mheusing Fri, 02/15/2008 - 08:40
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Strictly speaking CoS is used in 802.1Q frames to apply markings, which can be used for DiffServ QoS treatment of traffic. The respective standard is 802.1p.

The TOS byte is found in an IP header and was defined in RFC 760. The purpose was the same as with the CoS bits in dot1Q frames: marking traffic for special QoS treatment.

The usage of the bits in the TOS byte has been redefined in several RFCs and today typically IP precedence or DiffServ markings as EF or AF11, etc. are used (see RFC 2474, 2475, 2597 and 2598).

Often COS and TOS are used in conversations interchangably only to mean a "field for QoS related markings" or "a group of packets to be treated with QoS in a specific manner".

So depending on the context they could mean the same.

Hope this helps! Please rate all posts.

Regards, Martin

Joseph W. Doherty Sat, 02/16/2008 - 05:54
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Although Martin has the details, simple version:

CoS -- L2

Tos -- L3

Also, Ethernet CoS has 3 bits, IP ToS has 8 bits, so the latter can convey more information or be more granular.


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