Network Working Group W A Simpson Internet Draft Daydreamer expires in six months August 1993 PPP over Circuit-Switched ISDN Status of this Memo This document is the product of the Point-to-Point Protocol Working Group of the Internet Engineering Task Force (IETF). Comments should be submitted to the ietf-ppp@ucdavis.edu mailing list. Distribution of this memo is unlimited. This document is an Internet Draft. Internet Drafts are working documents of the Internet Engineering Task Force (IETF), its Areas, and its Working Groups. Note that other groups may also distribute working documents as Internet Drafts. Internet Drafts are draft documents valid for a maximum of six months. Internet Drafts may be updated, replaced, or obsoleted by other documents at any time. It is not appropriate to use Internet Drafts as reference material or to cite them other than as a ``working draft'' or ``work in progress.'' Please check the 1id-abstracts.txt listing contained in the internet-drafts Shadow Directories on nic.ddn.mil, nnsc.nsf.net, nic.nordu.net, ftp.nisc.sri.com, or munnari.oz.au to learn the current status of any Internet Draft. Abstract The Point-to-Point Protocol (PPP) [1] provides a standard method for transporting multi-protocol datagrams over point-to-point links. This document describes the use of PPP over Integrated Services Digital Network (ISDN) switched circuits. Simpson expires in six months [Page i] DRAFT PPP over ISDN August 1993 1. Introduction PPP was designed as a standard method of communicating over point- to-point links. Initial deployment has been over short local lines, leased lines, and plain-old-telephone-service (POTS) using modems. As new packet services and higher speed lines are introduced, PPP is easily deployed in these environments as well. This specification is primarily concerned with the use of the PPP encapsulation over ISDN B-channel links. Since the ISDN B-channel is by definition a point-to-point circuit, PPP is well suited to use over ISDN B-channels. The ISDN Primary Rate Interface (PRI) may support hundreds of concurrent B-channel links. The PPP LCP and NCP mechanisms are particularly useful in this situation in reducing or eliminating hand configuration, and facilitating ease of communication between diverse implementations. The ISDN D-channel can also be used for sending packets, but is limited in bandwidth and often restricts communication links to a local switch. 2. Physical Layer Requirements PPP treats ISDN channels as bit-oriented synchronous links. These links MUST be full-duplex, but MAY be either dedicated or circuit- switched. Interface Format PPP presents an octet interface to the physical layer. There is no provision for sub-octets to be supplied or accepted. Transmission Rate PPP does not impose any restrictions regarding transmission rate, other than that of the particular ISDN channel interface. Control Signals PPP does not require the use of control signals. When available, using such signals can allow greater functionality and performance. Implications are discussed in [3]. Simpson expires in six months [Page 1] DRAFT PPP over ISDN August 1993 Encoding By default, it is recommended that a bit-synchronous NRZ encoding be used directly over the ISDN B-channel interface. By prior configuration, PPP may also be used with NRZI encoding, and with asynchronous terminal adapters conforming to V.120 [2]. Due to the use of these alternate encoding techniques, it is possible to fail to interoperate. Those implementations which desire to interoperate with multiple encodings MAY choose to detect those encodings automatically. The only reliable method of detection available is to switch modes between the various encodings. The mode switch SHOULD occur at twice the Restart Timer interval; that is, if the interval is 300 milliseconds, the mode would be switched each 600 milliseconds. Max-Configure MUST be set such that the cumulative attempts result in no more than 59 seconds of time before disconnect. It is preferable that the default limit of 30 seconds be observed. Simpson expires in six months [Page 2] DRAFT PPP over ISDN August 1993 3. Framing For B-channels, in the absence of prior configuration, the implementation MUST first use "PPP HDLC Framing" [3], as opposed to other framings, for initial link establishment. This assumes that circuit-switched communications are generally [host | router] to [host | router]. The implementation MAY change to X.25 or Frame Relay framing, when those frames are detected. For D-channels, by default no data service is expected. By prior configuration, "PPP in X.25" [4] or "PPP in Frame Relay" [5] framing MAY be used. Despite the fact that