Pseudowire Congestion Considerations
RFC 7893
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
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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
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