Network Working Group M. Lepinski, Ed.
Internet-Draft BBN
Intended status: Standards Track July 16, 2012
Expires: January 17, 2013
BGPSEC Protocol Specification
draft-ietf-sidr-bgpsec-protocol-04
Abstract
This document describes BGPSEC, an extension to the Border Gateway
Protocol (BGP) that provides security for the AS-PATH attribute in
BGP update messages. BGPSEC is implemented via a new optional non-
transitive BGP path attribute that carries a digital signature
produced by each autonomous system on the AS-PATH.
Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [8].
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 17, 2013.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
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publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. BGPSEC Negotiation . . . . . . . . . . . . . . . . . . . . . . 3
3. The BGPSEC_Path_Signatures Attribute . . . . . . . . . . . . . 6
3.1. Secure_Path . . . . . . . . . . . . . . . . . . . . . . . 8
3.2. Additional_Info . . . . . . . . . . . . . . . . . . . . . 9
3.3. Signature_Block . . . . . . . . . . . . . . . . . . . . . 11
4. Generating a BGPSEC Update . . . . . . . . . . . . . . . . . . 12
4.1. Originating a New BGPSEC Update . . . . . . . . . . . . . 13
4.2. Propagating a Route Advertisement . . . . . . . . . . . . 16
4.3. Reconstructing the AS_Path Attribute . . . . . . . . . . . 19
4.4. Processing Instructions for Confederation Members . . . . 19
5. Processing a Received BGPSEC Update . . . . . . . . . . . . . 21
5.1. Validation Algorithm . . . . . . . . . . . . . . . . . . . 22
6. Algorithms and Extensibility . . . . . . . . . . . . . . . . . 26
6.1. Algorithm Suite Considerations . . . . . . . . . . . . . . 26
6.2. Extensibility Considerations . . . . . . . . . . . . . . . 27
7. Security Considerations . . . . . . . . . . . . . . . . . . . 28
8. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 31
8.1. Authors . . . . . . . . . . . . . . . . . . . . . . . . . 31
8.2. Acknowledgements . . . . . . . . . . . . . . . . . . . . . 32
9. Normative References . . . . . . . . . . . . . . . . . . . . . 32
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 33
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1. Introduction
This document describes BGPSEC, a mechanism for providing path
security for Border Gateway Protocol (BGP) [1] route advertisements.
That is, a BGP speaker who receives a valid BGPSEC update has
cryptographic assurance that the advertised route has the following
two properties:
1. The route was originated by an AS that has been explicitly
authorized by the holder of the IP address prefix to originate
route advertisements for that prefix.
2. Every AS listed in the AS_Path attribute of the update explicitly
authorized the advertisement of the route to the subsequent AS in
the AS_Path.
This document specifies a new optional (non-transitive) BGP path
attribute, BGPSEC_Path_Signatures. It also describes how a BGPSEC-
compliant BGP speaker (referred to hereafter as a BGPSEC speaker) can
generate, propagate, and validate BGP update messages containing this
attribute to obtain the above assurances.
BGPSEC relies on the Resource Public Key Infrastructure (RPKI)
certificates that attest to the allocation of AS number and IP
address resources. (For more information on the RPKI, see [6] and
the documents referenced therein.) Any BGPSEC speaker who wishes to
send BGP update messages to external peers (eBGP) containing the
BGPSEC_Path_Signatures must have an RPKI end-entity certificate (as
well as the associated private signing key) corresponding to the
BGPSEC speaker's AS number. Note, however, that a BGPSEC speaker
does not require such a certificate in order to validate update
messages containing the BGPSEC_Path_Signatures attribute.
2. BGPSEC Negotiation
This document defines a new BGP capability [4]that allows a BGP
speaker to advertise to its neighbors the ability to send and/or
receive BGPSEC update messages (i.e., update messages containing the
BGPSEC_Path_Signatures attribute).
This capability has capability code : TBD
The capability length for this capability MUST be set to 5.
The three octets of the capability value are specified as follows.
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Capability Value:
0 1 2 3 4 5 6 7
+---------------------------------------+
| Send | Receive | Reserved | Version |
+---------------------------------------+
| AFI |
+---------------------------------------+
| |
+---------------------------------------+
| Reserved |
+---------------------------------------+
| SAFI |
+---------------------------------------+
The high order bit (bit 0) of the first octet is set to 1 to indicate
that the sender is able to send BGPSEC update messages, and is set to
zero otherwise. The next highest order bit (bit 1) of this octet is
set to 1 to indicate that the sender is able to receive BGPSEC update
messages, and is set to zero otherwise. The next two bits of the
capability value (bits 2 and 3) are reserved for future use. These
reserved bits should be set to zero by the sender and ignored by the
receiver.
The four low order bits (4, 5, 6 and 7) of the first octet indicate
the version of BGPSEC for which the BGP speaker is advertising
support. This document defines only BGPSEC version 0 (all four bits
set to zero). Other versions of BGPSEC may be defined in future
documents. A BGPSEC speaker MAY advertise support for multiple
versions of BGPSEC by including multiple versions of the BGPSEC
capability in its BGP OPEN message.
If there does not exist at least one version of BGPSEC that is
supported by both peers in a BGP session, then the use of BGPSEC has
not been negotiated. (That is, in such a case, messages containing
the BGPSEC_Path_Signatures MUST NOT be sent.)
If version 0 is the only version of BGPSEC for which both peers (in a
BGP session) advertise support, then the use of BGPSEC has been
negotiated and the BGPSEC peers MUST adhere to the specification of
BGPSEC provided in this document. (If there are multiple versions of
BGPSEC which are supported by both peers, then the behavior of those
peers is outside the scope of this document.)
The second and third octets contain the 16-bit Address Family
Identifier (AFI) which indicates the address family for which the
BGPSEC speaker is advertising support for BGPSEC. This document only
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specifies BGPSEC for use with two address families, IPv4 and IPv6,
AFI values 1 and 2 respectively. BGPSEC for use with other address
families may be specified in future documents.
The fourth octet in the capability is reserved. It is anticipated
that this octet will not be used until such a time as the reserved
octet in the Multi-protocol extensions capability advertisement [2]
is specified for use. The reserved octet should be set to zero by
the sender and ignored by the receiver.
The fifth octet in the capability contains the 8-bit Subsequent
Address Family Identifier (SAFI). This value is encoded as in the
BGP multiprotocol extensions [2].
Note that if the BGPSEC speaker wishes to use BGPSEC with two
different address families (i.e., IPv4 and IPv6) over the same BGP
session, then the speaker must include two instances of this
capability (one for each address family) in the BGP OPEN message. A
BGPSEC speaker SHOULD NOT advertise the capability of BGPSEC support
for any <AFI, SAFI> combination unless it has also advertises the
multiprotocol extension capability for the same <AFI, SAFI>
combination [2].
