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Versions: 00 01 02 03 04 05 RFC 4732

Network Working Group                                    M. Handley (ed)
Internet-Draft                                                       UCL
Expires: January 21, 2007                               E. Rescorla (ed)
                                                       Network Resonance
                                             Internet Architecture Board
                                                                     IAB
                                                           July 20, 2006


               Internet Denial of Service Considerations
                          draft-iab-dos-05.txt

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Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   This document provides an overview of possible avenues for denial-of-
   service attack on Internet systems.  The aim is to encourage protocol
   designers and network engineers towards designs that are more robust.
   We discuss partial solutions that reduce the effectiveness of
   attacks, and how some solutions might inadvertently open up



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   alternative vulnerabilities.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  An Overview of Denial-of-Service Threats . . . . . . . . . . .  6
     2.1.  DoS Attacks on End-systems . . . . . . . . . . . . . . . .  6
       2.1.1.  Exploiting Poor Software Quality . . . . . . . . . . .  6
       2.1.2.  Application Resource Exhaustion  . . . . . . . . . . .  7
       2.1.3.  Operating System Resource Exhaustion . . . . . . . . .  8
       2.1.4.  Triggered Lockouts and Quota Exhaustion  . . . . . . .  9
     2.2.  DoS Attacks on Routers . . . . . . . . . . . . . . . . . .  9
       2.2.1.  Attacks on Routers through Routing Protocols . . . . . 10
       2.2.2.  IP Multicast-based DoS Attacks . . . . . . . . . . . . 11
       2.2.3.  Attacks on Router Forwarding Engines . . . . . . . . . 12
     2.3.  Attacks on Ongoing Communications  . . . . . . . . . . . . 13
     2.4.  Attacks using the Victim's Own Resources . . . . . . . . . 14
     2.5.  DoS Attacks on Local Hosts or Infrastructure . . . . . . . 14
     2.6.  DoS Attacks on Sites though DNS  . . . . . . . . . . . . . 16
     2.7.  DoS Attacks on Links . . . . . . . . . . . . . . . . . . . 17
     2.8.  DoS attacks on firewalls . . . . . . . . . . . . . . . . . 18
     2.9.  DoS attacks on IDS systems . . . . . . . . . . . . . . . . 19
     2.10. DoS attacks on or via NTP  . . . . . . . . . . . . . . . . 19
     2.11. Physical DoS . . . . . . . . . . . . . . . . . . . . . . . 20
     2.12. Social Engineering DoS . . . . . . . . . . . . . . . . . . 20
     2.13. Legal DoS  . . . . . . . . . . . . . . . . . . . . . . . . 20
     2.14. Spam and Black-hole Lists  . . . . . . . . . . . . . . . . 20
   3.  Attack Amplifiers  . . . . . . . . . . . . . . . . . . . . . . 22
     3.1.  Methods of Attack Amplification  . . . . . . . . . . . . . 22
     3.2.  Strategies to Mitigate Attack Amplification  . . . . . . . 24
   4.  DoS Mitigation Strategies  . . . . . . . . . . . . . . . . . . 25
     4.1.  Protocol Design  . . . . . . . . . . . . . . . . . . . . . 25
       4.1.1.  Don't Hold State for Unverified Hosts  . . . . . . . . 25
       4.1.2.  Make it Hard to Simulate a Legitimate User . . . . . . 25
       4.1.3.  Graceful Routing Degradation . . . . . . . . . . . . . 26
       4.1.4.  Autoconfiguration and Authentication . . . . . . . . . 27
     4.2.  Network Design and Configuration . . . . . . . . . . . . . 27
       4.2.1.  Redundancy and Distributed Service . . . . . . . . . . 28
       4.2.2.  Authenticate Routing Adjacencies . . . . . . . . . . . 28
       4.2.3.  Isolate Router-to-Router Traffic . . . . . . . . . . . 28
     4.3.  Router Implementation Issues . . . . . . . . . . . . . . . 28
       4.3.1.  Checking Protocol Syntax and Semantics . . . . . . . . 29
       4.3.2.  Consistency Checks . . . . . . . . . . . . . . . . . . 29
       4.3.3.  Enhance Router Robustness through Operational
               Adjustments  . . . . . . . . . . . . . . . . . . . . . 30
       4.3.4.  Proper Handling of Router Resource Exhaustion  . . . . 31
     4.4.  End-System Implementation Issues . . . . . . . . . . . . . 31



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       4.4.1.  State Lookup Complexity  . . . . . . . . . . . . . . . 31
       4.4.2.  Operational Issues . . . . . . . . . . . . . . . . . . 32
   5.  Conclusions  . . . . . . . . . . . . . . . . . . . . . . . . . 33
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 34
   7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 35
   8.  Normative References . . . . . . . . . . . . . . . . . . . . . 36
   9.  Informative References . . . . . . . . . . . . . . . . . . . . 37
   Appendix A.  IAB Members at the time of this writing . . . . . . . 40
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 41
   Intellectual Property and Copyright Statements . . . . . . . . . . 42









































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1.  Introduction

   A Denial-of-Service (DoS) attack is an attack in which one or more
   machines target a victim and attempt to prevent the victim from doing
   useful work.  The victim can be a network server, client or router, a
   network link or an entire network, an individual Internet user or a
   company doing business using the Internet, an Internet Service
   Provider (ISP), country, or any combination of or variant on these.
   Denial of service attacks may involve gaining unauthorized access to
   network or computing resources, but for the most part in this
   document we focus on the cases where the denial-of-service attack
   itself does not involve a compromise of the victim's computing
   facilities.

   Because of the closed context of the original ARPAnet and NSFnet, no
   consideration was given to denial-of-service attacks in the original
   Internet Architecture.  As a result, almost all Internet services are
   vulnerable to denial-of-service attacks of sufficient scale.  In most
   cases, sufficient scale can be achieved by compromising enough end-
   hosts (typically using a virus, worm, or remotely controlled "bots")
   or routers, and using those compromised hosts to perpetrate the
   attack.  Such an attack is known as a Distributed Denial of Service
   attack (DDoS).  However, there are also many cases where a single
   well- connected end-system can perpetrate a successful DoS attack.

   This document is intended to serve several purposes:

   o To highlight possible avenues for attack, and by so doing encourage
   protocol designers and network engineers towards designs that are
   more robust.

   o To discuss partial solutions that reduce the effectiveness of
   attacks.

   o To highlight how some partial solutions can be taken advantage of
   by attackers to perpetrate alternative attacks.

   This last point appears to be a recurrent theme in DoS, and
   highlights the lack of proper architectural solutions.  It is our
   hope that this document will help initiate informed debate about
   future architectural solutions that might be feasible and cost-
   effective for deployment.

   In addition it is our hope that this document will spur discussion
   leading to architectural solutions that reduce the succeptibility of
   all Internet systems to denial-of-service attacks.

   We note that in principle it is not possible to distinguish between a



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   sufficiently subtle DoS attack and a flash-crowd, where unexpected
   heavy but non-malicious traffic has the same effect as a DoS attack.
   Whilst this is true, such malicious attacks are usually more
   expensive to launch than many of the crude attacks that have been
   seen to date.  Thus defending against DoS is not about preventing all
   possible attacks, but rather is largely a question of raising the bar
   sufficiently high for malicious traffic.

   However, it is also important to note that not all DoS problems are
   malicious.  Failed links, flash crowds, misconfigured bots, and
   numerous other causes can result in resource exhaustion problems, and
   so the overall goal should be to be robust to all forms of overload.







































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2.  An Overview of Denial-of-Service Threats

   In this section we will discuss a wide range of possible DoS attacks.
   This list cannot be exhaustive, but the intent is to provide a good
   overview of the spectrum of possibilities that need to be defended
   against.

   We do not provide descriptions of any attacks that are not already
   publicly well documented.

2.1.  DoS Attacks on End-systems

   We first discuss attacks on end-systems.  An end-system in this
   context is typically a PC or network server, but it can also include
   any communication endpoint.  For example, a router also is an end-
   system from the point of view of terminating TCP connections for BGP
   [32] or ssh [49].

2.1.1.  Exploiting Poor Software Quality

   The simplest DoS attacks on end-systems exploit poor software quality
   on the end-systems themselves, and cause that software to simply
   crash.  For example, buffer-overflow attacks might be used to
   compromise the end-system, but even if the buffer-overflow cannot be
   used to gain access, it will usually be possible to overwrite memory
   and cause the software to crash.  Such vulnerabilities can in
   principle affect any software that uses data supplied from the
   network.  Thus not only might a web server be potentially vulnerable,
   but it might also be possible to crash the back-end software (such as
   a database) to which a web server provides data.

