User Manual

Moxa’s Managed Switch Next Generation OS (v3.x) User Manual
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Moxa switch traffic prioritization is based on two standards:
• IEEE 802.1p—a layer 2 QoS marking scheme
• Differentiated Services (DiffServ)—a layer 3 QoS marking scheme.
IEEE 802.1p Class of Service
The IEEE Std 802.1D 2005 Edition marking scheme, which is an enhancement to IEEE Std 802.1D, enables
Quality of Service on the LAN. Traffic service levels are defined in the IEEE 802.1Q 4-byte tag, which is used
to carry VLAN identification as well as IEEE 802.1p priority information. The IEEE 802.1p occupying 3 bits of
the tag follows the destination MAC address and Source MAC address.
The IEEE Std 802.1D 2005 Edition priority marking scheme assigns an IEEE 802.1p priority level between 0
and 7 to each frame, which specifies the level of service that the associated packets shall be handled. The
table below shows an example of how different traffic types can be mapped to the eight IEEE 802.1p priority
levels.
IEEE 802.1p Priority Level
IEEE 802.1D Traffic Type
0
Best Effort
1
Background (lowest priority)
2
Reserved
3
Excellent Effort (business critical)
4
Controlled Load (streaming multimedia)
5
Video (interactive media)
6
Voice (interactive voice)
7
Network Control Reserved traffic
Even though the IEEE 802.1p standard is the most widely used prioritization scheme for LAN environments,
it still has some restrictions:
• It requires an additional 4-byte tag in the frame, which is normally optional for Ethernet networks.
Without this tag, the scheme cannot work.
• The tag is part of the IEEE 802.1Q header, so to implement QoS at layer 2, the entire network must
implement IEEE 802.1Q VLAN tagging.
• It is only supported within a LAN and does not cross the WAN boundaries, since the IEEE 802.1Q tags
will be removed when the packets pass through a router.
Differentiated Services (DiffServ) Traffic Marking
DiffServ is a Layer 3 marking scheme that uses the DiffServ Code Point (DSCP) field in the IP header to
specify the packet priority. DSCP is an advanced intelligent method of traffic marking that allows you to
choose how your network prioritizes different types of traffic. The DSCP field can be set from 0 to 63 to map
to user-defined service levels, enabling users to regulate and categorize traffic by applications with different
service levels.
The advantages of DiffServ over IEEE 802.1Q are as follows:
• You can prioritize and assign different traffic with appropriate latency, throughput, or reliability by each
port.
• No extra tags are required.
• The DSCP priority tags are carried in the IP header, which can pass the WAN boundaries and through
the Internet.
• DSCP is backwards compatible with IPv4 ToS (Type of Service), which allows operation with legacy
devices that use IPv4 layer 3.