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Versions: 00 01 02

Network Working Group                                           R. White
Internet-Draft                                             Cisco Systems
Expires: December 17, 2006                                 June 15, 2006


Architecture and Deployment Considerations for Secure Origin BGP (soBGP)
                   draft-white-sobgp-architecture-02

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Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   There is a great deal of concern over the security of internetworks
   built using the Border Gateway Protocol to provide routing
   information to autonomous systems connected to the internetwork.
   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 scenarios, starting with what we believe will
   be the most common deployment option.




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1.  Motivation


2.  Overview

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

   o  Adjacencies between devices running the routing protocol.
   o  Information carried within the routing protocol.

   While security between [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
   problem.

   This draft proposes a possible solution to securing the information
   within BGP, using the certificates and protocol extensions proposed
   in [SOBGP-BGPTRANSPORT], [SOBGP-CERTIFICATE], and [SOBGP-RADIUS].


3.  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.

   A valid AS Path, in this document, is a path that has the following
   attributes:

   o  Each autonomous system listed in the AS Path is an actual
      participant in the internetwork.
   o  Each pair of autonomous systems listed in the AS Path are actually
      interconnected.
   o  Starting from the first autonomous system (the origin AS), and
      passing through each autonomous system listed in the AS Path,
      actually results in reaching the advertising peer's AS.

   As shown in [PATH-CONSIDER], it isn't possible to verify an AS more
   than one AS hop away has authorized the advertisement of specific
   reachability information based on the AS Path.  The concept of
   policy, and soBGP's interaction with policy, is considered more fully
   in a later section in this draft.



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   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
   certificates MAY be distributed using the mechanisms described in
   [SOBGP-BGPTRANSPORT], or some other mechanism.  Once these
   certificates have 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 originate
      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,
      below.

   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.

   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.


4.  soBGP Operation

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

4.1.  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
   internetwork running soBGP and providing interconnection information)
   are possible.  If every AS within the internetwork is providing
   interconnection information, one graph can be built containing all



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   the internetwork's interconnections.

   The process of creating this graph is:

   o  Begin with the local AS, or any AS for which an ASPolicycert is
      available.
   o  Examine the list of connected autonomous systems advertised by the
      current AS.
   o  Examine the ASPolicycert of each AS the current AS is advertising
      as connected, and determine if that AS is advertising a connection
      back to the current AS.  This is termed the two way connectivity
      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 security
      preference will be assigned to routes containing this AS pair in
      their AS Path.
   o  Apply any policies indicated by either of the two autonomous
      systems in their ASPolicycert.  This could include, for instance,
      noting the connected autonomous system MUST NOT be used for
      transiting traffic.
   o  Repeat this process for each ASPolicycert in the authorization
      database.

   The resulting graph is called the internetwork graph.

4.2.  Validating Routing Information (The Security Preference)

   soBGP provides a two tier evaluation of routes.  In the first stage,
   a BGP speaker evaluating received routing information would discard
   all routing information found to be false, or not accurately
   representing the internetwork as it exists.  Routing information not
   meeting this criteria SHOULD be discarded, as indicated in the
   processing steps outlined below.

   In the second tier, the BGP speaker assigns a Security Preference to
   the received routing information, indicating a locally significant
   trust level determined by examining the received routing information.
   The amount by which the Security Preference is increased or decreased
   for any operation described in this draft is locally significant to
   the autonomous system.  This allows the operator provide a finer
   granularity of security policy, from dropping routing information
   deemed invalid through simply preferring routes the operator deems
   "more secure."

   The operator MAY configure a lower bound.  Routes with Security
   Preferences under this lower bound SHOULD be discarded.  Any of the



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   following methods may be used to implement the Security Preference
   within an autonomous system:

   o  Assign the value of the Security Preference to any of the
      attributes used in the [BGP] decision process.  Care must be taken
      with attributes for which the lower value is preferred.
   o  Use a Cost Community [COST] and its associated methods to consider
      the Security Preference at any step in the Decision Process [BGP]
      without overloading other attributes.  Care must be taken as the
      lowest value in a Cost Community is preferred.

