Transport Services
charter-ietf-taps-00-03

Most Internet applications make use of the Transport Services
provided by TCP (a reliable, in-order stream protocol) or UDP
(an unreliable datagram protocol). We use the term "Transport
Service" to mean an end-to-end facility provided by the
transport layer. That service can only be provided correctly if
information is supplied from the application. The application may
determine the information to be supplied at design time,
compile time, or run time and may include guidance on whether
the facility in question is required or simply a preference by the
application. Four examples of Transport Services are reliable
delivery, ordered delivery of data, content privacy to in-path
devices, and minimal latency.

Transport protocols such as SCTP, DCCP, WebSockets, MPTCP, and
UDP-Lite and the LEDBAT congestion control mechanism extend the
set of available Transport Services beyond those provided to
applications by TCP and UDP. For example, SCTP provides potentially
faster reliable delivery for applications than TCP because it can
accept blocks of data out of order, and LEDBAT provides low-priority
"scavenger" communication.

However, application programmers face difficulty when they use
protocols other than TCP or UDP. Most network stacks ship with only
TCP and UDP as transport protocols. Many firewalls only pass TCP
and UDP and some only allow TCP when it carries HTTP as its payload.
As a result, using other transport protocols or building a new protocol
over raw sockets risks having an application not work in many
environments. Applications, therefore, must always be able to fall back
to TCP or UDP, or even to using HTTP as a transport substrate in some
cases, and once the application programmer has committed
to making an application work on TCP or UDP or HTTP, there is little
incentive to try other transport protocols before falling back.
Further, different protocols can provide the same services in different
ways. Layering decisions must also be made (for example, whether a
protocol is used natively or tunneled through UDP).

Because of these complications, programmers often resort to either
using TCP or HTTP or implementing their own customized "transport
services" over UDP. When application developers re-implement transport
features already available elsewhere they open the door to problems
that simply using TCP would have avoided and ensure that the
applications can't benefit from other transport protocols as they
become available. And, since even TCP and UDP are not guaranteed to
work in all environments today, some programmers simply give up on
using TCP or UDP directly, relying instead on "HTTP as a Substrate".
BCP 56 identified many issues with this strategy, but assuming that
if "any protocol is available on a given network path and on the hosts
that will be communicating, that protocol will be HTTP" is a reasonable
strategy for today's Internet. The IESG has agreed with this viewpoint
enough to publish the WebSocket protocol on the standards track.

The goal of the TAPS working group is to help application and network
stack programmers by describing an (abstract) interface for applications
to make use of Transport Services. The Working Group deliverables
emphasize defining Transport Services that are important to applications
followed by experimental mechanisms to a) determine if the protocols to
provide the requested services are available on the end points and
supported along the path in the network and b) fall back, i.e., select
alternate, possibly less-preferred, protocols when desired protocols
are not available. The Working Group will not define a richer set of
Transport Services for applications, although the TAPS deliverables
could inform proposals for future chartered work on Transport Services.

The Working Group will:

1) Define a set of Transport Services, identifying the
services provided by existing IETF protocols and congestion
control mechanisms. As a starting point, the working group will
consider services used
between two endpoints.

2) Specify the subset of those Transport Services, as identified
in item 1, that end systems supporting TAPS will provide, and
give guidance on choosing among available mechanisms and
protocols. Note that not all the capabilities of IETF Transport
protocols need to be exposed as Transport Services.

3) Specify experimental support mechanisms to provide the Transport
Services identified in work item 2. This document will explain
how to select and engage an appropriate protocol and how to
discover which protocols are available for the selected service
between a given pair of end points. Further, it will provide a
basis for incremental deployment. Work on this document will
begin when the TAPS Transport Services have been specified.

The following topics are out of scope for this Working Group:

• Signaling-based Quality-of-Service (QoS) mechanisms

• Definition of new encapsulations and tunneling mechanisms

• Extension, modification, or creation of transport protocols

• Language-specific APIs

TAPS is not chartered to perform detailed analysis of the security
aspects of transport protocols. While security can be a transport
service, TAPS is being chartered almost simultaneously with TCPINC,
which is developing the TCP extensions to provide unauthenticated
encryption and integrity protection of TCP streams, and TAPS will
work with TCPINC to ensure that TAPS will be able to accommodate
the protocol extensions that TCPINC defines.