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Network Working Group Bernard Aboba
INTERNET-DRAFT Microsoft Corporation
<draft-aboba-radius-tunnel-imp-01.txt> Glen Zorn
26 November 1996 Microsoft Corporation
Implementation of Mandatory Tunneling via RADIUS
1. Status of this Memo
This document is an Internet-Draft. Internet-Drafts are working docu-
ments of the Internet Engineering Task Force (IETF), its areas, and
its working groups. Note that other groups may also distribute work-
ing documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
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rial or to cite them other than as ``work in progress.''
To learn the current status of any Internet-Draft, please check the
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The distribution of this memo is unlimited. It is filed as <draft-
aboba-radius-tunnel-imp-01.txt> and expires June 1, 1997. Please
send comments to the authors.
2. Abstract
This document discusses implementation issues arising in the provi-
sioning of mandatory tunneling in dial-up networks using the PPTP and
L2TP protocols. This provisioning may be accomplished via the inte-
gration of RADIUS and tunneling protocols.
3. Terminology
Roaming "Roaming capability" may be loosely defined as the ability
to use any one of multiple Internet service providers
(ISPs), while maintaining a formal, customer-vendor rela-
tionship with only one. Examples of cases where roam-
ing capability might be required include ISP "confedera-
tions" and ISP-provided corporate network access support.
Shared Use Network
This is an IP dialup network whose use is shared by two or
more organizations. Shared use networks typically implement
distributed authentication and accounting in order to facil-
itate the relationship among the sharing parties. Since
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these facilities are also required for implementation of
roaming, implementation of shared use is frequently a first
step toward development of roaming capabilities. In fact,
one of the ways by which a provider may offer roaming ser-
vice is to conclude shared use agreements with multiple net-
works. However, to date the ability to accomplish this has
been hampered by lack of interoperability among shared use
implementations.
Tunnel Network Server
This is a server which terminates a tunnel. In PPTP termi-
nology, this is known as the PPTP Network Server (PNS). In
L2TP terminology, this is known as the L2TP Network Server
(LNS).
Network Access Server
The Network Access Server (NAS) is the device that clients
dial in order to get access to the network. In PPTP termi-
nology this is referred to as the PPTP Access Concentrator
(PAC). In L2TP terminology, the NAS is referred to as the
L2TP Access Concentrator (LAC).
RADIUS server
This is a server which provides for authentica-
tion/authorization via the protocol described in [3], and
for accounting as described in [4].
RADIUS proxy
In order to provide for the routing of RADIUS authentication
and accounting requests, a RADIUS proxy may employed. To the
NAS, the RADIUS proxy appears to act as a RADIUS server, and
to the RADIUS server, the proxy appears to act as a RADIUS
client.
Network Access Identifier
In order to provide for the routing of RADIUS authentication
and accounting requests, the userID field used in PPP and in
the subsequent RADIUS authentication and accounting
requests, known as the Network Access Identifier (NAI) may
contain structure. This structure provides a means by which
the RADIUS proxy will locate the RADIUS server that is to
receive the request. This same structure may also be used to
locate the tunnel endpoint when domain-based tunneling is
used.
4. Requirements language
This document uses the same words as RFC 1123 [4] for defining the
significance of each particular requirement. These words are:
MUST This word or the adjective "required" means that the item is
an absolute requirement of the specification.
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MUST NOT This phrase means that the definition is an absolute prohi-
bition of the specification.
SHOULD This word or the adjective "recommended" means that there
may exist valid reasons in particular circumstances to
ignore this item, but the full implications should be under-
stood and the case carefully weighed before choosing a dif-
ferent course.
MAY This word or the adjective "optional" means that this item
is truly optional. One vendor may choose to include the item
because a particular marketplace requires it or because it
enhances the product, for example; another vendor may omit
the same item.
Silently Discard
The implementation discards the datagram without further
processing, and without indicating an error to the sender.
The implementation SHOULD provide the capability of logging
the error, including the contents of the discarded datagram,
and SHOULD record the event in a statistics counter.
