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Network Working Group                           W A Simpson [DayDreamer]
Internet Draft
expires in six months                                        August 1998

             Applicability Statement for PPP over SONET/SDH

Status of this Memo

   This document is an Internet-Draft.  Internet Drafts are working doc-
   uments of the Internet Engineering Task Force (IETF), its Areas, and
   its Working Groups.  Note that other groups may also distribute work-
   ing 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 not appropriate to use Internet Drafts as refer-
   ence material, or to cite them other than as a ``working draft'' or
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   To learn the current status of any Internet-Draft, please check the
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   Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) William Allen Simpson (1994, 1997-1998).  All Rights


   The Point-to-Point Protocol (PPP) [RFC-1661] provides a standard
   method for transporting multi-protocol datagrams over point-to-point
   links.  This document describes the use of PPP over Synchronous Opti-
   cal Network (SONET) and Synchronous Digital Hierarchy (SDH) circuits.

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1.  Applicability Statement

   PPP over SONET/SDH is intended for those implementations that desire
   the PPP encapsulation over high speed private point-to-point links,
   such as intra-campus single-mode fiber that may already be installed
   and unused.

   Some installations have attempted to use commercial long-distance
   carriers.  Because these services were installed and tested for
   voice, and carriers have bid least-cost components that do not meet
   the needs of high speed data services, numerous interoperability
   problems have arisen.  The profile identifies many of these pitfalls,
   and recommends practices that avoid them.

1.1.  Terminology

   The various committees changing the SONET/SDH specifications have
   been inconsistent in their terminology.  This specification uses a
   few simplified terms:

   block            A fixed-size time-based multiplexing unit, carried
                    from interface to interface; also known as the SONET
                    Synchronous Transport Signal (STS-N) Frame, or the
                    SDH Synchronous Transport Module (STM-N) Frame
                    Structure.  This use of "frame" conflicts with link-
                    layer use of the same term.  The format has more in
                    common with fixed-length unit record equipment, such
                    as a magnetic tape, than with a variable-length

   byte             An 8-bit quantity; also known as "octet" in stan-

   envelope         A fixed-size time-based multiplexing unit, carried
                    within successive blocks along a path; also known as
                    the SONET Synchronous Payload Envelope (SPE), or the
                    SDH Administrative Unit Group (AUG).

   frame            A variable-length unit of transmission, which is
                    passed across the interface between the link-layer
                    and the physical-layer.  A single packet is usually
                    wrapped in a frame, unless link-layer packet frag-
                    mentation is performed, or multiple packets are
                    aggregated into a single frame.

   packet           The basic unit of link encapsulation, which is
                    passed across the interface between the network-

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                    layer and the link-layer.  The packet information is

   In this document, the key words "MAY", "MUST, "MUST NOT", "optional",
   "recommended", "SHOULD", and "SHOULD NOT", are to be interpreted as
   described in [RFC-2119].

   To remain consistent with standard Internet practice, and avoid con-
   fusion for people used to reading RFCs, all binary numbers in the
   following descriptions are in Most Significant Bit to Least Signifi-
   cant Bit order, from Most Significant Byte to Least Significant Byte,
   reading from left to right, unless otherwise indicated.  Note that
   this is contrary to ISO and ITU practice, which orders bits as trans-
   mitted (network bit order).  Keep this in mind when comparing this
   document with the other documents.

2.  SONET/SDH Profile

   "The nice thing about standards is that you have so many to choose
   from; furthermore, if you do not like any of them, you can just wait
   for next year's model."  [Tanenbaum]

   "The problem with standards is the last 's'."  [Kleinrock]

   SONET and SDH were "standardized" by at least 4 different organiza-

   -  Bellcore developed the original SONET specification, and has
      issued significantly reorganized and revised documents in 1991 and
      1995.  According the [SONET] Revision History: "Since TR-
      TSY-000253, Issue 1, there have been so many revisions to this
      material that the use of change bars is impractical."

   -  ANSI (T1X1 and T1M3) has a separate series of documents, that are
      continuously revised, and strongly influenced by a single vendor.

