Internet DRAFT - draft-purkayastha-sfc-service-indirection
draft-purkayastha-sfc-service-indirection
Network Working Group D. Purkayastha
Internet-Draft A. Rahman
Intended status: Informational D. Trossen
Expires: September 2, 2018 InterDigital Communications, LLC
Z. Despotovic
R. Khalili
Huawei
March 1, 2018
Alternative Handling of Dynamic Chaining and Service Indirection
draft-purkayastha-sfc-service-indirection-02
Abstract
Many stringent requirements are imposed on today's network, such as
low latency, high availability and reliability in order to support
several use cases such as IoT, Gaming, Content distribution, Robotics
etc. Networks need to be flexible and dynamic in terms of allocation
of services and resources. Network Operators should be able to
reconfigure the composition of a service and steer users towards new
service end points as user move or resource availability changes.
SFC allows network operators to easily create and reconfigure service
function chains dynamically in response to changing network
requirements. We discuss a use case where Service Function Chain can
adapt or self-organize as demanded by the network condition without
requiring SPI re-classification. This can be achieved, for example,
by decoupling the service consumer and service endpoint by a new
service function proposed in this draft. We describe few
requirements for this service function to enable dynamic switching
between consumer and end point.
Status of This Memo
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This Internet-Draft will expire on September 2, 2018.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Use Case Description . . . . . . . . . . . . . . . . . . . . 3
2.1. Data Center . . . . . . . . . . . . . . . . . . . . . . . 3
2.2. Third party cloud service provider . . . . . . . . . . . 4
2.3. ETSI MEC USE CASE . . . . . . . . . . . . . . . . . . . . 5
2.4. 3GPP . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.5. Use Case Analysis . . . . . . . . . . . . . . . . . . . . 6
3. NSH and Re-classification . . . . . . . . . . . . . . . . . . 8
3.1. Dynamic service chain creation using NSH . . . . . . . . 9
4. Challenges with dynamic indirection . . . . . . . . . . . . . 10
5. HTTP as a transport . . . . . . . . . . . . . . . . . . . . . 12
6. Service Request Routing (SRR) Service Function . . . . . . . 14
6.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 14
6.2. Details of SRR Function . . . . . . . . . . . . . . . . . 16
7. Protocol Consideration . . . . . . . . . . . . . . . . . . . 21
8. Next Steps . . . . . . . . . . . . . . . . . . . . . . . . . 21
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21
10. Security Considerations . . . . . . . . . . . . . . . . . . . 22
11. Informative References . . . . . . . . . . . . . . . . . . . 22
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 23
1. Introduction
The requirements on today's networks are very diverse, enabling
multiple use cases such as IoT, Content Distribution, Gaming, Network
functions such as Cloud RAN. Every use case imposes certain
requirements on the network. These requirements vary from one
extreme to other and often they are in a divergent direction.
Network operator and service providers are pushing many functions
towards the edge of the network in order to be closer to the users.
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This reduces latency and backhaul traffic, as user request can be
processed locally.
It becomes more challenging when network congestion, user mobility as
well as non-deterministic availability of compute and storage
resources are considered. The impact is felt most in the edge of the
network because as the users move, their point of attachment changes
frequently, which results in (at least partially) relocating the
service as well as the service endpoint. Furthermore, network
functions are pushed more and more towards the edge, where network,
compute and storage resources are constrained and availability is
non-deterministic. Constrained network resources may lead into
congestion in the network. Also, storage resources may need to be
moved where the user concentration is more in case of content
delivery applications.
We describe few use cases in the next section and derive the
requirement for composing new services and service path in a dynamic
edge network. We address this dynamicity by introducing a special
Service Function, called SRR (service request routing). We describe
the problems associated with today's network and Layer 3 based
approach to handle dynamicity in the network. We then discuss how
such new Service Function with certain capabilities can handle the
dynamicity better than these conventional methods.
2. Use Case Description
2.1. Data Center
The data center use case draft [I-D.ietf-sfc-dc-use-cases] describes
an East West traffic use case. This is the predominant traffic in
data centers today. Server virtualization has led to the new
paradigm where virtual machines can migrate from one server to
another across the data center. This explosion in east-west traffic
is leading to newer data center network fabric architectures that
provide consistent latencies from one point in the fabric to another.
SFCs applied in an enterprise or service provider data center can be
broadly categorized into two types:
o Access SFCs
o Application SFCs
Access SFCs are focused on servicing traffic entering and leaving the
data center while Application SFCs are focused on servicing traffic
destined to applications. Service providers deploy a single "Access
SFC" and multiple "Application SFCs" for each tenant. Enterprise
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data center operators on the other hand may not have a need for
Access SFCs depending on the size and requirements of the enterprise.
