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IETF RFC 4077
A Negative Acknowledgement Mechanism for Signaling Compression
Last modified on Monday, May 23rd, 2005
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Network Working Group A.B. Roach
Request for Comments: 4077 Estacado Systems
Category: Standards Track May 2005
A Negative Acknowledgement Mechanism for Signaling Compression
 Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright © The Internet Society (2005).
Abstract
This document describes a mechanism that allows Signaling Compression
(SigComp) implementations to report precise error information upon
receipt of a message which cannot be decompressed. This negative
feedback can be used by the recipient to make fine-grained
adjustments to the compressed message before retransmitting it,
allowing for rapid and efficient recovery from error situations.
Roach Standards Track PAGE 1 
RFC 4077 SigComp NACK May 2005
Table of Contents
1. Introduction ....................................................2
1.1. The Problem ................................................2
1.1.1. Compartment Disposal ................................3
1.1.2. Client Restart ......................................3
1.1.3. Server Failover .....................................3
1.2. The Solution ...............................................4
2. Node Behavior ...................................................4
2.1. Normal SigComp Message Transmission ........................4
2.2. Receiving a "Bad" SigComp Message ..........................5
2.3. Receiving a SigComp NACK ...................................6
2.3.1. Unreliable Transport ................................6
2.3.2. Reliable Transport ..................................6
2.4. Detecting Support for NACK .................................7
3. Message Format ..................................................7
3.1. Message Fields .............................................8
3.2. Reason Codes ...............................................9
4. Security Considerations ........................................13
4.1. Reflector Attacks .........................................13
4.2. NACK Spoofing .............................................13
5. IANA Considerations ............................................14
6. Acknowledgements ...............................................14
7. References .....................................................14
7.1. Normative References ......................................14
7.2. Informative References ....................................14
1. Introduction
Signaling Compression [1], often called "SigComp", defines a protocol
for transportation of compressed messages between two network
elements. One of the key features of SigComp is the ability of the
sending node to request that the receiving node store state objects
for later retrieval.
1.1. The Problem
While the "SigComp - Extended Operations" document [2] defines a
mechanism that allows for confirmation of state creation, operational
experience with the SigComp protocol has demonstrated that there are
still several circumstances in which a sender's view of the shared
state differs from the receiver's view. A non-exhaustive list
detailing the circumstances in which such failures may occur is
below.
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RFC 4077 SigComp NACK May 2005
1.1.1. Compartment Disposal
In SigComp, stored states are associated with compartments.
Conceptually, the compartments represent one instance of a remote
application. These compartments are used to limit the amount of
state that each remote application is allowed to store. Compartments
are created upon receipt of a valid SigComp message from a remote
application. In the current protocol, applications are expected to
signal when they are finished with a compartment so that it can be
deleted (by using the S-bit in requested feedback data).
Unfortunately, expecting the applications to be well-behaved is not
sufficient to prevent state from piling up. Unexpected client
failures, reboots, and loss of connectivity can cause compartments to
become "stuck" and never removed. To prevent this situation, it
becomes necessary to implement a scheme by which compartments that
appear disused may eventually be discarded.
While the preceding facts make such a practice necessary, discarding
compartments without explicit signaling can have the unfortunate side
effect that active compartments are sometimes discarded. This leads
to a different view of state between the server and the client.
1.1.2. Client Restart
The prime motivation for SigComp was compression of messages to be
sent over a radio interface. Consequently, most deployments of
SigComp will involve a mobile unit as one of the endpoints. Mobile
terminals are generally not guaranteed to be available for extended
durations of time. Node restarts (due to, for example, a battery
running out) will induce situations in which the network-based server
believes that the client contains several states that are no longer
actually available.
