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Transport Protocol Issues of In-Network Computing Systems
draft-kunze-coinrg-transport-issues-03

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This is an older version of an Internet-Draft whose latest revision state is "Expired".
Authors Ike Kunze , Klaus Wehrle , Dirk Trossen
Last updated 2020-11-02
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draft-kunze-coinrg-transport-issues-03
COINRG                                                          I. Kunze
Internet-Draft                                                 K. Wehrle
Intended status: Informational                    RWTH Aachen University
Expires: 6 May 2021                                           D. Trossen
                                    Huawei Technologies Duesseldorf GmbH
                                                         2 November 2020

       Transport Protocol Issues of In-Network Computing Systems
                 draft-kunze-coinrg-transport-issues-03

Abstract

   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 raises several questions
   regarding the use of transport protocols in connection with COIN.

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).  Note that other groups may also distribute
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   Drafts is at https://datatracker.ietf.org/drafts/current/.

   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."

   This Internet-Draft will expire on 6 May 2021.

Copyright Notice

   Copyright (c) 2020 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
   Provisions Relating to IETF Documents (https://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
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Addressing  . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Flow granularity  . . . . . . . . . . . . . . . . . . . . . .   4
   5.  Collective Communication  . . . . . . . . . . . . . . . . . .   5
   6.  Authentication  . . . . . . . . . . . . . . . . . . . . . . .   5
   7.  Security  . . . . . . . . . . . . . . . . . . . . . . . . . .   6
   8.  Transport Features  . . . . . . . . . . . . . . . . . . . . .   6
     8.1.  Reliability . . . . . . . . . . . . . . . . . . . . . . .   7
     8.2.  Flow/Congestion Control . . . . . . . . . . . . . . . . .   9
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   11. Conclusion  . . . . . . . . . . . . . . . . . . . . . . . . .  11
   12. Informative References  . . . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

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 be located on end-hosts at the edges of the
   network.  This choice is reflected in the end-to-end principle which
   states that end-hosts directly address each other and perform all
   relevant computations while the network only delivers 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, computing in
   the network (COIN) encourages explicit computations in the network
   which introduces an intertwined complexity as the computations on the
   end-hosts depend on the functionality deployed in the network.  It
   further challenges traditional end-to-end transport protocols as they
   are not designed to address in-network computation entities or to
   include more than two devices into a communication.  Some of these
   problems are already presented in [I-D.draft-kutscher-coinrg-dir-02].

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   This draft discusses 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 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.  Terminology

   COIN element: Device on which COIN functionality can be deployed

3.  Addressing

   The traditional addressing concept of the Internet is that end-hosts
   directly address each other with all computational intelligence
   residing at the network edges.  With COIN, computations move into the
   network and need to be integrated into the established
   infrastructure.  In systems where the whole network is under the
   control of the network operator this integration can be implemented
   by explicitly adjusting the communication schemes based on the COIN
   functionality.  Considering larger scales, this approach of manually
   adjusting traffic patterns and applications to correctly incorporate
   changes made by the network is not feasible.

   What is needed are ways to specify which kind of functionality should
   be applied to the transmitted data on the path inside the network and
   maybe even where or by whom the execution should take place.  Such
   functionality could for example be implemented using an indirection
   mechanism which routes a packet along a pre-defined or dynamically
   chosen path which then realizes the desired functionality.

   One possibility is to directly route on service or functionality
   identifiers instead of sending individual packets between locator-
   addressed network elements
   [I-D.draft-sarathchandra-coin-appcentres-03].  While this aligns the
   routing more clearly with the communication between computational
   elements, selecting the 'right' computational endpoint (out of
   possibly several ones) becomes critical to the proper functioning of
   the overall service.

   Further related concepts are Segment and Source Routing as well as
   (Service/Network) Function Chaining/Composition.

   Main challenges/questions are:

   1.  How should end-hosts address the COIN functionality?

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   2.  How can the treatment of the transmitted data, i.e., which COIN
       functionality to execute, be represented in the addressing of the
       request?

   3.  How can end-hosts direct computational requests to different
       computational endpoints of the same service in different network
       locations, i.e., decide where the COIN functionality is executed?

   4.  How to decide which computational endpoint to choose (from
       possibly several ones existing in the network)?

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

4.  Flow granularity

   Core networking hardware pipelines such as backbone switches are
   built to process incoming packets on a per-packet basis, keeping
   little to no state between them.  This is appropriate for the general
   task of forwarding packets, but might not be sufficient for COIN as
   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.  Summarizing, 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 is treated individually.  This respects the
       possibilities that are already offered by all networking
       equipment.

