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</style><div role="note" class="hatnote navigation-not-searchable">"Layer 5" redirects here. For the political layer sometimes included in the <a href="OSI_model" title="OSI model">OSI model</a>, see <a href="Layer_8" title="Layer 8">Layer 8</a>.</div>
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</style><table class="sidebar sidebar-collapse nomobile nowraplinks"><tbody><tr><th class="sidebar-title"><a href="OSI_model" title="OSI model">OSI model</a><br><span style="font-size:80%;">by <a href="Abstraction_layer" title="Abstraction layer">layer</a></span></th></tr><tr><td class="sidebar-content">
<div class="sidebar-list mw-collapsible mw-collapsed"><div class="sidebar-list-title" style="color: var(--color-base)">7. <a href="Application_layer" title="Application layer">Application layer</a></div><div class="sidebar-list-content mw-collapsible-content hlist">
<ul><li><a href="Network_News_Transfer_Protocol" title="Network News Transfer Protocol">NNTP</a></li>
<li><a href="Session_Initiation_Protocol" title="Session Initiation Protocol">SIP</a></li>
<li><a href="Simple_Sensor_Interface_protocol" title="Simple Sensor Interface protocol">SSI</a></li>
<li><a href="Domain_Name_System" title="Domain Name System">DNS</a></li>
<li><a href="File_Transfer_Protocol" title="File Transfer Protocol">FTP</a></li>
<li><a href="Gopher_(protocol)" title="Gopher (protocol)">Gopher</a></li>
<li><a href="Hypertext_Transfer_Protocol" class="mw-redirect" title="Hypertext Transfer Protocol">HTTP</a> (<a href="HTTP/3" title="HTTP/3">HTTP/3</a>)</li>
<li><a href="Network_File_System" title="Network File System">NFS</a></li>
<li><a href="Network_Time_Protocol" title="Network Time Protocol">NTP</a></li>
<li><a href="Short_Message_Peer-to-Peer" title="Short Message Peer-to-Peer">SMPP</a></li>
<li><a href="Secure_Shell" title="Secure Shell">SSH</a></li>
<li><a href="Simple_Mail_Transfer_Protocol" title="Simple Mail Transfer Protocol">SMTP</a></li>
<li><a href="Simple_Network_Management_Protocol" title="Simple Network Management Protocol">SNMP</a></li>
<li><a href="Telnet" title="Telnet">Telnet</a></li>
<li><a href="Dynamic_Host_Configuration_Protocol" title="Dynamic Host Configuration Protocol">DHCP</a></li>
<li><a href="NETCONF" title="NETCONF">NETCONF</a></li>
<li><i>more....</i></li></ul></div></div></td>
</tr><tr><td class="sidebar-content">
<div class="sidebar-list mw-collapsible mw-collapsed"><div class="sidebar-list-title" style="color: var(--color-base)">6. <a href="Presentation_layer" title="Presentation layer">Presentation layer</a></div><div class="sidebar-list-content mw-collapsible-content hlist">
<ul><li><a href="MIME" title="MIME">MIME</a></li>
<li><a href="External_Data_Representation" title="External Data Representation">XDR</a></li>
<li><a href="ASN.1" title="ASN.1">ASN.1</a></li>
<li><a href="ASCII" title="ASCII">ASCII</a></li>
<li><a href="Transport_Layer_Security" title="Transport Layer Security">TLS</a></li>
<li><a href="Pretty_Good_Privacy" title="Pretty Good Privacy">PGP</a></li></ul></div></div></td>
</tr><tr><td class="sidebar-content">
<div class="sidebar-list mw-collapsible mw-collapsed"><div class="sidebar-list-title" style="color: var(--color-base)">5. </div><div class="sidebar-list-content mw-collapsible-content hlist">
<ul><li><a href="Named_pipe" title="Named pipe">Named pipe</a></li>
<li><a href="NetBIOS" title="NetBIOS">NetBIOS</a></li>
<li><a href="Session_Announcement_Protocol" title="Session Announcement Protocol">SAP</a></li>
<li><a href="Point-to-Point_Tunneling_Protocol" title="Point-to-Point Tunneling Protocol">PPTP</a></li>
<li><a href="Real-time_Transport_Protocol" title="Real-time Transport Protocol">RTP</a></li>
<li><a href="SOCKS" title="SOCKS">SOCKS</a></li>
<li>X.225<sup id="cite_ref-1" class="reference"><a href="#cite_note-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup></li></ul></div></div></td>
</tr><tr><td class="sidebar-content">
