Independent Submission K. Wierenga
Request for Comments: 7593 Cisco Systems
Category: Informational S. Winter
ISSN: 2070-1721 RESTENA
Nicolaus Copernicus University
September 2015 The eduroam Architecture for Network Roaming
This document describes the architecture of the eduroam service for
federated (wireless) network access in academia. The combination of
IEEE 802.1X, the Extensible Authentication Protocol (EAP), and RADIUS
that is used in eduroam provides a secure, scalable, and deployable
service for roaming network access. The successful deployment of
eduroam over the last decade in the educational sector may serve as
an example for other sectors, hence this document. In particular,
the initial architectural choices and selection of standards are
described, along with the changes that were prompted by operational
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This is a contribution to the RFC Series, independently of any other
RFC stream. The RFC Editor has chosen to publish this document at
its discretion and makes no statement about its value for
implementation or deployment. Documents approved for publication by
the RFC Editor are not 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
6.3. Track Location of Users . . . . . . . . . . . . . . . . . 287. Security Considerations . . . . . . . . . . . . . . . . . . . 297.1. Man-in-the-Middle and Tunneling Attacks . . . . . . . . . 297.1.1. Verification of Server Name Not Supported . . . . . . 297.1.2. Neither Specification of CA nor Server Name Checks
during Bootstrap . . . . . . . . . . . . . . . . . . 297.1.3. User Does Not Configure CA or Server Name Checks . . 307.1.4. Tunneling Authentication Traffic to Obfuscate User
Origin . . . . . . . . . . . . . . . . . . . . . . . 307.2. Denial-of-Service Attacks . . . . . . . . . . . . . . . . 317.2.1. Intentional DoS by Malign Individuals . . . . . . . . 317.2.2. DoS as a Side-Effect of Expired Credentials . . . . . 328. References . . . . . . . . . . . . . . . . . . . . . . . . . 338.1. Normative References . . . . . . . . . . . . . . . . . . 338.2. Informative References . . . . . . . . . . . . . . . . . 34
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 36
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 371. Introduction
In 2002, the European Research and Education community set out to
create a network roaming service for students and employees in
academia [eduroam-start]. Now, over 10 years later, this service has
grown to more than 10,000 service locations, serving millions of
users on all continents with the exception of Antarctica.
This memo serves to explain the considerations for the design of
eduroam as well as to document operational experience and resulting
changes that led to IETF specifications such as RADIUS over TCP
[RFC6613] and RADIUS with TLS [RFC6614] and that promoted alternative
uses of RADIUS like in Application Bridging for Federated Access
Beyond web (ABFAB) [ABFAB-ARCH]. Whereas the eduroam service is
limited to academia, the eduroam architecture can easily be reused in
First, this memo describes the original architecture of eduroam
[eduroam-homepage]. Then, a number of operational problems are
presented that surfaced when eduroam gained wide-scale deployment.
Lastly, enhancements to the eduroam architecture that mitigate the
aforementioned issues are discussed.
This document uses identity management and privacy terminology from
[RFC6973]. In particular, this document uses the terms "Identity
Provider", "Service Provider", and "identity management".
1.2. Notational Conventions
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].
Note: Also, the policy to which eduroam participants subscribe
expresses the requirements for participation in RFC 2119 language.
1.3. Design Goals
The guiding design considerations for eduroam were as follows:
- Unique identification of users at the edge of the network
The access Service Provider (SP) needs to be able to determine
whether a user is authorized to use the network resources.
Furthermore, in case of abuse of the resources, there is a
requirement to be able to identify the user uniquely (with the
cooperation of the user's Identity Provider (IdP) operator).
- Enable (trusted) guest use
In order to enable roaming, it should be possible for users of
participating institutions to get seamless access to the networks
of other institutions.
Note: Traffic separation between guest users and normal users is
possible (for example, through the use of VLANs), and indeed
widely used in eduroam.
