61850.PACKET HEADER

8 位位组1	8 位位组2	8 位位组3~4	8位位组4~n
版本＝3 (vrsn)	保留 (reserved)	报文长度，ISO传输 层报文长度＋4 (packet length)	ISO传输层报文 (data)

where:
vrsn 8 bits,This field is always 3 for the version of the protocol described inthis memo.packet length 16 bits (min=7, max=65535),This field contains the length of entire packet in octets,including packet-header. This permits a maximum TPDU size of 65531 octets. Based on the size of the data transfer (DT) TPDU,this permits a maximum TSDU size of 65524 octets.The format of the TPDU is defined in [ISO8073]. Note that onlyTPDUs formatted for transport class 0 are exchanged (different transport classes may use slightly different formats).