   -  ETSI also has a separate series of documents, but has greatly ben-
      efited by close attention to detail, and coordination with

   -  CCITT (since renamed ITU-T) published the SDH specifications in
      1988.  Minor revisions occurred in 1993 and 1994, with a major
      reorganization in 1996.

   The latter organizations have not maintained consistent naming of
   fields and payloads, have made conflicting changes and extensions,
   and have been careless about conformity and backward compatibility

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   with existing deployment.

   This specification was developed utilizing carefully chosen documents
   that reflect a particular point in time, and that correspond to
   extant fielded implementations.  Where naming conventions are incon-
   sistent, or conflict with other network usage of the same terms, a
   simpler taxonomy is chosen.

   To promote interoperability, this document provides a condensed
   description of basic SONET/SDH functionality, together with some of
   the recent changes and their relevance, and a profile of recommended

2.1.  Bit Transmission

   All bytes are transmitted Most Significant Bit first.


      Care must be taken when converting from other media, such as
      serial links and ethernet, that are transmitted Least Significant
      Bit first.

2.1.1.  Electrical Bits

   SONET specifies an optional electrical transmission capability using
   a pair of 75 Ohm coaxial cables, one for each direction.

   At 156 Mbps, Coded Mark Inversion (CMI) is specified.

      single bits

         -         -                       ---
      | | |       | | |       |           |
      | | |       | | |       |           |
       -             -         ---
       "0"         "0"         "1"         "1"

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      A1A2 pattern

           ---     ---   -     ---   -   -   -     -   ---   -   -   -
      |   |   |   |   | | |   |   | | | | | | |   | | |   | | | | | | |
      |   |   |   |   | | |   |   | | | | | | |   | | |   | | | | | | |
       ---     ---     -   ---     -   -   -   ---   -     -   -   -
        1   1   1   1   0   1   1   0   0   0   1   0   1   0   0   0

   or its inverse.

   Lower signal rates (52 Mbps) use Bipolar with Three-Zero Substitution
   (B3ZS).  These rates are not relevant to PPP over SONET/SDH (see
   "Physical Layer Requirements").


      There are no recent changes that might affect interoperability.

      At the time of writing, there are no reported router implementa-
      tions using CMI electrical transmission to directly feed electro-
      optical section or line terminating equipment.

      Typically, pseudo-ECL signals over very short inter-component dis-
      tances are used to drive the optics with the same encoding pattern
      as the optical bits.


      As the cost of optical interfaces and cabling has rapidly
      decreased, the use of optical transmission is preferred, even for
      moderately short intra-office distances.

      For electrical intra-office router interconnection, use of
      100baseT (or future gigabit ethernet) is preferable to PPP over

2.1.2.  Optical Bits

   SONET specifies an optical transmission capability using a pair of
   fibers, one for each direction.  The use of bi-directional fiber
   transmission is "for further study".

   The optical line coding used for all system interfaces is binary Non-
   Return-to-Zero (NRZ).  The convention adopted for optical logic level

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      logical 1 -- emission of light
      logical 0 -- no emission of light

      single bits

      |           |
      |           |
       "0"         "1"

      A1A2 pattern

       ---------------     -------             ---     ---
                      |   |       |           |   |   |   |
                      |   |       |           |   |   |   |
                       ---         -----------     ---     -----------
        1   1   1   1   0   1   1   0   0   0   1   0   1   0   0   0

   To simplify the development of compatible multi-vendor systems, it is
   desirable to define a relatively small set of categories and corre-
   sponding optical interface specifications.

   Short Reach (intra-office)
      SONET optical sections having loss budgets of 0 dB to 7 dB; alter-
      natively, SDH interconnect distances less than 2 kilometers.

   Intermediate Reach (short haul inter-office)
      SONET optical sections having loss budgets of 0 dB to 12 dB;
      alternatively, SDH interconnect distances up to 15 kilometers.

   Long Reach (long haul inter-office)
      SONET optical sections having loss budgets of 10 dB to 28 dB;
      alternatively, SDH interconnect distances up to 40 kilometers at
      1310 nanometers, and up to 60 kilometers at 1550 nanometers.