   Software crashes due to poor coding not only affect application
   software, but also the operating system kernel itself.  A classic
   example is the so-called "Ping of Death", which became widely known
   in 1996 [10].  This exploit caused many popular operating systems to
   crash when sent a single fragmented ICMP echo request packet whose
   fragments totaled more than the 65535 bytes allowed in an IPv4
   packet.

   While DoS attacks such as the ping-of-death are a significant
   problem, they are not a significant architectural problem.  Once such
   an attack is discovered, the relevant code can easily be patched, and
   the problem goes away.  We should note though that as more and more
   software becomes embedded, it is important not to lose the
   possibility of upgrading the software in such systems.






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2.1.2.  Application Resource Exhaustion

   Network applications exist in a context that has finite resources.
   In processing network traffic, such an application uses these
   resources to do its intended task.  However, an attacker may be able
   to prevent the application from performing its intended task by
   causing the application to exhaust the finite supply of a specific
   resource.

   The obvious resources that might be exhausted include:

   o Available memory.

   o The CPU cycles available.

   o The disk space available to the application.

   o The number of processes or threads or both that the application is
   permitted to use.

   o The configured maximum number of simultaneous connections the
   application is permitted.

   This list is clearly not exhaustive, but it illustrates a number of
   points.

   Some resources are self-renewing: CPU cycles fall in this category -
   if the attack ceases, more CPU cycles become available.

   Some resources such as disk space require an explicit action to free
   up - if the application cannot do this automatically then the effects
   of the attack may be persistent after the attack has ceased.

   This problem has been understood for many years, and it is common
   practice for logs and incoming email to be stored in a separate disk
   partition (/var on Unix systems) in order to limit the impact of
   exhaustion.

   Some resources are constrained by configuration: the maximum number
   of processes and the maximum number of simultaneous connections are
   not normally hard limits, but rather are configured limits.  The
   purpose of such limits is clearly to allow the machine to perform
   other tasks in the event the application misbehaves.  However, great
   care needs to be taken to choose such limits appropriately.  For
   example, if a machine's sole task is to be an FTP server, then
   setting the maximum number of simultaneous connections to be
   significantly less than the machine can service makes the attacker's
   job easier.  But setting the limit too high may permit the attacker



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   to cause the machine to crash (due to poor OS design in handling
   resource exhaustion) or permit livelock (see below), which are
   generally even less desirable failure modes.

2.1.3.  Operating System Resource Exhaustion

   Conceptually OS resource exhaustion and application resource
   exhaustion are very similar.  However, in the case of application
   resource exhaustion, the operating system may be able to protect
   other tasks from being affected by the DoS attack.  In the case of
   the operating system itself running out of resources, the problem may
   be more catastrophic.

   Perhaps the best-known DoS attack on an operating system is the TCP
   SYN- flood [8], which is essentially a memory-exhaustion attack.  The
   attacker sends a flood of TCP SYN packets to the victim, requesting
   connection setup, but then does not complete the connection setup.
   The victim instantiates state to handle the incoming connections.  If
   the attacker can instantiate state faster than the victim times it
   out, then the victim will run out of memory that it can use to hold
   TCP state, and so it cannot service legitimate TCP connection setup
   attempts.  This issue was exacerbated in some implementations by the
   use of a small dedicated storage space for half-open connections,
   which made the attack easier than it might otherwise have been.  In
   the case of a poorly coded operating system, running out of resources
   may also cause a system crash.

   An alternative TCP DoS attack is the Ack-flood [12], which is
   essentially a CPU exhaustion attack on the victim.  The attacker
   floods the victim with TCP packets pretending to be from connections
   that have never been established.  A busy server that has a large
   number of outstanding connections needs to check which connection the
   packet corresponds to.  Some TCP implementations implemented this
   search rather inefficiently, and so the attacker could use all the
   victim's CPU resources servicing these packets rather than servicing
   legitimate requests.

   We note that strong authentication mechanisms do not mitigate against
   such CPU exhaustion attacks.  In fact poorly designed authentication
   mechanisms using cryptographic methods can exacerbate the problem.
   If such an authentication mechanism allows an attacker to present a
   packet to the victim that requires relatively expensive cryptographic
   authentication before the packet can be discarded, then this makes
   the attacker's CPU exhaustion attack easier.

   CPU exhaustion attacks can be also be exacerbated by poor OS handling
   of incoming network traffic.  In the absence of malicious traffic, an
   ideal OS should behave as follows:



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   o As incoming traffic increases, the useful work done by the OS
   should increase until some resource (such as the CPU) is saturated.

   o From this point on, as incoming traffic continues to increase the
   useful work done should be constant.

   However, this is often not the case.  Many systems suffer from
   livelock [29] where, after saturation, increasing the load causes a
   decrease in the useful work done.  One cause of this is that the
   system spends an increasing amount of time processing network
   interrupts for packets that will never be processed, and hence a
   decreasing amount of time is available for the application for which
   these packets were intended.

2.1.4.  Triggered Lockouts and Quota Exhaustion

   Many user-authentication mechanisms attempt to protect against
   password guessing attacks by locking the user out after a small
   number of failed authentications.  If an attacker can guess or
   discover a user's ID, they may be able to trigger such a mechanism,
   locking out the legitimate user.

   Another way to deny service using protection mechanisms is to cause a
   quota to be exhausted.  This is perhaps most common in the case of
   small web servers being commercially hosted, where the server has a
   contract with the hosting company allowing a fixed amount of traffic
   per day.  An attacker may be able to rapidly exhaust this quota, and
   cause service to be suspended.  Similar attacks may be possible
   against other forms of quota.

   In the absence of such quotas, if the victim is charged for their
   network traffic, a financial denial-of-service may be possible.

2.2.  DoS Attacks on Routers

   Many of the denial-of-service attacks that can be launched against
   end-systems can also be launched against the control processor of an
   IP router, for example by flooding the command and control access
   ports.  In the case of a router, these attacks may cause the router
   to stall, or may cause the router to cease processing routing
   packets.  Even if the router does not stop servicing routing packets,
   it may become sufficiently slow that routing protocols time out.  In
   any of these circumstances, the consequence of routing failure is not
   only that the router ceases to forward traffic, but also that it
   causes routing protocol churn that may have further side effects.

   An example of such a side effect is caused by BGP route flap damping
   [35], which is intended to reduce global routing churn.  If an



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   attacker can cause BGP routing churn, route flap damping may then
   cause the flapping routes to be suppressed [27].  This suppression
   likely causes the networks served by those routes to become
   unreachable.

   A DoS attack on the router control processor might also prevent the
   router from being managed effectively.  This may prevent actions
   being taken that would mitigate the DoS attack, and it might prevent
   diagnosis of the cause of the problem.

2.2.1.  Attacks on Routers through Routing Protocols

   In addition to their roles as end-systems, most routers run dynamic
   routing protocols.  The routing protocols themselves can be used to
   stage a DoS attack on a router or a network of routers.  This
   requires the ability to send traffic from addresses which might
   plausibly have generated the relevant routing messages, which is
   somewhat difficult with interior routing protocols but fairly easy
   with eBGP, for instance.

   The simplest attack on a network of routers is to overload the
   routing table with sufficiently many routes that the router runs out
   of memory, or the router has insufficient CPU power to process the
   routes [15].  We note that depending on the distribution and
   capacities of various routers around the network, such an attack
   might not overwhelm routers near to the attacking router, but might
   cause problems to show up elsewhere in the network.

   Some routing protocol implementations allow limits to be configured
   on the maximum number of routes to be heard from a neighbor [17].
   However, limits often make the problem worse rather than better, by
   making it possible for the attacker to push out legitimate routes
   with spoofed routes, thus creating an easy form of DoS attack.

   An alternative attack is to overload the routers on the network by
   creating sufficient routing table churn that routers are unable to
   process the changes.  Many routing protocols allow damping factors to
   be configured to avoid just such a problem.  However, as with table
   size, such a threshold applied inconsistently may allow the spoofed
   routes to merge with legitimate routes before the mechanism is
   applied, causing legitimate routes to be damped.

   The simplest routing attack on a specific destination is for an
   attacker to announce a spoofed desirable route to that destination.
   Such a route might be desirable because it has low metric, or because
   it is a more specific route than the legitimate route.  In any event,
   if the route is believed, it will cause traffic for the victim to be
   drawn towards the attacking router, where it will typically be



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   discarded.

   A more subtle denial-of-service attack might be launched against a
   network rather than against a destination.  Under some circumstances,
   the propagation of inconsistent routing information can cause traffic
   to loop.  If an attacker can cause this to happen on a busy path, the
   looping traffic might cause significant congestion, as well as fail
   to reach the legitimate destination.

