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Versions: 00 01 02 03 04 05 06 07 08 09 10 11 12            Experimental
          13 14                                         IPR declarations
Network Working Group                                           J. Zhu
Internet Draft                                                  Intel
Intended status: Experimental                             S. Kanugovi
Expires: April 21,2022                                          Nokia
                                                      October 21, 2021

            Generic Multi-Access (GMA) Encapsulation Protocol
                         draft-zhu-intarea-gma-12


Abstract

   A device can be simultaneously connected to multiple networks,
   e.g., Wi-Fi, LTE, 5G, and DSL. It is desirable to seamlessly
   combine the connectivity over these networks below the transport
   layer (L4) to improve quality of experience for applications that
   do not have built in multi-path capabilities. Such optimization
   requires additional control information, e.g., a sequence number,
   in each packet. This document presents a new light weight and
   flexible encapsulation protocol for this need. The solution has
   been developed by the authors based on their experiences in
   multiple standards bodies including the IETF and 3GPP, is not an
   Internet Standard and does not represent the consensus opinion of
   the IETF. This document will enable other developers to build
   interoperable implementations in order to experiment with the
   protocol.

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.

   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
   http://www.ietf.org/ietf/1id-abstracts.txt

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html



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   This Internet-Draft will expire on April 21, 2019.

Copyright Notice

   Copyright (c) 2021 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 ................................................. 2
      1.1. Scope of Experiment ....................................4
   2. Conventions used in this document ............................ 5
   3. Use Case ..................................................... 5
   4. GMA Encapsulation Methods .................................... 7
      4.1. Trailer-based IP Encapsulation .........................8
      4.2. Header-based IP Encapsulation .........................11
      4.3. (Header-based) non-IP Encapsulation ...................11
      4.4. IP Protocol Identifier ................................12
   5. Fragmentation ............................................... 12
   6. Concatenation ............................................... 14
   7. Security Considerations ..................................... 15
   8. IANA Considerations ......................................... 15
   9. References .................................................. 16
      9.1. Normative References ..................................16
      9.2. Informative References ................................16

1. Introduction

   A device can be simultaneously connected to multiple networks,
   e.g., Wi-Fi, LTE, 5G, and DSL. It is desirable to seamlessly
   combine the connectivity over these networks below the transport
   layer (L4) to improve quality of experience for applications that
   do not have built in multi-path capabilities.

   Figure 1 shows the Multi-Access Management Service (MAMS) user-
   plane protocol stack [MAMS], which has been used in today's multi-
   access solutions [ATSSS] [LWIPEP] [GRE1] [GRE2]. It consists of
   two layers: convergence and adaptation.

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   The convergence layer is responsible for multi-access operations,
   including multi-link (path) aggregation, splitting/reordering,
   lossless switching/retransmission, fragmentation, concatenation,
   etc. It operates on top of the adaptation layer in the protocol
   stack. From the perspective of a transmitter, a User Payload
   (e.g., IP packet) is processed by the convergence layer first, and
   then by the adaptation layer before being transported over a
   delivery connection; from the receiver's perspective, an IP packet
   received over a delivery connection is processed by the adaptation
   layer first, and then by the convergence layer.

          +-----------------------------------------------------+
          |   User Payload, e.g., IP Protocol Data Unit (PDU)   |
          +-----------------------------------------------------+
       +-----------------------------------------------------------+
       |  +-----------------------------------------------------+  |
       |  | Multi-Access (MX) Convergence Layer                 |  |
       |  +-----------------------------------------------------+  |
       |  +-----------------------------------------------------+  |
       |  | MX Adaptation   | MX Adaptation   | MX Adaptation   |  |
       |  | Layer           | Layer           | Layer           |  |
       |  +-----------------+-----------------+-----------------+  |
       |  | Access #1 IP    | Access #2 IP    | Access #3 IP    |  |
       |  +-----------------------------------------------------+  |
       |                            MAMS User-Plane Protocol Stack |
       +-----------------------------------------------------------+

             Figure 1: MAMS User-Plane Protocol Stack [MAMS]

   GRE (Generic Routing Encapsulation) can be used [LWIPEP] [GRE1]
   [GRE2] as the encapsulation protocol at the convergence layer to
   encode additional control information, e.g., Key, Sequence Number.
   However, there are two main drawbacks with this approach:

      o It is difficult to introduce new control fields because the
        GRE header formats are already defined,
      o IP-over-IP tunnelling (required for GRE) leads to higher
        overhead especially for small packet.

