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Versions: (draft-floyd-ccid4) 00 01 draft-ietf-dccp-ccid4

Internet Engineering Task Force                              Sally Floyd
INTERNET-DRAFT                                                      ICIR
draft-floyd-dccp-ccid4-00.txt                               Eddie Kohler
Expires: 21 April 2007                                              UCLA
                                                         21 October 2006


        Profile for Datagram Congestion Control Protocol (DCCP)
 Congestion ID 4: TCP-Friendly Rate Control for Small Packets (TFRC-SP)


Status of This Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
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   This Internet-Draft will expire on 21 April 2007.

Abstract

   This document contains the profile for Congestion Control Identifier
   4, the Small-Packet variant of TCP-Friendly Rate Control (TFRC), in
   the Datagram Congestion Control Protocol (DCCP).  CCID 4 is for
   experimental use, and uses TFRC-SP [TFRC-SP], a variant of TFRC
   designed for applications that send small packets.  The goal for
   TFRC-SP is to achieve roughly the same bandwidth in bits per second
   (bps) as a TCP flow using packets of up to 1500 bytes but



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   experiencing the same level of congestion.  CCID 4 is for
   experimental use for senders that send small packets and would like a
   TCP-friendly sending rate, possibly with Explicit Congestion
   Notification (ECN), while minimizing abrupt rate changes.















































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Table of Contents

   1. Introduction ....................................................5
   2. Conventions .....................................................5
   3. Usage ...........................................................6
      3.1. Relationship with TFRC .....................................6
      3.2. Example Half-Connection ....................................6
   4. Connection Establishment ........................................7
   5. Congestion Control on Data Packets ..............................7
      5.1. Response to Idle and Application-limited Periods ...........8
      5.2. Response to Data Dropped and Slow Receiver .................8
      5.3. Packet Sizes ...............................................9
   6. Acknowledgements ................................................9
      6.1. Loss Interval Definition ...................................9
      6.2. Congestion Control on Acknowledgements .....................9
      6.3. Acknowledgements of Acknowledgements .......................9
      6.4. Quiescence .................................................9
   7. Explicit Congestion Notification ................................9
   8. Options and Features ............................................9
      8.1. Window Counter Value ......................................10
      8.2. Elapsed Time Options ......................................11
      8.3. Receive Rate Option .......................................11
      8.4. Send Loss Event Rate Feature ..............................11
      8.5. Loss Event Rate Option ....................................11
      8.6. Loss Intervals Option .....................................11
      8.7. Dropped Packets Option ....................................11
      8.8. Send Dropped Packets Feature ..............................12
   9. Verifying Congestion Control Compliance With ECN ...............12
      9.1. Verifying the ECN Nonce Echo ..............................12
      9.2. Verifying the Reported Loss Intervals and Loss Event Rate
      ................................................................12
   10. Implementation Issues .........................................12
      10.1. Timestamp Usage ..........................................12
      10.2. Determining Loss Events at the Receiver ..................12
      10.3. Sending Feedback Packets .................................12
   11. Security Considerations .......................................12
   12. IANA Considerations ...........................................13
      12.1. Reset Codes ..............................................13
      12.2. Option Types .............................................13
      12.3. Feature Numbers ..........................................13
   13. Thanks ........................................................14
   Normative References ..............................................14
   Informative References ............................................14
   Authors' Addresses ................................................14
   Full Copyright Statement ..........................................15
   Intellectual Property .............................................15





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List of Tables

   Table 1: DCCP CCID 4 Options ......................................10
   Table 2: DCCP CCID 4 Feature Numbers ..............................10















































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1.  Introduction

   This document contains the profile for Congestion Control Identifier
   4, TCP-Friendly Rate Control for Small Packets (TFRC-SP), in the
   Datagram Congestion Control Protocol (DCCP) [RFC4340].  CCID 4
   differs from CCID 3 in that CCID 4 uses TFRC-SP, the Small-Packet
   variant of TFRC, while CCID 3 [RFC4342] uses standard TFRC [RFC3448].
   This document assumes that the reader is familiar with [RFC4342],
   instead of repeating from that document unnecessarily.

