Lecture 7 Application layer protocols_v1_Part3 PDF

Summary of E-mail Protocols • All three email protocols are TCP-based protocols to ensure reliable delivery of email messages. • POP3 and IMAP are for mail retrievals and SMPT is for sending email messages • IMAP is designed to store messages on the mail server. It stores incoming and outgoing messa...

Summary of E-mail Protocols • All three email protocols are TCP-based protocols to ensure reliable delivery of email messages. • POP3 and IMAP are for mail retrievals and SMPT is for sending email messages • IMAP is designed to store messages on the mail server. It stores incoming and outgoing messages on the server in folders. • POP3 is designed to store messages on the local device. 41 Outline • Domain Name System (DNS) • Hypertext Transfer Protocol (HTTP) • Simple Mail Transfer Protocol (SMTP) • File Transfer Protocol (FTP) • Telnet and Secure Socket Shell (SSH) • Dynamic Host Configuration Protocol (DHCP) 42 FTP • FTP is a client/server protocol used to transfer files and manage files across a network: – FTP uses TCP ports 20 and 21 • Port 21 is for users sending control commands • Port 20 is for transferring file data – It is not a secure protocol, as user credentials and data are sent in plaintext • Uses URL for an FTP site begins with ftp://… • The 3 most common ways to access an FTP site: – Through a browser – Using a FTP client program – Issuing FTP commands at a text-based command prompt 43 Is FTP Still Used? • FTP was a revolutionary development when it was first introduced in the 1970s. • Within the last 40+ years, FTP has served as the foundation for various methods of sending data • FTP is outdated and insecure. 44 Outline • Domain Name System (DNS) • Hypertext Transfer Protocol (HTTP) • Simple Mail Transfer Protocol (SMTP) • File Transfer Protocol (FTP) • Telnet and Secure Socket Shell (SSH) • Dynamic Host Configuration Protocol (DHCP) 45 Telnet and SSH • Telnet and Secure Shell (SSH) – allows a user log into another computer remotely – Example: use to connect to a managed switch or router – Uses a well-known port: 23 on the server side – Is not a secure protocol • SSH – Uses TCP port 22 – Provides an encrypted channel between the client and server 46 PuTTY • PuTTY is a client program that supports Telnet along with SSH and Rlogin (remote login) network protocols. 47 Remote Desktop Software • Remote desktop software – Remote Desktop Protocol (RDP) • Uses Graphic User Interface (GUI) to manage/access Windows computers remotely – Independent Computing Architecture (ICA) – Virtual network computing (VNC) 48 Outline • Domain Name System (DNS) • Hypertext Transfer Protocol (HTTP) • Simple Mail Transfer Protocol (SMTP) • File Transfer Protocol (FTP) • Telnet and Secure Socket Shell (SSH) • Dynamic Host Configuration Protocol (DHCP) 49 DHCP • DHCP is used to automatically assign IP addresses as needed within a network. – When a computer is turned on, it requests an address from a DHCP server that is configured as a DHCP server – The DHCP server assigns an address to this client for a specific amount of time (called a lease) • DHCP Servers listen on UDP port 67 for IP address releases. • DHCP clients use UDP port 68 for IP address requests 50 DHCP Server A DHCP server is composed of the following: • IP address scope – a range of IP addresses the server leases to clients – Scope options – IP settings: • The default gateway, DNS servers, and other options – Reservations – an IP address tied to a particular MAC address • When the client’s MAC address matches an address specified by a reservation, the reserved IP is leased to the client instead of getting it form the scope – Exclusions – one or more IP addresses excluded from the IP address range, e.g. • If the scope ranges from 192.168.1.1 to 192.168.1.100, you can exclude addresses 192.168.1.1-192.168.1.10 for static IP assignment 51 DHCP Lease Process 1. During the boot process, a DHCP client broadcasts a DHCPDISCOVER message that it’s looking for a DHCP server 2. The DHCP server reserves an IP address for the client and makes a lease offer by sending a DHCPOFFER message to the client via unicast. 52 DHCP Lease Process (cont.) 3. The client responds with a DHCPREQUEST message via broadcast to accept the offered IP address. – If several DHCP servers respond to the request, the client accepts the first offer that it receives. 4. The DHCP server whose offer was accepted responds with a DHCPACK message via unicast. – It acknowledges the lease acceptance and contains the client’s IP address lease and 53 other IP addressing information. DHCP Lease Renewal Process After an address is leased – A record of the lease is stored in a database, including a lease expiration time – When 50% of the lease time has elapsed, the computer attempts to renew the lease from the same DHCP server that originally responded – If no response, the computer waits until the lease reaches 87.5%, a broadcast DHCP renewal request is sent • If no response when lease expires, the computer broadcasts a DCHP request for a new IP address 54 DHCP Server • Benefit of using DHCP – In a large network, it can keep track of assigned addresses and to which machine they are assigned. – Computers can easily be moved and requested new IP configuration from a DHCP server on the network. – IP lease time can be controlled. – IP addresses can be reusable for other computer. • DHCP uses UDP – DHCP servers are usually located on the same network and DHCP messages are short. 55 Summary of DHCP • • • • DHCP functionality DHCP server elements DHCP lease process DHCP renewal process 56 End of Lecture