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Internet Engineering Task Force                                  A. Jain
Internet-Draft                                                 A. Terzis
Intended status: Informational                                    Google
Expires: March 10, 2016                                        H. Flinck
                                                             N. Sprecher
                                                          S. Arunachalam
                                                          Nokia Networks
                                                                K. Smith
                                                                Vodafone
                                                       September 7, 2015


          Mobile Throughput Guidance Inband Signaling Protocol
             draft-flinck-mobile-throughput-guidance-03.txt

Abstract

   The bandwidth available for end user devices in cellular networks can
   vary by an order of magnitude over a few seconds due to changes in
   the underlying radio channel conditions, as device mobility and
   changes in system load as other devices enter and leave the network.
   Furthermore, packets losses are not always signs of congestion.  The
   Transmission Control Protocol (TCP) can have difficulties adapting to
   these rapidly varying conditions leading to inefficient use of a
   cellular network's resources and degraded application performance.
   Problem statement, requirements and the architecture for a solution
   is documented in [Req_Arch_MTG_Exposure].

   This document proposes a mechanism and protocol elements that allow
   the cellular network to provide near real-time information on
   capacity available to the TCP server.  This "Throughput Guidance"
   (TG) information would indicate the throughput estimated to be
   available at the radio downlink interface (between the Radio Access
   Network (RAN) and the mobile device (UE)).  TCP server can use this
   TG information to ensure high network utilization and high service
   delivery performance.  The document describes the applicability of
   the proposed mechanism for video delivery over cellular networks; it
   also presents test results from live operator's environment.

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/.



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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 March 10, 2016.

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
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
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   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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Contributing Authors  . . . . . . . . . . . . . . . . . .   3
     1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
     1.3.  Acronyms and Abbreviations  . . . . . . . . . . . . . . .   3
     1.4.  Definitions . . . . . . . . . . . . . . . . . . . . . . .   4
     1.5.  Assumptions and Considerations for the Solution . . . . .   4
   2.  Protocol  . . . . . . . . . . . . . . . . . . . . . . . . . .   6
     2.1.  Common Kind-Length-Value header . . . . . . . . . . . . .   8
     2.2.  Plain text mode Throughput Guidance Options . . . . . . .  10
     2.3.  Encrypted mode  . . . . . . . . . . . . . . . . . . . . .  11
     2.4.  Nonce (Initialization Vector) . . . . . . . . . . . . . .  13
     2.5.  Authentication  . . . . . . . . . . . . . . . . . . . . .  14
   3.  Applicability to Video Delivery Optimization  . . . . . . . .  15
     3.1.  Test Results  . . . . . . . . . . . . . . . . . . . . . .  15
   4.  Manageability considerations  . . . . . . . . . . . . . . . .  16
   5.  Security considerations . . . . . . . . . . . . . . . . . . .  16
   6.  IANA considerations . . . . . . . . . . . . . . . . . . . . .  17
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  18
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  18
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  18
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  18
   Appendix A. . . . . . . . . . . . . . . . . . . . . . . . . . . .  19
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  19




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1.  Introduction

   The problem statement related to the behavior of the TCP in cellular
   networks is provdied in [Req_Arch_MTG_Exposure].  That same document
   specifies the requirements, reference architecture and proposed
   solution principles for a mobile throughput guidance exposure
   mechanism that can be used to assist TCP in cellular networks,
   ensuring high utilization and high service delivery performance.

   This document presents a set of considerations and assumptions for
   the development of a solution.  It specifies a protocol that
   addresses the requirements and the architecture stated in the
   [Req_Arch_MTG_Exposure].  This document describes also the
   applicability of the proposed mechanism to video delivery over
   cellular networks with test results from live production environment.

1.1.  Contributing Authors

   The editors gratefully acknowledge the following additional
   contributors: Peter Szilagyi/Nokia, Csaba Vulkan/Nokia, Ram Gopal/
   Nokia, Guenter Klas/Vodafone and Peter Cosimini/Vodafone.

1.2.  Terminology

   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 RFC 2119 [RFC2119].

