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2.25 DSCP Value Rewrite Function
DSCP value rewrite is a function to rewrite the DSCP value of IP packets specified. Delay within IP-VPN net can
be reduced if the DSCP value of data that is requested by voice and response using IP-VPN net is changed and
then sent. The function is enabled when connected with carrier VPN service (Super VPN etc) that controls
packet priority by DSCP value.
DSCP value rewrite function supported by this device is compliant with given below RFC (Request For
Comments).
RFC2474: Definition of the Differentiated Services Field (DS Field) in the IPv4 and IPv6 Headers
DSCP value rewrite function can control DSCP field from 8 bit 'Type Of Service (TOS)' field having IP packet
header and 8 bit 'traffic class' field having IPv6 packet header defined by IPv4 [RFC 791].
• RFC791 Internet Protocol
• RFC2460 Internet Protocol,Version 6 （IPv6）Specification
Destination IP address, destination port number, source IP address, source port number and protocol number
can be specified in 'rewrite' condition. DSCP value of packets that agree with this condition are re-written and
transmitted. When agreed with multiple conditions, condition with smaller definition number is used.
DSCP value of the packets which is not the target of 'rewrite', are not re-written.
Packets entered in this device are executed by DSCP value rewrite process, corresponding to "acl.ip" definition
(acl.ip6 definition in case of IPv6), of specified ACL definition and "acl.tcp", "acl.udp", or "acl.icmp" definition, in
this device.
When executing DSCP rewrite, determination method of output queue can be selected in 2 ways. 1 is, by
determination method on the basis of relation of user priority in packets with the corresponding queue. In this
case, DSCP rewrite is not influenced by determination of output queue. User priority is determined by user
priority according to upper 3 bit (Upper 3 bit of DSCP before rewrite) of TOS or TC when used by 'qos
classification' function, IEEE802.1p compliant CoS and priority level of default priority in untagged received
packets. Another 1 is, when 'changeQueue' function is used, output queue is determined according to DSCP
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