Internet DRAFT - draft-ietf-tsvwg-le-phb
draft-ietf-tsvwg-le-phb
Internet Engineering Task Force R. Bless
Internet-Draft Karlsruhe Institute of Technology (KIT)
Obsoletes: 3662 (if approved) March 11, 2019
Updates: 4594,8325 (if approved)
Intended status: Standards Track
Expires: September 12, 2019
A Lower Effort Per-Hop Behavior (LE PHB) for Differentiated Services
draft-ietf-tsvwg-le-phb-10
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. 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 DSCP value for the LE PHB. This specification obsoletes RFC
3662 and updates the DSCP recommended in RFC 4594 and RFC 8325 to use
the DSCP assigned in this specification.
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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Internet-Drafts are draft documents valid for a maximum of six months
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on September 12, 2019.
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Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 3
3. Applicability . . . . . . . . . . . . . . . . . . . . . . . . 3
4. PHB Description . . . . . . . . . . . . . . . . . . . . . . . 6
5. Traffic Conditioning Actions . . . . . . . . . . . . . . . . 7
6. Recommended DS Codepoint . . . . . . . . . . . . . . . . . . 7
7. Deployment Considerations . . . . . . . . . . . . . . . . . . 7
8. Remarking to other DSCPs/PHBs . . . . . . . . . . . . . . . . 8
9. Multicast Considerations . . . . . . . . . . . . . . . . . . 9
10. The Update to RFC 4594 . . . . . . . . . . . . . . . . . . . 10
11. The Update to RFC 8325 . . . . . . . . . . . . . . . . . . . 12
12. The Update to draft-ietf-tsvwg-rtcweb-qos . . . . . . . . . . 12
13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14
14. Security Considerations . . . . . . . . . . . . . . . . . . . 14
15. References . . . . . . . . . . . . . . . . . . . . . . . . . 15
15.1. Normative References . . . . . . . . . . . . . . . . . . 15
15.2. Informative References . . . . . . . . . . . . . . . . . 15
Appendix A. History of the LE PHB . . . . . . . . . . . . . . . 17
Appendix B. Acknowledgments . . . . . . . . . . . . . . . . . . 18
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Appendix C. Change History . . . . . . . . . . . . . . . . . . . 18
Appendix D. Note to RFC Editor . . . . . . . . . . . . . . . . . 21
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 21
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 the 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 packets that ought to consume network resources
only when no other traffic is demanding them. In this point of view,
packets forwarded by the LE PHB scavenge otherwise unused resources
only, which led to the name "scavenger service" in early Internet2
deployments (see Appendix A). Other commonly used names for LE PHB
type services are "Lower-than-best-effort" or "Less-than-best-
effort". In summary, with the mentioned feature above, 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
("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.
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
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
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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 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. It is application-
dependent when a lack of progress is considered being a failure
(e.g., if a transport connection fails due to timing out, the
application may try several times to re-establish 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 Differentiated Services (DS) domain or DS region.
Just like best-effort 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
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
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 re-establish failed
connections. 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 is also essential if LE traffic
is mapped to the default PHB in DS domains that do not support LE.
In this case, use of background transport protocols, e.g., similar to
LEDBAT [RFC6817], is expedient.
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
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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 ought not to be used for the general
case of downgraded traffic, but 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
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 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 of
multicast traffic 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 (cf.
Section 6 of [RFC3439]). 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. 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.
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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 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 (best-effort).
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 case of link resource contention LE traffic can be
starved completely, which may not be always desired by the network
operator's policy. The used scheduler 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 lower-
effort 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 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 CE marking for ECT marked packets 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 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. Remarking 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 also remarks with respect to
downgrading in Section 3 and Section 8.
6. Recommended DS Codepoint
The RECOMMENDED codepoint for the LE PHB is '000001'.
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
transition to use the unambiguous LE codepoint '000001' whenever
possible.
This particular codepoint was chosen due to measurements on the
currently observable DSCP remarking 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).
7. 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
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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 remark 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 domains 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.
8. 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 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 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 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 quality-of-service degradation of the
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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 within the
default PHB (i.e., forwarded as best-effort 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 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]) SHOULD remark 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 remarked by the default DSCP if the next domain
is applying Diffserv-Interconnection [RFC8100].
9. Multicast Considerations
Basically, the multicast considerations in [RFC3754] apply. However,
using the Lower Effort PHB for multicast requires paying special
attention to the way 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 and in the
worst case, these forwarding resources are missing for higher
prioritized 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 potential 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 5 times as long 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
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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, packet replication of 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 case
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 case of available resources at the output ports. The
previously mentioned redundancy data traffic could nicely use the
varying available residual bandwidth being utilized the by LE PHB,
but only if the specific requirements stated above for conditional
replication in the internal implementation of the network devices are
considered.
