PPP Working Group J. Carlson Internet Draft IronBridge Networks Updates RFC 1619 P. Langner expires in six months Lucent Technologies Microelectronics Group J. Manchester Lucent Technologies November 1998 PPP over Simple Data Link (SDL) using SONET/SDH with ATM-like framing <draft-ietf-pppext-sdl-00.txt> Status of this Memo This document is the product of the Point-to-Point Protocol Extensions Working Group of the Internet Engineering Task Force (IETF). Comments should be submitted to the ietf-ppp@merit.edu mailing list. Distribution of this memo is unlimited. This document is an Internet-Draft. 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. Internet-Drafts 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." To view the entire list of current Internet-Drafts, please check the "1id-abstracts.txt" listing contained in the Internet-Drafts Shadow Directories on ftp.is.co.za (Africa), ftp.nordu.net (Northern Europe), ftp.nis.garr.it (Southern Europe), munnari.oz.au (Pacific Rim), ftp.ietf.org (US East Coast), or ftp.isi.edu (US West Coast). Carlson, et al. expires May 1999 [Page 1]
INTERNET DRAFT PPP SDL on SONET/SDH November 1998 Abstract The Point-to-Point Protocol (PPP) [1] provides a standard method for transporting multi-protocol datagrams over point-to-point links, and RFCs 1662 [2] and 1619 [3] provide a means to carry PPP over Synchronous Optical Network (SONET) [5] and Synchronous Digital Hierarchy (SDH) [6] circuits. This document extends these standards to include a new encapsulation for PPP called Simple Data Link (SDL) [7]. SDL provides a very low overhead alternative to standard HDLC encapsulation for SONET/SDH links. This document is the product of the Point-to-Point Protocol Working Group of the Internet Engineering Task Force (IETF). Comments should be submitted to the ietf-ppp@merit.edu mailing list. Applicability This specification is intended for those implementations which desire to use the PPP encapsulation over high speed point-to-point circuits, both with so-called "dark fiber" and over public telecommunications networks. Because this enhanced PPP encapsulation has very low overhead, it is anticipated that significantly higher throughput can be attained compared to other SONET/SDH payload mappings, at a significantly lower cost for line termination equipment. Carlson, et al. expires May 1999 [Page 2]
INTERNET DRAFT PPP SDL on SONET/SDH November 1998 Table of Contents 1. Introduction ........................................ 4 2. Physical Layer Requirements ......................... 4 2.1. Payload Types ....................................... 4 2.2. Control Signals ..................................... 5 2.3. Synchronization Modes ............................... 6 2.4. Simple-Data-Link LCP Option ......................... 6 2.5. Framing ............................................. 7 2.6. Synchronization Procedure ........................... 9 2.7. Scrambler Operation ................................. 10 2.8. CRC Generation ...................................... 11 2.9. Error Correction .................................... 12 3. Performance Analysis ................................ 13 3.1 Mean Time To Frame (MTTF) ........................... 13 3.2 Mean Time To Synchronization (MTTS) ................. 14 3.3 Probability of False Frame (PFF) .................... 15 3.4 Probability of False Synchronization (PFS) .......... 15 3.5 Probability of Loss of Frame (PLS) .................. 15 4. Configuration Details ............................... 15 APPENDICES ..................................................... 16 A. CRC Generation ...................................... 16 B. Error Correction Tables ............................. 18 5. Security Considerations ............................. 20 6. References .......................................... 20 7. Acknowledgments ..................................... 20 8. Working Group and Chair Address ..................... 21 9. Authors' Addresses .................................. 21 Carlson, et al. expires May 1999 [Page 3]
