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INTAREA                                                          J. Zhu
Internet Draft                                                    Intel
Intended status: Standards Track                                 S. Seo
Expires: November 17, 2018                                Korea Telecom
                                                             S. Kanugovi
                                                                   Nokia
                                                                 S. Peng
                                                                  Huawei
                                                    May 17, 201817, 2018

        User-Plane Protocols for Multiple Access Management Service
                  draft-zhu-intarea-mams-user-protocol-05


Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on November 17, 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
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   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



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   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.

Abstract

   Today, a device can be simultaneously connected to multiple
   communication networks based on different technology implementations
   and network architectures like WiFi, LTE, and DSL. In such multi-
   connectivity scenario, it is desirable to combine multiple access
   networks or select the best one to improve quality of experience for
   a user and improve overall network utilization and efficiency. This
   document presents the u-plane protocols for a multi access
   management services (MAMS) framework that can be used to flexibly
   select the combination of uplink and downlink access and core
   network paths having the optimal performance, and user plane
   treatment for improving network utilization and efficiency and
   enhanced quality of experience for user applications.

Table of Contents

   1. Introduction...................................................3
   2. Terminologies..................................................3
   3. Conventions used in this document..............................3
   4  MAMS User-Plane Protocols......................................4
      4.1   MX Adaptation Sublayer...................................4
      4.2   Trailer-based MX Convergence Sublayer....................5
         4.2.1    Trailer-based MX PDU Format........................5
         4.2.2    MX Fragmentation...................................8
         4.2.3    MX Concatenation...................................9
      4.3   MPTCP-based MX Convergence Sublayer.....................10
      4.4   GRE as MX Convergence Sublayer..........................11
         4.4.1    Transmitter Procedures............................11
         4.4.2    Receiver Procedures...............................12
      4.5   Co-existence of MX Adaptation and MX Convergence Sublayers
            12
   5. MX Convergence Control Message................................12
      5.1   Keep-Alive Message......................................13
      5.2   Probe REQ/ACK Message...................................13
   5.3   Acknowledgement (ACK) Message..............................14
   5.4   First Sequence Number (FSN) Message........................15
   5.5   Coded MX SDU (CMS) Message.................................16
   6  Security Considerations.......................................18
   7  IANA Considerations...........................................18
   8  Contributing Authors..........................................18
   9  References....................................................18
      9.1   Normative References....................................18
      9.2   Informative References..................................18

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1. Introduction

   Multi Access Management Service (MAMS) [MAMS] is a programmable
   framework to select and configure network paths, as well as adapt to
   dynamic network conditions, when multiple network connections can
   serve a client device. It is based on principles of user plane
   interworking that enables the solution to be deployed as an overlay
   without impacting the underlying networks.

   This document presents the u-plane protocols for enabling the MAMS
   framework. It co-exists and complements the existing protocols by
   providing a way to negotiate and configure the protocols based on
   client and network capabilities. Further it allows exchange of
   network state information and leveraging network intelligence to
   optimize the performance of such protocols. An important goal for
   MAMS is to ensure that there is minimal or no dependency on the
   actual access technology of the participating links. This allows the
   scheme to be scalable for addition of newer access technologies and
   for independent evolution of the existing access technologies.

2. Terminologies

   Anchor Connection: refers to the network path from the N-MADP to the
   Application Server that corresponds to a specific IP anchor that has
   assigned an IP address to the client.

   Delivery Connection: refers to the network path from the N-MADP to
   the C-MADP.

   "Network Connection Manager" (NCM), "Client Connection Manager"
   (CCM), "Network Multi Access Data Proxy" (N-MADP), and "Client Multi
   Access Data Proxy" (C-MADP) in this document are to be interpreted
   as described in [MAMS].

3. 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 [RFC2119].

   The terminologies "Network Connection Manager" (NCM), "Client
   Connection Manager" (CCM), "Network Multi Access Data Proxy" (N-
   MADP), and "Client Multi Access Data Proxy" (C-MADP) in this
   document are to be interpreted as described in [MAMS].