The infrastructure that is created should scale to a large number
of users and organizations without requiring a lot of coordination
and other administrative procedures (possibly with the exception
of an initial setup). Specifically, it should not be necessary
for a user that visits another organization to go through an
- Easy to install and use
It should be easy for both organizations and users to participate
in the roaming infrastructure; otherwise, it may inhibit wide-
scale adoption. In particular, there should be no client
installation (or it should be easy) and only one-time
An important design criterion has been that there needs to be a
security association between the end user and their Identity
Provider, eliminating the possibility of credential theft. The
minimal requirements for security are specified in the eduroam
policy and subject to change over time. As an additional
protection against user errors and negligence, it should be
possible for participating Identity Providers to add their own
requirements for the quality of authentication of their own users
without the need for the infrastructure as a whole to implement
the same requirements.
- Privacy preserving
The design of the system should provide for user anonymization,
i.e., a possibility to hide the user's identity from any third
parties, including Service Providers.
- Standards based
In an infrastructure in which many thousands of organizations
participate, it is obvious that it should be possible to use
equipment from different vendors; therefore, it is important to
build the infrastructure using open standards.
1.4. Solutions That Were Considered
Three architectures were trialed: one based on the use of VPN
technology (deemed secure but not scalable), one based on Web
captive-portals (scalable but not secure), and one based on IEEE
802.1X, the latter being the basis of what is now the eduroam
architecture. An overview of the candidate architectures and their
relative merits can be found in [nrenroaming-select].
The chosen architecture is based on:
o IEEE 802.1X [IEEE.802.1X] as the port-based authentication
o EAP [RFC3748] for integrity-protected and confidential transport
of credentials and
o a RADIUS [RFC2865] hierarchy as the trust fabric.
2. Classic Architecture
Federations, like eduroam, implement essentially two types of direct
trust relations (and one indirect). The trust relation between an
end user and the IdP (operated by the home organization of the user)
and between the IdP and the SP (in eduroam, the operator of the
network at the visited location). In eduroam, the trust relation
between the user and IdP is through mutual authentication. IdPs and
the SP establish trust through the use of a RADIUS hierarchy.
These two forms of trust relations in turn provide the transitive
trust relation that makes the SP trust the user to use its network
Authentication in eduroam is achieved by using a combination of IEEE
802.1X [IEEE.802.1X] and EAP [RFC4372] (the latter carried over
RADIUS for guest access; see Section 2.2).
2.1.1. IEEE 802.1X
By using the IEEE 802.1X [IEEE.802.1X] framework for port-based
network authentication, organizations that offer network access (SPs)
for visiting (and local) eduroam users can make sure that only
authorized users get access. The user (or rather the user's
supplicant) sends an access request to the authenticator (Wi-Fi
Access Point or switch) at the SP, the authenticator forwards the
access request to the authentication server of the SP, that in turn
proxies the request through the RADIUS hierarchy to the
authentication server of the user's home organization (the IdP).
Note: The security of the connections between local wireless
infrastructure and local RADIUS servers is a part of the local
network of each SP; therefore, it is out of scope for this document.
For completeness, it should be stated that security between access
points and their controllers is vendor specific, and security between
controllers (or standalone access points) and local RADIUS servers is
based on the typical RADIUS shared secret mechanism.
In order for users to be aware of the availability of the eduroam
service, an SP that offers wireless network access MUST broadcast the
Service Set Identifier (SSID) 'eduroam', unless that conflicts with
the SSID of another eduroam SP, in which case, an SSID starting with
"eduroam-" MAY be used. The downside of the latter is that clients
will not automatically connect to that SSID, thus losing the seamless
Note: A direct implication of the common eduroam SSID is that the
users cannot distinguish between a connection to the home network and
a guest network at another eduroam institution (IEEE 802.11-2012 does
have the so-called "Interworking" to make that distinction, but it is
not widely implemented yet). Furthermore, without proper server
verification, users may even be tricked into joining a rogue eduroam
network. Therefore, users should be made aware that they should not
assume data confidentiality in the eduroam infrastructure.
