Network Working Group Ghyslain Pelletier
INTERNET-DRAFT Ericsson AB
Expires: April 2004
October 11, 2003
RObust Header Compression (ROHC):
Profiles for UDP-Lite
<draft-ietf-rohc-udp-lite-01.txt>
Status of this memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
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This document is an individual submission to the IETF. Comments
should be directed to the authors.
Abstract
This document defines ROHC (Robust Header Compression) profiles for
compression of RTP/UDP-Lite/IP packets (Real-Time Transport Protocol,
User Datagram Protocol Lite, Internet Protocol) and UDP-Lite/IP.
These profiles are defined based on their differences with the
profiles specified in [RFC-3095] for UDP [RFC-768].
Although both transport protocols are very similar, ROHC profiles
must be defined separately for robust compression of UDP-Lite headers
because it does not share protocol identifier with UDP. Also, the
UDP-Lite Checksum Coverage field does not share the semantics of the
corresponding UDP Length field and as a consequence cannot always be
inferred anymore.
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Table of contents
1. Introduction....................................................3
2. Terminology.....................................................3
3. Background......................................................4
3.1. Overview of the UDP-Lite protocol.........................4
3.2. Expected behaviours of UDP-Lite flows.....................5
3.3. Header field classification...............................6
4. Rationale behind the design of ROHC profiles for UDP-Lite.......6
4.1. Design motivations........................................6
4.2. ROHC considerations.......................................6
5. ROHC profiles for UDP-Lite......................................7
5.1. Context parameters........................................7
5.2. Initialization............................................8
5.2.1. Initialization of the UDP-Lite header [UDP-Lite]........8
5.2.2. Compressor and decompressor logic.......................9
5.3. Packet formats............................................9
5.3.1. General packet format...................................9
5.3.2. Packet type CE: CE(), CE(ON) and CE(OFF)...............10
5.4. Compression logic........................................11
5.5. Decompression logic......................................12
6. Security considerations........................................12
7. IANA considerations............................................12
8. Acknowledgements...............................................12
9. References.....................................................12
10. Authors address...............................................13
Appendix A. Detailed classification of header fields..............14
A.1. UDP-Lite header fields...................................14
A.2. Header compression strategies for UDP-Lite...............16
Appendix B. Detailed format of the CE packet type.................17
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1. Introduction
The ROHC WG has developed a header compression framework on top of
which various profiles can be defined for different protocol sets, or
for different compression strategies. Due to the demands of the
cellular industry for an efficient way of transporting voice over IP
over wireless, ROHC [RFC-3095] has mainly focused on compression of
IP/UDP/RTP headers, which are generous in size, especially compared
to the payloads often carried by packets with these 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 high
extent even when residual bit errors are present in compressed
headers delivered to the decompressor.
UDP-Lite [UDP-Lite] is a transport protocol similar to the UDP
protocol [RFC-768]. UDP-Lite is useful for applications that are
designed with the capability to tolerate errors in the payload and
for which receiving damaged data is better than dealing with the loss
of entire packets. This may be particularly suitable when packets are
transported over link technologies where data can be partially
damaged, such as wireless links.
Separate ROHC profiles are needed for UDP-Lite because it does not
share protocol identifier with UDP. Also, the UDP-Lite Checksum
Coverage field does not share the semantics of the corresponding UDP
Length field and cannot always be inferred.
This document defines two ROHC profiles for efficient compression of
UDP-Lite headers. The objectives of these profiles are to provide
simple modifications to the corresponding ROHC profiles for UDP, as
specified in [RFC-3095]. In addition, the ROHC profiles for UDP-Lite
support compression of multiple IP headers using the mechanisms
defined in [IP-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.
ROHC RTP : RTP/UDP/IP profile 0x0001 defined in [RFC-3095].
ROHC UDP : UDP/IP profile 0x0002 defined in [RFC-3095].
ROHC UDP-Lite : UDP-Lite/IP profile defined in this document.
ROHC RTP/UDP-Lite : RTP/UDP-Lite/IP profile defined in this document.
