One document matched: draft-ietf-pppext-sdl-00.txt
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
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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.
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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
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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.
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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
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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.
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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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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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
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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
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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.
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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
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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
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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
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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.
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(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)
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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;
}
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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;
}
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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(' ');
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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]);
}
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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.
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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
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