IPHC over PPP: IP Header Compression over PPP
How a PPP link negotiates IP header compression, and the nine packet types the link has to tell apart.
Koren, T., Casner, S., and C. Bormann, "IP Header Compression over PPP", RFC 3544, DOI 10.17487/RFC3544, July 2003, <https://www.rfc-editor.org/info/rfc3544>.
@misc{rfc3544,
series = {Request for Comments},
number = 3544,
howpublished = {RFC 3544},
publisher = {RFC Editor},
doi = {10.17487/RFC3544},
url = {https://www.rfc-editor.org/info/rfc3544},
author = {T. Koren and S. Casner and C. Bormann},
title = {{IP Header Compression over PPP}},
pagetotal = 14,
year = 2003,
month = jul,
} RFC text
Contents
Network Working Group T. Koren
Request for Comments: 3544 Cisco Systems
Obsoletes: 2509 S. Casner
Category: Standards Track Packet Design
C. Bormann
Universitaet Bremen TZI
July 2003
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.
Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
This document describes an option for negotiating the use of header
compression on IP datagrams transmitted over the Point-to-Point
Protocol (RFC 1661). It defines extensions to the PPP Control
Protocols for IPv4 and IPv6 (RFC 1332, RFC 2472). Header compression
may be applied to IPv4 and IPv6 datagrams in combination with TCP,
UDP and RTP transport protocols as specified in RFC 2507, RFC 2508
and RFC 3545.
1. Introduction
The IP Header Compression (IPHC) defined in [RFC2507] 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 [RFC2508] and [RFC3545] 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.
This document obsoletes RFC 2509, adding two new suboptions to the IP
header compression configuration option. One suboption negotiates
usage of Enhanced RTP-Compression (specified in [RFC3545]), and the
other suboption negotiates header compression for only TCP or only
non-TCP packets.
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 [RFC2507] 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 [RFC2686],
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. [RFC3241] updates RFC 1332 by
explicitly allowing the sending of multiple instances of the IP-
Compression-Protocol configuration option, each with a different
value for IP-Compression-Protocol. Each type of compression
protocol may independently establish its own parameters.
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 [RFC2472] 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. Successful negotiation of parameters
enables the use of Protocol Identifiers FULL_HEADER,
COMPRESSED_TCP, COMPRESSED_TCP_NODELTA, COMPRESSED_NON_TCP and
CONTEXT_STATE as specified in [RFC2507]. 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
2
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.
Inclusion of the RTP-Compression suboption enables use of additional
Protocol Identifiers COMPRESSED_RTP and COMPRESSED_UDP along with
additional forms of CONTEXT_STATE as specified in [RFC2508].
Description
Enable use of Protocol Identifiers COMPRESSED_RTP, COMPRESSED_UDP
and CONTEXT_STATE as specified in [RFC2508].
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type
1
Length
2
2.3. Enhanced RTP-Compression Suboption
To use the enhanced RTP header compression defined in [RFC3545], a
new sub-option 2 is added. Sub-option 2 is negotiated instead of,
not in addition to, sub-option 1.
Description
Enable use of Protocol Identifiers COMPRESSED_RTP and
CONTEXT_STATE as specified in [RFC2508]. In addition, enable use
of [RFC3545] compliant compression including the use of Protocol
Identifier COMPRESSED_UDP with additional flags and use of the C
flag with the FULL_HEADER Protocol Identifier to indicate use of
HDRCKSUM with COMPRESSED_RTP and COMPRESSED_UDP packets.
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type
2
Length
2
2.4. Negotiating header compression for only TCP or only non-TCP
packets
In RFC 2509 it was not possible to negotiate only TCP header
compression or only non-TCP header compression because a value of 0
in the TCP_SPACE or the NON_TCP_SPACE fields actually means that 1
context is negotiated.
A new suboption 3 is added to allow specifying that the number of
contexts for TCP_SPACE or NON_TCP_SPACE is zero, disabling use of the
corresponding compression.
Description
Enable header compression for only TCP or only non-TCP packets.
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 | Parameter |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type
3
Length
3
Parameter
The parameter is 1 byte with one of the following values:
1 = the number of contexts for TCP_SPACE is 0
2 = the number of contexts for NON_TCP_SPACE is 0
This suboption overrides the values that were previously assigned to
TCP_SPACE and NON_TCP_SPACE in the IP Header Compression option.
