Network Working Group W A Simpson [DayDreamer]
Internet Draft
expires in six months November 1997
PPP in Frame Relay
draft-ietf-pppext-framerelay-ds-00.txt
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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 Frame Relay for framing
PPP encapsulated packets.
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
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environments.
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1. Introduction
Frame Relay [Q.922] is a relative newcomer to the serial link commu-
nity. Like X.25, the protocol was designed to provide virtual cir-
cuits for connections between stations attached to the same Frame
Relay network. The improvement over X.25 is that Q.922 is restricted
to delivery of packets, and dispenses with sequencing and flow con-
trol, simplifying the service immensely.
At one time, it had been hoped that "PPP in HDLC-like Framing"
[RFC-1662] would co-exist with other Frame Relay transmissions on the
same links. Unfortunately, the Q.922 method for expanding the
address from 1 to 2 to 4 octets is not reliably distinguishable from
the ISO 3309 HDLC method, due to the structure of its Data Link Con-
nection Identifier (DLCI) subfields. Co-existance is precluded.
When Frame Relay is configured as a point-to-point circuit, PPP can
use Frame Relay as a framing mechanism, ignoring its other features.
This is equivalent to the technique used to carry SNAP headers over
Frame Relay [RFC-1490].
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 Frame Relay interfaces. The
only absolute requirement imposed by PPP is the provision of a bi-
directional full-duplex circuit, either dedicated (permanent) or
frame-switched, that can operate in either a bit-synchronous, or
octet-synchronous mode, transparent 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 Frame Relay interface.
Control Signals
Implementation of Frame Relay requires the provision of control
signals, that indicate when the link has become connected or dis-
connected. 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 speci-
fication.
While PPP will operate without regard to the underlying represen-
tation of the octet stream, bit-synchronous Frame Relay requires
NRZ encoding.
In addition, this specification permits octet-synchronous Frame
Relay, with the same stuffing conventions as HDLC [RFC-1662].
3. The Data Link Layer
This specification uses the principles, terminology, and frame
structure described in [RFC-1490].
The purpose of this specification is not to document what is
already standardized in [RFC-1490]. 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
As described in [RFC-1490], Q.922 header address and control
fields are followed by a Network Layer Protocol Identifier (NLPID)
to identify the encapsulated packet. This specification describes
the PPP Protocol encapsulation. These fields are transmitted from
left to right.
+-+-+-+-+-+-+-+-+
| Flag (0x7e) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Q.922 Address | Control | NLPID(0xcf) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 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 Frame Relay.
Address-and-Control-Field-Compression
Since Frame Relay Address and Control field values are not con-
stant, are variable size, 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 Frame Relay 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 rec-
ognizable, and are not acceptable for negotiation. The implemen-
tation 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 Frame Relay.
Protocol-Field-Compression
The default Frame Relay header does not align the PPP Informa-
tion field on a 32-bit boundary. Alignment to a 32-bit bound-
ary occurs when the NLPID is removed and the PPP Protocol field
is compressed to a single octet. When this improves through-
put, Protocol-Field-Compression SHOULD be negotiated.
4. In-Band Protocol Demultiplexing
The PPP NLPID (CF hex) and PPP Protocol fields easily distinguish
the PPP encapsulation from the other NLPID encapsulations
described in [RFC-1490].
The joining of the PPP and NLPID number space has an added advan-
tage, in that the LCP Protocol-Reject can be used to indicate
NLPIDs that are not recognized. This can eliminate "black-holes"
that occur when traffic is not supported.
For those network-layer protocols that have no PPP Protocol
assignment, or have not yet been implemented under the PPP encap-
sulation, or have not been successfully negotiated by a PPP NCP,
another method of encapsulation defined under [RFC-1490] SHOULD be
used.
Currently, there are no conflicts between NLPID and PPP Protocol
values. If a future implementation is configured to send a NLPID
value which is the same as a compressed Protocol field, that Pro-
tocol field MUST NOT be sent compressed.
