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IETF RFC 8372
MPLS Flow Identification Considerations
Last modified on Wednesday, May 2nd, 2018
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Internet Engineering Task Force (IETF) S. Bryant
Request for Comments: 8372 Huawei
Category: Informational C. Pignataro
ISSN: 2070-1721 Cisco
M. Chen
Z. Li
Huawei
G. Mirsky
ZTE Corp.
May 2018
MPLS Flow Identification Considerations
 Abstract
This document discusses aspects to consider when developing a
solution for MPLS flow identification. The key application that
needs this solution is in-band performance monitoring of MPLS flows
when MPLS is used to encapsulate user data packets.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
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/RFC 8372.
Bryant, et al. Informational PAGE 1 
RFC 8372 MPLS Flow Identification May 2018
Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Loss Measurement Considerations . . . . . . . . . . . . . . . 3
3. Delay Measurement Considerations . . . . . . . . . . . . . . 4
4. Units of Identification . . . . . . . . . . . . . . . . . . . 4
5. Types of LSP . . . . . . . . . . . . . . . . . . . . . . . . 6
6. Network Scope . . . . . . . . . . . . . . . . . . . . . . . . 7
7. Backwards Compatibility . . . . . . . . . . . . . . . . . . . 7
8. Data Plane . . . . . . . . . . . . . . . . . . . . . . . . . 7
9. Control Plane . . . . . . . . . . . . . . . . . . . . . . . . 9
10. Privacy Considerations . . . . . . . . . . . . . . . . . . . 9
11. Security Considerations . . . . . . . . . . . . . . . . . . . 9
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
13. Informative References . . . . . . . . . . . . . . . . . . . 10
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction
This document discusses the aspects that need to be considered when
developing a solution for MPLS flow identification. The key
application that needs this is in-band performance monitoring of MPLS
flows when MPLS is used to encapsulate user data packets.
There is a need to identify flows in MPLS networks for various
applications such as determining packet loss and packet delay
measurement. A method of loss and delay measurement in MPLS networks
was defined in [RFC 6374]. When used to measure packet loss,
[RFC 6374] depends on the use of injected Operations, Administration,
and Maintenance (OAM) packets to designate the beginning and the end
of the packet group over which packet loss is being measured. If the
misordering of packets from one group relative to the following group
Bryant, et al. Informational PAGE 2
RFC 8372 MPLS Flow Identification May 2018
or the misordering of any of the packets being counted relative to
the Loss Measurement packet [RFC 6374] occurs, then an error will
occur in the packet loss measurement.
In addition, [RFC 6374] did not support different granularities of
flow or address a number of multipoint cases in which two or more
ingress Label Switching Routers (LSRs) could send packets to one or
more destinations.
Due to the very low loss rate in normal operation, improvements in
link and transmission technologies have made it more difficult to
assess packet loss using active performance measurement methods with
synthetic traffic. That, together with more demanding service-level
requirements, means that network operators now need to be able to
measure the loss of the actual user data traffic using passive
performance measurement methods. Any technique deployed needs to be
transparent to the end user, and it needs to be assumed that they
will not take any active part in the measurement process. Indeed, it
is important that any flow identification technique be invisible to
them and that no remnant of the measurement process leaks into their
network.
Additionally, when there are multiple traffic sources, such as in
multipoint-to-point and multipoint-to-multipoint network
environments, there needs to be a method whereby the sink can
distinguish between packets from the various sources; that is to say,
a multipoint measurement model needs to be developed.
2. Loss Measurement Considerations
Modern networks, if not oversubscribed, generally drop relatively few
packets; thus, packet loss measurement is highly sensitive to the
common demarcation of the exact set of packets to be measured for
loss. Without some form of coloring or batch marking such as that
proposed in [RFC 8321], it may not be possible to achieve the required
accuracy in the loss measurement of customer data traffic. Thus,
when accurate measurement of packet loss is required, it may be
economically advantageous, or even be a technical requirement, to
include some form of marking in the packets to assign each packet to
a particular counter for loss measurement purposes.
