Internet DRAFT - draft-ietf-sfc-nsh
draft-ietf-sfc-nsh
Service Function Chaining P. Quinn, Ed.
Internet-Draft Cisco
Intended status: Standards Track U. Elzur, Ed.
Expires: May 7, 2018 Intel
C. Pignataro, Ed.
Cisco
November 3, 2017
Network Service Header (NSH)
draft-ietf-sfc-nsh-28
Abstract
This document describes a Network Service Header (NSH) imposed on
packets or frames to realize service function paths. The NSH also
provides a mechanism for metadata exchange along the instantiated
service paths. The NSH is the SFC encapsulation required to support
the Service Function Chaining (SFC) architecture (defined in
RFC7665).
Status of This Memo
This Internet-Draft is submitted 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 May 7, 2018.
Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved.
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to this document. Code Components extracted from this document must
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 4
1.2. Applicability . . . . . . . . . . . . . . . . . . . . . . 4
1.3. Definition of Terms . . . . . . . . . . . . . . . . . . . 5
1.4. Problem Space . . . . . . . . . . . . . . . . . . . . . . 6
1.5. NSH-based Service Chaining . . . . . . . . . . . . . . . 6
2. Network Service Header . . . . . . . . . . . . . . . . . . . 7
2.1. Network Service Header Format . . . . . . . . . . . . . . 7
2.2. NSH Base Header . . . . . . . . . . . . . . . . . . . . . 8
2.3. Service Path Header . . . . . . . . . . . . . . . . . . . 11
2.4. NSH MD Type 1 . . . . . . . . . . . . . . . . . . . . . . 12
2.5. NSH MD Type 2 . . . . . . . . . . . . . . . . . . . . . . 12
2.5.1. Optional Variable Length Metadata . . . . . . . . . . 13
3. NSH Actions . . . . . . . . . . . . . . . . . . . . . . . . . 14
4. NSH Transport Encapsulation . . . . . . . . . . . . . . . . . 16
5. Fragmentation Considerations . . . . . . . . . . . . . . . . 17
6. Service Path Forwarding with NSH . . . . . . . . . . . . . . 17
6.1. SFFs and Overlay Selection . . . . . . . . . . . . . . . 17
6.2. Mapping the NSH to Network Topology . . . . . . . . . . . 21
6.3. Service Plane Visibility . . . . . . . . . . . . . . . . 21
6.4. Service Graphs . . . . . . . . . . . . . . . . . . . . . 22
7. Policy Enforcement with NSH . . . . . . . . . . . . . . . . . 22
7.1. NSH Metadata and Policy Enforcement . . . . . . . . . . . 22
7.2. Updating/Augmenting Metadata . . . . . . . . . . . . . . 24
7.3. Service Path Identifier and Metadata . . . . . . . . . . 25
8. Security Considerations . . . . . . . . . . . . . . . . . . . 26
8.1. NSH Security Considerations from Operators' Environments 27
8.2. NSH Security Considerations from the SFC Architecture . . 28
8.2.1. Integrity . . . . . . . . . . . . . . . . . . . . . . 29
8.2.2. Confidentiality . . . . . . . . . . . . . . . . . . . 31
9. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 32
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 33
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 33
11.1. Network Service Header (NSH) Parameters . . . . . . . . 34
11.1.1. NSH Base Header Bits . . . . . . . . . . . . . . . . 34
11.1.2. NSH Version . . . . . . . . . . . . . . . . . . . . 34
11.1.3. MD Type Registry . . . . . . . . . . . . . . . . . . 34
11.1.4. MD Class Registry . . . . . . . . . . . . . . . . . 35
11.1.5. New IETF Assigned Optional Variable Length Metadata
Type Registry . . . . . . . . . . . . . . . . . . . 36
11.1.6. NSH Base Header Next Protocol . . . . . . . . . . . 36
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12. NSH-Related Codepoints . . . . . . . . . . . . . . . . . . . 37
12.1. NSH EtherType . . . . . . . . . . . . . . . . . . . . . 37
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 37
13.1. Normative References . . . . . . . . . . . . . . . . . . 37
13.2. Informative References . . . . . . . . . . . . . . . . . 38
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 40
1. Introduction
Service functions are widely deployed and essential in many networks.
These service functions provide a range of features such as security,
WAN acceleration, and server load balancing. Service functions may
be instantiated at different points in the network infrastructure
such as the wide area network, data center, and so forth.
Prior to development of the SFC architecture [RFC7665] and the
protocol specified in this document, current service function
deployment models have been relatively static and bound to topology
for insertion and policy selection. Furthermore, they do not adapt
well to elastic service environments enabled by virtualization.
New data center network and cloud architectures require more flexible
service function deployment models. Additionally, the transition to
virtual platforms demands an agile service insertion model that
supports dynamic and elastic service delivery. Specifically, the
following functions are necessary:
1. The movement of service functions and application workloads in
the network.
2. The ability to easily bind service policy to granular
information, such as per-subscriber state.
3. The capability to steer traffic to the requisite service
function(s).
The Network Service Header (NSH) specification defines a new data
plane protocol, which is an encapsulation for service function
chains. The NSH is designed to encapsulate an original packet or
frame, and in turn be encapsulated by an outer transport
encapsulation (which is used to deliver the NSH to NSH-aware network
elements), as shown in Figure 1:
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+------------------------------+
| Transport Encapsulation |
+------------------------------+
| Network Service Header (NSH) |
+------------------------------+
| Original Packet / Frame |
+------------------------------+
Figure 1: Network Service Header Encapsulation
The NSH is composed of the following elements:
1. Service Function Path identification.
2. Indication of location within a Service Function Path.
3. Optional, per packet metadata (fixed length or variable).
[RFC7665] provides an overview of a service chaining architecture
that clearly defines the roles of the various elements and the scope
of a service function chaining encapsulation. Figure 3 of [RFC7665]
depicts the SFC architectural components after classification. The
NSH is the SFC encapsulation referenced in [RFC7665].
1.1. Requirements Language
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 RFC 2119 [RFC2119].
1.2. Applicability
The NSH is designed to be easy to implement across a range of
devices, both physical and virtual, including hardware platforms.
The intended scope of the NSH is for use within a single provider's
operational domain. This deployment scope is deliberately
constrained, as explained also in [RFC7665], and limited to a single
network administrative domain. In this context, a "domain" is a set
of network entities within a single administration. For example, a
network administrative domain can include a single data center, or an
overlay domain using virtual connections and tunnels. A corollary is
that a network administrative domain has a well defined perimeter.
An NSH-aware control plane is outside the scope of this document.
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1.3. Definition of Terms
Byte: All references to "bytes" in this document refer to 8-bit
bytes, or octets.
Classification: Defined in [RFC7665].
Classifier: Defined in [RFC7665].
Metadata: Defined in [RFC7665]. The metadata, or context
information shared between classifiers and SFs, and among SFs, is
carried on the NSH's Context Headers. It allows summarizing a
classification result in the packet itself, avoiding subsequent
re-classifications. Examples of metadata include classification
information used for policy enforcement and network context for
forwarding post service delivery.
Network Locator: Data plane address, typically IPv4 or IPv6, used to
send and receive network traffic.
Network Node/Element: Device that forwards packets or frames based
on an outer header (i.e., transport encapsulation) information.
Network Overlay: Logical network built on top of existing network
(the underlay). Packets are encapsulated or tunneled to create
the overlay network topology.
NSH-aware: NSH-aware means SFC-encapsulation-aware, where the NSH
provides the SFC encapsulation. This specification uses NSH-aware
as a more specific term from the more generic term SFC-aware
[RFC7665].
Service Classifier: Logical entity providing classification
function. Since they are logical, classifiers may be co-resident
with SFC elements such as SFs or SFFs. Service classifiers
perform classification and impose the NSH. The initial classifier
imposes the initial NSH and sends the NSH packet to the first SFF
in the path. Non-initial (i.e., subsequent) classification can
occur as needed and can alter, or create a new service path.
