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COMS/CSEE 4140
Networking Laboratory
Lecture 04
Salman Abdul Baset
Spring 2008
Announcements
Lab 3 (5-7) and prelab 4 due next week before
your slot.
 Grades for prelab 1 &2 and lab report 1 & 2 will
be posted this week.
 Lab report format.

2
Previous Lecture

More on CIDR



Internet Protocol (IP)



Type/code, update routing tables
IP forwarding



Best effort datagram service.
Fragmentation, TTL, protocol de-multiplexing
Internet Control Message Protocol (ICMP)


Hierarchical addressing, route aggregation
Multi-homing
Forwarding and routing
Configure IP address, default gw, static, dynamic
Router architecture

Shared bus (1st generation), shared bus with line card cache (2nd
generation), switched architecture.
3
Agenda
CIDR multi-homing and IP forwarding
 The Internet
 Routing protocols
 Routing Information Protocol (RIP)

4
CIDR Multi-homing
Comp
1
129.21/16
AOL
129.0/8
My-ISP
129.15/16
208.20/16
Comp
2
Verizo
n
208.20.16/8
5
CIDR Multi-homing

My-ISP requests
Verizon to advertise
its prefixes.

Q: I have a 128.59/16
network. Can I
connect a
202.15.16.23 IP
address?
Typically no.
Announcements
smaller than /24 not
accepted.
Comp
1
129.21/16
AOL
129.0/8
My-ISP
129.15/16

208.20/16
129.15/16
Comp
2
Verizo
n
208.20.16/8
6
Routing and Forwarding
Control plane: run
routing protocols:
(RIP, OSPF, BGP)
RIB: routing information base
Data plane: forwarding
packets from incoming to
outgoing link
FIB: forwarding information base
7
Routing and Forwarding

Select the next-hop router.



Find the outgoing interface.
Find the MAC address of the next-hop router.
In Linux, you specify the IP address of the next-hop
router.

Longest-prefix first.

Default routing (implied by longest-prefix rule:
default has prefix of length 0).
8
Longest Prefix Match

Find route for 10.30.4.1
Host IP address: 10.0.2.41 netmask: 255.255.255.0
Bits mached
Network
prefix / host
Network mask
Interface
Next hop
11
10.0.2.0
255.255.255.0
eth1
16
10.30.0.0
255.255.0.0
eth2
10.30.0.1
24
10.30.4.0
255.255.255.0
eth3
10.30.4.4
0
0.0.0.0
0.0.0.0
eth1
10.0.2.1
9
default gw
Agenda
CIDR multi-homing and IP forwarding
 The Internet
 Routing protocols
 Routing Information Protocol (RIP)

10
A Definition (not the only one)

On October 24, 1995, the FNC unanimously passed a
resolution defining the term Internet.
•RESOLUTION: The Federal Networking Council (FNC) agrees that the
following language reflects our definition of the term "Internet".
"Internet" refers to the global information system that --
•(i) is logically linked together by a globally unique address space
based on the Internet Protocol (IP) or its subsequent
extensions/follow-ons;
•(ii) is able to support communications using the Transmission
Control Protocol/Internet Protocol (TCP/IP) suite or its subsequent
extensions/follow-ons, and/or other IP-compatible protocols; and
•(iii) provides, uses or makes accessible, either publicly or privately,
high level services layered on the communications and related
infrastructure described herein.
11
Applications of the Internet