In carrier networks, operators may deploy multiple data centers
dispersed geographically. Each data center may host different types
of service functions. For example, latency sensitive or high usage
service functions are deployed in regional data centers while other
latency tolerant, low usage service functions are deployed in global
or central data centers. In such deployments, SFCs may span multiple
data centers and enable operators to deploy services in a flexible
and inexpensive way.
It is clear that within the data center as well as in inter data
center scenarios, users are serviced by multiple SFs distributed
inside as well as outside a location. In this scenario, it is clear
that Service function chains should be able to reselect, redirect
traffic very fast. The draft identifies that Static service chains
do not allow for modifying the SFCs as they require the ability to
add SNs or remove SNs to scale up and down the service capacity.
Likewise the ability to dynamically pick one among the many SN
instance is not available.
2.2. Third party cloud service provider
This use case is related to an emerging business model, where
computational resources for edge cloud service are provided by
alternative facility providers that are non-traditional network
operators. This is due to the situation for many specific localized
use cases, where network operators may not have necessary real estate
available. They may even not be willing to spend on CAPEX and OPEX
for said point-of-presence, because there is no clear path for
sustainable cost recovery [UKNIC].
The industry is witnessing the emergence of real estate owners such
as building asset or management companies, cell tower owners, railway
companies or other facility owners willing to deploy edge cloud
resources. The facility provider, e.g. cell tower owner or building
management company, deploys edge computing resources throughout their
installation in the country. They have their own operation and
management software, which is capable of resource deployment, scale
up or scale down resources, deploy edge applications from third party
service providers . They are capable of offering service to more than
one network operator at a specific location, thus acting as a
"neutral host". The facility provider, which owns cloud resources
and provides application services, is referred to as "Third party
Edge Owner (TEO)".
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There is more than one stakeholder in this ecosystem, E.g. Network
Service Provider, Real estate owner, Cloud capability (compute and
storage resource) provider, Application/service provider. An entity
can assume more than one role. From network operators point of view
there may be "Cloud provider" or "Cloud service provider" depending
on the roles assumed by external entity.
"Cloud Providers" provide cloud resources (compute and storage) to
network operators. Network operators rent those resources and manage
MEC host by themselves. Network operator can set up application
traffic rules, so that traffic can be processed, by that host.
"Cloud Service Providers" not only make resources available to
network operators or service providers, but also provides management
and hosting service. They can host edge applications on behalf of
application service providers and sets up user plane traffic to be
steered towards the edge application.
Cloud Service Providers, as well as many organizations that need to
share and analyze a quickly growing amount of data, such as
retailers, manufacturers, telcos, financial services firms, and many
more, are turning to localized Micro Data Centers(MDC) installed on
the factory floor, in the telco central office, the back of a retail
outlet, etc. The solution applies to a broad base of applications
that require low latency, high bandwidth, or both.
As Micro Date centers are deployed at the edge of the network, common
deployment options are:
o Micro Data Centers are deployed on L2 in the edge of the network
o Instead of single internet Point Of Presence (POP) deployment,
multiple internet POP deployment is desirable to localize data
o Service is composed out of these multiple POP deployment of MDC,
where data exchange and collaboration is expected among these MDCs
o Due to mobility, changes in network condition (e.g. congestion,
load), service composition may change frequently to support
promised quality of experience
2.3. ETSI MEC USE CASE
Take the following video orchestration service example from ETSI MEC
Requirements document [ETSI_MEC]. The proposed use case of edge
video orchestration suggests a scenario where visual content can be
produced and consumed at the same location close to consumers in a
densely populated and clearly limited area. Such a case could be a
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sports event or concert where a remarkable number of consumers are
using their handheld devices to access user select tailored content.
The overall video experience is combined from multiple sources, such
as local recording devices, which may be fixed as well as mobile, and
master video from central production server. The user is given an
opportunity to select tailored views from a set of local video
sources.
2.4. 3GPP
3GPP Rel. 15 introduces the notion of the service-based interface
(SBI) as an alternative to the traditional call pattern invocation of
network functions. This introduction targets the support for
replication, e.g., driven by virtualized functions, as well as
supporting alternative interactions, e.g., for different vertical
market specific control planes, by making the discovery as well as
composition of new interactions more flexible.
We believe that SFC is a suitable framework for the interconnection
of such network functions through the new SBI. One of the
aforementioned driving forces, namely the replication of functions
aligns with our thinking in this draft in that indirections to new
vertical instances need to be dynamic in reacting to the appearance
of new virtual instances or to changes in policies for the selection
of specific instances by specific calling entities.
2.5. Use Case Analysis
SFC allows network operators as well as service providers to compose
new services by chaining individual service functions.
