Internet Engineering Task Force (IETF) T. Mizrahi

Internet Engineering Task Force (IETF) T. Mizrahi
Request for Comments: 7821 Marvell
Category: Experimental March 2016
ISSN: 2070-1721

UDP Checksum Complement in the Network Time Protocol (NTP)

Abstract

The Network Time Protocol (NTP) allows clients to synchronize to a
time server using timestamped protocol messages. To facilitate
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 Complement, 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 document is not an Internet Standards Track specification; it is
published for examination, experimental implementation, and
evaluation.

This document defines an Experimental Protocol for the Internet
community. This document is a product of the Internet Engineering
Task Force (IETF). It represents the consensus of the IETF
community. It has received public review and has been approved for
publication by the Internet Engineering Steering Group (IESG). Not
all documents approved by the IESG are a candidate for any level of
Internet Standard; see Section 2 of RFC 5741.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7821.

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Copyright Notice

Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.

Table of Contents

1. Introduction ....................................................3
1.1. Intermediate Entities ......................................3
1.2. Updating the UDP Checksum ..................................4
2. Conventions Used in This Document ...............................5
2.1. Terminology ................................................5
2.2. Abbreviations ..............................................6
3. Using the UDP Checksum Complement in NTP ........................6
3.1. Overview ...................................................6
3.2. Checksum Complement in NTP Packets .........................7
3.2.1. Using the Checksum Complement .......................7
3.2.2. Transmission of NTP with Checksum Complement ........8
3.2.3. Updates of NTP with Checksum Complement .............8
3.2.4. Reception of NTP with Checksum Complement ...........8
3.3. Interoperability with Existing Implementations .............9
3.4. The Checksum Complement and Authentication .................9
4. Security Considerations ........................................10
5. IANA Considerations ............................................10
6. References .....................................................11
6.1. Normative References ......................................11
6.2. Informative References ....................................11
Appendix A. Checksum Complement Usage Example .....................13
Acknowledgments ...................................................14
Author's Address ..................................................14

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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.

1.1. Intermediate Entities

In this document, we use the term "intermediate entity" to refer to
an entity that resides on the path between the sender and the
receiver of an NTP packet and that modifies this NTP packet en route.

In order to facilitate accurate timestamping, an implementation can
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 | |
| +-----+-----+ |
| | | +-----------------------+
| +-----+-----+ | / Intermediate entity |
| | Accurate | | / in charge of: |
ASIC/FPGA | | Timestamp | | /__ - Timestamping |
| | engine | | |- Updating checksum or |
| +-----------+ | | Checksum Complement |
| | | +-----------------------+
+---------+---------+
|
|NTP packets
|
___ v _
/ \_/ \__
/ \_
/ IP /
\_ Network /
/ \
\__/\_ ___/
\_/

ASIC: Application-Specific Integrated Circuit
FPGA: Field-Programmable Gate Array

Figure 1: Accurate Timestamping in NTP

The accuracy of clock synchronization over packet networks is highly
sensitive to delay jitters in the underlying network; this
dramatically affects clock accuracy. To address this challenge, the
Precision Time Protocol (PTP) [IEEE1588] defines Transparent Clocks
(TCs) -- switches and routers that improve end-to-end clock 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".

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 that cannot update

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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.

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].

This document defines the Checksum Complement for [NTPv4]. The
Checksum Complement is a 2-octet field that resides at the end of the
UDP payload. It allows intermediate entities to update NTP packets
and maintain the correctness of the UDP Checksum by modifying the
last 2 octets of the packet, instead of updating the UDP Checksum
field. This is performed by adding an NTP extension field at the end
of the packet, in which the last 2 octets are used as a Checksum
Complement.

The usage of the Checksum Complement can in some cases simplify the
implementation, because if the packet data is processed in 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. Note
that while it is not impossible to implement a hardware timestamper
that updates the UDP Checksum, using the Checksum Complement instead
can significantly simplify the implementation.

Note that the software layer and the intermediate entity (see
Figure 1) are two modules in a single NTP clock. It is assumed that
these two modules are in agreement regarding whether transmitted NTP
packets include the Checksum Complement or not.

[RFC7820] defines the Checksum Complement mechanism for the One-Way
Active Measurement Protocol (OWAMP) and the Two-Way Active
Measurement Protocol (TWAMP). A similar mechanism is presented in
Annex E of [IEEE1588].

