Network Working Group W A Simpson [DayDreamer]
Internet Draft
expires in six months November 1997
PPP over SONET/SDH
draft-ietf-pppext-sonet-ds-00.txt
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Abstract
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 Heirarchy (SDH) circuits.
Applicability
This specification is intended for those implementations which desire
to use 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. Because the PPP encapsulation has relatively
low overhead, it is anticipated that substantially higher throughput
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can be attained compared to other SONET/SDH payload mappings, at a
significantly lower cost for line termination equipment.
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1. Introduction
PPP was designed as a standard method of communicating over point-to-
point links. Initial deployment has been over short local lines,
leased lines, and plain-old-telephone-service (POTS) using modems.
As new packet services and higher speed lines are introduced, PPP is
easily deployed in these environments as well.
The Synchronous Optical Network [SONET] is a time division multiplex-
ing scheme that defines a family of standard rates and formats.
Despite the name, it is not limited to optical links. The ITU-T Syn-
chronous Digital Heirarchy (SDH) attempts to internationalize SONET
to provide interworking beteen networks based on different ple-
siochronous digital heirarchies and speech encoding regimes.
The SONET and SDH efforts specify an entire infrastructure, from
physical-layer through network-layer to application-layer. The upper
layers rely on ISO CLNP, TP4/CLNS, ASN.1, ASCE, CMISE, and an exten-
sive list of ITU-T X.committee specifications.
Where SONET/SDH is configured as a point-to-point circuit, PPP is
well suited to use over these links. PPP provides link-layer packet
encapsulation with framing, and treats SONET/SDH in its entirety
merely as an overly complicated physical-layer, ignoring its other
features.
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
packet.
byte An 8-bit quantity; also known as "octet" in stan-
dardese.
envelope A fixed-size time-based multiplexing unit, carried
within successive blocks along a path; also known as
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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-
layer and the link-layer. The packet information is
variable-length.
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. Physical Layer Requirements
PPP treats SONET/SDH transport as an octet-oriented synchronous link.
SONET/SDH links are bi-directional and full-duplex by definition.
Interface Format
PPP presents an octet interface to the physical layer. There is
no provision for sub-octets to be supplied or accepted.
The octet stream is mapped into the SONET/SDH envelope payload,
with the octet boundaries aligned with the payload byte bound-
aries.
Additional scrambling is not needed during insertion into the
envelope. Instead, byte sequences that might otherwise need
scrambling (for underperforming circuits) are escaped using octet-
stuffing [RFC-1662] as described in Appendix B.
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PPP Authentication is preferable to the use of Path Trace (J1) for
confirming connection between the correct peers.
The Path Signal Label (C2) is intended to indicate the contents of
the envelope. The value of 207 (cf hex) SHOULD be used to indi-
cate PPP. An implementation MUST also allow configuration of the
C2 field to 0 (Unequipped) or 1 (Non-Specific Payload).
The Multipurpose Position Indicator (H4) is currently unused, and
MUST be zero.
Transmission Rate
The SONET transmission rates are integral multiples of 51.840
Mbps, which may be used to carry T3/E3 signals. The allowed mul-
tiples are currently specified (in Mbps) as
STS-1 51.840 STS-18 933.120
STS-3 155.520 STS-24 1,244.160
STS-9 466.560 STS-36 1,866.240
STS-12 622.080 STS-48 2,488.320
SDH defines a subset of SONET transmission rates beginning at
155.520 Mbps [G.707]
SONET SDH equivalent
STS-3c STM-1
STS-12c STM-4
STS-48c STM-16
The basic rate chosen for this specification is that of
STS-3c/STM-1 at 155.520 Mbps. The available information bandwidth
is 149.760 Mbps, which is the STS-3c/STM-1 envelope payload with
overhead removed. This is the same super-rate mapping that is
used for ATM and FDDI.
Lower signal rates MUST use the SONET Virtual Tributary (VT) mech-
anism, also known as SDH Tributary Units (TU) and Virtual Contain-
ers (VC). This maps existing signals up to T3/E3 rates asyn-
chronously into the envelope, or uses available clocks for bit-
synchronous and byte-synchronous mapping.
Higher signal rates SHOULD conform to the SDH STM series, rather
than the SONET STS series, as equipment becomes available. The
STM series progresses in powers of 4 (instead of 3), and employs
fewer steps, which is likely to simplify multiplexing and integra-
tion, thereby promoting interoperability.
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Control Signals
PPP does not require the use of control signals. When available,
using such signals can allow greater functionality and perfor-
mance. Implications are discussed in [RFC-1662].
