6MAN Working Group G. Fioccola
Internet-Draft T. Zhou
Intended status: Standards Track Huawei
Expires: January 2, 2023 M. Cociglio
Telecom Italia
F. Qin
China Mobile
R. Pang
China Unicom
July 1, 2022
IPv6 Application of the Alternate Marking Method
draft-ietf-6man-ipv6-alt-mark-16
Abstract
This document describes how the Alternate Marking Method can be used
as a passive performance measurement tool in an IPv6 domain. It
defines an Extension Header Option to encode Alternate Marking
information in both the Hop-by-Hop Options Header and Destination
Options Header.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on January 2, 2023.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Alternate Marking application to IPv6 . . . . . . . . . . . . 3
2.1. Controlled Domain . . . . . . . . . . . . . . . . . . . . 5
2.1.1. Alternate Marking Measurement Domain . . . . . . . . 6
3. Definition of the AltMark Option . . . . . . . . . . . . . . 7
3.1. Data Fields Format . . . . . . . . . . . . . . . . . . . 7
4. Use of the AltMark Option . . . . . . . . . . . . . . . . . . 8
5. Alternate Marking Method Operation . . . . . . . . . . . . . 10
5.1. Packet Loss Measurement . . . . . . . . . . . . . . . . . 10
5.2. Packet Delay Measurement . . . . . . . . . . . . . . . . 12
5.3. Flow Monitoring Identification . . . . . . . . . . . . . 13
5.4. Multipoint and Clustered Alternate Marking . . . . . . . 16
5.5. Data Collection and Calculation . . . . . . . . . . . . . 16
6. Security Considerations . . . . . . . . . . . . . . . . . . . 16
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 20
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 21
9.1. Normative References . . . . . . . . . . . . . . . . . . 21
9.2. Informative References . . . . . . . . . . . . . . . . . 21
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 23
1. Introduction
[I-D.ietf-ippm-rfc8321bis] and [I-D.ietf-ippm-rfc8889bis] describe a
passive performance measurement method, which can be used to measure
packet loss, latency and jitter on live traffic. Since this method
is based on marking consecutive batches of packets, the method is
often referred to as the Alternate Marking Method.
This document defines how the Alternate Marking Method can be used to
measure performance metrics in IPv6. The rationale is to apply the
Alternate Marking methodology to IPv6 and therefore allow detailed
packet loss, delay and delay variation measurements both hop-by-hop
and end-to-end to exactly locate the issues in an IPv6 network.
The Alternate Marking is an on-path telemetry technique and consists
of synchronizing the measurements in different points of a network by
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switching the value of a marking bit and therefore dividing the
packet flow into batches. Each batch represents a measurable entity
recognizable by all network nodes along the path. By counting the
number of packets in each batch and comparing the values measured by
different nodes, it is possible to precisely measure the packet loss.
Similarly, the alternation of the values of the marking bits can be
used as a time reference to calculate the delay and delay variation.
The Alternate Marking operation is further described in Section 5.
This document introduces a TLV (type-length-value) that can be
encoded in the Options Headers (Hop-by-Hop or Destination), according
to [RFC8200], for the purpose of the Alternate Marking Method
application in an IPv6 domain.
The Alternate Marking Method MUST be applied to IPv6 only in a
controlled environment, as further described in Section 2.1.
Besides, [RFC8799] also discusses the trend towards network behaviors
that can be applied only within limited domains.
The threat model for the application of the Alternate Marking Method
in an IPv6 domain is reported in Section 6.
1.1. Terminology
This document uses the terms related to the Alternate Marking Method
as defined in [I-D.ietf-ippm-rfc8321bis] and
[I-D.ietf-ippm-rfc8889bis].
1.2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2. Alternate Marking application to IPv6
The Alternate Marking Method requires a marking field. Several
alternatives could be considered such as IPv6 Extension Headers, IPv6
Address and Flow Label. But, it is necessary to analyze the
drawbacks for all the available possibilities, more specifically:
Reusing existing Extension Header for Alternate Marking leads to a
non-optimized implementation;
Using the IPv6 destination address to encode the Alternate Marking
processing is very expensive;
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Using the IPv6 Flow Label for Alternate Marking conflicts with the
utilization of the Flow Label for load distribution purpose
([RFC6438]).
In the end, a Hop-by-Hop or a Destination Option is the best choice.
The approach for the Alternate Marking application to IPv6 specified
in this memo is compliant with [RFC8200]. It involves the following
operations:
o The source node is the only one that writes the Option Header to
mark alternately the flow (for both Hop-by-Hop and Destination
Option). The intermediate nodes and destination node MUST only
read the marking values of the option without modifying the Option
Header.
o In case of Hop-by-Hop Option Header carrying Alternate Marking
bits, it is not inserted or deleted, but can be read by any node
along the path. The intermediate nodes may be configured to
support this Option or not and the measurement can be done only
for the nodes configured to read the Option. As further discussed
in Section 4, the presence of the hop-by-hop option should not
affect the traffic throughput both on nodes that do not recognize
this option and on the nodes that support it. However, it is
worth mentioning that there is a difference between theory and
practice. Indeed, in a real implementation it can happen that
packets with hop-by-hop option could also be skipped or processed
in the slow path. While some proposals are trying to address this
problem and make Hop-by-Hop Options more practical
([I-D.ietf-v6ops-hbh], [I-D.ietf-6man-hbh-processing]), these
aspects are out of the scope for this document.
o In case of Destination Option Header carrying Alternate Marking
bits, it is not processed, inserted, or deleted by any node along
the path until the packet reaches the destination node. Note
that, if there is also a Routing Header (RH), any visited
destination in the route list can process the Option Header.
