Skip to main content

6. Control Connection Protocol Specification

The following control connection messages are used to establish, clear and maintain L2TP tunnels. All data is sent in network order (high order octets first). Any "reserved" or "empty" fields MUST be sent as 0 values to allow for protocol extensibility.

6.1 Start-Control-Connection-Request (SCCRQ)​

SCCRQ is a control message used to initialize a tunnel between an LNS and an LAC. It is sent by either the LAC or the LNS to begin the tunnel establishment process.

Required AVPs:

  • Message Type AVP
  • Protocol Version
  • Host Name
  • Framing Capabilities
  • Assigned Tunnel ID

Optional AVPs:

  • Bearer Capabilities
  • Receive Window Size
  • Challenge
  • Tie Breaker
  • Firmware Revision
  • Vendor Name

6.2 Start-Control-Connection-Reply (SCCRP)​

SCCRP is a control message sent in reply to a received SCCRQ message. SCCRP is used to indicate that the SCCRQ was accepted and establishment of the tunnel should continue.

Required AVPs:

  • Message Type
  • Protocol Version
  • Framing Capabilities
  • Host Name
  • Assigned Tunnel ID

Optional AVPs:

  • Bearer Capabilities
  • Firmware Revision
  • Vendor Name
  • Receive Window Size
  • Challenge
  • Challenge Response

6.3 Start-Control-Connection-Connected (SCCCN)​

SCCCN is a control message sent in reply to an SCCRP. SCCCN completes the tunnel establishment process.

Required AVP:

  • Message Type

Optional AVP:

  • Challenge Response

6.4 Stop-Control-Connection-Notification (StopCCN)​

StopCCN is a control message sent by either the LAC or LNS to inform its peer that the tunnel is being shutdown and the control connection should be closed. In addition, all active sessions are implicitly cleared (without sending any explicit call control messages). The reason for issuing this request is indicated in the Result Code AVP.

Required AVPs:

  • Message Type
  • Assigned Tunnel ID
  • Result Code

6.5 Hello (HELLO)​

The Hello (HELLO) message is an L2TP control message sent by either peer of a LAC-LNS control connection. This control message is used as a "keepalive" for the tunnel.

Required AVP:

  • Message Type

6.6 Incoming-Call-Request (ICRQ)​

ICRQ is a control message sent by the LAC to the LNS to indicate that an inbound call has been detected and to request that the LNS accept the call.

Required AVPs:

  • Message Type
  • Assigned Session ID
  • Call Serial Number

6.7 Incoming-Call-Reply (ICRP)​

ICRP is a control message sent by the LNS to the LAC to indicate that the inbound call will be accepted.

Required AVPs:

  • Message Type
  • Assigned Session ID

6.8 Incoming-Call-Connected (ICCN)​

ICCN is a control message sent by the LAC to the LNS in response to a received ICRP to complete the establishment of an incoming call.

Required AVPs:

  • Message Type
  • Tx Connect Speed

6.9 Outgoing-Call-Request (OCRQ)​

OCRQ is a control message sent by the LNS to the LAC to indicate that the LNS wishes the LAC to initiate an outgoing call to a remote system.

Required AVPs:

  • Message Type
  • Assigned Session ID
  • Call Serial Number
  • Minimum BPS
  • Maximum BPS
  • Bearer Type
  • Framing Type
  • Called Number

6.10 Outgoing-Call-Reply (OCRP)​

OCRP is a control message sent by the LAC to the LNS to indicate that the OCRQ has been accepted and the outgoing call will be initiated.

Required AVPs:

  • Message Type
  • Assigned Session ID

6.11 Outgoing-Call-Connected (OCCN)​

OCCN is a control message sent by the LAC to the LNS to complete the establishment of an outgoing call.

Required AVPs:

  • Message Type
  • Tx Connect Speed
  • Framing Type

6.12 Call-Disconnect-Notify (CDN)​

CDN is a control message sent by either the LAC or LNS to request disconnection of a specific call. It may be sent at any point during call establishment or after the establishment of a session.

Required AVPs:

  • Message Type
  • Result Code
  • Assigned Session ID

6.13 WAN-Error-Notify (WEN)​

WEN is a control message sent by the LAC to the LNS to indicate WAN error conditions.

Required AVPs:

  • Message Type
  • Call Errors

SLI is a control message sent by the LNS to the LAC to set PPP-negotiated options.

Required AVPs:

  • Message Type
  • ACCM

Note: For complete AVP field definitions and detailed formats, please refer to Chapter 4 AVP Definitions.

Last updated on