Service Function Chaining S. Homma
Internet-Draft K. Naito
Intended status: Informational NTT
Expires: December 10, 2015 D. R. Lopez
Telefonica I+D
M. Stiemerling
NEC/H-DA
D. Dolson
Sandvine
A. Gorbunov
Nokia
N. Leymann
Deutsche Telekom AG
June 8, 2015
Analysis on Forwarding Methods for Service Chaining
draft-homma-sfc-forwarding-methods-analysis-02
Abstract
Some working groups of the IETF and other Standards Developing
Organizations are now discussing use cases of a technology that
enables data packets to traverse appropriate service functions
located remotely through networks. This is called Service Chaining
in this document. (Also, in Network Functions Virtualisation (NFV),
a subject that forwarding packets to required service functions in
appropriate order is called VNF Forwarding Graph.) This draft does
not focus only on SFC method, and thus, use the term "Service
Chaining." SFC may be one of approaches to realize Service Chaining.
There are several Service Chaining methods to forward data packets to
service functions, and the applicable methods will vary depending on
the service requirements of individual networks.
This document presents the results of analyzing packet forwarding
methods and path selection patterns for achieving Service Chaining.
For forwarding data packets to the appropriate service functions,
distribution of route information and steering data packets following
the route information, are required. Examples of route information
are packet identifier and the routing configurations based on the
identifier. Also, forwarding functions are required to decide the
path according to the route information.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Definition of Terms . . . . . . . . . . . . . . . . . . . . . 4
3. Classification of Forwarding Methods and SP Decision Patterns 5
3.1. Forwarding Methods . . . . . . . . . . . . . . . . . . . 5
3.1.1. Method 1: Forwarding Based on Flow Identifiable
Information . . . . . . . . . . . . . . . . . . . . . 5
3.1.2. Method 2: Forwarding with Stacked Transport Headers . 6
3.1.3. Method 3: Forwarding Based on Service Chain
Identifiable Tags . . . . . . . . . . . . . . . . . . 8
3.2. Service Path Selection Patterns . . . . . . . . . . . . . 9
3.2.1. Pattern 1: Static Selection of End to End Service
Path . . . . . . . . . . . . . . . . . . . . . . . . 10
3.2.2. Pattern 2: Dynamic Selection of Segmented Service
Path . . . . . . . . . . . . . . . . . . . . . . . . 12
4. Consideration of Forwarding Methods and Paths Selection
Patterns . . . . . . . . . . . . . . . . . . . . . . . . . . 18
4.1. Analysis of 3.1. Forwarding Methods . . . . . . . . . . . 18
4.1.1. Analysis of Method 1 . . . . . . . . . . . . . . . . 18
4.1.2. Analysis of Method 2 . . . . . . . . . . . . . . . . 19
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4.1.3. Analysis of Method 3 . . . . . . . . . . . . . . . . 20
4.2. Analysis of 3.2. Service Paths Selection Patterns . . . . 21
4.2.1. Analysis of Pattern 1 . . . . . . . . . . . . . . . . 21
4.2.2. Analysis of Pattern 2 . . . . . . . . . . . . . . . . 22
4.3. Example of selecting Methods and Patterns . . . . . . . . 25
4.3.1. Example#1: Enterprise Datacenter Network . . . . . . 25
4.3.2. Example#2: Current Mobile Service Providers Network . 26
4.3.3. Example#3: Fixed and Mobile Converged Service
Providers Network . . . . . . . . . . . . . . . . . . 27
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 28
6. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 28
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 29
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 29
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 30
1. Introduction
Service Chaining is a technology to provide service oriented
forwarding which enables data packets to traverse the appropriate
service functions deployed in networks. This draft assumes that
Service Chaining is achieved by the following steps:
a. A classification function identifies data packets and determines
the set of services that will be provided for the packets and in
which order.
b. The path, that the packets will traverse for reaching the required
service functions, is established based on the result of step a.
The paths may be established in advance.
c. Forwarding functions determine the appropriate destination and
forward each packet to the next hop according to the path.
d. A service function provides services to received packets and
return each packet to the forwarding function.
e. Steps c and d are repeated until each packet has been transferred
to all required service functions.
f. After a packet has been transferred to all required Service
Functions, it is forwarded to its original destination.
There are several forwarding methods for Service Chaining, and they
can be classified into certain categories in terms of distribution of
information for setting the paths and decision of the paths. The
methods used to distribute the information and the patterns used to
decide the paths will affect the mechanism of Service Chaining as
well as service flexibility.
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The applicable methods vary depending on network requirements, and
thus, classifying and determining forwarding methods will be
important in designing the architecture of Service Function Chaining
(SFC). This document provides the results of analyzing forwarding
methods for Service Chaining.
OAM, security, and redundancy are outside the scope of this draft.
2. Definition of Terms
Term "Classification", "Classifier" referred to
[I-D.ietf-sfc-architecture]. Term "Service Function", "Service Node"
referred to [I-D.ietf-sfc-dc-use-cases].
Service Chaining: A technology that lets data packets traverse a
series of service functions.
Classification: Locally instantiated policy and customer/network/
service profile matching of traffic flows for identification of
appropriate outbound forwarding actions.
Classifier (CF): The entity that performs classification.
Service Function (SF): A function that is responsible for specific
treatment of received packets. A Service Function can act at
various layers of a protocol stack (e.g. at the network layer or
other OSI layers). A Service Function can be a virtual element or
be embedded in a physical network element. One of multiple
Service Functions can be embedded in the same network element.
Multiple occurrences of the Service Function can be enabled in the
same administrative domain.
One or more Service Functions can be involved in the delivery of
added-value services. A non-exhaustive list of Service Functions
includes: firewalls. WAN and application acceleration, Deep
Packet Inspection (DPI), LI (Lawful Intercept) module, server load
balancers, NAT44 [RFC3022], NAT64 [RFC6146], NPTv6 [RFC6296],
HOST_ID injection, HTTP Header Enrichment functions, TCP
optimizer, etc.