In a dynamic network environment, like the edge of a network, the
capability to dynamically compose new services from available
services as well as move a service instance is desirable. Dynamic
composition and relocation of services may be attributed to:
o Congestion in the network: Due to constrained network resources,
increase in the network load may create congestion in the network,
resulting in a congested Service Function Path. Service functions
may detect congestion and reconfigure the Service Function Path to
avoid it.
o In response to latency: in a dynamic network environment and with
the need for ultra-low latency communication, instantiation of new
service function endpoints might be the only remedy to combat the
increase of latency caused, e.g., by increased load on a previous
endpoint or mobility of the user and therefore increasing the
'distance' to the service function endpoint. Keeping the service
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function endpoint 'close' to the user allows for reducing latency,
segregating communication in localized islands of service
interaction.
o In response to user mobility: In a dynamic network environment
where service functions move frequently because of user movement,
load balancing or resource modification, service function chains
and the service end points need to be created and recreated
frequently
o Resource availability.: Availability of compute and storage
resources varies with network load, number and type of
applications running etc. In the edge of the network, due to
sudden increase of users, compute load may increase. In this
situation applications, running on the compute resources may be
moved to another location where more resources are available.
In SFC, there is a notion of logical chaining of SFs and chaining of
actual physical locations, known as Rendered Service Path (RSP). RSP
provides a static binding of SFs to their physical location. In
order to create a chain in dynamic fashion, late binding of SFs and
physical location may be desired. SFC is capable of modifying the
service chain to certain extent in response to network conditions,
but not a complete solution has been described
In order to route the service requests to service end points in a
dynamic manner, we identify the following desirable features in a
service function chain:
o Capability to trigger service chain reconfiguration based on
network information such as congestion indication, mobility,
degradation of user experience etc. Service Functions should be
able to process such network information, identify which section
of the chain needs to be reconfigured and take action
o Fast switching from one service instance to another by not relying
on the DNS for service location resolution. Instead of DNS, the
function should be able to identify the path, which will allow to
reach the service end point.
o Direct path mobility, where the path between the requester and the
responding service can be determined as being optimal (e.g.,
shortest path or direct path to a selected instance), is needed to
avoid the use of anchor points and further reduce service-level
latency
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o Indirect service requests at the network level, transparent to the
requesting client and without the involvement of the DNS. End
user is not aware of the decision made by the SF.
o New methods for forwarding, such as path-based forwarding, direct
path routing in mobility cases, path pinning for traffic steering
and simplified service-specific peering towards the Internet.
3. NSH and Re-classification
[RFC7498] captures the problems associated with existing service
deployments that are problematic. The problems are described below
at a high level:
o Network topology: Network service deployment is tightly coupled
with network topology thus reducing the flexibility in service
delivery. It adds complexity in deploying network service when
certain traffic types may need some service and other traffic
types do not need the same service.
o Configuration complexity is the direct result of dependency on
network topology.
o Limited availability of services
o Altering the order of a deployed chain is complex and cumbersome
o Coupling of service functions to topology may require service
functions to support many transport encapsulations or for a
transport gateway function to be present.
o In a dynamic environment like the Edge of a network service
delivery, routing changes fast. It may be difficult to deliver
service dynamically due to the risk and complexity of VLANs and/or
routing modifications.
These factors provide motivation for a simplified and flexible
service insertion model that addresses many of the current
shortcomings and provides new, much needed functionality to enable
service deployments in modern network environments. Service chaining
accomplishes this by considering service functions as resources, with
associated attributes, available for scheduled consumption.
Selective traffic, subject to policy, may then be "steered" to the
requisite service resources, along with any "extra" information
referred to as metadata. This metadata is used for policy
enforcement.
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A basic form of service chaining may be realized using existing
transport encapsulations. This method of chaining relies upon the
tunneling of selected data between service functions. Although this
form of service chaining achieves some level of abstraction from the
underlying topology, it does not truly create a service plane. NSH
[RFC8300] is a distinct identifiable plane that can be used across
all transports to create a service chain and exchange metadata along
the chain.
Fundamentally, however, the notion of "services" in SFC is tied into
specific service function endpoints, which lie along a well-defined
service function path (SFP) where the path is defined through lower
layer transport encapsulations. If any such service function
endpoint changes, the service chain needs to be adjusted; a procedure
we outline in the following sub-section.
3.1. Dynamic service chain creation using NSH
We revisit the dynamic service chain creation capability of NSH. NSH
defines a new service plane protocol [RFC8300]. A Network Service
Header (NSH) contains service path information and optionally
metadata that are added to a packet or frame and used to create a
service plane. A control plane is required in order to exchange NSH
values with participating nodes, and to provision the same nodes with
requisite information such as service path ID to overlay mapping.
The Network Service Header has three parts, Base header, Service Path
Header and Context Header. NSH Service Path Header is a 4-byte
service path header follows the base header and defines two fields
used to construct a service path:
o Service path identifier (SPI)
o Service index (SI)
The following figure depicts the service path header.
