Problem Statement of SAVI Beyond the First Hop
draft-bi-savi-problem-12

SAVI                                                               J. Bi
Internet-Draft                                                    B. Liu
Intended status: Informational                            Tsinghua Univ.
Expires: May 17, 2017                                  November 13, 2016

             Problem Statement of SAVI Beyond the First Hop
                        draft-bi-savi-problem-12

Abstract

   IETF Source Address Validation Improvements (SAVI) working group is
   chartered for source address validation within the first hop from end
   hosts, i.e., preventing a node from spoofing the IP source address of
   another node in the same IP link.  However, since SAVI requires the
   edge routers or switches to be upgraded, the deployment of SAVI will
   need a long time.  During this transition period, some source address
   validation techniques beyond the first hop (SAVI-BF) may be needed to
   complement SAVI and protect the networks from spoofing based attacks.
   In this document, we first propose three desired features of the
   SAVI-BF techniques.  Then we analyze the problems of the current
   SAVI-BF technique, ingress filtering.  Finally, we discuss the
   directions that we can explore to improve SAVI-BF.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Desired Features of SAVI-BF Techniques  . . . . . . . . . . .   3
     2.1.  High Deployment Incentives  . . . . . . . . . . . . . . .   3
     2.2.  Low Operational Risks . . . . . . . . . . . . . . . . . .   3
     2.3.  Low Cost  . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Problems of Ingress Filtering . . . . . . . . . . . . . . . .   4
     3.1.  IAL . . . . . . . . . . . . . . . . . . . . . . . . . . .   5
     3.2.  Strict/feasible RPF . . . . . . . . . . . . . . . . . . .   5
     3.3.  Loose RPF*  . . . . . . . . . . . . . . . . . . . . . . .   6
     3.4.  Other Reasons . . . . . . . . . . . . . . . . . . . . . .   6
   4.  Discussion  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     4.1.  Path based Techniques . . . . . . . . . . . . . . . . . .   7
     4.2.  End-to-end based Techniques . . . . . . . . . . . . . . .   7
     4.3.  Non-technical Proposals . . . . . . . . . . . . . . . . .   8
   5.  Acknowledgment  . . . . . . . . . . . . . . . . . . . . . . .   8
   6.  Informative References  . . . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   IETF Source Address Validation Improvements (SAVI) working group is
   chartered for source address validation within the first hop from the
   end hosts, so as to prevent a node from spoofing the IP source
   address of another node in the same IP link.  However, since SAVI
   requires the edge routers or switches to be upgraded, the deployment
   of SAVI will need a long time.  During this transition period, some

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   source address validation techniques beyond the first hop (SAVI-BF)
   may be needed to complement SAVI, so as to protect the networks from
   spoofing based attacks, which are prevalent DDoS attacks on the
   current Internet [Ground-Truth] [ARBOR-2010] [NANOG-Helpless]
   [DrDoS-300Gbps].

   In this document, we first propose three desired features of SAVI-BF
   techniques.  The first desired feature is high deployment incentives,
   i.e., by deploying a technique, an ISP should significantly increase
   its ability to protect its network from spoofing based attacks.  The
   second one is low operational risks.  If a technique may improperly
   drop legitimate packets (so called false positives), it introduces
   new risks to the network operation and management.  It is desired
   that the false positives (FP) is as low as possible.  The third one
   low cost.  It is desired that the technique requires minimum
   deployment investment and operational cost.

   We evaluate ingress filtering [BCP38] [BCP84], the best current
   practice in for SAVI-BF, against these three features.  Recent
   measurement shows that, the deployment of ingress filtering has not
   been improved over four years because the ISPs do not have incentives
   to deploy it [Efficacy], and sophisticated attackers exploit the
   spoofable networks to launch attacks.  We discuss the reasons why
   ingress filtering is still insufficiently applied by the ISPs despite
   that it has long been available in modern routers.

   Finally, we discuss the directions that we can explore to improve
   SAVI-BF.  We briefly survey two categories of SAVI-BF proposals, the
   path based techniques and the end-to-end based techniques.  We
   discuss their requirements, advantages and disadvantages.

2.  Desired Features of SAVI-BF Techniques

2.1.  High Deployment Incentives

   To motivate an ISP to deploy a technique, it is desirable that the
   technique can generate additional benefit for the ISP.  In terms of a
   SAVI-BF technique, it should provide the ISP with additional
   protection from spoofing based attacks.  Specifically, it should
   significantly decrease the volume of the spoofing based attacks
   targeting the ISP, or the number of networks where these attacks can
   be successfully launched, or the number of source addresses or
   destination addresses that can be used in these attacks.

