图3-1TheDoDandOSImodels图3-2PortNumbersTCPPort NumbersF T PTransport LayerT E L N E TD N SS N M PT F T PS M T PUDPApplication Layer2123255369161R I P520图3-…HostA102823…=23. -4IPDatagramVersion (4)DestinationIPAddress(32)Options(0or32ifany)Data(variesifany)1Bit0Bit15Bit16Bit31Header Length(4)Priority&Type ofService(8)TotalLength(16)Identification(16)Flags (3)Fragmentoffset(13)Timetolive(8)Protocol(8)Headerchecksum(16)SourceIPAddress(32)20 :=???-:.::=???- :.::=???-
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