   In the past there have been cases where different generations of
   routers interpreted a routing protocol specification differently.  In
   particular, BGP specifies that in the case of an error, the BGP
   peering should be dropped.  However, if some of the routers in a
   network treat a particular route as valid and other routers treat the
   route as invalid, then it may be possible to inject a BGP route at
   one point in the Internet and cause peerings to be dropped at many
   other places in the Internet.  Unlike many of the examples above,
   while such an issue might be a serious short-term problem, this is
   not a fundamental architectural problem.  Once the problem is
   understood, deploying patched routing code can permanently solve the
   issue.

2.2.2.  IP Multicast-based DoS Attacks

   There are essentially two forms of IP multicast: traditional Any-
   Source Multicast (ASM), as specified in RFC 1112 [19] where multiple
   sources can send to the same multicast group, and Source-Specific
   Multicast (SSM) where the receiver must specify both the IP source
   address and the group address.  The two forms of multicast provide
   rather different DoS possibilities.

   ASM protocols such as PIM-SM [21], MSDP [28] and DVMRP [38] typically
   cause some routers to instantiate routing state at the time a packet
   is sent to a multicast group.  They do this to ensure that the
   traffic goes to the group receivers and not to non-receivers.  Such
   protocols are particularly vulnerable to DoS attacks, as an attacker
   that sends to many multicast groups may cause both multicast routing
   table explosion (and hence control processor memory exhaustion) and
   multicast forwarding table exhaustion (and hence forwarding card
   memory exhaustion or thrashing).

   ASM also permits an attacker to send traffic to the same group as
   legitimate traffic, potentially causing network congestion and
   denying service to the legitimate group.

   SSM does not permit senders to send to arbitrary groups unless a
   receiver has requested the traffic.  Thus sender-based attacks on
   multicast routing state are not possible with SSM.  However, as with



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   ASM, a receiver can still join a large number of multicast groups
   causing routers to hold a large amount of multicast routing state,
   potentially causing memory exhaustion and hence denial-of-service to
   legitimate traffic.

   With IPv6, hosts are required to send ICMP Packet Too Big or
   Parameter Problem messages under certain circumstances, even if the
   destination address is a multicast address.  If the attacker can
   place himself in the appropriate position in the multicast tree, a
   packet with an unknown but mandatory Destination Option, for
   instance, could generate a very large number of responses to the
   claimed sender.

   With IPv4 the same problem exists with multicast ICMP Echo Request
   packets, but these are somewhat easier to filter.

   The examples above should not be taken as exhaustive.  These are
   actually specific cases of a general problem that can happen whene a
   multicast/broadcast request solicits a reply from a large number of
   nodes.

2.2.3.  Attacks on Router Forwarding Engines

   Router vendors implement many different mechanisms for packet
   forwarding, but broadly speaking they fall into two categories: ones
   that use a forwarding cache, and ones that do not.  With a forwarding
   cache, the forwarding engine does not hold the full routing table,
   but rather holds just the currently active subset of the forwarding
   table.

   Many modern routers use a loosely coupled architecture, where one or
   more control processors handle the routing protocols, and communicate
   over an internal network link to special-purpose forwarding engines,
   which actually forward the data traffic.  In such architectures it
   may be possible for an attacker to overwhelm the communications link
   between the control processor and the forwarding engine.  This is
   possible because the forwarding engines support very high speed
   links, and the control processor simply cannot handle a similar rate
   of traffic.

   There may be many ways in which an attacker can trigger communication
   between the forwarding engines and the control processor.  The
   simplest way is for the attacker to simply send to the router's IP
   address, but this should in principle be relatively easy to prevent
   using filtering on the forwarding engines.  Another way might be to
   cause the router to forward data packets using the "slow path".  This
   involves sending packets that require special attention from the
   forwarding router; if the forwarding engine is not smart enough to



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   perform such forwarding, then it will typically pass the packet to
   the control processor.  In a router using a forwarding cache, it may
   be possible to overload the internal communications by thrashing the
   forwarding cache.  Finally, any form of data-triggered communication
   between the forwarding engine and the control processor might cause
   such a problem.  Certain multicast routing protocols including PIM-SM
   contain many such data triggered events that could potentially be
   problematic.

   The effects of overloading such internal communications are hard to
   predict, and very implementation-dependent.  One possible effect
   might be that the forwarding table in the forwarding engine gets out
   of synchronization with the routing table in the control processor
   that reflects what the routing protocols believe is happening.  This
   might cause traffic to be dropped or to loop.

   Finally, if an attacker can generate traffic that causes a router to
   auto-install access control list (ACL) entries, perhaps by triggering
   a response from an intrusion detection system, then it may be
   possible to exhaust the ACL resources on the router.  This might
   prevent future attacks from being filtered, or worse, cause ACL
   processing to be handled by the route processor.

2.3.  Attacks on Ongoing Communications

   Instead of attacking the end-system itself, it is also possible for
   an attacker to disrupt ongoing communications.  If an attacker can
   observe a TCP connection, then it is relatively easy for them to
   spoof packets to either reset that connection or to de-synchronize it
   so that no further progress can be made [25].  Such attacks are not
   prevented by transport or application-level security mechanisms such
   as TLS [20] or ssh, because the authentication takes place after TCP
   has finished processing the packets.

   If an attacker cannot observe a TCP connection, but can infer that
   such a connection exists, it is theoretically possible to reset or
   desynchronize that connection by spoofing packets into the
   connection.  However, this might require an excessively large number
   of spoofed packets to guess both the port of the active end of the
   TCP connection (in most cases the port of the passive end is
   predictable) and the currently valid TCP sequence numbers.  However,
   as some operating systems have poorly implemented predictable
   algorithms for selecting either the dynamically selected port or the
   TCP initial sequence number [40] [9], then such attacks have been
   found to be feasible [30].  Advice as to how to reduce the
   vulnerability in the specific case of TCP is available in [34].

   An attacker might be able to significantly reduce the throughput of a



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   connection by sending spoofed ICMP source quench packets, although
   most modern operating systems should ignore such packets.  However,
   care should be taken in the design of future transport and signaling
   protocols to avoid the introduction of similar mechanisms that could
   be exploited.

2.4.  Attacks using the Victim's Own Resources

   Instead of directly overloading the victim, it may be possible to
   cause the victim or a machine on the same subnet as the victim to
   overload itself.

   An example of such an attack is documented in [7], where the attacker
   spoofs the source address on a packet sent to the victim's UDP echo
   port.  The source address is that of another machine that is running
   a UDP chargen server (a chargen server sends a character pattern back
   to the originating source).  The result is that the two machines
   bounce packets back and forth as fast as they can, overloading either
   the network between them or one of the end-systems itself.

2.5.  DoS Attacks on Local Hosts or Infrastructure

   There are a number of attacks that might only be performed by a local
   attacker.

   An attacker with access to a subnet may be able to prevent other
   local hosts from accessing the network at all by simply exhausting
   the address pool allocated by a DHCP server.  This requires being
   able to spoof the MAC address of an ethernet or wireless card, but
   this is quite feasible with certain hardware and operating systems.

   An alternative DHCP-based attack is simply to respond faster than the
   legitimate DHCP server, and to give out an address that is not useful
   to the victim.

   These sorts of bootstrapping attacks tend to be difficult to avoid
   because most of the time trust relationships are established after IP
   communication has already been established.

   Similar attacks are possible through ARP spoofing [4]; an attacker
   can respond to ARP requests before the victim and prevent traffic
   from reaching the victim.  Some brands of ethernet switch allow an
   even simpler attack - simply send from the victim's MAC address, and
   the switch will redirect traffic destined for the victim to the
   attacker's port.  This attack might also potentially be used to block
   traffic from the victim by engaging screening or flap-dampening
   algorithms in the switch, depending on the switch design.




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   It may be possible to cause broadcast storms [4] on a local LAN by
   sending a stream of unicast IP packets to the broadcast MAC address -
   some hosts on the LAN may then attempt to forward the packets to the
   correct MAC address greatly amplifying the traffic on the LAN.

   802.11 wireless networks provide many opportunities to deny service
   to other users.  In some cases, the lack of defenses against DoS was
   a deliberate choice--because 802.11 operates on unlicensed spectrum
   it was assumed that there would be sources of interference and that
   producing intentional radio-level jamming would be trivial.  Thus,
   the amount of DoS protection possible at higher levels was minimal.

   Nevertheless, some of the weaknesses of the protocols against more
   sophisticated attacks are worth noting.  The most prominent of these
   is that association is unprotected, thus allowing rogue APs to
   solicit notifications that would otherwise have gone to legitimate
   APs.

   The SSID field provides effectively no defense against this kind of
   attacks.  Unless encryption is enabled, it is trivial to announce the
   presence of a base station (or even of an ad-hoc mode host) with the
   same network name (SSID) as the legitimate basestation.  Even adding
   authentication and encryption a la 802.1X and 802.11i may not help
   much in this respect.  The SSID space is unmanaged, so everyone's
   free to put anything they want in the SSID field.  Most host stacks
   don't deal gracefully with this.  Moreover, SSIDs are very often set
   to the manufacturer's default, making them highly predictable.

