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Snom Phones: 547 Status: 401 Authentication Required / Settings

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Our Vodia PBX worked fine until a couple of days ago. For some unknown reason all Snom phones can't contact the PBX any more (Snom message: Network failure).

 

All of our softphones are working fine. The Snom phones are registered and can be called by the PBX (incoming calls), but not the other way around (outgoing calls).

 

There have been no updates or changes to the network infrastructure, there is no firewall involved.

 

We checked the traffic with wireshark and found this line:

<Server IP> to <Snom IP> SIP 547 Status: 401 Authentication Required

 

The Snom phones then CANCEL the sip request.

 

Could this be the cause of the "network failure" and what setting is involved?

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There is definitively something not all right. Anything in the log of the phones? Do you see the INVITE hitting the PBX, e.g. in the log? Maybe something with DNS? I guess there was no software update on the phones or the PBX?

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Today, after 5 days of malfunction, the Snom phones are accessing the PBX. We did not find any apparent reason for this change. However there is audio with all external numbers, but no audio with internal extensions.

 

Again the issue occurs with our SNOM phones only, not with any ot the other devices.

 

We checked the PCAP traces again and there was a significant change from yesterday (abreviated):

 

<SNOM IP> to <Server IP> 1326 Request: INVITE: sip xxx@pbx..

<Server IP> to <Snom IP> SIP547 351 Status: 100 Trying

<Server IP> to <Snom IP> SIP547 Status: 401 Authentication Required

<SNOM IP> to <Server IP> 471 Request: CANCEL sip: xxx@pbx...

 

TODAY there is a second INVITE in the PCAP trace and the phone is sending authentication data plus some other codec information.

 

Why would the SNOM phones and the VODIA pbx communicate that dífferently?

 

This is a very strange issue and also could be Snom related. I will contact them too.

<Server IP> to <Snom IP>

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Issue solved: Our IT provider changed the MTU from 1.500 to 1.200 and the Snoms are working again with the Vodia PBX. Is there any explanation from you side why they act so special vs all the other equipment?

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That is one of the major problems with SIP on UDP. The INVITE packets come dangerously close to the MTU size, e.g. if you have a long name and many codecs, you easily hit the 1500 bytes. If the provider goes down to 1200, then the probability dramatically increases.

 

The answer is, use TCP or better: TLS. SIP/UDP was a major mistake anyway.

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