By indicating support for receiving BGPSEC update messages, a BGP
speaker is, in particular, indicating that the following are true:
o The BGP speaker understands the BGPSEC_Path_Signatures attribute
(see Section 3).
o The BGP speaker supports 4-byte AS numbers (see RFC 4893).
Note that BGPSEC update messages can be quite large, therefore any
BGPSEC speaker announcing the capability to receive BGPSEC messages
SHOULD also announce support for the capability to receive BGP
extended messages [9].
A BGP speaker MUST NOT send an update message containing the
BGPSEC_Path_Signatures attribute within a given BGP session unless
both of the following are true:
o The BGP speaker indicated support for sending BGPSEC update
messages in its open message.
o The peer of the BGP speaker indicated support for receiving BGPSEC
update messages in its open message.
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3. The BGPSEC_Path_Signatures Attribute
The BGPSEC_Path_Signatures attribute is a new optional (non-
transitive) BGP path attribute.
This document registers a new attribute type code for this attribute
: TBD
The BGPSEC_Path_Signatures algorithm carries the secured AS Path
information, including the digital signatures that protect this AS
Path information. We refer to those update messages that contain the
BGPSEC_Path_Signatures attribute as "BGPSEC Update messages". The
BGPSEC_Path_Signatures attribute replaces the AS_PATH attribute, in a
BGPSEC update message. That is, update messages that contain the
BGPSEC_Path_Signatures attribute MUST NOT contain the AS_PATH
attribute.
The BGPSEC_Path_Signatures attribute is made up of several parts.
The following high-level diagram provides an overview of the
structure of the BGPSEC_Path_Signatures attribute:
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High-Level Diagram of the BGPSEC_Path_Signatures Attribute
BGPSEC_Path_Signatures
+---------------------------------------------------------+
| +-----------------+ |
| | Secure Path | +-----------------+ |
| +-----------------+ | Additional Info | |
| | AS X | +-----------------+ |
| | pCount X | | Info Type | |
| | Flags X | | Info Length | |
| | AS Y | | Info Value | |
| | pCount Y | +-----------------+ |
| | Flags Y | |
| | ... | |
| +-----------------+ |
| |
| +-----------------+ +-----------------+ |
| | Sig Block 1 | | Sig Block 2 | |
| +-----------------+ +-----------------+ |
| | Alg Suite 1 | | Alg Suite 2 | |
| | SKI X | | SKI X | |
| | Sig Length X | | Sig Length X | |
| | Signature X | | Signature X | |
| | SKI Length Y | | SKI Length Y | |
| | SKI Y | | SKI Y | |
| | Sig Length Y | | Sig Length Y | |
| | Signature Y | | Signature Y | |
| | ... | | .... | |
| +-----------------+ +-----------------+ |
| |
+---------------------------------------------------------+
The following is a more detailed explanation of the format of the
BGPSEC_Path_Signatures attribute.
BGPSEC_Path_Signatures Attribute
+-------------------------------------------------------+
| Secure_Path (variable) |
+-------------------------------------------------------+
| Additional_Info (variable) |
+-------------------------------------------------------+
| Sequence of one or two Signature_Blocks (variable) |
+-------------------------------------------------------+
The Secure_Path contains AS Path information for the BGPSEC update
message. This is logically equivalent to the information that would
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be contained in the AS_PATH attribute. A BGPSEC update message
containing the BGPSEC_PATH_SIGNATURES attribute MUST NOT contain the
AS_PATH attribute. The format of the Secure_Path is described below
in Section 3.1.
The Additional_Info contains additional signed information about the
update message. Additional_Info is specified as a type-length-value
field for future extensibility. However, this specification defines
only a single (null) type of Additional Info which has zero length.
It is anticipated that future specifications may specify semantics
for Info Types other than zero. See Section 3.2 below for more
detail.
The BGPSEC_Path_Signatures attribute will contain one or two
Signature_Blocks, each of which corresponds to a different algorithm
suite. Each of the Signature_Blocks will contain a signature segment
for each AS number (i.e, secure path segment) in the Secure_Path. In
the most common case, the BGPSEC_Path_Signatures attribute will
contain only a single Signature_Block. However, in order to enable a
transition from an old algorithm suite to a new algorithm suite, it
will be necessary to include two Signature_Blocks (one for the old
algorithm suite and one for the new algorithm suite) during the
transition period. (See Section 6.1 for more discussion of algorithm
transitions.) The format of the Signature_Blocks is described below
in Section 3.3.
3.1. Secure_Path
Here we provide a detailed description of the Secure_Path information
in the BGPSEC_Path_Signatures attribute.
Secure_Path
+-----------------------------------------------+
| Secure_Path Length (2 octets) |
+-----------------------------------------------+
| One or More Secure_Path Segments (variable) |
+-----------------------------------------------+
The Secure_Path Length contains the length (in octets) of the
variable-length sequence of Secure_Path Segments. As explained
below, each Secure_Path segment is six octets long. Note that this
means the Secure_Path Length is six times the number Secure_Path
Segments (i.e., the number of AS numbers in the path).
The Secure_Path contains one Secure_Path segment for each (distinct)
Autonomous System in the path to the NLRI specified in the update
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message.
Secure_Path Segment
+----------------------------+
| AS Number (4 octets) |
+----------------------------+
| pCount (1 octet) |
+----------------------------+
| Flags (1 octet) |
+----------------------------+
The AS Number is the AS number of the BGP speaker that added this
Secure_Path segment to the BGPSEC_Path_Signatures attribute. (See
Section 4 for more information on populating this field.)
The pCount field contains the number of repetitions of the associated
autonomous system number that the signature covers. This field
enables a BGPSEC speaker to mimic the semantics of adding multiple
copies of their AS to the AS_PATH without requiring the speaker to
generate multiple signatures.
The first bit of the Flags field is the Entering_Confed flag. The
Entering_Confed flag is set to one in the Secure_Path Segment
corresponding to the first Autonomous System in a confederation [3].
(That is, the Secure_Path Segment corresponding to the AS that would
otherwise have created an AS_Path segment of type AS_Confed_Sequence
in a non-BGSPEC update message.) In all other cases the
Entering_Confed flag is set to zero.
The remaining seven bits of the Flags field are reserved for future
use. These bits MUST be set to zero by the sender. The receiver
uses the entire Flags octet to verify the digital signature
(regardless of what value the reserved bits contain), but otherwise
ignores the reserved flags (see Section 4 for sender instructions and
Section 5 for receiver validation instructions).
