SIP and RTP: Overview of a VoIP Communication
Read this article, which discusses one of the most popular network applications: Voice over IP. There are quite a few industry protocols for Voice over IP; one of the most popular and widely accepted is IETF protocol SIP. As you read, pay special attention to the registering and calling process.
SIP and RTP : overview of a VoIP communication
We are working on a very simple network:
-
An OpenSER PBX → 192.168.0.30
-
Two X-Lite clients → 10.42.16.48 and 10.42.16.88
The reason why all hosts are not in the same IP range is that we are using a VPN between our two networks to avoid NAT problems… We will discuss this later.
Registering
The first thing that a user of a SIP Network does is registering himself with the SIP Registrar. In our example, we use an OpenSER server that acts as the SIP Registrar (and also the SIP Proxy, but for now it doesn't matter). So, Julien, the user who has the IP 10.42.16.48, launches his softphone and tries to register himself only by sending a REGISTER datagram such as this one:
No. Time Source Destination Protocol Info 74 15.788470 10.42.16.48 192.168.0.30 SIP Request: REGISTER sip:192.168.0.30
{......truncated...datagram......}
User Datagram Protocol, Src Port: 5061 (5061), Dst Port: 5060 (5060)
Source port: 5061 (5061)
Destination port: 5060 (5060)
Length: 434
Checksum: 0xc0db [correct]
Session Initiation Protocol
Request-Line: REGISTER sip:192.168.0.30 SIP/2.0
Method: REGISTER
[Resent Packet: False]
Message Header
Via: SIP/2.0/UDP 10.42.16.48:5061;rport;branch=z9hG4bK240C2422F5DFEF700B859D9BAD8F9063
From: julien <sip:julien@intra-calcman.org>;tag=1847976374
SIP Display info: julien
SIP from address: sip:julien@intra-calcman.org
SIP tag: 1847976374
To: julien <sip:julien@intra-calcman.org>
SIP Display info: julien
SIP to address: sip:julien@intra-calcman.org
Contact: "julien" <sip:julien@10.42.16.48:5061>
Contact Binding: "julien" <sip:julien@10.42.16.48:5061>
URI: "julien" <sip:julien@10.42.16.48:5061>
SIP Display info: "julien"
SIP contact address: sip:julien@10.42.16.48:5061
Call-ID: 16E70BC816CCE9FF71C7F405E6C4B56F@192.168.0.30
CSeq: 8933 REGISTER
Expires: 1800
Max-Forwards: 70
User-Agent: X-Lite release 1105d
Content-Length: 0
The message is very simple. The SIP data structure is sent in plain text and contains human readable information such as the local IP address, the Registrar IP address, several information and a CALL-ID. This last one is very important because UDP protocols do not maintain a network session. So, the Call-ID is used to refer to a session and will change when a new session is launched (a new registrar message, an invite message and so on…).
The Registrar returns a “100 Trying” message while processing the request. This message contains the same Call-ID.
But here is the interesting thing: after a very short time, the Registrar returns the following message to julien:
No. Time Source Destination Protocol Info
79 15.865100 192.168.0.30 10.42.16.48 SIP Status: 401 Unauthorized (1 bindings){......truncated...datagram......}
User Datagram Protocol, Src Port: 5060 (5060), Dst Port: 5061 (5061)
Source port: 5060 (5060)
Destination port: 5061 (5061)
Length: 536
Checksum: 0xcbdf [correct]
Session Initiation Protocol
Status-Line: SIP/2.0 401 Unauthorized
Status-Code: 401
[Resent Packet: False]
Message Header
Via: SIP/2.0/UDP 10.42.16.48:5061;rport;branch=z9hG4bK240C2422F5DFEF700B859D9BAD8F9063;received=10.42.16.48
From: julien <sip:julien@intra-calcman.org>;tag=1847976374
SIP Display info: julien
SIP from address: sip:julien@intra-calcman.org
SIP tag: 1847976374
To: julien <sip:julien@intra-calcman.org>;tag=as4c996f20
SIP Display info: julien
SIP to address: sip:julien@intra-calcman.org
SIP tag: as4c996f20
Call-ID: 16E70BC816CCE9FF71C7F405E6C4B56F@192.168.0.30
CSeq: 8933 REGISTER
User-Agent: OpenSER PBX
Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, SUBSCRIBE, NOTIFY
Contact: <sip:julien@intra-calcman.org>
Contact Binding: <sip:julien@intra-calcman.org>
URI: <sip:julien@intra-calcman.org>
SIP contact address: sip:julien@intra-calcman.org
WWW-Authenticate: Digest realm="intra-calcman.org", nonce="3f314b07"
Authentication Scheme: Digest
Realm: "intra-calcman.org"
Nonce Value: "3f314b07"
Content-Length: 0
Why is the registrar sending an Unauthorized message ? This could be quite disconcerting… but, in fact, this is the regular way to register!!! As defined in the RFC :
The WWW-Authenticate response-header field MUST be included in 401 (Unauthorized) response messages. The field value consists of at least one challenge that indicates the authentication scheme(s) and parameters applicable to the realm.
