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RFC 8622

A Lower-Effort Per-Hop Behavior (LE PHB) for Differentiated Services

Pages: 18
Proposed STD
Obsoletes:  3662
Updates:  45948325
Part 1 of 2 – Pages 1 to 10
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Internet Engineering Task Force (IETF)                          R. Bless
Request for Comments: 8622                                           KIT
Obsoletes: 3662                                                June 2019
Updates: 4594, 8325
Category: Standards Track
ISSN: 2070-1721


  A Lower-Effort Per-Hop Behavior (LE PHB) for Differentiated Services

Abstract

   This document specifies properties and characteristics of a Lower-
   Effort Per-Hop Behavior (LE 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, BE traffic has precedence over LE traffic
   and may preempt it.  Alternatively, packets forwarded by the LE PHB
   can be associated with a scavenger service class, i.e., they scavenge
   otherwise-unused resources only.  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 Differentiated Services Code Point (DSCP) value for the LE
   PHB.

   This specification obsoletes RFC 3662 and updates the DSCP
   recommended in RFCs 4594 and 8325 to use the DSCP assigned in this
   specification.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8622.
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Copyright Notice

   Copyright (c) 2019 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
   (https://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.

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.

Table of Contents

   1. Introduction ....................................................3
   2. Requirements Language ...........................................3
   3. Applicability ...................................................3
   4. PHB Description .................................................6
   5. Traffic-Conditioning Actions ....................................7
   6. Recommended DSCP ................................................7
   7. Deployment Considerations .......................................8
   8. Re-marking to Other DSCPs/PHBs ..................................9
   9. Multicast Considerations .......................................10
   10. The Updates to RFC 4594 .......................................11
   11. The Updates to RFC 8325 .......................................12
   12. IANA Considerations ...........................................13
   13. Security Considerations .......................................14
   14. References ....................................................15
      14.1. Normative References .....................................15
      14.2. Informative References ...................................15
   Appendix A. History of the LE PHB .................................18
   Acknowledgments ...................................................18
   Author's Address ..................................................18
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1.  Introduction

   This document defines a Differentiated Services (DS) per-hop behavior
   [RFC2474] called "Lower-Effort Per-Hop Behavior" (LE PHB), which is
   intended for traffic of sufficiently low urgency that all other
   traffic takes precedence over the LE traffic in consumption of
   network link bandwidth.  Low-urgency traffic has a low priority for
   timely forwarding; note, however, that this 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 packets that ought to consume network resources
   only when no other traffic is demanding them.  From this point of
   view, packets forwarded by the LE PHB scavenge otherwise-unused
   resources only; this led to the name "scavenger service" in early
   Internet2 deployments (see Appendix A).  Other commonly used names
   for LE PHB types of services are "Lower than best effort"
   [Carlberg-LBE-2001] or "Less than best effort" [Chown-LBE-2003].  In
   summary, with the above-mentioned feature, the LE PHB has two
   important properties: it should scavenge residual capacity, and it
   must be preemptable by the default PHB (or other elevated PHBs) in
   case they need more resources.  Consequently, the effect of this type
   of traffic on all other network traffic is strictly limited (the
   "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
   an LE DS PHB for handling this "optional" traffic in a DS node.

2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  Applicability

   An LE PHB is applicable for many applications that otherwise use BE
   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 needs to be able
   to tolerate short or (even very) long interruptions due to the
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   presence of severe congestion conditions during the transmission of
   the flow.  Thus, there ought to be an expectation that packets of the
   LE PHB could be excessively delayed or dropped when any other traffic
   is present.  Whether or not a lack of progress is considered to be a
   failure is application dependent (e.g., if a transport connection
   fails due to timing out, the application may try several times to
   reestablish the transport connection in order to resume the
   application session before finally giving up).  The LE PHB is
   suitable for sending traffic of low urgency across a DS domain or DS
   region.

