Request for Comments: 4362 G. Pelletier
Obsoletes: 3242 K. Sandlund
Category: Standards Track Ericsson
January 2006
RObust Header Compression (ROHC):
A Link-Layer Assisted Profile for IP/UDP/RTP
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 (2006).
Abstract
This document defines a ROHC (Robust Header Compression) profile for
compression of IP/UDP/RTP (Internet Protocol/User Datagram
Protocol/Real-Time Transport Protocol) packets, utilizing
functionality provided by the lower layers to increase compression
efficiency by completely eliminating the header for most packets
during optimal operation. The profile is built as an extension to
the ROHC RTP profile. It defines additional mechanisms needed in
ROHC, states requirements on the assisting layer to guarantee
transparency, and specifies general logic for compression and
decompression related to the usage of the header-free packet format.
This document is a replacement for RFC 3242, which it obsoletes.
Table of Contents
1. Introduction ....................................................2
1.1. Differences from RFC 3242 ..................................5
2. Terminology .....................................................5
3. Overview of the Link-Layer Assisted Profile .....................6
3.1. Providing Packet Type Identification .......................7
3.2. Replacing the Sequence Number ..............................7
3.3. CRC Replacement ............................................8
3.4. Applicability of This Profile ..............................8
4. Additions and Exceptions Compared to ROHC RTP ...................9
4.1. Additional Packet Types ....................................9
4.1.1. No-Header Packet (NHP) ..............................9
4.1.2. Context Synchronization Packet (CSP) ................9
4.1.3. Context Check Packet (CCP) .........................11
4.2. Interfaces Towards the Assisting Layer ....................12
4.2.1. Interface, Compressor to Assisting Layer ...........13
4.2.2. Interface, Assisting Layer to Decompressor .........13
4.3. Optimistic Approach Agreement .............................14
4.4. Fast Context Initialization, IR Redefinition ..............15
4.5. Feedback Option, CV-REQUEST ...............................16
4.6. Periodic Context Verification .............................16
4.7. Use of Context Identifier .................................16
5. Implementation Issues ..........................................17
5.1. Implementation Parameters and Signals .....................17
5.1.1. Implementation Parameters at the Compressor ........17
5.1.2. Implementation Parameters at the Decompressor ......19
5.2. Implementation over Various Link Technologies .............19
6. IANA Considerations ............................................20
7. Security Considerations ........................................20
8. Acknowledgements ...............................................20
9. References .....................................................20
9.1. Normative References ......................................20
9.2. Informative References ....................................21
1. Introduction
Header compression is a technique used to compress and transparently
decompress the header information of a packet on a per-hop basis,
utilizing redundancy within individual packets and between
consecutive packets within a packet stream. Over the years, several
protocols [VJHC, IPHC] have been developed to compress the network
and transport protocol headers [IPv4, IPv6, UDP, TCP], and these
schemes have been successful in improving efficiency over many wired
bottleneck links, such as modem connections over telephone networks.
In addition to IP, UDP, and TCP compression, an additional
compression scheme called Compressed RTP [CRTP] has been developed to
improve compression efficiency further for real-time traffic using
the Real-Time Transport Protocol [RTP].
The schemes mentioned above have all been designed by taking into
account normal assumptions about link characteristics, which
traditionally have been based on wired links only. However, with an
increasing number of wireless links in the Internet paths, these
assumptions are no longer generally valid. In wireless environments,
especially wide-coverage cellular environments, relatively high error
rates are tolerated in order to allow efficient usage of the radio
resources. For real-time traffic, which is more sensitive to delays
than to errors, such operating conditions will be norm over, for
example, 3rd generation cellular links, and header compression must
therefore tolerate packet loss. However, with the previously
mentioned schemes, especially for real-time traffic compressed by
CRTP, high error rates have been shown to significantly degrade
header compression performance [CRTPC]. This problem was the driving
force behind the creation of the RObust Header Compression (ROHC) WG
in the IETF.
The ROHC WG has developed a header compression framework on top of
which profiles can be defined for different protocol sets, or for
different compression strategies. Due to the limited packet-loss
robustness of CRTP and the demands of the cellular industry for an
efficient way of transporting voice over IP over wireless, the main
focus of ROHC has so far been on compression of IP/UDP/RTP headers,
which are generous in size, especially when compared to the payloads
often carried by packets with such headers.
ROHC RTP has become a very efficient, robust, and capable compression
scheme, able to compress the headers down to a total size of one
octet only. Also, transparency is guaranteed to an extremely great
extent, even when residual bit errors are present in compressed
headers delivered to the decompressor. The requirements for RTP
compression [RTP-REQ], defined by the WG before and during the
development process, have thus been fulfilled.
