The transport layer in the OSI model is responsible for ensuring end-to-end communication, reliability, and data flow control. When it comes to mobile environments, traditional transport layer protocols may face challenges due to factors like varying bandwidth, higher error rates, or latency introduced by handovers between different networks. To address these challenges, several mobile transport layer protocols and enhancements to existing protocols have been proposed. Here are some key ones:

TCP (Transmission Control Protocol):

  • Mobile TCP (M-TCP): An enhancement of standard TCP to handle frequent disconnections and long latency periods in mobile networks.
  • Indirect TCP (I-TCP): Divides TCP into two separate connections: one between the mobile device and a fixed host (proxy) and another between the fixed host and the destination. This ensures that local issues (like handovers) do not affect the overall TCP flow.
  • TCP over 2.5/3G: Modifications to TCP to optimize its performance over 2.5G and 3G networks.
  • Fast Retransmit/Recovery: Enhances TCP’s reaction to packet losses without waiting for a timeout.

UDP (User Datagram Protocol): While UDP itself is connectionless and does not ensure data delivery, certain mobile applications use UDP because of its lower overhead. Voice over IP (VoIP) and streaming media are examples where slight data loss might be acceptable, but lower latency is preferred.

SCTP (Stream Control Transmission Protocol): Initially designed for signaling in telecommunication networks, SCTP provides multi-streaming and multi-homing features, which can be beneficial in mobile environments for balancing loads and ensuring redundancy.

DCCP (Datagram Congestion Control Protocol): Designed for applications that need congestion control but not the strict reliability of TCP. DCCP is useful for streaming media in mobile environments where occasional data loss can be tolerated.

Mobile SCTP (mSCTP): An extension of SCTP designed specifically for mobility support. It enables seamless handovers by adding, deleting, and changing IP addresses during an ongoing session.

RTP/RTCP (Real-time Transport Protocol/Real-time Transport Control Protocol): Used primarily for real-time communication applications like video and voice streaming. RTP provides end-to-end delivery services for data with real-time characteristics, while RTCP provides monitoring of the data delivery and provides feedback on the quality of service.

In mobile scenarios, the choice of transport protocol or its variant depends on the application’s specific requirements, such as reliability, real-time constraints, and the nature of the mobile network being used.