RFC2363
Network Working Group G. Gross Request for Comments: 2363 Lucent Technologies Category: Standards Track M. Kaycee
Paradyne
A. Li
Shasta Networks
A. Malis
Ascend Communications
J. Stephens
Cayman Systems
July 1998
PPP Over FUNI
Status of this Memo
This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (1998). All Rights Reserved.
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 ATM Frame User Network Interface (FUNI) for framing PPP encapsulated packets.
Applicability
This specification is intended for those implementations which desire to use the 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 the peers, and are not designed for the multi- point relationships which are available in ATM and other multi-access environments.
Contents
- 1 Introduction
- 2 Conventions
- 3 FUNI Layer Service Interface
- 4 Multi-Protocol Encapsulation
- 5 Virtual Circuit Multiplexed PPP Over FUNI
- 6 LLC Encapsulated PPP Over FUNI
- 7 Out-Of-Band Control Plane Signaling
- 8 Detection And Recovery From Unsolicited PPP Encapsulation Transitions
- 9 LCP Configuration Options
Introduction
ATM FUNI protocol is designed to provide virtual connections between end stations attached to the same network. These connections offer a packet delivery service that includes error detection, but does not do error correction.
Most existing implementations of PPP use ISO 3309 HDLC as a basis for their framing [3].
When an ATM network is configured with point-to-point connections, PPP can use FUNI as a framing mechanism.
Conventions
The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD, SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this document, are to be interpreted as described in [10].
FUNI Layer Service Interface
The PPP layer treats the underlying ATM FUNI layer service as a bit- synchronous point-to-point link. In this context, the PPP link corresponds to an ATM FUNI virtual connection. The virtual connection MUST be full-duplex, point to point, and it MAY be either dedicated (i.e. permanent, set up by provisioning) or switched (set up on demand). In addition, the PPP/FUNI service interface boundary MUST meet the following requirements:
Interface Format - The PPP/FUNI layer boundary presents an octet
service interface to the FUNI layer. There is no provision for
sub-octets to be supplied or accepted.
Transmission Rate - The PPP layer does not impose any
restrictions regarding transmission rate or the underlying ATM
layer traffic descriptor parameters.
Control Signals - The FUNI layer MUST provide control signals to
the PPP layer which indicate when the virtual connection link
has become connected or disconnected. These provide the "Up"
and
"Down" events to the LCP state machine [1] within the PPP layer.
Multi-Protocol Encapsulation
This specification uses the principles, terminology, and frame structure described in "Multiprotocol Encapsulation over ATM Adaptation Layer 5" [4].
The purpose of this specification is not to document what is already standardized in [4], but to specify how the mechanisms described in [4] are to be used to map PPP onto a FUNI-based ATM network. Section 1 within [4] defines the two mechanisms for identifying the Protocol Data Unit (PDU) payload field's protocol type: virtual circuit based multiplexing, and Logical Link Control (LLC) encapsulation. In the former technique, the payload's protocol type is implicitly agreed to by the end points for each virtual circuit using provisioning or control plane procedures. When using the LLC encapsulation technique, the payload's protocol type is explicitly identified on a per PDU basis by an in-band LLC header, followed by the payload data.
When transporting a PPP payload over FUNI, an implementation:
1. MUST support virtual circuit multiplexed PPP payloads as
described in section 5 below by mutual configuration or
negotiation of both end points. This technique is referred to
as "VC-multiplexed PPP".
2. MUST support LLC encapsulated PPP payloads on PVCs as
described in section 6 below by mutual configuration or
negotiation of both end points. This technique is referred to
as "LLC encapsulated PPP".
3. For SVC set up, an implementation MUST negotiate using the
Q.2931 [9] Annex C procedure, encoding the Broadband Lower Layer
Interface (B-LLI) information element to signal either VC-
multiplexed PPP or LLC encapsulated PPP. The details of this
control plane procedure are described in section 7.
If an implementation is connecting through a Frame Relay/ATM FRF.8 [7]
service inter-working unit to an RFC 1973 [6] end point, then it MUST use LLC encapsulated PPP payloads. Frame Relay/ATM FRF.8 inter-working units are exempted from the requirement to support VC-multiplexed PPP. This exemption allows the FR/ATM IWU to remain compliant with FRF.8 when the PPP over FUNI end point is inter-operating with an RFC 1973 end point.
Virtual Circuit Multiplexed PPP Over FUNI
The FUNI protocol data unit (PDU) format [2] is as follows:
+-------------------------------+
| Flag |
+-------------------------------+---------
| FUNI Header | ^
+-------------------------------+ |
| | |
| | |
| User SDU | FUNI PDU
| | |
| | |
+-------------------------------+ |
| FUNI FCS (2 or 4 octets) | v
+-------------------------------+---------
| Flag |
+-------------------------------+
Figure 1
The FUNI Header includes a 10-bit or 24-bit Frame Address (a.k.a. VPI/VCI bits), a Congestion Notification bit, a Congestion Loss Priority bit, and four Reserved bits.