1.1.3. Server Failover
Many applications for which SigComp will be used (e.g., SIP [3]) use
DNS SRV records for server lookup. One of the important features of
DNS SRV records is the ability to specify multiple servers from which
clients will select at random, with probabilities determined by the
q-value weighting. The reason for defining this behavior for SRV
records is to allow load distribution through a set of equivalent
servers, and to permit clients to continue to function even if the
server with which they are communicating fails. When using protocols
that use SRV for such distribution, the traffic to a failed server is
typically sent by the client to an equivalent server that can serve
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RFC 4077 SigComp NACK May 2005
the same purpose. From an application perspective, this new server
often appears to be the same endpoint as the failed server, and will
consequently resolve to the same compartment.
Although SigComp state can be replicated amongst such a cluster of
servers, maintaining integrity of such states requires a two-phase
commit process that adds a great deal of complexity to the server and
can degrade performance significantly.
1.2. The Solution
Although SigComp allows returned SigComp parameters to signal that
all states have been lost (by setting "state_memory_size" to 0 for
one message in the reverse direction), such an approach provides an
incomplete solution to the problem. In addition to wiping out an
entire compartment when only one state is corrupt or missing, this
approach suffers from the unfortunate behavior that it requires a
message in the reverse direction that the remote application will
authorize. Unless a lower-layer security mechanism is employed
(e.g., TLS), this would typically mean that a compressed
application-level message in the reverse direction must be sent
before recovery can occur. In many cases (such as SIP-based mobile
terminals), these messages won't be sent often; in others (pure
client/server deployments), they won't ever be sent.
The proposed solution to this problem is a simple Negative
Acknowledgement (NACK) mechanism which allows the recipient to
communicate to the sender that a failure has occurred. This NACK
contains a reason code that communicates the nature of the failure.
For certain types of failures, the NACK will also contain additional
details that might be useful in recovering from the failure.
2. Node Behavior
The following sections detail the behavior of nodes sending and
receiving SigComp NACKs. The actual format and values are described
in Section 3.
2.1. Normal SigComp Message Transmission
Although normal in all other respects, SigComp implementations that
use the NACK mechanism need to calculate and store a SHA-1 hash for
each SigComp message that they send. This must be stored in such a
way that, given the SHA-1 hash, the implementation is able to locate
the compartment with which the sent message was associated.
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RFC 4077 SigComp NACK May 2005
In other words, if someone hands the SHA-1 hash back to the
compressor, it needs to be able to find the compartment with which it
was working when it sent the message with that hash. This only
requires that the compressor knows with which compartment it is
working when it sends a message (which is always the case), and that
the SHA-1 hash, when stored, points to that compartment in some way.
2.2. Receiving a "Bad" SigComp Message
When a received SigComp message causes a decompression failure, the
recipient forms and sends a SigComp NACK message. This NACK message
contains a SHA-1 hash of the received SigComp message that could not
be decompressed. It also contains the exact reason decompression
failed, as well as any additional details that might assist the NACK
recipient to correct any problems. See Section 3 for more
information about formatting the NACK message and its fields.
For a connection-oriented transport, such as TCP, the NACK message is
sent back to the originator of the failed message over that same
connection.
For a stream-based transport, such as TCP, the standard SigComp
delimiter of 0xFFFF is used to terminate the NACK message.
For a connectionless transport, such as UDP, the NACK message is sent
back to the originator of the failed message at the port and IP
address from which the message was sent. Note that this may or may
not be the same port on which the application would typically receive
messages. To accommodate implementations that use connect() or
similar constructs, the NACK will be sent from the IP address and
port to which the uninterpretable message was sent. From a practical
perspective, this is probably easiest to determine by binding
listening sockets to a specific interface; however, other mechanisms
may also be employed.
The behavior specified above is strictly necessary for any generally
useful form of a NACK mechanism. In the most general case, when an
implementation receives a message that it cannot decompress, it has
exactly three useful pieces of information: (1) the contents of the
message, (2) an indication of why the message cannot be decoded, and
(3) the IP address and port from which the message originated. Note
that none of these contains any indication of where the remote
application is listening for messages, if it differs from the sending
port.
