PPP has three main components:
- A method for encapsulating datagrams over serial links.
- Link Control Protocol (LCP) for establishing, configuring, and testing the data-link connection.
- A family of Network Control Protocols (NCPs) for establishing and configuring different network-layer protocols.
In order to establish communications over a point-to-point link, each end of the PPP link must first send LCP packets to configure and test the data link. After the link has been established and optional facilities have been negotiated as needed by the LCP, PPP must send NCP packets to choose and configure one or more network-layer protocols. Once each of the chosen network-layer protocols has been configured, datagrams from each network-layer protocol can be sent over the link.
In this document, the NCP for establishing and configuring the IPv6 over PPP is referred as the IPv6 Control Protocol (IPV6CP).
The link will remain configured for communications until explicit LCP or NCP packets close the link down, or until some external event occurs (power failure at the other end, carrier drop, etc.).
2. Sending IPv6 Datagrams
Before any IPv6 packets may be communicated, PPP MUST reach the Network-Layer Protocol phase, and the IPv6 Control Protocol MUST reach the Opened state.
Exactly one IPv6 packet is encapsulated in the Information field of PPP Data Link Layer frames where the Protocol field indicates type hex 0057 (Internet Protocol Version 6).
The maximum length of an IPv6 packet transmitted over a PPP link is the same as the maximum length of the Information field of a PPP data link layer frame. PPP links supporting IPv6 MUST allow the information field at least as large as the minimum link MTU size required for IPv6.
3. A PPP Network Control Protocol for IPv6
The IPv6 Control Protocol (IPV6CP) is responsible for configuring, enabling, and disabling the IPv6 protocol modules on both ends of the point-to-point link. IPV6CP uses the same packet exchange mechanism as the Link Control Protocol (LCP). IPV6CP packets may not be exchanged until PPP has reached the Network-Layer Protocol phase. IPV6CP packets received before this phase is reached should be silently discarded.
The IPv6 Control Protocol is exactly the same as the Link Control Protocol with the following exceptions:
Data Link Layer Protocol Field
Exactly one IPV6CP packet is encapsulated in the Information field of PPP Data Link Layer frames where the Protocol field indicates type hex 8057 (IPv6 Control Protocol).
Only Codes 1 through 7 (Configure-Request, Configure-Ack, Configure-Nak, Configure-Reject, Terminate-Request, Terminate-Ack, and Code-Reject) are used. Other Codes should be treated as unrecognized and should result in Code-Rejects.
IPV6CP packets may not be exchanged until PPP has reached the Network-Layer Protocol phase. An implementation should be prepared to wait for Authentication and Link Quality Determination to finish before timing out waiting for a Configure-Ack or other response. It is suggested that an implementation give up only after user intervention or a configurable amount of time.
Configuration Option Types
IPV6CP has a distinct set of Configuration Options.
4. IPV6CP Configuration Options
IPV6CP Configuration Options allow negotiation of desirable IPv6 parameters. IPV6CP uses the same Configuration Option format defined for LCP, with a separate set of Options. If a Configuration Option is not included in a Configure-Request packet, the default value for that Configuration Option is assumed.
Up-to-date values of the IPV6CP Option Type field are specified in the most recent “Assigned Numbers” RFC. Current values are assigned as follows:
The only IPV6CP options defined in this document are Interface-Identifier and IPv6-Compression-Protocol. Any other IPV6CP configuration options that can be defined over time are to be defined in separate documents.
This Configuration Option provides a way to negotiate a unique 64-bit interface identifier to be used for the address auto configuration at the local end of the link. A Configure-Request MUST contain exactly one instance of the Interface-Identifier option. The interface identifier MUST be unique within the PPP link; i.e. upon completion of the negotiation different Interface-Identifier values are to be selected for the ends of the PPP link. The interface identifier MAY also be unique over a broader scope.
Before this Configuration Option is requested, an implementation chooses its tentative Interface-Identifier. The non-zero value of the tentative Interface-Identifier SHOULD be chosen such that the value is both unique to the link and, if possible, consistently reproducible across initializations of the IPV6CP finite state machine (administrative Close and reOpen, reboots, etc). The rationale for preferring a consistently reproducible unique interface identifier to a completely random interface identifier is to provide stability to global scope addresses that can be formed from the interface identifier.
Assuming that interface identifier bits are numbered from 0 to 63 in canonical bit order where the most significant bit is the bit number 0, the bit number 6 is the “u” bit (universal/local bit in IEEE EUI-64 terminology) which indicates whether or not the interface identifier is based on a globally unique IEEE identifier (EUI-48 or EUI-64) (see the case 1 below). It is set to one (1) if a globally unique IEEE identifier is used to derive the interface identifier, and it is set to zero (0) otherwise.
The following are methods for choosing the tentative Interface Identifier in the preference order:
1) If an IEEE global identifier (EUI-48 or EUI-64) is available anywhere on the node, it should be used to construct the tentative Interface-Identifier due to its uniqueness properties. When extracting an IEEE global identifier from another device on the node, care should be taken to that the extracted identifier is presented in canonical ordering.
The only transformation from an EUI-64 identifier is to invert the “u” bit (universal/local bit in IEEE EUI-64 terminology).
In the case of an EUI-48 identifier, it is first converted to the EUI-64 format by inserting two bytes, with hexadecimal values of 0xFF and 0xFE, in the middle of the 48 bit MAC (between the company_id and extension-identifier portions of the EUI-48 value).
