Service Function Chaining Use Cases in Mobile Networks
draft-ietf-sfc-use-case-mobility-02
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| Last updated | 2015-01-10 | ||
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draft-ietf-sfc-use-case-mobility-02
Service Function Chaining W. Haeffner
Internet-Draft Vodafone
Intended status: Informational J. Napper
Expires: July 13, 2015 Cisco Systems
M. Stiemerling
NEC
D. Lopez
Telefonica I+D
J. Uttaro
AT&T
January 9, 2015
Service Function Chaining Use Cases in Mobile Networks
draft-ietf-sfc-use-case-mobility-02
Abstract
This document provides some exemplary use cases for service function
chaining in mobile service provider networks. The objective of this
draft is not to cover all conceivable service chains in detail.
Rather, the intention is to localize and explain the application
domain of service chaining within mobile networks as far as it is
required to complement the problem statement
[ietf-sfc-problem-statement] and architecture framework
[ietf-sfc-arch] of the working group.
Service function chains typically reside in a LAN segment which links
the mobile access network to the actual application platforms located
in the carrier's datacenters or somewhere else in the Internet.
Service function chains (SFC) ensure a fair distribution of network
resources according to agreed service policies, enhance the
performance of service delivery or take care of security and privacy.
SFCs may also include Value Added Services (VAS). Commonly, SFCs are
typical middlebox services.
General considerations and specific use cases are presented in this
document to demonstrate the different technical requirements of these
goals for service function chaining in mobile service provider
networks.
The specification of service function chaining for mobile networks
must take into account an interaction between service function chains
and the 3GPP Policy and Charging Control (PCC) environment.
Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on July 13, 2015.
Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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Table of Contents
1. Introduction
1.1. Terminology and abbreviations
1.2. General scope of mobile service chains
1.3. General structure of end-to-end carrier networks
2. Mobile network overview
2.1. Building blocks of 3GPP mobile LTE networks
2.2. Overview of mobile service chains
2.3. The most common classification scheme
2.4. More sophisticated classification schemes
3. Example use cases specific to mobile networks
3.1. Service chain model for Internet HTTP services
3.1.1. Weaknesses of current approaches
3.2. Service chain for TCP optimization
3.2.1. Weaknesses of current approaches
3.3. HTTP header enrichment in mobile networks
4. Remarks on QoS in mobile networks
5. Weaknesses of current implementations
5.1. Granularity of the classification scheme
5.2. Service chain implementations
6. Operator requirements
6.1. Simplicity of service function chain instantiation
6.2. Extensions
6.3. Reflections on Metadata
6.4. Delimitations
7. Security Considerations
8. IANA Considerations
9. Acknowledgments
10. References
10.1. Normative References
10.2. Informative References
Authors' Addresses
1. Introduction
1.1. Terminology and abbreviations
Much of the terminology used in this document has been defined by the
3rd Generation Partnership Project (3GPP), which defines standards
for mobile service provider networks. Although a few terms are
defined here for convenience, further terms can be found in
[RFC6459].
UE User equipment like tablets or smartphones
eNB enhanced NodeB, radio access part of the LTE system
S-GW Serving Gateway, primary function is user plane mobility
P-GW Packet Gateway, actual service creation point, terminates 3GPP
mobile network, interface to Packet Data Networks (PDN)
HSS Home Subscriber System (control plane element)
MME Mobility Management Entity (control plane element)
GTP GPRS (General Packet Radio Service) Tunnel Protocol
S-IP Source IP address
D-IP Destination IP address
IMSI The International Mobile Subscriber Identity that identifies a
mobile subscriber
(S)Gi Egress termination point of the mobile network (SGi in case of
LTE, Gi in case of UMTS/HSPA). The internal data structure of
this interface is not standardized by 3GPP
PCRF 3GPP standardized Policy and Charging Rules Function
PCEF Policy and Charging Enforcement Function
IDS Intrusion Detection System
FW Firewall
ACL Access Control List
PEP Performance Enhancement Proxy
1.2. General scope of mobile service chains
Mobile access networks terminate at a mobile service creation point
(Packet Gateway) typically located at the edge of an operator IP
backbone. From the user equipment (UE) up to the Packet Gateway
(P-GW) everything is fully standardized by the 3rd Generation
Partnership Project (3GPP) e.g., in [TS.23.401]. Within the mobile
network, the user payload is encapsulated in 3GPP specific tunnels
terminating eventually at the P-GW. In many cases application-
specific IP traffic is not directly exchanged between the original
mobile network, more specific the P-GW, and an application platform,
but will be forced to pass a set of service functions. Those
application platforms are, for instance, a web server environment, a
video platform, a social networking platform or some other multimedia
platform. Network operators use these service functions to
differentiate their services to their subscribers. Service function
chaining is thus integral to the business model of operators.
