<Tr> <Td_colspan="6"> If an excessive number of PDUs are unacknowledged . </Td> </Tr> <P> An easy way to visualize the transport layer is to compare it with a post office, which deals with the dispatch and classification of mail and parcels sent . A post office inspects only the outer envelope of mail to determine its delivery . Higher layers may have the equivalent of double envelopes, such as cryptographic presentation services that can be read by the addressee only . Roughly speaking, tunneling protocols operate at the transport layer, such as carrying non-IP protocols such as IBM's SNA or Novell's IPX over an IP network, or end - to - end encryption with IPsec . While Generic Routing Encapsulation (GRE) might seem to be a network - layer protocol, if the encapsulation of the payload takes place only at the endpoint, GRE becomes closer to a transport protocol that uses IP headers but contains complete Layer 2 frames or Layer 3 packets to deliver to the endpoint . L2TP carries PPP frames inside transport segments . </P> <P> Although not developed under the OSI Reference Model and not strictly conforming to the OSI definition of the transport layer, the Transmission Control Protocol (TCP) and the User Datagram Protocol (UDP) of the Internet Protocol Suite are commonly categorized as layer - 4 protocols within OSI . </P> <P> The session layer controls the dialogues (connections) between computers . It establishes, manages and terminates the connections between the local and remote application . It provides for full - duplex, half - duplex, or simplex operation, and establishes checkpointing, adjournment, termination, and restart procedures . The OSI model made this layer responsible for graceful close of sessions, which is a property of the Transmission Control Protocol, and also for session checkpointing and recovery, which is not usually used in the Internet Protocol Suite . The session layer is commonly implemented explicitly in application environments that use remote procedure calls . </P>

The tcp and udp protocols both exist at what layer of the osi model