Université de Liège. Is ATM a Continuity or a Discontinuity for the LAN Environment? Systèmes et Automatique. André Danthine, Olivier Bonaventure

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1 Université de Liège Faculté des Sciences Appliquées Systèmes et Automatique Institut d'electricité Montefiore, B28 Université de Liège au Sart Tilman B-4000 Liège (Belgique) Is ATM a Continuity or a Discontinuity for the LAN Environment? André Danthine, Olivier Bonaventure University of Liège Date: April 1996 Status: Public R Published in : High Speed Networks for Multimedia Applications W. Effelsberg, O. Spaniol, A. Danthine, D. Ferrari, Eds. Kluwer Academic Publishers, 1996 ISBN :

2 IS ATM A CONTINUITY OR A DISCONTINUITY FOR THE LAN ENVIRONMENT?* A. Danthine, O. Bonaventure 1 Institut d Electricité Montefiore, B-28, Université de Liège, B-4000, LIEGE, Belgium danthine@vm1.ulg.ac.be ABSTRACT The introduction of ATM has raised a lot of interest and controversy in the data communication world. After the presentation of the ITU view of the ATM and of the legacy LANs, the two paradigms for the introduction of ATM are discussed. The replacement of the access method of the legacy LANs by an access control based on a policing function on the network side and on a shaping function on the user side is a clear discontinuity. However, the authors show that the need for a control of the access rate was already present in large networks involving the interconnection of legacy LANs through the wide area. The final conclusion is that trying to hide the characteristics of ATM to the applications is a short term view where opportunities may be missed. Keywords: ATM, legacy LANs, QoS, rate control 1 INTRODUCTION ATM, adopted by the ITU in 1988 as the basic technology for the B-ISDN [DeP 95], [HHS 94], has immediately raised a lot of interest in the data communications world and soon appeared as the technology of the future for * This work has been done in the framework of the RACE project ACCOPI. 1 Research Assistant of the University of Liège

3 the LAN environment. The extraordinary success of the ATM Forum, created at the end of 1991, is a clear indication of this growing interest. However, the number of different solutions which have already been proposed and, for some of them, introduced, the number of pending problems still in discussion in the ATM Forum as well as the disappointing results of some experiments, have raised questions, about the easiness of the migration from today legacy LANs, to the ATM in the LAN environment. In this short paper, we will present some approaches and see if the ATM LAN must be considered as a clear discontinuity. 2 THE ITU VIEW OF THE ATM The public operators, faced with the multiplicity of services to be provided and with the difficulty to convince the users to support multiple network interfaces, have been searching for integration. ISDN is an integrated interface allowing access to many types of services which are still supported by different networks. B-ISDN is more ambitious. Its goal, in the long run, is to provide an integrated infrastructure able to support all communication services including the ones not yet foreseen today. Between the circuit switching orientation of the telephone network and the packet switching orientation of the X25 network, the choice of the cell switching for the ATM appears as the most reasonable compromise. The basic service is a connection-oriented cell relay service. Cells do not carry source or destination addresses, but a local identifier of the connection. On each link between switches, all the cells belonging to the same virtual circuit carry the same identifier (VPI/VCI) and a switch will use the VPI/VCI information of an incoming cell to select an outgoing link after having introduced, in the cell, the new VPI/VCI which identifies the virtual circuit on the outgoing link. The switch operates on the fly, with a table which has been set during the establishment of the virtual circuit. This operation is done by signalling for

4 switched virtual circuits (SVC) or by management for permanent or semipermanent virtual circuits (PVC). The basic ATM service is associated with the principle of a contract between the service user and the service provider. This contract involves the peak cell rate (PCR). The service user guarantees that its access throughput will not be higher than the declared PCR and the service provider guarantees that a stream of cells which does not violate the peak cell rate will be delivered in sequence with a cell loss rate below an announced value and, in some cases, with a maximum value of the transit delay. To guarantee the requested throughput, the ATM service must reserve resources. This means that the sum of the peak cell rates of the virtual circuits multiplexed on an output link of a switch must be, at most, equal to the capacity of this output link. Otherwise, cells will be lost and the guaranteed QoS will be violated. peak rate 0.33 T peak rate=1/tmin Figure 1. Four CBR sources entering a switch In figure 1, we represented four CBR (Constant Bit Rate) sources entering the switch by different input links. As the unit of time, we used the transmission time of one cell on the physical line. In that case the peak rate of the CBR source indicates the ratio of the throughput used by this source with respect to the capacity of the physical line. The sum of the peak rates of the four sources,