   CCID 4 differs from CCID 3 only in the following respects:

   o  Header size: For TFRC-SP, the allowed transmit rate in bytes per
      second is reduced by a factor that accounts for packet header
      size.  This is specified for TFRC-SP in Section 4.2 of [TFRC-SP],
      and described for CCID 4 in Section 5 below.

   o  Minimum sending rate: TFRC-SP enforces a minimum interval of
      10 milliseconds between data packets.  This is specified for TFRC-
      SP in Section 4.3 of [TFRC-SP], and described for CCID 4 in
      Section 5 below.

   o  Loss rates for short loss intervals: For short lost intervals of
      at most two round-trip times, the loss rate is computed by
      counting the actual number of packets lost or marked.  For such a
      short loss interval with N data packets, including K lost or
      marked data packets, the loss interval length is calculated as
      N/K, instead of as N.  This is specified for TFRC-SP in Section
      4.4 of [TFRC-SP].  The CCID 3 Dropped Packets option [CCID3-DP] is
      thus mandated above and beyond to CCID 3's Loss Intervals option,
      as specified in 8.7 below.  This section also describes the use of
      the Dropped Packets option in calculating the loss event rate.
      The computation of the loss rate by the receiver for the Loss
      Event Rate option is described for CCID 4 in Section 8.4 below.

   o  The nominal segment size: In TFRC-SP, the nominal segment size
      used by the TCP throughput equation is set to 1460 bytes.  This is
      specified for TFRC-SP in Section 4.5 of [RFC3448], and described
      for CCID 4 in Section 5 below.

2.  Conventions

   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 [RFC2119].

   Additional terminology is described in Section 2 of [RFC4342].




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3.  Usage

   Like CCID 3, CCID 4's congestion control is appropriate for flows
   that would prefer to minimize abrupt changes in the sending rate,
   including streaming media applications with small or moderate
   receiver buffering before playback.

   CCID 4 is designed to be used either by applications that use a small
   fixed segment size, or by applications that change their sending rate
   by varying the segment size.  If CCID 4 is used by an application
   that varies its segment size in response to changes in the allowed
   sending rate in bps, we note that CCID 4 doesn't dictate the segment
   size to be used by the application; this is done by the application
   itself.  The CCID 4 sender determines the allowed sending rate in
   bps, in response to on-going feedback from the CCID 4 receiver, and
   the application can use information about the current allowed sending
   rate to decide whether to change the current segment size.

   We note that in some environments there will be a feedback loop, with
   changes in the packet size or in the sending rate in bps affecting
   congestion along the path, therefore affecting the allowed sending
   rate in the future.

3.1.  Relationship with TFRC

   The congestion control mechanisms described here follow the TFRC-SP
   mechanism specified in [TFRC-SP].  As with CCID 3, conformant CCID 4
   implementations MAY track updates to the TCP throughput equation
   directly, as updates are standardized in the IETF, rather than
   waiting for revisions of this document.  However, conformant
   implementations SHOULD wait for explicit updates to CCID 4 before
   implementing other changes to TFRC congestion control.

3.2.  Example Half-Connection

   This example shows the typical progress of a half-connection using
   CCID 4's TFRC Congestion Control, not including connection initiation
   and termination.  The example is informative, not normative.  This
   example differs from that for CCID 3 in [RFC4342] only in that the
   allowed transmit rate is determined by [TFRC-SP] as well as by
   [RFC3448].

   1. The sender transmits DCCP-Data packets, where the sending rate is
      governed by the allowed transmit rate as specified in [TFRC-SP].
      Each DCCP-Data packet has a sequence number, and the DCCP header's
      CCVal field contains the window counter value, used by the
      receiver in determining when multiple losses belong in a single
      loss event.



