RFC 6038 PROPOSED STANDARD MEASUREMENT · TWAMP · EXTENSIONS
Two-Way Active Measurement Protocol (TWAMP) Reflect Octets and Symmetrical Size Features
Two optional TWAMP add-ons: bounce chosen bytes back to the sender, and keep test packets the same size in both directions.
RFC EDITOR
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, <https://www.rfc-editor.org/info/rfc6038>.
BIBTEX
@misc{rfc6038,
series = {Request for Comments},
number = 6038,
howpublished = {RFC 6038},
publisher = {RFC Editor},
doi = {10.17487/RFC6038},
url = {https://www.rfc-editor.org/info/rfc6038},
author = {A. Morton and L. Ciavattone},
title = {{Two-Way Active Measurement Protocol (TWAMP) Reflect Octets and Symmetrical Size Features}},
pagetotal = 18,
year = 2010,
month = oct,
} LINEAGE · FROM THE RFC INDEXOpen full graph →
RFC text
Contents
Internet Engineering Task Force (IETF) A. Morton
Request for Comments: 6038 L. Ciavattone
Updates: 5357 AT&T Labs
Category: Standards Track October 2010
ISSN: 2070-1721
Two-Way Active Measurement Protocol (TWAMP) Reflect Octets and
Symmetrical Size Features
Abstract
This memo describes two closely related features for the core
specification of the Two-Way Active Measurement Protocol (TWAMP): an
optional capability where the responding host returns some of the
command octets or padding octets to the sender, and an optional
sender packet format that ensures equal test packet sizes are used in
both directions.
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/rfc6038.
Copyright Notice
Copyright (c) 2010 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.
This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
10, 2008. The person(s) controlling the copyright in some of this
material may not have granted the IETF Trust the right to allow
modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.
Table of Contents
1. Introduction ....................................................3
2. Requirements Language ...........................................3
3. Purpose and Scope ...............................................4
4. TWAMP Control Extensions ........................................4
4.1. Connection Setup with New Features .........................5
4.2. Reflect Octets: Request-TW-Session Packet Format ...........6
4.3. Reflect Octets: Accept Session Packet Format ...............7
4.4. Additional Considerations ..................................9
5. Extended TWAMP Test .............................................9
5.1. Sender Behavior ............................................9
5.1.1. Packet Timings ......................................9
5.1.2. Reflect Octets: Packet Formats and Contents .........9
5.1.3. Reflect Octets: Interaction with Padding
Truncation .........................................11
5.1.4. Symmetrical Size: Session-Sender Packet Format .....13
5.1.5. Symmetrical Size AND Reflect Octets:
Session-Sender Packet ..............................14
5.2. Reflector Behavior ........................................14
5.2.1. Reflect Octets: Session-Reflector Packet
Format and Contents ................................15
5.2.2. Symmetrical Size: Session-Reflector Packet Format ..16
5.2.3. Symmetrical Size AND Reflect Octets:
Session-Sender Packet Format .......................16
6. Security Considerations ........................................17
7. IANA Considerations ............................................17
7.1. Registry Specification ....................................17
7.2. Registry Contents .........................................17
8. Acknowledgements ...............................................17
9. Normative References ...........................................18
1. Introduction
The Two-Way Active Measurement Protocol (TWAMP) [RFC5357] is an
extension of the One-way Active Measurement Protocol (OWAMP)
[RFC4656]. The TWAMP specification gathered wide review as it
approached completion, and the by-products were several
recommendations for new features in TWAMP.
This memo describes two closely related features for TWAMP.
One is the OPTIONAL capability for the responder host to return a
limited number of unassigned (padding) octets to the Control-Client
or Session-Sender entities in both the TWAMP-Control and TWAMP-Test
protocols. With this capability, the Control-Client or Session-
Sender can embed octets of information it deems useful and have the
assurance that the corresponding reply/test packet will contain that
information when it is reflected and returned (by the Server or
Session-Reflector).
