Service Function Chaining D. Dolson
Internet-Draft Sandvine
Intended status: Informational S. Homma
Expires: January 7, 2016 NTT
D. Lopez
Telefonica I+D
M. Boucadair
Orange Group
July 6, 2015
Hierarchical Service Chaining
draft-dolson-sfc-hierarchical-02
Abstract
Hierarchical Service Function Chaining (hSFC) is a network
architecture allowing an organization to compartmentalize a large-
scale network into multiple domains of administration.
The goals of hSFC are to make a large-scale network easier to reason
about, simpler to control and to support independent functional
groups within large operators.
Status of This Memo
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document authors. All rights reserved.
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Provisions Relating to IETF Documents
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Hierarchical Service Function Chaining (hSFC) . . . . . . . . 3
2.1. Top Level . . . . . . . . . . . . . . . . . . . . . . . . 4
2.2. Lower Levels . . . . . . . . . . . . . . . . . . . . . . 5
3. Internal Boundary Node (IBN) . . . . . . . . . . . . . . . . 6
3.1. IBN Path Configuration . . . . . . . . . . . . . . . . . 7
3.1.1. Flow-Stateful IBN . . . . . . . . . . . . . . . . . . 7
3.1.2. Encoding Upper-Level Paths in Metadata . . . . . . . 8
3.1.3. Using Unique Paths per Upper-Level Path . . . . . . . 9
3.2. Gluing Levels Together . . . . . . . . . . . . . . . . . 9
4. Sub-domain Classifier . . . . . . . . . . . . . . . . . . . . 10
5. Control Plane Elements . . . . . . . . . . . . . . . . . . . 10
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 11
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
8. Security Considerations . . . . . . . . . . . . . . . . . . . 11
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
9.1. Normative References . . . . . . . . . . . . . . . . . . 12
9.2. Informative References . . . . . . . . . . . . . . . . . 12
Appendix A. Examples of Hierarchical Service Function Chaining . 13
A.1. Simplify SFC management . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 15
1. Introduction
Service Function Chaining (SFC) is a technique for prescribing
differentiated traffic forwarding policies within the SFC domain.
SFC is described in detail in the SFC architecture document
[I-D.ietf-sfc-architecture], and is not repeated here.
In this document we consider the difficult problem of implementing
SFC across a large, geographically dispersed network comprised of
millions of hosts and thousands of network forwarding elements,
involving multiple operational teams (with varying functional
responsibilities). We expect asymmetrical routing is inherent in the
network, while recognizing that some Service Functions (SFs) require
bidirectional traffic for transport-layer sessions (e.g., NATs,
firewalls). We assume that some paths need to be selected on the
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basis of application-specific data visible to the network, with
5-tuple stickiness to specific Service Function instances.
Note: in this document, the notion of the "path" of a packet is the
series of SF instances traversed by a packet. The means of
delivering packets between SFs (the forwarding mechanisms of the
underlay network) is not relevant to the current discussion.
Difficult problems are often made easier by decomposing them in a
hierarchical (nested) manner. So instead of considering an
omniscient SFC Control Plane that can manage (create, withdraw,
supervise, etc.) complete paths from one end of the network to the
other, we decompose the network into smaller sub-domains. Each sub-
domain may support a subset of the network applications or a subset
of the users. The criteria for determining decomposition into SFC-
enabled sub-domains are beyond the scope of this document.
Note that decomposing a network into multiple SFC-enabled domains
should permit end-to-end visibility of Service Functions and Service
Function Paths. Decomposition should also be implemented with
special care to ease monitoring and troubleshooting of the network as
a whole.
An example of simplifying a network by using multiple SF domains is
further discussed in [I-D.ietf-sfc-dc-use-cases].
We assume the SF technology uses NSH [I-D.ietf-sfc-nsh] or a similar
labeling mechanism.
The "domains" discussed in this document are assumed to be under
control of a single organization, such that here is a strong trust
relationship between the domains. The intention of creating multiple
domains is to improve the ability to operate a network. It is
outside of the scope of the document to consider domains operated by
different organizations.
