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Network Working Group W A Simpson
Internet Draft Daydreamer
expires in six months March 1993
PPP over X.25
Status of this Memo
This memo is the product of the Point-to-Point Protocol Working Group
of the Internet Engineering Task Force (IETF). Comments on this memo
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 of
encapsulating Network Layer protocol information over point-to-point
links.
This document defines a method for using PPP to transport multi-
protocol datagrams over X.25 circuits.
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1. Introduction
PPP has three main components:
1. A method for encapsulating datagrams over serial links.
2. A Link Control Protocol (LCP) for establishing, configuring,
and testing the data link connection.
3. A family of Network Control Protocols (NCPs) for establishing
and configuring different network layer protocols.
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.
One protocol to carry them all.
One protocol to mind them.
One protocol to link them all,
and in the network bind them.
PPP was intended to replace X.25. There are still ISO-lated pockets
of existing X.25 links, and some interest in bringing the advantages
of the PPP multi-protocol datagram service to this venue. When X.25
emulates a point-to-point circuit, PPP is well suited to use over
X.25.
2. Encapsulation
PPP provides an encapsulation protocol over both bit-oriented
synchronous links and asynchronous links with 8 bits of data and no
parity. These links MUST be full-duplex, but MAY be either dedicated
or circuit-switched. This fits the X.25 model.
PPP uses HDLC [2] as a basis for the default encapsulation. X.25 is
also in the family of HDLC derivatives, and the X.25 header may be
easily substituted for the smaller HDLC header.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+
| Flag (0x7e) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address | Control |D|Q| SVC# (hi) | SVC# (lo) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|p(r) |M|p(s) |0| Pad (0) | PPP Protocol |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Unfortunately, the X.25 header is 5 octets in length. Therefore, a
single octet of zero padding is used to align the header to a more
convenient boundary. The use of this zero padding is conformant with
both the ISO Network Layer Protocol Identifier (NLPID) Null Network
layer, and the PPP protocol field extension mechanism.
LCP negotiation MAY permit the Pad and Protocol fields to be
compressed to a single octet.
3. In-Band Protocol Detection
When Out-of-Band signaling is not used to configure call setup for
the circuit, or the Null encapsulation is indicated, the PPP Protocol
field may be easily distinguished from other NLPID values. Initial
LCP packets will contain the sequence 00-c0-21 following the header.
Older implementations [3] might contain the NLPID value CC hex.
Other ISO conformant implementations might contain other NLPID
values, such as 80 hex (SNAP), or 81 hex (CLNP). Such packets
indicate that the link is not properly configured for PPP operation,
and MUST generate a Protocol-Reject.
4. Out-of-Band signaling
The first octet in the Call User Data (CUD) Field (the first data
octet in the Call Request packet) is used for protocol
demultiplexing, in accordance with the Subsequent Protocol Identifier
(SPI) in ISO/IEC TR 9577. This field contains a one octet Network
Layer Protocol Identifier (NLPID), which identifies the network layer
protocol encapsulated over the X.25 virtual circuit. The CUD field
MAY contain more than one octet of information, and receivers MUST
ignore all extraneous octets in the field.
The PPP encapsulation SHOULD be indicated by a value of C0 hex. When
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DRAFT PPP over X.25 March 1993
specifically configured to do so, the implementation MAY also accept
a value of 00.
Other values of the CUD are beyond the scope of this specification.
5. Configuration Details
The standard LCP configuration defaults apply to X.25 links.
The following Configurations Options are recommended:
Magic Number
Link Quality Monitoring
Address and Control Field Compression
Protocol Field Compression
A maximum PDU size of 1600 is commonly available. This translates to
a PPP MRU size of 1598. Since the typical network feeding the link
is unlikely to have a MRU of greater than 1500, it is not expected to
be worth the trouble to negotiate a higher MRU.
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DRAFT PPP over X.25 March 1993
Security Considerations
Security issues are not discussed in this memo.
References
[1] Simpson, W. A., "The Point-to-Point Protocol", RFC 1331, May
1992.
[2] International Organization For Standardization, ISO Standard
3309-1979, "Data communication - High-level data link control
procedures - Frame structure", 1979.
[3] Malis, A., Robinson, D., Ullman R., "Multiprotocol Interconnect
on X.25 and ISDN in the Packet Mode", RFC 1356, August 1992.
Acknowledgments
Chair's Address
The working group can be contacted via the current chair:
Brian Lloyd
B.P. Lloyd & Associates
3420 Sudbury Road
Cameron Park, California 95682
Phone: (916) 676-1147
EMail: brian@lloyd.com
Author's Address
Questions about this memo can also be directed to:
William Allen Simpson
Daydreamer
Computer Systems Consulting Services
P O Box 6205
East Lansing, MI 48826-6205
EMail: Bill.Simpson@um.cc.umich.edu
Simpson expires in six months [Page 4]
DRAFT PPP over X.25 March 1993
Table of Contents
1. Introduction .......................................... 1
2. Encapsulation ......................................... 1
3. In-Band Protocol Detection ............................ 2
4. Out-of-Band signaling ................................. 2
5. Configuration Details ................................. 3
SECURITY CONSIDERATIONS ...................................... 4
REFERENCES ................................................... 4
ACKNOWLEDGEMENTS ............................................. 4
CHAIR'S ADDRESS .............................................. 4
AUTHOR'S ADDRESS ............................................. 4
Bill.Simpson@um.cc.umich.edu