For more information on the specification of the SONET/SDH interface,
see the Appendices.
3. The Data Link Layer
By default, octet-synchronous HDLC-like framing [RFC-1662] MUST be
implemented.
As an option, octet-synchronous Frame Relay [RFC-1973bis] MAY be
implemented.
The framed PPP packet MUST be mapped directly into the envelope pay-
load by row, skipping a single column for Path Overhead (POH), and
filling any fixed-stuff columns. Because packets are variable in
length, the frames are allowed to carry over envelope boundaries.
Interleaving and separating VT/TU PPP packet streams over multiple
circuit paths are beyond the scope of this specification.
4. Configuration Details
The standard LCP 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
Link Quality Monitoring
32-bit FCS
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A. 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 3 different organiza-
tions.
- 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.
- 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
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 Appendix provides a condensed
description of basic SONET/SDH functionality, together with some of
the recent changes and their relevance, and a profile of recommended
practices.
A.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.
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A.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"
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").
Discussion
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.
Recommendations
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.
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For electrical intra-office router interconnection, use of
100baseT (or future gigabit ethernet) is preferable to PPP over
SONET/SDH.
A.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
is
logical 1 -- emission of light
logical 0 -- no emission of light
Care must be taken when comparing signals from other media, such as
high speed serial links, where "no signal" is "logical 1".
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;
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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.
Discussion
There are too many recent changes that impede interoperability to
detail in this short overview.
At the time of writing, the majority of reported implementations
use "intermediate" level 1310 nanometer optics with single mode
fiber. This can accomodate multi-mode fiber and short reach
(intra-office) interconnection with the addition of transmitter
attenuation.
Upgrading to future higher speeds would be facilitated by
installing single mode fiber instead of multi-mode fiber.
Recommendations
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.
A.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.
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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
byte following the block overhead. The scrambler runs continuously
from that bit onward, throughout the remainder of the transmission
block.
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
hex):
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
Discussion
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.
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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).
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 number of bits without a transition to 127
bytes.
Recommendations
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).
A.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
recovery.
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
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condition is easily distinguished from an "all-zeros pattern".
For optical (and component) interfaces, the no light pulses condition
corresponds to an "all-zeros pattern". For testing 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."
Second, as a supplier option, equipment may also indicate 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).
Note that the secondary method is in addition to the primary method,
not in lieu of it, and is not required to be implemented.
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).
Discussion
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
SONET.
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 some interoperability
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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-
tions.
Recommendations
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 will detect the fail-
ure.
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.
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A.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".
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).
A.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).
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+----+----+----+----+----+----+----+----+----+----------
| 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:
(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 reciept.
Since the block is transmitted by row, the overhead columns are
interspersed with envelope columns, complicating generation and
recovery.
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An additional complication is added by the section-level bit scram-
bling. Overhead bytes of the first row (A1, A2, J0, Z0) are not
scrambled.
Discussion
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
SDH.
These changes might affect interoperability between international
facilities.
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.
Recommendations
PPP envelopes SHOULD be sent with the H1H2 offset of 522. How-
ever, every implementation MUST be prepared to receive a variable
offset.
A.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
recovery.
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.
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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).
Discussion
The SDH specification is incomplete, and is missing some time
intervals.
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.
Recommendations
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.
A.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
data.
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+----+---------
| 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
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-
tary.
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
recovery.
Discussion
The envelope payload provides the physical-layer interface for
insertion of PPP encapsulated packets in HDLC-like frames (or
Frame Relay).
The J1 16-byte E.164 number format is rarely applicable, as there
is no voice circuit involved. Neither have fielded
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implementations observed strict adherance 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 : 127.255.255.255
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
received.
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.
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.
Recommendations
See "Physical Layer Requirements".
A.2.4. Payload Insertion
A.2.5. 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
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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.
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.
Discussion
This time domain switching facility wastes entire backup links
against the rare possibility that a link might fail. Conversely,
Internet routing utilizes all available circuits concurrently, and
gracefully degrades while providing best-effort packet switching
around points of failure.
Since this specification is intended for carrying PPP packets over
private 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 might interact badly with routing protocols.
Recommendations
Where SONET/SDH circuit path configuration is under control of the
user, APS/MSP SHOULD be disabled.
B. Payload Scrambling
Several suggestions have been made for reducing the possibility that
a maliciously chosen payload can cause a long sequence of one or zero
bits, resulting in the Loss of Signal (LOS) indication. Implementa-
tions strictly conforming to original SONET specification are not
subject to this problem, and the recommendations in the foregoing
profile completely prevent this problem.
