Network Working Group L-E. Jonsson
INTERNET-DRAFT K. Sandlund
Expires: July 2006 G. Pelletier
P. Kremer
Ericsson
January 23, 2006
The RFC 3095 Implementer's Guide
<draft-ietf-rohc-rtp-impl-guide-17.txt>
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Abstract
RFC 3095 defines the RObust Header Compression (ROHC) framework and
profiles for IP, UDP, RTP, and ESP. This document clarifies
ambiguities and common misinterpretations of RFC 3095, and it also
corrects some actual errors in the specification. The corrections and
clarifications provided herein must be followed to get interoperable
implementations, i.e. this document updates RFC 3095. Further, minor
interpretation details of RFC 3241 (ROHC over PPP), RFC 3843 (ROHC IP
profile) and RFC 4109 (ROHC UPD-Lite profiles) are also addressed.
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Table of Contents
1. Introduction.....................................................3
2. CRC calculation and coverage issues..............................3
2.1. CRC calculation.............................................3
2.2. Padding octet in CRC........................................4
2.3. CRC coverage in CRC feedback options........................4
2.4. CRC coverage of the ESP NULL header.........................4
3. Enhanced mode transition procedures..............................4
3.1. Modified transition logic for enhanced transitions..........5
3.2. Transition from Reliable to Optimistic mode.................6
3.3. Transition to Unidirectional mode...........................6
4. Timestamp encoding considerations................................7
4.1. Encoding used for compressed TS bits........................7
4.2. (De)compression of TS without transmitted TS bits...........7
4.3. Interpretation intervals for TS encoding....................8
4.4. TS_STRIDE for scaled timestamp encoding.....................8
4.5. TS wraparound with scaled timestamp encoding................9
4.6. Recalculating TS_OFFSET.....................................9
4.7. TS_STRIDE and the Tsc flag in Extension 3..................10
4.8. Using timer-based compression..............................10
5. List compression issues.........................................10
5.1. Generic extension header list..............................10
5.2. CSRC list items in RTP dynamic chain.......................11
5.3. RTP dynamic chain..........................................11
5.4. Compressed lists in UO-1-ID packets........................11
5.5. Bit masks in list compression..............................11
5.6. Headers compressed with list compression...................12
5.7. ESP NULL header list compression...........................12
5.8. Translation tables and indexes for IP extension headers....12
5.9. Reference list.............................................12
6. Updating properties.............................................13
6.1. Implicit updates...........................................13
6.2. Updating properties of UO-1*...............................13
7. Context management and CID/context re-use.......................14
7.1. The decompressor MUST keep MAX_CID contexts................14
7.2. CID/context re-use.........................................14
7.2.1. Re-using a CID/context with the same profile..........14
7.2.2. Re-using a CID/context with a different profile.......15
7.3. Context updating properties for IR packets.................15
8. Other protocol clarifications...................................15
8.1. Meaning of NBO.............................................15
8.2. IP-ID......................................................15
8.3. Extension-3 in UO-1-ID packets.............................16
8.4. Extension-3 in UOR-2* packets..............................16
8.5. Multiple SN options in one feedback packet.................16
8.6. Multiple CRC options in one feedback packet................17
8.7. Packet decoding during mode transition.....................17
8.8. How to respond to lost feedback links?.....................17
8.9. What does "presumed zero if absent" mean on page 88?.......18
8.10. UOR-2 in profile 2 (UDP) and profile 3 (ESP)..............18
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8.11. Sequence number LSB's in IP extension headers.............18
8.12. Expecting UOR-2 ACKs in O-mode............................18
8.13. Compression of SN in AH and GRE extension headers.........19
9. ROHC negotiation clarifications.................................19
10. PROFILES suboption in ROHC-over-PPP............................20
11. Constant IP-ID encoding in IP-only and UPD-Lite profiles.......20
12. Security considerations........................................20
13. IANA considerations............................................20
14. Acknowledgment.................................................20
15. References.....................................................21
15.1. Normative References......................................21
15.2. Informative References....................................21
16. Authors' Addresses.............................................21
Appendix A - Sample CRC algorithm..................................22
1. Introduction
RFC 3095 [1] defines the RObust Header Compression (ROHC) framework
and profiles for IP, UDP, RTP, and ESP. During implementation and
interoperability testing of RFC 3095 some ambiguities and common
misinterpretations have been identified, as well as a few actual
errors.
This document has been created to summarize all identified issues,
and provide the clarifications and corrections needed to make
creation of interoperable implementations possible. It should thus be
noted that it is essential that implementers of RFC 3095 follow the
corrections and clarifications provided herein, i.e. this document
constitute an update to RFC 3095. When referring to RFC 3095, this
document should therefore also be referenced.
A few minor interpretation details of RFC 3241 [2] (ROHC over PPP),
RFC 3843 [3] (ROHC IP-only profile), and RFC 4019 [5] (ROHC UDP-Lite
profiles) are also addressed in this document (chapter 10-11).
Note that all section and chapter references in this document refer
to [1], where not stated otherwise.
2. CRC calculation and coverage issues
2.1. CRC calculation
ROHC uses CRC checksum in order to provide some protection against
bit errors. CRC is used in the segmentation protocol and in the
compressed packets, as well.
Section 5.2.5.2 describes the segmentation protocol and refers to
[3], which describes a well-defined CRC algorithm for 32 bit
checksums. Although, Section 5.9 only defines the polynomials for 3,
7 and 8-bit long checksum, the same algorithm can be used in these
cases as well.
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A PERL implementation of the algorithm (written by Carsten Bormann)
can be found in Appendix A of this document.
