INTERNET DRAFT J.M.Pullen Expiration: 25 September 1997 George Mason U. M.Myjak U.of Central Florida C.Bouwens SAIC, Inc. 24 March 1997 Limitations of Internet Protocol Suite for Distributed Simulation in the Large Multicast Environment draft-ietf-lsma-limitations-01.txt Status of this Memo This document is an Internet-Draft. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet- Drafts as reference material or to cite them other than as ``work in progress.'' To learn the current status of any Internet-Draft, please check the ``1id-abstracts.txt'' listing contained in the Internet- Drafts Shadow Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe), munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or ftp.isi.edu (US West Coast). Abstract The Large-Scale Multicast Applications (LSMA) working group was chartered to produce two Internet-Drafts aimed at a consensus-based development of the Internet protocols to support large scale, real-time distributed simulation. This draft defines aspects of the Internet protocols that LSMA has found may need further development in order to meet that goal. 1. The Large Multicast Environment The Large-Scale Multicast Applications working group (LSMA) was formed to create a consensus-based requirement for Internet Protocols to support Distributed Interactive Simulation (DIS), its successor the High Level Architecture for simulation (HLA), and related applications. The applications are characterized by the need to distribute a real-time application over a shared wide-area network in a scalable manner such that numbers of hosts from a few to tens of thousands are able to interchange state data with sufficient reliability and timeliness to sustain a three- dimensional virtual, visual environment containing large numbers of moving objects. The network supporting such an system necessarily will be capable of multicast. Distributed Interactive Simulation is the name of a family of protocols used to exchange information about a virtual environment among hosts in a distributed system that are simulating the behavior of objects in that environment. The objects are capable of physical interactions and can sense each other by visual and other means (infrared, etc.). DIS was developed by the U.S. Department of Defense (DoD) to implement system for military training, rehearsal, and other purposes. More information on DIS can be found in the references. The feature of DIS that drives network requirements is that it is intended to work with output to and input from humans across distributed simulators in real time. This places tight limits on latency between hosts. It also means that any practical network will require multicasting to implement the required distribution of all data to all participating simulators. Large DIS configurations are expected to group hosts on multicast groups based on sharing the same sensor inputs in the virtual environment. This can mean a need for hundreds of multicast groups where objects may move between groups in large numbers at high rates. The overall total data rate (the sum of all multicast groups) is bounded, but the required data rate in any particular group cannot be predicted, and may change quite rapidly during the simulation. DIS real time flow consists of packets of length around 2000 bits at rates from .2 per second per simulator to 15 per second per simulator. This information is intentionally redundant and is normally transmitted with a best-effort transport protocol (UDP), and in some cases also is compressed. Required accuracy both of latency and of physical simulation varies with the intended purpose but generally must be at least sufficient to satisfy human perception, for example in tightly coupled simulations such as high performance aircraft maximum acceptable latency is 100 milliseconds between any two hosts. At relatively rare intervals events (e.g. collisions) may occur which require reliable transmission of some data on a unicast basis, to any other host in the system. DoD has a goal to build DIS systems with up to 100,000 simulated objects, many of them computer-generated forces that run with minimal human intervention, acting as opposing force or simulating friendly forces that are not available to participate. DoD would like to carry out such simulations using a shared WAN. Beyond DoD many people see a likelihood that DIS-like capabilities may be commercialized as entertainment. The scope of such an entertainment system is hard to predict but conceivably could be larger than the DoD goal of 100,000. The High Level Architecture (HLA) is a development beyond DIS that aims at bringing DIS and other forms of distributed simulation into a unifying system paradigm. Thus HLA has network requirements at least as demanding as DIS. HLA is still under development, however it is clear that its network requirements will be similar to those of DIS because it must achieve the same functions as DIS. 2. DIS network requirements. a. real-time packet delivery, with low packet loss (less than 2%), predictable latency on the order of a few hundred milliseconds, after buffering to account for jitter (variation of latency) such that less than 2% of packets fail to arrive within the specified latency, in a shared network b. multicasting with thousands of multicast groups that can sustain join/leave in less than one second at rates of hundreds of join/leaves per second c. multicasting using a many-to-many paradigm in which 90% or more of the group members act as receivers and senders within any given multicast group d. support for resource reservation because of the impracticality of over-provisioning the WAN and the LAN; it is important to be able to reserve an overall capacity that can be dynamically allocated among the multicast groups e. support for secure networking, needed for classified military simulations 3. Internet Protocol Suite facilities needed and not yet available for large-scale DIS in shared networks. These derive from the need for real-time multicast with established quality of service in a shared network. 3.1 Resource reservation available in production systems The standardized portion of the Internet protocol suite has featured only best effort protocols, which do not entail a commitment on the part of the network to perform particular quality of service. It is likely that simulation exercises using the Internet protocols would need to reserve a maximum overall networking capacity that would could be dynamically shared among various groups of information flows. Information flows will need to be dynamically grouped in relation to multicast groups, regions of interest, or some possibly some other paradigm as the exercise evolves. The Resource reSerVation Protocol (RSVP) is aimed at providing setup and flow-based information for managing information flows at pre- committed performance levels. This capability is generally seen as needed in real-time systems such as the HLA Run Time Infrastructure (RTI). However, RSVP is not fully available in production routers, nor has the current experimental version been approved as a Proposed Standard protocol within the IETF. The current experimental version of RSVP that is available in some routers does not support aggregation of reservation resources for groups of flows, nor does it support highly dynamic flow control changes. 