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].
2. Hierarchical Service Function Chaining (hSFC)
A hierarchy has multiple levels. The top-most level encompasses the
entire network domain to be managed, and lower levels encompass
portions of the network.
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2.1. Top Level
Considering the example in Figure 1, a top-level network domain
includes SFC components distributed over a wide area, including:
o classifiers (CFs),
o Service Function Forwarders (SFFs) and
o Sub-domains.
For the sake of clarity, components of the underlay network are not
shown; an underlay network is assumed to provide connectivity between
SFC components.
Top-level service function paths carry packets from classifiers
through a series of SFFs and sub-domains, with the operations within
sub-domains being opaque to the higher levels.
We expect the system to include a top-level control-plane having
responsibility for configuring forwarding and classification. The
top-level Service Chaining control-plane manages end-to-end service
chains and associated service function paths from network edge points
to sub-domains and configuring top-level classifiers at a coarse
level (e.g., based on source or destination host) to forward traffic
along paths that will transit appropriate sub-domains. The figure
shows one possible service chain passing from edge, through two sub-
domains, to network egress. The top-level control plane does NOT
configure classification or forwarding within the sub-domains.
At this network-wide level, the number of SFPs required is a linear
function of the number of ways in which a packet is required to
traverse different sub-domains and egress the network. Note that the
various paths which may be taken within a sub-domain are not
represented by distinct network-wide SFPs; specific policies at the
ingress nodes of each sub-domain bind flows to sub-domain paths.
Packets are classified at the edge of the network to select the paths
by which sub-domains are to be traversed. At the ingress of each
sub-domain, paths are reclassified to select the paths by which SFs
in the sub-domain are to be traversed. At the egress of each sub-
domain, packets are returned to the top-level paths. Contrast this
with an approach requiring the top-level classifier to select paths
to specify all of the SFs in each sub-domains.
It should be assumed that some service functions in the network
require bidirectional symmetry of paths (see more in Section 4).
Therefore the classifiers at the top level must be configured with
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policies ensuring server-to-client packets take the reverse path of
client-to-server packet through sub-domains. (Recall the "path"
denotes the series of service functions; the precise physical network
path within the underlay network is not relevant here.)
+------------+
|Sub-domain#1|
| in DC1 |
+----+-------+
|
.---- SFF1 ------. +--+
+--+ / / | \--|CF|
--->|CF|--/---->' | \ +--+
+--+ / SC#1 | \
| | |
| V .------>|--->
| / / |
\ | / /
+--+ \ | / / +--+
|CF|---\ | / /---|CF|
+--+ '---- SFF2 ------' +--+
|
+----+-------+
|Sub-domain#2|
| in DC2 |
+------------+
One path is shown from edge classifier to SFF1 to Sub-domain#1
(residing in data-center1) to SFF1 to SFF2 (residing in data-center
2) to Sub-domain#2 to SFF2 to network egress.
Figure 1: Network-wide view of Top Level of Hierarchy
2.2. Lower Levels
Each of the sub-domains in Figure 1 is an SFC domain.
Unlike the top level, however, data packets entering the sub-domain
are already encapsulated within SFC transport. Figure 2 shows a sub-
domain interfaced with a higher-level domain by means of an Internal
Boundary Node (IBN). It is the purpose of the IBN to remove packets
from the SFC encapsulation, apply Classification rules, and direct
the packets to the selected local service function paths terminating
at an egress IBN. The egress SFC Domain Gateway finally restores
packets to the original SFC transport and hands them off to SFFs.
Each sub-domain intersects a subset of the total paths that are
possible in the higher-level domain. An IBN is concerned with
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higher-level paths, but only those traversing the sub-domain. A top-
level controller may configure the IBN as an SF (the IBN plays the SF
role in the top-level domain).
We expect each sub-domain to have a control-plane that can operate
independently of the top-level control-plane. The sub-domain
control-plane configures the classification and forwarding rules in
the sub-domain. The classification rules reside in the IBN, where
packets are moved from SFC encapsulation of the top-level domain to
and from SFC encapsulation of the lower-level domain.