Service Function (SF): Defined in [RFC7665].
Service Function Chain (SFC): Defined in [RFC7665].
Service Function Forwarder (SFF): Defined in [RFC7665].
Service Function Path (SFP): Defined in [RFC7665].
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Service Plane: The collection of SFFs and associated SFs creates a
service-plane overlay in which all SFs and SFC Proxies reside
[RFC7665].
SFC Proxy: Defined in [RFC7665].
1.4. Problem Space
The NSH addresses several limitations associated with service
function deployments. [RFC7498] provides a comprehensive review of
those issues.
1.5. NSH-based Service Chaining
The NSH creates a dedicated service plane; more specifically, the NSH
enables:
1. Topological Independence: Service forwarding occurs within the
service plane, so the underlying network topology does not
require modification. The NSH provides an identifier used to
select the network overlay for network forwarding.
2. Service Chaining: The NSH enables service chaining per [RFC7665].
The NSH contains path identification information needed to
realize a service path. Furthermore, the NSH provides the
ability to monitor and troubleshoot a service chain, end-to-end
via service-specific OAM messages. The NSH fields can be used by
administrators (via, for example, a traffic analyzer) to verify
(account, ensure correct chaining, provide reports, etc.) the
path specifics of packets being forwarded along a service path.
3. The NSH provides a mechanism to carry shared metadata between
participating entities and service functions. The semantics of
the shared metadata is communicated via a control plane, which is
outside the scope of this document, to participating nodes.
[I-D.ietf-sfc-control-plane] provides an example of such in
Section 3.3. Examples of metadata include classification
information used for policy enforcement and network context for
forwarding post service delivery. Sharing the metadata allows
service functions to share initial and intermediate
classification results with downstream service functions saving
re-classification, where enough information was enclosed.
4. The NSH offers a common and standards-based header for service
chaining to all network and service nodes.
5. Transport Encapsulation Agnostic: The NSH is transport
encapsulation-independent, meaning it can be transported by a
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variety of encapsulation protocols. An appropriate (for a given
deployment) encapsulation protocol can be used to carry NSH-
encapsulated traffic. This transport encapsulation may form an
overlay network and if an existing overlay topology provides the
required service path connectivity, that existing overlay may be
used.
2. Network Service Header
An NSH is imposed on the original packet/frame. This NSH contains
service path information and optionally metadata that are added to a
packet or frame and used to create a service plane. Subsequently, an
outer transport encapsulation is imposed on the NSH, which is used
for network forwarding.
A Service Classifier adds the NSH. The NSH is removed by the last
SFF in the service chain or by an SF that consumes the packet.
2.1. Network Service Header Format
The NSH is composed of a 4-byte Base Header, a 4-byte Service Path
Header and optional Context Headers, as shown in Figure 2 below.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Base Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Service Path Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Context Header(s) ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: Network Service Header
Base header: Provides information about the service header and the
payload protocol.
Service Path Header: Provides path identification and location within
a service path.
Context header: Carries metadata (i.e., context data) along a service
path.
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2.2. NSH Base Header
Figure 3 depicts the NSH base header:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Ver|O|U| TTL | Length |U|U|U|U|MD Type| Next Protocol |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: NSH Base Header
Base Header Field Descriptions:
Version: The version field is used to ensure backward compatibility
going forward with future NSH specification updates. It MUST be set
to 0x0 by the sender, in this first revision of the NSH. If a packet
presumed to carry an NSH header is received at an SFF, and the SFF
does not understnad the version of the protocol as indicated in the
base header, the packet MUST be discarded, and the event SHOULD be
logged. Given the widespread implementation of existing hardware
that uses the first nibble after an MPLS label stack for equal-cost
multipath (ECMP) decision processing, this document reserves version
01b. This value MUST NOT be used in future versions of the protocol.
Please see [RFC7325] for further discussion of MPLS-related
forwarding requirements.
O bit: Setting this bit indicates an Operations, Administration, and
Maintenance (OAM, see [RFC6291]) packet. The actual format and
processing of SFC OAM packets is outside the scope of this
specification (see for example [I-D.ietf-sfc-oam-framework] for one
approach).
The O bit MUST be set for OAM packets and MUST NOT be set for non-OAM
packets. The O bit MUST NOT be modified along the SFP.
SF/SFF/SFC Proxy/Classifier implementations that do not support SFC
OAM procedures SHOULD discard packets with O bit set, but MAY support
a configurable parameter to enable forwarding received SFC OAM
packets unmodified to the next element in the chain. Forwarding OAM
packets unmodified by SFC elements that do not support SFC OAM
procedures may be acceptable for a subset of OAM functions, but can
result in unexpected outcomes for others; thus, it is recommended to
analyze the impact of forwarding an OAM packet for all OAM functions
prior to enabling this behavior. The configurable parameter MUST be
disabled by default.
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TTL: Indicates the maximum SFF hops for an SFP. This field is used
for service plane loop detection. The initial TTL value SHOULD be
configurable via the control plane; the configured initial value can
be specific to one or more SFPs. If no initial value is explicitly
provided, the default initial TTL value of 63 MUST be used. Each SFF
involved in forwarding an NSH packet MUST decrement the TTL value by
1 prior to NSH forwarding lookup. Decrementing by 1 from an incoming
value of 0 shall result in a TTL value of 63. The packet MUST NOT be
forwarded if TTL is, after decrement, 0.
This TTL field is the primary loop prevention mechanism. This TTL
mechanism represents a robust complement to the Service Index (see
Section 2.3), as the TTL is decrement by each SFF. The handling of
incoming 0 TTL allows for better, although not perfect,
interoperation with pre-standard implementations that do not support
this TTL field.
Length: The total length, in 4-byte words, of the NSH including the
Base Header, the Service Path Header, the Fixed Length Context Header
or Variable Length Context Header(s). The length MUST be 0x6 for MD
Type equal to 0x1, and MUST be 0x2 or greater for MD Type equal to
0x2. The length of the NSH header MUST be an integer multiple of 4
bytes, thus variable length metadata is always padded out to a
multiple of 4 bytes.
Unassigned bits: All other flag fields, marked U, are unassigned and
available for future use, see Section 11.1.1. Unassigned bits MUST
be set to zero upon origination, and MUST be ignored and preserved
unmodified by other NSH supporting elements. At reception, all
elements MUST NOT modify their actions based on these unknown bits.
Metadata (MD) Type: Indicates the format of the NSH beyond the
mandatory Base Header and the Service Path Header. MD Type defines
the format of the metadata being carried. Please see the IANA
Considerations Section 11.1.3.
This document specifies the following four MD Type values:
0x0 - This is a reserved value. Implementations SHOULD silently
discard packets with MD Type 0x0.
0x1 - This indicates that the format of the header includes a fixed
length Context Header (see Figure 5 below).
0x2 - This does not mandate any headers beyond the Base Header and
Service Path Header, but may contain optional variable length Context
Header(s). With MD Type 0x2, a Length of 0x2 implies there are no
Context Headers. The semantics of the variable length Context
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Header(s) are not defined in this document. The format of the
optional variable length Context Headers is provided in
Section 2.5.1.
0xF - This value is reserved for experimentation and testing, as per
[RFC3692]. Implementations not explicitly configured to be part of
an experiment SHOULD silently discard packets with MD Type 0xF.
The format of the Base Header and the Service Path Header is
invariant, and not affected by MD Type.
The NSH MD Type 1 and MD Type 2 are described in detail in Sections
2.4 and 2.5, respectively. NSH implementations MUST support MD types
0x1 and 0x2 (where the length is 0x2). NSH implementations SHOULD
support MD Type 0x2 with length greater than 0x2. Devices that do
not support MD Type 0x2 with length greater than 0x2 MUST ignore any
optional context headers and process the packet without them; the
base header length field can be used to determine the original
payload offset if access to the original packet/frame is required.