1.3.  Acronyms and Abbreviations






















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   This document uses the following acronyms:

   ECGI     E-UTRAN Cell Global Identifier format
   ECN      Explicit Congestion Notification
   HMAC     Hash-based Message Authentication Code
   HTTP     Hypertext Transfer Protocol
   HTTPS    Hypertext Transfer Protocol Secure
   IP       Internet Protocol
   IV       Initialization Vector
   LTE      Long Term Evolution
   MTG      Mobile Throughput Guidance
   RAN      Radio Access Network
   RCTP     RTP Control Protocol
   RTT      Round Trip Time
   SACK     Selective Acknowledgement
   TCP      Transmission Control Protocol
   TCP-EDO  TCP Extended Data option
   TG       Throughput Guidance
   UE       User Equipment

1.4.  Definitions

   Throughput Guidance Provider:

      A functional element in the RAN that signals to the TCP server the
      information on the (near-real time) throughput estimated to be
      available at the radio downlink interface

1.5.  Assumptions and Considerations for the Solution

   This document specifies a solution protocol that is compliant with
   the requirements and architecture specified in
   [Req_Arch_MTG_Exposure].  The protocol is used by the cellular
   network to provide throughput guidance information to the TCP server;
   this information indicates the throughput estimated to be available
   at the radio downlink interface for the TCP connection.  The protocol
   allows the information to be provided in near real time in situations
   where the network conditions are changing frequently or the user is
   moving.

   While the implementation details can vary according to the access
   technology, the resource allocation is abstracted as the capacity of
   the "radio link" between the RAN and the UE.  For example, in the
   case of an LTE network, the number of physical resource blocks
   allocated to a UE, along with the modulation scheme and coding rate
   used, can be translated into radio link capacity in Megabits per
   second (Mbit/s).  From the derived UE's total throughput and with the




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   UE's TCP flow information, Throughput guidance for the TCP connection
   can be computed.

   The TCP server can use this explicit information to inform several
   congestion control decisions.  For example: (1) selecting the initial
   congestion window size, (2) deciding the value of the congestion
   window during the congestion avoidance phase, and (3) adjusting the
   size of the congestion window when the conditions on the "radio link"
   change.  In other words, with this additional information, TCP
   neither has to congest the network when probing for available
   resources (by increasing its congestion window), nor rely on
   heuristics to decide how much it should reduce its sending rate after
   a congestion episode.

   The same explicit information can also be used to optimize
   application behavior given the available resources.  For example,
   when video is encoded in multiple bitrates, the application server
   can select the highest encoding rate that the network can deliver.

   This solution specified in this document also satisfies the following
   assumptions and considerations:

   o  The end-to-end traffic is delivered via HTTP.

   o  The end-to-end traffic is encrypted (through HTTPS), thus HTTP
      header enrichment cannot be used by intermediate elements between
      the client and the server.

   o  TCP is used to deliver the HTTPS traffic.

   o  The Real-time Transport Protocol (RTP) network protocol is not
      used for traffic delivery.

   The protocol specified in this document assumes that a trustful
   relationship between the Throughput Guidance Provider and the TCP
   server has been formed using the means discussed in the Security
   considerations section.

   The solution in this document satisfies the considerations and the
   assumptions presented above, and proposes an in-band exposure
   mechanism where the throughput guidance information is added to the
   TCP headers of the relevant upstream packets.  HTTP and TCP are the
   most prevalent protocols in the Internet, used even by the most
   popular streaming application.  Throughput guidance at TCP level can
   be shared among multiple applications; it is not limited to any
   particular application level optimization only but it offers a
   generic approach that works even if application level end-to-end
   encryption, e.g HTTPS, is applied.



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   In particular, the Throughput Guidance Providers adds the throughput
   guidance information to the Options field of the TCP header (see RFC
   0793 [RFC0793]) of packets from the TCP client to the TCP server.  An
   in-band mechanism is proposed because it does not require a separate
   interface, reference value, or correlation mechanism that would be
   needed with out of band approaches such as with RCTP that is limited
   to only certain types of applications.  Furthermore, an in-band
   mechanism can keep up with the rapid changes in the underlying radio
   link throughput.  The proposed scheme is similar to existing
   mechanisms such as ECN, where an ECN- aware router sets a mark in the
   IP header in order to signal impending congestion (see [RFC3168]).
   Note, however, that the proposed scheme provides explicit
   information, (termed "Throughput Guidance") about the estimated
   throughput available for the TCP connection at the radio link between
   the RAN and the UE.