EDITOR'S NOTE: The unused portion of the signed flags field provides
the possibility of adding in the future (in a backwards compatible
fashion) a new feature that requires some per-AS signed bits. For
example, one could use a couple bits from this flag field to mark
some property of the connection between two ASes.
3.2. Additional_Info
Here we provide a detailed description of the Additional_Info in the
BGPSEC_Path_Signatures attribute.
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Additional_Info
+---------------------------------------------+
| Info Type (1 octet) |
+---------------------------------------------+
| Info Length (1 octet) |
+---------------------------------------------+
| Info Value (variable) |
+---------------------------------------------+
The Info Type field is a one-octet value that identifies the type of
additional information included in the Info Value field. This
specification defines a single (null) type of Additional_Info. The
Info Type for this null type is zero.
The Info Length field contains the length in octets of the Info Value
field. For the (null) Info Type zero specified in this document, the
Info Length MUST be zero.
The syntax and semantics contained in the Info Value field depends on
the type contained in the Info Type field. For the (null) Info Type
zero specified in this document, the Info Value field is empty (since
the Info Length field must be zero).
Implementations compliant with this specification MUST set the Info
Type to zero in BGPSEC update messages for route advertisements that
they originate (see Section 4.1 for more details). When an
implementation compliant with this specification receives a BGPSEC
update message with an Info Type field that it does not understand
(i.e., an Info Type other than zero), the implementation MUST use the
Additional_Info when it verifies digital signatures (as per Section
5.1). However, other than signature verification, the implementation
MUST ignore the Info Value field when it does not understand the Info
Type.
EDITOR'S NOTE: In a previous version of this document there was an
Expire Time that was used to provide protection against replay of old
(stale) digital signatures or failure to propagate a withdrawal
message. This mechanism was removed from the current version of the
document. Please see the SIDR mailing list for discussions related
to protection against replay attacks. Depending on the result of
discussions within the SIDR working group this Additional Info field
could at some future point be used to re-introduce Expire Time, or
some other octets used in a future replay protection mechanism. The
authors believe that the current instructions whereby the sender uses
a null Additional_Info type and the receiver ignores Additional_Info
types that it does not understand provides an opportunity to use
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these octets in the future in a backwards-compatible fashion.
3.3. Signature_Block
Here we provide a detailed description of the Signature_Blocks in the
BGPSEC_Path_Signatures attribute.
Signature_Block
+---------------------------------------------+
| Algorithm Suite Identifier (1 octet) |
+---------------------------------------------+
| Signature_Block Length (2 octets) |
+---------------------------------------------+
| Sequence of Signature Segments (variable) |
+---------------------------------------------+
The Algorithm Suite Identifier is a one-octet identifier specifying
the digest algorithm and digital signature algorithm used to produce
the digital signature in each Signature Segment. An IANA registry of
algorithm identifiers for use in BGPSEC is created in the BGPSEC
algorithms document[12].
The Signature_Block Length is the total number of octets in all
Signature Segments (i.e., the total size of the variable-length
portion of the Signature_Block.)
A Signature_Block has exactly one Signature Segment for each
Secure_Path Segment in the Secure_Path portion of the
BGPSEC_Path_Signatures Attribute. (That is, one Signature Segment
for each distinct AS on the path for the NLRI in the Update message.)
Signature Segments
+---------------------------------------------+
| Subject Key Identifier (20 octets) |
+---------------------------------------------+
| Signature Length (2 octets) |
+---------------------------------------------+
| Signature (variable) |
+---------------------------------------------+
The Subject Key Identifier contains the value in the Subject Key
Identifier extension of the RPKI end-entity certificate that is used
to verify the signature (see Section 5 for details on validity of
BGPSEC update messages).
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The Signature Length field contains the size (in octets) of the value
in the Signature field of the Signature Segment.
The Signature contains a digital signature that protects the NLRI and
the BGPSEC_Path_Signatures attribute (see Sections 4 and 5 for
details on generating and verifying this signature, respectively).
4. Generating a BGPSEC Update
Sections 4.1 and 4.2 cover two cases in which a BGPSEC speaker may
generate an update message containing the BGPSEC_Path_Signatures
attribute. The first case is that in which the BGPSEC speaker
originates a new route advertisement (Section 4.1). That is, the
BGPSEC speaker is constructing an update message in which the only AS
to appear in the BGPSEC_Path_Signatures is the speaker's own AS. The
second case is that in which the BGPSEC speaker receives a route
advertisement from a peer and then decides to propagate the route
advertisement to an external (eBGP) peer (Section 4.2). That is, the
BGPSEC speaker has received a BGPSEC update message and is
constructing a new update message for the same NLRI in which the
BGPSEC_Path_Signatures attribute will contain AS number(s) other than
the speaker's own AS.
In the remaining case where the BGPSEC speaker is sending the update
message to an internal (iBGP) peer, the BGPSEC speaker populates the
BGPSEC_Path_Signatures attribute by copying the
BGPSEC_Path_Signatures attribute from the received update message.
That is, the BGPSEC_Path_Signatures attribute is copied verbatim.
Note that in the case that a BGPSEC speaker chooses to forward to an
iBGP peer a BGPSEC update message that has not been successfully
validated (see Section 5), the BGPSEC_Path_Signatures attribute
SHOULD NOT be removed. (See Section 7 for the security ramifications
of removing BGPSEC signatures.)
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 to multiple BGP peers, it MUST generate a separate BGPSEC
update message for each unique peer AS to which the update message is
sent.
A BGPSEC update message MUST advertise a route to only a single NLRI.
This is because a BGPSEC speaker receiving an update message with
multiple NLRI would be unable to construct a valid BGPSEC update
message (i.e., valid path signatures) containing a subset of the NLRI
in the received update. If a BGPSEC speaker wishes to advertise
routes to multiple NLRI, then it MUST generate a separate BGPSEC
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update message for each NLRI.
Note that 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 Resource PKI end-entity certificate whose
AS number resource extension includes the BGPSEC speaker's AS number
[11]. 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). Therefore, a
BGPSEC speaker who only sends BGPSEC update messages to peers within
its own AS, it does not need to possess any private signature keys.
4.1. Originating a New BGPSEC Update
In an update message that originates a new route advertisement (i.e.,
an update whose path will contain only a single AS number), when
sending the route advertisement to an external, BGPSEC-speaking peer,
the BGPSEC speaker creates a new BGPSEC_Path_Signatures attribute as
follows.
