CLNS
Encyclopedia
CLNS is an abbreviation of Connectionless Network Service.
It is an OSI
Network Layer
service that (unlike CONS
) does not require a circuit to be established before data is transmitted. CLNS routes messages to their destinations
independently of any other messages.
In an OSI protocol deployment, CLNS would be the service provided by CLNP (Connectionless Network Protocol)
and used by TP4 (Transport Protocol Class 4). However CLNP is not used on the Internet
, instead its function
is provided by IP
. CLNP is still widely used today in many telecommunications networks around the world. This is because IS-IS
(an OSI
layer 3 protocol) is mandated by the ITU-T as the protocol for management of the SDH (Synchronous Digital Hierarchy) elements.
CLNP uses IS-IS
(developed by DEC
) for routing. CLNP type supports two values; 1 and 28. 1 is used to indicate Error Report. All the other packets use the value 28 (Hex 0x1c) (TP4, TARP
). CLNP does not have a Protocol field like IP. The NSEL field in the NSAP
is used for forwarding to the appropriate protocol handler. In IP it is always assumed that the end detecting and handling errors in the packet will send an error packet (via ICMP
) back to the source. CLNP PDU
s (PDUs are the OSI equivalent of packets) will only elicit error reports if the Error Report Requested flag is set. The CLNP header does not require the messages to be padded to the nearest 32 bit boundary. The length of a CLNP header can go up to 254 octets.
CLNP has separate fields for indicating total length and segment length which helps in reassembly.
CLNP is standardized in ISO
8473. TP4 is standardized in ISO 8073.
It is an OSI
Open Systems Interconnection
Open Systems Interconnection is an effort to standardize networking that was started in 1977 by the International Organization for Standardization , along with the ITU-T.-History:...
Network Layer
Network Layer
The network layer is layer 3 of the seven-layer OSI model of computer networking.The network layer is responsible for packet forwarding including routing through intermediate routers, whereas the data link layer is responsible for media access control, flow control and error checking.The network...
service that (unlike CONS
CONS
CONS, Connection-Oriented Network Service, is one of the two OSI network layer protocols, the other being CLNS...
) does not require a circuit to be established before data is transmitted. CLNS routes messages to their destinations
NSAP address
A Network Service Access Point address , defined in ISO/IEC 8348, is an identifying label for a Service Access Point used in OSI networking....
independently of any other messages.
In an OSI protocol deployment, CLNS would be the service provided by CLNP (Connectionless Network Protocol)
and used by TP4 (Transport Protocol Class 4). However CLNP is not used on the Internet
Internet
The Internet is a global system of interconnected computer networks that use the standard Internet protocol suite to serve billions of users worldwide...
, instead its function
is provided by IP
Internet Protocol
The Internet Protocol is the principal communications protocol used for relaying datagrams across an internetwork using the Internet Protocol Suite...
. CLNP is still widely used today in many telecommunications networks around the world. This is because IS-IS
IS-IS
Intermediate System To Intermediate System , is a routing protocol designed to move information efficiently within a computer network, a group of physically connected computers or similar devices....
(an OSI
OSI
- General :* OSI Pharmaceuticals, an American pharmaceutical company* OSI Restaurant Partners, the restaurant and entertainment group that includes Outback Steakhouse...
layer 3 protocol) is mandated by the ITU-T as the protocol for management of the SDH (Synchronous Digital Hierarchy) elements.
CLNP uses IS-IS
IS-IS
Intermediate System To Intermediate System , is a routing protocol designed to move information efficiently within a computer network, a group of physically connected computers or similar devices....
(developed by DEC
Digital Equipment Corporation
Digital Equipment Corporation was a major American company in the computer industry and a leading vendor of computer systems, software and peripherals from the 1960s to the 1990s...
) for routing. CLNP type supports two values; 1 and 28. 1 is used to indicate Error Report. All the other packets use the value 28 (Hex 0x1c) (TP4, TARP
Terminal Identifier - Address Resolution Protocol
Terminal Identifier - Address Resolution Protocol is a protocol defined in Telcordia Technologies Generic Requirements document for SONET and used in OSS to resolve a TL1 TID to a CLNP address of a Network Element . The protocol is used in a number of network devices. Steven R...
). CLNP does not have a Protocol field like IP. The NSEL field in the NSAP
NSAP address
A Network Service Access Point address , defined in ISO/IEC 8348, is an identifying label for a Service Access Point used in OSI networking....
is used for forwarding to the appropriate protocol handler. In IP it is always assumed that the end detecting and handling errors in the packet will send an error packet (via ICMP
Internet Control Message Protocol
The Internet Control Message Protocol is one of the core protocols of the Internet Protocol Suite. It is chiefly used by the operating systems of networked computers to send error messages indicating, for example, that a requested service is not available or that a host or router could not be...
) back to the source. CLNP PDU
Protocol data unit
In telecommunications, the term protocol data unit has the following meanings:#Information that is delivered as a unit among peer entities of a network and that may contain control information, address information, or data....
s (PDUs are the OSI equivalent of packets) will only elicit error reports if the Error Report Requested flag is set. The CLNP header does not require the messages to be padded to the nearest 32 bit boundary. The length of a CLNP header can go up to 254 octets.
CLNP has separate fields for indicating total length and segment length which helps in reassembly.
CLNP is standardized in ISO
International Organization for Standardization
The International Organization for Standardization , widely known as ISO, is an international standard-setting body composed of representatives from various national standards organizations. Founded on February 23, 1947, the organization promulgates worldwide proprietary, industrial and commercial...
8473. TP4 is standardized in ISO 8073.
TP4: ISO Transport Protocol Class 4 In Conjunction with CLNS
Transport Protocol Class 4 (TP4), one of the five transport layer protocols existing in the OSI suite, offers error recovery, performs segmentation and reassembly, and supplies multiplexing and demultiplexing of data streams over a single virtual circuit. TP4 sequences PDUs and retransmits them or re-initiates the connection if an excessive number are unacknowledged. TP4 provides reliable transport service and functions with either connection-oriented or connectionless network service. TP4 is the most commonly used of all the OSI transport protocols, which is similar to the Transmission Control Protocol (TCP) in the TCP/IP suite.Protocols providing CLNS
Some protocols that provide the CLNS service:- CLNP, as specified in ITU-T Recommendation X.233 is a Public Data NetworkPublic Data NetworkA public data network is a network established and operated by a telecommunications administration, or a recognized private operating agency, for the specific purpose of providing data transmission services for the public....
protocol that provides the connectionless mode network service as described in ITU-T Recommendation X.213.
- Signalling Connection Control Part (SCCP), as specified in ITU-T Recommendation Q.711 is a Signaling System 7 protocol that provides the connectionless mode network service as described in ITU-T Recommendation X.213.
- Internet ProtocolInternet ProtocolThe Internet Protocol is the principal communications protocol used for relaying datagrams across an internetwork using the Internet Protocol Suite...
and User Datagram ProtocolUser Datagram ProtocolThe User Datagram Protocol is one of the core members of the Internet Protocol Suite, the set of network protocols used for the Internet. With UDP, computer applications can send messages, in this case referred to as datagrams, to other hosts on an Internet Protocol network without requiring...
essentially provide the connectionless mode network service as described in ITU-T Recommendation X.213.