6. Internet Protocol (IP) Concepts
• Delivery refers to the way a packet is handled
by the underlying networks under the control
of the network layer.
Ex. direct and indirect delivery
• Forwarding refers to the way a packet is
delivered to the next station.
7. Delivery
• Direct delivery - the final destination of the
packet is a host connected to the same
physical network as the deliverer.
How can the sender determine if the delivery is direct?
8. Delivery
• Indirect delivery - the packet goes from router
to router until it reaches the one connected to
the same physical network as its final
destination.
9. Forwarding
• Forwarding means to deliver the packet to the
next hop.
• Traditionally, forwarding requires a host or a
router to have a routing table.
10. Forwarding Techniques
• Next-hop method - the routing table holds
only the address of the next hop instead of
information about the complete route.
11. Forwarding Techniques
• Network-specific method - treat all hosts
connected to same network as one single entity.
• Host-Specific Method - the destination host
address is given in the routing table.
13. Forwarding Techniques
• Default Method - instead of listing all networks
in the entire Internet, host A can just have one
entry called the default (i.e. network address
0.0.0.0).
14. Forwarding with Classful Addressing
• Existence of a default mask in a classful
address makes the forwarding process simple.
15. Forwarding with Classful Addressing
1. The destination address of the packet is extracted.
2. A copy of the destination address is used to find
the class of the address. This is done by shifting the
copy of the address 28 bits to the right. The result is
a 4-bit number between 0 and 15. If the result is
a. 0 to 7, the class is A.
b. 8 to 11, the class is B.
c. 12 or 13, the class is C
d. 14, the class is D.
e. 15, the class is E.
16. Forwarding with Classful Addressing
3. The result of Step 2 for class A, B, or C and the
destination address are used to extract the
network address.
4. The class of the address and the network
address are used to find next-hop information.
5. The ARP module uses the next-hop address
and the interface number to find the physical
address of the next router.
18. Solution
• Some entries in the next-hop address column
are empty because in these cases, the
destination is in the same network to which the
router is connected (direct delivery).
19. Example
• Router R1 in receives a packet with destination
address 192.16.7.14. Show how the packet is
forwarded.
20. Solution
• The destination address in binary is 11000000
00010000 00000111 00001110.
• A copy of the address is shifted 28 bits to the
right. The result is 00000000 00000000
00000000 00001100 or 12. The destination
network is class C.
• The network address is extracted by masking
off the leftmost 24 bits of the destination
address; the result is 192.16.7.0.
21. Solution
• The table for Class C is searched.
• The network address is found in the first row.
The next-hop address 111.15.17.32. and the
interface m0 are passed to ARP.
22. Drill
• Router R1 in receives a packet with destination
address 167.24.160.5. Show how the packet is
forwarded.
23. Solution
• The destination address in binary is 10100111
00011000 10100000 00000101.
• A copy of the address is shifted 28 bits to the
right. The result is 00000000 00000000
00000000 00001010 or 10. The class is B.
• The network address can be found by masking
off 16 bits of the destination address, the
result is 167.24.0.0. The table for Class B is
searched.
24. Solution
• No matching network address is found. The
packet needs to be forwarded to the default
router.
• The next-hop address 111.30.31.18 and the
interface number m0 are passed to ARP.
25. Forwarding with Subnetting
• The routers that handle subnetting are either
at the border of the organization site or inside
the site boundary.
26. Forwarding with Subnetting
1. The module extracts the destination address
of the packet.
2. If the destination address matches any of the
host-specific addresses in the table, the next-
hop and the interface number is extracted from
the table.
3. The destination address and the mask are
used to extract the subnet address.
27. Forwarding with Subnetting
4. The table is searched using the subnet
address to find the next-hop address and the
interface number. If no match is found, the
default is used.
5. The next-hop address and the interface
number are given to ARP.
29. Example
• The site address is 145.14.0.0/16 (a class B).
• Every packet with destination address in the
range 145.14.0.0 to 145.14.255.255 is
delivered to the interface m4 and distributed
to the final destination subnet by the router.
• x.y.z.t/n for the interface m4 because we do
not know to which network this router is
connected.
• The table has a default entry for packets that
are to be sent out of the site.
30. Drill
• The router in previous figure receives a packet
with destination address 145.14.32.78. Show
how the packet is forwarded.
• Answer:
The mask is /18. After applying the mask, the
subnet address is 145.14.0.0. The packet is
delivered to ARP with the next-hop address
145.14.32.78 and the outgoing interface m0.
31. Drill
• The router in previous figure has a packet to send
to the host with address 7.22.67.91. Show how
the packet is routed.
• Answer:
The router receives the packet and applies the mask
(/18). The network address is 7.22.64.0. The table is
searched and the address is not found. The router
uses the address of the default router and sends
the packet to that router.
32. Forwarding with Classless Addressing
• In classless addressing, the whole address
space is one entity; there are no classes.
