Skip to main content


The information below is for an old version of the document that is already published as an RFC.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 6613.
Author Alan DeKok
Last updated 2015-10-14 (Latest revision 2010-10-12)
Replaces draft-dekok-radext-tcp-transport
RFC stream Internet Engineering Task Force (IETF)
Intended RFC status Experimental
Additional resources Mailing list discussion
Stream WG state WG Document
Document shepherd (None)
IESG IESG state Became RFC 6613 (Experimental)
Action Holders
Consensus boilerplate Unknown
Telechat date (None)
Responsible AD Benoît Claise
IESG note
Send notices to (None)
Network Working Group                                           A. DeKok
INTERNET-DRAFT                                                FreeRADIUS
Category: Experimental
Expires: April 12, 2011
12 October 2010

                            RADIUS Over TCP


   The Remote Authentication Dial In User Server (RADIUS) Protocol has
   until now required the User Datagram Protocol (UDP) as the underlying
   transport layer.  This document defines RADIUS over the Transmission
   Control Protocol (RADIUS/TCP), in order to address handling issues
   related to RADIUS over Transport Layer Security (RADIUS/TLS).  It
   permits TCP to be used as a transport protocol for RADIUS only when a
   transport layer such as TLS or IPsec provides confidentialy and

   Status of this Memo

      This Internet-Draft is submitted to IETF in full conformance with
      the provisions of BCP 78 and BCP 79.

      Internet-Drafts are working documents of the Internet Engineering
      Task Force (IETF), its areas, and its working groups.  Note that
      other groups may also distribute working documents as Internet-

      Internet-Drafts are draft documents valid for a maximum of six
      months and may be updated, replaced, or obsoleted by other
      documents at any time.  It is inappropriate to use Internet-Drafts
      as reference material or to cite them other than as "work in

      The list of current Internet-Drafts can be accessed at

      The list of Internet-Draft Shadow Directories can be accessed at

      This Internet-Draft will expire on April 12, 2011

   Copyright Notice

DeKok, Alan                   Experimental                      [Page 1]
INTERNET-DRAFT               RADIUS Over TCP             12 October 2010

      Copyright (c) 2010 IETF Trust and the persons identified as the
      document authors.  All rights reserved.

      This document is subject to BCP 78 and the IETF Trust's Legal
      Provisions Relating to IETF Documents
      ( in effect on the date of
      publication of this document.  Please review these documents
      carefully, as they describe your rights and restrictions with
      respect to this document.  Code Components extracted from this
      document must include Simplified BSD License text as described in
      Section 4.e of the Trust Legal Provisions and are provided without
      warranty as described in the Simplified BSD License.

DeKok, Alan                   Experimental                      [Page 2]
INTERNET-DRAFT               RADIUS Over TCP             12 October 2010

   Table of Contents

1.  Introduction .............................................    4
   1.1.  Applicability of Reliable Transport .................    5
   1.2.  Terminology .........................................    6
   1.3.  Requirements Language ...............................    7
2.  Changes to RADIUS ........................................    7
   2.1.  Packet Format .......................................    8
   2.2.  Assigned Ports for RADIUS/TCP .......................    8
   2.3.  Management Information Base (MIB) ...................    9
   2.4.  Detecting Live Servers ..............................    9
   2.5.  Congestion Control Issues ...........................   10
   2.6.  TCP Specific Issues .................................   10
      2.6.1.  Duplicates and Retransmissions .................   11
      2.6.2.  Head of Line Blocking ..........................   12
      2.6.3.  Shared Secrets .................................   12
      2.6.4.  Malformed Packets and Unknown Clients ..........   12
      2.6.5.  Limitations of the ID Field ....................   13
      2.6.6.  EAP Sessions ...................................   14
      2.6.7.  TCP Applications are not UDP Applications ......   15
3.  Diameter Considerations ..................................   15
4.  IANA Considerations ......................................   15
5.  Security Considerations ..................................   15
6.  References ...............................................   16
   6.1.  Normative References ................................   16
   6.2.  Informative References ..............................   16

DeKok, Alan                   Experimental                      [Page 3]
INTERNET-DRAFT               RADIUS Over TCP             12 October 2010

1.  Introduction

   The RADIUS Protocol is 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 up to 4096 octets in length.  These packets are larger
      than the common Internet MTU (576), resulting in fragmentation of
      the packets at the IP layer when they are proxied over the
      Internet.  Transport of fragmented UDP packets appears to 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

      * Connectionless transport.  Neither clients nor servers receive
      positive statements that a "connection" is down.  This information
      has to be deduced instead from the absence of a reply to a

      * Lack of congestion control.  Clients can send arbitrary amounts
      of traffic with little or no feedback.  This lack of feedback can
      result in congestive collapse of the network.