   Note that once standardized and implemented, TCP Extended Data option
   (TCP-EDO) can be used to carry the throughput guidance information as
   specified in [tcp-edo] and simplify the use of the TCP Option fields
   by extending the space available for TCP options.  Currently the TCP-
   EDO is still work in progress and not available in production.
   Therefore, the use of TCP-EDO to carry throughput guidance is left
   for the later drafts.

2.  Protocol

   This section describes the protocol mechanism and the information
   element that needs to be communicated from the RAN to the TCP remote
   endpoint.  We describe the protocol mechanism and message format for
   throughput guidance.  The protocol mechanism is defined in an
   extensible way to allow additional information to be specified and
   communicated.  The protocol specification is based on the existing
   experiments and running code.  It is recommended to insert the
   throughput guidance information to the TCP segments that flow from
   client to server (see reasoning in "Assumptions and Considerations"
   section).  Most of the time, TCP segments are ACK packets from a
   client to the server and hence packets are unlikely to be fragmented.
   However, the described protocol solution can deal with fragmentation.

   The Mobile Throughput Guidance Signaling message conveys information
   on the throughput estimated to be available at the down link path for
   a given TCP connection.  The information is sent to the uplink end-
   point of the connection (i.e, the TCP server).  The TCP server MAY
   use this information to adapt TCP behavior and to adjust application-
   level behavior to the link conditions as defined in
   [Req_Arch_MTG_Exposure].





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   A good example is a content optimizer or a cache that can adapt the
   application-level coding to match the indicated downlink radio
   conditions.  As radio link conditions may change rapidly, this
   guidance information is best carried in-band using TCP options
   headers rather than through an out-of-band protocol.

   Using the TCP options to carry throughput guidance associates the
   guidance information with an ongoing TCP connection and explicitly
   avoids separate session identification information.  The proposed
   mechanism neither impacts the TCP state machine nor the congestion
   control algorithms of the TCP protocol.

   The Options field enables information elements to be inserted into
   each packet with a 40-byte overall limit; this needs to be shared
   with the standardized and widely-used option elements, such as the
   TimeStamp and SACK.  (Use of TCP-EDO will lift this constraint once
   available and deployed).  The TCP Options field uses a Kind-Length-
   Value structure that enables TCP implementations to interpret or
   ignore information elements in the Options field based on the Kind.

   In this draft, we define a Kind-Length-Value structure for encoding
   information about the estimated capacity of a radio access link
   between the RAN and the UE which is traversed by a TCP connection.
   The intention is to define a generic container to convey in-band
   information within the limited TCP Option space with optional
   authentication and/or encryption capabilities.  Throughput guidance
   is the conveyed information in this document.  Additional information
   can be specified in future.

   The Throughput Guidance Provider functional element inserts Mobile
   Throughput Guidance TCP options only if there is enough space in the
   TCP header.  The Throughput Guidance Provider resides on top of a
   radio network element see [Req_Arch_MTG_Exposure]).

   Confidential information must be delivered in a secure way.  The
   information can be provided as plain text in a secure and closed
   network.  In other cases, the information should be authenticated and
   encrypted at the TCP-header level (between the Throughput Guidance
   Provider and the TCP server).  An acceptable level of authentication
   and encryption (according to best common practices) may require more
   data than fits into a single TCP header (maximum of 40 bytes if no
   other options are present).  As described below, fragmenting
   information across multiple packets will be used is such a case.

   Two transfer modes are defined to deal with data confidentiality in
   this document; namely, plain-text mode and authenticated encryption
   mode.  A third mode, authentication-only mode, is equally feasible.
   A third mode, authentication-only mode, is equally feasible and may



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   use TCP Authentication Option (TCP-AO) (see RFC 5935 [RFC5935]).  We
   will describe the authentication-only mode in detail in future
   version of this draft.  Both modes share a common Kind-Length-Value
   "option header" structure with a flag field separating the two cases.

