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Versions: (draft-chan-pcn-encoding-comparison) 00 01 02 03 04 05 06 07 08 09 RFC 6627

PCN                                                       G. Karagiannis
Internet-Draft                                      University of Twente
Intended status: Informational                                   K. Chan
Expires: September 08, 2012                                   Consultant
                                                            T. Moncaster
                                                 University of Cambridge
                                                                M. Menth
                                                 University of Tuebingen
                                                              P. Eardley
                                                              B. Briscoe
                                                                      BT
                                                          March 08, 2012


            Overview of Pre-Congestion Notification Encoding
                 draft-ietf-pcn-encoding-comparison-09

Abstract

   The objective of Pre-Congestion Notification (PCN) is to protect the
   quality of service (QoS) of inelastic flows within a Diffserv domain.
   On every link in the PCN domain, the overall rate of the PCN-traffic
   is metered, and PCN-packets are appropriately marked when certain
   configured rates are exceeded.  Egress nodes provide decision points
   with information about the PCN-marks of PCN-packets which allows them
   to take decisions about whether to admit or block a new flow request,
   and to terminate some already admitted flows during serious pre-
   congestion.

   The PCN Working Group explored a number of approaches for encoding
   this pre-congestion information into the IP header.  This document
   provides details of all those approaches along with an explanation of
   the constraints that had to be met by any solution.


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
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   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 September 08, 2012.





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Copyright Notice

   Copyright (c) 2012 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
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.






































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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  General PCN Encoding Requirements  . . . . . . . . . . . . . .  4
     2.1.  Metering and Marking Algorithms  . . . . . . . . . . . . .  5
     2.2.  Approaches for PCN Based Admission Control and Flow
           Termination  . . . . . . . . . . . . . . . . . . . . . . .  5
       2.2.1.  Dual Marking (DM)  . . . . . . . . . . . . . . . . . .  5
       2.2.2.  Single Marking (SM)  . . . . . . . . . . . . . . . . .  6
       2.2.3.  Packet Specific Dual Marking (PSDM)  . . . . . . . . .  7
       2.2.4.  Preferential Packet Dropping . . . . . . . . . . . . .  8
   3.  Encoding Constraints . . . . . . . . . . . . . . . . . . . . .  8
     3.1.  Structure of the DS Field  . . . . . . . . . . . . . . . .  8
     3.2.  Constraints from the DSCP  . . . . . . . . . . . . . . . .  8
       3.2.1.  General Scarcity of DSCPs  . . . . . . . . . . . . . .  8
       3.2.2.  Handling of the DSCP in Tunneling Rules                 9
       3.2.3.  Restoration of Original DSCPs at the Egress Node . . .  9
     3.3.  Constraints from the ECN Field . . . . . . . . . . . . . . 10
       3.3.1.  Structure and Use of the ECN Field . . . . . . . . . . 10
       3.3.2.  Redefinition of the ECN Field  . . . . . . . . . . . . 10
       3.3.3.  Handling of the ECN Field in Tunneling Rules . . . . . 11
       3.3.4.  Restoration of the Original ECN Field at the
               PCN-Egress-Node . . . . . . . . . . . . . . . . . . .  13

   4.  Comparison of Encoding Options . . . . . . . . . . . . . . . . 13
     4.1.  Baseline Encoding  . . . . . . . . . . . . . . . . . . . . 14
     4.2.  Encoding with 1 DSCP Providing 3 States  . . . . . . . . . 14
     4.3.  Encoding with 2 DSCPs Providing 3 or More States . . . . . 15
     4.4.  Encoding for Packet Specific Dual Marking (PSDM) . . . . . 15
     4.5.  Standardized Encodings . . . . . . . . . . . . . . . . . . 15
   5.  Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . 15
   6.  Security Implications  . . . . . . . . . . . . . . . . . . . . 16
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 16
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 16
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
      9.1.  Normative References . . . . . . . . . . . . . . . . . .  16
      9.2.  Informative References . . . . . . . . . . . . . . . . .  16

















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1.  Introduction

   The objective of Pre-Congestion Notification (PCN) [RFC5559] is to
   protect the quality of service (QoS) of inelastic flows within a
   Diffserv domain, in a simple, scalable, and robust fashion.  Two
   mechanisms are used: admission control, to decide whether to admit or
   block a new flow request, and flow termination to terminate some
   existing flows during serious pre-congestion.  To achieve this, the
   overall rate of PCN-traffic is metered on every link in the domain,
   and PCN-packets are appropriately marked when certain configured
   rates are exceeded.  These configured rates are below the rate of the
   link. Thus boundary nodes are notified of a potential overload before
   any real congestion occurs (hence "pre-congestion notification").

   [RFC5670] provides for two metering and marking functions that are
   configured with reference rates.  Threshold-marking marks all PCN
   packets once their traffic rate on a link exceeds the configured
   reference rate (PCN-threshold-rate).  Excess-traffic-marking marks
   only those PCN packets that exceed the configured reference rate
   (PCN-excess-rate).

