Pseudowire Congestion Considerations
RFC 7893

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Internet Engineering Task Force (IETF)                        Y(J) Stein
Request for Comments: 7893                       RAD Data Communications
Category: Informational                                         D. Black
ISSN: 2070-1721                                          EMC Corporation
                                                              B. Briscoe
                                                                      BT
                                                               June 2016

                  Pseudowire Congestion Considerations

Abstract

   Pseudowires (PWs) have become a common mechanism for tunneling
   traffic and may be found in unmanaged scenarios competing for network
   resources both with other PWs and with non-PW traffic, such as TCP/IP
   flows.  Thus, it is worthwhile specifying under what conditions such
   competition is acceptable, i.e., the PW traffic does not
   significantly harm other traffic or contribute more than it should to
   congestion.  We conclude that PWs transporting responsive traffic
   behave as desired without the need for additional mechanisms.  For
   inelastic PWs (such as Time Division Multiplexing (TDM) PWs), we
   derive a bound under which such PWs consume no more network capacity
   than a TCP flow.  For TDM PWs, we find that the level of congestion
   at which the PW can no longer deliver acceptable TDM service is never
   significantly greater, and is typically much lower, than this bound.
   Therefore, as long as the PW is shut down when it can no longer
   deliver acceptable TDM service, it will never do significantly more
   harm than even a single TCP flow.  If the TDM service does not
   automatically shut down, a mechanism to block persistently
   unacceptable TDM pseudowires is required.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are a candidate for any level of Internet
   Standard; see Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7893.

Stein, et al.                 Informational                     [Page 1]
RFC 7893                  Pseudowire Congestion                June 2016

Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  PWs Comprising Elastic Flows  . . . . . . . . . . . . . . . .   6
   4.  PWs Comprising Inelastic Flows  . . . . . . . . . . . . . . .   7
   5.  Conclusions . . . . . . . . . . . . . . . . . . . . . . . . .  19
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  19
   7.  Informative References  . . . . . . . . . . . . . . . . . . .  19
   Appendix A.  Loss Probabilities for TDM PWs . . . . . . . . . . .  22
   Appendix B.  Effect of Packet Loss on Voice Quality for
                Structure-Aware TDM PWs  . . . . . . . . . . . . . .  23
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  27

Stein, et al.                 Informational                     [Page 2]
RFC 7893                  Pseudowire Congestion                June 2016

1.  Introduction

   A pseudowire (PW) (see [RFC3985]) is a construct for tunneling a
   native service, such as Ethernet or TDM, over a Packet Switched
   Network (PSN), such as IPv4, IPv6, or MPLS.  The PW packet
   encapsulates a unit of native service information by prepending the
   headers required for transport in the particular PSN (which must
   include a demultiplexer field to distinguish the different PWs) and
   preferably the 4-byte Pseudowire Emulation Edge-to-Edge (PWE3)
   control word.

   PWs have no bandwidth reservation or control mechanisms, meaning that
   when multiple PWs are transported in parallel, and/or in parallel
   with other flows, there is no defined means for allocating resources