RFC2615 - PPP over SONET/SDH

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　　Network Working Group A. Malis
Request for Comments: 2615 Ascend Communications, Inc.
Obsoletes: 1619 W. Simpson
Category: Standards Track DayDreamer
June 1999

PPP over SONET/SDH

Status of this Memo

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.

Copyright Notice

Copyright (C) The Internet Society (1999). 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.
This document describes the use of PPP over Synchronous Optical
Network (SONET) and Synchronous Digital Hierarchy (SDH) circuits.

This document replaces and obsoletes RFC1619. See section 7 for a
summary of the changes from RFC1619.

Table of Contents

1. Introduction .......................................... 2
2. Physical Layer Requirements ........................... 3
3. Framing ............................................... 4
4. X**43 + 1 Scrambler Description ....................... 4
5. Configuration Details ................................. 6
6. Security Considerations ............................... 6
7. Changes from RFC1619 ................................. 7
8. Intellectual Property ................................. 7
9. Acknowledgments ....................................... 8
10. References ............................................ 8
11. Authors' Addresses .................................... 9
12. Full Copyright Statement .............................. 10

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.

Real differences between SONET and SDH (other than terminology) are
minor; for the purposes of encapsulation of PPP over SONET/SDH, they
are inconsequential or irrelevant.

For the convenience of the reader, we list the equivalent terms below:

SONET SDH
---------------------------------------------
SPE VC
STS-SPE Higher Order VC (VC-3/4/4-Nc)
STS-1 frame STM-0 frame (rarely used)
STS-1-SPE VC-3
STS-1 payload C-3
STS-3c frame STM-1 frame, AU-4
STS-3c-SPE VC-4
STS-3c payload C-4
STS-12c/48c/192c frame STM-4/16/64 frame, AU-4-4c/16c/64c
STS-12c/48c/192c-SPE VC-4-4c/16c/64c
STS-12c/48c/192c payload C-4-4c/16c/64c

The only currently supported SONET/SDH SPE/VCs are the following:

SONET SDH
----------------------------------------
STS-3c-SPE VC-4
STS-12c-SPE VC-4-4c
STS-48c-SPE VC-4-16c
STS-192c-SPE VC-4-64c

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.

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 [3][5].

The octet stream is mapped into the SONET STS-SPE/SDH Higher Order
VC, with the octet boundaries aligned with the SONET STS-SPE/SDH
Higher Order VC octet boundaries.

Scrambling is performed during insertion into the SONET STS-
SPE/SDH Higher Order VC to provide adequate transparency and
protect against potential security threats (see Section 6). For
backwards compatibility with RFC1619 (STS-3c-SPE/VC-4 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 (SONET STS-
SPE/SDH Higher Order VC 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 SONET STS-
SPE/SDH Higher Order VC. The value of 22 (16 hex) is used to
indicate PPP with X^43 + 1 scrambling [4].

For compatibility with RFC1619 (STS-3c-SPE/VC-4 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.

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 SONET STS-SPE/SDH Higher
Order VC payload. Because frames are variable in length, the frames
are allowed to cross SONET STS-SPE/SDH Higher Order VC boundaries.

4. X**43 + 1 Scrambler Description

The X**43 + 1 scrambler transmitter and receiver operation are as
follows:

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...

The scrambler operates continuously through the bytes of the SONET
STS-SPE/SDH Higher Order VC, 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 SONET
STS-SPE/SDH Higher Order VC is the state at the end of the previous
SONET STS-SPE/SDH Higher Order VC. 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

Other than the FCS length (see below), the standard LCP sync
configuration defaults apply to SONET/SDH links.

The following Configuration Options are RECOMMENDED for STS-3c-
SPE/VC-4:

Magic Number
No Address and Control Field Compression
No Protocol Field Compression

For operation at STS-12c-SPE/VC-4-4c and above, Address and Control
Field Compression and Protocol Field Compression are NOT RECOMMENDED.
The Magic Number option remains RECOMMENDED.

Regarding the FCS length, with one exception, the 32-bit FCS MUST be
used for all SONET/SDH rates. For STS-3c-SPE/VC-4 only, the 16-bit
FCS MAY be used, although the 32-bit FCS is RECOMMENDED. The FCS
length is set by provisioning and is not negotiated.

6. Security Considerations

The major change from RFC1619 is the addition of payload scrambling
when inserting the HDLC-like framed PPP packets into the SONET STS-
SPE/SDH Higher Order VC. RFC1619 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
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.

A known security issue is bandwidth reduction by intentional
transmission of characters or sequences requiring transparency
processing by including flag and/or escape characters in user data. A
user may cause up to a 100% increase in the bandwidth required for
transmitting his or her packets by filling the packet with flag
and/or escape characters.

7. Changes from RFC1619

As mentioned in the previous section, the major change from RFC1619
was the addition of payload scrambling when inserting the HDLC-like
framed PPP packets into the SONET STS-SPE/SDH Higher Order VC. Other
changes were:

The terminology was updated to better match that used by ANSI and
ITU-T.

The specification's applicability is now specifically restricted to:

SONET SDH
----------------------------------------
STS-3c-SPE VC-4
STS-12c-SPE VC-4-4c
STS-48c-SPE VC-4-16c
STS-192c-SPE VC-4-64c

The Path Signal Label (C2) is set to 22 (16 hex) when using X^43 + 1
scrambling.

The 32-bit FCS is required except for operation with STS-3c-SPE/VC-4,
in which case the 16-bit FCS is allowed (but the 32-bit FCS is still
recommended).

The Security Considerations section was added.

8. Intellectual Property

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.

9. Acknowledgments

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. The authors would also like to thank the members
of the IETF's Point-to-Point Protocol Extensions Working Group for
their many suggestions and improvements to the text.

10. References

[1] Simpson, W., Editor, "The Point-to-Point Protocol (PPP)", STD
51, RFC1661, Daydreamer, July 1994.

[2] Simpson, W., Editor, "PPP in HDLC-like Framing", STD 51, 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, RFC2119, March 1997.

11. Authors' Addresses

Andrew G. Malis
Ascend Communications, Inc.
1 Robbins Road
Westford, MA 01810 USA

Phone: +1 978 952 7414
EMail: malis@ascend.com

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

EMail: wsimpson@GreenDragon.com

12. Full Copyright Statement

Copyright (C) The Internet Society (1999). All Rights Reserved.

This document and translations of it may be copied and furnished to
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Acknowledgement

Funding for the RFCEditor function is currently provided by the
Internet Society.

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