When doing IPC using TCP/IP sockets using the loopback address, do common networking stacks skip framing the message in lower-level PDUs?
In some environments such as Java, it's natural to use TCP/IP sockets to pass messages between processes on the same host using the 'localhost' address (127.0.0.1 in IPv4, or ::1 in IPv6). (Because Java tends not to expose other IPC mechanisms in its API).
Clearly, this has the potential to be a lot slower than IPC via message passing over pipes, or IPC using shared-memory.
On the other hand, if the TCP/IP networking stack realised that both ends of the connection were on the loopback interface, it might be able to do a fair bit of optimisation so that the efficiency might not differ much from using pipes.
But do common operating systems (Windows, Linux) implement such optimisations in their TCP/IP stacks?
Answer
Yes. When a packet/data to a Loopback address(127.x.x.x) is received, the IP layer of the TCP/IP uses the Loopback route to route the packet to itself.
Looback Route
Network Destination || Netmask || Gateway || Interface || Metric
127.0.0.0 |||||||||||||||||||||| 255.0.0.0 || 127.0.0.1|| 127.0.0.1 || 1
After routing it to itsef, at TCP/UDP layer with the help of protocol control blocks(per connection data structure) the corresponding socket and its owner process will be identified to deliver the packet/data.
Bottom line, All the tasks at Data Link layers and Physical layers(of OSI Model) will be avoided.