1. Communication Networks NETW 501
Lecture 3
Layered Architecture of Communication Networks: TCP/IP Model
Course Instructor: Dr.-Ing. Maggie Mashaly
C3.220

2. Emergence of TCP/IP Problem: Solution:
When satellite and radio networks were added, existing protocols had trouble inter-working the three networks together
ARPANET was a research network supported by US DoD (Department of Defense) interconnecting universities and government installations using telephone leased lines.
And they had a solution for the problem!
Solution:
TCP/IP Reference Model:
The ability to connect multiple networks together in a seamless way
One of the main objectives of DoD was that communications can survive hardware failures

3. TCP/IP: Inter-connecting Networks
Cellular Network
ARPANET
SRC
DEST
Ethernet
Satellite Network
Ethernet

4. TCP/IP Reference Model
TCP/IP vs OSI Reference Model
Application
Presentation
Session
Transport
Network
Data Link
Physical
Application
Transport
Internet
Network Interface
OSI Reference Model
TCP/IP Reference Model

5. TCP/IP Reference Model
HTTP
SMTP
DNS
RTP
Application
Transport
Internet
Network Interface
TCP
UDP IP
Network Interface 1
Network Interface 2
Network Interface 3

6. Application Layer Incorporates functions of the top three OSI layers
Transport
Internet
Network Interface
Incorporates functions of the top three OSI layers
Example Protocols:
HTTP (Hypertext Transfer Protocol)
Publishing and receiving HTML pages
SMTP (Simple Mail Transfer Protocol)
Sending s towards a server
DNS (Domain Name System)
Helps to map ASCII strings (URLs) to IP addresses
RTP (Real-time Transport Protocol)
Delivering audio and video over the Internet

7. Transport Layer TCP (Transport Control Protocol)
Application
Transport
Internet
Network Interface
TCP (Transport Control Protocol)
Reliable connection-oriented transfer of a byte stream
Applies error recovery and flow control mechanisms
UDP (User Datagram Protocol)
Unreliable best-effort connectionless transfer of individual messages
No message sequencing or flow control. Leaves the upper layer to decide how to deal with these situations
Used by applications that require quick delivery and has some tolerance to information loss

8. Internet Layer
Application
Transport
Internet
Network Interface
Handles transfer of datagrams (packets) across multiple networks
Best-effort connectionless packet transfer
Best-effort means best-effort The IP layer attempts to send the datagrams towards the destination
It does not guarantee the transfer would be successful
It does not guarantee the datagrams would arrive in sequence
Handles routing and congestion control

9. Network Interface
Application
Transport
Internet
Network Interface
It is not really a layer Rather it defines an interface to underlying networks
TCP/IP assumes that data-link and physical layers are part of the underlying network design
TCP/IP reference model is only concerned with inter-connecting functional networks

10. Example: Web Page Download
Network 1
Network 2
cleo.guc.edu.eg
1.4
2.1
1.1
1.3
2.4
1.2
2.2
2.3
CNN.com
Assume a host on cleo.guc.edu.eg connected to Network 1 wants to read the news on CNN.com whose server is located on Network 2
Let us simplify the IP addresses to x.y where
x is the network id
y is the terminal id of a particular network
HOW DOES THE DOWNLOAD OCCUR?

11. HTTP Web browser runs an HTTP client program
Get …
200 OK …
HTML Content
Web browser runs an HTTP client program
Web server runs an HTTP server program
HTTP Protocol
HTTP client sends its request message: Get …
Relevant information associated with Get request
Server’s IP address : 2.3
Filename : index.html
Host’s IP address : 1.1
HTTP server sends a status response
HTTP server sends HTML data content

12. Establish TCP Connection
Q: What is Port?
HTTP Client
It is a logical interface to help the transport layer identify which application process should be contacted
Establish a TCP Connection from port z of 1.1. to port 80 of 2.3
Connection Established
Port z
Port 80
TCP Connection Request
from 1.1 port z to 2.3 port 80
TCP Client
TCP Server
ACK, TCP Connection Request
from 2.3 port 80 to 1.1 port z
ACK
Before the HTTP client can send anything, a TCP connection must be established
TCP connection uses a three-way handshake
Note that at this phase, we have yet to get the HTTP server involved in any communications

