connection control information is thus co-located with the
              end points of the communication, so it will be lost only
              if an end point fails.

         (c)  Routing complexity should be in the gateways.

              Routing is a complex and difficult problem, and ought to
              be performed by the gateways, not the hosts.  An important




 



RFC1122                       INTRODUCTION                  October 1989


              objective is to insulate host software from changes caused
              by the inevitable evolution of the Internet routing
              architecture.

         (d)  The System must tolerate wide network variation.

              A basic objective of the Internet design is to tolerate a
              wide range of network characteristics -- e.g., bandwidth,
              delay, packet loss, packet reordering, and maximum packet
              size.  Another objective is robustness against failure of
              individual networks, gateways, and hosts, using whatever
              bandwidth is still available.  Finally, the goal is full
              "open system interconnection": an Internet host must be
              able to interoperate robustly and effectively with any
              other Internet host, across diverse Internet paths.

              Sometimes host implementors have designed for less
              ambitious goals.  For example, the LAN environment is
              typically much more benign than the Internet as a whole;
              LANs have low packet loss and delay and do not reorder
              packets.  Some vendors have fielded host implementations
              that are adequate for a simple LAN environment, but work
              badly for general interoperation.  The vendor justifies
              such a product as being economical within the restricted
              LAN market.  However, isolated LANs seldom stay isolated
              for long; they are soon gatewayed to each other, to
              organization-wide internets, and eventually to the global
              Internet system.  In the end, neither the customer nor the
              vendor is served by incomplete or substandard Internet
              host software.

              The requirements spelled out in this document are designed
              for a full-function Internet host, capable of full
              interoperation over an arbitrary Internet path.


      1.1.3  Internet Protocol Suite

         To communicate using the Internet system, a host must implement
         the layered set of protocols comprising the Internet protocol
         suite.  A host typically must implement at least one protocol
         from each layer.

         The protocol layers used in the Internet architecture are as
         follows [INTRO:4]:


         o  Application Layer




 



RFC1122                       INTRODUCTION                  October 1989


              The application layer is the top layer of the Internet
              protocol suite.  The Internet suite does not further
              subdivide the application layer, although some of the
              Internet application layer protocols do contain some
              internal sub-layering.  The application layer of the
              Internet suite essentially combines the functions of the
              top two layers -- Presentation and Application -- of the
              OSI reference model.

              We distinguish two categories of application layer
              protocols:  user protocols that provide service directly
              to users, and support protocols that provide common system
              functions.  Requirements for user and support protocols
              will be found in the companion RFC [INTRO:1].

              The most common Internet user protocols are:

                o  Telnet (remote login)
                o  FTP    (file transfer)
                o  SMTP   (electronic mail delivery)

              There are a number of other standardized user protocols