To protect over-the-air confidentiality of user data, IEEE 802.11
wireless networks of eduroam SPs MUST deploy WPA2+AES, and they MAY
additionally support Wi-Fi Protected Access with the Temporal Key
Integrity Protocol (WPA/TKIP) as a courtesy to users of legacy
The use of the Extensible Authentication Protocol (EAP) [RFC4372]
serves two purposes. In the first place, a properly chosen EAP
method allows for integrity-protected and confidential transport of
the user credentials to the home organization. Secondly, by having
all RADIUS servers transparently proxy access requests, regardless of
the EAP method inside the RADIUS packet, the choice of EAP method is
between the 'home' organization of the user and the user. In other
words, in principle, every authentication form that can be carried
inside EAP can be used in eduroam, as long as they adhere to minimal
requirements as set forth in the eduroam Policy Service Definition
,----------\ | | ,---------\
| SP | | eduroam | | IdP |
| +----+ trust fabric +---+ |
`------+---' | | '-----+---'
| | | |
| \ / |
| \ / |
| \ / |
| \ / |
+----+ +-----+ +----+
| | | |
+-+------+-+ ___________________________ | |
| | O__________________________ ) +------+
Host (supplicant) EAP tunnel Authentication server
Figure 1: Tunneled EAP
Proxying of access requests is based on the outer identity in the
EAP-Message. Those outer identities MUST be a valid user identifier
with a mandatory realm as per [RFC7542], i.e., be of the form
something@realm or just @realm, where the realm part is the domain
name of the institution that the IdP belongs to. In order to
preserve credential protection, participating organizations MUST
deploy EAP methods that provide mutual authentication. For EAP
methods that support outer identity, anonymous outer identities are
recommended. Most commonly used in eduroam are the so-called
tunneled EAP methods that first create a server-authenticated TLS
[RFC5246] tunnel through which the user credentials are transmitted.
As depicted in Figure 1, the use of a tunneled EAP method creates a
direct logical connection between the supplicant and the
authentication server, even though the actual traffic flows through
the RADIUS hierarchy.
2.2. Federation Trust Fabric
The eduroam federation trust fabric is based on RADIUS. RADIUS trust
is based on shared secrets between RADIUS peers. In eduroam, any
RADIUS message originating from a trusted peer is implicitly assumed
to originate from a member of the roaming consortium.
Note: See also the security considerations for a discussion on RADIUS
security that motivated the work on RADIUS with TLS [RFC6614].
The eduroam trust fabric consists of a proxy hierarchy of RADIUS
servers (organizational, national, global) that is loosely based on
the DNS hierarchy. That is, typically an organizational RADIUS
server agrees on a shared secret with a national server, and the
national server in turn agrees on a shared secret with the root
server. Access requests are routed through a chain of RADIUS proxies
towards the Identity Provider of the user, and the access accept (or
reject) follows the same path back.
Note: In some circumstances, there are more levels of RADIUS servers
(for example, regional or continental servers), but that doesn't
change the general model. Also, the packet exchange that is
described below requires, in reality, several round-trips.
o The Access Point forwards the EAP message to its Authentication
Server (the UTK RADIUS server).
o The UTK RADIUS server checks the realm to see if it is a local
realm; since it isn't, the request is proxied to the .edu RADIUS
o The .edu RADIUS server verifies the realm; since it is not in a
.edu subdomain, it proxies the request to the root server.
o The root RADIUS server proxies the request to the .nl RADIUS
server, since the ".nl" domain is known to the root server.
o The .nl RADIUS server proxies the request to the surfnet.nl
server, since it knows the SURFnet server.
o The surfnet.nl RADIUS server decapsulates the EAP message and
verifies the user credentials, since the user is known to SURFnet.
o The surfnet.nl RADIUS server informs the utk.edu server of the
outcome of the authentication request (Access-Accept or Access-
Reject) by proxying the outcome through the RADIUS hierarchy in
o The UTK RADIUS server instructs the UTK Access Point to either
accept or reject access based on the outcome of the
Note: The depiction of the root RADIUS server is a simplification.
