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Network Working Group                                           R. WhiteRequest for Comments: 5123                                      B. AkyolCategory: Informational                                    Cisco Systems                                                           February 2008Considerations in Validating the Path in BGPStatus 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.  SeeRFC 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.White & Akyol                Informational                      [Page 1]

RFC 5123             Path Validation Considerations        February 2008   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.White & Akyol                Informational                      [Page 2]

RFC 5123             Path Validation Considerations        February 20081.2.  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.White & Akyol                Informational                      [Page 3]

RFC 5123             Path Validation Considerations        February 2008   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 beWhite & Akyol                Informational                      [Page 4]

RFC 5123             Path Validation Considerations        February 2008   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 aWhite & Akyol                Informational                      [Page 5]

RFC 5123             Path Validation Considerations        February 2008   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 inSection 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 aWhite & Akyol                Informational                      [Page 6]

RFC 5123             Path Validation Considerations        February 2008   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, sinceWhite & Akyol                Informational                      [Page 7]

RFC 5123             Path Validation Considerations        February 2008      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 asSection 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:White & Akyol                Informational                      [Page 8]

RFC 5123             Path Validation Considerations        February 2008   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 withWhite & Akyol                Informational                      [Page 9]

RFC 5123             Path Validation Considerations        February 2008      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 directWhite & Akyol                Informational                     [Page 10]

RFC 5123             Path Validation Considerations        February 2008   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}.White & Akyol                Informational                     [Page 11]

RFC 5123             Path Validation Considerations        February 2008   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.White & Akyol                Informational                     [Page 12]

RFC 5123             Path Validation Considerations        February 20082.5.  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,White & Akyol                Informational                     [Page 13]

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.White & Akyol                Informational                     [Page 14]

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: riw@cisco.com   Bora Akyol   Cisco Systems   EMail: bora@cisco.comWhite & Akyol                Informational                     [Page 15]

RFC 5123             Path Validation Considerations        February 2008Full Copyright Statement   Copyright (C) The IETF Trust (2008).   This document is subject to the rights, licenses and restrictions   contained inBCP 78 and at www.rfc-editor.org/copyright.html, and   except as set forth therein, the authors retain all their rights.   This document and the information contained herein are provided on an   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND   THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS   OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF   THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Intellectual Property   The IETF takes no position regarding the validity or scope of any   Intellectual Property Rights or other rights that might be claimed to   pertain to the implementation or use of the technology described in   this document or the extent to which any license under such rights   might or might not be available; nor does it represent that it has   made any independent effort to identify any such rights.  Information   on the procedures with respect to rights in RFC documents can be   found inBCP 78 andBCP 79.   Copies of IPR disclosures made to the IETF Secretariat and any   assurances of licenses to be made available, or the result of an   attempt made to obtain a general license or permission for the use of   such proprietary rights by implementers or users of this   specification can be obtained from the IETF on-line IPR repository athttp://www.ietf.org/ipr.   The IETF invites any interested party to bring to its attention any   copyrights, patents or patent applications, or other proprietary   rights that may cover technology that may be required to implement   this standard.  Please address the information to the IETF at   ietf-ipr@ietf.org.White & Akyol                Informational                     [Page 16]