RADIUS Fragmentation: Support of Fragmentation of RADIUS Packets

Internet Engineering Task Force (IETF)              A. Perez-Mendez, Ed.
Request for Comments: 7499                                R. Marin-Lopez
Category: Experimental                              F. Pereniguez-Garcia
ISSN: 2070-1721                                          G. Lopez-Millan
                                                    University of Murcia
                                                                D. Lopez
                                                          Telefonica I+D
                                                                A. DeKok
                                                          Network RADIUS
                                                              April 2015


               Support of Fragmentation of RADIUS Packets

Abstract

   The Remote Authentication Dial-In User Service (RADIUS) protocol is
   limited to a total packet size of 4096 bytes.  Provisions exist for
   fragmenting large amounts of authentication data across multiple
   packets, via Access-Challenge packets.  No similar provisions exist
   for fragmenting large amounts of authorization data.  This document
   specifies how existing RADIUS mechanisms can be leveraged to provide
   that functionality.  These mechanisms are largely compatible with
   existing implementations, and they are designed to be invisible to
   proxies and "fail-safe" to legacy RADIUS Clients and Servers.

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/rfc7499.

Copyright Notice

   Copyright (c) 2015 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   described in the Simplified BSD License.

Table of Contents

   1. Introduction ....................................................4
      1.1. Requirements Language ......................................6
   2. Status of This Document .........................................6
   3. Scope of This Document ..........................................7
   4. Overview .......................................................10
   5. Fragmentation of Packets .......................................13
      5.1. Pre-Authorization .........................................14
      5.2. Post-Authorization ........................................18
   6. Chunk Size .....................................................21
   7. Allowed Large Packet Size ......................................22
   8. Handling Special Attributes ....................................23
      8.1. Proxy-State Attribute .....................................23
      8.2. State Attribute ...........................................24
      8.3. Service-Type Attribute ....................................25
      8.4. Rebuilding the Original Large Packet ......................25
   9. New T Flag for the Long Extended Type Attribute Definition .....26
   10. New Attribute Definition ......................................26
      10.1. Frag-Status Attribute ....................................27
      10.2. Proxy-State-Length Attribute .............................28
      10.3. Table of Attributes ......................................29
   11. Operation with Proxies ........................................29
      11.1. Legacy Proxies ...........................................29
      11.2. Updated Proxies ..........................................29
   12. General Considerations ........................................31
      12.1. T Flag ...................................................31
      12.2. Violation of RFC 2865 ....................................32
      12.3. Proxying Based on User-Name ..............................32
      12.4. Transport Behavior .......................................33
   13. Security Considerations .......................................33
   14. IANA Considerations ...........................................34
   15. References ....................................................35
      15.1. Normative References .....................................35
      15.2. Informative References ...................................35
   Acknowledgements ..................................................37
   Authors' Addresses ................................................37

1.  Introduction

   The RADIUS [RFC2865] protocol carries authentication, authorization,
   and accounting information between a RADIUS Client and a RADIUS
   Server.  Information is exchanged between them through RADIUS
   packets.  Each RADIUS packet is composed of a header, and zero or
   more attributes, up to a maximum packet size of 4096 bytes.  The
   protocol is a request/response protocol, as described in the
   operational model ([RFC6158], Section 3.1).

   The intention of the above packet size limitation was to avoid UDP
   fragmentation as much as possible.  Back then, a size of 4096 bytes
   seemed large enough for any purpose.  Now, new scenarios are emerging
   that require the exchange of authorization information exceeding this
   4096-byte limit.  For instance, the Application Bridging for
   Federated Access Beyond web (ABFAB) IETF working group defines the
   transport of Security Assertion Markup Language (SAML) statements
   from the RADIUS Server to the RADIUS Client [SAML-RADIUS].  This
   assertion is likely to be larger than 4096 bytes.

   This means that peers desiring to send large amounts of data must
   fragment it across multiple packets.  For example, RADIUS-EAP
   [RFC3579] defines how an Extensible Authentication Protocol (EAP)
   exchange occurs across multiple Access-Request / Access-Challenge
   sequences.  No such exchange is possible for accounting or
   authorization data.  [RFC6158], Section 3.1 suggests that exchanging
   large amounts of authorization data is unnecessary in RADIUS.
   Instead, the data should be referenced by name.  This requirement
   allows large policies to be pre-provisioned and then referenced in an
   Access-Accept.  In some cases, however, the authorization data sent
   by the RADIUS Server is large and highly dynamic.  In other cases,
   the RADIUS Client needs to send large amounts of authorization data
   to the RADIUS Server.  Neither of these cases is met by the
   requirements in [RFC6158].  As noted in that document, the practical
   limit on RADIUS packet sizes is governed by the Path MTU (PMTU),
   which may be significantly smaller than 4096 bytes.  The combination
   of the two limitations means that there is a pressing need for a
   method to send large amounts of authorization data between RADIUS
   Client and Server, with no accompanying solution.

   [RFC6158], Section 3.1 recommends three approaches for the
   transmission of large amounts of data within RADIUS.  However, they
   are not applicable to the problem statement of this document for the
   following reasons:

   o  The first approach (utilization of a sequence of packets) does not
      talk about large amounts of data sent from the RADIUS Client to a
      RADIUS Server.  Leveraging EAP (request/challenge) to send the
      data is not feasible, as EAP already fills packets to PMTU, and
      not all authentications use EAP.  Moreover, as noted for the
      NAS-Filter-Rule attribute ([RFC4849]), this approach does not
      entirely solve the problem of sending large amounts of data from a
      RADIUS Server to a RADIUS Client, as many current RADIUS
      attributes are not permitted in Access-Challenge packets.

   o  The second approach (utilization of names rather than values) is
      not usable either, as using names rather than values is difficult
      when the nature of the data to be sent is highly dynamic (e.g., a
      SAML statement or NAS-Filter-Rule attributes).  URLs could be used
      as a pointer to the location of the actual data, but their use
      would require them to be (a) dynamically created and modified,
      (b) securely accessed, and (c) accessible from remote systems.
      Satisfying these constraints would require the modification of
      several networking systems (e.g., firewalls and web servers).
      Furthermore, the setup of an additional trust infrastructure
      (e.g., Public Key Infrastructure (PKI)) would be required to allow
      secure retrieval of the information from the web server.

   o  PMTU discovery does not solve the problem, as it does not allow
      the sending of data larger than the minimum of (PMTU or 4096)
      bytes.

   This document provides a mechanism to allow RADIUS peers to exchange
   large amounts of authorization data exceeding the 4096-byte limit by
   fragmenting it across several exchanges.  The proposed solution does
   not impose any additional requirements to the RADIUS system
   administrators (e.g., need to modify firewall rules, set up web
   servers, configure routers, or modify any application server).  It
   maintains compatibility with intra-packet fragmentation mechanisms
   (like those defined in [RFC3579] or [RFC6929]).  It is also
   transparent to existing RADIUS proxies, which do not implement this
   specification.  The only systems needing to implement this RFC are
   the ones that either generate or consume the fragmented data being
   transmitted.  Intermediate proxies just pass the packets without
   changes.  Nevertheless, if a proxy supports this specification, it
   may reassemble the data in order to examine and/or modify it.

   A different approach to deal with RADIUS packets above the 4096-byte
   limit is described in [RADIUS-Larger-Pkts], which proposes to extend
   RADIUS over TCP by allowing the Length field in the RADIUS header to
   take values up to 65535 bytes.  This provides a simpler operation,
   but it has the drawback of requiring every RADIUS proxy in the path
   between the RADIUS Client and the RADIUS Server to implement the
   extension as well.

1.1.  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 RFC 2119 [RFC2119].
   When these words appear in lower case, they have their natural
   language meaning.

