168 CHAPTER 10 Network Troubleshooting TROUBLESHOOTING THE PRESENTATION LAYER When troubleshooting the presentation layer, the functions will often actually be performed by a protocol that you would normally think of as functioning at the application layer, so that troubleshooting will often take place with the same steps described in the application layer section. When troubleshooting gateways, an effective approach is to restart or reinstall the gateway service. Some examples of problems you may find at the presentation layer include the following: An image file becomes garbled or corrupted when it's sent through e-mail from one person to another. E-mail messages between two different server types (for example, exchange and groupwise) become scrambled or unreadable. You are unable to copy or move files between two different network types, usually Microsoft and Novell, or you are unable to open a file once it's been copied. TROUBLESHOOTING THE APPLICATION LAYER The application layer doesn't provide services to any other OSI layer. Instead, it provides network services to user applications. Troubleshooting the application layer. Utilize Telnet to connect to an application layer services by specifying the port that's used by the service. Verify name resolution services, such as DNS and WINS are functioning properly. Some of the common protocols that operate at the application layer are as follows: FTP FTP is used for copying files from one computer to another. SNMP SNMP provides a way to gather statistical and troubleshooting infor- mation about devices such as PCs, routers, switches, and hubs. An SNMP management system sends requests to an SNMP agent, and the informa- tion is stored in an MIB, which is a database that holds information about a networked computer. Telnet Telnet is a TCP/IP-based service that allows users to log onto a com- puter from a remote location, run character-mode or command-line utilities on the remote computer or device, and view files on a remote device. The Telnet server service uses TCP port 23 to listen for Telnet requests. Simple Mail Transfer Protocol (SMTP) SMTP is used for sending e-mail messages, typically across the Internet. Most e-mail client programs use SMTP for sending e-mail only, and either POP3 (Post Office Protocol version 3) or Internet Message Access Protocol (IMAP) for storing any messages that are received by an e-mail server. The SMTP service uses TCP port 25 to send messages using SMTP.