In reality, the root server is distributed over three continents and
each maintains a list of the top-level realms that a specific root
server is responsible for. This also means that, for
intercontinental roaming, there is an extra proxy step from one root
server to the other. Also, the physical distribution of nodes
doesn't need to mirror the logical distribution of nodes. This helps
with stability and scalability.
3. Issues with Initial Trust Fabric
While the hierarchical RADIUS architecture described in the previous
section has served as the basis for eduroam operations for an entire
decade, the exponential growth of authentications is expected to lead
to, and has in fact in some cases already led to, performance and
operations bottlenecks on the aggregation proxies. The following
sections describe some of the shortcomings and the resulting
3.1. Server Failure Handling
In eduroam, authentication requests for roaming users are statically
routed through preconfigured proxies. The number of proxies varies:
in a national roaming case, the number of proxies is typically 1 or 2
(some countries deploy regional proxies, which are in turn aggregated
by a national proxy); in international roaming, 3 or 4 proxy servers
are typically involved (the number may be higher along some routes).
RFC 2865 [RFC2865] does not define a failover algorithm. In
particular, the failure of a server needs to be deduced from the
absence of a reply. Operational experience has shown that this has
detrimental effects on the infrastructure and end-user experience:
1. Authentication failure: the first user whose authentication path
is along a newly failed server will experience a long delay and
2. Wrongly deduced states: since the proxy chain is longer than one
hop, a failure further along in the authentication path is
indistinguishable from a failure in the next hop.
3. Inability to determine recovery of a server: only a "live"
authentication request sent to a server that is believed to be
inoperable can lead to the discovery that the server is in
working order again. This issue has been resolved with RFC 5997
The second point can have significant impact on the operational state
of the system in a worst-case scenario: imagine one realm's home
server being inoperable. A user from that realm is trying to roam
internationally and tries to authenticate. The RADIUS server on the
hotspot location may assume its own national proxy is down because it
does not reply. That national server, being perfectly alive, in turn
will assume that the international aggregation proxy is down, which
in turn will believe the home country proxy national server is down.
None of these assumptions are true. Worse yet: in case of failover
to a back-up next-hop RADIUS server, also that server will be marked
as being defunct, since through that server no reply will be received
from the defunct home server either. Within a short time, all
redundant aggregation proxies might be considered defunct by their
In the absence of proper next-hop state derivation, some interesting
concepts have been introduced by eduroam participants -- the most
noteworthy being a failover logic that considers up/down states not
per next-hop RADIUS peer, but instead per realm (See [dead-realm] for
details). Recently, implementations of RFC 5997 [RFC5997] and
cautious failover parameters make false "downs" unlikely to happen,
as long as every hop implements RFC 5997. In that case, dead realm
detection serves mainly to prevent proxying of large numbers of
requests to known dead realms.
3.2. No Signaling of Error Conditions
The RADIUS protocol lacks signaling of error conditions, and the IEEE
802.1X standard does not allow conveying of extended failure reasons
to the end user's device. For eduroam, this creates two issues:
o The home server may have an operational problem, for example, its
authentication decisions may depend on an external data source
such as a SQL server or Microsoft's Active Directory, and the
external data source is unavailable. If the RADIUS interface is
still functional, there are two options for how to reply to an
Access-Request that can't be serviced due to such error
1. Do Not Reply: The inability to reach a conclusion can be
handled by not replying to the request. The upside of this
approach is that the end user's software doesn't come to wrong
conclusions and won't give unhelpful hints such as "maybe your
password is wrong". The downside is that intermediate proxies
may come to wrong conclusions because their downstream RADIUS
server isn't responding.
2. Reply with Reject: In this option, the inability to reach a
conclusion is treated like an authentication failure. The
upside of this approach is that intermediate proxies maintain
a correct view on the reachability state of their RADIUS peer.
