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Keywords: MPLS, Flow, Label

Internet Engineering Task Force (IETF) S. Bryant
Request for Comments: 8957 Futurewei Technologies Inc.
Category: Standards Track M. Chen
ISSN: 2070-1721 Huawei
G. Swallow
Southend Technical Center
S. Sivabalan
Ciena Corporation
G. Mirsky
ZTE Corp.
January 2021
Synonymous Flow Label Framework
Abstract
RFC 8372 ("MPLS Flow Identification Considerations") describes the
requirement for introducing flow identities within the MPLS
architecture. This document describes a method of accomplishing this
by using a technique called "Synonymous Flow Labels" in which labels
that mimic the behavior of other labels provide the identification
service. These identifiers can be used to trigger per-flow
operations on the packet at the receiving label switching router.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8957.
Copyright Notice
Copyright (c) 2021 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
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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. Requirements Language
3. Synonymous Flow Labels
4. User Service Traffic in the Data Plane
4.1. Application Label Present
4.1.1. Setting TTL and the Traffic Class Bits
4.2. Single-Label Stack
4.2.1. Setting TTL and the Traffic Class Bits
4.3. Aggregation of SFL Actions
5. Equal-Cost Multipath Considerations
6. Privacy Considerations
7. Security Considerations
8. IANA Considerations
9. References
9.1. Normative References
9.2. Informative References
Contributors
Authors' Addresses
1. Introduction
[RFC8372] ("MPLS Flow Identification Considerations") describes the
requirement for introducing flow identities within the MPLS
architecture. This document describes a method of providing the
required identification by using a technique called "Synonymous Flow
Labels (SFLs)" in which labels that mimic the behavior of other MPLS
labels provide the identification service. These identifiers can be
used to trigger per-flow operations on the packet at the receiving
label switching router.
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.
3. Synonymous Flow Labels
An SFL is defined to be a label that causes exactly the same behavior
at the egress Label Edge Router (LER) as the label it replaces,
except that it also causes one or more additional actions that have
been previously agreed between the peer LERs to be executed on the
packet. There are many possible additional actions, such as
measuring the number of received packets in a flow, triggering an IP
Flow Information Export (IPFIX) [RFC7011] capture, triggering other
types of deep packet inspection, or identifying the packet source.
For example, in a Performance Monitoring (PM) application, the agreed
action could be recording the receipt of the packet by incrementing a
packet counter. This is a natural action in many MPLS
implementations, and where supported, this permits the implementation
of high-quality packet loss measurement without any change to the
packet-forwarding system.
To illustrate the use of this technology, we start by considering the
case where there is an "application" label in the MPLS label stack.
As a first example, let us consider a pseudowire (PW) [RFC3985] on
which it is desired to make packet loss measurements. Two labels,
synonymous with the PW labels, are obtained from the egress
terminating provider edge (T-PE). By alternating between these SFLs
and using them in place of the PW label, the PW packets may be
batched for counting without any impact on the PW forwarding behavior
[RFC8321] (note that strictly only one SFL is needed in this
application, but that is an optimization that is a matter for the
implementor). The method of obtaining these additional labels is
outside the scope of this text; however, one control protocol that
provides a method of obtaining SFLs is described in
[MPLS-SFL-CONTROL].
Next, consider an MPLS application that is multipoint to point, such
as a VPN. Here, it is necessary to identify a packet batch from a
specific source. This is achieved by making the SFLs source
specific, so that batches from one source are marked differently from
batches from another source. The sources all operate independently
and asynchronously from each other, independently coordinating with
the destination. Each ingress LER is thus able to establish its own
SFL to identify the subflow and thus enable PM per flow.
Finally, we need to consider the case where there is no MPLS
application label such as occurs when sending IP over a Label
Switched Path (LSP), i.e., there is a single label in the MPLS label
stack. In this case, introducing an SFL that was synonymous with the
LSP label would introduce network-wide forwarding state. This would
not be acceptable for scaling reasons. Therefore, we have no choice
but to introduce an additional label. Where penultimate hop popping
(PHP) is in use, the semantics of this additional label can be
similar to the LSP label. Where PHP is not in use, the semantics are
similar to an MPLS Explicit NULL [RFC3032]. In both of these cases,
the label has the additional semantics of the SFL.
Note that to achieve the goals set out above, SFLs need to be
allocated from the platform label table.
4. User Service Traffic in the Data Plane
As noted in Section 3, it is necessary to consider two cases:
1. Application label is present
2. Single-label stack
4.1. Application Label Present
Figure 1 shows the case in which both an LSP label and an application
label are present in the MPLS label stack. Traffic with no SFL
function present runs over the "normal" stack, and SFL-enabled flows
run over the SFL stack with the SFL used to indicate the packet
batch.
