Network Working Group Y. Rekhter
Request for Comments: 1771 T.J. Watson Research Center, IBM Corp.
Obsoletes: 1654 T. Li
Category: Standards Track cisco Systems
Editors
March 1995
A Border Gateway Protocol 4 (BGP-4)
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Abstract
This document, together with its companion document, "Application of
the Border Gateway Protocol in the Internet", define an inter-
autonomous system routing protocol for the Internet.
1. Acknowledgements
This document was originally published as RFC 1267 in October 1991,
jointly authored by Kirk Lougheed (cisco Systems) and Yakov Rekhter
(IBM).
We would like to express our thanks to Guy Almes (ANS), Len Bosack
(cisco Systems), and Jeffrey C. Honig (Cornell University) for their
contributions to the earlier version of this document.
We like to explicitly thank Bob Braden (ISI) for the review of the
earlier version of this document as well as his constructive and
valuable comments.
We would also like to thank Bob Hinden, Director for Routing of the
Internet Engineering Steering Group, and the team of reviewers he
assembled to review the previous version (BGP-2) of this document.
This team, consisting of Deborah Estrin, Milo Medin, John Moy, Radia
Perlman, Martha Steenstrup, Mike St. Johns, and Paul Tsuchiya, acted
with a strong combination of toughness, professionalism, and
courtesy.
Rekhter & Li [Page 1]RFC 1771 BGP-4 March 1995
This updated version of the document is the product of the IETF IDR
Working Group with Yakov Rekhter and Tony Li as editors. Certain
sections of the document borrowed heavily from IDRP [7], which is the
OSI counterpart of BGP. For this credit should be given to the ANSI
X3S3.3 group chaired by Lyman Chapin (BBN) and to Charles Kunzinger
(IBM Corp.) who was the IDRP editor within that group. We would also
like to thank Mike Craren (Proteon, Inc.), Dimitry Haskin (Bay
Networks, Inc.), John Krawczyk (Bay Networks, Inc.), and Paul Traina
(cisco Systems) for their insightful comments.
We would like to specially acknowledge numerous contributions by
Dennis Ferguson (MCI).
The work of Yakov Rekhter was supported in part by the National
Science Foundation under Grant Number NCR-9219216.
2. Introduction
The Border Gateway Protocol (BGP) is an inter-Autonomous System
routing protocol. It is built on experience gained with EGP as
defined in RFC 904 [1] and EGP usage in the NSFNET Backbone as
described in RFC 1092 [2] and RFC 1093 [3].
The primary function of a BGP speaking system is to exchange network
reachability information with other BGP systems. This network
reachability information includes information on the list of
Autonomous Systems (ASs) that reachability information traverses.
This information is sufficient to construct a graph of AS
connectivity from which routing loops may be pruned and some policy
decisions at the AS level may be enforced.
BGP-4 provides a new set of mechanisms for supporting classless
interdomain routing. These mechanisms include support for
advertising an IP prefix and eliminates the concept of network
"class" within BGP. BGP-4 also introduces mechanisms which allow
aggregation of routes, including aggregation of AS paths. These
changes provide support for the proposed supernetting scheme [8, 9].
To characterize the set of policy decisions that can be enforced
using BGP, one must focus on the rule that a BGP speaker advertise to
its peers (other BGP speakers which it communicates with) in
neighboring ASs only those routes that it itself uses. This rule
reflects the "hop-by-hop" routing paradigm generally used throughout
the current Internet. Note that some policies cannot be supported by
the "hop-by-hop" routing paradigm and thus require techniques such as
source routing to enforce. For example, BGP does not enable one AS
to send traffic to a neighboring AS intending that the traffic take a
different route from that taken by traffic originating in the
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RFC 1771 BGP-4 March 1995
neighboring AS. On the other hand, BGP can support any policy
conforming to the "hop-by-hop" routing paradigm. Since the current
Internet uses only the "hop-by-hop" routing paradigm and since BGP
can support any policy that conforms to that paradigm, BGP is highly
applicable as an inter-AS routing protocol for the current Internet.
A more complete discussion of what policies can and cannot be
enforced with BGP is outside the scope of this document (but refer to
the companion document discussing BGP usage [5]).
BGP runs over a reliable transport protocol. This eliminates the
need to implement explicit update fragmentation, retransmission,
acknowledgement, and sequencing. Any authentication scheme used by
the transport protocol may be used in addition to BGP's own
authentication mechanisms. The error notification mechanism used in
BGP assumes that the transport protocol supports a "graceful" close,
i.e., that all outstanding data will be delivered before the
connection is closed.
BGP uses TCP [4] as its transport protocol. TCP meets BGP's
transport requirements and is present in virtually all commercial
routers and hosts. In the following descriptions the phrase
"transport protocol connection" can be understood to refer to a TCP
connection. BGP uses TCP port 179 for establishing its connections.
This document uses the term `Autonomous System' (AS) throughout. The
classic definition of an Autonomous System is a set of routers under
a single technical administration, using an interior gateway protocol
and common metrics to route packets within the AS, and using an
exterior gateway protocol to route packets to other ASs. Since this
classic definition was developed, it has become common for a single
AS to use several interior gateway protocols and sometimes several
sets of metrics within an AS. The use of the term Autonomous System
here stresses the fact that, even when multiple IGPs and metrics are
used, the administration of an AS appears to other ASs to have a
single coherent interior routing plan and presents a consistent
picture of what destinations are reachable through it.
The planned use of BGP in the Internet environment, including such
issues as topology, the interaction between BGP and IGPs, and the
enforcement of routing policy rules is presented in a companion
document [5]. This document is the first of a series of documents
planned to explore various aspects of BGP application. Please send
comments to the BGP mailing list (bgp@ans.net).
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RFC 1771 BGP-4 March 1995
3. Summary of Operation
Two systems form a transport protocol connection between one another.
They exchange messages to open and confirm the connection parameters.
The initial data flow is the entire BGP routing table. Incremental
updates are sent as the routing tables change. BGP does not require
periodic refresh of the entire BGP routing table. Therefore, a BGP
speaker must retain the current version of the entire BGP routing
tables of all of its peers for the duration of the connection.
KeepAlive messages are sent periodically to ensure the liveness of
the connection. Notification messages are sent in response to errors
or special conditions. If a connection encounters an error
condition, a notification message is sent and the connection is
closed.
The hosts executing the Border Gateway Protocol need not be routers.
A non-routing host could exchange routing information with routers
via EGP or even an interior routing protocol. That non-routing host
could then use BGP to exchange routing information with a border
router in another Autonomous System. The implications and
applications of this architecture are for further study.
If a particular AS has multiple BGP speakers and is providing transit
service for other ASs, then care must be taken to ensure a consistent
view of routing within the AS. A consistent view of the interior
routes of the AS is provided by the interior routing protocol. A
consistent view of the routes exterior to the AS can be provided by
having all BGP speakers within the AS maintain direct BGP connections
with each other. Using a common set of policies, the BGP speakers
arrive at an agreement as to which border routers will serve as
exit/entry points for particular destinations outside the AS. This
information is communicated to the AS's internal routers, possibly
via the interior routing protocol. Care must be taken to ensure that
the interior routers have all been updated with transit information
before the BGP speakers announce to other ASs that transit service is
being provided.
