NetcomRouter is an add-on product for the following TSG products:
Netcom II
NetcomRouter is an LLI Streams driver which allows X.25 Switched Virtual Circuits or to carry TCP/IP traffic.
Normally, NetcomRouter is invisible to the UNIX user. To keep it that way, someone has to spend some time installing and configuring the software. This manual describes network design issues and how to make changes to NetcomRouter so that it keeps pace with your network as it evolves and changes.
NetcomRouter conforms to RFC 877, "A Standard for the Transmission of IP Datagrams over Public Data Networks" making it compatible with all other equipment conforming to that specification. NetcomRouter also conforms to the X.25-specific portions of RFC 1356, "Multi-protocol Interconnect on X.25 and ISDN in the Packet Mode", which replaces RFC 877. NetcomRouter can interchange information with other systems using the algorithms defined in RFC 1356, notably Cisco® Systems Gateway devices.
NetcomRouter allows your site to act as a Level III router. That is, it operates at Level III of the OSI reference model. With NetcomRouter, some of the Switched Virtual Circuits on an X.25 data link become available for carrying IP traffic.
NetcomRouter addresses two common communications requirements:
workstation connection
NetcomRouter can be installed on a single workstation to provide a small office with connectivity to corporate TCP/IP network resources. In this environment, an X.25 data link replaces the traditional Ethernet connection; all the workstation's TCP/IP traffic is carried on one or more X.25connections.
gateway
NetcomRouter acts as a gateway between TCP/IP networks. Equipping a machine with an Ethernet interface (or other network medium), as well as NetcomRouter, gives workstations on that network the ability to exchange information with other network-connected TCP/IP networks or workstations.
NetcomRouter offers the following features:
Several IP addresses can be associated with one physical link.
Often in TCP/IP networks, a local IP address and IP network are assigned to each physical interface. This is done with an ifconfig command. However, with NetcomRouter devices, the IP address and the /etc/x25hosts file determine the board and link number used, separate from the xinet interface used. That is, the NetcomRouter devices represent logical interfaces; they are not directly associated with any physical connection. It is therefore possible to have several IP addresses (that is, several ifconfig statements) associated with one physical link. Likewise, it is possible to have one ifconfig statement that sets one IP address for several physical links.
The user can control the number of virtual circuits to be used for each IP connection. When packet and window sizes are limited (such as the 128/2 default in most public data networks), the ability to use more than one virtual circuit for each IP connection increases throughput considerably.
NetcomRouter automatically maintains X.25 connections. It establishes new virtual circuits (to a user-specified limit) whenever it cannot send IP packets because of X.25 flow control. It disconnects virtual circuits that have been idle for a period of time.
NetcomRouter normally uses statically mapped X.121 addresses when connecting virtual circuits. The user can select between statically mapped and DDN-style X.121 address generation. With static mapping, X.121 addresses are user-specified and assigned by the packet switch manager. DDN-style addresses are generated automatically from the internet address of an interface.
Note: X.121 is the CCITT specification for the structure of addresses on a data network, and is the technically correct term for the address field. However, X.25 address is a more familiar term, and so is used throughout the rest of this manual.
Using large IP packets avoids spending CPU cycles and buffer space segmenting and reassembling packets. However, some network bridges and gateways may be unable to carry large IP packets. For example, RFC 1356 specifies a maximum IP packet size of 1500 bytes, which ensures compatibility with all gateways on the Internet.
The default maximum IP packet size for NetcomRouter is 1500 bytes. You may change it to better suit your networking environment. Generally, the larger the better, subject to limits imposed by the streams buffer resources in your kernel, and the other equipment being used in your network.
Note that IP segment length affects response time. Using a large IP segment length, while reducing IP header overheads, can also cause perceptible delays. It can take appreciable time to assemble a large IP segment from a sequence of full X.25 data packets travelling over a slow (9600 bps) link.
As installed, NetcomRouter limits IP communications to hosts whose X.25 addresses are stored in its table of X.25/IP address equivalences. NetcomRouter will not accept calls when the Incoming Call packet does not contain a Calling DNA field which is present in the table (or when the Calling DNA field is not present in the Incoming Call packet). For proper operation in a point-to-point or test environment, configure the X.25 link to supply a Calling DNA in Call Request packets.
In a public network X.32 environment where the Calling DNA is random (as a result of public dial ports being part of a telephone hunt group), a system receiving a call will not recognize the Calling DNA, and thus will not accept an incoming call. Identifying Hosts That Call You describes how NetcomRouter can be configured to avoid this problem.
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