2.2. Padding octet in CRC
According to Section 5.9.1, in case of IR and IR-DYN packets the CRC
"is calculated over the entire IR or IR-DYN packet, excluding Payload
and including CID or Add-CID octet". Padding isn't meant to be
meaningful part of a packet and not included in CRC calculation. As
a result, CRC doesn't cover the Add-CID octet for CID 0, either.
2.3. CRC coverage in CRC feedback options
Section 5.7.6.3 states "The CRC option contains an 8-bit CRC computed
over the entire feedback payload, without the packet type and code
octet, but including any CID fields, using the polynomial of section
5.9.1". However, it does not mention how the "size" field is handled,
if present. Since the "size" field can be considered an extension of
the "code" field, it must be treated as the "code" field, i.e. the
"size" field is not covered by the CRC.
2.4. CRC coverage of the ESP NULL header
Section 5.8.7 gives the CRC coverage of the ESP NULL header as
"Entire ESP header". This should be interpreted as being only the
initial part of the header (i.e. SPI and Sequence number), and not
the trailer part at the end of the payload. Therefore, the ESP NULL
header will have the same CRC coverage as the ESP header used in the
ESP profile (section 5.7.7.7).
3. Enhanced mode transition procedures
To reduce transmission overhead and computational complexity
(including CRC calculation) associated with feedback packets sent for
each decompressed packet during mode transition, a decompressor can
be implemented with slightly modified mode transition procedures,
compared to those defined in [1].
These modifications affect transitions to Optimistic and
Unidirectional modes of operation, i.e. the transitions described in
sections 5.6.5 and 5.6.6, and make those transition diagrams more
consistent with the diagram describing the transition to R-mode.
However, the differences between the original diagrams of [1] were
motivated by robustness concerns for mode transitions to Optimistic
and Unidirectional modes. To avoid deadlock situations in mode
transitions, these aspects must be addressed also when a decompressor
implements the enhanced transition procedures, and that is done by
following a slightly modified definition of the decompressor
transition states. All aspects related to implementation of the
enhanced transition procedures are described in subsequent chapters.
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Note that these modified operations should be seen only as an
improved decompressor implementation, since interoperability is not
at all affected. A decompressor implemented according to the
optimized procedures would be able to interoperate with an RFC3095
compressor, as well as a decompressor implemented according to the
procedures described in RFC3095 would do.
3.1. Modified transition logic for enhanced transitions
The intent with these enhanced transition procedures is to allow the
decompressor to stop sending feedback packets for all packets
decompressed during the second half of the transition procedure, i.e.
after an appropriate IR/IR-DYN/UOR-2 packet has been received from
the compressor. In the transition diagrams, sections 3.2 and 3.3
below, this is realized by allowing the decompressor transition
parameter (D_TRANS) to be set to P (Pending) at that stage. However,
as mentioned above, there are robustness concerns related to this
optimization, and to avoid deadlock situations with never completed
transitions as a result of feedback losses, the decompressor must
continue to send feedback at least periodically, also when in Pending
transition state. That would be the equivalence of enhancing the
D_TRANS parameter definition in section 5.6.1, to include a
definition of Pending state operation.
- D_TRANS:
Possible values for the D_TRANS parameter are (I)NITIATED,
(P)ENDING and (D)ONE. D_TRANS MUST be initialized to D, and a mode
transition can be initiated only when D_TRANS is D. While D_TRANS
is I, the decompressor sends a NACK or ACK carrying a CRC option
for each packet received. When D_TRANS is set to P, the
decompressor do not have to send a NACK or ACK for each packet
received, but it MUST continue to send feedback on a regular
basis, and all feedback packets sent MUST include the CRC option.
This ensures that all mode transitions will be completed also in
case of feedback losses.
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3.2. Transition from Reliable to Optimistic mode
The enhanced procedure for transition from Reliable to Optimistic
mode is shown below:
Compressor Decompressor
----------------------------------------------
| |
| ACK(O)/NACK(O) +-<-<-<-| D_TRANS = I
| +-<-<-<-<-<-<-<-+ |
C_TRANS = P |-<-<-<-+ |
C_MODE = O | |
|->->->-+ IR/IR-DYN/UOR-2(SN,O) |
| +->->->->->->->-+ |
|->-.. +->->->-| D_TRANS = P
|->-.. | D_MODE = O
| ACK(SN,O) +-<-<-<-|
| +-<-<-<-<-<-<-<-+ |
C_TRANS = D |-<-<-<-+ |
| |
|->->->-+ UO-0, UO-1* |
| +->->->->->->->-+ |
| +->->->-| D_TRANS = D
| |
3.3. Transition to Unidirectional mode
The enhanced procedure for transition to Unidirectional mode is shown
on the following figure:
Compressor Decompressor
----------------------------------------------
| |
| ACK(U)/NACK(U) +-<-<-<-| D_TRANS = I
| +-<-<-<-<-<-<-<-+ |
C_TRANS = P |-<-<-<-+ |
C_MODE = U | |
|->->->-+ IR/IR-DYN/UOR-2(SN,U) |
| +->->->->->->->-+ |
|->-.. +->->->-| D_TRANS = P
|->-.. |
| ACK(SN,U) +-<-<-<-|
| +-<-<-<-<-<-<-<-+ |
C_TRANS = D |-<-<-<-+ |
| |
|->->->-+ UO-0, UO-1* |
| +->->->->->->->-+ |
| +->->->-| D_TRANS = D
| | D_MODE= U
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4. Timestamp encoding considerations
4.1. Encoding used for compressed TS bits
RTP Timestamp values (TS) are always encoded using W-LSB encoding,
both when sent scaled and when sent unscaled. For TS values sent in
Extension 3, W-LSB encoded values are sent using the self-describing
variable-length format (section 4.5.6), and this applies to both
scaled and unscaled values.
