INTERNET-DRAFT                                                 R. Bless
Expires: March 2000                                            K. Wehrle
                                             Universitaet Karlsruhe (TH)
                                                          September 1999

                A Lower Than Best-Effort Per-Hop Behavior


Status of this Memo

     This document is an Internet-Draft and is in full conformance
     with all provisions of Section 10 of RFC2026.

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     This document describes a Lower than Best-Effort (LBE) per-hop
     behavior (PHB) for use within and between Differentiated
     Services domains [3].  The primary objective of this LBE PHB
     is to separate LBE traffic from best-effort traffic in
     congestion situations, i.e., when resources become scarce.
     Furthermore, LBE traffic gets a minimal share of bandwidth so
     that it will not be fully preempted by best-effort traffic.
     This is achieved by discarding LBE packets more aggressively
     than best-effort packets while trying to enqueue them in case
     of congestion.

     There are numerous uses for this PHB, e.g., for transmission
     of background traffic such as bulk data transfers of minor
     importance, backup data traffic during working hours or
     traffic caused by web search engines while gathering
     information from web servers.  Thus, it constitutes the
     network equivalent to a background priority for processes in
     an operating system.  Moreover, it is particularly useful in
     cases when packets of a better service are re-marked by a node

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     for subsequently receiving a forwarding treatment that is
     equivalent to the best-effort service.  In this situation the
     LBE PHB helps to protect other best-effort packets from
     experiencing unfair forwarding treatment because it avoids
     their preemption by re-marked packets.  For instance, this
     case occurs when heterogeneous multicast groups should be

1  Purpose and Overview

In some situations changing a per-hop forwarding behavior (PHB) of an
incoming packet is desired or required, so that the packet is
subsequently forwarded with the lowest available priority.  Usually,
this forwarding behavior would be equivalent to the Default PHB
resulting in a best-effort service.  But, simply re-marking the DS
codepoint (DSCP) to the DSCP value of the Default PHB will probably
result in some unfair share of this re-marked traffic relating to
best-effort traffic.  This is due to the fact that nearly all packets
which previously experienced a better service enter the behavior
aggregate (BA) of the Default behavior.  Consequently, other incoming
packets carrying a DSCP of the Default PHB will be preempted by those
re-marked packets if not enough resources are available for the combined
traffic.  This unfairness against existing best-effort traffic should be

The basic concept behind the proposed Lower-Than-Best-Effort per-hop
behavior (LBE PHB) is to discard those re-marked packets more
aggressively than packets belonging to the default PHB if resources
become scarce.  Merely discarding those packets more drastically in the
re-marking node is not sufficient, because currently there may be enough
resources available in this node, but maybe not in subsequent downstream
nodes.  Therefore, this re-marked traffic should be identifyable by a
separate codepoint.

For instance, a re-marking of packets is required when a receiver joins
a multicast group and does not want to or even is not able to receive
the currently used better service within this group (e.g., 1 Mbit/s
Premium service).  Instead of this, the receiver wants to get the
traffic of this group without any quality of service guarantee, i.e., a
best-effort service.  The node which connects the new subtree of this
receiver to the already existing multicast delivery tree must therefore
degrade the quality of service of the incoming packet stream for
replicated packets which are sent downstream on the output link of the
newly joined subtree (see Fig. 1).

Moreover, specific excess traffic of any kind may be marked with the LBE
codepoint.  For example, this excess traffic could consist of packets
belonging to non-responsive flows (e.g., UDP flows).  Thus, other
best-effort traffic (e.g., responsive TCP flows) is segregated from this
excess traffic, consequently not being adversely affected by it.