2.2.  Low Operational Risks

   If a technique may improperly drop legitimate packets, so called
   false positives (FP), it introduces new operational risks.

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   Apparently, it is desired that the FP is as low as possible.  The
   higher the FP, the more limited its application scope.  For example,
   if the attack is not very severe, the network administrator would not
   apply a technique with high FP, since its operation risks may even
   surpass the loss caused by the attack.

2.3.  Low Cost

   It is desired that the technique requires minimum investment on the
   device and system upgrading.  It should also adapt to network
   dynamics rather than require manual intervention, which is costly,
   slow, and often the source of errors (misconfiguration).

3.  Problems of Ingress Filtering

   Ingress filtering is the best current practice for SAVI-BF.  Ingress
   filtering was proposed in 2000 [BCP38] , and updated in 2004 [BCP84].
   Ingress access lists (IALs) and unicast reverse path forwarding
   (uRPF) are two general ways to implement ingress filtering.  An IAL
   is a filter that checks the source address of every packet received
   on a network interface against a list of acceptable prefixes,
   dropping any packet that does not match the filter.  IALs are
   typically maintained manually.  Upon network dynamics (topology
   change or routing change), the IALs should be updated accordingly to
   avoid dropping legitimate packets. uRPF is an automated tool which
   can adapt with the network dynamics.  On the receipt of a packet,
   uRPF checks its source address against the forwarding table or
   routing table for validation.  uRPF has four variants, strict RPF,
   feasible RPF, loose RPF and loose RPF ignoring default routes.
   Readers are referred to [BCP84] for the details of these variants.

   Today, many modern routers are capable of ingress filtering.  Many
   network administrators have turned on uRPF on their routers or been
   actively maintaining IALs to filter spoofing traffic.  The fraction
   of networks where spoofing is possible is significantly limited
   [MIT-Spoofer].  However, as shown by the recent measurement, the
   deployment of ingress filtering has not been improved over four
   years.  Sophisticated attackers can still exploit the networks where
   spoofing is possible to launch spoofing based attacks, and IP
   spoofing remains a viable attack vector on the current Internet
   [Efficacy].

   In this section, we evaluate ingress filtering against the desired
   features, and analyze the reasons why ingress filtering is not
   sufficiently deployed, and why its deployment has not been improved
   over years.  We classify the five variants of ingress filtering (IAL
   and four variants of uRPF) into three categories according to their
   technical similarities, i.e., IALs, strict/feasible RPF, and loose

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   RPF* (including loose RPF and loose RPF ignoring default routes).
   The evaluation results are summarized in Table 1, which will be
   explained in detail in the following subsections.

             +---------------------+-----------+------+------+
             |      Techniques     | Incentive | Risk | Cost |
             +---------------------+-----------+------+------+
             |         IAL         |    low    | low  | low  |
             |          -          |     -     |  -   |  -   |
             | Strict/feasible RPF |    high   | high | low  |
             |          -          |     -     |  -   |  -   |
             |      Loose RPF*     |    low    | low  | low  |
             +---------------------+-----------+------+------+

                 Table 1: Evaluation of Ingress Filtering

3.1.  IAL

   IAL can be applied at network border routers and enforced on outbound
   packets.  All the outbound packets whose source addresses do not
   belong to the local network are identifed as spoofed.  Since it only
   filters outbound packets, and cannot protect the local network from
   receiving spoofing packets, it provides little deployment incentives.
   When IAL is enforced on inbound packets, there are two typical
   scenarios.  In the first scenario, it is applied on the routers'
   ingress interface connecting a stub network.  In this case, the
   source address space of the stub network can be configured on this
   interface, and all inbound packets whose source addresses fall out of
   this address space are identified as spoofed.  Since the address
   space size of directly connected stub networks is very small compared
   to the entire routable address space, the effectiveness is low.  In
   the second scenario, the directly connected network is not stub, and
   thus the valid address space of inbound packets is hard to determine.
   In this case, IAL can only safely drop the inbound packets whose
   source addresses belong to the local network.  It is very ineffective
   since the address space size of the local network very small compared
   to the entire routable address space.  To summize, IAL provides low
   deployment incentives in all cases.  If well maintained, false
   positives can also be avoided.  IAL can be implemented using the
   existing functions on routers (access control lists, ACLs), and thus
   do not require additional investment.  Overall, the deployment cost
   is low except in the "stub-network" scenario, where the address space
   of all stubs need to be carefully maintained, which may require heavy
   manual configuration if there are many directly connected stub
   networks.