So, the client has to re-send a Register request but, this time, it has to include an authorization method. Let's see the next message:
No. Time Source Destination Protocol Info 81 15.869887 10.42.16.48 192.168.0.30 SIP Request: REGISTER sip:192.168.0.30
{......truncated...datagram......}
User Datagram Protocol, Src Port: 5061 (5061), Dst Port: 5060 (5060)
Source port: 5061 (5061)
Destination port: 5060 (5060)
Length: 575
Checksum: 0xab44 [correct]
Session Initiation Protocol
Request-Line: REGISTER sip:192.168.0.30 SIP/2.0
Method: REGISTER
[Resent Packet: False]
Message Header
Via: SIP/2.0/UDP 10.42.16.48:5061;rport;branch=z9hG4bK4FD30F8A1D5F0DE817436029ACE18888
From: julien <sip:julien@intra-calcman.org>;tag=1847976374
SIP Display info: julien
SIP from address: sip:julien@intra-calcman.org
SIP tag: 1847976374
To: julien <sip:julien@intra-calcman.org>
SIP Display info: julien
SIP to address: sip:julien@intra-calcman.org
Contact: "julien" <sip:julien@10.42.16.48:5061>
Contact Binding: "julien" <sip:julien@10.42.16.48:5061>
URI: "julien" <sip:julien@10.42.16.48:5061>
SIP Display info: "julien"
SIP contact address: sip:julien@10.42.16.48:5061
Call-ID: 16E70BC816CCE9FF71C7F405E6C4B56F@192.168.0.30
CSeq: 8934 REGISTER
Expires: 1800
Authorization: Digest username="julien",realm="intra-calcman.org",nonce="3f314b07",response="3ef916fd68651deaf5dd74b4473aa641",uri="sip:192.168.0.30"
Authentication Scheme: Digest
Username: "julien"
Realm: "intra-calcman.org"
Nonce Value: "3f314b07"
Digest Authentication Response: "3ef916fd68651deaf5dd74b4473aa641"
Authentication URI: "sip:192.168.0.30"
Max-Forwards: 70
User-Agent: X-Lite release 1105d
Content-Length: 0
As expected, this Register request contains an authorization field. Because this is not a new Registrar session, the Call-ID is still the same and as defined in the RFC, this authorization response includes the realm and the nonce provided by the Registrar.
The nonce is a temporary random number used to avoid replay attacks.
The generation of the “Digest Authentication Response” field is defined in RFC 3617. It uses the realm, the nonce, the URI, the username and the password to generate a md5 sum.
The Registrar sends a “100 Trying” message while processing the request and after some milliseconds, it sends an Options request:
No. Time Source Destination Protocol Info 84 15.960925 192.168.0.30 10.42.16.48 SIP Request: OPTIONS sip:julien@10.42.16.48:5061
The Registrar has not yet accepted the registration. This allows the Registrar to query the client about what functionalities it supports. What is interesting is that this message uses a new Call-ID:
Call-ID: 0b51f8be3c5396be74caa22c52f26340@192.168.0.30
Almost at the same time, the Registrar sends the “200 OK” message which validates the registration.
So, the client is logged in now. In response to the Options request, it sends the “200 OK” message which lists the SIP methods it supports. (The code could be 486 if the client was here but not ready to accept a call).