As mentioned above, the 3rd generation cellular systems, where IP
will be used end-to-end, have been one of the driving forces behind
ROHC RTP, and the scheme has also been designed to suit new cellular
air interfaces, such as WCDMA, making it possible to run even speech
services with spectrum efficiency insignificantly lower than for
existing one-service circuit switched solutions [VTC2000]. However,
other air interfaces (such as those based on GSM and IS-95) will also
be used in all-IP networks, with further implications for the header
compression issue. These older air interfaces are less flexible,
with radio bearers optimized for specific payload sizes. This means
that not even a single octet of header can be added without using the
next higher fixed packet size supported by the link, something that
is obviously very costly. For the already deployed speech vocoders,
the spectrum efficiency over these links will thus be low compared to
existing circuit-switched solutions. To achieve high spectrum
efficiency overall with any application, more flexible air interfaces
must be deployed, and then the ROHC RTP scheme will perform
excellently, as shown for WCDMA [MOMUC01]. However, for deployment
reasons, it is important to also provide a suitable header
compression strategy for already existing vocoders and air
interfaces, such as for GERAN and for CDMA2000, with minimal effects
on spectral efficiency.
This document describes a link-layer-assisted ROHC RTP profile,
originally defined by [LLA], extending ROHC RTP (profile 0x0001)
[ROHC], and compliant with the ROHC 0-byte requirements [0B-REQ].
The purpose of this profile is to provide a header-free packet format
that, for a certain application behavior, can replace a majority of
the 1-octet header ROHC RTP packets during normal U/O-mode operation,
while still being fully transparent and complying with all the
requirements of ROHC RTP [RTP-REQ]. For other applications,
compression will be carried out as with normal ROHC RTP.
To completely eliminate the compressed header, all functionality
normally provided by the 1-octet header has to be provided by other
means, typically by utilizing functionality provided by the lower
layers and sacrificing efficiency for less-frequently occurring
larger compressed headers. The latter is not a contradiction, since
the argument for eliminating the last octet for most packets is not
overall efficiency in general. It is important to remember that the
purpose of this profile is to provide efficient matching of existing
applications to existing link technologies, not efficiency in
general. The additional complexity introduced by this profile,
although minimized by a tight integration with already-existing ROHC
functionality, implies that it should therefore only be used to
optimize performance of specific applications over specific links.
When implementing this profile over various link technologies, care
must be taken to guarantee that all the functionality needed is
provided by ROHC and the lower layers together. Therefore,
additional documents should specify how to incorporate this profile
on top of various link technologies.
The profile defined by this document was originally specified by RFC
3242 [LLA], but to address one technical flaw and clarify one
implementation issue, this document has been issued to replace RFC
3242, which becomes obsolete.
1.1. Differences from RFC 3242
This section briefly summarizes the differences of this document from
RFC 3242. Acronyms and terminology can be found in Section 2.
The format of the CSP packet, as defined in [LLA], was identified as
non-interoperable when carrying a RHP header with a 3-bit or 7-bit
CRC. This problem occurs because the payload has been dropped by the
compressor, and the decompressor is supposed to use the payload
length to infer certain fields in the uncompressed header. These
fields are the IPv4 total length, the IPv6 payload length, the UDP
length, and the IPv4 header checksum field (all INFERRED fields in
[ROHC]). To correct this flaw, the CSP packet must carry information
about the payload length of the RHP packet. Therefore, the length of
the RTP payload has been included in the CSP packet.
This document also clarifies an unclear referencing in RFC 3242,
where Section 4.1.3 of [LLA] states that upon CRC failure, the
actions of [ROHC], Section 5.3.2.2.3 MUST be taken. That section
specifies that detection of SN wraparound and local repair must be
performed, but neither of these steps apply when the failing packet
is a CCP. Therefore, upon CRC failure, actions to be taken are the
ones specified in Section 5.3.2.2.3, but steps a-d only.
2. Terminology
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 [RFC2119].
CCP Context Check Packet
CRC Cyclic Redundancy Check
CSP Context Synchronization Packet
LLA Link Layer Assisted ROHC RTP profile
NHP No Header Packet
ROHC RObust Header Compression
RHP ROHC Header Packet (a non-NHP packet; i.e., RRP, CSP, or CCP)
RRP ROHC RTP Packet as defined in [ROHC, profile 0x0001]
Assisting layer
"Assisting layer" refers to any entity implementing the interface
to ROHC (Section 4.2). It may, for example, refer to a sub-layer
used to adapt the ROHC implementation and the physical link layer.
