Network Working Group G. McGregor

Network Working Group G. McGregor
Request for Comments: 1332 Merit
Obsoletes: RFC 1172 May 1992

The PPP Internet Protocol Control Protocol (IPCP)

Status of this Memo

This RFC specifies an IAB standards track protocol for the Internet
community, and requests discussion and suggestions for improvements.
Please refer to the current edition of the "IAB Official Protocol
Standards" 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 of
encapsulating Network Layer protocol information over point-to-point
links. PPP also defines an extensible Link Control Protocol, and
proposes a family of Network Control Protocols (NCPs) for
establishing and configuring different network-layer protocols.

This document defines the NCP for establishing and configuring the
Internet Protocol [2] over PPP, and a method to negotiate and use Van
Jacobson TCP/IP header compression [3] with PPP.

This RFC is a product of the Point-to-Point Protocol Working Group of
the Internet Engineering Task Force (IETF).

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Table of Contents

1. Introduction .......................................... 1

2. A PPP Network Control Protocol (NCP) for IP ........... 2
2.1 Sending IP Datagrams ............................ 2

3. IPCP Configuration Options ............................ 4
3.1 IP-Addresses .................................... 5
3.2 IP-Compression-Protocol ......................... 6
3.3 IP-Address ...................................... 8

4. Van Jacobson TCP/IP header compression ................ 9
4.1 Configuration Option Format ..................... 9

APPENDICES ................................................... 11

A. IPCP Recommended Options .............................. 11

SECURITY CONSIDERATIONS ...................................... 11

REFERENCES ................................................... 11

ACKNOWLEDGEMENTS ............................................. 11

CHAIR'S ADDRESS .............................................. 12

AUTHOR'S ADDRESS ............................................. 12

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RFC 1332 PPP IPCP May 1992

1. Introduction

PPP has three main components:

1. A method for encapsulating datagrams over serial links.

2. A Link Control Protocol (LCP) for establishing, configuring,
and testing the data-link connection.

3. A family of Network Control Protocols (NCPs) for establishing
and configuring different network-layer protocols.

In order to establish communications over a point-to-point link, each
end of the PPP link must first send LCP packets to configure and test
the data link. After the link has been established and optional
facilities have been negotiated as needed by the LCP, PPP must send
NCP packets to choose and configure one or more network-layer
protocols. Once each of the chosen network-layer protocols has been
configured, datagrams from each network-layer protocol can be sent
over the link.

The link will remain configured for communications until explicit LCP
or NCP packets close the link down, or until some external event
occurs (an inactivity timer expires or network administrator
intervention).

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2. A PPP Network Control Protocol (NCP) for IP

The IP Control Protocol (IPCP) is responsible for configuring,
enabling, and disabling the IP protocol modules on both ends of the
point-to-point link. IPCP uses the same packet exchange machanism as
the Link Control Protocol (LCP). IPCP packets may not be exchanged
until PPP has reached the Network-Layer Protocol phase. IPCP packets
received before this phase is reached should be silently discarded.

The IP Control Protocol is exactly the same as the Link Control
Protocol [1] with the following exceptions:

Data Link Layer Protocol Field

Exactly one IPCP packet is encapsulated in the Information field
of PPP Data Link Layer frames where the Protocol field indicates
type hex 8021 (IP Control Protocol).

Code field

Only Codes 1 through 7 (Configure-Request, Configure-Ack,
Configure-Nak, Configure-Reject, Terminate-Request, Terminate-Ack
and Code-Reject) are used. Other Codes should be treated as
unrecognized and should result in Code-Rejects.

Timeouts

IPCP packets may not be exchanged until PPP has reached the
Network-Layer Protocol phase. An implementation should be
prepared to wait for Authentication and Link Quality Determination
to finish before timing out waiting for a Configure-Ack or other
response. It is suggested that an implementation give up only
after user intervention or a configurable amount of time.

Configuration Option Types

IPCP has a distinct set of Configuration Options, which are
defined below.

2.1. Sending IP Datagrams

Before any IP packets may be communicated, PPP must reach the
Network-Layer Protocol phase, and the IP Control Protocol must reach
the Opened state.

Exactly one IP packet is encapsulated in the Information field of PPP
Data Link Layer frames where the Protocol field indicates type hex
0021 (Internet Protocol).

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The maximum length of an IP packet transmitted over a PPP link is the
same as the maximum length of the Information field of a PPP data
link layer frame. Larger IP datagrams must be fragmented as
necessary. If a system wishes to avoid fragmentation and reassembly,
it should use the TCP Maximum Segment Size option [4], and MTU
discovery [5].

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3. IPCP Configuration Options

IPCP Configuration Options allow negotiatiation of desirable Internet
Protocol parameters. IPCP uses the same Configuration Option format
defined for LCP [1], with a separate set of Options.

The most up-to-date values of the IPCP Option Type field are specified
in the most recent "Assigned Numbers" RFC [6]. Current values are
assigned as follows:

1 IP-Addresses
2 IP-Compression-Protocol
3 IP-Address

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3.1. IP-Addresses

Description

The use of the Configuration Option IP-Addresses has been
deprecated. It has been determined through implementation
experience that it is difficult to ensure negotiation convergence
in all cases using this option. RFC 1172 [7] provides information
for implementations requiring backwards compatability. The IP-
Address Configuration Option replaces this option, and its use is
preferred.

This option SHOULD NOT be sent in a Configure-Request if a
Configure-Request has been received which includes either an IP-
Addresses or IP-Address option. This option MAY be sent if a
Configure-Reject is received for the IP-Address option, or a
Configure-Nak is received with an IP-Addresses option as an
appended option.

Support for this option MAY be removed after the IPCP protocol
status advances to Internet Draft Standard.

