Separation of Data Path and Data Flow Sublayers in the Transport Layer
draft-asai-tsvwg-transport-review-01

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Author Hirochika Asai 
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Network Working Group                                            H. Asai
Internet-Draft                         Preferred Networks / WIDE Project
Intended status: Standards Track                        22 February 2021
Expires: 26 August 2021

 Separation of Data Path and Data Flow Sublayers in the Transport Layer
                  draft-asai-tsvwg-transport-review-01

Abstract

   This document reviews the architectural design of the transport
   layer.  In particular, this document separates the transport layer
   into two sublayers; the data path and the data flow layers.  The data
   path layer provides functionality on the data path, such as
   connection handling, path quality and trajectory monitoring, waypoint
   management, and congestion control.  The data flow layer provides
   additional functionality upon the data path layer, such as flow
   control for the receive buffer management, retransmission for
   reliable data delivery, and transport layer security.  The data path
   layer multiplexes multiple data flow layer protocols and provides
   data path information to the data flow layer to control data
   transmissions, such as prioritization and inverse multiplexing for
   multipath protocols.

Status of This Memo

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   This Internet-Draft will expire on 26 August 2021.

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
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   Please review these documents carefully, as they describe your rights
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   provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Transport Layer Functionality Review  . . . . . . . . . . . .   3
     2.1.  Conventional Layering . . . . . . . . . . . . . . . . . .   3
     2.2.  Data Path-aware Networking  . . . . . . . . . . . . . . .   4
     2.3.  Resource Management: Flow Control and Congestion
           Control . . . . . . . . . . . . . . . . . . . . . . . . .   5
     2.4.  Communication Models and Transport Layer Protocols  . . .   6
     2.5.  Multipath Protocols . . . . . . . . . . . . . . . . . . .   7
     2.6.  Reliable Data Communication . . . . . . . . . . . . . . .   7
     2.7.  Security  . . . . . . . . . . . . . . . . . . . . . . . .   8
     2.8.  Summary . . . . . . . . . . . . . . . . . . . . . . . . .   8
   3.  Specifications of Data Path Layer and Data Flow Layer . . . .   9
     3.1.  Data Path Layer . . . . . . . . . . . . . . . . . . . . .   9
       3.1.1.  In-band trajectory monitoring . . . . . . . . . . . .   9
       3.1.2.  Waypoint management . . . . . . . . . . . . . . . . .  10
       3.1.3.  Bidirectional connection establishment  . . . . . . .  11
       3.1.4.  Data path quality (congestion) monitoring and
               congestion control  . . . . . . . . . . . . . . . . .  11
       3.1.5.  Data flow multiplexing  . . . . . . . . . . . . . . .  11
       3.1.6.  Packet duplication  . . . . . . . . . . . . . . . . .  12
       3.1.7.  Data path security  . . . . . . . . . . . . . . . . .  12
     3.2.  Data Flow Layer . . . . . . . . . . . . . . . . . . . . .  12
       3.2.1.  Retransmission for reliable data communication  . . .  12
       3.2.2.  Flow control for receive-buffer management  . . . . .  13
       3.2.3.  Flow prioritization . . . . . . . . . . . . . . . . .  13
       3.2.4.  Data flow and end-to-end security . . . . . . . . . .  13
       3.2.5.  Inverse multiplexing for multipath protocols  . . . .  13
   4.  Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . .  13
     4.1.  Multipath Transport Protocols . . . . . . . . . . . . . .  14
     4.2.  Congestion Control Acceleration . . . . . . . . . . . . .  15
     4.3.  In-Network Computing  . . . . . . . . . . . . . . . . . .  16
     4.4.  Flow Arbitration  . . . . . . . . . . . . . . . . . . . .  17
   5.  Implementation Considerations . . . . . . . . . . . . . . . .  18
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  18
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  18
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  19
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  19

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     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  19
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  21
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  22

1.  Introduction

   This document specifies two sublayers of the transport layer; the
   data path and the data flow layers.  In this document, the transport
   layer's data path functionality, such as bidirectional connection
   handling, waypoint management, and congestion control, is separated
   from the data flow functionality, such as flow control for the
   receive buffer management, retransmission for reliable data delivery,
   and transport layer security.  This document reviews the transport
   layer's functionality from the viewpoint of data paths and data flows
   in Section 2.  It then specifies the data path layer and the data
   flow layer in Section 3.  The data path and data flow layers provide
   the data path and the data flow functionality, respectively.

   This document reviews the transport layer to clarify the transport
   layer's functionality and to invent data flow layer protocols for
   advanced Internet technologies, such as middleboxes and in-network
   computing.  Hence, this document does not intend to obsolete the
   transport layer or violate the current Internet architecture.

   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].

2.  Transport Layer Functionality Review

   This section reviews the conventional layering and transport layer
   functionality.  It then summarizes the transport layer functionality
   by distinguishing data paths and data flows.

2.1.  Conventional Layering

   RFC 1122 [RFC1122] defines three communication layers, link layer,
   Internet layer, transport layer, and the interfaces between these
   layers.  Link-layer protocols provide hop-by-hop data communications.
   Internet layer protocols such as the Internet Protocol (IP), the
   Internet Control Message Protocol (ICMP), and the Internet Group
   Management Protocol (IGMP) provide fragmentation, hop-by-hop datagram
   forwarding, and end-to-end datagram delivery.  RFC 1123 [RFC1123]
   defines the interface between the application layer and the transport
   layer.

