Network Working Group G. Pelletier

Network Working Group G. Pelletier
Request for Comments: 4224 L-E. Jonsson
Category: Informational K. Sandlund
Ericsson
January 2006

RObust Header Compression (ROHC):
ROHC over Channels That Can Reorder Packets

Status of This Memo

This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (2006).

Abstract

RObust Header Compression (ROHC), RFC 3095, defines a framework for
header compression, along with a number of compression protocols
(profiles). One operating assumption for the profiles defined in RFC
3095 is that the channel between compressor and decompressor is
required to maintain packet ordering. This document discusses
aspects of using ROHC over channels that can reorder packets. It
provides guidelines on how to implement existing profiles over such
channels, as well as suggestions for the design of new profiles.

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Table of Contents

1. Introduction ....................................................3
2. Terminology .....................................................4
3. Applicability of This Document to ROHC Profiles .................5
3.1. Profiles within Scope ......................................5
3.2. Profiles with Special Considerations .......................5
3.3. Profiles Incompatible with Reordering ......................6
4. Background ......................................................6
4.1. Reordering Channels ........................................6
4.2. Robustness Principles of ROHC ..............................6
4.2.1. Optimistic Approach (U/O-mode) ......................7
4.2.2. Secure Reference Principle (R-mode) .................7
5. Problem Description .............................................7
5.1. ROHC and Reordering Channels ...............................7
5.1.1. LSB Interpretation Interval and Reordering ..........7
5.1.2. Reordering of Packets in R-mode .....................9
5.1.2.1. Updating Packets ...........................9
5.1.2.2. Non-Updating Packets ......................10
5.1.3. Reordering of Packets in U/O-mode ..................10
5.1.4. Reordering on the Feedback Channel .................11
5.1.5. List Compression ...................................11
5.1.6. Reordering and Mode Transitions ....................12
5.2. Consequences of Reordering ................................13
5.2.1. Functionality Incompatible with Reordering .........13
5.2.2. Context Damage (Loss of Synchronization) ...........13
5.2.3. Detected Decompression Failures (U/O/R-mode) .......13
5.2.4. Undetected Decompression Failures (R-mode only) ....14
6. Making ROHC Tolerant against Reordering ........................14
6.1. Properties of ROHC Implementations ........................14
6.1.1. Compressing Headers with Robustness against
Reordering .........................................14
6.1.1.1. Reordering and the Optimistic Approach ....15
6.1.1.2. Reordering and the Secure
Reference Principle .......................15
6.1.1.3. Robust Selection of Compressed Header .....15
6.1.2. Implementing a Reordering-Tolerant Decompressor ....16
6.1.2.1. Decompressor Feedback Considerations ......16
6.1.2.2. Considerations for Local Repair
Mechanisms ................................17
6.2. Specifying ROHC Profiles with Robustness against
Reordering ................................................17
6.2.1. Profiles with Interpretation Interval
Offset p = -1 ......................................17
6.2.2. Modifying the Interpretation Interval Offset .......18
6.2.2.1. Example Profile for Handling Reordering ...18
6.2.2.2. Defining the Values of p for New
Profiles ..................................18

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7. Security Considerations ........................................19
8. Acknowledgements ...............................................19
9. Informative References .........................................19

1. Introduction

RObust Header Compression (ROHC), RFC 3095 [1], defines a framework
for header compression, along with a number of compression protocols
(profiles). One operating assumption for the profiles defined in RFC
3095 is that the channel between compressor and decompressor is
required to maintain packet ordering for each compressed flow. The
motivation behind this assumption was that the primary candidate
channels considered did guarantee in-order delivery of header-
compressed packets. This assumption made it possible to meet the
design objectives that were on top of the requirements list at the
time when ROHC was being designed, namely to improve the compression
efficiency and the tolerance to packet losses.

Since the publication of RFC 3095 in 2001, the question about ROHC
operation over channels that do not guarantee in-order delivery has
surfaced several times; arguments that ROHC cannot perform adequately
over such channels have been heard. Specifically, this has been
raised as a weakness when compared to other header compression
alternatives, as RFC 3095 explicitly states its inability to operate
if in-order delivery is not guaranteed. For those familiar with the
details of ROHC and of other header compression schemes, it is clear
that this is a misconception, but it can also be easily understood
that the wording used in RFC 3095 can lead to such interpretation.