4.2. (De)compression of TS without transmitted TS bits
RFC 3095 explains that SO-state provides the most efficient
compression within ROHC RTP. In this state, apart from packet type
identification and the error detection CRC, only RTP sequence number
(SN) bits have to be transmitted in RTP compressed headers. All other
fields are then omitted either because they are unchanged or because
they can be reconstructed through a function from the SN (i.e. by
combining the transmitted SN bits with state information from the
context). Although it is never spelled out explicitly what fields are
inferred from the SN in this way, one should be able to figure out
that this principle applies to the IP Identification (IP-ID) field
and the RTP Timestamp (TS) field.
IP-ID compression and decompression, both with and without
transmitted IP-ID bits in the compressed header, are well defined in
section 4.5.5 (see section 8.2 of this document). However, for TS it
is only defined how to decompress based on actual TS bits in the
compressed header, either scaled or unscaled, but not how to infer
the TS from the SN, i.e. the SO-state operation. Although the general
idea is simple, the actual operation must be clearly defined to
ensure interoperability. There are also inconsistent text pieces that
might confuse an implementer and result in non-interoperable
implementations. This section therefore provides the necessary
clarifications to SN-to-TS decompression, i.e. decompression of TS
(scaled or unscaled) when no TS bits are transmitted in the
compressed header.
When no TS bits are transmitted in the compressed header, the encoded
TS value (scaled or unscaled) is to be decompressed assuming a linear
extrapolation from the SN, i.e. delta_TS = delta_SN * default-slope.
Section 5.7 defines the potential values for default-slope as:
If value(Tsc) = 1, Scaled RTP Timestamp encoding is used before
compression (see section 4.5.3), and default-slope(TS) = 1.
If value(Tsc) = 0, the Timestamp value is compressed as-is, and
default-slope(TS) = value(TS_STRIDE).
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What must be noted here is that no slope value is used other than the
default-slope value, as defined in section 5.7. There is confusing
text in section 5.5.1.2 that might mistakenly be interpreted as if
the slope can have different values and be "learned", which is
incorrect. The default-slope from 5.7 is always the value used when
decompressing TS based on SN.
4.3. Interpretation intervals for TS encoding
Section 4.5.4 defines the interpretation interval, p, for timer-based
compression of the RTP timestamp. However, Section 5.7 defines a
different interpretation interval, which is defined as the
interpretation interval to use for all TS values. It is thus unclear
which p-value to use, at least for timer-based compression.
The way this should be interpreted is that the p-value differs
depending on whether timer-based compression is enabled or not. For
timer-based compression, the interpretation interval of section
4.5.4, p = 2^(k-1) - 1, is used for TS. Otherwise, the interval of
section 5.7, p = 2^(k-2) - 1, is used for TS with regular W-LSB
encoding.
Since two different p-values are used, the compressor must take this
into account during the process of enabling timer-based compression.
Before timer-based compression can be used, the decompressor will
have to inform the compressor (on a per-channel basis) about its
clock resolution. Further, the compressor has to send (on a per-
context basis) a non-zero TIME_STRIDE to the decompressor.
When the compressor is confident that the decompressor has received
the TIME_STRIDE value, it can switch to timer-based compression.
During this transition from window-based compression to timer-based
compression, it is necessary that the compressor keep k large enough
to cover both interpretation intervals.
4.4. TS_STRIDE for scaled timestamp encoding
The timestamp stride (TS_STRIDE) is defined as the expected increase
in the timestamp value between two RTP packets with consecutive
sequence numbers. TS_STRIDE is set by the compressor and explicitly
communicated to the decompressor, and it is used either as the
scaling factor for scaled TS encoding, or constitutes the default-
slope used when decompressing an unscaled TS through a linear
extrapolation from the SN (see also section 4.2 above).
The relation between TS and TS_SCALED, given by the following
equality in section 4.5.3, defines the mathematical meaning of
TS_STRIDE:
TS = TS_SCALED * TS_STRIDE + TS_OFFSET
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In the compression step explained following this core equality of
section 4.5.3, TS_SCALED is incorrectly written as TS / TS_STRIDE.
This formula is incorrect both because it excludes TS_OFFSET, and
because it would prevent a TS_STRIDE value of 0. If "/" were a
generally unambiguously defined operation meaning "the integral part
of the result from dividing X by Y", the absence of TS_OFFSET could
be explained, but the formula still lacks a proper output for
TS_STRIDE equal to 0. As the core equality above does not prevent
setting TS_STRIDE to 0, and there is no reason not to allow a
compressor to do that, the formula of "2. Compression" should not be
read as having any formal meaning.
4.5. TS wraparound with scaled timestamp encoding
In the scaled timestamp encoding section, 4.5.3, it is said in point
4 and 5 that the compressor is not required to initialize TS_OFFSET
at wraparound, but that it is required to increase the number of bits
sent for the scaled TS value when there is a TS wraparound. The
decompressor is also required to detect and cope with TS wraparound,
including updating TS_OFFSET. This has been found to be non-trivial
to do, as well as hard to make interoperable and robust. The gain is
also insignificant, as TS wraparound happens very seldom.
It is therefore recommended not to follow point 4-5 of section 4.5.3,
and instead the compressor is recommended to reinitialize TS_OFFSET
upon TS wraparound, by sending unscaled TS. This is equivalent of
replacing point 4-5 with:
4. Offset at wraparound: If the value of TS_STRIDE is not equal
to a power of two, wraparound of the unscaled 32-bit TS will
change the value of TS_OFFSET. When this happens, the
compressor must reinitialize TS_OFFSET by sending unscaled TS,
as in 1 above.
