RTDelay_Passive_IP-TCP_RFC8912sec10_Seconds_Min
All section numbers below refer to RFC 8912 

10.1. Summary

   This category includes multiple indexes to the Registry Entries: the element ID 
   and Metric Name.

10.1.1. ID (Identifier)

   23

10.1.2. Name

   RTDelay_Passive_IP-TCP_RFC8912sec10_Seconds_Min

10.1.3. URI

   URL: https://www.iana.org/assignments/performance-metrics/RTDelay_Passive_IP-TCP_RFC8912sec10_Seconds_Min

10.1.4. Description

   RTDelay:
   This metric assesses the round-trip delay of TCP packets constituting a single 
   connection, exchanged between two hosts. We consider the measurement of round-trip 
   delay based on a single Observation Point (OP) [RFC7011] somewhere in the network. 
   The output is the round-trip delay for all successfully exchanged packets expressed 
   as the minimum of their conditional delay distribution.

10.1.5. Change Controller

   IETF

10.1.6. Version (of Registry Format)

   1.0

10.2. Metric Definition

   This category includes columns to prompt the entry of all necessary details related 
   to the metric definition, including the RFC reference and values of input factors, 
   called "Fixed Parameters".

10.2.1. Reference Definition

   Almes, G., Kalidindi, S., and M.  Zekauskas, "A Round-trip Delay
   Metric for IPPM", RFC 2681, DOI 10.17487/RFC2681, September 1999,
   <https://www.rfc-editor.org/info/rfc2681>.  [RFC2681]

   Although there is no RFC that describes Passive Measurement of round-
   trip delay, the parallel definition for Active Measurement is
   provided in [RFC2681].

   This metric definition uses the term “wire time” as defined in Section 10.2
   of RFC [RFC2330], and the terms "singleton" and "sample" as
   defined in Section 11 of [RFC2330]. (Section 2.4 of [RFC2681]
   provides the reference definition of the singleton (single value)
   round-trip delay metric.  Section 3.4 of [RFC2681] provides the
   reference definition expanded to cover a multi-singleton sample.)

   With the OP [RFC7011] typically located between the hosts
   participating in the TCP connection, the round-trip delay metric
   requires two individual measurements between the OP and each host,
   such that the Spatial Composition [RFC6049] of the measurements yields
   a round-trip delay singleton (we are extending the composition of
   one-way subpath delays to subpath round-trip delay).

   Using the direction of TCP SYN transmission to anchor the
   nomenclature, host A sends the SYN, and host B replies with SYN-ACK
   during connection establishment.  The direction of SYN transfer is
   considered the Forward direction of transmission, from A through the
   OP to B (the Reverse direction is B through the OP to A).

   Traffic Filters reduce the packet streams at the OP to a Qualified
   bidirectional flow of packets.

   In the definitions below, Corresponding Packets are transferred in
   different directions and convey a common value in a TCP header field
   that establishes correspondence (to the extent possible).  Examples
   may be found in the TCP timestamp fields.

   For a real number, RTD_fwd, >> the round-trip delay in the Forward
   direction from the OP to host B at time T' is RTD_fwd << it is
   REQUIRED that the OP observed a Qualified Packet to host B at
   wire time T', that host B received that packet and sent a
   Corresponding Packet back to host A, and the OP observed the
   Corresponding Packet at wire time T' + RTD_fwd.

   For a real number, RTD_rev, >> the round-trip delay in the Reverse
   direction from the OP to host A at time T'' is RTD_rev << it is
   REQUIRED that the OP observed a Qualified Packet to host A at
   wire time T'', that host A received that packet and sent a
   Corresponding Packet back to host B, and that the OP observed the
   Corresponding Packet at wire time T'' + RTD_rev.

   Ideally, the packet sent from host B to host A in both definitions
   above SHOULD be the same packet (or, when measuring RTD_rev first,
   the packet from host A to host B in both definitions should be the
   same).

   The REQUIRED Composition Function for a singleton of round-trip delay
   at time T (where T is the earliest of T' and T'' above) is:

   RTDelay = RTD_fwd + RTD_rev

   Note that when the OP is located at host A or host B, one of the
   terms composing RTDelay will be zero or negligible.

