| Internet-Draft | ASPA-based AS_PATH Verification | March 2023 |
| Azimov, et al. | Expires 7 September 2023 | [Page] |
This document describes procedures that make use of Autonomous System Provider Authorization (ASPA) objects in the Resource Public Key Infrastructure (RPKI) to verify the Border Gateway Protocol (BGP) AS_PATH attribute of advertised routes. This type of AS_PATH verification provides detection and mitigation of route leaks and improbable AS paths. It also to some degree provides protection against forged-origin prefix hijacks.¶
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶
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The Border Gateway Protocol (BGP) as originally designed is known to be vulnerable to prefix (or route) hijacks and BGP route leaks [RFC7908]. Some existing BGP extensions are able to partially solve these problems. For example, Resource Public Key Infrastructure (RPKI) based route origin validation (RPKI-ROV) [RFC6480] [RFC6482] [RFC6811] [RFC9319] can be used to detect and filter accidental mis-originations. [RFC9234] or [I-D.ietf-grow-route-leak-detection-mitigation] can be used to detect and mitigate accidental route leaks. While RPKI-ROV can prevent accidental prefix hijacks, malicious forged-origin prefix hijacks can still occur [RFC9319]. RFC9319 includes some recommendations for reducing the attack surface for forged-origin prefix hijacks.¶
This document describes procedures that make use of Autonomous System Provider Authorization (ASPA) objects [I-D.ietf-sidrops-aspa-profile] in the RPKI to verify the BGP AS_PATH attribute of advertised routes. These new ASPA-based procedures automatically detect invalid AS_PATHs in announcements that are received from customers, lateral peers (defined in [RFC7908]), transit providers, IXP Route Servers (RS), RS-clients, and siblings. This type of AS_PATH verification provides detection and mitigation of route leaks and improbable AS paths. It also to some degree provides protection against forged-origin prefix hijacks. The protections provided by these procedures (together with RPKI-ROV) are based on cryptographic techniques, and they are effective against a majority of accidental and malicious actions.¶
ASPA objects are cryptographically signed registrations of customer-to-provider relationships and stored in a distributed database [I-D.ietf-sidrops-aspa-profile]. ASPA-based path verification is an incrementally deployable technique and provides benefits to early adopters in the context of limited deployment.¶
The procedures described in this document are applicable only for BGP routes with {AFI, SAFI} combinations {AFI 1 (IPv4), SAFI 1} and {AFI 2 (IPv6), SAFI 1} [IANA-AF]. SAFI 1 represents NLRI used for unicast forwarding [IANA-SAF].¶
For brevity, the term "provider" is often used instead of "transit provider" in this document; they mean the same.¶
Both route leaks and hijacks have similar effects on ISP operations - they redirect traffic which can result in denial of service (DoS), eavesdropping, increased latency, and packet loss. But the level of risk depends significantly on the extent of propagation of the anomalies. For example, a route leak or hijack that is propagated only to customers may cause bottlenecking within a particular ISP's customer cone, but if the anomaly propagates through lateral (i.e., non-transit) peers and transit providers, or reaches global distribution through transit-free networks, then the ill effects will likely be amplified and experienced across continents.¶
The ability to constrain propagation of BGP anomalies to transit providers and lateral peers - without requiring support from the source of the anomaly (which is critical if the source has malicious intent) - should significantly improve the robustness of the global inter-domain routing system.¶
For path verification purposes in this document, the BGP roles an AS can have in relation to a neighbor AS are customer, provider, lateral peer, RS, RS-client [RFC9234], and sibling. Sibling ASes export everything (both customer and non-customer routes) to each other, i.e., consider each other as a customer. Thus, the customer-to-provider relationship applies in each direction.¶
