IEEE TR.INSACTIOSS OX COMMUNIC~LTIOKS, VOL. COM-22, NO. 5, MAY 1974 A Protocol For Packet Network Intercommunication VINTON G. CERF AND ROBERT E. ICAHN, MEMBER, IEEE 637 Absfract-A protocol that supports the sharing of resources that exist in different packet switching networks is presented. The proto- col provides for variation in individual network packet sizes, trans- mission failures, sequencing, flow control, end-to-end error checking, and the creation and destruction of logical process-to-process con- nections. Some implementation issues are considered, and problems such as internetwork routing, accounting, and timeouts are exposed. INTRODUCTION I" 1 THE LAST few years considerable effort has been expended on the design and implementation of packet switching net\vorl<s [l]-[7],[14],[17]. A principle reason for developing such not\vorks has been to facilitate the sharing of computer resources. A packet communication network includes a transportation mechanism for dcliver- ing data between computers or between computers and terminals. To make the data meaningful, computers and tcrminals share a common protocol (i.c., a set of agreed upon conventions). Several protocols have already been developed for this purpose [S]-[12],[16]. However, these protocols have addressed only the problem of com- munication on the same nct\vork. In this paper we prcscnt a protocol design and philosophy that supports the sharing of resources that exist in different packct switching net- works. After a brief introduction to internetwork protocol issues, we describe the function of a GATEWAY as an intcr- face bctwccn nctn-orks and discuss its role in the protocol. We then consider thc various det,ails of the protocol, including addressing, formatting, buffering, scquoncing, floxv control, error control, and so forth. Wc close with a description of an interprocess communication nxchanism and show how it can be supported by the internet\\-ork protocol. Even though many different and complex problems must be solved in the design of an individual packet switching network, these problems are manifestly com- pounded when dissimilar networks arc interconnected. Issues arise which may have no direct counterpart in an individual network and which strongly influence the way in which internetwork communication can take place. A typical packet switching network is composed of a tions of the IEEE Communications Society for publication without Paper approved by the Associate Isditor for Data Communica- oral presentation. Manuscript received Novemtxr 5, 1973. The research reported in this paper was supported in part hy the Ad- vanced Research Projects Agency of the Department of Ihfense under Contract DAHC 15-73-C-0370. trim1 Engineering, Stanford University, Stanford, Calif. V. G. Cerf is with the Department of Computer Science and Elec- It. E. Kahn is with the Information Processing Technology Office, Advanced Research Projects Agency, Department of De- fense, Arlington, Va. set of computer resources called HOSTS, a set of one or more packet switches, and a collcction of communication media that interconnect the packct switches. Within each HOST, wc assume that there exist processes which must communicate with processes in their own or other HOSTS. Any current definition of a process will be adequate for our purposes [13]. These processes are generally the ultimate source and destination of data in the network. Typically, within an individual network, there exists a protocol for communication between any source and destination process. Only the source and destination processes require kno\\-ledge of this convention for com- munication to talx place. Processes in two distinct nct- works would ordinarily use different protocols for this purpose. The ensemble of packet switches and com- munication media is called the paclxt 'switching subnet. Fig. 1 illustrates these idcas. In a typical packet switching subnet, data of a fixed maximum size arc accepted from a source HOST, togethcr with a formatted destination address which is used to route the data in a store and forward fashion. The transmit time for this data is usually dependent upon internal net\\-ork paramctcrs such as communication media dat>a ratcs, buffering and signaling strategies, routing, propa- gation delays, etc. In addition, somc mechanism is gen- erally prcscnt for error handling and determination of status of the networks components. Individual pacltct switching nctn;orl<s may differ in their implementations as follows. 1) Each net\vorlt may have distinct ways of addressing the rcccivcr, thus requiring that a uniform addressing schemc be created Tvhich can be undcrstood by each individual nctworlt. 2) Each nct\vorl< may accept data of different maximum size, thus requiring nct\vorl<s to deal in units of the smallest maximum size (which may he impractically small) or requiring procedures which allow data crossing a network boundary to bc rcformatted into smaller picccs. 3) The success or failure of a transmission and its pcr- formancc in each network is governed by different time dclays in accepting, delivering, and transporting the data. This requires careful development of intersetwork timing procedures to insurc that data can be successfully dc- livcred through tho various nctworlts. 4) Within each nct\vorl;, communication may be dis- ruptcd due to unrccoverahlc mStation of the data or missing data. End-to-cnd restoration proccduros are desirable to allow complete recovery from these con- ditions.636 IEEE TRAKSACTIONS ON COMMUNICATIONS. MAY 1074 /n\ PACKET-SWITCHING SUBNETWORK (-) PS I PS intact the internal operation of each individual network This is easily achieved if two networks interconnect a: if each were a HOST to the other network, but withoul utilizing or indeed incorporating any elaborate HOS~ protocol transformations. It is thus apparent that the interface between network; must play a central role in the development of any net work interconnection strategy. We give a special name tc this interface