| Rfc | 2823 |
|---|
| Title | PPP over Simple Data Link (SDL) using SONET/SDH with ATM-like
framing |
|---|
| Author | J. Carlson, P. Langner, E. Hernandez-Valencia, J.
Manchester |
|---|
| Date | May 2000 |
|---|
| Format: | TXT, HTML |
|---|
| Status: | EXPERIMENTAL |
|---|
|
Network Working Group J. Carlson
Request for Comments: 2823 Sun Microsystems, Inc.
Category: Experimental P. Langner
Lucent Technologies Microelectronics Group
E. Hernandez-Valencia
J. Manchester
Lucent Technologies
May 2000
PPP over Simple Data Link (SDL)
using SONET/SDH with ATM-like framing
Status of this Memo
This memo defines an Experimental Protocol for the Internet
community. It does not specify an Internet standard of any kind.
Discussion and suggestions for improvement are requested.
Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2000). All Rights Reserved.
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 2615 [3] provide a means to carry PPP over
Synchronous Optical Network (SONET) [4] and Synchronous Digital
Hierarchy (SDH) [5] circuits. This document extends these standards
to include a new encapsulation for PPP called Simple Data Link (SDL)
[6]. SDL provides a very low overhead alternative to HDLC-like
encapsulation, and can also be used on SONET/SDH links.
Applicability
This specification is intended for those implementations that use PPP
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 and good hardware
scaling characteristics, it is anticipated that significantly higher
throughput can be attained when compared to other possible SONET/SDH
payload mappings, and at a significantly lower cost for line
termination equipment.
SDL is defined over other media types and for other data link
protocols, but this specification covers only the use of PPP over SDL
on SONET/SDH.
The use of SDL requires the presentation of packet length information
in the SDL header. Thus, hardware implementing SDL must have access
to the packet length when generating the header, and where a router's
input link does not have this information (that is, for non-SDL input
links), the router may be required to buffer the entire packet before
transmission. "Worm-hole" routing is thus at least problematic with
SDL, unless the input links are also SDL. This, however, does not
appear to be a great disadvantage on modern routers due to the
general requirement of length information in other parts of the
system, notably in queuing and congestion control strategies such as
Weighted Fair Queuing [7] and Random Early Detect [8].
This document is not a replacement for the existing HDLC-like framing
mandated by RFC 2615 [3]. Instead, the authors intend to gain
implementation experience with this technique for operational and
performance evaluation purposes, and would like to hear from others
either considering or using the protocol as described in this
document. Please see Section 14 of this document for contact
information.
Table of Contents
1. Introduction ............................................... 4
2. Compliance ................................................. 4
3. Physical Layer Requirements ................................ 5
3.1. Payload Types ............................................ 5
3.2. Control Signals .......................................... 6
3.3. Synchronization Modes .................................... 7
3.4. Simple-Data-Link LCP Option .............................. 7
3.5. Framing .................................................. 8
3.6. Framing Example .......................................... 11
3.7. Synchronization Procedure ................................ 11
3.8. Scrambler Operation ...................................... 12
3.9. CRC Generation ........................................... 12
3.10. Error Correction ........................................ 13
4. Performance Analysis ....................................... 14
4.1. Mean Time To Frame (MTTF) ................................ 14
4.2. Mean Time To Synchronization (MTTS) ...................... 15
4.3. Probability of False Frame (PFF) ......................... 16
4.4. Probability of False Synchronization (PFS) ............... 16
4.5. Probability of Loss of Frame (PLF) ....................... 16
5. The Special Messages ....................................... 16
5.1. Scrambler State .......................................... 17
5.2. A/B Message .............................................. 17
6. The Set-Reset Scrambler Option ............................. 17
6.1. The Killer Packet Problem ................................ 17
6.2. SDL Set-Reset Scrambler .................................. 18
6.3. SDL Scrambler Synchronization ............................ 18
6.4. SDL Scrambler Operation .................................. 19
7. Configuration Details ...................................... 20
7.1. Default LCP Configuration ................................ 20
7.2. Modification of the Standard Frame Format ................ 21
8. Implementation Details ..................................... 21
8.1. CRC Generation ........................................... 21
8.2. Error Correction Tables .................................. 23
9. Security Considerations .................................... 25
10. References ................................................ 25
11. Acknowledgments ........................................... 26
12. Working Group and Chair Address ........................... 26
13. Intellectual Property Notices ............................. 26
14. Authors' Addresses ........................................ 27
15. Full Copyright Statement .................................. 28
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
requirements for PPP. While O-S framing on SONET/SDH has a fixed
seven octet overhead per frame plus a worst-case overhead of 100% of
all data octets transmitted, SDL has a fixed eight octet per frame
overhead with zero data overhead. Unlike O-S framing, SDL also
provides positive indication of link synchronization.