   2.  Every packet is 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.

   3.  Every packet is treated as part of a byte stream.  Here, all
       previous packets and potentially even all subsequent 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.

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   Along those options above, the question arises how shorter-term
   'messages' (or transactions) of the computation should be handled
   compared to the often longer-term management of the network resources
   needed to transmit the packets across one or more such messages.  For
   instance, error control may be best applied to the individual
   messages between computational endpoints, while congestion control
   may be applied across several messages at the level of the relation
   between the network elements hosting the computational endpoints.  In
   this view, the notion of a 'flow' may separate message or transaction
   handling from the resource management aspect, where a flow may be
   divided into sub-flows (said messages or transactions) with error
   control being applied to those sub-flows but resource management
   being applied to the overall flow.  Such choice of flow granularity
   would consequently have a significant impact on how and where
   computations can be performed as well as ensuring that end-hosts know
   who has altered the data and how.

5.  Collective Communication

   COIN scenarios may exhibit a collective communication semantic, i.e.,
   a communication between one and more computational endpoints, as is
   for example illustrated by use cases in
   [I-D.draft-sarathchandra-coin-appcentres-03].  With this, unicast and
   multicast transmissions become almost equal forms of communication,
   as also observed in work on information-centric networking (ICN).

   Yet, these many-point relations may be ephemeral down to the
   granularity of individual service requests between computational
   endpoints which questions the viability of stateful routing and
   transport approaches used for long-lived multicast scenarios such as
   liveTV transmissions.

   This is particularly pertinent for the transport layer where
   reliability and flow control among a quickly changing set of
   receivers is a challenging problem.  The ability to divide receiver
   groups with the support of in-network COIN elements may provide
   solutions that will cater to the possible dynamics of collective
   communication among computational endpoints.

6.  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.  This 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 potentially cause harm
   to the end-hosts.  What is needed are mechanisms with which the

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   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 3) and
   subsequently authenticated.  The Proof of Transit
   [I-D.draft-ietf-sfc-proof-of-transit-08] concept of the SFC WG could
   be applicable for proving that a packet has indeed passed the desired
   COIN elements, although it does not provide means to validate which
   changes were made by the known nodes.

   Main challenges/questions are:

   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 are changes made by the COIN elements authenticated?

7.  Security

   Many early COIN concepts require an unencrypted transmission of data.
   At the same time, there is a general tendency towards more and more
   security features in communication protocols.  QUIC, e.g., encrypts
   all payload data and almost all header content already inside the
   transport layer.  This makes current COIN concepts infeasible in
   settings where QUIC connections are used as the COIN elements do not
   have access to any packet content.  Using COIN thus also depends on
   how well security mechanisms like encryption can be integrated into
   COIN frameworks.

8.  Transport Features

   Depending on application needs, different transport protocols provide
   different features.  These features shape the behavior of the
   protocol and have to be taken into account when developing COIN
   functionality.  In this section, we focus on the impact of
   reliability as well as flow and congestion control to create
   awareness for the multifaceted interaction between the transport
   protocols and COIN elements.

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8.1.  Reliability

   Applications require a reliable transport whenever it is important
   that all data is transmitted successfully.  TCP[RFC0793] provides
   such a reliable communication as it first sets up a dedicated
   connection and then ensures the successful reception of all data.  In
   contrast, UDP[RFC0768] is a connectionless protocol without
   guarantees and COIN elements working on UDP transmissions must be
   robust to lost information.  This is not the case for applications on
   top of TCP, but the retransmissions and the TCP state, which TCP uses
   to achieve the reliability, make packet processing for COIN more
   complex due to at least three reasons.

   The concept of retransmissions bases on the end-to-end principle as
   retransmissions are performed by the sender if it has determined that
   the receiver did not receive the corresponding original message.
   Both participants can then act knowing that parts of the overall data
   are still missing.  For simple COIN elements, which are not aware of
   the involved TCP states and which do not track sequence numbers, it
   is difficult to identify (a) that a packet in the sequence is missing
   and (b) that a packet is a retransmission.  One question is whether
   COIN elements should incorporate an understanding for retransmissions
   on the basis of existing transport mechanisms or if a COIN-capable
   transport should include dedicated signals for the COIN elements.

