path: root/doc/rfc/rfc9506.txt

Internet Engineering Task Force (IETF)                       M. Cociglio
Request for Comments: 9506                          Telecom Italia - TIM
Category: Informational                                      A. Ferrieux
ISSN: 2070-1721                                              Orange Labs
                                                             G. Fioccola
                                                     Huawei Technologies
                                                             I. Lubashev
                                                     Akamai Technologies
                                                           F. Bulgarella
                                                                 M. Nilo
                                                    Telecom Italia - TIM
                                                            I. Hamchaoui
                                                             Orange Labs
                                                                R. Sisto
                                                   Politecnico di Torino
                                                            October 2023


         Explicit Host-to-Network Flow Measurements Techniques

Abstract

   This document describes protocol-independent methods called Explicit
   Host-to-Network Flow Measurement Techniques that can be applicable to
   transport-layer protocols between the client and server.  These
   methods employ just a few marking bits inside the header of each
   packet for performance measurements and require the client and server
   to collaborate.  Both endpoints cooperate by marking packets and,
   possibly, mirroring the markings on the round-trip connection.  The
   techniques are especially valuable when applied to protocols that
   encrypt transport headers since they enable loss and delay
   measurements by passive, on-path network devices.  This document
   describes several methods that can be used separately or jointly
   depending of the availability of marking bits, desired measurements,
   and properties of the protocol to which the methods are applied.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are candidates for any level of Internet
   Standard; see Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9506.

Copyright Notice

   Copyright (c) 2023 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
   Provisions Relating to IETF Documents
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   publication of this document.  Please review these documents
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   include Revised BSD License text as described in Section 4.e of the
   Trust Legal Provisions and are provided without warranty as described
   in the Revised BSD License.

Table of Contents

   1.  Introduction
   2.  Latency Bits
     2.1.  Spin Bit
     2.2.  Delay Bit
       2.2.1.  Generation Phase
       2.2.2.  Reflection Phase
       2.2.3.  T_Max Selection
       2.2.4.  Delay Measurement Using the Delay Bit
         2.2.4.1.  RTT Measurement
         2.2.4.2.  Half-RTT Measurement
         2.2.4.3.  Intra-domain RTT Measurement
       2.2.5.  Observer's Algorithm
       2.2.6.  Two Bits Delay Measurement: Spin Bit + Delay Bit
   3.  Loss Bits
     3.1.  T Bit -- Round-Trip Loss Bit
       3.1.1.  Round-Trip Loss
       3.1.2.  Setting the Round-Trip Loss Bit on Outgoing Packets
       3.1.3.  Observer's Logic for Round-Trip Loss Signal
       3.1.4.  Loss Coverage and Signal Timing
     3.2.  Q Bit -- sQuare Bit
       3.2.1.  Q Block Length Selection
       3.2.2.  Upstream Loss
       3.2.3.  Identifying Q Block Boundaries
         3.2.3.1.  Improved Resilience to Burst Losses
     3.3.  L Bit -- Loss Event Bit
       3.3.1.  End-To-End Loss
         3.3.1.1.  Loss Profile Characterization
       3.3.2.  L+Q Bits -- Loss Measurement Using L and Q Bits
         3.3.2.1.  Correlating End-to-End and Upstream Loss
         3.3.2.2.  Downstream Loss
         3.3.2.3.  Observer Loss
     3.4.  R Bit -- Reflection Square Bit
       3.4.1.  Enhancement of Reflection Block Length Computation
       3.4.2.  Improved Resilience to Packet Reordering
         3.4.2.1.  Improved Resilience to Burst Losses
       3.4.3.  R+Q Bits -- Loss Measurement Using R and Q Bits
         3.4.3.1.  Three-Quarters Connection Loss
         3.4.3.2.  End-To-End Loss in the Opposite Direction
         3.4.3.3.  Half Round-Trip Loss
         3.4.3.4.  Downstream Loss
     3.5.  E Bit -- ECN-Echo Event Bit
       3.5.1.  Setting the ECN-Echo Event Bit on Outgoing Packets
       3.5.2.  Using E Bit for Passive ECN-Reported Congestion
               Measurement
       3.5.3.  Multiple E Bits
   4.  Summary of Delay and Loss Marking Methods
     4.1.  Implementation Considerations
   5.  Examples of Application
   6.  Protocol Ossification Considerations
   7.  Security Considerations
     7.1.  Optimistic ACK Attack
     7.2.  Delay Bit with RTT Obfuscation
   8.  Privacy Considerations
   9.  IANA Considerations
   10. References
     10.1.  Normative References
     10.2.  Informative References
   Acknowledgments
   Contributors
   Authors' Addresses

1.  Introduction

   Packet loss and delay are hard and pervasive problems of day-to-day
   network operation.  Proactively detecting, measuring, and locating
   them is crucial to maintaining high QoS and timely resolution of end-
   to-end throughput issues.

   Detecting and measuring packet loss and delay allows network
   operators to independently confirm trouble reports and, ideally, be
   proactively notified of developing problems on the network.  Locating
   the cause of packet loss or excessive delay is the first step to
   resolving problems and restoring QoS.

   Network operators wishing to perform quantitative measurement of
   packet loss and delay have been heavily relying on information
   present in the clear in transport-layer headers (e.g., TCP sequence
   and acknowledgment numbers).  By passively observing a network path
   at multiple points within one's network, operators have been able to
   either quickly locate the source the problem within their network or
   reliably attribute it to an upstream or downstream network.

   With encrypted protocols, the transport-layer headers are encrypted
   and passive packet loss and delay observations are not possible, as
   also noted in [TRANSPORT-ENCRYPT].  Nevertheless, accurate
   measurement of packet loss and delay experienced by encrypted
   transport-layer protocols is highly desired, especially by network
   operators who own or control the infrastructure between the client
   and server.

   The measurement of loss and delay experienced by connections using an
   encrypted protocol cannot be based on a measurement of loss and delay
   experienced by connections between the same or similar endpoints that
   use an unencrypted protocol because different protocols may utilize
   the network differently and be routed differently by the network.
   Therefore, it is necessary to directly measure the packet loss and
   delay experienced by users of encrypted protocols.

   The Alternate-Marking method [AltMark] defines a consolidated method
   to perform packet loss, delay, and jitter measurements on live
   traffic.  However, as mentioned in [IPv6AltMark], [AltMark] mainly
   applies to a network-layer-controlled domain managed with a Network
   Management System (NMS), where the Customer Premises Equipment (CPE)
   or the Provider Edge (PE) routers are the starting or the ending
   nodes.  [AltMark] provides measurement within a controlled domain in
   which the packets are marked.  Therefore, applying [AltMark] to end-
   to-end transport-layer connections is not easy because packet
   identification and marking by network nodes is prevented when
   encrypted transport-layer headers (e.g., QUIC, TCP with TLS) are
   being used.

   This document defines Explicit Host-to-Network Flow Measurement
   Techniques that are specifically designed for encrypted transport
   protocols.  According to the definitions of [IPPM-METHODS], these
   measurement methods can be classified as Hybrid.  They are to be
   embedded into a transport-layer protocol and are explicitly intended
   for exposing delay and loss rate information to on-path measurement
   devices.  Unlike [AltMark], most of these methods require
   collaborative endpoint nodes.  Since these measurement techniques
   make performance information directly visible to the path, they do
   not rely on an external NMS.

   The Explicit Host-to-Network Flow Measurement Techniques described in
   this document are applicable to any transport-layer protocol
   connecting a client and a server.  In this document, the client and
   the server are also referred to as the endpoints of the transport-
   layer protocol.

   The different methods described in this document can be used alone or
   in combination.  Each technique uses few bits and exposes a specific
   measurement.  It is assumed that the endpoints are collaborative in
   the sense of the measurements, indeed both the client and server need
   to cooperate.

   Following the recommendation in [RFC8558] of making path signals
   explicit, this document proposes adding some dedicated measurement
   bits to the clear portion of the transport protocol headers.  These
   bits can be added to an unencrypted portion of a transport-layer
   header, e.g., UDP surplus space (see [UDP-OPTIONS] and [UDP-SURPLUS])
   or reserved bits in a QUIC v1 header, as already done with the
   latency Spin bit (see Section 17.4 of [QUIC-TRANSPORT]).  Note that
   this document does not recommend the use of any specific bits, as
   these would need to be chosen by the specific protocol
   implementations (see Section 5).

   The Spin bit, Delay bit, and loss bits explained in this document are
   inspired by [AltMark], [QUIC-MANAGEABILITY], [QUIC-SPIN],
   [TSVWG-SPIN], and [IPPM-SPIN].

   Additional details about the performance measurements for QUIC are
   described in the paper [ANRW19-PM-QUIC].

2.  Latency Bits

   This section introduces bits that can be used for round-trip latency
   measurements.  Whenever this section of the specification refers to
   packets, it is referring only to packets with protocol headers that
   include the latency bits.

   In [QUIC-TRANSPORT], Section 17.4 introduces an explicit, per-flow
   transport-layer signal for hybrid measurement of RTT.  This signal
   consists of a Spin bit that toggles once per RTT.  Section 4 of
   [QUIC-SPIN] discusses an additional two-bit Valid Edge Counter (VEC)
   to compensate for loss and reordering of the Spin bit and to increase
   fidelity of the signal in less than ideal network conditions.

   This document introduces a standalone single-bit delay signal that
   can be used by passive observers to measure the RTT of a network
   flow, avoiding the Spin bit ambiguities that arise as soon as network
   conditions deteriorate.

