Internet DRAFT - draft-fioccola-rfc8321bis
draft-fioccola-rfc8321bis
Network Working Group G. Fioccola, Ed.
Internet-Draft Huawei Technologies
Obsoletes: 8321 (if approved) M. Cociglio
Intended status: Standards Track Telecom Italia
Expires: October 7, 2022 G. Mirsky
Ericsson
T. Mizrahi
T. Zhou
Huawei Technologies
April 5, 2022
Alternate-Marking Method
draft-fioccola-rfc8321bis-04
Abstract
This document describes the Alternate-Marking technique to perform
packet loss, delay, and jitter measurements on live traffic. This
technology can be applied in various situations and for different
protocols. It could be considered Passive or Hybrid depending on the
application. This document obsoletes [RFC8321].
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on October 7, 2022.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Overview of the Method . . . . . . . . . . . . . . . . . . . 4
3. Detailed Description of the Method . . . . . . . . . . . . . 5
3.1. Packet Loss Measurement . . . . . . . . . . . . . . . . . 5
3.2. One-Way Delay Measurement . . . . . . . . . . . . . . . . 9
3.2.1. Single-Marking Methodology . . . . . . . . . . . . . 9
3.2.2. Double-Marking Methodology . . . . . . . . . . . . . 10
3.3. Delay Variation Measurement . . . . . . . . . . . . . . . 11
4. Alternate Marking Functions . . . . . . . . . . . . . . . . . 12
4.1. Marking the Packets . . . . . . . . . . . . . . . . . . . 12
4.2. Counting and Timestamping Packets . . . . . . . . . . . . 12
4.3. Data Collection and Correlation . . . . . . . . . . . . . 13
5. Synchronization and Timing . . . . . . . . . . . . . . . . . 14
6. Packet Fragmentation . . . . . . . . . . . . . . . . . . . . 16
7. Results of the Alternate Marking Experiment . . . . . . . . . 16
7.1. Controlled Domain requirement . . . . . . . . . . . . . . 18
8. Compliance with Guidelines from RFC 6390 . . . . . . . . . . 18
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20
10. Security Considerations . . . . . . . . . . . . . . . . . . . 20
11. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 22
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 22
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 22
13.1. Normative References . . . . . . . . . . . . . . . . . . 22
13.2. Informative References . . . . . . . . . . . . . . . . . 23
Appendix A. Changes Log . . . . . . . . . . . . . . . . . . . . 25
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 26
1. Introduction
Most Service Providers' networks carry traffic with contents that are
highly sensitive to packet loss [RFC7680], delay [RFC7679], and
jitter [RFC3393].
Service Providers need methodologies and tools to monitor and
accurately measure network performance, in order to constantly
control the quality of experience perceived by their customers.
Performance monitoring also provides useful information for improving
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network management (e.g., isolation of network problems,
troubleshooting, etc.).
RFC 7799 [RFC7799] defines Passive and Hybrid Methods of Measurement.
In particular, Passive Methods of Measurement are based solely on
observations of an undisturbed and unmodified packet stream of
interest; Hybrid Methods are Methods of Measurement that use a
combination of Active Methods and Passive Methods.
[RFC7276] provides a good overview of existing Operations,
Administration, and Maintenance (OAM) mechanisms defined in the IETF,
ITU-T, and IEEE. In the IETF, a lot of work has been done on fault
detection and connectivity verification, while little has been thus
far dedicated to performance monitoring. The IETF has defined
standard metrics to measure network performance; however, its methods
mainly focus on Active measurement techniques.For example, [RFC6374]
defines mechanisms for measuring packet loss, one-way and two-way
delay, and delay variation in MPLS networks, but its applicability to
Passive measurements has some limitations, especially for connection-
less networks.
This document proposes a Passive performance monitoring technique,
potentially applicable to any kind of packet-based traffic, including
Ethernet, IP, and MPLS, both unicast and multicast. The method
addresses primarily packet loss measurement, but it can be easily
extended to one-way or two-way delay and delay variation measurements
as well.
The method has been explicitly designed for Passive measurements, but
it can also be used with Active probes. Passive measurements are
usually more easily understood by customers and provide much better
accuracy, especially for packet loss measurements.
Therefore, the Alternate-Marking Method could be considered Hybrid or
Passive, depending on the case. In the case where the marking method
is obtained by changing existing field values of the packets the
technique is Hybrid. In the case where the marking field is
dedicated, reserved, and included in the protocol specification, the
Alternate-Marking technique can be considered as Passive.
1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
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2. Overview of the Method
In order to perform packet loss measurements on a production traffic
flow, different approaches exist. The most intuitive one consists in
numbering the packets so that each router that receives the flow can
immediately detect a packet that is missing. This approach, though
very simple in theory, is not simple to achieve: it requires the
insertion of a sequence number into each packet, and the devices must
be able to extract the number and check it in real time. Such a task
can be difficult to implement on live traffic: if UDP is used as the
transport protocol, the sequence number is not available; on the
other hand, if a higher-layer sequence number (e.g., in the RTP
header) is used, extracting that information from each packet and
processing it in real time could overload the device.
An alternate approach is to count the number of packets sent on one
end, count the number of packets received on the other end, and
compare the two values. This operation is much simpler to implement,
but it requires the devices performing the measurement to be in sync:
in order to compare two counters, it is required that they refer
exactly to the same set of packets. Since a flow is continuous and
cannot be stopped when a counter has to be read, it can be difficult
to determine exactly when to read the counter. A possible solution
to overcome this problem is to virtually split the flow in
consecutive blocks by periodically inserting a delimiter so that each
counter refers exactly to the same block of packets. The delimiter
could be, for example, a special packet inserted artificially into
the flow. However, delimiting the flow using specific packets has
some limitations. First, it requires generating additional packets
within the flow and requires the equipment to be able to process
those packets. In addition, the method is vulnerable to out-of-order
reception of delimiting packets and, to a lesser extent, to their
loss.
