Internet DRAFT - draft-bryant-mpls-sfl-framework
draft-bryant-mpls-sfl-framework
MPLS Working Group S. Bryant
Internet-Draft M. Chen
Intended status: Informational Z. Li
Expires: December 28, 2017 Huawei
G. Swallow
S. Sivabalan
Cisco Systems
G. Mirsky
Ericsson
June 26, 2017
Synonymous Flow Label Framework
draft-bryant-mpls-sfl-framework-05
Abstract
draft-ietf-mpls-flow-ident 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 which mimic the behaviour of
other labels provide the identification service. These identifiers
can be used to trigger per-flow operations on the on the packet at
the receiving label switching router.
Status of This Memo
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This Internet-Draft will expire on December 28, 2017.
Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 2
3. Synonymous Flow Labels . . . . . . . . . . . . . . . . . . . 3
4. User Service Traffic in the Data Plane . . . . . . . . . . . 4
4.1. Applications Label Present . . . . . . . . . . . . . . . 4
4.1.1. Setting TTL and the Traffic Class Bits . . . . . . . 5
4.2. Single Label Stack . . . . . . . . . . . . . . . . . . . 5
4.2.1. Setting TTL and the Traffic Class Bits . . . . . . . 6
4.3. Aggregation of SFL Actions . . . . . . . . . . . . . . . 6
5. Equal Cost Multipath Considerations . . . . . . . . . . . . . 7
6. Privacy Considerations . . . . . . . . . . . . . . . . . . . 8
7. Security Considerations . . . . . . . . . . . . . . . . . . . 8
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
9.1. Normative References . . . . . . . . . . . . . . . . . . 8
9.2. Informative References . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
[I-D.ietf-mpls-flow-ident] 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 (SFL) (see (Section 2)) in
which labels which mimic the behaviour 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.
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
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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
behaviour at the egress Label Switching Router (LSR) as the label it
replaces, but in addition also causes an agreed action to take place
on the packet. There are many possible additional actions such as
the measurement of the number of received packets in a flow,
triggering IPFIX inspection, triggering other types of Deep Packet
Inspection, or identification of the packet source. In, for example,
a Performance Monitoring (PM) application, the agreed action could be
the recording of 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.
Consider an MPLS application such as a pseudowire (PW), and consider
that it is desired to use the approach specified in this document to
make a packet loss measurement. By some method outside the scope of
this text, 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 behaviour (note that strictly only one SFL is needed in
this application, but that is an optimization that is a matter for
the implementor).
Now consider an MPLS application that is multi-point 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 co-ordinating with
the destination. Each ingress is thus able to establish its own SFL
to identify the sub-flow 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 an LSP. 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. We therefore 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
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an MPLS explicit NULL. In both of these cases the label has the
additional semantics of the SFL.
Note that to achieve the goals set out in Section 1 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. Applications label present
2. Single label stack
4.1. Applications 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 | <May be PHPed
| Label | | Label |
+-----------------+ +-----------------+
| | | |
| Application | | Synonymous Flow |
| Label | | Label |
+-----------------+ +-----------------+ <= 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 LSR the LSP label is popped (if present). Then the SFL
is processed in exactly the same way as the corresponding application
label would have been processed.
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4.1.1. Setting TTL and the Traffic Class Bits
The TTL and the Traffic Class bits [RFC5462] in the SFL 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 recognised that there
may be an applications need to set the SFL 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 LSR to first push the SFL and then to push
the LSP label.
+-----------------+
| |
| LSP | <= May be PHPed
| Label |
+-----------------+ +-----------------+
| | | | <= Synonymous with
| LSP | | Synonymous Flow | Explicit NULL
| Label | | Label |
+-----------------+ +-----------------+ <= 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 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 processed exactly as it would
normally processed and then it is popped. This reveals the SFL which
in the case of [RFC6374] measurements is simply counted and then
discarded. In this respect the processing of the SFL is synonymous
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with an 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. Either
the SFL can be treated as an LSP label that was not PHPed and the
additional associated SFL action is taken when the label is
processed. Alternatively, it can be treated as an explicit NULL with
associated SFL actions. From the perspective of the measurement
system described in this document the behaviour of two approaches are
indistinguishable and 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 consequently the increase in the number of
allocated labels needed to support the SFL actions consequently
becomes 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 a existing label, but is somewhat
similar to the single label case shown in Section 4.2, and the same
signalling, management and configuration tools would be applicable.
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+-----------------+
| |
| LSP | < May be PHPed
| Label |
+-----------------+ +-----------------+
| | | |
| LSP | | Aggregate |
| Label | | SFL |
+-----------------+ +-----------------+
| | | |
| Application | | Application |
| Label | | Label |
+-----------------+ +-----------------+ <= 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 to 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 is turn may invalidate the 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 can elect to always run with the SFL in place in the
MPLS label stack.
2. The operator can elect to use [RFC6790] Entropy Labels 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 4.2 of [RFC6790]. In networks
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in which the ECMP decision is independent of both the value of
any other label in the label stack, and the MPLS payload, the
path of the flow with the SFL will be congruent with the path
without the SFL.
6. Privacy Considerations
Recent 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. 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 other than by inspection of the network at the
point of ingress, or inspection of the control protocol packets.
This privacy threat may be mitigated by encrypting the control
protocol packets, regularly changing the synonymous labels and by
concurrently using a number of such labels. Minimizing the scope of
the identity indication can be useful in minimizing the observability
of the flow characteristics.
7. Security Considerations
The issue noted in Section 6 is a security consideration. There are
no other new security issues associated with the MPLS dataplane. Any
control protocol used to request SFLs will need to ensure the
legitimacy of the request.
8. IANA Considerations
This draft makes no IANA requests.
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,
<http://www.rfc-editor.org/info/rfc2119>.
[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, <http://www.rfc-editor.org/info/rfc5462>.
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[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <http://www.rfc-editor.org/info/rfc8174>.
9.2. Informative References
[I-D.ietf-mpls-flow-ident]
Bryant, S., Pignataro, C., Chen, M., Li, Z., and G.
Mirsky, "MPLS Flow Identification Considerations", draft-
ietf-mpls-flow-ident-04 (work in progress), February 2017.
[RFC6374] Frost, D. and S. Bryant, "Packet Loss and Delay
Measurement for MPLS Networks", RFC 6374,
DOI 10.17487/RFC6374, September 2011,
<http://www.rfc-editor.org/info/rfc6374>.
[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,
<http://www.rfc-editor.org/info/rfc6790>.
[RFC7258] Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May
2014, <http://www.rfc-editor.org/info/rfc7258>.
Authors' Addresses
Stewart Bryant
Huawei
Email: stewart.bryant@gmail.com
Mach Chen
Huawei
Email: mach.chen@huawei.com
Zhenbin Li
Huawei
Email: lizhenbin@huawei.com
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George Swallow
Cisco Systems
Email: swallow@cisco.com
Siva Sivabalan
Cisco Systems
Email: msiva@cisco.com
Gregory Mirsky
Ericsson
Email: gregory.mirsky@eicsson.com
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