Internet Engineering Task Force (IETF) S. Hartman
Request for Comments: 7930 Painless Security
Updates: 6613 August 2016
Larger Packets for RADIUS over TCP
The RADIUS-over-TLS experiment described in RFC 6614 has opened
RADIUS to new use cases where the 4096-octet maximum size limit of a
RADIUS packet proves problematic. This specification extends the
RADIUS-over-TCP experiment (RFC 6613) to permit larger RADIUS
packets. This specification compliments other ongoing work to permit
fragmentation of RADIUS authorization information. This document
registers a new RADIUS code, an action that required IESG approval.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for examination, experimental implementation, and
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 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
The experiment with Remote Authentication Dial-In User Service
(RADIUS) over Transport Layer Security (TLS) [RFC6614] provides
strong confidentiality and integrity for RADIUS [RFC2865]. This
enhanced security has opened new opportunities for using RADIUS to
convey additional authorization information. As an example,
[RFC7833] describes a mechanism for using RADIUS to carry Security
Assertion Markup Language (SAML) messages in RADIUS. Many attributes
carried in these SAML messages will require confidentiality or
integrity such as that provided by TLS.
These new use cases involve carrying additional information in RADIUS
packets. The maximum packet length of 4096 octets is proving
insufficient for some SAML messages and for other structures that may
be carried in RADIUS.
One approach is to fragment a RADIUS message across multiple packets
at the RADIUS layer. RADIUS fragmentation [RFC7499] provides a
mechanism to split authorization information across multiple RADIUS
messages. That mechanism is necessary in order to split
authorization information across existing unmodified proxies.
However, there are some significant disadvantages to RADIUS
fragmentation. First, RADIUS is a lock-step protocol, and only one
fragment can be in transit at a time as part of a given request.
Also, there is no current mechanism to discover the Path Maximum
Transmission Unit (PMTU) across the entire path that the fragment
will travel. As a result, fragmentation is likely both at the RADIUS
layer and at the transport layer. When TCP is used, much better
transport characteristics can be achieved by fragmentation only at
the TCP layer. This specification provides a mechanism to achieve
these better transport characteristics when TCP is used. As part of
this specification, a new RADIUS code is registered.
This specification is published as an Experimental specification
because the TCP extensions to RADIUS are currently experimental. The
need for this specification arises from operational experience with
the TCP extensions. However, this specification introduces no new
experimental evaluation criteria beyond those in the base TCP
specification; this specification can be evaluated along with that
one for advancement on the Standards Track.
1.1. Requirements Notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
2. Changes to Packet Processing
The maximum length of a RADIUS message is increased from 4096 to
65535. A RADIUS Server implementing this specification MUST be able
to receive a RADIUS packet of maximum length. Servers MAY have a
maximum size over which they choose to return an error, as discussed
in Section 5, rather than processing a received packet; this size
MUST be at least 4096 octets.
Clients implementing this specification MUST be able to receive a
RADIUS packet of maximum length; that is, clients MUST NOT close a
TCP connection simply because a large packet is sent over it.
Clients MAY include the Response-Length attribute defined in
Section 6 to indicate the maximum size of a packet that they can
successfully process. Clients MAY silently discard a packet greater
than some configured size; this size MUST be at least 4096 octets.
Clients MUST NOT retransmit an unmodified request whose response is
larger than the client can process, as subsequent responses will
likely continue to be too large.
Proxies MUST be able to receive a RADIUS packet of maximum length
without closing the TCP connection. Proxies SHOULD be able to
process and forward packets of maximum length. When a proxy receives
a request over a transport with a 4096-octet maximum length and the
proxy forwards that request over a transport with a larger maximum
length, the proxy MUST include the Response-Length attribute with a
value of 4096.
2.1. Status-Server Considerations
This section extends processing of Status-Server messages as
described in Sections 4.1 and 4.2 of [RFC5997].
Clients implementing this specification SHOULD include the Response-
Length attribute in Status-Server requests. Servers are already
required to ignore unknown attributes received in this message. By
including the attribute, the client indicates how large of a response
it can process to its Status-Server request. It is very unlikely
that a response to Status-Server is greater than 4096 octets.