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3. Background
3.1. Overview of the UDP-Lite protocol
UDP-Lite is a transport protocol defined as an independent variant of
the UDP transport protocol. UDP-Lite is very similar to UDP, and
allow applications that can tolerate errors in the payload to use a
checksum with an optional partial coverage. This is particularly
useful with IPv6 [RFC-2460], where the use of the transport-layer
checksum is mandatory.
UDP-Lite replaces the Length field of the UDP header with a Checksum
Coverage field. This field indicates the number of octets covered by
the 16-bit checksum, and it is applied on a per-packet basis. The
coverage area must always include the UDP-Lite header and may cover
the entire packet, in which case UDP-Lite becomes semantically
identical to UDP. UDP-Lite and UDP do not share protocol identifier.
The UDP-Lite header format:
0 15 16 31
+--------+--------+--------+--------+
| Source | Destination |
| Port | Port |
+--------+--------+--------+--------+
| Checksum | |
| Coverage | Checksum |
+--------+--------+--------+--------+
| |
: Payload :
| |
+-----------------------------------+
The UDP-Lite checksum, like the UDP checksum, is an end-to-end
mechanism against erroneous delivery of error sensitive data.
However, as opposed to UDP, the UDP-Lite checksum may not be
transmitted as all zeroes and cannot be disabled for IPv4 [RFC-791].
For UDP, in the case where the checksum is disabled (IPv4 only), the
Checksum field maintains a constant value and is normally not sent by
the header compression scheme. In the case where the UDP checksum is
enabled (mandatory for IPv6), such an unpredictable field cannot be
compressed and is sent uncompressed. The UDP Length field, however,
is always redundant and can be provided by the IP module. Header
compression schemes do not normally transmit any bits of information
for this field, as its value can be inferred from the link layer.
For UDP-Lite, the checksum also has unpredictable values and this
field must always be included as-is in the compressed header, for
both IPv4 and IPv6. Furthermore, as the UDP Length field is redefined
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as the Checksum Coverage field by UDP-Lite, this leads to different
properties for this field from a header compression perspective.
The following summarizes the relationship between UDP and UDP-Lite:
- UDP-Lite and UDP have different protocol identifiers;
- The UDP-Lite checksum cannot be disabled for IPv4;
- UDP-Lite redefines the UDP Length field as the Checksum
Coverage field, with different semantics;
- UDP-Lite is semantically equivalent to UDP when the Checksum
Coverage field indicates the total length of the packet.
The next section provides a more detailed discussion of the behavior
of the Checksum Coverage field of UPD-Lite in relation to header
compression.
3.2. Expected behaviours of UDP-Lite flows
Per-packet behavior
As mentioned in the previous section, the checksum coverage value
is applied independently of other packets that may belong to the
same flow. Specifically, the value of the checksum coverage may
indicate that the UDP-Lite packet is either entirely covered by the
checksum, or covered up to some boundary less than the packet size
but including the UDP-Lite header.
Inter-packet behavior
In relation to each other, UDP-Lite packets may exhibit either one
of three possible change patterns, where within a sequence of
packets the value of the Checksum Coverage field is:
1. changing, while covering the entire packet;
2. unchanging, covering up to a fixed boundary within the packet;
3. changing, but does not follow any specific pattern.
The first pattern above corresponds to the semantics of UDP, when
the UDP checksum is enabled. For this case, the checksum coverage
field varies according to the packet length and may be inferred
from the IP module similarly as for the UDP Length field.
The second pattern corresponds to the case where the coverage is
the same from one packet to another within a particular sequence.
For this case, the Checksum Coverage field may be a static value
defined in the context and it does not need to be sent in the
compressed header.
For the third case, no useful change pattern can be identified
from packet to packet for the value of the checksum coverage field,
and it must be included in the compressed header.
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Per-flow behavior
Finally, it can be expected that any one of these change patterns
for sequences of packets may be predominant at any time during the
lifetime of the UDP-Lite flow. A flow that predominantly follows
the first two change patterns described above may provide
opportunities for compressing the Checksum Coverage field for most
of the packets.
3.3. Header field classification
In relation to the header field classification from [RFC-3095], the
first two patterns represent the case where the value of the
Checksum Coverage field behavior is fixed and may be either
INFERRED (pattern 1) or STATIC (pattern 2); pattern 3 is for the
case where the value varies unpredictably, the field is CHANGING
and the value must be sent along with every packet.
