Network Working Group K. Hedayat
Internet Draft Brix Networks
Expires: Jan 2009 R. Krzanowski
Intended Status:Standards Track Verizon
A. Morton
AT&T Labs
K. Yum
Juniper Networks
J. Babiarz
Nortel Networks
July 30, 2008
A Two-way Active Measurement Protocol (TWAMP)
draft-ietf-ippm-twamp-09
Status of this Memo
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Copyright Notice
Copyright (C) The IETF Trust (2008).
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Abstract
The One-way Active Measurement Protocol [RFC4656] (OWAMP) provides
a common protocol for measuring one-way metrics between network
devices. OWAMP can be used bi-directionally to measure one-way
metrics in both directions between two network elements. However,
it does not accommodate round-trip or two-way measurements. This
memo specifies a Two-way Active Measurement Protocol (TWAMP), based
on the OWAMP, that adds two-way or round-trip measurement
capabilities. The TWAMP measurement architecture is usually
comprised of two hosts with specific roles, and this allows for
some protocol simplifications, making it an attractive alternative
in some circumstances.
Table of Contents
1. Introduction..................................................3
1.1 Relationship of Test and Control Protocols................3
1.2 Logical Model.............................................4
1.3 Pronunciation Guide.......................................5
2. Protocol Overview.............................................5
3. TWAMP Control.................................................6
3.1 Connection Setup..........................................6
3.2 Integrity Protection......................................7
3.3 Value of the Accept Fields................................8
3.4 TWAMP Control Commands....................................8
3.5 Creating Test Sessions....................................8
3.6 Send Schedules...........................................10
3.7 Starting Test Sessions...................................11
3.8 Stop-Sessions............................................11
3.9 Fetch-Session............................................12
4. TWAMP Test...................................................12
4.1 Sender Behavior..........................................13
4.2 Reflector Behavior.......................................13
5. Implementers Guide...........................................21
6. Security Considerations......................................21
7. Acknowledgements.............................................22
8. IANA Considerations..........................................22
8.1 Registry Specification...................................23
8.2 Registry Management......................................23
8.3 Experimental Numbers.....................................23
8.4 Initial Registry Contents................................23
9. Internationalization Considerations..........................24
10. APPENDIX I - TWAMP Light (Informative)......................24
11. References..................................................25
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11.1 Normative References....................................25
11.2 Informative References..................................26
1. Introduction
The Internet Engineering Task Force (IETF) has completed a Proposed
standard for the round-trip delay [RFC2681] metric. IETF has also
completed a protocol for the control and collection of one-way
measurements, the One-way Active Measurement Protocol (OWAMP)
[RFC4656]. However, OWAMP does not accommodate round-trip or two-
way measurements.
Two-way measurements are common in IP networks, primarily because
synchronization between local and remote clocks is unnecessary for
round-trip delay, and measurement support at the remote end may be
limited to a simple echo function. However, the most common
facility for round-trip measurements is the ICMP Echo Request/Reply
(used by the ping tool), and issues with this method are documented
in section 2.6 of [RFC2681]. This memo specifies the Two-way Active
Measurement Protocol, or TWAMP. TWAMP uses the methodology and
architecture of OWAMP [RFC4656] to define an open protocol for
measurement of two-way or round-trip metrics (henceforth in this
document the term two-way also signifies round-trip), in addition
to the one-way metrics of OWAMP. TWAMP employs time stamps applied
at the echo destination (reflector) to enable greater accuracy
(processing delays can be accounted for). The TWAMP measurement
architecture is usually comprised of only two hosts with specific
roles, and this allows for some protocol simplifications, making it
an attractive alternative to OWAMP in some circumstances.
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].
1.1 Relationship of Test and Control Protocols
Similar to OWAMP [RFC4656], TWAMP consists of two inter-related
protocols: TWAMP-Control and TWAMP-Test. The relationship of these
protocols is as defined in section 1.1 of OWAMP [RFC4656].
TWAMP-Control is used to initiate, start, and stop test sessions,
whereas TWAMP-Test is used to exchange test packets between two
TWAMP entities.
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1.2 Logical Model
The role and definition of the logical entities are as defined in
section 1.2 of OWAMP [RFC4656] with the following exceptions:
- The Session-Receiver is called the Session-Reflector in the
TWAMP architecture. The Session-Reflector has the capability
to create and send a measurement packet when it receives a
measurement packet. Unlike the Session-Receiver, the
Session-Reflector does not collect any packet information.
- The Server is an end system that manages one or more TWAMP
sessions, and is capable of configuring per-session state in
the end-points. However, a Server associated with a
Session-Reflector would not have the capability to return the
results of a test session, and this is a difference from OWAMP.
- The Fetch-Client entity does not exist in the TWAMP
architecture, as the Session-Reflector does not collect any
packet information to be fetched. Consequently there is no
need for the Fetch-Client.
