h arriving packet to the proper application. These identifiers are referred to as TCP port numbers, and each IP-based application uses one. For instance, datagrams with the destination TCP port number 80 are delivered to the HTTP application, whereas datagrams sent to the TCP port number 23 are handled by the Telnet [RFC 854] application (Figure 3-3). User Datagram Protocol (UDP) UDP [RFC 768] provides unreliable datagram delivery; that is, it does not ensure that a given datagram will arrive at its destination by any means. UDP merely performs IP traffic demultiplexing based on UDP port numbers. Both UDP and TCP are 16-bit port numbers. After traffic demultiplexing, UDP provides a checksum that allows end systems to check that the datagrams received were not corrupted by the network. For instance, if Bob sends the same message as before using UDP, he cannot be sure Laura will receive it. To find out, he'll have to check with Laura herself. If a datagram containing a UDP packet gets lost, Bob will also have to retransmit it himself; UDP does not provide such functionality. Stream Control Transmssion Protocol (SCTP) The Stream Control Transmission Protocol (SCTP) [RFC 2960] is a newly developed transport protocol. It is foreseen that SCTP will be widely implemented, but currently, it is less available than TCP and UDP. The Internet Multimedia Conferencing Architecture Figure 3-3 HTTP application Telnet application VolP application Application layer Delivery of IP port 80 port 23 port 20000 packets to the proper application. Transport layer IP interface Network layer IP packets Real-Time Services in the Internet User services are implemented using application layer protocols. Two of the most familiar Internet services are the Web and e-mail. The Web uses the application layer protocol HTTP and e-mail is implemented using, among other protocols, Simple Mail Transport Protocol (SMTP) [RFC 821] and Internet Message Access Protocol (IMAP) [RFC 2060]. These services, and others like them, have boosted the Internet devel