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Versions: (draft-ietf-pwe3-congcons) 00 01 02 RFC 7893

PALS                                                           YJ. Stein
Internet-Draft                                   RAD Data Communications
Intended status: Informational                                  D. Black
Expires: October 22, 2016                                EMC Corporation
                                                              B. Briscoe
                                                                      BT
                                                          April 20, 2016


                  Pseudowire Congestion Considerations
                      draft-ietf-pals-congcons-02

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.  It is thus 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 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 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 Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on October 22, 2016.



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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
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  PWs Comprising Elastic Flows  . . . . . . . . . . . . . . . .   4
   4.  PWs Comprising Inelastic Flows  . . . . . . . . . . . . . . .   6
   5.  Conclusions . . . . . . . . . . . . . . . . . . . . . . . . .  18
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  18
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  18
   8.  Informative References  . . . . . . . . . . . . . . . . . . .  19
   Appendix A.  Loss Probabilities for TDM PWs . . . . . . . . . . .  20
   Appendix B.  Effect of Packet Loss on Voice Quality for Structure
                Aware TDM PWs  . . . . . . . . . . . . . . . . . . .  22
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  24

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
   for any particular PW, or for preventing negative impact of a
   particular PW on neighboring flows.  The case where the service
   provider network provisions a PW with sufficient capacity is well
   understood and will not be discussed further here.  Concerns arise



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   when PWs share network capacity with elastic or congestion-responsive
   traffic, whether that capacity sharing was planned by a service
   provider or results from PW deployment by an end-user.

   PWs are most often placed in MPLS tunnels, but we herein restrict
   ourselves to PWs in IPv4 or IPv6 PSNs; MPLS PSNs are beyond the scope
   of this document.  There are several mechanisms that enable
   transporting of PWs over an IP infrastructure, including:

   o  UDP/IP encapsulations as defined for TDM PWs
      ([RFC4553][RFC5086][RFC5087]),
   o  L2 tunneling protocol (L2TPv3) based PWs,
   o  MPLS PWs directly over IP according to RFC 4023 [RFC4023],
   o  MPLS PWs over Generic Routing Encapsulation (GRE) over IP
      according to RFC 4023 [RFC4023].

   Whenever PWs are transported over IP, they may compete for network
   resources with neighboring congestion-responsive flows (e.g., TCP
   flows).  In this document we study the effect of PWs on such
   neighboring flows, and discover that the negative impact of PW
   traffic is generally no worse than that of congestion-responsive
   flows ([RFC2914],[RFC5033]}.

   At first glance one may consider a PW transported over IP to be
   considered as a single flow, on a par with a single TCP flow.  Were
   we to accept this tenet, we would require a PW to back off under
   congestion to consume no more bandwidth than a single TCP flow under
   such conditions (see [RFC5348]).  However, since PWs may carry
   traffic from many users, it makes more sense to consider each PW to
   be equivalent to multiple TCP flows.

   The following two sections consider PWs of two types.

   Elastic Flows:  Section 3 concludes that the response to congestion
           of a PW carrying elastic (e.g., TCP) flows is no different
           from the aggregated behaviours of the individual elastic
           flows were they not encapsulated within a PW.
   Inelastic Flows:  Section 4 considers the case of inelastic constant
           bit-rate (CBR) TDM PWs ([RFC4553][RFC5086] [RFC5087])
           competing with TCP flows.  Such PWs require a preset amount
           of bandwidth, that may be lower or higher than that consumed
           by an otherwise unconstrained TCP flow under the same network
           conditions.  In any case, such a PW is unable to respond to
           congestion in a TCP-like manner; although admittedly the
           total bandwidth it consumes remains constant and does not
           increase to consume additional bandwidth as TCP rates back
           off.  For TDM services we will show that TDM service quality
           degradation generally occurs before the TDM PW becomes TCP-



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           unfriendly.  For TDM services that do not automatically shut
           down when they persistently fail to comply with acceptable
           TDM service criteria, a transport circuit breaker
           [I-D.ietf-tsvwg-circuit-breaker] may be employed as a last
           resort to shut down a TDM pseudowire that can no longer
           deliver acceptable service.