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RFC 4077 SigComp NACK May 2005
2.3. Receiving a SigComp NACK
The first action taken upon receipt of a NACK is an attempt to find
the message to which the NACK corresponds. This search is performed
using the 20-byte SHA-1 hash contained in the NACK. Once the
matching message is located, further operations are performed based
on the compartment that was associated with the sent message.
Further behavior of a node upon receiving a SigComp NACK depends on
whether a reliable or unreliable transport is being used.
2.3.1. Unreliable Transport
When SigComp is used over an unreliable transport, the application
has no reasonable expectation that the transport layer will deliver
any particular message. It then becomes the application layer's
responsibility to ensure that data is retransmitted as necessary. In
these circumstances, the NACK mechanism relies on such behavior to
ensure delivery of the message, and never performs retransmissions on
the application's behalf.
When a NACK is received for a message sent over an unreliable
transport, the NACK recipient uses the contained information to make
appropriate adjustments to the compressor associated with the proper
compartment. The exact nature of these adjustments are specific to
the compression scheme being used, and will vary from implementation
to implementation. The only requirement on these adjustments is that
they must have the effect of compensating for the error that has been
indicated (e.g., by removing the state that the remote node indicates
it cannot retrieve).
In particular, when an unreliable transport is used, the original
message must not be retransmitted by the SigComp layer upon receipt
of a NACK. Instead, the next application-initiated transmission of a
message will take advantage of the adjustments made as a result of
processing the NACK.
2.3.2. Reliable Transport
When a reliable transport is employed, the application makes a basic
assumption that any message passed down the stack will be
retransmitted as necessary to ensure that the remote node receives
it, unless a failure is indicated by the transport layer. Because
SigComp acts as a shim between the transport-layer and the
application, it becomes the responsibility of the SigComp
implementation to ensure that any failure to transmit a message is
communicated to the application.
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RFC 4077 SigComp NACK May 2005
When a NACK is received for a message sent over a reliable transport,
the SigComp layer must indicate to the application that an error has
occurred. In general, the application should react in the same way
as it does for any other transport layer error, such as a TCP
connection reset. For most applications, this reaction will
initially be an attempt to reset and re-establish the connection, and
re-initiate the failed transaction. The SigComp layer should also
use the information contained in the NACK to make appropriate
adjustments to the compressor associated with the proper compartment
(similar to the adjustments made for unreliable transport). Thus, if
the compartment is not reset by resetting the TCP connection, the
next message will take advantage of the adjustments.
2.4. Detecting Support for NACK
Detection of support for the NACK mechanism may be beneficial in
certain circumstances. For example, with the current definition of
SigComp, acknowledgment of state receipt is required before a sender
can reference such state. When multiple messages are sent before a
response is received, the need to wait for such responses can cause
significant decreases in message compression efficiency. If it is
known that the receiver supports the NACK mechanism, the sender can
instead optimistically assume that the state created by a sent
message has been created, and is allowed to be referenced. If such
an assumption turns out to be false (due to, for example, packet loss
or packet reordering), the sender can recover upon receipt of a NACK.
In order to facilitate such detection, any implementation that will
send NACK messages upon decompression failure will indicate a SigComp
version number of 0x02 in its Universal Decompressor Virtual Machine
(UDVM). The bytecodes sent to such an endpoint can check the version
number, and send appropriate indication back to their compressor as
requested feedback. Except for the NACK mechanism described in this
document, implementations advertising a version of 0x02 behave
exactly like those advertising a version number of 0x01.
3. Message Format
SigComp NACK packets are syntactically valid SigComp messages which
have been specifically designed to be safely ignored by
implementations that do not support the NACK mechanism.
In particular, NACK messages are formatted as the second variant of a
SigComp message (typically used for code upload) with a "code_len"
field of zero. The NACK information (message identifier, reason for
failure, and error details) is encoded in the "remaining SigComp
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RFC 4077 SigComp NACK May 2005
message" area, typically used for input data. Further, the
"destination" field is used as a version identifier to indicate which
version of NACK is being employed.
