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638 INTRODUCTION TO PACKET NETWORKS AND VoIPLayer 6 (presentation layer). This layer is concerned with the way data is presentedto the application layer, such as encoding schemes (e.g., ASCII or binary),compression, and encryption.Layer 7 (application layer). This layer is concerned with overall management andcontrol of the communication process that transfers data between hosts. This layerinterfaces directly with the user, be that a human or a software program. Examplesof Layer 7 protocols are HTTP (Hypertext Transfer Protocol), e-mail programs, andthe signaling protocols for VoIP (discussed in Section 20.3 and in the next chapter).Layers 1 through 4 are concerned with the transport of data between hosts.Layers 5 through 7 are concerned with functions internal to hosts and play theirrole before a packet is sent and/or after it has been delivered.It needs to be pointed out that the OSI model is used mainly as a reference frameworkand that implementations often skip layers, merge layers, or split layers intosublayers. Layers 5 and 6, in particular, are often skipped and/or their functionsare merged into Layer 7.Layers and sublayers generally correspond to separate communication protocols,which, when describing a communication process, are depicted on top of each otherto form a protocol stack.Types of Packet Communication. The transport layer (L4) supports two typesof communication:. Connectionless communication. Connection-oriented communicationWith connectionless communication each packet sent by a host is treated as an independententity, with no feedback from the destination and no automatic retransmissionin case of packet loss.With connection-oriented communication the source and destination hosts establisha mutual agreement to communicate before data-carrying packets can beexchanged, thus creating a logical connection. The logical connection is maintainedfor the duration of the packet flow and released at the end of it. Establishment andrelease of the (logical) connection is done by exchanging packets that carry controlinformation. It should be noted that, even with connection-oriented communication,no resources or paths are booked or reserved between source and destination; sourceand destination are the only entities aware of the connection. Intermediate nodes,which operate only up to Layer 3, are not aware of connections, which are Layer 4functions. A similar distinction between connectionless and connection-oriented isfound in common-channel signaling with the SCCP protocol (Chapter 15).Reliability of Packet Communication. Some applications require reliabledelivery of data, while others do not. Connectionless transport is inherently unreliable,since there is no way for the transport layer to ensure that packets are delivered