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Network Working Group M. Lepinski, Ed.
Internet-Draft BBN
Intended status: Standards Track February 25, 2013
Expires: August 25, 2013
BGPSEC Protocol Specification
draft-ietf-sidr-bgpsec-protocol-07
Abstract
This document describes BGPSEC, an extension to the Border Gateway
Protocol (BGP) that provides security for the path of autonomous
systems through which a BGP update message passes. BGPSEC is
implemented via a new optional non-transitive BGP path attribute that
carries a digital signature produced by each autonomous system that
propagates the update message.
Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" are to be interpreted as described in RFC 2119 [1] only
when they appear in all upper case. They may also appear in lower or
mixed case as English words, without normative meaning
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 April 25, 2013.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
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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
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
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. BGPSEC Negotiation . . . . . . . . . . . . . . . . . . . . . . 3
2.1. BGPSEC Send Capability . . . . . . . . . . . . . . . . . . 3
2.2. BGPSEC Receive Capability . . . . . . . . . . . . . . . . 4
2.3. Negotiating BGPSEC Support . . . . . . . . . . . . . . . . 5
3. The BGPSEC_Path Attribute . . . . . . . . . . . . . . . . . . 6
3.1. Secure_Path . . . . . . . . . . . . . . . . . . . . . . . 8
3.2. Signature_Block . . . . . . . . . . . . . . . . . . . . . 9
4. Generating a BGPSEC Update . . . . . . . . . . . . . . . . . . 11
4.1. Originating a New BGPSEC Update . . . . . . . . . . . . . 12
4.2. Propagating a Route Advertisement . . . . . . . . . . . . 14
4.3. Processing Instructions for Confederation Members . . . . 18
4.4. Reconstructing the AS_PATH Attribute . . . . . . . . . . . 20
5. Processing a Received BGPSEC Update . . . . . . . . . . . . . 21
5.1. Overview of BGPSEC Validation . . . . . . . . . . . . . . 23
5.2. Validation Algorithm . . . . . . . . . . . . . . . . . . . 24
6. Algorithms and Extensibility . . . . . . . . . . . . . . . . . 28
6.1. Algorithm Suite Considerations . . . . . . . . . . . . . . 28
6.2. Extensibility Considerations . . . . . . . . . . . . . . . 28
7. Security Considerations . . . . . . . . . . . . . . . . . . . 29
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 32
9. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 32
9.1. Authors . . . . . . . . . . . . . . . . . . . . . . . . . 32
9.2. Acknowledgements . . . . . . . . . . . . . . . . . . . . . 34
10. Normative References . . . . . . . . . . . . . . . . . . . . . 34
11. Informative References . . . . . . . . . . . . . . . . . . . . 35
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 35
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1. Introduction
This document describes BGPSEC, a mechanism for providing path
security for Border Gateway Protocol (BGP) [2] 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 on the path of ASes through which the update message
passes has explicitly authorized the advertisement of the route
to the subsequent AS in the path.
This document specifies a new optional (non-transitive) BGP path
attribute, BGPSEC_Path. 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 [7] and
the documents referenced therein.) Any BGPSEC speaker who wishes to
send BGP update messages to external peers (eBGP) containing the
BGPSEC_Path needs to have the private key associated with an RPKI
router certificate [10] that corresponds to the BGPSEC speaker's AS
number. Note, however, that a BGPSEC speaker does not need such a
certificate in order to validate update messages containing the
BGPSEC_Path attribute.
2. BGPSEC Negotiation
This document defines a new BGP capability [6] that allows a BGP
speaker to advertise to a neighbor the ability to send or to receive
BGPSEC update messages (i.e., update messages containing the
BGPSEC_Path attribute).
2.1. The BGPSEC Capability
This capability has capability code : TBD
The capability length for this capability MUST be set to 3.
The three octets of the capability value are specified as follows.
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BGPSEC Send Capability Value:
0 1 2 3 4 5 6 7
+---------------------------------------+
| Version | Dir | Reserved |
+---------------------------------------+
| |
+------ AFI -----+
| |
+---------------------------------------+
The first four bits 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.
The fifth bit of the first octet is a direction bit which indicates
whether the BGP speaker is advertising the capability to send BGPSEC
update message or receive BGPSEC update messages. The BGP speaker
sets this bit to 0 to indicate the capability to receive BGPSEC
update messages. The BGP speaker sets this bit to 1 to indicate the
capability to send BGPSEC update messages.
