services expected in an operating system(e.g. suppo Protocol (UDP) was created to provide a application- for timers. )So XNET was designed to run directly level interface to the basic datagram service of Internet p of the datagram service provided by Internet The architecture did not wish to assume that the Another service which did not fit tCP was real time underlying networks themselves support multiple types delivery of digitized speech, which was needed to of services, because this would violate the goal of using support the teleconferencing aspect of command and existing networks. Instead, the hope was that multiple control applications. In real time digital speech, the types of service could be constructed out of the basic primary requirement is not a reliable service, but a datagram building block using algorithms within the service which minimizes and smoothes the delay in the host and the gateway. For example, (although this is not delivery of packets. The application layer is digitizing done in most current implementations) it is possible to e an ng the resulting bits, and take datagrams which are associated with a controlled sending them out across the network on a regular basis delay but unreliable service and place them at the head They must arrive at the receiver at a regular basis in of the transmission queues unless their lifetime has order to be converted back to the analog signal. If expired, in which case they would be discarded; while packets do not arrive when expected, it is impossible to packets associated with reliable streams would be reassemble the signal in real time. A surprising placed at the back of the queues, but never discarded observation about the control of variation in delay is no matter how long they had been in the net that the most serious source of delay in networks is the mechanism to provide reliable delivery. a typical It proved more difficult than first hoped to provide reliable transport protocol responds to a missing packet multiple types of service without explicit support from by requesting a retransmission and delaying the delivery the underlying networks. The most serious problem was of any subsequent packets until the lost packet has been that networks designed with one particular type of retransmitted. It then delivers that packet and all service in mind were not flexible enough to support maining ones in sequence. The delay while this occurs other services. Most commonly, a network will have can be many times the round trip delivery time of the been designed under the assumption that it should net, and may completely disrupt the speech reassembly deliver reliable service, and will inject delays as a part algorithm. In contrast, it is very easy to cope with an of producing reliable service, whether or not this occasional missing packet. The missing speech can reliability is desired. The interface behavior defined by simply be replaced by a short period of silence, which X 25, for example, implies reliable delivery, and there in most cases does not impair the intelligibility of the is no way to turn this feature off. Therefore, although speech to the listening human. If it does, high level error Internet operates successfully over X 25 networks it correction can occur. and the listener can ask the cannot deliver the desired variability of type service in speaker to repeat the da that context. Other networks which have an intrinsic datagram service are much more flexible in the type of It was thus decided, fairly early in the development of service they will permit, but these networks are much the Internet architecture, that more than one transport less common, especially in the long-haul context. service would be required, and the architecture must be prepared to tolerate simultaneously transports which 6. Varieties of Networks wish to constrain reliability, delay, or bandwidth, at a It was very important for the success of the Internet architecture that it be able to incorporate and utilize a This goal caused TCP and IP, which originally had been wide variety of network technologies, including military single protocol in the architecture, to be separated into and commercial facilities. The Internet architecture has two layers. TCP provided one particular type of service, been very successful in meeting this goal; it is operated the reliable sequenced data stream, while IP attempted over a wide variety of networks, including long haul to provide a basic building block out of which a variety nets( the ARPANET itself and various X 25 networks of types of service could be built. This building block local area nets (Ethernet, ringnet, etc. ) broadcast was the datagram, which had also been adopted to satellite nets (the DARPa Atlantic Satellite support survivability. Since the reliability associated Network,operating at 64 kilobits per second and the with the delivery of a datagram was not guaranteed, but DARPA Experimental Wideband Satellite Net, best effort, it was possible to build out of the operating within the United States at 3 megabits per datagram a service that was reliable(by acknowledging second), packet radio networks(the DARPa packet and retransmitting at a higher level) or a service which radio network, as well as an experimental British packet traded reliability for the primitive delay characteristics radio net and a network developed by amateur radio of the underlying network substrate. The User Datagram operators), a variety of serial links, ranging from 1200 ACM SIGCOMM Computer Communication ReviewACM SIGCOMM -4- Computer Communication Review 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 fit TCP was real time delivery of digitized speech, which was needed to support the teleconferencing aspect of command and control applications. In real time digital speech, the primary requirement is not a reliable service, but a service which minimizes and smoothes 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 datagrams 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 Network14,15 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