   Some 802.11 basestations have limited memory for the number of
   associations they can support.  If this is exceeded, they may drop
   all associations.  In an attempt to forestall this problem, some APs
   advertise their load so as to enable stations to choose APs that are
   less loaded.  However, crude implementations of these algorithms can
   result in instability.

   Finally, as the authentication in 802.11 takes place at a
   comparatively high level in the stack, it is possible to simply
   deauthenticate or disassociate the victim from the basestation, even
   if WEP is in use [26].  Bellardo and Savage [3] describe some simple
   remedies that reduce the effectiveness of such attacks.  While IEEE
   802.11w will protect Deauthenticate or Disassociate frames, this
   attack is still possible via forging of Association frames.

   What all these attacks have in common is that they exploit
   vulnerabilities in the link auto-configuration mechanisms.  In a
   wireless network it is necessary for a station to detect the presence
   of APs in order to choose which one to connect to.  In 802.11 this is
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   The Beacon cannot easily be encrypted, because the station needs to
   utilize them prior to authentication in order to discover which APs
   it may wish to communicate with.  Since authentication can only occur
   after interpreting the Beacon, an encrypted Beacon would present a
   chicken-egg problem: you can't obtain a key to decrypt the Beacon
   until completing authentication, and you may not be able to figure
   out which AP to authenticate with prior to decrypting the Beacon.
   Note that in principle you could encrypt the Beacons with a shared
   (per-AP) key but this would require each station to trial-decrypt
   beacons until it find one that matches up to whatever shared
   authentication secret it had.  This is not particularly convenient.

   As a result, discussions of Beacon frame security have largely
   focused on authentication of Beacon frames, not encryption.  Even
   here, solutions are difficult.  While it may be possible to for a
   station to validate a Beacon *after* authentication (either by
   checking a MIC computed with the group key provided by the AP, or
   verifying the Beacon parameters during the 4-way handshake), doing so
   *before* authentication may require synchronization of keys between
   APs within an SSID.

2.6.  DoS Attacks on Sites though DNS

   In today's Internet, DNS is of sufficient importance that if access
   to a site's DNS servers is denied, the site is effectively
   unreachable, even if there is no actual communication problem with
   the suite itself.

   Many of the attacks on end-systems described above can be perpetrated
   on DNS servers.  As servers go, DNS servers are not particularly
   vulnerable to DoS.  So long as a DNS server has sufficient memory, a
   modern host can usually respond very rapidly to DNS requests for
   which it is authoritative.  This was demonstrated in October 2002
   when the root nameservers were subjected to a very large DoS attack
   [36].  A number of the root nameservers have since been replicated
   using anycast [1] to further improve their resistance to DoS.
   However it is important for authoritative servers to have relaying
   disabled, or it is possible for an attacker to force the DNS servers
   to hold state [39].

   Many of the routing attacks can also be used against DNS servers by
   targeting the routing for the server.  If the DNS server is co-
   located with the site for which is authoritative, then the fact that
   the DNS server is also unavailable is of secondary importance.
   However, if all the DNS servers are made unavailable, this may cause
   email to that site to bounce rather than being stored while the mail
   servers are unreachable, so distribution of DNS server locations is
   important.



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   Causing network congestion on links to and from a DNS server can have
   similar effects to end-system attacks or routing attacks, causing DNS
   to fail to obtain an answer, and effectively denying access to the
   site being served.

   We note that if an attacker can deny external access to all the DNS
   servers for a site, this will not only cause email to that site to be
   dropped, but will also cause email from that site to be dropped.
   This is because recent versions of mail transfer agents such as
   sendmail will drop email if the mail originates from a domain that
   does not exist.  This is a classic example of unexpected
   consequences.  Sendmail performs this check as an anti-spam measure,
   and spam itself can be viewed as a form of DoS attack.  Thus
   defending against one DoS attack opens up the vulnerability that
   allows another DoS attack.  If a receiving implementation is using a
   blackhole list (see Section 2.14) served by DNS an attacker can also
   mount a DoS attack by attacking the blackhole server.

   Finally, a data corruption attack is possible if a site's nameserver
   is permitted to relay requests from untrusted third parties [39].
   The attacker issues a query for the data he wishes to corrupt, and
   the victim's nameserver relays the request to the authoritative
   nameserver.  The request contains a 16-bit ID that is used to match
   up the response with the request.  If the attacker spoofs sufficient
   response packets from the authoritative nameserver just before the
   official response will arrive, each containing a forged response and
   a different DNS ID, then there is a reasonable chance that one of the
   forged responses will have the correct DNS ID.  The incorrect data
   will then be believed and cached by the victim's nameserver, so
   giving the incorrect response to future queries.  The probability of
   the attack can further be increased if the attacker issues many
   different requests for the same data with different DNS IDs, because
   many nameserver implementations will issue relayed requests with
   different DNS IDs, and so the response only has to match any one of
   these request IDs [6] [33].

   The use of anycast for DNS services makes it even more vulnerable to
   spoofing attacks.  An attacker who can convince the ISP to accept an
   anycast route to his fake DNS server can arrange to receive requests
   and generate fake responses.  Anycast DNS also makes DoS attacks on
   DNS easier.  The idea is to disable one of the DNS servers while
   maintaining the BGP route to that server.  This creates failures for
   any client which is routed to the (now defunct) server.

2.7.  DoS Attacks on Links

   The simplest DoS attack is to simply send enough non-congestion-
   controlled traffic such that a link becomes excessively congested,



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   and legitimate traffic suffers unacceptably high packet loss.

   Under some circumstances the effect of such a link DoS can be much
   more extensive.  We have already discussed the effects of denying
   access to a DNS server.  Congesting a link might also cause a routing
   protocol to drop an adjacency if sufficient routing packets are lost,
   potentially greatly amplifying the effects of the attack.  Good
   router implementations will prioritize the transmission of routing
   packets, but this is not a total panacea.  If routers are peered
   across a shared medium such as ethernet, it may be possible to
   congest the medium sufficiently that routing packets are still lost.

   Even if a link DoS does not cause routing packets to be lost, it may
   prevent remote access to a router using ssh or SNMP.  This might make
   the router unmanageable, or prevent the attack from being correctly
   diagnosed.

   The prioritization of routing packets can itself cause a DoS problem.
   If the attacker can cause a large amount of routing flux, it may be
   possible for a router to send routing packets at a high enough rate
   that normal traffic is effectively excluded.  This is however
   unlikely except on low bandwidth links.

   Finally, it may be possible to an attacker to deny access to a link
   by causing the router to generate sufficient monitoring or report
   traffic that the link is filled.  SNMP traps are one possible vector
   for such an attack, as they are not normally congestion controlled.

   Attackers with physical access to multiple access links can easily
   bring down the link.  This is particularly easy to mount and
   difficult to counter with wireless networks.

2.8.  DoS attacks on firewalls

   Firewalls are intended to defend the systems behind them against
   attack.  In that they restrict the traffic that can reach those
   systems, they may also aid in defending against denial-of-service
   attacks.  However, under some circumstances the firewall itself may
   also be used as a weapon in a DoS attack.

   There are many different types of firewall, but generally speaking
   they fall into stateful and stateless classes.  The state here refers
   to whether the firewall holds state for the active flows traversing
   the firewall.  Stateless firewalls generally can only be attacked by
   attempting to exhaust the processing resources of the firewall.
   Stateful firewalls can be attacked by sending traffic that causes the
   firewall to hold excessive state or state which has pathological
   structure.



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   In the case of excessive state, the firewall simply runs out of
   memory, and can no longer instantiate the state required to pass
   legitimate flows.  Most firewalls will then fail disconnected,
   causing denial-of-service to the systems behind the firewall.

   In the case of pathological structure, the attacker sends traffic
   that causes the firewall's data structures to exhibit worst case
   behaviour.  An example of this would be when the firewall uses hash
   tables to look up forwarding state, and the attacker can predict the
   hash function used.  The attacker may then be able to cause a large
   amount of flow state to hash to the same bucket, which causes the
   firewall's lookup performance to change from O(1) to O(n), where n is
   the number of flows the attacker can instantiate [18].  Thus the
   attacker can cause forwarding performance to degrade to the point
   where service is effectively denied to the legitimate traffic
   traversing the firewall.

2.9.  DoS attacks on IDS systems

   Intrusion detection systems (IDS) suffer from similar problems to
   firewalls.  It may be possible for an attacker to cause the IDS to
   exhaust its available processing power, to run out of memory, or to
   instantiate state with pathological structure.  Unlike a firewall, an
   IDS will normally fail open, which will not deny service to the
   systems protected by the IDS.  However it may mean that subsequent
   attacks that the IDS would have detected will be missed.