An example of possible relationship scenarios between these roles
are presented below. In this example different logical roles are
played on different hosts. Unlabeled links in the figure are
unspecified by this document and may be proprietary protocols.
+----------------+ +-------------------+
| Session-Sender |<-TWAMP-Test-->| Session-Reflector |
+----------------+ +-------------------+
^ ^
| |
| |
| |
| +----------------+<----------------+
| | Server |
| +----------------+
| ^
| |
| TWAMP-Control
| |
v v
+----------------+
| Control-Client |
+----------------+
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As in OWAMP [RFC4656], different logical roles can be played by the
same host. For example, in the figure above, there could be
actually two hosts: one playing the roles of Control-Client and
Session-Sender, and the other playing the roles of Server and
Session-Reflector. This example is shown below.
+-----------------+ +-------------------+
| Control-Client |<--TWAMP Control-->| Server |
| | | |
| Session-Sender |<--TWAMP-Test----->| Session-Reflector |
+-----------------+ +-------------------+
Throughout this memo, the bits marked MBZ (Must Be Zero) MUST be
set to zero by senders and MUST be ignored by receivers.
1.3 Pronunciation Guide
The acronym OWAMP is usually pronounced in two syllables, Oh-wamp.
The acronym TWAMP is also pronounced in two syllables, Tee-wamp.
2. Protocol Overview
The Two-way Active Measurement Protocol is an open protocol for
measurement of two-way metrics. It is based on OWAMP [RFC4656] and
adheres to its overall architecture and design. The TWAMP-control
and TWAMP-Test protocols accomplish their testing tasks as outlined
below:
- The Control-Client initiates a TCP connection on TWAMP's well-
known port, and the Server (its role now established) responds
with its greeting message indicating the security/integrity
mode(s) it is willing to support.
- The Control-Client responds with the chosen mode of
communication and information supporting integrity protection
and encryption, if the mode requires them. The Server responds
to accept the mode and start time. This completes the control
connection setup.
- The Control-Client requests (and describes) a test session with
a unique TWAMP-Control message. The Server responds with its
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acceptance and supporting information. More than one test
session may be requested with additional messages.
- The Control-Client initiates all requested testing with a start
sessions message, and the Server acknowledges.
- The Session-Sender and the Session-Reflector exchange test
packets according to the TWAMP-Test protocol for each active
session.
- When appropriate, the Control-Client sends a message to stop all
test sessions.
There are two recognized extension mechanisms in the TWAMP
Protocol. The Modes field is used to establish the communication
options during TWAMP-Control Connection Setup. The TWAMP-Control
Command Number is another intended extension mechanism, allowing
additional commands to be defined in the future. TWAMP-Control
protocol addresses different levels of support between Control-
Client and Server.
All multi-octet quantities defined in this document are represented
as unsigned integers in network byte order unless specified
otherwise.
3. TWAMP Control
TWAMP-Control is a derivative of the OWAMP-Control for two-way
measurements. All TWAMP Control messages are similar in format and
follow similar guidelines to those defined in section 3 of OWAMP
[RFC4656] with the exceptions outlined in the following sections.
One such exception is the Fetch Session command, which is not used
in TWAMP.
3.1 Connection Setup
Connection establishment of TWAMP follows the same procedure
defined in section 3.1 of OWAMP [RFC4656]. The Modes field is a
recognized extension mechanism in TWAMP, and the current mode
values are identical to those used in OWAMP. The only exception is
the well-known port number for TWAMP-control. A client opens a TCP
connection to the server on well-known port N (Refer to the IANA
Considerations section below for the TWAMP-control port number
assignment). The host that initiates the TCP connection takes the
roles of Control-Client and (in the two-host implementation) the
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Session-Sender. The host that acknowledges the TCP connection
accepts the roles of Server and (in the two-host implementation)
the Session Reflector.
The Control-Client MAY set a desired code-point in the Diffserv
Code Point (DSCP) field in the IP header for ALL packets of a
specific control connection. The Server SHOULD use the DSCP of the
Control-Client's TCP SYN in ALL subsequent packets on that
connection (avoiding any ambiguity in case of re-marking).
The possibility exists for Control-Client failure after TWAMP-
Control connection establishment, or the path between the Control-
Client and Server may fail while a connection is in-progress. The
Server MAY discontinue any established control connection when no
packet associated with that connection has been received within
SERVWAIT seconds. The Server SHALL suspend monitoring control
connection activity after receiving a Start-Sessions command, and
SHALL resume after receiving a Stop-Sessions command (IF the
SERVWAIT option is supported). Note that the REFWAIT time-out
(described below) covers failures during test sessions. The default
value of SERVWAIT SHALL be 900 seconds, and this waiting time MAY
be configurable. This time-out allows a Server to free-up resources
in case of failure.