2.1.  Common Kind-Length-Value header

   Mobile Throughput Guidance Signaling uses the common TCP options
   structure as in [RFC0793] with experimental identifier as defined in
   [RFC6994].  To make Mobile Throughput Guidance Signaling extendible
   to different use cases a common Kind-Length-Value structure is
   defined below.  To make Mobile Throughput Guidance Signaling
   extendible to different use cases a common Kind-Length-Value header
   is defined below.


       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |Kind | Length | ExID |Flags|     variable length data          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


                                 Figure 1

    Kind:

      Code point 253 for Experimental Opition for 16-bit ExID [RFC6994].
      The size of this field is 1 byte.

    Length:

      A 1 byte field, length of the option in bytes as defined in
      RFC793.

    ExID:

      Two bytes Experimental Identifier according to [RFC6994].  Code
      point 0x6006.

    Flags:

      One byte of MTG protocol flag field as defined below.










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           0 1 2 3 4 5 6 7
          +-+-+-+-+-+-+-+-+
          | Seq |Frag |P|T|
          +-+-+-+-+-+-+-+-+


               Flag field of common Kind-Lenght-Value header

                                 Figure 2

    Seq:

      Three-bit sequence number that maintains context across different
      packet types as defined by P- and T-bits below.  The scope of the
      sequence number is to protect against packet reordering, not to
      provide a globally unique identifier or sequence number.  The use
      of these bits are reserved for possible transfer mode extensions.

    Frag:

      Three bits that provide information about how to reassemble
      information if fragmented into multiple packets.  If no
      fragmentation across multiple TCP packet headers is needed, these
      bits are set to zero.  Otherwise, Frag is a counter starting from
      1 and incremented by 1 for each subsequent packet of the same type
      (see P- and T-bits below).  For the last fragment, the Fragment is
      always 7 (binary 111) to indicate that the information is
      complete.

    P and T bits:

      These two bits encode the packet type: Plaintext (P=0, T= 0),
      Cipher text (P=0, T=1), Nonce (IV) (P=1, T=0) or Authentication
      (P=1, T=1).  For Plaintext, the Fragment bits are always zero.

     Variable length data:

      The variable length content (i.e. option data) in <type, value>
      format.  The content depends of the transfer mode as defined in
      the following sections of this document.  If the option data is
      fragmented across multiple headers the first fragment (marked with
      Frag=001 in the Flags-field) contains "Total Length of Data"-field
      that is the length of the variable data of MTG in all the
      fragments.  Total Length of Data field is followed the content in
      <type, value>-format

   As an example for the use of the Flags-field, consider a cipher text
   of a single block.  For it the T-bit is set to one, P-bit is set to



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   zero, Fragment and Seq-fields are zero in the Flags-field.  In case
   the cipher text option cannot fit into a single TCP packet option,
   the cipher text is fragmented across multiple TCP headers.  The first
   fragment has value Frag= 001, and the value is incremented for each
   subsequent fragment.  The first fragment contains the "Total Length
   of Data"-field indicating the total length of the data to be
   fragmented.  Last fragment is marked with all Frag-bits set to 1
   (Frag= 111 for the last fragment).  Therefore, the maximum number of
   fragments is seven.  Details follow in the next sections.

2.2.  Plain text mode Throughput Guidance Options

   The plain text mode can be used in secure and closed networks or with
   information that has no confidentiality requirement.  The plain text
   mode is made of one or more type-value pairs.  The type determines
   the length of the following value.

       Table of Type Value pairs of Throughput Guidance option data

       +---------------------+------+----------+------------------+
       |         Name        | Type |  Length  | Unit of the type |
       +---------------------+------+----------+------------------+
       | Throughput Guidance |  1   | 2 bytes  |     Mbits/s      |
       +---------------------+------+----------+------------------+

                       Table 1: MTG type-vale pairs

   The Type 1 element carries the actual throughput estimate in the
   16-bit value field The throughput value is encoded using a fixed-
   point number representation.  The 12 most significant bits are used
   for the integer value while the bottom 4 bits correspond to the
   decimal portion of the throughput value.  Throughput is expressed in
   Megabits per second.

   The type-value pair elements are laid out consecutively in the
   header.  At the end padding (i.e., the NO-OP TCP Option header with
   kind equal to 1, or the End of Option List TCP Option header with
   kind equal to 0) may be required to align the header size to the
   multiple of 4 bytes (required by the TCP standard).  All bits in the
   Flag field are set to zero.











