RFC 5072

IP Version 6 over PPP

📅 published September 2007, from ietf

RFC 5072: IP Version 6 over PPP ✨

okay bestie, this is RFC 5072 💖 written by S. Varada (Ed.) from Transwitch, with D. Haskins and E. Allen. It's a Standards Track doc from September 2007 🦋. It obsoletes RFC 2472, which is the older spec this one replaces, and it doesn't list any updates 🎀.

Status of This Memo

okay so this is the official vibe check 🧚. It's an Internet standards track protocol for the Internet community, and it's literally asking everyone to come discuss it and suggest improvements 💬. For the standardization state and status of this protocol, check the current edition of "Internet Official Protocol Standards" (STD 1) 📜. Distribution of this memo is unlimited 🌈.

Abstract

okay this part is the highlight reel ✨. PPP (the Point-to-Point Protocol) is the standard way to wrap network-layer info up and send it over point-to-point links 🎁. It also has an extensible Link Control Protocol, plus a whole family of Network Control Protocols (NCPs) for setting up and configuring different network-layer protocols 🔥.

This document is where IPv6 gets its PPP glow-up 💅. It defines how IPv6 packets get sent over PPP links, the NCP that sets up and configures IPv6 over PPP, and how IPv6 link-local addresses are formed on PPP links 🦋.

It also lays out the conditions for doing Duplicate Address Detection on IPv6 global unicast addresses that get configured for PPP links, whether that happens through stateful or stateless address autoconfiguration 🔍.

And yes, this document obsoletes RFC 2472 👑.

Table of Contents

   1. Introduction ....................................................2
      1.1. Specification of Requirements ..............................3
   2. Sending IPv6 Datagrams ..........................................3
   3. A PPP Network Control Protocol for IPv6 .........................3
   4. IPV6CP Configuration Options ....................................4
      4.1. Interface Identifier .......................................4
   5. Stateless Autoconfiguration and Link-Local Addresses ............9
   6. Security Considerations ........................................11
   7. IANA Considerations ............................................11
   8. Acknowledgments ................................................11
   9. References .....................................................12
      9.1. Normative References ......................................12
      9.2. Informative references ....................................12
   Appendix A:  Global Scope Addresses................................14
   Appendix B:  Changes from RFC-2472.................................14

1. Introduction 🎀

okay so PPP has three main components 💖:

  1. A method for encapsulating datagrams over serial links.
  2. A Link Control Protocol (LCP) for establishing, configuring, and testing the data-link connection.
  3. A family of Network Control Protocols (NCPs) for establishing and configuring different network-layer protocols.

the setup goes like this 🧚. To get communications going over a point-to-point link, each end of the PPP link must first send LCP packets to configure and test the data link. Once the link is up and the LCP has negotiated any optional facilities that are needed, PPP must send NCP packets to choose and configure one or more network-layer protocols. After each chosen network-layer protocol is configured, its datagrams can finally be sent over the link ✨.

in this document, the NCP that sets up and configures IPv6 over PPP is called the IPv6 Control Protocol (IPV6CP) 💜.

the link stays configured for communications until explicit LCP or NCP packets shut it down, or until some external event happens (power failure at the other end, carrier drop, etc.) 🙈.

this document obsoletes the earlier spec from RFC 2472 [7] 👑. The changes from RFC 2472 are listed in Appendix B 📝.

1.1. Specification of Requirements 🌸

ngl this part is the rulebook 📖. Several words in this document are used to show requirement levels:

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [6].

2. Sending IPv6 Datagrams 🦋

okay this is the core moment 🔥. Before any IPv6 packets can be communicated, PPP MUST reach the network-layer protocol phase, and the IPv6 Control Protocol MUST reach the Opened state 🚦.

Exactly one IPv6 packet gets 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 sent 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 to be at least as large as the minimum link MTU size required for IPv6 [2] 💪.

3. A PPP Network Control Protocol for IPv6 🧚

the IPv6 Control Protocol (IPV6CP) is in charge of 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 LCP 🔁. IPV6CP packets may not be exchanged until PPP has reached the network-layer protocol phase 🙅. IPV6CP packets that get received before this phase is reached should be silently discarded 🤫.

