Transcript Robust Header Compression

Robust Header Compression
(ROHC)
An introduction
Jonathan Shufelt
[email protected]
What is Robust Header
Compression?
• First specified in RFC 3095, updated by
RFCs 4815 and 3759 created by the
ROHC charter.
• Utilizes the significant redundancy
between header fields in consecutive
packets from a packet stream.
• Static field information is only sent initially.
• Predictability of other fields is used to
reduce header size.
What is Robust Header
Compression?
• Relevant information from past packets in the stream is
used to maintain a context.
• This context information is used to compress and
decompress subsequent packets.
• Certain events will cause the context to be updated.
• Impairments may cause loss of context, or Context
Damage.
• Context Damage leads to incorrect decompression.
• Provides mechanisms to avoid and recover from Context
Damage.
ROHC Terminology
• Channel – Logical link over which the ROHC
header-compressed packets flow.
• Context Identifiers (CID) – Uniquely identify a
stream, useful when a channel is transporting
more than one stream simultaneously.
• Context Damage – Situation where
decompressor is unable to decompress headers
without error.
ROHC – how is it negotiated?
• The use of ROHC can be manually
configured, or negotiated at the time of the
channel’s establishment.
• State information and compression profiles
available can be established at the time
the channel is established, or may be
predefined.
• Per context parameters are initialized with
IR packets, updated with IR-DYN packets.
Compression and Decompression
states
• ROHC can be characterized as an interaction
between two state machines, compressor and
decompressor.
• State machine pairs are associated with a
context identifier (CID).
• The compressor and decompressor each have 3
possible states of operation.
• The compressor and decompressor states are
related to each other.
• Both machines start in the lowest compression
state.
Compression and Decompression
states
• State transitions occur in both the
compressor and decompressor.
• State transitions are not necessarily
synchronized.
• In normal operation, the compressor will
temporarily transition to a lower state.
• The decompressor will only transition to a
lower state when context damage is
detected.
Compressor States
• Initialization and Refresh (IR) – Compressor will
start here and transition to higher compression
states such as FO and SO.
• First Order (FO) – Efficiently communicates
irregularities in the packet stream.
• Second Order (SO) – Optimized compression is
used and only minimal header information is
sent in each packet.
Compressor States
Compressor States: Initialization
and Refresh (IR)
• Used to initialize static parts of context at
decompresser, or to recover after failure.
• Complete header information, static and
non-static fields, is sent from the
compressor in this state.
• Compressor will remain in this state until it
is fairly confident that the decompresser
has correctly received the static
information.
Compressor States:
First Order (FO)
• Provides efficient communication of irregularities of
packet stream.
• Information about dynamic fields is rarely sent in this
state.
• Sent information in this state is usually partially
compressed.
• Only a few static fields can be updated in this state.
• Compressor may enter this state from IR or SO states.
• Transition to FO from IR is a normal step in the state
machine transition.
Compressor States:
First Order (FO)
• Transition to FO from SO will occur when a pattern
change is present in the packet stream headers.
• Compressor will remain in FO state until it is confident
that decompresser has acquired all parameters of packet
stream headers.
• Changes in fields that are always irregular are
communicated in all packets.
• Some or all packets sent carry context updating
information.
• Detection of corruption of FO state packets is very
important to avoid erroneous updates and context
inconsistencies.
Compressor States:
Second Order (SO)
• State of optimal compression.
• Compressor enters this state when header is
completely predictable given the sequence
number (SN), and confidence that the
decompressor has acquired the functions of SN
to other fields.
• Correct decompression of the SN is required to
prevent Context Damage.
• Compressor will transition to lower states when
the header no longer conforms to uniform
pattern established upon previous context
information.
Decompressor States
• No Context – Initial and lowest compression
state. Used for initializing and refreshing context.
• Static Context – Intermediate compression state.
Used when some header fields are still dynamic
in nature.