• Thus, forwarding requires one row of
information for each block involved.
• In classful addressing we can have a routing
table with three columns;
• In classless addressing, we need at least four
columns.
33. Forwarding with Classless Addressing
• Network address extraction is done at the
same time as table searching.
34. Example
• Make a routing table for router R1 using the
configuration.
36. Example
• Show the forwarding process if a packet
arrives at R1 with the destination address
180.70.65.140.
37. Solution
• The first mask (/26) is applied to the destination
address. The result is 180.70.65.128, which does
not match the corresponding network address.
• The second mask (/25) is applied to the
destination address. The result is 180.70.65.128,
which matches the corresponding network
address.
• The next-hop address and the interface number
m0 are passed to ARP.
38. Drill
• Show the forwarding process if a packet
arrives at R1 with the destination address
201.4.22.35.
39. Solution
1. The first mask (/26) is applied to the destination
address. The result is 201.4.22.0, which does not
match the corresponding network address (row 1).
2. The second mask (/25) is applied to the destination
address. The result is 201.4.22.0, which does not
match the corresponding network address (row 2).
3. The third mask (/24) is applied to the destination
address. The result is 201.4.22.0, which matches the
corresponding network address. The destination
address of the packet and the interface number m3
are passed to ARP.
40. Example
• Can we find the configuration of a router, R1,
if we know only its routing table?
41. What do we know?
• Three networks directly connected to router.
• Two networks indirectly connected to router.
• The router has three interfaces: m0, m1, & m2.
• There must be at least three other routers
involved.
• One router, the default router, is connected to
the rest of the Internet.
• But, we don’t know the …
44. Address Aggregation
• It alleviates the problem of increased size of
the table which results in an increase in the
amount of time needed to search the table.
Ex..
45. Address Aggregation
• The blocks of addresses for four organizations
are aggregated into one larger block.
• R2 would have a longer routing table if each
organization had addresses that could not be
aggregated into one block.
46. Drill
• What happens if one of the organizations in
the previous example is not geographically
close to the other three?
• For example, if organization 4 cannot be
connected to router R1 for some reason, can
we still use the idea of address aggregation
and still assign block 140.24.7.192/26 to
organization 4?
47. Answer
• Yes because routing in classless addressing
uses another principle, longest mask
matching which states that the routing table is
sorted from the longest mask to the shortest
mask.
• Ex. Let there be three masks, /27, /26, and
/24, the mask /27 must be the first entry and
/24 must be last.
49. Hierarchical Routing
• To solve the problem of gigantic routing
tables, we can create a sense of hierarchy in
the routing tables.
• Ex. A local ISP can be assigned a single, but
large, block of addresses with a certain prefix
length. The local ISP can divide this block into
smaller blocks of different sizes, and assign
these to individual users and organizations,
both large and small.
51. Geographical Routing
• To decrease the size of the routing table even
further, we need to extend hierarchical
routing to include geographical routing.
• We assign a block to America, a block to
Europe, a block to Asia, a block to Africa, and
so on.
• The routers of ISPs outside of Asia will have
only one entry for packets to Asia in their
routing tables.
52. Routing Table Search Algorithms
• In classful addressing, the routing table is
organized as a list, divided into three tables
(sometimes called buckets), one for each class.
• In classless addressing, there is no network
information in the destination address. The
simplest, but not the most efficient, search
method is called the longest prefix match.
53. Forwarding Based on Destination
Address and Label
• A connectionless network (datagram
approach), a router forwards a packet based
on the destination address in the header of
packet.
• A connection-oriented network (virtual-circuit
approach), a switch forwards a packet based
on the label attached to a packet.
56. Structure of a Router
• A router has four components: input ports,
output ports, the routing processor, and the
switching fabric.
57. Input Port
• An input port performs the physical and data
link layer functions of the router.
• The bits are constructed from the received
signal. The packet is decapsulated from the
frame.
• It has buffers (queues) to hold the packets
before they are directed to the switching
fabric.
58. Output Port
• An output port performs the same functions
as the input port, but in the reverse order.
• First the outgoing packets are queued, then
the packet is encapsulated in a frame.
• Then, physical layer functions are applied to
the frame to create the signal to be sent on
the line.
59. Routing Processor
• The routing processor performs the functions
of the network layer.
• The destination address is used to find the
address of the next hop and, at the same
time, the output port number from which the
packet is sent out.
• This activity is sometimes referred to as table
lookup because the routing processor
searches the routing table.
60. Switching Fabrics
• Switching fabrics move the packet from the
input queue to the output queue.
• The simplest type of switching fabric is the
crossbar switch:
A crossbar switch connects n inputs to n outputs in a
grid, using electronic microswitches at each crosspoint.
61. Switching Fabrics
• A banyan switch is a multistage switch with
microswitches at each stage that route the
packets based on the output port represented
as a binary string.
64. In Summary
• Routers connect LANs, and switches connect
computers.