If suboption 3 is included multiple times with parameter 1 and 2,
compression is disabled for all packets.
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 [RFC2507] 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 [RFC2508] 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 [RFC2508] Section
3.3.1. This is the same as the FULL_HEADER specified in [RFC2507]
Section 5.3.
Value: 0061 (hex)
COMPRESSED_TCP
The frame contains a datagram with a compressed header with the
format as specified in [RFC2507] Section 6a.
Value: 0063 (hex)
COMPRESSED_TCP_NODELTA
The frame contains a datagram with a compressed header with the
format as specified in [RFC2507] Section 6b.
Value: 2063 (hex)
COMPRESSED_NON_TCP
The frame contains a datagram with a compressed header with the
format as specified in either Section 6c or Section 6d of
[RFC2507].
Value: 0065 (hex)
COMPRESSED_RTP_8
The frame contains a datagram with a compressed header with the
format as specified in [RFC2508] Section 3.3.2, using 8-bit CIDs.
Value: 0069 (hex)
COMPRESSED_RTP_16
The frame contains a datagram with a compressed header with the
format as specified in [RFC2508] Section 3.3.2, using 16-bit CIDs.
Value: 2069 (hex)
COMPRESSED_UDP_8
The frame contains a datagram with a compressed header with the
format as specified in [RFC2508] Section 3.3.3 or as specified in
[RFC3545] Section 2.1, using 8-bit CIDs.
Value: 0067 (hex)
COMPRESSED_UDP_16
The frame contains a datagram with a compressed header with the
format as specified in [RFC2508] Section 3.3.3 or as specified in
[RFC3545] Section 2.1, using 16-bit CIDs.
Value: 2067 (hex)
CONTEXT_STATE
The frame is a link-level message sent from the decompressor to
the compressor as specified in [RFC2508] Section 3.3.5.
Value: 2065 (hex)
5. Changes from RFC 2509
Two new suboptions are specified. See Sections 2.3 and 2.4.
6. References
6.1. Normative References
[RFC1144] Jacobson, V., "Compressing TCP/IP Headers for low-speed
serial links", RFC 1144, February 1990.
[RFC1332] McGregor, G., "The PPP Internet Protocol Control Protocol
(IPCP)", RFC 1332, May 1992.
[RFC2472] Haskin, D. and E. Allen, "IP Version 6 over PPP", RFC
2472, December 1998.
[RFC2507] Degermark, M., Nordgren, B. and S. Pink, "Header
Compression for IP", RFC 2507, February 1999.
[RFC2508] Casner, S. and V. Jacobson, "Compressing IP/UDP/RTP
Headers for Low-Speed Serial Links", RFC 2508, February
1999.
[RFC3241] Bormann, C., "Robust Header Compression (ROHC) over PPP",
RFC 3241, April 2002.
[RFC3545] Koren, T., Casner, S., Geevarghese, J., Thompson, B. and
P. Ruddy, "Enhanced Compressed RTP (CRTP) for Links with
High Delay, Packet Loss and Reordering", RFC 3545, July
2003.
6.2. Informative References
[RFC1661] Simpson, W., Ed., "The Point-To-Point Protocol (PPP)", STD
51, RFC 1661, July 1994.
[RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 2434,
October 1998.
[RFC2686] Bormann, C., "The Multi-Class Extension to Multi-Link
PPP", RFC 2686, September 1999.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R. and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", RFC 3550, July 2003.
7. IANA Considerations
This document does not require any additional allocations from
existing namespaces in the IANA Point-to-Point Protocol Field
Assignments registry. However, there are three namespaces that were
defined by RFC 1332, RFC 2472, and RFC 2509 but not created in the
registry. Those three namespaces, described below, have been added
to the PPP registry. This document specifies two additional
allocations in the third one.
Section 3.2 of RFC 1332 specifies an IP-Compression-Protocol
Configuration Option for the PPP IP Control Protocol and defines one
value for the IP-Compression-Protocol type field in that option. An
IANA registry has been created to allocate additional values for that
type field. As stated in RFC 1332, the values for the IP-
Compression-Protocol type field are always the same as the (primary)
PPP DLL Protocol Number assigned to packets of the particular
compression protocol. Assignment of additional IP-Compression-
Protocol type values is through the IETF consensus procedure as
specified in [RFC2434].
