IPPM M. Cociglio Internet-Draft Telecom Italia Intended status: Experimental G. Fioccola Expires: September 10, 2020 Huawei Technologies M. Nilo F. Bulgarella Telecom Italia R. Sisto Politecnico di Torino March 9, 2020 Client-Server Spin bit enabled Performance Measurements draft-cfb-ippm-spinbit-measurements-00 Abstract This document introduces an additional single bit signal to enhance the spin bit [I-D.trammell-ippm-spin] performance in presence of network impairments and application limited flow. In addition, it defines two new explicit per-flow transport-layer signals for hybrid measurement of connection loss rate. The former is a spin-bit dependent signal and uses a single bit. The latter is a standalone solution based on a two bits loss signal and on alternate marking RFC 8321 [RFC8321]. Requirements Language The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119]. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at https://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on September 10, 2020. Cociglio, et al. Expires September 10, 2020 [Page 1] Internet-Draft March 2020 Copyright Notice Copyright (c) 2020 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 (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Spin bit and Delay bit mechanism . . . . . . . . . . . . . . 4 2.1. Delay Sample generation . . . . . . . . . . . . . . . . . 5 2.1.1. The recovery process . . . . . . . . . . . . . . . . 6 2.2. Delay Sample reflection . . . . . . . . . . . . . . . . . 6 3. Using the Spin bit and Delay bit for Hybrid RTT Measurement . 7 3.1. End-to-end RTT measurement . . . . . . . . . . . . . . . 7 3.2. Half-RTT measurement . . . . . . . . . . . . . . . . . . 8 3.3. Intra-domain RTT measurement . . . . . . . . . . . . . . 8 4. Observer's algorithm and Waiting Interval . . . . . . . . . . 8 5. Adding a Loss signal for Packet loss measurement . . . . . . 10 5.1. Round Trip Packet Loss measurement . . . . . . . . . . . 11 6. Packet Loss using one bit loss signal . . . . . . . . . . . . 11 6.1. Observer's logic for one bit loss signal . . . . . . . . 12 7. Two Bits packet loss measurement using alternate marking . . 13 7.1. Setting the square bit (Q) on outgoing packets . . . . . 13 7.2. Setting the reflection square bit (R) on outgoing packets 13 7.2.1. Determining the completion of an incoming marking period . . . . . . . . . . . . . . . . . . . . . . . 14 7.3. Observer's logic and passive loss measurements . . . . . 15 7.3.1. Upstream one-way loss . . . . . . . . . . . . . . . . 15 7.3.2. Three-quarters connection loss . . . . . . . . . . . 16 7.3.3. Full one-way loss in the opposite direction . . . . . 17 7.3.4. Half round-trip loss . . . . . . . . . . . . . . . . 17 7.3.5. Downstream one-way loss . . . . . . . . . . . . . . . 18 7.4. Enhancement of reflection period size computation . . . . 19 8. Protocols . . . . . . . . . . . . . . . . . . . . . . . . . . 19 8.1. QUIC . . . . . . . . . . . . . . . . . . . . . . . . . . 19 8.2. TCP . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 9. Security Considerations . . . . . . . . . . . . . . . . . . . 20 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 20 Cociglio, et al. Expires September 10, 2020 [Page 2] Internet-Draft March 2020 11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20 12. References . . . . . . . . . . . . . . . . . . . . . . . . . 20 12.1. Normative References . . . . . . . . . . . . . . . . . . 20 12.2. Informative References . . . . . . . . . . . . . . . . . 21 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 21 1. Introduction Both [I-D.trammell-tsvwg-spin] and [I-D.trammell-ippm-spin] define an explicit per-flow transport-layer signal for hybrid measurement of end-to-end RTT. This signal consists of three bits: a spin bit, which oscillates once per end-to-end RTT, and a two-bit Valid Edge Counter (VEC), which compensates for loss and reordering of the spin bit to increase fidelity of the signal in less than ideal network conditions. In this document it is introduced the delay bit, that is a single bit signal that can be used together with the spin bit by passive observers to measure the RTT of a network flow, avoiding the spin bit ambiguities that arise as soon as network conditions deteriorate. Unlike the spin bit, which is actually set in every packet transmitted on the network, the delay bit is set only once per round trip. Regarding loss rate measurement, two new algorithms are introduced. The first algorithm enables end-to-end round trip loss rate measurement using a single bit signal called loss bit. This signal is used to mark a train of packets (a portion of traffic) which bounces back an forth two times between endpoints, realizing a two round trip reflection. A passive on-path observer, placed on whatever direction, can trivially count and compare