3.2 Resource-sensitive multicast routing RSVP provides support only for communicating specifications of the required information flows between simulators and the network, and within the network. Distributing routing information among the routers within the network is a different function altogether, performed by routing protocols such as Multicast Open Shortest Path First (MOSPF). In order to make the overall network function, it may be necessary to have a routing protocol that determines paths through the network within the context of a quality of service requirement. An example is the proposed Quality Of Service Path First (QOSPF) routing protocol. 3.3 IP multicast that is capable of taking advantage of all multicast-capable data link protocols Multicast takes advantage of the efficiency obtained when the network can recognize and replicate information packets that are destined to a group of locations. Under these circumstances, the network can take on the job of providing duplicate copies to all destinations, thereby greatly reducing the amount of information flowing into and through the network. When IP multicast operates over Ethernet in a LAN and all subnets are interconnected using the Internet Group Management Protocol (IGMP), this is exactly what happens. However, with the new high performance wide-area technology Asynchronous Transfer Mode (ATM), the ability to take advantage of data link layer multicast capability is not yet available beyond a single LIS. This appears to be due to the fact that (1) the switching models of IP and ATM are sufficiently different that this capability will require a rather complex solution, and (2) there has been no clear application requirement for IP multicast over ATM multicast that provides for packet replication across multiple Logical IP Subnets (LIS). 3.4 A hybrid transmission protocol In general the Internet protocol suite uses the Transmission Control Protocol (TCP) for reliable end-to-end transport, and the User Datagram Protocol (UDP) for best-effort end-to-end transport, including all multicast transport services. The design of TCP is only capable of unicast use. In the HLA environment, three different transport needs have been identified: best-effort multicast of most data, lightweight reliable multicast of critical reference data, and low-latency, reliable unicast of occasional data among arbitrary members of the multicast group. All this takes place with in the same large-scale multicasting environment. Cohen has shown the need for these capabilities, while Pullen and Laviano have showed the value of integrating these three transport types into a single Selectively Reliable Transmission Protocol (SRTP) for DIS multicast. Recently the IETF has seen proposals for several reliable multicast transport protocols. A general issue with reliable transport for multicast is the congestion problem associated with delivery acknowledgments, which has made real-time reliable multicast transport infeasible to date. Of the roughly 15 attempts to develop a reliable multicast transport, all have shown to have some problem relating to datagram receipt acknowledgments (ACK), or requesting datagram retransmissions through the selected use of negative acknowledgments (NAK). Approaches involving distributed logging seem to hold promise for the distributed simulation application. In any event, its seems clear that there is not likely to be a single solution for reliable multicast, but rather a number of solutions tailored to different application domains. 3.5 Network management for distributed systems Coordinated, integrated network management is one of the more difficult aspects of a large distributed simulation exercise. The Internet network management techniques have been used successfully to support the growth of the Internet for the past several years. The technique is based on a primitive called a Management Information Base (MIB) being periodically polled at very low data rates. The receiver of the poll is called an Agent and is collocated with the remote process being monitored. The agent is simple so as to not absorb very many resources. The requesting process is called a Manager, and is typically located elsewhere on a separate workstation. The Manager communicates to all of the agents in a given domain using the Simple Network Management Protocol (SNMP). It appears that SNMP is well adapted to the purpose of distributed simulation management, in addition to managing the underlying simulation network resources. Creating a standard distributed simulation MIB format would make it possible for the simulation community to make use of the collection of powerful, off-the-shelf network management tools that have been created around SNMP. 3.6 A session protocol to start, pause, and stop a distributed simulation exercise in an IP network Coordinating start, stop, and pause of large distributed exercises is a complex and difficult task. The IETF has developed the Multiparty MUltimedia Session Control (MMUSIC) protocol which serves a similar purpose for managing large scale multimedia conferences. It is possible that MMUSIC's work could be adapted to distributed simulation, although it was designed around a multicast environment which requires principally a one-to-many transport service. If MMUSIC adaptation does not prove possible, surely the lessons learned in developing MMUSIC could help to develop a similar protocol for distributed simulation exercises. 3.7 An integrated security architecture A shortcoming of the current Internet Protocol (IPv4) implementation is the lack of integrated security. The new IPv6 protocol requires implementers to follow an integrated security architecture that provides the required integrity, authenticity, and confidentiality for use of the Internet by communities with stringent security demands, such as the financial community. The possibility that the IPv6 security architecture may meet military needs, when combined either with military cryptography or government- certified commercial cryptography, merits further study. 4. References Cohen, D., "Back to Basics", Proceedings of the 11th Workshop on Standards for Distributed Interactive Simulation, 1994 DIS Steering Committee, "The DIS Vision", Institute for Simulation and Training, University of Central Florida, May 1994 IEEE 1278.1-1995, Standard for Distributed Interactive Simulation - Application Protocols IEEE 1278.2-1995, Standard for Distributed Interactive Simulation - Communication services and Profiles Pullen, J. and V. Laviano, "A Selectively Reliable Transport Protocol for Distributed Interactive Simulation" Proceedings of the 13th Workshop on Standards for Distributed Interactive Simulation, 1995 RFC1667, "Modeling and Simulation Requirements for IPng" August 1994 Seidensticker, S., W. Smith and M. Myjak, draft-ietf-lsma-scenarios-00.txt, "Scenarios and Appropriate Protocols for Distributed Interactive Simulation", work in progress (companion Internet Draft) Zhang, Z., C. Sanchez, W. Salkecicz, and E. Crawley, draft-zhang-qos-ospf-00.txt, "Quality of Service Path First Routing Protocol", work in progress 5. Authors' Addresses J. Mark Pullen Computer Science/4A5 George Mason University Fairfax, VA 22032 Michael Myjak Institute for Simulation and Training University of Central Florida Orlando, FL Christina Bouwens ASSET Group, SAIC Inc. Orlando FL Expiration: 25 September 1997