   Just like BE traffic, LE traffic SHOULD be congestion controlled
   (i.e., use a congestion controlled transport or implement an
   appropriate congestion control method [RFC2914] [RFC8085]).  Since LE
   traffic could 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
   starvation situation.  An appropriate reaction would be to resume the
   transfer instead of aborting it, i.e., an LE-optimized transport
   ought to use appropriate retry strategies (e.g., exponential back-off
   with an upper bound) as well as corresponding retry and timeout
   limits in order to avoid the loss of the connection due to the
   above-mentioned starvation periods.  While it is desirable to achieve
   a quick resumption of the transfer as soon as resources become
   available again, it may be difficult to achieve this in practice.  In
   the case of a lack of a transport protocol and congestion control
   that are adapted to LE, applications can also use existing common
   transport protocols and implement session resumption by trying to
   reestablish failed connections.  Congestion control is not only
   useful for letting the flows within the LE Behavior Aggregate (BA)
   adapt to the available bandwidth, which may be highly fluctuating; it
   is also essential if LE traffic is mapped to the default PHB in DS
   domains that do not support LE.  In this case, the use of background
   transport protocols, e.g., similar to Low Extra Delay Background
   Transport (LEDBAT) [RFC6817], is expedient.

   The use of the LE PHB might assist a network operator in moving
   certain kinds of traffic or users to off-peak times.  Furthermore,
   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 and
   packets SHOULD NOT 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 during certain periods.
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   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 ought not be used for the general
   case of downgraded traffic, but it could 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 the
   forwarding of multicast traffic from untrusted sources.  Multicast
   forwarding is currently enabled within domains only for specific
   sources within a domain -- not for sources from anywhere in the
   Internet.  One major problem is that multicast routing creates
   traffic sources at (mostly) unpredictable branching points within a
   domain, potentially leading to congestion and packet loss.  In the
   case where multicast traffic packets from untrusted sources are
   forwarded as LE traffic, they will not harm traffic from non-LE BAs.
   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 (cf.
   Section 6 of [RFC3439]).  LE-marked traffic can utilize the normally
   unused capacity and will be preempted automatically in the case of
   link failure when 100% of the link capacity is required for all other
   traffic.  Ideally, applications mark their packets as LE traffic,
   because they know the urgency of flows.  Since LE traffic may be
   starved for longer periods of time, it is probably less suitable for
   real-time and interactive applications.

   Example uses for the LE PHB:

   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 reporting errors or telemetry data from operating systems or
      applications.

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

   o  For content distribution transfers between caches.
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   o  For preloading or prefetching objects from web sites.

   o  For network news 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.

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

4.  PHB Description

   The LE PHB is defined in relation to the default PHB (BE).  A packet
   forwarded with the LE PHB SHOULD have lower precedence than packets
   forwarded with the default PHB, i.e., in the case of congestion,
   LE-marked traffic SHOULD be dropped prior to dropping any default PHB
   traffic.  Ideally, LE packets would be forwarded only when no packet
   with any other PHB is awaiting transmission.  This means that in the
   case of link resource contention LE traffic can be starved
   completely, which may not always be desired by the network operator's
   policy.  A scheduler used to implement the LE PHB may reflect this
   policy accordingly.

   A straightforward implementation could be a simple priority scheduler
   serving the default PHB queue with higher priority than the LE PHB
   queue.  Alternative implementations may use scheduling algorithms
   that assign a very small weight to the LE class.  This, however,
   could sometimes cause better service for LE packets compared to BE
   packets in cases when the BE share is fully utilized and the LE share
   is 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 (ECN) [RFC3168] SHOULD be used for
   LE packets, too.  More specifically, an LE implementation SHOULD also
   apply Congestion Experienced (CE) marking for ECT-marked packets
   ("ECT" stands for ECN-Capable Transport), and transport protocols
   used for LE SHOULD support and employ ECN.  For more information on
   the benefits of using ECN, see [RFC8087].
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5.  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
   also be performed at the first DS boundary node according to the DS
   domain's own policies (e.g., as a protection measure against
   untrusted sources).  However, non-LE traffic (e.g., BE traffic)
   SHOULD NOT be re-marked to LE.  Re-marking traffic from another PHB
   results in that traffic being "downgraded".  This changes the way the
   network treats this traffic, and it is important not to violate the
   operational objectives of the original PHB.  See Sections 3 and 8 for
   notes related to downgrading.

6.  Recommended DSCP

   The RECOMMENDED codepoint for the LE PHB is '000001'.

   Earlier specifications (e.g., [RFC4594]) recommended the use of Class
   Selector 1 (CS1) as the 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 transition to use the unambiguous LE
   codepoint '000001' whenever possible.