INTERNET DRAFT PPP SDL on SONET/SDH November 1998 1. Introduction The Path Signal Label (SONET/SDH overhead byte named C2; referred to as PSL in this document) is intended to indicate the type of data carried on the path. This data, in turn, is referred to as the SONET Synchronous Payload Envelope (SPE) or SDH Administrative Unit Group (AUG). The experimental PSL value of decimal 207 (CF hex) is currently [3] used to indicate that the SPE contains PPP framed using RFC 1662 Octet Synchronous (O-S) framing and transmission without scrambling, and the value 22 (16 hex) is used to indicated PPP framed using O-S framing and transmission with ATM-style X^43+1 scrambling. This document describes a method to enable the use of SDL framing for PPP over SONET/SDH, and describes the framing technique and require- ments for PPP. While O-S framing has a worst-case octet overhead of 100% of all data octets transmitted, SDL has a fixed eight octet per frame overhead with zero data overhead. This mapping is similar to the earlier "Ether-like Framing" proposal, found in a separate work- in-progress. [4] SDL is being submitted by Lucent, IronBridge, and others to ANSI sub- committee T1A1.5 for eventual international telecommunications stan- dardization. Also, a Bellcore Digest article on SDL will appear in September. Note: This document describes a new PSL value 23 (17 hex). This value has not been allocated by any applicable standards body, and SDL must not be used on public networks until a standard value is allocated. A joint contribution will be made to ANSI subcommittee T1X1.5 requesting the assignment of 17 hex as a PSL for an SDL over SONET mapping in T1.105. 2. Physical Layer Requirements PPP treats SONET/SDH transport as octet-oriented synchronous links. No provision is made to transmit partial octets. Also, SONET/SDH links are full-duplex by definition. 2.1. Payload Types Only synchronous payloads STS-1 and higher are considered in this document. Plesiochronous payload mappings, such as T1 and T3, are defined for SONET/SDH and for SDL, but, since standard HDLC is defined for PPP on those media, PPP over SDL is not defined. "Packet-over-fiber" mode is also defined for SDL, but is for future study in PPP. Carlson, et al. expires May 1999 [Page 4]
INTERNET DRAFT PPP SDL on SONET/SDH November 1998 2.2. Control Signals The PPP over SONET/SDH mapping allows the use of the PSL as a control signal. Not all equipment, however, is capable of setting or detect- ing this value, and any standard must take this into account. Equip- ment employing only SDL MUST be capable of transmitting PSL with value 23, but need not be capable of detecting the peer's value or capable of changing its own value. There are two methods to enable SDL, an LCP-negotiated method and a prior-arrangement method. The former allows for easier configuration and compatibility with existing equipment, while the latter allows general use with separate SONET/SDH transmission equipment with PSL limitations. Both types of implementations will freely interoperate given the procedures below. LCP-negotiated systems MUST be capable of changing their transmitted PSL value and detecting the peer's value. Equipment without these features MUST NOT support LCP negotiation of SDL. When SDL is negotiated by LCP, LCP negotiation MUST be started with the PSL value initially set to 22 or 207 and the corresponding non- SDL O-S PPP encapsulation MUST be used. The SDL LCP option is then placed in the LCP Configure-Request messages transmitted. On recep- tion of LCP Configure-Request with an SDL LCP option or when the peer's transmitted PSL value is received as 23, the implementation MUST shut down LCP by sending a Down event to its state machine, then switch its transmitted PSL value to 23, switch encapsulation mode to SDL, wait for SDL synchronization, and then restart LCP by sending an Up event into LCP. Otherwise, if the peer does not transmit PSL value 23 and it does not include the SDL LCP option in its LCP Configure-Request messages, then operation using non-SDL O-S PPP encapsulation continues. If the received PSL value subsequently received reverts from 23 to any other value, then this is treated as a Down event into the LCP state machine, and an Up event MUST be gen- erated if the new value is recognized as a valid PPP framing mode. When SDL is enabled by prior arrangement, the PSL SHOULD be transmit- ted as 23. Any other value may also be used by prior external arrangement with the peer, although the values 22 and 207 are discouraged. (Such use is enforced by an administrator, and is out- side the scope of this specification.) When SDL is enabled by prior arrangement, the SDL LCP option SHOULD NOT be negotiated by the peers. An implementation-specific configuration option SHOULD exist to enable the use of prior-arrangement versus LCP-negotiated modes. This option SHOULD be presented to an administrator, and SHOULD Carlson, et al. expires May 1999 [Page 5]
