Network Working Group R. Braden
Request for Comments: 1636 ISI
Category: Informational D. Clark
MIT Laboratory for Computer Science
S. Crocker
Trusted Information Systems, Inc.
C. Huitema
INRIA, IAB Chair
June 1994
Report of IAB Workshop on
Security in the Internet Architecture
February 8-10, 1994
Status of this Memo
This memo provides information for the Internet community. This memo
does not specify an Internet standard of any kind. Distribution of
this memo is unlimited.
Abstract
This document is a report on an Internet architecture workshop,
initiated by the IAB and held at USC Information Sciences Institute
on February 8-10, 1994. This workshop generally focused on security
issues in the Internet architecture.
This document should be regarded as a set of working notes containing
ideas about security that were developed by Internet experts in a
broad spectrum of areas, including routing, mobility, realtime
service, and provider requirements, as well as security. It contains
some significant diversity of opinions on some important issues.
This memo is offered as one input in the process of developing viable
security mechanisms and procedures for the Internet.
Braden, Clark, Crocker & Huitema [Page 1]RFC 1636 IAB Workshop Report June 1994
Table of Contents
1. INTRODUCTION .................................................. 2
2. OVERVIEW ...................................................... 4
2.1 Strategic and Political Issues ........................... 4
2.2 Security Issues .......................................... 4
2.3 DNS Names for Certificates ............................... 7
3. FIREWALL ARCHITECTURE ......................................... 9
3.1 Introduction ............................................. 9
3.2 Application-Layer Firewalls .............................. 11
3.3 IP-Layer Firewalls ....................................... 12
4. SECURE QOS FORWARDING ......................................... 21
4.1 The Requirement for Setup ................................ 21
4.2 Securing the Setup Process. .............................. 22
4.3 Validating an LLID ....................................... 24
4.4 Dynamics of Setup ........................................ 28
4.5 Receiver-Initiated Setup ................................. 30
4.6 Other Issues ............................................. 30
5. AN AUTHENTICATION SERVICE ..................................... 35
5.1 Names and Credentials .................................... 36
5.2 Identity-Based Authorization ............................. 37
5.3 Choosing Credentials ..................................... 38
6. OTHER ISSUES .................................................. 39
6.1 Privacy and Authentication of Multicast Groups ........... 39
6.2 Secure Plug-and-Play a Must .............................. 41
6.3 A Short-Term Confidentiality Mechanism ................... 42
7. CONCLUSIONS ................................................... 44
7.1 Suggested Short-Term Actions ............................. 44
7.2 Suggested Medium-Term Actions ............................ 46
7.3 Suggested Long-Term Actions .............................. 46
APPENDIX A -- Workshop Organization .............................. 48
Security Considerations .......................................... 52
Authors' Addresses ............................................... 52
1. INTRODUCTION
The Internet Architecture Board (IAB) holds occasional workshops
designed to consider long-term issues and strategies for the
Internet, and to suggest future directions for the Internet
architecture. This long-term planning function of the IAB is
complementary to the ongoing engineering efforts performed by working
groups of the Internet Engineering Task Force (IETF), under the
leadership of the Internet Engineering Steering Group (IESG) and area
directorates.
An IAB-initiated workshop on the role of security in the Internet
Architecture was held on February 8-10, 1994 at the Information
Sciences Institute of the University of Southern California, in
Braden, Clark, Crocker & Huitema [Page 2]RFC 1636 IAB Workshop Report June 1994
Marina del Rey, California. This RFC reports the results of the
workshop.
In addition to the IAB members, attendees at this meeting included
the IESG Area Directors for the relevant areas (Internet, Transport,
Security, and IPng) and a group of 15 other experts in the following
areas: IPng, routing, mobility, realtime service, and security (see
Appendix for a list of attendees). The IAB explicitly tried to
balance the number of attendees from each area of expertise.
Logistics limited the attendance to about 30, which unfortunately
meant that many highly qualified experts were omitted from the
invitation list.
In summary, the objectives of this workshop were (1) to explore the
interconnections between security and the rest of the Internet
architecture, and (2) to develop recommendations for the Internet
community on future directions with respect to security. These
objectives arose from a conviction in the IAB that the two most
important problem areas for the Internet architecture are scaling and
security. While the scaling problems have led to a flood of
activities on IPng, there has been less effort devoted to security.
Although some came to the workshop eager to discuss short-term
security issues in the Internet, the workshop program was designed to
focus more on long-term issues and broad principles. Thus, the
meeting began with the following ground rule: valid topics of
discussion should involve both security and at least one from the
list: (a) routing (unicast and multicast), (b) mobility, and (c)
realtime service. As a basis for initial discussion, the invitees
met via email to generate a set of scenarios (see Appendix)
satisfying this ground rule.
The 30 attendees were divided into three "breakout" groups, with each
group including experts in all the areas. The meeting was then
structured as plenary meetings alternating with parallel breakout
group sessions (see the agenda in Appendix). On the third day, the
groups produced text summarizing the results of their discussions.
This memo is composed of that text, somewhat rearranged and edited
into a single document.
The meeting process determined the character of this document. It
should be regarded as a set of working notes produced by mostly-
autonomous groups, containing some diversity of opinions as well as
duplication of ideas. It is not the output of the "security
community", but instead represents ideas about security developed by
a broad spectrum of Internet experts. It is offered as a step in a
process of developing viable security mechanisms and procedures for
the Internet.
Braden, Clark, Crocker & Huitema [Page 3]
RFC 1636 IAB Workshop Report June 1994
2. OVERVIEW
2.1 Strategic and Political Issues
Despite the workshop emphasis on architectural issues, there was
considerable discussion of the real-politik of security.
For a number of years, the IETF, with IAB backing, has worked on
developing PEM, which provides email security with a great deal of
functionality. A question was repeatedly raised at the workshop:
why has user acceptance of PEM been slow? A number of answers to
this question were suggested.
(a) High-quality implementations have been slow in coming.
(b) The use of a patented technology, the RSA algorithm, violates
social conventions of the Internet.
(c) Export restrictions dampen vendor enthusiasm.
(d) PEM currently depends upon a certificate hierarchy for its
names, and certificates form a new and complex name space.
There is no organizational infrastructure in place for creat-
ing and managing this name space.
(e) There is no directory infrastructure available for looking up
certificates.
The decision to use X.500 has been a complete failure, due to
the slow deployment of X.500 in the Internet. Because of UDP
packet size restrictions, it is not currently feasible to
store certificates in the DNS, even if the DNS were expanded
to hold records for individual email users.
It seems probable that more than one, and possibly all, of these
reasons are at work to discourage PEM adoption.
The baleful comment about eating: "Everything I enjoy is either
immoral, illegal, or fattening" seems to apply to the cryptography
technology that is required for Internet security.
2.2 Security Issues
Almost everyone agrees that the Internet needs more and better
security. However, that may mean different things to different
people. Four top-level requirements for Internet security were
identified: end-to-end security, end-system security, secure QOS,
and secure network infrastructure.
Braden, Clark, Crocker & Huitema [Page 4]RFC 1636 IAB Workshop Report June 1994
A. End-to-End Security
One requirement is to support confidentiality, authentication
and integrity for end-to-end communications. These security
services are best provided on an end-to-end basis, in order
to minimize the number of network components that users must
trust. Here the "end" may be the end system itself, or a
proxy (e.g., a firewall) acting on behalf of an end system.
For point-to-point applications, the workshop felt that
existing security techniques are well suited to support
confidentiality, authentication and integrity services
efficiently. These existing techniques include symmetric
encryption applied on an end-to-end basis, message digest
functions, and key management algorithms. Current work in
these areas in the IETF include the PEM and Common
Authentication Technologies working groups.
The group favored a strategic direction for coping with
export restrictions: separate authentication from privacy
(i.e., confidentiality). This will allow work to proceed on
authentication for the Internet, despite government
restrictions on export of privacy technology. Conversely, it
will allow easy deployment of privacy without authentication,
where this is appropriate.
The workshop explored the implications of multicasting for
end-to-end security. Some of the unicast security techniques
can be applied directly to multicast applications, while
others must be modified. Section 6.2 contains the results of
these discussions; in summary, the conclusions were:
a) Existing technology is adequate to support
confidentiality, authentication, and integrity at the
level of an entire multicast group. Supporting
authentication and integrity at the level of an
individual multicast source is performance-limited and
will require technology advances.
b) End-to-end controls should be based on end system or
user identifiers, not low level identifiers or locator
information. This requirement should spawn engineering
work which consists of applying known key distribution
Braden, Clark, Crocker & Huitema [Page 5]RFC 1636 IAB Workshop Report June 1994
and cryptographic techniques.
B. End-System Security
Every host has its own security defenses, but the strength of
these defenses depends upon the care that is taken in
administering them. Careful host security administration
means plugging security holes in the kernel and applications
as well as enforcing discipline on users to set good (hard to
crack) passwords.
Good security administration is labor-intensive, and
therefore organizations often find it difficult to maintain
the security of a large number of internal machines. To
protect their machines from outside subversion, organizations
often erect an outer security wall or "perimeter". Machines
inside the perimeter communicate with the rest of the
Internet only through a small set of carefully managed
machines called "firewalls". Firewalls may operate at the
application layer, in which case they are application relays,
or at the IP layer, in which case they are firewall routers.
The workshop spent considerable time on the architecture of
firewall routers. The results are contained in Section 3.
C. Secure QOS
The Internet is being extended to provide quality-of-service
capabilities; this is the topic called "realtime service" in
the workshop. These extensions raise a new set of security
issues for the architecture, to assure that users are not
allowed to attach to resources they are not authorized to
use, both to prevent theft of resources and to prevent denial
of service due to unauthorized traffic. The resources to be
protected include link shares, service classes or queues,
multicast trees, and so on. These resources are used as
virtual channels within the network, where each virtual
channel is intended to be used by a particular subset or
"class" of packets.
Secure QOS, i.e., protection against improper virtual channel
usage, is a form of access control mechanism. In general it
will be based on some form of state establishment (setup)
that defines authorized "classes". This setup may be done
via management configuration (typically in advance and for
aggregates of users), or it may be done dynamically via
control information in packets or special messages (typically
at the time of use by the source or receiver(s) of the
Braden, Clark, Crocker & Huitema [Page 6]RFC 1636 IAB Workshop Report June 1994
flow/data). In addition to state establishment, some form of
authentication will be needed to assure that successive
packets belong to the established class. The general case to
be solved is the multicast group, since in general the
multicast problem includes the two-party case as a subset.
The workshop developed an approach to the secure QOS problem,
which appears in Section 4 below.
D. Secure Network Infrastructure
Network operation depends upon the management and control
protocols used to configure and operate the network
infrastructure, including routers and DNS servers. An attack
on the network infrastructure may cause denial-of-service
from the user viewpoint, but from the network operators'
viewpoint, security from attack requires authentication and
integrity for network control and management messages.
