RFC9341: Alternate-Marking Method
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Obsoletes:
RFC8321
Internet Engineering Task Force (IETF) G. Fioccola, Ed.
Request for Comments: 9341 Huawei Technologies
Obsoletes: 8321 M. Cociglio
Category: Standards Track Telecom Italia
ISSN: 2070-1721 G. Mirsky
Ericsson
T. Mizrahi
T. Zhou
Huawei Technologies
December 2022
Alternate-Marking Method
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. According to the classification defined in RFC 7799, it
could be considered Passive or Hybrid depending on the application.
This document obsoletes RFC 8321.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9341.
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Table of Contents
1. Introduction
1.1. Summary of Changes from RFC 8321
1.2. Requirements Language
2. Overview of the Method
3. Detailed Description of the Method
3.1. Packet-Loss Measurement
3.2. One-Way Delay Measurement
3.2.1. Single-Marking Methodology
3.2.2. Double-Marking Methodology
3.3. Delay Variation Measurement
4. Alternate-Marking Functions
4.1. Marking the Packets
4.2. Counting and Timestamping Packets
4.3. Data Collection and Correlation
5. Synchronization and Timing
6. Packet Fragmentation
7. Recommendations for Deployment
7.1. Controlled Domain Requirement
8. Compliance with Guidelines from RFC 6390
9. IANA Considerations
10. Security Considerations
11. References
11.1. Normative References
11.2. Informative References
Acknowledgements
Contributors
Authors' Addresses
1. Introduction
Most Service Providers' networks carry traffic with contents that are
highly sensitive to packet loss [RFC7680], delay [RFC7679], and
jitter [RFC3393].
Methodologies and tools are therefore needed to monitor and
accurately measure network performance, in order to constantly
control the quality of experience perceived by the end customers.
Performance monitoring also provides useful information for improving
network management (e.g., isolation of network problems,
troubleshooting, etc.).
[RFC7799] defines Active, Passive, and Hybrid Methods of Measurement.
In particular, Active Methods of Measurement depend on a dedicated
measurement packet stream; 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.
This document proposes a performance monitoring technique, called the
"Alternate-Marking Method", which is potentially applicable to any
kind of packet-based traffic, both point-to-point unicast and
multicast, including Ethernet, IP, and MPLS. The method primarily
addresses packet-loss measurement, but it can be easily extended to
one-way or two-way delay and delay variation measurements as well.
The Alternate-Marking methodology, described in this document, allows
the synchronization of the measurements at different points by
dividing the packet flow into batches. So it is possible to get
coherent counters and timestamps in every marking period and
therefore measure the Performance Metrics for the monitored flow.
The method has been explicitly designed for Passive or Hybrid
measurements as stated in [RFC8321]. But, according to the
definitions of [RFC7799], the Alternate-Marking Method can be
classified as Hybrid Type I. Indeed, Alternate Marking can be
implemented by using reserved bits in the protocol header, and the
change in value of these marking bits at the domain edges (and not
along the path) is formally considered a modification of the stream
of interest.
It is worth mentioning that this is a methodology document, so the
mechanism that can be used to transmit the counters and the
timestamps is out of scope here. Additional details about the
applicability of the Alternate-Marking methodology are described in
[IEEE-NETWORK-PNPM].
1.1. Summary of Changes from RFC 8321
This document defines the Alternate-Marking Method, addressing
ambiguities and building on its experimental phase that was based on
the original specification [RFC8321].
The relevant changes are:
* Added the recommendations about the methods to employ in case one
or two flag bits are available for marking (Section 7).
* Changed the structure to improve the readability.
* Removed the wording about the initial experiments of the method
and considerations that no longer apply.
* Extended the description of detailed aspects of the methodology,
e.g., synchronization, timing, packet fragmentation, and marked
and unmarked traffic handling.
It is important to note that all the changes are totally backward
compatible with [RFC8321] and no new additional technique has been
introduced in this document compared to [RFC8321].
1.2. 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.
