Network Working Group                                           R. White
Request for Comments: 5123                                      B. Akyol
Category: Informational                                    Cisco Systems
                                                          February 2008


             Considerations in Validating the Path in BGP

Status of This Memo

  This memo provides information for the Internet community.  It does
  not specify an Internet standard of any kind.  Distribution of this
  memo is unlimited.

IESG Note

  After consultation with the RPSEC WG, the IESG thinks that this work
  is related to IETF work done in WG RPSEC, but this does not prevent
  publishing.

  This RFC is not a candidate for any level of Internet Standard.  The
  IETF disclaims any knowledge of the fitness of this RFC for any
  purpose and in particular notes that the decision to publish is not
  based on IETF review for such things as security, congestion control,
  or inappropriate interaction with deployed protocols.  The RFC Editor
  has chosen to publish this document at its discretion.  Readers of
  this document should exercise caution in evaluating its value for
  implementation and deployment.  See RFC 3932 for more information.

Abstract

  This document examines the implications of hop-by-hop forwarding,
  route aggregation, and route filtering on the concept of validation
  within a BGP Autonomous System (AS) Path.

1.  Background

  A good deal of thought has gone into, and is currently being given
  to, validating the path to a destination advertised by BGP.  The
  purpose of this work is to explain the issues in validating a BGP AS
  Path, in the expectation that it will help in the evaluation of
  schemes seeking to improve path validation.  The first section
  defines at least some of the types of questions a BGP speaker
  receiving an update from a peer not in the local autonomous system
  (AS) could ask about the information within the routing update.  The
  following sections examine the answers to these questions in
  consideration of specific deployments of BGP.




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  The examples given in this document are intended to distill
  deployments down to their most critical components, making the
  examples easier to understand and consider.  In many situations, the
  specific path taken in the example may not be relevant, but that does
  not nullify the principles considered in each example.  It has been
  suggested that these examples are "red herrings", because they do not
  illustrate actual problems with specific policies.  On the contrary,
  these examples are powerful because they are simple.  Any topology in
  which one of these example topologies is a subtopology will exhibit
  the characteristics explained in this document.  Rather than focusing
  on a specific topology, then dismissing that single topology as a
  "corner case", this document shows the basic issues with assertions
  about the AS Path attribute within BGP.  These generalized issues can
  then be applied to more specific cases.

  With the heightened interest in network security, the security of the
  information carried within routing systems running BGP, as described
  in [RFC4271], is being looked at with great interest.  While there
  are techniques available for securing the relationship between two
  devices exchanging routing protocol information, such as [BGP-MD5],
  these techniques do not ensure various aspects of the information
  carried within routing protocols are valid or authorized.

  The following small internetwork is used to examine the concepts of
  validity and authorization within this document, providing
  definitions used through the remainder of the document.

  10.1.1.0/24--(AS65000)---(AS65001)--(AS65002)

  Assume a BGP speaker in AS65002 has received an advertisement for
  10.1.1.0/24 from a BGP speaker in AS65001, with an AS Path of {65000,
  65001}.

1.1.  Is the Originating AS Authorized to Advertise Reachability to the
     Destination?

  The most obvious question the receiving BGP speaker can ask about
  this advertisement is whether or not the originating AS, in this case
  AS65000, is authorized to advertise the prefix contained within the
  advertisement, in this case 10.1.1.0/24.  Whether or not a BGP
  speaker receiving a route to 10.1.1.0/24 originating in AS65000 can
  verify that AS65000 is, indeed, authorized to advertise 10.1.1.0/24
  is outside the scope of this document.








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1.2.  Is the Path Contained in the Advertised Routing Information Valid?

  If a BGP speaker receives an advertisement from a peer outside the
  local autonomous system (AS), the peer sending the update has a path
  to the destination prefix in the update.  Specifically, are the
  autonomous systems within the internetwork connected in such a way
  that the receiver, following the AS Path listed in the BGP update
  itself, can reach the originating AS listed in the received AS Path?
  Within this document, this is called path validation.

  Path validation, in the context of this small internetwork, asserts
  that when a BGP speaker in AS65002 receives an advertisement from a
  BGP speaker in AS65001 with the AS Path {65000, 65001}, the speaker
  can assume that AS65001 is attached to the local AS, and that AS65001
  is also attached to AS65000.