Calling
OK, Now our client is registered and he can call his friends. To explain this part, let me welcome david@intra-calcman.org. Julien is going to call david. To do this, he sends an INVITE request to the proxy (which is also the registrar in our case, but can be a different one). The following datagram is the INVITE request:
No. Time Source Destination Protocol Info 95 30.398482 10.42.16.48 192.168.0.30 SIP/SDP Request: INVITE sip:david@intra-calcman.org, with session description
{......truncated...datagram......}
User Datagram Protocol, Src Port: 5060 (5060), Dst Port: 38276 (38276)
Source port: 5060 (5060)
Destination port: 38276 (38276)
Length: 831
Checksum: 0x25eb [correct]
Session Initiation Protocol
Request-Line: INVITE sip:david@intra-calcman.org:38276;rinstance=90eafe1f95fbbfa3 SIP/2.0
Method: INVITE
[Resent Packet: False]
Message Header
Via: SIP/2.0/UDP 192.168.0.30:5060;branch=z9hG4bK01a93c8b;rport
From: "julien" <sip:julien@intra-calcman.org>;tag=as1ea41f9e
SIP Display info: "julien"
SIP from address: sip:julien@intra-calcman.org
SIP tag: as1ea41f9e
To: <sip:david@intra-calcman.org:38276;rinstance=90eafe1f95fbbfa3>
SIP to address: sip:david@intra-calcman.org:38276
Contact: <sip:julien@intra-calcman.org>
Contact Binding: <sip:julien@intra-calcman.org>
URI: <sip:julien@intra-calcman.org>
SIP contact address: sip:julien@intra-calcman.org
Call-ID: 48f934056782cf6f581c024128f6c29c@192.168.0.30
CSeq: 102 INVITE
User-Agent: OpenSER PBX
Max-Forwards: 70
Date: Mon, 02 Oct 2006 14:26:13 GMT
Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, SUBSCRIBE, NOTIFY
Content-Type: application/sdp
Content-Length: 259
Message body
Session Description Protocol
Session Description Protocol Version (v): 0
Owner/Creator, Session Id (o): root 3725 3725 IN IP4 192.168.0.30
Owner Username: root
Session ID: 3725
Session Version: 3725
Owner Network Type: IN
Owner Address Type: IP4
Owner Address: 192.168.0.30
Session Name (s): session
Connection Information (c): IN IP4 10.42.16.88
Connection Network Type: IN
Connection Address Type: IP4
Connection Address: 10.42.16.88
Time Description, active time (t): 0 0
Session Start Time: 0
Session Stop Time: 0
Media Description, name and address (m): audio 14318 RTP/AVP 0 3 8 101
Media Type: audio
Media Port: 14318
Media Proto: RTP/AVP
Media Format: ITU-T G.711 PCMU
Media Format: GSM 06.10
Media Format: ITU-T G.711 PCMA
Media Format: 101
Media Attribute (a): rtpmap:0 PCMU/8000
Media Attribute Fieldname: rtpmap
Media Format: 0
MIME Type: PCMU
MIME type: PCMU
Media Attribute (a): rtpmap:3 GSM/8000
Media Attribute Fieldname: rtpmap
Media Format: 3
MIME Type: GSM
MIME type: GSM
Media Attribute (a): rtpmap:8 PCMA/8000
Media Attribute Fieldname: rtpmap
Media Format: 8
MIME Type: PCMA
MIME type: PCMA
Media Attribute (a): rtpmap:101 telephone-event/8000
Media Attribute Fieldname: rtpmap
Media Format: 101
MIME Type: telephone-event
MIME type: telephone-event
Media Attribute (a): fmtp:101 0-16
Media Attribute Fieldname: fmtp
Media Format: 101 [telephone-event]
Media format specific parameters: 0-16
Media Attribute (a): silenceSupp:off - - - -
Media Attribute Fieldname: silenceSupp
Media Attribute Value: off - - - -
Once again, reading a SIP message is very simple (this is one of the aims of the protocol). We can see david's address and port:
INVITE sip:david@intra-calcman.org:38276;rinstance=90eafe1f95fbbfa3 SIP/2.0
and many other pieces of useful information such as the Via parameter, including the “branch” transaction identifier. The via parameter is needed to identify where the response is to be sent.
Via: SIP/2.0/UDP 192.168.0.30:5060;branch=z9hG4bK01a93c8b;rport
We haven't already talked about the SDP protocol. Session Description Protocol is the purpose of the RFC 4566 (july 2006).
From the RFC:
SDP is intended to be general purpose so that it can be used in a wide range of network environments and applications. However, it is not intended to support negotiation of session content or media encodings: this is viewed as outside the scope of session description.