The memo also adds an OPTIONAL capability to ensure that reflected
test packets are the same size in both directions of transmission.
This is accomplished by specifying a new TWAMP-Test Session-Sender
packet format. Although TWAMP [RFC5357] recommends padding
truncation to achieve symmetrical sizes (to compensate for the
Session-Reflector's larger test packet header), it's not guaranteed
that the Session-Reflector will truncate the padding, and it's not
possible if there's insufficient padding.
This memo is an update to the TWAMP core protocol specified in
[RFC5357]. Measurement systems are not required to implement the
features described in this memo to claim compliance with [RFC5357].
Throughout this memo, the bits marked MBZ (Must Be Zero) MUST be set
to zero by senders and MUST be ignored by receivers. Also, the HMAC
(Hashed Message Authentication Code) MUST be calculated as defined in
Section 3.2 of [RFC4656].
2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
3. Purpose and Scope
The purpose of this memo is to define two OPTIONAL closely related
features for TWAMP [RFC5357]. The features enhance the TWAMP host's
capabilities to perform a simple operation on control and test
packets: the reflection of octets or padding, and the capability to
ensure symmetrical size TWAMP-Test packets. Motivations include
permitting the controller host to tag packets with an index for
simplified identification, and/or assert that the same size test
packets will be used in each direction.
The scope of the memo is limited to specifications of the following
features:
o Reflect Octets: the capability of the Server/Session-Reflector to
reflect specific octets back to the Client/Session-Sender, and a
similar service provided by the Client and Session-Sender.
o Symmetrical Size: the capability to ensure that TWAMP-Test
protocol uses the same packet size in both directions through
support of a new TWAMP-Test Session-Sender test packet format in
both the Session-Sender and the Session-Reflector. Only the
Session-Sender test packet format is new.
This memo extends the modes of operation through assignment of two
new values in the Modes field (see Section 3.1 of [RFC4656] for the
format of the Server Greeting message), while retaining backward
compatibility with the core TWAMP [RFC5357] implementations. The two
new values correspond to the two features defined in this memo.
When the Server and Control-Client have agreed to use the Reflect
Octets mode during control connection setup, then the Control-Client,
the Server, the Session-Sender, and the Session-Reflector MUST all
conform to the requirements of that mode, as identified below.
When the Server and Control-Client have agreed to use the Symmetrical
Size mode during control connection setup, then the Control-Client,
the Server, the Session-Sender, and the Session-Reflector MUST all
conform to the requirements of that mode, as identified below.
4. TWAMP Control Extensions
TWAMP-Control protocol [RFC5357] uses the Modes field to identify and
select specific communication capabilities, and this field is a
recognized extension mechanism. The following sections describe two
such extensions.
4.1. Connection Setup with New Features
TWAMP connection establishment follows the procedure defined in
Section 3.1 of [RFC4656] and Section 3.1 of [RFC5357]. The new
features require two new bit positions (and values) to identify the
ability of the Server/Session-Reflector to reflect specific octets
back to the Control-Client/Session-Sender, and to support the new
Session-Sender packet format in the TWAMP-Test protocol. See the
IANA section for details on the assigned values and bit positions.
The Server sets one or both of the new bit positions in the Modes
field of the Server Greeting message to indicate its capabilities and
willingness to operate in either of these modes (or both) if desired.
If the Control-Client intends to operate all test sessions invoked
with this control connection using one or both of the new modes, it
MUST set the Mode field bit corresponding to each function in the
Setup Response message. With this and other extensions, the Control-
Client MAY set multiple Mode field bits in the Setup Response
message.
4.2. Reflect Octets: Request-TW-Session Packet Format
The bits designated for the Reflect Octets feature in the Request-TW-
Session command are as shown in the packet format below.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 5 | MBZ | IPVN | Conf-Sender | Conf-Receiver |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Number of Schedule Slots |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. ... Many fields (66 octets) not shown ... .