   RADIUS has been widely deployed for well over a decade, and continues
   to be widely deployed.  Experience shows that these issues have been
   minor in some use-cases, and problematic in others.  For use-cases
   such as inter-server proxying, an alternative transport and security
   model -- RADIUS/TLS, as defined in [RADIUS/TLS].  That document
   describes the transport implications of running RADIUS/TLS.

   The choice of TCP as a transport protocol is largely driven by the
   desire to improve the security of RADIUS by using RADIUS/TLS.  For
   practical reasons, the transport protocol (TCP) is defined separately
   from the security mechanism (TLS).

   Since "bare" TCP does not provide for confidentiality or enable
   negotiation of credible ciphersuites, its use is not appropriate for
   inter-server communications where strong security is required.  As a
   result "bare" TCP transport MUST NOT be used without TLS, IPsec, or
   other secure upper layer.

DeKok, Alan                   Experimental                      [Page 4]
INTERNET-DRAFT               RADIUS Over TCP             12 October 2010

   "Bare" TCP transport MAY, however, be used when another method such
   as IPSec [RFC4301] is used to provide additional confidentiality and
   security.  Should experience show that such deployments are useful,
   this specification could be moved to standards track.

1.1.  Applicability of Reliable Transport

   The intent of this document is to address transport issues related to
   RADIUS/TLS [RADIUS/TLS] in inter-server communications scenarios,
   such as inter-domain communication between proxies.  These situations
   benefit from the confidentiality and ciphersuite negotiation that can
   be provided by TLS. Since TLS is already widely available within the
   operating systems used by proxies, implementation barriers are low.

   In scenarios where RADIUS proxies exchange a large volume of packets,
   it is likely that there will be sufficient traffic to enable the
   congestion window to be widened beyond the minimum value on a long-
   term basis, enabling ACK piggy-backing.  Through use of an
   application-layer watchdog as described in [RFC3539], it is possible
   to address the objections to reliable transport described in
   [RFC2865] Section 2.4 without substantial watchdog traffic, since
   regular traffic is expected in both directions.

   In addition, use of RADIUS/TLS has been found to improve operational
   performance when used with multi-round trip authentication mechanisms
   such as EAP over RADIUS  [RFC3579].  In such exchanges, it is typical
   for EAP fragmentation to increase the number of round-trips required.
   For example, where EAP-TLS authentication [RFC5216] is attempted and
   both the EAP peer and server utilize certificate chains of 8KB, as
   many as 15 round-trips can be required if RADIUS packets are
   restricted to the common Ethernet MTU (1500 octets) for EAP over LAN
   (EAPoL) use-cases.  Fragmentation of RADIUS/UDP packets is generally
   inadvisable due to lack of fragmentation support within intermediate
   devices such as filtering routers, firewalls and NATs.  However,
   since RADIUS/UDP implementations typically do not support MTU
   discovery, fragmentation can occur even when the maximum RADIUS/UDP
   packet size is restricted to 1500 octets.

   These problems disappear if a 4096 application-layer payload can be
   used alongside RADIUS/TLS.  Since most TCP implementations support
   MTU discovery, the TCP MSS is automatically adjusted to account for
   the MTU, and the larger congestion window supported by TCP may allow
   multiple TCP segments to be sent within a single window.  Even those
   few TCP stacks which do not perform path MTU discovery can already
   support arbitrary payloads.

   Where the MTU for EAP packets is large, RADIUS/EAP traffic required
   for an EAP-TLS authentication with 8KB certificate chains may be

DeKok, Alan                   Experimental                      [Page 5]
INTERNET-DRAFT               RADIUS Over TCP             12 October 2010

   reduced to 7 round-trips or less, resulting in substantially reduced
   authentication times.

