One document matched: draft-ietf-pppext-pppoversonet-update-01.txt
Differences from draft-ietf-pppext-pppoversonet-update-00.txt
PPP Extensions Working Group A. Malis
INTERNET DRAFT Ascend Communications, Inc.
Expires: August 1999 February 1999
<draft-ietf-pppext-pppoversonet-update-01.txt>
Obsoletes: RFC 1619
PPP over SONET/SDH
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet- Drafts as reference
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The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Abstract
The Point-to-Point Protocol (PPP) [1] provides a standard method for
transporting multi-protocol datagrams over point-to-point links.
This document describes the use of PPP over Synchronous Optical
Network (SONET) and Synchronous Digital Heirarchy (SDH) circuits.
This document is the product of the Point-to-Point Protocol
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Extensions Working Group of the Internet Engineering Task Force
(IETF). Comments should be submitted to the ietf-ppp@merit.edu
mailing list.
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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.
This specification is primarily concerned with the use of the PPP
encapsulation over SONET/SDH links. Since SONET/SDH is by definition
a point-to-point circuit, PPP is well suited to use over these links.
The Synchronous Optical Network (SONET) [3] is an octet-synchronous
multiplex scheme that defines a family of standard rates and formats.
The transmission rates are integral multiples of 51.840 Mbps, which
may be used to carry T3/E3 bit-synchronous signals. The current
commonly-used multiples are specified as
STS-1c 51.840
STS-3c 155.520
STS-12c 622.080
STS-48c 2,488.320
STS-192c 9,953.280
The ITU Synchronous Digital Heirarchy (SDH) defines an equivalent set
of transmission rates beginning at 155.520 Mbps [5]:
SONET SDH equivalent
STS-3c STM-1
STS-12c STM-4
STS-48c STM-16
STS-192c STM-64
This document specifies PPP over SONET/SDH for rates up to and
including STS-192c/STM-64.
The keywords MUST, MUST NOT, MAY, OPTIONAL, REQUIRED, RECOMMENDED,
SHALL, SHALL NOT, SHOULD, and SHOULD NOT are to be interpreted as
defined in [6].
2. Physical Layer Requirements
PPP treats SONET/SDH transport as octet oriented synchronous links.
SONET/SDH links are full-duplex by definition.
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Interface Format
PPP in HDLC-like framing 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 Synchronous Payload
Envelope (SPE), with the octet boundaries aligned with the SPE
octet boundaries.
Scrambling is performed during insertion into the SPE to provide
adequate transparency and protect against potential security
threats (see Section 6). For backwards compatibility with RFC
1619 (STS-3c/STM1 only), the scrambler MAY have an on/off
capability where the scrambler is bypassed entirely when it is in
the off mode. If this capability is provided, the default MUST be
set to scrambling enabled.
For PPP over SONET/SDH, the entire SONET/SDH payload (SPE minus
the path overhead and any fixed stuff) is scrambled using a self-
synchronous scrambler of polynomial X**43 + 1. See Section 4 for
the description of the scrambler.
The proper order of operation is:
When transmitting:
IP -> PPP -> FCS generation -> Byte stuffing -> Scrambling ->
SONET/SDH framing
When receiving:
SONET/SDH framing -> Descrambling -> Byte destuffing -> FCS
detection -> PPP -> IP
The Path Signal Label (C2) indicates the contents of the SPE. The
value of 22 (16 hex) is used to indicate PPP with X^43 + 1
scrambling.
For compatibility with RFC 1619 (STS-3c/STM1 only), if scrambling
has been configured to be off, then the value 207 (CF hex) is used
for the Path Signal Label to indicate PPP without scrambling.
The Multiframe Indicator (H4) is unused, and MUST be zero.
Transmission Rate
The basic rate for PPP over SONET/SDH is that of STS-3c/STM-1 at
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155.520 Mbps. The available information bandwidth is 149.760
Mbps, which is the STS-3c/STM-1 SPE with section, line and path
overhead removed. This is the same super-rate mapping that is
used for ATM and FDDI [4].
Lower signal rates MUST use the Virtual Tributary (VT) mechanism
of SONET/SDH. This maps existing signals up to T3/E3 rates
asynchronously into the SPE, or uses available clocks for bit-
synchronous and byte-synchronous mapping.
This document specifies PPP over SONET/SDH for rates up to and
including STS-192c/STM-64.
Control Signals
PPP does not require the use of control signals. When available,
using such signals can allow greater functionality and
performance. Implications are discussed in [2].
3. Framing
The framing for octet-synchronous links is described in "PPP in
HDLC-like Framing" [2].
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.
