RFC 8205
BGPsec Protocol Specification
4. BGPsec UPDATE Messages
Section 4.1 provides general guidance on the creation of BGPsec UPDATE messages -- that is, UPDATE messages containing the BGPsec_PATH attribute. Section 4.2 specifies how a BGPsec speaker generates the BGPsec_PATH attribute to include in a BGPsec UPDATE message. Section 4.3 contains special processing instructions for members of an AS confederation [RFC5065]. A BGPsec speaker that is not a member of such a confederation MUST NOT set the Confed_Segment flag in its Secure_Path Segment (i.e., leave the Confed_Segment flag at the default value of 0) in all BGPsec UPDATE messages it sends. Section 4.4 contains instructions for reconstructing the AS_PATH attribute in cases where a BGPsec speaker receives an UPDATE message with a BGPsec_PATH attribute and wishes to propagate the UPDATE message to a peer who does not support BGPsec.4.1. General Guidance
The information protected by the signature on a BGPsec UPDATE message includes the AS number of the peer to whom the UPDATE message is being sent. Therefore, if a BGPsec speaker wishes to send a BGPsec UPDATE message to multiple BGP peers, it MUST generate a separate BGPsec UPDATE message for each unique peer AS to whom the UPDATE message is sent. A BGPsec UPDATE message MUST advertise a route to only a single prefix. This is because a BGPsec speaker receiving an UPDATE message with multiple prefixes would be unable to construct a valid BGPsec UPDATE message (i.e., valid path signatures) containing a subset of the prefixes in the received update. If a BGPsec speaker wishes to advertise routes to multiple prefixes, then it MUST generate a separate BGPsec UPDATE message for each prefix. Additionally, a BGPsec UPDATE message MUST use the MP_REACH_NLRI attribute [RFC4760] to encode the prefix.
The BGPsec_PATH attribute and the AS_PATH attribute are mutually exclusive. That is, any UPDATE message containing the BGPsec_PATH attribute MUST NOT contain the AS_PATH attribute. The information that would be contained in the AS_PATH attribute is instead conveyed in the Secure_Path portion of the BGPsec_PATH attribute. In order to create or add a new signature to a BGPsec UPDATE message with a given algorithm suite, the BGPsec speaker MUST possess a private key suitable for generating signatures for this algorithm suite. Additionally, this private key must correspond to the public key in a valid RPKI end entity certificate whose AS number resource extension includes the BGPsec speaker's AS number [RFC8209]. Note also that new signatures are only added to a BGPsec UPDATE message when a BGPsec speaker is generating an UPDATE message to send to an external peer (i.e., when the AS number of the peer is not equal to the BGPsec speaker's own AS number). The RPKI enables the legitimate holder of IP address prefix(es) to issue a signed object, called a Route Origin Authorization (ROA), that authorizes a given AS to originate routes to a given set of prefixes (see RFC 6482 [RFC6482]). It is expected that most Relying Parties (RPs) will utilize BGPsec in tandem with origin validation (see RFC 6483 [RFC6483] and RFC 6811 [RFC6811]). Therefore, it is RECOMMENDED that a BGPsec speaker only originate a BGPsec UPDATE message advertising a route for a given prefix if there exists a valid ROA authorizing the BGPsec speaker's AS to originate routes to this prefix. If a BGPsec router has received only a non-BGPsec UPDATE message containing the AS_PATH attribute (instead of the BGPsec_PATH attribute) from a peer for a given prefix, then it MUST NOT attach a BGPsec_PATH attribute when it propagates the UPDATE message. (Note that a BGPsec router may also receive a non-BGPsec UPDATE message from an internal peer without the AS_PATH attribute, i.e., with just the Network Layer Reachability Information (NLRI) in it. In that case, the prefix is originating from that AS, and if it is selected for advertisement, the BGPsec speaker SHOULD attach a BGPsec_PATH attribute and send a signed route (for that prefix) to its external BGPsec-speaking peers.) Conversely, if a BGPsec router has received a BGPsec UPDATE message (with the BGPsec_PATH attribute) from a peer for a given prefix and it chooses to propagate that peer's route for the prefix, then it SHOULD propagate the route as a BGPsec UPDATE message containing the BGPsec_PATH attribute.