Connections between BGP speakers of different ASs are referred to as
"external" links. BGP connections between BGP speakers within the
same AS are referred to as "internal" links. Similarly, a peer in a
different AS is referred to as an external peer, while a peer in the
same AS may be described as an internal peer.
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RFC 1771 BGP-4 March 1995
3.1 Routes: Advertisement and Storage
For purposes of this protocol a route is defined as a unit of
information that pairs a destination with the attributes of a path to
that destination:
- Routes are advertised between a pair of BGP speakers in UPDATE
messages: the destination is the systems whose IP addresses are
reported in the Network Layer Reachability Information (NLRI)
field, and the the path is the information reported in the path
attributes fields of the same UPDATE message.
- Routes are stored in the Routing Information Bases (RIBs):
namely, the Adj-RIBs-In, the Loc-RIB, and the Adj-RIBs-Out. Routes
that will be advertised to other BGP speakers must be present in
the Adj-RIB-Out; routes that will be used by the local BGP speaker
must be present in the Loc-RIB, and the next hop for each of these
routes must be present in the local BGP speaker's forwarding
information base; and routes that are received from other BGP
speakers are present in the Adj-RIBs-In.
If a BGP speaker chooses to advertise the route, it may add to or
modify the path attributes of the route before advertising it to a
peer.
BGP provides mechanisms by which a BGP speaker can inform its peer
that a previously advertised route is no longer available for use.
There are three methods by which a given BGP speaker can indicate
that a route has been withdrawn from service:
a) the IP prefix that expresses destinations for a previously
advertised route can be advertised in the WITHDRAWN ROUTES field
in the UPDATE message, thus marking the associated route as being
no longer available for use
b) a replacement route with the same Network Layer Reachability
Information can be advertised, or
c) the BGP speaker - BGP speaker connection can be closed, which
implicitly removes from service all routes which the pair of
speakers had advertised to each other.
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3.2 Routing Information Bases
The Routing Information Base (RIB) within a BGP speaker consists of
three distinct parts:
a) Adj-RIBs-In: The Adj-RIBs-In store routing information that has
been learned from inbound UPDATE messages. Their contents
represent routes that are available as an input to the Decision
Process.
b) Loc-RIB: The Loc-RIB contains the local routing information
that the BGP speaker has selected by applying its local policies
to the routing information contained in its Adj-RIBs-In.
c) Adj-RIBs-Out: The Adj-RIBs-Out store the information that the
local BGP speaker has selected for advertisement to its peers. The
routing information stored in the Adj-RIBs-Out will be carried in
the local BGP speaker's UPDATE messages and advertised to its
peers.
In summary, the Adj-RIBs-In contain unprocessed routing information
that has been advertised to the local BGP speaker by its peers; the
Loc-RIB contains the routes that have been selected by the local BGP
speaker's Decision Process; and the Adj-RIBs-Out organize the routes
for advertisement to specific peers by means of the local speaker's
UPDATE messages.
Although the conceptual model distinguishes between Adj-RIBs-In,
Loc-RIB, and Adj-RIBs-Out, this neither implies nor requires that an
implementation must maintain three separate copies of the routing
information. The choice of implementation (for example, 3 copies of
the information vs 1 copy with pointers) is not constrained by the
protocol.
4. Message Formats
This section describes message formats used by BGP.
Messages are sent over a reliable transport protocol connection. A
message is processed only after it is entirely received. The maximum
message size is 4096 octets. All implementations are required to
support this maximum message size. The smallest message that may be
sent consists of a BGP header without a data portion, or 19 octets.
Rekhter & Li [Page 6]RFC 1771 BGP-4 March 1995
4.1 Message Header Format
Each message has a fixed-size header. There may or may not be a data
portion following the header, depending on the message type. The
layout of these fields is shown below:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ +
| Marker |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length | Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Marker:
This 16-octet field contains a value that the receiver of the
message can predict. If the Type of the message is OPEN, or if
the OPEN message carries no Authentication Information (as an
Optional Parameter), then the Marker must be all ones.
Otherwise, the value of the marker can be predicted by some a
computation specified as part of the authentication mechanism
(which is specified as part of the Authentication Information)
used. The Marker can be used to detect loss of synchronization
between a pair of BGP peers, and to authenticate incoming BGP
messages.
Length:
This 2-octet unsigned integer indicates the total length of the
message, including the header, in octets. Thus, e.g., it
allows one to locate in the transport-level stream the (Marker
field of the) next message. The value of the Length field must
always be at least 19 and no greater than 4096, and may be
further constrained, depending on the message type. No
"padding" of extra data after the message is allowed, so the
Length field must have the smallest value required given the
rest of the message.
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Type:
This 1-octet unsigned integer indicates the type code of the
message. The following type codes are defined:
1 - OPEN
2 - UPDATE
3 - NOTIFICATION
4 - KEEPALIVE
4.2 OPEN Message Format
After a transport protocol connection is established, the first
message sent by each side is an OPEN message. If the OPEN message is
acceptable, a KEEPALIVE message confirming the OPEN is sent back.
Once the OPEN is confirmed, UPDATE, KEEPALIVE, and NOTIFICATION
messages may be exchanged.
In addition to the fixed-size BGP header, the OPEN message contains
the following fields:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+
| Version |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| My Autonomous System |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Hold Time |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| BGP Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Opt Parm Len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Optional Parameters |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Version:
This 1-octet unsigned integer indicates the protocol version
number of the message. The current BGP version number is 4.
My Autonomous System:
This 2-octet unsigned integer indicates the Autonomous System
number of the sender.
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Hold Time:
This 2-octet unsigned integer indicates the number of seconds
that the sender proposes for the value of the Hold Timer. Upon
receipt of an OPEN message, a BGP speaker MUST calculate the
value of the Hold Timer by using the smaller of its configured
Hold Time and the Hold Time received in the OPEN message. The
Hold Time MUST be either zero or at least three seconds. An
implementation may reject connections on the basis of the Hold
Time. The calculated value indicates the maximum number of
seconds that may elapse between the receipt of successive
KEEPALIVE, and/or UPDATE messages by the sender.
BGP Identifier:
This 4-octet unsigned integer indicates the BGP Identifier of
the sender. A given BGP speaker sets the value of its BGP
Identifier to an IP address assigned to that BGP speaker. The
value of the BGP Identifier is determined on startup and is the
same for every local interface and every BGP peer.
Optional Parameters Length:
This 1-octet unsigned integer indicates the total length of the
Optional Parameters field in octets. If the value of this field
is zero, no Optional Parameters are present.
Optional Parameters:
This field may contain a list of optional parameters, where
each parameter is encoded as a <Parameter Type, Parameter
Length, Parameter Value> triplet.
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...
| Parm. Type | Parm. Length | Parameter Value (variable)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...
Parameter Type is a one octet field that unambiguously
identifies individual parameters. Parameter Length is a one
octet field that contains the length of the Parameter Value
field in octets. Parameter Value is a variable length field
that is interpreted according to the value of the Parameter
Type field.
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This document defines the following Optional Parameters:
a) Authentication Information (Parameter Type 1):
This optional parameter may be used to authenticate a BGP
peer. The Parameter Value field contains a 1-octet
Authentication Code followed by a variable length
Authentication Data.
