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44Protocols GuideTCP/IP - Application Layer ProtocolsProtocol NameSNTP: Simple Network TimeProtocolReceive Timestamp (64)Transmit Timestamp (64)Key Identifier (optional) (32)Message digest (optional) (128)Protocol DescriptionThe Simple Network Time Protocol (SNTP) Version 4 is an adaptationof the Network Time Protocol (NTP) used to synchronizecomputer clocks in the Internet. SNTP can be used whenthe ultimate performance of the full NTP implementation is notneeded or justified. When operating with current and previousNTP and SNTP versions, SNTP Version 4 involves no changesto the NTP specification or known implementations, but rather aclarification of certain design features of NTP which allow operationin a simple, stateless remote-procedure call (RPC) modewith accuracy and reliability expectations similar to the UDP/TIME protocol.It is strongly recommended that SNTP be used only at the extremitiesof the synchronization subnet. SNTP clients shouldoperate only at the leaves (highest stratum) of the subnet andin configurations where no NTP or SNTP client is dependent onanother SNTP client for synchronization. SNTP servers shouldoperate only at the root (stratum 1) of the subnet and then onlyin configurations where no other source of synchronization otherthan a reliable radio or modem time service is available. Thefull degree of reliability ordinarily expected of primary serversis possible only using the redundant sources, diverse subnetpaths and crafted algorithms of a full NTP implementation. Thisextends to the primary source of synchronization itself in theform of multiple radio or modem sources and backup paths toother primary servers should all sources fail or the majority deliversincorrect time. Therefore, the use of SNTP rather than NTPin primary servers should be carefully considered.The only significant protocol change in SNTP Version 4 overprevious versions of NTP and SNTP is a modified header interpretationto accommodate Internet Protocol Version 6 (IPv6)and OSI addressing. However, SNTP Version 4 includes certainoptional extensions to the basic Version 3 model, including ananycast mode and an authentication scheme designed specificallyfor multicast and anycast modes.Protocol StructureSNTP message has the same format as the NTP:2 5 8 16 24 32bitLI VN Mode Stratum Poll PrecisionRoot DelayRoot DispersionReference IdentifierReference timestamp (64)Originate Timestamp (64)LIVNLeap Indicator warning of impending leap-secondto be inserted at the end of the last day ofthe current month.Version number indicating the version number.Mode - The mode: This field can contain the followingvalues:0 Reserved.1 Symmetric active.3 Client.4 Server.5 Broadcast.6 NTP control message.StratumAn integer identifying the stratum level of the local clock.PollSigned integer indicating the maximum interval betweensuccessive messages, in seconds to the nearest powerof 2.PrecisionSigned integer indicating the precision of the local clock,in seconds to the nearest power of 2.Root DelaySigned fixed-point number indicating the total roundtripdelay to the primary reference source, in seconds withfraction point between bits 15 and 16.Root DispersionUnsigned fixed-point number indicating the nominal errorrelative to the primary reference source, in seconds withfraction point between bits 15 and 16.Reference IdentifierIdentifying the particular reference source.Originate TimestampThis is the time at which the request departed the client forthe server, in 64-bit timestamp format.Receive TimestampThis is the time at which the request arrived at the server,in 64-bit timestamp format.Transmit TimestampThis is the time at which the reply departed the server forthe client, in 64-bit timestamp format.Authenticator (optional)When the NTP authentication scheme is implemented,the Key Identifier and Message Digest fields contain themessage authentication code (MAC) information defined.
45Protocols GuideTCP/IP - Application Layer ProtocolsRelated <strong>protocols</strong>NTP, UDPSponsor SourceSNTP is defined by IETF (http://www.ietf.org) in RFC 2030.Referencehttp://www.javvin.com/protocol/rfc2030.<strong>pdf</strong>Simple Network Time Protocol (SNTP) Version 4 for IPv4, IPv6and OSI.
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102Protocols GuideSecurity and VPNP
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104Protocols GuideSecurity and VPN
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120Protocols GuideVoice Over IP(VOI
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304Protocols GuideSS7 / C7 Protocol
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318Protocols GuideOther Protocols
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324Network Protocols Dictionary: Fr
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342Network Communication Protocols
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