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TCP Maintenance and Minor Extensions                    R. Scheffenegger
(tcpm)                                                      NetApp, Inc.
Internet-Draft                                             M. Kuehlewind
Updates: 1323 (if approved)                      University of Stuttgart
Intended status: Experimental                           October 31, 2011
Expires: May 3, 2012


        Additional negotiation in the TCP Timestamp Option field
                        during the TCP handshake
           draft-scheffenegger-tcpm-timestamp-negotiation-03

Abstract

   A number of TCP enhancements in so diverse fields as congestion
   control, loss recovery or side-band signaling could be improved by
   allowing both ends of a TCP session to interpret the values carried
   in the Timestamp option.  Further enhancements are enabled by
   changing the receiver side processing of timestamps in the presence
   of Selective Acknowledgements.

   This documents updates RFC1323 and specifies a backwards compatible
   way of negotiating for Timestamp capabilities, and lists a number of
   benefits and drawbacks of this approach.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   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 progress."

   This Internet-Draft will expire on May 3, 2012.

Copyright Notice

   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal



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   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  5
   3.  Overview . . . . . . . . . . . . . . . . . . . . . . . . . . .  6
     3.1.  Overview of the TCP Timestamp Option . . . . . . . . . . .  6
     3.2.  Overview of the Timestamp Capabilities . . . . . . . . . .  7
   4.  Problem statement  . . . . . . . . . . . . . . . . . . . . . .  8
   5.  Signaling  . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     5.1.  Capability Flags . . . . . . . . . . . . . . . . . . . . . 10
     5.2.  Version 0 specific fields  . . . . . . . . . . . . . . . . 11
     5.3.  Timestamp Capability Negotiation . . . . . . . . . . . . . 15
       5.3.1.  Implicit extended negotiation  . . . . . . . . . . . . 16
       5.3.2.  Interaction with the Retransmission Timer  . . . . . . 17
   6.  Possible use cases . . . . . . . . . . . . . . . . . . . . . . 18
     6.1.  One-way delay variation measurement  . . . . . . . . . . . 18
     6.2.  Early spurious retransmit detection  . . . . . . . . . . . 19
     6.3.  Early lost retransmission detection  . . . . . . . . . . . 20
     6.4.  Integrity of the Timestamp value . . . . . . . . . . . . . 22
     6.5.  Disambiguation with slow Timestamp clock . . . . . . . . . 22
     6.6.  Masked timestamps as segment digest  . . . . . . . . . . . 23
     6.7.  Timestamp value as covert channel  . . . . . . . . . . . . 23
   7.  Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . 25
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 26
   9.  Updates to Existing RFCs . . . . . . . . . . . . . . . . . . . 26
   10. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 26
   11. Security Considerations  . . . . . . . . . . . . . . . . . . . 28
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 28
     12.1. Normative References . . . . . . . . . . . . . . . . . . . 28
     12.2. Informative References . . . . . . . . . . . . . . . . . . 28
   Appendix A.  Possible Extension  . . . . . . . . . . . . . . . . . 30
     A.1.  Capability Flags . . . . . . . . . . . . . . . . . . . . . 31
     A.2.  Range Negotiation  . . . . . . . . . . . . . . . . . . . . 32
   Appendix B.  Revision history  . . . . . . . . . . . . . . . . . . 34
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 35






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1.  Introduction

   The timestamp option originally introduced in [RFC1323] was designed
   solely for two-way delay measurement and to support a particular TCP
   algorithm (Reno).  It would be useful to be able to support one-way
   delay measurement and to take advantage of developments since TCP
   Reno, such as selective acknowledgements (SACK) [RFC2018].

   This specification defines a protocol for the two ends of a TCP
   session to negotiate alternative semantics for the timestamps they
   will exchange during the rest of the session.  It updates RFC1323 but
   it is backwards compatible with implementations of RFC1323 timestamp
   options.

   The RFC1323 timestamp protocol presents the following problems when
   trying to extend it for alternative uses:

   a.  Unclear meaning of the value in a timestamp.

       *  A timestamp value (TSval) as defined in [RFC1323] is
          deliberately only meaningful to the end that sends it.  The
          other end is merely meant to echo the value without
          understanding it.  This is fine if one end is trying to
          measure two-way delay (round trip time).  However, to measure
          one-way delay, timestamps from both ends need to be compared
          by one end, which needs to relate the values in timestamps
          from both ends to a notion of the passage of time that both
          ends share.

   b.  No control over which timestamp to echo.

       *  A host implementing [RFC1323] is meant to echo the timestamp
          value of the most recent in-order segment received.  This was
          fine for TCP Reno, but it is not the best choice for TCP
          sessions using selective acknowledgement (SACK) [RFC2018].

       *  A [RFC1323] host is meant to echo the timestamp value of the
          earliest unacknowledged segment, e.g. if a host delays ACKs
          for one segment, it echoes the first timestamp not the second.
          It is desirable to include delay due to ACK withholding when a
          host is conservatively measuring RTT.  However, is not useful
          to include the delay due to ACK withholding when measuring
          one-way delay.

   c.  Alternative protection against wrapped sequence numbers.

       *  [RFC1323] also points out that the timestamps it specifies
          will always strictly monotonically increase in each window so



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          they can be used to protect against wrapped sequence numbers
          (PAWS).  If the endpoints negotiate an alternative timestamp
          scheme in which timestamps may not monotonically increase per
          window, then it needs to be possible to negotiate alternative
          protection against wrapped sequence numbers.

   To solve these problems this specification changes the wire protocol
   of the TCP timestamp option in two main ways:

   1.  It updates [RFC1323] to add the ability to negotiate the
       semantics of timestamp options.  The initiator of a TCP session
       starts the negotiation in the TSecr field in the first <SYN>,
       which is currently unused.  This specification defines the
       semantics of the TSecr field in a segment with the SYN flag set.
       A version number is included to allow further extension of
       capability negotiation in future.

   2.  A version independent ability to mask a specified number of the
       lower significant bits of the timestamp values is present.  These
       masked bits are not considered for timestamp calculations, or in
       an algorithm to protect against wrapped sequence numbers.  Future
       extensions can thereby change the timestamp signaling without
       changing the modified treatment on the receiver side.

   3.  It updates [RFC1323] to define version 0 of timestamp
       capabilities to include:

       *  the duration in seconds of a tick of the timestamp clock using
          a floating point representation

       *  agreement that both ends will echo the timestamp on the most
          recently received segment, rather than the one that would be
          echoed by an [RFC1323] host.  There is no specific option to
          request this behavior, however it is implied by successful
          negotiation of both SACK and timestamp capabilities.

   With this new wire protocol, a number of new use-cases for the TCP
   timestamp option become possible.  Section 6 gives some examples.
   Further extensions might be required in future.  Appendix A gives an
   example of a further version of timestamp capability negotiation that
   could be defined in the future.










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2.  Terminology

   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].

   The reader is expected to be familiar with the definitions given in
   [RFC1323].

   Further terminology used within this document:

   Timestamp clock interval
       The Timestamp value is derived from a clock source running at a
       reasonable constant frequency.  The interval between two ticks of
       that clock is signaled during the timestamp capability
       negotiation.  Note that the timestamp clock is not required to be
       identical with the TCP clock, even though most implementations
       use the same clock for practical purposes.

   Timestamp option
       This refers to the entire TCP timestamp option, including both
       TSval and TSecr fields.

   Timestamp capabilities
       Refers only to the values and bits carried in the TSecr field of
       <SYN> and <SYN,ACK> segments during a TCP handshake.  For
       signaling purposes, the timestamp capabilities are sent in clear
       with the <SYN> segment, and in an encoded form (see Section 5 for
       details) in the <SYN,ACK> segment.






















