💾 Archived View for gemini.bortzmeyer.org › rfc-mirror › rfc8321.txt captured on 2023-04-26 at 13:55:37.
⬅️ Previous capture (2021-11-30)
-=-=-=-=-=-=-
Internet Engineering Task Force (IETF) G. Fioccola, Ed.
Request for Comments: 8321 A. Capello
Category: Experimental M. Cociglio
ISSN: 2070-1721 L. Castaldelli
Telecom Italia
M. Chen
L. Zheng
Huawei Technologies
G. Mirsky
ZTE
T. Mizrahi
Marvell
January 2018
Alternate-Marking Method for Passive and Hybrid Performance Monitoring
Abstract
This document describes a method to perform packet loss, delay, and
jitter measurements on live traffic. This method is based on an
Alternate-Marking (coloring) technique. A report is provided in
order to explain an example and show the method applicability. This
technology can be applied in various situations, as detailed in this
document, and could be considered Passive or Hybrid depending on the
application.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for examination, experimental implementation, and
evaluation.
This document defines an Experimental Protocol for the Internet
community. This document is a product of the Internet Engineering
Task Force (IETF). It represents the consensus of the IETF
community. It has received public review and has been approved for
publication by the Internet Engineering Steering Group (IESG). Not
all documents approved by the IESG are a candidate for any level of
Internet Standard; see Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8321.
Fioccola, et al. Experimental [Page 1]
RFC 8321 Alternate-Marking Method January 2018
Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 5
2. Overview of the Method . . . . . . . . . . . . . . . . . . . 5
3. Detailed Description of the Method . . . . . . . . . . . . . 6
3.1. Packet Loss Measurement . . . . . . . . . . . . . . . . . 6
3.1.1. Coloring the Packets . . . . . . . . . . . . . . . . 11
3.1.2. Counting the Packets . . . . . . . . . . . . . . . . 12
3.1.3. Collecting Data and Calculating Packet Loss . . . . . 13
3.2. Timing Aspects . . . . . . . . . . . . . . . . . . . . . 13
3.3. One-Way Delay Measurement . . . . . . . . . . . . . . . . 15
3.3.1. Single-Marking Methodology . . . . . . . . . . . . . 15
3.3.2. Double-Marking Methodology . . . . . . . . . . . . . 17
3.4. Delay Variation Measurement . . . . . . . . . . . . . . . 18
4. Considerations . . . . . . . . . . . . . . . . . . . . . . . 18
4.1. Synchronization . . . . . . . . . . . . . . . . . . . . . 19
4.2. Data Correlation . . . . . . . . . . . . . . . . . . . . 19
4.3. Packet Reordering . . . . . . . . . . . . . . . . . . . . 20
5. Applications, Implementation, and Deployment . . . . . . . . 21
5.1. Report on the Operational Experiment . . . . . . . . . . 22
5.1.1. Metric Transparency . . . . . . . . . . . . . . . . . 24
6. Hybrid Measurement . . . . . . . . . . . . . . . . . . . . . 24
7. Compliance with Guidelines from RFC 6390 . . . . . . . . . . 25
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 27
9. Security Considerations . . . . . . . . . . . . . . . . . . . 27
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 28
10.1. Normative References . . . . . . . . . . . . . . . . . . 28
10.2. Informative References . . . . . . . . . . . . . . . . . 29
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 32
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 32
Fioccola, et al. Experimental [Page 2]
RFC 8321 Alternate-Marking Method January 2018
1. Introduction
Nowadays, most Service Providers' networks carry traffic with
contents that are highly sensitive to packet loss [RFC7680], delay
[RFC7679], and jitter [RFC3393].
In view of this scenario, Service Providers need methodologies and
tools to monitor and measure network performance with an adequate
accuracy, in order to constantly control the quality of experience
perceived by their customers. On the other hand, performance
monitoring provides useful information for improving network
management (e.g., isolation of network problems, troubleshooting,
etc.).
A lot of work related to Operations, Administration, and Maintenance
(OAM), which also includes performance monitoring techniques, has
been done by Standards Developing Organizations (SDOs): [RFC7276]
provides a good overview of existing 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 a minor effort
has been thus far dedicated to performance monitoring. The IPPM WG
has defined standard metrics to measure network performance; however,
the methods developed in this WG mainly refer to focus on Active
measurement techniques. More recently, the MPLS WG has defined
mechanisms for measuring packet loss, one-way and two-way delay, and
delay variation in MPLS networks [RFC6374], but their applicability
to Passive measurements has some limitations, especially for pure
connection-less networks.