Securing the routing protocols seems to be a straightforward
engineering task. The workshop concluded the following.
a) All routing information exchanges should be
authenticated between neighboring routers.
b) The sources of all route information should be
authenticated.
c) Although authenticating the authority of an injector of
route information is feasible, authentication of
operations on that routing information (e.g.,
aggregation) requires further consideration.
Securing router management protocols (e.g., SNMP, Telnet,
TFTP) is urgent, because of the currently active threats.
Fortunately, the design task should be a straightforward
application of existing authentication mechanisms.
Securing DNS is an important issue, but it did not receive
much attention at the workshop.
2.3 DNS Names for Certificates
As noted in Section 2.1, work on PEM has assumed the use of X.509
distinguished names as the basis for issuing certificates, with
public-key encryption. The most controversial discussion at the
workshop concerned the possibility of using DNS (i.e., domain)
names instead of X.509 distinguished names as (at least) an
interim basis for Internet security.
Braden, Clark, Crocker & Huitema [Page 7]RFC 1636 IAB Workshop Report June 1994
The argument in favor of DNS names is that they are simple and
well understood in the Internet world. It is easy for a computer
operating in the Internet to be identified this way, and users who
receive email on such machines already have DNS mailbox names. In
contrast, introducing X.509 distinguished names for security will
add a new layer of names. Most importantly, there is an existing
administrative model for assigning DNS names. There is no
administrative infrastructure for assigning X.509 distinguished
names, and generating them may be too complex for early
acceptance. The advocates of DNS names for certificates hope that
using DNS names would encourage the widespread use of security in
the Internet. It is expected that DNS names can be replaced later
by a more capable naming mechanism such as X.509-based
certificates.
The basic argument against DNS names as a basis for security is
that they are too "weak". Their use may lead to confusion in many
instances, and this confusion can only grow as more organizations
and individuals attach to the Internet. Some commercial email
systems employ numeric mailbox names, and in many organizations
there are uncertainties such as whether "bumber@foo.edu" belongs
to Bill Umber or Tom Bumber. While it is feasible to make DNS
names more descriptive, there is a concern that the existing
infrastructure, with millions of short, non-descriptive names,
will be an impediment to adoption of more descriptive names.
It was noted that the question of what name space to use for
certificates is independent of the problem of building an
infrastructure for retrieving those names. Because of UDP packet
size restrictions, it would not be feasible to store certificates
in the DNS without significant changes, even if the DNS were
expanded to hold records for individual email users.
The group was unable to reach a consensus on the issue of using
DNS names for security; further discussion in the Internet
community is needed.
Braden, Clark, Crocker & Huitema [Page 8]RFC 1636 IAB Workshop Report June 1994
3. FIREWALL ARCHITECTURE
3.1 Introduction
A firewall may be used to isolate a specific connected segment of
Internet topology. When such a segment has multiple links to the
rest of the Internet, coordinated firewall machines are required
on all the links.
Firewalls may be implemented at different layers in the protocol
stack. They are most commonly implemented at the application
layer by forwarding (application) gateways, or at the IP
(Internet) layer by filtering routers. Section 3.2 discusses
application gateways. Section 3.3 concerns Internet-layer
firewalls, which filter IP datagrams entering or leaving a
security perimeter.
The general architectural model for a firewall should separate
policy, i.e., determining whether or not the requester of a
service should be granted access to that service, from control,
i.e., limiting access to resources to those who have been granted
access.
3.1.1 The Use for Firewalls
Firewalls are a very emotional topic in the Internet community.
Some community members feel the firewall concept is very
powerful because firewalls aggregate security functions in a
single place, simplifying management, installation and
configuration. Others feel that firewalls are damaging for the
same reason: they provide "a hard, crunchy outside with a soft
chewy center", i.e., firewalls foster a false sense of
security, leading to lax security within the firewall
perimeter. They observe that much of the "computer crime" in
corporate environments is perpetrated by insiders, immune to
the perimeter defense strategy. Firewall advocates counter
that firewalls are important as an additional safeguard; they
should not be regarded as a substitute for careful security
management within the perimeter. Firewall detractors are also
concerned about the difficulty of using firewalls, requiring
multiple logins and other out-of-band mechanisms, and their
interference with the usability and vitality of the Internet.
However, firewalls are a fact of life in the Internet today.
They have been constructed for pragmatic reasons by
organizations interested in a higher level of security than may
be possible without them. This section will try to outline
some of the advantages and disadvantages of firewalls, and some
Braden, Clark, Crocker & Huitema [Page 9]RFC 1636 IAB Workshop Report June 1994
instances where they are useful.
Consider a large organization of thousands of hosts. If every
host is allowed to communicate directly with the outside world,
attackers will attempt to penetrate the organization by finding
the weakest host in the organization, breaching its defenses,
and then using the resources of that host to extend the
penetration further within the organization. In some sense,
firewalls are not so much a solution to a security problem as
they are a reaction to a more basic software
engineering/administration problem: configuring a large number
of host systems for good security. If this more basic problem
could be solved, firewalls would generally be unnecessary.
It is interesting to consider the effect that implementing a
firewall has upon various individuals in the organization.
Consider first the effect upon an organization's most secure
host. This host basically receives little or no extra
protection, because its own perimeter defenses are as strong or
stronger than the firewall. In addition, the firewall will
probably reduce the connectivity available to this host, as
well as the reliability of the communications path to the
outside world, resulting in inconvenience to the user(s) of
this host. From this (most secure) user's point of view, the
firewall is a loss.
On the other hand, a host with poor security can "hide" behind
the firewall. In exchange for a more limited ability to
communicate with the outside world, this host can benefit from
the higher level of security provided by the firewall, which is
assumed to be based upon the best security available in the
entire organization. If this host only wants to communicate
with other hosts inside the organization, the outside
communications limitations imposed by the firewall may not even
be noticed. From this host's viewpoint, better security has
been gained at little or no cost.
Finally, consider the point of view of the organization as a
whole. A firewall allows the extension of the best security in
the organization across the whole organization. This is a
benefit (except in the case where all host perimeter defenses
in the organization are equal). Centralized access control
also becomes possible, which may be either a benefit or a cost,
depending upon the organization. The "secure" hosts within the
organization may perceive a loss, while the "unsecure" hosts
receive a benefit. The cost/benefit ratio to the organization
as a whole thus depends upon the relative numbers of "secure"
and "unsecure" hosts in the organization.
Braden, Clark, Crocker & Huitema [Page 10]RFC 1636 IAB Workshop Report June 1994
Consider some cases where firewalls do not make sense. An
individual can be thought of as an organization of one host.
The security of all the host(s) is thus (trivially) identical,
and by definition the best available to the organization. In
this case the choice of firewall is simple. Does this
individual wish to communicate with the outside or not? If
not, then the "perfect" firewall is implemented (by complete
disconnection). If yes, then the host perimeter will be the
same as the firewall perimeter, so a firewall becomes
unnecessary.
Another interesting case is an organization that consists of
individuals with few shared interests. This might be the case
of a service provider that sells public access to the network.
An unrelated community of subscribers should probably be
considered as individuals, rather than an organization.
Firewalls for the whole organization may make little sense in
this case.
To summarize, the benefit of a firewall depends upon the nature
of the organization it protects. A firewall can be used to
extend the best available protection within the organization
across the entire organization, and thus be of benefit to large
organizations with large numbers of poorly administered hosts.
A firewall may produce little or no perceived benefit, however,
to the individuals within an organization who have strong host
perimeters already.
3.2 Application-Layer Firewalls
An application-layer firewall can be represented by the following
diagram.
C <---> F <---> S
Here the requesting client C opens its transport connection to the
firewall F rather than directly to the desired server S. One
mechanism for redirecting C's request to F's IP address rather
than S's could be based on the DNS. When C attempts to resolve
S's name, its DNS lookup would return a "service redirection"
record (analogous to an MX record) for S. The service redirection
record would return the IP address of F.
C enters some authentication conversation to identify itself to F,
and specifies its intention to request a specific service from S.
F then decides if C is authorized to invoke this service. If C is
authorized, F initiates a transport layer connection to S and
begins the operation, passing requests and responses between C and
Braden, Clark, Crocker & Huitema [Page 11]RFC 1636 IAB Workshop Report June 1994
S.
A major advantage of this scenario over an IP-layer firewall is
that raw IP datagrams are never passed through the firewall.
Because the firewall operates at the application layer, it has the
opportunity to handle and verify all data passing through it, and
it may be more secure against illicit rendezvous attacks (see
below).
Application layer firewalls also have important disadvantages.
For full benefit, an application level firewall must be coded
specifically for each application. This severely limits the
deployment of new applications. The firewall also represents a
new point of failure; if it ceases to be reachable, the
application fails. Application layer firewalls also may affect
performance more than IP-layer firewalls, depending on specific
mechanisms in use.
3.3 IP-Layer Firewalls
Our model of an IP-layer firewall is a multi-ported IP router that
applies a set of rules to each incoming IP datagram, to decide
whether it will be forwarded. It is said to "filter" IP
datagrams, based on information available in the packet headers.
A firewall router generally has a set of filtering rules, each of
which specifies a "packet profile" and an "action". The packet
profile specifies values for particular header fields, e.g.,
source and destination IP address, protocol number, and other
suitable source and destination identifying information (for
instance, port numbers). The set of possible information that may
be used to match packets is called an "association". The exact
nature of an association is an open issue.
The high-speed datagram forwarding path in the firewall processes
every arriving packet against all the packet profiles of all
active rules, and when a profile matches, it applies the
corresponding action. Typical actions may include forwarding,
dropping, sending a failure response, or logging for exception
tracking. There may be a default rule for use when no other rule
matches, which would probably specify a drop action.
In addition to the packet profile, some firewalls may also use
some cryptographic information to authenticate the packet, as
described below in section 3.3.2.
Braden, Clark, Crocker & Huitema [Page 12]RFC 1636 IAB Workshop Report June 1994
3.3.1 Policy Control Level
This section presents a model for the control of a firewall
router, with some examples of specific mechanisms that might be
used.
1. A client C attempts to access a service S. (Client here
can mean either a person or a process - that also is an
issue to be resolved.)
2. The initiation of access to that service may result in an
attempt to cross one or more boundaries of protection via
firewall router(s).
3. The policy control level sets filters in the firewall
router(s), to permit or deny that attempt.
The policy control level consists of two distinct functions,
authentication and authorization. Authentication is the
function of verifying the claimed identity of a user. The
authentication function should be distributed across the
Internet, so that a user in one organization can be
authenticated to another organization. Once a user is
authenticated, it is then the job of the authorization service
local to the resource being requested to determine if that user
is authorized to access that resource. If authorization is
granted, the filter in the firewall can be updated to permit
that access.
