Network Address Translators (NAT) and firewalls create problems for end-to-end connectivity on the Internet. This not only affects P2P-SIP but also client-server SIP. In this article I post some example numbers to illustrate the point.
These numbers are for example only: suppose there are 10% public Internet nodes, 30% nodes behind good (cone or address restricted) NAT, 30% nodes behind bad (symmetric) NAT and 30% nodes behind UDP blocking firewalls (F). Let's denote these as P=10%, G=30%, B=30%, F=30%. Here the public Internet nodes are typically from universities and research institutes, those behind good NAT are usually from residential DSL/Cable access, those behind bad NAT are partly from residential and partly from enterprise environment, and those behind UDP blocking firewalls are from enterprise and corporate networks. Suppose a call event between any two pair of nodes is independent of each other for the probability analysis purpose and nodes are equally likely to call any other node. Thus, percentage of calls between two public Internet nodes is (10%)^2 = 0.01 = 1%.
Now let us enumerate the NAT and firewall traversal techniques available to SIP. STUN helps with good NAT, whereas TURN relay is needed for bad NAT. ICE is used to negotiate the connectivity using STUN or TURN bindings. A TCP-based relay (or even HTTP relay) is needed for UDP blocking and very restricted firewalls. (what about TCP hole punching and other techniques?) A STUN server is light in terms of bandwidth utilization, whereas a TURN relay needs high network bandwidth and hence costs the service provider more money. Same is the case with TCP-based relay.
In a call if one participant is behind a UDP blocking firewall (F), then the call must use a TCP relay. This amounts to 1-(1-F)^2 = 51% calls going through TCP relay.
In a call if both participants are behind bad NAT, then we need a TURN relay. This amounts to B^2 = 9% of the calls.
If one participant in a call is either on public Internet or good NAT and other is on public Internet, good NAT or bad NAT, then the media can go end-to-end using STUN bindings. This amounts to 40% of the calls.
In conclusion, the VoIP provider will need to host UDP or TCP relays for 51+9=60% of the calls. This is not a good proposition.
In real world, the call events are not independent of each other: probability of a corporate user calling another corporate user within the same corporation is high. Also probability of a home user calling another home user is also high. For example, a SIP service targeted towards consumers can expect to have most of the calls among residential users. Thus, the percentage of calls that can be end-to-end is much higher than 40%. Similarly, an enterprise VoIP system can expect to have mostly internal intra-enterprise calls, which do not need to cross the enterprise firewall. Hence the percentage of calls needing the relay is not as high as 60%. Let us analyze these two use cases separately.
Suppose, for a consumer SIP service, the distribution of nodes is P=15%, G=50%, B=30%, F=5%, i.e., less number of users are from bad NAT or UDP blocking firewalls. In this scenario about 20% calls need a relay whereas 80% calls don't.
In an enterprise VoIP system, suppose 60% calls are intra-office and 40% are with outside the office network, then only those 40% calls need a relay whereas 60% calls don't. In a properly engineered enterprise VoIP system, appropriate ports are opened for UDP as well as appropriate media relays are installed in DMZ which facilitates smooth media path for inter-office communication.
While we can play with these numbers as much as we want, the fact remains that a significant percentage of calls need media relay, either UDP TURN relays or TCP relays. This puts unnecessary burden on the VoIP service provider to install and manage relays and buy network bandwidth for those relays, or simply disallow calls that require relay (in which case they may lose customers).
In a peer-to-peer system with super nodes such as Skype, these super nodes can act as media relays and hence save a lot of bandwidth and maintenance cost for the provider. There are some things to consider though: a node behind public Internet can become UDP as well as TCP relay for any call, whereas a node behind good NAT can become only UDP relay with some workaround, but not a TCP relay. This puts too much burden on nodes behind public Internet.
Let us consider the original example with P=10%, G=30%, B=30%, F=30%. In this case the 51% of calls that require TCP relay must use one of the 10% P nodes. When acting as a relay, the bandwidth requirement at the relay is twice that of when the node is in a call. Suppose each node makes N calls a day, and generally speaking needs bandwidth for N calls. However, a public Internet node not only needs bandwidth for its own N calls, but also for relaying 5xN calls of other users which amounts to total bandwidth for 11xN calls. Thus, while the super-node architecture is beneficial to the provider, it heavily punishes users on the public Internet. (My guess is that number of public nodes using VoIP are about 4-5%, which further burdens the public nodes).
A managed P2P-SIP infrastructure can be a good alternative, where corporations and universities donate hosts/bandwidth on high speed network to act as relays/super-nodes. Alternatively, one can have an incentive system to promote hosts to become relays and super-nodes.