Network Working Group N. Zong
Internet-Draft L. Dunbar
Intended status: Informational Huawei Technologies
Expires: January 2, 2015 M. Shore
No Mountain Software
D. Lopez
Telefonica
G. Karagiannis
University of Twente
July 1, 2014
Virtualized Network Function (VNF) Pool Problem Statement
draft-zong-vnfpool-problem-statement-06
Abstract
Network functions are traditionally implemented on specialized
hardware rather than on general purpose servers, but there is a clear
trend to implement a number of network functions, such as firewall or
load balancer, as software on virtualized computing platforms. These
virtualized functions are called Virtualized Network Functions
(VNFs), which can be used to build network services. The use of VNFs
to build network services introduces additional challenges on
reliability, such as additional points of failure and the need to
coordinate various VNFs.
This document introduces a general idea of VNF Pool to support
reliable function provision by the VNFs. We then highlight the
reliability challenges and issues when using the VNFs to build
services. Related IETF works are also briefly described.
Status of This Memo
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This Internet-Draft will expire on January 2, 2015.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Background . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. From Specialized Hardware to Virtualized Network Function 4
3.2. Concept of VNF Set . . . . . . . . . . . . . . . . . . . 5
3.3. Challenges to reliability . . . . . . . . . . . . . . . . 6
4. VNF Pool . . . . . . . . . . . . . . . . . . . . . . . . . . 6
5. Challenges and Open Issues . . . . . . . . . . . . . . . . . 8
5.1. Redundancy model inside VNF . . . . . . . . . . . . . . . 8
5.2. State synchronization inside VNF . . . . . . . . . . . . 8
5.3. Interaction between VNF and Service Control Entity . . . 8
5.4. Reliable transport . . . . . . . . . . . . . . . . . . . 9
5.5. Scope Considerations . . . . . . . . . . . . . . . . . . 9
6. Related Works . . . . . . . . . . . . . . . . . . . . . . . . 9
6.1. Reliable Server Pooling (RSerPool) . . . . . . . . . . . 9
6.2. Virtual Router Redundancy Protocol (VRRP) . . . . . . . . 10
6.3. Service Function Chaining (SFC) . . . . . . . . . . . . . 10
7. Security Considerations . . . . . . . . . . . . . . . . . . . 10
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 11
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
10.1. Normative References . . . . . . . . . . . . . . . . . . 11
10.2. Informative References . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction
Network functions such as firewall, load balancer, WAN optimizer are
conventionally deployed as specialized hardware servers in both
network operators' networks and data center networks, as the building
blocks of the network services.
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A Virtualized Network Function (VNF) provides such network function
through its implementation as software instances running on general
purpose servers via a virtualization layer (i.e., hypervisor). VNFs
potentially offer benefits such as elastic service offering, reduced
operational and equipment costs [NFV-WP].
There is a trend to move network functions from specialized hardware
servers to general purpose servers based on virtualized computing
platforms, in order to build network services by using VNFs. For
example, in Service Function Chaining (SFC), a network service can be
built using a set of sequentially connected VNF instances deployed at
different points in the network [SFC].
Nevertheless, the use of VNFs can pose additional challenges on the
reliability of the provided services. For a VNF instance, it
typically would not have built-in reliability mechanisms on its host
(i.e., a general purpose server). Instead, there are more factors of
risk such as software failure at various levels including hypervisors
and virtual machines, hardware failure, and instance migration that
may make a VNF instance unreliable.
In order to achieve higher reliability, a VNF may adopt a pooling
mechanism, where a number of VNF instances with the same function can
be grouped as a pool to provide the function. We call such a pool a
VNF Pool. Conceptually, a Pool Manager is used to manage a VNF Pool,
e.g., selects active/standby VNF instances, and potentially interacts
with a Service Control Entity. A Service Control Entity is an entity
that combines and orchestrates a set of network functions, e.g.,
VNFs, to build network services. The major benefit of using VNF Pool
is that the reliability mechanisms such as redundancy management are
achieved by the VNF Pool inside the VNF and thus transparent to the
Service Control Entity. A VNF Pool-enabled VNF still acts as a
normal VNF when orchestrated by the Service Control Entity.