   Egress nodes monitor the PCN-marks of received PCN-packets and
   provide information about the PCN-marks to decision points which take
   decisions about flow admission and termination on this basis
   [I-D.ietf-pcn-cl-edge-behaviour], [I-D.ietf-pcn-sm-edge-behaviour].

   This PCN information has to be encoded into the IP header. This PCN
   information has to be encoded into the IP header. This requires at
   least three different codepoints: one for PCN traffic that has not
   been marked, one for traffic that has been marked by the threshold
   meter and one for traffic that has been marked by the excess-traffic-
   meter.
   Since unused codepoints are not available for that purpose in the IP
   header (version 4 and 6), already used codepoints must be re-used
   which imposes additional constraints on design and applicability of
   PCN-based admission control (AC) and flow termination (FT). This
   document summarizes these issues as a record of the PCN WG
   discussions and for the benefit of the wider IETF community.

   In Section 2, we briefly point out PCN encoding requirement imposed
   by metering and marking algorithms, and by special packet drop
   strategies.  The Differentiated Services Codepoint (6 bits) and the
   ECN field (2 bits) have been selected to be re-used for encoding of
   PCN marks (PCN encoding).  In Section 3, we briefly explain the
   constraints imposed by this decision.  In Section 4, we review
   different PCN encodings supported by the PCN working group that allow
   different implementations of PCN-based admission control and flow
   termination which have different pros and cons.

2.  General PCN Encoding Requirements

   The choice of metering and marking algorithms and the way they are
   applied to PCN-based AC and FT impose certain requirements on PCN
   encoding.

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2.1.  Metering and Marking Algorithms

   Two different metering and marking algorithms are defined in
   [RFC5670]: excess-traffic-marking and threshold-marking.  They are
   both configured with reference rates which are termed PCN-excess-rate
   and PCN-threshold-rate, respectively.  When traffic for PCN flows
   enter a PCN domain, the PCN ingress node sets a codepoint in the IP
   header indicating that the packet is subject to PCN metering and
   marking and that it is not-marked (NM).  The two metering and marking
   algorithms possibly re-mark PCN packets as PCN and excess-traffic-
   marked (ETM) or threshold-marked (ThM).

   Excess-traffic-marking leaves a rate of PCN traffic equal to the PCN-
   excess-rate to be not-ETM marked if possible.  To that end, the
   algorithm needs to know whether a PCN packet has already been ETM
   marked or not.  Threshold-marking re-marks all not-marked PCN traffic
   to ThM when the rate of PCN traffic exceeds the PCN-threshold-rate.
   Therefore, it does not need knowledge of the prior marking state of
   the packet for metering, but it needs it for packet re-marking.

2.2.  Approaches for PCN-Based Admission Control and Flow Termination

   We briefly review three different approaches to implement PCN-based
   AC and FT and derive their requirements for PCN encoding.

2.2.1.  Dual Marking (DM)

   The intuitive approach for PCN-based AC and FT requires that
   threshold and excess-traffic-marking are simultaneously activated on
   all links of a PCN domain and their reference rate is configured with
   the PCN-admissible-rate (AR) and the PCN-supportable-rate (SR),
   respectively.  Threshold-marking meters all PCN traffic, but re-marks
   only not-marked traffic (NM) to ThM.  Excess-traffic-marking meters
   only non-ETM traffic and re-marks either not-marked (NM) or
   threshold-marked (ThM) PCN traffic to ETM.  Thus, both meters and
   markers need to identify PCN packets and their exact PCN codepoint.
   We call this marking behavior dual marking (DM) and Figure 1
   illustrates all possible re-marking actions.

              NM -----------> ThM
                \             /
                 \           /
                  \         /
                    > ETM <

     Figure 1: PCN Codepoint Re-Marking Diagram for Dual Marking (DM)

   Dual marking is used to support the Controlled-Load PCN (CL-PCN) edge
   behavior [I-D.ietf-pcn-cl-edge-behaviour].  We briefly summarize the
   concept.  All actions are performed on per ingress-egress-aggregate
   basis.  The egress node measures the rate of NM-, ThM-, and ETM-
   traffic in regular intervals and sends them as PCN egress reports to
   the AC and FT decision point.

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   If the proportion of re-marked (ThM- and ETM-) PCN traffic is larger
   than a defined threshold, called CLE-limit, the decision point blocks
   new flow requests until new PCN egress reports are received,
   otherwise it admits them.  With CL-PCN, AC is rather robust with
   regard to the value chosen for the CLE-limit. FT works as follows. If
   the ETM-traffic rate is positive, the decision point triggers the
   ingress node to send a newly measured rate of the sent PCN traffic.
   The decision point calculates the rate of PCN traffic that needs to
   be terminated by:

   termination-rate = PCN-ingress-rate - (rate-of-NM-traffic + rate-of-
   ThM-traffic)

   and terminates an appropriate set of flows.  CL-PCN is accurate
   enough for most application scenarios and its implementation
   complexity is acceptable, therefore, it is a preferred implementation
   option for PCN-based AC and FT.

