One document matched: draft-dekok-radext-tcp-transport-00.txt
Network Working Group A. DeKok
INTERNET-DRAFT FreeRADIUS
Category: Proposed Standard
<draft-dekok-radext-tcp-transport-00.txt>
Expires: January 03, 2009
3 July 2008
RADIUS Over TCP
draft-dekok-radext-tcp-transport-00
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Copyright Notice
Copyright (C) The IETF Trust (2008).
Abstract
The Remote Authentication Dial In User Server (RADIUS) Protocol has
traditionally used the User Datagram Protocol (UDP) as it's
underlying transport layer. This document defines RADIUS over the
Transport Control Protocol (TCP).
DeKok, Alan Proposed Standard [Page 1]
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Table of Contents
1. Introduction ............................................. 3
1.1. Benefits of Reliable Transport ...................... 3
1.2. Drawbacks of Reliable Transport ..................... 4
1.3. Terminology ......................................... 4
1.4. Requirements Language ............................... 5
2. Changes to RADIUS ........................................ 5
2.1. Packet Format ....................................... 5
2.2. TCP Ports ........................................... 5
2.3. Management Information Base (MIB) ................... 6
2.4. Interaction with RadSec ............................. 6
2.4.1. Applicability .................................. 6
2.5. RADIUS Proxies ...................................... 7
2.6. TCP Specific Issues ................................. 7
2.6.1. Duplicates and Retransmissions ................. 8
2.6.2. Shared Secrets ................................. 9
2.6.3. Malformed Packets and Unknown Clients .......... 9
2.6.4. Limits of the ID Field ......................... 10
2.6.5. EAP Sessions ................................... 10
2.6.6. TCP Applications are not UDP Applications ...... 11
3. Diameter Considerations .................................. 11
4. IANA Considerations ...................................... 11
5. Security Considerations .................................. 11
6. References ............................................... 12
6.1. Normative References ................................ 12
6.2. Informative References .............................. 12
Full Copyright Statement ..................................... 14
DeKok, Alan Proposed Standard [Page 2]
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1. Introduction
The RADIUS Protocol has been defined in [RFC2865] as using the User
Datagram Protocol (UDP) for the underlying transport layer. While
there are a number of benefits to using UDP as outlined in [RFC2865]
Section 2.4, there are also some limitations:
* Unreliable transport. As a result, systems using RADIUS have to
implement application-layer timers and re-transmissions, as
described in [RFC5080] Section 2.2.1.
* Packet fragmentation. [RFC2865] Section 3 permits RADIUS
packets to up to 4096 octets in length. These packets are larger
than the default Internet MTU (576), resulting in fragmentation of
the packets at the IP layer. Transport of fragmented UDP packets
appearsto be a poorly tested code path on network devices. Some
devices appear to be incapable of transporting fragmented UDP
packets, making it difficult to deploy RADIUS in a network where
those devices are deployed.
* Connectionless transport. Neither clients no servers can
reliably detect when the other is down. This information has be
deduced from the absence of a reply to a request.
As RADIUS is widely deployed, and has been widely deployed for well
over a decade, these issues are relatively minor. However, new
systems may be interested in choosing a different set of trade-offs
than those outlined in [RFC2865] Section 2.4. For those systems, we
define RADIUS over TCP.
1.1. Benefits of Reliable Transport
There are a number of benefits to using a reliable transport. For
example, when RADIUs is used to carry EAP conversions [RFC3579], the
EAP exchanges may involve 10 round trips at the RADIUS application
layer. If we assume a 0.1% probability of packet loss in each
direction, then approximately 2% (1 - 0.999^20) of the authentication
attempts will have a lost packet. If we assume a 0.01% packet loss,
then 0.2% of authentication attempts will result in a lost packet.
These lost packets require the supplicant and/or the NAS to re-
transmit packets at the application layer. The difficulty with this
approach is that retransmission implementations have historically
been poor. Some implementations retransmit packets, others do not.
Some implementations are incapable of detecting EAP retransmissions,
and will instead treat the retransmitted packet as an error.