The lack of adequate tools to measure packet loss with the desired
accuracy drove an effort to design a new method for the performance
monitoring of live traffic, which is easy to implement and deploy.
The effort led to the method described in this document: basically,
it is 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.
Fioccola, et al. Experimental [Page 3]
RFC 8321 Alternate-Marking Method January 2018
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.
Taking into consideration these definitions, 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 (e.g., the Differentiated Services Code
Point (DSCP) field), 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 (e.g., Synonymous Flow Label as described in [SFL-FRAMEWORK]
or OAM Marking Bits as described in [PM-MM-BIER]).
The advantages of the method described in this document are:
o easy implementation: it can be implemented by using features
already available on major routing platforms, as described in
Section 5.1, 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 packet loss measurement: 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 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 to experiment
and test the method (as described in Section 5.1) are available on
all current routing platforms. Both a centralized or distributed
solution can be used to harvest data from the routers.
Fioccola, et al. Experimental [Page 4]
RFC 8321 Alternate-Marking Method January 2018
The method doesn't raise any specific need for protocol extension,
but it could be further improved by means of some extension to
existing protocols. Specifically, the use of Diffserv bits for
coloring the packets could not be a viable solution in some cases: a
standard method to color the packets for this specific application
could be beneficial.
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.
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.
Fioccola, et al. Experimental [Page 5]
RFC 8321 Alternate-Marking Method January 2018
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
(see also [IP-MULTICAST-PM] and [OPSAWG-P3M], where this technique
was introduced).
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
packet loss occurred in any single block between any two points.
Fioccola, et al. Experimental [Page 6]
RFC 8321 Alternate-Marking Method January 2018
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
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 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.
Fioccola, et al. Experimental [Page 7]
RFC 8321 Alternate-Marking Method January 2018
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 down 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
still counter of the previous color, so the possibility of reading a
running counter instead of a still one is minimized. The drawback is
that the longer the duration of the block, the less frequent the
measurement can be taken.
Fioccola, et al. Experimental [Page 8]
RFC 8321 Alternate-Marking Method January 2018
The following table shows how the counters can be used to calculate
the packet loss between R1 and R2. The first column lists the
sequence of traffic blocks, while the other columns contain the
counters of A-colored packets and B-colored packets for R1 and R2.
In this example, we assume that the values of the counters are reset
to zero whenever a block ends and its associated counter has been
read: with this assumption, the table shows only relative values,
which is the exact number of packets of each color within each block.
If the values of the counters were not reset, the table would contain
cumulative values, but the relative values could be determined simply
by the difference from the value of the previous block of the same
color.
The color is switched on the basis of a fixed timer (not shown in the
table), so the number of packets in each block is different.
+-------+--------+--------+--------+--------+------+
| Block | C(A)R1 | C(B)R1 | C(A)R2 | C(B)R2 | Loss |
+-------+--------+--------+--------+--------+------+
| 1 | 375 | 0 | 375 | 0 | 0 |
| 2 | 0 | 388 | 0 | 388 | 0 |
| 3 | 382 | 0 | 381 | 0 | 1 |
| 4 | 0 | 377 | 0 | 374 | 3 |
| ... | ... | ... | ... | ... | ... |
| 2n | 0 | 387 | 0 | 387 | 0 |
| 2n+1 | 379 | 0 | 377 | 0 | 2 |
+-------+--------+--------+--------+--------+------+
Table 1: Evaluation of Counters for Packet Loss Measurements
During an A block (blocks 1, 3, and 2n+1), all the packets are
A-colored; therefore, the C(A) counters are incremented to the number
seen on the interface, while C(B) counters are zero. Conversely,
during a B block (blocks 2, 4, and 2n), all the packets are
B-colored: C(A) counters are zero, while C(B) counters are
incremented.
When a block ends (because of color switching), the relative counters
stop incrementing; it is possible to read them, compare the values
measured on routers R1 and R2, and calculate the packet loss within
that block.