The remaining three bits of the first octet are reserved for future
use. These bits are set to zero by the sender of the capability and
ignored by the receiver of the capability.
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
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.
2.2. Negotiating BGPSEC Support
In order to indicate that a BGP speaker is willing to send BGPSEC
update messages (for a particular address family), a BGP speaker
sends the BGPSEC Capability (see Section 2.1) with the Direction bit
(the fifth bit of the first octet) set to 1. In order to indicate
that the speaker is willing to receive BGP update messages containing
the BGPSEC_Path attribute (for a particular address family), a BGP
speaker sends the BGPSEC capability with the Direction bit set to 0.
In order to advertise the capability to both send and receive BGPSEC
update messages, the BGP speaker sends two copies of the BGPSEC
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capability (one with the direction bit set to 0 and one with the
direction bit set to 1).
Similarly, if a BGP speaker wishes to use BGPSEC with two different
address families (i.e., IPv4 and IPv6) over the same BGP session,
then the speaker includes two instances of this capability (one for
each address family) in the BGP OPEN message. A BGP speaker SHOULD
NOT advertise the capability of BGPSEC support for a particular AFI
unless it has also advertised the multiprotocol extension capability
for the same AFI combination [3].
In a session where BGP session, a peer is permitted to send update
messages containing the BGPSEC_Path attribute if, and only if:
o The given peer has sent the BGPSEC capability for a particular
version of BGPSEC and a particular address family with the
Direction bit set to 1; and
o The other peer has sent the BGPSEC capability for the same version
of BGPSEC and the same address family with the Direction bit set
to 0.
In such a session, we say that the use of (the particular version of)
BGPSEC has been negotiated (for a particular address family). BGP
update messages without the BGPSEC_PATH attribute MAY be sent within
a session regardless of whether or not the use of BGPSEC is
successfully negotiated. However, if BGPSEC is not successfully
negotiated, then BGP update messages containing the BGPSEC_PATH
attribute MUST NOT be sent.
This document defines the behavior of implementations in the case
where BGPSEC version zero is the only version that has been
successfully negotiated. If there exist multiple versions have
BGPSEC that are negotiated for a particular session, the behavior of
the peers (e.g., which version of BGPSEC shall actually be used) will
be specified in a future document.
BGPSEC cannot provide meaningful security guarantees without support
for four-byte AS numbers. Therefore, any BGP speaker that announces
the BGPSEC capability, MUST also announce the capability for four-
byte AS support [4]. If a BGP speaker sends the BGPSEC capability
but not the four-byte AS support capability then BGPSEC has not been
successfully negotiated, and update messages containing the
BGPSEC_Path attribute MUST NOT be sent within such a session.
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
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extended messages [9].
3. The BGPSEC_Path Attribute
The BGPSEC_Path 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 attribute carries the secured information regarding
the path of ASes through which an update message passes. This
includes the digital signatures used to protect this information. We
refer to those update messages that contain the BGPSEC_Path attribute
as "BGPSEC Update messages". The BGPSEC_Path attribute replaces the
AS_PATH attribute in a BGPSEC update message. That is, update
messages that contain the BGPSEC_Path attribute MUST NOT contain the
AS_PATH attribute, and vice versa.
The BGPSEC_Path attribute is made up of several parts. The following
high-level diagram provides an overview of the structure of the
BGPSEC_Path attribute:
High-Level Diagram of the BGPSEC_Path Attribute
+---------------------------------------------------------+
| +-----------------+ |
| | Secure Path | |
| +-----------------+ |
| | AS X | |
| | pCount X | |
| | Flags X | |
| | AS Y | |
| | pCount Y | |
| | Flags Y | |
| | ... | |
| +-----------------+ |
| |
| +-----------------+ +-----------------+ |
| | Sig Block 1 | | Sig Block 2 | |
| +-----------------+ +-----------------+ |
| | Alg Suite 1 | | Alg Suite 2 | |
| | SKI X1 | | SKI X1 | |
| | Signature X1 | | Signature X1 | |
| | SKI Y1 | | SKI Y1 | |
| | Signature Y1 | | Signature Y1 | |
| | ... | | .... | |
| +-----------------+ +-----------------+ |
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| |
+---------------------------------------------------------+
The following is the specification of the format for the BGPSEC_Path
attribute.
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BGPSEC_Path Attribute
+-------------------------------------------------------+
| Secure_Path (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 is
contained in a non-BGPSEC AS_PATH attribute. A BGPSEC update message
containing the BGPSEC_PATH attribute MUST NOT contain the AS_PATH
attribute. The Secure_Path is used by BGPSEC speakers in the same
way that information from the AS_PATH is used by non-BGPSEC speakers.
