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Network Working Group P. Martin
INTERNET-DRAFT Pacific Softworks Inc
May 1997
PPP/IPCP Extension for Word Alignment of IP Datagrams
<draft-rfced-info-martin-00.txt>
Status of this Memo
This document is an Internet-Draft. 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
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To learn the current status of any Internet-Draft, please check the
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ftp.isi.edu (US West Coast).
Abstract
The Point-to-Point Protocol (PPP) [1] provides a standard method for
transporting multi-protocol datagrams over point-to-point links.
The PPP/IPCP extension for 32 bit Alignment of IP datagrams provides
a method to negotiate and align IP datagrams on a 32 bit boundary.
This document describes the use of the Internet Protocol Control
Protocol (IPCP) [2] option and the PPP framing that is required for
alignment of all IP datagrams.
Table of Contents
1. Introduction .......................................... 2
1.1. Specification of Requirements ......................... 2
2.1 Configuration Option Format ........................... 3
3.1 Frame Format .......................................... 4
SECURITY CONSIDERATIONS ...................................... 5
REFERENCES ................................................... 5
CHAIR'S ADDRESS ........................................... 5
AUTHORS' ADDRESS ............................................. 5
Martin Informational [Page 1]
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I/D PPP IP Padding May 1997
1. Introduction
Many processors today are 32 bit word bound. This is especially
true for processors like the TI TMS320C3X. Other processors are
16 bit word bound like the Motorola 68000. Today with the ever
increasing use of microcontrollers that are used to convey IP
datagrams to the home in consumer electronic devices, it is
becoming more and more important to leave an IP datagram aligned
on a 32 bit boundary for performance. The cost of building these
consumer electronic devices is so sensitive that the minimum
amount of RAM, ROM and processing power are used. For these devices
an addition to the IPCP protocol needed to be defined that will add
a simple padding character to the data stream to allow the IP
datagram to remain on a word bit boundary. The proposed solution is
designed to require the minimum overhead in code, ram, and
processing requirements to achieve the goal of leaving the IP
datagram in it's orignal word boundary state. The effects of
compression on the packet (like Protocol Field, Address/Control
field and Van-Jacobson compression), cause the microcontroller to
either spend a lot of time processing the packet in order to get it
aligned or force the microcontroller not to gain the benefits of
these compression techniques.
1.1. Specification of Requirements
A new IPCP option SHALL be added to negotiate the desire to have IP
datagrams aligned on a negotiated octet boundary. Upon successful
negotiation of the IP Padding Option, the Acknowledger of the
request SHALL pad prior to the IP datagram with zero to the
negotiated alignment value - 1 octets with the negotiated the pad
character for all PPP frames containing an IP datagram. These MUST
be identified by the protocol field containing the following values:
0021 IP Datagram
002D VJ Compressed TCP/IP Datagram
002E Uncompressed TCP/IP Datagram
The implementation MAY be extended to provide for other compressed
IP Datagrams not yet defined.
The requesting system SHOULD discard the pad character(s) when
passing the datagram to the upper layers. The requesting system MAY
also check the IP Datagram to verify that it is on the word boundary
that was requested.
Martin Informational [Page 2]
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I/D PPP IP Padding May 1997
2.1. IPCP Option Negotiation
A new negotiation option is added for 32 bit padding. If the
receiver of such an option acknowledges this option then the
receiver SHALL send all IP datagrams with the padding character
sufficient to force the IP datagram (after unstuffing and
decompression) to a 32 bit aligned value within the frame.
IP Padding Option
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Alignment | Pad Character |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type
64 (0x40)
Length
4
Alignment
Number of octets to align to:
1 No Alignment
2 16 bit Alignment
4 32 Bit Alignment
8 64 Bit Alignment
16 128 bit Alignment
Pad Character
The character to be used for padding
An example for 32 bit alignment using 0xFE for the PAD Character
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x40 | 0x04 | 0x04 | 0xFE |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Martin Informational [Page 3]
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I/D PPP IP Padding May 1997
3.1. Frame Format
If the IP Padding Option is successfully negotiated, then the
acknowledger of that option SHALL send all IP datagrams with
the following format. This will allow the system requesting
the IP Padding Option to assume that all IP datagrams before
passing to the network layer will be aligned to a negotiated
boundary.
The PPP frame is padded following the Protocol Field so that the
beginning of the IP datagram is placed on the requested word
boundary after all unstuffing and decompression take place.
The pad character to be used SHALL be the one negotiated in the
IPCP option 64.
Examples of Padding taking place with 32 bit alignment and the pad
character being 0xFE.
1. Protocol Field Compression
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| FF | 03 | 21 | FE (PAD) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IP Datagram
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
2. Protocol & Address/Control Field Compression
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 21 | FE (PAD) | FE (PAD) | FE (PAD) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IP Datagram
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
3. Van-Jacobson Compression
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| FF | 03 | 00 | 2D |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| FE (PAD) | VJ Compressed TCP/IP Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TCP Payload... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Martin Informational [Page 4]
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I/D PPP IP Padding May 1997
Security Considerations
Security issues are not discussed in this memo.
References
[1] Simpson, W., Editor, "The Point-to-Point Protocol (PPP)", STD
51, RFC 1661, Daydreamer, July 1994.
[2] McGregor, G., "The (PPP) Internet Protocol Control Protocol
(IPCP)", RFC 1332, Merit, May 1992.
[3] Postel, J., "Internet Protocol", RFC 791, USC/Information
Sciences Institute, September 1981.
[4] Jacobson, V., "Compressing TCP/IP Headers", RFC 1144, January
1990.
[5] Reynolds, J., and Postel, J., "Assigned Numbers", STD 2, RFC
1340, USC/Information Sciences Institute, July 1992.
Chair's Address
The working group can be contacted via the current chair:
Karl F. Fox
Ascend Communications
3518 Riverside Dr., Suite 101
Columbus, Ohio 43221
(614) 451-1883
EMail: karl@ascend.ComAuthor's Address
Questions about this memo can also be directed to:
Paul S. Martin
Pacific Softworks Inc.
4000 Via Pescador
Camarillo, CA 93012
(805)484-2128
Email: paulmart@microsoft.com
INTERNET-DRAFT EXPIRES JANUARY 1998 INTERNET-DRAFT