0 1 2 3 4 5 6 7 8
+-+-+-+-+-+-+-+-+
| Auth. Code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Authentication Data |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Authentication Code:
This 1-octet unsigned integer indicates the
authentication mechanism being used. Whenever an
authentication mechanism is specified for use within
BGP, three things must be included in the
specification:
- the value of the Authentication Code which indicates
use of the mechanism,
- the form and meaning of the Authentication Data, and
- the algorithm for computing values of Marker fields.
Note that a separate authentication mechanism may be
used in establishing the transport level connection.
Authentication Data:
The form and meaning of this field is a variable-
length field depend on the Authentication Code.
The minimum length of the OPEN message is 29 octets (including
message header).
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4.3 UPDATE Message Format
UPDATE messages are used to transfer routing information between BGP
peers. The information in the UPDATE packet can be used to construct
a graph describing the relationships of the various Autonomous
Systems. By applying rules to be discussed, routing information
loops and some other anomalies may be detected and removed from
inter-AS routing.
An UPDATE message is used to advertise a single feasible route to a
peer, or to withdraw multiple unfeasible routes from service (see
3.1). An UPDATE message may simultaneously advertise a feasible route
and withdraw multiple unfeasible routes from service. The UPDATE
message always includes the fixed-size BGP header, and can optionally
include the other fields as shown below:
+-----------------------------------------------------+
| Unfeasible Routes Length (2 octets) |
+-----------------------------------------------------+
| Withdrawn Routes (variable) |
+-----------------------------------------------------+
| Total Path Attribute Length (2 octets) |
+-----------------------------------------------------+
| Path Attributes (variable) |
+-----------------------------------------------------+
| Network Layer Reachability Information (variable) |
+-----------------------------------------------------+
Unfeasible Routes Length:
This 2-octets unsigned integer indicates the total length of
the Withdrawn Routes field in octets. Its value must allow the
length of the Network Layer Reachability Information field to
be determined as specified below.
A value of 0 indicates that no routes are being withdrawn from
service, and that the WITHDRAWN ROUTES field is not present in
this UPDATE message.
Withdrawn Routes:
This is a variable length field that contains a list of IP
address prefixes for the routes that are being withdrawn from
service. Each IP address prefix is encoded as a 2-tuple of the
form <length, prefix>, whose fields are described below:
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+---------------------------+
| Length (1 octet) |
+---------------------------+
| Prefix (variable) |
+---------------------------+
The use and the meaning of these fields are as follows:
a) Length:
The Length field indicates the length in bits of the IP
address prefix. A length of zero indicates a prefix that
matches all IP addresses (with prefix, itself, of zero
octets).
b) Prefix:
The Prefix field contains IP address prefixes followed by
enough trailing bits to make the end of the field fall on an
octet boundary. Note that the value of trailing bits is
irrelevant.
Total Path Attribute Length:
This 2-octet unsigned integer indicates the total length of the
Path Attributes field in octets. Its value must allow the
length of the Network Layer Reachability field to be determined
as specified below.
A value of 0 indicates that no Network Layer Reachability
Information field is present in this UPDATE message.
Path Attributes:
A variable length sequence of path attributes is present in
every UPDATE. Each path attribute is a triple <attribute type,
attribute length, attribute value> of variable length.
Attribute Type is a two-octet field that consists of the
Attribute Flags octet followed by the Attribute Type Code
octet.
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Attr. Flags |Attr. Type Code|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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The high-order bit (bit 0) of the Attribute Flags octet is the
Optional bit. It defines whether the attribute is optional (if
set to 1) or well-known (if set to 0).
The second high-order bit (bit 1) of the Attribute Flags octet
is the Transitive bit. It defines whether an optional
attribute is transitive (if set to 1) or non-transitive (if set
to 0). For well-known attributes, the Transitive bit must be
set to 1. (See Section 5 for a discussion of transitive
attributes.)
The third high-order bit (bit 2) of the Attribute Flags octet
is the Partial bit. It defines whether the information
contained in the optional transitive attribute is partial (if
set to 1) or complete (if set to 0). For well-known attributes
and for optional non-transitive attributes the Partial bit must
be set to 0.
The fourth high-order bit (bit 3) of the Attribute Flags octet
is the Extended Length bit. It defines whether the Attribute
Length is one octet (if set to 0) or two octets (if set to 1).
Extended Length may be used only if the length of the attribute
value is greater than 255 octets.
The lower-order four bits of the Attribute Flags octet are .
unused. They must be zero (and must be ignored when received).
The Attribute Type Code octet contains the Attribute Type Code.
Currently defined Attribute Type Codes are discussed in Section
5.
If the Extended Length bit of the Attribute Flags octet is set
to 0, the third octet of the Path Attribute contains the length
of the attribute data in octets.
If the Extended Length bit of the Attribute Flags octet is set
to 1, then the third and the fourth octets of the path
attribute contain the length of the attribute data in octets.
The remaining octets of the Path Attribute represent the
attribute value and are interpreted according to the Attribute
Flags and the Attribute Type Code. The supported Attribute Type
Codes, their attribute values and uses are the following:
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a) ORIGIN (Type Code 1):
ORIGIN is a well-known mandatory attribute that defines the
origin of the path information. The data octet can assume
the following values:
Value Meaning
0 IGP - Network Layer Reachability Information
is interior to the originating AS
1 EGP - Network Layer Reachability Information
learned via EGP
2 INCOMPLETE - Network Layer Reachability
Information learned by some other means
Its usage is defined in 5.1.1
b) AS_PATH (Type Code 2):
AS_PATH is a well-known mandatory attribute that is composed
of a sequence of AS path segments. Each AS path segment is
represented by a triple <path segment type, path segment
length, path segment value>.
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The path segment type is a 1-octet long field with the
following values defined:
Value Segment Type
1 AS_SET: unordered set of ASs a route in the
UPDATE message has traversed
2 AS_SEQUENCE: ordered set of ASs a route in
the UPDATE message has traversed
The path segment length is a 1-octet long field containing
the number of ASs in the path segment value field.
The path segment value field contains one or more AS
numbers, each encoded as a 2-octets long field.
Usage of this attribute is defined in 5.1.2.
c) NEXT_HOP (Type Code 3):
This is a well-known mandatory attribute that defines the IP
address of the border router that should be used as the next
hop to the destinations listed in the Network Layer
Reachability field of the UPDATE message.
Usage of this attribute is defined in 5.1.3.
d) MULTI_EXIT_DISC (Type Code 4):
This is an optional non-transitive attribute that is a four
octet non-negative integer. The value of this attribute may
be used by a BGP speaker's decision process to discriminate
among multiple exit points to a neighboring autonomous
system.
Its usage is defined in 5.1.4.
e) LOCAL_PREF (Type Code 5):
LOCAL_PREF is a well-known discretionary attribute that is a
four octet non-negative integer. It is used by a BGP speaker
to inform other BGP speakers in its own autonomous system of
the originating speaker's degree of preference for an
advertised route. Usage of this attribute is described in
5.1.5.
