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INTERNET-DRAFT                                                   Xia Yin
Intended Status: Proposed Standard                        Tsinghua Univ.
Expires: January 3, 2012                                      Yang Xiang
                                                          Tsinghua Univ.
                                                           Zhiliang Wang
                                                          Tsinghua Univ.
                                                             Jianping Wu
                                                          Tsinghua Univ.
                                                            July 2, 2011

               Efficient Secure BGP AS Path using FS-BGP


   This draft proposes Fast Secure BGP (FS-BGP), an efficient mechanism
   for securing AS paths and preventing prefix hijacking by signing
   critical AS path segments (i.e., adjacent AS triples). FS-BGP can
   achieve similar level of security as S-BGP, but with much higher

Status of this Memo

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Copyright and License Notice

   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors. All rights reserved.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Background . . . . . . . . . . . . . . . . . . . . . . . . . .  3
   4.  Secure Feasible AS Paths . . . . . . . . . . . . . . . . . . .  5
   5.  FS-BGP: Fast Secure BGP  . . . . . . . . . . . . . . . . . . .  6
     5.1.  Signing Critical AS Path Segments  . . . . . . . . . . . .  6
     5.2.  Prevent Effective Hijacking in FS-BGP  . . . . . . . . . .  7
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . .  9
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  9
   8.  Conclusions  . . . . . . . . . . . . . . . . . . . . . . . . .  9
   9  References  . . . . . . . . . . . . . . . . . . . . . . . . . .  9
     9.1  Normative References  . . . . . . . . . . . . . . . . . . . 10
     9.2  Informative References  . . . . . . . . . . . . . . . . . . 11
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11

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1.  Introduction

   In order to improve the security of BGP, several extensions have been
   proposed, which fall into two categories: anomaly detection and
   cryptographic based authentication. However, anomaly detection
   approaches [Whisper] [PGBGP] can not guarantee security and
   correctness. Cryptographic approaches, which are being pursued by the
   SIDR WG, use the Public Key Infrastructure (PKI) to authenticate
   routing announcements. There are a bunch of solutions including S-BGP
   [S-BGP] [I-D.lepinski-bgpsec-protocol] and many others. However, S-
   BGP may consume significant resources of computation and storage. The
   other solutions either compromise in the security [IRV] [I-D. ng-
   sobgp-bgp-extensions] [psBGP] [SPV], or bring in more complexity on
   certification distribution [SA].

   Towards these unsolved issues, we propose an efficient approach, FS-
   BGP (Fast Secure BGP), to secure AS path. Through signing critical AS
   path segments (i.e., adjacent AS triples), FS-BGP can achieve similar
   level of security as S-BGP, but with much higher efficiency.
   Analysis, evaluations, and more discussions can be found in our
   recent technical report [TR-FSBGP].

2.  Terminology

   (i):           AS i
   <n, ..., 0>:   AS path from AS n to the origin AS 0
   <n, ..., 0>f:  AS path of prefix f
   <i+1, i, i-1>: critical AS path segment, adjacent AS triple in a path
   <1, 0, f>:     origin critical AS path segment in a path of prefix f
   {msg}i:        signature on msg generated by AS i

3.  Background

   In BGP, UPDATE messages can not be validated, so neither the origin
   AS nor the AS path is guaranteed to be correct. Secure BGP (S-BGP)
   [SBGP] is the dominant solution to this problem, and it uses a PKI to
   help authenticating involved parties and messages. Specifically, S-
   BGP uses Route Attestations (RAs) for path authentication.

   As shown in Figure 1, a RA is all signatures signed by ASes along the
   path to authenticate the existence and position of ASes in the path.
   We define {msg}i as the signature on msg generated with AS i's
   private key. In Figure 1, each AS i equivalently signs the
   corresponding extended AS path <i+1, i, ..., 0> and the prefix f. The
   inclusion of the recipient AS i+1 in each signature is necessary to
   prevent cut-and-paste attack.

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    | (n+1) <-- (n) <-- ... <-- (i) <-- ... <-- (1) <-- (0)       |
    |       s_0             s_0             s_0     s_0           |
    |       s_1             s_1             s_1       \\          |
    |         .               .               \\       {1, 0, f}0 |
    |         .               .               {2, 1, s_0}1        |
    |         .               .                \\                 |
    |       s_i             s_i                {2, 1, 0, f}1      |
    |         .               \\                                  |
    |         .               {i+1, i, s_i-1}i                    |
    |         .                \\                                 |
    |       s_n                {i+1, i, i-1, ..., 1, 0, f}i       |
    |         \\                                                  |
    |         {n+1, n, s_n-1}n                                    |
    |          \\                                                 |
    |          {n+1, n, n-1, ..., 1, 0, f}n                       |
                        Figure 1. RAs in S-BGP.