It should be noted that by following this recommendation for the
compressor to reinitialize TS_OFFSET at wraparound, there will be no
problems interacting with a decompressor that still tries to follow
4.5.3 points 4-5. For a decompressor that assumes the compressor will
follow the above recommendation, there is a risk of the decompressor
context becoming invalid. Considering the size if the TS number
space, and thus the number of packets between each TS wraparound, the
potential cost of this is considered negligible.
4.6. Recalculating TS_OFFSET
TS can be sent unscaled if the TS value change does not match the
established TS_STRIDE, but the TS_STRIDE might still stay unchanged.
To ensure correct decompression of subsequent packets, the
decompressor should therefore always recalculate the RTP TS modulo,
TS_OFFSET, when a packet with an unscaled TS value is received.
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4.7. TS_STRIDE and the Tsc flag in Extension 3
The Tsc flag in Extension 3 indicates whether TS is scaled or not. It
must be noted that the Tsc value apply to all TS bits, also if there
are no TS bits in the extension itself.
When a compressor uses Extension 3 to re-establish a new value for
TS_STRIDE, it must send unscaled TS together with TS_STRIDE for some
packets until decompressor confidence is obtained.
When Tsc=1, TS must be scaled using context(TS_STRIDE) and not
value(TS_STRIDE), which is incorrectly stated in the legend for
Extension 3 in section 5.7.5. Instead, if the decompressor receives
an Extension 3 with TS_STRIDE included while Tsc=1, the decompressor
must simply ignore/discard value(TS_STRIDE). Since an unscaled TS is
needed together with TS_STRIDE to recalculate TS_OFFSET, it is thus
meaningless to include TS_STRIDE in Extension 3 if Tsc is set to 1.
4.8. Using timer-based compression
Timer-based compression of the RTP timestamp, as described in section
4.5.4, may be used to reduce the number of transmitted timestamp bits
(bytes) needed when the timestamp can not be inferred from the SN. It
should thus be noted that timer-based compression has no influence on
decompression of packets where no timestamp bits are sent, in that
case the timestamp is just linearly inferred from the SN (see section
4.2 of this document).
Whether to use timer-based compression or not is controlled by the
TIME_STRIDE control field, which can be set either by an IR, an IR-
DYN, or by a compressed packet with extension 3. The compressor turns
on timer-based compression by setting TIME_STRIDE to a value > 0, but
that can be done first after the decompressor has declared its clock
resolution, which is done by sending a CLOCK feedback option for any
CID on the channel.
5. List compression issues
5.1. Generic extension header list
Section 5.7.7.4 defines the static and dynamic parts of the IPv4
header. This section indicates a 'Generic extension header list'
field in the dynamic chain, which has a variable length. The
detailed description of this field can be found in Section 5.8.6.1.
The generic extension header list starts with an octet that is always
present, so its length is one octet, at least. If the 'GP' bit in
the first octet is set to 1 then the length of the list is two
octets, even if the list is empty.
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5.2. CSRC list items in RTP dynamic chain
Section 5.7.7.6 defines the static and dynamic parts of the RTP
header. This section indicates a 'Generic CSRC list' field in the
dynamic chain, which has a variable length. This field uses the same
encoding rules as the 'Generic extension header list' in the IPv4
header, so the same rules apply to its length.
5.3. RTP dynamic chain
Section 5.7.7.6 defines the static and dynamic parts of the RTP
header. In the dynamic part, a 'CC' field indicates the number of
CSRC items present in the 'Generic CSRC list'. A 'CC' field can also
be found within the 'Generic CSRC list' (when present), and these
fields would then have the same meaning. In order to decode a
compressed packet correctly it's necessary to know the 'CC' value
because it has serious impact on the packet's length. In normal
case, the values of the fields are equal.
Proposed behavior if the values are different:
Both fields are within the RTP dynamic part but only the second
'CC' field resides inside the 'Generic CSRC list' together with
the XI items. Since the 'CC' value determines the number of XI
items in the CSRC list and isn't used otherwise, the first CC
field should be ignored and only the second field (inside the CSRC
list) should be used for incoming packets. For outgoing packets
both fields should be set to the same value.
5.4. Compressed lists in UO-1-ID packets
This section describes the situation when a UO-1-ID packet carries a
compressed list. Compressed lists are encoded using Encoding Type 0
(section 5.7.5 and 5.8.6.1) and every list may have a unique
identifier (gen_id). The identifier is present in U/O-mode when the
compressor decides that it may use this list as a future reference.
On one hand, the decompressor shouldn't save the list because UO-1-ID
packets doesn't update the context. On the other hand, the
decompressor updates its translation table whenever an (Index, item)
pair is received (section 5.8.1). The decompressor must be able to
handle such a packet, thus it has to behave as described in the
latter case. According to section 5.8.1.2, the compressor must
increment Counter by 1.
5.5. Bit masks in list compression
A 7-bit or 15-bit mask may be used in the insertion and/or removal
schemes for compressed lists. It should be noted that even if a list
has more than 7 items, a 7-bit mask could be used as long as changes
are only applied in the first part of the reference list, and items
with an index not covered by the 7-bit mask are thus kept unchanged.
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5.6. Headers compressed with list compression
In section 5.8, it is stated that headers which can be part of
extension header chains INCLUDE AH, null ESP, minimal encapsulation
(MINE), GRE, and IPv6 extensions. This list of headers which can be
compressed is correct, but the word INCLUDE should not be there,
since only the header types listed can actually be handled. It should
further be noted that for the Minimal Encapsulation (MINE) header,
there is no explicit discussion of how to compress it, as the header
is either sent uncompressed or fully compressed away.