An implementation is not compliant if it fails to satisfy
one or more of the MUST or MUST NOT requirements for the
protocols it implements. An implementation that satisfies
all the MUST and all the SHOULD requirements for its proto-
cols is said to be "unconditionally compliant"; one that
satisfies all the MUST requirements but not all the SHOULD
requirements for its protocols is said to be "conditionally
compliant."
5. Introduction
Many of the most interesting applications of tunneling protocols such
as PPTP and L2TP involve dial-up network access. Some, such as the
provisioning of secure access to corporate intranets via the Internet,
are characterized by voluntary tunneling: the tunnel is created at the
request of the user for a specific purpose. Other applications involve
compulsory tunneling: the tunnel is created automatically, without any
action from the user and more importantly, without allowing the user
any choice in the matter.
Examples of applications that might be implemented using compulsory
tunnels are Internet software upgrade servers, software registration
servers and banking services. These are all services which, without
mandatory tunneling, would probably be provided using dedicated net-
works or at least dedicated network access servers (NAS), since they
are characterized by the need to limit user access to specific hosts.
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Given the existence of widespread support for mandatory tunneling,
however, these types of services could be accessed via virtually any
Internet service provider (ISP). The most popular means of authoriz-
ing dial-up network users today is through the RADIUS protocol. The
use of RADIUS allows the dial-up users' authorization and authentica-
tion data to be maintained in a central location, rather than being
subject to manual configuration. It makes sense to use RADIUS to cen-
trally administer compulsory tunneling, since RADIUS is widely
deployed and was designed to carry this type of information. New
RADIUS attributes are needed to carry the tunneling information from
the RADIUS server to the NAS and to transfer accounting data from the
NAS to the RADIUS server; those attributes are defined in [7].
This document focuses on implementation issues arising in the provi-
sioning of mandatory tunneling in dialup networks. The use of RADIUS
in provisioning of mandatory tunneling has several advantages. These
include:
Auditing capabilities
Per-user tunneling
Telephone-number based authentication and accounting
5.1. Auditing Capabilities
The integration of RADIUS and tunnel protocols allows the ISP and the
corporation to synchronize their accounting activities so that each
side receives a record of the user's resource consumption. This pro-
vides the corporation with the ability to audit ISP bills. This capa-
bility is particularly important when the user is taking advantage of
roaming capabilities in order to connect to the corporate intranet
from a remote location.
As described in [7], auditing requires implementation of number of new
RADIUS attributes. These new attributes allow the RADIUS server to
provide information within the Accounting-Request packet that will
allow matching with the corresponding tunnel server Accounting-Request
packet. This information includes the Connection-Identifier, the tun-
nel protocol (PPTP, L2F, or L2TP), tunnel-media (X.25, ATM, Frame
Relay, IP) the address of the remote tunnel endpoint, and the tunnel
client address.
5.1.1. Connection-Identifier
In L2TP the Call-Serial-Number is used as the RADIUS Connection Iden-
tifier, and the tuple (userID, tunnel-client-endpoint address, tunnel-
server-endpoint address, Call-Serial-Number) is used to uniquely iden-
tify the call. In order to protect against wrapping due to reboots or
call volume, it is recommended that a 64-bit NTP timestamp be used as
the Call-Serial Number.
While the PPTP specification states that the Call-Serial-Number SHOULD
be unique, it does not mandate this. In addition, no method for
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determining the Call-Serial-Number is specified, which leaves open the
possibility of wrapping after a reboot. Finally since the Call-Serial-
Number is a 16-bit value, the Call-Serial-Number is not sufficient to
distinguish a given call from all other calls over an extended time
period. For example, let us assume that the Call Serial Number is
assigned monotonically, and that the NAS in question has 96 ports. If
the NAS ports are kept continually busy and the average call is of 20
minutes duration, then the Call Serial Number will wrap within
65536/(96 * 3 calls/hour * 24 hours/day) = 9.48 days.
In order to rectify this, it is recommended that another message be
added to PPTP so that the NAS could provide the tunnel server with a
unique Connection Identifier. It is recommended that a 64-bit NTP
timestamp be used for this purpose. This is not an issue in L2TP since
the Call-Serial-Number string may be of variable length.