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   The Internet design follows the end-to-end principle to keep the
   simplicity of the Internet layer.  Thus, the main functionality of
   the Internet layer is to provide end-to-end reachability.  It does
   not guarantee datagram integrity, which means that packet loss,
   duplication, corruption, or reordering may occur.  Over the Internet
   layer, the transport layer implements such functionality to provide
   datagram integrity on the end-hosts to achieve end-to-end
   communications.

   +-------------------+                           +-------------------+
   | Application layer |<------------------------->| Application layer |
   +-------------------+                           +-------------------+
   | Transport layer   |<------------------------->| Transport layer   |
   +-------------------+   +-------------------+   +-------------------+
   | Internet layer    |<->| Internet layer    |<->| Internet layer    |
   +-------------------+   +-------------------+   +-------------------+
   | Link layer        |<->| Link layer        |<->| Link layer        |
   +-------------------+   +-------------------+   +-------------------+
     End-host                Router                  End-host

          Figure 1: Conventional layering of Internet architecture

   The transport layer provides various functions over the IP.  User
   Datagram Protocol (UDP) [RFC0768] and Transmission Control Protocol
   (TCP) [RFC0793] are the most commonly used transport layer protocols.
   UDP is a connectionless transport protocol with a minimum header.
   TCP implements flow control according to the receiver's buffer
   capacity, retransmission for reliable communication, congestion
   control by a packet delivery status such as packet loss and delay.
   The transport layer may implement an end-to-end security function,
   Transport Layer Security (TLS) [RFC8446].

   Figure 1 illustrates the conventional layering of Internet
   architecture.  A router forwards an IP datagram to a next-hop router
   corresponding to the destination address.  In this way, the Internet
   layer protocol, IP, provides end-to-end reachability through hop-by-
   hop routing.  The transport layer provides additional functions for
   end-to-end communications.

2.2.  Data Path-aware Networking

   As described above, the Internet layer's main functionality has been
   end-to-end reachability with hop-by-hop packet forwarding based on
   destination IP address.  Therefore, it is unaware of data paths,
   i.e., trajectories of packets.  Best-effort communications over
   ``dumb'' networks without the quality of service (QoS) have been the
   Internet principle [RFC2768].  This end-to-end principle of the
   Internet has achieved a scalable architecture.  However, computer

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   networks' advancement introduced QoS and middleboxes to achieve high-
   quality data communication and optimize communication over
   distributed and heterogeneous computer networks.  These technologies
   require to be aware of data paths associated with data flows.

   The Internet layer has been extended to support QoS.  Differentiated
   Services, DiffServ [RFC2474], is one of the technologies to implement
   QoS.  It enables autonomous and scalable service discrimination using
   an IP header field, differentiated services codepoint (DSCP), to
   implement QoS for a data path.  Segment Routing [RFC8402] enables
   data path configuration for individual flows by leveraging the source
   routing paradigm.  Thus, Segment Routing is another technology to
   treat QoS.

   Middleboxes are more complex than QoS.  They add various functions
   such as firewalls, TCP offloading, transcoding, and content caches to
   a data path.  These functions require to be aware of waypoints to be
   activated.  Routing technologies with packet classification using the
   IP and the transport protocol headers have collectively been
   leveraged to route traffic through the middleboxes as the IP layer is
   not aware of data paths.  If a middlebox is transparent to end-hosts,
   it should be installed at the network gateway or redirected by
   policy-based routing or segment routing to activate the function.
   Otherwise, an end-host should specify the middlebox as a target end-
   host.  In addition to these middleboxes, distributed computing
   technologies, such as in-network computing and multi-access edge
   computing (MEC), also require to be aware of data paths.

   In summary, advanced computer networks require to treat data paths
   for QoS, middleboxes, and distributed computing.  Packet
   classification for data path treatment may use the header information
   of transport layer protocols such as port numbers as well as IP
   header fields.

2.3.  Resource Management: Flow Control and Congestion Control

   As the Internet layer does not provide resource management
   functionality, transport layer protocols may implement it, such as
   flow control and congestion control.  Both flow control and
   congestion control mechanisms control packet transmission for
   resource management.  However, the target resource is different.
   They control packet transmission according to the receiver's buffer
   capacity and the network bandwidth capacity, respectively.

   For example, in TCP's flow control, a receiver announces the
   remaining receive buffer size as the window size.  Hence, flow
   control is not aware of network bandwidth capacity.  On the other
   hand, congestion control is performed based on data communication

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   quality information, such as packet loss and delay.  The underlying
   Internet layer may provide congestion information by the Explicit
   Congestion Notification (ECN) [RFC3168].  In this manner, transport
   layer protocols control congestion depending on the data path's
   network resources.  Therefore, congestion control is associated with
   a data path, while flow control is associated with the end-hosts.

   As discussed above, congestion control should be performed on the
   associated data path.  However, in current transport layer protocols
   such as TCP, Datagram Congestion Control Protocol (DCCP) [RFC4340],
   and Stream Control Transmission Protocol (SCTP) [RFC4960], an
   individual flow independently performs congestion control even if the
   same data path multiplexes multiple flows.  Therefore, multiple flows
   cannot collectively perform congestion control for the data path.
   For example, when a data path multiplexes a TCP and a UDP flows, the
   TCP flow's congestion control may affect the other UDP flow's
   quality.  Moreover, transport layer protocols utilize network
   resources on a fair basis.  Thus, flow prioritization cannot be
   implemented at the transport layer, although a QoS technology such as
   DiffServ may be leveraged.