   For example, the overhead of IP-over-IP/GRE tunnelling with both
   Key and Sequence Number is 32 Bytes (20 Bytes IP header + 12 Bytes
   GRE header), which is 80% of a 40 Bytes TCP ACK packet.

   This document presents a light-weight GMA (Generic Multi-Access)
   encapsulation protocol for the convergence layer. It supports
   three encapsulation methods: trailer-based IP encapsulation,

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   header-based IP encapsulation, and non-IP encapsulation.
   Particularly, the IP encapsulation methods avoid IP-over-IP
   tunneling overhead (20 Bytes), which is 50% of a 40 Bytes TCP ACK
   packet. Moreover, it introduces new control fields to support
   fragmentation and concatenation, which are not available in GRE-
   based solutions [LWIPEP] [GRE1] [GRE2].

   The GMA protocol only operates between endpoints that have been
   configured to use GMA. This configuration can be through any
   control messages and procedures, including, for example, Multi-
   Access Management Services [MAMS]. Moreover, UDP or IPSec
   tunneling can be used at the adaptation sublayer to protect GMA
   operation from intermediate nodes.

   The solution described in this document was been developed by the
   authors based on their experiences in multiple standards bodies
   including the IETF and 3GPP. However, it is not an Internet
   Standard and does not represent the consensus opinion of the IETF.
   This document presents the protocol specification to enable
   experimentation as described in Section 1.1 and to facilitate
   other interoperable implementations.

1.1. Scope of Experiment

   The protocol described in this document is an experiment. The
   objective of the experiment is to determine whether the protocol
   meets the requirements, can be safely used, and has support for
   deployment.

   Section 4 describes three possible encapsulation methods that are
   enabled by this protocol. Part of this experiment is to assess
   whether all three mechanisms are necessary, or whether, for
   example, all implementations are able to support the main
   "trailer-based" IP encapsulation method. Similarly, the experiment
   will investigate the relative merits of the IP and non-IP
   encapsulation methods.

   It is expected that this protocol experiment can be conducted on
   the Internet since the GMA packets are identified by an IP
   protocol number and the protocol is intended for single hop
   propagation: devices should not be forwarding packet and if they
   do they will not need to examine the payload, while destination
   systems that do not support this protocol should not receive such
   packets and will handle them as unknown payloads according to
   normal IP processing. Thus, experimentation is conducted between
   consenting end systems that have been mutually configured to
   participate in the experiment as described in Section 7.


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   Note that this experiment "re-uses" the IP protocol identifier 114
   as described in Section 4.4. Part of this experiment is to assess
   the safety of doing this. The experiment should consider the
   following safety mechanisms:

      o GMA endpoints SHOULD detect non-GMA IP packets that also use
        114 and log an error to report the situation (although such
        error logging MUST be subject to rate limits).
      o GMA endpoints SHOULD stop using 114 and switch to non-IP
        (UDP) based encapsulation (Sec 4.3, Figure 7) after detecting
        any non-GMA usage of 114.

   The experiment SHOULD use packet tracing tool, e.g., WireShark,
   TCPDUMP, to monitor both ingress and egress traffic at GMA
   endpoints and ensure the above safety mechanisms are implemented.

   Path quality measurements (one-way-delay, loss, etc.) and
   congestion detection are performed by receiver based on the GMA
   control fields, e.g., sequence number, timestamp, etc. Receiver
   will then dynamically control how to split or steer traffic over
   multiple delivery connections accordingly. GMA control protocol
   [GMAC] MAY be used for signaling between GMA endpoints. Another
   objective of the experiment is to evaluate the usage of various
   receiver-based algorithms [GCC] [MPIP] in multi-path traffic
   management, and the impact on the e2e performance (throughput,
   delay, etc.) of higher layer (transport) protocols, e.g., TCP,
   QUIC, WebRTC, etc.

   The authors will continually assess the progress of this
   experiment and encourage other implementers to contact them to
   report the status of their implementations and their experiences
   with the protocol.

2. Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
   NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED",
   "MAY", and "OPTIONAL" in this document are to be interpreted as
   described in BCP 14 [RFC2119] [RFC8174] when, and only when, they
   appear in all capitals, as shown here.

