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IETF RFCs 2474 and 2475 define “services” basically as different ways to treat a network’s different traffic subsets at the Internet Protocol (IP) layer, Layer 3. For example, some packets might be routed to expedite delivery and minimize delay, with other packets routed to minimize loss or cost. Redefining an octet in the Layer 3 headers for IP versions 4, or IPv4 and 6, or IPv6 provides the differentiation between these services (Differentiated Services). IPv4 calls this octet a Type of Service (TOS) octet while IPv6 calls this octet a Traffic Class. In both cases, the octet is interpreted differently than it was originally defined. With Differentiated Services, bits 0 through 5 of the octet identify a Differentiated Services Code Point (DSCP). The DSCP identifies a procedure to be used to handle that packet on a per-hop basis. Bits 6 and 7 of the octet are currently unused, and DSCP-compliant routers ignore them.
With DiffServ, the default DSCP is all zeroes, and represents “no special handling.” RFC 2474 also defines eight “Class Selector Codepoints,” which are the eight DSCP encodings that can be represented by xxx000, where “x” represents one bit. These Code Selector Codepoints are considered prioritized, with the larger numeric values having a higher relative order. DSCP-compliant routers should give the associated packets of larger-valued DSCPs a “probability of timely forwarding” greater than a packet with a lower-valued DSCP. In addition to the eight Class Selector Codepoints, a network can define its own DSCPs by defining encodings that do not terminate in 000. The specific treatment intended by these custom DSCPs will not necessarily be carried out by routers outside the customer’s own network.
The 4600 Series IP Telephone Installation Guide describes the Local Administrative Option for QoS. This option allows you to specify Diffserv values for Layer 3 audio (“L3QAUD”) and signaling traffic (“L3QSIG”) on a telephone-by-telephone basis.
The Avaya H.323 IP Telephones’ DiffServ values change to the values administered on the media server as soon as the telephone registers. For more information, see the document titled Administration for Network Connectivity (555-233-504). Unless there is a specific need in your enterprise LAN, we do not recommend you change the default values.
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