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3.  Overview

3.1.  Overview of the TCP Timestamp Option

   The TCP Timestamp option (TSopt) provides timestamp echoing for
   round-trip time (RTT) measurements.  TSopt is widely deployed and
   activated by default in many systems.  [RFC1323] specifies TSopt the
   following way:

         Kind: 8

         Length: 10 bytes

         +-------+-------+---------------------+---------------------+
         |Kind=8 |  10   |   TS Value (TSval)  |TS Echo Reply (TSecr)|
         +-------+-------+---------------------+---------------------+
             1       1              4                     4

                          Figure 1: RFC1323 TSopt

      "The Timestamps option carries two four-byte timestamp fields.
      The Timestamp Value field (TSval) contains the current value of
      the timestamp clock of the TCP sending the option.

      The Timestamp Echo Reply field (TSecr) is only valid if the ACK
      bit is set in the TCP header; if it is valid, it echos a times-
      tamp value that was sent by the remote TCP in the TSval field of a
      Timestamps option.  When TSecr is not valid, its value must be
      zero.  The TSecr value will generally be from the most recent
      Timestamp option that was received; however, there are exceptions
      that are explained below.

      A TCP may send the Timestamps option (TSopt) in an initial <SYN>
      segment (i.e., segment containing a SYN bit and no ACK bit), and
      may send a TSopt in other segments only if it received a TSopt in
      the initial <SYN> segment for the connection."

   The comparison of the timestamp in the TSecr field to the current
   timestamp clock gives an estimation of the two-way delay (RTT).  With
   [RFC1323] the receiver is not supposed to interpret the TSVal field
   for timing purposes, e.g. one-way delay measurments, but only to echo
   the content in the TSecr field.  [RFC1323] specifies various cases
   when more than one timestamp is available to echo.  The approach
   taken by [RFC1323] is not always be the best choice, i.e. when the
   TCP Selective Acknowledgment option (SACK) is used in conjunction.






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3.2.  Overview of the Timestamp Capabilities

   This document specifies a way of negotiating the timestamp
   capabilities available between the end hosts.  This is enabled by
   using the TSecr field in the TCP <SYN> segment.  In order to remain
   backwards compatible, a receiver capable of timestamp capability
   negotiation has to XOR the receivers (local) capabilities flags with
   the received TSval, before echoing the result back in the TSecr
   field.  During the initial handshake, the sender has to store the
   sent initial TSval, in order to determine if the receiver can support
   this timestamp capability negotiation.

   As there exist some benefit to change the receiver side treatment of
   which timestamp value to echo, the negotiation protocol itself must
   also provide some backwards compatibility.  Therefore, even when a
   sender tries to negotiate for a higher version than supported by the
   receiver, the receiver MUST respond with at least version 0.  Also, a
   future protocol enhancement MUST make sure that any extension is
   compatible with at least version 0.

   As the importance of the timestamp option increases by using it in
   more aspects of a TCP sender's operation e.g. congestion control, so
   increases the importance of maintaining the integrity of the
   reflected timestamps.  At the same time this must not inhibit the
   receiver to interpret a received timestamp in TSval.

   This is achieved by indicating how many LSB bits of the timestamp
   value MUST NOT be interpreted by the receiver.  Apart from the
   purpose of maintaining timestamp integrity for the use as input
   signal into congestion control algorithms, this also allows the use
   of timestamp based methods to discriminate at the earliest possible
   moment (within 1 RTT after the retransmission) between spurious
   retransmissions and genuine loss even when using slow running TCP
   timestamp clocks.

   In addition, by using synergistic signaling between timestamps
   [RFC1323] and selective acknowledgments [RFC2018], enhancements in
   loss recovery are possible by removing any remaining retransmission
   and acknowledgment ambiguity.  See Section 6 for a detailed
   discussion.

   As an optional extension, a timestamp clock interval range
   negotiation is also briefly introduced in Appendix A.  This is only
   included as one potential example of further enhancements.







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4.  Problem statement

   Timestamp values are carried in each segment if negotiated for.
   However, the content of this values is to be treated as an unmutable
   and largely uninterpreted entity by the receiver.  This document
   describes an enhancement to the timestamp negotiation, and must meet
   the following criteria:

   o  Indicate the (approximate) timestamp clock interval used by the
      sender in a wide range.  The longest interval should be around 10
      seconds, while the shorted interval should allow unique timestamps
      per segment, even at extremely high link speeds.  At the time of
      writing, the shortest meaningful duration was found to be a 64
      byte packets (i.e.  ACK segment) sent at a rate of 100 Gbit/s.
      This corresponds to a maximum timestamp clock rate of around 200
      MHz, or an interval between clock ticks of around 5 ns.

   o  Allow for timestamps that are not directly related to real time
      (i.e. segment counting, or use of the timestamp value as a true
      extension of sequence numbers).

   o  Provide means to prevent or at least detect tampering with the
      echoed timestamp value, allowing for basic integrity and
      consistency checks.

   o  Allow for future extensions that may use some of the timestamp
      value bits for other signaling purposes during the remainder of
      the session.

   o  Signaling must be backwards compatible with existing TCP stacks
      implementing basic [RFC1323] timestamps.  Current methods for
      timestamp value generation must be supported.

   o  Allow to state timing information explicitly during the initial
      handshake, to avoid a training phase extending beyond the initial
      handshake.

   o  Provide a means to disambiguate between resent <SYN> segments.

   o  Cater for broken implementations, that either send a non-zero
      TSecr value in the initial <SYN>, or a zero TSecr value in
      <SYN,ACK>.

   Some legacy implementations exist that violate [RFC1323] in that the
   TSecr field in a <SYN> is not cleared (see
   [I-D.ietf-tcpm-tcp-security].  The protocol should have some
   resiliency in the presence of such misbehaving senders, and must not
   lead to an unfair advantage for such wrongly negotiated sessions.



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   As there exist some benefit to change the receiver side treatment of
   which timestamp value to echo, the negotiation protocol itself must
   also provide some backwards compatibility.  Therefore, even when a
   sender tries to negotiate for a higher version than supported by the
   receiver, the receiver MUST respond with at least version 0.  Also, a
   future protocol enhancement MUST make sure that any extension is
   compatible with at least version 0.












































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5.  Signaling

5.1.  Capability Flags

   In order to signal the supported capabilities, both the sender and
   the receiver will idependently generate a timestamp capability
   negotiation field, as indicated below.  The TSecr value field of the
   [RFC1323] TSopt is overloaded with the following flags and fields
   during the initial <SYN> and <SYN,ACK> segments.  The connection
   initiator will send the timestamp capabilities in plain, as with
   [RFC1323] the TSecr is not used in the inital <SYN>.  The receiver
   will XOR the local timestamp capabilities with the TSVal received
   from the sender and send the result in the TSecr field.  The
   initiating host of a session with timestamp capability negotiation
   has to keep minimal state to decode the returned capabilities XOR'ed
   with the sent TSval.

       Kind: 8

       Length: 10 bytes

       +-------+-------+---------------------+---------------------+
       |Kind=8 |  10   |   TS Value (TSval)  |TS Echo Reply (TSecr)|
       +-------+-------+---------------------+---------------------+
           1       1              4          |           4         |
                                            /                      |
       .-----------------------------------'                       |
      /                                                             \
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |E|   |         #                                               |
     |X|VER|   MSK   #           version specific contents           |
     |O|   |         #                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                   Figure 2: Timestamp Capability flags

   Common fields to all versions:

   EXO - Extended Options (1 bit)
       Indicates that the sender supports extended timestamp
       capabilities as defined by this document, and MUST be set to one
       by a compliant implementation.  This flag also enables the
       immediate echoing of the TSval with the next ACK, if both
       timestamp capabilities and selective acknowledgement [RFC2018]
       are successful negotiated during the initial handshake (see
       Section 5.3.1).  This change in semantics is independent of the
       version in the signaled timestamp capabilities.



