Network Working Group A. Malis
Request for Comments: 2615 Ascend Communications, Inc.
Obsoletes: 1619 W. Simpson
Category: Standards Track DayDreamer
 June 1999
 PPP over SONET/SDH
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 (1999). 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 PPP over Synchronous Optical
 Network (SONET) and Synchronous Digital Hierarchy (SDH) circuits.
 This document replaces and obsoletes RFC 1619. See section 7 for a
 summary of the changes from RFC 1619.
Table of Contents
 1. Introduction .......................................... 2
 2. Physical Layer Requirements ........................... 3
 3. Framing ............................................... 4
 4. X**43 + 1 Scrambler Description ....................... 4
 5. Configuration Details ................................. 6
 6. Security Considerations ............................... 6
 7. Changes from RFC 1619 ................................. 7
 8. Intellectual Property ................................. 7
 9. Acknowledgments ....................................... 8
 10. References ............................................ 8
 11. Authors' Addresses .................................... 9
 12. Full Copyright Statement .............................. 10
1. Introduction
 PPP was designed as a standard method of communicating over
 point-to-point links. Initial deployment has been over short local
 lines, leased lines, and plain-old-telephone-service (POTS) using
 modems. As new packet services and higher speed lines are introduced,
 PPP is easily deployed in these environments as well.
 This specification is primarily concerned with the use of the PPP
 encapsulation over SONET/SDH links. Since SONET/SDH is by definition
 a point-to-point circuit, PPP is well suited to use over these links.
 Real differences between SONET and SDH (other than terminology) are
 minor; for the purposes of encapsulation of PPP over SONET/SDH, they
 are inconsequential or irrelevant.
 For the convenience of the reader, we list the equivalent terms below:
 SONET SDH
 ---------------------------------------------
 SPE VC
 STS-SPE Higher Order VC (VC-3/4/4-Nc)
 STS-1 frame STM-0 frame (rarely used)
 STS-1-SPE VC-3
 STS-1 payload C-3
 STS-3c frame STM-1 frame, AU-4
 STS-3c-SPE VC-4
 STS-3c payload C-4
 STS-12c/48c/192c frame STM-4/16/64 frame, AU-4-4c/16c/64c
 STS-12c/48c/192c-SPE VC-4-4c/16c/64c
 STS-12c/48c/192c payload C-4-4c/16c/64c
 The only currently supported SONET/SDH SPE/VCs are the following:
 SONET SDH
 ----------------------------------------
 STS-3c-SPE VC-4
 STS-12c-SPE VC-4-4c
 STS-48c-SPE VC-4-16c
 STS-192c-SPE VC-4-64c
 The keywords MUST, MUST NOT, MAY, OPTIONAL, REQUIRED, RECOMMENDED,
 SHALL, SHALL NOT, SHOULD, and SHOULD NOT are to be interpreted as
 defined in [6].
2. Physical Layer Requirements
 PPP treats SONET/SDH transport as octet oriented synchronous links.
 SONET/SDH links are full-duplex by definition.
 Interface Format
 PPP in HDLC-like framing presents an octet interface to the
 physical layer. There is no provision for sub-octets to be
 supplied or accepted [3][5].
 The octet stream is mapped into the SONET STS-SPE/SDH Higher Order
 VC, with the octet boundaries aligned with the SONET STS-SPE/SDH
 Higher Order VC octet boundaries.
 Scrambling is performed during insertion into the SONET STS-
 SPE/SDH Higher Order VC to provide adequate transparency and
 protect against potential security threats (see Section 6). For
 backwards compatibility with RFC 1619 (STS-3c-SPE/VC-4 only), the
 scrambler MAY have an on/off capability where the scrambler is
 bypassed entirely when it is in the off mode. If this capability
 is provided, the default MUST be set to scrambling enabled.
 For PPP over SONET/SDH, the entire SONET/SDH payload (SONET STS-
 SPE/SDH Higher Order VC minus the path overhead and any fixed
 stuff) is scrambled using a self-synchronous scrambler of
 polynomial X**43 + 1. See Section 4 for the description of the
 scrambler.
 The proper order of operation is:
 When transmitting:
 IP -> PPP -> FCS generation -> Byte stuffing -> Scrambling ->
 SONET/SDH framing
 When receiving:
 SONET/SDH framing -> Descrambling -> Byte destuffing -> FCS
 detection -> PPP -> IP
 The Path Signal Label (C2) indicates the contents of the SONET STS-
 SPE/SDH Higher Order VC. The value of 22 (16 hex) is used to
 indicate PPP with X^43 + 1 scrambling [4].
