draft-ietf-idr-rfc4893bis-06.txt   draft-ietf-idr-rfc4893bis-07.txt 
Network Working Group Q. Vohra Network Working Group Q. Vohra
Internet Draft Juniper Networks Internet Draft Juniper Networks
Obsoletes: 4893 (if approved) E. Chen Obsoletes: 4893 (if approved) E. Chen
Intended Status: Standards Track Cisco Systems Intended Status: Standards Track Cisco Systems
Expiration Date: Oct 24, 2012 April 23, 2012 Expiration Date: December 21, 2012 June 20, 2012
BGP Support for Four-octet AS Number Space BGP Support for Four-octet AS Number Space
draft-ietf-idr-rfc4893bis-06.txt draft-ietf-idr-rfc4893bis-07.txt
Status of this Memo Status of this Memo
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skipping to change at page 1, line 33 skipping to change at page 1, line 33
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Copyright Notice Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the Copyright (c) 2012 IETF Trust and the persons identified as the
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Abstract Abstract
The Autonomous System (AS) number is encoded as a two-octet entity in The Autonomous System number is encoded as a two-octet entity in the
the base BGP specification. This document describes extensions to BGP base BGP specification. This document describes extensions to BGP to
to carry the Autonomous System numbers as four-octet entities. This carry the Autonomous System numbers as four-octet entities. This
document obsoletes RFC 4893. document obsoletes RFC 4893.
1. Introduction 1. Introduction
In the base BGP specification [RFC4271] the Autonomous System number In the base BGP specification [RFC4271] the Autonomous System (AS)
is encoded as a two-octet entity. To prepare for the anticipated number is encoded as a two-octet entity. To prepare for the
exhaustion of the two-octet AS numbers, this document describes anticipated exhaustion of the two-octet AS numbers, this document
extensions to BGP to carry the Autonomous System numbers as four- describes extensions to BGP to carry the AS numbers as four-octet
octet entities. entities.
More specifically, this document defines a BGP capability, "support More specifically, this document defines a BGP capability, "support
for 4-octet AS number capability", to be used by a BGP speaker to for four-octet AS number capability", to be used by a BGP speaker to
indicate its support for the four-octet AS numbers. Two attributes, indicate its support for the four-octet AS numbers. Two attributes,
AS4_PATH and AS4_AGGREGATOR, are introduced that can be used to AS4_PATH and AS4_AGGREGATOR, are introduced that can be used to
propagate four-octet based AS path information across BGP speakers propagate four-octet based AS path information across BGP speakers
that do not support the four-octet AS numbers. This document also that do not support the four-octet AS numbers. This document also
specifies mechanisms for constructing the AS path information from specifies mechanisms for constructing the AS path information from
the AS_PATH attribute and the AS4_PATH attribute. the AS_PATH attribute and the AS4_PATH attribute.
The extensions specified in this document allow a gradual transition The extensions specified in this document allow a gradual transition
from 2-octet AS numbers to 4-octet AS numbers. from two-octet AS numbers to four-octet AS numbers.
This document obsoletes RFC 4893, and a comparison with RFC 4893 is This document obsoletes RFC 4893, and a comparison with RFC 4893 is
provided in Appendix A. provided in Appendix A.
2. Specification of Requirements 2. Specification of Requirements
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119]. document are to be interpreted as described in [RFC2119].
3. Protocol Extensions 3. Protocol Extensions
For the purpose of this document we define a BGP speaker that does For the purpose of this document we define a BGP speaker that does
not support the new 4-octet AS number extensions as an OLD BGP not support the new four-octet AS number extensions as an OLD BGP
speaker, and a BGP speaker that supports the new 4-octet AS number speaker, and a BGP speaker that supports the new four-octet AS number
extensions as a NEW BGP speaker. extensions as a NEW BGP speaker.
BGP carries the Autonomous System numbers in the "My Autonomous BGP carries the AS numbers in the "My Autonomous System" field of the
System" field of the OPEN message, in the AS_PATH attribute of the OPEN message, in the AS_PATH attribute of the UPDATE message, and in
UPDATE message, and in the AGGREGATOR attribute of the UPDATE the AGGREGATOR attribute of the UPDATE message. BGP also carries the
message. BGP also carries the Autonomous System numbers in the BGP AS numbers in the BGP Communities attribute.
Communities attribute.
