A Lower Effort Per-Hop Behavior (LE PHB)
draft-ietf-tsvwg-le-phb-02

Internet Engineering Task Force                                 R. Bless
Internet-Draft                   Karlsruhe Institute of Technology (KIT)
Obsoletes: 3662 (if approved)                              June 30, 2017
Updates: 4594 (if approved)
Intended status: Standards Track
Expires: January 1, 2018

                A Lower Effort Per-Hop Behavior (LE PHB)
                       draft-ietf-tsvwg-le-phb-02

Abstract

   This document specifies properties and characteristics of a Lower
   Effort (LE) per-hop behavior (PHB).  The primary objective of this LE
   PHB is to protect best-effort (BE) traffic (packets forwarded with
   the default PHB) from LE traffic in congestion situations, i.e., when
   resources become scarce, best-effort traffic has precedence over LE
   traffic and may preempt it.  There are numerous uses for this PHB,
   e.g., for background traffic of low precedence, such as bulk data
   transfers with low priority in time, non time-critical backups,
   larger software updates, web search engines while gathering
   information from web servers and so on.  This document recommends a
   standard DSCP value for the LE PHB.

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
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Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Applicability . . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Deployment Considerations . . . . . . . . . . . . . . . .   5
     1.3.  Requirements Language . . . . . . . . . . . . . . . . . .   6
   2.  PHB Description . . . . . . . . . . . . . . . . . . . . . . .   6
   3.  Traffic Conditioning Actions  . . . . . . . . . . . . . . . .   7
   4.  Recommended DS Codepoint  . . . . . . . . . . . . . . . . . .   7
   5.  Remarking to other DSCPs/PHBs . . . . . . . . . . . . . . . .   7
   6.  Changes to RFC 4594 . . . . . . . . . . . . . . . . . . . . .   8
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  10
   Appendix A.  History of the LE PHB  . . . . . . . . . . . . . . .  11
   Appendix B.  Acknowledgments  . . . . . . . . . . . . . . . . . .  11
   Appendix C.  Change History . . . . . . . . . . . . . . . . . . .  11
   Appendix D.  Note to RFC Editor . . . . . . . . . . . . . . . . .  12
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  12

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1.  Introduction

   This document defines a Differentiated Services per-hop behavior
   [RFC2474] called "Lower Effort" (LE) which is intended for traffic of
   sufficiently low urgency that all other traffic takes precedence over
   LE traffic in consumption of network link bandwidth.  Low urgency
   traffic has a low priority for timely forwarding, which does not
   necessarily imply that it is generally of minor importance.  From
   this viewpoint, it can be considered as a network equivalent to a
   background priority for processes in an operating system.  There may
   or may not be memory (buffer) resources allocated for this type of
   traffic.

   Some networks carry traffic for which delivery is considered
   optional; that is, packets of this type of traffic ought to consume
   network resources only when no other traffic is present.
   Alternatively, the effect of this type of traffic on all other
   network traffic is strictly limited ("no harm" property).  This is
   distinct from "best- effort" (BE) traffic since the network makes no
   commitment to deliver LE packets.  In contrast, BE traffic receives
   an implied "good faith" commitment of at least some available network
   resources.  This document proposes a Lower Effort Differentiated
   Services per-hop behavior (LE PHB) for handling this "optional"
   traffic in a differentiated services node.

1.1.  Applicability

   A Lower Effort PHB is applicable for many applications that otherwise
   use best-effort delivery.  More specifically, it is suitable for
   traffic and services that can tolerate strongly varying throughput
   for their data flows, especially periods of very low throughput or
   even starvation (i.e., long interruptions due to significant or even
   complete packet loss).  Therefore, an application sending an LE
   marked flow must be able to tolerate short or (even very) long
   interruptions due to the presence of severe congestion conditions
   during the transmission of the flow.  Thus, there should be an
   expectation that packets of the LE PHB may be excessively delayed or
   dropped when any other traffic is present.  The LE PHB is suitable
   for sending traffic of low urgency across a Differentiated Services
   (DS) domain or DS region.