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f) ATOMIC_AGGREGATE (Type Code 6)
ATOMIC_AGGREGATE is a well-known discretionary attribute of
length 0. It is used by a BGP speaker to inform other BGP
speakers that the local system selected a less specific
route without selecting a more specific route which is
included in it. Usage of this attribute is described in
5.1.6.
g) AGGREGATOR (Type Code 7)
AGGREGATOR is an optional transitive attribute of length 6.
The attribute contains the last AS number that formed the
aggregate route (encoded as 2 octets), followed by the IP
address of the BGP speaker that formed the aggregate route
(encoded as 4 octets). Usage of this attribute is described
in 5.1.7
Network Layer Reachability Information:
This variable length field contains a list of IP address
prefixes. The length in octets of the Network Layer
Reachability Information is not encoded explicitly, but can be
calculated as:
UPDATE message Length - 23 - Total Path Attributes Length -
Unfeasible Routes Length
where UPDATE message Length is the value encoded in the fixed-
size BGP header, Total Path Attribute Length and Unfeasible
Routes Length are the values encoded in the variable part of
the UPDATE message, and 23 is a combined length of the fixed-
size BGP header, the Total Path Attribute Length field and the
Unfeasible Routes Length field.
Reachability information is encoded as one or more 2-tuples of
the form <length, prefix>, whose fields are described below:
+---------------------------+
| Length (1 octet) |
+---------------------------+
| Prefix (variable) |
+---------------------------+
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The use and the meaning of these fields are as follows:
a) Length:
The Length field indicates the length in bits of the IP
address prefix. A length of zero indicates a prefix that
matches all IP addresses (with prefix, itself, of zero
octets).
b) Prefix:
The Prefix field contains IP address prefixes followed by
enough trailing bits to make the end of the field fall on an
octet boundary. Note that the value of the trailing bits is
irrelevant.
The minimum length of the UPDATE message is 23 octets -- 19 octets
for the fixed header + 2 octets for the Unfeasible Routes Length + 2
octets for the Total Path Attribute Length (the value of Unfeasible
Routes Length is 0 and the value of Total Path Attribute Length is
0).
An UPDATE message can advertise at most one route, which may be
described by several path attributes. All path attributes contained
in a given UPDATE messages apply to the destinations carried in the
Network Layer Reachability Information field of the UPDATE message.
An UPDATE message can list multiple routes to be withdrawn from
service. Each such route is identified by its destination (expressed
as an IP prefix), which unambiguously identifies the route in the
context of the BGP speaker - BGP speaker connection to which it has
been previously been advertised.
An UPDATE message may advertise only routes to be withdrawn from
service, in which case it will not include path attributes or Network
Layer Reachability Information. Conversely, it may advertise only a
feasible route, in which case the WITHDRAWN ROUTES field need not be
present.
4.4 KEEPALIVE Message Format
BGP does not use any transport protocol-based keep-alive mechanism to
determine if peers are reachable. Instead, KEEPALIVE messages are
exchanged between peers often enough as not to cause the Hold Timer
to expire. A reasonable maximum time between KEEPALIVE messages
would be one third of the Hold Time interval. KEEPALIVE messages
MUST NOT be sent more frequently than one per second. An
implementation MAY adjust the rate at which it sends KEEPALIVE
Rekhter & Li [Page 17]RFC 1771 BGP-4 March 1995
messages as a function of the Hold Time interval.
If the negotiated Hold Time interval is zero, then periodic KEEPALIVE
messages MUST NOT be sent.
KEEPALIVE message consists of only message header and has a length of
19 octets.
4.5 NOTIFICATION Message Format
A NOTIFICATION message is sent when an error condition is detected.
The BGP connection is closed immediately after sending it.
In addition to the fixed-size BGP header, the NOTIFICATION message
contains the following fields:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Error code | Error subcode | Data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Error Code:
This 1-octet unsigned integer indicates the type of
NOTIFICATION. The following Error Codes have been defined:
Error Code Symbolic Name Reference
1 Message Header Error Section 6.1
2 OPEN Message Error Section 6.2
3 UPDATE Message Error Section 6.3
4 Hold Timer Expired Section 6.5
5 Finite State Machine Error Section 6.6
6 Cease Section 6.7
Error subcode:
This 1-octet unsigned integer provides more specific
information about the nature of the reported error. Each Error
Code may have one or more Error Subcodes associated with it.
Rekhter & Li [Page 18]RFC 1771 BGP-4 March 1995
If no appropriate Error Subcode is defined, then a zero
(Unspecific) value is used for the Error Subcode field.
Message Header Error subcodes:
1 - Connection Not Synchronized.
2 - Bad Message Length.
3 - Bad Message Type.
OPEN Message Error subcodes:
1 - Unsupported Version Number.
2 - Bad Peer AS.
3 - Bad BGP Identifier. '
4 - Unsupported Optional Parameter.
5 - Authentication Failure.
6 - Unacceptable Hold Time.
UPDATE Message Error subcodes:
1 - Malformed Attribute List.
2 - Unrecognized Well-known Attribute.
3 - Missing Well-known Attribute.
4 - Attribute Flags Error.
5 - Attribute Length Error.
6 - Invalid ORIGIN Attribute
7 - AS Routing Loop.
8 - Invalid NEXT_HOP Attribute.
9 - Optional Attribute Error.
10 - Invalid Network Field.
11 - Malformed AS_PATH.
Data:
This variable-length field is used to diagnose the reason for
the NOTIFICATION. The contents of the Data field depend upon
the Error Code and Error Subcode. See Section 6 below for more
details.
Note that the length of the Data field can be determined from
the message Length field by the formula:
Message Length = 21 + Data Length
The minimum length of the NOTIFICATION message is 21 octets
(including message header).
Rekhter & Li [Page 19]RFC 1771 BGP-4 March 1995
5. Path Attributes
This section discusses the path attributes of the UPDATE message.
Path attributes fall into four separate categories:
1. Well-known mandatory.
2. Well-known discretionary.
3. Optional transitive.
4. Optional non-transitive.
Well-known attributes must be recognized by all BGP implementations.
Some of these attributes are mandatory and must be included in every
UPDATE message. Others are discretionary and may or may not be sent
in a particular UPDATE message.
All well-known attributes must be passed along (after proper
updating, if necessary) to other BGP peers.
In addition to well-known attributes, each path may contain one or
more optional attributes. It is not required or expected that all
BGP implementations support all optional attributes. The handling of
an unrecognized optional attribute is determined by the setting of
the Transitive bit in the attribute flags octet. Paths with
unrecognized transitive optional attributes should be accepted. If a
path with unrecognized transitive optional attribute is accepted and
passed along to other BGP peers, then the unrecognized transitive
optional attribute of that path must be passed along with the path to
other BGP peers with the Partial bit in the Attribute Flags octet set
to 1. If a path with recognized transitive optional attribute is
accepted and passed along to other BGP peers and the Partial bit in
the Attribute Flags octet is set to 1 by some previous AS, it is not
set back to 0 by the current AS. Unrecognized non-transitive optional
attributes must be quietly ignored and not passed along to other BGP
peers.
