802.1ad Drop Precedence
Architecture Proposal v3
Stephen Haddock
March 16, 2004
Bridge Model
Relay
DE
DE
PRI
MAC
PRI
EISS
Network



MAC
Network
Assume that the EISS carries a 3-bit Priority parameter
(PRI) and a single bit Drop Eligible parameter (DE).
PRI is identical to the current priority parameter.
Discuss how these parameters are generated at each
port later – focus on the drop precedence functionality in
the Relay first.
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Objectives
 Maintain current frame ordering constraints
 The probability of dropping a yellow frame (DE set)
shall be greater than or equal to the probability of
dropping a green frame (DE clear) in the same traffic
class.
 Never promote yellow (DE set) to green (DE clear).
 Relative priority between any two frames will never
be reversed. (mapping to equal priority is OK).
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Drop Precedence Relay Model
DE
DE
PRI
PRI
Ingress
0 or more
Ingress
Flow
Meters
Forwarding
Queuing
1 to 8
Traffic
Class
Queues
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Transmission
Scheduler
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Ingress Rules
(discussion points)
1. Zero or more flow meters may be implemented per ingress
port.
2. Meters do not change the PRI value.
3. No restrictions on how an individual packet is directed to a
specific flow meter (e.g. may be based on S-VID, PRI, a
combination thereof, or something else).
4. If flow meters are implemented, not all flows are required to go
through a meter.
5. The DE value shall not be changed for packets not going
through a meter.
6. Flow meters may be buffered (shaper) or unbuffered (policer).
7. Flow meters may set the DE parameter and may drop
packets, but shall not clear the DE parameter.
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Ingress Rules
(incorporate in 8.6.1)
1. Zero or more flow meters may be implemented per ingress
port.
2. Meters do not change the PRI value.
3. No restrictions on how an individual packet is directed to a
specific flow meter (e.g. may be based on S-VID, PRI, a
combination thereof, or something else).
4. A bridge supporting metering at a given port shall be capable
of metering at line rate.
5. Not all flows are required to go through a meter, but at a
minimum, a bridge supporting metering at a given port should
be capable of metering all the frames received on that port.
Finer grain metering may be supported.
6. The DE value shall not be changed for packets not going
through a meter.
7. Flow meters may be buffered (shaper) or unbuffered (policer).
8. Flow meters may set the DE parameter and may drop
Slide 6
July 2003
packets, but shall
not
clear
the DE
parameter.
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Queuing Rules
(incorporate in 8.6.5)
1. One to eight queues may be implemented per egress port.
2. Individual packets are directed to a specific queue according
the PRI bits and the priority-to-traffic-class mapping table
currently specified in 802.1D-2004.
3. Some or all queues may implement a drop precedence aware
queue management algorithm (e.g. queue depth threshold for
packets with DE set, RED, WRED, …)
4. Queues may discard packets. When drop precedence is
supported, the probability of dropping a packet with DE set
shall be greater than the probability of dropping a packet with
DE clear.
5. Queues shall not change the PRI value or the DE value.
6. There may be meters in front of the queues. If these meters
are implemented, they may set the DE value.
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Transmission Rules
(incorporate in 8.6.6)
1. As specified in 802.1D-2004 a strict priority scheduling
algorithm shall be supported, and other scheduling algorithms
may be supported.
2. The scheduler shall not change the PRI value.
3. An optional scheduling algorithm may incorporate a flow meter
(i.e. rate-based scheduling or shaping). Such a scheduler
may set the DE parameter. Otherwise The DE parameter is
not modified by the scheduler.
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Minimal Implementation
 Minimal compliant implementation has no drop
precedence awareness:



Zero flow meters at any ingress port.
One to eight queues at each egress port with no drop
precedence aware queue management algorithms.
No rate based scheduling algorithms, or at least no
algorithms that modify the DE value.
 Therefore the PRI and DE values pass through the
Relay unchanged.
 Only change from an 802.1D-2004 compliant bridge is
the ability to carry the DE value through the Relay.
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Implementation Consideration