2.4.  Communication Models and Transport Layer Protocols

   This subsection summarizes four communication models between end-
   hosts at the Internet layer; unicast, anycast [RFC7094], broadcast
   [RFC0919], and multicast [RFC1112], and then discusses transport
   layer protocols for these communication models.

   Unicast is the most fundamental communication model that performs a
   data communication between two end-hosts.  Most transport layer
   protocols, such as UDP [RFC0768] and TCP [RFC0793], are designed for
   unicast.  Unicast may be unidirectional, but some transport layer
   protocols only support a bidirectional data communication.

   Anycast is a special communication model extended from unicast.
   Therefore, the same transport layer protocols as unicast are often
   used.  Anycast performs a one-to-one data communication, but a
   destination end-host is not uniquely identified by its IP address
   because multiple end-hosts share an identical IP address.  Instead, a
   destination end-host is determined by IP routing in anycast.  Thus,
   anycast needs to take into account data path changes for stateful
   connections as the destination end-host may change by IP routing
   during a long-lived transaction [RFC4786].

   Broadcast and multicast are a communication model that one end-host
   sends a packet to multiple end-hosts.  A difference between broadcast
   and multicast is that multicast is to deliver a packet to multiple
   end-hosts that join a specific multicast group while broadcast

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   delivers packets to all end-hosts within a specified hop count.  In
   other words, multicast is referred to as a group data communication.
   In multicast, IGMP and Protocol Independent Multicast (PIM) (e.g.,
   PIM-DM [RFC3973] and PIM-SM [RFC7761]) are used to build data paths
   (i.e., multicast tree) from a source end-host to destination end-
   hosts for a group.  Therefore, data paths are controlled by the
   Internet layer.  Reliable transport protocols may be used in the
   control plane [RFC6559].  Over the data paths, stateless and non-
   reliable transport layer protocols such as UDP are usually used.  For
   TCP-friendly multicast, [RFC4654] defines a congestion control
   protocol for multicast.  To achieve reliable multicast, Multicast
   Transport Protocol (MTP) [RFC1301] defines a transport layer protocol
   with flow control, QoS and error recovery for multicast.  However, it
   has been pointed out that MTP has a potential scalability issue for
   flow handling [RFC1458].

   In these four communication models, IP routing provides end-to-end
   reachability and determines data paths.  Transport layer protocols
   are not basically aware of data paths, although operational
   considerations exist in anycast for uncertainty of data paths
   [RFC4786].  As pointed out in [RFC1458], data flow management
   requires more resources compared to data path functionality such as
   congestion control.  It implies that it is more significant in one-
   to-many communication in multicast cases than unicast cases discussed
   in Section 2.3.

2.5.  Multipath Protocols

   Multipath TCP [RFC8684] and SCTP utilize multiple data paths over
   multiple endpoint addresses.  As these multipath protocols are
   unaware of data paths, they distinguish the data paths by endpoint IP
   addresses.  Accordingly, multiple flows of these protocols may use an
   identical data path without recognizing it.

   Multipath protocols are also responsible for inverse multiplexing to
   split a data stream into multiple data paths.  This inverse
   multiplexing is independent of data paths except for congestion
   control.

2.6.  Reliable Data Communication

   Transport layer protocols may implement retransmission and reordering
   functions to recover lost or reordered datagrams for reliable data
   communication.  This functionality is independent of data paths, and
   consequently, should be implemented over data paths.  Instead,
   ``smart'' end-hosts or middleboxes may implement it.

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   An end-host may transmit a duplicate packet for improving reliability
   [RFC8655].  This functionality is also independent of data paths.
   However, a router in a data path may be capable of duplicating a
   packet because the duplication process does not require significant
   computing resources, unlike retransmission.

2.7.  Security

   The transport layer may implement an end-to-end security function,
   such as Transport Layer Security (TLS) [RFC8446] Datagram Transport
   Layer Security (DTLS) [RFC6347].  As TLS does not encrypt the
   underlying transport protocol header, middleboxes such as TCP
   offloading can still work.  However, some protocols implementing a
   transport layer protocol over DTLS, such as SCTP over DTLS used in
   WebRTC Data Channel [RFC8831], encrypt the transport layer header,
   and consequently, have difficulty cooperating with these middleboxes.

   In this way, transport layer's security functions have focused on
   end-to-end security.  However, middleboxes that terminate security
   function, such as [mcTLS], have been proposed.  Moreover, distributed
   computing paradigms such as in-network computing and MEC may also
   introduce middleboxes allowed to terminate security functions to
   deploy a computing function.  Therefore, security should be
   considered at three levels.  data path security, data flow security,
   and end-to-end security.  Note that data link security such as Wi-Fi
   Protected Access (WPA) is out of the scope of this document.

2.8.  Summary

   As described above, the transport layer functionality is summarized
   by distinguishing data paths and data flows as follows:

   The following functionality is categorized as data path functions.

   *  In-band trajectory monitoring

   *  Waypoint management

   *  Bidirectional connection establishment

   *  Data path quality (congestion) monitoring and congestion control

   *  Data flow multiplexing

   *  Packet duplication

   *  Data path security

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   The following functionality is categorized as data flow functions.