The method proposed in this document follows the second approach, but
it doesn't use additional packets to virtually split the flow in
blocks. Instead, it "marks" the packets so that the packets
belonging to the same block will have the same color, whilst
consecutive blocks will have different colors. Each change of color
represents a sort of auto-synchronization signal that guarantees the
consistency of measurements taken by different devices along the
path.
Figure 1 represents a very simple network and shows how the method
can be used to measure packet loss on different network segments: by
enabling the measurement on several interfaces along the path, it is
possible to perform link monitoring, node monitoring, or end-to-end
monitoring. The method is flexible enough to measure packet loss on
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any segment of the network and can be used to isolate the faulty
element.
Traffic Flow
========================================================>
+------+ +------+ +------+ +------+
---<> R1 <>-----<> R2 <>-----<> R3 <>-----<> R4 <>---
+------+ +------+ +------+ +------+
. . . . . .
. . . . . .
. <------> <-------> .
. Node Packet Loss Link Packet Loss .
. .
<--------------------------------------------------->
End-to-End Packet Loss
Figure 1: Available Measurements
3. Detailed Description of the Method
This section describes, in detail, how the method operates. A
special emphasis is given to the measurement of packet loss, which
represents the core application of the method, but applicability to
delay and jitter measurements is also considered.
3.1. Packet Loss Measurement
The basic idea is to virtually split traffic flows into consecutive
blocks: each block represents a measurable entity unambiguously
recognizable by all network devices along the path. By counting the
number of packets in each block and comparing the values measured by
different network devices along the path, it is possible to measure
if packet loss occurred in any single block between any two points.
As discussed in the previous section, a simple way to create the
blocks is to "color" the traffic (two colors are sufficient), so that
packets belonging to different consecutive blocks will have different
colors. Whenever the color changes, the previous block terminates
and the new one begins. Hence, all the packets belonging to the same
block will have the same color and packets of different consecutive
blocks will have different colors. The number of packets in each
block depends on the criterion used to create the blocks:
o if the color is switched after a fixed number of packets, then
each block will contain the same number of packets (except for any
losses); and
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o if the color is switched according to a fixed timer, then the
number of packets may be different in each block depending on the
packet rate.
The rest of the document assumes that the blocks are created
according to a fixed timer. The switching after a fixed number of
packets is an additional possibility but its detailed specification
is out of scope.
The following figure shows how a flow looks like when it is split in
traffic blocks with colored packets.
A: packet with A coloring
B: packet with B coloring
| | | | |
| | Traffic Flow | |
------------------------------------------------------------------->
BBBBBBB AAAAAAAAAAA BBBBBBBBBBB AAAAAAAAAAA BBBBBBBBBBB AAAAAAA
------------------------------------------------------------------->
... | Block 5 | Block 4 | Block 3 | Block 2 | Block 1
| | | | |
Figure 2: Traffic Coloring
Figure 3 shows how the method can be used to measure link packet loss
between two adjacent nodes.
Referring to the figure, let's assume we want to monitor the packet
loss on the link between two routers: router R1 and router R2.
According to the method, the traffic is colored alternatively with
two different colors: A and B. Whenever the color changes, the
transition generates a sort of square-wave signal, as depicted in the
following figure.
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Color A ----------+ +-----------+ +----------
| | | |
Color B +-----------+ +-----------+
Block n ... Block 3 Block 2 Block 1
<---------> <---------> <---------> <---------> <--------->
Traffic Flow
===========================================================>
Color ...AAAAAAAAAAA BBBBBBBBBBB AAAAAAAAAAA BBBBBBBBBBB AAAAAAA...
===========================================================>
Figure 3: Computation of Link Packet Loss
Traffic coloring can be done by R1 itself if the traffic is not
already colored. R1 needs two counters, C(A)R1 and C(B)R1, on its
egress interface: C(A)R1 counts the packets with color A and C(B)R1
counts those with color B. As long as traffic is colored as A, only
counter C(A)R1 will be incremented, while C(B)R1 is not incremented;
conversely, when the traffic is colored as B, only C(B)R1 is
incremented. C(A)R1 and C(B)R1 can be used as reference values to
determine the packet loss from R1 to any other measurement point down
the path. Router R2, similarly, will need two counters on its
ingress interface, C(A)R2 and C(B)R2, to count the packets received
on that interface and colored with A and B, respectively. When an A
block ends, it is possible to compare C(A)R1 and C(A)R2 and calculate
the packet loss within the block; similarly, when the successive B
block terminates, it is possible to compare C(B)R1 with C(B)R2, and
so on, for every successive block.
Likewise, by using two counters on the R2 egress interface, it is
possible to count the packets sent out of the R2 interface and use
them as reference values to calculate the packet loss from R2 to any
measurement point downstream from R2.
Using a fixed timer for color switching offers better control over
the method: the (time) length of the blocks can be chosen large
enough to simplify the collection and the comparison of measures
taken by different network devices. It's preferable to read the
value of the counters not immediately after the color switch: some
packets could arrive out of order and increment the counter
associated with the previous block (color), so it is worth waiting
for some time. A safe choice is to wait L/2 time units (where L is
the duration for each block) after the color switch, to read the
counter of the previous color. The drawback is that the longer the
duration of the block, the less frequently the measurement can be
taken.
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Two different strategies that can be used when implementing the
method:
o flow-based: the flow-based strategy is used when only a limited
number of traffic flows need to be monitored. According to this
strategy, only a subset of the flows is colored. Counters for
packet loss measurements can be instantiated for each single flow,
or for the set as a whole, depending on the desired granularity.