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 2
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Service Path ID | Service Index |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: NSH Path Header
The service path identifier (SPI) is used to identify the service
path that interconnects the needed service functions. It allows
nodes to utilize the identifier to select the appropriate network
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transport protocol and forwarding techniques. The service index (SI)
identifies the location of a packet within a service path. As
packets traverse a service path, the SI is decremented post-service.
SPI represents the service path and altering the path identifier
results in a change of a service path. A change in SPI value is a
result of re-classification. It means a node in the service path
determined, based on policy, that the initial classification was
incorrect or incomplete. If the updated classification results in
the necessity of a new service path, the node updates the SPI and SI
fields accordingly. The new identifier is then used to select the
appropriate overlay topology. This allows service functions to alter
the path of a packet without having to participate in the network
topology and its associated control plane(s). The method to
determine that an existing classification is incorrect and how to
determine the new classification is not defined.
4. Challenges with dynamic indirection
The emerging trend in today's network is to deploy network functions,
services and applications at the edge of the network to support
latency requirements, computational offload, traffic optimization
etc. As users are moving, application or services being used by
users, may need to be moved closer to the user's new location. This
implies another instance of the service function may need to be
instantiated close to the user's new location. It may result in re-
establishing service path from the newly instantiated service
function to other service instances. It is also possible that the
newly instantiated service function may be redirected to a new
service end point (e.g. Application Server) for various reasons,
such as incomplete content, proximity to data store, load balancing
etc. In another scenario, a single instance of the service function
may not handle all users due to latency or load constraints. A
single service function may be instantiated more than once to balance
user load. As the number of instances increase and along with
mobility, the complexity of service routing increases. It is
anticipated that there may be a constant action of function chaining,
re-chaining occurring in the network.
The challenge of dynamic indirection may be better described by
analyzing the working of CDNs, which dynamically (re-)direct user-
initiated requests towards the most appropriate content instance.
This task becomes more difficult if granularity of the instance
placement increases. For instance, in case of a CDN being realized
close to end users, specifically in edge of the network, the specific
content instance might need to be selected dynamically. After
initial selection, the instance may change during service execution.
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In a conventional network, an instance of a service is found and
selected using DNS. The subsequent service request is then routed
through the network between the client and the service. If the user
is doing a DNS lookup to access content served by a CDN then the DNS
service will maintain a list of IP addresses that can be returned for
a given domain name and will try to return an IP address of a node
geographically close to the client. Should the service provider want
to replace an instance of their service with another one at a
different IP address (and potentially a different physical location
for various reasons such as load balancing, reliability etc.) then
the DNS tables must be updated, i.e., the service needs to be
(re-)registered quickly. This is done by updating the local
authoritative DNS server which then propagates the new mapping to DNS
services across the world. DNS propagation can take up to 48 hours
so fast and dynamic switching from one service instance to another is
not possible in conventional networks; even in more localized
scenarios, the propagation of DNS updates might still be
insufficient. When relying on many surrogate service endpoints to
exist in the edge network, there is a clear issue of certain
resources not being available in one surrogate instance while
existing in another so that changes in redirection might be
desirable, while also changes in local load drive the need for such
change in redirection. With the emergence of container-based
virtualization platforms, service function endpoints can be
established in a matter of seconds and we therefore believe that the
'reachability' of such said service instance, i.e., the possibility
of route service requests to it from a client that was previously
served elsewhere, must follow a similar timeline, i.e., a few seconds
or even less.
The other issue in conventional network lies with mobility management
procedure. These procedures use an anchor point, which terminates a
session at the network edge. As user moves around, traffic is
redirected from the anchor point to the new point of attachment.
Relying on typical mobility management approaches found in IP
networks, usually leads to inefficient 'triangular' routing of
requests through this common 'anchor' point. This triangular routing
increases the latency in reaching the new service function or service
end points as users move.
Traffic steering is a common procedure in managed networks,
particularly at the edge, due to desired subscriber-centric traffic
policies (e.g., related to pricing structures), resource requirements
(e.g., related to using particular paths in the network) or mobility
(e.g., users moving in a cellular network). Today's methods for
traffic steering include anchor-based mobility management as well as
traffic classification, for instance, in packet gateways of cellular
systems (using, e.g., deep packet inspection as well as port and
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address classification). While the former leads to inefficient
'triangular' traffic forwarding, the latter often requires additional
state in the forwarders to differentiate traffic from one user to
another.
The analysis of CDN network shows that dynamic indirection is a
necessary requirement, which needs to be supported by the networks.
The goal for this indirection is to provide user applications lowest
possible latency. But as discussed above, relying on today's
technique does not help in guaranteeing same latency to user
applications. On the other hand, there is a high possibility that
latency may increase if we rely on Layer 3 based service redirection
techniques.
