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Versions: 00 01 02 03 04 05 rfc2581                      Standards Track
TCP Implementation Working Group                               M. Allman
INTERNET DRAFT                              NASA Lewis/Sterling Software
File: draft-ietf-tcpimpl-cong-control-05.txt                   V. Paxson
                                                              W. Stevens
                                                          February, 1999

                        TCP Congestion Control

Status of this Memo

    This document is an Internet-Draft and is in full conformance with
    all provisions of Section 10 of RFC2026.  Internet-Draft.
    Internet-Drafts are working documents of the Internet Engineering
    Task Force (IETF), its areas, and its working groups.  Note that
    other groups may also distribute working documents as

    Internet-Drafts are draft documents valid for a maximum of six
    months and may be updated, replaced, or obsoleted by other documents
    at any time.  It is inappropriate to use Internet-Drafts as
    reference material or to cite them other than as ``work in

    To view the entire list of current Internet-Drafts, please check the
    "1id-abstracts.txt" listing contained in the Internet-Drafts Shadow
    Directories on ftp.is.co.za (Africa), ftp.nordu.net (Northern
    Europe), ftp.nis.garr.it (Southern Europe), munnari.oz.au (Pacific
    Rim), ftp.ietf.org (US East Coast), or ftp.isi.edu (US West Coast).


    This document defines TCP's four intertwined congestion control
    algorithms: slow start, congestion avoidance, fast retransmit, and
    fast recovery.  In addition, the document specifies how TCP should
    begin transmission after a relatively long idle period, as well as
    discussing various acknowledgment generation methods.

1   Introduction

    This document specifies four TCP [Pos81] congestion control
    algorithms: slow start, congestion avoidance, fast retransmit and
    fast recovery.  These algorithms were devised in [Jac88] and
    [Jac90].  Their use with TCP is standardized in [Bra89].

    This document is an update of [Ste97].  In addition to specifying
    the congestion control algorithms, this document specifies what TCP
    connections should do after a relatively long idle period, as well
    as specifying and clarifying some of the issues pertaining to TCP
    ACK generation.

    Note that [Ste94] provides examples of these algorithms in action
    and [WS95] provides an explanation of the source code for the BSD

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    implementation of these algorithms.

    This document is organized as follows.  Section 2 provides various
    definitions which will be used throughout the document.  Section 3
    provides a specification of the congestion control algorithms.
    Section 4 outlines concerns related to the congestion control
    algorithms and finally, section 5 outlines security considerations.

    The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
    document are to be interpreted as described in [Bra97].

2   Definitions

    This section provides the definition of several terms that will be
    used throughout the remainder of this document.

        A segment is ANY TCP/IP data or acknowledgment packet (or both).

        The SMSS is the size of the largest segment that the sender can
        transmit.  This value can be based on the maximum transmission
        unit of the network, the path MTU discovery [MD90] algorithm,
        RMSS (see next item), or other factors.  The size does not
        include the TCP/IP headers and options.

        The RMSS is the size of the largest segment the receiver is
        willing to accept.  This is the value specified in the MSS
        option sent by the receiver during connection startup.  Or, if
        the MSS option is not used, 536 bytes [Bra89].  The size does
        not include the TCP/IP headers and options.

        A segment that contains the maximum number of data bytes
        permitted (i.e., a segment containing SMSS bytes of data).

        The most recently advertised receiver window.

        A TCP state variable that limits the amount of data a TCP can
        send.  At any given time, a TCP MUST NOT send data with a
        sequence number higher than the sum of the highest acknowledged
        sequence number and the minimum of cwnd and rwnd.

        The initial window is the size of the sender's congestion window
        after the three-way handshake is completed.

        The loss window is the size of the congestion window after a TCP
        sender detects loss using its retransmission timer.

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        The restart window is the size of the congestion window after a
        TCP restarts transmission after an idle period (if the slow
        start algorithm is used; see section 4.1 for more discussion).

        The amount of data that has been sent but not yet acknowledged.