2.  Status of This Document

   This document is an Experimental RFC.  It defines a proposal to allow
   the sending and receiving of data exceeding the 4096-byte limit in
   RADIUS packets imposed by [RFC2865], without requiring the
   modification of intermediary proxies.

   The experiment consists of verifying whether the approach is usable
   in a large-scale environment, by observing the uptake, usability, and
   operational behavior it shows in large-scale, real-life deployments.
   In that sense, so far the main use case for this specification is the
   transportation of large SAML statements defined within the ABFAB
   architecture [ABFAB-Arch].  Hence, it can be tested wherever an ABFAB
   deployment is being piloted.

   Besides, this proposal defines some experimental features that will
   need to be tested and verified before the document can be considered
   for the Standards Track.  The first one of them is the requirement of
   updating [RFC2865] in order to relax the sentence defined in
   Section 4.1 of that document that states that "An Access-Request MUST
   contain either a User-Password or a CHAP-Password or a State."  This
   specification might generate Access-Request packets without any of
   these attributes.  Although all known implementations have chosen the
   philosophy of "be liberal in what you accept," we need to gain more
   operational experience to verify that unmodified proxies do not drop
   these types of packets.  More details on this aspect can be found in
   Section 12.2.

   Another experimental feature of this specification is that it
   requires proxies to base their routing decisions on the value of the
   RADIUS User-Name attribute.  Our experience is that this is the
   common behavior; thus, no issues are expected.  However, it needs to
   be confirmed after using different implementations of intermediate
   proxies.  More details on this aspect can be found in Section 12.3.

   Moreover, this document requires two minor updates to Standards Track
   documents.  First, it modifies the definition of the Reserved field
   of the Long Extended Type attribute [RFC6929] by allocating an
   additional flag called the T (Truncation) flag.  No issues are
   expected with this update, although some proxies might drop packets
   that do not have the Reserved field set to 0.  More details on this
   aspect can be found in Section 12.1.

   The other Standards Track document that requires a minor update is
   [RFC6158].  It states that "attribute designers SHOULD NOT assume
   that a RADIUS implementation can successfully process RADIUS packets
   larger than 4096 bytes," something no longer true if this document
   advances.

   A proper "Updates" clause will be included for these modifications
   when/if the experiment is successful and this document is reissued as
   a Standards Track document.

3.  Scope of This Document

   This specification describes how a RADIUS Client and a RADIUS Server
   can exchange data exceeding the 4096-byte limit imposed by one
   packet.  However, the mechanism described in this specification
   SHOULD NOT be used to exchange more than 100 kilobytes of data.  Any
   more than this may turn RADIUS into a generic transport protocol,
   such as TCP or the Stream Control Transmission Protocol (SCTP), which
   is undesirable.  Experience shows that attempts to transport bulk
   data across the Internet with UDP will inevitably fail, unless these
   transport attempts reimplement all of the behavior of TCP.  The
   underlying design of RADIUS lacks the proper retransmission policies
   or congestion control mechanisms that would make it a competitor
   of TCP.

   Therefore, RADIUS/UDP transport is by design unable to transport bulk
   data.  It is both undesirable and impossible to change the protocol
   at this point in time.  This specification is intended to allow the
   transport of more than 4096 bytes of data through existing RADIUS/UDP
   proxies.  Other solutions such as RADIUS/TCP MUST be used when a
   "green field" deployment requires the transport of bulk data.

   Section 7, below, describes in further detail what is considered to
   be a reasonable amount of data and recommends that administrators
   adjust limitations on data transfer according to the specific
   capabilities of their existing systems in terms of memory and
   processing power.

   Moreover, its scope is limited to the exchange of authorization data,
   as other exchanges do not require such a mechanism.  In particular,
   authentication exchanges have already been defined to overcome this
   limitation (e.g., RADIUS-EAP).  Moreover, as they represent the most
   critical part of a RADIUS conversation, it is preferable to not
   introduce into their operation any modification that may affect
   existing equipment.

   There is no need to fragment accounting packets either.  While the
   accounting process can send large amounts of data, that data is
   typically composed of many small updates.  That is, there is no
   demonstrated need to send indivisible blocks of more than 4 kilobytes
   of data.  The need to send large amounts of data per user session
   often originates from the need for flow-based accounting.  In this
   use case, the RADIUS Client may send accounting data for many
   thousands of flows, where all those flows are tied to one user
   session.  The existing Acct-Multi-Session-Id attribute defined in
   [RFC2866], Section 5.11 has been proven to work here.

   Similarly, there is no need to fragment Change-of-Authorization (CoA)
   [RFC5176] packets.  Instead, according to [RFC5176], the CoA client
   will send a CoA-Request packet containing session identification
   attributes, along with Service-Type = Additional-Authorization, and a
   State attribute.  Implementations not supporting fragmentation will
   respond with a CoA-NAK and an Error-Cause of Unsupported-Service.

   The above requirement does not assume that the CoA client and the
   RADIUS Server are co-located.  They may, in fact, be run on separate
   parts of the infrastructure, or even by separate administrators.
   There is, however, a requirement that the two communicate.  We can
   see that the CoA client needs to send session identification
   attributes in order to send CoA packets.  These attributes cannot be
   known a priori by the CoA client and can only come from the RADIUS
   Server.  Therefore, even when the two systems are not co-located,
   they must be able to communicate in order to operate in unison.  The
   alternative is for the two systems to have differing views of the
   users' authorization parameters; such a scenario would be a security
   disaster.

   This specification does not allow for fragmentation of CoA packets.
   Allowing for fragmented CoA packets would involve changing multiple
   parts of the RADIUS protocol; such changes introduce the risk of
   implementation issues, mistakes, etc.

   Where CoA clients (i.e., RADIUS Servers) need to send large amounts
   of authorization data to a CoA server (i.e., RADIUS Client), they
   need only send a minimal CoA-Request packet containing a Service-Type
   of Authorize Only, as per [RFC5176], along with session
   identification attributes.  This CoA packet serves as a signal to the
   RADIUS Client that the users' session requires re-authorization.
   When the RADIUS Client re-authorizes the user via Access-Request, the
   RADIUS Server can perform fragmentation and send large amounts of
   authorization data to the RADIUS Client.

   The assumption in the above scenario is that the CoA client and
   RADIUS Server are co-located, or at least strongly coupled.  That is,
   the path from CoA client to CoA server SHOULD be the exact reverse of
   the path from RADIUS Client to RADIUS Server.  The following diagram
   will hopefully clarify the roles:

                              +----------------+
                              | RADIUS   CoA   |
                              | Client  Server |
                              +----------------+
                                 |        ^
                 Access-Request  |        |   CoA-Request
                                 v        |
                              +----------------+
                              | RADIUS   CoA   |
                              | Server  Client |
                              +----------------+

   Where there is a proxy involved:

                              +----------------+
                              | RADIUS   CoA   |
                              | Client  Server |
                              +----------------+
                                 |        ^
                 Access-Request  |        |   CoA-Request
                                 v        |
                              +----------------+
                              | RADIUS   CoA   |
                              | Proxy   Proxy  |
                              +----------------+
                                 |        ^
                 Access-Request  |        |   CoA-Request
                                 v        |
                              +----------------+
                              | RADIUS   CoA   |
                              | Server  Client |
                              +----------------+

   That is, the RADIUS and CoA subsystems at each hop are strongly
   connected.  Where they are not strongly connected, it will be
   impossible to use CoA-Request packets to transport large amounts of
   authorization data.

   This design is more complicated than allowing for fragmented CoA
   packets.  However, the CoA client and the RADIUS Server must
   communicate even when not using this specification.  We believe that
   standardizing that communication and using one method for exchange of
   large data are preferred to unspecified communication methods and
   multiple ways of achieving the same result.  If we were to allow
   fragmentation of data over CoA packets, the size and complexity of
   this specification would increase significantly.

