Table of Contents

1. TCP
2. UDP

TCP (Transmission Control Protocal)

* Description :

    ``` 
    TCP provides reliable, ordered, and error-checked delivery of a stream of octets (bytes) between applications running on hosts communicating via an IP network
    ```
* Its a persistent connection
* Three way handshare(at the begining)
    ```
    step1 : Client sends -> I want to talk -> Server
    step2 : Client recieves <- "Sure. Are you Ready? <- Server
    step3 : Client sends -> "Yes I am Ready" -> Server

        <- TCP Connection established ->
    ```
* Its Dominant protocal & Used for
    * Web (http and https)
    * Telnet
    * FTP
    * email

UDP(User Datagram Protocal)

* Description - provides a connectionless datagram service that emphasizes reduced latency over reliability. 
* Sends Data to server; does not care whether it reached destination or not;
* Its faster.

    ![TCP vs IDP](https://techtaste.in/img/tcp-udp.png)
* Used for 
    * Live streaming audio or video
    * DNS Queries, VoiceOverIP(VoIP),  Dynamic Host Configuration Protocol (DHCP)

• HTTP

  • verion : HTTP/1.x (aka H1)

    • Features

      • Contents of each stream are HTTP 1.1 requests and responses
      • Single persistent connection(TCP)
      • Semantics like HTTP methods, HTTP Headers & Response Status Codes
        

    • Flaws

      • No Server Push; Low performance
      • Messages are not converted to binary format
        

  • version : HTTP/2.0 (aka H2; supports only with SSL/TLS)

    • Features
      • Protocol negotiation mechanism — protocol electing, eg. HTTP/1.1, HTTP/2 or other.
      • Server push
      • Compression of request headers(Using HPACK)
      • Multiplexing over a single TCP connection
      • Binary protocal
      • Stream concurrency: Carrying multiple streams over a single TCP connection
      • Flow Control: receiver signals the sender for the maximal amount of data it is allowed to transmit (over a stream/TCP connection)
      • Stream priority: Sender signals the receiver for the priority of this stream, compared to others
      • Stream dependency: Sender signals the receiver on recommended order for processing the streams
        

    e.g. e.g.

  • Ref : https://medium.com/@factoryhr/http-2-the-difference-between-http-1-1-benefits-and-how-to-use-it-38094fa0e95b

• HTTPS(SSL/TLS)

  • Works on HTTP protocal with encrypted TLS/SSL connection

    • Browser requests https cert & gets certficate & signature of the ssl cert.
    • Browser verifies the cert’s integrity & identity
    • Browser creates a symetric key, encrypts it usnig server’s public key and sends to server.
    • Both Browser & Server uses this symetric key to encrypt the messages.
      

  • Some of SSL cert types

    • Domain Validation SSL
    • Wildcard SSL
    • Multidomain SLL certs Note: All browsers enforce SSL certs to be used with HTTP/2.0 protocal.

• Ping

  • protocal - Internet Control Message Protocol (ICMP)
  • Sends echo request packet to an address, then waits for a reply.
  • The ping is successful only if:
    • the echo request gets to the destination, and
    • the destination is able to get an echo reply back to the source within a predetermined time called a timeout. To be more precise ICMP type 8 (echo message) and type 0 (echo reply message) are used. ICMP has no ports!
  • port - No Ports. Does not use TCP or UDP.

• SSH (port# 22)

  • protocal - SSH (its a communication protocal just like http, https, ftp…etc)
  • SSH is client(/usr/bin/ssh)

  • SSHD is server(openssh-server)

    -- Install openssh-server
    $ sudo apt-get install openssh-server
            
    -- sshd (ssh daemon server)
    $ systemctl start ssh
    

  • Authenticatoin methods(ssh user@192.168.56.101)

    • password

    • Public & Private key

      • ssh-keygen to generate ssh public & private keys
      • ~/.ssh/id_rsa (Private Key)
      • ~/.ssh/id_rsa.pub (public Key)
      • public key goes into “authorized_keys” file
        

       $cat ~/.ssh/authorized_keys
        ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABgQC6rsvFUW/CD3x/HLp/6WJKPFwqMqex5LnEvMsQvrPtOs4rarQs4JZqwfBOhGpBB3MGU8OfDvBYjL2JMOyw/YqNtbhnSH2n5/Sco5Sge11SJuy1HbjrkVJE/fyZqxzyypsI8/7HqOYRxArCHHdg56Oz0MAZfeTLNIkvvL7CDyoc5snoiCWrLaQnDmOkKArrdVq6VoABWUFHfEN3/AERh6SOfwSHl5y/7uvcW7Yaemv6mm1MnEqCqi/Wo8ratQ9WFEvm2md8a4Hk4gD+VVyqNTr/P8dAnLqCYY9XpAczHAOp9NFqAeTo9LhLOXfm7/d89kB5ZIbobiuo6GfdOR9Nb0EpRsN8Pr3MYYFxj06N0+FoQJjL/1BNnvjrbSS4BA7KX/Z3c08lcpxPUc7QoAuS8qkEIJyhPtVc4jtEBT7CkyKvi3UVFFLRawSbAwjrLYxdL1HwXCcpTw3iw0QAK5CYPklXphikiPTPy7JnvkxE9hH3/ALioWD8LG4rwYWf24iT+yM= INTEL@DESKTOP-G5I1PT9
      

      • Refer : ssh-add command

    • host based

• telnet (port# 23)

  -- Verify whether given port is open on a host
  $ telnet [host [port]]
  rchari:~$ telnet techtaste.in 443
  Trying 185.199.111.153...
  Connected to techtaste.in.
  Escape character is '^]'.
      
  ^]
  Connection closed by foreign host.
  

• DNS resolution (Port# 53)

  • DNS uses TCP for Zone transfer and UDP for name queries either regular (primary) or reverse

  • Host to IP(Forward lookup)

    • Client sends dns query to ISP(Resolver)

    • ISP asks ROOT Server(top DNS) & ROOT server redirects to ISP to TLD(Top level Domain Server)

      Note: TLD(Top level domain server) contains address of top level domains like .com, .net, .org.. etc.
      

    • TLD redirects ISP to authoritative name servers(ns1.yahoo.xxx)

    • ISP gets ip of the host from name server(ns1.yahoo.xxx) & ISP caches the ip.

    • ISP sends the ip of the domain to client(browser)

  • IP to Host(reverse lookup)

    • using nslookup”

  • protocal - uses either UDP or TCP

    A client computer will always send a DNS Query using UDP Protocol over Port 53. If a client computer does not get response from a DNS Server, it must re-transmit the DNS Query using the TCP after 3-5 seconds of interval. 
    

• FTP [Uses two ports# 21(for command) & 20(for data transfer)]