As an aid to understanding the issues, we introduce a
particular detailed mechanism. We emphasize that this
mechanism is intended only as an illustrative example; actual
engineering of the mechanism will no doubt lead to many
changes. Our mechanism is illustrated by the following sketch.
Here a user wishes to connect from a computer C behind firewall
F1, to a server S behind firewall F2. A1 is a particular
authentication server and Z1 is a particular authorization
server.
C <-------> F1 <-------> F2 <-------> S
\ /
\_____ /
\ \ /
A1 Z1
C attempts to initiate its conversation by sending an initial
packet to S. C uses a normal DNS lookup to resolve S's name,
and uses normal IP routing mechanisms. C's packet reaches
Braden, Clark, Crocker & Huitema [Page 13]RFC 1636 IAB Workshop Report June 1994
firewall router F1, which rejects the packet because it does
not match any acceptable packet profile. F1 returns an
"Authentication Required" error indication to C, including a
list of authentication/authorization servers that F1 trusts.
This indication might be a new type of ICMP Destination
Unreachable packet, or some other mechanism for communicating
with C.
When C receives the error indication, authenticates itself with
A1, one of the authentication servers listed in the error
indication, after validating A1's identity. C then requests
authorization from server Z1 (using a ticket provided by A1),
informs Z1 of the application it wishes to perform, and
provides a profile for the packets it wishes to pass through
F1. Z1 then performs an authorization function to decide
whether to allow C to penetrate F1. If C is to be allowed, Z1
then informs the firewall F1 to allow packets matching the
packet profile to pass through the firewall F1.
After C's packets penetrate F1, they may again be rejected by a
second firewall F2. C could perform the same procedures with
authentication server A2 and authorization server Z2, which F2
trusts. This is illustrated by the following schematic diagram
of the sequence of events.
Braden, Clark, Crocker & Huitema [Page 14]RFC 1636 IAB Workshop Report June 1994
----------+--------+--------+------------+------------+----
| C | A1 | Z1 | F1 | F2 | S
----------+--------+--------+------------+------------+----
| Sends pkt| | | | |
| to S ----------------------->Intercept;| |
| | | | requires | |
| | | |authenticat'n |
| <------------------------------- | |
|Auth'cate | | | | |
| C to A1 ----> | | | |
| |Provide | | | |
| <------- ticket| | | |
| Request | | | | |
|authoriz'n| | | | |
| -------------------> Is C| | |
| | |allowed?| | |
| | | OK ---------> | |
|Resend | | | Set filter | |
| first pkt| | | | |
| to S -------------------------->(OK)------>Intercept;|
| | | | | requires |
| | | | |authenticat'n
| <------------------------------------------- |
| (Repeat | | | | |
|procedure | | | | |
with A2,Z2)| | | | |
| ... | | | | |
|Resend | | | | |
| first pkt| | | | |
| ------------------------------>(OK)--------(OK)------>
| | | | | |
-----------+--------+--------+------------+------------+----
Again, we emphasize that this is only intended as a partial
sketch of one possible mechanism. It omits some significant
issues, including the possibility of asymmetric routes (see
3.3.3 below), and the possibility that the profiles may be
different in the two directions between C and S.
We could imagine generalizing this to an arbitrary sequence of
firewalls. However, security requires that each of the
firewalls be able to verify that data packets actually come
from C. This packet authentication problem, which is discussed
in the next section, could be extremely difficult if the data
must traverse more than one or possibly two firewalls in
sequence.
Braden, Clark, Crocker & Huitema [Page 15]RFC 1636 IAB Workshop Report June 1994
A firewall router may require re-authentication because:
* it has been added to the path by a routing change, or
* it has timed out the profile entry, or
* it has been newly re-activated, perhaps after a crash that
lost its list of acceptable profiles.
If C contacts authentication and authorization servers that S
trusts, C may utilize tickets given it by these servers when
initiating its use of S, and avoid re-authenticating itself to
S.
Although the authentication server A1 and the authorization
server Z1 are conceptually separate, they may run on the same
computer or router or even be separate aspects of a single
program. The protocol that C speaks to an An, the protocol
that C speaks to a Zn, and the protocol that Zn speaks to Fn
are not specified in these notes. The authentication mechanism
used with An and the packet profile required by a firewall Fn
are considered matters of policy.
3.3.2 Source Authentication
We next consider how to protect against spoofing the IP source
address, i.e., injecting packets that are alleged from come
from C but do not. There are three classes of mechanisms to
prevent such spoofing of IP-level firewalls. The mechanisms
outlined here are also discussed in Section 4.3 below.
o Packet Profile Only
The lowest level of security consists of allowing the IP-
layer firewall to filter packets purely on the basis of
the packet profile. This is essentially the approach used
by filtering routers today, with the addition of (1)
authentication and authorization servers to control the
filtering profiles, and (2) the automatic "Authentication
Required" notification mechanism. This approach provides
almost no security; it does not prevent other computers
from spoofing packets that appear to be transmitted by C,
or from taking over C's transport level connection to S.
o Sealed Packets
In the second level of security, each packet is "sealed"
with a secure hash algorithm. An authentication server Ai
Braden, Clark, Crocker & Huitema [Page 16]RFC 1636 IAB Workshop Report June 1994
chooses a secret and shares it with the source host S and
also with the authorization server Zi, which shares the
secret with the firewall Fi. Every packet that C
transmits contains a hash value that depends upon both the
contents of the packet and the secret value. The firewall
Fi can compute the same hash function and verify that the
packet was originated by a computer that knew the shared
secret.
This approach does raise issues of how much C trusts Zi
and Fi. Since they know C's secret, Zi or Fi could spoof
C. If C does not trust all Z's and F's in its path, a
stronger mechanism (see below) is needed.
A more difficult problem arises in authenticating C's
packets when more than one firewall lies in the path.
Carrying a separate seal for each firewall that is
penetrated would be costly in terms of packet size. On
the other hand, in order to use a single seal, all the
firewalls would have to cooperate, and this might require
a much more complex mechanism than the one sketched in the
previous section. Morever, it may require mutual trust
among all of the authentication servers Ai and
authorization servers Zi; any of these servers could
undermine all the others. Another possibility to be
investigated is to use hop-by-hop rather than end-to-end
authentication of C's packets. That is, each firewall
would substitute into the packet the hash needed by the
next firewall.
Multi-firewall source authentication is a difficult
problem that needs more investigation.
o Packet Signatures
In the third level of security, each packet is "signed"
using a public/private key algorithm. C shares its public
key with Zn, which shares it with Fn. In this scenario, C
can safely use one pair of keys for all authorization
servers and firewalls. No authorization server or
firewall can spoof C because they cannot sign packets
correctly.
Although packet signing gives a much higher level of
security, it requires public key algorithms that are
patented and currently very expensive to compute; their
time must be added to that for the hash algorithm. Also,
signing the hash generally makes it larger.
Braden, Clark, Crocker & Huitema [Page 17]RFC 1636 IAB Workshop Report June 1994
3.3.3 Other Firewall Issues
o Performance
An Internet-layer firewall has the advantage of generality
and flexibility. However, filtering introduces a
potential performance problem. Performance may depend
upon the number and position of the packet fields used for
filtering, and upon the number of rules against which a
packet has to be matched.
Denial of service attacks require that the per-packet rule
matching and the drop path be able to keep up with the
interface speed.
o Multicasting
To allow multicast traffic to penetrate a firewall, the
rule that is needed should be supplied by the receiver
rather than the sender. However, this will not work with
the challenge mechanism outlined in Section 3.3.1, since
"Authentication Required" notifications would be sent to
the sender, not to the receiver(s).
Multicast conversations may use any of the three levels of
security described in the previous section, but all
firewalls will have to share the same secret with the
originator of the data stream. That secret would have to
be provided to the receivers through other channels and
then passed to the firewalls at the receivers' initiative
(in much the same way that resources are reserved at
receiver's initiative in RSVP).
o Asymmetric Routing
Given a client computer C utilizing a service from another
computer C through a firewall F: if the packets returning
from S to C take a different route than packets from C to
S, they may encounter another firewall F' which has not
been authorized to pass packets from S to C (unlike F,
which has been). F' will challenge S rather than C, but S
may not have credentials to authenticate itself with a
server trusted by F'.
Fortunately, this asymmetric routing situation is not a
problem for the common case of single homed administrative
domains, where any asymmetric routes converge at the
firewall.
Braden, Clark, Crocker & Huitema [Page 18]RFC 1636 IAB Workshop Report June 1994
o Illicit Rendezvous
None of these mechanisms prevent two users on opposite
sides of a firewall from rendezvousing with a custom
application written over a protocol that may have been
authorized to run through a firewall.
For example, if an organization has a policy that certain
information is sensitive and must not be allowed outside
its premises, a firewall will not be enough to enforce
this policy if users are able to attach sensitive
information to mail and send it outside to arbitrary
parties. Similarly, a firewall will not prevent all
problems with incoming data. If users import programs and
execute them, the programs may have Trojan horses which
disclose sensitive information or modify or delete
important data. Executable code comes in many, many
forms, including PostScript files, scripts for various
interpreters, and even return addresses for sendmail. A
firewall can detect some of these and scan for some forms
of potentially hazardous code, but it cannot stop users
from transforming things that look like "data" into
programs.
We consider these problems to be somewhat outside the
scope of the firewall router mechanism. It is a matter of
the policies implemented by the organization owning the
firewalls to address these issues.
o Transparency for Security Packets
For the mechanisms described above to operate, the
"Authentication Required" notification and the
authentication/authorization protocol that is used between
the client computer and the authentication and
authorization servers trusted by a firewall, must be
passed by all firewalls automatically. This might be on
the basis of the packet profiles involved in security.
Alternatively, firewall routers might serve as
application-layer firewalls for these types of
communications. They could then validate the data they
pass to avoid spoofing or illicit rendezvous.
3.3.4 Firewall-Friendly Applications
Firewall routers have problems with certain communication
patterns where requests are initiated by the server, including
callbacks and multiple connections (e.g., FTP). It was
Braden, Clark, Crocker & Huitema [Page 19]RFC 1636 IAB Workshop Report June 1994
suggested that it would be useful to have guidelines to
application designers to help them to build 'firewall-friendly
applications'. The following guidelines were suggested:
1) no inbound calls (the xterm problem),
2) fixed port numbers (no portmapper or tcpmux),
3) integral redirection is good (application gateways),
4) no redirection in the protocol,
5) 32 bit sequence numbers that are crypto-strong random #'s,
and
6) fixed length and number of header fields.
Type fields are good, but they may not be needed if there are
fixed port numbers.
3.3.5 Conclusions
Compared to an application-layer firewall, an IP-layer firewall
scheme could provide a number of benefits:
- No extra authentication is required for end hosts.
- A single authentication protocol can be used for all
intended applications.
- An IP-layer firewall causes less performance degradation.
- An IP-layer firewall may be able to crash and recover
state without disturbing open TCP connections.
