Postgres frontend–backend protocol messages

Chr	Message name	Who issue	Bind command.
B	Bind	(F)	Bind command.
C	Close	(F)	Close command.
f	CopyFail	(F)	COPY-failure indicator.
D	Describe	(F)	Describe command.
E	Execute	(F)	Execute command.
H	Flush	(F)	Flush command.
F	FunctionCall	(F)	function call.
p	GSSResponse	(F)	GSSAPI or SSPI response. Note that this is also used for SASL and password response messages. The exact message type can be deduced from the context.
P	Parse	(F)	Parse command.
p	PasswordMessage	(F)	password response. Note that this is also used for GSSAPI, SSPI and SASL response messages. The exact message type can be deduced from the context.
Q	Query	(F)	simple query.
p	SASLInitialResponse	(F)	initial SASL response. Note that this is also used for GSSAPI, SSPI and password response messages. The exact message type is deduced from the context.
p	SASLResponse	(F)	SASL response. Note that this is also used for GSSAPI, SSPI and password response messages. The exact message type can be deduced from the context.
S	Sync	(F)	Sync command.
X	Terminate	(F)	termination.
d	CopyData	(F & B)	COPY data.
c	CopyDone	(F & B)	COPY-complete indicator.
R	AuthenticationOk	(B)	authentication request.
R	AuthenticationKerberosV5	(B)	authentication request.
R	AuthenticationCleartextPassword	(B)	authentication request.
R	AuthenticationMD5Password	(B)	authentication request.
R	AuthenticationSCMCredential	(B)	authentication request.
R	AuthenticationGSS	(B)	authentication request.
R	AuthenticationSSPI	(B)	authentication request.
R	AuthenticationGSSContinue	(B)	authentication request.
R	AuthenticationSASL	(B)	authentication request.
R	AuthenticationSASLContinue	(B)	authentication request.
R	AuthenticationSASLFinal	(B)	authentication request.
K	BackendKeyData	(B)	cancellation key data. The frontend must save these values if it wishes to be able to issue CancelRequest messages later.
2	BindComplete	(B)	Bind-complete indicator.
3	CloseComplete	(B)	Close-complete indicator.
C	CommandComplete	(B)	command-completed response.
G	CopyInResponse	(B)	Start Copy In response. The frontend must now send copy-in data (if not prepared to do so, send a CopyFail message).
H	CopyOutResponse	(B)	Start Copy Out response. This message will be followed by copy-out data.
W	CopyBothResponse	(B)	Start Copy Both response. This message is used only for Streaming Replication.
D	DataRow	(B)	data row.
I	EmptyQueryResponse	(B)	response to an empty query string. (This substitutes for CommandComplete.)
E	ErrorResponse	(B)	error.
V	FunctionCallResponse	(B)	function call result.
v	NegotiateProtocolVersion	(B)	protocol version negotiation message.
n	NoData	(B)	no-data indicator.
N	NoticeResponse	(B)	notice.
A	NotificationResponse	(B)	notification response.
t	ParameterDescription	(B)	parameter description.
S	ParameterStatus	(B)	run-time parameter status report.
1	ParseComplete	(B)	Parse-complete indicator.
s	PortalSuspended	(B)	portal-suspended indicator. Note this only appears if an Execute message's row-count limit was reached.
Z	ReadyForQuery	(B)	ReadyForQuery is sent whenever the backend is ready for a new query cycle.
T	RowDescription	(B)	row description.