PPP Extensions Working Group A. Malis INTERNET DRAFT Ascend Communications, Inc. Expires: August 1999 February 1999 <draft-ietf-pppext-pppoversonet-update-01.txt> Obsoletes: RFC 1619 PPP over SONET/SDH Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet- Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. 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. 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 Heirarchy (SDH) circuits. This document is the product of the Point-to-Point Protocol Malis Expires 8/99 [Page i]
INTERNET-DRAFT PPP over SONET/SDH February 1999 Extensions Working Group of the Internet Engineering Task Force (IETF). Comments should be submitted to the ietf-ppp@merit.edu mailing list. Malis Expires 8/99 [Page ii]
INTERNET-DRAFT PPP over SONET/SDH February 1999 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. The Synchronous Optical Network (SONET) [3] is an octet-synchronous multiplex scheme that defines a family of standard rates and formats. The transmission rates are integral multiples of 51.840 Mbps, which may be used to carry T3/E3 bit-synchronous signals. The current commonly-used multiples are specified as STS-1c 51.840 STS-3c 155.520 STS-12c 622.080 STS-48c 2,488.320 STS-192c 9,953.280 The ITU Synchronous Digital Heirarchy (SDH) defines an equivalent set of transmission rates beginning at 155.520 Mbps [5]: SONET SDH equivalent STS-3c STM-1 STS-12c STM-4 STS-48c STM-16 STS-192c STM-64 This document specifies PPP over SONET/SDH for rates up to and including STS-192c/STM-64. 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. Malis Expires 8/99 [Page 1]
INTERNET-DRAFT PPP over SONET/SDH February 1999 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. The octet stream is mapped into the SONET/SDH Synchronous Payload Envelope (SPE), with the octet boundaries aligned with the SPE octet boundaries. Scrambling is performed during insertion into the SPE to provide adequate transparency and protect against potential security threats (see Section 6). For backwards compatibility with RFC 1619 (STS-3c/STM1 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 (SPE 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 SPE. The value of 22 (16 hex) is used to indicate PPP with X^43 + 1 scrambling. For compatibility with RFC 1619 (STS-3c/STM1 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. Transmission Rate The basic rate for PPP over SONET/SDH is that of STS-3c/STM-1 at Malis Expires 8/99 [Page 2]
INTERNET-DRAFT PPP over SONET/SDH February 1999 155.520 Mbps. The available information bandwidth is 149.760 Mbps, which is the STS-3c/STM-1 SPE with section, line and path overhead removed. This is the same super-rate mapping that is used for ATM and FDDI [4]. Lower signal rates MUST use the Virtual Tributary (VT) mechanism of SONET/SDH. This maps existing signals up to T3/E3 rates asynchronously into the SPE, or uses available clocks for bit- synchronous and byte-synchronous mapping. This document specifies PPP over SONET/SDH for rates up to and including STS-192c/STM-64. 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 SPE payload. Because frames are variable in length, the frames are allowed to cross SPE boundaries. 4. X**43 + 1 Scrambler Description The X**43 + 1 scrambler transmitter and receiver operation are as follows: Malis Expires 8/99 [Page 3]
INTERNET-DRAFT PPP over SONET/SDH February 1999 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... Malis Expires 8/99 [Page 4]
INTERNET-DRAFT PPP over SONET/SDH February 1999 The scrambler operates continuously through the bytes of the SPE, 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 SPE is the state at the end of the previous SPE. 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 The standard LCP sync configuration defaults apply to SONET/SDH links. The following Configuration Options are RECOMMENDED: Magic Number No Address and Control Field Compression No Protocol Field Compression 32-bit FCS Regarding the FCS length, the 32-bit FCS MUST be supported for all SONET/SDH rates. For STS-3c ONLY, the 16-bit FCS MAY be used, but is NOT RECOMMENDED due to the error multiplication that can result from scrambling. 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/SDH SPE. 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 Malis Expires 8/99 [Page 5]
INTERNET-DRAFT PPP over SONET/SDH February 1999 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. 7. Intellectual Propoerty 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. 8. Acknowledgments Bill Simpson, Daydreamer, was the editor for this document's predecessor, RFC 1619, and much of his original text remains. 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. Malis Expires 8/99 [Page 6]
INTERNET-DRAFT PPP over SONET/SDH February 1999 9. References [1] Simpson, W., Editor, "The Point-to-Point Protocol (PPP)", RFC 1661, Daydreamer, July 1994. [2] Simpson, W., Editor, "PPP in HDLC-like Framing", 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, Harvard University, March 1997. 10. Author's Address Andrew G. Malis Ascend Communications, Inc. 1 Robbins Road Westford, MA 01810 USA Phone: +1 978 952 7414 Email: malis@ascend.com 11. Full Copyright Statement Copyright (C) The Internet Society (1999). 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 Malis Expires 8/99 [Page 7]
INTERNET-DRAFT PPP over SONET/SDH February 1999 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. Malis Expires 8/99 [Page 8]
INTERNET-DRAFT PPP over SONET/SDH February 1999 Table of Contents 1. Introduction .......................................... 1 2. Physical Layer Requirements ........................... 1 3. Framing ............................................... 3 4. X**43 + 1 Scrambler Description ....................... 3 5. Configuration Details ................................. 5 6. Security Considerations ............................... 5 7. Intellectual Propoerty ................................ 6 8. Acknowledgments ....................................... 6 9. References ............................................ 7 10. Author's Address ...................................... 7 11. Full Copyright Statement .............................. 7 Malis Expires 8/99 [Page iii]
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