NOTE: I have just overhauled this page in an attempt to bring it up to date. I have not been at UNC since 1995, so I have very little knowledge of the present situation regarding ISIS and the dialup lines. Please correct me if any of the tips on this page are inaccurate. Thanks, and I hope folks find this page useful. -- Will (07-SEP-97)

Using PPP emulators at UNC

DISCLAIMER: The UNC Office of Information Technology (OIT) in no way endorse any PPP emulator, and this page should in no way be construed as an endorsement of any PPP emulator by OIT or UNC. OIT does not support PPP emulators, and OIT will not assist users experiencing problems with PPP emulators.

1. What is a PPP emulator?

A PPP emulator is a program that allows users of UNIX (or UNIX-clone) shell accounts to run a pseudo-PPP (Point to Point Protocol) link through their shell accounts. PPP emulators thus give users the flexibility of using TCP/IP programs written for PCs and Macintosh computers that would ordinarily require a hardware connection to the Internet or access to a real PPP account through an ISP while also allowing users to use the features of their shell accounts. More importantly for ISIS users, it allows you to run a PPP connection through your ISIS email account instead of contracting with a local ISP for PPP access, which is the method UNC would prefer you use to access the Internet with PPP. PPP emulators are used in conjunction with a TCP/IP stack that has PPP support, such as Trumpet Winsock or the Dial-Up Networking feature of Windows 95. Once installed and running, a PPP emulator provides a virtual onramp to the Information Highway, making it possible to use programs such as Netscape and Microsoft's Internet Explorer that would otherwise require a standard PPP account.

2. What PPP emulators are currently available?

SLiRP
SLiRP is a free SLIP/CSLIP/PPP emulator that appeared shortly after the debut of TIA in 1995, which was the first SLIP/CSLIP emulator to appear on the scene (TIA is no longer available). SLiRP has matured into a robust and reliable piece of software, so do not let its "freeness" turn you off. PPP support was added some time ago, and although I have only used PPP with SLiRP once or twice myself, I was impressed with how well SLiRP emulated the protocol.

3. How do I get SLiRP?

You can download the source code for the latest version of SLiRP from the SLiRP Homepage There are several sites from which you can download SLiRP listed on the page. You can either download the source code for SLiRP or you can download one of the pre-compiled binaries, which is a good idea if you are not familiar with compiling programs on a UNIX machine. Precompiled binaries can be found on www.ibc.wust l.edu in the USE_AT_YOUR_OWN_RISK directory. ISIS runs IBM AIX, so get the file with the name like slirp.10b.aix41.tar.Z (version 1.0b was the most recent version of the program as of the time of writing. To use Lynx to download SLiRP , highlight the AIX binary, and hit "D" to download. Go out to the command prompt and uncompress the file with command ("#" is your command prompt):

# uncompress slirp.10b.aix41.tar.Z

Then untar it with the command:

# tar -xvf slirp.10b.aix41.tar

This will create a directory hirerarchy called slirp in your home directory. The SLiRP binary can be found in the slirp/bin directory.

4. Great! Now I have SLiRP installed on my ISIS account. What do I need to do to run it properly?

In order to use SLiRP properly, you will have to log into ISIS and enter a sequence of commands to set up the dialup port to pass-through TCP/IP frames, which often contain 8-bit data, without editing or bit-stripping.

Dial into the UNC Broadband as you normally would.

NOTE: If you are dialing in with a terminal program, make sure the modem configuration string does not contain a command to hang up when the TR line drops. Check your modem's manual on the exact command to do this and make any necessary changes to the modem configuration string in your terminal software package.

Select option 18 (expert mode) from the dialup menu and press ENTER. You should then see the "OIT INET>" prompt.

Enter the following command sequence at the "OIT INET>" prompt:
set port local %
connect email.unc.edu

Log into your ISIS account as usual and go to a shell command prompt with menu option 6 on the ISIS menu (assuming you have not already disabled the !@%@%$@ menu!). At the shell command prompt, invoke your SLIP/CSLIP emulator by cding into the slirp/bin directory and entering
# ./slirp

If all goes well, you should see your emulator's startup message and an instruction to start your SLIP software. When you see this message, press the "%" key, and you should return to the "OIT INET>" prompt. When you are back at this prompt, enter the following commands:
set session passall
fg
You should now see a message that you are now reconnected to ISIS. At this point you can start your SLIP software on your computer.

4. What parameters should I use when configuring my TCP/IP stack to use with SLiRP?

As far as your IP address, you can use just about any IP address you want with SLiRP since the emulator uses ISIS's IP address when it sends TCP/IP frames out onto the Internet. SLiRP recommends that you use an address from the 10.0.2.x block, since 10.x.x.x addresses are reserved for private internets. As far as the other stack parameters, conventional wisdom says that smaller is generally better for serial links such as PPP connections. Here are some "optimum" parameters for Trumpet Winsock and a 14.4/28.8 kbaud modem someone posted awhile back:

MTU: 576
Rwin: 2144
MSS: 536

I have gotten away with using the following parameters on my own PPP link:

MTU: 1500
Rwin: 4096
MSS: 1024

Also, SLiRP supports SLIP and CSLIP, which are older protocols which perform the same function as PPP. PPP is now the standard, so I suggest you stick with it.

IP address of one of the nameservers at UNC is 152.2.21.1 (hermes.oit.unc.edu). Enter that IP address into the Nameserver field in your TCP/IP stack.

J. William Snyder, Jr. ())
Last modified: May 20, 2005

Back to Will's Home Page