Internet DRAFT - draft-mizrahi-ippm-checksum-trailer
draft-mizrahi-ippm-checksum-trailer
Network Working Group T. Mizrahi
Internet Draft Marvell
Intended status: Informational
Expires: June 2015 December 15, 2014
UDP Checksum Trailer in OWAMP and TWAMP
draft-mizrahi-ippm-checksum-trailer-02.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 Trailer, 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 June 15, 2015.
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Copyright Notice
Copyright (c) 2014 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
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described in the Simplified BSD License.
Table of Contents
1. Introduction ................................................. 2
2. Conventions used in this document ............................ 4
2.1. Terminology ............................................. 4
2.2. Abbreviations ........................................... 4
3. Using the UDP Checksum Trailer in OWAMP and TWAMP ............ 5
3.1. Overview ................................................ 5
3.2. OWAMP / TWAMP Test Packets with Checksum Trailer ........ 5
3.2.1. Transmission of OWAMP/TWAMP with Checksum Trailer .. 8
3.2.2. Intermediate Updates of OWAMP/TWAMP with Checksum
Trailer ................................................... 9
3.2.3. Reception of OWAMP/TWAMP with Checksum Trailer ..... 9
3.3. Interoperability with Existing Implementations........... 9
3.4. Using the Checksum Trailer with or without Authentication 9
4. Security Considerations ..................................... 10
5. IANA Considerations ......................................... 10
6. Acknowledgments ............................................. 10
7. References .................................................. 10
7.1. Normative References ................................... 10
7.2. Informative References ................................. 11
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.
Delay and delay variation are two of the metrics that OWAMP/TWAMP can
measure. This measurement is performed using timestamped test
packets.
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The accuracy of delay measurements relies on the timestamping method
and its implementation. In order to facilitate accurate timestamping,
an implementation MAY 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.
OWAMP/TWAMP-enabled Node
+-------------------+
| |
| +-----------+ |
Software | |OWAMP/TWAMP| |
| | protocol | |
| +-----+-----+ |
| | |
| +-----+-----+ |
| | Accurate | |
ASIC/FPGA | | Timestamp | |
| | engine | |
| +-----------+ |
| | |
+---------+---------+
|
|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 can assign a value
of zero to the checksum field, causing the receiver to ignore the
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checksum field. UDP over IPv6, as defined in [IPv6], does not allow a
zero checksum, and requires the UDP checksum field to contain a
correct checksum of the UDP payload.
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 Trailer 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 Trailer 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 Trailer rather than to update the timestamp and
then update the UDP checksum, residing at the UDP header.
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
OWAMP One-Way Active Measurement Protocol
PTP Precision Time Protocol
TWAMP Two-Way Active Measurement Protocol
UDP User Datagram Protocol
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3. Using the UDP Checksum Trailer in OWAMP and TWAMP
3.1. Overview
The UDP Checksum Trailer is a two-octet trailer 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 Trailer (2 octets) |
v +--------------------------------+
Figure 2 Checksum Trailer in OWAMP/TWAMP Test Packet
3.2. OWAMP / TWAMP Test Packets with Checksum Trailer
The One-Way Active Measurement Protocol [OWAMP], and the Two-Way
Active Measurement Protocol [TWAMP] both make use of timestamped test
packets. The formats of these packets are defined in [OWAMP] and in
[TWAMP].
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.
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 Trailer. In this case the Checksum Trailer is
always the last 2 octets of the UDP payload, and thus the trailer 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 trailer.
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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 | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
. Packet Padding .
. .
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | Checksum Trailer |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3 Checksum Trailer in OWAMP Test Packets
Figure 4 illustrates the TWAMP test packet format including the UDP
checksum trailer.
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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 Trailer |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4 Checksum Trailer 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 Trailer is included, the <Padding Length> MUST be
sufficiently long to include the Checksum Trailer:
o In OWAMP the padding length is at least 2 octets, allowing the
sender to incorporate the checksum trailer in the last 2 octets of
the padding.
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o In TWAMP the padding length is at least 29 octets. The additional
padding is required since the header of reflector test packets is
27 octets longer than the header of sender test packets. Thus, the
padding in reflector test packets is 27 octets shorter than in
sender packet. Using 29 octets of padding in sender test packets
allows both the sender and the reflector to use a 2-octet checksum
trailer.
Note: the 27-octet difference between the sender packet and the
reflector packet is specifically in unauthenticated mode, whereas
in authenticated mode the difference between the sender and
receiver packets is 56 octets. As specified in Section 3.4. , the
checksum trailer should only be used in unauthenticated mode.
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 Trailer 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
Trailer 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 Trailer
The transmitter of an OWAMP/TWAMP test packet MAY include a Checksum
Trailer field, incorporated in the last 2 octets of the Packet
Padding.
A transmitter that includes a Checksum Trailer in its outgoing test
packets MUST include a Packet Padding in these packets, the length of
which MUST be sufficient to include the checksum trailer. The length
of the padding field is negotiated during session initiation, as
described in Section 3.2.
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3.2.2. Intermediate Updates of OWAMP/TWAMP with Checksum Trailer
An intermediate entity that receives and alters an OWAMP/TWAMP test
packet MAY alter the Checksum Trailer field in order to maintain the
correctness of the UDP checksum value.
3.2.3. Reception of OWAMP/TWAMP with Checksum Trailer
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 Trailer 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 an OWAMP receiver or a
TWAMP reflector, since the existence of the checksum trailer is
transparent from the perspective of the receiver/reflector. Thus, the
functionality described in this document allows interoperability with
existing implementations that comply to [OWAMP] or [TWAMP].
3.4. Using the Checksum Trailer with or without Authentication
Both OWAMP and TWAMP may use authentication, as defined in [OWAMP] or
[TWAMP]. A Checksum Trailer SHOULD NOT be used when authentication is
enabled. The Checksum Trailer is effective in unauthenticated mode,
allowing the intermediate entity to perform serial processing of the
packet without storing-and-forwarding it.
On the other hand, when message authentication is used, an
intermediate entity that alters test packets must also re-compute the
Message Authentication Code (MAC) accordingly. The MAC update
typically requires the intermediate entity to store the packet, re-
compute its MAC, and then forward it. Thus, the benefit of the
checksum trailer is effectively irrelevant when a MAC is used.
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
Trailer SHOULD NOT be used in this case.
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4. Security Considerations
This document describes how a Checksum Trailer 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
trailer 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 mode. As described in Section 3.4. , the benefits of
the Checksum Trailer do not apply when authentication is enabled.
5. IANA Considerations
There are no IANA actions required by this document.
RFC Editor: please delete this section before publication.
6. Acknowledgments
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.
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[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.
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., Cui, Y., Zhang, E., "Network
Performance Measurement for IPsec", draft-ietf-ippm-
ipsec (work in progress), September 2014.
Authors' Addresses
Tal Mizrahi
Marvell
6 Hamada St.
Yokneam, 20692 Israel
Email: talmi@marvell.com
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