• Routers work with logical (IP) addresses rather
than physical (MAC) addresses, as switches do.
• Routers work with packets rather than the
frames that switches work with.
• Routers don’t forward broadcast packets, but
switches do.
• Routers use routing tables, and switches use
switching tables.
65. Datagram
• Packets in the network (internet) layer
• a variable-length packet consisting of two
parts: header and data.
• the header contains information essential to
routing and delivery.
67. IP Header Contents
• Version (VER). This 4-bit field defines the
version of the IP protocol. Ex. version 4
• Header length (HLEN). This 4-bit field defines
the total length (20-60 bytes) of the datagram
header in 4-byte words. Ex. 5 to 15
• Service type or type of service (TOS).
68. IP Header Contents
• Total length. This is a 16-bit field that defines
the total length (header plus data) of the IP
datagram in bytes.
Length of data = total length − header length
• Thus, the total length of the IP datagram is
limited to:
65,535 (216 − 1) bytes
69. IP Header Contents
• Identification, Flags, and Fragmentation offset.
used in fragmentation
• Time to live. used to control the maximum
number of hops (routers) visited by the
datagram.
What if the source wants to confine the packet
to the local network?
70. IP Header Contents
• Protocol. This 8-bit field defines the higher-
level protocol that uses services of the IP layer.
• An IP datagram can encapsulate data from
several higher level protocols such as TCP,
UDP, ICMP, and IGMP.
71. IP Header Contents
• Source address. This 32-bit field defines the IP
address of the source.
• Destination address. This 32-bit field defines
the IP address of the destination.
• Checksum. Error detection. It is formed by
adding bit streams using one’s complement
arithmetic and then complementing the
result.
72. Drill
• An IP packet has arrived with the first 8 bits as
shown:
01000010
Will the receiver accept or discard the packet?
Why?
73. Drill
• An IP packet has arrived with the first few
hexadecimal digits as shown below:
45000028000100000102 . . .
• How many hops can this packet travel before
being dropped?
• The data belong to what upper layer protocol?
74. Fragmentation
• The division of a packet into smaller units to
accommodate a protocol’s MTU.
• Maximum transfer unit (MTU) The largest size
data unit a specific network can handle.
• Ex. Ethernet LAN = 1500 bytes,
FDDI LAN = 4352 bytes, PPP = 296 bytes, etc.
75. Fragmentation
• In order to make the IP protocol independent
of the physical network, the maximum length
of the IP datagram was set to 65,535 bytes.
• Thus, for other physical networks, we must
divide the datagram to make it possible to
pass through these networks.
• Only data in a datagram is fragmented.
77. Drill
• A packet has arrived with an M bit value of 0.
Is this the first fragment, the last fragment, or
a middle fragment? Do we know if the packet
was fragmented?
FLAG BITS
78. Drill
• A packet has arrived with an M bit value of 1.
Is this the first fragment, the last fragment, or
a middle fragment? Do we know if the packet
was fragmented?
79. Drill
• A packet has arrived with an M bit value of 1
and a fragmentation offset value of zero. Is
this the first fragment, the last fragment, or a
middle fragment?
83. Option Types
• The header of the IP datagram is made of two
parts: a fixed part and a variable part.
• The fixed part or base header is 20 bytes long
and the variable part (at most 40 bytes).
• Options format:
84. Option Types
• No-operation option – a 1-byte option used as
a filler between options.
• End-of-option option is also a 1-byte option
used for padding at the end of the option field.
85. • Record-route option - used to record the
Internet routers that handle the datagram.
* Pointer points to the first available entry.
*
86. • Strict-source-route option is used by the
source to predetermine a route for the
datagram as it travels through the Internet.
88. Drill
• Which of the six options must be copied to
each fragment?
a. No operation
b. End of option
c. Record route
d. Strict source route
e. Loose source route
f. Timestamp
89. Answer
• We look at the first (left-most) bit of the type
for each option.
a. No operation: type is 00000001; not copied.
b. End of option: type is 00000000; not copied.
c. Record route: type is 00000111; not copied.
d. Strict source route: type is 10001001; copied.
e. Loose source route: type is 10000011; copied.
f. Timestamp: type is 01000100; not copied.
90. Drill
• Which of the six options are used for datagram
control and which are used for debugging and
management?
a. No operation
b. End of option
c. Record route
d. Strict source route
e. Loose source route
f. Timestamp
91. Answer
• We look at the second and third (left-most) bits of the
type.
a. No operation: type is 00000001; datagram control.
b. End of option: type is 00000000; datagram control.
c. Record route: type is 00000111; datagram control.
d. Strict source route: type is 10001001; datagram
control.
e. Loose source route: type is 10000011; datagram
control.
f. Timestamp: type is 01000100; debugging and
management control.
92. ping utility
• An application program to determine the
reachability of a destination using an ICMP
echo request and reply.
ping dlsu.edu.ph
• Use the ping utility with the -R option to
implement the record route option and show
the interfaces and IP addresses.
