id	summary	reporter	owner	description	type	status	priority	component	version	resolution	keywords	cc
149	ntripcaster-2.0.37 UDP-based mountpoint time-out after RTP Sequence number overflow	emiravalls	stoecker	"Hello,

We are using the BKG Ntrip Caster in the following setup:

Corrections Data Source [IPv4 / UDP / RTP / RTCM] -> BKG Caster [TCP / NTRIP] <-> Web Proxy [IPv4 / TCP / TLS / NTRIP] <-> BNC

What we found is that the following check in store_udp_data() at main.c:997:

    if((int)(seq-con->udpbuffers->seq) > 0)


Is always false once the Corrections Data Source has sent more than 65535 messages, which is the maximum number of messages that can be sent without repeating sequence numbers in RTP (the sequence numbers wrap around 0). After some seconds, the mountpoint expires.

We believe the Caster should be able to support wrap around of sequence number for very long-lived mountpoints. Is that limit imposed in some standard?

The check could be changed to something like

    #define MAX_RTP_SEQNUM 0x00FFFFU
    #define MAX_PACKETS_IN_FLIGHT (MAX_RTP_SEQNUM/2)
    // choose a value between 1 and MAX_RTP_SEQNUM


    if ((seq != con->udpbuffers->seq) && (((seq - con->udpbuffers->seq) & MAX_RTP_SEQNUM) <= MAX_PACKETS_IN_FLIGHT))


so that it would allow jumps of upto MAX_PACKETS_IN_FLIGHT RTP messages. We have found that this proposed check is similar to the ones performed in rtp_recieve_datagram_buffered() in rtp.c.

Given that the old check allowed jumps from 0 to 65535, MAX_PACKETS_IN_FLIGHT could be set upto to MAX_RTP_SEQNUM, but this would allow accepting packets potentially from the past if there is a very high rate of packets per second, which it didn't with the original check. Thus a threshold less than half the range seems reasonable (and it would be very similar to the checks in rtp.c).

We would like some feedback on this issue and the proposed solution.

Thank you,
Kind regards.
"	defect	closed	major	Professional Caster		fixed