+----+ +-----+ +----------------------+ +-----+
| |SC#1| SFF | | IBN 1 | | SFF |
->| |================* *===============>
| | +-----+ | # (in DC 1) # | +-----+
| CF | | V # |
| | |+---+ +---+| Top domain
| | * * * * *||CF | * * * * * *|SFF|| * * * * *
| | * |+---+ +-+-+| *
+----+ * | | | | | | Sub *
* +-o-o--------------o-o-+ domain*
* SC#2 | |SC#1 ^ ^ #1 *
* +-----+ | | | *
* | V | | *
* | +---+ +------+ | | *
* | |SFF|->|SF#1.1|--+ | *
* | +---+ +------+ | *
* V | *
* +---+ +------+ +---+ +------+ *
* |SFF|->|SF#2.1|->|SFF|->|SF#2.2| *
* +---+ +------+ +---+ +------+ *
* * * * * * * * * * * * * * * * * * * * * *
*** Sub-domain boundary; === top-level chain; --- low-level chain.
Figure 2: Sub-domain within a higher-level domain
If desired, the pattern can be applied recursively. For example,
SF#1.1 in Figure 2 could be a sub-domain of the sub-domain.
3. Internal Boundary Node (IBN)
A network element termed "Internal Boundary Node" (IBN) bridges
packets between domains. It looks like an SF to the higher level,
and looks like a classifier and end-of-chain to the lower level.
To achieve the benefits of hierarchy, the IBN should be applying more
granular traffic classification rules at the lower level than the
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traffic passed to it. This means that the number of SF Paths within
the lower level is greater than the number of SF Paths arriving to
the IBN.
The IBN is also the termination of lower-level SF paths. This is
because the packets exiting lower-level SF paths must be returned to
the higher-level SF paths and forwarded to the next hop in the
higher-level domain.
3.1. IBN Path Configuration
An operator of a lower-level SF Domain may be aware of which high-
level paths transit their domain, or they may wish to accept any
paths.
When packets enter the sub-domain, the Path Identifier and Path Index
are re-marked according to the path selected by the classifier.
After exiting a path in the sub-domain, packets can be restored to an
upper-level SF path by these methods:
1. Stateful per flow,
2. Pushing path identifier into metadata,
3. Using unique lower-level paths per upper-level path.
3.1.1. Flow-Stateful IBN
An IBN can be flow-aware, returning packets to the correct higher-
level SF path on the basis of 5-tuple of packets exiting the lower-
level SF paths.
When packets are received by the IBN on a higher-level path, the
encapsulated packets are parsed for IP and transport-layer (TCP or
UDP) coordinates. State is created, indexed by the 5-tuple of
{source-IP, destination-IP, source-port, destination-port and
transport protocol}. The state contains critical fields of the
encapsulating SFC header (or perhaps the entire header).
The simplest approach has the packets return to the same IBN at the
end of the chain that classified the packet at the start of the
chain. This is because the required 5-tuple state is rapidly
changing and most efficiently kept locally. If the packet is
returned to a different IBN for egress, 5-tuple state must be
synchronized between the IBNs.
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When a packet returns to the IBN at the end of a chain, the SFC
header is removed, the packet is parsed for IP and transport-layer
coordinates, and state is retrieved by the 5-tuple of the packet.
The state contains the information required to forward the packet
within the higher-level service chain.
State cannot be created by packets arriving from the lower-level
chain; when state cannot be found for such packets, they MUST be
dropped.
This stateful approach is limited to use with SFs that retain the
5-tuple of the packet. This approach cannot be used with SFs that
modify the 5-tuple (e.g., as done by a NAT) or otherwise create
packets for new 5-tuples other than those received (e.g., as an HTTP
cache might do to retrieve content on behalf of the original flow).
In both cases, the fundamental problem is the inability to forward
packets when state cannot be found for the packet 5-tuples.