Service Node (SN): A virtual or physical device that hosts one or
more service functions, which can be accessed via the network
location associated with it.
Forwarder (FWD): The entity, responsible for forwarding data packets
along the service path, which includes delivery of traffic to the
connected service functions. FWD handles Forwarding Tables, which
is used for forwarding packets.
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Control Entity (CE): The entity responsible for managing service
topology and indicating forwarding configurations to Forwarders.
Service Chain (SC): A service chain defines an ordered list of
service functions that must be applied to user packets selected as
a result of classification. The implied order may not be a linear
progression as the architecture allows for nodes that copy to more
than one branch.
Service Path (SP): The instantiation of a service chain in the
network. Packets follow a service path through the requisite
service functions. Service path shows a specific path of
traversing SF instance. For example, SC is written as SF#1 ->
SF#2 -> SF#3 (This shows an ordered list of SFs), and SP is
written as SF#1_1(1_1 means instance 1 of SF1) -> SF#2_1 ->
SF#3_1.
Service Chaining Domain (SC Domain): The domain managed by one or a
set of CEs.
Service Path Information (SP Information): The information used to
forward packets to the appropriate SFs based on the selected
service. Examples of SP information include routing
configurations for Forwarders, transport headers for forwarding
packets to required SFs, and service/flow identifiable tags.
3. Classification of Forwarding Methods and SP Decision Patterns
3.1. Forwarding Methods
In Service Chaining, data packets are transferred to service
functions, which can be located outside the regular computed path to
the original destination. Therefore, a routing mechanism that is
different from general L2/L3 switching/routing may be required. The
routing mechanism can be classified into three methods in terms of
distribution of SP information and packet forwarding.
3.1.1. Method 1: Forwarding Based on Flow Identifiable Information
The mechanism of method 1 is shown in Figure 1. In this method,
routing configurations based on flow identifiable information, such
as 5-tuple (e.g. dst IP, src IP, dst port, src port, tcp) are
indicated to the CF and each FWD. There may be an CE to handle this.
The flow identifiable information can be constructed with some fields
of L2 or L3 or combination of those. The information can be
configured either before packets arrive, or at the time packets
arrive at CF and FWD. Each FWD identifies the packets with flow
identifiable information and forwards the packets to the SFs
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according to the configuration. This method does not require
changing any fields of the original packet frame.
*Distribution model of SP information*
+----------------+
| Control Entity |
+----------------+
^ | indication of routing configuration
| | based on packet identifiable information
| +---------------+-------------------------------+--------->
| | | |
| | | |
| v v v
+--------+ +-------+ +------+ +-------+
------>| CF |------> | FWD |------> | SF#1 |------>| FWD |----->
+--------+ +-------+ +------+ +-------+
////////////////////////////////////////////////////////////////////////
*Forwarding Tables*
Locate: [CF] [FWD] [FWD]
Table: 192.168.1.1 192.168.1.1 192.168.1.1
->FWD#1 ->SF#1 ->SF#2
10.0.1.1 10.0.1.1 10.0.1.1
->FWD#1 ->FWD#2 ->SF#2
... ... ...
////////////////////////////////////////////////////////////////////////
*Condition of Packet*
Locate: [CF] [FWD] [SF#1] [FWD]
+-------+ +-------+ +-------+ +-------+
Packet: | PDU | | PDU | | PDU | | PDU |
+-------+ +-------+ +-------+ +-------+
Figure 1: Forwarding Based on Flow Identifiable Information
3.1.2. Method 2: Forwarding with Stacked Transport Headers
The mechanism of method 2 is shown in Figure 2. In this method, the
CF classifies packets and stacks transport headers in which actual
network address is included, e.g., MPLS or GRE headers, onto the
packets based on the classification. This method does not require
any forwarding function for forwarding packets based on the service
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information. Forwarding functions of underlay networks forward the
packets to SFs following the outermost header. The outermost header
is removed after service process of the SF. The actions are repeated
until all headers are removed.
*Distribution model of SP information*
+----------------+
| Control Entity |
+----------------+
^ |
| | indication of
| | stacking headers
| v
+--------+ +-------+ +------+ +------+
-------->| CF |------>| SF#1 |------>| SF#2 |------>| SF#3 |------>
+--------+ +-------+ +------+ +------+
////////////////////////////////////////////////////////////////////////
*Forwarding Tables*
Locate: [CF]
Table: 192.168.1.1 __/__/__/__/__/__/__/__/__/__/__/__/__/
->Stack #1,2,3 __/ Packets are forwarded to SFs by __/
10.0.1.1 __/ the outermost transport header. __/
->Stack #1,3 __/__/__/__/__/__/__/__/__/__/__/__/__/
...
////////////////////////////////////////////////////////////////////////
*Condition of Packet*
Locate: [CF] [SF#1] [SF#2] [SF#3]
+--------+
Header: |To SF#1 |
+--------+ +--------+
|To SF#2 | |To SF#2 |
+--------+ +--------+ +--------+
|To SF#3 | |To SF#3 | |To SF#3 |
+--------+ +--------+ +--------+
: : : :
+--------+ +--------+ +--------+ +--------+
Packet: | PDU | | PDU | | PDU | | PDU |
+--------+ +--------+ +--------+ +--------+
Figure 2: Forwarding with Stacked Multiple Transport Headers
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3.1.3. Method 3: Forwarding Based on Service Chain Identifiable Tags
The mechanism of this method is shown in Figure 3. In this method, a
CF classifies each packet and attaches a tag for identifying the
service or flow on the packets based on the classification. The
routing configuration based on the tags is sent to each FWD (from
some CE) in advance. Each FWD forwards packets to the SFs following
the configuration and the tag. After a packet has traversed all SFs,
the tag is removed and the packet is transported to the original
destination.