   *  Retransmission for reliable data communication

   *  Flow control for receive-buffer management

   *  Flow prioritization

   *  Data flow and end-to-end security

   *  Inverse multiplexing for multipath protocols

3.  Specifications of Data Path Layer and Data Flow Layer

   As summarized in Section 2.8, this document separates data paths and
   data flows from the transport layer.  Hence, this document divides
   the transport layer into two sublayers; the data path and the data
   flow layers.  This section specifies the functionality of these two
   sublayers.

3.1.  Data Path Layer

   The data path layer provides the functionality to make the transport
   layer aware of data paths upon the Internet layer.  The data path
   layer forwards packets without manipulating the payload.  It means
   that the data path layer does not provide fragmentation,
   segmentation, or reassembly.  Instead, the Internet layer or the data
   flow layer provide these functions.

   The data path layer MAY implement in-band trajectory monitoring,
   bidirectional connection establishment, data path quality monitoring
   and congestion control, data flow multiplexing, and packet
   duplication.  A data path layer protocol MAY implement other
   functions related to data paths.

3.1.1.  In-band trajectory monitoring

   The data path layer MAY provide an in-band trajectory monitoring
   function.  The in-band trajectory monitoring function enables an
   upper-layer protocol to detect path change events and a single point
   of failure for multipath protocols.

   DPH: Data path layer protocol header

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   +--------+     *--------------*     *--------------*     +--------+
   | Host A |-----| DPR1 (ID: 3) |-----| DPR2 (ID: 4) |-----| Host B |
   +--------+     *--------------*     *--------------*     +--------+
                     |                    |
                     |                    v
                     |           +-----+-+-+----...----+
                     |           | DPH |3|4| Payload   |
                     |           +-----+-+-+----...----+
                     v
              +-----+-+----...----+
              | DPH |3| Payload   |
              +-----+-+----...----+

      Figure 2: Appending data path router identifiers to distinguish
                               the data path

   One approach to implement this functionality is prepending,
   appending, or XOR-ing device identifiers like in-band network
   telemetry or in-situ OAM [I-D.ietf-ippm-ioam-data] by data path layer
   protocol's routers.  Figure 2 shows an example of in-band trajectory
   monitoring.  A router that handles a data path layer protocol appends
   the router identifier to the header.  In case the upper-layer
   protocol does not require the path information but path change
   events, the router can apply an XOR operation with a hashed
   identifier to a fixed-length field.  Other alternative approaches MAY
   be used.

   The latter approach separates the control plane and the data plane.
   The control plane manages the data path, and the data plane monitors
   if packets pass through the dedicated data path.  The data path
   control MAY leverage underlay protocols such as Segment Routing.

3.1.2.  Waypoint management

   The data path layer MAY support waypoint management functionality.
   The in-band trajectory monitoring functionality described above does
   not provide the functionality to designate the data path's waypoints.
   However, some middleboxes such as firewalls and in-network computing
   require to designate waypoints to activate the in-network functions.

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   A data path layer protocol MAY define a specification to control the
   waypoints of a data path.  There are two approaches to designate
   waypoints over the Internet layer; 1) setting a waypoint as the
   destination IP address and 2) using routing technologies such as
   source routing and policy-based routing.  The former approach
   includes Network Address Translation (NAT) [RFC3022] and proxy
   services.  The latter approach MAY leverage underlay protocols to
   designate waypoints, such as Segment Routing and IPv6 Type 2 Routing
   Header [RFC6275].  Other alternative approaches MAY be used in a data
   path layer protocol.

3.1.3.  Bidirectional connection establishment

   The data path layer SHOULD support both unidirectional and
   bidirectional paths.  A bidirectional path MAY be symmetric or
   asymmetric.  As the characteristics of unidirectional and
   bidirectional paths are different, some data path layer protocols MAY
   only support either unidirectional or bidirectional paths.

   A data path protocol supporting bidirectional data paths SHOULD
   implement a handshake mechanism to establish a bidirectional
   connection, such as a 3-way handshake of TCP and a 4-way handshake of
   SCTP.  It MAY provide a 0-RTT connection establishment feature such
   as TLS 1.3 [RFC8446] and TCP Fast Open [RFC7413].

3.1.4.  Data path quality (congestion) monitoring and congestion control

   The data path layer SHOULD implement congestion control.  However,
   Data path layer protocols supporting only unidirectional paths MAY
   not implement congestion control because it cannot implement
   congestion or data path quality reporting.

   Congestion control is performed based on data path quality or
   congestion reporting from the receiver end-host or intermediate nodes
   that process a data path layer protocol.  Data path quality or
   congestion signals MAY be packet losses, delay, or ECN, as used in
   many transport layer protocols.  Other signals or metrics, such as
   in-band network telemetry information, MAY be used.

3.1.5.  Data flow multiplexing

   The data path layer enables us to collectively handle different
   characteristics (e.g., service level requirements) of transport layer
   protocols such as stream and datagram protocols.

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3.1.6.  Packet duplication

   On a lossy data path, a data path layer protocol MAY implement packet
   duplication for improving reliability.  However, the data path layer
   SHOULD NOT provide retransmission because it requires significant
   resources.

3.1.7.  Data path security

   A data path layer protocol MAY implement data path security.  Data
   path security provides a security function between data path routers.
   As pointed out in Section 2.7, the data path layer assumes
   middleboxes that terminate a security function.  Therefore, data
   (payload) encryption provided by data path security MAY be terminated
   at a data path router.  In addition to encryption, other security
   functions, such as authentication, authorization, and accounting for
   a data path router, MAY be implemented as data path security.

