Internet Engineering Task Force (IETF) G. Fioccola, Ed.
Request for Comments: 8889 Huawei Technologies
Category: Experimental M. Cociglio
ISSN: 2070-1721 Telecom Italia
A. Sapio
Intel Corporation
R. Sisto
Politecnico di Torino
August 2020
Multipoint Alternate-Marking Method for Passive and Hybrid Performance
Monitoring
Abstract
The Alternate-Marking method, as presented in RFC 8321, can only be
applied to point-to-point flows, because it assumes that all the
packets of the flow measured on one node are measured again by a
single second node. This document generalizes and expands this
methodology to measure any kind of unicast flow whose packets can
follow several different paths in the network -- in wider terms, a
multipoint-to-multipoint network. For this reason, the technique
here described is called "Multipoint Alternate Marking".
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 candidates 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/rfc8889.
Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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publication of this document. Please review these documents
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction
2. Terminology
2.1. Correlation with RFC 5644
3. Flow Classification
4. Multipoint Performance Measurement
4.1. Monitoring Network
5. Multipoint Packet Loss
6. Network Clustering
6.1. Algorithm for Clusters Partition
7. Timing Aspects
8. Multipoint Delay and Delay Variation
8.1. Delay Measurements on a Multipoint-Paths Basis
8.1.1. Single-Marking Measurement
8.2. Delay Measurements on a Single-Packet Basis
8.2.1. Single- and Double-Marking Measurement
8.2.2. Hashing Selection Method
9. A Closed-Loop Performance-Management Approach
10. Examples of Application
11. Security Considerations
12. IANA Considerations
13. References
13.1. Normative References
13.2. Informative References
Acknowledgements
Authors' Addresses
1. Introduction
The Alternate-Marking method, as described in RFC 8321 [RFC8321], is
applicable to a point-to-point path. The extension proposed in this
document applies to the most general case of multipoint-to-multipoint
path and enables flexible and adaptive performance measurements in a
managed network.
The Alternate-Marking methodology described in RFC 8321 [RFC8321]
allows the synchronization of the measurements in different points by
dividing the packet flow into batches. So it is possible to get
coherent counters and show what is happening in every marking period
for each monitored flow. The monitoring parameters are the packet
counter and timestamps of a flow for each marking period. Note that
additional details about the applicability of the Alternate-Marking
methodology are described in RFC 8321 [RFC8321] while implementation
details can be found in the paper "AM-PM: Efficient Network Telemetry
using Alternate Marking" [IEEE-Network-PNPM].
There are some applications of the Alternate-Marking method where
there are a lot of monitored flows and nodes. Multipoint Alternate
Marking aims to reduce these values and makes the performance
monitoring more flexible in case a detailed analysis is not needed.
For instance, by considering n measurement points and m monitored
flows, the order of magnitude of the packet counters for each time
interval is n*m*2 (1 per color). The number of measurement points
and monitored flows may vary and depends on the portion of the
network we are monitoring (core network, metro network, access
network) and the granularity (for each service, each customer). So
if both n and m are high values, the packet counters increase a lot,
and Multipoint Alternate Marking offers a tool to control these
parameters.
The approach presented in this document is applied only to unicast
flows and not to multicast. Broadcast, Unknown Unicast, and
Multicast (BUM) traffic is not considered here, because traffic
replication is not covered by the Multipoint Alternate-Marking
method. Furthermore, it can be applicable to anycast flows, and
Equal-Cost Multipath (ECMP) paths can also be easily monitored with
this technique.
In short, RFC 8321 [RFC8321] applies to point-to-point unicast flows
and BUM traffic, while this document and its Clustered Alternate-
Marking method is valid for multipoint-to-multipoint unicast flows,
anycast, and ECMP flows.
Therefore,the Alternate-Marking method can be extended to any kind of
multipoint-to-multipoint paths, and the network-clustering approach
presented in this document is the formalization of how to implement
this property and allow a flexible and optimized performance
measurement support for network management in every situation.
Without network clustering, it is possible to apply Alternate Marking
only for all the network or per single flow. Instead, with network
clustering, it is possible to use the partition of the network into
clusters at different levels in order to perform the needed degree of
detail. In some circumstances, it is possible to monitor a
multipoint network by analyzing the network clustering, without
examining in depth. In case of problems (packet loss is measured or
the delay is too high), the filtering criteria could be specified
more in order to perform a detailed analysis by using a different
combination of clusters up to a per-flow measurement as described in
RFC 8321 [RFC8321].
This approach fits very well with the Closed-Loop Network and
Software-Defined Network (SDN) paradigm, where the SDN orchestrator
and the SDN controllers are the brains of the network and can manage
flow control to the switches and routers and, in the same way, can
calibrate the performance measurements depending on the desired
accuracy. An SDN controller application can orchestrate how
accurately the network performance monitoring is set up by applying
the Multipoint Alternate Marking as described in this document.
