# FemtoTCP

## Stack architecture

 - No dynamic allocation (pre-allocated sockets and buffers)
 - Four-steps main loop function
 - Callback-based socket interface (allows implementing blocking BSD calls)



## Data structures

* Two types of circular buffers (fixed size):

   - "fifo" : contains entire frames, including a descriptor.
   - "queue" : contains pure data, indexed by byte. Used for TCP receive buffer
   only.

* One binary heap for timers



### FemtoTCP fifo

```
+---------------------------------------------------------------------------------------------------------------------------+
|               +-----+---+----+-----+------------------+-----+---+----+-----+------------------+                           | 
|               | De  | E | IP | TCP |      Payload     | De  | E | IP | TCP |      Payload     |                           |
|               | sc  | T |    |     |                  | sc  | T |    |     |                  |                           |
|* FREE SPACE * | ri  | H |    |     |                  | ri  | H |    |     |                  |   * FREE SPACE*           |
|               | pt  |   |    |     |                  | pt  |   |    |     |                  |                           |
|               | or  |   |    |     |                  | or  |   |    |     |                  |                           |
|               +-----+---+----+-----+------------------+-----+---+----+-----+------------------+                           |
+---------------------------------------------------------------------------------------------------------------------------+
                ^                                                                               ^                      
                |                                                                               |                      
                |                                                                               |                      
                |                                                                               |                      
                |Tail                                                                       Head|                      

```



### FemtoTCP queue

```
 +--------------+--------------------------------------------+---------------------------------------------------------------+
 |              |*------------------------------------------*|                                                               |
 |              ||                                          ||                                                               |
 |              ||                                          ||                                                               |
 |* FREE SPACE *||                DATA PAYLOAD              ||                           * FREE SPACE *                      |
 |              ||                                          ||                                                               |
 |              ||                                          ||                                                               |
 |              |*------------------------------------------*|                                                               |
 +--------------+--------------------------------------------+---------------------------------------------------------------+
                ^                                            ^                                                         
                |                                            |                                                         
                |                                            |                                                         
                |                                            |                                                         
                |Tail                                    Head|                                                         
```



## Sockets

### TCP socket

```
                                             +-------------+
                                             |Main loop TX |
                                             +-------------+
                                                        ^   
+----------------------------------+                    |   
|                                  |             +------+   
|  TCP Socket                      |             |          
|                                  |             |          
|                                  |             |          
|                                  |             |          
|                                +-----------------------+ 
|            +---------------+   |                       | 
>DATA OUT==>>|socket send()  |-->|  TX buffer (fifo)     | 
|            +---------------+   |                       | 
|                                +-----------------------+ 
|                                  |                        
|                                  |                        
|                                  |                        
|                                +-----------------------+ 
|             +-------------+    |                       | 
<DATA IN<<====|socket recv()|<---|  RX buffer (queue)    | 
|             +-------------+    |                       | 
|                                +-----------------------+ 
+----------------------------------+      ^                 
                                          |                 
                                          |                 
                                          |                 
                                    +--------------+                 
                                    |  tcp_recv()  |                 
                                    +--------------+
```