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.
Updating properties: CCP packets do not update context.
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. If C=1, the CRC field MUST be set to the 7-bit 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 to 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, steps a-d only]. 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.
When using the 7-bit CRC in the CCP packet to verify the context, the
decompressor needs to have access to the entire uncompressed header
of the latest packet decompressed. Some implementations of [ROHC]
might not save the values of INFERRED fields. An implementation of
ROHC LLA MUST save these fields in the decompressor context to be
able to successfully verify CCP packets.
The use of CCP by an assisting layer is optional and depends on the
characteristics of the actual link. Whether it is used 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 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 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 UO-0.
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 decompressor MUST receive the following from the
assisting layer:
- an indication for each packet loss over the link between the
compressing and decompressing sides for CID=0.
- the received packet together with an indication of whether the
packet received is an NHP.
Note that the context is updated from a packet loss indication.
4.3. Optimistic Approach Agreement
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, and the agreed principles MUST be enforced not
only by the compressor but also by the transmitting assisting layer.
If, for example, three consecutive updates are sent to convey a
header field change, the decompressor must know this and invalidate
the context if three or more consecutive physical packets are lost.
Note that the mechanism used to enforce the optimistic approach must
be reinitialized if a new field change needs to be conveyed while the
compressing side is already sending packets to convey non-linear
context updates.
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 to between the compressing side and the
decompressing side. It could be handled with a fixed principle, with
negotiation at startup, or by other means, but the method must be
unambiguously defined.
4.4. 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 that the payload has been discarded.
After an IR packet with D=0 has been processed by the decompressor,
the packet MUST be discarded.
4.5. 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 NHPs have been received for a long time and the context
therefore has not been verified recently.
+---+---+---+---+---+---+---+---+
| Opt Type = 8 | Opt Len = 0 |
+---+---+---+---+---+---+---+---+
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.
4.6. 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 that context updating packets be sent periodically,
even when the compressor logic allows NHP packets to be used.
4.7. 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 physical
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 section 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, useful to 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 ROHC padding
of one octet, minimum.
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. Note that in such cases, possible collisions of the
padding with the NHP payload must be avoided.
By default, this parameter is set to FALSE.
PREFERRED_PACKET_SIZES -- list of:
SIZE -- value: integer (octets)
RESTRICTED_TYPE -- values: [NHP_ONLY, RHP_ONLY, NO_RESTRICTION]
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 RESTRICTED_TYPE is set to one of NHP_ONLY or
RHP_ONLY for any of the preferred packet sizes, that size is
allowed only for packets of the specified type.
By default, no preferred packet sizes are specified. When sizes
are specified, the default value for RESTRICTED_TYPE is
NO_RESTRICTION.
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
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 4.6).
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 compressing and the decompressing
sides, 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_LINK_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, this 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
ROHC profile identifier 0x0005 has been reserved by the IANA for the
IP/UDP/RTP profile defined in this document.
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
using random CRC values onto the link, 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 Lila Madour, Ulises Olvera-Hernandez,
and Francis Lupien for input regarding the typical links in which LLA
can be applied. Thanks also to Mikael Degermark for fruitful
discussions that led to improvements of this profile, and to Zhigang
Liu for many valuable comments.
9. References
9.1. Normative References
[ROHC] 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.
[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", STD 64, RFC 3550, July 2003.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
9.2. Informative References
[LLA] Jonsson, L-E. and G. Pelletier, "RObust Header Compression
(ROHC): A Link-Layer Assisted Profile for IP/UDP/RTP", RFC
3242, April 2002.
[TCP] Postel, J., "Transmission Control Protocol", STD 7, RFC
793, September 1981.
[RTP-REQ] Degermark, M., "Requirements for robust IP/UDP/RTP header
compression", RFC 3096, July 2001.
[0B-REQ] Jonsson, L-E., "RObust Header Compression (ROHC):
Requirements and Assumptions for 0-byte IP/UDP/RTP
Compression", RFC 3243, April 2002.
[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,
"Evaluation of CRTP Performance over Cellular Radio
Networks", IEEE Personal Communications Magazine, Volume 7,
number 4, pp. 20-25, August 2000.