Traditional core applications:
Email
News
Remote login
File transfer
The killer application:
World-Wide Web (WWW)
New applications:
Video
Telephony
Mesh-ups
P2P applications
Social networks
12
Brief History of the Internet
Mid 1960:
End 1969s:
1974:
1980:
1983:
1984:
1995:
Papers on “Packet Switching” emerge.
ARPA sponsors the development of a packet-switching
network, called the ARPANET. First four nodes are
UCLA, SRI, U. Utah, UCSB.
The TCP/IP protocols and model are being proposed by
Cerf/Kahn.
IPv4 is introduced
ARPANET adopts TCP/IP. At this time, the ARPANET has
200 routers.
NSF funds a TCP/IP based backbone network. This
backbone grows into the NSFNET, which becomes the
successor of the ARPANET.
NSF stops funding of NSFNET. The Internet is
completely commercial.
13
A Brief Summary of the
Evolution of the Internet
First Vast
Computer
Network
Silicon Envisioned
Chip
A
1962
Mathematical 1958
Theory of
Communication
Memex
1948
Conceived
Packet
Switching
Invented
1964
Hypertext
Invented
1965
TCP/IP
Created
ARPANET
1972
1969
Mosaic
Created
WWW
Internet Created
1993
Named
1989
and
Goes
TCP/IP
1984
Age of
eCommerce
Begins
1995
1945
1945
Copyright 2002, William F. Slater, III, Chicago, IL, USA
1995
14
Internet History
Source:
15
Growth of the Internet
Source: Internet Software Consortium
16
Internet Infrastructure
Tier-1
Tier-2
Regional
Network
Backbone Network
Regional
Network
IXP
local ISP
IXP
Backbone Network
local ISP
Regional
Network
Tier-3
local ISP
IXP
corporate
network
Regional
Network
campus
network
17
Internet Infrastructure

The infrastructure of the Internet consists of a federation
of connected networks that are each independently
managed (“autonomous system”)



Note: Each “autonomous system may consist of multiple IP
networks
Autonomous systems have a number (AS number)
Hierarchy of network service providers (NSPs)



Tier-1: nation or worldwide network (US: less than 20)
Tier-2: regional networks (in US: less than 100)
Tier-3: local Internet service provider (in US: several thousand)
18
Internet Infrastructure
Location where a network (ISP, corporate
network, or regional network) gets access to the
Internet is called a Point-of-Presence (POP).
 Locations (Tier-1 or Tier-2) networks are
connected for the purpose of exchanging traffic
are called peering points.



Public peering: Traffic is swapped in a specific
location, called Internet exchange points (IXPs)
Private peering: Two networks establish a direct
link to each other.
19
Node degrees
Source: caida.org
20
Topology of a Tier-1 NSP
21
Organization of a single node in a Tier1 network
....
Peering points
Leased links
to customers
3Com
Bay Networks
Modem Bank
Modem Bank
Modem Bank
Modem Bank
Leased links
to customers
Leased links
to customers
Bay Networks
SD
3Com
....
3Com
Modem Bank
....
Links to other
nodes of the
network
....
SD
....
3Com
Metropolitan
area networks
Dial-up and
leased links
to customers
22
Who is Who on the Internet ?
ISOC
ICANN
-IP addresses
-port numbers
-RIRs
-domain
names
performs
IANA
functions
IESG
IANA
IAB
Approves
RFC
[Get port number]
Technical management
IETF
IRTF
RFC
Internet standard
23
Who is Who on the Internet ?





Internet Society (ISOC): Founded in 1992, an international nonprofit professional
organization that provides administrative support for the Internet. Founded in 1992,
ISOC is the organizational home for the standardization bodies of the Internet.
Internet Engineering Task Force (IETF): Forum that coordinates the
development of new protocols and standards. Organized into working groups that are
each devoted to a specific topic or protocol. Working groups document their work in
reports, called Request For Comments (RFCs).
IRTF (Internet Research Task Force): The Internet Research Task Force is a
composed of a number of focused, long-term and small Research Groups.
Internet Architecture Board (IAB): a technical advisory group of the Internet
Society, provides oversight of the architecture for the protocols and the
standardization process
The Internet Engineering Steering Group (IESG): The IESG is responsible for
technical management of IETF activities and the Internet standards process.
Standards. Composed of the Area Directors of the IETF working groups.
24
Internet Standardization Process



Working groups present their working of the Internet
which are published as RFC (Request for Comments).
RFCs are the basis for Internet standards.
Not all RFCs become Internet Standards ! (There are
>5000 RFCs and less than 70 Internet standards)
A typical (but not only) way of standardization is:





Internet Drafts
RFC
Proposed Standard
Draft Standard (requires 2 working implementation)
Internet Standard (declared by IAB)
25
Assigning Identifiers for the
Internet



Who gives the university the domain name “virginia.edu”
Who assigns it the network prefix “128.143.0.0/16”?
Who assigns port 80 as the default port for web servers?