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*Distribution model of SP information*
+----------------+
| Control Entity |
+----------------+
^ | indication of attached tag
| | and routing configuration based on tags
| +----------------+------------------------------+--------->
| | | |
| | | |
| v v v
+--------+ +-------+ +------+ +-------+
----->| CF |------> | FWD |------>| SF#1 |------>| FWD |----->
+--------+ +-------+ +------+ +-------+
//////////////////////////////////////////////////////////////////////
*Forwarding Tables*
Locate: [CF] [FWD] [FWD]
Table: 192.168.1.1 IF ID#1,3 IF ID#1,2,5
->Stack ID#1 ->SF#1 ->SF#2
10.0.1.1
->Stack ID#2
... ... ...
//////////////////////////////////////////////////////////////////////
*Condition of Packet*
Locate: [CF] [FWD] [SF#1] [FWD]
+-------+ +-------+ +-------+ +-------+
Tag: | ID#1 | | ID#1 | | ID#1 | | ID#1 |
+-------+ +-------+ +-------+ +-------+
Packet:| PDU | | PDU | | PDU | | PDU |
+-------+ +-------+ +-------+ +-------+
Figure 3: Forwarding Based on Service Chain Identifiable Tags
3.2. Service Path Selection Patterns
Since SC contains only logical information (e.g. series of services
that are applied to flows and their sequences), the actual instances,
which are called SPs, are needed in order for the forwarding process
to work. In this process, an instance of SP is created at certain
points during a packet's delivery. Therefore, to forward packets,
the SC needs to be turned into an SP, which indicates specific FWDs
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(or switches, routers) and SFs that the packets will be forwarded to.
From the perspective of points translating SC to SP, the methods that
establish SPs from end to end are classified into two patterns.
3.2.1. Pattern 1: Static Selection of End to End Service Path
The translation point is only a CF; that is, the SP is statically
pre-established as an end-to-end path and a CF inserts packets into
the appropriate path based on the result of the classification. Each
FWD on the route has a forwarding table to uniquely determine the
next destination of packets, and each FWD statically forwards the
received packets to the next destination based on the table. FWD
requires only a function to receive indications of forwarding
configurations from the CE. Pattern 1 can be achieved in the
following ways.
3.2.1.1. SF Shared Model
Figure 4 shows the mechanism of this model. In this model, an SF is
shared by multiple SPs. Therefore, FWDs require a function to
identify SP for each packet and insert the packets into the next
appropriate hop.
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*Path Structure*
+----+ +---+ +----+ +---+ +------+ +---+ +----+
| |SC#1 |FWD| |SF#1| |FWD| |SF#2_1| |FWD| |SF#3| SP#1
| |==============================================================>
| |SC#2 | | | | | | +------+ | | | | SP#2
| |============================# +------+ #======================>
| | | | +----+ | | # |SF#2_2| # | | +----+
| | | | | | #==========# | |
->| CF | +---+ +---+ +------+ +---+
| |
. .
. .
. .
+---+ +----+ +---+ +----+
| |SC#n |FWD| |SF#4| |FWD| |SF#5| SP#n
| |==============================================================>
+----+ +---+ +----+ +---+ +----+
SC:Service Chain SP:Service Path
///////////////////////////////////////////////////////////////////////
*Packet Flow*
Service Chain#1:
SP#1
[ CF ]---->[FWD]-->[SF#1]-->[FWD]-->[SF#2_1]-->[FWD]-->[SF#3]--->
Service Chain#2:
SP#2
[ CF ]---->[FWD]-->[SF#1]-->[FWD]-->[SF#2_2]-->[FWD]-->[SF#3]--->
:
Service Chain#n:
SP#n
[ CF ]---->[FWD]-->[SF#4]--------------------->[FWD]-->[SF#5]--->
Figure 4: SF Shared Model
3.2.1.2. SF Dedicated Model
Figure 5 shows the mechanism of this model. In this model, an SF
instance (or a set of SF instances) is used by only one single SP; in
other words, a set of SF instance is prepared for each SP. At each
FWD, incoming packets are statically forwarded to the single
predefined next hop.
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*Path Structure*
+----+ +---+ +------+ +---+ +------+ +---+ +------+
| |SC#1 |FWD| |SF#1_1| |FWD| |SF#2_1| |FWD| |SF#3_1| SP#1
| |=============================================================>
| | +---+ +------+ +---+ +------+ +---+ +------+
| | +---+ +------+ +---+ +------+ +---+ +------+
| |SC#2 |FWD| |SF#1_2| |FWD| |SF#2_2| |FWD| |SF#3_2| SP#2
| |=============================================================>
->| CF | +---+ +------+ +---+ +------+ +---+ +------+
| |
. .
. .
. .
+---+ +------+ +---+ +------+
| |SC#n |FWD| | SF#4 | |FWD| | SF#5 | SP#n
| |=============================================================>
+----+ +---+ +------+ +---+ +------+
SC:Service Chain SP:Service Path
//////////////////////////////////////////////////////////////////////
*How packets traverse*
Service Chain#1:
SP#1
[ CF ]--->[FWD]-->[SF#1_1]->[FWD]->[SF#2_1]->[FWD]->[SF#3_1]--->
Service Chain#2:
SP#2
[ CF ]--->[FWD]-->[SF#1_2]->[FWD]->[SF#2_2]->[FWD]->[SF#3_2]--->
:
Service Chain#n:
SP#n
[ CF ]--->[FWD]-->[ SF#4 ]------------------>[FWD]->[ SF#5 ]--->
Figure 5: SF Dedicated Model
3.2.2. Pattern 2: Dynamic Selection of Segmented Service Path
The mechanism of this pattern is shown in Figure 6. The translation
points are CFs and some FWDs. The SP is established by a series of
multiple paths, which are sectioned by CFs and FWDs. The path, which
is sectioned by CFs and FWDs, is referred to as a segmented path in
this draft. CFs or FWDs that select the next segmented path may
require notification of forwarding configurations from the CE.