Note that removing BGPsec signatures (i.e., propagating a route advertisement without the BGPsec_PATH attribute) has significant security ramifications. (See Section 8 for a discussion of the security ramifications of removing BGPsec signatures.) Therefore, when a route advertisement is received via a BGPsec UPDATE message, propagating the route advertisement without the BGPsec_PATH attribute is NOT RECOMMENDED, unless the message is sent to a peer that did not advertise the capability to receive BGPsec UPDATE messages (see Section 4.4). Furthermore, note that when a BGPsec speaker propagates a route advertisement with the BGPsec_PATH attribute, it is not attesting to the validation state of the UPDATE message it received. (See Section 8 for more discussion of the security semantics of BGPsec signatures.) If the BGPsec speaker is producing an UPDATE message that would, in the absence of BGPsec, contain an AS_SET (e.g., the BGPsec speaker is performing proxy aggregation), then the BGPsec speaker MUST NOT include the BGPsec_PATH attribute. In such a case, the BGPsec speaker MUST remove any existing BGPsec_PATH in the received advertisement(s) for this prefix and produce a traditional (non-BGPsec) UPDATE message. It should be noted that BCP 172 [RFC6472] recommends against the use of AS_SET and AS_CONFED_SET in the AS_PATH of BGP UPDATE messages. The case where the BGPsec speaker sends a BGPsec UPDATE message to an iBGP (internal BGP) peer is quite simple. When originating a new route advertisement and sending it to a BGPsec-capable iBGP peer, the BGPsec speaker omits the BGPsec_PATH attribute. When originating a new route advertisement and sending it to a non-BGPsec iBGP peer, the BGPsec speaker includes an empty AS_PATH attribute in the UPDATE message. (An empty AS_PATH attribute is one whose length field contains the value 0 [RFC4271].) When a BGPsec speaker chooses to forward a BGPsec UPDATE message to an iBGP peer, the BGPsec_PATH attribute SHOULD NOT be removed, unless the peer doesn't support BGPsec. In the case when an iBGP peer doesn't support BGPsec, then a BGP UPDATE message with AS_PATH is reconstructed from the BGPsec UPDATE message and then forwarded (see Section 4.4). In particular, when forwarding to a BGPsec-capable iBGP (or eBGP) peer, the BGPsec_PATH attribute SHOULD NOT be removed even in the case where the BGPsec UPDATE message has not been successfully validated. (See Section 5 for more information on validation and Section 8 for the security ramifications of removing BGPsec signatures.)
All BGPsec UPDATE messages MUST conform to BGP's maximum message size. If the resulting message exceeds the maximum message size, then the guidelines in Section 9.2 of RFC 4271 [RFC4271] MUST be followed.4.2. Constructing the BGPsec_PATH Attribute
When a BGPsec speaker receives a BGPsec UPDATE message containing a BGPsec_PATH attribute (with one or more signatures) from an (internal or external) peer, it may choose to propagate the route advertisement by sending it to its other (internal or external) peers. When sending the route advertisement to an internal BGPsec-speaking peer, the BGPsec_PATH attribute SHALL NOT be modified. When sending the route advertisement to an external BGPsec-speaking peer, the following procedures are used to form or update the BGPsec_PATH attribute. To generate the BGPsec_PATH attribute on the outgoing UPDATE message, the BGPsec speaker first generates a new Secure_Path Segment. Note that if the BGPsec speaker is not the origin AS and there is an existing BGPsec_PATH attribute, then the BGPsec speaker prepends its new Secure_Path Segment (places in first position) onto the existing Secure_Path. The AS number in this Secure_Path Segment MUST match the AS number in the Subject field of the RPKI router certificate that will be used to verify the digital signature constructed by this BGPsec speaker (see Section 3.1.1 in [RFC8209] and RFC 6487 [RFC6487]). The pCount field of the Secure_Path Segment is typically set to the value 1. However, a BGPsec speaker may set the pCount field to a value greater than 1. Setting the pCount field to a value greater than 1 has the same semantics as repeating an AS number multiple times in the AS_PATH of a non-BGPsec UPDATE message (e.g., for traffic engineering purposes). To prevent unnecessary processing load in the validation of BGPsec signatures, a BGPsec speaker SHOULD NOT produce multiple consecutive Secure_Path Segments with the same AS number. This means that to achieve the semantics of prepending the same AS number k times, a BGPsec speaker SHOULD produce a single Secure_Path Segment -- with a pCount of k -- and a single corresponding Signature Segment. A route server that participates in the BGP control plane but does not act as a transit AS in the data plane may choose to set pCount to 0. This option enables the route server to participate in BGPsec and obtain the associated security guarantees without increasing the length of the AS path. (Note that BGPsec speakers
compute the length of the AS path by summing the pCount values in the BGPsec_PATH attribute; see Section 5.) However, when a route server sets the pCount value to 0, it still inserts its AS number into the Secure_Path Segment, as this information is needed to validate the signature added by the route server. See [RFC8206] for a discussion of setting pCount to 0 to facilitate AS Number migration. Also, see Section 4.3 for the use of pCount=0 in the context of an AS confederation. See Section 7.2 for operational guidance for configuring a BGPsec router for setting pCount=0 and/or accepting pCount=0 from a peer. Next, the BGPsec speaker generates one or two Signature_Blocks. Typically, a BGPsec speaker will use only a single algorithm suite and thus create only a single Signature_Block in the BGPsec_PATH attribute. However, to ensure backwards compatibility during a period of transition from a 'current' algorithm suite to a 'new' algorithm suite, it will be necessary to originate UPDATE messages that contain a Signature_Block for both the 'current' and the 'new' algorithm suites (see Section 6.1). If the received BGPsec UPDATE message contains two Signature_Blocks and the BGPsec speaker supports both of the corresponding algorithm suites, then the new UPDATE message generated by the BGPsec speaker MUST include both of the Signature_Blocks. If the received BGPsec UPDATE message contains two Signature_Blocks and the BGPsec speaker only supports one of the two corresponding algorithm suites, then the BGPsec speaker MUST remove the Signature_Block corresponding to the algorithm suite that it does not understand. If the BGPsec speaker does not support the algorithm suites in any of the Signature_Blocks contained in the received UPDATE message, then the BGPsec speaker MUST NOT propagate the route advertisement with the BGPsec_PATH attribute. (That is, if it chooses to propagate this route advertisement at all, it MUST do so as an unsigned BGP UPDATE message. See Section 4.4 for more information on converting to an unsigned BGP UPDATE message.) Note that in the case where the BGPsec_PATH has two Signature_Blocks (corresponding to different algorithm suites), the validation algorithm (see Section 5.2) deems a BGPsec UPDATE message to be 'Valid' if there is at least one supported algorithm suite (and corresponding Signature_Block) that is deemed 'Valid'. This means that a 'Valid' BGPsec UPDATE message may contain a Signature_Block that is not deemed 'Valid' (e.g., contains signatures that BGPsec does not successfully verify). Nonetheless, such Signature_Blocks MUST NOT be removed. (See Section 8 for a discussion of the security ramifications of this design choice.)