   The main concern about deploying S-BGP in practice is the huge
   computational cost for signing and verifying signatures. The
   dominating barrier for adopting S-BGP is the overhead of processing
   RAs, that is to authenticate paths. Toward this direction, there are
   a bunch of solutions for reducing the overhead of path

   soBGP [I-D.ng-sobgp-bgp-extensions] maintains all authenticated AS
   edges in a database, but faces the problem of forged paths. IRV [IRV]
   builds an authentication server in each AS, but brings the problem of
   maintaining and inter-connecting these servers, and introduces query
   latencies. SPV [SPV] accelerates the signing process by pre-generated
   one-time signatures based on a single root value, but involves a
   significant amount of state information, and its security can only be
   guaranteed probabilistically. Signature Amortization (S-A) [SA] uses
   one bit vector for each neighbor of an AS to indicate the allowed
   recipients of a route, such that only one signing is needed for
   multiple recipients. However, each AS will need to pre-establish a
   neighbor list corresponding to the bit vector, and to distribute it
   to all other ASes.

   As we can see, existing methods usually compromise security, and most
   of them only improve the performance of signing. However,
   verification happens more frequently than signing, since one
   signature often needs to be verified at multiple places.

   According to the above analysis, it is important to design an
   efficient method to secure AS paths. Our solution, FS-BGP, builds on
   the assumption that a PKI is ready for use, and focuses on AS path

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4.  Secure Feasible AS Paths

   S-BGP can not prevent replay of outdated routes. It can only use
   expiration-date to roughly control the window of exposure to replay
   attack. As a result, though it only signs currently announcing path,
   it actually authenticates all announced feasible paths. Under a
   stable AS-level topology, we call a path feasible when the path
   satisfies the import and export policies of all ASes along the path.

   Since failures often occur in the global routing system, many
   feasible paths can be easily announced and become authenticated.
   Thus, if a protocol can guarantee that all authenticated paths are
   feasible path, then it can achieve similar level of security as S-
   BGP. So we wonder that is it possible to efficiently secure feasible
   paths but not blindly sign every currently announcing path.

   BGP is a policy-based routing protocol. An AS only exports a route to
   a neighbor if it is willing to forward traffic to the corresponding
   prefix from that neighbor. Although complex policies (i.e., route
   filters [RFC2622]) exist, AS usually does not differentiate between
   prefixes or nonadjacent ASes. For example, in Figure 2, when AS n
   decides whether routes learned from AS n-1 can be exported to AS n+1,
   it only considers its relation with the two neighbors, but does not
   consider other ASes along the path (<n-2, ..., 1, 0>). We call this
   the Neighbor Based Importing and Exporting (NBIE).

      |                              / ... (x_0) ... \           |
      |                             /        .        \          |
      |  (n+1) <-- (n) <-- (n-1) <--   ...   .   ...    <-- (0)  |
      |                             \        .        /          |
      |                              \ ... (x_k) ... /           |
      Figure 2. In S-BGP, AS n signs k paths which share a mutual
                     AS path segment <n+1, n, n-1>.

   NBIE abstracts the basic functionality of BGP. According to our
   measurement results in whois database, only a small portion of
   routing polices (route filters) violate the NBIE assumption.
   Nevertheless, the purpose of route filters is to protect the routing
   system against distribution of inaccurate routing information
   [RFC2622]. In other words, the use of route filters is mainly due to
   security considerations rather than policy requirements. We believe
   that under a security environment (i.e., FS-BGP or S-BGP), these
   filters are not needed any more. In deed, our schema can flexibly

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   support complicated routing polices [TR-FSBGP].

5.  FS-BGP: Fast Secure BGP

5.1.  Signing Critical AS Path Segments

   Following our key observation above, we propose Fast Secure BGP (FS-
   BGP) to grantee the authentication of feasible paths. Given a
   feasible path p=<n+1, n, ..., 0>, we define its set of critical path
   segments as c_i, 0<i<=n, where

                         / <1,0,f>      , for i=0
                   c_i =
                         \ <i+1,i,i-1>  , for 0<i<=n

   We call AS i the owner of c_i. Particularly, c_0 is called the
   originating critical path segment owned by AS 0. A critical path
   segment <i+1, i, i-1> actually describes an routing export policy of
   its owner AS i, and implies that AS i can export all routes imported
   from AS i-1 to AS i+1.

