No or distorted sound using SRTP and rtpengine

 

TL;DR;

SDES-pad 

Long version:

Recently had a chance of upgrading rtpengine from version 8 to version 12. 

For those, who don't know what it is, it's just one of the most popular RTP proxy in the opensource ecosystem. And usually coming with Kamailio and OpenSIPS as a default RTP Proxy option nowdays. 

And I must say, it's a really stable software with great backwards compatibility. I did not expected to get any issues on this upgrade, but quite fast, got into situation, when receiving a calls on Linphone (mobile, if it matters to someone), sound get distorted in a case of Android and completely absent in a case of iOS.

First I've blamed a new chiper suites, that had been added and disabled it. (SDES-no-<CHIPER> string in rtpengine_manage(), if someone interested). But that does not help.

In the process of searching I've found this issue on github. And yes, turning on SDES-pad resolved the issue, but I was really interested why this option was introduced in a first place.

So, according to the documentation,

pad

   RFC 4568 (section 6.1) is somewhat ambiguous regarding the base64 encoding format of a=crypto parameters added to an SDP body. The default interpretation is that trailing = characters used for padding should be omitted. With this flag set, these padding characters will be left in place.

Hm, let's read the RFC about it.

   When base64 decoding the key and salt, padding characters (i.e., one or two "=" at the end of the base64-encoded data) are discarded (see [RFC3548] for details).
   Base64 encoding assumes that the base64 encoding input is an integral number of octets.  If a given crypto-suite requires the use of a concatenated key and salt with a length that is not an integral number of octets, said crypto-suite MUST define a padding scheme that results in the base64 input being an integral number of octets.

What I can see here is reference to how base64 is made, just explicit remark, that padding characters are not carrying any data, just.. padding according to the corresponding RFC:

   In some circumstances, the use of padding ("=") in base encoded data is not required nor used.  In the general case, when assumptions on size of transported data cannot be made, padding is required to yield correct decoded data.
   Implementations MUST include appropriate pad characters at the end of encoded data unless the specification referring to this document explicitly states otherwise.

Really, I can't see any way, that padding is "optional" in base64 encoding by default, both RFC's expicitly saying, that padding is needed and this is the case for SDP option a=crypto.

As a takeway for me, "default" options even in poular software not meaning "right".

Sublime syntax for Kamailio

For a long time I've been using Visual Studio Code for writing code and it's still a great product, it's becoming more and more bloated with M$ telemetry and even using VSCodium does not help a lot (especially how hard to get Pylance working there).

So now it's a good time to "prepare a spare airfield" and every time I'm going back to Sublime. Yes, it's paid, but it's worth it. And Sublime Merge is the best git GUI over there for me.

One of the things that were missing for me, was the lack of syntax highlighting for Kamailio in Sublime Text.

Luckily, there is a syntax file for VSCode from Daniel-Constantin Mierla, so it was really easy to adapt it for Sublime.

The file itself is here. It's less advanced than the original file, mainly cause I need to dig into syntax format a bit more, but as a start, it will work.

Now it's a bit easier to write Kamailio configs in Sublime

Python and random task distribution in ThreadPoolExecutor

Not a usual article for me, but will leave this boilerplate here for myself.

So, the task was the following: send multiple PUBLISH messages over SIPP, but not overload the SIP Proxy, and distribute them in time, like not sending all at once. Yes, I do know SIPP can do this, but I wanted to have SIPP exit code after sending each message and try to re-send it in case of issues. Order of sending is whatever. Just need to be delivered.

Here is the boilerplate Python function that I've used to achieve this

#!/usr/bin/python
import logging
import time
import random

from concurrent.futures import ThreadPoolExecutor

logging.getLogger('paramiko').setLevel(logging.INFO)

logging.basicConfig(
    format = '[%(asctime)s.%(msecs)03d] %(threadName)s %(name)s %(levelname)s: %(message)s',
    level=logging.INFO,
)

def get_timer_delay():
    '''
    Generator to be passed in ThreadPool to have delays in multiple iterated value processes.
    random delay between 5 and 200 ms. When reaching 1 sec - resets back to 0
    '''
    num = 0.0
    while True:
        yield num
        if num < 1:
            num += float(random.randrange(5, 200, 3)) / 1000
        else:
            num = 0.0

def process_iterated_value(value, start_delay):
    '''
    This function is called in thread-wise way to have multiple values processed at the moment
    '''
    time.sleep(start_delay)