93. traceroute utility
• The traceroute utility can also be used to keep
track of the route of a packet.
94. Recall:
• IP is an unreliable and connectionless
datagram protocol—a best-effort delivery
95. ARP Revisited
• A protocol for obtaining the physical address
of a node when the Internet address is known.
• Position of ARP in TCP/IP protocol suite:
98. ARP Packet Fields
• Hardware type
16-bit field defining the type of the network on which
ARP is running. Each LAN has been assigned.
Ex. Ethernet: type 1
• Protocol type
16-bit field defining the protocol.
Ex. IPv4 protocol: 080016
• Hardware length
8-bit field defining the length of the physical address
in bytes. Ex. Ethernet: 6 bytes
99. ARP Packet Fields
• Protocol Length
8-bit field defining the length of the logical address
in bytes.
Ex. IPv4 protocol: 4
• Operation
16-bit field defining the type of packet
Ex. ARP request (1), ARP reply (2).
• Sender hardware address
variable-length field defining the physical address
of the sender.
Ex. Ethernet: 6 bytes
100. ARP Packet Fields
• Sender protocol address
variable-length field defining the logical
address of the sender. Ex. IPv4 protocol: 4 bytes
• Target hardware address
variable-length field defining the physical
address of the target. Ex. Ethernet: 6 bytes
• Target protocol address
variable-length field defining the logical
address of the target.
Ex. IPv4 protocol: 4bytes
101. Encapsulation of ARP packet
• An ARP packet is encapsulated directly into a
data link frame.
104. Example
A host with IP address 130.23.43.20 and physical
address B2 : 34 : 55 : 10 : 22 : 10 has a packet to
send to another host with IP address
130.23.43.25 and physical address A4 : 6E : F4 :
59 : 83 : AB (which is unknown to the first host).
The two hosts are on the same Ethernet
network. Show the ARP request and reply
packets encapsulated in Ethernet frames.
107. ARP Package
• ARP package involves five components: a
cache table, queues, an output module, an
input module, and a cache-control module.
• The package receives an IP datagram that
needs to be encapsulated in a frame that
needs the destination physical (hardware)
address.
109. ARP Components
• Cache Table - When a host or router receives the
corresponding physical address for an IP
datagram, the address can be saved in the cache
table.
• The address can be used for the datagrams
destined for the same receiver within the next
few minutes.
• Cache Table is implemented as an array of entries.
110. Cache Table Entries
• State. This column shows the state of the entry. It
can have one of three values:
FREE, PENDING, or RESOLVED.
- FREE state means that the time-to-live for this
entry has expired.
- PENDING state means a request for this entry has
been sent, but the reply has not yet been received.
- RESOLVED state means that the entry is complete.
The entry now has the physical (hardware) address of
the destination.
111. Cache Table Entries
• Hardware type, Protocol type, Hardware
length, Protocol length. This columns is the
same as the corresponding field in the ARP
packet.
• Interface number. A router can be connected
to different networks, each with a different
interface number. Each network can have
different hardware and protocol types.
112. Cache Table Entries
• Queue number. ARP uses numbered queues
to enqueue the packets waiting for address
resolution. Packets for the same destination
are usually enqueued in the same queue.
• Attempts. This column shows the number of
times an ARP request is sent out for this entry.
• Time-out. This column shows the lifetime of
an entry in seconds.
113. Cache Table Entries
• Hardware address. This column shows the
destination hardware address. It remains
empty until resolved by an ARP reply.
• Protocol address. This column shows the
destination IP address.
114. Queues
• ARP package maintains a set of queues, one
for each destination, to hold the IP packets
while ARP tries to resolve the hardware
address.
• Output module sends unresolved packets into
the corresponding queue.
• Input module removes a packet from a queue
and sends it, with the resolved physical
address, to the data link layer for
transmission.
115. Output Module
• Output module waits for an IP packet from the
IP software and checks the cache table to find
an entry corresponding to the destination IP
address of this packet.
• The destination IP address of the IP packet
must match the protocol address of the entry.
• If the entry is found and the state of the entry
is RESOLVED, the packet along with the
destination hardware address is passed to the
data link layer for transmission.
116. Output Module
• If the entry is found and the state of the entry is
PENDING, the packet waits until the destination
hardware address is found.
• If no entry is found, the module creates a queue
and enqueues the packet. A new entry with the
state of PENDING is created for this destination
and the value of the ATTEMPTS field is set to 1.
• An ARP request packet is then broadcast.
117. Output Module Pseudocode
1 ARP_Output_Module ( )
2 {
3 Sleep until an IP packet is received from IP
software.
4 Check cache table for an entry corresponding to
5 destination of IP packet.
6 If (entry is found)
7 {
8 If (the state is RESOLVED)
9 {
10 Extract the value of the hardware
address from the entry.