For example, looking at the table above, during the first block
(A-colored), C(A)R1 and C(A)R2 have the same value (375), which
corresponds to the exact number of packets of the first block (no
loss). Also, during the second block (B-colored), R1 and R2 counters
have the same value (388), which corresponds to the number of packets
of the second block (no loss). During the third and fourth blocks,
Fioccola, et al. Experimental [Page 9]
RFC 8321 Alternate-Marking Method January 2018
R1 and R2 counters are different, meaning that some packets have been
lost: in the example, one single packet (382-381) was lost during
block three, and three packets (377-374) were lost during block four.
The method applied to R1 and R2 can be extended to any other router
and applied to more complex networks, as far as the measurement is
enabled on the path followed by the traffic flow(s) being observed.
It's worth mentioning 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, a couple of
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
Fioccola, et al. Experimental [Page 10]
RFC 8321 Alternate-Marking Method January 2018
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.
3.1.1. Coloring the Packets
The coloring operation is fundamental in order to create packet
blocks. 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 3.2; so every node that is designated as a measurement
point along the path should be able to identify unambiguously the
colored packets. Furthermore, [MULTIPOINT-ALT-MM] 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 a specific bit 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. However, some applications are reported in Section 5.
Fioccola, et al. Experimental [Page 11]
RFC 8321 Alternate-Marking Method January 2018
3.1.2. Counting the 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 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: one counter for packets with color A and one counter for
packets with color B. For each flow (or group of flows) being
monitored and for every interface where the monitoring is Active, a
couple of counters are needed. For example, in order to separately
monitor three flows on a router with four interfaces involved, 24
counters are needed (two counters for each of the three flows on each
of the four interfaces). Furthermore, [MULTIPOINT-ALT-MM]
generalizes the counting for multipoint-to-multipoint flow.
In case of link-based measurements, the behavior is similar except
that coloring and counting 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: in order to count the exact number of packets of a
block, the routers must perform this operation when that block has
ended; in other words, the counter for color A must be read when the
current block has color B, in order to be sure that the value of the
counter is stable. This task can be accomplished in two ways. The
general approach suggests reading the counters periodically, many
times during a block duration, and comparing these successive
readings: when the counter stops incrementing, it means that the
current block has ended, and its value can be elaborated safely.
Alternatively, if the coloring operation is performed on the basis of
a fixed timer, it is possible to configure the reading of the
counters according to that timer: for example, reading the counter
for color A every period in the middle of the subsequent block with
color B is a safe choice. A sufficient margin should be considered
between the end of a block and the reading of the counter, in order
to take into account any out-of-order packets.
Fioccola, et al. Experimental [Page 12]
RFC 8321 Alternate-Marking Method January 2018
3.1.3. Collecting Data and Calculating Packet Loss
The nodes enabled to perform performance monitoring collect the value
of the counters, but they are not able to directly use this
information to measure packet loss, because they only have their own
samples. For this reason, an external Network Management System
(NMS) can be used to collect and elaborate data and to perform packet
loss calculation. The NMS compares the values of counters from
different nodes and can calculate if some packets were lost (even a
single packet) and where those packets were lost.
The value of the counters needs to be transmitted to the NMS as soon
as it has been read. This can be accomplished by using SNMP or FTP
and 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.
It would also be possible to use a protocol to exchange values of
counters between the two endpoints in order to let them perform the
packet loss calculation for each traffic direction.
A possible approach for the performance measurement (PM) architecture
is explained in [COLORING], while [IP-FLOW-REPORT] introduces new
information elements of IP Flow Information Export (IPFIX) [RFC7011].
3.2. Timing Aspects
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 will be considered in the rest of the
document.
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. But, to implement the methodology, they must be synchronized to
the same clock reference with an accuracy of +/- L/2 time units,
where L is the fixed time duration of the block. So each colored
packet can be assigned to the right batch by each router. This is
because the minimum time distance between two packets of the same
color but that belong to different batches is L time units.
Fioccola, et al. Experimental [Page 13]
RFC 8321 Alternate-Marking Method January 2018
In practice, in addition to clock errors, the delay between
measurement points also affects the implementation of the methodology
because each packet can be delayed differently, and this can produce
out of order at batch boundaries. This means that, without
considering clock error, we wait L/2 after color switching to be sure
to take a still counter.