the number of marked packets seen during the two reflections estimating statistically the loss rate experienced by the connection. The second algorithm uses a double square signal and RFC 8321 [RFC8321] to mark the whole traffic exchanged between endpoints. This solution enables different types of measurements providing a complete picture of connection loss events. This document defines a hybrid measurement RFC 7799 [RFC7799] path signal to be embedded into a transport layer protocol, explicitly intended for exposing end-to-end RTT to measurement devices on path. The document introduces a mechanism applicable to any transport-layer protocol, then explains how to bind the signal to a variety of IETF transport protocols, and in particular to QUIC and TCP. Cociglio, et al. Expires September 10, 2020 [Page 3] Internet-Draft March 2020 The application of the Spin bit to QUIC is described in [I-D.ietf-quic-spin-exp] which adds the spin bit to QUIC for experimentation purposes. Note that both the spin bit and the delay bit are inspired by RFC 8321 [RFC8321]. This is also mentioned in [I-D.trammell-quic-spin]. Note that additional details about the Performance Measurements for QUIC are also described in the paper [ANRW19-PM-QUIC]. 2. Spin bit and Delay bit mechanism The main idea is to have a single packet, with a second marked bit (the delay bit), that bounces between client and server during the entire connection life. This single packet is called Delay Sample. A simple observer placed in an intermediate point, tracking the delay sample and the relative timestamp in every spin bit period, can measure the end-to-end round trip delay of the connection. In the same way as seen with the spin bit, it is possible to carry out other types of measurements. The next paragraphs give an overview of the observer capabilities. In order to describe the delay sample working mechanism in detail, we have to distinguish two different phases which take part in the delay bit lifetime: 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 Delay bit mechanism: the first bit is the spin bit and the second one is the delay bit. +--------+ -- -- -- -- -- +--------+ | | -----------> | | | Client | | Server | | | <----------- | | +--------+ -- -- -- -- -- +--------+ (a) No traffic at beginning. +--------+ 00 00 01 -- -- +--------+ | | -----------> | | | Client | | Server | | | <----------- | | +--------+ -- -- -- -- -- +--------+ (b) The Client starts sending data and Cociglio, et al. Expires September 10, 2020 [Page 4] Internet-Draft March 2020 sets the first packet as Delay Sample. +--------+ 00 00 00 00 00 +--------+ | | -----------> | | | Client | | Server | | | <----------- | | +--------+ -- -- 01 00 00 +--------+ (c) The Server starts sending data and reflects the Delay Sample. +--------+ 10 10 11 00 00 +--------+ | | -----------> | | | Client | | Server | | | <----------- | | +--------+ 00 00 00 00 00 +--------+ (d) The Client inverts the spin bit and reflects the Delay Sample. +--------+ 10 10 10 10 10 +--------+ | | -----------> | | | Client | | Server | | | <----------- | | +--------+ 00 00 11 10 10 +--------+ (e) The Server reflects the Delay Sample. +--------+ 00 00 01 10 10 +--------+ | | -----------> | | | Client | | Server | | | <----------- | | +--------+ 10 10 10 10 10 +--------+ (f) The client reverts the spin bit and reflects the Delay Sample. Figure 1: Spin bit and Delay bit 2.1. Delay Sample generation During this first phase, endpoints play different roles. First of all a single delay sample must be bouncing per round trip period (and so per spin bit period). According to that statement and in order to simplify the general algorithm, the delay sample generation is in charge of just one of the two endpoints: Cociglio, et al. Expires September 10, 2020 [Page 5] Internet-Draft March 2020 o the Client, when connection starts and spin bit is set to 0, initializes the delay bit of the first packet to 1, so it becomes the delay sample for that marking period. Only this packet is marked with the delay bit set to 1 for this round trip period; the other ones will carry only the spin bit; o the server never initializes the delay bit to 1; its only task is to reflect the incoming delay bit into the next outgoing packet only if certain conditions occur. Theoretically, in absence of network impairments, the delay sample should bounce between client and server continuously, for the entire duration of the connection. Actually, that is highly unlikely mainly for two different reasons: 1) the packet carrying the delay bit might be lost during its journey on the network which is unreliable by definition; 2) one of the two endpoints could stop or delay sending data because the application is limiting the amount of traffic transmitted; To deal with these problems, the algorithm provides a procedure to regenerate the delay sample and to inform a possible observer that a problem has occurred, and then the measurement has to be restarted. 