   Several basic rules apply to all BGP speakers either evaluating the
   security level of received routing information, or using the Security
   Preference to determine which path to install in the local RIB:

   o  The method selected to implement the Security Preference MUST be
      consistent through the local autonomous system.
   o  All devices processing routes against soBGP information MUST use
      the same mechanisms and values of the Security Preference to
      ensure consistent routing within the autonomous system.
   o  The Security Preference value may be used to select among
      different routes for the same prefix; the higher value MUST be
      preferred.

   The process described below does not rule out additional policies
   added locally, or in some future draft.  For each route (prefix/
   attribute pair) 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.  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.  The Security Preference SHOULD be set to a level
      indicated by local policy.
   o  If there is an AS_SET in the AS_PATH, the following process MAY be
      followed for each AS_SET:
      *  For each AS in the AS_SET, examine the set of PrefixPolicycerts
         advertised by that AS.
      *  If a PrefixPolicycert is found authorizing at least one of the
         autonomous systems in the AS_SET to advertise some component of
         the prefix, the Security Preference MAY be increased or left at
         its current value.
      *  If a PrefixPolicycert is not found authorizing at least one of
         the autonomous systems in the AS_SET to advertise some
         component of the prefix, the Security Preference MAY be
         decreased or left at its current value.




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      *  If a path exists from the aggregator to each AS listed in the
         AS_SET, the Security Preference MAY be increased or left at its
         current value.
      *  If a path does not exist from the aggregator to each AS listed
         in the AS_SET, the Security Preference MAY be decreased or left
         at its current value.
   o  If there is an AS_SET in the AS_PATH, it is disregarded in all
      further processing.  The first AS contained in the AS_PATH not
      contained in the AS_SET is considered the originator of the route
      for the remainder of the processing.
   o  The second hop in the AS_PATH attribute is examined.
      *  If the second hop in the AS_PATH is advertised as connected by
         the originating AS, the Security Preference for this prefix
         SHOULD be increased.
      *  If the second hop in the AS_PATH is not advertised as connected
         by the originating AS, the Security Preference for this prefix
         SHOULD be decreased.
      *  If the second hop in the AS_PATH is not advertised as connected
         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.
      *  If a series of two way verified pairwise peerings exists,
         beginning with the first AS listed in the AS_PATH, and ending
         in the advertising AS, the Security Preference SHOULD be
         increased.
      *  If a series of pairwise peerings exists, beginning with the
         first AS listed in the AS_PATH, and ending in the advertising
         AS, the Security Preference MAY be increased.  This case allows
         for the inclusion of one-way advertised AS interconnections in
         the graph.
      *  If the AS_PATH described is not contained within the
         internetwork graph, and the originator indicated the AS_PATH
         MUST be checked, the prefix SHOULD be removed from further
         consideration.
      *  Otherwise, the Security Preference SHOULD be decreased.
   o  The Authcert chosen at the first step is examined.
      *  If the authorized AS in the Authcert matches the originating AS
         in the AS_PATH, the Security Preference SHOULD be increased.
      *  If the authorized AS in the Authcert does not match the
         originating AS in the AS_PATH, the prefix SHOULD be removed
         from further consideration.

4.3.  Validating Received BGP UPDATES

   As BGP UPDATES are received, they MAY be processed at one of several
   points:




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

4.4.  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
   certificates.

   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 IPsec ([ESP,
      [AH]), or through some other current, effective means of
      protecting these sessions from being hijacked, or otherwise
      abused.
   o  For each received route, the last (most recently added) autonomous
      system MUST be compared to the autonomous system of the BGP
      speaker advertising the route.  If the last (most recently added)
      AS in the AS Path does not match the autonomous system of the
      transmitting speaker, the route MUST be discarded.