First, the BGPSEC speaker constructs the Secure_Path with a single
Secure_Path Segment. The AS in this path is the BGPSEC speaker's own
AS number. In particular, this AS number MUST match the AS number in
the AS number resource extension field of the Resource PKI end-entity
certificate(s) that will be used to verify the digital signature(s)
constructed by this BGPSEC speaker.
Note that the BGPSEC_Path_Signatures attribute and the AS4_Path
attribute are mutually exclusive. That is, any update message
containing the BGPSEC_Path_Signatures attribute MUST NOT contain the
AS4_Path attribute nor the AS_Path attribute. The information that
would be contained in the AS4_Path (or AS_Path) attribute is instead
conveyed in the Secure_Path portion of the BGPSEC_Path_Signatures
attribute.
Note that the Resource PKI enables the legitimate holder of IP
address prefix(es) to issue a signed object, called a Route
Origination Authorization (ROA), that authorizes a given AS to
originate routes to a given set of prefixes (see [7]). Note that
validation of a BGPSEC update message will fail (i.e., the validation
algorithm, specified in Section 5.1, returns 'Not Good') unless there
exists a valid ROA authorizing the first AS in the Secure_Path
portion of the BGPSEC_Path_Signatures attribute to originate routes
to the prefix being advertised. Therefore, a BGPSEC speaker SHOULD
NOT originate a BGPSEC update advertising a route for a given prefix
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unless there exists a valid ROA authorizing the BGPSEC speaker's AS
to originate routes to this prefix.
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 one 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). Setting the pCount field to a value
greater than one permits this repetition without requiring a separate
digital signature for each repetition.
If the BGPSEC speaker is not a member of an autonomous system
confederation [3], then the Flags field of the Secure_Path Segment
MUST be set to zero. (Members of a confederation should follow the
special processing instructions for confederation members in Section
4.4.)
The BGPSEC speaker next constructs the Additional_Info portion of the
BGPSEC_Path_Signatures attribute. The Info Type MUST be set to zero
and the Info Length MUST also be set to zero. The Info Value field
is empty (has length zero). It is anticipated that future
specifications may specify values of Info Type other than zero.
Therefore, BGPSEC receivers compliant with this specification must be
able to accept Additional_Info fields with non-zero Info Type. Such
receivers will use the Additional_Field to verify digital signatures
(see Section 5) but will otherwise ignore Additional_Field non-zero
Info Fields.
Typically, a BGPSEC speaker will use only a single algorithm suite,
and thus create only a single Signature_Block in the
BGPSEC_Path_Signatures 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).
When originating a new route advertisement, each Signature_Block MUST
consist of a single Signature Segment. The following describes how
the BGPSEC speaker populates the fields of the Signature_Block.
The Subject Key Identifier field (see Section 3) is populated with
the identifier contained in the Subject Key Identifier extension of
the RPKI end-entity certificate used by the BGPSEC speaker. This
Subject Key Identifier will be used by recipients of the route
advertisement to identify the proper certificate to use in verifying
the signature.
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The Signature field contains a digital signature that binds the NLRI
and BGPSEC_Path_Signatures attribute to the RPKI end-entity
certificate used by the BGPSEC speaker. The digital signature is
computed as follows:
o Construct a sequence of octets by concatenating the Target AS
Number, the Secure_Path (Origin AS, pCount, and Flags), the
Additional_Info (Info Type, Info Length, and Info Value),
Algorithm Suite Identifier, and NLRI. 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 is sent.)
Sequence of Octets to be Signed
+------------------------------------+
| Target AS Number (4 octets) |
+------------------------------------+
| Origin AS Number (4 octets) | ---\
+------------------------------------+ \
| pCount (1 octet) | > Secure_Path
+------------------------------------+ /
| Flags (1 octet) | ---/
+------------------------------------+
| Info Type (1 octet) | ---\
+------------------------------------+ \
| Info Length (1 octet) | > Additional_Info
+------------------------------------+ /
| Info Value (variable) | ---/
+------------------------------------+
| Algorithm Suite Id. (1 octet) |
+------------------------------------+
| NLRI Length (1 octet) |
+------------------------------------+
| NLRI Prefix (variable) |
+------------------------------------+
o Apply to this octet sequence 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 with this digital
signature.
The Signature Length field is populated with the length (in octets)
of the Signature field.
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4.2. Propagating a Route Advertisement
When a BGPSEC speaker receives a BGPSEC update message containing a
BGPSEC_Path_Signatures attribute (with one or more signatures) from
an (internal or external) peer, it may choose to propagate the route
advertisement by sending to its (internal or external) peers by
creating a new BGPSEC advertisement for the same prefix.
If a BGPSEC router has received only non-BGPSEC update messages
(without the BGPSEC_Path_Signatures attribute), containing the
AS_Path attribute, from a peer for a given prefix and if it chooses
to propagate that peer's route for the prefix, then it MUST NOT
attach any BGPSEC_Path_Signatures attribute to the corresponding
update being propagated. (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 NLRI in it. In that case, the prefix
is originating from that AS and hence the BGPSEC speaker SHOULD sign
and forward the update to its external peers, as specified in Section
4.1.)
Conversely, if a BGPSEC router has received a BGPSEC update message
(with the BGPSEC_Path_Signatures 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_Signatures attribute. However, the BGPSEC
speaker MAY propagate the route as a (unsigned) BGP update message
without the BGPSEC_Path_Signatures attribute.
Note that removing BGPSEC signatures (i.e., propagating a route
advertisement without the BGPSEC_Path_Signatures attribute) has
significant security ramifications. (See Section 7 for 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_Signatures attribute is NOT RECOMMENDED. Furthermore,
note that when a BGPSEC speaker propagates a route advertisement with
the BGPSEC_Path_Signatures attribute it is not attesting to the
validation state of the update message it received. (See Section 7
for more discussion of the security semantics of BGPSEC signatures.)
If the BGPSEC speaker is producing an update message which 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_Signatures attribute. In such a case, the
BGPSEC speaker must remove any existing BGPSEC_Path_Signatures in the
received advertisement(s) for this prefix and produce a standard
(non-BGPSEC) update message. It should be noted that BCP 172 [5]
recommends against the use of AS_SET and AS_CONFED_SET in AS_PATH in
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BGP updates.
To generate the BGPSEC_Path_Signatures attribute on the outgoing
update message, the BGPSEC speaker first prepends a new Secure_Path
Segment (places in first position) to the Secure_Path. The AS number
in this Secure_Path segment MUST match the AS number in the AS number
resource extension field of the Resource PKI end-entity
certificate(s) that will be used to verify the digital signature(s)
constructed by this BGPSEC speaker.