The IPv6 Control Protocol is exactly the same as the LCP [1], 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) 🎀.

Code field

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 🙅‍♀️.

Timeouts

IPV6CP packets may not be exchanged until PPP has reached the network-layer protocol phase ⏳. An implementation should be ready to wait for Authentication and Link Quality Determination to finish before it times out while waiting for a Configure-Ack or another response 🫶. It is suggested that an implementation give up only after user intervention or after a configurable amount of time 🌈.

Configuration Option Types

IPV6CP has its own distinct set of Configuration Options 💜.

4. IPV6CP Configuration Options 🍓

IPV6CP Configuration Options let the two sides negotiate desirable IPv6 parameters 🤝. IPV6CP uses the same Configuration Option format defined for LCP [1], but 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 in the online "Assigned Numbers" database maintained at IANA [9] 📚. The current value assignment looks like this 👑:

      1 Interface-Identifier

The only IPV6CP option defined in this document is the interface identifier 🫧. Any other IPV6CP configuration options that get defined over time will be defined in separate documents 📄.

4.1. Interface Identifier 🌟

Description

This Configuration Option gives a way to negotiate a unique, 64-bit interface identifier 💖. That identifier gets used for the address autoconfiguration [3] at the local end of the link (see Section 5) 🔗. A Configure-Request MUST contain exactly one instance of the interface-identifier option [1] 🚨. The interface identifier MUST be unique within the PPP link, meaning that once the negotiation is done, the ends of the PPP link have to end up with different interface-identifier values 🙅‍♀️. The interface identifier may also be unique over a broader scope 🌍.

Before this Configuration Option is requested, an implementation picks its tentative interface identifier 🎯. The non-zero value of the tentative interface identifier SHOULD be chosen so that it's unique to the link and, preferably, consistently reproducible across initializations of the IPV6CP finite state machine (administrative Close and reOpen, reboots, etc.) 🔄. The reason we prefer a consistently reproducible unique identifier over a completely random one is stability: it keeps the global scope addresses (see Appendix A) that get formed from the interface identifier steady 🧠✨.

Okay now the bit math, bestie 🤓. Say the interface identifier bits are numbered from 0 to 63 in canonical bit order, where the most significant bit is bit number 0. Then bit number 6 is the "u" bit (universal/local bit in IEEE EUI-64 [4] terminology) 🧮. It tells you whether or not the interface identifier is based on a globally unique IEEE identifier (EUI-48 or EUI-64 [4]) (see case 1 below) 🔍. It's set to one (1) if a globally unique IEEE identifier is used to derive the interface identifier, and it's set to zero (0) otherwise 🎀.

These are the methods for choosing the tentative interface identifier, in preference order 🏆:

  1. If an IEEE global identifier (EUI-48 or EUI-64) is available anywhere on the node, it should be used to build the tentative interface identifier, because of its uniqueness properties 💎. When extracting an IEEE global identifier from another device on the node, take care that the extracted identifier is presented in canonical ordering [14] 🫶.

The only transformation from an EUI-64 identifier is to invert the "u" bit (universal/local bit in IEEE EUI-64 terminology) 🙃.

For example, here's a globally unique EUI-64 identifier 🦋:

   most-significant                                    least-significant
   bit                                                               bit
   |0              1|1              3|3              4|4              6|
   |0              5|6              1|2              7|8              3|
   +----------------+----------------+----------------+----------------+

   |cccccc0gcccccccc|cccccccceeeeeeee|eeeeeeeeeeeeeeee|eeeeeeeeeeeeeeee|
   +----------------+----------------+----------------+----------------+

where "c" are the bits of the assigned company_id, "0" is the value of the universal/local bit to indicate global scope, "g" is the group/individual bit, and "e" are the bits of the extension identifier. The IPv6 interface identifier would then look like this 💅:

   most-significant                                    least-significant
   bit                                                               bit
   |0              1|1              3|3              4|4              6|
   |0              5|6              1|2              7|8              3|
   +----------------+----------------+----------------+----------------+

   |cccccc1gcccccccc|cccccccceeeeeeee|eeeeeeeeeeeeeeee|eeeeeeeeeeeeeeee|
   +----------------+----------------+----------------+----------------+

The only change is inverting the value of the universal/local bit 🔁✨.