3.2.  Data Flow Layer

   The data flow layer MAY implement various functions for end-to-end
   data communication over the data path layer.  Data flow layer
   protocols MAY be stream, datagram, or message-oriented protocols.
   Middleboxes, MEC nodes, or in-network computing nodes MAY process
   data flow layer protocols.  Therefore, a node processing data flow
   layer protocols SHOULD be as ``smart'' as end-hosts.

   A data flow layer protocol MAY implement a retransmission function
   for reliable data communication, a flow control mechanism for
   receive-buffer management, flow prioritization for effective network
   resource utilization, a security extension such as TLS and DTLS, and
   a multipath transport protocol using multiple bidirectional paths
   over the data path layer.

3.2.1.  Retransmission for reliable data communication

   A data flow layer protocol MAY provide retransmission for reliable
   data communication.  To this end, a node implementing the data flow
   layer protocol MUST equip a transmit buffer for retransmission to
   retransmit lost corrupted packets.  It MUST also equip a receive
   buffer to reassemble a stream and a message from a sequence of
   packets in a stream and message-oriented data flow layer protocols.
   The receive buffer also handles packet reordering and duplication.

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3.2.2.  Flow control for receive-buffer management

   Flow control is necessary for receive-buffer management.  Therefore,
   a data flow layer protocol MAY implement the flow control mechanism.
   A receiver node advertises the available receive-buffer size in the
   data flow layer, like TCP's window size, when implementing the flow
   control mechanism.  A sender node controls the transmission so that
   transmitted data do not exceed the receive-buffer size.

3.2.3.  Flow prioritization

   The prioritization of data flows multiplexed in a data path layer
   protocol MAY be implemented on the data flow layer using the data
   path layer information, such as monitored data path quality or
   congestion.  A data flow layer protocol MAY provide a service code
   point or priority so that nodes processing the data flow protocol
   discriminate a flow.

3.2.4.  Data flow and end-to-end security

   A data flow layer protocol MAY implement a data flow or end-to-end
   security function.  TLS and DTLS can be used for stream and datagram
   data flows, respectively.  In such case, data flow security is
   identical to end-to-end security.  If we consider a mechanism that
   terminates a security function for a data flow, such as [mcTLS], data
   flow security is handled by a middlebox.

3.2.5.  Inverse multiplexing for multipath protocols

   A multipath data flow protocol MAY leverage multiple bidirectional
   data paths established by data path layer protocols.  The data flow
   layer is responsible for the inverse multiplexing functionality.
   Therefore, the multipath data flow protocol divides a stream, a
   message, or a datagram into these multiple data paths.  Note that the
   data path layer performs congestion control for each data path, while
   the data flow layer performs flow control.

4.  Use Cases

   This document does not specify any data path protocols or data flow
   protocols, but the data path and data flow layers' architectural
   design.  For better understanding, this section describes the use
   cases of the data path and the data flow layers.  In the use cases, a
   data path router (DPR) and a data flow layer node (DFN) denote a
   router that processes a data path layer protocol and a node that
   processes a data flow layer protocol, respectively.

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4.1.  Multipath Transport Protocols

   Hosts A and B communicate with each other over multiple data paths;
   Path R1-DPR3-DPR2 and Path DPR4-DPR2.  R1 is an IP router.  DPR2,
   DPR3, and DPR4 are data path routers that treat data path layer
   protocols.

                     *----*     *------*
                  +--| R1 |-----| DPR3 |--+
      +--------+_/   *----*     *------*   \_*------*     +--------+
      | Host A |_                           _| DPR2 |-----| Host B |
      +--------+ \              *------*   / *------*     +--------+
                  +-------------| DPR4 |--+
                                *------*

            Figure 3: An example of multipath data communication

   Figure 3 shows an example of multipath data communication over two
   data paths.  The end-hosts A and B establish two bidirectional data
   paths; A-R1-DPR3-R2-B and A-DPR4-DPR2-B, and they use a data flow
   layer protocol over these data paths for end-to-end communication.
   The data path layer protocols monitor the data path quality and
   perform congestion control for each data path.  The data flow layer
   protocol performs flow control and inverse multiplexing into these
   data paths.

   Multipath transport protocols MAY leverage in-band trajectory
   monitoring to detect a shared waypoint.  We assume that the data path
   routers manipulate the header of a data path layer protocol.  They
   add the router identifier to the data path layer protocol's header to
   report the trajectory to the end-hosts.  When sending packets from
   end-host A to end-host B over these paths, each packet reports
   trajectory A-DPR3-DPR2-B or A-DPR4-DPR2-B.  In this way, end-host B
   recognizes two different data paths and detects a shared data path
   router, DPR2, on the multiple paths, potentially a single point of
   failure for end-to-end communication.

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4.2.  Congestion Control Acceleration

   Some TCP congestion control acceleration functions proxy TCP sessions
   to shorten the perspective latency.  The acceleration functions
   acknowledge receipt of packets.  Using a loss-based congestion
   control algorithm, both congestion control and flow control use the
   acknowledgments (ACKs).  For congestion control, missing ACKs report
   packet losses, and consequently, they present the data path quality
   to control the transmission rate to avoid congestion.  For flow
   control, ACKs report successful data delivery.  Then, the sender can
   release part of the transmit buffer.  Otherwise, the sender
   retransmits the lost data.