   The above requirement solves a number of issues.  It clearly
   separates session identification from authorization.  Without this
   separation, it is difficult to both identify a session and change its
   authorization using the same attribute.  It also ensures that the
   authorization process is the same for initial authentication and
   for CoA.

4.  Overview

   Authorization exchanges can occur either before or after end-user
   authentication has been completed.  An authorization exchange before
   authentication allows a RADIUS Client to provide the RADIUS Server
   with information that MAY modify how the authentication process will
   be performed (e.g., it may affect the selection of the EAP method).
   An authorization exchange after authentication allows the RADIUS
   Server to provide the RADIUS Client with information about the end
   user, the results of the authentication process, and/or obligations
   to be enforced.  In this specification, we refer to
   "pre-authorization" as the exchange of authorization information
   before the end-user authentication has started (from the RADIUS
   Client to the RADIUS Server), whereas the term "post-authorization"
   is used to refer to an authorization exchange happening after this
   authentication process (from the RADIUS Server to the RADIUS Client).

   In this specification, we refer to the "size limit" as the practical
   limit on RADIUS packet sizes.  This limit is the minimum between
   4096 bytes and the current PMTU.  We define below a method that uses
   Access-Request and Access-Accept in order to exchange fragmented
   data.  The RADIUS Client and Server exchange a series of
   Access-Request / Access-Accept packets, until such time as all of the
   fragmented data has been transported.  Each packet contains a
   Frag-Status attribute, which lets the other party know if
   fragmentation is desired, ongoing, or finished.  Each packet may also
   contain the fragmented data or may instead be an "ACK" to a previous
   fragment from the other party.  Each Access-Request contains a
   User-Name attribute, allowing the packet to be proxied if necessary
   (see Section 11.1).  Each Access-Request may also contain a State
   attribute, which serves to tie it to a previous Access-Accept.  Each
   Access-Accept contains a State attribute, for use by the RADIUS
   Client in a later Access-Request.  Each Access-Accept contains a
   Service-Type attribute with the "Additional-Authorization" value.
   This indicates that the service being provided is part of a
   fragmented exchange and that the Access-Accept should not be
   interpreted as providing network access to the end user.

   When a RADIUS Client or RADIUS Server needs to send data that exceeds
   the size limit, the mechanism proposed in this document is used.
   Instead of encoding one large RADIUS packet, a series of smaller
   RADIUS packets of the same type are encoded.  Each smaller packet is
   called a "chunk" in this specification, in order to distinguish it
   from traditional RADIUS packets.  The encoding process is a simple
   linear walk over the attributes to be encoded.  This walk preserves
   the order of the attributes of the same type, as required by
   [RFC2865].  The number of attributes encoded in a particular chunk
   depends on the size limit, the size of each attribute, the number of
   proxies between the RADIUS Client and RADIUS Server, and the overhead
   for fragmentation-signaling attributes.  Specific details are given
   in Section 6.  A new attribute called Frag-Status (Section 10.1)
   signals the fragmentation status.

   After the first chunk is encoded, it is sent to the other party.  The
   packet is identified as a chunk via the Frag-Status attribute.  The
   other party then requests additional chunks, again using the
   Frag-Status attribute.  This process is repeated until all the
   attributes have been sent from one party to the other.  When all the
   chunks have been received, the original list of attributes is
   reconstructed and processed as if it had been received in one packet.

   The reconstruction process is performed by simply appending all of
   the chunks together.  Unlike IPv4 fragmentation, there is no Fragment
   Offset field.  The chunks in this specification are explicitly
   ordered, as RADIUS is a lock-step protocol, as noted in Section 12.4.
   That is, chunk N+1 cannot be sent until all of the chunks up to and
   including N have been received and acknowledged.

   When multiple chunks are sent, a special situation may occur for Long
   Extended Type attributes as defined in [RFC6929].  The fragmentation
   process may split a fragmented attribute across two or more chunks,
   which is not permitted by that specification.  We address this issue
   by using the newly defined T flag in the Reserved field of the Long
   Extended Type attribute format (see Section 9 for further details on
   this flag).

   This last situation is expected to be the most common occurrence in
   chunks.  Typically, packet fragmentation will occur as a consequence
   of a desire to send one or more large (and therefore fragmented)
   attributes.  The large attribute will likely be split into two or
   more pieces.  Where chunking does not split a fragmented attribute,
   no special treatment is necessary.

   The setting of the T flag is the only case where the chunking process
   affects the content of an attribute.  Even then, the Value fields of
   all attributes remain unchanged.  Any per-packet security attributes,
   such as Message-Authenticator, are calculated for each chunk
   independently.  Neither integrity checks nor security checks are
   performed on the "original" packet.

   Each RADIUS packet sent or received as part of the chunking process
   MUST be a valid packet, subject to all format and security
   requirements.  This requirement ensures that a "transparent" proxy
   not implementing this specification can receive and send compliant
   packets.  That is, a proxy that simply forwards packets without
   detailed examination or any modification will be able to proxy
   "chunks".

5.  Fragmentation of Packets

   When the RADIUS Client or the RADIUS Server desires to send a packet
   that exceeds the size limit, it is split into chunks and sent via
   multiple client/server exchanges.  The exchange is indicated via the
   Frag-Status attribute, which has value More-Data-Pending for all but
   the last chunk of the series.  The chunks are tied together via the
   State attribute.

   The delivery of a large fragmented RADIUS packet with authorization
   data can happen before or after the end user has been authenticated
   by the RADIUS Server.  We can distinguish two phases, which can be
   omitted if there is no authorization data to be sent:

   1.  Pre-authorization.  In this phase, the RADIUS Client MAY send a
       large packet with authorization information to the RADIUS Server
       before the end user is authenticated.  Only the RADIUS Client is
       allowed to send authorization data during this phase.

   2.  Post-authorization.  In this phase, the RADIUS Server MAY send a
       large packet with authorization data to the RADIUS Client after
       the end user has been authenticated.  Only the RADIUS Server is
       allowed to send authorization data during this phase.

   The following subsections describe how to perform fragmentation for
   packets for these two phases.  We give the packet type, along with a
   RADIUS Identifier, to indicate that requests and responses are
   connected.  We then give a list of attributes.  We do not give values
   for most attributes, as we wish to concentrate on the fragmentation
   behavior rather than packet contents.  Attribute values are given for
   attributes relevant to the fragmentation process.  Where "long
   extended" attributes are used, we indicate the M (More) and T
   (Truncation) flags as optional square brackets after the attribute
   name.  As no "long extended" attributes have yet been defined, we use
   example attributes, named as "Example-Long-1", etc.  For the sake of
   simplicity, the maximum chunk size is established in terms of the
   number of attributes (11).

5.1.  Pre-Authorization

   When the RADIUS Client needs to send a large amount of data to the
   RADIUS Server, the data to be sent is split into chunks and sent to
   the RADIUS Server via multiple Access-Request / Access-Accept
   exchanges.  The example below shows this exchange.

   The following is an Access-Request that the RADIUS Client intends to
   send to a RADIUS Server.  However, due to a combination of issues
   (PMTU, large attributes, etc.), the content does not fit into one
   Access-Request packet.

   Access-Request
       User-Name
       NAS-Identifier
       Calling-Station-Id
       Example-Long-1 [M]
       Example-Long-1 [M]
       Example-Long-1 [M]
       Example-Long-1 [M]
       Example-Long-1 [M]
       Example-Long-1 [M]
       Example-Long-1 [M]
       Example-Long-1 [M]
       Example-Long-1
       Example-Long-2 [M]
       Example-Long-2 [M]
       Example-Long-2

                     Figure 1: Desired Access-Request

   The RADIUS Client therefore must send the attributes listed above in
   a series of chunks.  The first chunk contains eight (8) attributes
   from the original Access-Request, and a Frag-Status attribute.  Since
   the last attribute is "Example-Long-1" with the M flag set, the
   chunking process also sets the T flag in that attribute.  The
   Access-Request is sent with a RADIUS Identifier field having
   value 23.  The Frag-Status attribute has value More-Data-Pending, to
   indicate that the RADIUS Client wishes to send more data in a
   subsequent Access-Request.  The RADIUS Client also adds a
   Service-Type attribute, which indicates that it is part of the
   chunking process.  The packet is signed with the
   Message-Authenticator attribute, completing the maximum number of
   attributes (11).