Additional information regarding the analysis of the behavior of
the UDP-Lite fields may be found in the Appendix A.
4. Rationale behind the design of ROHC profiles for UDP-Lite
4.1. Design motivations
Simplicity is a strong motivation for the design of the UDP-Lite
header compression profiles. The profiles defined for UDP-Lite should
entail only a few simple modifications to the corresponding profiles
defined for UDP in [RFC-3095]. In addition, whenever UDP-Lite is used
in a manner that is semantically identical to UDP, the compression
efficiency should be similar.
4.2. ROHC considerations
The simplest approach to the definition of ROHC profiles for UDP-Lite
is to treat the Checksum Coverage field as an irregular value, and to
send it uncompressed for every packet. This may be achieved simply by
adding the field to the definition of the general packet format [RFC-
3095]. However, the compression efficiency would then always be less
than for UDP.
Some care should be given to achieve similar compression efficiency
for UDP-Lite as for UDP when the Checksum Coverage field behaves like
the UDP Length field. This requires the possibility to infer the
Checksum Coverage field when it is equal to the length of the packet.
This would otherwise put the UDP-Lite protocol at a disadvantage over
links where header compression is used, when its behavior is made
similar to the semantics of UDP.
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A mechanism to detect the presence of the Checksum Coverage field in
compressed headers is thus needed. This is achieved by defining a new
packet type, using the unused identifiers from [RFC-3095].
5. ROHC profiles for UDP-Lite
This section describes two ROHC profiles:
- RTP/UDP-Lite/IP compression (profile 0x0007)
- UDP-Lite/IP compression (profile 0x0008)
These profiles build on the specifications found in [RFC-3095] with
as little modifications as possible. Unless explicitly stated
otherwise, the profiles defined herein follow the specifications of
ROHC UDP and ROHC RTP, respectively.
Note that this document also reuses the notation found in [RFC-3095].
5.1. Context parameters
As described in [RFC-3095], information relevant to previous packets
is maintained in a context. This includes information describing the
packet stream, or parameters. While the UDP and UDP-Lite protocols
share many commonalities, the differences in semantics as described
earlier renders the following parameter inapplicable:
The parameter context(UDP Checksum)
The UDP-Lite checksum cannot be disabled, as opposed to UDP. The
parameter context(UDP Checksum) of [RFC-3095, section 5.7] is
therefore not used for compression of UDP-Lite.
In addition, the UDP-Lite checksum is always sent as-is in every
compressed packet. However, the Checksum Coverage field may not
always be sent in each compressed packet, and the following context
parameter is used to indicate whether or not the field is sent:
The parameter context(UDP-Lite Coverage Field Present)
Whether the UDP-Lite Checksum Coverage field is present or not in
the general packet format (see 5.3.1.) is controlled by the value
of the Coverage Field Present (CFP) flag in the context.
If context(CFP) is nonzero, the Checksum Coverage field is not
compressed and it is present within compressed packets. If
context(CFP) is zero, the Checksum Coverage field is compressed and
it is not sent. This is the case when the value of the Checksum
Coverage field follows a stable inter-packet change pattern; the
field has either a constant value or it has a value equal to the
packet length for most packets in a sequence (see 3.2.).
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Finally, the following context parameter is needed to indicate
whether the field should be inferred or taken from a value previously
saved in the context:
The parameter context(UDP-Lite Coverage Field Inferred)
When the UDP-Lite Checksum Coverage field is not present in the
compressed header (CFP=0), whether it is inferred or not is
controlled by the value of the Coverage Field Inferred (CFI) flag
in the context.
If context(CFI) is nonzero, the Checksum Coverage field is inferred
from the packet length, similarly as for the UDP Length field in
ROHC RTP. If context(CFI) is zero, the Checksum Coverage field is
decompressed using context(UDP-Lite Checksum Coverage). Therefore,
when context(CFI) is updated to a nonzero value, the value of the
Checksum Coverage field stored in the context must also be updated.
5.2. Initialization
Unless stated otherwise, the mechanisms of ROHC RTP and ROHC UDP
found in [RFC-3095] are used also for the ROHC RTP/UDP-Lite and the
ROHC UDP-Lite profiles, respectively.
