The following advanced kernel features help make PC/TCP and PC/TCP OnNet both the most reliable and the most versatile of the TCP/IP-for-PCs packages on the market today:
As a data packet traverses a wide area network, it must pass through various transmission media, each of which allows a different maximum packet size (also called maximum transmission unit). MTU discovery finds the right packet size to fit all the different media.
For example, the largest packet on a Token Ring can be 1962 bytes, but the largest packet on an Ethernet LAN is 1460 bytes. When a big packet on a Token Ring gets switched onto an Ethernet, the router between the two networks must subdivide the single packet from the Token Ring into two packets for the Ethernet, and then it must re-transmit them both. This causes a delay for each oversized packet sent, and there may be hundreds in a typical data transmission.
Fortunately, a router can return to the sending host a warning: an Internet Control Message Protocol (ICMP) notification of oversized packets. Every router understands the MTU Discovery protocol (based on RFC 1191). When a router encounters an oversized packet, it sends an ICMP packet to the sending host, telling it that oversized packets will always be fragmented unless the host sends all future packets with a size smaller than the MTU. If the sending host can understand the ICMP message (and not all non-PC/TCP hosts can), then the delay is limited to the transmission of one packet, and one packet only.
If the host sending oversized packets is using PC/TCP or PC/TCP OnNet, then the Kernel reacts to the ICMP message by sending smaller packets. Now, the router does not have to fragment and reassemble every packet coming from our host, and data transfer performance improves substantially.
You can manually configure both the PC/TCP Kernel and the PC/TCP OnNet Kernel to use up to three routers or gateways. The Kernel, running the Gateway Fallback protocol (as defined in RFC 1122) can respond to a failure in its primary router by attempting to switch transmissions across secondary routers.
If the primary router fails in any way, the application on the PC/TCP host is unaffected because the Kernel identifies the failure of the main router and begins to use the second or third (if need be) router in its configuration. In fact, Gateway Fallback may attempt to use additional routers, identified through Router Discovery.
Gateway Fallback increases the fault tolerance of the TCP/IP network; its use lessens the need for reconfiguration of client PCs when the network router configuration changes. This leads to long term reductions in the administrative cost of operating a TCP/IP network.
You can configure both the PC/TCP Kernel and the PC/TCP OnNet Kernel to use the Router Discovery option (based on RFC 1256). With Router Discovery set, the Kernel at startup sends out a Router Discovery ICMP packet that essentially says: "If you receive this and you're a router, please identify yourself to the sending host."
When the Kernel receives the router responses, it automatically compares their addresses to those already configured manually. If the Kernel discovers extra routers are available, it automatically configures up to three additional routers.
When the primary configured router fails for any reason, our Kernel (using Router Discovery as a supplemental part of its Gateway Fallback function) attempts to use up to 5 backup routers: 2 "static" (configured) routers plus 3 "ephemeral" (discovered) routers. Note that this is a maximum number; the actual number depends on user configuration and the availability of routers.
This means that end users do not have to worry about manually updating their initial router configurations, because Router Discovery automatically configures up to 3 extra routers. The likelihood that all static and all ephemeral routers will simultaneously become inaccessible is extremely small.
Router Discovery helps not only the end user but also the system administrator, who can change routers at will and not have to worry about the connectivity of end users. In supplementing other Gateway Fallback features, Router Discovery dramatically lowers the administrative cost of operating a TCP/IP network.
IP Multicast (as defined in RFC 1112) uses a special kind of IP address to send data streams (video, voice, etc.) to multiple recipients. Broadcasting beyond the local subnet (except broadcasting mail to a list of recipients) is a little unusual; ordinarily, TCP/IP applications consist of point to point connections, and normally the only messages that are truly sent via broadcast are control messages that are limited to the Local Area Network.
Although routers on a private network isolated from the Internet can still use IP Multicast, the primary use of IP Multicast occurs on the Multicast Backbone of the Internet. Users of this type of IP Multicast must have access to one of those subnets on the worldwide Internet that comprise the Multicast Backbone or MBone; 750 such subnets exist as of January 1994. In order to have access to the MBone, a user must agree to provide routers that support multicast routing.
Currently the primary tools that use IP Multicast are NV (net video), VAT (visual audio tool), and WB (whiteboard). These tools are freely available for a variety of platforms from sources throughout the Internet. Some commercial applications also take advantage of IP Multicast capabilities, such as video conferencing and document sharing applications. This adds multi-user and work group capabilities to existing network applications. For example, video conferencing applications can now have any number of participants. Dozens of other applications using this new broadcast technology are likely to emerge over the coming years.
FTP Software is the first to offer a TCP/IP-for-PCs package that supports IP Multicast. While other vendors are beginning to follow our example, we have the most experience with IP Multicast from a PC, and it shows in the reliability of our Kernel when used as a platform for multicast applications.
No other TCP/IP-for-the-PC package on the market today offers all these advanced features. Not only is PC/TCP and PC/TCP OnNet the most diverse in user applications, the most improved in speed of response, and the most user-accessible package available, it is also the most versatile in its reliability. It's there when you need it!
For more information, please contact your sales representative.
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29-Nov-94
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