   Access-Request (ID = 23)
       User-Name
       NAS-Identifier
       Calling-Station-Id
       Example-Long-1 [M]
       Example-Long-1 [M]
       Example-Long-1 [M]
       Example-Long-1 [M]
       Example-Long-1 [MT]
       Frag-Status = More-Data-Pending
       Service-Type = Additional-Authorization
       Message-Authenticator

                    Figure 2: Access-Request (Chunk 1)

   Compliant RADIUS Servers (i.e., servers implementing fragmentation)
   receiving this packet will see the Frag-Status attribute and will
   postpone all authorization and authentication handling until all of
   the chunks have been received.  This postponement also applies to the
   verification that the Access-Request packet contains some kind of
   authentication attribute (e.g., User-Password, CHAP-Password, State,
   or other future attribute), as required by [RFC2865] (see
   Section 12.2 for more information on this).

   Non-compliant RADIUS Servers (i.e., servers not implementing
   fragmentation) should also see the Service-Type requesting
   provisioning for an unknown service and return Access-Reject.  Other
   non-compliant RADIUS Servers may return an Access-Reject or
   Access-Challenge, or they may return an Access-Accept with a
   particular Service-Type other than Additional-Authorization.
   Compliant RADIUS Client implementations MUST treat these responses as
   if they had received Access-Reject instead.

   Compliant RADIUS Servers who wish to receive all of the chunks will
   respond with the following packet.  The value of the State here is
   arbitrary and serves only as a unique token for example purposes.  We
   only note that it MUST be temporally unique to the RADIUS Server.

   Access-Accept (ID = 23)
       Frag-Status = More-Data-Request
       Service-Type = Additional-Authorization
       State = 0xabc00001
       Message-Authenticator

                     Figure 3: Access-Accept (Chunk 1)

   The RADIUS Client will see this response and use the RADIUS
   Identifier field to associate it with an ongoing chunking session.
   Compliant RADIUS Clients will then continue the chunking process.
   Non-compliant RADIUS Clients will never see a response such as this,
   as they will never send a Frag-Status attribute.  The Service-Type
   attribute is included in the Access-Accept in order to signal that
   the response is part of the chunking process.  This packet therefore
   does not provision any network service for the end user.

   The RADIUS Client continues the process by sending the next chunk,
   which includes an additional six (6) attributes from the original
   packet.  It again includes the User-Name attribute, so that
   non-compliant proxies can process the packet (see Section 11.1).  It
   sets the Frag-Status attribute to More-Data-Pending, as more data is
   pending.  It includes a Service-Type, for the reasons described
   above.  It includes the State attribute from the previous
   Access-Accept.  It signs the packet with Message-Authenticator, as
   there are no authentication attributes in the packet.  It uses a new
   RADIUS Identifier field.

   Access-Request (ID = 181)
       User-Name
       Example-Long-1 [M]
       Example-Long-1 [M]
       Example-Long-1 [M]
       Example-Long-1
       Example-Long-2 [M]
       Example-Long-2 [MT]
       Frag-Status = More-Data-Pending
       Service-Type = Additional-Authorization
       State = 0xabc000001
       Message-Authenticator

                    Figure 4: Access-Request (Chunk 2)

   Compliant RADIUS Servers receiving this packet will see the
   Frag-Status attribute and look for a State attribute.  Since one
   exists and it matches a State sent in an Access-Accept, this packet
   is part of a chunking process.  The RADIUS Server will associate the
   attributes with the previous chunk.  Since the Frag-Status attribute
   has value More-Data-Request, the RADIUS Server will respond with an
   Access-Accept as before.  It MUST include a State attribute, with a
   value different from the previous Access-Accept.  This State MUST
   again be globally and temporally unique.

   Access-Accept (ID = 181)
       Frag-Status = More-Data-Request
       Service-Type = Additional-Authorization
       State = 0xdef00002
       Message-Authenticator

                     Figure 5: Access-Accept (Chunk 2)

   The RADIUS Client will see this response and use the RADIUS
   Identifier field to associate it with an ongoing chunking session.
   The RADIUS Client continues the chunking process by sending the next
   chunk, with the final attribute(s) from the original packet, and
   again includes the original User-Name attribute.  The Frag-Status
   attribute is not included in the next Access-Request, as no more
   chunks are available for sending.  The RADIUS Client includes the
   State attribute from the previous Access-Accept.  It signs the packet
   with Message-Authenticator, as there are no authentication attributes
   in the packet.  It again uses a new RADIUS Identifier field.

   Access-Request (ID = 241)
       User-Name
       Example-Long-2
       State = 0xdef00002
       Message-Authenticator

                    Figure 6: Access-Request (Chunk 3)

   On reception of this last chunk, the RADIUS Server matches it with an
   ongoing session via the State attribute and sees that there is no
   Frag-Status attribute present.  It then processes the received
   attributes as if they had been sent in one RADIUS packet.  See
   Section 8.4 for further details on this process.  It generates the
   appropriate response, which can be either Access-Accept or
   Access-Reject.  In this example, we show an Access-Accept.  The
   RADIUS Server MUST send a State attribute, which allows linking the
   received data with the authentication process.

   Access-Accept (ID = 241)
       State = 0x98700003
       Message-Authenticator

                     Figure 7: Access-Accept (Chunk 3)

   The above example shows in practice how the chunking process works.
   We reiterate the implementation and security requirements here.

   Each chunk is a valid RADIUS packet (see Section 12.2 for some
   considerations about this), and all RADIUS format and security
   requirements MUST be followed before any chunking process is applied.

   Every chunk except for the last one from a RADIUS Client MUST include
   a Frag-Status attribute, with value More-Data-Pending.  The last
   chunk MUST NOT contain a Frag-Status attribute.  Each chunk except
   for the last one from a RADIUS Client MUST include a Service-Type
   attribute, with value Additional-Authorization.  Each chunk MUST
   include a User-Name attribute, which MUST be identical in all chunks.
   Each chunk except for the first one from a RADIUS Client MUST include
   a State attribute, which MUST be copied from a previous
   Access-Accept.

   Each Access-Accept MUST include a State attribute.  The value for
   this attribute MUST change in every new Access-Accept and MUST be
   globally and temporally unique.

5.2.  Post-Authorization

   When the RADIUS Server wants to send a large amount of authorization
   data to the RADIUS Client after authentication, the operation is very
   similar to the pre-authorization process.  The presence of a
   Service-Type = Additional-Authorization attribute ensures that a
   RADIUS Client not supporting this specification will treat that
   unrecognized Service-Type as though an Access-Reject had been
   received instead ([RFC2865], Section 5.6).  If the original large
   Access-Accept packet contained a Service-Type attribute, it will be
   included with its original value in the last transmitted chunk, to
   avoid confusion with the one used for fragmentation signaling.  It is
   RECOMMENDED that RADIUS Servers include a State attribute in their
   original Access-Accept packets, even if fragmentation is not taking
   place, to allow the RADIUS Client to send additional authorization
   data in subsequent exchanges.  This State attribute would be included
   in the last transmitted chunk, to avoid confusion with the ones used
   for fragmentation signaling.

   Clients supporting this specification MUST include a Frag-Status =
   Fragmentation-Supported attribute in the first Access-Request sent to
   the RADIUS Server, in order to indicate that they would accept
   fragmented data from the server.  This is not required if the
   pre-authorization process was carried out, as it is implicit.