This document discusses the various aspects of implementing ROHC over
channels that can reorder header-compressed packets. It explains
different ways of implementing the profiles found in RFC 3095, as
well as other profiles based on those profiles, over reordering
channels. This can be achieved either by ensuring that compressor
implementations use compressed headers that are sufficiently robust
to the expected possible reordering and/or by modifying decompressor
implementations to tolerate reordered packets. Ideas regarding how
existing profiles could be updated and how new profiles can be
defined to cope efficiently with reordering are also discussed.

In some scenarios, there might be external means (such as a sequence
number) to detect and potentially correct reordering. That is, for
example, the case when running compression over an IPsec
Encapsulating Security Payload (ESP) tunnel. With such external
means to detect reordering, the decompressor can be modified to make
use of the external information provided, and reordering can then be
handled. How to make use of external means to address reordering is,
however, out of scope for this document.

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2. Terminology

This document uses terminology consistent with RFC 3759 [2], and is
in itself only informative. Although it does discuss technical
aspects of implementing the ROHC specifications in particular
environments, it does not specify any new technology.

ROHC

The term "ROHC" herein refers to the following profiles:

- 0x0001, 0x0002, and 0x0003 defined in RFC 3095 [1];
- 0x0004 for compression of IP-only headers [3];
- 0x0007 and 0x0008 for compression of UDP-Lite headers [4].

The term "ROHC" excludes the following profiles, which are either
not affected by reordering or have the assumption of in-order
delivery as a fundamental requirement for their proper operation:

- 0x0000 (uncompressed) [1];
- 0x0005 (Link-Layer Assisted (LLA)) [5] and 0x0105
(R-mode extension to LLA) [6];

Reordering

A type of transmission taking place between compressor and
decompressor where in-order delivery of header-compressed packets
is not guaranteed.

Reordering channel

A connection over which reordering, as defined above, can occur.

Sequentially early packet

A packet that reaches the decompressor before one or several
packets of the same context identifier (CID) that were delayed on
the link. At the time of the arrival of a sequentially early
packet, the packet(s) delayed on the link cannot be differentiated
from lost packet(s).

Sequentially late packet

A packet is late within its sequence if it reaches the
decompressor after one or several other packets belonging to the
same CID have been received, although the sequentially late packet
was sent from the compressor before the other packet(s).

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Updating packet

A packet that updates the context of the decompressor, e.g., all
packets except R-0 and R-1* in RFC 3095 [1].

Non-updating packet

A packet that does not update the context of the decompressor,
e.g., only R-0 and R-1* in RFC 3095 [1].

Change packet

A packet that updates one or more fields of the context other than
the fields pertaining to the functions established with respect to
the sequence number (SN). Specifically, it is a packet that
updates fields other than the SN, the IPv4 identifier (IP-ID), the
sequence number of an extension header or the RTP timestamp (TS).

3. Applicability of This Document to ROHC Profiles

This document addresses general reordering issues for ROHC profiles.
The foremost objectives are to ensure that ROHC implementations do
not forward packets with incorrectly decompressed headers to upper
layers, as well as to limit the possible increase in the rate of
decompression failures or in events leading to context damage, when
compression is applied over reordering channels.

3.1. Profiles within Scope

The following sections outline solutions that are generally
applicable to profiles 0x0001 (RTP), 0x0002 (UDP), and 0x0003 (ESP)
defined in RFC 3095 [1]. Profile 0x0000 (uncompressed) is not
affected by reordering, as the headers are sent uncompressed. The
solutions also apply to profiles for IP-only (0x0004) [3] and for
UDP-Lite (0x0007 and 0x0008) [4]. These profiles are based on the
profiles of RFC 3095 [1] and inherently make the same in-order
delivery assumption.

3.2. Profiles with Special Considerations

Special considerations are needed to make some of the implementation
solutions of sections 6.1 and 6.2 applicable to profiles 0x0002 (UDP)
[1], 0x0004 (IP-only) [3], and 0x0008 (UDP-Lite) [4]. For these
profiles, the SN is generated at the compressor, as it is not present
in headers being compressed. For the least significant bit (LSB)
encoding method, the interpretation interval offset (p) is always
p = -1 (see section 5.1.1) when interpreting the SN. The SN is thus

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required to increase for each packet received at the decompressor,
which means that reordered packets cannot be decompressed.