New transitive optional attributes may be attached to the path by the
originator or by any other AS in the path. If they are not attached
by the originator, the Partial bit in the Attribute Flags octet is
set to 1. The rules for attaching new non-transitive optional
attributes will depend on the nature of the specific attribute. The
documentation of each new non-transitive optional attribute will be
expected to include such rules. (The description of the
MULTI_EXIT_DISC attribute gives an example.) All optional attributes
(both transitive and non-transitive) may be updated (if appropriate)
by ASs in the path.
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The sender of an UPDATE message should order path attributes within
the UPDATE message in ascending order of attribute type. The
receiver of an UPDATE message must be prepared to handle path
attributes within the UPDATE message that are out of order.
The same attribute cannot appear more than once within the Path
Attributes field of a particular UPDATE message.
5.1 Path Attribute Usage
The usage of each BGP path attributes is described in the following
clauses.
5.1.1 ORIGIN
ORIGIN is a well-known mandatory attribute. The ORIGIN attribute
shall be generated by the autonomous system that originates the
associated routing information. It shall be included in the UPDATE
messages of all BGP speakers that choose to propagate this
information to other BGP speakers.
5.1.2 AS_PATH
AS_PATH is a well-known mandatory attribute. This attribute
identifies the autonomous systems through which routing information
carried in this UPDATE message has passed. The components of this
list can be AS_SETs or AS_SEQUENCEs.
When a BGP speaker propagates a route which it has learned from
another BGP speaker's UPDATE message, it shall modify the route's
AS_PATH attribute based on the location of the BGP speaker to which
the route will be sent:
a) When a given BGP speaker advertises the route to another BGP
speaker located in its own autonomous system, the advertising
speaker shall not modify the AS_PATH attribute associated with the
route.
b) When a given BGP speaker advertises the route to a BGP speaker
located in a neighboring autonomous system, then the advertising
speaker shall update the AS_PATH attribute as follows:
1) if the first path segment of the AS_PATH is of type
AS_SEQUENCE, the local system shall prepend its own AS number
as the last element of the sequence (put it in the leftmost
position).
Rekhter & Li [Page 21]RFC 1771 BGP-4 March 1995
2) if the first path segment of the AS_PATH is of type AS_SET,
the local system shall prepend a new path segment of type
AS_SEQUENCE to the AS_PATH, including its own AS number in that
segment.
When a BGP speaker originates a route then:
a) the originating speaker shall include its own AS number in
the AS_PATH attribute of all UPDATE messages sent to BGP
speakers located in neighboring autonomous systems. (In this
case, the AS number of the originating speaker's autonomous
system will be the only entry in the AS_PATH attribute).
b) the originating speaker shall include an empty AS_PATH
attribute in all UPDATE messages sent to BGP speakers located
in its own autonomous system. (An empty AS_PATH attribute is
one whose length field contains the value zero).
5.1.3 NEXT_HOP
The NEXT_HOP path attribute defines the IP address of the border
router that should be used as the next hop to the destinations listed
in the UPDATE message. If a border router belongs to the same AS as
its peer, then the peer is an internal border router. Otherwise, it
is an external border router. A BGP speaker can advertise any
internal border router as the next hop provided that the interface
associated with the IP address of this border router (as specified in
the NEXT_HOP path attribute) shares a common subnet with both the
local and remote BGP speakers. A BGP speaker can advertise any
external border router as the next hop, provided that the IP address
of this border router was learned from one of the BGP speaker's
peers, and the interface associated with the IP address of this
border router (as specified in the NEXT_HOP path attribute) shares a
common subnet with the local and remote BGP speakers. A BGP speaker
needs to be able to support disabling advertisement of external
border routers.
A BGP speaker must never advertise an address of a peer to that peer
as a NEXT_HOP, for a route that the speaker is originating. A BGP
speaker must never install a route with itself as the next hop.
When a BGP speaker advertises the route to a BGP speaker located in
its own autonomous system, the advertising speaker shall not modify
the NEXT_HOP attribute associated with the route. When a BGP speaker
receives the route via an internal link, it may forward packets to
the NEXT_HOP address if the address contained in the attribute is on
a common subnet with the local and remote BGP speakers.
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5.1.4 MULTI_EXIT_DISC
The MULTI_EXIT_DISC attribute may be used on external (inter-AS)
links to discriminate among multiple exit or entry points to the same
neighboring AS. The value of the MULTI_EXIT_DISC attribute is a four
octet unsigned number which is called a metric. All other factors
being equal, the exit or entry point with lower metric should be
preferred. If received over external links, the MULTI_EXIT_DISC
attribute may be propagated over internal links to other BGP speakers
within the same AS. The MULTI_EXIT_DISC attribute is never
propagated to other BGP speakers in neighboring AS's.
5.1.5 LOCAL_PREF
LOCAL_PREF is a well-known discretionary attribute that shall be
included in all UPDATE messages that a given BGP speaker sends to the
other BGP speakers located in its own autonomous system. A BGP
speaker shall calculate the degree of preference for each external
route and include the degree of preference when advertising a route
to its internal peers. The higher degree of preference should be
preferred. A BGP speaker shall use the degree of preference learned
via LOCAL_PREF in its decision process (see section 9.1.1).
A BGP speaker shall not include this attribute in UPDATE messages
that it sends to BGP speakers located in a neighboring autonomous
system. If it is contained in an UPDATE message that is received from
a BGP speaker which is not located in the same autonomous system as
the receiving speaker, then this attribute shall be ignored by the
receiving speaker.
5.1.6 ATOMIC_AGGREGATE
ATOMIC_AGGREGATE is a well-known discretionary attribute. If a BGP
speaker, when presented with a set of overlapping routes from one of
its peers (see 9.1.4), selects the less specific route without
selecting the more specific one, then the local system shall attach
the ATOMIC_AGGREGATE attribute to the route when propagating it to
other BGP speakers (if that attribute is not already present in the
received less specific route). A BGP speaker that receives a route
with the ATOMIC_AGGREGATE attribute shall not remove the attribute
from the route when propagating it to other speakers. A BGP speaker
that receives a route with the ATOMIC_AGGREGATE attribute shall not
make any NLRI of that route more specific (as defined in 9.1.4) when
advertising this route to other BGP speakers. A BGP speaker that
receives a route with the ATOMIC_AGGREGATE attribute needs to be
cognizant of the fact that the actual path to destinations, as
specified in the NLRI of the route, while having the loop-free
property, may traverse ASs that are not listed in the AS_PATH
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RFC 1771 BGP-4 March 1995
attribute.
5.1.7 AGGREGATOR
AGGREGATOR is an optional transitive attribute which may be included
in updates which are formed by aggregation (see Section 9.2.4.2). A
BGP speaker which performs route aggregation may add the AGGREGATOR
attribute which shall contain its own AS number and IP address.
6. BGP Error Handling.
This section describes actions to be taken when errors are detected
while processing BGP messages.
When any of the conditions described here are detected, a
NOTIFICATION message with the indicated Error Code, Error Subcode,
and Data fields is sent, and the BGP connection is closed. If no
Error Subcode is specified, then a zero must be used.
The phrase "the BGP connection is closed" means that the transport
protocol connection has been closed and that all resources for that
BGP connection have been deallocated. Routing table entries
associated with the remote peer are marked as invalid. The fact that
the routes have become invalid is passed to other BGP peers before
the routes are deleted from the system.
Unless specified explicitly, the Data field of the NOTIFICATION
message that is sent to indicate an error is empty.