For each Signature_Block corresponding to an algorithm suite that the BGPsec speaker does support, the BGPsec speaker MUST add a new Signature Segment to the Signature_Block. This Signature Segment is prepended to the list of Signature Segments (placed in the first position) so that the list of Signature Segments appears in the same order as the corresponding Secure_Path Segments. The BGPsec speaker populates the fields of this new Signature Segment as follows. The Subject Key Identifier field in the new segment is populated with the identifier contained in the Subject Key Identifier extension of the RPKI router certificate corresponding to the BGPsec speaker [RFC8209]. This Subject Key Identifier will be used by recipients of the route advertisement to identify the proper certificate to use in verifying the signature. The Signature field in the new segment contains a digital signature that binds the prefix and BGPsec_PATH attribute to the RPKI router certificate corresponding to the BGPsec speaker. The digital signature is computed as follows: o For clarity, let us number the Secure_Path and corresponding Signature Segments from 1 to N, as follows. Let Secure_Path Segment 1 and Signature Segment 1 be the segments produced by the origin AS. Let Secure_Path Segment 2 and Signature Segment 2 be the segments added by the next AS after the origin. Continue this method of numbering, and ultimately let Secure_Path Segment N and Signature Segment N be those that are being added by the current AS. The current AS (Nth AS) is signing and forwarding the UPDATE message to the next AS (i.e., the (N+1)th AS) in the chain of ASes that form the AS path. o In order to construct the digital signature for Signature Segment N (the Signature Segment being produced by the current AS), first construct the sequence of octets to be hashed as shown in Figure 8. This sequence of octets includes all the data that the Nth AS attests to by adding its digital signature in the UPDATE message that is being forwarded to a BGPsec speaker in the (N+1)th AS. (For the design rationale for choosing the specific structure in Figure 8, please see [Borchert].)
+------------------------------------+ | Target AS Number | +------------------------------------+----\ | Signature Segment : N-1 | \ +------------------------------------+ | | Secure_Path Segment : N | | +------------------------------------+ \ ... > Data from +------------------------------------+ / N Segments | Signature Segment : 1 | | +------------------------------------+ | | Secure_Path Segment : 2 | | +------------------------------------+ / | Secure_Path Segment : 1 | / +------------------------------------+---/ | Algorithm Suite Identifier | +------------------------------------+ | AFI | +------------------------------------+ | SAFI | +------------------------------------+ | NLRI | +------------------------------------+ Figure 8: Sequence of Octets to Be Hashed The elements in this sequence (Figure 8) MUST be ordered exactly as shown. The 'Target AS Number' is the AS to whom the BGPsec speaker intends to send the UPDATE message. (Note that the 'Target AS Number' is the AS number announced by the peer in the OPEN message of the BGP session within which the UPDATE message is sent.) The Secure_Path and Signature Segments (1 through N-1) are obtained from the BGPsec_PATH attribute. Finally, the Address Family Identifier (AFI), Subsequent Address Family Identifier (SAFI), and NLRI fields are obtained from the MP_REACH_NLRI attribute [RFC4760]. Additionally, in the Prefix field within the NLRI field (see Section 5 in RFC 4760 [RFC4760]), all of the trailing bits MUST be set to 0 when constructing this sequence. o Apply to this octet sequence (in Figure 8) the digest algorithm (for the algorithm suite of this Signature_Block) to obtain a digest value. o Apply to this digest value the signature algorithm (for the algorithm suite of this Signature_Block) to obtain the digital signature. Then populate the Signature field (in Figure 7) with this digital signature.