1.3.  Is the Advertisement Authorized?

  There are at least three senses in which the readvertisement of a
  received advertisement can be authorized in BGP:

  o  The transmitter is authorized to advertise the specific routing
     information contained in the route.  This treats the routing
     information as a single, atomic unit, regardless of the
     information the route actually contains.  A route to 10.1.1.0/24
     and another route to 10.1.0.0/16 are considered completely
     different advertisements of routing information, so an AS may be
     authorized to advertise 10.1.0.0/16 without regard to its
     authorization to advertise 10.1.1.0/24, since these are two
     separate routes.  This is called route authorization throughout
     this document.

  o  The transmitter is authorized to advertise the specific reachable
     destination(s) contained in the route.  This treats the routing
     information as a set of destinations. 10.1.1.0/24 is contained
     within 10.1.0.0/16, and authorization to advertise the latter
     implies authorization to advertise the former.  This is called
     reachability authorization throughout this document.

  o  The transmitter is authorized to transit traffic to the
     destinations contained within the route.  This ties the concepts
     of the route to what the route is used for.  If a BGP speaker is
     advertising reachability to 10.1.1.0/24, it is authorized to
     transit traffic to all reachable destinations within 10.1.1.0/24
     along the path advertised.  This is called transit authorization
     throughout this document.





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  There is considerable tension between these three definitions of
  authorization; much of this document is built around exploring the
  relationships between these different types of authorization, and how
  they may, or may not, work in various internetworks.  One of the
  conclusions reached by this document is that route authorization,
  reachability authorization, and transit authorization are often at
  odds with each other.  Showing one type of authorization to be true
  does not show any other types of authorization to be true, and route
  authorization is of questionable value because of the intertwined
  nature of these three types of authorization in a routing system.

1.4.  Will Traffic Forwarded to an Advertising Speaker Follow the
     Described AS Path?

  If a BGP speaker receives an advertisement from a peer not in the
  local AS, will traffic forwarded to the peer advertising the update
  follow the path described in the AS Path?  In this document, this is
  called forwarding consistency.

  In terms of the small example internetwork, if a BGP speaker in
  AS65002 receives an advertisement from a peer in AS65001 for the
  destination 10.1.1.0/24, with an AS Path {65000, 65001}, will traffic
  forwarded to the BGP speaker in AS65001 actually be forwarded through
  routers within AS65001, then AS65000, to reach its destination?

1.5.  Is a Withdrawing Speaker Authorized to Withdraw the Routing
     Information?

  If an advertisement is withdrawn, the withdrawing BGP peer was
  originally advertising the prefix (or was authorized to advertise the
  prefix).  This assertion is out of the scope of this document.

2.  Analysis

  To begin, we review some of the concepts of routing, since we need to
  keep these concepts fixed firmly in place while we examine these
  questions.  After this, four examples will be undertaken with BGP to
  show the tension between the various types of authorization in a path
  vector routing system.

2.1.  A Short Analysis of Routing

  Routing protocols are designed, in short, to discover a set of
  loop-free paths to each reachable destination within a network (or
  internetwork).  The loop-free path chosen to reach a specific
  destination may not be the shortest path, and it may not always be





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  the "best" path (depending on the definition of "best"), but it
  should always be a loop-free path, otherwise the routing protocol has
  failed.

  This sheds some light on the purpose of the path included in a path
  vector protocol's routing update: the path is there to prove the path
  is loop free, rather than to provide any other information.  While
  Dijkstra's Sender Policy Framework (SPF) and the Diffusing Update
  Algorithm (DUAL) both base their loop-free path calculations on the
  cost of a path, path vector protocols, such as BGP, prove a path is
  loop free by carrying a list of nodes the advertisement itself has
  traversed.  BGP specifically uses an AS Path-based mechanism to prove
  loop freeness for any given update so each AS along the path may
  implement local policy without risking a loop in the routing system
  caused by competing metrics.

  We need to keep this principle in mind when considering the use of
  the path carried in a path-vector protocol, and its use by a
  receiving BGP speaker for setting policy or gauging the route's
  security level.

