Service Function Chaining Guanwen Li
Internet Draft Guanglei Li
Intended status: Standards Track Taixin Li
Expires: September 27, 2019 Qi Xu
Bohao Feng
Huachun Zhou
Beijing Jiaotong University
March 26, 2019
Multi-domain Service Forwarding For NSH
draft-li-sfc-nsh-multi-domain-06
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Abstract
This document describes the mechanism to achieve multi-domain service
forwarding for NSH. The proposed mechanism adopts a horizontal
deployment structure, which divides a multi-domain SFC into several
segmental SFCs in the control plane and each domain creates its own
SFP independently in data plane. A label is proposed to identify
different cross-domain flows at the classifier by extending the
metadata of the NSH encapsulation.
Table of Contents
1. Introduction ................................................ 2
1.1. Requirement Language ................................... 3
2. Definition Of Terms ......................................... 4
3. Multi-domain Service Forwarding Mechanism ................... 3
3.1. Service Function Chaining Segmentation ................. 4
3.2. Inter-domain Service Forwarding ........................ 5
3.3. Classification and Intra-domain Service Forwarding ..... 5
4. Multi-domain Service Forwarding Encapsulation ............... 5
5. Multi-domain Service Forwarding Example ..................... 6
6. Security Considerations ..................................... 8
7. IANA Considerations ......................................... 8
8. Conclusions ................................................. 8
9. References .................................................. 8
9.1. Normative References ................................... 8
9.2. Informative References ................................. 9
10. Acknowledgments ............................................ 9
1. Introduction
Service Function Chaining (SFC) [RFC7665] is an architecture proposed
to decouple traditional network service functions with corresponding
physical resources. It is flexible and convenient for the network
operator to deploy on-demand service functions and steer the traffic
through them in sequence.
Network Service Header (NSH) [RFC8300] is defined as a data plane
protocol to create dynamic service function chains. According to the
NSH encapsulation, the flow can pass along pre-defined Service
Function Path and exchange metadata among the Service Classifier, the
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Service Function and the Service Function Forwarder for information
sharing.
Network service forwarding in SFC is based on the combination of
Service Path Identifier (SPI) and Service index (SI), which are
defined in [RFC8300]. [I-D.kumar-sfc-nsh-forwarding] analyzes the NSH
forwarding shortcomings and further discusses the separation of the
service forwarding and the service delivery. However, it focuses on
infrastructure service forwarding for NSH in a single domain.
[RFC8459] proposes a hierarchical way to achieve multi-domain
forwarding for SFC, which can be regarded as a vertical approach.
Contrast to the vertical approach, a horizontal one is easy to scale
in and out. Besides, the horizontal approach can decrease the
overhead of the control plane, because it maintains more service
traffic in the data plane.
Therefore, this document proposes a horizontal approach to achieve
multi-domain service forwarding for NSH. The main idea is to divide a
multi-domain SFC into several segmental SFCs according to domain
partitions in the control plane and create corresponding SFPs in each
domain by its classifier independently. A label is proposed to
identify different cross-domain flows, which is encapsulated in the
metadata.
1.1. Requirement 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].
2. Multi-domain Service Forwarding Mechanism
+---------------------------------------------------+
| |
| Control Plane |
| |
+-----+----------------+----------------------+-----+
| | |
|Segmental |Segmental |Segmental
|SFC-1 |SFC-2 |SFC-N
| | |
+-----v-----+ +-----v-----+ +-----v-----+
| | | | | |
----> Domain 1 +----> Domain 2 +->......--> Domain N +--->
| | | | | |
+-----------+ +-----------+ +-----------+
Figure 1 Multi-domain SFC Divide
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As shown in Figure 1, according to requirements, it divides the whole
SFC in the control plane and issues cross-domain forwarding tables to
corresponding classifiers initially. Multi-domain service forwarding
includes two aspects: the inter-domain service forwarding and the
intra-domain service forwarding. The former is based on a unique
label for each flow, and the latter is performed by the classifier.
To achieve a unified service forwarding mechanism in multi-domain,
this mechanism uses metadata in NSH encapsulation to carry the
necessary forwarding information among different forwarding elements,
such as the Service Classifier and the Service Function Forwarder.
3. Definition Of Terms
This document uses some terms defined in SFC architecture [RFC7665]
and NSH [RFC8300] drafts for ease of understanding and the reader is
advised to refer to those documents for up to date and additional
information.
Segmental SFC: The cross-domain SFC is divided into several segmental
SFCs according to the domain partition. Each segmental SFC is
assigned to its corresponding domain.