   Unfortunately, the current proliferation of specifications falls far
   short of this goal, having different category definitions (as seen
   above) and more than 20 total pages of parameter tables.


      There are too many recent changes that impede interoperability to
      detail in this short overview.

      Care must be taken when comparing signals from other media, such

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      as high speed serial links, where "no signal" is "logical 1".

      At the time of writing, the majority of reported implementations
      use "intermediate" level 1310 nanometer optics with single mode
      fiber.  This can accommodate multi-mode fiber and short reach
      (intra-office) interconnection with the addition of transmitter

      Upgrading to future higher speeds would be facilitated by
      installing single mode fiber instead of multi-mode fiber.


      The greatest interoperability and economies of scale will be
      achieved with deployment of a few general interface choices, all
      for single mode fiber:

         Intermediate Reach equipment at 1310 nanometers for each STM
         line speed.

         Long Reach equipment at 1550 nanometers for 2 Gbps and above.

2.1.3.  Bit Scrambling

   Optical interface signals use NRZ binary line coding (described
   above).  A series of repeated bits will not feature any signal level
   transitions.  Such transitions (zeros to ones and ones to zeros) are
   needed at the receiver to dynamically correct bit timing variance for
   line rate clock recovery.  Therefore, although it has no effect on
   random data, the signal is scrambled against the possibility that a
   very long series of ones or zeros might naturally occur in the trans-
   mitted bit stream.

   Electrical interface signals use line encoding (B3ZS and CMI) that
   assures adequate transitions.  Scrambling is not needed.  However,
   they are also scrambled for consistency between the electrical and
   optical component interfaces.

   In both cases, a 7-bit linear feedback shift register (LFSR) is iden-
   tically applied at the transmitter and receiver.  The sequence of
   bits from the x**7 stage of the LFSR is exclusive-or'd (in most sig-
   nificant bit order) with the output bits before transmission, and
   with the corresponding input bits after signal recovery.

   The first row of the STM-N transmission block overhead (N * 9 bytes
   described later) is not scrambled.  The 7-bit LFSR state is reset to
   bit value 1111111 (127 decimal) on the most significant bit of the

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   byte following the block overhead.  The scrambler runs continuously
   from that bit onward, throughout the remainder of the transmission

   The generating polynomial for the LFSR is x**7 + x**6 + 1.  The
   resulting 127-bit sequence repeats in a 127 byte stream (shown in

      fe 04 18 51 e4 59 d4 fa 1c 49 b5 bd 8d 2e e6 55
      fc 08 30 a3 c8 b3 a9 f4 38 93 6b 7b 1a 5d cc ab
      f8 10 61 47 91 67 53 e8 71 26 d6 f6 34 bb 99 57
      f0 20 c2 8f 22 ce a7 d0 e2 4d ad ec 69 77 32 af
      e0 41 85 1e 45 9d 4f a1 c4 9b 5b d8 d2 ee 65 5f
      c0 83 0a 3c 8b 3a 9f 43 89 36 b7 b1 a5 dc ca bf
      81 06 14 79 16 75 3e 87 12 6d 6f 63 4b b9 95 7f
      02 0c 28 f2 2c ea 7d 0e 24 da de c6 97 73 2a


      There are no recent changes that might affect interoperability.

      The scrambling mechanism is defective in a packet transmission
      context, where adjacent bytes are sequentially related.  Appar-
      ently, the short polynomial assumed unrelated data that is
      obtained from DS0 through DS3 circuits.  This is characteristic
      for legacy technology such as Asynchronous Transfer Mode (ATM),
      interleaved virtual circuits, or voice traffic.

      Since the feedback is based on the internal state of the LFSR, and
      is not dependent on its interaction with previous bytes, any sub-
      set of the bit sequence that exactly matches the phase of the LFSR
      will generate a series of zeros.  The bitwise inverse of the bit
      sequence will generate a series of ones.

      This effect is not limited to direct PPP over SONET/SDH.  Any
      packet oriented transmissions carried by SONET/SDH without inter-
      leaving would have the same result.

      Moreover, although the 127 byte stream may have been appropriate
      for the original STS-1 (90 bytes per row), this length is mani-
      festly insufficient for STM-N (N * 270 bytes per row).