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4  MAMS User-Plane Protocols

Figure 1 shows the MAMS u-plane protocol stack as specified in
[MAMS_CP].
             +-----------------------------------------------------+
             |      User Payload (e.g. IP PDU)                     |
             |-----------------------------------------------------|
          +--|-----------------------------------------------------|--+
          |  |-----------------------------------------------------|  |
          |  | Multi-Access (MX) Convergence Sublayer              |  |
          |  |-----------------------------------------------------|  |
          |  |-----------------------------------------------------|  |
          |  | MX Adaptation  | MX Adaptation | MX Adaptation      |  |
          |  | Sublayer       | Sublayer      | Sublayer           |  |
          |  | (optional)     | (optional)    | (optional)         |  |
          |  |-----------------------------------------------------|  |
          |  | Access #1 IP   | Access #2 IP  | Access #3 IP       |  |
          |  +-----------------------------------------------------+  |
          +-----------------------------------------------------------+
                 Figure 1: MAMS U-plane Protocol Stack


It consists of the following two Sublayers:

o Multi-Access (MX) Convergence Sublayer: This layer performs multi-
  access specific tasks, e.g., access (path) selection, multi-link
  (path) aggregation, splitting/reordering, lossless switching,
  fragmentation, concatenation, keep-alive, and probing etc.
o Multi-Access (MX) Adaptation Sublayer: This layer performs functions
  to handle tunneling, network layer security, and NAT.

The MX convergence sublayer operates on top of the MX adaptation
sublayer in the protocol stack. From the Transmitter perspective, a
User Payload (e.g. IP PDU) is processed by the convergence sublayer
first, and then by the adaptation sublayer before being transported
over a delivery access connection; from the Receiver perspective, an IP
packet received over a delivery connection is processed by the MX
adaptation sublayer first, and then by the MX convergence sublayer.

4.1  MX Adaptation Sublayer

The MX adaptation sublayer supports the following mechanisms and
protocols while transmitting user plane packets on the network path:



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o UDP Tunneling: The user plane packets of the anchor connection can be
  encapsulated in a UDP tunnel of a delivery connection between the N-
  MADP and C-MADP.
o IPsec Tunneling: The user plane packets of the anchor connection are
  sent through an IPsec tunnel of a delivery connection.
o Client Net Address Translation (NAT): The Client IP address of user
  plane packet of the anchor connection is changed, and sent over a
  delivery connection.
o Pass Through: The user plane packets are passing through without any
  change over the anchor connection.

The MX adaptation sublayer also supports the following mechanisms and
protocols to ensure security of user plane packets over the network
path.

o IPsec Tunneling: An IPsec [RFC7296] tunnel is established between the
  N-MADP and C-MADP on the network path that is considered untrusted.
o DTLS: If UDP tunneling is used on the network path that is considered
  "untrusted", DTLS (Datagram Transport Layer Security) [RFC6347] can
  be used.

The Client NAT method is the most efficient due to no tunneling
overhead. It SHOULD be used if a delivery connection is "trusted" and
without NAT function on the path.

The UDP or IPsec Tunnelling method SHOULD be used if a delivery
connection has a NAT function placed on the path.

4.2  Trailer-based MX Convergence Sublayer

4.2.1 Trailer-based MX PDU Format

Trailer-based MX convergence integrates multiple connections into a
single e2e IP connection. It operates between Layer 2 (L2) and Layer 3
(network/IP).

                   <-- MX Data PDU Payload ------->
          +------------------------------------------------------+
          | IP hdr |        IP payload             | MX Trailer  |
          +------------------------------------------------------+
         Figure 2: Trailer-based Multi-Access (MX) Data PDU Format

Figure 2 shows the trailer-based Multi-Access (MX) PDU (Protocol Data
Unit) format. A MX PDU MAY carry multiple IP PDUs in the payload if
concatenation is supported, and MAY carry a fragment of the IP PDU if
fragmentation is supported.


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The MX trailer may consist of the following fields:

