Internet Engineering Task Force (IETF) J. Hedin

Internet Engineering Task Force (IETF) J. Hedin
Request for Comments: 7750 G. Mirsky
Updates: 5357 S. Baillargeon
Category: Standards Track Ericsson
ISSN: 2070-1721 February 2016

Differentiated Service Code Point and Explicit Congestion Notification
Monitoring in the Two-Way Active Measurement Protocol (TWAMP)

Abstract

This document describes an optional extension for Two-Way Active
Measurement Protocol (TWAMP) allowing the monitoring of the
Differentiated Service Code Point and Explicit Congestion
Notification fields with the TWAMP-Test protocol.

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 5741.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7750.

Copyright Notice

Copyright (c) 2016 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
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.

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RFC 7750 DSCP and ECN Monitoring in TWAMP February 2016

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Conventions Used in This Document . . . . . . . . . . . . 3
1.1.1. Terminology . . . . . . . . . . . . . . . . . . . . . 3
1.1.2. Requirements Language . . . . . . . . . . . . . . . . 3
2. TWAMP Extensions . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Setting Up Connection to Monitor DSCP and ECN . . . . . . 3
2.2. TWAMP-Test Extension . . . . . . . . . . . . . . . . . . 4
2.2.1. Session-Reflector Packet Format for DSCP and ECN
Monitoring . . . . . . . . . . . . . . . . . . . . . 4
2.2.2. DSCP and ECN Monitoring with Extensions from RFC 6038 8
2.2.3. Consideration for TWAMP Light Mode . . . . . . . . . 8
3. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
4. Security Considerations . . . . . . . . . . . . . . . . . . . 9
5. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
5.1. Normative References . . . . . . . . . . . . . . . . . . 9
5.2. Informative References . . . . . . . . . . . . . . . . . 10
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11

1. Introduction

The One-Way Active Measurement Protocol (OWAMP) [RFC4656] defines the
Type-P Descriptor field and negotiation of its value in the OWAMP-
Control protocol. The Two-Way Active Measurement Protocol (TWAMP)
[RFC5357] states that only a Differentiated Services Code Point
(DSCP) value (see [RFC2474], [RFC3168], and [RFC3260]) can be defined
by Type-P Descriptor, and the negotiated value must be used by both
the Session-Sender and Session-Reflector. The TWAMP specification
also states that the same DSCP value (found in the Session-Sender
packet) MUST be used in the test packet reflected by the Session-
Reflector. However, the TWAMP-Test protocol does not specify any
methods to determine or report when the DSCP value has changed or is
different than expected in the forward or reverse direction. Re-
marking the DSCP (changing its original value) in IP networks is
possible and often accomplished by a Differentiated Services policy
configured on a single node along the IP path. In many cases, a
change of the DSCP value indicates an unintentional or erroneous
behavior. At best, the Session-Sender can detect a change of the
DSCP reverse direction, assuming such a change is actually
detectable.

This document describes an OPTIONAL feature for TWAMP. It is called
DSCP and ECN Monitoring. It allows the Session-Sender to know the
actual DSCP value received at the Session-Reflector. Furthermore,
this feature tracks the Explicit Congestion Notification (ECN) value
(see [RFC2474], [RFC3168], and [RFC3260]) received at the Session-

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RFC 7750 DSCP and ECN Monitoring in TWAMP February 2016

Reflector. This is helpful to determine if the ECN is actually
operating or if an ECN-capable node has detected congestion in the
forward direction.

1.1. Conventions Used in This Document

1.1.1. Terminology

DSCP: Differentiated Services Code Point

ECN: Explicit Congestion Notification

IPPM: IP Performance Metrics

TWAMP: Two-Way Active Measurement Protocol

OWAMP: One-Way Active Measurement Protocol

1.1.2. 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
[RFC2119].

2. TWAMP Extensions

TWAMP connection establishment follows the procedure defined in
Section 3.1 of [RFC4656] and Section 3.1 of [RFC5357] where the Modes
field is used to identify and select specific communication
capabilities. At the same time, the Modes field is recognized and
used as an extension mechanism [RFC6038]. The new feature requires a
new flag to identify the ability of a Session-Reflector to return the
values of received DSCP and ECN values back to a Session-Sender, and
to support the new Session-Reflector packet format in the TWAMP-Test
protocol. See Section 3 for details on the assigned bit position.

2.1. Setting Up Connection to Monitor DSCP and ECN

The Server sets the DSCP and ECN Monitoring flag in the Modes field
of the Server Greeting message to indicate its capabilities and
willingness to monitor them. If the Control-Client agrees to monitor
DSCP and ECN on some or all test sessions invoked with this control
connection, it MUST set the DSCP and ECN Monitoring flag in the Modes
field in the Setup Response message.