Hop-by-Hop Option Header is also useful to signal to routers on the
path to process the Alternate Marking. However, as said, routers
will only examine this option if properly configured.
The optimization of both implementation and scaling of the Alternate
Marking Method is also considered and a way to identify flows is
required. The Flow Monitoring Identification field (FlowMonID), as
introduced in Section 5.3, goes in this direction and it is used to
identify a monitored flow.
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The FlowMonID is different from the Flow Label field of the IPv6
Header ([RFC6437]). The Flow Label field in the IPv6 header is used
by a source to label sequences of packets to be treated in the
network as a single flow and, as reported in [RFC6438], it can be
used for load-balancing/equal cost multi-path (LB/ECMP). The reuse
of Flow Label field for identifying monitored flows is not considered
because it may change the application intent and forwarding behavior.
Also, the Flow Label may be changed en route and this may also
invalidate the integrity of the measurement. Those reasons make the
definition of the FlowMonID necessary for IPv6. Indeed, the
FlowMonID is designed and only used to identify the monitored flow.
Flow Label and FlowMonID within the same packet are totally disjoint,
have different scope, are used to identify flows based on different
criteria, and are intended for different use cases.
The rationale for the FlowMonID is further discussed in Section 5.3.
This 20 bit field allows easy and flexible identification of the
monitored flow and enables improved measurement correlation and finer
granularity since it can be used in combination with the traditional
TCP/IP 5-tuple to identify a flow. An important point that will be
discussed in Section 5.3 is the uniqueness of the FlowMonID and how
to allow disambiguation of the FlowMonID in case of collision.
The following section highlights an important requirement for the
application of the Alternate Marking to IPv6. The concept of the
controlled domain is explained and it is considered an essential
precondition, as also highlighted in Section 6.
2.1. Controlled Domain
IPv6 has much more flexibility than IPv4 and innovative applications
have been proposed, but for security and compatibility reasons, some
of these applications are limited to a controlled environment. This
is also the case of the Alternate Marking application to IPv6 as
assumed hereinafter. In this regard, [RFC8799] reports further
examples of specific limited domain solutions.
The IPv6 application of the Alternate Marking Method MUST be deployed
in a controlled domain. It is not common that the user traffic
originates and terminates within the controlled domain, as also noted
in Section 2.1.1. For this reason, it will typically only be
applicable in an overlay network, where user traffic is encapsulated
at one domain border, decapsulated at the other domain border and the
encapsulation incorporates the relevant extension header for
Alternate Marking. This requirement also implies that an
implementation MUST filter packets that carry Alternate Marking data
and are entering or leaving the controlled domain.
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A controlled domain is a managed network where it is required to
select, monitor and control the access to the network by enforcing
policies at the domain boundaries in order to discard undesired
external packets entering the domain and check the internal packets
leaving the domain. It does not necessarily mean that a controlled
domain is a single administrative domain or a single organization. A
controlled domain can correspond to a single administrative domain or
can be composed by multiple administrative domains under a defined
network management. Indeed, some scenarios may imply that the
Alternate Marking Method involves more than one domain, but in these
cases, it is RECOMMENDED that the multiple domains create a whole
controlled domain while traversing the external domain by employing
IPsec [RFC4301] authentication and encryption or other VPN technology
that provides full packet confidentiality and integrity protection.
In a few words, it must be possible to control the domain boundaries
and eventually use specific precautions if the traffic traverse the
Internet.
The security considerations reported in Section 6 also highlight this
requirement.
2.1.1. Alternate Marking Measurement Domain
The Alternate Marking measurement domain can overlap with the
controlled domain or may be a subset of the controlled domain. The
typical scenarios for the application of the Alternate Marking Method
depend on the controlled domain boundaries, in particular:
the user equipment can be the starting or ending node, only in
case it is fully managed and if it belongs to the controlled
domain. In this case the user generated IPv6 packets contain the
Alternate Marking data. But, in practice, this is not common due
to the fact that the user equipment cannot be totally secured in
the majority of cases.
the CPE (Customer Premises Equipment) and the PE (Provider Edge)
router are most likely to be the starting or ending node since
they can border a controlled domain. For instance, the CPE, which
connects the user's premises with the service provider's network,
belongs to a controlled domain only if it is managed by the
service provider and if additional security measures are taken to
keep it trustworthy. Typically the CPE or the PE can encapsulate
a received packet in an outer IPv6 header which contains the
Alternate Marking data. They can also be able to filter and drop
packets from outside of the domain with inconsistent fields to
make effective the relevant security rules at the domain
boundaries, for example a simple security check can be to insert
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the Alternate Marking data if and only if the destination is
within the controlled domain.
3. Definition of the AltMark Option
The definition of a TLV for the Options Extension Headers, carrying
the data fields dedicated to the Alternate Marking method, is
reported below.
3.1. Data Fields Format
The following figure shows the data fields format for enhanced
Alternate Marking TLV (AltMark). This AltMark data can be
encapsulated in the IPv6 Options Headers (Hop-by-Hop or Destination
Option).