o MX flags (e.g. 1 Byte): Bit 0 is the most significant bit, bit 7 is
  the least significant bit. Bit 6 and 7 are reserved for future.
     + Next Header Present (bit 0): If the Next Header Present bit is
       set to 1, then the Next Header field is present and contains
       valid information.
     + Connection ID Present (bit 1): If the Connection ID Present bit
       is set to 1, then the Connection ID field is present and
       contains valid information.
     + Traffic Class Present (bit 2): If the Traffic Class Present bit
       is set to 1, then the Traffic Class field is present and
       contains valid information.
     + Sequence Number Present (bit 3): If the Sequence Number Present
       bit is set to 1, then the Sequence Number field is present and
       contains valid information.
     + Packet Length Present (bit 4): If the Packet Length Present bit
       is set to 1, then the First SDU (Service Data Unit) Length field
       is present and contains valid information.
     + Fragmentation Control Present (bit 5): If the Fragmentation
       Control Present bit is set to 1, then the Fragmentation Control
       field is present and contains valid information.
     + Bit 6~7: reserved
o Next Header (e.g. 1 Byte): the IP protocol type of the (first) IP
  packet in a MX PDU
o Connection ID (e.g.1 Byte): an unsigned integer to identify the
  anchor connection of the IP packets in a MX PDU
o Traffic Class (TC) ID (e.g. 1 Byte): an unsigned integer to identify
  the traffic class of the IP packets in a MX PDU, for example Data
  Radio Bearer (DRB) ID [LWIPEP] for a cellular (e.g. LTE) connection
o Sequence Number (e.g. 2 Bytes): an auto-incremented integer to
  indicate order of transmission of the MX SDU (e.g. IP packet), needed
  for lossless switching or multi-link (path) aggregation or
  fragmentation. Sequence Number SHALL be generated on a per Connection
  and per Traffic Class (TC) basis.
o First SDU Length (e.g. 2 Bytes): the length of the first IP packet,
  only included if a MX PDU contains multiple IP packets, i.e.
  concatenation.
o Fragmentation Control (FC) (e.g. 1 Byte): to provide necessary
  information for re-assembly, only needed if a MX PDU carries
  fragments, i.e. fragmentation.

Figure 3 shows the MX trailer format with all the fields present. The
MX flags are always encoded in the last octet of the MX Trailer at the
end of a MX PDU. Hence, the Receiver SHOULD first decode the MX flags
field to determine the length of the MX trailer, and then decode each
MX field accordingly.


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      0                   1                   2                   3

      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     | Next Header   | Connection ID |   TC ID       |   Sequence

     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       Number        |      First SDU Length         |      FC       |

     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     |  MX Flags     |

     +-+-+-+-+-+-+-+-+

                        Figure 3: MX Trailer Format

Moreover, the following field of the IP header of the MX PDU SHOULD be
changed:

o  Protocol Type: "114" to indicate that the presence of MX trailer
(i.e. the trailer based MAMS u-plane protocol is a "0-hop" protocol,
not subject to IP routing)

If the MX PDU is transported with the MX adaptation method of IPSec
tunnelling, Client NAT, or Pass Through, the following fields of the IP
header of the MX PDU SHOULD also be changed:

o  IP length: add the length of "MX Trailer" to the length of the
original IP packet
o  IP checksum: recalculate after changing "Protocol Type" and "IP
Length"

If the MX adaptation method is UDP tunnelling and "MX header
optimization" in the "MX_UP_Setup_Configuration_Request" message [MAMS]
is true, the "IP length" and "IP checksum" header fields of the MX PDU
SHOULD remain unchanged.

The MX u-plane protocol can support multiple Anchor connections
simultaneously, each of which is uniquely identified by Connection ID.
It can also support multiple traffic classes per connection, each of
which is identified by Traffic Class ID.

Moreover, the MX trailer format MAY be negotiated dynamically between
NCM and CCM. For example, NCM can send a control message to indicate

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which of the above fields SHOULD be included for individual delivery
connection, on downlink and uplink, respectively.

4.2.2 MX Fragmentation

The Trailer-based MX Convergence Layer SHOULD support MX fragmentation
if a delivery connection has a smaller maximum transmission unit (MTU)
than the original IP packet (MX SDU), and IP fragmentation is not
supported or enabled on the connection. The MX fragmentation procedure
is similar to IP fragmentation [RFC791] in principle, but with the
following two differences for less overhead:

o The fragment offset field is expressed in number of fragments not 8-
  bytes blocks
o The maximum number of fragments per MX SDU is 2^7 (=128)

The Fragmentation Control (FC) field in the MX Trailer contains the
following bits:

o Bit #7: a More Fragment (MF) flag to indicate if the fragment is the
  last one (0) or not (1)
o Bit #0~#6: Fragment Offset (in units of fragments) to specify the
  offset of a particular fragment relative to the beginning of the MX
  SDU

A MX PDU carries a whole MX SDU without fragmentation if the FC field
is set to all "0"s or the FC field is not present in the trailer.
Otherwise, the MX PDU contains a fragment of the MX SDU.

The Sequence Number (SN) field in the trailer is used to distinguish
the fragments of one MX SDU from those of another. The Fragment Offset
(FO) field tells the receiver the position of a fragment in the
original MX SDU. The More Fragment (MF) flag indicates the last
fragment.