Important use cases classes for service function chains generally
include:
1. functions to protect the carrier network and the privacy of its
users(IDS, FW, ACL, encryption, decryption, etc.),
2. functions that ensure the contracted quality of experience using
e.g., performance enhancement proxies (PEP) like video
optimizers, TCP optimizers or functions guaranteeing fair service
delivery based on policy based QoS mechanisms,
3. functions like HTTP header enrichment that may be used to
identify and charge subscribers real time,
4. functions like CG-NAT/PAT, which are required solely for
technical reasons, and
5. functions like parental control or malware detection that may be
a cost option of a service offer.
1.3. General structure of end-to-end carrier networks
Although this memo is focused on the Service Function Chaining use
cases for mobile carrier networks, such as 3GPP- based ones, a number
of other, different carrier networks exists that share similarities
in the structure of the access networks and the service functions
with mobile networks.
Figure 1 shows a simplified schematic view of 4 different access
service networks to indicate similarities with respect to Service
Functions and their Chaining. These service networks consist of
access-specific user equipment, a dedicated access network, a related
service creation point and finally a (LAN) infrastructure hosting
Service Functions which eventually interconnect to application
platforms in the Internet or in the carrier's own datacenter (DC).
From top to down, there is a 3GPP mobile network terminating at the
P-GW, a xDSL network with its PPP tunnels terminating at a BNG
(Broadband Network Gateway), a FTTH network terminating at an OLT
(Optical Line Terminal) and finally a cable TV network terminating at
a CMTS (Cable Modem Termination System).
Access Service Functions (Categories)
Services +---------------------------+
+--+ *~~~~~~~* +-----+ |+--1---+ +--2---+ +--3---+|| +---------+
|UE|--| 3GGP |---| P-GW|--|| NAT | | MWD | | TCP || |Internal |
+--+ *~~~~~~~* +-----+ || . | | | | Opt. ||-|Appl. |
|| FW | | Par. | | . || |Platforms|
+--+ *~~~~~~~* +-----+ || . | | Ctrl | | Video || |(e.g.IMS)|
|UE|--| xDSL |---| BNG |--|| LB | | . | | Opt. || +---------+
+--+ *~~~~~~~* +-----+ || . | | LI | | . ||
|| DPI | | . | | Head. ||
+--+ *~~~~~~~* +-----+ || . | | . | | Enr. || +---------+
|UE|--| FTTH |---| OLT |--|| . | | | | . || | |
+--+ *~~~~~~~* +-----+ || . | | | | . || | |
|| . | | | | ||-|Internet |
+--+ *~~~~~~~* +-----+ || | | | | || | |
|UE|--| CATV |---| CMTS|--|| | | | | || | |
+--+ *~~~~~~~* +-----+ |+------+ +------+ +-------+| +---------+
+---------------------------+
Figure 1: Various end-to-end carrier networks and service functions
sorted into categories 1, 2 and 3.
Category 1 of service functions like NAT or DPI may be used by all of
these service networks mainly just (but not exclusively) for
technical reasons. The same is true for category 2, Value Added
Services (VAS) like parental control, malware detection and
elimination (MWD) or Lawful Interception (LI). TCP optimization is
basically only seen in mobile networks only. The same may be true
for video optimizers or HTTP header enrichment; i.e., category 3 as a
rule mainly belongs to mobile networks only.
In our view, 3GPP-based mobile networks seem to have the largest
demand for service functions and service function chains. Service
Function Chains used in other access networks are very likely a
subset of what one can expect in 3GPP-based mobile networks.
Typical data center use cases are described in
[ietf-sfc-dc-use-cases]. Some technical aspects for the handling of
long-lived flows are discussed in
[ietf-sfc-long-lived-flow-use-cases].
2. Mobile network overview
For simplicity we only describe service function chaining in the
context of LTE (Long Term Evolution) networks. But indeed our
service chaining model also applies to earlier generations of mobile
networks, such as purely UMTS-based mobile networks.
2.1. Building blocks of 3GPP mobile LTE networks
The major functional components of a LTE network are shown in
Figure 2 and include user equipment (UE) like smartphones or tablets,
the LTE radio unit named enhanced NodeB (eNB), the serving gateway
(S-GW) which together with the mobility management entity (MME) takes
care of mobility and the packet gateway (P-GW), which finally
terminates the actual mobile service. These elements are described
in detail in [TS.23.401]. Other important components are the home
subscriber system (HSS) and the policy and charging rule function
(PCRF), which are described in [TS.23.203]. The P-GW interface
towards the SGi-LAN is called the SGi-interface, which is described
in [TS.29.061]. Finally, the SGi-LAN is the home of service function
chains (SFC), which are not standardized by 3GPP.