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       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |Kind | Length |ExID|Flags |Type1|Value-1| Type2|Value-2|     ...     |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
           Kind, Length, ExID remains same as described in section 2.1.
           Options data constitutes the Flags and the variable length data.
           Flags: P- and T-bits set to zero

     Layout of plain text option data in the TCP header options space.

                                 Figure 3

2.3.  Encrypted mode

   Encryption requires authentication for integrity protection, as it is
   insecure to use encryption without it.  Thus, the encrypted mode
   contains authentication as well.  Encryption and authentication must
   use different keys.  The following diagram shows the encryption
   process.



                      +-+-+-+-+        +-+-+-+-+-+
     +-+-+-+-+-+      | key1  |        |IV(Nonce)|
     |key index| -->  +-+-+-+-+        +-+-+-+-+-+
     +-+-+-+-+-+      | key 2 |             |
                      +-+-+-+-+  key  +-+-+-V-+-+-+-+
                          ...   ----> |  AES 128-CNT|
                      +-+-+-+-+       +-+-+-+-+-+-+-+
                      | key n |             |
                      +-+-+-+-+             +<------ Plain text
                                            |
                                      +-+-+-V-+-+-+-+
                                      | Cipher text |
                                      +-+-+-+-+-+-+-+


                             Encryption method

                                 Figure 4

   The encryption uses Advanced Encryption Standard (AES), 128 bits (16
   bytes) block size, 128 bits (16 bytes) key size, Counter (CTR) block
   cipher mode.  Integrity protection with CTR mode is MUST; this is
   provided via HMAC based message authentication (see Authentication
   section below).

   The plaintext contains type-value pair elements of the variable
   length data.  The plaintext is divided into blocks of 16 bytes.  A



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   block of plain text MUST not exceed 16 bytes in a single run.
   Encryption takes a key (16 bytes), an IV or Nonce (16 bytes), the
   plain-text (at most 16 bytes) and produces a cipher text of 16 bytes.
   Note: multiple keys, at most 256, may be available (can be exchanged
   via an out-of-band key management mechanism such as Diffie-Hellman
   key exchange; this is out of scope of this document) for encryption
   key index.  The keys MUST be different from those used for
   authentication.

   The Nonce is 16 bytes.  A unique Nonce is generated for each
   encrypted block.  The same Initialization Vector, IV or Nonce MUST
   NOT be used with the same encryption key more than once.  This is to
   be enforced by the Throughput Guidance Provider; otherwise security
   scheme will be broken.

   The resulting cipher text is in blocks of 16 bytes.  The cipher text
   blocks are packed into the option space together with the used Key
   Index in a following way if they fit into single option space of a
   single TCP header.



       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |Kind | Length |ExID| Flags | Key Index |first block of 16 bytes |    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       Kind, Length, ExID remains same as described in section 2.1.
       Options data constitutes the Flags and the variable length data.
           Flags: Type of cipher text T-bit set to 1, only one block Frag= 000.
       Key Index is the index used in encryption


        Cipher text layout in the TCP options without fragmentation

                                 Figure 5

   The flag field of the common option header indicates that the content
   is cipher text by having the T bit set to one.  Since the ciphered
   block is not fragmented the Frag-bits of the flag field are set to
   zero (Frag= 000).  (Use of Seq bits is left for later submissions).
   If there is not enough space to accommodate the 16 bytes in the
   option data, the data is fragmented.

   If there are multiple cipher text blocks of 16 bytes, the flag field
   shows the type of the option being cipher text with the T-bit set to
   one, and by Frag-field showing the fragment number starting from 001
   and incremented by one for each subsequent fragment of a packet of




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   the same type.  For the last fragment, the Frag-field is always
   binary 111 to indicate the last fragment.