Both the server and the client use the same mappings from KeyIDs to
shared secrets. The server, being prepared to conduct sessions
with more than one client, uses KeyIDs to choose the appropriate
secret key; a client would typically have different secret keys for
different servers. The shared secret is a passphrase. To maximize
passphrase interoperability, the passphrase character set MUST be
encoded using Appendix B of [RFC 5198] (the ASCII Network Virtual
Terminal Definition) It MUST not contain newlines (any combination
of Carriage-Return (CR) and/or Line-Feed (LF) characters), and
control characters SHOULD be avoided.
3.2 Integrity Protection
Integrity protection of TWAMP follows the same procedure defined in
section 3.2 of OWAMP [RFC4656]. As in OWAMP, each HMAC sent covers
everything sent in a given direction between the previous HMAC (but
not including it) and up to the beginning of the new HMAC. This
way, once encryption is set up, each bit of the TWAMP-Control
connection is authenticated by an HMAC exactly once.
Note that the Server-Start message (sent by a Server during the
initial control connection exchanges) does not terminate with an
HMAC field. Therefore, the HMAC in the first Accept-Session message
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also covers the Server-Start message and includes the Start-Time
field in the HMAC calculation.
Also, in authenticated and encrypted modes, the HMAC in TWAMP-
Control packets is encrypted.
3.3 Value of the Accept Fields
Accept values used in TWAMP are the same as the values defined in
section 3.3 of OWAMP [RFC4656].
3.4 TWAMP Control Commands
TWAMP control commands conform to the rules defined in section 3.4
of OWAMP [RFC4656]
The following commands are available for the Control-client:
Request-TW-Session, Start-Sessions, and Stop-Sessions. The Server
can send specific messages in response to the commands it receives
(as described in the sections that follow).
Note that the OWAMP Request-Session command is replaced by the
TWAMP Request-TW-Session command, and the Fetch-Session command
does not appear in TWAMP.
3.5 Creating Test Sessions
Test session creation follows the same procedure as defined in
section 3.5 of OWAMP [RFC4656]. The Request-TW-Session command is
based on the OWAMP Request-Session command, and uses the message
format as described in OWAMP secition 3.5, but without the schedule
slot description field(s) and uses one HMAC. The description of the
Request-TW-Session format follows.
In TWAMP, the first octet is referred to as the Command Number, and
the Command Number is a recognized extension mechanism. Readers are
encouraged to consult the TWAMP-Control Command Number Registry to
determine if there have been additional values assigned.
The Command Number value of 5 indicates a Request-TW-Session
Command, and the Server MUST interpret this command as a request
for a two-way test session using the TWAMP-Test protocol.
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If a TWAMP Server receives an unexpected command number, it MUST
respond with the Accept field set to 3 (meaning "Some aspect of
request is not supported") in the Accept-Session message. Command
numbers that are Forbidden (and possibly numbers that are Reserved)
are unexpected.
In OWAMP, the Conf-Sender field is set to 1 when the
Request-Session message describes a task where the Server will
configure a one-way test packet sender. Likewise, the
Conf-Receiver field is set to 1 when the message describes the
configuration for a Session-Receiver. In TWAMP, both endpoints
perform in these roles, with the Session-Sender first sending and
then receiving test packets. The Session-Reflector first receives
the test packets, and returns each test packet to the
Session-Sender as fast as possible.
Both Conf-Sender field and Conf-Receiver field MUST be set to 0
since the Session-Reflector will both receive and send packets, and
the roles are established according to which host initiates the TCP
connection for control. The server MUST interpret any non-zero
value as an improperly formatted command, and MUST respond with the
Accept field set to 3 (meaning "Some aspect of request is not
supported") in the Accept-Session message.
The Session-Reflector in TWAMP does not process incoming test
packets for performance metrics and consequently does not need to
know the number of incoming packets and their timing schedule.
Consequently the Number of Scheduled Slots and Number of Packets
MUST be set to 0.
The Sender Port is the UDP port from which TWAMP-Test packets will
be sent and the port to which TWAMP-Test packets will be sent by
the Session-Reflector (Session-Sender will use the same UDP port to
send and receive packets). Receiver Port is the desired UDP port
to which TWAMP test packets will be sent by the Session-Sender (the
port where the Session-Reflector is asked to receive test packets).
Receiver Port is also the UDP port from which TWAMP test packets
will be sent by the Session-Reflector (Session-Reflector will use
the same UDP port to send and receive packets).
The Sender Address and Receiver Address fields contain,
respectively, the sender and receiver addresses of the endpoints of
the Internet path over which a TWAMP test session is requested.
They MAY be set to 0, in which case the IP addresses used for the
Control-Client to Server TWAMP-Control Message exchange MUST be
used in the test packets.
The Session Identifier (SID) is as defined in OWAMP [RFC4656].
Since the SID is always generated by the receiving side, the Server
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determines the SID, and the SID in the Request-TW-Session message
MUST be set to 0.
The Start Time is as defined in OWAMP [RFC4656].