3.3. Profiles Incompatible with Reordering

The ROHC LLA profiles defined in RFC 3242 [5] and RFC 3408 [6] have
been explicitly designed with in-order delivery as a fundamental
requirement to their proper operation. Profiles 0x0005 and 0x0105
can therefore not be implemented over channels where reordering can
occur; this document therefore does not apply to these profiles.

4. Background

ROHC was designed with the assumption that packets are delivered in
order from compressor to decompressor. This was considered as a
reasonable working assumption for links where it was expected that
ROHC would be used. However, many have expressed that it would be
desirable to use ROHC also over connections where in-order delivery
is not guaranteed [7].

4.1. Reordering Channels

The reordering channels that are potential candidates to use ROHC are
single-hop channels and multi-hop virtual channels.

A single-hop channel is a point-to-point link that constitutes a
single IP hop. Note that one IP hop could be one or multiple
physical links. For example, a single-hop reordering channel could
be a wireless link that applies error detection and performs
retransmissions to guarantee error-free delivery of all data.
Another example could be a wireless connection that performs
bicasting of data during a handoff procedure.

A multi-hop virtual channel is a virtual point-to-point link that
traverses multiple IP hops. A multi-hop virtual channel would
typically be an IP tunnel, where compression is applied over the
tunnel by the endpoints of the tunnel (not to be confused with single
link compression of tunneled packets).

4.2. Robustness Principles of ROHC

Robustness is based on the optimistic approach in the unidirectional
and optimistic modes of operation (U/O-mode), and on the secure
reference principle in the bidirectional reliable mode (R-mode).
Both approaches have different characteristics in the presence of
reordering between compressor and decompressor. However, in any
mode, decompression of sequentially early packets will generally be

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handled quite well since they will be perceived and treated by the
decompressor as if there had been one or more packet losses.

4.2.1. Optimistic Approach (U/O-mode)

A ROHC compressor uses the optimistic approach to reduce header
overhead when performing context updates in U/O-mode. The compressor
normally repeats the same update until it is fairly confident that
the decompressor has successfully received the information. The
number of consecutive packets needed to obtain this confidence is
open to implementations, and this number is normally related to the
packet loss characteristics of the link where header compression is
used (see also [1], section 5.3.1.1.1).

All packet types used in U/O-mode are context updating.

4.2.2. Secure Reference Principle (R-mode)

A ROHC compressor uses the secure reference principle in R-mode to
ensure that context synchronization between ROHC peers cannot be lost
due to packet losses. The compressor obtains its confidence that the
decompressor has successfully updated the context from a packet
carrying a 7- or 8-bit Cyclic Redundancy Check (CRC) based on
acknowledgements received from the decompressor (see also [1],
section 5.5.1.2).

The secure reference principle makes it possible for a compressor to
use packets that do not update the context (i.e., R-0 and R-1* [1]).

5. Problem Description

5.1. ROHC and Reordering Channels

This section reviews different aspects of ROHC susceptible of being
impacted by reordering of compressed packets between ROHC peers.

5.1.1. LSB Interpretation Interval and Reordering

The least significant bit (LSB) encoding method defined in RFC 3095
([1], section 5.7) specifies the interpretation interval offset,
called p, as follows:

For profiles 0x0001, 0x0003, and 0x0007:

p = 1, when bits(SN) <= 4;
p = 2^(bits(SN)-5) - 1 otherwise.

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The resulting table describing the interpretation interval is as
follows:

+-----------+--------------+--------------+
| bits (SN) | Offset p | (2^k-1) - p |
| k | (reordering) | (losses) |
+-----------+--------------+--------------+
| 4 | 1 | 14 |
| 5 | 0 | 31 |
| 6 | 1 | 62 |
| 7 | 3 | 124 |
| 8 | 7 | 248 |
| 9 | 15 | 496 |
+-----------+--------------+--------------+

As shown in the table above, the ability for ROHC to handle
sequentially late packets depends on the number of bits sent in
each packet. For example, a sequentially late packet of type 0
(with either 4 or 6 bits of SN) sets the limit to one packet out
of sequence for successful decompression to be possible.