5.2. Per-user Tunneling
Current proposals for routing of tunnel requests include static tun-
neling, where all users are automatically tunneled to a given end-
point, and realm-based tunneling, where the tunnel endpoint is deter-
mined from the realm portion of the userID. Per-user tunneling as pro-
vided by integration of RADIUS and tunnel protocols offers significant
advantages over both of these approaches.
Static tunneling requires dedication of a NAS device to the purpose.
In the case of an ISP, this is undesirable because it requires them to
dedicate a NAS to tunneling service for a given corporate customer,
rather than allowing them to use existing NASes deployed in the field.
As a result static tunneling is likely to be costly for deployment of
a global service.
Realm-based tunneling assumes that all users within a given realm wish
to be treated the same way. This limits a corporation's flexibility in
managing the account rights of their users. For example, BIGCO may
wish to provide Janet with an account that allows access to both the
Internet and the intranet, with Janet's intranet access provided by a
tunnel server located in the engineering department. However BIGCO may
wish to provide Fred with an account that provides only access to the
intranet, with Fred's intranet access provided by a tunnel network
server located in the sales department. Such a situation cannot be
accommodated with realm-based tunneling.
On the other hand, per-user tunneling as enabled by the attributes
defined in [7] provides BIGCO with the flexibility to administer tun-
neling behavior on a per-user basis. When deployed in concert with
roaming, per-user tunneling offers corporations the ability to provide
their users with access to the corporate Intranet on a global basis.
As described in [7], provisioning of mandatory tunneling requires no
new RADIUS messages, and implementation of three new RADIUS
attributes. During authentication, the new attributes allow the RADIUS
server to indicate the tunnel protocol (PPTP, L2F, or L2TP), the
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tunnel medium (X.25, ATM, Frame Relay, IP) and the address of the
remote tunnel endpoint. During accounting, the new attributes allow
the NAS to provide the RADIUS server with these same attributes, as
well as the tunnel client address and Connection Identifier. Together
the Connection Identifier and tunnel client and endpoint addresses
uniquely identify the call, and allow linking of the RADIUS server and
tunnel server accounting records.
5.3. Telephone-number based authentication and accounting
Using the Calling-Station-ID and Called-Station-ID RADIUS attributes,
authorization and subsequent tunnel attributes can be based on the
phone number originating the call, or the number being called. This
allows the RADIUS server to authorize users based on the calling phone
number (with or without a userID/password combination), or to select
the tunnel type, tunnel medium, or tunnel endpoint address based on
the calling or called phone number. Similarly, the tunnel server may
choose to reject or accept the call based on the Dialed Number and
Dialing Number included in the Incoming-Call-Request packet sent by
the NAS.
The use of RADIUS accounting by the NAS and/or the tunnel server
allows for accounting to take place based on the Calling-Station-ID
and/or Called-Station-ID.
6. Alternatives for implementation of mandatory tunneling
There are several alternatives for integration of RADIUS and tunnel-
ing:
Authenticate at the NAS
Authenticate at the NAS, and forward the RADIUS reply
Authenticate at the tunnel network server
Authenticate at both the NAS and the tunnel network server
We discuss each of these alternatives in turn.
6.1. Authenticate at the NAS
With this approach, authentication and authorization (including tun-
neling information) occurs once, at the NAS. The advantages of this
approach are that it disallows network access for unauthorized NAS
users; and allows RADIUS accounting to be used at the NAS. Disadvan-
tages are that it requires that the tunnel network server trust the
NAS, since no user authentication occurs on the tunnel network server
end of the tunnel. Another disadvantage is that this does not allow
LCP parameters to be re-negotiated by the tunnel network server. Also,
due to the lack of user authentication, accounting cannot take place
at the tunnel network server with strong assurance that the correct
party is being billed.
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6.2. Authenticate at the NAS, and forward the RADIUS reply
With this approach, authentication and authorization occurs once, at
the NAS end of the tunnel and the RADIUS reply is forwarded to the
tunnel network server. This approach disallows network access for
unauthorized NAS users; does not require trust between the NAS and
tunnel network server ends of the tunnel; and allows for RADIUS
accounting to be used at both ends of the tunnel. However, it also
requires that both ends share the same secret with the RADIUS server,
since that's the only way that the tunnel network server could check
the RADIUS reply. Also, the tunnel network server must share secrets
with all the NASes and associated RADIUS servers. Other disadvantages
include lack of provision for LCP renegotiation by the tunnel network
server; and the tunnel network server must know how to handle and ver-
ify RADIUS Access-Accept messages.
