Independent Submission T. Mizrahi

Independent Submission T. Mizrahi
Request for Comments: 9192 Huawei
Category: Informational I. Yerushalmi
ISSN: 2070-1721 D. Melman
Marvell
R. Browne
Intel
February 2022

Network Service Header (NSH) Fixed-Length Context Header Allocation

Abstract

The Network Service Header (NSH) specification defines two possible
methods of including metadata (MD): MD Type 0x1 and MD Type 0x2. MD
Type 0x1 uses a fixed-length Context Header. The allocation of this
Context Header, i.e., its structure and semantics, has not been
standardized. This memo defines the Timestamp Context Header, which
is an NSH fixed-length Context Header that incorporates the packet's
timestamp, a sequence number, and a source interface identifier.

Although the definition of the Context Header presented in this
document has not been standardized by the IETF, it has been
implemented in silicon by several manufacturers and is published here
to facilitate interoperability.

Status of This Memo

This document is not an Internet Standards Track specification; it is
published for informational purposes.

This is a contribution to the RFC Series, independently of any other
RFC stream. The RFC Editor has chosen to publish this document at
its discretion and makes no statement about its value for
implementation or deployment. Documents approved for publication by
the RFC Editor are not candidates for any level of Internet Standard;
see Section 2 of RFC 7841.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9192.

Copyright Notice

Copyright (c) 2022 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
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Table of Contents

1. Introduction
2. Terminology
2.1. Requirements Language
2.2. Abbreviations
3. NSH Timestamp Context Header Allocation
4. Timestamping Use Cases
4.1. Network Analytics
4.2. Alternate Marking
4.3. Consistent Updates
5. Synchronization Considerations
6. IANA Considerations
7. Security Considerations
8. References
8.1. Normative References
8.2. Informative References
Acknowledgments
Authors' Addresses

1. Introduction

The Network Service Header (NSH), defined in [RFC8300], is an
encapsulation header that is used as the service encapsulation in the
Service Function Chaining (SFC) architecture [RFC7665].

In order to share metadata (MD) along a service path, the NSH
specification [RFC8300] supports two methods: a fixed-length Context
Header (MD Type 0x1) and a variable-length Context Header (MD Type
0x2). When using MD Type 0x1, the NSH includes 16 octets of Context
Header fields.

The NSH specification [RFC8300] has not defined the semantics of the
16-octet Context Header, nor does it specify how the Context Header
is used by NSH-aware Service Functions (SFs), Service Function
Forwarders (SFFs), and proxies. Several Context Header formats are
defined in [NSH-TLV]. Furthermore, some allocation schemes were
proposed in the past to accommodate specific use cases, e.g.,
[NSH-DC-ALLOC], [NSH-BROADBAND-ALLOC], and [RFC8592].

This memo presents an allocation for the MD Type 0x1 Context Header,
which incorporates the timestamp of the packet, a sequence number,
and a source interface identifier. Note that other allocation schema
for MD Type 0x1 might be specified in the future. Although such
schema are currently not being standardized by the SFC Working Group,
a consistent format (allocation schema) should be used in an SFC-
enabled domain in order to allow interoperability.

In a nutshell, packets that enter the SFC-enabled domain are
timestamped by a classifier [RFC7665]. Thus, the timestamp, sequence
number, and source interface are incorporated in the NSH Context
Header. As discussed in [RFC8300], if reclassification is used, it
may result in an update to the NSH metadata. Specifically, when the
Timestamp Context Header is used, a reclassifier may either leave it
unchanged or update the three fields: Timestamp, Sequence Number, and
Source Interface.

The Timestamp Context Header includes three fields that may be used
for various purposes. The Timestamp field may be used for logging,
troubleshooting, delay measurement, packet marking for performance
monitoring, and timestamp-based policies. The source interface
identifier indicates the interface through which the packet was
received at the classifier. This identifier may specify a physical
interface or a virtual interface. The sequence numbers can be used
by SFs to detect out-of-order delivery or duplicate transmissions.
Note that out-of-order and duplicate packet detection is possible
when packets are received by the same SF but is not necessarily
possible when there are multiple instances of the same SF and
multiple packets are spread across different instances of the SF.
The sequence number is maintained on a per-source-interface basis.