In particular, the considerations of ROHC UDP regarding the UDP SN
taking the role of the RTP Sequence Number applies to ROHC UDP-Lite.
Also, the static context for ROHC UDP-Lite may be initialized by
reusing an existing context belonging to a stream compressed using
ROHC RTP/UDP-Lite (profile 0x0007), similarly as for ROHC UDP.
5.2.1. Initialization of the UDP-Lite header [UDP-Lite]
The structure of the IR and IR-DYN packets and the initialization
procedures are the same as for the ROHC profiles for UDP [RFC-3095],
with the exception of the dynamic part as specified for UDP. A 2-
octet field containing the checksum coverage is added before the
Checksum field. This affects the format of dynamic chains in both IR
and IR-DYN packets.
Dynamic part:
+---+---+---+---+---+---+---+---+
/ Checksum Coverage / 2 octets
+---+---+---+---+---+---+---+---+
/ Checksum / 2 octets
+---+---+---+---+---+---+---+---+
CRC-DYNAMIC: Checksum Coverage field, Checksum field (octets 5-8).
CRC-STATIC: All other fields (octets 1-4).
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5.2.2. Compressor and decompressor logic
The following logic must be used by both the compressor and the
decompressor for assigning values to the parameters context(CFP) and
context(CFI) during initialization:
Context(CFP)
During context initialization, the value of context(CFP) MUST be
set to a nonzero value if the Checksum Coverage field differs from
the length of the UDP-Lite packet, for any one IR or IR-DYN packet
sent (compressor) or received (decompressor); otherwise the value
MUST be set to zero.
Context(CFI)
During context initialization, the value of context(CFI) MUST be
set to a nonzero value if the Checksum Coverage field is equal to
the length of the UDP-Lite packet within an IR or an IR-DYN packet
sent (compressor) or received (decompressor); otherwise the value
MUST be set to zero.
5.3. Packet formats
The general packet format as defined in [RFC-3095] is modified to
include an additional field for the UDP-Lite checksum coverage. A
packet type is also defined to handle the specific semantics and
characteristics of this field.
5.3.1. General packet format
The general packet format of a compressed ROHC UDP-Lite header is
similar to the compressed ROHC RTP header [RFC-3095, section 5.7],
with modifications to the Checksum field, as well as additional
fields for handling multiple IP headers [IP-ONLY, section 3.3.] and
for the UDP-Lite checksum coverage:
--- --- --- --- --- --- --- ---
: List of :
/ dynamic chains / variable, given by static chain
: for additional IP headers : see [IP-ONLY, section 3.3].
--- --- --- --- --- --- --- ---
: : 2 octets,
+ UDP-Lite Checksum Coverage + if context(CFP) = 1 or
: : if packet type = CE (see 5.3.2.)
--- --- --- --- --- --- --- ---
: :
+ UDP-Lite Checksum + 2 octets
: :
--- --- --- --- --- --- --- ---
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Note that the order of the fields following the optional extension of
the general ROHC packet format is the same as the order between the
fields in the uncompressed header.
Note also that when calculating the CRC for this profile, the
Checksum Coverage field is CRC-DYNAMIC.
5.3.2. Packet type CE: CE(), CE(ON) and CE(OFF)
The ROHC profiles for UDP-Lite defines a packet type to handle the
various possible change patterns of the checksum coverage. This
packet type may be used to manipulate the context values that control
the presence of the Checksum Coverage field within the general packet
format, i.e. context(CFP), and how the field is decompressed, i.e.
context(CFI). The 2-octet Checksum Coverage field is always present
within the format of this packet (see 5.3.1.).
This packet is named Coverage Extension, or CE, and its updating
properties depend on the final two bits of the packet type octet (see
format below). A naming scheme of the form CE(<some property>) is
used to uniquely identify the properties of a particular CE packet.
Although this packet type defines its own format, it may be
considered as an extension mechanism for packets of type 2, 1 or 0
[RFC-3095]. This is achieved by substitution of the packet type
identifier of the first octet of the base header (the "outer"
identifier) with one of the unused packet types from [RFC-3095]. The
substituted identifier is then moved to the first octet of the
remainder of the base header (the "inner" identifier).