It is important to underline that, as an extension of RFC 8321
[RFC8321], this is a methodology document, so the mechanism that can
be used to transmit the counters and the timestamps is out of scope
here, and the implementation is open. Several options are possible
-- e.g., see "Enhanced Alternate Marking Method"
[ENHANCED-ALTERNATE-MARKING].
Note that the fragmented packets case can be managed with the
Alternate-Marking methodology only if fragmentation happens outside
the portion of the network that is monitored. This is always true
for both RFC 8321 [RFC8321] and Multipoint Alternate Marking, as
explained here.
2. Terminology
The definitions of the basic terms are identical to those found in
Alternate Marking [RFC8321]. It is to be remembered that RFC 8321
[RFC8321] is valid for point-to-point unicast flows and BUM traffic.
The important new terms that need to be explained are listed below:
Multipoint Alternate Marking: Extension to RFC 8321 [RFC8321], valid
for multipoint-to-multipoint unicast flows, anycast, and ECMP
flows. It can also be referred to as Clustered Alternate Marking.
Flow definition: The concept of flow is generalized in this
document. The identification fields are selected without any
constraints and, in general, the flow can be a multipoint-to-
multipoint flow, as a result of aggregate point-to-point flows.
Monitoring network: Identified with the nodes of the network that
are the measurement points (MPs) and the links that are the
connections between MPs. The monitoring network graph depends on
the flow definition, so it can represent a specific flow or the
entire network topology as aggregate of all the flows.
Cluster: Smallest identifiable subnetwork of the entire monitoring
network graph that still satisfies the condition that the number
of packets that go in is the same as the number that go out.
Multipoint metrics: Packet loss, delay, and delay variation are
extended to the case of multipoint flows. It is possible to
compute these metrics on the basis of multipoint paths in order to
associate the measurements to a cluster, a combination of
clusters, or the entire monitored network. For delay and delay
variation, it is also possible to define the metrics on a single-
packet basis, and it means that the multipoint path is used to
easily couple packets between input and output nodes of a
multipoint path.
The next section highlights the correlation with the terms used in
RFC 5644 [RFC5644].
2.1. Correlation with RFC 5644
RFC 5644 [RFC5644] is limited to active measurements using a single
source packet or stream. Its scope is also limited to observations
of corresponding packets along the path (spatial metric) and at one
or more destinations (one-to-group) along the path.
Instead, the scope of this memo is to define multiparty metrics for
passive and hybrid measurements in a group-to-group topology with
multiple sources and destinations.
RFC 5644 [RFC5644] introduces metric names that can be reused here
but have to be extended and rephrased to be applied to the Alternate-
Marking schema:
a. the multiparty metrics are not only one-to-group metrics but can
be also group-to-group metrics;
b. the spatial metrics, used for measuring the performance of
segments of a source to destination path, are applied here to
group-to-group segments (called clusters).
3. Flow Classification
A unicast flow is identified by all the packets having a set of
common characteristics. This definition is inspired by RFC 7011
[RFC7011].
As an example, by considering a flow as all the packets sharing the
same source IP address or the same destination IP address, it is easy
to understand that the resulting pattern will not be a point-to-point
connection, but a point-to-multipoint or multipoint-to-point
connection.
In general, a flow can be defined by a set of selection rules used to
match a subset of the packets processed by the network device. These
rules specify a set of Layer 3 and Layer 4 header fields
(identification fields) and the relative values that must be found in
matching packets.
The choice of the identification fields directly affects the type of
paths that the flow would follow in the network. In fact, it is
possible to relate a set of identification fields with the pattern of
the resulting graphs, as listed in Figure 1.
A TCP 5-tuple usually identifies flows following either a single path
or a point-to-point multipath (in the case of load balancing). On
the contrary, a single source address selects aggregate flows
following a point-to-multipoint, while a multipoint-to-point can be
the result of a matching on a single destination address. In the
case where a selection rule and its reverse are used for
bidirectional measurements, they can correspond to a point-to-
multipoint in one direction and a multipoint-to-point in the opposite
direction.
So the flows to be monitored are selected into the monitoring points
using packet selection rules, which can also change the pattern of
the monitored network.
Note that, more generally, the flow can be defined at different
levels based on the potential encapsulation, and additional
conditions that are not in the packet header can also be included as
part of matching criteria.