This specification does not disallow the MD Type value from changing
along an SFP; however, the specification of the necessary mechanism
to allow the MD Type to change along an SFP are outside the scope of
this document and would need to be defined for that functionality to
be available. Packets with MD Type values not supported by an
implementation MUST be silently dropped.
Next Protocol: indicates the protocol type of the encapsulated data.
The NSH does not alter the inner payload, and the semantics on the
inner protocol remain unchanged due to NSH service function chaining.
Please see the IANA Considerations section below, Section 11.1.6.
This document defines the following Next Protocol values:
0x1: IPv4
0x2: IPv6
0x3: Ethernet
0x4: NSH
0x5: MPLS
0xFE: Experiment 1
0xFF: Experiment 2
The functionality of hierarchical NSH using a Next Protocol value of
0x4 NSH is outside the scope of this specification. Packets with
Next Protocol values not supported SHOULD be silently dropped by
default, although an implementation MAY provide a configuration
parameter to forward them. Additionally, an implementation not
explicitly configured for a specific experiment [RFC3692] SHOULD
silently drop packets with Next Protocol values 0xFE and 0xFF.
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2.3. Service Path Header
Figure 4 shows the format of the Service Path Header:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Service Path Identifier (SPI) | Service Index |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Service Path Identifier (SPI): 24 bits
Service Index (SI): 8 bits
Figure 4: NSH Service Path Header
The meaning of these fields is as follows:
Service Path Identifier (SPI): Uniquely identifies a service function
path. Participating nodes MUST use this identifier for Service
Function Path selection (SFP). The initial classifier MUST set the
appropriate SPI for a given classification result.
Service Index (SI): Provides location within the SFP. The initial
classifier for a given SFP SHOULD set the SI to 255, however the
control plane MAY configure the initial value of SI as appropriate
(i.e., taking into account the length of the service function path).
The Service Index MUST be decremented by a value of 1 by Service
Functions or by SFC Proxy nodes after performing required services
and the new decremented SI value MUST be used in the egress packet's
NSH. The initial Classifier MUST send the packet to the first SFF in
the identified SFP for forwarding along an SFP. If re-classification
occurs, and that re-classification results in a new SPI, the
(re)classifier is, in effect, the initial classifier for the
resultant SPI.
The SI is used in conjunction the with Service Path Identifier for
Service Function Path Selection and for determining the next SFF/SF
in the path. The SI is also valuable when troubleshooting or
reporting service paths. While the TTL provides the primary SFF
based loop prevention for this mechanism, SI decrement by SF serves
as a limited loop prevention mechanism. NSH packets, as described
above, are discarded when an SFF decrements the TTL to 0. In
addition, an SFF which is not the terminal SFF for a Service Function
Path will discard any NSH packet with an SI of 0, as there will be no
valid next SF information.
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2.4. NSH MD Type 1
When the Base Header specifies MD Type = 0x1, a Fixed Length Context
Header (16-bytes) MUST be present immediately following the Service
Path Header, as per Figure 5. The value of a Fixed Length Context
Header that carries no metadata MUST be set to zero.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Ver|O|U| TTL | Length |U|U|U|U|MD Type| Next Protocol |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Service Path Identifier | Service Index |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Fixed Length Context Header |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: NSH MD Type=0x1
This specification does not make any assumptions about the content of
the 16 byte Context Header that must be present when the MD Type
field is set to 1, and does not describe the structure or meaning of
the included metadata.
An SFC-aware SF or SFC Proxy needs to receive the data structure and
semantics first in order to process the data placed in the mandatory
context field. The data structure and semantics include both the
allocation schema and order, and the meaning of the included data.
How an SFC-aware SF or SFC Proxy gets the data structure and
semantics is outside the scope of this specification.
An SF or SFC Proxy that does not know the format or semantics of the
Context Header for an NSH with MD Type 1 MUST discard any packet with
such an NSH (i.e., MUST NOT ignore the metadata that it cannot
process), and MUST log the event at least once per the SPI for which
the event occurs (subject to thresholding).
[I-D.guichard-sfc-nsh-dc-allocation] and
[I-D.napper-sfc-nsh-broadband-allocation] provide specific examples
of how metadata can be allocated.
2.5. NSH MD Type 2
When the base header specifies MD Type = 0x2, zero or more Variable
Length Context Headers MAY be added, immediately following the
Service Path Header (see Figure 6). Therefore, Length = 0x2,
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indicates that only the Base Header followed by the Service Path
Header are present. The optional Variable Length Context Headers
MUST be of an integer number of 4-bytes. The base header Length
field MUST be used to determine the offset to locate the original
packet or frame for SFC nodes that require access to that
information.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Ver|O|U| TTL | Length |U|U|U|U|MD Type| Next Protocol |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Service Path Identifier | Service Index |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Variable Length Context Headers (opt.) ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6: NSH MD Type=0x2
2.5.1. Optional Variable Length Metadata
The format of the optional variable length Context Headers, is as
depicted in Figure 7.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Metadata Class | Type |U| Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Variable Metadata |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 7: Variable Context Headers
Metadata Class (MD Class): Defines the scope of the 'Type' field to
provide a hierarchical namespace. The IANA Considerations
Section 11.1.4 defines how the MD Class values can be allocated to
standards bodies, vendors, and others.
Type: Indicates the explicit type of metadata being carried. The
definition of the Type is the responsibility of the MD Class owner.
Unassigned bit: One unassigned bit is available for future use. This
bit MUST NOT be set, and MUST be ignored on receipt.
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Length: Indicates the length of the variable metadata, in bytes. In
case the metadata length is not an integer number of 4-byte words,
the sender MUST add pad bytes immediately following the last metadata
byte to extend the metadata to an integer number of 4-byte words.
The receiver MUST round up the length field to the nearest 4-byte
word boundary, to locate and process the next field in the packet.
The receiver MUST access only those bytes in the metadata indicated
by the length field (i.e., actual number of bytes) and MUST ignore
the remaining bytes up to the nearest 4-byte word boundary. The
Length may be 0 or greater.
A value of 0 denotes a Context Header without a Variable Metadata
field.
This specification does not make any assumption about Context Headers
that are mandatory-to-implement or those that are mandatory-to-
process. These considerations are deployment-specific. However, the
control plane is entitled to instruct SFC-aware SFs with the data
structure of context header together with its scoping (see e.g.,
Section 3.3.3 of [I-D.ietf-sfc-control-plane]).
Upon receipt of a packet that belongs to a given SFP, if a mandatory-
to-process context header is missing in that packet, the SFC-aware SF
MUST NOT process the packet and MUST log an error at least once per
the SPI for which the mandatory metadata is missing.
If multiple mandatory-to-process context headers are required for a
given SFP, the control plane MAY instruct the SFC-aware SF with the
order to consume these Context Headers. If no instructions are
provided and the SFC-aware SF will make use of or modify the specific
context header, then the SFC-aware SF MUST process these Context
Headers in the order they appear in an NSH packet.
If multiple instances of the same metadata are included in an NSH
packet, but the definition of that context header does not allow for
it, the SFC-aware SF MUST process the first instance and ignore
subsequent instances. The SFC-aware SF MAY log or increase a counter
for this event.
3. NSH Actions
NSH-aware nodes, which include service classifiers, SFFs, SFs and SFC
proxies, may alter the contents of the NSH headers. These nodes have
several possible NSH-related actions:
1. Insert or remove the NSH: These actions can occur respectively at
the start and end of a service path. Packets are classified, and
if determined to require servicing, an NSH will be imposed. A
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service classifier MUST insert an NSH at the start of an SFP. An
imposed NSH MUST contain both a valid Base Header and Service
Path Header. At the end of a service function path, an SFF MUST
remove the NSH before forwarding or delivering the un-
encapsulated packet. It is therefore the last node operating on
the service header.