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3.2. IP-Compression-Protocol

Description

This Configuration Option provides a way to negotiate the use of a
specific compression protocol. By default, compression is not
enabled.

A summary of the IP-Compression-Protocol Configuration Option format
is shown below. The fields are transmitted from left to right.

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 | IP-Compression-Protocol |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Data ...
+-+-+-+-+

Type

2

Length

>= 4

IP-Compression-Protocol

The IP-Compression-Protocol field is two octets and indicates the
compression protocol desired. Values for this field are always
the same as the PPP Data Link Layer Protocol field values for that
same compression protocol.

The most up-to-date values of the IP-Compression-Protocol field
are specified in the most recent "Assigned Numbers" RFC [6].
Current values are assigned as follows:

Value (in hex) Protocol

002d Van Jacobson Compressed TCP/IP

Data

The Data field is zero or more octets and contains additional data
as determined by the particular compression protocol.

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Default

No compression protocol enabled.

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3.3. IP-Address

Description

This Configuration Option provides a way to negotiate the IP
address to be used on the local end of the link. It allows the
sender of the Configure-Request to state which IP-address is
desired, or to request that the peer provide the information. The
peer can provide this information by NAKing the option, and
returning a valid IP-address.

If negotiation about the remote IP-address is required, and the
peer did not provide the option in its Configure-Request, the
option SHOULD be appended to a Configure-Nak. The value of the
IP-address given must be acceptable as the remote IP-address, or
indicate a request that the peer provide the information.

By default, no IP address is assigned.

A summary of the IP-Address Configuration Option format is shown
below. The fields are transmitted from left to right.

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 | IP-Address
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
IP-Address (cont) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type

3

Length

6

IP-Address

The four octet IP-Address is the desired local address of the
sender of a Configure-Request. If all four octets are set to
zero, it indicates a request that the peer provide the IP-Address
information.

Default

No IP address is assigned.

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4. Van Jacobson TCP/IP header compression

Van Jacobson TCP/IP header compression reduces the size of the TCP/IP
headers to as few as three bytes. This can be a significant improvement
on slow serial lines, particularly for interactive traffic.

The IP-Compression-Protocol Configuration Option is used to indicate the
ability to receive compressed packets. Each end of the link must
separately request this option if bi-directional compression is desired.

The PPP Protocol field is set to the following values when transmitting
IP packets:

Value (in hex)

0021 Type IP. The IP protocol is not TCP, or the packet is a
fragment, or cannot be compressed.

002d Compressed TCP. The TCP/IP headers are replaced by the
compressed header.

002f Uncompressed TCP. The IP protocol field is replaced by
the slot identifier.

4.1. Configuration Option Format

A summary of the IP-Compression-Protocol Configuration Option format
to negotiate Van Jacobson TCP/IP header compression is shown below.
The fields are transmitted from left to right.

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 | IP-Compression-Protocol |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Max-Slot-Id | Comp-Slot-Id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type

2

Length

6

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IP-Compression-Protocol

002d (hex) for Van Jacobson Compressed TCP/IP headers.

Max-Slot-Id

The Max-Slot-Id field is one octet and indicates the maximum slot
identifier. This is one less than the actual number of slots; the
slot identifier has values from zero to Max-Slot-Id.

Note: There may be implementations that have problems with only
one slot (Max-Slot-Id = 0). See the discussion in reference
[3]. The example implementation in [3] will only work with 3
through 254 slots.

Comp-Slot-Id

The Comp-Slot-Id field is one octet and indicates whether the slot
identifier field may be compressed.

0 The slot identifier must not be compressed. All compressed
TCP packets must set the C bit in every change mask, and
must include the slot identifier.

1 The slot identifer may be compressed.

The slot identifier must not be compressed if there is no ability
for the PPP link level to indicate an error in reception to the
decompression module. Synchronization after errors depends on
receiving a packet with the slot identifier. See the discussion
in reference [3].

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A. IPCP Recommended Options

The following Configurations Options are recommended:

IP-Compression-Protocol -- with at least 4 slots, usually 16
slots.

IP-Address -- only on dial-up lines.

Security Considerations

Security issues are not discussed in this memo.

References

[1] Simpson, W., "The Point-to-Point Protocol", RFC 1331, May 1992.

[2] Postel, J., "Internet Protocol", RFC 791, USC/Information
Sciences Institute, September 1981.

[3] Jacobson, V., "Compressing TCP/IP Headers", RFC 1144, January
1990.

[4] Postel, J., "The TCP Maximum Segment Size Option and Related
Topics", RFC 879, USC/Information Sciences Institute, November
1983.

[5] Mogul, J., and S. Deering, "Path MTU Discovery", RFC 1191,
November 1990.

[6] Reynolds, J., and J. Postel, "Assigned Numbers", RFC 1060,
USC/Information Sciences Institute, March 1990.

[7] Perkins, D., and R. Hobby, "Point-to-Point Protocol (PPP)
initial configuration options", RFC 1172, August 1990.

Acknowledgments

Some of the text in this document is taken from RFCs 1171 & 1172, by
Drew Perkins of Carnegie Mellon University, and by Russ Hobby of the
University of California at Davis.

Information leading to the expanded IP-Compression option provided by
Van Jacobson at SIGCOMM '90.

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Bill Simpson helped with the document formatting.

Chair's Address

The working group can be contacted via the current chair:

Brian Lloyd
Lloyd & Associates
3420 Sudbury Road
Cameron Park, California 95682

Phone: (916) 676-1147

EMail: [email protected]

Author's Address

Questions about this memo can also be directed to:

Glenn McGregor
Merit Network, Inc.
1071 Beal Avenue
Ann Arbor, MI 48109-2103

Phone: (313) 763-1203

EMail: [email protected]

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