PPP in X.25
This specification is intended for those implementations which desire
to use facilities which are defined for PPP, such as the Link Control
Protocol, Network-layer Control Protocols, authentication, and
compression. These capabilities require a point-to-point
relationship between peers, and are not designed for multi-point or
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Table of Contents
1. Introduction .......................................... 1
2. Physical Layer Requirements ........................... 2
3. The Data Link Layer ................................... 2
3.1 Frame Format .................................... 3
3.2 Modification of the Basic Frame ................. 3
4. Call Setup ............................................ 4
5. Configuration Details ................................. 5
SECURITY CONSIDERATIONS ...................................... 6
REFERENCES ................................................... 6
ACKNOWLEDGEMENTS ............................................. 6
CHAIR'S ADDRESS .............................................. 7
AUTHOR'S ADDRESS ............................................. 7
CCITT recommendation 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
PPP also uses ISO 3309 HDLC as a basis for its framing .
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
equivalent to the technique used to carry SNAP headers over X.25 .
At one time, it had been hoped that PPP HDLC frames and X.25 frames
would co-exist 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 protocol such as Frame Relay, and
reconstruct the X.25 header at the final hop. Co-existance and
gradual migration are precluded.
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2. Physical Layer Requirements
PPP treats X.25 framing as a bit synchronous link. The link MUST be
full-duplex, but MAY be either dedicated (permanent) or switched.
PPP presents an octet interface to the physical layer. There is
no provision for sub-octets to be supplied or accepted.
PPP does not impose any restrictions regarding transmission rate,
other than that of the particular X.25 interface.
Implementation of X.25 requires the provision of control signals,
which indicate when the link has become connected or disconnected.
These in turn provide the Up and Down events to the LCP state
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 .
The definition of various encodings is the responsibility of the
DTE/DCE equipment in use, and is outside the scope of this
While PPP will operate without regard to the underlying
representation of the bit stream, X.25 requires NRZ encoding.
3. The Data Link Layer
This specification uses the principles, terminology, and frame
structure described in "Multiprotocol Interconnect on X.25 and ISDN
in the Packet Mode" .
The purpose of this specification is not to document what is already
standardized in . Instead, this document attempts to give a
concise summary and point out specific options and features used by
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3.1. Frame Format
Since both "PPP in HDLC Framing"  and X.25 use ISO 3309 as a basis
for framing, the X.25 header is easily substituted for the smaller
HDLC header. The fields are transmitted from left to right.
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| PPP Protocol |
The PPP Protocol field and the following Information and Padding
fields are described in the Point-to-Point Protocol Encapsulation
3.2. Modification of the Basic Frame
The Link Control Protocol can negotiate modifications to the basic
frame structure. However, modified frames will always be clearly
distinguishable from standard frames.
Because the 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 MUST NOT be
Note that unlike the HDLC framing, the X.25 framing does not align
the 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 negotiated.
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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
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 . 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
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  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 which
use the Zero NLPID in call setup, as described in . When such a
circuit exists concurrently with PPP encapsulated circuits, only
network layer traffic which has not been negotiated by the associated
NCP is sent over the Zero NLPID circuit.
Using call setup to determine if PPP is supported should be
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inexpensive, 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:
Protocol Field Compression
The standard LCP configuration defaults apply to X.25 links, except
To ensure interoperability with existing X.25 implementations, the
initial Maximum-Receive-Unit (MRU) is 1600 octets . 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
Maximum-Transmission-Unit (MTU) at the network layer SHOULD NOT
exceed 1500, unless a peer MRU of 2048 or greater is specifically
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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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.
 Simpson, W., Editor, "The Point-to-Point Protocol (PPP)", RFC
1548, December 1993.
 CCITT Recommendation X.25, "Interface Between Data Terminal
Equipment (DTE) and Data Circuit Terminating Equipment (DCE)
for Terminals Operating in the Packet Mode on Public Data
Networks", Vol. VIII, Fascicle VIII.2, Rec. X.25.
 Simpson, W., Editor, "PPP in HDLC Framing", RFC 1549, December
 Malis, A., Robinson, D., and R. Ullmann, "Multiprotocol
Interconnect on X.25 and ISDN in the Packet Mode", RFC 1356,
 ISO/IEC TR 9577, "Information technology - Telecommunications
and Information exchange between systems - Protocol
Identification in the network layer", 1990 (E) 1990-10-15.
This design was inspired by the paper "Parameter Negotiation for the
Multiprotocol Interconnect", Keith Sklower and Clifford Frost,
University of California, Berkeley, 1992, unpublished.
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The working group can be contacted via the current chair:
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