INTERNET-DRAFT Carsten Bormann
Expires: May 2001 TZI/Uni Bremen
November 2000
ROHC over PPP
draft-ietf-rohc-over-ppp-00.txt
Status of this memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC 2026.
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 working documents as Internet-
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Internet-Drafts are draft documents valid for a maximum of six months
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The list of current Internet-Drafts can be accessed at
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The list of Internet-Draft Shadow Directories can be accessed at
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This document is a submission to the IETF ROHC WG. Comments should
be directed to its mailing list, rohc@cdt.luth.se.
Abstract
This document describes an option for negotiating the use of robust
header compression (ROHC) 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].
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1. Introduction
Robust Header Compression (ROHC) as defined in [ROHC] may be used for
compression of both IPv4 and IPv6 datagrams or packets encapsulated
with multiple IP headers. The initial version of ROHC focuses on
compression of the packet headers in RTP streams, while supporting
compression of other UDP flows; however, it also defines a framework
into which further header compression mechanisms can be plugged as
new profiles. Planned additions to the set of profiles supported by
ROHC will be capable of compressing TCP transport protocol headers as
well.
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.
ROHC does not rely on any link layer's ability to indicate the types
of datagrams carried in the link layer frames. Therefore, in this
document a single new type for the PPP Data Link Layer Protocol Field
is defined.
ROHC assumes that the link layer delivers packets in sequence. PPP
normally does not reorder packets. 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.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119.
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
ROHC 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
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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 Robust Header Compression. The option format is summarized
below. The fields are transmitted from left to right.
Robust Header Compression (ROHC) 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 | IP-Compression-Protocol |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MAX_CID | MRRU |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 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
00XX (hex) [to be assigned]
MAX_CID
The MAX_CID field is two octets and indicates the maximum value
of a context identifier.
Suggested value: 15
MAX_CID must be at least 0 and at most 16383 (The value 0 implies
having one context).
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MRRU
The MRRU field is two octets and indicates the maximum
reconstructed reception unit (see [ROHC], section 5.1.1).
Suggested value: 0
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.
Figure
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. LARGE_CIDS Suboption
Without specifying further options, the LARGE_CIDS per-channel
parameter (see [ROHC], section 5.1.1) is false.
Description
Set LARGE_CIDS to true.
LARGE_CIDS suboption
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type
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1
Length
2
2.3. PROFILES Suboption
The set of profiles to be enabled is subject to negotiation. Most
initial implementations of ROHC implement profiles 0 to 3. This
option MUST be supplied.
Description
Define the set of profiles supported by the decompressor.
PROFILES suboption
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 | Profiles...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type
2
Length
n+2
Value
n octets in ascending order, each specifying a profile
supported.
3. Multiple Network Control Protocols
The ROHC protocol is able to compress both IPv6 and IPv4 datagrams.
Both IPCP and IPV6CP are able to negotiate option parameter values
for ROHC. 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.
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4. Demultiplexing of Datagrams
The ROHC specification [ROHC] defines a single header format for all
different types of compressed headers. One PPP Data Link Layer
Protocol Field value is specified below.
ROHC
The frame contains a ROHC packet as defined in [ROHC].
Value: 00XX (hex) [to be assigned -- same XX as above]
5. Security Considerations
Negotiation of the option defined here imposes no additional security
considerations beyond those that otherwise apply to PPP [RFC1661].
The security considerations of ROHC [ROHC] apply.
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 [ROHC]. 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
[RFC1889]. This method is appropriate when the UDP and RTP header
information need not be kept confidential.
6. Acknowledgments
The present document borrows heavily from [RFC2509].
7. References
[ROHC] Carsten Bormann (ed.) et al., "RObust Header Compression
(ROHC)", work in progress, November 2000 (draft-ietf-
rohc-rtp-06.txt).
[RFC2023] Haskin, E. and E. Allan, "IP Version 6 over PPP", RFC
2023, October 1996.
[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.
[RFC1889] Schulzrinne, H., Casner, S., Frederick, R. and V.
Jacobson, "RTP: A Transport Protocol for real-time
applications", RFC 1889, January 1996.
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[RFC1661] Simpson, W., Ed., "The Point-To-Point Protocol (PPP)",
STD 51, RFC 1661, July 1994.
[RFC2686] Bormann, C., "The Multi-Class Extension to Multi-Link
PPP", RFC 2686, September 1999.
[RFC2509] M. Engan, S. Casner, C. Bormann, "IP Header Compression
over PPP", RFC 2509, February 1999.
8. Authors' addresses
Carsten Bormann
Universitaet Bremen FB3 TZI
Postfach 330440
D-28334 Bremen, GERMANY
cabo@tzi.org
phone +49.421.218-7024
fax +49.421.218-7000
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