Transport layer
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In computer networking, the transport layer is a conceptual division of methods in the layered architecture of protocols in the network stack in the Internet protocol suite and the OSI model. The protocols of this layer provide end-to-end communication services for applications.[1]: §1.1.3 It provides services such as connection-oriented communication, reliability, flow control, and multiplexing.
The details of implementation and semantics of the transport layer of the Internet protocol suite,[1] which is the foundation of the Internet, and the OSI model of general networking are different. The protocols in use today in this layer for the Internet all originated in the development of TCP/IP. In the OSI model the transport layer is often referred to as Layer 4, or L4,[2] while numbered layers are not used in TCP/IP.
The best-known transport protocol of the Internet protocol suite is the
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Services
Transport layer services are conveyed to an application via a programming interface to the transport layer protocols. The services may include the following features:[4]
- Connection-oriented communication:[5] It is normally easier for an application to interpret a connection as a data stream rather than having to deal with the underlying connection-less models, such as the datagram model of the User Datagram Protocol (UDP) and of the Internet Protocol (IP).
- Same order delivery: The network layer doesn't generally guarantee that packets of data will arrive in the same order that they were sent, but often this is a desirable feature. This is usually done through the use of segment numbering, with the receiver passing them to the application in order. This can cause head-of-line blocking.
- NACK message to the sender. Automatic repeat requestschemes may be used to retransmit lost or corrupted data.
- Flow control: The rate of data transmission between two nodes must sometimes be managed to prevent a fast sender from transmitting more data than can be supported by the receiving data buffer, causing a buffer overrun. This can also be used to improve efficiency by reducing buffer underrun.
- packets. For example, automatic repeat requests may keep the network in a congested state; this situation can be avoided by adding congestion avoidance to the flow control, including slow start. This keeps the bandwidth consumption at a low level in the beginning of the transmission, or after packet retransmission.
- TCP/IP model, but of the session layerin the OSI model.
Analysis
The transport layer is responsible for delivering data to the appropriate application process on the host computers. This involves
Some transport layer protocols, for example TCP, but not UDP, support virtual circuits, i.e. provide connection-oriented communication over an underlying packet-oriented datagram network. A byte-stream is delivered while hiding the packet mode communication for the application processes. This involves connection establishment, dividing of the data stream into packets called segments, segment numbering and reordering of out-of-order data.
Finally, some transport layer protocols, for example TCP, but not UDP, provide end-to-end reliable communication, i.e.
UDP is a very simple protocol and does not provide virtual circuits, nor reliable communication, delegating these functions to the application program. UDP packets are called datagrams, rather than segments.
TCP is used for many protocols, including
Many non-IP-based networks, such as X.25, Frame Relay and ATM, implement the connection-oriented communication at the network or data link layer rather than the transport layer. In X.25, in telephone network modems and in wireless communication systems, reliable node-to-node communication is implemented at lower protocol layers.
The OSI connection-mode transport layer protocol specification defines five classes of transport protocols: TP0, providing the least error recovery, to TP4, which is designed for less reliable networks.
Due to
Protocols
This list shows some protocols that are commonly placed in the transport layers of the Internet protocol suite, the OSI protocol suite, NetWare's IPX/SPX, AppleTalk, and Fibre Channel.
- ATP, AppleTalk Transaction Protocol
- CUDP, Cyclic UDP[9]
- DCCP, Datagram Congestion Control Protocol
- FCP, Fibre Channel Protocol
- IL, IL Protocol
- MPTCP, Multipath TCP
- NORM, NACK-Oriented Reliable Multicast
- RDP, Reliable Data Protocol
- RUDP, Reliable User Datagram Protocol
- SCTP, Stream Control Transmission Protocol
- SPX, Sequenced Packet Exchange
- SST, Structured Stream Transport
- TCP, Transmission Control Protocol
- UDP, User Datagram Protocol
- UDP-Lite
- µTP, Micro Transport Protocol
Comparison of Internet transport layer protocols
Feature | UDP | UDP-Lite | TCP | Multipath TCP | SCTP | DCCP | RUDP[a] |
---|---|---|---|---|---|---|---|
Packet header size | 8 bytes | 8 bytes | 20–60 bytes | 50–90 bytes | 12 bytes[b] | 12 or 16 bytes | 14+ bytes |
Typical data-packet overhead | 8 bytes | 8 bytes | 20 bytes | ?? bytes | 44–48+ bytes[c] | 12 or 16 bytes | 14 bytes |
Transport-layer packet entity | Datagram | Datagram | Segment | Segment | Datagram | Datagram | Datagram |
Connection-oriented | No | No | Yes | Yes | Yes | Yes | Yes |
Reliable transport | No | No | Yes | Yes | Yes | No | Yes |
Unreliable transport | Yes | Yes | No | No | Yes | Yes | Yes |
Preserve message boundary | Yes | Yes | No | No | Yes | Yes | Yes |
Delivery | Unordered | Unordered | Ordered | Ordered | Ordered / Unordered | Unordered | Unordered |
Data checksum | Optional | Yes | Yes | Yes | Yes | Yes | Optional |
Checksum size | 16 bits | 16 bits | 16 bits | 16 bits | 32 bits | 16 bits | 16 bits |
Partial checksum | No | Yes | No | No | No | Yes | No |
Path MTU | No | No | Yes | Yes | Yes | Yes | ? |
Flow control | No | No | Yes | Yes | Yes | No | Yes |
Congestion control
|
No | No | Yes | Yes | Yes | Yes | ? |
Explicit Congestion Notification | No | No | Yes | Yes | Yes | Yes | ? |
Multiple streams | No | No | No | No | Yes | No | No |
Multi-homing
|
No | No | No | Yes | Yes | No | No |
Bundling / Nagle | No | No | Yes | Yes | Yes | No | ? |
- ^ RUDP is not officially standardized. There have been no standard-related developments since 1999.