The functions associated with the assignment of numbers is
referred to as Internet Assigned Number Authority (IANA).

IANA used to be managed by Jon Postel at ISI

Since the 1990s, IP addresses and domain name allocation are
delegated to independent organizations. Different organizations are
responsible for allocating domain names and IP addresses
26
The IANA Function

The functions associated with the assignment of
numbers in the Internet is referred to as Internet
Assigned Number Authority (IANA).

IANA serves as a registry that keeps records of
assigned numbers:




IP addresses
Protocol numbers
Domain names (until 1992)
There is no charge for allocation.
27
Regional Internet Registries
(RIRs)



Registration and management of IP address is done by Regional
Internet Registries (RIRs)
Where do RIRs get their addresses from: IANA maintains a high-level
registry that distributes large blocks to RIRs
RIRs administer allocation of:




IPv4 address blocks
IPv6 address blocks
Autonomous system (AS) numbers
There are currently 5 RIRs worldwide:





APNIC (Asia/Pacific Region),
ARIN (North America and Sub-Sahara Africa),
LACNIC (Latin America and some Caribbean Islands)
RIPE NCC (Europe, the Middle East, Central Asia)
AfriNIC (African continent)
28
Transitioning of Domain Name
Registration





Until 1992: Domain name registration done as part of IANA
1992: InterNIC was created in a partnership between US
government and companies to organize and maintain the growing
DNS registry and services. The company Network Solutions ran the
administration of InterNIC. Until 1998, Network Solutions had a
monopoly for domain names.
1995: InterNIC started harging for domain names ($100 for 2
years)
1997: President Clinton directs the Secretary of Commerce to
privatize the management of the domain name system (DNS) in a
manner that increases competition and facilitates international
participation in its management.
1998: ICANN was created in response to a policy statement issued
by the US Department of Commerce that called for the formation of
a private sector not-for-profit Internet stakeholder to administer
policy for the Internet name and address system. ICANN operates
under a renewable 3-year contract with the US Department of the
Commerce.

ICANN accredits domain-name registrar for .com, .org., .net (and other29
domain)
ICANN

The Internet Corporation for Assigned Names and Numbers
(ICANN) is an internationally organized, non-profit corporation that
has responsibility for Internet Protocol (IP) address space
allocation, protocol identifier assignment, Top-Level Domain name
system management, and root server system management functions.




ICANN performs the IANA functions
ICANN accredits domain-name registrar for .com, .org., .net (and other
domain)
Since ICANN performs the IANA functions, it is in charge for
allocating all numbers. However, the main concern is the allocation
of domain names.
ICANN role is to oversee the domain-name registration system's
transition from government hands to private hands and to
coordinate its decentralization and the integration into a global
community.
30
Who is Who on the Internet ?
ISOC
ICANN
-IP addresses
-port numbers
-RIRs
-domain
names
performs
IANA
functions
IESG
IANA
IAB
Approves
RFC
[Get port number]
Technical management
IETF
IRTF
RFC
Internet standard
31
Agenda
CIDR multi-homing and IP forwarding
 The Internet
 Routing protocols
 Routing Information Protocol (RIP)

32
Routing Protocols

Recall: There are two parts to routing IP packets:
1. How to pass a packet from an input interface to the
output interface of a router (forwarding) ?
2. How to find and setup a route (routing) ?

We already discussed the packet forwarding part.


Longest prefix match
There are two approaches for calculating the routing
tables:


Static routing: manually modify the routes.
Dynamic routing: routes are calculated by a routing protocol.
33
Routing Protocols vs. Routing
Algorithms

Routing protocols establish routing tables at routers.

