Chapter 11
Unicast
Routing
Protocols
TCP/IP Protocol Suite
1
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
11-1 INTRODUCTION
An internet is a combination of networks connected
by routers. When a datagram goes from a source to
a destination, it will probably pass through many
routers until it reaches the router attached to the
destination network.
TCP/IP Protocol Suite
2
11-2 INTER- AND INTRA-DOMAIN
ROUTING
Today, an internet can be so large that one routing
protocol cannot handle the task of updating the
routing tables of all routers. For this reason, an
internet is divided into autonomous systems. An
autonomous system (AS) is a group of networks and
routers under the authority of a single administration.
Routing inside an autonomous system is called intradomain routing. Routing between autonomous
systems is called inter-domain routing
TCP/IP Protocol Suite
3
Intra-and Interdomain Routing
• An internet is divided into autonomous systems. An
autonomous system (AS) is a group of networks and
routers under the authority of a single administration
• Routing inside an autonomous system is referred to as
intradomain routing
• Routing between autonomous system is referred to as
interdomain routing
Popular Routing Protocols
• In this chapter, we discuss two intradomain routing protocols:
distance vector and link state
• For the interdomain routing protocol, we introduce path vector
• Routing Information Protocol (RIP) is the implementation of the
distance vector protocol
• Open Shortest Path First (OSPF) is the implementation of the link
state protocol
• Border Gateway Protocol (BGP) is the implementation of the path
vector protocol
Distance Vector Routing
• In distance vector routing, the least cost route between any two
nodes is the route with minimum distance
• In this protocol each node maintains a vector (table) of minimum
distances to every node
• The table at each node also guides the packets to the desired node
by showing the next stop in the route (next-hop routing)
Initialization
• At the beginning, each node can know only the
distance between itself and its immediate
neighbors, those directly connected to it
Sharing
• The whole idea of distance vector routing is the sharing
of information between neighbors
• Although node A does not know about node E, node C
does. So if node C shares its routing table with A, node
A can also know how to reach node E
Note:
In distance vector routing, each node shares its
routing table with its immediate neighbors
periodically and when there is a change.
Figure 11.4
TCP/IP Protocol Suite
The fact behind Bellman-Ford algorithm
9
Updating
•
When a node receives a 2-column table from a
neighbor, it needs to update its routing table.
Updating takes 3 steps:
1. The receiving node needs to add the cost between
itself and the sending node to each value in the
second column
2. The receiving node needs to add the name of the
sending node to each row as the 3rd column if the
receiving node uses information from any row. The
sending node is the next node in the route
Updating (cont.)
3. The receiving node needs to compare each
row of its old table with the corresponding row
of the modified version of the received table.
a) If the next-node entry is different, the receiving node
chooses the row with the smaller cost. If there is a
tie, the old one is kept
b) If the next-node entry is the same, the receiving
node chooses the new row. For example, if node C
has previously advertised a route to node X with
distance 3. Suppose that now there is no path
between C and X; node C now advertises this route
with a distance of infinity. Node A must not ignore
this value even though its old entry is smaller.
Updating (cont.)
When to Share
•
•
•
The table is sent both periodically and when there is a
change in the table
Periodic Update. A node sends its routing table,
normally every 30 seconds, in a periodic update. The
period depends on the protocol that is using distance
vector routing
Triggered Update. A node sends its 2-column routing
table to its neighbors any time there is a change in its
routing table. This is called a triggered update. The
change can result from the following.
1. A node receive a table from a neighbor resulting in changes in
its own table after updating
2. A node detects some failure in the neighboring links which
results in a distance change to infinity
Example 11.2
Now assume router A sends four records to its neighbors,
routers B, D, and C. Figure 11.6 shows the changes in B’s
routing table when it receives these records. We leave the
changes in the routing tables of other neighbors as exercise.
TCP/IP Protocol Suite
14
Figure 11.6
Example 11.2
4
2
3
Net5 , 1Net4 , 1Net2 , 1
TCP/IP Protocol Suite
15
Initial routing tables in a small
autonomous system
Final routing tables for the previous figure
Two-Node Loop Instability
• A problem with distance vector routing is
instability, which means that a network using this
protocol can become unstable
Solutions to two-node instability
• Defining Infinity. Most implementation of the
distance vector protocol define the distance
between each node to be 1 and define 16 as
infinity. Therefore, the distance vector cannot be
used in large systems
• Split Horizon. In this strategy, instead of
flooding the table through each interface, each
node sends only part of its table through each
interface. If node B thinks that the optimum route
to reach X is via A, it does not need to advertise
this piece of information to A.
Solutions to two-node instability
(cont.)
Solutions to two-node instability
(cont.)
• Split Horizon and Poison Reverse. This
strategy is a combination between split
horizon and poison reverse where node B
can still advertise the value of X, but if the
source of information is A, it can replace
the distance with infinity as a warning:
“Don’t use this value; what I know about
this route comes from you.”
Solutions to two-node instability
(cont.)