The Timeout is interpreted differently from the definition in OWAMP
[RFC4656]. In TWAMP, Timeout is the interval that the
Session-Reflector MUST wait after receiving a Stop-Sessions
message. In case there are test packets still in transit, the
Session Reflector MUST reflect them if they arrive within the
timeout interval following the reception of the Stop-Sessions
message. The Session-Reflector MUST NOT reflect packets that are
received beyond the timeout.
Type-P descriptor is as defined in OWAMP [RFC4656]. The only
capability of this field is to set the Differentiated Services Code
Point (DSCP) as defined in [RFC2474]. The same value of DSCP MUST
be used in test packets reflected by the Session-Reflector.
Since there are no Schedule Slot Descriptions, the Request-TW-
Session Message is completed by MBZ (Must Be Zero) and HMAC (Hash
Message Authentication Code) fields. This completes one logical
message, referred to as the Request-TW-Session Command.
The Session-Reflector MUST respond to each Request-TW-Session
Command with an Accept-Message as defined in OWAMP [RFC4656]. When
the Accept Field = 0, the Port field confirms (repeats) the port to
which TWAMP test packets are sent by the Session-Sender toward the
Session-Reflector. In other words, the Port field indicates the
port number where the Session-Reflector expects to receive packets
from the Session-Sender.
When the requested Receiver Port is not available (e.g., port in
use), the Server at the Session-Reflector MAY suggest an alternate
and available port for this session in the Port Field. The
Session-Sender either accepts the alternate port, or composes a new
Session-Request message with suitable parameters. Otherwise, the
Server at the Session-Reflector uses the Accept Field to convey
other forms of session rejection or failure and MUST NOT suggest an
alternate port. In this case the Port Field MUST be set to zero.
3.6 Send Schedules
The Send Schedule for test packets defined in section 3.6 of OWAMP
[RFC4656] is not used in TWAMP. The Control-Client and
Session-Sender MAY autonomously decide the Send Schedule. The
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Session-Reflector SHOULD return each test packet to the
Session-Sender as quickly as possible.
3.7 Starting Test Sessions
The procedure and guidelines for Starting test sessions is the same
as defined in section 3.7 of OWAMP [RFC4656].
3.8 Stop-Sessions
The procedure and guidelines for Stopping test sessions is the same
as defined in section 3.8 of OWAMP [RFC4656]. The Stop-Sessions
command can only be issued by the Control-Client. The message MUST
NOT contain any session description records or skip ranges. The
message is terminated with a single block HMAC, to complete the
Stop-Sessions Command. Since the TWAMP Stop-Sessions command does
not convey SIDs, it applies to all sessions previously requested
and started with a Start-Sessions command.
Thus, the TWAMP Stop-Sessions command is constructed as follows:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 3 | Accept | MBZ |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Number of Sessions |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MBZ (8 octets) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| HMAC (16 octets) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Above, the Command Number in the first octet (3) indicates that
this is the Stop-Sessions command.
Non-zero Accept values indicate a failure of some sort. Zero
values indicate normal (but possibly premature) completion. The
full list of available Accept values is described in Section 3.3 of
[RFC4656], "Values of the Accept Field".
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If Accept had a non-zero value, results of all TWAMP-Test sessions
spawned by this TWAMP-Control session SHOULD be considered invalid.
If the Accept message was not transmitted at all (for whatever
reason, including failure of the TCP connection used for TWAMP-
Control), the results of all TWAMP-Test sessions spawned by this
TWAMP-control session MAY be considered invalid.
Number of Sessions indicates the number of sessions that the
Control-Client intends to stop.
Number of Sessions MUST contain the number of send sessions started
by the Control-Client that have not been previously terminated by a
Stop-Sessions command (i.e., the Control-Client MUST account for
each accepted Request-Session). If the Stop-Sessions message does
not account for exactly the number of sessions in-progress, then it
is to be considered invalid and the TWAMP-Control connection SHOULD
be closed and any results obtained considered invalid.
Upon receipt of a TWAMP-Control Stop-Sessions command, the Session-
Reflector MUST discard any TWAMP-Test packets that arrive at the
current time plus the Timeout (in the Request-TW-Session command).
3.9 Fetch-Session
One purpose of TWAMP is measurement of two-way metrics. Two-way
measurement methods do not require packet level data to be
collected by the Session-Reflector (such as sequence number,
timestamp, and TTL) because this data is communicated in the
"reflected" test packets. As such the protocol does not require
the retrieval of packet level data from the Server and the OWAMP
Fetch-Session command is not used in TWAMP.
4. 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 defines two different test packet formats, one
for packets transmitted by the Session-Sender and one for packets
transmitted by the Session-Reflector. As with OWAMP [RFC4656] test
protocol there are three modes: unauthenticated, authenticated, and
encrypted.
