Network Working Group G. Gross
Request for Comments: 2364 LUCent Technologies
Category: Standards Track M. Kaycee
Paradyne
A. Lin
Shasta Networks
A. Malis
Ascend Communications
J. Stephens
Cayman Systems
July 1998
PPP Over AAL5
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 Adaptation Layer 5 (AAL5) 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.
1. Introduction
ATM AAL5 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 AAL5 as a framing mechanism.
2. 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].
3. AAL5 Layer Service Interface
The PPP layer treats the underlying ATM AAL5 layer service as a bit-
synchronous point-to-point link. In this context, the PPP link
corresponds to an ATM AAL5 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/AAL5 service interface boundary
MUST meet the following requirements:
Interface Format - The PPP/AAL5 layer boundary presents an octet
service interface to the AAL5 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 AAL5 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.
4. 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 an AAL5-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 AAL5, 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 RFC1973 [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 AAL5 end point is inter-
operating with an RFC1973 end point.
5. Virtual Circuit Multiplexed PPP Over AAL5
The AAL5 PDU format is shown in figure 1:
AAL5 CPCS-PDU Format
+-------------------------------+
.
.
CPCS-PDU Payload
up to 2^16 - 1 octets)
.
+-------------------------------+
PAD ( 0 - 47 octets)
+-------------------------------+ -------
CPCS-UU (1 octet ) ^
+-------------------------------+
CPI (1 octet )
+-------------------------------+CPCS-PDU Trailer
Length (2 octets)
+-------------------------------
CRC (4 octets) V
+-------------------------------+ -------
Figure 1
The Common Part Convergence Sub-layer (CPCS)-PDU Payload field
contains user information up to 2^16 - 1 octets.
The PAD field pads the CPCS-PDU to fit exactly into the ATM cells
such that the last 48 octet cell payload created by the SAR sublayer
will have the CPCS-PDU Trailer right justified in the cell.
The CPCS-UU (User-to-User indication) field is used to transparently
transfer CPCS user to user information. The field has no function
under the multi-protocol ATM encapsulation described in this memo and
can be set to any value.
The CPI (Common Part Indicator) field aligns the CPCS-PDU trailer to
64 bits. Possible additional functions are for further study in
ITU-T. When only the 64 bit alignment function is used, this field
shall be coded as 0x00.
The Length field indicates the length, in octets, of the Payload
field. The maximum value for the Length field is 65535 octets. A
Length field coded as 0x00 is used for the abort function.
The CRC field protects the entire CPCS-PDU except the CRC field
itself.
A VC-multiplexed PPP frame SHALL constitute the CPCS-PDU payload and
is defined as:
+-------------+-------------+---------+
Protocol ID Information Padding
8/16 bits
+-------------+-------------+---------+
Figure 2
Each of these fields are specifically defined in [1].
6. LLC Encapsulated PPP Over AAL5
LLC encapsulated PPP over AAL5 is the alternative technique to VC-
multiplexed PPP over AAL5.
The AAL5 CPCS-PDU 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
+-------------------------+ --------
PAD ( 0 - 47 octets)
+-------------------------+ --------
CPCS-UU (1 octet ) ^
+-------------------------+
CPI (1 octet )
+-------------------------+CPCS-PDU Trailer
Length (2 octets)
+-------------------------
CRC (4 octets) V
+-------------------------+ --------
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.
7. Out-Of-Band Control Plane Signaling
When originating a switched virtual circuit AAL5 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 AAL5 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 RFC1755 [8] appendix A for an
example. By definition, the first bytes of the AAL5 frame's payload
field will contain an LLC header, followed by a NLPID and the PPP
payload.
8. 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 AAL5 frame. In the case of
VC-multiplexed PPP, initial LCP packets contain the sequence c0-21.
This sequence constitutes the first 2 octets of an AAL5 frame. 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.
9. 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 AAL5 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 AAL5 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
AAL5 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", RFC1973, 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, RFC1661, 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", RFC1483,
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", RFC1973, 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", RFC1755,
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, RFC2119, 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: karl@ascend.com
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: gmgross@lucent.com
Manu Kaycee
Paradyne Corporation
21 Bear Meadow Road
Londonderry, NH 03053-2168
Phone: +1.603.434.6088
EMail: mjk@nj.paradyne.com
Arthur Lin
Shasta Networks Inc.
249 Humboldt Court
Sunnyvale, CA 94089-1300
Phone: +1.408.747.5051
EMail: alin@shastanets.com
Andrew Malis
Ascend Communications, Inc.
1 Robbins Road
Westford, MA 01886
Phone: +1.978.952.7414
EMail: malis@ascend.com
John Stephens
Cayman Systems, Inc.
100 Maple Street
Stoneham, MA 02180
Phone: +1.617.279.1101
EMail: john@cayman.com
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