Network Working Group C. Bormann
Request for Comments: 3241 TZI/Uni Bremen
Updates: 1332 April 2002
Category: Standards Track
Robust Header Compression (ROHC) 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 (C) The Internet Society (2002). All Rights Reserved.
This document describes an option for negotiating the use of robust
header compression (ROHC) on IP datagrams transmitted over the
Point-to-Point Protocol (PPP). It defines extensions to the PPP
Control Protocols for IPv4 and IPv6.
Robust Header Compression (ROHC) as defined in [RFC3095] 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.
Bormann Standards Track [Page 1]RFC 3241 ROHC over PPP April 2002
ROHC does not require that the link layer be able to indicate the
types of datagrams carried in the link layer frames. However, there
are two basic types of ROHC headers defined in the ROHC framework:
small-CID headers (zero or one bytes are used to identify the
compression context) and large-CID headers (one or two bytes are used
for this purpose). To keep the PPP packets self-describing, in this
document two new types for the PPP Data Link Layer Protocol Field are
defined, one for small-CID ROHC packets and one for large-CID ROHC
packets. (This also avoids a problem that would occur if PPP were to
negotiate which of the formats to use in each of IPCP and IPV6CP and
the two negotiation processes were to arrive at different results.)
A PPP ROHC sender may send packets in either small-CID or large-CID
format at any time, i.e., the LARGE_CIDS parameter from [RFC3095] is
not used. Any PPP ROHC receiver MUST be able to process both small-
CID and large-CID ROHC packets, therefore no negotiation of this
function is required.
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. (Note that in certain cases, reordering may be acceptable
to ROHC, such as within a sequence of packets that all do not change
the decompression context.)
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
It may be worth repeating [RFC1332], section 4: "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." I.e.,
the option describes the capabilities of the decompressor (receiving
side) of the peer that sends the Configure-Request.