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 the UDP Checksum Complement in NTP

3.1. Overview

The UDP Checksum Complement is a 2-octet field 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
Complement extension.

+--------------------------------+
| IPv4/IPv6 Header |
+--------------------------------+
| UDP Header |
+--------------------------------+
^ | |
| | NTP packet |
| | |
| +--------------------------------+
UDP | Optional NTP Extension Fields |
Payload +--------------------------------+
| | UDP Checksum Complement |
| | Extension Field (28 octets) |
v +--------------------------------+

Figure 2: Checksum Complement in NTP Packets

The Checksum Complement is used to compensate for changes performed
in the NTP packet by intermediate entities, as described in the
Introduction (Section 1). An example of the usage of the Checksum
Complement is provided in Appendix A.

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3.2. Checksum Complement 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.

NTP packets may include one or more extension fields, as defined in
[NTPv4]. The Checksum Complement in NTP packets resides in a
dedicated NTP extension field, as shown in Figure 3.

If the NTP packet includes more than one extension field, the
Checksum Complement extension is always the last extension field.
Thus, the Checksum Complement is the last 2 octets in the UDP payload
and is located at (UDP Length - 2 octets) after the beginning of the
UDP header. Note that the Checksum Complement is not used in
authenticated NTP packets, as further discussed in Section 3.4.

3.2.1. Using the Checksum Complement

As described in Section 1, an intermediate entity that updates the
timestamp in the NTP packet can use the Checksum Complement in order
to maintain the correctness of the UDP Checksum field. Specifically,
if the value of the timestamp is updated, this update yields a change
in the UDP Checksum value; thus, the intermediate entity assigns a
new value in the Checksum Complement that cancels this change,
leaving the current value of the UDP Checksum correct. An example of
the usage of the Checksum Complement is provided in Appendix A.

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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| MBZ |
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | Checksum Complement |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 3: NTP Checksum Complement Extension Field

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Field Type

A dedicated Field Type value is used to identify the Checksum
Complement extension. See Section 5.

Length

The Checksum Complement 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.
[RFC7822] provides further details about the length of an
extension field in the absence of a MAC.

MBZ

The extension field includes a 22-octet MBZ (MUST be zero) field.
This field MUST be set to 0 and MUST be ignored by the recipient.
The MBZ field is used for padding the extension field to
28 octets.

Checksum Complement

The Checksum Complement extension includes the Checksum Complement
field, residing in the last 2 octets of the extension.

3.2.2. Transmission of NTP with Checksum Complement

The transmitter of an NTP packet MAY include a Checksum Complement
extension field.

3.2.3. Updates of NTP with Checksum Complement

An intermediate entity that receives and alters an NTP packet
containing a Checksum Complement extension MAY use the Checksum
Complement to maintain a correct UDP Checksum value.

3.2.4. Reception of NTP with Checksum Complement

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
Complement extension field.

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3.3. Interoperability with Existing Implementations

The behavior defined in this document does not impose new
requirements on the reception of NTP packets beyond the requirements
defined in [RFC7822]. Note that, as defined in [RFC7822], a host
that receives an NTP message with an unknown extension field SHOULD
ignore the extension field and MAY drop the packet if policy requires
it. Thus, transmitters and intermediate entities that support the
Checksum Complement can transparently interoperate with receivers
that are not Checksum Complement compliant, as long as these
receivers ignore unknown extension fields. It is noted that existing
implementations that discard packets with unknown extension fields
cannot interoperate with transmitters that use the Checksum
Complement.

It should be noted that when hardware-based timestamping is used, it
will likely be used at both ends, and thus both hosts that take part
in the protocol will support the functionality described in this
memo. If only one of the hosts uses hardware-based timestamping,
then the Checksum Complement can only be used if it is known that the
peer host can accept the Checksum Complement.

3.4. The Checksum Complement and Authentication

A Checksum Complement MUST NOT be used when authentication is
enabled. The Checksum Complement is useful 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 recompute the
Message Authentication Code (MAC) accordingly. In this case, it is
not possible to update the Checksum Complement; updating the Checksum
Complement would result in having to recalculate the MAC, and there
would be a cyclic dependency between the MAC and the Checksum
Complement. Hence, when updating the MAC, it is necessary to update
the UDP Checksum field, making the Checksum Complement field
unnecessary in the presence of authentication.