The Signature Length field (in Figure 7) is populated with the length (in octets) of the value in the Signature field.4.3. Processing Instructions for Confederation Members
Members of AS confederations [RFC5065] MUST additionally follow the instructions in this section for processing BGPsec UPDATE messages. When a BGPsec speaker in an AS confederation receives a BGPsec UPDATE message from a peer that is external to the confederation and chooses to propagate the UPDATE message within the confederation, it first adds a signature signed to its own Member-AS (i.e., the 'Target AS Number' is the BGPsec speaker's Member-AS Number). In this internally modified UPDATE message, the newly added Secure_Path Segment contains the public AS number (i.e., Confederation Identifier), the segment's pCount value is set to 0, and Confed_Segment flag is set to 1. Setting pCount=0 in this case helps ensure that the AS path length is not unnecessarily incremented. The newly added signature is generated using a private key corresponding to the public AS number of the confederation. The BGPsec speaker propagates the modified UPDATE message to its peers within the confederation. Any BGPsec_PATH modifications mentioned below in the context of propagation of the UPDATE message within the confederation are in addition to the modification described above (i.e., with pCount=0). When a BGPsec speaker sends a BGPsec UPDATE message to a peer that belongs within its own Member-AS, the confederation member SHALL NOT modify the BGPsec_PATH attribute. When a BGPsec speaker sends a BGPsec UPDATE message to a peer that is within the same confederation but in a different Member-AS, the BGPsec speaker puts its Member-AS Number in the AS Number field of the Secure_Path Segment that it adds to the BGPsec UPDATE message. Additionally, in this case, the Member-AS that generates the Secure_Path Segment sets the Confed_Segment flag to 1. Further, the signature is generated with a private key corresponding to the BGPsec speaker's Member-AS Number. (Note: In this document, intra-Member-AS peering is regarded as iBGP, and inter-Member-AS peering is regarded as eBGP. The latter is also known as confederation-eBGP.) Within a confederation, the verification of BGPsec signatures added by other members of the confederation is optional. Note that if a confederation chooses not to verify digital signatures within the confederation, then BGPsec is not able to provide any assurances about the integrity of the Member-AS Numbers placed in Secure_Path Segments where the Confed_Segment flag is set to 1.
When a confederation member receives a BGPsec UPDATE message from a
peer within the confederation and propagates it to a peer outside the
confederation, it needs to remove all of the Secure_Path Segments
added by confederation members as well as the corresponding Signature
Segments. To do this, the confederation member propagating the route
outside the confederation does the following:
o First, starting with the most recently added Secure_Path Segment,
remove all of the consecutive Secure_Path Segments that have the
Confed_Segment flag set to 1. Stop this process once a
Secure_Path Segment that has its Confed_Segment flag set to 0 is
reached. Keep a count of the number of segments removed in this
fashion.
o Second, starting with the most recently added Signature Segment,
remove a number of Signature Segments equal to the number of
Secure_Path Segments removed in the previous step. (That is,
remove the K most recently added Signature Segments, where K is
the number of Secure_Path Segments removed in the previous step.)
o Finally, add a Secure_Path Segment containing, in the AS field,
the AS Confederation Identifier (the public AS number of the
confederation) as well as a corresponding Signature Segment. Note
that all fields other than the AS field are populated as per
Section 4.2.
Finally, as discussed above, an AS confederation MAY optionally
decide that its members will not verify digital signatures added by
members. In such a confederation, when a BGPsec speaker runs the
algorithm in Section 5.2, the BGPsec speaker, during the process of
signature verifications, first checks whether the Confed_Segment flag
in a Secure_Path Segment is set to 1. If the flag is set to 1, the
BGPsec speaker skips the verification for the corresponding signature
and immediately moves on to the next Secure_Path Segment. Note that
as specified in Section 5.2, it is an error when a BGPsec speaker
receives, from a peer who is not in the same AS confederation, a
BGPsec UPDATE message containing a Confed_Segment flag set to 1.
4.4. Reconstructing the AS_PATH Attribute
BGPsec UPDATE messages do not contain the AS_PATH attribute.