All roles are configured locally and used for registration of ASPA objects (Section 3, Section 4) and/or for path verification (Section 6). The procedure of BGP Role capability [RFC9234] in the BGP OPEN message to verify the role with a neighbor is RECOMMENDED. The procedure is not applied for verifying a sibling-to-sibling role since it is not specified in [RFC9234]. However, there is little concern about a pair of sibling ASes since they have a trusted relationship. In fact, they are neighbors typically managed by a single entity.¶
An ASPA record is a digitally signed object that binds a set of Provider AS numbers to a Customer AS (CAS) number (in terms of BGP announcements) and is signed by the CAS [I-D.ietf-sidrops-aspa-profile]. The CAS can choose to specify an AFI (i.e., afiLimit = 1 for IPv4 or 2 for IPv6) in the ASPA or it may omit it in which case the ASPA applies to both IPv4 and IPv6. The ASPA attests that the CAS has a Set of Provider ASes (SPAS) as specified in the ASPA. The definition of Provider AS in given in Section 1 of [I-D.ietf-sidrops-aspa-profile]. A function of a Provider AS is to propagate a CAS's route announcements onward, i.e., to the Provider's upstream providers, lateral peers, or customers. Another function is to offer outbound (customer to Internet) data traffic connectivity to the Customer. The ASPA object profile is described in [I-D.ietf-sidrops-aspa-profile].¶
In this document, the notation (AS1, [AS2,...], AFI), where the AFI (afiLimit) is optional, is used to represent the ASPA object for a CAS denoted as AS1. In this example, AS2 and any other ASes listed in the square brackets represent the transit provider ASes attested by the CAS (i.e., AS1). A CAS is expected to register a single ASPA listing all its Provider ASes (see Section 4). If a CAS has a single ASPA, then the SPAS for the CAS are the Provider ASes listed in that ASPA. In case a CAS has multiple ASPAs, then the SPAS is the union of the Provider ASes listed in all ASPAs of the CAS.¶
Verified ASPA Payload (VAP) refers to the payload in a cryptographically verified (i.e., X.509 valid [RFC3779] [RFC5280]) ASPA object. In the procedures for the AS path verification described in this document (Section 5, Section 6), SPAS refers to the set of providers derived from the VAP(s) of the CAS in consideration.¶
It is RECOMMENDED that the afiLimit parameter in the ASPA object be left unspecified (unless there is compelling reason to specify) so that the ASPA applies to both IPv4 and IPv6 prefixes. This gives the CAS significant flexibility, e.g., the need to scramble to modify the ASPA registrations can be averted when adding or moving IPv4 and IPv6 route announcements across different providers.¶
In the specifications that follow, it is assumed that the entity registering the ASPA is mindful of the above recommendation regarding the afiLimit.¶
An ASPA object showing only AS 0 as a provider AS is referred to as an AS0 ASPA. It is a statement by the registering AS that it has no transit providers. An ASPA that is not an AS 0 ASPA is referred to as a normal ASPA. As mentioned before, the set of provider ASes contained in the VAP(s) is referred to as the SPAS of the AS registering the ASPA(s). Any AS, including an RS AS, MUST have an ASPA. An AS SHOULD NOT have more than one ASPA.¶
If an AS does not have any transit providers and is also not an RS-client at a non-transparent Route Server AS (RS AS), then it MUST register an AS 0 ASPA. Otherwise, the AS MUST include in its SPAS, all its provider ASes and any non-transparent RS AS(es) at which it is an RS-client. An RS AS MUST register an AS 0 ASPA.¶
Each of the two ASes in a sibling pair MUST register its ASPA including the other AS in its SPAS.¶
The ASes on the boundary of an AS Confederation MUST register ASPAs using the Confederation's global ASN as the CAS.¶