    First fragment:

     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
     |Kind |Length|ExID|Flags| Total Length|KeyIndex|1. block|fragmented block|
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

       Kind, Length, ExID remains same as described in section 2.1
           Options data constitutes the Flags, Total Length, Key Index  and the variable length data.
           Flags: Type of cipher text T-bit = 1, Frag field = 001 first fragment
       Total Length: total number of bytes of option data to be fragmented
       Key Index is the index used in encryption

    Second fragment if the last one:

     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |Kind | Length | ExID |Flags| Key Index | Rest of the fragmented block  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

            Kind, Length, ExID remains same as described in section 2.1
           Options data constitutes the Flags, Key Index and the variable length data.
           Flags: Type of cipher text T-bit = 1,  Frag field = 111 last fragment, otherwise 010.
       Total Length: total number of bytes in the fragments
       Key Index is the index used in encryption


          Cipher text layout extending to two consecutive headers

                                 Figure 6

2.4.  Nonce (Initialization Vector)

   The 16 byte Nonce (or IV) is transmitted along with the cipher text
   to protect against de-synchronization between the encryption-
   decryption points.












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       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |Kind | Length | ExID |Flags| Key Index |    Nonce (IV)  16 bytes |   |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

        Kind, Length, ExID remains same as described in section 2.1
           Options data constitutes the Flags and the variable length data.
           Flags: Type of IV/Nonce P-bit set to 1, only one block Frag= 000
       Key Index is the index used in encryption


                       Nonce (IV) in a single header

                                 Figure 7

   If the Nonce (IV) doesn't fit into the remaining free bytes of the
   option field it needs to be fragmented using the Frag-field in the
   same way as cipher text layout is extending across two or more
   consecutive TCP headers but with the option type field set to
   indicate Nonce/IV by P-bit set to 1.

2.5.  Authentication

   The authentication covers the cipher text, the Nonce (IV) and
   includes additional TCP protocol header fields to protect against
   replay attacks.  The authentication uses HMAC codes (e.g.  HMAC-
   SHA2-224), 128 bits (16 bytes) key size, 224 bits (28 bytes) digest
   size.  Multiple keys (at most 256) for authentication with the same
   information receiver can be used.  The keys MUST be different from
   those used for encryption.  Truncation is possible but at least 160
   bits (20 bytes) must be used from the digest to meet the typical
   security level of mobile networks.

   Authentication takes a key, the input (arbitrary length) and produces
   a 28 byte long digest, which is truncated to 20 bytes (keeping the
   most significant bytes).  The HMAC algorithm and truncation can be
   negotiated via key management (out of scope of this document).

   The authentication covers the TCP sequence number, ACK number, and
   TimeStamp (TSval, TSecr not the possible 2 bytes of padding) fields
   of the TCP header as well as the Common Kind-Length-ExID-header with
   its data in all cipher text option and IV/Nonce option packets.  (The
   Authentication type options itself cannot be covered by the
   authentication.)

   The order in which the fields are included into the message
   authentication code is the following.  From the TCP header: TCP Seq,
   ACK, TSval, TSecr.  Followed by the following fields from the
   ciphered text: Kind, Length, ExID, Flags, Key Index, cipher text, and



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   from the IV/Nonce type of option packets TCP Seq, ACK, TSval, TSecr
   (note cipher text and IV/Nonce type of options may be in different
   TCP packets) followed by Kind, Length, ExID, Flags, Key Index, Nonce/
   IV.

   In case the option packets used as input to the HMAC are fragmented
   into multiple TCP headers, they are processed so that headers with
   cipher text option are processed first, followed by IV/Nonce option
   packets.

   The options containing the result of the HMAC are marked by setting
   both P- and T-bits of the flag-field to one.  Key Index is set to
   point to the used authentication key, followed by the resulting
   authentication code.  If the option doesn't fit into the free option
   space in the TCP header, it is fragmented across multiple TCP headers
   in the same way as the cipher text options.

3.  Applicability to Video Delivery Optimization

   The applicability of the protocol specified in this document to
   mobile video delivery optimization has been evaluated and tested in
   different network load scenarios.

   In this use case, TCP traffic, for which throughput guidance
   information is required, passes through a Radio Analytics application
   which resides in a Mobile-edge Computing (MEC) server (see
   [MEC_White_Paper]).  This Radio Analytics application acts as the
   Throughput Guidance Provider and sends throughput guidance
   information for a TCP connection using the Options field in the TCP
   header (according to the message specification provided in section
   2).  The TCP server MAY use this information to assist TCP congestion
   control decisions as described above.  The information MAY also be
   used to select the application level coding so that it matches the
   estimated capacity at the radio downlink for that TCP connection.