However, the AS_PATH attribute can be reconstructed from the
BGPsec_PATH attribute. This is necessary in the case where a route
advertisement is received via a BGPsec UPDATE message and then
propagated to a peer via a non-BGPsec UPDATE message (e.g., because
the latter peer does not support BGPsec). Note that there may be
additional cases where an implementation finds it useful to perform
this reconstruction. Before attempting to reconstruct an AS_PATH for
the purpose of forwarding an unsigned (non-BGPsec) UPDATE message to a peer, a BGPsec speaker MUST perform the basic integrity checks listed in Section 5.2 to ensure that the received BGPsec UPDATE message is properly formed. The AS_PATH attribute can be constructed from the BGPsec_PATH attribute as follows. Starting with a blank AS_PATH attribute, process the Secure_Path Segments in order from least recently added (corresponding to the origin) to most recently added. For each Secure_Path Segment, perform the following steps: 1. If the Secure_Path Segment has pCount=0, then do nothing (i.e., move on to process the next Secure_Path Segment). 2. If the Secure_Path Segment has pCount greater than 0 and the Confed_Segment flag is set to 1, then look at the most recently added segment in the AS_PATH. * In the case where the AS_PATH is blank or in the case where the most recently added segment is of type AS_SEQUENCE, add (prepend to the AS_PATH) a new AS_PATH segment of type AS_CONFED_SEQUENCE. This segment of type AS_CONFED_SEQUENCE shall contain a number of elements equal to the pCount field in the current Secure_Path Segment. Each of these elements shall be the AS number contained in the current Secure_Path Segment. (That is, if the pCount field is X, then the segment of type AS_CONFED_SEQUENCE contains X copies of the Secure_Path Segment's AS Number field.) * In the case where the most recently added segment in the AS_PATH is of type AS_CONFED_SEQUENCE, then add (prepend to the segment) a number of elements equal to the pCount field in the current Secure_Path Segment. The value of each of these elements shall be the AS number contained in the current Secure_Path Segment. (That is, if the pCount field is X, then add X copies of the Secure_Path Segment's AS Number field to the existing AS_CONFED_SEQUENCE.) 3. If the Secure_Path Segment has pCount greater than 0 and the Confed_Segment flag is set to 0, then look at the most recently added segment in the AS_PATH. * In the case where the AS_PATH is blank or in the case where the most recently added segment is of type AS_CONFED_SEQUENCE, add (prepend to the AS_PATH) a new AS_PATH segment of type AS_SEQUENCE. This segment of type AS_SEQUENCE shall contain a number of elements equal to the pCount field in the current Secure_Path Segment. Each of these elements shall be the AS
number contained in the current Secure_Path Segment. (That
is, if the pCount field is X, then the segment of type
AS_SEQUENCE contains X copies of the Secure_Path Segment's AS
Number field.)
* In the case where the most recently added segment in the
AS_PATH is of type AS_SEQUENCE, then add (prepend to the
segment) a number of elements equal to the pCount field in the
current Secure_Path Segment. The value of each of these
elements shall be the AS number contained in the current
Secure_Path Segment. (That is, if the pCount field is X, then
add X copies of the Secure_Path Segment's AS Number field to
the existing AS_SEQUENCE.)
As part of the procedure described above, the following additional
actions are performed in order not to exceed the size limitations of
AS_SEQUENCE and AS_CONFED_SEQUENCE. While adding the next
Secure_Path Segment (with its prepends, if any) to the AS_PATH being
assembled, if it would cause the AS_SEQUENCE (or AS_CONFED_SEQUENCE)
at hand to exceed the limit of 255 AS numbers per segment [RFC4271]
[RFC5065], then the BGPsec speaker would follow the recommendations
in RFC 4271 [RFC4271] and RFC 5065 [RFC5065] of creating another
segment of the same type (AS_SEQUENCE or AS_CONFED_SEQUENCE) and
continue filling that.
Finally, one special case of reconstruction of AS_PATH is when the
BGPsec_PATH attribute is absent. As explained in Section 4.1, when a
BGPsec speaker originates a prefix and sends it to a BGPsec-capable
iBGP peer, the BGPsec_PATH is not attached. So, when received from a
BGPsec-capable iBGP peer, no BGPsec_PATH attribute in a BGPsec UPDATE
message is equivalent to an empty AS_PATH [RFC4271].
5. Processing a Received BGPsec UPDATE Message
Upon receiving a BGPsec UPDATE message from an external (eBGP) peer,
a BGPsec speaker SHOULD validate the message to determine the
authenticity of the path information contained in the BGPsec_PATH
attribute. Typically, a BGPsec speaker will also wish to perform
origin validation (see RFC 6483 [RFC6483] and RFC 6811 [RFC6811]) on
an incoming BGPsec UPDATE message, but such validation is independent
of the validation described in this section.
