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Network Working Group                                        A. Sabatini
Internet-Draft                                Broker Communications Inc.
Intended Status: Standards Track                                       .
Expires: February 14, 2013                               August 15, 2012

       Highly Efficient Selective Acknowledgement (SACK) for TCP

Status of this Memo

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   This memo expands on the Selective Acknowledgement Protocol described
   in RFC2018 to improve its performance and efficiency while reducing
   the delay involved in recovering lost segments.  This leads to very
   reliable and efficient communications regardless of transit delay or
   high levels of lost segments due to noise or congestion.  It
   introduces a fundamentally new way of looking at Selective
   Acknowledgement and uses this concept to improve the performance of
   the RFC2018 protocol.  This memo proposes an implementation of the
   improved SACK and discusses its performance and related issues.


   Much of the text in this document is taken directly from RFC2018 "TCP
   Selective Acknowledgement Options" by M. Mathis, J. Mahdavi, S. Floyd
   and A. Romanow and RFC1072 "TCP Extensions for Long-Delay Paths" by
   B. Braden and V. Jacobson.

1.  Introduction

   This revision to the SACK protocol has its roots in a similar, HDLC
   based protocol I designed and implemented for secure financial
   transactions.  That protocol, being designed for use on a worldwide
   basis, was born out of the need for a protocol that would handle any
   communications environment no matter how noisy or how much delay
   (including multiple satellite hops) was in the path.  In later years
   its properties were found valuable in congestion situations where
   packets were dropped.

   Multiple packet losses from a window of data can have a catastrophic
   effect on TCP throughput. TCP [Postel81] uses a cumulative
   acknowledgment scheme in which received segments that are not at the
   left edge of the receive window are not acknowledged.  This forces
   the sender to either wait a round-trip time to find out about each
   lost packet, or to unnecessarily retransmit segments which have been
   correctly received [Fall95].  With the cumulative acknowledgment
   scheme, multiple dropped segments generally cause TCP to lose its
   ACK-based clock, reducing overall throughput.

   Selective Acknowledgment (SACK) is a strategy which corrects this
   behavior in the face of multiple dropped segments.  With selective
   acknowledgments, the data receiver can inform the sender about all
   segments that have arrived successfully, so the sender need
   retransmit only the segments that have actually been lost.  The
   compatible extensions to RFC2018 proposed here enhance the protocol
   by changing retransmission from a worst case timer basis to a
   deterministic, state driven basis which responds rapidly to link

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   I propose modifications to the SACK options as proposed in RFC2018.
   Specifically, I add a transmit state to each transmitted message and
   return that transmit state when each acknowledgement is sent.  By
   using the returned transmit state I can tell what messages have been
   transmitted after the information in the acknowledgement and thus
   rebuild the current state of the receiver at the transmitter.  I also
   propose changes to the way SACK blocks are reported to insure that
   the oldest, and thus the most critical, are transmitted expeditiously
   without jeopardizing the multiple repetition of SACK information
   which gives the current protocol its reliability.  Additionally since
   the space to store acknowledgements in IPv4 is limited and may not be
   able to accommodate all of the acknowledgement pairs, I propose a
   method of sending the complete receiver state by sending multiple
   acknowledgements when it becomes evident that transmission has
   stalled due to loss of multiple ACKs.

   The RFC2018 selective acknowledgment extension uses two TCP options.
   The first is an enabling option, "SACK-permitted", which may be sent
   in a SYN segment to indicate that the SACK option can be used once
   the connection is established.  This option is extended to both
   indicate that this newer version of the protocol is being used and to
   establish an initial value for transmit state.  The other is the SACK
   option itself, which may be sent over an established connection once
   permission has been given by SACK-permitted.  This has also been
   extended to add both the transmit state implicit in the message and
   the transmit state that was received at the far end (now called
   "Returned State").