The format of the ROHC UDP-Lite CE packet type:
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 1 1 1 1 1 0 F | K | Outer packet type identifier
+===+===+===+===+===+===+===+===+
: : (with inner type identifier)
/ Inner Base header / variable number of bits, given by
: : the inner packet type identifier
+---+---+---+---+---+---+---+---+
F,K: F,K = 00 is reserved at framework level (IR-DYN);
F,K = 01 indicates CE();
F,K = 10 indicates CE(ON);
F,K = 11 indicates CE(OFF).
Updating properties: The updating properties of the inner packet
type carried within any of the CE packets are always maintained. In
addition, CE(ON) always update context(CFP); CE(OFF) always update
context(CFP), context(CFI) and context(UDP-Lite Checksum Coverage).
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Appendix B provides an expanded view of the resulting format of the
CE packet type.
Properties of CE():
Aside from the updating properties of the inner packet type carried
within CE(), this packet does not update any other context values.
CE() thus is mode-agnostic, e.g. it can extend any of packet types
2, 1 and 0, regardless of the current mode of operation [RFC-3095].
CE() may be used when the checksum coverage deviates from the
change pattern assumed by the compressor, while the field could
previously be compressed. This packet is useful if the occurrence
of such deviations are seldom.
Properties of CE(ON):
In addition to the updating properties of the inner packet type,
CE(ON) updates context(CFP) to a nonzero value, i.e. it effectively
turns on the presence of the Checksum Coverage field within the
general packet format. This is useful when the predominant change
pattern of the checksum coverage preclude its compression.
CE(ON) can extend any of the context updating packets of type 2, 1
and 0, that is packets with a compressed header containing a CRC
[RFC-3095]. Specifically, R-0 and R-1* headers MUST NOT be extended
using CE(ON).
Properties of CE(OFF):
In addition to the updating properties of the inner packet type,
CE(OFF) updates context(CFP) to a value of zero, i.e. it
effectively turns off the presence of the Checksum Coverage field
within the general packet format. This is useful when the change
pattern of the checksum coverage seldom deviates from the pattern
assumed by the compressor.
CE(OFF) also updates context(CFI) to a nonzero value, if field(UDP-
Lite Checksum Coverage) is equal to the packet length; otherwise it
must be set to zero. Finally, context(UDP-Lite Checksum Coverage)
is also updated by CE(OFF).
Similarly to CE(ON), CE(OFF) can extend any of the context updating
packets of type 2, 1 and 0 [RFC-3095].
5.4. Compressor logic
Should hdr(UDP-Lite Checksum Coverage) be different from context(UDP-
Lite Checksum Coverage) and different from the packet length when
context(CFP) is zero, the Checksum Coverage field cannot be
compressed. In addition, should hdr(UDP-Lite Checksum Coverage) be
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different from the packet length when context(CFP) is zero and
context(CFI) is nonzero, the Checksum Coverage field cannot be
compressed either. For both cases, the field must be sent
uncompressed using a CE packet or the context must be reinitialized
using an IR packet.
5.5. Decompressor logic
For packet types other than IR, IR-DYN and CE that are received when
the value of context(CFP) is zero, the Checksum Coverage field must
be decompressed using the value stored in the context if the value of
context(CFI) is zero; otherwise the field is inferred from the length
of the UDP-Lite packet derived from the IP module.
6. Security considerations
The security considerations of [RFC-3095] apply integrally to this
document without modifications.
7. IANA considerations
A ROHC profile identifier must be reserved by the IANA for each of
the profiles defined in this document, preferably as listed below:
Profile Document Usage
Identifier
0x0007 RFCthis ROHC RTP/UDP-Lite
0x0008 RFCthis ROHC UDP-Lite
8. Acknowledgements
The author would like to thank Lars-Erik Jonsson, Mats Nordberg for
reviews and discussions around this document.
9. References
[RFC-3095] Bormann, C., Burmeister, C., Degermark, M., Fukushima,
H., Hannu, H., Jonsson, L., Hakenberg, R., Koren, T.,
Le, K., Liu, Z., Martensson, A., Miyazaki, A., Svanbro,
K., Wiebke, T., Yoshimura, T. and H. Zheng, "RObust
Header Compression (ROHC): Framework and four profiles:
RTP, UDP, ESP, and uncompressed", RFC 3095, July 2001.