A relevant problem with this approach is the necessity to know in
advance the path followed by flows that are subject to
measurement. Path rerouting and traffic load-balancing increase
the issue complexity, especially for unicast traffic. The problem
is easier to solve for multicast traffic, where load-balancing is
seldom used and static joins are frequently used to force traffic
forwarding and replication.
o link-based: measurements are performed on all the traffic on a
link-by-link basis. The link could be a physical link or a
logical link. Counters could be instantiated for the traffic as a
whole or for each traffic class (in case it is desired to monitor
each class separately), but in the second case, two counters are
needed for each class.
As mentioned, the flow-based measurement requires the identification
of the flow to be monitored and the discovery of the path followed by
the selected flow. It is possible to monitor a single flow or
multiple flows grouped together, but in this case, measurement is
consistent only if all the flows in the group follow the same path.
Moreover, if a measurement is performed by grouping many flows, it is
not possible to determine exactly which flow was affected by packet
loss. In order to have measures per single flow, it is necessary to
configure counters for each specific flow. Once the flow(s) to be
monitored has been identified, it is necessary to configure the
monitoring on the proper nodes. Configuring the monitoring means
configuring the rule to intercept the traffic and configuring the
counters to count the packets. To have just an end-to-end
monitoring, it is sufficient to enable the monitoring on the first-
and last-hop routers of the path: the mechanism is completely
transparent to intermediate nodes and independent from the path
followed by traffic flows. On the contrary, to monitor the flow on a
hop-by-hop basis along its whole path, it is necessary to enable the
monitoring on every node from the source to the destination. In case
the exact path followed by the flow is not known a priori (i.e., the
flow has multiple paths to reach the destination), it is necessary to
enable the monitoring system on every path: counters on interfaces
traversed by the flow will report packet count, whereas counters on
other interfaces will be null.
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3.2. One-Way Delay Measurement
The same principle used to measure packet loss can be applied also to
one-way delay measurement. There are three alternatives, as
described hereinafter.
Note that, for all the one-way delay alternatives described in the
next sections, by summing the one-way delays of the two directions of
a path, it is always possible to measure the two-way delay (round-
trip "virtual" delay).
3.2.1. Single-Marking Methodology
The alternation of colors can be used as a time reference to
calculate the delay. Whenever the color changes (which means that a
new block has started), a network device can store the timestamp of
the first packet of the new block; that timestamp can be compared
with the timestamp of the same packet on a second router to compute
packet delay. When looking at Figure 2, R1 stores the timestamp
TS(A1)R1 when it sends the first packet of block 1 (A-colored), the
timestamp TS(B2)R1 when it sends the first packet of block 2
(B-colored), and so on for every other block. R2 performs the same
operation on the receiving side, recording TS(A1)R2, TS(B2)R2, and so
on. Since the timestamps refer to specific packets (the first packet
of each block), we are sure that timestamps compared to compute delay
refer to the same packets. By comparing TS(A1)R1 with TS(A1)R2 (and
similarly TS(B2)R1 with TS(B2)R2, and so on), it is possible to
measure the delay between R1 and R2. In order to have more
measurements, it is possible to take and store more timestamps,
referring to other packets within each block. The number of
measurements could be increased by considering multiple packets in
the block: for instance, a timestamp could be taken every N packets,
thus generating multiple delay measurements. Taking this to the
limit, in principle, the delay could be measured for each packet by
taking and comparing the corresponding timestamps (possible but
impractical from an implementation point of view).
In order to coherently compare timestamps collected on different
routers, the clocks on the network nodes must be in sync.
Furthermore, a measurement is valid only if no packet loss occurs and
if packet misordering can be avoided; otherwise, the first packet of
a block on R1 could be different from the first packet of the same
block on R2 (for instance, if that packet is lost between R1 and R2
or it arrives after the next one). Since packet misordering is
generally undetectable it is not possible to check whether the first
packet on R1 is the same on R2 and this is part of the intrinsic
error in this measurement.
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3.2.1.1. Mean Delay
As mentioned before, the method previously exposed for measuring the
delay is sensitive to out-of-order reception of packets. In order to
overcome this problem, a different approach has been considered: it
is based on the concept of mean delay. The mean delay is calculated
by considering the average arrival time of the packets within a
single block. The network device locally stores a timestamp for each
packet received within a single block: summing all the timestamps and
dividing by the total number of packets received, the average arrival
time for that block of packets can be calculated. By subtracting the
average arrival times of two adjacent devices, it is possible to
calculate the mean delay between those nodes. When computing the
mean delay, the measurement error could be augmented by accumulating
the measurement error of a lot of packets. This method is robust to
out-of-order packets and also to packet loss (only a small error is
introduced). Moreover, it greatly reduces the number of timestamps
(only one per block for each network device) that have to be
collected by the management system. On the other hand, it only gives
one measure for the duration of the block, and it doesn't give the
minimum, maximum, and median delay values [RFC6703]. This limitation
could be overcome by reducing the duration of the block (for
instance, from 5 minutes to a few seconds), which implies a highly
optimized implementation of the method.
3.2.2. Double-Marking Methodology
The Single-Marking methodology for one-way delay measurement is
sensitive to out-of-order reception of packets. The first approach
to overcome this problem has been described before and is based on
the concept of mean delay. But the limitation of mean delay is that
it doesn't give information about the delay value's distribution for
the duration of the block. Additionally, it may be useful to have
not only the mean delay but also the minimum, maximum, and median
delay values and, in wider terms, to know more about the statistic
distribution of delay values. So, in order to have more information
about the delay and to overcome out-of-order issues, a different
approach can be introduced; it is based on a Double-Marking
methodology.