   The SACK option is to be included in a segment sent from a TCP that
   is receiving data to the TCP that is sending that data; we will refer
   to these TCP's as the data receiver and the data sender,
   respectively.  We will consider a particular simplex data flow; any
   data flowing in the reverse direction over the same connection can be
   treated independently.

2.  Underlying concepts

   In order for a sender to know how to optimally transmit messages to a
   receiver the sender must recreate the state of the receiver as of the
   last acknowledgement received (which segments have been received and
   acknowledged, which segments have not) and then "age" or modify that
   state by updating it based upon the messages transmitted since the
   state implicit in the acknowledgement was current.  In order to do
   this the sender must maintain a transmission order list which
   contains entries for the segment ranges of each message as it is
   sent.  We called the index into the transmission order list "Send
   State" and transmit this state variable with each message.  The
   receiver, after correctly receiving the message, saves this value and

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   returns it (now called "Returned State") and the list of selectively
   acknowledged segments with each acknowledgement.  When the sender
   receives this information it is then capable of constructing a list
   of missing segments by taking its unacknowledged segment range list
   and modifying it on the basis of the received selective
   acknowledgements and then removing from that list all segments that
   have been transmitted since the message which caused the
   acknowledgement which is all segments sent with indexes between the
   current "send state" and the "receive state" in the acknowledgement

   To accommodate the issue of receiving segments out of order at the
   receiver, or those packets delayed by alternate routing, the sender
   does not instantly update its Current Returned State value from the
   incoming ACK (which could trigger a false retransmission) but rather
   puts it on a timer queue for a length of time ("Reordering Time")
   appropriate to the delay randomness in the arrival path (typically 20
   to 100 ms based on media, speed and distance), which when the timer
   entry expires, causes the update of the Current Returned State value.
   If the updated Current Returned State value shows blocks that remain
   unacknowledged after this time out they are assumed to be lost and
   they are queued for retransmission.

   Thus by transmitting the complete acknowledgement information through
   the SACK blocks from the receiver along with an indicator to the
   sender as to its state current at the time of the acknowledgement the
   sender can accurately recreate the current status of the receiver
   assuming all "in flight" messages were received and thus only send
   the unacknowledged messages starting with the oldest followed by any
   new messages whose transmission is requested.

3. Enhanced Sack-Permitted Option

   This document is designed to be an extension of RFC2018 and any
   implementation of it must be designed to fall back to handling
   RFC2018 when he other paty is not capable of handling the enhanced

   Although Enhanced SACK is a compatible extension of standard SACK it
   is recognized that certain middleware boxes are not RFC compliant as
   to extensions and therefore will fail if, as would properly be done,
   Enhanced SACK was handled as such.  Therefore Enhanced SACK is
   transmitted as two options both in the SYN packets as well as data
   and ACK packets.

   The first option of the pair MUST be the standard SACK option (Kind =
   4) if this endpoint desires a SACK session of any kind.  The second
   four or six byte option may be sent in a SYN by a TCP that has been

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   extended to receive (and presumably process) the Enhanced SACK option
   in order to indicate its willingness to enter into an Enhanced SACK

   This option MUST NOT be transmitted on non-SYN segments in the
   current protocol, it is left to future study as to its use for
   transmitting long sequences of acknowledgements in one frame.

       TCP SACK-Permitted Option:

       Kind: 4

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
                                      | Kind=4        | Length=2      |

       TCP Enhanced SACK Setup Option:

       Kind: X1

      |               |               |P|R|P|R|                       |
      | Kind=X1       | Length=6 or 8 |T|T|X|X| Reserved              |
      |               |               |O|O|T|T|                       |
      if RXT = 0 not requesting extended state
      | Send State Token              |
      if RXT = 1 requesting extended state
      | Extended Send State Token                     | Reserved      |

   Control Bits: 4 bits (from left to right):

       PTO:  Tokens Permitted, the sender supports the requirements of
       this extension

       RTO:  Request Tokens, sender requests link use the protocol
       outlined in this extension

       PXT:  Extended Tokens Supported

       RXT:  Request Extended Tokens: sender requests using extended

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   Reserved: 12 bits

       Reserved for future use, must be set to zero

   If RTO is set then the Send State immediately follows, 16 bits if RXT
   is not set and 24 bits if it is.  If necessary the option is padded
   with a binary zero byte so that length is an even number.  In the
   case that one end can only support 16 bit tokens only the right most
   16 bits of the extended field is used.

