Drop Precedence for Ethernet Frames November 9, 2003 Ali Sajassi sajassi@cisco. com 1
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 2
Implicit v. s. Explicit DP • Two ways of doing Drop Precedence a) Explicitly by using CFI bit b) Implicitly by using. 1 p code points 3
Advantages of Explicit Indication • Can have all eight Co. S • No packet re-ordering if default configuration is used • Can have drop precedence on all eight classes if needed 4
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 L 3 Qo. S marking is reflected in L 2 Co. S 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. 5
Advantages for Implicit Indication • Can be currently supported with deployed bridges w/ implicit marking • Compatible with existing deployed router operation where L 3 Qo. S marking is reflected in L 2 Co. S field • Compatible when peering with MPLS/IP networks • Frames don’t get discarded when there is no congestion 6
Disadvantages for Implicit Indication • Fewer than eight classes (e. g. , seven or six classes). BUT even IEEE 802. 1 D 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 E 2 E Qo. S) Possibility can be minimize by proper assignment of DP to priority classes 7
Implicit Drop Precedence • 7 classes – one of which w/ DP • 6 classes – two of which w/ DP • 5 classes – three of which w/ DP 8
802. 1 D – Appendix G, Table G-2 user_priority Acronym 1 2 0 (Default) 3 4 5 6 7 BK BE EE CL VI VO NC Traffic type Background Spare Best Effort Excellent Effort Controlled Load Video, < 100 ms delay Voice, <10 ms delay Network Control 9
802. 1 D – Appendix G, Table G-1 # of Qs 1 2 3 4 5 6 7 Traffic Types {BK, BE, EE, CL, VI, VO, NC} {BK, BE, EE} {CL, VI} {VO, NC} {BK} {BE, EE} {CL} {VI} {VO, NC} {BK} {BE} {EE} {CL} {VI} {VO} {NC} 10
802. 1 D – Appendix G, Table G-3 # of Qs 1 2 3 4 5 6 7 8 BK BK BK Defining Traffic Type BE BE VO BE CL VI VO BE EE CL VI VO NC 11
Frame Re-ordering To minimize frame re-ordering, we choose the following classes for DP • If there are 7 classes - use P 1 & P 2 as one class - P 1 indicates lower DP • If there are 6 classes - use {P 1, P 2} and {P 7, P 8} as two classes - P 1 and P 7 would indicate lower DP 12
Frame Re-odering - Continue • With this assignment: w/ 7 Co. S, no re-ordering occurs for bridges with 1 to 7 queues w/ 6 Co. S, no re-odering occurs for bridges with 1 to 6 queues 13
Encoding w/ 7 Class of Services one of which with DP Application Typical L 3 Classification IPP PHB DSCP L 2 Co. S MPLS Exp SP Routing & Control 6 CS 6 48 6 6 Voice 5 EF 46 5 5 Streaming Video (future) 4 CS 4 32 4 4 Call Signaling 3 CS 3 24 3 3 Network Management 2 C 16 2 2 Mission-Critical Data 3 AF 31 26 3 3 CIR (Committed Information Rate) 2 AF 11 10 2 2 PIR (Peak Information Rate) 1 AF 12 12 1 1 Best Effort 0 0 0 14
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