A NEW BGP speaker uses BGP Capability Advertisements [RFC5492] to A NEW BGP speaker uses BGP Capability Advertisements [RFC5492] to
advertise to its neighbors (either internal or external) that it advertise to its neighbors (either internal or external) that it
supports 4-octet AS number extensions, as specified in this document. supports four-octet AS number extensions, as specified in this
document.
The Capability that is used by a BGP speaker to convey to its BGP The Capability that is used by a BGP speaker to convey to its BGP
peer the 4-octet Autonomous System number capability, also carries peer the four-octet Autonomous System number capability, also carries
the Autonomous System number (encoded as a 4-octet entity) of the the AS number (encoded as a four-octet entity) of the speaker in the
speaker in the Capability Value field of the Capability Optional Capability Value field of the Capability Optional Parameter. The
Parameter. The Capability Length field of the Capability is set to Capability Length field of the Capability is set to 4.
4.
The AS path information exchanged between NEW BGP speakers are The AS path information exchanged between NEW BGP speakers are
carried in the existing AS_PATH attribute, except that each AS number carried in the existing AS_PATH attribute, except that each AS number
in the attribute is encoded as a 4-octet entity (instead of a 2-octet in the attribute is encoded as a four-octet entity (instead of a two-
entity). The same applies to the AGGREGATOR attribute - the same octet entity). The same applies to the AGGREGATOR attribute - the
attribute is used between NEW BGP speakers, except that the AS number same attribute is used between NEW BGP speakers, except that the AS
carried in the attribute is encoded as a 4-octet entity. number carried in the attribute is encoded as a four-octet entity.
The AS_PATH attribute and the AGGREGATOR attribute carried between a The AS_PATH attribute and the AGGREGATOR attribute carried between a
NEW BGP speaker and an OLD BGP speaker will continue to contain NEW BGP speaker and an OLD BGP speaker will continue to contain two-
2-octet AS numbers. octet AS numbers.
To preserve the AS path information with 4-octet AS numbers across To preserve the AS path information with four-octet AS numbers across
OLD BGP speakers, this document defines a new AS path attribute, OLD BGP speakers, this document defines a new AS path attribute,
called AS4_PATH. This is an optional transitive attribute that called AS4_PATH. This is an optional transitive attribute that
contains the AS path encoded with 4-octet AS numbers. The AS4_PATH contains the AS path encoded with four-octet AS numbers. The
attribute has the same semantics as the AS_PATH attribute, except AS4_PATH attribute has the same semantics as the AS_PATH attribute,
that it is optional transitive, and it carries 4-octet AS numbers. except that it is optional transitive, and it carries four-octet AS
numbers.
To prevent the possible propagation of confederation path segments To prevent the possible propagation of confederation path segments
outside of a confederation, the path segment types AS_CONFED_SEQUENCE outside of a confederation, the path segment types AS_CONFED_SEQUENCE
and AS_CONFED_SET [RFC5065] are declared invalid for the AS4_PATH and AS_CONFED_SET [RFC5065] are declared invalid for the AS4_PATH
attribute, and MUST NOT be included in the AS4_PATH attribute of an attribute, and MUST NOT be included in the AS4_PATH attribute of an
UPDATE message. UPDATE message.
Similarly, this document defines a new aggregator attribute called Similarly, this document defines a new aggregator attribute called
AS4_AGGREGATOR, which is optional transitive. The AS4_AGGREGATOR AS4_AGGREGATOR, which is optional transitive. The AS4_AGGREGATOR
attribute has the same semantics as the AGGREGATOR attribute, except attribute has the same semantics as the AGGREGATOR attribute, except
that it carries a 4-octet AS number. that it carries a four-octet AS number.
Currently assigned 2-octet Autonomous System numbers are converted Currently assigned two-octet AS numbers are converted into four-octet
into 4-octet Autonomous System numbers by setting the two high-order AS numbers by setting the two high-order octets of the four-octet
octets of the 4-octet field to zero. Such a 4-octet AS number is field to zero. Such a four-octet AS number is said to be mappable to
said to be mappable to a 2-octet AS number. a two-octet AS number.