   +-------------------+                           +-------------------+
   | Application layer |<------------------------->| Application layer |
   +-------------------+                           +-------------------+
   | Data flow layer   |<------------------------->| Data flow layer   |
   +-------------------+   +-------------------+   +-------------------+
   | Data path layer   |<->| Data path layer   |<->| Data path layer   |
   +-------------------+   +-------------------+   +-------------------+
   | Internet layer    |<->| Internet layer    |<->| Internet layer    |
   +-------------------+   +-------------------+   +-------------------+
   | Link layer        |<->| Link layer        |<->| Link layer        |
   +-------------------+   +-------------------+   +-------------------+
     Sender                  Accelerator             Receiver

    Figure 4: An example of a communication model for congestion control
                                acceleration

   Figure 4 illustrates an example of a communication model for
   congestion control acceleration using the data path layer.  The
   accelerator or the receiver reports the data path quality to the
   sender on the data path layer to perform congestion control.  There
   are various ways to measure data path quality.  For example, data
   path routers MAY use the buffer capacity in the same way as ECN.  If
   a data path router can detect packet losses for a connection of data
   path layer protocols, the packet losses or statistics MAY be used to
   report the data path quality.  The data path quality report by the
   accelerator enables the fast response of congestion control
   algorithms.  The data flow layer is responsible for retransmission
   for reliable communication and flow control for receive-buffer
   management.

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4.3.  In-Network Computing

   In-network computing [I-D.kunze-coin-industrial-use-cases] is a novel
   distributed computing paradigm.  It requires to be aware of the data
   path's waypoints because computing components are placed in between
   end-hosts.  The message-oriented protocol MAY adopt the pub/sub
   communication model, widely used in machine-to-machine communication.

   The data path layer establishes a data path while designating the
   waypoints to perform in-network computing.  The data flow layer over
   the data path implements a message-oriented protocol or a stream
   protocol to feed data into in-network computing nodes.  The message-
   oriented protocol MAY adopt the pub/sub communication model, widely
   used in machine-to-machine communication.

   App., DF, DP, IP, and Link denote the application layer, the data
   flow layer, the data path layer, the IP, and the link layer,
   respectively.  H1 and H2 are end-hosts, C1 and C2 are in-network
   computing nodes, P1 is a data path router, and R1 is an IP router.

   +------+                                               +------+
   | App. |<--------------------------------------------->| App. |
   +------+              +------+              +------+   +------+
   | DF   |<------------>| DF   |<------------>| DF   |<->| DF   |
   +------+   +------+   +------+              +------+   +------+
   | DP   |<->| DP   |<->| DP   |<------------>| DP   |<->| DP   |
   +------+   +------+   +------+   +------+   +------+   +------+
   | IP   |<->| IP   |<->| IP   |<->| IP   |<->| IP   |<->| IP   |
   +------+   +------+   +------+   +------+   +------+   +------+
   | Link |<->| Link |<->| Link |<->| Link |<->| Link |<->| Link |
   +------+   +------+   +------+   +------+   +------+   +------+

    +----+     *----*     *----*     *----*     *----*     +----+
    | H1 |-----| P1 |-----| C1 |-----| R1 |-----| C2 |-----| H2 |
    +----+     *----*     *----*     *----*     *----*     +----+

    Figure 5: An example of communication model of in-network computing

   Figure 5 illustrates an example of the communication model of in-
   network computing.  A data path is established between H1 and H2.  A
   data path layer protocol designates C1 and C2 as the waypoints.  Over
   the established data path, H1 transmits messages to H2 using a data
   flow layer protocol.  C1 and C2 process the messages according to the
   programs running on C1 and C2.

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4.4.  Flow Arbitration

   The separation of the data path and the data flow layer enables flow-
   level prioritization.  As the data path layer is responsible for
   congestion control, the multiplexed data flows over a data path can
   collectively perform resource arbitration for the data path bandwidth
   capacity.

   For example, a data path multiplexes two data flows; one of them
   requires a constant data rate, and the other is tolerant of delayed
   data delivery.  Flow control can prioritize the former data flow and
   decrease the transmission rate when the data path layer detects
   congestion.  This flow arbitration is crucial in coexisting deadline-
   based and best-effort flows.

   H1, H2, H3, and H4 are end-hosts.  R1, R2, and R3 are IP routers.  P1
   is a data path router that support the data path quality monitoring
   function.

   +----+                           *----*     +----+
   | H1 |--+                     +--| R2 |-----| H3 |
   +----+   \_*----*     *----*_/   *----*     +----+
             _| R1 |-----| P1 |_
   +----+   / *----*     *----* \   *----*     +----+
   | H2 |--+                     +--| R3 |-----| H4 |
   +----+                           *----*     +----+

     Figure 6: An example of flow arbitration over multiple data paths

   Moreover, the data path layer MAY support data path quality
   monitoring for multiple data paths.  If data path routers monitor
   multiple data paths' bandwidth capacity, they can report the
   estimated capacity to arbitrate multiple flows over the data paths.
   Figure 6 shows an example flow arbitration over multiple data paths.
   Suppose data flows X and Y are over the data paths H1-R1-P1-R2-H3 and
   H2-R1-P1-R3-H4, respectively; the data path router P1 can monitor the
   data path quality such as packet losses for these data paths.  If the
   bandwidth utilization reaches the capacity, the data path router
   requests resource arbitration.  A flow MAY reduce the data rate using
   the flow control mechanism in the data flow layer if its priority is
   not high.  Thus, these two flows can autonomously perform the flow
   arbitration.  This flow arbitration MAY fail and perform best-effort
   communication, such in case both flows are the high priority and do
   not reduce the data rate.