The pCount is typically set to the value 1. A BGPSEC speaker may set
the pCount field to a value greater than 1. (See Section 4.1 for a
discussion of setting pCount to a value greater than 1.) A route
server that participates in the BGP control path, 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 effective
length of the AS path. (Note that BGPSEC speakers compute the
effective length of the AS path by summing the pCount values in the
BGPSEC_Path_Signatures 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. Note that the
option of setting pCount to 0 is intended only for use by route
servers that desire not to increase the effective AS-PATH length of
routes they advertise. The pCount field SHOULD NOT be set to 0 in
other circumstances. BGPSEC speakers SHOULD drop incoming update
messages with pCount set to zero in cases where the BGPSEC speaker
does not expect its peer to set pCount to zero (i.e., cases where the
peer is not acting as a route server).
If the BGPSEC speaker is not a member of an autonomous system
confederation [3], then the Flags field of the Secure_Path Segment
MUST be set to zero. (Members of a confederation should follow the
special processing instructions for confederation members in Section
4.4.)
The BGPSEC speaker next copies the Additional_Info portion of the
BGPSEC_Path_Signatures directly from the received update message to
the new update message (that it is constructing). Note that the
BGPSEC speaker MUST NOT change the Additional_Info as any change to
Additional_Info will cause the new BGPSEC update message to fail
validation (see Section 5).
If the received BGPSEC update message contains two Signature_ Blocks
and the BGPSEC speaker supports both of the corresponding algorithms
suites, then the new update message generated by the BGPSEC speaker
SHOULD include both of the Signature_Blocks. If the received BGPSEC
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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_Signatures attribute (i.e., propagate it as an unsigned
BGP update message).
Note that in the case where there are two Signature_Blocks
(corresponding to different algorithm suites) that the validation
algorithm (see Section 5.1) deems a BGPSEC update message to be
'Good' if there is at least one supported algorithm suite (and
corresponding Signature_Block) that is deemed 'Good'. This means
that a 'Good' BGPSEC update message may contain a Signature_Block
which is not deemed 'Good' (e.g., contains signatures that the BGPSEC
does not successfully verify). Nonetheless, such Signature_Blocks
MUST NOT be removed. (See Section 7 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 then adds 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 in the Secure_Path
portion of the BGPSEC_Path_Signatures attribute. 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 end-entity certificate used by the BGPSEC speaker. 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 NLRI and BGPSEC_Path_Signatures attribute to the RPKI
end-entity certificate used by the BGPSEC speaker. The digital
signature is computed as follows:
o Construct a sequence of octets by concatenating the Target AS
number, the Secure_Path segment that is being added by the BGPSEC
speaker constructing the signature, and the signature field of the
most recent Signature Segment (the one corresponding to AS from
whom the BGPSEC speaker's AS received the announcement). Note
that the Target AS number is the AS number announced by the peer
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in the OPEN message of the BGP session within which the BGPSEC
update message is sent.
Sequence of Octets to be Signed
+--------------------------------------+
| Target AS Number (4 octets) |
+--------------------------------------+
| Signer's AS Number (4 octets) | ---\
+--------------------------------------+ \
| pCount (1 octet) | > Secure_Path
+--------- ----------------------------+ /
| Flags (1 octet) | ---/
+--------------------------------------+
| Most Recent Sig Field (variable) |
+--------------------------------------+
o Apply to this octet sequence 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 with this digital
signature.
The Signature Length field is populated with the length (in octets)
of the Signature field.
4.3. Reconstructing the AS_Path Attribute
EDITOR'S NOTE: This is a place-holder section. Given that BGPSEC
update messages do not contain the AS_Path attribute, this document
needs to include a clearly specified algorithm for reconstructing the
AS_Path attribute from the data in the BGPSEC_Path_Signatures
attribute. (For example, when propagating a path received via BGPSEC
to a non-BGPSEC peer.) This algorithm for reconstructing the AS_Path
will appear in the next version of this document. In essence, the
algorithm is: For each Secure_Path Segment put into the AS_Path
pCount copies of the AS number field of the segment --- and if you
see the Entering_Confed flag is set to one, then add an
AS_Confed_Sequence to the AS_Path.
4.4. Processing Instructions for Confederation Members
Members of autonomous system confederations [3] must additionally
follow the instructions in this section for processing BGPSEC update
messages.
When a confederation member sends a BGPSEC update message to a peer
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who is a member of the same confederation, the confederation member
puts its (private) Member-AS Number (as opposed to the public AS
Confederation Identifier) in the AS Number field of the Secure_Path
Segment that it adds to the BGPSEC update message.
Furthermore, when sending a BGPSEC update message to a peer who is a
member of the same confederation, the first confederation member to
add a Secure_Path Segment to a BGPSEC update message sets the
Entering_Cofed flag in the Flags field to be one in the Secure_Path
Segment that it produces. In the case where the route advertisement
originates within a confederation, the member AS that originates the
route and sends it to a peer is the member AS that sets its
Entering_Confed flag to one. In the case where the route
advertisement is received from outside the confederation, it is the
member AS that receives the route advertisement from a peer outside
the confederation and propagates it to a peer inside the
confederation that sets its Entering_Confed flag to one. Note that
this is the same of the confederation member who would have added a
new AS_Path segment of type AS_Confed_Sequence to the AS_Path
attribute in a non-BGPSEC update message.
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 must 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, search through the Secure_Path, going from most recently
added segment to least recently added segment, and find first
Secure_Path Segment with the Entering_Confed flag to set to one.
(That is, of all the Secure_Path Segments with the Entering_Confed
flag set to one, find the one that was most recently added.)
o Second, remove the Secure_Path Segment found in previous step
along with all more recently added Secure_Path Segments. Keep a
count of the number of segments removed in this fashion.
o Third, 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 that the AS field are populated as per
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Sections 4.1 and 4.2.
When validating a received BGPSEC update message, confederation
members must make the following adjustment to the algorithm presented
in Section 5.1. That is, when a confederation member is processing
(validating) a Signature Segment and its corresponding Secure_Path
Segment, the confederation member must note that when a signature is
produced by a BGPSEC speaker outside of a confederation, the Target
AS will always be the AS Confederation Identifier (the public AS
number of the confederation) as opposed to the Member-AS Number. To
handle this case, when processing a current Secure_Path Segment, if
the next most recently added Secure_Path segment has the
Entering_Confed flag set then ,when computing the digest for the
current Secure_Path segment, take the Target AS Number to be the AS
Confederation Identifier of the validating BGPSEC speaker's own
confederation. (Note that the algorithm in Section 5.1 processes
Secure_Path Segments in order from most recently added to least
recently added, therefore the algorithm encounters the
Entering_Confed flag immediately before it encounters the Secure_Path
segment that requires using the AS Confederation Identifier to
validate.)