HDLC, LAPB, LAPD, and LAPF are nominally distinguishable, multiple methods of framing SHOULD NOT be used concurrently on the same link. 4. In-Band Format Detection It is possible to connect an ISDN circuit to an X.25 or Frame Relay circuit. This results in the receipt of a different type of frame. The PPP HDLC framing is easily distinguished from X.25 or Frame Relay frames. Initial PPP frames always start with the sequence ff-03-c0- 21. Initial X.25 frames will likely have a first octet of 1 or 3. If an X.25 frame is detected, the implementation MUST either change to the "PPP in X.25" [4] framing method, or report a link misconfiguration. Initial Frame Relay frames have a first octet which is even (the least significant bit is 0). If a Frame Relay frame is detected, the implementation MUST either change to the "PPP in Frame Relay" [5] framing method, or report a link misconfiguration. The accidental connection of an ISDN link to feed an X.25 or Frame Relay multipoint network MUST result in a misconfiguration indication. This can be detected by multiple responses to the LCP Configure-Request with the same Identifier, coming from different framing addresses. 5. Out-of-Band signaling Experience has shown that the LLC Information Element is not reliably transmitted end to end. The deployment of compatible switches is too limited, and the subscription policies of the providers are too Simpson expires in six months [Page 3] DRAFT PPP over ISDN August 1993 diverse. No values which pertain to PPP have been assigned, as yet. Therefore, transmission of the LLC-IE SHOULD NOT be relied upon. 6. Configuration Details For 64 Kbps links, the Restart Timer is recommended to be 300 milliseconds, and Max-Configure is recommended to be 100 attempts. This results in 30 seconds before failure is detected. The LCP recommended sync configuration options apply to ISDN links. The standard LCP sync configuration defaults apply to ISDN links. The typical network feeding the link is likely to have a MRU of either 1500, or 2048 or greater. To avoid fragmentation, the Maximum-Transmission-Unit (MTU) at the network layer SHOULD NOT exceed 1500, unless a peer MRU of 2048 or greater is specifically negotiated. Simpson expires in six months [Page 4] DRAFT PPP over ISDN August 1993 Security Considerations Security issues are not discussed in this memo. References [1] Simpson, W. A., "The Point-to-Point Protocol (PPP)", work in progress. [2] CCITT, "Recommendation V.120: Data Communications over the Telephone Network", Blue Book, ITU 1988 [3] Simpson, W.A., "PPP HLDC Framing", work in progress. [4] Simpson, W.A., "PPP in X.25", work in progress. [5] Simpson, W.A., "PPP in Frame Relay", work in progress. Acknowledgments This design was inspired by the paper "Parameter Negotiation for the Multiprotocol Interconnect", Keith Sklower and Clifford Frost, University of California, Berkeley, 1992, unpublished. Other details were gleaned from "Determination of Encapsulation of Multi-protocol Datagrams in Circuit-switched Environments", Keith Sklower, University of California, Berkeley, IETF IPLPDN WG draft, July 1993. That paper credits previous work "A Subnetwork Control Protocol for ISDN Circuit-Switching", Leifer, D., Sheldon, S. and Gorsline, B.; IETF IPLPDN WG draft, March 1991; and "A Negotiation Protocol for Multiple Link-Protocol over ISDN Circuit-Switching", Muramaki, K. and Sugawara, T.; IETF IPLPDN WG draft, May 1992. Simpson expires in six months [Page 5] DRAFT PPP over ISDN August 1993 Chair's Address The working group can be contacted via the current chair: Fred Baker Advanced Computer Communications 315 Bollay Drive Santa Barbara, California, 93111 EMail: fbaker@acc.com Author's Address Questions about this memo can also be directed to: William Allen Simpson Daydreamer Computer Systems Consulting Services 1384 Fontaine Madison Heights, Michigan 48071 EMail: Bill.Simpson@um.cc.umich.edu Simpson expires in six months [Page 6] DRAFT PPP over ISDN August 1993 Table of Contents 1. Introduction .......................................... 1 2. Physical Layer Requirements ........................... 1 3. Framing ............................................... 3 4. In-Band Format Detection .............................. 3 5. Out-of-Band signaling ................................. 3 6. Configuration Details ................................. 4 SECURITY CONSIDERATIONS ...................................... 5 REFERENCES ................................................... 5 ACKNOWLEDGEMENTS ............................................. 5 CHAIR'S ADDRESS .............................................. 6 AUTHOR'S ADDRESS ............................................. 6