   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
      checks described later in this document against that local
      database.
   o  Pass received routing information to a locally maintained server
      for validation against that server's copy of the authorization
      database.  [SOBGP-RADIUS] describes one such possible access
      mechanism, although others are possible.



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

4.5.  Logging Requirements

   Any system valildating received routing information using an soBGP
   database built using the mechanisms described in this draft SHOULD
   log:

   o  Any change in the Security Preference of any processed route, and
      the reason for the change in Security Preference.
   o  Any route that is discarded from further processing, and the
      reason for the discarding of the route.
   o  Any route that is marked as unverified.
   o  The verification of any certificate received by an soBGP speaker.
   o  Failure to verify any certificate received by an soBGP speaker,
      and why the certificate failed to be verified.


5.  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.

5.1.  Deploying soBGP on Distributed Registry Servers

   This deployment scenario 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
   10.1.1.0/24---A---B-----C---D-----E---F
   | 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
   interconnected to distribute the certificates described in [SOBGP-
   CERTIFICATE] between them (possibly using the mechanism outlines in



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   [SOBGP-TRANSPORT], but other transport mechanisms are possible).

   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: A local server (or a set of servers) builds the
      required databases based on received certificates, and distributes
      certificates throughout the routing system.
   o  Locally Controlled: Each local server (or set of servers) 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 existing
      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, through [SOBGP-RADIUS], or through
      some other mechanism).

5.2.  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

         A--------B-----C-----D--------E

                  ^           ^
                  |           |
                  +--(iBGP)---+

   In this network, A is sending certificates it has learned from other
   sources to B using the mechanisms described in [SOBGP-BGPTRANSPORT].
   B is passing these certificates to D via iBGP, and D is passing these
   certificates 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.




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   B has two choices with regard to the certificates it receives from D.
   It can assume these certificates have been validated before they were
   transmitted by D, or it can assume these certificates were not
   validated before being transmitted by D. If B assumes D is validating
   certificates before transmitting them, then B can place any
   certificates received from D, an iBGP peer, directly into its local
   databases.  If B assumes D is not validating certificates before
   transmitting them, then B can validate any received certificates
   before placing them in its local database.  These two options are
   determined within the autonomous system, and do no impact soBGP's
   inter-AS operation, nor the overall system operation.

5.3.  Multihoming Deployment

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

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

   Assume No-AS has obtained a block of addresses, 10.1.1.0/24, 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 No-AS to obtain an AS number, and fully
   participate in soBGP, but barring that, what other solutions are
   there?

   o  AS65401 could issue a certificate allowing AS65402 to originate
      just the prefix in question, 10.1.1.0/24.  AS6402 could then
      advertise this certificate.
   o  AS65401 could list AS65402 in the certificate covering 10.1.1.0/24
      as an authorized originator for this address space (as multiple
      authorized originators are allowed).

   These options are also applicable to the case where No-AS receives an



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   address allocation, perhaps provided with a certificate as described
   in [RFC3779].  No-AS can use these certificates, provided by the
   authorizing entity to create and sign Authcerts containing the
   autonomous system number of each of its service providers (or two
   Authcerts, one for each service provider).

5.4.  Proxy Advertisement of Certificates

   Note there is no requirement for a given entity which originates
   routes into the routing system to actually originate the
   corresponding 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---)
                |                      |
                 \                    /
                  \-----+      +-----/
                        |      |
                     (--C------D--)
                    (              )
                     (  AS65403   )
                      (----------)

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


6.  Other Considerations

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

6.1.  Certificate Generation and Private Key Protection

   There is only one private/public key pair per autonomous system;
   certificates are generated as determined by local policy and as



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   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 information, 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.

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

6.2.  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
      Entitycerts within the routing system would be the number
      participating autonomous systems multiplied by the number of
      outstanding 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
      PrefixPolicycerts 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 processing
   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
   used.

