NTP Working Group T. Mizrahi
Internet Draft Marvell
Intended status: Experimental
Expires: January 2015 July 21, 2014
Using UDP Checksum Trailers in the Network Time Protocol (NTP)
draft-ietf-ntp-checksum-trailer-00.txt
Abstract
The Network Time Protocol (NTP) allows clients to synchronize to a
time server using timestamped protocol messages. To faciliate
accurate timestamping, some implementations use hardware-based
timestamping engines that integrate the accurate transmission time
into every outgoing NTP packet during transmission. Since these
packets are transported over UDP, the UDP checksum field is then
updated to reflect this modification. This document proposes an
extension field that includes a 2-octet Checksum Trailer, allowing
timestamping engines to reflect the checksum modification in the last
2 octets of the packet rather than in the UDP checksum field. The
behavior defined in this document is interoperable with existing NTP
implementations.
Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with the
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This Internet-Draft will expire on January 21, 2015.
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Copyright Notice
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Table of Contents
1. Introduction ................................................. 2
1.1. Intermediate Entities ................................... 3
1.2. Updating the UDP Checksum ............................... 5
2. Conventions used in this document ............................ 5
2.1. Terminology ............................................. 5
2.2. Abbreviations ........................................... 6
3. Using UDP Checksum Trailers in NTP ........................... 6
3.1. Overview ................................................ 6
3.2. Checksum Trailer in NTP Packets ......................... 6
3.2.1. Transmission of NTP with Checksum Trailer........... 8
3.2.2. Intermediate Updates of NTP with Checksum Trailer .. 8
3.2.3. Reception of NTP with Checksum Trailer ............. 8
3.3. Interoperability with Existing Implementations........... 8
3.4. Using the Checksum Trailer with or without Authentication 8
4. Security Considerations ...................................... 9
5. IANA Considerations .......................................... 9
6. Acknowledgments .............................................. 9
7. References ................................................... 9
7.1. Normative References .................................... 9
7.2. Informative References ................................. 10
1. Introduction
The Network Time Protocol [NTPv4] allows clients to synchronize their
clocks to a time server by exchanging NTP packets. The increasing
demand for highly accurate clock synchronization motivates
implementations that provide accurate timestamping.
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1.1. Intermediate Entities
In this document we use the term 'intermediate entity', referring to
an entity that reside on the path between the sender and the receiver
of an NTP packet, that modifies this NTP packet en-route. Two
examples of intermediate entities are presented below.
In order to facilitate accurate timestamping, an implementation MAY
use a hardware based timestamping engine, as shown in Figure 1. In
such cases, NTP packets are sent and received by a software layer,
whereas a timestamping engine modifies every outgoing NTP packet by
incorporating its accurate transmission time into the <Transmit
Timestamp> field in the packet.
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NTP client/server
+-------------------+
| |
| +-----------+ |
Software | | NTP | |
| | protocol | |
| +-----+-----+ |
| | |
| +-----+-----+ |
| | Accurate | |
ASIC/FPGA | | Timestamp | |
| | engine | |
| +-----------+ |
| | |
+---------+---------+
|
|NTP packets
|
___ v _
/ \_/ \__
/ \_
/ IP /
\_ Network /
/ \
\__/\_ ___/
\_/
Figure 1 Accurate Timestamping in NTP
The accuracy of clock synchronization over packet networks is highly
sensitive to delay jitters in the underlying network, which
dramatically affects the clock accuracy. To address this challenge,
the Precision Time Protocol (PTP) [IEEE1588] defines Transparent
Clocks (TCs), intermediate switches and routers that improve the end-
to-end accuracy by updating a "Correction Field" in the PTP packet by
adding the latency caused by the current TC. In NTP no equivalent
entity is currently defined, but future versions of NTP may define an
intermediate node that modifies en-route NTP packets using a
"Correction Field".
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1.2. Updating the UDP Checksum
When the UDP payload is modified by an intermediate entity, the UDP
Checksum field needs to be updated to maintain its correctness. When
using UDP over IPv4 ([UDP]), an intermediate entity can assign a
value of zero in the checksum field, causing the receiver to ignore
the 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 [NTP], allowing
intermediate entities to update NTP packets and maintain the
correctness of the UDP checksum by modifying the last 2 octets of the
packet. This is performed by adding an NTP extension field at the end
of the packet, in which the last two bytes are used as a checksum
trailer.
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].
