The receive procedure is executed upon arrival of an NTP message. It validates the message, interprets the various modes and calls other procedures to filter the data and select the synchronization source. If the version number in the packet does not match the current version, the message may be discarded; however, exceptions may be advised on a case-by-case basis at times when the version is changed. If the NTP control messages described in Appendix B are implemented and the packet mode is 6 (control), the control-message procedure is called. The source and destination Internet addresses and ports in the IP and UDP headers are matched to the correct peer. If there is no match a new instantiation of the protocol machine is created and the association mobilized.
begin receive procedure
if ( pkt.version != NTP.VERSION) exit; #ifdef (control messages implemented) if ( pkt.mode =6) call control-message; #endef for (all associations) /* access control goes here */ match addresses and ports to associations; if (no matching association) call receive-instantiation procedure; /* create association */
The call to decrypt is implemented only if authentication is implemented.
#ifdef (authentication implemented) /* see Appendix C */ call decrypt; #endef
If the packet mode is nonzero, this becomes the value of mode used in the following step; otherwise, the peer is an old NTP version and mode is determined from the port numbers as described in Section 3.3.
if (pkt.mode = 0) /* for compatibility with old versions */ mode <-- (see Section 3.3); else mode <-- pkt.mode;
Table 5 shows for each combination of peer.mode and mode the resulting case labels.
case (mode, peer.hostmode) /* see Table 5 */
If error the packet is simply ignored and the association demobilized, if not previously configured. error: if (peer.config = 0) demobilize association; /* see no evil */ break;
If recv the packet is processed and the association marked reachable if tests five through eight (valid header) enumerated in the packet procedure succeed. If, in addition, tests one through four succeed (valid data), the clock-update procedure is called to update the local clock. Otherwise, if the association was not previously configured, it is demobilized.
recv: call packet; /* process packet */ if (valid header) begin /* if valid header, update local clock */ peer.reach <-- peer.reach | 1; if (valid data) call clock-update; endif else if (peer.config = 0) demobilize association; break;
If xmit the packet is processed and an immediate reply is sent. The association is then demobilized if not previously configured.
If pkt the packet is processed and the association marked reachable if tests five through eight (valid header) enumerated in the packet procedure succeed. If, in addition, tests one through four succeed (valid data), the clock-update procedure is called to update the local clock. Otherwise, if the association was not previously configured, an immediate reply is sent and the association demobilized.
pkt: call packet; /* process packet */ if (valid header) begin /* if valid header, update local clock */ peer.reach <-- peer.reach | 1>; if (valid data) call clock-update; endif else if ( peer.config = 0) begin peer.hostpoll <-- peer.peerpoll; /* send immediate reply */ call poll-update; call transmit; demobilize association; endif endcase end receive procedure;