Moreover, some FWDs require functions to select the destination of
packets from various alternatives and to retrieve the information for
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selecting the next path. For example, each FWD obtains metric
information or load conditions of servers and selects an optimal
segmented path based on the information. The CE may have the
selection mechanism and may notify CFs or FWDs of it.
*Path Structure*
+----+ +---+ +----+ +---+ +------+ +---+ +----+
| |SC#1 |FWD| |SF#1| |FWD| |SF#2_1| |FWD| |SF#3| SP#1
| |========================*=====================================>
| | | | | | | # | +------+ | | | | SP#2
| | | | | | | # | +------+ #======================>
| | | | +----+ | # | |SF#2_2| # | | +----+
| | | | | #==============# | |
->| CF | +---+ +---+ +------+ +---+
| |
. .
. .
. .
+---+ +----+ +---+ +----+
| |SC#n |FWD| |SF#4| |FWD| |SF#5| SP#m
| |==============================================================>
+----+ +---+ +----+ +---+ +----+
SC:Service Chain SP:Service Path
///////////////////////////////////////////////////////////////////////
*How packets traverse*
Service Chain#1:
SP#1
[ CF ]---->[FWD]-->[SF#1]-->[FWD]-->[SF#2_1]-->[FWD]-->[SF#3]--->
SP#2
[ CF ]---->[FWD]-->[SF#1]-->[FWD]-->[SF#2_2]-->[FWD]-->[SF#3]--->
:
Service Chain#n:
SP#m
[ CF ]---->[FWD]-->[SF#4]--------------------->[FWD]-->[SF#5]--->
Figure 6: Dynamic Selection of Segmented Service Path
In addition, this pattern accepts establishment of hierarchical
domains as following:
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3.2.2.1. Hierarchical Service Path Domains
Complex problems often become manageable with a hierarchical
approach. This pattern allows network-wide orchestration of Service
Chaining to be relatively simple, while hiding the complexities of
fine-grained policy-based path selection within sub-domains. Each
sub-domain can be independently administered and orchestrated. This
architecture is described in [I-D.dolson-sfc-hierarchical].
Figure 7 shows two levels of hierarchy in a service provider's
network. At the top level in the hierarchy, Service Chaining
components are:
1. Edge-classifiers (Edge CF) that reside near the edge of a service
provider's domain and
2. SF sub-domains that reside in data centers.
3. SF Domain Gateways that reside in data centers, linking together
the levels of the hierarchy. To the higher level, this is an SF.
To the lower level, this is a classifier and FWD.
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*How packets traverse*
+----+ +-----+ +----------------------+ +-----+
| |SC#1| FWD | |SF Domain Gateway#1 | | FWD |
->| |================* *=====================>
| | +-----+ | # (in DC#1) # | +-----+
| | | V # |
|Edge| |+---+ +---+| Top domain
| CF | * * * * *||CF | * * * * * *|FWD|| * * * * *
| | * |+---+ +-+-+| *
| | * | | | | | | Sub *
| | * +-o-o--------------o-o-+ domain*
| | * SC#1.2 | |SC#1.1 ^ ^ #1 *
| | * +-----+ | | | *
| | * | V | | *
| | * | +---+ +------+ | | *
| | * | |FWD|->|SF#1_1|--+ | *
| | * | +---+ +------+ | *
| | * V | *
| | * +---+ +------+ +---+ +------+ *
| | * |FWD|->|SF#1_2|->|FWD|->|SF#2_1| *
| | * +---+ +------+ +---+ +------+ *
. * * * * * * * * * * * * * * * * * * * * * *
.
| | +-----+ +---------------------+ +-----+
| |SC#n| FWD | | SF Domain Gateway#q | | FWD |
| |=======================================================>
| | +-----+ | (in DC#m) | +-----+
+----+ +---------------------+
(Details of sub-domain #q not shown)
Figure 7: Service Chain Hierarchy in Service Provider Network
The components within an SF sub-domain are opaque at the top level;
each SF domain gateway acts as a single SF node in the top-level
domain. A service path in the top-level domain may visit multiple
sub-domains.
At the lower level in the hierarchy, each sub-domain contains an
independently administrated Service Chaining network, generally
comprised of multiple instances of multiple types of hosts, most
likely (but not necessarily) within the same data center. There is
no need for knowledge of the "big picture" at the level of the SF-
sub-domain except as required to forward packets to the other SFs
that are the next hop of each chain.
Note that different encapsulation methods can be used at each layer
in the hierarchy, provided the SF domain-Proxy can translate between
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them. For example, MPLS could be used to deliver packets from
network edge to the SF clusters within data centers, and NSH
[I-D.ietf-sfc-nsh] could be used within the data center.
Details of Top Level of Hierarchy
In this pattern, referring to Figure 8, network-wide Service Chaining
orchestration is only concerned with creating service paths from
network edge points to sub-domains within data centers and
configuring classifiers at a coarse level to get the correct hosts'
traffic onto paths that will arrive at appropriate sub-domains. The
figure shows one possible service chain passing from edge, through
two sub-domains, to network egress.
This top level of orchestration may attach meta-data to provide
context from the network edge into the data center.
+------------+
|Sub-domain#1|
| in DC1 |
+----+-------+
|
.------+---------. +--+
+--+ / / | \--|CF|
--->|CF|--/---->' | \ +--+
+--+ / SC#1 | \
| | |
| | .------>|--->
| / / |
\ | / /
+--+ \ | / / +--+
|CF|---\ V / /---|CF|
+--+ '------+---------' +--+
|
+----+-------+
|Sub-domain#2|
| in DC2 |
+------------+
Figure 8: Network-wide view of Top Level of Hierarchy
The orchestration at this top level must ensure bidirectional path
symmetry so that inbound packets traverse sub-domains in the reverse
order as outbound packets.