2.2.2.  Single Marking (SM)

   Single-marking uses only excess-traffic-marking whose reference rate
   is set to the PCN-admissible-rate (AR) on all links of the PCN
   domain.  Figure 2 illustrates all possible re-marking actions.

                NM --------> ETM

    Figure 2: PCN Codepoint Re-Marking Diagram for Single Marking (SM)

   Single marking is used to support the single-marking PCN (SM-PCN)
   edge behavior [I-D.ietf-pcn-sm-edge-behaviour].  We briefly summarize
   the concept.  AC works essentially in the same way as with CL-PCN but
   AC is sensitive to the value of the CLE-limit.  Also FT
   works similarly to CL-PCN. The PCN-supportable-rate (SR) is not
   configured on any link, but is implicitly:


   SR=u*AR

   in the PCN domain using a network-wide constant u.  The decision
   point triggers FT only if the rate-of-NM-traffic * u < rate-of-NM-
   traffic + rate-of-ETM-traffic, requests the PCN-sent-rate from the
   corresponding PCN-ingress-node, calculates the amount of PCN traffic
   to be terminated by

   termination-rate = PCN-sent-rate - rate-of-NM-traffic * u,

   and terminates an appropriate set of flows.

   SM-PCN has two major benefits: it requires only two PCN codepoints
   and only excess-traffic-marking is needed which means that it might
   be earlier to the market than CL-PCN since some chipsets do not yet
   support threshold-marking.

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   However, it only works well when ingress-egress-aggregates have a
   high PCN packet rate which is not always the case.  Otherwise, over-
   admission and over-termination may occur [Menth12] [Menth10q].

2.2.3.  Packet Specific Dual Marking (PSDM)

   Packet-specific dual marking (PSDM) uses threshold-marking and
   excess-traffic-marking whose reference rates are configured with the
   PCN-admissible-rate and the PCN-supportable-rate, respectively.
   There are two different types of not-marked packets: those that are
   subject to threshold-marking (not-ThM) and those that are subject to
   excess-traffic-marking (not-ETM). Both not-Thm and not-ETM have the
   same NM-marking and are distinguished by higher layer information
   (see below). Threshold-marking meters all PCN traffic and re-marks
   only not-ThM packets to PCN-marked (PM).  In contrast, excess-
   traffic-marking meters only not-ETM packets and possibly re-marks
   them to PM, too.  Again, both meters and markers need to identify PCN
   packets and their exact PCN codepoint.  Figure 3 illustrates all
   possible re-marking actions.


            not-ThM        not-ETM
                \            /
                 \          /
                  \        /
                    > PM <


    Figure 3: PCN Codepoint Re-Marking Diagram for Packet Specific Dual
                              Marking (PSDM)

   An edge behavior for PSDM has been presented in [Menth09f].  We call
   it PSDM-PCN.  In contrast to CL-PCN and SM-PCN, AC is realized by re-
   using marked signaling messages for probing. The assumption is that
   admission requests are triggered by an external end-to-end signaling
   protocol, e.g. RSVP (RFC2205). Signaling traffic for a flow is also
   labeled as PCN traffic and if an initial signaling traverses the PCN
   domain and is re-marked, then the corresponding flow is blocked. This
   is a light-weight probing mechanism which does not generate extra
   traffic and does not introduce probing delay [draft-menth-pcn-marked-
   signaling-ac]. In PSDM-PCN, PCN-ingress-nodes label initial signaling
   messages as not-ThM and threshold-marking configured with admissible
   rates possibly re-marks them to PM. Data packets are labeled with
   not-ETM and excess-traffic-marking configured with supportable rates
   possibly re-marks them to PM, too, so that the same algorithms for FT
   may be used as for CL-PCN and SM-PCN.

   Disadvantages of this approach are that every end-to-end signaling
   protocol, e.g. RSVP, needs to be adapted that it denies admission if
   initial request messages are re-marked to PM.
   Advantages are that the AC algorithm is more accurate than the one of
   CL-PCN and SM-PCN [Menth12], that only a single DSCP is needed,
   and that the new tunneling rules in RFC6040 are not needed for
   deployment.

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2.2.4.  Preferential Packet Dropping

   The termination algorithms described in [I-D.ietf-pcn-cl-edge
   behaviour] and [I-D.ietf-pcn-sm-edge-behaviour] require the
   preferential dropping of ETM-marked packets to avoid over-termination
   in the case of packet loss. An analysis explaining this phenomenon
   can be found in Section 4 of [Menth10q]. Thus, preferential dropping
   of ETM-marked packets is "RECOMMENDED" in [RFC5670].  As a
   consequence, droppers must have access to the exact marking
   information of PCN-packets.


