Network Working Group Ghyslain Pelletier, Ericsson INTERNET-DRAFT Lars-Erik Jonsson, Ericsson Expires: May 2005 Kristofer Sandlund, Effnet November 12, 2004 RObust Header Compression (ROHC): ROHC over Channels that can Reorder Packets Status of this memo By submitting this Internet-Draft, I (we) certify that any applicable patent or other IPR claims of which I am (we are) aware have been disclosed, and any of which I (we) become aware will be disclosed, in accordance with RFC 3668 (BCP 79). By submitting this Internet-Draft, I (we) accept the provisions of Section 3 of RFC 3667 (BCP 78). Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or cite them other than as "work in progress". The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/lid-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html This document is an individual submission to the IETF. Comments should be directed to the authors. 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. Pelletier, et al. [Page 1] INTERNET-DRAFT ROHC over Reordering Channels October 12, 2004 Table of Contents 1. Introduction.....................................................3 2. Terminology......................................................3 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.......................5 4. Background.......................................................6 4.1. Reordering Channels.........................................6 4.2. Robustness Principles of ROHC...............................6 4.2.1. Optimistic Approach (U/O-mode).........................6 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..............................9 5.1.3. Reordering of Packets in U/O-mode.....................10 5.1.4. Reordering on the Feedback Channel....................10 5.1.5. List Compression......................................10 5.1.6. Reordering and Mode Transitions.......................10 5.2. Consequences of Reordering.................................11 5.2.1. Functionality Incompatible with Reordering............11 5.2.2. Context Damage (Loss of Synchronization)..............12 5.2.3. Detected Decompression Failures (U/O/R-mode)..........12 5.2.4. Undetected Decompression Failures (R-mode only).......12 6. Making ROHC Tolerant against Reordering.........................13 6.1. Properties of ROHC Implementations.........................13 6.1.1. Compressing Headers with Robustness against Reordering13 6.1.1.1. Reordering and the Optimistic Approach...........13 6.1.1.2. Reordering and the Secure Reference Principle....14 6.1.1.3. Robust Selection of Compressed Header............14 6.1.2. Implementing a Reordering Tolerant Decompressor.......15 6.1.2.1. Bi-directional Reliable Mode (R-mode)............15 6.1.2.2. Decompressor Feedback Considerations.............16 6.1.2.3. Considerations for Local Repair Mechanisms.......16 6.2. Specifying ROHC Profiles with Robustness against Reordering16 6.2.1. Profiles with Interpretation Interval Offset p = -1...16 6.2.2. Modifying the Interpretation Interval Offset..........17 6.2.2.1. Example profile for handling reordering..........17 6.2.2.2. Defining the values of p for new profiles........17 6.2.3. TCP Profile Considerations............................18 7. Security Consideration..........................................18 8. IANA Considerations.............................................18 9. Acknowledgments.................................................18 10. Authors' Addresses.............................................18 11. References.....................................................19 Pelletier, et. al [Page 2] INTERNET-DRAFT ROHC over Reordering Channels October 12, 2004 1. Introduction RObust Header Compression (ROHC), RFC 3095 [2], 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; making this assumption made it possible to improve the compression efficiency and the tolerance to packet loss, objecti ves that were on top of the requirements list at the time. 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 even 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. 2. Terminology This document uses terminology consistent with RFC 3759 [3], 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. However, the document discusses possible ways of modifying existing ROHC implementations and/or specifications to address its objectives. In those parts of the document, the key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD, "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as described in RFC 2119 [1]. Pelletier, et. al [Page 3] INTERNET-DRAFT ROHC over Reordering Channels October 12, 2004 ROHC The term "ROHC" herein refers to the following profiles: - 0x0001, 0x0002 and 0x0003 defined in RFC 3095 [2]; - 0x0004 for compression of IP-only headers [5]; - 0x0007 and 0x0008 for compression of UDP-Lite headers [6]. The term "ROHC" excludes the following profiles, which are either not affected by reordering or that have the assumption of in-order delivery as a fundamental requirement for their proper operation: - 0x0000 (uncompressed) [2]; - 0x0005 (LLA) [7] and 0x0105 (R-mode extension to LLA) [8]; 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 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). Updating packet A packet that updates the context of the decompressor, i.e. all packets carrying a CRC calculated over the uncompressed header. Non-updating packet A packet that carries a CRC calculated over the uncompressed header updates the context of the decompressor when it is successfully decompressed. A packet without such a CRC is thus referred to as a non-updating packet. Pelletier, et. al [Page 4] INTERNET-DRAFT ROHC over Reordering Channels October 12, 2004 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, IP-ID or 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 will 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 [2]. Profile 0x0000 (uncompressed) is not affected by reordering, as the headers are sent uncompressed. The solutions also apply to profiles for IP-only (0x0004) [5] and for UDP-Lite (0x0007 and 0x0008) [6]. These profiles are based on the profiles of RFC 3095 [2] 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) [2], 0x0004 (IP-only) [5], and 0x0008 (UDP-Lite) [6]. 