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Versions: (draft-haeffner-sfc-use-case-mobility) 00 01 02 03 04 05 06 07 08

Service Function Chaining                                    W. Haeffner
Internet-Draft                                                  Vodafone
Intended status: Informational                                 J. Napper
Expires: November 15, 2018                                 Cisco Systems
                                                          M. Stiemerling
                                                    Hochschule Darmstadt
                                                                D. Lopez
                                                          Telefonica I+D
                                                               J. Uttaro
                                                                    AT&T
                                                            May 14, 2018


         Service Function Chaining Use Cases in Mobile Networks
                  draft-ietf-sfc-use-case-mobility-08

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 SFC problem statement and architecture
   framework 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 middle box based 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



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   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
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   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 November 15, 2018.

Copyright Notice

   Copyright (c) 2018 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
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.


















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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Terminology and abbreviations  . . . . . . . . . . . . . .  4
     1.2.  General scope of mobile service chains . . . . . . . . . .  5
     1.3.  General structure of end-to-end carrier networks . . . . .  6
   2.  Mobile network overview  . . . . . . . . . . . . . . . . . . .  7
     2.1.  Building blocks of 3GPP mobile LTE networks  . . . . . . .  7
     2.2.  Overview of mobile service chains  . . . . . . . . . . . .  8
     2.3.  The most common classification scheme  . . . . . . . . . . 11
     2.4.  More sophisticated classification schemes  . . . . . . . . 12
   3.  Example use cases specific to mobile networks  . . . . . . . . 14
     3.1.  Service chain model for Internet HTTP services . . . . . . 14
       3.1.1.  Weaknesses of current approaches . . . . . . . . . . . 17
     3.2.  Service chain for TCP optimization . . . . . . . . . . . . 18
       3.2.1.  Weaknesses of current approaches . . . . . . . . . . . 18
     3.3.  HTTP header enrichment in mobile networks  . . . . . . . . 18
       3.3.1.  HTTP header enrichment in legacy mobile data
               networks . . . . . . . . . . . . . . . . . . . . . . . 19
       3.3.2.  HTTP header enrichment in modern mobile data
               networks . . . . . . . . . . . . . . . . . . . . . . . 19
       3.3.3.  HTTP header enrichment security condiderations . . . . 19
   4.  Remarks on QoS in mobile networks  . . . . . . . . . . . . . . 20
   5.  Weaknesses of current implementations  . . . . . . . . . . . . 20
     5.1.  Granularity of the classification scheme . . . . . . . . . 20
     5.2.  Service chain implementations  . . . . . . . . . . . . . . 20
   6.  Operator requirements  . . . . . . . . . . . . . . . . . . . . 21
     6.1.  Simplicity of service function chain instantiation . . . . 21
     6.2.  Extensions . . . . . . . . . . . . . . . . . . . . . . . . 22
     6.3.  Reflections on Metadata  . . . . . . . . . . . . . . . . . 23
     6.4.  Delimitations  . . . . . . . . . . . . . . . . . . . . . . 23
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 24
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 24
   9.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 24
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 24
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 24
     10.2. Informative References . . . . . . . . . . . . . . . . . . 24
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 26













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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 Server (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  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

   TDF  Traffic Detection Function

   TSSF  Traffic Steering Support Function

   IDS  Intrusion Detection System






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   FW Firewall

   ACL  Access Control List

   PEP  Performance Enhancement Proxy

   IMS  IP Multimedia Subsystem

   LI Lawful Interception

1.2.  General scope of mobile service chains

   Mobile access networks terminate at a mobile service creation point
   (called Packet Gateway) typically located at the edge of an operator
   IP backbone.  From the user equipment (UE) up to the Packet Gateway
   (P-GW) or, if deployed, the Traffic Detection Function (TDF)
   everything is fully standardized by the 3rd Generation Partnership
   Project (3GPP) e.g., in [TS.23.401] and in [TS.23.203].  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 specifically 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 case classes for service function chains generally
   include:

   o  functions to protect the carrier network and the privacy of its
      users(IDS, FW, ACL, encryption, decryption, etc.),

   o  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
      built upon policy based QoS mechanisms,

   o  functions like HTTP header enrichment that may be used to identify
      and charge subscribers real time,

   o  functions like Carrier Grade NAT (CG-NAT) and NAPT, which are
      required solely for technical reasons, and





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   o  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 (or TDF), an 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 CATV (cable TV) network terminating at a CMTS
   (Cable Modem Termination System).