2.1.  Spin Bit

   This section is a small recap of the Spin bit working mechanism.  For
   a comprehensive explanation of the algorithm, see Section 3.8.2 of
   [QUIC-MANAGEABILITY].

   The Spin bit is a signal generated by Alternate-Marking [AltMark],
   where the size of the alternation changes with the flight size each
   RTT.

   The latency Spin bit is a single-bit signal that toggles once per
   RTT, enabling latency monitoring of a connection-oriented
   communication from intermediate observation points.

   A "Spin bit period" is a set of packets with the same Spin bit value
   sent during one RTT time interval.  A "Spin bit period value" is the
   value of the Spin bit shared by all packets in a Spin bit period.

   The client and server maintain an internal per-connection spin value
   (i.e., 0 or 1) used to set the Spin bit on outgoing packets.  Both
   endpoints initialize the spin value to 0 when a new connection
   starts.  Then:

   *  when the client receives a packet with the packet number larger
      than any number seen so far, it sets the connection spin value to
      the opposite value contained in the received packet; and

   *  when the server receives a packet with the packet number larger
      than any number seen so far, it sets the connection spin value to
      the same value contained in the received packet.

   The computed spin value is used by the endpoints for setting the Spin
   bit on outgoing packets.  This mechanism allows the endpoints to
   generate a square wave such that, by measuring the distance in time
   between pairs of consecutive edges observed in the same direction, a
   passive on-path observer can compute the round-trip network delay of
   that network flow.

   Spin bit enables round-trip latency measurement by observing a single
   direction of the traffic flow.

   Note that packet reordering can cause spurious edges that require
   heuristics to correct.  The Spin bit performance deteriorates as soon
   as network impairments arise as explained in Section 2.2.

2.2.  Delay Bit

   The Delay bit has been designed to overcome accuracy limitations
   experienced by the Spin bit under difficult network conditions:

   *  packet reordering leads to generation of spurious edges and errors
      in delay estimation;

   *  loss of edges causes wrong estimation of Spin bit periods and
      therefore wrong RTT measurements; and

   *  application-limited senders cause the Spin bit to measure the
      application delays instead of network delays.

   Unlike the Spin bit, which is set in every packet transmitted on the
   network, the Delay bit is set only once per round trip.

   When the Delay bit is used, a single packet with a marked bit (the
   Delay bit) bounces between a client and a server during the entire
   connection lifetime.  This single packet is called the "delay
   sample".

   An observer placed at an intermediate point, observing a single
   direction of traffic and tracking the delay sample and the relative
   timestamp, can measure the round-trip delay of the connection.

   The delay sample lifetime comprises two phases: initialization and
   reflection.  The initialization is the generation of the delay
   sample, while the reflection realizes the bounce behavior of this
   single packet between the two endpoints.

   The next figure describes the elementary Delay bit mechanism.

                 +--------+   -   -   -   -   -   +--------+
                 |        |      ----------->     |        |
                 | Client |                       | Server |
                 |        |     <-----------      |        |
                 +--------+   -   -   -   -   -   +--------+

                 (a) No traffic at beginning.

                 +--------+   0   0   1   -   -   +--------+
                 |        |      ----------->     |        |
                 | Client |                       | Server |
                 |        |     <-----------      |        |
                 +--------+   -   -   -   -   -   +--------+

                 (b) The Client starts sending data and sets
                     the first packet as the delay sample.

                 +--------+   0   0   0   0   0   +--------+
                 |        |      ----------->     |        |
                 | Client |                       | Server |
                 |        |     <-----------      |        |
                 +--------+   -   -   -   1   0   +--------+

                 (c) The Server starts sending data
                     and reflects the delay sample.

                 +--------+   0   1   0   0   0   +--------+
                 |        |      ----------->     |        |
                 | Client |                       | Server |
                 |        |     <-----------      |        |
                 +--------+   0   0   0   0   0   +--------+

                 (d) The Client reflects the delay sample.

                 +--------+   0   0   0   0   0   +--------+
                 |        |      ----------->     |        |
                 | Client |                       | Server |
                 |        |     <-----------      |        |
                 +--------+   0   0   0   1   0   +--------+

                 (e) The Server reflects the delay sample
                     and so on.

                       Figure 1: Delay Bit Mechanism

2.2.1.  Generation Phase

   Only the client is actively involved in the Generation Phase.  It
   maintains an internal per-flow timestamp variable (ds_time) updated
   every time a delay sample is transmitted.

   When connection starts, the client generates a new delay sample
   initializing the Delay bit of the first outgoing packet to 1.  Then
   it updates the ds_time variable with the timestamp of its
   transmission.

   The server initializes the Delay bit to 0 at the beginning of the
   connection, and its only task during the connection is described in
   Section 2.2.2.

   In absence of network impairments, the delay sample should bounce
   between the client and server continuously for the entire duration of
   the connection.  However, that is highly unlikely for two reasons:

   1.  The packet carrying the Delay bit might be lost.

   2.  An endpoint could stop or delay sending packets because the
       application is limiting the amount of traffic transmitted.

   To deal with these problems, the client generates a new delay sample
   if more than a predetermined time (T_Max) has elapsed since the last
   delay sample transmission (including reflections).  Note that T_Max
   should be greater than the max measurable RTT on the network.  See
   Section 2.2.3 for details.

2.2.2.  Reflection Phase

   Reflection is the process that enables the bouncing of the delay
   sample between a client and a server.  The behavior of the two
   endpoints is almost the same.

   *  Server-side reflection: When a delay sample arrives, the server
      marks the first packet in the opposite direction as the delay
      sample.

   *  Client-side reflection: When a delay sample arrives, the client
      marks the first packet in the opposite direction as the delay
      sample.  It also updates the ds_time variable when the outgoing
      delay sample is actually forwarded.

   In both cases, if the outgoing delay sample is being transmitted with
   a delay greater than a predetermined threshold after the reception of
   the incoming delay sample (1 ms by default), the delay sample is not
   reflected, and the outgoing Delay bit is kept at 0.

   By doing so, the algorithm can reject measurements that would
   overestimate the delay due to lack of traffic at the endpoints.
   Hence, the maximum estimation error would amount to twice the
   threshold (e.g., 2 ms) per measurement.

2.2.3.  T_Max Selection

   The internal ds_time variable allows a client to identify delay
   sample losses.  Considering that a lost delay sample is regenerated
   at the end of an explicit time (T_Max) since the last generation,
   this same value can be used by an observer to reject a measure and
   start a new one.

   In other words, if the difference in time between two delay samples
   is greater or equal than T_Max, then these cannot be used to produce
   a delay measure.  Therefore, the value of T_Max must also be known to
   the on-path network probes.

   There are two alternatives to selecting the T_Max value so that both
   the client and observers know it.  The first one requires that T_Max
   is known a priori (T_Max_p) and therefore set within the protocol
   specifications that implements the marking mechanism (e.g., 1 second,
   which usually is greater than the max expected RTT).  The second
   alternative requires a dynamic mechanism able to adapt the duration
   of the T_Max to the delay of the connection (T_Max_c).

   For instance, the client and observers could use the connection RTT
   as a basis for calculating an effective T_Max.  They should use a
   predetermined initial value so that T_Max = T_Max_p (e.g., 1 second)
   and then, when a valid RTT is measured, change T_Max accordingly so
   that T_Max = T_Max_c.  In any case, the selected T_Max should be
   large enough to absorb any possible variations in the connection
   delay.  This also helps to prevent the mechanism from failing when
   the observer cannot recognize sudden changes in RTT exceeding T_Max.

   T_Max_c could be computed as two times the measured RTT plus a fixed
   amount of time (100 ms) to prevent low T_Max values in the case of
   very small RTTs.  The resulting formula is: T_Max_c = 2RTT + 100 ms.
   If T_Max_c is greater than T_Max_p, then T_Max_c is forced to the
   T_Max_p value.  Note that the value of 100 ms is provided as an
   example, and it may be chosen differently depending on the specific
   scenarios.  For instance, an implementer may consider using existing
   protocol-specific values if appropriate.

   Note that the observer's T_Max should always be less than or equal to
   the client's T_Max to avoid considering as a valid measurement what
   is actually the client's T_Max.  To obtain this result, the client
   waits for two consecutive incoming samples and computes the two
   related RTTs.  Then it takes the largest of them as the basis of the
   T_Max_c formula.  At this point, observers have already measured a
   valid RTT and then computed their T_Max_c.

2.2.4.  Delay Measurement Using the Delay Bit

   When the Delay bit is used, a passive observer can use delay samples
   directly and avoid inherent ambiguities in the calculation of the RTT
   as can be seen in Spin bit analysis.

2.2.4.1.  RTT Measurement

   The delay sample generation process ensures that only one packet
   marked with the Delay bit set to 1 runs back and forth between two
   endpoints per round-trip time.  To determine the RTT measurement of a
   flow, an on-path passive observer computes the time difference
   between two delay samples observed in a single direction.

   To ensure a valid measurement, the observer must verify that the
   distance in time between the two samples taken into account is less
   than T_Max.

              =======================|======================>
              = **********     -----Obs---->     ********** =
              = * Client *                       * Server * =
              = **********     <------------     ********** =
              <==============================================

                        (a) client-server RTT

              ==============================================>
              = **********     ------------>     ********** =
              = * Client *                       * Server * =
              = **********     <----Obs-----     ********** =
              <======================|=======================

                        (b) server-client RTT

                Figure 2: Round-Trip Time (Both Directions)

2.2.4.2.  Half-RTT Measurement

   An observer that is able to observe both forward and return traffic
   directions can use the delay samples to measure "upstream" and
   "downstream" RTT components, also known as the half-RTT measurements.
   It does this by measuring the time between a delay sample observed in
   one direction and the delay sample previously observed in the
   opposite direction.

