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Versions: 00 01 02

Network Working Group                                        G. Lencse
Internet Draft                       Budapest Univ. of Tech. and Econ.
Intended status: Experimental                             Sz. Szilagyi
Expires: December 2018                                        F. Fejes
                                                University of Debrecen
                                                          M. Georgescu
                                                               RCS&RDS
                                                         June 11, 2018



                    MPT Network Layer Multipath Library
                       draft-lencse-tsvwg-mpt-02.txt


Abstract

   Although several contemporary IT devices have multiple network
   interfaces, communication sessions are restricted to use only one of
   them at a time due to the design of the TCP/IP protocol stack: the
   communication endpoint is identified by an IP address and a TCP or
   UDP port number. The simultaneous use of these multiple interfaces
   for a communication session would improve user experience through
   higher throughput and improved resilience to network failures.

   MPT is a network layer multipath solution, which provides a tunnel
   over multiple paths using the GRE-in-UDP specification, thus being
   different from both MPTCP and Huawei's GRE Tunnel Bonding Protocol.

   MPT can also be used as a router, routing the packets among several
   networks between the tunnel endpoints, thus establishing a multipath
   site-to-site connection.

   The version of tunnel IP and the version of path IP are independent
   from each other, therefore MPT can also be used for IPv6 transition
   purposes.

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), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-
   Drafts.





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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."

   The list of current Internet-Drafts can be accessed at
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   The list of Internet-Draft Shadow Directories can be accessed at
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   This Internet-Draft will expire on December 11, 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.

Table of Contents

   1. Introduction ................................................. 3
      1.1. Design Assumptions ...................................... 3
      1.2. MPT in the Networking Stack ............................. 3
      1.3. Terminology ............................................. 4
      1.4. MPT Concept ............................................. 5
   2. Conventions Used in this Document ............................ 6
   3. Operation Overview ........................................... 6
   4. MPT Control .................................................. 8
      4.1. Configuration Information ............................... 8
         4.1.1. General Information for the MPT Server ............. 8
         4.1.2. Connection Specifications .......................... 9
      4.2. MPT Configuration Commands ............................. 13
   5. Possible Mappings of the Tunnel Traffic to Paths ............ 14
      5.1. Per Packet Based Mapping ............................... 17
      5.2. Flow Based Mapping ..................................... 18
      5.3. Combined Mapping ....................................... 19
   6. Packet Reordering ........................................... 19


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   7. Why MPT is Considered Experimental? ......................... 20
      7.1. Published Results ...................................... 21
         7.1.1. MPT Concept and First Implementation .............. 21
         7.1.2. Estimation of the Channel Aggregation Capabilities  21
         7.1.3. Demonstrating the Resilience of an MPT Connection . 21
      7.2. Open questions ......................................... 21
         7.2.1. Parameters......................................... 21
         7.2.2. Development of Further Mapping Algorithms.......... 21
         7.2.3. Performance Issues ................................ 22
   8. Security Considerations ..................................... 22
   9. IANA Considerations ......................................... 22
   10. Conclusions ................................................ 23
   11. References ................................................. 23
      11.1. Normative References .................................. 23
      11.2. Informative References ................................ 23
   12. Acknowledgments ............................................ 24
   Appendix A. Sample C code for packet reordering ................ 26



1. Introduction

   MPT is a multipath extension of the GRE-in-UDP encapsulation
   [RFC8086].

1.1. Design Assumptions

   MPT is intended to be used as a preconfigured tunnel and the
   application of MPT does not require any modifications to the
   applications using the TCP/IP socket interface API.

1.2. MPT in the Networking Stack

   The layer architecture of MPT is shown in Fig. 1. MPT extends the
   GRE-in-UDP [RFC8086] architecture by allowing multiple physical
   paths. To that end it can be compared to MPTCP [RFC6824], but unlike
   MPTCP, MPT uses UDP in the underlying layer, builds on GRE-in-UDP,
   and provides a tunnel IP layer, over which both UDP and TCP can be
   used. The aim of Huawei's GRE tunnel bonding protocol [RFC8157] is
   also similar to that of MPT: it targets to bonded access to wired
   and wireless network in customer premises. However, it uses GRE (not
   GRE-in-UDP) which is less supported in ISP networks than UDP, and it
   seems to limit the number of physical interfaces to two. For the
   comparison of MPT with other multipath solutions, please refer to
   [Alm2017].




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         +---------------------------------------------+
         |               Application (Tunnel)          |
         +---------------------------------------------+
         |                TCP/UDP (Tunnel)             |
         +---------------------------------------------+
         |               IPv4/IPv6 (Tunnel)            |
         +---------------------------------------------+
         |                 GRE-in-UDP                  |   +-----+
         +----------------------+----------------------+<--| MPT |
         |    UDP (Physical)    |    UDP (Physical)    |   +-----+
         +----------------------+----------------------+
         | IPv4/IPv6 (Physical) | IPv4/IPv6 (Physical) |
         +----------------------+----------------------+
         |    Network Access    |   Network Access     |
         +----------------------+----------------------+

                     Figure 1: MPT Layer Architecture

1.3. Terminology

   This document uses a number of terms that are either MPT specific or
   have defined meaning in the context of MPT as follows:

   MPT server: An MPT server is a software that implements network
      layer multipath communication by providing an UDP tunnel (named
      "connection" in the MPT terminology) over several underlying
      "paths".