As specified earlier, a compliant AS should maintain a single ASPA object that includes all its provider ASes. Such a practice will help prevent race conditions during ASPA updates that might affect prefix propagation. The software that provides hosting for ASPA records SHOULD support enforcement of this practice. During a transition process between different certificate authority (CA) registries, the ASPA records SHOULD be kept identical in all registries.¶
Let AS(i) and AS(j) represent adjacent unique ASes in an AS_PATH, and thus (AS(i), AS(j)) represents an AS hop. A hop-check function, hop(AS(i), AS(j), AFI), checks if the ordered pair of ASNs, (AS(i), AS(j)), has the property that AS(j) is an attested provider of AS(i) per SPAS corresponding to AS(i)'s ASPA for the specified AFI. This function is specified as follows:¶
/
| "Provider" if AS(i)'s SPAS
| includes AS(j) for mentioned AFI
|
hop(AS(i), AS(j), AFI) = / "Not Provider" if AS(i)'s SPAS does
\ not include AS(j) for mentioned AFI
|
| "no Attestation" if AS(i)
| does not have ASPA (i.e., VAP)
| for mentioned AFI
\
The hop-check function is AFI dependent because an AS may have different SPAS for different AFI. This function is used in the ASPA-based AS_PATH verification algorithms described in Section 6.1 and Section 6.2. For simplicity while describing the algorithms, the function hop(AS(i), AS(j), AFI) is replaced with hop(AS(i), AS(j)) by dropping the AFI since it is understood that the algorithms are run for a specific AFI at a time (AFI = 1 or AFI = 2).¶
The procedures described in this document are applicable only to four-octet AS number compatible BGP speakers [RFC6793]. If such a BGP speaker receives both AS_PATH and AS4_PATH attributes in an UPDATE, then the procedures are applied on the reconstructed AS path (Section 4.2.3 of [RFC6793]). So, the term AS_PATH is used in this document to refer to the usual AS_PATH [RFC4271] as well as the reconstructed AS path.¶
If an attacker creates a route leak intentionally, they may try to strip their AS from the AS_PATH. To partly guard against that, a check is necessary to match the most-recently added AS in the AS_PATH to the BGP neighbor's ASN. This check MUST be performed as specified in Section 6.3 of [RFC4271]. If the check fails, then the AS_PATH is considered a Malformed AS_PATH and the UPDATE is considered to be in error (Section 6.3 of [RFC4271]). The case of transparent RS MUST also be appropriately taken care of (e.g., by suspending the neighbor ASN check). The check fails also when the AS_PATH is empty (zero length) and such UPDATEs will also be considered to be in error.¶
[I-D.ietf-idr-deprecate-as-set-confed-set] specifies that "treat-as-withdraw" error handling SHOULD be applied to routes with AS_SET in the AS_PATH. In this document, routes with AS_SET are given Invalid evaluation in the AS_PATH verification procedures (Section 6.1 and Section 6.2).¶
Wherever AFI is mentioned in the AS_PATH verification algorithms, it refers to the AFI of the prefix in the route for which the AS_PATH verification is performed. When an AS_PATH is evaluated as Valid, Invalid, or Unknown, it pertains only to the AFI for which the verification was performed. The same AS_PATH can have a different verification outcome for a different AFI. Since it is understood that the algorithms described here are run for a single AFI at a time (pertaining to the route(s) being verified), the AFI in the function hop(AS(i), AS(j), AFI) is not shown explicitly for simplicity for the reader.¶
The upstream verification algorithm described here is applied when a route is received from a customer or lateral peer, or received by an RS from an RS-client, or received by an RS-client from an RS. In all these cases, the receiving/validating AS expects the AS_PATH to consist of only customer-to-provider hops successively from the origin AS to the neighbor AS (most-recently added).¶
The basic principles of the upstream verification algorithm are stated here. Let the sequence {AS(N), AS(N-1),..., AS(2), AS(1)} represent the AS_PATH in terms of unique ASNs, where AS(1) is the origin AS and AS(N) is the most-recently added AS and neighbor of the receiving/validating AS. For each hop AS(i-1) to AS(i)in this sequence, the hop-check function, hop(AS(i-1), AS(i)), must equal "Provider" (Section 5) for the AS_PATH to be Valid . If the hop-check function for at least one of those hops is "Not-Provider", then the AS_PATH would be Invalid. If the AS_PATH verification outcome is neither Valid nor Invalid (per above principles), then it would be evaluated as Unknown.¶