For profiles 0x0002, 0x0004, and 0x0008:

p = - 1, independently of bits(SN).

A value of p = -1 means that the interpretation interval offset
can only take positive values and that no sequentially late packet
can be decompressed if reordering occurs over the link.

The trade-off between reordering and robustness

The ability of ROHC to handle sequentially late packets is limited
by the interpretation interval offset of the sliding window used
for LSB encoding. This offset has a very small value for packets
with a small number of sequence number (SN) bits, but grows with
the number of SN bits transmitted.

For channels where both packet losses and reordering can occur,
modifications to the interpretation interval face a trade-off
between the amount of reordering and the number of consecutive
packet losses that can be handled by the decompressor. If the
negative offset (i.e., p) is increased to handle a larger amount
of reordering, the value of the positive offset of the
interpretation interval must be decreased. This may impact the
compression efficiency when the channel has a high loss rate.

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This is shown in the figure:

<--- interpretation interval (size is 2^k) ---->
|------------------+---------------------------|
Lower v_ref Upper
Bound Bound
<--- reordering --> <--------- losses --------->
max delta(SN) = p max delta(SN) = (2^k-1) - p

where v_ref is the reference value as per [1], section 4.5.1.

In practice, the maximum variation in SN value (max delta(SN)) due
to reordering that can be handled will normally correspond to the
maximum number of packets that can be reordered. The same applies
to the maximum number of consecutive packet losses covered by the
robustness interval.

Timer-based compression of RTP TS (see [1], section 4.5.4) provides
means to reduce the number of timestamp bits needed in compressed
headers after longer gaps in the packet stream (e.g., for an audio
stream, this is typically due to silence suppression). To use
timer-based compression, an upper limit on the inter-arrival jitter
must be reliably estimated by the compressor. It should be noted
that although the risk of reordering of course means there is a more
significant jitter on the path between the compressor and the
decompressor, there are no special reordering considerations for
timer-based compression. It all still boils down to the task of
estimating the jitter, requiring channel characteristics knowledge at
the compressor, and/or jitter estimation figures received from the
decompressor.

5.1.2. Reordering of Packets in R-mode

5.1.2.1. Updating Packets

The compressor always adds references in the sliding window for all
updating packets sent. The compressor removes values older than
values for which it has received an acknowledgement to shrink the
window and thereby increase the compression efficiency.

The decompressor always updates the context when receiving an
updating packet and uses the new reference for decompression.
Acknowledgements are sent to allow the compressor to shrink its
sliding window.

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Reordering between updating packets

The decompressor can update its context from the reception of a
sequentially late updating packet. The decompressor reference is
then updated with a value that is no longer in the sliding window
of the compressor. This "missing reference" can be caused by
reordering when operating in R-mode.

The result is that the compressor and the decompressor lose
synchronization with each other. When the decompressor
acknowledges the sequentially late packet, the compressor might
already have discarded the reference to this sequence number, and
continue to compress packets based on more recent references (in
packet arrival time). Decompression will then be attempted using
the wrong reference.

5.1.2.2. Non-Updating Packets

Reordering between non-updating packets only

A non-updating packet that reaches the decompressor out of
sequence only with respect to other non-updating packets can
always be decompressed properly.

Reordering between non-updating packets and updating packets

When a non-updating packet is reordered and becomes sequentially
late with respect to an updating packet, the decompressor may have
already updated the context with a new reference when the late
packet is received. It is thus possible for a non-updating packet
to be decompressed based on the wrong reference because of
reordering when operating in R-mode.

Since decompression of non-updating packets cannot be verified,
this can lead to a packet erroneously decompressed to be forwarded
to upper layers.

5.1.3. Reordering of Packets in U/O-mode

Reordering between non-change packets only

When only non-change packets are reordered with respect to each
other, decompression of sequentially late packets is limited by
the offset p of the interpretation interval (see section 5.1.1).
Decompression of a sequentially late packet with SN = x is
possible if the value of the SN of the packet that last updated
the context was less than or equal to x + p.

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Problems occur if context(SN) has increased by more than p with
respect to field(SN) carried within the packet to decompress.

This means that for a well-behaved stream with a constant unit
increase in the RTP SN, a packet can arrive up to p packets out of
sequence and still be correctly decompressed. Otherwise, it
cannot be properly decompressed. It also means that if the
compressor sends two consecutive packets with SN(packet1)=100 and
SN(packet2)=108 when p=7, packet1 cannot be decompressed if it
arrives even one packet late due to reordering.