 For compatibility with RFC 1619 (STS-3c-SPE/VC-4 only), if scrambling
 has been configured to be off, then the value 207 (CF hex) is used
 for the Path Signal Label to indicate PPP without scrambling.
 The Multiframe Indicator (H4) is unused, and MUST be zero.
 Control Signals
 PPP does not require the use of control signals. When available,
 using such signals can allow greater functionality and
 performance. Implications are discussed in [2].
3. Framing
 The framing for octet-synchronous links is described in "PPP in
 HDLC-like Framing" [2].
 The PPP frames are located by row within the SONET STS-SPE/SDH Higher
 Order VC payload. Because frames are variable in length, the frames
 are allowed to cross SONET STS-SPE/SDH Higher Order VC boundaries.
4. X**43 + 1 Scrambler Description
 The X**43 + 1 scrambler transmitter and receiver operation are as
 follows:
 Transmitter schematic:
 Unscrambled Data
 |
 v
 +-------------------------------------+ +---+
 +->| --> 43 bit shift register --> |--->|xor|
 | +-------------------------------------+ +---+
 | |
 +-----------------------------------------------+
 |
 v
 Scrambled Data
 Receiver schematic:
 Scrambled Data
 |
 +-----------------------------------------------+
 | |
 | v
 | +-------------------------------------+ +---+
 +->| --> 43 bit shift register --> |--->|xor|
 +-------------------------------------+ +---+
 |
 v
 Unscrambled Data
 Note: While the HDLC FCS is calculated least significant bit first as
 shown:
 <- <- <- <-
 A B C D
 (that is, the FCS calculator is fed as follows: A[0], A[1], ... A[7],
 B[0], B[1], etc...), scrambling is done in the opposite manner, most
 significant bit first, as shown:
 -> -> -> ->
 A B C D.
 That is, the scrambler is fed as follows: A[7], A[6], ... A[0], B[7],
 B[6], etc...
 The scrambler operates continuously through the bytes of the SONET
 STS-SPE/SDH Higher Order VC, bypassing bytes of SONET Path Overhead
 and any fixed stuff (see Figure 20 of ANSI T1.105 [3] or Figure 10-17
 of ITU G.707 [5]). The scrambling state at the beginning of a SONET
 STS-SPE/SDH Higher Order VC is the state at the end of the previous
 SONET STS-SPE/SDH Higher Order VC. Thus, the scrambler runs
 continuously and is not reset per frame. The initial seed is randomly
 chosen by transmitter to improve operational security (see Section
 6). Consequently, the first 43 transmitted bits following startup or
 reframe operation will not be descrambled correctly.
5. Configuration Details
 Other than the FCS length (see below), the standard LCP sync
 configuration defaults apply to SONET/SDH links.
 The following Configuration Options are RECOMMENDED for STS-3c-
 SPE/VC-4:
 Magic Number
 No Address and Control Field Compression
 No Protocol Field Compression
 For operation at STS-12c-SPE/VC-4-4c and above, Address and Control
 Field Compression and Protocol Field Compression are NOT RECOMMENDED.
 The Magic Number option remains RECOMMENDED.
 Regarding the FCS length, with one exception, the 32-bit FCS MUST be
 used for all SONET/SDH rates. For STS-3c-SPE/VC-4 only, the 16-bit
 FCS MAY be used, although the 32-bit FCS is RECOMMENDED. The FCS
 length is set by provisioning and is not negotiated.
6. Security Considerations
 The major change from RFC 1619 is the addition of payload scrambling
 when inserting the HDLC-like framed PPP packets into the SONET STS-
 SPE/SDH Higher Order VC. RFC 1619 was operationally found to permit
 malicious users to generate packets with bit patterns that could
 create SONET/SDH-layer low-transition-density synchronization
 problems, emulation of the SDH set-reset scrambler pattern, and
 replication of the STM-N frame alignment word.
