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
偶尔怀念Status of this Memo
This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for
improvements. Plea 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 Rerved.
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 u of PPP over Synchronous Optical
Network (SONET) and Synchronous Digital Hierarchy (SDH) circuits.
This document replaces and obsoletes RFC 1619. See ction 7 for a
summary of the changes from RFC 1619.
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 RFC 1619 (7)
8. Intellectual Property (7)
9. Acknowledgments (8)
10. References (8)
11. Authors’ Address (9)
12. Full Copyright Statement (10)
Malis & Simpson Standards Track [Page 1]
1. Introduction
PPP was designed as a standard method of communicating over
point-to-point links. Initial deployment has been over short local
lines, lead lines, and plain-old-telephone-rvice (POTS) using
modems. As new packet rvices and higher speed lines are introduced, PPP is easily deployed in the environments as well.
This specification is primarily concerned with the u of the PPP
encapsulation over SONET/SDH links. Since SONET/SDH is by definition a point-to-point circuit, PPP is well suited to u over the links. Real differences between SONET and SDH (other than terminology) are
minor; for the purpos of encapsulation of PPP over SONET/SDH, they
are inconquential or irrelevant.
For the convenience of the reader, we list the equivalent terms below: SONET SDH
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SPE VC
STS-SPE Higher Order VC (VC-3/4/4-Nc)
STS-1 frame STM-0 frame (rarely ud)
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].
Malis & Simpson Standards Track [Page 2]
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 prents 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 inrtion into the SONET STS-
SPE/SDH Higher Order VC to provide adequate transparency and
protect against potential curity threats (e Section 6). For
一切刚刚好backwards compatibility with RFC 1619 (STS-3c-SPE/VC-4 only), the scrambler MAY have an on/off capability where the scrambler is
bypasd entirely when it is in the off mode. If this capability is provided, the default MUST be t 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 lf-synchronous scrambler of
polynomial X**43 + 1. See Section 4 for the description of the
scrambler.
The proper order of operation is:
When transmitting:
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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 ud to
indicate PPP with X^43 + 1 scrambling [4].
For compatibility with RFC 1619 (STS-3c-SPE/VC-4 only), if scrambling has been configured to be off, then the value 207 (CF hex) is ud
for the Path Signal Label to indicate PPP without scrambling.
Malis & Simpson Standards Track [Page 3]
The Multiframe Indicator (H4) is unud, and MUST be zero.
Control Signals
PPP does not require the u of control signals. When available, using such signals can allow greater functionality and
performance. Implications are discusd 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. Becau 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|
| +-------------------------------------+ +---+
| |
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+-----------------------------------------------+
|
v
Scrambled Data
Malis & Simpson Standards Track [Page 4]
皮的笔顺怎么写Receiver schematic:
Scrambled Data
|
+-----------------------------------------------+
| |
| v
| +-------------------------------------+ +---+
+->| --> 43 bit shift register --> |--->|xor|
+-------------------------------------+ +---+
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|
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], ), 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],
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 (e 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 ret per frame. The initial ed is randomly chon by transmitter to improve operational curity (e Section
6). Conquently, the first 43 transmitted bits following startup or reframe operation will not be descrambled correctly.
Malis & Simpson Standards Track [Page 5]
5. Configuration Details
Other than the FCS length (e below), the standard LCP sync
configuration defaults apply to SONET/SDH links.
The following Configuration Options are RECOMMENDED for STS-3c-
SPE/VC-4:
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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 ud for all SONET/SDH rates. For STS-3c-SPE/VC-4 only, the 16-bit
FCS MAY be ud, although the 32-bit FCS is RECOMMENDED. The FCS
length is t by provisioning and is not negotiated.
6. Security Considerations
The major change from RFC 1619 is the addition of payload scrambling when inrting the HDLC-like framed PPP packets into the SONET STS-
SPE/SDH Higher Order VC. RFC 1619 was operationally found to permit malicious urs to generate packets with bit patterns that could
create SONET/SDH-layer low-transition-density synchronization
problems, emulation of the SDH t-ret scrambler pattern, and
replication of the STM-N frame alignment word.
The u of the x^43 + 1 lf-synchronous scrambler was introduced to alleviate the potential curity 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 cau
SONET/SDH-layer problems. This ems reasonably cure for this
application.
This scrambler is also ud when transmitting ATM over SONET/SDH, and public network carriers have considerable experience with its u. Malis & Simpson Standards Track [Page 6]
