Chapter 58, Fundamentals of TCP/IP Networks

Objectives

We would like to :

learn enough to deal with internet sockets.

Notes

DataComm is a course
And a PhD study field.
It is very old,
- Standards were adopted in the 70s
- That means work was done in the 60s and 70s.
- The network was much different then.
Model
- The network is broken into a number of different layers.
  - Application layer: how do our programs work?
    - A writer sends a message to the server in the format
```
from_username:POST:to_unsername/ALL:Message 
```
    - A reader sends a message to the server in the format:
```
username:CONNECT
username:DISCONNECT 
```
    - The server sends a message to a reader in the form:
```
from_username:POST:Message 
```
  - Note, this is a protocol I made up, but it is still a protocol.
  - This logic is entirely contained in my program.
- Since we are dealing with datagrams, my program hands the message to the transport layer.
  - This is implemented in the kernel.
  - Known as UDP
  - Also at this layer is TCP
  - And other protocols.
  - Each wrap the message in their protocol.
- Each of these hands the message to the IP layer
  - This is still in the kernel
  - IP possibly breaks the message into fragments
  - Wraps each in the IP protocol format
  - And sends it to the network layer.
- The network layer is the hardware
  - I don't know for sure, but I bet a combination of device driver and hardware wrap the massage again.
  - And send it off on the physical device.
- Look at pictures on 1181, 1183
Encapsulation: Wrapping data within protocol specific layers.
- See the wikipedia page for the TCP packet structure.
- Or the UDP structure.
- The picture on 1183 shows this.
With encapsulation, we have a system where each layer doesn't need to know what is happening below, it just sends and receives messages.
With this system we can go through many different forms of technology.
- The physical and link layers might be different.
- It might come up to the IP layer depending on the hardware involved.
- A picture I stole from telecomhall.net.
The physical layer
- Ethernet, wireless protocols, satellite protocols ...
- Physical wire (many forms), fiber optic, ...
- This is physically how the data is moved.
- Not really a concern to us.
Data Link
- Interacts with the hardware.
- MTU Maximum Transmission Unit
- Data moves in frames, not packets.
- Not really a concern to us.
IP (internet protocol) or the Network layer
- IP Does the following
  - Breaks the data into small enough fragments to send it through the hardware.
  - Routes data across the internet
  - Provides services to the transport layer.
- There have been multiple IP protocols over the years
  - IPV4 provides a 32 bit address space.
    - And was designed in a different era,
    - With little/no security
    - But has been adopted for now.
  - IPV6 is the current and future standard
    - 128 bit address space
    - Improved security
- Connectionless and unreliable
- May fragment and reassemble packets.
IP (V4) Addresses
- 147.64.242.52/23
- Two parts, the network part and the host id
  - 147.65.242 is the network part
  - 0.52 is the machine part
  - Netmask: 255.255.254.0
  - (ifconfig)
- Assigned by (Local up to IANA)
- Default route: where to send packets by default
- Other routes:
We will need to find the IP address of any machine we wish to communicate with.
Transport Layer (UDP, TCP)
- Ports and port numbers
  - Port numbers are like mail boxes for a system.
  - The IP address tells us what machine we are talking to
  - The port number tells us what application.
- There are a set of reserved ports 0-1023 with assigned applications
- Others through 49151 may have a purpose
- 49152 - 65535 are for dynamic or private use.
  - We will build some servers in this range.
  - But applications will pick some of these ports as well
- And more in /etc/services
- A full connection is two ip addresses and two ports.
UPD
- Connectionless , unreliable
- There is an error detection checksum, but this is primitive.
- To avoid IP fragmentation, datagrams should be kept under 512 bytes.
TCP
- Established connections.
- Look at the picture on 1190 for an idea.
- Acknowledgment of packets, retransmission of lost packets.
- Sequence numbers so fragments can be reassembled with certainty of no loss.
- Flow and congestion control.
RFC : request for comment.
- The way the internet is "built" and run.
  - Proposal submitted
  - Debated and adopted or dismissed.
  - Eventually may be replaced
  - Some are informational or experimental and not standards
  - But some are the internet standards.