< draft-ietf-rohc-rtp-lla-02.txt		draft-ietf-rohc-rtp-lla-03.txt >
			A new Request for Comments is now available in online RFC libraries.
	Network Working Group Lars-Erik Jonsson		RFC 3242
	INTERNET-DRAFT Ghyslain Pelletier
	Expires: March 2002 Ericsson
	September 12, 2001
	A Link-Layer Assisted ROHC Profile for IP/UDP/RTP
	<draft-ietf-rohc-rtp-lla-02.txt>
	Status of this memo
	This document is an Internet-Draft and is in full conformance with
	all provisions of Section 10 of RFC2026.
	Internet-Drafts are working documents of the Internet Engineering
	Task Force (IETF), its areas, and its working groups. Note that other
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	This document is a submission of the IETF ROHC WG. Comments should be
	directed to its mailing list, rohc@cdt.luth.se.
	Abstract
	This document defines a ROHC profile for compression of IP/UDP/RTP
	packets, utilizing functionality provided by the lower layers to
	increase compression efficiency by completely eliminating the header
	for most packets during normal operation. The profile is built as an
	extension to the ROHC RTP profile [ROHC]. It defines additional
	mechanisms needed in ROHC, states requirements on the assisting layer
	to guarantee transparency, and specifies general logic for
	compression and decompression making use of this header-free packet.
	Table of contents
	1. Introduction....................................................3
	2. Terminology.....................................................5
	3. Overview of the link-layer assisted profile.....................5
	3.1. Providing packet type identification.....................6
	3.2. Replacing the sequence number............................6
	3.3. CRC replacement..........................................7
	3.4. Applicability of this profile............................7
	4. Additions and exceptions compared to ROHC RTP...................8
	4.1. Additional packet types..................................8
	4.1.1. No-Header Packet (NHP)..........................8
	4.1.2. Context Synchronization Packet (CSP)............8
	4.1.3. Context Check Packet (CCP)......................9
	4.2. Interfaces towards the assisting layer..................10
	4.2.1. Interface, compressor to assisting layer.......11
	4.2.2. Interface, assisting layer to decompressor.....12
	4.3. Optimistic approach agreement (U/O-mode)................12
	4.4. Specific notes on reliable mode.........................13
	4.5. Fast context initialization, IR redefinition............13
	4.6. Feedback option, CV-REQUEST.............................14
	4.7. Periodic context verification...........................14
	4.8. Use of context identifier...............................15
	5. Implementation issues..........................................15
	5.1. Implementation parameters and signals...................15
	5.1.1. Implementation parameters at the compressor....15
	5.1.2. Implementation parameters at the decompressor..17
	5.2. Implementation over various link technologies...........17
	6. IANA considerations............................................18
	7. Security considerations........................................18
	8. Acknowledgements...............................................18
	9. References.....................................................18
	10. Authors' addresses.............................................19
	1. Introduction
	Header compression is a technique used to compress and transparently
	decompress the header information of a packet on a per-hop basis,
	utilizing redundancy within individual packets and between
	consecutive packets within a packet stream. Over the years, several
	protocols [VJHC, IPHC] have been developed to compress the network
	and transport protocol headers [IPv4, IPv6, UDP, TCP], and these
	schemes have been successful in improving efficiency over many wired
	bottleneck links, such as modem connections over telephone networks.
	In addition to IP, UDP and TCP compression, an additional compression
	scheme called Compressed RTP [CRTP] has been developed to further
	improve compression efficiency for the case of real-time traffic
	using the Real-time Transport Protocol [RTP].
	The schemes mentioned above have all been designed taking into
	account normal assumptions about link characteristics, which
	traditionally have been based on wired links only. However, with an
	increasing number of wireless links in the Internet paths, these
	assumptions are no longer generally valid. In wireless environments,
	especially wide coverage cellular environments, relatively high error
	rates are tolerated in order to allow efficient usage of the radio
	resources. For real-time traffic, which is more sensitive to delays
	than to errors, such operating conditions will be norm over, for
	example, 3rd generation cellular links, and header compression must
	therefore tolerate packet loss. However, with the previously
	mentioned schemes, especially for real-time traffic compressed by
	CRTP, high error rates have been shown to significantly degrade
	header compression performance [CRTPC]. This problem was the driving
	force behind the creation of the RObust Header Compression (ROHC) WG
	in the IETF.
	The ROHC WG has developed a header compression framework on top of
	which profiles can be defined for different protocol sets, or for
	different compression strategies. Due to the limited packet loss
	robustness of CRTP, and the demands of the cellular industry for an
	efficient way of transporting voice over IP over wireless, the main
	focus of ROHC has so far been on compression of IP/UDP/RTP headers,
	which are generous in size, especially compared to the payloads often
	carried by packets with such headers.