The Alternate-Marking method is applicable only to a single path (and
partially to a one-to-one multipath), so the extension proposed in
this document is suitable also for the most general case of
multipoint-to-multipoint, which embraces all the other patterns of
Figure 1.
point-to-point single path
+------+ +------+ +------+
---<> R1 <>----<> R2 <>----<> R3 <>---
+------+ +------+ +------+
point-to-point multipath
+------+
<> R2 <>
/ +------+ \
/ \
+------+ / \ +------+
---<> R1 <> <> R4 <>---
+------+ \ / +------+
\ /
\ +------+ /
<> R3 <>
+------+
point-to-multipoint
+------+
<> R4 <>---
/ +------+
+------+ /
<> R2 <>
/ +------+ \
+------+ / \ +------+
---<> R1 <> <> R5 <>---
+------+ \ +------+
\ +------+
<> R3 <>
+------+ \
\ +------+
<> R6 <>---
+------+
multipoint-to-point
+------+
---<> R1 <>
+------+ \
\ +------+
<> R4 <>
/ +------+ \
+------+ / \ +------+
---<> R2 <> <> R6 <>---
+------+ / +------+
+------+ /
<> R5 <>
/ +------+
+------+ /
---<> R3 <>
+------+
multipoint-to-multipoint
+------+ +------+
---<> R1 <> <> R6 <>---
+------+ \ / +------+
\ +------+ /
<> R4 <>
+------+ \
+------+ \ +------+
---<> R2 <> <> R7 <>---
+------+ \ / +------+
\ +------+ /
<> R5 <>
/ +------+ \
+------+ / \ +------+
---<> R3 <> <> R8 <>---
+------+ +------+
Figure 1: Flow Classification
The case of unicast flow is considered in Figure 1. The anycast flow
is also in scope, because there is no replication and only a single
node from the anycast group receives the traffic, so it can be viewed
as a special case of unicast flow. Furthermore, an ECMP flow is in
scope by definition, since it is a point-to-multipoint unicast flow.
4. Multipoint Performance Measurement
By using the Alternate-Marking method, only point-to-point paths can
be monitored. To have an IP (TCP/UDP) flow that follows a point-to-
point path, we have to define, with a specific value, 5
identification fields (IP Source, IP Destination, Transport Protocol,
Source Port, Destination Port).
Multipoint Alternate Marking enables the performance measurement for
multipoint flows selected by identification fields without any
constraints (even the entire network production traffic). It is also
possible to use multiple marking points for the same monitored flow.
4.1. Monitoring Network
The monitoring network is deduced from the production network by
identifying the nodes of the graph that are the measurement points,
and the links that are the connections between measurement points.
There are some techniques that can help with the building of the
monitoring network (as an example, see [ROUTE-ASSESSMENT]). In
general, there are different options: the monitoring network can be
obtained by considering all the possible paths for the traffic or
periodically checking the traffic (e.g. daily, weekly, monthly) and
updating the graph as appropriate, but this is up to the Network
Management System (NMS) configuration.
So a graph model of the monitoring network can be built according to
the Alternate-Marking method: the monitored interfaces and links are
identified. Only the measurement points and links where the traffic
has flowed have to be represented in the graph.
Figure 2 shows a simple example of a monitoring network graph:
+------+
<> R6 <>---
/ +------+
+------+ +------+ /
<> R2 <>---<> R4 <>
/ +------+ \ +------+ \
/ \ \ +------+
+------+ / +------+ \ +------+ <> R7 <>---
---<> R1 <>---<> R3 <>---<> R5 <> +------+
+------+ \ +------+ \ +------+ \
\ \ \ +------+
\ \ <> R8 <>---
\ \ +------+
\ \
\ \ +------+
\ <> R9 <>---
\ +------+
\
\ +------+
<> R10 <>---
+------+
Figure 2: Monitoring Network Graph
Each monitoring point is characterized by the packet counter that
refers only to a marking period of the monitored flow.
The same is also applicable for the delay, but it will be described
in the following sections.
5. Multipoint Packet Loss
Since all the packets of the considered flow leaving the network have
previously entered the network, the number of packets counted by all
the input nodes is always greater than, or equal to, the number of
packets counted by all the output nodes. Noninitial fragments are
not considered here.
The assumption is the use of the Alternate-Marking method. In the
case of no packet loss occurring in the marking period, if all the
input and output points of the network domain to be monitored are
measurement points, the sum of the number of packets on all the
ingress interfaces equals the number on egress interfaces for the
monitored flow. In this circumstance, if no packet loss occurs, the
intermediate measurement points only have the task of splitting the
measurement.
It is possible to define the Network Packet Loss of one monitored
flow for a single period. In a packet network, the number of lost
packets is the number of packets counted by the input nodes minus the
number of packets counted by the output nodes. This is true for
every packet flow in each marking period.