. .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Padding Length (4 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Start Time (8 octets) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Timeout (8 octets) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type-P Descriptor |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Octets to be reflected | Length of padding to reflect |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MBZ (4 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| HMAC (16 octets) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
It is important to note that the Padding Length field continues to
specify the number of padding octets that the Session-Sender will
append to ALL TWAMP-Test packets associated with this test session.
See below for considerations on the minimum length of the padding
octets following the definitions of the two new fields that follow
the Type-P Descriptor.
Note that the number of padding octets appended to the Session-
Reflector's test packet depends on support for the truncation process
that TWAMP recommends, see Section 4.2.1 of [RFC5357].
The Octets to be reflected field SHALL be 2 octets long, as shown,
and contain the octets that the Server MUST reflect in the Accept
Session message as specified below.
The Length of padding to reflect field SHALL be 2 octets long and
contain an unsigned binary value in units of octets. This field
communicates the length of the padding in the TWAMP-Test Packet that
the Session-Sender expects to be reflected and the length of octets
that the Session-Reflector SHALL return in its TWAMP-Test packet
format (see Section 5.2). By including this length field in the
Request-TW-Session message, a Server is able to determine if it can
comply with a specific request to reflect padding in the TWAMP-Test
packets and to arrange for the Session-Reflector processing in
advance.
The Padding Length SHOULD be >= 27 octets when specifying a test
session using the Unauthenticated TWAMP-Test mode to allow for the
truncation process that TWAMP recommends, see Section 4.2.1 of
[RFC5357].
The Padding Length SHOULD be >= 56 octets when specifying a test
session using the Authenticated or Encrypted TWAMP-Test modes to
allow for the truncation process that TWAMP recommends, see Section
4.2.1 of [RFC5357].
The Padding Length SHALL be > the Length of padding to reflect when
specifying a test session using the OPTIONAL Reflect Octets mode.
In Unauthenticated TWAMP-Test mode, the Padding Length SHALL be >= 27
+ Length of padding to reflect octets when specifying a test session
using both the OPTIONAL Reflect Octets mode and the truncation
process that TWAMP recommends, see Section 4.2.1 of [RFC5357].
In Authenticated or Encrypted TWAMP-Test modes, the Padding Length
SHALL be >= 56 + Length of padding to reflect octets when specifying
a test session using both the OPTIONAL Reflect Octets mode and the
truncation process that TWAMP recommends, see Section 4.2.1 of
[RFC5357].
4.3. Reflect Octets: Accept Session Packet Format
The bits designated for the Reflect Padding feature in the Accept
Session command are as shown in the packet format below.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Accept | MBZ | Port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
| |
| SID (16 octets) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reflected octets | Server octets |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MBZ (8 octets) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| HMAC (16 octets) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Reflected octets field SHALL contain the octets from the Request-
TW-Session Octets to be reflected field and be 2 octets long, as
shown.
The Server octets field SHALL contain information that the Server
intends to be returned in the TWAMP-Test Packet Padding (to be
reflected) field, OR SHALL be zero, and be 2 octets long, as shown.
Although the Server determines the SID, this field is very long (16
octets) and does not normally appear in TWAMP-Test packets. The
following items MUST be part of compliant implementations using the
Reflect Octets feature:
o When a Server does not require octets to be returned in TWAMP-Test
packets, it MUST send all zeros in the Server octets field.
o When a Server intends octets to be returned in TWAMP-Test packets,
it MUST send a non-zero value in the Server octets field, and the
TWAMP-Test Sender MUST include those octets at the beginning of
the Packet Padding (to be reflected) field.
o Section 4.1.2 defines how Server octets MUST be included in the
TWAMP-Test packet padding when this service is desired by the
Server (indicated in the second of two figures in this section).
When performing the truncation process that TWAMP recommends, see
Section 4.2.1 of [RFC5357], if calculations on the Padding lengths
reveal that there are insufficient octets supplied to produce equal-
length Session-Sender and Session-Reflector test packets, then the
Accept field MUST be set to 3 to indicate that some aspect of the
request is not supported.