However, the client also indicates support for this specification,
which triggers the server behavior below.
If a server implementing this specification receives a Response-
Length attribute in a Status-Server request, it MUST include a
Response-Length attribute indicating the maximum size request it can
process in its response to the Status-Server request.
3. Forward and Backward Compatibility
An implementation of [RFC6613] will silently discard any RADIUS
packet larger than 4096 octets and will close the TCP connection.
This section provides guidelines for interoperability with these
implementations. These guidelines are stated at the SHOULD level.
In some environments, support for large packets will be important
enough that roaming or other agreements will mandate their support.
In these environments, all implementations might be required to
support this specification, thus removing the need for
interoperability with RFC 6613. It is likely that these guidelines
will be relaxed to the MAY level and support for this specification
made a requirement if RADIUS over TLS and TCP are moved to the
Standards Track in the future.
Clients SHOULD provide configuration for the maximum size of a
request sent to each server. Servers SHOULD provide configuration
for the maximum size of a response sent to each client. If dynamic
discovery mechanisms are supported, configuration SHOULD be provided
for the default maximum size of RADIUS packets sent to clients and
servers. If an implementation provides more granular configuration
for some classes of dynamic resources, then the implementation SHOULD
also provide configuration of default maximum packet sizes at the
same granularity. As an example, an implementation that provided
granular configuration for resources using a particular trust anchor
or belonging to a particular roaming consortium SHOULD provide
default packet size configuration at the same granularity.
If a client sends a request larger than 4096 octets and the TCP
connection is closed without a response, the client SHOULD treat the
request as if a "Request Too Big" error (Section 5) specifying a
maximum size of 4096 is received. Clients or proxies sending
multiple requests over a single TCP connection without waiting for
responses SHOULD implement capability discovery as discussed in
By default, a server SHOULD NOT generate a response larger than 4096
octets. The Response-Length attribute MAY be included in a request
to indicate that larger responses are acceptable. Other attributes
or configurations MAY be used as an indicator that large responses
are likely to be acceptable.
A proxy that implements both this specification and RADIUS
fragmentation [RFC7499] SHOULD use RADIUS fragmentation when the
following conditions are met:
1. A RADIUS packet is being forwarded towards a next hop whose
configuration does not support a packet that large.
2. RADIUS fragmentation can be used for the packet in question.
The interoperability challenge appears at first significant. This
specification proposes to introduce behavior where new
implementations will fail to function with existing implementations.
However, these capabilities are introduced to support new use cases.
If an implementation has 10000 octets of attributes to send, it
cannot, in general, trim down the response to something that can be
sent. Under this specification, a large packet would be generated
that will be silently discarded by an existing implementation.
Without this specification, no packet is generated because the
required attributes cannot be sent.
The biggest risk to interoperability would be if requests and
responses are expanded to include additional information that is not
strictly necessary. So, avoiding creating situations where large
packets are sent to existing implementations is mostly an operational
matter. Interoperability is most impacted when the size of packets
in existing use cases is significantly increased and least impacted
when large packets are used for new use cases where the deployment is
likely to require updated RADIUS implementations.
There is a special challenge for proxies or clients with a high
request volume. When an implementation of RFC 6613 receives a packet
that is too large, it closes the connection and does not respond to
any requests in process. Such a client would lose requests and might
find it difficult to distinguish "Request Too Big" situations from
other failures. In these cases, the discovery mechanism described in
Section 3.2 can be used.
Also, RFC 6613 is an experiment. Part of running that experiment is
to evaluate whether additional changes are required to RADIUS. A
lower bar for interoperability should apply to changes to
Experimental protocols than Standard protocols.
This specification provides good facilities to enable implementations
to understand packet size when proxying to/from Standards Track UDP
As discussed in Section 2.1, a client MAY send a Status-Server
message to discover whether an authentication or accounting server
supports this specification. The client includes a Response-Length
attribute; this signals the server to include a Response-Length
attribute indicating the maximum packet size the server can process.