- Routes can shift without disturbing open TCP connections.
- There is no single point of failure.
- It is independent of application.
However, there are substantial difficult design issues to be
solved, particularly in the areas of multiple firewalls,
assymmetric routes, multicasting, and performance.
Braden, Clark, Crocker & Huitema [Page 20]RFC 1636 IAB Workshop Report June 1994
4. SECURE QOS FORWARDING
When the Internet supports special qualities-of-service (QOS) for
particular packet flows, there will be a new set of security
problems. There will be a need to authenticate and authorize users
asking for those QOS values that are expensive in network resources,
and it will be necessary to prevent theft of these resources and
denial-of-service attacks by others. This section contains a
conceptual model for these problems, which we may call secure QOS
forwarding. The issues here differ from end-to-end security and
firewalls, because QOS forwarding security may need to be enforced at
every router along a path.
It was noted that this is not a new problem; it was stated and solved
in a theoretical way in a thesis by Radia Perlman.
4.1 The Requirement for Setup
Setup is an essential part of any QOS mechanism. However, it may
be argued that there are also good engineering reasons for setup
in any Internet-layer security mechanism, even without QOS
support. In the abstract, one could imagine a pure datagram model
in which each IP packet separately carried the necessary
authorizations for all the stages in the forwarding path.
Realistically, this is not practical, since the security
information may be both unacceptably large and computationally
demanding for inclusion in every packet. This seems to imply the
need for some form of state setup for security.
Thus, we presume a two stage process that moves somewhat away from
the pure datagram model. In the first stage, the setup stage,
some state is established in the routers (and other network
elements) that describes how a subsequent stream of packets is to
be treated. In the second stage, the classification stage, the
arriving packets are matched with the correct state information
and processed. The terminology in use today calls these different
state descriptions "classes", and the process of sorting
"classification".
Setup can take many forms. It could be dynamic, invoked across
the network by an application as described above. The setup
process could also be the manual configuration of a router by
means of a protocol such as SNMP or remote login. For example, a
network link, such as a link across the Atlantic, might be shared
by a number of users who purchase it jointly. They might
implement this sharing by configuring a router with
specifications, or filters, which describe the sorts of packets
that are permitted to use each share. Whether the setup is
Braden, Clark, Crocker & Huitema [Page 21]RFC 1636 IAB Workshop Report June 1994
dynamic or manual, short-lived or semi-permanent, it has the same
effect: it creates packet classes in the router and defines how
packets are to be classified as they arrive.
Much of the current research on extensions to IP for QOS, such as
realtime service, has assumed an explicit setup phase and a
classification stage. The setup stage is accomplished using
protocols such as RSVP or ST-II, which also specify how the
subsequent classification is to be done. Security at the setup
stage would thus simply be an extension to such a protocol. It
should be noted that there are alternative proposals for realtime
QOS, based on an implicit setup process.
4.2 Securing the Setup Process.
To secure the setup process, we require that a setup request be
accompanied by user credentials that provide a trustworthy
assurance that the requester is known and is authorized to make
the request in question. We refer to the credentials used in the
setup phase as the high-level identification (HLID).
A simple version of this authorization would be a password on the
management interface to a router (the limitations of such a
password scheme are well known and not the issue here). In the
case of setup requests made by individual applications, some
user-specific authorization must be assumed.
While there could be any number of ways to organize the HLIDs, the
objective of scaling suggests that a global framework for user
naming and authentication would be useful. The choice of naming
framework is discussed further in Section 5. Note that this
discussion, which concerns controlling access to network resources
and security devices, is distinct from end-to-end authentication
and access control; however, the same authentication
infrastructure could be used for both.
In general, while significant engineering effort will be required
to define a setup architecture for the Internet, there is no need
to develop new security techniques. However, for the security
aspects of the classification process, there are significant
problems related to performance and cost. We thus focus on that
aspect of the overall framework in more detail.
Above, we defined the high-level ID (HLID) as that set of
information presented as part of a setup request. There may also
be a "low-level ID" (LLID), sometimes called a "cookie", carried
in each packet to drive classification. In current proposals for
IP extensions for QOS, packets are classified based on existing
Braden, Clark, Crocker & Huitema [Page 22]RFC 1636 IAB Workshop Report June 1994
packet fields, e.g., source and destination addresses, ports, and
protocol type.
It is important to note that the LLID is distinct from the address
of the user, at least conceptually. By stressing this distinction
we make the point that the privileges of the user are not
determined by the address in use. If the user's address changes,
the privileges do not.
The LLID in a packet acts as a form of tag that is used by some or
all routers along a path to make decisions about the sort of QOS
that shall be granted to this packet. An LLID might refer to a
data stream between a single source-destination address pair, or
it might be more general and encompass a range of data streams.
There is no requirement that the LLID embody a syntax that permits
a router to discern the QOS parameters that it represents, but
there also is no prohibition against imposing such a structure.
We propose that an IP datagram contain one LLID, which can be used
at various stages of the network to map the packet to a class. We
reject the alternative that the packet should have a variable
number of LLIDs, each one for a different point in the net.
Again, this is not just a security comment, but it has security
implications.
The attributes of the LLID should be picked to match as broad a
range of requirements as possible.
* Its duration (discussed below) must match both the needs of
the security protocol, balancing robustness and efficiency,
and the needs of the application, which will have to deal
with renewal of the setup when the LLID expires. A useful
end-node facility would be a service to renew setup requests
automatically.
* The degree of trust must be high enough to meet the most
stringent requirement we can reasonably meet.
* The granularity of the LLID structure must permit packet
classification into classes fine-grained enough for any
resource selection in the network. We should therefore
expect that each separate stream of packets from an
application will have a distinct LLID. There will be little
opportunity for aggregating multiple streams under one LLID
or one authenticator.
Braden, Clark, Crocker & Huitema [Page 23]RFC 1636 IAB Workshop Report June 1994
4.3 Validating an LLID
At a minimum, it is necessary to validate the use of an LLID in
context, i.e., to ensure that it is being asserted in an
authorized fashion. Unauthorized use of an LLID could result in
theft of service or denial-of-service attacks, where packets not
emitted by an authorized sender are accorded the QOS treatment
reserved for that sender (or for a group of which the sender is a
member). Thus, use of an LLID should be authenticated by routers
that make QOS decisions based on that LLID. (Note that not all
routers may "pay attention" to the LLID.)
In principle, the validity of an LLID assertion needs to be
checked on every packet, though not necessarily at every router;
it may be possible to restrict the checks to security perimeters.
At those routers that must validate LLIDs, there is an obvious
concern over the performance impact. Therefore, a router may
adopt a less rigorous approach to LLID validation. For example, a
router may elect to sample a data stream and validate some, but
not all, packets. It may also elect to forward packets first and
perform selective validation as a background activity. In the
least stringent approach, a router might log selected packets and
validate them as part of an audit activity much later.
There are several candidate techniques for validating the use of
LLIDs. We have identified three basic techniques, which differ in
terms of computational performance, bandwidth overhead, and
effectiveness (resistance to various forms of attack).
* Digital Signatures
The first technique entails the use of public key
cryptography and digital signatures. The sender of each
packet signs the packet (header and payload) by computing a
one-way hash over the packet and transforming the hash value
using a private key associated with the LLID. The resulting
authenticator value is included in the packet header. The
binding between the public key and the LLID is established
through a connection setup procedure that might make use of
public keys that enjoy a much longer lifetime. Using public
key technology yields the advantage that any router can
validate a packet, but no router is entrusted with data that
would enable it to generate a packet with a valid
authenticator (i.e., which would be viewed as valid by other
routers.) This characteristic makes this technique ideal
from the standpoint of the "principle of least privilege."
Braden, Clark, Crocker & Huitema [Page 24]RFC 1636 IAB Workshop Report June 1994
Public key cryptosystems such as RSA have the advantage that
validation of a signature is much faster than signing, which
reduces the router processing burden. Nonetheless, this
approach is not likely to be feasible for anything other than
selective checking by routers, given current public key
algorithm performance.
* Sealing
The next technique is based on the use of the same type of
one-way hash function used for digital signatures, but it
does not require signing the hash value. Here the sender
computes a one-way hash with a secret quantity (essentially a
"key") appended to the packet. This process is an example of
what is sometimes referred to more generically as
cryptographic "sealing." The inclusion of this key at the
end of the hash computation results in a hash value that is
not predictable by any entity not possessing the key. The
resulting hash value is the authenticator and is included in
the packet header. A router validates a packet by
recomputing the hash value over the received packet with the
same secret quantity appended. If the transmitted hash value
matches the recomputed hash value, the packet is declared
valid. Unlike the signature technique, sealing implies that
all routers capable of verifying a seal are also capable of
generating (forging) a seal. Thus, this technique requires
that the sender trust the routers not to misuse the key.
This technique has been described in terms of a single secret
key shared between the sender and all the routers that need
to validate packets associated with an LLID. A related
alternative strategy uses the same authenticator technique,
but shares the secret key on a pairwise basis, e.g., between
the sender and the first router, between the first router and
the next, etc. This avoids the need to distribute the secret
key among a large group of routers, but it requires that the
setup mechanism enable Router A to convince his neighbor
(Router B) that Router A is authorized to represent traffic
on a specific LLID or set of LLIDs. This might best be done
by encapsulating the packet inside a wrapper that both ends
of the link can validate. Once this strategy is in place, it
may even be most efficient for routers to aggregate traffic
between them, providing authentication not on a per-LLID
basis, since the router pairs are prepared to "trust" one
another to accurately represent the data stream LLIDs.
For a unicast data stream, the use of pairwise keying between
routers does not represent a real change in the trust
Braden, Clark, Crocker & Huitema [Page 25]RFC 1636 IAB Workshop Report June 1994
required of the routers or of the setup mechanism, because of
the symmetric sharing of the secret key. However, for a
multicast connection, this pairwise keying approach is
superior in that it prevents a router at one point in a
multicast tree from being able to generate traffic that could
be inserted at another point in the tree. At worst, a router
can generate spurious, but authenticatable, traffic only for
routers "below" it in the multicast tree.
Note that the use of network management fault isolation
techniques, e.g., sampling router traffic statistics at
different points along a data stream, should permit post hoc
detection of packet forgery attacks mounted by rogue routers
along a data stream path. Use of this technique could
provide a deterrent to such activity by routers, further
arguing for the pairwise keying approach.
The sealing technique is faster than the digital signature
technique, because the incremental hash calculation
(including the appended secret quantity) is much faster than
the cryptographic transformation required to sign a hash.
The processing burden is symmetric here, i.e., the sender and
each router devote the same amount of processing power to
seal a packet and to verify the seal. Also, a sealed hash
may be smaller than a signed hash, even if the same function
is used in both cases. (This is because the modulus size of
the public key signature algorithm and any ancillary
parameters tend to increase the size of the signed hash
value.) Moreover, one could use a hash function with a
"wide" value and truncate that value, if necessary to reduce
overhead; this option is not available when the authenticator
is a signed hash value.