      Finally, the most egregious flaw in this part of the SONET/SDH
      specification, the number of bits permitted without a transition
      is not explicitly defined.  Instead, the value is implicitly
      derived from the fixed length scrambling sequence versus the vari-
      able length Loss of Signal (LOS) condition based on the link speed
      (described later).

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      Note that the complete scrambling byte stream cannot be repre-
      sented in a valid PPP packet.  The pair "fd 03" is not conformant,
      limiting the maximum sequence without a transition to 127 bytes.


      Packet transmissions carried at rates less than 156 Mbps MUST be
      interleaved under the VT/TU scheme (see "Physical Layer Require-
      ments").  This is consistent with prior use of bit-synchronous
      HDLC (and Frame Relay) over DS0 through DS3 circuits.

      Line rate clock recovery SHOULD be sufficiently stable to with-
      stand periods of 1016 repeated one or zero bits (127 bytes).

2.1.4.  Loss of Signal (LOS)

   To detect a failure that occurs at the transmitter (such as laser
   failure) or within the transmission facility (such as a fiber cut),
   all incoming signals (optical and electrical) are monitored for Loss
   of Signal (LOS) before descrambling, by detecting a lack of suffi-
   cient signal level transitions to ensure accurate line rate clock

   No transitions on the incoming signal is not considered LOS when the
   condition lasts 2.3 microseconds or less, and is considered LOS when
   the condition lasts 100 microseconds or more.  The treatment of no
   transitions lasting between 2.3 microseconds and 100 microseconds for
   the purpose of LOS detection is not specified (left to the choice of
   the equipment designer).

   For electrical (B3ZS and CMI) interfaces, the no voltage transitions
   condition is easily distinguished from an "all-zeros pattern".

   For SONET optical (and component) interfaces, the no light pulses
   condition corresponds to an "all-zeros pattern".  For testing SONET
   compliance with the LOS detection requirement, it suffices to apply
   an "all-zeros pattern" lasting at most 2.3 microseconds, and to apply
   an "all-zeros pattern" lasting at least 100 microseconds.  Optical
   equipment shall enter the LOS state within 100 microseconds of the
   onset of the incoming "all-zeros pattern."

   For SDH optical (and component) interfaces, equipment may also indi-
   cate LOS within 3 milliseconds of the time the received signal level
   drops below an implementation-determined threshold (based on receiver
   sensitivity).  The threshold should be selected such that LOS will
   not be indicated while the Bit Error Rate (BER) is still acceptable.
   For SDH, the BER is defined to be 1 in 10**(-3).

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   Note that the secondary method is in addition to the primary method,
   not in lieu of it, and is not required to be implemented in SONET

   The equipment shall terminate the LOS condition when the incoming
   signal has two consecutive valid block alignment patterns (described
   later), and during the intervening time (125 microseconds) sufficient
   transitions prevent another LOS defect.

   For trouble isolation purposes, especially for intermittent failures,
   it is important to distinguish between LOS and other signal failure
   conditions, such as Loss of Frame (LOF).


      There are significant differences between SONET, ANSI, and SDH;
      SDH does not describe the primary method of detection.  Circa
      1994, this resulted in the addition of the secondary method to

      There is no explicit requirement given for the "all-ones pattern"
      that would indicate a stuck clock or laser transmitter.  The same
      difficulty with lack of transitions will apply.

      There is no rationale given for the selection of 2.3 us and 100
      us.  Presumably, 100 us was chosen to allow sufficient variance
      between 125 us block alignment patterns.  However, 100 us will
      cause multiple errors in the block overhead (described later),
      while 2.3 us is too short to cause any significant damage.

      The wide disparity between 2.3 us and 100 us, and leaving the
      error treatment to the vendor, has caused many interoperability
      problems.  Incorrect LOS values as low as 1 +-0.750 us have been
      discovered.  Typical values for valid designs are 20 +-3 us, and
      26 +-1 us.  No values higher than 30 us have been found.