   Using the abbreviation HS to refer to the TCP handshake: when the Qualified and
   Corresponding Packets are a TCP-SYN and a TCP-SYN-ACK, 
   RTD_fwd == RTD_HS_fwd.

   When the Qualified and Corresponding Packets are a TCP-SYN-ACK and a 
   TCP-ACK, RTD_rev == RTD_HS_rev.

   The REQUIRED Composition Function for a singleton of round-trip delay
   for the connection handshake is:

   RTDelay_HS = RTD_HS_fwd + RTD_HS_rev


10.2.2. Fixed Parameters

   Traffic Filters:

      IPv4 header values:
         DSCP:  Set to 0
         Protocol:  Set to 06 (TCP)

      IPv6 header values:
         DSCP:  Set to 0
         Hop Count:  Set to 255
         Next Header:  Set to 6 (TCP)
         Flow Label:  Set to 0
         Extension Headers:  None

      TCP header values:
         Flags:  ACK, SYN, FIN, set as required
         Timestamps Option (TSopt):  Set.  See Section 3.2 of [RFC7323]

10.3. Method of Measurement

   This category includes columns for references to relevant sections of
   the RFC(s) and any supplemental information needed to ensure an
   unambiguous method for implementations.

10.3.1. Reference Methods

   The foundational methodology for this metric is defined in Section 4
   of [RFC7323] using the Timestamps option with modifications that
   allow application at a mid-path OP [RFC7011].  Further details and
   applicable heuristics were derived from [Strowes] and [Trammell-14].

   The Traffic Filter at the OP is configured to observe a single TCP
   connection.  When the SYN/SYN-ACK/ACK handshake occurs, it offers the
   first opportunity to measure both RTD_fwd (on the SYN to SYN-ACK
   pair) and RTD_rev (on the SYN-ACK to ACK pair).  Label this singleton
   of RTDelay as RTDelay_HS (composed using the Forward and Reverse
   measurement pair).  RTDelay_HS SHALL be treated separately from other
   RTDelays on data-bearing packets and their ACKs.  The RTDelay_HS
   value MAY be used as a consistency check on the composed values of RTDelay
   for payload-bearing packets.

   For payload-bearing packets, the OP measures the time interval
   between observation of a packet with sequence number "s" and the
   corresponding ACK with the same sequence number.  When the payload is
   transferred from host A to host B, the observed interval is RTD_fwd.

   Because many data transfers are unidirectional (say, in the Forward
   direction from host A to host B), it is necessary to use pure ACK
   packets with Timestamp (TSval) and packets with the Timestamp value
   echo to perform a RTD_rev measurement.  The time interval between
   observation of the ACK from B to A, and the Corresponding Packet with
   a Timestamp Echo Reply (TSecr) field [RFC7323], is the RTD_rev.


   Delay Measurement Filtering Heuristics:

   *  If data payloads were transferred in both Forward and Reverse
      directions, then the Round-Trip Time Measurement rule in
      Section 4.1 of [RFC7323] could be applied.  This rule essentially
      excludes any measurement using a packet unless it makes progress
      in the transfer (advances the left edge of the send window,
      consistent with [Strowes]).

   *  A different heuristic from [Trammell-14] is to exclude any RTD_rev
      that is larger than previously observed values.  This would tend
      to exclude Reverse measurements taken when the application has no
      data ready to send, because considerable time could be added to
      RTD_rev from this source of error.

   *  Note that the above heuristic assumes that host A is sending data.
      Host A expecting a download would mean that this heuristic should
      be applied to RTD_fwd.

   *  The statistic calculations to summarize the delay (RTDelay) SHALL
      be performed on the conditional distribution, conditioned on
      successful Forward and Reverse measurements that follow the
      heuristics.

   Sources of Error:

   *  The principal source of RTDelay error is the host processing time
      to return a packet that defines the termination of a time
      interval.  The heuristics above intend to mitigate these errors by
      excluding measurements where host processing time is a significant
      part of RTD_fwd or RTD_rev.

10.3.2. Packet Stream Generation

   N/A

10.3.3. Traffic Filtering (Observation) Details

   The Fixed Parameters above give a portion of the Traffic Filter. 
   Other aspects will be supplied as Runtime Parameters (below).