   MPT client: An MPT client is a software tool, which is used to
      control the local MPT server (e.g. start/stop connections, add
      paths to connections, etc.).

   Connection: An MPT connection (also referred as communication
      session) is an UDP tunnel between two MPT servers, which can be
      used to carry user data. A connection can be established over one
      or more paths. A connection is initiated on the basis of a
      "connection specification".

   Path: A path is used to refer to the pair of the network cards of
      the end nodes (identified by the pair of IP addresses of the
      cards). Using a specified path, the packet transmission runs
      between the given pair of network cards.

   Connection specification: A connection specification is stored in a
      configuration file and it is used by an MPT server to establish
      an MPT connection with another MPT server. It contains all the
      configuration information for the connection (e.g. endpoint IP


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      versions and addresses, number of paths and configuration
      information for all paths). The precise definition of the
      connection specification can be found in Section 4.1.2.

   Data port: Data port means the GRE-in-UDP port defined in [RFC8086]
      as 4754. It is used for transmitting data packets.

   Local command port: An MPT server accepts commands from the MPT
      client at the local command port.

   Remote command port: An MPT server MAY accept commands from other
      MPT servers at the remote command port.

   Data plane: The parts and functions of MPT, which are responsible
      for handling user data packets.

   Control plane: All parts and functions of MPT except the data plane.
      E.g.: handling connections and paths, all the communication
      through local or remote command ports, etc.

1.4. MPT Concept

   When an MPT server is started, it reads its configuration files, and
   depending on its contents, it MAY wait for and accept connection(s)
   initiated by other MPT server(s) and/or it MAY initiate one or more
   MPT connection(s) with other MPT server(s). In the simplest case,
   the MPT server uses the connection specifications described in its
   configuration files for initiating connections. In addition to that,
   new connections MAY be established, connections MAY be closed, the
   parameters of the connections MAY be changed later (e.g. some paths
   may be switched on and off) dynamically by issuing the appropriate
   commands using an MPT client.

   MPT connections between MPT servers implement tunnels. The traffic
   comes from the tunnel interface is distributed over the active paths
   of the MPT connection by the MPT server. There are three possible
   mappings (see Section 5 for details and illustrative examples):

   o per packet based mapping, where a mapping decision is made for
      every single packet

   o flow based mapping, where the flows, identified by the usual five
      tuple, are always mapped to a given path.

   o combined mapping, where the flows, identified by the usual five
      tuple, are always mapped to a given connection and a mapping
      decision is made for every single packet of each connections.


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   The peer MPT server receives and de-encapsulates the traffic from
   the different paths and restores the tunnel traffic using the
   optional GRE sequence numbers for packet reordering if necessary.

2. Conventions Used in this Document

   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].

   In this document, these words will appear with that interpretation
   only when in ALL CAPS. Lower case uses of these words are not to be
   interpreted as carrying significance described in RFC 2119.

3. Operation Overview

   In this Section, we describe the operation of the data plane,
   whereas the operation of the control plane can be found in Section
   4.

   The data packet transmission and receive mechanism of MPT is
   summarized in Fig. 2. Now, we shall follow the route and processing
   of data packets.

   When a packet is read from the tunnel interface, the MPT software
   looks up the appropriate connection specification, which determines
   the mapping of the packets to the paths. The connection
   specification determines the behavior of the multipath
   communication, especially the distribution of the packets among the
   paths (see Section 5 for possible mapping methods). The path is
   selected and the user data packet encapsulated into a GRE-in-UDP
   data unit, which may optionally contain GRE Sequence Numbers for
   reordering. The simplest GRE header contains 4 octets: 16 bits of
   zeros and 16 bits of protocol type identification value (i.e. 0x86DD
   in the case of using IPv6 on the tunnel interface, or 0x0800 in the
   case of IPv4). Then the GRE-in-UDP data unit is encapsulated into
   the UDP/IP data unit of the selected path, where the destination UDP
   port number is the 4754 GRE-in-UDP port and the IP addresses (either
   both IPv4 or both IPv6) are determined by the path definition.
   Finally, the encapsulated packet is transmitted through the physical
   interface. The encapsulation of the different protocol data units is
   shown in Fig. 3.