As a variant on this technique, one could imagine a
"clearinghouse" that would receive, from the sender, the
secret key used to generate and validate authenticators. A
router needing to validate a packet would send a copy of the
packet to the clearinghouse, which would check the packet and
indicate to the router whether it was a valid packet
associated with the LLID in question. Obviously, this
variant is viable only if the router is performing
infrequent, selective packet validation. However, it does
avoid the need to share the authenticator secret among all
the routers that must validate packets.
For both of these techniques, there is a residual
vulnerability to denial-of-service attacks based on replay of
valid packets during the lifetime of a data stream. Unless
Braden, Clark, Crocker & Huitema [Page 26]RFC 1636 IAB Workshop Report June 1994
packets carry sequence numbers and routers track a sequence
number window for each data stream, an (external) attacker
can copy valid packets and replay them. It may be easiest to
protect against this form of attack by aggregating all
traffic between a pair of routers into a single flow and
providing replay protection for the flow as a whole, rather
than on a per data stream basis.
* Temporary Passwords
The final technique explored in the workshop takes a very
different tack to packet validation. The preceding
techniques compute a function of the bits in a packet and
transform that value in a fashion that prevents an intruder
from generating packets with valid authenticators. The
ability to generate packets with valid authenticators for a
given LLID requires access to a secret value that is
available only to the sender, or to the sender and to routers
participating in a given data stream.
In contrast, this third technique calls for the authenticator
to be a short term, secret quantity that is carried in the
packet header, without benefit of further protection. In
essence, this technique incorporates a short term "password"
into each packet header. This approach, like its
predecessor, requires that all of the routers validating the
LLID be privy to this authenticator. Moreover, the
authenticator is visible to any other router or other
equipment along the path, and thus this technique is much
more vulnerable than the previous ones.
Here the same authenticator may be applied to all packets
with the same LLID, since the authenticator is not a function
of the packet it authenticates. In fact, this suggests that
it is feasible to use the LLID as the authenticator.
However, adopting this tack would not be consistent with the
two previous techniques, each of which requires an explicit,
separate authenticator, and so we recommend against this
optimization.
Nonetheless, the fact that the authenticator is independent
of the packet context makes it trivial to generate (forge)
apparently authentic packets if the authenticator is
intercepted from any legitimate packet. Also, if the
authenticator can be guessed, an attacker need not even
engage in passive wiretapping to defeat this scheme. This
latter observation suggests that the authenticator must be of
sufficient size to make guessing unlikely, and making the
Braden, Clark, Crocker & Huitema [Page 27]RFC 1636 IAB Workshop Report June 1994
LLID and the authenticator separate further supports this
requirement.
The major advantage of this approach is one of performance.
The authenticator can be validated very quickly through a
simple comparison. Consistent with the need to protect
against guessing attacks, the authenticator need not consume
a significant amount of space in the packet header.
The use of a sequence number visible to the routers is an
interesting technique to explore to make these somewhat
vulnerable methods more robust. If each stream (each source
of packets) numbers its packets, then an intruder attempting
to use the network resource must delete the legitimate
packets, which in many cases would be difficult. Otherwise,
the router being attacked would notice duplicate sequence
numbers and similar anomalies. The exact details of the
numbering would have to be worked out, since for the
legitimate stream packets might be lost, which would cause
holes in the sequence space.
We do not consider here the issues of collusion, in which a user
with a given LLID and authenticator deliberately shares this with
another unauthorized user. This possibility should be explored,
to see if there is a practical advantage to this act, and thus a
real threat.
4.4 Dynamics of Setup
o Duration of LLID's
A key question in the use of LLIDs is how long they remain
valid. At one extreme, they last only a very short time,
perhaps seconds. This limits the damage that can be done if
the authenticator for the LLID is stolen. At the other
extreme, LLIDs are semi-permanent, like credit card numbers.
The techniques proposed above for securing the LLID traded
strength for efficiency, under the assumption that the peril
was limited by the limited validity of the LLID.
The counterbalancing advantage of long-term or semi-permanent
LLIDs is that it becomes practical to use primitive setup
techniques, such as manual configuration of routers to
establish packet classes. This will be important in the
short run, since deployment of security and dynamic resource
allocation protocols may not exactly track in time.
Braden, Clark, Crocker & Huitema [Page 28]RFC 1636 IAB Workshop Report June 1994
We conclude that the correct short-term action is to design
LLIDs under the assumption that they are fairly short lived,
and to tolerate, in the short run, a longer period of
validity. This would imply that we will get an acceptable
long-term mechanism in place, which operationally will have a
lower level of security at first. As we get better tools for
automatic setup, we can shorten the duration of validity on a
individual basis, without replacing mechanism in the packet
forwarding path.
o Setup Latency
The tradition of the Internet is not to impose any setup
latency in the communication path between end nodes. This
supports the classic datagram model for quick transactions,
etc., and it is a feature that should be preserved.
For setup that is done "in advance", either through a
management interface or by an end-node in the background, the
issue of latency does not arise. The latency issue occurs
for dynamic reservations made in response to a specific
application request.
We observe that while latency is a key issue, it is not
materially influenced by security concerns. The designers of
resource reservation protocols such as RSVP and ST-II are
debating the latency of these protocols today, absent
security. Adding an authenticator to the request message
will increase the processing needed to validate the request,
and might even imply a message exchange with an
authentication service, but should not substantially change
the real time of the setup stage, which might already take
time on the order of a round-trip delay. But the design of
the high level authentication and authorization methods for
the setup protocol should understand that this process, while
not demanding at the level of the per-packet processing, is
still somewhat time-critical.
One way of dealing with an expensive setup process is to set
up the request provisionally and perform the validation in
the background. This would limit the damage from one bad
setup request to a short period of time. Note, however, that
the system is still vulnerable to an attack that uses a
sequence of setup requests, each of which allows unauthorized
usage for at least a short period of time.
Note also that a denial-of-service attack can be mounted by
flooding the setup process with invalid setup requests, all
Braden, Clark, Crocker & Huitema [Page 29]RFC 1636 IAB Workshop Report June 1994
of which need to be processed and rejected. This could
prevent a valid user from setting up any state. However,
denial-of-service attacks based upon flooding leave very
large "finger prints"; they should not normally be an
important threat. If it is a problem, it may be possible to
incorporate a mechanism at the level of setup processing that
is equivalent to "fair queueing", to limits the damage from a
flooding attack at the packet level.
4.5 Receiver-Initiated Setup
Recent work on a QOS extension for the Internet, embodied in the
RSVP protocol, uses the model that the receiver will reserve
resources. This scheme is consistent with the current IP
multicast paradigm, which requires the receiver to join the
multicast group. The receiver reserves the resources to insure
that the multicast traffic reaches the receiver with the desired
QOS. In this case, it is the credentials (the HLIDs) of the
receivers that will be presented to the setup phase.
Note that receiver initiation requires an explicit setup phase.
Suppose setup were implicit, driven by pre-existing fields in the
packet. Then there would be no way to associate a packet with a
particular receiver, since in multicast, the address of the
receiver never appears in the packet.
Further, it is impossible in this case to perform a setup "in
advance", unless the sender and the receiver are very tightly co-
ordinated; otherwise, the receiver will not know in advance what
LLID will be in the packet. It is certainly impossible, in this
case, for the receiver to set up "semi-permanent" reservations for
multicast traffic coming to it. This, again, is not a security
issue; the problem exists without adding security concerns, but
the security architecture must take it into account.
4.6 Other Issues
4.6.1 Encrypting Firewalls and Bypass
Our view of security, both end node and network protection,
includes the use of firewalls, which partition the network into
regions of more or less trust. This idea has something in
common with the encrypting-firewall model used in the
military/intelligence community: red (trusted) networks
partitioned from black (untrusted) networks. The very
significant difference is that, in the military model, the
partition uses an encryption unit that encodes as much as
possible of the packet for its trip across the black network to
Braden, Clark, Crocker & Huitema [Page 30]RFC 1636 IAB Workshop Report June 1994
another red network. That is, the purpose of the encryption
unit, among others, is to provide a very high degree of
protection against disclosure for data housed within the red
networks. In contrast, our version of a firewall is more to
protect the trusted (red) region of the network from outside
attacks. It is concerned both with what comes in and with what
goes out. It does permit communication between a node on the
trusted and nodes in the untrusted parts of the network.
We would like to be able to adapt our model of secure QOS to
the case of military-style encrypting firewalls. However, this
use of encryption raises a problem with our model of secure
resource management, discussed above, which was based on a
two-stage process of setup and classification. This model is
problematic because it requires information to pass from the
red region to the black region in the clear. This information
includes both the setup packets themselves, if setup is done
dynamically from the end node, and the classification fields
(the LLIDs) in the data packets. Obviously, this information
cannot be encrypted when leaving the red region of the network,
since it would then be meaningless to the black net, so that
the black network would be unable to make resource allocation
decisions based on it.
To make this sort of control scheme work, it is necessary for
the encryption device to be programmed to permit certain
packets and fields in packets to pass through the encryptor in
the clear. This bypass of the encryption is considered highly
undesirable. In a high security situation, the process
generating the bypassing information might be corrupted, with
the result that information that should be controlled is
removed from the secure network by hiding it in the bypassed
fields of the packets.
We concluded, however, that this bypass problem is not
insurmountable. The key idea, as in all cases of bypass, is to
limit, rather than wholly outlaw, the information passing in
the clear. To limit the information needed for bypass, one can
either perform the setup as a management function totally
within the black environment, or divide the process into two
stages. The first stage, again totally in the black context,
defines a limited number of setup situations. The second stage
involves sending from the red net a very small message that
selects one request to be instantiated from among the pre-
defined set.
Perhaps the more difficult issue is the LLID in the packet
header. If the LLID is an explicit field (as we have discussed
Braden, Clark, Crocker & Huitema [Page 31]RFC 1636 IAB Workshop Report June 1994
so far, but see below), it represents a new field in each
packet, with perhaps as many as 32 bits. Again, the solution
is to limit the way this field can be used. When the end-node
performs a setup, it will specify the value of the LLID to be
used. This fact can be observed by the red/black encryption
unit, which can then limit the components of this field to the
values currently in use. To further improve the situation, the
encryption unit might be able to aggregate a number of flows
onto one flow for the purpose of crossing the black net, which
would permit a further reduction in the number of distinct
LLIDs that must escape the red region.
The details of this proposal, including some important issues
such as the time duration of LLIDs in this case, must be
considered further. However, the initial conclusion that
bypass can be incorporated into a general resource control
framework is very encouraging, since it suggests that both
military and commercial forms of security can be built out of
the same building blocks.
4.6.2 The Principle of Consistent Privilege
A well understood principle of security is the principle of
least privilege, which states that a system is most robust when
it is structured to demand the least privilege from its
components.