3.1. Message Fields
The format of the NACK message and the use of the fields within it
are shown in Figure 1.
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 1 1 1 1 1 | T | 0 |
+---+---+---+---+---+---+---+---+
| |
: returned feedback item :
| |
+---+---+---+---+---+---+---+---+
| code_len = 0 |
+---+---+---+---+---+---+---+---+
| code_len = 0 | version = 1 |
+---+---+---+---+---+---+---+---+
| Reason Code |
+---+---+---+---+---+---+---+---+
| OPCODE of failed instruction |
+---+---+---+---+---+---+---+---+
| PC of failed instruction |
| |
+---+---+---+---+---+---+---+---+
| |
: SHA-1 Hash of failed message :
| |
+---+---+---+---+---+---+---+---+
| |
: Error Details :
| |
+---+---+---+---+---+---+---+---+
Figure 1: SigComp NACK Message Format
o "Reason Code" is a one-byte value that indicates the nature of the
decompression failure. The specific codes are given in
Section 3.2.
o "OPCODE of failed instruction" is a one-byte field that includes
the opcode to which the PC was pointing when the failure occurred.
If failure occurred before the UDVM began executing any code, this
field is set to 0.
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RFC 4077 SigComp NACK May 2005
o "PC of failed instruction" is a two-byte field containing the
value of the program counter when failure occurred (i.e., the
memory address of the failed UDVM instruction). The field is
encoded with the most significant byte of the PC first (i.e., in
network or big endian order). If failure occurred before the UDVM
began executing any code, this field is set to 0.
o "SHA-1 Hash of failed message" contains the full 20-byte SHA-1
hash of the SigComp message that could not be decompressed. This
information allows the NACK recipient to locate the message that
failed to decompress so that adjustments to the correct
compartment can be performed. When performing this hash, the
entire SigComp message is used, from the header byte (binary
11111xxx) to the end of the input. Any lower-level protocol
headers (such as UDP or IP) and message delimiters (the 0xFFFF
that marks message boundaries in stream protocols) are not
included in the hash. When used over a stream based protocol, any
0xFFxx escape sequences are un-escaped before performing the hash
operation.
o "Error Details" provides additional information that might be
useful in correcting the problem that caused decompression
failure. Its meaning is specific to the "Reason Code". See
Section 3.2 for specific information on what appears in this
field.
o "Code_len" is the "code_len" field from a standard SigComp
message. It is always set to "0" for NACK messages.
o "Version" gives the version of the NACK mechanism being employed.
This document defines version 1.
3.2. Reason Codes
Note that many of the status codes are more useful in debugging
interoperability problems than with on-the-fly correction of errors.
The "STATE_NOT_FOUND" error is a notable exception: it will generally
cause the NACK recipient to encode future messages so as to not use
the indicated state.
Upon receiving the other status messages, an implementation would
typically be expected either to use a different set of bytecodes or,
if that is not an option, to send that specific message uncompressed.
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RFC 4077 SigComp NACK May 2005
Error Code Details
-------------------------- ---- ---------------------------
STATE_NOT_FOUND 1 State ID (6 - 20 bytes)
CYCLES_EXHAUSTED 2 Cycles Per Bit (1 byte)
USER_REQUESTED 3
SEGFAULT 4
TOO_MANY_STATE_REQUESTS 5
INVALID_STATE_ID_LENGTH 6
INVALID_STATE_PRIORITY 7
OUTPUT_OVERFLOW 8
STACK_UNDERFLOW 9
BAD_INPUT_BITORDER 10
DIV_BY_ZERO 11
SWITCH_VALUE_TOO_HIGH 12
TOO_MANY_BITS_REQUESTED 13
INVALID_OPERAND 14
HUFFMAN_NO_MATCH 15
MESSAGE_TOO_SHORT 16
INVALID_CODE_LOCATION 17
BYTECODES_TOO_LARGE 18 Memory size (2 bytes)
INVALID_OPCODE 19
INVALID_STATE_PROBE 20
ID_NOT_UNIQUE 21 State ID (6 - 20 bytes)
MULTILOAD_OVERWRITTEN 22
STATE_TOO_SHORT 23 State ID (6 - 20 bytes)
INTERNAL_ERROR 24
FRAMING_ERROR 25
Only the five errors "STATE_NOT_FOUND", "CYCLES_EXHAUSTED",
"BYTECODES_TOO_LARGE", "ID_NOT_UNIQUE", and "STATE_TOO_SHORT" contain
details; for all other error codes, the "Error Details" field has
zero length.