When this level of accuracy is required and the traffic between a
source-destination pair is subject to Equal-Cost Multipath (ECMP), a
demarcation mechanism is needed to group the packets into batches.
Once a batch is correlated at both ingress and egress, the packet
accounting mechanism is then able to operate on the batch of packets
that can be accounted for at both the packet ingress and the packet
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RFC 8372 MPLS Flow Identification May 2018
egress. Errors in the accounting are particularly acute in Label
Switched Paths (LSPs) subjected to ECMP because the network transit
time will be different for the various ECMP paths since:
1. the packets may traverse different sets of LSRs;
2. the packets may depart from different interfaces on different
line cards on LSRs; and
3. the packets may arrive at different interfaces on different line
cards on LSRs.
A consideration with this solution on modifying the identity label
(the MPLS label ordinarily used to identify the LSP, Virtual Private
Network, Pseudowire, etc.) to indicate the batch is the impact that
this has on the path chosen by the ECMP mechanism. When the member
of the ECMP path set is chosen by deep packet inspection, a change of
batch represented by a change of identity label will have no impact
on the ECMP path. If the path member is chosen by reference to an
entropy label [RFC 6790], then changing the batch identifier will not
result in a change to the chosen ECMP path. ECMP is so pervasive in
multipoint-to-(multi)point networks that some method of avoiding
accounting errors introduced by ECMP needs to be supported.
3. Delay Measurement Considerations
Most of the existing delay measurement methods are active methods
that depend on the extra injected test packet to evaluate the delay
of a path. With the active measurement method, the rate, numbers,
and interval between the injected packets may affect the accuracy of
the results. Due to ECMP (or link aggregation techniques), injected
test packets may traverse different links from the ones used by the
data traffic. Thus, measuring the delay of the real traffic is
required.
For combined loss and delay measurements, both the loss and the delay
considerations apply.
4. Units of Identification
The most basic unit of identification is the identity of the node
that processed the packet on its entry to the MPLS network. However,
the required unit of identification may vary depending on the use
case for accounting, performance measurement, or other types of
packet observations. In particular, note that there may be a need to
impose identity at several different layers of the MPLS label stack.
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RFC 8372 MPLS Flow Identification May 2018
This document considers the identification of the following traffic
components:
o Per source LSR: everything from one source is aggregated
o Per group of LSPs chosen by an ingress LSR: an ingress LSP
aggregates a group of LSPs (e.g., all the LSPs of a tunnel)
o Per LSP: the basic form
o Per flow [RFC 6790] within an LSP: a fine-grained method
Note that a fine-grained identity resolution is needed when there is
a need to perform these operations on a flow not readily identified
by some other element in the label stack. Such a fine-grained
resolution may be possible by deep packet inspection. However, this
may not always be possible, or it may be desired to minimize
processing costs by doing this only on entry to the network. Adding
a suitable identifier to the packet for reference by other network
elements minimizes the processing needed by other network elements.
An example of such a fine-grained case might be traffic belonging to
a certain service or from a specific source, particularly if matters
related to service level agreement or application performance were
being investigated.
We can thus characterize the identification requirement in the
following broad terms:
o There needs to be some way for an egress LSR to identify the
ingress LSR with an appropriate degree of scope. This concept is
discussed further in Section 6.
o There needs to be a way to identify a specific LSP at the egress
node. This allows for the case of instrumenting multiple LSPs
operating between the same pair of nodes. In such cases, the
identity of the ingress LSR is insufficient.
o In order to conserve resources such as labels, counters, and/or
compute cycles, it may be desirable to identify an LSP group so
that an operation can be performed on the group as an aggregate.
o There needs to be a way to identify a flow within an LSP. This is
necessary when investigating a specific flow that has been
aggregated into an LSP.
The unit of identification and the method of determining which
packets constitute a flow will be specific to the application or use
case and are out of scope of this document.