We are specifically concerned with the reliability of an individual
VNF based on the VNF Pool managed inside the VNF. For example, how
to manage the redundancy model, e.g., select active/standby for a VNF
instance in a VNF Pool, considering the policy and the infrastructure
conditions? How are the service states of a VNF instance held and
accessed for efficient synchronization with backup instances in a VNF
Pool? What pool states need to be maintained to support the pooling
mechanism itself, and how are such states maintained? We also
consider the information exchanged between the VNF and Service
Control Entity. For example, how can a VNF Pool be addressed by the
Service Control Entity? After a VNF instance failover, how does the
Pool Manager notify the Service Control Entity of some characteristic
changes of the VNF, e.g., capacity change, but without disclosure of
the pooling procedure?
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Note that we do not address the reliability related control or
routing between adjacent VNFs that can form a network service, as
such coordination could be done by the Service Control Entity.
This document introduces a general idea of VNF Pool to support
reliable functions provision by the VNFs. We then highlight the
reliability challenges and issues when using the VNFs to build
services. Related IETF works are also briefly described.
2. Terminology
Reliability: capability of a functional entity to consistently
provide its function under various dynamic and even unexpected
conditions such as fault, overload, etc.
Service Control Entity: an entity of the service provider that
decides how to combine and orchestrate the network functions to build
network services. Examples of Service Control Entity are
orchestrator of DC services, SFC control plane, etc.
Virtualized Network Function (VNF): a VNF provides the same
functional behavior and interfaces as the equivalent network
function, but is deployed as software instance(s) building on top of
a virtualization layer [NFV-TERM].
VNF Pool: a number of VNF instances providing the same network
function.
VNF Pool Element: a VNF instance inside a VNF pool.
VNF Pool Manager: an entity that manages a VNF pool, and interacts
with the Service Control Entity to provide the network function.
VNF Set: a general set of VNF instances that can be grouped into
multiple VNF Pools, where each pool corresponds to a specific VNF and
different pools provide different functions.
3. Background
3.1. From Specialized Hardware to Virtualized Network Function
Network functions are traditionally implemented on specialized
hardware. There is a trend to implement a number of network
functions as software instances on general purpose servers, via
virtualized computing platforms. These virtualized functions are
called Virtualized Network Functions (VNFs). For example, in
Figure 1, virtual firewall (vFW) can be deployed as software
instances on general purpose servers, which could be located in Data
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Center (DC) networks, network operators' networks, or end user
premises. Compared with traditional FW deployed as "standalone box"
built by specialized hardware and software, vFW has potential
advantages such as agility, scalability [NFV-WP].
FW vFW vFW vFW
+-------------+ +-----------+ +-----------+ +-----------+
| Specialized | |FW Software| |FW Software| |FW Software| ...
| Hardware |----\ +-----------+ +-----------+ +-----------+
| + |----/ +------------------------------------------+
| Software | | Virtualization Platform |
+-------------+ +------------------------------------------+
+-----------------+ +-----------------+
| General Purpose | | General Purpose |
| Server | | Server | ...
+-----------------+ +-----------------+
Figure 1: Example of vFW.
3.2. Concept of VNF Set
We call a general set of VNF instances a VNF set. A VNF set can
include a single or multiple types of VNF, and each type of VNF may
have a number of instances providing the same function. The
following examples are all valid VNF sets.
1. n vFW instances: {vFW#1,vFW#2,...,vFW#n}.
2. m vFW instances and k virtual load balancer (vLB) instances:
{vFW#1,...,vFW#m,vLB#1,...,vLB#k}.
To be more generic, we denote VNF-A#x the xth instance of a VNF of
type A (e.g., vFW), VNF-B#y the yth instance of a VNF of type B
(e.g., vLB), and so on.
A VNF set can be used as part of a Service Function Chaining (SFC)
[SFC], where the instances of various functions are sequentially
connected to build a network service. A simple example is shown in
Figure 2.
Network Service
+----------+ +----------+ +----------+
| VNF-A#x | data conn | VNF-B#y | data conn | VNF-C#z |
| |-----------| |-----------| |
+----------+ +----------+ +----------+
Figure 2: A VNF set used as part of a SFC.
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Alternatively, a VNF set can be also used merely as a set of VNFs,
where the instances provide network functions in a parallel way. An
example is shown in Figure 3.