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                           || Multicast Group Flow
                           || 1 Mbit/s Premium service
                       |   ||    |
                       |   ||    |
                       |  // \*) |           *) replicates are re-marked
                       +-//---\--+              with the LBE PHB
                        //     \
             1 Mbit/s  ||       |
             Premium   ||       | Lower than Best-Effort (LBE) service
             service   \/       V
                       .         \
                       .          \
                     // \\         .
                    //   \\         .
                    .     .       +---+
                   .       .      | R | New receiver wants no QoS

Figure 1:  A new receiver without QoS requirements joining a multicast
group that uses Premium service

As a further example, the LBE can be used as a separate service class
for background traffic (e.g., bulk transfers of low priority) that
should not impact the usual best-effort traffic.  Examples for this kind
of background traffic are backup data traffic sent over the network
during normal working hours and traffic from web search engines
gathering data from web servers.  Additionally, applications in DS aware
end-systems are able to pre-mark packets of minor importance, while
allowing to use the traditional inexpensive best-effort service for all
other more important packets.

The LBE PHB solves the above mentioned problems by discarding LBE
packets more rigorously than those of the best-effort BA in case of
resource contention for residual bandwidth, therefore limiting the
amount of packets in the LBE class in relation to the best-effort class.
This is described more detailed in sections 2.1 and 2.3.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
document are to be interpreted as described in [4].

2  Description of LBE per-hop behavior

A lower than best-effort service can be used in general to protect
traffic in the best-effort service class from unfairness that would
otherwise have been caused by those packets which are now in the LBE.
The latter packets may stem from excess traffic or re-marked traffic
that has been experiencing a better service before.  Therefore, in order

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to restrict the impact of those packets on packets in the Default PHB
BA, the LBE PHB provides forwarding of IP packets with a higher drop
precedence than Default PHB packets.

It is not necessary to have an LBE PHB group comprising more than one
PHB, because within its BA, LBE PHB essentially resembles best-effort
behavior that does not distinguish different types of traffic (e.g.,
responsive and non-responsive flows), too.

The Lower than Best-Effort per-hop behavior is intended for general use.
There may be many cases in which the LBE PHB can be applied.  Three of
them were already mentioned in section 1.

Using this PHB between two adjacent service providers is also useful,
because the upstream domain possibly had enough resources left to carry
the additional LBE traffic volume, whereas the downstream domain has not
enough resources, therefore being in need of discarding some portion of
this traffic.

2.1  Interaction with Other PHB Groups

The LBE PHB is essentially defined by its relation to the default PHB:
an LBE packet is of minor relative importance compared to a best-effort
packet.  Thus, in case of contention for bandwidth (i.e., a congestion
situation), packets receiving best-effort treatment preempt packets of
the LBE behavior aggregate down to a certain share.  This share must be
considerably lower than that of the best-effort BA, but should not be
equal to zero in order to retain a low portion of LBE traffic even when
best-effort traffic takes up all available residual bandwidth.
Therefore, a minimum configured bandwidth share for LBE traffic exists
as a lower bound that guarantees transport of LBE packets even in case
of congestion.  On the other hand, this bound also constitutes an upper
limit for the share of LBE traffic during congestion.  This upper bound
may be static, that is, fixed in relation to the overall available
bandwidth on a particular link (a constant value for 100-Y in Fig. 2),
or it may be dynamic, i.e., fixed relative to the BE traffic share, thus
variable in relation to the overall available bandwidth, because BE
traffic may consume resources currently not used by other BAs (a dynamic
value for Y that depends on the amount of BE traffic; corresponding to a
constant ratio of (100-Y)/(100-X) in Fig. 2).

0%                       X%                            Y%     100%
| Other BAs              |        Best-Effort          |   LBE   |
                                                       |  Bound  |

Figure 2:  Bandwidth share limits for LBE traffic.  Other behavior
aggregates (BA) currently use X% of total available bandwidth, while
best-effort and LBE traffic share the residual bandwidth.

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If enough resources are available for other BAs and BE traffic, i.e.,
residual bandwidth exists that is not used by best-effort traffic, LBE
packets may use more than the previously defined bound on their share of
total bandwidth, because this limit is only needed in case of congestion
(see Fig. 3 and 4).  Therefore, any remaining resources can be used to
forward excess LBE traffic, because all BE demand is already met.  In
this case, there is no reason to discard any of the LBE packets until
the residual bandwidth is exhausted by them, because their presence does
not adversely affect any of the best-effort packets.

