Transcript Layered Architecture of Communication Networks (2)

1. Layered Architecture of Communication
Networks: TCP/IP Model
Emergence of TCP/IP
 ARPANET was a research network supported by US DoD
(Department of Defense) interconnecting universities and
government installations using telephone leased lines
 When satellite and radio networks were added, existing
protocols had trouble inter-working the three networks
together
 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
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TCP/IP: Inter-connecting Networks
Cellular Network
ARPANET
DEST
SRC
Ethernet
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Satellite Network
Ethernet
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TCP/IP Reference Model
DNS
SMTP
HTTP
TCP
RTP
UDP
IP
Network
Interface 1
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Network
Interface 2
Network
Interface 3
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Application Layer
 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 emails 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
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Transport Layer
 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
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Internet Layer
 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 datagtams would arrive in sequence
 Handles routing and congestion control
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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
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TCP/IP vs OSI Reference Model
Application
Presentation
Application
Session
Transport
Transport
Network
Internet
Data Link
Network Interface
Physical
OSI Reference
Model
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TCP/IP Reference
Model
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Example: Web Page Download
Network 2
Network 1
cleo.guc.edu.eg
1.4
2.1
1.3
1.1
1.2
2.4
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?
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HTTP
www.cnn.
com
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 send HTML data content
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Establish TCP Connection
Q: What is Port?
HTTP Client
Establish a TCP
Connection from
port z of 1.1. to
port 80 of 2.3
It is a logical interface to help
the transport layer identify
which application process
should be contacted
Connection
Established
Port z
Port 80
TCP Connection Request
from 1.1 port z to 2.3 port 80
TCP Client
ACK, TCP Connection Request
from 2.3 port 80 to 1.1 port z
TCP Server
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
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HTTP Data Transfer using TCP Connection
HTTP Client
HTTP Server
OK, 200
Get …
OK, 200
Get …
HTML Content
Port z
HTML Content
Port 80
TCP Send (80, z, Get)
TCP Client
ACK
TCP Send (z, 80, Response)
TCP Server
ACK
 HTTP uses the TCP connection to transfer data in both
directions
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Interfacing TCP and IP Protocols
TCP Server
TCP Client
Send this
datagram to
2.3
Send this
datagram to
1.1
Datagram
Received
IP
IP
Network
Datagram
Received
IP
 The role of the IP layer is to attempt its best (best-effort
service) to forward datagrams towards their target
destination
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IP Routing: Sending Packets from 1.1 to 2.3
Network 2
Network 1
cleo.guc.edu.eg
1.1
2.3
1.4
TCP Data
1.3
1.1
2.1
1.1
2.3
2.4
TCP Data
Router C
2.2
1.2
2.3
CNN.com
Destination
Next Hop
Interface
1.2
1.2
1.1
1.3
1.3
1.1
2.2
1.4
1.1
2.3
1.4
1.1
2.4
1.4
1.1
Routing Table of cleo.guc.edu.eg
Destination
Next Hop
Interface
1.1
1.1
1.4
1.2
1.2
1.4
1.3
1.3
1.4
2.2
2.2
2.1
2.3
2.3
2.1
2.4
2.4
2.1
Routing Table of Router C
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IP Routing

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
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Sending IP Packets over LANs
cleo.guc.edu.eg
ARP Message:
What is the MAC address of 1.4?
Router C
ARP Message:
IP Address 1.4 corresponds to MAC address 3f
ARP Message:
What is the MAC address of 2.3?
Router C
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ARP Message:
IP Address 2.3 corresponds to MAC address 1g
CNN.com
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Sending IP Packets over LAN
 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, Argon must first translate the IP address 1.4 into
a MAC address.
 The translation of addressed is performed via the Address
Resolution Protocol (ARP)
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Summary: Sending the TCP Request
Cleo.guc.edu.eg
CNN.com
HTTP
TCP
IP
Frame is an IP
datagram
Establish a connection to 2.3 at port 80
Open TCP connection to 2.3
port 80
IP datagram is a TCP
segment for port 80
Send
IP data-gram
to
Send a datagram (which
contains
a connection
Send IP datagram
to
2.3
IP
2.3
request) to 2.3
Send the datagram to 1.4
Ethernet
Send Ethernet frame
to 3f
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HTTP
Send HTTP Request
to CNN.com
Ethernet
Router C
1.4
Frame is an IP
datagram
Ethernet
TCP
IP
Send the datagram
Ethernet
to 2.3
Router
C Ethernet frame
Send
2.3to 1g
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How the layers work together
Internet
 User clicks on http://www.nytimes.com/
 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
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“Communications Networks 2nd Edition”, A. Leon-Garcia, and I. Widjaja, McGraw Hill, 2003
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Top Pane
Ethereal
windows
shows
frame/packet
sequence
Middle Pane
shows
encapsulation for
a given frame
Bottom Pane shows hex & text
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“Communications Networks 2nd Edition”, A. Leon-Garcia, and I. Widjaja, McGraw Hill, 2003
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Top pane: frame sequence
DNS
Query
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TCP
Connection
Setup
HTTP
Request &
Response
“Communications Networks 2nd Edition”, A. Leon-Garcia, and I. Widjaja, McGraw Hill, 2003
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Middle pane: Encapsulation
Ethernet Frame
Protocol Type
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Ethernet
Destination and
Source
Addresses
“Communications Networks 2nd Edition”, A. Leon-Garcia, and I. Widjaja, McGraw Hill, 2003
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Middle pane: Encapsulation
And a lot of
other stuff!
IP Packet
IP Source and
Destination
Addresses
Protocol Type
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“Communications Networks 2nd Edition”, A. Leon-Garcia, and I. Widjaja, McGraw Hill, 2003
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Middle pane: Encapsulation
TCP Segment
Source and
Destination Port
Numbers
GET
HTTP
Request
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“Communications Networks 2nd Edition”, A. Leon-Garcia, and I. Widjaja, McGraw Hill, 2003
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