 function of the 'presentation layer', or a Skype call may be initiated between two physical end devices, an example of a 'session layer' function. Each application is specified through a well-defined protocol. It may be possible for an application to communicate with another based on the protocol. Some applications layer protocols are developed through the Internet Engineering Task Force (IETF) and/or the World Wide Web Consortium (W3C) and their structure is in the public domain. Examples are Simple Mail Transfer Protocol (SMTP), Hypertext Transfer Protocol (HTTP) and File Transfer Protocol (FTP). There also exist many proprietary protocols whose internal structure is not disclosed. Examples of these applications include Skype, Twitter and YouTube, all of which communicate using the IP suites. Transport Layer Protocols Transport layer protocols provide host-to-host connectivity to deliver information generated by an application in one device to an application in another device. The function carried out in this layer ensures data are passed on to the host's application reliably and in the correct order. The prominent internet transport layer protocol is the TCP, which is often used as part of the TCP/IP suite. The TCP protocol enables communications between applications within two Broadband Telecommunications Technologies and Management 16-31 Source port Destination port 32-63 Sequence number 64-95 Reserved Various Window size offset 96-128 Checksum Urgent pointer 127-159 Options Data (up to 1500 bytes) Figure 8.8 TCP header and data fields 16-31 Source port Destination port 32-63 Length Checksum 64-65635 (max) Figure 8.9 UDP header and data fields remote devices, structures the data output from the application, and delivers it to the IP layer for transportation. TCP also facilitates flow control including adjustment of transmission speed in response to the physical link transmission capability. It further provides a mechanism to check whether the received data are correctly received and if SO send an acknowledg