These retransmissions have a high likelihood of causing the entire
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authentication session to fail. For systems with millions to tens of
millions of users, such a high authentication failure rate (0.2% to
2%) may be unacceptable.
Using TCP as an underlying reliable transport means that the RADIUS
implementations can remove all of the application-layer
retransmissions, and instead rely on the Operating System (OS)
kernel's well-tested TCP transport.
1.2. Drawbacks of Reliable Transport
No protocol is perfect for all uses. RADIUS over TCP has some
drawbacks, as noted in [RFC2865] Section 2.4. [RFC3539] Section 2
discusses further issues with using TCP as a transport for
Authentication, Authorization, and/or Accounting (AAA) protocols such
as RADIUS.
The impact of these issues is dicussed in more detail, below.
1.3. Terminology
This document uses the following terms:
Network Access Server (NAS)
A device that provides an access service for a user to a network.
RADIUS server
A RADIUS authentication, authorization, and/or accounting (AAA)
server is an entity that provides one or more AAA services to a
NAS.
RADIUS proxy
A RADIUS proxy acts as a RADIUS server to the NAS, and a RADIUS
client to the RADIUS server.
RADIUS request packet
A packet originated by a RADIUS client to a RADIUS server. e.g.
Access-Request, Accounting-Request, CoA-Request, or Disconnect-
Request.
RADIUS response packet
A packet sent by a RADIUS server to a RADIUS client, in response to
a RADIUS request packet. e.g. Access-Accept, Access-Reject,
Access-Challenge, Accounting-Response, CoA-ACK, etc.
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1.4. Requirements Language
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 [RFC2119].
2. Changes to RADIUS
Adding TCP as a RADIUS transport has a number of impacts on the
protocol, on applications using the protocol, and on networks that
deploy the protocol. This section outlines those impacts, and
defines behaviors.
2.1. Packet Format
The RADIUS packet format is unchanged from [RFC2865], [RFC2866], and
[RFC5176]. Specifically, all of the following portions of RADIUS
MUST be unchanged when using RADIUS over TCP:
* Packet format
* Permitted codes
* Request Authenticator calculation
* Response Authenticator calculation
* Minimum packet length
* Maximum packet length
* Attribute format
* Vendor-Specific Attribute (VSA) format
* Permitted data types
* Calculations of dynamic attributes such as CHAP-Challenge,
or Message-Authenticator.
The changes to RADIUS implementations required to implement this
specification are largely limited to the code that sends and receives
packets on the network.
2.2. TCP Ports
IANA has already assigned TCP ports for RADIUS transport, as outlined
below:
* radius 1812/udp
* radius-acct 1813/tcp
* radius-dynauth 3799/tcp
These ports are unused by existing RADIUS applications.
Implementations SHOULD use the assigned values as the default ports
for RADIUS over TCP.
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The early deployment of RADIUS was done using UDP port number 1645,
which conflicts with the "datametrics" service. Implementations
using RADIUS over TCP MUST NOT use TCP ports 1645 or 1646 as the
default ports for this specification.
2.3. Management Information Base (MIB)
The MIB definitions in [RFC4668], [RFC4669], [RFC4670], [RFC4671],
[RFC4672], and [RFC4673] each contain only one reference to UDP.
These references are in the DESCRIPTION field of the MIB definition,
and are in the form of "The UDP port" or "the UDP destination port".
Implementations of RADIUS over TCP MAY re-use these MIBs to perform
statistics counting for RADIUS over TCP connections. However,
implementors are warned that there is no way for these MIBs to
distinguish between packets sent over UDP or over TCP transport.
Similarly, there is no requirement in RADIUS that the RADIUS services
offered over UDP on a particular IP address and port are identical to
the RADIUS services offered over TCP on a particular IP address and
the same (numerical) port.
2.4. Interaction with RadSec
IANA has already assigned TCP ports for RadSec transport, as outlined
below:
* radsec 2083/tcp
This value SHOULD be used as the default port for RADIUS over TLS
(i.e. RadSec). The "radius" port (1812/tcp) SHOULD NOT be used for
RadSec.