5.7. ESP NULL header list compression
Due to the offset of the fields in the trailer part of the ESP
header, a compressor must only compress packets containing at most
one NULL ESP header.
5.8. Translation tables and indexes for IP extension headers
Section 5.8.4 aims at describing how list indexes are associated to
list items and how table lists are built for IP extension headers.
However, the text is not very clear, and it is actually wrong to just
say that an index per type is used, since the same type can appear
several times with different content in one single chain.
In IP extension header list compression, an index is associated with
each individual extension header of an extension header chain. When
there are multiple occurrences of the same extension type (Protocol
Number) within a header chain, each will have to be given its own
index (assuming they are not identical). When content of extension
headers changes, an implementation can choose to either initiate a
new index, or update the existing one. Note that some extensions can
be compressed away also when some fields change, as there are other
means to explicitly or implicitly convey the changes.
When there is more than one IP header, there is more than one list of
extension headers, and a translation table is maintained for each
list independently of one another.
5.9. Reference list
A list compressed using encoding type 1 (insertion), type 2 (removal)
or type 3 (removal/insertion) uses a coding scheme that is based on
the use of a reference list in the context (identified as ref_id).
It is noted that while it could seem a fair choice to send a type 1
list when ref_id is an empty list, there is no gain in doing so with
respect to using a type 0 list. Sending a type 2 list when ref_id is
an empty list would lead to a failure, while sending a type 3 list
has very little meaning. All these alternatives are currently
possible with how list compression is specified in RFC 3095.
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Because of this, a decompressor becomes required to maintain a
sliding window of ref_id lists in R-mode even for the case where no
items are sent as compressed list. To avoid this, the sending of type
1, 2, and 3 compressed list using an empty reference list should thus
be disallowed. Therefore empty lists are not needed to be stored in
the sliding window in the decompressor.
More specifically, when the compressor uses list encoding of type 1,
type 2, and type 3, the ref_id used must refer to a non-empty
reference list, regardless of the operating mode.
6. Updating properties
6.1. Implicit updates
A context updating packet that contains compressed sequence number
information may also carry information about other fields; in such
case, these fields are updated according to the content of the
packet. The updating packet also implicitly updates inferred fields
(e.g. RTP timestamp) according to the current mode and the
appropriate mapping function of the updated and the inferred fields.
An updating packet thus updates the reference values of all header
fields, either explicitly or implicitly, with an exception for the
UO-1-ID packet, which only updates TS, SN, IP-ID, and sequence
numbers of IP extension headers (see 5.7.3). In UO-mode, all packets
are updating packets, while in R-mode all packets with a CRC are
updating packets.
For example, a UO-0 packet contains the compressed RTP sequence
number (SN). Such a packet also implicitly updates RTP timestamp,
IPv4 ID, and sequence numbers of IP extension headers.
6.2. Updating properties of UO-1*
In section 5.7.3, the updating properties of UO-1* are stated:
"Values provided in extensions, except those in other SN, TS,
or IP-ID fields, do not update the context."
However, also sequence number fields of extension headers must be
updated, which means the updating properties should be rephrased as:
"The only values provided in extensions that update the context are
the additional bits for the SN, TS, or IP-ID fields. Other values
provided in extensions do not update the context. Note that
sequence number fields of extension headers are also updated."
See also section 5.4 of this document.
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7. Context management and CID/context re-use
7.1. The decompressor MUST keep MAX_CID contexts
As part of the negotiated channel parameters, compressor and
decompressor have through the MAX_CID parameter agreed on the highest
context identification (CID) number to be used. By agreeing on
MAX_CID, the decompressor also agrees to provide memory resources to
host at least MAX_CID+1 contexts, and an established context with a
CID within this negotiated space MUST be kept by the decompressor
until either the CID gets re-used, or the channel is taken down or
re-negotiated.
7.2. CID/context re-use
As part of the channel negotiation, the maximal number of active
contexts supported is negotiated between the compressor and the
decompressor through the MAX_CID parameter. The value of MAX_CID can
of course vary enormously between different link scenarios, as well
as the load in terms of actual packet streams to compress. Depending
on link technology, the ROHC channel lifetime will also vary from
almost permanent to rather short-lived. However, in general it is not
expected that resources will be allocated for more contexts than what
can reasonably be expected to be active concurrently over the link.
As a consequence hereof, context identifiers (CIDs) and context
memory are resources that will have to be re-used by the compressor
as part of what can be considered normal operation.
How context resources are re-used is in RFC 3095 [1] and subsequent
ROHC standards basically left unspecified and up to implementation.
However, re-using a CID and its allocated memory is not always as
simple as initiating a contexts with a previously unused CID. Because
some profiles can be operating in various modes where packet formats
vary depending on current mode, care has to be taken to ensure that
the old context data will be completely and safely overwritten,
eliminating the risk of undesired side effects from interactions
between old and new context data.
On a high level, CID/context re-use can be of two kinds, either re-
use for a new context based on the same profile as the old context,
or for a new context based on a different profile. These cases, are
discussed separately in the following two subsections.
7.2.1. Re-using a CID/context with the same profile
For multi-mode profiles, such as those defined in RFC 3095 [1], when
a CID/context is re-used for a new context based on the same profile
as the old context, the current mode of operation MUST be inherited
from the old to the new context. The reason for this is that there is
no reliable way for the compressor to inform the decompressor that a
CID/context re-use is happening. The decompressor can thus not be
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expected to flush the context memory for the CID, and there is no way
to trigger a safe mode switching, which requires a decompressor-
initiated handshake procedure, as defined in section 5.6.