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   VER - Version (2 bits)
       Version of the capabilities fields definition.  This document
       specifies codepoint 0.  With the exception of the immediate
       mirroring - simplifying the receiver side processing - and the
       masking of some LSB bits before performing the Protection Against
       Wrapped Sequence Numbers (PAWS) test, hosts must not interpret
       the received timestamps and not use a timestamp value as input
       into advanced heuristics, if the version received is not
       supported.  This is an identical requirement as with current
       [RFC1323] compliant implementations.  The lower 3 octets of the
       timestamp capability flags MUST be ignored if an unsupported
       version is received.  It is expected, that a host will implement
       at least version 0.  A receiver MUST respond with the appropriate
       (equal or version 0) version when responding to a new session
       request.

   MSK - Mask Timestamps (5 bits)
       The MaSK field indicates how many least significant bits should
       be excluded by the receiver, before further processing the
       timestamp (i.e.  PAWS, or for timing purposes).  The unmasked
       portion of a TSval has to comply with the constraints imposed by
       [RFC1323] on the generation of valid timestamps, e.g. must be
       monotone increasing between segments, and strict monotone
       increasing for each TCP window.  Note that this does not impact
       the reflected timestamp in any way - TSecr will always be equal
       to an appropriate TSval.  This field MUST be present in all
       future version of timestamp capability fields.  A value of 31
       (all bits set) MUST be interpreted by a receiver that the full
       TSval is to be ignored by any legacy heuristics, including PAWS.
       For PAWS to be effective, at least 2 bits are required to
       discriminate between an increase (and roll-over) versus outdated
       segments.

5.2.  Version 0 specific fields

















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       Kind: 8

       Length: 10 bytes

       +-------+-------+---------------------+---------------------+
       |Kind=8 |  10   |   TS Value (TSval)  |TS Echo Reply (TSecr)|
       +-------+-------+---------------------+---------------------+
           1       1              4          |           4         |
                                            /                      |
       .-----------------------------------'                       |
      /                                                             \
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |E|   |         #               |         |                     |
     |X|VER|   MSK   #      RES      |   ADJ   |         INT         |
     |O|   |         #               |         |                     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

             Figure 3: Timestamp Capability flags - version 0

   RES - Reserved (8 bits)
       Reserved for future use, and MUST be zero ("0") with version 0.
       If timestamp capabilities are received with version set to 0, but
       some of these bits set, the receiver MUST ignore the extended
       options field and react as if the TSecr was zero (compatibility
       mode).

   ADJ - Adjustment factor (5 bits)
       The scaling factor by which the signaled interval has to be left-
       shifted.  This is similar to the way the Window Scale option is
       defined in [RFC1323].  All values between zero and 31 are valid.
       This allows timestamp clock ticks of up to 15.99 s.
       See Section 6.1 for details.

   INT - Interval (11 bits)
       The integer part of the timestamp clock interval can be signaled
       with up to 11 bits of precision.  This allows a range with the
       highest resolution to cover clock intervals between 7.45 ns
       (INT=0x400, ADJ=0) and 15.99 s (INT=0x7FF, ADJ=31).  If a sender
       is using a less precice clock source, fewer significant bits can
       be used to implicitly signal this.  For example, a timestamp
       clock interval of approximately 1 ms (1/1024th sec) can be
       represented by both (INT=0x001, ADJ=28) and (INT=0x400, ADJ=18).
       A more accurate representation of 1 ms would be (INT=0x418,
       ADJ=18).  The latter representation carries more significant
       bits, indicating a more stable clock source with low jitter.
       Only non-zero values are valid when ADJ is non-zero.  An invalid
       combination of ADJ and INT MUST be treaded as if no timestamp



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       capability negotiation is attempted.  A compliant sender can
       choose the value of the <SYN> TSval in such a way, that either
       the EXO bit, or some of the RES bits are set, or all the INT bits
       are cleared, in the encoded response from the receiver.  A
       receiver that does not reflect the initial TSval in it's
       <SYN,ACK> and instead sends a zero value in TSecr, will not
       erraneously negotiate for timestamp capabilities.

   Conceptually, the timestamp clock interval can be represented as a
   unsigned integer with 42 bits length.  In this form, the least
   significant bit represents an interval of 2^-38 sec (3.64 ps), while
   still allowing a maximum interval of 16 sec.  This value is then
   shifted to the right, until it can be represented by only 11
   significant bits, and the number of shift operations is stored as
   scaling adjustment factor (ADJ).

   A value of zero (both ADJ and INT are set to zero) is supported and
   indicates, that the timestamp values are NOT correlated to wall-clock
   time (i.e. the sender may perform some form of segment counting or
   sequence number extension instead).  A host receiving an interval of
   zero from the other end host MUST NOT perform time-based heuristics
   which take the received TSval into account, but SHOULD apply the
   regular PAWS test.

   Timestamp clock periods faster than 1 ms SHOULD be implemented by
   inserting the timestamp "late" before transmitting a segment to avoid
   unnecessary timing jitter.  Shortest clock periods, with intervals of
   only a few microseconds or less, are provided for hardware-assisted
   implementations.

   The range of possible values runs from 15.99 s to 7.45 ns with
   highest precision, and down to 3.64 ps with reducing precision, which
   is also the shortest difference in tick duration, that could be
   resolved.  This equates to clock frequencies of 0.06 Hz, 134 MHz and
   275 GHz respectively.

   Despite the provision of such a large dynamic range, a receiver
   should consider, that a timestamp clock may deviate from the
   indicated rate by a large fraction.  Similarily, a sender SHOULD
   refrain from signaling the clock interval with too much precision
   (significan bits), if the clock can not be sampled with low variance
   over time.

   Example for an timestamp capability negotiation, to indicate that the
   senders timestamp clock (tcp clock) is running with 1 ms per tick,
   and using a clock source of typical quality (e.g. software timer
   interrupt):




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   SYN, TSval=<X>, TSecr=EXO|MSK|ADJ=22|INT=0x041

   +-----------+------------+--------------------+---------------------+
   |      tick |       tick |     encoding at    |  encoding at lowest |
   |  interval |  frequency |  highest precision |      precision      |
   +-----------+------------+--------------------+---------------------+
   |     16 s  |   0.06 Hz  |  ADJ=31, INT=0x7FF |  ADJ=31, INT=0x7FF  |
   |      1 s  |      1 Hz  |  ADJ=28, INT=0x400 |  ADJ=31, INT=0x080  |
   |    0.5 s  |      2 Hz  |  ADJ=27, INT=0x400 |  ADJ=31, INT=0x040  |
   |    100 ms |     10 Hz  |  ADJ=24, INT=0x666 |  ADJ=31, INT=0x00C  |
   |     10 ms |    100 Hz  |  ADJ=21, INT=0x51F |  ADJ=31, INT=0x001  |
   |      4 ms |    250 Hz  |  ADJ=20, INT=0x419 |  ADJ=30, INT=0x001  |
   |      1 ms |      1 kHz |  ADJ=18, INT=0x418 |  ADJ=28, INT=0x001  |
   |    200 us |      5 kHz |  ADJ=15, INT=0x68E |  ADJ=25, INT=0x001  |
   |     50 us |     20 kHz |  ADJ=13, INT=0x68E |  ADJ=23, INT=0x001  |
   |      1 us |      1 MHz |   ADJ=8, INT=0x432 |  ADJ=18, INT=0x001  |
   |     60 ns |   16.7 MHz |   ADJ=4, INT=0x407 |  ADJ=14, INT=0x001  |
   +-----------+------------+--------------------+---------------------+

            Table 1: Common used TCP Timestamp Clock intervals

   The wide range of indicated timestamp clock intervals (spanning 9
   orders of (decimal) magnitude, or 28 binary digits, and the
   limitation to no more than 24 bits requires the use of a logarithmic
   encoding.  Since the precision of the timestamp clock value is most
   valuable at low frequencies (long tick durations), the clock rate is
   encoded as a time duration.  This results in full precision for
   common used timestamp clock tick durations, while allowing even
   shorter intervals at reduced precision.  A format was chosen that is
   simple to implement and poses no risk of confusion with common
   floating point representations.