Basically, the idea is to use the first marking to create the
alternate flow and, within this colored flow, a second marking to
select the packets for measuring delay/jitter. The first marking is
needed for packet loss and mean delay measurement. The second
marking creates a new set of marked packets that are fully identified
over the network, so that a network device can store the timestamps
of these packets; these timestamps can be compared with the
timestamps of the same packets on a second router to compute packet
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delay values for each packet. The number of measurements can be
easily increased by changing the frequency of the second marking.
But the frequency of the second marking must not be too high in order
to avoid out-of-order issues. Between packets with the second
marking, there should be a security time gap (e.g., this gap could
be, at the minimum, the mean network delay calculated with the
previous methodology) to avoid out-of-order issues and also to have a
number of measurement packets that are rate independent. If a
second-marking packet is lost, the delay measurement for the
considered block is corrupted and should be discarded.
Mean delay is calculated on all the packets of a sample and is a
simple computation to be performed for a Single-Marking Method. In
some cases, the mean delay measure is not sufficient to characterize
the sample, and more statistics of delay extent data are needed,
e.g., percentiles, variance, and median delay values. The
conventional range (maximum-minimum) should be avoided for several
reasons, including stability of the maximum delay due to the
influence by outliers. RFC 5481 [RFC5481], Section 6.5 highlights
how the 99.9th percentile of delay and delay variation is more
helpful to performance planners. To overcome this drawback, the idea
is to couple the mean delay measure for the entire batch with a
Double-Marking Method, where a subset of batch packets is selected
for extensive delay calculation by using a second marking. In this
way, it is possible to perform a detailed analysis on these double-
marked packets. Please note that there are classic algorithms for
median and variance calculation, but they are out of the scope of
this document. The comparison between the mean delay for the entire
batch and the mean delay on these double-marked packets gives useful
information since it is possible to understand if the Double-Marking
measurements are actually representative of the delay trends.
3.3. Delay Variation Measurement
Similar to one-way delay measurement (both for Single Marking and
Double Marking), the method can also be used to measure the inter-
arrival jitter. We refer to the definition in RFC 3393 [RFC3393].
The alternation of colors, for a Single-Marking Method, can be used
as a time reference to measure delay variations. In case of Double
Marking, the time reference is given by the second-marked packets.
Considering the example depicted in Figure 2, R1 stores the timestamp
TS(A)R1 whenever it sends the first packet of a block, and R2 stores
the timestamp TS(B)R2 whenever it receives the first packet of a
block. The inter-arrival jitter can be easily derived from one-way
delay measurement, by evaluating the delay variation of consecutive
samples.
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The concept of mean delay can also be applied to delay variation, by
evaluating the average variation of the interval between consecutive
packets of the flow from R1 to R2.
4. Alternate Marking Functions
4.1. Marking the Packets
The coloring operation is fundamental in order to create packet
blocks and marked packets. This implies choosing where to activate
the coloring and how to color the packets.
In case of flow-based measurements, the flow to monitor can be
defined by a set of selection rules (e.g., header fields) used to
match a subset of the packets; in this way, it is possible to control
the number of involved nodes, the path followed by the packets, and
the size of the flows. It is possible, in general, to have multiple
coloring nodes or a single coloring node that is easier to manage and
doesn't raise any risk of conflict. Coloring in multiple nodes can
be done, and the requirement is that the coloring must change
periodically between the nodes according to the timing considerations
in Section 5; so every node that is designated as a measurement point
along the path should be able to identify unambiguously the colored
packets. Furthermore, [I-D.fioccola-rfc8889bis] generalizes the
coloring for multipoint-to-multipoint flow. In addition, it can be
advantageous to color the flow as close as possible to the source
because it allows an end-to-end measure if a measurement point is
enabled on the last-hop router as well.
For link-based measurements, all traffic needs to be colored when
transmitted on the link. If the traffic had already been colored,
then it has to be re-colored because the color must be consistent on
the link. This means that each hop along the path must (re-)color
the traffic; the color is not required to be consistent along
different links.
Traffic coloring can be implemented by setting specific flags in the
packet header and changing the value of that bit periodically. How
to choose the marking field depends on the application and is out of
scope here.
4.2. Counting and Timestamping Packets
For flow-based measurements, assuming that the coloring of the
packets is performed only by the source nodes, the nodes between
source and destination (included) have to count and timestamp the
colored packets that they receive and forward: this operation can be
enabled on every router along the path or only on a subset, depending
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on which network segment is being monitored (a single link, a
particular metro area, the backbone, or the whole path). Since the
color switches periodically between two values, two counters (one for
each value) are needed for each flow and for every interface being
monitored. The number of timestamps to be stored depends on the
method for delay measurement that is applied. Furthermore,
[I-D.fioccola-rfc8889bis] generalizes the counting for multipoint-to-
multipoint flow.
In case of link-based measurements, the behavior is similar except
that coloring, counting and timestamping operations are performed on
a link-by-link basis at each endpoint of the link.
Another important aspect to take into consideration is when to read
the counters or when to select the packets to be double-marked for
delay measurement. It involves timing aspects to consider that are
further described in Section 5.
4.3. Data Collection and Correlation
The nodes enabled to perform performance monitoring collect the value
of the counters and timestamps, but they are not able to directly use
this information to measure packet loss and delay, because they only
have their own samples.
Data collection enables the transmission of the counters and
timestamps as soon as it has been read. While, data correlation is
the mechanism to compare counters and timestamps for packet loss,
delay, and delay variation calculation.