In the case of an EUI-48 identifier, it first gets 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) 🧩. For example, for a globally unique 48-bit EUI-48 identifier of the form:

  most-significant                   least-significant
  bit                                              bit
  |0              1|1              3|3              4|
  |0              5|6              1|2              7|
  +----------------+----------------+----------------+
  |cccccc0gcccccccc|cccccccceeeeeeee|eeeeeeeeeeeeeeee|
  +----------------+----------------+----------------+

where c is the bits of the assigned company_id, 0 is the universal/local bit set to indicate global scope, g is the group/individual bit, and e is the bits of the extension identifier. The IPv6 interface identifier then looks like this 💅:

   most-significant                                    least-significant
   bit                                                               bit
   |0              1|1              3|3              4|4              6|
   |0              5|6              1|2              7|8              3|
   +----------------+----------------+----------------+----------------+

   |cccccc1gcccccccc|cccccccc11111111|11111110eeeeeeee|eeeeeeeeeeeeeeee|
   +----------------+----------------+----------------+----------------+
  1. Okay, plan B 🧚. If there's no IEEE global identifier, you can grab a different source of uniqueness instead. Good examples 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). 🫶
  2. Plan C: if you can't find a good source of uniqueness at all, it is recommended that you generate a random number. In this case too, the "u" bit of the interface identifier MUST be set to zero (0). 🎲

Good sources [1] of uniqueness or randomness are required for the interface identifier negotiation to succeed, so don't skip this part bestie 😭. If you can't come up with a unique number or a random number, it is recommended that you transmit a zero value for the interface identifier in the Configure-Request. In that case, the PPP peer might give you a valid non-zero interface identifier in its response, as described below. Also, if at least one of the PPP peers can generate separate non-zero numbers for itself and its peer, the identifier negotiation will succeed ✨.

When a Configure-Request comes in with the Interface-Identifier Configuration Option, and the receiving peer implements this option, the received interface identifier gets compared with the interface identifier of the last Configure-Request that was sent to the peer. Depending on how that comparison goes, an implementation MUST respond in one of the following ways 👑:

  • If the two interface identifiers are different but the received interface identifier is zero, send a Configure-Nak with a non-zero interface identifier value suggested for use by the remote peer. That suggested interface identifier MUST be different from the interface identifier of the last Configure-Request sent to the peer. It is recommended that the suggested value 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. That means you send a Configure-Ack with the requested interface identifier, which tells the other peer you agree with the identifier it asked for 🦋.
  • If the two interface identifiers are equal and are not zero, 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 suggested value 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 identifier's negotiation MUST be terminated by transmitting the Configure-Reject with the interface-identifier value set to zero. In this case, a unique interface identifier cannot be negotiated 💀.

If a Configure-Request comes in with the Interface-Identifier Configuration Option, and the receiving peer does not implement this option, send a Configure-Reject 🙈.

A new Configure-Request SHOULD NOT be sent to the peer until normal processing would cause it to be sent. That means waiting until a Configure-Nak is received or the Restart timer runs out [1]. Patience is a whole vibe 🌸.

A new Configure-Request MUST NOT contain the interface-identifier option if a valid Interface-Identifier Configure-Reject has been received.

Receiving a Configure-Nak with a suggested interface identifier that's different from the one in the last Configure-Nak sent to the peer means the identifier is unique 🎉. 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 that 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 happen a few times, but it is extremely unlikely to keep happening over and over. More likely, the interface identifiers chosen at each end will quickly diverge, and that ends 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. In other words, 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 💖.

Here's a quick look at the Interface-Identifier Configuration Option format. The fields are sent from left to right 🍓.

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length     | Interface-Identifier (MS Bytes)
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                        Interface-Identifier (cont)
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   Interface-Identifier (LS Bytes) |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  • Type: 1
  • Length: 10
  • Interface-Identifier: The 64-bit interface identifier, which is very likely to be unique on the link, or zero if a good source of uniqueness cannot be found.
  • Default: If no valid interface identifier can be successfully negotiated, no default interface-identifier value should be assumed. The procedures for recovering from such a case are unspecified. One approach is to manually configure the interface identifier of the interface 🛠️.