The format of the Secure_Path is described below in Section 3.1.
The BGPSEC_Path 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 one AS
number (i.e, secure path segment) in the Secure_Path. In the most
common case, the BGPSEC_Path 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 (without a flag day), 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 3.2.
3.1. Secure_Path
Here we provide a detailed description of the Secure_Path information
in the BGPSEC_Path 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 entire
Secure_Path (including the two octets used to express this length
field). As explained below, each Secure_Path segment is six octets
long. Note that this means the Secure_Path Length is two greater
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than 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 originating AS of the NLRI
specified in the update 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 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 prepending
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 Confed_Segment flag. The
Confed_Segment flag is set to one to indicate that the BGPSEC speaker
that constructed this Secure_Path segment is sending the update
message to a peer AS within the same Autonomous System confederation
[5]. (That is, the Confed_Segment flag is set in a BGPSEC update
message whenever in a non-BGPSEC update message the BGP speaker's AS
would appear in a AS_PATH segment of type AS_CONFED_SEQUENCE.) In
all other cases the Confed_Segment flag is set to zero.
The remaining seven bits of the Flags MUST be set to zero by the
sender, and ignored by the receiver. Note, however, that the
signature is computed over all eight bits of the flags field.
3.2. Signature_Block
Here we provide a detailed description of the Signature_Blocks in the
BGPSEC_Path attribute.
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Signature_Block
+---------------------------------------------+
| Signature_Block Length (2 octets) |
+---------------------------------------------+
| Algorithm Suite Identifier (1 octet) |
+---------------------------------------------+
| Sequence of Signature Segments (variable) |
+---------------------------------------------+
The Signature_Block Length is the total number of octets in the
Signature_Block (including the two octets used to express this length
field).
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[11].
A Signature_Block has exactly one Signature Segment for each
Secure_Path Segment in the Secure_Path portion of the BGPSEC_Path
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 router certificate [10] that is used
to verify the signature (see Section 5 for details on validity of
BGPSEC update messages).
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 attribute (see Sections 4 and 5 for details on
signature generation and validation, respectively).
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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 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 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 attribute will contain AS
number(s) other than the speaker's own AS.
The remaining case is where the BGPSEC speaker sends the update
message to an internal (iBGP) peer. When originating a new route
advertisement and sending it to an internal peer, the BGPSEC speaker
creates a new BGPSEC_Path attribute with zero Secure_Path segments
and zero Signature Segments. When propagating a received route
advertisement to an internal peer, the BGPSEC speaker populates the
BGPSEC_Path attribute by copying the BGPSEC_Path attribute from the
received update message. That is, the BGPSEC_Path 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 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
update message for each NLRI.
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
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key in a valid Resource PKI end-entity certificate whose AS number
resource extension includes the BGPSEC speaker's AS number [10]. 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 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 an AS number in
the AS number resource extension field of the Resource PKI router
certificate(s) [10] that will be used to verify the digital
signature(s) constructed by this BGPSEC speaker.
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.
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 [8]). Note that validation of a
BGPSEC update message will fail (i.e., the validation algorithm,
specified in Section 5.2, returns 'Not Valid') unless there exists a
valid ROA authorizing the first AS in the Secure_Path portion of the
BGPSEC_Path 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 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
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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 [5], 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.)
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).
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 router certificate corresponding to the BGPSEC speaker[10].
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 contains a digital signature that binds the NLRI
and BGPSEC_Path attribute to the RPKI router corresponding to 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), 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.)
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Sequence of Octets to be Signed
+------------------------------------+
| Target AS Number (4 octets) |
+------------------------------------+
| Origin AS Number (4 octets) | ---\
+------------------------------------+ \
| pCount (1 octet) | > Secure_Path
+------------------------------------+ /
| Flags (1 octet) | ---/
+------------------------------------+
| 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.
4.2. Propagating a Route Advertisement
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 to its (internal or external) peers by creating a new
BGPSEC advertisement for the same prefix.
If a BGPSEC router has received only a non-BGPSEC update message
(without the BGPSEC_Path 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 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.)
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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. However, the BGPSEC speaker MAY propagate the
route as a (unsigned) BGP update message without 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 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 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
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 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 standard (non-BGPSEC)
update message. It should be noted that BCP 172 [12] recommends
against the use of AS_SET and AS_CONFED_SET in AS_PATH in BGP
updates.