Section 5.1 provides an overview of BGPsec validation, and
Section 5.2 provides a specific algorithm for performing such
validation. (Note that an implementation need not follow the
specific algorithm in Section 5.2 as long as the input/output
behavior of the validation is identical to that of the algorithm in
Section 5.2.) During exceptional conditions (e.g., the BGPsec
speaker receives an incredibly large number of UPDATE messages at once), a BGPsec speaker MAY temporarily defer validation of incoming BGPsec UPDATE messages. The treatment of such BGPsec UPDATE messages, whose validation has been deferred, is a matter of local policy. However, an implementation SHOULD ensure that deferment of validation and status of deferred messages is visible to the operator. The validity of BGPsec UPDATE messages is a function of the current RPKI state. When a BGPsec speaker learns that the RPKI state has changed (e.g., from an RPKI validating cache via the RPKI-Router protocol [RFC8210]), the BGPsec speaker MUST rerun validation on all affected UPDATE messages stored in its Adj-RIB-In [RFC4271]. For example, when a given RPKI router certificate ceases to be valid (e.g., it expires or is revoked), all UPDATE messages containing a signature whose SKI matches the SKI in the given certificate MUST be reassessed to determine if they are still valid. If this reassessment determines that the validity state of an UPDATE message has changed, then, depending on local policy, it may be necessary to rerun best path selection. BGPsec UPDATE messages do not contain an AS_PATH attribute. The Secure_Path contains AS path information for the BGPsec UPDATE message. Therefore, a BGPsec speaker MUST utilize the AS path information in the Secure_Path in all cases where it would otherwise use the AS path information in the AS_PATH attribute. The only exception to this rule is when AS path information must be updated in order to propagate a route to a peer (in which case the BGPsec speaker follows the instructions in Section 4). Section 4.4 provides an algorithm for constructing an AS_PATH attribute from a BGPsec_PATH attribute. Whenever the use of AS path information is called for (e.g., loop detection or the use of the AS path length in best path selection), the externally visible behavior of the implementation shall be the same as if the implementation had run the algorithm in Section 4.4 and used the resulting AS_PATH attribute as it would for a non-BGPsec UPDATE message.5.1. Overview of BGPsec Validation
Validation of a BGPsec UPDATE message makes use of data from RPKI router certificates. In particular, it is necessary that the recipient have access to the following data obtained from valid RPKI router certificates: the AS Number, Public Key, and Subject Key Identifier from each valid RPKI router certificate. Note that the BGPsec speaker could perform the validation of RPKI router certificates on its own and extract the required data, or it could receive the same data from a trusted cache that performs RPKI
validation on behalf of (some set of) BGPsec speakers. (For example, the trusted cache could deliver the necessary validity information to the BGPsec speaker by using the Router Key PDU (Protocol Data Unit) for the RPKI-Router protocol [RFC8210].) To validate a BGPsec UPDATE message containing the BGPsec_PATH attribute, the recipient performs the validation steps specified in Section 5.2. The validation procedure results in one of two states: 'Valid' and 'Not Valid'. It is expected that the output of the validation procedure will be used as an input to BGP route selection. That said, BGP route selection, and thus the handling of the validation states, is a matter of local policy and is handled using local policy mechanisms. Implementations SHOULD enable operators to set such local policy on a per-session basis. (That is, it is expected that some operators will choose to treat BGPsec validation status differently for UPDATE messages received over different BGP sessions.) BGPsec validation need only be performed at the eBGP edge. The validation status of a BGP signed/unsigned UPDATE message MAY be conveyed via iBGP from an ingress edge router to an egress edge router via some mechanism, according to local policy within an AS. As discussed in Section 4, when a BGPsec speaker chooses to forward a (syntactically correct) BGPsec UPDATE message, it SHOULD be forwarded with its BGPsec_PATH attribute intact (regardless of the validation state of the UPDATE message). Based entirely on local policy, an egress router receiving a BGPsec UPDATE message from within its own AS MAY choose to perform its own validation.5.2. Validation Algorithm
This section specifies an algorithm for validation of BGPsec UPDATE messages. A conformant implementation MUST include a BGPsec update validation algorithm that is functionally equivalent to the externally visible behavior of this algorithm. First, the recipient of a BGPsec UPDATE message performs a check to ensure that the message is properly formed. Both syntactical and protocol violation errors are checked. The BGPsec_PATH attribute MUST be present when a BGPsec UPDATE message is received from an external (eBGP) BGPsec peer and also when such an UPDATE message is propagated to an internal (iBGP) BGPsec peer (see Section 4.2). The error checks specified in Section 6.3 of [RFC4271] are performed, except that for BGPsec UPDATE messages the checks on the AS_PATH attribute do not apply and instead the following checks on the BGPsec_PATH attribute are performed:
1. Check to ensure that the entire BGPsec_PATH attribute is
syntactically correct (conforms to the specification in this
document).
2. Check that the AS number in the most recently added Secure_Path
Segment (i.e., the one corresponding to the eBGP peer from which
the UPDATE message was received) matches the AS number of that
peer as specified in the BGP OPEN message. (Note: This check is
performed only at an ingress BGPsec router where the UPDATE
message is first received from a peer AS.)
3. Check that each Signature_Block contains one Signature Segment
for each Secure_Path Segment in the Secure_Path portion of the
BGPsec_PATH attribute. (Note that the entirety of each
Signature_Block MUST be checked to ensure that it is well formed,
even though the validation process may terminate before all
signatures are cryptographically verified.)
4. Check that the UPDATE message does not contain an AS_PATH
attribute.
5. If the UPDATE message was received from a BGPsec peer that is not
a member of the BGPsec speaker's AS confederation, check to
ensure that none of the Secure_Path Segments contain a Flags
field with the Confed_Segment flag set to 1.