To represent 4-octet AS numbers (which are not mapped from 2-octets) To represent four-octet AS numbers (which are not mapped from two-
as 2-octet AS numbers in the AS path information encoded with 2-octet octets) as two-octet AS numbers in the AS path information encoded
AS numbers, this document reserves a 2-octet AS number. We denote with two-octet AS numbers, this document reserves a two-octet AS
this special AS number as AS_TRANS for ease of description in the number. We denote this special AS number as AS_TRANS for ease of
rest of this specification. This AS number is also placed in the "My description in the rest of this specification. This AS number is
Autonomous System" field of the OPEN message originated by a NEW BGP also placed in the "My Autonomous System" field of the OPEN message
speaker, if the speaker does not have a (globally unique) 2-octet AS originated by a NEW BGP speaker, if and only if the speaker does not
number. have a (globally unique) two-octet AS number.
4. Operations 4. Operations
4.1. Interaction Between NEW BGP Speakers 4.1. Interaction Between NEW BGP Speakers
A BGP speaker that supports 4-octet Autonomous System numbers SHALL A BGP speaker that supports four-octet AS numbers SHALL advertise
advertise this to its peers using the BGP Capability Advertisements. this to its peers using the BGP Capability Advertisements. The AS
The Autonomous System number of the BGP speaker MUST be carried in number of the BGP speaker MUST be carried in the capability value
the capability value field of the "support for 4-octet AS number field of the "support for four-octet AS number capability".
capability".
When a NEW BGP speaker processes an OPEN message from another NEW BGP When a NEW BGP speaker processes an OPEN message from another NEW BGP
speaker, it MUST use the Autonomous System number encoded in the speaker, it MUST use the AS number encoded in the Capability Value
Capability Value field of the Capability in lieu of the "My field of the Capability in lieu of the "My Autonomous System" field
Autonomous System" field of the OPEN message. of the OPEN message.
A BGP speaker that advertises such capability to a particular peer, A BGP speaker that advertises such capability to a particular peer,
and receives from that peer the advertisement of such capability MUST and receives from that peer the advertisement of such capability MUST
encode Autonomous System numbers as 4-octet entities in both the encode AS numbers as four-octet entities in both the AS_PATH and the
AS_PATH and the AGGREGATOR attributes in the updates it sends to the AGGREGATOR attributes in the updates it sends to the peer, and MUST
peer, and MUST assume that these attributes in the updates received assume that these attributes in the updates received from the peer
from the peer encode Autonomous System numbers as 4-octet entities. encode AS numbers as four-octet entities.
The new attributes, AS4_PATH and AS4_AGGREGATOR MUST NOT be carried The new attributes, AS4_PATH and AS4_AGGREGATOR MUST NOT be carried
in an UPDATE message between NEW BGP speakers. A NEW BGP speaker in an UPDATE message between NEW BGP speakers. A NEW BGP speaker
that receives the AS4_PATH attribute or the AS4_AGGREGATOR attribute that receives the AS4_PATH attribute or the AS4_AGGREGATOR attribute
in an UPDATE message from another NEW BGP speaker MUST discard the in an UPDATE message from another NEW BGP speaker MUST discard the
path attribute and continue processing the UPDATE message. path attribute and continue processing the UPDATE message.
4.2. Interaction Between NEW and OLD BGP Speakers 4.2. Interaction Between NEW and OLD BGP Speakers
4.2.1. BGP Peering 4.2.1. BGP Peering
Note that peering between a NEW BGP speaker and an OLD one is Note that peering between a NEW BGP speaker and an OLD BGP speaker is
possible only if the NEW BGP speaker has a 2-octet AS number. possible only if the NEW BGP speaker has a two-octet AS number.
However, this document does not assume that an Autonomous System with However, this document does not assume that an Autonomous System with
NEW speakers has to have a globally unique 2-octet AS number -- NEW speakers has to have a globally unique two-octet AS number --
AS_TRANS could be used instead (even if a multiple Autonomous System AS_TRANS MUST be used when the NEW speaker does not have a two-octet
would use it). AS number (even if multiple Autonomous Systems would use it).
4.2.2. Generating Updates 4.2.2. Generating Updates
When communicating with an OLD BGP speaker, a NEW speaker MUST send When communicating with an OLD BGP speaker, a NEW speaker MUST send
the AS path information in the AS_PATH attribute encoded with 2-octet the AS path information in the AS_PATH attribute encoded with two-
AS numbers. The NEW speaker MUST also send the AS path information octet AS numbers. The NEW speaker MUST also send the AS path
in the AS4_PATH attribute (encoded with 4-octet AS numbers), except information in the AS4_PATH attribute (encoded with four-octet AS
for the case where the entire AS path information is composed of numbers), except for the case where the entire AS path information is
2-octet mappable AS numbers only. In this case, the NEW speaker MUST composed of two-octet mappable AS numbers only. In this case, the
NOT send the AS4_PATH attribute. NEW speaker MUST NOT send the AS4_PATH attribute.