INTERNET DRAFT PPP SDL on SONET/SDH November 1998 default to LCP-negotiated if the hardware permits. Otherwise, if the hardware implementation precludes non-SDL modes of operation, then it MUST default to prior-arrangement mode. 2.3. Synchronization Modes Unlike O-S encapsulation, SDL provides a positive indication that it has achieved synchronization with the peer. An SDL PPP implementa- tion MUST provide a means to temporarily suspend PPP data transmis- sion (both user data and negotiation traffic) if synchronization loss is detected. An SDL PPP implementation SHOULD also provide a confi- gurable timer that is started when SDL is initialized and restarted on the loss of synchronization. If this timer expires, implementation-dependent action should be taken to report the hardware failure. 2.4. Simple-Data-Link LCP Option A new LCP Configuration Option is used to request Simple Data Link (SDL) [7] operation for the PPP link. A summary of the Simple-Data-Link Configuration Option format for the Link Control Protocol (LCP) is shown below. The fields are transmit- ted from left to right. 0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Type tbd Length 2 This option is used only as a hint to the peer that SDL operation is preferred by the sender. If the current encapsulation mode is not SDL, then the only appropriate response to reception of this option by an SDL speaker is to then switch the encapsulation mode to SDL (as detailed in the section above) and restart LCP. Non SDL-speakers SHOULD instead send LCP Configure-Reject for the option. Carlson, et al. expires May 1999 [Page 6]
INTERNET DRAFT PPP SDL on SONET/SDH November 1998 If either LCP Configure-Nak or LCP Configure-Reject is received for this option, then the next transmitted LCP Configure-Request MUST NOT include this option. If LCP Configure-Ack with this option is received, it MUST NOT be treated as a request for SDL mode. If the received LCP Configure-Request message does not contain an SDL LCP option, an implementation SHOULD NOT send an unsolicited Configure- Nak for the option. (An implementation of SDL that is already in SDL framing mode and receives this option in an LCP Configure-Request message MAY, both for clarity and for convergence reasons, elect to send LCP Configure-Ack. It MUST NOT restart LCP nor change framing modes in this case.) 2.5. Framing 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. Bytes marked as "overhead" or "fixed stuff" in SONET/SDH documentation for concatenated streams are not used as payload bytes. When SDL framing for PPP is employed, the SDL "Datagram Offset" is fixed at 4, and the "A" and "B" messages are never used. These optional features of SDL are not described in this document, but are rather described in Lucent's SDL specification [7]. Fixing the Datagram Offset to 4 allows a PPP MRU/MTU of 65536 using SDL. SDL framing is in general accomplished by the use of a four octet header on the packet. This fixed-length header allows the use of a simple framer to detect synchronization as described in section 2.6. For use with PPP, this header precedes each raw PPP packet as fol- lows: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Packet Length | Header CRC | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | PPP packet (beginning with address and control fields) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ..... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Packet CRC | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Carlson, et al. expires May 1999 [Page 7]
INTERNET DRAFT PPP SDL on SONET/SDH November 1998 The four octet length header is DC balanced by exclusive-OR (also known as "modulo 2 addition") with the hex value B6AB31E0. This is the maximum transition, minimum sidelobe, Barker-like sequence of length 32. No other scrambling is done on the header itself. Packet Length is an unsigned 16 bit number in network byte order. Unlike the standard PPP FCS, the Header CRC is a CRC-16 generated with initial value zero and transmitted in standard network byte order. The PPP packet is scrambled, and begins with the standard address and control fields, and may be any integral octet length (i.e., it is not padded unless the Self Describing Padding option is used). The Packet CRC is also