A related rule we observe is the principle of consistent
privilege. This can be illustrated simply in the case of
denial of service, where it is particularly relevant. For a
particular route, no assumption of service can be justified
unless we trust the routers to deliver the packets. If a
router is corrupted and will not forward packets, the only
solution is to find another route not involving this router.
We do not concern ourselves here with protocols for finding new
routes in the presence of a corrupted router, since this topic
is properly part of another topic, securing the network
infrastructure. We only observe that either we will get
service from the router or we will not. If the router is
corrupted, it does not matter how it chooses to attack us.
Thus, as long as the router is part of a forwarding path (most
generally a multicast forwarding tree), we should not hesitate
to trust it in other ways, such as by giving it shared resource
keys or LLID verifiers.
This illustrates the principle of consistent privilege. This
principle is exploited in the scheme for hop-by-hop or pairwise
use of secrets to validate LLIDs in a multicast tree. If a
Braden, Clark, Crocker & Huitema [Page 32]RFC 1636 IAB Workshop Report June 1994
single key is issued for the whole tree, then the privilege is
not consistent. We only need to trust a router with respect to
the nodes "below" it in the tree. If it fails to forward
traffic, it can affect only those nodes. But if we give it the
group key, then it can generate bogus traffic and inject it
into the tree at any point, affecting traffic for other parts
of the tree. If, on the other hand, we use pairwise keys, then
a corrupt node can only generate bogus traffic with the key for
traffic it would directly receive, which is the part of the
tree it could damage anyway.
Another requirement we must place on the network concerns
routing. If a firewall is in place, we must trust the routing
architecture not to bypass that firewall. One way to
accomplish this is to eliminate any physical path between the
regions other than those that go through the firewall.
Operational experience will be required to see if this simple
physical limit is an acceptable constraint.
4.6.3 Implicit LLID's
We stress the importance of a strong conceptual distinction
between the addresses in a packet and the LLID which is used to
classify the packet. The conceptual distinction is important,
but under limited circumstances it may be possible to overload
some of the packet fields and create an LLID from the current
packet header. For example, current packet classifiers for
IPv4, which are not secure but which seem to work for
classifying the packets into service classes, use a number of
the packet fields together as a form of LLID: the source and
destination IP addresses and ports plus the protocol type.
This sort of "implicit" LLID must be short-lived, especially if
the host can change its IP address as it moves. But if the
LLID is established by some sort of dynamic setup protocol, it
should be possible reestablish the LLID as needed.
The current IPv4 header has no authenticator field to validate
the LLID. An authenticator field could be optionally carried
in an option; adding it gives robustness to network
reservations. Any of the schemes described above for creating
an authenticator could be used, except that if the simple
password-style authenticator is used, it must be an explicit
separate field, since the LLID cannot be picked randomly.
Braden, Clark, Crocker & Huitema [Page 33]RFC 1636 IAB Workshop Report June 1994
4.6.4 Security without Setup
As we describe this architecture, the setup phase is an
essential part of the sequence. This suggests that the current
Internet, which has no setup protocols, cannot be secured
against denial-of-service attacks. It is important to explore
the limits of this point. As we stressed above, setup can
occur in many ways. Routers today offer management options to
classify packets based on protocol types and other fields found
in the header, and to use this classification to create a few
fair queueing classes that can prevent one class from
overloading the net to the exclusion of the others.
There are two problem here. The first is that for a setup done
using a management interface, the secret that is shared among
the source and the routers to validate the LLID must remain
valid for a long time, and it must be manually configured. The
second problem is that the granularity of the categories may be
coarse. However, it has been proposed, in a thesis by Radia
Perlman, that a router might create a separate fair queueing
class implicitly for each source address. This approach, which
uses the addresses as an implicit LLID, must have some form of
authenticator for robustness. But if the LLID can be trusted,
this scheme provides classification of traffic based only on an
implicit setup operation. The granularity of classification is
not sufficient to provide any QOS distinction. The only
objective is to prevent the traffic from one source from
flooding the net to the exclusion of another.
4.6.5 Validating Addresses
We make a claim here that if the LLID and the addresses in the
packet are conceptually distinct, and if there is a suitable
means to validate the LLID, then there is no reason to validate
the addresses. For example, a packet constructed with a false
source address does not seem to represent any security problem,
if its LLID can be validated.
An exception to this might possibly lie in communication with
mobile hosts, but it will require a complete model of threats
and requirements in the mobile environment to be sure.
However, we make the claim, as a starting point for discussion,
that if LLIDs are distinguished from addresses, many of the
security concerns with mobility are mitigated and perhaps
removed. This point should be validated by more detailed
consideration of the mobility problem.
Braden, Clark, Crocker & Huitema [Page 34]RFC 1636 IAB Workshop Report June 1994
4.6 Conclusions
a) It is important to conceptually separate a LLID (Low-Level
IDentifier) carried in a packet from addresses in the packet.
b) There will be a single LLID carried in each packet. Although
this might imply some additional state in the routers than if
multiple LLIDs were used, using only one LLID choice is more
scalable.
c) Hop-by-hop LLID authentication mechanisms might provide a
highly scalable approach that limits the distribution of
secrets. However, the robustness limitations must be
investigated thoroughly.
d) Statistical sampling or after-the-fact detection mechanisms
may be employed by routers to address performance concerns.
5. AN AUTHENTICATION SERVICE
The purpose of an authentication service is simply to verify names,
or more precisely to verify the origin of "messages". It differs
from the authorization service, which determines what services are
available to an authenticated name. We expect that authentication
will be an Internet-wide service, while authorization will be
specific to the resources to which access is being authorized.
This "identification" function can be used in several contexts, for
example:
* One-time passwords: "it is really <huitema@inria.fr> that is
responding to this challenge".
* Access to a firewall: "it is really <huitema@inria.fr> that is
trying to send data to host-A at port-a".
There are many Internet objects that we may want to name, e.g.,:
domain names: sophia.inria.fr
machine names: jupiter.inria.fr
service names: www.sophia.inria.fr
(in fact, a data base)
users: huitema@sophia.inria.fr
Braden, Clark, Crocker & Huitema [Page 35]RFC 1636 IAB Workshop Report June 1994
processes: p112.huitema@sophia.inria.fr
p112.sophia.inria.fr
universal resource locators:
http//www.sophia.inria.fr:222/tmp/foobar
One could be tempted to believe that the authentication service will
only be concerned with naming humans, as only humans are
"responsible"; a process obtains some access rights because it is
acting on behalf of a person. However, this is too reductive and
potentially misleading. We may have to authenticate "machines" or
hardware components. For example:
* When a machine boots it needs to access resources for
configuring itself, but it is not yet "used" by a person; there
is no user.
* On a "distributed processor", component CPUs may need to
authenticate each other.
Machines do differ from users; machines cannot keep their "secrets"
in the same way that people do. However, there is a big value in
having a simple and extensible name space.
5.1 Names and Credentials
We make the hypothesis that the authorization services will
generally use "access control lists" (ACLs), i.e., some definition
of a set of authorized users. A compact way to represent such a
set would be to allow "wildcard" authorizations, e.g., "anybody at
<Bellcore.com>", or "any machine at <INRIA.FR>". The
authentication service should be designed to facilitate the
realization of the authorization service and should support
"wildcards".
However, wildcards are not general enough. Assuming that we have
a hierarchical name space, a wildcarded entry is limited to the
naming hierarchy. For example, a name like
<huitema@sophia.inria.fr> could be matched by the wildcard
<*@sophia.inria.fr> or <*.inria.fr> or <*.fr>. This is useful as
long as one stays at INRIA, but does not solve the generic
problem. Suppose that an IETF file server at CNRI is to be
accessible by all IAB members: its ACL will explicitly list the
members by name.
The classic approach to naming, as exemplified in the X.500 model,
is to consider that people have "distinguished names". Once one
has discovered such a name through some "white pages" service, can
Braden, Clark, Crocker & Huitema [Page 36]RFC 1636 IAB Workshop Report June 1994
use it as an access key in a global directory service.
An individual may acquire authorizations from a variety of
sources. Using a pure, identity-based access control system, the
user would have to acquire multiple identities (i.e.,
distinguished names), corresponding to the roles in which she is
authorized to access different services. We discuss this approach
in the next section.
An alternative approach is for the user to have a very small
number of identities, and to have the grantors of authorizations
issue (signed) credentials granting permissions to the user,
linked to her ID. These additional signed credentials are known
as "capabilities". The user can then establish her identity
through a generic identity credential, e.g., an X.509 certificate,
and can establish authorization by presenting capabilities as
required. This is somewhat analogous to a person acquiring credit
cards linked to the name on a driver's license, and presenting the
appropriate credit card, plus the license for picture verification
of identity.
5.2 Identity-Based Authorization
Let's open the wallet of an average person: we find several
"credit cards" in it. We all have many "credit cards", e.g.,
company cards, credit cards, airline frequent flyers memberships,
driver licenses. Each of these cards is in fact a token asserting
the existence of a relation: the bank certifies that checks
presented by the bearer will be paid, the traffic authorities
certifies that the bearer has learned how to drive, etc. This is
an example of identity-based authorization, in which an individual
is given different names corresponding to different relations
entered into by that individual.
If we imagine that the name space is based upon DNS (domain)
names, then for example, the person mentioned above could be
authenticated with the names:
customer@my-big-bank.com
customer@frequent-flyer.airline.com
The model we used here is that "the name is an association". This
is consistent with name verification procedures, in which that one
builds a "chain of trust" between the user and the "resource
agent". By following a particular path in the trust graph, one
can both establish the trust and show that the user belongs to an
"authorized group".
Braden, Clark, Crocker & Huitema [Page 37]RFC 1636 IAB Workshop Report June 1994
The existence of "multiple names" for a person may or may not
imply the existence of an "equivalence" relation. It may be
useful to know that <huitema@sophia.inria.fr> and
<huitema@iab.isoc.org> are two names for the same person, but
there are many cases where the user does not want to make all his
tokens visible.
5.3 Choosing Credentials
Let's consider again the example of Christian Huitema accessing a
file at CNRI. He will have to interact with INRIA's outgoing
firewall and with CNRI's incoming controls. Regardless of whether
authorization depends upon capabilities or upon multiple
association names, a different credential may be needed in each
firewall on the path. For example, assuming multiple names are
used, he will use an INRIA name, <huitema@sophia.inria.fr>, to be
authorized by INRIA to use network resources, and he will use an
IAB name, <huitema@iab.isoc.org>, to access the file server. Thus
comes an obvious problem: how does he choose the credential
appropriate to a particular firewall? More precisely, how does
the computer program that manages the connection discover that it
should use one credential in response to INRIA's firewall
challenge and another in response to CNRI's request?
There are many possible answers. The program could simply pass
all the user's credentials and let the remote machine pick one.