========================================|  40%   upper bound



==========|                   10% upper bound
LLLLLLLLLLLLLLL               15% admitted traffic
output link bandwidth

O: Other BAs, B: Best-Effort BA, L: LBE BA

Figure 3:  Other BAs fully exploit their allotted resources, LBE BA gets
residual bandwidth that best-effort doesn't use.

OOOOOOOOOOOOOOOOOOO                        15% offered traffic
========================================|  40% upper bound
OOOOOOOOOOOOOOOOOOO                        15% admitted traffic



==========|                              10% upper bound
LLLLLLLLLLLLLLLLLLLLLLLLLLLLL            30% admitted traffic
output link bandwidth

O: Other BAs, B: Best-Effort BA, L: LBE BA

Figure 4:  Other BAs do not fully use their allotted resources,
best-effort utilizes unused bandwidth of the other BAs, LBE gets
residual bandwidth

The primary objective of the LBE PHB is to segregate packets of the
best-effort behavior aggregate from packets which should receive a lower
than best-effort treatment in case of congestion.  This is achieved by

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using a higher drop precedence for LBE packets than for best-effort
packets, and by limiting the overall amount of LBE traffic in relation
to the best-effort behavior aggregate (see Fig. 5).  Therefore, a
congested node tries to protect best-effort packets from being lost by
preferably discarding LBE packets.

========================================|  40% upper bound



==========|                               10% upper bound
LLLLLLLLLLL                               10% admitted traffic
output link bandwidth

O: Other BAs, B: Best-Effort BA, L: LBE BA

Figure 5:  Other BAs do not fully use their allotted resources,
best-effort utilizes unused bandwidth of the other BAs, LBE is limited
to its specified upper bound.

With exception of the Default PHB, the LBE PHB is entirely independent
of all other existing PHB specifications.  Thus, any other PHB groups
may co-exist with the LBE PHB in the same DS domain, because the LBE PHB
does not preempt forwarding resources of other PHB groups.

If only a part of all packets belonging to a microflow are marked as
LBE, the probability for reordering this microflow's packets may be
increased in dependence on the relation of the prior PHB to the LBE PHB
and may depend on the actual implementation.  However, because the LBE
PHB is defined by a special relation to the Default PHB, it is
recommended that packets of a microflow are not reordered if they are
marked by codepoints associated with these two PHBs.

A realization of the LBE PHB may use an already existing PHB of higher
drop precedence (e.g., AFx2 or AFx3 from the AF PHB group [2]), if its
actual implementation fulfills the requirements of this specification.

2.2  Traffic Conditioning Actions

Usually, the amount of LBE traffic is implicitly limited by queueing
mechanisms and related discard actions.  Therefore, there is normally no
need to meter and police LBE traffic explicitly.  However, a DS domain
MAY control the amount of LBE traffic that enters or exits the domain.
Traffic conditioning actions MAY include traffic shaping and discarding
of packets.

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2.3  Queueing and Discard Behavior

All packets of an arbitrary microflow that are marked for the LBE PHB
MUST NOT be reordered.  The dropping algorithm MUST treat all packets
within the LBE BA identically, i.e., the discard rate of a particular
microflow's packets will be proportional to that flow's percentage of
the total amount of traffic with this LBE BA.

It is recommended that LBE packets use the same queue as best-effort
packets in order to avoid reordering of a microflow's packets that are
marked intermittently for best-effort or LBE forwarding.

One way to achieve the above objectives is to use a common queue for
best-effort and LBE packets that is actively managed by a Random Early
Detection (RED) scheme [5].  A separate RED parameter set is managed for
each traffic type.  If the "current" queue length (usually a weighted
average value) grows beyond a lower threshold, new arriving LBE packets
for this queue are going to be dropped randomly with increasing
probability, until the queue length reaches an upper threshold.  Beyond
this upper threshold every new incoming LBE packet for this queue is
going to be discarded.  If the threshold values for LBE packets are
lower than those for best-effort packets, the RED queue will start
discarding packets earlier.

2.4  Recommended Codepoint for this PHB

As long as this a PHB proposal, the temporary recommended code point is
taken from the experimental/local use (EXP/LU) portion of the codepoint
space.  The recommended temporary codepoint is '101011' (see section 3
of [1] for the meaning of this notation).