To fragment a long MX SDU, the MADP transmitter creates two MX PDUs and
copies the content of the IP header fields from the long MX PDU into
the IP header of both MX PDUs. The length field in the IP header of MX
PDU SHOULD be changed to the length of the MX PDU, and the protocol
type SHOULD be changed to "114", indicating the presence of the MX
trailer.

The data of the long MX SDU is divided into two portions based on the
MTU size of the delivery connection. The first portion of the data is
placed in the first MX PDU. The MF flag is set to "1", and the FO field
is set to "0". The second portion of the data is placed in the second
MX PDU. The MF flag is set to "0", and the FO field is set to "1". This


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procedure can be generalized for an n-way split, rather than the two-
way split described the above.

To assemble the fragments of a MX SDU, the MADP receiver combines MX
PDUs that all have the same MX Sequence Number (in the trailer). The
combination is done by placing the data portion of each fragment in the
relative order indicated by the Fragment Offset in that fragment's MX
trailer. The first fragment will have the Fragment Offset set to "0",
and the last fragment will have the More-Fragments flag reset to "0".

4.2.3 MX Concatenation

The Trailer-based MX Convergence Layer MAY support MX concatenation if
a delivery connection has a larger maximum transmission unit (MTU) than
the original IP packet (MX SDU). Only the MX SDUs with the same client
address, the same anchor connection and the same Traffic Class MAY be
concatenated.

If the MX adaptation method is IPSec tunnelling, Client NAT, or Pass
Through, The First SDU Length (FSL) field SHOULD be included in the MX
Trailer to indicate the length of the first MX SDU.

If the MX adaptation method is UDP tunneling and "MX header
optimization" in the "MX_UP_Setup_Configuration_Request" message [MAMS]
is true, the FSL field SHOULD not be present, or the entire MX trailer
MAY not be present. The MADP receiver compares the IP length field of
the MX PDU and the actual length of the MX PDU to determine if the MX
PDU contains multiple MX SDUs. If the MX PDU is larger than what the IP
length field indicates, the MX PDU contains multiple MX SDUs;
otherwise, the MX PDU contains only one MX SDU. To concatenate two or
more MX SDUs, the MADP transmitter creates one MX PDU and copies the
content of the IP header field from the first MX SDU into the IP header
of the MX PDU. The data of the first MX SDU is placed in the first
portion of the data of the MX PDU. The whole second MX SDU is then
placed in the second portion of the data of the MX PDU (Figure 4). The
procedure continues till the MX PDU size reaches the MTU of the
delivery connection. If the FSL field is present in the MX Trailer, the
IP length field of the MX PDU SHOULD be updated to include all
concatenated SDUs and the trailer, and the IP checksum field SHOULD be
recalculated.

To disaggregate a MX PDU, the MADP receiver first obtains the length of
the first MX SDU from the FSL field in the trailer, and decodes the
first MX SDU. If the FSL field or the MX Trailer is not present, the
MADP receiver obtains the length of the first MX SDU directly from the
IP length field of the MX PDU. The MADP receiver then obtains the
length of the second MX SDU based on the length field in the second MX


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SDU IP header, and decodes the second MX SDU. The procedure continues
till no byte is left in the MX PDU.

If a MX PDU contains multiple SDUs, the SN field in the MX trailer is
for the last MX SDU, and the SN of other SDU carried by the same PDU
can be obtained according to its order in the PDU. For example, if the
SN field is 6 and a MX PDU contains 3 SDUs (IP packets), then the SN is
4, 5, and 6 for the first, second, and the last IP packet in the PDU,
respectively.

              <---- MX Data PDU Payload ------------>
     +------------------------------------------------------------+
     | IP hdr | IP payload  | IP hdr |  IP payload  | MX Trailer  |
     +------------------------------------------------------------+
                Figure 4: MX PDU Format with Concatenation

4.3  MPTCP-based MX Convergence Sublayer

Figure 5 shows the MAMS u-plane protocol stack based on MPTCP. Here,
MPTCP is reused as the "MX Convergence Sublayer" protocol. Multiple
access networks are combined into a single MPTCP connection. Hence, no
new u-plane protocol or PDU format is needed in this case.