The monitored network packet loss with n input nodes and m output
nodes is given by:
PL = (PI1 + PI2 +...+ PIn) - (PO1 + PO2 +...+ POm)
where:
PL is the network packet loss (number of lost packets)
PIi is the number of packets flowed through the i-th input node in
this period
POj is the number of packets flowed through the j-th output node in
this period
The equation is applied on a per-time-interval basis and a per-flow
basis:
The reference interval is the Alternate-Marking period, as defined
in RFC 8321 [RFC8321].
The flow definition is generalized here. Indeed, as described
before, a multipoint packet flow is considered, and the
identification fields can be selected without any constraints.
6. Network Clustering
The previous equation can determine the number of packets lost
globally in the monitored network, exploiting only the data provided
by the counters in the input and output nodes.
In addition, it is also possible to leverage the data provided by the
other counters in the network to converge on the smallest
identifiable subnetworks where the losses occur. These subnetworks
are named "clusters".
A cluster graph is a subnetwork of the entire monitoring network
graph that still satisfies the packet loss equation (introduced in
the previous section), where PL in this case is the number of packets
lost in the cluster. As for the entire monitoring network graph, the
cluster is defined on a per-flow basis.
For this reason, a cluster should contain all the arcs emanating from
its input nodes and all the arcs terminating at its output nodes.
This ensures that we can count all the packets (and only those)
exiting an input node again at the output node, whatever path they
follow.
In a completely monitored unidirectional network (a network where
every network interface is monitored), each network device
corresponds to a cluster, and each physical link corresponds to two
clusters (one for each device).
Clusters can have different sizes depending on the flow-filtering
criteria adopted.
Moreover, sometimes clusters can be optionally simplified. For
example, when two monitored interfaces are divided by a single router
(one is the input interface, the other is the output interface, and
the router has only these two interfaces), instead of counting
exactly twice, upon entering and leaving, it is possible to consider
a single measurement point. In this case, we do not care about the
internal packet loss of the router.
It is worth highlighting that it might also be convenient to define
clusters based on the topological information so that they are
applicable to all the possible flows in the monitored network.
6.1. Algorithm for Clusters Partition
A simple algorithm can be applied in order to split our monitoring
network into clusters. This can be done for each direction
separately. The clusters partition is based on the monitoring
network graph, which can be valid for a specific flow or can also be
general and valid for the entire network topology.
It is a two-step algorithm:
1. Group the links where there is the same starting node;
2. Join the grouped links with at least one ending node in common.
Considering that the links are unidirectional, the first step implies
listing all the links as connections between two nodes and grouping
the different links if they have the same starting node. Note that
it is possible to start from any link, and the procedure will work.
Following this classification, the second step implies eventually
joining the groups classified in the first step by looking at the
ending nodes. If different groups have at least one common ending
node, they are put together and belong to the same set. After the
application of the two steps of the algorithm, each one of the
composed sets of links, together with the endpoint nodes, constitutes
a cluster.
In our monitoring network graph example, it is possible to identify
the clusters partition by applying this two-step algorithm.
The first step identifies the following groups:
1. Group 1: (R1-R2), (R1-R3), (R1-R10)
2. Group 2: (R2-R4), (R2-R5)
3. Group 3: (R3-R5), (R3-R9)
4. Group 4: (R4-R6), (R4-R7)
5. Group 5: (R5-R8)
And then, the second step builds the clusters partition (in
particular, we can underline that Groups 2 and 3 connect together,
since R5 is in common):
1. Cluster 1: (R1-R2), (R1-R3), (R1-R10)
2. Cluster 2: (R2-R4), (R2-R5), (R3-R5), (R3-R9)
3. Cluster 3: (R4-R6), (R4-R7)
4. Cluster 4: (R5-R8)
The flow direction here considered is from left to right. For the
opposite direction, the same reasoning can be applied, and in this
example, you get the same clusters partition.
In the end, the following 4 clusters are obtained:
Cluster 1
+------+
<> R2 <>---
/ +------+
/
+------+ / +------+
---<> R1 <>---<> R3 <>---
+------+ \ +------+
\
\
\
\
\
\
\
\
\ +------+
<> R10 <>---
+------+
Cluster 2
+------+ +------+
---<> R2 <>---<> R4 <>---
+------+ \ +------+
\
+------+ \ +------+
---<> R3 <>---<> R5 <>---
+------+ \ +------+
\
\
\
\
\ +------+
<> R9 <>---
+------+
Cluster 3
+------+
<> R6 <>---
/ +------+
+------+ /
---<> R4 <>
+------+ \
\ +------+
<> R7 <>---
+------+
Cluster 4
+------+
---<> R5 <>
+------+ \
\ +------+
<> R8 <>---
+------+
Figure 3: Clusters Example
There are clusters with more than two nodes as well as two-node
clusters. In the two-node clusters, the loss is on the link (Cluster
4). In more-than-two-node clusters, the loss is on the cluster, but
we cannot know in which link (Cluster 1, 2, or 3).