It should be noted that the rule of mode inheritance applies also
when the CONTEXT_REINITIALIZATION signal is used to reinitiate an
entire context.
7.2.2. Re-using a CID/context with a different profile
When a CID is re-used for a new context based on a different profile
than the old context, operation with that context MUST start in the
initial mode of the profile (if it is a multi-mode profile). This
applies both to IR-initiated new contexts and profile downgrades with
IR-DYN (e.g. the IP/UDP/RTP->IP/UDP downgrade in [1]section 5.11.1).
A CID for an R-mode operating context SHOULD NOT be re-used for a new
context based on a different profile than the old context, because of
the R-0/R-1 misinterpretation risk (these packets have no CRC). If a
compressor still wants or has to do this, the compressor must be very
careful to minimize the misinterpretation risk, e.g. by not using
packets of types beginning with 00 or 10 before the compressor is
highly confident that the new context has successfully been initiated
at the decompressor.
7.3. Context updating properties for IR packets
It should be noted that an IR does not flush the whole context, but
updates all fields carried in the IR header. Similarly, an IR without
a dynamic chain simply updates the static part of the context, while
the rest of the context is left unchanged.
A consequence of this is that fields that cannot be updated by the IR
packet, e.g. the Translation Tables for list compression, MUST NOT be
invalidated by the decompressor when it assumes context damage.
8. Other protocol clarifications
8.1. Meaning of NBO
In general, an unset flag indicates the normal operation and a set
flag indicates unusual behavior. However, in IPv4 dynamic part
(Section 5.7.7.4), if the 'NBO' bit is set, it means that network
byte order is used.
8.2. IP-ID
According to Section 5.7 IP-ID means the compressed value of the IPv4
header's 'Identification' field. Compressed packets contain this
compressed value (IP-ID), while IR packets with dynamic chain and IR-
DYN packets transmit the original, uncompressed Identification field
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value. The IP-ID field always represents the Identification value of
the innermost IPv4 header whose corresponding RND flag is not 1.
If RND or RND2 is set to 1, the corresponding IP-ID(s) is(are) sent
as 16-bit original Identification value(s) at the end of the
compressed base header, according to the IP-ID description (see the
beginning of section 5.7). When there is no compressed IP-ID, i.e.
for IPv6 or when all IP Identification information is sent as-is (as
indicated by RND/RND2 being set to 1), the decompressor ignores IP-ID
bits sent within compressed base headers.
It should be noted that when RND*=0, IP-ID is always compressed, i.e.
expressed as an SN offset and byte-swapped if NBO=0. This is the case
even when 16 bits of IP-ID is sent in extension 3.
When RND=0 but no IP-ID bits are sent in the compressed header, the
SN offset for IP-ID stays unchanged, meaning that Offset_m equals
Offset_ref, as described in Section 4.5.5. This is further expressed
in a slightly different way (with the same meaning) in Section 5.7,
where it is said that "default-slope(IP-ID offset) = 0", meaning that
if no bits are sent for IP-ID, its SN offset slope defaults to 0.
8.3. Extension-3 in UO-1-ID packets
Extension-3 is applied to give values and indicate changes to fields
other than SN, TS and IP-ID in IP header(s) and RTP header. In case
of UO-1-ID packets, it should be noted that values provided in
extensions do not update the context, with an exception for SN, TS
and IP-ID fields, which always update the context (Section 5.7.3.).
It should also be noted that a UO-1-ID packet with Extension 3 must
never be sent with RND flags that changes the packet interpretation,
i.e. that would violate the base condition for UO-1-ID (at least one
RND value must be 0). See also section 5.4 of this document.
Besides, usage of Extension-3 in UO-1-ID can be useful to compress a
transient change in a packet stream. For example, if a field's value
changes in the current packet but in the next packet it returns to
its regular behavior, e.g. changes in TTL.
8.4. Extension-3 in UOR-2* packets
If Extension-3 is used in a UOR-2* packet then the information of the
extension updates the context (Section 5.7.4). Some flags of the IP
header in the extension (e.g. NBO or RND) changes the interpretation
of fields in UOR-2* packets. In these cases, when a flag changes in
Extension-3, a decompressor should re-parse the UOR-2* packet.
8.5. Multiple SN options in one feedback packet
The length of the sequence number field in the original ESP header is
32 bits. A decompressor can't send back all the 32 bits in a
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feedback packet unless it uses multiple SN options in one feedback
packet. Section 5.7.6.1 declares that a FEEDABCK-2 packet can
contain variable number of feedback options and the options can
appear in any order.
A compressor should be able to process multiple SN options in one
feedback packet.
8.6. Multiple CRC options in one feedback packet
Although it is never required to have more than one single CRC option
in a feedback packet, having multiple CRC options is still allowed.
If multiple CRC options are included, all such CRC options will be
identical, as they will be calculated over the same header.
8.7. Packet decoding during mode transition
Each ROHC profile defines its own set of packet formats, and also its
own feedback packets. The use of various operational modes is also
defined by each specific profile. A decompressor can therefore not
initiate a mode transfer request before at least one packet of a new
context has been correctly decompressed, establishing the context
based on one specific profile (as specified in IR packets). First
then the context has been established, the decompressor knows the
profile used, which modes are defined by that profile, and the
feedback packet formats available.