3  Congestion Control Algorithms

    This section defines the four congestion control algorithms: slow
    start, congestion avoidance, fast retransmit and fast recovery,
    developed in [Jac88] and [Jac90].  In some situations it may be
    beneficial for a TCP sender to be more conservative than the
    algorithms allow, however a TCP MUST NOT be more aggressive than the
    following algorithms allow (that is, MUST NOT send data when the
    value of cwnd computed by the following algorithms would not allow
    the data to be sent).

3.1 Slow Start and Congestion Avoidance

    The slow start and congestion avoidance algorithms MUST be used by a
    TCP sender to control the amount of outstanding data being injected
    into the network.  To implement these algorithms, two variables are
    added to the TCP per-connection state.  The congestion window (cwnd)
    is a sender-side limit on the amount of data the sender can transmit
    into the network before receiving an acknowledgment (ACK), while the
    receiver's advertised window (rwnd) is a receiver-side limit on the
    amount of outstanding data.  The minimum of cwnd and rwnd governs
    data transmission.

    Another state variable, the slow start threshold (ssthresh), is used
    to determine whether the slow start or congestion avoidance
    algorithm is used to control data transmission, as discussed below.

    Beginning transmission into a network with unknown conditions
    requires TCP to slowly probe the network to determine the available
    capacity, in order to avoid congesting the network with an
    inappropriately large burst of data.  The slow start algorithm is
    used for this purpose at the beginning of a transfer, or after
    repairing loss detected by the retransmission timer.

    IW, the initial value of cwnd, MUST be less than or equal to 2*SMSS
    bytes and MUST NOT be more than 2 segments.

    We note that a non-standard, experimental TCP extension allows that
    a TCP MAY use a larger initial window (IW), as defined in equation 1

               IW = min (4*SMSS, max (2*SMSS, 4380 bytes))           (1)

    With this extension, a TCP sender MAY use a 3 or 4 segment initial

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    window, provided the combined size of the segments does not exceed
    4380 bytes.  We do NOT allow this change as part of the standard
    defined by this document.  However, we include discussion of (1) in
    the remainder of this document as a guideline for those
    experimenting with the change, rather than conforming to the present
    standards for TCP congestion control.

    The initial value of ssthresh MAY be arbitrarily high (for example,
    some implementations use the size of the advertised window), but it
    may be reduced in response to congestion.  The slow start algorithm
    is used when cwnd < ssthresh, while the congestion avoidance
    algorithm is used when cwnd > ssthresh.  When cwnd and ssthresh are
    equal the sender may use either slow start or congestion avoidance.

    During slow start, a TCP increments cwnd by at most SMSS bytes for
    each ACK received that acknowledges new data.  Slow start ends when
    cwnd exceeds ssthresh (or, optionally, when it reaches it, as noted
    above) or when congestion is observed.

    During congestion avoidance, cwnd is incremented by 1 full-sized
    segment per round-trip time (RTT).  Congestion avoidance continues
    until cwnd congestion is detected.  One formula commonly used to
    update cwnd during congestion avoidance is given in equation 2:

                          cwnd += SMSS*SMSS/cwnd                     (2)

    This adjustment is executed on every incoming non-duplicate ACK.
    Equation (2) provides an acceptable approximation to the underlying
    principle of increasing cwnd by 1 full-sized segment per RTT.  (Note
    that for a connection in which the receiver acknowledges every data
    segment, (2) proves slightly more aggressive than 1 segment per RTT,
    and for a receiver acknowledging every-other packet, (2) is less

    Implementation Note: Since integer arithmetic is usually used in TCP
    implementations, the formula given in equation 2 can fail to
    increase cwnd when the congestion window is very large (larger than
    SMSS*SMSS).  If the above formula yields 0, the result SHOULD be
    rounded up to 1 byte.

    Implementation Note: older implementations have an additional
    additive constant on the right-hand side of equation (2).  This is
    incorrect and can actually lead to diminished performance [PAD+98].

    Another acceptable way to increase cwnd during congestion avoidance
    is to count the number of bytes that have been acknowledged by ACKs
    for new data.  (A drawback of this implementation is that it
    requires maintaining an additional state variable.)  When the number
    of bytes acknowledged reaches cwnd, then cwnd can be incremented by
    up to SMSS bytes.  Note that during congestion avoidance, cwnd MUST
    NOT be increased by more than the larger of either 1 full-sized
    segment per RTT, or the value computed using equation 2.