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option Type | Opt Data Len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| FlowMonID |L|D| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
o Option Type: 8-bit identifier of the type of Option that needs to
be allocated. Unrecognized Types MUST be ignored on processing.
For Hop-by-Hop Options Header or Destination Options Header,
[RFC8200] defines how to encode the three high-order bits of the
Option Type field. The two high-order bits specify the action
that must be taken if the processing IPv6 node does not recognize
the Option Type; for AltMark these two bits MUST be set to 00
(skip over this Option and continue processing the header). The
third-highest-order bit specifies whether the Option Data can
change en route to the packet's final destination; for AltMark the
value of this bit MUST be set to 0 (Option Data does not change en
route). In this way, since the three high-order bits of the
AltMark Option are set to 000, it means that nodes can simply skip
this Option if they do not recognize and that the data of this
Option do not change en route, indeed the source is the only one
that can write it.
o Opt Data Len: 4. It is the length of the Option Data Fields of
this Option in bytes.
o FlowMonID: 20-bit unsigned integer. The FlowMon identifier is
described in Section 5.3. As further discussed below, it has been
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picked as 20 bits since it is a reasonable value and a good
compromise in relation to the chance of collision. It MUST be set
pseudo randomly by the source node or by a centralized controller.
o L: Loss flag for Packet Loss Measurement as described in
Section 5.1;
o D: Delay flag for Single Packet Delay Measurement as described in
Section 5.2;
o Reserved: is reserved for future use. These bits MUST be set to
zero on transmission and ignored on receipt.
4. Use of the AltMark Option
The AltMark Option is the best way to implement the Alternate Marking
method and it is carried by the Hop-by-Hop Options header and the
Destination Options header. In case of Destination Option, it is
processed only by the source and destination nodes: the source node
inserts and the destination node processes it. While, in case of
Hop-by-Hop Option, it may be examined by any node along the path, if
explicitly configured to do so.
It is important to highlight that the Option Layout can be used both
as Destination Option and as Hop-by-Hop Option depending on the Use
Cases and it is based on the chosen type of performance measurement.
In general, it is needed to perform both end to end and hop by hop
measurements, and the Alternate Marking methodology allows, by
definition, both performance measurements. In many cases the end-to-
end measurement is not enough and it is required the hop-by-hop
measurement, so the most complete choice can be the Hop-by-Hop
Options Header.
IPv6, as specified in [RFC8200], allows nodes to optionally process
Hop-by-Hop headers. Specifically the Hop-by-Hop Options header is
not inserted or deleted, but may be examined or processed by any node
along a packet's delivery path, until the packet reaches the node (or
each of the set of nodes, in the case of multicast) identified in the
Destination Address field of the IPv6 header. Also, it is expected
that nodes along a packet's delivery path only examine and process
the Hop-by-Hop Options header if explicitly configured to do so.
Another scenario that can be mentioned is the presence of a Routing
Header, in particular it is possible to consider SRv6. A type of
Routing Header, referred as Segment Routing Header (SRH), has been
defined in [RFC8754] for SRv6. Like any other use case of IPv6, Hop-
by-Hop and Destination Options are usable when SRv6 header is
present. Because SRv6 is implemented through the SRH, Destination
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Options before the Routing Header are processed by each destination
in the route list, that means, in case of SRH, by every SR node that
is identified by the SR path. If segment list compression is in use,
as described in [I-D.ietf-spring-srv6-srh-compression], Destination
Options or Hop-by-Hop Options can always be applicable. More details
about the SRv6 application are described in
[I-D.fz-spring-srv6-alt-mark].
In summary, it is possible to list the alternative possibilities:
o Destination Option not preceding a Routing Header => measurement
only by node in Destination Address.
o Hop-by-Hop Option => every router on the path with feature
enabled.
o Destination Option preceding a Routing Header => every destination
node in the route list.
In general, Hop-by-Hop and Destination Options are the most suitable
ways to implement Alternate Marking.
It is worth mentioning that Hop-by-Hop Options are not strongly
recommended in [RFC7045] and [RFC8200], unless there is a clear
justification to standardize it, because nodes may be configured to
ignore the Options Header, drop or assign packets containing an
Options Header to a slow processing path. In case of the AltMark
data fields described in this document, the motivation to standardize
a Hop-by-Hop Option is that it is needed for OAM (Operations,
Administration, and Maintenance). An intermediate node can read it
or not, but this does not affect the packet behavior. The source
node is the only one that writes the Hop-by-Hop Option to mark
alternately the flow, so, the performance measurement can be done for
those nodes configured to read this Option, while the others are
simply not considered for the metrics.
The Hop-by-Hop Option defined in this document is designed to take
advantage of the property of how Hop-by-Hop options are processed.
Nodes that do not support this Option would be expected to ignore it
if encountered, according to the procedures of [RFC8200]. This can
mean that, in this case, the performance measurement does not account
for all links and nodes along a path. The definition of the Hop-by-
Hop Options in this document is also designed to minimize throughput
impact both on nodes that do not recognize the Option and on node
that support it. Indeed, the three high-order bits of the Options
Header defined in this draft are 000 and, in theory, as per [RFC8200]
and [I-D.ietf-6man-hbh-processing], this means "skip if do not
recognize and data do not change en route". [RFC8200] also mentions
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that the nodes only examine and process the Hop-by-Hop Options header
if explicitly configured to do so. For these reasons, this Hop-by-
Hop Option should not affect the throughput. However, in practice,
it is important to be aware that the things may be different in the
implementation and it can happen that packets with Hop-by-Hop are
forced onto the slow path, but this is a general issue, as also
explained in [I-D.ietf-6man-hbh-processing]. It is also worth
mentioning that the application to a controlled domain should avoid
the risk of arbitrary nodes dropping packets with Hop-by-Hop Options.