   Thus, in both cases, pseudowires will not inflict significant harm on
   neighboring TCP flows, as in one case they respond adequately to
   congestion, and in the other they would be shut down due to being
   unable to deliver acceptable service before harming neighboring
   flows.

   Note: This document contains a large number of graphs that are
   necessary for its understanding, but could not be rendered in ASCII.
   It is suggested that only the PDF version be consulted.

2.  Terminology

   The following acronyms used in this document :

   AIS     Alarm Indication Signal (see G.775)
   BER     Bit Error Rate [G826]
   BW      bandwidth
   CBR     Constant Bit Rate
   ES      Errored Second [G826]
   ESR     Errored Second Rate [G826]
   GRE     Generic Routing Encapsulation (see RFC 2890)
   L2TPv3  Layer 2 Tunneling Protocol Version 3 (see RFC 3931)
   MOS     Mean Opinion Score (see ITU-T P.800)
   MPLS    Multiprotocol Label Switching (see RFC 3031)
   NSP     Native Service Processing (see RFC 3985)
   PLR     Packet Loss Ratio
   PSN     Packet Switched Network [RFC3985]
   PW      pseudowire [RFC3985]
   SAToP   Structure Agnostic TDM over Packet [RFC4553]
   SES     Severely Errored Seconds [G826]
   SESR    Severely Errored Seconds Ratio [G826]
   TCP     Transmission Control Protocol
   TDM     Time Division Multiplexing (see G.703)
   UDP     User Datagram Protocol

3.  PWs Comprising Elastic Flows

   In this section we consider Ethernet PWs that primarily carry
   congestion-responsive traffic.  We expand on the remark in
   Section 6.5 (Congestion Considerations) of [RFC4553], and show that




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   the desired congestion avoidance behavior is automatically obtained
   and additional mechanisms are not needed.

   Let us assume that an Ethernet PW aggregating several TCP flows is
   flowing alongside several TCP/IP flows.  Each Ethernet PW packet
   carries a single Ethernet frame that carries a single IP packet that
   carries a single TCP segment.  Thus, if congestion is signaled by an
   intermediate router dropping a packet, a single end-user TCP/IP
   packet is dropped, whether or not that packet is encapsulated in the
   PW.

   The result is that the individual TCP flows inside the PW experience
   the same drop probability as the non-PW TCP flows.  Thus the behavior
   of a TCP sender (retransmitting the packet and appropriately reducing
   its sending rate) is the same for flows directly over IP and for
   flows inside the PW.  In other words, individual TCP flows are
   neither rewarded nor penalized for being carried over the PW.  An
   elastic PW does not behave as a single TCP flow, as it will consume
   the aggregated bandwidth of its component flows; yet if its component
   TCP flows backs off by some percentage, the bandwidth of the PW as a
   whole will be reduced by the very same percentage, purely due to the
   combined effect of its component flows.

   This is, of course, precisely the desired behavior.  Were individual
   TCP flows rewarded for being carried over a PW, this would create an
   incentive to create PWs for no operational reason.  Were individual
   flows penalized, there would be a deterrence that could impede
   pseudowire deployment.

   There have been proposals to add additional TCP-friendly mechanisms
   to PWs, for example by carrying PWs over DCCP.  In light of the above
   arguments, it is clear that this would force the PW down to the
   bandwidth of a single flow, rather than N flows, and penalize the
   constituent TCP flows.  In addition, the individual TCP flows would
   still back off due to their end points being oblivious to the fact
   that they are carried over a PW.  This would further degrade the
   flow's throughput as compared to a non-PW-encapsulated flow, in
   contradiction to desirable behavior.

   We have limited our treatment to the case of TCP traffic carried by
   Ethernet PWs (which are by far the most commonly deployed packet-
   carrying pseudowires) but it is not overly difficult to show that our
   result is equally valid for other PW types, such as ATM or frame
   relay pseudowires.







