Internet Draft Pankaj K. Jha draft-jha-optical-sdl-00.txt Cypress Semiconductor Expiration Date: April, 2002 November, 2001 Payload-independent Delineation for Simple Data Link (SDL) Framing 1. Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026 [1]. 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. 2. Abstract Simple Data Link (SDL) framing specification (rfc2823) defines a [length, length CRC] based construct with a tail-end payload CRC for delineating frames over digital links. Many protocols, however, already have built-in CRC mechanisms, and a tail-end CRC is not always needed. This draft proposes a modification to allow SDL receivers to delineate all types of payload without having to analyze payload protocol header to determine presence of CRC. 3. Conventions used in this document The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC-2119 [2]. TABLE OF CONTENTS 1. Status of this Memo ............................................1 2. Abstract .......................................................1 3. Conventions used in this document ..............................1 4. Introduction ...................................................2 5. Organization of this Draft .....................................2 Jha Expires April, 2002 [Page 1] INTERNET-DRAFT Payload-independent SDL November, 2001 6. An Overview of Frame Delineation Methods .......................3 6.1 HDLC .........................................................3 6.2 SDL (Simple Data Link, rfc2823) ..............................3 6.2.1 Length (LHdr) ............................................4 6.2.2 Length HEC (LHEC) ........................................4 6.2.3 Payload CRC (pCRC) .......................................4 7. SDL for a single Service Transport .............................5 8. SDL for a Multiservice Transport ...............................5 9. SDL Engine Limitations .........................................5 10. Proposal for Length Field Modification .......................5 11. Intellectual Property Considerations .........................6 12. Author's Address .............................................6 13. Full Copyright Statement .....................................6 14. References ...................................................7 4. Introduction Simple Data Link (SDL) framing specification (rfc2823) defines a [length, length CRC] construct for delineating frames over digital links. Current SDL protocol specifies a payload CRC at the end for all types of payloads. Many protocols, however, already have built-in CRC mechanisms, and a tail end CRC is not always needed. Consequently, with the current SDL specification a frame delineation engine on the receive must include logic to process payload protocol fields. This draft proposes a modification to so it can delineate all types of payload data without having to depend on the type of payload being carried. The SDL-style delineation scheme is superior to an HDLC frame, with length field indicating number of bytes in the payload. Current scheme would require complex SDL delineation engines that must also analyze a multiservice transport header to determine if a tail end CRC is present. A 4-byte CRC field that an SDL delineation engine skips before restarting hunt for next frame follows payload. Those payloads that don't have a tail end CRC or have a built-in CRC don't work well this mechanism. 5. Organization of this Draft As a backgrounder, this draft first presents an overview of SDL, with a brief comparative analysis of HDLC (High-level Data Link Control) protocol for frame delineation. Jha Expires April, 2002 [Page 2] INTERNET-DRAFT Payload-independent SDL November, 2001 This is followed by an analysis of SDL operation with current specification and with suggested modification to the protocol. 6. An Overview of Frame Delineation Methods 6.1 HDLC HDLC-based delineation is currently the most popular method for sending frames on serial links, optical fiber, and over optical fiber using SONET/SDH (HDLC-based delineation over SONET/SDH is described in Packet-over-SONET (POS) as defined in rfc2615 [4]). +====+---------+---------+-----------------+------+====+ | 7E | Address | Control | ....Payload.... | pCRC | 7E | +====+---------+---------+-----------------+------+====+ 1 1 1 4 1 (bytes) Figure 1: HDLC Framing Delineation Data is framed using a 0x7E pattern at both ends. Conflicting control bytes in payload are byte-stuffed to avoid hitting a 0x7E or any of the control characters within the frame. With increasing speeds and larger frame sizes, limitations of HDLC are becoming apparent, making it less favorable for optical networks. Some of the issues with HDLC are as follows: o During transmission, every outgoing byte must be monitored, and byte-by-byte stuffing decision needs to be made to prevent flag emulation by data bytes. And the receiver needs to monitor every incoming byte to perform the de-stuffing. Processing every byte at speeds of OC-48/192/768 requires high-speed hardware logic for every channel. o Malicious long packets (killer packets) can defeat scrambling, causing loss of synchronization. This is more likely now with large packet sizes. o No advance knowledge of length prevents efficient use of variable- size queues for handling packets on incoming ports. Length of HDLC frame can only be determined when terminating 0x7E is encountered. Since nodes have no knowledge of frame length a priori, they must allocate buffer with the highest size for every incoming frame. o HDLC implicitly is bandwidth-inefficient as byte stuffing causes the number of bytes transmitted to be much larger than the actual number of bytes in the packet. 