3.  Encoding Constraints

   The PCN WG decided to use the DS field (i.e., combination of the DSCP
   and ECN field) for the encoding of the PCN Marks, see [RFC5696].
   This section describes the criteria that are used to compare the
   resulting encoding options described in section 4.

3.1.  Structure of the DS Field

   Figure 4 shows the structure of the DS field.  [RFC0793]
   defined the 8 bit ToS field and [RFC2474] redefined it as DS field.
   It consists of a 6 bit DS codepoint (DSCP, see [RFC2474]) and the 2
   bit ECN field (see [RFC3168]).


           0   1   2   3   4   5   6   7
         +---+---+---+---+---+---+---+---+
         |         DSCP          |  ECN  |
         +---+---+---+---+---+---+---+---+

           DSCP: Differentiated Services codepoint [RFC2474]
           ECN:  ECN field [RFC3168]


                Figure 4: The Structure of the DS Field

3.2.  Constraints from the DSCP

   The Differentiated Services codepoint (DSCP) indicates the per-hop
   behavior (PHB), i.e., the treatment IP packets receive from nodes in
   a DS domain.  Multiple DSCPs may indicate the same PHB.  PCN traffic
   is high-priority traffic and requires a special DSCP that indicate a
   PHB with preferred treatment.

3.2.1.  General Scarcity of DSCPs

   As the number of unused DSCPs is small, PCN encoding should use only
   a single DSCP if possible, in any case not more than two DSCPs.
   Therefore, the DSCP should be used to indicate that traffic is
   subject to PCN metering and marking, but not to differentiate
   different PCN markings.

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3.2.2.  Handling of the DSCP in Tunneling Rules

   PCN encoding must be chosen in such a way that PCN traffic can be
   tunneled within a PCN domain without any impact on PCN metering and
   re-marking.  In the following, the "inner header" refers to the
   header of the encapsulated packet and the "outer header" refers to
   the encapsulating header.

   [RFC2983] provides two tunneling modes for Differentiated Services
   networks.  The uniform model copies the DSCP from the inner header to
   the outer header upon encapsulation and it copies the DSCP from the
   outer header to the inner header upon decapsulation.  This assures
   that changes applied to the DSCP field survive encapsulation and
   decapsulation.  In contrast, the pipe model ignores the content of
   the DSCP field in the outer header upon decapsulation.  Therefore,
   decapsulation erases changes applied to the DSCP along the tunnel.
   As a consequence, only the uniform model may be used for tunneling
   PCN traffic within a PCN domain, if PCN encoding uses more than a
   single DSCP.

3.2.3.  Restoration of Original DSCPs at the Egress Node

   If PCN-marking does not alter the original DSCP, the traffic leaves
   the PCN-domain with its original DSCP.
   However, if the PCN-marking alters the DSCP, then some additional
   technique is needed to restore the original DSCP. A few possibilities
   are discussed:

   1. Each Diffserv class using PCN uses a different set of DSCPs.
      Therefore, if there are M DSCPs using PCN and PCN encoding uses N
      different DSCPs, N*M DSCPs are needed. This solution may work well
      in IP networks. However, when PCN is applied to MPLS networks or
      other layers restricted to 8 QoS classes and codepoints, this
      solution fails due to the extreme shortage of available DSCPs.

   2. The original DSCP for the packets of a flow is signaled to the
      egress node. No suitable signaling protocol has been developed and
      therefore, it is not clear whether this approach could work.

   3. PCN-traffic is tunneled across the PCN-domain. The pipe tunneling
      model is applied and so the original DSCP is restored after
      decapsulation. However, tunneling across a PCN domain adds an
      additional IP header and reduces the maximum transfer unit (MTU)
      from the perspective of the user. GRE, MPLS, or Ethernet using
      Pseudo-Wires are potential solutions that scale well also in
      backbone networks.

   The most appropriate option depends on the specific circumstances an
   operator faces.

      o) Option 1 is most suitable unless there is a shortage of
         available DSCPs.

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      o) Option 3 is suitable where the reduction of MTU is not liable
         to cause issues.

3.3.  Constraints from the ECN Field

   This section briefly reviews the structure and use of the ECN field.
   The ECN field may be redefined, but certain constraints must be met
   [RFC4774]. The impact on PCN deployment is discussed, as well as the
   constraints imposed by various tunneling rules on the persistence of
   PCN marks after decapsulation and its impact on possible re-marking
   actions.

3.3.1.  Structure and Use of the ECN Field

   Some transport protocols, like TCP, can typically use packet drops as
   an indication of congestion in the Internet. The idea of Explicit
   Congestion Notification (ECN) [RFC3168] is that routers provide a
   congestion indication for incipient congestion, where the
   notification can sometimes be through ECN marking (and re-marking)
   packets rather than dropping them. Figure 5 summarizes the ECN
   codepoints defined [RFC3168].


