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rfc:rfc8622

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.

Bless Standards Track [Page 1] RFC 8622 Lower-Effort PHB June 2019

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

Bless Standards Track [Page 2] RFC 8622 Lower-Effort PHB June 2019

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

Bless Standards Track [Page 3] RFC 8622 Lower-Effort PHB June 2019

 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].

Bless Standards Track [Page 9] RFC 8622 Lower-Effort PHB June 2019

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.

Bless Standards Track [Page 10] RFC 8622 Lower-Effort PHB June 2019

10. The Updates to RFC 4594

 [RFC4594] recommended the use of CS1 as the codepoint in its
 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 the original purpose of LE.
 Therefore, every occurrence of the CS1 DSCP is replaced by the
 LE DSCP.
 Changes:
 o  This update to RFC 4594 removes the following entry from its
    Figure 3:
 |---------------+---------+-------------+--------------------------|
 | Low-Priority  |  CS1    |   001000    | Any flow that has no BW  |
 |     Data      |         |             | assurance                |
  ------------------------------------------------------------------
    and replaces it with the following entry:
 |---------------+---------+-------------+--------------------------|
 | Low-Priority  |   LE    |   000001    | Any flow that has no BW  |
 |     Data      |         |             | assurance                |
  ------------------------------------------------------------------
 o  This update to RFC 4594 extends the Notes text below Figure 3 that
    currently states "Notes for Figure 3: Default Forwarding (DF) and
    Class Selector 0 (CS0) provide equivalent behavior and use the
    same DS codepoint, '000000'." to state "Notes for Figure 3:
    Default Forwarding (DF) and Class Selector 0 (CS0) provide
    equivalent behavior and use the same DSCP, '000000'.  The prior
    recommendation to use the CS1 DSCP for Low-Priority Data has been
    replaced by the current recommendation to use the LE DSCP,
    '000001'."

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 o  This update to RFC 4594 removes the following entry from its
    Figure 4:
 |---------------+------+-------------------+---------+--------+----|
 | Low-Priority  | CS1  | Not applicable    | RFC3662 |  Rate  | Yes|
 |     Data      |      |                   |         |        |    |
  ------------------------------------------------------------------
    and replaces it with the following entry:
 |---------------+------+-------------------+----------+--------+----|
 | Low-Priority  | LE   | Not applicable    | RFC 8622 |  Rate  | Yes|
 |     Data      |      |                   |          |        |    |
  -------------------------------------------------------------------
 o  Section 2.3 of [RFC4594] specifies the following: "In network
    segments that use IP precedence marking, only one of the two
    service classes can be supported, High-Throughput Data or
    Low-Priority Data.  We RECOMMEND that the DSCP value(s) of the
    unsupported service class be changed to 000xx1 on ingress and
    changed back to original value(s) on egress of the network segment
    that uses precedence marking.  For example, if Low-Priority Data
    is mapped to Standard service class, then 000001 DSCP marking MAY
    be used to distinguish it from Standard marked packets on egress."
    This document removes this recommendation, because by using the LE
    DSCP defined herein, such re-marking 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 8.  That removed text is replaced by the following: "In
    network segments that use IP Precedence marking, the Low-Priority
    Data service class receives the same Diffserv QoS as the Standard
    service class when the LE DSCP is used for Low-Priority Data
    traffic.  This is acceptable behavior for the Low-Priority Data
    service class, although it is not the preferred behavior."
 o  This document removes the following line in Section 4.10 of
    RFC 4594: "The RECOMMENDED DSCP marking is CS1 (Class
    Selector 1)." and replaces it with the following text:
    "The RECOMMENDED DSCP marking is LE (Lower Effort), which replaces
    the prior recommendation for CS1 (Class Selector 1) marking."

11. The Updates to RFC 8325

 Section 4.2.10 of RFC 8325 [RFC8325] specifies that "[RFC3662] and
 [RFC4594] both recommend Low-Priority Data be marked CS1 DSCP."  This
 is updated to "[RFC3662] recommends that Low-Priority Data be marked
 CS1 DSCP.  [RFC4594], as updated by RFC 8622, recommends that
 Low-Priority Data be marked LE DSCP."