However, the profile recommends testing for non-conforming installa-
tions. When the recommended installation test detects that line rate
clock recovery is not sufficiently stable to meet the requirements of
this specification, Prophylactic Octet Stuffing MAY be configured by
the sending peer. There is no requirement that this method be
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implemented.
B.1. Prophylactic Octet Stuffing
The installation test SHOULD determine the maximum number of bytes
that can be safely sent, and configure the allowance at one less.
The transmitter checks the outgoing bytes, and adds an escape when-
ever the allowance is exceeded.
The principal advantage of this method is that it is fully backward
compatable. The receiver does not need to make any changes, as
octet-stuffing escapes are already handled.
Also, there are no additional error patterns introduced that are
undetectable by the FCS.
Moreover, this method operates on octets rather than bits, and is
parallelizable in the same fashion as HDLC-like framing.
Finally, the resulting protection is complete, rather than probablis-
tic.
/* Detect SONET/SDH section scrambler pattern in PPP data.
* 1996 May, William Allen Simpson
* based on a suggestion by Garry Epps (Cisco),
* with a pattern supplied by Steve Lang (PMC-Sierra).
*/
typedef unsigned char uint8;
/* Combining the patterns for all-zeros and all-ones, each
* byte in this table contains the next byte in the pattern.
* There are 2 bytes which do not appear in the patterns
* (00 and ff). These are both directed to 7d, as the series
* "7d 0e" is not a feasible PPP construct.
*/
uint8 pattern[256] =
{ 0x7d, 0xfb, 0x0c, 0xf7, 0x18, 0xe3, 0x14, 0xef,
0x30, 0xcb, 0x3c, 0xc7, 0x28, 0xd3, 0x24, 0xdf,
0x61, 0x9a, 0x6d, 0x96, 0x79, 0x82, 0x75, 0x8e,
0x51, 0xaa, 0x5d, 0xa6, 0x49, 0xb2, 0x45, 0xbe,
0xc2, 0x39, 0xce, 0x35, 0xda, 0x21, 0xd6, 0x2d,
0xf2, 0x09, 0xfe, 0x05, 0xea, 0x11, 0xe6, 0x1d,
0xa3, 0x58, 0xaf, 0x54, 0xbb, 0x40, 0xb7, 0x4c,
0x93, 0x68, 0x9f, 0x64, 0x8b, 0x70, 0x87, 0x7c,
0x7e, 0x85, 0x72, 0x89, 0x66, 0x9d, 0x6a, 0x91,
0x4e, 0xb5, 0x42, 0xb9, 0x56, 0xad, 0x5a, 0xa1,
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0x1f, 0xe4, 0x13, 0xe8, 0x07, 0xfc, 0x0b, 0xf0,
0x2f, 0xd4, 0x23, 0xd8, 0x37, 0xcc, 0x3b, 0xc0,
0xbc, 0x47, 0xb0, 0x4b, 0xa4, 0x5f, 0xa8, 0x53,
0x8c, 0x77, 0x80, 0x7b, 0x94, 0x6f, 0x98, 0x63,
0xdd, 0x26, 0xd1, 0x2a, 0xc5, 0x3e, 0xc9, 0x32,
0xed, 0x16, 0xe1, 0x1a, 0xf5, 0x0e, 0xf9, 0x02,
0xfd, 0x06, 0xf1, 0x0a, 0xe5, 0x1e, 0xe9, 0x12,
0xcd, 0x36, 0xc1, 0x3a, 0xd5, 0x2e, 0xd9, 0x22,
0x9c, 0x67, 0x90, 0x6b, 0x84, 0x7f, 0x88, 0x73,
0xac, 0x57, 0xa0, 0x5b, 0xb4, 0x4f, 0xb8, 0x43,
0x3f, 0xc4, 0x33, 0xc8, 0x27, 0xdc, 0x2b, 0xd0,
0x0f, 0xf4, 0x03, 0xf8, 0x17, 0xec, 0x1b, 0xe0,
0x5e, 0xa5, 0x52, 0xa9, 0x46, 0xbd, 0x4a, 0xb1,
0x6e, 0x95, 0x62, 0x99, 0x76, 0x8d, 0x7a, 0x81,
0x83, 0x78, 0x8f, 0x74, 0x9b, 0x60, 0x97, 0x6c,
0xb3, 0x48, 0xbf, 0x44, 0xab, 0x50, 0xa7, 0x5c,
0xe2, 0x19, 0xee, 0x15, 0xfa, 0x01, 0xf6, 0x0d,
0xd2, 0x29, 0xde, 0x25, 0xca, 0x31, 0xc6, 0x3d,
0x41, 0xba, 0x4d, 0xb6, 0x59, 0xa2, 0x55, 0xae,
0x71, 0x8a, 0x7d, 0x86, 0x69, 0x92, 0x65, 0x9e,
0x20, 0xdb, 0x2c, 0xd7, 0x38, 0xc3, 0x34, 0xcf,
0x10, 0xeb, 0x1c, 0xe7, 0x08, 0xf3, 0x04, 0x7d
};
int pattern_allowed = 7; /* maximum number of bytes
permitted before escaping,
default 7 */
int pattern_found = 0; /* count of bytes matched */
uint8 pattern_next = 0x7e; /* next byte in pattern,
assume start of HDLC frame */
/* Check a byte stream for a pattern matching sequence
* exceeding the allowed match length. Return true/false.