There are two main possibilities to perform both data collection and
correlation depending on the Alternate-Marking application and use
case:
o Use of a centralized solution using Network Management System
(NMS) to correlate data. This can be done in Push Mode or Polling
Mode. In the first case, each router periodically sends the
information to the NMS; in the latter case, it is the NMS that
periodically polls routers to collect information. In any case,
the NMS has to collect all the relevant values from all the
routers within one cycle of the timer.
o Definition of a protocol-based distributed solution to exchange
values of counters and timestamps between the endpoints. This can
be done by introducing a new protocol or by extending the existing
protocols (e.g., the Two-Way Active Measurement Protocol (TWAMP)
as defined in RFC 5357 [RFC5357] or the One-Way Active Measurement
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Protocol (OWAMP) as defined in RFC 4656 [RFC4656]) in order to
communicate the counters and timestamps between nodes.
In the following paragraphs, an example data correlation mechanism is
explained and could be used independently of the adopted solutions.
When data is collected on the upstream and downstream nodes, e.g.,
packet counts for packet loss measurement or timestamps for packet
delay measurement, and is periodically reported to or pulled by other
nodes or an NMS, a certain data correlation mechanism SHOULD be in
use to help the nodes or NMS tell whether any two or more packet
counts are related to the same block of markers or if any two
timestamps are related to the same marked packet.
The Alternate-Marking Method described in this document literally
splits the packets of the measured flow into different measurement
blocks. An implementation MAY use a Block Number (BN) for data
correlation. The BN MAY be assigned to each measurement block and
associated with each packet count and timestamp reported to or pulled
by other nodes or NMSs. When the nodes or NMS see, for example, the
same BNs associated with two packet counts from an upstream and a
downstream node, respectively, it considers that these two packet
counts correspond to the same block. The assumption of this BN
mechanism is that the measurement nodes are time synchronized. This
requires the measurement nodes to have a certain time synchronization
capability (e.g., the Network Time Protocol (NTP) [RFC5905] or the
IEEE 1588 Precision Time Protocol (PTP) [IEEE-1588]).
5. Synchronization and Timing
This document introduces two color-switching methods: one is based on
a fixed number of packets, and the other is based on a fixed timer.
But the method based on a fixed timer is preferable because it is
more deterministic, and it is considered in the document.
Color switching is the reference for all the network devices, and the
only requirement to be achieved is that all network devices have to
recognize the right batch along the path.
In general, clocks in network devices are not accurate and for this
reason, there is a clock error between the measurement points R1 and
R2. And, to implement the methodology, they must be synchronized to
the same clock reference with an adequate accuracy in order to
guarantee that all network devices consistently match the marking bit
to the correct block. Additionally, in practice, besides clock
errors, packet reordering is also very common in a packet network due
to equal-cost multipath (ECMP). In particular, the delay between
measurement points is the main cause of out of order because each
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packet can be delayed differently. If the block is sufficiently
large, packet reordering occurs only at the edge of adjacent blocks
and it can be easy to assign reordered packets to the right interval
blocks.
In summary, we need to take into account two contributions: clock
error between network devices and the interval we need to wait to
avoid packets being out of order because of network delay.
The following figure explains both issues.
...BBBBBBBBB | AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA | BBBBBBBBB...
|<======================================>|
| L |
...=========>|<==================><==================>|<==========...
| L/2 L/2 |
|<===>| |<===>|
d | | d
|<==========================>|
available counting interval
Figure 4: Timing Aspects
Where L is the time duration of each block.
It is assumed that all network devices are synchronized to a common
reference time with an accuracy of +/- A/2. Thus, the difference
between the clock values of any two network devices is bounded by A.
The network delay between the network devices can be represented as a
data set and 99.7% of the samples are within 3 standard deviation of
the average.
The guard band d is given by:
d = A + D_avg + 3*D_stddev,
where A is the clock accuracy, D_avg is the average value of the
network delay between the network devices, and D_stddev is the
standard deviation of the delay.
The available counting interval is L - 2d that must be > 0.
The condition that must be satisfied and is a requirement on the
synchronization accuracy is:
d < L/2.
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It is worth mentioning that, if the time duration L of each block is
not so small, the synchronization requirement could be satisfied even
with a relatively inaccurate synchronization method. This is true
for packet loss and two-way delay measurement, but not for one-way
delay measurement, where clock synchronization must be accurate.
Therefore, a system that uses only packet loss and two-way delay
measurement may not require a very precise synchronization. This is
because the value of the clocks of network devices does not affect
the computation of the two-way delay measurement.
6. Packet Fragmentation
Fragmentation can be managed with the Alternate-Marking Method and in
particular it is possible to give the following guidance:
Marking nodes MUST mark all fragments if there are flag bits to
use (i.e. it is in the specific encapsulation), as if they were
separate packets.
Nodes that fragment packets within the measurement domain SHOULD,
if they have the capability to do so, ensure that only one
resulting fragment carries the marking bit(s) of the original
packet. Failure to do so can introduce errors into the
measurement.
Measurement points MAY simply ignore unmarked fragments and count
marked fragments as full packets. However, if resources allow,
measurement points MAY make note of both marked and unmarked
initial fragments and only increment the corresponding counter if
(a) other fragments are also marked, or (b) it observes all other
fragments and they are unmarked.
The proposed approach allows the marking node to mark all the
fragments except in the case of fragmentation within the network
domain, in that event it is suggested to mark only the first
fragment.
7. Results of the Alternate Marking Experiment
The methodology described in the previous sections can be applied to
various performance measurement problems, as explained in [RFC8321].
The only requirement is to select and mark the flow to be monitored;
in this way, packets are batched by the sender, and each batch is
alternately marked such that it can be easily recognized by the
receiver.
Either one or two flag bits might be available for marking in
different deployments:
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One flag: packet loss measurement SHOULD be done as described in
Section 3.1, while delay measurement MAY be done according to the
single-marking method described in Section 3.2.1. Mean delay
(Section 3.2.1.1) is NOT RECOMMENDED since it implies more
computational load.