The upstream path verification procedure is specified as follows:¶
The downstream verification algorithm described here is applied when a route is received from a transit provider or sibling AS. As described in Section 4, a sending sibling AS acts towards its receiving sibling AS in a manner similar to that of a provider towards its customer. So, the downstream verification algorithm applies also in the case when the receiving AS has the role of a sibling AS.¶
It is not essential, but the reader may take a look at the illustrations and formal proof in [sriram1] to develop a clearer understanding of the algorithm described here.¶
Here again (as in Section 6.1), let the AS_PATH be simplified and represented by the ordered sequence of unique ASNs as {AS(N), AS(N-1),..., AS(2), AS(1)}.¶
If 1 <= N <= 2, then the AS_PATH is trivially Valid.¶
In the following descriptions regarding determination of Invalid, Valid, or Unknown, it is assumed that the AS_PATH contains 3 or more unique ASNs (N >= 3).¶
Determination of Invalid AS_PATH:¶
Given the above ordered sequence, if there exist indices u and v such that (1) u <= v, (2) hop(AS(u-1), AS(u)) = "not Provider", and (3) hop(AS(v+1), AS(v)) = "not Provider", then the AS_PATH is Invalid.¶
Determination of Valid AS_PATH:¶
As shown in Figure 2, assume that the ASes in the AS_PATH are in the same physical (locational) order as in the sequence representation {AS(N), AS(N-1),..., AS(2), AS(1)}, i.e., AS(N) is the left most and AS(1) the right most.¶
AS(L) ............. AS(K)
/ \
AS(L+1) AS(K-1)
. .
. .
(down-ramp) . . (up-ramp)
. .
. .
AS(N-1) AS(2)
/ \
AS(N) AS(1)
/ (Origin AS)
Receiving & Validating AS
Each ramp has consecutive ASPA-attested
customer-to-provider hops in the bottom-to-top direction
Looking at Figure 2, the UPDATE is received from a provider or sibling (i.e., AS(N) is a provider or sibling). The AS_PATH may have both an up-ramp (on the right starting at AS(1)) and a down-ramp (on the left starting at AS(N)). An explanation of what the ramps mean is as follows. The sequence of ASes in either of these ramps consists of consecutive customer-to-provider hops. The up-ramp starts at AS(1) and each AS hop, (AS(i), AS(i+1)), in it has the property that hop(AS(i), AS(i+1)) = "Provider" for i = 1, 2,... , K-1. If such a K does not exist, then K is set to 1. The up-ramp ends (reaches its apex) at AS(K) because hop(AS(K), AS(K+1)) = "Not Provider" or "no Attestation". The down-ramp runs backwards from AS(N) to AS(L). Each AS hop, (AS(j), AS(j-1)), in it has the property that hop(AS(j), AS(j-1)) = "Provider" for j = N, N-1,... , L+1. If such an L does not exist, then L is set to N. The down-ramp ends at AS(L) because hop(AS(L), AS(L-1)) = "Not Provider" or "no Attestation". Thus, the apex of the down-ramp is AS(L).¶
If there is an up-ramp that runs across all ASes in the AS_PATH (i.e., K = N), then clearly the AS_PATH is Valid. Similarly, if there is a down-ramp that runs across all ASes in the AS_PATH (i.e., L = 1), then also the AS_PATH is Valid. However, if both ramps exist in an AS_PATH with K < N and L > 1, then the AS_PATH is Valid if and only if L-K <= 1. Note that K could be greater than L (i.e., L-K has a negative value) which means that the up-ramp and down-ramp overlap, and that could happen when some adjacent AS pairs in the AS_PATH have mutually registered sibling relationships (i.e., include each other in their respective SPAS) (see Section 4). If L-K = 0, it means that the apexes of the up-ramp and down-ramp are at the same AS. If L-K = 1, it means that the apexes are at adjacent ASes. In summary, the AS_PATH is Valid if L-K is 0 or 1 or has a negative value.¶
Determination of Unknown AS_PATH:¶