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 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 [7] and RFC 3408 [8] 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. Pelletier, et. al [Page 5] INTERNET-DRAFT ROHC over Reordering Channels October 12, 2004 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 [9]. 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 bi-directional 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 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 Pelletier, et. al [Page 6] INTERNET-DRAFT ROHC over Reordering Channels October 12, 2004 packet loss characteristics of the link where header compression is used (see also [2], 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 CRC based on acknowledgements received from the decompressor (see also [2], 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* [2]). 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 LSB encoding method defined in RFC 3095 ([2], section 5.7) specifies the interpretation interval offset, called p, as follow: For profiles 0x0001, 0x0003 and 0x0007: p = 1, when bits(SN) <= 4; p = 2^(bits(SN)-5) - 1 otherwise. The resulting table describing the interpretation interval is: +-----------+--------------+--------------+ | 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 | +-----------+--------------+--------------+ Pelletier, et. al [Page 7] INTERNET-DRAFT ROHC over Reordering Channels October 12, 2004 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 interval interpretation 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 faces a trade-off between the amount of reordering and the number of consecutive packets 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. 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 [2]. 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. Pelletier, et. al [Page 8] INTERNET-DRAFT ROHC over Reordering Channels October 12, 2004 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 smaller 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. 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 with respect to other non-updating packets only, 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 being forwarded to upper layers. Pelletier, et. al [Page 9] INTERNET-DRAFT ROHC over Reordering Channels October 12, 2004 5.1.3. Reordering of Packets in U/O-mode Sequentially late packets The ability to decompress 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. 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. Decompression can always be verified since all U/O-mode packet types are context updating. Consequently, reordering of packets is not deemed problematic when operating in U/O-mode. For channels known to reorder packets, the 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 ([2], 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 <# Editor's Note: This is still for further study. #> <# #> <# #> <# #> 5.1.6. Reordering and Mode Transitions Pelletier, et. al [Page 10] INTERNET-DRAFT ROHC over Reordering Channels October 12, 2004 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 [2], 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, while 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. 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 follow: - 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 are some optional ROHC functions that cannot work in the presence of reordering between ROHC peers. The ROHC segmentation scheme (see [2], 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 is discarded by the decompressor. Therefore segmentation should not be used if there can be reordering between the ROHC peers. Pelletier, et. al [Page 11] INTERNET-DRAFT ROHC over Reordering Channels October 12, 2004 Timer-based compression of RTP TS (see [2], section 4.5.4) is built on an assumption of timely (minimal jitter) delivery. When using this encoding method, the decompressor discards packets arriving with an arrival time that is outside the estimated upper bound for the estimated jitter. Therefore it should be used with care over links where reordering can occur, with respect to the amount of jitter that can be introduced by reordering. The use of these optional features 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 caused by reordering and discard the packet. Note that local repairs, subject to the limitations stated in [2] section 5.3.2.3, 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. Pelletier, et. al [Page 12] INTERNET-DRAFT ROHC over Reordering Channels October 12, 2004 6. Making ROHC Tolerant against Reordering This chapter 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 t he specifications 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 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 damages causing 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. 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 a least one change packet in the sequence of repetitions has reached the decompressor before the first packet sent after this sequence. Pelletier, et. al [Page 13] INTERNET-DRAFT ROHC over Reordering Channels October 12, 2004 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. It can also be based on the knowledge that the decompressor implements the context updating logic of section 6.1.2.1 (e.g. by means of standardization). 