* On true, the caller will generate a two byte escape sequence,
* calling this routine again with both values.
*/
int pattern_escape_needed( uint8 current_byte )
{
if ( current_byte != pattern_next )
{
pattern_found = 0;
}
else
{
pattern_found++;
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}
pattern_next = pattern[current_byte];
return ((pattern_found > pattern_allowed)
&& (current_byte != 0x5e)
&& (current_byte != 0x7d)
&& (current_byte != 0x7e));
}
B.2. Rejected Alternatives
Several recurrent suggestions are less desirable than octet stuffing.
Most of these suggestions do not provide backward compatibility.
Moreover, none of these suggestions scale well. They do not accomo-
date parallel processing, as they are bit-oriented.
Furthermore, for the LFSR techniques, the resulting protection is
probablistic, not a complete solution.
ATM employs a payload LFSR scrambler that affects only the data por-
tion of the ATM cell, and does not include the ATM header. The gen-
erating polynomial for the LFSR is x**43 + 1.
The equivalent technique applied to PPP encapsulated packets in HDLC-
like frames (or Frame Relay) as they are inserted into the payload
would not include the framing octets.
This is rejected, as the scrambled data could mimic the frame delim-
iting flag sequence, resulting in incorrect frame detection.
The ATM (x**43 + 1) LFSR could be applied to the entire frame as it
is inserted into the payload.
This is rejected, as the polynomial has an undesirable interaction
with the HDLC 16-bit FCS (x**16 + x**12 + x**5 + 1). Analysis has
shown [FC97] that there exist undetectable burst error patterns, and
that protection against errors in general is reduced.
Other LFSR polynomials have been proposed. These have a similar
error multiplication effect.
An enhancement to the octet-stuffing technique has been suggested
that checks the scrambler output for the all-zeros pattern, and sig-
nals an escape insertion to the HDLC-like framer (or Frame Relay).
This is only useful for highly integrated devices, and protects only
the first section. Other payload alignments could occur on later
sections in the path.
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Security Considerations
This protocol introduces no known security risks.
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.
Acknowledgements
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).
Garry Epps (Cisco), Steve Lang (PMC-Sierra), Peter Lothberg (Sprint),
Subhash Roy (TranSwitch), Stuart Venters (Adtran), and Eric Verwillow
(Juniper) provided useful critiques of earlier versions of this docu-
ment.
Special thanks to Ascend Communications (nee Morning Star Technolo-
gies) for providing computing resources and network access support
for writing this specification.
References
[RFC-1661]
Simpson, W., Editor, "The Point-to-Point Protocol (PPP)",
STD-51, July 1994.
[RFC-1662]
Simpson, W., Editor, "PPP in HDLC-like Framing", STD-51,
July 1994.
[RFC-2119]
Bradner, S., "Key words for use in RFCs to Indicate Require-
ment Levels", BCP 14, Harvard University, March 1997.
[SONET] "Synchronous Optical Network (SONET) Transport Systems: Com-
mon Generic Criteria", Bellcore, TR-NWT-000253 Issue 2,
December 1991.
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[G.707] CCITT Recommendation G.707, "Synchronous Digital Hierarchy
Bit Rates", June 1992.
Contacts
Comments about this document should be discussed on the
pppsdh@greendragon.com or ietf-ppp@merit.edu mailing lists.
This document is a submission to 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
3518 Riverside Drive Suite 101
Columbus, Ohio 43221
karl@Ascend.com
Questions about this document can also be directed to:
William Allen Simpson
DayDreamer
Computer Systems Consulting Services
1384 Fontaine
Madison Heights, Michigan 48071
wsimpson@UMich.edu
wsimpson@GreenDragon.com (preferred)
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