3. Use Case

   As shown in Figure 2, a client device (e.g., Smartphone, Laptop,
   etc.) may connect to the Internet via both Wi-Fi and LTE
   connections, one of which (e.g., LTE) may operate as the anchor
   connection, and the other (e.g., Wi-Fi) may operate as the
   delivery connection. The anchor connection provides the IP address

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   and connectivity for end-to-end Internet access, and the delivery
   connection provides an additional path between client and Multi-
   Access Gateway for multi-access optimizations.

                         Multi-Access Aggregation

                    +---+                        +---+
                    | |A|--- LTE Connection -----|C| |
                    |U|-|                        |-|S| Internet
                    | |B|--- Wi-Fi Connection ---|D| |
                    +---+                        +---+
                   Client                Multi-Access Gateway

         A: The adaptation layer endpoint of the LTE connection
         resides in the client

         B: The adaptation layer endpoint of the Wi-Fi connection
         resides in the client

         C: The adaptation layer endpoint of the LTE connection
         resides in the Multi-Access Gateway, aka N-MADP (Network
         Multi-Access Data Proxy) in [MAMS]

         D: The adaptation layer endpoint of the Wi-Fi connection
         resides in the Multi-Access Gateway

         U: The convergence layer endpoint resides in the client

         S: The convergence layer endpoint resides in the Multi-
         Access Gateway

               Figure 2: GMA-based Multi-Access Aggregation

   For example, per-packet aggregation allows a single IP flow to use
   the combined bandwidth of the two connections. In another example,
   packets lost due to a temporarily link outage may be
   retransmitted. Moreover, packets may be duplicated over multiple
   connections to achieve high reliability and low latency, where
   duplicated packets are eliminated by the receiving side. Such
   multi-access optimization requires additional control information,
   e.g., a sequence number, in each packet, which can be supported by
   the GMA encapsulation protocol described in this document.

   The GMA protocol described in this document is designed for
   multiple connections, but it may also be used when there is only
   one connection between two endpoints. For example, it may be used
   for loss detection and recovery. In another example, it may be

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   used to concatenate multiple small packets and reduce per packet
   overhead.

4. GMA Encapsulation Methods

   The GMA encapsulation protocol supports the following three
   methods:

      o Trailer-based IP Encapsulation (4.1)
      o Header-based IP Encapsulation (4.2)
      o (Header-based) non-IP Encapsulation (4.3)

   Trailer-based IP encapsulation MUST be used if it is supported by
   GMA endpoints.

   Header-based encapsulation MUST be used if the trailer-based
   method is not supported by either Client or Multi-Access Gateway.
   In this case, if the adaptation layer, e.g., UDP tunnelling,
   supports non-IP packet format, non-IP encapsulation MUST be used;
   otherwise, header-based IP encapsulation MUST be used.

   If non-IP encapsulation is configured, a GMA header MUST be
   present in every packet. In comparison, if IP encapsulation is
   configured, a GMA header or trailer may be added dynamically on
   per-packet basis, and it indicates the presence of GMA header (or
   trailer) to set the protocol type of the GMA PDU to "114" (see
   Section 4.4).

   The GMA endpoints MAY configure the GMA encapsulation method
   through control signalling or pre-configuration. For example, the
   "MX UP Setup Configuration Request" message as specified in Multi-
   Access Management Service [MAMS] includes "MX Convergence Method
   Parameters", which provides the list of parameters to configure
   the convergence layer, and can be extended to indicate the GMA
   encapsulation method.

   GMA endpoint MUST discard a received packet and MAY log an error
   to report the situation (although such error logging MUST be
   subject to rate limits) under any of the following conditions:

      . the GMA version number in the GMA header (or trailer) is not
        understood or supported by the GMA endpoint
      . a Flag bit in the GMA header (or trailer) not understood or
        supported by the GMA endpoint is set to "1"





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4.1. Trailer-based IP Encapsulation

          |<---------------------GMA PDU ----------------------->|
          +------------------------------------------------------+
          | IP hdr |        IP payload             | GMA Trailer |
          +------------------------------------------------------+
          |<------- GMA SDU (user payload)-------->|


       Figure 3: GMA PDU Format with Trailer-based IP Encapsulation

   Figure 3 shows the trailer-based IP encapsulation GMA PDU
   (protocol data unit) format. A (GMA) PDU may carry one or multiple
   IP packets, aka (GMA) SDUs (service data unit), in the payload, or
   a fragment of the SDU.

