Last updated 2014-07-26
Transport Services (taps)
The vast majority of 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 can only be correctly provided with information from the application. This information may be provided at design time, compile time, or run time. Four examples of Transport Services are reliable delivery, no guarantee of order preservation, content privacy to in-path devices, and minimal latency.
Other transport protocols such as SCTP, DCCP, MPTCP, 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 that can accept blocks of data out of order, and LEDBAT provides low-priority "scavenger" communication.
Application programmers face difficulty when they use protocols other than TCP or UDP. Most network stacks only support TCP and UDP, and many firewalls only pass TCP and UDP, so using other transport protocols risks having an application not work in many environments. Applications, therefore, must always be able to fall back to TCP or UDP, and once the application programmer has committed to making an application work on TCP or UDP, 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 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 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 TCP would have avoided, and ensure that the applications can't benefit from other transport protocols as they become available.
Alternatively, programmers may simply give up on using transport protocols 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 Websockets protocol on the standards track.
The goal of the TAPS working group is to help application and network stack programmers by describing an (initially abstract) interface for applications to make use of Transport Services. The Working Group deliverables emphasize definition of Transport Services important to applications followed by experimental mechanisms to a) determine if those Transport Services are available on the end points and along the path in the network and b) fall back when desired services 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, prioritizing those provided by existing IETF protocols and congestion control mechanisms. As a starting point, the working group will consider services used between two endpoints.
2) Note that not all capabilities of IETF Transport protocols need to be exposed as Transport Services. This document will recommend the minimal set of Transport Services that support mechanisms must provide (such as those in work item 3). The resulting document will also provide guidance on making a choice among available mechanisms and protocols to obtain a certain Transport Service.
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 a given connection. 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, but 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.
Milestones:
M9: Submit summary of the services provided by IETF transport protocols and congestion control mechanisms to IESG.
M15: Submit end system transport services to IESG.
M18: Submit specification of how the transport services can be provided to IESG.
M18: Recharter or close.