	ROHC RTP has become a very efficient, robust and capable compression
	scheme, able to compress the headers down to a total size of one
	octet only. Also, transparency is guaranteed to an extremely great
	extent even when residual bit errors are present in compressed
	headers delivered to the decompressor. The requirements for RTP
	compression [RTP-REQ], defined by the WG before and during the
	development process, have thus been fulfilled.
	As mentioned above, the 3rd generation cellular systems, where IP
	will be used end-to-end, have been one of the driving forces behind
	ROHC RTP, and the scheme has been designed to also suit new cellular
	air interfaces, such as WCDMA, making it possible to run even speech
	services with spectrum efficiency insignificantly lower than for
	existing one-service circuit switched solutions [VTC2000]. However,
	other air interfaces such as those based on GSM and IS-95 will also
	be used in all-IP networks, with further implications for the header
	compression issue. These older air interfaces are less flexible, with
	radio bearers optimized for specific payload sizes. This means that
	not even a single octet of header can be added without using the next
	higher fixed packet size supported by the link, something which is
	obviously very costly. For the already deployed speech vocoders, the
	spectrum efficiency over these links will thus be low compared to
	existing circuit switched solutions. To achieve high spectrum
	efficiency overall with any application, more flexible air interfaces
	must be deployed, and then the ROHC RTP scheme will perform
	excellently, as shown for WCDMA [MOMUC01]. However, for deployment
	reasons, it is however important to also provide a suitable header
	compression strategy for already existing vocoders and air
	interfaces, such as for GERAN and for CDMA2000, with minimal effects
	on spectral efficiency.
	This document defines a new link-layer assisted ROHC RTP profile
	extending ROHC RTP (profile #1) [ROHC], compliant with the ROHC 0-
	byte requirements [0B-REQ]. The purpose of this new profile is to
	provide a header-free packet format that, for a certain application
	behavior, can replace a majority of the 1-octet header ROHC RTP
	packets during normal operation, while still being fully transparent
	and complying with all the requirements of ROHC RTP [RTP-REQ]. For
	other applications, compression will be carried out as with normal
	ROHC RTP.
	To completely eliminate the compressed header, all functionality
	normally provided by the 1-octet header has to be provided by other
	means, typically by utilizing functionality provided by the lower
	layers and sacrificing efficiency for less frequently occurring
	larger compressed headers. The latter is not a contradiction since
	the argument for eliminating the last octet for most packets is not
	overall efficiency in general. It is important to remember that the
	purpose of this profile is to provide efficient matching of existing
	applications to existing link technologies, not efficiency in
	general. The additional complexity introduced by this profile,
	although minimized by a tight integration with already existing ROHC
	functionality, implies that it should therefore only be used to
	optimize performance of specific applications over specific links.
	When implementing this profile over various link technologies, care
	must be taken to guarantee that all the functionality needed is
	provided by ROHC and the lower layers together. Therefore, additional
	documents should specify how to incorporate this profile on top of
	various link technologies.
	2. Terminology
	The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
	"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
	document are to be interpreted as described in RFC 2119.
	CCP Context Check Packet
	CRC Cyclic Redundancy Check
	CSP Context Synchronization Packet
	LLA Link Layer Assisted ROHC RTP Profile
	NHP No Header Packet
	ROHC RObust Header Compression
	RHP ROHC Header Packet (a non-NHP packet, i.e. RRP, CSP or CCP)
	RRP ROHC RTP Packet as defined in [ROHC, profile 1]
	Assisting layer
	"Assisting layer" refers to any entity implementing the
	interface to ROHC (section 4.2). It may, for example,
	refer to a sub-layer used to adapt the ROHC implementation
	and the physical link layer. This layer is assumed to have
	knowledge of the physical layer synchronization.
	Compressing side
	"Compressing side" refers to the combination of the header
	compressor, operating with the LLA profile, and its associated
	assisting layer.
	Lower layers
	"Lower layers" in this document refers to entities located below
	ROHC in the protocol stack, including the assisting layer.
	ROHC RTP
	"ROHC RTP" in this document refers to the IP/UDP/RTP profile
	(profile #1) as defined in [ROHC].
	3. Overview of the link-layer assisted profile
	This ROHC IP/UDP/RTP profile is designed to be used over channels
	that have been optimized for specific payload sizes and therefore
	cannot efficiently accommodate header information when transmitted
	together with payloads corresponding to these optimal sizes.
	The LLA profile extends, and thus also inherits all functionality
	from, the ROCH RTP profile by defining some additional functionality
	and an interface from the ROHC component towards an assisting lower
	layer.
	+---------------------------------------+
	| |
	The LLA ROHC | ROHC RTP, |
	profile | Profile #1 +-----------------+
	| | LLA Additions |
	+---------------------+-----------------+
	By imposing additional requirements on the lower layers compared to
	[ROHC], it is possible to infer the information needed to maintain
	robust and transparent header compression even though the headers are
	completely eliminated during most of the operation time.