   The following is an Access-Accept that the RADIUS Server intends to
   send to a RADIUS Client.  However, due to a combination of issues
   (PMTU, large attributes, etc.), the content does not fit into one
   Access-Accept packet.

   Access-Accept
       User-Name
       EAP-Message
       Service-Type = Login
       Example-Long-1 [M]
       Example-Long-1 [M]
       Example-Long-1 [M]
       Example-Long-1 [M]
       Example-Long-1 [M]
       Example-Long-1 [M]
       Example-Long-1 [M]
       Example-Long-1 [M]
       Example-Long-1
       Example-Long-2 [M]
       Example-Long-2 [M]
       Example-Long-2
       State = 0xcba00003

                      Figure 8: Desired Access-Accept

   The RADIUS Server therefore must send the attributes listed above in
   a series of chunks.  The first chunk contains seven (7) attributes
   from the original Access-Accept, and a Frag-Status attribute.  Since
   the last attribute is "Example-Long-1" with the M flag set, the
   chunking process also sets the T flag in that attribute.  The
   Access-Accept is sent with a RADIUS Identifier field having value 30,
   corresponding to a previous Access-Request not depicted.  The
   Frag-Status attribute has value More-Data-Pending, to indicate that
   the RADIUS Server wishes to send more data in a subsequent
   Access-Accept.  The RADIUS Server also adds a Service-Type attribute
   with value Additional-Authorization, which indicates that it is part
   of the chunking process.  Note that the original Service-Type is not
   included in this chunk.  Finally, a State attribute is included to
   allow matching subsequent requests with this conversation, and the
   packet is signed with the Message-Authenticator attribute, completing
   the maximum number of attributes (11).

   Access-Accept (ID = 30)
       User-Name
       EAP-Message
       Example-Long-1 [M]
       Example-Long-1 [M]
       Example-Long-1 [M]
       Example-Long-1 [M]
       Example-Long-1 [MT]
       Frag-Status = More-Data-Pending
       Service-Type = Additional-Authorization
       State = 0xcba00004
       Message-Authenticator

                     Figure 9: Access-Accept (Chunk 1)

   Compliant RADIUS Clients receiving this packet will see the
   Frag-Status attribute and suspend all authorization handling until
   all of the chunks have been received.  Non-compliant RADIUS Clients
   should also see the Service-Type indicating the provisioning for an
   unknown service and will treat it as an Access-Reject.

   RADIUS Clients who wish to receive all of the chunks will respond
   with the following packet, where the value of the State attribute is
   taken from the received Access-Accept.  They will also include the
   User-Name attribute so that non-compliant proxies can process the
   packet (Section 11.1).

   Access-Request (ID = 131)
       User-Name
       Frag-Status = More-Data-Request
       Service-Type = Additional-Authorization
       State = 0xcba00004
       Message-Authenticator

                    Figure 10: Access-Request (Chunk 1)

   The RADIUS Server receives this request and uses the State attribute
   to associate it with an ongoing chunking session.  Compliant RADIUS
   Servers will then continue the chunking process.  Non-compliant
   RADIUS Servers will never see a response such as this, as they will
   never send a Frag-Status attribute.

   The RADIUS Server continues the chunking process by sending the next
   chunk, with the final attribute(s) from the original packet.  The
   value of the Identifier field is taken from the received
   Access-Request.  A Frag-Status attribute is not included in the next
   Access-Accept, as no more chunks are available for sending.  The
   RADIUS Server includes the original State attribute to allow the
   RADIUS Client to send additional authorization data.  The original
   Service-Type attribute is included as well.

   Access-Accept (ID = 131)
       Example-Long-1 [M]
       Example-Long-1 [M]
       Example-Long-1 [M]
       Example-Long-1
       Example-Long-2 [M]
       Example-Long-2 [M]
       Example-Long-2
       Service-Type = Login
       State = 0xfda000003
       Message-Authenticator

                    Figure 11: Access-Accept (Chunk 2)

   On reception of this last chunk, the RADIUS Client matches it with an
   ongoing session via the Identifier field and sees that there is no
   Frag-Status attribute present.  It then processes the received
   attributes as if they had been sent in one RADIUS packet.  See
   Section 8.4 for further details on this process.

6.  Chunk Size

   In an ideal scenario, each intermediate chunk would be exactly the
   size limit in length.  In this way, the number of round trips
   required to send a large packet would be optimal.  However, this is
   not possible for several reasons.

   1.  RADIUS attributes have a variable length and must be included
       completely in a chunk.  Thus, it is possible that, even if there
       is some free space in the chunk, it is not enough to include the
       next attribute.  This can generate up to 254 bytes of spare space
       in every chunk.

   2.  RADIUS fragmentation requires the introduction of some extra
       attributes for signaling.  Specifically, a Frag-Status attribute
       (7 bytes) is included in every chunk of a packet, except the last
       one.  A RADIUS State attribute (from 3 to 255 bytes) is also
       included in most chunks, to allow the RADIUS Server to bind an
       Access-Request with a previous Access-Challenge.  User-Name
       attributes (from 3 to 255 bytes) are included in every chunk the
       RADIUS Client sends, as they are required by the proxies to route
       the packet to its destination.  Together, these attributes can
       generate from up to 13 to 517 bytes of signaling data, reducing
       the amount of payload information that can be sent in each chunk.

   3.  RADIUS packets SHOULD be adjusted to avoid exceeding the network
       MTU.  Otherwise, IP fragmentation may occur, with undesirable
       consequences.  Hence, maximum chunk size would be decreased from
       4096 to the actual MTU of the network.

   4.  The inclusion of Proxy-State attributes by intermediary proxies
       can decrease the availability of usable space in the chunk.  This
       is described in further detail in Section 8.1.

7.  Allowed Large Packet Size

   There are no provisions for signaling how much data is to be sent via
   the fragmentation process as a whole.  It is difficult to define what
   is meant by the "length" of any fragmented data.  That data can be
   multiple attributes and can include RADIUS attribute header fields,
   or it can be one or more "large" attributes (more than 256 bytes in
   length).  Proxies can also filter these attributes, to modify, add,
   or delete them and their contents.  These proxies act on a "packet by
   packet" basis and cannot know what kind of filtering actions they
   will take on future packets.  As a result, it is impossible to signal
   any meaningful value for the total amount of additional data.

   Unauthenticated end users are permitted to trigger the exchange of
   large amounts of fragmented data between the RADIUS Client and the
   RADIUS Server, having the potential to allow denial-of-service (DoS)
   attacks.  An attacker could initiate a large number of connections,
   each of which requests the RADIUS Server to store a large amount of
   data.  This data could cause memory exhaustion on the RADIUS Server
   and result in authentic users being denied access.  It is worth
   noting that authentication mechanisms are already designed to avoid
   exceeding the size limit.

   Hence, implementations of this specification MUST limit the total
   amount of data they send and/or receive via this specification.  Its
   default value SHOULD be 100 kilobytes.  Any more than this may turn
   RADIUS into a generic transport protocol, which is undesirable.  This
   limit SHOULD be configurable, so that it can be changed if necessary.

   Implementations of this specification MUST limit the total number of
   round trips used during the fragmentation process.  Its default value
   SHOULD be 25.  Any more than this may indicate an implementation
   error, misconfiguration, or DoS attack.  This limit SHOULD be
   configurable, so that it can be changed if necessary.