A routing protocol specifies:




What messages are sent between routers.
Under what conditions the messages are sent.
How messages are processed to compute routing tables.
At the heart of any routing protocol is a routing algorithm
that determines the path from a source to a destination.
34
Overview of Routing Protocols
Routing protocol
Routing Algorithm
Routing information protocol (RIP)
Distance vector
Interior gateway routing protocol
(IGRP, Cisco proprietary)
Distance vector
Open shortest path first (OSPF)
Link state
Intermediate System-to-Intermediate
System (IS-IS)
Link state
Border gateway protocol (BGP)
Path vector
Intra-domain routing protocol
Inter-domain routing protocol
RIP, OSPF, IGRP, IS-IS
BGP
35
Domain not to be confused with domain name. Here domain refers to administrative domain.
Intra-domain Routing vs. Inter-domain
Routing

Recall Internet is a network of networks.

Administrative autonomy



Internet = network of networks
each network admin. may want to control routing in its own
network
Scale: with 550 million destinations:

can’t store all destination IP addresses in routing tables!


perhaps yes but then …
routing table exchange would swamp links.
36
Autonomous Systems
Ethernet
Router
Ethernet
Ethernet
Autonomous
System 1
Router
Router
Router
Ethernet
Router


Ethernet
Autonomous
System 2
Router
Ethernet
aggregate routers into regions, “autonomous systems”
(AS) or domain
routers in the same AS run the same routing protocol.


“intra-AS” or intra-domain routing protocol
routers in different AS can run different intra-AS routing
protocol
37
Autonomous Systems

An autonomous system is a region of the Internet that is administered
by a single entity.

Examples of autonomous regions are:



Columbia campus network
AT&T backbone network
Regional Internet Service Provider

Routing is done differently within an autonomous system
(intradomain routing) and between autonomous system
(interdomain routing).

RIP, OSPF, IGRP, and IS-IS are intra-domain routing protocols.

BGP is the only inter-domain routing protocol.
38
Components of a Routing
Algorithm

A procedure for sending and receiving reachability
information about network to other routers.

A procedures for reacting to and advertising topology
changes.

A procedure for calculating optimal routes.

Routes are calculated using a shortest path algorithm:

Goal: Given a network were each link is assigned a cost. Find the
path with the least cost between two networks with minimum cost.
39
Approaches to Shortest Path
Routing
There are two basic routing algorithms found on the Internet.
1. Distance Vector Routing





Each node knows the distance (=cost) to its directly connected neighbors.
A node sends periodically a list of routing updates to its neighbors.
If all nodes update their distances, the routing tables eventually converge.
New nodes advertise themselves to their neighbors.
2. Link State Routing



Each node knows the distance to its neighbors.
The distance information (=link state) is broadcast to all nodes in the
network.
Each node calculates the routing tables independently after receiving the
broadcast.
40
Distance Vector Routing

Variations of Bellman-Ford algorithm.

Each router starts by knowing:



Each router advertises only to its neighbors:



All prefixes it knows about.
Its distance from them.
Each router learns:



Prefixes of its attached networks (“zero” distance).
Its next hop routers (how to find them?)
All prefixes its neighbors know about.
Their distance from them.
Each router figures out, for each destination prefix:


The “distance” (how far away it is).
The “vector” (the next hop router).
41
Distance Vector Routing Properties

DV computes the shortest path

“Routing by rumor”

Each router believes what its neighbors tell it.

In steady-state, each router has the “shortest” (smallest
metric) path to the destination.

Convergence time is (on the average) proportional to the
diameter of the network.

Any link change affects the entire network.
42
Distance Vector Algorithm

A decentralized algorithm



A router knows physically-connected neighbors and link costs to
neighbors
A router does not have a global view of the network
Path computation is iterative and mutually dependent.




A router sends its known distances to each destination (distance
vector) to its neighbors.
A router updates the distance to a destination from all its
neighbors’ distance vectors
A router sends its updated distance vector to its neighbors.
The process repeats until all routers’ distance vectors do not
change (this condition is called convergence).
43
Bellman-Ford Algorithm
Bellman-Ford Equation
Define
dx(y) := cost of the least-cost path from x to y
Then
 dx(y) = minv{c(x,v) + dv(y) }, where min is taken over
all neighbors of node x
44
Distance Vector Algorithm:
Initialization

Let Dx(y) be the estimate of least cost from x to y

Initialization:



Each node x knows the cost to each neighbor: c(x,v).
For each neighbor v of x, Dx(v) = c(x,v)
Dx(y) to other nodes are initialized as infinity.
Each node x maintains a distance vector (DV):

Dx = [Dx(y): y in N ]
45
Distance Vector Algorithm: Updates

Each node x sends its distance vector to its neighbors,
either periodically, or triggered by a change in its DV.

