PPP Extensions Working Group George Gross, Lucent Technologies
INTERNET DRAFT Manu Kaycee, Paradyne
Expires October 5, 1998 Arthur Lin, Shasta Networks
Andrew Malis, Ascend Communications
John Stephens, Cayman Systems
April 5th, 1998
PPP Over AAL5
<draft-ietf-pppext-aal5-05.txt>
Status Of This Memo
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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
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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. Specification of Requirements
In this document, several words are used to signify the requirements of
the specification. These words are often capitalized.
1. MUST This word, or the terms "REQUIRED" or "SHALL", mean that
the definition is an absolute requirement of the specification.
2. MUST NOT This phrase, or the phrase "SHALL NOT", mean that the
definition is an absolute prohibition of the specification.
3. SHOULD This word, or the adjective "RECOMMENDED", mean that
there may exist valid reasons in particular circumstances to ignore
a particular item, but the full implications must be understood and
carefully weighed before choosing a different course.
4. SHOULD NOT This phrase, or the phrase "NOT RECOMMENDED" mean
that there may exist valid reasons in particular circumstances when
the particular behavior is acceptable or even useful, but the full
implications should be understood and the case carefully weighed
before implementing any behavior described with this label.
5. MAY This word, or the adjective "OPTIONAL", mean that an item
is truly optional. One vendor may choose to include the item
because a particular marketplace requires it or because the vendor
feels that it enhances the product while another vendor may omit
the same item. An implementation which does not include a
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particular option MUST be prepared to interoperate with another
implementation which does include the option, though perhaps with
reduced functionality. In the same vein an implementation which
does include a particular option MUST be prepared to interoperate
with another implementation which does not include the option
(except, of course, for the feature the option provides.)
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.
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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 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 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
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+-------------------------------+ -------
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).
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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
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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 RFC 1755 [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
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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.
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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", 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.
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[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] M. Perez, F. Liaw, A. Mankin, E. Hoffman, D. Grossman, 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)
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
Author's Address
Questions about this memo can also be directed to:
George Gross
Lucent Technologies, Inc
184 Liberty Corner Road
Warren, NJ 07059
Tel: +1.908.580.4589
Email: gmgross@lucent.com
Manu Kaycee
Paradyne Corporation
21 Bear Meadow Road
Londonderry, NH 03053-2168
Tel: +1.603.434.6088
Email: mjk@nj.paradyne.com
Arthur Lin
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Shasta Networks Inc.
249 Humboldt Court
Sunnyvale, CA 94089-1300
Tel: +1.408.747.5051
Email: alin@shastanets.com
Andrew Malis
Ascend Communications, Inc.
1 Robbins Road
Westford, MA 01886
Tel: +1.978.952.7414
Email: malis@ascend.com
John Stephens
Cayman Systems, Inc.
100 Maple Street
Stoneham, MA 02180
Tel: +1.617.279.1101
Email: john@cayman.com
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