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5.  Implementation Considerations

   Although this document does not specify the data path layer and the
   data flow layer protocols, it discusses the implementation of these
   protocols' specifications.  The data path layer MAY be implemented
   over UDP for interoperability with the current Internet operations
   because other protocols than ICMP, TCP, and UDP MAY be filtered on
   the Internet.  Data path layer protocols MUST implement the maximum
   transmission unit (MTU) discovery [RFC8899].

   LH: Link-layer protocol header, IPH: IP header, UDPH: UDP header,
   DPH: Data path layer protocol header, DFH: Data flow layer protocol
   header

   +----+-----+------+-----+-----+------------------+-----------+
   | LH | IPH | UDPH | DPH | DFH | Payload          | (Trailer) |
   +----+-----+------+-----+-----+------------------+-----------+

        Figure 7: An implementation of data path layer and data flow
                          layer protocols over UDP

   Figure 7 illustrates an example packet format of data path layer and
   data flow layer protocols over UDP.  In this figure, the UDP header
   is not intended to provide the transport layer's functionality but is
   introduced for interoperability with existing middleboxes on the
   Internet.

   A data path layer protocol MAY use hop-by-hop UDP connections to
   designate waypoints like an overlay network.  Otherwise, data path
   routers require to leverage routing technologies such as Segment
   Routing and policy-based routing to support waypoint management.

6.  IANA Considerations

   This document does not have IANA Considerations.

7.  Security Considerations

   End-to-end encryption becomes critical to Internet protocols for
   privacy and security.  The data path layer MUST be visible to
   intermediate nodes, such as routers and middleboxes, by design to
   process and manipulate a data path layer protocol.  Therefore, this
   document proposes to implement transport layer security at the data
   flow layer.  It exposes data path information.  Therefore, data path
   layer protocols MUST NOT include privacy-sensitive information in the
   protocol header.

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   Data path layer protocols may be invisible to intermediate nodes if
   an underlay protocol encrypts the payload.  For example, IPsec
   [RFC6071] encrypts the payload of IP datagrams.  In such cases,
   intermediate nodes cannot process or manipulate data path layer
   protocols.

8.  Acknowledgements

   We thank Kenichi Murata, Ryokichi Onishi, Yusuke Doi, and Masahiro
   Ishiyama for their comments and review of this document.

9.  References

9.1.  Normative References

   [RFC0768]  Postel, J., "User Datagram Protocol", STD 6, RFC 768,
              DOI 10.17487/RFC0768, August 1980,
              <https://www.rfc-editor.org/info/rfc768>.

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

   [RFC0919]  Mogul, J., "Broadcasting Internet Datagrams", STD 5,
              RFC 919, DOI 10.17487/RFC0919, October 1984,
              <https://www.rfc-editor.org/info/rfc919>.

   [RFC1112]  Deering, S., "Host extensions for IP multicasting", STD 5,
              RFC 1112, DOI 10.17487/RFC1112, August 1989,
              <https://www.rfc-editor.org/info/rfc1112>.

   [RFC1122]  Braden, R., Ed., "Requirements for Internet Hosts -
              Communication Layers", STD 3, RFC 1122,
              DOI 10.17487/RFC1122, October 1989,
              <https://www.rfc-editor.org/info/rfc1122>.

   [RFC1123]  Braden, R., Ed., "Requirements for Internet Hosts -
              Application and Support", STD 3, RFC 1123,
              DOI 10.17487/RFC1123, October 1989,
              <https://www.rfc-editor.org/info/rfc1123>.

   [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>.

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   [RFC2474]  Nichols, K., Blake, S., Baker, F., and D. Black,
              "Definition of the Differentiated Services Field (DS
              Field) in the IPv4 and IPv6 Headers", RFC 2474,
              DOI 10.17487/RFC2474, December 1998,
              <https://www.rfc-editor.org/info/rfc2474>.

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

   [RFC4340]  Kohler, E., Handley, M., and S. Floyd, "Datagram
              Congestion Control Protocol (DCCP)", RFC 4340,
              DOI 10.17487/RFC4340, March 2006,
              <https://www.rfc-editor.org/info/rfc4340>.

   [RFC4786]  Abley, J. and K. Lindqvist, "Operation of Anycast
              Services", BCP 126, RFC 4786, DOI 10.17487/RFC4786,
              December 2006, <https://www.rfc-editor.org/info/rfc4786>.

   [RFC4960]  Stewart, R., Ed., "Stream Control Transmission Protocol",
              RFC 4960, DOI 10.17487/RFC4960, September 2007,
              <https://www.rfc-editor.org/info/rfc4960>.

   [RFC6275]  Perkins, C., Ed., Johnson, D., and J. Arkko, "Mobility
              Support in IPv6", RFC 6275, DOI 10.17487/RFC6275, July
              2011, <https://www.rfc-editor.org/info/rfc6275>.

   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
              January 2012, <https://www.rfc-editor.org/info/rfc6347>.

   [RFC7761]  Fenner, B., Handley, M., Holbrook, H., Kouvelas, I.,
              Parekh, R., Zhang, Z., and L. Zheng, "Protocol Independent
              Multicast - Sparse Mode (PIM-SM): Protocol Specification
              (Revised)", STD 83, RFC 7761, DOI 10.17487/RFC7761, March
              2016, <https://www.rfc-editor.org/info/rfc7761>.