4.4. Additional Considerations
The value of the Modes field sent by the Server in the Server
Greeting message is the bit-wise OR of the mode values that it is
willing to support during this session.
With the publication of this memo as an RFC, the last 7 bit positions
of the Modes 32-bit field are used. A Control-Client conforming to
this extension of [RFC5357] MAY ignore the values in the higher bits
of the Modes field, or it MAY support other features that are
communicated in those bit positions. The other bits are available
for future protocol extensions.
5. Extended TWAMP Test
The TWAMP-Test protocol is similar to the OWAMP [RFC4656] test
protocol with the exception that the Session-Reflector transmits test
packets to the Session-Sender in response to each test packet it
receives. TWAMP, see Section 4 of [RFC5357], defines two additional
test packet formats for packets transmitted by the Session-Reflector.
The appropriate format depends on the security mode chosen. The new
modes specified here utilize some of the padding octets within each
test packet format or require truncation of those octets depending on
the security mode in use.
5.1. Sender Behavior
This section describes extensions to the behavior of the TWAMP
Session-Sender.
5.1.1. Packet Timings
The Send Schedule is not utilized in TWAMP, and this is unchanged in
this memo.
5.1.2. Reflect Octets: Packet Formats and Contents
The Session-Sender packet format and content follow the same
procedure and guidelines as defined in Section 4.1.2 of [RFC4656] (as
indicated in Section 4.1.2 of [RFC5357]).
The Reflect Octets mode re-designates the original TWAMP-Test Packet
Padding field (see Section 4.1.2 of [RFC4656]), as shown below 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 | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| |
| Packet Padding (to be reflected) |
. (length in octets specified in command) .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. Additional Packet Padding .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Packet Padding (to be reflected) field SHALL correspond to the
length of octets specified in the Request-TW-Session Length of
padding to reflect field to this test session. These are the octets
that the Session-Sender expects will be returned by the Session-
Reflector.
The length of the Additional Packet Padding field is the difference
between two fields in the Request-TW-Session command, as follows:
Additional Packet Padding, in octets =
Padding Length - Length of padding to reflect
When a Server intends octets to be returned in TWAMP-Test packets, it
MUST send a non-zero value in the Server octets field, and the TWAMP-
Test Session-Sender MUST include those octets in the first 2 octets
of the Packet Padding (to be reflected) field as shown below:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Server octets |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Remaining Packet Padding (to be reflected) |
. (total length in octets specified in command) .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Server octets field contains the same information that the Server
returned to the Control-Client in the Accept-Session message
corresponding to this specific test session (see Section 4.3). At
the Session-Reflector, these octets MUST be reflected the same as the
rest of the Packet Padding (to be reflected) field.
Note that it is permissible for the Session-Sender to insert the same
octets used in the Octets to be reflected field of the Request-TW-
Session command elsewhere in the Packet Padding (to be reflected)
field.
5.1.3. Reflect Octets: Interaction with Padding Truncation
When the Reflect Octets mode is selected, and performing the
truncation process that TWAMP recommends, see Section 4.2.1 of
[RFC5357], the Session-Sender MUST anticipate a minimum padding
required to achieve equal-size test packets in both directions. The
amount of padding needed to achieve symmetrical packet size depends
on both the security mode (Unauthenticated/Authenticated/Encrypted)
and whether the Reflect Octets mode is selected simultaneously.