In the AS_PATH attribute encoded with 2-octet AS numbers, non- In the AS_PATH attribute encoded with two-octet AS numbers, non-
mappable 4-octet AS numbers are represented by the well-known 2-octet mappable four-octet AS numbers are represented by the well-known two-
AS number, AS_TRANS. This will preserve the path length property of octet AS number, AS_TRANS. This will preserve the path length
the AS path information and also help in updating the AS path property of the AS path information and also help in updating the AS
information received on a NEW BGP speaker from an OLD speaker, as path information received on a NEW BGP speaker from an OLD speaker,
explained in the next section. as explained in the next section.
The NEW speaker constructs the AS4_PATH attribute from the AS path The NEW speaker constructs the AS4_PATH attribute from the AS path
information. Whenever the AS path information contains the information. Whenever the AS path information contains the
AS_CONFED_SEQUENCE or AS_CONFED_SET path segment, the NEW BGP speaker AS_CONFED_SEQUENCE or AS_CONFED_SET path segment, the NEW BGP speaker
MUST exclude such path segments from the AS4_PATH attribute being MUST exclude such path segments from the AS4_PATH attribute being
constructed. constructed.
The AS4_PATH attribute, being optional transitive, will be carried The AS4_PATH attribute, being optional transitive, will be carried
across a series of OLD BGP speakers without modification and will across a series of OLD BGP speakers without modification and will
help preserve the non-mappable 4-octet AS numbers in the AS path help preserve the non-mappable four-octet AS numbers in the AS path
information. information.
Similarly, if the NEW speaker has to send the AGGREGATOR attribute, Similarly, if the NEW speaker has to send the AGGREGATOR attribute,
and if the aggregating Autonomous System's AS number is a non- and if the aggregating Autonomous System's AS number is a non-
mappable 4-octet AS number, then the speaker MUST use the mappable four-octet AS number, then the speaker MUST use the
AS4_AGGREGATOR attribute, and set the AS number field in the existing AS4_AGGREGATOR attribute, and set the AS number field in the existing
AGGREGATOR attribute to the reserved AS number, AS_TRANS. Note that AGGREGATOR attribute to the reserved AS number, AS_TRANS. Note that
if the AS number is 2-octets mappable, then the AS4_AGGREGATOR if the AS number is two-octets mappable, then the AS4_AGGREGATOR
attribute MUST NOT be sent. attribute MUST NOT be sent.
4.2.3. Processing Received Updates 4.2.3. Processing Received Updates
When a NEW BGP speaker receives an update from an OLD one, it MUST be When a NEW BGP speaker receives an update from an OLD BGP speaker, it
prepared to receive the AS4_PATH attribute along with the existing MUST be prepared to receive the AS4_PATH attribute along with the
AS_PATH attribute. If the AS4_PATH attribute is also received, both existing AS_PATH attribute. If the AS4_PATH attribute is also
the attributes will be used to construct the exact AS path received, both the attributes will be used to construct the exact AS
information, and therefore the information carried by both the path information, and therefore the information carried by both the
attributes will be considered for AS path loop detection. attributes will be considered for AS path loop detection.
Note that a route may have traversed a series of autonomous systems Note that a route may have traversed a series of autonomous systems
with 2-octet AS numbers and OLD BGP speakers only. In that case, if with two-octet AS numbers and OLD BGP speakers only. In that case,
the route carries the AS4_PATH attribute, this attribute would have if the route carries the AS4_PATH attribute, this attribute would
remained unmodified since the route left the last NEW BGP speaker. have remained unmodified since the route left the last NEW BGP
The trailing AS path information (representing autonomous systems speaker. The trailing AS path information (representing autonomous
with 2-octet AS numbers and OLD BGP speakers only) is contained only systems with two-octet AS numbers and OLD BGP speakers only) is
in the current AS_PATH attribute (encoded in the leading part of the contained only in the current AS_PATH attribute (encoded in the
AS_PATH attribute). leading part of the AS_PATH attribute).