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   +------------------------------------------------+
   |                Tunnel Interface                |
   +------------------------------------------------+
               ||                      /\
               ||                      ||
   ############||######################||############
   #           \/                      ||           #
   # +--------------------+  +--------------------+ #
   # |     data packet    |  | forwarding to the  | #  +----------+
   # |       reading      |  | tunnel interface   | #  | Control  |
   # +--------------------+  +--------------------+ #  | Protocol |
   #           ||                      /\           #  |   PDU    |
   #           \/                      ||           #  +--------- +
   # +--------------------+  +--------------------+ #    ||
   # | checking connection|  | packet reordering  | #    ||
   # |    specification   |  |     (optional)     | #    || ###########
   # +--------------------+  +--------------------+ #    \/ #   MPT   #
   #           ||                      /\           #<======# Control #
   #           \/                      ||           #       #         #
   # +--------------------+  +--------------------+ #       ###########
   # |   path selection,  |  |        data        | #
   # | GRE-in-UDP encaps. |  |      checking      | #
   # +--------------------+  +--------------------+ #
   #           ||                      /\           #
   #           \/                      ||           #
   # +--------------------+  +--------------------+ #
   # |   physical data    |  |     data packet    | #
   # |    transmission    |  |       reading      | #
   # +--------------------+  +--------------------+ #
   #           ||                      /\           #
   ############||######################||############
               ||                      ||
               \/                      ||
   +------------------------------------------------+
   |              Physical Interface                |
   +------------------------------------------------+

         Figure 2: Conceptual architecture of MPT working mechanism

   When a packet is read from the physical interface, its destination
   UDP port number is the 4754 GRE-in-UDP port. MPT reads the packet,
   identifies the connection the packet belongs to (by the source and
   destination IP addresses of the tunnel IP header) and runs checking
   mechanisms (e.g. connection validity check, GRE sequence number
   check or GRE Key value check, if present). If all the checking
   mechanisms finish successfully and no reordering is necessary, then
   the packet is promptly transmitted to the Transport and Application


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   Layers through the tunnel interface. If reordering is on and GRE
   sequence number indicates that one or more data unit(s) are missing,
   then the packet is placed into a buffer array for reordering
   purposes. (Reordering is discussed in Section 6.)

   +----------+-----------+---------+-----------+---------+-----------+
   | path IP  | path UDP  | GRE-in- | tunnel IP | tunnel  |application|
   | v4 or v6 |(port 4754)|   UDP   | v4 or v6  | TCP/UDP |   data    |
   +----------+-----------+---------+-----------+---------+-----------+

         Figure 3: PDU encapsulation of the MPT data communication

4. MPT Control

   A connection can be established between two MPT servers in two ways:

   1. When the MPT server is started, it establishes the connection on
      the basis of a connection specification from the configuration
      files. In this case, the connection specification contains all
      the necessary parameters. MPT client commands still can be used
      to modify the parameters, switch off and on paths, etc. as
      described in Section 4.2.

   2. The connection is established by using MPT client commands. In
      this case the command line arguments of the MPT commands and
      configuration files contain the necessary parameters.

4.1. Configuration Information

   The MPT configuration files contain various pieces of information.
   They can be divided into two groups:

   1. general information for the MPT server

   2. connections specification(s)

4.1.1. General Information for the MPT Server

   The MPT configuration file is made up of sections. The "general"
   section MUST be present and it contains general information for the
   operation of the MPT server, whereas there MAY several sections
   follow, each of which describes a different tunnel.

   The general section MUST contain the following elements:

   o tunnel number: the number of tunnels to create (they are to be
      described in separate sections)


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   o accept remote: it is a key (yes/no) whether this MPT server
      should accept commands from other MPT servers to build up
      connections, which are not defined in the local configuration
      files

   o local command port: the port number on which the local MPT client
      software can give commands to the MPT server

   o command timeout: the timeout value for the MPT client

   For each tunnel, a separate section is to be used to describe the
   following parameters:

   o name: The name is used by the operating system to access to the
      interface.

   o MTU: The maximum transmission unit of the tunnel interface. For
      the Ethernet environment, the value should be set between 1436
      and 1468 (depending on the additional header sizes, used by the
      actual system). It can be calculated as: 1500-
      Path_IP_header_size-UDP_header_size-GRE_header_size.

   o ipv4_addr: IPv4 address and mask

   o ipv6_addr: IPv6 address and mask

   Note that both ipv4_addr and ipv6_addr MAY be present. At least one
   of them MUST be present.

   It is important that the same tunnel may be used by several
   connections. A connection can be uniquely identified by the IP
   addresses of the two endpoints, which have to be of the same type
   (IPv4 or IPv6).

4.1.2. Connection Specifications

   A connection specification is made up of sections. The "connection"
   section contains parameters that are to be specified only ones for
   each connection. The "paths" section contains one or more path
   definitions. The optional "networks" section contains network
   definitions for routing purposes.

   The general section (called "connection") MUST contain the following
   elements:

   o name: The unique name of the connection. If we use multiple
      connections, the name must uniquely identify the connection.