Service Label: the label used to identify different flows can help
the classifier create the SFP by its corresponding segmental SFC,
which is issued from SFC control plane.
Cross-domain Forwarding Table: There are three columns in the table:
Service Label, Next Classifier, and Segmental SFC. The table matches
a specific flow with its Service Label. The cross-domain forwarding
of the flow is depended on the address of Next Classifier. The Cross-
domain Forwarding Table is maintained by SFC control plane and issued
to the corresponding classifier according to the Segmental SFC
partitions.
3.1. Service Function Chaining Segmentation
At first, the control plane creates an SFC with a unique Service
Label for the flow. Then, the SFC is divided into several segmental
SFCs according to physical resource constraints. It is important to
note that the control plane MUST NOT specific the SFP directly. The
control plane is only responsible to indicate what service functions
are required in each domain, and the corresponding SFP MUST be
specified by the service classifier.
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3.2. Inter-domain Service Forwarding
The Service Label is proposed to identify different flows when
packets need to be cross-domain forwarded. When the service
classifier receives packets with NSH encapsulations, it checks the
Service Label of the first packet to look up the address of the next
classifier in its cross-domain forwarding table, which is issued by
SFC control plane. Then the information of next classifier is written
into the metadata and will be used by the last SFF in the Segmental
SFC. Once the last SFF receives the packet from last SF, which
changes the SI to zero, it forwards the packets to next classifier
directly without any modification of their NSH encapsulations.
The Service Label is only carried by the first packet of a certain
flow with specific SPI. It's beneficial to decrease header cost and
improve forwarding efficiency.
3.3. Classification and Intra-domain Service Forwarding
The service classifier creates SFP for each flow according to the
segmental SFC in its cross-domain forwarding table. After the control
plane assigns segmental SFCs for different domains, the corresponding
table is issued to the classifier in each domain. When the packet
arrives at the classifier, its Service Label is used to find out the
next segmental SFC. Then, the classifier creates an SFP for the flow
according to that segmental SFC.
In this situation, the classifier in a segmental SFC SHOULD set the
SI to the length of the segmental SFC.
4. Multi-domain Service Forwarding Encapsulation
In order to reduce the overhead of metadata, the context header with
MD Type = 0x2 is chosen to support multi-domain service forwarding,
Figure 2 shows the allocation of the metadata.
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 (SPI) | Service Index |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Metadata Class=0x20 | Type=0x1 |C| Length=0x4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|D|U|U|U| Service Label | Next Classifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2 Multi-domain Service Forwarding Encapsulation
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Context Header Allocation Descriptions:
Metadata Class: It MUST be set to 0x20 as requested in Section 7.
C bit: It SHOULD be set indicating critical metadata exists.
Type: It MUST be set to 0x1 for multi-domain service forwarding.
Length: Because the length of the context header is constant, it MUST
be set to 0x4.
D bit: The default value is zero, which means the metadata SHOULD be
ignored. When set to 0x1 indicates that this packet needs to be for-
warded in multiple domains.
Service Label: Identifies different flows with different labels. The
service classifier decides the service forwarding path of the packet
according to its domain label with a forwarding table issued by the
control plane.
Next Classifier: Indicates the identifier of the classifier in next
domain. Because of the SFC segmentation, each classifier only works
in the SFC domain it belongs to. When the current segmental SFC is
terminated, the last SFF will query the next segmental domain by this
identifier.
All other flag fields are reserved for future use. Unassigned bits(U)
MUST be set to zero and MUST be ignored upon receipt.
5. Multi-domain Service Forwarding Example
+----------+ +----------+ +----------+
| | | | | |
Domain1 | SF-a | | SF-b | Domain2 | SF-c |
| | | | | |
+---^--+---+ +---^--+---+ +---^--+---+
| | | | | |
+-------+ +---+--v---+ +---+--v---+ +-------+ +---+--v---+
| | | | | | | | | |
---> CF1 +-> SFF1-1 +-> SFF1-2 +----> CF2 +-> SFF2-1 +--->
| | | | | | | | | |
+-------+ +----------+ +----------+ +-------+ +----------+
Figure 3 Multi-domain Service Forwarding Example
This section describes the scenario shown in Figure 3, a packet flow
pass through three service functions deployed in two domains. The
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Domain1 is consist of CF1, SFF1-1, SFF1-2, SF-a and SF-b; the Domain2
is consist of CF2, SFF2-1 and SF-c. The workflow is as follow:
1.SFC control plane creates the SFC with a Service Label for the
specific flow and divides the whole SFC into several segmental SFCs.
2.The cross-domain forwarding table is issued to its corresponding
classifier with specific Service Label, Next Classifier and Segmental
SFC.