5. Processing a Received BGPSEC Update
Validation of a BGPSEC update messages makes use of data from RPKI
certificates and signed Route Origination Authorizations (ROA). In
particular, to validate update messages containing the
BGPSEC_Path_Signatures attribute, it is necessary that the recipient
have access to the following data obtained from valid RPKI
certificates and ROAs:
o For each valid RPKI end-entity certificate containing an AS Number
extension, the AS Number, Public Key and Subject Key Identifier
are required,
o For each valid ROA, the AS Number and the list of IP address
prefixes.
Note that the BGPSEC speaker could perform the validation of RPKI
certificates and ROAs 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. (The latter
case in analogous to the use of the RPKI-RTR protocol [13] for origin
validation.)
To validate a BGPSEC update message containing the
BGPSEC_Path_Signatures attribute, the recipient performs the
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validation steps specified in Section 5.1. The validation procedure
results in one of two states: 'Good' and 'Not Good'.
It is expected that the output of the validation procedure will be
used as an input to BGP route selection. However, BGP route
selection and thus the handling of the two validation states is a
matter of local policy, and shall be handled using existing local
policy mechanisms. It is expected that BGP peers will generally
prefer routes received via 'Good' BGPSEC update messages over routes
received via 'Not Good' BGPSEC update messages as well as routes
received via update messages that do not contain the
BGPSEC_Path_Signatures attribute. However, BGPSEC specifies no
changes to the BGP decision process and leaves to the operator the
selection of an appropriate policy mechanism to achieve the
operator's desired results within the BGP decision process.
BGPSEC validation needs only be performed at eBGP edge. The
validation status of a BGP signed/unsigned update MAY be conveyed via
iBGP from an ingress edge router to an egress edge router. Local
policy in the AS determines the specific means for conveying the
validation status through various pre-existing mechanisms (e.g.,
modifying an attribute). 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_Signatures
attribute intact (regardless of the validation state of the update
message). Based entirely on local policy settings, an egress router
MAY trust the validation status conveyed by an ingress router or it
MAY perform its own validation.
Upon receiving a BGPSEC update message, a BGPSEC speaker SHOULD sum
the pCount values within BGPSEC_Path_Signatures attribute to
determine the effective length of the AS Path. The BGPSEC speaker
SHOULD use this sum of pCount values in precisely the same way as it
uses the length of the AS Path in non-BGPSEC update messages.
5.1. 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 external
behavior of this algorithm.
First, the recipient of a BGPSEC update message performs a check to
ensure that the message is properly formed. Specifically, the
recipient performs the following checks:
o Check to ensure that the entire BGPSEC_Path_Signatures attribute
is syntactically correct (conforms to the specification in this
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document).
o Check that each Signature_Block contains one Signature segment for
each Secure_Path segment in the Secure_Path portion of the
BGPSEC_Path_Signatures 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.)
If there are two Signature_Blocks within the BGPSEC_Path_Signatures
attribute and one of them is poorly formed (or contains the wrong
number of Signature segments) , then the recipient should log that an
error occurred, strip off that particular Signature_Block and process
the update message as though it arrived with a single
Signature_Block. If the BGPSEC_Path_Signatures attribute contains a
syntax error that is not local to one of two Signature_Blocks, then
the recipient should log that an error occurred and drop the update
message containing the error. Similarly, if an update message
contains both the BGPSEC_Path_Signatures attribute and either an
AS_Path or AS4_Path attribute, then the recipient should log that an
error occurred and drop the update message containing the error.
Next, the BGPSEC speaker verifies that the origin AS is authorized to
advertise the prefix in question. To do this, consult the valid ROA
data to obtain a list of AS numbers that are associated with the
given IP address prefix in the update message. Then locate the last
(least recently added) AS number in the Secure_Path portion of the
BGPSEC_Path_Signatures attribute. If the origin AS in the
Secure_Path is not in the set of AS numbers associated with the given
prefix, then the BGPSEC update message is 'Not Good' and the
validation algorithm terminates.
Finally, the BGPSEC speaker examines the Signature_Blocks in the
BGPSEC_Path_Signatures attribute. A Signature_Block corresponding to
an algorithm suite that the BGPSEC speaker does not support is not
considered in validation. If there does not exist a Signature_Block
corresponding to an algorithm suite that the BGPSEC speaker supports,
then the BGPSEC speaker MUST treat the update message in the same
manner that the BGPSEC speaker would treat an (unsigned) update
message that arrived without a BGPSEC_Path_Signatures attribute.
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_Signatures attribute. The following steps
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make use of this correspondence.
o (Step I): Locate the public key needed to verify the signature (in
the current Signature segment). To do this, consult the valid
RPKI end-entity certificate data and look up all valid (AS, SKI,
Public Key) 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-List Block as 'Not Good' and proceed to
the next Signature-List Block.
o (Step II): Compute the digest function (for the given algorithm
suite) on the appropriate data. If the segment is not the (least
recently added) segment corresponding to the origin AS, then the
digest function should be computed on the following sequence of
octets:
Sequence of Octets to be Hashed
+-------------------------------------------+
| AS Number of Target AS (4 octets) |
+-------------------------------------------+
| AS Number (4 octets) | ---\
+-------------------------------------------+ \
| pCount (1 octet) | > Secure_Path
+-------------------------------------------+ /
| Flags (1 octet) | ---/
+-------------------------------------------+
| Sig Field in the Next Segment (variable) |
+-------------- ----------------------------+
For the first segment to be processed (the most recently added
segment), the 'AS Number of Target AS' is 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 was received.
For each other Signature Segment, the 'AS Number of Target AS' is 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.)
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The AS Number, pCount and Flags fields are taken from the Secure_Path
segment that corresponds to the Signature segment currently being
processed. The 'Signature Field in the Next Segment' is the
Signature field found in the Signature segment that is next to be
processed (that is, the next most recently added Signature Segment).
Alternatively, if the segment being processed corresponds to the
origin AS (i.e., if it is the least recently added segment), then the
digest function should be computed on the following sequence of
octets:
Sequence of Octets to be Hashed
+------------------------------------+
| AS Number of Target AS (4 octets) |
+------------------------------------+
| Origin AS Number (4 octets) | ---\
+------------------------------------+ \
| pCount (1 octet) | > Secure_Path
+------------------------------------+ /
| Flags (1 octet) | ---/
+------------------------------------+
| Info Type (1 octet) | ---\
+------------------------------------+ \
| Info Length (1 octet) | > Additional_Info
+------------------------------------+ /
| Info Value (variable) | ---/
+------------------------------------+
| Algorithm Suite Id. (1 octet) |
+------------------------------------+
| NLRI Length (1 octet) |
+------------------------------------+
| NLRI Prefix (variable) |
+------------------------------------+
The NLRI Length, NLRI Prefix, Additional_Info, and Algorithm Suite
Identifier are all obtained in a straight forward manner from the
NLRI of the update message or the BGPSEC_Path_Signatures attribute
being validated. The Origin AS Number, pCount, and Flags fields are
taken from the Secure_Path segment corresponding to the Signature
Segment currently being processed.