Note: This document describes two new SONET/SDH Path Signal Label
(PSL) values; 23 (17 hex) for SDL with the proposed self synchronous
scrambler and 25 (19 hex) for SDL with the proposed set-reset
scrambler. These values have been allocated by ANSI T1X1.5 and ITU-T
SG-15 for use with SDL over SONET and SDH, and will appear in
subsequent updates of T1.105 (Table 8) and Recommendation G.707
(Table 7).
2. Compliance
In this document, the words that are used to define the significance
of each particular requirement are capitalized.
These words are:
* "MUST"
This word means that the item is an absolute requirement of the
specification.
* "MUST NOT"
This phrase means that the item is an absolute prohibition of the
specification.
* "SHOULD"
This word means that there may exist valid reasons in particular
circumstances to ignore this item, but the full implications
should be understood and the case carefully weighed before
choosing a different course.
* "SHOULD NOT"
This phrase means that there may exist valid reasons in particular
circumstances to apply this item, but the full implications should
be understood and the case carefully weighed before choosing a
different course.
* "MAY"
This word means that this item is truly optional. One vendor may
choose to include the item because a particular marketplace
requires it or because it enhances the product, for example;
another vendor may omit the same item.
An implementation is not compliant if it fails to satisfy one or more
of the MUST or MUST NOT requirements for this protocol. An
implementation that satisfies all of the MUST, MUST NOT, SHOULD, and
SHOULD NOT requirements for this protocol is said to be
"unconditionally compliant". One that satisfies all the MUST and
MUST NOT requirements but not all the SHOULD or SHOULD NOT
requirements is said to be "conditionally compliant".
3. 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.
3.1. Payload Types
Only synchronous payloads STS-1 and higher are considered in this
document. Lower speed synchronous, such as VT1.5-SPE/VC-11, and
plesiochronous payload mappings, such as T1 and T3, are defined for
SONET/SDH and for the SDL algorithm itself, but, since HDLC-like
framing is defined for PPP on those media, PPP over SDL is not
defined.
SDL is separately defined as a PPP transport for use on raw fiber
without SONET/SDH framing for use as an alternative to bit-
synchronous HDLC. Please see the separate work-in-progress for
details.
3.2. 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
detecting this value, and any use must take this into account.
Equipment employing only SDL MUST be capable of transmitting PSL with
value 23, and MAY also be capable of transmitting PSL with value 25,
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
reception of LCP Configure-Request with an SDL LCP option or when the
peer's transmitted PSL value is received as 23 (or 25), the
implementation MUST shut down LCP by sending a Down event to its
state machine, then switch its transmitted PSL value to 23 (or 25),
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 (or 25) 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 (or 25) to
any other value, then this is treated as a Down event into the LCP
state machine, and an Up event MUST be generated if the new value is
recognized as a valid PPP framing mode.
When SDL is enabled by prior arrangement, the PSL SHOULD be
transmitted as 23 (or 25). 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
outside 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
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.
The LCP-negotiated method of operation is compatible with the current
version of G.783 [12]. This method may not be compatible, however,
with some non-intrusive SDH path monitoring equipment based on
obsolete versions of G.783. The change in PSL value indicated by the
LCP negotiation method will cause this equipment to declare an alarm
condition on the path. For this reason, the prior-arrangement method
MUST be used on any SDH network that is using such monitoring
equipment.