Reordering involving change packets

When a packet is reordered and becomes sequentially late with
respect to a change packet, decompression of the late packet may
eventually fail, as the context information required for
successful decompression may not be available anymore.

Decompression can always be verified since all U/O-mode packet types
are context updating. Consequently, a failure to decompress a packet
that is caused by reordering can be detected, and context
invalidation due to reordering can thus be avoided. The risk of
forwarding incorrectly decompressed packets to upper layers is
therefore small when operating in U/O-mode. For channels known to
reorder packets, U/O-mode should therefore be the preferred mode of
operation. The additional risk of losing context synchronization, or
for erroneous packet to be delivered to upper layers, is limited.

5.1.4. Reordering on the Feedback Channel

For R-mode, upon reception of an acknowledgement, the compressor
searches the sliding window to locate an updating packet with the
corresponding SN; if it is not found, the acknowledgement is invalid
and is discarded ([1], section 5.5.1.2). In other words, feedback
received out of order either is still useful or is discarded.

In U/O-mode, if the compressor updates its context based on feedback,
the same logic as for R-mode applies in practice.

Reordering on the feedback channel has thus no impact in either mode.

5.1.5. List Compression

ROHC list compression is an additional compression scheme for RTP
contributing source (CSRC) lists and IP extension header chains. The
base is called table-based item compression, and it is almost
completely independent from the rest of the ROHC compression logic.
Therefore, this part of the scheme does not exhibit any special

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vulnerabilities when it comes to reordering, assuming a reasonable
optimistic approach is used in U/O-mode. Specifically, it does not
suffer significantly from the "missing reference" problem when
operating in R-mode.

On top of the table-based item compression mechanism, an additional
compression technique may be used, called reference based list
compression. Reference based list compression however has a logic
that is similar to the rest of the ROHC compression logic, and
therefore it suffers from similar reordering vulnerabilities,
especially the "missing reference" problem of R-mode. Note, however,
that the generation identifier used in U/O-mode makes that scheme
more robust to reordering.

When using list encoding type 1, 2, or 3, which makes use of
reference lists, decompression will succeed only if all individual
items are known by the decompressor, along with the correct reference
list required to properly decompress the packet. List compression
using the "Generic scheme", also known as "Encoding type 0", is not
using reference based list compression, and type 0 decompression will
thus succeed as long as all individual items are known by the
decompressor. Because of this, type 0 list compression should be the
preferred method used when operating over reordering channels.

5.1.6. Reordering and Mode Transitions

Transition from U/O-mode to R-mode

This transition can be affected by reordering if a packet type 0
(UO-0) is reordered and delayed by at least one round-trip time
(RTT). If the decompressor initiates a mode change request to
R-mode in the meantime, the reordered UO-0 packet may be handled
as an R-0 packet; it can be erroneously decompressed and forwarded
to upper layers. This is because the decompressor can switch to
R-mode as soon as it sends the acknowledgement Ack(SN, R) to the
compressor (see also [1], section 5.6).

Transition from R-mode to U/O-mode

A similar situation as above can occur during this transition.
However, because the outcome of the decompression is always
verified using a CRC verification in U/O-mode, the reordered
packet will most likely fail decompression and will be discarded.

The above situation, although it is not deemed to occur frequently,
is still possible; thus, mode transitions from U/O-mode to R-mode
should be avoided when reordering can occur.

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5.2. Consequences of Reordering

The context updating properties of the packets exchanged between ROHC
peers are the most important factors to consider when deriving the
impacts of reordering. For this reason, the robustness properties of
the U/O-mode and of the R-mode are affected differently.

The effects of reordering on ROHC can be summarized as follows:

- Functionality incompatible with reordering;
- Increased probability of context damage (loss of synchronization);
- Increased number of decompression failures - Detected (U/O/R-mode);
- Increased number of decompression failures - Undetected (R-mode).

5.2.1. Functionality Incompatible with Reordering

There is one optional ROHC function that cannot work in the presence
of reordering between ROHC peers.