When a node x receives a new DV estimate from a
neighbor v, it updates its own DV using B-F equation:

If c(x,v) + Dv(y) < Dx(y) then




Dx(y) = c(x,v) + Dv(y)
Sets the next hop to reach the destination y to the neighbor v
Notify neighbors of the change
The estimate Dx(y) will converge to the actual least cost
dx(y)
46
Distance vector algorithm: an
example
1
1
1
1
1
1
1
Time = 0
1
47
Distance vector algorithm: an
example
Time = 1
48
Distance vector algorithm: an
example
Time = 2 (End)
49
How to map the abstract graph to the
physical network
c(v,w)
Net(v,w)
w
v
Net
c(v,n)
Net(v,n)
n



Nodes (e.g., v, w, n) are routers, identified by IP addresses, e.g.
10.0.0.1
Nodes are connected by either a directed link or a broadcast link
(Ethernet)
Destinations are IP networks, represented by the network prefixes,
e.g., 10.0.0.0/16


Net(v,n) is the network directly connected to router v and n.
Costs (e.g. c(v,n)) are associated with network interfaces.



Router1(config)# router rip
Router1(config-router)# offset-list 0 out 10 Ethernet0/0
Router1(config-router)# offset-list 0 out 10 Ethernet0/1
50
Distance Vector Algorithm:
Routing Table
c(v,w): cost to transmit on the
interface to network Net(v,w)
Net(v,w): Network address of the network between v
and w
The network can be a link, but could also be a LAN
RoutingTable of node v
Dest
v
Net(v,w)
c(v,w)
Net(v,n)
c(v,n)
via
(next hop)
cost
w
Net
Net
n
D(v,Net)
n
51
Distance Vector Algorithm:
Messages
RoutingTable of node v
Dest
Net
via
(next hop)
n
cost
D(v,Net)
• Nodes send messages to their neighbors which contain
routing table entries
v
[Net , D(v,Net)]
n
• A message has the format: [Net , D(v,Net)] means“My cost to
go to Net is D (v,Net)”
52
Distance Vector Algorithm:
Sending Updates
RoutingTable of node v
Dest
via
(next hop)
cost
Net1
m
D(v,Net 1)
Net2
n
D(v,Net 2)
NetN
w
D(v,Net N)
Periodically, each node v
sends the content of its routing
table to its neighbors:
m
[Net1,D(v,Net1)]
[Net1,D(v,Net1)]
[NetN,D(v,NetN)]
[NetN,D(v,NetN)]
v
w
[Net1,D(v,Net1)]
[NetN,D(v,NetN)]
n
53
Initiating Routing Table I
Suppose a new node v becomes active.
 The cost to access directly connected networks is
zero:

 D (v, Net(v,m)) = 0
 D (v, Net(v,w)) = 0
 D (v, Net(v,n)) = 0
RoutingTable
c(v,m)
Net(v,m)
m
c (v,w)
Net(v,w)
v
Dest
via
(next hop)
cost
w
c(v,n)
Net(v,n)
Net(v,m)
m
0
Net(v,w)
w
0
Net(v,n)
n
0
n
54
Initiating Routing Table II
RoutingTable
Dest

via
(next hop)
cost
Net(v,m)
m
0
Net(v,w)
w
0
Net(v,n)
n
0
New node v sends the routing table entry to all
its neighbors:
[n,0]
[Net(v,n),0]
[w,0]
[Net(v,w),0]
m
[n,0]
[Net(v,n),0]
[m,0]
[Net(v,m),0]
v
w
[m,0]
[Net(v,m),0]
[w,0]
[Net(v,w),0]
n
55
Initiating Routing Table III