   As with RTT measurement, the observer must verify that the distance
   in time between the two samples taken into account is less than
   T_Max.

   Note that upstream and downstream sections of paths between the
   endpoints and the observer (i.e., observer-to-client vs. client-to-
   observer and observer-to-server vs. server-to-observer) may have
   different delay characteristics due to the difference in network
   congestion and other factors.

              =======================>
              = **********     ------|----->     **********
              = * Client *          Obs          * Server *
              = **********     <-----|------     **********
              <=======================

                     (a) client-observer half-RTT

                                     =======================>
                **********     ------|----->     ********** =
                * Client *          Obs          * Server * =
                **********     <-----|------     ********** =
                                     <=======================

                     (b) observer-server half-RTT

              Figure 3: Half Round-Trip Time (Both Directions)

2.2.4.3.  Intra-domain RTT Measurement

   Intra-domain RTT is the portion of the entire RTT used by a flow to
   traverse the network of a provider.  To measure intra-domain RTT, two
   observers capable of observing traffic in both directions must be
   employed simultaneously at the ingress and egress of the network to
   be measured.  Intra-domain RTT is the difference between the two
   computed upstream (or downstream) RTT components.

           =========================================>
           = =====================>
           = = **********      ---|-->           ---|-->      **********
           = = * Client *         Obs               Obs       * Server *
           = = **********      <--|---           <--|---      **********
           = <=====================
           <=========================================

                    (a) client-observer RTT components (half-RTTs)

                                  ==================>
               **********      ---|-->           ---|-->      **********
               * Client *         Obs               Obs       * Server *
               **********      <--|---           <--|---      **********
                                  <==================

                    (b) the intra-domain RTT resulting from the
                        subtraction of the above RTT components

     Figure 4: Intra-domain Round-Trip Time (Client-Observer: Upstream)

2.2.5.  Observer's Algorithm

   An on-path observer maintains an internal per-flow variable to keep
   track of the time at which the last delay sample has been observed.
   The flow characterization should be part of the protocol.

   If the observer is unidirectional or in case of asymmetric routing,
   then upon detecting a delay sample:

   *  if a delay sample was also detected previously in the same
      direction and the distance in time between them is less than T_Max
      - K, then the two delay samples can be used to calculate RTT
      measurement.  K is a protection threshold to absorb differences in
      T_Max computation and delay variations between two consecutive
      delay samples (e.g., K = 10% T_Max).

   If the observer can observe both forward and return traffic flows,
   and it is able to determine which direction contains the client and
   the server (e.g., by observing the connection handshake), then upon
   detecting a delay sample:

   *  if a delay sample was also detected in the opposite direction and
      the distance in time between them is less than T_Max - K, then the
      two delay samples can be used to measure the observer-client half-
      RTT or the observer-server half-RTT, according to the direction of
      the last delay sample observed.

   Note that the accuracy can be influenced by what the observer is
   capable of observing.  Additionally, the type of measurement differs,
   as described in the previous sections.

2.2.6.  Two Bits Delay Measurement: Spin Bit + Delay Bit

   The Spin and Delay bit algorithms work independently.  If both
   marking methods are used in the same connection, observers can choose
   the best measurement between the two available:

   *  when a precise measurement can be produced using the Delay bit,
      observers choose it; and

   *  when a Delay bit measurement is not available, observers choose
      the approximate Spin bit one.

3.  Loss Bits

   This section introduces bits that can be used for loss measurements.
   Whenever this section of the specification refers to packets, it is
   referring only to packets with protocol headers that include the loss
   bits -- the only packets whose loss can be measured.

   T:   The "round-Trip loss" bit is used in combination with the Spin
        bit to measure round-trip loss.  See Section 3.1.

   Q:   The "sQuare" bit is used to measure upstream loss.  See
        Section 3.2.

   L:   The "Loss Event" bit is used to measure end-to-end loss.  See
        Section 3.3.

   R:   The "Reflection square" bit is used in combination with the Q
        bit to measure end-to-end loss.  See Section 3.4.

   Loss measurements enabled by T, Q, and L bits can be implemented by
   those loss bits alone (T bit requires a working Spin bit).  Two-bit
   combinations Q+L and Q+R enable additional measurement opportunities
   discussed below.

   Each endpoint maintains appropriate counters independently and
   separately for each identifiable flow (or each sub-flow for multipath
   connections).

   Since loss is reported independently for each flow, all bits (except
   for the L bit) require a certain minimum number of packets to be
   exchanged per flow before any signal can be measured.  Therefore,
   loss measurements work best for flows that transfer more than a
   minimal amount of data.

3.1.  T Bit -- Round-Trip Loss Bit

   The round-Trip loss bit is used to mark a variable number of packets
   exchanged twice between the endpoints realizing a two round-trip
   reflection.  A passive on-path observer, observing either direction,
   can count and compare the number of marked packets seen during the
   two reflections, estimating the loss rate experienced by the
   connection.  The overall exchange comprises:

   *  the client selects and consequently sets the T bit to 1 in order
      to identify a first train of packets;

   *  upon receiving each packet included in the first train, the server
      sets the T bit to 1 and reflects to the client a respective second
      train of packets of the same size as the first train received;

   *  upon receiving each packet included in the second train, the
      client sets the T bit to 1 and reflects to the server a respective
      third train of packets of the same size as the second train
      received; and

   *  upon receiving each packet included in the third train, the server
      sets the T bit to 1 and finally reflects to the client a
      respective fourth train of packets of the same size as the third
      train received.

   Packets belonging to the first round trip (first and second train)
   represent the Generation Phase, while those belonging to the second
   round trip (third and fourth train) represent the Reflection Phase.

   A passive on-path observer can count and compare the number of marked
   packets seen during the two round trips (i.e., the first and third or
   the second and the fourth trains of packets, depending on which
   direction is observed) and estimate the loss rate experienced by the
   connection.  This process is repeated continuously to obtain more
   measurements as long as the endpoints exchange traffic.  These
   measurements can be called round-trip losses.

   Since the packet rates in two directions may be different, the number
   of marked packets in the train is determined by the direction with
   the lowest packet rate.  See Section 3.1.2 for details on packet
   generation.

3.1.1.  Round-Trip Loss

   Since the measurements are performed on a portion of the traffic
   exchanged between the client and the server, the observer calculates
   the end-to-end Round-Trip Packet Loss (RTPL) that, statistically,
   will correspond to the loss rate experienced by the connection along
   the entire network path.

              =======================|======================>
              = **********     -----Obs---->     ********** =
              = * Client *                       * Server * =
              = **********     <------------     ********** =
              <==============================================

                        (a) client-server RTPL

              ==============================================>
              = **********     ------------>     ********** =
              = * Client *                       * Server * =
              = **********     <----Obs-----     ********** =
              <======================|=======================

                        (b) server-client RTPL

             Figure 5: Round-Trip Packet Loss (Both Directions)

   This methodology also allows the half-RTPL measurement and the Intra-
   domain RTPL measurement in a way similar to RTT measurement.

              =======================>
              = **********     ------|----->     **********
              = * Client *          Obs          * Server *
              = **********     <-----|------     **********
              <=======================

                     (a) client-observer half-RTPL

                                     =======================>
                **********     ------|----->     ********** =
                * Client *          Obs          * Server * =
                **********     <-----|------     ********** =
                                     <=======================

                     (b) observer-server half-RTPL

          Figure 6: Half Round-Trip Packet Loss (Both Directions)

                              =========================================>
                                                =====================> =
           **********      ---|-->           ---|-->      ********** = =
           * Client *         Obs               Obs       * Server * = =
           **********      <--|---           <--|---      ********** = =
                                                <===================== =
                              <=========================================

                (a) observer-server RTPL components (half-RTPLs)

                              ==================>
           **********      ---|-->           ---|-->      **********
           * Client *         Obs               Obs       * Server *
           **********      <--|---           <--|---      **********
                              <==================

                (b) the intra-domain RTPL resulting from the
                    subtraction of the above RTPL components

      Figure 7: Intra-domain Round-Trip Packet Loss (Observer-Server)

3.1.2.  Setting the Round-Trip Loss Bit on Outgoing Packets

   The round-Trip loss signal requires a working Spin bit signal to
   separate trains of marked packets (packets with T bit set to 1).  A
   "pause" of at least one empty Spin bit period between each phase of
   the algorithm serves as such a separator for the on-path observer.
   The connection between T bit and Spin bit helps the observer
   correlate packet trains.

   The client maintains a "generation token" count that is set to zero
   at the beginning of the session and is incremented every time a
   packet is received (marked or unmarked).  The client also maintains a
   "reflection counter" that starts at zero at the beginning of the
   session.

   The client is in charge of launching trains of marked packets and
   does so according to the algorithm:

   1.  Generation Phase.  The client starts generating marked packets
       for two consecutive Spin bit periods.  When the client transmits
       a packet and a "generation token" is available, the client marks
       the packet and retires a "generation token".  If no token is
       available, the outgoing packet is transmitted unmarked.  At the
       end of the first Spin bit period spent in generation, the
       reflection counter is unlocked to start counting incoming marked
       packets that will be reflected later.

   2.  Pause Phase.  When the generation is completed, the client pauses
       till it has observed one entire Spin bit period with no marked
       packets.  That Spin bit period is used by the observer as a
       separator between generated and reflected packets.  During this
       marking pause, all the outgoing packets are transmitted with T
       bit set to 0.  The reflection counter is still incremented every
       time a marked packet arrives.