    Implementation Note: some implementations maintain cwnd in units of

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    bytes, while others in units of full-sized segments.  The latter
    will find equation (2) difficult to use, and may prefer to use the
    counting approach discussed in the previous paragraph.

    When a TCP sender detects segment loss using the retransmission
    timer, the value of ssthresh MUST be set to no more than the value
    given in equation 3:

                   ssthresh = max (FlightSize / 2, 2*SMSS)            (3)

    As discussed above, FlightSize is the amount of outstanding data in
    the network.

    Implementation Note: an easy mistake to make is to simply use cwnd,
    rather than FlightSize, which in some implementations may
    incidentally increase well beyond rwnd.

    Furthermore, upon a timeout cwnd MUST be set to no more than the
    loss window, LW, which equals 1 full-sized segment (regardless of
    the value of IW).  Therefore, after retransmitting the dropped
    segment the TCP sender uses the slow start algorithm to increase the
    window from 1 full-sized segment to the new value of ssthresh, at
    which point congestion avoidance again takes over.

3.2 Fast Retransmit/Fast Recovery

    A TCP receiver SHOULD send an immediate duplicate ACK when an
    out-of-order segment arrives.  The purpose of this ACK is to inform
    the sender that a segment was received out-of-order and which
    sequence number is expected.  From the sender's perspective,
    duplicate ACKs can be caused by a number of network problems.
    First, they can be caused by dropped segments.  In this case, all
    segments after the dropped segment will trigger duplicate ACKs.
    Second, duplicate ACKs can be caused by the re-ordering of data
    segments by the network (not a rare event along some network paths
    [Pax97]).  Finally, duplicate ACKs can be caused by replication of
    ACK or data segments by the network.  In addition, a TCP receiver
    SHOULD send an immediate ACK when the incoming segment fills in all
    or part of a gap in the sequence space.  This will generate more
    timely information for a sender recovering from a loss through a
    retransmission timeout, a fast retransmit, or an experimental loss
    recovery algorithm, such as NewReno [FH98].

    The TCP sender SHOULD use the "fast retransmit" algorithm to detect
    and repair loss, based on incoming duplicate ACKs.  The fast
    retransmit algorithm uses the arrival of 3 duplicate ACKs (4
    identical ACKs without the arrival of any other intervening packets)
    as an indication that a segment has been lost.  After receiving 3
    duplicate ACKs, TCP performs a retransmission of what appears to be
    the missing segment, without waiting for the retransmission timer to

    After the fast retransmit algorithm sends what appears to be the
    missing segment, the "fast recovery" algorithm governs the

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    transmission of new data until a non-duplicate ACK arrives.  The
    reason for not performing slow start is that the receipt of the
    duplicate ACKs not only indicates that a segment has been lost, but
    also that segments are most likely leaving the network (although a
    massive segment duplication by the network can invalidate this
    conclusion).  In other words, since the receiver can only generate a
    duplicate ACK when a segment has arrived, that segment has left the
    network and is in the receiver's buffer, so we know it is no longer
    consuming network resources.  Furthermore, since the ACK "clock"
    [Jac88] is preserved, the TCP sender can continue to transmit new
    segments (although transmission must continue using a reduced cwnd).

    The fast retransmit and fast recovery algorithms are usually
    implemented together as follows.

    1.  When the third duplicate ACK is received, set ssthresh to no
        more than the value given in equation 3.

    2.  Retransmit the lost segment and set cwnd to ssthresh plus
        3*SMSS.  This artificially "inflates" the congestion window by
        the number of segments (three) that have left the network and
        which the receiver has buffered.

    3.  For each additional duplicate ACK received, increment cwnd by
        SMSS.  This artificially inflates the congestion window in order
        to reflect the additional segment that has left the network.

    4.  Transmit a segment, if allowed by the new value of cwnd and the
        receiver's advertised window.

    5.  When the next ACK arrives that acknowledges new data, set cwnd
        to ssthresh (the value set in step 1).  This is termed
        "deflating" the window.