Node v receives the routing tables from other
nodes and builds up its routing table
[Net1,D(m,Net1)]
[Net1,D(w,Net1)]
[NetN,D(m,NetN)]
[NetN,D(w,NetN)]
m
v
w
[Net1,D(n,Net1)]
[NetN,D(n,NetN)]
n
56
Updating Routing Tables I
• Suppose node v receives a message from node m: [Net,D(m,Net)]
[Net,D(m,Net)]
Net
m
c(v,m)
Net(v,m)
v
w
n
Node v updates its routing table and sends out further
messages if the message reduces the cost of a route:
if ( D(m,Net) + c (v,m) < D (v,Net) ) {
Dnew (v,Net) := D (m,Net) + c (v,m);
Update routing table;
send message [Net, Dnew (v,Net)] to all neighbors
}
57
Updating Routing Tables II
• Before receiving the message:
RoutingTable
[Net,D(m,Net)]
Net
m
c(v,m)
Net(v,m)
Dest
v
via
(next hop)
cost
w
Net
??
D(v,Net)
n
• Suppose D (m,Net) + c (v,m) < D (v,Net):
RoutingTable
Dest
[Net,Dnew (v,Net)]
Net
m
c(v,m)
Net(v,m)
v
w
Net
via
(next hop)
m
cost
Dnew(v,Net)
[Net,Dnew (v,Net)]
n
58
Assume: - link cost is 1, i.e., c(v,w) = 1
- all updates, updates occur simultaneously
- Initially, each router only knows the cost of
connected interfaces
Example
10.0.3.0/24
10.0.4.0/24
.1
.1
.1
Net
via
cost
Router A
t=0:
10.0.1.0 10.0.2.0 -
0
0
t=1:
10.0.1.0 10.0.2.0 10.0.3.0 10.0.2.2
t=2:
10.0.1.0
10.0.2.0
10.0.3.0
10.0.4.0
10.0.2.2
10.0.2.2
.2
Router B
Net
via
.2
Router C
Net
via
0
0
t=0:
10.0.3.0 10.0.4.0 -
0
0
0
0
1
t=1:
10.0.1.0
10.0.2.0
10.0.3.0
10.0.4.0
1
0
0
1
t=1:
10.0.2.0
10.0.3.0
10.0.4.0
10.0.5.0
1
0
0
1
0
0
1
2
t=2:
10.0.1.0
10.0.2.0
10.0.3.0
10.0.4.0
10.0.5.0
1
0
0
1
2
t=2:
10.0.1.0
10.0.2.0
10.0.3.0
10.0.4.0
10.0.5.0
10.0.2.1
10.0.3.2
10.0.3.2
10.0.3.1
10.0.4.2
10.0.3.1
10.0.3.1
10.0.4.2
.1
Router D
t=0:
10.0.2.0 10.0.3.0 -
10.0.2.1
10.0.3.2
10.0.5.0/24
2
1
0
0
1
Net
via
cost
.2
cost
.2
10.0.2.0/24
cost
10.0.1.0/24
t=0:
10.0.4.0 10.0.5.0 -
0
0
t=1:
10.0.3.0 10.0.4.1
10.0.4.0 10.0.5.0 -
1
0
0
t=2:
10.0.2.0
10.0.3.0
10.0.4.0
10.0.5.0
10.0.4.1 2
10.0.4.1 1
0
59
0
Example
10.0.3.0/24
10.0.4.0/24
.1
.1
.1
Net
t=2:
10.0.1.0
10.0.2.0
10.0.3.0
10.0.4.0
t=3:
10.0.1.0
10.0.2.0
10.0.3.0
10.0.4.0
10.0.5.0
via
10.0.2.2
10.0.2.2
10.0.2.2
10.0.2.2
10.0.2.2
Router B
cost
Router A
.2
Net
0
0
1
2
0
0
1
2
3
via
.2
Router C
t=2:
10.0.1.0
10.0.2.0
10.0.3.0
10.0.4.0
10.0.5.0
10.0.2.1
10.0.3.2
10.0.3.2
1
0
0
1
2
t=3:
10.0.1.0
10.0.2.0
10.0.3.0
10.0.4.0
10.0.5.0
10.0.2.1
10.0.3.2
10.0.3.2
1
0
0
1
2
Net
t=2:
10.0.1.0
10.0.2.0
10.0.3.0
10.0.4.0
10.0.5.0
t=3:
10.0.1.0
10.0.2.0
10.0.3.0
10.0.4.0
10.0.5.0
via
10.0.3.1
10.0.3.1
10.0.4.2
10.0.3.1
10.0.3.1
10.0.4.2
Now, routing tables have converged !
10.0.5.0/24
.1
Router D
2
1
0
0
1
2
1
0
0
1
Net
via
cost
.2
cost
.2
10.0.2.0/24
cost
10.0.1.0/24
t=2:
10.0.2.0
10.0.3.0
10.0.4.0
10.0.5.0
10.0.4.1
10.0.4.1
-
2
1
0
0
t=3:
10.0.1.0
10.0.2.0
10.0.3.0
10.0.4.0
10.0.5.0
10.0.4.1
10.0.4.1
10.0.4.1
-
3
2
1
0
0
60
Characteristics of Distance Vector
Routing