6.3.  soBGP Impact on Internetwork Convergence

   We generally assume that adding a security infrastructure on top of
   an operating system will dramatically decrease the performance of



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   that system.  However, much depends on the system being modified,
   itself, and how closely to perfectly efficient that system already
   performs.  We've already examined, in prior sections, the impact of
   soBGP on memory and processor utilization in devices running these
   extensions, but we've not examined the impact of soBGP on another
   aspect of an internetwork's operation, convergence time.  In this
   section, we will examine some possible side effects of deploying
   soBGP using the following small internetwork as an example.

   +--B--C--D--+
   |           |
   A---E---F---G---K
   |           |
   +-----H-----+

   In this network, assume that:

   o  A prefers the path through {H,G} to K.
   o  E prefers the path through {F,G} to K.
   o  B prefers the path through {C,D,G} to K.

   In this network, if the link from G to K fails:

   o  A will first receive a withdraw from H, and begin to prefer the
      path through {E,F,G} to K.
   o  A will then receive a withdraw from E, and begin to prefer the
      path through {C,D,E,G} to K.
   o  A will finally receive a withdraw from C, and remove the route to
      K from its local tables.

   This processing pattern is well documented through multiple studies
   in the operations of [BGP] in large scale internetworks.  The most
   obvious answer to resolve this problem is for G to include some sort
   of information in its withdraw indicating the nature of the failure,
   so A can directly remove all paths through the link {G,K} on
   receiving the first withdraw.  This is more problematic than it
   appears, however, because [BGP] is designed for protocol efficiency,
   and withdraws are often removed from the internetwork, along with any
   information they might contain, at an early point in the convergence
   process.

   The mechanism soBGP uses to build a graph of the interconnections
   between the autonomous systems in the internetwork, however, provide
   another place where this sort of direct information about changes in
   the topology of the internetwork can be distributed.  If this network
   were running soBGP, G would be able to reissue its certificate
   claiming connectivity to K, or use some specific policy indicator to
   note the link {G,K} has failed.  On receiving this certificate, all



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   the autonomous systems could remove all routes with the link {G,K} in
   their AS Paths, and the network would converge with much less
   distribution and processing of routing information.

   We believe there are probably several performance enhancements that
   may be gained through the laying of a connectivity graph on top of
   the current [BGP] provided view of an internetwork.  These types of
   efficiency gains may overcome or fully offset the added costs of
   deploying soBGP as a security system.

6.4.  Aggregation

   Aggregation is a diificult problem within any system attempting to
   validate routes in an internetwork running BGP.  The primary purpose
   of aggregation is to remove information from the routing system, and
   information removed from the system cannot be validated or verified.
   This appears to be a simple observation, but it has a number of far
   reaching impacts.

   (   AS1   )  (AS4) (AS5)
   10.1.0.0/24----A-----+
                        |
   (   AS2   )          |  (AS5)
   10.1.1.0/24----------B----C
                        |
   (   AS3   )          |
   10.1.2.0/24----------+

   In this small internetwork, B could be:

   o  Reoriginating 10.1.0.0/22 towards C. This means that rather than
      building a BGP aggregate, B is simply generating 10.1.0.0.0/22
      locally, and filtering all longer prefix components of this
      aggregate.  This is a common, normally recommended, practice, in
      many situations.  In this case, C will receive 10.1.0.0/22 with an
      AS Path of {B}.
   o  Aggregating 10.1.0.0/22, using the aggregation procedure described
      in [BGP].  In this case, B will generate an AS Set containing the
      contributing autonomous system numbers.  In this case, C will
      receive 10.1.0.0/22 with an AS Path of {(1,2,3),4}
   If B is reoriginating 10.1.0.0/22, C will not know this route is an
   aggregate, and MUST treat the route as it does any other received
   routing information.