   For brevity in this document only the lesser, 16 bit format is shown.

4. SACK Option Format

   As with the SYN options, Enhanced SACK information must be
   transmitted as a seperate option in order to accomodate non RFC
   compliant middleware boxes.  By its nature it must precede the TCP
   SACK Option.

       TCP Enhanced SACK Option:

       Kind: X2

       Length: 6 (or 8 if extended token)

                         | Kind=X2 | Len=6 |
       | Send State      | Returned State  |

       TCP SACK Option:

       Kind: 5

       Length: Variable

                         | Kind=5 | Length |
       |      Left Edge of 1st Block       |
       |      Right Edge of 1st Block      |
       |                                   |

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       /            . . .                  /
       |                                   |
       |      Left Edge of nth Block       |
       |      Right Edge of nth Block      |

   The SACK option is to be sent by a data receiver to inform the data
   sender of non-contiguous blocks of data that have been received and
   queued.  The data receiver awaits the receipt of data (perhaps by
   means of retransmissions) to fill the gaps in sequence space between
   received blocks.  When missing segments are received, the data
   receiver acknowledges the data normally by advancing the left window
   edge in the Acknowledgement Number Field of the TCP header.  The SACK
   option does not change the meaning or use of the Acknowledgement
   Number field.

   This option contains a list of some of the blocks of contiguous
   sequence space occupied by data that has been received and queued
   within the window.

   Each contiguous block of data queued at the data receiver is defined
   in the SACK option by two 32-bit unsigned integers in network byte

   *    Left Edge of Block

        This is the first sequence number of this block.

   *    Right Edge of Block

        This is the sequence number immediately following the last
   sequence number of this block.

   Each block represents received bytes of data that are contiguous and
   isolated; that is, the bytes just below the block, (Left Edge of
   Block - 1), and just above the block, (Right Edge of Block), have not
   been received.

   A SACK option that specifies n blocks will have a length of 8*n+6
   bytes, so the 40 bytes available for TCP options can specify a
   maximum of 4 blocks.  It is suggested that the Enhanced SACK will
   provide the time-stamp information used for RTTM [Jacobson92].

5.  Generating Sack Options: Data Receiver Behavior

   If the data receiver has received a SACK-Permitted option on the SYN

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   for this connection, the data receiver MAY elect to generate SACK
   options as described below.  If the data receiver generates SACK
   options under any circumstance, it MUST generate them under all
   permitted circumstances.  If the data receiver has not received a
   SACK-Permitted option for a given connection, it MUST NOT send SACK
   options on that connection.

   If sent at all, SACK options MUST be included in all ACKs which do
   not ACK the highest sequence number in the data receiver's queue.  In
   this situation the network has lost or mis-ordered data, such that
   the receiver holds non-contiguous data in its queue.  RFC 1122,
   Section, discusses the reasons for the receiver to send ACKs
   in response to additional segments received in this state.  The
   receiver MUST send an ACK for every valid segment that arrives
   containing new data, and each of these "duplicate" ACKs SHOULD bear a
   SACK option.

   The purpose of the SACK blocks is to recreate the status of the
   receiver at the transmitter.  To that end the most important
   information is (1) new or changed blocks, (2) the second transmission
   of new or changed blocks, (3) a complete enumeration of all received
   blocks starting from the oldest first.