+-----------------+ +-----------------+
| LSP | | LSP |
| Label | | Label |
| (May be PHPed) | | (May be PHPed) |
+-----------------+ +-----------------+
| | | |
| Application | | Synonymous Flow |
| Label | | Label |
+-----------------+ <= BoS +-----------------+ <= Bottom of Stack
| | | |
| Payload | | Payload |
| | | |
+-----------------+ +-----------------+
"Normal" Label Stack Label Stack with SFL
Figure 1: Use of Synonymous Labels in a Two-Label MPLS Label Stack
At the egress LER, the LSP label is popped (if present). Then, the
SFL is processed executing both the synonymous function and the
corresponding application function.
4.1.1. Setting TTL and the Traffic Class Bits
The TTL and the Traffic Class bits [RFC5462] in the SFL label stack
entry (LSE) would normally be set to the same value as would have
been set in the label that the SFL is synonymous with. However, it
is recognized that, if there is an application need, these fields in
the SFL LSE MAY be set to some other value. An example would be
where it was desired to cause the SFL to trigger an action in the TTL
expiry exception path as part of the label action.
4.2. Single-Label Stack
Figure 2 shows the case in which only an LSP label is present in the
MPLS label stack. Traffic with no SFL function present runs over the
"normal" stack, and SFL-enabled flows run over the SFL stack with the
SFL used to indicate the packet batch. However, in this case, it is
necessary for the ingress Label Edge Router (LER) to first push the
SFL and then to push the LSP label.
+-----------------+
| LSP |
| Label |
| (May be PHPed) |
+-----------------+ +-----------------+
| LSP | | | <= Synonymous with
| Label | | Synonymous Flow | Explicit NULL
| (May be PHPed) | | Label |
+-----------------+ <= BoS +-----------------+ <= Bottom of Stack
| | | |
| Payload | | Payload |
| | | |
+-----------------+ +-----------------+
"Normal" Label Stack Label Stack with SFL
Figure 2: Use of Synonymous Labels in a Single-Label MPLS Label Stack
At the receiving Label Switching Router (LSR), it is necessary to
consider two cases:
1. Where the LSP label is still present
2. Where the LSP label is penultimate hop popped
If the LSP label is present, it is processed exactly as it would
normally be processed, and then it is popped. This reveals the SFL,
which, in the case of the measurements defined in [RFC6374], is
simply counted and then discarded. In this respect, the processing
of the SFL is synonymous with an MPLS Explicit NULL. As the SFL is
the bottom of stack, the IP packet that follows is processed as
normal.
If the LSP label is not present due to PHP action in the upstream
LSR, two almost equivalent processing actions can take place. The
SFL can be treated either 1) as an LSP label that was not PHPed and
the additional associated SFL action is taken when the label is
processed or 2) as an MPLS Explicit NULL with associated SFL actions.
From the perspective of the measurement system described in this
document, the behavior of the two approaches is indistinguishable;
thus, either may be implemented.
4.2.1. Setting TTL and the Traffic Class Bits
The TTL and the Traffic Class considerations described in
Section 4.1.1 apply.
4.3. Aggregation of SFL Actions
There are cases where it is desirable to aggregate an SFL action
against a number of labels, for example, where it is desirable to
have one counter record the number of packets received over a group
of application labels or where the number of labels used by a single
application is large and the resultant increase in the number of
allocated labels needed to support the SFL actions may become too
large to be viable. In these circumstances, it would be necessary to
introduce an additional label in the stack to act as an aggregate
instruction. This is not strictly a synonymous action in that the
SFL is not replacing an existing label but is somewhat similar to the
single-label case shown in Section 4.2, and the same signaling,
management, and configuration tools would be applicable.
+-----------------+
| LSP |
| Label |
| (May be PHPed) |
+-----------------+ +-----------------+
| LSP | | |
| Label | | Aggregate |
| (May be PHPed) | | SFL |
+-----------------+ +-----------------+
| | | |
| Application | | Application |
| Label | | Label |
+-----------------+ <=BoS +-----------------+ <= Bottom of Stack
| | | |
| Payload | | Payload |
| | | |
+-----------------+ +-----------------+
"Normal" Label Stack Label Stack with SFL
Figure 3: Aggregate SFL Actions
The aggregate SFL is shown in the label stack depicted in Figure 3 as
preceding the application label; however, the choice of position
before or after the application label will be application specific.