   3.  Reflection Phase.  The client starts transmitting marked packets,
       decrementing the reflection counter for each transmitted marked
       packet until the reflection counter has reached zero.  The
       "generation token" method from the Generation Phase is used
       during this phase as well.  At the end of the first Spin bit
       period spent in reflection, the reflection counter is locked to
       avoid incoming reflected packets incrementing it.

   4.  Pause Phase 2.  The Pause Phase is repeated after the Reflection
       Phase and serves as a separator between the reflected packet
       train and a new packet train.

   The generation token counter should be capped to limit the effects of
   a subsequent sudden reduction in the other endpoint's packet rate
   that could prevent that endpoint from reflecting collected packets.
   A cap value of 1 is recommended.

   A server maintains a "marking counter" that starts at zero and is
   incremented every time a marked packet arrives.  When the server
   transmits a packet and the "marking counter" is positive, the server
   marks the packet and decrements the "marking counter".  If the
   "marking counter" is zero, the outgoing packet is transmitted
   unmarked.

   Note that a choice of 2 RTT (two Spin bit periods) for the Generation
   Phase is a trade-off between the percentage of marked packets (i.e.,
   the percentage of traffic monitored) and the measurement delay.
   Using this value, the algorithm produces a measurement approximately
   every 6 RTT (2 generations, ~2 reflections, 2 pauses), marking ~1/3
   of packets exchanged in the slower direction (see Section 3.1.4).
   Choosing a Generation Phase of 1 RTT, we would produce measurements
   every 4 RTT, monitoring ~1/4 of packets in the slower direction.

   It is worth mentioning that problems can happen in some cases,
   especially if the rate suddenly changes, but the mechanism described
   here worked well with normal traffic conditions in the
   implementation.

3.1.3.  Observer's Logic for Round-Trip Loss Signal

   The on-path observer counts marked packets and separates different
   trains by detecting Spin bit periods (at least one) with no marked
   packets.  The Round-Trip Packet Loss (RTPL) is the difference between
   the size of the Generation train and the Reflection train.

   In the following example, packets are represented by two bits (first
   one is the Spin bit, second one is the round-Trip loss bit):

           Generation          Pause           Reflection       Pause
      ____________________ ______________ ____________________ ________
     |                    |              |                    |        |
      01 01 00 01 11 10 11 00 00 10 10 10 01 00 01 01 10 11 10 00 00 10

                  Figure 8: Round-Trip Loss Signal Example

   Note that 5 marked packets have been generated, of which 4 have been
   reflected.

3.1.4.  Loss Coverage and Signal Timing

   A cycle of the round-Trip loss signaling algorithm contains 2 RTTs of
   Generation phase, 2 RTTs of Reflection Phase, and 2 Pause Phases at
   least 1 RTT in duration each.  Hence, the loss signal is delayed by
   about 6 RTTs since the loss events.

   The observer can only detect the loss of marked packets that occurs
   after its initial observation of the Generation Phase and before its
   subsequent observation of the Reflection Phase.  Hence, if the loss
   occurs on the path that sends packets at a lower rate (typically ACKs
   in such asymmetric scenarios), 2/6 (1/3) of the packets will be
   sampled for loss detection.

   If the loss occurs on the path that sends packets at a higher rate,
   lowPacketRate/(3*highPacketRate) of the packets will be sampled for
   loss detection.  For protocols that use ACKs, the portion of packets
   sampled for loss in the higher rate direction during unidirectional
   data transfer is 1/(3*packetsPerAck), where the value of
   packetsPerAck can vary by protocol, by implementation, and by network
   conditions.

3.2.  Q Bit -- sQuare Bit

   The sQuare bit (Q bit) takes its name from the square wave generated
   by its signal.  This method is based on the Alternate-Marking method
   [AltMark], and the Q bit represents the "packet color" that can be
   switched between 0 and 1 in order to mark consecutive blocks of
   packets with different colors.  This method does not require
   cooperation from both endpoints.

   [AltMark] introduces two variations of the Alternate-Marking method
   depending on whether the color is switched according to a fixed timer
   or after a fixed number of packets.  Cooperating and synchronized
   observers on either end of a network segment can use the fixed-timer
   method to measure packet loss on the segment by comparing packet
   counters for the same packet blocks.  The time length of the blocks
   can be chosen depending on the desired measurement frequency, but it
   must be long enough to guarantee the proper operation with respect to
   clock errors and network delay issues.

   The Q bit method described in this document chooses the color-
   switching method based on a fixed number of packets for each block.
   This approach has the advantage that it does not require cooperating
   or synchronized observers or network elements.  Each probe can
   measure packet loss autonomously without relying on an external NMS.
   For the purpose of the packet loss measurement, all blocks have the
   same number of packets, and it is necessary to detect only the loss
   event and not to identify the exact block with losses.

   Following the method based on fixed number of packets, the square
   wave signal is generated by the switching of the Q bit: every
   outgoing packet contains the Q bit value, which is initialized to 0
   and inverted after sending N packets (a sQuare Block or simply Q
   Block).  Hence, Q Period is 2*N.

   Observation points can estimate upstream losses by watching a single
   direction of the traffic flow and counting the number of packets in
   each observed Q Block, as described in Section 3.2.2.

3.2.1.  Q Block Length Selection

   The length of the block must be known to the on-path network probes.
   There are two alternatives to selecting the Q Block length.  The
   first one requires that the length is known a priori and therefore
   set within the protocol specifications that implement the marking
   mechanism.  The second requires the sender to select it.

   In this latter scenario, the sender is expected to choose N (Q Block
   length) based on the expected amount of loss and reordering on the
   path.  The choice of N strikes a compromise -- the observation could
   become too unreliable in case of packet reordering and/or severe loss
   if N is too small, while short flows may not yield a useful upstream
   loss measurement if N is too large (see Section 3.2.2).

   The value of N should be at least 64 and be a power of 2.  This
   requirement allows an observer to infer the Q Block length by
   observing one period of the square signal.  It also allows the
   observer to identify flows that set the loss bits to arbitrary values
   (see Section 6).

   If the sender does not have sufficient information to make an
   informed decision about Q Block length, the sender should use N=64,
   since this value has been extensively tried in large-scale field
   tests and yielded good results.  Alternatively, the sender may also
   choose a random power-of-2 N for each flow, increasing the chances of
   using a Q Block length that gives the best signal for some flows.

   The sender must keep the value of N constant for a given flow.

3.2.2.  Upstream Loss

   Blocks of N (Q Block length) consecutive packets are sent with the
   same value of the Q bit, followed by another block of N packets with
   an inverted value of the Q bit.  Hence, knowing the value of N, an
   on-path observer can estimate the amount of upstream loss after
   observing at least N packets.  The upstream loss rate (uloss) is one
   minus the average number of packets in a block of packets with the
   same Q value (p) divided by N (uloss=1-avg(p)/N).

   The observer needs to be able to tolerate packet reordering that can
   blur the edges of the square signal, as explained in Section 3.2.3.

             =====================>
             **********     -----Obs---->     **********
             * Client *                       * Server *
             **********     <------------     **********

               (a) in client-server channel (uloss_up)

             **********     ------------>     **********
             * Client *                       * Server *
             **********     <----Obs-----     **********
                                  <=====================

               (b) in server-client channel (uloss_down)

                          Figure 9: Upstream Loss

3.2.3.  Identifying Q Block Boundaries

   Packet reordering can produce spurious edges in the square signal.
   To address this, the observer should look for packets with the
   current Q bit value up to X packets past the first packet with a
   reverse Q bit value.  The value of X, a "Marking Block Threshold",
   should be less than N/2.

   The choice of X represents a trade-off between resiliency to
   reordering and resiliency to loss.  A very large Marking Block
   Threshold will be able to reconstruct Q Blocks despite a significant
   amount of reordering, but it may erroneously coalesce packets from
   multiple Q Blocks into fewer Q Blocks if loss exceeds 50% for some Q
   Blocks.

3.2.3.1.  Improved Resilience to Burst Losses

   Burst losses can affect the accuracy of Q measurements.  Generally,
   burst losses can be absorbed and correctly measured if smaller than
   the established Q Block length.  If the entire Q Block length of
   packets is lost in a burst, however, the observer may be left
   completely unaware of the loss.

   To improve burst loss resilience, an observer may consider a received
   Q Block larger than the selected Q Block length as an indication of a
   burst loss event.  The observer would then compute the loss as three
   times the Q Block length minus the measured block length.  By doing
   so, the observer can detect burst losses of less than two blocks
   (e.g., less than 128 packets for a Q Block length of 64 packets).  A
   burst loss of two or more consecutive periods would still remain
   unnoticed by the observer (or underestimated if a period longer than
   Q Block length were formed).

3.3.  L Bit -- Loss Event Bit

   The Loss Event bit uses an Unreported Loss counter maintained by the
   protocol that implements the marking mechanism.  To use the Loss
   Event bit, the protocol must allow the sender to identify lost
   packets.  This is true of protocols such as QUIC, partially true for
   TCP and Stream Control Transmission Protocol (SCTP) (losses of pure
   ACKs are not detected), and is not true of protocols such as UDP and
   IPv4/IPv6.

   The Unreported Loss counter is initialized to 0, and the L bit of
   every outgoing packet indicates whether the Unreported Loss counter
   is positive (L=1 if the counter is positive, and L=0 otherwise).

   The value of the Unreported Loss counter is decremented every time a
   packet with L=1 is sent.

