An Overview Of Future Routing Technologies Robert Smith Armin Eberlein Fred Halsall Department of Electrical & Electronic Engineering, University of Wales Singleton Park, Swansea, SA2 8PP, U.K., Tel: +44 1792 295541, Fax: +44 1792 295686 e-mail:
[email protected] Abstract Current Internet routing technology is not capable of supporting the continuous growth of traffic and the high bandwidth demands of multimedia applications. The future Internet will need to be able to support applications such as videoconferencing, by increasing router performance and providing some guarantees regarding the quality of service (QoS) the application receives. This paper reviews the proposed future routing technologies put forward by several networking companies which aim to meet these demands.
1. Introduction Today’s Internet comprises of a collection of subnetworks (subnets), each of which supports communications between devices attached to it. Routers are required not only to route traffic within these domains but also to interconnect subnets that are running different protocols. The Internet Protocol (IP) is the most widely used of these. Increasing strain is placed on routing technologies to improve processing speeds to meet the demands of continuous traffic growth and multimedia applications. IP at present will not be able to cope with this demand. Future multimedia applications, such as videoconferencing, will also require guaranteed bandwidth per session (i.e. QoS). Today’s Internet is not capable of providing such guarantees. Asynchronous transfer mode (ATM) is a technology that has come about over the last ten years, which utilises switching. Switches have very low latencies, typically one tenth that of routers, thus providing improved performance. One major problem with ATM is that it is not compatible with existing internetwork devices and applications. Most of today’s protocols, such as IP, are based on a connection-less, best-effort technology yet ATM is connection-oriented. It is proposed that the future Internet will need to be a broadband multiservice network that can offer QoS support by integrating ATM switching with IP routing and forwarding. This combines the flexibility of IP routing with the speed of switching. In this paper we describe several new technologies currently proposed to improve router performance.
2. IP Switch Ipsilon’s IP Switch is a device which combines IP software with ATM hardware. The connection-oriented nature of the ATM protocol stack is discarded, removing the signalling and routing protocols. Standard IP software is implemented directly on top of the ATM hardware resulting in a device which provides the flexibility of routing with the high speed capacity of ATM switching hardware. Fig. 1 shows the basic design of the IP Switch.
IP Software
ATM Forum Software
IP Software
MAC Layer Hardware
ATM Hardware
ATM Hardware
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IP Switch
IP Router
Fig. 1 The IP Switch
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The IP Switch can operate as a standard router forwarding packets on a hop-by-hop basis, but also makes a decision whether future IP packets that are part of a particular flow should be switched by the ATM hardware. When a new flow arrives, the IP packet header is examined and the IP Switch identifies longer lasting flows, such as file transfer protocol (FTP) data or multimedia data, and allows them to be switched in hardware. This cuts out the need for network layer IP processing resulting in increased performance. Short lived flows such as network management messages, are forwarded using normal IP processing. Routing decisions are based on IP protocols so IP Switches are fully compatible with existing networks. Support for the next generation of protocols, such as IPv6, is planned. When an IP subnet replaces existing routers with IP Switches the network’s performance will improve, providing a throughput of about 5.3 million packets per second [1]. However, non-IP protocols like IPX, SNA, NetBIOS, DECnet are not supported. IP tunnelling or encapsulation is required to support these. Another disadvantage of the IP Switch is that it is unable to interconnect with standard ATM switches. Internetworking with IP Switches therefore is a problem, which needs further investigation.
3. Cell Switch Router (CSR) Toshiba’s CSR contains both the cell switching fabric and packet switching fabric. High throughput IP packet transmission can be provided using of a cell switching fabric. The CSR can forward some IP packet flows while bypassing the packet assembly/reassembly and IP header processing. Throughput is approximately 40 times larger than that of current high end routers [2]. To perform this ‘cut through’ forwarding CSRs exchange information about how packet flows are aggregated into ATM virtual circuits (VCs). This cut through forwarding reduces both the IP packet processing delay and the queuing delay at the router. Longer life flows are switched by the cell switching fabric based on the value in the cell header (i.e. VPI/VCI). The CSR can support ATM switched networks and can interconnect with all kinds of datalink platforms. The CSR has a multiprotocol capability enabling it to interconnect subnets even if they are of different standards. The CSR can therefore be used not only within subnets to improve performance but also as an internetworking router. It does not suffer from the problems existing routers have with processing IP packets when interconnecting IP over ATM segments. Fig. 2 shows the internetworking of IP over ATM segments through CSRs.
