[INTRO:4] and many private user protocols.

              Support protocols, used for host name mapping, booting,
              and management, include SNMP, BOOTP, RARP, and the Domain
              Name System (DNS) protocols.


         o  Transport Layer

              The transport layer provides end-to-end communication
              services for applications.  There are two primary
              transport layer protocols at present:

                o Transmission Control Protocol (TCP)
                o User Datagram Protocol (UDP)

              TCP is a reliable connection-oriented transport service
              that provides end-to-end reliability, resequencing, and
              flow control.  UDP is a connectionless ("datagram")
              transport service.

              Other transport protocols have been developed by the
              research community, and the set of official Internet
              transport protocols may be expanded in the future.

              Transport layer protocols are discussed in Chapter 4.




 



RFC1122                       INTRODUCTION                  October 1989


         o  Internet Layer

              All Internet transport protocols use the Internet Protocol
              (IP) to carry data from source host to destination host.
              IP is a connectionless or datagram internetwork service,
              providing no end-to-end delivery guarantees. Thus, IP
              datagrams may arrive at the destination host damaged,
              duplicated, out of order, or not at all.  The layers above
              IP are responsible for reliable delivery service when it
              is required.  The IP protocol includes provision for
              addressing, type-of-service specification, fragmentation
              and reassembly, and security information.

              The datagram or connectionless nature of the IP protocol
              is a fundamental and characteristic feature of the
              Internet architecture.  Internet IP was the model for the
              OSI Connectionless Network Protocol [INTRO:12].

              ICMP is a control protocol that is considered to be an
              integral part of IP, although it is architecturally
              layered upon IP, i.e., it uses IP to carry its data end-
              to-end just as a transport protocol like TCP or UDP does.
              ICMP provides error reporting, congestion reporting, and
              first-hop gateway redirection.

              IGMP is an Internet layer protocol used for establishing
              dynamic host groups for IP multicasting.

              The Internet layer protocols IP, ICMP, and IGMP are
              discussed in Chapter 3.


         o  Link Layer

              To communicate on its directly-connected network, a host
              must implement the communication protocol used to
              interface to that network.  We call this a link layer or
              media-access layer protocol.

              There is a wide variety of link layer protocols,
              corresponding to the many different types of networks.
              See Chapter 2.


      1.1.4  Embedded Gateway Code

         Some Internet host software includes embedded gateway
         functionality, so that these hosts can forward packets as a




 



RFC1122                       INTRODUCTION                  October 1989


         gateway would, while still performing the application layer
         functions of a host.

         Such dual-purpose systems must follow the Gateway Requirements