Table 6-2 Layer 3 Qos Example - NEC Sl2100 Networking Manual

SL2100
6.3 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
Type of Service Field (Diffserv)
Differentiated Services Code Point
Type of Service Field (IP Precedence - Ref. RFC 1349)
3 Bits
IP Precedence Value
Listed below are the fields used in
Version – the version of IP currently used.
IP Header Length (IHL) – datagram header length. Points to the beginning of the data. The minimum
value for a correct header is 5.
Type-of-Service – Indicates the quality of service desired by specifying how an upper-layer protocol
would like a current datagram to be handled, and assigns datagrams various levels of importance.
This field is used for the assignment of Precedence, Delay, Throughput and Reliability.
Total Length – Specifies the length, in bytes, of the entire IP packet, including the data and header.
The maximum length specified by this field is 65,535 bytes. Typically, hosts are prepared to accept
datagrams up to 576 bytes.
Identification – Contains an integer that identifies the current datagram. This field is assigned by
sender to help receiver to assemble the datagram fragments.
Flags – Consists of a 3-bit field of which the two low-order (least-significant) bits control fragmentation.
The low-order bit specifies whether the packet can be fragmented. The middle bit specifies whether
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Table 6-2 Layer 3 QoS Example

8 Bits
Type of Service
Protocol
Source Address
Destination Address
Option + Padding
6 Bits
1 Bit 1 Bit
Delay Throughput
Table 6-2 Layer 3 QoS Example on page
Flags
Header Checksum
Data
ECN (Not QoS related)
1 Bit 1 Bit
Reliability Cost
Network Design Considerations
ISSUE 1.0
16 Bits
Total Length
Fragment Offset
2 Bits
1 Bit
MBZ (must be zero)
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