   Some IDSs are reactive; that is on detection of a hostile event they
   react to block subsequent traffic from the hostile system, or to
   terminate an ongoing connection from that system.  It may be possible
   for an attacker to spoof packets from a legitimate system, and hence
   cause the IDS to believe that system is hostile.  The IDS will then
   cause traffic from the legitimate system to be blocked, hence denying
   service to it.  The effect can be particularly bad if the legitimate
   system is a router, DNS server, or other system whose performance is
   essential for the operation of a large number of other systems.

2.10.  DoS attacks on or via NTP

   Network time servers are generally not considered security-critical
   services, but under some circumstances NTP servers might be used to
   perpetrate a DoS attack.

   The most obvious such attack is to DoS the NTP servers themselves.
   Many end systems have rather poor clock accuracy and so, without
   access to network time, their clock will naturally drift.  This can
   cause problems with distributed systems that rely on good clocks.
   For example one commonly used revision control system can fail if it



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   perceives the modification timestamp to be in the future.

   If the NTP servers relied on by a host can be subverted, either
   through compromising or impersonating them, then the attacker may be
   able to control the host's system clock.  This can cause many
   unexpected consequences, including the premature expiry of dated
   resources such as encryption or authentication keys.  This in turn
   can prevent access to other more critical services.

2.11.  Physical DoS

   The discussion thus far has centered on denial-of-service attacks
   perpetrated using the network.  However, computer systems are only as
   resilient as the weakest link.  It may be easier to deny service by
   causing a power failure, by cutting network cables, or by simply
   switching a system off, and so physical security is at least as
   important as network security.  Physical attacks can also serve as
   entry points for non-physical DoS, for instance by reducing the
   resources available to deal with overcapacity.

2.12.  Social Engineering DoS

   The weakest link may also be human.  In defending against DoS, the
   possibility of denial-of-service through social engineering should
   not be neglected, such as convincing an employee to make a
   configuration change that prevents normal operation.

2.13.  Legal DoS

   Computer systems cannot be considered in isolation from the social
   and legal systems in which they operate.  This document focuses
   primarily on the technical issues, but we note that "cease and
   desist" letters, government censorship, and other legal mechanisms
   also touch on denial-of-service issues.

2.14.  Spam and Black-hole Lists

   Unsolicited commercial email, also known as "spam", can effectively
   cause denial-of-service to email systems.  While the intent is not
   denial-of-service, the large amount of unwanted mail can waste the
   recipient's time, or cause legitimate email to fail to be noticed
   amongst all the background noise.  If spam filtering software is
   used, some level of false positives is to be expected, and so these
   messages are effectively denied service.

   One mechanism to reduce spam is the use of black-hole lists.  The IP
   addresses of dial-up ISPs or mail servers used to originate or relay
   spam are added to black-hole lists.  The recipients of mail choose to



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   consult these lists and reject spam if it originates or is relayed by
   systems on the list.  One significant problem with such lists is that
   it may be possible for an attacker to cause a victim to be black-hole
   listed, even if the victim was not responsible for relaying spam.
   Thus the black-hole list itself can be a mechanism for effecting a
   DoS attack.  Note that every black-hole list has its own policy
   regarding additions, and some are less susceptible to this DoS attack
   than others.  Consumers of black-hole list technology are advised to
   investigate these policies before they subscribe.  Similar
   considerations apply to feeds of bad BGP bad route advertisements.









































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3.  Attack Amplifiers

   Many of the attacks described above rely on sending sufficient
   traffic to overwhelm the victim.  Such attacks are made much easier
   by the existence of "attack amplifiers", where an attacker can send
   traffic from the spoofed source address of the victim and cause
   larger responses to be returned to the victim.  A detailed discussion
   of such reflection attacks can be found in [31].

3.1.  Methods of Attack Amplification

   The simplest such attack was the "smurf" attack [11], where an ICMP
   echo request packet with the spoofed source address of the victim is
   sent to the subnet-broadcast address of a network to be used as an
   amplifier.  Every system on that subnet then responds with an ICMP
   echo response that returns to the victim.  Smurf attacks are no
   longer such a serious problem, as these days routers usually drop
   such packets and end-systems do not respond to them.

   An alternative form of attack amplifier is typified by a DNS
   reflection attack.  An attacker sends a DNS request to a DNS server
   requesting resolution of a domain name.  Again the source address of
   the request is the spoofed address of the victim.  The request is
   carefully chosen so that the size of the response is significantly
   greater than the size of the request, thereby providing the
   amplification.  As an aside, it is interesting to note that the
   largest DNS responses tend to be those incorporating DNSsec
   authentication information.  This attack amplifier can only be used
   by an attacker with the ability to spoof the source address of the
   victim.  However, we note that if the victim's DNS server is
   configured to relay requests from external clients, it may be
   possible to cause it to congest its own incoming network link.

   Another variant of attack amplifier involves amplification through
   retransmission.  This is typified by a TCP amplification attack known
   as "bang.c".  The attacker sends a spoofed TCP SYN with the source
   address of the victim to an arbitrary TCP server.  The server will
   respond with a SYN|ACK which is sent to the victim, and when no final
   ACK is received to complete the handshake, the SYN|ACK will be
   retransmitted a number of times.  Typically this attack uses a very
   large list of arbitrarily chosen servers as reflectors.  For the
   attack to be successful, the reflector must not receive a RST from
   the victim in response to the SYN|ACK - however if the attack traffic
   sufficiently overwhelms the server or access link to the server, then
   packet loss will ensure that many reflectors do not receive a RST in
   response to their SYN|ACK, and so continue to retransmit.  The attack
   can be exacerbated by firewalls that silently drop the incoming SYN|
   ACK without sending a RST.



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   Care must also be taken with services that relay requests.  If an
   attacker can send a request to a proxy, and that proxy now attempts
   to connect to a victim whose address is chosen by the attacker then,
   if the proxy repeatedly resends the request when receiving no answer,
   this can also serve as an attack amplifier.

   Another variant of amplification occurs in protocols that include,
   within the protocol payload, an IP address or name of host to which
   subsequent messages should be sent.  An example of such a protocol is
   the Session Initiation Protocol (SIP), which carries a payload
   defined by the Session Description Protocol (SDP).  The SDP payload
   of the SIP message conveys the IP address and port to which media
   packets, typically encoded using the Real Time Transport Protocol
   (RTP), are sent.

   To launch this attack, an attacker sends a protocol message, and sets
   the IP address within the payload to point to the attack target.  The
   recipient of the message will generate subsequent traffic to that IP
   address.  Depending on the protocol, this attack can provide
   substantial amplification properties.  In the specific case of SIP,
   if a caller makes calls to high bandwidth media sources (such as a
   video server or streaming audio server), a single SIP INVITE packet,
   typically a few hundred bytes, can result in a nearly continuous
   stream of media packets at rates anywhere from a few kbits per second
   up to megabits per second.  This particular attack is called the
   "voice hammer".

   Unlike the other techniques described above, this technique does not
   require the attacker to modify packets or even spoof their source IP
   address.  This makes it easier to launch.

   This attack is prevented through careful protocol design.  Protocols
   should, whenever possible, avoid including IP addresses or hostnames
   within protocol payloads as addresses to which subsequent messaging
   should be sent.  Rather, when possible, messages should be sent to
   the source IP from which the protocol packet came.  If such a design
   is not possible, the protocol should include a handshake whereby it
   can be positively determined that the protocol entity at that IP
   address or hostname does, in fact, wish to receive that subsequent
   messaging.  That handshake itself needs to be lightweight (to avoid
   being the source of another DoS attack), and secured against the
   spoofing of the handshake response.

   Finally, a somewhat similar attack is possible with some protocols
   where where one message leads to another message that is not sent as
   a reply to the source address of the first message.  This can be an
   issue with protocols to enable mobility for example, and might permit
   an attacker to avoid ingress filtering.  Such protocols are



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   notoriously difficult to get right.

3.2.  Strategies to Mitigate Attack Amplification

   In general, the architectural lessons to be learnt are simple:

   o As far as possible, perform ingress filtering [22] [37] to prevent
   source address spoofing.

   o Avoid designing protocols or mechanisms that can return
   significantly larger responses than the size of the request, unless a
   handshake is performed to validate the client's source address.  Such
   a handshake needs to incorporate an unpredictable nonce that is
   secure enough to mitigate the amplification effects of the protocol.

   o All retransmission during initial connection setup should be
   performed by the client.

   o Proxies should not arbitrarily relay requests to destinations
   chosen by a client.

   o Avoid signaling third-party connections.  Any unavoidable third-
   party connections setup by a signaling protocol should incorporate
   lightweight validation before sending significant data.



























