| Description | This attribute specifies the AND mask for the IP TOS
byte for overwriting an IPv4 packet's TOS value or IPv6
packet's Traffic Class value. The IP packet TOS byte
is bitwise ANDed with TosAndMask, then the result
is bitwise ORed with TosORMask and the result is written
to the IP packet TOS byte.
A value of 'FF'H for TosAndMask and a value of '00'H for
TosOrMask means that the IP Packet TOS byte is not overwritten.
This combination is reported if the referenced parameter
is not present in a QOS Parameter Set.
The IP TOS octet as originally defined in RFC 791 has been
superseded by the 6-bit Differentiated Services
Field (DSField, RFC 3260) and the 2-bit Explicit Congestion
Notification Field (ECN field, RFC 3168).
The IPv6 Traffic Class octet RFC 2460 is consistent with
that new definition.
Network operators SHOULD avoid specifying values of
TosAndMask and TosORMask that would result in the modification
of the ECN bits.
In particular, operators should not use values of TosAndMask
that have either of the least-significant
two bits set to 0. Similarly, operators should not use
values of TosORMask that have either of the least-significant
two bits set to 1.
Even though this attribute is only enforced by the CMTS,
the CM reports the value as signaled in the referenced
parameter. |