Subject: Re: [tsvwg] Considerations for assigning new DSCPs

From: <[email-masked]>
Date: Tue, 23 Feb 2021 07:56:46 +0000

Hi Ana,

thanks for your draft.

The text as is reviews the standards and operational situation fairly good. There's one particular scenario, which I'd like to be mentioned: a network operator may have deployed an own operational DiffServ scheme, which works with a set of DSCPs. If DSCP based classification is configured for any router within that domain, the network operator needs to decide, how to treat packets received with such a DSCP at an interconnection link without an an operational DSCP.

Similar, a network operator may operate an interconnection agreement with a DiffServ related SLA in place. A set of DSCPs is agreed upon for exchange then. Again, a network operator will have to determine, how to ensure that only suitable traffic carries the DSCPs agreed between both parties at the interconnection.

I'd appreciate if the above problem is included in the text. DiffServ isn't "greenfield deployment".

As an aside, router vendors often support default behaviours to set the MPLS TC. A quite common one is to copy the most significant DSCP bits into the MPLS TC field. Without further action, the local behaviour may be default forwarding. If the next hop classifies on MPLS TC, this might not be true there too. That means operators wishing to support different policies have to configure a suitable policy (leaving the DSCP unchanged and setting MPLS TC to Default PHB is one of them).

Finally, from my experience, DSCP to PHB assignment isn't fully compatible between any two network providers. Default is the only exception, EF DSCP 46 (decimal) and CS6 for a network management PHB are often seen too. That also means, the same DSCP may be deployed with different PHBs in separate domains supporting DiffServ.

Regards,

Ruediger