2.5  Configuration and Management Issues

There is no need for providing any resource-based admission control
mechanisms for this PHB. As described in section 2.1, a configurable
minimal bandwidth share, respectively upper bound, exists for bandwidth
used by the LBE BA if no residual bandwidth is left and all other
bandwidth is used for best-effort.

3  Security Considerations

Basically, the LBE PHB causes no other security implications besides the
ones already mentioned in [1].  Because LBE PHB provides a quality that
is even lower compared to the usual best-effort delivery, there is now
one more possible PHB to reduce a packet's service.

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On the other hand, there is currently no PHB providing a worse service,
so LBE packets cannot be further reduced in service by re-marking.
Consequently, re-marking the inner header's codepoint of LBE packets at
the egress of a tunnel with a codepoint of another PHB would have only a
positive effect on these packets.  However, this would possibly
eliminate the primary objective of the LBE and lead to depletion of
forwarding resources for other traffic streams in congestion situations.

4  Tunneling

When LBE packets are tunneled, the tunneling packets must also be marked
as LBE.

5  Implications on Services

As described in section 2.1, traffic using the current best-effort
service should be segregated and protected from LBE traffic in
congestion situations.  Therefore, performance of the common best-effort
service is increased under those conditions.

6  Mapping to link-layer QoS mechanisms

In a shared medium LBE traffic has a very good counterpart in the
link-layer QoS mechanisms as defined by IEEE 802.1p:  the background
traffic type.  Therefore, packets carrying a DSCP value of the LBE PHB
can be mapped to 802.1p background traffic priority, i.e., setting the
"user_priority" field to value 1 in all link-layer frames that carry a
part of an LBE packet as payload.  In order to achieve a real support
for LBE packets, link-layer frames containing best-effort packets should
use the default user_priority of 0 for indicating traffic type "Best

LBE packets within a switched link-layer could also use available means
to indicate a higher drop precedence for LBE packets.  For instance,
when using ATM as link-layer, the value encoded in the Cell Loss
Priority (CLP) field of the ATM cell header [6] may be set to 1 in all
ATM cells carrying a part of an LBE packet as payload, whereas cells
carrying a part of a best-effort packet as payload should use a CLP
value of 0.

As another example, when using Frame Relay as link-layer, the Discard
Eligibility Indicator (DE) bit can be set to 1 for frames containing an
LBE packet as payload, indicating that this frame should be discarded in
preference to other frames in a congestion situation [7].  All frames
carrying a part of a best-effort packet as payload should use a DE bit
value of 0.

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7  Acknowledgments

Thanks to Jochen Schiller for discussion of the LBE PHB.


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

[2] F. Baker, J. Heinanen, W. Weiss, and J. Wroclawski. Assured
    Forwarding PHB Group. RFC2597, June 1999.

[3] S. Blake, D. Black, M. Carlson, E. Davies, Z. Wang, and W. Weiss.
    An Architecture for Differentiated Services. RFC 2475, Dec. 1998.

[4] S. Bradner. Key words for use in RFCs to Indicate Requirement
    Levels. RFC 2119, Mar. 1997.

[5] S. Floyd and V. Jacobson. Random Early Detection Gateways for
    Congestion Avoidance. IEEE/ACM Transaction on Networking,
    1(4):397--413, Aug. 1993.

[6] ITU-T. B-ISDN ATM Layer Specification. ITU-T Recommendation I.361,

[7] ITU-T. ISDN Data Link Layer Specification for Frame Mode Bearer
    Services. ITU-T Recommendation Q.922, 1992.

8  Authors' addresses

Comments and questions related to this draft can be addressed to one of
the authors listed below.

Roland Bless
Institute of Telematics
Universitaet Karlsruhe (TH)
Zirkel 2
D-76128 Karlsruhe
Tel.:  +49 721 608 6396

Klaus Wehrle
Institute of Telematics
Universitaet Karlsruhe (TH)
Zirkel 2
D-76128 Karlsruhe
Tel.:  +49 721 608 6414

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