IP Switch Network
CSR
MPOA Network
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RFC1577 LANE Network CSR IP Switch Network
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Fig. 2 Internetworking of IP over ATM segments
4. Tag Switching Cisco’s Tag Switching is a product that works with existing Cisco routers. It improves packet forwarding performance and adds routing functionality to support multicast, allowing more flexible control when routing traffic. This gives network managers better control over a flow of packets within router based internets. It assigns tags to multiprotocol frames for transport across either packet or cell based networks. Each frame carries a short, fixed length label that tells each switching node how to process the frame. At the edge of a Tag switched network there are Tag Edge Routers which assign network labels (tags) to each packet or cell based on its destination. This tag is an index into the routing table and represents a shorthand way for the router to determine what rules it should apply to forward a given packet [3].
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Tag switching can easily be implemented into an ATM switch, making it appear as a router to adjacent routers. Tag switching therefore simplifies the integration of IP and ATM by enabling ATM switches to be fully integrated into Internet core networks. Tag switching addresses QoS by marking packets belonging to a particular class after they have been classified for the first time. This is done by using information stored in the network layer or higher layers. Tags are assigned to the packets allowing them to pass through other tag switching routers without requiring IP processing. This improves performance by simplifying forwarding decisions.
5. Aggregate Route Based IP Switching (ARIS) ARIS is a protocol developed by IBM. It can be implemented into IP subnets and uses the standard routing protocols to establish switched paths through a network. It can be extended to any switching technology but in this case ATM is considered. An Integrated Switch Router (ISR) is an ATM switch implemented with IP routing capability and the ARIS protocol. An ISR at the edge of a network has a next hop routing table that is extended to include a reference to a VC. Each VC may have an endpoint at a neighbouring router or it may transverse a series of ISRs along the best IP forwarding path to an egress ISR [4]. Datagrams can now be switched through hardware in an ISR network. The main advantages of ARIS are: (a) only a few VCs are set up due to its routing base and ability to merge VCs, (b) all IP traffic is switched and (c) it is capable of multiprotocol support and can be extended to other switching environments other than ATM.
6. Fast IP Fast IP is 3Com’s solution in collaboration with IBM and Cascade for providing IP switching technology. It allows desktops and servers in both packet and cell based LANs to have an active role in requesting the services they require. The technology requires no change to existing switches and routers and is simply installed in the desktops and servers. When layer 3 communication begins Fast IP investigates whether there is a faster layer 2 path available. End nodes use the next hop resolution protocol (NHRP) to determine a route to the destination point. If both end systems support Fast IP an end-to-end layer 2 path is established and automatically moves the communication between the desktop and the server over to the faster layer 2 path [5]. If a layer 2 path cannot be established the communication will continue over the original layer 3 path.
7. Switch Node The Switch Node is a device designed by Bay Networks which acts like a ‘routing switch’. It combines the speed and latency of switches with the flexibility and scalability of routers. It is capable of delivering IP and IPX traffic at layer 2 switch speeds. It provides layer 3 switching to existing routers using ‘IP Autolearn’, a method of automatically building forwarding tables for subnets and virtual LANs connected to the Switch Node [6]. It achieves this without having to run a routing protocol enabling transparent integration into existing routers. As these forwarding tables are built, traffic can gradually be off loaded from the router. The benefits of a layer 3 switch is that it is able to forward both layer 2 and layer 3 traffic with the speed of a switch. The Switch Node is not intended to replace existing routers, they do not have to be altered in any way, no configuration or address changes are required. Future real time applications demand high forwarding speeds, low latencies, support for multicast and QoS. All these can be provided by the Switch Node.
