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.
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>.
@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,
} 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
ISO Plain Language
On this page
§1Two Optional Extensions
The Two-Way Active Measurement Protocol of RFC 5357 is an extension of the One-way Active Measurement Protocol of RFC 4656. The TWAMP specification gathered wide review as it approached completion, and the by-products were several recommendations for new features in TWAMP, which this memo describes.
One feature is the OPTIONAL capability for the responder host to return a limited number of unassigned, that is 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 or 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, accomplished by specifying a new TWAMP-Test Session-Sender packet format. Although TWAMP recommends padding truncation to achieve symmetrical sizes, to compensate for the Session-Reflector's larger test packet header, it is not guaranteed that the Session-Reflector will truncate the padding, and it is not possible if there is insufficient padding.
This memo is an update to the TWAMP core protocol of RFC 5357. Measurement systems are not required to implement the features described here to claim compliance with RFC 5357. Throughout the memo, bits marked MBZ, Must Be Zero, MUST be set to zero by senders and ignored by receivers, and the HMAC MUST be calculated as defined in Section 3.2 of RFC 4656.
Both features are optional, and a measurement system that ignores them is still a conforming TWAMP system. The first one lets a sender mark a packet and get the mark back; the second makes the reply packet exactly the same size as the one that was sent.
§3Purpose and Scope
The purpose of the memo is to define two closely related OPTIONAL features for TWAMP. 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 asserting that the same size test packets will be used in each direction.
The scope is limited to two features. Reflect Octets is the capability of the Server and Session-Reflector to reflect specific octets back to the Client and Session-Sender, and the similar service provided by the Client and Session-Sender. Symmetrical Size is the capability to ensure that the 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.
The memo extends the modes of operation through the assignment of two new values in the Modes field, while retaining backward compatibility with the core TWAMP implementations. The two new values correspond to the two features. When the Server and the Control-Client have agreed to use the Reflect Octets mode during control connection setup, the Control-Client, the Server, the Session-Sender and the Session-Reflector MUST all conform to the requirements of that mode, and the same applies to the Symmetrical Size mode.
Both features are negotiated once, on the control connection, and then every session set up under it follows the same rules. The two are close enough to be defined together, because the second changes how much padding there is and the first uses padding to carry information.
§4.1Agreeing to Use the Features
TWAMP connection establishment follows the procedure defined in Section 3.1 of RFC 4656 and Section 3.1 of RFC 5357. The new features need two new bit positions, and their values, to identify the ability of the Server and Session-Reflector to reflect specific octets back to the Control-Client and Session-Sender, and to support the new Session-Sender packet format in the TWAMP-Test protocol.
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 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.
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 32-bit Modes field are used. A Control-Client conforming to this extension MAY ignore the values in the higher bits of the Modes field, or it MAY support other features communicated in those bit positions, and the other bits remain available for future protocol extensions.
The Server says in its greeting which of the two features it can do, and the Control-Client answers with the ones it wants to use. The choice is made once for the whole control connection, so every session started on it behaves the same way.
§4.2The Reflect Octets Control Packets
In the Request-TW-Session command, the Reflect Octets feature adds two fields after the Type-P Descriptor. The Octets to be reflected field is 2 octets long and contains the octets that the Server MUST reflect in the Accept Session message. The Length of padding to reflect field is 2 octets long and holds an unsigned binary value in units of octets. It communicates the length of the padding in the TWAMP-Test packet that the Session-Sender expects to be reflected, and the length of octets the Session-Reflector SHALL return in its TWAMP-Test packet format. By including the length in the Request-TW-Session message, a Server is able to determine whether it can comply with a specific request to reflect padding, and to arrange for the Session-Reflector processing in advance.
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 the test session. It SHOULD be at least 27 octets in Unauthenticated TWAMP-Test mode, and at least 56 octets in the Authenticated or Encrypted modes, to allow for the truncation process that TWAMP recommends. It SHALL be greater than the Length of padding to reflect when using the OPTIONAL Reflect Octets mode. In Unauthenticated mode it SHALL be at least 27 plus the Length of padding to reflect octets when using both the Reflect Octets mode and the truncation process, and in Authenticated or Encrypted modes at least 56 plus that length.
In the Accept Session command, the Reflected octets field contains the octets from the Request-TW-Session Octets to be reflected field and is 2 octets long. The Server octets field contains information that the Server intends to be returned in the TWAMP-Test packet padding, or SHALL be zero, and is 2 octets long. Compliant implementations using the Reflect Octets feature MUST send all zeros in the Server octets field when the Server does not require octets to be returned in test packets, MUST send a non-zero value when it does, and in that case the TWAMP-Test Sender MUST include those octets at the beginning of the padding field. When performing the truncation process, if the padding calculations show there are insufficient octets to produce equal-length test packets, the Accept field MUST be set to 3 to indicate that some aspect of the request is not supported.
The session request carries two bytes the sender wants back, and a number saying how much padding should come back with them. The answer carries the two bytes echoed, plus two bytes of the Server's own that it wants reflected in the test packets. The padding has to be long enough for the feature to work, and if it is not, the Server refuses the session.
- Request-TW-SessionCarries the octets to be reflected and how much padding to return.
- Accept SessionEchoes the octets and adds the Server's own two octets.
- Test packetThe padding to be reflected sits at the start of the padding.
- Test packet with padding returnedThe designated octets come back, and the padding is trimmed to match.