   The value of the Unreported Loss counter is incremented for every
   packet that the protocol declares lost, using whatever loss detection
   machinery the protocol employs.  If the protocol is able to rescind
   the loss determination later, a positive Unreported Loss counter may
   be decremented due to the rescission.  In general, it should not
   become negative due to the rescission, but it can happen in few
   cases.

   This loss signaling is similar to loss signaling in [ConEx], except
   that the Loss Event bit is reporting the exact number of lost
   packets, whereas the signal mechanism in [ConEx] is reporting an
   approximate number of lost bytes.

   For protocols, such as TCP [TCP], that allow network devices to
   change data segmentation, it is possible that only a part of the
   packet is lost.  In these cases, the sender must increment the
   Unreported Loss counter by the fraction of the packet data lost (so
   the Unreported Loss counter may become negative when a packet with
   L=1 is sent after a partial packet has been lost).

   Observation points can estimate the end-to-end loss, as determined by
   the upstream endpoint, by counting packets in this direction with the
   L bit equal to 1, as described in Section 3.3.1.

3.3.1.  End-To-End Loss

   The Loss Event bit allows an observer to estimate the end-to-end loss
   rate by counting packets with L bit values of 0 and 1 for a given
   flow.  The end-to-end loss ratio is the fraction of packets with L=1.

   The assumption here is that upstream loss affects packets with L=0
   and L=1 equally.  If some loss is caused by tail-drop in a network
   device, this may be a simplification.  If the sender's congestion
   controller reduces the packet send rate after loss, there may be a
   sufficient delay before sending packets with L=1 that they have a
   greater chance of arriving at the observer.

3.3.1.1.  Loss Profile Characterization

   The Loss Event bit allows an observer to characterize the loss
   profile, since the distribution of observed packets with the L bit
   set to 1 roughly corresponds to the distribution of packets lost
   between 1 RTT and 1 retransmission timeout (RTO) before (see
   Section 3.3.2.1).  Hence, observing random single instances of the L
   bit set to 1 indicates random single packet loss, while observing
   blocks of packets with the L bit set to 1 indicates loss affecting
   entire blocks of packets.

3.3.2.  L+Q Bits -- Loss Measurement Using L and Q Bits

   Combining L and Q bits allows a passive observer watching a single
   direction of traffic to accurately measure:

   upstream loss:  sender-to-observer loss (see Section 3.2.2)

   downstream loss:  observer-to-receiver loss (see Section 3.3.2.2)

   end-to-end loss:  sender-to-receiver loss on the observed path (see
      Section 3.3.1) with loss profile characterization (see
      Section 3.3.1.1)

3.3.2.1.  Correlating End-to-End and Upstream Loss

   Upstream loss is calculated by observing packets that did not suffer
   the upstream loss (Section 3.2.2).  End-to-end loss, however, is
   calculated by observing subsequent packets after the sender's
   protocol detected the loss.  Hence, end-to-end loss is generally
   observed with a delay of between 1 RTT (loss declared due to multiple
   duplicate acknowledgments) and 1 RTO (loss declared due to a timeout)
   relative to the upstream loss.

   The flow RTT can sometimes be estimated by timing the protocol
   handshake messages.  This RTT estimate can be greatly improved by
   observing a dedicated protocol mechanism for conveying RTT
   information, such as the Spin bit (see Section 2.1) or Delay bit (see
   Section 2.2).

   Whenever the observer needs to perform a computation that uses both
   upstream and end-to-end loss rate measurements, it should consider
   the upstream loss rate leading the end-to-end loss rate by
   approximately 1 RTT.  If the observer is unable to estimate RTT of
   the flow, it should accumulate loss measurements over time periods of
   at least 4 times the typical RTT for the observed flows.

   If the calculated upstream loss rate exceeds the end-to-end loss rate
   calculated in Section 3.3.1, then either the Q Period is too short
   for the amount of packet reordering or there is observer loss,
   described in Section 3.3.2.3.  If this happens, the observer should
   adjust the calculated upstream loss rate to match end-to-end loss
   rate, unless the following applies.

   In case of a protocol, such as TCP or SCTP, that does not track
   losses of pure ACK packets, observing a direction of traffic
   dominated by pure ACK packets could result in measured upstream loss
   that is higher than measured end-to-end loss if said pure ACK packets
   are lost upstream.  Hence, if the measurement is applied to such
   protocols, and the observer can confirm that pure ACK packets
   dominate the observed traffic direction, the observer should adjust
   the calculated end-to-end loss rate to match upstream loss rate.

3.3.2.2.  Downstream Loss

   Because downstream loss affects only those packets that did not
   suffer upstream loss, the end-to-end loss rate (eloss) relates to the
   upstream loss rate (uloss) and downstream loss rate (dloss) as
   (1-uloss)(1-dloss)=1-eloss.  Hence, dloss=(eloss-uloss)/(1-uloss).

3.3.2.3.  Observer Loss

   A typical deployment of a passive observation system includes a
   network tap device that mirrors network packets of interest to a
   device that performs analysis and measurement on the mirrored
   packets.  The observer loss is the loss that occurs on the mirror
   path.

   Observer loss affects the upstream loss rate measurement since it
   causes the observer to account for fewer packets in a block of
   identical Q bit values (see Section 3.2.2).  The end-to-end loss rate
   measurement, however, is unaffected by the observer loss since it is
   a measurement of the fraction of packets with the L bit value of 1,
   and the observer loss would affect all packets equally (see
   Section 3.3.1).

   The need to adjust the upstream loss rate down to match the end-to-
   end loss rate as described in Section 3.3.2.1 is an indication of the
   observer loss, whose magnitude is between the amount of such
   adjustment and the entirety of the upstream loss measured in
   Section 3.2.2.  Alternatively, a high apparent upstream loss rate
   could be an indication of significant packet reordering, possibly due
   to packets belonging to a single flow being multiplexed over several
   upstream paths with different latency characteristics.

3.4.  R Bit -- Reflection Square Bit

   R bit requires a deployment alongside Q bit.  Unlike the square
   signal for which packets are transmitted in blocks of fixed size, the
   number of packets in Reflection square blocks (also an Alternate-
   Marking signal) varies according to these rules:

   *  when the transmission of a new block starts, its size is set equal
      to the size of the last Q Block whose reception has been
      completed; and

   *  if the reception of at least one further Q Block is completed
      before transmission of the block is terminated, the size of the
      block is updated to be the average size of the further received Q
      Blocks.

   The Reflection square value is initialized to 0 and is applied to the
   R bit of every outgoing packet.  The Reflection square value is
   toggled for the first time when the completion of a Q Block is
   detected in the incoming square signal (produced by the other
   endpoint using the Q bit).  The number of packets detected within
   this first Q Block (p), is used to generate a reflection square
   signal that toggles every M=p packets (at first).  This new signal
   produces blocks of M packets (marked using the R bit) and each of
   them is called "Reflection Block" (Reflection Block).

   The M value is then updated every time a completed Q Block in the
   incoming square signal is received, following this formula:
   M=round(avg(p)).

   The parameter avg(p), the average number of packets in a marking
   period, is computed based on all the Q Blocks received since the
   beginning of the current Reflection Block.

   The transmission of a Reflection Block is considered complete (and
   the signal toggled) when the number of packets transmitted in that
   block is at least the latest computed M value.

   To ensure a proper computation of the M value, endpoints implementing
   the R bit must identify the boundaries of incoming Q Blocks.  The
   same approach described in Section 3.2.3 should be used.

   By looking at the R bit, unidirectional observation points have an
   indication of loss experienced by the entire unobserved channel plus
   the loss on the path from the sender to them.

   Since the Q Block is sent in one direction, and the corresponding
   reflected R Block is sent in the opposite direction, the reflected R
   signal is transmitted with the packet rate of the slowest direction.
   Namely, if the observed direction is the slowest, there can be
   multiple Q Blocks transmitted in the unobserved direction before a
   complete Reflection Block is transmitted in the observed direction.
   If the unobserved direction is the slowest, the observed direction
   can be sending R Blocks of the same size repeatedly before it can
   update the signal to account for a newly completed Q Block.

3.4.1.  Enhancement of Reflection Block Length Computation

   The use of the rounding function used in the M computation introduces
   errors that can be minimized by storing the rounding applied each
   time M is computed and using it during the computation of the M value
   in the following Reflection Block.

   This can be achieved by introducing the new r_avg parameter in the
   computation of M.  The new formula is Mr=avg(p)+r_avg; M=round(Mr);
   r_avg=Mr-M where the initial value of r_avg is equal to 0.

3.4.2.  Improved Resilience to Packet Reordering

   When a protocol implementing the marking mechanism is able to detect
   when packets are received out of order, it can improve resilience to
   packet reordering beyond what is possible by using methods described
   in Section 3.2.3.

   This can be achieved by updating the size of the current Reflection
   Block while it is being transmitted.  The Reflection Block size is
   then updated every time an incoming reordered packet of the previous
   Q Block is detected.  This can be done if and only if the
   transmission of the current Reflection Block is in progress and no
   packets of the following Q Block have been received.

3.4.2.1.  Improved Resilience to Burst Losses

   Burst losses can affect the accuracy of R measurements similar to how
   they affect accuracy of Q measurements.  Therefore, recommendations
   in Section 3.2.3.1 apply equally to improving burst loss resilience
   for R measurements.

3.4.3.  R+Q Bits -- Loss Measurement Using R and Q Bits

   Since both sQuare and Reflection square bits are toggled at most
   every N packets (except for the first transition of the R bit as
   explained before), an on-path observer can count the number of
   packets of each marking block and, knowing the value of N, can
   estimate the amount of loss experienced by the connection.  An
   observer can calculate different measurements depending on whether it
   is able to observe a single direction of the traffic or both
   directions.