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 This document removes the following paragraph in Section 4.2.10 of
 [RFC8325], because this document makes the anticipated change: "Note:
 This marking recommendation may change in the future, as [LE-PHB]
 defines a Lower Effort (LE) PHB for Low-Priority Data traffic and
 recommends an additional DSCP for this traffic."
 Section 4.2.10 of RFC 8325 [RFC8325] specifies that "therefore, it is
 RECOMMENDED to map Low-Priority Data traffic marked CS1 DSCP to
 UP 1", which is updated to "therefore, it is RECOMMENDED to map
 Low-Priority Data traffic marked with LE DSCP or legacy CS1 DSCP
 to UP 1".
 This update to RFC 8325 replaces the following entry from its
 Figure 1:
 +---------------+------+----------+------------+--------------------+
 | Low-Priority  | CS1  | RFC 3662 |     1      | AC_BK (Background) |
 |     Data      |      |          |            |                    |
 +-------------------------------------------------------------------+
 with the following entries:
 +---------------+------+----------+------------+--------------------+
 | Low-Priority  | LE   | RFC 8622 |     1      | AC_BK (Background) |
 |     Data      |      |          |            |                    |
 +-------------------------------------------------------------------+
 | Low-Priority  | CS1  | RFC 3662 |     1      | AC_BK (Background) |
 | Data (legacy) |      |          |            |                    |
 +-------------------------------------------------------------------+

12. IANA Considerations

 This document assigns the Differentiated Services Field Codepoint
 (DSCP) '000001' from the "Differentiated Services Field Codepoints
 (DSCP)" registry (https://www.iana.org/assignments/dscp-registry/)
 ("DSCP Pool 3 Codepoints", Codepoint Space xxxx01, Standards Action)
 [RFC8126] to the LE PHB.  This document uses a DSCP from Pool 3 in
 order to avoid problems for other PHB-marked flows, where they could
 become accidentally re-marked as LE PHB, e.g., due to partial DSCP
 bleaching.  See [RFC8436] regarding reclassifying Pool 3 for
 Standards Action.

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 IANA has updated this registry as follows:
 o  Name: LE
 o  Value (Binary): 000001
 o  Value (Decimal): 1
 o  Reference: RFC 8622

13. Security Considerations

 There are no specific security exposures for this PHB.  Since it
 defines a new class that is of low forwarding priority, re-marking
 other traffic as LE traffic may lead to QoS 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].
 With respect to privacy, an attacker could use the information from
 the DSCP to infer that the transferred (probably even encrypted)
 content is considered of low priority or low urgency by a user if the
 DSCP was set per the user's request.  On the one hand, this disclosed
 information is useful only if correlation with metadata (such as the
 user's IP address) and/or other flows reveal a user's identity.  On
 the other hand, it might help an observer (e.g., a state-level actor)
 who is interested in learning about the user's behavior from observed
 traffic: LE-marked background traffic (such as software downloads,
 operating system updates, or telemetry data) may be less interesting
 for surveillance than general web traffic.  Therefore, the LE marking
 may help the observer to focus on potentially more interesting
 traffic (however, the user may exploit this particular assumption and
 deliberately hide interesting traffic in the LE aggregate).  Apart
 from such considerations, the impact of disclosed information by the
 LE DSCP is likely negligible in most cases, given the numerous
 traffic analysis possibilities and general privacy threats (e.g., see
 [RFC6973]).

Bless Standards Track [Page 14] RFC 8622 Lower-Effort PHB June 2019

14. References

14.1. 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,
            <https://www.rfc-editor.org/info/rfc2119>.
 [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,
            <https://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,
            <https://www.rfc-editor.org/info/rfc2475>.
 [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in
            RFC 2119 Key Words", BCP 14, RFC 8174,
            DOI 10.17487/RFC8174, May 2017,
            <https://www.rfc-editor.org/info/rfc8174>.

14.2. Informative References

 [Carlberg-LBE-2001]
            Carlberg, K., Gevros, P., and J. Crowcroft, "Lower than
            best effort: a design and implementation", ACM SIGCOMM
            Computer Communication Review, Volume 31 Issue 2
            supplement, DOI 10.1145/844193.844208, April 2001,
            <https://dl.acm.org/citation.cfm?doid=844193.844208>.
 [Chown-LBE-2003]
            Chown, T., Ferrari, T., Leinen, S., Sabatino, R., Simar,
            N., and S. Venaas, "Less than Best Effort: Application
            Scenarios and Experimental Results", Proceedings of the
            Second International Workshop on Quality of Service in
            Multiservice IP Networks (QoS-IP 2003), Lecture Notes in
            Computer Science, vol 2601, Springer, Berlin, Heidelberg,
            Pages 131-144, DOI 10.1007/3-540-36480-3_10,
            February 2003, <https://link.springer.com/chapter/
            10.1007%2F3-540-36480-3_10>.

