Internet DRAFT - draft-ietf-ippm-checksum-trailer
draft-ietf-ippm-checksum-trailer
Network Working Group T. Mizrahi
Internet Draft Marvell
Intended status: Experimental
Expires: August 2016 February 9, 2016
UDP Checksum Complement in OWAMP and TWAMP
draft-ietf-ippm-checksum-trailer-06.txt
Abstract
The One-Way Active Measurement Protocol (OWAMP) and the Two-Way
Active Measurement Protocol (TWAMP) are used for performance
monitoring in IP networks. Delay measurement is performed in these
protocols by using timestamped test packets. Some implementations use
hardware-based timestamping engines that integrate the accurate
transmission timestamp into every outgoing OWAMP/TWAMP test packet
during transmission. Since these packets are transported over UDP,
the UDP checksum field is then updated to reflect this modification.
This document proposes to use the last 2 octets of every test packet
as a Checksum Complement, allowing timestamping engines to reflect
the checksum modification in the last 2 octets rather than in the UDP
checksum field. The behavior defined in this document is completely
interoperable with existing OWAMP/TWAMP implementations.
Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on August 9, 2016.
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Copyright Notice
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document authors. All rights reserved.
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Table of Contents
1. Introduction...................................................2
2. Conventions used in this document..............................5
2.1. Terminology...............................................5
2.2. Abbreviations.............................................5
3. Using the UDP Checksum Complement in OWAMP and TWAMP...........6
3.1. Overview..................................................6
3.2. OWAMP / TWAMP Test Packets with Checksum Complement.......6
3.2.1. Transmission of OWAMP/TWAMP with Checksum Complement.9
3.2.2. Intermediate Updates of OWAMP/TWAMP with Checksum
Complement.................................................10
3.2.3. Reception of OWAMP/TWAMP with Checksum Complement...10
3.3. Interoperability with Existing Implementations...........10
3.4. Using the Checksum Complement with or without Authentication
..............................................................10
3.4.1. Checksum Complement in Authenticated Mode...........10
3.4.2. Checksum Complement in Encrypted Mode...............11
4. Security Considerations.......................................11
5. IANA Considerations...........................................12
6. Acknowledgments...............................................12
7. References....................................................12
7.1. Normative References.....................................12
7.2. Informative References...................................13
Appendix A. Checksum Complement Usage Example....................13
1. Introduction
The One-Way Active Measurement Protocol ([OWAMP]) and the Two-Way
Active Measurement Protocol ([TWAMP]) are used for performance
monitoring in IP networks.
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Delay and delay variation are two of the metrics that OWAMP/TWAMP can
measure. This measurement is performed using timestamped test
packets. In some use cases, such as carrier networks, these two
metrics are an essential aspect of the Service Level Agreement (SLA),
and therefore must be measured with a high degree of accuracy. If
packets are timestamped in hardware as they exit the host, then
greater accuracy is possible in comparison to higher-layer timestamps
(as explained further below).
The accuracy of delay measurements relies on the timestamping method
and its implementation. In order to facilitate accurate timestamping,
an implementation can use a hardware based timestamping engine, as
shown in Figure 1. In such cases, the OWAMP/TWMAP packets are sent
and received by a software layer, whereas the timestamping engine
modifies every outgoing test packet by incorporating its accurate
transmission time into the <Timestamp> field in the packet.
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OWAMP/TWAMP-enabled Node
+-------------------+
| |
| +-----------+ |
Software | |OWAMP/TWAMP| |
| | protocol | |
| +-----+-----+ |
| | | +---------------------+
| +-----+-----+ | / Intermediate entity |
| | Accurate | | / in charge of: |
ASIC/FPGA | | Timestamp | | /__ -Timestamping |
| | engine | | |-Updating checksum or|
| +-----------+ | | Checksum Complement |
| | | +---------------------+
+---------+---------+
|
|test packets
|
___ v _
/ \_/ \__
/ \_
/ IP /
\_ Network /
/ \
\__/\_ ___/
\_/
Figure 1 Accurate Timestamping in OWAMP/TWAMP
OWAMP/TWAMP test packets are transported over UDP. When the UDP
payload is changed by an intermediate entity such as the timestamping
engine, the UDP Checksum field must be updated to reflect the new
payload. When using UDP over IPv4 ([UDP]), an intermediate entity
that cannot update the value of the UDP checksum has no choice except
to assign a value of zero to the checksum field, causing the receiver
to ignore the checksum field and potentially accept corrupted
packets. UDP over IPv6, as defined in [IPv6], does not allow a zero
checksum, except in specific cases [ZeroChecksum]. As discussed in
[ZeroChecksum], the use of a zero checksum is generally not
recommended, and should be avoided to the extent possible.