 The use of the x^43 + 1 self-synchronous scrambler was introduced to
 alleviate these potential security problems. Predicting the output
 of the scrambler requires knowledge of the 43-bit state of the
 transmitter as the scrambling of a known input is begun. This
 requires knowledge of both the initial 43-bit state of the scrambler
 when it started and every byte of data scrambled by the device since
 it was started. The odds of guessing correctly are 1/2**43, with the
 additional probability of 1/127 that a correct guess will leave the
 frame properly aligned in the SONET/SDH payload, which results in a
 probability of 9e-16 against being able to deliberately cause
 SONET/SDH-layer problems. This seems reasonably secure for this
 application.
 This scrambler is also used when transmitting ATM over SONET/SDH, and
 public network carriers have considerable experience with its use.
 A known security issue is bandwidth reduction by intentional
 transmission of characters or sequences requiring transparency
 processing by including flag and/or escape characters in user data. A
 user may cause up to a 100% increase in the bandwidth required for
 transmitting his or her packets by filling the packet with flag
 and/or escape characters.
7. Changes from RFC 1619
 As mentioned in the previous section, the major change from RFC 1619
 was the addition of payload scrambling when inserting the HDLC-like
 framed PPP packets into the SONET STS-SPE/SDH Higher Order VC. Other
 changes were:
 The terminology was updated to better match that used by ANSI and
 ITU-T.
 The specification's applicability is now specifically restricted to:
 SONET SDH
 ----------------------------------------
 STS-3c-SPE VC-4
 STS-12c-SPE VC-4-4c
 STS-48c-SPE VC-4-16c
 STS-192c-SPE VC-4-64c
 The Path Signal Label (C2) is set to 22 (16 hex) when using X^43 + 1
 scrambling.
 The 32-bit FCS is required except for operation with STS-3c-SPE/VC-4,
 in which case the 16-bit FCS is allowed (but the 32-bit FCS is still
 recommended).
 The Security Considerations section was added.
8. Intellectual Property
 The IETF takes no position regarding the validity or scope of any
 intellectual property or other rights that might be claimed to
 pertain to the implementation or use of the technology described in
 this document or the extent to which any license under such rights
 might or might not be available; neither does it represent that it
 has made any effort to identify any such rights. Information on the
 IETF's procedures with respect to rights in standards-track and
 standards-related documentation can be found in BCP-11. Copies of
 claims of rights made available for publication and any assurances of
 licenses to be made available, or the result of an attempt made to
 obtain a general license or permission for the use of such
 proprietary rights by implementors or users of this specification can
 be obtained from the IETF Secretariat.
 The IETF invites any interested party to bring to its attention any
 copyrights, patents or patent applications, or other proprietary
 rights which may cover technology that may be required to practice
 this standard. Please address the information to the IETF Executive
 Director.
9. Acknowledgments
 The scrambler description was provided by J. Manchester, S. Davida,
 B. Doshi, and J. Anderson of Lucent Technologies, R. Broberg of Cisco
 Systems, and Peter Lothberg of Sprint Corporation. The security
 analysis was provided by Iain Verigin of PMC-Sierra and Larry McAdams
 of Cisco Systems. The authors would also like to thank the members
 of the IETF's Point-to-Point Protocol Extensions Working Group for
 their many suggestions and improvements to the text.
10. References
 [1] Simpson, W., Editor, "The Point-to-Point Protocol (PPP)", STD
 51, RFC 1661, Daydreamer, July 1994.
 [2] Simpson, W., Editor, "PPP in HDLC-like Framing", STD 51, RFC
 1662, Daydreamer, July 1994.
 [3] American National Standards Institute, "Synchronous Optical
 Network (SONET) - Basic Description including Multiplex
 Structure, Rates and Formats," ANSI T1.105-1995.
 [4] American National Standards Institute, "Synchronous Optical
 Network (SONET)--Payload Mappings," T1.105.02-1998.
 [5] ITU Recommendation G.707, "Network Node Interface For The
 Synchronous Digital Hierarchy", 1996.
 [6] Bradner, S., "Key words for use in RFCs to indicate Requirement
 Levels", BCP 14, RFC 2119, March 1997.
11. Authors' Addresses
 Andrew G. Malis
 Ascend Communications, Inc.
 1 Robbins Road
 Westford, MA 01810 USA
 Phone: +1 978 952 7414
 EMail: malis@ascend.com
 William Allen Simpson
 DayDreamer
 Computer Systems Consulting Services
 1384 Fontaine
 Madison Heights, Michigan 48071
 EMail: wsimpson@GreenDragon.com
12. Full Copyright Statement
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Acknowledgement
 Funding for the RFC Editor function is currently provided by the
 Internet Society.