Network Working Group W A Simpson [DayDreamer]
Internet Draft
expires in six months August 1998
PPP in X.25
draft-ietf-pppext-x25-ds-00.txt
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Copyright Notice
Copyright (C) William Allen Simpson (1993-1994, 1996-1998). All
Rights Reserved.
Abstract
The Point-to-Point Protocol (PPP) [RFC-1661] provides a standard
method for transporting multi-protocol datagrams over point-to-point
links. This document describes the use of X.25 for framing PPP
encapsulated packets.
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Applicability
This specification is intended for those implementations that desire
to use facilities which are defined for PPP, such as the Link Control
Protocol, Network-layer Control Protocols, authentication, and com-
pression. These capabilities require a point-to-point relationship
between peers, and are not designed for multi-point or multi-access
environments.
Use of X.25 as a secondary framing mechanism (for example, with asyn-
chronous HDLC-like frames tunnelled through X.3) is outside the scope
of this specification.
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1. Introduction
CCITT recommendation X.25 [X.25] describes a network layer protocol
providing error-free, sequenced, flow controlled, virtual circuits.
X.25 includes a data link layer, X.25 LAPB, which uses ISO 3309, 4335
and 6256.
At one time, it had been hoped that "PPP in HDLC-like Framing"
[RFC-1662] would co-exist with other X.25 transmissions on the same
links. Equipment could gradually be converted to PPP. Subsequently,
it has been learned that some switches actually remove the X.25
header, transport packets to another switch using a different proto-
col such as Frame Relay, and reconstruct the X.25 header at the final
hop. Co-existance and gradual migration are precluded.
When X.25 is configured as a point-to-point circuit, PPP can use X.25
as a framing mechanism, ignoring its other features. This is equiva-
lent to the technique used to carry SNAP headers over X.25
[RFC-1356].
1.1. Terminology
In this document, the key words "MAY", "MUST, "MUST NOT", "optional",
"recommended", "SHOULD", and "SHOULD NOT", are to be interpreted as
described in [RFC-2119].
To remain consistent with standard Internet practice, and avoid con-
fusion for people used to reading RFCs, all binary numbers in the
following descriptions are in Most Significant Bit to Least Signifi-
cant Bit order, from Most Significant Byte to Least Significant Byte,
reading from left to right, unless otherwise indicated. Note that
this is contrary to ISO and ITU practice, which orders bits as trans-
mitted (network bit order). Keep this in mind when comparing this
document with the other documents.
2. Physical Layer Requirements
PPP is capable of operating across most X.25 interfaces. The only
absolute requirement imposed by PPP is the provision of a bi-
directional full-duplex circuit, either dedicated (permanent) or cir-
cuit-switched, that can operate in a bit-synchronous mode, transpar-
ent to PPP Data Link Layer frames.
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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 X.25 interface.
Control Signals
Implementation of X.25 requires the provision of control signals,
that indicate when the link has become connected or disconnected.
These in turn provide the Up and Down events to the PPP LCP state
machine.
Because PPP does not normally require the use of control signals,
the failure of such signals MUST NOT affect correct operation of
PPP. Implications are discussed in [RFC-1662].
2.1. Transmission Considerations
The definition of various encodings is the responsibility of the
DTE/DCE equipment in use, and is outside the scope of this specifica-
tion.
While PPP will operate without regard to the underlying representa-
tion of the bit stream, bit-synchronous X.25 requires NRZ encoding.
3. The Data Link Layer
This specification uses the principles, terminology, and frame struc-
ture described in [RFC-1356].
The purpose of this specification is not to document what is already
standardized in [RFC-1356]. Instead, this document attempts to give
a concise summary and point out specific options and features used by
PPP.
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3.1. Frame Header
Since both "PPP in HDLC-like Framing" [RFC-1662] and X.25 use ISO
3309 as a basis for framing, the X.25 header is easily substituted
for the smaller HDLC header. These fields are transmitted from left
to right.
+-+-+-+-+-+-+-+-+
| Flag (0x7e) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+?+?+-+-+-+-+-+-+-+-+-+-+-+-+
| Address | Control |0|0| SVC# (hi) | SVC# (lo) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|p(r) |M|p(s) |0| PPP Protocol |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The PPP Protocol field and the following Information and Padding
fields are described in the Point-to-Point Protocol Encapsulation
[RFC-1661].
3.2. Modification of the Basic Frame
The Link Control Protocol can negotiate modifications to the basic
frame structure. This is not compatible with X.25.
Address-and-Control-Field-Compression
Since the X.25 Address and Control field values are not constant,
and are modified as the frame is transported by the network
switching fabric, Address-and-Control-Field-Compression cannot
affect the frame format.
FCS-Alternatives
Since X.25 requires a 16-bit FCS, which is modified as the frame
is transported by the network switching fabric, FCS-Alternatives
cannot affect the frame format.
In general, framing-related LCP Configuration Options are not recog-
nizable, and are not acceptable for negotiation. The implementation
MUST NOT send ineffectual options in a Configure-Request, and SHOULD
respond to such requested options with a Configure-Reject. See [RFC-
ffff] for details.
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3.3. Modification of the Basic Packet
The Link Control Protocol can negotiate modifications to the basic
packet structure. These are transparent to X.25.
Protocol-Field-Compression
The X.25 framing does not align the PPP Information field on a
32-bit boundary. Alignment to a 16-bit boundary occurs when the
Protocol field is compressed to a single octet. When this
improves throughput, Protocol-Field-Compression SHOULD be negoti-
ated.
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4. Call Setup
When the link is configured as a Permanent Virtual Circuit (PVC),
support for Switched Virtual Circuit (SVC) call setup and clearing is
not required. Calls are Established and Terminated using PPP LCP
packets.
