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THE DESIGN PHILOSOPHY OF THE DARPA INTERNET PROTOCOL~S
David D. Clark
Massachusetts Institute of Technology Laboratory for Computer Science
Cambridge, Ma. 02139
The Internet protocol suite, TCP/IP, was first proposed fifteen years ago. It was developed by the Defense Advanced Research Projects Agency (DARPA), and has been used widely in military and commercial systems. While there have been papers and specifications that describe how the protocols work, it is sometimes difficult to deduce from these why the protocol is as it is. For example, the Internet protocol is based on a connectionless or datagram mode of service. The motivation for this has been greatly misunderstood. This paper attempts to capture some of the early reasoning which shaped the Internet protocols.
I. Int reduction
For the last 15 years I, the Advanced Research Projects Agency of the U.S. Department of Defense has been developing a suite of protocols for packet switched networking. These protocols, which include the Internet Protocol (IP), and the Transmission Control Protocol (TCP), are now U.S. Department of Defense standards for intemetworking, and are in wide use in the commercial networking environment. The ideas developed in this effort have also influenced other protocol suites, most importantly the connectionless configuration of the IS0 protocols’, 3* 4.
This work was support4 in part by the Defense Advanced Research Projects Agency (DARPA) under Contract No. NOOOIJ-83-K-0125.
Permission 10 copy without fee all or part of this material is granted provided that the copies are not made or distributed for direct commercial advantage. the ACM copyright notice and the title of the publication and its date appear. and notice is given that copying is by permission of the Association for Computing Machinery. To copy otherwise. or to republish. requires a fee and/ or specific permission.
o 1988 ACM 0-89791-279-9/88/008/0106 $1.50
While specific information on the DOD protocols is fairly generally available’, 6. ‘, it is sometimes difficult to determine the motivation and reasoning which led to the design.
In fact, the design philosophy has evolved considerably from the first proposal to the current standards. For example, the idea of the datagram, or connectionless service, does not receive particular emphasis in the first paper, but has come to be the defining characteristic of the protocol. Another example is the layering of the architecture into the IP and TCP layers. This seems basic to the design, but was also not a part of the original proposal. These changes in the Internet design arose through the repeated pattern of implementation and testing that occurred before the standards were set.
The Internet architecture is still evolving. Sometimes a new extension challenges one of the design principles, but in any case an understanding of the history of the design provides a necessary context for current design extensions. The connectionless configuration of IS0 protocols has also been colored by the history of the Internet suite, so an understanding ‘of the Internet design philosophy may be helpful to those working with ISO.
This paper catalogs one view of the original objectives of the Internet architecture, and discusses the relation between these goals and the important features of the pnXocols.
2. Fundamental Goal
The top level goal for the DARPA Internet Architecture was to develop an effective technique for multiplexed utilization of existing interconnected networks. Some elaboration is appropriate to make clear the meaning of that goal.
The components of the Internet were networks, which were to be interconnected to provide some larger service. The original goal was to connect together the ori
ARPANET’ with the ARPA packet radio network’. ‘, in order to give users on the packet radio network access to the large service machines on the ARPANET. At the time it was assumed that there would be other sorts of
networks to interconnect, although the local area network had not yet emerged.
An alternative to interconnecting existing networks would have been to design a unified system which incorporated a variety of different transmission media, a multi-media network. While this might have permitted a higher degree of integration, and thus better performance, it was felt that it was necessary to incorporate the then existing network architectures if Internet was to be useful in a practical sense. Further, networks represent administrative boundaries of control, and it was an ambition of this project to come to grips with the problem of integrating a number of separately administrated entities into a common utility.
The technique selected for multiplexing was packet switching. Au alternative such as circuit switching could have been considered, but the applications being supported, such as remote login, were naturally served by the packet switching paradigm, and the networks which were to be integrated together in this project were packet switching networks. So packet switching was accepted as a fundamental component of the Internet architecture.
The fmal aspect of this fundamental goal was the assumption of the particular technique for interconnecting these networks. Since the technique of store and forward packet switching, as demonstrated in the previous DARPA project, the ARPANET, was well understood, the top level assumption was that networks would be interconnected by a layer of Internet packet switches, which were called gateways.
From these assumptions comes the fundamental structure of the Internet: a packet switched communications facility in which a number of distinguishable networks am connected together using packet communications processors called gateways which implement a store arid forward packet forwarding algorithm.