   If B is building an AS SET, C can examine the aggregator (the first
   AS listed after the AS Path), and treat this AS as the originating
   AS, verifying the route as it does any other received routing
   information.  If the internetwork's local policy rules require all



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   participants to run soBGP, and does not allow any AS to filter soBGP
   certificates, C can also use the AS interconnection graph to verify B
   is actually connected to each AS listed in the AS Set.


7.  Incremental Deployment of soBGP

   One of the primary concerns with any security system is the ability
   of users to incrementally deploy the system without impacting current
   network operations.  As the security system is deployed, it should
   provide greater security.  In theory, the amount of additional
   security offered verses the additional work required should be fairly
   balanced.

   There are two aspects of incremental deployment that need to be
   considered:

   o  The impact of some of the participants in the system deploying the
      security system, but not all participants deploy the system.
   o  The impact of some part of the system being deployed widely, but
      not all of the system.

7.1.  Not All Connected Networks Participate

   The first consideration in incremental deployment of soBGP is asking
   what happens if all of the autonomous systems in an internetwork
   don't run soBGP.  Is there any advantage to partial deployments of
   soBGP in this sense?

   Throughout this section, we will assume soBGP certificates are
   received by all autonomous systems running soBGP, even if they are
   separated by multiple hops which are not running soBGP.  This is not
   an unreasonable assumption, since soBGP certificates can be shared in
   multiple ways, including multihop BGP sessions across non-
   participating autonomous systems.

   Assume we have the following small internetwork, what impact will
   incrementally deploying soBGP through this network have?

   (AS1) (AS2) (AS3) (AS4             )
     A-----B-----C-----D---10.1.1.0/24

   Assume AS3 and AS4 deploy soBGP, but not AS1 and AS2; is there any
   value in this partial deployment?  When AS3 receives routes from AS4,
   it can verify AS4 is authorized to advertise 10.1.1.0/24.  Further,
   any routes AS3 forwards to AS4 from AS1 or AS2 can be validated, to
   some degree, by AS4.  The AS Path can be checked to make certain AS2
   is actually connected to AS3 (since AS3 is advertising its



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   connectivity to AS3).  If some route is advertised from AS2 showing
   an AS hop in the middle of those two autonomous systems, it can be
   safely discarded by AS4 as an invalid AS Path.

   We can make an alternate assumption, that AS1 and AS4 have deployed
   soBGP, while AS2 and AS3 have not.  In this case, what gains would be
   made by deploying soBGP?  Assume Router A receives a route from
   Router B with an AS Path of {B,C,D}.  If Router A has access to
   Router D's certificates, it can:

   o  Check the origin AS (the first AS in the AS Path, in this case
      AS4) is authorized to advertise the address space (in this case
      10.1.1.0/24).
   o  Check the first hop in the AS Path (in this case AS3) is actually
      attached to AS4, as advertised by AS4.
   o  Since Router A knows it is connected to AS2, through B, it can
      also validate the last AS listed in the AS Path.

   There is some gain, then, in deploying soBGP in both of these
   situations.  The gain is obviously more in the second scenario than
   the first.

7.2.  Deploying Parts of soBGP

   The second question concerning incremental deploying is if
   implementing some part of soBGP, without the remainder, would be
   useful.  This question is generally placed in the context of
   validating the origination authorization of routes, and possibly the
   first hop in the AS Path, but not the entire AS Path.

   o  soBGP Authcerts could be advertised or published (for instance, on
      a Web page), to provide authorization for each origin AS to
      advertise specific address blocks.  These certificates could be
      self signed, in the most relaxed case, or signed by the entity
      authorizing the AS to advertise the address block.
   o  soBGP PrefixPolicycerts could be advertised or published (for
      instance, on a web page), to provide authorization and first hop
      checking for received routes.  The Authcert within the
      PrefixPolicycert contains the information required to validate the
      origin's authorization to originate a route.  The list of MAY
      TRANSIT autonomous systems contained in the PrefixPolicycert would
      provide the ability to check the first hop in the AS Path of any
      received route.
   o  soBGP PrefixPolicycerts and ASPolicycerts could be advertised to
      provide authorization to advertise a route from within an address
      block, and also provide the ability to validate the first hop in
      the AS Path.  The Authcert, within the PrefixPolicycert, contains
      the information required to validate the origin's authorization to



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      originate a route.  The list of connected autonomous systems
      within the ASPolicycert provides the information required to
      validate the first hop in the AS Path of any received route.

