The diagram shows the species and its relationship with other species, e.g., whether A uses B or whether A and B are friendly. Due to space constraint, some items are grouped together, e.g., all the audio/video codecs, and some relationships are missing, e.g., RTMP is friendly with Speex. Ideally, we need a multi-dimensional representation to show multiple aspects of the jungle and how they are related. The following text lists the protocols that serve similar or common functions, and usually are competing within that function.
| Function | Protocols |
| Structured data encoding | XML, ASN.1, RFC822, others |
| Audio encoding | G.711, G.723.1, G.722, G.726, G.728, G.729, MP3, Speex, Nellymoser, AMR, Silk, GIPS, etc. |
| Video encoding | H.261, H.263, H.264, MPEG, Sorenson, Vidyo, etc. |
| Media transport | RTP/RTCP, SRTP, ZRTP, Skype, IAX, RTMP, RTMFP |
| Rendezvous | SIP, H.323, Skype, Stratus/RTMFP |
| Session description | SDP, H.245, Jingle |
| Session negotiation | SIP/SDP, H.245, Jingle, Skype, RTMFP |
| Call signaling and control | SIP, H.225/Q.931, Skype, IAX, MGCP, SCCP (Skinny), RTMFP |
| Streaming media control | RTSP, RTMP |
| Session announcement | SAP |
| Connectivity | ICE/STUN/TURN, Skype, RTMFP |
| Remote Procedure Call | SOAP, XMLRPC, REST, RTMP |
| Programming calls | CGI, CPL, CCXML, MSCML |
| Programming voice dialog | VoiceXML |
| Instant messaging | XMPP, SIMPLE, MSRP |
| Presence | XMPP, SIMPLE |
| Shared resource access | REST, XMPP, XCAP |
| Shared state | XMPP, RTMP, HTTP |
As you can see that a SIP system typically employs one protocol for one task or a few related tasks, but integrated monolithic systems such as those based on RTMP/RTMFP, Skype or IAX tend to combine multiple functions in the single protocol. I have not listed H.32x protocols other than H.323 because those are intended for non-IP networks. Nevertheless, there are several H.32x systems, e.g., for room based video conferencing or for carrying voice among carriers.
Interworking
With multiple protocols available for the same function, interoperability or interworking among those becomes important. I have talked about SIP and XMPP interworking in the last post. I have hands-on experience with several of the interworking scenarios among protocols shown in the diagram.
H.323-H.324: One of my projects in my first job was interworking between H.323 and H.324. Since both these systems use H.245 as the main session description and negotiation, the interworking task is relatively simple. I also worked on part of H.320 system to try to build H.323-H.320 interworking, but did not complete.
SIP-H.323: One of my first project during my M.S. at Columbia University was SIP-H.323 interworking. I have written sip323 software and couple of internet drafts and papers [1] on this. My PhD thesis gives a complete interworking procedure for basic call setup and registration. The conclusion was that while basic call setup and registration are easy to interwork, the full interworking of all the supplementary services is not feasible and not even needed in many cases. Since both SIP and H.323 use RTP/RTCP for media transport and can use the same set of codecs, the signaling gateway is efficient. The company SIPquest which productized my software demonstrated 10k simultaneous calls (this article).
SIP-RTSP: These protocols serve different purposes, but it is possible to build a system that needs both these functions in a standard compliant way. The sipum software is a voice mail and answering machine that uses SIP for calls and RTSP for recording and playback of media. Since both these use RTP/RTCP for media transport and can use the same set of codecs, the software is efficient as the media path can bypass the software. Please see my papers [1] for details.
SIP-RTMP: There have been several attempts at implementing Flash based SIP systems and SIP-RTMP translator is one of the approach. Some existing projects that implement these are siprtmp, gtalk2voip, red5phone and flaphone. Since RTMP is an integrated streaming protocol which can also do control and RPC, the translator is inefficient since it needs to incorporate the media path as well.
SIP-Skype: Being a proprietary protocol, it is not easy to interwork with Skype. However, Skype itself uses SIP to allow trunking with PSTN providers, and recently there was some news about SIP-based Skype gateway for enterprise.
SIP-IAX: Although IAX is open, it is an integrated protocol that combines media and signaling in the same connection, hence suffers from the same scalability problem as other integrated protocols like RTMP. Asterix also has a SIP gateway so that it can talk to SIP-enabled devices, especially carrier equipments.
SIP-XMPP: There is a interest group that discusses this in depth. My last post gives more links about the interworking scenarios using a gateway or co-location in the client.
SIP-RTMFP: Given the P2P promise of RTMFP, a gateway between these two protocols will be able to connect the proprietary Adobe protocol with the rest of the world for a true web-based end-to-end media path. I haven't seen any system that does this.
SIP-H.320: This gateway is particularly useful for existing room based video conferencing systems that want to connect with more Internet-style SIP devices. The idea is similar to SIP-H.323 translator, and in fact a real deployment may use two gateways: SIP-H.323 and H.323-H.320 in practice.
RTMP-XMPP: Since RTMP and XMPP serve two completely different functions, there is no need to interoperate. However, people have built systems that use XMPP for messaging and signaling while using RTMP for media path. Unfortunately since Jingle extension wants to define its own end-to-end session, it becomes not so useful for exchanging RTMP server session information. In particular use XMPP custom extensions based on presence and message to rendezvous, but do session control and call management in RTMP itself.
XMPP-SIMPLE: The SIP-XMPP interest group is also looking at SIMPLE-XMPP translation. However, given the disconnect between the two protocols, it is likely that all the presence and message updates go through the gateway and hence not as efficient as one would want for presence and instant messages.
RTMP-Skype: Now this is going to be really tough because firstly Skype is still a proprietary protocol, and secondly, both these are integrated protocols hence requiring complete conversion of signaling and media. An specific example could be allowing people to access Skype from web pages, e.g., by having a simple RTMP server in the Skype application itself. This works if Skype is running on your local computer. Alternatively, you need the Flash application to connect to Skype super-nodes running on public computers using RTMP. This poses security risk and is inefficient. Why inefficient? because RTMP over TCP means that only the applications on public Internet will be able to receive the connection, and RTMP is not really good for real-time interactive communication because of its latency and buffering. However, if such gateway are incorporated in Skype, then it truly become ubiquitous to web applications.
RTMP-RTSP: These are two competing streaming protocols. Instead of having a gateway that translates between the two protocols, it might be better to build an integrated client or integrated server -- you can record using RTMP and view using Quicktime (RTSP), or you can use the same client to access real-time streams from RTMP or RTSP. Since RTMP incorporates RPC along with streaming control and media path, whether as RTSP is just streaming control, a complete translation of all the functions may not be feasible.
ASN.1-XML: There has been effort to standardize this, e.g., XER. The proposed H.325 standard by ITU-T will use XML while allowing compatibility with some of the predecessors which are in ASN.1 PER. ASN.1 and XML are just data formats and for the purpose of P2P-SIP, they are not very significant.
If you have data about the usage in real deployment for particular protocol(s), feel free to post your comment.
[1] My publication page http://kundansingh.com/#papers
1 comment:
Great diagrama, Thx.
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