              +-----+-----+
              | ECN FIELD |
              +-----+-----+
                 0     0         Not-ECT
                 0     1         ECT(1)
                 1     0         ECT(0)
                 1     1         CE


               Figure 5: ECN Codepoints within the ECN field

   ECT stands for "ECN-capable transport" and indicates that the sender
   and receivers of a flow understand ECN semantics.  Packets of other
   flows are labeled with not-ECT.  To indicate congestion to a
   receiver, routers may re-mark ECT(1) or ECT(0) labeled packets to CE
   which stands for "congestion experienced".  Two different ECT
   codepoints were introduced "to protect against accidental or
   malicious concealment of marked packets from the TCP sender" which
   may be the case with cheating receivers [RFC3540].

3.3.2. Redefinition of the ECN Field

   The ECN field may be redefined for other purposes and [RFC4774] gives
   guidelines for that. Essentially, not-ECT-marked packets must never
   be re-marked to ECT or CE because not-ECT-capable end systems do not
   reduce their transmission rate when receiving CE-marked packets.
   This is a threat to the stability of the Internet.



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   Moreover, CE-marked packets must not be re-marked to not-ECT or ECT,
   because then ECN-capable end systems cannot reduce their transmission
   rate. The re-use of the ECN field for PCN encoding has some impact on
   the deployment of PCN. First, routers within a PCN domain must not
   apply ECN re-marking when the ECN field has PCN semantics.
   Second, before a PCN packet leaves the PCN domain, the egress nodes
   must either (A) reset the ECN field of the packet to the contents it
   had when entering the PCN domain or (B) reset its ECN field to not-
   ECT. According to Section 3.3.3, tunneling ECN traffic through a PCN
   domain may help to implement (A). When (B) applies, CE-marked
   packets must never become PCN packets within a PCN domain as the
   egress node resets their ECN field to not-ECT. The ingress node may
   drop such traffic instead.

3.3.3.  Handling of the ECN Field in Tunneling Rules

   When packets are encapsulated, the ECN field of the inner header may
   or may not be copied to the ECN field of the outer header and upon
   decapsulation, the ECN field of the outer header may or may not be
   copied from the ECN field of the outer header to the ECN field of the
   inner header.  Various tunneling rules with different treatment of
   the ECN field exist. Two different modes are defined in [RFC3168]
   for IP-in-IP tunnels and a third one in [RFC4301] for IP-in-IPsec
   tunnels. [RFC6040] updates both these RFCs to rationalize them
   into one consistent approach.

3.3.3.1.  Limited Functionality Option

   The limited-functionality option has been defined in [RFC3168].  Upon

   encapsulation, the ECN field of the outer header is generally set to
   not-ECT.  Upon decapsulation, the ECN field of the inner header
   remains unchanged.

   Since this tunneling mode loses information upon encapsulation and
   decapsulation, it cannot be used for tunneling PCN traffic within a
   PCN domain.  However, the PCN ingress may use this mode to tunnel
   traffic with ECN semantics to the PCN egress to preserve the ECN
   field in the inner header while the ECN field of the outer header is
   used with PCN semantics within the PCN domain.

3.3.3.2.  Full Functionality Option

   The full-functionality option has been defined in [RFC3168].  Upon
   encapsulation, the ECN field of the inner header is copied to the
   outer header unless the ECN field of the inner header carries CE.  In
   that case, the ECN field of the outer header is set to ECT(0).  This
   choice has been made for security reasons, to disable the ECN fields
   of the outer header as a covert channel.  Upon decapsulation, the ECN
   field of the inner header remains unchanged unless the ECN field of
   the outer header carries CE.  In that case, the ECN field of the
   inner header is also set to CE.


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   This mode imposes the following constraints on PCN metering and
   marking.  First, PCN must re-mark the ECN field only to CE because
   any other information is not copied to the inner header upon
   decapsulation and will be lost.  Second, CE information in
   encapsulated packet headers is invisible for routers along a tunnel.
   Threshold marking does not require information about whether PCN
   packets have already been marked and would work when CE denotes that
   packets are marked.  In contrast, excess-traffic- marking requires
   information about already excess-traffic-marked packets and cannot be
   supported with this tunneling mode.  Furthermore, this tunneling mode
   cannot be used when marked or not-marked packets should be
   preferentially dropped because the PCN marking information is
   possibly not visible in the outer header of a packet.

3.3.3.3.  Tunneling with IPSec

   Tunneling has been defined in Section 5.1.2.1 of [RFC4301].  Upon
   encapsulation, the ECN field of the inner header is copied to the ECN
   field of the outer header.  Decapsulation works as for the full-
   functionality option in Section 3.3.3.2.  Tunneling with IPsec also
   requires that PCN re-marks the ECN field only to CE because any other
   information is not copied to the inner header upon decapsulation and
   lost.  In contrast to Section 3.3.3.2, with IPsec tunnels, CE marks
   of tunneled PCN traffic remain visible for routers along the tunnel
   and to their meters, markers, and droppers.