This layer is assumed to have knowledge of the physical layer
synchronization.
Compressing side
"Compressing side" refers to the combination of the header
compressor, operating with the LLA profile, and its associated
assisting layer.
Lower layers
"Lower layers", in this document, refers to entities located below
ROHC in the protocol stack, including the assisting layer.
ROHC RTP
"ROHC RTP" refers to the IP/UDP/RTP profile as defined in [ROHC].
3. Overview of the Link-Layer Assisted Profile
The ROHC IP/UDP/RTP profile defined in [LLA] and updated by this
document, profile 0x0005 (hex), is designed to be used over channels
that have been optimized for specific payload sizes and that
therefore cannot efficiently accommodate header information when
transmitted together with payloads corresponding to these optimal
sizes.
The LLA profile extends, and thus also inherits all functionality
from, the ROCH RTP profile by defining some additional functionality
and an interface from the ROHC component towards an assisting lower
layer.
+---------------------------------------+
| |
The LLA | ROHC RTP, |
profile | Profile #1 +-----------------+
| | LLA Additions |
+---------------------+-----------------+
By imposing additional requirements on the lower layers compared to
[ROHC], it is possible to infer the information needed to maintain
robust and transparent header compression, even though the headers
are completely eliminated during most of the operation time.
Basically, this profile replaces the smallest and most frequent ROHC
U/O-mode headers with a no-header format, for which the header
functionality must be provided by other means.
Smallest header in Smallest header in
ROHC RTP (profile #1) LLA (profile #5)
+--+--+--+--+--+--+--+--+ ++
| 1 octet | -----> || No Header
+--+--+--+--+--+--+--+--+ ++
|
| Header field functionality
+-------------------> provided by other means
The fields present in the ROHC RTP headers for U/O-mode PT0 are the
packet type identifier, the sequence number, and the CRC. The
subsequent sections elaborate more on how the functionality of these
fields is replaced for NHP.
3.1. Providing Packet Type Identification
All ROHC headers carry a packet type identifier, indicating to the
decompressor how the header should be interpreted. This is a
function that must be provided by some means in 0-byte header
compression. It will be possible to distinguish ROHC RTP packets
with compressed headers thanks to the packet type identifier, but a
mechanism is needed to separate packets with a header from packets
without a header. This function MUST therefore be provided by the
assisting layer in one way or another.
3.2. Replacing the Sequence Number
From the sending application, the RTP sequence number is increased by
one for each packet sent. The purpose of the sequence number is to
cope with packet reordering and packet loss. If reordering or loss
has occurred before the transmission point, the compressing side, if
needed, can easily avoid problems by not allowing the use of a
header-free packet.
However, at the transmission point, loss or reordering that may occur
over the link can not be anticipated and covered for. Therefore, for
NHP, the assisting layer MUST guarantee in-order delivery over the
link (already assumed by [ROHC]), and at the receiving side, it MUST
provide an indication for each packet loss over the link. This is
basically the same principle as that which the VJ header compression
[VJHC] relies on.
Note that guaranteeing in-order delivery and packet loss indication
over the link not only makes it possible to infer the sequence number
information, but also supersedes the main function of the CRC, which
normally takes care of errors due to link losses and bit errors in
the compressed sequence number.
3.3. CRC Replacement
All context-updating RRP packets carry a CRC calculated over the
uncompressed header. The CRC is used by the decompressor to verify
that the updated context is correct. This verification serves three
purposes in U/O-mode:
1) Detection of longer losses than can be covered by the sequence
number LSBs.
2) Protection against failures caused by residual bit errors in
compressed headers.
3) Protection against faulty implementations and other causes of
error.
Since this profile defines an NHP packet without this CRC, care must
be taken to fulfill these purposes by other means when an NHP is used
as a replacement for a context-updating packet. Detection of long
losses (1) is already covered, since the assisting layer MUST provide
an indication of all packet losses. Furthermore, the NHP packet has
one important advantage over RHP packets in that residual bit errors
(2) cannot damage a header that is not even sent.
It is thus reasonable to assume that compression and decompression
transparency can be assured with high confidence, even without a CRC
in header-free packets. However, to provide additional protection
against damage propagation due to undetected residual bit errors in
context-updating packets (2) or other unexpected errors (3), periodic
context verifications SHOULD be performed (see Section 4.6).
3.4. Applicability of This Profile
The LLA profile can be used with any link technology capable of
providing the required functionality described in previous sections.