    # Call SIPP process here (with exit code control)

    logging.info(f"Processed value {value} with delay {start_delay}")

#### ---------------- Script start ---------------- ########

execute_timeout = get_timer_delay()
iterated_value = range(0, 30)

logging.info("Process start")

with ThreadPoolExecutor(max_workers=5) as executor:
    executor.map(process_iterated_value, iterated_value, execute_timeout)

logging.info("Process end")

Output is looks like this. All values are processed in a more-or-less distributed way that can be seen by timestamps

[00:06:43,375.375] MainThread root INFO: Process start
[00:06:43,375.375] ThreadPoolExecutor-0_0 root INFO: Processed value 0 with delay 0.0
[00:06:43,504.504] ThreadPoolExecutor-0_0 root INFO: Processed value 1 with delay 0.128
[00:06:43,620.620] ThreadPoolExecutor-0_1 root INFO: Processed value 2 with delay 0.244
[00:06:43,794.794] ThreadPoolExecutor-0_2 root INFO: Processed value 3 with delay 0.417
[00:06:43,928.928] ThreadPoolExecutor-0_3 root INFO: Processed value 4 with delay 0.5509999999999999
[00:06:44,053.053] ThreadPoolExecutor-0_4 root INFO: Processed value 5 with delay 0.6759999999999999
[00:06:44,334.334] ThreadPoolExecutor-0_0 root INFO: Processed value 6 with delay 0.828
[00:06:44,334.334] ThreadPoolExecutor-0_0 root INFO: Processed value 11 with delay 0.0
[00:06:44,475.475] ThreadPoolExecutor-0_0 root INFO: Processed value 12 with delay 0.14
[00:06:44,590.590] ThreadPoolExecutor-0_1 root INFO: Processed value 7 with delay 0.968
[00:06:44,689.689] ThreadPoolExecutor-0_0 root INFO: Processed value 13 with delay 0.21400000000000002
[00:06:44,769.769] ThreadPoolExecutor-0_2 root INFO: Processed value 8 with delay 0.973
[00:06:44,914.914] ThreadPoolExecutor-0_3 root INFO: Processed value 9 with delay 0.984
[00:06:44,941.941] ThreadPoolExecutor-0_1 root INFO: Processed value 14 with delay 0.35100000000000003
[00:06:45,185.185] ThreadPoolExecutor-0_0 root INFO: Processed value 15 with delay 0.494
[00:06:45,185.185] ThreadPoolExecutor-0_4 root INFO: Processed value 10 with delay 1.13
[00:06:45,413.413] ThreadPoolExecutor-0_2 root INFO: Processed value 16 with delay 0.643
[00:06:45,641.641] ThreadPoolExecutor-0_3 root INFO: Processed value 17 with delay 0.726
[00:06:45,686.686] ThreadPoolExecutor-0_1 root INFO: Processed value 18 with delay 0.743
[00:06:45,686.686] ThreadPoolExecutor-0_1 root INFO: Processed value 23 with delay 0.0
[00:06:45,740.740] ThreadPoolExecutor-0_1 root INFO: Processed value 24 with delay 0.053
[00:06:45,810.810] ThreadPoolExecutor-0_1 root INFO: Processed value 25 with delay 0.07
[00:06:45,886.886] ThreadPoolExecutor-0_1 root INFO: Processed value 26 with delay 0.07500000000000001
[00:06:45,952.952] ThreadPoolExecutor-0_0 root INFO: Processed value 19 with delay 0.766
[00:06:46,021.021] ThreadPoolExecutor-0_1 root INFO: Processed value 27 with delay 0.134
[00:06:46,071.071] ThreadPoolExecutor-0_4 root INFO: Processed value 20 with delay 0.885
[00:06:46,142.142] ThreadPoolExecutor-0_0 root INFO: Processed value 28 with delay 0.19
[00:06:46,354.354] ThreadPoolExecutor-0_1 root INFO: Processed value 29 with delay 0.33299999999999996
[00:06:46,376.376] ThreadPoolExecutor-0_2 root INFO: Processed value 21 with delay 0.962
[00:06:46,721.721] ThreadPoolExecutor-0_3 root INFO: Processed value 22 with delay 1.078
[00:06:46,721.721] MainThread root INFO: Process end
 

For sure, all values need to be adjusted after, but the idea is there.

The only thing that bothers, I'm not 100% sure how generator value num would behave in a multithread environment due to Python GIL. But here I'm asking for someone who has a Python knowledge to comment.