   Any of these modes of operation could be mixed with a full deployment
   of soBGP, and provides checks for the first hop and origination of
   received routes.


8.  Policy Interactions with soBGP

   Beyond simply securing the information contained within the routing
   database [BGP] builds, it's also desirable to have a secure mechanism
   for an autonomous system to advertise policy information.  For
   instance, an autonomous system may not want a specific peer to
   transit traffic, or an originator may want routing information to be
   advertised only to a specific number of AS hops away from the origin.

   The sections below examine some various policies of this type, and
   possible solutions within soBGP.

8.1.  Indicating Do Not Transit

   In the following small internetwork, A would like to enforce a policy
   preventing C from transiting traffic from B to A.

        A-------B--------D
        |       |
        +---C---+

   A may attempt to prevent C from transiting traffic from B to A by
   advertising its routing information to C in such a way that C cannot
   readvertise that routing information to B. The problem with this
   approach is that B must assume the lack of specific routing
   information from C indicates A has a local policy forbidding C from
   transiting traffic to A. Unfortunately, because of the nature of
   address space assignment, aggregation, filtering, and other factors,
   B cannot make this assumption.  For instance, C may receive a
   superset of the routing information A is advertising, and advertise
   those routes to B instead, in which case A will find there's no
   effective way to enforce its policy towards C.

   We find, however, that the interconnection graph laid on top of the
   routing information transmitted by each autonomous system provides a
   point where A may communicate its nontransit policy towards C
   directly to B. Using its ASPolicycert, A may indicate B is not a
   transit AS, allowing B to mark routes with the AS pair {B,A} in their
   AS Path with a lower security preference, or possibly even discarding



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   such routing information altogether.

   This is a simple application of the policies available in the soBGP
   certificates; more complex policies may be expressed through similar
   means.  The certificates described in [SOBGP-CERTIFICATE] are built
   so policies may be added in the future, as well.


9.  Acknowledgements

   A large number of people contributed to this draft either by
   contributing text, ideas, or comments; 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, Iljitsch van Beijnum, Bora Akyol, Tony
   Li, Sue Hares, and Victor P. Long.


10.  Security Considerations


11.  IANA Considerations


12.  References

12.1.  Normative References

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

   [SOBGP-CERTIFICATE]
              Weiss, B., "Secure Origin BGP (soBGP) certificates",
              draft-weis-sobgp-certificates-01.txt (work in progress),
              October 2003.

12.2.  Informative References

   [COST]     Retana, A. and R. White, "BGP Custom Decision Process",
              draft-retana-bgp-custom-decision-00.txt (work in
              progress), October 2002.

   [PATH-CONSIDER]
              White, R., "Considerations in Validating the Path in
              Routing Protocols", draft-white-pathconsiderations-02.txt
              (work in progress), April 2004.

   [SOBGP-BGPTRANSPORT]
              Ng, J., "Extensions to BGP Transport soBGP certificates",



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              draft-ng-sobgp-bgp-extensions-01.txt (work in progress),
              April 2004.

   [SOBGP-RADIUS]
              Lonvick, C., "RADIUS Attributes for soBGP Support",
              draft-lonvick-sobgp-radius-04.txt (work in progress),
              February 2004.












































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Author's Address

   Russ White, editor
   Cisco Systems

   Email: riw@cisco.com













































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