated port for RIP is UDP port 520.

Two types of messages:

Request messages


used to ask neighboring nodes for an update
Response messages

contains an update
14
Routing with RIP

Initialization: Send a request packet (command = 1, address
family=0..0) on all interfaces:






RIPv1 uses broadcast if possible,
RIPv2 uses multicast address 224.0.0.9, if possible
requesting routing tables from neighboring routers
Request received: Routers that receive above request send their
entire routing table
Response received: Update the routing table
Regular routing updates: Every 30 +/- 5 seconds, send all or
part of the routing tables to every neighbor in an response message
Triggered Updates: Whenever the metric for a route change, send
entire routing table.
15
Agenda
Routing Information Protocol (RIPv2)
 Link state protocols
 Open Shortest Path First (OSPF)
 Autonomous Systems

16
Link State Routing

Based on Dijkstra’ s Shortest-Path-First algorithm.

Each router starts by knowing:



Each router advertises to the entire network (flooding):





Prefixes of its attached networks.
Links to its neighbors.
Key idea: synchronize state with directly connected routers
Key idea: ACK the flooded messages
Prefixes of its directly connected networks
Active links to its neighbors.
Each router learns:

A complete topology of the network (routers, links).

Each router computes shortest path to each destination.

In a stable situation, all routers have the same graph, and compute
the same paths.
17
Dijkstra’s Shortest Path Algorithm for
a Graph
Input: Graph (N,E) with
N the set of nodes and E the set of edges
cvw
link cost (cvw = 1 if (v,w)  E, cvv = 0)
s
source node.
Output: Dn
cost of the least-cost path from node s to node n
M = {s};
for each n  M
Dn = csn;
while (M  all nodes) do
Find w  M for which Dw = min{Dj ; j  M};
Add w to M;
for each neighbor n of w and n  M
Dn = min[ Dn, Dw + cwn ];
Update route;
end for
end while
end for
18
Link state routing: graphical illustration
Global view:
b
3
a
a’s view:
3
a
d
b
6
b
d’s view:
c
c
1
a
c’s view:
2
c
6
a
b’s view: 3
1
2
d
c
b
1
c
2
d
6
19
Collecting all views yield a global & complete view of the network!
Operation of a Link State Routing
Protocol
Received
LSAs
Link State
Database
Dijkstra’s
Algorithm
IP Routing
Table
LSAs are flooded
to other interfaces
LSA: link-state advertisement
20
Link State Routing: Properties

Each node requires complete topology
information

Link state information must be flooded to all
nodes

Guaranteed to converge
21
Distance Vector vs. Link State Routing

With distance vector routing, each node has information only about
the next hop:






Node A: to reach F go to B
Node B: to reach F go to D
Node D: to reach F go to E
Node E: go directly to F
Distance vector routing makes
poor routing decisions if
directions are not completely
correct
(e.g., because a node is down).
A
B
C
D
E
F
If parts of the directions incorrect, the routing may be incorrect until the
routing algorithms has re-converged.
22
Distance Vector vs. Link State
Routing

In link state routing, each node has a complete map of
the topology
A


If a node fails, each
node can calculate
the new route
B
C
D
E
A
F
A
Difficulty: All nodes need to
have a consistent view of the
network
A
B
C
D
E
B
C
D
E
A
F
B
C
D
E
B
C
D
E
A
F
B
C
D
E
F
F
A
F
B
C
D
E
23
F
Distance Vector vs. Link State
Routing
Link State
•
•
•
•
•
•
Topology information is
flooded within the routing
domain
Best end-to-end paths are
computed locally at each
router.
Best end-to-end paths
determine next-hops.
Based on minimizing some
notion of distance
Works only if policy is
shared and uniform
Examples: OSPF, IS-IS
Vectoring
•
•
•
•
•
•
Each router knows little
about network topology
Only best next-hops are
chosen by each router for
each destination network.
Best end-to-end paths
result from composition of
all next-hop choices
Does not require any notion
of distance
Does not require uniform
policies at all routers
Examples: RIP, BGP
24
Agenda
Routing Information Protocol (RIPv2)
 Link state protocols
 Open Shortest Path First (OSPF)
 Autonomous Systems

25
OSPF



OSPF = Open Shortest Path First (Why Open?)
The OSPF routing protocol is the most important link
state routing protocol on the Internet (another link state
routing protocol is IS-IS (intermediate system to
intermediate system)
The complexity of OSPF is significant