	Basically, what this profile does is to replace the smallest and most
	frequent ROHC headers (PT0, 1 octet) with a no-header format by
	providing the header functionality by other means.
	Smallest header in Smallest header in
	ROHC RTP (profile #1) LLA ROHC RTP profile
	+--+--+--+--+--+--+--+--+ ++
	: 1 octet : -----> || No Header
	+--+--+--+--+--+--+--+--+ ++
	|
	| Header field functionality
	+-------------------> provided by other means
	The fields present in the ROHC RTP headers for PT0 are the packet
	type identifier, the sequence number and the CRC (not present in PT
	R-0). The subsequent sections elaborate more on the replacement of
	the functionality of these fields.
	3.1. Providing packet type identification
	All ROHC headers carry a packet type identifier, indicating to the
	decompressor how the header should be interpreted. This is a function
	that must be provided by some means in 0-byte header compression.
	ROHC RTP packets with compressed headers will be possible to
	distinguish thanks to the packet type identifier, but a mechanism is
	needed to separate packets with a header from packets without a
	header. This function MUST therefore be provided by the assisting
	layer in one way or another.
	3.2. Replacing the sequence number
	From the sending application, the RTP sequence number is increased by
	one for each packet sent. The purpose of the sequence number is to
	cope with packet reordering and packet loss. If reordering or loss
	has occurred before the compression point, if needed the compressing
	side can easily avoid problems by not allowing the use of a header-
	free packet.
	However, the compressor cannot anticipate loss or reordering that may
	occur between compressor and decompressor. Therefore, the assisting
	layer MUST guarantee in-order delivery (already assumed by [ROHC])
	and it MUST provide an indication for each packet loss over the link.
	This is basically the same principle as VJ header compression [VJHC]
	relies on.
	Note that guaranteeing in-order delivery and packet loss indication
	between compressor and decompressor not only makes it possible to
	infer the sequence number information, but also supersedes the main
	function of the CRC, which normally takes care of errors due to long
	losses and bit errors in the compressed sequence number.
	3.3. CRC replacement
	All context updating RRP packets carry a CRC calculated over the
	uncompressed header. The CRC is used by the decompressor to verify
	that the updated context is correct. This verification serves three
	purposes:
	1) Detection of longer losses than can be covered by the sequence
	number LSBs (this applies to U/O-mode only)
	2) Protection against failures caused by residual bit errors in
	compressed headers
	3) Protection against faulty implementations and other causes of
	error
	Since this profile defines an NHP packet without this CRC, care must
	be taken to fulfill these purposes by other means, when an NHP is
	used as a replacement for a context updating packet. Detection of
	long losses (1) is already covered since the assisting layer MUST
	provide indication of all packet losses. Furthermore, the NHP packet
	has one important advantage over RHP packets in that residual bit
	errors (2) cannot damage a header that is not even sent.
	It is thus reasonable to assume that compression and decompression
	transparency can be assured with high confidence even without a CRC
	in header-free packets. However, to provide additional protection
	against damage propagation due to undetected residual bit errors in
	context updating packets (2) or other unexpected errors (3),
	periodic context verifications SHOULD be performed (see section 4.7).
	3.4. Applicability of this profile
	The LLA profile can be used with any link technology capable of
	providing the required functionality described in previous sections.
	Whether LLA ROHC RTP or ROHC RTP should be implemented thus depends
	on the characteristics of the link itself. For most RTP packet
	streams, LLA will work exactly as ROHC RTP, while it will be more
	efficient for packet streams with certain characteristics. LLA will
	never be less efficient than ROHC RTP.
	Note as well that LLA, like all other ROHC profiles, is fully
	transparent to any packet stream reaching the compressor. LLA does
	not make any assumptions about the packet stream but will perform
	optimally for packet streams with certain characteristics, e.g.
	synchronized streams exactly timed with the assisting link over which
	the LLA profile is implemented.
	The LLA profile is obviously not applicable if the UDP checksum (2
	bytes) is enabled, which is always the case for UDP/IPv6. For
	UDP/IPv4, the sender may choose to disable the UDP checksum.
	4. Additions and exceptions compared to ROHC RTP
	4.1. Additional packet types
	The LLA profile defines three new packet types to be used in addition
	to the RRP packet types defined by [ROHC]. The following sections
	describe these packet types and their purpose in detail.
	4.1.1. No-Header Packet (NHP)
	A No-Header Packet (NHP), i.e. a packet consisting only of a payload,
	is defined and MAY be used instead of ROHC RTP packet type 0 (PT0)
	with one-octet header. Note that the requirement for using PT0 in the
	first place is basically that all header fields must be unchanged or
	follow the currently established change pattern. In addition, there
	are some considerations for the use of the NHP (see 4.3, 4.4, 4.6 and
	4.7). The context updating properties of NHP packets are the same as
	for corresponding PT0 packets and depend on the mode of operation.
