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Versions: 00 01 02                                                      
Network Working Group                                         Russ White
Internet Draft                                                  (editor)
Expiration Date: October 2004                              Cisco Systems
File Name: draft-white-sobgparchitecture-00.txt               April 2004

Architecture and Deployment Considerations for Secure Origin BGP (soBGP)

   Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet Drafts are working documents of the Internet Engineering
   Task Force (IETF), its Areas, and its Working Groups. Note that other
   groups may also distribute working documents as Internet Drafts.

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   The list of current Internet-Drafts can be accessed at

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   There is a great deal of concern over the security of the Border
   Gateway Protocol, which is used to provide routing information to the
   Internet and other large internetworks. This draft provides an
   architecture for a secure distributed registry of routing information
   to address these concerns. The draft begins with an overview of the
   operation of this system, and then follows with various deployment
   scenerios, starting with what we believe will be the most common
   deployment option.

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1. Background

   There are two fundamental pieces of a routing system that need to be

   o    Adjacencies between devices running the routing protocol

   o    Information carried within the routing protocol.

   While security between BGP [BGP] speakers has been addressed in a
   number of ways, including cryptographic authentication [BGP-MD5] and
   limiting the attack radius through TTL mechanisms [GTSH], security
   for the information carried within BGP is not considered a solved

   This draft proposes a possible solution to securing the information
   within BGP, using the certificates and protocol extensions proposed

   A large number of people contributed to this draft; we've tried to
   include all of them here (but might have missed a few): James Ng, Tim
   Gage, Alvaro Retana, Dave Cook, Brian Weis, and Iljitsch van Beijnum.

2. General Theory

   soBGP provides a secure registry mechanism against which a BGP
   speaker can check:

   o    The authorization of the AS listed as the originating AS in any
        received update to advertise reachability to the prefix listed
        in the update.

   o    The validity of the AS Path contained in the update.

   We use the term validity in reference to the AS Path, in this docu-
   ment, to indicate the plausibility of the AS Path listed. As shown in
   [PATH-CONSIDER], it isn't possible to communicate authorization
   through an AS Path; only the existence or nonexistance of the AS Path
   listed can be proven.

   soBGP operates by distributing a set of signed certificates,
   described in [SOBGP-CERTIFICATE], containing the information required
   to validate the two pieces of information given above. These certifi-
   cates MAY be distributed using the mechanisms described in [SOBGP-
   BGPTRANSPORT], or some other mechanism. Once these certificates have

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   been received and processed (signatures validated, etc, as described
   in [SOBGP-CERTIFICATE], they form a database containing:

   o    A listing of IP address blocks and the AS authorized to ori-
        ginate them.

   o    Policies related to specific prefixes and blocks of addresses.

   o    A list of autonomous systems connected to each autonomous system
        within the internetwork. This connection list is used to build a
        graph of AS interconnectivity within the internetwork, as
        described in the section Building the AS Connectivity Graph,

   This effectively forms a secure registry of routing information which
   can be used to check the validity of routing information received
   from BGP peers. This database is termed the "authorization database."
   No assumption about the location of the authorization database is
   made within this document.

   When soBGP is supported, a BGP speaker MUST have access to the
   authorization database. Possible methods of access include:

   o    Have a local copy of this authorization database, and perform
        the checkes described later in this document against that local

   o    Pass received routing information to a locally maintained server
        for validation against that server's copy of the authorization

   o    Accept filters built from a copy of the authorization database
        contained on a locally maintained server.

   As BGP updates are processed, a security preference is assigned to
   each prefix, as described further in the Security Preference section
   of this document. BGP update processing is described in the Receiving
   and Processing Updates section of this document.

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3. soBGP Operation

   Each section below provides detailed information on some aspect of
   soBGP operation.

3.1. The Security Preference

   Rather than simply noting a given prefix should be dropped (not
   trusted) or retained (trusted), soBGP extends the concept of locally
   generated and maintained policy in BGP by assigning each prefix a
   Security Preference. This allows the local operator to drop prefixes
   not meeting certain security criteria, while simply lowering their
   preference for prefixes meeting some security criteria. This allows
   operators some flexibility in their implementation of security poli-
   cies, especially as the security system is being tested, or while the
   security system isn't fully deployed.

   While the amount by which the Security Preference is increased or
   decreased for any operation described in this draft is locally signi-
   ficant to the autonomous system. All devices processing routes
   against soBGP information MUST use the same mechanisms and values of
   the Security Preference to ensure consistent routing within the auto-
   nomous system.

