RADIUS over TCP
RFC 6613

Internet Engineering Task Force (IETF)                          A. DeKok
Request for Comments: 6613                                    FreeRADIUS
Category: Experimental                                          May 2012
ISSN: 2070-1721

                            RADIUS over TCP

Abstract

   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
   confidentiality and security.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for examination, experimental implementation, and
   evaluation.

   This document defines an Experimental Protocol for the Internet
   community.  This document is a product of the Internet Engineering
   Task Force (IETF).  It represents the consensus of the IETF
   community.  It has received public review and has been approved for
   publication by the Internet Engineering Steering Group (IESG).  Not
   all documents approved by the IESG are a candidate for any level of
   Internet Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6613.

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Copyright Notice

   Copyright (c) 2012 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
   (http://trustee.ietf.org/license-info) 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.

Table of Contents

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

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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 retransmissions, 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 deployed.

      *  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 request.

      *  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, is defined in [RFC6614].  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
   another secure upper layer.

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   However, "bare" TCP transport MAY 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 the Standards Track.

1.1.  Applicability of Reliable Transport

   The intent of this document is to address transport issues related to
   RADIUS/TLS [RFC6614] 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 piggybacking.  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 the Extensible Authentication Protocol (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 8 KB, 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-octet application-layer payload
   can be used alongside RADIUS/TLS.  Since most TCP implementations
   support MTU discovery, the TCP Maximum Segment Size (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 that do
   not perform Path MTU discovery can already support arbitrary
   payloads.

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   Where the MTU for EAP packets is large, RADIUS/EAP traffic required
   for an EAP-TLS authentication with 8-KB certificate chains may be
   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 (see Section 1.2) implementations have exhibited poor
   retransmission behavior.  Some implementations retransmit packets,
   others do not, and others send new packets rather than 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 likelihood 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 Network Access
   Server (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
   Round-Trip Time (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.

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1.2.  Terminology

   This document uses the following terms:

   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.  For
      example, 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.  For example, Access-Accept, Access-
      Reject, Access-Challenge, Accounting-Response, or CoA-ACK.

   RADIUS/UDP
      RADIUS over UDP, as defined in [RFC2865].

   RADIUS/TCP
      RADIUS over TCP, as defined in this document.

   RADIUS/TLS
      RADIUS over TLS, as defined in [RFC6614].

1.3.  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

   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.

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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 in Section 2.6, of (IP address, port,
   transport protocol).

   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
   below:

      * 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
   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/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
   RADIUS/TLS.

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 reuse 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 they 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".

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   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 the connection is still active,
   clients 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
   proactively 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 they 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.

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   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 in Section 2.6.4.

   RADIUS/TLS implementations MUST support receiving RADIUS packets over
   both UDP and TCP transports originating from the same endpoint.
   RADIUS packets received over UDP MUST be replied to over UDP; RADIUS
   packets received over TCP MUST be replied to over TCP.  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 failover 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 failover, load-balancing,
   etc.

   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
   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 the processing of any
   requests associated with that TCP connection.  No response to these
   requests can be sent over the TCP connection, so any further

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   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
   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 retransmitted 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 failover 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 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, at the start of this section (i.e.,
   IP address, port, transport protocol).

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2.6.4.  Malformed Packets and Unknown Clients

   The RADIUS specifications ([RFC2865], and many others) 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.

   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".  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"
   upon receipt:

      *  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 they 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.

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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
   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
   connection.

   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
   specification.

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 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 Media Access Control (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 the source port and
   transport protocol.

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   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.

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
   concepts.

   Clients and servers SHOULD implement configurable connection limits.
   Clients and servers SHOULD implement configurable limits on
     connection lifetime and idle timeouts.  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.  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 [RFC6614] for issues related to the
   security of RADIUS/TLS, and [RFC5246] for issues related to the
   security of the TLS protocol.

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RFC 6613                     RADIUS over TCP                    May 2012

   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
   another secure upper layer.

   There are no (at this time) other known security issues for RADIUS-
   over-TCP transport.

5.  References

5.1.  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. and J. Wood, "Authentication, Authorization
                and Accounting (AAA) Transport Profile", RFC 3539, June
                2003.

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

   [RFC6614]    Winter, S., McCauley, M., Venaas, S., and K. Wierenga,
                "Transport Layer Security (TLS) Encryption for RADIUS",
                RFC 6614, May 2012.

5.2.  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 K. Seo, "Security Architecture for the
                Internet Protocol", RFC 4301, December 2005.

   [RFC4668]    Nelson, D., "RADIUS Authentication Client MIB for IPv6",
                RFC 4668, August 2006.

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RFC 6613                     RADIUS over TCP                    May 2012

   [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]    De Cnodder, S., Jonnala, N., and M. Chiba, "RADIUS
                Dynamic Authorization Client MIB", RFC 4672, September
                2006.

   [RFC4673]    De Cnodder, S., Jonnala, N., and M. Chiba, "RADIUS
                Dynamic Authorization Server MIB", RFC 4673, September
                2006.

   [RFC5080]    Nelson, D. and A. DeKok, "Common Remote Authentication
                Dial In User Service (RADIUS) Implementation Issues and
                Suggested Fixes", RFC 5080, December 2007.

   [RFC5176]    Chiba, M., Dommety, G., Eklund, M., Mitton, D., and B.
                Aboba, "Dynamic Authorization Extensions to Remote
                Authentication Dial In User Service (RADIUS)", RFC 5176,
                January 2008.

   [RFC5216]    Simon, D., Aboba, B., and R. Hurst, "The EAP-TLS
                Authentication Protocol", RFC 5216, March 2008.

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

Author's Address

   Alan DeKok
   The FreeRADIUS Server Project
   http://freeradius.org/

   EMail: aland@freeradius.org

DeKok                         Experimental                     [Page 16]