Figure 2: SigComp NACK Reason Codes
1. STATE_NOT_FOUND
A state that was referenced cannot be found. The state may have
been referenced by the UDVM executing a STATE-ACCESS
instruction; it also may have been referenced by the "partial
state identifier" field in a SigComp message. The "details"
field contains the state identifier for the state that could not
be found. This is also the proper error to return in the case
that a unique state item was matched but fewer bytes of state ID
were sent than required by the minimum_access_length.
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RFC 4077 SigComp NACK May 2005
2. CYCLES_EXHAUSTED
Decompression of the message has taken more cycles than were
allocated to it. The "details" field contains a one-byte value
that communicates the number of cycles per bit. The cycles per
bit is represented as an unsigned 8-bit integer (i.e., not
encoded).
3. USER_REQUESTED
The DECOMPRESSION-FAILURE opcode has been executed.
4. SEGFAULT
An attempt to read from or write to memory that is outside of
the UDVM's memory space has been attempted.
5. TOO_MANY_STATE_REQUESTS
More than four requests to store or delete state objects have
been requested.
6. INVALID_STATE_ID_LENGTH
A state id length less than 6 or greater than 20 has been
specified.
7. INVALID_STATE_PRIORITY
A state priority of 65535 has been specified when attempting to
store a state.
8. OUTPUT_OVERFLOW
The decompressed message is too large to be decoded by the
receiving node.
9. STACK_UNDERFLOW
An attempt to pop a value off the UDVM stack was made with a
stack_fill value of 0.
10. BAD_INPUT_BITORDER
An INPUT-BITS or INPUT-HUFFMAN instruction was encountered with
the "input_bit_order" register set to an invalid value (i.e.,
one of the upper 13 bits is set).
11. DIV_BY_ZERO
A DIVIDE or REMAINDER opcode was encountered with a divisor of
0.
12. SWITCH_VALUE_TOO_HIGH
The input to a SWITCH opcode exceeds the number of branches
defined.
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RFC 4077 SigComp NACK May 2005
13. TOO_MANY_BITS_REQUESTED
An INPUT-BITS or INPUT-HUFFMAN instruction was encountered that
attempted to input more than 16 bits.
14. INVALID_OPERAND
An operand for an instruction could not be resolved to an
integer value (e.g., a literal or reference operand beginning
with 11111111).
15. HUFFMAN_NO_MATCH
The input string does not match any of the bitcodes in the
INPUT-HUFFMAN opcode.
16. MESSAGE_TOO_SHORT
When attempting to decode a SigComp message, the recipient
determined that there were not enough bytes in the message for
it to be valid.
17. INVALID_CODE_LOCATION
The "code location" field in the SigComp message was set to the
invalid value of 0.
18. BYTECODES_TOO_LARGE
The bytecodes that a SigComp message attempted to upload exceed
the amount of memory available in the receiving UDVM. The
details field is a two-byte expression of the
DECOMPRESSION_MEMORY_SIZE of the receiving UDVM. This value is
communicated most-significant-byte first.
19. INVALID_OPCODE
The UDVM attempted to identify an undefined byte value as an
instruction.