6.1 Message Header error handling.
All errors detected while processing the Message Header are indicated
by sending the NOTIFICATION message with Error Code Message Header
Error. The Error Subcode elaborates on the specific nature of the
error.
The expected value of the Marker field of the message header is all
ones if the message type is OPEN. The expected value of the Marker
field for all other types of BGP messages determined based on the
presence of the Authentication Information Optional Parameter in the
BGP OPEN message and the actual authentication mechanism (if the
Authentication Information in the BGP OPEN message is present). If
the Marker field of the message header is not the expected one, then
a synchronization error has occurred and the Error Subcode is set to
Connection Not Synchronized.
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RFC 1771 BGP-4 March 1995
If the Length field of the message header is less than 19 or greater
than 4096, or if the Length field of an OPEN message is less than
the minimum length of the OPEN message, or if the Length field of an
UPDATE message is less than the minimum length of the UPDATE message,
or if the Length field of a KEEPALIVE message is not equal to 19, or
if the Length field of a NOTIFICATION message is less than the
minimum length of the NOTIFICATION message, then the Error Subcode is
set to Bad Message Length. The Data field contains the erroneous
Length field.
If the Type field of the message header is not recognized, then the
Error Subcode is set to Bad Message Type. The Data field contains
the erroneous Type field.
6.2 OPEN message error handling.
All errors detected while processing the OPEN message are indicated
by sending the NOTIFICATION message with Error Code OPEN Message
Error. The Error Subcode elaborates on the specific nature of the
error.
If the version number contained in the Version field of the received
OPEN message is not supported, then the Error Subcode is set to
Unsupported Version Number. The Data field is a 2-octet unsigned
integer, which indicates the largest locally supported version number
less than the version the remote BGP peer bid (as indicated in the
received OPEN message).
If the Autonomous System field of the OPEN message is unacceptable,
then the Error Subcode is set to Bad Peer AS. The determination of
acceptable Autonomous System numbers is outside the scope of this
protocol.
If the Hold Time field of the OPEN message is unacceptable, then the
Error Subcode MUST be set to Unacceptable Hold Time. An
implementation MUST reject Hold Time values of one or two seconds.
An implementation MAY reject any proposed Hold Time. An
implementation which accepts a Hold Time MUST use the negotiated
value for the Hold Time.
If the BGP Identifier field of the OPEN message is syntactically
incorrect, then the Error Subcode is set to Bad BGP Identifier.
Syntactic correctness means that the BGP Identifier field represents
a valid IP host address.
If one of the Optional Parameters in the OPEN message is not
recognized, then the Error Subcode is set to Unsupported Optional
Parameters.
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RFC 1771 BGP-4 March 1995
If the OPEN message carries Authentication Information (as an
Optional Parameter), then the corresponding authentication procedure
is invoked. If the authentication procedure (based on Authentication
Code and Authentication Data) fails, then the Error Subcode is set to
Authentication Failure.
6.3 UPDATE message error handling.
All errors detected while processing the UPDATE message are indicated
by sending the NOTIFICATION message with Error Code UPDATE Message
Error. The error subcode elaborates on the specific nature of the
error.
Error checking of an UPDATE message begins by examining the path
attributes. If the Unfeasible Routes Length or Total Attribute
Length is too large (i.e., if Unfeasible Routes Length + Total
Attribute Length + 23 exceeds the message Length), then the Error
Subcode is set to Malformed Attribute List.
If any recognized attribute has Attribute Flags that conflict with
the Attribute Type Code, then the Error Subcode is set to Attribute
Flags Error. The Data field contains the erroneous attribute (type,
length and value).
If any recognized attribute has Attribute Length that conflicts with
the expected length (based on the attribute type code), then the
Error Subcode is set to Attribute Length Error. The Data field
contains the erroneous attribute (type, length and value).
If any of the mandatory well-known attributes are not present, then
the Error Subcode is set to Missing Well-known Attribute. The Data
field contains the Attribute Type Code of the missing well-known
attribute.
If any of the mandatory well-known attributes are not recognized,
then the Error Subcode is set to Unrecognized Well-known Attribute.
The Data field contains the unrecognized attribute (type, length and
value).
If the ORIGIN attribute has an undefined value, then the Error
Subcode is set to Invalid Origin Attribute. The Data field contains
the unrecognized attribute (type, length and value).
If the NEXT_HOP attribute field is syntactically incorrect, then the
Error Subcode is set to Invalid NEXT_HOP Attribute. The Data field
contains the incorrect attribute (type, length and value). Syntactic
correctness means that the NEXT_HOP attribute represents a valid IP
host address. Semantic correctness applies only to the external BGP
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RFC 1771 BGP-4 March 1995
links. It means that the interface associated with the IP address, as
specified in the NEXT_HOP attribute, shares a common subnet with the
receiving BGP speaker and is not the IP address of the receiving BGP
speaker. If the NEXT_HOP attribute is semantically incorrect, the
error should be logged, and the the route should be ignored. In this
case, no NOTIFICATION message should be sent.
The AS_PATH attribute is checked for syntactic correctness. If the
path is syntactically incorrect, then the Error Subcode is set to
Malformed AS_PATH.
If an optional attribute is recognized, then the value of this
attribute is checked. If an error is detected, the attribute is
discarded, and the Error Subcode is set to Optional Attribute Error.
The Data field contains the attribute (type, length and value).
If any attribute appears more than once in the UPDATE message, then
the Error Subcode is set to Malformed Attribute List.
The NLRI field in the UPDATE message is checked for syntactic
validity. If the field is syntactically incorrect, then the Error
Subcode is set to Invalid Network Field.
6.4 NOTIFICATION message error handling.
If a peer sends a NOTIFICATION message, and there is an error in that
message, there is unfortunately no means of reporting this error via
a subsequent NOTIFICATION message. Any such error, such as an
unrecognized Error Code or Error Subcode, should be noticed, logged
locally, and brought to the attention of the administration of the
peer. The means to do this, however, lies outside the scope of this
document.
6.5 Hold Timer Expired error handling.
If a system does not receive successive KEEPALIVE and/or UPDATE
and/or NOTIFICATION messages within the period specified in the Hold
Time field of the OPEN message, then the NOTIFICATION message with
Hold Timer Expired Error Code must be sent and the BGP connection
closed.
6.6 Finite State Machine error handling.
Any error detected by the BGP Finite State Machine (e.g., receipt of
an unexpected event) is indicated by sending the NOTIFICATION message
with Error Code Finite State Machine Error.
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RFC 1771 BGP-4 March 1995
6.7 Cease.
In absence of any fatal errors (that are indicated in this section),
a BGP peer may choose at any given time to close its BGP connection
by sending the NOTIFICATION message with Error Code Cease. However,
the Cease NOTIFICATION message must not be used when a fatal error
indicated by this section does exist.
6.8 Connection collision detection.
If a pair of BGP speakers try simultaneously to establish a TCP
connection to each other, then two parallel connections between this
pair of speakers might well be formed. We refer to this situation as
connection collision. Clearly, one of these connections must be
closed.