          |-----------------------------------------------------|
          |                       MPTCP                         |
          |-----------------------------------------------------|
          |  TCP           |   TCP         |      TCP           |
          |-----------------------------------------------------|
          | MX Adaptation  | MX Adaptation | MX Adaptation      |
          | Sublayer       | Sublayer      | Sublayer           |
          | (optional)     | (optional)    | (optional)         |
          |-----------------------------------------------------|
          | Access #1 IP   | Access #2 IP  | Access #3 IP       |
          +-----------------------------------------------------+
    Figure 5: MAMS U-plane Protocol Stack with MPTCP as MX Convergence
                                   Layer


If NCM determines that N-MADP is to be instantiated with MPTCP as the
MX Convergence Protocol, it exchanges the support of MPTCP capability
in the discovery and capability exchange procedures [MAMS_CP]. MPTCP
proxy protocols [MPProxy][MPPlain] SHOULD be used to manage traffic
steering and aggregation over multiple delivery connections.





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4.4  GRE as MX Convergence Sublayer

Figure 6 shows the MAMS u-plane protocol stack based on GRE (Generic
Routing Encapsulation) [GRE2784]. Here, GRE is reused as the "MX
Convergence sub-layer" protocol. Multiple access networks are combined
into a single GRE connection. Hence, no new u-plane protocol or PDU
format is needed in this case.

          +-----------------------------------------------------+
          |      User Payload (e.g. IP PDU)                     |
          |-----------------------------------------------------|
          |              GRE as MX Convergence Sublayer         |
          |-----------------------------------------------------|
          |        GRE Delivery Protocol (e.g. IP)              |
          |-----------------------------------------------------|
          | MX Adaptation  | MX Adaptation | MX Adaptation      |
          | Sublayer       | Sublayer      | Sublayer           |
          | (optional)     | (optional)    | (optional)         |
          |-----------------------------------------------------|
          | Access #1 IP   | Access #2 IP  | Access #3 IP       |
          +-----------------------------------------------------+
     Figure 6: MAMS U-plane Protocol Stack with GRE as MX Convergence
                                   Layer


If NCM determines that N-MADP is to be instantiated with GRE as the MX
Convergence Protocol, it exchanges the support of GRE capability in the
discovery and capability exchange procedures [MAMS_CP].

4.4.1            Transmitter Procedures

Transmitter is the N-MADP or C-MADP instance, instantiated with GRE as
the  convergence  protocol  that  transmits  the  GRE  packets.  The
Transmitter receives the User Payload (e.g. IP PDU), encapsulates it
with a GRE header and Delivery Protocol (e.g. IP) header to generate
the GRE Convergence PDU.

When IP is used as the GRE delivery protocol, the IP header information
(e.g. IP address) can be created using the IP header of the user
payload or a virtual IP address. The "Protocol Type" field of the
delivery header is set to 47 (or 0X2F, i.e. GRE)[IANA].

The GRE header fields are set as specified below,

  - If the transmitter is a C-MADP instance, then sets the LSB 16 bits
     to the value of Connection ID for the Anchor Connection associated
     with the user payload or sets to 0xFFFF if no Anchor Connection ID
     needs to be specified.

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  - All other fields in the GRE header including the remaining bits in
     the key fields are set as per [GRE_2784][GRE_2890].

4.4.2            Receiver Procedures

Receiver is the N-MADP or C-MADP instance, instantiated with GRE as the
convergence  protocol  that  receives  the  GRE  packets.  The  receiver
processes  the  received  packets  per  the  GRE  procedures  [GRE_2784,
GRE_2890] and retrieves the GRE header.

  - If the Receiver is an N-MADP instance,
       o Unless the LSB 16 Bits of the Key field are 0xFFFF, they are
          interpreted as the Connection ID of Anchor Connection for the
          user payload. This is used to identify the network path over
          which the User Payload (GRE Payload) is to be transmitted.
  - All other fields in the GRE header, including the remaining bits
     in the Key fields, are processed as per [GRE_2784][GRE_2890].

The GRE Convergence PDU is passed onto the MX Adaptation Layer (if
present) before delivery over one of the network paths.

4.5   Co-existence of MX Adaptation and MX Convergence Sublayers

MAMS u-plane protocols support multiple combinations and instances of
user plane protocols to be used in the MX Adaptation and the
Convergence sublayers.

For example, one instance of the MX Convergence Layer can be MPTCP
Proxy [MPProxy][MPPlain] and another instance can be Trailer-based. The
MX Adaptation for each can be either UDP tunnel or IPsec. IPsec may be
set up for network paths considered as untrusted by the operator, to
protect the TCP subflow between client and MPTCP proxy traversing that
network path.