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
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 guard band d is given by:
d = A + D_max - D_min,
where A is the clock accuracy, D_max is an upper bound on the network
delay between the network devices, and D_min is a lower bound on 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.
Fioccola, et al. Experimental [Page 14]
RFC 8321 Alternate-Marking Method January 2018
3.3. 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).
3.3.1. 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.
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).
The following table shows how timestamps can be used to calculate the
delay between R1 and R2. The first column lists the sequence of
blocks, while other columns contain the timestamp referring to the
first packet of each block on R1 and R2. The delay is computed as a
difference between timestamps. For the sake of simplicity, all the
values are expressed in milliseconds.
Fioccola, et al. Experimental [Page 15]
RFC 8321 Alternate-Marking Method January 2018
+-------+---------+---------+---------+---------+-------------+
| Block | TS(A)R1 | TS(B)R1 | TS(A)R2 | TS(B)R2 | Delay R1-R2 |
+-------+---------+---------+---------+---------+-------------+
| 1 | 12.483 | - | 15.591 | - | 3.108 |
| 2 | - | 6.263 | - | 9.288 | 3.025 |
| 3 | 27.556 | - | 30.512 | - | 2.956 |
| | - | 18.113 | - | 21.269 | 3.156 |
| ... | ... | ... | ... | ... | ... |
| 2n | 77.463 | - | 80.501 | - | 3.038 |
| 2n+1 | - | 24.333 | - | 27.433 | 3.100 |
+-------+---------+---------+---------+---------+-------------+
Table 2: Evaluation of Timestamps for Delay Measurements
The first row shows timestamps taken on R1 and R2, respectively, and
refers to the first packet of block 1 (which is A-colored). Delay
can be computed as a difference between the timestamp on R2 and the
timestamp on R1. Similarly, the second row shows timestamps (in
milliseconds) taken on R1 and R2 and refers to the first packet of
block 2 (which is B-colored). By comparing timestamps taken on
different nodes in the network and referring to the same packets
(identified using the alternation of colors), it is possible to
measure delay on different network segments.
For the sake of simplicity, in the above example, a single
measurement is provided within a block, taking into account only the
first packet of each block. The number of measurements can be easily
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).
3.3.1.1. 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
Fioccola, et al. Experimental [Page 16]
RFC 8321 Alternate-Marking Method January 2018
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 (for instance, 5 minutes),
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 implicates a highly optimized implementation of the
method.
3.3.2. 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
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.
Fioccola, et al. Experimental [Page 17]
RFC 8321 Alternate-Marking Method January 2018
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.
3.4. 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.
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. Considerations
This section highlights some considerations about the methodology.
Fioccola, et al. Experimental [Page 18]
RFC 8321 Alternate-Marking Method January 2018
4.1. Synchronization
The Alternate-Marking technique does not require a strong
synchronization, especially for packet loss and two-way delay
measurement. Only one-way delay measurement requires network devices
to have synchronized clocks.
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.
If the length of the measurement period is L time units, then all
network devices must be synchronized to the same clock reference with
an accuracy of +/- L/2 time units (without considering network
delay). This level of accuracy guarantees that all network devices
consistently match the color bit to the correct block. For example,
if the color is toggled every second (L = 1 second), then clocks must
be synchronized with an accuracy of +/- 0.5 second to a common time
reference.
This synchronization requirement can 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 does not require synchronization. This is because the
value of the clocks of network devices does not affect the
computation of the two-way delay measurement.
4.2. Data Correlation
Data correlation is the mechanism to compare counters and timestamps
for packet loss, delay, and delay variation calculation. It could be
performed in several ways depending on the Alternate-Marking
application and use case. Some possibilities are to:
o use a centralized solution using NMS to correlate data; and
o define a protocol-based distributed solution by introducing a new
protocol or by extending the existing protocols (e.g., see RFC
6374 [RFC6374] or the Two-Way Active Measurement Protocol (TWAMP)
as defined in RFC 5357 [RFC5357] or the One-Way Active Measurement
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.
Fioccola, et al. Experimental [Page 19]
RFC 8321 Alternate-Marking Method January 2018
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; in addition, a Block Number (BN) could be assigned to each
such measurement block. The BN is generated each time a node reads
the data (packet counts or timestamps) and is associated with each
packet count and timestamp reported to or pulled by other nodes or
NMSs. The value of a BN could be calculated as the modulo of the
local time (when the data are read) and the interval of the marking
time period.