SFC handles indirection through the use of SPI. A packet needs to be
reclassified and the intermediate node changes the SPI. Following
are the typical steps that happens in order to implement the
indirection.
o A packet arrives at a particular node
o The node contacts the policy manager
o Identifies the current classification is incorrect
o Reclassifies the packet, i.e. change the SPI
o Inserts the packet in the pipe, possibly towards the SFF
The indirection mechanism in SFC involves certain steps to process
policy information and change the SPI in the packet header, making it
suitable to handle dynamic indirection requirements. Our proposed SF
in this document provides an additional method to handle dynamic
indirection of service requests, not relying on the reclassification
mechanism. Combining these two techniques may provide flexibility
and improvement over single method.
5. HTTP as a transport
With the extensive use of "web technology", "distributed services"
and availability of heterogeneous network, HTTP has effectively
transitioned into the common transport for name-based E2E
communication across the web. In the context of SFC and SF, HTTP
requests and response are considered as the "Service Request (SR)".
This use case describes how these SRs are directed towards correct SF
in a fast and dynamic way. The routing and indirection of SRs are
abstracted at HTTP level, instead of the traditional approach where
routing decision for a service request is made at Layer 3.
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If we abstract HTTP as a transport, HTTP requests, such as GET, PUT
and POST can be routed based on the URI associated with the request,
with the URI being simply the name of a resource or the invocation
point for a service transaction. Based on the name of the resource
requested, the appropriate HTTP request can be routed to the suitable
service endpoint. If Service Functions (SF) could be identified
using URI or name, HTTP requests to an SF would be routed or directed
using name based routing. With that, the redirection to the most
suitable service instance is purely done based on named services with
HTTP being a specific (application layer) transport service.
The ongoing EU H2020 efforts like FLAME [H2020FLAME] are driven by
city-scale many-POP deployments of compute infrastructure, all SDN-
connected and OpenStack managed. Localized media use cases drive the
need for name-based (HTTP as the main transport protocol here)
service instances being chained with the relationship between
specific virtual instances being controlled at the underlying
routing/switching level.
The notion of 'HTTP as-a transport', utilizing URLs as addressing
scheme, can be used to create SFP as shown in Fig 2., i.e.,
192.168.x.x -> www.example.com -> 192.168.x.x -> www.example2.com ->
192.168.x.x -> ... -> www.exampleN.com. It is this 'name-based'
relationship that we see possibly realized through specific
replicated instances, where in turn the routing towards those
specific instances is realized by the SRR.
+--------+
| |
|-------------------------|------------------+ SRR +
| | | |
| | +---/|\--+
| | |
+---\|/--+ +---------+ +--\|/--+ +------+ +----+---+
| | | | | | | | | |
+ Client +-->+ SRR +-->+ Media +-->+ SRR +-->+ Media +
| | | | | Fn1 | | | | Fn2 |
+--------+ +---------+ +-------+ +------+ +--------+
SFP:192.168.x.x-->www.example.com-->192.168.x.x
-->www.example2.com-->192.168.x.x-->www.exampleN.com
Figure 2: SFP with new HTTP-based Transport option
In a pure SFC architectural framework, Classifier function may
interact with SRR to obtain an SE (Service Encapsulation). E.g. the
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Classifier function may look into the network locator map in Fig 2
and determine the next SF is www.example.com. It provides this
information to SRR to obtain the next hop information. SRR returns
the SE for next hop, which can be a "bitfield" information that is
being used in the overlay routing for this part of the SFP. The
Classifier function uses this SE to route the incoming packet
directly at the transport network level.
6. Service Request Routing (SRR) Service Function
6.1. Overview
The following diagram shows the application of the new proposed SRR
service function in an example of media clients connecting to media
servers. There may be more than one media functions to support CDN
like architecture, Surrogate servers to handle mobility and load
balancing.
