Appendix B. Changes to RFC 5966

This document obsoletes [RFC5966] and differs from it in several respects. An overview of the most substantial changes/updates that implementors should take note of is given below.

1. A Terminology section (Section 3) is added defining several new concepts.

2. Paragraph 3 of Section 5 puts TCP on a more equal footing with UDP than RFC 5966 does. For example, it states:

   1. TCP MAY be used before sending any UDP queries.

   2. TCP ought to be considered a valid alternative transport to UDP, not purely a fallback option.

3. Section 6.2.1 adds a new recommendation that TCP connection reuse SHOULD be supported.

4. Section 6.2.1.1 adds a new recommendation that DNS clients SHOULD pipeline their queries and DNS servers SHOULD process pipelined queries concurrently.

5. Section 6.2.2 adds new recommendations on the number and usage of TCP connections for client/server interactions.

6. Section 6.2.3 adds a new recommendation that DNS clients SHOULD close idle sessions unless using a signalling mechanism.

7. Section 7 clarifies that servers are RECOMMENDED to prepare TCP responses in parallel and send answers out of order. It also clarifies how TCP queries and responses should be matched by clients.

8. Section 8 adds a new recommendation about how DNS clients and servers should handle the 2-byte message length field for TCP messages.

9. Section 9 adds a non-normative discussion of the use of TCP Fast Open.

10. Section 10 adds new advice regarding DoS mitigation techniques.