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4.1 Sender Behavior
The sender behavior is determined by the configuration of the
Session-Sender and is not defined in this standard. Further, the
Session-Reflector does not need to know the Session-Sender behavior
to the degree of detail as needed in OWAMP [RFC4656].
Additionally the Session-Sender collects and records the necessary
information provided from the packets transmitted by the
Session-Reflector for measuring two-way metrics. The information
recording based on the received packet by the Session-Sender is
implementation dependent.
4.1.1 Packet Timings
Since the Send Schedule is not communicated to the
Session-Reflector, there is no need for a standardized computation
of packet timing.
Regardless of any scheduling delays, each packet that is actually
sent MUST have the best possible approximation of its real time of
departure as its timestamp (in the packet).
4.1.2 Packet Format and Content
The Session-Sender packet format and content follow the same
procedure and guidelines as defined in section 4.1.2 of OWAMP
[RFC4656] (with the exception of the reference to the Send
Schedule).
Note that the Reflector test packet formats are larger than the
Sender's formats. The Session-Sender MAY append sufficient Packet
Padding to allow the same IP packet payload lengths to be used in
each direction of transmission (this is usually desirable). To
compensate for the Reflector's larger test packet format, the
Sender appends at least 27 octets of padding in unauthenticated
mode, and at least 56 octets in authenticated and encrypted modes.
4.2 Reflector Behavior
TWAMP requires the Session-Reflector to transmit a packet to the
Session-Sender in response to each packet it receives.
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As packets are received the Session-Reflector will,
- Timestamp the received packet. Each packet that is actually
received MUST have the best possible approximation of its real
time of arrival entered as its timestamp (in the packet).
- In authenticated or encrypted mode, decrypt the appropriate
sections of the packet body (first block (16 octets) for
authenticated, 96 octets for encrypted), and then check
integrity of sections covered by the HMAC.
- Copy the packet sequence number into the corresponding reflected
packet to the Session-Sender.
- Sender TTL value is extracted from the TTL/Hop Limit value of
received packets. Session-Reflector Implementations SHOULD
fetch the TTL/Hop Limit value from the IP header of the packet,
replacing the value of 255 set by the Session-Sender. If an
implementation does not fetch the actual TTL value (the only
good reason not to do so is an inability to access the TTL
field of arriving packets), it MUST set the Sender TTL value as
255.
- In authenticated and encrypted modes, the HMAC MUST be
calculated first, then the appropriate portion of the packet
body is encrypted.
- Transmit a test packet to the Session-Sender in response to
every received packet. The response MUST be generated as
immediately as possible. The format and content of the test
packet is defined in section 4.2.1. Prior to the transmission
of the test packet, the Session-Reflector MUST enter the best
possible approximation of its actual sending time of as its
Timestamp (in the packet). This permits the determination of
the elapsed time between the reception of the packet and its
transmission.
- Packets not received within the Timeout (following the Stop-
Session command) MUST be ignored by the Reflector. The
Session-Reflector MUST NOT generate a test packet to the
Session-Sender for packets that are ignored.
The possibility exists for Session-Sender failure during a session,
or the path between the Session-Sender and Session-Reflector may
fail while a test session is in-progress. The Session-Reflector MAY
discontinue any session which has been Started when no packet
associated with that session has been received for REFWAIT seconds.
The default value of REFWAIT SHALL be 900 seconds, and this waiting
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time MAY be configurable. This time-out allows a Session-Reflector
to free-up resources in case of failure.
4.2.1 TWAMP-Test Packet Format and Content
The Session-Reflector MUST transmit a packet to the Session-Sender
in response to each packet received. The Session-Reflector SHOULD
transmit the packets as immediately as possible. The
Session-Reflector SHOULD set the TTL in IPV4 (or Hop Limit in IPv6)
in the UDP packet to 255.
The test packet will have the necessary information for calculating
two-way metrics by the Session-Sender. The format of the test
packet depends on the mode being used. The two formats are
presented below.
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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 | |
+-+-+-+-+-+-+-+-+ +
| |
. .
. Packet Padding .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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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 | |
+-+-+-+-+-+-+-+-+ +
| |
| |
| MBZ (15 octets) |
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
| HMAC (16 octets) |
| |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
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| |
. .
. Packet Padding .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Note that all Timestamps have the same format as OWAMP [RFC4656] as
follows:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Integer part of seconds |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Fractional part of seconds |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Sequence Number is the sequence number of the test packet according
to its transmit order. It starts with zero and is incremented by
one for each subsequent packet. The Sequence Number generated by
the Session-Reflector is independent from the sequence number of
the arriving packets.
Timestamp and Error Estimate are the Session-Reflector's transmit
timestamp and error estimate for the reflected test packet,
respectively. The format of all timestamp and error estimate
fields follow the definition and formats defined by OWAMP section
4.1.2, in [RFC4656].
Sender Timestamp and Sender Error Estimate are exact copies of the
timestamp and error estimate from the Session-Sender test packet
that corresponds to this test packet.