   The timestamp clock values a host is using must not necessarily run
   synchronous with the internal TCP clock.  Different clock sources,
   such as a NTP stratum, RTC, CPU cycle counters, or other independent
   clocks can be used to derive the TSval.  This allows the de-coupling
   of the coarse-grained TCP clock used for retransmission and delayed
   ACK timeouts, from the clock frequency indicated in the TSval itself.
   Since [RFC1323] timestamp clocks used to be only useful for RTT
   measurement, and calculation of the RTO, the straight forward use of
   the TCP timer directly seemed natural to minimize subsequent RTT
   calculations.

   Most stacks will at first not be able to dynamically adjust their
   timestamp clock interval.  Therefore, the indicated clock duration
   can be a static, compile time value.  To use the indicated clock
   interval, for example to perform one-way delay variation
   calculations, simple integer operations can be used after an initial



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   conversion of the wire presentation to longer (i.e. 32 or 64 bit)
   integer values.

5.3.  Timestamp Capability Negotiation

   During the initial TCP three-way handshake, timestamp capabilities
   are negotiated using the TSecr field.  Timestamp capabilities MAY
   only be negotiated in TSecr when the SYN bit is set.  A host detects
   the presence of timestamp capability flags when the EXO bit is set in
   the TSecr field of the received <SYN> segment.  When receiving a
   session request (<SYN> segment with timestamp capabilities), a
   compliant TCP receiver is required to XOR the received TSval with the
   receivers timestamp capabilities.  The resulting value is then sent
   in the <SYN,ACK> response.

   To support these design goals stated in Section 4, only the TSecr
   field in the initial <SYN> can be used directly.  The response from
   the receiver has to be encoded, since no unused field is available in
   the <SYN,ACK>.  The most straightforward encoding is a XOR with a
   value that is known to the sender.  Therefore, the receiver also uses
   TSecr to indicate it's capabilities, but calculates the XOR sum with
   the received TSval.  This allows the receiver to remain stateless and
   functionalities like syncache (see [RFC4987]) can be maintained with
   no change.

   If a sender has to retransmit the <SYN>, this encoding also allows to
   detect which segment was received and responeded to.  This is
   possible by changing the timestamp clock offset between
   retransmissions in such a way, that the decoding on the sender side
   would result in an invalid timestamp capability negotiation field
   (e.g. some RES bits are set).  If the sender does not require the
   capability to differentiate which <SYN> was received, the timestamp
   clock offset for each new <SYN> can be set in such a way, that the
   TSopt of the <SYN> is identical for each retransmission.

   As a receiver MAY report back a zero value at any time, in particular
   during the <SYN,ACK>, the sender is slightly constrained in it's
   selection of an initial Timestamp value.  The Timestamp value sent in
   the <SYN> should be selected in such a way, that it does not resemble
   a valid Timestamp capabilities field.  This prevents a compliant
   sender to erraneously detect a compliant receiver, if the returned
   TSecr value is zero.

   A host initiating a TCP session must verify if the partner also
   supports timestamp capability negotiation and a supported version,
   before using enhanced algorithms.  Note that this change in semantics
   does not necessarily change the signaling of timestamps on the wire
   after initial negotiation.



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   To mitigate the effect from misbehaving TCP senders appearing to
   negotiate for timestamp capabilities, a receiver MUST verify that one
   specific bit (EXO) is set, and any reserved bits (currently 8, RES
   field) are cleared.  This limits the chance for a receiver to
   mistakenly negotiate for version 0 capabilities to around 0.05%.
   However, as a receiver has to use changed semantics when reflecting
   TSval also for higher values in the version field, a misbehaving
   sender negotiating for SACK, but not properly clearing TSecr, may
   have a 37.5% chance of receiving timestamp values with modified
   receiver behavior.  This may lead to an increased number of spurious
   retransmission timeouts, putting such a session to a disadvantage.

   Once timestamp capabilities are successfully negotiated, the receiver
   MUST ignore an indicated number of masked, low-order bits, before
   applying the heuristics defined in [RFC1323].  The monotonic increase
   of the timestamp value for each new segment could be violated if the
   full 32 bit field, including the masked bits, are used.  This
   conflicts with the constraints imposed by PAWS.  The use of generic
   (secure) hash algorithms makes it possible to protect the integrity
   of the timestamp value, without any compromise to fulfill the PAWS
   requirement of monotonic increasing values.

   The presented distribution of the common three fields, EXO, VER and
   MASK, that MUST be present regardless of which version is implemented
   in a compliant TCP stack, is a result of the previously mentioned
   design goals.  The lower three octets MAY be redefined freely with
   subsequent versions of the timestamp capability negotiation protocol.
   This allows a future version to be implemented in such a way, that a
   receiver can still operate with the modified behavior, and a minimum
   amount of processing (PAWS)

5.3.1.  Implicit extended negotiation

   If both Timestamp capabilities and Selective Acknowledgement options
   [RFC2018] are negotiated (both hosts send these options in their
   respective segments), both hosts MUST echo the timestamp value of the
   last received segment, irrespective of the order of delivery.  Note
   that this is in conflict with [RFC1323], where only the timestamp of
   the last segment received in sequence is mirrored.  As SACK allows
   discrimination of reordered or lost segments, the reflected
   timestamps are not required to convey the most conservative
   information.  If SACK indicates lost or reordered packets at the
   receiver, the sender MUST take appropriate action such as ignoring
   the received timestamps for calculating the round trip time, or
   assuming a delayed packet (with appropriate handling).  An updated
   algorithm to calculate the retransmission timeout timer (RTO) is not
   discribed in this document.




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   The immediate echoing of the last received timestamp value allowed by
   the synergistic use of the timestamp option with the SACK option
   enables enhancements to improve loss recovery, round trip time (RTT)
   and one-way delay (OWD) variation measurements (see Section 6) even
   during loss or reordering episodes.  This is enabled by removing any
   retransmission ambiguity using unique timestamps for every
   retransmission, while simultaneously the SACK option indicates the
   ordering of received segments even in the presence of ACK loss or
   reordering.

   The use of RTT and OWD measurements during loss episodes is an open
   research topic.  A sender has to accomodate for the changed timestamp
   semantics in order to maintain a conservative estimate of the
   Retransmission Timer as defined in [RFC6298], when a TCP sender has
   negotiated for an immideate reflection of the timestamp triggering an
   ACK (i.e. both timestamp capability negotiation and Selective
   Acknowledgements are enabled for the session).  As the presence of a
   SACK option in an ACK signals an ongoing reordering or loss episode,
   timestamps conveyed in such segments MUST NOT be used to update the
   retransmission timeout.  Also note that the presence of a SACK option
   alleviates the need of the receiver to reflect the last in-order
   timestamp, as lost ACKs can no longer cause erraneous updates of the
   retransmission timeout.