Each of the instances of MAMS user plane, i.e. combination of MX
Convergence and MX Adaptation layer protocols, can coexist
simultaneously and independently handle different traffic types.

5. MX Convergence Control Message

A UDP connection may be configured between C-MADP and N-MADP to
exchange control messages for keep-alive or path quality estimation.
The N-MADP end-point IP address and UDP port number of the UDP
connection is used to identify MX control PDU. Figure 7 shows the MX
control PDU format with the following fields:

o Type (1 Byte): the type of the MX control message
     + 0: Keep-Alive

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     + 1: Probe REQ/ACK
     + Others: reserved
o CID (1 Byte): the connection ID of the delivery connection for
  sending out the MX control message
o MX Control Message (variable): the payload of the MX control message

Figure 8 shows the MX convergence control protocol stack, and MX
control PDU goes through the MX adaptation sublayer the same way as MX
data PDU.

                        <----MX Control PDU Payload --------------->
+------------------------------------------------------------------+
| IP header | UDP Header| Type | CID |       MX Control Message    |
+------------------------------------------------------------------+
                      Figure 7: MX Control PDU Format

          |-----------------------------------------------------|
          |          MX Convergence Control Messages            |
          |-----------------------------------------------------|
          |                  UDP/IP                             |
          |-----------------------------------------------------|
          | MX Adaptation  | MX Adaptation | MX Adaptation      |
          | Sublayer       | Sublayer      | Sublayer           |
          | (optional)     | (optional)    | (optional)         |
          |-----------------------------------------------------|
          | Access #1 IP   | Access #2 IP  | Access #3 IP       |
          +-----------------------------------------------------+
              Figure 8: MX Convergence Control Protocol Stack

5.1  Keep-Alive Message

The "Type" field is set to "0" for Keep-Alive messages. C-MADP may send
out Keep-Alive message periodically over one or multiple delivery
connections, especially if UDP tunneling is used as the adaptation
method for the delivery connection with a NAT function on the path.

A Keep-Alive message is 2 Bytes long, and consists of the following
fields:

  o Keep-Alive Sequence Number (2 Bytes): the sequence number of the
     keep-alive message

5.2  Probe REQ/ACK Message

The "Type" field is set to "1" for Probe REQ/ACK messages.



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N-MADP may send out the Probe REQ message for path quality estimation.
In response, C-MADP may send back the Probe ACK message.

A Probe REQ message consists of the following fields:

  o Probing Sequence Number (2 Bytes): the sequence number of the
     Probe REQ message
  o Probing Flag (1 Byte):
       + Bit #0: a Probe ACK flag to indicate if the Probe ACK message
          is expected (1) or not (0);
       + Bit #1: a Probe Type flag to indicate if the Probe REQ/ACK
          message is sent during the initialization phase (0) when the
          network path is not included for transmission of user data or
          the active phase (1) when the network path is included for
          transmission of user data;
       + Bit #2: a bit flag to indicate the presence of the Reverse
          Connection ID (R-CID) field.
       + Bit #3~7: reserved
  o Reverse Connection ID (1 Byte): the connection ID of the delivery
     connection for sending out the Probe ACK message on the reverse
     path
  o Padding (variable)

The "R-CID" field is only present if both Bit #0 and Bit #2 of the
"Probing Flag" field are set to "1". Moreover, Bit #2 of the "Probing
Flag" field SHOULD be set to "0" if the Bit #0 is "0", indicating the
Probe ACK message is not expected.

If the "R-CID" field is not present but the Bit #0 of the "Probing
Flag" field is set to "1", the Probe ACK message SHOULD be sent over
the same delivery connection as the Probe REQ message.

The "Padding" field is used to control the length of Probe REQ message.

C-MADP SHOULD send out the Probe ACK message in response to a Probe REQ
message with the Probe ACK flag set to "1".

A Probe ACK message is 2 Bytes long, and consists of the following
fields:

  o Probing Acknowledgement Number (2 Bytes): the sequence number of
     the corresponding Probe REQ message

5.3  Acknowledgement (ACK) Message

The "Type" field is set to "2" for ACK messages.



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C-MADP may send out the ACK messages to report lost MX SDU for example
during handover. In response, C-MADP may retransmit the lost MX SDU
accordingly.