3.CF1(the classifier in domain1) creates the SFP and the NSH
encapsulation for the first packet of a certain flow according to its
segmental SFC in domain1. It sets 'D' flag to 1 and fills in the
Service Label, and Next Classifier. The SI is set to the length of
this segmental SFC.
4.CF1 forwards the packet to SFF1-1.
5.SFF1-1 determines SF-a as the next hop and forwards the packet.
6.After SF-a processes the packet, the packet is forwarded back to
SFF with decremented SI.
7.SFF1-1 forwards the received packet to SFF1-2.
8.SFF1-2 determines SF-a as the next hop and forwards the packet.
9.Similar to SF-a, SF-b forwards the packet to SFF1-2 and decrements
SI to zero.
10.When SFF1-2 receives the packet, it finds out that the segmental
SFC is terminate in this domain because of the SI, and then SFF1-2
forwards the packet to next classifier without modification of the
NSH encapsulation. The following packets of this flow (in the same
SPI in this domain) are forwarded with the same routing information.
11.When CF2(the classifier in domain2) receives the first packet with
an NSH encapsulation, CF2 will check its Service Label.
12.According to the Service Label, CF2 looks up segmental SFC in the
cross-domain forwarding table and creates the corresponding SFP and
the NSH encapsulation.
13.CF2 sets the 'D' flag to zero and forwards the packet to SFF2-1.
The action of its following packets is similar.
13.SFF2-1 determines SF-c as the next hop and forwards the packet.
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14.SFC processes the packet and forwards it back to SFF2-1 with
decremented SI.
15.SFF2-1 finds out that the SFP is terminate and forwards the packet
to the final Receiver.
6. Security Considerations
As with many other protocols, metadata of the NSH encapsulation can
be spoofed or otherwise modified. It is important to protect the
cross-domain packet from malicious modification because the metadata
contains sensitive information about the user and environment.
Therefore, it is significate to ensure the integrity of the metadata
and provide the protection of the user privacy.
7. IANA Considerations
IANA is requested to allocate a new class from the TLV Class defined
in [RFC8300].
0x20 Multi-domain Forwarding Type
IANA is requested to set up a registry of "NSH Multi-domain Service
Forwarding TLV Types". These are 7-bit values. Registry entries
are assigned by using the "IETF Review" policy defined in [RFC8126].
IANA is requested to allocate two new types as follows:
o Type = 0x00 Reserved
o Type = 0x01 Multi-domain Service Forwarding
8. Conclusions
This document proposes a mechanism for multi-domain service forward-
ing based on a unique label. In order to relieve the pressure of the
control plane, the multi-domain SFC is divided into segmental SFC
according to the domain partitions, and the SFP in each domain is
created independently.
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
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[RFC8300] P. Quinn, Ed., U. Elzur, Ed. and C. Pignataro, Ed. "Network
Service Header (NSH)", January 2018.
9.2. Informative References
[RFC8126] M. Cotton, B. Leiba and T. Narten, "Guidelines for Writing
an IANA Considerations Section in RFCs", BCP 26, RFC 8126,
June 2017.
[RFC7665] J. Halpern, Ed. and C. Pignataro, Ed. "Service Function
Chaining (SFC) Architecture", October 2015.
[RFC8459] D. Dolson, S. Homma, D. Lopez, et al. "Hierarchical Service
Function Chaining", September 2018.
[I-D.kumar-sfc-nsh-forwarding]
S. Kumar, K. Leung, P. Bosch, et al. "Infrastructure
Service Forwarding For NSH", draft-kumar-sfc-nsh-
forwarding-01, August 2016.
10. Acknowledgments
This work in this document was supported by National High Technology
of China ("863 program") under Grant No.2015AA015702.
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Authors' Addresses
Guanwen Li
Beijing Jiaotong University
Beijing 100044, P.R. China
Email: 16111011@bjtu.edu.cn
Guanglei Li
Beijing Jiaotong University
Beijing 100044, P.R. China
Email: 15111035@bjtu.edu.cn
Taixin Li
Beijing Jiaotong University
Beijing 100044, P.R. China
Email: 14111040@bjtu.edu.cn
Qi Xu
Beijing Jiaotong University
Beijing 100044, P.R. China
Email: 15111046@bjtu.edu.cn
Bohao Feng
Beijing Jiaotong University
Beijing 100044, P.R. China
Email: bhfeng@bjtu.edu.cn
Huachun Zhou
Beijing Jiaotong University
Beijing 100044, P.R. China
Email: hchzhou@bjtu.edu.cn
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