The ROHC segmentation scheme (see [1], section 5.2.5) relies entirely
on the in-order delivery of each segment, as there is no sequencing
information in the segments. A segmented packet for which one (or
more) segment is received out of order cannot be decompressed, and it
is discarded by the decompressor. Therefore, segmentation should not
be used if there can be reordering between the ROHC peers.

The use of this optional feature is open to implementations and is
local to the compressor only; it does not impact the decompressor.

5.2.2. Context Damage (Loss of Synchronization)

Reordering of packets between ROHC peers can impact the robustness
properties of the optimistic approach (U/O-mode) as well as the
reliability of the secure reference principle (R-mode).

The successful decompression of a sequentially late change packet
(U/O-mode) and/or updating packet (R-mode) can update the context of
the decompressor in a manner unexpected by the compressor. This can
lead to a loss of context synchronization between the ROHC peers.

5.2.3. Detected Decompression Failures (U/O/R-mode)

Reordering of packets between ROHC peers can lead to an increase in
the number of decompression failures for context updating packets
(see sections 5.1.2.1 and 5.1.3). Fortunately, as the outcome of the
decompression of updating packets can be verified, the decompressor
can reliably detect decompression failures, including those caused by
reordering, and discard the packet. Note that local repairs, subject

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to the limitations stated in [1] section 5.3.2.2.3, can still be
performed.

5.2.4. Undetected Decompression Failures (R-mode only)

Reordering of packets between ROHC peers can lead to an increase in
the number of decompression errors for non-updating packets. For
R-mode, decompression of R-0 and R-1* packets cannot be verified. If
reordering occurs and decompression is performed using the wrong
secure reference (see section 5.1.2.1 and 5.1.2.2), the decompressor
cannot reliably detect such errors. As a result, erroneous packets
may be forwarded to upper layers.

6. Making ROHC Tolerant against Reordering

This section describes different approaches that can improve the
performance of ROHC when used over reordering channels and minimize
the effects of reordering. Examples are provided to guide
implementers and designers of new profiles. The solutions target
either the properties of ROHC implementations or the specification of
profiles. This is covered by sections 6.1 and 6.2, respectively.

6.1. Properties of ROHC Implementations

Existing ROHC profiles can be implemented with the capability to
properly handle packet reordering. The methods described in this
section conform with, and thus do not require any modifications to,
the ROHC specifications within scope of this document (see section
3). Specifically, the methods presented in this section can be
implemented without any impairment to interoperability with other
ROHC implementations that do not use these methods.

The methods suggested here may, however, lower the compression
efficiency, and these modifications should not be used when
reordering is known not to occur. Some of these methods aim to
increase the decompression success rate at the decompressor, while
others aim to avoid context damage that would cause a loss of context
synchronization between compressor and decompressor.

The methods proposed are each addressing specific issues listed in
section 5 and can be combined to achieve better robustness against
reordering.

6.1.1. Compressing Headers with Robustness against Reordering

The methods described in this section are methods local only to the
compressor implementation. They can be used without modifications or
impact to the decompressor.

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6.1.1.1. Reordering and the Optimistic Approach

The optimistic approach is affected by the reordering characteristics
of the channel when operating over a reordering channel. Compressor
implementations should therefore adjust their optimistic approach
strategy to match both packet loss and reordering characteristics.

For example, the number of repetitions for each context update can be
increased. The compressor should ensure that each update is repeated
until it is reasonably confident that at least one change packet in
the sequence of repetitions has reached the decompressor before the
first packet sent after this sequence.

6.1.1.2. Reordering and the Secure Reference Principle

Fundamental to the secure reference principle is that only values
acknowledged by the decompressor can be used as reference for
compression. In addition, some of the packet types used in R-mode do
not include a CRC over the original uncompressed header, and the
decompressor has no means to verify the outcome of the decompression.

Decompression of non-updating packet types thus entirely relies on
the cumulative effect of previous updates to the secure reference,
and the compressed data is based on the current value of the
reference. This reference must be synchronized between ROHC peers.
For R-0 and R-1* packets, the reception of the encoded bits applied
to the secure reference is sufficient for correct decompression, but
only when in-order delivery between ROHC peers is guaranteed.

Avoiding the "missing reference" problem (section 5.1.2.1)

A compressor implementation can delay the advance in the sliding
window to a reference acknowledged by the decompressor, until it
has confidence that no acknowledgement for any of the values that
could be discarded can be received. This confidence can be based
on the maximum delay that reordering can introduce over the
channel.