Based on the value of the BGP Identifier a convention is established
for detecting which BGP connection is to be preserved when a
collision does occur. The convention is to compare the BGP
Identifiers of the peers involved in the collision and to retain only
the connection initiated by the BGP speaker with the higher-valued
BGP Identifier.
Upon receipt of an OPEN message, the local system must examine all of
its connections that are in the OpenConfirm state. A BGP speaker may
also examine connections in an OpenSent state if it knows the BGP
Identifier of the peer by means outside of the protocol. If among
these connections there is a connection to a remote BGP speaker whose
BGP Identifier equals the one in the OPEN message, then the local
system performs the following collision resolution procedure:
1. The BGP Identifier of the local system is compared to the BGP
Identifier of the remote system (as specified in the OPEN
message).
2. If the value of the local BGP Identifier is less than the
remote one, the local system closes BGP connection that already
exists (the one that is already in the OpenConfirm state), and
accepts BGP connection initiated by the remote system.
3. Otherwise, the local system closes newly created BGP connection
(the one associated with the newly received OPEN message), and
continues to use the existing one (the one that is already in the
OpenConfirm state).
Comparing BGP Identifiers is done by treating them as (4-octet
long) unsigned integers.
Rekhter & Li [Page 28]RFC 1771 BGP-4 March 1995
A connection collision with an existing BGP connection that is in
Established states causes unconditional closing of the newly
created connection. Note that a connection collision cannot be
detected with connections that are in Idle, or Connect, or Active
states.
Closing the BGP connection (that results from the collision
resolution procedure) is accomplished by sending the NOTIFICATION
message with the Error Code Cease.
7. BGP Version Negotiation.
BGP speakers may negotiate the version of the protocol by making
multiple attempts to open a BGP connection, starting with the highest
version number each supports. If an open attempt fails with an Error
Code OPEN Message Error, and an Error Subcode Unsupported Version
Number, then the BGP speaker has available the version number it
tried, the version number its peer tried, the version number passed
by its peer in the NOTIFICATION message, and the version numbers that
it supports. If the two peers do support one or more common
versions, then this will allow them to rapidly determine the highest
common version. In order to support BGP version negotiation, future
versions of BGP must retain the format of the OPEN and NOTIFICATION
messages.
8. BGP Finite State machine.
This section specifies BGP operation in terms of a Finite State
Machine (FSM). Following is a brief summary and overview of BGP
operations by state as determined by this FSM. A condensed version
of the BGP FSM is found in Appendix 1.
Initially BGP is in the Idle state.
Idle state:
In this state BGP refuses all incoming BGP connections. No
resources are allocated to the peer. In response to the Start
event (initiated by either system or operator) the local system
initializes all BGP resources, starts the ConnectRetry timer,
initiates a transport connection to other BGP peer, while
listening for connection that may be initiated by the remote
BGP peer, and changes its state to Connect. The exact value of
the ConnectRetry timer is a local matter, but should be
sufficiently large to allow TCP initialization.
If a BGP speaker detects an error, it shuts down the connection
and changes its state to Idle. Getting out of the Idle state
Rekhter & Li [Page 29]RFC 1771 BGP-4 March 1995
requires generation of the Start event. If such an event is
generated automatically, then persistent BGP errors may result
in persistent flapping of the speaker. To avoid such a
condition it is recommended that Start events should not be
generated immediately for a peer that was previously
transitioned to Idle due to an error. For a peer that was
previously transitioned to Idle due to an error, the time
between consecutive generation of Start events, if such events
are generated automatically, shall exponentially increase. The
value of the initial timer shall be 60 seconds. The time shall
be doubled for each consecutive retry.
Any other event received in the Idle state is ignored.
Connect state:
In this state BGP is waiting for the transport protocol
connection to be completed.
If the transport protocol connection succeeds, the local system
clears the ConnectRetry timer, completes initialization, sends
an OPEN message to its peer, and changes its state to OpenSent.
If the transport protocol connect fails (e.g., retransmission
timeout), the local system restarts the ConnectRetry timer,
continues to listen for a connection that may be initiated by
the remote BGP peer, and changes its state to Active state.
In response to the ConnectRetry timer expired event, the local
system restarts the ConnectRetry timer, initiates a transport
connection to other BGP peer, continues to listen for a
connection that may be initiated by the remote BGP peer, and
stays in the Connect state.
Start event is ignored in the Active state.
In response to any other event (initiated by either system or
operator), the local system releases all BGP resources
associated with this connection and changes its state to Idle.
Active state:
In this state BGP is trying to acquire a peer by initiating a
transport protocol connection.
If the transport protocol connection succeeds, the local system
clears the ConnectRetry timer, completes initialization, sends
an OPEN message to its peer, sets its Hold Timer to a large
Rekhter & Li [Page 30]RFC 1771 BGP-4 March 1995
value, and changes its state to OpenSent. A Hold Timer value
of 4 minutes is suggested.
In response to the ConnectRetry timer expired event, the local
system restarts the ConnectRetry timer, initiates a transport
connection to other BGP peer, continues to listen for a
connection that may be initiated by the remote BGP peer, and
changes its state to Connect.
If the local system detects that a remote peer is trying to
establish BGP connection to it, and the IP address of the
remote peer is not an expected one, the local system restarts
the ConnectRetry timer, rejects the attempted connection,
continues to listen for a connection that may be initiated by
the remote BGP peer, and stays in the Active state.
Start event is ignored in the Active state.
In response to any other event (initiated by either system or
operator), the local system releases all BGP resources
associated with this connection and changes its state to Idle.
OpenSent state:
In this state BGP waits for an OPEN message from its peer.
When an OPEN message is received, all fields are checked for
correctness. If the BGP message header checking or OPEN
message checking detects an error (see Section 6.2), or a
connection collision (see Section 6.8) the local system sends a
NOTIFICATION message and changes its state to Idle.
If there are no errors in the OPEN message, BGP sends a
KEEPALIVE message and sets a KeepAlive timer. The Hold Timer,
which was originally set to a large value (see above), is
replaced with the negotiated Hold Time value (see section 4.2).
If the negotiated Hold Time value is zero, then the Hold Time
timer and KeepAlive timers are not started. If the value of
the Autonomous System field is the same as the local Autonomous
System number, then the connection is an "internal" connection;
otherwise, it is "external". (This will effect UPDATE
processing as described below.) Finally, the state is changed
to OpenConfirm.
If a disconnect notification is received from the underlying
transport protocol, the local system closes the BGP connection,
restarts the ConnectRetry timer, while continue listening for
connection that may be initiated by the remote BGP peer, and
goes into the Active state.
Rekhter & Li [Page 31]RFC 1771 BGP-4 March 1995
If the Hold Timer expires, the local system sends NOTIFICATION
message with error code Hold Timer Expired and changes its
state to Idle.
In response to the Stop event (initiated by either system or
operator) the local system sends NOTIFICATION message with
Error Code Cease and changes its state to Idle.
Start event is ignored in the OpenSent state.
In response to any other event the local system sends
NOTIFICATION message with Error Code Finite State Machine Error
and changes its state to Idle.
Whenever BGP changes its state from OpenSent to Idle, it closes
the BGP (and transport-level) connection and releases all
resources associated with that connection.
OpenConfirm state:
In this state BGP waits for a KEEPALIVE or NOTIFICATION
message.