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4.  PWs Comprising Inelastic Flows

   Inelastic PWs, such as TDM PWs ([RFC4553][RFC5086][RFC5087]), are
   potentially more problematic than the elastic PWs of the previous
   section.  As mentioned in Section 8 (Congestion Control) of
   [RFC4553], being constant bit-rate (CBR), TDM PWs can't incrementally
   respond to congestion in a TCP-like fashion.  On the other hand,
   being CBR, TDM PWs do not make things worse by attempting to capture
   additional bandwidth when neighboring TCP flows back off.

   Since a TDM PW consumes a constant amount of bandwidth, if the
   bandwidth occupied by a TDM PW endangers the network as a whole, it
   might seem that the only recourse is to shut it down, denying service
   to all customers of the TDM native service.  Nonetheless, under
   certain conditions it may be possible to reduce the bandwidth
   consumption of an emulated TDM service.  A prevalent case is that of
   a TDM native service that carries voice channels that may not all be
   active.  The AAL2 mode of [RFC5087] (perhaps along with connection
   admission control) can enable bandwidth adaptation, at the expense of
   more sophisticated native service processing (NSP).

   In the following we will focus on structure-agnostic TDM PWs
   [RFC4553] although similar analysis can be readily applied to
   structure-aware PWs (see Appendix B).  We will show that, for many
   cases of interest, a TDM PW, even when treated as a single flow, will
   behave in a reasonable manner without any additional mechanisms.  We
   also show that, at the level of congestion when a TDM PW can no
   longer deliver acceptable TDM service, a single unconstrained TCP
   flow would typically still consume more capacity than a whole TDM PW.
   Therefore, to ensure that a TDM PW does not inflict significantly
   more harm than a TCP flow, it suffices to shut down a TDM PW that is
   persistently unable to deliver acceptable TDM service.  This shutting
   down could be accomplished by employing a managed transport circuit
   breaker, by which we mean an automatic mechanism for terminating an
   unresponsive flow during persistently high levels of congestion
   [I-D.ietf-tsvwg-circuit-breaker].  Note that a transport circuit
   breaker is intended as a protection mechanism of last resort, just as
   an electrical circuit breaker is only triggered when absolutely
   necessary.

   For the avoidance of doubt, the above does not say that a TDM PW
   should be shut down when it becomes TCP-unfriendly.  It merely says
   that the act of shutting down a TDM PW that can no longer deliver
   acceptable TDM service ensures that the PW does not contribute to
   congestion significantly more than a TCP flow would.  Also note that
   being unable to deliver acceptable TDM service for a short amount of
   time is insufficient justification for shutting down a TDM PW.  While
   TCP flows react within a round trip time, service commissioning and



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   decommissioning are generally time consuming processes that should
   only be undertaken when it becomes clear that the congestion is not
   transient.

   In order to quantitatively compare TDM PWs to TCP flows, we will
   compare the effect of TDM PW traffic with that of TCP traffic having
   the same packet size and delay.  This is potentially an overly
   pessimistic comparison, as TDM PW packets are frequently configured
   to be short in order to minimize latency, while TCP packets are free
   to be much larger.

   There are two network parameters relevant to our discussion, namely
   the one-way delay (D) and the packet loss ratio (PLR).  The one-way
   delay of a native TDM service consists of the physical time-of-flight
   plus 125 microseconds for each TDM switch traversed; and is thus very
   small as compared to typical PSN network-crossing latencies.  Since
   TDM services are designed with this low latency in mind, emulated TDM
   services are usually required to have similar low end-to-end delay.
   In our comparisons we will only consider one-way delays of a few
   milliseconds.