   The Protocol Type field in the IP header of the GMA PDU MUST be
   changed to 114 (Any 0-Hop Protocol) (see Section 4.4) to indicate
   the presence of the GMA trailer.

   If the original IP packet is IPv4, the following three IP header
   fields MUST be changed:

     o IP Length: add the length of "GMA Trailer" to the length of
        the original IP packet
     o Time To Live (TTL): set to "1"
     o IP checksum: recalculate after changing "Protocol Type", "TTL"
        and "IP Length"

   If the original IP packet is IPv6, the following two IP header
   fields MUST be changed:

     o IP Length: add the length of "GMA Trailer" to the length of
        the original IP packet
     o Hop-Limit (HL): set the HL field to "0"

   The GMA (Generic Multi-Access) trailer MUST consist of two
   mandatory fields (the last 3 bytes): Next Header and Flags,
   defined as follows:

     o Next Header (1 Byte): the IP protocol type of the (first) SDU
        in a PDU, and it stores the value before it was overwritten to
        114.
     o Flags (2 Bytes): Bit 0 is the most significant bit (MSB), and
        Bit 15 is the least significant bit (LSB)
        + Checksum Present (bit 0): If the Checksum Present bit is set
        to 1, then the Checksum field is present

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        + Concatenation Present (bit 1): If the Concatenation Present
        bit is set to 1, then the PDU carries multiple SDUs, and the
        First SDU Length field is present
        + Connection ID Present (bit 2): If the Connection ID Present
        bit is set to 1, then the Connection ID field is present
        + Flow ID Present (bit 3): If the Flow ID Present bit is set
        to 1, then the Flow ID field is present
        + Fragmentation Present (bit 4): If the Fragmentation Present
        bit is set to 1, then the PDU carry a fragment of the SDU and
        the Fragmentation Control field is present
        + Delivery SN Present (bit 5): If the Delivery SN (Sequence
        Number) Present bit is set to 1, then the Delivery SN field is
        present and contains the valid information
        + Flow SN Present (bit 6): If the Flow SN Present bit is set
        to 1, then the Sequence Number field is present
        + Timestamp Present (bit 7): If the Timestamp Present bit is
        set to 1, then the Timestamp field is present
        + TTL Present (bit 8): If the TTL Present bit is set to 1,
        then the TTL field is present
        + Reserved (bit 9-12): set to "0" and ignored on receipt
        + Version (bit 13~15): GMA version number, set to 0 for the
        GMA encapsulation protocol specified in this document.

   The Flags field is at the end of the PDU, and the Next Header
   field is the second last. The Receiver SHOULD first decode the
   Flags field to determine the length of the GMA trailer, and then
   decode each optional field accordingly. The GMA (Generic Multi-
   Access) trailer MAY consist of the following optional fields:

     o Checksum (1 Byte): to contain the (one's complement) checksum
        sum of all the 8 bits in the trailer. For purposes of
        computing the checksum, the value of the checksum field is
        zero. This field is present only if the Checksum Present bit
        is set to one.
     o First SDU Length (2 Bytes): the length of the first IP packet
        in the PDU, only included if a PDU contains multiple IP
        packets. This field is present only if the Concatenation
        Present bit is set to one.
     o Connection ID (1 Byte): an unsigned integer to identify the
        anchor and delivery connection of the GMA PDU. This field is
        present only if the Connection ID Present bit is set to one.
        + Anchor Connection ID (MSB 4 Bits): an unsigned integer to
        identify the anchor connection
        + Delivery Connection ID (LSB 4 Bits): an unsigned integer to
        identify the delivery connection
     o Flow ID (1 Byte): an unsigned integer to identify the IP flow
        that a PDU belongs to, for example Data Radio Bearer (DRB) ID