While this scheme may be workable if the reply comes directly from a
RADIUS server, it would become unmanageable if a RADIUS proxy is
involved, since the reply would be authenticated using the secret
shared by the client and proxy, rather than the RADIUS server.
6.3. Authenticate at the tunnel network server
In this scheme, authentication and authorization occurs once at the
tunnel network server. This requires that the NAS determine that the
user needs to be tunneled (through a RADIUS request, manual configura-
tion, etc.). Since the RADIUS specification [3] requires that either a
CHAP or PAP password be present in an Access-Request message, but
doesn't require that it be non-empty, this scheme probably requires
either attribute-specific processing on the RADIUS server, or addition
of a new "Authorize-Only" RADIUS message.
In attribute-specific processing an attribute is used to signal tunnel
initiation. For example, tunnel attributes can be sent back if the PAP
password is empty (or "tunnel" or "L2TP"). Alternatively, a user
could prefix his NAI with some special character ('*') if he wanted to
be tunneled. This particular solution does not support compulsory tun-
neling. Another solution involves keying on the domain portion of the
NAI; all users in domain 'X' would be tunneled to address 'Y'. This
proposal supports compulsory tunneling, but does not provide for per-
user tunneling.
An Authorize-Only message would not include a CHAP or PAP password;
nevertheless, in response the RADIUS server would return the attribute
list. In order to prevent password guessing attacks, an Authorize-Only
message would need to be authenticated via the RADIUS shared secret.
This could be accomplished by adding a field within the Authorization-
Only message for an MD5 hash over the contents of the message (with
the authenticator field set to zero) and the shared secret.
Note that when either attribute-specific processing or authorization-
only messages are used, the tunnel network server would need to rene-
gotiate LCP. Note also that in order for the NAS to start accounting
on the connection, it would need to use the identity claimed by the
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user in authenticating to the tunnel network server, since it did not
verify the identity via RADIUS. However, in order for that to be of
any use in accounting, the tunnel endpoint needs to have an account
relationship with the NAS owner. Thus even if a user has an account
with the NAS owner, they cannot use this account for tunneling unless
the tunnel endpoint also has a business relationship with the NAS
owner. Without putting in place a public key infrastructure, it is not
clear how the NAS would be able to verify the existence of such a
business relationship in a scalable manner. Thus this approach nulli-
fies many of the benefits of roaming.
6.4. Authenticate at both the NAS and the tunnel network server
In this scheme, authentication occurs both at the NAS and the tunnel
network server. This requires the dial-up PPP peer to handle dual
authentications, with attendant LCP re-negotiations. In order to allow
the NAS and tunnel network server to authenticate against the same
database, this requires RADIUS client capability on the tunnel network
server, and possibly a RADIUS proxy on the NAS end.
Advantages of this scheme are that it allows secure authentication and
accounting at both ends of the tunnel; allows the use of a single
userID/password pair via implementation of RADIUS on the tunnel net-
work server; and does not require attribute-specific processing or
addition of an Authorize-Only message on the RADIUS server. This
scheme allows for accounting records to be generated on both the NAS
and tunnel server ends allowing for auditing. Also, in contrast to the
previous scheme, the tunnel endpoint does not need to have an account
relationship with the NAS owner. Thus this scheme is compatible with
roaming. Disadvantages are that LCP must be renegotiated, and that
dual authentications are required.
Considering all the advantages and disadvantages of the various
schemes, we believe that this scheme is the best choice, and as a
result, the rest of this document focuses on the dual authentication
scheme.