   For instance, let's imagine that the RADIUS Server wants to transport
   a SAML assertion that is 15000 bytes long to the RADIUS Client.  In
   this hypothetical scenario, we assume that there are three
   intermediate proxies, each one inserting a Proxy-State attribute of
   20 bytes.  Also, we assume that the State attributes generated by the
   RADIUS Server have a size of 6 bytes and the User-Name attribute
   takes 50 bytes.  Therefore, the amount of free space in a chunk for
   the transport of the SAML assertion attributes is as follows:
   Total (4096 bytes) - RADIUS header (20 bytes) - User-Name (50 bytes)
   - Frag-Status (7 bytes) - Service-Type (6 bytes) - State (6 bytes) -
   Proxy-State (20 bytes) - Proxy-State (20 bytes) - Proxy-State
   (20 bytes) - Message-Authenticator (18 bytes), resulting in a total
   of 3929 bytes.  This amount of free space allows the transmission of
   up to 15 attributes of 255 bytes each.

   According to [RFC6929], a Long-Extended-Type provides a payload of
   251 bytes.  Therefore, the SAML assertion described above would
   result in 60 attributes, requiring four round trips to be completely
   transmitted.

8.  Handling Special Attributes

8.1.  Proxy-State Attribute

   RADIUS proxies may introduce Proxy-State attributes into any
   Access-Request packet they forward.  If they are unable to add this
   information to the packet, they may silently discard it rather than
   forward it to its destination; this would lead to DoS situations.
   Moreover, any Proxy-State attribute received by a RADIUS Server in an
   Access-Request packet MUST be copied into the corresponding reply
   packet.  For these reasons, Proxy-State attributes require special
   treatment within the packet fragmentation mechanism.

   When the RADIUS Server replies to an Access-Request packet as part of
   a conversation involving a fragmentation (either a chunk or a request
   for chunks), it MUST include every Proxy-State attribute received in
   the reply packet.  This means that the RADIUS Server MUST take into
   account the size of these Proxy-State attributes in order to
   calculate the size of the next chunk to be sent.

   However, while a RADIUS Server will always know how much space MUST
   be left in each reply packet for Proxy-State attributes (as they are
   directly included by the RADIUS Server), a RADIUS Client cannot know
   this information, as Proxy-State attributes are removed from the
   reply packet by their respective proxies before forwarding them back.
   Hence, RADIUS Clients need a mechanism to discover the amount of
   space required by proxies to introduce their Proxy-State attributes.
   In the following paragraphs, we describe a new mechanism to perform
   such a discovery:

   1.  When a RADIUS Client does not know how much space will be
       required by intermediate proxies for including their Proxy-State
       attributes, it SHOULD start using a conservative value (e.g.,
       1024 bytes) as the chunk size.

   2.  When the RADIUS Server receives a chunk from the RADIUS Client,
       it can calculate the total size of the Proxy-State attributes
       that have been introduced by intermediary proxies along the path.
       This information MUST be returned to the RADIUS Client in the
       next reply packet, encoded into a new attribute called
       Proxy-State-Length.  The RADIUS Server MAY artificially increase
       this quantity in order to handle situations where proxies behave
       inconsistently (e.g., they generate Proxy-State attributes with a
       different size for each packet) or where intermediary proxies
       remove Proxy-State attributes generated by other proxies.
       Increasing this value would make the RADIUS Client leave some
       free space for these situations.

   3.  The RADIUS Client SHOULD respond to the reception of this
       attribute by adjusting the maximum size for the next chunk
       accordingly.  However, as the Proxy-State-Length offers just an
       estimation of the space required by the proxies, the RADIUS
       Client MAY select a smaller amount in environments known to be
       problematic.

8.2.  State Attribute

   This RADIUS fragmentation mechanism makes use of the State attribute
   to link all the chunks belonging to the same fragmented packet.
   However, some considerations are required when the RADIUS Server is
   fragmenting a packet that already contains a State attribute for
   other purposes not related to the fragmentation.  If the procedure
   described in Section 5 is followed, two different State attributes
   could be included in a single chunk.  This is something explicitly
   forbidden in [RFC2865].

   A straightforward solution consists of making the RADIUS Server send
   the original State attribute in the last chunk of the sequence
   (attributes can be reordered as specified in [RFC2865]).  As the last
   chunk (when generated by the RADIUS Server) does not contain any
   State attribute due to the fragmentation mechanism, both situations
   described above are avoided.

   Something similar happens when the RADIUS Client has to send a
   fragmented packet that contains a State attribute in it.  The RADIUS
   Client MUST ensure that this original State is included in the first
   chunk sent to the RADIUS Server (as this one never contains any State
   attribute due to fragmentation).

8.3.  Service-Type Attribute

   This RADIUS fragmentation mechanism makes use of the Service-Type
   attribute to indicate that an Access-Accept packet is not granting
   access to the service yet, since an additional authorization exchange
   needs to be performed.  Similarly to the State attribute, the RADIUS
   Server has to send the original Service-Type attribute in the last
   Access-Accept of the RADIUS conversation to avoid ambiguity.

8.4.  Rebuilding the Original Large Packet

   The RADIUS Client stores the RADIUS attributes received in each chunk
   in a list, in order to be able to rebuild the original large packet
   after receiving the last chunk.  However, some of these received
   attributes MUST NOT be stored in that list, as they have been
   introduced as part of the fragmentation signaling and hence are not
   part of the original packet.

   o  State (except the one in the last chunk, if present)

   o  Service-Type = Additional-Authorization

   o  Frag-Status

   o  Proxy-State-Length

   Similarly, the RADIUS Server MUST NOT store the following attributes
   as part of the original large packet:

   o  State (except the one in the first chunk, if present)

   o  Service-Type = Additional-Authorization

   o  Frag-Status

   o  Proxy-State (except the ones in the last chunk)

   o  User-Name (except the one in the first chunk)

9.  New T Flag for the Long Extended Type Attribute Definition

   This document defines a new field in the Long Extended Type attribute
   format.  This field is one bit in size and is called "T" for
   Truncation.  It indicates that the attribute is intentionally
   truncated in this chunk and is to be continued in the next chunk of
   the sequence.  The combination of the M flag and the T flag indicates
   that the attribute is fragmented (M flag) but that all the fragments
   are not available in this chunk (T flag).  Proxies implementing
   [RFC6929] will see these attributes as invalid (they will not be able
   to reconstruct them), but they will still forward them, as
   Section 5.2 of [RFC6929] indicates that they SHOULD forward unknown
   attributes anyway.

   As a consequence of this addition, the Reserved field is now 6 bits
   long (see Section 12.1 for some considerations).  The following
   figure represents the new attribute format:

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     Type      |    Length     | Extended-Type |M|T| Reserved  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     Value ...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

          Figure 12: Updated Long Extended Type Attribute Format

10.  New Attribute Definition

   This document proposes the definition of two new extended type
   attributes, called Frag-Status and Proxy-State-Length.  The format of
   these attributes follows the indications for an Extended Type
   attribute defined in [RFC6929].

10.1.  Frag-Status Attribute

   This attribute is used for fragmentation signaling, and its meaning
   depends on the code value transported within it.  The following
   figure represents the format of the Frag-Status attribute:

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |   Type        |    Length     | Extended-Type |     Code
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                        Code (cont)                   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Figure 13: Frag-Status Format

   Type

      241

   Length

      7

   Extended-Type

      1

   Code

      4 bytes.  Integer indicating the code.  The values defined in this
      specification are:

         0 - Reserved

         1 - Fragmentation-Supported

         2 - More-Data-Pending

         3 - More-Data-Request

   This attribute MAY be present in Access-Request, Access-Challenge,
   and Access-Accept packets.  It MUST NOT be included in Access-Reject
   packets.  RADIUS Clients supporting this specification MUST include a
   Frag-Status = Fragmentation-Supported attribute in the first
   Access-Request sent to the RADIUS Server, in order to indicate that
   they would accept fragmented data from the server.