+--------------------------------------------+
| Control Plane (C) [HSS] | [OTT Appl. Platform]
| | | |
| +--------[MME] [PCRF]--+--------+ Internet
| | | | | | |
| [UE-C] -- [eNB-C] == [S-GW-C] == [P-GW-C] | | |
+=====|=========|==========|============|====+ +-----+----+-------+
| | | | | | | | | |
| [UE-U] -- [eNB-U] == [S-GW-U] == [P-GW-U]-+--+----[SGi-LAN] |
| | | | |
| | | | |
| | | [Appl. Platform] |
| | | |
| User Plane (U) | | |
+--------------------------------------------+ +------------------+
|<---------- 3GPP Mobile Network -------->| |<-- IP Backbone ->|
Figure 2 shows the end to end context including all major components
of a LTE network. The actual 3GPP mobile network includes the
elements from the user equipment [UE] to the packet gateway [P-GW].
Figure 2: End to end context including all major components of a LTE
network. The actual 3GPP mobile network includes the elements from
the user equipment [UE] to the packet gateway [P-GW].
The radio-based IP traffic between the UE and the eNB is encrypted
according 3GPP standards. Between eNB, S-GW and P-GW user IP packets
as well as control plane packets are encapsulated in 3GPP-specific
tunnels. In some mobile carrier networks the 3GPP-specific tunnels
between eNB and S-GW are even additionally IPSec-encrypted. More
precisely, IPSec originates/terminates at the eNB and on the other
side at an IPSec-GW often placed just in front of the S-GW. For more
details see [TS.29.281], [TS.29.274] and [TS.33.210].
Service function chains act on user plane IP traffic only. But the
way these act on user traffic may depend on subscriber, service or
network specific control plane metadata.
2.2. Overview of mobile service chains
The original user IP packet, including the Source-IP-Address (S-IP)
of the UE and the Destination-IP-Address (D-IP) of the addressed
application platform, leaves the Packet Gateway from the mobile
network via the so-called Gi-interface (3G service, e.g., UMTS)
respectively SGi-interface (4G service, e.g., LTE). Between this
(S)Gi-interface and the actual application platform the user
generated upstream IP packets and the corresponding downstream IP
packets are typically forced to pass an ordered set of service
functions, loosely called a Service Function Chain (SFC).
The set of all available service functions (physical or virtualized)
which can be used to establish different Service Function Chains for
different services is often called a Gi-LAN for 2G/3G services and
SGi-LAN for 4G services.
The (S)Gi-interface towards the (S)Gi-LAN itself is discussed in
[TS.29.061], but service function chaining is not specified by 3GPP.
The (S)Gi-LAN service functions can use subscriber and service
related metadata delivered from the mobile control plane, such as the
PCRF, or from the user plane e.g., via HTTP Header Enrichment to
process the flows according to service related policies. Some
service functions may even use network performance data describing
the actual momentary state of a network segment.
In short, the (S)Gi-LAN service area is presently used by mobile
service providers to differentiate their services to their
subscribers and reflect the business model and of mobile operators.
For different applications (e.g., Appl. 1,2,3) upstream and
downstream user plane IP flows will be forced to pass a sequence of
service functions which is called a Service Function Chain specific
to a given application. In the simple example sketched in Figure 3,
the service chains for applications 1, 2 and 3 may be just classified
by a unique interface-ID of the egress P-GW interfaces where the
service chains for application 1, 2 and 3 are attached.
+------------------------------------------------------------------+
| Control Plane Environment [HSS] [MME] [PCRF] [others] |
+------------------------------------------------|-----------------+
+--------------------+
+---------------------------|--------------------|-----------------+
| User Plane Environment | | |
| | +------(S)Gi-LAN --+-----+ |
| | | | |
| | | +---[SF1]-[SF3]-[SF5]---[Appl. 1] |
| | | / | |
| [UE]---[eNB]===[S-GW]===[P-GW]-----[SF2]-[SF4]-[SF6]--------+ |
| | \ | | |
| | +---[SF7]-[SF8]-[SF9]-----+ | |
| | | | | |
| +------------------------+ | | |
| | | |
+----------------------------------------------------------|--|----+
| |
OTT Internet Applications
| |
[Appl. 2] [Appl. 3]
Figure 3: Typical service chain topology.
Service functions typically observe, alter or even terminate and re-
establish application session flows between mobile user equipment and
application platforms. Control plane metadata supporting policy
based traffic handling may be linked to individual service functions
SFn. Because in Figure 3 the P-GW classifies service chains, we
consider the P-GW as a component of the service chaining environment.