6. If the UPDATE message was received from a BGPsec peer that is a
member of the BGPsec speaker's AS confederation, check to ensure
that the Secure_Path Segment corresponding to that peer contains
a Flags field with the Confed_Segment flag set to 1.
7. If the UPDATE message was received from a peer that is not
expected to set pCount=0 (see Sections 4.2 and 4.3), then check
to ensure that the pCount field in the most recently added
Secure_Path Segment is not equal to 0. (Note: See Section 7.2
for router configuration guidance related to this item.)
8. Using the equivalent of AS_PATH corresponding to the Secure_Path
in the UPDATE message (see Section 4.4), check that the local AS
number is not present in the AS path (i.e., rule out an AS loop).
If any of these checks fail, it is an error in the BGPsec_PATH
attribute. BGPsec speakers MUST handle any syntactical or protocol
errors in the BGPsec_PATH attribute by using the "treat-as-withdraw"
approach as defined in RFC 7606 [RFC7606]. (Note: Since the AS
number of a transparent route server does appear in the Secure_Path
with pCount=0, the route server MAY check to see if its local AS is
listed in the Secure_Path, and this check MAY be included in the loop-detection check listed above.) Next, the BGPsec speaker examines the Signature_Blocks in the BGPsec_PATH attribute. A Signature_Block corresponding to an algorithm suite that the BGPsec speaker does not support is not considered in the validation process. If there is no Signature_Block corresponding to an algorithm suite that the BGPsec speaker supports, then in order to consider the UPDATE message in the route selection process, the BGPsec speaker MUST strip the Signature_Block(s), reconstruct the AS_PATH from the Secure_Path (see Section 4.4), and treat the UPDATE message as if it were received as an unsigned BGP UPDATE message. For each remaining Signature_Block (corresponding to an algorithm suite supported by the BGPsec speaker), the BGPsec speaker iterates through the Signature Segments in the Signature_Block, starting with the most recently added segment (and concluding with the least recently added segment). Note that there is a one-to-one correspondence between Signature Segments and Secure_Path Segments within the BGPsec_PATH attribute. The following steps make use of this correspondence: o Step 1: Let there be K AS hops in a received BGPsec_PATH attribute that is to be validated. Let AS(1), AS(2), ..., AS(K+1) denote the sequence of AS numbers from the origin AS to the validating AS. Let Secure_Path Segment N and Signature Segment N in the BGPsec_PATH attribute refer to those corresponding to AS(N) (where N = 1, 2, ..., K). The BGPsec speaker that is processing and validating the BGPsec_PATH attribute resides in AS(K+1). Let Signature Segment N be the Signature Segment that is currently being verified. o Step 2: Locate the public key needed to verify the signature (in the current Signature Segment). To do this, consult the valid RPKI router certificate data and look up all valid (AS Number, Public Key, Subject Key Identifier) triples in which the AS matches the AS number in the corresponding Secure_Path Segment. Of these triples that match the AS number, check whether there is an SKI that matches the value in the Subject Key Identifier field of the Signature Segment. If this check finds no such matching SKI value, then mark the entire Signature_Block as 'Not Valid' and proceed to the next Signature_Block. o Step 3: Compute the digest function (for the given algorithm suite) on the appropriate data.
In order to verify the digital signature in Signature Segment N,
construct the sequence of octets to be hashed as shown in Figure 9
(using the notations defined in Step 1). (Note that this sequence
is the same sequence that was used by AS(N) that created the
Signature Segment N (see Section 4.2 and Figure 8).)
+------------------------------------+
| Target AS Number |
+------------------------------------+----\
| Signature Segment : N-1 | \
+------------------------------------+ |
| Secure_Path Segment : N | |
+------------------------------------+ \
... > Data from
+------------------------------------+ / N Segments
| Signature Segment : 1 | |
+------------------------------------+ |
| Secure_Path Segment : 2 | |
+------------------------------------+ /
| Secure_Path Segment : 1 | /
+------------------------------------+---/
| Algorithm Suite Identifier |
+------------------------------------+
| AFI |
+------------------------------------+
| SAFI |
+------------------------------------+
| NLRI |
+------------------------------------+
Figure 9: Sequence of Octets to Be Hashed for Signature Verification
of Signature Segment N; N = 1,2, ..., K, Where K Is the Number of
AS Hops in the BGPsec_PATH Attribute
The elements in this sequence (Figure 9) MUST be ordered exactly
as shown. For the first segment to be processed (the
most recently added segment (i.e., N = K) given that there are K
hops in the Secure_Path), the 'Target AS Number' is AS(K+1), the
AS number of the BGPsec speaker validating the UPDATE message.
Note that if a BGPsec speaker uses multiple AS Numbers (e.g., the
BGPsec speaker is a member of a confederation), the AS number used
here MUST be the AS number announced in the OPEN message for the
BGP session over which the BGPsec UPDATE message was received.
For each other Signature Segment (N smaller than K), the 'Target
AS Number' is AS(N+1), the AS number in the Secure_Path Segment
that corresponds to the Signature Segment added immediately after
the one being processed (that is, in the Secure_Path Segment that
corresponds to the Signature Segment that the validator just
finished processing).
