o Pipelining: the sending of multiple queries and responses over a single TCP connection but not waiting for any outstanding replies before sending another query.
o Out-of-Order Processing: The processing of queries concurrently and the returning of individual responses as soon as they are available, possibly out of order. This will most likely occur in recursive servers; however, it is possible in authoritative servers that, for example, have different backend data stores.
6.1. Current Practices
Other more modern protocols (e.g., HTTP/1.1 [RFC7230], HTTP/2 [RFC7540]) have support by default for persistent TCP connections for all requests. Connections are then normally closed via a 'connection close' signal from one party.
6.2.1. Connection Reuse
To amortise connection setup costs, both clients and servers SHOULD support connection reuse by sending multiple queries and responses over a single persistent TCP connection.
When sending multiple queries over a TCP connection, clients MUST NOT reuse the DNS Message ID of an in-flight query on that connection in order to avoid Message ID collisions. This is especially important if the server could be performing out-of-order processing (see Section 7).
6.2.1.1. Query Pipelining
In order to achieve performance on par with UDP, DNS clients SHOULD pipeline their queries.
It is likely that DNS servers need to process pipelined queries concurrently and also send out-of-order responses over TCP in order to provide the level of performance possible with UDP transport.
DNS servers (especially recursive) MUST expect to receive pipelined queries. The server SHOULD process TCP queries concurrently, just as it would for UDP. The server SHOULD answer all pipelined queries, even if they are received in quick succession. The handling of responses to pipelined queries is covered in Section 7.
7. Response Reordering
Authoritative servers and recursive resolvers are RECOMMENDED to support the preparing of responses in parallel and sending them out of order, regardless of the transport protocol in use.
In order to achieve performance on par with UDP, recursive resolvers SHOULD process TCP queries in parallel and return individual responses as soon as they are available, possibly out of order.
[HTTP/1.1 pipelining can only do ordered responses]
3. Terminology
o Pipelining: the sending of multiple queries and responses over a single TCP connection but not waiting for any outstanding replies before sending another query.
o Out-of-Order Processing: The processing of queries concurrently and the returning of individual responses as soon as they are available, possibly out of order. This will most likely occur in recursive servers; however, it is possible in authoritative servers that, for example, have different backend data stores.
6.1. Current Practices
Other more modern protocols (e.g., HTTP/1.1 [RFC7230], HTTP/2 [RFC7540]) have support by default for persistent TCP connections for all requests. Connections are then normally closed via a 'connection close' signal from one party.
6.2.1. Connection Reuse
To amortise connection setup costs, both clients and servers SHOULD support connection reuse by sending multiple queries and responses over a single persistent TCP connection.
When sending multiple queries over a TCP connection, clients MUST NOT reuse the DNS Message ID of an in-flight query on that connection in order to avoid Message ID collisions. This is especially important if the server could be performing out-of-order processing (see Section 7).
6.2.1.1. Query Pipelining
In order to achieve performance on par with UDP, DNS clients SHOULD pipeline their queries.
It is likely that DNS servers need to process pipelined queries concurrently and also send out-of-order responses over TCP in order to provide the level of performance possible with UDP transport.
DNS servers (especially recursive) MUST expect to receive pipelined queries. The server SHOULD process TCP queries concurrently, just as it would for UDP. The server SHOULD answer all pipelined queries, even if they are received in quick succession. The handling of responses to pipelined queries is covered in Section 7.
7. Response Reordering
Authoritative servers and recursive resolvers are RECOMMENDED to support the preparing of responses in parallel and sending them out of order, regardless of the transport protocol in use.
In order to achieve performance on par with UDP, recursive resolvers SHOULD process TCP queries in parallel and return individual responses as soon as they are available, possibly out of order.
[HTTP/1.1 pipelining can only do ordered responses]