   More specifically, FS-BGP uses Critical Segment Attestations (CSA) to
   authenticate paths. A CSA is simply the signature of the critical
   path segment signed by its owner. In a path p=<n+1, n, ..., 0>, the
   CSA s_i signed by AS i is defined as:

                         / {1,0,f}0      , for i=0
                   s_i =
                         \ {i+1,i,i-1}i  , for 0<i<=n

   The inclusion of the prefixes f in s_0 is necessary, because AS 0
   might be multi-homing and only announces part of its prefixes to AS
   1. Figure 3 and Figure 1 compare the signatures in FS-BGP and S-BGP.
   Obviously, the number of distinct critical path segments is far less
   than the number of distinct paths. As a result, the number of signing
   and verification operations in FS-BGP can be greatly reduced, after
   using a small cache. In Figure 2, AS n needs to sign each of the k
   paths individually in S-BGP. However, in FS-BGP, all the k different
   paths can reuse one signature of the common critical segment <n+1, n,

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     |  (n+1) <-- (n) <-- ... <-- (i) <-- ... <-- (1) <-- (0)     |
     |        s_0             s_0             s_0     s_0         |
     |        s_1             s_1             s_1       \\        |
     |          .               .              \\        {1,0,f}0 |
     |          .               .              {2,1,0}1           |
     |          .               .                                 |
     |        s_i             s_i                                 |
     |          .              \\                                 |
     |          .              {i+1,i,i-1}i                       |
     |          .                                                 |
     |        s_n                                                 |
     |         \\                                                 |
     |         {n+1,n,n-1}n                                       |
                       Figure 3.        CSAs in FS-BGP.

   We argue that, under the NBIE rule, if every AS along a path signs
   its critical path segment, then the path can be authenticated as a
   feasible path [TR-FSBGP]. However, since not all feasible paths are
   actually announced, it is possible to forge a path if the security
   mechanism relies on CSA only, as shown in Section 5.2. We will
   provide effective solution to this problem.

5.2.  Prevent Effective Hijacking in FS-BGP

   In FS-BGP, an AS using FS-BGP can forge paths that are not actually
   announced by others, but avoids CSA based detection. Forged paths can
   be constructed by concatenating critical segments, and used for
   prefix hijacking.

   Although forging paths in FS-BGP is possible, there are still some
   restrictions on how paths can be forged. First, a path can only be
   forged by combining non-forged paths which share mutual segments.
   Second, some part of a forged path must be treated as sub-optimal or
   suppressed by some AS along the path. Third, forged paths are still
   feasible, and can only be used for prefixes associated with the
   originating critical segment in the path. Last, forged paths can not
   be very short [TR-FSBGP].

   Although there are limitations on forged paths, prefix hijacking is
   still possible. In this section, we discuss solutions to prevent
   prefix hijacking. We only concern effective hijacking, in a sense
   that, the recipient of a forged route indeed changes its forwarding

   Firstly, we divide all feasible paths into three categories:

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      Optimal path: the best path that passes all the decision steps in

      Sub-optimal path: paths with the same Local Preference as the
      optimal path, but not chosen as the best one.

      Suppressed path: paths with lower Local Preferences than the
      optimal and sub-optimal paths. For example, paths that are more
      expensive (i.e., through a provider), are often suppressed by a
      low preference.

   We argue that, if a forged path is no shorter than the non-forged
   path BGP should announce, it can not be used for effective hijacking
   [TR-FSBGP]. Under a stable AS-level topology, a router will use its
   optimal path for every prefix. If BGP is purely a shortest path
   routing protocol (optimal path is always the shortest one),
   manipulator can not effectively hijack any prefix by forging paths.
   However, policy routing makes hijacking possible.

   We know only suppressed path can be shorter than the optimal path
   (since a sub-optimal path has the same local preference as the
   optimal path, its length can not be shorter). Thus, if there is a
   mechanism to guarantee that all suppressed paths are no shorter than
   their corresponding optimal paths, manipulator can no longer
   effectively hijack a prefix either. This idea can be implemented by
   using AS Path Pre-pending (ASPP).

   We call such a mechanism Suppressed Path Padding (SPP), and Figure 4
   depicts the pseudo code for deciding how many times an AS i should
   pad itself in a path. If a path is imported from a neighbor AS i-1
   with the highest local preference, AS i only appears once (line 1 and
   2). Otherwise, the number of occurrences k_i must be large enough
   such that no suppressed path can be shorter than the corresponding
   optimal path. Given a path p, denote the optimal path to the same
   prefix as p by opt(p), then k_i is set as the largest Path Length
   difference between any suppressed path p imported from this neighbor
   and the corresponding opt(p) (line 4 to 7).