           Access                  Service Functions
          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.




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   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 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].


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 an 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), the Policy and Charging Rule Function (PCRF)
   and the optional components: the Traffic Detection Function (TDF) and
   the Traffic Support Steering Function (TSSF), 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].  The TDF resides on
   this interface.  Finally, the SGi-LAN is the home of service function
   chains (SFC), which are not standardized by 3GPP.










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   +--------------------------------------------+
   | 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: End to end context including all major components of an LTE
    network. The actual 3GPP mobile network includes the elements from
    the user equipment [UE] to the packet gateway [P-GW]. Labels ending
    with -C denote control plane functionality and ending with -U user
     plane functionality, respectively. Separation of control and user
        plane is presented for a logical view and is not currently
                           standardized by 3GPP.

   The radio-based IP traffic between the UE and the eNB is encrypted
   according to 3GPP standards.  Between eNB, S-GW and P-GW user IP
   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 (see Section 2.4 for a
   discussion of metadata in the context of this document).

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 (or any host in the Internet in general), 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



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   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], and service function chaining classification or traffic
   steering is discussed in [TS.23.203].

   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.

   If a network operator utilizes HTTP Header Enrichment, care must be
   taken that privacy is ensured by some mechanism especially when IP
   service flows leave the operator's network towards a third party (see
   Section 3.3.3).

   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 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 or TDF where
   the service chains for application 1, 2 and 3 are attached.














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   +------------------------------------------------------------------+
   | Control Plane Environment   [HSS]   [MME]   [PCRF]   [others]    |
   +------------------------------------------------|-----------------+
                               +--------------------+
   +---------------------------|--------------------|-----------------+
   | User Plane Environment    |                    |                 |
   |                           | /------(S)Gi-LAN --+-----\           |
   |                           | |                        |           |
   |                           | |  +---[SF1]-[SF3]-[SF5]---[Appl. 1] |
   |                           | | /                      |           |
   | [UE]---[eNB]===[S-GW]===[P-GW/TDF]--[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 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.






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2.3.  The most common classification scheme

   Mobile user equipment like smartphones, tablets or other mobile
   devices use Access Point Names (APNs) to address a service network or
   service platform.  APNs are DNS host names comparable to FQDN (Fully
   Qualified Domain Name) host names.  While an 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 Virtual LAN ID (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





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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., vendor), 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 Control Architecture, [TS.23.203]) architecture.  Such a
   TDF, when deployed in the network, resides on the SGi interface, can
   inspect the user traffic after leaving the P-GW function (see
   Figure 4) and can maintain connections to the charging
   infrastructure: Online Charging System (OCS) and Offline Charging
   System (OFCS).  The TDF can be used to classify traffic originating
   from an APN into more detailed services.  This can be used to
   classify traffic into different Service Functions.

   The Traffic Steering Support Function (TSSF) has also been defined
   recently (since Rel. 13) as part of the 3GPP PCC architecture to
   support classification of traffic into different Service Functions.
   The TSSF does not have any connections into the charging
   infrastructure (OFS or OFCS), but does maintain an interface (St)
   into the PCRF.  Over this interface, the PCRF can provision, modify
   and remove classification rules for steering traffic into different
   Service Functions.