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   o permissions: There MAY be SEND and RECEIVE permissions, which
      allow sending and receiving connection updates. The term SEND
      means that the local MPT environment is allowed to start
      configuration change to the peer. The term RECEIVE means that the
      peer is allowed to start a configuration change, and the local
      MPT environment will accept it. (The actual execution of the
      requested change depends on further conditions, e.g. successful
      authentication.)

   o IP version: its possible values are 4 or 6.

   o local IP address: must be of the IP version specified above and
      must be the same as defined for the tunnel.

   o remote IP address: the IP address of the remote peer, must be of
      the IP version specified above

   o local data port number: used for data communication, SHOULD be
      set to the 4754 GRE-in-UDP port number

   o remote data port number: used for data communication, SHOULD be
      set to the 4754 GRE-in-UDP port number

   o remote command port number: The UDP port number of the peer,
      which is used to accept control commands. If the local MPT client
      starts an MPT command (e.g. turning off a path usage), the MPT
      server will communicate this action to the peer by using the
      remote command port number as the destination port number.

   o path count: The key is an integer P, denoting the number of paths
      defined for this connection. The minimum value is 1, the maximum
      value is implementation dependent, e.g. 20. This configuration
      file MUST have P sections (usually named [path_n]), where 0<=n<P,
      describing all paths of the connection.

   o network count: The MPT environment can be used to establish a
      tunnel between networks (i.e. not only the tunnel peers can use
      the tunnel for communication). The minimum value is 0, the
      maximum value is implementation dependent, e.g. 20. This key is
      an integer L, denoting the number of networks on which the actual
      connection is able to route. This configuration file MUST have L
      sections (usually named [net_n]), where 0<=n<L, describing all
      networks that belong to the connection.

   o status: The key indicates the initial status of the connection.
      The value 0 means OK.



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   o authentication type: The MPT system uses control communication
      between the tunnel endpoints. The control communication can be
      requested to use authentication. The value 0 means no
      authentication.

   The following elements are OPTIONAL:

   o reorder window: The reorder window value specifies the length (or
      size) of the buffer-array for the optional packet reordering on
      the basis of the GRE sequence numbers, see Section 6 for more
      information. The value of 0 (which is the default value when
      omitting the key) means, that no sorting will be performed at the
      receiver (the packets are transferred to the tunnel interface
      immediately when they arrive).

   o maximum buffer delay: This key is used only if we require ordered
      packet transmission (i.e. reorder window > 0). If ordered packet
      transmission is required, maximum buffer delay specifies the
      maximum time (in milliseconds) while the packet may be stored in
      the buffer-array.

   o authentication key: The authentication key contains the key value
      of the control communication authentication. Some algorithms do
      not need authentication keys. In this case the specification of
      the authentication key is not necessary, or will be ignored.

   A path definition section MUST contain the following elements:

   o interface name: The value is the name of the physical interface
      used by the given path for packet forwarding (e.g. eth0, wlan0).

   o IP version: Specifies the version of IP used by the path. The
      value can be 4 or 6.

   o public IP address: Specifies the public IP address of the
      interface used for the tunnel communication. If the host is
      placed into the Global Address Realm, the public IP address is
      the IP address of the interface, otherwise (i.e. when the host is
      behind a NAT-Box) it is the public address assigned by the NAT-
      Box to the tunnel communication session. If the path uses IPv4
      and NAT, then the special address value of 0.0.0.0 can be used to
      force the MPT server program to determine the public IP address
      automatically.

   o remote IP address: Indicates the public IP address of the remote
      endpoint.



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   o gateway IP address: The IP address of the gateway, used to reach
      the peer (i.e. remote IP address) using the given path. If the
      operating system uses the Network Manager (nmcli) software for
      network configuration, then the value of 0.0.0.0 can be used to
      find the gateway of the named interface automatically.

   o weight out: This is the "weight of the path" in the system
      expressing the estimated transmission capacity of the path. The
      MPT server program distributes the outgoing packets between the
      available paths according to their weights, if per packet based
      mapping is used. The value must be between 1 and 10,000.

   o status: This key means the initial state of the path after
      starting the MPT server. The value "up" means that the path is
      usable (working), and the state of the path is OK. If required,
      may be set initially as "down".

   A path definition section MAY contain the following elements:

   o private IP address: The IP address of the physical interface. Can
      be omitted, if the public IP address is assigned directly to the
      interface. When using IPv4 and NAT, the special value of 0.0.0.0
      can be used to force the MPT server application to read and use
      the first IPv4 address assigned to the interface.

   o keepalive time: The MPT system monitors the availability of each
      path by sending keepalive messages regularly. The key specifies
      the frequency (i.e. the time between the keepalive messages in
      seconds) that the MPT server uses for sending keepalives. The
      value of zero (which is the default value) means switching off
      the keepalive mechanism.

   o dead time: If the keepalive mechanism is active, and the host
      does not receive any keepalive message on the given path from the
      peer for dead time seconds, then the path is considered as "dead"
      and will not be used for data transmission. (The default value is
      3*keepalive time.)

   o weight in: This field is used at the "mpt path up" command (see
      Section 4.2) to set the outgoing weight of the corresponding path
      at the peer. The default value is 1.

   o command default: This key can be used to specify one path as the
      default path for control command communication. In the case of
      receiving the control command of "create connection", the system
      will use this path for the control communication.



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   The optional "networks" section contains network definitions for
   routing purposes. Each network definition begins with its name in
   the [net_n] format and contains the following parameters:

   o IP version: Specifies the version of IP used in the network
      definitions. The value can be 4 or 6.

   o source address: specifies the source network and its prefix
      length in the CIDR notation.

   o destination address: specifies the destination network and its
      prefix length in the CIDR notation.