11 Send the packet and the hardware address to data
12 link layer.
13 Return
14 } // end if
15 If (the state is PENDING)
16 {
17 Enqueue the packet to the
corresponding queue.
18 Return
19 }//end if
20 }//end if
21 If (entry is not found)
22 {
23 Create a cache entry with
state set to PENDING and
24 ATTEMPTS set to 1.
25 Create a queue.
26 Enqueue the packet.
27 Send an ARP request.
28 Return
29 }//end if
30 } //end module
118. Input Module
• The input module waits until an ARP packet
(request or reply) arrives.
• The input module checks the cache table to
find an entry corresponding to this ARP
packet.
• The target protocol address should match the
protocol address of the entry.
119. Input Module
• If the entry is found and the state of the entry
is PENDING, the module updates the entry by
copying the target hardware address in the
packet to the hardware address field of the
entry and changing the state to RESOLVED.
• If the entry is found and the state is
RESOLVED, the module still updates the entry.
• This is because the target hardware address
could have been changed.
120. Input Module
• If the entry is not found, the module creates a
new entry and adds it to the table.
• The protocol requires that any information
received is added to the table for future use.
• Then, the module checks to see if the arrived
ARP packet is a request. If it is, the module
immediately creates an ARP reply message
and sends it to the sender.
121. Input Module Pseudocode
1 ARP_Input_Module ( )
2 {
3 Sleep until an ARP packet
(request or reply) arrives.
4 Check the cache table to find the
corresponding entry.
5 If (found)
6 {
7 Update the entry.
8 If (the state is PENDING)
9 {
10 While (the queue is not empty)
11 {
12 Dequeue one packet.
13 Send the packet and the hardware address.
14 }//end if
15 }//end if
16 }//end if
17 If (not found)
18 {
19 Create an entry.
20 Add the entry to the table.
21 }//end if
22 If (the packet is a request)
23 {
24 Send an ARP reply.
25 }//end if
26 Return
27 }//end module
122. Cache Module
• It periodically checks the cache table, entry by
entry.
• If the state of the entry is FREE, it continues to the
next entry.
• If the state is PENDING, the module increments
the value of the attempts field by 1.
• If the state of the entry is RESOLVED, the module
decrements the value of the time-out field by the
amount of time elapsed since the last check.
123. Cache Module Pseudocode
1 ARP_Cache_Control_Module ( )
2 {
3 Sleep until the periodic timer matures.
4 Repeat for every entry in the cache table
5 {
6 If (the state is FREE)
7 {
8 Continue.
9 }//end if
10 If (the state is PENDING)
11 {
124. Cache Module Pseudocode
12 Increment the value of attempts by 1.
13 If (attempts greater than maximum)
14 {
15 Change the state to FREE.
16 Destroy the corresponding queue.
17 }// end if
18 else
19 {
20 Send an ARP request.
21 }//end else
22 continue.
23 }//end if
125. Cache Module Pseudocode
12 Increment the value of attempts by 1.
13 If (attempts greater than maximum)
14 {
15 Change the state to FREE.
16 Destroy the corresponding queue.
17 }// end if
18 else
19 {
20 Send an ARP request.
21 }//end else
22 continue.
23 }//end if
126. Cache Module Pseudocode
24 If (the state is RESOLVED)
25 {
26 Decrement the value of time-out.
27 If (time-out less than or equal 0)
28 {
29 Change the state to FREE.
30 Destroy the corresponding queue.
31 }//end if
32 }//end if
33 }//end repeat
34 Return.
35 }//end module
127. Example
• The ARP output module receives an IP datagram with
the destination address 114.5.7.89. It checks the cache
table and finds that an entry exists for this destination
with the RESOLVED state (R).
• It extracts the hardware address, which is
457342ACAE32, and sends the packet and the address
to the data link layer for transmission.
128. Drill
• Twenty seconds later, the ARP output module receives
an IP datagram (from the IP layer) with the destination
address 116.1.7.22.
129. Answer
• It checks the cache table and does not find this
destination in the table.
• The module adds an entry to the table with the state
PENDING and the Attempt value 1.
• It creates a new queue for this destination and
enqueues the packet.
• It then sends an ARP request to the data link layer for
this destination.
131. RARP
• Reverse Address Resolution Protocol (RARP)-
a version of ARP designed to provide the IP
address for a booted computer.
• ARP maps an IP address to a physical address:
RARP maps a physical address to an IP address.
• RARP used the broadcast service of the data
link layer, which means that a RARP server
must be present in each network.
• RARP can provide only the IP address of the
computer
132. Routing Protocols
• Routing protocols have been created in
response to the demand for dynamic routing
tables.
• A routing protocol is a combination of rules
and procedures that lets routers in the
internet inform each other of changes.
• Ex. The sharing of information allows a router
in Greenhills to know about the failure of a
network in Paranaque.
133. Intra- and Inter-Domain Routing
• Routing inside an autonomous system is
referred to as intra-domain routing.