5.3.2.  Interaction with the Retransmission Timer

   The above stated rule, to ignore timestamps as soon as a SACK option
   is present, is fully consistent with the guidance given in [RFC1323],
   even though most implementations skip over such an additional
   verification step in the precense of the SACK option.

   To address the additional delay imposed by delayed ACKs, a compliant
   sender SHOULD modify the update procedure when receiving normal, in-
   sequence ACKs that acknowledge more than SMSS bytes, so that the
   input (denoted R in [RFC6298]) is calculated as

   R = RTT * ( 1 + 1/(cwnd/smss) )

   If RTT (as measured in units of the timestamp clock) is smaller than
   the congestion window measured in full sized segments, the above
   heuristic MAY be bypassed before updating the retransmission timeout
   value.









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6.  Possible use cases

6.1.  One-way delay variation measurement

   New congestion control algorithms are currently proposed, that react
   on the measured one-way delay variation (i.e.
   [I-D.ietf-ledbat-congestion], [Chirp]).  This control variable is
   updated after each received ACK:

   C(t) = TSval(t) - TSecr(t)

   V(t) = C(t) - C(t-1)

   provided that the timestamp clocks at both ends are running at
   roughly the same rate.  Without prior knowledge of the timestamp
   clock interval used by the partner, a sender can try to learn this
   interval by observing the exchanged segments for a duration of a few
   RTTs.  However, such a scheme fails if the partner uses some form of
   implicit integrity check of the timestamp values, which would appear
   as either random scrambling of LSB bits in the timestamp, or give the
   impression of much shorter clock intervals than what is actually
   used.  If the partner uses some form of segment counting as timestamp
   value, without any direct relationship to the wall-clock time, the
   above formula will fail to yield meaningful results.  Finally the
   network conditions need to remain stable during any such training
   phase, so that the sender can arrive at reasonable estimates of the
   partners timestamp clock tick duration.

   This note addresses these concerns by providing a means by which both
   host are required to use a timestamp clock that is closely related to
   the wall-clock time, with known clock rate, and also provides means
   by which a host can signal the use of a few LSB bits for timestamp
   value integrity checks.  To arrive at a valid one-way delay (OWD)
   variation, first the timestamp received from the partner has to be
   right-shifted by a known amount of bits as defined by the mask field.
   Next the local and remote timestamp values need to be normalized to a
   common base clock interval (typically, the local clock interval):

                                                         remote interval
   C  = (TSecr >> local mask) - (TSval >> remote mask) * ---------------
    t                                                    local interval

   V(t) = C(t) - C(t-1)

   The adjustment factor can be calculated once during the timestamp
   capability negotiation phase, and pure integer arithmetic can be used
   during per-segment processing:




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   EXP.min = min(EXP.loc, EXP.rem)

   EXP.rem -= EXP.min

   EXP.loc -= EXP.min

   FRAC.rem = (0x800 | FRAC.rem) << EXP.rem

   FRAC.loc = (0x800 | FRAC.loc) << EXP.loc

   and assuming that the local clock tick duration is lower

   ADJ = FRAC.rem / FRAC.loc

   with ADJ being a integer variable.  For higher precision, two
   appropriately calculated integers can be used.

   Any previously required training on the remote clock interval can be
   removed, resulting in a simpler and more dependable algorithm.
   Furthermore, transient network effects during the training phase
   which may result in a wrong inference of the remote clock interval
   are eliminated completely.

6.2.  Early spurious retransmit detection

   Using the provided timestamp negotiation scheme, clients utilizing
   slow running timestamp clocks can set aside a small number of least
   significant bits in the timestamps.  These bits can be used to
   differentiate between original and retransmitted segments, even
   within the same timestamp clock tick (i.e. when RTT is shorter than
   the TCP timestamp clock interval).  It is recommended to use only a
   single bit (mask = 1), unless the sender can also perform lost
   retransmission detection.  Using more than 2 bits for this purpose is
   discouraged due to the diminishing probability of loosing
   retransmitted packets more than one time.  A simple scheme could send
   out normal data segments with the so masked bits all cleared.  Each
   advance of the timestamp clock also clears those bits again.  When a
   segment is retransmitted without the timestamp clock increasing,
   these bits increased by one for each consecutive retry of the same
   segment, until the maximum value is reached.  Newly sent segments
   (during the same clock interval) should maintain these bits, in order
   to maintain monotonically increasing values, even though compliant
   end hosts do not require this property.  This scheme maintains
   monotonically increasing timestamp values - including the masked
   bits.  Even without negotiating the immediate mirroring of timestamps
   (done by simultaneously doing timestamp capabilities negotiation, and
   selective acknowledgments), this extends the use of the Eifel
   Detection [RFC3522] and Eifel Response [RFC4015] algorithm to detect



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   and react to spurious retransmissions under all circumstances.  Also,
   currently experimental schemes such as ER-SRTO [Cho08] could be
   deployed without requiring the receiver to explicitly support that
   capability.

                  Seg0 Seg1 Seg2 Seg3 Seg4
                  TS00 TS00 TS00 TS00 TS00
                         X

                       Seg1                Seg5
                       TS01                TS01

                                                Seg6 Seg7
                                                TS01 TS10


           Figure 4: timestamp for spurious retranmit detection

   Masked bits are the 2nd digit, the timestamp value is represented by
   the first digit.  The timestamp clock "ticks" between segment 6 and
   7.

6.3.  Early lost retransmission detection

   During phases where multiple segments in short succession (but not
   necessarily successive segments) are lost, there is a high likelihood
   that at least one segment is retransmitted, while the cause of loss
   (i.e. congestion, fading) is still persisting.  The best current
   algorithms can recover such a lost retransmission with a few
   constraints, for example, that the session has to have at least
   DupThresh more segments to send beyond the current recovery phase.
   During loss recovery, when a retransmission is lost again, currently
   the timestamp can also not be used as means of conveying additional
   information, to allow more rapid loss recovery while maintaining
   packet conservation principles.  Only the timestamp of the last
   segment preceding the continuous loss will be reflected.  Using the
   extended timestamp option negotiation together with selective
   acknowledgements, the receiver will immediately reflect the timestamp
   of the last seen segment.  Using both SACK and TS information
   synergistically, a sender can infer the exact order in which original
   and retransmitted segments are received.  This allows a slightly less
   conservative and faster approach to retransmit lost retransmitted
   segments.

   This can be implemented in combination with the masked bit approach
   described in the previous paragraph, or without.  However, if the
   timestamp clock interval is lower than 1/2 RTT, both the original and
   the retransmitted segment may carry an identical timestamp.  If the



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   sender cannot discriminate between the original and the retransmitted
   segments, is MUST refrain from taking any action before such a
   determination can be made.

   In this example, masked bits are used, with a simple marking method.
   As the timestamp value of the retransmission itself is already
   different from the original segments, such an additional
   discrimination would not strictly be required here.  The timestamp
   clock ticks in the first digit and the dupthresh value is 3.