§5.1.3Padding, Truncation and the Reflector's Reply
In Reflect Octets mode the original TWAMP-Test packet padding field is re-designated as the padding to be reflected, and its length corresponds to the Length of padding to reflect field of the Request-TW-Session command. Those are the octets the Session-Sender expects back from the Session-Reflector. The length of the additional padding is the difference between the Padding Length and the Length of padding to reflect. When the Server intends octets to be returned in test packets, the Session-Sender MUST include them in the first 2 octets of the padding to be reflected field, and the Session-Reflector MUST reflect those octets as it does the rest of that field.
When the Reflect Octets mode is selected and the truncation process that TWAMP recommends is used, the Session-Sender MUST anticipate the minimum padding needed to achieve equal-size test packets in both directions, and that amount depends on the security mode and on whether Reflect Octets is selected at the same time. With the truncation process alone, the Session-Sender MUST append sufficient padding to allow the same IP packet payload lengths in each direction, and to compensate for the Session-Reflector's larger test packet format it MUST append at least 27 octets of padding in Unauthenticated mode and at least 56 octets in Authenticated and Encrypted modes. The document tabulates the sizes: the reflector header is 41 octets in Unauthenticated mode and 104 in Authenticated or Encrypted mode, the sender header is 14 and 48 octets, and the truncated padding is 27 and 56 octets.
With Reflect Octets selected at the same time, the Session-Sender MUST append at least 27 octets of padding plus the Length of padding to reflect in Unauthenticated mode, and at least 56 octets plus that length in Authenticated and Encrypted modes.
On the reflector side, the designated octets in the padding field of the Session-Sender's test packet MUST be included in the Session-Reflector's test packet. The padding from the Session-Sender field in the reflector's packet MUST be the same octets as the padding to be reflected field of the sender's packet, and so MUST conform to the length specified in the Request-TW-Session message. When the Server returned a non-zero Server octets field, the Session-Reflector SHALL reflect those octets as well. When using the recommended truncation process together with Reflect Octets, the Session-Reflector MUST reflect the designated octets and MAY reuse additional padding from the Session-Sender, and it MUST truncate the padding so that the highest-numbered octets are discarded and the test packet length equals the Session-Sender's packet length, which means truncating exactly 27 octets in Unauthenticated mode and exactly 56 octets in Authenticated and Encrypted modes.
The reflector's reply has a bigger header than the sender's packet, so it trims padding to make up the difference. With the reflect feature in use, the padding it trims is the part the sender did not ask to have returned, and the numbers say how much padding the sender must add so there is always enough to trim.
| Octets in | Unauthenticated mode | Authenticated or encrypted mode |
|---|---|---|
| Reflector header | 41 | 104 |
| Sender header | 14 | 48 |
| Truncated padding | 27 | 56 |
§5.1.4The New Session-Sender Packet Format
When the Symmetrical Size mode is selected, the Session-Sender SHALL use a new TWAMP-Test packet format in Unauthenticated mode. The packet keeps the sequence number, the timestamp and the error estimate, and then carries 27 octets that must be zero before the packet padding begins. The padding octets are separated from the information fields, which is what allows the two directions to end up the same size.
This feature requires only a new Session-Sender test packet format; the Session-Reflector test packet format is unchanged.
When both the Symmetrical Size mode and the Reflect Octets mode are selected, the Session-Sender uses the same format with the padding to be reflected following the 27 must-be-zero octets, followed by the additional packet padding. In Authenticated or Encrypted modes the padding to be reflected follows 56 must-be-zero octets instead.
On the reflector side, when Symmetrical Size mode is selected, the Session-Reflector packet formats for Unauthenticated and Authenticated or Encrypted modes are identical to the core TWAMP specification, so the reflector's format is unchanged. The Session-Reflector MUST construct its test packet using the information in the Session-Sender's test packet, and the length of the reflector's test packet SHALL equal the length of the sender's test packet. When both modes are selected, the Session-Reflector MUST operate using the Session-Sender packet format with the padding octets separated from the information fields and the padding to be reflected preceding the additional padding, and it SHALL use the same packet format as the Reflect Octets case.
Instead of hoping the reflector trims its padding, the sender builds a packet that already has room for the reflector's bigger header. The result is that the reply packet comes back exactly the same length as the one that was sent, with no reliance on the reflector doing anything special.
§6Security Considerations
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 as well, and the reader is referred to RFC 4656 and RFC 5357.
The document finds no new attack surface in either feature. The usual cautions about running active measurements on a live network still apply.
§7IANA Considerations
The memo adds two modes to the IANA registry for the TWAMP Modes field, and describes the behavior when the new modes are used. The field is a recognized extension mechanism for TWAMP. IANA has created a TWAMP-Modes registry, as requested in RFC 5618. The modes are specified in TWAMP Server Greeting and Setup Response messages, and are indicated by setting bits in the 32-bit Modes field that correspond to values in the registry. New features are expected to be assigned increasing registry values that correspond to single bit positions, unless there is a good reason not to.
The registry is augmented with two values. Value 32 is Reflect Octets Capability, a new bit position, number 5. Value 64 is Symmetrical Size Sender Test Packet Format, a new bit position, number 6.
Each feature gets one bit in the Modes field: bit 5 for reflecting octets and bit 6 for the symmetrical size format. The rest of the field is left for future extensions.
- 32Reflect Octets Capability — A new bit position, number 5.
- 64Symmetrical Size Sender Test Packet Format — A new bit position, number 6.