The 'AS Number of Target AS' is the AS Number from the Secure_Path
segment that was added immediately after the Secure_Path segment
containing the Origin AS Number. (That is, the Secure_Path segment
corresponding to the Signature segment that the receiver just
finished processing prior to the current Signature segment.)
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o (Step III): 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 II above; and
the public key obtained from the valid RPKI data in Step I above.
If the signature validation algorithm determines that the
signature is invalid, then mark the entire Signature-List Block as
'Not Good' 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-List Block).
If all Signature-Segments within a Signature-List Block pass
validation (i.e., all segments are processed and the Signature-List
Block has not yet been marked 'Not Good'), then the Signature_Block
is marked as 'Good'.
If at least one Signature_Block is marked as 'Good', then the
validation algorithm terminates and the BGPSEC update message is
deemed to be 'Good'. (That is, if a BGPSEC update message contains
two Signature_Blocks then the update message is deemed 'Good' if the
first Signature_Block is marked 'Good' OR the second Signature_Block
is marked 'Good'.)
6. Algorithms and Extensibility
6.1. Algorithm Suite Considerations
Note that there is currently no support for bilateral negotiation
between BGPSEC peers to use of a particular (digest and signature)
algorithm suite using BGP capabilities. 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 he 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, a mandatory algorithm suites document will be created
which specifies a mandatory-to-use 'current' algorithm suite for use
by all BGPSEC speakers [12]. Additionally, the document specifies an
additional 'new' algorithm suite that is recommended to implement.
It is anticipated that in the future the mandatory algorithm suites
document will be updated to specify a transition from the 'current'
algorithm suite to the 'new' algorithm suite. During the period of
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transition (likely a small number of years), 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_Signatures attribute can contain signatures, in
parallel, for two algorithm suites.) Once the transition is
complete, use of the old 'current' algorithm will be deprecated, use
of the 'new' algorithm will be mandatory, and a subsequent 'even
newer' algorithm suite may be specified as recommend 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. Extensibility Considerations
This section discusses potential changes to BGPSEC that would require
substantial changes to the processing of the BGPSEC_Path_Signatures
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_SIG_TWO, which is designed to accommodate
the desired changes to BGPSEC. In such a case, the mandatory
algorithm suites document would be updated to specify algorithm
suites appropriate for the new version of BGPSEC.
At this point a transition would begin which is analogous to the
algorithm transition discussed in Section 6.2. During the transition
period all BGPSEC speakers SHOULD simultaneously include both the
BGPSEC_PATH_SIGNATURES attribute and the new BGPSEC_PATH_SIG_TWO
attribute. Once the transition is complete, the use of
BGPSEC_PATH_SIGNATURES could then be deprecated, at which point
BGPSEC speakers SHOULD include only the new BGPSEC_PATH_SIG_TWO
attribute. Such a process could facilitate a transition to a new
BGPSEC semantics in a backwards compatible fashion.
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7. Security Considerations
For discussion of the BGPSEC threat model and related security
considerations, please see [10].
A BGPSEC speaker who receives a valid BGPSEC update message,
containing a route advertisement for a given prefix, is provided with
the following security guarantees:
o The origin AS number corresponds to an autonomous system that has
been authorized by the IP address space holder to originate route
advertisements for the given prefix.
o For each AS number in the AS Path, a BGPSEC speaker authorized by
the holder of the AS number intentionally chose (in accordance
with local policy) to propagate the route advertisement to the
next AS in the Secure_Path.
That is, the recipient of a valid BGPSEC Update message is assured
that the Secure_Path corresponds to a sequence of autonomous systems
who have all agreed in principle to forward packets to the given
prefix along the indicated path. (It should be noted that BGPSEC
does not offer a precise guarantee that the data packets would
propagate along the indicated path; it only guarantees that the BGP
update conveying the path indeed propagated along the indicated
path.) Furthermore, the recipient is assured that this path
terminates in an autonomous system that has been authorized by the IP
address space holder as a legitimate destination for traffic to the
given prefix.
Note that although BGPSEC provides a mechanism for an AS to validate
that a received update message has certain security properties, the
use of such a mechanism to influence route selection is completely a
matter of local policy. Therefore, a BGPSEC speaker can make no
assumptions about the validity of a route received from an external
BGPSEC peer. That is, a compliant BGPSEC peer may (depending on the
local policy of the peer) send update messages that fail the validity
test in Section 5. Thus, a BGPSEC speaker MUST completely validate
all BGPSEC update messages received from external peers. (Validation
of update messages received from internal peers is a matter of local
policy, see Section 5).
Note that there may be cases where a BGPSEC speaker deems 'Good' (as
per the validation algorithm in Section 5.1) a BGPSEC update message
that contains both a 'Good' and a 'Not Good' Signature_Block. That
is, the update message contains two sets of signatures corresponding
to two algorithm suites, and one set of signatures verifies correctly
and the other set of signatures fails to verify. In this case, the
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protocol specifies that if the BGPSEC speaker propagates the route
advertisement received in such an update message then the BGPSEC
speaker SHOULD add its signature to each of the Signature_Blocks
using both the corresponding algorithm suite. Thus the BGPSEC
speaker creates a signature using both algorithm suites and creates a
new update message that contains both the 'Good' and the 'Not Good'
set of signatures (from its own vantage point).
To understand the reason for such a design decision consider the case
where the BGPSEC speaker receives an update message with both a set
of algorithm A signatures which are 'Good' and a set of algorithm B
signatures which are 'Not Good'. In such a case it is possible
(perhaps even quite likely) that some of the BGPSEC speaker's peers
(or other entities further 'downstream' in the BGP topology) do not
support algorithm A. Therefore, if the BGPSEC speaker were to remove
the 'Not Good' set of signatures corresponding to algorithm B, such
entities would treat the message as though it were unsigned. By
including the 'Not Good' set of signatures when propagating a route
advertisement, the BGPSEC speaker ensures that 'downstream' entities
have as much information as possible to make an informed opinion
about the validation status of a BGPSEC update.