In the stateful approach, there are issues caused by the state, such
as how long the state should be maintained (it MUST time out
eventually), as well as whether the state needs to be replicated to
other devices to create a highly available network.
It is valid to consider the state disposable after failure, since it
can be re-created by each new packet arriving from the higher-level
domain. For example, if an IBN loses all flow state, the state is
re-created by an end-point retransmitting a TCP packet.
If an SFC domain handles multiple network regions (e.g., multiple
private networks), the 5-tuple may be augmented with a 6th parameter,
perhaps using some metadata to identify the network region.
In this stateful approach, it is not necessary for the sub-domain's
control-plane to modify paths when higher-level paths are changed.
The complexity of the higher-level domain does not cause complexity
in the lower-level domain.
3.1.2. Encoding Upper-Level Paths in Metadata
An IBN can push the upper-level service path identifier (SPI) and
service index (SI) (or encoding thereof) into a metadata field of the
lower-level encapsulation (e.g., placing upper-level path information
into a metadata field of NSH). When packets exit the lower-level
path, the upper-level SPI and SI can be restored from the metadata
retrieved from the packet.
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This approach requires the SFs in the path to be capable of
forwarding the metadata and appropriately attaching metadata to any
packets injected for a flow.
Using new metadata may inflate packet size when variable-length
metadata (type 2 from NSH [I-D.ietf-sfc-nsh]) is used.
It is conceivable that the MD-type 1 Mandatory Context Header fields
of NSH [I-D.ietf-sfc-nsh] are not all relevant to the lower-level
domain. In this case, one of the metadata slots of the Mandatory
Context Header could be repurposed within the lower-level domain, and
restored when leaving.
In this metadata approach, it is not necessary for the sub-domain's
controller to modify paths when higher-level paths are changed. The
complexity of the higher-level domain does not cause complexity in
the lower-level domain.
3.1.3. Using Unique Paths per Upper-Level Path
In this approach, paths within the sub-domain are constrained so that
a path identifier (of the sub-domain) unambiguously indicates the
egress path (of the upper domain). This allows the original path
information to be restored at sub-domain egress from a look-up table
using the sub-domain path identifier.
Whenever the upper-level domain provisions a path via the lower-level
domain, the lower-level domain controller must provision
corresponding paths to traverse the lower-level domain.
A down-side of this approach is that the number of paths in the
lower-level domain is multiplied by the number of paths in the
higher-level domain that traverse the lower-level domain. I.e., a
sub-path must be created for each combination of upper Path
identifier and lower path.
3.2. Gluing Levels Together
The path identifier or metadata on a packet received by the IBN may
be used as input to reclassification and path selection within the
lower-level domain.
In some cases the meanings of the various path IDs and metadata must
be coordinated between domains.
One approach is to use well-known identifier values in metadata,
communicated by some organizational registry.
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Another approach is to use well-known labels for path identifiers or
metadata, as an indirection to the actual identifiers. The actual
identifiers can be assigned by control-plane systems. For example, a
sub-domain classifier could have a policy, "if pathID=classA then
chain packet to path 1234"; the higher-level controller would be
expected to configure the concrete higher-level pathID for classA.
4. Sub-domain Classifier
Within the sub-domain (referring to Figure 2), after the IBN removes
higher-level encapsulation from incoming packets, it sends the
packets to the classifier, which selects the encapsulation for the
packet within the sub-domain.
One of the goals of the hierarchical approach is to make it easy to
have transport-flow-aware service chaining with bidirectional paths.
For example, it is desired that for each TCP flow, the client-to-
server packets traverse the same SFs as the server-to-client packets,
but in the opposite sequence. We call this bidirectional symmetry.
If bidirectional symmetry is required, it is the responsibility of
the control-plane to be aware of symmetric paths and configure the
classifier to chain the traffic in a symmetric manner.
Another goal of the hierarchical approach is to simplify the
mechanisms of scaling in and scaling out service functions. All of
the complexities of load-balancing among multiple SFs can be handled
within a sub-domain, under control of the classifier, allowing the
higher-level domain to be oblivious to the existence of multiple SF
instances.