Multiple logical classifiers may exist within a given service
path. Non-initial classifiers may re-classify data and that re-
classification MAY result in the selection of a different Service
Function Path. When the logical classifier performs re-
classification that results in a change of service path, it MUST
replace the existing NSH with a new NSH with the Base Header and
Service Path Header reflecting the new service path information
and MUST set the initial SI. The O bit, the TTL field, as well
as unassigned flags, MUST be copied transparently from the old
NSH to a new NSH. Metadata MAY be preserved in the new NSH.
2. Select service path: The Service Path Header provides service
path information and is used by SFFs to determine correct service
path selection. SFFs MUST use the Service Path Header for
selecting the next SF or SFF in the service path.
3. Update the NSH: SFs MUST decrement the service index by one. If
an SFF receives a packet with an SPI and SI that do not
correspond to a valid next hop in a valid Service Function Path,
that packet MUST be dropped by the SFF.
Classifiers MAY update Context Headers if new/updated context is
available.
If an SFC proxy is in use (acting on behalf of an NSH-unaware
service function for NSH actions), then the proxy MUST update
Service Index and MAY update contexts. When an SFC proxy
receives an NSH-encapsulated packet, it MUST remove the NSH
before forwarding it to an NSH-unaware SF. When the SFC Proxy
receives a packet back from an NSH-unaware SF, it MUST re-
encapsulate it with the correct NSH, and MUST decrement the
Service Index by one.
4. Service policy selection: Service Functions derive policy (i.e.,
service actions such as permit or deny) selection and enforcement
from the NSH. Metadata shared in the NSH can provide a range of
service-relevant information such as traffic classification.
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Figure 8 maps each of the four actions above to the components in the
SFC architecture that can perform it.
+-----------+-----------------------+-------+---------------+-------+
| | Insert, remove, or |Forward| Update |Service|
| | replace the NSH |the NSH| the NSH |policy |
| | |Packets| |sel. |
|Component +-------+-------+-------+ +-------+-------+ |
| | | | | |Dec. |Update | |
| |Insert |Remove |Replace| |Service|Context| |
| | | | | |Index |Header | |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| | + | | + | | | + | |
|Classifier | | | | | | | |
+-----------+-------+-------+-------+-------+-------+-------+-------+
|Service | | + | | + | | | |
|Function | | | | | | | |
|Forwarder | | | | | | | |
|(SFF) | | | | | | | |
+-----------+-------+-------+-------+-------+-------+-------+-------+
|Service | | | | | + | + | + |
|Function | | | | | | | |
|(SF) | | | | | | | |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| | + | + | | | + | + | |
|SFC Proxy | | | | | | | |
+-----------+-------+-------+-------+-------+-------+-------+-------+
Figure 8: NSH Action and Role Mapping
4. NSH Transport Encapsulation
Once the NSH is added to a packet, an outer transport encapsulation
is used to forward the original packet and the associated metadata to
the start of a service chain. The encapsulation serves two purposes:
1. Creates a topologically independent services plane. Packets are
forwarded to the required services without changing the
underlying network topology.
2. Transit network nodes simply forward the encapsulated packets
without modification.
The service header is independent of the transport encapsulation
used. Existing transport encapsulations can be used. The presence
of an NSH is indicated via a protocol type or another indicator in
the outer transport encapsulation.
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5. Fragmentation Considerations
The NSH and the associated transport encapsulation header are "added"
to the encapsulated packet/frame. This additional information
increases the size of the packet.
Within a managed administrative domain, an operator can ensure that
the underlay MTU is sufficient to carry SFC traffic without requiring
fragmentation. Given that the intended scope of the NSH is within a
single provider's operational domain, that approach is sufficient.
However, although explicitly outside the scope of this specification,
there might be cases where the underlay MTU is not large enough to
carry the NSH traffic. Since the NSH does not provide fragmentation
support at the service plane, the transport encapsulation protocol
ought to provide the requisite fragmentation handling. For instance,
Section 9 of [I-D.ietf-rtgwg-dt-encap] provides exemplary approaches
and guidance for those scenarios.
When the transport encapsulation protocol supports fragmentation, and
fragmentation procedures needs to be used, such fragmentation is part
of the transport encapsulation logic. If, as it is common,
fragmentation is performed by the endpoints of the transport
encapsulation, then fragmentation procedures are performed at the
sending NSH entity as part of the transport encapsulation, and
reassembly procedures are performed at the receiving NSH entity
during transport de-encapsulation handling logic. In no case would
such fragmentation result in duplication of the NSH header.
For example, when the NSH is encapsulated in IP, IP-level
fragmentation coupled with Path MTU Discovery (PMTUD) (e.g.,
[RFC8201]) is used. Since PMTUD relies on ICMP messages, an operator
should ensure ICMP packets are not blocked. When, on the other hand,
the underlay does not support fragmentation procedures, an error
message SHOULD be logged when dropping a packet too big. Lastly,
NSH-specific fragmentation and reassembly methods may be defined as
well, but these methods are outside the scope of this document, and
subject for future work.
6. Service Path Forwarding with NSH
6.1. SFFs and Overlay Selection
As described above, the NSH contains a Service Path Identifier (SPI)
and a Service Index (SI). The SPI is, as per its name, an
identifier. The SPI alone cannot be used to forward packets along a
service path. Rather the SPI provides a level of indirection between
the service path/topology and the network transport encapsulation.
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Furthermore, there is no requirement, or expectation of an SPI being
bound to a pre-determined or static network path.
The Service Index provides an indication of location within a service
path. The combination of SPI and SI provides the identification of a
logical SF and its order within the service plane, and is used to
select the appropriate network locator(s) for overlay forwarding.
The logical SF may be a single SF, or a set of eligible SFs that are
equivalent. In the latter case, the SFF provides load distribution
amongst the collection of SFs as needed.
SI serves as a mechanism for detecting invalid service function
paths. In particular, an SI value of zero indicates that forwarding
is incorrect and the packet must be discarded.
This indirection -- SPI to overlay -- creates a true service plane.
That is, the SFF/SF topology is constructed without impacting the
network topology but more importantly, service plane only
participants (i.e., most SFs) need not be part of the network overlay
topology and its associated infrastructure (e.g., control plane,
routing tables, etc.) SFs need to be able to return a packet to an
appropriate SFF (i.e., has the requisite NSH information) when
service processing is complete. This can be via the overlay or
underlay and in some cases require additional configuration on the
SF. As mentioned above, an existing overlay topology may be used
provided it offers the requisite connectivity.
The mapping of SPI to transport encapsulation occurs on an SFF (as
discussed above, the first SFF in the path gets an NSH encapsulated
packet from the Classifier). The SFF consults the SPI/ID values to
determine the appropriate overlay transport encapsulation protocol
(several may be used within a given network) and next hop for the
requisite SF. Table 1 below depicts an example of a single next-hop
SPI/SI to network overlay network locator mapping.
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+------+------+---------------------+-------------------------+
| SPI | SI | Next hop(s) | Transport Encapsulation |
+------+------+---------------------+-------------------------+
| 10 | 255 | 192.0.2.1 | VXLAN-gpe |
| | | | |
| 10 | 254 | 198.51.100.10 | GRE |
| | | | |
| 10 | 251 | 198.51.100.15 | GRE |
| | | | |
| 40 | 251 | 198.51.100.15 | GRE |
| | | | |
| 50 | 200 | 01:23:45:67:89:ab | Ethernet |
| | | | |
| 15 | 212 | Null (end of path) | None |
+------+------+---------------------+-------------------------+
Table 1: SFF NSH Mapping Example
Additionally, further indirection is possible: the resolution of the
required SF network locator may be a localized resolution on an SFF,
rather than a service function chain control plane responsibility, as
per Table 2 and Table 3 below.
Please note: VXLAN-gpe and GRE in the above table refer to
[I-D.ietf-nvo3-vxlan-gpe] and [RFC2784] [RFC7676], respectively.