             Seg0 Seg1 Seg2 Seg3 Seg4 Seg5 Seg6 Seg7
             TS00 TS10 TS10 TS10 TS10 TS10 TS10 TS20
                    X    X    X    *

                  Seg1 Seg2 Seg3 Seg4
                  TS21 TS30 TS30 TS30
                    X

                  Seg1                               Seg8 Seg9
                  TS31                               TS31 TS40

                      Figure 5: timestamp under loss

   If Seg1,TS00 is lost twice, and Seg4,TS10 is also lost, the sender
   could resend Seg1 once more after seeing dupthresh number of segments
   sent after the first retransmission of Seg1 being received (ie, when
   Seg4 is SACKed).  However, there is a ambiguity between retransmitted
   segments and original segments, as the sender cannot know, if a SACK
   for one particular segment was due to the retransmitted segment, or a
   delayed original segment.  The timestamp value will not help in this
   case, as per RFC1323 it will be held at TS00 for the entire loss
   recovery episode.  Therefore, currently a sender has to assume that
   any SACKed segments may be due to delayed original sent segments, and
   can only resolve this conflict by injecting additional, previously
   unsent segments.  Once dupthresh newly injected segments are SACKed,
   continuous loss (and not further delay) of Seg1 can safely be
   assumed, and that segment be resent.  This approach is conservative
   but constrained by the requirement that additional segments can be
   sent, and thereby delayed in the response.

   With the synergistic use of timestamp extended options together with
   selective acknowledgments, the receiver would immediately reflect
   back the timestamp of the last received segment.  This allows the
   sender to discriminate between a SACK due to a delayed Seg4,TS10, or
   a SACK because of Seg4,TS30.  Therefore, the appropriate decision
   (retransmission of Seg1 once more, or addressing the observed
   reordering/delay accordingly [I-D.blanton-tcp-reordering] can be
   taken with high confidence.



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6.4.  Integrity of the Timestamp value

   If the timestamp is used for congestion control purposes, an
   incentive exists for malicious receivers to reflect tampered
   timestamps, as demonstrated with some exploits [CUBIC].

   One way to address this is to not use timestamp information directly,
   but to keep state in the sender for each sent segment, and track the
   round trip time independent of sent timestamps.  Such an approach has
   the drawback, that it is not straightforward to make it work during
   loss recovery phases for those segments possibly lost (or reordered).
   In addition there is processing and memory overhead to maintain
   possibly extensive lists in the sender that need to be consulted with
   each ACK.  Despite these drawbacks, this approach is currently
   implemented due to lack of alternatives (see [Linux], and [BSD10]).

   The preferred approach is that the sender MAY choose to protect
   timestamps from such modifications by including a fingerprint (secure
   hash of some kind) in some of the least significant bits.  However,
   doing so prevents a receiver from using the timestamp for other
   purposes, unless the receiver has prior knowledge about this use of
   some bits in the timestamp value.  Furthermore, strict monotonic
   increasing values are still to be maintained.  That constraint
   restricts this approach somewhat and limits or inhibits the use of
   timestamp values for direct use by the receiver (i.e. for one-way
   delay variation measurement, as the hash bits would look like random
   noise in the delay measurement).

6.5.  Disambiguation with slow Timestamp clock

   In addition, but somewhat orthogonal to maintaining timestamp value
   integrity, there is a use case when the sender does not support a
   timestamp clock interval that can guarantee unique timestamps for
   retransmitted segments.  This may happen whenever the TCP timestamp
   clock interval is higher than the round-trip time of the path.  For
   unambiguously identifying regular from retransmitted segments, the
   timestamp must be unique for otherwise identical segments.  Reserving
   the least significant bits for this purpose allows senders with slow
   running timestamp clocks to make use of this feature.  However,
   without modifying the receiver behavior, only limited benefits can be
   extracted from such an approach.  Furthermore the use of this option
   has implications in the protection against wrapped sequence numbers
   (PAWS - [RFC1323]), as the more bits are set aside for tamper
   prevention, the faster the timestamp number space cycles.

   Using Timestamp capabilities to explicitly negotiate mask bits, and
   set aside a (low) number of least significant bits for the above
   listed purposes, allows a sender to use more reliable integrity



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   checks.  These masked bits are not to be considered part of the
   timestamp value, for the purposes described in [RFC1323] (i.e.  PAWS)
   and subsequent heuristics using timestamp values (i.e.  Eifel
   Detection), thereby lifting the strict requirement of always
   monotonically increasing timestamp values.  However, care should be
   taken to not mask too many bits, for the reasons outlined in
   [RFC1323].  Using a mask value higher than 8 is therefore
   discouraged.

   The reason for having 5 bits for the mask field nevertheless is to
   allow the implementation of this protocol in conjunction with TCP
   cookie transaction (TCPCT) extended timestamps [RFC6013].  That
   allows for nearly a quarter of a 128 bit timestamp to be set aside.

6.6.  Masked timestamps as segment digest

   After making TCP alternate checksums historic (see [RFC6247]), there
   still remains a need to address increased corruption probabilities
   when segment sizes are increased (see
   [I-D.ietf-tcpm-anumita-tcp-stronger-checksum]).

   Utilizing a completely masked TSval field allows the sender to
   include a stronger CRC32, with semantics independent of the fixed TCP
   header fields.  However, such a use would again exclude the use of
   PAWS on the receiver side, and a receiver would need to know the
   specifics of the digest for processing.  It is assumed, that such a
   digest would only cover the data payload of a TCP segment.  In order
   to allow disambiguation of retransmissions, a special TSval can be
   defined (e.g.  TSval=0) which bypasses regular CRC processing but
   allows the identification of retransmitted segments.

   The full semantics of such a data-only CRC scheme are beyond the
   scope of this document, but would require a different version of the
   timestamp capability.  Nevertheless, allowing the full TSval to
   remain unprocessed by the receiver for the purpose of PAWS even in
   version 0 could still allow the successful negotiation of sender-side
   enhancements such as loss recovery improvements (see Section 6.2, and
   Section 6.3).

   In effect, the masked portion of the timestamp value represent an
   unreliable out of band signal channel, that could also be used for
   other purposes than solely performing timestamp integrity checks (for
   example, this would allow ER-SRTO algorithms [Cho08]).

6.7.  Timestamp value as covert channel

   Covert channels SHOULD NOT be implemented by using the mask field, as
   the explicit masking clearly points to such a channel.  As the



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   regular operation of the timestamp clock is still maintained, covert
   channels working by artificially delaying data segments in an
   application (and thereby influencing the timestamp inserted into the
   segment) work unaffected.  The received TSval would need to be
   shifted by the appropriate number of bits, before extracting the data
   from the covert channel by the receiver.













































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7.  Discussion

   RTT and OWD variation during loss episodes is not deeply researched.
   Current heuristics ([RFC1122], [RFC1323], Karn's algorithm [RFC2988])
   explicitly exclude (and prevent) the use of RTT samples when loss
   occurs.  However, solving the retransmission ambiguity problem - and
   the related reliable ACK delivery problem - would enable new
   functionality to improve TCP processing.  Also, having an immediate
   echo of the last received timestamp value would enable new research
   to distinguish between corruption loss (assumed to have no RTT / OWD
   impact) and congestion loss (assumed to have RTT / OWD impact).
   Research into this field appears to be rather neglected, especially
   when it comes to large scale, public internet investigations.  Due to
   the very nature of this, passive investigations without signals
   contained within the headers are only of limited use in empirical
   research.

   Retransmission ambiguity detection during loss recovery would allow
   an additional level of loss recovery control without reverting to
   timer-based methods.  As with the deployment of SACK, separating
   "what" to send from "when" to send it could be driven one step
   further.  In particular, less conservative loss recovery schemes
   which do not trade principles of packet conservation against
   timeliness, require a reliable way of prompt and best possible
   feedback from the receiver about any delivered segment and their
   ordering.  [RFC2018] SACK alone goes quite a long way, but using
   timestamp information in addition could remove any ambiguity.
   However, the current specs in [RFC1323] make that use impossible,
   thus a modified semantic (receiver behavior) is a necessity.