<div class="sidebar-list mw-collapsible mw-collapsed"><div class="sidebar-list-title" style="color: var(--color-base)">4. <a href="Transport_layer" title="Transport layer">Transport layer</a></div><div class="sidebar-list-content mw-collapsible-content hlist">
<ul><li><a href="Transmission_Control_Protocol" title="Transmission Control Protocol">TCP</a></li>
<li><a href="User_Datagram_Protocol" title="User Datagram Protocol">UDP</a></li>
<li><a href="Stream_Control_Transmission_Protocol" title="Stream Control Transmission Protocol">SCTP</a></li>
<li><a href="Datagram_Congestion_Control_Protocol" title="Datagram Congestion Control Protocol">DCCP</a></li>
<li><a href="QUIC" title="QUIC">QUIC</a></li>
<li><a href="IPX/SPX" title="IPX/SPX">SPX</a></li></ul></div></div></td>
</tr><tr><td class="sidebar-content">
<div class="sidebar-list mw-collapsible mw-collapsed"><div class="sidebar-list-title" style="color: var(--color-base)">3. <a href="Network_layer" title="Network layer">Network layer</a></div><div class="sidebar-list-content mw-collapsible-content hlist">
<ul><li><a href="Internet_Protocol" title="Internet Protocol">IP</a>
<ul><li><a href="IPv4" title="IPv4">IPv4</a></li>
<li><a href="IPv6" title="IPv6">IPv6</a></li></ul></li>
<li><a href="Internet_Control_Message_Protocol" title="Internet Control Message Protocol">ICMP</a> (<a href="ICMPv6" title="ICMPv6">ICMPv6</a>)</li>
<li><a href="IPsec" title="IPsec">IPsec</a></li>
<li><a href="Internet_Group_Management_Protocol" title="Internet Group Management Protocol">IGMP</a></li>
<li><a href="Internetwork_Packet_Exchange" title="Internetwork Packet Exchange">IPX</a></li>
<li><a href="IS-IS" title="IS-IS">IS-IS</a></li>
<li><a href="AppleTalk" title="AppleTalk">AppleTalk</a></li>
<li><a href="X.25" title="X.25">X.25</a></li>
<li><a href="Packet_Layer_Protocol" title="Packet Layer Protocol">PLP</a></li></ul></div></div></td>
</tr><tr><td class="sidebar-content">
<div class="sidebar-list mw-collapsible mw-collapsed"><div class="sidebar-list-title" style="color: var(--color-base)">2. <a href="Data_link_layer" title="Data link layer">Data link layer</a></div><div class="sidebar-list-content mw-collapsible-content hlist">
<ul><li><a href="Asynchronous_Transfer_Mode" title="Asynchronous Transfer Mode">ATM</a></li>
<li><a href="Address_Resolution_Protocol" title="Address Resolution Protocol">ARP</a></li>
<li><a href="Synchronous_Data_Link_Control" title="Synchronous Data Link Control">SDLC</a></li>
<li><a href="High-Level_Data_Link_Control" title="High-Level Data Link Control">HDLC</a></li>
<li><a href="Serial_Line_Internet_Protocol#CSLIP" title="Serial Line Internet Protocol">CSLIP</a></li>
<li><a href="Serial_Line_Internet_Protocol" title="Serial Line Internet Protocol">SLIP</a></li>
<li><a href="Generic_Framing_Procedure" title="Generic Framing Procedure">GFP</a></li>
<li><a href="Parallel_Line_Internet_Protocol" title="Parallel Line Internet Protocol">PLIP</a></li>
<li><a href="IEEE_802" title="IEEE 802">IEEE 802</a></li>
<li><a href="Logical_link_control" title="Logical link control">LLC</a></li>
<li><a href="Medium_access_control" title="Medium access control">MAC</a></li>
<li><a href="Layer_2_Tunneling_Protocol" title="Layer 2 Tunneling Protocol">L2TP</a></li>
<li><a href="Frame_Relay" title="Frame Relay">Frame Relay</a></li>
<li><a href="G.hn" title="G.hn">ITU-T G.hn DLL</a></li>
<li><a href="Point-to-Point_Protocol" title="Point-to-Point Protocol">PPP</a></li>
<li><a href="X.25" title="X.25">X.25</a> <a href="LAPB" title="LAPB">LAPB</a></li>
<li><a href="Link_Access_Procedure_for_Frame_Relay" title="Link Access Procedure for Frame Relay">Q.922 LAPF</a></li></ul></div></div></td>
</tr><tr><td class="sidebar-content">
<div class="sidebar-list mw-collapsible mw-collapsed"><div class="sidebar-list-title" style="color: var(--color-base)">1. <a href="Physical_layer" title="Physical layer">Physical layer</a></div><div class="sidebar-list-content mw-collapsible-content hlist">
