RFC2509 - IP Header Compression over PPP

Network Working Group

Request for Comments: 2509 M. Engan

Category: Standards Track Effnet

S. Casner

Cisco Systems

C. Bormann

Universitaet Bremen TZI

February 1999

IP Header Compression over PPP

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.

Abstract

This document describes an option for negotiating the use of header

compression on IP datagrams transmitted over the Point-to-Point

Protocol [RFC1661]. It defines extensions to the PPP Control

Protocols for IPv4 and IPv6 [RFC1332, RFC2023]. Header compression

may be applied to IPv4 and IPv6 datagrams in combination with TCP,

UDP and RTP transport protocols as specified in [IPHC] and [CRTP].

1. IntrodUCtion

The IP Header Compression (IPHC) defined in [IPHC] may be used for

compression of both IPv4 and IPv6 datagrams or packets encapsulated

with multiple IP headers. IPHC is also capable of compressing both

TCP and UDP transport protocol headers. The IP/UDP/RTP header

compression defined in [CRTP] fits within the framework defined by

IPHC so that it may also be applied to both IPv4 and IPv6 packets.

In order to establish compression of IP datagrams sent over a PPP

link each end of the link must agree on a set of configuration

parameters for the compression. The process of negotiating link

parameters for network layer protocols is handled in PPP by a family

of network control protocols (NCPs). Since there are separate NCPs

for IPv4 and IPv6, this document defines configuration options to be

used in both NCPs to negotiate parameters for the compression scheme.

IPHC relies on the link layer's ability to indicate the types of

datagrams carried in the link layer frames. In this document nine new

types for the PPP Data Link Layer Protocol Field are defined along

with their meaning.

In general, header compression schemes that use delta encoding of

compressed packets require that the lower layer does not reorder

packets between compressor and decompressor. IPHC uses delta encoding

of compressed packets for TCP and RTP. The IPHC specification [IPHC]

includes methods that allow link layers that may reorder packets to

be used with IPHC. Since PPP does not reorder packets these

mechanisms are disabled by default. When using reordering mechanisms

such as multiclass multilink PPP [MCML], care must be taken so that

packets that share the same compression context are not reordered.

2. Configuration Option

This document specifies a new compression protocol value for the IPCP

IP-Compression-Protocol option as specified in [RFC1332]. The new

value and the associated option format are described in section 2.1.

The option format is structured to allow future extensions to the

IPHC scheme.

NOTE: The specification of link and network layer parameter

negotiation for PPP [RFC1661], [RFC1331], [RFC1332] does not

prohibit multiple instances of one configuration option but states

that the specification of a configuration option must eXPlicitly

allow multiple instances. From the current specification of the

IPCP IP-Compression-Protocol configuration option [RFC1332, p 6]

it follows that it can only be used to select a single compression

protocol at any time.

NOTE: [RFC1332] is not explicit about whether the option

negotiates the capabilities of the receiver or of the sender. In

keeping with current practice, we assume that the option describes

the capabilities of the decompressor (receiving side) of the peer

that sends the Config-Req.

2.1. Configuration Option Format

Both the network control protocol for IPv4, IPCP [RFC1332] and the

IPv6 NCP, IPV6CP [RFC2023] may be used to negotiate IP Header

Compression parameters for their respective protocols. The format of

the configuration option is the same for both IPCP and IPV6CP.

Description

This NCP configuration option is used to negotiate parameters for

IP Header Compression. The option format is summarized 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

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

TCP_SPACE NON_TCP_SPACE

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

F_MAX_PERIOD F_MAX_TIME

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

MAX_HEADER suboptions...

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type

Length

>= 14

The length may be increased if the presence of additional

parameters is indicated by additional suboptions.

IP-Compression-Protocol

0061 (hex)

TCP_SPACE

The TCP_SPACE field is two octets and indicates the maximum value

of a context identifier in the space of context identifiers

allocated for TCP.

Suggested value: 15

TCP_SPACE must be at least 0 and at most 255 (The value 0 implies

having one context).