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4. Security Considerations

This document describes how a Checksum Complement extension can be
used for maintaining the correctness of the UDP Checksum. The
security considerations of time protocols in general are discussed in
[SecTime], and the security considerations of NTP are discussed in
[NTPv4].

The purpose of this extension is to ease the implementation of
accurate timestamping engines, as illustrated in Figure 1. The
extension is intended to be used internally in an NTP client or
server. This extension is not intended to be used by 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 man-in-the-middle (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 attacker 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 mode. As discussed in Section 3.4, if a
cryptographic security mechanism is used, then the Checksum
Complement does not simplify the implementation compared to using the
conventional Checksum, and therefore the Checksum Complement is not
used.

5. IANA Considerations

IANA has allocated a new value in the "NTP Extension Field Types"
registry:

0x2005 Checksum Complement

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6. References

6.1. Normative References

[Checksum] Rijsinghani, A., Ed., "Computation of the Internet
Checksum via Incremental Update", RFC 1624,
DOI 10.17487/RFC1624, May 1994,
<http://www.rfc-editor.org/info/rfc1624>.

[IPv6] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460,
December 1998, <http://www.rfc-editor.org/info/rfc2460>.

[KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.

[NTPv4] Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
"Network Time Protocol Version 4: Protocol and Algorithms
Specification", RFC 5905, DOI 10.17487/RFC5905,
June 2010, <http://www.rfc-editor.org/info/rfc5905>.

[RFC7822] Mizrahi, T. and D. Mayer, "Network Time Protocol
Version 4 (NTPv4) Extension Fields", RFC 7822,
DOI 10.17487/RFC7822, March 2016,
<http://www.rfc-editor.org/info/rfc7822>.

[UDP] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
DOI 10.17487/RFC768, August 1980,
<http://www.rfc-editor.org/info/rfc768>.

6.2. Informative References

[IEEE1588] IEEE, "IEEE Standard for a Precision Clock
Synchronization Protocol for Networked Measurement and
Control Systems", IEEE Std 1588-2008,
DOI 10.1109/IEEESTD.2008.4579760, July 2008.

[RFC7820] Mizrahi, T., "UDP Checksum Complement in the One-Way
Active Measurement Protocol (OWAMP) and Two-Way Active
Measurement Protocol (TWAMP)", RFC 7820,
DOI 10.17487/RFC7820, March 2016,
<http://www.rfc-editor.org/info/rfc7820>.

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[SecTime] Mizrahi, T., "Security Requirements of Time Protocols in
Packet Switched Networks", RFC 7384,
DOI 10.17487/RFC7384, October 2014,
<http://www.rfc-editor.org/info/rfc7384>.

[ZeroChecksum]
Fairhurst, G. and M. Westerlund, "Applicability Statement
for the Use of IPv6 UDP Datagrams with Zero Checksums",
RFC 6936, DOI 10.17487/RFC6936, April 2013,
<http://www.rfc-editor.org/info/rfc6936>.

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Appendix A. Checksum Complement Usage Example

Consider an NTP packet sent by an NTP client to an NTP server.

The client's software layer (see Figure 1) generates an NTP packet
with an Origin 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 Origin Timestamp T.

Recall that the client's software emits the NTP packet with a
Checksum Complement extension field, which resides at the end of the
PTP packet. It is assumed that the client initially assigns zero to
the value of the Checksum Complement.

The client's timestamping engine updates the Origin Timestamp field
to the accurate time, changing its value from T to T'. The engine
also updates the Checksum Complement field from zero to a new value
C, such that:

checksum(C) = checksum(T) - checksum(T') (2)

When the NTP packet is transmitted by the client'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 NTP packet reaches the NTP server, the server performs a
conventional UDP Checksum computation, and the computed value is U.
Since the Checksum Complement is part of the extension field, its
value (C) is transparently included in the computation, as per
Equation (3), without requiring special treatment by the server.

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Acknowledgments

The author gratefully thanks Danny Mayer, Miroslav Lichvar, Paul
Kyzivat, Suresh Krishnan, and Brian Haberman for their review and
helpful comments.

Author's Address

Tal Mizrahi
Marvell
6 Hamada St.
Yokneam, 20692
Israel

Email: [email protected]

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