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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.


 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


 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
 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
 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


 2.1  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
         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
         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
    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 "" 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


 3.1  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
    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
       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
        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

    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
    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
       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
              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

  1. ———+——–+——–+————+————+—-

| C | A1 | Z1 | F1 | F2 | S

  1. ———+——–+——–+————+————+—-

| 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

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
       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
            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
            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

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
            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,
       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:
  1. No extra authentication is required for end hosts.
  1. A single authentication protocol can be used for all

intended applications.

  1. An IP-layer firewall causes less performance degradation.
  1. An IP-layer firewall may be able to crash and recover

state without disturbing open TCP connections.

  1. Routes can shift without disturbing open TCP connections.
  1. There is no single point of failure.
  1. 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


 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
    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
    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
  • 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
         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
         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
       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
    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.


 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

responding to this challenge".

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:
         machine names:
         service names:
                         (in fact, a data base)

Braden, Clark, Crocker & Huitema [Page 35] RFC 1636 IAB Workshop Report June 1994

         universal resource locators:
 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
    <>", or "any machine at <INRIA.FR>".  The
    authentication service should be designed to facilitate the
    realization of the authorization service and should support
    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
    <> could be matched by the wildcard
    <*> or <*> 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:
    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 <> and
    <> 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, <>, to be
    authorized by INRIA to use network resources, and he will use an
    IAB name, <>, 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.


 6.1  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
    (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:
  1. Obtain a HLID from a smart card, smart disk, or similar


  1. Authenticate itself with the first plug-and-play server using

its HLID, to register a name and to find the location of

         other services.
  1. Discover services available on the network (e.g., printers,

routers, file servers, etc.) based on its HLID.

  1. 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.


 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

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
    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:

Braden, Clark, Crocker & Huitema [Page 48] 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
        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
        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
 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,
 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
      "".   (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
 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
 Steve Crocker
 Trusted Information Systems, Inc.
 3060 Washington Road (Rte 97)
 Glenwood, MD 21738
 Phone: (301) 854-6889
 Christian Huitema
 INRIA, Sophia-Antipolis
 2004 Route des Lucioles
 BP 109
 F-06561 Valbonne Cedex
 Phone:  +33 93 65 77 15
 EMail: Christian.Huitema@MIRSA.INRIA.FR

Braden, Clark, Crocker & Huitema [Page 52]

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