Tag Archives: SDP

Kamailio Bytes – Extracting SDP Parameters with Kamailio

So the other day I needed to extract the IP and Port parameters from an SDP body – Not the whole line mind, but the values themselves.

As with so many things in Kamailio, there’s a lot of ways to achieve an outcome, but here’s how I approached this problem.

Using the SDPops module we can get a particular line in the SDP, for example, we can get the media line with:

#Get SDP line starting with m= and put it into AVP $avp(mline)
sdp_get_line_startswith("$avp(mline)", "m=")
#Print value of $avp(mline)
xlog("m-line: $avp(mline)\n");

This gets us the line, but now we need to extract the data, in the example from the screenshot the M line has the value:

m=audio 4002 RTP/AVP 8 101

But we only want the port from the M line.

This is where I’ve used the Kamailio Dialplan module and regex to extract the port from this line.

With a fairly simple regex pattern, we can get a group match for the Port from the m= line.

So I took this regular expression, and put it into the Kamailio Dialplan database with dialplan ID 400 for this example:

INSERT INTO `dialplan` VALUES (4,400,10,1,'m=audio (\\d*)',0,'m=audio (\\d*)','\\1','SDP M Port Stripper');

Now using Dialplan ID 400 we can translate an inputted m= SDP line, and get back the port used, so let’s put that into practice:

        if(sdp_get_line_startswith("$avp(mline)", "m=")) {
            xlog("m-line: $avp(mline)\n");
            xlog("raw: $avp(mline)");
            xlog("Extracting Port from Media Line");
            dp_translate("400", "$avp(mline)/$avp(m_port_b_leg)");
            xlog("Translated m_port_b_leg is: $avp(m_port_b_leg)");

Now we have an AVP called $avp(m_port_b_leg) which contains the RTP Port from the SDP.

Now we’ve got a few other values we might want to get, such as the IP the RTP is to go to, etc, we can extract this in the same way, with Dialplans and store them as AVPs:

        #Print current SDP Values and store as Vars
        if(sdp_get_line_startswith("$avp(mline)", "m=")) {
            xlog("m-line: $avp(mline)\n");
            xlog("raw: $avp(mline)");
            xlog("Extracting Port from Media Line");
            dp_translate("400", "$avp(mline)/$avp(m_port_b_leg)");
            xlog("Translated m_port_b_leg is: $avp(m_port_b_leg)");

        if(sdp_get_line_startswith("$avp(oline)", "o=")) {
            xlog("o-line: $avp(oline)\n");
            dp_translate("401", "$avp(oline)/$avp(o_line_port_1)");
            xlog("O Line Port 1: $avp(o_line_port_1)");
            dp_translate("402", "$avp(oline)/$avp(o_line_port_2)");
            xlog("O Line Port 2: $avp(o_line_port_2)");
            dp_translate("403", "$avp(oline)/$avp(o_ip_b_leg)");
            xlog("O IP: $avp(o_ip_b_leg)");

And all the Regex you’ll need:

(4,400,10,1,'m=audio (\\d*)',0,'m=audio (\\d*)','\\1','SDP M Port Stripper'),
(5,401,10,1,'o=[^ ]* (\\d*) (\\d*) IN IP4 (\\d*.d*.\\d*.\\d*)',0,'o=[^ ]* (\\d*) (\\d*) IN IP4 (\\d*.d*.\\d*.\\d*)','\\1','O Port 1'),
(6,402,10,1,'o=[^ ]* (\\d*) (\\d*) IN IP4 (\\d*.d*.\\d*.\\d*)',0,'o=[^ ]* (\\d*) (\\d*) IN IP4 (\\d*.d*.\\d*.\\d*)','\\2','O Port 2'),
(7,403,10,1,'o=[^ ]* (\\d*) (\\d*) IN IP4 (\\d*.d*.\\d*.\\d*)',0,'o=[^ ]* (\\d*) (\\d*) IN IP4 (\\d*[.]\\d*[.]\\d*[.]\\d*)','\\3','O IP');

SIP SDP – ptime

ptime is the packetization timer in VoIP, it’s set in the SDP message and defines the length of each RTP packet that’s sent;

This gives the length of time in milliseconds represented by the media in a packet. This is probably only meaningful for audio data, but may be used with other media types if it makes sense. It should not be necessary to know ptime to decode RTP or vat audio, and it is intended as a recommendation for the encoding/packetisation of audio. It is a media-level attribute, and it is not dependent on charset.