5. Alternate Marking Method Operation
This section describes how the method operates.
[I-D.ietf-ippm-rfc8321bis] introduces several applicable methods
which are reported below, and an additional field is introduced to
facilitate the deployment and improve the scalability.
5.1. Packet Loss Measurement
The measurement of the packet loss is really straightforward in
comparison to the existing mechanisms, as detailed in
[I-D.ietf-ippm-rfc8321bis]. The packets of the flow are grouped into
batches, and all the packets within a batch are marked by setting the
L bit (Loss flag) to a same value. The source node can switch the
value of the L bit between 0 and 1 after a fixed number of packets or
according to a fixed timer, and this depends on the implementation.
The source node is the only one that marks the packets to create the
batches, while the intermediate nodes only read the marking values
and identify the packet batches. By counting the number of packets
in each batch and comparing the values measured by different network
nodes along the path, it is possible to measure the packet loss
occurred in any single batch between any two nodes. Each batch
represents a measurable entity recognizable by all network nodes
along the path.
Both fixed number of packets and fixed timer can be used by the
source node to create packet batches. But, as also explained in
[I-D.ietf-ippm-rfc8321bis], the timer-based batches are preferable
because they are more deterministic than the counter-based batches.
There is no definitive rule for counter-based batches, differently
from timer-based batches. Using a fixed timer for the switching
offers better control over the method, indeed the length of the
batches can be chosen large enough to simplify the collection and the
comparison of the measures taken by different network nodes. In the
implementation the counters can be sent out by each node to the
controller that is responsible for the calculation. It is also
possible to exchange this information by using other on-path
techniques. But this is out of scope for this document.
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Packets with different L values may get swapped at batch boundaries,
and in this case, it is required that each marked packet can be
assigned to the right batch by each router. It is important to
mention that for the application of this method there are two
elements to consider: the clock error between network nodes and the
network delay. These can create offsets between the batches and out-
of-order of the packets. The mathematical formula on timing aspects,
explained in section 5 of [I-D.ietf-ippm-rfc8321bis], must be
satisfied and it takes into considerations the different causes of
reordering such as clock error and network delay. The assumption is
to define the available counting interval where to get stable
counters and to avoid these issues. Specifically, if the effects of
network delay are ignored, the condition to implement the methodology
is that the clocks in different nodes MUST be synchronized to the
same clock reference with an accuracy of +/- B/2 time units, where B
is the fixed time duration of the batch. In this way each marked
packet can be assigned to the right batch by each node. Usually the
counters can be taken in the middle of the batch period to be sure to
read quiescent counters. In a few words this implies that the length
of the batches MUST be chosen large enough so that the method is not
affected by those factors. The length of the batches can be
determined based on the specific deployment scenario.
L bit=1 ----------+ +-----------+ +----------
| | | |
L bit=0 +-----------+ +-----------+
Batch n ... Batch 3 Batch 2 Batch 1
<---------> <---------> <---------> <---------> <--------->
Traffic Flow
===========================================================>
L bit ...1111111111 0000000000 11111111111 00000000000 111111111...
===========================================================>
Figure 1: Packet Loss Measurement and Single-Marking Methodology
using L bit
It is worth mentioning that the duration of the batches is considered
stable over time in the previous figure. In theory, it is possible
to change the length of batches over time and among different flows
for more flexibility. But, in practice, it could complicate the
correlation of the information.
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5.2. Packet Delay Measurement
The same principle used to measure packet loss can be applied also to
one-way delay measurement. Delay metrics MAY be calculated using the
two possibilities:
1. Single-Marking Methodology: This approach uses only the L bit to
calculate both packet loss and delay. In this case, the D flag
MUST be set to zero on transmit and ignored by the monitoring
points. The alternation of the values of the L bit can be used
as a time reference to calculate the delay. Whenever the L bit
changes and a new batch starts, a network node can store the
timestamp of the first packet of the new batch, that timestamp
can be compared with the timestamp of the first packet of the
same batch on a second node to compute packet delay. But this
measurement is accurate only if no packet loss occurs and if
there is no packet reordering at the edges of the batches. A
different approach can also be considered and it is based on the
concept of the mean delay. The mean delay for each batch is
calculated by considering the average arrival time of the packets
for the relative batch. There are limitations also in this case
indeed, each node needs to collect all the timestamps and
calculate the average timestamp for each batch. In addition, the
information is limited to a mean value.
2. Double-Marking Methodology: This approach is more complete and
uses the L bit only to calculate packet loss and the D bit (Delay
flag) is fully dedicated to delay measurements. The idea is to
use the first marking with the L bit to create the alternate flow
and, within the batches identified by the L bit, a second marking
is used to select the packets for measuring delay. The D bit
creates a new set of marked packets that are fully identified
over the network, so that a network node can store the timestamps
of these packets; these timestamps can be compared with the
timestamps of the same packets on a second node to compute packet
delay values for each packet. The most efficient and robust mode
is to select a single double-marked packet for each batch, in
this way there is no time gap to consider between the double-
marked packets to avoid their reorder. Regarding the rule for
the selection of the packet to be double-marked, the same
considerations in Section 5.1 apply also here and the double-
marked packet can be chosen within the available counting
interval that is not affected by factors such as clock errors.