3. Second Level Goals
The top level goal stated in the previous section contains the word “effective,” without offering any definition of what an effective interconnection must achieve. The following list summarizes a more detailed set of goals which were established for the Internet architecture.
1. Internet commuuication must continue despite loss of networks or gateways.
2. The Internet must support multiple types of communications service.
3. The Internet architecture must accommodate a variety of networks.
4. The Internet architecture must permit distributed management of its resources.
5. The Internet architecture must be cost effective.
6. The Internet architecture must permit host attachment with a low level of effort.
7. The resources used in the iutemet architecture must be accountable.
This set of goals might seem to be nothing more than a checklist of all the desirable network features. It is important to understand that these goals are in order of importance, and an entirely different network architecture would result if the order were changed. For example, since this network was designed to operate in a military context, which implied the possibility of a hostile environment, survivability was put as a first goal, and accountability as a last goal. During wartime. one is less concerned with detailed accounting of resources used than with mustering whatever resources are available and rapidly deploying them it-i an operational manner. While the architects of the Internet were mindful of accountability, the problem received very little attention during the early stages of the design. aud is only now being considered. An architecture primarily for commercial deployment would clearly place these goals at the opposite end of the list.
Similarly, the goal that the architecture be cost effective is clearly on the list, but below certain other goals, such as distributed management, or support of a wide variety of networks. Other protocol suites, including some of the more popular commercial architectures, have been optimized to a particular kind of network, for example a long haul store and forward network built of medium speed telephone lines, and deliver a very cost effective solution in this context, in exchange for dealing somewhat poorly with other kinds of nets, such as local area nets.
The reader should consider carefully the above list of goals, and recognize that this is not a “motherhood” list, but a set of priorities which strongly colored the design decisions within the Internet architecture. The following sections discuss the relationship between this list and the features of the Internet.
4. Surirability in the Face of Failure
The most important goal on the list is that the Internet should continue to supply communications service, even though networks and gateways are failing. In particular, this goal was interpreted to mean that if two entities are commuuicating over the Internet. and some failure causes the Internet to be temporarily disrupted and reconfigured to reconstitute the service, then the entities communicating should be able to continue without having to reestablish or reset the high level state of their conversation. More concretely, at the service interface of the transport layer, this architecture provides no facility to communicate to the client of the transport service that
the synchronization between the sender and the receiver may have been lost. It was an assumption in this architecture that synchronization would never be lost unless there was no physical path over which any sort of communication could be achieved. In other words, at the top of transport, there is only one failure, and it is total partition. The architecture was to mask completely any transient failure.
To achieve this goal, the state information which describes the on-going conversation must be protected. Specific examples of state information would be the number of packets transmitted, the number of packets acknowledged, or the number of outstanding flow control permissions. If the lower layers of the architecture lose this information, they will not be able to tell if data has been lost, and the application layer will have to cope with the loss of synchrony. This architecture insisted that this disruption not occur, which meant that the state information must be protected from loss.
In some network architectures, this state is stored in the intermediate packet switching nodes of the network. In this case, to protect the information from loss, it must replicated. Because of the distributed nature of the replication, algorithms to ensure robust replication are themselves difficult to build, and few networks with distributed state information provide any sort of protection against failure. The alternative, which this architecture chose, is to take this information and gather it at the endpoint of the net, at the entity which is utilizing the service of the network. reliability
I call this approach to “fate-sharing.” The fate-sharing model
suggests that it is acceptable to lose the state information associated with an entity if, at the same time, the entity itself is lost. Specifically, information about transport level synchronization is stored in the host which is attached to the net and using its communication service.
There are two important advantages to fate-sharing over replication. First, fate-sharing protects against any number of intermediate failures, whereas replication can only protect against a certain number (less than the number of replicated copies). Second, fate-sharing is much easier to engineer than replication.
There are two consequences to the fate-sharing approach to survivability. First. the intermediate packet switching nodes, or gateways, must not have any essential state information about on-going connections. Instead, they are stateless packet switches, a class of network design sometimes called a “datagram” network. Secondly, rather more trust is placed in the host machine than in an architecture where the network ensures the reliable delivery of data. If the host resident algorithms that ensure the sequencing and acknowledgment of data fail, applications on that machine are prevented from operation.
Despite the the fact that survivability is the first goal in the list, it is still second to the top level goal of interconnection of existing networks. A more survivable
technology might have resulted from a single multi- media network design. For example, the Internet makes very weak assumptions about the ability of a network to report that it has failed. Internet is thus forced to detect network failures using Internet level mechanisms, with the potential for a slower and less specific error detection.