2.2.  First Example: Manual Intervention in the Path Choice

  In the small network:

                  +---(AS65002)---+
  (AS65000)--(AS65001)          (AS65004)--10.1.1.0/24
                  +---(AS65003)---+

  A BGP speaker in AS65000 may receive an advertisement from a peer
  that 10.1.1.0/24 is reachable along the path {65004, 65002, 65001}.
  Based on this information, the BGP speaker may forward packets to its
  peer in AS65001, expecting them to take the path described.  However,
  within AS65001, the network administrator may have configured a
  static route making the next hop to 10.1.1.0/24 the edge router with
  AS65003.

  It's useful to note that while [RFC4271] states: "....we assume that
  a BGP speaker advertises to its peers only those routes that it
  itself uses...", the definition of the term "use" is rather loose in
  all known widely deployed BGP implementations.  Rather than meaning:
  "A BGP speaker will only advertise destinations the BGP process on
  the speaker has installed in the routing table", it generally means:
  "A BGP speaker will only advertise destinations that the local
  routing table has a matching route installed for, no matter what
  process on the local router installed the route".  A naive reaction
  may be to simply change the BGP specifications and all existing
  implementations so a BGP speaker will only advertise a route to a



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  peer if the BGP process on the router actually installed the route in
  the local routing table.  This, however, ignores the complex
  interactions between interior and exterior gateway protocols, which
  most often run on the same device, and the complexities of route
  origination.

  Although this is an "extreme" example, since we can hardly claim the
  information within the routing protocol is actually insufficient, we
  still find this example instructive in light of the questions
  outlined in Section 1:

  o  Is the AS Path valid?  The AS Path the receiving BGP speaker in
     AS65000 receives from its peer in AS65001, {65004, 65002, 65001),
     does exist, and is valid.

  o  Is the advertisement authorized?  Since we have no knowledge of
     any autonomous system level policy within this network, we have no
     way of answering this question.  We can assume that AS65001 has
     both route and reachability authorization, but it is difficult to
     see how transit authorization can be accomplished in this
     situation.  Even if the BGP speaker in AS65000 could verify
     AS65001 is authorized to transit AS65002 to reach 10.1.1.0/24,
     this implies nothing about the authorization to transit traffic
     through the path traffic will actually take, which is through
     AS65003.

  o  Is the AS Path consistent with the forwarding path (does
     forwarding consistency exist)?  No, the advertised AS Path is
     {65004, 65002, 65001}, while the actual path is {65004, 65003,
     65001}.

  From this example, we can see forwarding consistency is not possible
  to validate in a deployed network; just because a BGP speaker
  advertises a specific path to reach a given destination, there is no
  reason to assume traffic forwarded to the speaker will actually
  follow the path advertised.  In fact, we can reason this from the
  nature of packet-forwarding networks; each router along a path makes
  a completely separate decision about where to forward received
  traffic.  Any router along the path could actually change the path
  due to network conditions without notifying, in any way, upstream
  routers.  Therefore, at any given time, an upstream router may be
  forwarding traffic along a path that no longer exists, or is no
  longer optimal, and downstream routers could be correcting the
  forwarding decision by placing the traffic on another available path
  unknown to the upstream router.

  As a corollary, we can see that authorizing transit through a
  specific AS Path is not possible, either.  If the source of a



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  specific flow cannot know what path the traffic within that flow will
  take to reach the destination, then there is no meaningful sense in
  which downstream routers can authorize the source to use available
  paths for transiting traffic.

2.3.  Second Example: An Unintended Reachable Destination

  In this internetwork, we assume a single policy: the system
  administrator of AS65000 would not like the destination 10.1.1.0/24
  to be reachable from any autonomous system beyond AS65001.  In other
  words, 10.1.1.0/24 should be reachable within AS65001, but not to
  autonomous systems connected to AS65001, such as AS65002.

  10.1.1.0/24---(AS65000)---(AS65001)---(AS65002)

  The system administrator can implement this policy by causing BGP
  speakers within AS65000 to advertise 10.1.1.0/24 to peers within
  AS65001 with a signal that the BGP speakers in AS65001 should not
  readvertise the reachability of this routing information.  For
  example, BGP speakers in AS65000 could advertise the route to
  10.1.1.0/24 with the NO_ADVERTISE community attached, as described in
  [RFC4271].  If the BGP speakers in AS65001 are configured to respond
  to this community (and we assume they are for the purposes of this
  document) correctly, they should accept this advertisement, but not
  readvertise reachability to 10.1.1.0/24 into AS65002.