   In addition, experience indicates that EAP sessions transported over
   RADIUS/TLS are less likely to abort unsuccessfully.  Historically,
   RADIUS over UDP implementations have exhibited poor retransmission
   behavior.  Some implementations retransmit packets, others do not,
   and others send new packets rather then performing retransmission.
   Some implementations are incapable of detecting EAP retransmissions,
   and will instead treat the retransmitted packet as an error.  As a
   result, within RADIUS/UDP implementations, retransmissions have a
   high likeilhood of causing an EAP authentication session to fail.
   For a system with a million logins a day running EAP-TLS mutual
   authentication with 15 round-trips, and having a packet loss
   probability of P=0.01%, we expect that 0.3% of connections will
   experience at least one lost packet.  That is, 3,000 user sessions
   each day will experience authentication failure.  This is an
   unacceptable failure rate for a mass-market network service.

   Using a reliable transport method such as TCP means that RADIUS
   implementations can remove all application-layer retransmissions, and
   instead rely on the Operating System (OS) kernel's well-tested TCP
   transport to ensure Path MTU discovery and reliable delivery.  Modern
   TCP implementations also implement anti-spoofing provisions, which is
   more difficult to do in a UDP application.

   In contrast, use of TCP as a transport between a NAS and a RADIUS
   server is usually a poor fit.  As noted in [RFC3539] Section 2.1, for
   systems originating low numbers of RADIUS request packets, inter-
   packet spacing is often larger than the packet RTT, meaning that, the
   congestion window will typically stay below the minimum value on a
   long-term basis. The result is an increase in packets due to ACKs as
   compared to UDP, without a corresponding set of benefits.  In
   addition, the lack of substantial traffic implies the need for
   additional watchdog traffic to confirm reachability.

   As a result, the objections to reliable transport indicated in
   [RFC2865] Section 2.4 continue to apply to NAS-RADIUS server
   communications and UDP SHOULD continue to be used as the transport
   protocol in this scenario.  In addition, it is recommended that
   implementations of "RADIUS Dynamic AUthorization Extensions"
   [RFC5176] SHOULD continue to utilize UDP transport, since the volume
   of dynamic authorization traffic is usually expected to be small.

1.2.  Terminology

   This document uses the following terms:

DeKok, Alan                   Experimental                      [Page 6]
INTERNET-DRAFT               RADIUS Over TCP             12 October 2010

RADIUS client
     A device that provides an access service for a user to a network.
     Also referred to as a Network Access Server, or NAS.

RADIUS server
     A device that provides one or more of authentication,
     authorization, and/or accounting (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-

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.

     RADIUS over UDP, as defined in [RFC2865].

     RADIUS over TCP, as defined in this document.

     RADIUS over TLS, as defined in [RADIUS/TLS].

1.3.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

2.  Changes to RADIUS

   RADIUS/TCP involves sending RADIUS application messages over a TCP
   connection.  In the sections that follow, we discuss the implications
   for the RADIUS packet format (Section 2.1), port usage (Section 2.2),
   RADIUS MIBs (Section 2.3) and RADIUS proxies (Section 2.5).  TCP-
   specific issues are discussed in Section 2.6.

DeKok, Alan                   Experimental                      [Page 7]
INTERNET-DRAFT               RADIUS Over TCP             12 October 2010

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/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.
      * Calculation of "encrypted" attributes such as Tunnel-Password.

   The use of TLS transport does not change the calculation of security-
   related fields (such as the Response-Authenticator) in RADIUS
   [RFC2865] or RADIUS Dynamic Authorization [RFC5176].  Calculation of
   attributes such as User-Password [RFC2865] or Message-Authenticator
   [RFC3579] also does not change.

   Clients and servers MUST be able to store and manage shared secrets
   based on the key described above, of (IP address, port, transport

   The changes to RADIUS implementations required to implement this
   specification are largely limited to the portions that send and
   receive packets on the network.

2.2.  Assigned Ports for RADIUS/TCP

   IANA has already assigned TCP ports for RADIUS transport, as outlined

      * radius          1812/tcp
      * radius-acct     1813/tcp
      * radius-dynauth  3799/tcp

   Since these ports are unused by existing RADIUS implementations, the
   assigned values MUST be used as the default ports for RADIUS over

   The early deployment of RADIUS was done using UDP port number 1645,
   which conflicts with the "datametrics" service.  Implementations

DeKok, Alan                   Experimental                      [Page 8]
INTERNET-DRAFT               RADIUS Over TCP             12 October 2010

   using RADIUS/TCP MUST NOT use TCP ports 1645 or 1646 as the default
   ports for this specification.