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      In the typical case of an ECN-capable half-connection, each DCCP-
      Data and DCCP-DataAck packet is sent as ECN-Capable, with either
      the ECT(0) or the ECT(1) codepoint set.  The use of the ECN Nonce
      with TFRC is described in Section 9.

   2. The receiver sends DCCP-Ack packets at least once per round-trip
      time acknowledging the data packets, unless the sender is sending
      at a rate of less than one packet per round-trip time, as
      indicated by the TFRC specification [RFC3448] (Section 6).  Each
      DCCP-Ack packet uses a sequence number, identifies the most recent
      packet received from the sender, and includes feedback about the
      recent loss intervals experienced by the receiver.

   3. The sender continues sending DCCP-Data packets as controlled by
      the allowed transmit rate.  Upon receiving DCCP-Ack packets, the
      sender updates its allowed transmit rate as specified in [RFC3448]
      (Section 4.3)  and [TFRC-SP].  This update is based upon a loss
      event rate calculated by the sender, based on the receiver's loss
      intervals feedback.  If it prefers, the sender can also use a loss
      event rate calculated and reported by the receiver.

   4. The sender estimates round-trip times and calculates a nofeedback
      time, as specified in [RFC3448] (Section 4.4).  If no feedback is
      received from the receiver in that time (at least four round-trip
      times), the sender halves its sending rate.

4.  Connection Establishment

   The connection establishment is as specified in Section 4 of
   [RFC4342].

5.  Congestion Control on Data Packets

   CCID 4 uses the congestion control mechanisms of TFRC [RFC3448] and
   TFRC-SP [TFRC-SP].  [TFRC-SP] should be considered normative except
   where specifically indicated.

   Loss Event Rate

   As with CCID 3, the basic operation of CCID 4 centers around the
   calculation of a loss event rate: the number of loss events as a
   fraction of the number of packets transmitted, weighted over the last
   several loss intervals.  For CCID 4, this loss event rate, a round-
   trip time estimate, and a nominal packet size of 1460 bytes are
   plugged into the TCP throughput equation, as specified in RFC 3448
   (Section 3.1) and [TFRC-SP].





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   Because CCID 4 is intended for applications that send small packets,
   the allowed transmit rate derived from the TCP throughput equation is
   reduced by a factor that accounts for packet header size, as
   specified in Section 4.2 of [TFRC-SP].  The header size on data
   packets is estimated as 32 bytes (20 bytes for the IP header, and 12
   bytes for the DCCP-Data header with 24-bit sequence numbers).  If the
   DCCP sender is sending N-byte data packets, the allowed transmit rate
   is reduced by N/(N+32).  CCID 4 senders are limited to this fair
   rate.

   The loss event rate itself is calculated in CCID 4 using recent loss
   interval lengths reported by the receiver.  Loss intervals are
   precisely defined in Section 6.1 of [RFC4342],  with the modification
   in [TFRC-SP] (Section 3) for loss intervals of at most two round-trip
   times.  In summary, a loss interval is up to 1 RTT of possibly lost
   or ECN-marked data packets, followed by an arbitrary number of non-
   dropped, non-marked data packets.  The CCID 3 Loss Intervals option
   is used to report loss interval lengths; see Section 8.6.

   For loss intervals of at most two round-trip times, CCID 4 calculates
   the loss event rate for that interval by counting the number of
   packets lost or marked, as described in Section 4.4 of [TFRC-SP].
   Thus, for such a short loss interval with N data packets, including K
   lost or marked data packets, the loss interval length is calculated
   as N/K, instead as N.  The CCID 3 Dropped Packets option is used to
   report K, the count of lost or marked data packets.

   Unlike CCID 3, the CCID 4 sender enforces a minimum interval of 10 ms
   between data packets, regardless of the allowed transmit rate.

   Other Congestion Control Mechanisms

   The other congestion control mechanisms such as slow-start, feedback
   packets, and the like are exactly as in CCID 3, and are described in
   the subsection on "Other Congestion Control Mechanisms" of Section 5
   in [RFC4342].