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             +--------------------------+
             |           PCRF           |
             +----+---------+-------+---+
                  |         |       |
                Gx-IF     St-IF   Sd-IF
                  |         |       |
             +----+-----+   |       |
   ==========O  [PCEF]  O--------------------[SFC 1] ---- [Appl. 1]
             |   P-GW   O--------------------[SFC 2] ---- [Appl. 2]
             +----------+   |       |
                            |       |
             +----------+   |  +----+---+
   ==========O   P-GW   O------O [TDF]  O----[SFC 1] ---- [Appl. 1]
             |          |   |  |        O----[SFC 2] ---- [Appl. 2]
             +----------+   |  +--------+
                            |
             +----------+  ++-------+
   ==========O   P-GW   O--O [TSSF] O--------[SFC 1] ---- [Appl. 1]
             |          |  |        O--------[SFC 2] ---- [Appl. 2]
             +----------+  +--------+
                        *
   3GPP Bearer         SGi

   Figure 5: 3GPP combined service chaining classification and steering
    points. The Policy and Charging Enforcement Function (PCEF) and the
    Traffic Detection Function (TDF) enforce policies received from the
     PCRF and, in the other direction, the PCEF provides the PCRF with
   subscriber and access information via Gx interface. The TSSF can also
      be used by the PCRF over the St interface to steer traffic into
                            Service Functions.

   In general, there are several possibilities to specify classification
   and steering in mobile networks.  Figure 5 demonstrates combined
   classification and steering deployments.  There are also separated
   classification and steering deployments. 3GPP considers these
   approaches: [TS.23.203]:

   o  Classification and steering by the P-GW alone with rules received
      from Gx.

   o  Classification and steering by the TDF alone with rules received
      from Sd.

   o  Classification and steering by the TSSF alone with rules received
      from St.

   o  Initial classification by the P-GW (with rules received from Gx)
      and steering by the TDF (with rules received from Sd).



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   o  Initial classification by the TDF (with rules received from Sd)
      and steering by the TSSF (with rules received from St interface).


3.  Example use cases specific to mobile networks

   HTTP via TCP port 80 (or TCP port 443 for HTTPS) is by far the most
   common Internet traffic class.  Therefore, 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 (PEPs).  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.













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                          [Cache]
                             |
   [P-GW/TDF]---[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
   contributes 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 also
   avoid potential large jitter imported from the Internet.

   An example for non HTTP port 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.

   A typical PEP environment for video optimization consists of three
   basic service functions as 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 one possible way for 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 does not necessarily reflect



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   every existing vendor-specific proprietary implementation.

   Specifically, the Steering Proxy includes a TCP proxy and an ICAP
   (Internet Content Adaptation Protocol) 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 may have reclassification or off-
   loading on the fly.



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   Assume a video is streamed 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
   Gateway GPRS Support Node (GGSN) in case of 3G traffic) via an
   Authorization, Authentication and Accounting (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.  Bypassing certain Service
   Functions is also known as off-loading.

   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 or TDF 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),
   possibly directed by a TSSF (not shown), 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.







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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 on the radio network side because buffer sizes are
   typically set proportional to the latency (rule of thumb: 2 x 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 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






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3.3.1.  HTTP header enrichment in legacy mobile data 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.

3.3.2.  HTTP header enrichment in modern mobile data networks

   Today, in 3G and 4G mobile networks HTTP header enrichment is done by
   the Gateway GPRS Support Node (GGSN)/P-GW/TDF 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

3.3.3.  HTTP header enrichment security condiderations

   In today's networks HTTP header enrichment is commonly used across
   operator and ISP boundaries.  In such cases one must implement
   security mechanisms, e.g., solutions which are based on a one-time,
   session-based key exchanged between user equipment and third party
   over the top (OTT) service platforms.




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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 Diameter-based Gx or Sd reference point.
   Diameter is specified in [RFC6733], Gx/Sd in [TS.29.212] and St in
   [TS.23.203].

   Service functions within the (S)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 to product introductions or modifications.  This very often
   ends in a mix of static IP links, policy based routing or individual
   VRF (Virtual Routing and Forwarding) implementations, etc. to enforce



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   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 difficult.


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.



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   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, classical hardware "boxes" and bare metal based systems
   (i.e., computes without using virtualization) 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 these data will be injected by
   the mobile control plane, commonly but not exclusively by the PCRF



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   via a Diameter-based 3GPP Sd/St reference point.