3.2.  Strict/feasible RPF

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   Strict and feasible RPF can adapt themselves to the routing dynamics
   and determine the incoming directions of source prefixes
   automatically.  They are more effective than IAL in detecting and
   filtering spoofing traffic, and thus provide higher deployment
   incentives to the ISPs.  However, they often drop legitimate packets
   under routing asymmetry, which is very prevalent with the existence
   of local routing policies, multi-homing and traffic engineering.
   This makes strict and feasible very risky [NANOG-Risk], and hence the
   operation needs to be very careful.  Feasible RPF has lower FP than
   strict RPF, since it can apply multiple interfaces as acceptable
   incoming directions for a source prefix.  But feasible RPF cannot
   avoid all FP in practice, since currently there is no practical way
   to generate and configure all possible incoming directions in the
   routers.  For example, in a link-state routing environment (IS-IS or
   OSPF), equal-cost multi-path (ECMP) [ECMP] is often used to generate
   the multiple acceptable incoming directions.  However, in practice,
   there can be many (tens of) ECMPs for a prefix, but the
   implementation of a router can only store several (e.g., 4 or 8) of
   them.  Thus, the ECMPs for the prefix installed in the forwarding
   table may be different in different routers, which eventually causes
   the FP of feasible RPF.  And in BGP, the directions where BGP
   announcements for a source address prefix have been received can be
   considered as acceptable incoming directions [IDPF].  However, an ISP
   may choose not to announce a prefix via a path but still send traffic
   through it due to its local routing policy.  In this case, feasible
   RPF also causes FP.  The basic function of strict and feasible RPF is
   supported in most modern routers.  So deploying them doesn't require
   investment on upgrading devices.

3.3.  Loose RPF*

   Instead of validating the incoming direction of a source address,
   loose RPF* only checks the existence of the source address in the
   forwarding table.  Loose RPF* is only useful for filtering Martian
   addresses and unroutable addresses.  However, sophisticated attackers
   can evade loose RPF* checking by simply using routable source
   addresses.  Thus the incentive to deploy loose RPF is low.  On the
   other hand, its operational risk is also low.  Loose RPF* is also
   available in most modern routers, making it cheap to deploy.

3.4.  Other Reasons

   There are other reasons why the deployment of ingress filtering
   hasn't been improved in four years.  First, although most modern
   routers are capable of ingress filtering, some legacy routers are
   incapable [NANOG-Equipment].  Hence, even at the locations where no
   risk exists (e.g. stub or single-homed networks), ingress filtering
   may not be applied.  Another reason is called inertia.  Since ingress

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   filtering is not enabled on routers by default, some network
   administrators just won't bother to turn it on
   [NANOG-PowerOfDefaults].

4.  Discussion

   In this section, we discuss the directions that we can explore to
   improve SAVI-BF.  We briefly survey two categories of SAVI-BF
   proposals, path based techniques and end-to-end based techniques.  We
   discuss their requirements, advantages and disadvantages.  We only
   focus on the techniques that are implemented on the routers.  The
   techniques implemented on end hosts, such as [IPSec], [HCF] and
   [IP-Puzzles], are not covered here.

4.1.  Path based Techniques

   The path based techniques essentially require that a router R knows
   the forwarding paths that each source prefix S uses toward its
   destinations, and subsequently knows the incoming directions/
   interfaces of S. Sometimes, this information is available to R. For
   example, in a pure link-state routing protocol environment (e.g., IS-
   IS, OSPF), all nodes have the same view of the network.  Thus, R can
   compute the paths from S to all destinations, and then infer the
   incoming directions of S. One exception is that, when there are
   multiple equally best paths, R may not determine which one S will
   use.  On the other hand, if a distance-vector (e.g., RIP) or a path-
   vector (BGP) routing protocol is used, it is even harder for R to
   determine the paths of S, since the sufficient routing information is
   missed.  [SAVE] and [IDPF] are tow proposals which validate source
   addresses by inferring their forwarding paths.