• Routing between autonomous systems is
referred to as inter-domain routing.
• Note: An autonomous system (AS) is a group
of networks and routers under the authority
of a single administration.
135. Routing Protocols
• Unicast Routing Protocols:
- Routing Information Protocol (RIP) - based on the
distance vector routing algorithm.
- Open shortest path first (OSPF) - interior
routing protocol based on link state routing.
- Border Gateway Protocol (BGP) - interautonomous
system routing protocol based on path vector routing.
• Multicasting and Multicast Routing Protocols
136. Routing Protocols
• RIP and OSPF are interior routing protocols;
BGP is an exterior routing protocol.
137. Distance Vector Routing
• This method sees an AS, with all routers and
networks, as a graph, a set of nodes and lines
(edges) connecting the nodes.
• The graph theory used an algorithm called
Bellman-Ford (also called Ford-Fulkerson) for a
while to find the shortest path between nodes
in a graph given the distance between nodes.
138. Bellman-Ford Algorithm
• If we know the cost between each pair of
nodes, we can use the algorithm to find the
least cost (shortest path) between any two
nodes.
• Ex.
139. Bellman-Ford Algorithm
• Principle: “If all neighbors of node i know the
shortest distance to node j, then the shortest
distance between node i and j can be found by
adding the distance between node i and each
neighbor to the neighbor’s shortest distance to
node j and then select the minimum.”
140. Bellman-Ford Algorithm
• Create a shortest distance table (vector) for each
node:
1. The shortest distance and the cost between a
node and itself is initialized to 0.
2. The shortest distance between a node and any
other node is set to infinity. The cost between a
node and any other node should be given (can be
infinity if the nodes are not connected).
3. The algorithm repeat until there is no more
change in the shortest distance vector.
141. Bellman-Ford Algorithm Definitions
• Find shortest paths from given node subject to constraint that
paths contain at most one link
• Find the shortest paths with a constraint of paths of at most two
links
• And so on
• s = source node
• w(i, j) = link cost from node i to node j
– w(i, i) = 0
– w(i, j) = if the two nodes are not directly connected
– w(i, j) 0 if the two nodes are directly connected
• h = maximum number of links in path at current stage of the
algorithm
• Lh(n) = cost of least-cost path from s to n under constraint
of no more than h links
142. Bellman-Ford Algorithm
• Step 1 [Initialization]
– L0(n) = , for all n s
– Lh(s) = 0, for all h
• Step 2 [Update]
• For each successive h 0
– For each n ≠ s, compute
– Lh+1(n)=min
j[Lh(j)+w(j,n)]
• Connect n with predecessor node j that achieves minimum
• Eliminate any connection of n with different predecessor node
formed during an earlier iteration
• Path from s to n terminates with link from j to n
143. Bellman-Ford Pseudocode
1 Bellman_Ford ( )
2 {
3 // Initialization
4 for (i = 1 to N; for j = 1 to N)
5 {
6 if(i == j) Dij = 0 cij = 0
7 else Dij = ∞ ; cij = cost
between i and j
8 }
9 // Updating
10 repeat
11 {
12 for (i = 1 to N; for j = 1 to N)
13 {
14 Dij ← minimum [(ci1 + D1j) ... (ciN +
DNj)]
15 } // end for
16 } until (there was no change in
previous iteration)
17 } // end Bellman-Ford
147. ARPANET Routing Strategies
• First Generation
– 1969
– Distributed adaptive
– Estimated delay as performance criterion
– Bellman-Ford algorithm
– Node exchanges delay vector with neighbors
– Update routing table based on incoming info
– Doesn't consider line speed, just queue length
– Queue length not a good measurement of delay
– Responds slowly to congestion
148. ARPANET Routing Strategies
• Second Generation
– 1979
– Uses delay as performance criterion
– Delay measured directly
– Uses Dijkstra’s algorithm
– Good under light and medium loads
– Under heavy loads, little correlation between
reported delays and those experienced
149. Dijkstra’s Algorithm Definitions
• Find shortest paths from given source node to all other nodes,
by developing paths in order of increasing path length
N = set of nodes in the network
s = source node
T = set of nodes so far incorporated by the algorithm
• w(i, j) = link cost from node i to node j
– w(i, i) = 0
– w(i, j) = if the two nodes are not directly connected
– w(i, j) 0 if the two nodes are directly connected
• L(n) = cost of least-cost path from node s to node n currently
known
– At termination, L(n) is cost of least-cost path from s to n
150. Dijkstra’s Algorithm Method
• Step 1 [Initialization]
– T = {s} Set of nodes so far incorporated consists of only
source node
– L(n) = w(s, n) for n ≠ s
– Initial path costs to neighboring nodes are simply link
costs
• Step 2 [Get Next Node]
– Find neighboring node not in T with least-cost path
from s
– Incorporate node into T
– Also incorporate the edge that is incident on that node
and a node in T that contributes to the path
151. Dijkstra’s Algorithm Method
• Step 3 [Update Least-Cost Paths]
– L(n) = min[L(n), L(x) + w(x, n)] for all n T
– If latter term is minimum, path from s to n is path
from s to x concatenated with edge from x to n
• Algorithm terminates when all nodes have been
added to T
• At termination, value L(x) associated with each node x
is cost (length) of least-cost path from s to x.