Periodic Updates: Updates to the routing tables are
sent at the end of a certain time period. A typical value is
90 seconds.
Triggered Updates: If a metric changes on a link, a
router immediately sends out an update without waiting
for the end of the update period.
Full Routing Table Update: Most distance vector
routing protocol send their neighbors the entire routing
table (not only entries which change).
Route invalidation timers: Routing table entries are
invalid if they are not refreshed. A typical value is to
invalidate an entry if no update is received after 3-6
61
update periods.
The Count-to-Infinity 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
62
Count-to-Infinity
The reason for the count-to-infinity problem is
that each node only has a “next-hop-view”
 For example, in the first step, A did not realize
that its route (with cost 2) to C went through
node B


How can the count-to-infinity problem be
solved?
63
Count-to-Infinity




The reason for the count-to-infinity problem is that each
node only has a “next-hop-view”.
For example, in the first step, A did not realize that its
route (with cost 2) to C went through node B.
How can the Count-to-Infinity problem be solved?
Solution 1: Always advertise the entire path in an
update message (Path vectors).
 If routing tables are large, the routing messages
require substantial bandwidth.
 BGP uses this solution.
64
Count-to-Infinity




The reason for the count-to-infinity problem is that each
node only has a “next-hop-view”
For example, in the first step, A did not realize that its
route (with cost 2) to C went through node B
How can the Count-to-Infinity problem be solved?
Solution 2: Never advertise the cost to a neighbor if
this neighbor is the next hop on the current path (Split
Horizon)


Example: A would not send the first routing update to B, since B
is the next hop on A’s current route to C
Split Horizon does not solve count-to-infinity in all cases!
65
RIP - Routing Information Protocol






A simple intradomain protocol
Straightforward implementation of distance vector
routing.
Each router advertises its distance vector every 30
seconds (or whenever its routing table changes) to all of
its neighbors
RIP always uses 1 as link metric.
Maximum hop count is 15, with “16” equal to “”
Routes are timeout (set to 16) after 3 minutes if they are
not updated.
66
RIP - History

Late 1960s :

Mid-1970s:


1982
1988

1993

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
Current version of RIPv2 (RFC 2453)
67
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
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)
68
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
69
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)
70
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)
2: RIPv2
one route entry
(20 bytes)
IP header UDP header
Up to 24 more routes (each 20 bytes)
71
32 bits
RIP Messages

This is the operation of RIP in routed.
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
72
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 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.
73
RIP Security


Issue: Sending bogus routing updates to a router
RIPv1: No protection
RIPv2: Simple authentication scheme
2: plaintext
password
RIPv2 Message
IP header UDP header
Command Version
Set to 00.00
0xffff
Authentication Type
Password (Bytes 0 - 3)
Password (Bytes 4 - 7)
Password (Bytes 8- 11)
Password (Bytes 12 - 15)
Authetication

Up to 24 more routes (each 20 bytes)
74
32 bits
RIP Problems

RIP takes a long time to stabilize


RIP has all the problems of distance vector
algorithms, e.g., count-to-Infinity


Even for a small network, it takes several minutes
until the routing tables have settled after a change
RIP uses split horizon to avoid count-to-infinity
The maximum path in RIP is 15 hops
75