In this one instance, Response-Length indicates the size of a request
that can be processed rather than a response.
4. Protocol-Error Code
This document defines a new RADIUS code, 52, called Protocol-Error.
This packet code may be used in response to any request packet, such
as Access-Request, Accounting-Request, CoA-Request, or Disconnect-
Request. It is a response packet sent by a server to a client. The
packet indicates to the client that the server is unable to process
the request for some reason.
A Protocol-Error packet MUST contain an Original-Packet-Code
attribute, along with an Error-Cause attribute. Other attributes MAY
be included if desired. The Original-Packet-Code contains the code
from the request that generated the protocol error so that clients
can disambiguate requests with different codes and the same ID.
Regardless of the original packet code, the RADIUS Server calculates
the Message-Authenticator attribute as if the original packet were an
Access-Request packet. The identifier is copied from the original
Clients processing Protocol-Error MUST ignore unknown or unexpected
This RADIUS code is hop by hop. Proxies MUST NOT forward a Protocol-
Error packet they receive.
5. Too Big Response
When a RADIUS Server receives a request that is larger than can be
processed, it generates a Protocol-Error response as follows:
The code is Protocol-Error.
The Response-Length attribute MUST be included and its value is
the maximum size of request that will be processed.
The Error-Cause attribute is included with a value of 601.
The Original-Packet-Code attribute is copied from the request.
Clients will not typically be able to adjust and resend requests when
this error is received. In some cases, the client can fall back to
RADIUS fragmentation. In other cases, this code will provide for
better client error reporting and will avoid retransmitting requests
guaranteed to fail.
6. IANA Considerations
A new RADIUS Packet Type Code is registered in the "RADIUS Packet
Type Codes" registry discussed in Section 2.1 of RFC 3575 [RFC3575].
The name is "Protocol-Error" and the code is 52.
The following RADIUS attribute Type values [RFC3575] are assigned.
The assignment rules in Section 10.3 of [RFC6929] are used.
| Name | Attribute | Description |
| Response-Length | 241.3 | An attribute of type "integer" |
| | | per Section 5 of RFC 2865 |
| | | containing maximum response |
| | | length. |
| | | |
| Original-Packet-Code | 241.4 | An integer attribute |
| | | containing the code from a |
| | | packet resulting in a |
| | | Protocol-Error response. |
The Response-Length attribute MAY be included in any RADIUS request.
In this context, it indicates the maximum length of a response the
client is prepared to receive. Values are between 4096 and 65535.
The attribute MAY also be included in a response to a Status-Server
message. In this case, the attribute indicates the maximum size
RADIUS request that is permitted.
A new Error-Cause value is registered in the "Values for RADIUS
Attribute 101, Error-Cause Attribute" registry at
<http://www.iana.org/assignments/radius-types> for "Response Too Big"
with value 601. The range of valid values for the Error-Cause
attribute in the "Values for RADIUS Attribute 101, Error-Cause
Attribute" registry originally defined in RFC 5176 are extended. Two
new ranges are defined:
6xx fatal errors committed by a RADIUS server
7xx fatal errors committed by a RADIUS client
8.2. Informative References
[RFC7499] Perez-Mendez, A., Ed., Marin-Lopez, R., Pereniguez-Garcia,
F., Lopez-Millan, G., Lopez, D., and A. DeKok, "Support of
Fragmentation of RADIUS Packets", RFC 7499,
DOI 10.17487/RFC7499, April 2015,
[RFC7833] Howlett, J., Hartman, S., and A. Perez-Mendez, Ed., "A
RADIUS Attribute, Binding, Profiles, Name Identifier
Format, and Confirmation Methods for the Security
Assertion Markup Language (SAML)", RFC 7833,
DOI 10.17487/RFC7833, May 2016,
Sam Hartman's time on this document was funded by JANET as part of
Alan DeKok provided valuable review and text for the Protocol-Error
Alejandro Perez Mendez provided valuable review comments.