If a double-marked packet is lost, the delay measurement for the
considered batch is simply discarded, but this is not a big
problem because it is easy to recognize the problematic batch and
skip the measurement just for that one. So in order to have more
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information about the delay and to overcome out-of-order issues
this method is preferred.
In summary the approach with double marking is better than the
approach with single marking. Moreover, the two approaches provide
slightly different pieces of information and the data consumer can
combine them to have a more robust data set.
Similar to what said in Section 5.1 for the packet counters, in the
implementation the timestamps can be sent out to the controller that
is responsible for the calculation or could also be exchanged using
other on-path techniques. But this is out of scope for this
document.
L bit=1 ----------+ +-----------+ +----------
| | | |
L bit=0 +-----------+ +-----------+
D bit=1 + + + + +
| | | | |
D bit=0 ------+----------+----------+----------+------------+-----
Traffic Flow
===========================================================>
L bit ...1111111111 0000000000 11111111111 00000000000 111111111...
D bit ...0000010000 0000010000 00000100000 00001000000 000001000...
===========================================================>
Figure 2: Double-Marking Methodology using L bit and D bit
Likewise to packet delay measurement (both for Single Marking and
Double Marking), the method can also be used to measure the inter-
arrival jitter.
5.3. Flow Monitoring Identification
The Flow Monitoring Identification (FlowMonID) identifies the flow to
be measured and is required for some general reasons:
First, it helps to reduce the per node configuration. Otherwise,
each node needs to configure an access-control list (ACL) for each
of the monitored flows. Moreover, using a flow identifier allows
a flexible granularity for the flow definition, indeed, it can be
used together with other identifiers (e.g. 5-tuple).
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Second, it simplifies the counters handling. Hardware processing
of flow tuples (and ACL matching) is challenging and often incurs
into performance issues, especially in tunnel interfaces.
Third, it eases the data export encapsulation and correlation for
the collectors.
The FlowMonID MUST only be used as a monitored flow identifier in
order to determine a monitored flow within the measurement domain.
This entails not only an easy identification but improved correlation
as well.
The FlowMonID allocation procedure can be stateful or stateless. In
case of a stateful approach, it is required that each source node
stores and keeps track of the FlowMonID historic information in order
to assign unique values within the domain. This may imply a complex
procedure and it is considered out of scope for this document. The
stateless approach is described hereinafter where FlowMonID values
are pseudo randomly generated.
The value of 20 bits has been selected for the FlowMonID since it is
a good compromise and implies a low rate of ambiguous FlowMonIDs that
can be considered acceptable in most of the applications. The
disambiguation issue can be solved by tagging the pseudo randomly
generated FlowMonID with additional flow information. In particular,
it is RECOMMENDED to consider the 3-tuple FlowMonID, source and
destination addresses:
o If the 20 bit FlowMonID is set independently and pseudo randomly
in a distributed way there is a chance of collision. Indeed, by
using the well-known birthday problem in probability theory, if
the 20 bit FlowMonID is set independently and pseudo randomly
without any additional input entropy, there is a 50% chance of
collision for 1206 flows. So, for more entropy, FlowMonID is
combined with source and destination addresses. Since there is a
1% chance of collision for 145 flows, it is possible to monitor
145 concurrent flows per host pairs with a 1% chance of collision.
o If the 20 bits FlowMonID is set pseudo randomly but in a
centralized way, the controller can instruct the nodes properly in
order to guarantee the uniqueness of the FlowMonID. With 20 bits,
the number of combinations is 1048576, and the controller should
ensure that all the FlowMonID values are used without any
collision. Therefore, by considering source and destination
addresses together with the FlowMonID, it can be possible to
monitor 1048576 concurrent flows per host pairs.
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A consistent approach MUST be used in the Alternate Marking
deployment to avoid the mixture of different ways of identifying.
All the nodes along the path and involved into the measurement SHOULD
use the same mode for identification. As mentioned, it is
RECOMMENDED to use the FlowMonID for identification purpose in
combination with source and destination addresses to identify a flow.
By considering source and destination addresses together with the
FlowMonID it can be possible to monitor 145 concurrent flows per host
pairs with a 1% chance of collision in case of pseudo randomly
generated FlowMonID, or 1048576 concurrent flows per host pairs in
case of centralized controller. It is worth mentioning that the
solution with the centralized control allows finer granularity and
therefore adds even more flexibility to the flow identification.
The FlowMonID field is set at the source node, which is the ingress
point of the measurement domain, and can be set in two ways:
a. It can be algorithmically generated by the source node, that can
set it pseudo-randomly with some chance of collision. This
approach cannot guarantee the uniqueness of FlowMonID since
conflicts and collisions are possible. But, considering the
recommendation to use FlowMonID with source and destination
addresses the conflict probability is reduced due to the
FlowMonID space available for each endpoint pair (i.e. 145 flows
with 1% chance of collision).
b. It can be assigned by the central controller. Since the
controller knows the network topology, it can allocate the value
properly to avoid or minimize ambiguity and guarantee the
uniqueness. In this regard, the controller can verify that there
is no ambiguity between different pseudo-randomly generated
FlowMonIDs on the same path. The conflict probability is really
small given that the FlowMonID is coupled with source and
destination addresses and up to 1048576 flows can be monitored
for each endpoint pair. When all values in the FlowMonID space
are consumed, the centralized controller can keep track and
reassign the values that are not used any more by old flows.