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Since an intermediate entity only modifies a specific field in the
packet, i.e. the timestamp field, the UDP checksum update can be
performed incrementally, using the concepts presented in [Checksum].
A similar problem is addressed in Annex E of [IEEE1588]. When the
Precision Time Protocol (PTP) is transported over IPv6, two octets
are appended to the end of the PTP payload for UDP checksum updates.
The value of these two octets can be updated by an intermediate
entity, causing the value of the UDP checksum field to remain
correct.
This document defines a similar concept for [OWAMP] and [TWAMP],
allowing intermediate entities to update OWAMP/TWAMP test packets and
maintain the correctness of the UDP checksum by modifying the last 2
octets of the packet.
The term Checksum Complement is used throughout this document and
refers to the 2 octets at the end of the UDP payload, used for
updating the UDP checksum by intermediate entities.
The usage of the Checksum Complement can in some cases simplify the
implementation, since if the packet data is processed in a serial
order, it is simpler to first update the timestamp field, and then
update the Checksum Complement rather than to update the timestamp
and then update the UDP checksum, residing at the UDP header.
The Checksum Complement mechanism is also defined for the Network
Time Protocol in [NTPComp].
2. Conventions used in this document
2.1. Terminology
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 [KEYWORDS].
2.2. Abbreviations
HMAC Hashed Message Authentication Code
OWAMP One-Way Active Measurement Protocol
PTP Precision Time Protocol
TWAMP Two-Way Active Measurement Protocol
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UDP User Datagram Protocol
3. Using the UDP Checksum Complement in OWAMP and TWAMP
3.1. Overview
The UDP Checksum Complement is a two-octet field that is piggybacked
at the end of the test packet. It resides in the last 2 octets of the
UDP payload.
+----------------------------------+
| IPv4 / IPv6 Header |
+----------------------------------+
| UDP Header |
+----------------------------------+
^ | |
| | OWAMP / TWAMP |
UDP | packet |
Payload +----------------------------------+
| |UDP Checksum Complement (2 octets)|
v +----------------------------------+
Figure 2 Checksum Complement in OWAMP/TWAMP Test Packet
The Checksum Complement is used to compensate for changes performed
in the packet by intermediate entities, as described in the
introduction. An example of the usage of the Checksum Complement is
provided in Appendix A.
3.2. OWAMP / TWAMP Test Packets with Checksum Complement
The One-Way Active Measurement Protocol [OWAMP], and the Two-Way
Active Measurement Protocol [TWAMP] both make use of timestamped test
packets. A Checksum Complement MAY be used in the following cases:
o In OWAMP test packets, sent by the sender to the receiver.
o In TWAMP test packets, sent by the sender to the reflector.
o In TWAMP test packets, sent by the reflector to the sender.
OWAMP/TWAMP test packets are transported over UDP, either over IPv4
or over IPv6. This document applies to both OWAMP/TWAMP over IPv4 and
over IPv6.
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OWAMP/TWAMP test packets contain a Packet Padding field. This
document proposes to use the last 2 octets of the Packet Padding
field as the Checksum Complement. In this case the Checksum
Complement is always the last 2 octets of the UDP payload, and thus
the field is located UDP Length - 2 octets after the beginning of the
UDP header.
Figure 3 illustrates the OWAMP test packet format including the UDP
Checksum Complement.
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 .
. .
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | Checksum Complement |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3 Checksum Complement in OWAMP Test Packets
Figure 4 illustrates the TWAMP test packet format including the UDP
Checksum Complement.
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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 .
. .
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | Checksum Complement |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4 Checksum Complement in TWAMP Test Packets
The length of the Packet Padding field in test packets is announced
during the session initiation through the <Padding Length> field in
the Request-Session message [OWAMP], or in the Request-TW-Session
[TWAMP].
When a Checksum Complement is included, the <Padding Length> MUST be
sufficiently long to include the Checksum Complement:
o In OWAMP the padding length is at least 2 octets, allowing the
sender to incorporate the Checksum Complement in the last 2 octets
of the padding.
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o In TWAMP the padding length is at least 29 octets in
unauthenticated mode, and at least 58 octets long in authenticated
mode. The additional padding is required since the header of
reflector test packets is longer than the header of sender test
packets. The difference between the sender packet and the
reflector packet is 27 octets in unauthenticated mode, and 56
octets in authenticated mode.
Thus, the padding in reflector test packets is shorter than in
sender packet. Using at least 29 octets of padding (58 in
authenticated mode) in sender test packets allows both the sender
and the reflector to use a 2-octet Checksum Complement.