6.1.1.3. Robust Selection of Compressed Header

Packet formats can be chosen with an interpretation interval for the
LSB encoded sequence number that allows for larger negative offsets
(see section 5.1.1). This provides the capability to decompress
sequentially late packets with a greater amount of reordering.

To achieve this, the compressor should be implemented conservatively
in terms of the choice of packet types to send, by transmitting
packets with more sequence number bits. As shown in the table in

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section 5.1.1, using 8 bits of SN allows a packet to be decompressed
when the reordering leads to up to 7 units in sequence number
variation (i.e., delta(SN)). Increasing the number of SN bits (i.e.,
using a larger SN_k [1]) transmitted will make ROHC even more
tolerant to reordering.

For example, a conservative compressor implementation could use the
packet types as shown in the table below:

+----------------------+-------------------------+
| Optimal Packet Type | Alternative Packet Type |
| (without reordering) | (reordering possible) |
+----------------------+-------------------------+
| UO-0 | UOR-2*-ext0 |
| R-0 | R-1*-ext0 |
| R-0-CRC | UOR-2*-ext0 |
| R-1* | R-1*-ext0 |
| UO-1 | UOR-2-ext0 |
| UO-1-TS | UOR-2-TS-ext0 |
| UO-1-ID | UO-1-ID-ext3 (with S=1) |
| | UOR-2-ID-ext0 |
| UOR-2* | UOR-2*-ext0 |
+----------------------+-------------------------+

Such a compressor implementation would thus always be sending at
least 3 octets (R-mode) or 4 octets (U/O-mode). This is a trade-off
when compared to the 1 octet that can be sent by a more aggressive
implementation operating on a channel with no reordering.

Note that since the interpretation interval for profiles 0x0002,
0x0004, and 0x0008 is always p = -1 independently of bits(SN), the
methods suggested in this section will not work for these profiles
unless this value is modified (section 6.2.1).

6.1.2. Implementing a Reordering-Tolerant Decompressor

The methods described in this section are methods local only to the
decompressor implementation. They can be used without modifications
or impact to the compressor.

6.1.2.1. Decompressor Feedback Considerations

Reducing the feedback rate when the flow behaves linearly

The decompressor should reduce its feedback rate when a large
number of UOR-2 packets with extensions are received, when the
flow behaves linearly (i.e., when only fields pertaining to the

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functions established with respect to the sequence number are
changing).

In particular, if the compressor implementation makes a more
conservative selection of packet types (section 6.1.1.3) in order
to handle reordering, the decompressor should try to avoid sending
more feedback than it would for the case where the more optimal
packet types are used. This can be useful to minimize the usage
of the feedback channel, thereby improving efficiency of the link.

Note that even if the decompressor does not make this adjustment
to its feedback rate, packet losses or context damages will not
increase.

Acknowledgements and sequentially late packets

Reordered feedback (or feedback for packets received out of order)
will not cause problems (see section 5.1.4). However, the
decompressor should not send acknowledging feedback for a packet
that can be identified as being sequentially late (e.g., based on
the sequence number of the packet), as the current state of the
context will better reflect the compressor context than the
content of the reordered packet.

6.1.2.2. Considerations for Local Repair Mechanisms

When decompression fails, and if reordering can be assumed to be the
cause of this failure, subsequent decompressions may be attempted for
sequentially late packets by going backward in the interpretation
interval (as opposed to moving forward for local repair). If one of
the decompression attempts is successful, the late packet may be
passed on to upper layers with or without updating the decompressor
context. If the subsequent decompression attempt fails, the packet
should be handled according to [1] section 5.3.2.2.3.

6.2. Specifying ROHC Profiles with Robustness against Reordering

6.2.1. Profiles with Interpretation Interval Offset p = -1

New revisions of profiles 0x0002 (UDP) [1], 0x0004 (IP-only) [3], and
0x0008 (UDP-Lite) [4] should redefine how the value of the offset p
is determined, and use the same algorithm as in profile 0x0001 [1]
instead of p = -1 independently of bits(SN) (section 5.1.1).

While such a change would make these updated profiles slightly less
robust to packet losses, they would still be no less robust than
profile 0x0001.