   This particular codepoint was chosen due to measurements on the
   currently observable Differentiated Services Code Point (DSCP)
   re-marking behavior in the Internet [IETF99-Secchi].  Since some
   network domains set the former IP Precedence bits to zero, it is
   possible that some other standardized DSCPs get mapped to the LE PHB
   DSCP if it were taken from the DSCP Standards Action Pool 1 (xxxxx0)
   [RFC2474] [RFC8436].
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7.  Deployment Considerations

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

   o  A BA 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 could 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 re-mark incoming
      LE traffic.

   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 5 for further
      discussion.

   Operators of DS domains that cannot or do not want to implement the
   LE PHB (e.g., because there is no separate LE queue available in the
   corresponding nodes) SHOULD NOT drop packets marked with the LE DSCP.
   They SHOULD map packets with this DSCP to the default PHB and SHOULD
   preserve the LE DSCP marking.  DS domain operators that do not
   implement the LE PHB should be aware that they violate the "no harm"
   property of LE.  See also Section 8 for further discussion of
   forwarding LE traffic with the default PHB instead of the LE PHB.
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8.  Re-marking 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 to protect BE traffic from LE traffic
   (the "no harm" property).  In the case that a DS domain does not
   support the LE PHB, its nodes 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 re-marking to DSCP '000000'.  This is
   because DS domains that are traversed later may then still have the
   opportunity to treat such packets according to the LE PHB.

   Operators of DS domains that forward LE traffic within the BE
   aggregate need to be aware of the implications, i.e., induced
   congestion situations and QoS degradation of the original BE traffic.
   In this case, the LE property of not harming other traffic is no
   longer fulfilled.  To limit the impact in such cases, traffic
   policing of the LE aggregate MAY be used.

   In the case that LE-marked packets are effectively carried with the
   default PHB (i.e., forwarded as BE traffic), they get a better
   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.  In the case that
   LE traffic is mapped to the default PHB, LE traffic may compete with
   BE traffic for the same resources and thus adversely affect the
   original BE aggregate.  Applications that want to ensure the lower
   precedence compared to BE traffic even in such cases SHOULD
   additionally use a corresponding lower-than-BE 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]) SHOULD re-mark traffic to the LE DSCP
   '000001' 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 re-marked by the default DSCP if the next
   domain is applying Diffserv-Interconnection [RFC8100].
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9.  Multicast Considerations

   Basically, the multicast considerations in [RFC3754] apply.  However,
   using the LE PHB for multicast requires paying special attention to
   how packets get replicated inside routers.  Due to multicast packet
   replication, resource contention may actually occur even before a
   packet is forwarded to its output port.  In the worst case, these
   forwarding resources are missing for higher-priority multicast or
   even unicast packets.

   Several forward error correction coding schemes, such as fountain
   codes (e.g., [RFC5053]), allow reliable data delivery even in
   environments with a potentially high amount of packet loss in
   transmission.  When used, for example, over satellite links or other
   broadcast media, this means that receivers that lose 80% of packets
   in transmission simply need five times longer to receive the complete
   data than those receivers experiencing no loss (without any receiver
   feedback required).

   Superficially viewed, it may sound very attractive to use IP
   multicast with the LE PHB to build this type of opportunistic
   reliable distribution in IP networks, but it can only be usefully
   deployed with routers that do not experience forwarding/replication
   resource starvation when a large amount of packets (virtually) need
   to be replicated to links where the LE queue is full.

   Thus, a packet replication mechanism for LE-marked packets should
   consider the situation at the respective output links: it is a waste
   of internal forwarding resources if a packet is replicated to output
   links that have no resources left for LE forwarding.  In those cases,
   a packet would have been replicated just to be dropped immediately
   after finding a filled LE queue at the respective output port.  Such
   behavior could be avoided -- for example, by using a conditional
   internal packet replication: a packet would then only be replicated
   in cases where the output link is not fully used.  This conditional
   replication, however, is probably not widely implemented.

   While the resource contention problem caused by multicast packet
   replication is also true for other Diffserv PHBs, LE forwarding is
   special, because often it is assumed that LE packets only get
   forwarded in the case of available resources at the output ports.
   The previously mentioned redundancy data traffic could suitably use
   the varying available residual bandwidth being utilized by the LE
   PHB, but only if the specific requirements stated above for
   conditional replication in the internal implementation of the network
   devices are considered.


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