+------+-----+----------------+
| SPI | SI | Next hop(s) |
+------+-----+----------------+
| 10 | 3 | SF2 |
| | | |
| 245 | 12 | SF34 |
| | | |
| 40 | 9 | SF9 |
+------+-----+----------------+
Table 2: NSH to SF Mapping Example
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+------+-------------------+-------------------------+
| SF | Next hop(s) | Transport Encapsulation |
+------+-------------------+-------------------------+
| SF2 | 192.0.2.2 | VXLAN-gpe |
| | | |
| SF34 | 198.51.100.34 | UDP |
| | | |
| SF9 | 2001:db8::1 | GRE |
+------+-------------------+-------------------------+
Table 3: SF Locator Mapping Example
Since the SPI is a representation of the service path, the lookup may
return more than one possible next-hop within a service path for a
given SF, essentially a series of weighted (equally or otherwise)
paths to be used (for load distribution, redundancy, or policy), see
Table 4. The metric depicted in Table 4 is an example to help
illustrated weighing SFs. In a real network, the metric will range
from a simple preference (similar to routing next-hop), to a true
dynamic composite metric based on some service function-centric state
(including load, sessions state, capacity, etc.)
+------+-----+--------------+---------+
| SPI | SI | NH | Metric |
+------+-----+--------------+---------+
| 10 | 3 | 203.0.113.1 | 1 |
| | | | |
| | | 203.0.113.2 | 1 |
| | | | |
| 20 | 12 | 192.0.2.1 | 1 |
| | | | |
| | | 203.0.113.4 | 1 |
| | | | |
| 30 | 7 | 192.0.2.10 | 10 |
| | | | |
| | | 198.51.100.1 | 5 |
+------+-----+--------------+---------+
(encapsulation type omitted for formatting)
Table 4: NSH Weighted Service Path
The information contained in Tables 1-4 may be received from the
control plane, but the exact mechanism is outside the scope of this
document.
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6.2. Mapping the NSH to Network Topology
As described above, the mapping of SPI to network topology may result
in a single path, or it might result in a more complex topology.
Furthermore, the SPI to overlay mapping occurs at each SFF
independently. Any combination of topology selection is possible.
Please note, there is no requirement to create a new overlay topology
if a suitable one already exists. NSH packets can use any (new or
existing) overlay provided the requisite connectivity requirements
are satisfied.
Examples of mapping for a topology:
1. Next SF is located at SFFb with locator 2001:db8::1
SFFa mapping: SPI=10 --> VXLAN-gpe, dst-ip: 2001:db8::1
2. Next SF is located at SFFc with multiple network locators for
load distribution purposes:
SFFb mapping: SPI=10 --> VXLAN-gpe, dst_ip:203.0.113.1,
203.0.113.2, 203.0.113.3, equal cost
3. Next SF is located at SFFd with two paths from SFFc, one for
redundancy:
SFFc mapping: SPI=10 --> VXLAN-gpe, dst_ip:192.0.2.10 cost=10,
203.0.113.10, cost=20
In the above example, each SFF makes an independent decision about
the network overlay path and policy for that path. In other words,
there is no a priori mandate about how to forward packets in the
network (only the order of services that must be traversed).
The network operator retains the ability to engineer the network
paths as required. For example, the overlay path between SFFs may
utilize traffic engineering, QoS marking, or ECMP, without requiring
complex configuration and network protocol support to be extended to
the service path explicitly. In other words, the network operates as
expected, and evolves as required, as does the service plane.
6.3. Service Plane Visibility
The SPI and SI serve an important function for visibility into the
service topology. An operator can determine what service path a
packet is "on", and its location within that path simply by viewing
NSH information (packet capture, IPFIX, etc.) The information can be
used for service scheduling and placement decisions, troubleshooting,
and compliance verification.
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6.4. Service Graphs
While a given realized service function path is a specific sequence
of service functions, the service as seen by a user can actually be a
collection of service function paths, with the interconnection
provided by classifiers (in-service path, non-initial
reclassification). These internal reclassifiers examine the packet
at relevant points in the network, and, if needed, SPI and SI are
updated (whether this update is a re-write, or the imposition of a
new NSH with new values is implementation specific) to reflect the
"result" of the classification. These classifiers may, of course,
also modify the metadata associated with the packet.
[RFC7665], Section 2.1 describes Service Graphs in detail.
7. Policy Enforcement with NSH
7.1. NSH Metadata and Policy Enforcement
As described in Section 2, NSH provides the ability to carry metadata
along a service path. This metadata may be derived from several
sources. Common examples include:
Network nodes/devices: Information provided by network nodes can
indicate network-centric information (such as VRF or tenant) that
may be used by service functions or conveyed to another network
node post service path egress.
External (to the network) systems: External systems, such as
orchestration systems, often contain information that is valuable
for service function policy decisions. In most cases, this
information cannot be deduced by network nodes. For example, a
cloud orchestration platform placing workloads "knows" what
application is being instantiated and can communicate this
information to all NSH nodes via metadata carried in the context
header(s).
Service Functions: A classifier co-resident with Service Functions
often perform very detailed and valuable classification.
Regardless of the source, metadata reflects the "result" of
classification. The granularity of classification may vary. For
example, a network switch, acting as a classifier, might only be able
to classify based on a 2-tuple, or based on a 5-tuple, while a
service function may be able to inspect application information.
Regardless of granularity, the classification information can be
represented in the NSH.
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Once the data is added to the NSH, it is carried along the service
path, NSH-aware SFs receive the metadata, and can use that metadata
for local decisions and policy enforcement. Figure 9 and Figure 10
highlight the relationship between metadata and policy:
+-------+ +-------+ +-------+
| SFF )------->( SFF |------->| SFF |
+---+---+ +---+---+ +---+---+
^ | |
,-|-. ,-|-. ,-|-.
/ \ / \ / \
( Class ) ( SF1 ) ( SF2 )
\ ify / \ / \ /
`---' `---' `---'
5-tuple: Permit Inspect
Tenant A Tenant A AppY
AppY
Figure 9: Metadata and Policy
+-----+ +-----+ +-----+
| SFF |---------> | SFF |----------> | SFF |
+--+--+ +--+--+ +--+--+
^ | |
,-+-. ,-+-. ,-+-.
/ \ / \ / \
( Class ) ( SF1 ) ( SF2 )
\ ify / \ / \ /
`-+-' `---' `---'
| Permit Deny AppZ
+---+---+ employees
| |
+-------+
External
system:
Employee
AppZ
Figure 10: External Metadata and Policy
In both of the examples above, the service functions perform policy
decisions based on the result of the initial classification: the SFs
did not need to perform re-classification; instead, they rely on a
antecedent classification for local policy enforcement.
Depending on the information carried in the metadata, data privacy
impact needs to be considered. For example, if the metadata conveys
tenant information, that information may need to be authenticated
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and/or encrypted between the originator and the intended recipients
(which may include intended SFs only); one approach to an optional
capability to do this is explored in [I-D.reddy-sfc-nsh-encrypt].
The NSH itself does not provide privacy functions, rather it relies
on the transport encapsulation/overlay. An operator can select the
appropriate set of transport encapsulation protocols to ensure
confidentiality (and other security) considerations are met.
Metadata privacy and security considerations are a matter for the
documents that define metadata format.
7.2. Updating/Augmenting Metadata
Post-initial metadata imposition (typically performed during initial
service path determination), the metadata may be augmented or
updated:
1. Metadata Augmentation: Information may be added to the NSH's
existing metadata, as depicted in Figure 11. For example, if the
initial classification returns the tenant information, a
secondary classification (perhaps co-resident with DPI or SLB)
may augment the tenant classification with application
information, and impose that new information in NSH metadata.
The tenant classification is still valid and present, but
additional information has been added to it.
2. Metadata Update: Subsequent classifiers may update the initial
classification if it is determined to be incorrect or not
descriptive enough. For example, the initial classifier adds
metadata that describes the traffic as "Internet" but a security
service function determines that the traffic is really "attack".
Figure 12 illustrates an example of updating metadata.
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+-----+ +-----+ +-----+
| SFF |---------> | SFF |----------> | SFF |
+--+--+ +--+--+ +--+--+
^ | |
,---. ,---. ,---.