10.2.  Proxy-State-Length Attribute

   This attribute indicates to the RADIUS Client the length of the
   Proxy-State attributes received by the RADIUS Server.  This
   information is useful for adjusting the length of the chunks sent by
   the RADIUS Client.  The format of this Proxy-State-Length attribute
   is as follows:

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |   Type        |    Length     | Extended-Type |     Value
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                  Value (cont)                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                   Figure 14: Proxy-State-Length Format

   Type

      241

   Length

      7

   Extended-Type

      2

   Value

      4 bytes.  Total length (in bytes) of received Proxy-State
      attributes (including headers).  As the RADIUS Length field cannot
      take values over 4096 bytes, values of Proxy-State-Length MUST be
      less than that maximum length.

   This attribute MAY be present in Access-Challenge and Access-Accept
   packets.  It MUST NOT be included in Access-Request or Access-Reject
   packets.

10.3.  Table of Attributes

   The following table shows the different attributes defined in this
   document, along with the types of RADIUS packets in which they can be
   present.

                            |     Type of Packet    |
                            +-----+-----+-----+-----+
      Attribute Name        | Req | Acc | Rej | Cha |
      ----------------------+-----+-----+-----+-----+
      Frag-Status           | 0-1 | 0-1 |  0  | 0-1 |
      ----------------------+-----+-----+-----+-----+
      Proxy-State-Length    | 0   | 0-1 |  0  | 0-1 |
      ----------------------+-----+-----+-----+-----+

11.  Operation with Proxies

   The fragmentation mechanism defined above is designed to be
   transparent to legacy proxies, as long as they do not want to modify
   any fragmented attribute.  Nevertheless, updated proxies supporting
   this specification can even modify fragmented attributes.

11.1.  Legacy Proxies

   As every chunk is indeed a RADIUS packet, legacy proxies treat them
   as they would the rest of the packets, routing them to their
   destination.  Proxies can introduce Proxy-State attributes into
   Access-Request packets, even if they are indeed chunks.  This will
   not affect how fragmentation is managed.  The RADIUS Server will
   include all the received Proxy-State attributes in the generated
   response, as described in [RFC2865].  Hence, proxies do not
   distinguish between a regular RADIUS packet and a chunk.

11.2.  Updated Proxies

   Updated proxies can interact with RADIUS Clients and Servers in order
   to obtain the complete large packet before starting to forward it.
   In this way, proxies can manipulate (modify and/or remove) any
   attribute of the packet or introduce new attributes, without worrying
   about crossing the boundaries of the chunk size.  Once the
   manipulated packet is ready, it is sent to the original destination
   using the fragmentation mechanism (if required).  The example in
   Figure 15 shows how an updated proxy interacts with the RADIUS Client
   to (1) obtain a large Access-Request packet and (2) modify an
   attribute, resulting in an even larger packet.  The proxy then
   interacts with the RADIUS Server to complete the transmission of the
   modified packet, as shown in Figure 16.

     +-+-+-+-+-+                                          +-+-+-+-+-+
     | RADIUS  |                                          | RADIUS  |
     | Client  |                                          | Proxy   |
     +-+-+-+-+-+                                          +-+-+-+-+-+
         |                                                    |
         | Access-Request(1){User-Name,Calling-Station-Id,    |
         |        Example-Long-1[M],Example-Long-1[M],        |
         |        Example-Long-1[M],Example-Long-1[M],        |
         |        Example-Long-1[MT],Frag-Status(MDP)}        |
         |--------------------------------------------------->|
         |                                                    |
         |                     Access-Challenge(1){User-Name, |
         |                           Frag-Status(MDR),State1} |
         |<---------------------------------------------------|
         |                                                    |
         | Access-Request(2){User-Name,State1,                |
         |        Example-Long-1[M],Example-Long-1[M],        |
         |        Example-Long-1[M],Example-Long-1}           |
         |--------------------------------------------------->|

              Proxy Modifies Attribute Data, Increasing Its
                 Size from 9 Fragments to 11 Fragments

           Figure 15: Updated Proxy Interacts with RADIUS Client

     +-+-+-+-+-+                                          +-+-+-+-+-+
     | RADIUS  |                                          | RADIUS  |
     | Proxy   |                                          | Server  |
     +-+-+-+-+-+                                          +-+-+-+-+-+
         |                                                    |
         | Access-Request(3){User-Name,Calling-Station-Id,    |
         |        Example-Long-1[M],Example-Long-1[M],        |
         |        Example-Long-1[M],Example-Long-1[M],        |
         |        Example-Long-1[MT],Frag-Status(MDP)}        |
         |--------------------------------------------------->|
         |                                                    |
         |                     Access-Challenge(1){User-Name, |
         |                           Frag-Status(MDR),State2} |
         |<---------------------------------------------------|
         |                                                    |
         | Access-Request(4){User-Name,State2,                |
         |        Example-Long-1[M],Example-Long-1[M],        |
         |        Example-Long-1[M],Example-Long-1[M],        |
         |        Example-Long-1[MT],Frag-Status(MDP)}        |
         |--------------------------------------------------->|
         |                                                    |
         |                     Access-Challenge(1){User-Name, |
         |                           Frag-Status(MDR),State3} |
         |<---------------------------------------------------|
         |                                                    |
         | Access-Request(5){User-Name,State3,Example-Long-1} |
         |--------------------------------------------------->|

           Figure 16: Updated Proxy Interacts with RADIUS Server

12.  General Considerations

12.1.  T Flag

   As described in Section 9, this document modifies the definition of
   the Reserved field of the Long Extended Type attribute [RFC6929] by
   allocating an additional flag called the T flag.  The meaning and
   position of this flag are defined in this document, and nowhere else.
   This might cause an issue if subsequent specifications want to
   allocate a new flag as well, as there would be no direct way for them
   to know which parts of the Reserved field have already been defined.

   An immediate and reasonable solution for this issue would be
   declaring that this RFC updates [RFC6929].  In this way, [RFC6929]
   would include an "Updated by" clause that will point readers to this
   document.  Another alternative would be creating an IANA registry for
   the Reserved field.  However, the RADIUS Extensions (RADEXT) working
   group thinks that would be overkill, as a large number of
   specifications extending that field are not expected.

   In the end, the proposed solution is that this experimental RFC
   should not update RFC 6929.  Instead, we rely on the collective mind
   of the working group to remember that this T flag is being used as
   specified by this Experimental document.  If the experiment is
   successful, the T flag will be properly assigned.

12.2.  Violation of RFC 2865

   Section 5.1 indicates that all authorization and authentication
   handling will be postponed until all the chunks have been received.
   This postponement also applies to the verification that the
   Access-Request packet contains some kind of authentication attribute
   (e.g., User-Password, CHAP-Password, State, or other future
   attribute), as required by [RFC2865].  This checking will therefore
   be delayed until the original large packet has been rebuilt, as some
   of the chunks may not contain any of them.

   The authors acknowledge that this specification violates the "MUST"
   requirement of [RFC2865], Section 4.1 that states that "An
   Access-Request MUST contain either a User-Password or a CHAP-Password
   or a State."  We note that a proxy that enforces that requirement
   would be unable to support future RADIUS authentication extensions.
   Extensions to the protocol would therefore be impossible to deploy.
   All known implementations have chosen the philosophy of "be liberal
   in what you accept."  That is, they accept traffic that violates the
   requirement of [RFC2865], Section 4.1.  We therefore expect to see no
   operational issues with this specification.  After we gain more
   operational experience with this specification, it can be reissued as
   a Standards Track document and can update [RFC2865].

12.3.  Proxying Based on User-Name

   This proposal assumes that legacy proxies base their routing
   decisions on the value of the User-Name attribute.  For this reason,
   every packet sent from the RADIUS Client to the RADIUS Server (either
   chunks or requests for more chunks) MUST contain a User-Name
   attribute.

12.4.  Transport Behavior

   This proposal does not modify the way RADIUS interacts with the
   underlying transport (UDP).  That is, RADIUS keeps following a
   lock-step behavior that requires receiving an explicit
   acknowledgement for each chunk sent.  Hence, bursts of traffic
   that could congest links between peers are not an issue.