scrambled, and has a mode-dependent length (described below), and is located only on an octet boundary; no alignment of this field may be assumed. When the Packet Length value is 4 or greater, the distance in octets between one message header and the next in SDL is the sum of Packet Length field, Datagram Offset value, and the fixed size of the Packet CRC field. The Datagram Offset is a configurable SDL parameter, which is set to the fixed value 4 for PPP. When the Packet Length is 0, the distance to the next header is 4 octets. This is the idle fill header. When the Packet Length is 1 to 3, the distance to the next header is 12 octets. These headers are used for special SDL messages described below. General SDL, like PPP, allows the use of no CRC, ITU-T CRC-16, or ITU-T CRC-32 for the packet data. However, because the Packet Length field does not include the CRC length, synchronization cannot be maintained if the CRC type is changed per RFC 1570, because frame- to-frame distance is, as described above, calculated including the CRC length. Thus, this PPP over SDL specification fixes the CRC type to CRC-32 (four octets), and all SDL implementations MUST reject any LCP FCS Alternatives Option [8] requested by the peer when in SDL mode. PPP over SDL implementations MAY allow a configuration option to set different CRC types for use by prior arrangement. Any such configur- able option MUST default to CRC-32, and MUST NOT be include LCP nego- tiation of FCS Alternatives. With the SDL Datagram Offset set to 4, the value placed in the Packet Length field is exactly the length in octets of the PPP frame itself, including the address and control fields but not including the FCS field. Because Packet Lengths below 4 are reserved, the Packet Length MUST be 4 or greater for any legal PPP packet. PPP packets with fewer octets, which are not possible without address/control or protocol Carlson, et al. expires May 1999 [Page 8]
INTERNET DRAFT PPP SDL on SONET/SDH November 1998 field compression, MUST be padded to length 4 for SDL. Inter-packet time fill is accomplished by sending the four octet length header with the Packet Length set to zero. No provision is made for intra-packet time fill. All octets in the PPP packet data following the SDL header through the final packet CRC are scrambled using an x^48+x^28+x^27+x+1 independent scrambler initialized to all ones. The scrambler is reset to all ones if the shift register ever contains all zeros. The special value of 1 for Packet Length is reserved to transfer the scrambler state from the transmitter to the receiver. In this case, the SDL header is followed by six octets (48 bits) of scrambler state plus two octets of CRC-16 over the scrambler state. None of these eight octets are scrambled. The special values of 2 and 3 for Packet Length are reserved for "A" and "B" messages, which are also six octets in length followed by two octets of CRC-16. Each of these eight octets are scrambled. No use for these messages with PPP SDL is defined. 2.6. Synchronization Procedure SDL synchronization consists of two components, link and scrambler synchronization. Both must be completed before PPP data flows on the link. The link synchronization procedure is similar to the I.432 section 4.5.1.1 ATM HEC delineation procedure [9], but simpler because the SDL messages are variable length. The machine starts in HUNT state until a four octet sequence in the data stream with a valid CRC-16 is found. (Note that the CRC-16 single-bit error correction technique described in section 2.9 is not employed until the machine is in in SYNC state.) Such a valid sequence is a candidate SDL header. On finding the valid sequence, the machine enters PRESYNCH state. Any one invalid SDL header in PRESYNC state returns the link to HUNT state. If a valid SDL header is seen in PRESYNCH state, then the link enters SYNCH state, and the scrambler synchronization sequence is started. If an invalid SDL header is detected, then the link is returned to HUNT state, and PPP transmission is suspended. When scrambler synchronization is started, a scrambler state message is sent (Packet Length set to 1 and six octets of scrambler state in network byte order follow the SDL header). This message is sent Carlson, et al. expires May 1999 [Page 9]
