Unable to match on the updated DSCP mark given by meter

[EnasAhmad]

Hi,

I am facing yet another problem in the DSCP remark meters. 

consider this scenario: I have two tables and one meter. 

Table 0 directs packets into meter 1, and then into table 1. 

Meter 1 changes the DSCP mark of packets that are over the meter's rate from DSCP X to DSCP Y.

Table 1 has a flow entry that matches on DSCP Y.

Although packets are correctly remarked to DSCP Y as shown in the receiver's Wireshark capture, the flow entry in table 1 does not match any packets. If I originally send packets with DSCP Y with no metering, the flow entry is correctly matched.

It seems it is related somehow to the bug I have previously reported. The old DSCP mark is taken into account when performing the matching in the following tables instead of the new DSCP mark given by the meter.

At what point is the DSCP mark changed in the switch processing flow? It seems to me that the mark is changed after (or right before) the packet is queued and not inside the OpenFlow pipeline?

Note: I am only trying to match on the new meter DSCP mark as a workaround for the previous problem of incorrect queue selection, however, this also does not work :(  

Thanks,

Enas

[Abhay_B]

Hello Enas,

The behavior you see is expected. The lookup for DSCP in a packet on any OpenFlow table is not affected by the remarked value of the DSCP in the packet. Lets us just say they are read and written into 2 different locations.

The new DSCP value is only updated at the egress when all OpenFlow related processing is done on the packet. 

Thanks!

Abhay

[EnasAhmad]

Hi Abhay,

Thank you for the clarification. 

I assumed that updating the DSCP is similar to the set field action. Becuase according to the Openflow standard as shown in the flow diagram below, a packet should be updated with new fields values before moving to the next table in the pipeline, thus, the update happens in the Openflow pipeline and not afterward.

Also, in section 5.3 : "The packet match fields represent the packet in its current state, if actions applied in a previous table using the Apply-Actions changed the packet headers, those changes are reflected in the packet match fields"

I have not tested the set_field action in the HP switch yet, but does it also update the fields at the egress? 

Thank you very much,

Enas