+----------+ +----------+ +----------+
| VNF-A#x | | VNF-B#y | | VNF-C#z |
+----------+ +----------+ +----------+
\ | /
data conn \ |data /data conn
\ |conn /
\ | /
+---------------+
| Client |
+---------------+
Figure 3: A VNF set used as multiple VNFs.
Some more detailed use cases of VNFs are documented in other drafts
[VNFPOOL-UC1] [VNFPOOL-UC2] [VNFPOOL-UC3].
3.3. Challenges to reliability
The use of VNFs introduces additional challenges to the reliability
of the provided network services. For a VNF instance, it typically
would not have built-in reliability mechanisms on its host (i.e., a
general purpose server). Instead, there are more factors of risk
that may make VNF instance unreliable.
1. Instance failure due to hardware failure or status change such
as server overload.
2. Instance failure due to software failure at various levels
including hypervisor, Virtual Machine (VM), VNF.
3. Instance migration caused by instance performance downgrade
caused by load (e.g., CPU, memory, disk I/O), server consolidation
or other service requirement changes. This is distinct from a
hard failure, although it may give the appearance of one.
4. VNF Pool
There are a number of existing technologies for providing reliable
functions, such as Reliable Server Pooling (RSerPool) [RFC5351],
Virtual Router Redundancy Protocol (VRRP) [RFC5798], amongst many
others. Both technologies provide the service with an abstract
object (e.g., pool handle in RSerPool, virtual router ID in VRRP)
representing a group of identical functional instances. The dynamic
mapping of such abstract object to the actual serving instance is
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managed internally in the group to cover the failover procedure. The
advantage is to provide reliable functions in a transparent manner
for both end-hosts and service control entities.
We adopt the similar idea of VNF Pool to provide reliable network
functions, as shown in figure 4.
+------------------------+
| Service Control Entity |
+------------------------+
^ ^
| |
+-----------+ +------------+
| |
v v
+ - - - - - - - - - - - - - - - + + - - - - - - - - - - - - - - - +
| VNF-A +--------------+ | | VNF-B +--------------+ |
| | Pool Manager | | | | Pool Manager | |
| +--------------+ | | +--------------+ |
| + - - - - - - - - - - - - - + | | + - - - - - - - - - - - - - + |
| |+---------+ +---------+| | | |+---------+ +---------+| |
| || VNF-A#1 | ... | VNF-A#n || | | || VNF-B#1 | ... | VNF-B#m || |
| |+---------+ +---------+| | | |+---------+ +---------+| |
| | VNF-A Pool | | | | VNF-B Pool | |
| + - - - - - - - - - - - - - + | | + - - - - - - - - - - - - - + |
+ - - - - - - - - - - - - - - - + + - - - - - - - - - - - - - - - +
Figure 4: VNF Pool Architecture.
In VNF Pool architecture, each VNF has a VNF Pool containing a number
of VNF instances (or VNF Pool Elements) providing the same function.
In this sense, a VNF set can be grouped into multiple VNF Pools,
where each pool corresponds to a specific VNF, thus different pools
provide different functions. Each VNF also has a Pool Manager that
manages the VNF instances in the VNF Pool. Pool Manager interacts
with the Service Control Entity to provide the network function.
The main benefit of using VNF Pool is that the pooling mechanisms
such as redundancy management are achieved by the VNF Pool inside the
VNF and thus transparent to the Service Control Entity. The Service
Control Entity simply interacts with the Pool Manager in each VNF to
request and orchestrate the network functions with desired
reliability level. In another word, a VNF Pool-enabled VNF still
acts as a normal VNF when orchestrated by the Service Control Entity.
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5. Challenges and Open Issues
5.1. Redundancy model inside VNF
Before a live VNF instance fails, one or more backup instances in the
same VNF Pool need to be selected. How to select such backup
instances? Moreover, there are policies influencing the appropriate
selection of backup instance. For example, it should be avoided that
a live VNF instance and its backup instances are placed in a single
physical server, or locations with shared risks in the network. On
the other hand, it would be desirable to place the live and backup
instances in geographically closed locations. Information from the
underlying network may need to be collected via - e.g., the interface
with Application Layer Traffic Optimization (ALTO) [ALTO], or
Interface to Routing System (I2RS) [I2RS]. Various infrastructure
conditions may also need to be considered for appropriate placement
of instances.