5 is smaller than 1 which by our definition of the unit of time is the maximum peak cell rate of the output physical line. Therefore the cells from these four streams may be multiplexed into a single output link (without losses) as only 85 per cent of the total capacity will be used. With CBR sources, each incoming stream has cells regularly spaced as shown in the figure 1. However, when several cells for the same output link arrive at the same time, they will have to leave one after the other. Therefore the multiplexing will be achieved without losses if enough cell buffers have been allocated by the switch to the cells having to leave by this output link. However, if we look at the various cell streams on the output link, it appears that the regularity of the traffic on each VC has not been preserved (Fig. 2). Figure 2. Four "CBR" sources leaving a switch Such a multiplexing effect may also happen at a source and that is the reason why the traffic contract is not only based on the cell peak rate but will also involve a cell delay variation (CDV) tolerance fixed by the service provider. Let us point out that the peak cell rate of a VC must not be confused with the peak cell rate of the access line of a switch. To fulfil the contract, the ATM service user will have to shape its traffic in order not to violate the peak cell rate. This is done by the scheduling of the transmission of the cells. This traffic shaping function is an essential part of the ATM and it is matched on the service provider side by a policing function. This

6 policing function uses an algorithm based on the PCR and on the CDV tolerance to allow for some cell jitter at the source, resulting for instance from the multiplexing of the cells associated with several VCs. If it detects a violation of the contract, the policing function will take some action such as dropping the cell or marking the cell as a low priority cell which may be discarded later on. It is clear that a constant bit rate source such as a voice or audio source or an uncompressed video signal will have no problem with the need of defining the characteristics of the source and with the traffic shaping function. In such an environment, the ATM service will allow a fine granularity in the setting of the peak cell rate. The classical data applications in general do not evaluate their "needs". They adapt their throughput to what is available and the idea of a contract has not been accepted by many data communications specialists. Before leaving the ITU view, let us point out an interesting technological discontinuity between ISDN and B-ISDN raised in [LVA 94]. ISDN switches require, for their operation, input demultiplexers and output multiplexers while the ATM switches of the B-ISDN are able to operate without them. However, the main point of difference is that ISDN offers a unified but inflexible interface to access several services while ATM offers a unified service on an open-ended list of interfaces. It is this characteristic which paves the way for a unique communication infrastructure. Using this ATM service, B- ISDN will offer various types of services by introducing several adaptation layers such as AAL1 for isochronous traffic and AAL3/4 and AAL5 for data communications. 3 THE LEGACY LANs The legacy LANs are based on the principle of a shared medium associated with an access method. Gaining access means that the total bandwidth of the LAN is, during a short period of time, entirely available for the user who has gained the access.

7 The CSMA/CD and the token passing schemes have a property in common. The various sources of packets are sharing the resources in a more or less fair way and nobody will be denied access but everybody will have to accept the reduction of bandwidth resulting from incoming new users. When loaded, the network, through its access method, prevents the source to transmit waiting packets and therefore an increase of traffic in a legacy LAN is felt by a back pressure to the source of the traffic. It is already clear that an ATM LAN is not just another LAN of the legacy type - all legacy LANs are connectionless while an ATM network is connectionoriented - all legacy LANs are shared medium based, with an access method while an ATM network is shared throughput based, with an access control - an additional service user in a legacy LAN will get its share of the resources with a corresponding reduction for the other service users, while in an ATM network an additional service user may be denied service if the resources are exhausted. The access control in an ATM network protects the set of connections which are sharing the available throughput while the access method of a legacy LAN protects the shared medium by allowing access to one source at a time in a fair way. In an ATM network, a source description is associated to each connection and each source must comply with the traffic characteristics defined in the contract while each node may protect itself by dropping cells not conforming with the contract. In a legacy LAN there is no source traffic description, there is no constraint on the behaviour of the source, but no guarantee of access for it. The access method avoids packet losses by preventing them to enter the network while, in an ATM network, the access control will drop the non conforming cells (which means packet losses) to protect conforming traffic. Last but not least, the reference model for the LAN based architecture is the OSI Reference Model, while the reference model for an ATM network is the B- ISDN Reference Model with a clear separation between the control plane and the user plane.

8 4 THE ATM LAN PARADIGMS For the IETF, the view towards ATM LAN may be summarised in the following way : - maintain TCP and IP unchanged - ATM is seen as a subnet, i.e. a connected communication network consisting of a single networking technology - TCP/IP will use ATM, just like it uses any other subnet technology - IP entities will be the way to access the local ATM subnet - IP routers will be the way to interconnect ATM subnet with legacy LANs. The approach, known as "classical IP over ATM" or "classical IP", is at the base of two models ; the local model and the end-to-end model. Application TCP UDP Socket Interface IP Signalling RFC1577 LLC/SNAP ATM Driver AAL-CS SAR Cell emission ATM Adapter Physical media Figure 3. Implementation Architecture of the "Classical IP over ATM" The local model focuses on the direct replacement of a local LAN segment. It solves the problem of access to an ATM LAN without modifying the access of the application to the TCP/IP sockets and without any modification of the TCP/IP implementation. All modifications are in the driver and in the ATM adapter board. The figure 3 shows a possible implementation where all the AAL and ATM functions are directly implemented in the adapter. As ATM does not