/ \ / \ / \
( Class ) ( SF1 ) ( SF2 )
\ / \ / \ /
`-+-' `---' `---'
| Inspect Deny
+---+---+ employees employee+
| | Class=AppZ appZ
+-------+
External
system:
Employee
Figure 11: Metadata Augmentation
+-----+ +-----+ +-----+
| SFF |---------> | SFF |----------> | SFF |
+--+--+ +--+--+ +--+--+
^ | |
,---. ,---. ,---.
/ \ / \ / \
( Class ) ( SF1 ) ( SF2 )
\ / \ / \ /
`---' `---' `---'
5-tuple: Inspect Deny
Tenant A Tenant A attack
--> attack
Figure 12: Metadata Update
7.3. Service Path Identifier and Metadata
Metadata information may influence the service path selection since
the Service Path Identifier values can represent the result of
classification. A given SPI can be defined based on classification
results (including metadata classification). The imposition of the
SPI and SI results in the packet being placed on the newly specified
SFP at the position indicated by the imposed SPI and SI.
This relationship provides the ability to create a dynamic service
plane based on complex classification without requiring each node to
be capable of such classification, or requiring a coupling to the
network topology. This yields service graph functionality as
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described in Section 6.4. Figure 13 illustrates an example of this
behavior.
+-----+ +-----+ +-----+
| SFF |---------> | SFF |------+---> | SFF |
+--+--+ +--+--+ | +--+--+
| | | |
,---. ,---. | ,---.
/ \ / SF1 \ | / \
( SCL ) ( + ) | ( SF2 )
\ / \SCL2 / | \ /
`---' `---' +-----+ `---'
5-tuple: Inspect | SFF | Original
Tenant A Tenant A +--+--+ next SF
--> DoS |
V
,-+-.
/ \
( SF10 )
\ /
`---'
DoS
"Scrubber"
Figure 13: Path ID and Metadata
Specific algorithms for mapping metadata to an SPI are outside the
scope of this document.
8. Security Considerations
NSH security must be considered in the contexts of the SFC
architecture and operators' environments. One important
characteristic of NSH is that it is not an end-to-end protocol. As
opposed to a protocol that "starts" on a host, and "ends" on a server
or another host, NSH is typically imposed by a network device on
ingress to the SFC domain and removed at the egress of the SFC
domain. As such, and as with any other network-centric protocol
(e.g., IP Tunneling, Traffic Engineering, MPLS, or Provider
Provisioned Virtual Private Networks) there an underlying trust that
the network devices responsible for imposing, removing and acting on
NSH information are trusted.
The following sections detail an analysis and present a set of
requirements and recommendations in those two areas.
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8.1. NSH Security Considerations from Operators' Environments
Trusted Devices
All classifiers, SFFs and SFSs (hereinafter referred to as "SFC
devices") within an operator's environment are assumed to have
been selected, vetted, and actively maintained, therefore trusted
by that operator. This assumption differs from the oft held view
that devices are untrusted, often refered to as zero trust model.
Operators SHOULD regularly monitor (i.e. continuously audit) these
devices to help ensure complaint behavior. This trust, therefore,
extends into NSH operations: SFC devices are not, themselves,
considered as attack vectors. This assumption, and the resultant
conclusion is reasonable since this is the very basis of an
operator posture; the operator depends on this reality to
function. If these devices are not trusted, and indeed
compromised, almost the entirety of the operator's standard-based
IP and MPLS protocol suites are vulnerable, and therefore the
operation of the entire network is compromised. Although there
are well documented monitoring-based methods for detecting
compromise, such as include continous monitoring, audit and log
review, these may not be sufficient to contain damage by a
completely compromised element.
Methods and best practices to secure devices are also widely
documented and outside the scope of this document.
Single Domain Boundary
As per [RFC7665], NSH is designed for use within a single
administrative domain. This scoping provides two important
characteristics:
i) Clear NSH boundaries
NSH egress devices MUST strip the NSH headers before they send the
users' packets or frames out of the NSH domain.
Means to prevent leaking privacy-related information outside an
administrative domain are natively supported by the NSH given that
the last SFF of a service path will systematically remove the NSH
encapsulation before forwarding a packet exiting the service path.
The second step in such prevention is to filter the transport
encapsulation protocol used by NSH at the domain edge. The
transport encapsulation protocol MUST be filtered and MUST NOT
leave the domain edge.
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Depending upon the transport encapsulation protocol used for NSH,
this can either be done by completely blocking the transport
encapsulation (e.g., if MPLS is the chosen NSH transport
encapsulation protocol, it is therefore never allowed to leave the
domain) or by examining the carried protocol with the transport
encapsulation (e.g., if VxLAN-gpe is used as the NSH transport
encapsulation protocol, all domain edges need to filter based on
the carried protocol in the VxLAN-gpe.)
The other consequence of this bounding is that ingress packets
MUST also be filtered to prevent attackers from sending in NSH
packets with service path identification and metadata of their own
selection. The same filters as described above for both the NSH
at SFC devices and for the transport encapsulation protocol as
general edge protections MUST be applied on ingress.
In summary, packets originating outside the SFC-enabled domain
MUST be dropped if they contain an NSH. Similarly, packets
exiting the SFC-enabled domain MUST be dropped if they contain an
NSH.
ii) Mitigation of external threats
As per the trusted SFC devices points raised above, given that NSH
is scoped within an operator's domain, that operator can ensure
that the environment, and its transitive properties, comply to
that operator's required security posture. Continuous audits for
assurance are recommended with this reliance on a fully trusted
environment. The term 'continuous audits' describes a method
(automated or manual) of checking security control compliance on a
regular basis, at some set period of time.
8.2. NSH Security Considerations from the SFC Architecture
The SFC architecture defines functional roles (e.g., SFF), as well as
protocol element (e.g. Metadata). This section considers each role
and element in the context of threats posed in the areas of integrity
and confidentiality. As with routing, the distributed computation
model assumes a distributed trust model.
An important consideration is that NSH contains mandatory to mute
fields, and further, the SFC architecture describes cases where other
fields in NSH change, all on a possible SFP hop-by-hop basis. This
means that any cryptographic solution requires complex key
distribution and lifecycle operations.
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8.2.1. Integrity
SFC devices
SFC devices MAY perform various forms of verification on received
NSH packets such as only accepting NSH packets from expected
devices, checking that NSH SPI and SI values received from
expected devices conform to expected values and so on.
Implementation of these additional checks are a local matter and
thus out of scope of this document.
NSH Base and Service Path Headers
Attackers who can modify packets within the operator's network may
be able to modify the service function path, path position, and /
or the metadata associated with a packet.
One specific concern is an attack in which a malicious
modification of the SPI/SI results in an alteration of the path to
avoid security devices. The options discussed in this section
help twart that attack, and so does the use of the optional "Proof
of Transit" method [I-D.brockners-proof-of-transit].
As stated above, SFC devices are trusted; in the case where an SFC
device is compromised, NSH integrity protection would be subject
to forging (in many cases) as well.
NSH itself does not mandate protocol-specific integrity
protection. However, if an operator deems protection required,
several options are viable:
1. SFF/SF NSH verification
Although strictly speaking not integrity protection, some of
the techniques mentioned above such as checking expected NSH
values are received from expected SFC device(s) can provide a
form of verification without incurring the burden of a full-
fledged integrity protection deployment.
2. Transport Security
NSH is always encapsulated by an outer transport encapsulation
as detailed in Section 4 of this specification, and as
depicted in Figure 1. If an operator deems cryptographic
integrity protection necessary due to their risk analysis,
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then an outer transport encapsulation that provides such
protection [RFC6071], such as IPsec, MUST be used.
Although the threat model and recommendations of BCP 72
[RFC3552] Section 5 would normally require cryptographic data
origin authentication for the header, this document does not
mandate such mechanisms in order to reflect the operational
and technical realities of deployment.