If L-K >= 2, then the AS_PATH is either Invalid (route leak) or Unknown (see illustrations and proof in [sriram1]). However, if L-K >= 2 and an Invalid outcome was not found by the process described earlier in this section, then the AS_PATH is determined to be Unknown.¶
The downstream path verification procedure is formally specified as follows:¶
In the above procedure, the computations in Steps 4, 5, and 6 can be done at the same time.¶
Conforming implementations of this specification are not required to implement the AS_PATH verification procedures (step-wise lists) exactly as described in Section 6.1 and Section 6.2, but MUST provide functionality equivalent to the external behavior resulting from those procedures. In other words, the algorithms used in a specific implementation may differ, for example, for computational efficiency purposes but the AS_PATH verification outcomes MUST be identical to those obtained by the procedures described in Section 6.1 and Section 6.2.¶
The above applies to eBGP routers in general including those on the boundary of an AS Confederation facing external ASes. However, the procedures for ASPA-based AS_PATH verification in this document are NOT RECOMMENDED for use on eBGP links internal to the Confederation.¶
The procedures described in this document MUST be applied to BGP routes with {AFI, SAFI} combinations {AFI 1 (IPv4), SAFI 1} and {AFI 2 (IPv6), SAFI 1} [IANA-AF]. The procedures MUST NOT be applied to other address families by default.¶
If the AS_PATH is determined to be Invalid based on the verification procedures specified above (Section 6), then the route MUST be rejected.¶
The ASPA-based path verification procedures are able to check routes received from customers, lateral peers, transit providers, RSes, RS-clients, and siblings. These procedures combined with BGP Roles [RFC9234] and RPKI-ROV [RFC6811] [RFC9319] can provide a fully automated solution to detect and filter hijacks and route leaks, including malicious ones. Malicious AS_PATH modifications are also detected but not all (see discussion and examples in Section 12). The procedures also detect and mitigate a majority of forged-origin hijacks, especially those in UPDATEs received from customers and lateral peers. Forged-origin hijacks received from providers are detected and mitigated to a lesser degree (see Section 12).¶
If any two ISP ASes register ASPAs and implement the procedures, then any route received from one of them and leaked to the other by a common customer AS (ASPA compliant or not) will be detected and mitigated. In effect, if most major ISPs are compliant, the propagation of route leaks in the Internet will be severely limited.¶
The procedures specified in this document are compatible only with BGP implementations that support 4-byte ASNs in the AS_PATH. This limitation should not have a real effect on operations since legacy BGP routers are rare, and it is highly unlikely that they support integration with the RPKI.¶
ASPA issuers should be aware of the implications of the ASPA-based AS path verification. Network operators must keep their ASPA objects correct and up to date. Otherwise, for example, if a provider AS is left out of the Set of Provider ASes (SPAS) in the ASPA, then routes containing the CAS (in the ASPA) and said provider AS may be incorrectly labeled as route leaks and rejected.¶
BGPsec [RFC8205] was designed to solve the problem of AS_PATH verification by including cryptographic signatures in BGP Update messages. It offers protection against unauthorized path modifications and assures that the BGPsec Update actually traveled the path shown in the BGPsec_PATH Attribute. However, it does not detect route leaks (valley-free violations). In comparison, the ASPA-based path verification described in this document detects if the AS path is improbable and focuses on detecting route leaks (including malicious cases) and forged-origin hijacks.¶
BGPsec and ASPA are complementary technologies.¶