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4.  DoS Mitigation Strategies

   A general problem with DoS defense is that it is not in principle
   possible to distinguish between a flash-crowd and a DoS attack.
   Indeed, having your site taken down by a flash crowd is probably a
   more common experience than having it DoS-ed --- so common it's
   acquired its own names: being Slashdotted or Farked, after the web
   sites that are common sources of flash crowds.  Thus, the first line
   of defense against DoS attacks must be to provision your service so
   that it can handle a foreseeable legitimate peak load.
   Underprovisioned sites are the easiest to take down.

   Specific strategies for DoS defense fall into two broad categories:

   1.  Avoiding allowing attacks that are better than generic resource
       consumption.

   2.  Minimizing the extent to which generic resource consumption
       attacks crowd out legitimate users.

   In the remainder of this section, we consider specific applications
   of these two approaches at a variety of levels of network system
   architecture.

4.1.  Protocol Design

4.1.1.  Don't Hold State for Unverified Hosts

   From an end-system server point of view, one simple aim is to avoid
   instantiating state without having completed a handshake with the
   client to validate their address, and as far as possible to push work
   and stateholding to client.  There are a number of techniques that
   might be used to do this, including SYN-cookies [5] [2].  All client-
   server protocols should probably be designed to allow such techniques
   to be used, but the enabling of the mechanism should normally be at
   the server's discretion to avoid unnecessary work under normal
   circumstances.

4.1.2.  Make it Hard to Simulate a Legitimate User

   Other than having massive overcapacity, the only real defense against
   resource consumption attacks is to preferentially discriminate
   against attackers.  The general idea is to find something that
   legitimate users can do but attackers can't.  The most commonly
   proposed approaches include:

   1.  Puzzles: force the attacker to do some computation that would not
       be onerous for a single user but is too expensive to do en masse.



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       [2]

   2.  Reverse Turing Tests: specialized puzzles that are hard for
       machines to do but easy for humans, thus making automated attacks
       hard.[42]

   3.  Reachability testing: force the proposed client to demonstrate
       that it can receive traffic at a given IP address.  This makes it
       easier to trace attackers.

   All of these techniques have substantial limitations.  Puzzles tend
   to discriminate against legitimate users with slow computers.  In
   addition, the wide availability of "bots" means that attackers have
   ample computing power at their disposal.  There has been substantial
   work in attacking Reverse Turing Tests automatically, thus making
   them of limited applicability.  Finally, reachability testing is
   substantially weakened by bots because the attacker does not need to
   hide his source address.

4.1.3.  Graceful Routing Degradation

   A goal with routing protocols is that of graceful degradation in
   overload, and automatic recovery after the source of the overload has
   been remedied.  Some routing protocols satisfy this goal more than
   others.  Although RIP doesn't scale well, if a router runs out of
   memory when receiving a RIP route, it can just drop the route and
   send an infinite metric to its peers.  The route will later be
   refreshed, and if the original source of the problem has been
   resolved, the router will now be able to process it correctly.

   On the other hand, BGP is stateful in the sense that a peer assumes
   you have processed or chosen to filter any route that it sent you.
   There is no mechanism to refresh state in the base BGP spec, and even
   the later route refresh option [16] is hard to use usefully in the
   presence of overload.  A BGP router that cannot store a route it
   received has two choices: completely restart BGP, or shutdown one or
   more peerings [15].  This means that the effects of a BGP overload
   are rather more severe than they need to be, and so amplifies the
   effect of any attack.

   In general, few routing protocol designs actively consider the
   possible behaviour of routers under overload conditions; this should
   be an explicit part of future routing protocol designs.  Although
   precise details should clearly be left to implementors, the protocol
   design needs to give them the capability to do their job properly.






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4.1.4.  Autoconfiguration and Authentication

   Autoconfiguration mechanisms greatly ease deployment, and are
   increasingly necessary as the number of networked devices grows
   beyond what can be managed manually.  However, it should be
   recognised that unauthenticated autoconfiguration opens up many
   avenues for attack.  There is a clear tension between ease of
   configuration and security of configuration, especially because there
   are environments in which it is desirable for units to operate with
   effectively no authentication (e.g., airport hotspots).  Future
   autoconfiguration protocols should consider the need to allow
   different end-systems to operate at different points in this spectrum
   within the same autoconfiguration framework.  However, this also
   implies that the network elements should avoid acting for
   unauthenticated hosts, instead just letting them access the network
   more or less directly.

4.2.  Network Design and Configuration

   In general, networks should be provisioned with private, out-of-band
   access to console or control ports so that such control facilities
   will be available in the face of a DoS attack launched against either
   the control or data plane of the (in-band) network.  Typically such
   out-of-band networks are provisioned on a separate infrastructure for
   exactly this purpose.  Out-of-band access is a crucial capability for
   DoS mitigation, since many of the typical redundancy and capacity
   management techniques (such as prioritizing routing or network
   management traffic) fail during such attacks.  In addition, many
   redundancy protocols such as VRRP [50] can fail during such attacks
   as they may be unable to keep adjacencies alive.

   There are several default configuration settings that can also be
   exploited to generate several of the attacks outlined in this
   document.  For example, some vendors may have features such as IP
   redirect, directed broadcast, and proxy ARP enabled by default.
   Similar defaults, such as publicly readable SNMP [51] communities
   (e.g., "public") can be used to reveal otherwise confidential
   information to a prospective attacker.  Finally, other
   unauthenticated configuration management protocols such as TFTP
   [RFC1350] should be avoided if possible; at the very least access to
   TFTP configuration archives should be protected and TFTP should be
   filtered at administrative boundaries.  Finally, since many of the
   password encryption techniques used by router vendors are reversible,
   keeping such passwords on a configuration archive (as part of a
   configuration file), even in the encrypted form written by the
   router, can lead to unauthorized access if the archive is
   compromized.




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4.2.1.  Redundancy and Distributed Service

   A basic principle of designing systems to handle failure to have
   redundant servers which can take over when one fails.  This is
   equally true in the case of DoS attacks, which often focus on a given
   server and/or link.  If service delivery points can be distributed
   across the network, then it becomes much harder to attack the entire
   service.  In particular, this makes attacks on a single network link
   more difficult.

4.2.2.  Authenticate Routing Adjacencies

   In general, cryptographic authentication mechanisms are too costly to
   form the main part in DoS prevention.  However, routing adjacencies
   are too important to risk an attacker being able to inject bad
   routing information, which can affect more than the router in
   question.  Additional non-cryptographic mechanisms should then be
   used to avoid arbitrary end-systems being able to cause the router to
   spend CPU cycles on validating authentication data.

   For BGP, at the very least, this implies the use of TCP MD5 [24] or
   IPsec authentication, combined with the GTSM [23] to prevent EBGP
   association with non-immediate neighbours.  In future, this will
   likely imply better authentication of the routing information itself.

4.2.3.  Isolate Router-to-Router Traffic

   As far as is feasible, router-to-router traffic should be isolated
   from data traffic.  How this should be implemented depends on the
   precise technologies available, both in the router and at the link-
   layer.  The goal should be that failure of the link for data traffic
   should also cause failure for the routing traffic, but that an
   attacker cannot directly send packets to the control processor of the
   routers.

   A downside of this is that some diagnostic techniques (such as
   pinging consecutive routers to find the source of a delay) may no
   longer be possible.  Ideally, alternative mechanisms (which do not
   open up additional avenues for DoS) should be designed to replace
   such lost techniques.

4.3.  Router Implementation Issues

   Because a router can be considered as an end-system, it can
   potentially benefit from all the prevention mechanisms prescribed for
   end-system implementation.  However one basic distinction between a
   router and a host is that the former implements routing protocols and
   forwards data, which in turn lead to additional router specific



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   implementation considerations.  The issues described below are meant
   to be illustrative and not exhaustive.

4.3.1.  Checking Protocol Syntax and Semantics

   Protocol syntax defines the formation of the protocol messages and
   the rules of exchanges.  The questions addressed by protocol syntax
   checking includes, but is not limited to, the following:

   1.  Who sent the message?

   2.  Does the content conform to the protocol format?

   3.  Was the message sent with correct timing?

   The first step in protocol syntax verification is to ensure that an
   incoming message was sent by a legitimate party.  There are multiple
   ways to perform this check.  One can verify the source IP address and
   even the MAC address of the message.  Utilizing the fact that eBGP
   peers are normally directly connected, one can also check the TTL
   value in a packet and discard any BGP updates packet whose TTL is
   less than some maximum value (typically max TTL - 1) [23].
   Cryptographic authentication should also be used whenever available
   to verify that an incoming message is indeed from an expected sender.
   For BGP, at the very least, this implies the use of TCP MD5 [24] or
   IPsec authentication.

   In addition to the sender verification, it is also important to check
   the syntax of a received routing message, as opposed to assuming that
   all messages came in a correct format.  It happened in the past that
   routers crashed upon receiving ill-formed routing messages.  Such
   faults will be prevented by performing rigorous syntax checking.