3.3. Synchronization Modes
Unlike O-S encapsulation, SDL provides a positive indication that it
has achieved synchronization with the peer. An SDL PPP
implementation MUST provide a means to temporarily suspend PPP data
transmission (both user data and negotiation traffic) if
synchronization loss is detected. An SDL PPP implementation SHOULD
also provide a configurable timer that is started when SDL is
initialized and restarted on the loss of synchronization, and is
terminated when link synchronization is achieved. If this timer
expires, implementation-dependent action should be taken to report
the hardware failure.
3.4. Simple-Data-Link LCP Option
A new LCP Configuration Option is used to request Simple Data Link
(SDL) [6] 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
transmitted 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
29
Length
2
This option is used only as a hint to the peer that SDL over
SONET/SDH 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.
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 to switch into SDL
mode. If the received LCP Configure-Request message does not contain
an SDL LCP option, an implementation MUST 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.)
3.5. 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.
With reference to the Lucent SDL specification [6] when SDL framing
for PPP is employed, the SDL "Datagram Offset" feature is set to the
value 4. This corresponds to the fixed overhead value 4 in the
description below. 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.
Fixing the Datagram Offset value described in the Lucent
documentation to 4 allows a PPP MRU/MTU up to 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 3.7.
For use with PPP, this fixed-length header precedes each PPP/HDLC
packet as follows:
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) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ..... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SDL CRC |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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 PPP FCS, the Header CRC is a CRC-16 generated with initial
value zero and transmitted in network byte order. The PPP packet is
scrambled, begins with the 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 8 plus
the Packet Length field. The value 8 represents a fixed overhead of
4 octets plus the fixed length of the Packet CRC field. 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 used only with optional scrambling and
management modes. See section 5 for details of the messages.
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 [9], 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 [9] 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
configurable option MUST default to CRC-32, and MUST NOT include LCP
negotiation of FCS Alternatives.
Setting the SDL Datagram Offset value to 4 accounts for the 4 octet
SDL header overhead. 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 CRC field (the RFC 1662 PPP FCS field is
not used with SDL). Note again that the Datagram Offset is just an
arithmetic value; it does not occupy bits in the message itself.
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
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.
By default, an independent, self-synchronous x^43+1 scrambler is used
on the data portion of the message including the 32 bit CRC. This is
done in exactly the same manner as with the ATM x^43+1 scrambler on
an ATM channel. The scrambler is not clocked when SDL header bits
are transmitted. Thus, the data scrambling MAY be implemented in an
entirely independent manner from the SDL framing, and the data stream
may be prescrambled before insertion of SDL framing marks.
Optionally, by prior arrangement, SDL links MAY use a set-reset
scrambler as described in section 6. If this option is provided, it
MUST be configurable by the administrator, and the option MUST
default to the self-synchronous scrambler.
3.6. Framing Example
To help clarify this structure, the following example may be helpful.
First we have an LCP Configure-Request message that we wish to
transmit over SDL:
FF 03 C0 21 01 01 00 04
Next, we create an SDL header for the length of this packet (8
octets), a header CRC, and an SDL CRC.
00 08 81 08 FF 03 C0 21 01 01 00 04 D1 F5 21 5E
Finally, we DC-balance the header with the barker-like sequence:
B6 A3 B0 E8 FF 03 C0 21 01 01 00 04 D1 F5 21 5E
Note that the final length of the message is 8 (original message
length) plus 4 (fixed datagram offset value) plus 4 (fixed CRC
length), or 16 octets.
3.7. Synchronization Procedure
The link synchronization procedure is similar to the I.432 section
4.5.1.1 ATM HEC delineation procedure [10], except that 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 3.10 is not employed until the machine is in in
SYNCH state. The header must have no bit errors in order to leave
HUNT 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 PRESYNCH state returns the link to HUNT
state.
If a second valid SDL header is seen after entering PRESYNCH state,
then the link enters SYNCH state and PPP transmission is enabled. If
an invalid SDL header is detected, then the link is returned to HUNT
state without enabling PPP transmission.