INTERNET DRAFT PPP SDL on SONET/SDH November 1998 once. At this point, PPP transmission is enabled. Scrambler state messages are periodically transmitted to keep the peers in synchronization. A period of once per eight transmitted packets is suggested, and it SHOULD be configurable. Excessive packet CRC errors detected indicates an extended loss of synchroniza- tion and should trigger link resynchronization. On reception of a scrambler state message, an SDL implementation MUST compare the received 48 bits of state with the receiver's scrambler state. If any of these bits differ, then a synchronization slip error is declared. After such an error, the next valid scrambler state message received MUST be loaded into the receiver's scrambler, and the error condition is then cleared. 2.7. Scrambler Operation The transmit and receive scramblers are shift registers with 48 stages that are initialized to all-ones when the link is initialized. Each is refilled with all one bits if the value in the shift register ever becomes all zeros. This scrambler is not reset at the beginning of each frame, as is the SONET/SDH X^7+X^6+1 scrambler, nor is it modified by the transmitted data, as is the ATM self-synchronous scrambler. Instead it is kept in synchronization using special SDL messages. +----XOR<--------------XOR<---XOR<----------------+ | ^ ^ ^ | | | | | | +->D0-+->D1-> ... ->D26-+->D27-+->D28-> ... ->D47-+ | v OUT Each XOR is an exclusive-or gate; also known as a modulo-2 adder. Each Dn block is a D-type flip-flop clocked on the appropriate data clock. The scrambler is clocked once after transmission of each bit of SDL data, whether or not the transmitted bit is scrambled. When scram- bling is enabled for a given octet, the OUT bit is exclusive-ored with the raw data bit to produce the transmitted bit. Bits within an octet are transmitted MSB-first. Reception of scrambled data is identical to transmission. Each received bit is exclusive-ored with the output of the separate receive data scrambler. Carlson, et al. expires May 1999 [Page 10]
INTERNET DRAFT PPP SDL on SONET/SDH November 1998 To generate a scrambler state message, the contents of D47 through D0 are snapshot at the point where the first scrambler state bit is sent. D47 is transmitted as the first bit of the output. The first octet transmitted contains D47 through D40, the second octet D39 through D32, and the sixth octet D7 through D0. The receiver of a scrambler state message MUST first run the CRC-16 check and correct algorithm over this message. If the CRC-16 message check detects multiple bit errors, then the message is dropped and is not processed further. Otherwise, it then should compare the contents of the entire receive scrambler state D47:D0 with the corrected message. (By pipelining the receiver with multiple clock stages between SDL Header error- correction block and the descrambling block, the receive descrambler will be on the correct clock boundary when the message arrives at the descrambler. This means that the decoded scrambler state can be treated as immediately available at the beginning of the D47 clock cycle into the receive scrambler.) If any of the received scrambler state bits is different from the corresponding shift register bit, then a soft error flag is set. If the flag was already set when this occurs, then a synchronization slip error is declared. This error SHOULD be counted and reported through implementation-defined network management procedures. When the receiver has this soft error flag set, any scrambler state mes- sage that passes the CRC-16 message check without multiple bit errors is clocked directly into the receiver's state register after the com- parison is done, and the soft error flag is then cleared. Otherwise, while uncorrectable scrambler state messages are received, the soft error flag state is maintained. (The intent of this mechanism is to reduce the likelihood that a falsely corrected scrambler state message with multiple bit errors can corrupt the running scrambler state.) 2.8. CRC Generation The CRC-16 and CRC-32 generator polynomials used by SDL are the ITU-T standard polynomials [10]. These are: x^16+x^12+x^5+1 x^32+x^26+x^23+x^22+x^16+x^12+x^11+x^10+x^8+x^7+x^5+x^4+x^2+x+1 The SDL Header CRC and the CRC-16 used for each of the three special messages (scrambler state, message A, and message B) are all Carlson, et al. expires May 1999 [Page 11]