2.1.1. The recovery process In order to relieve the server from tasks that go beyond the mere reflection of the sample, even in this case the recovery process belongs to the client. A fundamental assumption is that a delay sample is strictly related to its spin bit period. Considering this rule, the client verifies that every spin bit period ends with its delay sample. If that does not happen and a marking period terminates without a delay sample, the client waits a further empty period; then, in the following period, it reinitializes the mechanism by setting the delay bit of the first outgoing packet to 1, making it the new delay sample. The empty period is needed to inform the intermediate points that there was an issue and a new delay measurement session is starting. 2.2. Delay Sample reflection The reflection is the process that enables the bouncing of the delay sample between client and server. The behavior of the two endpoints is slightly different. With the exception of the client that, as previously exposed, generates a new delay sample, by default the delay bit is set to 0. Cociglio, et al. Expires September 10, 2020 [Page 6] Internet-Draft March 2020 Server side reflection: when a packet with the delay bit set to 1 arrives, the server marks the first packet in the opposite direction as the delay sample, if it has the same spin bit value. While if it has the opposite spin bit value this sample is considered lost. Client side reflection: when a packet with delay bit set to 1 arrives, the client marks the first packet in the opposite direction as the delay sample, if it has the opposite spin bit value. While if it has the same spin bit value this sample is considered lost. In both cases, if the outgoing marked packet is transmitted with a delay greater than a predetermined threshold after the reception of the incoming delay sample (1ms by default), reflection is aborted and this sample is considered lost. Note that reflection takes place for the packet that is carrying the delay bit regardless of its position within the period. For this reason it is necessary to introduce that condition of validation in order to identify and discard those samples that, due to reordering, might move to a contiguous period. Furthermore, by introducing a threshold for the retransmission delay of the sample, it is possible to eliminate all those measurements which, due to lack of traffic on the endpoints, would be overestimated and not true. Thus, the maximum estimation error, without considering any other delays due to flow control, would amount to twice the threshold (e.g. 2ms) per measurement, in the worst case. 3. Using the Spin bit and Delay bit for Hybrid RTT Measurement Unlike what happens with the spin bit for which it is necessary to validate or at least heuristically evaluate the goodness of an edge, the delay sample can be used by an intermediate observer as a simple demarcator between a period and the following one eliminating the ambiguities on the calculation of the RTT found with the analysis of the spin-bit only. The measurement types, that can be done from the observation of the delay sample, are exactly the same achievable with the spin bit only. 3.1. End-to-end RTT measurement The delay sample generation process ensures that only one packet marked with the delay bit set to 1 runs back and forth on the wire between two endpoints per round trip time. Therefore, in order to determine the end-to-end RTT measurement of a QUIC flow, an on-path passive observer can simply compute the time difference between two delay samples observed in a single direction. Note that a measurement, to be valid, must take into account the difference in time between the timestamps of two consecutive delay samples Cociglio, et al. Expires September 10, 2020 [Page 7] Internet-Draft March 2020 belonging to adjacent spin-bit periods. For this reason, an observer, in addition to intercepting and analyzing the packets containing the delay bit set to 1, must maintain awareness of each spin period in such a way as to be able to assign each delay sample to its period and, at the same time, identifying those periods that do not contain it. 3.2. Half-RTT measurement An on-path passive observer that is sniffing traffic in both directions -- from client to server and from server to client -- can also use the delay sample to measure "upstream" and "downstream" RTT components. Also known as the half-RTT measurement, it represents the components of the end-to-end RTT concerning the paths between the client and the observer (upstream), and the observer and the server (downstream). It does this by measuring the delay between a delay sample observed in the downstream direction and the one observed in the upstream direction, and vice versa. Also in this case, it should verify that the two delay samples belong to two adjacent periods, for the upstream component, or to the same period for the downstream component. 