	The assisting layer MAY send the NHP for RTP SN = X only if an NHP
	was delivered by the LLA compressor AND the assisting layer can
	guarantee that the decompressor will infer the proper sequencing for
	this NHP. This guarantee is based on the confidence that the
	decompressor
	a) has the means to infer proper sequencing for the packet
	corresponding to SN = X-1, AND
	b) has either received a loss indication or the packet itself for the
	packet corresponding to SN = X-1.
	4.1.2. Context Synchronization Packet (CSP)
	The case where the packet stream overruns the channel bandwidth may
	lead to data being discarded, which may result in decompressor
	context invalidation. It might therefore be beneficial to send a
	packet with only the header information and discard the payload. This
	would be helpful to maintain synchronization of the decompressor
	context, while efficiently using the available bandwidth.
	This case can be handled with the Context Synchronization Packet
	(CSP), which has the following format:
	0 1 2 3 4 5 6 7
	+---+---+---+---+---+---+---+---+
	| 1 1 1 1 1 0 1 0 | Packet type identifier
	+===+===+===+===+===+===+===+===+
	: ROHC header without padding :
	: see [ROHC, section 5.7] :
	+---+---+---+---+---+---+---+---+
	The CSP is defined by one of the unused packet type identifiers from
	ROHC RTP, carried in the one-octet base header. As for any ROHC
	packet, except the NHP, the packet may begin with ROHC padding and/or
	feedback. It may also carry context identification after the packet
	type identifier. It is possible to have two CID fields present, one
	after the packet type ID and one within the encapsulated ROHC header.
	If a decompressor receives a CSP with two non-equal CID values
	included, the packet MUST be discarded. ROHC segmentation may also be
	applied to the CSP.
	Note that when the decompressor has received and processed a CSP, the
	packet (including any possible data following the CSP encapsulated
	compressed header) MUST be discarded.
	4.1.3. Context Check Packet (CCP)
	A Context Check Packet (CCP), which does not carry any payload but
	only an optional CRC value in addition to the packet type identifier,
	is defined.
	The purpose of the CCP is to provide a useful packet that MAY be sent
	by a synchronized physical link layer in the case where data must be
	sent at fixed intervals, even if no compressed packet is available.
	Whether the CCP is sent over the link and delivered to the
	decompressor is decided by the assisting layer. The CCP has the
	following format:
	0 1 2 3 4 5 6 7
	+---+---+---+---+---+---+---+---+
	| 1 1 1 1 1 0 1 1 | Packet type identifier
	+===+===+===+===+===+===+===+===+
	| C | CRC |
	+---+---+---+---+---+---+---+---+
	C: C = 0 indicates that the CRC field is not used;
	C = 1 indicates that a valid CRC is present.
	The CCP is defined by one of the unused packet type identifiers from
	ROHC RTP, carried in the first octet of the base header. The first
	bit of the second octet, the C bit, indicates whether the CRC field
	is used or not. If C=1, the CRC field MUST be set to the 7-bits CRC
	calculated over the original uncompressed header defined in [ROHC
	section 5.9.2]. As for any ROHC packet, except NHP, the packet MAY
	begin with ROHC padding and/or carry context identification.
	The use of the CRC field to perform decompressor context verification
	is optional and is therefore a compressor implementation issue.
	However, a CCP MUST always be made available to the assisting layer.
	If the assisting layer receives CCPs with the C-bit set (C=1) from
	the compressor, it MUST use the last CCP received if a CCP is to be
	sent, i.e. the CCP corresponding to the last non-CCP packet sent
	(NHP, RRP or CSP). An assisting layer MAY use the CCP for other
	purposes, such as signaling a packet loss before the link.
	The decompressor is REQUIRED to handle a CCP received with the C bit
	set (C=1), indicating a valid CRC field, and perform context
	verification. The received CRC MUST then be applied to the last
	decompressed packet, unless a packet loss indication was previously
	received. Upon CRC failure, actions MUST be taken as specified in
	[ROHC, section 5.3.2.2.3]. A CCP received with C=0 MUST be ignored by
	the decompressor. The decompressor is not allowed to make any further
	interpretation of the CCP.
	The use of CCP by an assisting layer is optional and depends on the
	characteristics of the actual link. Whether it is used or not MUST
	therefore be specified in link layer implementation specifications
	for this profile.
	4.2. Interfaces towards the assisting layer
	This profile relies on the lower layers to provide the necessary
	functionality to allow NHP packets to be sent. This interaction
	between LLA and the assisting layer is defined as interfaces between
	the ROHC LLA compressor/decompressor and the LLA applicable link
	technology.
	| |
	+ +
	+-------------------------+ +-------------------------+
	| ROHC RTP HC | | ROHC RTP HD |
	+-------------------------+ +-------------------------+
	| LLA profile | | LLA profile |
	+=========================+ +=========================+
	| Interface | | Interface |
	| ROHC to assisting layer | | Assisting layer to ROHC |
	+=========================+ +=========================+
	| Applicable | | Applicable |
	| link technology | | link technology |
	+=========================+ +=========================+
	| |
	+------>---- CHANNEL ---->-----+
	The figure above shows the various levels, as defined in [ROHC] and
	this document, constituting a complete implementation of the LLA
	profile. The figure also underlines the need for additional documents
	to specify how to implement these interfaces for a link technology
	for which this profile is relevant.