      Line rate clock stability has been a related interoperability
      problem.  Early 156 Mbps implementations were very robust, and
      could handle periods of 2,000 bits with no transitions.  Some sam-
      ple and/or low quality implementations have not been as robust;
      failure has been noted as low as 72 to 80 bits without transi-

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      Using the minimum and maximum specification of LOS, line rate
      clock recovery MUST be sufficiently stable to withstand periods of
      repeated one or zero bits:

              Mbps    min bits    max bits
               156         358      15,552
               622       1,431      62,208
             2,488       5,724     248,832

      Implementations compatible with this specification SHOULD observe
      a stricter range of LOS detection, between 13.89 us and 27.26 us:

              Mbps    min bits    max bits
               156       2,161       4,240
               622       8,641      16,960
             2,488      34,561      67,840

      The lower limit affects no more than one row of overhead, while
      the upper limit includes at least one row of overhead.

      When a circuit path is divided into multiple line sections, the
      LOS is not propagated end to end.  A hidden section can fail inde-
      pendently.  Therefore, the LOS indication is only advisory, and
      the (possibly improperly) decoded signal bits MUST be passed to
      the next section.  The framed PPP packet FCS is used to detect the

      Some SONET framers provide a test feature to send an "all-zeroes
      pattern" (post scrambling) that forces the LOS alarm downstream.
      Alternatively, the pattern can be simulated with software.  This
      SHOULD be used to test for compatible equipment during the instal-
      lation of the circuit, by generating progressively longer zero
      patterns, to verify that the circuit meets the requirements of
      this profile.

2.2.  Block Transmission

   SONET/SDH transmits each block at precise 125 microsecond intervals.
   This corresponds to the traditional 4 kHz voice digital sampling rate
   used for DS0.

   Each block is divided into 9 logical "rows".

   For "concatenated" STS/STM, each row consists of some multiple of 270
   byte "columns".

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   Block transmission overhead is arranged into layers, which correspond
   to different equipment interfaces that might occur in the path of a
   single circuit.  Although SONET and SDH use inconsistent terms, the
   layers have a general equivalence.

   SONET divides its Network Element (NE) interface overhead into Sec-
   tion (SOH), Line (LOH), and Path (POH).

   SDH has a corresponding Network Node Interface (NNI) overhead called
   Regenerator-Section (RSOH), Multiplex-Section (MSOH), and Path (POH).

   There are 3 rows of (Regenerator) Section Overhead (SOH or RSOH), and
   6 rows of Line or Multiplex-Section Overhead (LOH or MSOH).

2.2.1.  Block Overhead

   The STS-3c/STM-1 transmission block overhead consists of 9 byte
   columns at the beginning of each 270 byte row, with the remaining 261
   bytes used for the data envelope(s).

      | A1   A1   A1   A2   A2   A2   J0   Z0   Z0   envelope
      | B1   ++   ++   E1   ++   ??   F1   **   **   envelope
      | D1   ++   ++   D2   ++   ??   D3   ??   ??   envelope
      | H1   H1#  H1#  H2   H2#  H2#  H3   H3   H3   envelope
      | B2   B2   B2   K1   ??   ??   K2   ??   ??   envelope
      | D4   ??   ??   D5   ??   ??   D6   ??   ??   envelope
      | D7   ??   ??   D8   ??   ??   D9   ??   ??   envelope
      | D10  ??   ??   D11  ??   ??   D12  ??   ??   envelope
      | S1   Z1   Z1   Z2   Z2   M1   E2   **   **   envelope

       ++ media dependent.
       ** reserved for national use.
       ?? reserved for future use, SHOULD be zero.

   Values relevant to this specification:

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   (A1)             bit value 1111 0110 (f6 hex)

   (A2)             bit value 0010 1000 (28 hex)

   (H1) and (H2)    Each of the 3 consecutive pairs of H1H2 is taken as
                    a 16-bit mask, with various combinations of bits
                    determining the offset (in multiples of 3*N) to the
                    start of the data envelope.  An offset of 0 indi-
                    cates that the envelope immediately follows the last
                    H3 overhead byte.  An offset of 522 indicates that
                    the envelope immediately follows the last Z0 over-
                    head byte in the next transmission block.

