Internet Draft Pankaj K. Jha draft-jha-optical-sdl-01.txt Cypress Semiconductor Expiration Date: September, 2002 March, 2002 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 a tail-end 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]. 4. Introduction Simple Data Link (SDL) framing specification (rfc2823) defines a {length, length CRC} based 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. Jha Expires September, 2002 [Page 1] INTERNET-DRAFT Payload-independent SDL March, 2002 Consequently, with the current SDL specification a frame delineation engine must include additional payload protocol-specific logic to process payload protocol header to be able to determine presence of a payload CRC at the end of frame. 5. An Overview of Simple Data Link (SDL, rfc2823) 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 calculated on the 16-bit length field, as shown below: +--------->------>---------------+ | v +=====+======+-----------+------++=====+======+-----------+------+ |LHdr | LHEC | .Payload. | pCRC ||LHdr | LHEC | .Payload. | pCRC | +=====+======+-----------+------++=====+======+-----------+------+ 2 2 N 4 2 2 N 4 Figure 1: SDL Framing Delineation 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 an SDL receive 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. 5.1 Length (LHdr) The current SDL protocol specifies the length field to include all bytes following the LHEC (length CRC) field up to the end of payload but not including pCRC. 5.2 Length HEC (LHEC) This field is 16 bits, and it contains ITU-T CRC-16 calculated on the 16-bit LHdr field. 5.3 Payload CRC (pCRC) Jha Expires September, 2002 [Page 2] INTERNET-DRAFT Payload-independent SDL March, 2002 A payload CRC is OPTIONAL. The SDL specification mandates an advance negotiation or provisioning for presence or absence of pCRC in all frames. 6. 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. 7. 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. In addition, in many systems a frame delineation engine is always separate from a payload CRC computation/verification logic block. Multiservice transport framing protocols must 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 8. 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. 9. Proposal for Length Field Definition To de-couple frame delineation from payload CRC dependence, this draft proposes that the LHdr length value MUST include payload plus Jha Expires September, 2002 [Page 3] INTERNET-DRAFT Payload-independent SDL March, 2002 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. 10. 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 11. Full Copyright Statement "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 Jha Expires September, 2002 [Page 4] INTERNET-DRAFT Payload-independent SDL March, 2002 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. 12. 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. 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. 4. Generic Framing Procedure, Revision 4, ANSI, http://www.t1.org T1X1.5/200-024R4.doc Jha Expires September, 2002 [Page 5]