Under certain conditions, it may not be possible to reconstruct the Under certain conditions, it may not be possible to reconstruct the
entire AS path information from the AS_PATH and the AS4_PATH entire AS path information from the AS_PATH and the AS4_PATH
attributes of a route. This occurs, for example, when two or more attributes of a route. This occurs, for example, when two or more
routes that carry the AS4_PATH attribute are aggregated by an OLD BGP routes that carry the AS4_PATH attribute are aggregated by an OLD BGP
speaker, and the AS4_PATH attribute of at least one of these routes speaker, and the AS4_PATH attribute of at least one of these routes
carries at least one 4-octet AS number (as opposed to a 2-octet AS carries at least one four-octet AS number (as opposed to a two-octet
number that is encoded in 4 octets). Depending on the AS number that is encoded in 4 octets). Depending on the
implementation, either the AS4_PATH attribute would be lost during implementation, either the AS4_PATH attribute would be lost during
route aggregation, or both the AS_PATH attribute and the AS4_PATH route aggregation, or both the AS_PATH attribute and the AS4_PATH
attribute would contain valid, partial information that cannot be attribute would contain valid, partial information that cannot be
combined seamlessly, resulting in incomplete AS path information in combined seamlessly, resulting in incomplete AS path information in
these cases. these cases.
A NEW BGP speaker MUST also be prepared to receive the AS4_AGGREGATOR A NEW BGP speaker MUST also be prepared to receive the AS4_AGGREGATOR
attribute along with the AGGREGATOR attribute from an OLD BGP attribute along with the AGGREGATOR attribute from an OLD BGP
speaker. When both the attributes are received, if the AS number in speaker. When both the attributes are received, if the AS number in
the AGGREGATOR attribute is not AS_TRANS, then: the AGGREGATOR attribute is not AS_TRANS, then:
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AS_PATH attribute, and then prepending them to the AS4_PATH attribute AS_PATH attribute, and then prepending them to the AS4_PATH attribute
so that the AS path information has an identical number of AS numbers so that the AS path information has an identical number of AS numbers
as the AS_PATH attribute. Note that a valid AS_CONFED_SEQUENCE or as the AS_PATH attribute. Note that a valid AS_CONFED_SEQUENCE or
AS_CONFED_SET path segment SHALL be prepended if it is either the AS_CONFED_SET path segment SHALL be prepended if it is either the
leading path segment or adjacent to a path segment that is prepended. leading path segment or adjacent to a path segment that is prepended.
5. Handling BGP Communities 5. Handling BGP Communities
As specified in [RFC1997], when the high-order two-octets of the As specified in [RFC1997], when the high-order two-octets of the
community attribute is neither 0x0000 nor 0xffff, these two octets community attribute is neither 0x0000 nor 0xffff, these two octets
encode the Autonomous System number. Quite clearly this would not encode the AS number. Quite clearly this would not work for a NEW
work for a NEW BGP speaker with a non-mappable 4-octet AS number. BGP speaker with a non-mappable four-octet AS number. Such BGP
Such BGP speakers should use the Four-octet AS Specific Extended speakers should use the four-octet AS Specific Extended Communities
Communities [RFC5668] instead. [RFC5668] instead.
6. Error Handling 6. Error Handling
This section provides an update to RFC 4271 [RFC4271] with respect to
the error conditions noted here and their handling.
Given that the two-octet AS numbers dominate during the transition, Given that the two-octet AS numbers dominate during the transition,
and are carried in the AS_PATH attribute by an OLD BGP speaker, in and are carried in the AS_PATH attribute by an OLD BGP speaker, in
this document the "attribute discard" approach is chosen to handle a this document the "attribute discard" approach is chosen to handle a
malformed AS4_PATH attribute. malformed AS4_PATH attribute.
Similarly, as the AS4_AGGREGATOR is just informational, the Similarly, as the AS4_AGGREGATOR is just informational, the
"attribute discard" approach is chosen to handle a malformed "attribute discard" approach is chosen to handle a malformed
AS4_AGGREGATOR attribute. AS4_AGGREGATOR attribute.
The AS4_PATH attribute and AS4_AGGREGATOR attribute MUST NOT be The AS4_PATH attribute and AS4_AGGREGATOR attribute MUST NOT be
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malformed under the following conditions: malformed under the following conditions:
- the attribute length is not a multiple of two, or is too small - the attribute length is not a multiple of two, or is too small
(i.e., less than 6) for the attribute to carry at least one (i.e., less than 6) for the attribute to carry at least one
AS number, or AS number, or
- the path segment length in the attribute is either zero, or - the path segment length in the attribute is either zero, or
is inconsistent with the attribute length, or is inconsistent with the attribute length, or
- the path segment type in the attribute is not one of the - the path segment type in the attribute is not one of the
types defined: AS_SEQUENCE, AS_SET. types defined: AS_SEQUENCE, AS_SET, AS_CONFED_SEQUENCE
and AS_CONFED_SET.