Bryant, et al. Informational PAGE 5
RFC 8372 MPLS Flow Identification May 2018
5. Types of LSP
We need to consider a number of types of LSP. The two simplest types
to monitor are point-to-point LSPs and point-to-multipoint LSPs. The
ingress LSR for a point-to-point LSP, such as those created using the
Resource Reservation Protocol - Traffic Engineering (RSVP-TE)
[RFC 5420] signaling protocol or those that conform to the MPLS
Transport Profile (MPLS-TP) [RFC 5654], may be identified by
inspection of the top label in the stack because, at any provider-
edge (PE) or provider (P) router on the path, the top label is unique
to the ingress-egress pair at every hop at a given layer in the LSP
hierarchy. Provided that Penultimate Hop Popping (PHP) is disabled,
the identity of the ingress LSR of a point-to-point LSP is available
at the egress LSR; thus, determining the identity of the ingress LSR
must be regarded as a solved problem. Note, however, that the
identity of a flow cannot to be determined without further
information being carried in the packet or gleaned from some aspect
of the packet payload.
In the case of a point-to-multipoint LSP, and in the absence of PHP,
the identity of the ingress LSR may also be inferred from the top
label. However, it may not possible to adequately identify the flow
from the top label alone; thus, further information may need to be
carried in the packet or gleaned from some aspect of the packet
payload. In designing any solution, it is desirable that a common
flow identification solution be used for both point-to-point and
point-to-multipoint LSP types. Similarly, it is desirable that a
common method of LSP group identification be used. In the above
cases, a context label [RFC 5331] needs to be used to provide the
required identity information. This is a widely supported MPLS
feature.
A more interesting case is the case of a multipoint-to-point LSP. In
this case, the same label is normally used by multiple ingress or
upstream LSRs; hence, source identification is not possible by
inspection of the top label by the egress LSRs. It is therefore
necessary for a packet to be able to explicitly convey any of the
identity types described in Section 4.
Similarly, in the case of a multipoint-to-multipoint LSP, the same
label is normally used by multiple ingress or upstream LSRs; hence,
source identification is not possible by inspection of the top label
by egress LSRs. The various identity types described in Section 4
are again needed. Note, however, that the scope of the identity may
be constrained to be unique within the set of multipoint-to-
multipoint LSPs terminating on any common node.
Bryant, et al. Informational PAGE 6
RFC 8372 MPLS Flow Identification May 2018
6. Network Scope
The scope of identification can be constrained to the set of flows
that are uniquely identifiable at an ingress LSR or some aggregation
thereof. There is no need for an ingress LSR to seek assistance from
outside the MPLS protocol domain.
In any solution that constrains itself to carrying the required
identity in the MPLS label stack rather than in some different
associated data structure, constraints on the choice of label and
label stack size imply that the scope of identity resides within that
MPLS domain. For similar reasons, the identity scope of a component
of an LSP is constrained to the scope of that LSP.
7. Backwards Compatibility
In any network, it is unlikely that all LSRs will have the same
capability to support the methods of identification discussed in this
document. It is therefore an important constraint on any flow
identity solution that it is backwards compatible with deployed MPLS
equipment to the extent that deploying the new feature will not
disable anything that currently works on the legacy equipment.
This is particularly the case when the deployment is incremental or
the feature is not required for all LSRs or all LSPs. Thus, the flow
identification design must support the coexistence of LSRs that can
identify the traffic components described in Section 4 and those that
cannot. In addition, the identification of the traffic components
described in Section 4 must be an optional feature that is disabled
by default. As a design simplification, a solution may require that
all egress LSRs of a point-to-multipoint or a multipoint-to-
multipoint LSP support the identification type in use so that a
single packet can be correctly processed by all egress devices. The
corollary of this last point is that either all egress LSRs are
enabled to support the required identity type or none of them are.
