The Internet has a long history of employing performance enhancing proxies (PEPs, RFC 3135) to improve performance over paths with links of varying quality. Today's PEPs often interact deeply and "transparently" (intrusively) with end-to-end transport and application layer protocols. This practice is coming to an end with increasing deployment of encryption.
At the same time, networks are becoming more complex, and network nodes are becoming more powerful. It is becoming more viable to trade processing power in network nodes against path quality, in particular for expensive path segments. Transport protocols and their implementations are moving towards playing better with forwarding node functions such as ECN marking and AQM.
LOOPS (Local Optimizations on Path Segments) attempts to capture opportunities opened by these developments, enabling optimizations within some segments of an end-to-end path. Typically, these segments will be delimited as tunnels maintained by dedicated overlay nodes (tunnel endpoints), which allows a local optimization protocol to run between these nodes. Many end-to-end flows can be aggregated into each such tunnel flow being optimized.
Initially, LOOPS will focus on path segments that do not include either end host. Also, multipath forwarding will not be specifically addressed and is left for future consideration.
The selection of the segments to be optimized and the nodes that will run LOOPS optimization is deployment-specific and is out of scope; the assumption is that this will be performed as part of the network configuration, which can also supply further parameters controlling LOOPS. LOOPS will define a simple key/value configuration data set that can be used by configuration protocols.
The functions to be addressed by LOOPS include:
Local recovery: Packet losses on the path segments are recovered autonomously, removing the need to burden the entire end-to-end path with the recovery, and decreasing the latency by which these recoveries can be effected. The protocol will be designed so that local recovery can be based on forward error correction (FEC) and/or (non-persistent) retransmission. The initial focus will be on retransmission, but a "default" FEC scheme will be included in order to exercise the protocol mechanisms for selecting FEC and a specific FEC scheme (more state-of-the-art FEC schemes can then be registered later).
Local measurement: To properly parameterize the LOOPS algorithms (e.g., RTO, FEC rate) and to trigger state transitions, measurements are continuously performed of the path segment between the tunnel endpoints.
Interaction with end-to-end congestion control: As well as Congestion Experienced (CE) events on the path, losses are relayed as congestion events to the end-to-end congestion control. Initially, LOOPS will focus on the use of ECN for this and therefore only supports ECN-enabled end-to-end flows. Circuit breakers (RFC 8084) will be employed on the LOOPS tunnel to further protect against congestion collapse.
The LOOPS protocol will need to run embedded into a tunneling protocol. Initially, this will be Geneve (Generic Network Virtualization Encapsulation, NVO3 WG). To facilitate future support of further encapsulations, LOOPS will be defined with a separation in mind between a generic information model level, and a set of protocol bindings that will start out as the Geneve embedding.
LOOPS will actively consult ICCRG, for congestion control issues, and NWCRG, for FEC encoding and encapsulation issues. In particular, documents of the latter will serve as a basis for the FEC-based components of LOOPS.
In addition to NWCRG mentioned above, TSVWG will be consulted with respect to their FEC-related work. (TSVWG and TCPM will also be important sources of information on recent developments on mechanisms used in LOOPS itself, such as ECN, time-based recovery, as well as, with QUIC and AVTcore, on the characteristics of end-to-end transport flows.) LOOPS will interact (in a user role) with NVO3 as the home of the Geneve tunneling protocol.
The main deliverable will be a LOOPS specification (one or split between two documents, to be decided by the WG). Also, the working group will adopt a "problems and opportunities" document as the basis of this work; this document then does not necessarily need to be published by itself as an RFC.