   Single directional observer:
      upstream loss in the observed direction:  the loss between the
         sender and the observation point (see Section 3.2.2)

      "three-quarters" connection loss:  the loss between the receiver
         and the sender in the unobserved direction plus the loss
         between the sender and the observation point in the observed
         direction

      end-to-end loss in the unobserved direction:  the loss between the
         receiver and the sender in the opposite direction

   Two directions observer (same metrics seen previously applied to
   both direction, plus):
      client-observer half round-trip loss:  the loss between the client
         and the observation point in both directions

      observer-server half round-trip loss:  the loss between the
         observation point and the server in both directions

      downstream loss:  the loss between the observation point and the
         receiver (applicable to both directions)

3.4.3.1.  Three-Quarters Connection Loss

   Except for the very first block in which there is nothing to reflect
   (a complete Q Block has not been yet received), packets are
   continuously R-bit marked into alternate blocks of size lower or
   equal than N.  By knowing the value of N, an on-path observer can
   estimate the amount of loss that has occurred in the whole opposite
   channel plus the loss from the sender up to it in the observation
   channel.  As for the previous metric, the three-quarters connection
   loss rate (tqloss) is one minus the average number of packets in a
   block of packets with the same R value (t) divided by N
   (tqloss=1-avg(t)/N).

           =======================>
           = **********     -----Obs---->     **********
           = * Client *                       * Server *
           = **********     <------------     **********
           <============================================

               (a) in client-server channel (tqloss_up)

             ============================================>
             **********     ------------>     ********** =
             * Client *                       * Server * =
             **********     <----Obs-----     ********** =
                                  <=======================

               (b) in server-client channel (tqloss_down)

                 Figure 10: Three-Quarters Connection Loss

   The following metrics derive from this last metric and the upstream
   loss produced by the Q bit.

3.4.3.2.  End-To-End Loss in the Opposite Direction

   End-to-end loss in the unobserved direction (eloss_unobserved)
   relates to the "three-quarters" connection loss (tqloss) and upstream
   loss in the observed direction (uloss) as
   (1-eloss_unobserved)(1-uloss)=1-tqloss.  Hence,
   eloss_unobserved=(tqloss-uloss)/(1-uloss).

             **********     -----Obs---->     **********
             * Client *                       * Server *
             **********     <------------     **********
             <==========================================

               (a) in client-server channel (eloss_down)

             ==========================================>
             **********     ------------>     **********
             * Client *                       * Server *
             **********     <----Obs-----     **********

               (b) in server-client channel (eloss_up)

            Figure 11: End-To-End Loss in the Opposite Direction

3.4.3.3.  Half Round-Trip Loss

   If the observer is able to observe both directions of traffic, it is
   able to calculate two "half round-trip" loss measurements -- loss
   from the observer to the receiver (in a given direction) and then
   back to the observer in the opposite direction.  For both directions,
   "half round-trip" loss (hrtloss) relates to "three-quarters"
   connection loss (tqloss_opposite) measured in the opposite direction
   and the upstream loss (uloss) measured in the given direction as
   (1-uloss)(1-hrtloss)=1-tqloss_opposite.  Hence,
   hrtloss=(tqloss_opposite-uloss)/(1-uloss).

           =======================>
           = **********     ------|----->     **********
           = * Client *          Obs          * Server *
           = **********     <-----|------     **********
           <=======================

         (a) client-observer half round-trip loss (hrtloss_co)

                                  =======================>
             **********     ------|----->     ********** =
             * Client *          Obs          * Server * =
             **********     <-----|------     ********** =
                                  <=======================

         (b) observer-server half round-trip loss (hrtloss_os)

             Figure 12: Half Round-Trip Loss (Both Directions)

3.4.3.4.  Downstream Loss

   If the observer is able to observe both directions of traffic, it is
   able to calculate two downstream loss measurements using either end-
   to-end loss and upstream loss, similar to the calculation in
   Section 3.3.2.2, or "half round-trip" loss and upstream loss in the
   opposite direction.

   For the latter, dloss=(hrtloss-uloss_opposite)/(1-uloss_opposite).

                                  =====================>
             **********     ------|----->     **********
             * Client *          Obs          * Server *
             **********     <-----|------     **********

                (a) in client-server channel (dloss_up)

             **********     ------|----->     **********
             * Client *          Obs          * Server *
             **********     <-----|------     **********
             <=====================

                (b) in server-client channel (dloss_down)

                         Figure 13: Downstream Loss

3.5.  E Bit -- ECN-Echo Event Bit

   While the primary focus of this document is on exposing packet loss
   and delay, modern networks can report congestion before they are
   forced to drop packets, as described in [ECN].  When transport
   protocols keep ECN-Echo feedback under encryption, this signal cannot
   be observed by the network operators.  When tasked with diagnosing
   network performance problems, knowledge of a congestion downstream of
   an observation point can be instrumental.

   If downstream congestion information is desired, this information can
   be signaled with an additional bit.

   E:  The "ECN-Echo Event" bit is set to 0 or 1 according to the
      Unreported ECN-Echo counter, as explained below in Section 3.5.1.

3.5.1.  Setting the ECN-Echo Event Bit on Outgoing Packets

   The Unreported ECN-Echo counter operates identically to Unreported
   Loss counter (Section 3.3), except it counts packets delivered by the
   network with Congestion Experienced (CE) markings, according to the
   ECN-Echo feedback from the receiver.

   This ECN-Echo signaling is similar to ECN signaling in [ConEx].  The
   ECN-Echo mechanism in QUIC provides the number of packets received
   with CE marks.  For protocols like TCP, the method described in
   [ConEx-TCP] can be employed.  As stated in [ConEx-TCP], such feedback
   can be further improved using a method described in [ACCURATE-ECN].

3.5.2.  Using E Bit for Passive ECN-Reported Congestion Measurement

   A network observer can count packets with the CE codepoint and
   determine the upstream CE-marking rate directly.

   Observation points can also estimate ECN-reported end-to-end
   congestion by counting packets in this direction with an E bit equal
   to 1.

   The upstream CE-marking rate and end-to-end ECN-reported congestion
   can provide information about the downstream CE-marking rate.  The
   presence of E bits along with L bits, however, can somewhat confound
   precise estimates of upstream and downstream CE markings if the flow
   contains packets that are not ECN capable.

3.5.3.  Multiple E Bits

   Some protocols, such as QUIC, support separate ECN-Echo counters.
   For example, Section 13.4.1 of [QUIC-TRANSPORT] describes separate
   counters for ECT(0), ECT(1), and ECN-CE.  To better support such
   protocols, multiple E bits can be used, one per a corresponding ECN-
   Echo counter.

4.  Summary of Delay and Loss Marking Methods

   This section summarizes the marking methods described in this
   document, which proposes a toolkit of techniques that can be used
   separately, partly, or all together depending on the need.

   For the delay measurement, it is possible to use the Spin bit and/or
   the Delay bit.  A unidirectional or bidirectional observer can be
   used.

   +===============+======+=====================+=============+========+
   | Method        | # of |   Available Delay   | Impairments |  # of  |
   |               | bits |       Metrics       |  Resiliency | meas.  |
   |               |      +==========+==========+             |        |
   |               |      |  UniDir  |  BiDir   |             |        |
   |               |      | Observer | Observer |             |        |
   +===============+======+==========+==========+=============+========+
   | S: Spin Bit   |  1   |   RTT    | x2, Half |     low     |  very  |
   |               |      |          |   RTT    |             |  high  |
   +---------------+------+----------+----------+-------------+--------+
   | D: Delay      |  1   |   RTT    | x2, Half |     high    | medium |
   | Bit           |      |          |   RTT    |             |        |
   +---------------+------+----------+----------+-------------+--------+
   | SD: Spin      |  2   |   RTT    | x2, Half |     high    |  very  |
   | Bit & Delay   |      |          |   RTT    |             |  high  |
   | Bit *         |      |          |          |             |        |
   +---------------+------+----------+----------+-------------+--------+

                         Table 1: Delay Comparison

   x2    Same metric for both directions

   *     Both bits work independently; an observer could use less
         accurate Spin bit measurements when Delay bit ones are
         unavailable.

   For the Loss measurement, each row in Table 2 represents a loss-
   marking method.  For each method, the table specifies the number of
   bits required in the header, the available metrics using a
   unidirectional or bidirectional observer, applicable protocols,
   measurement fidelity, and delay.