The interface identifier of IPv6 unicast addresses [5] of a PPP interface SHOULD be negotiated in the IPV6CP phase of the PPP connection setup (see Section 4.1). If no valid interface identifier has been successfully negotiated, the procedures for recovering from that are unspecified. One approach is to manually configure the interface identifier of the interface 🥹.

The local end of the PPP link uses the negotiated interface identifier to autoconfigure an IPv6 link-local unicast address for the PPP interface. However, it SHOULD NOT be assumed that the same interface identifier is used when configuring global unicast addresses for the PPP interface using IPv6 stateless address autoconfiguration [3]. The PPP peer MAY generate one or more interface identifiers, for instance using a method described in [8], to autoconfigure one or more global unicast addresses 🌈.

As long as the interface identifier is negotiated in the IPV6CP phase of the PPP connection setup, it is redundant to perform duplicate address detection (DAD) as part of the IPv6 Stateless Address Autoconfiguration protocol [3] on the IPv6 link-local address generated by the PPP peer. It may also be redundant to perform DAD on any global unicast addresses configured for the interface (using an interface identifier that is either negotiated during IPV6CP or generated, for instance, as per [8]) as part of the IPv6 Stateless Address Autoconfiguration protocol [3], provided that the following two conditions are met 🧐:

  1. The prefixes advertised through the Router Advertisement messages by the access router terminating the PPP link are exclusive to the PPP link.
  2. The access router terminating the PPP link does not autoconfigure any IPv6 global unicast addresses from the prefixes that it advertises.

Therefore, it is RECOMMENDED that, for PPP links with the IPV6CP interface-identifier option enabled and satisfying the two conditions above, the default value of the DupAddrDetectTransmits autoconfiguration variable [3] is set to zero by the system management 🎀. 3GPP2 networks are an example of a technology that uses PPP to enable a host to obtain an IPv6 global unicast address and satisfies the two conditions above [10]. 3GPP networks are another example ([11] [13]) 🛰️.

Link-local addresses

Link-local addresses of PPP interfaces have the following format 🌷:

   | 10 bits  |        54 bits         |          64 bits            |
   +----------+------------------------+-----------------------------+
   |1111111010|           0            |    Interface-Identifier     |
   +----------+------------------------+-----------------------------+

The most significant 10 bits of the address are the Link-Local prefix FE80::. 54 zero bits pad out the address between the Link-Local prefix and the interface-identifier fields 🦄.

6. Security Considerations 🔐

okay bestie, this one is seriously important 😭 lack of link security (like authentication) totally triggers the security concerns from [3] when you use stateless address autoconfiguration to make global unicast IPv6 addresses out of interface identifiers. Those identifiers could be negotiated through the IPV6CP, or generated with a method described in [8]. So the mechanisms that actually help with PPP link security are covered below, along with a reference to a generic threat model. 💅

The mechanisms that are appropriate for ensuring PPP link security are:

  1. Access Control Lists that apply filters on traffic received over the link, to enforce an admission policy 🚪
  2. An Authentication protocol that helps peers negotiate an authentication method (for example, MD5 [16]) to validate the peer [15] 🪪
  3. An Encryption protocol that helps peers negotiate encryption algorithms (or crypto-suites) to keep the data confidential [17] 🔒
⚠️ Not gonna lie, there are still threats on a PPP link even when you have one or more of the measures above. For example, using the MD5 authentication method [16] exposes you to replay attacks, where an attacker could intercept and replay a station's identity and password hash to get access to a network. 😱 If you're using this spec, you're advised to check out [15], which presents a generic threat model so you can understand the threats to the security of a link. [15] also gives you a framework for specifying requirements for choosing an authentication method for a given application. 🧠

7. IANA Considerations 🧾

IANA has assigned value 1 for the Type field of the IPv6 datagram interface-identifier option specified in this document. The current assignment is kept up-to-date at [9]. 🌸

8. Acknowledgments 🫶

This document borrows from the Magic-Number LCP option, so it's partially based on earlier work done by the PPP working group. 🎀