RIP (RFC 2453 ~ 40 pages)
OSPF (RFC 2328 ~ 250 pages)
History:





1989: RFC 1131 OSPF Version 1
1991: RFC 1247 OSPF Version 2
1994: RFC 1583 OSPF Version 2 (revised)
1997: RFC 2178 OSPF Version 2 (revised)
1998: RFC 2328 OSPF Version 2 (current version)
26
Features of OSPF

Provides authentication of routing messages

Enables load balancing by allowing traffic to be split
evenly across routes with equal cost (problem:
reordering)

Type-of-Service routing allows to setup different routes
dependent on the TOS field

Supports subnetting

Supports multicasting

Allows hierarchical routing
27
Hierarchical OSPF
28
Hierarchical OSPF

Two-level hierarchy: local area, backbone.
 Link-state advertisements only in area
 each nodes has detailed area topology; only know
direction (shortest path) to nets in other areas.

Area border routers: “summarize” distances to
nets in own area, advertise to other Area Border
routers.

Backbone routers: run OSPF routing limited to
backbone.
29
Example Network
10.1.1.2
.1
4
.2
.2
3
.4
.4
3
.3
5
1
.5
.5
10.1.5.0/24
10.1.2.3
.6
.5
.3
.3
.6
10.1.7.0 / 24
.2
2
1
.4
10.1.4.0 / 24
10.1.1.0 / 24
Router IDs can be
selected
independent of
interface addresses,
but usually chosen to
be the smallest
interface address
2
10.1.3.0 / 24
.1
10.1.7.6
10.1.4.4
10.1.6.0 / 24
10.1.1.1
10.1.5.5
• Link costs are called Metric
• Metric is in the range [0 , 216]
• Metric can be asymmetric
30
Link State Advertisement (LSA)
10.1.1.1
10.1.1.2
10.1.1.0 / 24
3
2
.2
.2
.4
.4
.3
.5
.3
.6
.5
.5
10.1.5.0/24
10.1.2.3
.6
10.1.7.0 / 24
10.1.4.0 / 24
.3

.4
10.1.6.0 / 24
.1
.2
10.1.3.0 / 24
4
.1
10.1.7.6
10.1.4.4
10.1.5.5
The LSA of router 10.1.1.1 is as follows:
Link State ID:
10.1.1.1 = Router ID
Advertising Router:
10.1.1.1 = Router ID
Number of links:
3 = 2 links plus router itself
Description of Link 1:
Description of Link 2:
Description of Link 3:
Link ID = 10.1.1.2, Metric = 4
Link ID = 10.1.2.2, Metric = 3
Link ID = 10.1.1.1, Metric = 0
31
Network and Link State Database
10.1.1.1
10.1.1.0 / 24
Each router has a
database which
contains the LSAs
from all other routers
.2
.2
.4
10.1.4.0 / 24
.4
.4
.3
.5
.3
.3
.6
10.1.7.0 / 24
10.1.6.0 / 24
.1
.2
10.1.7.6
10.1.4.4
10.1.3.0 / 24
.1
10.1.1.2
.6
.5
.5
10.1.5.0/24
10.1.5.5
10.1.2.3
LS Type
Link StateID
Adv. Router
Checksum
LS SeqNo
LS Age
Router-LSA
10.1.1.1
10.1.1.1
0x9b47
0x80000006
0
Router-LSA
10.1.1.2
10.1.1.2
0x219e
0x80000007
1618
Router-LSA
10.1.2.3
10.1.2.3
0x6b53
0x80000003
1712
Router-LSA
10.1.4.4
10.1.4.4
0xe39a
0x8000003a
20
Router-LSA
10.1.5.5
10.1.5.5
0xd2a6
0x80000038
18
Router-LSA
10.1.7.6
10.1.7.6
0x05c3
0x80000005
1680
32
Link State Database

The collection of all LSAs is called the link-state database

Each router has an identical link-state database

Useful for debugging: Each router has a complete description of
the network

If neighboring routers discover each other for the first
time, they will exchange their link-state databases