10.3.4. Sampling Distribution

   This metric requires a complete sample of all packets that qualify 
   according to the Traffic Filter criteria.

10.3.5. Runtime Parameters and Data Format

   Runtime Parameters are input factors that must be determined,
   configured into the measurement system, and reported with the results
   for the context to be complete.

   Src:  The IP address of the host in the host A Role (format
      ipv4-address-no-zone value for IPv4 or ipv6-address-no-zone value
      for IPv6; see Section 4 of [RFC6991]).

   Dst:  The IP address of the host in the host B Role (format
      ipv4-address-no-zone value for IPv4 or ipv6-address-no-zone value
      for IPv6; see Section 4 of [RFC6991]).

   T0:  A time, the start of a measurement interval (format "date-time"
      as specified in Section 5.6 of [RFC3339]; see also "date-and-time"
      in Section 3 of [RFC6991]).  The UTC Time Zone is required by
      Section 6.1 of [RFC2330].  When T0 is "all-zeros", a start time is
      unspecified and Tf is to be interpreted as the duration of the
      measurement interval.  The start time is controlled through other
      means.

   Tf:  Optionally, the end of a measurement interval (format
      "date-time" as specified in Section 5.6 of [RFC3339]; see also
      "date-and-time" in Section 3 of [RFC6991]), or the duration (see
      T0).  The UTC Time Zone is required by Section 6.1 of [RFC2330].
      Alternatively, the end of the measurement interval MAY be
      controlled by the measured connection, where the second pair of
      FIN and ACK packets exchanged between host A and host B
      effectively ends the interval.

   TTL or Hop Limit:  Set at desired value.

10.3.6. Roles

   host A:  Launches the SYN packet to open the connection.  The Role of
      "host A" is synonymous with the IP address used at host A.

   host B:  Replies with the SYN-ACK packet to open the connection.  The
      Role of "host B" is synonymous with the IP address used at host B.

10.4. Output

   This category specifies all details of the output of measurements 
   using the metric.

10.4.1. Type

   RTDelay Types are discussed in the subsections below.

10.4.2. Reference Definition

   For all output types:

   T0:  The start of a measurement interval (format "date‑time" 
      as specified in Section 5.6 of [RFC3339]; see also "date‑and‑time" in 
      Section 3 of [RFC6991]). The UTC Time Zone is required by Section 6.1 
      of [RFC2330].

   Tf:  The end of a measurement interval (format "date‑time" as specified in 
      Section 5.6 of [RFC3339]; see also "date‑and‑time" in Section 3 of [RFC6991]). 
      The UTC Time Zone is required by Section 6.1 of [RFC2330]. The end of the 
      measurement interval MAY be controlled by the measured connection, where the 
      second pair of FIN and ACK packets exchanged between host A and host B 
      effectively ends the interval.

   The minimum SHALL be calculated using the conditional distribution of all 
   packets with a finite value of round-trip delay (undefined delays are excluded) 
   -- a single value, as follows:

   See Section 4.1 of [RFC3393] for details on the conditional distribution to 
   exclude undefined values of delay, and see Section 5 of [RFC6703] for 
   background on this analysis choice.

   See Section 4.3.2 of [RFC6049] for details on calculating this statistic; see 
   also Section 4.3.3 of [RFC6049].

   Min:
   The time value of the result is expressed in units of seconds, as a positive 
   value of type decimal64 with fraction digits = 9 (see Section 9.3 of [RFC6020]) 
   with a resolution of 0.000000001 seconds (1.0 ns), and with lossless conversion 
   to/from the 64-bit NTP timestamp as per Section 6 of [RFC5905].

10.4.3. Metric Units

   The minimum of round-trip delay is expressed in seconds.

10.4.4. Calibration

   Passive Measurements at an OP could be calibrated against an Active
   Measurement (with loss emulation) at host A or host B, where the
   Active Measurement represents the ground truth.

10.5. Administrative Items

10.5.1. Status

   Current

10.5.2. Requester

   RFC 8912

10.5.3. Revision

   1.0

10.5.4. Revision Date

   2021-11-17

10.6. Comments and Remarks

   None