   Regarding packet loss, the relevant RFCs specify actions to be
   carried out upon detecting a lost packet.  Structure-agnostic
   transport has no alternative to outputting an "all-ones" Alarm
   Indication Signal (AIS) pattern towards the TDM circuit, which, when
   long enough in duration, is recognized by the receiving TDM device as
   a fault indication (see Appendix A).  TDM standards (such as [G826])
   place stringent limits on the number of such faults tolerated.
   Calculations presented in the appendix show that only loss
   probabilities in the realm of fractions of a percent are relevant for
   structure-agnostic transport (see Appendix A).  Structure-aware
   transport regenerates frame alignment signals thus avoiding AIS
   indications resulting from infrequent packet loss.  Furthermore, for
   TDM circuits carrying voice channels the use of packet loss
   concealment algorithms is possible (such algorithms have been
   previously described for TDM PWs).  However, even structure-aware
   transport ceases to provide a useful service at about 2 percent loss
   probability.  Hence, in our comparisons we will only consider PLRs of
   1 or 2 percent.

   RFC 5348 on TCP Friendly Rate Control (TFRC) [RFC5348] provides a
   simplified formula for TCP throughput as a function of round-trip
   delay and packet loss ratio.

                                    S
       X     = ------------------------------------------------
                 R  ( sqrt(2p/3) + 12 sqrt(3p/8) p (1+32p^2) )




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   where

      X is average sending rate in Bytes per second,
      S is the segment (packet payload) size in Bytes,
      R is the round-trip time in seconds,
      p is the packet loss probability (i.e., PLR/100).

   We can now compare the bandwidth consumed by TDM pseudowires with
   that of a TCP flow for given packet loss ratio and one-way end-to-end
   delay (taken to be half the round-trip delay R).  The results are
   depicted in the accompanying figures (available only in the PDF
   version of this document).  In Figures 1 and 2 we see the
   conventional rate vs. packet loss plot for low-rate TDM (both T1 and
   E1) traffic, as well as TCP traffic with the same payload size (64 or
   256 Bytes respectively).  Since the TDM rates are constant (T1 and E1
   having payload throughputs of 1.544 Mbps and 2.048 Mbps
   respectively), and Structure-Agnostic TDM over packet (SAToP) can
   only faithfully emulate a TDM service up to a PLR of about half a
   percent, the T1 and E1 pseudowires occupy line segments on the graph.
   On the other hand, the TCP rate equation produces rate curves
   dependent on both one-way delay and packet loss.

   For large packet sizes, short one-way delays, and low packet loss
   ratios, the TDM pseudowires typically consume much less bandwidth
   than TCP would under identical conditions.  For small packets, long
   one-way delays, and high packet loss ratios, TDM PWs potentially
   consume more bandwidth, but only marginally.  Furthermore, our
   "apples to apples" comparison forced the TCP traffic to use packets
   of sizes smaller than would be typical.

   Similarly, in Figures 3 and 4 we repeat the exercise for higher rate
   E3 and T3 (rates 34.368 and 44.736 Mbps respectively) pseudowires,
   allowing delays and PLRs suitable for these signals.  We see that the
   TDM pseudowires consume much less bandwidth than TCP, for all
   reasonable parameter combinations.
















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   Figure 1 E1/T1 PWs vs. TCP for segment size 64B































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   Figure 2 E1/T1 PWs vs. TCP for segment size 256B































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   Figure 3 E3/T3 PWs vs. TCP for segment size 536B































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   Figure 4 E3/T3 PWs vs. TCP for segment size 1024B































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   We can use the TCP rate equation to determine precise conditions
   under which a TDM PW consumes no more bandwidth than a TCP flow
   between the same endpoints under identical conditions.  Replacing the
   round-trip delay with twice the one-way delay D, setting the
   bandwidth to that of the TDM service BW, and the segment size to be
   the TDM fragment (taking into account the PWE3 control word), we
   obtain the following condition for a TDM PW.

              4 S
       D < -----------
             BW f(p)

   where

      D is the one-way delay,
      S is the TDM segment size (packet excluding overhead) in Bytes,
      BW is TDM service bandwidth in bits per second,
      f(p) = sqrt(2p/3) + 12 sqrt(3p/8) p (1+32p^2).