3.3. Intra-domain RTT measurement Taking advantage of the half-RTT measurements it is also possible to calculate the intra-domain RTT which is the portion of the entire RTT used by a QUIC flow to traverse the network of a provider (or part of it). To achieve this result two observers, able to watch traffic in both directions, must be employed simultaneously at ingress and egress of the network to be measured. At this point, to determine the delay between the two observers, it is enough to subtract the two computed upstream (or downstream) RTT components. The spin bit is an alternate marking generated signal and the only difference than RFC 8321 [RFC8321] is the size of the alternation that will change with the flight size each RTT. So it can be useful to segment the RTT and deduce the contribution to the RTT of the portion of the network between two on-path observers and it can be easily performed by calculating the delay between two or more measurement points on a single direction by applying RFC 8321 [RFC8321]. 4. Observer's algorithm and Waiting Interval Given below is a formal summary of the functioning of the observer every time a delay sample is detected. A packet containing the delay bit set to 1: Cociglio, et al. Expires September 10, 2020 [Page 8] Internet-Draft March 2020 o if it has the same spin bit value of the current period and no delay sample was detected in the previous period, then it can be used as a left edge (i.e. to start measuring an RTT sample), but not as a right edge (i.e. to complete and RTT measurement since the last edge). If the observation point is symmetric (i.e. it can see both upstream and downstream packets in the flow) and in the current period a delay sample was detected in the opposite direction (i.e. in the upstream direction), the packet can also be used to compute the downstream RTT component. o if it has the same spin bit value of the current period and a delay sample was detected in the previous period, then it can be used at the same time as a left or right edge, and to compute RTT component in both directions. Like stated previously, every time an empty period is detected, the observer must restart the measurement process and consider the next delay sample that will come as the beginning of a new measure, then as a left edge. As a result, being able to assign the delay sample to the corresponding spin period becomes a crucial factor for the proper functioning of the entire algorithm. Considering that the division into periods is realized by exploiting the spin bit square wave, it is easy to understand that the presence of spurious spin edges -- caused by packet reordering -- would inevitably lead the observer to overestimate the amount of periods actually present in the transmission. This results in a greater number of empty periods detected and the consequent decrease of the actual RTT samples achievable. Therefore, in order to maximize the performance of the whole algorithm, the observer must implement a mechanism to filter out spurious spin edges. To face this problem the waiting interval has to be introduced. Basically, every time a spin bit edge is detected, the observer sets a time interval during which it rejects every potential spurious edges observed on the wire. While, at the end of the interval it starts again to accept changes in the spin bit value. This guarantees a proper protection against the spurious edges in relation to the size of the interval itself. For instance, an interval of 5ms is able to filter out edges that have been reordered by a maximum of 5ms. Clearly, the mechanism does its job for intervals smaller than the RTT of the observed connection (if RTT is smaller than the waiting interval the observer can't measure the RTT). Cociglio, et al. Expires September 10, 2020 [Page 9] Internet-Draft March 2020 5. Adding a Loss signal for Packet loss measurement It is possible to introduce a mechanism to evaluate also the packet loss together with the delay measurement. This can be achieved by introducing the loss signal, a single or two bits signal whose purpose is to mark a variable number of packets (from live traffic) which are exchanged two times between the endpoints realizing a two round-trip reflection. The overall exchange comprises: o The client first selects, generates and consequent transmits to the server a first train of packets, by marking the loss bit to 1; o The server, upon reception from the client of each one of the packets included in the first train, reflects to the client a respective second train of