Just as a 802.1D bridge may support fewer than 8 traffic
classes, and 802.1ad bridge may support fewer than 8 traffic
classes and only a subset of those traffic classes support drop
precedence.
If the number of PRI:DE combinations that are supported is 8
or fewer, an implementation may choose to carry PRI:DE
through the Relay in a 3 bit field.
There is a potential loss of information in encoding PRI:DE to
a 3 bit field, but no more so than occurs when encoding
PRI:DE as a 3 bit field in a S-tag.

Therefore the difference between this implementation and the
architectural model is not externally observable, so it is an
allowed implementation.
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Slide 10
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Bridge Model
DE
PRI
Relay
DE
PRI
S-TAG
MAC
S-TAG
EISS
Network

MAC
Network
Now consider how to encode PRI:DE in the S-TAG.
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Encoding: Conclusions from conf calls



If figure out how to make PRI:DE encoded in 3 bit field work,
then should be simple to add option use CFI for DE.
Encoding issues are very simple if every bridge uses the same
encoding, but need to consider the case where connecting
“domains” that use different encoding.
Having a restricted set of allowed mappings is acceptable.
There should also be specified default mappings (similar to
the current priority-to-traffic-class mapping table).
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Proposed default DP mapping:
Encoded
Value
PRI only
Bridge
PRI w/ DP
Bridge
PRI only
Bridge
7:
6:
5:
4:
3:
2:
1:
0:
7
6
5
4
3
2
1
0
6/7 green
6/7 yellow
4/5 green
4/5 yellow
0/3 green
1/2 green
1/2 yellow
0/3 yellow
7
6
5
4
3
2
1
0
Identical to 802.1Q
Traffic Class Mapping
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802.1Q-2003
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802.1D – Appendix G, Table G-2
user_priority
Acronym
Traffic type
1
BK
Background
2
-
Spare
0 (Default)
BE
Best Effort
3
EE
Excellent Effort
4
CL
Controlled Load
5
VI
Video, < 100 ms delay
6
VO
Voice, <10 ms delay
7
NC
Network Control
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802.1D Table G-3
User
Pri
8tc
1
BK
2
--
0
BE
BE
BE
3
EE
EE
EE
4
CL
CL
CL
CL
5
VI
VI
VI
VI
6
VO
VO
7
NC
NC
VO
VO
7tc
6tc
BK
BK
5tc
4tc
3tc
2tc
BE
BE
1tc
BK
BE
BE
BE
CL
CL
CL
VO
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VO
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Allowable pairings for drop precedence
8tc
w/
0dp
1
2
7tc
w/
1dp
2
6tc
w/
2dp
1
1
1
2
2
2
0
0
3
3
0
0
3
3
4
4
4
5
5
5
6
6
6
6
7
7
7
7
8q
7q
0
4
2
0
4
4
5
5
6
2
0
3
1
2
0
4
4
6
6
6
7
7
7
2
0
3
5tc
w/
3dp
1
1
2
2
0
4
4
5
5
6
6
0
3
4
6
6q
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2
0
4
6
7
2
0
4
5
6
5q
2
0
4tc
w/
4dp
1
2
2
0
0
4
4
4
6
6
6
3
4q
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If we can change Table 8-2 …
 Swap priority 0 and 2

Current table has two traffic classes that are lower priority
than the default priority