3.3.3.4.  ECN Tunneling

   New tunneling rules for ECN are specified in [RFC6040], which
   updates [RFC3168] and [RFC4301]. These rules provide a consistent and
   rational approach to encapsulation and decapsulation.

   With the normal mode, the ECN field of the inner header is copied to
   the ECN field of the outer header on encapsulation.
   In compatibility mode, the ECN field of the outer header is reset to
   not-ECT.

   Upon decapsulation, the scheme specified in [RFC6040] and shown in
   Figure 6 is applied.  Thus, re-marking encapsulated not-ECT packets
   to any other codepoint would not survive decapsulation.  Therefore,
   not-ECT cannot be used for PCN encoding.  Furthermore, re-marking
   encapsulated ECT(0) packets to ECT(1) or CE survives decapsulation,
   but not vice-versa, and re-marking encapsulated ECT(1) packets to CE
   also survives decapsulation, but not vice-versa.
   Certain combinations of inner and outer ECN fields cannot result from
   any transition in any current or previous ECN tunneling
   specification.  These currently unused (CU) combinations are
   indicated in Figure 6 by '(!!!)' or '(!)', where '(!!!)' means the
   combination is CU and always potentially dangerous, while '(!)' means
   it is CU and possibly dangerous.




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            +---------+------------------------------------------------+
            |Arriving |            Arriving Outer Header               |
            |   Inner +---------+------------+------------+------------+
            |  Header | Not-ECT | ECT(0)     | ECT(1)     |     CE     |
            +---------+---------+------------+------------+------------+
            | Not-ECT | Not-ECT |Not-ECT(!!!)|Not-ECT(!!!)| <drop>(!!!)|
            |  ECT(0) |  ECT(0) | ECT(0)     | ECT(1)     |     CE     |
            |  ECT(1) |  ECT(1) | ECT(1) (!) | ECT(1)     |     CE     |
            |    CE   |      CE |     CE     |     CE(!!!)|     CE     |
            +---------+---------+------------+------------+------------+


    The ECN field in the outgoing header is set to the codepoint at the
    intersection of the appropriate arriving inner header (row) and
    arriving outer header (column), or the packet is dropped where
    indicated.  Currently unused combinations are indicated by '(!!!)'
    or '(!)'. ([RFC6040]: '(!!!)' means the combination is CU and always
    potentially dangerous, while '(!)' means it is CU and possibly
    dangerous.)

      Figure 6: New IP in IP Decapsulation Behavior (from [RFC6040])

3.3.4.  Restoration of the Original ECN Field at the PCN-Egress-Node

   As ECN is an end-to-end service, it is desirable that the egress node
   of a PCN domain restores the ECN field a PCN packet had at the
   ingress node.  There are basically two options.  PCN traffic may be
   tunneled between ingress and egress node using limited functionality
   tunnels (see Section 3.3.3.1).  Then, PCN marking is applied only to
   the outer header, and the original ECN field is restored after
   decapsulation.  However, this reduces the MTU from the perspective of
   the user.  Another option is to use some intelligent encoding that
   preserves the ECN codepoints.  However, a viable solution is not
   known.

4.  Comparison of Encoding Options

   The PCN WG has studied four different PCN encodings, which redefine
   the ECN field. Figure 7 summarizes these PCN encodings. One or at
   most two different DSCPs are used to indicate PCN traffic, and only
   for these DSCPs the semantics of the ECN field are redefined within
   the PCN domain.
   When a PCN-ingress-node classifies a packet as a PCN-packet it sets
   its PCN-codepoint to not-marked (NM). Non-PCN traffic can also to be
   sent with the PCN-specific DSCP, by setting the Not-PCN codepoint.
   Special per hop behavior, defined in [RFC5670], applies to PCN-
   traffic.







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 -----------------------------------------------------------------------
| ECN Bits     ||    00    |    10    |    01    |    11    ||   DSCP  |
|==============++==========+==========+==========+==========++=========|
| RFC 3168     || Not-ECT  |  ECT(0)  |  ECT(1)  |    CE    ||   Any   |
|==============++==========+==========+==========+==========++=========|
| Baseline     || Not-PCN  |    NM    |   EXP    |    PM    ||   PCN-n |
|==============++==========+==========+==========+==========++=========|
| 3-In-1       || Not-PCN  |    NM    |   ThM    |   ETM    ||   PCN-n |
|==============++==========+==========+==========+==========++=========|
| 3-In-2       || Not-PCN  |    NM    |    CU    |   ThM    ||   PCN-n |
|              ||----------+----------+----------+----------++---------|
|              || Not-PCN  |    CU    |    CU    |   ETM    ||   PCN-m |
|==============++==========+==========+==========+==========++=========|
| PSDM         || Not-PCN  |  Not-ETM |  Not-ThM |    PM    ||   PCN-n |
 -----------------------------------------------------------------------

   Notes: PCN-n, PCN-m under the DSCP column denotes PCN compatible
   DSCPs which may be chosen by the network operator. Not-PCN means that
   packets are not PCN-enabled. NM means Not-Marked to signal a not-pre-
   congested path. CU means Currently Unused.