Two flags: packet loss measurement SHOULD be done as described in
Section 3.1, while delay measurement SHOULD be done according to
double-marking method Section 3.2.2. In this case single-marking
MAY also be used in combination with double-marking and the two
approaches provide slightly different pieces of information that
can be combined to have a more robust data set.
The experiment with Alternate Marking methodologies confirmed the
following benefits:
o easy implementation: it can be implemented by using features
already available on major routing platforms, or by applying an
optimized implementation of the method for both legacy and newest
technologies;
o low computational effort: the additional load on processing is
negligible;
o accurate loss and delay measurements: single packet loss
granularity is achieved with a Passive measurement;
o potential applicability to any kind of packet-based or frame-based
traffic: Ethernet, IP, MPLS, etc., and both unicast and multicast;
o robustness: the method can easily tolerate out-of-order packets,
and it's not based on "special" packets whose loss could have a
negative impact;
o flexibility: all the timestamp formats are allowed, because they
are managed out of band. The format (the Network Time Protocol
(NTP) [RFC5905] or the IEEE 1588 Precision Time Protocol (PTP)
[IEEE-1588]) depends on the precision you want; and
o no interoperability issues: the features required are available on
all current routing platforms. Both a centralized or distributed
solution can be used to harvest data from the routers.
A deployment of the Alternate-Marking Method SHOULD also take into
account how to handle and recognize marked and unmarked traffic
depending on whether the technique is applied as Hybrid or Passive.
In the case where the marking method is applied by changing existing
fields of the packets, it is RECOMMENDED to use an additional flag or
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some out-of-band signaling to indicate if the measurement is
activated or not in order to inform the measurement points. While,
in the case where the marking field is dedicated, reserved, and
included in a protocol extension, the measurement points can learn
whether the measurement is activated or not by checking if the
specific extension is included or not within the packets.
It is worth mentioning some related work: in particular
[IEEE-Network-PNPM] explains the Alternate-Marking method together
with new mechanisms based on hashing techniques as also further
described in [I-D.mizrahi-ippm-marking]; while
[I-D.zhou-ippm-enhanced-alternate-marking] extends the Alternate-
Marking Data Fields, to provide enhanced capabilities and allow
advanced functionalities.
7.1. Controlled Domain requirement
The Alternate Marking Method is an example of a solution limited to a
controlled domain [RFC8799].
A controlled domain is a managed network that selects, monitors, and
controls access by enforcing policies at the domain boundaries, in
order to discard undesired external packets entering the domain and
check internal packets leaving the domain. It does not necessarily
mean that a controlled domain is a single administrative domain or a
single organization. A controlled domain can correspond to a single
administrative domain or multiple administrative domains under a
defined network management. It must be possible to control the
domain boundaries, and use specific precautions if traffic traverses
the Internet.
For security reasons, the Alternate Marking Method is RECOMMENDED
only for controlled domains.
8. Compliance with Guidelines from RFC 6390
RFC 6390 [RFC6390] defines a framework and a process for developing
Performance Metrics for protocols above and below the IP layer (such
as IP-based applications that operate over reliable or datagram
transport protocols).
This document doesn't aim to propose a new Performance Metric but
rather a new Method of Measurement for a few Performance Metrics that
have already been standardized. Nevertheless, it's worth applying
guidelines from [RFC6390] to the present document, in order to
provide a more complete and coherent description of the proposed
method. We used a combination of the Performance Metric Definition
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template defined in Section 5.4 of [RFC6390] and the Dependencies
laid out in Section 5.5 of that document.
o Metric Name / Metric Description: as already stated, this document
doesn't propose any new Performance Metrics. On the contrary, it
describes a novel method for measuring packet loss [RFC7680]. The
same concept, with small differences, can also be used to measure
delay [RFC7679] and jitter [RFC3393]. The document mainly
describes the applicability to packet loss measurement.
o Method of Measurement or Calculation: according to the method
described in the previous sections, the number of packets lost is
calculated by subtracting the value of the counter on the source
node from the value of the counter on the destination node. Both
counters must refer to the same color. The calculation is
performed when the value of the counters is in a steady state.
The steady state is an intrinsic characteristic of the marking
method counters because the alternation of color makes the counter
associated with a color inactive for the duration of a marking
period.
o Units of Measurement: the method calculates and reports the exact
number of packets sent by the source node and not received by the
destination node.
o Measurement Point(s) with Potential Measurement Domain: the
measurement can be performed between adjacent nodes, on a per-link
basis, or along a multi-hop path, provided that the traffic under
measurement follows that path. In case of a multi-hop path, the
measurements can be performed both end-to-end and hop-by-hop.
o Measurement Timing: the method has a constraint on the frequency
of measurements. This is detailed in Section 5, where it is
specified that the marking period and the guard band interval are
strictly related each other to avoid out-of-order issues. That is
because, in order to perform a measurement, the counter must be in
a steady state, and this happens when the traffic is being colored
with the alternate color.
o Implementation: the method uses one or two marking bits to color
the packets; this enables the use of policy configurations on the
router to color the packets and accordingly configure the counter
for each color. The path followed by traffic being measured
should be known in advance in order to configure the counters
along the path and be able to compare the correct values.
o Verification: both in the lab and in the operational network, the
methodology has been tested and experimented for packet loss and
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delay measurements by using traffic generators together with
precision test instruments and network emulators.
o Use and Applications: the method can be used to measure packet
loss with high precision on live traffic; moreover, by combining
end-to-end and per-link measurements, the method is useful to
pinpoint the single link that is experiencing loss events.
o Reporting Model: the value of the counters has to be sent to a
centralized management system that performs the calculations; such
samples must contain a reference to the time interval they refer
to, so that the management system can perform the correct
correlation; the samples have to be sent while the corresponding
counter is in a steady state (within a time interval); otherwise,
the value of the sample should be stored locally.
o Dependencies: the values of the counters have to be correlated to
the time interval they refer to.
o Organization of Results: the Method of Measurement produces
singletons.
o Parameters: currently, the main parameter of the method is the
time interval used to alternate the colors and read the counters.
