Computer science and telecommunications groups from fifteen Bay Area organizations have formed a testbed to develop and deploy the infrastructure needed to support a diverse set of distributed applications in a large-scale, metropolitan IP over ATM network environment. These organizations are:
+Apple Computer +DEC - Palo Alto Systems Research Center
+Hewlett-Packard Laboratories +International Computer Science Institute
+Lawrence Berkeley Laboratory +Lawrence Livermore National Laboratory
+NASA Ames Research Center +Pacific Bell-Broadband Development Group
+Sandia National Laboratories +Silicon Graphics, Inc.
+SRI International +Stanford University
+Sun Microsystems, Inc. +University of California, Berkeley
+Xerox Palo Alto Research Center
This paper describes the architecture of the testbed, and some of the issues of running IP over ATM.
The architecture of the Internet is that of collections of hot potato packet routers that communicate with each other regarding the probable best way to get packets to their destination. These routers communicate with each other over a very heterogeneous collection of interconnects, and over LANs between routers and end hosts.
To first order, there are arguably two basic Internet services: unicast and multicast. In the former, packets are labeled with source and destination addresses of communicating end systems (hosts). TCP (transmission control protocol) provides logical circuits between processes, and UPD (user datagram protocol) and RTP (realtime transport protocol) provide messaging. IP multicast is an addressing mechanism whereby many hosts can receive the same datagram, and the real trick is figuring out the minimum number of routers that have to replicate datagrams in order that all interested parties receive them. IP multicast is not supported by many of the commercial routers used in the Internet so a IP multicast backbone (mbone) overlays the larger IP network by having multicast-capable routers that are separated by incapable routers communicate with each other via TCP. See .
Asynchronous Transfer Mode (as opposed to the synchronous operation of SONET, which is the typical physical-layer protocol for ATM) is a circuit-oriented, micro-packet (cell) protocol. ATM is circuit oriented in the sense that ATM cells follow the same path for the life-time of the connection. So, like a telephone call, an elaborate signaling process precedes data transmission in order to identify the entire path that data carrying cells will follow. This circuit-establishing signaling process comes in two flavors, permanent virtual circuits (PVCs) and switched virtual circuits (SVCs). As the names imply, PVCs are virtual wires and changed infrequently, and SVCs are dynamic (they may occur at the frequency of TCP connection establishment). ATM defines a number of transport capabilities called adaptation layers (AALs), one of which (AAL-5) was tailored for IP.
2.0 BAGNet Architecture
Pacific Bells CalREN program has awarded to each participant a grant covering the cost of two years of OC-3 (155 Mbits/sec), ATM service. The service is defined as best-effort virtual circuits to the other BAGNet sites, and up to a 15 db loss from each BAGNet site to the Pacific Bell equipment.
2.1 The Physical-level
The network backbone consists of several Pacific Bell operated, Central Office ATM switches connected via a SONET network. SONET is a topic of its own, but for the purposes of this discussion a few brief comments are in order.
At the BAGNet sites there are several different strategies for connecting to the Pacific Bell network. The important issue is that Pacific Bell OC-3 ATM service is provided on single mode fiber. Single mode fiber is used almost exclusively in wide area and metropolitan area fiber-based networks, as opposed to the multimode fiber common in LAN (site) environments. Multimode fiber is used, or example, to support campus-wide FDDI and Ethernet. Several different types of ATM LAN equipment (switches and workstations) make up the customer premise equipment at the BAGNet sites. This equipment, even if SONET, OC-3 based, usually uses multimode fiber. So there is typically a special piece of equipment needed to attach to the Pacific Bell network interface (the carrier network - user network interface, or UNI). Figure 2 illustrates the two most common strategies. A third strategy, not illustrated, is to use a single-mode to multi-mode SONET converter. While this should be a simple device compared to other SONET and ATM equipment, there are not currently any volume commercial sources of this device, so it tends to be an expensive solution to the UNI problem.
2.2 The Link-level
The Pacific Bell, Bay Area ATM network is currently PVC based and fully meshed.This means that there is a fixed set of virtual circuits between every communicating pair of end-nodes in the network. The reason for using PVCs instead of SVC is that the much more complex mechanisms needed to support SVCs have not yet been implemented by all of the ATM switch vendors, and for SVCs to work all of the switches have to talk to each other during the circuit setup phase.
2.3 The Network-level
ATM Adaptation Layer - 5 (AAL-5) is used to carry IP packets. AAL-5 defines structure that organizes collections of cells to carry larger unites of data. AAL-5 has no multiplexing and no cell sequence numbers. The basic idea is to provide an efficient and reliable way to transport a protocol data unit (PDU). A PDU of 1 to 65535 bytes is structured in a cell sequence marked by a trailer. Cells are not sequence numbered within the PDU under the assumption that cells may be dropped, but not reordered by the ATM network. The entire PDU is protected by a 32 bit CRC.
2.4 The Internet-level
There are a number of ways that the semantics of IP and its associated protocols might map to the ATM environment, and this is an area of active research in the computer science community. The BAGNet testbed group elected to use the model proposed in RFC - 1577: Classical IP and ARP over ATM . This RFC specifies one way that the model of a classical IP network can be implemented in an ATM environment. Adhering to this model will provide us with insights into how ATM networks will work in the Internet environment. The essential elements of the model are:
More information about BAGNet, including an expanded version of this paper, may be found at http://www-itg.lbl.gov/BAGNet.html.
The work in BAGnet is done by many people from the fifteen constituent organizations. This note is a brief report on some of that work. The BAGNet executive is co-chaired by Bill Johnston, LBL (email@example.com), Marjory Johnson, NASA Ames (firstname.lastname@example.org), and Dan Swinehart, Xerox PARC (email@example.com. The IP task group is chaired by Berry Kercheval, Xerox PARC (firstname.lastname@example.org).
M. Macedonia and D. Brutzman, MBone Provides Audio and Video Across the Internet, IEEE COMPUTER magazine, pp. 30-36, April 1994. (Available from ftp://taurus.cs.nps.navy.mil/pub/mbmg/mbone.html .)