In the case described in Section 4.1, by definition, the SFL has the
full application context. In this case, the positioning will depend
on whether the SFL action needs the full context of the application
to perform its action and whether the complexity of the application
will be increased by finding an SFL following the application label.
5. Equal-Cost Multipath Considerations
The introduction of an SFL to an existing flow may cause that flow to
take a different path through the network under conditions of Equal-
Cost Multipath (ECMP). This, in turn, may invalidate certain uses of
the SFL, such as performance measurement applications. Where this is
a problem, there are two solutions worthy of consideration:
1. The operator MAY elect to always run with the SFL in place in the
MPLS label stack.
2. The operator can elect to use entropy labels [RFC6790] in a
network that fully supports this type of ECMP. If this approach
is adopted, the intervening MPLS network MUST NOT load balance on
any packet field other than the entropy label. Note that this is
stricter than the text in Section 4.3 of [RFC6790].
6. Privacy Considerations
IETF concerns on pervasive monitoring are described in [RFC7258].
The inclusion of originating and/or flow information in a packet
provides more identity information and hence potentially degrades the
privacy of the communication to an attacker in a position to observe
the added identifier. Whilst the inclusion of the additional
granularity does allow greater insight into the flow characteristics,
it does not specifically identify which node originated the packet
unless the attacker can inspect the network at the point of ingress
or inspect the control protocol packets. This privacy threat may be
mitigated by encrypting the control protocol packets by regularly
changing the synonymous labels or by concurrently using a number of
such labels, including the use of a combination of those methods.
Minimizing the scope of the identity indication can be useful in
minimizing the observability of the flow characteristics. Whenever
IPFIX or other deep packet inspection (DPI) technique is used, their
relevant privacy considerations apply.
7. Security Considerations
There are no new security issues associated with the MPLS data plane.
Any control protocol used to request SFLs will need to ensure the
legitimacy of the request, i.e., that the requesting node is
authorized to make that SFL request by the network operator.
8. IANA Considerations
This document has no IANA actions.
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC3032] Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack
Encoding", RFC 3032, DOI 10.17487/RFC3032, January 2001,
<https://www.rfc-editor.org/info/rfc3032>.
[RFC5462] Andersson, L. and R. Asati, "Multiprotocol Label Switching
(MPLS) Label Stack Entry: "EXP" Field Renamed to "Traffic
Class" Field", RFC 5462, DOI 10.17487/RFC5462, February
2009, <https://www.rfc-editor.org/info/rfc5462>.
[RFC6790] Kompella, K., Drake, J., Amante, S., Henderickx, W., and
L. Yong, "The Use of Entropy Labels in MPLS Forwarding",
RFC 6790, DOI 10.17487/RFC6790, November 2012,
<https://www.rfc-editor.org/info/rfc6790>.
[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>.
9.2. Informative References
[MPLS-SFL-CONTROL]
Bryant, S., Swallow, G., and S. Sivabalan, "A Simple
Control Protocol for MPLS SFLs", Work in Progress,
Internet-Draft, draft-bryant-mpls-sfl-control-09, 7
December 2020, <https://tools.ietf.org/html/draft-bryant-
mpls-sfl-control-09>.
[RFC3985] Bryant, S., Ed. and P. Pate, Ed., "Pseudo Wire Emulation
Edge-to-Edge (PWE3) Architecture", RFC 3985,
DOI 10.17487/RFC3985, March 2005,
<https://www.rfc-editor.org/info/rfc3985>.
[RFC6374] Frost, D. and S. Bryant, "Packet Loss and Delay
Measurement for MPLS Networks", RFC 6374,
DOI 10.17487/RFC6374, September 2011,
<https://www.rfc-editor.org/info/rfc6374>.
[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>.
[RFC7258] Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May
2014, <https://www.rfc-editor.org/info/rfc7258>.
[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>.
[RFC8372] Bryant, S., Pignataro, C., Chen, M., Li, Z., and G.
Mirsky, "MPLS Flow Identification Considerations",
RFC 8372, DOI 10.17487/RFC8372, May 2018,
<https://www.rfc-editor.org/info/rfc8372>.
Contributors
Zhenbin Li
Huawei
Email: lizhenbin@huawei.com
Authors' Addresses
Stewart Bryant
Futurewei Technologies Inc.
Email: sb@stewartbryant.com
Mach(Guoyi) Chen
Huawei
Email: mach.chen@huawei.com
George Swallow
Southend Technical Center
Email: swallow.ietf@gmail.com
Siva Sivabalan
Ciena Corporation
Email: ssivabal@ciena.com
Gregory Mirsky
ZTE Corp.
Email: gregimirsky@gmail.com