2) If an IEEE global identifier is not available a different source of uniqueness should be used. Suggested sources of uniqueness include link-layer addresses, machine serial numbers, et cetera.
In this case, the “u” bit of the interface identifier MUST be set to zero (0).
3) If a good source of uniqueness cannot be found, it is recommended that a random number be generated. In this case, the “u” bit of the interface identifier MUST be set to zero (0).
Good sources of uniqueness or randomness are required for the Interface-Identifier negotiation to succeed. If neither a unique number or a random number can be generated it is recommended that a zero value be used for the Interface-Identifier transmitted in the Configure-Request. In this case, the PPP peer may provide a valid non-zero Interface-Identifier in its response as described below.
Note that if at least one of the PPP peers is able to generate separate non-zero numbers for itself and its peer, the identifier negotiation will succeed.
When a Configure-Request is received with the Interface-Identifier Configuration Option and the receiving peer implements this option, the received Interface-Identifier is compared with the Interface-Identifier of the last Configure-Request sent to the peer. Depending on the result of the comparison an implementation MUST respond in one of the following ways:
If the two Interface-Identifiers are different but the received Interface-Identifier is zero, a Configure-Nak is sent with a non-zero Interface-Identifier value suggested for use by the remote peer. Such a suggested Interface-Identifier MUST be different from the Interface-Identifier of the last Configure-Request sent to the peer. It is recommended that the value suggested be consistently reproducible across initializations of the IPV6CP finite state machine (administrative Close and reOpen, reboots, etc). The “u” universal/local) bit of the suggested identifier MUST be set to zero (0) regardless of its source unless the globally unique EUI-48/EUI-64 derived identifier is provided for the exclusive use by the remote peer.
If the two Interface-Identifiers are different and the received Interface-Identifier is not zero, the Interface-Identifier MUST be acknowledged, i.e. a Configure-Ack is sent with the requested Interface-Identifier, meaning that the responding peer agrees with the Interface-Identifier requested.
If the two Interface-Identifiers are equal and are not zero, a Configure-Nak MUST be sent specifying a different non-zero Interface-Identifier value suggested for use by the remote peer. It is recommended that the value suggested be consistently reproducible across initializations of the IPV6CP finite state machine (administrative Close and reOpen, reboots, etc). The “u” universal/local) bit of the suggested identifier MUST be set to zero (0) regardless of its source unless the globally unique EUI-48/EUI-64 derived identifier is provided for the exclusive use by the remote peer.
If the two Interface-Identifiers are equal to zero, the Interface-Identifiers negotiation MUST be terminated by transmitting the Configure-Reject with the Interface-Identifier value set to zero. In this case, a unique Interface-Identifier can not be negotiated.
If a Configure-Request is received with the Interface-Identifier Configuration Option and the receiving peer does not implement this option, Configure-Rej is sent. A new Configure-Request SHOULD NOT be sent to the peer until normal processing would cause it to be sent (that is, until a Configure-Nak is received or the Restart timer runs out). A new Configure-Request MUST NOT contain the Interface-Identifier option if a valid Interface-Identifier Configure-Reject is received.
Reception of a Configure-Nak with a suggested Interface-Identifier different from that of the last Configure-Nak sent to the peer indicates a unique Interface-Identifier. In this case, a new Configure-Request MUST be sent with the identifier value suggested in the last Configure-Nak from the peer. But if the received Interface-Identifier is equal to the one sent in the last Configure-Nak, a new Interface-Identifier MUST be chosen. In this case, a new Configure-Request SHOULD be sent with the new tentative Interface-Identifier. This sequence (transmit Configure-Request, receive Configure-Request, transmit Configure-Nak, receive Configure-Nak) might occur a few times, but it is extremely unlikely to occur repeatedly. More likely, the Interface-Identifiers chosen at either end will quickly diverge, terminating the sequence.
If negotiation of the Interface-Identifier is required, and the peer did not provide the option in its Configure-Request, the option SHOULD be appended to a Configure-Nak. The tentative value of the Interface-Identifier given must be acceptable as the remote Interface-Identifier; i.e. it should be different from the identifier value selected for the local end of the PPP link. The next Configure-Request from the peer may include this option. If the next Configure-Request does not include this option the peer MUST NOT send another Configure-Nak with this option included. It should assume that the peer’s implementation does not support this option.
By default, an implementation SHOULD attempt to negotiate the Interface-Identifier for its end of the PPP connection. A summary of the Interface-Identifier Configuration Option format is shown below. The fields are transmitted from left to right.
This Configuration Option provides a way to negotiate the use of a specific IPv6 packet compression protocol. The IPv6-Compression-Protocol Configuration Option is used to indicate the ability to receive compressed packets. Each end of the link must separately request this option if bi-directional compression is desired. By default, compression is not enabled.
IPv6 compression negotiated with this option is specific to IPv6 datagrams and is not to be confused with compression resulting from negotiations via Compression Control Protocol (CCP), which potentially affect all datagrams.
A summary of the IPv6-Compression-Protocol Configuration Option format is shown below. The fields are transmitted from left to right.
The IPv6-Compression-Protocol field is two octets and indicates the compression protocol desired. Values for this field are always the same as the PPP Data Link Layer Protocol field values for that same compression protocol. No IPv6-Compression-Protocol field values are currently assigned. Specific assignments will be made in documents that define specific compression algorithms.
The Data field is zero or more octets and contains additional data as determined by the particular compression protocol.
No IPv6 compression protocol enabled.
Reference: RFC 2472