Section 4.2 of RFC 2472 specifies an IPv6-Compression-Protocol
Configuration Option for the PPP IPv6 Control Protocol and defines
one value for the IPv6-Compression-Protocol type field in that
option. An IANA registry has been created to allocate additional
values for that type field. The IPv6-Compression-Protocol
Configuration Option has the same structure as the IP-Compression-
Protocol Configuration Option defined in RFC 1332, but the set of
values defined for the type field may be different. As stated in RFC
2472, the values for the IPv6-Compression-Protocol type field are
always the same as the (primary) PPP DLL Protocol Number assigned to
packets of the particular compression protocol. Assignment of
additional IPv6-Compression-Protocol type values is through the IETF
consensus procedure as specified in [RFC2434].
Section 2.1 of RFC 2509 specifies an additional type value to be
registered for both the IP-Compression-Protocol Configuration Option
and the IPv6-Compression-Protocol Configuration Option to indicate
use of the "IP Header Compression" protocol. The specification of
that type value is repeated in Section 2.1 of this document which
obsoletes RFC 2509. In conjunction with the additional type value,
the format for the variable-length option is specified. The format
includes a suboption field that may contain one or more suboptions.
Each suboption begins with a suboption type value. An IANA registry
has been created for the suboption type values; and is titled, "IP
Header Compression Configuration Option Suboption Types".
Section 2.2 of RFC 2509 (and this document) defines one suboption
type. Sections 2.3 and 2.4 of this document define two additional
suboption types. It is expected that the number of additional
suboptions that will need to be defined is small. Therefore, anyone
wishing to define new suboptions is required to produce a revision of
this document to be vetted through the normal Internet Standards
process, as specified in [RFC2434].
RFC 2509 also defines nine PPP Data Link Layer Protocol Field values
which are already listed in the IANA registry of Point-to-Point
Protocol Field Assignments. Section 4 of this document repeats the
specification of those values without change.
8. Security Considerations
Negotiation of the option defined here imposes no additional security
considerations beyond those that otherwise apply to PPP [RFC1661].
The use of header compression can, in rare cases, cause the
misdelivery of packets. If necessary, confidentiality of packet
contents should be assured by encryption.
Encryption applied at the IP layer (e.g., using IPSEC mechanisms)
precludes header compression of the encrypted headers, though
compression of the outer IP header and authentication/security
headers is still possible as described in [RFC2507]. For RTP
packets, full header compression is possible if the RTP payload is
encrypted by itself without encrypting the UDP or RTP headers, as
described in [RFC3550]. This method is appropriate when the UDP and
RTP header information need not be kept confidential.
9. Intellectual Property Rights Notice
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.
10. Acknowledgements
Mathias Engan was the primary author of RFC 2509, of which this
document is a revision.
11. Authors' Addresses
Tmima Koren
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 95134-1706
United States
EMail: tmima@cisco.com
Stephen L. Casner
Packet Design
3400 Hillview Avenue, Building 3
Palo Alto, CA 94304
United States
EMail: casner@packetdesign.com
Carsten Bormann
Universitaet Bremen FB3 TZI
Postfach 330440
D-28334 Bremen, GERMANY
Phone: +49.421.218-7024
Fax: +49.421.218-7000
EMail: cabo@tzi.org
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ISO Plain Language
On this page
§1Introduction
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. Negotiating link parameters for network layer protocols is handled in PPP by a family of network control protocols. Since there are separate control protocols for IPv4 and IPv6, this document defines configuration options to be used in both of them to negotiate parameters for the compression scheme.
The document obsoletes RFC 2509 and adds two new suboptions to the IP header compression configuration option. One suboption negotiates the use of enhanced RTP compression, and the other negotiates header compression for only TCP or only non-TCP packets.