This works, but poses some efficiency problems: passing all
possible names is bulky, looking through many names is long.
Advertising many names is also very undesirable for privacy and
security reasons: one does not want remote servers to collect
statistics on all the credentials that a particular user may have.
Another possibility is to let the agent that requests an
authorization pass the set of credentials that it is willing to
accept, e.g., "I am ready to serve CNRI employees and IAB
members". This poses the same privacy and security problems as
the previous solutions, although to a lesser degree. In fact, the
problem of choosing a name is the same as the generic "trust path"
model. The name to choose is merely a path in the authentication
graph, and network specialists are expected to know how to find
paths in graphs.
In the short term, it is probably possible to use a "default name"
or "principal name", at least for local transactions, and to count
on the user to "guess" the credential that is required by remote
services. To leave the local environment we need only the local
credentials; to contact a remote server we need only the
destination credentials. So we need one or maybe two credentials,
Braden, Clark, Crocker & Huitema [Page 38]RFC 1636 IAB Workshop Report June 1994
which may be derived from the destination. It will be very often
the case that the generic credential is enough; then wildcards;
then "FTP provided" tokens.
6. OTHER ISSUES
6.1 Privacy and Authentication of Multicast Groups
Multicast applications are becoming an increasingly important part
of Internet communications. Packet voice, video and shared
whiteboard can be powerful productivity tools for users. For
these applications to have maximum value to their users, a variety
of security services will be required.
Existing techniques are directly applicable to providing privacy
for a private teleconference. If each member of the conference
shares a single key for a symmetric encryption algorithm (such as
DES), existing point-to-point security techniques can be extended
to protect communication within the group from outsiders.
However, slight modifications to existing techniques are required
to accommodate the multicast environment. Each packet will
require independent cryptographic processing to ensure that
packets from multiple sources can be independently decrypted by
the numerous receivers, particularly in the presence of lost
packets. N-party authentication and key management will be
required to establish the shared key among the proper group
members. This can be done by extending existing two-party key
management techniques pairwise. For example, the conference
manager may provide the key to each member following individual
authentication; for example, this could be implemented trivially
using PEM technology. The overhead experienced by each host
computer in the conference will be similar to that of existing
point-to-point encryption applications, This overhead is be low
enough that, today, software encryption can offer adequate
performance to secure whiteboard and voice traffic, while hardware
encryption is adequate for video.
The nature of multicast communication adds an additional
requirement. Existing multicast conferences provide gradual
degradation in quality as the packet loss rate increases. To be
acceptable, authentication protocols must tolerate lost packets.
Techniques to accomplish this efficiently need to be developed.
One initial sketch is outlined below. Engineering work will be
required to validate the practicality of this approach.
Braden, Clark, Crocker & Huitema [Page 39]RFC 1636 IAB Workshop Report June 1994
The use of symmetric encryption provides the members of the
conference with effective protection from outsiders. However,
because all members of the conference share a single key, it does
not provide a means of authenticating individual conference
members. In principle, existing techniques, based on one-way hash
functions coupled with digital signatures based on asymmetric
encryption algorithms, can provide individual authentication.
One-way hash functions such as MD5 are comparable in cost to
symmetric encryption. However, digital signatures are
considerably more costly, both in computation and in communication
size. The degree of overhead depends on the quality of
authentication required.
In summary, realtime authentication at the granularity of group
membership is easy and cheap, but individual authentication is
costly in time and space. Over time, the costs of both
communications and processing are expected to decline. It is
possible that this will help make authentication at the level of
individual conference participants. There are two conflicting
trends: (1) increasing CPU speeds to provide symmetric
encryption, and (2) increasing communication data rates. If both
technologies increase proportionally, there will be no net gain,
at least if the grain size is measured in terms of bits, rather
than as a period in seconds.
The group felt that the correct approach to end-to-end controls is
the use of encryption, as discussed above. The alternative is to
control the ability of a user to join a multicast group as a
listener, or as a speaker. However, we are not comfortable with
the level of assurance that we can offer if we attempt to ensure
end-to-end semantics using these means. Any passive penetration
of the network, i.e., any wire-tap, can compromise the privacy of
the transmitted information. We must acknowledge, however, that
problems with deployment of encryption code and hardware, and
especially problems of export controls, will create a pressure to
use the tools described in Section 4 to implement a form of end-
to-end control. Such a decision would raise no new issues in
security technology. The shared key now used for encrypting the
data could instead be used as the basis for authenticating a
multicast group join request. This would require modification of
the multicast packet format, but nothing more. Our concern is not
the technical difficulty of this approach, but the level of
assurance we can offer the user.
Braden, Clark, Crocker & Huitema [Page 40]RFC 1636 IAB Workshop Report June 1994
6.2 Secure Plug-and-Play a Must
Plug-and-play is the ability to plug a new device into a network
and have it obtain the information it needs to communicate with
other devices, without requiring any new configuration
information. Secure plug-and-play is an important Internet
requirement, and a central architectural issue is whether it can
be made to scale well.
For plug-and-play operation, a new machine that is "plugged" into
the network needs to:
(1) Obtain an locator so it can communicate with other devices
(2) Register or obtain a name to be identified by (e.g., machine
name)
(3) Discover services available on the network (e.g., printers,
routers, file servers, etc.)
(4) Discover other systems on the network so it can communicate
with them.
In some environments, no security mechanisms are required because
physical security and local knowledge of the users are sufficient
protection. At the other end of the spectrum is a large network
with many groups of users, different types of outside connections,
and levels of administrative control. In such environments,
similar plug-and-play capabilities are needed, but the new device
must be "authenticated" before it can perform these functions. In
each step in the discovery process the new device must
authenticate itself prior to learning about services.
The steps might be:
- Obtain a HLID from a smart card, smart disk, or similar
device.
- Authenticate itself with the first plug-and-play server using
its HLID, to register a name and to find the location of
other services.
- Discover services available on the network (e.g., printers,
routers, file servers, etc.) based on its HLID.
- Discover other systems on the network so it can communicate
with them.
Braden, Clark, Crocker & Huitema [Page 41]RFC 1636 IAB Workshop Report June 1994
The problem of taking a system out of the box and initially
configuring it is similar to the problem of a mobile or portable
machine that a human wants to connect to a local network
temporarily in order to receive services on that network. How can
the local network authenticate the human (and therefore the
human's machine) and know which services this visiting machine is
permitted to use?
The human must be endowed with a high level identifier (HLID)
which acts as his/her passport and can be verified by the local
network. This high level identifier must be globally unique and
registered/assigned by some recognized authority.
When the human plugs the machine onto a local net, the machine
identifies itself to the net with the human's high level
identifier. If local net has a policy of permitting anyone to
plug and play on its network, it will ignore the HLID and assign
an address (locator), permitting the visitor unrestricted access
and privileges. More likely, the local net will authenticate the
HLID prior to granting the visitor an address or any privileges.
At this point, the HLID has only authenticated the visitor to the
local network; the issue of which services or resources the
visitor is entitled to use has not been addressed. It is
desirable to develop a low-overhead approach to granting
authentications to new users. This will help in the case of
visitors to a site, as well as new users joining a facility.
6.3 A Short-Term Confidentiality Mechanism
Authentication has customarily been achieved using passwords. In
the absence of active attacks, the greatest threat to computer
system security may be the ease with which passwords can be
"snooped" by the promiscuous monitoring of shared-media networks.
There are known security techniques for achieving authentication
without exposing passwords to interception, for example the
techniques implemented in the well-known Kerberos system.
However, authentication systems such as Kerberos currently operate
only in isolation within organizational boundaries. Developing
and deploying a global authentication infrastructure is an
important objective, but it will take some years. Another useful
approach in the short term is the use of a challenge-response user
authentication scheme (e.g., S/Key).
One of the groups explored another interim approach to guarding
passwords: introducing a readily-used confidentiality mechanism
based on an encrypted TCP connection. This would operate at the
IP level to encrypt the IP payload, including the TCP header, to
Braden, Clark, Crocker & Huitema [Page 42]RFC 1636 IAB Workshop Report June 1994
allow the nature as well of the contents of the communication to
be kept private. It could be implemented to provide either
"strict" protection (the connection fails if the other side cannot
decrypt your data stream) or "loose" protection (falling back to
non-private TCP if decryption fails).
Loose protection would allow interoperability with older hosts in
a seamless (non-user-intrusive) manner.
One-time keys may be exchanged during the SYN handshake that
starts the TCP connection. Using one-time keys avoids a need for
infrastructure support and does not require trust between the
organizations on the two ends of the connection. Tieing the key
exchange to the SYN handshake will avoid the possibility of having
the connection fully open without knowing the state of encryption
on both ends of the connection. Although it may still be
theoretically possible to intercept the SYN exchange and subvert
the connection by an active "man-in-the-middle" attack, in
practice such attacks on TCP connections are quite difficult
unless the routing protocols have been subverted.
The keys could be exchanged using a new option that specifies the
key exchange protocol, the data encryption algorithm, and the key
to be used to decrypt the connection. It could be possible to
include multiple options in the same SYN segment, specifying
different encryption models; the far end would then need to
acknowledge the option that it is willing to use. In this case,
the lack of an acknowledgement would imply disinterest in
decrypting the datastream. If a loose privacy policy were in
force, the connection could continue even without an
acknowledgment. The policy, "strict" or "loose", would be set by
either the user or the default configuration for the machine.
One must however observe that a TCP option can carry only a
limited amount of data. Efficient protection against crypto-
analysis of the Diffie-Hellmann scheme may require the use of a
very long modulus, e.g., 1024 bits, which cannot be carried in the
40 bytes available for TCP options. One would thus have either to
define an "extended option" format or to implement encryption in a
separate protocol layered between TCP and IP, perhaps using a
version of "IP security". The detailed engineering of such a
solution would have to be studied by a working group.
A TCP connection encryption mechanism such as that just outlined
requires no application changes, although it does require kernel
changes. It has important drawbacks, including failure to provide
privacy for privacy for UDP, and the great likelihood of export
control restrictions. If Diffie-Hellman were used, there would
Braden, Clark, Crocker & Huitema [Page 43]RFC 1636 IAB Workshop Report June 1994
also be patent issues.
7. CONCLUSIONS
As a practical matter, security must be added to the Internet
incrementally. For example, a scheme that requires, as a
precondition for any improvement, changes to application code, the
DNS, routers and firewalls all at once will be very hard to deploy.
One of the reasons the workshop explored schemes that are local to
the IP layer is that we surmise that they might be easier to deploy
in practice.
There are two competing observations that must shape planning for
Internet security. One is the well known expression: "the best is
the enemy of the good." The other is the observation that the
attacks are getting better.
Finally, it should noted that the principle of least privilege, which
was mentioned above, may be in contradiction to the principle of
least cost.