Once the link enters SYNCH state, the SDL header single bit error
correction logic is enabled (see section 3.10). Any unrecoverable
header CRC error returns the link to HUNT state, disables PPP
transmission, and disables the error correction logic.
3.8. Scrambler Operation
The transmit and receive scramblers are shift registers with 43
stages that MAY be initialized to all-ones when the link is
initialized. Synchronization is maintained by the data itself.
Transmit Receive
DATA-STREAM (FROM PPP) IN (FROM SDL FRAMER)
| |
v |
XOR<-------------------------+ +->D0-+->D1-> ... ->D41->D42-+
| | | |
+->D0-+->D1-> ... ->D41->D42-+ XOR<-------------------------+
| |
v v
OUT (TO SDL FRAMER) DATA-STREAM (TO PPP)
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 or reception of each
bit of payload and before the next bit is applied as input. Bits
within an octet are, per SONET/SDH practice, transmitted and received
MSB-first.
3.9. CRC Generation
The CRC-16 and CRC-32 generator polynomials used by SDL are the ITU-T
polynomials [11]. 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; see section 5)
are all 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 except by prior arrangement) uses the 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. Alternatively, an implementation may stop CRC
calculation before processing the appended CRC itself, and do a
direct comparison.
The 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, most significant bit first, unlike
the optional PPP 32 bit FCS, which is transmitted in reverse order.
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 section 8.1.
3.10. 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 CRC-16 remainder 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
calculation over the four octet SDL header to correct bit errors in
the header and to detect multiple bit errors. For the optional set-
reset scrambler, a table look-up is similarly 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: No error correction is performed for the payload.
Note: This error correction technique is used only when the link has
entered SYNCH state. While in HUNT or PRESYNCH state, error
correction should not be performed, and only messages with syndrome
0000 are accepted. If the calculated syndrome does not appear in
this table, then an unrecoverable error has occurred. Any such error
in the SDL header will return the link to HUNT state.
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
eight octet table.
Eight octet (64 bit) single bit error syndrome table (in
hexadecimal):
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.
Note that eight octet messages are used only for the optional set-
reset scrambling mode, described in section 6.
Corresponding C code to generate this table is found in section 8.2.
4. 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 [6].
4.1. Mean Time To Frame (MTTF)
This metric measures the amount of time required to establish 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 failed to
detect the next incoming SDL frame in the data stream.
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
candidate 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
slightly larger than Internet average 354 byte packets, are:
Number of Unavailability Unavailability
Framers 64KB packets 354 byte pkts
1 3.679E-1 5.373E-3
2 3.083E-2 1.710E-6
3 2.965E-3 9.712E-10
4 2.532E-4 4.653E-13
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
summarized as:
Number of Flat region Flat region
Framers 64KB packets 354 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
approximately 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.
4.2. Mean Time To Synchronization (MTTS)
The MTTS for SDL with a self-synchronous scrambler is the same as the
MTTF, or 1.5 packets.
The MTTS for SDL using the optional set-reset scrambler 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.
(The probability of receiving a bad scrambling state transmission
should also be included in this calculation. The probability of
random corruption of this short message is shown in the SDL document
[6] to be small enough that it can be neglected for this
calculation.)
4.3. Probability of False Frame (PFF)
The PFF is 2.328E-10 (2^-32), since false framing requires two
consecutive headers with falsely correct CRC-16.
4.4. Probability of False Synchronization (PFS)
The PFS for SDL with the self-synchronous scrambler is the same as
the PFF, or 2.328E-10 (2^-32).
The PFS for SDL with the set-reset scrambler is 5.421E-20 (2^-64),
and is calculated as the PFF above multiplied by the probability of a
falsely detected scrambler state message, which itself contains two
independent CRC-16 calculations.
4.5. Probability of Loss of Frame (PLF)
The PLF is a function of the BER, and for SDL is approximately the
square of the BER multiplied by 500, 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 PLF is 5E-8.
5. The Special Messages
When the SDL Packet Length field has any value between 0000 and 0003,
the message following the header has a special, pre-defined length.