   Another benefit of the lock-step nature of RADIUS is that there are
   no security issues with overlapping fragments.  Each chunk simply has
   a length, with no Fragment Offset field as with IPv4.  The order of
   the fragments is determined by the order in which they are received.
   There is no ambiguity about the size or placement of each chunk, and
   therefore no security issues associated with overlapping chunks.

13.  Security Considerations

   As noted in many earlier specifications ([RFC5080], [RFC6158], etc.),
   RADIUS security is problematic.  This specification changes nothing
   related to the security of the RADIUS protocol.  It requires that all
   Access-Request packets associated with fragmentation are
   authenticated using the existing Message-Authenticator attribute.
   This signature prevents forging and replay, to the limits of the
   existing security.

   The ability to send bulk data from one party to another creates new
   security considerations.  RADIUS Clients and Servers may have to
   store large amounts of data per session.  The amount of this data can
   be significant, leading to the potential for resource exhaustion.  We
   therefore suggest that implementations limit the amount of bulk data
   stored per session.  The exact method for this limitation is
   implementation-specific.  Section 7 gives some indications of what
   could be reasonable limits.

   The bulk data can often be pushed off to storage methods other than
   the memory of the RADIUS implementation.  For example, it can be
   stored in an external database or in files.  This approach mitigates
   the resource exhaustion issue, as RADIUS Servers today already store
   large amounts of accounting data.

14.  IANA Considerations

   The Internet Assigned Numbers Authority (IANA) has registered the
   Attribute Types and Attribute Values defined in this document in the
   RADIUS namespaces as described in the "IANA Considerations" section
   of [RFC3575], in accordance with BCP 26 [RFC5226].  For RADIUS
   packets, attributes, and registries created by this document, IANA
   has updated <http://www.iana.org/assignments/radius-types>
   accordingly.

   In particular, this document defines two new RADIUS attributes,
   entitled "Frag-Status" (value 241.1) and "Proxy-State-Length"
   (value 241.2), which have been allocated from the short extended
   space as described in [RFC6929]:

   Type     Name                 Length  Meaning
   ----     ----                 ------  -------
   241.1    Frag-Status          7       Signals fragmentation
   241.2    Proxy-State-Length   7       Indicates the length of the
                                         received Proxy-State attributes

   The Frag-Status attribute also defines an 8-bit "Code" field, for
   which IANA has created and now maintains a new sub-registry entitled
   "Code Values for RADIUS Attribute 241.1, Frag-Status".  Initial
   values for the RADIUS Frag-Status "Code" registry are given below;
   future assignments are to be made through "RFC Required" [RFC5226].
   Assignments consist of a Frag-Status "Code" name and its associated
   value.

         Value    Frag-Status Code Name           Definition
         ----     ------------------------        ----------
         0        Reserved                        See Section 10.1
         1        Fragmentation-Supported         See Section 10.1
         2        More-Data-Pending               See Section 10.1
         3        More-Data-Request               See Section 10.1
         4-255    Unassigned

   Additionally, IANA has allocated a new Service-Type value for
   "Additional-Authorization".

         Value    Service Type Value              Definition
         ----     ------------------------        ----------
         19       Additional-Authorization        See Section 5.1

15.  References

15.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC2865]  Rigney, C., Willens, S., Rubens, A., and W. Simpson,
              "Remote Authentication Dial In User Service (RADIUS)",
              RFC 2865, June 2000, <http://www.rfc-editor.org/
              info/rfc2865>.

   [RFC3575]  Aboba, B., "IANA Considerations for RADIUS (Remote
              Authentication Dial In User Service)", RFC 3575,
              July 2003, <http://www.rfc-editor.org/info/rfc3575>.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008, <http://www.rfc-editor.org/info/rfc5226>.

   [RFC6158]  DeKok, A., Ed., and G. Weber, "RADIUS Design Guidelines",
              BCP 158, RFC 6158, March 2011,
              <http://www.rfc-editor.org/info/rfc6158>.

   [RFC6929]  DeKok, A. and A. Lior, "Remote Authentication Dial In User
              Service (RADIUS) Protocol Extensions", RFC 6929,
              April 2013, <http://www.rfc-editor.org/info/rfc6929>.

15.2.  Informative References

   [ABFAB-Arch]
              Howlett, J., Hartman, S., Tschofenig, H., Lear, E., and J.
              Schaad, "Application Bridging for Federated Access Beyond
              Web (ABFAB) Architecture", Work in Progress,
              draft-ietf-abfab-arch-13, July 2014.

   [RADIUS-Larger-Pkts]
              Hartman, S., "Larger Packets for RADIUS over TCP", Work in
              Progress, draft-ietf-radext-bigger-packets-03, March 2015.

   [RFC2866]  Rigney, C., "RADIUS Accounting", RFC 2866, June 2000,
              <http://www.rfc-editor.org/info/rfc2866>.

   [RFC3579]  Aboba, B. and P. Calhoun, "RADIUS (Remote Authentication
              Dial In User Service) Support For Extensible
              Authentication Protocol (EAP)", RFC 3579, September 2003,
              <http://www.rfc-editor.org/info/rfc3579>.

   [RFC4849]  Congdon, P., Sanchez, M., and B. Aboba, "RADIUS Filter
              Rule Attribute", RFC 4849, April 2007,
              <http://www.rfc-editor.org/info/rfc4849>.

   [RFC5080]  Nelson, D. and A. DeKok, "Common Remote Authentication
              Dial In User Service (RADIUS) Implementation Issues and
              Suggested Fixes", RFC 5080, December 2007,
              <http://www.rfc-editor.org/info/rfc5080>.

   [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, <http://www.rfc-editor.org/info/rfc5176>.

   [SAML-RADIUS]
              Howlett, J., Hartman, S., and A. Perez-Mendez, Ed., "A
              RADIUS Attribute, Binding, Profiles, Name Identifier
              Format, and Confirmation Methods for SAML", Work in
              Progress, draft-ietf-abfab-aaa-saml-10, February 2015.

Acknowledgements

   The authors would like to thank the members of the RADEXT working
   group who have contributed to the development of this specification
   by either participating in the discussions on the mailing lists or
   sending comments about our RFC.

   The authors also thank David Cuenca (University of Murcia) for
   implementing a proof-of-concept implementation of this RFC that has
   been useful to improve the quality of the specification.

   This work has been partly funded by the GEANT GN3+ SA5 and CLASSe
   (<http://www.um.es/classe/>) projects.

Authors' Addresses

   Alejandro Perez-Mendez (editor)
   University of Murcia
   Campus de Espinardo S/N, Faculty of Computer Science
   Murcia  30100
   Spain

   Phone: +34 868 88 46 44
   EMail: alex@um.es


   Rafa Marin-Lopez
   University of Murcia
   Campus de Espinardo S/N, Faculty of Computer Science
   Murcia  30100
   Spain

   Phone: +34 868 88 85 01
   EMail: rafa@um.es


   Fernando Pereniguez-Garcia
   University of Murcia
   Campus de Espinardo S/N, Faculty of Computer Science
   Murcia  30100
   Spain

   Phone: +34 868 88 78 82
   EMail: pereniguez@um.es
   Gabriel Lopez-Millan
   University of Murcia
   Campus de Espinardo S/N, Faculty of Computer Science
   Murcia  30100
   Spain

   Phone: +34 868 88 85 04
   EMail: gabilm@um.es


   Diego R. Lopez
   Telefonica I+D
   Don Ramon de la Cruz, 84
   Madrid  28006
   Spain

   Phone: +34 913 129 041
   EMail: diego@tid.es


   Alan DeKok
   Network RADIUS SARL
   57bis Boulevard des Alpes
   Meylan  38240
   France

   EMail: aland@networkradius.com
   URI:   http://networkradius.com