diffserv, dscp & wrr - What layer(s) do they operate?

[voiper_99]

Hi, I am interested to know which layer(s) diffserv, dscp & wrr operate at.

I believe diffserv and dscp are the same thing, though diffserv only has priority bits ranging from 0-7 but dscp extends that to more than 60. However, what I don't know what layer the two operate nor do I know what layer wrr operates at.

If someone could enlighten me it would be much appreciated.

Thank you.

[Timor_SSS]

Diffserv is a QOS technology for dealing many issues. One of it's components is marking traffic classes. One of the marking methods is DSCP.

If you look inside IP header, you'll see a whole BYTE for QOS, it's called TOS (Type Of Service Byte). In this Byte the first 6 bits are used by DSCP, however, only the first 3 are considered most significant, the other three are used to mark drop probability for the packet and rarely used for actual marking. The first three bits give you 8 values, the same bits are used by other marking methods as well, thus making DSCP backward compatible.

QOS is a big topic and a very important one... My advice for you is to spend some time learning it.

[voiper_99]

Thank you for the reply. I am currently studying QoS (amongst other things). So what layer is Diffserv and WRR? Are they both layer 3?

[Timor_SSS · Accepted solution]

Diffserv is QOS technology that applies both on L2 and L3 layers. In case of DSCP it is considered to be a layer 3 feature because it uses TOS byte of the IP header. In case of CoS (class of service), it uses L2 markings. WRR is bandwidth allocation method for various queues (each queue at a time). It can be layer 2 feature on L2 switches (every queue gets certain transmission period), and also can be L3 feature that used by CQ (custom queuing) - one of the queuing methods of the Diffserv models.

HTH

Tim

[voiper_99]

Thanks for the great explanation, it is much appreciated.

[voiper_99]

I just have one other question. Why would one want to map a DSCP value to a CoS value? Would it be because the layer 3 device would be using DSCP but a directly connected layer 2 device would be using CoS so the layer 3 device needs a way to convert CoS to DSCP for incoming traffic and also convert DSCP to CoS for outgoing traffic?

[Edison Ortiz · Accepted solution]

That is exactly right.

[voiper_99]

Excellent, thank you for the confirmation.