5.1.  Response to Idle and Application-limited Periods

   This is described in Section 5.1 of [RFC4342].

5.2.  Response to Data Dropped and Slow Receiver

   This is described in Section 5.2 of [RFC4342].







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5.3.  Packet Sizes

   CCID 4 is intended for applications that use a fixed small segment
   size, or that vary their segment size in response to congestion.

   The CCID 4 sender uses a segment size of 1460 bytes in the TCP
   throughput equation.  This gives the CCID 4 sender roughly the same
   sending rate in bytes per second as a TFRC flow using 1460-byte
   segments but experiencing the same packet drop rate.

6.  Acknowledgements

   The acknowledgements are as specified in Section 6 of [RFC4342] with
   the exception of the Loss Interval lengths specified below.

6.1.  Loss Interval Definition

   The loss interval definition is as defined in Section 6.1 of
   [RFC4342].

6.2.  Congestion Control on Acknowledgements

   The congestion control on acknowledgements is as specified in Section
   6.2 of [RFC4342].

6.3.  Acknowledgements of Acknowledgements

   Procedures for the acknowledgement of acknowledgements are as
   specified in Section 6.3 of [RFC4342].

6.4.  Quiescence

   The procedure for detecting that the sender has gone quiescent is as
   specified in Section 6.4 of [RFC4342].

7.  Explicit Congestion Notification

   Procedures for the use of Explicit Congestion Notification (ECN) are
   as specified in Section 7 of [RFC4342].

8.  Options and Features

   CCID 4 can make use of DCCP's Ack Vector, Timestamp, Timestamp Echo,
   and Elapsed Time options, and its Send Ack Vector and ECN Incapable
   features.  CCID 4 also imports the currently defined CCID 3-specific
   options and features [RFC4342], augmented by the Dropped Packets
   options and features [CCID3-DP].  Each CCID 4-specific option and
   feature contains the same data as the corresponding CCID 3 option or



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   feature, and is interpreted in the same way, except as specified
   elsewhere in this document.

                  Option                        DCCP-   Section
         Type     Length     Meaning            Data?  Reference
         -----    ------     -------            -----  ---------
        128-191              Reserved
          192        6       Loss Event Rate      N      8.5
          193     variable   Loss Intervals       N      8.6
          194        6       Receive Rate         N      8.3
          195     variable   Dropped Packets      N      8.7
        196-255              Reserved

                      Table 1: DCCP CCID 4 Options
   The "DCCP-Data?" column indicates that all currently defined
   CCID 4-specific options MUST be ignored when they occur on DCCP-Data
   packets.

   As with CCID 3, the following CCID-specific features are also
   defined.

                                       Rec'n Initial        Section
     Number   Meaning                  Rule   Value  Req'd Reference
     ------   -------                  -----  -----  ----- ---------
     128-191  Reserved
       192    Send Loss Event Rate      SP      0      N      8.4
     193-194  Reserved
       195    Send Dropped Packets      SP      0      N
     196-255  Reserved

                  Table 2: DCCP CCID 4 Feature Numbers
   More information is available in Section 8 of [RFC4342] and in
   [CCID3-DP].

8.1.  Window Counter Value

   The use of the Window Counter Value in the DCCP generic header's
   CCVal field is as specified in Section 8.1 of [RFC4342].  In addition
   to their use described in CCID 3, the CCVal counters are used by the
   receiver in CCID 4 to determine when the length of a loss interval is
   at most two round-trip times.  None of these procedures require the
   receiver to maintain an explicit estimate of the round-trip time.
   However, Section 8.1 of [RFC4342] gives a procedure that implementors
   may use if they wish to keep such an RTT estimate using CCVal.







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8.2.  Elapsed Time Options

   The use of the Elapsed Time option is defined in Section 8.2 of
   [RFC4342].