7. Initiation Sequence
The provisioning of a mandatory tunnel involves an interaction between
the client, NAS, Tunnel Server, and RADIUS server. The following
events occur in initiation of the connection:
Client and NAS: PPP LCP negotiation
Client and NAS: PPP authentication
NAS to RADIUS Server: RADIUS Access-request
RADIUS server to NAS: RADIUS Access-reply/Access-Reject
NAS to Tunnel Server: PPTP/L2TP Incoming-Call-Request
Tunnel Server to NAS: PPTP/L2TP Incoming-Call-Reply
NAS to Tunnel Server: PPTP/L2TP Incoming-Call-Connected
Client and Tunnel Server: PPP LCP re-negotiation
Client and Tunnel Server: PPP re-authentication
Tunnel Server to RADIUS Server: RADIUS Access-request (optional)
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RADIUS server to Tunnel Server: RADIUS Access-reply/Access-Reject
Client and Tunnel Server: IPCP negotiation
NAS to RADIUS Server: RADIUS Accounting-Request/START
RADIUS Server to NAS: RADIUS Accounting-Reply
Tunnel Server to RADIUS Server: RADIUS Accounting-Request/START (Optional)
RADIUS Server to Tunnel Server: RADIUS Accounting-Reply
The process begins with an incoming call to the NAS. The client and
NAS then begin LCP negotiation. Subsequently the PPP authentication
phase starts, and the NAS sends a RADIUS Access-Request message to the
RADIUS server. If the authentication is successful, the RADIUS server
responds with a RADIUS Access-Reply indicating a ACK and including the
following attributes: the tunnel type (L2F, PPTP, L2TP), tunnel medium
type (X.25, ATM, Frame Relay, IP), and the address of the remote tun-
nel endpoint.
It is possible that RADIUS proxies may be used to forward the Access-
Reply message from the RADIUS server to the NAS. In order to ensure
that the mandatory tunnel attributes arrive without modification,
RADIUS proxies present in the path MUST NOT modify these attributes in
any way.
Since this is a mandatory tunnel, address assignment will be handled
by the tunnel server rather than the NAS. As a result, if tunnel
attributes are present, the NAS MUST ignore any address assignment
attributes sent by the RADIUS server. In addition, the NAS and client
MUST NOT begin IPCP negotiation, since this could create a time window
in which the client will be capable of sending packets to the Inter-
net, which is not permitted under mandatory tunneling.
If the authentication is unsuccessful, the RADIUS server sends a
RADIUS Access-Reply indicating a NAK. In this case, the NAS MUST dis-
connect the user.
The NAS will now bring up a control connection to the tunnel server if
none existed before. This is accomplished by sending a PPTP/L2TP
Start-Control-Connection-Request message to the tunnel server. The
tunnel server replies with a PPTP/L2TP Start-Control-Connection-Reply.
If this message indicates an error, or if the control connection is
terminated at any future time, then the NAS MUST disconnect the user.
The NAS will then proceed to send a PPTP/L2TP Incoming-Call-Request
message to the tunnel server. Among other things, this message will
contain the Call Serial Number, which along with the IP addresses of
the NAS and tunnel server, is used to uniquely identify the call. The
tunnel server will reply with a PPTP/L2TP Incoming-Call-Reply message.
If this message indicates an error, then the NAS MUST disconnect the
user. The NAS then replies with an Incoming-Call-Connected message.
At this point, data begins to flow through the tunnel, and the client
and tunnel server must renegotiate LCP and go through another round of
PPP authentication. At the time that this renegotiation begins, the
client and NAS should have concluded LCP negotiation, but must not
have begun IPCP negotiation. When LCP negotiation has been concluded,
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the IPCP phase will begin, and the tunnel server will assign an IP
address to the client.
In performing the PPP authentication, the tunnel server may access its
own user database, or it may issue a RADIUS Access-Request to a RADIUS
server. The latter approach is useful in cases where authentication
forwarding is enabled, such as with roaming or shared use networks. In
this case, the RADIUS server and tunnel server are under the same
administration and are typically located close together, possibly on
the same LAN. Therefore having the tunnel server act as a RADIUS
client provides a means for unifying user administration. Other meth-
ods are also available, such as use of LDAP. Note that the tunnel
server's RADIUS Access-Request is typically sent directly to the local
RADIUS server rather than being forwarded via the authentication rout-
ing procedure described in [8].