	This section defines the information to be exchanged between the LLA
	compressor and the assisting layer for this profile to operate
	properly. While it does define semantics, it does not specify how
	these interfaces are to be implemented.
	4.2.1. Interface, compressor to assisting layer
	This section defines the interface semantics between the compressor
	and the assisting layer, providing rules for packet delivery from the
	compressor.
	The interface defines the following parameters: RRP, RRP segmentation
	flag, CSP, CSP segmentation flag, NHP and RTP Sequence Number. All
	parameters, except the NHP, MUST always be delivered to the assisting
	layer. This leads to two possible delivery scenarios:
	a. RRP, CSP, CCP, NHP and RTP Sequence Number are delivered, along
	with the corresponding segmentation flags set accordingly.
	This corresponds to the case when the compressor allows sending of
	an NHP packet, with or without segmentation being applied to the
	corresponding RRP/CSP packets.
	Recall that delivery of an NHP packet occurs when the ROHC RTP
	compressor would have used a ROHC PT0.
	b. RRP, CSP, CCP and RTP Sequence Number are delivered, along with
	the corresponding segmentation flags set accordingly.
	This corresponds to the case when the compressor does not allow
	sending of an NHP packet. Segmentation might be applied to the
	corresponding RRP and CSP packets.
	Segmentation may be applied independently to an RRP or a CSP packet
	if its size exceeds the largest value provided in the PREFERRED
	PACKET_SIZES list and if the LARGE_PACKET_ALLOWED parameter is set to
	false. The segmentation flags are explicitly stated in the interface
	definition to emphasize that the RRP and the CSP may be delivered by
	the compressor as segmented packets.
	The RTP SN MUST be delivered for each packet by the compressor to
	allow the assisting layer to maintain the necessary sequencing
	information.
	4.2.2. Interface, assisting layer to decompressor
	Here the interface semantics between the assisting layer and the
	decompressor are defined, providing simple rules for the delivery of
	received packets to the decompressor. The decompressor needs a way to
	distinguish NHP packets from RHP packets. Also, when receiving
	packets without a header, the decompressor needs a way to infer the
	sequencing information to keep synchronization between the received
	payload and the sequence information of the decompressed headers. To
	achieve this, the assisting layer MUST provide the following to the
	decompressor:
	- an indication for each packet loss between compressor and
	decompressor for CID=0
	- the received packet together with an indication whether the packet
	received is an NHP or not
	Note that in U/O-mode the context is updated from a packet loss
	indication.
	4.3. Optimistic approach agreement (U/O-mode)
	ROHC defines an optimistic approach for updates to reduce the header
	overhead. This approach is fully exploited in the Optimistic and
	Unidirectional modes of operation. Due to the presence of a CRC in
	all compressed headers, the optimistic approach is defined as a
	compressor issue only because the decompressor will always be able to
	detect an invalid context through the CRC verification.
	However, no CRC is present in the NHP packet defined by the LLA
	profile. Therefore the loss of an RHP packet updating the context may
	not always be detected. To avoid this problem, the compressing and
	decompressing sides must agree on the principles for the optimistic
	approach. If, for example, three consecutive updates are sent to
	convey a header field change, the decompressor must know this and
	invalidate the context in case of three or more consecutive physical
	packet losses.
	When operating in U/O-mode, an LLA decompressor MUST use the
	optimistic approach knowledge to detect possible context loss events.
	If context loss is suspected it MUST invalidate the context and not
	forward any packets before the context has been synchronized.
	It is REQUIRED that all documents describing how the LLA profile is
	implemented over a certain link technology define how the optimistic
	approach is agreed between compressor and decompressor. It could be
	handled with a fixed principle, negotiation at startup, or by other
	means, but the method must be unambiguously defined.
	4.4. Specific notes on reliable mode
	For the R-mode, this profile extends ROHC RTP by performing a mapping
	of the R-0 packet to the NHP packet.
	R-mode relies on the secure reference principle [ROHC, section 5.5]
	which states that only packets carrying a 7- or 8-bit CRC can update
	the context and be used for decompression of subsequent packets. As
	no CRC field is present in the one-octet packet for R-mode (i.e. R-
	0), only the function related to the RTP SN needs to be replaced.
	Consequently, the secure reference principle is not affected in any
	way by this mapping, and there is no loss of robustness in the LLA
	profile compared to [ROHC].
	As opposed to U/O-mode, NHP packets in R-mode do not update either
	the compressor or the decompressor context. Specifically, RTP SN
	reference values in the compressor context are not updated by NHP
	packets. This follows naturally from the updating properties of R-0
	packets [ROHC, section 5.7].