The downside is that EAP supplicants on end-user devices often
react with either false advice ("your password is wrong") or
even trigger permanent configuration changes (e.g., the
Windows built-in supplicant will delete the credential set
from its registry, prompting the user for their password on
the next connection attempt). The latter case of Windows is a
source of significant help-desk activity; users may have
forgotten their password after initially storing it but are
suddenly prompted again.
There have been epic discussions in the eduroam community as well as
in the IETF RADEXT Working Group as to which of the two approaches is
more appropriate, but they were not conclusive.
Similar considerations apply when an intermediate proxy does not
receive a reply from a downstream RADIUS server. The proxy may
either choose not to reply to the original request, leading to
retries and its upstream peers coming to wrong conclusions about its
own availability; or, it may decide to reply with Access-Reject to
indicate its own liveliness, but again with implications for the end
The ability to send Status-Server watchdog requests is only of use
after the fact, in case a downstream server doesn't reply (or hasn't
been contacted in a long while, so that its previous working state is
stale). The active link-state monitoring of the TCP connection with,
e.g., RADIUS/TLS (see Section 4.1), gives a clearer indication
whether there is an alive RADIUS peer, but it does not solve the
defunct back-end problem. An explicit ability to send Error-Replies,
on the RADIUS level (for other RADIUS peer information) and EAP level
(for end-user supplicant information), would alleviate these problems
but is currently not available.
3.3. Routing Table Complexity
The aggregation of RADIUS requests based on the structure of the
user's realm implies that realms ending with the same top-level
domain are routed to the same server, i.e., to a common
administrative domain. While this is true for country code Top-Level
Domains (ccTLDs), which map into national eduroam federations, it is
not true for realms residing in generic Top-Level Domains (gTLDs).
Realms in gTLDs were historically discouraged because the automatic
mapping "realm ending" -> "eduroam federation's server" could not be
applied. However, with growing demand from eduroam realm
administrators, it became necessary to create exception entries in
the forwarding rules; such realms need to be mapped on a realm-by-
realm basis to their eduroam federations. Example: "kit.edu"
(Karlsruher Institut fuer Technologie) needs to be routed to the
German federation server, whereas "iu.edu" (Indiana University) needs
to be routed to the USA federation server.
While the ccTLDs occupy only approximately 50 routing entries in
total (and have an upper bound of approximately 200), the potential
size of the routing table becomes virtually unlimited if it needs to
accommodate all individual entries in .edu, .org, etc.
In addition to that, all these routes need to be synchronized between
three international root servers, and the updates need to be applied
manually to RADIUS server configuration files. The frequency of the
required updates makes this approach fragile and error-prone as the
number of entries grows.
3.4. UDP Issues
RADIUS is based on UDP, which was a reasonable choice when its main
use was with simple Password Authentication Protocol (PAP) requests
that required only exactly one packet exchange in each direction.
When transporting EAP over RADIUS, the EAP conversations require
multiple round-trips; depending on the total payload size, 8-10
round-trips are not uncommon. The loss of a single UDP packet will
lead to user-visible delays and might result in servers being marked
as dead due to the absence of a reply. The proxy path in eduroam
consists of several proxies, all of which introduce a very small
packet loss probability; that is, the more proxies needed, the higher
the failure rate is going to be.
For some EAP types, depending on the exact payload size they carry,
RADIUS servers and/or supplicants may choose to put as much EAP data
into a single RADIUS packet as the supplicant's Layer 2 medium allows
-- typically 1500 bytes. In that case, the RADIUS encapsulation
around the EAP-Message will add more bytes to the overall RADIUS
payload size and in the end exceed the 1500-byte limit, leading to
fragmentation of the UDP datagram on the IP layer. While in theory
this is not a problem, in practice there is evidence of misbehaving
firewalls that erroneously discard non-first UDP fragments; this
ultimately leads to a denial of service for users with such EAP types
and that specific configuration.