5.2. State synchronization inside VNF
Service states related to the specific function performed by a VNF
instance, e.g., NAT translation table, TCP connection states, should
be synchronized between a live VNF instance and its backup instances
for stateful failover. Who is responsible for and how to collect,
hold, and access such service states to achieve efficient
synchronization? A VNF instance should provide negotiated level of
state sharing with the necessary performance to fulfill the service
requirements - e.g., state synchronization method, format of state
data, location and mechanism to access state data.
Other than service states, pool states could be operational
information of VNF pool itself, e.g. redundancy settings, backup
location/status, etc. What pool states need to be maintained to
support the pooling mechanism itself, and how are such states
maintained?
5.3. Interaction between VNF and Service Control Entity
Some information needs to be exchanged between a VNF and the Service
Control Entity when the Service Control Entity orchestrates a VNF
Pool-enable VNF. For example, how can a VNF Pool be addressed by the
Service Control Entity? A Pool Manager can advertise the locator
(e.g., IP address) of the active instance - subject to dynamic due to
failover. It is also possible to use a virtual address for the whole
VNF Pool (similar to RSerPool or VRRP), and map between virtual and
actual addresses. Moreover, after a VNF instance failover, how does
the Pool Manager notify the Service Control Entity of some
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characteristic changes of the VNF, e.g., capacity change, but without
disclosure of the pooling procedure?
5.4. Reliable transport
The transport mechanism used to carry the pool control messages,
e.g., redundancy management, should provide reliable message
delivery. Transport redundancy mechanisms such as Multipath TCP
(MPTCP) [MPTCP] and the Stream Control Transmission Protocol (SCTP)
[RFC3286] will need to be evaluated for applicability. Latency
requirements for pool control message delivery must also be
evaluated.
5.5. Scope Considerations
Ideally, the reliability goal is that the network service provided by
the VNFs will continue throughout an interruption within the VNFs ,
and VNF instances failure or migration will not be visible to the
external entities. Our work of VNF Pool initially focuses on several
reliability mechanisms that are mainly associated with a redundancy
model based on a VNF Pool. Additional mechanisms may include pool
state maintenance only for pooling purpose. Service state
synchronization is out of scope for this phase.
We currently assume that a VNF Pool contains the instances of same
functional type, e.g., FW, LB, etc. Different types of VNFs are
envisioned to be held in separate VNF Pools. VNF Pool composed of
both virtualized and non-virtualized functional instances may be
included after further use case and requirements study.
We are specifically concerned with the reliability of an individual
VNF based on the VNF Pool managed inside the VNF. We do not address
the reliability related control or routing between adjacent VNFs that
can form a network service, as such coordination could be done by the
Service Control Entity.
We do not intend to resolve the service availability that usually
involves more factors including the interruptions in various OSI
layers, and even user perception on service performance.
6. Related Works
6.1. Reliable Server Pooling (RSerPool)
RSerPool supports high availability and scalability of the
applications through the use of pools of servers [RFC5351]. The main
functions of RSerPool involve server pool management, as well as
receiving requests from a client to bind to a desired server. The
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applicability and gaps of RSerPool to our work of VNF Pool are
described in another draft [VNFPOOL-RSP].
6.2. Virtual Router Redundancy Protocol (VRRP)
VRRP specifies an election protocol that dynamically assigns
responsibility of a virtual router to one of the VRRP routers called
master on a LAN [RFC5798]. The election process provides dynamic
failover in the forwarding responsibility should the Master become
unavailable. The advantage of VRRP is a higher availability default
path without requiring configuration of dynamic routing or router
discovery protocols on every end-host.
6.3. Service Function Chaining (SFC)
A service chain defines an ordered set of service functions that must
be applied to packets [SFC]. Although the VNFs can be used as part
of a SFC, SFC and our work of VNF Pool have different focus.
As mentioned in the section of scope consideration, we mostly
consider the reliability of an individual VNF based on the VNF Pool
inside the VNF. We do not address the reliability related control or
routing between adjacent VNFs in the forwarding graph. Moreover,
according to VNF Pool architecture and principles, the VNF Pools will
be orthogonal to and invisible to the SFC. A VNF Pool-enabled VNF
still acts as a normal VNF when orchestrated by the SFC. Just like
the communication between any pool users and VNF Pool, the
information exchanged between the VNF Pool and the SFC may include
some operational information of the VNF Pool.