The Secure_Path and Signature Segment are obtained from the
BGPsec_PATH attribute. The AFI, SAFI, and NLRI fields are
obtained from the MP_REACH_NLRI attribute [RFC4760].
Additionally, in the Prefix field within the NLRI field (see
Section 5 in RFC 4760 [RFC4760]), all of the trailing bits MUST be
set to 0 when constructing this sequence.
o Step 4: Use the signature validation algorithm (for the given
algorithm suite) to verify the signature in the current segment.
That is, invoke the signature validation algorithm on the
following three inputs: the value of the Signature field in the
current segment, the digest value computed in Step 3 above, and
the public key obtained from the valid RPKI data in Step 2 above.
If the signature validation algorithm determines that the
signature is invalid, then mark the entire Signature_Block as
'Not Valid' and proceed to the next Signature_Block. If the
signature validation algorithm determines that the signature is
valid, then continue processing Signature Segments (within the
current Signature_Block).
If all Signature Segments within a Signature_Block pass validation
(i.e., all segments are processed and the Signature_Block has not yet
been marked 'Not Valid'), then the Signature_Block is marked as
'Valid'.
If at least one Signature_Block is marked as 'Valid', then the
validation algorithm terminates and the BGPsec UPDATE message is
deemed 'Valid'. (That is, if a BGPsec UPDATE message contains two
Signature_Blocks, then the UPDATE message is deemed 'Valid' if the
first Signature_Block is marked 'Valid' OR the second Signature_Block
is marked 'Valid'.)
6. Algorithms and Extensibility
6.1. Algorithm Suite Considerations
Note that there is currently no support for bilateral negotiation
(using BGP capabilities) between BGPsec peers to use a particular
(digest and signature) algorithm suite. This is because the
algorithm suite used by the sender of a BGPsec UPDATE message MUST be
understood not only by the peer to whom it is directly sending the
message but also by all BGPsec speakers to whom the route
advertisement is eventually propagated. Therefore, selection of an
algorithm suite cannot be a local matter negotiated by BGP peers but
instead must be coordinated throughout the Internet.
To this end, [RFC8208] specifies a mandatory-to-use 'current' algorithm suite for use by all BGPsec speakers. It is anticipated that, in the future, [RFC8208] or its successor will be updated to specify a transition from the 'current' algorithm suite to a 'new' algorithm suite. During the period of transition, all BGPsec UPDATE messages SHOULD simultaneously use both the 'current' algorithm suite and the 'new' algorithm suite. (Note that Sections 3 and 4 specify how the BGPsec_PATH attribute can contain signatures, in parallel, for two algorithm suites.) Once the transition is complete, the use of the old 'current' algorithm will be deprecated, the use of the 'new' algorithm will be mandatory, and a subsequent 'even newer' algorithm suite may be specified as "recommended to implement". Once the transition has successfully been completed in this manner, BGPsec speakers SHOULD include only a single Signature_Block (corresponding to the 'new' algorithm).6.2. Considerations for the SKI Size
Depending on the method of generating key identifiers [RFC7093], the size of the SKI in an RPKI router certificate may vary. The SKI field in the BGPsec_PATH attribute has a fixed size of 20 octets (see Figure 7). If the SKI is longer than 20 octets, then use the leftmost 20 octets of the SKI (excluding the tag and length) [RFC7093]. If the SKI value is shorter than 20 octets, then pad the SKI (excluding the tag and length) to the right (least significant octets) with octets having "0" values.6.3. Extensibility Considerations
This section discusses potential changes to BGPsec that would require substantial changes to the processing of the BGPsec_PATH and thus necessitate a new version of BGPsec. Examples of such changes include: o A new type of signature algorithm that produces signatures of variable length o A new type of signature algorithm for which the number of signatures in the Signature_Block is not equal to the number of ASes in the Secure_Path (e.g., aggregate signatures) o Changes to the data that is protected by the BGPsec signatures (e.g., attributes other than the AS path) In the case that such a change to BGPsec were deemed desirable, it is expected that a subsequent version of BGPsec would be created and that this version of BGPsec would specify a new BGP path attribute --
let's call it "BGPsec_PATH_Two" -- that is designed to accommodate the desired changes to BGPsec. In such a case, [RFC8208] or its successor would be updated to specify algorithm suites appropriate for the new version of BGPsec. At this point, a transition would begin that is analogous to the algorithm transition discussed in Section 6.1. During the transition period, all BGPsec speakers SHOULD simultaneously include both the BGPsec_PATH attribute and the new BGPsec_PATH_Two attribute. Once the transition is complete, the use of BGPsec_PATH could then be deprecated, at which point BGPsec speakers should include only the new BGPsec_PATH_Two attribute. Such a process could facilitate a transition to a new BGPsec semantics in a backwards-compatible fashion.
(page 29 continued on part 3)
