COINRG I. Kunze
Internet-Draft K. Wehrle
Intended status: Informational RWTH Aachen University
Expires: March 12, 2021 September 08, 2020
Transport Protocol Issues of In-Network Computing Systems
draft-kunze-coinrg-transport-issues-02
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 discusses selected
characteristics of transport protocols which have to be adapted to
support COIN functionality.
Status of This Memo
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This Internet-Draft will expire on March 12, 2021.
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document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Addressing . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Flow granularity . . . . . . . . . . . . . . . . . . . . . . 4
5. Authentication . . . . . . . . . . . . . . . . . . . . . . . 4
6. Security . . . . . . . . . . . . . . . . . . . . . . . . . . 5
7. Advanced Transport Features . . . . . . . . . . . . . . . . . 5
7.1. Reliability . . . . . . . . . . . . . . . . . . . . . . . 6
7.2. Flow/Congestion Control . . . . . . . . . . . . . . . . . 8
8. Security Considerations . . . . . . . . . . . . . . . . . . . 9
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
10. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . 9
11. Informative References . . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
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. 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?
2. How can the end-hosts influence where or by whom the
functionality is executed?
3. How can devices which do not implement COIN functionality be
integrated into the systems without breaking the COIN or legacy
functionality?
Another question is how the transmitted data is to be treated by the
devices implementing the functionality.
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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.
The flow granularity consequently has a significant impact on how
computations can be performed and where. Apart from how the COIN
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.
5. 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
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to the end-hosts. What is needed are mechanisms 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 3) and
subsequently authenticated. The Proof of Transit
[I-D.draft-ietf-sfc-proof-of-transit-06] 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?
6. 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.
Together, the four aspects presented in Section 3 to Section 6 form a
set of fundamental properties for a basic transport-compatible
realization of COIN. In the following, we briefly discuss selected
additional transport features to create awareness for the
multifaceted interaction between the transport protocols and COIN
elements.
7. Advanced 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.
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7.1. 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. In both 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
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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.
Challenges/questions regarding reliability are:
1. Should COIN elements be aware of retransmissions?
2. How can COIN elements identify missing packets or
retransmissions?
3. Should COIN elements be explicitly notified about
retransmissions?
4. To what extent should COIN elements be capable of incorporating
retransmissions into their computation schemes?
5. How much storage capabilities are required for incorporating
retransmissions?
6. How can COIN elements incorporate missing packets into their
computations?
7. How to deal with state changes in COIN elements caused by data
lost later in the communication chain and then retransmitted?
8. Should COIN elements be capable of requesting retransmissions/
answering retransmission requests?
9. Which devices should perform retransmissions?
10. Do COIN elements have to keep transport state?
11. How much transport state do COIN elements have to keep?
Another aspect is flow and congestion control to avoid overloading
the receiving end-host and the network.
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7.2. 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.
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?
All in all, there is a wide range of non-essential transport features
which offer improved performance in certain settings and for certain
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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. 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.
Overall, the general question summarizing all of the other
challenges/question is:
Which transport features should be supported by COIN, how can they be
identified, and how can they be provided to application designers?
8. 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.
9. IANA Considerations
N/A
10. 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.
11. Informative References
[I-D.draft-ietf-sfc-proof-of-transit-06]
Brockners, F., Bhandari, S., Mizrahi, T., Dara, S., and S.
Youell, "Proof of Transit", draft-ietf-sfc-proof-of-
transit-06 (work in progress), June 2020.
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[I-D.draft-kutscher-coinrg-dir-02]
Kutscher, D., Karkkainen, T., and J. Ott, "Directions for
Computing in the Network", draft-kutscher-coinrg-dir-02
(work in progress), July 2020.
[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
Aachen D-50274
Germany
Phone: +49-241-80-21422
Email: kunze@comsys.rwth-aachen.de
Klaus Wehrle
RWTH Aachen University
Ahornstr. 55
Aachen D-50274
Germany
Phone: +49-241-80-21401
Email: wehrle@comsys.rwth-aachen.de
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