Sender TTL is 255 when transmitted by the Session Sender. Sender
TTL is set to the Time To Live (or Hop Count) value of the received
packet from the IP packet header when transmitted by the Session
Reflector.
Receive Timestamp is the time the test packet was received by the
reflector. The difference between Timestamp and Receive Timestamp
is the amount of time the packet was in transition in the
Session-Reflector. The Error Estimate associated with the
Timestamp field also applies to the Receive Timestamp.
Sender Sequence Number is a copy of the Sequence Number of the
packet transmitted by the Session-Sender that caused the
Session-Reflector to generate and send this test packet.
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The HMAC field in TWAMP-Test packets covers the same fields as the
AES encryption. Thus, in authenticated mode, HMAC covers the first
block (16 octets). In encrypted mode, HMAC covers the first six
blocks (96 octets). In TWAMP-Test, the HMAC field MUST NOT be
encrypted.
Packet Padding in TWAMP-Test SHOULD be pseudo-random (it MUST be
generated independently of any other pseudo-random numbers
mentioned in this document). However, implementations MUST provide
a configuration parameter, an option, or a different means of
making Packet Padding consist of all zeros. Packet Padding MUST NOT
be covered by the HMAC and MUST NOT be encrypted.
The minimum data segment length of TWAMP-Test packets in
unauthenticated mode is 41 octets, and 104 octets in both
authenticated mode and encrypted modes.
Note that the Session-Reflector Test Packet Formats are larger than
the Sender's formats. The Session-Reflector SHOULD reduce the
length of the Sender's Packet Padding to achieve equal IP packet
payload lengths in each direction of transmission, when sufficient
padding is present. The Session-Reflector MAY re-use the Sender's
Packet Padding (since the requirements for padding generation are
the same for each), and in this case the Session-Reflector SHOULD
truncate the padding such that the highest number octets are
discarded.
In unauthenticated mode, encryption or authentication MUST NOT be
applied.
The TWAMP-Test packet layout is identical in authenticated and
encrypted modes. The encryption operation for a Session-Sender
packet follows the same rules of Session-Sender packets as defined
in OWAMP section 4.1.2 of [RFC4656].
The main difference between authenticated mode and encrypted mode
is the portions of the test packets that are covered by HMAC and
encrypted. Authenticated mode permits the timestamp to be fetched
after a portion of the packet is encrypted, but in encrypted mode
all the sequence numbers and timestamps are fetched before
encryption to provide maximum data integrity protection.
In authenticated mode, only the sequence number in the first block
is encrypted and the subsequent timestamps and sequence numbers are
sent in clear text. Sending the timestamp in clear text allows one
to reduce the time between when a timestamp is obtained by a
Session-Reflector and when that packet is sent out. This
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potentially improves the timestamp accuracy, because the sequence
number can be encrypted before the timestamp is fetched.
In encrypted mode, the reflector MUST fetch the timestamps,
generate the HMAC and encrypt the packet, then send it.
Obtaining the keys and encryption methods follow the same procedure
as OWAMP as described below. Each TWAMP-Test session has two keys
an AES Session-key and an HMAC Session-key, and the keys are
derived from the TWAMP-Control keys and the SID.
The TWAMP-Test AES Session-key is obtained as follows: the
TWAMP-Control AES Session-key (the same AES Session-key as used for
the corresponding TWAMP-Control session) is encrypted with the 16-
octet session identifier (SID) as the key, using a single-block
AES-ECB encryption. The result is the TWAMP-Test AES Session-key to
use in encrypting (and decrypting) the packets of the particular
TWAMP-Test session. Note that the TWAMP-Test AES Session-key,
TWAMP-Control AES Session-key, and the SID are all comprised of 16
octets.
The TWAMP-Test HMAC Session-key is obtained as follows: the
TWAMP-Control HMAC Session-key (the same HMAC Session-key as used
for the corresponding TWAMP-Control session) is encrypted using
AES-CBC with the 16-octet session identifier (SID) as the key. This
is a two-block CBC encryption always performed with IV=0. Note that
the TWAMP-Test HMAC Session-key and TWAMP-Control HMAC Session-key
are comprised of 32 octets, while the SID is 16 octets.
In authenticated mode, the first block (16 octets) of each TWAMP-
Test packet is encrypted using AES Electronic Codebook (ECB) mode.
This mode does not involve any chaining, and lost, duplicated, or
reordered packets do not cause problems with deciphering any packet
in a TWAMP-Test session.
In encrypted mode, the first six blocks (96octets) are encrypted
using AES CBC mode. The AES Session-key to use is obtained in the
same way as the key for authenticated mode. Each TWAMP-Test packet
is encrypted as a separate stream, with just one chaining
operation; chaining does not span multiple packets so that lost,
duplicated, or reordered packets do not cause problems. The
initialization vector for the CBC encryption is a value with all
bits equal to zero.
