The main transport layer protocols (TCP, UDP) have been active for 40 years.

TCP is not the best in particular environments.

• TCP is mainly used for reliable data transfer
• Alternative is UDP and implementing reliability in application layer

Some poor workloads for TCP include:

• Long, fat pipes (large data transfers, long distance)
  • Utilization for TCP is $\frac{L/R}{RTT + L/R}$ (single packet)
    • For large $R$ and large RTT, this ratio can be small (link is underutilized, as TCP link cannot understand what is happening)
  • Challenge - many packets are in-flight, loss will shut down pipeline
• Wireless networks
  • Similar problem with poor utilization
    • Bit errors end up being treated similarly to congestion with TCP
  • Challenges - loss due to noisy, wireless links, mobility
• Long-delay links
  • Challenges - extremely long RTTs
• Data center networks
  • Challenges - latency sensitive
• Background traffic flows
  • Challenges - low priority, “background” TCP flows

To deal with these scenarios, transport layers functions were moved to application layer and built on top of UDP

• e.g. HTTP/3 QUIC uses UDP

QUIC - Quick UDP Internet Connections

QUIC is an application-layer protocol built on top of UDP.

• Goal is to increase the performance of HTTP
• Deployed on many Google servers, apps (Chrome, YouTube)
• Referred to as both an application-layer and transport-layer protocol