However, more sophisticated classification schemes are possible and
discussed later.
Care must be taken in classifying and directing flows in different
directions (upstream versus downstream) or different flows from the
same subscriber when Service Functions observe or alter session
flows. Such functions can maintain local state that is necessary to
the correct functioning of session flows (e.g., non-transparent
proxies) or to enforcing the policies of the service provider (e.g.,
fair-use policies). Such stateful service functions can require
steering both upstream and downstream directions of a flow through a
single service function instance (e.g., from a set of identical
service function instances deployed for scale) or for steering all
flows with a common criteria (e.g., belonging to the same subscriber)
through such a single service function instance.
2.3. The most common classification scheme
Mobile user equipment like smartphones, tablets or other mobile
devices address use Access Point Names (APNs) to address a service
network or service platform. APNs are DNS host names and comparable
to FQDN host names. While a FQDN refers to an Internet IP address,
an APN (loosely speaking) specifies a P-GW IP address. These APNs
are used to distinguish certain user groups and their traffic, e.g.,
there can be an APN for a mobile service offered to the general
public while enterprise customers get their own APN. For APN details
see [TS.23.003].
Operators often associate a designated VLAN-ID with an APN. A VLAN-
ID n then may classify the service function chain n (SFC n) related
to an application platform n (Appl. n), as shown in the following
Figure 4.
+----------+
| |
| IF-1 O [APN 1 => VLAN-ID 1] ---- [SFC 1] ---- [Appl. 1]
| |
=====| P-GW O . . . .
| |
| IF-n O [APN n => VLAN-ID n] ---- [SFC n] ---- [Appl. n]
| |
+----------+
Figure 4: Association of a service chain to an application platform.
Examples for an APN are, e.g.:
+------------+-----------------+
| APN: | web.vodafone.de |
| User Name: | not required |
| Password: | not required |
+------------+-----------------+
Table 1: Example APN for Vodafone Germany
+------------+------------------+
| APN: | internet.telekom |
| User Name: | t-mobile |
| Password: | tm |
+------------+------------------+
Table 2: Example APN for Deutsche Telekom/T-Mobile
2.4. More sophisticated classification schemes
More sophisticated classifications are feasible using metadata that
is UE related, subscriber and service related, as well as network
related metadata. Typical metadata and their sources are:
UE: terminal type (e.g., HTC one), IMSI (country, carrier, user)
GTP tunnel endpoint: eNB-Identifier, time, and many more
PCRF: subscriber info, APN (service name), QoS, policy rules
Mobile operator defined subscriber, service or network specific
policies are typically encoded in the 3GPP-based Policy and Charging
Rules Function (PCRF), see [TS.23.203]. For instance, a PCRF may
encode the rules that apply to pre-paid and post-paid users, users
with a classification of gold, silver, or bronze, or even as detailed
as describing rules that apply to "gold users, wishing to download a
video file, while these subscribers are subjected to a fair-usage
policy". It is up to the mobile service providers to encode the
precise mappings between its subscriber classes and the associated
service chains.
The Traffic Detection Function (TDF) is part of the 3GPP PCC (Policy
and Charging Architecture, [TS.23.203]). Such a TDF inspects the
user traffic after leaving the P-GW function (see Figure 4). The TDF
can be used to classify traffic originating from an APN into more
detailed services. This could be used to classify traffic into
different Service Functions.
+-------------------------+
| PCRF |
+----+--------------+-----+
| |
Gx-IF Sd-IF
| |
+----+-----+ +----+-----+
==========O [PCEF] | | O--------[SFC 1] ---- [Appl. 1]
| | | O--------[SFC 2] ---- [Appl. 2]
==========O P-GW O---O [TDF] O--------[SFC 3] ---- [Appl. 3]
| | | O--------[SFC 4] ---- [Appl. 4]
==========O | | O--------[SFC n] ---- [Appl. n]
+----------+ +----------+
* *
3GPP Bearer SGi SGi
Figure 5: 3GPP Traffic Detection Function (TDF) for classification.
The Policy and Charging Enforcement Function (PCEF) enforces policies
received from the PCRF and, in the other direction, provides the PCRF
with subscriber and access information via Gx interface.
The TDF will typically observe the traffic on all layers. On
application start and stop the TDF provides feedback to the PCRF for
further actions to be taken on a particular flow. The PCRF can
request that the TDF apply application and detection controls to
application flows including charging and usage monitoring. The TDF
can also act without any interaction with the PCRF taking care of
gating (firewalling), traffic redirection, bandwidth management or
charging.
