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Ibid., pg. 3. SIP vs. H.323, A Business Analysis.
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5
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means a gatekeeper must hold its TCP connections for the entire duration of a call. This can pose serious scalability problems for large gatekeepers. Conferencing is another concern for scalability. H.323 supports multiparty conferences with multicast data distribution. However, it requires a central control point (called an MC) for processing all signaling, even for the smallest conferences. This presents several difficulties. Firstly, should the user providing the MC functionality leave the conference and exit their application, the entire conference terminates. In addition, since MC and gatekeeper functionality is optional, H.323 cannot support even three party conferences in some cases. We note that the MC is a bottleneck for larger conferences. H.323 Version 2 has defined the concept of cascaded MCs, allowing for a very limited, application-layer, multicast distribution tree of control messaging. This improves scaling somewhat, but for even larger conferences, the H.332 protocol defines additional procedures. This means that three distinct mechanisms exist to support conferences of different sizes. SIP, however, scales to all different conference sizes. There is no requirement for a central MC; conference coordination is fully distributed. This improves scalability and complexity. Furthermore, as it can use UDP as well as TCP, SIP supports native multicast signaling, enabling a single protocol to scale from sessions with two to millions of members. Table 5-4 compares the SIP and H.323 call features. Feedback is another concern when comparing H.323 and SIP. H.245 defines procedures that enable receivers to control media encodings, trans-
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Table 5-4 Comparison of SIP and H.323 call features
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Feature Blind transfer Operator-assisted transfer Hold Multicast conferences Multiunicast conferences Bridged conferences Forward Call park Directed call pickup SIP Yes Yes Yes, through SDP Yes Yes Yes Yes Yes Yes H.323 Yes No No Yes Yes Yes Yes No No
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Source: Schulzrenne and Rosenberg
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SIP: Alternative Softswitch Architecture
mission rates, and error recovery. This kind of feedback makes sense in point-to-point scenarios, but it ceases to be functional in multipoint conferencing. SIP, instead, relies on RTCP for providing feedback on reception quality (and also for obtaining group membership lists). RTCP, like SIP, operates in a fully distributed fashion. The feedback it provides automatically scales from a two-person point-to-point conference to huge broadcaststyle conferences with millions of participants.16
Extensibility
Extensibility is a key metric for measuring an IP telephony signaling protocol. Telephony is a tremendously popular, critical service, and Internet telephony is poised to supplant the existing circuit-switched infrastructure developed to support it. As with any heavily used service, the features provided evolve over time as new applications are developed. This makes compatibility among versions a complex issue. As the Internet is an open, distributed, and evolving entity, one can expect extensions to IP telephony protocols to be widespread and uncoordinated. This makes it critical to include powerful extension mechanisms from the outset. SIP has learned the lessons of HTTP and SMTP (both of which are widely used protocols that have evolved over time) and built in a rich set of extensibility and compatibility functions. By default, unknown headers and values are ignored. Using the Require header, clients can indicate named feature sets that the server must understand. When a request arrives at a server, it checks the list of named features in the Requires header. If any of them are not supported, the server returns an error code and lists the set of features it does understand. The client can then determine the problematic feature and fall back to a simpler operation. To further enhance extensibility, numerical error codes are hierarchically organized, as in HTTP. Six basic classes exist, each of which is identified by the hundreds digit in the response code (refer to Table 5-3). Basic protocol operation is dictated solely by the class, and terminals need only understand the class of the response. The other digits provide additional information, usually useful but not critical. This allows for additional features to be added by defining semantics for the error codes in a class while achieving compatibility. The textual encoding means that header fields are selfdescribing.
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