      Figure 7: Semantics of the ECN field for various encoding types

4.1.  Baseline Encoding

   With baseline encoding [RFC5696], the NM codepoint can be re-marked
   only to PCN-marked (PM).  Excess-traffic-marking uses PM as ETM,
   threshold-marking uses PM as ThM, and only one of the two marking
   schemes can be used.

   The 01-codepoint is reserved for experimental purposes (EXP) and the
   other defined PCN encoding schemes can be seen as extensions of
   baseline encoding by appropriate redefinition of EXP.  Baseline
   encoding [RFC5696] works well with IPsec tunnels (see Section
   3.3.3.3).

4.2.  Encoding with 1 DSCP Providing 3 States

   PCN 3-state encoding extension in a single DSCP (3-in-1 encoding,
   [I-D.ietf-pcn-3-in-1-encoding] extends the baseline encoding and
   supports the simultaneous use of both excess-traffic-marking and
   threshold-marking. 3-in-1 encoding well supports the preferred CL-PCN
   and also SM-PCN.

   The problem with 3-in-1 encoding is that the 10-codepoint does not
   survive decapsulation with the tunneling options in Section 3.3.3.1 -
   3.3.3.3.  Therefore, 3-in-1 encoding may be used only for PCN domains
   implementing the new rules for ECN tunneling [RFC6040], see Section
   3.3.3.4), or where it is known that there are no tunnels in the PCN
   domain.  Currently it is not clear how fast the new tunneling rules
   will be deployed, but the applicability of 3-in-1-encoding depends on
   that.

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4.3.  Encoding with 2 DSCPs Providing 3 or More States

   PCN encoding using 2 DSCPs to provide 3 or more states (3-in-2
   encoding, [I-D.ietf-pcn-3-state-encoding] uses two different DSCPs to
   accommodate the three required codepoints NM, ThM, and ETM.  It
   leaves some codepoints currently unused (CU) and proposes also one
   way how to reuse them to store some information about the content of
   the ECN field before the packet entered the PCN domain. 3-in-2
   encoding works well with IPsec tunnels (see Section 3.3.3.3). This
   type of encoding can support both CL-PCN and SM-PCN schemes.

   The disadvantage of 3-in-2 encoding is that it consumes two DSCPs.
   Moreover, the direct application of this encoding scheme to other
   technologies like MPLS, where even fewer bits are available for the
   encoding of DSCPs is more difficult.

4.4.  Encoding for Packet Specific Dual Marking (PSDM)

   PCN encoding for packet-specific dual marking (PSDM) is designed to
   support PSDM-PCN outlined in Section 2.2.3.  It is the only proposal
   that supports PCN-based AC and FT with only a single DSCP
   [I-D.ietf-pcn-psdm-encoding] in the presence of IPsec tunnels (see
   Section 3.3.3.3).  PSDM encoding also supports SM-PCN.

4.5. Standardized encodings

   The baseline encoding described in section 4.1 was published as a
   draft Internet Standard [RFC5696]. The intention was to allow for
   experimental encodings to build upon this baseline. However,
   following the publication of [RFC6040], the WG decided to change
   approach and instead standardize only one encoding (the 3-in-1
   encoding described in 4.2 [I-D.ietf-pcn-3-in-1-encoding]. Rather than
   defining the 3-in-1 encoding as a standards track extension to the
   existing baseline encoding [RFC5696], it was agreed that it was best
   to define a new standards track document that obsoletes [RFC5696].


5.  Conclusion

   This document summarizes the PCN Working Group's exploration of a
   number of approaches for encoding pre-congestion information into the
   IP header. It is presented as an informational archive. It provides
   details of all those approaches along with an explanation of the
   constraints that have to be met. The Working Group has concluded that
   the "3-in-1" encoding should be published as a standards-track RFC
   that obsoletes the encoding specified in [RFC5696].
   The reasoning is as follows. During the early life of the working
   group, we decided on an approach of a standardized "baseline
   encoding" [RFC5696] plus a series of experimental encodings that
   would all build on the baseline encoding and each of which would be
   useful in specific circumstances. However, after the tunneling of
   ECN was standardized in [RFC6040], the PCN WG decided on a different
   approach - to recommend just one encoding, the "3-in-1 encoding".

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   Although in theory "3-in-1" could be specified as a standards-track
   extension to the "baseline" encoding, the WG decided that it would be
   cleaner to obsolete [RFC5696] and specify "3-in-1" encoding in a new
   stand-alone RFC.


6.  Security Implications

   [RFC5559] provides a general description of the security
   considerations for PCN. This memo does not introduce additional
   security considerations.


7.  IANA Considerations

   This memo includes no request to IANA.

8.  Acknowledgements

   We would like to acknowledge the members of the PCN working group and
   Gorry Fairhust for the discussions that generated and improved the
   contents of this memo.