For our purpose, we only need to know that SDP syntax is used to define call parameters. In the previous datagram, we see that RTP will be used between david and julien. While OpenSER delivers the datagram to david, a “100 Trying” message is sent to julien.
So, what happens on david's side after he has received this datagram?
No. Time Source Destination Protocol Info
33 172.296544 10.42.16.88 192.168.0.30 SIP Status: 180 Ringing
His softphone (we are using x-lite) sent a "180 Ringing" message, which is exactly what it means… The Ringing response contains all the SIP informations to identify the call:
User Datagram Protocol, Src Port: 38276 (38276), Dst Port: 5060 (5060)
Source port: 38276 (38276)
Destination port: 5060 (5060)
Length: 435
Checksum: 0xb210 [correct]
Session Initiation Protocol
Status-Line: SIP/2.0 180 Ringing
Status-Code: 180
[Resent Packet: False]
Message Header
Via: SIP/2.0/UDP 192.168.0.30:5060;branch=z9hG4bK01a93c8b;rport=5060
Contact: <sip:david@intra-calcman.org:38276;rinstance=90eafe1f95fbbfa3>
Contact Binding: <sip:david@intra-calcman.org:38276;rinstance=90eafe1f95fbbfa3>
URI: <sip:david@intra-calcman.org:38276;rinstance=90eafe1f95fbbfa3>
SIP contact address: sip:david@intra-calcman.org:38276
To: <sip:david@intra-calcman.org:38276;rinstance=90eafe1f95fbbfa3>;tag=79182b50
SIP to address: sip:david@intra-calcman.org:38276
SIP tag: 79182b50
From: "julien"<sip:julien@intra-calcman.org>;tag=as1ea41f9e
SIP Display info: "julien"
SIP from address: sip:julien@intra-calcman.org
SIP tag: as1ea41f9e
Call-ID: 48f934056782cf6f581c024128f6c29c@192.168.0.30
CSeq: 102 INVITE
User-Agent: X-Lite release 1003l stamp 30942
Content-Length: 0
OpenSER, the proxy, receives this response and sends a “180 Ringing” message to julien.
To establish the call, david sends a “200 OK” datagram:
No. Time Source Destination Protocol Info 37 175.961865 10.42.16.88 192.168.0.30 SIP/SDP Status: 200 OK, with session description
{......truncated...datagram......}
User Datagram Protocol, Src Port: 38276 (38276), Dst Port: 5060 (5060)
Source port: 38276 (38276)
Destination port: 5060 (5060)
Length: 785
Checksum: 0xb573 [correct]
Session Initiation Protocol
Status-Line: SIP/2.0 200 OK
Status-Code: 200
[Resent Packet: False]
Message Header
Via: SIP/2.0/UDP 192.168.0.30:5060;branch=z9hG4bK01a93c8b;rport=5060
Contact: <sip:david@intra-calcman.org:38276;rinstance=90eafe1f95fbbfa3>
Contact Binding: <sip:david@intra-calcman.org:38276;rinstance=90eafe1f95fbbfa3>
URI: <sip:david@intra-calcman.org:38276;rinstance=90eafe1f95fbbfa3>
SIP contact address: sip:david@intra-calcman.org:38276
To: <sip:david@intra-calcman.org:38276;rinstance=90eafe1f95fbbfa3>;tag=79182b50
SIP to address: sip:david@intra-calcman.org:38276
SIP tag: 79182b50
From: "julien"<sip:julien@intra-calcman.org>;tag=as1ea41f9e
SIP Display info: "julien"
SIP from address: sip:julien@intra-calcman.org
SIP tag: as1ea41f9e
Call-ID: 48f934056782cf6f581c024128f6c29c@192.168.0.30
CSeq: 102 INVITE
Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY, MESSAGE, SUBSCRIBE, INFO
Content-Type: application/sdp
User-Agent: X-Lite release 1003l stamp 30942
Content-Length: 239
Message body
Session Description Protocol
Session Description Protocol Version (v): 0
Owner/Creator, Session Id (o): - 4 2 IN IP4 10.42.16.88
Owner Username: -
Session ID: 4
Session Version: 2
Owner Network Type: IN
Owner Address Type: IP4
Owner Address: 10.42.16.88
Session Name (s): CounterPath eyeBeam 1.5
Connection Information (c): IN IP4 10.42.16.88
Connection Network Type: IN
Connection Address Type: IP4
Connection Address: 10.42.16.88
Time Description, active time (t): 0 0
Session Start Time: 0
Session Stop Time: 0
Media Description, name and address (m): audio 21008 RTP/AVP 0 3 8 101
Media Type: audio
Media Port: 21008
Media Proto: RTP/AVP
Media Format: ITU-T G.711 PCMU
Media Format: GSM 06.10
Media Format: ITU-T G.711 PCMA
Media Format: 101
Media Attribute (a): fmtp:101 0-15
Media Attribute Fieldname: fmtp
Media Format: 101
Media format specific parameters: 0-15
Media Attribute (a): rtpmap:101 telephone-event/8000
Media Attribute Fieldname: rtpmap
Media Format: 101
MIME Type: telephone-event
MIME type: telephone-event
Media Attribute (a): sendrecv