<ul><li><a href="RS-232" title="RS-232">RS-232</a></li>
<li><a href="RS-449" title="RS-449">RS-449</a></li>
<li><a href="List_of_ITU-T_V-series_recommendations" title="List of ITU-T V-series recommendations">ITU-T V-Series</a></li>
<li><a href="I.430" title="I.430">I.430</a></li>
<li><a href="I.431" title="I.431">I.431</a></li>
<li><a href="Plesiochronous_digital_hierarchy" title="Plesiochronous digital hierarchy">PDH</a></li>
<li><a href="Synchronous_optical_networking" title="Synchronous optical networking">SONET/SDH</a></li>
<li><a href="Passive_optical_network" title="Passive optical network">PON</a></li>
<li><a href="Optical_Transport_Network" class="mw-redirect" title="Optical Transport Network">OTN</a></li>
<li><a href="Digital_subscriber_line" title="Digital subscriber line">DSL</a></li>
<li><a href="IEEE_802" title="IEEE 802">IEEE 802</a></li>
<li><a href="IEEE_1394" title="IEEE 1394">IEEE 1394</a></li>
<li><a href="G.hn" title="G.hn">ITU-T G.hn PHY</a></li>
<li><a href="USB" title="USB">USB</a></li>
<li><a href="Bluetooth" title="Bluetooth">Bluetooth</a></li>
<li><a href="X.21" title="X.21">X.21</a></li></ul></div></div></td>
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<p>In the seven-layer <a href="OSI_model" title="OSI model">OSI model</a> of <a href="Computer_network" title="Computer network">computer networking</a>, the <b>session layer</b> is <b>layer 5</b>.
</p><p>The session layer provides the mechanism for opening, closing and managing a <a href="Session_(computer_science)" title="Session (computer science)">session</a> between end-user application processes, i.e., a semi-permanent dialogue. Communication sessions consist of requests and responses that occur between applications. Session-layer services are commonly used in application environments that make use of <a href="Remote_procedure_call" title="Remote procedure call">remote procedure calls</a> (RPCs).<sup id="cite_ref-2" class="reference"><a href="#cite_note-2"><span class="cite-bracket">[</span>2<span class="cite-bracket">]</span></a></sup>
</p><p>An example of a session-layer protocol is the <a href="OSI_protocol_suite" class="mw-redirect" title="OSI protocol suite">OSI protocol suite</a> session-layer protocol, also known as X.225 or ISO 8327. In case of a connection loss this protocol may try to recover the connection. If a connection is not used for a long period, the session-layer protocol may close it and re-open it. It provides for either <a href="Duplex_(telecommunications)" title="Duplex (telecommunications)">full duplex</a> or <a href="Duplex_(telecommunications)" title="Duplex (telecommunications)">half-duplex</a> operation and provides <a href="Synchronization_point" class="mw-redirect" title="Synchronization point">synchronization points</a> in the stream of exchanged messages.<sup id="cite_ref-3" class="reference"><a href="#cite_note-3"><span class="cite-bracket">[</span>3<span class="cite-bracket">]</span></a></sup>
</p><p>Other examples of session layer implementations include <a href="Zone_Information_Protocol" class="mw-redirect" title="Zone Information Protocol">Zone Information Protocol</a> (ZIP) – the <a href="AppleTalk" title="AppleTalk">AppleTalk</a><sup id="cite_ref-4" class="reference"><a href="#cite_note-4"><span class="cite-bracket">[</span>4<span class="cite-bracket">]</span></a></sup> protocol that coordinates the name binding process, and Session Control Protocol (SCP)<sup id="cite_ref-5" class="reference"><a href="#cite_note-5"><span class="cite-bracket">[</span>5<span class="cite-bracket">]</span></a></sup> – the <a href="DECnet" title="DECnet">DECnet</a> Phase IV session-layer protocol.
</p><p>Within the service layering semantics of the OSI network architecture, the session layer responds to service requests from the <a href="Presentation_layer" title="Presentation layer">presentation layer</a> and issues service requests to the <a href="Transport_layer" title="Transport layer">transport layer</a>.