In this way, the calculation of packet loss can be made on a cluster
basis. Note that the packet counters for each marking period permit
calculating the packet rate on a cluster basis, so Committed
Information Rate (CIR) and Excess Information Rate (EIR) could also
be deduced on a cluster basis.
Obviously, by combining some clusters in a new connected subnetwork
(called a "super cluster"), the packet-loss rule is still true.
In this way, in a very large network, there is no need to configure
detailed filter criteria to inspect the traffic. You can check a
multipoint network and, in case of problems, go deep with a step-by-
step cluster analysis, but only for the cluster or combination of
clusters where the problem happens.
In summary, once a flow is defined, the algorithm to build the
clusters partition is based on topological information; therefore, it
considers all the possible links and nodes crossed by the given flow,
even if there is no traffic. So, if the flow does not enter or
traverse all the nodes, the counters have a nonzero value for the
involved nodes and a zero value for the other nodes without traffic;
but in the end, all the formulas are still valid.
The algorithm described above is an iterative clustering algorithm,
but it is also possible to apply a recursive clustering algorithm by
using the node-node adjacency matrix representation
[IEEE-ACM-ToN-MPNPM].
The complete and mathematical analysis of the possible algorithms for
clusters partition, including the considerations in terms of
efficiency and a comparison between the different methods, is in the
paper [IEEE-ACM-ToN-MPNPM].
7. Timing Aspects
It is important to consider the timing aspects, since out-of-order
packets happen and have to be handled as well, as described in RFC
8321 [RFC8321]. However, in a multisource situation, an additional
issue has to be considered. With multipoint path, the egress nodes
will receive alternate marked packets in random order from different
ingress nodes, and this must not affect the measurement.
So, if we analyze a multipoint-to-multipoint path with more than one
marking node, it is important to recognize the reference measurement
interval. In general, the measurement interval for describing the
results is the interval of the marking node that is more aligned with
the start of the measurement, as reported in Figure 4.
Note that the mark switching approach based on a fixed timer is
considered in this document.
time -> start stop
T(R1) |-------------|
T(R2) |-------------|
T(R3) |------------|
Figure 4: Measurement Interval
In Figure 4, it is assumed that the node with the earliest clock (R1)
identifies the right starting and ending times of the measurement,
but it is just an assumption, and other possibilities could occur.
So, in this case, T(R1) is the measurement interval, and its
recognition is essential in order to make comparisons with other
active/passive/hybrid Packet Loss metrics.
When we expand to multipoint-to-multipoint flows, we have to consider
that all source nodes mark the traffic, and this adds more
complexity.
Regarding the timing aspects of the methodology, RFC 8321 [RFC8321]
already describes two contributions that are taken into account: the
clock error between network devices and the network delay between
measurement points.
But we should now consider an additional contribution. Since all
source nodes mark the traffic, the source measurement intervals can
be of different lengths and with different offsets, and this mismatch
m can be added to d, as shown in Figure 5.
...BBBBBBBBB | AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA | BBBBBBBBB...
|<======================================>|
| L |
...=========>|<==================><==================>|<==========...
| L/2 L/2 |
|<=><===>| |<===><=>|
m d | | d m
|<====================>|
available counting interval
Figure 5: Timing Aspects for Multipoint Paths
So the misalignment between the marking source routers gives an
additional constraint, and the value of m is added to d (which
already includes clock error and network delay).
Thus, three different possible contributions are considered: clock
error between network devices, network delay between measurement
points, and the misalignment between the marking source routers.
In the end, the condition that must be satisfied to enable the method
to function properly is that the available counting interval must be
> 0, and that means:
L - 2m - 2d > 0.
This formula needs to be verified for each measurement point on the
multipoint path, where m is misalignment between the marking source
routers, while d, already introduced in RFC 8321 [RFC8321], takes
into account clock error and network delay between network nodes.
Therefore, the mismatch between measurement intervals must satisfy
this condition.
Note that the timing considerations are valid for both packet loss
and delay measurements.
8. Multipoint Delay and Delay Variation
The same line of reasoning can be applied to delay and delay
variation. Similarly to the delay measurements defined in RFC 8321
[RFC8321], the marking batches anchor the samples to a particular
period, and this is the time reference that can be used. It is
important to highlight that both delay and delay-variation
measurements make sense in a multipoint path. The delay variation is
calculated by considering the same packets selected for measuring the
delay.
In general, it is possible to perform delay and delay-variation
measurements on the basis of multipoint paths or single packets:
* Delay measurements on the basis of multipoint paths mean that the
delay value is representative of an entire multipoint path (e.g.,
the whole multipoint network, a cluster, or a combination of
clusters).