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 notional "color",
whilst consecutive blocks will have different colors. Each change of
color represents a sort of auto-synchronization signal that enhances
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
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 alternate 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:
* 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
* 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 use of a fixed timer for the creation of blocks is REQUIRED when
implementing this specification. The switching after a fixed number
of packets is an additional possibility, but its detailed
specification is out of scope. An example of application is in
[EXPLICIT-FLOW-MEASUREMENTS].
The following figure shows how a flow appears when it is split into
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.
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.
The 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
(Section 5). The drawback is that the longer the duration of the
block, the less frequently the measurement can be taken.
Two different strategies that can be used when implementing the
method are:
flow-based: the flow-based strategy is used when well-defined
traffic flows need to be monitored. According to this strategy,
only the specified flow 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. With this
approach, it is necessary to know in advance the path followed by
flows that are subject to measurement. Path rerouting and traffic
load balancing need to be taken into account.
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.
The flow-based strategy is REQUIRED when implementing this
specification. It 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 of 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 on
every path: counters on interfaces traversed by the flow will report
packet count, whereas counters on other interfaces will be null.
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 two methodologies, 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). The Network Time Protocol (NTP) [RFC5905] or
the IEEE 1588 Precision Time Protocol (PTP) [IEEE-1588] (as discussed
in the previous section) can be used for the timestamp formats
depending on the needed precision.
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), in the case where no packet loss or misordering
exists, we would be 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 (Section 5).
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.
3.2.1.1. Mean Delay
The method previously exposed for measuring the delay is sensitive to
out-of-order reception of packets. In order to overcome this
problem, an approach based on the concept of mean delay can be
considered. 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. This method greatly reduces the number of
timestamps that have to be collected (only one per block for each
network device), and it is robust to out-of-order packets with only a
small error introduced in case of packet loss. But, when computing
the mean delay, the measurement error could be augmented by
accumulating the measurement error of a lot of packets.
Additionally, 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 minutes to seconds), which
implies a highly optimized implementation of the method. For this
reason, the mean delay calculation may not be so viable in some
cases.
3.2.2. Double-Marking Methodology
As mentioned above, the Single-Marking methodology for one-way delay
measurement has some limitations, since it is sensitive to out-of-
order reception of packets, and even the mean delay calculation is
limited because it doesn't give information about the delay value's
distribution for the duration of the block. Actually, 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
statistical 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, and 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 may be used for 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 the next node to compute packet
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 an adequate time gap to avoid out-of-order
issues and also to have a number of measurement packets that are rate
independent. This gap may be, at the minimum, the mean network delay
calculated with the previous methodology. Therefore, it is possible
to choose a proper time gap to guarantee a fixed number of double-
marked packets uniformly spaced in each block. If packets with the
second marking are lost, it is easy to recognize the loss since the
number of double-marked packets is known for each block. Based on
the spacing between these packets, it can also be possible to
understand which packet of the second marking sequence has been lost
and perform the measurements only for the remaining packets. But
this may be complicated if more packets are lost. In this case, an
implementation may simply discard the delay measurements for the
corrupted block and proceed with the next block.
An efficient and robust mode is to select a single packet with the
second marking for each block; in this way, there is no time gap to
consider between the double-marked packets to avoid their reorder.
In addition, it is also easier to identify the only double-marked
packet in each block and skip the delay measurement for the block if
it is lost.
The Double-Marking methodology can also be used to get more
statistics of delay extent data, e.g., percentiles, variance, and
median delay values. Indeed, a subset of batch packets is selected
for extensive delay calculation by using the second marking, and it
is possible to perform a detailed analysis on these double-marked
packets. It is worth noting that there are classic algorithms for
median and variance calculation, but they are out of the scope of
this document. The conventional range (maximum-minimum) should be
avoided for several reasons, including stability of the maximum delay
due to the influence by outliers. In this regard, Section 6.5 of
[RFC5481] highlights how the 99.9th percentile of delay and delay
variation is more helpful to performance planners.
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 [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.
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 nodes involved, 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, [RFC9342] 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 (inclusive) 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
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, [RFC9342]
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 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. 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:
* Use of a centralized solution using the 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.