+--------+
| |
|-------------------------|------------------+ SRR +
| | | |
| | +---/|\--+
| | |
+---\|/--+ +---------+ +--\|/--+ +------+ +----+---+
| | | | | | | | | |
+ Client +-->+ IP +-->+ Media +-->+ SRR +-->+ Media +
| | | Routing | | Fn1 | | | | Fn2 |
+--------+ +---------+ +-------+ +------+ +--------+
Figure 3: General SFC with SRR Flexible Chaining, initiated via IP
Routed Client Connection
The clients are connected to media functions through frontend routed
network, e.g., relying on standard IP routing, while media functions
are chained via the new proposed service request routing (SRR)
function. Alternatively, we also envision to utilize the SRR
function directly between client SF and media function SF, as
outlined in the figure below
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+--------+
| |
|-------------------------|------------------+ SRR +
| | | |
| | +---/|\--+
| | |
+---\|/--+ +---------+ +--\|/--+ +------+ +----+---+
| | | | | | | | | |
+ Client +-->+ SRR +-->+ Media +-->+ SRR +-->+ Media +
| | | | | Fn1 | | | | Fn2 |
+--------+ +---------+ +-------+ +------+ +--------+
Figure 4: General SFC with SRR Flexible Chaining, initiated via SRR
Chained Client
For our considerations, we assume that each SF is realized by at
least one or more service function endpoints (SFEs). Hence, instead
of looking at "chaining" as a concept that connects specific SFEs
along a well-defined SFP, we propose to look at "chaining" at the
level of "named" service functions rather than their specific
endpoint instances. With this in mind, the SRR service function
lifts the relationship between the connecting SFs to the level of
"logical" service functions rather than their specific realizing
endpoints. Instead of relying on dynamic re-chaining in case of any
dynamically changing relationship between specific SFEs, the SRR
provides the selection of suitable SFEs while maintaining the logical
relationship between the SFs. In Section 6.3, we will present the
necessary extensions to the SFP concept to support this higher
abstraction of "chaining" via "named" logical SFs. The SRR
introduces the flexibility in routing service requests from client to
specific SFEs. In the edge network, where users are moving and
service end points may also change, having flexibility to decide and
steer service requests directly helps in guaranteeing the same
latency to user applications. Clearly, that is achieved by reducing
the switching time from SF to another. As service end point changes,
the routing functions makes instantaneous decision to route the
request to the appropriate media server.
The SRR introduces the flexibility in routing service requests from
client to specific SFEs in response to conditions such as congestion
in the network, user mobility etc. In the edge network, where users
are moving and service end points may also change, having flexibility
to decide and steer service requests directly helps in guaranteeing
the same latency to user applications. The edge of the network maybe
congested due to limited network resources. The SRR may be able to
determine network congestion and quickly route service requests to
other Service End point, which is not experiencing congestion. In
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addition, application-layer control functions might utilize latency
measurements to ensure that suitable service instances are being
created during runtime of the scenario such as to ensure that service
function endpoints are available 'nearby' (possibly) moving so as to
keep a desired latency under a desired value.
Clearly, that is achieved by reducing the switching time from one SF
endpoint to another. As the service end point changes, the routing
functions makes instantaneous decision to route the request to the
appropriate media server.
The possible improvements of using SRR within an SFC framework are
listed below:
o Fast (between 10 and 20ms) switching times from one service
instance to another by not relying on the DNS for service
discovery and directly routing service requests at the level of
the transport network.
o The capability to indirect service requests at the network level
will help in reducing latency, when service end points change.
E.g. when a service request is being sent to one surrogate
instance but results in a HTTP 404 or 5xx error response, the
original request is redirected to another alternative surrogate
with minimal latency, i.e., right at the destination of said
failed service request. Nesting these operations effectively
leads to a net-level 'search' among all available surrogate
instances until the search is exhausted (with a negative result)
or the resource is found.
o New methods for forwarding, such as path-based forwarding, will
enable direct path routing in mobility cases, path pinning for
traffic steering and simplified service-specific peering towards
the Internet. Such capability would allow for localizing traffic,
reduce latency and costs.
6.2. Details of SRR Function
Assuming such introduction of an HTTP-level transport notion, the SRR
function can be decomposed further as shown in Fig 5.
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+--------+
| |
|-------------------------|------------------+ SRR +
| | | |
| | +---/|\--+
| | |
+---\|/--+ +---------+ +--\|/--+ +------+ +----+----+
| | | | | | | | | |
+ Client +-->+ SRR +-->+Service+-->+ SRR +-->+ Service +
| | | | | Fn1 | | | | Fn2 |
+--------+ +---------+ +-------+ +------+ +---------+
/ \
/ \
/ \
+--------------------------------------+
| +------------------+ |
| | +-----+ +----+ | +-----+ |
|---> | SFC | | SR | | | SR |----->
| | |Proxy| | | | | | |
| | +-----+ +----+ | +-/|\-+ |
| | Use Proxy if NAP| | |
| | is not SFC | | |
| | enabled | | |
| +-------/|\--------+ | |
| | | |
| | | |
| | +----------+ | |
| |->| tSFF1 |----- |
| +---/|\----+ |
| | |
| | |
| +----------+ | |
| | | | |
| + PCE +---- +-----+ |
| | |--------| RT | |
| +----------+ +-----+ |
| |
+--------------------------------------+
Figure 5: SRR decomposition
Another option for the two functions routing via the SRR could be
entirely link-local, i.e., there's another simple tSFF2 between
client and SRR as well as SF1 and SRR that is simply a link-local
transport. The following figure describes this alternate option.