9. IANA Considerations
This document has no IANA actions.
10. Security Considerations
This document specifies a method to perform measurements in the
context of a Service Provider's network and has not been developed to
conduct Internet measurements, so it does not directly affect
Internet security nor applications that run on the Internet.
However, implementation of this method must be mindful of security
and privacy concerns.
There are two types of security concerns: potential harm caused by
the measurements and potential harm to the measurements.
o Harm caused by the measurement: the measurements described in this
document are Passive, so there are no new packets injected into
the network causing potential harm to the network itself and to
data traffic. Nevertheless, the method implies modifications on
the fly to a header or encapsulation of the data packets: this
must be performed in a way that doesn't alter the quality of
service experienced by packets subject to measurements and that
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preserves stability and performance of routers doing the
measurements. One of the main security threats in OAM protocols
is network reconnaissance; an attacker can gather information
about the network performance by passively eavesdropping on OAM
messages. The advantage of the methods described in this document
is that the marking bits are the only information that is
exchanged between the network devices. Therefore, Passive
eavesdropping on data-plane traffic does not allow attackers to
gain information about the network performance.
o Harm to the Measurement: the measurements could be harmed by
routers altering the marking of the packets or by an attacker
injecting artificial traffic. Authentication techniques, such as
digital signatures, may be used where appropriate to guard against
injected traffic attacks. Since the measurement itself may be
affected by routers (or other network devices) along the path of
IP packets intentionally altering the value of marking bits of
packets, as mentioned above, the mechanism specified in this
document can be applied just in the context of a controlled
domain; thus, the routers (or other network devices) are locally
administered and this type of attack can be avoided.
It is worth highlighting that an attacker can't gain information
about network performance from a single monitoring point; it must use
synchronized monitoring points at multiple points on the path,
because they have to do the same kind of measurement and aggregation
that Service Providers using Alternate Marking must do.
Attacks on the data collection and reporting of the statistics
between the monitoring points and the network management system can
interfere with the proper functioning of the system. Hence, the
channels used to report back flow statistics MUST be secured.
The privacy concerns of network measurement are limited because the
method only relies on information contained in the header or
encapsulation without any release of user data. Although information
in the header or encapsulation is metadata that can be used to
compromise the privacy of users, the limited marking technique in
this document seems unlikely to substantially increase the existing
privacy risks from header or encapsulation metadata. It might be
theoretically possible to modulate the marking to serve as a covert
channel, but it would have a very low data rate if it is to avoid
adversely affecting the measurement systems that monitor the marking.
Delay attacks are another potential threat in the context of this
document. Delay measurement is performed using a specific packet in
each block, marked by a dedicated color bit. Therefore, a
man-in-the-middle attacker can selectively induce synthetic delay
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only to delay-colored packets, causing systematic error in the delay
measurements. As discussed in previous sections, the methods
described in this document rely on an underlying time synchronization
protocol. Thus, by attacking the time protocol, an attacker can
potentially compromise the integrity of the measurement. A detailed
discussion about the threats against time protocols and how to
mitigate them is presented in RFC 7384 [RFC7384].
11. Contributors
Xiao Min
ZTE Corp.
Email: xiao.min2@zte.com.cn
Mach(Guoyi) Chen
Huawei Technologies
Email: mach.chen@huawei.com
Alessandro Capello
Telecom Italia
Email: alessandro.capello@telecomitalia.it
12. Acknowledgements
The authors would like to thank Alberto Tempia Bonda, Luca
Castaldelli and Lianshu Zheng for their contribution to the
experimentation of the method.
The authors would also thank Martin Duke and Tommy Pauly for their
assistance and their detailed and precious reviews.
13. References
13.1. Normative References
[IEEE-1588]
IEEE, "IEEE Standard for a Precision Clock Synchronization
Protocol for Networked Measurement and Control Systems",
IEEE Std 1588-2008.
[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>.
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[RFC5905] Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
"Network Time Protocol Version 4: Protocol and Algorithms
Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010,
<https://www.rfc-editor.org/info/rfc5905>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
13.2. Informative References
[I-D.fioccola-rfc8889bis]
Fioccola, G., Cociglio, M., Sapio, A., Sisto, R., and T.
Zhou, "Multipoint Alternate-Marking Method", draft-
fioccola-rfc8889bis-03 (work in progress), February 2022.
[I-D.mizrahi-ippm-marking]
Mizrahi, T., Fioccola, G., Cociglio, M., Chen, M., and G.
Mirsky, "Marking Methods for Performance Measurement",
draft-mizrahi-ippm-marking-00 (work in progress), October
2021.
[I-D.zhou-ippm-enhanced-alternate-marking]
Zhou, T., Fioccola, G., Liu, Y., Cociglio, M., Lee, S.,
and W. Li, "Enhanced Alternate Marking Method", draft-
zhou-ippm-enhanced-alternate-marking-09 (work in
progress), February 2022.
[IEEE-Network-PNPM]
IEEE Network, "AM-PM: Efficient Network Telemetry using
Alternate Marking", DOI 10.1109/MNET.2019.1800152, 2019.
[RFC3393] Demichelis, C. and P. Chimento, "IP Packet Delay Variation
Metric for IP Performance Metrics (IPPM)", RFC 3393,
DOI 10.17487/RFC3393, November 2002,
<https://www.rfc-editor.org/info/rfc3393>.
[RFC4656] Shalunov, S., Teitelbaum, B., Karp, A., Boote, J., and M.