   The network configuration can also be used to provide multipath
   Internet connection by specifying 0.0.0.0/0 as destination address
   and prefix length. (The source is our tunnel address in this case.)

4.2. MPT Configuration Commands

   The same control interface is used for the local administration of
   the MPT server (by the MPT client accessing the MPT server at the
   local command port through the loopback interface) and for the
   communication of the local MPT server with the remote MPT server
   (accessing it at its remote command port).

   Now, some client commands will follow. Although some of the syntax
   of our MPT implementation will be used, the focus is not on their
   syntax, which may be implementation dependent, but rather on their
   functionalities. The execution of these commands may also involve
   communication between the local MPT server and a/the remote MPT
   server.



   mpt address {add|del} IPADDRESS/PREFIX dev INTERFACE

   An IPv4 or IPv6 address can be added to or deleted from a (local)
   interface.



   mpt interface INTERFACE {up|down}

   The specified interface is turned up or down plus all the paths,
   that are based on the given local physical interface are also turned
   on or off by starting the "mpt path {up|down}" command (see below)
   for each considered path.


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   mpt path PATH {up|down}

   This command can be used to turn on or off a specified path. If the
   path status is changed to down, then it is not used by the
   connection, (i.e. no data is sent through that path by the MPT
   software).



   mpt connection CONNECTION {create|delete}

   This command can be used to establish or tear down a connection
   between the local and a remote MPT server. (The parameters are taken
   from local configuration files.) If the remote server is configured
   so, then it accepts the parameters of the connection from the local
   server.



   mpt save [FILENAME]

   The current configuration can be changed during runtime by remote
   peers. (This can be enabled with the accept remote key and with the
   permissions key.) This command is used to write these connection
   changes to the configuration files, so the new settings will remain
   after server startup or after mpt reload.



   mpt reload [FILENAME]

   Warm restart: the MPT server build up its connections according to
   its configuration files. (Our implementation only establishes, but
   it does not tears down connections.)



5. Possible Mappings of the Tunnel Traffic to Paths

   The data packets coming from the tunnel interface must be forwarded
   through one of the active paths of the connection. Three possible
   mapping solutions are proposed:





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   o Per packet based mapping means that the tunnel traffic is
      distributed among the paths on the basis of the parameters of the
      paths only, and regardless of what network flow a given packet
      belongs to.

   o Flow based mapping means that packets which belong to a given
      network flow, identified by the usual five tuple of source IP
      address, destination IP address, source port number, destination
      port number, and protocol number (TCP or UDP), or three tuple of
      source IP address, destination IP address, and protocol number
      (TCP, UDP or ICMP), are always mapped to the same path.

   o Combined mapping means the combinations of the two above in the
      way that packets which belong to a given network flow, identified
      by the way described above, are always mapped to the same
      connection. And the packets that belong to a connection are
      distributed among the paths of that connection by per packet
      decisions on the basis of the parameters of the paths of the
      connection.

   We illustrate the three mapping solutions by examples.

   Definitions for the examples:

   Computers A and B are interconnected by 3 different paths:

   path_1: 100Base-TX Ethernet

   path_2: 802.11g WiFi

   path_3: LTE

   Connection_1 has 3 paths with the following weight out values:

   path_1: 5

   path_2: 2

   path_3: 3



   Example 1 (Per packet based mapping)

   All the traffic between the two computers is distributed among the
   three paths of Connection_1 proportionally to their weight out



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   values. A decision is made about every single packet as described in
   Section 5.1, regardless of the fact what application it belongs to.

   Advantage: The transmission capacity of all the paths can be
   utilized.

   Disadvantage: There is no possibility to use different mappings for
   different applications.



   Example 2 (Per flow based mapping)

   Based on the destination port number or port range, the traffic of
   different applications are mapped to paths as follows:

   HTTP, VoD: path_1

   FTP, Bit-Torrent: path_2

   VoIP: path3

   Advantage: Application can be differentiated: e.g. the delay
   critical VoIP can use LTE, whereas the free WiFi is satisfactory for
   the non-mission critical Bit-Torrent.

   Disadvantage: The mapping of the traffic is too rigid, all the
   traffic of applications of a given type is mapped to a single path,
   therefore, the applications (and thus their users) do not experience
   the benefits of multipath transmission.



   Example 3 (Combined mapping)

   We define further two connections:

   Connection_2

   path_1: 5

   path_2: 2

   Connection_2

   path_1: 5



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   path_3: 3

   Based on the destination port number or port range, the traffic of
   different applications are mapped to paths as follows:

   HTTP: connection_1

   FTP, Bit-Torrent: connecton_2

   VoIP, VoD: connection_3

   Advantage: The applications may benefit from the multipath
   transmission, whereas each types of applications use those paths,
   which are beneficial and affordable for them.

   Disadvantage: The price of the above resilience is the time and
   computational complexity of the execution of both algorithms.

   Conclusion: The appropriate choice of the mapping algorithm depends
   on the expectations of the user.

