Links TCP http://www.webopedia.com/TERM/T/TCP.html
Content 11.1 TCP/IP Transport Layer 11.1.8 User Datagram Protocol (UDP) User Datagram Protocol (UDP) is the connectionless transport protocol in the TCP/IP protocol stack. UDP is a simple protocol that exchanges datagrams, without acknowledgments or guaranteed delivery. Error processing and retransmission must be handled by higher layer protocols. UDP uses no windowing or acknowledgments so reliability, if needed, is provided by application layer protocols. UDP is designed for applications that do not need to put sequences of segments together. The protocols that use UDP include: The following are the definitions of the fields in the UDP segment: Web Links User Datagram Protocol http://www.webopedia.com/TERM/U/ User_Datagram_ Protocol.html
Content 11.1 TCP/IP Transport Layer 11.1.9 TCP and UDP port numbers Both TCP and UDP use port (socket) numbers to pass information to the upper layers. Port numbers are used to keep track of different conversations crossing the network at the same time. Application software developers agree to use well-known port numbers that are issued by the Internet Assigned Numbers Authority (IANA). Any conversation bound for the FTP application uses the standard port numbers 20 and 21. Port 20 is used for the data portion and port 21 is used for control. Conversations that do not involve an application with a well-known port number are assigned port numbers randomly from within a specific range above 1023. Some ports are reserved in both TCP and UDP, but applications might not be written to support them. Port numbers have the following assigned ranges: End systems use port numbers to select the proper application. The source host dynamically assigns originating source port numbers. These numbers are always greater than 1023. Web Links Well-Known TCP Port Numbers http://www.webopedia.com/quick_ref/ portnumbers.asp
Content 11.2 The Application Layer 11.2.1 Introduction to the TCP/IP application layer When the TCP/IP model was designed, the session and presentation layers from the OSI model were bundled into the application layer of the TCP model. This means that issues of representation, encoding, and dialog control are handled in the application layer rather than in separate lower layers as in the OSI model. This design assures that the TCP/IP model provides maximum flexibility at the application layer for developers of software. The TCP/IP protocols that support file transfer, e-mail, and remote login are probably the most familiar to users of the Internet. These protocols include the following applications: Web Links Application Layer http://searchnetworking.techtarget.com/ sDefinition/0,,sid7_ gci211579,00.html
Content 11.2 The Application Layer 11.2.2 DNS The Internet is built on a hierarchical addressing scheme. This scheme allows for routing to be based on classes of addresses rather than based on individual addresses. The problem this creates for the user is associating the correct address with the Internet site. It is very easy to forget an IP address to a particular site because there is nothing to associate the contents of the site with the address. Imagine the difficulty of remembering the IP addresses of tens, hundreds, or even thousands of Internet sites. A domain naming system was developed in order to associate the contents of the site with the address of that site. The Domain Name System (DNS) is a system used on the Internet for translating names of domains and their publicly advertised network nodes into IP addresses. A domain is a group of computers that are associated by their geographical location or their business type. A domain name is a string of characters, number, or both. Usually a name or abbreviation that represents the numeric address of an Internet site will make up the domain name. There are more than 200 top-level domains on the Internet, examples of which include the following: .us – United States
.uk – United Kingdom There are also generic names, which examples include the following: .edu – educational sites
.com – commercial sites
.gov – government sites
.org – non-profit sites
.net – network service See Figure for a detailed explanation of these domains. Web Links Domain Name System http://searchwebservices.techtarget.com/ sDefinition/ 0,,sid26_gci213908,00.html
Content 11.2 The Application Layer 11.2.3 FTP and TFTP FTP is a reliable, connection-oriented service that uses TCP to transfer files between systems that support FTP. The main purpose of FTP is to transfer files from one computer to another by copying and moving files from servers to clients, and from clients to servers. When files are copied from a server, FTP first establishes a control connection between the client and the server. Then a second connection is established, which is a link between the computers through which the data is transferred. Data transfer can occur in ASCII mode or in binary mode. These modes determine the encoding used for data file, which in the OSI model is a presentation layer task. After the file transfer has ended, the data connection terminates automatically. When the entire session of copying and moving files is complete, the command link is closed when the user logs off and ends the session. TFTP is a connectionless service that uses User Datagram Protocol (UDP). TFTP is used on the router to transfer configuration files and Cisco IOS images and to transfer files between systems that support TFTP. TFTP is designed to be small and easy to implement. Therefore, it lacks most of the features of FTP. TFTP can read, write, or mail files to or from a remote server but it cannot list directories and currently has no provisions for user authentication. It is useful in some LANs because it operates faster than FTP and in a stable environment it works reliably. Web Links File Transfer Protocol http://whatis.techtarget.com/definition/ 0,,sid9_ gci213976,00.htm
Content 11.2 The Application Layer 11.2.4 HTTP Hypertext Transfer Protocol (HTTP) works with the World Wide Web, which is the fastest growing and most used part of the Internet. One of the main reasons for the