   [RFC8402]  Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
              Decraene, B., Litkowski, S., and R. Shakir, "Segment
              Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
              July 2018, <https://www.rfc-editor.org/info/rfc8402>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.

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   [RFC8655]  Finn, N., Thubert, P., Varga, B., and J. Farkas,
              "Deterministic Networking Architecture", RFC 8655,
              DOI 10.17487/RFC8655, October 2019,
              <https://www.rfc-editor.org/info/rfc8655>.

   [RFC8684]  Ford, A., Raiciu, C., Handley, M., Bonaventure, O., and C.
              Paasch, "TCP Extensions for Multipath Operation with
              Multiple Addresses", RFC 8684, DOI 10.17487/RFC8684, March
              2020, <https://www.rfc-editor.org/info/rfc8684>.

   [RFC8831]  Jesup, R., Loreto, S., and M. Tüxen, "WebRTC Data
              Channels", RFC 8831, DOI 10.17487/RFC8831, January 2021,
              <https://www.rfc-editor.org/info/rfc8831>.

   [RFC8899]  Fairhurst, G., Jones, T., Tüxen, M., Rüngeler, I., and T.
              Völker, "Packetization Layer Path MTU Discovery for
              Datagram Transports", RFC 8899, DOI 10.17487/RFC8899,
              September 2020, <https://www.rfc-editor.org/info/rfc8899>.

9.2.  Informative References

   [RFC1301]  Armstrong, S., Freier, A., and K. Marzullo, "Multicast
              Transport Protocol", RFC 1301, DOI 10.17487/RFC1301,
              February 1992, <https://www.rfc-editor.org/info/rfc1301>.

   [RFC1458]  Braudes, R. and S. Zabele, "Requirements for Multicast
              Protocols", RFC 1458, DOI 10.17487/RFC1458, May 1993,
              <https://www.rfc-editor.org/info/rfc1458>.

   [RFC2768]  Aiken, B., Strassner, J., Carpenter, B., Foster, I.,
              Lynch, C., Mambretti, J., Moore, R., and B. Teitelbaum,
              "Network Policy and Services: A Report of a Workshop on
              Middleware", RFC 2768, DOI 10.17487/RFC2768, February
              2000, <https://www.rfc-editor.org/info/rfc2768>.

   [RFC3022]  Srisuresh, P. and K. Egevang, "Traditional IP Network
              Address Translator (Traditional NAT)", RFC 3022,
              DOI 10.17487/RFC3022, January 2001,
              <https://www.rfc-editor.org/info/rfc3022>.

   [RFC3973]  Adams, A., Nicholas, J., and W. Siadak, "Protocol
              Independent Multicast - Dense Mode (PIM-DM): Protocol
              Specification (Revised)", RFC 3973, DOI 10.17487/RFC3973,
              January 2005, <https://www.rfc-editor.org/info/rfc3973>.

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   [RFC4654]  Widmer, J. and M. Handley, "TCP-Friendly Multicast
              Congestion Control (TFMCC): Protocol Specification",
              RFC 4654, DOI 10.17487/RFC4654, August 2006,
              <https://www.rfc-editor.org/info/rfc4654>.

   [RFC6071]  Frankel, S. and S. Krishnan, "IP Security (IPsec) and
              Internet Key Exchange (IKE) Document Roadmap", RFC 6071,
              DOI 10.17487/RFC6071, February 2011,
              <https://www.rfc-editor.org/info/rfc6071>.

   [RFC6559]  Farinacci, D., Wijnands, IJ., Venaas, S., and M.
              Napierala, "A Reliable Transport Mechanism for PIM",
              RFC 6559, DOI 10.17487/RFC6559, March 2012,
              <https://www.rfc-editor.org/info/rfc6559>.

   [RFC7094]  McPherson, D., Oran, D., Thaler, D., and E. Osterweil,
              "Architectural Considerations of IP Anycast", RFC 7094,
              DOI 10.17487/RFC7094, January 2014,
              <https://www.rfc-editor.org/info/rfc7094>.

   [RFC7413]  Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP
              Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014,
              <https://www.rfc-editor.org/info/rfc7413>.

   [I-D.ietf-ippm-ioam-data]
              Brockners, F., Bhandari, S., and T. Mizrahi, "Data Fields
              for In-situ OAM", Work in Progress, Internet-Draft, draft-
              ietf-ippm-ioam-data-11, 22 November 2020,
              <http://www.ietf.org/internet-drafts/draft-ietf-ippm-ioam-
              data-11.txt>.

   [I-D.kunze-coin-industrial-use-cases]
              Kunze, I., Wehrle, K., and D. Trossen, "Use Cases for In-
              Network Computing", Work in Progress, Internet-Draft,
              draft-kunze-coin-industrial-use-cases-04, 2 November 2020,
              <http://www.ietf.org/internet-drafts/draft-kunze-coin-
              industrial-use-cases-04.txt>.

   [mcTLS]    Naylor, D., Schomp, K., Varvello, M., Leontiadis, I.,
              Blackburn, J., López, D., Papagiannaki, K., Rodriguez, P.,
              and P. Steenkiste, "Multi-Context TLS (mcTLS) Enabling
              Secure In-Network Functionality in TLS",  SIGCOMM Comput.
              Commun. Rev., vol. 45, no. 4, pp. 199-212, 2015.

Author's Address

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   Hirochika Asai
   Preferred Networks, Inc. / WIDE Project
   1-6-1 Otemachi, Chiyoda, Tokyo
   100-0004
   Japan

   Email: panda@wide.ad.jp

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