	It cannot be assumed that packets will necessarily be transmitted in
	sequence (following the RTP sequencing) over the link, due to loss
	and/or reordering before the link. As a consequence, the compressor
	delivers an NHP if the use of R-0 would normally be allowed. When
	using R-0-CRC, the compressing side is not allowed to start sending
	NHP packets before an acknowledgement of the R-0-CRC has been
	received from the decompressor, with an exception for the case when
	an R-0-CRC is sent instead of an NHP during a monotonic sequence.
	An NHP packet is decompressed in the same way as the R-0, with the
	exception that the RTP SN field is decompressed using the NHP
	sequencing information derived from the interface and maintained as
	sequencing state. This state is defined as the sum of the number of
	packets indicated as lost by the assisting layer and the number of
	non-context-updating packets received by the decompressor since the
	last context update.
	4.5. Fast context initialization, IR redefinition
	As initial IR packets might overrun the channel bandwidth and
	significantly delay decompressor context establishment, it might be
	beneficial to initially discard the payload. This allows state
	transitions and higher compression efficiency to be achieved with
	minimal delay.
	To serve this purpose, the D-bit from the basic structure of the ROHC
	RTP IR packet [ROHC section 5.7.7.1] is redefined for the LLA
	profile. For D=0 (no dynamic chain), the meaning of the D-bit is
	extended to indicate that the payload has been discarded when
	assembling the IR packet. All other fields keep their meanings as
	defined for ROHC RTP.
	The resulting structure, using small CIDs and CID=0, becomes:
	0 1 2 3 4 5 6 7
	+---+---+---+---+---+---+---+---+
	| 1 | 1 | 1 | 1 | 1 | 1 | 0 | D |
	+---+---+---+---+---+---+---+---+
	| Profile | 1 octet
	+---+---+---+---+---+---+---+---+
	| CRC | 1 octet
	+---+---+---+---+---+---+---+---+
	| Static | variable length
	| chain |
	- - - - - - - - - - - - - - - -
	| Dynamic | not present if D = 0
	| chain | present if D = 1, variable length
	- - - - - - - - - - - - - - - -
	| Payload | not present if D = 0
	| | present if D = 1, variable length
	- - - - - - - - - - - - - - - -
	D: D = 0 indicates that the dynamic chain is not present
	and the payload has been discarded.
	After an IR packet with D=0 has been processed by the decompressor,
	the packet MUST be discarded.
	4.6. Feedback option, CV-REQUEST
	The CV-REQUEST option MAY be used by the decompressor to request an
	RRP or CSP for context verification. This option should be used if
	only NHP have been received for a long time and the context therefore
	has not been verified recently. If the compressor receives a feedback
	packet with this option, the next packet compressed SHOULD NOT be
	delivered to the assisting layer as an NHP.
	+---+---+---+---+---+---+---+---+
	| Opt Type = 8 | Opt Len = 0 |
	+---+---+---+---+---+---+---+---+
	4.7. Periodic context verification
	As described in section 3.3, transparency is expected to be
	guaranteed by the functionality provided by the lower layers. This
	ROHC profile would therefore be at least as reliable as the older
	header compression schemes [VJHC, IPHC, CRTP], which do not make use
	of a header compression CRC. However, since ROHC RTP normally is
	extremely safe to use from a transparency point of view, it would be
	desirable to be able to achieve this with LLA also.
	To provide an additional guarantee for transparency and also catch
	unexpected errors, such as errors due to faulty implementations, it
	is RECOMMENDED to periodically send context updating packets, even
	when the compressor logic allows NHP packets to be used.
	4.8. Use of context identifier
	Since an NHP cannot carry a context identifier (CID), there is a
	restriction on how this profile may be used, related to context
	identification. Independent of which CID size has been negotiated,
	NHP packets can only be used for CID=0. If the decompressor receives
	an NHP packet, it can only belong to CID=0.
	Note that if multiple packet streams are handled by a compressor
	operating using LLA, the assisting layer must in case of packet loss
	be able to tell for which CID the loss occurred, or at least it MUST
	be able to tell if packets with CID=0 (packet stream with NHPs) have
	been lost.
	5. Implementation Issues
	This document specifies mechanisms for the protocol and leaves
	details on the use of these mechanisms to the implementers. The
	present chapter aims to provide guidelines, ideas and suggestions for
	implementation of LLA.
	5.1. Implementation parameters and signals
	As described in [ROHC, section 6.3], implementations use parameters
	to set up configuration information and to stipulate how a ROHC
	implementation is to operate. The following parameters are additions,
	required by LLA, to the parameter set defined for ROHC RTP
	implementations. Note that if the PREFERRED_PACKET_SIZES parameters
	defined here are used, they obsolete all PACKET_SIZE and PAYLOAD_SIZE
	parameters of ROHC RTP.