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        [LWIPEP] for a cellular (e.g., LTE) connection. This field is
        present only if the Flow ID Present bit is set to one.
     o Fragmentation Control (FC) (1 Byte): to provide necessary
        information for re-assembly, only needed if a PDU carries
        fragments. This field is present only if the Fragmentation
        Present bit is set to one. Please refer to section 5 for its
        detailed format and usage.
     o Delivery SN (1 Byte): an auto-incremented integer to indicate
        the GMA PDU transmission order on a delivery connection.
        Delivery SN is needed to measure packet loss of each delivery
        connection and therefore generated per delivery connection per
        flow. This field is present only if the Delivery SN Present
        bit is set to one.
     o Flow SN (3 Bytes): an auto-incremented integer to indicate the
        GMA SDU (IP packet) order of a flow. Flow SN is needed for
        retransmission, reordering, and fragmentation. It SHALL be
        generated per flow. This field is present only if the Flow SN
        Present bit is set to one.
     o Timestamp (4 Bytes): to contain the current value of the
        timestamp clock of the transmitter in the unit of 1
        millisecond. This field is present only if the Timestamp
        Present bit is set to one.
     o TTL (1 Byte): to contain the TTL value of the original IP
        header if the GMA SDU is IPv4, or the Hop-Limit value of the
        IP header if the GMA SDU is IPv6. This field is present only
        if the TTL Present bit is set to one.

   Figure 4 shows the GMA trailer format with all the fields present,
   and the order of the GMA control fields SHALL follow the bit order
   in the Flags field. Note that the bits in the Flags field are
   ordered with the first bit transmitted being bit 0 (MSB). All
   fields are transmitted in regular network byte order and appear in
   reverse order to their corresponding flag bits. If a flag bit is
   clear, the corresponding optional field is absent.

   For example, Bit 0 (the MSB) of the Flags field is the Checksum
   Present bit, and the Checksum field is the last in the trailer
   except of the two mandatory fields. Bit 1 is the Concatenation
   present bit, and the FSL field is the second last.

 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|     TTL       |                   Timestamp
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                |                   Flow SN                     |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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|  Delivery SN  |    FC         |   Flow ID     | Connection ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|      First SDU Length (FSL)   |   Checksum    |  Next Header  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|         Flags                 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Figure 4: GMA Trailer Format with all Optional Fields Present

4.2. Header-based IP Encapsulation

   Figure 5 shows the header-based IP encapsulation format. Here, the
   GMA header is inserted right after the IP header of the GMA SDU,
   and the IP header fields of the GMA PDU MUST be changed the same
   way as in trailered-based IP encapsulation.

          +-----------------------------------------------+
          |IP hdr | GMA Header  |       IP payload        |
          +-----------------------------------------------+
       Figure 5: GMA PDU Format with Header-based IP Encapsulation

   Figure 6 shows the GMA header format. In comparison to GMA
   trailer, the only difference is that the Flags field is now in the
   front so that the Receiver can first decode the Flags field to
   determine the GMA header length.

   "TTL" field MUST be included and the "TTL" bit in the GMA header
   (or Trailer) MUST be set to 1 if (Trailer or Header based) IP
   Encapsulation is used.

       +------------------------------------------------------+
       | Flags | other fields (TTL, Timestamp, Flow SN, etc.) |
       +------------------------------------------------------+
                       Figure 6: GMA Header Format

4.3. (Header-based) non-IP Encapsulation

   Figure 7 shows the header-based non-IP encapsulation format. Here,
   "UDP Tunnelling" is configured at the MX adaptation layer. The
   ports for "UDP Tunnelling" at Client are chosen from the Dynamic
   Port range, and the ports for "UDP Tunnelling" at Multi-Access
   Gateway are configured and provided to Client through additional
   control messages, e.g., [MAMS].




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   "TTL", "FSL", and "Next Header" are no longer needed, and MUST not
   be included. Moreover, the IP header fields of the GMA SDU remain
   unchanged.

    +-------------------------------------------------------------+
    | IP hdr | UDP hdr  | GMA Header | IP hdr |    IP payload     |
    +-------------------------------------------------------------+
                                    |<------- GMA SDU------------>|
                        |<------------------- GMA PDU------------>|

            Figure 7: GMA PDU Format with Non-IP Encapsulation

4.4. IP Protocol Identifier

   As described in Section 4.1, IP encapsulated GMA PDUs are
   indicated using the IP Protocol Type 114. This is designated and
   recorded by IANA [IANA] to indicate "any 0-Hop Protocol". No
   reference is given in the IANA registry for the definition of this
   Protocol Type, and IANA has no record of why the assignment was
   made or how it is used, although it was probably assigned before
   1999 [IANA1999].