Note: if the minimal length requirement is not met, the reflector
cannot use a Checksum Complement in the reflected test packets,
but the sender can use a Checksum Complement in the test packets
it transmits.
o Two optional TWAMP features are defined in [RFC6038]: octet
reflection and symmetrical size. When at least one of these
features is enabled, the Request-TW-Session includes the <Padding
Length> field, as well as a <Length of padding to reflect> field.
In this case both fields must be sufficiently long to allow at
least 2 octets of padding in both sender test packets and
reflector test packets.
Specifically, when octet reflection is enabled, the two length
fields must be defined such that the padding expands at least 2
octets beyond the end of the reflected octets.
As described in Section 1. , the extensions described in this
document are implemented by two logical layers, a protocol layer and
a timestamping layer. It is assumed that the two layers are
synchronized about whether the usage of the Checksum Complement is
enabled or not; since both logical layers reside in the same network
device, it is assumed there is no need for a protocol that
synchronizes this information between the two layers. When Checksum
Complement usage is enabled, the protocol layer must take care to
verify that test packets include the necessary padding, and avoiding
the need for the timestamping layer to verify that en-route test
packets include the necessary padding.
3.2.1. Transmission of OWAMP/TWAMP with Checksum Complement
The transmitter of an OWAMP/TWAMP test packet MAY include a Checksum
Complement field, incorporated in the last 2 octets of the Packet
Padding.
A transmitter that includes a Checksum Complement in its outgoing
test packets MUST include a Packet Padding in these packets, the
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length of which MUST be sufficient to include the Checksum
Complement. The length of the padding field is negotiated during
session initiation, as described in Section 3.2.
3.2.2. Intermediate Updates of OWAMP/TWAMP with Checksum Complement
An intermediate entity that receives and alters an OWAMP/TWAMP test
packet can alter either the UDP Checksum field or the Checksum
Complement field in order to maintain the correctness of the UDP
checksum value.
3.2.3. Reception of OWAMP/TWAMP with Checksum Complement
This document does not impose new requirements on the receiving end
of an OWAMP/TWAMP test packet.
The UDP layer at the receiving end verifies the UDP Checksum of
received test packets, and the OWAMP/TWAMP layer should treat the
Checksum Complement as part of the Packet Padding.
3.3. Interoperability with Existing Implementations
The behavior defined in this document does not impose new
requirements on the reception behavior of OWAMP/TWAMP test packets.
The protocol stack of the receiving host performs the conventional
UDP checksum verification, and thus the existence of the Checksum
Complement is transparent from the perspective of the receiving host.
Therefore, the functionality described in this document allows
interoperability with existing implementations that comply to [OWAMP]
or [TWAMP].
3.4. Using the Checksum Complement with or without Authentication
Both OWAMP and TWAMP may use authentication or encryption, as defined
in [OWAMP] and [TWAMP].
3.4.1. Checksum Complement in Authenticated Mode
OWAMP and TWAMP test packets can be authenticated using an HMAC
(Hashed Message Authentication Code). The HMAC covers some of the
fields in the test packet header. The HMAC does not cover the
Timestamp field and the Packet Padding field.
A Checksum Complement MAY be used when authentication is enabled. In
this case an intermediate entity can timestamp test packets and
update their Checksum Complement field without modifying the HMAC.
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3.4.2. Checksum Complement in Encrypted Mode
When OWAMP and TWAMP are used in encrypted mode, the Timestamp field
is encrypted.
A Checksum Complement SHOULD NOT be used in encrypted mode. The
Checksum Complement is effective in unauthenticated and in
authenticated mode, allowing the intermediate entity to perform
serial processing of the packet without storing-and-forwarding it.
On the other hand, in encrypted mode an intermediate entity that
timestamps a test packet must also re-encrypt the packet accordingly.
Re-encryption typically requires the intermediate entity to store the
packet, re-encrypt it, and then forward it. Thus, from an
implementer's perspective, the Checksum Complement has very little
value in encrypted mode, as it does not necessarily simplify the
implementation.
Note: while [OWAMP] and [TWAMP] include an inherent security
mechanism, these protocols can be secured by other measures, e.g.,
[IPPMIPsec]. For similar reasons as described above, a Checksum
Complement SHOULD NOT be used in this case.
4. Security Considerations
This document describes how a Checksum Complement extension can be
used for maintaining the correctness of the UDP checksum.