  However, unknown to the system administrator of AS65000, AS65001 is
  actually advertising a default route to AS65002 with an AS Path of
  {65001}, and not a full routing table.  If some host within AS65002,
  then, originates a packet destined to 10.1.1.1, what will happen?
  The packet will be routed according to the default route from AS65002
  into AS65001.  Routers within AS65001 will forward the packet along
  the 10.1.1.0/24 route, eventually forwarding the traffic into
  AS65000.

  o  Is the AS Path valid?  This is a difficult question to answer,
     since there are actually two different advertisements in the
     example.  From the perspective of the BGP speaker in AS65002
     receiving a default route in an advertisement from a peer in
     AS65001, the AS Path to the default route is valid.  However,
     there is no route to 10.1.1.0/24 for the BGP speaker in AS65002 to
     examine for validity, so the question appears to be out of scope
     for this example.

  o Is the AS Path consistent with the forwarding path (is there
     forwarding consistency)?  From the perspective of a BGP speaker in
     AS65002, traffic forwarded to AS65001 towards a destination within
     10.1.1.0/24 is going to actually terminate within AS65001, since



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     that is the entire AS Path for the default route.  However, this
     traffic actually transits AS65001 towards AS65000.  Therefore,
     forwarding consistency does not exist in this example, in which we
     are dealing with a case of aggregation, and as Section 9.1.4 of
     [RFC4271], in reference to aggregated routing information, states:
     "Forwarding along such a route does not guarantee that IP packets
     will actually traverse only ASes listed in the AS_PATH attribute
     of the route".

2.3.1.  Advertisement Authorization

  Is the advertisement authorized?  This example higlights the tension
  between the three different types of authorization.  The three
  following sections discuss issues with different advertisements
  AS65001 may send to AS65002.

2.3.1.1.  Valid Unauthorized Aggregates

  The first issue that comes up in this example is the case where there
  is no expectation of authorization for aggregation.  The most common
  example of this is the advertising and accepting of the default route
  (0/0).  This is a common practice typically done by agreement between
  the two parties.  Obviously, there is not an authorization process
  for such an advertisement.  This advertisement may extend
  reachability beyond the originator's intention (along the lines of
  the previous example).  It may cause packets to take paths not known
  by the sender (since the path on 0/0 is simply the advertising AS).
  It may violate other security constraints.  This places limits on the
  power and applicability of efforts to secure the AS path and AS
  policies.  It does not vitiate the underlying value in such efforts.

2.3.1.2.  Owner Aggregation

  In the current instantiation of IP address allocation, an AS may
  receive authorization to advertise 10.1.0.0/16, for instance, and may
  authorize some other party to use (or own) 10.1.1.0/24, a subblock of
  the space authorized.  This is called a suballocation.

  For instance, in this example, if AS65001 were authorized to
  originate 10.1.0.0/16, it could advertise 10.1.0.0/16 towards
  AS65002, rather than a default route.  Assume there is some form of
  authorization mechanism AS65002 can consult to verify AS65001 is
  authorized to advertise 10.1.0.0/16.  In this case, AS65002 could
  examine the authorization of AS65001 to originate 10.1.0.0/16, and
  assume that if AS65002 is authorized to advertise 10.1.0.0/16, it is
  also authorized to transit traffic towards every possible subblock of
  (every destination within) 10.1.0.0/16.  To put this in more distinct
  terms:



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  o  AS65002 verifies route authorization by examining the
     authorization of AS65001 to advertise 10.1.0.0/16.

  o  AS65002 assumes destination authorization, that AS65001 has the
     authorization to advertise every possible subblock of 10.1.0.0/16,
     because AS65001 is authorized to advertise 10.1.0.0/16.

  o  AS65002 assumes transit authorization, that AS65001 has the
     authorization to transit traffic to every possible subblock of
     10.1.0.0/16, because AS65001 is authorized to advertise
     10.1.0.0/16.

  From the example given, however, it is obvious route authorization
  does not equal destination or transit authorization.  While AS65001
  does have route authorization to advertise 10.1.0.0/16, it does not
  have destination authorization to advertise 10.1.1.0/24, nor transit
  authorization for destinations with 10.1.1.0/24.