   +============+====+==========================+====+=================+
   | Method     |Bits|  Available Loss Metrics  |Prto|   Measurement   |
   |            |    |                          |    |     Aspects     |
   |            |    +============+=============+    +==========+======+
   |            |    |   UniDir   |    BiDir    |    | Fidelity |Delay |
   |            |    |  Observer  |   Observer  |    |          |      |
   +============+====+============+=============+====+==========+======+
   | T: Round-  | $1 |     RT     | x2, Half RT | *  |Rate by   |~6 RTT|
   | Trip Loss  |    |            |             |    |sampling  |      |
   | Bit        |    |            |             |    |1/3 to    |      |
   |            |    |            |             |    |1/(3*ppa) |      |
   |            |    |            |             |    |of pkts   |      |
   |            |    |            |             |    |over 2    |      |
   |            |    |            |             |    |RTT       |      |
   +------------+----+------------+-------------+----+----------+------+
   | Q: sQuare  | 1  |  Upstream  |      x2     | *  |Rate over |N pkts|
   | Bit        |    |            |             |    |N pkts    |(e.g.,|
   |            |    |            |             |    |(e.g.,    |64)   |
   |            |    |            |             |    |64)       |      |
   +------------+----+------------+-------------+----+----------+------+
   | L: Loss    | 1  |    E2E     |      x2     | #  |Loss      |Min:  |
   | Event Bit  |    |            |             |    |shape     |RTT,  |
   |            |    |            |             |    |(and      |Max:  |
   |            |    |            |             |    |rate)     |RTO   |
   +------------+----+------------+-------------+----+----------+------+
   | QL: sQuare | 2  |  Upstream  |      x2     | #  |see Q     |see Q |
   | + Loss Ev. |    +------------+-------------+----+----------+------+
   | Bits       |    | Downstream |      x2     | #  |see Q|L   |see L |
   |            |    +------------+-------------+----+----------+------+
   |            |    |    E2E     |      x2     | #  |see L     |see L |
   +------------+----+------------+-------------+----+----------+------+
   | QR: sQuare | 2  |  Upstream  |      x2     | *  |Rate over |see Q |
   | + Ref. Sq. |    +------------+-------------+----+N*ppa     +------+
   | Bits       |    |   3/4 RT   |      x2     | *  |pkts (see |N*ppa |
   |            |    +------------+-------------+----+Q bit for |pkts  |
   |            |    |    !E2E    |     E2E,    | *  |N)        |(see Q|
   |            |    |            | Downstream, |    |          |bit   |
   |            |    |            |   Half RT   |    |          |for N)|
   +------------+----+------------+-------------+----+----------+------+

                          Table 2: Loss Comparison

   *     All protocols

   #     Protocols employing loss detection (with or without pure ACK
         loss detection)

   $     Require a working Spin bit

   !     Metric relative to the opposite channel

   x2    Same metric for both directions

   ppa   Packets-Per-Ack

   Q|L   See Q if Upstream loss is significant; L otherwise

   E2E   End to end

4.1.  Implementation Considerations

   By combining the information of the two tables above, it can be
   deduced that the solutions with 3 bits (i.e., QL or QR + S or D) or 4
   bits (i.e., QL or QR + SD) allow having more complete and resilient
   measurements.

   The methodologies described in the previous sections are transport
   agnostic and can be applied in various situations.  The choice of the
   methods also depends on the specific protocol.  For example, QL is a
   good combination; however, if a protocol does not support, or cannot
   set, the L bit, QR is the only viable solution.

5.  Examples of Application

   This document describes several measurement methods, but it is not
   expected that all methods will be implemented together.  For example,
   only some of the methods described in this document (i.e., sQuare bit
   and Spin bit) are utilized in [CORE-COAP-PM].  Also, the binding of a
   delay signal to QUIC is partially described in Section 17.4 of
   [QUIC-TRANSPORT], which adds only the Spin bit to the first byte of
   the short packet header, leaving two reserved bits for future use
   (see Section 17.2.2 of [QUIC-TRANSPORT]).

   All signals discussed in this document have been implemented in
   successful experiments for both QUIC and TCP.  The application
   scenarios considered allow the monitoring of the interconnections
   inside a data center (Intra-DC), between data centers (Inter-DC), as
   well as end-to-end large-scale data transfers.  For the application
   of the methods described in this document, it is assumed that the
   monitored flows follow stable paths and traverse the same measurement
   points.

   The specific implementation details and the choice of the bits used
   for the experiments with QUIC and TCP are out of scope for this
   document.  A specification defining the specific protocol application
   is expected to discuss the implementation details depending on which
   bits will be implemented in the protocol, e.g., [CORE-COAP-PM].  If
   bits used for specific measurements can also be used for other
   purposes by a protocol, the specification is expected to address ways
   for on-path observers to disambiguate the signals or to discuss
   limitations on the conditions under which the observers can expect a
   valid signal.

6.  Protocol Ossification Considerations

   Accurate loss and delay information is not required for the operation
   of any protocol, though its presence for a sufficient number of flows
   is important for the operation of networks.

   The delay and loss bits are amenable to "greasing" described in
   [RFC8701] if the protocol designers are not ready to dedicate (and
   ossify) bits used for loss reporting to this function.  The greasing
   could be accomplished similarly to the latency Spin bit greasing in
   Section 17.4 of [QUIC-TRANSPORT].  For example, the protocol
   designers could decide that a fraction of flows should not encode
   loss and delay information, and instead, the bits would be set to
   arbitrary values.  Setting any of the bits described in this document
   to arbitrary values would make the corresponding delay and loss
   information resemble noise rather than the expected signal for the
   flow, and the observers would need to be ready to ignore such flows.

7.  Security Considerations

   The methods described in this document are transport agnostic and
   potentially applicable to any transport-layer protocol, and
   especially valuable for encrypted protocols.  These methods can be
   applied to both limited domains and the Internet, depending on the
   specific protocol application.

   Passive loss and delay observations have been a part of the network
   operations for a long time, so exposing loss and delay information to
   the network does not add new security concerns for protocols that are
   currently observable.

   In the absence of packet loss, Q and R bits signals do not provide
   any information that cannot be observed by simply counting packets
   transiting a network path.  In the presence of packet loss, Q and R
   bits will disclose the loss, but this is information about the
   environment and not the endpoint state.  The L bit signal discloses
   internal state of the protocol's loss-detection machinery, but this
   state can often be gleaned by timing packets and observing the
   congestion controller response.

   The measurements described in this document do not imply that new
   packets injected into the network can cause potential harm to the
   network itself and to data traffic.  The measurements could be harmed
   by an attacker altering the marking of the packets or injecting
   artificial traffic.  Authentication techniques may be used where
   appropriate to guard against these traffic attacks.

   Hence, loss bits do not provide a viable new mechanism to attack data
   integrity and secrecy.

   The measurement fields introduced in this document are intended to be
   included in the packets.  However, it is worth mentioning that it may
   be possible to use this information as a covert channel.

   This document does not define a specific application, and the
   described techniques can generally apply to different communication
   protocols operating in different security environments.  A
   specification defining a specific protocol application is expected to
   address the respective security considerations and must consider
   specifics of the protocol and its expected operating environment.
   For example, security considerations for QUIC, discussed in
   Section 21 of [QUIC-TRANSPORT] and Section 9 of [QUIC-TLS], consider
   a possibility of active and passive attackers in the network as well
   as attacks on specific QUIC mechanisms.

7.1.  Optimistic ACK Attack

   A defense against an optimistic ACK attack, described in Section 21.4
   of [QUIC-TRANSPORT], involves a sender randomly skipping packet
   numbers to detect a receiver acknowledging packet numbers that have
   never been received.  The Q bit signal may inform the attacker which
   packet numbers were skipped on purpose and which had been actually
   lost (and are, therefore, safe for the attacker to acknowledge).  To
   use the Q bit for this purpose, the attacker must first receive at
   least an entire Q Block of packets, which renders the attack
   ineffective against a delay-sensitive congestion controller.

   A protocol that is more susceptible to an optimistic ACK attack with
   the loss signal provided by the Q bit and that uses a loss-based
   congestion controller should shorten the current Q Block by the
   number of skipped packets numbers.  For example, skipping a single
   packet number will invert the square signal one outgoing packet
   sooner.

   Similar considerations apply to the R bit, although a shortened
   Reflection Block along with a matching skip in packet numbers does
   not necessarily imply a lost packet, since it could be due to a lost
   packet on the reverse path along with a deliberately skipped packet
   by the sender.

7.2.  Delay Bit with RTT Obfuscation

   Theoretically, delay measurements can be used to roughly evaluate the
   distance of the client from the server (using the RTT) or from any
   intermediate observer (using the client-observer half-RTT).  As
   described in [RTT-PRIVACY], connection RTT measurements for
   geolocating endpoints are usually inferior to even the most basic IP
   geolocation databases.  It is the variability within RTT measurements
   (the jitter) that is most informative, as it can provide insight into
   the operating environment of the endpoints as well as the state of
   the networks (queuing delays) used by the connection.

   Nevertheless, to further mask the actual RTT of the connection, the
   Delay bit algorithm can be slightly modified by, for example,
   delaying the client-side reflection of the delay sample by a fixed,
   randomly chosen time value.  This would lead an intermediate observer
   to measure a delay greater than the real one.

   This Additional Delay should be randomly selected by the client and
   kept constant for a certain amount of time across multiple
   connections.  This ensures that the client-server jitter remains the
   same as if no Additional Delay had been inserted.  For example, a new
   Additional Delay value could be generated whenever the client's IP
   address changes.

   Despite the Additional Delay, this Hidden Delay technique still
   allows an accurate measurement of the RTT components (observer-
   server) and all the intra-domain measurements used to distribute the
   delay in the network.  Furthermore, unlike the Delay bit, the Hidden
   Delay bit does not require the use of the client reflection threshold
   (1 ms by default).  Removing this threshold may lead to increasing
   the number of valid measurements produced by the algorithm.

   Note that the Hidden Delay bit does not affect an observer's ability
   to measure accurate RTT using other means, such as timing packets
   exchanged during the connection establishment.

8.  Privacy Considerations

   To minimize unintentional exposure of information, loss bits provide
   an explicit loss signal -- a preferred way to share information per
   [RFC8558].

   New protocols commonly have specific privacy goals, and loss
   reporting must ensure that loss information does not compromise those
   privacy goals.  For example, [QUIC-TRANSPORT] allows changing
   Connection IDs in the middle of a connection to reduce the likelihood
   of a passive observer linking old and new sub-flows to the same
   device (see Section 5.1 of [QUIC-TRANSPORT]).  A QUIC implementation
   would need to reset all counters when it changes the destination (IP
   address or UDP port) or the Connection ID used for outgoing packets.
   It would also need to avoid incrementing the Unreported Loss counter
   for loss of packets sent to a different destination or with a
   different Connection ID.