The link-state databases are synchronized using reliable
flooding (flooded packets are acknowledged using ‘Link
State Acknowledgement’ packet)
33
OSPF Packet Format
OSPF Message
IP header
OSPF Message
Header
OSPF packets are not
carried as UDP payload!
OSPF has its own IP
protocol number: 89
TTL: set to 1 (in most cases)
Body of OSPF Message
Message Type
Specific Data
LSA
LSA
Header
Destination IP: neighbor’s IP address or 224.0.0.5
(ALLSPFRouters) or 224.0.0.6 (AllDRouters)
LSA
... ...
LSA
LSA
Data
34
OSPF Packet Format
OSPF Message
Header
2: current version
is OSPF V2
version
Message types:
1: Hello (tests reachability)
2: Database description
3: Link state request
4: Link state update
5: Link state acknowledgement
Standard IP checksum taken
over entire packet
Authentication passwd = 1:
Authentication passwd = 2:
Body of OSPF Message
type
message length
source router IP address
ID of the Area
from which the
packet originated
Area ID
checksum
authentication type
authentication
authentication
32 bits
64 cleartext password
0x0000 (16 bits)
KeyID (8 bits)
Length of MD5 checksum (8 bits)
Nondecreasing sequence number (32 bits)
0: no authentication
1: Cleartext
password
2: MD5 checksum
(added to end
packet)
Prevents replay
35
attacks
OSPF LSA Format
LSA
Link Age
LSA
Header
LSA
Header
LSA
Data
Link Type
Link State ID
advertising router
link sequence number
checksum
length
Link ID
Link 1
Link Data
Link Type #TOS metrics
Metric
Link ID
Link 2
Link Data
Link Type #TOS metrics
Metric
36
Discovery of Neighbors


Routers multicasts OSPF Hello packets on all OSPF-enabled
interfaces.
If two routers share a link, they can become neighbors, and
establish an adjacency
10.1.10.1
10.1.10.2
Scenario:
Router 10.1.10.2 restarts
OSPF Hello
OSPF Hello: I heard 10.1.10.2

After becoming a neighbor, routers exchange their link state
databases
37
Neighbor discovery and database
synchronization
Scenario:
Router 10.1.10.2
restarts
Discovery of
adjacency
10.1.10.1
10.1.10.2
OSPF Hello
OSPF Hello: I heard 10.1.10.2
After neighbors are discovered the nodes exchange their databases
Database Description: Sequence = X
Sends database
description.
(description only
contains LSA
headers)
Acknowledges
receipt of
description
Database Description: Sequence = X, 5 LSA headers =
Router-LSA, 10.1.10.1, 0x80000006
Router-LSA,
10.1.10.2, 0x80000007
Router-LSA,
10.1.10.3, 0x80000003
Router-LSA,
10.1.10.4, 0x8000003a
Router-LSA,
10.1.10.5, 0x80000038
Router-LSA,
10.1.10.6, 0x80000005
Database Description: Sequence = X+1, 1 LSA header=
Router-LSA,
10.1.10.2, 0x80000005
Database Description: Sequence = X+1
Sends empty
database
description
Database
description of
10.1.10.2
38
Regular LSA exchanges
10.1.10.1
Link State Request packets, LSAs =
Router-LSA,
10.1.10.1,
Router-LSA,
10.1.10.2,
Router-LSA,
10.1.10.3,
Router-LSA,
10.1.10.4,
Router-LSA,
10.1.10.5,
Router-LSA,
10.1.10.6,
10.1.10.1 sends
requested LSAs
10.1.10.2
10.1.10.2 explicitly
requests each LSA
from 10.1.10.1
Link State Update Packet, LSAs =
Router-LSA, 10.1.10.1, 0x80000006
Router-LSA, 10.1.10.2, 0x80000007
Router-LSA, 10.1.10.3, 0x80000003
Router-LSA, 10.1.10.4, 0x8000003a
Router-LSA, 10.1.10.5, 0x80000038
Router-LSA, 10.1.10.6, 0x80000005
39
Dissemination of LSA-Update



A router sends and refloods LSA-Updates, whenever the topology or
link cost changes. (If a received LSA does not contain new
information, the router will not flood the packet)
Exception: Infrequently (every 30 minutes), a router will flood LSAs
even if there are not new changes.
Acknowledgements of LSA-updates:



explicit ACK, or
implicit via reception of an LSA-Update
Question: If a new node comes up, it could build the database from
regular LSA-Updates (rather than exchange of database
description). What role do the database description packets play?
40
Agenda
Routing Information Protocol (RIPv2)
 Link state protocols
 Open Shortest Path First (OSPF)
 Autonomous Systems

41
Autonomous Systems

An autonomous system (AS) is a region of the Internet that is
administered by a single entity and that has a unified routing policy

Each autonomous system is assigned an Autonomous System Number
(ASN).