4.3.2.  Consistency Checks

   Protocol semantics define the meaning of the message content, the
   interpretation of the values, and the actions to be taken according
   to the content.  Here is a simple example of using semantic checking.
   When a link failure causes a router in AS A to send a peer router B a
   withdrawal message for prefix P, B should make sure that any
   alternative path it finds to reach P does not go through A. This
   simple check is shown to significantly improve BGP convergence time
   in many cases [45].

   Another example of using semantic checking against false routing
   injection is described in [47].  The basic idea is to attach to the
   route announcement for prefix P a list of the valid origin ASes.  Due
   to the rich connectivity in today's Internet topology, a remote AS



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   will receive routing updates from multiple different paths and can
   check to see whether each update carries the identical origin AS
   list.  Although a false origin may announce reachability to P, or
   alter the origin AS list, it would be difficult, if not impossible,
   to block the correct updates from propagating out, thus remote ASes
   can detect the existence of false updates by observing the
   inconsistency of the received origin AS lists for P. Research studies
   show that the "allowed origin list" test can effectively detect
   majority of falsely originated updates.

   Generally speaking, verifying the validity of BGP routes can be
   challenging because BGP is policy driven and policies of individual
   ISPs are not known in most cases.  But assuming policies do not
   change in short time scale, in principle one could verify new updates
   against observed routes from recent past, which reflect the routing
   policies in place.  Research work is needed to explore this
   direction.

   Note that while the above steps are all fairly simple and don't
   really "bullet-proof" the protocol, each adds some degree of
   protection.  As such, the combination of the above techniques can
   result in an efffective reduction in the probability of undetected
   faults.

4.3.3.  Enhance Router Robustness through Operational Adjustments

   There exist a number of configuration tunings that can enhance
   robustness of BGP operations.  One example is to let BGP peers
   coordinate the setting of a limit on the number of prefixes which one
   BGP speaker will send to its peer [46].  Although such check does not
   validate the prefix owned by each peer, it can prevent false
   announcements of large number of invalid routes.  Had all BGP routers
   been configured with this simple checking earlier, several large
   scale routing outages in the past could have been prevented.  Note,
   however, that care must be taken with hard limits of this type
   because they can be used to mount a DoS because implementations often
   discard excess routes indiscriminately, thus potentially causing
   black-holing of correct routes.

   Another example of useful configuration tuning is to adjust the BGP's
   KeepAlive and Hold Timer values to minimize BGP peering session
   resets.  Previous measurements show that heavy traffic load, such as
   those caused by Worms, can cause BGP KeepAlive messages to be delayed
   or dropped, which in turn cause BGP peering session break down.  Such
   load-induced session breaks and re-establishments can lead to an
   excessive amount of BGP updates during the periods when stable
   routing is needed most.




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4.3.4.  Proper Handling of Router Resource Exhaustion

   In addition to the resource exhaustion problems that are generally
   apply to all end systems, as described in Section 2, router
   implementations must also take special care in handling resource
   exhaustions when they occur in order to keep the router operating
   despite the problem.  For example under normal operations a router
   does not require a large cache to hold outstanding ARP requests
   because the replies are normally received within a few milliseconds.
   However certain conditions can lead to ARP cache exhaustion, for
   example during a virus attack where many packets are sent to non-
   existing IP addresses, thus there are no ARP replies to the requests
   for those addresses.  Such phenomena have happened in the past and
   led to routers failing to forward packets.

   Another example is queue management.  Many high-end routers are
   designed so that most packets can be handled purely in specialized
   processors (ASICs, FPGAs, etc.).  However, exceptional packets must
   be routed to a supporting general purpose CPU for handling.  On some
   such systems, it may be possible mount a low-effort DoS attack by
   saturating the queues between the specialized hardware and the
   supporting processor.

   So the attack vector on routers/network devices is a low PPS queue
   saturation attack on the ASIC's raw queue structure.  The
   countermeasure here is to have multiple such queues designed in such
   a way that it's difficult for an attacker to arrange to fill multiple
   queues [41].

4.4.  End-System Implementation Issues

4.4.1.  State Lookup Complexity

   Any system that instantiates per-connection state should take great
   care to implement state-lookup mechanisms in such a way that
   performance can not be controlled by the attacker.  One way to
   achieve this is to use hash-tables where the hash mechanism is keyed
   in such a way that the attacker cannot instantiate a large number of
   flows in the same hash bucket.

4.4.1.1.  Avoid Livelock

   Most operating systems use network interrupts to receive data from
   the network, which is a good solution if the host spends only a small
   amount of its time handling network traffic.  However, this leaves
   the host open to livelock [29], where under heavy load the OS spends
   all its time handling interrupts and no time doing the work needed to
   handle the traffic at the application level.  Server operating



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   systems should consider using network polling at times of heavy load,
   rather than being interrupt-driven, and should be carefully
   architected so that as far as reasonably possible, traffic received
   by the OS is processed to completion, or very cheaply discarded.

4.4.1.2.  Use Unpredictable Values for Session IDs

   Most recent TCP implementations use fairly good random mechanisms for
   allocating the TCP initial sequence numbers.  In general, any
   dynamically allocated value used purely to identify a communication
   session should be allocated using an unpredictable mechanism, as this
   increases the search space for an attacker that wishes to disrupt
   ongoing communications.  Thus the dynamically allocated port of the
   active end of a TCP connection might also be randomly allocated.

   With DNS, the ID which is used to match responses with requests
   should also be randomly generated.  However, as the ID field is only
   16 bits, the protection is rather limited.

4.4.2.  Operational Issues

4.4.2.1.  Eliminate Bad Traffic Early

   Many DoS attacks are generic bandwidth consumption attacks that
   operate by clogging the link that connects the victim server to the
   Internet.  Filtering these attacks at the server does no good because
   the traffic has already traversed the link which is the scarce
   resource.  Such flows need to be filtered at some point closer to the
   attacker.  Where possible, operators should filter out obviously bad
   traffic.  In particular, they should perform ingress filtering [22].

4.4.2.2.  Establish a Monitoring Framework

   Network operators are strongly encouraged to establish a monitoring
   framework to detect and log abnormal network activity.  One can not
   defend against an attack one doesn't detect or understand.  Such
   monitoring tools can be used to set a baseline of "normal" traffic,
   and can be used to detect aberrant flows and determine the type and
   source of the aberrant flows.  This is extremely helpful when
   responding to distributed DoS attacks or a flash crowd, and should be
   in place prior to the event.










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5.  Conclusions

   In this document we have highlighted possible avenues for DoS attacks
   on networks and networked systems, with the aim of encouraging
   protocol designers and network engineers towards designs that are
   more robust.  We have discussed partial solutions that reduce the
   effectiveness of attacks, and highlighted how some partial solutions
   can be taken advantage of by attackers to perpetrate alternative
   attacks.

   Our focus has primarily been on protocol and network architecture
   issues, but there are many things that network and service operators
   can do to lessen the threat.  Further advice and information for
   network operators can be found in [13] [37] [14].

   It is our hope that this document will spur discussion leading to
   architectural solutions that reduce the succeptibility of all
   Internet systems to denial-of-service attacks.

































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6.  Security Considerations

   This entire document is about security.
















































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7.  Acknowledgements

   We are very grateful to Vern Paxson, Paul Vixie, Rob Thomas, Dug
   Song, George Jones, Jari Arkko, and Geoff Huston for their
   constructive comments on earlier versions of this document.














































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8.  Normative References

   [1] J. Abley, "Hierarchical Anycast for Global Service Distribution",
   http://www.isc.org/tn/isc-tn-2003-1.txt

   [5] D.J. Bernstein, "SYN Cookies", http://cr.yp.to/syncookies.html

   [16] E. Chen, "Route Refresh Capability for BGP-4", RFC 2918,
   September 2000

   [19] S. Deering, "Host extensions for IP multicasting", RFC 1112, Aug
   1989.

   [20] T. Dierks, C. Allen, "The TLS Protocol, Version 1.0", RFC 2246,
   Jan 1999.

   [21] D. Estrin, D. Farinacci, A. Helmy, D. Thaler, S. Deering, M.
   Handley, V. Jacobson, C. Liu, P. Sharma, L. Wei, "Protocol
   Independent Multicast-Sparse Mode (PIM-SM): Protocol Specification",
   RFC 2362, June 1998.

   [22] P. Ferguson, D. Senie, "Network Ingress Filtering: Defeating
   Denial of Service Attacks which employ IP Source Address Spoofing",
   RFC 2827, May 2000.

   [23] V. Gill, J. Heasley, D. Meyer "The Generalized TTL Security
   Mechanism (GTSM)", RFC 3682, February 2004.