7. Security Considerations
Any technology which allows the insertion, deletion, reordering, or
manipulation of network functions has the potential to be subverted
by an attacker, with serious consequences. Distributed VNFs
introduce an additional attack vector, in which bad actors join
several VNFs of a service. Replay attacks have the potential to
create denials of service, reordering, adding, or removing VNFs. VNF
reliability technologies must provide cryptographic protections
against spoofing and insertion attacks as well as replay attacks, in
the form of client authentication, origin authentication on VNF
reliability management (control plane) traffic, and replay
protections. There may be circumstances under which an attacker
masquerading as a VNF manager can introduce data leakage or similar
attacks, and consequently server authentication would be required, as
well.
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Failing over a VNF or otherwise transferring service state raises
issues related to the transfer of security state, including VNF
element identity and credentials, session-associated cryptographic
state, and so on. Where possible, transfer of security state should
be avoided as a matter of good practice, and this will require
particular attention as solutions are drafted.
8. IANA Considerations
This document has no actions for IANA.
9. Acknowledgements
The authors would like to thank Chidung Lac from Orange, Daniel King
from Lancaster University, Lingli Deng, Zhen Cao from China Mobile,
Richard Yang from Yale University, Hidetoshi Yokota from KDDI,
Mukhtiar Shaikh from Brocade, Qiang Zu from Ericsson, Marco Liebsch
from NEC, Kapil Sood from Intel, Adrian Farrel, and Susan Hares for
their valuable comments.
10. References
10.1. Normative References
TBD.
10.2. Informative References
[NFV-WP] NFV Whitepaper: "Network Function Virtualization", issue 1,
2012, http://portal.etsi.org/NFV/NFV_White_Paper.pdf.
[SFC] "Service Function Chaining (SFC)",
.
[NFV-TERM] ETSI GS NFV 003: "Terminology for Main Conceptional
Entities in NFV", Version 0.0.4, 2013.
[VNFPOOL-UC1] L. Xia, Q. Wu, D. King, H. Yokota, and N. Khan,
"Requirements and Use Cases for Virtual Network Functions", draft-
xia-vnfpool-use-cases-00, February 2014.
[VNFPOOL-UC2] D. King, M. Liebsch, P. Willis and J. Ryoo,
"Virtualization of Mobile Core Network Use Case", draft-king-vnfpool-
mobile-use-case-00, February 2014.
[VNFPOOL-UC3] S. Hares and K. Subramaniam, "Use Cases for Resource
Pools with Virtual Network Functions (VNFs)", draft-hares-vnf-pool-
use-case-00, January 2014.
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[ALTO] "Application-Layer Traffic Optimization (alto)",
.
[I2RS] "Interface to the Routing System (i2rs)",
.
[MPTCP] "Multipath TCP (mptcp)", .
[RFC3286] L. Ong and J. Yoakum, "An Introduction to the Stream
Control Transmission Protocol (SCTP)", RFC3286, May 2002.
[NFV-REL] ETSI GS NFV REL 001: "Network Function Virtualization;
Resiliency Requirements", Version 0.0.7, 2014.
[NFV-SWA] ETSI GS NFV SWA 001: "Network Function Virtualization; SW
Architecture; Virtual Network Functions Architecture", Version 0.1.0,
2014.
[RFC5351] P. Lei, L. Ong, M. Tuexen and T. Dreibholz, "An
Overview of Reliable Server Pooling Protocols", RFC5351, September
2008.
[RFC5798] S. Nadas, "Virtual Router Redundancy Protocol (VRRP)
Version 3 for IPv4 and IPv6", RFC5798, March 2010.
[VNFPOOL-RSP] T. Dreibholz, M. Tuexen, M. Shore and N. Zong, "The
Applicability of Reliable Server Pooling (RSerPool) for Virtual
Network Function Resource Pooling (VNFPOOL)", draft-dreibholz-
vnfpool-rserpool-applic-00, October 2013.
Authors' Addresses
Ning Zong
Huawei Technologies
Email: zongning@huawei.com
Linda Dunbar
Huawei Technologies
Email: linda.dunbar@huawei.com
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Melinda Shore
No Mountain Software
Email: melinda.shore@nomountain.net
Diego Lopez
Telefonica
Email: diego@tid.es
Georgios Karagiannis
University of Twente
Email: g.karagiannis@utwente.nl
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