One EAP type proved to be particularly problematic: EAP-TLS. While
it is possible to configure the EAP server to send smaller chunks of
EAP payload to the supplicant (e.g., 1200 bytes, to allow for another
300 bytes of RADIUS overhead without fragmentation), very often the
supplicants that send the client certificate do not expose such a
configuration detail to the user. Consequently, when the client
certificate is over 1500 bytes in size, the EAP-Message will always
make use of the maximum possible Layer 2 chunk size, and this
introduces fragmentation on the path from EAP peer to EAP server.
Both of the previously mentioned sources of errors (packet loss and
fragment discard) lead to significant frustration for the affected
users. Operational experience of eduroam shows that such cases are
hard to debug since they require coordinated cooperation of all
eduroam administrators on the authentication path. For that reason,
the eduroam community is developing monitoring tools that help to
locate fragmentation problems.
Note: For more detailed discussion of these issues, please refer to
Section 1.1 of [RFC6613].
3.5. Insufficient Payload Encryption and EAP Server Validation
The RADIUS protocol's design foresaw only the encryption of select
RADIUS attributes, most notably User-Password. With EAP methods
conforming to the requirements of [RFC4017], the user's credential is
not transmitted using the User-Password attribute, and stronger
encryption than the one for RADIUS User-Password is in use (typically
Still, the use of EAP does not encrypt all personally identifiable
details of the user session, as some are carried inside cleartext
RADIUS attributes. In particular, the user's device can be
identified by inspecting the Calling-Station-ID attribute; and the
user's location may be derived from observing NAS-IP-Address, NAS-
Identifier, or Operator-Name attributes. Since these attributes are
not encrypted, even IP-layer third parties can harvest the
corresponding data. In a worst-case scenario, this enables the
creation of mobility profiles. Pervasive passive surveillance using
this connection metadata such as the recently uncovered incidents in
the US National Security Agency (NSA) and the UK Government
Communications Headquarters (GCHQ) becomes possible by tapping RADIUS
traffic from an IP hop near a RADIUS aggregation proxy. While this
is possible, the authors are not aware whether this has actually been
These profiles are not necessarily linkable to an actual user because
EAP allows for the use of anonymous outer identities and protected
credential exchanges. However, practical experience has shown that
many users neglect to configure their supplicants in a privacy-
preserving way or their supplicants don't support that. In
particular, for EAP-TLS users, the use of EAP-TLS identity protection
is not usually implemented and cannot be used. In eduroam, concerned
individuals and IdPs that use EAP-TLS are using pseudonymous client
certificates to provide for better privacy.
One way out, at least for EAP types involving a username, is to
pursue the creation and deployment of preconfigured supplicant
configurations that make all the required settings in user devices
prior to their first connection attempt; this depends heavily on the
remote configuration possibilities of the supplicants though.
A further threat involves the verification of the EAP server's
identity. Even though the cryptographic foundation, TLS tunnels, is
sound, there is a weakness in the supplicant configuration: many
users do not understand or are not willing to invest time into the
inspection of server certificates or the installation of a trusted
certification authority (CA). As a result, users may easily be
tricked into connecting to an unauthorized EAP server, ultimately
leading to a leak of their credentials to that unauthorized third
Again, one way out of this particular threat is to pursue the
creation and deployment of preconfigured supplicant configurations
that make all the required settings in user devices prior to their
first connection attempt.
Note: There are many different and vendor-proprietary ways to
preconfigure a device with the necessary EAP parameters (examples
include Apple, Inc.'s "mobileconfig" and Microsoft's "EAPHost" XML
schema). Some manufacturers even completely lack any means to
distribute EAP configuration data. We believe there is value in
defining a common EAP configuration metadata format that could be
used across manufacturers, ideally leading to a situation where IEEE
802.1X network end users merely need to apply this configuration file
to configure any of their devices securely with the required
Another possible privacy threat involves transport of user-specific
attributes in a Reply-Message. If, for example, a RADIUS server
sends back a hypothetical RADIUS Vendor-Specific-Attribute "User-Role
= Student of Computer Science" (e.g., for consumption of an SP RADIUS
server and subsequent assignment into a "student" VLAN), this
information would also be visible for third parties and could be
added to the mobility profile.