   A synergistic signaling with immediate timestamp value echoes would
   however break legacy, per-packet RTT measurements.  The reason is,
   that delayed ACKs would not be covered.  Research has shown, that
   per-packet updates of the RTT estimation (for the purpose of
   calculating a reasonable RTO value) are only of limited benefit (see
   [Path99], and [PH04]).  This is the most serious implication of the
   proposed synergistic signaling scheme with directly echoing the
   timestamp value of the segment triggering the ACK.  Even when using
   the directly reflected timestamp values in an unmodified RTT
   estimator, the immediate impact would be limited to causing premature
   RTOs when the sending rate suddenly drops below two segments per RTT.
   That is, assuming the receiver implements delayed ACK and sending one
   ACK for every other data segment received.  If the receiver has
   D-SACK [RFC2883] enabled, such premature RTOs can be detected and
   mitigated by the sender (for example, by increasing minRTO for low
   bandwidth flows).





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8.  Acknowledgements

   The authors would like to thank Dragana Damjanovic for some initial
   thoughts around Timestamps and their extended potential use.

   The editor would like to thank Bob Briscoe for his insightful
   comments, and the gratuitous donation of text, that have resulted in
   a substantially improved document.


9.  Updates to Existing RFCs

   Care has been taken to make sure the updates in this specification
   can be deployed incrementally.

   Updates to existing [RFC1323] implementations are only REQUIRED if
   they do not clear the TSecr value in the initial <SYN> segment.  This
   is a misinterpretation of [RFC1323] and may leak data anyway (see
   [I-D.ietf-tcpm-tcp-security]).  Otherwise, there will be no need to
   update an RFC1323-compliant TCP stack unless the timestamp
   capabilities negotiation is to be used.

   Implementations compliant with the definitions in this document shall
   be prepared to encounter misbehaving senders, that don't clear TSecr
   in their initial <SYN>.  It is believed, that checking the reserved
   bits to be all zero provides enough protection against misbehaving
   senders.

   In the unlikely case of an erraneous negotiation of timestamp
   capabilities between a compliant receiver, and a misbehaving sender,
   the proposed semantic changes will trigger a higher rate of spurious
   retransmissions, while time-based heuristics on the receiver side may
   further negatively impact congestion control decisions.  Overall,
   misbehaving receivers will suffer from self-inflicted reductions in
   TCP performance.


10.  IANA Considerations

   With this document, the IANA is requested to establish a new registry
   to record the timestamp capability flags defined with future versions
   (codepoints 1, 2 and 3).

   The lower 24 bits (3 octets) of the timestamp capabilities field may
   be freely assigned in future versions.  The first octet must always
   contain the EXO, VER and MASK fields for compatibility, and the MASK
   field MUST be set to allow interoperation with a version 0 receiver.




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   This document specifies version 0 and the use of the last three
   octets to signal the senders timestamp clock interval to the
   receiver.
















































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11.  Security Considerations

   The algorithm presented in this paper shares security considerations
   with [RFC1323] (see [I-D.ietf-tcpm-tcp-security]).

   An implementation can address the vulnerabilities of [RFC1323], by
   dedicating a few low-order bits of the timestamp fields for use with
   a (secure) hash, that protects against malicious modification of
   returned timestamp value by the receiver.  A MASK field has been
   provided to explicitly notify the receiver about that alternate use
   of low-order bits.  This allows the use of timestamps for purposes
   requiring higher integrity and security while allowing the receiver
   to extract useful information nevertheless.


12.  References

12.1.  Normative References

   [RFC1323]  Jacobson, V., Braden, B., and D. Borman, "TCP Extensions
              for High Performance", RFC 1323, May 1992.

   [RFC2018]  Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP
              Selective Acknowledgment Options", RFC 2018, October 1996.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

12.2.  Informative References

   [BSD10]    Hayes, D., "Timing enhancements to the FreeBSD kernel to
              support delay and rate based TCP mechanisms", Feb 2010, <h
              ttp://caia.swin.edu.au/reports/100219A/
              CAIA-TR-100219A.pdf>.

   [CUBIC]    Rhee, I., Ha, S., and L. Xu, "CUBIC: A New TCP-Friendly
              High-Speed TCP Variant", Feb 2005, <http://
              citeseerx.ist.psu.edu/viewdoc/
              download?doi=10.1.1.153.3152&rep=rep1&type=pdf>.

   [Chirp]    Kuehlewind, M. and B. Briscoe, "Chirping for Congestion
              Control -  Implementation Feasibility", Nov 2010, <http://
              bobbriscoe.net/projects/netsvc_i-f/chirp_pfldnet10.pdf>.

   [Cho08]    Cho, I., Han, J., and J. Lee, "Enhanced Response Algorithm
              for Spurious TCP Timeout (ER-SRTO)", Jan 2008, <http://
              ubinet.yonsei.ac.kr/v2/publication/hpmn_papaers/ic/
              2008_Enhanced%20Response%20Algorithm%20for%20Spurious%



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              20TCP.pdf>.

   [I-D.blanton-tcp-reordering]
              Blanton, E., Dimond, R., and M. Allman, "Practices for TCP
              Senders in the Face of Segment Reordering",
              draft-blanton-tcp-reordering-00 (work in progress),
              February 2003.

   [I-D.ietf-ledbat-congestion]
              Shalunov, S., Hazel, G., Iyengar, J., and M. Kuehlewind,
              "Low Extra Delay Background Transport (LEDBAT)",
              draft-ietf-ledbat-congestion-08 (work in progress),
              October 2011.

   [I-D.ietf-tcpm-anumita-tcp-stronger-checksum]
              Biswas, A., "Support for Stronger Error Detection Codes in
              TCP for Jumbo Frames",
              draft-ietf-tcpm-anumita-tcp-stronger-checksum-00 (work in
              progress), May 2010.

   [I-D.ietf-tcpm-tcp-security]
              Gont, F., "Security Assessment of the Transmission Control
              Protocol (TCP)", draft-ietf-tcpm-tcp-security-02 (work in
              progress), January 2011.

   [Linux]    Sarolahti, P., "Linux TCP", Apr 2007,
              <http://www.cs.clemson.edu/~westall/853/linuxtcp.pdf>.

   [PH04]     Eckstroem, H. and R. Ludwig, "The Peak-Hopper: A New End-
              to-End Retransmission Timer for Reliable Unicast
              Transport", Apr 2004, <citeseerx.ist.psu.edu/viewdoc/
              download?doi=10.1.1.76.2748&rep=rep1&type=pdf>.

   [Path99]   Allman, M. and V. Paxson, "On Estimating End-to-End
              Network Path Properties", Sep 1999,
              <http://www.icir.org/mallman/papers/estimation.ps>.

   [RFC1122]  Braden, R., "Requirements for Internet Hosts -
              Communication Layers", STD 3, RFC 1122, October 1989.

   [RFC2883]  Floyd, S., Mahdavi, J., Mathis, M., and M. Podolsky, "An
              Extension to the Selective Acknowledgement (SACK) Option
              for TCP", RFC 2883, July 2000.

   [RFC2988]  Paxson, V. and M. Allman, "Computing TCP's Retransmission
              Timer", RFC 2988, November 2000.

   [RFC3522]  Ludwig, R. and M. Meyer, "The Eifel Detection Algorithm



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              for TCP", RFC 3522, April 2003.

   [RFC4015]  Ludwig, R. and A. Gurtov, "The Eifel Response Algorithm
              for TCP", RFC 4015, February 2005.