   LE traffic SHOULD be congestion controlled.  Since LE traffic may be
   starved completely for a longer period of time, transport protocols
   or applications (and their related congestion control mechanisms)
   SHOULD be able to detect and react to such a situation and should
   resume the transfer as soon as possible.  Congestion control is not
   only useful to let the flows within the LE behavior aggregate adapt
   to the available bandwidth that may be highly fluctuating, but also

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   in case that LE traffic is mapped to the default PHB in DS domains
   that do not support LE.

   Use of the LE PHB might assist a network operator in moving certain
   kinds of traffic or users to off-peak times.  Alternatively, or in
   addition, packets can be designated for the LE PHB when the goal is
   to protect all other packet traffic from competition with the LE
   aggregate while not completely banning LE traffic from the network.
   An LE PHB SHOULD NOT be used for a customer's "normal internet"
   traffic nor should packets be "downgraded" to the LE PHB instead of
   being dropped, particularly when the packets are unauthorized
   traffic.  The LE PHB is expected to have applicability in networks
   that have at least some unused capacity at certain periods.

   The LE PHB allows networks to protect themselves from selected types
   of traffic as a complement to giving preferential treatment to other
   selected traffic aggregates.  LE should not be used for the general
   case of downgraded traffic, but may be used by design, e.g., to
   protect an internal network from untrusted external traffic sources.
   In this case there is no way for attackers to preempt internal (non
   LE) traffic by flooding.  Another use case in this regard is
   forwarding of multicast traffic from untrusted sources.  Multicast
   forwarding is currently enabled within domains only for specific
   sources within a domain, but not for sources from anywhere in the
   Internet.  A main problem is that multicast routing creates traffic
   sources at (mostly) unpredictable branching points within a domain,
   potentially leading to congestion and packet loss.  In case multicast
   packets from untrusted sources are forwarded as LE traffic, they will
   not harm traffic from non-LE behavior aggregates.  A further related
   use case is mentioned in [RFC3754]: preliminary forwarding of non-
   admitted multicast traffic.

   There is no intrinsic reason to limit the applicability of the LE PHB
   to any particular application or type of traffic.  It is intended as
   an additional traffic engineering tool for network administrators.
   For instance, it can be used to fill protection capacity of
   transmission links that is otherwise unused.  Some network providers
   keep link utilization below 50% to ensure that all traffic is
   forwarded without loss after rerouting caused by a link failure.  LE
   marked traffic can utilize the normally unused capacity and will be
   preempted automatically in case of link failure when 100% of the link
   capacity is required for all other traffic.  Ideally, applications
   mark their packets as LE traffic, since they know the urgency of
   flows.

   Example uses for the LE PHB:

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   o  For traffic caused by world-wide web search engines while they
      gather information from web servers.

   o  For software updates or dissemination of new releases of operating
      systems.

   o  For backup traffic or non-time critical synchronization or
      mirroring traffic.

   o  For content distribution transfers between caches.

   o  For preloading or prefetching objects from web sites.

   o  For Netnews and other "bulk mail" of the Internet.

   o  For "downgraded" traffic from some other PHB when this does not
      violate the operational objectives of the other PHB or the overall
      network.

   o  For multicast traffic from untrusted (e.g., non-local) sources.

1.2.  Deployment Considerations

   In order to enable LE support, DS nodes typically only need

   o  A BA classifier (Behavior Aggregate classifier, see [RFC2475])
      that classifies packets according to the LE DSCP

   o  A dedicated LE queue

   o  A suitable scheduling discipline, e.g., simple priority queueing

   Alternatively, implementations may use active queue management
   mechanisms instead of a dedicated LE queue, e.g., dropping all
   arriving LE packets when certain queue length or sojourn time
   thresholds are exceeded.

   Internet-wide deployment of the LE PHB is eased by the following
   properties:

   o  No harm to other traffic: since the LE PHB has the lowest
      forwarding priority it does not consume resources from other PHBs.
      Deployment across different provider domains with LE support
      causes no trust issues or attack vectors to existing (non LE)
      traffic.  Thus, providers can trust LE markings from end-systems,
      i.e., there is no need to police or remark incoming LE traffic.

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   o  No PHB parameters or configuration of traffic profiles: the LE PHB
      itself possesses no parameters that need to be set or configured.
      Similarly, since LE traffic requires no admission or policing, it
      is not necessary to configure traffic profiles.

   o  No traffic conditioning mechanisms: the LE PHB requires no traffic
      meters, droppers, or shapers.  See also Section 3 for further
      discussion.