   There is some risk associated with "re-using" Protocol Type 114
   because there may be implementations of other protocols also using
   this Protocol Type. However, because the protocol described in
   this document is used only between adjacent devices specifically
   configured for this purpose, the use of Protocol Type 114 should
   be safe.

   As described in Section 1.1, one of the purposes of the experiment
   described in this document is to verify the safety of using this
   Protocol Type. Deployments should be aware of the risk of a clash
   with other uses of this Protocol Type.

5. Fragmentation

   If the MTU size of the anchor connection (for GMA SDU) is
   configured such that the corresponding GMA PDU length adding GMA
   header (or trailer) and other overhead (UDP tunneling) MAY exceed
   the MTU of a delivery connection, GMA endpoints MUST be configured
   to support fragmentation through additional control messages
   [MAMS].

   The fragmentation procedure at the convergence sublayer is similar
   to IP fragmentation [RFC791] in principle, but with the following
   two differences for less overhead:


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     o The fragment offset field is expressed in number of fragments
     o The maximum number of fragments per SDU is 2^7 (=128)

   The Fragmentation Control (FC) field in the GMA trailer (or
   header) 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 SDU

   A PDU carries a whole 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 PDU contains a fragment of the SDU.

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

   To fragment a long SDU, the transmitter creates n PDUs and copies
   the content of the IP header fields from the long PDU into the IP
   header of all the PDUs. The length field in the IP header of PDU
   SHOULD be changed to the length of the PDU, and the protocol type
   SHOULD be changed to 114.

   The data of the long SDU is divided into n portions based on the
   MTU size of the delivery connection. The first portion of the data
   is placed in the first PDU. The MF flag is set to "1", and the FO
   field is set to "0". The i-th portion of the data is placed in the
   i-th PDU. The MF flag is set to "0" if it is the last fragment and
   set to "1" otherwise. The FO field is set to i-1.

   To assemble the fragments of a SDU, the receiver combines PDUs
   that all have the same Flow SN. 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 GMA trailer (or header).
   The first fragment will have the Fragment Offset set to "0", and
   the last fragment will have the More-Fragments flag set to "0".

   GMA fragmentation operates above the IP layer of individual access
   connection (Wi-Fi, LTE) and between the two end points of
   convergence layer. The convergence layer end points (client,
   multi-access gateway) SHOULD obtain the MTU of individual
   connection through either manual configuration or implementing
   PMTUD as suggested in [RFC8900].

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6. Concatenation

   The convergence sublayer MAY support concatenation if a delivery
   connection has a larger maximum transmission unit (MTU) than the
   original IP packet (SDU). Only the SDUs with the same client IP
   address, and the same Flow ID MAY be concatenated.

   If the (trailer or header based) IP encapsulation method is used,
   the First SDU Length (FSL) field SHOULD be included in the GMA
   trailer (or header) to indicate the length of the first SDU.
   Otherwise, the FSL field SHOULD not be included.

     +-----------------------------------------------------------+
     |IP hdr| IP payload    |IP hdr|   IP payload  | GMA Trailer |
     +-----------------------------------------------------------+
        Figure 8: Example of GMA PDU Format with Concatenation and
                      Trailer-based IP Encapsulation

   To concatenate two or more SDUs, the transmitter creates one PDU
   and copies the content of the IP header field from the first SDU
   into the IP header of the PDU. The data of the first SDU is placed
   in the first portion of the data of the PDU. The whole second SDU
   is then placed in the second portion of the data of the PDU
   (Figure 8). The procedure continues till the PDU size reaches the
   MTU of the delivery connection. If the FSL field is present, the
   IP length field of the PDU SHOULD be updated to include all
   concatenated SDUs and the trailer (or header), and the IP checksum
   field SHOULD be recalculated if the packet is IPv4.

   To disaggregate a PDU, if the (header or trailer based) IP
   encapsulation method is used, the receiver first obtains the
   length of the first SDU from the FSL field and decodes the first
   SDU. The receiver then obtains the length of the second SDU based
   on the length field in the second SDU IP header and decodes the
   second SDU. The procedure continues till no byte is left in the
   PDU. If the non-IP encapsulation method (Figure 7) is used, the IP
   header of the first SDU will not change during the encapsulation
   process, and the receiver SHOULD obtain the length of the first
   SDU directly from its IP header (Figure 9).

