Important Abstraction: Datagrams

"Best-effort Delivery"

Features:

  1. has a "TO addres" (a computer's address)
  2. has a "FROM address"
  3. short text (~1kbyte, <1.5 kbyte)

Possibilities a datagram could have

From address (IPv4 32 bits/ IPv6 128 bits)
To address (v4 32 bits/ v6 128 bits)
Message: "hello"
  • posibility:
    1. delivered, quickly
    2. delivered, wrong text (truncate/disordered)
    3. Delivered, really late
    4. Delivered, to wrong address / from wrong address
    5. Never delivered
    6. Delivered, tampered text
    7. Delivered, multiple times
    8. Lives forever (Can not happen, using TTL to limit)

Address

  • We may Run out of address 2^32(IPv4)

  • Calculate 104.196.238.229:

    • 229 + 238*256 + 196*256 *256 + 104 * 256* 256* 256 = 1757736677
    • telnet 104.196.238.229 == telnet 1757736677 [amazing!]
    • address is unique in a network
    • IP address is from network (no from the manufacturer)
    • IP address has distributed, hiearchiral structure(eg, MIT distribute IP address accoring to the computer's building)

mtr(traceroute)

Time to Live (TTL) : number of step the datagram allows to have

  • How to traceroute?
    • using TTL
    • when the TTL = 0, send a message back to the sender
  • ==Competition: find a mtr where steps than 35?==

Byte Stream

Byte Stream: [another abstraction]

  • A writer: "abc"
  • A reader: "abc", get same bit to the same order

Lab0 Task: build a Byte stream from a datagram

Network Stack/Layers

layers / "network stack" /Internet 4 layes model：

response/request
^
[HTTP] [Application layer] 
^
Byte stream
^
[Transmission Control Protocol (TCP)] [Transport Layer] 
^
datagram [Internet Layer]
^
network interface 
^
wave pocket/frame== [link layer]

5 layers model:

response/request
^
[TLS(Transport Layer Security)]
^
[HTTP] [Application layer] 
^
Byte stream
^
[Transmission Control Protocol (TCP)] [Transport Layer] 
^
datagram [Internet Layer]
^
network interface 
^
wave pocket/frame [link layer]

how to prentend to be another "from address":

Web HTTP proxy

rr        proxy         To address
------------------------------------
HTTP   HTTP -> HTTP     HTTP
⬇      ^       ⬇        ^ 
BS      BS      BS       BS  
⬇      ^       ⬇        ^
TCP     TCP     TCP      TCP
⬇      ^       ⬇        ^
DG  ->  DG      DG  ->   DG

Virtual private network(VPN)

use a different DG (with another "from address")

Request/Response
^
[HTTP]
^
BS
^
[TCP]
^
DG1 / DG 2

TCP proxy (Tor, onion router)

Talk about it later.

Multiplexing

multiplexing：simply put a signal in each layer to tell which upper layers it should goes to.

eg: The upper layer of IP might be ICMP(a control protocol) or TCP. Using a mark in the Datagram to distinguish