   The "radsec" port (2083/tcp) SHOULD be used as the default port for
   RADIUS/TLS.  The "radius" port (1812/tcp) SHOULD NOT be used for

2.3.  Management Information Base (MIB)

   The MIB Module definitions in [RFC4668], [RFC4669], [RFC4670],
   [RFC4671], [RFC4672], and [RFC4673] are intended to be used for
   RADIUS over UDP.  As such, they do not support RADIUS/TCP, and will
   need to be updated in the future.  Implementations of RADIUS/TCP
   SHOULD NOT re-use these MIB Modules to perform statistics counting
   for RADIUS/TCP connections.

2.4.  Detecting Live Servers

   As RADIUS is a "hop by hop" protocol, a RADIUS proxy 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.

   Within RADIUS as defined in [RFC2865], proxies typically only forward
   traffic between the NAS and RADIUS server, and do not generate their
   own responses.  As a result, when a NAS does not receive a response
   to a request, this could be the result of packet loss between the NAS
   and proxy, a problem on the proxy, loss between the RADIUS proxy and
   server, or a problem with the server.

   When UDP is 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 (OKAY state in [RFC3539] Appendix A).
   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/TLS, it is RECOMMENDED that implementations utilize the
   existence of a TCP connection along with the application layer
   watchdog defined in [RFC3539] Section 3.4 to determine that the
   server is "live".

   RADIUS clients using RADIUS/TCP MUST mark a connection DOWN if the
   network stack indicates that the connection is no longer active.  If
   the network stack indicates that connection is still active, Clients

DeKok, Alan                   Experimental                      [Page 9]
INTERNET-DRAFT               RADIUS Over TCP             12 October 2010

   MUST NOT decide that it is down until the application layer watchdog
   algorithm has marked it DOWN ([RFC3539] Appendix A).  RADIUS clients
   using RADIUS/TCP MUST NOT decide that a RADIUS server is unresponsive
   until all TCP connections to it have been marked DOWN.

   The above requirements do not forbid the practice of a client pro-
   actively closing connections, or marking a server as DOWN due to an
   administrative decision.

2.5.  Congestion Control Issues

   Additional issues with RADIUS proxies involve transport protocol
   changes where the proxy receives packets on one transport protocol,
   and forwards them on a different transport protocol.  There are
   several situations in which the law of "conservation of packets"
   could be violated on an end-to-end basis (e.g. where more packets
   could enter the system than could leave it on a short-term basis):

      * Where TCP is used between proxies, it is possible that the
      bandwidth consumed by incoming UDP packets destined to a given
      upstream server could exceed the sending rate of a single TCP
      connection to that server, based on the window size/RTT estimate.

      * It is possible for the incoming rate of TCP packets destined to
      a given realm to exceed the UDP throughput achievable using the
      transport guidelines established in [RFC5080].  This could happen,
      for example, where the TCP window between proxies has opened, but
      packet loss is being experienced on the UDP leg, so that the
      effective congestion window on the UDP side is 1.

   Intrinsically, proxy systems operate with multiple control loops
   instead of one end-to-end loop, and so are less stable.  This is true
   even for TCP-TCP proxies.  As discussed in [RFC3539], the only way to
   achieve stability equivalent to a single TCP connection is to mimic
   the end-to-end behavior of a single TCP connection.  This typically
   is not achievable with an application-layer RADIUS implementation,
   regardless of transport.

2.6.  TCP Specific Issues

   The guidelines defined in [RFC3539] for implementing a AAA protocol
   over reliable transport are applicable to RADIUS/TLS.

   The Application Layer Watchdog defined in [RFC3539] Section 3.4 MUST
   be used.  The Status-Server packet [RFC5997] MUST be used as the
   application layer watchdog message.  Implementations MUST reserve one
   RADIUS ID per connection for the application layer watchdog message.
   This restriction is described further below in Section 2.6.4.

DeKok, Alan                   Experimental                     [Page 10]
INTERNET-DRAFT               RADIUS Over TCP             12 October 2010

   RADIUS/TLS Implementations MUST support receiving RADIUS packets over
   both UDP and TLS transports originating from the same endpoint.
   RADIUS packets received over UDP MUST be replied to over UDP; RADIUS
   packets received over TLS MUST be replied to over TLS.  That is,
   RADIUS clients and servers MUST be treated as unique based on a key
   of the three-tuple (IP address, port, transport protocol).
   Implementations MUST permit different shared secrets to be used for
   UDP and TCP connections 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,

   Whenever the underlying network stack permits the use of TCP
   keepalive socket options, their use is RECOMMENDED.

