Autosave: 2024-09-06 10:00:02

zk/Application_Layer_of_Internet_Protocol.md

@@ -1,7 +1,23 @@
 ---
 title: Application_Layer_of_Internet_Protocol
-tags: []
+tags: [internet, networks]
 created: Friday, September 06, 2024
 ---
 
 # Application Layer of the Internet Protocol
+
+The final, topmost layer of the Internet Protocol suite.
+
+![Data at the Application Layer](../img/application-layer-data.png)
+
+The Application Layer is where we get to the protocols that describe the
+behaviour of applications. All the preceding lower levels are effectively
+**plumbing that enables applications to do the things they want accross the
+internet**.
+
+Whereas the lower levels facilitate communication accross the internet, the
+Application Layer is concerned with achieving a specific task once the ability
+to communicate is assured.
+
+Application Layer protocols include HTTP, the Simple Mail Transfer Protocol
+(SMTP), and FTP.

zk/Example_scenario_internet_data_transfer.md

@@ -0,0 +1,43 @@
+---
+title: Example_scenario_internet_data_transfer
+tags: [networks, internet]
+created: Friday, September 06, 2024
+---
+
+# Example scenario of data transfer accross the internet
+
+![Path of a network request accross the internet](../img/network-request-layers.png)
+
+![All layers of the IP](../img/all-layers-of-IP.png)
+
+- A client device is connected to a wireless WiFi network.
+- This network is connected to the internet via a router.
+- Elsewhere on the internet is a server also connected to the internet via
+  router.
+- A user of the client device opens a web browser and requests a web page hosted
+  on the server (we will assume it already knows the IP address)
+- The web browser knows HTTP - a protocol of the
+  [Application Layer](Application_Layer_of_Internet_Protocol.md) - and forms an
+  HTTP request intended for the destination server.
+- The browser hands off the HTTP request to the TCP/IP software stack of the
+  device's [operating system](Basic_model_of_the_operating_system.md), asking
+  that the data be delivered to the server on its IP address and at port 80.
+- The TCP/IP software stack on the client OS encapsulates the HTTP payload in a
+  TCP segment (at the
+  [Transport Layer](Transport_Layer_of_Internet_Protocol.md)), setting the
+  destination port to 80 in the segment header.
+- The [Internet Layer](Internet_Layer_of_Internet_Protocol.md) software on the
+  client then wraps the TCP segment in an IP packet, which includes the
+  destination IP address of the server in the packet header.
+- At the [Link Layer](Link_Layer_of_Internet_Protocol.md) of the client device,
+  the IP packet is encapsulated in a frame with the
+  [MAC_address](MAC_addresses.md) of the local router in its header. The frame
+  is wirelessly transmitted by the client device's WiFi hardware.
+- The wireless access point receives the frame and passes to the router. The
+  router examines the internet layer packet to determine the destination IP
+  address.
+- To reach the server, the request needs to travel through multiple routers on
+  the internet and the process is replicated (looking up MAC address of the
+  device and determining IP target). Eventually, the request reaches the router
+  on the network where the server is connected.
+- The last router

zk/Internet_fundamentals.md

@@ -37,12 +37,14 @@ protocols. From the top:
   communication accross networks. It is responsible for identifying hosts on the
   inter-network by their addresses and routing data from network to network.
 
-- The **Transport Layer** come back to this
+- The [Transport Layer](Transport_Layer_of_Internet_Protocol.md) creates a
+  commmunication channel between two hosts using ports and sockets.
 
-- The **Application Layer** is concerned with software that utilises information
-  received and sent over the internet, for instance web pages and emails. This
-  is the highest level of abstraction for which all the lower levels serve as
-  "plumbing". Examples of protocols operative at the Application Layer include
-  HTTP and SSH.
+- The [Application Layer](Application_Layer_of_Internet_Protocol.md) concerned
+  with software that utilises information received and sent over the internet,
+  for instance web pages and emails. This is the highest level of abstraction
+  for which all the lower levels serve as "plumbing". Examples of protocols
+  operative at the Application Layer include HTTP and SSH.
 
-  ![Path of a network request accross the internet](../img/network-request-layers.png)
+See:
+[Example_scenario_internet_data_transfer](Example_scenario_internet_data_transfer.md)