   All of these improvements aim to enhance the quality of experience of
   the end user by reducing the time-to-start of the content as well as
   video stall occurrences.

3.1.  Test Results

   Nokia Networks and Google tested the video delivery optimization use
   case in a live production LTE network.  Google server was placed
   close to the packet core network of LTE (SGi-interface of LTE).
   Different network load scenarios were taken into consideration.  TCP
   Cubic was used in these tests [MTG_ICCRG].





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                      Field trial preformance results

   +-------------------+-----------------------+-----------------------+
   |    Performance    |     Difference of     |      Diff of 99th     |
   |       metric      |      Averages (%)     |      percentiles      |
   +-------------------+-----------------------+-----------------------+
   |    Time to play   |         -8.0%         |          -12%         |
   | Number of formats |         +4.1%         |         +29.9%        |
   |  Client bandwidth |         +0.7%         |         +8.0%         |
   |     Ave Video     |         +6.2%         |         +5.6%         |
   |     resolution    |                       |                       |
   |   Re-buffer time  |         -19.7%        |         -5.1%         |
   +-------------------+-----------------------+-----------------------+

                         Table 2: Performance Data

   These user experience improvements results into better video play and
   are likely to offer longer battery life.

4.  Manageability considerations

   The application in the RAN SHOULD be configured with a list of
   destinations to which throughput guidance should be provided.  The
   application in RAN will supply mobile throughput guidance information
   to more than one TCP server simultaneously based on the list of
   destinations.

   In addition, it SHOULD be possible to configure the frequency (in
   milliseconds) at which throughput guidance needs to be signaled as
   well as the required security level and parameters for the encryption
   and the authentication if supported.

5.  Security considerations

   Throughput guidance is considered confidential information and it
   SHOULD be provided in a secure way.  The information can be provided
   as plain text in a secure and closed network (e.g. inside operator
   network).  In other cases, the information should be authenticated
   and encrypted at the TCP-header level (between the Throughput
   Guidance Provider and the TCP server).

   Section 2 described how the TCP Header information can be signed and
   encrypted for security purposes.  An out-of-band mechanism is
   currently used to agree upon the set of keys used to encrypt and
   authenticate the messages exchanged between the endpoint and the
   network element that generates the throughput guidance headers.





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   As stated in [Req_Arch_MTG_Exposure], the policy configuration of the
   Throughput Guidance Provider and the server endpoint, as well as the
   key management and the encryption algorithm are beyond the scope of
   this protocol definition.  The protocol assumes that a trustful
   relationship has been formed between the Throughput Guidance Provider
   and the TCP server and that the required security level is already
   configured by the operator and agreed between the entities ( i.e.
   authentication, encryption or both).

   The identity of the Mobile Throughput Guidance provider that injects
   the throughput guidance header must be explicitly known to the
   endpoint receiving the information.  Omitting such information would
   enable malicious third parties to inject erroneous information.

   Fortunately, the issue of malicious disinformation can be easily
   addressed using well known techniques.  First, the network entity
   responsible for injecting the throughput guidance header can encrypt
   the header and include a cryptographically secure message
   authentication code.  In this way the transport endpoint that
   receives the throughput guidance header can check that the
   information was sent by a legitimate entity and that the information
   has not been tampered with.

   Furthermore, the throughput guidance information should be treated
   only as an estimate to the congestion control algorithm running at
   the transport endpoint.  The endpoint that receives this information
   should not assume that it is always correct and accurate.
   Specifically, endpoints should check the validity of the information
   received and if they find it erroneous they should discard it and
   possibly take other corrective actions (e.g., discard all future
   throughput guidance information from a particular IP prefix).

   The impact of TCP Authentication Option (TCP-AO) with encrypted TCP
   segment payload [tcp-ao-encrypt] implies that the Throughput Guidance
   Provider functional element acts as a full back to back TCP proxy.
   This case is left for later stages as the work [tcp-ao-encrypt] is
   still at draft stage.