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[IP-ONLY] Jonsson, L. and G. Pelletier, "RObust Header Compression
(ROHC): A compression profile for IP", Internet draft
(work in progress), September 2003, <draft-ietf-rohc-ip-
only-04.txt>
[RFC-791] Postel, J., "Internet Protocol", STD 5, RFC 791,
September 1981.
[RFC-2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
[RFC-768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
August 1980.
[UDP-Lite] Larzon, L., Degermark, M., Pink, S., Jonsson, L. and G.
Fairhurst, "The UDP-Lite Protocol", Internet draft (work
in progress), August 2003, <draft-ietf-tsvwg-udp-lite-
02.txt>
[RFC-1889] Schulzrinne, H., Casner S., Frederick, R. and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", RFC 1889, January 1996.
10. Authors address
Ghyslain Pelletier
Ericsson AB
Box 920
SE-971 28 Lulea, Sweden
Phone: +46 920 20 24 32
Fax: +46 920 20 20 99
Email: ghyslain.pelletier@ericsson.com
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Appendix A. Detailed classification of header fields
This section summarizes the difference from the classification found
in the corresponding appendix in [RFC-3095], and similarly provides
conclusions about how the various header fields should be handled by
the header compression scheme to optimize compression and
functionality. These conclusions are separated based on the behavior
of the UDP-Lite Checksum Coverage field and uses the expected change
patterns described in section 3.2 of this document.
A.1. UDP-Lite header fields
The following table summarizes a possible classification for the UDP-
Lite header fields in comparison with the classification for UDP,
using the same classes as in [RFC-3095].
Header fields of UDP-Lite and UDP:
+-------------------+-------------+
| UDP-Lite | UDP |
+-------------------+--------+-------------------+-------------+
| Header | Size | Class | Class |
| Field | (bits) | | |
+-------------------+--------+-------------------+-------------+
| Source Port | 16 | STATIC-DEF | STATIC-DEF |
| Destination Port | 16 | STATIC-DEF | STATIC-DEF |
| Checksum Coverage | 16 | INFERRED | |
| | | STATIC | |
| | | CHANGING | |
| Length | 16 | | INFERRED |
| Checksum | 16 | CHANGING | CHANGING |
+-------------------+--------+-------------------+-------------+
Source and Destination Port
Same as for UDP. Specifically, these fields are part of the
definition of a stream and must thus be constant for all packets in
the stream. The fields are therefore classified as STATIC-DEF.
Checksum Coverage
This field specifies which part of the UDP-Lite datagram is covered
by the checksum. It may have a value of zero or equal to the
datagram length if the checksum covers the entire datagram, or it
may have any value between eight octets and the length of the
datagram to specify the number of octets protected by the checksum,
calculated from the first octet of the UDP-Lite header. The value
of this field may vary for each packet, and this makes the value
unpredictable from a header compression perspective.
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Checksum
The information used for the calculation of the UDP-Lite checksum
is governed by the value of the checksum coverage, and minimally
includes the UDP-Lite header. The checksum is a changing field that
must always be sent as-is.
The total size of the fields in each class, for each expected change
patterns (see section 3.2), is summarized in the tables below:
Pattern 1:
+------------+---------------+
| Class | Size (octets) |
+------------+---------------+
| INFERRED | 2 | Checksum Coverage
| STATIC-DEF | 4 | Source Port / Destination Port
| CHANGING | 2 | Checksum
+------------+---------------+
Pattern 2:
+------------+---------------+
| Class | Size (octets) |
+------------+---------------+
| STATIC-DEF | 4 | Source Port / Destination Port
| STATIC | 2 | Checksum Coverage
| CHANGING | 2 | Checksum
+------------+---------------+
Pattern 3:
+------------+---------------+
| Class | Size (octets) |
+------------+---------------+
| STATIC-DEF | 4 | Source Port / Destination Port
| CHANGING | 4 | Checksum Coverage / Checksum
+------------+---------------+
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INTERNET-DRAFT ROHC Profiles for UDP-Lite October 11, 2003
A.2. Header compression strategies for UDP-Lite
The following table revisits the corresponding table (table A.1) for
UDP from [RFC-3095, section A.2] and classifies the changing fields,
based on the change patterns previously identified in section 3.2.
