Issue 6.0 NEC SL11006 - 20 Network Design Considerations6.3 Layer 3 QoSQoS is most commonly implemented at Layer 3. This allows the VoIP packetsto 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 an8-bit field in the header of the IP packet. The field can be used by Diffserv orIP Precedence. Although these are two different standards, the actual field inthe 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 VoIPpackets). It is an end-to-end process and requires configuration on allnetworking devices.Packet Marking is the first step in this process and is often the only step thatthe NEC dealer performs.Figure 6-7 Layer 3 QoS ExampleProtocol Structure - IP/Pv4 Header (Internet Protocol Version 4)4 Bits 4 Bits 8 Bits 16 BitsVersion IHL Type of Service Total LengthIdentification Flags Fragment OffsetTime to Live Protocol Header ChecksumSource AddressDestination AddressOption + PaddingDataType of Service Field (Diffserv)6 Bits 2 BitsDifferentiated Services Code Point ECN(Not QoS related)Type of Service Field (IP Precedence - Ref. RFC 1349)3 Bits 1 Bit 1 Bit 1 Bit 1 Bit 1 BitIP Precedence Value Delay Throughput Reliability Cost MBZ(must bezero)