The only way to mitigate all information leakage to third parties is
by protecting the entire RADIUS packet payload so that IP-layer third
parties cannot extract privacy-relevant information. RADIUS as
specified in RFC 2865 does not offer this possibility though. This
motivated [RFC6614]; see Section 4.1.
4. New Trust Fabric
The operational difficulties with an ever-increasing number of
participants (as documented in the previous section) have led to a
number of changes to the eduroam architecture that in turn have led
to IETF specifications (as mentioned in the introduction).
Note: The enhanced architecture components are fully backwards
compatible with the existing installed base and are, in fact,
gradually replacing those parts of it where problems may arise.
Whereas the user authentication using IEEE 802.1X and EAP has
remained unchanged (i.e., no need for end users to change any
configurations), the issues as reported in Section 3 have resulted in
a major overhaul of the way EAP messages are transported from the
RADIUS server of the SP to that of the IdP and back. The two
fundamental changes are the use of TCP instead of UDP and reliance on
TLS instead of shared secrets between RADIUS peers, as outlined in
4.1. RADIUS with TLS
The deficiencies of RADIUS over UDP as described in Section 3.4
warranted a search for a replacement of RFC 2865 [RFC2865] for the
transport of EAP. By the time this need was understood, the
designated successor protocol to RADIUS, Diameter, was already
specified by the IETF in its intial version [RFC3588]. However,
within the operational constraints of eduroam (listed below), no
single combination of software could be found (and that is believed
to still be true, more than ten years and one revision of Diameter
[RFC6733] later). The constraints are:
o reasonably cheap to deploy on many administrative domains
o supporting the application of Network Access Server Requirements
o supporting EAP application
o supporting Diameter Redirect
o supporting validation of authentication requests of the most
popular EAP types (EAP Tunneled Transport Layer Security
(EAP-TTLS), Protected EAP (PEAP), and EAP-TLS)
o possibility to retrieve these credentials from popular back-ends
such as MySQL or Microsoft's Active Directory.
In addition, no Wi-Fi Access Points at the disposal of eduroam
participants supported Diameter, nor did any of the manufacturers
have a roadmap towards Diameter support (and that is believed to
still be true, more than 10 years later). This led to the open
question of lossless translation from RADIUS to Diameter and vice
versa -- a question not satisfactorily answered by NASREQ.
After monitoring the Diameter implementation landscape for a while,
it became clear that a solution with better compatibility and a
plausible upgrade path from the existing RADIUS hierarchy was needed.
The eduroam community actively engaged in the IETF towards the
specification of several enhancements to RADIUS to overcome the
limitations mentioned in Section 3. The outcome of this process was
[RFC6614] and [DYN-DISC].
With its use of TCP instead of UDP, and with its full packet
encryption, while maintaining full packet format compatibility with
RADIUS/UDP, RADIUS/TLS [RFC6614] allows any given RADIUS link in
eduroam to be upgraded without the need of a "flag day".
In a first upgrade phase, the classic eduroam hierarchy (forwarding
decision made by inspecting the realm) remains intact. That way,
RADIUS/TLS merely enhances the underlying transport of the RADIUS
datagrams. But, this already provides some key advantages:
o explicit peer reachability detection using long-lived TCP sessions
o protection of user credentials and all privacy-relevant RADIUS
RADIUS/TLS connections for the static hierarchy could be realized
with the TLS-PSK [RFC4279] operation mode (which effectively provides
a 1:1 replacement for RADIUS/UDP's "shared secrets"), but since this
operation mode is not widely supported as of yet, all RADIUS/TLS
links in eduroam are secured by TLS with X.509 certificates from a
set of accredited CAs.