2.4.1. Applicability
As noted in [RFC3539] Section 2.1, for systems originating low
numbers of RADIUS request packets, inter-packet spacing is often
larger than the RTT. In those situations, RADIUS over TCP SHOULD NOT
be used.
In general, RADIUS clients generating small amounts of RADIUS traffic
SHOULD NOT use TCP. This suggestion will usually apply to most
NASes, and to most clients that originate CoA-Request and Disconnect-
Request packets.
RADIUS over TCP is most applicable to RADIUS proxies that exchange a
large volume of packets with RADIUS clients and servers (10's to
1000's of packets per second). In those situations, RADIUS over TCP
is a good fit, and may result in increased network stability and
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performance.
2.5. RADIUS Proxies
As RADIUS is a "hop by hop" protocol, a RADIUS proxy effectively
shields the client from any information about downstream servers.
While the client may be able to deduce the operational state of the
local server (i.e. proxy), it cannot make any determination about the
operational state of the downstream servers.
If a request is proxied through intermediate proxies, it is not
possible to detect which of the later hops is responsible for the
absence of a reply. An intermediate proxy also cannot signal that
the outage lies in a later hop because RADIUS does not have the
ability to carry such signalling information. This issue is further
exacerbated by some proxy implementations that do not reply to a
client if they do not recieve a reply to a proxied request.
When UDP was used as a transport protocol, the absence of a reply can
cause a client to deduce (incorrectly) that the proxy is unavailable.
The client could then fail over to another server, or conclude that
no "live" servers are available. This situation is made even worse
when requests are sent through a proxy to multiple destinations.
Failures in one destination may result in service outages for other
destinations, if the client erroneously believes that the proxy is
unresponsive.
For RADIUS over TCP, the continued existence of the TCP connection
SHOULD be used to deduce that the service on the other end of the
connection is still responsive. Further, the application layer
watchdog defined in [RFC3539] Section 3.4 enables clients to
determine that the server is "live", even though it may not have
responded recently to other, non-watchdog requests.
RADIUS clients using RADIUS over TCP MUST NOT decide that a
connection is down until the application layer watchdog algorithm has
marked it DOWN ([RFC3539] Appendix A). RADIUS clients using RADIUS
over TCP MUST NOT decide that a RADIUS server is unresponsive until
all TCP connections to it have been marked DOWN.
2.6. TCP Specific Issues
The guidelines defined in [RFC3539] for implementing an AAA protocol
operating over a reliable transport MUST be followed by implementors
of this specification.
The Application Layer Watchdog defined in [RFC3539] Section 3.4 MUST
be used. The Status-Server packet [STATUS] MUST be used as the
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application layer watchdog message. Implementations MUST reserve one
RADIUS ID per connection for the application layer watchdog message.
This restriction is described further below.
Implementations MUST NOT confuse UDP and TCP transport. That is,
RADIUS clients and servers MUST be treated as unique based on a key
of (IP address, port, transport protocol). Implementations MUST be
configurable to have different shared secrets for UDP and TCP to the
same destination IP address and numerical port.
This requirement does not forbid the traditional practice of using
primary and secondary servers in a fail-over relationship. Instead,
it requires that two services sharing an IP address and numerical
port, but differing in transport protocol, MUST be treated as
independent services for the purpose of fail-over, load-balancing,
etc.
Whenever the underlying operating system permits the use of TCP
keepalive socket options, their use is RECOMMENDED.
2.6.1. Duplicates and Retransmissions
As TCP is a reliable transport, implementors of this specification
MUST NOT retransmit RADIUS packets over the same TCP connection.
Similarly, if there is no response to a RADIUS packet over one TCP
connection, implementations MUST NOT retransmit that packet over a
different TCP connection to the same destination IP address and port.
However, if the TCP connection is broken or closed, the above
requirement can be relaxed somewhat. RADIUS request packets that
have not yet received a response MAY be transmitted by a RADIUS
client over a new TCP connection. As this procedure involves using a
new source port, the ID of the packet MAY change. If the ID changes,
any security attributes such as Message-Authenticator MUST be
recalculated.
