Connectivity
Connectivity and managing Your Comcast Business network
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SIP TCP/5060 through DPC3939B

We have a DPC3939B gateway on our Comcast Business account. It blocks our SIP services that use TCP (not UDP) port 5060. Information on the web, such as this knowledgebase article (https://help.close.io/customer/portal/articles/1457963-nat-configuration-settings) indicate that this is a flaw with the gateway.

 

I have examined the settings of the gateway and cannot find any way to disable SIP ALG. Can Comcast make adjustments?

 

How can we get this problem solved with the DPC3939B? Or do we need a different router from Comcast?

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Re: SIP TCP/5060 through DPC3939B

I was able to perform a capture last week and the issue is clear. SIP works perfectly on every combination of transport (TCP, UDP, TLS) and port *EXCEPT* TCP/5060 which is the most important one to us. I can't post that data here because it is sensitive, but the evidence is conclusive.

 

We spoke with a Comcast tech support rep who investigated the gateway and was unable to find any settings to control. However, he was friendly and offered that a different modem may be suitable for us.

 

Another poster in this form who mentioned "VoIP Consistency?" seems to be experiencing the very same issues with the DPC3939, or was, until he replaced his modem. We will do the same. 

 

I think Comcast should address this issue with the DPC3939 gateway.

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Re: SIP TCP/5060 through DPC3939B

We replaced our DPC3939B with an Arris SB6183 and TP-Link wireless router. The VoIP is working great and another problem we were having--disrupted VPN sessions--is also solved.

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Re: SIP TCP/5060 through DPC3939B

Hello billsimon and welcome,

 

Comcast DPC3939B does not block any SIP Ports especially ALG 5060. This Comcast gateway, like all the reset, does not have any SIP parameter facilities to control. If you are having an issue with one of your SIP devices where you believe the embedded SIP subpacket within the Ethernet Packet is being blocked, I would first use your SIP analyzer to make sure that SIP ALG is disabled on the SIP device. Then simply use either DHCP or Static IP Port forwarding control facility open or block that port.

 

Hope this helps you out.  

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Re: SIP TCP/5060 through DPC3939B

Hi Rich, thanks for your input but I do have packet captures demonstrating the problem. Several packets get through and then the traffic is blocked. Additionally, I posted an example of one knowledgebase article on the web discussing SIP TCP 5060 problems through the DPC3939B and there are several others. Unless you have experienced and/or solved this particular problem, I'm not sure you can authoritatively say that the gateway is not blocking anything. I will await Comcast's response.

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Re: SIP TCP/5060 through DPC3939B

Interesting commentary from a respected tech forum: http://www.dslreports.com/forum/r29788636-Comcast-Equip-DPC3939

 

Comcast, can this gateway be reconfigured with firmware that does not interfere with SIP?

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Re: SIP TCP/5060 through DPC3939B

I read all the articles posted in this thread which are do not really have too much technical backbone about what the REAL ALG port 5060 issue really is and where it is. You might want to check out this more technically credible article which tells it like it technically really is. Also, please be advised that although the DPC3939B does have 2 ATA SIP interconnection ports, Comcast DOES NOT facilitate any telephony support for its business class customers, whatsoever. The firmware is ONLY intrinsic internet control, processing, and routing. Lastly, please note in the article that I referenced that  ONLY the CISCO 800 device is with SIP ALG support.

 

As a result of the above information, I would be VERY interested in your sharing your specific Ethernet SIP subpacket analysis/captures. I happen to know MANY Comcast customers currently using the DPC3939B with a VOIP SIP server, phones, etc. quite successfully. As my orgiinal post tried to get across, some of these customers had to make sure that specific SIP devices used definitive open ports and not the default 5060 as indicated here in the IETF SIP Standard.

 

In conclusion, Comcast DPC3939B does not intentionally block or open ports that are specific to any implementation, with the exception of this information . Perhaps you need to examine the SIP subpackets a little closer to insure that the presribed implementation ports a being used AND are all correctly opened on the VOIP devices and in the DPC3939B IP address devices. 

 

Hope this helps you out.

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Re: SIP TCP/5060 through DPC3939B

SIP over UDP works fine so the majority of VoIP users would not experience a problem here. It is the TCP version of the protocol that we require and that is not functioning correctly through the gateway. I'll post the evidence when back in the office.

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Visitor

Re: SIP TCP/5060 through DPC3939B

I was able to perform a capture last week and the issue is clear. SIP works perfectly on every combination of transport (TCP, UDP, TLS) and port *EXCEPT* TCP/5060 which is the most important one to us. I can't post that data here because it is sensitive, but the evidence is conclusive.

 

We spoke with a Comcast tech support rep who investigated the gateway and was unable to find any settings to control. However, he was friendly and offered that a different modem may be suitable for us.

 

Another poster in this form who mentioned "VoIP Consistency?" seems to be experiencing the very same issues with the DPC3939, or was, until he replaced his modem. We will do the same. 

 

I think Comcast should address this issue with the DPC3939 gateway.

View solution in original post

0 Kudos
Highlighted
Visitor

Re: SIP TCP/5060 through DPC3939B

We replaced our DPC3939B with an Arris SB6183 and TP-Link wireless router. The VoIP is working great and another problem we were having--disrupted VPN sessions--is also solved.

View solution in original post

0 Kudos