Because classifiers must have rules to handle any traffic passing
through the network, we believe that a useful approach to
classification will be to assign traffic to service function paths on
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the basis of coarse classification like subscriber tier, tenant or
VRF identifier. These classification rules could be relatively
static, changing in response to provisioning but not in response to
traffic.
In some networks it might be possible to create a rule per
residential subscriber, resulting in rule updates when subscribers
are assigned IP addresses. However, with judicious allocation of IP
blocks, entire classes of subscribers could be classified with IP-
prefix rules. Similarly, in a mobile network path selection could be
based on APN.
Hence, there are methods of globally managing very large networks by
choosing a suitable classification granularity.
Details of Lower Level of Hierarchy
Within each SF sub-domain, there are:
1. An SF domain-gateway to receive incoming data packets on any of
the configured service chains and load-balance (if necessary)
traffic to classifiers,
2. Classifier(s) to select internal service chain to use,
potentially based on stateful flow analysis, DPI, etc.
3. Service components comprised of FWD and SF.
Local Service Chaining orchestration is concerned with providing
viable paths to various functions, providing failure recovery, NFV
elasticity, etc.
Classification within each sub-domain can be concerned with
determining the local service paths for individual transport-layer
flows based on ports, DPI and meta-data provided by the higher-level
chain.
For any classifier that is transport-layer-stateful, it is most
efficient for the same classifier instance to handle traffic in both
directions of a bidirectional connection. State tracking may require
that service function paths begin and end at the same node with the
flow state, where the same classifier instance can be used for both
directions of traffic.
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4. Consideration of Forwarding Methods and Paths Selection Patterns
This chapter presents the results of analyzing the forwarding methods
and architecture patterns in chapter 3.
4.1. Analysis of 3.1. Forwarding Methods
4.1.1. Analysis of Method 1
Data Plane Aspects
This method can achieve Service Chaining without changing packet
format, such as attaching any header on packets, so it may not
cause any increase in packet size or be subject to MTU
restrictions. Furthermore, this method does not require
additional functions for SFs to apply or handle any header because
data packets are transported in original format. Therefore, it
will be easier to use legacy SFs for network operators.
On the other hand, it is difficult to forward a packet to same
FWDs several times because flow identifiable information is not
basically chainged in the forwarding processes. For example,
distinction of incoming ports will be required for FWD to decide
the next hop appropriately when a packet traverse it several
times.
Control Plane Aspects
This method requires FWDs to set forwarding entries for each flow.
For example, if there are 10,000 flows to be handled at a CF/FWD,
the forwarding table for each CF/FWD uses 10,000 flow entries at
most. Therefore, it might not be feasible for large-scale
networks such as carrier networks that handle a SC per user (which
means that individual users have their own policies), because some
large carriers have over a million users and even more flows.
Another concern is increase of control signaling because route
setting is required for each flow. Moreover, it may be hard to
use this method if some SFs modify header fields of a packet or
frame, for example, NAT/NAPT, in a chain. For example, if a NAT
changes the IP address of packets dynamically, the FWDs that
follow need to renew their forwarding tables.
The results of the above analysis suggest that, although this method
is beneficial in terms of impact to existing network, it would not be
scalable. Therefore, this method might be suitable for networks with
a limited number of flows.
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Measurements taken in multiple residential service providers'
networks indicate that for each 1Gbps of traffic the sustained rate
of new flows can range from 1,000 flows/s to 30,000 flows/s. From
this, for example, there would be between 10,000 and 300,000 new
flows/s on a 10 Gbps link. Therefore, in some networks at some times
of day, this method using 5-tuple as flow identifiable information
would require sustaining up to 300,000 table updates per second for
each FWD. This incurs a significant amount of control traffic and
computational effort.
4.1.2. Analysis of Method 2
Data Plane Aspects
In this method, SP information is attached on each packet as
transport headers, and the number of the headers increases
depending on the number of SFs which the packet will traverse.
This means that size of each packet increases. Packet sizes may
be restricted by the minimal available MTU of any link in the
network and exceeding the MTU will require to fragment the
original packets. Fragmentation adds a new source of errors and
may require forwarding processes to be more complex. For example,
the whole original packet will get discarded even if one of
fragments of the packet gets lost, or in terms of SF equipment, it
would be very wasteful of CPU if fragmented packets need to be
reassembled at every SF resources, and some equipment has
restricted resources and memory for reassembly. Fragmentation
will also cause an increase in traffic as more packets have to be
processed by the network.
Moreover, this method requires SF to be applied to the headers
because they receive packets with optional headers. Therefore SFs
will be required to be able to recognize the headers, or proxy
functions, which remove the tags before inserting packets into SFs
and reattaches the appropriate tag on the returned packet, will be
required. In addition, when a SF is used by multiple SCs, it will
be challenging for SFs to process packets because header length
attached on each packet may vary and SFs are required to have a
mechanism to recognize the header length for each packet.
Control Plane Aspects
In this method, none of the FWDs require any specific forwarding
tables for Service Chaining or interface to receive indications of
forwarding configuration. Also, no CEs will be required to manage
the forwarding configuration of FWDs, so the control plane might
become simple.
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On the other hand, some relay nodes such as switches or SFs are
required to have a function to remove the outermost header from
the received packets. FWDs also don't have identify flow or
service so can not change the following SPs. Moreover, CF must
grasp all of addresses of relay nodes which packets will traverse,
and it will require any CE to manage addresses of relay nodes and
a link between CF and the CE. There are already several
technologies proposed that can be used to achieve this method,
such as segment routing.