Header compression strategies for UDP-Lite:
+----------+---------+-------------+-----------+-----------+
| Field | Pattern | Value/Delta | Class | Knowledge |
+==========+=========+=============+===========+===========+
| | #1 | Value | CHANGING | INFERRED |
| Checksum |---------+-------------+-----------+-----------+
| Coverage | #2 | Value | RC | UNKNOWN |
| |---------+-------------+-----------+-----------+
| | #3 | Value | IRREGULAR | UNKNOWN |
+----------+---------+-------------+-----------+-----------+
| Checksum | All | Value | IRREGULAR | UNKNOWN |
+----------+---------+-------------+-----------+-----------+
A.2.1. Transmit initially, but be prepared to update
UDP-Lite Checksum Coverage (Patterns #1 and #2)
A.2.2. Transmit as-is in all packets
UDP-Lite Checksum
UDP-Lite Checksum Coverage (Pattern #3)
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INTERNET-DRAFT ROHC Profiles for UDP-Lite October 11, 2003
Appendix B. Detailed format of the CE packet type
This section provides an expanded view of the format of the CE
packet, based on the general ROHC RTP compressed header [RFC-3095]
and the general format of a compressed header [IP-ONLY]. The
modifications necessary to carry the base header of a packet of type
2, 1 or 0 [RFC-3095] within the CE packet format along with the
additional fields to properly handle compression of multiple IP
headers results in the following structure for the CE packet type:
0 1 2 3 4 5 6 7
--- --- --- --- --- --- --- ---
: Add-CID octet : if for small CIDs and CID 1-15
+---+---+---+---+---+---+---+---+
| 1 1 1 1 1 0 F | K | Outer packet type identifier
+---+---+---+---+---+---+---+---+
: :
/ 0, 1, or 2 octets of CID / 1-2 octets if large CIDs
: :
+---+---+---+---+---+---+---+---+
| First octet of base header | (with "inner" type indication)
+---+---+---+---+---+---+---+---+
/ Remainder of base header / variable number of bits
+---+---+---+---+---+---+---+---+
: :
/ Extension / See [RFC-3095], section 5.7.
: :
--- --- --- --- --- --- --- ---
: :
+ IP-ID of outer IPv4 header + See [RFC-3095], section 5.7.
: :
--- --- --- --- --- --- --- ---
/ AH data for outer list / See [RFC-3095], section 5.7.
--- --- --- --- --- --- --- ---
: :
+ GRE checksum + See [RFC-3095], section 5.7.
: :
--- --- --- --- --- --- --- ---
: :
+ IP-ID of inner IPv4 header + See [RFC-3095], section 5.7.
: :
--- --- --- --- --- --- --- ---
/ AH data for inner list / See [RFC-3095], section 5.7.
--- --- --- --- --- --- --- ---
: :
+ GRE checksum + See [RFC-3095], section 5.7.
: :
--- --- --- --- --- --- --- ---
: List of :
/ dynamic chains / See [IP-ONLY], section 3.3.
: for additional IP headers :
Pelletier [Page 17]
INTERNET-DRAFT ROHC Profiles for UDP-Lite October 11, 2003
--- --- --- --- --- --- --- ---
: :
+ UDP-Lite Checksum Coverage + 2 octets
: :
+---+---+---+---+---+---+---+---+
: :
+ UDP-Lite Checksum + 2 octets
: :
+---+---+---+---+---+---+---+---+
F,K: F,K = 00 is reserved at framework level (IR-DYN);
F,K = 01 indicates CE();
F,K = 10 indicates CE(ON);
F,K = 11 indicates CE(OFF).
Note that this document does not define (F,K) = 00, as this would
collide with the IR-DYN packet type already reserved at the ROHC
framework level.
Full Copyright Statement
Copyright (C) The Internet Society (2003). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
This Internet-Draft expires April 11, 2004.
Pelletier [Page 18]