If the original transition procedures in sections 5.6.5 and 5.6.6 are
followed (and not the enhanced procedures described in section 3 of
this document), it is important to note that type 0 or type 1 packets
may be received by the decompressor during the first half of the
transition procedure, and these packets must not mistakenly be
interpreted as the packets sent by the compressor to indicate
completed transition. The decompressor side must therefore keep track
of the transition status, e.g. with an additional parameter. If the
enhanced transition procedures described in section 3 of this
document are used, the D_TRANS parameter can serve this purpose since
its definition and usage is slightly modified.
8.8. How to respond to lost feedback links?
Althought this is neither desirable or expected, it may happen that a
link used to carry feedback between two associated instances become
unavailable. If the compressor can be notified of such event, the
most suitable response in such case is for the compressor to restart
compression for each flow going over the ROHC channel, except for
flows that are operating in U/O-mode or flows using a CID associated
with the uncompressed profile (profile 0x0000). When restarting
compression, the compressor should use a different CID for each flow
being restarted; this is useful to avoid that packet types for which
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both U/O-mode and R-mode share the same type identifier gets
misinterpreted when restarting the flow in U-mode.
Generally, feedback links are not expected to disappear when once
present, but it should be noted that this might be the case for
certain link technologies.
8.9. What does "presumed zero if absent" mean on page 88?
On page 88, RFC 3095 says that R-P contains the absolute value of RTP
Padding bit and it's presumed zero if absent. It could be absent
from RTP header flags and fields, from the extension type 3 or from
the ROHC packet. It's been agreed that the RTP padding bit is
presumed zero if absent from the RTP header flags.
8.10. UOR-2 in profile 2 (UDP) and profile 3 (ESP)
One single new format is defined for UOR-2 in profile 2 and profile
3, which replaces all three (UOR-2, UOR-2-ID, UOR-2-TS) formats from
profile 1. The same UOR-2 format is thus used independent of if there
are IP headers with a corresponding RND=1 or not.
8.11. Sequence number LSB's in IP extension headers
In section 5.8.5, formats are defined for compression of IP extension
header fields. These include compressed sequence number fields, and
it is said that these fields contain "LSB of sequence number". This
means these sequence numbers are not "LSB-encoded" as e.g. the RTP
sequence number with an interpretation interval, but are actually
just the LSB's of the uncompressed fields.
8.12. Expecting UOR-2 ACKs in O-mode
It should be noted that the use of UOR-2 ACKs in O-mode, as discussed
in section 5.4.1.1.2, is indeed optional, and a decompressor can send
ACKs for other purposes than actually acking the UOR-2, without then
having to continue sending them for all UOR-2. Similarly, compressor
implementations can totally ignore UOR-2 ACKs for the purpose of
adapting the optimistic approach strategies. Current implementation
experience also suggests using that approach, and the recommendation
is thus to not make use of the optional ACK mechanism in O-mode,
neither in compressor nor in decompressor implementations.
For implementers who still want to make use of the optional O-mode
acks, the following clarifications should be taken into account.
Section 5.4.1.1.2 discusses the use of optional UOR-2 ACKs in O-mode,
and explains a conceptual algorithm for a compressor to determine
whether such ACKs can be expected from the decompressor, simply using
a condition based on unspecified parameters k_3 and n_3. However,
what is not clearly pointed out is the importance of being very
careful when implementing this confidence algorithm, as using an
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incorrect expectation on UOR-2 ACKs as a basis for compressor
behavior will significantly degrade the compression performance. If a
compressor implementation wants to adapt its optimistic approach
behavior on potential UOR-2 ACK usage, the confidence algorithm for
determining UOR-2 ACK usage must be carefully designed, with k_3 and
n_3 having values much larger than 1, as UOR-2 ACKs can be sent from
a decompressor for other purposes than to actually acknowledge the
UOR-2 packet, e.g. to send feedback data such as clock resolution, or
to initiate a mode transfer. Before adapting a compressor to the
potential use of UOR-2 ACKs, the implementer must ensure all aspects
are considered by the confidence algorithm.
8.13. Compression of SN in AH and GRE extension headers
The AH and GRE sequence numbers are compressed exactly as the ESP
sequence number. Specifically, the principle for when to include or
exclude the AH and GRE sequence numbers is the same as for ESP, i.e.
the following rule from section 5.8.4.3 applies to all these sequence
numbers:
"Sequence Number: Not sent when the offset from the sequence number
of the compressed header is constant. When the
offset is not constant, the sequence number is
compressed by sending LSBs. See 5.8.4."
9. ROHC negotiation clarifications
According to section 4.1, the link layer must provide means to
negotiate e.g. the channel parameters listed in section 5.1.1. One
of these parameters is the PROFILES parameter, which is said to be a
set of non-negative integers where each integer indicates a profile
supported by the decompressor. This can be interpreted as if it is
sufficient to have a mechanism to announce profile support from
decompressor to compressor. However, things are a little bit more
complicated than that.
Each profile is identified by a 16-bit value, where the 8 LSB bits
indicate the actual profile, and the 8 MSB bits indicate the variant
of that profile (see chapter 8). In the ROHC headers sent over the
link, the profile used is identified only with the 8 LSB bits, which
means that the compressor and decompressor must have agreed on which
variant to use for each profile. This can be done in various ways,
but the negotiation protocol must provide means to do it.
In conclusion, the negotiation protocol must be able to communicate
to the compressor the set of profiles supported by the decompressor,
and when multiple variants of the same profile are available, also
provide means for the decompressor to know which variant will be used
by the compressor. This basically means that the PROFILES set after
negotiation must not include more than one variant of a profile.
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10. PROFILES suboption in ROHC-over-PPP
The logical union of suboptions for IPCP and IPV6CP negotiations, as
specified by ROHC over PPP [2], can not be used for the PROFILES
suboption, as the whole union would then have to be considered within
each of the two IPCP negotiations, to avoid getting an ambiguous
profile set. An implementation of RFC 3241 must therefore ensure the
same profile set is negotiated for both IPv4 and IPv6 (IPCP/IPV6CP).