Media Attribute (a): x-rtp-session-id:9A236EF6D78A42C39FFB8460582A5DE0
Media Attribute Fieldname: x-rtp-session-id
Media Attribute Value: 9A236EF6D78A42C39FFB8460582A5DE0
As you can see, david also sends some information for Session Description. In fact, david chooses several parameters among those sent by julien and returns them. Those parameters will define the session. We can also note that the branch value is still the same.
Voice transport
David gets the call and sends a RTCP message. RTCP (Real Time Control Protocol) is a protocol used to manage RTP (Real Time Protocol) communication. Both protocols are defined in the RFC 3550. I couldn't present it better than the way the RFC does it:
I will deal with RTP after, but for now we have to study the RTCP packet sent by david just before he has accepted the call. Why did he send a RTCP packet before he sent a “200 OK” packet for accepting the call?
There's no official reason so we have to assume that's an X-lite developers' choice…
RTCP has been designed to manage the RTP protocol. It performs four functions:
-
Provide feedback on the quality of the data distribution;
-
Carry a persistent identifier for an RTP source: the canonical name or CNAME;
-
Use the first two functions to know the exact number of participants, and then calculates the rate at which packets are sent;
-
Provide minimal information about the participants. This last function is optional.
There is several packets types for RTCP:
-
SR : sender report (transmission/reception statistics from active senders);
-
RR : ACK for SR when there are more than 31 sources;
-
SDES : Source Description, including CNAME;
-
BYE : a participant has left the session;
-
APP : application specific function;
So, now, let's see the RTCP packet sent by david:
No. Time Source Destination Protocol Info
36 175.934060 10.42.16.88 10.42.16.48 RTCP Receiver Report
{......truncated...datagram......}
User Datagram Protocol, Src Port: 21009 (21009), Dst Port: 14319 (14319)
Source port: 21009 (21009)
Destination port: 14319 (14319)
Length: 140
Checksum: 0x3bf0 [correct]
Real-time Transport Control Protocol (Receiver Report)
[Stream setup by SDP (frame 34)]
[Setup frame: 34]
[Setup Method: SDP]
10.. .... = Version: RFC 1889 Version (2)
..0. .... = Padding: False
...0 0000 = Reception report count: 0
Packet type: Receiver Report (201)
Length: 1
Sender SSRC: 739353178
Real-time Transport Control Protocol (Source description)
[Stream setup by SDP (frame 34)]
[Setup frame: 34]
[Setup Method: SDP]
10.. .... = Version: RFC 1889 Version (2)
..0. .... = Padding: False
...0 0001 = Source count: 1
Packet type: Source description (202)
Length: 30
Chunk 1, SSRC/CSRC 739353178
Identifier: 739353178
SDES items
Type: CNAME (user and domain) (1)
Length: 61
Text: 7260F2D3A8994DF8B7C89FD1A725211A@unique.z4140B17CA7EF45E8.org
Type: PRIV (private extensions) (8)
Length: 49
Prefix length: 16
Prefix string: x-rtp-session-id
Text: 9A236EF6D78A42C39FFB8460582A5DE0
Type: END (0)
It's a SDES packet, so it provides the CNAME of david. This last is generated randomly using the user name and the host name. In fact, the CNAME is not the primary identifier of an RTP communication. This functionality is supplied by the SSRC number (which will be described later). But because the SSRC number could change during a session, the CNAME is used to identify a participant in any case. For david, CNAME is: 7260F2D3A8994DF8B7C89FD1A725211A@unique.z4140B17CA7EF45E8.org
David, who is very talkative, also sent the first RTP packet:
No. Time Source Destination Protocol Info
38 175.989523 10.42.16.88 192.168.0.30 RTP Payload type=ITU-T G.711 PCMU, SSRC=739353178, Seq=1424, Time=2097700, Mark
{......truncated...datagram......}
User Datagram Protocol, Src Port: 21008 (21008), Dst Port: 14318 (14318)
Source port: 21008 (21008)
Destination port: 14318 (14318)
Length: 180
Checksum: 0x78d0 [correct]
Real-Time Transport Protocol
10.. .... = Version: RFC 1889 Version (2)
..0. .... = Padding: False
...0 .... = Extension: False
.... 0000 = Contributing source identifiers count: 0
1… .... = Marker: True
Payload type: ITU-T G.711 PCMU (0)
Sequence number: 1424
Timestamp: 2097700
Synchronization Source identifier: 739353178
Payload: FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF...