packets of the same size as the first train received, by marking the loss bit to 1; o The client, upon reception from the server of each one of the packets included in the second train, reflects to the server a respective third train of packets of the same size as the second train received, by marking the loss bit to 1; o The server, upon reception from the client of each one of the packets included in the third train, finally reflects to the client a respective fourth train of packets of the same size as the third train received, by marking the loss bit to 1. Packets belonging to the first round (first and second train) represent the Generation Phase while those belonging to the second round (third and fourth train) represent the Reflection Phase. A passive on-path observer, placed on whatever direction, can trivially count and compare the number of marked packets seen during the two mentioned phases (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(RT) losses The general algorithm shown above gives an idea of its underlying principles but is not enough to make the whole process working properly. Firstly, there is the issue that packet rates in the two directions may be different. Therefore, the right number of packets to be marked has to be chosen in order to avoid their congestion on the slowest traffic direction. As a consequence, this number is Cociglio, et al. Expires September 10, 2020 [Page 10] Internet-Draft March 2020 inevitably equal to the amount of packets transited, indeed, on the slowest direction. This problem can be easily addressed by a method wherein the two endpoints of a communication exchange marked packets interleaved with unmarked packets. From an implementation point of view, this result can be achieved by introducing a single token system that adjusts the number of outgoing marked packets. Basically, the token is enabled every time a packet arrives and disabled when a marked packet is transmitted. Since the creation of the initial train of marked packets is carried out by the client, the management and use of this single token is also assigned to it, which in fact "calculates" the correct number of packets to be marked each time. Secondly, a mechanism to individually identify each train of packets must be provided to enable the observer to distinguish between trains belonging to different phases (Generation and Reflection). 5.1. Round Trip Packet Loss measurement Since the measurements are performed on a portion of the traffic exchanged between client and server, the observer calculates the end- to-end Round Trip Packet Loss that, statistically, will be equal to the loss rate experienced by the connection along the entire network path. So this measurement can be simply referred as the Round Trip Packet Loss (RTPL). In addition, this methodology allows the Half-RTPL measurement and the Intra-domain RTPL measurement, in the same way as described in the previous sections for RTT measurement. 6. Packet Loss using one bit loss signal The single bit loss signal is implemented using just one bit: marked packets have this bit set to 1, whereas unmarked ones have it set to 0. This solution requires a working spin-bit signal used to separate different trains of packets. In particular, a "pause" of at least one empty spin-bit period is introduced between each phase of the algorithm. An on-path observer can determine in this way if a phase (and therefore a train of packets) is ended and a new one is starting. The client is in charge of almost the entire complexity of the algorithm. Its task can be summarized in 4 different points: 1. The client starts generating marked packets for two consecutive spin-bit periods; it maintains a generation token that is enabled every time a packet arrives and disabled when another one is forwarded. When this token is disabled, the generation process Cociglio, et al. Expires September 10, 2020 [Page 11] Internet-Draft March 2020 is paused (i.e. outgoing packets are transmitted unmarked) and resumes as soon as its value returns true, and that happens as soon as a packet is received. In addition, at the end of the first spin-bit period spent in generation, the reflection counter is unlocked to start counting incoming marked packets which will be later reflected; 2. When the generation is completed, the client waits to see in input an empty spin-bit period so as to be sure that everyone has seen at least that empty period. This one will be used by the observer as a divider between generated and reflected packets. During this phase, all the outgoing packets are forwarded with the loss bit set to 0. The reflection counter is still incremented every time a marked packet arrives; 3. The client starts reflecting marked packets until the reflection counter is zeroed; the generation token is also used (in the same