The editor is so grateful for the input from the IPv6 community, which was all about updating RFC 2472. 💖 Big thanks to Pete Barany and Karim El Malki for their technical contributions, and to Alex Conta for a thorough review. Stephen Kent helped with the security aspects, and Spencer Dawkins and Pekka Savola caught the nits. ✨ Finally, the author is grateful to Jari Arkko for kicking things off to bring closure to this specification. 👑

9. References 📚

9.1. Normative References

  • [1] Simpson, W., Ed., "The Point-to-Point Protocol (PPP)", STD 51, RFC 1661, July 1994.
  • [2] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", RFC 2460, December 1998.
  • [3] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless Address Autoconfiguration", RFC 4862, September 2007.
  • [4] IEEE, "Guidelines For 64-bit Global Identifier (EUI-64)", http://standards.ieee.org/regauth/oui/tutorials/EUI64.html
  • [5] Hinden, R. and S. Deering, "IP Version 6 Addressing Architecture", RFC 4291, February 2006.
  • [6] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
  • [7] Haskin, D. and E. Allen, "IP Version 6 over PPP", RFC 2472, December 1998.
  • [8] Narten T., Draves, R., and S. Krishnan, "Privacy Extensions for Stateless Address Autoconfiguration in IPv6", RFC 4941, September 2007.

9.2. Informative references

  • [9] IANA, "Assigned Numbers," http://www.iana.org/numbers.html
  • [10] 3GPP2 X.S0011-002-C v1.0, "cdma2000 Wireless IP Network Standard: Simple IP and Mobile IP Access Services," September 2003.
  • [11] 3GPP TS 29.061 V6.4.0, "Interworking between the Public Land Mobile Network (PLMN) Supporting packet based services and Packet Data Networks (PDN) (Release 6)," April 2005.
  • [12] Droms, R., Ed., Bound, J., Volz, B., Lemon, T., Perkins, C., and M. Carney, "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)", RFC 3315, July 2003.
  • [13] 3GPP TS 23.060 v6.8.0, "General Packet Radio Service (GPRS); Service description; Stage 2 (Release 6)," March 2005.
  • [14] Narten, T. and C. Burton, "A Caution On The Canonical Ordering Of Link-Layer Addresses", RFC 2469, December 1998.
  • [15] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H. Levkowetz, Ed., "Extensible Authentication Protocol (EAP)", RFC 3748, June 2004.
  • [16] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321, April 1992.
  • [17] Meyer, G., "The PPP Encryption Control Protocol (ECP)", RFC 1968, June 1996.

Appendix A: Global Scope Addresses 🌍

A node on the PPP link creates global unicast addresses through either stateless or stateful address autoconfiguration mechanisms. 🛰️ With stateless autoconfiguration [3], the node relies on sub-net prefixes advertised by the router in Router Advertisement messages, and builds global unicast addresses from an interface identifier. With stateful autoconfiguration, the host relies on a Stateful Server, like DHCPv6 [12], to get its global unicast addresses. 🧚

Appendix B: Changes from RFC 2472 🦋

Here's what changed from RFC 2472 "IPv6 over PPP":

  • Minor updates to Sections 3 and 4 ✏️
  • Updated the text in Section 4.1 to include the reference to Appendix A, plus some minor text clarifications.
  • Removed Section 4.2 on IPv6-Compression-Protocol based on IESG recommendation. A new standards-track document now covers negotiating the IPv6 datagram compression protocol using IPV6CP. 🗂️
  • Updated the text in Section 5 to: (a) allow the use of one or more interface identifiers generated by a peer, in addition to the interface identifier negotiated between the peers of the link, when creating global unicast addresses for the local PPP interface, and (b) identify cases against the DAD of created non-link-local addresses. 🔍
  • Added new and updated references. 📎
  • Added Appendix A. 🎉

Authors' Addresses 💌

Dimitry Haskin
Ed Allen

Srihari Varada (Editor)
TranSwitch Corporation
3 Enterprise Dr.
Shelton, CT 06484. US.

Phone: +1 203 929 8810
EMail: varada@ieee.org

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