                                    |<-------1st GMA SDU------------
   +---------------------------------------------------------------+
   | IP hdr | UDP hdr  | GMA Header | IP hdr |       IP payload    |
   +---------------------------------------------------------------+
            | IP hdr |           IP payload    |
   +-------------------------------------------+
   -------->|<-------2nd GMA SDU--------------->

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    Figure 9: Example of GMA PDU Format with Concatenation and Header-
                     based Non-IP (UDP) Encapsulation

   If a PDU contains multiple SDUs, the Flow SN field is for the last
   SDU, and the Flow 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 PDU contains 3 SDUs (IP packets), the SN is 4, 5,
   and 6 for the first, second, and last SDU respectively.

   GMA concatenation can be used for packing small packets of a
   single application, e.g., TCP ACKs, or from multiple applications.
   Notice that a single GMA flow may carry multiple application flows
   (TCP, UDP, etc.).

   GMA endpoint MUST NOT perform concatenation and fragmentation in a
   single PDU. If a GMA PDU carries a fragmented SDU, it MUST NOT
   carry any other (fragmented or whole) SDU.

7. Security Considerations

   Security in a network using GMA should be relatively similar to
   security in a normal IP network. GMA is unaware of IP or higher
   layer end-to-end security as it carries the IP packets as opaque
   payload. Deployers are encouraged to not consider that GMA adds
   any form of security and to continue to use IP or higher layer
   security as well as link-layer security.

   The GMA protocol at the convergence sublayer is a 0-hop protocol
   and relies on the security of the underlying network transport
   paths. When this cannot be assumed, appropriate security protocols
   (IPsec, DTLS, etc.) SHOULD be configured at the adaptation
   sublayer. On the other hand, packet filtering requires either that
   a firewall looks inside the GMA packet or that the filtering is
   done on the GMA endpoints. In those environments in which this is
   considered to be a security issue it may be desirable to terminate
   the GMA operation at the firewall.

   Local-only packet leak prevention (HL=0, TTL=1) SHOULD be on
   preventing the leak of the local-only GMA PDUs outside the "local
   domain" to the Internet or to another domain which could use the
   same IP protocol type, i.e. 114.

8. IANA Considerations

   This document makes no requests of IANA




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9. References

9.1. Normative References

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

   [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
             2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174
             May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [GRE1] Dommety, G., "Key and Sequence Number Extensions to GRE",
             <https://www.rfc-editor.org/info/rfc2890> .

9.2. Informative References

   [MAMS] S. Kanugovi, F. Baboescu, J. Zhu, and S. Seo "Multi-Access
             Management Services
             (MAMS)https://tools.ietf.org/rfc/rfc8743.txt

   [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

   [ATSSS] 3GPP TR 23.793, Study on access traffic steering, switch
             and splitting support in the 5G system architecture.

   [GRE2] RFC 8157, Huawei's GRE Tunnel Bonding Protocol, May 2017

   [RFC8900] Bonica, R., Baker, F., Huston, G., Hinden, R., Troan,
             O., and F. Gont, "IP Fragmentation Considered Fragile",
             BCP 230, RFC 8900, DOI 10.17487/RFC8900, September 2020,
             <https://www.rfc-editor.org/info/rfc8900>.

   [IANA1999]https://web.archive.org/web/19990203044112/http://www.is
             i.edu:80/in-notes/iana/assignments/protocol-numbers





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   [GCC]  S. Holmer, et al. A Google Congestion Control Algorithm for
             Real-Time Communication,
             https://www.ietf.org/archive/id/draft-ietf-rmcat-gcc-
             02.txt

   [MPIP] L. Sun, et al. Multipath IP Routing on End Devices:
             Motivation, Design, and
             Performance,  https://eeweb.engineering.nyu.edu/faculty/
             yongliu/docs/MPIP_Tech.pdf

   [GMAC] J. Zhu M. Zhang, UDP-based GMA Control
             Protocol,  https://www.ietf.org/archive/id/draft-zhu-
             intarea-gma-control-00.txt



Authors' Addresses

   Jing Zhu

   Intel

   Email: jing.z.zhu@intel.com

   Satish Kanugovi

   Nokia

   Email: satish.k@nokia.com




















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