   DS domains that cannot or do not want to support the LE PHB should be
   aware that they violate the "no harm" property of LE.  DS domains
   without LE PHB support SHOULD NOT drop LE marked packets, but rather
   map them to the default PHB and keep the LE DSCP.  See also Section 5
   for further discussion of forwarding LE traffic with the default PHB
   instead.

1.3.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

2.  PHB Description

   The LE PHB is defined in relation to the default PHB (best-effort).
   A packet forwarded with the LE PHB SHOULD have lower precedence than
   packets forwarded with the default PHB, i.e., in case of congestion,
   LE marked traffic SHOULD be dropped prior to dropping any default PHB
   traffic.  Ideally, LE packets SHOULD be forwarded only if no packet
   with any other PHB is awaiting transmission.

   A straightforward implementation could be a simple priority scheduler
   serving the default PHB queue with higher priority than the lower-
   effort PHB queue.  Alternative implementations may use scheduling
   algorithms that assign a very small weight to the LE class.  This,
   however, may sometimes cause better service for LE packets compared
   to BE packets in cases when the BE share is fully utilized and the LE
   share not.

   If a dedicated LE queue is not available, an active queue management
   mechanism within a common BE/LE queue could also be used.  This could
   drop all arriving LE packets as soon as certain queue length or
   sojourn time thresholds are exceeded.

   Since congestion control is also useful within the LE traffic class,
   Explicit Congestion Notification [RFC3168] SHOULD be used for LE
   packets, too.

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3.  Traffic Conditioning Actions

   If possible, packets SHOULD be pre-marked in DS-aware end systems by
   applications due to their specific knowledge about the particular
   precedence of packets.  There is no incentive for DS domains to
   distrust this initial marking, because letting LE traffic enter a DS
   domain causes no harm.  Thus, any policing such as limiting the rate
   of LE traffic is not necessary at the DS boundary.

   As for most other PHBs an initial classification and marking can be
   also performed at the first DS boundary node according to the DS
   domain's own policies (e.g., as protection measure against untrusted
   sources).  However, non-LE traffic (e.g., BE traffic) SHOULD NOT be
   remarked to LE on a regular basis without consent or knowledge of the
   user.  See also remarks with respect to downgrading in Section 1.1.

4.  Recommended DS Codepoint

   The RECOMMENDED codepoint for the LE PHB is '000010'.

   Earlier specifications [RFC4594] recommended to use CS1 as codepoint
   (as mentioned in [RFC3662]).  This is problematic since it may cause
   a priority inversion in DiffServ domains that treat CS1 as originally
   proposed in [RFC2474], resulting in forwarding LE packets with higher
   precedence than BE packets.  Existing implementations SHOULD
   therefore use the unambiguous LE codepoint '000010' whenever
   possible.

5.  Remarking to other DSCPs/PHBs

   "DSCP bleaching", i.e., setting the DSCP to '000000' (default PHB) is
   NOT RECOMMENDED for this PHB.  This may cause effects that are in
   contrast to the original intent in protecting BE traffic from LE
   traffic (no harm property).  In case DS domains do not support the LE
   PHB, they SHOULD treat LE marked packets with the default PHB instead
   (by mapping the LE DSCP to the default PHB), but they SHOULD do so
   without remarking to DSCP '000000'.  The reason for this is that
   later traversed DS domains may then have still the possibility to
   treat such packets according the LE PHB.  However, operators of DS
   domains that forward LE traffic within the BE aggregate should be
   aware of the implications, i.e., induced congestion situations and
   quality-of-service degradation of the original BE traffic.  In this
   case, the LE property of not harming other traffic is no longer
   fulfilled.  In order to limit the impact in such cases, traffic
   policing of the LE aggregate may be used.

   In case LE marked packets are effectively carried within the default
   PHB (i.e., forwarded as best-effort traffic) they get a better

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   forwarding treatment than expected.  For some applications and
   services, it is favorable if the transmission is finished earlier
   than expected.  However, in some cases it may be against the original
   intention of the LE PHB user to strictly send the traffic only if
   otherwise unused resources are available, i.e., LE traffic may
   compete with BE traffic for the same resources and thus adversely
   affect the original BE aggregate.  In some cases users want to be
   sure that their LE marked traffic actually fulfills the "no harm"
   property.

