I am new to asterisk and would like to ask you a question about receiving a service from SIP without using a middle server. In the fist scenario, two agents communication with each other using a middle-way SIP server which has asterisk installed on it. However, this method is not reliable as if the server goes down, communication is lost. In the other scenario, I want SIP to be installed on the two agents willing to communicate with each other, without using any middle-way SIP servers. is the latter case more reliable and beneficial than the former? what are the pros and cons of each model? and unfortunately, as I am newbie I couldn't find any direct material about implementing the second scenario with asterisk. could you give me some hints please.
receiving service from SIP without using a server
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Asterix is a Server solution that can host multiple parties to talk to each other over SIP. So in the case of exploring Peer to Peer Communications, there is no purpose for the Asterix.
For the Peer to Peer, there are lot of SIP Client software that can exactly do what you want. Set the IP's of other Peer and start having a call. But this is inherently not scalable. Not knowing other peers connections or rather managing them becomes too cumbersome for the client.
A central server can massage the signaling towards a peer and in a world where different users using different client software can all interwork. Big Advantage to widen the user base.
It all depends whats the business case you are working towards. There are cases where the peer to peer is simpler, but has limitations.
the CS Columbia Articulates pointed by @Saman articulated the issues on a very deep level.
SIP has been conceived to be used as a peer to peer protocol, but in this case both agents need to know each other IP addresses (this works only if both addresses are public or both agents are in the same LAN). If a server is used, then both agents need to know only the server IP address. In any case, once the communication is established between the two agents, the RTP packets are exchanged directly, without a server in the middle. In most cases a SIP server (like Asterisk) is needed in order to handle all possible situations (remote agent not available, voice mail, etc.).
In theory, it is possible to have peer to peer VoIP protocols even when both agents are on separate LANs, however this technology is in a very early stage (e.g. Retroshare http://retroshare.sourceforge.net).