   It is also worth highlighting that, if these techniques are not
   widely deployed, an endpoint that uses them may be fingerprinted
   based on their usage.  However, since there is no release of user
   data, the techniques seem unlikely to substantially increase the
   existing privacy risks.

   Furthermore, if there is experimental traffic with these bits set on
   the network, a network operator could potentially prioritize this
   marked traffic by placing it in a priority queue.  This may result in
   the delivery of better service, which could potentially mislead an
   experiment intended to benchmark the network.

9.  IANA Considerations

   This document has no IANA actions.

10.  References

10.1.  Normative References

   [ECN]      Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
              of Explicit Congestion Notification (ECN) to IP",
              RFC 3168, DOI 10.17487/RFC3168, September 2001,
              <https://www.rfc-editor.org/info/rfc3168>.

   [IPPM-METHODS]
              Morton, A., "Active and Passive Metrics and Methods (with
              Hybrid Types In-Between)", RFC 7799, DOI 10.17487/RFC7799,
              May 2016, <https://www.rfc-editor.org/info/rfc7799>.

   [QUIC-TRANSPORT]
              Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
              Multiplexed and Secure Transport", RFC 9000,
              DOI 10.17487/RFC9000, May 2021,
              <https://www.rfc-editor.org/info/rfc9000>.

   [RFC8558]  Hardie, T., Ed., "Transport Protocol Path Signals",
              RFC 8558, DOI 10.17487/RFC8558, April 2019,
              <https://www.rfc-editor.org/info/rfc8558>.

   [TCP]      Eddy, W., Ed., "Transmission Control Protocol (TCP)",
              STD 7, RFC 9293, DOI 10.17487/RFC9293, August 2022,
              <https://www.rfc-editor.org/info/rfc9293>.

10.2.  Informative References

   [ACCURATE-ECN]
              Briscoe, B., Kühlewind, M., and R. Scheffenegger, "More
              Accurate Explicit Congestion Notification (ECN) Feedback
              in TCP", Work in Progress, Internet-Draft, draft-ietf-
              tcpm-accurate-ecn-26, 24 July 2023,
              <https://datatracker.ietf.org/doc/html/draft-ietf-tcpm-
              accurate-ecn-26>.

   [AltMark]  Fioccola, G., Ed., Cociglio, M., Mirsky, G., Mizrahi, T.,
              and T. Zhou, "Alternate-Marking Method", RFC 9341,
              DOI 10.17487/RFC9341, December 2022,
              <https://www.rfc-editor.org/info/rfc9341>.

   [ANRW19-PM-QUIC]
              Bulgarella, F., Cociglio, M., Fioccola, G., Marchetto, G.,
              and R. Sisto, "Performance measurements of QUIC
              communications", Proceedings of the Applied Networking
              Research Workshop (ANRW '19), Association for Computing
              Machinery, DOI 10.1145/3340301.3341127, July 2019,
              <https://doi.org/10.1145/3340301.3341127>.

   [ConEx]    Mathis, M. and B. Briscoe, "Congestion Exposure (ConEx)
              Concepts, Abstract Mechanism, and Requirements", RFC 7713,
              DOI 10.17487/RFC7713, December 2015,
              <https://www.rfc-editor.org/info/rfc7713>.

   [ConEx-TCP]
              Kuehlewind, M., Ed. and R. Scheffenegger, "TCP
              Modifications for Congestion Exposure (ConEx)", RFC 7786,
              DOI 10.17487/RFC7786, May 2016,
              <https://www.rfc-editor.org/info/rfc7786>.

   [CORE-COAP-PM]
              Fioccola, G., Zhou, T., Nilo, M., Milan, F., and F.
              Bulgarella, "Constrained Application Protocol (CoAP)
              Performance Measurement Option", Work in Progress,
              Internet-Draft, draft-ietf-core-coap-pm-01, 19 October
              2023, <https://datatracker.ietf.org/doc/html/draft-ietf-
              core-coap-pm-01>.

   [IPPM-SPIN]
              Trammell, B., Ed., "An Explicit Transport-Layer Signal for
              Hybrid RTT Measurement", Work in Progress, Internet-Draft,
              draft-trammell-ippm-spin-00, 9 January 2019,
              <https://datatracker.ietf.org/doc/html/draft-trammell-
              ippm-spin-00>.

   [IPv6AltMark]
              Fioccola, G., Zhou, T., Cociglio, M., Qin, F., and R.
              Pang, "IPv6 Application of the Alternate-Marking Method",
              RFC 9343, DOI 10.17487/RFC9343, December 2022,
              <https://www.rfc-editor.org/info/rfc9343>.

   [QUIC-MANAGEABILITY]
              Kühlewind, M. and B. Trammell, "Manageability of the QUIC
              Transport Protocol", RFC 9312, DOI 10.17487/RFC9312,
              September 2022, <https://www.rfc-editor.org/info/rfc9312>.

   [QUIC-SPIN]
              Trammell, B., Ed., De Vaere, P., Even, R., Fioccola, G.,
              Fossati, T., Ihlar, M., Morton, A., and S. Emile, "Adding
              Explicit Passive Measurability of Two-Way Latency to the
              QUIC Transport Protocol", Work in Progress, Internet-
              Draft, draft-trammell-quic-spin-03, 14 May 2018,
              <https://datatracker.ietf.org/doc/html/draft-trammell-
              quic-spin-03>.

   [QUIC-TLS] Thomson, M., Ed. and S. Turner, Ed., "Using TLS to Secure
              QUIC", RFC 9001, DOI 10.17487/RFC9001, May 2021,
              <https://www.rfc-editor.org/info/rfc9001>.

   [RFC8701]  Benjamin, D., "Applying Generate Random Extensions And
              Sustain Extensibility (GREASE) to TLS Extensibility",
              RFC 8701, DOI 10.17487/RFC8701, January 2020,
              <https://www.rfc-editor.org/info/rfc8701>.

   [RTT-PRIVACY]
              Trammell, B. and M. Kühlewind, "Revisiting the Privacy
              Implications of Two-Way Internet Latency Data", Passive
              and Active Measurement, pp. 73-84, Springer International
              Publishing, DOI 10.1007/978-3-319-76481-8_6,
              ISBN 9783319764801, March 2018,
              <https://doi.org/10.1007/978-3-319-76481-8_6>.

   [TRANSPORT-ENCRYPT]
              Fairhurst, G. and C. Perkins, "Considerations around
              Transport Header Confidentiality, Network Operations, and
              the Evolution of Internet Transport Protocols", RFC 9065,
              DOI 10.17487/RFC9065, July 2021,
              <https://www.rfc-editor.org/info/rfc9065>.

   [TSVWG-SPIN]
              Trammell, B., Ed., "A Transport-Independent Explicit
              Signal for Hybrid RTT Measurement", Work in Progress,
              Internet-Draft, draft-trammell-tsvwg-spin-00, 2 July 2018,
              <https://datatracker.ietf.org/doc/html/draft-trammell-
              tsvwg-spin-00>.

   [UDP-OPTIONS]
              Touch, J., "Transport Options for UDP", Work in Progress,
              Internet-Draft, draft-ietf-tsvwg-udp-options-23, 15
              September 2023, <https://datatracker.ietf.org/doc/html/
              draft-ietf-tsvwg-udp-options-23>.

   [UDP-SURPLUS]
              Herbert, T., "UDP Surplus Header", Work in Progress,
              Internet-Draft, draft-herbert-udp-space-hdr-01, 8 July
              2019, <https://datatracker.ietf.org/doc/html/draft-
              herbert-udp-space-hdr-01>.

Acknowledgments

   The authors would like to thank the QUIC WG for their contributions,
   Christian Huitema for implementing Q and L bits in his picoquic
   stack, and Ike Kunze for providing constructive reviews and helpful
   suggestions.

Contributors

   The following people provided valuable contributions to this
   document:

   Marcus Ihlar
   Ericsson
   Email: marcus.ihlar@ericsson.com


   Jari Arkko
   Ericsson
   Email: jari.arkko@ericsson.com


   Emile Stephan
   Orange
   Email: emile.stephan@orange.com


   Dmitri Tikhonov
   LiteSpeed Technologies
   Email: dtikhonov@litespeedtech.com


Authors' Addresses

   Mauro Cociglio
   Telecom Italia - TIM
   Via Reiss Romoli, 274
   10148 Torino
   Italy
   Email: mauro.cociglio@outlook.com


   Alexandre Ferrieux
   Orange Labs
   Email: alexandre.ferrieux@orange.com


   Giuseppe Fioccola
   Huawei Technologies
   Riesstrasse, 25
   80992 Munich
   Germany
   Email: giuseppe.fioccola@huawei.com


   Igor Lubashev
   Akamai Technologies
   Email: ilubashe@akamai.com


   Fabio Bulgarella
   Telecom Italia - TIM
   Via Reiss Romoli, 274
   10148 Torino
   Italy
   Email: fabio.bulgarella@guest.telecomitalia.it


   Massimo Nilo
   Telecom Italia - TIM
   Via Reiss Romoli, 274
   10148 Torino
   Italy
   Email: massimo.nilo@telecomitalia.it


   Isabelle Hamchaoui
   Orange Labs
   Email: isabelle.hamchaoui@orange.com


   Riccardo Sisto
   Politecnico di Torino
   Email: riccardo.sisto@polito.it