   If the Security Preference is set to a value precluding a route from
   further consideration in the decision process, the route should be
   discarded at that point, rather than continuing with the decision

   The Security Preference value may be used to select among different
   routes for the same prefix; the higher value MUST be preferred. Any
   of the following methods may be used:

   A    Consider the Security Preference prior to calculating the degree
        of preference [BGP] for a prefix.

   B    Assign the value of the Security Preference to any of the attri-
        butes used in the Decision Process [BGP]. Care must be taken
        with attributes for which the lower value is preferred.

   C    Use a Cost Community [COST] and its associated methods to con-
        sider the Security Preference at any step in the Decision Pro-
        cess [BGP] without overloading other attributes. Care must be
        taken as the lowest value in a Cost Community is preferred.

   The method selected MUST be consistent through the local Autonomous

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3.2. Building the AS Connectivity Graph

   Each ASPolicyCert advertised by a member of the internetwork contains
   a list of the autonomous systems the advertising AS is connected to,
   along with possible policy information about that connection. From
   this information, a graph of AS connectivity within the internetwork
   is built.

   Any AS can be used as the starting point for building this graph,
   thus multiple disconnected graphs (representing section of the inter-
   network running soBGP and providing interconnection information) are
   possible. If every AS within the internetwork is providing intercon-
   nection information, one graph can be built containing all the
   internetwork's interconnections.

   The process of creating this graph is:

   o    Examine the list of connected autonomous systems advertised by
        the current AS.

   o    Examine the ASPolicyCert of each AS the current AS is advertis-
        ing as connected, and determine if that AS is advertising a con-
        nection back to the current AS. This is termed the two way con-
        nectivity check.

   o    If the two way connectivity check passes, the connection SHOULD
        be added to the interconnection graph, and marked as trustable.

   o    If the two way connectivity check fails, the connection MAY be
        added to the interconnection graph, but marked so a lower secu-
        rity preference will be assigned to AS_PATHs traversing this

   o    Repeat this process for each ASPolicyCert in the authorization

        The resulting graph is called the internetwork graph.

3.3. Validating Routing Information

   For each prefix within a given BGP UPDATE message:

   o    The local authorization database is examined, and the AuthCert
        with the longest prefix length encompassing the range of
        addresses described by the prefix is chosen.

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   o    If there is no entry in the local authorization database which
        encompasses the range of addresses described by the prefix, then
        the route is said to be unverified, and should be handled
        according to local policy (either discarded, or have its secu-
        rity preference lowered). The rest of this process is ignored in
        these cases.

   o    The second hop in the AS_PATH attribute is examined.

      o    If the second hop in the AS_PATH is advertised as connected
           by the originating AS, the Security Preference for this pre-
           fix SHOULD be increased.

      o    If the second hop in the AS_PATH is not advertised as con-
           nected by the originating AS, the Security Preference for
           this prefix SHOULD be decreased.

      o    If the second hop in the AS_PATH is not advertised as con-
           nected by the originating AS and the originator's policy
           indicates the second hop MUST be validated, the prefix should
           be removed from further consideration.

   o    The AS_PATH attribute is compared to the internetwork graph.

      o    If the AS_PATH described is contained within the internetwork
           graph, the Security Preference SHOULD be increased.

      o    If the AS_PATH described is not contained within the inter-
           network graph, the Security Preference SHOULD be decreased.

      o    If the AS_PATH traverses a connection which is only described
           by one of the two autonomous systems, this is a one way con-
           nection. Local policy may be used to determine if the secu-
           rity preference should be increased in this case.

      o    If the AS_PATH described is not contained within the inter-
           network graph, and the originator indicated the AS_PATH MUST
           be checked, the prefix should be removed from further con-

   o    The AuthCert chosen at the first step is examined.

      o    If the authorized AS in the AuthCert matches the originating
           AS in the AS_PATH, the Security Preference SHOULD be

      o    If the authorized AS in the AuthCert does not mathc the ori-
           ginating AS in the AS_PATH, the Security Preference SHOULD be

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           set low enough to cause the route to be discarded.

   o    Other policies contained in the local authorization database
        should be applied as directed by the policy.

3.4. Validating Received BGP UPDATES

   As BGP UPDATES are received, they may be processed in one of several

   o    Each prefix may be validated according to the process outlined
        in Validating Routing Information before they are installed in
        the ADj-RIB-IN.

   o    Each prefix may be validated according to the process outlined
        in Validating Routing Information after they are installed in
        the Adj-RIB-In, but before they are considered in the BGP Best
        Path calculation.

   o    Each prefix may be validated according to the process outlined
        in Validating Routing Information after they are run through the
        Best Path algorithm, but before they are installed in the local

   o    Routes may be installed in the local RIB, and then validated
        using the process outlined in Validating Routing Information.
        Once validation is accomplished, adjustments to the local RIB
        and routes advertised to BGP peers may need to be adjusted.