INTERNET DRAFT PPP SDL on SONET/SDH November 1998 generated using an initial remainder value of 0000 hex. The optional CRC-16 on the payload data (this mode is not used with PPP over SDL) uses the standard initial remainder value of FFFF hex for calculation and the bits are complemented before transmission. The final CRC remainder, however, is transmitted in network byte order, unlike the regular PPP FCS. If the CRC-16 algorithm is run over all of the octets including the appended CRC itself, then the remainder value on intact packets will always be E2F0 hex. Alterna- tively, an implemenation may stop CRC calculation before processing the appended CRC itself, and do a direct comparison. The standard CRC-32 on the payload data (used for PPP over SDL) uses the initial remainder value of FFFFFFFF hex for calculation and the bits are complemented before transmission. The CRC, however, is transmitted in network byte order, unlike the optional PPP 32 bit FCS. The remainder value on intact packets when the appended CRC value is included in the calculation is 38FB2284. C code to generate these CRCs is found in Appendix A. 2.9. Error Correction The error correction technique is based on the use of a Galois number field, as with the ATM HEC correction. In a Galois number field, f(a+b) = f(a) + f(b). Since the CRC-16 used for SDL forms such a field, we can state that CRC(message+error) = CRC(message) + CRC(error). Since the SDL CRC-16 of a properly formed message is always zero, this means that, for the N distinct "error" strings corresponding to a single bit error, there are N distinct CRC(error) values, where N is the number of bits in the message. A table look-up is thus applied to the CRC-16 residue after calcula- tion over the four octet SDL header to correct bit errors in the header and to detect multiple bit errors. A table look-up is simi- larly applied to the CRC-16 residue after calculation over the eight octet scrambler state message to correct bit errors and to detect multiple bit errors. (This second correction is also used for the special SDL A and B messages, which are not used for PPP over SDL.) Note: This error correction technique is used only when the link has entered synchronization state. While hunting for SDL framing and when in PRESYNC state, error correction should not be performed, and only messages with syndrome 0000 are accepted. Since the CRC calculation is started with zero, the two tables can be merged. The four octet table is merely the last 32 entries of the Carlson, et al. expires May 1999 [Page 12]
INTERNET DRAFT PPP SDL on SONET/SDH November 1998 eight octet table. Eight octet (64 bit) single bit error syndrome table (in hex): FD81 F6D0 7B68 3DB4 1EDA 0F6D 8FA6 47D3 ABF9 DDEC 6EF6 377B 93AD C1C6 60E3 B861 D420 6A10 3508 1A84 0D42 06A1 8B40 45A0 22D0 1168 08B4 045A 022D 8906 4483 AA51 DD38 6E9C 374E 1BA7 85C3 CAF1 ED68 76B4 3B5A 1DAD 86C6 4363 A9A1 DCC0 6E60 3730 1B98 0DCC 06E6 0373 89A9 CCC4 6662 3331 9188 48C4 2462 1231 8108 4084 2042 1021 Thus, if the syndrome 6EF6 is seen on an eight octet message, then the third bit (hex 20) of the second octet is in error. Similarly, if 48C4 is seen on an eight octet message, then the second bit (hex 40) in the eighth octet is in error. For a four octet message, the same two syndromes would indicate a multiple bit error for 6EF6, and a single bit error in the second bit of the fourth octet for 48C4. Corresponding C code to generate this table is found in Appendix B. 3. Performance Analysis There are five general statistics that are important for framing algorithms. These are: MTTF Mean time to frame MTTS Mean time to synchronization PFF Probability of false frame PFS Probability of false synchronization PLF Probability of loss of frame The following sections summarize each of these statistics for SDL. Details and mathematic development can be found in the Lucent SDL documentation [7]. 3.1. Mean Time To Frame (MTTF) This metric measures the amount of time required to discover correct framing in the input data. This may be measured in any convenient units, such as seconds or bytes. For SDL, the relevant measurement is in packets, since fragments of packets are not useful. In order to calculate MTTF, we must first determine how often the frame detection state machine is "unavailable" because it has Carlson, et al. expires May 1999 [Page 13]
