Every tutorial I read talks about how the layers "communicate" with each other, but never mentions what hardware/software receives, adds on, and sends each layer of control data.
What piece of hardware and software recieves/encapsulates/sends the data pertaining to each layer of IP/TCP?
For example, what layer does my router work on? What software translates a text file into SMTP? What/Where in my computer is the software for flow control/ transportation?
Layer 1 - Physical - Signaling
Layer 2 - Data Link - Switching (Ethernet frames)
Layer 3 - Network - Routing (IP packets)
Layer 4 - Transport - Multiplexing (TCP segments)
Layer 5 - Session
Layer 6 - Presentation
Layer 7 - Application
The reason why specific software/hardware implementations are not mentioned is because that is the whole point of developing the standard. Each layer does not care how the layer above and beneath takes care of its function, as long as the data is presented according to the standard. It can be in software, hardware, firmware, by way of a mechanical non-electronic device, or even manually by human beings (google "TCP over carrier pigeon" for a non-serious example).
The specific software and hardware involved vary widely as well in the real world. It is entirely possible to perform all functions in software (cheap but slow). It is also entirely possible to do all of them in hardware (fast but expensive). Dedicated network devices such as Cisco routers, etc. will perform more functions in hardware than say, a PC running Linux configured as a router.
Generally Layer 1 and mostly Layer 2 is guaranteed to be performed in hardware. Layer 3 is mostly done in hardware on an enterprise-level router such as those from Cisco. Many NICs come with a feature called "TCP Offload Engine" that can accelerate most of Layer 3 and 4 via hardware on the NIC. Your consumer-level routers from Wal-mart usualy do the routing function entirely in software. Layer 5, 6, and 7 are usually not done in hardware, and if they are, the devices that do so are termed "accelerators".
What piece of hardware and software recieves/encapsulates/sends the data pertaining to each layer of IP/TCP?
Ethernet hardware is typically only used at the lowest layers, the physical and link layers. Based on its MAC address, the hardware will only accept unicast frames that match. The Ethernet device driver is typically responsible for providing the software interface to this device. The device can typically read and write complete Ethernet frames from and to buffers arranged in a linked (circular) ring, and the driver either services interrupts or polls status for any change in buffer ring status (i.e. read or write frame operation complete).
The software is collectively known as the protocol stack. It is a collection of procedures and functions built into the kernel. They are invoked just like any hierarchy of software. When the user program makes a socket R/W system call, a top-layer application routine is invoked. This subroutine/procedure hierarchy is just like any other well-constructed software that enforces modularity, loose coupling and high cohesion.
Each layer in the protocol is performed by one or more (nested) procedure calls, until the data packet is stored in a frame buffer, ready for output by hardware. On successful write completion, the status is returned to the user's call.
For example, what layer does my router work on?
The Internet is described by the TCP/IP model :
Your router directs packets based on IP address, which is the Internet layer. But the router has its own protocol stack to perform other services like its web-based configuration utility. Every host on the network will probably have a complete protocol stack; the layers are not distributed among network hosts/devices.
What software translates a text file into SMTP?
Your mail program interfaces with the Application layer.
What/Where in my computer is the software for flow control/ transportation?
Probably the Transport layer, where TCP operates.