8.3.  Receive Rate Option

   The Receive Rate option is as specified in Section 8.3 of [RFC4342].

8.4.  Send Loss Event Rate Feature

   The Send Loss Event Rate feature is as defined in Section 8.4 of
   [RFC4342].

   See [RFC3448], Section 5 and [TFRC-SP], Section 4.4 for a normative
   calculation of the loss event rate.  Section 4.4 of [TFRC-SP]
   modifies the calculation of the loss interval size for loss intervals
   of at most two round-trip times.

   If the CCID 4 receiver is using the Loss Event Rate option, the
   receiver needs to be able to determine if a loss interval is short,
   of at most two round-trip times.  The receiver can heuristically
   detect a short loss interval by using the Window Counter in arriving
   data packets.  The sender increases the Window Counter by 1 every
   quarter of a round-trip time, with the caveat that the Window Counter
   is never increased by more than five, modulo 16, from one data packet
   to the next.  Using the Window Counter to detect loss intervals of at
   most two round-trip times could result in some false positives, with
   some longer loss intervals incorrectly identified as short ones.

8.5.  Loss Event Rate Option

   The Loss Event Rate option is as specified in Section 8.5 of
   [RFC4342].

   See [RFC3448] (Section 5) and [TFRC-SP] for a normative calculation
   of loss event rate.

8.6.  Loss Intervals Option

   The Loss Intervals option is as specified in Section 8.6 of
   [RFC4342].

8.7.  Dropped Packets Option

   The Dropped Packets option is as specified in [CCID3-DP].  CCID 4
   receivers MUST always include Dropped Packets options on their
   feedback packets, regardless of the value of the Send Dropped Packets



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   feature.  If, nevertheless, a feedback packet does not include a
   relevant Dropped Packets option, a CCID 4 sender MUST act as if the
   relevant loss intervals' Drop Counts equal the corresponding Loss
   Lengths, as specified in [CCID3-DP].

8.8.  Send Dropped Packets Feature

   The Send Dropped Packets feature is as specified in [CCID3-DP].

9.  Verifying Congestion Control Compliance With ECN

   Verifying congestion control compliance with ECN is as discussed in
   Section 9 of [RFC4342].

9.1.  Verifying the ECN Nonce Echo

   Procedures for verifying the ECN Nonce Echo are as specified in
   Section 9.1 of [RFC4342].

9.2.  Verifying the Reported Loss Intervals and Loss Event Rate

   Section 9.2 of [RFC4342] discusses the sender's possible verification
   of loss intervals and loss event rate information reported by the
   receiver.

10.  Implementation Issues

10.1.  Timestamp Usage

   The use of the Timestamp option is as discussed in Section 10.1 of
   [RFC4342].

10.2.  Determining Loss Events at the Receiver

   The use of the window counter by the receiver to determine if
   multiple lost packets belong to the same loss event is as described
   in Section 10.2 of [RFC4342].

10.3.  Sending Feedback Packets

   The procedure for sending feedback packets is as described in Section
   10.3 of [RFC4342].


11.  Security Considerations

   Security considerations include those discussed in Section 11 of
   [RFC4342]. There are no new security considerations introduced by



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   CCID 4.

12.  IANA Considerations

   This specification defines the value 4 in the DCCP CCID namespace
   managed by IANA.

   CCID 4 also uses three sets of numbers whose values should be
   allocated by IANA, namely CCID 4-specific Reset Codes, option types,
   and feature numbers.  This document makes no particular allocations
   from the Reset Code range, except for experimental and testing use
   [RFC3692].  We refer to the Standards Action policy outlined in
   [RFC2434].

12.1.  Reset Codes

   Each entry in the DCCP CCID 4 Reset Code registry contains a
   CCID 4-specific Reset Code, which is a number in the range 128-255; a
   short description of the Reset Code; and a reference to the RFC
   defining the Reset Code.  Reset Codes 184-190 and 248-254 are
   permanently reserved for experimental and testing use.  The remaining
   Reset Codes -- 128-183, 191-247, and 255 -- are currently reserved,
   and should be allocated with the Standards Action policy, which
   requires IESG review and approval and standards-track IETF RFC
   publication.