   One may view this condition as defining a 'friendly' operating
   envelope for a TDM PW, as a TDM PW that occupies no more bandwidth
   than a TCP flow causes no more congestion than that TCP flow.  Under
   this condition it is acceptable to place the TDM PW alongside
   congestion-responsive traffic such as TCP.  On the other hand, were
   the TDM PW to consume significantly more bandwidth than a TCP flow,
   it could contribute disproportionately to congestion, and its mixture
   with congestion-responsive traffic might be inappropriate.  Note that
   we are sidestepping any debate over the validity of the TCP-
   friendliness concept, and merely saying that there can be no question
   that a TDM PW is acceptable if it causes no more congestion than a
   single TCP flow.

   We derived this condition assuming steady-state conditions, and thus
   two caveats are in order.  First, the condition does not specify how
   to treat a TDM PW that initially satisfies the condition, but is then
   faced with a deteriorating network environment.  In such cases one
   additionally needs to analyze the reaction times of the responsive
   flows to congestion events.  Second, the derivation assumed that the
   TDM PW was competing with long-lived TCP flows, because under this
   assumption it was straightforward to obtain a quantitative comparison
   with something widely considered to offer a safe response to
   congestion.  Short-lived TCP flows may find themselves disadvantaged
   as compared to a long-lived TDM PW satisfying the above condition.

   We see in Figures 5 and 6 that TDM pseudowires carrying T1 or E1
   native services satisfy the condition for all parameters of interest
   for large packet sizes (e.g., S=512 Bytes of TDM data).  For the
   SAToP default of 256 Bytes, as long as the one-way delay is less than



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   10 milliseconds, the loss probability can exceed 0.3 or 0.6 percent.
   For packets containing 128 or 64 Bytes the constraints are more
   troublesome, but there are still parameter ranges where the TDM PW
   consumes less than a TCP flow under similar conditions.  Similarly,
   Figures 7 and 8 demonstrate that E3 and T3 native services with the
   SAToP default of 1024 Bytes of TDM per packet satisfy the condition
   for a broad spectrum of delays and PLRs.


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   Figure 5 TCP Compatibility areas for T1 SAToP






















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   Figure 6 TCP Compatibility areas for E1 SAToP































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   Figure 7 TCP Compatibility areas for E3 SAToP































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   Figure 8 TCP Compatibility areas for T3 SAToP































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5.  Conclusions

   The figures presented in the previous section demonstrate that TDM
   service quality degradation generally occurs before the TDM PW would
   consume more bandwidth that a comparable TCP flow.  Thus while TDM
   PWs are unable to respond to congestion in a TCP-like manner, TDM PWs
   that are able to deliver acceptable TDM service do not contribute to
   congestion significantly more than a TCP flow.

   Combined with our earlier determination that Ethernet PWs
   automatically respond in TCP-like fashion (see Section 3), our final
   conclusion is that PW-specific congestion-avoidance mechanisms are
   generally not required.  This is true even for TDM PWs, assuming that
   the TDM management plane initiates service shut down when service
   parameters are persistently below levels required by the relevant TDM
   standards.  If the TDM service does not automatically shut down, a
   mechanism to block persistently unacceptable TDM pseudowires is
   required, or a transport circuit breaker
   [I-D.ietf-tsvwg-circuit-breaker] may be triggered as a last resort.

6.  Security Considerations

   This document does not introduce any new congestion-specific
   mechanisms and thus does not introduce any new security
   considerations above those present for PWs in general.

7.  IANA Considerations

   This document requires no IANA actions.






















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8.  Informative References

   [RFC2914]  Floyd, S., "Congestion Control Principles", BCP 41,
              RFC 2914, DOI 10.17487/RFC2914, September 2000,
              <http://www.rfc-editor.org/info/rfc2914>.

   [RFC3985]  Bryant, S., Ed. and P. Pate, Ed., "Pseudo Wire Emulation
              Edge-to-Edge (PWE3) Architecture", RFC 3985,
              DOI 10.17487/RFC3985, March 2005,
              <http://www.rfc-editor.org/info/rfc3985>.