3.5. Aggregation

   Aggregation is a difficult problem with any method which attempts to
   verify the origin of any given prefix, since aggregation removes the
   relationship between prefixes originated and originators. Prefixes
   may only be aggregated by an entity which is otherwise authorized to
   advertise the aggregated prefix.

3.6. Requirements for Systems Running soBGP

   This section describes requirements for autonomous systems running
   soBGP, requirements for BGP speakers forming external adjacencies
   from within such autonomous systems, and devices exchanging soBGP

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   o    Any peering session along the border of an autonomous system
        running soBGP SHOULD be authenticated through some means such as
        [BGP-MD5], IPsec ([ESP], [AH]), or through some other current,
        effective means of protecting BGP sessions from being hijacked,
        or otherwise abused.

   o    Any peering session along which soBGP certificates are exchanged
        SHOULD be authenticated through some means such as [BGP-MD5],
        IPsec ([ESP, [AH]), or through some other current, effective
        means of protecting BGP sessions from being hijacked, or other-
        wise abused.

   o    The AS_PATH of any routing information received from any BGP
        peer outside the autonomous system MUST be checked to validate
        the next hop AS is the AS the update was received from. If the
        next hop AS in any received update does not match the configured
        AS the route is learned from, the update MUST be discarded.

4. soBGP Deployment

   This section begins by describing what we believe to be the most
   practical deployment of this secure registry of routing information.
   Following sections describe some other deployment options that may
   prove useful in some situations, or may prove to be more practical
   than the deployment outlined in this section.

4.1. Deploying soBGP on Distributed Registry Servers

   This deployment scenerio works within three constraints:

   o    It may not be not desirable to combine routing and cryptographic
        processing of soBGP certificates on the same device.

   o    The system should be distributed, using as few centralized
        resources as possible.

   o    Trust relationships should be based on existing business and
        working relationships, rather than building new relationships
        specifically for securing the routing system.

   Assume we have a small internetwork, as shown below:

     S1 - - - - - - - - - - -S2 - - - -S3

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   | AS65000              | AS65001 | AS65002

   In this network, we assume each AS has an soBGP server locally within
   their AS, marked as S1, S2, and S3, above. These servers are inter-
   connected in a way similar to eBGP peering between AS65000, AS65001,
   and AS65002; S1 and S2 are using the mechanisms described in [SOBGP-
   BGPEXT] to distribute the certificates described in [SOBGP-
   CERTIFICATE] between them.

   Each server then processes the certificates as described in [SOBGP-
   CERTIFICATE], and either provides a set of filters or a mechanism
   through which the eBGP peering routers can authenticate routing
   information, such as described in [SOBGP-RADIUS]. This deployment
   technique provides BGP route validation that is:

   o    Fully Distributed: Local server (or set of servers) which builds
        the required databases based on received certificates, and dis-
        tributes certificates throughout the routing system.

   o    Locally Controlled: Each local server (or set of server) is
        maintained and managed by autonomous systems participating in
        the internetwork.

   o    Based on Existing Business Relationships: Peering autonomous
        systems also peer their soBGP servers, so the system uses exist-
        ing business relationships to provide the deployment and long
        term maintenance of the system.

   o    Very Little Impact on the Existing Routing System: The current
        processing and distribution of routing information through [BGP]
        isn't impacted in any way.  The only additional requirements on
        existing equipment are to compare the routing information to the
        database results provided by the local servers (i.e., receiving
        and processing filter lists, or through [SOBGP-RADIUS]).

4.2. Certificate Processing on Edge Peering Routers

   soBGP can also be deployed entirely within BGP speakers at the edge
   of an Autonomous System (AS).

   +-(eBGP)-+           +-(eBGP)-+
   |        |           |        |
   v        v           v        V


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            ^           ^
            |           |

   In this network, A is sending certificates it has learned from other
   sources to B using the mechanisms described in [SOBGP-BGPEXT]. It is
   passing these certificates to D via iBGP, and D is passing these cer-
   tificates to E via eBGP. Each edge router, B and D, process these
   certificates locally, building the databases required to validate
   received routing information from them.

4.3. Multihoming Deployment

   Multihoming presents a special challenge to the deployment of soBGP
   within a large scale internetwork.