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+--------+
| |
|-------------------------|------------------+ SRR +
| | | |
| | +---/|\--+
| | |
+---\|/--+ +---------+ +--\|/--+ +------+ +----+---+
| | | | | | | | | |
+ Client +-->+ SRR +-->+Service+-->+ SRR +-->+Service +
| | | | | Fn1 | | | | Fn2 |
+--------+ +---------+ +-------+ +------+ +--------+
/ \
/ \
/ \
+-----+ +---------------------------------+
|tSFF2|--------->+----+ +-----+ | +--------+
+-----+ | | SR | | SR |----->| tSFF2 |-->
| | | | | | +--------+
| +----+ +-/|\-+ |
| | | |
| | | |
| | | |
| | | |
| | +-------+ | |
| |---->| tSFF1 |--- |
| +--/|\--+ |
| | |
| | |
| +-------+ | |
| | | | |
| + PCE +--- +----+ |
| | |--------| RT | |
| +-------+ +----+ |
| |
+---------------------------------+
Figure 6: SRR decomposition using link-local client/function
communication
The SRR function may be composed of the following functions:
o Service Router(SR) at the ingress, terminates on the client side
Layer 3 and above protocols, such as TCP
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o Service Router(SR) at the egress, terminates any transport
protocol on the outgoing (server) side
o PCE, Path Computation Element function is responsible for
selecting the correct next SF, also possibly realizing path policy
enforcement. The result of the selection is a path identifier
which is delivered to the ingress SR upon initial path computation
request (i.e., when sending a request to a specific URL on the SFP
for the first time). The path identifier is utilized for any
future request for a given URL-based SF. In case of another SF
instance becoming available, indicated to the PCE through a
registration procedure, the PCE will instruct all ingress SRs to
invalidate path identifiers to the specific URL of the SF,
resulting in an initial path computation request at the next SF
request forwarding. Through this, the newly registered SF
instance might be utilized if the policy-governed path computation
will select said SF instance.
o Reclassification Trigger Handler (RT) : Network measurement
information, such as latency, packet loss or network congestion,
etc. could be processed by the handler. This may trigger
reconfiguration of the specific service function endpoint chain
over which the SFC is being executed. The handler forwards the
information about the chain reconfiguration to PCE.
o Transport-derived SFF (tSFF1): the communication between ingress/
egress SRs as well as SRs to PCE is realized via a transport-
derived SFF. We outline here three possible tSFFs
* SDN-based: This option utilizes path-based forwarding through
SDN-based wildcard matching fields, supported with
OF1.2+[Reed2016]. It can be embedded into slicing approach of
underlying transport infrastructure by leaving typical slicing
fields available (e.g., VLAN tags). The forwarding utilizes
the Ethernet frame format at Layer 2, representing the
topological links of a specific forwarding path in the
transport network as unique bits in a fixed size bit array.
For the latter, the approach utilizes the IPv6 source and
destination fields for storing the bit array information (in a
simple version for this forwarding, this limits the topology to
256 links but extensions schemes are possible, which are left
out of this document at this stage). AS mentioned, the SDN
forwarding decision action is a simple wildcard matching,
supported with OF1.2+, with the wildcard representing the
unique bit of a switch-specific output port. With that, the
switch needs to consider as many forwarding rules as switch
local output ports - see [Reed2016] for more information. Fig.
xx illustrate this forwarding solution, including the ability
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to create ad-hoc multicast relations by simply ORing individual
bitarrays representing unicast paths.
* Another approach is outlined in [I-D.ietf-bier-use-cases] where
the SFF is suggested to be realized via a BIER overlay, in turn
realized over a BIER-compliant underlay, such as MPLS. BIER
utilizes a similar bit array approach for representing a
forwarding path in the overlay network but unlike [Reed2016],
the bit fields indicate the egress BIER-compliant router that
the packet is supposed to reach.
* As yet another alternative, the tSFF may utilize a flow
aggregation approach, outlined in [Khalili2016], called edge
switch classification (ESC). In this approach, a path from an
ingress to egress SR is described as a so-called edge
classification vector (ECV), which combines information on the
aggregated flow (following [Khalili2016]) and the switch-local
endpoint. The representation has similar bitarray
characteristics as the previous two approaches
o NOTE: with the ingress and egress SRs terminating SF Layer 3
connections and the utilization of bitarray-based tSFFs, the
transmission of packets can effectively take place as an ad-hoc
Layer multicast while the SFC itself is denoted as an n-times
unicast SFC. As an example, consider the chaining of a set of n
clients to a single video server. Each sub-SFC from an individual
client to the video server will semantically result in a unicast
response from the server back to the client (e.g., carrying the
video chunk for a MPEG DASH-based video stream). When combining
the sub-SFCs to the single SFC with n times unicast relations to
the server, the SRR will deliver the responses from the server via
one or more multicast responses to one or more clients. The size
of the individual multicast groups will depend on the
synchronicity of the client requests (and therefore on the
synchronicity of the server responses). Note that the multicast
relations here are ad-hoc created by ORing the bitarrays
representing the specific clients to which the responses are meant
to be sent. This is illustrated in the figure below. The HTTP
multicast use case is being presented in the BIER use case draft
[I-D.ietf-bier-use-cases]albeit without specific a SFC relation.
