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As defined in IETF RFCs 2474 and 2475, “services” are basically ways of treating different subsets of a network’s traffic in different ways at the Internet Protocol (IP) layer, Layer 3. For example, some packets might be routed in such a way as to expedite delivery (minimize delay), while others are routed to minimize loss, minimize cost, etc. The differentiation between these services (that is, Differentiated Services) is provided by a redefinition of an octet in the Layer 3 headers for IP versions 4 and 6, also termed IPv4 and IPv6, respectively. This octet is called a Type of Service (TOS) octet in IPv4 and a Traffic Class octet in IPv6, but 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) that 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 are ignored by DSCP-compliant routers.
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 numerical values having a higher relative order. DSCP-compliant routers should treat larger-valued DSCPs in such a way as to give the associated packets a “probability of timely forwarding” greater than a packet with a lower-valued DSCP. In addition to the eight Class Selector Codepoints, a network may 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.
In the 4600 Series IP Telephone Installation Guide, the Local Administrative Option for QoS allows you to specify Diffserv values for Layer 3 audio (“L3QAUD”) and signaling traffic (“L3QSIG”) on a phone-by-phone basis if necessary.
The Avaya IP Telephones’ DiffServ values will be changed to the values administered on the media server as soon as the phone is registered. 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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