If a TCP connection is broken or closed, any cached RADIUS response
packets ([RFC5080] Section 2.2.2) associated with that connection
MUST be discarded. A RADIUS server SHOULD stop processing any "live"
requests associated with that TCP connection. No response to these
requests cannot be sent over the TCP connection, so any further
processing is pointless. A RADIUS proxy that has a client close it's
TCP connection SHOULD silently discard any responses it recieves to a
proxied requests that is associated with the original client request.
Despite the above requirement, RADIUS servers SHOULD still perform
duplicate detection on received packets, as described in [RFC5080]
Section 2.2.2. This effort can prevent duplicate processing of
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packets from non-conformant clients.
As noted above, RADIUS packets SHOULD NOT be re-transmitted to the
same destination IP and numerical port, but over a different
transport layer. There is no guarantee in RADIUS that the two ports
are in any way related. This requirement does not forbid the
practice of putting multiple servers into a fail-over or load-balance
pool.
Much of the discussion in this section can be summarized by the
following requirement. RADIUS requests MAY be re-transmitted
verbatim only if the following 5-tuple (Client IP address, Client
port, Transport Protocol, Server IP address, Server port) remains the
same. If any field of that 5-typle changes, the packet MUST NOT be
considered to be a re-transmission. Instead, the packet MUST be
considered to be a new request, and be treated accordingly. (e.g.
header calculations, packet signatures, associated timers and
counters, etc.)
The above requirement is necessary, but not sufficient in all cases.
Other specifications give additional situations where the packet is
to be considered as a new request. Those recommendations MUST be
followed.
2.6.2. Shared Secrets
The use of shared secrets in calculating the Response Authenticator,
and other attributes such as User-Password or Message-Authenticator
[RFC3579] MUST be unchanged from previous specifications.
Clients and servers MUST be able to store and manage shared secrets
based on the key described above, of (IP address, port, transport).
2.6.3. Malformed Packets and Unknown Clients
The original specifications say that an implement should "silently
discard" a packet in a number of circumstances. This action has no
further consequences for UDP transport, as the "next" packet is
completely independent of the previous one.
When TCP is used as a transport, decoding the "next" packet on a
connection depends on the proper decoding of the previous packet. As
a result, the behavior with respect to discarded packets has to
change.
Implementations of this specification SHOULD treat the "silently
discard" texts referenced above as "silently discard and close the
connection." Specifically, the TCP connection MUST be closed if any
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of the following circumstances are seen:
* Packet from an unknown client (using the key as defined above)
* Packet with an invalid code field
* Packet that is less than the minimim RADIUS packet length
* Packet that is more than the minimim RADIUS packet length
* A packet that is otherwise malformed, e.g. Attribute Length of
0 or 1
* Packet where the Request Authenticator fails validation
(if applicable)
* Packet where the Response Authenticator fails validation
* Packet where the Message-Authenticator fails validation
* Response packets that do not match any outstanding request
These requirements minimize the possibilty for a misbehaving client
or server to wreak havoc on the network.
2.6.4. Limits of the ID Field
The RADIUS ID field is one octet in size. As a result, any one TCP
connection can have only 256 "in flight" RADIUS packets at a time.
If more than 256 simultaneous "in flight" packets are required,
additional TCP connections will need to be opened. This limitation
is also noted in [RFC3539] Section 2.4.
An additional limit is the requirement to send a Status-Server packet
over the same TCP connection as is used for normal requests. As
noted in [STATUS], the response to a Status-Server packet is either
an Access-Accept, or an Accounting-Response. If all IDs were
allocated to normal requests, then there would be no free Id to use
for the Status-Server packet, and it could not be sent over the
connection.
Implementations SHOULD reserve ID zero on each TCP connection for
Status-Server packets. This value was picked arbitrarily, as there
is no reason to choose any one value over another for this use.
It is tempting to extend RADIUS to permit more than 256 outstanding
packets on one connection. However, doing so will likely require
fundamental changes to the RADIUS protocol, and as such, are outside
of the scope of this specification.
2.6.5. EAP Sessions
When RADIUS clients send EAP requests using RADIUS over TCP, they
SHOULD choose the same TCP connection for all packets related to one
EAP conversation. A simple method that may often work is hashing the
contents of the Calling-Station-Id attribute, which normally contains
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the MAC accress. The output of that hash can be used to select a
particular TCP connection.