158. Dijkstra’s vs. Bellman-Ford
– Bellman-Ford
• Calculation for node n involves knowledge of link cost to all
neighboring nodes plus total cost to each neighbor from s
• Each node can maintain set of costs and paths for every other
node
• Can exchange information with direct neighbors
• Can update costs and paths based on information from
neighbors and knowledge of link costs
– Dijkstra
• Each node needs complete topology
• Must know link costs of all links in network
• Must exchange information with all other nodes
159. Distance Vector Routing Algorithm
• In distance vector routing, the cost is normally
hop counts. So the cost between any two
neighbors is set to 1.
• Each router needs to update its routing table
asynchronously, whenever it has received
some information from its neighbors.
• After a router has updated its routing table, it
should send the result to its neighbors so that
they can also update their routing table.
160. Distance Vector Routing Algorithm
• Each router should keep at least three pieces
of information for each route: destination
network, the cost, and the next hop.
• We refer to information about each route
received from a neighbor as R (record), which
has only two pieces of information: R.dest and
R.cost.
163. Routing Information Protocol (RIP)
• RIP is defined in RFC 1058, 1388, 1723 (RIP2)
• An intradomain (interior) routing protocol used
inside an autonomous system.
• It is a very simple protocol based on distance
vector routing.
• RIP implements distance vector routing directly.
• The distance is defined as the number of links
(networks) that have to be used to reach the
destination. The metric in RIP is called a hop count.
164. Routing Information Protocol (RIP)
• Infinity is defined as 16, which means that any
route in an autonomous system using RIP
cannot have more than 15 hops.
• The destination in a routing table is a network,
which means the first column defines a
network address.
• The next node column defines the address of
the router to which the packet is to be sent to
reach its destination.
166. RIP Algorithm
If (destination not in the routing table)
Add the advertised information to the table.
Else
If (next -hop field is the same)
Replace entry in the table with the advertised one.
Else
If (advertised hop count smaller than one in the
table)
Replace entry in the routing table.
Return
Update Algorithm: Add one hop to the hop count for
each advertised destination
167. RIP Message Format
• Command. This 8-bit field specifies the type of
message: request (1) or response (2).
• Version. This 8-bit field defines the version.
• Family. This 16-bit field defines the family of
the protocol used. For TCP/IP the value is 2.
168. RIP Message Format
• Network address. The address field defines
the address of the destination network.
• RIP has allocated 14 bytes for this field to be
applicable to any protocol.
• Distance. This 32-bit field defines the hop
count (cost) from the advertising router to the
destination network.
169. Requests and Responses
• Request
A request message is sent by a router that has
just come up or by a router that has some time-
out entries.
170. Requests and Responses
• Response
A response can be either solicited or unsolicited.
• 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, on the other hand, is
sent periodically, every 30 seconds or when
there is a change in the routing table.
• The response is sometimes called an update
packet.
172. 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.
173. RIP Version 2
• RIPv1 uses classful addressing while RIPv2 uses
classless addressing .
• Authentication is added to protect the message
against unauthorized advertisement.
174. RIP Version 2
• Version 1 of RIP uses broadcasting to send RIP
messages to every neighbor. All the routers on
the network receive the packets, as well as the
hosts.
• RIP version 2 uses the all-router multicast
address to send the RIP messages only to RIP
routers in the network.
175. Encapsulation
• RIP messages are encapsulated in UDP user
datagrams.
• A RIP message does not include a field that
indicates the length of the message. This can
be determined from the UDP packet.
• RIP uses the services of UDP on well-known
port 520.
176. Link State Routing
• If each node in the domain has the entire
topology of the domain— the list of nodes
and links, how they are connected including
the type, cost (metric), and the condition of
the links (up or down)—the node can use the
Dijkstra algorithm to build a routing table.
178. Link State Routing
• The topology must be dynamic, representing
the latest situation of each node and each
link.
• Link state routing is based on the assumption
that, although the global knowledge about the
topology is not clear, each node has partial
knowledge: it knows the state (type,
condition, and cost) of its links.
179. Building Routing Tables
• In link state routing, four sets of actions are
required:
1. Creation of the states of the links by each node,
called the link state packet or LSP.
2. Dissemination of LSPs to every other router,
called flooding.
3. Formation of a shortest path tree for each
node.
4. Calculation of a routing table based on the
shortest path tree.
180. Link State Packet (LSP)
• LSP carries the node identity, the list of links,
a sequence number, and age.