</p>
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<div class="mw-heading mw-heading2"><h2 id="Services">Services</h2></div>
<div class="mw-heading mw-heading3"><h3 id="Connection_establishment_and_release">Connection establishment and release</h3></div>
<p>At the minimum, the session layer allows the two sides to establish and use a connection, called a session, and allows orderly release of the connection.
</p><p>In the OSI model, the transport layer is not responsible for an orderly release of a connection. Instead, the session layer is responsible for that. However, in modern TCP/IP networks, TCP already provides orderly closing of connections at the transport layer.
</p><p>After a session connection is released, the underlying transport connection may be reused for another session connection. Also, a session connection may make use of multiple consecutive transport connections. For example, if, during a session, the underlying transport connection has a failure, the session layer may try to re-establish a transport connection to continue the session.
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<div class="mw-heading mw-heading3"><h3 id="Dialogue_control">Dialogue control</h3></div>
<p>The session layer may provide three different dialogue types - two way simultaneous (full-duplex), two way alternate (half-duplex), and one way (simplex). It also provides the mechanisms to negotiate the type of the dialogue, and controls which side has the "turn" or "token" to send data or to perform some control functions.
</p><p>Dialogue control is not implemented in TCP/IP, and is left to the application layer to handle, if necessary. In the widely used HTTP/1.1 protocol, the client and the server typically work in a half-duplex way. HTTP/1.1 also supports <a href="HTTP_pipelining" title="HTTP pipelining">HTTP pipelining</a> for full-duplex operation, but many servers/proxies couldn't handle it correctly, and there was no dialogue negotiation mechanism to check whether full-duplex is usable or not, so its support was eventually dropped by most browsers.
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<div class="mw-heading mw-heading3"><h3 id="Synchronization_points_and_resynchronization">Synchronization points and resynchronization</h3></div>
<p>The session layer may also allow the two sides to insert <i>synchronization points</i> into the dialogue, and allow them to do a <i>resynchronization</i>, which aborts the current transmission, sets the synchronization point to a certain value, and restarts transmission from that point.
</p><p>This may be used in real-time audio/video transmission. Synchronization points can be used to insert timestamps to the data flow, and a resynchronization may be used to reset the transmission to start from a new timestamp. For example, if the video stream lags behind the audio stream too much, the receiving side may issue a resynchronization request on the video stream, restarting its transmission from a later timestamp.
</p><p>This may also be used by the application to do checkpointing. Synchronization points can be used to indicate that a checkpoint has been committed by the application, and after an application crash or a power failure, a resynchronization can be used to indicate that the application has recovered from a checkpoint and the transmission can be resumed from that point.
</p><p>This may also be used to interrupt / resume a dialogue at any time, not due to an application failure, but as planned by the application. The application may interrupt a dialogue, start another dialogue in the same session, and resume the previous dialogue in the same session or in another session.
</p><p>The session layer may also provide explicit support for managing multiple interruptible dialogues over one or more sessions. These dialogues are called <i>activities</i>. Activities can be interrupted and resumed explicitly. Compared to implicitly interrupting and resuming dialogues by resynchronization, activity support gives the application simpler control of these dialogues.
