<Standard Message Header>

Each administrative or application message is preceded by a standard header. The header identifies the message type, length, destination, sequence number, origination point and time

Two fields help with resending messages. The PossDupFlag (43) is set to Y when resending a message as the result of a session level event (i.e. the retransmission of a message reusing a sequence number). The PossResend (97) is set to Y when reissuing a message with a new sequence number (e.g. resending an order). The receiving application should process these messages as follows:

PossDupFlag - if a message with this sequence number has been previously received, ignore message, if not, process normally.
PossResend - forward message to application and determine if previously received (i.e. verify order id and parameters).

Note that if OnBehalfOfCompID (115) or DeliverToCompID (128) message source identification/routing is used for a FIX session, then it must be used on all Application messages transmitted via that session accordingly ( Reject (3) message if not).

The following table provides examples regarding the use of SenderCompID (49) , TargetCompID (56) , DeliverToCompID (128) , and OnBehalfOfCompID (115) when using a single point-to-point FIX session between two firms. Assumption (A=sellside, B =buyside):

	SenderCompID (49)	OnBehalfOfCompID (115)	TargetCompID (56)	DeliverToCompID (128)
A to B directly	A		B
B to A directly	B		A

The following table provides examples regarding the use of SenderCompID (49) , TargetCompID (56) , DeliverToCompID (128) , and OnBehalfOfCompID (115) when using a single FIX session to represent multiple firms. Assumption (A=sellside, B and C=buyside, Q=third party):

		SenderCompID (49)	OnBehalfOfCompID (115)	TargetCompID (56)	DeliverToCompID (128)	OnBehalfOfSendingTime (370)
Send from A to B via Q
1)	A sends to Q	A		Q	B
2)	Q sends to B	Q	A	B		A's SendingTime
B responds to A via Q
1)	B sends to Q	B		Q	A
2)	Q sends to A	Q	B	A		B's SendingTime
Send from A to B AND C via Q
1)	A sends to Q	A		Q	B
2)	Q sends to B	Q	A	B		A's SendingTime
3)	A sends to Q	A		Q	C
4)	Q sends to C	Q	A	C		A's SendingTime
B AND C send to A via Q
1)	B sends to Q	B		Q	A
2)	Q sends to A	Q	B	A		B's SendingTime
3)	C sends to Q	C		Q	A
4)	Q sends to A	Q	C	A		C's SendingTime

Used in all messages

Tag	Field Name	FIXML	Req'd	Comments
8	BeginString		Y	FIX.4.2 (Always unencrypted, must be first field in message)
9	BodyLength		Y	(Always unencrypted, must be second field in message)
35	MsgType	ApplicationMessage	Y	(Always unencrypted, must be third field in message)
49	SenderCompID	CompID	Y	(Always unencrypted)
56	TargetCompID	CompID	Y	(Always unencrypted)
115	OnBehalfOfCompID	CompID	N	Trading partner company ID used when sending messages via a third party (Can be embedded within encrypted data section.)
128	DeliverToCompID	CompID	N	Trading partner company ID used when sending messages via a third party (Can be embedded within encrypted data section.)
90	SecureDataLen		C	Required to identify length of encrypted section of message. (Always unencrypted)
91	SecureData		C	Required when message body is encrypted. Always immediately follows SecureDataLen (90) field.
34	MsgSeqNum		Y	(Can be embedded within encrypted data section.)
50	SenderSubID	SubID	N	(Can be embedded within encrypted data section.)
142	SenderLocationID	LocationID	N	Sender's LocationID (i.e. geographic location and/or desk) (Can be embedded within encrypted data section.)
57	TargetSubID	SubID	N	"ADMIN" reserved for administrative messages not intended for a specific user. (Can be embedded within encrypted data section.)
143	TargetLocationID	LocationID	N	Trading partner LocationID (i.e. geographic location and/or desk) (Can be embedded within encrypted data section.)
116	OnBehalfOfSubID	SubID	N	Trading partner SubID used when delivering messages via a third party. (Can be embedded within encrypted data section.)
144	OnBehalfOfLocationID	LocationID	N	Trading partner LocationID (i.e. geographic location and/or desk) used when delivering messages via a third party. (Can be embedded within encrypted data section.)
129	DeliverToSubID	SubID	N	Trading partner SubID used when delivering messages via a third party. (Can be embedded within encrypted data section.)
145	DeliverToLocationID	LocationID	N	Trading partner LocationID (i.e. geographic location and/or desk) used when delivering messages via a third party. (Can be embedded within encrypted data section.)
43	PossDupFlag	PossDupFlag	N	Always required for retransmitted messages, whether prompted by the sending system or as the result of a resend request. (Can be embedded within encrypted data section.)
97	PossResend	PossResend	N	Required when message may be duplicate of another message sent under a different sequence number. (Can be embedded within encrypted data section.)
52	SendingTime	SendingTime	Y	(Can be embedded within encrypted data section.)
122	OrigSendingTime		N	Required for message resent as a result of a Resend Request (2) . If data is not available set to same value as SendingTime (52) (Can be embedded within encrypted data section.)
212	XmlDataLen		C	Required when specifying XmlData (213) to identify the length of a XmlData message block. (Can be embedded within encrypted data section.)
213	XmlData		C	Can contain a XML formatted message block (e.g. FIXML). Always immediately follows XmlDataLen field. (Can be embedded within encrypted data section.)
347	MessageEncoding		N	Type of message encoding (non-ASCII characters) used in a message's "Encoded" fields. Required if any "Encoding" fields are used.
369	LastMsgSeqNumProcessed		N	The last MsgSeqNum (34) value received and processed. Can be specified on every message sent. Useful for detecting a backlog with a counterparty.
370	OnBehalfOfSendingTime		N	Used when a message is sent via a "hub" or "service bureau". If A sends to Q (the hub) who then sends to B via a separate FIX session, then when Q sends to B the value of this field should represent the SendingTime (52) on the message A sent to Q. (always expressed in UTC (Universal Time Coordinated, also known as "GMT")