   Apart from challenges in identifying retransmissions, there is also
   the fact that they are sent out of order with the original packet
   sequence.  Depending on the chosen flow granularity (see Section 4),
   COIN elements might have to hold contextual information for a
   prolonged time once they identify an impeding retransmission.
   Moreover, they might have to postpone or cancel computations if data
   is missing and instead schedule later computations.  The main
   question arising from this is: to what extent should COIN elements be
   capable of incorporating retransmissions into their computation
   schemes and how much additional storage capabilities are required for
   this?

   When incorporating COIN elements into the retransmission mechanisms,
   it is also an interesting question whether it should be possible to
   request or perform retransmissions from COIN elements.  Considering a
   setting with COIN elements that are capable of detecting missing
   packets and retransmission requests, it might improve the overall
   performance if the COIN element directly requests or performs the
   retransmission instead of forwarding the packet/request through the
   complete sequence of elements.  This is especially interesting in the
   context of collective communication where reliability mechanisms
   could make use of the multi-source nature of the communication and
   leverage the presence of many COIN elements in the network, for

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   instance by using network coding techniques, which in turn may
   benefit from COIN elements participating in the reliability
   mechanism.  In all cases, the aforementioned storage capabilities are
   relevant so that the COIN elements can store enough information.  The
   general question, i.e., which nodes in the sequence should do the
   retransmission, has already been worked on in the context of
   multicast transport protocols.

   Depending on the extent of realization of the presented
   retransmission features, COIN elements might almost have to implement
   some of TCP's state to fulfil their tasks.  Considering that
   different COIN elements have different computational and storage
   capacities, 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 will
   hence certainly affect the types of transport protocols that can be
   operated on the COIN networks.

   Another aspect to consider is the 'unit' that needs to be reliably
   transferred.  In stream-based transport protocols, such as TCP,
   packets represent the smallest unit of transfer.  However, different
   choices in the flow granularity and a possible move to larger-than-
   a-packet messages or transactions, as suggested in Section 4, might
   make other approaches to reliability viable that operate on the basis
   of such messages.

   Challenges/questions regarding reliability are:

   1.   What is the unit of reliable transfer?

   2.   How to utilize more than one computational endpoint in the
        reliability mechanism?

   3.   Should COIN elements be aware of retransmissions?

   4.   How can COIN elements identify missing packets or
        retransmissions?

   5.   Should COIN elements be explicitly notified about
        retransmissions?

   6.   To what extent should COIN elements be capable of incorporating
        retransmissions into their computation schemes?

   7.   How much storage capabilities are required for incorporating
        retransmissions?

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   8.   How can COIN elements incorporate missing packets into their
        computations?

   9.   How to deal with state changes in COIN elements caused by data
        lost later in the communication chain and then retransmitted?

   10.  Should COIN elements be capable of requesting retransmissions/
        answering retransmission requests?

   11.  Which devices should perform retransmissions?

   12.  Do COIN elements have to keep transport state?

   13.  How much transport state do COIN elements have to keep?

8.2.  Flow/Congestion Control

   TCP incorporates mechanisms to avoid overloading the receiving host
   (flow control) and the network (congestion control) and determines
   its sending rate as the minimum value of what both mechanisms
   determine as feasible for the system.  This approach is based on the
   notion that computing and forwarding hosts are separated and is
   challenged by the inclusion of COIN elements, i.e., computing nodes
   in the network.

   Flow control bases on explicit end-host information as the
   participating end-hosts notify each other about how much data they
   are capable of processing and consequently do not transmit more data
   as the other host can handle.  This only changes if one of the end-
   hosts updates its flow control information.

   Congestion control, on the other hand, interprets volatile feedback
   from the network to guess a sending rate that is possible given the
   current network conditions.  Most congestion control algorithms
   hereby follow a cyclical procedure where the sending end-hosts
   constantly increase their sending rate until they detect network
   congestion.  They then decrease their sending rate once and start to
   increase it again.

   In this traditional two-fold approach, loss, delay, or any other
   congestion signal (depending on the congestion control algorithm)
   induced by COIN elements (only in case that they are the bottleneck)
   is interpreted as network congestion and thus accounted for in the
   congestion control mechanism.  This means that the sending end-host
   may repeatedly overload the computational capabilities of the COIN
   elements when probing for the current network conditions instead of
   respecting general device capabilities as is done by flow control.

