User's Manual Part 1
Release8OverviewofCycloneNetworks
Issue2,November2007 Draft5forRegulatoryReview 87
A Cyclone module prioritizes traffic by
◦ reading the Low Latency bit (Bit 3) in the IPv4 Type of Service (ToS) byte in a
received packet.
◦ reading the 802.1p field of the 802.1Q header in a received packet, where VLAN
is enabled on the module.
◦ comparing the 6-bit Differentiated Services Code Point (DSCP) field in the ToS
byte of a received packet to a corresponding value in the DiffServe tab of the
Configuration page of the module.
Low Latency Bit
Bit 3 is set by a device outside the Cyclone system. In the uplink frame, the SM monitors
Bit 3. If this bit is set, then
• the SM prioritizes this traffic in its high-priority queue according to AP
configuration settings for the high-priority channel.
• the system sends the packet on the high-priority channel and services this
channel before any normal traffic.
802.1P Field
See
Priority on VLANs (802.1p) on Page 166.
DSCP Field
Like Bit 3 of the original IPv4 ToS byte, the DSCP field (Bits 0 through 5) in the redefined
ToS byte is set by a device outside the Cyclone system. A packets contains no flag that
indicates whether the encoding is for the Low Latency bit or the DSCP field. For this
reason, you must ensure that all elements in your trusted domain, including routers and
endpoints, set and read the ToS byte with the same scheme.
Cyclone modules monitor ToS bytes with DSCP fields, but with the following differences:
◦ The 6-bit length of the field allows it to specify one of 64 service differentiations.
◦ These correlate to 64 individual (CodePoint) parameters in the DiffServe tab of
the Configuration page.
◦ Per RFC 2474, 3 of these 64 are preset and cannot be changed. (See
http://www.faqs.org/rfcs/rfc1902.html
.)
◦ For any or all of the remaining 61 CodePoint parameters, you can specify a value
of
• 0 through 3 for low-priority handling.
• 4 through 7 for high-priority handling.
RECOMMENDATION:
Ensure that your Differentiated Services domain boundary nodes mark any
entering packet, as needed, so that it specifies the appropriate Code Point for
that traffic and domain. This prevents theft of service level.