• Full Context – Highest compression state. Used
when all header fields are known from previous
packets, or can be predicted from the SN.
Modes of Operation
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Unidirectional – U-mode
Bidirectional Optimistic – O-mode
Bidirectional Reliable – R-mode
States and Modes abstractions are orthogonal to each
other.
• State abstraction is the same for all modes.
• Mode controls the logic of state transitions and what
actions to perform in each state.
• The optimal mode of operation will depend on
environmental characteristics of the compression
protocol. These include feedback abilities, error
probabilities and distributions, and effects of header size
and variation.
Unidirectional mode – U-mode
• Packets are sent in one direction only, from compressor to
decompressor.
• Useful in situations where the return path over communications link
is unavailable or undesirable.
• Transitions between compressor states result from periodic timeouts
or irregularities in the header field change patterns of the packet
stream.
• Least efficient mode due to periodic refreshes and lack of feedback.
• Slightly higher probability of loss propagation as compared to
bidirectional modes (O-mode and R-mode) of operation.
• U-mode must be used to start compression.
• Transition to bidirectional modes will occur when feedback packet
indicating a desire for transition is received from decompressor.
Bidirectional Optimistic mode –
O-mode
• Utilizes a feedback channel from decompressor to compressor.
• Feedback channel is used to send error recovery requests and
optionally acknowledgements of significant context updates.
• Periodic refreshes are not used in O-mode.
• Maximizes compression while minimizing usage of the feedback
channel.
• Reduces number of damaged headers delivered to upper layers due
to residual errors or context invalidation.
• Frequency of context invalidation may be higher than in R-mode.
• Prone to context invalidation when long loss or burst errors are
present on link.
Bidirectional Reliable mode –
R-mode
• More intensive use of feedback channel when
compared to U-mode and O-mode.
• Stricter logic at compressor and decompressor
prevents loss of context except in the case of
very high residual Bit Error Rate (BER)
conditions.
• Feedback is sent to acknowledge all context
updates, including SN updates.
• Not every feedback packet is sent to update
context in R-mode.
Encoding Methods
• Least Significant Bits (LSB) – Used for header fields
whose values are subject to small value changes.
• Window-based LSB (W-LSB) – Adds robustness to
the LSB encoding algorithm by the compressor
using a sliding window reference approach based
upon feedback from decompressor.
• Scaled RTP Timestamp (TS) – Scaled counter
method that minimizes the encoding field size.
Applied to frame-based codecs.
• Offset IP-ID – In the case of IPv4 the compressor
will compress the offset of the IP-ID value
subtracted from the RTP SN instead of the IP-ID
value itself.
Encoding Methods
• Timer-based compression of RTP Timestamp –
Utilizes fixed sampling intervals, constant sampling
rate, and the fact that packets are generated lockstep with sampling to make use of a linear time-ofday relationship and decompressor’s local clock to
approximate the scaled TS in the header.
• Self-describing variable-length values – Compressor
examines the values of up to the first 3 bits, and chooses
the number of octets to use based upon their values.
• Encoded values across several fields in compressed
headers – Compressor examines for extension headers,
compares the bits available to the bits in the value, and
chooses the appropriate packet type.
Residual Errors
• Packets may become damaged in transit
between compressor and decompressor.
• CRC values calculated by compressor and
included in packet are used by
decompressor to detect damaged packets.
• Damage to packets may cause
misinterpretation of packet content.
• Undetected damaged packets may be
passed to upper layers.
Impairment Considerations
• Impairments can cause loss or damage to individual
headers, context invalidation, loss propagation, and
damage propagation.
• Impairments to headers can be classified as follows:
–
Lower layer decoding failure, and subsequent discarding of
packet.
– Lower layer error detection, and subsequent discarding of
packet.
– ROHC error detection, and subsequent discarding of packet.
– ROHC error detection failure, and subsequent passing of
packet to upper layers.
Per-context parameters, profiles
• Established with IR headers, which contain a profile identifier.