13. HTTP Data Transfer using TCP Connection
HTTP Client
HTTP Server
Get …
OK, 200
HTML Content
Get …
OK, 200
HTML Content
Port z
Port 80
TCP Send (80, z, Get)
TCP Client
TCP Server
ACK
TCP Send (z, 80, Response)
ACK
HTTP uses the TCP connection to transfer data in both directions

14. Interfacing TCP and IP Protocols
TCP Client
TCP Server
Send this datagram to
2.3
Datagram Received
Send this datagram to
1.1
Datagram Received IP
IP Network IP
The role of the IP layer is to attempt its best (best-effort service) to forward datagrams towards their target destination

15. IP Routing: Sending Packets from 1.1 to 2.3
Network 1
Network 2
cleo.guc.edu.eg
1.4
2.1
1.1
1.3
2.4
Router C
1.2
2.2
2.3
CNN.com
Destination
Next Hop
Interface
1.2
1.1
1.3
2.2
1.4
2.3
2.4
Destination
Next Hop
Interface
1.1
1.4
1.2
1.3
2.2
2.1
2.3
2.4
Routing Table of cleo.guc.edu.eg
Routing Table of Router C

16. IP Routing How does it work?
Terminal 1.1 may only send a packet directly to destinations within the same network
How would 1.1 know if the destination is in the same network?
By looking at the prefix of the IP address
Terminal 2.3 is not on the same network as 1.1
The IP layer of cleo.guc.edu.eg has a routing table that says:
If you wish to reach 2.3 then you have to go through router (gateway) 1.4
Router C has multiple interfaces each of which corresponds to an IP address
The routing table of Router C indicates that if you want to reach 2.3 then you will need to send over the interface which has the IP address 2.1

17. Sending IP Packets over LANs
cleo.guc.edu.eg
Router C
ARP Message:
What is the MAC address of 1.4?
ARP Message:
IP Address 1.4 corresponds to MAC address 3f
ARP Message:
What is the MAC address of 2.3?
Router C
CNN.com
ARP Message:
IP Address 2.3 corresponds to MAC address 1g

18. Sending IP Packets over LAN
How does it work?
To send an IP datagram to IP Address 1.4, cleo.guc.edu.eg puts the IP datagram in an Ethernet frame, and transmits the frame.
However, Ethernet uses different addresses, so-called Media Access Control (MAC) addresses (also called: physical address, hardware address)
Therefore, we must first translate the IP address 1.4 into a MAC address.
The translation of addresses is performed via the Address Resolution Protocol (ARP)

19. Summary: Sending the TCP Request
Cleo.guc.edu.eg
CNN.com
HTTP
Send HTTP Request to CNN.com
HTTP
Establish a connection to 2.3 at port 80
TCP
Open TCP connection to 2.3 port 80
TCP
IP datagram is a TCP segment for port 80
Send a datagram (which contains a connection request) to 2.3
Send IP data-gram to 2.3
Send IP datagram to 2.3 IP IP IP
Frame is an IP datagram
Frame is an IP datagram
Send the datagram to 1.4
Send the datagram to 2.3
Ethernet
Ethernet
Ethernet
Ethernet
Send Ethernet frame to 3f
Send Ethernet frame to 1g
Router C
Router C
2.3
1.4

20. How the layers work together?
Internet
User clicks on
Ethereal network analyzer captures all frames observed by its Ethernet NIC
Sequence of frames and contents of frame can be examined in detail down to individual bytes

21. Ethereal Windows Top Pane shows frame/packet sequence
Middle Pane shows encapsulation for a given frame
Ethereal Windows
Bottom Pane shows hex & text

22. HTTP Request & Response
Top Pane: Frame sequence
TCP Connection Setup
DNS Query
HTTP Request & Response

23. Ethernet Destination and Source Addresses
Middle Pane: Encapsulation
Ethernet Frame
Ethernet Destination and Source Addresses
Protocol Type

24. IP Source and Destination Addresses
And a lot of other stuff!
Middle Pane: Encapsulation
IP Packet
IP Source and Destination Addresses
Protocol Type

25. Source and Destination Port Numbers
Middle Pane: Encapsulation
TCP Segment
Source and Destination Port Numbers
GET
HTTP Request

26. References NETW 501 Lectures slides by Assoc. Prof. Tallal El-Shabrawy
“Communication Networks 2nd Edition”, A. Leon-Garcia and I. Widjaja, McGraw Hill, 2013
“Computer Networks 4th Edition”, A. S. Tanenbaum, Pearson International