* Delay measurements on a single-packet basis mean that you can use
a multipoint path just to easily couple packets between input and
output nodes of a multipoint path, as described in the following
sections.
8.1. Delay Measurements on a Multipoint-Paths Basis
8.1.1. Single-Marking Measurement
Mean delay and mean delay-variation measurements can also be
generalized to the case of multipoint flows. It is possible to
compute the average one-way delay of packets in one block, a cluster,
or the entire monitored network.
The average latency can be measured as the difference between the
weighted averages of the mean timestamps of the sets of output and
input nodes. This means that, in the calculation, it is possible to
weigh the timestamps by considering the number of packets for each
endpoints.
8.2. Delay Measurements on a Single-Packet Basis
8.2.1. Single- and Double-Marking Measurement
Delay and delay-variation measurements relative to only one picked
packet per period (both single and double marked) can be performed in
the multipoint scenario, with some limitations:
Single marking based on the first/last packet of the interval
would not work, because it would not be possible to agree on the
first packet of the interval.
Double marking or multiplexed marking would work, but each
measurement would only give information about the delay of a
single path. However, by repeating the measurement multiple
times, it is possible to get information about all the paths in
the multipoint flow. This can be done in the case of a point-to-
multipoint path, but it is more difficult to achieve in the case
of a multipoint-to-multipoint path because of the multiple source
routers.
If we would perform a delay measurement for more than one picked
packet in the same marking period, and especially if we want to get
delay measurements on a multipoint-to-multipoint basis, neither the
single- nor the double-marking method is useful in the multipoint
scenario, since they would not be representative of the entire flow.
The packets can follow different paths with various delays, and in
general it can be very difficult to recognize marked packets in a
multipoint-to-multipoint path, especially in the case when there is
more than one per period.
A desirable option is to monitor simultaneously all the paths of a
multipoint path in the same marking period; for this purpose, hashing
can be used, as reported in the next section.
8.2.2. Hashing Selection Method
RFCs 5474 [RFC5474] and 5475 [RFC5475] introduce sampling and
filtering techniques for IP packet selection.
The hash-based selection methodologies for delay measurement can work
in a multipoint-to-multipoint path and can be used either coupled to
mean delay or stand-alone.
[ALTERNATE-MARKING] introduces how to use the hash method (RFCs 5474
[RFC5474] and 5475 [RFC5475]) combined with the Alternate-Marking
method for point-to-point flows. It is also called Mixed Hashed
Marking: the coupling of a marking method and hashing technique is
very useful, because the marking batches anchor the samples selected
with hashing, and this simplifies the correlation of the hashing
packets along the path.
It is possible to use a basic-hash or a dynamic-hash method. One of
the challenges of the basic approach is that the frequency of the
sampled packets may vary considerably. For this reason, the dynamic
approach has been introduced for point-to-point flows in order to
have the desired and almost fixed number of samples for each
measurement period. Using the hash-based sampling, the number of
samples may vary a lot because it depends on the packet rate that is
variable. The dynamic approach helps to have an almost fixed number
of samples for each marking period, and this is a better option for
making regular measurements over time. In the hash-based sampling,
Alternate Marking is used to create periods, so that hash-based
samples are divided into batches, which allows anchoring the selected
samples to their period. Moreover, in the dynamic hash-based
sampling, by dynamically adapting the length of the hash value, the
number of samples is bounded in each marking period. This can be
realized by choosing the maximum number of samples (NMAX) to be
caught in a marking period. The algorithm starts with only a few
hash bits, which permits selecting a greater percentage of packets
(e.g., with 0 bits of hash all the packets are sampled, with 1 bit of
hash half of the packets are sampled, and so on). When the number of
selected packets reaches NMAX, a hashing bit is added. As a
consequence, the sampling proceeds at half of the original rate, and
also the packets already selected that do not match the new hash are
discarded. This step can be repeated iteratively. It is assumed
that each sample includes the timestamp (used for delay measurement)
and the hash value, allowing the management system to match the
samples received from the two measurement points. The dynamic
process statistically converges at the end of a marking period, and
the final number of selected samples is between NMAX/2 and NMAX.
Therefore, the dynamic approach paces the sampling rate, allowing to
bound the number of sampled packets per sampling period.
In a multipoint environment, the behavior is similar to a point-to-
point flow. In particular, in the context of a multipoint-to-
multipoint flow, the dynamic hash could be the solution for
performing delay measurements on specific packets and overcoming the
single- and double-marking limitations.