RFC 4556 – SDP: Session Description Protocol, Section 6
SDP body showing ptime value of 20ms

What it’s all about

A lower ptime value leads to more packet per second, while longer ptime leads to fewer packets per second.

In a Toll Quality (TDM) network 8000 samples per second are taken, this is reflected in PCM (Pulse Code Modulation) encoding of the data, see in PCMA / G.711 a-law for example.

But if each of these 8,000 samples per second were sent on an individual packet, we’d be seeing a huge number of tiny RTP packets where the header is a lot larger than the payload.

Instead endpoints generally wait until they’ve got a certain number of theses samples and then send them at once, every X milliseconds as defined by the ptime value.

  • A ptime of 1000ms would mean 1 packet per second.
  • A ptime of 20ms would mean 50 packets per second.
  • A ptime of 50ms would mean 20 packets per second.

ptime headaches

Some VoIP endpoints have issues with varied ptime (*cough Cisco SPA series cough*), and if you’re interconnecting with other carrier networks you have no real control as to what ptime endpoints use (except if you have a B2Bua that can resample / restuff the packets, or you use maxptime which really just limits more than fixes) so it’s worth understanding well.

International carrier trunks often have higher ptime values as they're often dealing with lower quality links, so they want to cut down the packets per second and often have jitter buffers in place to compensate for poor quality links.

RFC4566 (the second version of SDP) introduced the maxptime value.

This optional header in the SDP body allows an endpoint to specify the maximum ptime value it supports.

Older endpoints often don’t have much memory or processing power, so have very small buffers to store the received audio in before playing it to the user, and store the audio to be transmitted before sending it down the wire.

Mismatched ptime or a ptime that’s out of bounds for one endpoint can lead to some strange issues. Often an endpoint will ring, answer the call and even get a 200 OK, but immediately followed by a BYE from the incompatible end instead of an ACK.

In the initial INVITE ptime is not mandatory, meaning you may not know the caller has limits to the ptime values they can support, and the endpoint hangs up the calls straight after the 200 OK.

Identifying these issues may take some time, but here’s some good places to look:

  • SDP ptime value on INVITE and 200 OK
  • Time between RTP packets
  • Timestamp difference between RTP packets

Although it seems pretty self evident, if your endpoint only supports up to 20ms ptime, set the maxptime header to 20ms. You’d be surprised how often this isn’t the case.

You can read more about SDP on my Overview of SDP post and the RFC – RFC4566. You can lean about manipulating SDP headers in Kamailio in my post on SDPops.

Kamailio Bytes – SDP Manipulation with SDPops

I’m not a fan of Transcoding. It costs resources, often leads to reduced quality and adds latency.

Through some fancy SDP manipulating footwork we can often rejig the SDP order or limit the codecs we don’t support to cut down, or even remove entirely, the need for transcoding in the network.

If you’re not yet familiar with SDP have a read over my post on SDP Overview.


There are no module parameters for SDP ops, we’ve just got to load the module with loadmodule “sdpops.so”

Use in Routing Logic

We’ll pickup where we left off on the Basic Stateless SIP Proxy use case (You can grab the basic code from that post), but this time we’ll remove PCMU (Aka G.711 μ-law) from the SDP body:

loadmodule "sdpops.so"

####### Routing Logic ########

/* Main SIP request routing logic
 * - processing of any incoming SIP request starts with this route
 * - note: this is the same as route { ... } */
request_route {

        sl_reply("200", "Ok");

        xlog("Received $rm to $ru - Forwarding");

        append_hf("X-Proxied: You betcha\r\n");

        #Remove PCMU (G.711 u-law) by it's SDP Payload ID

        #Remove PCMU by name

        #Forard to new IP


        xlog("Got a reply $rs");
        append_hf("X-Proxied: For the reply\r\n");

We can remove the codec either by it’s name (PCMU) or by it’s payload ID.

Before traversing the Proxy
After traversing the proxy

For removing it by name we just specify the name:

#Remove PCMU by name

And by payload ID:

#Remove PCMU (G.711 u-law) by it's SDP Payload ID

We may want to remove all but one codec, again super simple:

#Only keep PCMA codec

If you can’t help but transcode RTPengine now has this functionality, have a read of Transcoding with RTPengine and Kamailio and it may be worth looking over Virtualized Transcoding Dimensioning for an idea of how powerful your box is going to have to be.