Implementation note: Naturally, the key schedule for each
TWAMP-Test session MUST be set up at most once per session, not
once per packet.
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5. Implementers Guide
This section serves as guidance to implementers of TWAMP. The
example architecture presented here is not a requirement. Similar
to OWAMP [RFC4656], TWAMP is designed with enough flexibility to
allow different architectures that suit multiple system
requirements.
In this example the roles of Control-Client and Session-Sender are
implemented in one host referred to as the controller and the roles
of Server and Session-Reflector are implemented in another host
referred to as the responder.
controller responder
+-----------------+ +-------------------+
| Control-Client |<--TWAMP-Control-->| Server |
| | | |
| Session-Sender |<--TWAMP-Test----->| Session-Reflector |
+-----------------+ +-------------------+
This example provides an architecture that supports the full TWAMP
standard. The controller establishes the test session with the
responder through the TWAMP-Control protocol. After the session is
established the controller transmits test packets to the responder.
The responder follows the Session-Reflector behavior of TWAMP as
described in section 4.2.
Appendix I provides an example for purely informational purposes.
It suggests an incremental path to adopting TWAMP, by implementing
the TWAMP-Test protocol first.
6. Security Considerations
Fundamentally TWAMP and OWAMP use the same protocol for
establishment of Control and Test procedures. The main difference
between TWAMP and OWAMP is the Session-Reflector behavior in TWAMP
vs. the Session-Receiver behavior in OWAMP. This difference in
behavior does not introduce any known security vulnerabilities that
are not already addressed by the security features of OWAMP. The
entire security considerations of OWAMP [RFC4656] applies to TWAMP.
The Server Greeting message (defined in OWAMP, section 3.1
[RFC4656]) includes a "Count" field to specify the iteration
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counter used in PKCS #5 to generate keys from shared secrets. OWAMP
recommends a lower limit of 1024 iterations, but no upper limit.
The Count field provides an opportunity for a DOS attack because it
is 32 bits long. If an attacking system set the maximum value in
Count (2**32), it could stall a system attempting to generate keys
for a significant period of time. Therefore, TWAMP-compliant
systems SHOULD have a configuration control to limit the maximum
Count value. The default maximum Count value SHOULD be 32768. As
suggested in OWAMP, this value MAY be increased when greater
computing power becomes common. If a Control-Client receives a
Server Greeting Message with Count greater that its maximum
configured value, it SHOULD close the control connection.
7. Acknowledgements
We would like to thank Nagarjuna Venna, Sharee McNab, Nick Kinraid,
Stanislav Shalunov, Matt Zekauskas, Walt Steverson, Jeff Boote, and
Murtaza Chiba for their comments, suggestions, reviews, helpful
discussion and proof-reading. Lars Eggert, Sam Hartman, and Tim
Polk contributed very useful AD-level reviews, and the authors
thank them for their contributions to the memo.
8. IANA Considerations
IANA has allocated a well-known TCP port number (861) for the
OWAMP-Control part of the OWAMP [RFC4656] protocol.
...
owamp-control 861/tcp OWAMP-Control
owamp-control 861/udp OWAMP-Control
# [RFC4656]
# 862-872 Unassigned
IANA is requested to allocate a well-known TCP/UDP port number for
the TWAMP-Control protocol. It would be ideal if the port number
assignment was adjacent to the OWAMP assignment. The recommended
Keyword for this entry is "twamp-control" and the Description is
"Two-way Active Measurement Protocol (TWAMP) Control".
During final editing, port N in section 3.1 should be replaced with
the assigned port number.
Since TWAMP adds an additional Control command to the OWAMP-Control
specification, and describes behavior when this control command is
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used, this memo requests creation an IANA registry for the TWAMP
Command Number field. The field is not explicitly named in
[RFC4656] but is called out for each command. This field is a
recognized extension mechanism for TWAMP.
8.1 Registry Specification
IANA will create an TWAMP-Control Command Number registry. TWAMP-
Control commands are specified by the first octet in OWAMP-Control
messages as shown in section 3.5 of [RFC4656], and modified by this
document. Thus this registry may contain sixteen possible values.
8.2 Registry Management
Because the registry may only contain sixteen values, and because
OWAMP and TWAMP are IETF protocols, this registry must only be
updated by "IETF Consensus" as specified in [RFC2434] -- an RFC
documenting the use that is approved by the IESG. We expect that
new values will be assigned as monotonically increasing integers in
the range [0-15], unless there is a good reason to do otherwise.
8.3 Experimental Numbers
[RFC3692] recommends allocating an appropriate number of values for
experimentation and testing. It is not clear to the authors
exactly how many numbers might be useful in this space, nor if it
would be useful that they were easily distinguishable or at the
"high end" of the number range. Two might be useful, say one for
session control, and one for session fetch. On the other hand, a
single number would allow for unlimited extension, because the
format of the rest of the message could be tailored, with
allocation of other numbers done once usefulness has been proven.