IP header compression relies on the link layer's ability to indicate the types of datagrams carried in its frames, and this document defines nine new types for the PPP Data Link Layer Protocol Field 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 the compressor and the decompressor. IP header compression uses delta encoding of compressed packets for TCP and RTP, and the compression specification includes methods that allow link layers that may reorder packets to be used with it. Since PPP does not reorder packets, those mechanisms are disabled by default. When using reordering mechanisms such as multiclass multilink PPP, care must be taken so that packets that share the same compression context are not reordered.
Compression here is not a new idea: it is a scheme defined elsewhere, and this document is about the PPP-specific parts. Those are the option that negotiates it and the packet types the link carries, because the link has to say which kind of compressed header each frame holds.
- TCP, UDP or RTPThe transport headers that the compression targets
- IPv4 or IPv6 headerThe network header that is compressed
- IPHCThe compression scheme, with the parameters negotiated over PPP
- PPP linkThe point-to-point link that carries the frames
§2The Configuration Option
The document specifies a new compression protocol value for the IP-Compression-Protocol option of the IP control protocol, and describes the associated option format. The option format is structured to allow future extensions to the compression scheme.
Both the IPv4 control protocol and the IPv6 control protocol may be used to negotiate IP header compression parameters for their respective protocols, and the format of the configuration option is the same for both. Successful negotiation enables the use of the protocol identifiers FULL_HEADER, COMPRESSED_TCP, COMPRESSED_TCP_NODELTA, COMPRESSED_NON_TCP and CONTEXT_STATE.
The option has a type field of 2, a length of 14 octets or more, which may be increased when additional parameters are indicated by additional suboptions, and the IP-Compression-Protocol value 0061 in hexadecimal. It then carries four parameter fields and a list of suboptions.
TCP_SPACE is two octets and gives the maximum value of a context identifier in the space allocated for TCP. The suggested value is 15, and it must be at least 0 and at most 255, where a value of 0 implies having one context. NON_TCP_SPACE is two octets and gives the maximum value of a context identifier in the space allocated for non-TCP; these identifiers are carried in the COMPRESSED_NON_TCP, COMPRESSED_UDP and COMPRESSED_RTP packet headers. The suggested value is 15, and it must be at least 0 and at most 65535, where 0 again implies one context.
F_MAX_PERIOD is the maximum interval between full headers: no more than F_MAX_PERIOD COMPRESSED_NON_TCP headers may be sent between FULL_HEADER headers. The suggested value is 256, and a value of zero means infinity, with no limit on the number of consecutive compressed headers. F_MAX_TIME is the maximum time interval between full headers, so that compressed headers may not be sent more than F_MAX_TIME seconds after the last full header. The suggested value is 5 seconds, and zero again means infinity. MAX_HEADER is the largest header size in octets that may be compressed, with a suggested value of 168 octets. It should be large enough that at least the outer network layer header can be compressed, and to increase compression efficiency it should cover common combinations of network and transport layer headers.
The suboptions field holds zero or more suboptions, each of a type field, a length field and zero or more parameter octets, where the length includes the type and length fields themselves. The document notes that the PPP specification does not prohibit several instances of one configuration option unless the option's own specification says so, and that RFC 3241 updates the IPv4 control protocol to allow the sending of multiple instances of the IP-Compression-Protocol option with different values, so that each type of compression protocol can establish its own parameters. It also assumes, in line with current practice, that the option describes the capabilities of the decompressor of the peer that sends the configuration request.
The option has a fixed part of 14 bytes and then a list of suboptions. The fixed part says how many short identifiers are available for TCP and for everything else, how many compressed packets may be sent before a full header has to be sent again, how long that may take, and how large a header may be before it is too big to compress.
| Field | Name | Size | Value | Description |
|---|---|---|---|---|
TYPE | Type | 1 byte | 2 | 2, the IP-Compression-Protocol option. |
LENGTH | Length | 1 byte | 14 | 14 or more, when suboptions are present. |
IP-COMPRESSION-PROTOCOL | IP-Compression-Protocol | 2 bytes | 0061 | 0061 in hexadecimal, the IP header compression protocol. |
TCP-SPACE | TCP_SPACE | 2 bytes | 15 | The highest context identifier in the space allocated for TCP. Suggested value 15. |
NON-TCP-SPACE | NON_TCP_SPACE | 2 bytes | 15 | The highest context identifier in the space allocated for non-TCP. Suggested value 15. |
F-MAX-PERIOD | F_MAX_PERIOD | 2 bytes | 256 | The most compressed non-TCP headers between full headers. Suggested value 256; zero means no limit. |
F-MAX-TIME | F_MAX_TIME | 2 bytes | 5 | The longest gap between full headers, in seconds. Suggested value 5; zero means no limit. |
MAX-HEADER | MAX_HEADER | 2 bytes | 168 | The largest header that may be compressed, in octets. Suggested value 168. |
- TCP_SPACEHow many short identifiers are available for TCP. Suggested value 15, from 0 to 255.