- ^ Excluding data chunk headers and overhead chunks. Without embedded chunks, an SCTP packet is essentially useless.
- ^ Counted as follows: 12 bytes SCTP header + 16 bytes DATA chunk header or 20 bytes I-DATA chunk header + 16+ bytes SACK chunk. Additional non-data chunks (e.g. AUTH) and/or headers for additional data chunks, which might easily increase the overhead with 50 bytes or more, not counted.
Comparison of OSI transport protocols
ISO/IEC 8073/ITU-T Recommendation X.224, "Information Technology - Open Systems Interconnection - Protocol for providing the connection-mode transport service", defines five classes of connection-mode transport protocols designated class 0 (TP0) to class 4 (TP4). Class 0 contains no error recovery and was designed for use on network layers that provide error-free connections. Class 4 is closest to TCP, although TCP contains functions, such as the graceful close, which OSI assigns to the session layer. All OSI connection-mode protocol classes provide expedited data and preservation of record boundaries. Detailed characteristics of the classes are shown in the following table:[10]
Service | TP0 | TP1 | TP2 | TP3 | TP4 |
---|---|---|---|---|---|
Connection-oriented network | Yes | Yes | Yes | Yes | Yes |
Connectionless network | No | No | No | No | Yes |
Concatenation and separation | No | Yes | Yes | Yes | Yes |
Segmentation and reassembly | Yes | Yes | Yes | Yes | Yes |
Error recovery | No | Yes | No | Yes | Yes |
Reinitiate connection (if an excessive number of PDUs are unacknowledged) | No | Yes | No | Yes | No |
Multiplexing and demultiplexing over a single virtual circuit | No | No | Yes | Yes | Yes |
Explicit flow control | No | No | Yes | Yes | Yes |
Retransmission on timeout | No | No | No | No | Yes |
Reliable Transport Service | No | Yes | No | Yes | Yes |
There is also a connectionless transport protocol, specified by ISO/IEC 8602/ITU-T Recommendation X.234.[11]
References
- ^ .
- ^ "Introducing the Internet Protocol Suite". System Administration Guide, Volume 3.
- ^ "X.225 : Information technology – Open Systems Interconnection – Connection-oriented Session protocol: Protocol specification". Archived from the original on February 1, 2021. Retrieved March 10, 2023.
- ^ "Transport Layer" (PDF). Galgotias University.
- ^ Heena, Khera. "Data Communication and networking" (PDF). Galgotias University. p. 9.
- ^ Papastergiou et al. 2017, p. 620-621.
- ^ Papastergiou et al. 2017, p. 623-624.
- ^ Corbet 2018.
- ^ Brian C. Smith, Cyclic-UDP: A Priority-Driven Best-Effort Protocol (PDF), retrieved February 23, 2020
- ^ "ITU-T Recommendation X.224 (11/1995) ISO/IEC 8073". Itu.int. Retrieved January 17, 2017.
- ^ "ITU-T Recommendation X.234 (07/1994) ISO/IEC 8602". Itu.int. Retrieved January 17, 2017.
Bibliography
- Corbet, Jonathan (January 29, 2018). "QUIC as a solution to protocol ossification". LWN.net.
- Papastergiou, Giorgos; Fairhurst, Gorry; Ros, David; Brunstrom, Anna; Grinnemo, Karl-Johan; Hurtig, Per; Khademi, Naeem; Tüxen, Michael; Welzl, Michael; Damjanovic, Dragana; Mangiante, Simone (2017). "De-Ossifying the Internet Transport Layer: A Survey and Future Perspectives". S2CID 1846371.