1=“background” and 2=“spare” are lower than 0=“best effort”
Change would make only one traffic class lower priority
than the default
 Select new default for the cases where there are 7, 6,
and 5 traffic classes
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If we can change table 8-2 …
8tc
w/
0dp
1
0
7tc
w/
1dp
0
6tc
w/
2dp
1
1
1
0
0
0
2
2
3
3
2
2
3
3
4
4
4
5
5
5
6
6
6
6
7
7
7
7
8q
2
4
7q
0
2
4
4
5
5
6
0
2
3
1
0
2
4
4
6
6
6
7
7
7
0
2
3
5tc
w/
3dp
1
1
0
0
2
4
4
5
5
6
6
2
3
4
6
6q
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0
2
4
6
7
5q
0
2
4
5
6
0
2
4tc
w/
4dp
1
0
0
2
2
4
4
4
6
6
6
3
4q
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Default drop precedence table
8tc 7tc 6tc 5tc 4tc
w/
w/
w/
w/
w/
0dp 1dp 2dp 3dp 4dp
1
1
1
0
0
0
2
2
3
3
0
0
2
2
2
4
4
4
4
6
6
6
6
7
7
7
7
4
5
6
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New table G-3 (with color)
8tc 7tc 6tc 5tc 4tc 3tc 2tc 1tc
w/
w/
w/
w/
w/
w/
w/
w/
0dp 1dp 2dp 3dp 4dp 3dp 2dp 1dp
BK
BK
BK
BE
BE
BE
--
--
EE
EE
CL
VI
CL
EE
BE
BE
BE
EE
BE
EE
BE
CL
CL
VO
VO
VO
VO
NC
NC
NC
NC
CL
CL
CL
VO
VO
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New default EISS mapping:
PRI : DE
Egress EISS
Encoded
Tag Field
7:G
7:Y
6:G
6:Y
5:G
5:Y
4:G
4:Y
3:G
3:Y
2:G
2:Y
1:G
1:Y
0:G
0:Y
7
6
5
4
3
2
1
0
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PRI : DE
Ingress EISS
6:G
6:Y
4:G
4:Y
2:G
2:Y
0:G
0:Y
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Old Slides
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Encoding PRI:DE in the S-tag

Using the old CFI bit for DE in the S-tag allows PRI:DE to be
carried without loss of information.


Encoding PRI:DE in what is currently the 3-bit priority field of
the S-tag (call it the Priority/Drop-Precedence or PDP field) is
friendlier to existing equipment.



But does not “grandfather in” existing equipment.
All ports connected to a LAN must use the same mapping
between PDP and PRI:DE.
There will be a loss of information bridging between LANs that
use different mappings between PDP and PRI:DE.
It is possible to allow both as configurable options.

Is support of one mode required and the other optional? If so,
which is required?
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Mapping observations -- 1

Things will “just work” if constrain all ports use the same
mapping between PDP and PRI:DE.



PDP  PRI:DE  PDP mapping does not result in any lost
information.
If ports on each end of a network link need to use the same
mapping , the it follows that all ports of all bridges on a
network use the same mapping.
Interoperability only assured if all switches must support all
possible mappings, or if a constrained set of required
mappings is specified.
But are we willing to live with the constraint that all ports of all
bridges connected in a network must have the same
mapping? (Particularly problematic at connection between two
different Service Providers.)
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Proposed Mapping Rules
1. Packet order within a priority shall be maintained

Means two values of PRI:DE that differ only in the DE value
cannot be mapped to two PDP values that are interpreted as
having different priority.
2. Relative priority between packets shall be maintained through
all mapping tables

No packet that is initially tagged as higher priority than a
second packet shall ever be mapped in a fashion that tags it a
lower priority than the second packet.
3. When two values of PRI:DE that differ only in DE are mapped
to a single PDP, packets with DE set may be transmitted
(effectively clearing DE) or discarded.