	5.1.1. Implementation parameters at the compressor
	ALWAYS_PAD -- value: boolean
	This parameter may be set by an external entity to specify to the
	compressor that every RHP packet MUST be padded with a minimum of
	one octet ROHC padding.
	The assisting layer MUST provide a packet type identification. If
	no field is available for this purpose from the protocol at the
	link layer, then a leading sequence may be used to distinguish
	RHP packets from NHP packets. Although the use of a leading
	sequence is obviously not efficient, since it sacrifices
	efficiency for RHP packets, the efficiency loss should be
	insignificant because the leading sequence applies only to
	packets with headers in order to favor the use of packets without
	headers. If a leading sequence is desired for RHP identification,
	the lower layer MAY use ROHC padding for the leading sequence by
	setting the ALWAYS_PAD parameter.
	By default, this parameter is set to FALSE.
	PREFERRED PACKET SIZES -- list of: SIZE -- value: integer (octets)
	ONLY_NHP -- value: boolean
	This parameter set governs which packet sizes are preferred by
	the assisting layer. If this parameter set is used, all RHP
	packets MUST be padded to fit the smallest possible preferred
	size. If the size of the unpadded packet (or, in the case of
	ALWAYS_PAD being set, the packet with minimal one octet padding)
	is larger than the maximal preferred packet size, the compressor
	has two options. Either, it may deliver this larger packet with
	an arbitrary size, or it may split the packet into several
	segments using ROHC segmentation and pad each segment to one of
	the preferred sizes. Which method to use depends on the value of
	the LARGE_PACKETS_ALLOWED parameter below.
	NHP packets can be delivered to the lower layer only if the
	payload size is part of the preferred packet size set.
	Furthermore, if ONLY_NHP is set to TRUE for any of the preferred
	packet sizes, that size is allowed only for NHP packets.
	By default, no preferred packet sizes are specified. When sizes
	are specified, the default value for ONLY_NHP is FALSE.
	LARGE_PACKETS_ALLOWED -- value: boolean
	This parameter may be set by an external entity to specify how to
	handle packets that do not fit any of the preferred packet sizes
	specified. If it is set to TRUE, the compressor MUST deliver the
	larger packet as-is and MUST NOT use segmentation. If it is set
	to FALSE, the ROHC segmentation scheme MUST be used to split the
	packet into two or more segments, and each segment MUST further
	be padded to fit one of the preferred packet sizes.
	By default, this parameter is set to TRUE, which means that
	segmentation is disabled.
	VERIFICATION_PERIOD -- value: integer (octets)
	This parameter may be set by an external entity to specify to the
	compressor the minimum frequency with which a packet validating
	the context must be sent. This tells the compressor that a packet
	containing a CRC field MUST be sent at least once every N
	packets, where N=VERIFICATION_PERIOD (see section 4.7).
	By default, this parameter is set to 0, which indicates that
	periodical verifications are disabled.
	5.1.2. Implementation parameters at the decompressor
	NHP_PACKET -- value: boolean
	This parameter informs the decompressor that the packet being
	delivered is an NHP packet. The decompressor MUST accept this
	packet type indicator from the lower layer. An assisting layer
	MUST set this indicator to true for every NHP packet delivered,
	and to false for any other packet.
	PHYSICAL_PACKET_LOSS -- signal
	This signal indicates to the decompressor that a packet has been
	lost on the link between the compressor and the decompressor, due
	to a physical link error. The signal is given once for each
	packet that was lost, and a decompressor must increase the
	sequence number accordingly when this signal is received.
	PRE_HC_PACKET_LOSS -- signal
	This signal tells the decompressor to increase the sequence
	number due to a gap in the sequencing, not related to a physical
	link error. A receiving assisting layer may for example use this
	signal to indicate to the decompressor that a packet was lost
	before the compressor, or that a packet was discarded by the
	transmitting assisting layer.
	5.2. Implementation over various link technologies
	This document provides the semantics and requirements of the
	interface needed from the ROHC compressor and decompressor towards
	the assisting layer to perform link-layer assisted header
	compression.
	However, the document does not provide any link layer specific
	operational information, except for some implementation suggestions.
	Further details about how this profile is to be implemented over
	various link technologies must be described in other documents, where
	specific characteristics of each link layer can be taken into account
	to provide optimal usage of this profile.
	These specifications MAY use a packet type bit pattern unused by this
	profile to implement signaling on the lower layer. The pattern
	available to lower layer implementations is [11111001].
	6. IANA considerations
	A ROHC profile identifier must be reserved by the IANA for the
	IP/UDP/RTP profile defined in this document. Since this additional
	profile will be used concurrently with the ROHC IP/UDP/RTP profile in
	[ROHC] and is part of the IETF standards track, an ordinary
	identifier in the range from 4 to 127 should be reserved.
