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## Traffic class and flow label
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The Traffic Class in every IPv6 packet is a byte also known as the
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Differentiated Services field. It is treated in every respect exactly
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like the same field in every IPv4 packet (originally named the TOS octet
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in [RFC791](https://www.rfc-editor.org/info/rfc791)). It contains six
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bits of
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[differentiated services](https://www.rfc-editor.org/info/rfc2474) code
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point followed by two
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[ECN (Explicit Congestion Notification)](https://www.rfc-editor.org/info/rfc3168)
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bits. [RFC8100](https://www.rfc-editor.org/info/rfc8100) gives a good
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overview of current differentiated service interconnection practices for
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ISPs. [RFC5127](https://www.rfc-editor.org/info/rfc5127),
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[RFC4594](https://www.rfc-editor.org/info/rfc4594),
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[RFC5865](https://www.rfc-editor.org/info/rfc5865),
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[RFC8622](https://www.rfc-editor.org/info/rfc8622) and
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[RFC8837](https://www.rfc-editor.org/info/rfc8837) also describe current
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practice.
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ECN is intended for use by transport protocols to support congestion
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control.
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The Flow Label is a 20 bit field in every IPv6 packet, although as it
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name indicates it is only relevant to sustained traffic flows. The
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sender of a packet should fill it with a pseudo-random non-zero value
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unique to a given traffic flow, such as a given TCP connection. It can
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then be used downstream in support of load balancing. By definition, the
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20 bits have no semantics, although some deployments are known to have
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broken this guideline, which would interfere with load balancing. See
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[IPv6 Flow Label Specification](https://www.rfc-editor.org/info/rfc6437),
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[Using the IPv6 Flow Label for Equal Cost Multipath Routing and Link Aggregation in Tunnels](https://www.rfc-editor.org/info/rfc6438)
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and
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[Using the IPv6 Flow Label for Load Balancing in Server Farms](https://www.rfc-editor.org/info/rfc7098).
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<!-- Link lines generated automatically; do not delete -->
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### [<ins>Previous</ins>](Extension%20headers%20and%20options.md) [<ins>Chapter Contents</ins>](2.%20IPv6%20Basic%20Technology.md)
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