IPPM Working Group M. Cociglio
Internet-Draft Telecom Italia
Intended status: Experimental G. Fioccola
Expires: September 1, 2019 Huawei Technologies
F. Bulgarella
R. Sisto
Politecnico di Torino
February 28, 2019
New Spin bit enabled measurements with one or two more bits
draft-cfb-ippm-spinbit-new-measurements-00
Abstract
This document introduces additional measurements by using the same
spin bit signal as defined in [I-D.trammell-ippm-spin]. The spin bit
signal alone is not enough to evaluate correctly in every network
condition the RTT of a flow. In order to solve this problem, it is
theorized the possibility of introducing an additional validation
signal called delay bit, similar to what is done done by the Valid
Edge Counter (VEC), but using just one bit instead of two. An
alternative with two bits is also introduced with a so called loss
bit.
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
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on September 1, 2019.
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Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Spin bit and Delay bit mechanism . . . . . . . . . . . . . . 3
2.1. Delay Sample generation . . . . . . . . . . . . . . . . . 5
2.1.1. The recovery process . . . . . . . . . . . . . . . . 5
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 . . . . . . . . . . . . . . . . . . 7
3.3. Intra-domain RTT measurement . . . . . . . . . . . . . . 7
4. Observer's algorithm and Waiting Interval . . . . . . . . . . 8
5. Adding a Loss bit to Delay bit and Spin bit . . . . . . . . . 9
5.1. Round Trip Packet Loss measurement . . . . . . . . . . . 9
6. Protocols . . . . . . . . . . . . . . . . . . . . . . . . . . 10
6.1. QUIC . . . . . . . . . . . . . . . . . . . . . . . . . . 10
6.2. TCP . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
7. Security Considerations . . . . . . . . . . . . . . . . . . . 10
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
10.1. Normative References . . . . . . . . . . . . . . . . . . 10
10.2. Informative References . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction
[I-D.trammell-ippm-spin] defines 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.
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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.
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.
The application of the Spin bit to QUIC is described in
[I-D.ietf-quic-spin-exp] which adds the spin bit only (without the
VEC) 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].
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 and the VEC, 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.
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+--------+ -- -- -- -- -- +--------+
| | -----------> | |
| Client | | Server |
| | <----------- | |
+--------+ -- -- -- -- -- +--------+
(a) No traffic at beginning.
+--------+ 00 00 01 -- -- +--------+
| | -----------> | |
| Client | | Server |
| | <----------- | |
+--------+ -- -- -- -- -- +--------+
(b) The Client starts sending data and
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 +--------+
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(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:
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
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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.
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.
It is noteworthy that differently from what happens with the VEC for
which the reflection always concerns the edge of the period, in this
case 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.
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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 (with or without the VEC).
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
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
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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.
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:
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
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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).
5. Adding a Loss bit to Delay bit and Spin bit
It is possible to introduce a mechanism to evaluate also the packet
loss together with the delay measurement. In particular, the Client
can select and mark a train of packets for this purpose, by using a
loss bit, additionally to the spin bit and delay bit.
These packets bounce between Client and Server to complete two rounds
and an Observer counts the marked packets during the two rounds and
compares the counters to find Round Trip(RT) losses.
The problem to be solved is to choose the right number of packets to
mark to avoid marked packets congestion on the slowest traffic
direction. But the solution is simple, because it is enough to
choose the number of packets that transit on the slowest direction
during an RTT.
5.1. Round Trip Packet Loss measurement
The Client generates a train of marked packets (Packet Loss Samples)
by using the additional bit called Loss bit. The marked packets are
generated at the slowest direction rate (only when a packet arrives
the Client marks an outgoing packet). The Server reflects these
packets accordingly and, as a consequence, it could insert some not-
marked packets. Then the client reflects the marked packets and the
server reflects the marked packets again. The Client generates a new
train of marked packets and so on.
The Packet Loss calculation can be made after the comparison of
counters taken by the on-path passive observer. Indeed the Observer
in the middle (upstream or downstream) sees the packet train twice
and so it calculates the Observer Round Trip Packet Loss that,
statistically, will be equal to the end-to-end Round Trip Packet
Loss. So this measurement can be simply referred as Round Trip
Packet Loss (RTPL).
In addition, this methodology allows Half-RTPL measurement and Intra-
domain RTPL measurement, in the same way as described in the previous
Sections for RTT measurement.
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The method allows the packet loss calculation for a portion of the
traffic but it is useful to perform RT Packet Loss measurement that
gives useful information coupled with RTT.
6. Protocols
6.1. QUIC
The binding of this signal to QUIC is partially described in
[I-D.ietf-quic-spin-exp], which adds the spin bit only to QUIC.
From an implementation point of view, the delay bit is placed in the
partially unencrypted (but authenticated) QUIC header, alongside the
spin bit, occupying one of the two bits left reserved for future
experiments. As things stand, according to
[I-D.ietf-quic-transport], the proposed scheme of the first header's
byte would be 01SDRKPP.
6.2. 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 loss bit.
7. Security Considerations
The privacy considerations for the hybrid RTT measurement signal are
essentially the same as those for passive RTT measurement in general.
8. Acknowledgements
tbc
9. IANA Considerations
tbc
10. References
10.1. 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.
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[I-D.ietf-quic-transport]
Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
and Secure Transport", draft-ietf-quic-transport-18 (work
in progress), January 2019.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC7799] 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>.
[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, <https://www.rfc-editor.org/info/rfc8321>.
10.2. Informative References
[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.
Authors' Addresses
Mauro Cociglio
Telecom Italia
Via Reiss Romoli, 274
Torino 10148
Italy
Email: mauro.cociglio@telecomitalia.it
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Giuseppe Fioccola
Huawei Technologies
Riesstrasse, 25
Munich 80992
Germany
Email: giuseppe.fioccola@huawei.com
Fabio Bulgarella
Politecnico di Torino
Email: fabio.bulgarella@guest.telecomitalia.it
Riccardo Sisto
Politecnico di Torino
Corso Duca degli Abruzzi, 24
Torino 10129
Italy
Email: riccardo.sisto@polito.it
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