   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 are 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
   typically reside 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 plane environment by means of
      individual standardized interfaces,

   o  synchronously, piggypacked with the user IP packet:

      *  by means of a to-be-defined metadata header

      *  or carried with http header enrichments 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.





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7.  Security Considerations

   Organizational security policies must apply to ensure the integrity
   of the SFC environment.

   SFC will very likely handle user traffic and user specific
   information in greater detail than the current service environments
   do today.  This is reflected in the considerations of carrying more
   metadata through the service chains and the control systems of the
   service chains.  This metadata will contain sensitive information
   about the user and the environment in which the user is situated.
   This will require proper considerations in the design, implementation
   and operations of such environments to preserve the privacy of the
   user and also the integrity of the provided metadata.


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, Dirk von Hugo, Konstantin Livanos, Praveen
   Muley, Ron Parker, Nirav Salot and Takeshi Usui for valuable
   discussions and contributions.

   We especially thank Narseo Vallina Rodriguez (ICSI Berkeley
   University) for multiple discussions on HTTP header extensions and
   network security.


10.  References

10.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/
              RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

10.2.  Informative References

   [RFC6459]  Korhonen, J., Ed., Soininen, J., Patil, B., Savolainen,
              T., Bajko, G., and K. Iisakkila, "IPv6 in 3rd Generation
              Partnership Project (3GPP) Evolved Packet System (EPS)",
              RFC 6459, DOI 10.17487/RFC6459, January 2012,



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              <https://www.rfc-editor.org/info/rfc6459>.

   [RFC6733]  Fajardo, V., Ed., Arkko, J., Loughney, J., and G. Zorn,
              Ed., "Diameter Base Protocol", RFC 6733, DOI 10.17487/
              RFC6733, October 2012,
              <https://www.rfc-editor.org/info/rfc6733>.

   [TS.23.003]
              "Numbering, addressing and identification", 3GPP TS 23.003
              14.1.0, September 2016.

   [TS.23.203]
              "Policy and charging control architecture", 3GPP TS 23.203
              14.1.0, September 2016.

   [TS.23.401]
              "General Packet Radio Service (GPRS) enhancements for
              Evolved Universal Terrestrial Radio Access Network
              (E-UTRAN) access", 3GPP TS 23.401 14.1.0, September 2016.

   [TS.29.061]
              "Interworking between the Public Land Mobile Network
              (PLMN) supporting packet based services and Packet Data
              Networks (PDN)", 3GPP TS 29.061 14.1.0, September 2016.

   [TS.29.212]
              "Policy and Charging Control (PCC); Reference points",
              3GPP TS 29.212 14.1.0, September 2016.

   [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 14.1.0,
              September 2016.

   [TS.29.281]
              "General Packet Radio System (GPRS) Tunnelling Protocol
              User Plane (GTPv1-U)", 3GPP TS 29.281 13.2.0, June 2016.

   [TS.33.210]
              "3G security; Network Domain Security (NDS); IP network
              layer security", 3GPP TS 33.210 13.0.0, December 2015.

   [ietf-sfc-dc-use-cases]
              Kumar, S., Tufail, M., Majee, S., Captari, C., and S.
              Homma, "Service Function Chaining Use Cases In Data
              Centers", I-D draft-ietf-sfc-dc-use-cases-05 (work in
              progress), August 2016.



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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 17-19
   Amsterdam  1101 CH
   NL

   Email: jenapper@cisco.com


   Martin Stiemerling
   Hochschule Darmstadt

   Email: mls.ietf@gmail.com


   Diego R. Lopez
   Telefonica I+D
   Editor Jose Manuel Lara, 9
   Seville,   41013
   ES

   Phone: +34 682 051 091
   Email: diego.r.lopez@telefonica.com


   Jim Uttaro
   AT&T
   200 South Laurel Ave
   Middletown, NJ  07748
   US

   Email: uttaro@att.com





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