        This ACK should be the acknowledgment elicited by the
        retransmission from step 1, one RTT after the retransmission
        (though it may arrive sooner in the presence of significant
        out-of-order delivery of data segments at the receiver).
        Additionally, this ACK should acknowledge all the intermediate
        segments sent between the lost segment and the receipt of the
        third duplicate ACK, if none of these were lost.

    Note: This algorithm is known to generally not recover very
    efficiently from multiple losses in a single flight of packets
    [FF96].  One proposed set of modifications to it to address this
    problem can be found in [FH98].

4   Additional Considerations

4.1 Re-starting Idle Connections

    A known problem with the TCP congestion control algorithms described
    above is that they allow a potentially inappropriate burst of
    traffic to be transmitted after TCP has been idle for a relatively

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    long period of time.  After an idle period, TCP cannot use the ACK
    clock to strobe new segments into the network, as all the ACKs have
    drained from the network.  Therefore, as specified above, TCP can
    potentially send a cwnd-size line-rate burst into the network after
    an idle period.

    [Jac88] recommends that a TCP use slow start to restart transmission
    after a relatively long idle period.  Slow start serves to restart
    the ACK clock, just as it does at the beginning of a transfer.  This
    mechanism has been widely deployed in the following manner.  When
    TCP has not received a segment for more than one retransmission
    timeout, cwnd is reduced to the value of the restart window (RW)
    before transmission begins.

    For the purposes of this standard, we define RW = IW.

    We note that the non-standard experimental extension to TCP defined
    in [AFP98] defines RW = min(IW, cwnd), with the definition of IW
    adjusted per equation (1) above.

    Using the last time a segment was received to determine whether or
    not to decrease cwnd fails to deflate cwnd in the common case of
    persistent HTTP connections [HTH98].  In this case, a WWW server
    receives a request before transmitting data to the WWW browser.  The
    reception of the request makes the test for an idle connection fail,
    and allows the TCP to begin transmission with a possibly
    inappropriately large cwnd.

    Therefore, a TCP SHOULD set cwnd to no more than RW before beginning
    transmission if the TCP has not sent data in an interval exceeding
    the retransmission timeout.

4.2 Generating Acknowledgments

    The delayed ACK algorithm specified in [Bra89] SHOULD be used by a
    TCP receiver.  When used, a TCP receiver MUST NOT excessively delay
    acknowledgments.  Specifically, an ACK SHOULD be generated for at
    least every second full-sized segment, and MUST be generated within
    500 ms of the arrival of the first unacknowledged packet.

    The requirement that an ACK "SHOULD" be generated for at least every
    second full-sized segment is listed in [Bra89] in one place as a
    SHOULD and another as a MUST.  Here we unambiguously state it is a
    SHOULD.  We also emphasize that this is a SHOULD, meaning that an
    implementor should indeed only deviate from this requirement after
    careful consideration of the implications.  See the discussion of
    "Stretch ACK violation" in [PAD+98] and the references therein for a
    discussion of the possible performance problems with generating ACKs
    less frequently than every second full-sized segment.

    In some cases, the sender and receiver may not agree on what
    constitutes a full-sized segment.  An implementation is deemed to
    comply with this requirement if it sends at least one acknowledgment
    every time it receives 2*RMSS bytes of new data from the sender,

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    where RMSS is the Maximum Segment Size specified by the receiver to
    the sender (or the default value of 536 bytes, per [Bra89], if the
    receiver does not specify an MSS option during connection
    establishment).  The sender may be forced to use a segment size less
    than RMSS due to the maximum transmission unit (MTU), the path MTU
    discovery algorithm or other factors.  For instance, consider the
    case when the receiver announces an RMSS of X bytes but the sender
    ends up using a segment size of Y bytes (Y < X) due to path MTU
    discovery (or the sender's MTU size).  The receiver will generate
    stretch ACKs if it waits for 2*X bytes to arrive before an ACK is
    sent.  Clearly this will take more than 2 segments of size Y bytes.
    Therefore, while a specific algorithm is not defined, it is
    desirable for receivers to attempt to prevent this situation, for
    example by acknowledging at least every second segment, regardless
    of size.  Finally, we repeat that an ACK MUST NOT be delayed for
    more than 500 ms waiting on a second full-sized segment to arrive.