11. Constant IP-ID encoding in IP-only and UPD-Lite profiles
In the ROHC IP-only profile, section 3.3 of RFC 3843 [3], a mechanism
for encoding of a constant Identification value in IPv4 (constant IP-
ID) is defined. This mechanism is also used by the ROHC UDP-Lite
profiles, RFC 4019 [5].
It should be noted that the "Constant IP-ID" mechanism applies to
both the inner and the outer IP header, when present , meaning that
there will be both a SID and a SID2 context value.
12. Security considerations
This document provides a number of clarifications to [1], but it does
not make any changes or additions to the protocol. As a consequence,
the security considerations of [1] apply without additions.
13. IANA considerations
This document does not require any IANA actions.
14. Acknowledgment
The authors would like to thank Vicknesan Ayadurai, Carsten Bormann,
Mikael Degermark, Zhigang Liu, Abigail Surtees, Mark West, Tommy
Lundemo, Alan Kennington and Remi Pelland for their contributions and
comments. Thanks also to the committed document reviewers, Carl
Knutsson and Biplab Sarkar, who reviewed the document during working
group last-call.
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15. References
15.1. Normative References
[1] C. Bormann, et al., "RObust Header Compression (ROHC) Framework
and four profiles: RTP, UDP, ESP, and uncompressed",
RFC 3095, July 2001.
[2] C. Bormann, "Robust Header Compression (ROHC) over PPP",
RFC 3241, April 2002.
[3] L-E. Jonsson & G. Pelletier, "RObust Header Compression (ROHC):
A Compression Profile for IP", RFC 3843, June 2004.
[4] W. Simpson, "PPP in HDLC-like Framing", RFC 1662, July 1994.
15.2. Informative References
[5] G. Pelletier, "RObust Header Compression (ROHC): Profiles for
User Datagram Protocol (UDP) Lite", RFC 4019, April 2005.
16. Authors' Addresses
Lars-Erik Jonsson
Ericsson AB
Box 920
SE-971 28 Lulea, Sweden
Phone: +46 8 404 29 61
EMail: lars-erik.jonsson@ericsson.com
Kristofer Sandlund
Ericsson AB
Box 920
SE-971 28 Lulea, Sweden
Phone: +46 8 404 41 58
EMail: kristofer.sandlund@ericsson.com
Ghyslain Pelletier
Ericsson AB
Box 920
SE-971 28 Lulea, Sweden
Phone: +46 8 404 29 43
EMail: ghyslain.pelletier@ericsson.com
Peter Kremer
Conformance and Software Test Laboratory
Ericsson Hungary
H-1300 Bp. 3., P.O. Box 107, HUNGARY
Phone: +36 1 437 7033
EMail: peter.kremer@ericsson.com
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Appendix A - Sample CRC algorithm
#!/usr/bin/perl -w
use strict;
#=================================
#
# ROHC CRC demo - Carsten Bormann cabo@tzi.org 2001-08-02
#
# This little demo shows the three types of CRC in use in RFC 3095,
# the robust header compression standard. Type your data in
# hexadecimal form and then press Control+D.
#
#---------------------------------
#
# utility
#
sub dump_bytes($) {
my $x = shift;
my $i;
for ($i = 0; $i < length($x); ) {
printf("%02x ", ord(substr($x, $i, 1)));
printf("\n") if (++$i % 16 == 0);
}
printf("\n") if ($i % 16 != 0);
}
#---------------------------------
#
# The CRC calculation algorithm.
#
sub do_crc($$$) {
my $nbits = shift;
my $poly = shift;
my $string = shift;
my $crc = ($nbits == 32 ? 0xffffffff : (1 << $nbits) - 1);
for (my $i = 0; $i < length($string); ++$i) {
my $byte = ord(substr($string, $i, 1));
for( my $b = 0; $b < 8; $b++ ) {
if (($crc & 1) ^ ($byte & 1)) {
$crc >>= 1;
$crc ^= $poly;
} else {
$crc >>= 1;
}
$byte >>= 1;
}
}
printf "%2d bits, ", $nbits;
printf "CRC: %02x\n", $crc;
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}
#---------------------------------
#
# Test harness
#
$/ = undef;
$_ = <>; # read until EOF
my $string = ""; # extract all that looks hex:
s/([0-9a-fA-F][0-9a-fA-F])/$string .= chr(hex($1)), ""/eg;
dump_bytes($string);
#---------------------------------
#
# 32-bit segmentation CRC
# Note that the text implies this is complemented like for PPP
# (this differs from 8, 7, and 3-bit CRC)
#
# C(x) = x^0 + x^1 + x^2 + x^4 + x^5 + x^7 + x^8 + x^10 +
# x^11 + x^12 + x^16 + x^22 + x^23 + x^26 + x^32
#
do_crc(32, 0xedb88320, $string);
#---------------------------------
#
# 8-bit IR/IR-DYN CRC
#
# C(x) = x^0 + x^1 + x^2 + x^8
#
do_crc(8, 0xe0, $string);
#---------------------------------
#
# 7-bit FO/SO CRC
#
# C(x) = x^0 + x^1 + x^2 + x^3 + x^6 + x^7
#
do_crc(7, 0x79, $string);
#---------------------------------
#
# 3-bit FO/SO CRC
#
# C(x) = x^0 + x^1 + x^3
#
do_crc(3, 0x6, $string);
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This Internet-Draft expires July 23, 2006.
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