10. IANA Considerations
This chapter defines various parameters that require assignment and management by IANA (Internet Assigned Numbers Authority) for the L2TP protocol.
10.1 AVP Attributes
IANA is responsible for maintaining the L2TP AVP attribute type registry. The AVP attribute type is a 16-bit field.
Registration Requirements:
- Value Range 0-1023: Assigned by IETF Consensus (requires publication of an RFC).
- Value Range 1024-65535: Assigned using "First Come First Served" policy.
Assigned Standard AVP Attribute Types:
| Attribute Type | AVP Name | Reference |
|---|---|---|
| 0 | Message Type | RFC 2661 Section 4.4.1 |
| 1 | Result Code | RFC 2661 Section 4.4.2 |
| 2 | Protocol Version | RFC 2661 Section 4.4.3 |
| 3 | Framing Capabilities | RFC 2661 Section 4.4.3 |
| 4 | Bearer Capabilities | RFC 2661 Section 4.4.3 |
| 5 | Tie Breaker | RFC 2661 Section 4.4.3 |
| 6 | Firmware Revision | RFC 2661 Section 4.4.3 |
| 7 | Host Name | RFC 2661 Section 4.4.3 |
| 8 | Vendor Name | RFC 2661 Section 4.4.3 |
| 9 | Assigned Tunnel ID | RFC 2661 Section 4.4.3 |
| 10 | Receive Window Size | RFC 2661 Section 4.4.3 |
| 11 | Challenge | RFC 2661 Section 4.4.3 |
| 12 | Q.931 Cause Code | RFC 2661 Section 4.4.4 |
| 13 | Challenge Response | RFC 2661 Section 4.4.3 |
| 14 | Assigned Session ID | RFC 2661 Section 4.4.4 |
| 15 | Call Serial Number | RFC 2661 Section 4.4.4 |
| 16 | Minimum BPS | RFC 2661 Section 4.4.4 |
| 17 | Maximum BPS | RFC 2661 Section 4.4.4 |
| 18 | Bearer Type | RFC 2661 Section 4.4.4 |
| 19 | Framing Type | RFC 2661 Section 4.4.4 |
| 20 | Packet Processing Delay | RFC 2661 Section 4.4.6 |
| 21 | Called Number | RFC 2661 Section 4.4.4 |
| 22 | Calling Number | RFC 2661 Section 4.4.4 |
| 23 | Sub-Address | RFC 2661 Section 4.4.4 |
| 24 | (Tx) Connect Speed | RFC 2661 Section 4.4.4 |
| 25 | Physical Channel ID | RFC 2661 Section 4.4.4 |
| 26 | Initial Received LCP CONFREQ | RFC 2661 Section 4.4.5 |
| 27 | Last Sent LCP CONFREQ | RFC 2661 Section 4.4.5 |
| 28 | Last Received LCP CONFREQ | RFC 2661 Section 4.4.5 |
| 29 | Proxy Authen Type | RFC 2661 Section 4.4.5 |
| 30 | Proxy Authen Name | RFC 2661 Section 4.4.5 |
| 31 | Proxy Authen Challenge | RFC 2661 Section 4.4.5 |
| 32 | Proxy Authen ID | RFC 2661 Section 4.4.5 |
| 33 | Proxy Authen Response | RFC 2661 Section 4.4.5 |
| 34 | Call Errors | RFC 2661 Section 4.4.6 |
| 35 | ACCM | RFC 2661 Section 4.4.6 |
| 36 | Random Vector | RFC 2661 Section 4.3 |
| 37 | Private Group ID | RFC 2661 Section 4.4.4 |
| 38 | (Rx) Connect Speed | RFC 2661 Section 4.4.4 |
| 39 | Sequencing Required | RFC 2661 Section 4.4.4 |
Vendor-Specific AVPs:
Vendor-specific AVPs use the Vendor ID field (based on SMI Network Management Private Enterprise Codes) to distinguish extensions from different vendors.
10.2 Message Type AVP Values
The Message Type AVP (attribute type 0) value is used to identify the type of L2TP control message.
Assigned Message Type Values:
| Value | Message Type | Abbreviation | Reference |
|---|---|---|---|
| 0 | (Reserved) | ||
| 1 | Start-Control-Connection-Request | SCCRQ | RFC 2661 Section 6.1 |
| 2 | Start-Control-Connection-Reply | SCCRP | RFC 2661 Section 6.2 |
| 3 | Start-Control-Connection-Connected | SCCCN | RFC 2661 Section 6.3 |
| 4 | Stop-Control-Connection-Notification | StopCCN | RFC 2661 Section 6.4 |
| 5 | (Reserved) | ||
| 6 | Hello | HELLO | RFC 2661 Section 6.5 |
| 7 | Outgoing-Call-Request | OCRQ | RFC 2661 Section 6.9 |
| 8 | Outgoing-Call-Reply | OCRP | RFC 2661 Section 6.10 |
| 9 | Outgoing-Call-Connected | OCCN | RFC 2661 Section 6.11 |
| 10 | Incoming-Call-Request | ICRQ | RFC 2661 Section 6.6 |
| 11 | Incoming-Call-Reply | ICRP | RFC 2661 Section 6.7 |
| 12 | Incoming-Call-Connected | ICCN | RFC 2661 Section 6.8 |
| 13 | (Reserved) | ||
| 14 | Call-Disconnect-Notify | CDN | RFC 2661 Section 6.12 |
| 15 | WAN-Error-Notify | WEN | RFC 2661 Section 6.13 |
| 16 | Set-Link-Info | SLI | RFC 2661 Section 6.14 |
Registration Policy:
Assignment of new message type values requires publication of an IETF standards track RFC or an informational RFC approved by the IESG.