6.  IANA considerations

   In the current version of the document and for field tests, the
   experimental value 253 is used for the "Throughput Guidance" TCP
   option kind.  ExpID SHOULD be set to 0x6006 (16 bits)








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7.  Acknowledgements

8.  References

8.1.  Normative References

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, DOI 10.17487/RFC0793, September 1981,
              <http://www.rfc-editor.org/info/rfc793>.

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

   [RFC6994]  Touch, J., "Shared Use of Experimental TCP Options",
              RFC 6994, DOI 10.17487/RFC6994, August 2013,
              <http://www.rfc-editor.org/info/rfc6994>.

8.2.  Informative References

   [I-D.narten-iana-considerations-rfc2434bis]
              Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", draft-narten-iana-
              considerations-rfc2434bis-09 (work in progress), March
              2008.

   [MEC_White_Paper]
              ETSI, "Mobile-Edge Computing - Introductory Technical
              White Paper", 2014.

   [MTG_ICCRG]
              Szilagyi, P., and Terzis, A., "Mobile Content Delivery
              Optimization based on Throughput Guidance", Presentation
              at ICCRG meeting IETF93 (work in progress), July 2015.

   [Req_Arch_MTG_Exposure]
              Jain, A., , Terzis, A., , Sprecher, N., , Arunachalam, S.,
              , Smith, K., , and G. Klas, "Requirements and reference
              architecture for Mobile Throughput Guidance Exposure",
              draft-sprecher-mobile-tg-exposure-req-arch-01.txt (work in
              progress), February 2015.

   [RFC2629]  Rose, M., "Writing I-Ds and RFCs using XML", RFC 2629,
              DOI 10.17487/RFC2629, June 1999,
              <http://www.rfc-editor.org/info/rfc2629>.





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   [RFC3168]  Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
              of Explicit Congestion Notification (ECN) to IP",
              RFC 3168, DOI 10.17487/RFC3168, September 2001,
              <http://www.rfc-editor.org/info/rfc3168>.

   [RFC3552]  Rescorla, E. and B. Korver, "Guidelines for Writing RFC
              Text on Security Considerations", BCP 72, RFC 3552,
              DOI 10.17487/RFC3552, July 2003,
              <http://www.rfc-editor.org/info/rfc3552>.

   [RFC4413]  West, M. and S. McCann, "TCP/IP Field Behavior", RFC 4413,
              DOI 10.17487/RFC4413, March 2006,
              <http://www.rfc-editor.org/info/rfc4413>.

   [RFC5935]  Ellison, M. and B. Natale, "Expressing SNMP SMI Datatypes
              in XML Schema Definition Language", RFC 5935,
              DOI 10.17487/RFC5935, August 2010,
              <http://www.rfc-editor.org/info/rfc5935>.

   [tcp-ao-encrypt]
              Touch, J., , "A TCP Authentication Option Extension for
              Payload Encryption", draft-touch-tcp-ao-encrypt-
              02.txt (work in progress), November 2014.

   [tcp-edo]  Touch, J., and Eddy, W., "TCP Extended Data Offset
              Option", draft-ietf-tcpm-tcp-edo-01.txt (work in
              progress), October 2013.

Appendix A.

Authors' Addresses

   Ankur Jain
   Google
   1600 Amphitheatre Parkway
   Mountain View, CA  94043
   US

   Phone: +1-925-526-5879
   Email: jankur@google.com











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   Andreas Terzis
   Google
   1600 Amphitheatre Parkway
   Mountain View, CA  94043
   US

   Phone: +1-650-214-5270
   Email: aterzis@google.com


   Hannu Flinck
   Nokia Networks
   Espoo
   FI

   Phone: +358504839522
   Email: hannu.flinck@nokia.com


   Nurit Sprecher
   Nokia Networks
   Hod HaSharon
   IL

   Phone: +97297751229
   Email: nurit.sprecher@nokia.com


   Swaminathan Arunachalam
   Nokia Networks
   Irving, TX
   US

   Phone: +19723303204
   Email: swaminathan.arunachalam@nokia.com


   Kevin Smith
   Vodafone
   One Kingdom Street, Paddington Central
   London  W2 6BY
   UK

   Phone: +19723303204
   Email: kevin.smith@vodafone.com






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