This document presents the Timestamp Context Header but does not
specify the functionality of the SFs that receive the Context Header.
Although a few possible use cases are described in this document, the
SF behavior and application are outside the scope of this document.

Key Performance Indicator (KPI) stamping [RFC8592] defines an NSH
timestamping mechanism that uses the MD Type 0x2 format. This memo
defines a compact MD Type 0x1 Context Header that does not require
the packet to be extended beyond the NSH. Furthermore, the
mechanisms described in [RFC8592] and this memo can be used in
concert, as further discussed in Section 4.1.

Although the definition of the Context Header presented in this
document has not been standardized by the IETF, it has been
implemented in silicon by several manufacturers and is published here
to facilitate interoperability.

2. Terminology

2.1. Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.

2.2. Abbreviations

The following abbreviations are used in this document:

KPI Key Performance Indicator [RFC8592]

MD Metadata [RFC8300]

NSH Network Service Header [RFC8300]

SF Service Function [RFC7665]

SFC Service Function Chaining [RFC7665]

SFF Service Function Forwarder [RFC8300]

3. NSH Timestamp Context Header Allocation

This memo defines the following fixed-length Context Header
allocation, as presented in Figure 1.

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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Interface |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Timestamp |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 1: NSH Timestamp Allocation

The NSH Timestamp allocation defined in this memo MUST include the
following fields:

Sequence Number: A 32-bit sequence number. The sequence number is
maintained on a per-source-interface basis. Sequence numbers can
be used by SFs to detect out-of-order delivery or duplicate
transmissions. The classifier increments the sequence number by 1
for each packet received through the source interface. This
requires the classifier to maintain a per-source-interface
counter. The sequence number is initialized to a random number on
startup. After it reaches its maximal value (2^32-1), the
sequence number wraps around back to zero.

Source Interface: A 32-bit source interface identifier that is
assigned by the classifier. The combination of the source
interface and the classifier identity is unique within the context
of an SFC-enabled domain. Thus, in order for an SF to be able to
use the source interface as a unique identifier, the identity of
the classifier needs to be known for each packet. The source
interface is unique in the context of the given classifier.

Timestamp: A 64-bit field that specifies the time at which the
packet was received by the classifier. Two possible timestamp
formats can be used for this field: the two 64-bit recommended
formats specified in [RFC8877]. One of the formats is based on
the timestamp format defined in [IEEE1588], and the other is based
on the format defined in [RFC5905].

The NSH specification [RFC8300] does not specify the possible
coexistence of multiple MD Type 0x1 Context Header formats in a
single SFC-enabled domain. It is assumed that the Timestamp Context
Header will be deployed in an SFC-enabled domain that uniquely uses
this Context Header format. Thus, operators SHOULD ensure that
either a consistent Context Header format is used in the SFC-enabled
domain or there is a clear policy that allows SFs to know the Context
Header format of each packet. Specifically, operators are expected
to ensure the consistent use of a timestamp format across the whole
SFC-enabled domain.

The two timestamp formats that can be used in the Timestamp field are
as follows:

Truncated Timestamp Format [IEEE1588]: This format is specified in
Section 4.3 of [RFC8877]. For the reader's convenience, this
format is illustrated in Figure 2.

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Seconds |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Nanoseconds |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 2: Truncated Timestamp Format (IEEE 1588)

NTP 64-bit Timestamp Format [RFC5905]: This format is specified in
Section 4.2.1 of [RFC8877]. For the reader's convenience, this
format is illustrated in Figure 3.

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Seconds |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Fraction |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 3: NTP 64-Bit Timestamp Format (RFC 5905)

Synchronization aspects of the timestamp format in the context of the
NSH Timestamp allocation are discussed in Section 5.

