Segment Routed Time Sensitive Networking

Document Type Active Internet-Draft (individual)
Author Yaakov Stein 
Last updated 2021-02-22
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DetNet Working Group                                          Y(J) Stein
Internet-Draft                                                       RAD
Intended status: Informational                         February 22, 2021
Expires: August 26, 2021

                Segment Routed Time Sensitive Networking


   Routers perform two distinct user-plane functionalities, namely
   forwarding (where the packet should be sent) and scheduling (when the
   packet should be sent).  One forwarding paradigm is segment routing,
   in which forwarding instructions are encoded in the packet in a stack
   data structure, rather than programmed into the routers.  Time
   Sensitive Networking and Deterministic Networking provide several
   mechanisms for scheduling under the assumption that routers are time
   synchronized.  The most effective mechanisms for delay minimization
   involve per-flow resource allocation.

   SRTSN is a unified approach to forwarding and scheduling that uses a
   single stack data structure.  Each stack entry consists of a
   forwarding portion (e.g., IP addresses or suffixes) and a scheduling
   portion (deadline by which the packet must exit the router).  SRTSN
   thus fully implements network programming for time sensitive flows,
   by prescribing to each router both to-where and by-when each packet
   should be sent.

Status of This Memo

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   This Internet-Draft will expire on August 26, 2021.

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1.  Introduction

   Packet Switched Networks (PSNs) use statistical multiplexing to fully
   exploit link data rate.  On the other hand, statistical multiplexing
   in general leads to end-to-end propagation latencies significantly
   higher than the minimum physically possible, due to packets needing
   to reside in queues waiting for their turn to be transmitted.

   Recently Time Sensitive Networking (TSN) and Deterministic Networking
   (DetNet) technologies have been developed to reduce this queueing
   latency for time sensitive packets [RFC8557].  Novel TSN mechanisms
   are predicated on the time synchronization of all forwarding elements
   (Ethernet switches, MPLS Label Switched Routers, or IP routers, to be
   called here simply routers).  Once routers agree on time to high
   accuracy, it is theoretically possible to arrange for time sensitive
   packets to experience "green waves", that is, never to wait in
   queues.  For example, scheduling timeslots for particular flows
   eliminates packet interference, but eliminates the statistical
   multiplexing advantage of PSNs.  In addition, the scheduling
   calculation and programming of the network to follow this calculation
   doesn't scale well to large networks.

   Segment Routing (SR) technologies provide a scalable method of
   network programming, but until now has not been applied to
   scheduling.  The SR instructions are contained within a packet in the
   form of a first-in first-out stack dictating the forwarding decisions
   of successive routers.  Segment routing may be used to choose a path
   sufficiently short to be capable of providing sufficiently low end-
   to-end latency but does not influence the queueing of individual
   packets in each router along that path.

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