3. Example use cases specific to mobile networks
Because HTTP via TCP port 80 (or TCP port 443 for HTTPS) is by far
the most common Internet traffic class, we discuss two typical
examples of an associated service function chaining model in some
more detail.
The models presented below are simplified compared to real life
service function chain implementations because we do not discuss
differentiated traffic handling based on different subscriber-
specific service level agreements and price plans or even actual
network load conditions.
3.1. Service chain model for Internet HTTP services
With the increase of Internet traffic in mobile networks mobile
operators have started to introduce Performance Enhancement Proxies
(PEPs) to optimize network resource utilization. PEPs are more or
less integrated platforms that ensure the best possible Quality of
Experience (QoE). Their service functions include but are not
limited to Deep Packet Inspection (DPI), web and video optimizations,
subscriber and service policy controlled dynamic network adaption,
analytics and management support.
A simple service function chain model for mobile Internet upstream
and downstream traffic is shown in Figure 6 below. The function
chain includes Load Balancers (LB), which split HTTP over TCP port 80
away from the rest of the internet traffic. Beside basic web
content, this traffic class includes more and more video. To act on
this traffic type we force this traffic to pass Performance
Enhancement Proxies. The firewall function (FW) protects the carrier
network from the outside and Network Address Translation (NAT) maps
the private IP address space dedicated to user equipment to a public
IP address.
[Cache]
|
[P-GW]---[LB]-----------[PEP]--[LB]--[FW]---[NAT]---[Internet]
| HTTP:80 |
| |
| |
| non HTTP:80 |
+---------------------+
Figure 6: Service function chain for HTTP traffic over TCP port 80.
The first application in this Service Chain example caches web
content to help reduce Round Trip Times (RTT) and therefore
contribute to improved web page load times. Optimizing RTT and
thereby improving Quality of Experience (QoE) is generally more
important for mobile service providers than reducing Internet peering
costs. Similar arguments hold for cached video content. We avoid
potential large jitter imported from the Internet.
An example for non HTTP:80 traffic in Figure 6 is UDP-encapsulated
IPsec traffic, which is dedicated to port 10000. Note that in a real
environment not only port 80 but for example additional traffic via
port 8080, 25 for SMTP, 110 for POP3 or 143 for IMAP may be forced to
pass a service chain.
A second application is video optimization. Video content from the
Internet may not fit to the size of mobile device displays or simply
would overload the mobile network when used natively. Therefore
mobile operators adapt internet-based video content to ensure the
best QoE.
Video content optimization very often is also an additional premium-
related component in service offers and price plans.
Our PEP environment for video optimization consists of three basic
service functions shown in Figure 7: a steering proxy which is able
to redirect HTTP traffic, a DPI-based controller which checks for
video content and an optimizer which transcodes videos to an
appropriate format on the fly in real time.
[PEP for video] ==>> [Steering Proxy]---[DPI Contr.]---[Optimizer]
Figure 7: Service functions required for video optimization.
In Figure 8 we show the detailed HTTP flows and their redirection in
some more detail. The intention here is to show every elementary
functional step in a chain as a separate physical or virtualized
item, but this state diagram must not necessarily reflect every
existing vendor-specific proprietary implementation.
Specifically, the Steering Proxy includes a TCP proxy and an ICAP
client which communicates with an ICAP server residing in the
controller function which is supported by a L7 DPI.
The video optimization process acts on the downstream flow only.
[UE]----[Steering Proxy]----[DPI Contr.]----[Optimizer]----[Content]
|-- HTTP GET ->|-------------- HTTP GET ----------------------->|
|<------------- HTTP Response -------------------|
|-- Is it Video? ->|
|<-- Video found --|
|<--- HTTP ----|
Redirect
|-- HTTP GET ->|-----HTTP GET ---------------->|
|-- HTTP GET -->|
Video
|<--- HTTP -----|
Response
Orig. Video
{Optimize}
|<------- HTTP Response --------|
Transcoded Video
|-- Is it Video? ->|
|<-- Video found --|
|<--- HTTP ----|
Response
Transcoded Video
Figure 8: Flow diagram between UE and video optimization PEP.
In such an application scenario one can have reclassification or off-
loading on the fly.
Assume a video streams within a 4G LTE radio cell. The video
optimizer would then apply a transcoding scheme appropriate to the
abilities of the 4G network. If one is now leaving the 4G cell and
entering a 3G radio cell, the network conditions will most probably
become different and the video optimizer has to use another
transcoding scheme to keep a certain QoE. This requires that the
video optimizer service function is aware of the Radio Access
Technology (RAT) in use. One may transfer RAT type from the P-GW (or
GGSN in case of 3G traffic) via an AAA Proxy to the service function
chain. The RAT information will then be embedded in an appropriate
Radius message. Other 3GPP steering mechanisms may apply as well.