                    When used for this specification, the second and
                    third H1#H2# pairs are always bit value 1001 xx11
                    1111 1111 (93ff hex), indicating a "concatenated"
                    envelope, where xx is ignored on receipt.

   Since the block is transmitted by row, the overhead columns are
   interspersed with envelope columns, complicating generation and

   An additional complication is added by the section-level bit scram-
   bling.  Overhead bytes of the first row (A1, A2, J0, Z0) are not


      J0 was formerly called C1.  Z0 was formerly additional copies of
      C1, but these bytes are now reserved for national use.

      The xx bit values in H1# were originally 00 for SONET and 10 for

      These changes might affect interoperability between international

      The scrambler needs a reset signal to indicate the position of the
      beginning of the transmission block and length of the overhead.
      Failure of the reset signal timing could prevent recognition of
      the block.

      Teleopolists seem inordinately fond of groups of 3.  This was evi-
      dent in early digital efforts (24 DS0 in T1), and continues into
      SONET/SDH (3 T3/E3 in STM-1).  Note that all of the values,
      including 261 and 270, are divisible by 3.  This does not scale
      well in a digital processing environment.

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      Envelopes SHOULD be sent with the H1H2 offset of 522.  However,
      every implementation MUST be prepared to receive a variable off-

2.2.2.  Block Synchronization

   Timing alignment is detected by searching for (a subset of) the A1
   and A2 overhead pattern.  There are two levels of synchronization

   When synchronization has been lost for 3 milliseconds or more (or has
   never been achieved), long-term synchronization recovery requires
   detection of a minimum of eight (8) successive error-free A1 and A2
   overhead patterns.  The maximum recovery time is 3 milliseconds (24
   error-free patterns).

   Once alignment has been achieved, short-term synchronization loss is
   declared when four (4) or more consecutive A1 and A2 pattern errors
   have been detected.  The maximum detection time is 625 microseconds.
   The algorithm used shall be such that, under normal operation, a
   10**(-3) Poisson distribution of bit errors will not cause a false
   loss indication more than once every 6 minutes.

   Recovery from a short-term synchronization loss requires detection of
   only two (2) successive error-free A1 and A2 overhead patterns.  Any
   recovery circuitry is acceptable that achieves resynchronization
   within the 250 microsecond interval implied by this definition.

   Failure to obtain resynchronization within 24 blocks (3 milliseconds)
   results in a long-term synchronization loss declaration, called Loss
   of Frame (LOF).


      The SDH specification is incomplete, and is missing some time

      Conversely, SDH has a somewhat stricter requirement for resynchro-
      nization after a short-term loss; the algorithm used shall be such
      that, with a random signal, the probability for false recovery is
      no more than 10**(-5) per 250 microsecond interval.

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      Because of the interaction between block synchronization detec-
      tion, termination of the Loss of Signal (LOS) condition, and the
      first row reset of the section-level bit scrambling, the detection
      circuitry MUST be independent (upstream) of the scrambler, and
      trigger the subsequent scrambler reset timing.

2.2.3.  Envelope Overhead

   A series of envelopes appear within the transmission block.  These
   envelopes float to allow precise timing alignment and jitter correc-
   tion as the payload is carried over the circuit path.  The position
   of the envelope(s) head within the block is specified by the H1 and
   H2 pair(s).  The envelope(s) tail can carry over into the next trans-
   mission block.

   The STS-3c/STM-1 Path Overhead (POH for both SONET and SDH) consists
   of a 1 byte column, with the remaining bytes used for the payload

      | J1   payload
      | B3   payload
      | C2   payload
      | G1   payload
      | F2   payload
      | H4   payload
      | Z3   payload
      | Z4   payload
      | Z5   payload

   Values relevant to this specification:

   (J1)             Path Trace.  Carries one byte at a time of a repeat-
                    ing 64-byte fixed-length string (SONET) or a 16-byte
                    E.164 number (SDH), so that a path receiving termi-
                    nal can verify its continued connection to the

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                    intended transmitter.

   (C2)             Path Signal Label.  Indicates the composition, con-
                    struction and content of the envelope payload.