This first deployment phase does not yet solve the routing table
complexity problem (see Section 3.3); this aspect is covered by
introducing dynamic discovery for RADIUS/TLS servers.
4.2. Dynamic Discovery
When introducing peer discovery, two separate issues had to be
1. how to find the network address of a responsible RADIUS server
for a given realm
2. how to verify that this realm is an authorized eduroam
4.2.1. Discovery of Responsible Server
Issue 1 can relatively simply be addressed by putting eduroam-
specific service discovery information into the global DNS tree. In
eduroam, this is done by using NAPTR records as per the S-NAPTR
specification [RFC3958] with a private-use NAPTR service tag
("x-eduroam:radius.tls"). The usage profile of that NAPTR resource
record is that exclusively "S" type delegations are allowed and that
no regular expressions are allowed.
A subsequent lookup of the resulting SRV records will eventually
yield hostnames and IP addresses of the authoritative server(s) of a
Example (wrapped for readability):
> dig -t naptr education.example.
;; ANSWER SECTION:
education.example. 43200 IN NAPTR 100 10 "s"
> dig -t srv _radsec._tcp.eduroam.example.
;; ANSWER SECTION:
_radsec._tcp.eduroam.example. 43200 IN SRV 0 0 2083
> dig -t aaaa tld1.eduroam.example.
;; ANSWER SECTION:
tld1.eduroam.example. 21751 IN AAAA 2001:db8:1::2
Figure 3: SRV Record Lookup
From the operational experience with this mode of operation, eduroam
is pursuing standardization of this approach for generic AAA use
cases. The current RADEXT working group document for this is
Note: It is worth mentioning that this move to a more complex,
flexible system may make the system as a whole more fragile, as
compared to the static set up.
4.2.2. Verifying Server Authorization
Any organization can put "x-eduroam" NAPTR entries into their Domain
Name Server, pretending to be the eduroam Identity Provider for the
corresponding realm. Since eduroam is a service for a heterogeneous,
but closed, user group, additional sources of information need to be
consulted to verify that a realm with its discovered server is
actually an eduroam participant.
The eduroam consortium has chosen to deploy a separate PKI that
issues certificates only to authorized eduroam Identity Providers and
eduroam Service Providers. Since certificates are needed for RADIUS/
TLS anyway, it was a straightforward solution to reuse the PKI for
that. The PKI fabric allows multiple CAs as trust roots (overseen by
a Policy Management Authority) and requires that certificates that
were issued to verified eduroam participants are marked with
corresponding "X509v3 Policy OID" fields; eduroam RADIUS servers and
clients need to verify the existence of these OIDs in the incoming
The policies and OIDs can be retrieved from the "eduPKI Trust Profile
for eduroam Certificates" [eduPKI].
4.2.3. Operational Experience
The discovery model is currently deployed in approximately 10
countries that participate in eduroam, making more than 100 realms
discoverable via their NAPTR records. Experience has shown that the
model works and scales as expected, the only drawback being that the
additional burden of operating a PKI that is not local to the
national eduroam administrators creates significant administrative
complexities. Also, the presence of multiple CAs and regular updates
of Certificate Revocation Lists makes the operation of RADIUS servers
4.2.4. Possible Alternatives
There are two alternatives to this approach to dynamic server
discovery that are monitored by the eduroam community:
1. DNSSEC + DNS-Based Authentication of Named Entities (DANE) TLSA
2. ABFAB Trust Router
For DNSSEC+DANE TLSA, the biggest advantage is that the certificate
data itself can be stored in the DNS -- possibly obsoleting the PKI
infrastructure *if* a new place for the server authorization checks
can be found. Its most significant downside is that the DANE
specifications only include client-to-server certificate checks,
while RADIUS/TLS requires also server-to-client verification.
For the ABFAB Trust Router, the biggest advantage is that it would
work without certificates altogether (by negotiating TLS-PSK keys ad
hoc). The downside is that it is currently not formally specified
and not as thoroughly understood as any of the other solutions.