INTERNET DRAFT J.M.Pullen Expires in six months George Mason U. M.Myjak U.of Central Florida C.Bouwens SAIC 7 February 1998 Limitations of Internet Protocol Suite for Distributed Simulation in the Large Multicast Environment draft-ietf-lsma-limitations-02.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 Internet-Drafts aimed at a consensus-based development of the Internet protocols to support large scale multicast applications including real-time distributed simulation. This draft defines aspects of the Internet protocols that LSMA has found to 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) [DIS94], its successor the High Level Architecture for simulation (HLA) [DMSO96], 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 [IEEE95a,IEEE95b]. 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 [SSM96]. The feature of distributed simulation 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 distributed simulation 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. The U.S. DoD has a goal to build distributed simulation 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 distributed simulation 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. From a distributed systems standpoint HLA is considerably more sophisticated than DIS. For example attributes of distributed objects may be controlled by different simulators. From the standpoint of the supporting network the primary difference between HLA and DIS is that HLA does not call for redundant transmission of object attributes; instead it specifies a "Run Time Infrastructure" (RTI) that is responsible to transmit data reliably, and may choose to do so by various means including redundant transmission using best-effort protocols. It is reasonable to say that any network that can meet the needs of DIS can support HLA by DIS-like redundant transmission, however this approach ignores the possibility that under HLA some mixture of redundant and reliable transmission can make significantly better use of network resources than is possible using DIS. 2. Distributed Simulation (DIS and HLA) 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 host group join in less than one second at rates of hundreds of joins per second, (leave need not be so rapid but must also be fast because holding groups open may delay opening other groups) 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 for large distributed simulations, it is important to be able to reserve an overall capacity that can be dynamically allocated among the multicast groups e. support for a mixture of best-effort and reliable low-latency multicast, where best-effort predominates in the mixture f. support for secure networking, needed for classified military simulations 3. Internet Protocol Suite facilities needed and not yet available for large-scale distributed simulation in shared networks. These derive from the need for real-time multicast with established quality of service in a shared network. (Implementation questions are not included in this discussion. For example, it is not clear that implementations of IP multicast exist that will support the required scale of multicast group changes for LSMA, but this appears to be a question of implementation, not a limitation of IP multicast.) 3.1 Large-scale resource reservation in shared networks 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 RTI. While RSVP has not been deployed on the scale of the LSMA, its architecture does not appear to pose any barriers to scaling to that level. However, the current RSVP draft standard does not support aggregation of reservation resources for groups of flows, nor does it support highly dynamic flow control changes. Further, 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 provide effective resource reservation in a large shared 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 [ZSSC97]. Unfortunately the requirement for resource-sensitive routing will be difficult to define before LSMA networks are deployed with RSVP. 3.2 IP multicast that is capable of taking advantage of all common link layer protocols (in particular, ATM) 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 Logical IP Subnet (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 LIS. Distributed simulation is an application with such a requirement. 3.3 Hybrid transmission of best-effort and reliable multicast 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 transmission. Recently the IETF has seen proposals for several reliable multicast transport protocols (see [Mont97] for a summary). 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 positive receipt acknowledgments (ACK) or negative acknowledgments (NAK). 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. Approaches involving distributed logging seem to hold particular promise for the distributed simulation application. In the DIS/HLA environment, five different transmission needs can be identified: (1) best-effort low-latency multicast of object attributes that often change continuously, for example position of mobile objects; (2) low-latency reliable multicast of object attributes that do not change continuously but may change at arbitrary times during the simulation, for example object appearance (An important characteristic of this category is that only the latest value of any attribute is needed.); (3) low-latency, reliable unicast of occasional data among arbitrary members of the multicast group (This form of transmission was specified for DIS "collisions"; it is not in the current HLA specification but might profitably be included there. The requirement is for occasional transaction-like exchange of data between two arbitrary hosts in the multicast group, with a low latency that makes TCP connection impractical.); (4) reliable but not