A NEW BGP speaker that receives a malformed AS4_PATH attribute in an A NEW BGP speaker that receives a malformed AS4_PATH attribute in an
UPDATE message from an OLD BGP speaker MUST discard the attribute, UPDATE message from an OLD BGP speaker MUST discard the attribute,
and continue processing the UPDATE message. The error SHOULD be and continue processing the UPDATE message. The error SHOULD be
logged locally for analysis. logged locally for analysis.
The AS4_AGGREGATOR attribute in an UPDATE message SHALL be considered The AS4_AGGREGATOR attribute in an UPDATE message SHALL be considered
malformed if the attribute length is not 8. malformed if the attribute length is not 8.
A NEW BGP speaker that receives a malformed AS4_AGGREGATOR attribute A NEW BGP speaker that receives a malformed AS4_AGGREGATOR attribute
in an UPDATE message from an OLD BGP speaker MUST discard the in an UPDATE message from an OLD BGP speaker MUST discard the
attribute, and continue processing the UPDATE message. The error attribute, and continue processing the UPDATE message. The error
SHOULD be logged locally for analysis. SHOULD be logged locally for analysis.
7. Transition 7. Transition
The scheme described in this document allows a gradual transition When an Autonomous System is using a two-octet AS number, then the
from 2-octet AS numbers to 4-octet AS numbers. One can upgrade one BGP speakers within that Autonomous System MAY be upgraded to support
Autonomous System or one BGP speaker at a time. the four-octet AS number extensions on a piecemeal basis. There is
no requirement for a coordinated upgrade of the four-octet AS number
To simplify transition, this document assumes that an Autonomous capability in this case. However, if an Autonomous System wishes to
System could start using a 4-octet AS number only after all the BGP use a four-octet AS number as its own AS number, then this document
speakers within that Autonomous System have been upgraded to support assumes that an Autonomous System can use a four-octet AS number only
4-octet AS numbers. after all the BGP speakers within that Autonomous System have been
upgraded to support four-octet AS numbers.
A non-mappable 4-octet AS number cannot be used as a "Member AS A non-mappable four-octet AS number cannot be used as a "Member AS
Number" of a BGP Confederation until all the BGP speakers within the Number" of a BGP Confederation until all the BGP speakers within the
Confederation have transitioned to support 4-octet AS numbers. Confederation have transitioned to support four-octet AS numbers.
In an environment where an Autonomous System that has OLD BGP In an environment where an Autonomous System that has OLD BGP
speakers peers with two or more Autonomous Systems that have NEW BGP speakers peers with two or more Autonomous Systems that have NEW BGP
speakers and use AS_TRANS (rather than having a globally unique speakers and use AS_TRANS (rather than having a globally unique two-
2-octet mappable AS number), use of Multi-Exit Discriminators by the octet mappable AS number), use of Multi-Exit Discriminators by the
Autonomous System with the OLD speakers may result in a situation Autonomous System with the OLD speakers may result in a situation
where Multi-Exit Discriminator will influence route selection among where Multi-Exit Discriminator will influence route selection among
the routes that were received from different neighboring Autonomous the routes that were received from different neighboring Autonomous
Systems. Systems.
Under certain conditions, it may not be possible to reconstruct the Under certain conditions, it may not be possible to reconstruct the
entire AS path information from the AS_PATH and the AS4_PATH entire AS path information from the AS_PATH and the AS4_PATH
attributes of a route. This occurs when two or more routes that attributes of a route. This occurs when two or more routes that
carry the AS4_PATH attribute are aggregated by an OLD BGP speaker, carry the AS4_PATH attribute are aggregated by an OLD BGP speaker,
and the AS4_PATH attribute of at least one of these routes carries at and the AS4_PATH attribute of at least one of these routes carries at
least one 4-octet AS number (as opposed to a 2-octet AS number that least one four-octet AS number (as opposed to a two-octet AS number
is encoded in 4 octets). When such aggregation results in creating a that is encoded in 4 octets). When such aggregation results in
route that is less specific than any of the component routes (route creating a route that is less specific than any of the component
whose Network Layer Reachability Information (NLRI) covers NLRI of routes (route whose Network Layer Reachability Information (NLRI)
all the component routes), loss of the AS path information does not covers NLRI of all the component routes), loss of the AS path
create a risk of a routing loop. In all other cases, loss of the AS information does not create a risk of a routing loop. In all other
path information does create a risk of a routing loop. cases, loss of the AS path information does create a risk of a
routing loop.
