Internet Engineering Task Force (IETF) A. Perez-Mendez, Ed.

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.

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Copyright Notice

Copyright (c) 2015 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.

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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

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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.

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[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.

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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.

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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.

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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.

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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 |
+----------------+

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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

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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.

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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".

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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).

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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).

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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)

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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.

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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.

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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.

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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).

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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

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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.

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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.

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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

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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.

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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)

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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].

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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.

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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.

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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.

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+-+-+-+-+-+ +-+-+-+-+-+
| 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

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+-+-+-+-+-+ +-+-+-+-+-+
| 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

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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.

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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.

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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

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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>.

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[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.

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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: [email protected]

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: [email protected]

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: [email protected]

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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: [email protected]

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

Phone: +34 913 129 041
EMail: [email protected]

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

EMail: [email protected]
URI: http://networkradius.com

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