4. Timestamping Use Cases

4.1. Network Analytics

Per-packet timestamping enables coarse-grained monitoring of network
delays along the Service Function Chain. Once a potential problem or
bottleneck is detected (for example, when the delay exceeds a certain
policy), a highly granular monitoring mechanism can be triggered (for
example, using the hop-by-hop measurement data defined in [RFC8592]
or [IOAM-DATA]), allowing analysis and localization of the problem.

Timestamping is also useful for logging, troubleshooting, and flow
analytics. It is often useful to maintain the timestamp of the first
and last packet of the flow. Furthermore, traffic mirroring and
sampling often require a timestamp to be attached to analyzed
packets. Attaching the timestamp to the NSH provides an in-band
common time reference that can be used for various network analytics
applications.

4.2. Alternate Marking

A possible approach for passive performance monitoring is to use an
Alternate-Marking Method [RFC8321]. This method requires data
packets to carry a field that marks (colors) the traffic, and enables
passive measurement of packet loss, delay, and delay variation. The
value of this marking field is periodically toggled between two
values.

When the timestamp is incorporated in the NSH, it can intrinsically
be used for Alternate Marking. For example, the least significant
bit of the timestamp Seconds field can be used for this purpose,
since the value of this bit is inherently toggled every second.

4.3. Consistent Updates

The timestamp can be used for making policy decisions, such as
'Perform action A if timestamp>=T_0'. This can be used for enforcing
time-of-day policies or periodic policies in SFs. Furthermore,
timestamp-based policies can be used for enforcing consistent network
updates, as discussed in [DPT]. It should be noted that, as in the
case of Alternate Marking, this use case alone does not require a
full 64-bit timestamp but could be implemented with a significantly
smaller number of bits.

5. Synchronization Considerations

Some of the applications that make use of the timestamp require the
classifier and SFs to be synchronized to a common time reference --
for example, using the Network Time Protocol [RFC5905] or the
Precision Time Protocol [IEEE1588]. Although it is not a requirement
to use a clock synchronization mechanism, it is expected that,
depending on the applications that use the timestamp, such
synchronization mechanisms will be used in most deployments that use
the Timestamp allocation.

6. IANA Considerations

This document has no IANA actions.

7. Security Considerations

The security considerations for the NSH in general are discussed in
[RFC8300]. The NSH is typically run within a confined trust domain.
However, if a trust domain is not enough to provide the operator with
protection against the timestamp threats as described below, then the
operator SHOULD use transport-level protection between SFC processing
nodes as described in [RFC8300].

The security considerations of in-band timestamping in the context of
the NSH are discussed in [RFC8592]; this section is based on that
discussion.

In-band timestamping, as defined in this document and [RFC8592], can
be used as a means for network reconnaissance. By passively
eavesdropping on timestamped traffic, an attacker can gather
information about network delays and performance bottlenecks. An on-
path attacker can maliciously modify timestamps in order to attack
applications that use the timestamp values, such as performance-
monitoring applications.

Since the timestamping mechanism relies on an underlying time
synchronization protocol, by attacking the time protocol an attack
can potentially compromise the integrity of the NSH Timestamp. A
detailed discussion about the threats against time protocols and how
to mitigate them is presented in [RFC7384].

8. References

8.1. Normative References

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

[RFC7665] Halpern, J., Ed. and C. Pignataro, Ed., "Service Function
Chaining (SFC) Architecture", RFC 7665,
DOI 10.17487/RFC7665, October 2015,
<https://www.rfc-editor.org/info/rfc7665>.

[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.

[RFC8300] Quinn, P., Ed., Elzur, U., Ed., and C. Pignataro, Ed.,
"Network Service Header (NSH)", RFC 8300,
DOI 10.17487/RFC8300, January 2018,
<https://www.rfc-editor.org/info/rfc8300>.