The 0 value is a time-fill on an idle link, and no other data
follows. The next octet on the link is the first octet of the next
SDL header.
The values 1 through 3 are defined in the following subsections.
These special messages each consist of a six octet data portion
followed by another CRC-16 over that data portion, as with the SDL
header, and this CRC is used for single bit error correction.
5.1. Scrambler State
The special value of 1 for Packet Length is reserved to transfer the
scrambler state from the transmitter to the receiver for the optional
set-reset scrambler. In this case, the SDL header is followed by six
octets (48 bits) of scrambler state. Neither the scrambler state nor
the CRC are scrambled.
5.2. A/B Message
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. These messages are
reserved for use by link maintenance protocols, in a manner analogous
to ATM's OAM cells.
6. The Set-Reset Scrambler Option
PPP over SDL uses a self-synchronous scrambler. SDL implementations
MAY also employ a set-reset scrambler to avoid some of the possible
inherent problems with self-synchronous scramblers.
6.1. The Killer Packet Problem
Scrambling in general solves two problems. First, SONET and SDH
interfaces require a minimum density of bit transitions in order to
maintain hardware clock recovery. Since data streams frequently
contain long runs of all zeros or all ones, scrambling the bits using
a pseudo-random number sequence breaks up these patters. Second, all
link-layer synchronization mechanisms rely on detecting long-range
patterns in the received data to detect framing.
Self-synchronous scramblers are an easy way to partially avoid these
problems. One problem that is inherent with self-synchronous,
however, is that long user packets from malicious sites can make use
of the known properties of these scramblers to inject either long
strings of zeros or other synchronization-destroying patterns into
the link. For public networks, where the data presented to the
network is usually multiplexed (interleaved) with multiple unrelated
streams, the clocking problem does not pose a significant threat to
the public network. It does, however, pose a threat to the PPP-
speaking device, and it poses a threat to long lines that are
unchannelized.
Such carefully constructed packets are called "killer packets".
6.2. SDL Set-Reset Scrambler
An alternative to the self-synchronous scrambler is the externally
synchronized or "set-reset" scrambler. This is a free-running
scrambler that is not affected by the patterns in the user data, and
therefore minimizes the possibility that a malicious user could
present data to the network that mimics an undesirable data pattern.
The option set-reset scrambler defined for SDL is an
x^48+x^28+x^27+x+1 independent scrambler initialized to all ones when
the link enters PRESYNCH state and reinitialized if the value ever
becomes all zero bits. As with the self-synchronous scrambler, all
octets in the PPP packet data following the SDL header through the
final packet CRC are scrambled.
This mode MAY be detected automatically. If a scrambler state
message is received (as described in the following section), an SDL
implementation that includes the set-reset scrambler option may
switch from self-synchronous into set-reset mode automatically. An
SDL implementation that does not include the set-reset scrambler MUST
NOT send scrambler state messages.
6.3. SDL Scrambler Synchronization
As described in the previous section, 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.
SDL synchronization consists of two components, link and scrambler
synchronization. Both must be completed before PPP data flows on the
link.
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). When a scrambler
synchronization message is received from the peer, 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
synchronization 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.
6.4. SDL 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
scrambling 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.
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
message that passes the CRC-16 message check without multiple bit
errors is clocked directly into the receiver's state register after
the comparison 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.)
7. Configuration Details
7.1. Default LCP Configuration
The 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)
This configuration means that PPP over SDL generally presents a 32-
bit aligned datagram to the network layer. With the address,
control, and protocol field intact, the PPP overhead on each packet
is four octets. If the SDL framer presents the SDL packet header to
the PPP input handling in order to communicate the packet length (the
Lucent implementation does not do this, but other hardware
implementations may), this header is also four octets, and alignment
is preserved.
7.2. Modification of the Standard Frame Format
Since SDL does take the place of HDLC as a transport for PPP, it is
at least tempting to remove the HDLC-derived overhead. This is not
done for PPP over SDL in order to preserve the message alignment and
to allow for the future possibility interworking with other services
(e.g., Frame Relay).