	7. Security considerations
	The security considerations of ROHC RTP [ROHC section 7] apply also
	to this document with one addition: in the case of a denial-of-
	service attack scenario where an intruder injects bogus CCP packets
	onto the link using random CRC values, the CRC check will fail for
	incorrect reasons at the decompressor side. This would obviously
	greatly reduce the advantages of ROHC and any extra efficiency
	provided by this profile due to unnecessary context invalidation,
	feedback messages and refresh packets. However, the same remarks
	related to the presence of such an intruder apply.
	8. Acknowledgements
	The authors would like to thank Ulises Olvera-Hernandez and Francis
	Lupien for inputs about the typical links that LLA can be applied to.
	Thanks also to Mikael Degermark for fruitful discussions that led to
	improvements of this profile, and to Zhigang Liu for valuable inputs
	especially regarding R-mode operation.
	9. References
	[ROHC] Bormann, C., et. al., "Robust Header Compression
	(ROHC)", RFC 3095, July 2001.
	[IPv4] Postel, J., "Internet Protocol", STD 5, RFC 791,
	September 1981.
	[IPv6] Deering, S. and R. Hinden, "Internet Protocol, Version 6
	(IPv6) Specification", RFC 2460, December 1998.
	[UDP] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
	August 1980.
	[RTP] Schulzrinne, H., Casner S., Frederick R. and V.
	Jacobson, "RTP: A Transport Protocol for Real-Time
	Applications", RFC 1889, January 1996.
	[TCP] Postel, P., "Transmission Control Protocol", RFC 793,
	September 1981.
	[RTP-REQ] Degermark, M., "Requirements for IP/UDP/RTP Header
	Compression", RFC 3096, July 2001.
	[0B-REQ] Jonsson, L-E., "Requirements and Assumptions for ROHC 0-
	byte Header Compression", Internet Draft, work in
	progress, August 2001.
	<draft-ietf-rohc-rtp-0-byte-requirements-01.txt
	[VJHC] Jacobson, V., "Compressing TCP/IP Headers for Low-Speed
	Serial Links", RFC 1144, February 1990.
	[IPHC] Degermark, M., Nordgren, B. and S. Pink, "IP Header
	Compression", RFC 2507, February 1999.
	[CRTP] Casner, S. and V. Jacobson, "Compressing IP/UDP/RTP
	Headers for Low-Speed Serial Links", RFC 2508, February
	1999.
	[CRTPC] Degermark, M., Hannu, H., Jonsson, L-E. and K. Svanbro,		Title: RObust Header Compression (ROHC):
	"Evaluation of CRTP Performance over Cellular Radio		A Link-Layer Assisted Profile for IP/UDP/RTP
	Networks", IEEE Personal Communications Magazine, Volume		Author(s): L-E. Jonsson, G. Pelletier
	7, number 4, pp. 20-25, August 2000.		Status: Standards Track
			Date: April 2002
			Mailbox: lars-erik.jonsson@ericsson.com,
			ghyslain.pelletier@epl.ericsson.se
			Pages: 21
			Characters: 49007
			Updates/Obsoletes/SeeAlso: None
	[VTC2000] Svanbro, K., Hannu, H., Jonsson, L-E. and M. Degermark,		I-D Tag: draft-ietf-rohc-rtp-lla-03.txt
	"Wireless real time IP-services enabled by header
	compression", proceedings of IEEE VTC2000, May 2000.
	[MOMUC01] Liu, G., et al., "Experimental field trials results of		URL: ftp://ftp.rfc-editor.org/in-notes/rfc3242.txt
	Voice-over IP over WCDMA links", MoMuC'01 - The
	International Workshop on Mobile Multimedia
	Communications, Conference proceedings, February 2001.
	10. Authors' addresses		This document defines a ROHC (Robust Header Compression) profile for
			compression of IP/UDP/RTP (Internet Protocol/User Datagram
			Protocol/Real-Time Transport Protocol) packets, utilizing
			functionality provided by the lower layers to increase compression
			efficiency by completely eliminating the header for most packets
			during optimal operation. The profile is built as an extension to the
			ROHC RTP profile. It defines additional mechanisms needed in ROHC,
			states requirements on the assisting layer to guarantee transparency,
			and specifies general logic for compression and decompression making
			use of this header-free packet.
	Lars-Erik Jonsson Tel: +46 920 20 21 07		This document is a product of the Robust Header Compression Working
	Ericsson Erisoft AB Fax: +46 920 20 20 99		Group of the IETF.
	Box 920
	SE-971 28 Lulea
	Sweden E-mail: lars-erik.jonsson@ericsson.com
	Ghyslain Pelletier Tel: +46 920 20 24 32		This is now a Proposed Standard Protocol.
	Ericsson Erisoft AB Fax: +46 920 20 20 99
	Box 920
	SE-971 28 Lulea
	Sweden E-mail: ghyslain.pelletier@epl.ericsson.se
	Full copyright statement		This document specifies an Internet standards track protocol for the
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