If the FlowMonID is set by the source node, the intermediate nodes
can read the FlowMonIDs from the packets in flight and act
accordingly. While, if the FlowMonID is set by the controller, both
possibilities are feasible for the intermediate nodes which can learn
by reading the packets or can be instructed by the controller.
The FlowMonID setting by the source node may seem faster and more
scalable than the FlowMonID setting by the controller. But, it is
supposed that the controller does not slow the process since it can
enable Alternate Marking method and its parameters (like FlowMonID)
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together with the flow instantiation, as further described in
[I-D.ietf-idr-sr-policy-ifit] and [I-D.chen-pce-pcep-ifit].
5.4. Multipoint and Clustered Alternate Marking
The Alternate Marking method can be extended to any kind of
multipoint to multipoint paths. [I-D.ietf-ippm-rfc8321bis] only
applies to point-to-point unicast flows, while the Multipoint
Alternate Marking Clustered method, introduced in
[I-D.ietf-ippm-rfc8889bis], is valid for multipoint-to-multipoint
unicast flows, anycast and ECMP flows.
[I-D.ietf-ippm-rfc8889bis] describes the network clustering approach
which allows a flexible and optimized performance measurement. A
Cluster is the smallest identifiable subnetwork (composed by more
than one node) of the entire Network graph that still satisfies the
condition that the number of packets that goes in is the same that
goes out. 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.
For Multipoint Alternate Marking, FlowMonID can identify in general a
multipoint-to-multipoint flow and not only a point-to-point flow.
5.5. Data Collection and Calculation
The nodes enabled to perform performance monitoring collect the value
of the packet counters and timestamps. There are several
alternatives to implement Data Collection and Calculation, but this
is not specified in this document.
There are documents on the control plane mechanisms of Alternate
Marking, e.g. [I-D.ietf-idr-sr-policy-ifit],
[I-D.chen-pce-pcep-ifit].
6. Security Considerations
This document aims to apply a method to perform measurements that
does not directly affect Internet security nor applications that run
on the Internet. However, implementation of this method must be
mindful of security and privacy concerns.
There are two types of security concerns: potential harm caused by
the measurements and potential harm to the measurements.
Harm caused by the measurement: Alternate Marking implies the
insertion of an Option Header to the IPv6 packets by the source node,
but this must be performed in a way that does not alter the quality
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of service experienced by the packets and that preserves stability
and performance of routers doing the measurements. As already
discussed in Section 4, the design of the AltMark Option has been
chosen with throughput in mind, such that it can be implemented
without affecting the user experience.
Harm to the measurement: Alternate Marking measurements could be
harmed by routers altering the fields of the AltMark Option (e.g.
marking of the packets, FlowMonID) or by a malicious attacker adding
AltMark Option to the packets in order to consume the resources of
network devices and entities involved. As described above, the
source node is the only one that writes the Option Header while the
intermediate nodes and destination node only read it without
modifying the Option Header. But, for example, an on-path attacker
can modify the flags, whether intentionally or accidentally, or
deliberately insert an option to the packet flow or delete the option
from the packet flow. The consequent effect could be to give the
appearance of loss or delay or invalidate the measurement by
modifying option identifiers, such as FlowMonID. The malicious
implication can be to cause actions from the network administrator
where an intervention is not necessary or to hide real issues in the
network. Since the measurement itself may be affected by network
nodes intentionally altering the bits of the AltMark Option or
injecting Options headers as a means for Denial of Service (DoS), the
Alternate Marking MUST be applied in the context of a controlled
domain, where the network nodes are locally administered and this
type of attack can be avoided. For this reason, the implementation
of the method is not done on the end node if it is not fully managed
and does not belong to the controlled domain. Packets generated
outside the controlled domain may consume router resources by
maliciously using the HbH Option, but this can be mitigated by
filtering these packets at the controlled domain boundary. This can
be done because, if the end node does not belong to the controlled
domain, it is not supposed to add the AltMark HbH Option, and it can
be easily recognized.
An attacker that does not belong to the controlled domain can
maliciously send packets with AltMark Option. But if Alternate
Marking is not supported in the controlled domain, no problem happens
because the AltMark Option is treated as any other unrecognized
option and will not be considered by the nodes since they are not
configured to deal with it, so the only effect is the increased
packet size (by 48 bits). While if Alternate Marking is supported in
the controlled domain, it is also necessary to avoid that the
measurements are affected and external packets with AltMark Option
MUST be filtered. As any other Hop-by-Hop Options or Destination
Options, it is possible to filter AltMark Options entering or leaving
the domain e.g. by using ACL extensions for filtering.
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The flow identifier (FlowMonID), together with the two marking bit (L
and D), comprises the AltMark Option. As explained in Section 5.3,
there is a chance of collision if the FlowMonID is set pseudo
randomly but that there is a solution for this issue. In general
this may not be a problem and a low rate of ambiguous FlowMonIDs can
be acceptable, since this does not cause significant harm to the
operators or their clients and this harm may not justify the
complications of avoiding it. But, for large scale measurements, a
big number of flows could be monitored and the probability of a
collision is higher, thus the disambiguation of the FlowMonID field
can be considered.