10.3 Result Code AVP Values
The Result Code AVP (attribute type 1) is used to indicate the reason for control connection or session termination.
10.3.1 Result Code Field Values
General Result Codes:
| Value | Meaning | Scope |
|---|---|---|
| 0 | Reserved | |
| 1 | General request to clear control connection | StopCCN |
| 2 | General error | StopCCN, CDN |
| 3 | Control channel already exists | StopCCN |
| 4 | Requester is not authorized | StopCCN |
| 5 | Protocol version not supported | StopCCN |
| 6 | Requester is being shut down | StopCCN |
| 7 | Finite State Machine error | StopCCN |
Call Disconnect Result Codes:
| Value | Meaning | Scope |
|---|---|---|
| 1 | Lost carrier | CDN |
| 2 | General error | CDN |
| 3 | Administrative reason | CDN |
| 4 | Temporary lack of appropriate facilities | CDN |
| 5 | Permanent lack of appropriate facilities | CDN |
| 6 | Invalid destination | CDN |
| 7 | No carrier detected | CDN |
| 8 | Busy signal | CDN |
| 9 | No dial tone | CDN |
| 10 | Timeout waiting for carrier | CDN |
| 11 | No framing detected | CDN |
10.3.2 Error Code Field Values
The error code field provides additional details about the error.
| Value | Error Message |
|---|---|
| 0 | No general error |
| 1 | No control connection exists yet for this pair |
| 2 | Length is wrong |
| 3 | One of the field values was out of range |
| 4 | Insufficient resources to handle this operation now |
| 5 | Invalid Session ID |
| 6 | A generic vendor-specific error occurred |
| 7 | Try another (LNS/LAC) |
| 8 | Session or tunnel was shutdown due to receipt of an unknown AVP with M-bit set |
Registration Policy:
Assignment of new result code and error code values requires IETF Consensus (requires publication of an RFC).
10.4 Framing Capabilities & Bearer Capabilities
Framing Capabilities AVP (attribute type 3) and Bearer Capabilities AVP (attribute type 4) use bitmasks to indicate supported capabilities.
Framing Capabilities Bit Definitions:
| Bit | Meaning |
|---|---|
| 0 | Asynchronous Framing supported |
| 1 | Synchronous Framing supported |
| 2-31 | Reserved |
Bearer Capabilities Bit Definitions:
| Bit | Meaning |
|---|---|
| 0 | Analog access supported |
| 1 | Digital access supported |
| 2-31 | Reserved |
Registration Policy:
Assignment of new capability bits requires IETF Consensus (requires publication of an RFC).
10.5 Proxy Authen Type AVP Values
Proxy Authen Type AVP (attribute type 29) is used to indicate the authentication type used by the LAC.
Assigned Authentication Type Values:
| Value | Authentication Type | Reference |
|---|---|---|
| 0 | Reserved | |
| 1 | Textual username/password exchange | RFC 1334 (PAP) |
| 2 | PPP CHAP | RFC 1994 |
| 3 | PPP PAP | RFC 1334 |
| 4 | No Authentication | |
| 5 | Microsoft CHAP Version 1 | RFC 2433 |
| 6 | Reserved | |
| 7 | Microsoft CHAP Version 2 | RFC 2759 |
Registration Policy:
Assignment of new authentication type values uses "First Come First Served" policy.
10.6 AVP Header Bits
The first 6 bits of the AVP header are used as a bitmask to control AVP behavior.
Defined AVP Header Bits:
| Bit | Name | Meaning | Reference |
|---|---|---|---|
| 0 | M (Mandatory) | This AVP must be understood | RFC 2661 Section 4.1 |
| 1 | H (Hidden) | AVP value is hidden | RFC 2661 Section 4.3 |
| 2-5 | Reserved | Reserved, must be set to 0 | RFC 2661 Section 4.1 |
Registration Policy:
Assignment of reserved bits requires Standards Action - i.e., publication of an IETF standards track RFC.
10.7 L2TP UDP Port
Assigned Port:
- Port Number: 1701
- Protocol: UDP
- Purpose: L2TP
- Reference: RFC 2661
IANA has assigned UDP port 1701 for L2TP for both control connections and data sessions.
10.8 L2TP Protocol Number
Although the current specification defines L2TP operating over UDP, L2TP can also operate directly over other packet transport protocols.
IP Protocol Number:
- L2TP Protocol Number: 115
- Name: L2TP
- Reference: RFC 3931 (L2TPv3, directly over IP)
IANA Registry Maintenance:
IANA maintains the following L2TP-related registries:
-
L2TP AVP Attributes Registry
-
L2TP Message Types Registry
- Contains all control message types
-
L2TP Result Codes Registry
- Contains result codes and error codes
-
L2TP Proxy Authen Types Registry
- Contains authentication type values
-
L2TP Ports and Protocol Numbers
- UDP port and IP protocol number assignments
Updates and Extensions:
Subsequent RFCs may define new AVPs, message types, or other parameters. All new assignments must follow the registration policies defined in this section.
Important extensions include:
- RFC 3931: Layer Two Tunneling Protocol - Version 3 (L2TPv3)
- RFC 4591: Frame Relay over L2TP
- RFC 5515: Layer Two Tunneling Protocol (L2TP) Access Concentrator Configuration