6.1.1.3. Robust Selection of Compressed Header The interpretation interval for the LSB encoded sequence number can be adjusted to allow for larger negative offsets (see section 5.1.1). This would provide 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 of section 5.1.1, using eight bits of SN allows a packet to be decompressed when the reordering leads to up to seven units in sequence number variation (i.e. delta(SN)). Increasing the number of SN bits (i.e. using a larger SN_k [2]) transmitted will make ROHC even more tolerant to reordering. Pelletier, et. al [Page 14] INTERNET-DRAFT ROHC over Reordering Channels October 12, 2004 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. Bi-directional Reliable Mode (R-mode) The "missing reference" problem described in section 5.1.2.1 can be avoided. If the decompressor can detect when two updating packets (packets including CRCs) are reordered with respect to each other, it should not update the context with the values of the sequentially late update packet. Pelletier, et. al [Page 15] INTERNET-DRAFT ROHC over Reordering Channels October 12, 2004 6.1.2.2. 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 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 effiency of the link. Note that 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). How ever, the decompressor should not send feedback for sequentially late packets, as the current state of the context will better reflect the compressor context than the content of the reordered packet. 6.1.2.3. Considerations for Local Repair Mechanisms When decompression fails, and if reordering can be the cause of this failure, a local repair may be attempted for the sequentially late packet by going backward in the interpretation interval (as opposed to moving forward as for packet losses). 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) [2], 0x0004 (IP-only) [5], and 0x0008 (UDP-Lite) [6] should redefine how the value of the offset p is determined, and use the same algorithm as in profile 0x0001 [2] 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. Pelletier, et. al [Page 16] INTERNET-DRAFT ROHC over Reordering Channels October 12, 2004 6.2.2. Modifying the Interpretation Interval Offset The interpretation interval offset p could be modified for existing profile to handle reordering while improving the compression efficiency when compared to the solution of 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 RFC3095, the interpretation interval when sending k bits of SN is defined as: 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, while 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 reordering and packet losses, including how these correlate with each Pelletier, et. al [Page 17] INTERNET-DRAFT ROHC over Reordering Channels October 12, 2004 other, should be considered when 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). 6.2.3. TCP Profile Considerations The current draft for the ROHC TCP profile [4] contains packet formats that allow sending as little as 1 bit of MSN (master sequence number). Since the MSN is used in the same fashion as the sequence number in profile 0x0002, it will not be possible to decompress reordered packets if used over a reordering channel. The work on the ROHC-TCP profile should consider using more bits of MSN to enable simple implementation modifications when operating over a reordering channel. 7. Security Consideration This document does not include additional security risks to [2]. 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. IANA Considerations This document does not require any IANA action. 9. Acknowledgments The authors would appreciate feedback on this document, as input from others would certainly help us improve it significantly. Thanks to Ramin Rezaiifar and to Gorry Fairhurst for their editorial comments. 10. 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: ghyslain.pelletier@ericsson.com Pelletier, et. al [Page 18] INTERNET-DRAFT ROHC over Reordering Channels October 12, 2004 Lars-Erik Jonsson Ericsson AB Box 920 SE-971 28 Lulea, Sweden Phone: +46 8 404 29 61 Fax: +46 920 996 21 EMail: lars-erik.jonsson@ericsson.com Kristofer Sandlund Effnet AB Stationsgatan 69 S-972 34 Lulea Sweden Phone: +46 920 609 17 Fax: +46 920 609 27 EMail: kristofer.sandlund@effnet.com 11. References [1] S. Bradner, "Key words for use in RFCs to Indicate Requirement Levels", RFC 2119, March 1997. [2] C. Bormann, et. al, "RObust Header Compression (ROHC): Framework and four profiles: RTP, UDP, ESP, and uncompressed", RFC 3095, July 2001. [3] Jonsson, L-E., " RObust Header Compression (ROHC): Terminology and Channel Mapping Examples", RFC 3759, April 2004. [4] G. Pelletier, et. al, "RObust Header Compression (ROHC): TCP/IP Profile (ROHC-TCP)", Internet-Draft (work in progress), , October 2004. [5] Jonsson, L-E. and G. Pelletier, "RObust Header Compression (ROHC): A compression profile for IP", RFC 3843, June 2004. [6] G. Pelletier, "RObust Header Compression (ROHC): Profiles for UDP-Lite", Internet draft (work in progress), , June 2004. [7] Jonsson, L-E. and G. Pelletier, "RObust Header Compression (ROHC): A Link-Layer Assisted Profile for IP/UDP/RTP", RFC 3242, April 2002. Pelletier, et. al [Page 19] INTERNET-DRAFT ROHC over Reordering Channels October 12, 2004 [8] Liu, Z. and K. Le, "Zero-byte Support for Bidirectional Reliable More (R-mode) in Extended Link-Layer Assisted Profile for RObust Header Compression (ROHC) Profile", RFC 3408, December 2002. [9] Ash, J., Goode B. and J. Hand, "Requirements for Header Compression over MPLS", Internet draft (work in progress), , June 2004. Pelletier, et. al [Page 20] INTERNET-DRAFT ROHC over Reordering Channels October 12, 2004 Intellectual Property Statement The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the IETF's procedures with respect to rights in IETF Documents can be found in RFC 3667 (BCP 78) and RFC 3668 (BCP 79). Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf- ipr@ietf.org. Copyright Statement Copyright (C) The Internet Society (2004). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. Disclaimer of Validity This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM 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 May 12, 2005. Pelletier, et. al [Page 21]