The privacy concerns also need to be analyzed even if the method only
relies on information contained in the Option Header without any
release of user data. Indeed, from a confidentiality perspective,
although AltMark Option does not contain user data, the metadata can
be used for network reconnaissance to compromise the privacy of users
by allowing attackers to collect information about network
performance and network paths. AltMark Option contains two kinds of
metadata: the marking bits (L and D bits) and the flow identifier
(FlowMonID).
The marking bits are the small information that is exchanged
between the network nodes. Therefore, due to this intrinsic
characteristic, network reconnaissance through passive
eavesdropping on data-plane traffic is difficult. Indeed, an
attacker cannot gain information about network performance from a
single monitoring point. The only way for an attacker can be to
eavesdrop on multiple monitoring points at the same time, because
they have to do the same kind of calculation and aggregation as
Alternate Marking requires.
The FlowMonID field is used in the AltMark Option as the
identifier of the monitored flow. It represents a more sensitive
information for network reconnaissance and may allow a flow
tracking type of attack because an attacker could collect
information about network paths.
Furthermore, in a pervasive surveillance attack, the information that
can be derived over time is more. But, as further described
hereinafter, the application of the Alternate Marking to a controlled
domain helps to mitigate all the above aspects of privacy concerns.
At the management plane, attacks can be set up by misconfiguring or
by maliciously configuring AltMark Option. Thus, AltMark Option
configuration MUST be secured in a way that authenticates authorized
users and verifies the integrity of configuration procedures.
Solutions to ensure the integrity of AltMark Option are outside the
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scope of this document. Also, attacks on the reporting of the
statistics between the monitoring points and the network management
system (e.g. centralized controller) can interfere with the proper
functioning of the system. Hence, the channels used to report back
flow statistics MUST be secured.
As stated above, the precondition for the application of the
Alternate Marking is that it MUST be applied in specific controlled
domains, thus confining the potential attack vectors within the
network domain. A limited administrative domain provides the network
administrator with the means to select, monitor and control the
access to the network, making it a trusted domain. In this regard it
is expected to enforce policies at the domain boundaries to filter
both external packets with AltMark Option entering the domain and
internal packets with AltMark Option leaving the domain. Therefore,
the trusted domain is unlikely subject to hijacking of packets since
packets with AltMark Option are processed and used only within the
controlled domain.
As stated, the application to a controlled domain ensures the control
over the packets entering and leaving the domain, but despite that,
leakages may happen for different reasons, such as a failure or a
fault. In this case, nodes outside the domain are expected to ignore
packets with AltMark Option since they are not configured to handle
it and should not process it.
Additionally, it is to be noted that the AltMark Option is carried by
the Options Header and it will have some impact on the packet sizes
for the monitored flow and on the path MTU, since some packets might
exceed the MTU. However, the relative small size (48 bit in total)
of these Option Headers and its application to a controlled domain
help to mitigate the problem.
It is worth mentioning that the security concerns may change based on
the specific deployment scenario and related threat analysis, which
can lead to specific security solutions that are beyond the scope of
this document. As an example, the AltMark Option can be used as Hop-
by-Hop or Destination Option and, in case of Destination Option,
multiple administrative domains may be traversed by the AltMark
Option that is not confined to a single administrative domain. In
this case, the user, aware of the kind of risks, may still want to
use Alternate Marking for telemetry and test purposes but the
controlled domain must be composed by more than one administrative
domains. To this end, the inter-domain links need to be secured
(e.g., by IPsec, VPNs) in order to avoid external threats and realize
the whole controlled domain.
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It might be theoretically possible to modulate the marking or the
other fields of the AltMark Option to serve as a covert channel to be
used by an on-path observer. This may affect both the data and
management plane, but, here too, the application to a controlled
domain helps to reduce the effects.
The Alternate Marking application described in this document relies
on a time synchronization protocol. Thus, by attacking the time
protocol, an attacker can potentially compromise the integrity of the
measurement. A detailed discussion about the threats against time
protocols and how to mitigate them is presented in [RFC7384].
Network Time Security (NTS), described in [RFC8915], is a mechanism
that can be employed. Also, the time, which is distributed to the
network nodes through the time protocol, is centrally taken from an
external accurate time source, such as an atomic clock or a GPS
clock. By attacking the time source it can be possible to compromise
the integrity of the measurement as well. There are security
measures that can be taken to mitigate the GPS spoofing attacks and a
network administrator should certainly employ solutions to secure the
network domain.
7. IANA Considerations
The Option Type should be assigned in IANA's "Destination Options and
Hop-by-Hop Options" registry.
This draft requests the following IPv6 Option Type assignment from
the Destination Options and Hop-by-Hop Options sub-registry of
Internet Protocol Version 6 (IPv6) Parameters
(https://www.iana.org/assignments/ipv6-parameters/).
Hex Value Binary Value Description Reference
act chg rest
----------------------------------------------------------------
TBD 00 0 tbd AltMark [This draft]
8. Acknowledgements
The authors would like to thank Bob Hinden, Ole Troan, Martin Duke,
Lars Eggert, Roman Danyliw, Alvaro Retana, Eric Vyncke, Warren
Kumari, Benjamin Kaduk, Stewart Bryant, Christopher Wood, Yoshifumi
Nishida, Tom Herbert, Stefano Previdi, Brian Carpenter, Greg Mirsky,
Ron Bonica for the precious comments and suggestions.