* 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 [RFC5357] or the One-Way Active Measurement Protocol
(OWAMP) as defined in [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 MUST 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 NTP [RFC5905] or the IEEE 1588 PTP
[IEEE-1588]).
5. Synchronization and Timing
Color switching is the reference for all the network devices acting
as measurement points, and the only requirement to be achieved is
that they have to recognize the right batch along the path in order
to get the related information of counters and timestamps.
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 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 packets because
each 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
normal distribution and 99.7% of the samples are within 3 standard
deviations 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, which must be > 0.
The condition that MUST be satisfied and is a requirement on the
synchronization accuracy is:
d < L/2.
This is the fundamental rule for deciding when to read the counters
and when to select the packets to be double-marked; indeed, packet
counters and double-marked packets MUST respectively be taken and
chosen within the available counting interval that is not affected by
error factors.
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.
6. Packet Fragmentation
Fragmentation can be managed with the Alternate-Marking Method using
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 SHOULD 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. Recommendations for Deployment
The methodology described in the previous sections can be applied to
various performance measurement problems. 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. [RFC8321] reports
experimental examples, and [IEEE-NETWORK-PNPM] also includes some
information about the deployment experience.
Either one or two flag bits might be available for marking in
different deployments:
One flag: packet-loss measurement MUST be done as described in
Section 3.1, while delay measurement MUST be done according to the
Single-Marking Method described in Section 3.2.1. Mean delay
(Section 3.2.1.1) MAY also be used but it could imply more
computational load.
Two flags: packet-loss measurement MUST be done as described in
Section 3.1, while delay measurement MUST be done according to the
Double-Marking Method as described in 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.
There are some operational guidelines to consider for the purpose of
deciding to follow the recommendations above and to use one or two
flags.
* The Alternate-Marking Method utilizes specific flags in the packet
header, so an important factor is the number of flags available
for the implementation. Indeed, if there is only one flag
available, then there is no other way; if two flags are available,
then the option with two flags is certainly more complete.
* The duration of the Alternate-Marking period affects the frequency
of the measurement, and this is a parameter that can be decided on
the basis of the required temporal sampling. But it cannot be
freely chosen, as explained in Section 5.
* The Alternate-Marking methodologies enable packet loss, delay, and
delay variation calculation, but in accordance with the method
used (e.g., Single Marking or Double Marking), there is a
different kind of information that can be derived. For example,
to get more statistics of extent data, the option with two flags
is desirable. For this reason, the type of data needed in the
specific scenario is an additional element to take into account.
* The Alternate-Marking Methods imply different computational load
depending on the method employed. Therefore, the available
computational resources on the measurement points can also
influence the choice. As an example, mean delay calculation may
require more processing, and it may not be the best option to
minimize the computational load.
The experiment with Alternate-Marking methodologies confirmed the
benefits already described in [RFC8321].
A deployment of the Alternate-Marking Method should also take into
account how to handle and recognize marked and unmarked traffic.
Since Alternate Marking normally employs a marking field that 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.
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
to 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 to ensure
authentication, encryption, and integrity protection if traffic
traverses the Internet.
For security reasons, the Alternate-Marking Method MUST only be
applied to controlled domains.
8. Compliance with Guidelines from 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. The mechanisms described in this document use a combination
of the Performance Metric Definition template defined in Section 5.4
of [RFC6390] and the Dependencies laid out in Section 5.5 of that
document.
* 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.
* 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.
* 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.
* 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.
* 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 to 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.
* 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.
* Verification: the methodology has been tested and deployed
experimentally in both lab and operational network scenarios
performing packet loss and delay measurements on traffic patterns
created by traffic generators together with precision test
instruments and network emulators.
* 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.
* 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.
* Dependencies: the values of the counters have to be correlated to
the time interval they refer to.
* Organization of Results: the Method of Measurement produces
singletons, according to the definition of [RFC2330].
* Parameters: the main parameters of the method are the information
about the flow or the link to be measured, the time interval
chosen to alternate the colors and to read the counters, and the
type of method selected for packet-loss and delay measurements.