On reception, the first octet following the Control field is exam-
ined:
- If the octet is zero, it MUST be assumed that the packet is
formatted according to [RFC-1490].
- Initial LCP packets contain the sequence cf-c0-21 following the
Control field. When a LCP Configure-Request packet is received
and recognized, the PPP link enters Link Establishment phase.
- If the octet is not the PPP NLPID value, and Protocol-Field-
Compression is enabled, and the associated NCP has been
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negotiated, then it is expected to be a compressed PPP Protocol
value.
- Otherwise, it MUST be assumed that the packet is formatted
according to [RFC-1490].
Once PPP has entered the Link Establishment phase, packets with
other NLPID values MUST NOT be sent, and on receipt such packets
MUST be silently discarded, until the PPP link enters the Network-
Layer Protocol phase.
Once PPP has entered the Network-Layer Protocol phase, and suc-
cessfully negotiated a particular NCP for a PPP Protocol, if a
frame arrives using another equivalent data encapsulation defined
in [RFC-1490], the PPP Link MUST re-enter Link Establishment phase
and send a new LCP Configure-Request. This prevents "black-holes"
that occur when the peer loses state.
An implementation that requires PPP link configuration, and other
PPP negotiated features (such as authentication), MAY enter Termi-
nation phase when configuration fails. Otherwise, when the Con-
figure-Request sender reaches the Max-Configure limit, it MUST
fall back to send only frames encapsulated according to
[RFC-1490].
Implementation Notes
The PPP Protocol field value 0x00cf is not allowed (reserved)
to avoid ambiguity when Protocol-Field-Compression is enabled.
For consistency, the NLPID value 0xcf MAY be treated as a com-
pressed PPP Protocol which indicates that another PPP Protocol
packet follows.
The accidental connection of a link to feed a multipoint net-
work (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.
5. Out-of-Band signaling
There is no generally agreed method of out-of-band signalling.
Until such a method is universally available, an implementation
MUST use In-Band Protocol Demultiplexing for both Permanent and
Switched Virtual Circuits.
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6. Configuration Details
The following Configuration Options are recommended:
Magic Number
Protocol Field Compression
The standard LCP configuration defaults apply to Frame Relay
links, except Maximum-Receive-Unit (MRU).
To ensure interoperability with existing Frame Relay implementa-
tions, the initial MRU is 1600 octets [RFC-1490]. 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 Max-
imum-Transmission-Unit (MTU) at the network layer SHOULD NOT
exceed 1500, unless a peer MRU of 2048 or greater is specifically
negotiated.
Some Frame Relay switches are only capable of 262 octet frames.
It is not recommended that anyone deploy or use a switch that is
capable of less than 1600 octet frames. However, PPP implementa-
tions MUST be configurable to limit the size of LCP packets that
are sent to 259 octets (leaving room for the NLPID and PPP Proto-
col fields), until LCP negotiation is complete.
XID negotiation is not required to be supported for links that are
capable of PPP negotiation.
Inverse ARP is not required to be supported for PPP links. That
function is provided by PPP NCP negotiation.
Security Considerations
This specification introduces no known security risks.
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Acknowledgements
This design was inspired by the paper "Parameter Negotiation for
the Multiprotocol Interconnect", Keith Sklower and Clifford Frost,
University of California, Berkeley, 1992, unpublished.
Use of octet-synchronous interfaces, such as SONET/SDH, was first
proposed by John Bartell (BellSouth).
References
[Q.922] CCITT Recommendation Q.922, "ISDN Data Link Layer Speci-
fication for Frame Mode Bearer Services", International
Telegraph and Telephone Consultative Committee, 1992.
[RFC-1490]
Bradley, T., Brown, C., and Malis, A., "Multiprotocol
Interconnect over Frame Relay", July 1993.
[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.
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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)
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