The User SDU field contains user information up to 4096 (optionally up to 64K) octets.
The FCS field protects the entire FUNI PDU except for the FCS field itself.
A VC-multiplexed PPP frame SHALL constitute the User Service Data Unit (SDU) field and is defined as shown in figure 2:
+-------------+-------------+---------+
| Protocol ID | Information | Padding |
| 8/16 bits | | |
+-------------+-------------+---------+
Figure 2
Each of these fields are specifically defined in [1].
LLC Encapsulated PPP Over FUNI
LLC encapsulated PPP over FUNI is the alternative technique to VC- multiplexed PPP over FUNI.
The FUNI SDU payload field is encoded as shown in figure 3. The pertinent fields in that diagram are:
1. LLC header: 2 bytes encoded to specify a source SAP and
destination SAP of routed OSI PDU (values 0xFE 0xFE), followed
by an Un-numbered Information (UI) frame type (value 0x03).
2. Network Layer Protocol IDentifier (NLPID) representing PPP,
(value 0xCF).
3. the PPP protocol identifier field, which can be either 1 or 2
octets long. See reference [1].
4. followed by the PPP information field as per Figure 2.
+-------------------------+ --------
| Destination SAP (0xFE) | ^
+-------------------------+ |
| Source SAP (0xFE) | LLC header
+-------------------------+ |
| Frame Type = UI (0x03) | V
+-------------------------+ --------
| NLPID = PPP (0xCF) |
+-------------------------+ --------
| Protocol Identifier | ^
| (8 or 16 bits) | |
+-------------------------+ PPP payload
| . | |
| . | |
| PPP information field | |
| . | |
| . | |
+-------------------------+ |
| padding | V
+-------------------------+ --------
| FUNI FCS (2 or 4 octets)| FUNI trailer
+-------------------------+---------
Figure 3
The end points MAY be bi-laterally provisioned to send other
LLC-encapsulated protocols besides PPP across the same virtual
connection. However, they MUST NOT send packets belonging to
any protocol that has an active NCP within the PPP session.
Implementations SHOULD do packet scheduling that minimizes the
performance impact on the quality of service commitments
associated with both the LLC-encapsulated PPP and non-PPP
protocol flows.
Out-Of-Band Control Plane Signaling
When originating a switched virtual circuit FUNI connection, the
caller MUST request in the SETUP message either VC-multiplexed
PPP, LLC-encapsulated PPP, or else both VC-multiplexed and LLC-
encapsulated PPP. When a caller is offering both techniques,
the two B-LLI IEs are encoded within a Broadband Repeat
Indicator IE in the order of their preference. The called
implementation MUST be able to accept an incoming call that
offers LLC-encapsulated PPP in the caller's request. The called
implementation MUST reject a call set up request that only
offers an encapsulation that it does not support.
Implementations originating a call offering both protocol
encapsulation techniques MUST be able to negotiate the use of
LLC-encapsulated PPP.
When originating a virtual circuit multiplexed call that is to
carry a PPP payload, the ITU Q.2931 [9] B-LLI element user
information layer 3 protocol field is encoded to select ISO/IEC
TR 9577 [5] in octet 7. The extension octets specify an IPI
value of PPP (0xCF). By definition, the first bytes of the FUNI
frame's payload field will always contain a PPP header followed
by a packet.
When originating an LLC encapsulated call that is to carry a PPP
payload, the ITU Q.2931 B-LLI element user information layer 2
protocol field is encoded to select LAN Logical Link Control
(ISO/IEC8802-2) in octet 6. See RFC 1755 [8] appendix A for an
example. By definition, the first bytes of the FUNI frame's
payload field will contain an LLC header, followed by a NLPID
and the PPP payload.
Detection And Recovery From Unsolicited PPP Encapsulation Transitions
When the virtual connection loses state, the PPP encapsulation
technique may uni-laterally and unexpectedly change across such
transitions. Detection and recovery procedures are defined for
the following state transitions:
VC-multiplexed PPP changing to LLC encapsulated PPP
LLC encapsulated PPP changing to VC-multiplexed PPP
When LLC-encapsulated PPP is being used, the inital 6 octets of the LCP packets contain the sequence: fe-fe-03-cf-c0-21. This sequence constitutes the first 6 octets of the FUNI frame. In the case of VC-multiplexed PPP, initial LCP packets contain the sequence c0-21.
In the case of FUNI, this sequence follows the FUNI Header. When a LCP Configure-Request packet is received and recognized, the PPP link enters Link Establishment phase.
Once PPP has entered the Network-layer Protocol phase, and successfully negotiated a particular NCP for a PPP Protocol, if a frame arrives using an alternate but equivalent data encapsulation as defined in [4], then the PPP Link MUST:
For a SVC, immediately clear the call with the cause value 111,
"protocol error, unspecified".
For a PVC: tear down the active NCPs, SHOULD generate an error
message, enter the Termination state, and silently drop all
received packets.