   (H4)             Multipurpose Position Indicator.  Used by specific
                    payload mappings, such as Floating Virtual Tribu-

   Since the envelope is transmitted by row, the path overhead columns
   are interspersed with payload columns, and the combination are inter-
   spersed with block overhead columns, complicating generation and


      The envelope payload provides the physical-layer interface for
      insertion of PPP encapsulated packets.

      The J1 16-byte E.164 number format is rarely applicable, as there
      is no voice circuit involved in packets.  Neither have fielded
      implementations observed strict adherence to the 64-byte limita-
      tion.  A widely used implementation has generated a string con-
      sisting of the following pattern, each line CRLF terminated:

        Remote hostname : sl-bb10
        Remote interface: POS9
        Remote IP addr  :
        Remote Rx(K1/K2): 00/00  Tx(K1/K2): 00/00

      The C2 value is primarily informational.  It is difficult to use
      for applying alternate processing for adjacent envelopes, as it
      occurs after two rows of the envelope have already been processed.
      Although there is no need for intermediate transit equipment to
      interpret the value, some equipment fails upon encountering an
      unrecognized value.

      At first glance this floating envelope may appear to be a highly
      efficient technique to avoid interface buffering of synchronized
      incoming data.  Unfortunately, due to interspersing the overhead
      with the data, it is impossible to determine where the envelope
      begins until at least a third of the transmission block has been

      Also, the envelope head can begin much later in the same block, or
      even in the next block.  An envelope tail can carry over into a
      block that is two blocks later than the H1H2 overhead that indi-
      cates its head.

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      Thus, in a packet switched network, no matter how fast the link
      speed, a minimum of 125 microseconds is added at every hop, and
      cut-through routing is difficult or impossible.


      See [RFC-xxxx] "Physical Layer Requirements".

2.2.4.  Envelope Ordering

   SONET and SDH differ in the ordering of envelope rows.  Several
   schemes are described, including:

   J1 ...
   B3 ...
   C2 ...
   J1 ...
   B3 ...
   C2 ...
   J1 ...
   B3 ...
   C2 ...

   J1 ... J1 ... J1 ...
   B3 ... B3 ... B3 ...
   C2 ... C2 ... C2 ...

   J1 J1 J1 ...
   B3 B3 B3 ...
   C2 C2 C2 ...

   Another difficulty has been the interleaving of lower rate channels
   within envelopes.  These are time division multiplexed.

   In the SONET and SDH specifications, much of the complexity is due to
   these multi-stage multiplexing schemes.


      After optical specifications, this has been the worst impediment
      to interoperability.

      As carriers have upgraded equipment to STS-3c/STM-1 and
      STS-12c/STM-4, some have merely reconnected older equipment with

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      coax (see "Electrical Bits").  This is often haphazard.  Although
      this has no effect on voice, where each byte is a separate chan-
      nel, this is unacceptable for packets.


      Higher data rate envelopes, and the bytes within envelopes, MUST
      be sequentially ordered by row.

      SDH VC-3/VC-4/VC-4-Xc ordering MUST be supported.

      Envelope data MUST be carried from source to sink in sequential
      order.  That is, bytes inserted in the order "1234" MUST arrive in
      the same order "1234".

      At the current setup and monthly charges of hundreds of thousands
      of US Dollars, installations can insist that equipment be care-
      fully configured, or will need to find an alternative carrier.

2.2.5.  Payload Insertion

   Various ordering schemes differ only in the location of duplicate
   POH.  These redundant POH bytes are specified as "fixed stuff", and
   the values are not examined.

   For example, in the previous section, the latter schemes interleave
   the redundant POH bytes throughout the row (VC-3/VC-4), or concate-
   nate the redundant POH bytes at the beginning of the row (VC-4-Xc).


      See [RFC-xxxx] "The Data Link Layer".  The specification finesses
      the issue by filling the "fixed stuff" with packet data.  This
      make VC-4 and VC-4-Xc identical with respect to packet processing.

2.2.6.  Protection Switching

   SONET provides Automatic Protection Switching (APS), also known as
   the SDH Multiplex Section Protection Function (MSP), to switch from
   one circuit to another spare circuit whenever a problem is detected.
   This switch can take place based on Signal Fail (SF), Signal Degrade
   (SD), or by manual intervention.  Once switching is initiated, it can
   take up to 50 milliseconds to complete the switch.