- NON_TCP_SPACEHow many are available for everything else. Suggested value 15, from 0 to 65535.
- F_MAX_PERIODHow many compressed headers may be sent before a full header has to be sent again. Suggested value 256.
- F_MAX_TIMEHow long that may take, in seconds. Suggested value 5.
- MAX_HEADERThe largest header that may be compressed. Suggested value 168 octets.
- SuboptionsType 1 for RTP compression, type 2 for enhanced RTP compression, type 3 to turn off TCP or non-TCP compression.
§2.2The Suboptions
The RTP-Compression suboption is included in the compression option when IP, UDP and RTP compression is to be enabled. Including it enables the use of the protocol identifiers COMPRESSED_RTP and COMPRESSED_UDP, along with additional forms of CONTEXT_STATE. Its type is 1 and its length is 2.
The Enhanced RTP-Compression suboption has type 2 and length 2, and is used to enable the enhanced RTP header compression defined in RFC 3545. It enables the protocol identifiers COMPRESSED_RTP and CONTEXT_STATE, and in addition enables compression compliant with RFC 3545, including the use of the COMPRESSED_UDP protocol identifier with additional flags, and the use of the C flag with the FULL_HEADER protocol identifier to indicate the use of the header checksum with COMPRESSED_RTP and COMPRESSED_UDP packets. Sub-option 2 is negotiated instead of, not in addition to, sub-option 1.
The third suboption, with type 3 and length 3, allows header compression to be negotiated for only TCP or only non-TCP packets. It exists because in RFC 2509 it was not possible to negotiate only TCP header compression or only non-TCP header compression, since a value of 0 in the TCP_SPACE or NON_TCP_SPACE field actually means that one context is negotiated. The new suboption has a one-byte parameter: 1 sets the number of contexts for TCP_SPACE to 0, and 2 sets the number of contexts for NON_TCP_SPACE to 0. It overrides the values previously assigned to those fields in the compression option. If suboption 3 is included several times with parameter 1 and 2, compression is disabled for all packets.
Three suboptions tune the scheme. One turns on compression for RTP and UDP traffic. Another turns on the newer, enhanced version of RTP compression, and replaces the first rather than adding to it. The third is for turning compression off for one kind of traffic, which the old fields could not express because a zero there meant one context rather than none.
| Suboption | Type | What it does |
|---|---|---|
| RTP-Compression | 1 | Enables the COMPRESSED_RTP and COMPRESSED_UDP protocol identifiers and further forms of CONTEXT_STATE. |
| Enhanced RTP-Compression | 2 | Enables the enhanced compression of RFC 3545. Negotiated instead of suboption 1, not in addition to it. |
| TCP or non-TCP only | 3 | Parameter 1 sets the number of TCP contexts to zero, parameter 2 sets the number of non-TCP contexts to zero. |
§3Multiple Network Control Protocols
The compression scheme is able to compress both IPv6 and IPv4 datagrams, and both the IPv4 and the IPv6 control protocols are able to negotiate option parameter values for it. Those values apply to the compression of packets whose outer header is an IPv4 header and an IPv6 header respectively.
For the compression and decompression of IPv4 and IPv6 datagram headers the context identifier space is shared. While the parameter values are negotiated independently, sharing the identifier spaces becomes more complex when the values differ. Since the compressed packets share the identifier space, the compression engine must allocate identifiers out of a common pool, and 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 the non-TCP, UDP and RTP traffic. Doing so would need extra mechanisms to make sure that no error can occur when switching from using an identifier for TCP to non-TCP.