   [RFC4987]  Eddy, W., "TCP SYN Flooding Attacks and Common
              Mitigations", RFC 4987, August 2007.

   [RFC6013]  Simpson, W., "TCP Cookie Transactions (TCPCT)", RFC 6013,
              January 2011.

   [RFC6247]  Eggert, L., "Moving the Undeployed TCP Extensions RFC
              1072, RFC 1106, RFC 1110, RFC 1145, RFC 1146, RFC 1379,
              RFC 1644, and RFC 1693 to Historic Status", RFC 6247,
              May 2011.

   [RFC6298]  Paxson, V., Allman, M., Chu, J., and M. Sargent,
              "Computing TCP's Retransmission Timer", RFC 6298,
              June 2011.


Appendix A.  Possible Extension

   This section is not intended as normative description of an
   extension, but merely as an example of a possible extension.  Future
   extensions MUST set the common fields in such a way that a receiver
   capable of version 0 only can react appropriately.

   Certain hosts may want to negotiate a common optimal timestamp clock
   interval between each other for various purposes.  For example, the
   balance between PAWS ([RFC1323]) and the timestamp clock resolution
   should be more towards one or the other.  Also, if a hosts wants to
   have identical timestamp clock intervals both at the sender and
   receiver to simplify one-way delay variation calculation, negotiating
   the clock interval could be useful.  With identical timestamp clock
   intervals, instead of multiplications and divisions, only additions
   and subtractions are required for OWD variation calculation.

   Without a full three way handshake, full negotiation of the timestamp
   clock intervals is not possible.  For this reason, a special semantic
   is required during negotiation.  This allows both ends to know the
   exact timestamp clock interval with only two exchanged segments,
   while at the same time remaining compatible with version 0.

   For this purpose, the following extension (version 1) of this
   protocol is one suggestion.  Depending on the exact requirements, a
   different signaling may be more appropriate.  For example, only the
   two different EXP fields could be required, while a single, but



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   higher precision FRAC field for both low and high boundaries could
   suffice, and some additional signaling bits could be made available.

A.1.  Capability Flags

       Kind: 8

       Length: 10 bytes

       +-------+-------+---------------------+---------------------+
       |Kind=8 |  10   |   TS Value (TSval)  |TS Echo Reply (TSecr)|
       +-------+-------+---------------------+---------------------+
           1       1              4          |           4         |
                                            /                      |
       .-----------------------------------'                       |
      /                                                             \
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |E|   |         #         DUR12lo       |         DUR12hi       |
     |X|VER|  MASK   #-----------------------|-----------------------|
     |O|   |         # ADJ12lo |   INT12lo   | ADJ12hi |   INT12hi   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

               Figure 6: Timestamp Capability enhanced flags

   The following additional fields are defined:

   VER - version (2 bits)
       Version 1 could indicated that the sender is capable of adjusting
       the timestamp clock interval within the bounds of the two 12 bit
       fields (see Appendix A.2).  A receiver that only implements
       version 0 SHOULD NOT ignore the timestamp capability negotiation
       entirely when encountering an unsupported version, any SHOULD
       respond with a version 0 response nevertheless (see below) -
       thereby enabling enhanced uses of the timestamp value and the
       modification of the receiver side timestamp processing.

   DUR12lo  and

   DUR12hi - Duration (12 bits each)
       The sender provides a range of two timestamp clock intervals in
       the initial <SYN> to ask the receiver to operate preferred in
       this range.

   ADJ12lo  and






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   ADJ12hi - Adjustment factor (5 bits each)
       The scale adjustment factor indicating the possible timestamp
       clock ranges.  All values between zero and 31 are allowed, with
       the only limitation that ADJ12hi must be equal or greater than
       ADJ12lo.  As the base value representation is shorter by 4 bits
       than the single interval representation, the values need to be
       left shifted always by 4. left-

   INT12lo  and

   INT12hi - Base Interval (7 bits each)
       The integer part of the timestamp clock interval before being
       left-shifted.  A a value of zero would have a special meaning,
       and is not a valid number for range negotiation.  The properly
       scaled intervals MUST be given in the correct order (lower
       interval in DUR12lo and higher interval in DUR12hi).

A.2.  Range Negotiation

   Only the host initiating a TCP session MAY offer a timestamp clock
   interval, while the receiver SHOULD select a timestamp clock interval
   within these bounds.  If the receiver can not adjust it's timestamp
   clock to match the range, it MAY use a timestamp clock rate outside
   these bounds.  If the receiver indicated a timestamp clock interval
   within the indicated bounds, the sender MUST set it's timestamp clock
   interval to the negotiated rate.  If the receiver uses a timestamp
   clock interval outside the indicated bounds, the sender MUST set the
   local timestamp clock interval to the value indicated by the closer
   boundary.

   The following example sequence is provided to demonstrate how
   timestamp clock range negotiation works.  Both sender and receiver
   finally know the clock interval of their respective partner.

   SYN, TSopt=<X>, TSecr=EXO|VER=1|MSK|DUR12lo=1ms|DUR12hi=100ms

   SYN,ACK, TSopt=<Y>, TSecr=<X>^EXO|VER=0|MSK|DUR=10ms

   In this example, both hosts would run their respective timestamp
   clocks with one tick every 10 ms.

   SYN, TSopt=<X>, TSecr=EXO|VER=1|MSK|DUR12lo=1ms|DUR12hi=100ms

   SYN,ACK, TSopt=<Y>, TSecr=<X>^EXO|VER=0|MSK|DUR=1000ms

   In this example, the sender would set the timestamp clock interval to
   100 ms (closer to the receivers clock interval of 1 sec), while the
   receiver will have a timestamp clock interval running at 1 sec.



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   SYN, TSopt=<X>, TSecr=EXO|VER=1|MSK|DUR12lo=1ms|DUR12hi=100ms

   SYN,ACK, TSopt=<Y>, TSecr=<X>^EXO|VER=0|MSK|DUR=100us

   In this example, the sender would set the timestamp clock rate to one
   tick every 10 ms (closest to the receiver's clock interval of 100
   us), while the receiver will have the timestamp clock running at 100
   us per tick.











































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Appendix B.  Revision history

   00 ... initial draft, early submission to meet deadline.

   01 ... refined draft, focusing only on those capabilities that have
   an immediate use case.  Also excluding flags that can be substituted
   by other means (MIR - synergistic with SACK option only, RNG moved to
   appendix A, BIA removed and the exponent bias set to a fixed value.
   Also extended other paragraphs.

   02 ... updated document after IETF80 - referrals to "timestamp
   options" were seen to be ambiguous with "timestamp option", and
   therefore replaced by "timestamp capabilities".  Also, the document
   was reworked to better align with RFC4101.  Removed SGN and increased
   FRAC to allow higher precision.

   03 ... removed references to "opaque" and "transparent". substituted
   "timestamp clock interval" for all instances of rate.  Changed signal
   encoding to resemble a scale/value approach like what is done with
   Window Scaling.  As added benefit, clock quality can be implicitly
   signaled, since multiple representations can map to idential time
   intervals.  Added discussion around resilience against broken RFC1323
   implementations (Win95, Linux 2.3.41+), which deviate from expected
   Timestamp signaling behavior.



























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Authors' Addresses

   Richard Scheffenegger
   NetApp, Inc.
   Am Euro Platz 2
   Vienna,   1120
   Austria

   Phone: +43 1 3676811 3146
   Email: rs@netapp.com


   Mirja Kuehlewind
   University of Stuttgart
   Pfaffenwaldring 47
   Stuttgart  70569
   Germany

   Email: mirja.kuehlewind@ikr.uni-stuttgart.de
































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