Three-Node Instability
Routing Information Protocol (RIP)
•
•
•
The Routing Information Protocol (RIP) is an
intradomain routing protocol used inside an
autonomous system
It is a very simple protocol based on distance
vector routing
RIP implements distance vector directly with
some considerations:
1. In an autonomous, we are dealing with routers and
networks (links). The routers have routing tables,
networks don’t
2. The destination in a routing table is a network,
which means the first column defines a network
address
Routing Information Protocol (RIP)
(cont.)
3.
4.
5.
The metric used by RIP is the number of links that have to be used to
reach the destination which is called hop count
Infinity is defined as 16
The next node column defines the address of the router to which the
packet is to be sent to reach its destination
RIP Message Format
RIP Message Format (cont.)
• Command. This 8-bit field specifies the type of
message: request (1) or response (2)
• Version. This 8-bit defines the version. In the
textbook, we use version 1
• Family. This 16-bit field defines the family of the
protocol used. For TCP/IP the value is 2.
• Network address. RIP has allocated 14 bytes for
this field to be applicable to any protocol.
• Distance. This 32-bit field defines the hop count
from the advertising router to the destination
network
Request and Response
• Request. A request message is sent by a router
that has just come up or by a router that has
some time-out entries. A request can ask about
specific entries or all entries
• Response. A solicited response is sent only in
answer to a request. It contains information
about the destination specified in the
corresponding request. An unsolicited response
is sent periodically, every 30 s or when there is a
change in the routing table
Request Messages
Example 1
What is the periodic response sent by router
R1? Assume R1 knows about the whole
autonomous system.
Solution
R1 can advertise three networks 144.2.7.0,
144.2.9.0, and 144.2.12.0. The periodic response
(update packet) is shown below.
Example 2
Figure 14.11 shows the update message sent from router R1 to
router R2 in Figure 14.8. The message is sent out of interface
130.10.0.2.
See Next Slide
The message is prepared with the
combination of split horizon and
poison reverse strategy in mind.
Router
R1
has
obtained
information
about
networks
195.2.4.0, 195.2.5.0, and 195.2.6.0
from router R2. When R1 sends
an update message to R2, it
replaces the actual value of the
hop counts for these three
networks with 16 (infinity) to
prevent any confusion for R2. The
figure also shows the table
extracted from the message.
Router R2 uses the source address
of the IP datagram carrying the
RIP message from R1 (130.10.02)
as the next hop address.
Timer in RIP
• RIP uses 3 timers. The periodic timer controls
the sending of messages, the expiration timer
governs the validity of a route, and the garbage
collection timer advertises the failure of a route
Periodic Timer
• The periodic timer controls the advertising
of regular update messages
• The working model uses a random
number between 25 and 30 s
• This is to prevent any possible
synchronization and therefore overload on
an internet if routers update
simultaneously
Expiration Timer
• The expiration timer governs the validity of a
route
• When a router receives update information for a
route, the expiration timer is set to 180 s for that
particular route
• Every time a new update for the route is
received, the timer is reset
• If the timer is expired, the hop count of the route
is set to 16, which means the destination is
unreachable
Garbage Collection Timer
• When the information about a route
becomes invalid, the router does not
immediately purge that route from its table
• Instead, it continues to advertise the route
with a metric value of 16
• At the same time, the garbage collection
timer is set to 120 s for that route
• When the count reaches zero, the route is
purged from the table
Example 3
A routing table has 20 entries. It does not receive information
about five routes for 200 s. How many timers are running at
this time?
Solution
The 21 timers are listed below:
Periodic timer: 1
Expiration timer: 20 − 5 = 15
Garbage collection timer: 5
RIP Version 2
• RIP version 2 was designed to overcome
some of the shortcomings of version 1
• The designers of version 2 have not
augmented the length of the message for
each entry
• They have only replaced those fields in
version 1 that were filled with 0s for the
TCP/IP protocol with some new fields
Message Format Updates
• Route tag. This field carries information such as
the autonomous system number. It can be used
to enable RIP to receive information from an
interdomain routing protocol.
• Subnet mask. This is a 4-byte field that carries
the subnet mask. This means that RIP2 supports
classless addressing and CIDR.
• Next-hop address. If the sending router want to
specify another router IP address to be the next
hop router.
RIP Version 2 Enhancements
Supports VLSM and CIDR
RFC - 2453
New RIPV2 Packet Format
1
2
3
4
5
6
7
8
1
9 0 1 2
3
4
6
3
4
5
6
7
3
8 9 0 1
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+==+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=
Source Port = 0X0208
( 520 )
Dest. Port = 0X0208
( 520 ) UDP
UDP
Len. [ UDP + RIP = 0X0200 (512) ]
UDP Checksum
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+==+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=
RIP
Command
Version
Unused
Dest. 01
Address Family Identifier
Route Tag
Dest. 01
IP Address
Dest. 01
Subnet Mask
Dest. 01
Next Hop
Dest. 01
Metric
92


RIPv2 over UDP Packet format.
IP Address
Subnet Mask
Next Hop
Metric
=+=+=+=+=+==+=+=+=+=
=+=+=+=+=+==+=+=+=+=
Address Family Identifier
Route Tag
=+=+=+=+=+==+=+=+=+=+=+=+=+=++
64
68
72
572
5
2
7 8 9 0 1 2
Dest.
25
Dest.2
5
Dest.2
5
Dest.2
5
Dest.2
5
41