4. X**43 + 1 Scrambler Description
The X**43 + 1 scrambler transmitter and receiver operation are as
follows:
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Transmitter schematic:
Unscrambled Data
|
v
+-------------------------------------+ +---+
+->| --> 43 bit shift register --> |--->|xor|
| +-------------------------------------+ +---+
| |
+-----------------------------------------------+
|
v
Scrambled Data
Receiver schematic:
Scrambled Data
|
+-----------------------------------------------+
| |
| v
| +-------------------------------------+ +---+
+->| --> 43 bit shift register --> |--->|xor|
+-------------------------------------+ +---+
|
v
Unscrambled Data
Note: While the HDLC FCS is calculated least significant bit first as
shown:
<- <- <- <-
A B C D
(that is, the FCS calculator is fed as follows: A[0], A[1], ... A[7],
B[0], B[1], etc...), scrambling is done in the opposite manner, most
significant bit first, as shown:
-> -> -> ->
A B C D.
That is, the scrambler is fed as follows: A[7], A[6], ... A[0], B[7],
B[6], etc...
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The scrambler operates continuously through the bytes of the SPE,
bypassing bytes of SONET Path Overhead and any fixed stuff (see
Figure 20 of ANSI T1.105 [3] or Figure 10-17 of ITU G.707 [5]). The
scrambling state at the beginning of a SPE is the state at the end of
the previous SPE. Thus, the scrambler runs continuously and is not
reset per frame. The initial seed is randomly chosen by transmitter
to improve operational security (see Section 6). Consequently, the
first 43 transmitted bits following startup or reframe operation will
not be descrambled correctly.
5. Configuration Details
The standard LCP sync 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
32-bit FCS
Regarding the FCS length, the 32-bit FCS MUST be supported for all
SONET/SDH rates. For STS-3c ONLY, the 16-bit FCS MAY be used, but is
NOT RECOMMENDED due to the error multiplication that can result from
scrambling.
6. Security Considerations
The major change from RFC 1619 is the addition of payload scrambling
when inserting the HDLC-like framed PPP packets into the SONET/SDH
SPE. RFC 1619 was operationally found to permit malicious users to
generate packets with bit patterns that could create SONET/SDH-layer
low-transition-density synchronization problems, emulation of the SDH
set-reset scrambler pattern, and replication of the STM-N frame
alignment word.
The use of the x^43 + 1 self-synchronous scrambler was introduced to
alleviate these potential security problems. Predicting the output
of the scrambler requires knowledge of the 43-bit state of the
transmitter as the scrambling of a known input is begun. This
requires knowledge of both the initial 43-bit state of the scrambler
when it started and every byte of data scrambled by the device since
it was started. The odds of guessing correctly are 1/2**43, with the
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additional probability of 1/127 that a correct guess will leave the
frame properly aligned in the SONET/SDH payload, which results in a
probability of 9e-16 against being able to deliberately cause
SONET/SDH-layer problems. This seems reasonably secure for this
application.
This scrambler is also used when transmitting ATM over SONET/SDH, and
public network carriers have considerable experience with its use.
7. Intellectual Propoerty
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
copyrights, patents or patent applications, or other proprietary
rights which may cover technology that may be required to practice
this standard. Please address the information to the IETF Executive
Director.
8. Acknowledgments
Bill Simpson, Daydreamer, was the editor for this document's
predecessor, RFC 1619, and much of his original text remains.
The scrambler description was provided by J. Manchester, S. Davida,
B. Doshi, and J. Anderson of Lucent Technologies, R. Broberg of Cisco
Systems, and Peter Lothberg of Sprint Corporation. The security
analysis was provided by Iain Verigin of PMC-Sierra and Larry McAdams
of Cisco Systems.
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9. 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] American National Standards Institute, "Synchronous Optical
Network (SONET) - Basic Description including Multiplex
Structure, Rates and Formats," ANSI T1.105-1995.
[4] American National Standards Institute, "Synchronous Optical
Network (SONET)--Payload Mappings," T1.105.02-1998.
[5] ITU Recommendation G.707, "Network Node Interface For The
Synchronous Digital Hierarchy", 1996.
[6] Bradner, S., "Key words for use in RFCs to indicate Requirement
Levels", BCP 14, RFC 2119, Harvard University, March 1997.
10. Author's Address
Andrew G. Malis
Ascend Communications, Inc.
1 Robbins Road
Westford, MA 01810 USA
Phone: +1 978 952 7414
Email: malis@ascend.com
11. Full Copyright Statement
Copyright (C) The Internet Society (1999). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
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the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
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Table of Contents
1. Introduction .......................................... 1
2. Physical Layer Requirements ........................... 1
3. Framing ............................................... 3
4. X**43 + 1 Scrambler Description ....................... 3
5. Configuration Details ................................. 5
6. Security Considerations ............................... 5
7. Intellectual Propoerty ................................ 6
8. Acknowledgments ....................................... 6
9. References ............................................ 7
10. Author's Address ...................................... 7
11. Full Copyright Statement .............................. 7
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