If for example the 4G network has sufficient bandwidth, one could
also think of another, different use case. The rule could be that
only 3G video streams are forced to pass the video optimizer while
all 4G video traffic will be bypassed.
Additionally, network utilization information can be used to trigger
the behavior of the service function. The degree of video
compression applied could depend on the actual current network load.
Last but not least the behavior of the video optimizer service
function (or any other service function) could additionally depend on
the user-specific service contract (price plan, gold, silver, bronze)
or on individual on demand requests.
3.1.1. Weaknesses of current approaches
This use case model highlights the weakness of current service
deployments in the areas of traffic selection, reclassification, and
multi-vendor support. Traffic in this example is classified after
the P-GW and separated into separate flows based on whether it is (in
this example) TCP traffic destined to port 80. This classification
could be done by the load balancer (see Figure 6) if it initiates the
service chain selection, or the traffic can be reclassified at the
load balancer if the traffic is already embedded in a Service Chain
(e.g., when combined with other functions such as the TCP
optimization in the following use case). Multi-vendor support is
needed because every element in the use case can be provided by a
different vendor: load-balancer, HTTP proxy, firewall, and NAT.
3.2. Service chain for TCP optimization
The essential parameters influencing TCP behavior are latency, packet
loss and available throughput.
Content servers are mostly attached to fixed networks. These are
characterized by high bandwidth and low latency. On the other side,
Radio Access Networks (RANs) tend to have higher latency, packet
loss, and congestion.
The fixed network part will typically get a TCP Rx/Tx buffer size
different to that one on the radio network side, because buffer sizes
are typically set proportional to the latency (rule of thumb: 2x
latency x bandwidth).
TCP cannot handle such disparate end-to-end situations very well.
One way to mitigate this problem is to use split TCP. However, even
without congestion, packet losses on the mobile network side may
result in time outs and finally can cause the content server on the
fixed network side to stall.
Therefore mobile operators often use TCP optimization proxies in the
data path. These proxies monitor latency and throughput real-time
and dynamically optimize TCP parameters for each TCP connection to
ensure a better transmission behavior.
A rudimentary service chain setup for TCP optimization is shown in
Figure 9. Examples of non TCP flows are UDP/RTP voice or video
traffic.
[P-GW]---[LB]----------[TCP Opt.]---[LB]---[FW]---[NAT]---[Internet]
| TCP |
| |
| non-TCP |
+--------------------------+
Figure 9: Optimizing TCP parameterization in a mobile network.
3.2.1. Weaknesses of current approaches
This use case highlights weaknesses of current service deployments in
the areas of traffic selection, reclassification, and multi-vendor
support as in the previous use case presented in Section 3.1.
3.3. HTTP header enrichment in mobile networks
In legacy mobile networks WAP (Wireless Application Protocol)
gateways mediated between traditional mobile phones and the Internet
translating HTML web content into a WML (Wireless Markup Language)
and vice versa. By functionality, WAP-GWs fit also in the SFC
category.
Traditionally WAP-GWs use HTTP header enrichment to insert subscriber
related data into WAP and HTTP request headers in real time. These
data were (are) used to identify and charge subscribers on third
party web sites.
Today 3G and 4G mobile networks HTTP header enrichment is done by the
GGSN/P-GW or a dedicated transparent HTTP optimizer as most of the
data traffic on a mobile network no longer passes a WAP-GW.
Information typically added to the header includes:
o Charging Characteristics
o Charging ID
o Subscriber ID
o GGSN or PGW IP address
o Serving Gateway Support Node (SGSN) or SGW IP address
o International Mobile Equipment Identity (IMEI)
o International Mobile Subscriber Identity (IMSI)
o Mobile Subscriber ISDN Number (MSISDN)
o UE IP address
4. Remarks on QoS in mobile networks
As indicated in Figure 3, service functions may be linked to the
control plane to take care of additional subscriber or service
related metadata. In many cases the source of metadata would be the
PCRF and the link would be a Gx or Diameter-based Sd interface.
Diameter is specified in [RFC6733] and Gx in [TS.29.212].
Service functions within the SGi/Gi-LAN are less focused on the
explicit QoS steering of the actual mobile wireless network. QoS in
mobile networks is based on the 3GPP "Bearer" concept. A Bearer is
the essential traffic separation element enabling traffic separation
according to different QoS settings and represents the logical
transmission path between the User Equipment (UE) and the Packet
Gateway (P-GW).
5. Weaknesses of current implementations
In many operator environments every new service introduction can
result in a further dedicated (S)Gi-LAN service chain, because
service chaining has been deployed historically in an ad hoc manner.