When using the truncation process in TWAMP alone, see Section 4.2.1
of [RFC5357], the Session-Sender MUST append sufficient Packet
Padding octets to allow the same IP packet payload lengths to be used
in each direction of transmission (this is usually desirable). To
compensate for the Session-Reflector's larger test packet format, the
Session-Sender MUST append at least 27 octets of padding in
Unauthenticated mode, and at least 56 octets in Authenticated and
Encrypted modes. The sizes of TWAMP-Test protocol packets and the
resulting truncated padding to achieve equal packet sizes in both
directions are shown in the table below:
+-------------------+----------------------+---------------------+
| Octets in: | Unauthenticated Mode | Auth/Encrypted Mode |
+-------------------+----------------------+---------------------+
| Reflector Header | 41 | 104 |
| Sender Header | 14 | 48 |
| Truncated Padding | 27 | 56 |
+-------------------+----------------------+---------------------+
TWAMP-Test Padding Truncation
When using the Reflect Octets mode simultaneously with the truncation
process that TWAMP recommends in Section 4.2.1 of [RFC5357], the
Session-Sender MUST append at least 27 octets of padding plus the
Length of the padding to reflect octets when operating in
Unauthenticated mode. The Session-Sender MUST append at least 56
octets of padding plus the Length of the padding to reflect octets
when operating in Authenticated and Encrypted modes.
5.1.4. Symmetrical Size: Session-Sender Packet Format
When the Symmetrical Size mode is selected, the Session-Sender SHALL
use the following TWAMP-Test Packet Format in Unauthenticated mode:
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 (27 octets) |
| |
| |
| |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | |
+-+-+-+-+-+-+-+-+ +
. .
. Packet Padding .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
This feature REQUIRES only a new Session-Sender test packet format,
the Session-Reflector test packet format is unchanged.
5.1.5. Symmetrical Size AND Reflect Octets: Session-Sender Packet
Format
When both the Symmetrical Size mode and the Reflect Octets mode are
selected, the Session-Sender SHALL use the following TWAMP-Test
Packet Format in Unauthenticated mode:
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 (27 octets) |
| |
| |
| |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | |
+-+-+-+-+-+-+-+-+ +
| Packet Padding (to be reflected) |
. (length in octets specified in command) .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. Additional Packet Padding .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
In this combined mode, the Packet Padding to be reflected follows the
27 MBZ octets. In Authenticated or Encrypted modes, the Packet
Padding to be reflected follows the 56 MBZ octets.
5.2. Reflector Behavior
The TWAMP Reflector follows the procedures and guidelines in Section
4.2 of [RFC5357], with the following additional functions:
o Reflect Octets mode: Designated octets in the Packet Padding (to
be reflected) field of the Session-Sender's test packet MUST be
included in the Session-Reflector's test packet.
o Symmetrical Size mode: The Session-Reflector MUST operate using
the Session_Reflector Packet Format defined in Section 5.1.4,
where the Padding Octets are separated from the information
fields.
5.2.1. Reflect Octets: Session-Reflector Packet Format and Contents
The Reflect Padding feature re-designates the Packet Padding field,
as shown below. When the Reflect Octets mode is selected, the
Session-Sender SHALL use the following TWAMP-Test Packet Format in
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 | Packet Padding (from Session-Sender) |
+-+-+-+-+-+-+-+-+ +
. .
+ +-+-+-+-+-+-+-+-+
| Packet Padding (from Session-Sender) | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| |
| |
. Additional Packet Padding .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Packet Padding (from Session-Sender) field MUST be the same
octets as the Packet Padding (to be reflected) field in the Session-
Sender's test packet, and therefore MUST conform to the length
specified in the Request-TW-Session message.
When the Server has returned a non-zero value in the Server octets
field of the Accept Session message (as described in Section 4.3),
then the Session-Reflector SHALL reflect these octets the same as the
rest of the Packet Padding (to be reflected) field.
Section 4.2.1 of [RFC5357] recommends a padding truncation process
for use in TWAMP. When using that process in conjunction with the
Reflect Octets mode, the Session-Reflector MUST reflect the
designated octets from the Session-Sender's test packet in the Packet
Padding (from Session-Sender) field, and MAY re-use additional Packet
Padding from the Session-Sender. The Session-Reflector MUST truncate
the padding such that the highest number octets are discarded, and
the test packet length equals the Session-Sender's packet length.