way) during this phase to avoid congestion on the slowest traffic direction. In addition, at the end of the first spin-period spent in reflection, the reflection counter is locked to avoid incoming reflected packets incrementing it; 4. When the reflection is completed, the client waits to see in input an empty spin-bit period so as to be sure that everyone has seen at least that empty period. This one will be used by the observer as a divider between reflected and newly generated packets. During this phase, all the outgoing packets are forwarded with the loss bit set to 0. The whole process restarts going back to the first point. As previously anticipated, the server simply reflects each incoming marked packet sent by the client. It maintains a simple counter that is incremented every time a marked packet arrives and decremented when a marked one is sent in the opposite direction. 6.1. Observer's logic for one bit loss signal The on-path observer, placed in any direction, counts marked packets and separates different trains detecting empty spin-bit periods between them (one or more). Then, it simply computes the difference between a Generation train and a Reflection train to produce a statistical measurement of the Round Trip Packet Loss (RTPL) and of the connection end-to-end loss rate. Here is an example. Packets are represented by two digits (first one is the spin bit, second one is the loss bit): Cociglio, et al. Expires September 10, 2020 [Page 12] Internet-Draft March 2020 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 2: one bit loss signal example Note that 5 marked packets have been generated of which 4 reflected. 7. Two Bits packet loss measurement using alternate marking An alternative methodology, based on the classical alternate marking RFC 8321 [RFC8321], can be deployed to enable passive packet loss measurement in a connection oriented communication. This section explains its fundamentals and all the metrics that can be achieved by exploiting this mechanism. Two new loss bits are introduced: o Square Bit (Q): this bit is toggled every N outgoing packets generating a square signal as already seen in the alternate marking methodology RFC 8321 [RFC8321]. o Reflection Square Bit (R): this bit is used to reflect the incoming square signal (the one generated by the opposite endpoint) according to the algorithm explained in next Section; in a nutshell, it is used to report the losses found in the opposite transmission channel. 7.1. Setting the square bit (Q) on outgoing packets The sQuare value is initialized to 0 and is applied to the Q bit of every outgoing packet. The sQuare value is toggled after sending N packets (e.g. 64). By doing so, each endpoint splits its outgoing traffic into blocks of N packets with different "packet color" as defined by RFC 8321 [RFC8321]. A single block of N packets is called "marking period". Observation points can estimate upstream losses by counting the number of packets included in a marking period of the produced square signal. 7.2. Setting the reflection square bit (R) on outgoing packets Unlike the sQuare signal for which packets are transmitted into blocks of fixed size, the Reflection square signal (being an alternate marking signal too) produces blocks of packets whose size varies according to these simple rules: Cociglio, et al. Expires September 10, 2020 [Page 13] Internet-Draft March 2020 o when the transmission of a new block starts, its size is set equal to the size of the last marking period whose reception has been completed; o if, before transmission of the block is terminated, the reception of at least one further marking period is completed, the size of the block is updated to the average size of the further received marking periods. Implementation details follow. 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 marking period is detected in the incoming sQuare signal (produced by the opposite node using the Q bit). When this happens, the number of packets (p), detected within this first marking period, is used to generate a reflection square signal which 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 marking period". The M value is then updated every time a completed marking period in the incoming sQuare signal is received, following this formula: M=round(avg(p)). The parameter avg(p) is the average number of packets in a marking period computed considering all the marking periods received since the beginning of the current reflection marking period. Looking at the R bit, observation points have clear indication of losses experienced by the entire opposite channel plus those occurred in the path from the sender up to them (if losses occur in this latter portion of path). 