• Node identity and the list of links - are needed
to make the topology.
• Sequence number - facilitates flooding and
distinguishes new LSPs from old ones.
• Age - prevents old LSPs from remaining in the
domain for a long time.
181. Understand
• LSPs are generated on which occasions?
Answer:
1. When there is a change in the topology of
the domain.
2. On a periodic basis. (done to ensure that old
information is removed from the domain)
182. Flooding of LSPs
1. The creating node sends a copy of the LSP out
of each interface.
2. A node that receives an LSP compares it with
the copy it may already have.
a. It discards the old LSP and keeps the new one.
b. It sends a copy of it out of each interface
except the one from which the packet arrived.
183. Formation of Shortest Path Tree:
Dijkstra Algorithm
• Example:
Show the shortest path tree as seen by node C.
184. Routing Table from Shortest Path Tree
• Each node uses the shortest path tree to
construct its routing table.
185. Open Shortest Path First (OSPF)
• An intradomain routing protocol based on link
state routing.
• OSPF divides an autonomous system into
areas.
• An area is a collection of networks, hosts, and
routers all contained within an autonomous
system.
• All networks inside an area must be connected.
186. OSPF
• Routers inside an area flood the area with
routing information.
• At the border of an area, special routers called
area border routers summarize the information
about the area and send it to other areas.
• Backbone – a special area inside an AS where all
of the other areas must be connected.
187. OSPF
• The routers inside the backbone are called the
backbone routers.
• Each area has an area identification. The area
identification of the backbone is zero.
188. OSPF Metric
• OSPF protocol allows the administrator to assign
a cost, called the metric, to each route.
• The metric can be based on a type of service
(minimum delay, maximum throughput, and so
on).
• A router can have multiple routing tables, each
based on a different type of service.
189. OSPF Links
• In OSPF, a connection is called a link.
• Point-to-point link - connects two routers
without any other host or router in between.
190. OSPF Links
• Transient link is a network with several routers
attached to it.
• The data can enter through any of the routers
and leave through any router.
• All LANs and some WANs with two or more
routers are of this type.
191. OSPF Links
• A stub link is a network that is connected to
only one router.
• The data packets enter the network through
this single router and leave the network
through this same router.
• A virtual link is created by the administration
when the link between two routers is broken.
193. OSPF Packet Encapsulation
• OSPF packets are encapsulated in IP datagrams.
• They contain the acknowledgment mechanism
for flow and error control.
• They do not need a transport layer protocol to
provide these services.
194. Path Vector Routing
• An exterior routing protocol for interdomain or
inter-AS routing.
• Recall: Distance vector and link state routing are
both interior routing protocols.
• In distance vector routing, a router has a list of
networks that can be reached in the same AS
with the corresponding cost (number of hops).
• In path vector routing, a router has a list of
networks that can be reached with the path (list
of ASs to pass) to reach each one (path).
195. Analogy
• The difference between the distance vector
routing and path vector routing can be compared
to the difference between a national map and an
international map.
• A national map can tell us the road to each city
and the distance to be travelled if we choose a
particular route;
• An international map can tell us which cities exist
in each country and which countries should be
passed before reaching that city.
196. Reachability Information
• Each AS must have at least one path vector routing
that collects reachability information about each
network in that AS.
197. Routing Tables
• A path vector routing table for each router can
be created if ASs share their reachability list
with each other. Ex.
198. Path Vector Routing Features
• Loop prevention: When a router receives a
reachability information, it checks to see if its
autonomous system is in the path list to any
destination (hence a loop).
• Aggregation: The path vector routing protocols
normally support CIDR notation and the
aggregation of addresses.
• Policy routing: When a router receives a
message, it can check the path. If one of the
autonomous systems listed in the path is against
its policy, it can ignore that path and that
destination.
199. Border Gateway Protocol (BGP)
• An interdomain routing protocol using path
vector routing.
• BGP uses classless interdomain routing
addresses (prefix-based).
• The exchange of routing information between
two routers using BGP takes place in a session.
• A session is a connection that is established
between two BGP routers only for the sake of
exchanging routing information.
200. BGP Sessions
• To create a reliable environment, BGP uses the
services of TCP.
• BGP can have two types of sessions: external
BGP (E-BGP) and internal BGP (I-BGP)
sessions.
201. Reading Assignment
• The types of OSPF packets and their formats.
• The types of BGP packets and their formats.
Hint: There are four types of BGP messages:
open, update, keepalive, and notification.
• Multicasting and Multicast Routing Protocols
Multicast Link State Routing (MOSPF),
Multicast Distance Vector Routing, etc.
202. RFCs
• RIP is discussed in RFC1058, RFC1388,
RFC1723 and RFC 2453.
• OSPF is discussed in RFC 1583 and RFC 2328.
• BGP is discussed in RFC 1654, RFC 1771, RFC
1773, RFC 1997, RFC 2439, RFC 2918, and RFC
3392.