It typically requires placement of new functions in the operator's
data center, changing the actual wiring to include any new or changed
service function, configuration of the functions and network
equipment, and finally testing of the new configuration to ensure
that everything has been properly setup.
5.1. Granularity of the classification scheme
Often the coarse grained classification according to APNs is not fine
enough to uniquely select a service function chain or a processing
scheme within a service function chain required to support the
typical user-, service- or network- related policies which the
operator likes to apply to a specific user plane flow.
It is very likely that an APN, such as shown in Section 2.3, is
carrying an extremely diverse set of user traffic. This can range
from HTTP web traffic to real-time traffic.
5.2. Service chain implementations
In many carrier networks service chain LANs grow incrementally
according product introductions or product modifications. This very
often ends in a mix of static IP links, policy based routing or
individual VRF implementations etc. to enforce the required sequence
of service functions. Major weak points seen in many carrier
networks are:
o Very static service chain instances, hard-wired on the network
layer leads to no flexibility with respect to reusing, adding, and
removing service nodes and reprogramming service chains.
o Evolutionary grown "handcrafted" connectivity models require high
complexity to manage or maintain.
o Basic implementation paradigm is based on APNs (that is service
types) only, which requires individual injections of context-
related metadata to obtain granularity down to user/service level.
o There is currently no natural (or standardized) information
exchange on network status between services and the network,
complicating management of network resources based on subscriber
profiles.
o It is currently practically impossible to implement an automated
service provisioning and delivery platform.
o Scaling up flows or service chains with service or subscriber
related metadata is extremely diffcult.
6. Operator requirements
Mobile operators use service function chains to enable and optimize
service delivery, offer network related customer services, optimize
network behavior or protect networks against attacks and ensure
privacy. Service function chains are essential to their business.
Without these, mobile operators are not able to deliver the necessary
and contracted Quality of Experience (QoE) or even certain products
to their customers.
6.1. Simplicity of service function chain instantiation
Because product development cycles are very fast in mobile markets,
mobile operators are asking for service chaining environments which
allow them to instantly create or modify service chains in a very
flexible and very simple way. The creation of service chains should
be embedded in the business and operation support layers of the
company and on an abstract functional level, independent of any
network underlay. No knowledge about internetworking technology
should be required at all. The mapping of the functional model of a
service chain onto the actual underlay network should be done by a
provisioning software package similar to that shown in Figure 10.
Details of the architecture and design are the subject of forthcoming
standards and proprietary implementation details.
+------------------------------------------------------------------+
| Creation of an abstract service function chain |
+------------------------------------------------------------------+
|
V
+------------------------------------------------------------------+
| +----------------------------------------------------+ |
| | Service function chain compiler | |
| +----------------------------------------------------+ |
| | |
| V |
| +----------------------------------------------------+ |
| | Mediation device | |
| +----------------------------------------------------+ |
+------------------------------------------------------------------+
|
V
+------------------------------------------------------------------+
| Physical network underlay |
+------------------------------------------------------------------+
Figure 10: Creation and provisioning system for service function
chains.
Service functions can be physical or virtualized. In the near future
the majority of mobile service functions will typically reside in the
local cloud computing environment of a mobile core location.
Nevertheless, first trials have shown that (virtualized) Service
Function interconnects via WAN links require careful latency
considerations.
Last but not least, any implementation must take into account that in
the migration phase a mixed infrastructure including virtual machines
and bare metal based systems must be supported.
6.2. Extensions
A service function chain should be generalized by a directed graph
where the vertices (nodes) represent an elementary service function.
This model allows branching conditions at the vertices. Branching in
a graph could then be triggered by typical 3GPP specified mobile
metadata (see Section 2.3) and allow for more sophisticated steering
methods in a service chain. Typically this data will be injected by
the mobile control plane, commonly by the PCRF via a Diameter-based
3GPP Sd interface.
Service chain behavior and output should additionally depend on
actual network conditions. For example, the selection of a video
compression format could depend on the actual load of the mobile
network segment a mobile user is attached to. That is to say that
classification of flows may allow very dynamic inputs while
specification of such inputs from a 3GPP network will need to be done
by 3GPP or another standards body.
Although necessary metadata can be obtained from the PCRF, to avoid
Diameter signaling storms in the (S)Gi-LAN, individual service
functions should probably not be attached individually to the control
plane. A mechanism where such metadata is carried by a metadata
header can reduce requests to the PCRF, provided these extensions do
not increase the original payload size too much.