   If the data receiver chooses to send a SACK option, the following
   rules apply:

      * The data receiver first fills in "Send State" in the option from
      the current value of its "Send State".  The data receiver then
      fills in "Returned State" from its "Saved Send State" which was
      set by either the SYN option or the SACK option of the last TCP
      packet that contained a value which was logically greater than the
      current saved value.

      * SACK blocks representing all discontiguous segment ranges
      received where those ranges are logically over the Acknowledgement
      Number in the TCP header are kept in logically ascending by
      segment range list.  Additionally a count (a four byte binary for
      safety) is maintained in each block which represents the number of
      times it has been transmitted.  Each time a SACK block is added or
      changes (normally by being merged with another entry) the count is
      set to zero(0).

      * All SACK block slots SHOULD be filled on each each normal ACK
      transmitted, starting with those that have the lowest count
      (acknowledging the most recently received segments), followed by
      those with the next lowest count, and so on until all SACK block
      slots are filled.  As each SACK block is moved to a slot its count
      is incremented by one(1) (thus care needs to be taken on the

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      second and subsequent passes to skip those entries currently in
      SACK blocks slots). By always starting from the oldest we insure
      the most critical have the first chance at receiving a SACK block
      slot. SACK blocks are empty ONLY if there are less SACK blocks
      outstanding than there are available slots. This methodology
      assures a fair number of transmissions to all SACK blocks.

      * The receiver receives a Send State from the sender that is
      logically greater than any previously seen the receiver must
      generate an ACK regardless of whether any SACK blocks have
      changed.  Note that such a Send State change can come from an ACK
      produced by the sender as well as a message.

      * The definitions in RFC1022 are changed such that if there are
      entries on the SACK block list an ACK ALWAYS goes out in response
      to a received data segment.

      * To insure that the last added or changed SACK block is
      transmitted a second time, if the link goes idle for 2*Reordering
      Time the receiver SHOULD send another ACK following the rules

      * A timer is maintained based on the timestamp of the oldest SACK
      block and is set to RTT*1.25, it is reset each time a SACK block
      with a different segment start becomes the oldest SACK block.  At
      the expiration of this timer, since this and probably other
      segments have not been retransmitted to the receiver, the receiver
      resets the timer to .25*RTT and again sends sufficient
      acknowledgements to completely transmit all current SACK blocks
      starting from the one with the logically lowest segment start and
      proceeding in ascending sequence.  Note that this process is
      aborted by any action that changes the oldest SACK block.  This
      timer is used to assure that in case of a burst error the sender
      has enough information to restart properly.

6.  Interpreting the Sack Option and Retransmission Strategy: Data
   Sender Behavior

   As each transmission request from the calling program is processed
   and entry is made into the segment queue to handle the request and
   its buffering.  Entries are removed from the segment queue when the
   segment is completely acknowledged either through the Acknowledgement
   Number Field of the TCP header passing through the end of that
   segment or by a SACK block completing the acknowledgement of that
   segment.  The segment is also appended to the end of the
   retransmission queue and transmission restarted from that segment if
   transmission has stopped.

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   Before processing the SACK information the Acknowledgement Number
   Field of the TCP header is used to eliminate outdated entries from
   the segment queue, saved list and retransmission queue before new
   information is added.  The acknowledgement may split the first entry
   in either the segment queue or the retransmission queue in which case
   a pseudo entry is created in that queue for the unacknowledged
   remainder which additionally points to the saved original entry with
   an additional field which is the count of pseudo segments derived
   from it, set to one in this case.  The Acknowledgement Number Field
   of the TCP header may end up eliminating a pseudo entry in which case
   the pseudo segment count of the original saved entry is decremented
   and if zero the segment is then entirely removed from both the
   segment queue and the saved list.

