.1ad Drop Precedence Discussion

1
.1ad Drop Precedence Discussion

• Dinesh Mohan
• May 18, 2004

2
Allowable pairings for drop precedence
5tc w/ 3dp
4tc w/ 4dp
6tc w/ 2dp
7tc w/ 1dp
8tc w/ 0dp
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
7
7
7
7
7
7
7
7
3
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
• 1background and 2spare are lower than
  0best 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

• Swap priority 1 and 0
• No need to select any new defaults!

4
If we can change table 8-2
5tc w/ 3dp
4tc w/ 4dp
6tc w/ 2dp
7tc w/ 1dp
8tc w/ 0dp
0
0
0
0
0
0
0
1
0
0
0
0
0
0
0
0
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
0
2
2
2
2
2
2
1
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
7
7
7
7
7
7
7
7
5
Default drop precedence table
8tc w/ 0dp 7tc w/ 1dp 6tc w/ 2dp 5tc w/ 3dp 4tc w/ 4dp
0 0 0 0
1 1
2 2
3
4
5 5 5
6 6 6
7 7 7 7
0
2
2
2
3
3
3
4
5
6
6
New table G-3 (with color) - TBD
8tc w/ 0dp 7tc w/ 1dp 6tc w/ 2dp 5tc w/ 3dp 4tc w/ 4dp 3tc w/ 3dp 2tc w/ 2dp 1tc w/ 1dp
BK BK BK
BE BE BE
-- --
EE EE
CL
VI
VO VO VO VO
NC NC NC NC
7
New default EISS mapping
PRI DE Ingress EISS
Encoded Tag Field
PRI DE Egress EISS
7G 6G 5G 3G 3Y 2G 2Y 0Y
7 6 5 4 3 2 1 0
8
Discussion from IEEE 802.1 (Orlando)
9
Bridge Model
Relay
EISS

• 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.

10
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).

11
Drop Precedence Relay Model
Ingress
Transmission
Queuing
Forwarding
0 or more Ingress Flow Meters
1 to 8 Traffic Class Queues
Scheduler
12
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
   packets, but shall not clear the DE parameter.

13
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.

14
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.

15
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.

16
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 PRIDE combinations that are
  supported is 8 or fewer, an implementation may
  choose to carry PRIDE through the Relay in a 3
  bit field.
• There is a potential loss of information in
  encoding PRIDE to a 3 bit field, but no more so
  than occurs when encoding PRIDE 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.

17
Bridge Model
DE
Relay
PRI
EISS

• Now consider how to encode PRIDE in the S-TAG.

18
Encoding Conclusions from conf calls

• If figure out how to make PRIDE 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).

19
Proposed default DP mapping
PRI only Bridge
PRI w/ DP Bridge
Encoded Value
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
20
802.1Q-2003
21
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, lt 100 ms delay
6 VO Voice, lt10 ms delay
7 NC Network Control
22
802.1D Table G-3
User Pri 8tc 7tc 6tc 5tc 4tc 3tc 2tc 1tc
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
23
Allowable pairings for drop precedence
8tc w/ 0dp 7tc w/ 1dp 7tc w/ 1dp 7tc w/ 1dp 7tc w/ 1dp 6tc w/ 2dp 6tc w/ 2dp 6tc w/ 2dp 6tc w/ 2dp 6tc w/ 2dp 6tc w/ 2dp 5tc w/ 3dp 5tc w/ 3dp 5tc w/ 3dp 5tc w/ 3dp 4tc w/ 4dp
1 1 1 1 1 1 1 1
2 2 2 2 2 2 2 2
0 0 0 0 0 0 0 0
3 3 3 3 3 3 3 3
4 4 4 4 4 4 4 4
5 5 5 5 5 5 5 5
6 6 6 6 6 6 6 6
7 7 7 7 7 7 7 7
2
2
2
2
2
2
2
2
0
0
0
0
0
0
0
0
4
4
4
4
4
4
4
4
6
6
6
6
6
6
6
6
24
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
• 1background and 2spare are lower than
  0best 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

25
If we can change table 8-2
8tc w/ 0dp 7tc w/ 1dp 7tc w/ 1dp 7tc w/ 1dp 7tc w/ 1dp 6tc w/ 2dp 6tc w/ 2dp 6tc w/ 2dp 6tc w/ 2dp 6tc w/ 2dp 6tc w/ 2dp 5tc w/ 3dp 5tc w/ 3dp 5tc w/ 3dp 5tc w/ 3dp 4tc w/ 4dp
1 1 1 1 1 1 1 1
0 0 0 0 0 0 0 0
2 2 2 2 2 2 2 2
3 3 3 3 3 3 3 3
4 4 4 4 4 4 4 4
5 5 5 5 5 5 5 5
6 6 6 6 6 6 6 6
7 7 7 7 7 7 7 7
0
0
0
0
0
0
0
0
2
2
2
2
2
2
2
2
4
4
4
4
4
4
4
4
6
6
6
6
6
6
6
6
26
Default drop precedence table
8tc w/ 0dp 7tc w/ 1dp 6tc w/ 2dp 5tc w/ 3dp 4tc w/ 4dp
1 1 1
0 0 0
2 2
3 3
4
5
6 6 6 6
7 7 7 7
0
0
2
2
2
4
4
4
4
6
27
New table G-3 (with color)
8tc w/ 0dp 7tc w/ 1dp 6tc w/ 2dp 5tc w/ 3dp 4tc w/ 4dp 3tc w/ 3dp 2tc w/ 2dp 1tc w/ 1dp
BK BK BK
BE BE BE
-- --
EE EE
CL
VI
VO VO VO VO
NC NC NC NC
28
New default EISS mapping
PRI DE Ingress EISS
Encoded Tag Field
6G 6Y 4G 4Y 2G 2Y 0G 0Y
7 6 5 4 3 2 1 0
29
Allowable pairings for drop precedence
5tc w/ 3dp
4tc w/ 4dp
6tc w/ 2dp
7tc w/ 1dp
8tc w/ 0dp
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
7
7
7
7
7
7
7
7