Thus, whether LLA or ROHC RTP should be implemented depends on the
characteristics of the link itself. For most RTP packet streams, LLA
will work exactly as ROHC RTP, and it will have a higher compression
efficiency for packet streams with certain characteristics. LLA will
never have a lower compression efficiency than ROHC RTP.
Note as well that LLA, like all other ROHC profiles, is fully
transparent to any packet stream reaching the compressor. LLA does
not make any assumptions about the packet stream but will perform
optimally for packet streams with certain characteristics, e.g.,
synchronized streams exactly timed with the assisting link over which
the LLA profile is implemented.
The LLA profile is obviously not applicable if the UDP checksum (2
bytes) is enabled, which is always the case for IPv6/UDP. For
IPv4/UDP, the sender may choose to disable the UDP checksum.
4. Additions and Exceptions Compared to ROHC RTP
4.1. Additional Packet Types
The LLA profile defines three new packet types to be used in addition
to the RRP packet types defined by [ROHC]. The following sections
describe these packet types and their purpose in detail.
4.1.1. No-Header Packet (NHP)
A No-Header Packet (NHP) is a packet that consists only of the
payload of the original packet. The NHP MAY be used when only the
sequence information needs to be conveyed to the decompressor. In
other words, the NHP can be used when all header fields are either
unchanged or follow the currently established change pattern. In
addition, there are some considerations for the use of the NHP (see
sections 4.3, 4.5, and 4.6). An LLA compressor is not allowed to
deliver NHP packets when operating in R-mode.
The assisting layer MAY send the NHP for RTP SN = X only if an NHP
was delivered by the LLA compressor AND the assisting layer can
guarantee that the decompressor will infer the proper sequencing for
this NHP. This guarantee is based on the confidence that the
decompressor
a) has the means to infer proper sequencing for the packet
corresponding to SN = X-1, AND
b) has either received a loss indication or the packet itself for
the packet corresponding to SN = X-1.
Updating properties: NHP packets update context (RTP Sequence
Number).
4.1.2. Context Synchronization Packet (CSP)
The case where the packet stream overruns the channel bandwidth may
lead to discarded data, which may result in decompressor context
invalidation. It might therefore be beneficial to send a packet with
only the header information and to discard the payload. This would
be helpful to maintain synchronization of the decompressor context
while efficiently using the available bandwidth.
This case can be handled with the Context Synchronization Packet
(CSP), which has the following format:
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 1 1 1 1 1 0 1 0 | Packet type identifier
+===+===+===+===+===+===+===+===+
/ RTP Payload Length / 2 octets
+---+---+---+---+---+---+---+---+
: ROHC header without padding :
: see [ROHC, Section 5.7] :
+---+---+---+---+---+---+---+---+
RTP Payload Length: This field is the length of the payload carried
inside the RTP header, stored in network byte
order. That is, this field will be set by the
compressor to (UDP length - size of the UDP
header - size of the RTP header including CSRC
identifiers).
Updating properties: CSP maintains the updating properties of the
ROHC header it carries.
The CSP is defined by one of the unused packet type identifiers from
ROHC RTP, carried in the one-octet base header. As for any ROHC
packet, except the NHP, the packet may begin with ROHC padding and/or
feedback. It may also carry context identification after the packet
type identifier. It is possible to have two CID fields present, one
after the packet type ID and one within the encapsulated ROHC header.
If a decompressor receives a CSP with two non-equal CID values
included, the packet MUST be discarded. ROHC segmentation may also
be applied to the CSP.
In the CSP packet, the payload has been dropped by the compressor.
However, the decompressor is supposed to use the payload length to
infer certain fields in the uncompressed header (the IPv4 total
length, the IPv6 payload length, the UDP length, and the IPv4 header
checksum field). When dropping the payload, the CSP packet needs to
contain information about the payload length carried in the RHP
packet. Therefore, the length of the RTP payload is carried in the
CSP packet. When the decompressor receives a CSP packet, it can use
the RTP payload length field to calculate the value of fields
classified as INFERRED in [ROHC] when attempting to verify a 3- or
7-bit CRC carried in the RHP header enclosed in the CSP.
Note that when the decompressor has received and processed a CSP, the
packet (including any possible data following the CSP encapsulated
compressed header) MUST be discarded.
4.1.3. Context Check Packet (CCP)
A Context Check Packet (CCP), which does not carry any payload but
only an optional CRC value in addition to the packet type identifier,
is defined.
The purpose of the CCP is to provide a useful packet that MAY be sent