12.2.  Option Types

   Each entry in the DCCP CCID 4 option type registry contains a
   CCID 4-specific option type, which is a number in the range 128-255;
   the name of the option, such as "Loss Intervals"; and a reference to
   the RFC defining the option type.  The registry is initially
   populated using the values in Table 1, in Section 8.  This document
   allocates option types 192-195, and option types 184-190 and 248-254
   are permanently reserved for experimental and testing use.  The
   remaining option types -- 128-183, 191, 196-247, and 255 -- are
   currently reserved, and should be allocated with the Standards Action
   policy, which requires IESG review and approval and standards-track
   IETF RFC publication.

12.3.  Feature Numbers

   Each entry in the DCCP CCID 4 feature number registry contains a
   CCID 4-specific feature number, which is a number in the range
   128-255; the name of the feature, such as "Send Loss Event Rate"; and
   a reference to the RFC defining the feature number.  The registry is
   initially populated using the values in Table 2, in Section 8.  This
   document allocates feature numbers 192 and 195, and feature numbers



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   184-190 and 248-254 are permanently reserved for experimental and
   testing use.  The remaining feature numbers -- 128-183, 191, 193-194,
   196-247, and 255 -- are currently reserved, and should be allocated
   with the Standards Action policy, which requires IESG review and
   approval and standards-track IETF RFC publication.

13.  Thanks


Normative References

   [RFC2119]      S. Bradner. Key Words For Use in RFCs to Indicate
                  Requirement Levels. RFC 2119.

   [RFC2434]      T. Narten and H. Alvestrand.  Guidelines for Writing
                  an IANA Considerations Section in RFCs.  RFC 2434.

   [RFC3448]      M. Handley, S. Floyd, J. Padhye, and J. Widmer, TCP
                  Friendly Rate Control (TFRC): Protocol Specification,
                  RFC 3448, Proposed Standard, January 2003.

   [RFC3692]      T. Narten.  Assigning Experimental and Testing Numbers
                  Considered Useful.  RFC 3692.

   [RFC4340]      Kohler, E., Handley, M., and S. Floyd.  Datagram
                  Congestion Control Protocol (DCCP), RFC 4340, March
                  2006.

   [RFC4342]      Floyd, S., Kohler, E., and J. Padhye.  Profile for
                  Datagram Congestion Control Protocol (DCCP) Congestion
                  Control ID 3: TCP-Friendly Rate Control (TFRC), RFC
                  4342, March 2006.

   [TFRC-SP]      S. Floyd and E. Kohler.  TCP Friendly Rate Control
                  (TFRC): the Small-Packet (SP) Variant.  Internet-draft
                  draft-ietf-dccp-tfrc-voip-05.txt, March 2005.

   [CCID3-DP]     Kohler, E., Datagram Congestion Control Protocol
                  (DCCP) Congestion Control ID 3 Dropped Packets Option,
                  Internet-draft draft-kohler-dccp-ccid3-drops-00.txt,
                  August 2006.  URL "http://www.read.cs.ucla.edu/dccp/".

Informative References

Authors' Addresses

   Sally Floyd <floyd@icir.org>
   ICSI Center for Internet Research



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   1947 Center Street, Suite 600
   Berkeley, CA 94704
   USA

   Eddie Kohler <kohler@cs.ucla.edu>
   4531C Boelter Hall
   UCLA Computer Science Department
   Los Angeles, CA 90095
   USA

Full Copyright Statement

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   specification can be obtained from the IETF on-line IPR repository at
   http://www.ietf.org/ipr.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
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   this standard.  Please address the information to the IETF at ietf-
   ipr@ietf.org.




Floyd, et al.                                                  [Page 15]


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