What protocols operate at the transport layer?

Protocols that operate at the transport layer (L4) provide data transfer between applications and control exactly how that data is delivered.

In the simplest terms: TCP and UDP operate at the transport layer.

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#Main transport layer protocols

#1. TCP (Transmission Control Protocol)

The main reliable transport layer protocol.

TCP provides:

• connection establishment
• guaranteed delivery of data
• correct segment order
• retransmission on loss
• flow and congestion control

Used where reliability matters:

• HTTP / HTTPS
• FTP
• SMTP, POP3, IMAP
• SSH

In short:

TCP is slower, but reliable.

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#2. UDP (User Datagram Protocol)

A simple and unreliable transport layer protocol.

UDP:

• does not establish a connection
• does not acknowledge delivery
• does not track order
• creates almost no overhead

Used where speed and latency matter more than perfect delivery:

• DNS
• VoIP (calls)
• video streaming
• online games

In short:

UDP is fast, but without guarantees.

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#Additional protocols (rarer, but useful to know)

#3. SCTP (Stream Control Transmission Protocol)

• combines ideas from TCP and UDP
• supports multiple streams within one connection
• rarely used, in specialized systems (for example, telecom)

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#4. DCCP (Datagram Congestion Control Protocol)

• like UDP, but with congestion control
• almost never used in practice

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#A common interview question

Why is it said that there are "two protocols" at the transport layer?

Because:

• TCP and UDP cover 99% of real scenarios
• the rest exist, but are rarely used

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#In short, for the interview

TCP and UDP operate at the transport layer: TCP provides reliable data transfer with order and congestion control, while UDP provides fast transfer without connection establishment or delivery guarantees.