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, i.e., these two packet counts belong to the same
block of markers from the upstream and downstream nodes. 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]). Synchronization aspects are further discussed in
Section 4.1.
4.3. Packet Reordering
Due to ECMP, packet reordering is very common in an IP network. The
accuracy of a marking-based PM, especially packet loss measurement,
may be affected by packet reordering. Take a look at the following
example:
Block : 1 | 2 | 3 | 4 | 5 |...
--------|---------|---------|---------|---------|---------|---
Node R1 : AAAAAAA | BBBBBBB | AAAAAAA | BBBBBBB | AAAAAAA |...
Node R2 : AAAAABB | AABBBBA | AAABAAA | BBBBBBA | ABAAABA |...
Figure 5: Packet Reordering
In Figure 5, the packet stream for Node R1 isn't being reordered and
can be safely assigned to interval blocks, but the packet stream for
Node R2 is being reordered; so, looking at the packet with the marker
Fioccola, et al. Experimental [Page 20]
RFC 8321 Alternate-Marking Method January 2018
of "B" in block 3, there is no safe way to tell whether the packet
belongs to block 2 or block 4.
In general, there is the need to assign packets with the marker of
"B" or "A" to the right interval blocks. Most of the packet
reordering occurs at the edge of adjacent blocks, and they are easy
to handle if the interval of each block is sufficiently large. Then,
it can be assumed that the packets with different markers belong to
the block that they are closer to. If the interval is small, it is
difficult and sometimes impossible to determine to which block a
packet belongs.
To choose a proper interval is important, and how to choose a proper
interval is out of the scope of this document. But an implementation
SHOULD provide a way to configure the interval and allow a certain
degree of packet reordering.
5. Applications, Implementation, and Deployment
The methodology described in the previous sections can be applied in
various situations. Basically, the Alternate-Marking technique could
be used in many cases for performance measurement. 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.
Some recent Alternate-Marking Method applications are listed below:
o IP Flow Performance Measurement (IPFPM): this application of the
marking method is described in [COLORING]. As an example, in this
document, the last reserved bit of the Flag field of the IPv4
header is proposed to be used for marking, while a solution for
IPv6 could be to leverage the IPv6 extension header for marking.
o OAM Passive Performance Measurement: In [RFC8296], two OAM bits
from the Bit Index Explicit Replication (BIER) header are reserved
for the Passive performance measurement marking method.
[PM-MM-BIER] details the measurement for multicast service over
the BIER domain. In addition, the Alternate-Marking Method could
also be used in a Service Function Chaining (SFC) domain. Lastly,
the application of the marking method to Network Virtualization
over Layer 3 (NVO3) protocols is considered by [NVO3-ENCAPS].
o MPLS Performance Measurement: RFC 6374 [RFC6374] uses the Loss
Measurement (LM) packet as the packet accounting demarcation
point. Unfortunately, this gives rise to a number of problems
that may lead to significant packet accounting errors in certain
situations. [MPLS-FLOW] discusses the desired capabilities for
Fioccola, et al. Experimental [Page 21]
RFC 8321 Alternate-Marking Method January 2018
MPLS flow identification in order to perform a better in-band
performance monitoring of user data packets. A method of
accomplishing identification is Synonymous Flow Labels (SFLs)
introduced in [SFL-FRAMEWORK], while [SYN-FLOW-LABELS] describes
performance measurements in RFC 6374 with SFL.
o Active Performance Measurement: [ALT-MM-AMP] describes how to
extend the existing Active Measurement Protocol, in order to
implement the Alternate-Marking methodology. [ALT-MM-SLA]
describes an extension to the Cisco SLA Protocol Measurement-Type
UDP-Measurement.
An example of implementation and deployment is explained in the next
section, just to clarify how the method can work.
5.1. Report on the Operational Experiment
The method described in this document, also called Packet Network
Performance Monitoring (PNPM), has been invented and engineered in
Telecom Italia.
It is important to highlight that the general description of the
methodology in this document is a consequence of the operational
experiment. The fundamental elements of the technique have been
tested, and the lessons learned from the operational experiment
inspired the formalization of the Alternate-Marking Method as
detailed in the previous sections.