2.6.1.  Duplicates and Retransmissions

   As TCP is a reliable transport, implementations MUST NOT retransmit
   RADIUS request packets over a given 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, while the
   first connection is in the OKAY state ([RFC3539] Appendix A).

   However, if the TCP connection is broken or closed, retransmissions
   over new connections are permissible.  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

   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 of any
   requests associated with that TCP connection.  No response to these
   requests can be sent over the TCP connection, so any further
   processing is pointless.  This requirement applies not only to RADIUS
   servers, but also to proxies.  When a client's connection to a proxy
   server is closed, there may be responses from a home server that were
   supposed to be sent by the proxy back over that connection to the
   client.  Since the client connection is closed, those responses from
   the home server to the proxy server SHOULD be silently discarded by

DeKok, Alan                   Experimental                     [Page 11]
INTERNET-DRAFT               RADIUS Over TCP             12 October 2010

   the proxy.

   Despite the above discussion, RADIUS servers SHOULD still perform
   duplicate detection on received packets, as described in [RFC5080]
   Section 2.2.2.  This detection can prevent duplicate processing of
   packets from non-conformant clients.

   RADIUS packets SHOULD NOT be re-transmitted to the same destination
   IP and numerical port, but over a different transport protocol.
   There is no guarantee in RADIUS that the two ports are in any way
   related.  This requirement does not, however, forbid the practice of
   putting multiple servers into a fail-over or load-balancing pool.  In
   that situation, RADIUS request MAY be retransmitted to another server
   that is known to be part of the same pool.

2.6.2.  Head of Line Blocking

   When using UDP as a transport for RADIUS, there is no ordering of
   packets.  If a packet sent by a client is lost, that loss has no
   effect on subsequent packets sent by that client.

   Unlike UDP, TCP is subject to issues related to Head of Line (HoL)
   blocking.  This occurs when when a TCP segment is lost and a
   subsequent TCP segment arrives out of order.  While the RADIUS server
   can process RADIUS packets out of order, the semantics of TCP makes
   this impossible.  This limitation can lower the maximum packet
   processing rate of RADIUS/TCP.

2.6.3.  Shared Secrets

   The use of TLS transport does not change the calculation of security-
   related fields (such as the Response-Authenticator) in RADIUS
   [RFC2865] or RADIUS Dynamic Authorization [RFC5176].  Calculation of
   attributes such as User-Password [RFC2865] or Message-Authenticator
   [RFC3579] also does not change.

   Clients and servers MUST be able to store and manage shared secrets
   based on the key described above, of (IP address, port, transport

2.6.4.  Malformed Packets and Unknown Clients

   The RADIUS specifications ([RFC2865], etc.) say that an
   implementation 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.

DeKok, Alan                   Experimental                     [Page 12]
INTERNET-DRAFT               RADIUS Over TCP             12 October 2010

   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

   Implementations of this specification SHOULD treat the "silently
   discard" texts referenced above as "silently discard and close the
   connection."  That is, the TCP connection MUST be closed if any of
   the following circumstances are seen:

      * Connection from an unknown client
      * Packet where the RADIUS "length" field is less than the minimum
        RADIUS packet length
      * Packet where the RADIUS "length" field is more than the maximum
        RADIUS packet length
      * Packet that has an Attribute "length" field has value of zero
        or one (0 or 1).
      * Packet where the attributes do not exactly fill the packet
      * Packet where the Request Authenticator fails validation
        (where validation is required).
      * Packet where the Response Authenticator fails validation
        (where validation is required).
      * Packet where the Message-Authenticator attribute fails
        validation (when it occurs in a packet).

   After applying the above rules, there are still two situations where
   the previous specifications allow a packet to be "silently discarded"
   on reception:

      * Packets with an invalid code field
      * Response packets that do not match any outstanding request

   In these situations, the TCP connections MAY remain open, or MAY be
   closed, as an implementation choice.  However, the invalid packet
   MUST be silently discarded.

   These requirements reduce the possibility for a misbehaving client or
   server to wreak havoc on the network.

