Type of Service (TOS)

Hi there,

Say if I had a qos appliance installed on networks between a lan and a wan box would the qos policies be carried across wan end points (point to point connection)? In other words, will the router retain the TOS bits across to the other side of the wan connection to provide QoS-style priority for the packets or will it clear the TOS bits? BTW, the other side of the wan connection also has the qos appliance sitting between a lan and a wan box.

Just so that I'm clear, I'm not talking about an upstream neighbor being an ISP connection which I know they will likely ignore the TOS bits unless I pay them extra for the feature. The above scenario is a point to point connection to a remote site.

Any insight will be appreciated.


The answer is it depends. routers _usually_ honor the TOS bits unless
they are configured to clear or rewrite them. We use the TOS bits for
designating traffic classes so in some cases we rewrite the TOS bits set
by the host so in your case we would modify the TOS bits.

                            Scott C. McGrath


Do you know by default if the routers pass the TOS bits?


Scott McGrath wrote:

Cisco and Enterasys definitely pass the TOS bits by default. You need to
talk to your engineering group to see whether it is your site's
policy to propagate TOS bits to make sure the TOS bits set by your
appliance will arrive at their destination.

                            Scott C. McGrath

First of all, most routers that apply quality of service mechanisms don't set the type of service in the process. However, many can use the type of service as input for their QoS decisions.

Second, routers generally pass these bits unmodified, unless specifically instructed to change them.

Third, these bits are often interpreted as a diffserv code point rather than the original type of service meaning.

One setup I've used myself is set the type of service selectively using a rate limit on a Cisco router, and then do traffic shaping for packets matching this type of service on another router. AFAIK this isn't a particular common setup, though.