INTERNET DRAFT PPP SDL on SONET/SDH November 1998 detected an apparent framing header within the user data. Since the probability of a false header detection using CRC-16 in random data is 2^-16 and this rate is large compared to the allowable packet size, it is worthwhile to run multiple parallel frame- detection state machines. Each machine starts with a different can- didate framing point in order to reduce the probability of falsely detecting user data as a valid frame header. The results for this calculation, given maximal 64KB packets and average 384 byte packets, are: Number of Unavailability Unavailability Framers 64KB packets 384 byte pkts 1 367.9E-3 5.373E-3 2 30.83E-3 1.710E-6 3 2.965E-3 971.2E-12 4 253.2E-6 465.3E-15 Using these values, MTTF can be calculated as a function of the Bit Error Rate (BER). These plots show a characteristically flat region for all BERs up to a knee, beyond which the begins to rise sharply. In all cases, this knee point has been found to occur at a BER of approximately 1E-4, which is several orders of magnitude above that observed on existing SONET/SDH links. The flat rate values are sum- marized as: Number of Flat region Flat region Framers 64KB packets 384 bytes 1 3.58 1.52 2 1.595 1.5 3 1.52 1.5 4 1.5 1.5 Thus, for common packet sizes in an implementation with two parallel framers using links with a BER of 1E-4 or better, the MTTF is approx- imately 1.5 packets. This is also the optimal time, since it represents initiating framing at an average point half-way into one packet, and achieving good framing after seeing exactly one correctly framed packet. 3.2. Mean Time To Synchronization (MTTS) The MTTS for SDL is one half of the scrambling state transmission interval (in packets) plus the MTTF. For insertion at the default rate of one per eight packets, the MTTS is 5.5 packets. Carlson, et al. expires May 1999 [Page 14]
INTERNET DRAFT PPP SDL on SONET/SDH November 1998 (The probability of receiving a bad scrambling state transmission should also be included in this calculation. The probability of ran- dom corruption of this short message is shown in the SDL document [7] to be small enough that it can be neglected for this calculation.) 3.3. Probability of False Frame (PFF) The PFF is 232.8E-12 (2^-32), since false framing requires two con- secutive headers with falsely correct CRC-16. 3.4. Probability of False Synchronization (PFS) The PFS is 54.21E-21 (2^-64), and is calculated as the PFF above mul- tiplied by the probability of a falsely detected scrambler state mes- sage, which itself contains two independent CRC-16 calculations. 3.5. Probability of Loss of Frame (PLS) The PLS is a function of the BER, and for SDL is approximately BER multiplied by .005, which is the probability of two or more bit errors occurring within the 32 bit SDL header. Thus, at a BER of 1E-5, the PLS is 5E-8. 4. Configuration Details The standard LCP synchronous 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 No FCS alternatives (32-bit FCS default) Carlson, et al. expires May 1999 [Page 15]
INTERNET DRAFT PPP SDL on SONET/SDH November 1998 Appendix A: CRC Generation The following unoptimized code generates proper CRC-16 and CRC-32 values for SDL messages. Note that the polynomial bits are numbered in big-endian order for SDL CRCs; bit 0 is the MSB. typedef unsigned char u8; typedef unsigned short u16; typedef unsigned long u32; #define POLY16 0x1021 #define POLY32 0x04C11DB7 u16 crc16(u16 crcval, u8 cval) { int i; crcval ^= cval << 8; for (i = 8; i--; ) crcval = crcval & 0x8000 ? (crcval << 1) ^ POLY16 : crcval << 1; return crcval; } u32 crc32(u32 crcval, u8 cval) { int i; crcval ^= cval << 24; for (i = 8; i--; ) crcval = crcval & 0x80000000 ? (crcval << 1) ^ POLY32 : crcval << 1; return crcval; } u16 crc16_special(u8 *buffer, int len) { u16 crc; crc = 0; while (--len >= 0) crc = crc16(crc,*buffer++); return crc; } Carlson, et al. expires May 1999 [Page 16]
INTERNET DRAFT PPP SDL on SONET/SDH November 1998 u16 crc16_payload(u8 *buffer, int len) { u16 crc; crc = 0xFFFF; while (--len >= 0) crc = crc16(crc,*buffer++); return crc ^ 0xFFFF; } u32 crc32_payload(u8 *buffer, int len) { u32 crc; crc = 0xFFFFFFFFul; while (--len >= 0) crc = crc32(crc,*buffer++); return crc ^ 0xFFFFFFFFul; } void make_sdl_header(int packet_length, u8 *buffer) { u16 crc; buffer[0] = (packet_length >> 8) & 0xFF; buffer[1] = packet_length & 0xFF; crc = crc16_special(buffer,2); buffer[0] ^= 0xB6; buffer[1] ^= 0xAB; buffer[2] = ((crc >> 8) & 0xFF) ^ 0x31; buffer[3] = (crc & 0xFF) ^ 0xE0; } Carlson, et al. expires May 1999 [Page 17]