4.2.  End-to-end based Techniques

   There are, however, other proposals that don't rely on path
   information.  We call them end-to-end based techniques.  For example,
   [SPM] associates each source prefix (indeed, source AS number) with a
   key.  S, an SPM-enabled AS, will tag the key into outbound packets
   toward R at its border routers.  And R verifies the keys of the
   inbound packets whose source addresses belong to S at its border
   routers.  The routers will drop a packet if the key is incorrect.
   Thus, R manages to validate the source addresses of S without knowing
   its forwarding paths.  The end-to-end based the techniques typically
   need the cooperation between the source and the verification nodes,
   and require particular tags be carried in the data packets.  Further
   more, even the end-to-end based techniques require to distinguish
   inbound traffic and outbound traffic, which is not completely path-
   independent.

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4.3.  Non-technical Proposals

   There are also proposals which formulate source address validation as
   an economic problem [FaaS], or suggest that laws and governance
   should be enforced.  These directions, however, may be out of the
   scope of IETF.

5.  Acknowledgment

   The authors would like to thank Fred Baker and Joel M. Halpern for
   their comments.

   This document was generated using the xml2rfc tool.

6.  Informative References

   [ARBOR-2010]
              Dobbins, R. and C. Morales, "Network Infrastructure
              Security Report", February 2010.

   [BCP38]    Paul, P. and D. Senie, "Network Ingress Filtering:
              Defeating Denial of Service Attacks which employ IP Source
              Address Spoofing", RFC 2827, BCP 38, May 2000.

   [BCP84]    Baker, F. and P. Savola, "Ingress Filtering for Multihomed
              Networks", RFC 3704, BCP 84, March 2004.

   [DrDoS-300Gbps]
              Prince, M., "The DDoS That Almost Broke the Internet",
              March 2013.

   [ECMP]     Thaler, D. and C. Hopps, "Multipath Issues in Unicast and
              Multicast Next-Hop Selection", RFC 2991, November 2000.

   [Efficacy]
              Beverly, R., Berger, A., Hyun, Y., and k. claffy,
              "Understanding the Efficacy of Deployed Internet Source
              Address Validation Filtering", August 2009.

   [FaaS]     Liu, B., Bi, J., and X. Yang, "FaaS: Filtering IP Spoofing
              Traffic as a Service", 2012.

   [Ground-Truth]
              Labovitz, C., "Botnets, DDoS and Ground-Truth", October
              2010.

   [HCF]      Jin, C., Wang, H., and K. Shin, "Hop-count filtering: an
              effective defense against spoofed DDoS traffic", 2003.

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   [IDPF]     Duan, Z., Yuan, X., and J. Ch, "Controlling IP Spoofing
              Through Inter-Domain Packet Filters", 2008.

   [IP-Puzzles]
              Feng, W., Feng, W., and A. Luu, "The Design and
              Implementation of Network Puzzles", 2005.

   [IPSec]    Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", RFC 4301, December 2005.

   [MIT-Spoofer]
              Beverly, R., "Spoofer Project", January 2012,
              <http://spoofer.csail.mit.edu/>.

   [NANOG-Equipment]
              Bicknell, L., "BCP38 Deployment", March 2012, <http://
              mailman.nanog.org/pipermail/nanog/2012-March/047139.html>.

   [NANOG-Helpless]
              Bulk, F., "Are we really this helpless? (Re: isprime DOS
              in progress)", January 2009, <http://mailman.nanog.org/
              pipermail/nanog/2009-January/006996.html>.

   [NANOG-PowerOfDefaults]
              Donelan, S., "BCP38 Deployment", March 2012, <http://
              mailman.nanog.org/pipermail/nanog/2012-March/047147.html>.

   [NANOG-Risk]
              Gilmore, P., "BCP38 Deployment", March 2012, <http://
              mailman.nanog.org/pipermail/nanog/2012-March/047087.html>.

   [SAVE]     Li, J., Mirkovic, J., Wang, M., Reiher, P., and L. Zhang,
              "SAVE: Source Address Validity Enforcement Protocol",
              2002.

   [SPM]      Anat, A. and H. Hanoch, "Spoofing Prevention Method",
              March 2005.

Authors' Addresses

   Jun Bi
   Tsinghua University
   Network Research Center, Tsinghua University
   Beijing  100084
   China

   Email: junbi@tsinghua.edu.cn

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   Bingyang Liu
   Tsinghua University
   Network Research Center, Tsinghua University
   Beijing  100084
   China

   Email: liubingyang@tsinghua.edu.cn

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