Application protocols are transmitted in packets of TCP or UDP protocol. They are used for transmission of user (application) data. In addition to standard application protocols which are available (i.e. SMTP, POP3, HTTP, FTP, etc.), application programmers may use a custom (non-standard) method for communication.
Information in text format that the server stores at a client (Web browser). It is used for later identification of a user when the same server/site is opened again. Cookies can be misused for monitoring which sites have been visited by a user, or they can be used for visit counter.
A tool (usually a software product) for protection from intrusions and from data outflow. Two basic firewall types are available:
network firewall — protects computers of a network. Usually, it is used as a gateway (router) through which the particular network is connected to the Internet.
personal firewall — protects one computer (user's workstation). Unlike network firewalls, it can match network communication with a particular application, change its behavior accordingly to interaction with users, etc.
Note: In this guide the word firewall represents Kerio Personal Firewall.
ICMP (Internet Control Message Protocol) is a protocol used for transmission of control messages. Several types of such messages are available, such as a report that the destination is not available, redirection request or response request (used in the PING command).
IP (Internet Protocol) is a protocol transmitting all Internet protocols in its data part. The header of this protocol provides essential routing information, such as source and destination IP address (which computer sent the message and to which computer the message should be delivered).
The most essential information in TCP and UDP packet is the source and
destination port. The IP address identifies a computer in the Internet,
whereas a port identifies an application running on the computer. Ports
1-1023 are reserved for standard services and the operating
system, whereas ports 1024-65535 can be used by any
application. In a typical client to server connection, usually the
destination port is known (connection is established for this port or UDP
datagram is sent to it). The source port is then assigned by the operating
system automatically.
TCP (Transmission Control Protocol) is used for reliable data transmission through so called virtual channel (connection). It is used as a transmission protocol for most application protocols, such as SMTP, POP3, HTTP, FTP, Telnet, etc.
TCP/IP is a general term for protocols used in communication over the Internet. Data is divided into data items called packets within individual protocols. Each packet consists of a header and a data part. The header includes routing information (i.e. source and destination address) and the data part contains transmitted data.
The Internet protocol stack is divided into several levels. Packets of lower protocols encapsulate parts of higher-level protocols in their data parts (i.e. packets of TCP protocol are transmitted in IP packets).
UDP (User Datagram Protocol) is a so called connectionless protocol. This implies that it does not create any connection and data is transmitted in individual messages (so called datagrams). UDP does not warrant reliable data delivery (datagrams can be lost during transmission). However, unlike transmission through TCP protocol, it provides faster data transmission (it is not necessary to establish connections or provide reliability control, confirmation is not demanded, etc.). UDP protocol is used especially for transmission of DNS queries, audio files, video files, or other types of streaming media which promote speed over reliability.