necessarily real-time multicast distribution of supporting bulk data such as terrain databases and object enumerations; and (5) reliable unicast of control information between individual RTI components (this requirement is met by TCP). All of these transmissions take place within the same large-scale multicasting environment. The value of integrating categories (1) and (2) into a single selectively reliable protocol was proposed by Cohen [Cohe94]. Pullen and Laviano implemented this concept [PuLa95] and demonstrated it within the HLA framework [PLM97] as the Selectively Reliable Tranmission Protocol (SRTP) for categories (1) through (3). Category (4) could be supported by a reliable multicast protocol such as the commercial multicast FTP offering from Starburst [MRTW97], however adequate congestion control has not been demonstrated in any such protocol. There has been some discussion of using the Real-Time Streaming Protocol, RTSP, for this purpose, however as the databases must be transmitted reliably and RTSP uses a best-effort model, it does not appear to be applicable. In summary, it is clear that a hybrid of best-effort and reliable multicast (not necessarily all in the same protocol) is needed to support DIS and HLA, and that the low-latency, reliable part of this hybrid is not available in the Internet protocol suite. 3.4 Network management for distributed simulation systems Coordinated, integrated network management is one of the more difficult aspects of a large distributed simulation exercise. The network management techniques that have been used successfully to support the growth of the Internet for the past several years could be expanded to fill this need. The technique is based on a primitive called a Management Information Base (MIB) being polled periodically 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. 3.5 A session protocol to start, pause, and stop a distributed simulation exercise Coordinating start, stop, and pause of large distributed exercises is a complex and difficult task. The Session Initiation Protocol (SIP) recently proposed by the Multiparty MUltimedia Session Control (MMUSIC) working group serves a similar purpose for managing large scale multimedia conferences. As proposed, SIP appears to offer sufficient extensibility to be used for exercise session control, if standardized by the IETF. 3.6 An integrated security architecture It appears that this requirement will be met by IPv6 deployment. 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. 3.7 Low-latency multicast naming service Name-to-address mapping in the Internet is performed by the Domain Name Service (DNS). DNS has a distributed architecture tuned to the needs of unicast networking with reliable transmission (TCP) that typically has latency of at least several seconds. The requirement of distributed simulation for agile movement among multicast groups implies a need for name-to-multicast-address mapping with latency of under one second for the name resolution and group join combined. This problem has been circumvented in military simulations by using group IP addresses rather than names. However, if distributed simulation is to grow into the domain of commercial entertainment, a parallel to the need for DNS will arise. Thus a low-latency naming service will be required. 3.8 Inter-Domain Multicast Routing for LSMA While military LSMAs typically take place within a single administrative domain, future entertainment LSMAs can be expected to involve heavy inter-domain multicast traffic. Standardized protocols able to support large numbers of multicast flows across domain boundaries will be needed for this purpose. 4. References [Cohe94] Cohen, D., "Back to Basics", Proceedings of the 11th Workshop on Standards for Distributed Interactive Simulation, Orlando, FL, September 1994 [DIS94] DIS Steering Committee, "The DIS Vision", Institute for Simulation and Training, University of Central Florida, May 1994 [DMSO96] Defense Modeling and Simulation Office, High Level Architecture Rules Version 1.0, U.S. Department of Defense, August 1996 [IEEE95a] IEEE 1278.1-1995, Standard for Distributed Interactive Simulation - Application Protocols [IEEE95b] IEEE 1278.2-1995, Standard for Distributed Interactive Simulation - Communication services and Profiles [MRTW97] Miller, K, K. Robertson, A. Tweedly, and M. White, "StarBurst Multicast File Transfer Protocol (MFTP) Specification", draft-miller-mftp-spec-02.txt, work in progress, January 1997 [Mont97] Montgomery, T., Reliable Multicast Links webpage, http://research.ivv.nasa.gov/RMP/links.html [PuLa95] 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, Orlando, FL, September 1995 [PLM97] Pullen, J., V. Laviano and M. Moreau, "Creating A Light-Weight RTI As An Evolution Of Dual-Mode Multicast Using Selectively Reliable Transmission", Proceedings of the Second Simulation Interoperability Workshop, Orlando, FL, September 1997] [SPW94] Symington, S., J.M.Pullen and D. Wood, "Modeling and Simulation Requirements for IPng", RFC1667, August 1994 [SSM96] Seidensticker, S., W. Smith and M. Myjak, "Scenarios and Appropriate Protocols for Distributed Interactive Simulation", draft-ietf-lsma-scenarios-00.txt, work in progress 13 June 1996 (companion Internet Draft) [ZSSC97] Zhang, Z., C. Sanchez, W. Salkewicz, and E. Crawley, "Quality of Service Path First Routing Protocol", draft-zhang-qos-ospf-01.txt, work in progress September 1997 5. Authors' Addresses J. Mark Pullen mpullen@gmu.edu Computer Science/C3I Center MS 4A5 George Mason University Fairfax, VA 22032 Michael Myjak mmyjak@ist.ucf.edu Institute for Simulation and Training University of Central Florida Orlando, FL 32816 Christina Bouwens christina.bouwens@cpmx.mail.saic.com ASSET Group, SAIC Inc. Orlando FL Expiration: 7 August 1998