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RFC 7750 DSCP and ECN Monitoring in TWAMP February 2016

2.2. TWAMP-Test Extension

Monitoring of DSCP and ECN requires support by the Session-Reflector
and changes the test packet format in all the original modes
(unauthenticated, authenticated, and encrypted). Monitoring of DSCP
and ECN does not alter the Session-Sender test packet format, but
certain considerations must be taken when and if this mode is
accepted in combination with Symmetrical Size mode [RFC6038].

2.2.1. Session-Reflector Packet Format for DSCP and ECN Monitoring

When the Session-Reflector supports DSCP and ECN Monitoring, it
constructs the Sender DSCP and ECN (S-DSCP-ECN) field, presented in
Figure 1, for each test packet it sends to the Session-Sender
according to the following procedure:

o the six (least-significant) bits of the Differentiated Service
field MUST be copied from the received Session-Sender test packet
into the Sender DSCP (S-DSCP) field;

o the two bits of the ECN field MUST be copied from the received
Session-Sender test packet into the Sender ECN (S-ECN) field.

0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| S-DSCP | S-ECN |
+---+---+---+---+---+---+---+---+

Figure 1: Sender DSCP and ECN Field Format

Formats of the test packet transmitted by the Session-Reflector in
unauthenticated, authenticated, and encrypted modes have been defined
in Section 4.2.1 of [RFC5357]. For the Session-Reflector that
supports DSCP and ECN Monitoring, these formats are displayed in
Figures 2 and 3.

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RFC 7750 DSCP and ECN Monitoring in TWAMP February 2016

For unauthenticated mode:

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Timestamp |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Error Estimate | MBZ |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Receive Timestamp |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sender Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sender Timestamp |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sender Error Estimate | MBZ |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sender TTL | S-DSCP-ECN | MBZ |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Packet Padding ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 2: Session-Reflector Test Packet Format with DSCP and ECN
Monitoring in Unauthenticated Mode

The DSCP and ECN values (part of the Type-P Descriptor [RFC4656]) can
be provisioned through TWAMP-Control or by other means (command-line
interface (CLI) or Central Controller). The DSCP and ECN values are
often copied into reflected test packets with current TWAMP
implementations without TWAMP-Control protocol. With the DSCP and
ECN Monitoring extension, the Session-Reflector handles the DSCP as
follows:

o the Session-Reflector MUST extract the DSCP and ECN values from
the received packet and MUST use them to populate the S-DSCP-ECN
field of the corresponding reflected packet;

o the Session-Reflector MUST transmit each reflected test packet
with the DSCP set to the provisioned value;

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RFC 7750 DSCP and ECN Monitoring in TWAMP February 2016

o if the provisioned DSCP value is not known (e.g., TWAMP Light),
the choice of the DSCP is implementation specific. For instance,
the Session-Reflector MAY copy the DSCP value from the received
test packet and set it as the DSCP in a reflected packet.
Alternatively, the Session-Reflector MAY set the DSCP value to CS0
(zero) [RFC2474];

o if the provisioned ECN value is not known, ECN SHOULD be set to
Not-ECT codepoint value [RFC3168]. Otherwise, the provisioned ECN
value for the session SHALL be used.

A Session-Reflector in the DSCP and ECN Monitoring mode does not
analyze nor act on the ECN value of the received TWAMP test packet;
therefore, it ignores congestion indications from the network. It is
expected that sending rates are low enough, as TWAMP deployment
experience had demonstrated since TWAMP base (RFC 5357) was published
in 2008, that ignoring these congestion indications will not
significantly contribute to network congestion.

For authenticated and encrypted modes:

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| MBZ (12 octets) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Timestamp |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Error Estimate | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| MBZ (6 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Receive Timestamp |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MBZ (8 octets) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sender Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| MBZ (12 octets) |
| |

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RFC 7750 DSCP and ECN Monitoring in TWAMP February 2016

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sender Timestamp |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sender Error Estimate | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| MBZ (6 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sender TTL | S-DSCP-ECN | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| |
| MBZ (14 octets) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| HMAC (16 octets) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Packet Padding ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 3: Session-Reflector Test Packet Format with DSCP and ECN
Monitoring in Authenticated or Encrypted Modes

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2.2.2. DSCP and ECN Monitoring with Extensions from RFC 6038

[RFC6038] defined two extensions to TWAMP -- first, to ensure that
the Session-Sender and Session-Reflector exchange TWAMP-Test packets
of equal size; second, to specify the number of octets to be
reflected by Session-Reflector. If DSCP and ECN Monitoring and
Symmetrical Size and/or Reflects Octets modes are being negotiated
between Server and Control-Client in Unauthenticated mode, then,
because Sender DSCP and Sender ECN increase the size of the
unauthenticated Session-Reflector packet by 4 octets, the Padding
Length value SHOULD be greater than or equal to 28 octets to allow
for the truncation process that TWAMP recommends in Section 4.2.1 of
[RFC5357].

