Policy-based TOS Routing
SonicOS supports policy-based TOS (type of service) routing when defining policy-based routing (PBR) policies by Type of Service (TOS) and TOS mask values. When defined, the TOS and mask values are compared against the associated IP packet's TOS/DSCP field in the IP header when finding a route match.
The TOS value is compared to an 8-bit field in the IP packet header (for information about this header, see RFC 2474, Differentiated Services, and RFC 2168, Explicit Congestion Notification). The TOS value can be used to define services relating to quantitative performance requirements (for example, peak bandwidth) and those based on relative performance (for example, class differentiation).
TOS routing differs from existing SonicOS QoS marking, which does not affect the routing of a packet and cannot forward packets differently based on an inbound packet's TOS field. TOS Routing provides this capability by allowing policy routes to define a TOS Value/TOS Mask pair to be compared to inbound packets for differential forwarding. TOS routing only applies to packets as they enter the Security Appliance.
With TOS routing, it is possible to define multiple policy routes with identical source IP, destination IP, and service values, but differing TOS/TOS mask values. This allows packets with marked TOS fields to be forwarded differently based on the value of the TOS field in the inbound packet.
Any PBR policy routes defined before SonicOS have no values defined for the TOS/TOS mask. Likewise, the default values for TOS/TOS mask fields are zero (no values defined).
Policy routes with a TOS value other than zero are prioritized before all simple destination-only routes, but below any policy routes that define a source or service. When comparing two TOS Policy routes, and assuming both have the same set of source, destination, and service values either defined or not defined, the TOS route with the greater number of TOS mask bits set to 1 is prioritized before TOS routes with fewer TOS mask bits set.
The general prioritization (high to low) of PBR routes is as follows, based on the policy fields defined as anything other than Any or zero for TOS:
- Destination, Source, Service, TOS
- Destination, Source, Service
- Destination, Source, TOS
- Destination, Source
- Destination, Service, TOS
- Destination, Service
- Destination, TOS
- Source, Service, TOS
- Source, Service
- Source, TOS
- Service, TOS
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