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6.2.2. Modifying the Interpretation Interval Offset

The interpretation interval offset p could be modified for existing
profiles to handle reordering while improving the compression
efficiency when compared to the solution in section 6.1.1.3.

6.2.2.1. Example Profile for Handling Reordering

The value of the interpretation interval offset p can be adjusted to
achieve a robustness against reordering similar to the effect of
selecting packet types as suggested in section 6.1.1.3.

Consider a scenario where robustness against packet losses is kept a
priority, and for which of a value p=7 is deemed enough. In this
case, a ratio where the positive offset is about twice as large as
the negative offset can be used. This leaves a value of p = 2^k/ 3.

The resulting values are shown in the following table:

+-----------+--------------+----------------+
| bits (SN) | Offset p | Positive range |
| k | (reordering) | (losses) |
+-----------+--------------+----------------+
| 4 | 5 | 10 |
| 5 | 10 | 21 |
| 6 | 21 | 42 |
| 7 | 42 | 85 |
| 8 | 85 | 170 |
| 9 | 170 | 341 |
+-----------+--------------+----------------+

Using this value for p, a fair amount of reordering can be handled
without having to send UOR-2 packets most of the time. The trade-off
is that this is at the expense of robustness against packet losses.

6.2.2.2. Defining the Values of p for New Profiles

As described in RFC 3095 [1], the interpretation interval when
sending k bits of SN is defined as follows:

f(v_ref, k) = [v_ref - p, v_ref + (2^k - 1) - p]

The negative bound (v_ref - p) limits the ability to handle
reordering, and the positive bound (v_ref + (2^k - 1) - p) limits the
ability to handle packet losses.

Adjusting p will increase one of these ranges, while the other range
will decrease. This trade-off between the capability to handle

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reordering and packet losses, including how these correlate with each
other, should be considered in a ROHC profile that is meant to handle
reordering.

For example, if it is desirable for a profile to be as robust against
reordering (negative range) and against packet losses (positive
range), this range can be made equal by setting p near (2^k / 2).

7. Security Considerations

This document does not include additional security risks to [1]. In
addition, it may lower risks related to context damage in R-mode with
injected packets when sequentially late packets do not update the
context (section 6.1.2.1).

8. Acknowledgements

Thanks to the committed WG document reviewers, Carl Knutsson and Mark
West, for their review efforts. Thanks also to Aniruddha Kulkarni,
Ramin Rezaiifar, and Gorry Fairhurst for their constructive comments.

9. Informative References

[1] Bormann, C., Burmeister, C., Degermark, M., Fukushima, H.,
Hannu, H., Jonsson, L-E., 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.

[2] Jonsson, L-E., "RObust Header Compression (ROHC): Terminology
and Channel Mapping Examples", RFC 3759, April 2004.

[3] Jonsson, L-E. and G. Pelletier, "RObust Header Compression
(ROHC): A Compression Profile for IP", RFC 3843, June 2004.

[4] Pelletier, G., "RObust Header Compression (ROHC): Profiles for
User Datagram Protocol (UDP) Lite", RFC 4019, April 2005.

[5] Jonsson, L-E. and G. Pelletier, "RObust Header Compression
(ROHC): A Link-Layer Assisted Profile for IP/UDP/RTP", RFC 3242,
April 2002.

[6] Liu, Z. and K. Le, "Zero-byte Support for Bidirectional Reliable
Mode (R-mode) in Extended Link-Layer Assisted RObust Header
Compression (ROHC) Profile", RFC 3408, December 2002.

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[7] Ash, J., Goode, B., Hand, J., and R. Zhang, "Requirements for
Header Compression over MPLS", RFC 4247, November 2005.

Authors' Addresses

Ghyslain Pelletier
Ericsson AB
Box 920
SE-971 28 Lulea, Sweden

Phone: +46 8 404 29 43
Fax: +46 920 996 21
EMail: [email protected]

Lars-Erik Jonsson
Ericsson AB
Box 920
SE-971 28 Lulea, Sweden

Phone: +46 8 404 29 61
Fax: +46 920 996 21
EMail: [email protected]

Kristofer Sandlund
Ericsson AB
Box 920
SE-971 28 Lulea, Sweden

Phone: +46 8 404 41 58
Fax: +46 920 996 21
EMail: [email protected]

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