[RFC8877] Mizrahi, T., Fabini, J., and A. Morton, "Guidelines for
Defining Packet Timestamps", RFC 8877,
DOI 10.17487/RFC8877, September 2020,
<https://www.rfc-editor.org/info/rfc8877>.

8.2. Informative References

[DPT] Mizrahi, T. and Y. Moses, "The Case for Data Plane
Timestamping in SDN", IEEE INFOCOM Workshop on Software-
Driven Flexible and Agile Networking (SWFAN),
DOI 10.1109/INFCOMW.2016.7562197, 2016,
<https://ieeexplore.ieee.org/document/7562197>.

[IEEE1588] IEEE, "IEEE 1588-2008 - IEEE Standard for a Precision
Clock Synchronization Protocol for Networked Measurement
and Control Systems", DOI 10.1109/IEEESTD.2008.4579760,
<https://standards.ieee.org/standard/1588-2008.html>.

[IOAM-DATA]
Brockners, F., Ed., Bhandari, S., Ed., and T. Mizrahi,
Ed., "Data Fields for In-situ OAM", Work in Progress,
Internet-Draft, draft-ietf-ippm-ioam-data-17, 13 December
2021, <https://datatracker.ietf.org/doc/html/draft-ietf-
ippm-ioam-data-17>.

[NSH-BROADBAND-ALLOC]
Napper, J., Kumar, S., Muley, P., Hendericks, W., and M.
Boucadair, "NSH Context Header Allocation for Broadband",
Work in Progress, Internet-Draft, draft-ietf-sfc-nsh-
broadband-allocation-01, 19 June 2018,
<https://datatracker.ietf.org/doc/html/draft-ietf-sfc-nsh-
broadband-allocation-01>.

[NSH-DC-ALLOC]
Guichard, J., Ed., Smith, M., Kumar, S., Majee, S., and T.
Mizrahi, "Network Service Header (NSH) MD Type 1: Context
Header Allocation (Data Center)", Work in Progress,
Internet-Draft, draft-ietf-sfc-nsh-dc-allocation-02, 25
September 2018, <https://datatracker.ietf.org/doc/html/
draft-ietf-sfc-nsh-dc-allocation-02>.

[NSH-TLV] Wei, Y., Ed., Elzur, U., Majee, S., Pignataro, C., and D.
Eastlake, "Network Service Header Metadata Type 2
Variable-Length Context Headers", Work in Progress,
Internet-Draft, draft-ietf-sfc-nsh-tlv-13, 26 January
2022, <https://datatracker.ietf.org/doc/html/draft-ietf-
sfc-nsh-tlv-13>.

[RFC5905] 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,
<https://www.rfc-editor.org/info/rfc5905>.

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

[RFC8321] Fioccola, G., Ed., Capello, A., Cociglio, M., Castaldelli,
L., Chen, M., Zheng, L., Mirsky, G., and T. Mizrahi,
"Alternate-Marking Method for Passive and Hybrid
Performance Monitoring", RFC 8321, DOI 10.17487/RFC8321,
January 2018, <https://www.rfc-editor.org/info/rfc8321>.

[RFC8592] Browne, R., Chilikin, A., and T. Mizrahi, "Key Performance
Indicator (KPI) Stamping for the Network Service Header
(NSH)", RFC 8592, DOI 10.17487/RFC8592, May 2019,
<https://www.rfc-editor.org/info/rfc8592>.

Acknowledgments

The authors thank Mohamed Boucadair and Greg Mirsky for their
thorough reviews and detailed comments.

Authors' Addresses

Tal Mizrahi
Huawei
Israel
Email: [email protected]

Ilan Yerushalmi
Marvell
6 Hamada
Yokneam 2066721
Israel
Email: [email protected]

David Melman
Marvell
6 Hamada
Yokneam 2066721
Israel
Email: [email protected]

Rory Browne
Intel
Dromore House
Shannon
Co. Clare
Ireland
Email: [email protected]