It may be difficult to implement this suggestion in practice, as busy
servers may allocate all RADIUS IDs in one TCP connection in the time
between two subsequent EAP packets. It is difficult to suggest
simple and reasonable methods to address this issue.
2.6.6. TCP Applications are not UDP Applications
Implementors should be aware that programming a robust TCP
application can be a very different process than programming a robust
UDP application. We RECOMMEND that implementors of this
specification familiarize themselves with TCP application programming
concepts. We RECOMMEND also that existing TCP applications be
examined with an eye to robustness, performance, scalability, etc.
Clients and servers SHOULD implement configurable connection limits.
Allowing an unlimited number of connections may result in resource
exhaustion.
Further discussion of implementation issues is outside of the scope
of this document.
3. Diameter Considerations
This document defines TCP as a transport layer for RADIUS. It
defines no new RADIUS attributes or codes. The only interaction with
Diameter is in a RADIUS to Diameter, or in a Diameter to RADIUS
gateway. The RADIUS side of such a gateway MAY implement RADIUS over
TCP, but this change has no effect on Diameter.
4. IANA Considerations
This document requires no action by IANA.
5. Security Considerations
As the RADIUS packet format, signing, and client verification are
unchanged from prior specifications, all of the security issues
outlined in previous specifications for RADIUS over UDP are also
applicable here.
As noted above, clients and servers SHOULD support configurable
connection limits. Allowing an unlimited number of connections may
result in resource exhaustion.
There are no (at this time) other known security issues for RADIUS
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over TCP transport.
6. References
6.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2865] Rigney, C., Willens, S., Rubens, A., and W. Simpson, "Remote
Authentication Dial In User Service (RADIUS)", RFC 2865, June
2000.
[RFC3539] Aboba, B. et al., "Authentication, Authorization and
Accounting (AAA) Transport Profile", RFC 3539, June 2003.
6.2. Informative References
[RFC2866] Rigney, C., "RADIUS Accounting", RFC 2866, June 2000.
[RFC3579] Aboba, B. and P. Calhoun, "RADIUS (Remote Authentication Dial
In User Service) Support For Extensible Authentication
Protocol (EAP)", RFC 3579, September 2003.
[RFC4668] Nelson, D, "RADIUS Authentication Client MIB for IPv6", RFC
4668, August 2006.
[RFC4669] Nelson, D, "RADIUS Authentication Server MIB for IPv6", RFC
4669, August 2006.
[RFC4670] Nelson, D, "RADIUS Accounting Client MIB for IPv6", RFC 4670,
August 2006.
[RFC4671] Nelson, D, "RADIUS Accounting Server MIB for IPv6", RFC 4671,
August 2006.
[RFC4672] Nelson, D, "RADIUS Dynamic Authorization Client MIB", RFC
4672, August 2006.
[RFC4673] Nelson, D, "RADIUS Dynamic Authorization Server MIB", RFC
4673, August 2006.
[RFC5080] Nelson, D. and DeKok, A, "Common Remote Authentication Dial In
User Service (RADIUS) Implementation Issues and Suggested
Fixes", RFC 5080, December 2007.
[RFC5176] Chiba, M. et al., "Dynamic Authorization Extensions to Remote
Authentication Dial In User Service (RADIUS)", RFC 5176,
DeKok, Alan Proposed Standard [Page 12]
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January 2008.
[STATUS] DeKok, A., "Use of Status-Server Packets in the Remote
Authentication Dial In User Service (RADIUS) Protocol", draft-
ietf-radext-status-server-00.txt, June 2008.
Acknowledgments
None at this time.
Authors' Addresses
Alan DeKok
The FreeRADIUS Server Project
http://freeradius.org/
Email: aland@freeradius.org
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DeKok, Alan Proposed Standard [Page 14]
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Open issues
Open issues relating to this document are tracked on the following
web site:
http://www.drizzle.com/~aboba/RADEXT/
DeKok, Alan Proposed Standard [Page 15]
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