5. Types of Service
The second goal of the Internet architecture is that it should support, at the transport service level, a variety of types of service. Different types of service are distinguished by differing requirements for such things as speed, latency and reliability. The traditional type of service is the bidirectional reliable delivery of data. This service, which is sometimes called a “virtual circuit” service, is appropriate for such applications as remote login or tile transfer. It was the first service provided in the Internet architecture, using the Transmission Control Protocol (TCP)“. It was early recognized that even this service had multiple variants, because remote login required a service with low delay in delivery, but low requirements for bandwidth, while file transfer was less concerned with delay, but very concerned with high throughput. TCP attempted to provide both these types of service.
The initial concept of TCP was that it could be general enough to support any needed type of service. However, as the full range of needed services became clear, it seemed too difficult to build support for all of them into one protocol.
The first example of a service outside the range of TCP was support for XNET , I2 the cross-Internet debugger. TCP did not seem a suitable transport for XNET for several reasons. First, a debugger protocol should not be reliable. This conclusion may seem odd, but under conditions of stress or failure (which may be exactly when a debugger is needed) asking for reliable communications may prevent any communications at all. It is much better to build a service which can deal with whatever gets through, rather than insisting that every byte sent be delivered in order. Second, if TCP is general enough to deal with a broad range of clients, it is presumably somewhat complex. Again, it seemed wrong to expect support for this complexity in a debugging environment, which may lack even basic services expected in an operating system (e.g. support for timers.) So XNET was designed to run directly on top of the datagram service provided by Internet.
Another service which did not fu TCP was real time delivery of digitized speech, which was needed to support the teleconferencing aspect of command and control applications. III real time digital speech, the primary requirement is not a reliable service, but a service which minimizes and smooths the delay in the delivery of packets. The application layer is digitizing the analog speech, packetizing the resulting bits, and sending them out across the network on a regular basis. They must
arrive at the receiver at a regular basis in order to be converted back to the analog signal. If packets do not arrive when expected, it is impossible to reassemble the signal in real time. A surprising observation about the control of variation in delay is that the most serious source of delay in networks is the mechanism to provide reliable delivery. A typical reliable transport protocol responds to a missing packet by requesting a retransmission and delaying the delivery of any subsequent packets until the lost packet has been retransmitted. It then delivers that packet and all remaining ones in sequence. The delay while this occurs can be many times the round trip delivery time of the net, and may completely disrupt the speech reassembly algorithm. In contrast, it is very easy to cope with an occasional missing packet. The missing speech can simply be replaced by a short period of silence, which in most cases does not impair the intelligibility of the speech to the listening human. If it does, high level error correction can occur, and the listener can ask the speaker to repeat the damaged phrase.
It was thus decided, fairly early in the development of the Internet architecture, that more than one transport service would be required, and the architecture must be prepared to tolerate simultaneously transports which wish to constrain reliability, delay, or bandwidth. at a minimum.
This goal caused TCP and IP, which originally had been a single protocol in the architecture, to be separated into two layers. TCP provided one particular type of service, the reliable sequenced data stream, while IP attempted to provide a basic building block out of which a variety of types of service could be built. This building block was the datagram, which had also been adopted to support survivability. Since the reliability associated with the delivery of a datagram was not guaranteed, but “best effort,” it was possible to build out of the datagram a service that was reliable (by acknowledging and retransmitting at a higher level), or a service which traded reliability for the primitive delay characteristics of the underlying network substrate. The User Datagram Protocol (UDP)13 was created to provide a application- level interface to the basic datagram service of Internet.
The architecture did not wish to assume that the underlying networks themselves support multiple types of services, because this would violate the goal of using existing networks. Instead, the hope was that multiple types of service could be constructed out of the basic datagram building block using algorithms within the host and the gateway. For example, (although this is not done in most current implementations) it is possible to take datapams which are associated with a controlled delay but unreliable service and place them at the head of the transmission queues unless their lifetime has expired, in which case they would be discarded; while packets associated with reliable streams would be placed at the back of the queues, but never discarded, no matter how long they had been in the net.
