<Tr> <Td> Reliable transport service </Td> <Td> No </Td> <Td> Yes </Td> <Td> No </Td> <Td> Yes </Td> <Td> Yes </Td> </Tr> <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 endpoint, GRE becomes closer to a transport protocol that uses IP headers but contains complete frames or packets to deliver to an endpoint . L2TP carries PPP frames inside transport packet . </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>

Ipsec is defined at what layer of the osi model