7.2.1. Determining the completion of an incoming marking period A simple sQuare bit transition cannot be used to determine the completion of a marking period. Indeed, packet reordering can lead to the generation of spurious edges in the sQuare signal. To address this problem, a marking period is considered ended when at least X packets (e.g. 5) with reverse marking (i.e. belonging to the following marking period) have been received. This same approach can be used by observation points to clean both sQuare and Reflection square signals. Cociglio, et al. Expires September 10, 2020 [Page 14] Internet-Draft March 2020 7.3. Observer's logic and passive loss measurements 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 trivially 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. Different metrics can be measured depending on which direction the observer is looking to. One direction observer: o upstream one-way loss: the loss between the sender and the observation point o "three-quarters" connection loss: the loss between the receiver and the sender in the opposite direction plus the loss between the sender and the observation point in the observed direction o full one-way loss in the opposite 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): o client-observer half round-trip loss: the loss between the client and the observation point in both directions o observer-server half round-trip loss: the loss between the observation point and the server in both directions o downstream one-way loss: the loss between the observation point and the receiver (valid for both directions) 7.3.1. Upstream one-way loss Since packets are continuously Q bit marked into alternate blocks of size N, knowing the value of N, an on-path observer can estimate the amount of loss occurred from the sender up to it after observing at least N packets. The upstream one-way loss rate ("uowl") is one minus the average number of packets in a block of packets with the same Q value ("p") divided by N ("uowl=1-avg(p)/N"). Cociglio, et al. Expires September 10, 2020 [Page 15] Internet-Draft March 2020 =====================> ********** -----Obs----> ********** * Client * * Server * ********** <------------ ********** (a) in client-server channel (uowl_up) ********** ------------> ********** * Client * * Server * ********** <----Obs----- ********** <===================== (b) in server-client channel (uowl_down) Figure 3: Upstream one-way loss 7.3.2. Three-quarters connection loss Except for the very first block in which there is nothing to reflect (a complete marking period has not been yet received), packets are continuously R bit marked into alternate blocks of size lower or equal than N. Knowing the value of N, an on-path observer can estimate the amount of loss 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 ("tql") is one minus the average number of packets in a block of packets with the same R value ("t") divided by N ("tql=1-avg(t)/N"). =======================> = ********** -----Obs----> ********** = * Client * * Server * = ********** <------------ ********** ============================================< (a) in client-server channel (tql_up) ============================================> ********** ------------> ********** = * Client * * Server * = ********** <----Obs----- ********** = <======================= (b) in server-client channel (tql_down) Figure 4: Three-quarters connection loss Cociglio, et al. Expires September 10, 2020 [Page 16] Internet-Draft March 2020 The following metrics derive from these first two metrics. 7.3.3. Full one-way loss in the opposite direction Using the previous metrics, full one-way loss can be computed: fowl_down = tql_up - uowl_up fowl_up = tql_down - uowl_down ********** -----Obs----> ********** * Client * * Server * ********** <------------ ********** <========================================== (a) in client-server channel (fowl_down) ==========================================> ********** ------------> ********** * Client * * Server * ********** <----Obs----- ********** (b) in server-client channel (fowl_up) Figure 5: full one-way loss in the opposite direction 7.3.4. Half round-trip loss Using the previous metrics, the two half round-trip loss measurements can be computed: hrtl_co = tql_up - uowl_down hrtl_os = tql_down - uowl_up Cociglio, et al. Expires September 10, 2020 [Page 17] Internet-Draft March 2020 =======================> = ********** ------|-----> ********** = * Client * Obs * Server * = ********** <-----|------ ********** <======================= (a) client-observer half round-trip loss (hrtl_co) =======================> ********** ------|-----> ********** = * Client * Obs * Server * = ********** <-----|------ ********** = <======================= (b) observer-server half round-trip loss (hrtl_os) Figure 6: half round-trip loss (both direction) 7.3.5. Downstream one-way loss Using the previous metrics, downstream one-way loss can