    Out-of-order data segments SHOULD be acknowledged immediately, in
    order to accelerate loss recovery.  To trigger the fast retransmit
    algorithm, the receiver SHOULD send an immediate duplicate ACK when
    it receives a data segment above a gap in the sequence space.  To
    provide feedback to senders recovering from losses, the receiver
    SHOULD send an immediate ACK when it receives a data segment that
    fills in all or part of a gap in the sequence space.

    A TCP receiver MUST NOT generate more than one ACK for every
    incoming segment, other than to update the offered window as the
    receiving application consumes new data [page 42, Pos81][Cla82].

4.3 Loss Recovery Mechanisms

    A number of loss recovery algorithms that augment fast retransmit
    and fast recovery have been suggested by TCP researchers.  While
    some of these algorithms are based on the TCP selective
    acknowledgment (SACK) option [MMFR96], such as [FF96,MM96a,MM96b],
    others do not require SACKs [Hoe96,FF96,FH98].  The non-SACK
    algorithms use "partial acknowledgments" (ACKs which cover new data,
    but not all the data outstanding when loss was detected) to trigger
    retransmissions.  While this document does not standardize any of
    the specific algorithms that may improve fast retransmit/fast
    recovery, these enhanced algorithms are implicitly allowed, as long
    as they follow the general principles of the basic four algorithms
    outlined above.

    Therefore, when the first loss in a window of data is detected,
    ssthresh MUST be set to no more than the value given by equation
    (3).  Second, until all lost segments in the window of data in
    question are repaired, the number of segments transmitted in each
    RTT MUST be no more than half the number of outstanding segments
    when the loss was detected.  Finally, after all loss in the given
    window of segments has been successfully retransmitted, cwnd MUST be
    set to no more than ssthresh and congestion avoidance MUST be used
    to further increase cwnd.  Loss in two successive windows of data,
    or the loss of a retransmission, should be taken as two indications

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    of congestion and, therefore, cwnd (and ssthresh) MUST be lowered
    twice in this case.  The algorithms outlined in
    [Hoe96,FF96,MM96a,MM6b] follow the principles of the basic four
    congestion control algorithms outlined in this document.

5.  Security Considerations

    This document requires a TCP to diminish its sending rate in the
    presence of retransmission timeouts and the arrival of duplicate
    acknowledgments.  An attacker can therefore impair the performance
    of a TCP connection by either causing data packets or their
    acknowledgments to be lost, or by forging excessive duplicate
    acknowledgments.  Causing two congestion control events back-to-back
    will often cut ssthresh to its minimum value of 2*SMSS, causing the
    connection to immediately enter the slower-performing congestion
    avoidance phase.

    The Internet to a considerable degree relies on the correct
    implementation of these algorithms in order to preserve network
    stability and avoid congestion collapse.  An attacker could cause
    TCP endpoints to respond more aggressively in the face of congestion
    by forging excessive duplicate acknowledgments or excessive
    acknowledgments for new data.  Conceivably, such an attack could
    drive a portion of the network into congestion collapse.

6.  Changes Relative to RFC 2001

    This document has been extensively rewritten editorially and it is
    not feasible to itemize the list of changes between the two
    documents. The intention of this document is not to change any of
    the recommendations given in RFC 2001, but to further clarify cases
    that were not discussed in detail in 2001. Specifically, this
    document suggests what TCP connections should do after a relatively
    long idle period, as well as specifying and clarifying some of the
    issues pertaining to TCP ACK generation.  Finally, the allowable
    upper bound for the initial congestion window has also been raised
    from one to two segments.


    The four algorithms that are described were developed by Van

    Some of the text from this document is taken from "TCP/IP
    Illustrated, Volume 1: The Protocols" by W. Richard Stevens
    (Addison-Wesley, 1994) and "TCP/IP Illustrated, Volume 2: The
    Implementation" by Gary R. Wright and W.  Richard Stevens
    (Addison-Wesley, 1995).  This material is used with the permission
    of Addison-Wesley.