9. IANA Considerations
This document has no IANA actions.
10. Security Considerations
This document specifies a method to perform measurements that 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.
* 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
preserves stability and performance of routers doing the
measurements. One of the main security threats in Operations,
Administration, and Maintenance (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.
* 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.
An attacker that does not belong to the controlled domain can
maliciously send marked packets. However, no problems occur if
Alternate Marking is not supported in the controlled domain. If
Alternate Marking is supported in the controlled domain, it is
necessary to keep the measurements from being affected; therefore,
externally marked packets must be checked to see if they are marked
and eventually filtered or cleared.
The precondition for the application of the Alternate-Marking Method
is that it MUST be applied in specific controlled domains, thus
confining the potential attack vectors within the network domain. A
limited administrative domain provides the network administrator with
the means to select, monitor, and control the access to the network,
making it a trusted domain. In this regard, it is expected to
enforce policies at the domain boundaries to filter both external
marked packets entering the domain and internal marked packets
leaving the domain. Therefore, the trusted domain is unlikely
subject to the hijacking of packets since marked packets are
processed and used only within the controlled domain. But despite
that, leakages may happen for different reasons, such as a failure or
a fault. In this case, nodes outside the domain are expected to
ignore marked packets since they are not configured to handle it and
should not process it.
It might be theoretically possible to modulate the marking to serve
as a covert channel to be used by an on-path observer. This may
affect both the data and management plane, but, here too, the
application to a controlled domain helps to reduce the effects.
It is worth highlighting that an attacker can't gain information
about network performance from a single monitoring point; they 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 NMS 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, an on-path
attacker can selectively induce synthetic delay 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 [RFC7384].
11. References
11.1. Normative References
[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>.
[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>.
[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>.
[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>.
11.2. Informative References
[EXPLICIT-FLOW-MEASUREMENTS]
Cociglio, M., Ferrieux, A., Fioccola, G., Lubashev, I.,
Bulgarella, F., Nilo, M., Hamchaoui, I., and R. Sisto,
"Explicit Flow Measurements Techniques", Work in Progress,
Internet-Draft, draft-ietf-ippm-explicit-flow-
measurements-02, 13 October 2022,
<https://datatracker.ietf.org/doc/html/draft-ietf-ippm-
explicit-flow-measurements-02>.
[IEEE-1588]
IEEE, "IEEE Standard for a Precision Clock Synchronization
Protocol for Networked Measurement and Control Systems",
IEEE Std 1588-2008, DOI 10.1109/IEEESTD.2008.4579760, July
2008, <https://doi.org/10.1109/IEEESTD.2008.4579760>.
[IEEE-NETWORK-PNPM]
Mizrahi, T., Navon, G., Fioccola, G., Cociglio, M., Chen,
M., and G. Mirsky, "AM-PM: Efficient Network Telemetry
using Alternate Marking", IEEE Network Vol. 33, Issue 4,
DOI 10.1109/MNET.2019.1800152, July 2019,
<https://doi.org/10.1109/MNET.2019.1800152>.
[RFC2330] Paxson, V., Almes, G., Mahdavi, J., and M. Mathis,
"Framework for IP Performance Metrics", RFC 2330,
DOI 10.17487/RFC2330, May 1998,
<https://www.rfc-editor.org/info/rfc2330>.
[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>.
[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>.
[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>.
[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>.
[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>.
[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>.
[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>.
[RFC9342] Fioccola, G., Ed., Cociglio, M., Sapio, A., Sisto, R., and
T. Zhou, "Clustered Alternate-Marking Method", RFC 9342,
DOI 10.17487/RFC9342, December 2022,
<https://www.rfc-editor.org/info/rfc9342>.
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 like to thank Martin Duke and Tommy Pauly for
their assistance and their detailed and precious reviews.
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
Authors' Addresses
Giuseppe Fioccola (editor)
Huawei Technologies
Riesstrasse, 25
80992 Munich
Germany
Email: giuseppe.fioccola@huawei.com
Mauro Cociglio
Telecom Italia
Email: mauro.cociglio@outlook.com
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