From the UDT website http://udt.sourceforge.net
UDT: Breaking the Data Transfer Bottleneck
UDT is a reliable UDP based application level data transport
protocol for distributed data intensive applications over wide area
high-speed networks. UDT uses UDP to transfer bulk data with its own
reliability control and congestion control mechanisms. The new
protocol can transfer data at a much higher speed than TCP does. UDT
is also a highly configurable framework that can accommodate various
congestion control algorithms.
The main attraction for me, is by using existing UDP channels, it's much
easier to traverse firewalls. UDP is a connectionless protocol.
Additional performance may be obtained by using UDT in preference to TCP
over wide area networks when NAT traversal isn't required.
NAT is used to share a public IP address among a number of privately
addressed network computers.
Computer (A) behind a NAT can send data to the internet, the NAT re
writes packet headers, a remote computer (B) can respond, the response
packets are returned to computer (A) by the NAT.
If a River installation resides on a box in a private network behind a
firewall and exports a service, and a proxy for this service is at some
point obtained by a client residing outside the the private network,
then that remote client cannot contact the service, unless a port is
opened on the firewall and redirects packets to the service.
To cut a long story short, a service needs a port opened on a firewall,
providing a way for external clients to contact it.
This can be done in the following ways:
1. Configuring the firewall manually.
2. NAT-PMP (If supported by NAT gateway)
http://tools.ietf.org/html/draft-cheshire-nat-pmp-03
3. UPnP IGD (If supported by NAT gateway)
4. STUN http://tools.ietf.org/html/rfc5389
The limitation with NAT-PMP and UPnP is, they must be supported and
enabled on a router but cannot be used for multi layer router network
topographies.
The STUN protocol allows a network node residing behind a firewall to
discover it's public port on a firewall and to keep it open by
connecting to a public STUN server. Publicly accessible STUN servers
are available on the internet. STUN doesn't succeed in all cases.
Publicly accessible TURN servers, available on the internet, provide a
method of relaying packets when STUN doesn't work.
Work on NAT Traversal is more mature and less complex for UDP than TCP,
see http://tools.ietf.org/html/draft-ietf-mmusic-ice-tcp-14#appendix-A
It should be recognised that any connection obtained using these methods
is not secure, STUN could misinform a client of it's public address to
perform a substitution attack, a TURN server could perform a man in the
middle attack. These network connections should only be used with
secure Endpoints, utilising cryptographic authentication.
A ServerSocketFactory could be designed to perform the necessary steps
to open a firewall port, discover public address, port and keep the
connection open.
Cheers,
Peter.