When using the recommended truncation process, the Session-Reflector
MUST truncate exactly 27 octets of padding in Unauthenticated mode,
and exactly 56 octets in Authenticated and Encrypted modes.
In any case, the Session-Reflector MAY re-use the Sender's Packet
Padding (since the requirements for padding generation are the same
for each).
5.2.2. Symmetrical Size: Session-Reflector Packet Format
When Symmetrical Size mode is selected, the Session-Reflector packet
formats for Unauthenticated and Authenticated/Encrypted modes are
identical to the core TWAMP specification, see Section 4.2.1 of
[RFC5357]. Thus, the Session-Reflector test packet format is
unchanged.
The Session-Reflector MUST construct its test packet using the
information in the Session-Sender's test packet. The length of the
Session-Reflector's test packet SHALL equal the length of the
Session-Sender's test packet.
5.2.3. Symmetrical Size AND Reflect Octets: Session-Sender Packet
Format
When both the Symmetrical Size mode and the Reflect Octets mode are
selected, the Session-Reflector MUST operate using the Session-Sender
Packet Format defined in Section 5.1.5, where the Padding Octets are
separated from the information fields, and the Packet Padding (to be
reflected) field precedes the Additional Padding.
The Session-Reflector SHALL use the same TWAMP-Test Packet Format as
specified in Section 5.2.1, above.
6. Security Considerations
These extended modes of operation do not appear to permit any new
attacks on hosts communicating with core TWAMP [RFC5357].
The security considerations that apply to any active measurement of
live networks are relevant here as well. See [RFC4656] and
[RFC5357].
7. IANA Considerations
This memo adds two modes to the IANA registry for the TWAMP Modes
field, and describes behavior when the new modes are used. This
field is a recognized extension mechanism for TWAMP.
7.1. Registry Specification
IANA has created a TWAMP-Modes registry (as requested in [RFC5618]).
TWAMP-Modes are specified in TWAMP Server Greeting messages and Setup
Response messages, as described in Section 3.1 of [RFC5357],
consistent with Section 3.1 of [RFC4656], and extended by this memo.
Modes are indicated by setting bits in the 32-bit Modes field that
correspond to values in the Modes registry. For the TWAMP-Modes
registry, we expect that new features will be assigned increasing
registry values that correspond to single bit positions, unless there
is a good reason to do otherwise (more complex encoding than single-
bit positions may be used in the future to access the 2^32 value
space).
7.2. Registry Contents
The TWAMP-Modes registry has been augmented as follows:
Value Description Semantics Definition
--------------------------------------------------------
32 Reflect Octets this memo, Section 3.1
Capability new bit position (5)
64 Symmetrical Size this memo, Section 3.1
Sender Test Packet Format new bit position (6)
8. Acknowledgements
The authors thank Steve Baillargeon, Walt Steverson, and Stina Ross
for helpful review and comments.
9. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4656] Shalunov, S., Teitelbaum, B., Karp, A., Boote, J., and M.
Zekauskas, "A One-way Active Measurement Protocol
(OWAMP)", RFC 4656, September 2006.
[RFC5357] Hedayat, K., Krzanowski, R., Morton, A., Yum, K., and J.
Babiarz, "A Two-Way Active Measurement Protocol (TWAMP)",
RFC 5357, October 2008.
[RFC5618] Morton, A. and K. Hedayat, "Mixed Security Mode for the
Two-Way Active Measurement Protocol (TWAMP)", RFC 5618,
August 2009.
Authors' Addresses
Al Morton
AT&T Labs
200 Laurel Avenue South
Middletown, NJ 07748
USA
Phone: +1 732 420 1571
Fax: +1 732 368 1192
EMail: acmorton@att.com
URI: http://home.comcast.net/~acmacm/
Len Ciavattone
AT&T Labs
200 Laurel Avenue South
Middletown, NJ 07748
USA
Phone: +1 732 420 1239
EMail: lencia@att.com
AI ELI5
On this page
PROBLEMCore TWAMP lets the reflector shrink its reply by truncating padding, but that is not guaranteed and is impossible when there is too little padding. Senders also have no assured way to tag a packet and get the tag back.
