TWAMP DSCP and ECN: Differentiated Service Code Point and Explicit Congestion Notification Monitoring in the Two-Way Active Measurement Protocol (TWAMP)
An optional TWAMP extension that lets the reflector report which DSCP and ECN values its test packets arrived with.
Hedin, J., Mirsky, G., and S. Baillargeon, "Differentiated Service Code Point and Explicit Congestion Notification Monitoring in the Two-Way Active Measurement Protocol (TWAMP)", RFC 7750, DOI 10.17487/RFC7750, February 2016, <https://www.rfc-editor.org/info/rfc7750>.
@misc{rfc7750,
series = {Request for Comments},
number = 7750,
howpublished = {RFC 7750},
publisher = {RFC Editor},
doi = {10.17487/RFC7750},
url = {https://www.rfc-editor.org/info/rfc7750},
author = {J. Hedin and G. Mirsky and S. Baillargeon},
title = {{Differentiated Service Code Point and Explicit Congestion Notification Monitoring in the Two-Way Active Measurement Protocol (TWAMP)}},
pagetotal = 11,
year = 2016,
month = feb,
} RFC text
Contents
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
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
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-
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.
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.
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;
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) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 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
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
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>.
[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.
Authors' Addresses
Jonas Hedin
Ericsson
Email: jonas.hedin@ericsson.com
Greg Mirsky
Ericsson
Email: gregory.mirsky@ericsson.com
Steve Baillargeon
Ericsson
Email: steve.baillargeon@ericsson.com
AI ELI5
On this page
§1The Problem: Silent DSCP Changes
The Two-Way Active Measurement Protocol (TWAMP, RFC 5357) measures network performance by sending test packets between a Session-Sender and a Session-Reflector. It says that only a Differentiated Services Code Point (DSCP) value can be defined by the Type-P Descriptor, and that the same DSCP value found in the Session-Sender packet MUST be used in the reflected test packet.
What TWAMP-Test does not do is say how to detect or report that the DSCP value has changed, or differs from what was expected, in the forward or reverse direction. Re-marking the DSCP is possible in IP networks and is often done by a Differentiated Services policy configured on a single node along the path. In many cases a change indicates unintentional or erroneous behavior. At best, the Session-Sender can detect a change in the reverse direction.
This RFC adds an OPTIONAL feature called DSCP and ECN Monitoring. It tells the Session-Sender the DSCP value actually received at the Session-Reflector, and also tracks the Explicit Congestion Notification (ECN) value, which helps show whether ECN is operating or whether an ECN-capable node has detected congestion in the forward direction.
Packets carry a priority sticker, and a router somewhere might swap it without telling anyone. This extension lets the receiver say what sticker it saw.
§2Agreeing to Monitor During Setup
Connection setup follows RFC 4656 and RFC 5357, where the Modes field selects communication capabilities. The Modes field also works as an extension mechanism (RFC 6038), and this feature uses it: a new flag marks the ability of a Session-Reflector to return the received DSCP and ECN values and to use the new reflector packet format.
The Server sets the DSCP and ECN Monitoring flag in the Modes field of the Server Greeting to show that it can and is willing to monitor. If the Control-Client agrees to monitor DSCP and ECN on some or all test sessions of this control connection, it MUST set the same flag in the Modes field of the Setup Response message.
Before the game starts, the receiver says it can report stickers, and the sender says yes please. Both have to agree first.
- Server GreetingModes field has the DSCP and ECN Monitoring flag set.
- Setup ResponseFlag set in Modes to agree to monitoring.
§3The Sender DSCP and ECN Field
Monitoring needs support in the Session-Reflector and changes the reflected test packet format in all three original modes (unauthenticated, authenticated and encrypted). The Session-Sender test packet format is not altered.
The reflector builds a one-octet S-DSCP-ECN field for each packet it sends back. The six least-significant bits of the Differentiated Service field of the received packet MUST be copied into the Sender DSCP (S-DSCP) part, and the two ECN bits MUST be copied into the Sender ECN (S-ECN) part.
In the reflected packet the field sits right after the Sender TTL octet. In unauthenticated mode the layout is Sequence Number, Timestamp, Error Estimate, MBZ, Receive Timestamp, Sender Sequence Number, Sender Timestamp, Sender Error Estimate, MBZ, then Sender TTL, S-DSCP-ECN and MBZ, followed by Packet Padding. Authenticated and encrypted modes use the same idea with more MBZ space and a 16-octet HMAC.
The receiver copies the sticker's details onto a small note and tucks it into the reply, in a spot right next to the note about how many hops the packet had left.
§4How the Reflector Behaves
The DSCP and ECN values (part of the Type-P Descriptor) can be provisioned through TWAMP-Control or by other means such as a command-line interface or a central controller. With this extension 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 reflected packet. It MUST transmit each reflected packet with the DSCP set to the provisioned value.
If the provisioned DSCP is not known (for example in TWAMP Light), the choice is implementation specific: the reflector MAY copy the DSCP from the received packet, or MAY set it to CS0 (zero). If the provisioned ECN value is not known, ECN SHOULD be set to the Not-ECT codepoint; otherwise the provisioned ECN value SHALL be used.
The reflector does not analyze or act on the ECN value of the received packet, so it ignores congestion indications from the network. The RFC expects sending rates to be low enough, based on TWAMP deployment experience since 2008, that this will not significantly add to congestion.
The receiver writes down the sticker it saw, then sends its reply with the sticker it was told to use. It does not slow down just because the road looked busy, since these test packets are few.
§5Combining With RFC 6038 Modes and TWAMP Light
RFC 6038 defined Symmetrical Size and Reflects Octets modes. If either is negotiated together with DSCP and ECN Monitoring in unauthenticated mode, the extra field makes the unauthenticated Session-Reflector packet 4 octets larger. The Padding Length value therefore SHOULD be greater than or equal to 28 octets, so the truncation that RFC 5357 recommends still works.
For TWAMP Light, Appendix I of RFC 5357 does not say how the Type-P Descriptor is synchronized between sender and reflector. This RFC assumes that both have been informed by some means, such as configuration, to use the same DSCP value. The same configuration could tell the reflector to support the monitoring mode by copying data from received packets, and tell the Session-Sender to use the S-DSCP-ECN field in reflected packets.
If the reply gets a bit bigger because of the extra note, you need to leave a little more room in the envelope so everything still fits.
§6The Registered Mode Bit
In the TWAMP-Modes registry defined in RFC 5618, IANA reserved a new capability named DSCP and ECN Monitoring Capability at bit position 8, with its semantics defined in Section 2 of this RFC.
The new option got its own numbered switch on the list of options, so nobody else uses that switch for something different.
§7Security Considerations
The RFC says DSCP and ECN monitoring does not appear to add any security threat to hosts that communicate with TWAMP as defined in RFC 5357 and its existing extensions such as RFC 6038. The security considerations that apply to any active measurement of live networks are relevant here too, and the Security Considerations sections of RFC 4656 and RFC 5357 apply.
Writing down the sticker does not give a bad guy anything new to use, but the usual care about measuring real networks still applies.