<productname>PostgreSQL</productname> version 18. The server and the libpq

client library are backwards compatible with protocol version 3.0,

implemented in <productname>PostgreSQL</productname> 7.4 and later.

For descriptions of earlier protocol versions, see previous releases of the

<productname>PostgreSQL</productname> documentation.

</para>

<para>

A single server

can support multiple protocol versions. The initial startup-request

message tells the server which protocol version the client is attempting to

use. If the major version requested by the client is not supported by

the server, the connection will be rejected (for example, this would occur

if the client requested protocol version 4.0, which does not exist as of

this writing). If the minor version requested by the client is not

supported by the server (e.g., the client requests version 3.2, but the

server supports only 3.0), the server may either reject the connection or

may respond with a NegotiateProtocolVersion message containing the highest

minor protocol version which it supports. The client may then choose either

to continue with the connection using the specified protocol version or

to abort the connection.

</para>

<para>

The protocol negotiation was introduced in

<productname>PostgreSQL</productname> version 9.3.21. Earlier versions would

reject the connection if the client requested a minor version that was not

supported by the server.

</para>

<para>

server versions; the text format is usually the more portable choice.

</para>

</sect2>

<sect2 id="protocol-versions">

<title>Protocol versions</title>

<para>

The current, latest version of the protocol is version 3.2. However, for

backwards compatibility with old server versions and middleware that don't

support the version negotiation yet, libpq still uses protocol version 3.0

by default.

</para>

<para>

A single server can support multiple protocol versions. The initial

startup-request message tells the server which protocol version the client

is attempting to use. If the major version requested by the client is not

supported by the server, the connection will be rejected (for example,

this would occur if the client requested protocol version 4.0, which does

not exist as of this writing). If the minor version requested by the

client is not supported by the server (e.g., the client requests version

3.2, but the server supports only 3.0), the server may either reject the

connection or may respond with a NegotiateProtocolVersion message

containing the highest minor protocol version which it supports. The

client may then choose either to continue with the connection using the

specified protocol version or to abort the connection.

</para>

<para>

The protocol negotiation was introduced in

<productname>PostgreSQL</productname> version 9.3.21. Earlier versions

would reject the connection if the client requested a minor version that

was not supported by the server.

</para>

<table>

<title>Protocol versions</title>

<tgroup cols="2">

<thead>

<row>

<entry>Version</entry>

<entry>Supported by</entry>

<entry>Description</entry>

</row>

</thead>

<tbody>

<row>

<entry>3.2</entry>

<entry>PostgreSQL 18 and later</entry>

<entry>Current latest version. The secret key used in query

cancellation was enlarged from 4 bytes to a variable length field. The

BackendKeyData message was changed to accomodate that, and the CancelRequest

message was redefined to have a variable length payload.

</entry>

</row>

<row>

<entry>3.1</entry>

<entry>-</entry>

<entry>Reserved. Version 3.1 has not been used by any PostgreSQL

version, but it was skipped because old versions of the popular

pgbouncer application had a bug in the protocol negotiation which made

it incorrectly claim that it supported version 3.1.

</entry>

</row>

<row>

<entry>3.0</entry>

<entry>PostgreSQL 7.4 and later</entry>

</row>

<row>

<entry>2.0</entry>

<entry>up to PostgreSQL 13</entry>

<entry>See previous releases of

the <productname>PostgreSQL</productname> documentation for

details</entry>

</row>

</tbody>

</tgroup>

</table>

</sect2>

</sect1>

<sect1 id="protocol-flow">