To generate the BGPSEC_Path 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 router certificate(s)
that will be used to verify the digital signature(s) constructed by
this BGPSEC speaker[10].
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
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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 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 [5], then the Confed_Segment bit of 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 4.3.)
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
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).
Note that in the case where there are two Signature_Blocks
(corresponding to different algorithm suites) that 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
which is not deemed 'Valid' (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
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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 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 router corresponding to the BGPSEC speaker[10]. 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 attribute to the RPKI router
certificate corresponding to 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
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
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signature.
The Signature Length field is populated with the length (in octets)
of the Signature field.
4.3. Processing Instructions for Confederation Members
Members of autonomous system confederations [5] 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
that 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
a confederation member sends a BGPSEC update message to a peer that
is a member of the same confederation, the BGPSEC speaker that
generates the Secure_Path Segment sets the Confed_Segment flag to
one. Note that this means that in a BGPSEC update message, an AS
number appears in a Secure_Path Segment with the Confed_Segment flag
set to one, in precisely those circumstances where the AS number
would appear in a segment of type AS_CONFED_SEQUENCE in a non-BGPSEC
update message.
Within a confederation, the verification of BGPSEC signatures added
by other members of the confederation is optional. If a
confederation chooses to have its members not verify signatures added
by other confederation members, then when sending a BGPSEC update
message to a peer that is a member of the same confederation, the
confederation MAY set the Signature field within the
Signature_Segment that it generates to be zero (in lieu of
calculating the correct digital signature as described in Sections
4.1 and 4.2). Note that if a confederation chooses not to verify
digital signatures within the confederation, then BGPSEC is able to
provide no assurances about the integrity of the (private) Member-AS
Numbers placed in Secure_Path segments where the Confed_Segment flag
is set to one.
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 segments,
remove all of the consecutive Secure_Path segments that have the
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Confed_Segment flag set to one. Stop this process once a
Scure_Path segment is reached which has its Confed_Segment flag
set to zero. 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 that the AS field are populated as per
Sections 4.1 and 4.2.
When validating a received BGPSEC update message, confederation
members need to make the following adjustment to the algorithm
presented in Section 5.2. When a confederation member processes
(validates) a Signature Segment and its corresponding Secure_Path
Segment, the confederation member must note that for a signature
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 a BGPSEC speaker (that is a confederation
member) processes a current Secure_Path Segment that has the
Confed_Segment flag set to zero, if the next most recently added
Secure_Path segment has the Confed_Segment flag set to one then, when
computing the digest for the current Secure_Path segment, the BGPSEC
speaker takes the Target AS Number to be the AS Confederation
Identifier of the validating BGPSEC speaker's own confederation.
(Note that the algorithm in Section 5.2 processes Secure_Path
Segments in order from most recently added to least recently added,
therefore this special case will apply to the first Secure_Path
segment that the algorithm encounters that has the Confed_Segment
flag set to zero.)
Finally, as discussed above, an AS confederation may optionally
decide that its members will not verify digital signatures added by
members. In such a federation, when a confederation member runs the
algorithm in Section 5.2, when processing a Signature_Segment, the
confederation member first checks whether the Confed_Sequence flag in
the corresponding Secure_Path segment is set to one. If the
Confed_Sequence flag is set to one in the corresponding Secure_Path
segment, the confederation member does not perform any further checks
on the Signature_Segment and immediately moves on to the next
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Signature_Segment (and checks its corresponding Secure_Path segment).
Note that as specified in Section 5.2, it is an error for a BGPSEC
speaker to receive a BGPSEC update messages containing a Secure_Path
segment with the Confed_Sequence flag set to one from a peer who is
not a member of the same AS confederation. (Such an error is treated
in exactly the same way as receipt of a non-BGPSEC update message
containing an AS_CONFED_SEQUENCE from a peer that is not a member of
the same AS confederation.)
4.4. Reconstructing the AS_PATH Attribute
BGPSEC update messages do not contain the AS_PATH attribute. Note,
however, that 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. There may be
additional cases where an implementation finds it useful to perform
this reconstruction.
The AS_PATH attribute can be constructed from the BGPSEC_Path
attribute as follows. Starting with an empty 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 Confed_Segment flag in the Secure_Path segment is set to
one, then look at the most-recently added segment in the AS_PATH.
* In the case where the AS_PATH is empty or in the case where
the most-recently added segment is of type AS_SEQUENCE then
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.)
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2. If the Confed_Segment flag in the Secure_Path segment is set to
zero, then look at the most-recently added segment in the
AS_PATH.