5.1. Per Packet Based Mapping

   The aim of the "per packet based" mapping is to distribute the
   tunnel traffic to the paths proportionally to their transmission
   capacity. This mapping facilitates the aggregation of the
   transmission capacities of the paths.

   In MPT, the transmission capacity of the paths is represented by
   their WEIGT_OUT parameter.

   The following algorithm calculates the sending vector, which
   contains the indices of the paths in the order they are to be used
   for transmission.

   ALGORITHM calculate_sending_vector

   INPUT: W[i] (1 <= i <= N), the vector of the weights of the paths.

     (Note: We have N paths with indices (1, . , N)

   OUTPUT: O[j] ( 1 <= j <= M ), the sending vector containing the
   indices of the paths; where M is the length of the sending cycle.



     lcm := Least Common Multiple for  (W[1], . , W[N])


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     M := 0

     s[i] := 0, for all i (1 <= i <= N)

   (Note: s[i] will store the sum of the increments for path i, where
   the increment is lcm/W[i])



     WHILE TRUE DO

        z := min(s[1]+lcm/W[1], . , s[N]+lcm/W[N])

        k := The smallest index i, for which z == s[i]+lcm/W[i]

        M := M+1

        s[k] := z

        O[M] := k

        IF s[i] == z for all i (1 <= i <= N) THEN RETURN

     DONE

   END

   A sample C code can be found in the Appendix.

5.2. Flow Based Mapping

   The aim of the flow based mapping is to be able to distinguish the
   packets belong to different network flows and map them to the path
   that was set for them. (E.g. WiFi is used for Torrent traffic and
   LTE is used for VoIP calls.)

   Our current implementation realizes a port-based flow mapping. It is
   possible to select the interface for the outgoing traffic based on
   transport protocol and port. For communication between two MPT
   servers, you can precisely specify which flow mapped to which path.

   The configuration of the mechanism is simple. Four new values can be
   added for the definition of paths:

   tcp_dst - TCP destination port matches

   tcp_src - TCP source port matches


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   udp_dst - UDP destination port matches

   udp_src - UDP source port matches



   All of these are optional, they can be listed in many ports. Ports
   that are not defined will continue to be per-packet based. The
   current implementation of the MPT with flow based mapping can be
   found on [MptFlow]

   In the example below, each outgoing TCP packet with destination port
   80, 443, and 8080 and UDP packet with destination port 5901 will be
   sent on path_1. TCP packets with source ports 7880 and 56000 will be
   sent on path_2.

   Example (flow based mapping configuration snippet)

   [path_1]
   ...
   tcp_dst         = 80 443 8080
   udp_dst         = 5901



   [path_2]

   ...

   tcp_src         = 7880 56000



5.3. Combined Mapping

   TBD

6. Packet Reordering

   As the delay of the different paths can be different, packet
   reordering may appear in a packet sequence transmission. The MPT
   environment offers an optional feature to ensure the right ordered
   packet transmission for the tunnel communication. If this feature is
   enabled, the receiver uses a buffer-array to store the incoming
   (unordered) packets. Then the packets are sorted according to the
   GRE sequence numbers, so ensuring the ordered transmission to the
   receiver.s tunnel interface.


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   There are two parameters aimed to control the reordering. The
   reorder window parameter specifies the length of the buffer array
   used for reordering. The maximum buffer delay parameter specifies
   the maximum time (in milliseconds) while the packet is stored in the
   buffer-array. If the packet is delayed in the buffer-array for the
   specified time, it will be transmitted to the tunnel interface, even
   in the case, when some packets are missing before the considered
   packet. The missing packets are considered as lost packets (i.e. we
   will not wait more for a lost packet). The arrived packets are
   transferred to the tunnel interface according to their GRE sequence
   number, so the ordered delivery will be kept also in the case of
   packet loss.

   How to set the values of these parameters?

   As for maximum buffer delay, if its value is too small, then MPT may
   incorrectly consider a sequence number as lost, and if it arrives
   later, MPT has to drop it to keep on the order-right delivery.  If
   its value is too large, then the packet loss will be determined too
   late, and thus the communication performance may decrease. Our
   experience shows that a feasible choice could be: a few times the
   RTT (Round-Trip Time) of the slowest path.

   As for reorder window, it MUST be large enough to store packets
   arriving at maximum line rate from all the active paths of the given
   connection during a maximum buffer delay interval.

   The appropriate choice of these parameters is still subject of
   research.

7. Why MPT is Considered Experimental?

   We view MPT as a research area rather than a solution which is ready
   for deployment. We have an MPT implementation, which is workable,
   but it contains only the "per packet based" mapping of the tunnel
   traffic to the paths. One of our aims of writing this Internet Draft
   is to enable others to write MPT implementations. It is our hope
   that the experience gained with preparing other implementations as
   well as the results of their testing and performance analysis will
   lead to a better MPT specification, which may then serve as a
   standard track specification of an improved MPT, which will be ready
   for deployment.

   In this section, we summarize the most important results as well as
   the open questions of the MPT related research.




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7.1. Published Results

7.1.1. MPT Concept and First Implementation

   The conceptual architecture of MPT, comparison with other multipath
   solutions, some details of the first implementation and some test
   results are available in [Alm2017].

