TCP.protocol.txt

                                  ┏━━━━━━━━━┓
                                  ┃   TCP   ┃
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FEATURES ==>                      #Transport layer, i.e. add features over IP
                                  #Re-order out-of-order IP requests
                                  #Reliability, data corruption: error correction
                                  #Packet loss, duplicate packets???
                                  #Session: arbitrary large TCP request, despite being broken in multiple fixed-size IP requests

PACKET STRUCTURE ==>              #In bytes
                                  #0-1: source port
                                  #2-3: destination port
                                  #4-7: SEQ number
                                  #8-11: ACK number
                                  #12 (first 4 bits) (data offset): header size (including Options), divided by 4
                                  #12 (last 4 bits): reserved
                                  #13 (flags): CWR, ECE, URG, ACK, PSH, RST, SYN, FIN
                                  #14-15: window size
                                  #16-17: checksum
                                  #18-19: URG
                                  #20[-480]: Options

PORT ==>                          #Identifies the application layer protocol
                                  #Ranges:
                                  #  - 0-1023: standard
                                  #  - 1024-49151: registered, for custom protocol
                                  #  - 49152-65535: ephemeral, for applications
STANDARD PORTS ==>                #Among:
                                  #  - 21 (FTP)
                                  #  - 22 (SSH)
                                  #  - 23 (Telnet)
                                  #  - 25 (SMTP)
                                  #  - 53 (DNS)
                                  #  - 80 (HTTP)
                                  #  - 110 (POP3)
                                  #  - 143 (IMAP2)
                                  #  - 194 (IRC)
                                  #  - 443 (HTTPS)
PORT SCANNING ==>                 #Test each port on a host to know which is available
PORT SWEEPING ==>                 #Test a specific port on multiple hosts

SOCKET ==>                        #IP address (or host) + transport layer + application layer
                                  #Written [PROTOCOL://]IP|HOST[:PORT]

THREE-TIME HANDSHAKE ==>          #A -> B: SYN, SEQ1
                                  #A <- B: SYN, SEQ2, ACK1 (SEQ1 + 1)
                                  #A -> B: SEQ3 (ACK1), ACK2 (SEQ2 + 1)
DATA TRANSMISSION ==>             #No SYN|ACK flag
                                  #SEQ number:
                                  #  - last ACK number
                                  #  - i.e. number of bytes sent
                                  #ACK number:
                                  #  - last SEQ number + current packet size
                                  #  - i.e. number of bytes received
DISCONNECT ==>                    #A -> B: FIN
                                  #A <- B: ACK, FIN
                                  #A -> B: ACK

KEEPALIVE ==>                     #Poll the other socket with empty packet.
                                  #If no response, close connection.
                                  #Parameters:
                                  #  - interval NUM: poll frequency (e.g. 1s)
                                  #  - time NUM: max time to poll (e.g. 2h)
                                  #  - retry NUM: max times to retry on failure
                                  #Completely unrelated to HTTP keepalive

NAGLE'S ALGORITHM ==>             #Inverse is called "no-delay"
                                  #Increase packet size by buffering outgoing requests
                                  #  - until receiver acknowledges previous requests
                                  #Same pros|cons as MTU size (see its doc)
                                  #Even without it, underlying IOSTREAM still use buffer to keep up with high demand

SPOOFING ==>                      #Guessing SEQ numbers to steal session
DOS                               #Denial Of Service
                                  #Limit server availability by sending lot of requests
DDOS ==>                          #Distributed DOS, using a botnet
SYN FLOOD ==>                     #DOS using SYN requests with different SEQ numbers but no following ACK request
                                  #This makes server keep one new connexion for each request

telnet HOST PORT                  #CLI TCP client

BROWSER DEBUGGING ==>             #  - about:net-internals#sockets (Chrome)
                                  #  - about:networking#sockets (Firefox)