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9. References
9.1. Normative References
[I-D.ietf-ippm-rfc8321bis]
Fioccola, G., Cociglio, M., Mirsky, G., Mizrahi, T., and
T. Zhou, "Alternate-Marking Method", draft-ietf-ippm-
rfc8321bis-02 (work in progress), June 2022.
[I-D.ietf-ippm-rfc8889bis]
Fioccola, G., Cociglio, M., Sapio, A., Sisto, R., and T.
Zhou, "Multipoint Alternate-Marking Clustered Method",
draft-ietf-ippm-rfc8889bis-02 (work in progress), June
2022.
[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>.
[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>.
[RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", STD 86, RFC 8200,
DOI 10.17487/RFC8200, July 2017,
<https://www.rfc-editor.org/info/rfc8200>.
9.2. Informative References
[I-D.chen-pce-pcep-ifit]
Yuan, H., Zhou, T., Li, W., Fioccola, G., and Y. Wang,
"Path Computation Element Communication Protocol (PCEP)
Extensions to Enable IFIT", draft-chen-pce-pcep-ifit-06
(work in progress), February 2022.
[I-D.fz-spring-srv6-alt-mark]
Fioccola, G., Zhou, T., and M. Cociglio, "Segment Routing
Header encapsulation for Alternate Marking Method", draft-
fz-spring-srv6-alt-mark-02 (work in progress), February
2022.
[I-D.ietf-6man-hbh-processing]
Hinden, R. and G. Fairhurst, "IPv6 Hop-by-Hop Options
Processing Procedures", draft-ietf-6man-hbh-processing-00
(work in progress), January 2022.
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[I-D.ietf-idr-sr-policy-ifit]
Qin, F., Yuan, H., Zhou, T., Fioccola, G., and Y. Wang,
"BGP SR Policy Extensions to Enable IFIT", draft-ietf-idr-
sr-policy-ifit-03 (work in progress), January 2022.
[I-D.ietf-spring-srv6-srh-compression]
Cheng, W., Filsfils, C., Li, Z., Decraene, B., Cai, D.,
Voyer, D., Clad, F., Zadok, S., Guichard, J. N., Aihua,
L., Raszuk, R., and C. Li, "Compressed SRv6 Segment List
Encoding in SRH", draft-ietf-spring-srv6-srh-
compression-01 (work in progress), March 2022.
[I-D.ietf-v6ops-hbh]
Peng, S., Li, Z., Xie, C., Qin, Z., and G. Mishra,
"Operational Issues with Processing of the Hop-by-Hop
Options Header", draft-ietf-v6ops-hbh-01 (work in
progress), April 2022.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, DOI 10.17487/RFC4301,
December 2005, <https://www.rfc-editor.org/info/rfc4301>.
[RFC6437] Amante, S., Carpenter, B., Jiang, S., and J. Rajahalme,
"IPv6 Flow Label Specification", RFC 6437,
DOI 10.17487/RFC6437, November 2011,
<https://www.rfc-editor.org/info/rfc6437>.
[RFC6438] Carpenter, B. and S. Amante, "Using the IPv6 Flow Label
for Equal Cost Multipath Routing and Link Aggregation in
Tunnels", RFC 6438, DOI 10.17487/RFC6438, November 2011,
<https://www.rfc-editor.org/info/rfc6438>.
[RFC7045] Carpenter, B. and S. Jiang, "Transmission and Processing
of IPv6 Extension Headers", RFC 7045,
DOI 10.17487/RFC7045, December 2013,
<https://www.rfc-editor.org/info/rfc7045>.
[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>.
[RFC8754] Filsfils, C., Ed., Dukes, D., Ed., Previdi, S., Leddy, J.,
Matsushima, S., and D. Voyer, "IPv6 Segment Routing Header
(SRH)", RFC 8754, DOI 10.17487/RFC8754, March 2020,
<https://www.rfc-editor.org/info/rfc8754>.
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[RFC8799] Carpenter, B. and B. Liu, "Limited Domains and Internet
Protocols", RFC 8799, DOI 10.17487/RFC8799, July 2020,
<https://www.rfc-editor.org/info/rfc8799>.
[RFC8915] Franke, D., Sibold, D., Teichel, K., Dansarie, M., and R.
Sundblad, "Network Time Security for the Network Time
Protocol", RFC 8915, DOI 10.17487/RFC8915, September 2020,
<https://www.rfc-editor.org/info/rfc8915>.
Authors' Addresses
Giuseppe Fioccola
Huawei
Riesstrasse, 25
Munich 80992
Germany
Email: giuseppe.fioccola@huawei.com
Tianran Zhou
Huawei
156 Beiqing Rd.
Beijing 100095
China
Email: zhoutianran@huawei.com
Mauro Cociglio
Telecom Italia
Via Reiss Romoli, 274
Torino 10148
Italy
Email: mauro.cociglio@telecomitalia.it
Fengwei Qin
China Mobile
32 Xuanwumenxi Ave.
Beijing 100032
China
Email: qinfengwei@chinamobile.com
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Ran Pang
China Unicom
9 Shouti South Rd.
Beijing 100089
China
Email: pangran@chinaunicom.cn
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