  The naive reply to this would be to simply state that destination and
  transit authorization should not be assumed from route authorization.
  However, the problem is not that simple to resolve.  The assumption
  of destination authorization and transit authorization are not
  decisions AS65002 actually makes; they are embedded in the way the
  routing system works.  The route itself, within the design of
  routing, carries these implications.

  Why does routing intertwine these three types of authorization?  Most
  simply, because AS65002 does not have any information about subblocks
  that are part of 10.1.0.0/16.  There is no way for AS65002 to check
  for destination and transit authorization because this information is
  removed from the system altogether.  In order to show destination and
  transit authorization, this information must be reinjected into the
  routing system in some way.

  For instance, considering destination authorization alone, it is
  possible to envision a system where AS65001, when suballocating part
  of 10.1.0.0/16 to the originator, must also obtain permission to
  continue advertising the original address block as an aggregate, to
  attempt to resolve this problem.  However, this raises some other
  issues:

  o  If the originator did not agree to AS65001 advertising an
     aggregate containing 10.1.1.0/24, then AS65001 would be forced to
     advertise some collection of advertisements not containing the
     suballocated block.

  o  If AS65001 actually does obtain permission to advertise the
     aggregate, we must find some way to provide AS65002 with



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     information about this authorization for each possible subblock of
     10.1.0.0/16.

  In other words, either AS65002 must receive the actual routes for
  each suballocation of 10.1.0.0/16, or it must receive some form of
  authorization allowing AS65001 to advertise each suballocation of
  10.1.0.0/16.  This appears to defeat the purpose of aggregation,
  rendering routing systems much less scalable than current design
  allows.  Further, this does not resolve the issue of how AS65002
  would actually know what all the suballocations of 10.1.0.0/16
  actually are.  Some possible solutions could be:

  o  The suballocator must advertise all suballocations.  This could
     potentially expose business relationships and patterns that many
     large commercial providers do not want to expose, and degrades the
     hierarchical nature of suballocation altogether.

  o  The IP address space must be reconstructed so everyone using IP
     address space will know, based on the construction of the IP
     address space, what subblocks exist.  For instance, the longest
     allowed subblock could be set at a /24, and authorization must be
     available for every possible /24 in the address space, either for
     origination, or as part of an aggregate.  This sort of solution
     would be an extreme burden on the routing system.

2.3.1.3.  Proxy Aggregation

  It is also possible for AS65001 to have some form of agreement with
  AS65002 to aggregate blocks of address space it does not own towards
  AS65002.  This might be done to reduce the burden on BGP speakers
  within AS65002.  This is called proxy aggregation.  While proxy
  aggregation is rare, it is useful to examine the result of agreed
  upon proxy aggregation in this situation.

  Assume AS65001 has an advertisement for 10.1.0.0/24 from some unknown
  source, and decides to advertise both 10.1.0.0/24 and 10.1.1.0/24 as
  10.1.0.0/23 to AS65002.  If there exists an agreement for AS65001 to
  advertise proxy aggregates to AS65002, the latter will accept the
  advertisement regardless of any attached authorization to advertise.
  This may represent a security risk for AS65002, but it might be seen
  as an acceptable risk considering the trade-offs, from AS65002's
  perspective.

  The problem is, however, this also impacts the policies of AS65000,
  which is originating one of the two routes being aggregated by
  AS65001.  There is no way for AS65002 to know about this policy, nor
  to react to it, and there is actually no incentive for AS65002 to
  react to a security threat posed to AS65000, which it has no direct



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  relationship with.  There doesn't appear to be any immediately
  available solution to this problem, other than to disallow proxy
  aggregation, even between two cooperating autonomous systems.

2.3.2.  Implications

  The basic problem is that AS65002 assumes when AS65001 advertises an
  authorized route containing 10.1.1.0/24, either through a valid
  unauthorized aggregate, an owner aggregated route, or a proxy
  aggregation, AS65001 also has destination authorization for the
  subblock 10.1.1.0/24, and transit authorization for destinations
  within 10.1.1.0/24.  These are, in fact, invalid assumptions, but
  they are tied to the way routing actually works.  This shows the
  value of route authorization is questionable, unless there is some
  way to untie destination and transit authorization from route
  advertisements, which are deeply intertwined today.

2.4.  Third Example: Following a Specific Path

  This example is slightly more complex than the last two.  Given the
  following small network, assume that A and D have a mutually agreed
  upon policy of not allowing traffic to transit B to reach
  destinations within 10.1.1.0/25.