Bless Standards Track [Page 15] RFC 8622 Lower-Effort PHB June 2019

 [Diffserv-LBE-PHB]
            Bless, R. and K. Wehrle, "A Lower Than Best-Effort
            Per-Hop Behavior", Work in Progress,
            draft-bless-diffserv-lbe-phb-00, September 1999.
 [IETF99-Secchi]
            Secchi, R., Venne, A., and A. Custura, "Measurements
            concerning the DSCP for a LE PHB", Presentation held at
            the 99th IETF Meeting, TSVWG, Prague, July 2017,
            <https://datatracker.ietf.org/meeting/99/materials/
            slides-99-tsvwg-sessb-31measurements-concerning-
            the-dscp-for-a-le-phb-00>.
 [RFC2914]  Floyd, S., "Congestion Control Principles", BCP 41,
            RFC 2914, DOI 10.17487/RFC2914, September 2000,
            <https://www.rfc-editor.org/info/rfc2914>.
 [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,
            <https://www.rfc-editor.org/info/rfc3168>.
 [RFC3439]  Bush, R. and D. Meyer, "Some Internet Architectural
            Guidelines and Philosophy", RFC 3439,
            DOI 10.17487/RFC3439, December 2002,
            <https://www.rfc-editor.org/info/rfc3439>.
 [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,
            <https://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, <https://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,
            <https://www.rfc-editor.org/info/rfc4594>.
 [RFC5053]  Luby, M., Shokrollahi, A., Watson, M., and T. Stockhammer,
            "Raptor Forward Error Correction Scheme for Object
            Delivery", RFC 5053, DOI 10.17487/RFC5053, October 2007,
            <https://www.rfc-editor.org/info/rfc5053>.

Bless Standards Track [Page 16] RFC 8622 Lower-Effort PHB June 2019

 [RFC6297]  Welzl, M. and D. Ros, "A Survey of Lower-than-Best-Effort
            Transport Protocols", RFC 6297, DOI 10.17487/RFC6297,
            June 2011, <https://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,
            <https://www.rfc-editor.org/info/rfc6817>.
 [RFC6973]  Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
            Morris, J., Hansen, M., and R. Smith, "Privacy
            Considerations for Internet Protocols", RFC 6973,
            DOI 10.17487/RFC6973, July 2013,
            <https://www.rfc-editor.org/info/rfc6973>.
 [RFC8085]  Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage
            Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085,
            March 2017, <https://www.rfc-editor.org/info/rfc8085>.
 [RFC8087]  Fairhurst, G. and M. Welzl, "The Benefits of Using
            Explicit Congestion Notification (ECN)", RFC 8087,
            DOI 10.17487/RFC8087, March 2017,
            <https://www.rfc-editor.org/info/rfc8087>.
 [RFC8100]  Geib, R., Ed. and D. Black, "Diffserv-Interconnection
            Classes and Practice", RFC 8100, DOI 10.17487/RFC8100,
            March 2017, <https://www.rfc-editor.org/info/rfc8100>.
 [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
            Writing an IANA Considerations Section in RFCs", BCP 26,
            RFC 8126, DOI 10.17487/RFC8126, June 2017,
            <https://www.rfc-editor.org/info/rfc8126>.
 [RFC8325]  Szigeti, T., Henry, J., and F. Baker, "Mapping Diffserv to
            IEEE 802.11", RFC 8325, DOI 10.17487/RFC8325,
            February 2018, <https://www.rfc-editor.org/info/rfc8325>.
 [RFC8436]  Fairhurst, G., "Update to IANA Registration Procedures for
            Pool 3 Values in the Differentiated Services Field
            Codepoints (DSCP) Registry", RFC 8436,
            DOI 10.17487/RFC8436, August 2018,
            <https://www.rfc-editor.org/info/rfc8436>.

Bless Standards Track [Page 17] RFC 8622 Lower-Effort PHB June 2019

Appendix A. History of the LE PHB

 A first draft version of this PHB was suggested by Roland Bless and
 Klaus Wehrle in September 1999 [Diffserv-LBE-PHB], named "A Lower
 Than Best-Effort Per-Hop Behavior".  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].
 There are several other names in use for this type of PHB or
 associated service classes.  Well known is the QBone Scavenger
 Service (QBSS) that was proposed in March 2001 within the Internet2
 QoS Working Group.  Alternative names are "Lower than best effort"
 [Carlberg-LBE-2001] or "Less than best effort" [Chown-LBE-2003].

Acknowledgments

 Since text is partially borrowed from earlier Internet-Drafts and
 RFCs, the coauthors of previous specifications are acknowledged here:
 Kathie Nichols and Klaus Wehrle.  David Black, Olivier Bonaventure,
 Spencer Dawkins, Toerless Eckert, Gorry Fairhurst, Ruediger Geib, and
 Kyle Rose provided helpful comments and (partially also text)
 suggestions.

Author's Address

 Roland Bless
 Karlsruhe Institute of Technology (KIT)
 Institute of Telematics (TM)
 Kaiserstr. 12
 Karlsruhe  76131
 Germany
 Phone: +49 721 608 46413
 Email: roland.bless@kit.edu

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