8. Data Plane
There is a huge installed base of MPLS equipment; typically, this
type of equipment remains in service for an extended period of time,
and in many cases, hardware constraints mean that it is not possible
to upgrade its data-plane functionality. Changes to the MPLS data
plane are therefore expensive to implement, add complexity to the
network, and may significantly impact the deployability of a solution
that requires such changes. For these reasons, MPLS users have set a
very high bar to changes to the MPLS data plane, and only a very
small number have been adopted. Hence, it is important that the
method of identification must minimize changes to the MPLS data
Bryant, et al. Informational PAGE 7
RFC 8372 MPLS Flow Identification May 2018
plane. Ideally, method(s) of identification that require no changes
to the MPLS data plane should be given preferential consideration.
If a method of identification that makes a change to the data plane
is chosen, it will need to have a significant advantage over any
method that makes no change, and the advantage of the approach will
need to be carefully evaluated and documented. If a change to the
MPLS data plane proves necessary, it should be (a) as small a change
as possible and (b) a general-purpose method, so as to maximize its
use for future applications. It is imperative that, as far as can be
foreseen, any necessary change made to the MPLS data plane does not
impose any foreseeable future limitation on the MPLS data plane.
Stack size is an issue with many MPLS implementations both as a
result of hardware limitations and due to the impact on networks and
applications in which a large number of small payloads need to be
transported. In particular, one MPLS payload may be carried inside
another. For example, one LSP may be carried over another LSP, or a
Pseudowire (PW) or similar multiplexing construct may be carried over
an LSP, and identification may be required at both layers. Of
particular concern is the implementation of low-cost edge LSRs that,
for cost reasons, have a significant limit on the number of Label
Stack Entries (LSEs) that they can impose or dispose. Therefore, any
method of identity must not consume an excessive number of unique
labels and must not result in an excessive increase in the size of
the label stack.
The design of the MPLS data plane provides two types of special-
purpose labels: the original 16 reserved labels and the much larger
set of special-purpose labels defined in [RFC 7274]. The original
reserved labels need one LSE, and the newer special-purpose labels
[RFC 7274] need two LSEs. Given the tiny number of original reserved
labels, it is core to the MPLS design philosophy that this scarce
resource is only used when it is absolutely necessary. Using a
special-purpose label to encode flow identity requires two label
stack entries, one for the reserved label and one for the flow
identity. Use of extended special-purpose labels [RFC 7274] requires
a total of three label stack entries to encode the flow identity.
The larger set of [RFC 7274] labels requires two label stack entries
for the special-purpose label itself; hence, a total of three label
stack entries is needed to encode the flow identity.
The use of special-purpose labels [RFC 7274] as part of a method to
encode the identity information therefore has a number of undesirable
implications for the data plane. Thus, while a solution may use
special-purpose labels, methods that do not require special-purpose
labels need to be carefully considered.
Bryant, et al. Informational PAGE 8
RFC 8372 MPLS Flow Identification May 2018
9. Control Plane
Any flow identity design should both seek to minimize the complexity
of the control plane and minimize the amount of label coordination
needed amongst LSRs.
10. Privacy Considerations
The inclusion of originating and/or flow information in a packet
provides more identity information and hence potentially degrades the
privacy of the communication.
Recent IETF concerns on pervasive monitoring [RFC 7258] have resulted
in a preference for a solution that does not degrade the privacy of
user traffic below that of an MPLS network not implementing the flow
identification feature. The choice of using MPLS technology for this
OAM solution has a privacy advantage, as the choice of the label
identifying a flow is limited to the scope of the MPLS domain and
does not have any dependency on the identification of the user data.
This minimizes the observability of the flow characteristics.
11. Security Considerations
Any flow identification solution must not degrade the security of the
MPLS network below that of an equivalent network not deploying the
specified identity solution. In order to preserve present
assumptions about MPLS privacy properties, propagation of
identification information outside the MPLS network imposing it must
be disabled by default. Any solution should provide for the
restriction of the identity information to those components of the
network that need to know it. It is thus desirable to limit the
knowledge of the identify of an endpoint to only those LSRs that need
to participate in traffic flow. The choice of using MPLS technology
for this OAM solution, with MPLS encapsulation of user traffic,
provides for a key advantage over other data-plane solutions, as it
provides for a controlled access and trusted domain within a service
provider's network.
