Server Load Balancer:NLB listeners

Protocol

Description

Scenario

TCP

• A connection-oriented protocol that requires a logical connection to be established before data can be transmitted.

• Data is transmitted at a fast rate.

• Suitable for scenarios that prioritize reliability and data accuracy over transmission speed. These scenarios include file transmission, email sending and receiving, and remote logons.

• Web applications that do not have custom requirements.

For more information, see Add a TCP listener.

UDP

• A connectionless protocol. UDP directly transmits data packets instead of making a three-way handshake with the other party before sending data. UDP does not provide error recovery or data re-transmission.

• Fast data transmission but relatively low reliability.

Suitable for scenarios in which real-time transmission outweighs reliability, such as video conferencing and real-time quote services.

For more information, see Add a UDP listener.

TCPSSL

• Encrypted data transmission that prevents unauthorized access.

• Centralized certificate management service. You can upload certificates to NLB. Then, data decryption is offloaded from backend servers to NLB.

• One-way and mutual authentication are supported.

NLB can forward encrypted TCP requests. You can use SSL over TCP in scenarios that require TCP while ensuring high security, such as large-scale TLS offloading.

For more information, see Add a TCPSSL listener.

Port type

Description

Limit

Listening ports (frontend ports)

Used by NLB to receive and forward requests to backend servers.

For each NLB instance:

• TCP and UDP can use the same listening port. For example, you can specify port 80 for a TCP listener and a UDP listener.

• However, you cannot specify the same port for a TCP listener and a listener that uses SSL over TCP because both listeners listen for TCP requests. For example, you cannot specify port 80 for a TCP listener and a listener that uses SSL over TCP.

Service ports (backend ports)

Used by backend servers to receive requests.

An NLB instance can forward requests from a listening port to multiple backend ports that are used by different backend servers of the NLB instance.

For each NLB instance:

You can associate a backend port with listeners that use different protocols. For example, you can associate a backend port with a listener that listens for TCP requests on port 80 and a listener that listens for TCP requests secured with SSL on port 81.