9.  References

9.1. Normative References

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, September 1981.

   [RFC2474]  Nichols, K., Blake, S., Baker, F., and D. Black,
              "Definition of the Differentiated Services Field (DS
              Field) in the IPv4 and IPv6 Headers", RFC 2474,
              December 1998.

   [RFC3168]  Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
              of Explicit Congestion Notification (ECN) to IP",
              RFC 3168, September 2001.

   [RFC4774]  Floyd, S., "Specifying Alternate Semantics for the
              Explicit Congestion Notification (ECN) Field", BCP 124,
              RFC 4774, November 2006.


9.2. Informative References

   [I-D.ietf-pcn-cl-edge-behaviour]
              Charny, A., Huang, F., Karagiannis, G., Menth, M., and T.
              Taylor, "PCN Boundary Node Behaviour for the Controlled
              Load (CL) Mode of Operation",
              draft-ietf-pcn-cl-edge-behaviour-12 (work in progress),
              February 2012.

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   [I-D.ietf-pcn-sm-edge-behaviour]
              Charny, A., Karagiannis, G., Menth, M., and T. Taylor,
              "PCN Boundary Node Behaviour for the Single Marking (SM)
              Mode of Operation", draft-ietf-pcn-sm-edge-behaviour-09
              (work in progress), February 2012.

   [I-D.ietf-pcn-3-in-1-encoding]
              Briscoe, B., Moncaster, T., and M. Menth, "Encoding 3 PCN-
              States in the IP header using a single DSCP",
              draft-ietf-pcn-3-in-1-encoding-08 (work in progress),
              August 2011.

   [I-D.ietf-pcn-3-state-encoding]
              Briscoe, B., Moncaster, T., and M. Menth, "A PCN encoding
              using 2 DSCPs to provide 3 or more states",
              draft-ietf-pcn-3-state-encoding-01 (work in progress),
              February 2010.

   [I-D.ietf-pcn-psdm-encoding]
              Menth, M., Babiarz, J., Moncaster, T., and B. Briscoe,
              "PCN Encoding for Packet-Specific Dual Marking (PSDM
              Encoding)", draft-ietf-pcn-psdm-encoding-01 (work in
              progress), March 2010.

   [RFC6040]  Briscoe, B., "Tunnelling of Explicit Congestion
              Notification", RFC 6040, November 2010.

   [RFC2983]  Black, D., "Differentiated Services and Tunnels",
              RFC 2983, October 2000.

   [RFC3540]  Spring, N., Wetherall, D., and D. Ely, "Robust Explicit
              Congestion Notification (ECN) Signaling with Nonces",
              RFC 3540, June 2003.

   [RFC4301]  Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", RFC 4301, December 2005.

   [RFC5559]  Eardley, P., "Pre-Congestion Notification (PCN)
              Architecture", RFC 5559, June 2009.

   [RFC5670]  Eardley, P., "Metering and Marking Behaviour of PCN-
              Nodes", RFC 5670, November 2009.

   [RFC5696]  Moncaster, T., Briscoe, B., and M. Menth, "Baseline
              Encoding and Transport of Pre-Congestion Information",
              RFC 5696, November 2009.

   [Menth09f]
              Menth, M., Babiarz, J., and P. Eardley, "Pre-Congestion
              Notification Using Packet-Specific Dual Marking", IEEE
              Proceedings of the International Workshop on the Network
              of the Future (Future-Net) at Dresden Germany, June 2009.

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  [Menth12]
              Menth, M. and F. Lehrieder, " Performance of PCN-Based
              Admission Control under Challenging Conditions", accepted
              for publication IEEE/ACM Transactions on Networking in
              2012.

   [Menth10q]
              Menth, M. and F. Lehrieder, "PCN-Based Measured Rate
              Termination", Computer Networks Journal, vol. 54, no. 3,
              Sept. 2010


Authors' Addresses

   Georgios Karagiannis
   University of Twente
   P.O. Box 217
   7500 AE Enschede,
   The Netherlands

   Email: g.karagiannis@utwente.nl


   Kwok Ho Chan
   Consultant

   Email: khchan.work@gmail.com

   Toby Moncaster
   University of Cambridge Computer Laboratory,
   William Gates Building, J J Thomson Avenue,
   Cambridge, CB3 0FD.

   Email Toby.Moncaster@cl.cam.ac.uk

   Michael Menth
   Chair of Communication Networks
   University of Tuebingen
   Sand 13
   72076 Tuebingen
   Germany

   Email: menth@informatik.uni-tuebingen.de

   Philip Eardley
   BT
   B54/77, Sirius House Adastral Park Martlesham Heath
   Ipswich, Suffolk  IP5 3RE
   United Kingdom

   Email: philip.eardley@bt.com


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   Bob Briscoe
   BT
   B54/77, Sirius House Adastral Park Martlesham Heath
   Ipswich, Suffolk  IP5 3RE
   United Kingdom

   Email: bob.briscoe@bt.com















































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