8. IP Navigator IP Navigator is an approach from Cascade which combines IP routing and WAN switching. It is capable of providing QoS across an end-to-end IP network WAN connection. It uses multiprotocol gateways at the edge of networks enabling it to be combined with switched networks such as ATM or Frame Relay. A VC is established between end points ensuring end-to-end QoS. Data is then switched through the connection and layer 3 processing is carried out only at the end points. Based on Cascade’s Virtual Network Navigator (VNN), IP Navigator basically takes standard OSPF and adds to it the ability to provide bandwidth control and QoS across Frame Relay and ATM [7]. IP Navigator is simply installed into switches enabling them to operate as IP routers. When installed into existing IP subnets high speed switching is used to forward traffic. The VNN is responsible for establishing VCs using OSPF, and the IP Navigator constructs a routing table. This routing table differs from existing routing tables because it points to the actual destination instead of the next hop. This enables a path to be established between two end points. To maintain QoS for paths already established VNN prevents a new session from being set up if no suitable path exists. IP Navigator also minimises the number of VCs created by introducing multipoint-to-point tunnelling.
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Switches establish multicast circuits to all other switches in the network and data is forwarded over this circuit. Effectively the point-to-multipoint circuit is turned around so that there is essentially only one multicast circuit referenced for any switch destination. No time is wasted in setting up a VC, when IP traffic arrives at a switch, as this multipoint-to-point circuit is pre-established. IP Navigator also provides support for multicast and all existing routing standards enabling it to interwork with devices from the likes of Cisco and Ipsilon.
9. PowerIP RND’s approach to providing high throughput is based on the concept of allowing hosts on different IP subnets to communicate without needing to cross through a router along the way. PowerIP is available in two different models. The PowerIP Engine is designed to transform any LAN switch into an IP switch transforming existing networks. The PowerIP Switch is designed to be deployed as the backbone of networks when they are in the process of being built. PowerIP is based on RND’s Fast Intranet Routed Switching Technology (FIRST) [8]. It implements cut through forwarding over layer 2 switches and can be deployed across any network type. The PowerIP routing technology fully supports IP and IPX therefore it does not require its own routing protocol. When two hosts wish to communicate the fastest route is calculated by PowerIP and then a virtual path is opened. Once a path is set up all packet forwarding is done at layer 2, removing the delays associated with multi-hop routed paths.
10. NetFlow Switching NetFlow Switching is Cisco Systems’ software upgrade for its existing routers. It allows routers to perform high performance network layer switching. IP traffic flows between endpoints of a network are identified and switched on a connection-oriented basis. It identifies flows using both network and transport layer information allowing its Internetwork Operating System (IOS) services to be applied on a per user, per application basis. NetFlow Switching examines the first packet of a flow and all subsequent packets are then handled on a connectionoriented basis [9]. This significantly reduces the amount of IP processing required, improving performance. One advantage of NetFlow Switching is that it enables connection-less networks to provide the performance and services associated with a connection-oriented network. These services include improved security, QoS and traffic accounting.
11. Conclusion We have described how today’s Internet routing technology is not capable of supporting the ever increasing traffic and high bandwidth multimedia applications require. Numerous networking companies are trying to establish their ‘fast routing’ products as a solution for integrating layer 3 routing and layer 2 switching. The future Internet is likely to include some of the technologies presented in this paper, either by replacing the existing routers or by implementing software upgrades. The integration of IP and ATM therefore seems inevitable for providing the desired processing speeds, QoS, and other network services that future applications will demand.
References [1] Ipsilon Networks, “IP Switch ATM 1600 technical specifications”, March 1997. [2] H. Esaki et al., “Cell Switch Router: High performance packet forwarding architecture over internet and Intranet using ATM”, Toshiba Corp., Japan, November 1996. [3] Cisco Systems Inc. “Scaling The Internet With Tag Switching”, February 1997. [4] R. Woundy et al., “ARIS: Aggregate Route Based IP Switching”, IBM Corp., IETF Draft, November 1996. [5] J. Hart “Fast IP: The Foundation For 3-D Networking”, 3Com Corp., January 1997. [6] Bay Networks Inc., “Switch Node”, 1996. [7] Cascade Communications Corp., “White Paper: IP Navigator”, December 1996. [8] RND Networks, “PowerIP-Industry’s Fastest IP Switching Solution”, April 1997. [9] Cisco Systems “NetFlow Switching and Services: Extending Today’s Internetwork to Meet Tomorrow’s Requirements”, April 1996.
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