These policies prevent "black-holes" that occur when the peer loses state. An implementation which requires PPP link configuration, and other PPP negotiated features (such as authentication), MAY enter Termination state when configuration fails.
LCP Configuration Options
The Magic Number LCP configuration option is RECOMMENDED, and the Protocol Field Compression (PFC) option is NOT RECOMMENDED. An implementation MUST NOT request any of the following options, and MUST reject a request for such an option:
Field Check Sequence (FCS) Alternatives,
Address-and-Control-Field-Compression (ACFC),
Asynchronous-Control-Character-Map (ACCM)
The Maximum-Receive-Unit (MRU) option MUST NOT be negotiated to a larger size than the maximum CPCS-SDU size specified in the associated direction for the virtual connection's traffic contract.
When viewed peer to peer, a PPP link may be bridged over multiple physical layer sections. For each such FUNI section, the LCP framing options MUST be actively negotiated by the bridging convertors independently of the LCP framing options in use by other physical layer sections.
Implementation Note:
When an ATM FUNI PVC is in the "Stopped" state, it is
RECOMMENDED that the implementation wait for Configure-Requests.
See the implementation option in reference [1] section 4.2, the
"Stopped State" sub-section.
10. Security Considerations
Generally, ATM networks are virtual circuit based, and security is implicit in the public data networking service provider's administration of Permanent Virtual Circuits (PVCs) between the network boundaries. The probability of a security breach caused by mis-routed ATM cells is considered to be negligible.
When a public ATM network supports Switched Virtual Circuits, the protocol model becomes analogous to traditional voice band modem dial up over the Public Telephone Switched Network (PTSN). The same PAP/CHAP authentication protocols that are already widely in use for Internet dial up access are leveraged. As a consequence, PPP over FUNI security is at parity with those practices already established by the existing Internet infrastructure.
Those applications that require stronger security are encouraged to use authentication headers, or encrypted payloads, and/or ATM-layer security services.
When using LLC-encapsulated PPP over a virtual connection, an end point can not assume that the PPP session authentication and related security mechanisms also secure the other LLC encapsulated flows on that same virtual connection.
11. Acknowledgments
This design is based on work performed in ADSL Forum's Packet Mode Working Group. It is inspired by "PPP in Frame Relay", RFC 1973, by William Simpson. Special thanks to Phil Rakity of Flowpoint, Tim Kwok of Microsoft, and David Allan of Nortel for their constructive review and commentary.
12. References
[1] Simpson, W., Editor, "The Point-to-Point Protocol (PPP)", STD
51, RFC 1661, July 1994.
[2] The ATM Forum, "Frame based User-to-Network Interface (FUNI)
Specification v2", af-saa-0088.000, May 1997.
[3] Simpson, W., Editor, "PPP in HDLC-like Framing", STD 51, RFC
1662, July 1994.
[4] Heinanen, J., "Multiprotocol Interconnect over AAL5", RFC 1483,
July 1993.
[5] ISO/IEC DTR 9577.2, "Information technology -
Telecommunications and Information exchange between systems -
Protocol Identification in the network layer", 1995-08-16.
[6] Simpson, W., "PPP in Frame Relay", RFC 1973, June 1996.
[7] The Frame Relay Forum, "Frame Relay/ATM PVC Service Inter-
working Implementation Agreement", FRF.8, April 1995.
[8] Perez, M., Liaw, F., Mankin, A., Hoffman, E., Grossman, D., and
A. Malis, "ATM Signaling Support for IP over ATM", RFC 1755, February 1995.
[9] International Telecommunication Union, "Broadband Integrated
Service Digital Network (B-ISDN) Digital Subscriber Signaling
System No.2 (DSS2) User Network Interface Layer 3 Specification
for Basic Call/Connection Control", ITU-T Recommendation
Q.2931, (International Telecommunication Union: Geneva, 2/95)
[10] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
Chair's Address
The working group can be contacted via the current chair:
Karl Fox Ascend Communications 3518 Riverside Drive, Suite 101 Columbus, Ohio 43221
EMail: [email protected]
Authors' Addresses
Questions about this memo can also be directed to:
George Gross Lucent Technologies, Inc 184 Liberty Corner Road Warren, NJ 07059
Phone: +1.908.580.4589 EMail: [email protected]
Manu Kaycee Paradyne Corporation 21 Bear Meadow Road Londonderry, NH 03053-2168
Phone: +1.603.434.6088 EMail: [email protected]
Arthur Lin Shasta Networks Inc. 249 Humboldt Court Sunnyvale, CA 94089-1300
Phone: +1.408.747.5051 EMail: [email protected]
Andrew Malis Ascend Communications, Inc. 1 Robbins Road Westford, MA 01886
Phone: +1.978.952.7414 EMail: [email protected]
John Stephens Cayman Systems, Inc. 100 Maple Street Stoneham, MA 02180
Phone: +1.617.279.1101 EMail: [email protected]
Full Copyright Statement
Copyright (C) The Internet Society (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 included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to translate it into languages other than English.
The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS 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 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