MECHANISMThe RFC assigns two new bits in the TWAMP Modes field. Reflect Octets makes the Server and Session-Reflector return designated octets, and Symmetrical Size defines a new Session-Sender packet format that already has the reflector's larger header built in.
WHY IT MATTERSBoth features are OPTIONAL, and a system does not need them to claim compliance with RFC 5357. The RFC updates RFC 5357 and says the extensions do not appear to permit any new attacks.
§1Why Extend TWAMP
The Two-Way Active Measurement Protocol (TWAMP, RFC 5357) extends the One-way Active Measurement Protocol (OWAMP, RFC 4656). Review of TWAMP produced several recommendations for new features, and this memo describes two closely related ones.
Reflect Octets is an OPTIONAL capability for the responding host to return a limited number of unassigned (padding) octets to the Control-Client or Session-Sender. The sender can embed information it finds useful and be assured that the reply will contain it.
Symmetrical Size is an OPTIONAL sender packet format that ensures test packets are the same size in both directions. TWAMP already recommends padding truncation for this, but it is not guaranteed that the reflector truncates, and it cannot work if there is not enough padding.
Measurement systems are not required to implement either feature to claim compliance with RFC 5357. Throughout, bits marked MBZ (Must Be Zero) MUST be set to zero by senders and ignored by receivers.
Two extra tricks for a game where you send a message and get an echo. One lets you get your own note back. The other keeps the note and the echo the same size.
§4.1Agreeing to Use the New Modes
TWAMP-Control uses the Modes field to pick communication capabilities, and that field is a recognized extension mechanism. Connection setup still follows RFC 4656 and RFC 5357, but two new bit positions are used.
The Server sets one or both of the new bits in the Modes field of its Server Greeting to show what it is willing to do. If the Control-Client wants all test sessions on this control connection to use a new mode, it MUST set the matching bit in the Setup Response message. The Client MAY set several Mode bits at once.
The Modes value the Server sends is the bit-wise OR of the modes it supports. With this memo the last 7 bit positions of the 32-bit Modes field are in use, and the other bits stay available for future extensions. Once a mode is agreed, the Control-Client, Server, Session-Sender and Session-Reflector all MUST conform to it.
Before playing, each side says which special rules it knows. If both know a rule, they agree to play by it.
- 32Reflect Octets Capability — New bit position 5. Server and Session-Reflector return designated octets.
- 64Symmetrical Size Sender Test Packet Format — New bit position 6. Sender uses the new packet format so both directions are the same size.
§4.2Control Messages for Reflect Octets
The Request-TW-Session command gains two 2-octet fields after the Type-P Descriptor. Octets to be reflected holds the octets the Server MUST reflect in its Accept Session message. Length of padding to reflect is an unsigned value in octets that says how much padding the Session-Sender expects back in the test packets, so the Server can decide in advance whether it can comply.
The existing Padding Length field still gives the padding appended to ALL test packets. It SHALL be greater than the length of padding to reflect. With the recommended truncation it SHALL be at least 27 plus the reflected length in Unauthenticated mode, and at least 56 plus the reflected length in Authenticated or Encrypted modes.
The Accept Session message gains two 2-octet fields. Reflected octets carries back the requested octets. Server octets either holds information the Server wants returned in the test packets, or is all zeros if it does not need that. If the supplied padding is too short to produce equal-length packets, the Accept field MUST be set to 3, meaning some aspect of the request is not supported.
When you ask to play, you hand over a small tag to be echoed back and say how much of your note should come back. The host can say yes, add its own tag, or say it cannot do that.