The methodology has been used experimentally in Telecom Italia's
network and is applied to multicast IPTV channels or other specific
traffic flows with high QoS requirements (i.e., Mobile Backhauling
traffic realized with a VPN MPLS).
This technology has been employed by leveraging functions and tools
available on IP routers, and it's currently being used to monitor
packet loss in some portions of Telecom Italia's network. The
application of this method for delay measurement has also been
evaluated in Telecom Italia's labs.
This section describes how the experiment has been executed,
particularly, how the features currently available on existing
routing platforms can be used to apply the method, in order to give
an example of implementation and deployment.
The operational test, described herein, uses the flow-based strategy,
as defined in Section 3. Instead, the link-based strategy could be
applied to a physical link or a logical link (e.g., an Ethernet VLAN
or an MPLS Pseudowire (PW)).
Fioccola, et al. Experimental [Page 22]
RFC 8321 Alternate-Marking Method January 2018
The implementation of the method leverages the available router
functions, since the experiment has been done by a Service Provider
(as Telecom Italia is) on its own network. So, with current router
implementations, only QoS-related fields and features offer the
required flexibility to set bits in the packet header. In case a
Service Provider only uses the three most-significant bits of the
DSCP field (corresponding to IP Precedence) for QoS classification
and queuing, it is possible to use the two least-significant bits of
the DSCP field (bit 0 and bit 1) to implement the method without
affecting QoS policies. That is the approach used for the
experiment. One of the two bits (bit 0) could be used to identify
flows subject to traffic monitoring (set to 1 if the flow is under
monitoring, otherwise, it is set to 0), while the second (bit 1) can
be used for coloring the traffic (switching between values 0 and 1,
corresponding to colors A and B) and creating the blocks.
The experiment considers a flow as all the packets sharing the same
source IP address or the same destination IP address, depending on
the direction. In practice, once the flow has been defined, traffic
coloring using the DSCP field can be implemented by configuring an
access-list on the router output interface. The access-list
intercepts the flow(s) to be monitored and applies a policy to them
that sets the DSCP field accordingly. Since traffic coloring has to
be switched between the two values over time, the policy needs to be
modified periodically. An automatic script is used to perform this
task on the basis of a fixed timer. The automatic script is loaded
on board of the router and automatizes the basic operations that are
needed to realize the methodology.
After the traffic is colored using the DSCP field, all the routers on
the path can perform the counting. For this purpose, an access-list
that matches specific DSCP values can be used to count the packets of
the flow(s) being monitored. The same access-list can be installed
on all the routers of the path. In addition, network flow
monitoring, such as provided by IPFIX [RFC7011], can be used to
recognize timestamps of the first/last packet of a batch in order to
enable one of the alternatives to measure the delay as detailed in
Section 3.3.
In Telecom Italia's experiment, the timer is set to 5 minutes, so the
sequence of actions of the script is also executed every 5 minutes.
This value has shown to be a good compromise between measurement
frequency and stability of the measurement (i.e., the possibility of
collecting all the measures referring to the same block).
For this experiment, both counters and any other data are collected
by using the automatic script that sends these out to an NMS. The
NMS is responsible for packet loss calculation, performed by
Fioccola, et al. Experimental [Page 23]
RFC 8321 Alternate-Marking Method January 2018
comparing the values of counters from the routers along the flow
path(s). A 5-minute timer for color switching is a safe choice for
reading the counters and is also coherent with the reporting window
of the NMS.
Note that the use of the DSCP field for marking implies that the
method in this case works reliably only within a single management
and operation domain.
Lastly, the Telecom Italia experiment scales up to 1000 flows
monitored together on a single router, while an implementation on
dedicated hardware scales more, but it was tested only in labs for
now.
5.1.1. Metric Transparency
Since a Service Provider application is described here, the method
can be applied to end-to-end services supplied to customers. So it
is important to highlight that the method MUST be transparent outside
the Service Provider domain.
In Telecom Italia's implementation, the source node colors the
packets with a policy that is modified periodically via an automatic
script in order to alternate the DSCP field of the packets. The
nodes between source and destination (included) have to use an
access-list to count the colored packets that they receive and
forward.
Moreover, the destination node has an important role: the colored
packets are intercepted and a policy restores and sets the DSCP field
of all the packets to the initial value. In this way, the metric is
transparent because outside the section of the network under
monitoring, the traffic flow is unchanged.