  10.1.1.0/25--A---B---C---D
               |       |   |
               E-------F---G

  Assume the following:

  o  A advertises 10.1.1.0/25 to B, and 10.1.1.0/24 to E.

  o  B advertises 10.1.1.0/25 {B,A} to C.

  o  E advertises 10.1.1.0/24 {E,A} to F, filtering 10.1.1.0/25 {E,A}
     based on some local policy.

  o  F advertises 10.1.1.0/24 {F,E,A} to C and to G.

  o  C advertises 10.1.1.0/24 {C,F,E,A} to D, filtering 10.1.1.0/25
     {B,A} based on some local policy.

  o  G advertises 10.1.1.0/24 {G,F,E,A} to D.

  o  D chooses 10.1.1.0/24 {C,F,E,A} over 10.1.1.0/24 {G,F,E,A}.






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  What path will traffic forwarded to C destined to hosts within
  10.1.1.0/25 actually follow?

  o  D forwards this traffic to C, since its best path is through
     10.1.1.0/24 {C,F,E,A}.

  o  C forwards this traffic to B, since its best path is through
     10.1.1.0/25 {B,A}.

  o  B forwards this traffic to A, since its best path is through
     10.1.1.0/25 {A}.

  Considering this result:

  o  Is the AS Path valid?  Both {G, F, E, A} and {C, F, E, A} are
     valid AS Paths, so both AS Paths in this example are valid.

  o  Is the advertisement authorized?  Assuming A is authorized to
     advertise 10.1.1.0/24, and all the autonomous systems in the
     example are authorized to readvertise 10.1.1.0/24, the route is
     authorized.  However, C does not have destination nor transit
     authorization to 10.1.1.0/25, since B is the best path from C to
     10.1.1.0/25, and D and A have explicit policies not to transit
     this path.  In effect, then C does not have destination or transit
     authorization for 10.1.1.0/24, because it contains 10.1.1.0/25.

  o  Is the AS Path consistent with the forwarding path (is there
     forwarding consistency)?  While C is advertising the AS Path {C,
     F, E, A} to D to reach destinations within 10.1.1.0/24, it is
     actually forwarding along a different path to some destinations
     within this advertisement.  Forwarding consistency does not exist
     within this internetwork.

  In this example, A assumes that D will receive both the advertisement
  for 10.1.1.0/24 and the advertisement for 10.1.1.0/25, and will be
  able to use the included AS Path to impose their mutually agreed on
  policy not to transit B.  Information about 10.1.1.0/25, however, is
  removed from the routing system by policies outside the knowledge or
  control of A or D.  The information remaining in the routing system
  implies D may correctly route to destinations within 10.1.1.0/25
  through C, since 10.1.1.0/25 is contained within 10.1.1.0/24, which C
  is legally advertising.

  The tension between route authorization, destination authorization,
  and transit authorization can be seen clearly in this slightly more
  complex example.  Route authorization implies destination and transit
  authorization in routing, but route authorization does not include
  destination or prefix authorization in this example.



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2.5.  Fourth Example: Interior and Exterior Paths Mismatch

  This is the most complex example we will cover in this document.  It
  can be argued that the configuration described in this example is a
  misconfiguration, but we have chosen this example for its simplicity,
  as an illustration of the complexity of the interaction between
  interior and exterior gateway protocols within an autonomous system.
  BGP route reflectors, particularly when configured in a hierarchy,
  provide many examples of forwarding inconsistency, but they are much
  more complex than this small example.

   +-----F(9)---------------G(3)--------+
   |                         |          |
   |                  +------+          |
   |                  |                 |
   |        +---C(2)--+                 |
   |        |         |                 |
  A(1)-----B(2)       +----------------E(5)--10.1.1.0/24
   |        |         |                 |
   |        +---D(2)--+                 |
   |                                    |
   +------------------H(6)--J(7)--K(8)--+

  In this diagram, each router is labeled, with the AS in which it is
  contained, in parenthesis following the router label.  As its best
  path to 10.1.1.0/24:

     o  Router E is using its local (intra-AS) path.

     o  Router C is using the path through AS3.

     o  Router D is using the path through Router E.

     o  Router B is using the path through Router E.