   [RFC4023]  Worster, T., Rekhter, Y., and E. Rosen, Ed.,
              "Encapsulating MPLS in IP or Generic Routing Encapsulation
              (GRE)", RFC 4023, DOI 10.17487/RFC4023, March 2005,
              <http://www.rfc-editor.org/info/rfc4023>.

   [RFC4553]  Vainshtein, A., Ed. and YJ. Stein, Ed., "Structure-
              Agnostic Time Division Multiplexing (TDM) over Packet
              (SAToP)", RFC 4553, DOI 10.17487/RFC4553, June 2006,
              <http://www.rfc-editor.org/info/rfc4553>.

   [RFC5033]  Floyd, S. and M. Allman, "Specifying New Congestion
              Control Algorithms", BCP 133, RFC 5033,
              DOI 10.17487/RFC5033, August 2007,
              <http://www.rfc-editor.org/info/rfc5033>.

   [RFC5086]  Vainshtein, A., Ed., Sasson, I., Metz, E., Frost, T., and
              P. Pate, "Structure-Aware Time Division Multiplexed (TDM)
              Circuit Emulation Service over Packet Switched Network
              (CESoPSN)", RFC 5086, DOI 10.17487/RFC5086, December 2007,
              <http://www.rfc-editor.org/info/rfc5086>.

   [RFC5087]  Stein, Y(J)., Shashoua, R., Insler, R., and M. Anavi,
              "Time Division Multiplexing over IP (TDMoIP)", RFC 5087,
              DOI 10.17487/RFC5087, December 2007,
              <http://www.rfc-editor.org/info/rfc5087>.

   [RFC5348]  Floyd, S., Handley, M., Padhye, J., and J. Widmer, "TCP
              Friendly Rate Control (TFRC): Protocol Specification",
              RFC 5348, DOI 10.17487/RFC5348, September 2008,
              <http://www.rfc-editor.org/info/rfc5348>.

   [G775]     International Telecommunications Union, "Loss of Signal
              (LOS), Alarm Indication Signal (AIS) and Remote Defect
              Indication (RDI) defect detection and clearance criteria
              for PDH signals", ITU Recommendation G.775, October 1998.





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   [G826]     International Telecommunications Union, "Error Performance
              Parameters and Objectives for International Constant Bit
              Rate Digital Paths at or above Primary Rate",
              ITU Recommendation G.826, December 2002.

   [P862]     International Telecommunications Union, "Perceptual
              evaluation of speech quality (PESQ): An objective method
              for end-to-end speech quality assessment of narrow-band
              telephone networks and speech codecs", ITU Recommendation
              G.826, February 2001.

   [I-D.stein-pwe3-tdm-packetloss]
              Stein, Y(J). and I. Druker, "The Effect of Packet Loss on
              Voice Quality for TDM over Pseudowires", October 2003.

   [I-D.ietf-tsvwg-circuit-breaker]
              Fairhurst, G., "Network Transport Circuit Breakers",
              draft-ietf-tsvwg-circuit-breaker-15 (work in progress),
              April 2016.

Appendix A.  Loss Probabilities for TDM PWs

   ITU-T Recommendation G.826 [G826] specifies limits on the Errored
   Second Ratio (ESR) and the Severely Errored Second Ratio (SESR).  For
   our purposes, we will simplify the definitions and understand an
   Errored Second (ES) to be a second of time during which a TDM bit
   error occurred or a defect indication was detected.  A Severely
   Errored Second (SES) is an ES second during which the Bit Error Rate
   (BER) exceeded one in one thousand (10^-3).  Note that if the error
   condition AIS was detected according to the criteria of ITU-T
   Recommendation G.775 [G775] a SES was considered to have occurred.
   The respective ratios are the fraction of ES or SES to the total
   number of seconds in the measurement interval.

