Drop Precedence for Ethernet Frames by aah15699


									Drop Precedence for
  Ethernet Frames

    November 9, 2003

        Ali Sajassi
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

Implicit v.s. Explicit DP

  •   Two ways of doing Drop Precedence
      a) Explicitly by using CFI bit
      b) Implicitly by using .1p code points

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

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.

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

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

Implicit Drop Precedence

     • 7 classes – one of which w/ DP
     • 6 classes – two of which w/ DP
     • 5 classes – three of which w/ DP

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
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}

802.1D – Appendix G, Table G-3

# of Qs               Defining Traffic Type
    1                          BE
    2              BE                      VO
    3              BE              CL          VO
    4       BK          BE         CL          VO
    5       BK          BE      CL      VI     VO
    6       BK       BE EE CL           VI     VO
    7       BK       BE EE CL           VI   VO NC
    8     BK     -   BE EE CL           VI   VO NC

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

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

Encoding w/ 7 Class of Services one of which
with DP

                                     Typical L3 Classification    L2   MPLS
                                    IPP       PHB       DSCP     CoS    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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