COINRG                                                          I. Kunze
Internet-Draft                                                 K. Wehrle
Intended status: Informational                    RWTH Aachen University
Expires: May 7, 2020                                   November 04, 2019

       Transport Protocol Issues of In-Network Computing Systems


   Today's transport protocols offer a variety of functionality based on
   the notion that the network is to be treated as an unreliable
   communication medium.  Some, like TCP, establish a reliable
   connection on top of the unreliable network while others, like UDP,
   simply transmit datagrams without a connection and without guarantees
   into the network.  These fundamental differences in functionality
   have a significant impact on how COIN approaches can be designed and
   implemented.  Furthermore, traditional transport protocols are not
   designed for the multi-party communication principles that underlie
   many COIN approaches.  This document discusses selected
   characteristics of transport protocols which have to be adapted to
   support COIN functionality.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Addressing  . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Flow granularity  . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Authentication  . . . . . . . . . . . . . . . . . . . . . . .   5
   5.  Security  . . . . . . . . . . . . . . . . . . . . . . . . . .   5
   6.  Advanced Transport Features . . . . . . . . . . . . . . . . .   6
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   9.  Conclusion  . . . . . . . . . . . . . . . . . . . . . . . . .   7
   10. Informative References  . . . . . . . . . . . . . . . . . . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   A fundamental design choice of the Internet is that the network
   should be kept as simple as possible while complexity in the form of
   processing should ideally be located at the edges of the network,
   i.e., on end-hosts.  This choice is reflected in the widely known
   end-to-end principle which states that end-hosts directly address
   each other and perform all relevant computations while the only
   purpose of the network is to deliver the packets without modifying
   them.  Transport protocols are consequently designed to facilitate
   the direct communication between end-hosts.

   In practice, the end-to-end principle is often violated by
   intransparent middleboxes which alter transmitted packets, e.g., by
   dropping or changing header fields.  Contrary to that, COIN
   encourages explicit computations in the network which introduces an
   intertwined complexity as the concrete computations on the end-hosts
   critically depend on the functionality that is deployed in the
   network.  On another note, it challenges traditional end-to-end
   transport protocols as they lack means for addressing in-network
   computation entities and as they are generally not designed to
   include more than two devices into a communication.  Some of these
   problems are already presented in [I-D.draft-kutscher-coinrg-dir-00].
   This draft intends to discuss potential problems for traditional
   transport protocols in more detail to raise questions that the COIN
   community needs to solve before a widespread application of COIN

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   functionality is sensible.  Collaboration with other IRTF and IETF
   groups, such as PANRG, the IETF transport area in general, or the
   LOOPS BOF, can help in finding suitable solutions.

2.  Addressing

   The traditional addressing concept of the Internet are end-hosts
   directly addressing each other, with all computational intelligence
   residing at the network edges.  As other COIN drafts, such as
   [I-D.draft-he-coin-managed-networks-01] and
   [I-D.draft-kunze-coin-industrial-use-cases-00], and extensive
   publications, such as [DANG], [RUETH] and [SAPIO], in the last years
   have shown, performing some computations within the network offers
   the prospect of improved application performance.  In systems like
   data centers, which do not rely on the Internet and where the whole
   network is under the control of the network operator, integrating
   such functionality can be done by explicitly adjusting the
   communication schemes within these fully controlled systems based on
   the functionality that is executed in the network.

   With a widespread application of COIN and a consequent increase in
   computational capacities in the network, however, it might also
   become viable to deploy functionality in the 'wild' Internet so that
   everyday applications might also benefit from these computations.  At
   this point, it is no longer feasible to manually adjust the traffic
   patterns or the applications to correctly incorporate changes made by
   the network.

   It might thus make sense to make it possible to specify which kind of
   functionality should be applied to the transmitted data on the path
   inside the network, maybe even where or by whom exactly the execution
   should take place.  Such functionality could for example be
   implemented using an indirection mechanisms which routes a packet
   along a pre-defined or dynamically chosen path which then realizes
   the desired functionality.  Related concepts which might be of use
   are Segment and Source Routing as well as (Service/Network) Function

   The main challenges/questions at this point are:

   1.  How should end-hosts address the functionality within the

   2.  How exactly can the end-hosts influence where or by whom the
       functionality is executed?

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   3.  How can devices which do not implement COIN functionality be
       integrated into the systems without breaking the COIN or legacy

   Assuming that there is a suitable addressing scheme which allows to
   define which kinds of functionality should be applied to the
   transmitted data, a next question that arises is how the transmitted
   data is to be treated by the devices implementing the functionality.