By prior external arrangement or via LCP negotiation, any two SDL
implementations MAY agree to omit the address and control fields or
implement protocol field compression on a link. Such use is not
described by this document and MUST NOT be the default on any SDL
implementation.
8. Implementation Details
8.1. 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;
}
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;
}
8.2. 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
messages, 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
3.10. 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(' ');
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_error_position[crc]);
}
9. Security Considerations
The reliability of public SONET/SDH networks depends on well-behaved
traffic that does not disrupt the synchronous data recovery
mechanisms. 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.
10. References
[1] Simpson, W., Editor, "The Point-to-Point Protocol (PPP)", STD
51, RFC 1661, July 1994.
[2] Simpson, W., Editor, "PPP in HDLC-like Framing", STD 51, RFC
1662, July 1994.
[3] Malis, A. and W. Simpson, "PPP over SONET/SDH", RFC 2615, June
1999.
[4] "American National Standard for Telecommunications -
Synchronous Optical Network (SONET) Payload Mappings," ANSI
T1.105.02-1995.
[5] ITU-T Recommendation G.707, "Network Node Interface for the
Synchronous Digital Hierarchy (SDH)," March 1996.
[6] Doshi, B., Dravida, S., Hernandez-Valencia, E., Matragi, W.,
Qureshi, M., Anderson, J., Manchester, J.,"A Simple Data Link
Protocol for High Speed Packet Networks", Bell Labs Technical
Journal, pp. 85-104, Vol.4 No.1, January-March 1999.
[7] Demers, A., S. Keshav, and S. Shenker, "Analysis and simulation
of a fair queueing algorithm," ACM SIGCOMM volume 19 number 4,
pp. 1-12, September 1989.
[8] Floyd, S. and V. Jacobson, "Random Early Detection Gateways for
Congestion Avoidance," IEEE/ACM Transactions on Networking,
August 1993.
[9] Simpson, W., Editor, "PPP LCP Extensions", RFC 1570, January
1994.
[10] ITU-T Recommendation I.432.1, "B-ISDN User-Network Interface -
Physical Layer Specification: General Characteristics,"
February 1999.
[11] ITU-T Recommendation V.41, "Code-independent error-control
system," November 1989.
[12] ITU-T Recommendation G.783, "Characteristics of synchronous
digital hierarchy (SDH) equipment functional blocks," April
1997.
11. Acknowledgments
PPP over SONET was first proposed by Craig Partridge (BBN) and is
documented by Andrew Malis and William Simpson as RFC 2615.
Much of the material in this document was supplied by Lucent.
Other length-prefixed forms of framing for PPP have gone before SDL,
such as William Simpson's "PPP in Ether-like Framing" expired draft.
12. 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
Extant, Inc.
3496 Snouffer Road, Suite 100
Columbus, Ohio 43235
EMail: karl@extant.net
13. Intellectual Property Notices
The IETF takes no position regarding the validity or scope of any
intellectual property or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; neither does it represent that it
has made any effort to identify any such rights. Information on the
IETF's procedures with respect to rights in standards-track and
standards-related documentation can be found in BCP-11. Copies of
claims of rights made available for publication and any assurances of
licenses to be made available, or the result of an attempt made to
obtain a general license or permission for the use of such
proprietary rights by implementors or users of this specification can
be obtained from the IETF Secretariat.
The IETF invites any interested party to bring to its attention any
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Director.
14. Authors' Addresses
James Carlson
Sun Microsystems, Inc.
1 Network Drive MS UBUR02-212
Burlington MA 01803-2757
Phone: +1 781 442 2084
Fax: +1 781 442 1677
EMail: james.d.carlson@sun.com
Paul Langner
Lucent Technologies Microelectronics Group
555 Union Boulevard
Allentown PA 18103-1286
EMail: plangner@lucent.com
Enrique J. Hernandez-Valencia
Lucent Technologies
101 Crawford Corners Rd.
Holmdel NJ 07733-3030
EMail: enrique@lucent.com
James Manchester
Lucent Technologies
101 Crawford Corners Rd.
Holmdel NJ 07733-3030
EMail: sterling@hotair.hobl.lucent.com
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