* In the case where the AS_PATH is empty, and the pCount field
in the Secure_Path segment is greater than zero, 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.)
5. Processing a Received BGPSEC Update
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. 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.
The validity of BGPSEC update messages is a function of the current
RPKI state. When a BGPSEC speaker learns that RPKI state has changed
(e.g., from an RPKI validating cache via the RTR protocol), the
BGPSEC speaker MUST re-run validation on all affected update messages
stored in its ADJ-RIB-IN. That is, when a given RPKI certificate
ceases to be valid (e.g., it expires or revoked), all update messages
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containing a signature whose SKI matches the SKI in the given
certificate must be re-assessed to determine if they are still valid.
Note that this reassessment determines that the validity state of an
update has changed then, depending on local policy, it may be
necessary to re-run best path selection.
BGPSEC update messages do not contain an AS_PATH attribute.
Therefore, a BGPSEC speaker MUST utilize the AS path information in
the BGPSEC_Path attribute 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 use of 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.
Many signature algorithms are non-deterministic. That is, many
signature algorithms will produce different signatures each time they
are run (even when they are signing the same data with the same key).
Therefore, if an implementation receives a BGPSEC update from a peer
and later receives a second BGPSEC update message from the same peer,
the implementation SHOULD treat the second message as a duplicate
update message if it differs from the first update message only in
the Signature fields (within the BGPSEC_Path attribute). That is, if
all the fields in the second update are identical to the fields in
the first update message, except for the Signature fields, then the
second update message should be treated as a duplicate of the first
update message. Note that if other fields (e.g., the Subject Key
Identifier field) within a Signature segment differ between two
update messages then the two updates are not duplicates.
With regards to the processing of duplicate update messages, if the
first update message is valid, then an implementation SHOULD NOT run
the validation procedure on the second, duplicate update message
(even if the bits of the signature field are different). If the
first update message is not valid, then an implementation SHOULD run
the validation procedure on the second duplicate update message (as
the signatures in the second update may be valid even though the
first contained a signature that was invalid).
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5.1. Overview of BGPSEC Validation
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
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 router 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. (For example,
the trusted cache could deliver the necessary validity information to
the BGPSEC speaker using the router key PDU [15]for the RTR protocol
[14].)
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. However, BGP route
selection and thus the handling of the two validation states is a
matter of local policy, and shall be handled using local policy
mechanisms. It is expected that BGP peers will generally prefer
routes received via 'Valid' BGPSEC update messages over routes
received via 'Not Valid' BGPSEC update messages as well as routes
received via update messages that do not contain the BGPSEC_Path
attribute. However, BGPSEC specifies no changes to the BGP decision
process. (See [16] for related operational considerations.)
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 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
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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. Specifically, the
recipient performs the following checks:
1. Check to ensure that the entire BGPSEC_Path attribute is
syntactically correct (conforms to the specification in this
document).
2. 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.)
3. Check that the update message does not contain an AS_PATH
attribute.
4. If the update message was received from a 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_Sequence flag set to one.
5. If the update message was received from a peer that is not
expected to set pCount equal to zero (see Section 4.2) then check
to ensure that the pCount field in the most-recently added
Secure_Path segment is not equal to zero.
If any of these checks identify an error in the BGPSEC_Path
attribute, then the implementation should notify the operator that an
error has occurred and treat the update in a manner consistent with
other BGP errors (i.e., following RFC 4271[2] or any future updates
to that document).
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
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BGPSEC_Path 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 Valid' and the validation algorithm
terminates.
Finally, 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 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 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 attribute. The following steps 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 router 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_Block as 'Not Valid' and proceed to the
next Signature_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:
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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.)
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:
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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) | ---/
+------------------------------------+
| Algorithm Suite Id. (1 octet) |
+------------------------------------+
| NLRI Length (1 octet) |
+------------------------------------+
| NLRI Prefix (variable) |
+------------------------------------+
The NLRI Length, NLRI Prefix, and Algorithm Suite Identifier are all
obtained in a straight forward manner from the NLRI of the update
message or the BGPSEC_Path 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.)
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_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'.
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If at least one Signature_Block is marked as 'Valid', then the
validation algorithm terminates and the BGPSEC update message is
deemed to be '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
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 [11].
It is anticipated that in the future mandatory algorithm suites
document will be updated to specify a transition from the 'current'
algorithm suite to a 'new' algorithm suite. During the period of
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 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 and thus
necessitate a new version of BGPSEC. Examples of such changes
include:
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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, 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 6.1. 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.