Considering the requirements of bidirectional symmetry and load-
balancing, it is useful to have all packets entering a sub-domain to
be received by the same classifier or a coordinated cluster of
classifiers. There are both stateful and stateless approaches to
ensuring bidirectional symmetry.
5. Control Plane Elements
Controllers have been mentioned in this document without much
explanation. Although control protocols have not yet been
standardized, from the point of view of hierarchical service chaining
we have these expectations:
o Each control-plane instance manages a single level of hierarchy of
a single domain.
o Each control-plane is agnostic about other levels of hierarchy.
This aspect allows humans to reason about the system within a
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single domain and allows control-plane algorithms to use only
domain-local inputs. Top-level control does not need visibility
to sub-domain policies, nor does sub-domain control need
visibility to higher-level policies.
o Sub-domain control-planes are agnostic about control-planes of
other sub-domains. This allows both humans and machines to
manipulate sub-domain policy without considering policies of other
domains.
Recall that the IBN acts as an SF in the higher-level domain
(receiving SF instructions from the higher-level control-plane) and
as a classifier in the lower-level domain (receiving classification
rules from the sub-domain control-plane). In this view, it is the
IBN that glues the layers together.
The above expectations are not intended to prohibit network-wide
control. A control hierarchy can be envisaged to distribute
information and instructions to multiple domains and sub-domains.
Control hierarchy is outside the scope of this document.
6. Acknowledgements
The concept of Hierarchical Service Path Domains was introduced in
draft-homma-sfc-forwarding-methods-analysis-01
[I-D.homma-sfc-forwarding-methods-analysis] as a means to improve
scalability of service chaining in large networks.
The authors would like to thank the following individuals for taking
the time to read and provide valuable feedback:
Ron Parker
Christian Jacquenet
Dapeng Liu
7. IANA Considerations
This memo includes no request to IANA.
8. Security Considerations
Hierarchical service chaining makes use of service chaining
architecture, and hence inherits the security considerations
described in the architecture document.
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Furthermore, hierarchical service chaining inherits security
considerations of the data-plane protocols (e.g., NSH) and control-
plane protocols used to realize the solution.
The systems described in this document bear responsibility for
forwarding internet traffic. In some cases the systems are
responsible for maintaining separation of traffic in private
networks.
This document describes systems within different domains of
administration that must have consistent configurations in order to
properly forward traffic and to maintain private network separation.
Any protocol designed to distribute the configurations must be secure
from tampering.
All of the systems and protocols must be secure from modification by
untrusted agents.
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.
9.2. Informative References
[I-D.homma-sfc-forwarding-methods-analysis]
Homma, S., Kengo, K., Lopez, D., Stiemerling, M., and D.
Dolson, "Analysis on Forwarding Methods for Service
Chaining", draft-homma-sfc-forwarding-methods-analysis-01
(work in progress), January 2015.
[I-D.ietf-sfc-architecture]
Halpern, J. and C. Pignataro, "Service Function Chaining
(SFC) Architecture", draft-ietf-sfc-architecture-07 (work
in progress), March 2015.
[I-D.ietf-sfc-dc-use-cases]
Surendra, S., Tufail, M., Majee, S., Captari, C., and S.
Homma, "Service Function Chaining Use Cases In Data
Centers", draft-ietf-sfc-dc-use-cases-02 (work in
progress), January 2015.
[I-D.ietf-sfc-nsh]
Quinn, P. and U. Elzur, "Network Service Header", draft-
ietf-sfc-nsh-00 (work in progress), March 2015.
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Appendix A. Examples of Hierarchical Service Function Chaining
The advantage of hierarchical service function chaining compared with
normal or flat service function chaining is that it can reduce the
management complexity significantly. This section discusses examples
that show the advantage of hierarchical service chaining.
A.1. Simplify SFC management
In this use case, hierarchical service chaining is used to simplify
service function chaining management by reducing the number of
Service Function Paths.