SDP – Session Description Protocol – Overview

Content-Type application/sdp is something you’ll see a whole lot when using SIP for Voice over IP, especially in INVITEs and 200 OK responses.

This is because SIP uses SDP to negotiate the media setup.

While Voice over IP uses RTP for media, and SIP for signalling, the meat in this sandwich is SDP, used to negotiate the RTP parameters and payloads before going ahead.

Without SDP you’d just have random unidentified RTP streams going everywhere and no easy way to correlate them back to a Session (SIP) or guarantee both endpoints support the same codec (RTP payload).

Enter SDP, the Session Description Protocol, before any RTP is sent, SDP advertises capabilities (which codecs to use), contact information, port information (which port to send the RTP stream to) and attempts to negotiate a media session both endpoints can support.

SDP is designed to be lightweight, while SIP uses human readable headers like To and From, SDP does away with this in favour of single letters representing what that header contains.

As an interesting aside, SIP at one stage also offered one-letter headers to make it smaller on the wire, but this never really took off.

Here we can see what an SDP header looks like, showing the Session ID, Session Name, Connection Information and Media Descriptions.

SDP from an INVITE

Let’s dig a little deeper and have a look at what this SDP header actually shows that’s useful to us.

The SDP Offer

Session Identifiers

Session information

The Owner / Creator & Session ID header (abbreviated to o=) contains the SDP session ID and the session owner / creator information. This contains the SDP Session ID and the IP Address / FQDN of the owner or creator of this session. In this case the SDP Session ID is 777830 and the Session owner / creator is

Connection Information

Receiving / listening information

Next up we’ve got the connection information header (abbreviated to c=) which contains the IP Address we want the incoming RTP stream sent to. In this example it’s coming IN on IPv4 address

The Media Description header (m=) also contains the port we want to receive the audio on, 15246.

So in summary we’re telling the called party that we’ll be listening on IP Address on port 15246, so they should send their RTP audio stream to that address & port.

Media Attributes

Media attributes

The Media Description header (abbreviated to m=) contains a name and address, in this case it’s audio, and sent to address (port) 15246.

After that we’ve got the RTP Audio / Video profile numbers. Because SDP is designed to be lightweight instead of saying PCMA, PCMU here each codec is assigned a number by IANA that translates to a codec. The full list is here, but 8 is equal to PCMA and 0 is equal to PCMU.

So from the Media Description header we can learn that it’s an Audio session, with media to be sent to port 15246, via RTP using PCMA or PCMU.

Different codecs can have different bitrates, so by using the Media Attribute header (Abbreviated to a=) we can set the bitrates for each. In this case both PCMA and PCMU are using a bitrate of 8000.


So to summarise we’ve told the party we’re calling our session ID is 777830 and it’s owned / created by We support PCMA and PCMU at 8000Hz, and we’ll be listening on IPv4 on on port 15246 for them to send their audio stream to.

The SDP Answer

Next we’ll take a look at the SDP from a 200 OK response, and work out what our session will look like.

Codec Selection

We can see this device only supports PCMA, which makes codec selection pretty easy, it’s going to be PCMA as that was also supported in the SDP offer contained in the initial INVITE.

In the scenario where both devices support the same codecs, the order in which the codecs are listed defines what codec is selected.

Connection Information

Like in the SDP offer we can see that we’re requesting incoming RTP / media to be sent to, in this case we’re asking for the RTP / media on port 25328

Final Steps

Generally after the 200 OK is received an ACK is sent and media starts flowing in both directions between endpoints.

In this example will send their audio (aka media / RTP) to on port 15246 (called party to the caller) and will send their audio to on port 25328 (calling party to the called party).

It’s always worth keeping in mind that SIP doesn’t have to be used for Voice, nor does it have to use SDP, nor does SDP have to be used with SIP, it can be used with other protocols (IAX, H.323), and doesn’t have to negotiate RTP sessions, but could negotiate anything.

That said, the SIP – SDP – RTP sandwich is pretty ubiquitous for good reason, and while it’s true that none of these protocols require each other, the truth is, most of their usage is with one-another and it’s easier to just say “SIP uses SDP” and “SDP uses RTP” than continually saying “SIP can use SDP” and “SDP can use RTP” etc.