   In processing selective acknowledgements the transmitter applies each
   SACK block to first the segment queue and then the retransmission
   queue.  If the SACK block completely acknowledges a segment it is
   removed from the segment queue and moved to the saved list with a
   count of zero or completely if from the retransmission queue.  If the
   SACK block completely acknowledges a pseudo segment that segment is
   removed and if from the segment queue the pseudo segment count in the
   related saved entry is decremented.  If the SACK block acknowledges
   the beginning or end of a segment in either queue a pseudo entry is
   created with the adjusted unacknowledged remainder, if the segment
   was on the segment list the original segment is moved to the saved
   list and the pseudo count is set to one.  If the entry was already a
   pseudo segment and this SACK acknowledges the beginning or end of the
   segment, the segment limits are adjusted but no other action occurs.
   If this entry is already a pseudo entry and the SACK block splits the
   segment in two a second pseudo entry is created to handle the right
   hand side of the range and, if on the segment list, the pseudo
   segment count in the related save list entry is incremented by one.
   The original pseudo entry is modified to represent the left hand
   range created by the SACK.

   The sender maintains a Transmission List which an array of structures
   into which the segment start and end addresses of each transmitted
   block (be it a primary transmission or a retransmission) is placed.
   This list is, for optimal processing, a power of 2 in size and is, at
   a minimum, four times as large as the Maximum Number of Segments
   Outstanding.  As each segment is transmitted the current Transmit
   Token modulus the Transmission List size is used as an index into
   this structure to store the segment start and end and then the
   Transmit Token is incremented by one.

   When each each new SACK option is processed, its Returned Token is
   checked against the Current Returned Token and the Future Returned
   Token List, if logically greater than any of the above, it is

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   inserted into the Future Returned Token list in logical order along
   with the current time-stamp incremented by the Reordering Time.  If
   it is the first entry on the list a timer is started for the value
   token time stamp minus current time stamp.  When the timer expires
   the first entry on the Future Returned Token list set as the Current
   Returned Token and then it is removed from the list.  If there are
   more members on the Future Returned Token list the timer is restarted
   with a value of the time-stamp in that entry minus the current time-
   stamp.  A change in the Current Returned Token causes a recreation of
   the Retransmission Queue by first copying the Segment Queue and then
   removing from it all segments that have been transmitted
   subsequently, in a process identical to processing a SACK block
   starting at the segment identified by the Current Returned Token from
   the Transmission List and continuing through the Transmission List up
   to the (but not including) the Transmit Token.  The Transmission
   Pointer is then set to first incomplete entry in the Retransmission
   Queue and transmission restarted if it has stopped.

   Another method of implementation which allows quicker retransmission
   response at the expense of building the Retransmission Queue a second
   time is to retrieve the time stamp of the just receieved Returned
   Token if that Returned Token has not previously been seen (by
   comparing it with the Current Returned Token and the Future Returned
   Token list) and then subtracting from that timestamp the Reordering
   Time to allow for out of order messages.  This value is then used to
   select any entry on the Transmission List with an equal or greater
   timestamp.  The first version of the Retransmission Queue is created
   by copying the Segment Queue and then removing the segments that have
   since been retransmitted based on the adjusted timestamp.  When the
   timer on the Future Returned Token List expires the retransmission
   queue is recreated a second time as in the preceeding paragraph.

   Note: For processing efficiency we believe most people will implement
   the Retransmission Queue as additional fields in the Segment Queue.

6.1  Handling last segment problem

   If the sender side of the link goes idle for 2*Reordering Time and
   there are still unacknowledged segments the sender SHOULD send an ACK
   (which would have the updated Send State) so that the receiver may
   ultimately detect if the last message is missing and cause it to be
   transmitted (the receiver would pass the Send State back as Returned
   State and the sender would realize the segment is still outstanding).

6.2  Congestion Control Issues

   This document does not attempt to specify in detail the congestion
   control algorithms for implementations of TCP with SACK.  However,

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   the congestion control algorithms present in the de facto standard
   TCP implementations MUST be preserved [Stevens94].  This algorithm
   eliminates much unnecessary retransmission so is likely to lessen
   overall congestion.