Zekauskas, "A One-way Active Measurement Protocol
(OWAMP)", RFC 4656, DOI 10.17487/RFC4656, September 2006,
<https://www.rfc-editor.org/info/rfc4656>.
[RFC5357] Hedayat, K., Krzanowski, R., Morton, A., Yum, K., and J.
Babiarz, "A Two-Way Active Measurement Protocol (TWAMP)",
RFC 5357, DOI 10.17487/RFC5357, October 2008,
<https://www.rfc-editor.org/info/rfc5357>.
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[RFC5481] Morton, A. and B. Claise, "Packet Delay Variation
Applicability Statement", RFC 5481, DOI 10.17487/RFC5481,
March 2009, <https://www.rfc-editor.org/info/rfc5481>.
[RFC6374] Frost, D. and S. Bryant, "Packet Loss and Delay
Measurement for MPLS Networks", RFC 6374,
DOI 10.17487/RFC6374, September 2011,
<https://www.rfc-editor.org/info/rfc6374>.
[RFC6390] Clark, A. and B. Claise, "Guidelines for Considering New
Performance Metric Development", BCP 170, RFC 6390,
DOI 10.17487/RFC6390, October 2011,
<https://www.rfc-editor.org/info/rfc6390>.
[RFC6703] Morton, A., Ramachandran, G., and G. Maguluri, "Reporting
IP Network Performance Metrics: Different Points of View",
RFC 6703, DOI 10.17487/RFC6703, August 2012,
<https://www.rfc-editor.org/info/rfc6703>.
[RFC7276] Mizrahi, T., Sprecher, N., Bellagamba, E., and Y.
Weingarten, "An Overview of Operations, Administration,
and Maintenance (OAM) Tools", RFC 7276,
DOI 10.17487/RFC7276, June 2014,
<https://www.rfc-editor.org/info/rfc7276>.
[RFC7384] Mizrahi, T., "Security Requirements of Time Protocols in
Packet Switched Networks", RFC 7384, DOI 10.17487/RFC7384,
October 2014, <https://www.rfc-editor.org/info/rfc7384>.
[RFC7679] Almes, G., Kalidindi, S., Zekauskas, M., and A. Morton,
Ed., "A One-Way Delay Metric for IP Performance Metrics
(IPPM)", STD 81, RFC 7679, DOI 10.17487/RFC7679, January
2016, <https://www.rfc-editor.org/info/rfc7679>.
[RFC7680] Almes, G., Kalidindi, S., Zekauskas, M., and A. Morton,
Ed., "A One-Way Loss Metric for IP Performance Metrics
(IPPM)", STD 82, RFC 7680, DOI 10.17487/RFC7680, January
2016, <https://www.rfc-editor.org/info/rfc7680>.
[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>.
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[RFC8799] Carpenter, B. and B. Liu, "Limited Domains and Internet
Protocols", RFC 8799, DOI 10.17487/RFC8799, July 2020,
<https://www.rfc-editor.org/info/rfc8799>.
Appendix A. Changes Log
Changes from RFC 8321 include:
o Minor editorial changes
o Replacement of the section on "Applications, Implementation, and
Deployment" with "Finding of the Alternate Marking Implementations
and Deployments"
o Moved advantages and benefits of the method from "Introduction" to
the new section on "Finding of the Alternate Marking
Implementations and Deployments"
o Removed section on "Hybrid Measurement"
Changes in v-(01) include:
o Considerations on the reference: [IEEE-Network-PNPM]
o Clarified that the method based on a fixed timer is specified in
this document while the method based on a fixed number of packets
is only mentioned but not detailed.
o Explanation of the the intrinsic error in section 3.3.1 on
"Single-Marking Methodology"
o Deleted some parts in section 4 "Considerations" that no longer
apply
o New section on "Packet Fragmentation"
Changes in v-(02) include:
o Considerations on how to handle unmarked traffic in section 5 on
"Results of the Alternate Marking Experiment"
o Minor rewording in section 4.4 on "Packet Fragmentation"
Changes in v-(03) include:
o Deleted numeric examples in sections on "Packet Loss Measurement"
and on "Single-Marking Methodology"
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o New section on "Alternate Marking Functions"
o Moved sections 3.1.1 on "Coloring the Packets", 3.1.2 on "Counting
the Packets" and 3.1.3 on "Collecting Data and Calculating Packet
Loss" into the new section on "Alternate Marking Functions"
o Renamed sections 4.1 as "Marking the Packets", 4.2 as "Counting
and Timestamping Packets" and 4.3 as "Data Collection and
Correlation"
o Merged old section on "Data Correlation" with section 4.3 on "Data
Collection and Correlation"
o Moved and renamed section on "Timing Aspects" as "Synchronization
and Timing"
o Merged old section on "Synchronization" with section on
"Synchronization and Timing"
o Merged old section on "Packet Reordering" with section on
"Synchronization and Timing"
Changes in v-(04) include:
o Revised "Introduction" section
o Revised sections 4.2 "Counting and Timestamping Packets" and 4.3
on "Data Collection and Correlation"
o Revised section 5 on "Synchronization and Timing"
Authors' Addresses
Giuseppe Fioccola (editor)
Huawei Technologies
Riesstrasse, 25
Munich 80992
Germany
Email: giuseppe.fioccola@huawei.com
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Mauro Cociglio
Telecom Italia
Via Reiss Romoli, 274
Torino 10148
Italy
Email: mauro.cociglio@telecomitalia.it
Greg Mirsky
Ericsson
Email: gregimirsky@gmail.com
Tal Mizrahi
Huawei Technologies
Email: tal.mizrahi.phd@gmail.com
Tianran Zhou
Huawei Technologies
156 Beiqing Rd.
Beijing 100095
China
Email: zhoutianran@huawei.com
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