The results of the above analysis indicate that this method would be
appropriate when the number of SFs in a SC is small, and most SFs are
deployed in a single domain. On the other hand, it may be unsuitable
in cases where there are many SFs in a chain, or packets have to
traverse multiple domains.
4.1.3. Analysis of Method 3
Data Plane Aspects
In this method, a tag is defined for each SC and attached on each
packet. By adopting single fixed-length tag, this method can
prevent an increase in the amount of traffic, and can provide an
upper bound on packet size. (Problems which happen as a result of
exceeding MTU are stated in Section 4.1.2.) Also, FWDs recognize
the next hops of received packets from the tags independent of any
information of original packets. Therefore, SFs which modify
original packet format can be also used. In addition, it is easy
to change the following SPs on a route by renewing the tag.
On the other hand, this method requires SFs to be applied to the
tags because SFs receive packets with the tags. (Problems which
happens as result of inserting packet with optional tags into SF
are stated in Section 4.1.2) By using existing transport headers
as the tags or outer header for forwarding, effect on network
nodes such as existing router and switches might be restrained.
Control Plane Aspects
This method enables FWDs to save resources for managing forwarding
tables and all SPs may be established in advance in most of cases.
This prevents an increase of control signals, and also enables to
change the following SPs without changing forwarding
configurations of FWDs.
On the other hand, this method requires a new control mechanism
based on the tags, therefore, FWDs, CE and interface between them
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have to be updated to apply forwarding configuration based on the
tags.
The results of the above analysis indicate that this method has many
advantages in terms of scalability, and it might be appropriate for
use in large-scaled networks in which there are many SFs and flows.
By the way, if the tag handling mechanism is an entirely new
architecture such as SFC[I-D.ietf-sfc-architecture], renewal or
introduction of several equipment such as FWDs and CE will be
required.
4.2. Analysis of 3.2. Service Paths Selection Patterns
4.2.1. Analysis of Pattern 1
In this pattern, the mechanism of FWDs would be simpler than the one
in pattern 2 because FWDs do not require any functions to select
paths or retrieve any information for determination of the next hop.
Moreover, it is not necessary to maintain the state of each flow.
Therefore, existing protocols for virtualizing networks, such as
VxLAN or MPLS, can be used to achieve Service Chaining in this
pattern.
However, this pattern will impact the flexibility of the SCs, as
adding new SFs to a SC, removing SFs from a SC, or migrating SFs to
other locations requires an update or new creation of a path in the
Service Path. Furthermore, unified management of FWDs and SFs in an
SC domain would be required in setting end-to-end paths. Therefore,
the management system of SPs, for example, a CE, for wide-area
networks that include several segments may be massive and complex.
Figure 9 shows the case in which SPs are established across multiple
datacenters in pattern 1. In Figure 9, a CE manages multiple
datacenters as a single SC domain for establishing SPs across
multiple datacenters.
In pattern 4.2.1.2 (SF Dedicated Model), the number of flow entries
that FWDs hold can be extremely small, as FWDs hold only static next-
hop information. Also, the CF function would be simple, as the CF
only determines the gateway of each SP. However, because the SF
(instance) is settled for each SP, resource usage would be high if
there were many SPs.
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+--------------+
. . . . . . . . . . |Control Entity| . . . . .
. . +--------------+ .
. . .
* * . * * * * . * * * * * * * * * * * * * * * * . * * * * * * * * *
* . . . *
* . . . *
* . .-----. .-----------. .-----. *
* +----+ / DC#1 \ / WAN \ / DC#2 \ *
* | |=====================================================> SP#1 *
* | CF |=====================================================> SP#2 *
* : : : *
* | |=====================================================> SP#n *
* +----+ \ / \ / \ / *
* '-----' '-----------' '-----' *
* *
* SC Domain *
* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
Figure 9: Establishment of SPs Across Multiples DCs in Pattern 1
4.2.2. Analysis of Pattern 2
In this pattern, SPs are established with a combination of segmented
paths, so it enables SPs to be established flexibly (which means, CEs
do not need to constantly manage the entire end-to-end SP) based on
additional information such as the load condition of SFs.
Furthermore, as it is described in the previous section, in cases
where some SPs traverse multiple datacenters across a WAN, SPs could
be established with a combination of segmented paths that each
datacenter determines independently based on the Service Chain
information. Therefore, it might be possible to separate SC domains
into several small areas for WANs, which would enable a simpler
configuration of each CE. Figure 10 shows the case in which SPs are
established across multiple datacenters in pattern 2. In Figure 10,
each CE manages a single datacenter independently, and the CEs
synchronize the Service Chain information for establishing and
determining the appropriate segmented SPs in each domain.
However, the (fault) monitoring of the whole SC can get harder as
multiple domains are part of the SC. On the other hand, each domain
can perform its fault management as required (and probably better as
it is more specific). This will require an overarching (fault)
monitoring where information from multiple SC domains is collected
and aggregated to get a full view of the end-to-end service of the
SC.
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Moreover, in this pattern, some FWDs may require additional
mechanisms to select the next segmented path, and the FWDs must
maintain the states of each flow because some SFs require a stateful
process, and the FWDs need to insert packets into the same SF
instances in the same session.
In case that SC information is conveyed to some components via data
plane as any encapsulation, a new protocol such as SFC
[I-D.ietf-sfc-architecture] will be required.
Synchronization of
Service Chain info.
+--------------------------------------+
| |
v v
+--------+ +--------+
| CE#1 | | CE#2 |
+--------+ +--------+
. .