It proved more difficult than first hoped to provide multiple types of service without explicit support from the underlying networks. The most serious problem was that networks designed with one particular type of service in mind were not flexible enough to support other services. Most commonly, a network will have been designed under the assumption that it should deliver reliable service, and will inject delays as a part of producing reliable service, whether or not this reliability is desired. The interface behavior defined by X.25, for example, implies reliable delivery, and there is no way to turn this feature off. Therefore, although Internet operates successfully over X.25 networks it cannot deliver the desired variability of type service in that context. Other networks which have an intrinsic datagram service are much more flexible in the type of service they will permit. but these networks are much less common, especially in the long-haul context.
6. Varieties of Networks
It was very important for the success of the Internet architecture that it be able to incorporate and utilize a wide variety of network technologies, including military and commercial facilities. The Internet architecture has been very successful in meeting this goal: it is operated over a wide variety of networks, including long haul nets (the ARPANET itself and various X.25 networks), local area nets (Ethernet, ringnet, etc.), broadcast satellite nets (the DARPA Atlantic Satellite Network’“, I5 operating at 64 kilobits per second and the DARPA Experimental Wideband Satellite Net,16 operating within the United States at 3 megabits per second), packet radio networks (the DARPA packet radio network, as well as an experimental British packet radio net and a network developed by amateur radio operators), a variety of serial links, ranging from 1200 bit per second asynchronous connections to TI links, and a variety of other ad hoc facilities, including intercomputer busses and the transport service provided by the higher layers of other network suites, such as IBM’s HASP.
The Internet architecture achieves this flexibility by making a minimum set of assumptions about the function which the net will provide. The basic assumption is that network can transport a packet or datagram. The packet must be of reasonable size, perhaps 100 bytes minimum, and should be delivered with reasonable but not perfect reliability. The network must have some suitable form of addressing if it is more than a point to point link.
There are a number of services which are explicitly not assumed from the network. These include reliable or sequenced delivery, network level broadcast or multicast, priority ranking of transmitted packet, multiple types of service, and ’ mtemal knyJ&te ‘z; failures, speeds, or delays. If these services had been required, then in order to accommodate a network within the Internet, it would be necessary either that the network support these services directly, or that the network interface software provide enhancements to simulate
these services at the endpoint of the network. It was felt that this was an undesirable approach, because these services would have to be re-engineered and reimplemented for every single network and every single host interface to every network. By engineering these services at the transport, for example reliable delivery via TCP, the engineering must be done only once, and the implementation must be done only once for each host. After that, the implementation of interface software for a new network is usually very simple.
7. Other Goals
The three goals discussed so far were those which had the most profound impact on the design on the architecture. The remaining goals, because they were lower in importance, were perhaps less effectively met, or not so completely engineered. The goal of permitting distributed management of the Internet has certainly been met in certain respects. For example, not all of the gateways in the Internet are implemented and managed by the same agency. There are several different management centers within the deployed Internet, each operating a subset of the gateways, and there is a two- tiered routing algorithm which permits gateways from dilferent administrations to exchange routing tables, even though they do not completely trust each other, and a variety of private routing algorithms used among the gateways in a single administration. Similarly, the various organizations which manage the gateways are not necessarily the same organizations that manage the networks to which the gateways are attached.
On the other hand, some of the most significant problems with the Internet today relate to lack of sufficient tools for distributed management, especially in the area of routing. In the large intemet being currently operated, routing decisions need to be constrained by policies for resource usage. Today this can be done only in a very limited way, which requires manual setting of tables. This is error-prone and at the same time not sufficiently powerful. The most important change in the Internet architecture over the next few years will probably be the development of a new generation of tools for management of resources in the context of multiple administrations.
It is clear that in certain circumstances, the Internet architecture does not produce as cost effective a utilization of expensive communication resources as a more tailored architecture would. The headers of Internet packets am fairly long (a typical header is 40 bytes), and if short packets are sent, this overhead is apparent. The worse case, of course, is the single character remote login packets, which carry 40 bytes of header and one byte of data. Actually, it is very difficult for any protocol suite to claim that these sorts of interchanges are carried out with reasonable efficiency. At the other extreme, large packets for file transfer, with perhaps 1,000 bytes of data, have an overhead for the header of only four percent.