   The user manual of the first MPT implementation and the precompiled
   MPT libraries for Linux (both i386 and amd64) and Raspbian are
   available from [Mpt2017].

7.1.2. Estimation of the Channel Aggregation Capabilities

   The channel aggregation capabilities of an early MPT implementation,
   which did not use GRE-in-UDP, were analyzed up to twelve 100Mbps
   links in [Len2015].

   Some of the above tests were repeated with the current GRE-in-UDP
   based MPT implementation, and the path aggregation capabilities of
   MPT were compared to that of MPTCP in [Kov2016].

7.1.3. Demonstrating the Resilience of an MPT Connection

   The resilience property of the early MPT implementation, which did
   not use GRE-in-UDP, was demonstrated in [Alm2014] and in [Alm2015].

   The fast connection recovery of the GRE-in-UDP based MPT
   implementation was demonstrated in [Fej2016].

   Playout buffer length triggered path switching algorithm was
   developed for the GRE-in-UDP based MPT, and its effectiveness was
   demonstrated by the elimination of the stalling events on YouTube
   video playback [Fej2017].

7.2. Open questions

7.2.1. Parameters

   The optimal (or good enough) choice of the reorder window size and
   maximum buffer delay parameters are important questions, which
   should be solved before MPT can be deployed.

7.2.2. Development of Further Mapping Algorithms

   The current MPT implementation [Mpt2017] includes only the per
   packet base mapping. For a precise specification of the further two


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   mapping algorithms, we would like to use or experiences with them.
   There are some open questions e.g. how to handle the traffic that is
   neither TCP nor UDP?

7.2.3. Performance Issues

   The current MPT implementation [Mpt2017] works in user space. Thus,
   it is not surprising, that multipath transmission of the same amount
   of traffic by MPT results in higher CPU load than its multipath
   transmission by MPTCP [Kov2017]. How much CPU power could a kernel
   space MPT implementation save?

   It was also pointed out by [Kov2017], that MPT is not able to
   utilize the computing power of more than two CPU cores. It is so,
   because MPT uses only two threads (one for each direction). This is
   not a serious issue, when MPT is used on personal computers.
   However, when MPT is used to connect several networks, it is an
   important question, how MPT could utilize the computing power of the
   modern CPUs with several cores.

7.3. Implementation

   A sample implementation of the MPT software is available from
   [MptSrc] under GPLv3 license. It is intended for research and
   experimentation purposes only, as it has not been sufficiently
   tested to be used for commercial purposes.

8. Security Considerations

   Threats that apply to GRE-in-UDP tunneling, apply here, too. For the
   security considerations of GRE-in-UDP, please refer to Section 11 of
   [RFC8086].

   If an MPT server is configured so, its peer is allowed to build up
   connections. It may lead to resource exhaustion and thus successful
   DoS (Denial of Service) attacks.

   Authentication between MPT servers is optional, which may lead to
   security issues.

9. IANA Considerations

   Port numbers may be reserved for local command port and remote
   command port.





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10. Conclusions

   Hereby we publish the specifications of the MPT network layer
   multipath library in the hope, that it can be made better by the
   review and comments of the WG members and, after answering several
   open questions, one day MPT can mature to be a production tool. We
   seek for interested volunteers for a different implementation and we
   would be happy to take part in research cooperation. We welcome all
   kinds of feedback from anyone to make MPT better.

11. References

11.1. Normative References

   [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC8086] Young, L. (Editor), Crabbe, E., Xu, X., and T. Herbert,
             "GRE-in-UDP Encapsulation", RFC 8086, DOI:
             10.17487/RFC8086, March 2017.

11.2. Informative References

   [Alm2014] Almasi, B., "A solution for changing the communication
             interfaces between WiFi and 3G without packet loss., in
             Proc. 37th Int. Conf. on Telecommunications and Signal
             Processing (TSP 2014), Berlin, Germany, Jul. 1-3, 2014,
             pp. 73.77

   [Alm2015] Almasi, B., Kosa, M., Fejes, F., Katona, R., and L. Pusok,
             "MPT: a solution for eliminating the effect of network
             breakdowns in case of HD video stream transmission", in:
             Proc. 6th IEEE Conf. on Cognitive Infocommunications
             (CogInfoCom 2015), Gyor, Hungary, 2015, pp. 121.126, doi:
             10.1109/CogInfoCom.2015.7390576 .

   [Alm2017] Almasi, B., Lencse, G., and Sz. Szilagyi, "Investigating
             the Multipath Extension of the GRE in UDP Technology",
             Computer Communications (Elsevier), vol. 103, no. 1,
             (2017.) pp. 29-38, DOI: 10.1016/j.comcom.2017.02.002

   [Fej2016] Fejes, F., Katona, R., and L. Pusok, "Multipath strategies
             and solutions in multihomed mobile environments", in:
             Proc. 7th IEEE Conf. on Cognitive Infocommunications
             (CogInfoCom 2016), Wroclaw, Poland, 2016, pp. 79.84, doi:
             10.1109/CogInfoCom.2016.7804529



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   [Fej2017] Fejes, F., Racz, S., and G. Szabo, "Application agnostic
             QoE triggered multipath switching for Android devices",
             In: Proc. 2017 IEEE International Conference on
             Communications (IEEE ICC 2017), Paris, France, 21-25 May
             21-25, 2017. pp. 1585-1591.