After the tunnel has been brought up, the NAS and tunnel server may
start accounting. In the case of the NAS, this is done by sending a
RADIUS Accounting-Request/START packet to the RADIUS server. The
Accounting-Request packet MUST include the following attributes: Tun-
nel-Type, Tunnel-Medium-Type, Tunnel-Client-Endpoint, Tunnel-Server-
Endpoint, and Connection-Identifier.
The tunnel server may produce its own accounting records, or it may
send a RADIUS Accounting-Request/START packet to a local RADIUS
server. In the latter case, the RADIUS Accounting-Request/START packet
MUST contain the following attributes: Tunnel-Type, Tunnel-Medium-
Type, Tunnel-Client-Endpoint, Tunnel-Server-Endpoint, and Connection-
Identifier.
The interactions involved in initiation of a mandatory tunnel are sum-
marized below. In order to simplify the diagram that follows, we have
left out the client. However, it should be understood that the client
participates via PPP negotiation, authentication and subsequent data
interchange with the Tunnel Server.
INITIATION SEQUENCE
NAS Tunnel Server RADIUS Server
--- ------------- -------------
Call accepted
LCP starts
PPP authentication
phase starts
Send RADIUS
Access-Request
with username and
authentication data
IF authentication
succeeds
Send ACK with
Tunnel-Type, Tunnel-Medium-Type,
and Tunnel-Server-Endpoint
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ELSE Send NAK
IF NAK DISCONNECT
ELSE
IF no control
connection exists
Send
Start-Control-Connection-Request
message to Tunnel Server
Send
Start-Control-Connection-Reply
to NAS
ENDIF
Send
Incoming-Call-Request
message to Tunnel Server
Send Incoming-Call-Reply
to NAS
Send
Incoming-Call-Connected
message to Tunnel Server
Send data through the tunnel
Re-negotiate LCP,
authenticate user,
bring up IPCP,
start accounting
Send RADIUS
Accounting-Request (optional)
Send
RADIUS Accounting-Reply
Send a RADIUS
Accounting-Request message
with Acct-Status-Type
set to Start, containing
Tunnel-Type, Tunnel-Medium-Type,
Tunnel-Client-Endpoint,
Tunnel-Server-Endpoint, and
Connection-Identifier.
Send
RADIUS Accounting-Reply
ENDIF
8. Termination Sequence
As with initiation, the tear down of a mandatory tunnel involves an
interaction between the client, NAS, Tunnel Server, and RADIUS server.
The following events occur in termination of the connection:
Tunnel Server to NAS: PPTP/L2TP Call-Clear-Request (optional)
NAS to Tunnel Server: PPTP/L2TP Call-Disconnect-Notify
NAS to RADIUS Server: RADIUS Accounting-Request/STOP
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RADIUS Server to NAS: RADIUS Accounting-Reply
Tunnel Server to RADIUS Server: RADIUS Accounting-Request/STOP (Optional)
RADIUS Server to Tunnel Server: RADIUS Accounting-Reply
Tunnel termination may occur due to a client request (PPP termina-
tion), a tunnel server request (Call-Clear-Request), or a telco issue
(call disconnect).
In the case of a client-requested termination, the tunnel server will
terminate the PPP session. The tunnel server will subsequently send a
Call-Clear-Request to the NAS. The NAS will then send a Call-
Disconnect-Notify message to the tunnel server, and will disconnect
the call.
The NAS will also respond with a Call-Disconnect-Notify message and
disconnection if it receives a Call-Clear-Request from the tunnel
server without a client-requested termination.
In the case of a telco issue or user hangup, the NAS will send a Call-
Disconnect-Notify to the tunnel server. Both sides will then tear down
the call.
After call tear down is complete, the NAS and tunnel server may stop
accounting. In the case of the NAS, this is done by sending a RADIUS
Accounting-Request/STOP packet to the RADIUS server. The Accounting-
Request packet MUST include the following attributes: Tunnel-Type,
Tunnel-Medium-Type, Tunnel-Client-Endpoint, Tunnel-Server-Endpoint,
and Connection-Identifier.
The tunnel server may produce its own accounting records, or it may
send a RADIUS Accounting-Request/STOP packet to a local RADIUS server.