   One possible solution for a clear distinction in such cases would be
   to use two different codepoints, "LE-min = LE, better treatment
   allowed", "LE-strict = LE, better treatment NOT allowed".  However,
   since DSCPs are a scarce resource, applications that want to ensure
   the lower precedence compared to BE traffic SHOULD use additionally a
   corresponding Lower-than-Best-Effort transport protocol [RFC6297],
   e.g., LEDBAT [RFC6817].

   A DS domain that still uses DSCP CS1 for marking LE traffic
   (including Low Priority-Data as defined in [RFC4594] or the old
   definition in [RFC3662]) MUST remark traffic to the LE DSCP '000010'
   at the egress to the next DS domain.  This increases the probability
   that the DSCP is preserved end-to-end, whereas a CS1 marked packet
   may be remarked by the default DSCP if the next domain is applying
   DiffServ-intercon [RFC8100].

6.  Changes to RFC 4594

   [RFC4594] recommended to use CS1 as codepoint in section 4.10,
   whereas CS1 was defined in [RFC2474] to have a higher precedence than
   CS0, i.e., the default PHB.  Consequently, DiffServ domains
   implementing CS1 according to [RFC2474] will cause a priority
   inversion for LE packets that contradicts with the original purpose
   of LE.  Therefore, every occurrence of the CS1 DSCP is replaced by
   the LE DSCP.

   Changes:

   o  The Low-Priority Data row in Figure 3 is updated as follows:

    |---------------+---------+-------------+--------------------------|
    | Low-Priority  |   LE    |   000010    | Any flow that has no BW  |
    |     Data      |         |             | assurance                |
     ------------------------------------------------------------------

   o  The Low-Priority Data row in Figure 4 is updated as follows:

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    |---------------+------+-------------------+---------+--------+----|
    | Low-Priority  | LE   | Not applicable    | RFCXXXX |  Rate  | Yes|
    |     Data      |      |                   |         |        |    |
     ------------------------------------------------------------------

   o  Section 4.10: The RECOMMENDED DSCP marking is LE (Lower Effort).

   o  [RFC4594] recommended to remark Low-Priority Data to DSCP '000001'
      inside a DS domain that uses IP precedence marking.  By using the
      herein defined LE DSCP such remarking is not necessary, so even if
      Low-Priority Data is unsupported (i.e., mapped to the default PHB)
      the LE DSCP should be kept across the domain as RECOMMENDED in
      Section 5.

7.  IANA Considerations

   This document assigns the Differentiated Services Field Codepoint
   (DSCP) '000010' from the Differentiated Services Field Codepoints
   (DSCP) registry (https://www.iana.org/assignments/dscp-registry/dscp-
   registry.xml) to the LE PHB.  IANA is requested to update the
   registry as follows:

   o  Name: LE

   o  Value (Binary): 000010

   o  Value (Decimal): 2

   o  Reference: [RFC number of this memo]

8.  Security Considerations

   There are no specific security exposures for this PHB.  Since it
   defines a new class of low forwarding priority, remarking other
   traffic as LE traffic may lead to quality-of-service degradation of
   such traffic.  Thus, any attacker that is able to modify the DSCP of
   a packet to LE may carry out a downgrade attack.  See the general
   security considerations in [RFC2474] and [RFC2475].

9.  References

9.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

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   [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,
              DOI 10.17487/RFC2474, December 1998,
              <http://www.rfc-editor.org/info/rfc2474>.

   [RFC2475]  Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.,
              and W. Weiss, "An Architecture for Differentiated
              Services", RFC 2475, DOI 10.17487/RFC2475, December 1998,
              <http://www.rfc-editor.org/info/rfc2475>.

9.2.  Informative References

   [draft-bless-diffserv-lbe-phb-00]
              Bless, R. and K. Wehrle, "A Lower Than Best-Effort Per-Hop
              Behavior", draft-bless-diffserv-lbe-phb-00 (work in
              progress), September 1999, <https://tools.ietf.org/html/
              draft-bless-diffserv-lbe-phb-00>.