* * * * * * . * * * * * * * * * * * * . * * * * * *
* . * * . *
* .-------------. * * .------------. *
* / DC#1 \ * .------. * / DC#2 \ *
* +----+ +-----+ * / WAN \ * +-----+ | *
* | |=========>| | * | | * | CF/ |==========> SP#1 *
* | CF |=========>| FWD |===============>| FWD |==========> SP#2 *
* : : : * | | * : : : *
* | |=========>| | * \ / * | |==========> SP#n *
* +----+ +-----+ * '------' * +-----+ | *
* \ / * * \ / *
* '-------------' * * '-----------' *
* SC Domain#1 * * SC Domain#2 *
* * * * * * * * * * * * * * * * * * * * * * * * * * * *
Figure 10: Establishment of SPs Across Multiples DCs in pattern 2
Also, the detail analysis of establishment of "Hierarchical Service
Path domains" is shown in the following section.
4.2.2.1. Analysis of Hierarchical Service Path domains
The dynamic selection of SPs pattern allows multiple independent
domains of administration. (In the example, two levels were shown,
but the pattern could be extended to multiple levels.)
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This pattern allows even the largest networks to implement SC from
the edges of the network by using coarse-grained classification.
Classification choices can be made that are feasible within the
constraints of the edge classifiers and FWDs. There is no need to
maintain flow state or react to traffic at the top level.
This pattern allows control of sub-domains to be delegated to
different owners. Each domain is simpler to comprehend than would be
the case by dealing with a single flat network. Furthermore,
failures and errors are localized. (See Figure 11.)
+----------+
|Top-level | . . . . . . . . . . . . . . . . . . . . .
|Control | .
|Entity | +------------+ +--------+ .
+----------+ |sub-domain#1|. . .| CE#1 | .
. +-----+------+ +--------+ .
. | .
. .------+---------. +---+ .
. +---+ / \--|CF |. . . .
. . . .|CF |--/ \ |FWD| .
. |FWD| / \+---+ .
. +---+ | | .
. | | .
. | | .
. +---+ \ / .
. |CF | \ / +---+ .
. . . .|FWD|---\ /---|CF | . . .
+---+ '------+---------' |FWD|
| +---+
+--------+ +------------+
| CE#2 |. . .|sub-domain#2|
+--------+ +-----+------+
Figure 11: Multiple Control Entities in Hierarchical Service Chaining
This hierarchical model supports management of large networks by
adhering to these principles:
1. At higher levels of hierarchy packet classification is coarse, to
minimize state and control-plane chatter.
2. At lower levels of hierarchy packet classification can be more
granular because classifiers in the lower levels deal with a
subset of the entire network: fewer flows, lower bit-rate and a
subset of network policy.
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However, in this model, a new component that can proxy between the
different domains, termed "SF Domain Gateway," will be required. It
has some commonality with the legacy SF proxy discussed in
[I-D.song-sfc-legacy-sf-mapping].
This model also requires some coordination of path information within
the SF Domain Gateway component, since the gateway must map packets
back and forth between domains. Solving this probably requires
sharing metadata dictionaries among controllers and inventing a
scheme that provides a level of indirection by naming path
identifiers and metadata values.
4.3. Example of selecting Methods and Patterns
In this section, clarifications about the most suitable method and
pattern are made for the following example networks based on the
results of the above analysis.
4.3.1. Example#1: Enterprise Datacenter Network
The conditions of the target network are as follows:
Network type: Network with a single DC.
Intended service: For providing several network service to traffic
of one or several business offices.
Variation of service: A group of adopting network service varies per
office.
The number of SFs included in a service chain: Less than 5 (ref.
section 3.2.1. Sample north-south service function chains in
[I-D.ietf-sfc-dc-use-cases]).
Features of SFs: SFs are set statically, and SFs are exclusively
used for each service.
On the basis of the conditions "network type" and "features of SFs",
pattern 1 with SF dedicated model would be selected.
As the condition "variation of service" describes, such network
requires few flow entries for each FWD, so method 1 would be
applicable. Method 1 also does not require SFs to have any
additional mechanism to apply any header, thus the impact of
implementing this method would be smaller than other methods.
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4.3.2. Example#2: Current Mobile Service Providers Network
The conditions of the target network are as follows:
Network type: Network with a single DC (e.g., (S)Gi-LAN (3GPP,
[TS.23.203])).
Intended service: For providing network access service and several
network service to traffic of millions customers.
Variation of service: Service varies per user or applications.
The number of SFs included in a service chain: Around 5(ref.
examples of service in [I-D.ietf-sfc-use-case-mobility].).
Features of SFs: Many SFs are hardware equipment and they are set
statically. Also, many SFs are used for several service. A
function to inspect the user traffic in detail, such as TDF (3GPP,
[TS.23.203]), is located around the edge of the network, and it
might behave as a CF.
On the basis of the conditions "network type" and "features of SFs,"
pattern 1 with SF shared model would be selected. In such network,
classification based on deep packet inspection such as application
type inspections is done, and paths branching will not be happen.
As the other conditions describe, the operator must handle millions
of flows and the flows traverse multiple SFs, so method 3 would be
applicable. Configuring such amounts of flows among large scale
network might be too much work for operators.
The examples of concrete service of such network are described as
follows:
1. HTTP Modification
Packet Gateway(P-GW), which is defined in 3GPP (ref. [tS.23.203]),
detects traffic to the specific website and that traffic must be
sent through a special element to insert additional data to the
http header or advertisement to the HTTP traffic, so the
destination site can apply specific deals with the operator's
customer (simplify DRM, premium service, etc.) That would require
flow entries with mobile source IP, destination IP and port.
2. VoLTE Calls
VoLTE calls are sent via a special SP. The VoLTE control plane
selects all application network elements. But to reach
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application network elements it fully relies on standard routing
and switching protocols. With Service Chaining it is possible to
select the SP which can provide required QoS. That would require
to set flow entries with mobile source IP, destination IP and
port.
3. Secure Internet Access
Some customers' HTTP traffic are forwarded to one or more security
functions to inspect for malware. This case would require flow
entries with source IP, destination IP and port.