   [Kov2016] Kovacs, A., "Comparing the aggregation capability of the
             MPT communications library and multipath TCP", in: Proc.
             7th IEEE Conf. on Cognitive Infocommunications (CogInfoCom
             2016), Wroclaw, Poland, 2016, pp. 157.162, doi:
             10.1109/CogInfoCom.2016.7804542

   [Kov2017] Kovacs, A., "Evaluation of the Aggregation Capability of
             the MPT Communications Library and Multipath TCP",
             unpublished

   [Len2015] Lencse, G. and A. Kovacs, "Advanced Measurements of the
             Aggregation Capability of the MPT Multipath Communication
             Library", International Journal of Advances in
             Telecommunications, Electrotechnics, Signals and Systems,
             vol. 4. no. 2. (2015.) pp 41-48. DOI:
             10.11601/ijates.v4i2.112

   [Mpt2017] MPT - Multipath Communication Library,
             http://irh.inf.unideb.hu/user/szilagyi/mpt/

   [MptFlow] "MPT - Multi Path Tunnel", source code version with flow
             based packet to path mapping feature,
             https://github.com/spyff/mpt/tree/flow_mapping

   [MptSrc]  "MPT - Multi Path Tunnel", source code,
             https://github.com/spyff/mpt

   [RFC6824] Ford, A., Raiciu, C, Handley, M., and O Bonaventure, "TCP
             Extensions for Multipath Operation with Multiple
             Addresses", RFC 6824, DOI: 10.17487/RFC6824, January,
             2013.

   [RFC8157] Leymann, N., Heidemann, C., Zhang, M., Sarikaya, B, and M.
             Cullen, "Huawei's GRE Tunnel Bonding Protocol", RFC 8157,
             DOI: 10.17487/RFC8157, May, 2017

12. Acknowledgments

   The MPT Network Layer Multipath Library was invented by Bela Almasi,
   the organizer and original leader of the MPT development team.



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   This document was prepared using 2-Word-v2.0.template.dot.














































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Appendix A. Sample C code for calculating the packet sending order

   <CODE BEGINS>
   void calculate_pathselection(connection_type *con) {
       long long lcm;
       long gcd, min_inc, cinc;
       int i,j, min_idx;
       path_type *p;

       con->pathselectionlength = 0;
       gcd = con->mpath[0].weight_out;
       lcm = gcd;
       for (i = 0; i < M; i++)
         O[i] = NULL;

       for (i = 0; i < con->path_count; i++) {
           gcd = CALCULATE_GCD(gcd, con->mpath[i].weight_out);
           lcm = (lcm * con->mpath[i].weight_out ) / gcd;
           con->mpath[i].selection_increment = 0;
       }

       for (j = 0; j < M; j++) {
           min_idx = 0;
           min_inc = lcm + 1;
           for (i = 0; i < con->path_count; i++) {
               p = &con->mpath[i];
               cinc = p->selection_increment + (lcm / p->weight_out);
               if ((p->weight_out) && (cinc < min_inc)) {
                   min_idx = i;
                   min_inc = cinc;
               }
           }
           O[j] = &con->mpath[min_idx];
           con->mpath[min_idx].selection_increment = min_inc;

           for (i = 0; i < con->path_count; i++) // check if ready
               if (con->mpath[i].selection_increment != min_inc)
                   goto NEXT_SELECTION;
           break;

   NEXT_SELECTION:
           continue;
       }
       con->path_index = 0;
       con->pathselectionlength = j + 1;
   }
   <CODE ENDS>


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   Copyright (c) 2018 IETF Trust and the persons identified as authors
   of the code. All rights reserved.

   Redistribution and use in source and binary forms, with or without
   modification, is permitted pursuant to, and subject to the license
   terms contained in, the Simplified BSD License set forth in Section
   4.c of the IETF Trust.s Legal Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info).






































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Authors' Addresses

   Gabor Lencse
   Budapest University of Technology and Economics
   Magyar Tudosok korutja 2.
   H-1117 Budapest, Hungary

   Phone: +36 1 463 2055
   Email: lencse@hit.bme.hu


   Szabolcs Szilagyi
   University of Debrecen
   Egyetem ter 1.
   H-4032 Debrecen
   Hungary

   Phone: +36 52 512 900 / 75015
   Email: szilagyi.szabolcs@inf.unideb.hu


   Ferenc Fejes
   University of Debrecen
   Egyetem ter 1.
   H-4032 Debrecen
   Hungary

   Phone: +36 70 545 48 07
   Email: fejes@openmailbox.org


   Marius Georgescu
   RCS&RDS
   Strada Dr. Nicolae D. Staicovici 71-75
   Bucharest 030167
   Romania

   Phone: +40 31 005 0979
   Email: marius.georgescu@rcs-rds.ro









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