Given that NSH is transport independent, as mentioned above, a
secure transport, such as IPsec can be used for carry NSH.
IPsec can be used either alone, or in conjunction with other
transport encapsulation protocols in turn encapsulating NSH.
Operators MUST ensure the selected transport encapsulation
protocol can be supported by the transport encapsulation/
underlay of all relevant network segments as well as SFFs, SFs
and SFC proxies in the service path.
If connectivity between SFC-enabled devices traverses the
public Internet, then such connectivity MUST be secured at the
transport encapsulation layer. IPsec is an example of such a
transport.
3. NSH Variable Header-based Integrity
Lastly, NSH MD-Type 2 provides, via variable length headers,
the ability to append cryptographic integrity protection to
the NSH packet. The implementation of such a scheme is
outside the scope of this document.
NSH metadata
As with the base and service path headers, if an operator deems
cryptographic integrity protection needed, then an existing,
standard transport protocol MUST be used since the integrity
protection applies to entire encapsulated NSH packets. As
mentioned above, a risk assessment that deems dataplane traffic
subject to tampering will apply not only to NSH but to the
transport information and therefore the use of a secure transport
is likely needed already to protect the entire stack.
If an MD-Type 2 variable header integrity scheme is in place, then
the integrity of the metadata can be ensured via that mechanism as
well.
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8.2.2. Confidentiality
SFC devices
SFC devices can "see" (and need to use) NSH information.
NSH base and service path headers
SPI and other base/service path information does not typically
require confidentiality; however, if an operator does deem
confidentiality required, then, as with integrity, an existing
transport encapsulation that provides encryption MUST be utilized.
NSH metadata
An attacker with access to the traffic in an operator's network
can potentially observe the metadata NSH carries with packets,
potentially discovering privacy sensitive information.
Much of the metadata carried by NSH is not sensitive. It often
reflects information that can be derived from the underlying
packet or frame. Direct protection of such information is not
necessary, as the risks are simply those of carrying the
underlying packet or frame.
Implementers and operators MUST be aware that metadata can have
privacy implications, and those implications are sometimes hard to
predict. Therefore, attached metadata should be limited to that
necessary for correct operation of the SFP. Further, [RFC8165]
defines metadata considerations that operators can take into
account when using NSH.
Protecting NSH metadata information between SFC components can be
done using transport encapsulation protocols with suitable
security capabilities, along the lines discussed above. If a
security analysis deems these protections necessary, then security
features in the transport encapsulation protocol (such as IPsec)
MUST be used.
One useful element of providing privacy protection for sensitive
metadata is described under the "SFC Encapsulation" area of the
Security Considerations of [RFC7665]. Operators can and should
use indirect identification for metadata deemed to be sensitive
(such as personally identifying information) significantly
mitigating the risk of a privacy violation. In particular,
subscriber identifying information should be handled carefully,
and in general SHOULD be obfuscated.
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For those situations where obfuscation is either inapplicable or
judged to be insufficient, an operator can also encrypt the
metadata. An approach to an optional capability to do this was
explored in [I-D.reddy-sfc-nsh-encrypt]. For other situations
where greater assurance is desired, optional mechanisms such as
[I-D.brockners-proof-of-transit] can be used.
9. Contributors
This WG document originated as draft-quinn-sfc-nsh; the following are
its co-authors and contributors along with their respective
affiliations at the time of WG adoption. The editors of this
document would like to thank and recognize them and their
contributions. These co-authors and contributors provided invaluable
concepts and content for this document's creation.
o Jim Guichard, Cisco Systems, Inc.
o Surendra Kumar, Cisco Systems, Inc.
o Michael Smith, Cisco Systems, Inc.
o Wim Henderickx, Alcatel-Lucent
o Tom Nadeau, Brocade
o Puneet Agarwal
o Rajeev Manur, Broadcom
o Abhishek Chauhan, Citrix
o Joel Halpern, Ericsson
o Sumandra Majee, F5
o David Melman, Marvell
o Pankaj Garg, Microsoft
o Brad McConnell, Rackspace
o Chris Wright, Red Hat, Inc.
o Kevin Glavin, Riverbed
o Hong (Cathy) Zhang, Huawei US R&D
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o Louis Fourie, Huawei US R&D
o Ron Parker, Affirmed Networks
o Myo Zarny, Goldman Sachs
o Andrew Dolganow, Alcatel-Lucent
o Rex Fernando, Cisco Systems, Inc.
o Praveen Muley, Alcatel-Lucent
o Navindra Yadav, Cisco Systems, Inc.
10. Acknowledgments
The authors would like to thank Sunil Vallamkonda, Nagaraj Bagepalli,
Abhijit Patra, Peter Bosch, Darrel Lewis, Pritesh Kothari, Tal
Mizrahi and Ken Gray for their detailed review, comments and
contributions.
A special thank you goes to David Ward and Tom Edsall for their
guidance and feedback.
Additionally the authors would like to thank Larry Kreeger for his
invaluable ideas and contributions which are reflected throughout
this document.
Loa Andersson provided a thorough review and valuable comments, we
thank him for that.
Reinaldo Penno deserves a particular thank you for his architecture
and implementation work that helped guide the protocol concepts and
design.
The editors also acknowledge comprehensive reviews and respective
useful suggestions by Med Boucadair, Adrian Farrel, Juergen
Schoenwaelder, Acee Lindem, and Kathleen Moriarty.
Lastly, David Dolson has provides significant review, feedback and
suggestions throughout the evolution of this document. His
contributions are very much appreciated.
11. IANA Considerations
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11.1. Network Service Header (NSH) Parameters
IANA is requested to create a new "Network Service Header (NSH)
Parameters" registry. The following sub-sections request new
registries within the "Network Service Header (NSH) Parameters "
registry.
11.1.1. NSH Base Header Bits
There are five unassigned bits (U bits) in the NSH Base Header, and
one assigned bit (O bit). New bits are assigned via Standards Action
[RFC8126].
Bit 2 - O (OAM) bit
Bit 3 - Unassigned
Bits 16-19 - Unassigned
11.1.2. NSH Version
IANA is requested to setup a registry of "NSH Version". New values
are assigned via Standards Action [RFC8126].
Version 00b: Protocol as defined by this document.
Version 01b: Reserved. This document.
Version 10b: Unassigned.
Version 11b: Unassigned.
11.1.3. MD Type Registry
IANA is requested to set up a registry of "MD Types". These are
4-bit values. MD Type values 0x0, 0x1, 0x2, and 0xF are specified in
this document, see Table 5. Registry entries are assigned by using
the "IETF Review" policy defined in RFC 8126 [RFC8126].
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+----------+-----------------+---------------+
| MD Type | Description | Reference |
+----------+-----------------+---------------+
| 0x0 | Reserved | This document |
| | | |
| 0x1 | NSH MD Type 1 | This document |
| | | |
| 0x2 | NSH MD Type 2 | This document |
| | | |
| 0x3..0xE | Unassigned | |
| | | |
| 0xF | Experimentation | This document |
+----------+-----------------+---------------+
Table 5: MD Type Values
11.1.4. MD Class Registry
IANA is requested to set up a registry of "MD Class". These are
16-bit values. New allocations are to be made according to the
following policies:
0x0000 to 0x01ff: IETF Review
0x0200 to 0xfff5: Expert Review
0xfff6 to 0xfffe: Experimental
0xffff: Reserved
IANA is requested to assign the values as per Table 6::
+-----------+-----------------------------+------------+
| MD Class | Meaning | Reference |
+-----------+-----------------------------+------------+
| 0x0000 | IETF Base NSH MD Class | This.I-D |
+-----------+-----------------------------+------------+
Table 6: MD Class Value
A registry for Types for the MD Class of 0x0000 is defined in
Section 11.1.5.
Designated Experts evaluating new allocation requests from the
"Expert Review" range should principally consider whether a new MD
class is needed compared to adding MD types to an existing class.