     (---------)            (---------)
    (  AS65401  )          (  AS65402  )
   (             )        (             )
    (           )          (           )
      (---A---)              (---B---)
          |                      |
           \                    /
            \-----+      +-----/
                  |      |
              (              )
               (   No-AS    )

   Assume No-AS has obtained a block of addresses,, from
   AS65401, and would like to advertise that same block of addresses
   through AS65402. Since No-AS has no AS number, it cannot generate any
   soBGP certificates, and must rely on its upstream providers to work
   out the security impact in some way. The simplest solution would be,
   of course, for NOAS to obtain an AS number, and fully participate in
   soBGP, but barring that, what other solutions are there?

   AS65401 could issue a certificate allowing AS65402 to originate just
   the prefix in question,, or AS65401 could simply list
   AS65402 in the certificate covering as an authorized ori-
   ginator for this address space (as multiple authorized originators
   are allowed).

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4.4. Proxy Advertisement of Certificates

   Note there is no requirement for a given entity which originates
   routes into the routing system to actually originate the correspond-
   ing certificates required for the correct origination of the route to
   be validated, and the AS Path attached to the route to be verified.

            ( Other Third Party )
               /             \
              /               \
     (---------)            (---------)
    (  AS65401  )          (  AS65402  )
   (             )        (             )
    (           )          (           )
      (---A---)              (---B---)
          |                      |
           \                    /
            \-----+      +-----/
                  |      |
              (              )
               (  AS65403   )

   In this case, AS65401, AS65402, or some other third part may actually
   advertise the certificates necessary for AS65403 to originate vali-
   dated routes.

5. Other Deployment Considerations

   In this section, we move from specific deployment scenerios to other
   deployment considerations, such as key generation and protection, and
   memory utilization/impact.

5.1. Certificate Generation and Private Key Protection

   There is only one private/public key pair per autonomous system; cer-
   tificates are generated as determined by local policy and as required
   to account for changes in the network. Since the entity's private key
   is not used in any part of the operations verifying received informa-
   tion, or in generating information to transmit to other devices,
   these certificates could be generated on some secure central system
   in the AS, and the results, containing only public keys, can be
   transmitted throughout the network.

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   Securing the private key of each entity should be relatively easy in
   this environment, since the location of the private key can be care-
   fully constrained; no device other than the system which generates
   the required certificates needs use of the private key.

5.2. Impact on Performance and Memory Utilization

   Detailed performance and memory utilization characteristics of soBGP
   will be the subject of future investigation. However, as this is an
   important area of consideration, we present some suggested analysis
   below. (In other words, this is a guess).

   In terms of memory, each device running sobGP will need to store:

   o    Each of the Entitycerts Received. The maximum number of Enti-
        tycerts within the routing system would be the number partici-
        pating autonomous systems multiplied by the number of outstand-
        ing Entitycerts from each autonomous system.

   o    Each of the ASPolicycerts Received. The number of ASPolicycerts
        within the system will probably be similar to the number of
        Entitycerts within the system.

   o    Each of the PrefixPolicycerts Received. The number of PrefixPol-
        icyCerts within the system will depend on the number of address
        blocks each participant in the routing system advertises, and
        could double during key rollover.

   Performance will depend on the cryptographic processing requirements
   imposed by the certificate signature methods, as described in
   [SOBGP-CERTIFICATE]. However, all of this additional memory and pro-
   cessing would most likely be required on a distributed soBGP server,
   rather than on routers themselves.

   The primary impact on routers and routing protocol convergence will
   be the memory and processing requirements added from the additional
   route filters or processing as required by the deployment technique

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6. Normative References

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

        Ng J (editor), "Extensions to BGP to Support Secure Origin BGP
        (soBGP)",  draft-ng-sobgp-bgp-extensions-01.txt, April 2004

        Weis, Brian (editor), "Secure Origin BGP (soBGP) Certificates",
        draft-weis-sobgp-certificates-01.txt, October 2003

7. Informative References

     Lovnick, C, "RADIUS Attributes for soBGP Support", draft-lonvick-
     sobgp-radius-04.txt, February 2004

     White, Russ, "Considerations in Validating the Path in Routing Pro-
     tocols", draft-white-pathconsiderations-02.txt, April 2004

     Retana, A., White, R., "BGP Custom Decision Process", draft-
     retana-bgp-custom-decision-00, October 2002.

8. Editor's Address

   Russ White
   Cisco Systems
   7025 Kit Creek Road
   Research Triangle Park, NC 27709

White, et. all                                                 [Page 13]