   Signal Fail (SF) is defined as LOS, LOF, BER exceeding 10**(-3), and
   various other error conditions.

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   Signal Degrade (SD) is defined as BER exceeding a user selectable
   range from 10**(-5) to 10**(-9).

   The protection switch applies to all circuits in one direction simul-
   taneously.  Bidirectional switching is optional.


      This time domain switching facility wastes entire backup links
      against the rare possibility that a link might fail.  Worse, these
      links are frequently in the same cable, and backhoe fade kills all
      the links.

      Conversely, Internet routing utilizes all available circuits con-
      currently, and gracefully degrades while providing best-effort
      packet switching around points of failure.

      Since this specification is intended for carrying PPP packets over
      point-to-point links in a routed network, APS/MSP is superfluous.

      During intermittent failures, due to the many orders of magnitude
      time difference between activation (microseconds) and completion
      of switching (milliseconds), and the severe extended packet loss,
      APS/MSP link flapping can interact badly with routing protocols.

      It usually costs more, with no added benefit.


      Where SONET/SDH circuit path configuration is under control of the
      user, APS/MSP SHOULD be disabled.

Security Considerations

   This specification introduces no known security vulnerabilities.

   Section-level bit scrambling consists of a simple repeated XOR with a
   short, easily computed, LFSR bit stream (listed earlier).  This is
   not a pseudo-random number generator.  There is no practical crypto-
   graphic security.

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   PPP over SONET was first proposed by Craig Partridge (BBN).  Some
   information was obtained from the good folks at Bellcore.

   Technical assistance and information was also provided by Victor Dem-
   janenko (SUNY Buffalo).

   Considerable explanatory text was contributed by C. M. Heard (VVNet).

   Anonymous, Karl Fox (Ascend), Peter Lothberg (Sprint), Subhash Roy
   (TranSwitch), Stuart Venters (Adtran), and Eric Verwillow (Juniper)
   provided useful critiques of earlier versions of this document.

   Special thanks to Ascend Communications (nee Morning Star Technolo-
   gies) for providing computing resources and network access support
   for writing this specification.


   [RFC-1661]  Simpson, W., Editor, "The Point-to-Point Protocol (PPP)",
               STD-51, DayDreamer, July 1994.

   [RFC-1662]  Simpson, W., Editor, "PPP in HDLC-like Framing", STD-51,
               DayDreamer, July 1994.

   [RFC-2119]  Bradner, S., "Key words for use in RFCs to Indicate
               Requirement Levels", BCP-14, Harvard University, March

   [RFC-xxxx]  Simpson, W., Editor, "PPP over SONET/SDH", DayDreamer,
               work in progress.

   [SONET]     "Synchronous Optical Network (SONET) Transport Systems:
               Common Generic Criteria", Bellcore, TR-NWT-000253 Issue
               2, December 1991.

   [G.707]     CCITT Recommendation G.707, "Synchronous Digital Hierar-
               chy Bit Rates", June 1992.

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   Comments about this document should be discussed on the
   pppsdh@greendragon.com or ietf-ppp@merit.edu mailing lists.

   This document was reviewed by the Point-to-Point Protocol Working
   Group of the Internet Engineering Task Force (IETF).  The working
   group can be contacted via the current chair:

      Karl Fox
      Ascend Communications
      655 Metro Place South,  Suite 370
      Dublin, Ohio  43017


   Questions about this document can also be directed to:

      William Allen Simpson
      Computer Systems Consulting Services
      1384 Fontaine
      Madison Heights, Michigan  48071

          wsimpson@GreenDragon.com (preferred)

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Full Copyright Statement

   Copyright (C) William Allen Simpson (1994, 1997-1998).  All Rights

   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 doc-
   ument itself may not be modified in any way, except as required to
   translate it into languages other than English.

   This document and the information contained herein is provided on an
   "AS IS" basis and the author(s) DISCLAIM ALL WARRANTIES, EXPRESS OR

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