   Note that the enhanced protocol does not suffer from traditional
   congestion collapse even though it is more robust, since it does not
   use timers and is rate limited by the tokens.  Delayed and lost
   messages and ACKS make it slower but do not increase the traffic it
   sends significantly.

   The use of time-outs as a fall-back mechanism for detecting dropped
   packets is unchanged by the SACK option.  Because in normal operation
   acknowledgements will prevent retransmit timeout, when a retransmit
   timeout occurs the data sender SHOULD ignore prior SACK information
   in determining which data to retransmit.

   Future research into congestion control algorithms may take advantage
   of the additional information provided by SACK.  One such area for
   future research concerns modifications to TCP for a wireless or
   satellite environment where packet loss is not necessarily an
   indication of congestion.

7.  Efficiency and Worst Case Behavior

   Although this high efficiency improved SACK option sends more and
   larger SACK blocks and more acknowledgements than the previous
   version, with an active bi-directional link additional
   acknowledgements are often associated with data transmission and thus
   not a penalty.  If the SACK option needs to be used due to segment
   loss then the improved efficiency afforded with this protocol more
   than justifies the additional SACK blocks.

   The deployment of other TCP options may reduce the number of
   available SACK blocks to 2 or even to 1.  This will reduce the
   redundancy of SACK delivery in the presence of lost ACKs.  Even so,
   the exposure of TCP SACK in regard to the unnecessary retransmission
   of packets is strictly less than the exposure of current
   implementations of TCP.  The worst-case conditions necessary for the
   sender to needlessly retransmit data is discussed in more detail in a
   separate document [Floyd96].

   Older TCP implementations which do not have the SACK option will not
   be unfairly disadvantaged when competing against SACK-capable TCPs.
   This issue is discussed in more detail in [Floyd96].

8.  Time-stamping

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   One pleasant benefit of having a token which is returned by the far
   end on a deterministic basis is the easy calculation of round trip
   delay.  We can save a time stamp along with the segment information
   in our transmission order array.  This allows us to calculate round
   trip delay when we receive our "Returned State" value and use it to
   access the time-stamp.  Since more than one received message might
   have the same "Returned State" value we mark the time-stamp after use
   to indicate that the value should not be used again.  Note that if an
   acknowledgement is lost we will calculate a longer delay than is
   accurate therefore we must smooth the returned values, typically
   returning the sma llest out of the last N where N is typically four.

9. Data Receiver Reneging

   Since the Sender is recreating the state of the Receiver, the data
   Receiver MUST NOT discard data in its queue once that data has been
   reported in a SACK option.  The Receiver is responsible for
   allocating enough buffers so that the missing segments within the
   window may be properly received and processed.  Since enhanced SACK
   is event driven the lack of a new event for 2.50*RTT SHOULD trigger a
   connection reset to guard agains denial of service attacks.

10.  Security Considerations

   This document neither strengthens nor weakens TCP's current security

11. References

   [Jacobson88], Jacobson, V. and R. Braden, "TCP Extensions for Long-
   Delay Paths", RFC 1072, October 1988.

   [Jacobson92] Jacobson, V., Braden, R., and D. Borman, "TCP Extensions
   for High Performance", RFC 1323, May 1992.

   [Mathis96] Mathis, M., Mahdavi, J., Floyd, S., Romanow, J. "TCP
   Selective Acknowleddgement Options", RFC 2018, October 1996.

   [Postel81]  Postel, J., "Transmission Control Protocol - DARPA
   Internet Program Protocol Specification", RFC 793, DARPA, September

Author's Address

      Anthony Sabatini
      Broker Communications Inc.
      200 West 20th Street

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      Suite 1216
      New York, NY 10011
      Email: draft-sack@tsabatini.com

      The author is currently a master's degree candidate at -

      Hofstra University
      Hempstead, N.Y.

      His adviser is Dr. Xiang Fu

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