The management system receives the samples, including the timestamps
and the hash value, from all the MPs, and this happens for both
point-to-point and multipoint-to-multipoint flows. Then, the longest
hash used by the MPs is deduced and applied to couple timestamps from
either the same packets of 2 MPs of a point-to-point path, or the
input and output MPs of a cluster (or a super cluster or the entire
network). But some considerations are needed: if there isn't packet
loss, the set of input samples is always equal to the set of output
samples. In the case of packet loss, the set of output samples can
be a subset of input samples, but the method still works because, at
the end, it is easy to couple the input and output timestamps of each
caught packet using the hash (in particular, the "unused part of the
hash" that should be different for each packet).
Therefore, the basic hash is logically similar to the double-marking
method, and in the case of a point-to-point path, double-marking and
basic-hash selection are equivalent. The dynamic approach scales the
number of measurements per interval. It would seem that double
marking would also work well if we reduced the interval length, but
this can be done only for a point-to-point path and not for a
multipoint path, where we cannot couple the picked packets in a
multipoint path. So, in general, if we want to get delay
measurements on the basis of a multipoint-to-multipoint path, and
want to select more than one packet per period, double marking cannot
be used because we could not be able to couple the picked packets
between input and output nodes. On the other hand, we can do that by
using hashing selection.
9. A Closed-Loop Performance-Management Approach
The Multipoint Alternate-Marking framework that is introduced in this
document adds flexibility to Performance Management (PM), because it
can reduce the order of magnitude of the packet counters. This
allows an SDN orchestrator to supervise, control, and manage PM in
large networks.
The monitoring network can be considered as a whole or split into
clusters that are the smallest subnetworks (group-to-group segments),
maintaining the packet-loss property for each subnetwork. The
clusters can also be combined in new, connected subnetworks at
different levels, depending on the detail we want to achieve.
An SDN controller or a Network Management System (NMS) can calibrate
performance measurements, since they are aware of the network
topology. They can start without examining in depth. In case of
necessity (packet loss is measured or the delay is too high), the
filtering criteria could be immediately reconfigured in order to
perform a partition of the network by using clusters and/or different
combinations of clusters. In this way, the problem can be localized
in a specific cluster or a single combination of clusters, and a more
detailed analysis can be performed step by step by successive
approximation up to a point-to-point flow detailed analysis. This is
the so-called "closed loop".
This approach can be called "network zooming" and can be performed in
two different ways:
1) change the traffic filter and select more detailed flows;
2) activate new measurement points by defining more specified
clusters.
The network-zooming approach implies that some filters or rules are
changed and that therefore there is a transient time to wait once the
new network configuration takes effect. This time can be determined
by the Network Orchestrator/Controller, based on the network
conditions.
For example, if the network zooming identifies the performance
problem for the traffic coming from a specific source, we need to
recognize the marked signal from this specific source node and its
relative path. For this purpose, we can activate all the available
measurement points and better specify the flow filter criteria (i.e.,
5-tuple). As an alternative, it can be enough to select packets from
the specific source for delay measurements; in this case, it is
possible to apply the hashing technique, as mentioned in the previous
sections.
[IFIT-FRAMEWORK] defines an architecture where the centralized Data
Collector and Network Management can apply the intelligent and
flexible Alternate-Marking algorithm as previously described.
As for RFC 8321 [RFC8321], it is possible to classify the traffic and
mark a portion of the total traffic. For each period, the packet
rate and bandwidth are calculated from the number of packets. In
this way, the network orchestrator becomes aware if the traffic rate
surpasses limits. In addition, more precision can be obtained by
reducing the marking period; indeed, some implementations use a
marking period of 1 sec or less.
In addition, an SDN controller could also collect the measurement
history.
It is important to mention that the Multipoint Alternate Marking
framework also helps Traffic Visualization. Indeed, this methodology
is very useful for identifying which path or cluster is crossed by
the flow.
10. Examples of Application
There are application fields where it may be useful to take into
consideration the Multipoint Alternate Marking:
VPN: The IP traffic is selected on an IP-source basis in both
directions. At the endpoint WAN interface, all the output traffic
is counted in a single flow. The input traffic is composed of all
the other flows aggregated for source address. So, by considering
n endpoints, the monitored flows are n (each flow with 1 ingress
point and (n-1) egress points) instead of n*(n-1) flows (each
flow, with 1 ingress point and 1 egress point).
Mobile Backhaul: LTE traffic is selected, in the Up direction, by
the EnodeB source address and, in the Down direction, by the
EnodeB destination address, because the packets are sent from the
Mobile Packet Core to the EnodeB. So the monitored flow is only
one per EnodeB in both directions.
Over The Top (OTT) services: The traffic is selected, in the Down
direction, by the source addresses of the packets sent by OTT
servers. In the opposite direction (Up), it is selected by the
destination IP addresses of the same servers. So the monitoring
is based on a single flow per OTT server in both directions.