In such a case, thanks to this restoring technique, network elements
outside the Alternate-Marking monitoring domain (e.g., the two
Provider Edge nodes of the Mobile Backhauling VPN MPLS) are totally
unaware that packets were marked. So this restoring technique makes
Alternate Marking completely transparent outside its monitoring
domain.
6. Hybrid Measurement
The method has been explicitly designed for Passive measurements, but
it can also be used with Active measurements. In order to have both
end-to-end measurements and intermediate measurements (Hybrid
measurements), two endpoints can exchange artificial traffic flows
and apply Alternate Marking over these flows. In the intermediate
Fioccola, et al. Experimental [Page 24]
RFC 8321 Alternate-Marking Method January 2018
points, artificial traffic is managed in the same way as real traffic
and measured as specified before. So the application of the marking
method can also simplify the Active measurement, as explained in
[ALT-MM-AMP].
7. 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
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
counters associated with each color still one at a time 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.
Fioccola, et al. Experimental [Page 25]
RFC 8321 Alternate-Marking Method January 2018
o Measurement Timing: the method has a constraint on the frequency
of measurements. This is detailed in Section 3.2, 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. As an example, in the experiment of the
method, the time interval is set to 5 minutes, while other
optimized implementations can also use a marking period of a few
seconds.
o Implementation: the experiment of the method uses two encodings of
the DSCP field 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
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; moreover, because the experiment
of the method is based on DSCP values, there are significant
dependencies on the usage of the DSCP field: it must be possible
to rely on unused DSCP values without affecting QoS-related
configuration and behavior; moreover, the intermediate nodes must
not change the value of the DSCP field not to alter the
measurement.
o Organization of Results: the Method of Measurement produces
singletons.
Fioccola, et al. Experimental [Page 26]
RFC 8321 Alternate-Marking Method January 2018
o Parameters: currently, the main parameter of the method is the
time interval used to alternate the colors and read the counters.
8. IANA Considerations
This document has no IANA actions.
9. 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
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. In addition,
an attacker can't gain information about network performance from
Fioccola, et al. Experimental [Page 27]
RFC 8321 Alternate-Marking Method January 2018
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.
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
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].
10. References
10.1. 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>.
[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>.
Fioccola, et al. Experimental [Page 28]
RFC 8321 Alternate-Marking Method January 2018
[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>.
[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>.
10.2. Informative References
[ALT-MM-AMP]
Fioccola, G., Clemm, A., Bryant, S., Cociglio, M.,
Chandramouli, M., and A. Capello, "Alternate Marking
Extension to Active Measurement Protocol", Work in
Progress, draft-fioccola-ippm-alt-mark-active-01, March
2017.
[ALT-MM-SLA]
Fioccola, G., Clemm, A., Cociglio, M., Chandramouli, M.,
and A. Capello, "Alternate Marking Extension to Cisco SLA
Protocol RFC6812", Work in Progress, draft-fioccola-ippm-
rfc6812-alt-mark-ext-01, March 2016.
[COLORING] Chen, M., Zheng, L., Mirsky, G., Fioccola, G., and T.
Mizrahi, "IP Flow Performance Measurement Framework", Work
in Progress, draft-chen-ippm-coloring-based-ipfpm-
framework-06, March 2016.
[IP-FLOW-REPORT]
Chen, M., Zheng, L., and G. Mirsky, "IP Flow Performance
Measurement Report", Work in Progress, draft-chen-ippm-
ipfpm-report-01, April 2016.
Fioccola, et al. Experimental [Page 29]
RFC 8321 Alternate-Marking Method January 2018
[IP-MULTICAST-PM]
Cociglio, M., Capello, A., Bonda, A., and L. Castaldelli,
"A method for IP multicast performance monitoring", Work
in Progress, draft-cociglio-mboned-multicast-pm-01,
October 2010.
[MPLS-FLOW]
Bryant, S., Pignataro, C., Chen, M., Li, Z., and G.
Mirsky, "MPLS Flow Identification Considerations", Work in
Progress, draft-ietf-mpls-flow-ident-06, December 2017.
[MULTIPOINT-ALT-MM]
Fioccola, G., Cociglio, M., Sapio, A., and R. Sisto,
"Multipoint Alternate Marking method for passive and
hybrid performance monitoring", Work in Progress,
draft-fioccola-ippm-multipoint-alt-mark-01, October 2017.