§5.1What the Session-Sender Sends
With Reflect Octets, the test packet's padding is split. The Packet Padding (to be reflected) field has exactly the length given in Length of padding to reflect, and the rest is Additional Packet Padding. If the Server returned a non-zero Server octets value, the sender MUST place those octets in the first 2 octets of the reflected padding field.
When truncation is used, the sender MUST append enough padding to anticipate the reflector's larger header. The table in the RFC gives header sizes and the resulting truncated padding.
With Symmetrical Size, the sender uses a new Unauthenticated packet format: Sequence Number, Timestamp, Error Estimate, then MBZ (27 octets), then Packet Padding. The 27 zero octets stand in for the extra space the reflector's header will need. In Authenticated or Encrypted modes the equivalent is 56 MBZ octets. If both modes are on, the reflected padding follows the MBZ octets, before the Additional Packet Padding.
The sender writes its note with some blank space reserved, so that when the host writes a longer reply header, the reply is still the same size.
| Octets in | Unauthenticated Mode | Auth/Encrypted Mode |
|---|---|---|
| Reflector Header | 41 | 104 |
| Sender Header | 14 | 48 |
| Truncated Padding THIS RFC | 27 | 56 |
| Field | Name | Size | Value | Description |
|---|---|---|---|---|
SEQ | Sequence Number | 4 bytes | Sequence number of the test packet. | |
TIMESTAMP | Timestamp | 8 bytes | Time the packet was sent. | |
ERR | Error Estimate | 2 bytes | Error estimate for the timestamp. | |
MBZ | MBZ (27 octets) | 2 bytes | Zero octets that start here and run for 27 octets in total, standing in for the reflector's larger header. |
§5.2What the Session-Reflector Does
The reflector follows RFC 5357 with additions. In Reflect Octets mode, the designated octets from the sender's padding MUST appear in the reflector's packet, in a field called Packet Padding (from Session-Sender). If the Server gave a non-zero Server octets value, the reflector reflects it like the rest of that padding. The reflector MAY reuse other padding from the sender.
When recommended truncation is used together with Reflect Octets, the reflector MUST discard the highest-numbered octets so its packet length equals the sender's. It MUST truncate exactly 27 octets in Unauthenticated mode and exactly 56 octets in Authenticated and Encrypted modes.
In Symmetrical Size mode the reflector's packet format is unchanged from RFC 5357. Its packet length SHALL equal the sender's packet length, and it builds the packet from information in the sender's packet.
The host copies back the part of your note you asked for, and trims any extra so its reply is exactly as long as yours.
§7New TWAMP-Modes Registry Values
The memo adds two modes to the IANA TWAMP-Modes registry. Modes are indicated by setting bits in the 32-bit Modes field. The registry expects new features to get increasing values that correspond to single bit positions.
The new entries are 32 for the Reflect Octets Capability (new bit position 5) and 64 for the Symmetrical Size Sender Test Packet Format (new bit position 6).
Each special rule gets its own numbered switch on a control panel, and the rule book records which switch is which.
§6Security Considerations
The RFC says these extended modes of operation do not appear to permit any new attacks on hosts communicating with core TWAMP. The security considerations that apply to any active measurement of live networks are relevant here too, and the RFC points to RFC 4656 and RFC 5357 for them.
These add-ons do not seem to open any new doors for troublemakers, but the usual care for measuring real networks still applies.
§8References
[RFC2119]RFC 2119Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.March 1997[RFC4656]RFC 4656Shalunov, S., Teitelbaum, B., Karp, A., Boote, J., and M. Zekauskas, "A One-way Active Measurement Protocol (OWAMP)", RFC 4656, September 2006.September 2006[RFC5357]RFC 5357Hedayat, K., Krzanowski, R., Morton, A., Yum, K., and J. Babiarz, "A Two-Way Active Measurement Protocol (TWAMP)", RFC 5357, October 2008.October 2008[RFC5618]RFC 5618Morton, A. and K. Hedayat, "Mixed Security Mode for the Two-Way Active Measurement Protocol (TWAMP)", RFC 5618, August 2009.August 2009