Avaya 555-245-600 manual Comparison of Dscp with original TOS, IP precedence bits ToS bits

Quality of Service guidelines

Note that the 802.1Q tag changes the size and the format of the Ethernet frames. Because of this, many switches must be explicitly configured to accept 802.1Q tagged frames. Otherwise, these switches might reject the tagged frames. The two fields to be concerned with are the Priority and Vlan ID (VID) fields. The Priority field is the “p” in 802.1p/Q, and ranges in value from 0 to 7. (802.1p/Q is a common term that is used to indicate that the Priority field in the 802.1Q tag has significance. Prior to real-time applications, 802.1Q was used primarily for VLAN trunking, and the Priority field was not important.) The VID field is used as it always has been, to indicate the VLAN to which the Ethernet frame belongs.

The IP header with its 8-bit Type of Service (ToS) field, which was, and in some cases still is, originally used. This original scheme was not widely used, and the IETF developed a new Layer 3 CoS tagging method for IP called Differentiated Services (DiffServ, RFC 2474/2475). DiffServ uses the first 6 bits of the ToS field, and ranges in value from 0 to 63. Table 52: Comparison of DSCP with original TOS shows the original ToS scheme and DSCP in relation to the 8 bits of the ToS field.

Table 52: Comparison of DSCP with original TOS

8-bit ToS field
IP precedence bits			ToS bits				0
0	1	2	3	4	5	6	7
DSCP bits						0	0

Ideally, any DSCP value should map directly to a precedence and traffic parameter combination of the original scheme. This is not always the case, however, and it can cause problems on some older devices.

On any device, new or old, a nonzero value in the ToS field has no effect if the device is not configured to examine the ToS field. Problems arise on some legacy devices when the ToS field is examined, either by default or by enabling QoS. These legacy devices (network and endpoint) might contain code that implemented only the precedence portion of the original ToS scheme, with the remaining bits defaulted to zeros. This means that only DSCP values that are divisible by 8 (XXX000) can map to the original ToS scheme. For example, if an endpoint is tagging with DSCP 40, a legacy network device can be configured to look for precedence 5, because both values show up as 10100000 in the ToS field. However, a DSCP of 46 (101110) cannot be mapped to any precedence value alone. Another problem is if the existing code implemented precedence with only one traffic parameter permitted to be set high. In this case, a DSCP of 46 still does not work, because it requires 2 traffic parameter bits to be set high. When these mismatches occur, the older device mighty reject the DSCP tagged IP packet, or exhibit some other abnormal behavior. Most newer devices support both DSCP and the original ToS scheme.

316 Avaya Application Solutions IP Telephony Deployment Guide