Columbia campus network (AS14)
Rogers Cable Inc. (AS812)
Sprint (AS1239, AS1240, AS 6211, …)
Interdomain routing is concerned with determining paths between
autonomous systems (interdomain routing)
Routing protocols for interdomain routing are called exterior gateway
protocols (EGP)
42
Autonomous Systems (AS)
Ethernet
Router
Ethernet
Ethernet
Autonomous
System 1
Router
Router
Router
Ethernet
Router
Ethernet
Autonomous
System 2
Router
Ethernet
43
Interdomain and Intradomain
Routing
AS 2
AS 5
AS 1
AS 6
AS 7
AS 3
AS 4

Routing protocols for intradomain routing are called interior
gateway protocols (IGP)


Objective: shortest path
Routing protocols for interdomain routing are called exterior
gateway protocols (EGP)
44

Objective: satisfy policy of the AS
Interdomain vs. Intradomain
AS 2
AS 2
EGP (e.g., BGP)
IGP (e.g., OSPF)

Intradomain routing



IGP (e.g., RIP)
Routing is done based on metrics
Routing domain is one autonomous system
Interdomain routing


Routing is done based on policies
Routing domain is the entire Internet
45
Interdomain Routing


Interdomain routing is based on connectivity between
autonomous systems
Interdomain routing can ignore many details of router
interconnection
AS 1
AS 2
AS 3
46
AS Graphs
AT&T North America
From: T. Griffin, BGP Tutorial, ICNP 2002
47
Multiple Routing Protocols


Multiple routing
protocols can run on the
same router
Each routing protocol
updates the routing table
RIP
Process
BGP
Process
OSPF
Process
routing
protocol
routing
protocol
routing table updates
routing
table
routing table
lookup
incoming IP
datagrams
IP
Forwarding
outgoing IP
datagrams
48
Autonomous Systems Terminology
local traffic
= traffic with source or
destination in AS
 transit traffic = traffic that passes through
the AS
 Stub AS
= has connection to only one
AS, only
carry local traffic
 Multihomed AS = has connection to >1 AS,
but does
not carry transit traffic
 Transit AS
= has connection to >1 AS and
carries
transit traffic

49
Stub and Transit Networks
AS 1



AS 1, AS 2, and AS 5
are stub networks
AS 2 is a multihomed stub network
AS 3 and AS 4 are
transit networks
AS 2
AS 3
AS 4
AS 5
50
Selective Transit
Example:
AS 1
 Transit AS 3 carries traffic
between AS 1 and AS 4 and
between AS 2 and AS 4
 But AS 3 does not carry
traffic between AS 1 and AS
2

The example shows a
routing policy.
AS 2
AS 3
AS 4
51
Customer/Provider
AS 2
Customer/
Provider
Customer/
Provider
AS 4
Customer/
Provider
AS 6



AS 5
Customer/
Provider
AS 6
Customer/
Provider
AS 6
A stub network typically obtains access to the Internet
through a transit network.
Transit network that is a provider may be a customer for
another network
Customer pays provider for service
52
Customer/Provider and Peers
AS 1
AS 2
AS 3
Peers
Peers
Customer/
Provider
Customer/
Provider
Customer/
Provider
AS 4
AS 5
Customer/Provider
AS 6
Customer/
Provider
AS 6
AS 6




Transit networks can have a peer relationship
Peers provide transit between their respective customers
Peers do not provide transit between peers
Peers normally do not pay each other for service
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Shortcuts through peering
AS 1
AS 2
AS 3
Peers
Peers
Customer/
Provider
Customer/
Provider
AS 4
AS 5
Customer/
Provider
AS 6
Peers
Customer/Provider
Customer/
Provider
AS 6
AS 6



Note that peering reduces upstream traffic
Delays can be reduced through peering
But: Peering may not generate revenue
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This week’s lab
/etc/quagga/ripd.conf
 eth1 does not work on some machines (PC1 and
PC2 of rack 3)
 Set eth1 to a completely different IP address e.g.,
202.11.12.15 and use eth2
 Enable debugging – and observe
/etc/quagga/ripd.log
 Count-to-infinity


disable split-horizon, triggered updates and set holddown timer to zero.
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