Figure 6-7 Layer 3 Qos Example - NEC SL1100 Networking Manual

Issue 6.0
6.3
6 - 20
Layer 3 QoS
QoS is most commonly implemented at Layer 3. This allows the VoIP packets
to be prioritized by routers, before they are forwarded to their next hop.
Layer 3 QoS uses the Type of Service (ToS) field of the IP packet. This is an
8-bit field in the header of the IP packet. The field can be used by Diffserv or
IP Precedence. Although these are two different standards, the actual field in
the IP packet is the same – Only the method of evaluating the bits differs.
QoS does not function only by using the ToS field (i.e., Marking the VoIP
packets). It is an end-to-end process and requires configuration on all
networking devices.
Packet Marking is the first step in this process and is often the only step that
the NEC dealer performs.
Protocol Structure - IP/Pv4 Header (Internet Protocol Version 4)
4 Bits 4 Bits
Version IHL
Identification
Time to Live
Differentiated Services Code Point
Type of Service Field (IP Precedence - Ref. RFC 1349)
3 Bits
IP Precedence Value
8 Bits
Type of Service
Protocol
Source Address
Destination Address
Option + Padding
Data
Type of Service Field (Diffserv)
6 Bits
1 Bit 1 Bit
Delay Throughput Reliability

Figure 6-7 Layer 3 QoS Example

NEC SL1100
16 Bits
Total Length
Flags Fragment Offset
Header Checksum
2 Bits
ECN
(Not QoS related)
1 Bit 1 Bit
Cost
(must be
Network Design Considerations
1 Bit
MBZ
zero)