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     | Algorithm: Suppressed Path Padding                        |
     | INPUT:  local AS i, neighbor AS i-1                       |
     | OUTPUT: k_i: number of times that AS i needs to be padded |
     |         in the paths import from AS i-1                   |
     | 1:  IF AS i-1 has the highest local preference THEN       |
     | 2:      RETURN 1                                          |
     | 3:  k_i <- 1                                              |
     | 4:  FOR ALL path p imported from AS i-1 DO                |
     | 5:      opt(p) <- the optimal path corresponding to p     |
     | 6:      IF length(p) - length(opt(p)) > k_i THEN          |
     | 7:          k_i <- length(p) - length(opt(p))             |
     | 8:  RETURN k_i                                            |
                Figure 4. SPP (Suppressed Path Padding).

   It is worth noting that, SPP is quite general. When necessary, it can
   and also should be used even in S-BGP. Consider the case when the
   optimal route fails. At this time, S-BGP will announce a previously
   sub-optimal or suppressed path temporarily, and this path can be used
   later by the manipulator to launch an effective attack, if it is
   short enough. S-BGP can not prevent this attack, while our SPP works

6.  Security Considerations

   The entire document is about security consideration. More theoretical
   analysis and experiment results can be found in our technical report

7.  IANA Considerations

   This document requires no IANA actions.

8.  Conclusions

   This draft proposes Fast Secure BGP (FS-BGP), an efficient mechanism
   for securing feasible AS paths and preventing prefix hijacking by
   signing critical AS path segments. We believe that FS-BGP can achieve
   similar level of security as S-BGP. Our experiment results show that,
   FS-BGP has a much higher efficiency.

9  References

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9.1  Normative References

   [RFC2622]  Alaettinoglu, C., Villamizar, C., Gerich, E., Kessens, D.,
              Meyer, D., Bates, T., Karrenberg, D., and M. Terpstra,
              "Routing Policy Specification Language (RPSL)", RFC 2622,
              June 1999.

   [RFC4271]  Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
              Border Gateway Protocol 4 (BGP-4)", RFC 4271, January

   [S-BGP]    S. Kent, C. Lynn, J. Mikkelson, and K. Seo, "Secure Border
              Gateway Protocol (S-BGP)", IEEE Journal on Selected Areas
              in Communications, 18:103-116, 2000.

   [I-D.lepinski-bgpsec-protocol]  M. Lepinski, "BGPSEC Protocol
              Specification", draft-lepinski-bgpsec-protocol, work-in-
              progress, 2011.

   [I-D.ng-sobgp-bgp-extensions]  J. Ng, "Extensions to BGP to Support
              Secure Origin BGP (soBGP)", draft-ng-sobgp-bgp-extensions,

   [IRV]      G. Goodell, W. Aiello, T. Griffin, J. Ioannidis, P. D.
              McDaniel, and A. D. Rubin, "Working around BGP: An
              Incremental Approach to Improving Security and Accuracy in
              Interdomain Routing", In NDSS, 2003.

   [psBGP]    P. C. van Oorschot, T. Wan, and E. Kranakis, "On
              interdomain routing security and pretty secure BGP
              (psBGP)", ACM Trans. Inf. Syst. Secur., 10(3), 2007.

   [SPV]      Y.-C. Hu, A. Perrig, and M. A. Sirbu, "SPV: secure path
              vector routing for securing BGP", In SIGCOMM, pages 179-
              192, 2004.

   [SA]       D. M. Nicol, S. W. Smith, and M. Zhao, "Evaluation of
              efficient security for BGP route announcements using
              parallel simulation", Simulation Modeling Practice and
              Theory, 12(3-4):187-216, 2004.

   [TR-FSBGP]  Yang Xiang, Zhiliang Wang, Xia Yin, Xingang Shi, and
              Jianping Wu, "FS-BGP: An Efficient Approach to Securing AS
              Paths", Tsinghua University, Technical Report, THUTR-2011-
              FSBGP, 2011.

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9.2  Informative References

   [Whisper]  L. Subramanian, V. Roth, I. Stoica, S. Shenker, and R. H.
   Katz, "Listen and Whisper: Security Mechanisms for BGP", In NSDI,
   pages 127-140, 2004.

   [PGBGP]    J. Karlin, S. Forrest, and J. Rexford, "Pretty Good BGP:
   Improving BGP by Cautiously Adopting Routes", In ICNP, pages 290-299,

Authors' Addresses

   Xia Yin
   Tsinghua University, Beijing, 100084 P.R. China

   Email: yxia@csnet1.cs.tsinghua.edu.cn

   Yang Xiang
   Tsinghua University, Beijing, 100084 P.R. China

   Email: xiangy08@csnet1.cs.tsinghua.edu.cn

   Zhiliang Wang
   Tsinghua University, Beijing, 100084 P.R. China

   Email: wzl@csnet1.cs.tsinghua.edu.cn

   Jianping Wu
   Tsinghua University, Beijing, 100084 P.R. China

   Email: jianping@csnet1.cs.tsinghua.edu.cn

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