The purpose of this extension is to ease the implementation of
accurate timestamping engines, as described in Figure 1. The
extension is intended to be used internally in an OWAMP/TWAMP enabled
node, and not intended to be used by intermediate switches and
routers that reside between the sender and the receiver/reflector.
Any modification of a test packet by intermediate switches or routers
should be considered a malicious MITM attack.
It is important to emphasize that the scheme described in this
document does not increase the protocol's vulnerability to MITM
attacks; a MITM who maliciously modifies a packet and its Checksum
Complement is logically equivalent to a MITM attacker who modifies a
packet and its UDP Checksum field.
The concept described in this document is intended to be used only in
unauthenticated or in authenticated mode. As described in Section
3.4.2. , in encrypted mode using the Checksum Complement does not
simplify the implementation compared to using the conventional
Checksum, and therefore the Checksum Complement should not be used.
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5. IANA Considerations
There are no IANA actions required by this document.
RFC Editor: please delete this section before publication.
6. Acknowledgments
The authors gratefully acknowledge Al Morton, Greg Mirsky, Steve
Baillargeon, Brian Haberman, and Spencer Dawkins for their helpful
comments.
This document was prepared using 2-Word-v2.0.template.dot.
7. References
7.1. Normative References
[KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[IPv6] Deering, S., Hinden, R., "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
[Checksum] Rijsinghani, A., "Computation of the Internet Checksum
via Incremental Update", RFC 1624, May 1994.
[UDP] Postel, J., "User Datagram Protocol", RFC 768, August
1980.
[OWAMP] Shalunov, S., Teitelbaum, B., Karp, A., Boote, J., and
Zekauskas, M., "A One-way Active Measurement Protocol
(OWAMP)", RFC 4656, September 2006.
[TWAMP] Hedayat, K., Krzanowski, R., Morton, A., Yum, K., and
Babiarz, J., "A Two-Way Active Measurement Protocol
(TWAMP)", RFC 5357, October 2008.
[RFC6038] Morton, A., Ciavattone, L., "Two-Way Active
Measurement Protocol (TWAMP) Reflect Octets and
Symmetrical Size Features", RFC 6038, October 2010.
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7.2. Informative References
[IEEE1588] IEEE TC 9 Instrumentation and Measurement Society,
"1588 IEEE Standard for a Precision Clock
Synchronization Protocol for Networked Measurement and
Control Systems Version 2", IEEE Standard, 2008.
[IPPMIPsec] Pentikousis, K., Zhang, E., Cui, Y., "IKEv2-Derived
Shared Secret Key for the One-Way Active Measurement
Protocol (OWAMP) and Two-Way Active Measurement
Protocol (TWAMP)", RFC 7717, December 2015.
[NTPComp] Mizrahi, T., "UDP Checksum Complement in the Network
Time Protocol (NTP)", draft-ietf-ntp-checksum-trailer
(work in progress), October 2015.
[ZeroChecksum] Fairhurst, G., Westerlund, M., "Applicability
Statement for the Use of IPv6 UDP Datagrams with Zero
Checksums", RFC 6936, April 2013.
Appendix A. Checksum Complement Usage Example
Consider a session between an OWAMP sender and an OWAMP receiver, in
which the sender transmits test packets to the receiver.
The sender's software layer generates an OWAMP test packet with a
timestamp T, and a UDP checksum value U. The value of U is the
checksum of the UDP header, UDP payload, and pseudo-header. Thus, U
is equal to:
U = Const + checksum(T) (1)
Where 'Const' is the checksum of all the fields that are covered by
the checksum except the timestamp T.
Recall that the sender's software emits the test packet with a
Checksum Complement field, which is simply the last two bytes of the
padding. In this example it is assumed that the sender initially
assigns zero to these two bytes.
The sender's timestamping engine updates the timestamp field to the
accurate time, changing its value from T to T'. The sender also
updates the Checksum Complement field from zero to a new value C,
such that:
checksum(C) = checksum(T) - checksum(T') (2)
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When the test packet is transmitted by the sender's timestamping
engine, the value of the checksum remains U as before:
U = Const + checksum(T) = Const + checksum(T)+ checksum(T')-
checksum(T') = Const + checksum(T') + checksum(C) (3)
Thus, after the timestamping engine has updated the timestamp, U
remains the correct checksum of the packet.
When the test packet reaches the receiver, the receiver performs a
conventional UDP checksum computation, and the computed value is U.
Since the Checksum Complement is part of the padding, the value of
checksum(C) is transparently included in the computation, as per
Equation (3), without requiring special treatment by the receiver.
Authors' Addresses
Tal Mizrahi
Marvell
6 Hamada St.
Yokneam, 20692 Israel
Email: talmi@marvell.com
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