A ACK message consists of the following fields:

  o Connection ID (1 Byte): an unsigned integer to identify the anchor
     connection which the ACK message is for;
  o Traffic Class ID (1 Byte): an unsigned integer to identify the
     traffic class of the anchor connection which the ACK message is
     for;
  o ACK number (2 Bytes): the next (in-order) sequence number (SN)
     that the sender of the ACK Report message is expecting
  o Number of Loss Bursts (1 Byte)
     For each loss burst, include the following
       + Sequence Number of the first lost MX SDU in a burst (2 Bytes)
       + Number of consecutive lost MX SDUs in the burst (1 Byte)


          C-MADP                                             N-MADP
              |                                                 |
              |<------------------ MX SDU (data packets)--------|
              |                                                 |
             +---------------------------------+                |
             |Packet Loss detected             |                |
             +---------------------------------+                |
              |                                                 |
              |----- ACK Message ------------------------------>|
              |<-------------retransmit(lost)MX SDUs -----------|

                Figure 9: MAMS Retransmission Procedure

Figure 9 shows the MAMS retransmission procedure in an example where
the lost packet is found and retransmitted.

5.4  First Sequence Number (FSN) Message

The "Type" field is set to "3" for FSN messages.

N-MADP may send out the FSN messages to indicate the oldest MX SDU in
its buffer if a lost MX SDU is not found in the buffer after receiving
the ACK message from C-MADP. In response, C-MADP SHALL only report
packet loss with SN not smaller than FSN.

A FSN message consists of the following fields:


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  o Connection ID (1 Byte): an unsigned integer to identify the anchor
     connection which the FSN message is for;
  o Traffic Class ID (1 Byte): an unsigned integer to identify the
     traffic class of the anchor connection which the FSN message is
     for;
  o First Sequence Number (2 Bytes): the sequence number (SN) of the
     oldest MX SDU in the (retransmission) buffer of the sender of the
     FSN message.


Figure 10 shows the MAMS retransmission procedure in an example where
the lost packet is not found.

          C-MADP                                             N-MADP
              |                                                 |
              |<------------------ MX SDU (data packets)--------|
              |                                                 |
             +---------------------------------+                |
             |Packet Loss detected             |                |
             +---------------------------------+                |
              |                                                 |
              |----- ACK Message ------------------------------>|
              |                              +---------------------+
              |                              |Lost packet not found|
              |                              +---------------------+
              |<-------------FSN message -----------------------|

           Figure 10: MAMS Retransmission Procedure with FSN

5.5  Coded MX SDU (CMS) Message

The "Type" field is set to "4" for CMS messages.

N-MADP (or C-MADP) may send out the CMS message to support downlink (or
uplink) packet loss recovery through coding, e.g. [CRLNC], [CTCP]. A
coded MX SDU is generated by applying a coding algorithm to multiple
consecutive (uncoded) MX SDUs, and it is used for fast recovery without
retransmission if any of the MX SDUs is lost.

A Coded MX SDU message consists of the following fields:

  o Connection ID (1 Byte): an unsigned integer to identify the anchor
     connection of the coded MX SDU;
  o Traffic Class ID (1 Byte): an unsigned integer to identify the
     traffic class of the coded MX;


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  o Sequence Number (2 Bytes): the sequence number of the first
     (uncoded) MX SDU used to generate the coded MX SDU.
  o Fragmentation Control (FC) (1 Byte): to provide necessary
     information for re-assembly, only needed if the coded MX SDU is
     too long to transport in a single MX control PDU.
  o N (1 Byte): the number of consecutive MX SDUs used to generate the
     coded MX SDU
  o Coding Control (CC) 1 Byte
       + K1 (LSB 4 bits): the number of bits for the coding coefficient
          field
       + K2 (MSB 4 bits): the number of bits for the coding sequence
          field
  o Coding Coefficient ( N x K1 / 8 Bytes)
       + a(i): the coding coefficient of the i-th MX SDU (1 . i . N)
       + padding
  o Coding Sequence ( N x K2 / 8 Bytes)
       + b(i): the coding sequence/order of the i-th MX SDU (1 . i . N)
       + padding
  o Coded MX SDU (variable): the coded MX SDU

If N = 2, K1 = 0 and K2 = 0, the simple XOR method is used to generate
the Coded MX SDU from two consecutive uncoded MX SDUs, and the a(i) or
b(i) fields are not included in the message. If K2 =0, the coding
sequence follows the MX SDU Sequence Number, and therefore the b(i)
fields are not included.