   All TDM signals run at 8000 frames per second (higher rate TDM
   signals have longer frames).  So, assuming an integer number of TDM
   frames per TDM PW packet, the number of packets per second is given
   by packets per second = 8000 / (frames per packet).  Prevalent cases
   are 1, 2, 4 and 8 frames per packet, translating to 8000, 4000, 2000,
   and 1000 packets per second, respectively.

   For both E1 and T1 TDM circuits, G.826 allows ESR of 4% (0.04), and
   SESR of 0.2% (0.002).  For E3 and T3 the ESR must be no more than
   7.5% (0.075), while the SESR is unchanged.  Focusing on E1 circuits,
   the ESR of 4% translates, assuming the worst case of isolated exactly
   periodic packet loss, to a packet loss event no more than every 25
   seconds.  However, once a packet is lost, another packet lost in the
   same second doesn't change the ESR, although it may contribute to the



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   ES becoming a SES.  Thus for 1, 2, 4, and 8 frames per packet, the
   maximum allowed packet loss probability is 0.0005%, 0.001%, 0.002%,
   and 0.004% respectively.

   These extremely low allowed packet loss probabilities are only for
   the worst case scenario.  With tail-drop buffers, when packet loss is
   above 0.001%, it is likely that loss bursts will occur.  If the lost
   packets are sufficiently close together (we ignore the precise
   details here) then the permitted packet loss ratio increases by the
   appropriate factor, without G.826 being cognizant of any change.
   Hence the worst-case analysis is expected to be extremely pessimistic
   for real networks.  Next we will consider the opposite extreme and
   assume that all packet loss events are in periodic loss bursts.  In
   order to minimize the ESR we will assume that the burst lasts no more
   than one second, and so we can afford to lose in each burst no more
   than the number of packets transmitted in one second.  As long as
   such one-second bursts do not exceed four percent of the time, we
   still maintain the allowable ESR.  Hence the maximum permissible
   packet loss ratio is 4%.  Of course, this estimate is extremely
   optimistic, and furthermore does not take into consideration the SESR
   criteria.

   As previously explained, a SES is declared whenever AIS is detected.
   There is a major difference between structure-aware and structure-
   agnostic transport in this regards.  When a packet is lost SAToP
   outputs an "all-ones" pattern to the TDM circuit, which is
   interpreted as AIS according to G.775 [G775].  For E1 circuits, G.775
   specifies that AIS is detected when four consecutive TDM frames have
   no more than 2 alternations.  This means that if a PW packet or
   consecutive packets containing at least four frames are lost, and
   four or more frames of "all-ones" output to the TDM circuit, a SES
   will be declared.  Thus burst packet loss, or packets containing a
   large number of TDM frames, lead SAToP to cause high SESR, which is
   20 times more restricted than ESR.  On the other hand, since
   structure-aware transport regenerates the correct frame alignment
   pattern, even when the corresponding packet has been lost, packet
   loss will not cause declaration of SES.  This is the main reason that
   SAToP is much more vulnerable to packet loss than the structure-aware
   methods.

   For realistic networks, the maximum allowed packet loss for SAToP
   will be intermediate between the extremely pessimistic estimates and
   the extremely optimistic ones.  In order to numerically gauge the
   situation, we have modeled the network as a four-state Markov model,
   (corresponding to a successfully received packet, a packet received
   within a loss burst, a packet lost within a burst, and a packet lost
   when not within a burst).  This model is an extension of the widely
   used Gilbert model.  We set the transition probabilities in order to



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   roughly correspond to anecdotal evidence, namely low background
   isolated packet loss, and infrequent bursts wherein most packets are
   lost.  Such simulation shows that up to 0.5% average packet loss may
   occur and the recovered TDM still conforms to the G.826 ESR and SESR
   criteria.

Appendix B.  Effect of Packet Loss on Voice Quality for Structure Aware
             TDM PWs

   Packet loss in voice traffic cause audio artifacts such as choppy,
   annoying or even unintelligible speech.  The precise effect of packet
   loss on voice quality has been the subject of detailed study in the
   VoIP community, but VoIP results are not directly applicable to TDM
   PWs.  This is because VoIP packets typically contain over 10
   milliseconds of the speech signal, while multichannel TDM packets may
   contain only a single sample, or perhaps a very small number of
   samples.