9 offer the broadcast capability of the legacy LANs, the resolution of the addresses is performed by an ARP server. From this local model, it is possible to define a LIS (Logical IP Subnet) as an ATM LAN subnet with (at least) one ARP server. The interconnection of the LISs takes place through IP routers. In the view of IETF, these IP routers are essential in this end-to-end model and the idea of a direct ATM connection between end systems belonging to different LISs does not seem to be supported by the IETF. It is clear that the "Classical IP" solution is restricted to the IP environment which is of course important but not, by far, the only one network layer. The ATM Forum view is more general and may be used with IP and with any network architecture even the ones which do not provide a routing layer. The prime interest of the users is, in the view of the ATM Forum, to be able to use ATM technology in the today's legacy environment. ATM should appear as just another transmission technology and the ATM Forum choice is to emulate an or MAC service. AppleTalk IPX IP NetBios LLC LANE Signaling AAL ATM Physical Network Service MAC service AAL Service IP Classical IP Signaling ATM Physical AAL Figure 4. LAN Emulation versus Classical IP Both solutions are represented in figure 4. It has already been said that the advantage of the LAN emulation is that the characteristics of the ATM are

10 hidden to the application, but that the disadvantage of the LAN emulation is that the characteristics of the ATM are hidden to the application. This amusing comment reflects very well the present situation. For the legacy applications, the introduction of an ATM LAN must be as invisible as possible. But for some new applications based for instance on multimedia, using the legacy LANs associated with TCP/IP is far from comfortable. These new applications have a direct interest in the QoS offered by the ATM service and in the idea of a contract making their communication environment a little less hectic i.e. less dependent on the behaviour of the others users. The LAN emulation (LANE) solution proposed by the ATM Forum also allows an unmodified access for the legacy applications. The TCP/IP implementation is also unchanged as for the classical IP, all modifications are in the driver and in the ATM adapter (Fig. 5). Application TCP UDP Socket Interface IP LLC LANE Entity Signalling ATM Driver AAL-CS SAR Cell emission ATM Adapter Physical media Figure 5. Implementation Architecture of the LAN emulation The unsolved problem today is related to the contract aspect of the ATM service. In the both solutions of figure 4, we are able to set PVC by management or to use signalling for the establishment of SVC in order to get

11 the VPI/VCI identifier which is used by the SAR function of the AAL5. However, the setting of the declared value of the PCR associated with the establishment of the connection is still a problem as the signalling function does not have any request from the application. Notice that the most recent ATM adapters have already implemented the shaping function which operates on the PCR value of the contract. Today the LAN emulation is assumed to operate in UBR (Unspecified Bit Rate) mode. Tomorrow, the LAN emulation will operate in ABR (Available Bit Rate) mode. 5 VBR SOURCES In order to handle the bursty character associated with VBR (Variable Bit Rate) sources of most data communications and of encoded video, two parameters have been added to the PCR and the CDV to characterise the source : - the sustainable cell rate, and - the burst tolerance. These two values fix the maximum burst size (MBS) that can be transmitted at the peak rate as well as the minimum value of the silent period which has to follow such a burst to avoid non conforming cells. It should be noted that in some cases, by adequate buffering and scheduling in the source, the source characteristics may be replaced by a peak cell rate equal to the sustainable cell rate. However, instead of putting an additional buffering in the service user equipment, in order to smooth the traffic characteristics, some have proposed to drastically increase the amount of buffering in the ATM switches. Once again, we have here a problem of consistency of design. The view of the ITU is that the ATM layer must be able to support all types of services. Adding a lot of buffers inside the switches will ease the problems associated with the bursty character of the data communications, but will jeopardise the capacity of supporting voice traffic or any real-time traffic for which the delay of transit in the switch must be kept to a minimum.

12 6 TCP/IP AND ATM The two paradigms we discussed in section 3 allow the introduction of ATM LANs without disturbing the legacy applications and with a minimal modification in the work session. It must however be stressed that TCP/IP has been developed and additional mechanisms have been introduced later to overcome problems such as congestion which have been found in a packet switching environment. Some experiments of TCP/IP over ATM have put into evidence that some impedance matching problems have still to be solved to get the full benefit of the new technology [BDK 95]. 7 LAN INTERCONNECTION OVER THE WIDE AREA Today, the interconnection of LANs over the wide area is mainly done with leased lines or with virtual connections. The well-defined and inflexible throughput of such a leased line prevents sometimes the source to send at the highest possible rate. This bandwidth limitation of the leased line also implies some back pressure to the source. In such an environment, ATM is at least able to offer more flexibility in bandwidth selection, but once again the traffic shaping will have to be done at the access to the WAN. 8 NEW APPLICATIONS As already mentioned, legacy applications may begin to migrate in an ATM environment. However, the new applications such as video conferencing, video on the desk and CSCW have a lot to gain from accessing the QoS offered by the ATM service [DBL 92], [GrM 94], [IMI 95 ]. Some IP evangelists [Hui 95] are claiming that the requirements of these new applications may be satisfied if the network has enough capacity to offer at any time what is needed. If this was true, Van Jacobson would not have had to introduce its slow start algorithm to overcome congestion problems in the Internet. This algorithm introduces a variable access rate control while the shaping function of ATM introduces a fixed rate equivalent to the bandwidth of a leased line. The ABR (Available Bit