When the link is configured as a Switched Virtual Circuit (SVC), 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 [TR 9577]. This field contains a one octet Network Layer
Protocol Identifier (NLPID), which identifies the encapsulation in
use over the X.25 virtual circuit. The CUD field MAY contain more
than one octet of information.
The PPP encapsulation MUST be indicated by the PPP NLPID value (CF
hex). Any subsequent octet in this CUD is extraneous and MUST be
ignored.
Multipoint networks (or multicast groups) MUST refuse calls which
indicate the PPP NLPID in the CUD.
The accidental connection of a link to feed a multipoint network (or
multicast group) SHOULD 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. Some implementations might be physically unable to either
log or report such information.
Conformance with this specification requires that the PPP NLPID (CF)
be supported. In addition, conformance with [RFC-1356] requires that
the IP NLPID (CC) be supported, and does not require that other NLPID
values be supported, such as Zero (00), SNAP (80), CLNP (81) or ES-IS
(82).
When IP address negotiation and/or VJ header compression are desired,
the PPP call setup SHOULD be attempted first. If the PPP call setup
fails, the normal IP call setup MUST be used.
The PPP NLPID value SHOULD NOT be used to demultiplex circuits that
use the Zero NLPID in call setup, as described in [RFC-1356]. When
such a circuit exists concurrently with PPP encapsulated circuits,
only network layer traffic which has not been negotiated by the asso-
ciated NCP is sent over the Zero NLPID circuit.
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Rationale:
Using call setup to determine if PPP is supported should be inex-
pensive, when users aren't charged for failed calls.
Using the Zero NLPID call together with PPP could be expensive,
when users are charged per packet or for connect time, due to the
probing of PPP configuration packets at each call.
PPP configuration provides a direct indication of the availability
of service, and on that basis is preferred over the Zero NLPID
technique, which can result in "black-holes".
5. Configuration Details
The following Configuration Options are recommended:
Magic Number
Protocol Field Compression
The standard LCP configuration defaults apply to X.25 links, except
Maximum-Receive-Unit (MRU).
To ensure interoperability with existing X.25 implementations, the
initial MRU is 1600 octets [RFC-1356]. This only affects the minimum
required buffer space available for receiving packets, not the size
of packets sent.
The typical network feeding the link is likely to have a MRU of
either 1500, or 2048 or greater. To avoid fragmentation, the Maxi-
mum-Transmission-Unit (MTU) at the network layer SHOULD NOT exceed
1500, unless a peer MRU of 2048 or greater is specifically negoti-
ated.
The X.25 packet size is not directly related to the MRU. Instead,
Protocol Data Units (PDUs) are sent as X.25 "complete packet
sequences". That is, PDUs begin on X.25 data packet boundaries and
the M bit ("more data") is used to fragment PDUs that are larger than
one X.25 data packet in length.
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Security Considerations
Implementations MUST NOT consider PPP authentication on call setup
for one circuit between two systems to apply to concurrent call setup
for other circuits between those same two systems. This results in
possible security lapses due to over-reliance on the integrity and
security of switching systems and administrations. An insertion
attack might be undetected. An attacker which is able to spoof the
same calling identity might be able to avoid link authentication.
Acknowledgements
This design was inspired by the paper "Parameter Negotiation for the
Multiprotocol Interconnect", Keith Sklower and Clifford Frost, Uni-
versity of California, Berkeley, 1992, unpublished.
References
[RFC-1356] Malis, A., Robinson, D., Ullman R., "Multiprotocol Inter-
connect on X.25 and ISDN in the Packet Mode", August
1992.
[RFC-1661] Simpson, W., Editor, "The Point-to-Point Protocol (PPP)",
STD-51, DayDreamer, July 1994.
[RFC-1662] Simpson, W., Editor, "PPP in HDLC-like Framing", STD-51,
DayDreamer, July 1994.
[RFC-2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP-14, Harvard University, March
1997.
[RFC-ffff] Simpson, W., "PPP with Framing Conversion", work in
progress.
[TR 9577] ISO/IEC TR 9577:1990(E), "Information technology -
Telecommunications and Information exchange between sys-
tems - Protocol Identification in the network layer",
1990-10-15.
[X.25] CCITT Recommendation X.25, "Interface Between Data Termi-
nal Equipment (DTE) and Data Circuit Terminating Equip-
ment (DCE) for Terminals Operating in the Packet Mode on
Public Data Networks", International Telecommunication
Union, CCITT Red Book, Volume VIII, Fascicle VIII.2, Rec.
X.25, October 1984.
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Contacts
Comments about this document should be discussed on the ietf-
ppp@merit.edu mailing list.
This document was reviewed by the Point-to-Point Protocol Working
Group of the Internet Engineering Task Force (IETF). The working
group can be contacted via the current chair:
Karl Fox
Ascend Communications
655 Metro Place South, Suite 370
Dublin, Ohio 43017
karl@Ascend.com
Questions about this document can also be directed to:
William Allen Simpson
DayDreamer
Computer Systems Consulting Services
1384 Fontaine
Madison Heights, Michigan 48071
wsimpson@UMich.edu
wsimpson@GreenDragon.com (preferred)
Full Copyright Statement
Copyright (C) William Allen Simpson (1993-1994, 1996-1998). All
Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
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This document and the information contained herein is provided on an
"AS IS" basis and the author(s) DISCLAIM ALL WARRANTIES, EXPRESS OR
IMPLIED, INCLUDING (BUT NOT LIMITED TO) ANY WARRANTY THAT THE USE OF
THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
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