1. The OSI Model in Practice
The transmitting process generates data to be used
by the receiving process.
In each OSI layer along the way down the model,
data is added - and subsequently used by the
corresponding layer at the receiving end.
This data is normally in the form of a header at the
beginning of a data packet that arrives from the
layer above.
In the link layer, data is often added at the end of
the packet as well, in the form of a trailer.
Each OSI layer has its own data format protocol.
3. TCP/IP Protocol Architecture
Developed by the US Defense Advanced
Research Project Agency (DARPA) for its
packet switched network (ARPANET)
Used by the global Internet
No official model but a working one.
Application layer
Host to host or transport layer
Internet layer
Network access layer
Physical layer
7. OSI vs. TCP/IP
OSI Network:
Connectionless or
connection-oriented.
OSI Transport
Connectionless.
TCP/IP Network:
Connectionless.
TCP/IP
Transport:
Connectionless or
connection-oriented svc.
9. The Application Layer
The higher level protocols include the session
and presentation layer details.
The application layer handles high-level
protocols, issues of representation, encoding,
and dialog control.
This model combines all application-related
issues into one layer, and assures this data is
properly packaged for the next layer.
10. The Transport Layer
• This layer deals with the quality-of-service issues of
reliability, flow control, and error correction.
• One of its protocols, the transmission control
protocol (TCP), provides excellent and flexible ways
to create reliable, well-flowing, low-error network
communications.
• TCP is a connection-oriented protocol.
• It dialogues between source and destination while
packaging application layer information into units
called segments.
11. The Transport Layer - Cont
• Connection-oriented does not mean that a
circuit exists between the communicating
computers (that would be circuit switching).
• It does mean that the segments travel back
and forth between two hosts to acknowledge
the connection exists logically for some
period.
• This is known as packet switching.
• Another protocol at this layer is UDP.
12. The Internet/Network Layer
• The purpose of the Network layer is to send source packets from
any network on the inter-network and have them arrive at the
destination independent of the path and networks they took to
get there.
• The specific protocol that governs this layer is called the Internet
protocol (IP).
• Best path determination and packet switching occur at this layer.
• Think of it in terms of the postal system: When you mail a letter,
you do not know how it gets there (there are various possible
routes), but you do care that it arrives.
13. Internet Protocol
The IP in UDP/IP and TCP/IP
IP is the network layer
packet delivery service (host-to-host).
translation between different data-link
protocols.
14. IP Datagrams
IP provides connectionless, unreliable
delivery of IP datagrams.
Connectionless: each datagram is
independent of all others.
Unreliable: there is no guarantee that
datagrams are delivered correctly or at
all.
15. IP Addresses
IP addresses are not the
same as the underlying data-
link (MAC) addresses.
Why ?
16. IP Addresses
IP is a network layer - it must be
capable of providing communication
between hosts on different kinds of
networks (different data-link
implementations).
The address must include information
about what network the receiving host
is on. This makes routing feasible.
17. IP Addresses
IP addresses are logical addresses (not
physical)
32 bits.
Includes a network ID and a host ID.
Every host must have a unique IP address.
IP addresses are assigned by a central
authority (Internet Corporation for Assigned
Names and Numbers -- ICANN)
18. The four formats of IP Addresses
0 NetID
10
110 NetID
1110 Multicast Group ID
HostID
NetID HostID
HostID
Class
A
B
C
D
8 bits 8 bits 8 bits
8 bits
19. Class A
128 possible network IDs
over 4 million host IDs per network ID
Class B
16K possible network IDs
64K host IDs per network ID
Class C
over 2 million possible network IDs
about 256 host IDs per network ID
20. Network and Host IDs
A Network ID is assigned to an
organization by a global authority.
Host IDs are assigned locally by a
system administrator.
Both the Network ID and the Host ID
are used for routing.
21. IP Addresses
IP Addresses are usually shown in dotted
decimal notation:
1.2.3.4 00000001 00000010 00000011 00000100
cs.rpi.edu is 128.213.1.1
10000000 11010101 00000001 00000001
CS has a class B network
22. Host and Network Addresses
A single network interface is assigned
a single IP address called the host
address.
A host may have multiple interfaces,
and therefore multiple host addresses.
Hosts that share a network all have
the same IP network address (the
network ID).
23. IP Broadcast and Network Addresses
An IP broadcast addresses has a
host ID of all 1s.
IP broadcasting is not necessarily a
true broadcast, it relies on the
underlying hardware technology.
An IP address that has a host ID of
all 0s is called a network address and
refers to an entire network.
24. Virtual Circuits
Source-to-destination path behaves much like
telephone circuit.
Call setup for each call before data can flow.
Each packet carries VC identifier (not destination
host ID).
Every router on source-destination path maintains
“state” for each passing connection.
Link and router resources (bandwidth, buffers) may
be allocated to VC to get circuit-like performance.
25. Datagram networks:
the Internet model
No call setup at network layer.
Routers: no state about end-to-end
connections.
No network-level concept of “connection”.
Packets forwarded using destination host
address.
packets between same source-destination pair
may take different paths.
26. Routing
Goal: determine “good”
path (sequence of routers)
through network from
source to destination.
graph nodes are routers
graph edges are physical links
link cost: delay, $ cost, or congestion level.
good” path:
typically means minimum cost path.
A
E
D
C
B
F
2
2
1
3
1
1
2
5
3
5
27. Data Link Layer
The main tasks of the data link layer are:
Transfer data from the network layer of one machine to the
network layer of another machine
Convert the raw bit stream of the physical layer into groups
of bits (“frames”)
Broadcast Networks: All stations share a single
communication channel
Point-to-Point Networks: Pairs of hosts (or routers)
are directly connected
Typically, local area networks (LANs) are broadcast
and wide area networks (WANs) are point-to-point