• Profile identifiers determine the syntax and semantics of the packet
type identifiers, and the packet types used in conjunction with the
specific context.
• Profiles:
0x0000 – Uncompressed IP
0x0001 – RTP/UDP/IP
0x0002 – UDP/IP
0x0003 – ESP/IP
0x0004 – IP
0x0005 – Link Layer Assisted
0x0006 – TCP/IP
0x0007 – RTP/UDP-Lite/IP
0x0008 – UDP-Lite/IP
Contexts and context identifiers
• A context is:
- associated with each compressed flow.
- used by compressor and decompressor to compress
and decompress headers of a packet stream.
- identified by Context Identifier (CID).
• CIDs are sent along with compressed headers and
feedback information.
• CID space, size, for each channel direction may be
distinct.
• CID parameters are negotiated during channel
establishment.
ROHC packets and packet types
• Packet type design constraints:
- must be possible to use limited number of packet sizes.
- must be possible to send feedback information in separate ROHC
packets as well as piggybacked on forward packets.
- desirable to allow elimination of CID for one packet stream when
few packet streams share a channel.
- anticipated that some packets with large headers may be larger
than the MTU of very constrained lower layers.
• Packet design includes:
- optional padding
- a feedback packet type
- optional Add-CID octet
- simple segmentation and reassembly mechanism
ROHC feedback
• Carries feedback from decompressor to compressor.
• Principal kinds of feedback:
ACK – acknowledges successful decompression.
NACK – indicates that the dynamic context of the decompressor is out of
sync.
STATIC-NACK – indicates that the static context of the decompressor is
not valid or has not been established.
• Feedback for ROHC can be realized in many ways such as:
– Lower layer specific mechanisms
– Dedicated feedback-only channel (i.e. lower layer based, or timing
based)
– Interspersed among normal compressed packets traveling in same
direction
– Piggybacking of feedback information in compressed packets traveling
in same direction
Unidirectional Mode (U-mode) –
compressor state machine
• Transition logic for compression states in Unidirectional mode based
upon three principles:
- optimistic approach principle
- timeouts
- the need for updates
• Feedback channels may or may not be available. Mode is designed
to operate in the absence of a feedback channel.
Unidirectional Mode (U-mode) –
compressor state machine
• Optimistic approach, upwards transition – compressor will transit to
a higher compression state when it is fairly confident that the
decompressor has received enough information to decompress
packets.
• Timeouts/Updates, downwards transition – compressor must
periodically transition to lower compression states (FO and IR) to
ensure that the decompressor has the required information for
successful decompression of packets.
• Compression logic and packets used – Compressor will choose the
smallest packet that can communicate the desired changes, and has
the required number of bits for W-LSB encoding.
• Feedback – If available, decompressor may send
acknowledgements of successful decompression to the compressor.
The compressor may then disable, or increase the interval, between
periodic IR refreshes.
Unidirectional Mode (U-mode) –
decompressor state machine
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Successful decompression will always move the decompressor to the Full
Context state.
Repeated failures will force the decompressor to transit to a lower state
(Static Context (SC) or No Context (NC) ).
The decompressor does not attempt decompression in the lower states
without sufficient context information present in received packets.
If feedback is present and used by the decompressor, acknowledgements
may be used to control compressor state.
Unidirectional Mode (U-mode) –
decompressor state machine
• Decompression is carried out by following these steps:
- Decide whether decompression is allowed.
- Reconstruct and verify the header.
- If header verifies correctly, remain in current state.
- If header fails verification, attempt recovery,
transitioning to lower state upon recovery failure.
• Feedback – If available, the decompressor may send
acknowledgement packets reducing the frequency of IR
packets from the compressor. The decompressor may
send repeated acknowledgement packets, but should
not do so continuously.
Bidirectional Optimistic Mode
(O-mode)
– compressor state machine
• Transition logic for compression states in Bidirectional
Optimistic mode is based upon these principles:
- optimistic approach principle
- feedback
- the need for updates
• Feedback channel is available, and always used.
