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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.
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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].
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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.
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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
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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.
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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.
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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
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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