Another possible source of inefficiency is retransmission of lost packets. Since Internet does not insist that lost packets be recovered at the network level, it may be necessary to retransmit a lost packet from one end of the Internet to the other. This means that the retransmitted packet may cross several intervening nets a second time, whereas recovery at the network level would not generate this repeat traffic. This is an example of the tradeoff resulting from the decision, discussed above, of providing services from the end-points. The network interface code is much simpler, but the overall efficiency is potentially less. However, if the retransmission rate is low enough (for example, 1%) then the incremental cost is tolerable. As a rough rule of thumb for networks incorporated into the architecture, a loss of one packet in a hundred is quite reasonable, but a loss of one packet in ten suggests that reliability enhancements be added to the network if that type of service is required.
The cost of attaching a host to the Internet is perhaps somewhat higher than in other architectures, because all of the mechanisms to provide the desired types of service, such as acknowledgments and retransmission strategies, must be implemented in the host rather than in the network. Initially, to programmers who were not familiar with protocol implementation, the effort of doing this seemed somewhat daunting. Implementors tried such things as moving the transport protocols to a front end processor, with the idea that the protocols would be implemented only once, rather than again for every type of host. However, this required the invention of a host to front end protocol which some thought almost as complicated to implement as the original transport protocol. As experience with protocols increases, the anxieties associated with implementing a protocol suite within the host seem to be decreasing, and implementations are now available for a wide variety of machines, including personal computers and other machines with very limited computing resources.
A related problem arising from the use of host-resident mechanisms is that poor implementation of the mechanism may hurt the network as well as the host. This problem was tolerated, because the initial experiments involved a limited number of host implementations which could be controlled. However, as the use of Internet has grown, this problem has occasionally surfaced in a serious way. In this respect, the goal of robustness, which led to the method of fate-sharing, which led to host- resident algorithms, contributes to a loss of robusmess if the host misbehaves.
The last goal was accountability. In fact, accounting was discussed in the first paper by Cerf and Kahn as an important function of the protocols and gateways. However, at the present time, the Internet architecture contains few tools for accounting for packet flows. This problem is only now being studied, as the scope of the architecture is being expanded to include non-military consumers who are seriously concerned with understanding and monitoring the usage of the resources within the intemet.
8. Architecture and Implementation
The previous discussion clearly suggests that one of the goals of the Internet architecture was to provide wide flexibility in the service offered. Different transport protocols could be used to provide different types of service, and different networks could be incorporated. Put another way, the architecture tried very hard not to constrain the range of service which the Internet could be engineered to provide. This, in turn, means that to understand the service which can be offered by a particular implementation of an Internet, one must look not to the architecture, but to the actual engineering of the software within the particular hosts and gateways, and to the particular networks which have been incorporated. I will use the term “realization” to describe a particular set of networks, gateways and hosts which have been connected together in the context of the Internet architecture. Realizations can differ by orders of magnitude in the service which they offer. Realizations have been built out of 1900 bit per second phone lines, and out of networks only with speeds greater than 1 megabit per second. Clearly, the throughput expectations which one can have of these realizations differ by orders of magnitude. Similarly, some Internet realizations have delays measured in tens of milliseconds, where others have delays measured in seconds. Certain applications such as real time speech work fundamentally differently across these two realizations. Some Intemets have been engineered so that there is great redundancy in the gateways and paths. These Internets are survivable, because resources exist which can be reconfigured after failure. Other Internet realizations, to reduce cost, have single points of connectivity through the realization, so that a failure may partition the Internet into two halves.
The Internet architecture tolerates this variety of realization by design. However, it leaves the designer of a particular realization with a great deal of engineering to do. One of the major struggles of this architectural development was to understand how to give guidance to the designer of a realization, guidance which would relate the engineering of the realization to the types of service which would result. For example, the designer must answer the following sort of question. What sort of bandwidths must he in the underlying networks, if the overall service is to deliver a throughput of a certain rate? Given a certain model of possible failures within this realization, what sorts of redundancy ought to be engineered into the realization?
Most of the known network design aids did not seem helpful in answering these sorts of questions. Protocol verifiers, for example, assist in confirming that protocols meet specifications. However, these tools almost never deal with performance issues, which are essential to the idea of the type of service. Instead, they deal with the much more restricted idea of logical correctness of the protocol with respect to specification. While tools to verify logical correctness are useful, both at the specification and implementation stage. they do not help with the severe problems that often arise related to
performance. A typical implementation experience is that even after logical correctness has been demonstrated, design faults are discovered that may cause a performance degradation of an order of magnitude. Exploration of this problem has led to the conclusion that the difficulty usually arises, not in the protocol itself, but in the operating system on which the protocol runs. This being the case, it is difficult to address the problem within
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