In the latter case, the RADIUS Accounting-Request/STOP packet MUST
contain the following attributes: Tunnel-Type, Tunnel-Medium-Type,
Tunnel-Client-Endpoint, Tunnel-Server-Endpoint, and Connection-
Identifier.
The interactions involved in termination of a mandatory tunnel are
summarized below. In order to simplify the diagram that follows, we
have left out the client. However, it should be understood that the
client participates via PPP termination and disconnection.
TERMINATION SEQUENCE
NAS Tunnel Server RADIUS Server
--- ------------- -------------
IF user disconnected
send
Call-Disconnect-Notify
message to tunnel server
Tear down the call
stop accounting
Send RADIUS
Accounting-Request/STOP
message (optional)
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Send RADIUS
Accounting-Reply
ELSE IF client requests
termination
send
Call-Clear-Request
to the NAS
Send
Call-Disconnect-Notify
message to tunnel server
Disconnect the user
Tear down the call
stop accounting
Send RADIUS
Accounting-Request/STOP
message (optional)
Send RADIUS
Accounting-Reply
Send a RADIUS
Accounting-Request message
with Acct-Status-Type
set to Stop, containing
Tunnel-Type, Tunnel-Medium-Type,
Tunnel-Client-Endpoint,
Tunnel-Server-Endpoint, and
Connection-Identifier.
Send RADIUS
Accounting-Reply
ENDIF
9. Use of distinct RADIUS servers
In the case that the NAS and the tunnel server are using distinct
RADIUS servers, some interesting cases can arise in the provisioning
of mandatory tunnels.
9.1. Distinct userIDs
If distinct RADIUS servers are being used, it is likely that two dis-
tinct userID/password pairs will be required to complete the RADIUS
and tunnel authentications. One pair will be used in the initial PPP
authentication, and the second pair will be used for the tunnel
authentication.
However, in order to provide maximum ease of use in the case where the
userID/password pairs are identical, tunnel clients typically attempt
authentication with the same userID/password pair as was used in the
initial PPP negotiation. Only after this fails do they prompt the user
for the second pair.
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In this case, tunnel client implementations should take care not to
put up error messages indicating a bad password. Instead a dialog
should be presented requesting the tunnel userID/password combination.
In the case where token cards are being used for both authentications,
the tunnel client MUST NOT attempt to present the token used in the
first authentication during the second authentication, since these
will never be identical. Instead the user should be prompted to enter
the second token.
9.2. Multilink PPP issues
It is possible for the two RADIUS servers to return distinct MAX CHAN-
NEL attributes. For example, it is conceivable that the NAS RADIUS
server will only grant use of a single channel to the user, while the
tunnel RADIUS server will grant more than one channel. In this case,
the correct behavior is for the tunnel client to open a connection to
another NAS in order to bring up a multilink bundle on the tunnel
server. The client MUST NOT indicate to the NAS that this additional
link is being brought up as part of a multilink bundle; this will only
be indicated in the subsequent negotiation with the tunnel server.
It is also conceivable that the NAS RADIUS server will allow the
client to bring up dual channels, but that the tunnel RADIUS server
will only allow a single channel. In this case, the client should ter-
minate use of the second channel.
10. UserID Issues
In the provisioning of roaming and shared use networks, one of the
requirements is to be able to route the authentication request to the
user's home RADIUS server. This authentication routing is accomplished
based on the userID submitted by the user to the NAS in the initial
PPP authentication.
Similarly, references [5] and [6] refer to use of the userID in deter-
mining the tunnel endpoint. However, since none of these references
provide guidelines for how RADIUS or tunnel routing is to be accom-
plished, the possibility of conflicting interpretations has emerged.
To head off this conflict, we must come to agreement on the syntax and
semantics expected from the userID. This convergence must be achieved
to allow for use of mandatory tunneling in roaming or shared network
situations.
10.1. UserID convergence in per-user tunneling
Among other things, the use of RADIUS in provisioning of mandatory
tunneling relieves the userID from having to do double duty. Rather
than being used both for routing of the RADIUS authentica-
tion/authorization request as well for determination of the tunnel
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INTERNET-DRAFT 26 November 1996
endpoint, the userID is now used solely for routing of RADIUS authen-
tication/authorization requests. The response to that request is in
turn used to determine the tunnel endpoint.
