8. Questions about Serial Line Internet Protocol (SLIP) and Point-to-Point Protocol (PPP) Implementations

8.1. I seem to recall hearing that SLIP [8] will only run on synchronous serial lines.

Is this true? ... is there something about SLIP which precludes it's being implemented over async lines?

Other way around: SLIP is designed for async lines and is not a good fit on sync lines. PPP [9, 10(-> 16(-> 24))] works on either, and is what you should be implementing if you're implementing something.

8.2. Since we are very interested in standards in this area, could someone tell me were I can find more information on PPP?

Also, can this protocol be used in other fields than for the Internet (i.e., telecontrol, telemetering) where we see a profusion of proprietary incompatible and hard to maintain Point-to-Point Protocols?

PPP was designed to be useful for many protocols besides just IP. Whether it would be useful for your particular application should probably be discussed with the IETF's Point-to-Point Protocol Working Group discussion list. For general discussion: ietf- ppp@ucdavis.edu. To subscribe: ietf-ppp-request@ucdavis.edu

The PPP specification is available as RFC 1171(-> 1331(-> 1548(-> 1661^std51))) [9], and a PPP options specification is available as RFC 1172(-> 1332^prop | 1331(-> 1548(-> 1661^std51))) [10].

In UnixWorld of April 1990 (Vol. VII, No. 4, Pg. 85), Howard Baldwin writes:

"Point-to-Point Protocol (PPP) has just been submitted to the CCITT from the Internet Engineering Task Force. It specifies a standard for encapsulating Internet Protocol data and other network layer (level three on ISO's OSI Model) protocol information over point-to-point links; it also provides ways to test and configure lines and the upper level protocols on the OSI Model. The only requirement is a provision of a duplex circuit either dedicated or switched, that can operate in either an asynchronous or synchronous mode, transparent to the data-linklayer frame.

"According to Michael Ballard, director of network systems for Telebit, PPP is a direct improvement upon Serial Line Internet Protocol (SLIP), which had neither error correction nor a way to exchange network address."

8.3. Does anyone know if there is a way to run a SLIP program on a IBM computer running SCO Xenix/Unix, with a multi-port serial board?

SCO TCP/IP for Xenix supports SLIP. It works. However, be warned: SCO SLIP works *only* with SCO serial drivers, so it will *not* work with intelligent boards that come with their own drivers. If you want lots of SLIP ports, you'll need lots of dumb ports, perhaps with a multi-dumb-port board.

Here's the setup -- SunOS 3.5, with the 4.3BSD TCP, IP & SLIP distributions installed. Slip is running between the "ttya" ports of two Sun 3/60's. "ping", "rlogin", etc., works fine, but a NFS mount results in "server not responding: RPC Timed Out".

SunOS 3.5 turns the UDP checksum off, which is legal and works okay over interfaces such as ethernet which has link- level checksumming. On the other hand, SLIP doesn't perform checksums thus running NFS over SLIP requires you to turn the UDP checksum on. Otherwise, you'll experience erratic behavior such as the one described above.

         Save the older kernel and try,

            % adb -k -w /vmunix /dev/kmem udpcksum?w 1

         to patch up the kernel.