4. Content Optimizer
Based on the policy rules, a SC/SP with the content optimization
might be provided. Content optimization primarily affects video
and HTTP traffic, and saves valuable radio resources in the
specific radio cells during times of congestion. A controller
might monitor Key Performance Indicators (KPIs) of the radio
network to detect congestion. When congestion is detected, the
controller might apply content optimization policy for the users
on the congested radio cell. Most resource-expensive traffic can
be transcoded by a content optimizer to save bandwidth. Selecting
traffic for optimization would require to set flow entries with
mobile source IP, destination IP and port. Also, content
optimization might require changing SCs/SPs assigned to users
flows based on the result of KPI monitoring or the time of day.
On the other hand, method 1 might be also selected with pattern 1
with SF dedicated model. For example, the series of the above
service might be achieved by static configured flow entries, for
example, with incoming port. However, it will require many incoming
ports for FWDs when the operator would like to share a SF with
multiple SCs, and it will not be scalable.
4.3.3. Example#3: Fixed and Mobile Converged Service Providers Network
The conditions of the target network are as follows:
Network type: Network with multiple DCs (e.g., SFs are deployed at
multiple DCs based on their applications).
Intended service: For providing network access service or several
network service to traffic of millions customers.
Variation of service: Service varies per user. Also, the service
assigned to each flow might vary based on using applications.
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The number of SFs included in a service chain: More than 5.
(Various service such as enriched security service and value added
service would be provided)
Features of SFs: Many SFs are deployed as vNF, and some SFs are
shared with multiple SCs. Also, some SFs changes the following
SPs dynamically based on the result of the process.
On the basis of the conditions "network type" and "features of SFs,"
pattern 2 would be selected. Pattern 2 allows hierarchical approach
which enables operators to deploy SFs in multiple domains easily
based on service requirements. For example, operators can deploy SFs
into several domains based on application types. This concept is
introduced in [I-D.ietf-sfc-dc-use-cases].
From the above conditions describe, the operator must handle enormous
flows and paths branching, thus method 3 will be appreciable for such
network. Especially, security scenario sometimes requires paths
branching based on the result of packet inspection such as processes
of DPI or traffic analyzer. Some security functions such as web
application firewall (WAF) are specialized for each application, and
it might be inefficient to insert all traffic into such SFs.
Therefore, for inserting only target packets to appropriate security
functions, classifying and paths branching based packet inspection
would be required.
5. Acknowledgements
The authors would like to thank Konomi Mochizuki and Lily Guo for
their reviews and comments.
6. Contributors
The following people are active contributors to this document and
have provided review, content and concepts (listed alphabetically by
surname):
Chuong D. Pham
Telstra
Hiroshi Dempo
NEC
Ron Parker
Affirmed Networks
Paul Quinn
Cisco Systems
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7. IANA Considerations
This memo includes no request to IANA.
8. References
[I-D.dolson-sfc-hierarchical]
Dolson, D., Homma, S., and D. Lopez, "Hierarchical Service
Chaining", draft-dolson-sfc-hierarchical-00 (work in
progress), May 2015.
[I-D.ietf-sfc-architecture]
Halpern, J. and C. Pignataro, "Service Function Chaining
(SFC) Architecture", draft-ietf-sfc-architecture-08 (work
in progress), May 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.
[I-D.ietf-sfc-use-case-mobility]
Haeffner, W., Napper, J., Stiemerling, M., Lopez, D., and
J. Uttaro, "Service Function Chaining Use Cases in Mobile
Networks", draft-ietf-sfc-use-case-mobility-03 (work in
progress), January 2015.
[I-D.song-sfc-legacy-sf-mapping]
Song, H., You, J., Yong, L., Jiang, Y., Dunbar, L.,
Bouthors, N., and D. Dolson, "SFC Header Mapping for
Legacy SF", draft-song-sfc-legacy-sf-mapping-04 (work in
progress), December 2014.
[RFC3022] Srisuresh, P. and K. Egevang, "Traditional IP Network
Address Translator (Traditional NAT)", RFC 3022, January
2001.
[RFC6146] Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful
NAT64: Network Address and Protocol Translation from IPv6
Clients to IPv4 Servers", RFC 6146, April 2011.
[RFC6296] Wasserman, M. and F. Baker, "IPv6-to-IPv6 Network Prefix
Translation", RFC 6296, June 2011.
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[RFC7498] Quinn, P. and T. Nadeau, "Problem Statement for Service
Function Chaining", RFC 7498, April 2015.
Authors' Addresses
Shunsuke Homma
NTT, Corp.
3-9-11, Midori-cho
Musashino-shi, Tokyo 180-8585
Japan
Phone: +81 422 59 3486
Email: homma.shunsuke@lab.ntt.co.jp
Kengo Naito
NTT, Corp.
3-9-11, Midori-cho
Musashino-shi, Tokyo 180-8585
Japan
Email: naito.kengo@lab.ntt.co.jp
Diego R. Lopez
Telefonica I+D.
Don Ramon de la Cruz, Street
Madrid 28006
Spain
Phone: +34 913 129 041
Email: diego.r.lopez@telefonica.com
Martin Stiemerling
NEC Laboratories Europe / Hochschule Darmstadt
Kurfuerstenanlage 36
Heidelberg 69115
Germany
URI: ietf.stiemerling.org
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David Dolson
Sandvine
408 Albert Street
Waterloo, Ontario N2L 3V3
Canada
Email: ddolson@sandvine.com
Alexey Gorbunov
Nokia
6000 Connection Drive
Irving, Texas 75039
USA
Phone: +1 214 516 11 41
Email: Alexey.gorbunov82@gmail.com
Nicolai Leymann
DT
Winterfeldtstrasse 21-27
Berlin 10781
Germany
Phone: +49 (0)30 835392761
Email: n.leymann@telekom.de
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