    Neal Cardwell, Sally Floyd, Craig Partridge and Joe Touch
    contributed a number of helpful suggestions.

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    [AFP98] M. Allman, S. Floyd, C. Partridge, Increasing TCP's Initial
        Window Size, September 1998.  RFC 2414.

    [Bra89] B. Braden, ed., Requirements for Internet Hosts --
        Communication Layers, RFC 1122, Oct. 1989.

    [Bra97] S. Bradner, Key words for use in RFCs to Indicate
        Requirement Levels, BCP 14, RFC 2119, March 1997.

    [Cla82] D. Clark, Window and Acknowledgment Strategy in TCP, RFC
        813.  July 1982.

    [FF96] K. Fall, S. Floyd.  Simulation-based Comparisons of Tahoe,
        Reno and SACK TCP.  Computer Communication Review, July 1996.

    [FH98] S. Floyd, T. Henderson.  The NewReno Modification to TCP's
        Fast Recovery Algorithm.  Internet-Draft
        draft-ietf-tcpimpl-newreno-00.txt, November 1998.  (Work in

    [Flo94] S. Floyd, TCP and Successive Fast Retransmits. Technical
        report, October 1994.

    [Hoe96] J. Hoe, Improving the Start-up Behavior of a Congestion
        Control Scheme for TCP.  In ACM SIGCOMM, August 1996.

    [HTH98] A. Hughes, J. Touch, J. Heidemann.  Issues in TCP Slow-Start
        Restart After Idle.  Internet-Draft
        draft-ietf-tcpimpl-restart-00.txt, March 1998.  (Work in

    [Jac88] V. Jacobson, Congestion Avoidance and Control, Computer
        Communication Review, vol. 18, no. 4, pp. 314-329, Aug. 1988.

    [Jac90] V. Jacobson, Modified TCP Congestion Avoidance Algorithm,
        end2end-interest mailing list, April 30, 1990.

    [MD90] J. Mogul, S. Deering.  Path MTU Discovery, November 1990.
        RFC 1191.

    [MM96a] M. Mathis, J. Mahdavi, Forward Acknowledgment: Refining TCP
        Congestion Control, Proceedings of SIGCOMM'96, August, 1996,
        Stanford, CA.  Available from

    [MM96b] M. Mathis, J. Mahdavi, TCP Rate-Halving with Bounding
        Parameters.  Technical report.  Available from

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    [MMFR96] M. Mathis, J. Mahdavi, S. Floyd, A. Romanow, TCP Selective
        Acknowledgement Options, October 1996.  RFC 2018.

    [PAD+98] V. Paxson, M. Allman, S. Dawson, W. Fenner, J. Griner,
        I. Heavens, K. Lahey, J. Semke, B. Volz.  Known TCP
        Implementation Problems.  Internet-Draft
        draft-ietf-tcpimpl-prob-05.txt, November 1998.  (Work in

    [Pax97] V. Paxson, End-to-End Internet Packet Dynamics,
        Proceedings of SIGCOMM '97, Cannes, France, Sep. 1997.

    [Pos81] J. Postel, Transmission Control Protocol, September 1981.
        RFC 793.

    [Ste94] W. R. Stevens, TCP/IP Illustrated, Volume 1: The
        Protocols, Addison-Wesley, 1994.

    [Ste97] W. R. Stevens, "TCP Slow Start, Congestion Avoidance, Fast
        Retransmit, and Fast Recovery Algorithms", January 1997.  RFC

    [WS95] G. R. Wright, W. R. Stevens, TCP/IP Illustrated, Volume 2:
        The Implementation, Addison-Wesley, 1995.

Author's Address:

    Mark Allman
    NASA Lewis Research Center/Sterling Software
    21000 Brookpark Rd.  MS 54-2
    Cleveland, OH  44135

    Vern Paxson
    Network Research Group
    Lawrence Berkeley National Laboratory
    Berkeley, CA 94720

    W. Richard Stevens
    1202 E. Paseo del Zorro
    Tucson, AZ  85718

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