As shown in Figure 3, there are two domains each with different
concerns: a Security Domain that selects Service Functions based on
network conditions and an Optimization Domain that selects Service
Functions based on traffic protocol.
There are five security functions deployed in the Security Domain.
The Security Domain operator wants to enforce the five different
security policies, and the Optimization Domain operator wants to
apply different optimization (either cache or video optimization to
each of these two types of traffic. If we use flat SFC (normal
branching), 10 SFPs are needed in each domain. In contrast, if we
use hierarchical SFC, only 5 SFPs in Security Domain and 2 SFPs in
Optimization Domain will be required, as shown in Figure 4.
In the flat model, the number of SFPs is the product of the number of
functions in all of the domains. In the hSFC model, the number of
SFPs is the sum of the number of functions. For example, adding a
"bypass" path in the Optimization Domain would cause the flat model
to require 15 paths (5 more), but cause the hSFC model to require one
more path in the Optimization Domain.
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. . . . . . . . . . . . . . . . . . . . . . . . .
. Security Domain . . Optimization Domain .
. . . .
. +-1---[ ]----------------->[Cache ]------->
. | [ WAF ] . . .
. +-2-->[ ]----------------->[Video Opt.]---->
. | . . .
. +-3---[Anti ]----------------->[Cache ]------->
. | [Virus] . . .
. +-4-->[ ]----------------->[Video Opt.]---->
. | . . .
. +-5-->[ ]----------------->[Cache ]------->
[DPI]--->[CF]---| [ IPS ] . . .
. +-6-->[ ]----------------->[Video Opt.]---->
. | . . .
. +-7-->[ ]----------------->[Cache ]------->
. | [ IDS ] . . .
. +-8-->[ ]----------------->[Video Opt.]---->
. | . . .
. +-9-->[Traffic]--------------->[Cache ]------->
. | [Monitor] . . .
. +-10->[ ]--------------->[Video Opt.]---->
. . . . . . . . . . . . . . . . . . . . . . . . .
The classifier must select paths that determine the combination of
Security and Optimization concerns. 1:WAF+Cache, 2:WAF+VideoOpt,
3:AntiVirus+Cache, 4:AntiVirus+VideoOpt, 5: IPS+Cache,
6:IPS+VideoOpt, 7:IDS+Cache, 8:IDS+VideoOpt, 9:TrafficMonitor+Cache,
10:TrafficMonitor+VideoOpt
Figure 3: Flat SFC (Normal Branching)
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. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. Security Domain . . Optimization Domain .
. . . .
[CF]---->[ DPI(Sub-domain GW)]------[CF]->[ Sub-domain GW ]---->
. | ^ . . | ^ .
. +----->[ WAF ]-----+ . . +-->[ Cache ]---------+ .
. | | . . | | .
. +-->[Anti-Virus]---+ . . +-->[Video Opt]-------+ .
. | | . . .
. +----->[ IPS ]-----+ . . . . . . . . . . . . . . . .
. | | .
. +----->[ IDS ]-----+ .
. | | .
. +-->[ Traffic ]----+ .
. [ Monitor ] .
. . . . . . . . . . . . . . .
Figure 4: Simplified Path Management with Hierarchical SFC
Authors' Addresses
David Dolson
Sandvine
408 Albert Street
Waterloo, ON N2L 3V3
Canada
Phone: +1 519 880 2400
Email: ddolson@sandvine.com
Shunsuke Homma
NTT, Corp.
3-9-11, Midori-cho
Musashino-shi, Tokyo 180-8585
Japan
Email: homma.shunsuke@lab.ntt.co.jp
Diego R. Lopez
Telefonica I+D
Don Ramon de la Cruz, 82
Madrid 28006
Spain
Phone: +34 913 129 041
Email: diego.r.lopez@telefonica.com
Dolson, et al. Expires January 7, 2016 [Page 15]
Internet-Draft Hierarchical Service Chaining July 2015
Mohamed Boucadair
Orange Group
Rennes 35000
France
Email: mohamed.boucadair@orange.com
Dolson, et al. Expires January 7, 2016 [Page 16]