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+---------+ +---------+
| | | | +--------+
+IP only +---+ ICN + 00000010 | ICN |
|receiver | | SR1 | |--------| SR3 |
|UE | +----|----+ | +---||---+
+---------+ | 10010011 | ||
+-----|----+ +----------+ |-----||-----|
| | | | | Cloud |
|SDN Switch|---|SDN Switch| | |
| | | | |--||--|
+----|-----+ +----------+ ||
| 10100011 ||
+---------+ +---|-----+ +----||----+
| | | | | |
+IP only +---+ ICN + + IP only +
|sender UE| | SR2 | | Server |
+---------+ +---------+ +----------+
Figure 7: Illustration of Bitfield-based Forwarding using SDN
7. Protocol Consideration
For the operations outlined in the previous section, we foresee the
following protocol changes are required:
o SR-to-SR protocol for HTTP: HTTP based message exchange between
client and server SRs
o SR-PCE protocol: Used for path computation, obtaining routing
information as well as provide path updates
o Registration protocol: Used to register FQDN service endpoints
8. Next Steps
Feedback from the SFC WG on the validity of this solution and its
scope within the SFC WG. If such alternative to the re-
classification for service indirection is seen beneficial as well as
fitting with the charter of the WG, the next steps would be to update
the draft to outline potential protocol solutions required for the
realization of such SRR SF.
9. IANA Considerations
This document requests no IANA actions.
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10. Security Considerations
TBD.
11. Informative References
[ETSI_MEC]
ETSI, "Mobile Edge Computing (MEC), Technical
Requirements", GS MEC 002 1.1.1, March 2016,
<http://www.etsi.org/deliver/etsi_gs/
MEC/001_099/002/01.01.01_60/gs_MEC002v010101p.pdf>.
[H2020FLAME]
EU, "EU H2020 FLAME PROJECT", , March 2016,
<https://www.ict-flame.eu/>.
[I-D.ietf-bier-use-cases]
Kumar, N., Asati, R., Chen, M., Xu, X., Dolganow, A.,
Przygienda, T., Gulko, A., Robinson, D., Arya, V., and C.
Bestler, "BIER Use Cases", draft-ietf-bier-use-cases-06
(work in progress), January 2018.
[I-D.ietf-sfc-dc-use-cases]
Kumar, S., Tufail, M., Majee, S., Captari, C., and S.
Homma, "Service Function Chaining Use Cases In Data
Centers", draft-ietf-sfc-dc-use-cases-06 (work in
progress), February 2017.
[Khalili2016]
Khalili, R., Poe, W., Despotovic, Z., and A. Hecker,
"Reducing State of SDN Switches in Mobile Core Networks by
Flow Rule Aggregation", ICCCN, August, 2016.
[Reed2016]
Reed, M., Al-Naday, M., Thomas, N., Trossen, D., and S.
Spirou, "Reducing State of SDN Switches in Mobile Core
Networks by Flow Rule Aggregation", ICC 2016, 2016.
[RFC7498] Quinn, P., Ed. and T. Nadeau, Ed., "Problem Statement for
Service Function Chaining", RFC 7498,
DOI 10.17487/RFC7498, April 2015,
<https://www.rfc-editor.org/info/rfc7498>.
[RFC8300] Quinn, P., Ed., Elzur, U., Ed., and C. Pignataro, Ed.,
"Network Service Header (NSH)", RFC 8300,
DOI 10.17487/RFC8300, January 2018,
<https://www.rfc-editor.org/info/rfc8300>.
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[UKNIC] UK NIC, "5G Infrastructure Requirements in the UK", Final
Report 3.0, December 2016,
<https://www.gov.uk/government/uploads/system/uploads/
attachment_data/
file/577940/5G_Infrastructure_requirements_for_the_UK_-
_LS_Telcom_report_for_the_NIC.pdf>.
Authors' Addresses
Debashish Purkayastha
InterDigital Communications, LLC
Conshohocken
USA
Email: Debashish.Purkayastha@InterDigital.com
Akbar Rahman
InterDigital Communications, LLC
Montreal
Canada
Email: Akbar.Rahman@InterDigital.com
Dirk Trossen
InterDigital Communications, LLC
64 Great Eastern Street, 1st Floor
London EC2A 3QR
United Kingdom
Email: Dirk.Trossen@InterDigital.com
URI: http://www.InterDigital.com/
Zoran Despotovic
Huawei
Email: Zoran.Despotovic@huawei.com
URI: http://www.huawei.com/
Ramin Khalili
Huawei
Email: Ramin.khalili@huawei.com
URI: http://www.huawei.com/
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