Network Working Group W. Simpson
Internet Draft DayDreamer
expires in six months July 1997
PPP LCP CallBack
draft-ietf-pppext-callback-ds-01.txt
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Abstract
The Point-to-Point Protocol (PPP) [1] provides a standard method for
transporting multi-protocol datagrams over point-to-point links. PPP
defines an extensible Link Control Protocol (LCP) for establishing,
configuring, and testing the data-link connection. This document
defines the CallBack option.
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1. Introduction
CallBack is a facility that permits a call originating party (caller)
to request that the call terminating party (callee) originate a sub-
sequent call in the reverse direction. This might be used for many
diverse purposes, such as savings on toll charges.
Unlike most PPP operations, CallBack is not a strictly peer-to-peer
service. The semantics of CallBack differ depending on which peers
are the caller and callee.
The successful operation of CallBack requires a significant amount of
administrative configuration. Such configuration might include per-
mission to use the CallBack facility, locations and time of day for
which the CallBack is allowed, and time delays prior to and following
disconnection.
- The callee might disconnect quickly, and return the call as soon
as possible.
- CallBack might be used to re-establish service at later time, when
sufficient additional traffic arrives at the callee to warrant
another connection.
- When the user is roaming, CallBack might be used to register a
variable contact location.
Such policy considerations are beyond the scope of this specifica-
tion.
2. Additional LCP Configuration Options
The Configuration Option format and basic options are already defined
for LCP [1].
Up-to-date values of the LCP Option Type field are specified in the
most recent "Assigned Numbers" [2]. This document concerns the fol-
lowing values:
13 CallBack
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2.1. CallBack
Description
This Configuration Option provides a method for an implementation
to request a dial-up peer to call back. By default, no call back
occurs.
Nota Bene:
Like all such PPP options, this indicates that the installation
has the capability to be called back. If the option is
rejected, link operation continues normally [1 page 39].
Moreover, acknowlegement of the option does not require that
the peer take any additional action [ibid].
A peer that Configure-Acks this option SHOULD Configure-Request
the Authentication-Protocol option. The user information learned
during authentication can be used to determine the user location,
or limit a user to certain locations, or merely to determine whom
to bill for the service.
When CallBack is successfully negotiated, and the Authentication
phase is complete, the peer will eventually enter the Link Termi-
nation phase. At the conclusion of the Link Termination phase,
the peer will disconnect the link.
Then, after a suitable interval of time, the peer will re-
establish the link. When mutual authentication is desired, the
implementation SHOULD Configure-Request the Authentication-
Protocol option as it answers the call.
Implementation Notes:
The delay times between calls are characteristics of the call-
ing environment, and MUST be configured on a per user and loca-
tion basis.
Since the information is redundant, the implementation is not
required to Configure-Request the CallBack option as it answers
the call. However, inclusion MUST NOT affect the operation of
the protocol.
A summary of the CallBack Option format is shown below. The fields
are transmitted from left to right.
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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 | Operation | Message ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type
13
Length
>= 3
Operation
The Operation field is one octet and indicates the contents of the
Message field.
Up-to-date values of the CallBack Operation field are specified in
the most recent "Assigned Numbers" [2]. This document defines the
following values:
0 Identification from the Authentication phase will be used
for a database lookup to determine the callback parame-
ters. The Message field is not present.
This method is required to be supported in all conformant
implementations.
1 Dialing string, the format and contents of which assumes
configuration knowledge of the specific device that is
making the callback.
A North American example might be: 10222,,,(800)555-1212.
This method is commonly supported, but suffers from fre-
quent configuration error.
2 Location identifier, which may or may not be human read-
able, used together with Authentication phase information
for a database lookup to determine the callback parame-
ters.
3 E.164 number. The implementation converts this to an
appropriate signalling sequence.
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4 X.500 distinguished name, used together with Authentica-
tion phase information for a database lookup to determine
the callback parameters.
5 E.165 number. The implementation converts this to an
appropriate signalling sequence.
Message
The Message field is zero or more octets, and its general contents
are determined by the Operation field. The actual format of the
information is site or application specific, and a robust imple-
mentation SHOULD support the field as undistinguished octets. The
size is determined from the Length field.
It is intended that only an authorized user will have correct site
specific information to make use of the CallBack. The codifica-
tion of the range of allowed usage of this field is beyond the
scope of this specification.
Security Considerations
Unauthenticated and unrestricted use of CallBack could lead to a sig-
nificant denial of service, or excessive service charges. Authenti-
cation is required to be supported. In addition, it is recommended
that an implementation be capable of logging usage.
Acknowledgements
Special thanks to Ascend Communications for providing computing
resources and network access support for writing this specification.
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References
[1] Simpson, W., Editor, "The Point-to-Point Protocol (PPP)",
STD-51, RFC-1661, December 1993.
[2] Reynolds, J.K., Postel, J.B., "Assigned Numbers", RFC-1700,
July 1992.
[3] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, Harvard University, March 1997.
Contacts
Comments about this document should be discussed on the
ietf-ppp@merit.edu mailing list.
This document is a submission to the Point-to-Point Protocol Working
Group of the Internet Engineering Task Force (IETF). The working
group can be contacted via the current chair:
Karl Fox
Ascend Communications
3518 Riverside Drive Suite 101
Columbus, Ohio 43221
karl@MorningStar.com
karl@Ascend.com
Questions about this document can also be directed to:
William Allen Simpson
DayDreamer
Computer Systems Consulting Services
1384 Fontaine
Madison Heights, Michigan 48071
wsimpson@UMich.edu
wsimpson@GreenDragon.com (preferred)
bsimpson@MorningStar.com
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