RTP packets are very simple. Their main goal is to deliver payloads (data samples). So, the packet contains only the codec used (Payload Type), the sequence number (for reassembly), the Timestamp (packets are dropped after a too long time) and the SSRC we have already talked about. The SSRC is a 32bits number chosen randomly that identifies one specific participant, or more precisely a participant's session. This number will change in many cases and that is why the CNAME number exists. But if the line is clear, the SSRC will be the same until the end of the discussion.
The Payload is the voice, the video or anything you want, encoded with the codec. Closing the call
When david wants to leave the communication, he sends a BYE SIP datagram such as this one to the proxy:
No. Time Source Destination Protocol Info 3090 79.192225 10.42.16.88 192.168.0.30 SIP Request: BYE sip:julien@10.42.16.48:5060;transport=UDP
{.....truncated...datagram.......}
Session Initiation Protocol
Request-Line: BYE sip:julien@10.42.16.48:5060;transport=UDP SIP/2.0
Method: BYE
Resent Packet: False
Message Header
Max-Forwards: 70
From: <sip:david@intra-calcman.org>;tag=13701
SIP from address: sip:david@intra-calcman.org
SIP tag: 13701
To: <sip:julien@intra-calcman.org>;tag=6868
SIP to address: sip:julien@intra-calcman.org
SIP tag: 6868
CSeq: 602 BYE
Call-ID: 11665@10.42.16.88
Route: <sip:192.168.0.30;lr=on;ftag=13701>
Via: SIP/2.0/UDP 10.42.16.88:5060;rport;branch=z9hG4bK17673
Content-Length: 0
This is a classic SIP message, David sends a BYE Request to Julien via the OpenSER Proxy. The proxy forwards the BYE Request to julien and this last one answers the proxy with a 200 OK datagram. Finally, the proxy forwards the 200 OK datagram to david, so the call is closed.
No. Time Source Destination Protocol Info 3095 79.356865 10.42.16.24 10.6.0.108 SIP Status: 200 OK
{.....truncated...datagram.....}
User Datagram Protocol, Src Port: sip (5060), Dst Port: sip (5060)
Source port: sip (5060)
Destination port: sip (5060)
Length: 306
Checksum: 0x0432 [correct]
Session Initiation Protocol
Status-Line: SIP/2.0 200 OK
Status-Code: 200
Resent Packet: False
Message Header
Max-Forwards: 70
Record-Route: <sip:192.168.0.30;lr=on;ftag=13701>
From: <sip:julien@intra-calcman.org>;tag=13701
SIP from address: sip:julien@intra-calcman.org
SIP tag: 13701
To: <sip:david@intra-calcman.org>;tag=6868
SIP to address: sip:david@intra-calcman.org
SIP tag: 6868
CSeq: 602 BYE
Call-ID: 11665@10.42.16.88
Via: SIP/2.0/UDP 10.42.16.88:5060;rport=5060;branch=z9hG4bK17673
Content-Length: 0
Conclusion
This paper is an inexhaustive overview of SIP and RTP protocols. If you want to find more precise information, I recommend that you read the RFCs. Moreover, if you find mismatching values on this page, this is because I have not written it at one time. So, several packets come from different communications.
Source: Linuxwall, http://wiki.linuxwall.info/doku.php/en:ressources:dossiers:voip:sip_rtp
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