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 .1p code points
3
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
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 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.
5
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
6
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
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
802.1D – 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
802.1D – 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}
{BK} {BE, EE} {CL} {VI} {VO, NC}
{BK} {BE} {EE} {CL} {VI} {VO, NC}
{BK} {BE} {EE} {CL} {VI} {VO} {NC}
10
802.1D – Appendix G, Table G-3
# of Qs
1
2
3
4
5
6
7
8
BK
BK
BK
BK
BK
Defining Traffic Type
BE
BE
VO
BE
CL
VO
BE
CL
VO
BE
CL
VI
VO
BE EE CL
VI
VO
BE EE CL
VI
VO NC
BE EE CL
VI
VO NC
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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
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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
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Encoding w/ 7 Class of Services one of which
with DP
Application
Typical L3 Classification
IPP
PHB
DSCP
L2
CoS
MPLS
Exp
SP Routing & Control
6
CS6
48
6
6
Voice
5
EF
46
5
5
Streaming Video (future)
4
CS4
32
4
4
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
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