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2.2. Abbreviations
MAC Message Authentication Code
NTP Network Time Protocol
PTP Precision Time Protocol
UDP User Datagram Protocol
3. Using UDP Checksum Trailers in NTP
3.1. Overview
The UDP Checksum Trailer is a two-octet trailer that is appended at
the end of the UDP payload using an NTP extension field. Figure 2
illustrates the packet format of an NTP packet with a Checksum
Trailer extension. The figure illustrates an unauthenticated NTP
packet. Section 3.4. provides further details about using the
Checksum Trailer in authenticated packets.
+--------------------------------+
| IPv4 / IPv6 Header |
+--------------------------------+
| UDP Header |
+--------------------------------+
^ | |
| | NTP packet |
| | |
| +--------------------------------+
UDP | Optional NTP Extension Fields |
Payload +--------------------------------+
| | UDP Checksum Trailer |
| | Extension Field (28 octets) |
v +--------------------------------+
Figure 2 Checksum Trailer in NTP Unauthenticated Packets
3.2. Checksum Trailer in NTP Packets
NTP is transported over UDP, either over IPv4 or over IPv6. This
document applies to both NTP over IPv4, and NTP over IPv6.
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NTP packets may include one or more extension fields, as defined in
[NTPv4]. The Checksum Trailer in NTP packets resides in a dedicated
NTP extension field, as shown in Figure 2.
In unauthenticated mode, if the NTP packet includes more than one
extension field, the Checksum Trailer extension is always the last
extension field. Thus, when NTP authentication is disabled, the
Checksum Trailer is the last 2 octets in the UDP payload, and thus
the trailer is located at UDP Length - 2 octets after the beginning
of the UDP header.
When NTP authentication is enabled, the Checksum Trailer is the last
2 octets before the Message Authentication Code (MAC).
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Field Type | Length = 28 octets |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Padding |
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | Checksum Trailer |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3 NTP Checksum Trailer Extension Field
Field Type
A dedicated Field Type value is used to identify the Checksum
Trailer extension. See Section 6 for further details.
Length
The Checksum Trailer extension field length is 28 octets.
This length guarantees that the host that receives the packet
parses it correctly, whether the packet includes a MAC or not.
[NTP-Ext] provides further details about the length of an
extension field in the absence of a MAC.
Padding
The extension field includes 22 octets of padding. This field
SHOULD be set to 0, and SHOULD be ignored by the recipient.
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Checksum Trailer
Includes the UDP Checksum Trailer field.
3.2.1. Transmission of NTP with Checksum Trailer
The transmitter of an NTP packet MAY include a Checksum Trailer
extension field.
3.2.2. Intermediate Updates of NTP with Checksum Trailer
An intermediate node that receives and alters an NTP packet
containing a Checksum Trailer extension MAY use the Checksum Trailer
to maintain a correct UDP checksum value.
3.2.3. Reception of NTP with Checksum Trailer
This document does not impose new requirements on the receiving end
of an NTP packet.
The UDP layer at the receiving end verifies the UDP Checksum of
received NTP packets, and the NTP layer SHOULD ignore the Checksum
Trailer extension field.
3.3. Interoperability with Existing Implementations
The behavior defined in this document does not impose new
requirements on the reception of NTP packets. Thus, transmitters and
intermediate nodes that support the Checksum Trailer can
transparently interoperate with existing implementations.
3.4. Using the Checksum Trailer with or without Authentication
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 NTP 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.
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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 NTP client or
server, and not intended to be used by intermediate switches and
routers that reside between the client and the server. As opposed to
PTP [IEEE1588], NTP does not require intermediate switches or routers
to modify the content of NTP messages, and thus any such modification
should be considered as a malicious MITM attack.
It is important to emphasize that the scheme described in this
document does not increase the protocol's vulnerabitliy 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
IANA is requested to allocate an NTP extension Field Type value for
the Checksum Trailer extension.
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.
[NTPv4] Mills, D., Martin, J., Burbank, J., Kasch, W.,
"Network Time Protocol Version 4: Protocol and
Algorithms Specification", RFC 5905, June 2010.
7.2. Informative References
[IEEE1588] IEEE TC 9 Instrumentation and Measurement Society
2000, "1588 IEEE Standard for a Precision Clock
Synchronization Protocol for Networked Measurement and
Control Systems Version 2", IEEE Standard, 2008.
[NTP-Ext] Mizrahi, T., Mayer, D., "Using NTP Extension Fields
without Authentication", draft-mizrahi-ntp-extension-
field (work in progress), January 2014.
Authors' Addresses
Tal Mizrahi
Marvell
6 Hamada St.
Yokneam, 20692 Israel
Email: talmi@marvell.com
Mizrahi, T. Expires January 21, 2015 [Page 10]
