Drop Precedence for Ethernet Frames November 9, 2003 Ali Sajassi sajassi@cisco.com

1. Drop Precedence forEthernet FramesNovember 9, 2003Ali Sajassisajassi@cisco.com

2. Need for Drop Precedence • Drop Precedence if VBR type SLA is needed with CIR/PIR type thresholds • DP would enable proper marking of frames that exceed CIR threshold • During congestion, frames that are marked with higher DP, will be discarded before frames with lower DP for the same class • Packet re-ordering should NOT happen within a class

3. Implicit v.s. Explicit DP • Two ways of doing Drop Precedence • Explicitly by using CFI bit • Implicitly by using .1p code points

4. Advantages of Explicit Indication • Can have all eight CoS • No packet re-ordering if default configuration is used • Can have drop precedence on all eight classes if needed

5. Disadvantage of Explicit Indication • Requires H/W changes and thus new bridges • Is not compatible with existing deployed bridges where marking is done implicitly • Is not compatible with existing deployed router operation where L3 QoS marking is reflected in L2 CoS field • Is not compatible when peering with MPLS/IP networks since MPLS/IP doesn’t use explicit bit for Drop Precedence • Frames can be discarded even without any congestion in the network (in some bridges). This issue can be very pronounced in enterprise networks where because of low cost of facilities (links) and over-provisioning, there can be no congestion.

6. Advantages for Implicit Indication • Can be currently supported with deployed bridges w/ implicit marking • Compatible with existing deployed router operation where L3 QoS marking is reflected in L2 CoS field • Compatible when peering with MPLS/IP networks • Frames don’t get discarded when there is no congestion

7. Disadvantages for Implicit Indication • Fewer than eight classes (e.g., seven or six classes). BUT even IEEE 802.1D doesn’t define eight classes • Possibility of frames re-ordering • Possibility only exists if default mode is used and doesn’t exist if bridges need to be configured which is very much the case for Service Provider networks (to deliver E2E QoS) • Possibility can be minimize by proper assignment of DP to priority classes

8. Implicit Drop Precedence • 7 classes – one of which w/ DP • 6 classes – two of which w/ DP • 5 classes – three of which w/ DP

9. 802.1D – Appendix G, Table G-2

10. 802.1D – Appendix G, Table G-1

11. 802.1D – Appendix G, Table G-3

12. Frame Re-ordering To minimize frame re-ordering, we choose the following classes for DP • If there are 7 classes • use P1 & P2 as one class • P1 indicates lower DP • If there are 6 classes • use {P1, P2} and {P7, P8} as two classes • P1 and P7 would indicate lower DP

13. Frame Re-odering - Continue • With this assignment: • w/ 7 CoS, no re-ordering occurs for bridges with 1 to 7 queues • w/ 6 CoS, no re-odering occurs for bridges with 1 to 6 queues

14. Encoding w/ 7 Class of Services one of which with DP Application Typical L3 Classification L2 MPLS IPP PHB DSCP CoS Exp SP Routing & Control 6 CS6 48 6 6 Voice 5 EF 46 5 5 4 CS4 32 4 4 Streaming Video (future) Call Signaling 3 CS3 24 3 3 Network Management 2 C 16 2 2 Mission-Critical Data 3 AF31 26 3 3 CIR (Committed Information Rate) 2 AF11 10 2 2 PIR (Peak Information Rate) 1 AF12 12 1 1 Best Effort 0 0 0 0 0