Bidirectional Optimistic Mode
(O-mode)
– compressor state machine
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State transition logic has much in common with Unidirectional mode, but
does not use timeouts.
Feedback from decompressor to compressor causes transitions to lower
compression states, and may optionally be used to cause transitions to
higher compression states.
Negative acknowledgements (NACKs), downward transition:
- NACKs are also known as context requests.
- NACKs transitions the compressor to the FO state, and prompts the
compressor to send updates (IR-DYN, UOR-2, IR) to the decompressor.
- Reception of a STATIC-NACK from decompressor will cause the
compressor to transition to the IR state.
Optional acknowledgements, upwards transition – positive feedback (ACKs)
may be used to transition the compressor to a higher state.
Compression logic and packets used – same as those used for the
Unidirectional mode.
Bidirectional Optimistic Mode
(O-mode)
– decompressor state machine
• Same decompression states and state logic as the Unidirectional
case, with different decompression and feedback logic.
• Decompression logic, timer-based timestamp decompression – May
be used to improve compression efficiency when RTP timestamp
values are proportional to wall clock time.
• Feedback logic (O-mode):
Decompressor - Three principal kinds of feedback: ACK, NACK, and
STATIC-NACK. Use of optional feedback requires decompressor to
send optional feedback for the lifetime of the stream.
• Compressor – IR, IR-DYN, UOR-2, UO-1, and U0-0 may be sent to
the decompressor depending on context conditions and state.
• Decompressor State Actions – Are defined for NC, SC, and FC
states and received packet types.
Bidirectional Reliable Mode
(R-mode)
– compressor state machine
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Transition logic for compression states in Bidirectional Reliable Mode is
based on three principles:
- the secure reference principle
- the need for updates
- negative acknowledgements
Feedback logic is available, though more sparsely used.
Bidirectional Reliable Mode
(R-mode)
– compressor state machine
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Upwards transition – Determined by secure reference principle. The secure reference principle
uses acknowledgements from the decompressor to ensure that context is not lost due to packet
losses.
Downward transition – Triggered by the need for context updates, or by NACK or STATIC-NACK
packets received from the decompressor.
Compression logic and packets used – Compression logic starts in the IR state and makes a
direct transition to the FO state upon receipt of a valid ACK from the decompressor. Transition to
the SO state is dependent upon the presence of a string. The string contains information about the
slope and offset values used in compression of packets.
Decompressor states and logic – State and state transition logic are the same as in the
Unidirectional mode, but not the decompression and feedback logic.
Decompression logic – Rules for when decompression is allowed are the same as for U-mode.
ACKing scheme in R-mode guarantees that non-decompress able packets are never sent by
compressor. Residual errors may cause delivery of unexpected packets for which decompression
will not be attempted. CRCs are only carried by update packets from the compressor.
Feedback logic – Rules:
- Updating packets correctly decompressed cause ACK packets to be sent to compressor.
- Upon detection of context damage, send NACK if in FC state, or send STATIC-NACK if in SC
state.
- Upon receipt of a non-IR packet in NC state, send a STATIC-NACK unless the CRC fails.
- Feedback is never sent for packets not updating the context.
ROHC – mode transitions
• The decision to move from one compression
mode to another is taken by the decompressor
ROHC – mode transitions
• For each context, the compressor and decompressor maintain a
variable whose value is the current compression mode for that
context.
• The value of the variable controls, for the context in question,
parameters such as; which packet types to use, which actions to be
taken, etc.
• CRCs must be used during all mode transitions as a safeguard.
• Mode initiations must not be initiated by feedback that is not
protected by a CRC.
• Each compression mode has corresponding mode variables.
• The behavior of the compressor and decompressor must be
restricted during mode transition phases by exception parameters.
• Exception parameters names and values are all explicitly defined for
each case of mode transition.