10. The Update 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 This update to RFC 4594 removes the following entry from figure 3:
|---------------+---------+-------------+--------------------------|
| Low-Priority | CS1 | 001000 | Any flow that has no BW |
| Data | | | assurance |
------------------------------------------------------------------
and replaces this by the following entry:
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|---------------+---------+-------------+--------------------------|
| 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 DS codepoint, '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'."
o This update to RFC 4594 removes the following entry from figure 4:
|---------------+------+-------------------+---------+--------+----|
| Low-Priority | CS1 | Not applicable | RFC3662 | Rate | Yes|
| Data | | | | | |
------------------------------------------------------------------
and replaces this by the following entry:
|---------------+------+-------------------+---------+--------+----|
| Low-Priority | LE | Not applicable | RFCXXXX | Rate | Yes|
| Data | | | | | |
------------------------------------------------------------------
o Section 2.3 of [RFC4594] specifies: "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 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 8. That
removed text is replaced by: "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."
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o This document removes the following line of RFC 4594,
Section 4.10: "The RECOMMENDED DSCP marking is CS1 (Class Selector
1)." and replaces this 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 Update to RFC 8325
Section 4.2.10 of RFC 8325 [RFC8325] specifies "[RFC3662] and
[RFC4594] both recommend Low-Priority Data be marked CS1 DSCP."
which is updated to "[RFC3662] recommends that Low-Priority Data be
marked CS1 DSCP. [RFC4594] as updated by [RFCXXXX] recommends Low-
Priority Data be marked LE DSCP."
This document removes the following paragraph of RFC 8325,
Section 4.2.10 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 "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 figure 1:
+---------------+------+----------+-------------+--------------------+
| Low-Priority | CS1 | RFC 3662 | 1 | AC_BK (Background) |
| Data | | | | |
+--------------------------------------------------------------------+
by the following entries:
+---------------+------+----------+-------------+--------------------+
| Low-Priority | LE | RFCXXXX | 1 | AC_BK (Background) |
| Data | | | | |
+--------------------------------------------------------------------+
| Low-Priority | CS1 | RFC 3662 | 1 | AC_BK (Background) |
| Data (legacy) | | | | |
+--------------------------------------------------------------------+
12. The Update to draft-ietf-tsvwg-rtcweb-qos
Section 5 of [I-D.ietf-tsvwg-rtcweb-qos] describes the Recommended
DSCP Values for WebRTC Applications
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This update to [I-D.ietf-tsvwg-rtcweb-qos] replaces all occurrences
of CS1 with LE in Table 1:
+------------------------+-------+------+-------------+-------------+
| Flow Type | Very | Low | Medium | High |
| | Low | | | |
+------------------------+-------+------+-------------+-------------+
| Audio | LE | DF | EF (46) | EF (46) |
| | (1) | (0) | | |
| | | | | |
| Interactive Video with | LE | DF | AF42, AF43 | AF41, AF42 |
| or without Audio | (1) | (0) | (36, 38) | (34, 36) |
| | | | | |
| Non-Interactive Video | LE | DF | AF32, AF33 | AF31, AF32 |
| with or without Audio | (1) | (0) | (28, 30) | (26, 28) |
| | | | | |
| Data | LE | DF | AF11 | AF21 |
| | (1) | (0) | | |
+------------------------+-------+------+-------------+-------------+
and updates the following paragraph:
"The above table assumes that packets marked with CS1 are treated as
"less than best effort", such as the LE behavior described in
[RFC3662]. However, the treatment of CS1 is implementation
dependent. If an implementation treats CS1 as other than "less than
best effort", then the actual priority (or, more precisely, the per-
hop-behavior) of the packets may be changed from what is intended.
It is common for CS1 to be treated the same as DF, so applications
and browsers using CS1 cannot assume that CS1 will be treated
differently than DF [RFC7657]. However, it is also possible per
[RFC2474] for CS1 traffic to be given better treatment than DF, thus
caution should be exercised when electing to use CS1. This is one of
the cases where marking packets using these recommendations can make
things worse."
as follows:
"The above table assumes that packets marked with LE are treated as
lower effort (i.e., "less than best effort"), such as the LE behavior
described in [RFCXXXX]. However, the treatment of LE is
implementation dependent. If an implementation treats LE as other
than "less than best effort", then the actual priority (or, more
precisely, the per- hop-behavior) of the packets may be changed from
what is intended. It is common for LE to be treated the same as DF,
so applications and browsers using LE cannot assume that LE will be
treated differently than DF [RFC7657]. During development of this
document, the CS1 DSCP was recommended for "very low" application
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priority traffic; implementations that followed that recommendation
SHOULD be updated to use the LE DSCP instead of the CS1 DSCP."
