Network Working Group S. Varada, Ed.

Network Working Group S. Varada, Ed.
Request for Comments: 5072 Transwitch
Obsoletes: 2472 D. Haskins
Category: Standards Track E. Allen
September 2007

IP Version 6 over PPP

Status of This Memo

This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.

Abstract

The Point-to-Point Protocol (PPP) provides a standard method of
encapsulating network-layer protocol information over point-to-point
links. PPP also defines an extensible Link Control Protocol, and
proposes a family of Network Control Protocols (NCPs) for
establishing and configuring different network-layer protocols.

This document defines the method for sending IPv6 packets over PPP
links, the NCP for establishing and configuring the IPv6 over PPP,
and the method for forming IPv6 link-local addresses on PPP links.

It also specifies the conditions for performing Duplicate Address
Detection on IPv6 global unicast addresses configured for PPP links
either through stateful or stateless address autoconfiguration.

This document obsoletes RFC 2472.

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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

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.

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 to 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.).

This document obsoletes the earlier specification from RFC 2472 [7].
Changes from RFC 2472 are listed in Appendix B.

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1.1. Specification of Requirements

In this document, several words are used to signify the requirements
of the specification.

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

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 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 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 LCP. IPV6CP packets may not be exchanged until PPP has
reached the network-layer protocol phase. IPV6CP packets that are
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).

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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
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 [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 specified in
the online database of "Assigned Numbers" maintained at IANA [9].
The current value assignment is as follows:

1 Interface-Identifier

The only IPV6CP option defined in this document is the interface
identifier. Any other IPV6CP configuration options that can be
defined over time are to be defined in separate documents.

4.1. Interface Identifier

Description

This Configuration Option provides a way to negotiate a unique, 64-
bit interface identifier to be 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

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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 unique to the link and, preferably, 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 (see Appendix A) 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 [4] terminology), which indicates 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 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 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, for a globally unique EUI-64 identifier of the form:

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|
+----------------+----------------+----------------+----------------+

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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 be of the form:

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 a EUI-48 identifier, it is first converted to the
EUI-64 format by inserting two bytes, with hexa-decimal 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" 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 be of the form:

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|
+----------------+----------------+----------------+----------------+

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.

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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 [1] of uniqueness or randomness are required for the
interface identifier negotiation to succeed. If neither a unique
number nor 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,
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

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(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 is received with the Interface-Identifier
Configuration Option and the receiving peer does not implement this
option, Configure-Reject 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 [1]).

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.

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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.

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.

5. Stateless Autoconfiguration and Link-Local Addresses

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, procedures for recovering from such
a case are unspecified. One approach is to manually configure the
interface identifier of the interface.

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The negotiated interface identifier is used by the local end of the
PPP link 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 in 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 a 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 (using an interface
identifier that is either negotiated during IPV6CP or generated, for
instance, as per [8]) for the interface 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 aforementioned
two conditions, 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 aforementioned two conditions [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 |
+----------+------------------------+-----------------------------+

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The most significant 10 bits of the address is 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

Lack of link security, such as authentication, trigger the security
concerns raised in [3] when the stateless address autoconfiguration
method is employed for the generation of global unicast IPv6
addresses out of interface identifiers that are either negotiated
through the IPV6CP or generated, for instance, using a method
described in [8]. Thus, the mechanisms that are appropriate for
ensuring PPP link security are addressed below, together with the
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 for enforcing admission policy, 2) an Authentication
protocol that facilitates negotiations between peers [15] to select
an authentication method (e.g., MD5 [16]) for validation of the peer,
and 3) an Encryption protocol that facilitates negotiations between
peers to select encryption algorithms (or crypto-suites) to ensure
data confidentiality [17].

There are certain threats associated with peer interactions on a PPP
link even with one or more of the above security measures in place.
For instance, using the MD5 authentication method [16] exposes one to
replay attack, where an attacker could intercept and replay a
station's identity and password hash to get access to a network. The
user of this specification is advised to refer to [15], which
presents a generic threat model, for an understanding of the threats
posed to the security of a link. The reference [15] also gives a
framework to specify requirements for the selection of an
authentication method for a given application.

7. IANA Considerations

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

8. Acknowledgments

This document borrows from the Magic-Number LCP option and as such is
partially based on previous work done by the PPP working group.

The editor is grateful for the input provided by members of the IPv6
community in the spirit of updating RFC 2472. Thanks, in particular,

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go to Pete Barany and Karim El Malki for their technical
contributions. Also, thanks to Alex Conta for a thorough review,
Stephen Kent for helping with security aspects, and Spencer Dawkins
and Pekka Savola for the nits. Finally, the author is grateful to
Jari Arkko for his initiation 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.

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RFC 5072 IP Version 6 over PPP September 2007

[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.

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Appendix A: Global Scope Addresses

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

Appendix B: Changes from RFC 2472

The following changes were made 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 and minor text clarifications.

- Removed Section 4.2 on IPv6-Compression-Protocol based on IESG
recommendation, and created a new standards-track document to
cover negotiation of 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 use
of interface identifier negotiated between peers of the link, in
the creation of 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

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Authors' Addresses

Dimitry Haskin
Ed Allen

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

Phone: +1 203 929 8810
EMail: [email protected]

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