   [RFC3168]  Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
              of Explicit Congestion Notification (ECN) to IP",
              RFC 3168, DOI 10.17487/RFC3168, September 2001,
              <http://www.rfc-editor.org/info/rfc3168>.

   [RFC3662]  Bless, R., Nichols, K., and K. Wehrle, "A Lower Effort
              Per-Domain Behavior (PDB) for Differentiated Services",
              RFC 3662, DOI 10.17487/RFC3662, December 2003,
              <http://www.rfc-editor.org/info/rfc3662>.

   [RFC3754]  Bless, R. and K. Wehrle, "IP Multicast in Differentiated
              Services (DS) Networks", RFC 3754, DOI 10.17487/RFC3754,
              April 2004, <http://www.rfc-editor.org/info/rfc3754>.

   [RFC4594]  Babiarz, J., Chan, K., and F. Baker, "Configuration
              Guidelines for DiffServ Service Classes", RFC 4594,
              DOI 10.17487/RFC4594, August 2006,
              <http://www.rfc-editor.org/info/rfc4594>.

   [RFC6297]  Welzl, M. and D. Ros, "A Survey of Lower-than-Best-Effort
              Transport Protocols", RFC 6297, DOI 10.17487/RFC6297, June
              2011, <http://www.rfc-editor.org/info/rfc6297>.

   [RFC6817]  Shalunov, S., Hazel, G., Iyengar, J., and M. Kuehlewind,
              "Low Extra Delay Background Transport (LEDBAT)", RFC 6817,
              DOI 10.17487/RFC6817, December 2012,
              <http://www.rfc-editor.org/info/rfc6817>.

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   [RFC8100]  Geib, R., Ed. and D. Black, "Diffserv-Interconnection
              Classes and Practice", RFC 8100, DOI 10.17487/RFC8100,
              March 2017, <http://www.rfc-editor.org/info/rfc8100>.

Appendix A.  History of the LE PHB

   A first version of this PHB was suggested by Roland Bless and Klaus
   Wehrle in 1999 [draft-bless-diffserv-lbe-phb-00].  After some
   discussion in the DiffServ Working Group Brian Carpenter and Kathie
   Nichols proposed a bulk handling per-domain behavior and believed a
   PHB was not necessary.  Eventually, Lower Effort was specified as
   per-domain behavior and finally became [RFC3662].  More detailed
   information about its history can be found in Section 10 of
   [RFC3662].

Appendix B.  Acknowledgments

   Since text is borrowed from earlier Internet-Drafts and RFCs the co-
   authors of previous specifications are acknowledged here: Kathie
   Nichols and Klaus Wehrle.  David Black and Ruediger Geib provided
   helpful comments and suggestions.

Appendix C.  Change History

   This section briefly lists changes between Internet-Draft versions
   for convenience.

   Changes in Version 02:

   o  Applied many editorial suggestions from David Black

   o  Added Multicast traffic use case

   o  Clarified what is required for deployment in section 1.2
      (Deployment Considerations)

   o  Added text about implementations using AQMs and ECN usage

   o  Updated IANA section according to David Black's suggestions

   o  Revised text in the security section

   o  Changed copyright Notice to pre5378Trust200902

   Changes in Version 01:

   o  Now obsoletes RFC 3662.

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   o  Tried to be more precise in section 1.1 (Applicability) according
      to R.  Geib's suggestions, so rephrased several paragraphs.  Added
      text about congestion control

   o  Change section 2 (PHB Description) according to R.  Geib's
      suggestions.

   o  Added RFC 2119 language to several sentences.

   o  Detailed the description of remarking implications and
      recommendations in Section 5.

   o  Added Section 6 to explicitly list changes with respect to RFC
      4594, because this document will update it.

Appendix D.  Note to RFC Editor

   This section lists actions for the RFC editor during final
   formatting.

   o  Please replace the occurrence of RFCXXXX in Section 6 with the
      assigned RFC number for this document.

   o  Delete Appendix C.

   o  Delete this section.

Author's Address

   Roland Bless
   Karlsruhe Institute of Technology (KIT)
   Kaiserstr. 12
   Karlsruhe  76131
   Germany

   Phone: +49 721 608 46413
   Email: roland.bless@kit.edu

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