6.3. Reflections on Metadata
At the moment we see just two types of metadata classes. Metadata
which are static and related to subscriber and service policies
residing typically in the control plane environment and dynamic
metadata, which may reflect time and location dependent status
somewhere in the network or other service platforms, e.g., load
conditions or relevant network technology indicators. It may be
useful to have proper interfaces to inject these metadata into the
Service Function Chain domain.
Summarized, metadata may by injected into individual Service Chain
Functions:
o asynchronously from the control plain environment by means of
individual standardized interfaces,
o synchrounously, piggypacked with the user IP packet:
* by means of a to-be-defined metadata header
* or carried with http header enrichements within the user
payload.
6.4. Delimitations
A clear separation between service chaining functionality and 3GPP
bearer handling is required. This may be subject of forthcoming
studies.
7. Security Considerations
Organizational security policies must apply to ensure the integrity
of the SFC environment.
8. IANA Considerations
This document has no actions for IANA.
9. Acknowledgments
We thank Peter Bosch, Carlos Correia, Dave Dolson, Linda Dunbar, Alla
Goldner, Wim Hendericks, Konstantin Livanos, Praveen Muley, Ron
Parker, and Nirav Salot for valuable discussions and contributions.
10. References
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
10.2. Informative References
[RFC6459] Korhonen, J., Soininen, J., Patil, B., Savolainen, T.,
Bajko, G., and K. Iisakkila, "IPv6 in 3rd Generation
Partnership Project (3GPP) Evolved Packet System (EPS)",
RFC 6459, January 2012.
[RFC6733] Fajardo, V., Arkko, J., Loughney, J., and G. Zorn,
"Diameter Base Protocol", RFC 6733, October 2012.
[TS.23.003]
"Numbering, addressing and identification", 3GPP TS 23.003
12.1.0, December 2013.
[TS.23.203]
"Policy and charging control architecture", 3GPP TS 23.203
12.3.0, December 2013.
[TS.23.401]
"General Packet Radio Service (GPRS) enhancements for
Evolved Universal Terrestrial Radio Access Network
(E-UTRAN) access", 3GPP TS 23.401 12.3.0, December 2013.
[TS.29.061]
"Interworking between the Public Land Mobile Network
(PLMN) supporting packet based services and Packet Data
Networks (PDN)", 3GPP TS 29.061 12.4.0, December 2013.
[TS.29.212]
"Policy and Charging Control (PCC); Reference points",
3GPP TS 29.212 13.0.0, January 2015.
[TS.29.274]
"3GPP Evolved Packet System (EPS); Evolved General Packet
Radio Service (GPRS) Tunnelling Protocol for Control plane
(GTPv2-C); Stage 3", 3GPP TS 29.274 12.3.0, December 2013.
[TS.29.281]
"General Packet Radio System (GPRS) Tunnelling Protocol
User Plane (GTPv1-U)", 3GPP TS 29.281 11.6.0, March 2013.
[TS.33.210]
"3G security; Network Domain Security (NDS); IP network
layer security", 3GPP TS 33.210 12.2.0, December 2012.
[ietf-sfc-arch]
and , "Service Function Chaining (SFC) Architecture", I-D
draft-ietf-sfc-architecture-03 (work in progress), January
2015.
[ietf-sfc-dc-use-cases]
, , , , and , "Service Function Chaining Use Cases In Data
Centers", I-D draft-ietf-sfc-dc-use-cases-01 (work in
progress), July 2014.
[ietf-sfc-long-lived-flow-use-cases]
, , , , and , "SFC Long-lived Flow Use Cases", I-D draft-
ietf-sfc-long-lived-flow-use-cases-01 (work in progress),
December 2014.
[ietf-sfc-problem-statement]
and , "Service Function Chaining Problem Statement", I-D
draft-ietf-sfc-problem-statement-10 (work in progress),
August 2014.
Authors' Addresses
Walter Haeffner
Vodafone
Vodafone D2 GmbH
Ferdinand-Braun-Platz 1
Duesseldorf 40549
DE
Phone: +49 (0)172 663 7184
Email: walter.haeffner@vodafone.com
Jeffrey Napper
Cisco Systems
Cisco Systems, Inc.
Haarlerbergweg 13-19
Amsterdam 1101 CH
NL
Email: jenapper@cisco.com
Martin Stiemerling
NEC
NEC Europe Ltd.
Kurfuersten-Anlage 36
Heidelberg 69181
DE
Email: mls.ietf@gmail.com
Diego R. Lopez
Telefonica I+D
Don Ramon de la Cruz, 82
Madrid 28006
ES
Phone: +34 913 129 041
Email: diego@tid.es
Jim Uttaro
AT&T
200 South Laurel Ave
Middletown, NJ 07748
US
Email: uttaro@att.com