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Timestamp |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Error Estimate | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| |
| MBZ (28 octets) |
| |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| |
. .
. Packet Padding .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 4: Session-Sender Test Packet Format with DSCP and ECN
Monitoring and Symmetrical Test Packet in Unauthenticated Mode

2.2.3. Consideration for TWAMP Light Mode

Appendix I of [RFC5357] does not explicitly state how the value of
the Type-P Descriptor is synchronized between the Session-Sender and
Session-Reflector and whether different values are considered as
error conditions and should be reported. We assume that by some
means the Session-Sender and the Session-Reflector of the given
TWAMP-Test session have been informed to use the same DSCP value.
The same means, i.e., configuration, could be used to inform the

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Session-Reflector to support DSCP and ECN Monitoring mode by copying
data from received TWAMP test packets. Then Session-Sender may be
informed to use the Sender DSCP and ECN field in the reflected TWAMP
test packet.

3. IANA Considerations

In the TWAMP-Modes registry defined in [RFC5618], IANA has reserved a
new DSCP and ECN Monitoring Capability as follows:

+-----+---------------------------+---------------------+-----------+
| Bit | Description | Semantics | Reference |
| Pos | | Definition | |
+-----+---------------------------+---------------------+-----------+
| 8 | DSCP and ECN Monitoring | Section 2 | RFC 7750 |
| | Capability | | |
+-----+---------------------------+---------------------+-----------+

Table 1: New Type-P Descriptor Monitoring Capability

4. Security Considerations

Monitoring of DSCP and ECN does not appear to introduce any
additional security threat to hosts that communicate with TWAMP as
defined in [RFC5357] and existing extensions [RFC6038]. Sections
such as 3.2, 4, 4.1.2, 4.2, and 4.2.1 of [RFC5357] discuss
unauthenticated, authenticated, and encrypted modes in varying
degrees of detail. The security considerations that apply to any
active measurement of live networks are relevant here as well. See
the Security Considerations sections in [RFC4656] and [RFC5357].

5. References

5.1. 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>.

[RFC2474] Nichols, K., Blake, S., Baker, F., and D. Black,
"Definition of the Differentiated Services Field (DS
Field) in the IPv4 and IPv6 Headers", RFC 2474,
DOI 10.17487/RFC2474, December 1998,
<http://www.rfc-editor.org/info/rfc2474>.

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RFC 7750 DSCP and ECN Monitoring in TWAMP February 2016

[RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
of Explicit Congestion Notification (ECN) to IP",
RFC 3168, DOI 10.17487/RFC3168, September 2001,
<http://www.rfc-editor.org/info/rfc3168>.

[RFC4656] Shalunov, S., Teitelbaum, B., Karp, A., Boote, J., and M.
Zekauskas, "A One-way Active Measurement Protocol
(OWAMP)", RFC 4656, DOI 10.17487/RFC4656, September 2006,
<http://www.rfc-editor.org/info/rfc4656>.

[RFC5357] Hedayat, K., Krzanowski, R., Morton, A., Yum, K., and J.
Babiarz, "A Two-Way Active Measurement Protocol (TWAMP)",
RFC 5357, DOI 10.17487/RFC5357, October 2008,
<http://www.rfc-editor.org/info/rfc5357>.

[RFC5618] Morton, A. and K. Hedayat, "Mixed Security Mode for the
Two-Way Active Measurement Protocol (TWAMP)", RFC 5618,
DOI 10.17487/RFC5618, August 2009,
<http://www.rfc-editor.org/info/rfc5618>.

[RFC6038] Morton, A. and L. Ciavattone, "Two-Way Active Measurement
Protocol (TWAMP) Reflect Octets and Symmetrical Size
Features", RFC 6038, DOI 10.17487/RFC6038, October 2010,
<http://www.rfc-editor.org/info/rfc6038>.

5.2. Informative References

[RFC3260] Grossman, D., "New Terminology and Clarifications for
Diffserv", RFC 3260, DOI 10.17487/RFC3260, April 2002,
<http://www.rfc-editor.org/info/rfc3260>.

Acknowledgements

The authors greatly appreciate thorough review and thoughtful
comments by Bill Cerveny, Christofer Flinta, and Samita Chakrabarti.

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RFC 7750 DSCP and ECN Monitoring in TWAMP February 2016

Authors' Addresses

Jonas Hedin
Ericsson

Email: [email protected]

Greg Mirsky
Ericsson

Email: [email protected]

Steve Baillargeon
Ericsson

Email: [email protected]

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