Layer 4 - Transport Layer

Transport Layer

  • Layer 3 - Deliver individual packets across multiple hops
  • Layer 4 - Manage multiple packets
    • Create a connection
    • Keep packets in order
    • Re-transmit lost packets

Layer 4 Protocols

  • TCP - Transmission Control Protocol
  • HTTP - Hyper-Text Transport Protocol
  • SMTP - Simple Mail Transfer Protocol
  • IMAP Internet Message Access Protocol
  • FTP - File Transfer Protocol

Create a Connection

  • TCP Handshake
  • Step 1: Client creates a TCP Synchronize Packet (SYN)
    • Packet includes a sequence number
  • Step 2: Server sends back a two-in-one packet
    • Acknowledgement (ACK) of SYN packet
    • SYN packet with sequence, server-to-client
    • Called a SYN-ACK packet
  • Step 3: Client sends back an ACK packet
  • Connection is now established

Create a Connection

What Is Happening Here?

ACK Packets

ACK Packets

ACK Packets - Data Loss

ACK Packets

How long does this take?

  • Your "ping" time is the time for a packet to do one round-trip - client to server back to client.
  • If your "ping" time is 10 ms, it takes 0.010 seconds for a round trip.
  • That's about 100 data packets and 100 ACK packets.
  • At 1500 bytes in an Ethernet frame, you could at most transmit 150 kBps. Or 1.2 Mbps.
  • The ACK time limits your data rate to something too slow.

Improving ACKs

Sliding Window

Sliding Window Animations

Wireshark - Sliding Window

Close a Connection

Close a Connection

What's happening?

Buffering and Sending Data


How is this broken into packets?

Other TCP Flags

  • PSH - Push. Data was sent immediately without waiting for buffer to fill. Receiver should pass up to application layer immediately.
  • RST - Reset. An unexpected packet was received. e.g. A packet for a closed socket.

PSH and RST Packet

Hyper-Text Transport Protocol

  • TCP connection on port 80 (443 encrypted)
  • Can transfer more than HTML
    • PDF
    • Images (PNG, JPG, GIF)
    • JavaScript
    • JSON-Formatted Data
    • Files

HTTP Response Codes

  • 200 - OK
  • 304 Not modified
  • 3XX Redirect
  • 404 Not found
  • 500 Server error


Wireshark Following HTTP

Wireshark Following HTTP

Wireshark Following HTTP

Networking: Layer 4 Transport Layer

By Paul Craven

Networking: Layer 4 Transport Layer

  • 293