   [24] A. Heffernan, "Protection of BGP Sessions via the TCP MD5
   Signature Option", RFC 2385, August 1998.

   [32] Y. Rekhter, T. Li, "A Border Gateway Protocol 4 (BGP-4)", RFC
   1771, March 1995.

   [35] C. Villamizar, R. Chandra, R. Govindan, "BGP Route Flap
   Damping", RFC 2439, November 1998.

   [38] D. Waitzman, C. Partridge, S.E. Deering, "Distance Vector
   Multicast Routing Protocol", RFC 1075, Nov 1988.

   [42] L. von Ahn, M. Blum, N. Hopper, and J. Langford.  CAPTCHA: Using
   hard AI problems for security.  In Proceedings of Eurocrypt, 2003.









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9.  Informative References

   [2] T. Aura, P. Nikander, J. Leiwo, "DOS-resistant authentication
   with client puzzles", In B. Christianson, B. Crispo, and M. Roe,
   editors, Proceedings of the 8th International Workshop on Security
   Protocols, Lecture Notes in Computer Science, Cambridge, UK, April
   2000.

   [3] J. Bellardo, S. Savage, "802.11 Denial-of-Service Attacks: Real
   Vulnerabilities and Practical Solutions", Proceedings of the USENIX
   Security Symposium, Washington D.C., August 2003.

   [4] S.M. Bellovin, "Security Problems in the TCP/IP Protocol Suite",
   Computer Communication Review, Vol. 19, No. 2, pp. 32-48, April 1989.

   [6] CCAIS/RNP Alertas do Cais ALR-19112002a, "Vulnerability in the
   sending requests control of Bind versions 4 and 8 allows DNS
   spoofing", http://www.rnp.br/cais/alertas/2002/cais- ALR-
   19112002a.html

   [7] CERT Advisory CA-1996-01, "UDP Port Denial-of-Service Attack",
   Feb 1996.

   [8] CERT Advisory CA-1996-21, "TCP SYN Flooding and IP Spoofing
   Attacks", Sept 1996.

   [9] CERT Advisory CA-2001-09, "Statistical Weaknesses in TCP/IP
   Initial Sequence Numbers", May 2001.

   [10] CERT Advisory CA-1996-26, "Denial-of-Service Attack via ping",
   Dec 1996.

   [11] CERT Advisory CA-1998-01, "Smurf IP Denial-of-Service Attacks",
   http://www.cert.org/advisories/CA-1998-01.html, Jan 1998.

   [12] CERT Incident Note IN-2000-05, "'mstream' Distributed Denial of
   Service Tool", May 2000.

   [13] CERT/CC - "Managing the Threat of Denial of Service Attacks",
   http://www.cert.org/archive/pdf/Managing_DoS.pdf

   [14] CERT/CC - "Trends in Denial of Service Attack Technology",
   http://www.cert.org/archive/pdf/DoS_trends.pdf

   [15] D.F. Chang, R. Govindan, J. Heidemann, "An Empirical Study of
   Router Response to Large Routing Table Load", Proceedings of the 2nd
   Internet Measurement Workshop (IMW 2002), 2002.




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   [17] Cisco Systems, "Configuring the BGP Maximum-Prefix Feature",
   Cisco Document ID: 25160, http://www.cisco.com/warp/public/459/bgp-
   maximum-prefix.html

   [18] Scott A Crosby and Dan S Wallach, "Denial of Service via
   Algorithmic Complexity Attacks", Proceedings of the USENIX Security
   Symposium, Washington D.C., August 2003.

   [25] Laurent Joncheray, "Simple Active Attack Against TCP", 5th
   USENIX Security Symposium, 1995.

   [26] M. Lough, "A Taxonomy of Computer Attacks with Applications to
   Wireless", PhD thesis, Virginia Polytechnic Institute, April 2001.

   [27] Z. Mao, R. Govindan, G. Varghese, R. Katz, "Route Flap Dampening
   Exacerbates Internet Routing Convergence", Proceedings of ACM
   SIGCOMM, 2002.

   [28] D. Meyer, W. Fenner (Editors), "Multicast Source Discovery
   Protocol (MSDP)", RFC 3618, October 2003.

   [29] J. Mogul, KK.  Ramakrishnan, "Eliminating Receive Livelock in an
   Interrupt-driven Kernel", ACM Transactions on Computer Systems, Vol
   15, Number 3, pp. 217-252, 1997.

   [30] National Infrastructure Secuity Co-ordination Center (NISCC),
   Vulnerability Advisory 236929, April 2004,
   http://www.uniras.gov.uk/vuls/2004/236929/

   [31] V. Paxson, "An Analysis of Using Reflectors for Distributed
   Denial- of-Service Attacks", Computer Communication Review 31(3),
   July 2001.

   [33] Joe Stewart, "DNS Cache Poisoning - The Next Generation", Jan 27
   2003, http://www.securityfocus.com/guest/17905

   [34] R. Stewart (ed.), M. Dalal, (ed.)  "Improving TCP's Robustness
   to Blind In-Window Attacks",, Internet Draft, February 2006,
   draft-ietf-tcpm-tcpsecure-04.txt

   [36] P. Vixie, G. Sneeringer, M. Schleifer, "Events of 21-Oct-2002",
   http://f.root-servers.org/october21.txt

   [37] P. Vixie, "Securing the Edge",
   http://www.icann.org/committees/security/sac004.txt

   [39] D. Wessels, "Running An Authoritative-Only BIND Nameserver",
   http://www.isc.org/tn/isc-tn-2002-2.txt



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   [40] M. Zalewski, "Strange Attractors and TCP/IP Sequence Number
   Analysis", http://razor.bindview.com/publish/papers/tcpseq.html

   [41] J. Bellardo and S. Savage, "802.11 Denial-of-Service Attacks:
   Real Vulnerabilities and Practical Solutions", Proceedings of the
   12th USENIX Security Symposium, August 2003.

   [43] The whole world disappeared? http://www.merit.edu/mail.archives/
   nanog/1998-04/msg00181.html, Apr 1998.

   [44] Outage: MCI Worldcom nationwide ATM network.
   http://www.merit.edu/ mail.archives/nanog/1999-02/msg00077.html, Feb
   1999.

   [45] D. Pei, X. Zhao, L. Wang, D. Massey, A. Mankin, F. S. Wu, and L.
   Zhang.  Improving BGP Conver-gence Through Assertions Approach.  In
   Proc. of IEEE INFOCOM, June 2002.

   [46] Srikanth Chavali, Vasile Radoaca, Mo Miri, Luyuan Fang, and
   Susan Hares.  Peer prefix limits ex-change in bgp. http://
   www.ietf.org/internet-drafts/draft-chavali-bgp-prefixlimit-01.txt,
   April 2004.

   [47] X. Zhao, D. Massey, A. Mankin, S.F. Wu, D. Pei, L. Wang, L.
   Zhang, "BGP Multiple Origin AS (MOAS) Conflicts",
   http://nanog.org/mtg-0110/lixia.html, 2001.

   [48] Cisco Systems, "Building Security Into the Hardware",
   ftp://ftp-eng.cisco.com/cons/isp/security/CPN-Summit-2004/ Paris-
   Sept-04/SE14-BUILDING-SECURITY-INTO-THE HARDWARE-c1_8_30_04.pdf,
   2004.

   [49] T. Ylonen, C. Lonvick, Ed., "The Secure Shell (SSH) Protocol
   Architecture", RFC 4251, January 2006.

   [50] Knight, S., Weaver, D., Whipple, D., Hinden, R., Mitzel, D.,
   Hunt, P., Higginson, P., Shand, M. and A. Lindemn, "Virtual Router
   Redundancy Protocol", RFC 2338, April 1998.

   [51] D. Harrington, R. Presuhn, B. Wijnen., "An Architecture for
   Describing Simple Network Management Protocol (SNMP) Management
   Frameworks.", RFC 3411, December 2002.

   [52] K. Sollins., "The TFTP Protocol (Revision 2)", RFC 1784, July
   1992.






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Appendix A.  IAB Members at the time of this writing

   o  Bernard Aboba

   o  Loa Andersson

   o  Brian Carpenter

   o  Leslie Daigle

   o  Elwyn Davies

   o  Kevin Fall

   o  Olaf Kolkman

   o  Kurtis Lindvist

   o  David Meyer

   o  David Oran

   o  Eric Rescorla

   o  Dave Thaler

   o  Lixia Zhang
























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Authors' Addresses

   Mark J. Handley (ed)
   UCL
   Gower Street
   London  WC1E 6BT
   UK

   Email: M.Handley@cs.ucl.ac.uk


   Eric Rescorla (ed)
   Network Resonance
   2483 E. Bayshore #212
   Palo Alto  94303
   USA

   Email: ekr@networkresonance.com


   Internet Architecture Board
   IAB


   Email: iab@ietf.org


























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