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<div class="mw-heading mw-heading2"><h2 id="Protocols">Protocols</h2></div>
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<ul><li>ADSP, <a href="AppleTalk" title="AppleTalk">AppleTalk Data Stream Protocol</a></li>
<li>ASP, <a href="AppleTalk" title="AppleTalk">AppleTalk Session Protocol</a></li>
<li>H.245, <a href="H.245" title="H.245">Call Control Protocol for Multimedia Communication</a></li>
<li>ISO-SP, OSI session-layer protocol (X.225, ISO 8327)</li>
<li>iSNS, <a href="ISNS" class="mw-redirect" title="ISNS">Internet Storage Name Service</a></li>
<li>L2F, <a href="L2F" class="mw-redirect" title="L2F">Layer 2 Forwarding Protocol</a></li>
<li>L2TP, <a href="L2TP" class="mw-redirect" title="L2TP">Layer 2 Tunneling Protocol</a></li>
<li>NetBIOS, <a href="NetBIOS" title="NetBIOS">Network Basic Input Output System</a></li>
<li>PAP, <a href="Password_Authentication_Protocol" title="Password Authentication Protocol">Password Authentication Protocol</a></li>
<li>PPTP, <a href="PPTP" class="mw-redirect" title="PPTP">Point-to-Point Tunneling Protocol</a></li>
<li>RPC, <a href="Remote_procedure_call" title="Remote procedure call">Remote Procedure Call Protocol</a></li>
<li>RTCP, <a href="RTCP" class="mw-redirect" title="RTCP">Real-time Transport Control Protocol</a></li>
<li>SMPP, <a href="Short_Message_Peer-to-Peer" title="Short Message Peer-to-Peer">Short Message Peer-to-Peer</a></li>
<li>SCP, <a href="Secure_Copy_Protocol" class="mw-redirect" title="Secure Copy Protocol">Secure Copy</a></li>
<li>SOCKS, the <a href="SOCKS" title="SOCKS">SOCKS</a> internet protocol</li>
<li>ZIP, <a href="Zone_Information_Protocol" class="mw-redirect" title="Zone Information Protocol">Zone Information Protocol</a></li>
<li>SDP, <a href="Sockets_Direct_Protocol" title="Sockets Direct Protocol">Sockets Direct Protocol</a></li></ul>
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<div class="mw-heading mw-heading2"><h2 id="Comparison_with_TCP/IP_model">Comparison with TCP/IP model</h2></div>
<p>The <a href="TCP/IP_reference_model" class="mw-redirect" title="TCP/IP reference model">TCP/IP reference model</a> does not concern itself with the OSI model's details of application or transport protocol semantics and therefore does not consider a session layer. OSI's session management in connection with the typical transport protocols (TCP, SCTP), is contained in the <a href="Transport_layer" title="Transport layer">transport-layer</a> protocols, or otherwise considered the realm of the <a href="Application_layer" title="Application layer">application layer</a> protocols. TCP/IP's layers are <i>descriptions</i> of operating scopes (application, host-to-host, network, link) and not detailed <i>prescriptions</i> of operating procedures or data semantics.
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<div class="mw-heading mw-heading2"><h2 id="See_also">See also</h2></div>
<ul><li><a href="Session_(computer_science)" title="Session (computer science)">Session (computer science)</a></li></ul>
<div class="mw-heading mw-heading2"><h2 id="References">References</h2></div>
<div class="mw-references-wrap"><ol class="references">
<li id="cite_note-1"><span class="mw-cite-backlink"><b><a href="#cite_ref-1">^</a></b></span> <span class="reference-text"><style data-mw-deduplicate="TemplateStyles:r1238218222">
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</style><cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://www.itu.int/rec/T-REC-X.225-199511-I/en">"X.225 : Information technology – Open Systems Interconnection – Connection-oriented Session protocol: Protocol specification"</a>. <a rel="nofollow" class="external text" href="https://web.archive.org/web/20210201064044/https://www.itu.int/rec/T-REC-X.225-199511-I/en">Archived</a> from the original on February 1, 2021<span class="reference-accessdate">. Retrieved <span class="nowrap">March 10,</span> 2023</span>.</cite></span>
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<li id="cite_note-2"><span class="mw-cite-backlink"><b><a href="#cite_ref-2">^</a></b></span> <span class="reference-text"><cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://www.techtarget.com/searchapparchitecture/definition/Remote-Procedure-Call-RPC">"What Is Remote Procedure Call (RPC)? Definition from SearchAppArchitecture"</a>. <i>SearchAppArchitecture</i><span class="reference-accessdate">. Retrieved <span class="nowrap">August 4,</span> 2022</span>.</cite></span>
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<li id="cite_note-3"><span class="mw-cite-backlink"><b><a href="#cite_ref-3">^</a></b></span> <span class="reference-text"><a rel="nofollow" class="external text" href="http://www.itu.int/rec/T-REC-X.225/en/">ITU-T Recommendation X.225</a></span>
</li>
<li id="cite_note-4"><span class="mw-cite-backlink"><b><a href="#cite_ref-4">^</a></b></span> <span class="reference-text"><cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://developer.apple.com/library/archive/documentation/mac/pdf/Networking/ZIP.pdf">"ZIP"</a> <span class="cs1-format">(PDF)</span>. <i>Apple.developer.com</i>.</cite></span>
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<li id="cite_note-5"><span class="mw-cite-backlink"><b><a href="#cite_ref-5">^</a></b></span> <span class="reference-text"><cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://www.w3.org/Protocols/HTTP-NG/http-ng-scp.html">"Session Control Protocol (SCP)"</a>. <i>www.w3.org</i><span class="reference-accessdate">. Retrieved <span class="nowrap">August 4,</span> 2022</span>.</cite></span>
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