   The effect of packet loss on TDM PWs has been previously reported
   [I-D.stein-pwe3-tdm-packetloss].  In that study it was assumed that
   each packet carried a single sample of each TDM timeslot (although
   the extension to multiple samples is relatively straightforward and
   does not drastically change the results).  Four sample replacement
   algorithms were compared, differing in the value used to replace the
   lost sample:

   1.  replacing every lost sample by a preselected constant (e.g., zero
       or "AIS" insertion),
   2.  replacing a lost sample by the previous sample,
   3.  replacing a lost sample by linear interpolation between the
       previous and following samples,
   4.  replacing the lost sample by STatistically Enhanced INterpolation
       (STEIN).

   Only the first method is applicable to SAToP transport, as structure
   awareness is required in order to identify the individual voice
   channels.  For structure aware transport, the loss of a packet is
   typically identified by the receipt of the following packet, and thus
   the following sample is usually available.  The last algorithm posits
   the LPC speech generation model and derives lost samples based on
   available samples both before and after each lost sample.

   The four algorithms were compared in a controlled experiment in which
   speech data was selected from English and American English subsets of
   the ITU-T P.50 Appendix 1 corpus [P.50App1] and consisted of 16
   speakers, eight male and eight female.  Each speaker spoke either
   three or four sentences, for a total of between seven and 15 seconds.
   The selected files were filtered to telephony quality using modified



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   IRS filtering and down-sampled to 8 KHz.  Packet loss of 0, 0.25,
   0.5, 0.75, 1, 2, 3, 4 and 5 percent were simulated using a uniform
   random number generator (bursty packet loss was also simulated but is
   not reported here).  For each file the four methods of lost sample
   replacement were applied and the Mean Opinion Score (MOS) was
   estimated using PESQ [P862].  Figure 9 depicts the PESQ-derived MOS
   for each of the four replacement methods for packet drop
   probabilities up to 5%.


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   Figure 9 PESQ derived MOS as a function of packet drop probability

   For all cases the MOS resulting from the use of zero insertion is
   less than that obtained by replacing with the previous sample, which
   in turn is less than that of linear interpolation, which is slightly
   less than that obtained by statistical interpolation.

   Unlike the artifacts speech compression methods may produce when
   subject to buffer loss, packet loss here effectively produces
   additive white impulse noise.  The subjective impression is that of
   static noise on AM radio stations or crackling on old phonograph
   records.  For a given PESQ-derived MOS, this type of degradation is
   more acceptable to listeners than choppiness or tones common in VoIP.





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   If MOS>4 (full toll quality) is required, then the following packet
   drop probabilities are allowable:

      zero insertion - 0.05 %
      previous sample - 0.25 %
      linear interpolation - 0.75 %
      STEIN - 2 %


   If MOS>3.75 (barely perceptible quality degradation) is acceptable,
   then the following packet drop probabilities are allowable:

      zero insertion - 0.1 %
      previous sample - 0.75 %
      linear interpolation - 3 %
      STEIN - 6.5 %

   If MOS>3.5 (cell-phone quality) is tolerable, then the following
   packet drop probabilities are allowable:

      zero insertion - 0.4 %
      previous sample - 2 %
      linear interpolation - 8 %
      STEIN - 14 %

Authors' Addresses

   Yaakov (Jonathan) Stein
   RAD Data Communications
   24 Raoul Wallenberg St., Bldg C
   Tel Aviv  69719
   ISRAEL

   Phone: +972 (0)3 645-5389
   Email: yaakov_s@rad.com


   David L. Black
   EMC Corporation
   176 South St.
   Hopkinton, MA  69719
   USA

   Phone: +1 (508) 293-7953
   Email: david.black@emc.com






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   Bob Briscoe
   BT

   Email: ietf@bobbriscoe.net
   URI:   http://bobbriscoe.net/














































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