Note also that during a period of partial BGPSEC deployment, a
'downstream' entity might reasonably treat unsigned messages
different from BGPSEC updates that contain a single set of 'Not Good'
signatures. That is, by removing the set of 'Not Good' signatures
the BGPSEC speaker might actually cause a downstream entity to
'upgrade' the status of a route advertisement from 'Not Good' to
unsigned. Finally, note that in the above scenario, the BGPSEC
speaker might have deemed algorithm A signatures 'Good' only because
of some issue with RPKI state local to his AS (for example, his AS
might not yet have obtained a CRL indicating that a key used to
verify an algorithm A signature belongs to a newly revoked
certificate). In such a case, it is highly desirable for a
downstream entity to treat the update as 'Not Good' (due to the
revocation) and not as 'unsigned' (which would happen if the 'Not
Good' Signature_Blocks were removed).
A similar argument applies to the case where a BGPSEC speaker (for
some reason such as lack of viable alternatives) selects as his best
route to a given prefix a route obtained via a 'Not Good' BGPSEC
update message. (That is, a BGPSEC update containing only 'Not Good'
Signature-List Blocks.) In such a case, the BGPSEC speaker should
propagate a signed BGPSEC update message, adding his signature to the
'Not Good' signatures that already exist. Again, this is to ensure
that 'downstream' entities are able to make an informed decision and
not erroneously treat the route as unsigned. It may also be noted
here that due to possible differences in RPKI data at different
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vantage points in the network, a BGPSEC update that was deemed 'Not
Good' at an upstream BGPSEC speaker may indeed be deemed 'Good' at
another BGP speaker downstream.
Therefore, it is important to note that when a BGPSEC speaker signs
an outgoing update message, it is not attesting to a belief that all
signatures prior to its are valid. Instead it is merely asserting
that:
o The BGPSEC speaker received the given route advertisement with the
indicated NLRI and Secure_Path; and
o The BGPSEC speaker chose to propagate an advertisement for this
route to the peer (implicitly) indicated by the 'Target AS'
The BGPSEC update validation procedure is a potential target for
denial of service attacks against a BGPSEC speaker. To mitigate the
effectiveness of such denial of service attacks, BGPSEC speakers
should implement an update validation algorithm that performs
expensive checks (e.g., signature verification) after performing less
expensive checks (e.g., syntax checks). The validation algorithm
specified in Section 5.1 was chosen so as to perform checks which are
likely to be expensive after checks that are likely to be
inexpensive. However, the relative cost of performing required
validation steps may vary between implementations, and thus the
algorithm specified in Section 5.1 may not provide the best denial of
service protection for all implementations.
The mechanism of setting the pCount field to zero is included in this
specification to enable route servers in the control path to
participate in BGPSEC without increasing the effective length of the
AS-PATH. However, entities other than route servers could
conceivably use this mechanism (set the pCount to zero) to attract
traffic (by reducing the effective length of the AS-PATH)
illegitimately. This risk is largely mitigated if every BGPSEC
speaker drops incoming update messages that set pCount to zero but
come from a peer that is not a route server. However, note that a
recipient of a BGPSEC update message in which an upstream entity that
is two or more hops away set pCount to zero is unable to verify for
themselves whether pCount was set to zero legitimately.
Finally, BGPSEC does not provide protection against all attacks at
the transport layer. An adversary on the path between a BGPSEC
speaker and its peer is able to perform attacks such as modifying
valid BGPSEC updates to cause them to fail validation, injecting
(unsigned) BGP update messages without BGPSEC_Path_Signature
attributes, or injecting BGPSEC update messages with
BGPSEC_Path_Signature attributes that fail validation, or causing the
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peer to tear-down the BGP session. Therefore, BGPSEC implementations
MUST support appropriate transport security mechanisms.
EDITOR'S NOTE: Do we want to mandate a specific transport security
mechanism (e.g., TCP-AO)?
8. Contributors
8.1. Authors
Rob Austein
Dragon Research Labs
sra@hactrn.net
Steven Bellovin
Columbia University
smb@cs.columbia.edu
Randy Bush
Internet Initiative Japan
randy@psg.com
Russ Housley
Vigil Security
housley@vigilsec.com
Matt Lepinski
BBN Technologies
lepinski@bbn.com
Stephen Kent
BBN Technologies
kent@bbn.com
Warren Kumari
Google
warren@kumari.net
Doug Montgomery
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USA National Institute of Standards and Technology
dougm@nist.gov
Kotikalapudi Sriram
USA National Institute of Standards and Technology
kotikalapudi.sriram@nist.gov
Samuel Weiler
Cobham
weiler+ietf@watson.org
8.2. Acknowledgements
The authors would like to thank Luke Berndt, Sharon Goldberg, Ed
Kern, Chris Morrow, Doug Maughan, Pradosh Mohapatra, Russ Mundy,
Sandy Murphy, Keyur Patel, Mark Reynolds, Heather Schiller, Jason
Schiller, John Scudder, Ruediger Volk and David Ward for their
valuable input and review.
9. Normative References
[1] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A Border
Gateway Protocol 4", RFC 4271, January 2006.
[2] Bates, T., Chandra, R., Katz, D., and Y. Rekhter,
"Multiprotocol Extensions for BGP-4", RFC 4760, January 2007.
[3] Traina, P., McPherson, D., and J. Scudder, "Autonomous System
Confederations for BGP", RFC 5065, August 2007.
[4] Scudder, J. and R. Chandra, "Capabilities Advertisement with
BGP-4", RFC 5492, February 2009.
[5] Kumari, W. and K. Sriram, "Recommendation for Not Using AS_SET
and AS_CONFED_SET in BGP", RFC 6472, December 2011.
[6] Lepinski, M. and S. Kent, "Recommendation for Not Using AS_SET
and AS_CONFED_SET in BGP", RFC 6480, February 2012.
[7] Lepinski, M., Kent, S., and D. Kong, "Recommendation for Not
Using AS_SET and AS_CONFED_SET in BGP", RFC 6482,
February 2012.
[8] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
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[9] Patel, K., Ward, D., and R. Bush, "Extended Message support for
BGP", March 2011.
[10] Kent, S., "Threat Model for BGP Path Security", February 2012.
[11] Reynolds, M., Turner, S., and S. Kent, "A Profile for BGPSEC
Router Certificates, Certificate Revocation Lists, and
Certification Requests", December 2011.
[12] Turner, S., "BGP Algorithms, Key Formats, & Signature Formats",
March 2012.
[13] Bush, R. and R. Austein, "The RPKI/Router Protocol",
February 2012.
Author's Address
Matthew Lepinski (editor)
BBN
10 Moulton St
Cambridge, MA 55409
US
Phone: +1 617 873 5939
Email: mlepinski@bbn.com
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