Thus, this document will allocate one number, the next sequential
number 6, as designated for experimentation and testing.
8.4 Initial Registry Contents
TWAMP-Control Command Number Registry
Value Description Semantics Definition
0 Reserved
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1 Forbidden
2 Start-Sessions RFC4656, Section 3.7
3 Stop-Sessions RFC4656, Section 3.8
4 Reserved
5 Request-TW-Session this document, Section 3.5
6 Experimentation undefined, see Section 8.3.
9. Internationalization Considerations
The protocol does not carry any information in a natural language,
with the possible exception of the KeyID in TWAMP-Control, which is
encoded in UTF-8 [RFC3629, RFC5198].
10. APPENDIX I - TWAMP Light (Informative)
In this example the roles of Control-Client, Server, and
Session-Sender are implemented in one host referred to as the
controller and the role of Session-Reflector is implemented in
another host referred to as the responder.
controller responder
+-----------------+ +-------------------+
| Server |<----------------->| |
| Control-Client | | Session-Reflector |
| Session-Sender |<--TWAMP-Test----->| |
+-----------------+ +-------------------+
This example provides a simple architecture for responders where
their role will be to simply act as light test points in the
network. The controller establishes the test session with the
Server through non-standard means. After the session is
established the controller transmits test packets to the responder.
The responder follows the Session-Reflector behavior of TWAMP as
described in section 4.2 with the following exceptions.
In the case of TWAMP Light, the Session-Reflector does not
necessarily have knowledge of the session state. IF the
Session-Reflector does not have knowledge of the session state,
THEN the Session-Reflector MUST copy the Sequence Number of the
received packet to the Sequence Number field of the reflected
packet. The controller receives the reflected test packets and
collects two-way metrics. This architecture allows for collection
of two-way metrics.
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This example eliminates the need for the TWAMP-Control protocol and
assumes that the Session-Reflector is configured and communicates
its configuration with the Server through non-standard means. The
Session-Reflector simply reflects the incoming packets back to the
controller while copying the necessary information and generating
sequence number and timestamp values per section 4.2.1.
TWAMP Light introduces some additional security considerations. The
non-standard means to control the responder and establish test
sessions SHOULD offer the features listed below.
The non-standard responder control protocol SHOULD have an
authenticated mode of operation. The responder SHOULD be
configurable to accept only authenticated control sessions.
The non-standard responder control protocol SHOULD have a means to
activate the authenticated and encrypted modes of the TWAMP-Test
protocol.
When the TWAMP Light test sessions operate in authenticated or
encrypted mode, the Session-Reflector MUST have some mechanism for
generating keys (because the TWAMP-Control protocol normally plays
a role in this process, but is not present here). The specification
of the key generation mechanism is beyond the scope of this memo.
11. References
11.1 Normative References
[RFC4656] Shalunov, S., Teitelbaum, B., Karp, A., Boote, J.,
Zekauskas, M., "A One-way Active Measurement Protocol
(OWAMP)", RFC 4656, October 2004.
[RFC2681] Almes, G., Kalidindi, S., Zekauskas, M., "A
Round-Trip Delay Metric for IPPM". RFC 2681,
September 1999.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
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[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,
December 1998.
[RFC2434] Narten, T., Alvestrand, H., Guidelines for Writing
an IANA Considerations Section in RFCs, RFC 2434,
October 1998.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003.
[RFC5198] Klensin, J., Padlipsky, M., "Unicode Format for
Network Interchange", RFC 5198, March 2008.
11.2 Informative References
[RFC3692] Narten, T., Assigning Experimental and Testing Numbers
Considered Useful, RFC 3692, January 2004.
Authors' Addresses
Kaynam Hedayat
Brix Networks
285 Mill Road
Chelmsford, MA 01824
USA
EMail: khedayat@brixnet.com
URI: http://www.brixnet.com/
Roman M. Krzanowski, Ph.D.
Verizon
500 Westchester Ave.
White Plains, NY
USA
EMail: roman.krzanowski@verizon.com
URI: http://www.verizon.com/
Al Morton
AT&T Labs
Room D3 - 3C06
200 Laurel Ave. South
Middletown, NJ 07748
USA
Phone +1 732 420 1571
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EMail: acmorton@att.com
URI: http://home.comcast.net/~acmacm/
Kiho Yum
Juniper Networks
1194 Mathilda Ave.
Sunnyvale, CA
USA
EMail: kyum@juniper.net
URI: http://www.juniper.com/
Jozef Z. Babiarz
Nortel Networks
3500 Carling Avenue
Ottawa, Ont K2H 8E9
Canada
Email: babiarz@nortel.com
URI: http://www.nortel.com/
Full Copyright Statement
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Hedayat, et al. Expires Jan 2009 [Page 28]