7.1 Suggested Short-Term Actions
The general recommendation for short-term Internet security policy
was that the IETF should make a list of desirable short-term
actions and then reach out to work with other organizations to
carry them out. Other organizations include regionals, which may
be in a good position to provide site security counseling services
to their customers, vendors and other providers, and other
societies. We should also give input to the US government to
influence their posture on security in the direction desired by
the community.
A suggested preliminary list of short-term actions was developed.
o Perform external diagnostic security probes
Organizations should be encouraged to use CRACK and other
tools to check the robustness of their own passwords. It
would also be useful to run a variety of security probes from
outside. Since this is a very sensitive issue, some care
needs to be taken to get the proper auspices for such
probing.
Braden, Clark, Crocker & Huitema [Page 44]RFC 1636 IAB Workshop Report June 1994
Useful probe tools include:
ISS: Klaus (GA)
SATAN: Farmer Venema
ICEPICK: NRL
o Determine Security-Risk Publication Channels
What channels should be used for disseminating information of
security risks?
o Encourage use of one-time passwords.
Available packages: S/Key, SecurID, Enigma, Digital Pathways.
o Develop and publish guidelines for protocol developers, for
security-friendliness and firewall-friendliness.
o Control topology to isolate threats
o Set privacy policy:
* Always
* As much as possible
o Bring Site Security Handbook up to date
o Support use of Kerberos
The subject of the "Clipper chip" came up several times, but there
was not sufficient discussion of this very complex issue for this
grouip to reach a recommendation. It has been observed that there
are a number of quite differing viewpoints about Clipper.
o Some people accept the government's Clipper proposal,
including key escrow by the US government and the
requirement that encryption be in hardware.
o Some people don't mind key escrow by the government in
principle, but the object to the hardware requirement.
o Some people don't mind key escrow in principle, but
don't want the government to hold the keys. They would
be comfortable with having the organization which owns
the data hold the keys.
o Some people don't want key escrow at all.
Braden, Clark, Crocker & Huitema [Page 45]RFC 1636 IAB Workshop Report June 1994
o Some people don't mind the hardware or the key escrow,
but they don't think this will be acceptable to other
countries and thus will not work internationally.
This report takes no position on any of these viewpoints.
7.2 Suggested Medium-Term Actions
These actions require some protocol design or modification;
however, they use existing security technology and require no
research.
o Authentication Protocol
There is a problem of the choice of technology. Public key
technology is generally deemed superior, but it is patented
and can also induce relatively long computations. Symmetric
key technology (Needham-Schroeder algorithm, as used in
Kerberos) has some technical drawbacks but it is not
patented. A system based on symmetric keys and used only for
authentication would be freely exportable without being
subject to patents.
o Push Kerberos
Engineering is needed on Kerberos to allow it to interoperate
with mechanisms that use public key cryptography.
o Push PEM/RIPEM/PGP...
o Develop an authenticated DNS
o Develop a key management mechanism
o Set up a certificate server infrastructure
Possible server mechanisms include the DNS, Finger, SNMP,
Email, Web, and FTP.
o Engineer authentication for the Web
7.3 Suggested Long-Term Actions
In this category, we have situations where a threat has been
identified and solutions are imaginable, but closure has not been
reached on the principles.
Braden, Clark, Crocker & Huitema [Page 46]RFC 1636 IAB Workshop Report June 1994
o Executable Apps
o Router sabotage counter-measures
o Prevent Byzantine routing.
o Proxy Computing
o Decomposition of computers
o Are there "good" viruses?
Braden, Clark, Crocker & Huitema [Page 47]RFC 1636 IAB Workshop Report June 1994
APPENDIX A -- Workshop Organization
The following list of attendees indicates also the breakout group to
which they were assigned.
Breakout Groups
Group I.1 Leader:
1 Christian Huitema, INRIA (IAB)
1 Steve Bellovin, AT&T
1 Bob Braden, ISI (IAB)
1 John Curran, NEARNET
1 Phill Gross, ANS (IETF/IAB)
1 Stev Knowles, FTP Software (Internet AD)
1 Barry Leiner, USRA (IAB)
1 Paul Mockapetris, ISI
1 Yakov Rekhter, IBM (IAB)
1 Dave Sincoskie, Bellcore (IAB)
Group I.2 Leader:
2 Steve Crocker, TIS (Security AD)
2 Jon Crowcroft
2 Steve Deering, PARC
2 Paul Francis, NTT
2 Van Jacobson, LBL
2 Phil Karn, Qualcomm
2 Allison Mankin, NRL (Transport AD, IPng AD)
2 Radia Perlman, Novell
2 John Romkey, ELF (IAB)
2 Mike StJohns, ARPA (IAB)
Group I.3 Leader:
3 Dave Clark, MIT
3 Deborah Estrin, USC
3 Elise Gerich, Merit (IAB)
3 Steve Kent, BBN (IAB)
3 Tony Lauck, DEC (IAB)
3 Tony Li, CISCO
3 Bob Hinden, Sun (IESG->IAB liaison, Routing AD)
3 Jun Murai, WIDE (IAB)
3 Scott Shenker, PARC
3 Abel Weinrib, Bellcore
The following were able to attend only the third day, due to a
conflicting ISOC Board of Trustees meeting:
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RFC 1636 IAB Workshop Report June 1994
Scott Bradner, Harvard (IPng AD)
Jon Postel, ISI (IAB)
The workshop agenda was as follows.
Tues Feb 8
9:00 - 10:30 Plenary
Discuss facilities, meeting goals, agenda, organization.
Establish some minimal common understandings. Assign
scenarios to Breakout I groups.
10:30 - 13:00 Breakout I meetings
Each breakout group examine one or more scenarios and
formulate a list of design questions. Lunch available on
11th floor.
13:00 - 15:00 Plenary
Report, discuss. Collate and shorten list of design
issues. Organize Breakout II groups to work on these
issues.
15:00 - 17:30 Breakout IIa meetings
Work on design issues.
Wed Feb 9
9:00 - 10:00 Plenary
Report, discuss.
10:00 - 13:30 Breakout IIb meetings
More work on design questions, develop list of
requirements.
13:30 - 14:30 Plenary
Report, discuss.
15:30 - 17:30 Breakout III groups
Thurs Feb 10
9:00 - 9:30 Plenary
9:30 - 11:00 Breakout Groups (wrapup)
11:00 - 12:00 Plenary
Discuss possible short-term security recommendations
13:00 - 14:00 Plenary -- Discuss short-term security issues
14:00 - 14:30 Plenary -- Presentation by Steve Bellovin
Braden, Clark, Crocker & Huitema [Page 49]RFC 1636 IAB Workshop Report June 1994
14:30 - 16:00 Plenary -- Long- and Medium-term
Recommendations
The following scenarios were used as a starting point for
discussions. It distinguished security-S (security as a service to
the end systems) from security-M, security as a mechanism to support
other services. The workshop was intended to be primarily concerned
with interactions among the following different *services*:
o Security-S
o Routing
o Multi-destination delivery (mcast-S)
o Realtime Packet scheduling (realtime)
o Mobility
o Accounting
(and maybe large-scale?)
These categories were then applied to the following scenarios:
S1. Support a private teleconference among mobile hosts connected to
the Internet. [Security-S, mcast-S, realtime, mobility]
S2. The group in S1 is 1/3 the Internet, i.e., there are VERY severe
scaling problems. [Security-S, mcast-S, realtime, mobility,
large-scale]
S3. Charge for communication to support a video teleconference.
[Accounting, realtime, mcast-S]
S4. I am travelling with my laptop. I tune in to radio channel IP-
RADIO, pick-up the beacon and start using it. Who gets the
bill? Why do they believe this is me? Is "me" a piece of
hardware (IP address) or a certified user (PEM certificate)?
[Mobility, accounting (, realtime, mcast-S)]
S5. A Politically Important Person will mcast an Internet
presentation, without danger of interruptions from the audience.
S6. The travel industry wants to use Internet to deliver tickets to
customer premises directly in a secure way, but the customer has
only dial-up capability. [Security-S, mobility]
Braden, Clark, Crocker & Huitema [Page 50]RFC 1636 IAB Workshop Report June 1994
S7. I am traveling with my laptop and this friendly host is running
the autoconfiguration protocol. I immediately get an address as
"mac1.friendly.host.com". (What is the difference between my
laptop and a bona fide autoconfigured local station?)
[Security-S, mobility]
S8. Multiple people are connected to a subnetwork providing mobility
(e.g., cellular, packet radio). The subnetwork is connected to
multiple places in the "fixed" backbone. How can routing be done
efficiently? [Routing, mobility]
The following scenarios that were suggested do not fit into the
primary thrust of the workshop, generally because they are single-
issue topics. Most of them are pure security topics and are
concerned with the security perimeter. The last two do not fit into
our classification system at all.
S9. XYZ corporation has two major branches on opposite ends of the
world, and they want to communicate securely over the Internet,
with each branch having IP-level connectivity to the other (not
through application gateways).
S10. I am visiting XYZ corporation, with my laptop. I want to
connect it to their LAN to read my email remotely over the
Internet. Even though I am inside their corporate firewall,
they want to be protect their machines from me.
S11. XYZ corporation is trying to use the Internet to support both
private and public networking. It wants to provide full
connectivity internally between all of its resources, and to
provide public access to certain resources (analogous of
anonymous ftp servers)
S12. The travel industry wants to use Internet to deliver tickets to
customer premises directly in a secure way.
S13. Some hacker is deliberately subverting routing protocols,
including mobile and multicast routing. Design counter
measures.
S14. Part of the Internet is running IPv4 and part is running IPng
(i.e. the Internet is in transition). How can we assure
continued secure operation through such a transition?
S15. A corporation uses ATM to connect a number of its sites. It also
uses Internet. It wants to make use of the ATM as its primary
carrier, but also wants to utilize other networking technologies
as appropriate (e.g., mobile radio). It wants to support all
Braden, Clark, Crocker & Huitema [Page 51]RFC 1636 IAB Workshop Report June 1994
media (data, voice, video).
Security Considerations
This memo is entirely concerned with security issues.
Authors' Addresses
Bob Braden [Editor]
USC Information Sciences Institute
4676 Admiralty Way
Marina del Rey, CA 90292-6695
Phone: (310) 822-1511
EMail: Braden@ISI.EDU
David Clark
MIT Laboratory for Computer Science
545 Technology Square
Cambridge, MA 02139-1986
Phone: 617-253-6003
EMail: ddc@lcs.mit.edu
Steve Crocker
Trusted Information Systems, Inc.
3060 Washington Road (Rte 97)
Glenwood, MD 21738
Phone: (301) 854-6889
EMail: crocker@tis.com
Christian Huitema
INRIA, Sophia-Antipolis
2004 Route des Lucioles
BP 109
F-06561 Valbonne Cedex
France
Phone: +33 93 65 77 15
EMail: Christian.Huitema@MIRSA.INRIA.FR
Braden, Clark, Crocker & Huitema [Page 52]