Enterprise SD-WAN: SD-WAN allows connecting remote branch offices to
data centers and building higher-performance WANs. A centralized
controller is used to set policies and prioritize traffic. The
SD-WAN takes into account these policies and the availability of
network bandwidth to route traffic. This helps ensure that
application performance meets Service Level Agreements (SLAs).
This methodology can also help the path selection for the WAN
connection based on per-cluster and per-flow performance.
Note that the preceding list is just an example and is not
exhaustive. More applications are possible.
11. Security Considerations
This document specifies a method of performing measurements that does
not directly affect Internet security or applications that run on the
Internet. However, implementation of this method must be mindful of
security and privacy concerns, as explained in RFC 8321 [RFC8321].
12. IANA Considerations
This document has no IANA actions.
13. References
13.1. Normative References
[RFC5474] Duffield, N., Ed., Chiou, D., Claise, B., Greenberg, A.,
Grossglauser, M., and J. Rexford, "A Framework for Packet
Selection and Reporting", RFC 5474, DOI 10.17487/RFC5474,
March 2009, <https://www.rfc-editor.org/info/rfc5474>.
[RFC5475] Zseby, T., Molina, M., Duffield, N., Niccolini, S., and F.
Raspall, "Sampling and Filtering Techniques for IP Packet
Selection", RFC 5475, DOI 10.17487/RFC5475, March 2009,
<https://www.rfc-editor.org/info/rfc5475>.
[RFC5644] Stephan, E., Liang, L., and A. Morton, "IP Performance
Metrics (IPPM): Spatial and Multicast", RFC 5644,
DOI 10.17487/RFC5644, October 2009,
<https://www.rfc-editor.org/info/rfc5644>.
[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>.
13.2. Informative References
[ALTERNATE-MARKING]
Mizrahi, T., Arad, C., Fioccola, G., Cociglio, M., Chen,
M., Zheng, L., and G. Mirsky, "Compact Alternate Marking
Methods for Passive and Hybrid Performance Monitoring",
Work in Progress, Internet-Draft, draft-mizrahi-ippm-
compact-alternate-marking-05, 6 July 2019,
<https://tools.ietf.org/html/draft-mizrahi-ippm-compact-
alternate-marking-05>.
[ENHANCED-ALTERNATE-MARKING]
Zhou, T., Fioccola, G., Lee, S., Cociglio, M., and W. Li,
"Enhanced Alternate Marking Method", Work in Progress,
Internet-Draft, draft-zhou-ippm-enhanced-alternate-
marking-05, 13 July 2020, <https://tools.ietf.org/html/
draft-zhou-ippm-enhanced-alternate-marking-05>.
[IEEE-ACM-ToN-MPNPM]
Cociglio, M., Fioccola, G., Marchetto, G., Sapio, A., and
R. Sisto, "Multipoint Passive Monitoring in Packet
Networks", IEEE/ACM Transactions on Networking vol. 27,
no. 6, pp. 2377-2390, DOI 10.1109/TNET.2019.2950157,
December 2019,
<https://doi.org/10.1109/TNET.2019.2950157>.
[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, no. 4,
pp. 155-161, DOI 10.1109/MNET.2019.1800152, July 2019,
<https://doi.org/10.1109/MNET.2019.1800152>.
[IFIT-FRAMEWORK]
Song, H., Qin, F., Chen, H., Jin, J., and J. Shin, "In-
situ Flow Information Telemetry", Work in Progress,
April 2020, <https://tools.ietf.org/html/draft-song-
opsawg-ifit-framework-12>.
[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>.
[ROUTE-ASSESSMENT]
Alvarez-Hamelin, J., Morton, A., Fabini, J., Pignataro,
C., and R. Geib, "Advanced Unidirectional Route Assessment
(AURA)", Work in Progress, Internet-Draft, draft-ietf-
ippm-route-10, 13 August 2020,
<https://tools.ietf.org/html/draft-ietf-ippm-route-10>.
Acknowledgements
The authors would like to thank Al Morton, Tal Mizrahi, and Rachel
Huang for the precious contributions.
Authors' Addresses
Giuseppe Fioccola (editor)
Huawei Technologies
Riesstrasse, 25
80992 Munich
Germany
Email: giuseppe.fioccola@huawei.com
Mauro Cociglio
Telecom Italia
Via Reiss Romoli, 274
10148 Torino
Italy
Email: mauro.cociglio@telecomitalia.it
Amedeo Sapio
Intel Corporation
4750 Patrick Henry Dr.
Santa Clara, CA 95054
United States of America
Email: amedeo.sapio@intel.com
Riccardo Sisto
Politecnico di Torino
Corso Duca degli Abruzzi, 24
10129 Torino
Italy
Email: riccardo.sisto@polito.it