Simulation of IP over ATM over SONET Layered Architecture

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1 Simulation of IP over ATM over SONET Layered Architecture Vaibhav Shah and Girish P. Saraph Department of Electrical Engineering IIT-Bombay Mumbai ABSTRACT This paper gives an overview of widely used technique of IP/ATM/SONET to enable IP data transport over optical networks. Simulation results of two sample networks with prioritized IP and QoS support for voice channels are presented here. These simulations enable determination of important network performance measures, which can drive network planning and infrastructure investment or build out decisions by identifying bottlenecks in the network topology and quantifying their effects on the network performance. 1. INTRODUCTION Network traffic has seen exponential growth in recent years due to popularity of Internet and related applications. Data traffic constitutes about ¾th of the total network traffic and has quite different characteristics from the traditional voice traffic. It is bursty and has variable packet sizes and arrival times. Data applications (Internet, ftp, etc.) are not sensitive to delay or jitter and can tolerate packet losses by packet sequencing and retransmission (TCP/IP protocol stack). Whereas real-time applications such as voice and video connections require QoS guarantees in terms of maximum delay, delay jitter, throughput and error rates. As Asynchronous Transfer Mode (ATM) network is based on cell switching like circuit switching in telephone networks, it can provide required QoS guarantees for voice and video transport. Since early networks were setup for voice applications, ATM became common in WANs (Wide Area Networks). IP based access networks are common due to support of various applications, hence IP over ATM standards were developed to route IP data over ATM networks. In the core networks, large data handling capacity and highly synchronous networks are needed. Synchronous Optical NETwork (SONET) and Synchronous Digital Hierarchy (SDH) are used to transmit data on high-speed optical fiber link. They use fixed size data frame for sending data in synchronous time slots. High data rate SONET/SDH frames (OC-12/STM Mbps) are formed from time division multiplexing of lower data rate frames (4xOC-3/ 4xSTM-1,OC-3 STM-1 155Mbps). This SONET/SDH overhead signaling system enables core networks to be highly reliable, fault-tolerant and operational/management functionalities. ATM over SONET/SDH standard is developed to map ATM cells on SONET/SDH and enable SONET/SDH to provide the ATM-QoS facilities Internet Service Providers (ISPs) are making huge investments in optical networks to provide QoS guarantees to voice and video traffic along with prioritized data traffic. As the deploying cost of optical backbone and infrastructure is very high, it is better to first test the network for various performance parameters via simulation. This paper evaluates performance of IP/ATM/SONET networks using computer simulations for different network topologies. It is organized as follows. Section 2 provides review of interconnections of IP, ATM and SONET layers. Section 3, describes the protocol stack for IP/ATM/SONET. In section 4, simulation results of sample networks with prioritized data and voice support are presented IP Layer 2. INTERCONNECTION OF LAYERS At present two versions of the IP protocol exist: IP version 4 (IPv4) using 32-bit IP address described in RFC 791[1], and IP version 6 (IPv6) using 128-bit IP address described in

2 RFC 1883[2]. Most networks are currently using IPv4 and only few have migrated to IPv IP Packet Size Distribution For realistic network performance in the presented simulations, we have used IPv4 traffic patterns and characteristics described in [3]. IP packet size distribution is tabulated in table-1 and cumulative distribution is shown in figure 1. transfer. ATM circuits are of two types: virtual path, a bundle of virtual channels having a common VPI switched across the ATM network; and virtual channel, where all VCI and VPI are remapped at each switch. The basic operations of an ATM switch are: receive a cell across a link on a known VCI/VPI value; look up the local translation table to determine outgoing port and new VPI/VCI value of the connection; retransmit the cell on outgoing port with the new VPI/VCI values ATM Addressing Each ATM device has 20 bytes long unique ATM address, which is significant for signaling and making virtual connections. In simulations we have used E.164 NSAP address format [12]. Figure 1. Cumulative IP packet size distribution [3] Table 1: Approximate packet size distribution Packet-size Percentage (%) Used in TCP Ack TCP MSS Ethernet TCP MSS Telnet Others ATM Layer ATM Cell ATM cell is 53 bytes long, first 5 bytes are header information and rest 48 bytes are payload. Descriptions of ATM cells can be found in ATM Forum [4]. Some main fields used in simulation are as follows: Virtual Path Identifier (VPI) - one or more VCI can be in it. Virtual Channel Identifier (VCI)- helps cell transmission in the switch fabric. Payload Type Identifier (PTI)- used to indicate type of cell and congestion Virtual Connection Since ATM networks are connection oriented, a virtual circuit needs to be set up prior to any data ATM Signaling Figure 2. Signaling procedure in ATM [10] The ATM signaling UNI 3.1 specification [5] is based upon a public network signaling protocol Q.2931 [6]. All UNI signaling requests are sent on a well-known connection VPI=0, VCI=5. The steps used to set up a switched virtual connection from client A to B are shown in figure ATM Adaptation Layer: AAL5 Figure 3. Segmentation of IP packet into ATM cells AAL5 is primary adaptation layer that supports data transfer for classical IP over ATM [13].