7. Security Considerations
For discussion of the BGPSEC threat model and related security
considerations, please see [13].
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, in the RPKI, by the IP address space holder to
originate route advertisements for the given prefix.
o For each AS in the 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 subsequent AS
in the path.
That is, the recipient of a valid BGPSEC Update message is assured
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that the Secure_Path portion of the BGPSEC_Path attribute 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 any 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 'Valid' (as
per the validation algorithm in Section 5.2) a BGPSEC update message
that contains both a 'Valid' and a 'Not Valid' 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
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 'Valid' and the 'Not Valid'
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 'Valid' and a set of algorithm B
signatures which are 'Not Valid'. 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 Valid' set of signatures corresponding to algorithm B, such
entities would treat the message as though it were unsigned. By
including the 'Not Valid' set of signatures when propagating a route
advertisement, the BGPSEC speaker ensures that 'downstream' entities
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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
Valid' signatures. That is, by removing the set of 'Not Valid'
signatures the BGPSEC speaker might actually cause a downstream
entity to 'upgrade' the status of a route advertisement from 'Not
Valid' to unsigned. Finally, note that in the above scenario, the
BGPSEC speaker might have deemed algorithm A signatures 'Valid' 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 Valid' (due to the
revocation) and not as 'unsigned' (which would happen if the 'Not
Valid' 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 Valid' BGPSEC
update message. (That is, a BGPSEC update containing only 'Not
Valid' Signature_Blocks.) In such a case, the BGPSEC speaker should
propagate a signed BGPSEC update message, adding his signature to the
'Not Valid' 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
vantage points in the network, a BGPSEC update that was deemed 'Not
Valid' at an upstream BGPSEC speaker may indeed be deemed 'Valid' 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
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expensive checks (e.g., signature verification) after performing less
expensive checks (e.g., syntax checks). The validation algorithm
specified in Section 5.2 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 5.2 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 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 peer to tear-down the
BGP session. Therefore, BGPSEC sessions SHOULD be protected by
appropriate transport security mechanisms.
8. IANA Considerations
TBD: Need IANA to assign numbers for the two capabilities and the
BGPSEC_PATH attribute.
This document does not create any new IANA registries.
9. Contributors
9.1. Authors
Rob Austein
Dragon Research Labs
sra@hactrn.net
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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
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
Sparta
weiler+ietf@watson.org
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9.2. 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.
10. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A Border
Gateway Protocol 4", RFC 4271, January 2006.
[3] Bates, T., Chandra, R., Katz, D., and Y. Rekhter,
"Multiprotocol Extensions for BGP-4", RFC 4760, January 2007.
[4] Vohra, Q. and E. Chen, "BGP Support for Four-octet AS Number
Space", RFC 4893, May 2007.
[5] Traina, P., McPherson, D., and J. Scudder, "Autonomous System
Confederations for BGP", RFC 5065, August 2007.
[6] Scudder, J. and R. Chandra, "Capabilities Advertisement with
BGP-4", RFC 5492, February 2009.
[7] Lepinski, M. and S. Kent, "An Infrastructure to Support Secure
Internet Routing", RFC 6480, February 2012.
[8] Lepinski, M., Kent, S., and D. Kong, "A Profile for Route
Origin Authorizations (ROAs)", RFC 6482, February 2012.
[9] Patel, K., Ward, D., and R. Bush, "Extended Message support for
BGP", July 2012.
[10] Reynolds, M., Turner, S., and S. Kent, "A Profile for BGPSEC
Router Certificates, Certificate Revocation Lists, and
Certification Requests", April 2012.
[11] Turner, S., "BGP Algorithms, Key Formats, & Signature Formats",
March 2012.
11. Informative References
[12] Kumari, W. and K. Sriram, "Recommendation for Not Using AS_SET
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and AS_CONFED_SET in BGP", RFC 6472, December 2011.
[13] Kent, S., "Threat Model for BGP Path Security", February 2012.
[14] Bush, R. and R. Austein, "The RPKI/Router Protocol",
February 2012.
[15] Bush, R., Patel, K., and S. Turner, "Router Key PDU for RPKI-
Router Protocol", October 2012.
[16] Bush, R., "BGPsec Operational Considerations", May 2012.
Author's Address
Matthew Lepinski (editor)
BBN
10 Moulton St
Cambridge, MA 55409
US
Phone: +1 617 873 5939
Email: mlepinski.ietf@gmail.com
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