6.2 SDL (Simple Data Link, rfc2823) Jha Expires April, 2002 [Page 3] INTERNET-DRAFT Payload-independent SDL November, 2001 SDL framing delineation protocol (rfc2823, [1]) prefixes a payload with a 32-bit header. First 16 bits of this word (LHdr) hold the length of the payload and the other 16 bits (LHEC) contain CRC-16 (Cyclic Redundancy Check) calculated on the 16-bit length field, as shown below. +--------->------>---------------+ | | +=====+======+-----------+------++=====+======+-----------+------+ |LHdr | LHEC | .Payload. | pCRC ||LHdr | LHEC | .Payload. | pCRC | +=====+======+-----------+------++=====+======+-----------+------+ 2 2 N 4 2 2 N 4 Figure 2: SDL Framing Delineation SDL provides a robust CRC-16 based framed boundary delineation mechanism that solves all current HDLC issues like robustness in bad BER conditions, variable packet size expansion, and malicious long packet scrambler manipulation. Frames are located by hunting for a length/CRC match, much the same way as ATM cells are located by HEC synchronization. Next frame is located by jumping length bytes in the frame, skipping next four bytes (payload CRC is computed and verified by SDL engine) and again looking for a length/CRC match at the following byte location. Since there is no manipulation to the header bytes, link bandwidth is optimally utilized for sending frames using SDL. In case of data corruption at the location of a length CRC field, the hardware begins a byte-by-byte hunting for the length/CRC construct until a match is found. 6.2.1 Length (LHdr) The current specification the length field to include all bytes following the LHEC (length CRC) field up to the end of payload but not including pCRC. 6.2.2 Length HEC (LHEC) This field is 16 bits, and it contains ITU-T CRC-16 calculated on the 16-bit LHdr field. 6.2.3 Payload CRC (pCRC) Jha Expires April, 2002 [Page 4] INTERNET-DRAFT Payload-independent SDL November, 2001 The payload CRC is an OPTIONAL field. When present it MUST be an ITU- T CRC-32. The SDL specification mandates an advance negotiation or provisioning for presence or absence of pCRC in all frames. 7. SDL for a single Service Transport The current SDL specification for payload CRC scheme is sufficient for sending a single type of data. If the service being carried doesn't have a built-in CRC mechanism, an SDL engine can provide a payload CRC on transmission and check for a valid CRC on reception. For other protocols, such as Ethernet, the external CRC is omitted. In this case, the SDL receive engine performs the length/length-CRC hunting and passes the payload to an Ethernet MAC that checks the CRC embedded in the Ethernet frame. 8. SDL for a Multiservice Transport In a multiservice transport, however, some data types may not have an explicit CRC field present at the end of the payload. Examples of such data types are _ ATM cell(s), SONET/SDH frame(s), T1/T3/NxDS0 frames, etc. In addition, in many systems a frame delineation engine is always separate from a CRC computation/verification logic block. Multiservice transport framing protocols, such as Generic Framing Procedure (GFP, [6]) and Hybrid Data Transport (HDT, [3]), feature a bit in their header fields to indicate presence of a payload CRC. These framing protocols understand when to include or exclude payload CRC during transmit/receive 9. SDL Engine Limitations Dependence on payload protocol structure complicates design of SDL transmit/receive engines. For multiservice transport, devices must contain logic blocks that understand details of protocol that are carried inside the SDL payload area, and the engines must be able to process them for sending and receiving payload data with/without a CRC. Scalability of such SDL engines will be quite limited, especially in multi-channel environments where multiple instances of SDL engines need to run. Worse yet, each time a new multiservice transport protocol is to be transported, each of the engines need to modified, forcing a re-design of hardware device. 10. Proposal for Length Field Modification Jha Expires April, 2002 [Page 5] INTERNET-DRAFT Payload-independent SDL November, 2001 To de-couple frame delineation from payload CRC dependence, this draft proposes that the LHdr length value MUST include payload plus any payload CRC field, if present. With this scheme, a frame delineation mechanism becomes simpler - to get to the next frame, the delineation logic simply skips the number of bytes given by the length field (LHdr). On reception, the entire payload (including any payload CRC) is given to a payload-processing logic block. The payload-processing block is responsible for any generation and processing of payload CRC. During transmission, the payload-processing block sends payload and any payload CRC to the SDL framing logic. This logic simply adds a 4-byte header consisting of a 2-byte length field for payload and any CRC, and a 2-byte CRC for the length field. With this proposed change, an SDL delineating device becomes very efficient and highly scalable with increasing speed and density, as it only deals with inserting or removing the 4-byte header for all types of payload data. 11. Intellectual Property Considerations Cypress Semiconductor Corporation may own intellectual property on some of the technologies disclosed in this document. In the event that Cypress obtains such patent rights, Cypress intends to license them on reasonable and non-discriminatory terms in accordance with the intellectual property rights procedures of the IETF standards process. Lucent Technologies Inc. may own intellectual property on some of the technologies discussed and used in this document. 12. Author's Address Pankaj K Jha Cypress Semiconductor 3901 N First Street San Jose, CA 95134 USA Phone: 408 432 7091 Fax: 408 943 2949 Email: pkj@cypress.com 13. Full Copyright Statement Jha Expires April, 2002 [Page 6] INTERNET-DRAFT Payload-independent SDL November, 2001 "Copyright (C) The Internet Society. 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. 14. References 1. Carlson, J., Langner, P., Hernandez-Valencia, E.J., Manchester, J., PPP over Simple Data Link (SDL) using SONET/SDH with ATM- like framing, rfc2823.txt, May 2000. 2. Doshi, B., Dravida, S., Hernandez-Valencia, E., Matragi, W., Qureshi, M., Anderson, J., Manchester, J.,"A Simple Data Link Protocol for High Speed Packet Networks", Bell Labs Technical Journal, pp. 85-104, Vol.4 No.1, January-March 1999. 3. Jha, P., A Hybrid Data Transport Protocol for Optical Networks, IETF Draft (http://www.ietf.org), draft-jha-optical-hdt-01.txt, November 2001 4. Malis, A., Simpson, W., PPP over SONET/SDH, rfc2615, June 1999. 5. Anderson, J., Manchester, J., Rodriguez-Moral, A., Veeraraghavan, M.,"Protocols and Architectures for IP Optical Networking", Bell Labs Technical Journal, pp. 105-124, Vol.4 No.1, January - March 1999. 6. Generic Framing Procedure, Revision 4, ANSI, http://www.t1.org, 1x150244.pdf Jha Expires April, 2002 [Page 7]