3.  Flow granularity

   Core networking hardware pipelines such as backbone switches serving
   several tens of GBit/s are built to process incoming packets on a
   per-packet basis, keeping little to no state between them.  While
   this is appropriate for the general task of forwarding packets, it
   might not be sufficient for performing computations as related
   information that is needed for the computations can be spread across
   several packets.  In a TCP stream, for example, data is dynamically
   distributed across different segments which means that the data
   needed for application-level computations might also be split up.  In
   contrast to that the content of UDP datagrams is defined by the
   application itself which is why the datagrams could either be self-
   contained or information can be cleverly distributed onto different
   datagrams.  The common scheme is that different transport protocols
   induce different meanings to the packets that they send out which
   needs to be accounted for in COIN elements as they have to know how
   the received data is to be interpreted.  There are at least three
   options for this.

   1.  Every packet should be treated individually.  This, above all,
       perfectly meets the possibilities that are already offered by all
       networking equipment.

   2.  Every packet should be treated as part of a message.  In this
       setting, the packet alone does not have enough information for
       the computations.  Instead, it is important to know the content
       of the surrounding packets which together form the overall
       message and might hence also be relevant for the computations.

   3.  Every packet should be treated as part of a byte stream.  Here,
       all previous packets and potentially even all following packets
       need to be taken into consideration for the computations as the
       current packet could, e.g., be the first of a group of packets, a
       packet in the middle or the final packet of a sequence of

   The flow granularity consequently has a significant impact on how
   computations can be performed and where.  Apart from how the COIN

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   elements should treat the transmitted data, another important aspect
   is how it can be ensured that the end-hosts know who has altered the
   data and how.

4.  Authentication

   The realisation of COIN legitimizes and actively promotes that data
   transmitted from one host to another can be altered on the way inside
   the network.  While this can be beneficial if implemented correctly,
   it also opens the door for foul play as all intermediate network
   elements - no matter if they are malicious or misbehaving by
   accident, COIN elements, or 'traditional' middleboxes - could simply
   start altering parts of the original data and thus potentially cause
   harm to the end-hosts.  What is consequently needed is a mechanism
   with which the receiving host can verify (a) how and (b) by whom the
   data has been altered on the way.  In fact, these might very well be
   two distinct mechanisms as one (a) only focusses on the changes that
   are made to the data while (b) requires a scheme with which COIN
   elements can be uniquely identified (could very well relate to
   Section 2) and subsequently authenticated.

   The challenges at this point are thus the following:

   1.  How are changes to the data within the network communicated to
       the end-hosts?

   2.  How are the COIN elements that are responsible for the changes
       communicated to the end-hosts?

   3.  How is it verified that indeed the proclaimed COIN elements have
       performed the changes and not some impostor?

5.  Security

   Today, most, if not all, COIN concepts base on the fact that the data
   is transmitted in plain text as this makes working on the data easy.
   This is in contrast to a general development which sees more and more
   security features added to communication protocols, nicely
   highlighted by QUIC where the all payload data and almost all header
   content (except for the spin bit) is already encrypted inside the
   transport layer.  This, in turn, makes COIN concepts infeasible in
   settings where QUIC connections are used as the COIN elements do not
   have access to any packet content.  As waiving security features is
   generally not acceptable, the widespread success of COIN might very
   well also depend on how well security mechanisms like encryption can
   be integrated into COIN frameworks.

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   Together, the four aspects presented in Section 2 to Section 5 form a
   set of fundamental properties that should be taken into account for a
   basic transport-compatible realization of COIN.  What is not yet
   considered is the fact that different transport protocols typically
   differ in functionality and thus have a significantly different
   behavior.  In the following, we briefly discuss select additional
   transport features to create awareness for the multifaceted
   interaction between the transport protocols and COIN elements.