For a more comprehensive discussion of MPLS security and attack
mitigation techniques, please see "Security Framework for MPLS and
GMPLS Networks" [RFC 5920].
12. IANA Considerations
This document has no IANA considerations.
Bryant, et al. Informational PAGE 9
RFC 8372 MPLS Flow Identification May 2018
13. Informative References
[RFC 5331] Aggarwal, R., Rekhter, Y., and E. Rosen, "MPLS Upstream
Label Assignment and Context-Specific Label Space",
RFC 5331, DOI 10.17487/RFC 5331, August 2008,
<https://www.rfc-editor.org/info/RFC 5331>.
[RFC 5420] Farrel, A., Ed., Papadimitriou, D., Vasseur, JP., and A.
Ayyangarps, "Encoding of Attributes for MPLS LSP
Establishment Using Resource Reservation Protocol Traffic
Engineering (RSVP-TE)", RFC 5420, DOI 10.17487/RFC 5420,
February 2009, <https://www.rfc-editor.org/info/RFC 5420>.
[RFC 5654] Niven-Jenkins, B., Ed., Brungard, D., Ed., Betts, M., Ed.,
Sprecher, N., and S. Ueno, "Requirements of an MPLS
Transport Profile", RFC 5654, DOI 10.17487/RFC 5654,
September 2009, <https://www.rfc-editor.org/info/RFC 5654>.
[RFC 5920] Fang, L., Ed., "Security Framework for MPLS and GMPLS
Networks", RFC 5920, DOI 10.17487/RFC 5920, July 2010,
<https://www.rfc-editor.org/info/RFC 5920>.
[RFC 6374] Frost, D. and S. Bryant, "Packet Loss and Delay
Measurement for MPLS Networks", RFC 6374,
DOI 10.17487/RFC 6374, September 2011,
<https://www.rfc-editor.org/info/RFC 6374>.
[RFC 6790] Kompella, K., Drake, J., Amante, S., Henderickx, W., and
L. Yong, "The Use of Entropy Labels in MPLS Forwarding",
RFC 6790, DOI 10.17487/RFC 6790, November 2012,
<https://www.rfc-editor.org/info/RFC 6790>.
[RFC 7258] Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
Attack", BCP 188, RFC 7258, DOI 10.17487/RFC 7258, May
2014, <https://www.rfc-editor.org/info/RFC 7258>.
[RFC 7274] Kompella, K., Andersson, L., and A. Farrel, "Allocating
and Retiring Special-Purpose MPLS Labels", RFC 7274,
DOI 10.17487/RFC 7274, June 2014,
<https://www.rfc-editor.org/info/RFC 7274>.
[RFC 8321] 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/RFC 8321,
January 2018, <https://www.rfc-editor.org/info/RFC 8321>.
Bryant, et al. Informational PAGE 10
RFC 8372 MPLS Flow Identification May 2018
Acknowledgments
The authors thank Nobo Akiya, Nagendra Kumar Nainar, George Swallow,
and Deborah Brungard for their comments.
Authors' Addresses
Stewart Bryant
Huawei
Email: stewart.bryant@gmail.com
Carlos Pignataro
Cisco Systems, Inc.
Email: cpignata@cisco.com
Mach(Guoyi) Chen
Huawei
Email: mach.chen@huawei.com
Zhenbin Li
Huawei
Email: lizhenbin@huawei.com
Gregory Mirsky
ZTE Corp.
Email: gregimirsky@gmail.com
Bryant, et al. Informational PAGE 11
MPLS Flow Identification Considerations
RFC TOTAL SIZE: 26645 bytes
PUBLICATION DATE: Wednesday, May 2nd, 2018
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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