[CentOS] Centos 7: UPD packet checksum verification?

Wed Jan 29 09:10:48 UTC 2020
Nataraj <incoming-centos at rjl.com>

On 1/28/20 12:39 PM, hw wrote:
> On Tuesday, January 28, 2020 9:00:22 AM CET Nataraj wrote:
>> On 1/26/20 5:44 PM, hw wrote:
>>> On Sunday, January 26, 2020 11:18:36 PM CET Pete Biggs wrote:
>>>> First of all - disclaimer - I'm no network specialist, I just read and
>>>> am interested in it.  I may get things wrong!!
>>>>> Both physical interfaces show the same.  But does this mean it's on as
>>>>> in
>>>>> "rx- checksumming: on" or off as in "tx-checksum-ipv4: off [fixed]"?
>>>> As far as I understand it rx-checksum is the underlying wire
>>>> checksumming - and from what I've read about it, disabling that
>>>> disables the UDP checksums.
>>> You mean layer 1 checksumming?  Is there such a thing with ethernet?  I
>>> think I read something about encoding, when I was trying to understand
>>> what "bandwidth" actually means, being involved in signal transmissions;
>>> and I seem to remember that there was no checksumming involved and it had
>>> to do with identifying signals as a requirement for the very possibility
>>> to transmit something before anything could be transmitted at all.
>>>>> Assuming that I do not receive packets with invalid UPD checksums, then
>>>>> the
>>>>> packages must be somehow altered and their UPD checksums recalculated to
>>>>> arrive here.  Does bad hardware etc. do that?  Why would the UDP
>>>>> checksums
>>>>> just happen to get recalculated correctly but like randomly without
>>>>> intent?
>>>> I'm not sure I understand what you are asking.
>>> It is about VOIP calls via SRTP being interrupted at irregular intervals. 
>>> The intervals appear to depend on the time of day:  Such phone calls can
>>> last for a duration of about 5--25 minutes during the day to up to 1.5
>>> hours at around 3am before being interrupted.
>> My sense is you may be starting at too low of a level in trying to debug
>> this.
> One of the reasons I have to look into it is that it is usually good to know 
> more/better.
>> I have seen the same kind of problems with my voip service when
>> there is a problem with my Internet connection.  When this happens I
>> also see high retransmission rates for tcp connections and other signs
>> of network problem.
> How do you monitor such retransmissions to be able to see if and when they 
> occur?

netstat -s | grep -i retrans

>> If I check the modem for my Internet connection
>> there are issues with the signal levels and high error rates reported by
>> the modem.  If you believe your Internet connection is reliable, then if
>> you run managed switches, check your switch logs for any reported errors.
>> You could try tools like iperf to check for problems on your internal
>> network.  You could run some of the basic tools for testing voip
>> performance of your Inetnet connection and if necessary run iperf to a
>> cloud hosted system.
> Can you suggest useful tools to analyze VOIP performance, and how do you 
> define VOIP performance?

Well there used to be a number of speedtest like sites that use to
report more accurately , latency, jitter and packet loss.  It seems most
of them have now scaled down their output, but you could use ping.  mean
deviation is basically jitter.

I think a few of the tests listed on this site, still work.


There used to be sites that did a calculation for something called MOS
score, which is a measure of expected voice quality based on the
performance of a connection.  Don't know if anyone does that anymore. 
In the VOIP industry there is fancy/expensive equipment for measuring
end to end performance, but in practice simple ping output with regular
sampling from something like a cron job can tell you alot.

Basically, what you want is that if your phone system relies on your
Internet connection, the pop that your connecting too needs to be
relatively close and have minimal packet loss and similar latency/jitter
characteristics on both the up/down stream.  Generally that is not too
hard to find these days, but if the Internet connectivity to your voip
pop takes a route half way across the country over the Internet, that's
not it.

I have one of the lowest cost voip providers, voip.ms,  and I find the
voip quality to be excellent and call drop rate to be low except when I
have problems with my Internet provider.

> The performance is kinda acceptable as long as the calls are not interrupted.  
> It's still worlds apart from what it used to be 25 years ago, before VOIP was 
> used.  Back then, you never had to worry that calls could be interrupted or 
> that you couldn't hear someone or that you couldn't have a conversation 
> because the latency makes it impossible.  You could just talk to someone on a 
> phone, like it should be.  Nowadays, we get to pay 10 times as much and more, 
> plus all the expensive hardware, and it still doesn't work right and doesn't 
> even come close.

Well if your relying on the Internet, you are essentially relying on the
availability of burst bandwidth.  If you application needs higher
reliability then I would be looking at purchasing some kind of commited
bandwidth to a voip provider.  I don't follow the voip industry much
lately, but I'm guessing that people still provision dedicated bandwidth
into a voip provider with a nationwide backbone.

>> I think it is highly unlikely that you are only having issues with srtp
>> packets and I would look at the broader picture first to try to isolate
>> some other problem in your network or Internet connection.
> See it this way: It is highly likely that I don't have any issues with SRTP at 
> all.  Calls over the LAN work fine.  The only issue is with the VOIP provider.  
> What I have learned about SRTP so far tells me that, like everything else 
> does.
> How would you explain that the same problem occurs at two entirely unrelated 
> physical locations each having their own asterisk installations, using 
> entirely different hardware and entirely different internet connections from 
> entirely different ISPs, with the only thing in common being the VOIP 
> provider?
> If it was only my internet connection which is affected, I'd be talking to my 
> ISP (probably useless) instead of the VOIP provider (who will probably do 
> something about it).

Well it sounds like you know where your problem is then.  If your
current provider can't solve the problems to your satisfaction then you
probably need to find a different provider.