NON_TCP_SPACE

The NON_TCP_SPACE field is two octets and indicates the maximum

value of a context identifier in the space of context identifiers

allocated for non-TCP. These context identifiers are carried in

COMPRESSED_NON_TCP, COMPRESSED_UDP and COMPRESSED_RTP packet

headers.

Suggested value: 15

NON_TCP_SPACE must be at least 0 and at most 65535 (The value 0

implies having one context).

F_MAX_PERIOD

Maximum interval between full headers. No more than F_MAX_PERIOD

COMPRESSED_NON_TCP headers may be sent between FULL_HEADER

headers.

Suggested value: 256

A value of zero implies infinity, i.e. there is no limit to the

number of consecutive COMPRESSED_NON_TCP headers.

F_MAX_TIME

Maximum time interval between full headers. COMPRESSED_NON_TCP

headers may not be sent more than F_MAX_TIME seconds after sending

the last FULL_HEADER header.

Suggested value: 5 seconds

A value of zero implies infinity.

MAX_HEADER

The largest header size in octets that may be compressed.

Suggested value: 168 octets

The value of MAX_HEADER should be large enough so that at least

the outer network layer header can be compressed. To increase

compression efficiency MAX_HEADER should be set to a value large

enough to cover common combinations of network and transport layer

headers.

suboptions

The suboptions field consists of zero or more suboptions. Each

suboption consists of a type field, a length field and zero or

more parameter octets, as defined by the suboption type. The

value of the length field indicates the length of the suboption in

its entirety, including the lengths of the type and length fields.

0 1 2

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type Length Parameters...

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

2.2 RTP-Compression Suboption

The RTP-Compression suboption is included in the NCP

IP-Compression-Protocol option for IPHC if IP/UDP/RTP compression is

to be enabled.

After successful negotiation of parameters for IP Header Compression

the use of Protocol Identifiers FULL_HEADER, COMPRESSED_TCP,

COMPRESSED_TCP_NODELTA and COMPRESSED_NON_TCP is enabled, regardless

of the prescence of an RTP-Compression suboption.

Description

Enable use of Protocol Identifiers COMPRESSED_RTP, COMPRESSED_UDP and

CONTEXT_STATE as specified in [CRTP].

0 1

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type Length

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type

Length

3. Multiple Network Control Protocols

The IPHC protocol is able to compress both IPv6 and IPv4 datagrams.

Both IPCP and IPV6CP are able to negotiate option parameter values

for IPHC. These values apply to the compression of packets where the

outer header is an IPv4 header and an IPv6 header, respectively.

3.1. Sharing Context Identifier Space

For the compression and decompression of IPv4 and IPv6 datagram

headers the context identifier space is shared. While the parameter

values are independently negotiated, sharing the context identifier

spaces becomes more complex when the parameter values differ. Since

the compressed packets share context identifier space, the

compression engine must allocate context identifiers out of a common

pool; for compressed packets, the decompressor has to examine the

context state to determine what parameters to use for decompression.

Context identifier spaces are not shared between TCP and non-

TCP/UDP/RTP. Doing so would require additional mechanisms to ensure

that no error can occur when switching from using a context

identifier for TCP to non-TCP.

4. Demultiplexing of Datagrams

The IPHC specification [IPHC] defines four header formats for

different types of compressed headers. They are compressed TCP,

compressed TCP with no delta encoding, compressed non-TCP with 8 bit

CID and compressed non-TCP with 16 bit CID. The two non-TCP formats

may be distinguished by their contents so both may use the same

link-level identifier. A fifth header format, the full header is

distinct from a regular header in that it carries additional

information to establish shared context between the compressor and

decompressor.

The specification of IP/UDP/RTP Header Compression [CRTP] defines

four additional formats of compressed headers. They are for

compressed UDP and compressed RTP (on top of UDP), both with either

8- or 16-bit CIDs. In addition, there is an explicit error message

from the decompressor to the compressor.

The link layer must be able to indicate these header formats with

distinct values. Nine PPP Data Link Layer Protocol Field values are

specified below.

FULL_HEADER

The frame contains a full header as specified in [CRTP] Section

3.3.1. This is the same as the FULL_HEADER specified in [IPHC]

Section 5.3.

Value: 0061 (hex)

COMPRESSED_TCP