20. INVALID_STATE_PROBE
When attempting to retrieve state, the state_length operand is
set to 0 but the state_begin operand is non-zero.
21. ID_NOT_UNIQUE
A partial state identifier that was used to access state matched
more than one state item. Note that this error might be
returned as the result of executing a STATE-ACCESS instruction
or attempting to locate a unique piece of state as identified by
the "partial state identifier" in a SigComp message. The
"details" field contains the partial state identifier that was
requested.
22. MULTILOAD_OVERWRITTEN
A MULTILOAD instruction attempted to overwrite itself.
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RFC 4077 SigComp NACK May 2005
23. STATE_TOO_SHORT
A STATE-ACCESS instruction has attempted to copy more bytes from
a state item than the state item actually contains. The
"details" field contains the partial state identifier that was
requested. Implementors are cautioned to return only the
partial state identifier that was requested; if the NACK
contains any state identifier in addition to what was requested,
attackers may be able to use that additional information to
access the state.
24. INTERNAL_ERROR
The UDVM encountered an unexpected condition that prevented it
from decompressing the message.
25. FRAMING_ERROR
The UDVM encountered a framing error (unquoted 0xFF 80 .. 0xFF
FE in an input stream.) This error is applicable only to
messages received on a stream transport. In the case of a
framing error, a SHA-1 hash for a unique message cannot be
determined. Consequently, when a FRAMING_ERROR NACK is sent,
the "SHA-1 Hash of failed message" field should be set to all
zeros.
4. Security Considerations
4.1. Reflector Attacks
Because SigComp NACK messages are by necessity sent in response to
other messages, it is possible to trigger them by intentionally
sending malformed messages to a SigComp implementation with a spoofed
IP address. However, because such actions can only generate one
message for each message sent, they don't serve as amplifier attacks.
Further, due to the reasonably small size of NACK packets, there
cannot be a significant increase in the size of the packet generated.
It is worth noting that nearly all deployed protocols exhibit this
same behavior.
4.2. NACK Spoofing
Although it is possible to forge NACK messages as if they were
generated by a different node, the damage that can be caused is
minimal. Reporting a loss of state will typically result in nothing
more than the re-transmission of that state in a subsequent message.
Other failure codes would result in the next message being sent using
an alternate compression mechanism, or possibly uncompressed.
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RFC 4077 SigComp NACK May 2005
Although all of the above consequences result in slightly larger
messages, none of them have particularly catastrophic implications
for security.
5. IANA Considerations
This document defines a new value for the IANA registered attribute
SigComp_version.
Value (in hex): 02
Description: SigComp version 2 (NACK support)
Reference: [RFC 4077]
6. Acknowledgements
Thanks to Carsten Bormann, Zhigang Liu, Pekka Pessi, and Robert Sugar
for their comments and suggestions. Special thanks to Abigail
Surtees and Richard Price for several very detailed reviews and
suggestions.
7. References
7.1. Normative References
[1] Price, R., Bormann, C., Christoffersson, J., Hannu, H., Liu, Z.,
and J. Rosenberg, "Signaling Compression (SigComp)", RFC 3320,
January 2003.
[2] Hannu, H., Christoffersson, J., Forsgren, S., Leung, K.-C., Liu,
Z., and R. Price, "Signaling Compression (SigComp) - Extended
Operations", RFC 3321, January 2003.
7.2. Informative References
[3] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP:
Session Initiation Protocol", RFC 3261, June 2002.
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RFC 4077 SigComp NACK May 2005
Author's Address
Adam Roach
Estacado Systems
17210 Campbell Road
Suite 250
Dallas, TX 75252
US
EMail: adam@estacado.net
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RFC 4077 SigComp NACK May 2005
Full Copyright Statement
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A Negative Acknowledgement Mechanism for Signaling Compression
RFC TOTAL SIZE: 34250 bytes
PUBLICATION DATE: Monday, May 23rd, 2005
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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