be computed: dowl_up = hrtl_os - uowl_down dowl_down = hrtl_co - uowl_up =====================> ********** -----Obs----> ********** * Client * * Server * ********** <------------ ********** (a) in client-server channel (dowl_up) ********** ------------> ********** * Client * * Server * ********** <----Obs----- ********** <===================== (b) in server-client channel (dowl_down) Figure 7: Downstream one-way loss Cociglio, et al. Expires September 10, 2020 [Page 18] Internet-Draft March 2020 7.4. Enhancement of reflection period size computation The use of the rounding function used in the M computation introduces errors. However, these errors 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 marking period. This can be achieved introducing the new r_avg parameter in the previous M formula. The new formula is M=round(avg(p)+r_avg) where r_avg is computed as not rounded M minus rounded M; its initial value is equal to 0. 8. Protocols 8.1. QUIC The binding of the delay bit signal to QUIC is partially described in [I-D.ietf-quic-transport], which adds the spin bit to the first byte of the short packet header, leaving two reserved bits for future experiments. To implement the additional signals discussed in this document, the first byte of the short packet header can be modified as follows: the delay bit (D) can be placed in the first reserved bit (i.e. the fourth most significant bit _0x10_) while the loss bit in the second reserved bit (i.e. the fifth most significant bit _0x08_); the proposed scheme is: 0 1 2 3 4 5 6 7 +-+-+-+-+-+-+-+-+ |0|1|S|D|L|K|P|P| +-+-+-+-+-+-+-+-+ Figure 8: scheme 1 alternatively, the standalone two bits loss signal (QR) can be placed in both reserved bits; the proposed scheme, in this case, is: 0 1 2 3 4 5 6 7 +-+-+-+-+-+-+-+-+ |0|1|S|Q|R|K|P|P| +-+-+-+-+-+-+-+-+ Figure 9: scheme 2 Cociglio, et al. Expires September 10, 2020 [Page 19] Internet-Draft March 2020 This implies that an observer must be able to determine whether the spin bit is active and correctly spinning or not (choosing, accordingly, the right version of packet loss measurement to be used). 8.2. TCP The signal can be added to TCP by defining bit 4 of bytes 13-14 of the TCP header to carry the spin bit, and eventually bits 5 and 6 to carry additional information, like the delay bit and the 1 bit loss signal (or the two bits loss signal). 9. Security Considerations The privacy considerations for the hybrid RTT measurement signal are essentially the same as those for passive RTT measurement in general. 10. Acknowledgements tbc 11. IANA Considerations tbc 12. References 12.1. Normative References [I-D.ietf-quic-spin-exp] Trammell, B. and M. Kuehlewind, "The QUIC Latency Spin Bit", draft-ietf-quic-spin-exp-01 (work in progress), October 2018. [I-D.ietf-quic-transport] Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed and Secure Transport", draft-ietf-quic-transport-27 (work in progress), February 2020. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [RFC7799] Morton, A., "Active and Passive Metrics and Methods (with Hybrid Types In-Between)", RFC 7799, DOI 10.17487/RFC7799, May 2016, . Cociglio, et al. Expires September 10, 2020 [Page 20] Internet-Draft March 2020 [RFC8321] Fioccola, G., Ed., Capello, A., Cociglio, M., Castaldelli, L., Chen, M., Zheng, L., Mirsky, G., and T. Mizrahi, "Alternate-Marking Method for Passive and Hybrid Performance Monitoring", RFC 8321, DOI 10.17487/RFC8321, January 2018, . 12.2. Informative References [ANRW19-PM-QUIC] ACM/IRTF Applied Networking Research Workshop 2019 (ANRW'19), "Performance measurements of QUIC communications", DOI 10.1145/3340301.3341127, 2019. [I-D.trammell-ippm-spin] Trammell, B., "An Explicit Transport-Layer Signal for Hybrid RTT Measurement", draft-trammell-ippm-spin-00 (work in progress), January 2019. [I-D.trammell-quic-spin] Trammell, B., 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", draft-trammell-quic-spin-03 (work in progress), May 2018. [I-D.trammell-tsvwg-spin] Trammell, B., "A Transport-Independent Explicit Signal for Hybrid RTT Measurement", draft-trammell-tsvwg-spin-00 (work in progress), July 2018. Authors' Addresses Mauro Cociglio Telecom Italia Via Reiss Romoli, 274 Torino 10148 Italy Email: mauro.cociglio@telecomitalia.it Giuseppe Fioccola Huawei Technologies Riesstrasse, 25 Munich 80992 Germany Email: giuseppe.fioccola@huawei.com Cociglio, et al. Expires September 10, 2020 [Page 21] Internet-Draft March 2020 Massimo Nilo Telecom Italia Via Reiss Romoli, 274 Torino 10148 Italy Email: massimo.nilo@telecomitalia.it Fabio Bulgarella Telecom Italia Email: fabio.bulgarella@guest.telecomitalia.it Riccardo Sisto Politecnico di Torino Corso Duca degli Abruzzi, 24 Torino 10129 Italy Email: riccardo.sisto@polito.it Cociglio, et al. 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