INTERNET DRAFT PPP SDL on SONET/SDH November 1998 Appendix B: Error Correction Tables To generate the error correction table, the following implementation may be used. It creates a table called sdl_error_position, which is indexed on CRC residue value. The tables can be used to determine if no error exists (table entry is equal to FE hex), one correctable error exists (table entry is zero-based index to errored bit with MSB of first octet being 0), or more than one error exists, and error is uncorrectable (table entry is FF hex). To use for eight octet mes- sages, the bit index from this table is used directly. To use for four octet messages, the index is treated as an unrecoverable error if it is below 32, and as bit index plus 32 if it is above 32. The program also prints out the error syndrome table shown in section 2.9. This may be used as part of a "switch" statement in a hardware implementation. u8 sdl_error_position[65536]; /* Calculate new CRC from old^(byte<<8) */ u16 crc16_t8(u16 crcval) { u16 f1,f2,f3; f1 = (crcval>>8) | (crcval<<8); f2 = (crcval>>12) | (crcval&0xF000) | ((crcval>>7)&0x01E0); f3 = ((crcval>>3) & 0x1FE0) ^ ((crcval<<4) & 0xF000); return f1^f2^f3; } void generate_error_table(u8 *bptab, int nbytes) { u16 crc; int i, j, k; /* Marker for no error */ bptab[0] = 0xFE; /* Marker for >1 error */ for (i = 1; i < 65536; i++ ) bptab[i] = 0xFF; /* Mark all single bit error cases. */ printf("Error syndrome table:\n"); for (i = 0; i < nbytes; i++) { putchar(' '); Carlson, et al. expires May 1999 [Page 18]
INTERNET DRAFT PPP SDL on SONET/SDH November 1998 for (j = 0; j < 8; j++) { crc = 0; for (k = 0; k < i; k++) crc = crc16_t8(crc); crc = crc16_t8(crc ^ (0x8000>>j)); for (k++; k < nbytes; k++) crc = crc16_t8(crc); bptab[crc] = (i * 8) + j; printf(" %04X",crc); } putchar('\n'); } } int main(int argc, char **argv) { u8 buffer[8] = { 0x01,0x55,0x02,0xaa, 0x99,0x72,0x18,0x56 }; u16 crc; int i; generate_error_table(sdl_error_position,8); /* Run sample message through check routine. */ crc = 0; for (i = 0; i < 8; i++) crc = crc16_t8(crc ^ (buffer[i]<<8)); /* Output is 0000 64 -- no error encountered. */ printf("\nError test: CRC %04X, bit position %d\n", crc,sdl_message_error_position[crc]); } Carlson, et al. expires May 1999 [Page 19]
INTERNET DRAFT PPP SDL on SONET/SDH November 1998 5. Security Considerations The reliability of public SONET/SDH networks depends on well-behaved traffic which does not disrupt the synchronous data recovery mechan- isms. This document describes framing and scrambling options that are used to ensure the distribution of transmitted data such that SONET/SDH design assumptions are not likely to be violated. 6. 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] Simpson, W., Editor, "PPP over SONET/SDH," RFC 1619, Daydreamer, May 1994. [4] Simpson, W., "PPP in Ether-like Framing," Daydreamer, work in progress. [5] "American National Standard for Telecommunications - Synchronous Optical Network (SONET) Payload Mappings," ANSI T1.105.02-1993 draft. [6] ITU-T Recommendation G.707, "Synchronous Digital Hierarchy Bit Rates," June 1992. [7] Lucent Technologies, "SDL Framer/Frame Inserter," work in progress. [8] Simpson, W., Editor, "PPP LCP Extensions," RFC 1570, Daydreamer, January 1994. [9] ITU-T Recommendation I.432, "B-ISDN User-Network Interface - Physical Layer Specification," March 1993. [10] ITU-T Recommendation V.41, "Code-independent error-control system," November 1989. 7. Acknowledgments PPP over SONET was first proposed by Craig Partridge (BBN), and was last documented by William Simpson as RFC 1619. Much of the material in this document was supplied by Lucent. Carlson, et al. expires May 1999 [Page 20]
INTERNET DRAFT PPP SDL on SONET/SDH November 1998 8. Working Group and Chair Address The working group can be contacted via the mailing list (ietf- ppp@merit.edu; send mail to ietf-ppp-request@merit.edu to subscribe), or via the current chair: Karl Fox Ascend Communications 655 Metro Place South Suite 370 Dublin OH 43017-3390 Email: karl@ascend.com 9. Authors' Addresses James Carlson IronBridge Networks 55 Hayden Avenue Lexington MA 02421-7996 Phone: +1 781 372 8132 Fax: +1 781 372 8190 Email: carlson@ibnets.com Paul Langner Lucent Technologies Microelectronics Group 555 Union Boulevard Allentown PA 18103-1286 Email: plangner@lucent.com James Manchester Lucent Technologies 101 Crawford Corners Rd. Holmdel NJ 07733-3030 Email: sterling@hotair.hobl.lucent.com Carlson, et al. expires May 1999 [Page 21]