3 Here, IP packet is appended with a variablelength pad and 8-byte trailer (T in figure 3) to make AAL5-PDU, a multiple of 48-bytes. The trailer includes frame length and a 32-bit cyclic redundancy check (CRC) computed across PDU that allows receiving process to detect bit errors, out of sequence or lost cells. This PDU is divided into blocks of 48 bytes, and then the ATM layer puts each block sequentially in payload field of consecutive ATM cells. For all cells except the last, a bit in the Payload Type (PT) field of ATM header is set to 0, this is shown by header H0 in figure 3. For the last cell, the bit in the PT field is set to 1, indicating last cell of the frame, shown by header H IP over ATM The transport of any network layer protocol over an ATM network involves following aspects: Packet Encapsulation In this method, multiple protocols can be carried over a single ATM connection with the type of encapsulated protocol identified by a standard LLC/SNAP header [7]. This conserves the connection resource space, and saves on latency of connection setup, after first connection setup Address Resolution: It is a mechanism used to resolve IP address to corresponding ATM address. A protocol "Classical IP over ATM is defined in [8] to support address resolution, it introduces a notion of Logical IP Subnet (LIS). LIS is a group of IP nodes (hosts and routers) that are connected to a single ATM network. Each LIS supports a single ATMARP server for address resolution. Each node maintains a local table of IP to ATM address mapping. If a node encounters an unknown address it sends a query to the ATMARP server for address resolution SONET (Synchronous Optical NETwork) In our simulation, we have used STS-3c SONET frame, as shown in figure 4. The obtained results are also applicable for STM-1 SDH frame networks since both frames are quite similar. Figure 4. STS3c Frame [9] 3. IP over ATM over SONET PROTOCOL STACK The Protocol stack for simulation is: IP Encapsulation byte packets LLC/SNAP Logical Link Control ([7]). ATM PDU=8-byte overhead + IP packet AAL5 AAL5-PDU = ATM-PDU + padding + 8-byte trailer ATM AAL5 PDU split into 48 byte payloads. 53 byte ATM cell= 5 byte header + 48 byte payload SONET STS3c frame=2430 bytes per 125 sec =90 bytes overhead ATM cells 4.1. Assumptions 4. SIMULATION 1. IP Nodes are transmitting IP packets on their average data rate; IP packet size distribution of figure 1 is used and probability of a source s IP packet for any other node in network is equal. 2. TCP flow control is not implemented; nodes are transmitting at maximum data rate available. 3. All IP nodes are connected to LIS for a long time and know ATM addresses of each other. Hence ATMARP server is not needed. 4. ATM addressing is done hierarchal (PNNI signaling [11]) to facilitate routing according to highest prefix match with destination address. 5."Exclusive VPI" schema (Q.2931 signaling standards [6]) is used for signaling. 6. ATM switches have FIFO (First In First Out) output buffer queue of 500 ATM cell capacity. 7. Simulations are done for 1000 frames of STS- 3c (i.e.1000*125 Sec=0.125 second) 4.2. Simulation Details All simulation codes are written in C using multithreaded POSIX library on a linux platform. A thread is initialized for every node and switch. These nodes and switches