If the coded MX SDU is too long, it can be fragmented, and transported
by multiple MX control PDUs. The N, K1, K2, a(i), and b(i) fields are
only included in the MX PDU carrying the first fragment of the coded MX
SDU.

          C-MADP                                             N-MADP
              |                                                 |
              |<------------------ MX SDU #1 -------------------|
              |      lost<-------- MX SDU #2 -------------------|
              |<---- CMS Message (MX SDU #1 XOR MX SDU #2)------|
             +----------------------+                           |
             | MX SDU #2 recovered  |                           |
             +----------------------+                           |
              |                                                 |

       Figure 11: MAMS Packet Recovery Procedure with XOR Coding





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6  Security Considerations

User data in MAMS framework rely on the security of the underlying
network transport paths.  When this cannot be assumed, NCM configures
use of appropriate protocols for security, e.g. IPsec [RFC4301]
[RFC3948], DTLS [RFC6347].

7  IANA Considerations

TBD

8  Contributing Authors

The editors gratefully acknowledge the following additional
contributors in alphabetical order: Salil Agarwal/Nokia, Hema
Pentakota/Nokia.

9  References

9.1  Normative References

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

   [RFC4301] Kent, S. and K. Seo, "Security Architecture for the
             Internet Protocol", RFC 4301, DOI10.17487/RFC4301,
             December 2005, <http://www.rfc-editor.org/info/rfc4301>.

9.2  Informative References

   [RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
             Security Version 1.2", RFC 6347, January 2012,
             <http://www.rfc-editor.org/info/rfc6347>.

   [RFC7296] Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T.
             Kivinen, "Internet Key Exchange Protocol Version 2
             (IKEv2)", STD 79, RFC 7296, DOI 10.17487/RFC7296, October
             2014, <http://www.rfc-editor.org/info/rfc7296>.

   [RFC3948] Huttunen, A., Swander, B., Volpe, V., DiBurro, L., and M.
             Stenberg, "UDP Encapsulation of IPsec ESP Packets", RFC
             3948, DOI 10.17487/RFC3948, January 2005, <http://www.rfc-
             editor.org/info/rfc3948>.

   [MPProxy] X. Wei, C. Xiong, and E. Lopez, "MPTCP proxy mechanisms",
             https://tools.ietf.org/html/draft-wei-mptcp-proxy-
             mechanism-02


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   [MPPlain] M. Boucadair et al, "An MPTCP Option for Network-Assisted
             MPTCP", https://www.ietf.org/id/draft-boucadair-mptcp-
             plain-mode-09.txt

   [MAMS] S. Kanugovi, S. Vasudevan, F. Baboescu, and J. Zhu, "Multiple
             Access Management Protocol",
             https://tools.ietf.org/html/draft-kanugovi-intarea-mams-
             protocol-03

   [MAMS_CP] S. Kanugovi, et al., "Control Plane Protocols and
             Procedures for Multiple Access Management Services"

   [GRE2784] D. Farinacci, et al., "Generic Routing Encapsulation
             (GRE)", RFC 2784 March 2000, <http://www.rfc-
             editor.org/info/rfc2784>.

   [GRE2890] G. Dommety, "Key and Sequence Number Extensions to GRE",
             RFC 2890 September 2000, <http://www.rfc-
             editor.org/info/rfc2890>.

   [IANA]    https://www.iana.org/assignments/protocol-
             numbers/protocol-numbers.xhtml

   [LWIPEP] 3GPP TS 36.361, "Evolved Universal Terrestrial Radio Access
             (E-UTRA); LTE-WLAN Radio Level Integration Using Ipsec
             Tunnel (LWIP) encapsulation; Protocol specification"

   [RFC791] Internet Protocol, September 1981

   [CRLNC] S Wunderlich, F Gabriel, S Pandi, et al. Caterpillar RLNC
             (CRLNC): A Practical Finite Sliding Window RLNC Approach,
             IEEE Access, 2017

   [CTCP] M. Kim, et al. Network Coded TCP (CTCP), eprint
             arXiv:1212.2291, 2012

Authors' Addresses

   Jing Zhu

   Intel

   Email: jing.z.zhu@intel.com

   SungHoon Seo

   Korea Telecom


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   Email: sh.seo@kt.com

   Satish Kanugovi

   Nokia

   Email: satish.k@nokia.com

   Shuping Peng

   Huawei

   Email: pengshuping@huawei.com




































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