The Peerlock mechanism [Peerlock] [Flexsealing] has a similar objective as the APSA-based route leak protection mechanism described in this document. It is commonly deployed by large Internet carriers to protect each other from route leaks. Peerlock depends on a laborious manual process in which operators coordinate the distribution of unstructured Provider Authorizations through out-of-band means in a many-to-many fashion. On the other hand, ASPA's use of the RPKI allows for automated, scalable, and ubiquitous deployment, making the protection mechanism available to a wider range of network operators.¶
The ASPA mechanism implemented in router code (in contrast to Peerlock's AS_PATH regular expressions) also provides a way to detect anomalies propagated from transit providers and IX route servers. ASPA is intended to be a complete solution and replacement for existing Peerlock deployments.¶
This document includes no request to IANA.¶
While the ASPA-based mechanism is able to detect and mitigate the majority of mistakes and malicious activity affecting routes, it might fail to detect some malicious path modifications, especially for routes that are received from transit providers.¶
Since an upstream provider becomes a trusted point, in theory it might be able to propagate without detection some instances of forged-origin hijacked prefixes or routes with manipulated AS_PATHs. This can be illustrated with some examples. In Figure 3, normally the receiving/validating AS located at the lower left side should receive a route with AS_PATH {AS(5), AS(4), AS(3), AS(2), AS(1)} and it would be Valid (Section 6.2) given all the ASPAs that are shown in the figure. However, if AS(5) which is a transit provider to the validating AS acts maliciously and sends the route with a shortened AS_PATH such as {{AS(5), AS(3), AS(2), AS(1)} or {AS(5), AS(2), AS(1)}, such path manipulation would be undetectable (i.e., the AS_PATH would be considered Valid). Also, if AS(5) were to perform a forged-origin hijack by inserting an AS_PATH {{AS(5), AS(1)}, that would also be undetectable.¶
AS(4) - AS(3)
/ \
(down-ramp) / \ (up-ramp)
AS(5) AS(2)
/ \
/ AS(1)
/ (Origin AS)
Receiving & Validating AS
ASPAs: {AS(1), [AS(2)]}, {AS(2), [AS(3)]}, {AS(5), [AS(4)]},
{AS(3), [AS 0]}, {AS(4), [AS 0]}
While attacks like the examples above may happen in theory, it does not seem to be a realistic scenario. Normally a customer and its transit provider would have a signed agreement, and a policy violation (of the above kind) should have legal consequences or the customer can just drop the relationship with such a provider and remove the corresponding ASPA record.¶
This section is to be removed before publishing as an RFC.¶
This section records the status of known implementations of the protocol defined by this specification at the time of posting of this Internet-Draft. The inclusion of this section here follows the process described in [RFC7942]. The description of implementations in this section is intended to assist the IETF in its decision processes in progressing drafts to RFCs. Please note that the listing of any individual implementation here does not imply endorsement by the IETF. Furthermore, no effort has been spent to verify the information presented here that was supplied by IETF contributors. This is not intended as, and must not be construed to be, a catalog of available implementations or their features. Readers are advised to note that other implementations may exist.¶
According to [RFC7942], "this will allow reviewers and working groups to assign due consideration to documents that have the benefit of running code, which may serve as evidence of valuable experimentation and feedback that have made the implemented protocols more mature. It is up to the individual working groups to use this information as they see fit".¶
The authors wish to thank Claudio Jeker, Jakob Heitz, Ben Maddison, Russ Housley, Jeff Haas, Nan Geng, Nick Hilliard, Shunwan Zhuang, Doug Montgomery, Rich Compton, Andrei Robachevsky, and Iljitsch van Beijnum for comments, suggestions, and discussion on the path verification procedures or the text in the document. For the implementation and testing of the procedures in the document, the authors wish to thank Claudio Jeker and Theo Buehler [bgpd] as well as Kyehwan Lee and Oliver Borchert [BGP-SRx].¶