In fact, in per-user tunneling the userID and password submitted to
RADIUS need not even be valid at the tunnel endpoint. In proposals [5]
and [6] after the user is granted access to the NAS, an attempt is
made to bring up a tunnel using the userID and password submitted by
the user. However, if this userID and password is not valid on the
tunnel endpoint, the user will then be asked to submit another userID
and password. Thus it is not required that the user maintain the same
userID and password both for his ISP and tunnel endpoint.
Since the framework provided in [7] allows both ISPs and tunnel users
to authenticate users as well as account for resources consumed by
them, and provides for maintenance of two distinct userID/password
pairs, it is believed that this scheme offers the maximum degree of
flexibility. However, in a great number of situations, it is highly
desirable for the user to only have to remember a single userID and
password. This is made possible by the joint implementation of RADIUS
authentication routing and tunneling.
How will this work in practice? Typically the user will login with a
userID of the form userID [@ | : | / ] domain. Examples:
fred@bigco.com, fred:bigco.com, fred/bigco.com.
Let us assume that user Fred logs in to a NAS using his corporate
account, fred@bigco.com, and wishes to access BIGCO's Intranet.
Through a process described in [8], the RADIUS access-request is
routed to BIGCO's RADIUS server, which then returns tunnel attributes
in the RADIUS access-reply.
After it receives the tunnel attributes, the NAS will attempt to open
a tunnel to the tunnel server specified in the RADIUS access-reply,
using the specified tunnel-type and tunnel-media. Fred's client will
then attempt to authenticate against the tunnel server, with the
userID fred@bigco.com, and the original password. Assuming that
BIGCO's RADIUS and tunnel server go against the same user database,
then this authentication will succeed. This can be accomplished by
having the tunnel server act as a RADIUS client, as described previ-
ously.
11. Acknowledgements
Thanks to Gurdeep Singh Pall of Microsoft for many useful discussions
of this problem space.
12. References
[1] B. Aboba, L. Liu, J. Alsop, J. Ding. "Review of Roaming Imple-
mentations." draft-ietf-roamops-imprev-00.txt, Microsoft, Aimnet, i-
Pass Alliance, Asiainfo, September, 1996.
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INTERNET-DRAFT 26 November 1996
[2] B. Aboba, G. Zorn. "Dialup Roaming Requirements." draft-ietf-
roamops-roamreq-00.txt, Microsoft, October, 1996.
[3] C. Rigney, A. Rubens, W. A. Simpson, S. Willens. "Remote Authen-
tication Dial In User Service (RADIUS)." draft-ietf-radius-
radius-05.txt, Livingston, Merit, Daydreamer, July 1996.
[4] C. Rigney. "RADIUS Accounting." draft-ietf-radius-
accounting-05.txt, Livingston, July 1996.
[5] K. Hamzeh, T. Kolar, M. Littlewood, G. S. Pall, J. Taarud, A. J.
Valencia, W. Verthein. "Layer Two Tunneling Protocol -- L2TP." draft-
ietf-pppext-l2tp-00.txt, Ascend Communications, August, 1996.
[6] K. Hamzeh, G. S. Pall, J. Taarud, W. Verthein, J. Taarud, W. A.
Little. "Point-to-Point Tunneling Protocol -- PPTP." draft-ietf-
pppext-pptp-00.txt, Ascend Communications, June, 1996.
[7] G. Zorn. "RADIUS Attributes for Tunnel Protocol Support." draft-
zorn-radius-tunnel-auth-00.txt, Microsoft Corporation, October, 1996.
[8] B. Aboba. "The Network Access Identifier and Authentication Rout-
ing." draft-ietf-roamops-nai-00.txt, Microsoft Corporation, November,
1996.
13. Authors' Addresses
Bernard Aboba
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
Phone: 206-936-6605
EMail: bernarda@microsoft.com
Glen Zorn
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
Phone: 206-703-1559
EMail: glennz@microsoft.com
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