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   In the context of COIN, the granularity of flows may see a division
   into sub-flows or messages to better represent the used computational
   semantic as discussed in Section 4.  This raises the question whether
   flow and congestion control should be applied to longer term flows
   (of many sub-flows or messages) or directly to sub-flows.
   Eventually, this could possibly lead to a separation of error control
   (for sub-flows) and flow control (for longer-term flows).  A
   subsequent challenge is then how to reconcile the possible volatile
   nature of sub-flow relations (between computational endpoints) with
   the longer-term relationship between network endpoints that will see
   a flow of messages between them.  This is particularly pertinent in
   collective communication scenarios, where many forward unicast sub-
   flows may lead to a single multicast sub-flow response albeit only
   for that one response message.  Reconciling the various unicast
   resource regimes into a single (ephemeral) multicast one poses a
   significant challenge.

   Consequently, the question arises whether COIN elements should be
   able to participate in end-to-end flow control.

   Main challenges/questions are:

   1.  Should COIN elements be covered by congestion control?

   2.  Should COIN elements be able to participate in end-to-end flow
       control?

   3.  How could a resource constraint scheme similar to flow control be
       realized for COIN elements?

   4.  How to reconcile message-level flexibility in transport relations
       between computational endpoints with longer-term resource
       stability between network elements participating in the
       computational scenario?

9.  Security Considerations

   COIN changes the traditional paradigm of a simple network and the
   corresponding end-to-end principle as it encourages computations in/
   by the network.  Approaches designed to protect transmitted data,
   such as Transport Layer Security (TLS) which is even embedded into
   newer transport protocols like QUIC, rely on the end-to-end principle
   and are thus conceptually not compatible with COIN.  Additionally,
   COIN elements often do not support required cryptographic
   functionality.  Thus, there exist no out-of-the-box security
   solutions for COIN which means new security concepts have to be
   developed to allow for a secure use of COIN.

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10.  IANA Considerations

   N/A

11.  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.

   All in all, there is a wide range of transport features which offer
   improved performance in certain settings and for certain application
   combinations.  However, as presented, it is unlikely that all of the
   features/types of transport protocols can be supported on every COIN
   element.  It might make sense to define different classes of COIN-
   ready transport protocols which can then be deployed depending on the
   concretely available networking/hardware elements.  Alternatively,
   each of the features could be treated separately and they could then
   be composed based on the demands of an application at-hand.

   The general question summarizing this document is:

   Which transport features should be supported by COIN, how can they be
   identified, and how can they be provided to application designers?

12.  Informative References

   [I-D.draft-ietf-sfc-proof-of-transit-08]
              Brockners, F., Bhandari, S., Mizrahi, T., Dara, S., and S.
              Youell, "Proof of Transit", Work in Progress, Internet-
              Draft, draft-ietf-sfc-proof-of-transit-08, 1 November
              2020, <http://www.ietf.org/internet-drafts/draft-ietf-sfc-
              proof-of-transit-08.txt>.

   [I-D.draft-kutscher-coinrg-dir-02]
              Kutscher, D., Karkkainen, T., and J. Ott, "Directions for
              Computing in the Network", Work in Progress, Internet-
              Draft, draft-kutscher-coinrg-dir-02, 31 July 2020,
              <http://www.ietf.org/internet-drafts/draft-kutscher-
              coinrg-dir-02.txt>.

   [I-D.draft-sarathchandra-coin-appcentres-03]
              Trossen, D., Sarathchandra, C., and M. Boniface, "In-
              Network Computing for App-Centric Micro-Services", Work in
              Progress, Internet-Draft, draft-sarathchandra-coin-

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              appcentres-03, 23 October 2020, <http://www.ietf.org/
              internet-drafts/draft-sarathchandra-coin-appcentres-
              03.txt>.

   [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>.

Authors' Addresses

   Ike Kunze
   RWTH Aachen University
   Ahornstr. 55
   D-52074 Aachen
   Germany

   Email: kunze@comsys.rwth-aachen.de

   Klaus Wehrle
   RWTH Aachen University
   Ahornstr. 55
   D-52074 Aachen
   Germany

   Email: wehrle@comsys.rwth-aachen.de

   Dirk Trossen
   Huawei Technologies Duesseldorf GmbH
   Riesstr. 25C
   D-80992 Munich
   Germany

   Email: Dirk.Trossen@Huawei.com

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