why pcie need reorder

[diankexiaobai]

     I was asked by interviewer that why pcie need cpld package reorder. as all i konw , same tag cpld would not out of order, but different tag will be out of order. such as tag0(cpld0a,cpldb), tag(cpld1a,cpld1b) .  it may be receiver sequence is cpld0a , cpld1a, cpld1b, cpld0b. I know it will be out of order when host and endpoint has a switch . but why when switch will make tlp out of order. and there are there other factors? thanks!!! 

[deepthi]

CLPD's with different Traffic class can go out of ordering. There is no ordering required between traffic
flows through different Virtual Channels. At every service point (e.g., Switch) within the
fabric, Traffic Class labels are used to apply appropriate servicing policies. Each Traffic Class
label has a unique ordering domain. No ordering guarantee can be provided for packets that
has different Traffic Class labels.

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