8. IANA Considerations 8. IANA Considerations
[Remove this paragraph before publication: IANA should update the [Remove this paragraph before publication: IANA should update the
reference for the "32-bit Autonomous System Numbers" sub-registry to reference for the "32-bit Autonomous System Numbers" sub-registry to
cite this document once an RFC number has been assigned to it, cite this document once an RFC number has been assigned to it,
replacing the citation of RFC 4893.] replacing the citation of RFC 4893.]
This document expands the pool for AS numbers from 0 - 65535 to 0 - This document expands the pool for AS numbers from 0 - 65535 to 0 -
4294967295. The AS numbers are managed by the IANA "Autonomous 4294967295. The AS numbers are managed by the IANA "Autonomous
System Numbers" registry. Other than expanding the AS number pool, System Numbers" registry. Other than expanding the AS number pool,
this document does not propose any modifications to the existing this document does not propose any modifications to the existing
policies and procedures pertaining to the AS number allocation. policies and procedures pertaining to the AS number allocation.
This document uses a BGP Capability code to indicate that a BGP This document uses a BGP Capability code to indicate that a BGP
speaker supports the 4-octet AS numbers. The Capability Code 65 has speaker supports the four-octet AS numbers. The Capability Code 65
been assigned by IANA per [RFC5492]. has been assigned by IANA per [RFC5492].
In addition, this document introduces two BGP optional transitive In addition, this document introduces two BGP optional transitive
attributes, and their type codes have been assigned by the IANA. The attributes, and their type codes have been assigned by the IANA. The
first one is the AS4_PATH attribute, value 17, which preserves the AS first one is the AS4_PATH attribute, value 17, which preserves the AS
path information with 4-octet AS numbers across old BGP speakers. path information with four-octet AS numbers across old BGP speakers.
The second one is the AS4_AGGREGATOR attribute, value 18, which is The second one is the AS4_AGGREGATOR attribute, value 18, which is
similar in use to the current AGGREGATOR attribute, but it carries a similar in use to the current AGGREGATOR attribute, but it carries a
4-octet AS number. four-octet AS number.
Finally, this document introduces a reserved 2-octet AS number -- Finally, this document introduces a reserved two-octet AS number --
AS_TRANS. The AS number 23456 has been assigned by the IANA for AS_TRANS. The AS number 23456 has been assigned by the IANA for
AS_TRANS. AS_TRANS.
9. Security Considerations 9. Security Considerations
This extension to BGP does not change the underlying security issues This extension to BGP does not change the underlying security issues
inherent in the existing BGP, except for the following: inherent in the existing BGP, except for the following:
The inconsistency between the AS_PATH attribute and the AS4_PATH The inconsistency between the AS_PATH attribute and the AS4_PATH
attribute can create loss of the AS path information, and potential attribute can create loss of the AS path information, and potential
routing loops in certain cases as discussed in the document. This routing loops in certain cases as discussed in the document. This
could be exploited by an attacker. could be exploited by an attacker.
It is a misconfiguration to assign a non-mappable 4-octet AS number It is a misconfiguration to assign a non-mappable four-octet AS
as the "Member AS Number" in a BGP confederation before all the BGP number as the "Member AS Number" in a BGP confederation before all
speakers within the confederation have transitioned to support the BGP speakers within the confederation have transitioned to
4-octet AS numbers. Such a misconfiguration would weaken the AS path support four-octet AS numbers. Such a misconfiguration would weaken
loop detection within a confederation. the AS path loop detection within a confederation.
10. Acknowledgments 10. Acknowledgments
The authors would like to thank Yakov Rekhter, Chaitanya Kodeboyina, The authors would like to thank Yakov Rekhter, Chaitanya Kodeboyina,
and Jeffrey Haas for the numerous discussions that went into the and Jeffrey Haas for the numerous discussions that went into the
making of this document. making of this document.
The authors would also like to thank members of the IDR Working Group The authors would also like to thank members of the IDR Working Group
for their review and comments. for their review and comments.
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