What are the services provided by the transport layer?

The services provided by the transport layer ensure reliable end-to-end data delivery between applications running on different hosts. These services hide the complexities of the underlying network and provide a uniform interface to the session layer.

Address Mapping

Address mapping involves translating transport addresses to network addresses. When a session entity requests to send a Transport Service Data Unit (TSDU) to another session entity, it provides its transport service access point address for identification. The transport entity then determines the corresponding Network Service Access Point (NSAP) address to establish proper routing.

Assignment of Network Connection

The transport entity assigns network connections to carry Transport Protocol Data Units (TPDUs). It establishes and manages these connections throughout the communication session. Some transport protocols support recovery from network disconnections by reassigning TPDU transport to alternative network connections when failures occur.

Multiplexing of Transport Connections

To optimize network link utilization, the transport entity can create multiple end-to-end transport connections over a single network connection. This process is called multiplexing. The receiving transport entity identifies different TSDUs using the Transport Connection Endpoint Identifier (TCEPI) attached by the sender.

Splitting of Transport Connection

When network service quality is insufficient or greater resilience against failures is needed, the transport entity performs splitting. This means TPDUs from one transport connection are sent over multiple network connections. Splitting requires resequencing at the destination because TPDUs may arrive out of order.

Establishment of Transport Connection

The transport layer establishes connections using T-CONNECT service primitives. During establishment, the transport entity negotiates quality of service parameters and collects addressing information required for the connection.

Data Transfer

The transport layer provides two types of data transfer:

• Normal data transfer − Supports any number of octets with preserved user data boundaries and bidirectional communication.

• Expedited data transfer − Provides high-priority delivery with separate control flow, bypassing normal data queues. Limited to 16 octets maximum.

Segmentation and Concatenation of TPDUs

Segmentation divides large TSDUs into smaller TPDUs when the network service cannot handle the original size. Each TPDU receives a separate header with Protocol Control Information. The receiving end reassembles these segments.

Concatenation performs the reverse function by mapping multiple TPDUs into a single Network Service Data Unit (NSDU) to improve network efficiency. The transport entity at the receiving end separates the concatenated TPDUs.

Flow Control

The transport entity implements a modified sliding window protocol for flow control. The receiver controls the variable window size and sends credit allocations to the sender, indicating how many TPDUs can be accepted. This mechanism handles back pressure from the network layer.

Error Recovery

The transport layer handles various error types:

• TPDU errors − Lost, duplicated, reordered, or corrupted TPDUs

• Protocol errors − Result in connection release

• Network connection failures − Handled through reassignment and resynchronization

Error detection uses checksums in TPDUs, while acknowledgments and retransmissions recover from losses.

Sequence Numbering

Each TPDU receives a sequence number for flow control and error recovery purposes. Normal mode uses 7-bit sequence numbers, while extended mode supports 31-bit sequence numbers for longer connections.

Conclusion

The transport layer provides essential services including connection management, reliable data transfer, flow control, and error recovery. These services ensure end-to-end communication reliability while hiding network complexities from upper layers.