4 communicate with each other using thread synchronization utilities. A typical simulation of second with 12 IP nodes, 7 ATM switches took 3 minutes to complete Simple IP/ATM/SONET Link Each node having ATM cell IP data rate =100Mbps throughput throughput Network % % Network 1 (With % % Network % % Network 2 (With % % NOTE: In these simulations, we haven t used TCP/IP stack, which can effectively make the throughput close to 100%. We have tried to achieve congestion control in ATM layer. Figure 5. Network with 2 nodes and 2 switches This simulation is to evaluate maximum IP data rate a node can transmit. Theoretically it is Mbps [9], which is verified in simulation Simulation 1 Figure 8. Network 2, ATM switch queues at 150Mbps: (a) without congestion control, (b) with congestion control NOTE: In attached graphs, y-axis is divided in number of rows depending on number of switches in ascending order & each row is normalized to the total queue buffer size Simulation 2 Figure 6. Network 1 Figure 7. Network 2 NOTE: Network 1 and 2 only differs in one link between switch 2 and switch 4 present in network 2. Simulation results of cell and IP throughput for the above networks 1 and 2, are tabulated below: Each node having ATM cell IP data rate =150Mbps throughput throughput Network % % Network 1 (With % % Network % % Network 2 (With % % Now we simulate networks 1 and 2 for different service classes. We have made 4 priority levels for IP packets: 0(lowest), 1, 2 and 3(highest). Intermediate switch drops cells of IP packet having priority 0, 1, 2, and 3 at 70%, 80%, 90% and 100% of buffer queue-fill respectively. Simulation results for both networks are shown in following tables (assuming priority levels have equal probability): Network 1 Cell throughput IP throughput Priority % 48.96% Priority % 70.07% Priority % 91.25% Priority % 98.72% Network 2 Cell throughput IP throughput Priority % 43.16% Priority % 47.46% Priority % 74.18% Priority % 98.64% 4.6. Including VOICE Services Voice services can be provided in IP/ATM/ SONET layered structure by reserving resources at ATM switches along the path and transmitting the voice packets with high priority.

5 Intermediate switches switch them, without putting them waiting in FIFO queue. Figure 9. T2 voice channel in network 1 Steps for providing voice services: 1. Sender sends a SETUP request for voice connection to receiver. 2. In routing, every intermediate switch makes a reservation of resources on outgoing link for voice channel. The switch checks that the cumulative resource reservation is kept below a specified level (so that other data traffic is not starved) and then only accepts a new request. 3. Receiver sends call accept message, using CONNECT message. T2-voice channel data rate=6.312mbps Equivalent data bytes sent per frame of T2 channel=6.312*125/8=98.6 bytes Number of cells to be reserved per STS-3c frame=3 voice cells/sts-3c frame (approx.). 5. RESULTS and DISCUSSION 1. In IP/ATM/SONET layered architecture STS- 3c SONET frame can support a maximum of Mbps IP data rate. 2. Need of simulating an optical network before deploying is emphasized in simulation 1, as in network 2, the link between ATM switch 2 and switch 4 becomes a bottleneck and performance is dropped by ~10% compared to network IP throughput is lower than ATM throughput since the entire packet is dropped when one of the constituent ATM cell is dropped. 4. Differentiated services can be supported by setting priority levels for the IP traffic. The buffer management policy at the ATM switch determines the actual IP throughputs at different priority levels and this policy can be adjusted to suit the network performance requirements. 5. Although the ATM and IP throughputs for the low priority traffics are low, the actual throughput is likely to be higher due to the TCP flow control mechanism. In the present simulations, only the ATM flow control is used. 6. Resource reservations can be used to set up real-time voice and video connections requiring high QoS guarantees. However, a limit on the reservation should be set in order to avoid starvation of lower priority traffic. 7. Since SONET/SDH layer is working as only Data Link Layer of OSI reference model, all results are mainly dependent on IP and ATM layer integration. 6. REFERENCES [1] RFC 791: "Internet protocol", Sept [2] RFC 1883: "Internet Protocol, Version 6 (IPv6) Specification", Jan [3] K. Thompson, G. Miller, R. Wilder, "Wide- Area Internet Traffic Patterns and Characteristics", IEEE Network, Nov/Dec [4] Web-Site: [5] ATM Forum, "ATM User-Network Interface Specification Version 3.1" ATM Forum Specification, Sept [6] ITU-T Rec. Q.2931, B-ISDN Application Protocol for Access Signaling, Feb [7] RFC 1483: Multiprotocol Encapsulation over ATM Adaptation Layer 5", July [8] RFC 1577: Classical IP and ARP over ATM", Jan [9] John D. Cavanaugh. Protocol Overhead in IP/ATM Networks Tech. Report, Minnesota Supercomputer Center Inc., [10] Anthony Alles, ATM Internetworking, White Paper, Cisco Systems Inc., May [11] ATM Forum "ATM Forum R7: P- NNI Draft Specification," March [12] ITU-T Rec. E.164, The international public telecom. numbering plan, May [13] ITU-T Rec. I.363.5, B-ISDN ATM Adaptation Layer specification: Type 5 AAL, Aug 1996.