6.  Advanced Transport Features

   There is a variety of transport features which are only supported in
   some concrete transport implementations.  Still, they have a
   significant impact on the behavior and performance of the protocols.
   One aspect is that some protocols offer reliability while others do
   not.  This is for example visible when comparing UDP, a
   connectionless protocol without guarantees, to TCP which first sets
   up a dedicated connection and then ensures the successful reception
   of all data.  When facing UDP transmissions, COIN elements
   potentially have to cope with lost information while with TCP it is
   fairly save to say that the packets will reach them at some point.

   This, however, also makes it more difficult for COIN elements as TCP
   retransmissions, which are issued once a packet has been detected as
   lost, are sent drastically out of order with the original packet

   Thinking one step further, retransmissions are sent from the original
   sender of the packet.  In a communication setting with COIN elements
   in between, resending the packet through the complete sequence of
   elements might not be necessary if, e.g., a packet is lost at the
   last stage of a sequence of COIN elements.  Here, it might be enough
   if this last element resends the packet, although this in turn means
   that there have to be storage capabilities on the devices enabling
   them to store a certain number of packets and have them ready for
   retransmission.  The general question, i.e., which of the nodes in
   the sequence should actually do the retransmission, has already been
   worked on in the context of multicast transport protocols.

   Now focussing on the aspect of storage capabilities, it can be said
   that different COIN devices have different computational and storage
   capacities which can become a challenge if they have to hold some
   packets (potentially for retransmission) or if they are supposed to
   embed into TCP for which they might be forced to hold some form of
   TCP's state.  Consequently, it is very likely that not every form of
   transport integration into COIN can be supported by every available
   COIN platform.  The choice of devices included into the communication

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   will hence certainly affect the types of transport protocols that can
   be operated on the COIN networks.

   Another aspect is flow and congestion control to avoid overloading
   the receiving end-host and the network; it is included in TCP, but
   not in UDP, but has an impact on how the end-hosts can send their

   All in all, there is a wide range of non-essential transport features
   which nonetheless offer improved performance in certain settings and
   for certain application combinations.  However, as presented, it is
   likely that not all of the features/types of transport protocols can
   be supported on every COIN element.  A potential approach might be to
   define different classes of COIN-ready transport protocols which can
   then be deployed depending on the concretely available networking/
   hardware elements.

7.  Security Considerations


8.  IANA Considerations


9.  Conclusion

   The advent of COIN comes with many new use cases and promises
   improved solutions for various problems.  It is, however, not
   directly compatible with the end-to-end nature of transport
   protocols.  To enable a transport-based communication, it is thus
   important to answer key questions regarding COIN and transport
   protocols, some of which are raised in this document.

10.  Informative References

   [DANG]     Dang, HT., "NetPaxos: Consensus at Network Speed", DOI:
              10.1145/2774993.2774999, in SOSR, 2015.

              He, J., Li, A., and M. Montpetit, "In-Network Computing
              for Managed Networks: Use Cases and Research Challenges",
              draft-he-coin-managed-networks-01 (work in progress), July

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              Kunze, I., Rueth, J., and K. Wehrle, "Industrial Use Cases
              for In-Network Computing", draft-kunze-coin-industrial-
              use-cases-00 (work in progress), July 2019.

              Kutscher, D., Karkkainen, T., and J. Ott, "Directions for
              Computing in the Network", draft-kutscher-coinrg-dir-00
              (work in progress), July 2019.

              Montpetit, M., "In Network Computing Enablers for Extended
              Reality", draft-montpetit-coin-xr-03 (work in progress),
              July 2019.

   [RUETH]    Rueth, J., "Towards In-Network Industrial Feedback
              Control", DOI: 10.1145/3229591.3229592, in ACM SIGCOMM
              NetCompute, August 2018.

   [SAPIO]    Sapio, A., "Scaling Distributed Machine Learning with In-
              Network Aggregation", 2019,

Authors' Addresses

   Ike Kunze
   RWTH Aachen University
   Ahornstr. 55
   Aachen  D-50274

   Phone: +49-241-80-21422

   Klaus Wehrle
   RWTH Aachen University
   Ahornstr. 55
   Aachen  D-50274

   Phone: +49-241-80-21401

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