The Designated Experts should also encourage the existence of an
associated and publicly visible registry of MD types although this
registry need not be maintained by IANA.
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When evaluating a request for an allocation, the Expert should verify
that the allocation plan includes considerations to handle privacy
and security issues associated with the anticipated individual MD
Types allocated within this class. These plans should consider, when
appropriate, alternatives such as indirection, encryption, and
limited deployment scenarios. Information that can't be directly
derived from viewing the packet contents should be examined for
privacy and security implications.
11.1.5. New IETF Assigned Optional Variable Length Metadata Type
Registry
The Type values within the IETF Base NSH MD Class, i.e., when the MD
Class is set to 0x0000 (see Section 11.1.4), are the Types owned by
the IETF. This document requests IANA to create a registry for the
Type values for the IETF Base NSH MD Class called the "IETF Assigned
Optional Variable Length Metadata Type Registry", as specified in
Section 2.5.1.
The type values are assigned via Standards Action [RFC8126].
No initial values are assigned at the creation of the registry.
11.1.6. NSH Base Header Next Protocol
IANA is requested to set up a registry of "Next Protocol". These are
8-bit values. Next Protocol values 0, 1, 2, 3, 4 and 5 are defined
in this document (see Table 7. New values are assigned via "Expert
Reviews" as per [RFC8126].
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+---------------+--------------+---------------+
| Next Protocol | Description | Reference |
+---------------+--------------+---------------+
| 0x0 | Unassigned | |
| | | |
| 0x1 | IPv4 | This document |
| | | |
| 0x2 | IPv6 | This document |
| | | |
| 0x3 | Ethernet | This document |
| | | |
| 0x4 | NSH | This document |
| | | |
| 0x5 | MPLS | This document |
| | | |
| 0x6..0xFD | Unassigned | |
| | | |
| 0xFE | Experiment 1 | This document |
| | | |
| 0xFF | Experiment 2 | This document |
+---------------+--------------+---------------+
Table 7: NSH Base Header Next Protocol Values
Expert Review requests MUST include a single code point per request.
Designated Experts evaluating new allocation requests from this
registry should consider the potential scarcity of code points for an
8-bit value, and check both for duplications as well as availability
of documentation. If the actual assignment of the Next Protocol
field allocation reaches half of the range, that is when there are
128 unassigned values, IANA needs to alert the IESG. At this point,
a new more strict allocation policy SHOULD be considered.
12. NSH-Related Codepoints
12.1. NSH EtherType
An IEEE EtherType, 0x894F, has been allocated for NSH.
13. References
13.1. 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>.
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[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>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.
13.2. Informative References
[I-D.brockners-proof-of-transit]
Brockners, F., Bhandari, S., Dara, S., Pignataro, C.,
Leddy, J., Youell, S., Mozes, D., and T. Mizrahi, "Proof
of Transit", draft-brockners-proof-of-transit-04 (work in
progress), October 2017.
[I-D.guichard-sfc-nsh-dc-allocation]
Guichard, J., Smith, M., Kumar, S., Majee, S., Agarwal,
P., Glavin, K., Laribi, Y., and T. Mizrahi, "Network
Service Header (NSH) MD Type 1: Context Header Allocation
(Data Center)", draft-guichard-sfc-nsh-dc-allocation-07
(work in progress), August 2017.
[I-D.ietf-nvo3-vxlan-gpe]
Maino, F., Kreeger, L., and U. Elzur, "Generic Protocol
Extension for VXLAN", draft-ietf-nvo3-vxlan-gpe-05 (work
in progress), October 2017.
[I-D.ietf-rtgwg-dt-encap]
Nordmark, E., Tian, A., Gross, J., Hudson, J., Kreeger,
L., Garg, P., Thaler, P., and T. Herbert, "Encapsulation
Considerations", draft-ietf-rtgwg-dt-encap-02 (work in
progress), October 2016.
[I-D.ietf-sfc-control-plane]
Boucadair, M., "Service Function Chaining (SFC) Control
Plane Components & Requirements", draft-ietf-sfc-control-
plane-08 (work in progress), October 2016.
[I-D.ietf-sfc-oam-framework]
Aldrin, S., Pignataro, C., Kumar, N., Akiya, N., Krishnan,
R., and A. Ghanwani, "Service Function Chaining (SFC)
Operation, Administration and Maintenance (OAM)
Framework", draft-ietf-sfc-oam-framework-03 (work in
progress), September 2017.
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[I-D.napper-sfc-nsh-broadband-allocation]
Napper, J., Kumar, S., Muley, P., Henderickx, W., and M.
Boucadair, "NSH Context Header Allocation -- Broadband",
draft-napper-sfc-nsh-broadband-allocation-03 (work in
progress), July 2017.
[I-D.reddy-sfc-nsh-encrypt]
Reddy, T., Patil, P., Fluhrer, S., and P. Quinn,
"Authenticated and encrypted NSH service chains", draft-
reddy-sfc-nsh-encrypt-00 (work in progress), April 2015.
[RFC2784] Farinacci, D., Li, T., Hanks, S., Meyer, D., and P.
Traina, "Generic Routing Encapsulation (GRE)", RFC 2784,
DOI 10.17487/RFC2784, March 2000,
<https://www.rfc-editor.org/info/rfc2784>.
[RFC3552] Rescorla, E. and B. Korver, "Guidelines for Writing RFC
Text on Security Considerations", BCP 72, RFC 3552,
DOI 10.17487/RFC3552, July 2003,
<https://www.rfc-editor.org/info/rfc3552>.
[RFC3692] Narten, T., "Assigning Experimental and Testing Numbers
Considered Useful", BCP 82, RFC 3692,
DOI 10.17487/RFC3692, January 2004,
<https://www.rfc-editor.org/info/rfc3692>.
[RFC6071] Frankel, S. and S. Krishnan, "IP Security (IPsec) and
Internet Key Exchange (IKE) Document Roadmap", RFC 6071,
DOI 10.17487/RFC6071, February 2011,
<https://www.rfc-editor.org/info/rfc6071>.
[RFC6291] Andersson, L., van Helvoort, H., Bonica, R., Romascanu,
D., and S. Mansfield, "Guidelines for the Use of the "OAM"
Acronym in the IETF", BCP 161, RFC 6291,
DOI 10.17487/RFC6291, June 2011,
<https://www.rfc-editor.org/info/rfc6291>.
[RFC7325] Villamizar, C., Ed., Kompella, K., Amante, S., Malis, A.,
and C. Pignataro, "MPLS Forwarding Compliance and
Performance Requirements", RFC 7325, DOI 10.17487/RFC7325,
August 2014, <https://www.rfc-editor.org/info/rfc7325>.
[RFC7498] Quinn, P., Ed. and T. Nadeau, Ed., "Problem Statement for
Service Function Chaining", RFC 7498,
DOI 10.17487/RFC7498, April 2015,
<https://www.rfc-editor.org/info/rfc7498>.
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[RFC7676] Pignataro, C., Bonica, R., and S. Krishnan, "IPv6 Support
for Generic Routing Encapsulation (GRE)", RFC 7676,
DOI 10.17487/RFC7676, October 2015,
<https://www.rfc-editor.org/info/rfc7676>.
[RFC8165] Hardie, T., "Design Considerations for Metadata
Insertion", RFC 8165, DOI 10.17487/RFC8165, May 2017,
<https://www.rfc-editor.org/info/rfc8165>.
[RFC8201] McCann, J., Deering, S., Mogul, J., and R. Hinden, Ed.,
"Path MTU Discovery for IP version 6", STD 87, RFC 8201,
DOI 10.17487/RFC8201, July 2017,
<https://www.rfc-editor.org/info/rfc8201>.
Authors' Addresses
Paul Quinn (editor)
Cisco Systems, Inc.
Email: paulq@cisco.com
Uri Elzur (editor)
Intel
Email: uri.elzur@intel.com
Carlos Pignataro (editor)
Cisco Systems, Inc.
Email: cpignata@cisco.com
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