13 Rate) proposal of the ATM Forum is introducing a variable rate control at the cell level making the service again more flexible. The architectural model of B-ISDN implies multiple AALs to match the need of different services. This multi-protocol approach at the low level may be also envisaged in the network and transport layers. Today, some experiments are already going on with the application accessing directly through an API the ATM or the AAL service. In such an approach, the QoS required by the application may be easily passed to the connection management entity. This will avoid the limitation of the LAN emulation 2.0 where the necessary parameters for the connection management entity will not be derived from the application but linked to the ABR service. In the same line of thought, it would be interesting today, to ease the future evolution, to have for the segmentation and reassembly sublayer a well-defined service. This will allow to specify if, for the SAR-SDU, we are interested in : - error detection and correction - no error detection - no delivery if any cell loss occurred - delivery of the SAR-SDU with cell loss replaced by a known pattern of bytes, for instance with some encoded video traffic. This will be achieved if the SAR sublayer consists of a certain number of building blocks more or less modular and independent. Applications, directly or indirectly, would be able to select the building block most suitable for a given connection [Cou 95]. Such a modification will introduce more flexibility in the specification of the complete AAL services. 9 CONCLUSIONS The introduction of ATM gave rise to controversial discussions still going on today [Lea 92], [Cro 94], [KiW 95], [Sch 95]. A cold analysis shows that ATM is a discontinuity for the LAN environment but this discontinuity also offers possibilities of supporting new services in a more efficient way. It may be a mistake in the long run to try to hide this discontinuity instead of exploiting it.

14 REFERENCES [BDK 95] [Cou 95] [Cro 94] O. Bonaventure, A. Danthine, E. Klovning, O. Danthine, "TCP/IP and the European ATM Pilot", to be presented at the International Conference on Network Protocol (ICNP-95), Tokyo, Nov.7-10, 1995 J.-P. Coudreuse, "Paragraphs on ATM", Ann. Télécommun., Vol. 50, N 1, 1995, pp J. Crowcroft, "Why Lossy Internetworking and Lossless ABR ATM Services Do Not Go Together - RN/94/21, June 1994, 6 p. [DBL 92] A. Danthine, Y. Baguette, G. Leduc, L. Leonard, "The OSI95 Connection-mode Transport Service - The Enhanced QoS", IFIP Conf. on High Performance Networking, Liège, December 16-18, 1992, in: A. Danthine, O. Spaniol, eds., C14 High Performance Networking, Elsevier Science Publ. (North-Holland), Amsterdam, 1993, pp [DeP 95] M. De Prycker, Asynchronous Transfer Mode - Solution for Broadband ISDN, Second Edition, Ellis Horwood, 1995, 331 p. [GrM 94] D.J. Greaves, D.Mc Auley, "ATM Network Services for Workstations" in The OSI95 Transport Service with Multimedia Support, A. Danthine (Ed.), Springer Verlag, 1994, pp [HHS 94] R. Händel, M.N. Huber, S. Schröder, ATM Networks - Concepts, Protocols, Applications, Second Edition, Addison-Wesley, 1994, 287 p. [Hui 95] C. Huitema, Et Dieu créa l'internet, Eyrolles, 1995, 201 p. [IMI 95] [KiW 95] [Lea 92] [LVA 94] [Sch 95] A. Iwata, N. Mori, C. Ikeda, H. Suzuki, M. Ott, "ATM Connection and Traffic Management Schemes for Multimedia Internetworking", Communications of the ACM, Vol. 38, N 2, February 1995, pp B.G. Kim, P. Wang,, "ATM Network : Goals and Challenges", Communications of the ACM, Vol. 38, N 2, February 1995, pp C-T Lea, "What Should Be the Goal for ATM", IEEE Networks, Vol. 6, N 5, September 1992, pp T.F. La Porta, M. Veeraraghavan, E. Ayanoglu, M. Karol, R.D. Gitlin, "B-ISDN: A Technological Discontinuity", IEEE Communications Magazine, Vol. 32, N 10, October 1994, pp H. Schulzrinne, "ATM: Dangerous at any speed?", Gigabit Networking Workshop, April 1995