13. 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-
registry.xhtml) (Pool 3, Codepoint Space xxxx01, Standards Action) to
the LE PHB. This document suggests to use a DSCP from Pool 3 in
order to avoid problems for other PHB marked flows to become
accidentally remarked as LE PHB, e.g., due to partial DSCP bleaching.
See [RFC8436] for re-classifying Pool 3 for Standards Action.
IANA is requested to update the registry as follows:
o Name: LE
o Value (Binary): 000001
o Value (Decimal): 1
o Reference: [RFC number of this memo]
14. 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].
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, in
case the DSCP was set on 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 user
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
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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]).
15. References
15.1. 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>.
[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>.
[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>.
15.2. Informative References
[carlberg-lbe-2001]
Carlberg, K., Gevros, P., and J. Crowcroft, "Lower than
best effort: a design and implementation", SIGCOMM
Computer Communications Review Volume 31, Issue 2
supplement, April 2001,
<https://doi.org/10.1145/844193.844208>.
[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", In 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, February 2003,
<https://doi.org/10.1007/3-540-36480-3_10>.
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[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>.
[I-D.ietf-tsvwg-rtcweb-qos]
Jones, P., Dhesikan, S., Jennings, C., and D. Druta, "DSCP
Packet Markings for WebRTC QoS", draft-ietf-tsvwg-rtcweb-
qos-18 (work in progress), August 2016.
[ietf99-secchi]
Secchi, R., Venne, A., and A. Custura, "Measurements
concerning the DSCP for a LE PHB", Presentation held at
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,
<http://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,
<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>.
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[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>.
[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>.
[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, <http://www.rfc-editor.org/info/rfc8100>.
[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>.
Appendix A. History of the LE PHB
A first version of this PHB was suggested by Roland Bless and Klaus
Wehrle in September 1999 [draft-bless-diffserv-lbe-phb-00], named "A
Lower Than Best-Effort Per-Hop Behavior". After some discussion in
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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].
Appendix B. Acknowledgments
Since text is partially borrowed from earlier Internet-Drafts and
RFCs the co-authors 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.
Appendix C. Change History
This section briefly lists changes between Internet-Draft versions
for convenience.
Changes in Version 10: (incorporated comments from IESG discussion as
follows)
o Appended "for Differentiated Services" to the title as suggested
by Alexey.
o Addressed Deborah Brungard's discuss: changed phrase to "However,
non-LE traffic (e.g., BE traffic) SHOULD NOT be remarked to LE."
with additional explanation as suggested by Gorry.
o Fixed the sentence "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." according to Alice's and Mirja's
comments.
o Made reference to RFC8174 normative.
o Added hint for the RFC editor to apply changes from section
Section 12 and to delete it afterwards.
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o Incorporated Mirja's and Benjamin's suggestions.
o Editorial suggested by Gorry: In case => In the case that
Changes in Version 09:
o Incorporated comments from IETF Last Call:
* from Olivier Bonaventure: added a bit of text for session
resumption and congestion control aspects as well as ECN usage.
* from Kyle Rose: Revised privacy considerations text in Security
Considerations Section
Changes in Version 08:
o revised two sentences as suggested by Spencer Dawkins
Changes in Version 07:
o revised some text for clarification according to comments from
Spencer Dawkins
Changes in Version 06:
o added Multicast Considerations section with input from Toerless
Eckert
o incorporated suggestions by David Black with respect to better
reflect legacy CS1 handling
Changes in Version 05:
o added scavenger service class into abstract
o added some more history
o added reference for "Myth of Over-Provisioning" in RFC3439 and
references to presentations w.r.t. codepoint choices
o added text to update draft-ietf-tsvwg-rtcweb-qos
o revised text on congestion control in case of remarking to BE
o added reference to DSCP measurement talk @IETF99
o small typo fixes
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Changes in Version 04:
o Several editorial changes according to review from Gorry Fairhurst
o Changed the section structure a bit (moved subsections 1.1 and 1.2
into own sections 3 and 7 respectively)
o updated section 2 on requirements language
o added updates to RFC 8325
o tried to be more explicit what changes are required to RFCs 4594
and 8325
Changes in Version 03:
o Changed recommended codepoint to 000001
o Added text to explain the reasons for the DSCP choice
o Removed LE-min,LE-strict discussion
o Added one more potential use case: reporting errors or telemetry
data from OSs
o Added privacy considerations to the security section (not worth an
own section I think)
o Changed IANA considerations section
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:
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o Now obsoletes RFC 3662.
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 8.
o Added Section 10 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 Apply the suggested changes of section Section 12 and add a
normative reference in draft-ietf-tsvwg-rtcweb-qos to this RFC.
o Delete Section 12.
o Please replace the occurrences of RFCXXXX in Section 10 and
Section 11 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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