Do we specify discard vs. transmit in the standard, or let the
implementation decide, or mandate that an implementation
must be configurable to do either?
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Mapping Example 1
8/0 port
7
6
5
4
3
2
1
0
Priority7
Priority6
Priority5
Priority4
Priority3
Priority2
Priority1
Priority0
4/4 port
7
6
5
4
3
2
1
0
Gold-G
Gold-Y
Silver-G
Silver-Y
Bronze-G
Bronze-Y
Lead-G
Lead-Y



PRI information is permanently
lost going from 8/0 to 4/4.
DE information is lost (or
packets are discarded) going
from 4/4 to 8/0.
Traffic going between a 8/0
port and a 4/4 port get the
same effective behavior as if
both ports were 4/0.
Key:
m/n  m Priority values of which
n have Drop Precedence.
G (Green)  low discard probability.
Y (Yellow)  high discard probability.
Dashed arrow  map or discard
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Mapping Example 2
A 6/2 port
7
6
5
4
3
2
1
0
Control
VoIP
EF
AF1-G
AF1-Y
AF2-G
AF2-Y
Default
B 6/2 port
7
6
5
4
3
2
1
0
Control
Platinum
Gold-G
Gold-Y
Gaming
Silver-G
Silver-Y
Other


(Conceivable that traveling A  B  C
 B  A could result in a packet
originally at EF ending up at Control,
thus violating Mapping Rule 2.)

Key:
m/n  m Priority values of which
n have Drop Precedence.
G (Green)  low discard probability.
Y (Yellow)  high discard probability.
Dashed arrow  map or discard
PRI information is permanently
lost going from A port to B port.
EF traffic that travels from
network A through B and back
to A will end up higher priority
than EF traffic local to A.
Alternative mapping would
map EF to Gold and AF1 to
Gaming, resulting in a loss of
DE information instead of PRI.
(Should one of these mapping
alternatives be required by the
standard?)
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Issues

Investigating the consequences of going from any given
mapping to any other gets complex, and leads to network
behavior that is certainly not obvious and possibly not
desirable.




Are the Proposed Mapping Rules sufficient to preserve
interoperability (and hopefully sanity)?
Do we need further rules (such as “when collapsing two PRI
values to one PDP, always map to a PDP which is interpreted
as the higher of the two priorities”)?
Do we try to describe the network behavior resulting from
various mapping combinations, or do we let Service Providers
sort it all out?
Should we simplify the situation by specifying a small set of
mappings that must be supported, or would that undermine the
implicit objective of grandfathering existing equipment?
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Note on placement of PRI:DE – PDP
mapping function



I’ve assumed it is between the ISS and EISS (therefore in
section 6.x of 802.1ad) so the PRI:DE values are passed
across the EISS.
It could as easily go on the other side of the EISS in the
Ingress and Transmission portions of the Relay (sections 8.6.1
and 8.6.6 respectively) which means the PDP value is passed
across the EISS.
There is precedent for either solution:



The VID value passed across the EISS is taken exactly from the
packet if tagged or priority-tagged, or null if untagged. It is translated
to the default PVID or port-protocol ID in the Ingress portion of the
Relay.
The current priority value is taken from a tagged packet or, if packet is
untagged, is “regenerated” prior to being passed across the EISS.
The functionality is the same either way.
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802.1D – Appendix G, Table G-1
# of Qs
Traffic Types
1
{BK, BE, EE, CL, VI, VO, NC}
2
{BK, BE, EE} {CL, VI, VO, NC}
3
{BK, BE, EE} {CL, VI} {VO, NC}
4
{BK} {BE, EE} {CL, VI} {VO, NC}
5
{BK} {BE, EE} {CL} {VI} {VO, NC}
6
{BK} {BE} {EE} {CL} {VI} {VO, NC}
7
{BK} {BE} {EE} {CL} {VI} {VO} {NC}
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802.1D – Appendix G, Table G-3
# of Qs
Defining Traffic Type
1
BE
2
BE
3
BE
VO
4
BK
BE
5
BK
BE
6
BK
BE
7
BK
CL
VO
CL
VO
CL
VI
VO
EE
CL
VI
VO
BE
EE
CL
VI
VO
NC
8
BK
-
BE
EE
CL
VI
VO
NC
Priority 
1
2
0
3
4
5
6
7
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