Limitations
Some of the known limitations of the monitor subscriber feature are:
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This feature does not support SR/ICSR.
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Max of 4 mon sub sessions can be enabled per UPF.
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Monitoring for a given IMSI should not be enabled from both the SMF and the UP console.
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A combo call consists of 2 mon sub-sessions since it involves monitoring 2 PDNs.
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Incoming uplink packets (with GTPU) are not captured in VPP ingress in this phase.
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Restarting a trace immediately after quitting may result in fastpath files in
/records/pcap
the directory to be overwritten. Therefore, restart the session after a few seconds. -
When a MonSub trace stops, the tear down process may take a few seconds, so it is recommended to wait. It takes a maximum of (5 seconds, hexdump poll timer value in sec) before toggling the MonSub trace to start. Else, the operators may observe MAX TRACING SESSIONS REACHED momentarily.
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Monitor Subscriber fastpath sessions show CLI does not display the sessions that are being stopped. Hence, there is a transient period, where the new MonSub sessions can be rejected due to maximum sessions reached. However, show CLI shows fewer sessions. Therefore, wait for some time before starting a new MonSub trace session.
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Changing fastpath configuration options is only possible when UP Pcap Trace is set to OFF.
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When MT=ON in the Multi-PDN, then once MT=OFF, new PDN tracing is not started due to MAX TRACING REACHED, and then all other tracing is STOPPED. It happens because the first new PDN tracing starts and then all previous PDNs were STOPPED for MT=OFF case.
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Do not launch the same UE MonSub sessions from different CLIs.
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In slowpath PCAP, the egress DL packets do not show the GTP-U header, because the functionality to add GTP-U is available in fastpath. Hence, the ingress and egress DL packets show the duplicates, unless there are some packet modifications like HTTP X-headers applied over the ingress packets.
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Toggling C and D options does not impact the PCAP captures in UPF.
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For Multi-PDN, the fastpath file names do not use the Call ID. As by definition, the multi-PDN case has more than one call ID and hence a higher-level configuration such as IMSI is more suitable for naming the files.
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Only the named options that are explicitly mentioned in this document are supported from the hexdump-module file configuration.
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Number of streams that can be traced in fastpath is limited to 5000. A stream is defined as a TCP or UDP flow. It includes source IP address, destination IP address, source port, and destination port, transport protocol such as, TCP or UDP.
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Fastpath packets cannot be streamed to an external server. They are stored on the hard-disk and transferred either manually or by using transfer-mode options.
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The UP PCAP trace must be set to ON to capture fastpath and slowpath PCAP files.
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MonSub CLI option ‘<SPACE> Pause’ is only to pause console events. There is no impact on other tracing events (slowpath PCAP, fastpath PCAP, and protocol packets tracing in a text file in hexdump format) with this option.
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The UP trace PCAP file does not contain the initial PFCP Sx Request/Response, due to race condition.
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The ICMP Packets and first packets of TCP and UDP streams flow through both slowpath and fastpath. Default values of GTPU (option 26) and User L3 (Option 19) are set to OFF. As a result, these packets are not captured in slowpath captures. If Option 26 is set to ON, then these packets are captured in slowpath PCAP captures. Option 19 has no effect on slowpath PCAP capture.
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Data Events flag must be set to ON, to capture fastpath and slowpath PCAP files.
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The MonSub tracing is not supported for option Next-SAEGW Call on UP.
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The MonSub tracing is not supported for option Next call by APN for Pure-S call type.
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On an ASR-5500 setup with the default value of poll-timer, all the packets may not be captured due to a known issue. To avoid rejection for a large number of packets, change the poll-timer value to the lowest possible (10 ms).
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If context replacement occurs (if the same subscriber reattaches without a detach), then the slowpath captures for the new call remain in the old slowpath files.