Date: Sun, 12 Jan 97 20:27 CST From: uhclem@nemesis.lonestar.org (Frank Durda IV) To: freebsd-current@freebsd.org Cc: uhclem@nemesis.lonestar.org Subject: Re: User ppp not hanging up modem. Message-ID: <m0vjc7J-000tx1C@nemesis.lonestar.org>
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As Nate Williams wrote: [5]That's *NOT* the correct thing. What happens when for some reason [5]PPP happens to send the sequence '+++' to the modem? All of a sudden [5]it'll drop into command mode and you're screwed. User-PPP (as well as [5]all other PPP/SLIP implementations I've worked with) assumes that you've [5]disabled the escape sequence at least temporarily. [7]From: J Wunsch <j@uriah.heep.sax.de> [7]That's why they protect it by requiring 500 ms spaces before and after [7]the +++. I think AT&T even has a patent on this escaping scheme. Actually that is a Hayes patent, and the royalties are so high, many modem vendors DO NOT implement the pauses. There are other schemes, such as the Time Independent Escape Sequence (TIES) used by a dozen different modem vendors to avoid paying Hayes. TIES looks for this sequence with no timing requirements at all: 1. Any character other than the escape character. 2. Three of the escape characters. (Modem returns OK) 3. "AT" followed by 0x0d. (Modem returns OK and enters Command Mode). Ventel is the official keeper of the spec and will send you a copy if you are interested. They have an agreement they want you to sign, but all it says is that if you build any modems using TIES, you agree to comply with the latest revision of the specification. That's it. Despite the lack of timing pauses, TIES did pretty good compared to Hayes IESWGT system. Frequently TIES did better. Then there is one used by PerfectData/EasyData, which relies on anagram patterns, (+++ is considered a three character anagram but you can specify longer ones) and it is really easy have screw up. Some modems (from big vendors) don't even implement the Hayes escape sequence correctly, and scan characters coming from the DCE end when looking for the escape sequence. A great hackers tool. Disabling the in-band escape sequence and using DTR is the only safe way to avoid accidential "escapes" into Command Mode based on traffic content. Frank Durda IV <uhclem@nemesis.lonestar.org>|"The Knights who say "LETNi" or uhclem%nemesis@rwsystr.nkn.net | demand... A SEGMENT REGISTER!!!" |"A what?" or ...letni!rwsys!nemesis!uhclem |"LETNi! LETNi! LETNi!" - 1983
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