2.6.5.  Limitations 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

DeKok, Alan                   Experimental                     [Page 13]
INTERNET-DRAFT               RADIUS Over TCP             12 October 2010

   over the same TCP connection as is used for normal requests.  As
   noted in [RFC5997], 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

   Implementations SHOULD reserve ID zero (0) 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.

   Implementors may be tempted to extend RADIUS to permit more than 256
   outstanding packets on one connection.  However, doing so is a
   violation of a fundamental part of the protocol and MUST NOT be done.
   Making that extension here is outside of the scope of this

2.6.6.  EAP Sessions

   When RADIUS clients send EAP requests using RADIUS/TCP, they SHOULD
   choose the same TCP connection for all packets related to one EAP
   session.  This practice ensures that EAP packets are transmitted in
   order, and that problems with any one TCP connection do affect the
   minimum number of EAP sessions.

   A simple method that may work in many situations is to hash the
   contents of the Calling-Station-Id attribute, which normally contains
   the MAC address.  The output of that hash can be used to select a
   particular TCP connection.

   However, EAP packets for one EAP session can still be transported
   from client to server over multiple paths.  Therefore, when a server
   receives a RADIUS request containing an EAP request, it MUST be
   processed without considering the transport protocol.  For TCP
   transport, it MUST be processed without considering the source port.
   The algorithm suggested in [RFC5080] Section 2.1.1 SHOULD be used to
   track EAP sessions, as it is independent of source port and transport

   The retransmission requirements of Section 2.6.1, above, MUST be
   applied to RADIUS encapsulated EAP packets.  That is, EAP
   retransmissions MUST NOT result in retransmissions of RADIUS packets
   over a particular TCP connection.  EAP retransmissions MAY result in
   retransmission of RADIUS packets over a different TCP connection, but
   only when the previous TCP connection is marked DOWN.

DeKok, Alan                   Experimental                     [Page 14]
INTERNET-DRAFT               RADIUS Over TCP             12 October 2010

2.6.7.  TCP Applications are not UDP Applications

   Implementors should be aware that programming a robust TCP
   application can be very different from programming a robust UDP
   application.  It is RECOMMENDED that implementors of this
   specification familiarize themselves with TCP application programming

   Clients and servers SHOULD implement configurable connection limits.
   Clients and servers SHOULD implement configurable rate limiting on
   new connections.  Allowing an unbounded number or rate of TCP
   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/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/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.

   Implementors should consult [RADIUS/TLS] for issues related the
   security of RADIUS/TLS, and [RFC5246] for issues related to the
   security of the TLS protocol.

   Since "bare" TCP does not provide for confidentiality or enable
   negotiation of credible ciphersuites, its use is not appropriate for
   inter-server communications where strong security is required.  As a
   result "bare" TCP transport MUST NOT be used without TLS, IPsec, or
   other secure upper layer.

DeKok, Alan                   Experimental                     [Page 15]
INTERNET-DRAFT               RADIUS Over TCP             12 October 2010

   There are no (at this time) other known security issues for RADIUS
   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

[RFC3539] Aboba, B. et al., "Authentication, Authorization and
          Accounting (AAA) Transport Profile", RFC 3539, June 2003.

          Winter, S. et. al., "TLS encryption for RADIUS over TCP
          (RadSec)", draft-ietf-radext-radsec-07.txt, July 2010 (work in

[RFC5997] DeKok, A., "Use of Status-Server Packets in the Remote
          Authentication Dial In User Service (RADIUS) Protocol", RFC
          5997, August, 2010.

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.

[RFC4301] Kent, S. and R. Atkinson, "Security Architecture for the
          Internet Protocol", RFC 4301, December, 2005.

[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.

DeKok, Alan                   Experimental                     [Page 16]
INTERNET-DRAFT               RADIUS Over TCP             12 October 2010

[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,
          January 2008.

[RFC5216] Simon, D., etc al., "The EAP-TLS Authentication Protocol", RFC
          5216, March 2008.

[RFC5246] Dierks, T., Rescorla, E., "The Transport Layer Security (TLS)
          Protocol Version 1.2", RFC 5246, August 2008.


   None at this time.

Authors' Addresses

   Alan DeKok
   The FreeRADIUS Server Project


DeKok, Alan                   Experimental                     [Page 17]
INTERNET-DRAFT               RADIUS Over TCP             12 October 2010

Open issues

   Open issues relating to this document are tracked on the following
   web site:

DeKok, Alan                   Experimental                     [Page 18]