  Examining the case of Router B more closely, however, we discover
  that while Router B prefers the path it has learned from Router E,
  that path has been advertised with a next hop of Router E itself.
  However, Router B's best path to this next hop (i.e., Router E), as
  determined by the interior routing protocol, is actually through
  Router C.  Thus, Router B advertises the path {2, 5} to Router A, but
  traffic actually follows the path {2, 3, 5} when Router B receives
  it.

  The system administrator of AS1 has determined there is an attacker
  in AS3, and has set the policy on router A to avoid any route with
  AS3 in the AS Path.  So, beginning with this rule, it discards the
  path learned from AS9.  It now examines the two remaining paths,



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RFC 5123             Path Validation Considerations        February 2008


  learned from AS2 (B) and AS6, and determines the best path is {2, 5},
  through AS2 (B).  However, unknown to A, AS2 (B) is also connected to
  AS3, and is transiting traffic to AS5 via the path {2, 3, 5}.

  Returning to our questions:

  o  Is the AS Path valid?  The AS Path {2, 3, 5} is a valid AS Path.

  o  Is the route authorized?  Assuming each AS along the path is
     authorized to originate, or readvertise, 10.1.1.0/24, the route is
     authorized.  Destination authorization is also clear in this
     situation, since 10.1.1.0/24 is the single destination throughout
     the example.  Transit authorization is a little more difficult to
     determine, since the traffic doesn't actually flow along the AS
     Path contained in the routing advertisement.  It's impossible to
     claim the AS Path {2,3,5} is a valid path from either the route
     originator or the traffic originator, since that AS Path is not
     the AS Path advertised.  Essentially, Router E assumes transit
     authorization from route authorization, when there is no way to
     determine that AS3 is actually authorized to transit traffic to
     10.1.1.0/24.

  o  Is the AS Path consistent with the forwarding path (is there
     forwarding consistency)?  The advertised AS Path is {2, 5}, while
     the traffic forwarded to the destination actually transits {2, 3,
     5}.  Forwarding is not consistent in this example.

3.  Summary

  The examples given in this document are not the only possible
  examples of forwarding that is inconsistent with the advertised AS
  Path; [ROUTINGLOGIC] also provides some examples, as well.
  [ASTRACEROUTE] provides some interesting background on the practical
  impact of forwarding that is inconsistent with the advertised AS
  Path, in the context of attempting to trace the actual path of
  packets through a large internetwork, running BGP as an exterior
  gateway protocol.

  Routing strongly intertwines the concepts of route authorization,
  destination authorization, and transit authorization.  If a BGP
  speaker is authorized to advertise a specific route, routing assumes
  that it is also authorized to advertise every possible subblock
  within the destination prefix, and the advertiser is authorized to
  transit packets to every destination within the route.  By surveying
  these examples, we see that route authorization does not, in fact,
  equal destination authorization or transit authorization, calling
  into question the value of route authorization.




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RFC 5123             Path Validation Considerations        February 2008


  There are no easy or obviously scalable solutions to this problem.

4.  Acknowledgements

  We would like to thank Steve Kent for his contributions and comments
  on this document.  We would also like to thank Joel Halpern for his
  work in clarifying many sections of the document, including
  additional text in critical sections.

5.  Security Considerations

  This document does not propose any new extensions or additions to
  existing or proposed protocols, and so does not impact the security
  of any protocol.

6.  Informative References

  [ASTRACEROUTE] Feamster, N. and H. Balakrishnan, "Towards an Accurate
                 ASLevel Traceroute Tool", SIGCOMM ACM SIGCOMM, 2003.

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

  [RFC4271]      Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
                 Border Gateway Protocol 4 (BGP-4)", RFC 4271, January
                 2006.

  [ROUTINGLOGIC] Feamster, N. and H. Balakrishnan, "Towards a Logic for
                 Wide Area Routing", SIGCOMM ACM SIGCOMM Worshop on
                 Future Directions in Network Architecture, Germany,
                 August 2003.

  [SOBGP]        White, R., "Architecture and Deployment Considerations
                 for Secure Origin BGP (soBGP)", Work in Progress.

Authors' Addresses

  Russ White
  Cisco Systems

  EMail: [email protected]


  Bora Akyol
  Cisco Systems

  EMail: [email protected]




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RFC 5123             Path Validation Considerations        February 2008


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