IPv4 and IPv6 share one pool of short identifiers, because a compressed packet does not say which version of IP it carries. TCP and the other protocols get separate pools, so that the decompressor can tell from the identifier alone which kind of header it stands for.
§4Demultiplexing of Datagrams
The IP header compression specification defines four header formats for different types of compressed header: compressed TCP, compressed TCP with no delta encoding, compressed non-TCP with an 8-bit context identifier, and compressed non-TCP with a 16-bit context identifier. The two non-TCP formats may be distinguished by their contents, so both may use the same link-level identifier. A fifth format, the full header, is distinct in that it carries the additional information that establishes the shared context between the compressor and the decompressor.
The IP/UDP/RTP header compression specification defines four further formats: compressed UDP and compressed RTP on top of UDP, both with either 8-bit or 16-bit context identifiers, and an explicit error message from the decompressor to the compressor. The link layer must be able to indicate these formats with distinct values, and nine PPP Data Link Layer Protocol Field values are specified.
FULL_HEADER, value 0061 in hexadecimal, carries a full header as specified in the IP/UDP/RTP header compression document, which is the same as the full header in the IP header compression specification. COMPRESSED_TCP, value 0063, carries a datagram with a compressed header in the format of Section 6a of that specification. COMPRESSED_TCP_NODELTA, value 2063, carries a datagram with a compressed header in the format of Section 6b. COMPRESSED_NON_TCP, value 0065, carries a datagram with a compressed header in either the format of Section 6c or that of Section 6d.
COMPRESSED_RTP_8, value 0069, and COMPRESSED_RTP_16, value 2069, carry compressed RTP headers with 8-bit and 16-bit context identifiers. COMPRESSED_UDP_8, value 0067, and COMPRESSED_UDP_16, value 2067, carry compressed UDP headers with 8-bit and 16-bit context identifiers. CONTEXT_STATE, value 2065, is a link-level message sent from the decompressor to the compressor.
The link has to distinguish nine kinds of frame: a full header, three kinds of compressed TCP header, one kind of compressed header for everything else, and the compressed UDP and RTP headers in their 8-bit and 16-bit forms, plus a message the decompressor sends back to the compressor. Each has its own value in the PPP protocol field.
- 0061FULL_HEADER — A full header, carrying the information that establishes the shared context.
- 0063COMPRESSED_TCP — A compressed TCP header, with delta encoding.
- 2063COMPRESSED_TCP_NODELTA — A compressed TCP header without delta encoding.
- 0065COMPRESSED_NON_TCP — A compressed non-TCP header, with an 8 or 16 bit context identifier.
- 0069COMPRESSED_RTP_8 — A compressed RTP header with 8-bit context identifiers.
- 2069COMPRESSED_RTP_16 — A compressed RTP header with 16-bit context identifiers.
- 0067COMPRESSED_UDP_8 — A compressed UDP header with 8-bit context identifiers.
- 2067COMPRESSED_UDP_16 — A compressed UDP header with 16-bit context identifiers.
- 2065CONTEXT_STATE — A link-level message from the decompressor to the compressor.
§5Changes from RFC 2509
Two new suboptions are specified, in Sections 2.3 and 2.4: the enhanced RTP compression suboption, which enables the compression of RFC 3545, and the suboption that negotiates header compression for only TCP or only non-TCP packets.
Two things were added to what RFC 2509 had: a switch to the newer RTP compression, and the ability to compress only one kind of traffic.
§8Security Considerations
Negotiation of the option defined here imposes no additional security considerations beyond those that otherwise apply to PPP.
The use of header compression can, in rare cases, cause the misdelivery of packets. If necessary, confidentiality of packet contents should be assured by encryption.
Encryption applied at the IP layer, for example using IPsec mechanisms, precludes header compression of the encrypted headers, though compression of the outer IP header and the authentication and security headers is still possible. For RTP packets, full header compression is possible if the RTP payload is encrypted by itself without encrypting the UDP or RTP headers. That method is appropriate when the UDP and RTP header information does not need to be kept confidential.
Negotiating the option adds no security of its own. The one risk to be aware of is that compression can, rarely, deliver a packet to the wrong place, so anything that must stay secret should be encrypted. Encrypting at the IP layer means the encrypted headers cannot be compressed, though the outer header still can.