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Introduction
• Everyday we communicate across the Internet– What kinds of activities?
• What’s in common with all of these?– Servers
• All these interactions use standards & protocols
Client-Server
• Web Page Request– Client has web client software
• Browser- widely used client software
– Server• Host running an application that provides info or
services to a client
Client-Server
Web Site domain names to IP address translation
Remote login to device to control it
Send/Receive email
Assigns IP addresses
Web Site File Transfer
Standardized Communication
• Different computers & operating systems across the Internet– Communication not possible if they didn’t
share a protocol
• Phone call China to US– We have connectivity– A Chinese sales person couldn’t be understood
by an English speaking person– That’s why there’s protocols (rules) to follow
Licklider• DoD• Saw the need for
networked computers• 3 bidders w/ different
systems– They couldn’t
communicate with each other
– Came up with the idea of ARPAnet, which later became the Internet
Client Server Interaction
• A web server & client use specific protocols and standards in exchanging info to ensure that the messages are received & understood.
• Application, transport, Internetwork and network access protocols
Client Server Interaction
• Application Protocol• HTTP
– Rules for how browser & web server interact
• You request web page• Another protocol delivers it
Client Server Interaction
• Transport Protocol• TCP
– Manages the conversations/packet exchange– Splits message into segments– Flow control & acknowledgments between the
hosts– Retransmission
Client Server Interaction
• Internetwork Protocol• IP
– Assigns IP addresses– Encapsulate into packets for routing
Client Server Interaction
• Network Access Protocols– Get on the Ethernet cable!!!
• Data Link Management– Takes packets & encaps them into frames– MAC addresses are attached here
• Physical Transmission– How bits go on media (wired, wireless)
Review
• Which server…– Translates names to IP addresses?– Used for remote login?– Assigns IP addresses?
• Which protocol…– Splits data into segments?– Controls flow of data?– Acknowledges receipt of data?– Requests retransmission if a piece is missing?
End of Day One
Review
• Which protocol adds the IP addresses?
• Which category of protocols encapsulates packets into frames?
• Which protocol is for the rules of how a browser & web server communicate?
Transport Protocols
• IP addresses & routes data for all protocols
• Different applications use different protocols for transport of the data– How data is sent/exchanged
• TCP or UDP
Transmission Control Protocol
• Used when an application requires an acknowledgement– Like a registered letter in the mail
• Breaks messages into small numbered segments– If sender doesn’t get an ACK of message received, it
retransmits – Only portion lost is resent
• On receiving host, TCP reassembles data• FTP & HTTP are examples of protocols using
TCP• Look at 6.1.3.2 Example
User Datagram Protocol
• Used for faster transmissions• “Best effort delivery”
– Standard postal letter• No ACK or retransmission• Used for streaming audio or video, Internet
radio, VoIP
UDP & Internet Radio• If some of the message is lost, it is not
retransmitted. – You might hear a slight break in the sound.
• If TCP were used and the lost packets were resent, the transmission would pause to receive them and the disruption would be more noticeable.
Comparing TCP & UDP
• TCP– Slower– Segments– Acknowledgements– Retransmission– Reassembles– Connection-
oriented
• UDP– Faster– Best effort delivery– No
acknowledgements– No retransmit– Connectionless
Port Numbers & Communication
• When message is delivered, port # assigned
• Used to keep track of conversations and destination services requested
• Each message sent, has a source & destination port number
Port Numbers, Communication & Sockets
• The combo of the source & destination IP AND the source & destination port number is known as a socket. – Used to identify the server and service being
requested by the client. – Thousands of requests per day are tracked by this
Port Numbers
• Destination- tells about service requested– 80- HTTP– 25- SMTP– 53- DNS– 21- FTP– 23- Telnet
• Source Port– Random– Allows multiple conversations
Domain Name Service• Server with table of IP & host names• Request sent to port 53 (DNS)
– DNS server looks in table to translate it– If there, it lets client know– If not there, forwards to another DNS server
• If no learn, time out
Review
• What does www.gcit.org represent?– An IP address
• Which server would translate that name to an IP address?– DNS server
• Which server would assign IP addresses?– DHCP server
• Which Application protocol does FTP use?– TCP
Review• Why would an application use UDP?
– No disruption if dropped packets (fast)– Slight interruption, but it keeps going
• Which protocol is used for web sites?– HTTP
• Which port # is…– FTP
• 21– HTTP
• 80
Web Communication• You want www.cisco.com• DNS gets you the IP address of it• Your browser uses that IP & port 80 to request the
web page• Socket connection made with server• Web page comes back to your browser encoded
in HTML
FTP Clients & Servers
• Transfer files from host to host• Built in to OS & browsers• Also GUI based software available
• Client/Server• Uses TCP• Request to server on port 21• Port 20 to send files to client
• Email server receives & stores emails• You email client allows access to view
messages• [email protected] • Protocols used in email include SMTP,
POP3, IMAP4
Email Protocols- SMTP
• SMTP (25)– Used to send email from client to server– Email server to email server
Email Protocols- POP3
• POP3 (110)– Used to send client messages & deleted from
storage on the server• Mail on server until collected from client
Email Protocols- IMAP4
• IMAP4 (143)– Like POP3, but it keeps messages on server
until user deleted them
IM Protocols: Clients & Servers
• Real-time communication• Each IM service may use different
protocols & destination ports– So you must have compatible IM software
Voice Protocols: Clients & Servers
• VoIP• Can call similar clients
– Calls to landlines need public phone network
More About Port Numbers- 6.2.7.2
• 1-65,535• Well-known ports
– 1- 1023– Common applications
• Registered ports– 1024- 49,151– Can be source or destination ports– Used for specific applications like IM
• Private ports– 49,152 & above – For source ports
Review• Which email service protocol…
– Is used to send mail to server?• SMTP
– Is used to get mail from the server & keep it there until deleted?• IMAP4
– Is used to communicate from email server to email server?• SMTP
– Is used to get mail from the server & delete it?• POP3
Protocol Interaction
• They interact with each other– Protocol Stack
• Some protocols focus on content of message
• Others focus on moving the data
• Protocols visualized in layers
Sending a Message
• Web Server to client (you)– Application data broken into TCP segments
• Each segment has a header w/ source & dest ports
– Segment encapsulates HTTP & HTML data– Encapsulates into a packet w/ IP header
• IP header has source & dest IP addresses
– IP Packet sent to Ethernet protocol• Encapsulates into frame header & trailer• Source & destination MAC AND error checking
– Bits encoded onto the media
Receiving a Message• Last process in reverse
– Bits received by NIC & decoded• Destination MAC recognized
– Ethernet header & trailer removed• Source & dest MAC removed
– IP header removed• Source & dest IP removed
– TCP header removed• Source & dest ports removed
– Web page data passed to HTTP & browser– TCP segment received, reassembled, & page
displays
Activity• 192.168.5.101
• 80 or TCP or UDP
• 01-5A-FF-65-80-DC
• 1100101101000010
• DHCP or POP3 or HTTP