Inter net-Based Multiprotocol Remote Access In this scenario, the NAS will not answer incoming calls until the home gateway signals an OK. However, this setup can cause some problems when the intervening Internet becomes congested; a user's modem will continue to ring until it times out. From the user's perspective, it appears as if the modem initially didn't answer. Cisco's Route L2F provides similar services, though its technology is more flexible in the way tunneling is implemented. Cisco's L2F doesn't make any assumptions about the underlying transport layer carrying the PPP packets. Rather, it defines a generic encapsulation protocol that carries Layer 2 PPP frames. This system was created for implementation over non-IP based protocols. L2F call negotiation begins with the establishment of the PPP link. The N AS picks up the user name and determines if the call should be routed to a VDPN. If the call goes to a VDPN, the NAS contacts the home gateway and passes PPP frames for call negotiation; otherwise, the call is established at the ISP as normal. VDPN establishment is determined by the user name, including the address of the home gateway. This scheme lets you make more efficient use of your modem pool. ISPs or dial-up vendors benefit from economies of scale--the same modem pool can be shared across multiple connection types, and calls can be aggregated on multiple phone lines. That savings should be passed onto you, the customer. Configuring both the NAS and the home gateway of the L2F tunnel is like installing multiple interfaces in a router. Using a Cisco 4700 for the home gateway and a Cisco AS 5200 for the NAS, we created virtual templates that routed L2F packets from one device to another like other network traffic. The address pool in our test resided on the 4700, but could also have come from Terminal Access Control Access System+ (TACACS+), if necessary. Merge Ahead L2TP is a new IETF draft from the combined efforts of Ascend , Cisco, Microsoft and 3Com to bring together the best of PPTP and L2F. L2TP is still in the draft phase, with interoperability testing expected to begin in May. The standard should be ready for adoption by the fall, with products rolling soon after. L2TP incorporates the dynamic tunnel creation on information contained in the PPP Link Control Protocol (LCP) packets, such as user name. Using a RADIUS server, you can define any number of parameters for use in establishing VDPN tunnels. L2TP removes PPTP's reliance on the IP protocol suite by abstracting the underlying transport protocol to any packetized protocol, such as IP, X.25 or frame relay. The specific network protocol is left up to vendor implementation. L2TP also incorporates flow control between the NAS and the home gateway, helping to reduce network traffic and letting servers handle congestion. The Roa d Home VDPN is very attractive for large corporations looking to outsource their dial-up connections while maintaining network-access security. Configuring VDPN tunnels is similar to configuring remote-access ports, except you don't have to configure racks of modems. In the Cisco 4700, the home gateway is configured like any router interface. Anyone familiar with IOS, Cisco's Internetwork Operating System, will be able to configure both the NAS and the home gateway virtual interfaces. Some nationwide service providers, such as CompuServe, can provide you with end-to-end VDPN solutions. In most cases, the service provider will manage both the home gateway in addition to the NAS as customer premise equipment. You manage user access. Of course, you do give up a certain amount of control over your remote access. If your vendor can't deliver adequate quality of service, or if it oversells its bandwidth, your performance will suffer. In deciding whether this technology is right for you, consider the cos ts of potentially reduced service versus the higher costs of total control. Mike Fratto is a independent network consultant in the central New York area. He can be reached at mafratto@syr.edu.

Updated April 8, 1997