[NVO3-ENCAPS]
Boutros, S., Ganga, I., Garg, P., Manur, R., Mizrahi, T.,
Mozes, D., Nordmark, E., Smith, M., Aldrin, S., and I.
Bagdonas, "NVO3 Encapsulation Considerations", Work in
Progress, draft-ietf-nvo3-encap-01, October 2017.
[OPSAWG-P3M]
Capello, A., Cociglio, M., Castaldelli, L., and A. Bonda,
"A packet based method for passive performance
monitoring", Work in Progress, draft-tempia-opsawg-p3m-04,
February 2014.
[PM-MM-BIER]
Mirsky, G., Zheng, L., Chen, M., and G. Fioccola,
"Performance Measurement (PM) with Marking Method in Bit
Index Explicit Replication (BIER) Layer", Work in
Progress, draft-ietf-bier-pmmm-oam-03, October 2017.
[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>.
Fioccola, et al. Experimental [Page 30]
RFC 8321 Alternate-Marking Method January 2018
[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>.
[RFC7011] Claise, B., Ed., Trammell, B., Ed., and P. Aitken,
"Specification of the IP Flow Information Export (IPFIX)
Protocol for the Exchange of Flow Information", STD 77,
RFC 7011, DOI 10.17487/RFC7011, September 2013,
<https://www.rfc-editor.org/info/rfc7011>.
[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>.
[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>.
[RFC8296] Wijnands, IJ., Ed., Rosen, E., Ed., Dolganow, A.,
Tantsura, J., Aldrin, S., and I. Meilik, "Encapsulation
for Bit Index Explicit Replication (BIER) in MPLS and Non-
MPLS Networks", RFC 8296, DOI 10.17487/RFC8296, January
2018, <https://www.rfc-editor.org/info/rfc8296>.
[SFL-FRAMEWORK]
Bryant, S., Chen, M., Li, Z., Swallow, G., Sivabalan, S.,
and G. Mirsky, "Synonymous Flow Label Framework", Work in
Progress, draft-ietf-mpls-sfl-framework-00, August 2017.
Fioccola, et al. Experimental [Page 31]
RFC 8321 Alternate-Marking Method January 2018
[SYN-FLOW-LABELS]
Bryant, S., Chen, M., Li, Z., Swallow, G., Sivabalan, S.,
Mirsky, G., and G. Fioccola, "RFC6374 Synonymous Flow
Labels", Work in Progress, draft-ietf-mpls-rfc6374-sfl-01,
December 2017.
Acknowledgements
The previous IETF specifications describing this technique were:
[IP-MULTICAST-PM] and [OPSAWG-P3M].
The authors would like to thank Alberto Tempia Bonda, Domenico
Laforgia, Daniele Accetta, and Mario Bianchetti for their
contribution to the definition and the implementation of the method.
The authors would also thank Spencer Dawkins, Carlos Pignataro, Brian
Haberman, and Eric Vyncke for their assistance and their detailed and
precious reviews.
Authors' Addresses
Giuseppe Fioccola (editor)
Telecom Italia
Via Reiss Romoli, 274
Torino 10148
Italy
Email: giuseppe.fioccola@telecomitalia.it
Alessandro Capello
Telecom Italia
Via Reiss Romoli, 274
Torino 10148
Italy
Email: alessandro.capello@telecomitalia.it
Mauro Cociglio
Telecom Italia
Via Reiss Romoli, 274
Torino 10148
Italy
Email: mauro.cociglio@telecomitalia.it
Fioccola, et al. Experimental [Page 32]
RFC 8321 Alternate-Marking Method January 2018
Luca Castaldelli
Telecom Italia
Via Reiss Romoli, 274
Torino 10148
Italy
Email: luca.castaldelli@telecomitalia.it
Mach(Guoyi) Chen
Huawei Technologies
Email: mach.chen@huawei.com
Lianshu Zheng
Huawei Technologies
Email: vero.zheng@huawei.com
Greg Mirsky
ZTE
United States of America
Email: gregimirsky@gmail.com
Tal Mizrahi
Marvell
6 Hamada St.
Yokneam
Israel
Email: talmi@marvell.com
Fioccola, et al. Experimental [Page 33]