If the local system receives a KEEPALIVE message, it changes
its state to Established.
If the Hold Timer expires before a KEEPALIVE message is
received, the local system sends NOTIFICATION message with
error code Hold Timer Expired and changes its state to Idle.
If the local system receives a NOTIFICATION message, it changes
its state to Idle.
If the KeepAlive timer expires, the local system sends a
KEEPALIVE message and restarts its KeepAlive timer.
If a disconnect notification is received from the underlying
transport protocol, the local system changes its state to Idle.
In response to the Stop event (initiated by either system or
operator) the local system sends NOTIFICATION message with
Error Code Cease and changes its state to Idle.
Start event is ignored in the OpenConfirm state.
In response to any other event the local system sends
NOTIFICATION message with Error Code Finite State Machine Error
and changes its state to Idle.
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Whenever BGP changes its state from OpenConfirm to Idle, it
closes the BGP (and transport-level) connection and releases
all resources associated with that connection.
Established state:
In the Established state BGP can exchange UPDATE, NOTIFICATION,
and KEEPALIVE messages with its peer.
If the local system receives an UPDATE or KEEPALIVE message, it
restarts its Hold Timer, if the negotiated Hold Time value is
non-zero.
If the local system receives a NOTIFICATION message, it changes
its state to Idle.
If the local system receives an UPDATE message and the UPDATE
message error handling procedure (see Section 6.3) detects an
error, the local system sends a NOTIFICATION message and
changes its state to Idle.
If a disconnect notification is received from the underlying
transport protocol, the local system changes its state to Idle.
If the Hold Timer expires, the local system sends a
NOTIFICATION message with Error Code Hold Timer Expired and
changes its state to Idle.
If the KeepAlive timer expires, the local system sends a
KEEPALIVE message and restarts its KeepAlive timer.
Each time the local system sends a KEEPALIVE or UPDATE message,
it restarts its KeepAlive timer, unless the negotiated Hold
Time value is zero.
In response to the Stop event (initiated by either system or
operator), the local system sends a NOTIFICATION message with
Error Code Cease and changes its state to Idle.
Start event is ignored in the Established state.
In response to any other event, the local system sends
NOTIFICATION message with Error Code Finite State Machine Error
and changes its state to Idle.
Whenever BGP changes its state from Established to Idle, it
closes the BGP (and transport-level) connection, releases all
resources associated with that connection, and deletes all
Rekhter & Li [Page 33]RFC 1771 BGP-4 March 1995
routes derived from that connection.
9. UPDATE Message Handling
An UPDATE message may be received only in the Established state.
When an UPDATE message is received, each field is checked for
validity as specified in Section 6.3.
If an optional non-transitive attribute is unrecognized, it is
quietly ignored. If an optional transitive attribute is
unrecognized, the Partial bit (the third high-order bit) in the
attribute flags octet is set to 1, and the attribute is retained for
propagation to other BGP speakers.
If an optional attribute is recognized, and has a valid value, then,
depending on the type of the optional attribute, it is processed
locally, retained, and updated, if necessary, for possible
propagation to other BGP speakers.
If the UPDATE message contains a non-empty WITHDRAWN ROUTES field,
the previously advertised routes whose destinations (expressed as IP
prefixes) contained in this field shall be removed from the Adj-RIB-
In. This BGP speaker shall run its Decision Process since the
previously advertised route is not longer available for use.
If the UPDATE message contains a feasible route, it shall be placed
in the appropriate Adj-RIB-In, and the following additional actions
shall be taken:
i) If its Network Layer Reachability Information (NLRI) is identical
to the one of a route currently stored in the Adj-RIB-In, then the
new route shall replace the older route in the Adj-RIB-In, thus
implicitly withdrawing the older route from service. The BGP speaker
shall run its Decision Process since the older route is no longer
available for use.
ii) If the new route is an overlapping route that is included (see
9.1.4) in an earlier route contained in the Adj-RIB-In, the BGP
speaker shall run its Decision Process since the more specific route
has implicitly made a portion of the less specific route unavailable
for use.
iii) If the new route has identical path attributes to an earlier
route contained in the Adj-RIB-In, and is more specific (see 9.1.4)
than the earlier route, no further actions are necessary.
iv) If the new route has NLRI that is not present in any of the
routes currently stored in the Adj-RIB-In, then the new route shall
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be placed in the Adj-RIB-In. The BGP speaker shall run its Decision
Process.
v) If the new route is an overlapping route that is less specific
(see 9.1.4) than an earlier route contained in the Adj-RIB-In, the
BGP speaker shall run its Decision Process on the set of destinations
described only by the less specific route.
9.1 Decision Process
The Decision Process selects routes for subsequent advertisement by
applying the policies in the local Policy Information Base (PIB) to
the routes stored in its Adj-RIB-In. The output of the Decision
Process is the set of routes that will be advertised to all peers;
the selected routes will be stored in the local speaker's Adj-RIB-
Out.
The selection process is formalized by defining a function that takes
the attribute of a given route as an argument and returns a non-
negative integer denoting the degree of preference for the route.
The function that calculates the degree of preference for a given
route shall not use as its inputs any of the following: the
existence of other routes, the non-existence of other routes, or the
path attributes of other routes. Route selection then consists of
individual application of the degree of preference function to each
feasible route, followed by the choice of the one with the highest
degree of preference.
The Decision Process operates on routes contained in each Adj-RIB-In,
and is responsible for:
- selection of routes to be advertised to BGP speakers located in
the local speaker's autonomous system
- selection of routes to be advertised to BGP speakers located in
neighboring autonomous systems
- route aggregation and route information reduction
The Decision Process takes place in three distinct phases, each
triggered by a different event:
a) Phase 1 is responsible for calculating the degree of preference
for each route received from a BGP speaker located in a
neighboring autonomous system, and for advertising to the other
BGP speakers in the local autonomous system the routes that have
the highest degree of preference for each distinct destination.
Rekhter & Li [Page 35]RFC 1771 BGP-4 March 1995
b) Phase 2 is invoked on completion of phase 1. It is responsible
for choosing the best route out of all those available for each
distinct destination, and for installing each chosen route into
the appropriate Loc-RIB.
c) Phase 3 is invoked after the Loc-RIB has been modified. It is
responsible for disseminating routes in the Loc-RIB to each peer
located in a neighboring autonomous system, according to the
policies contained in the PIB. Route aggregation and information
reduction can optionally be performed within this phase.
9.1.1 Phase 1: Calculation of Degree of Preference
The Phase 1 decision function shall be invoked whenever the local BGP
speaker receives an UPDATE message from a peer located in a
neighboring autonomous system that advertises a new route, a
replacement route, or a withdrawn route.
The Phase 1 decision function is a separate process which completes
when it has no further work to do.
The Phase 1 decision function shall lock an Adj-RIB-In prior to
operating on any route contained within it, and shall unlock it after
operating on all new or unfeasible routes contained within it.
For each newly received or replacement feasible route, the local BGP
speaker shall determine a degree of preference. If the route is
learned from a BGP speaker in the local autonomous system, either the
value of the LOCAL_PREF attribute shall be taken as the degree of
preference, or the local system shall compute the degree of
preference of the route based on preconfigured policy information. If
the route is learned from a BGP speaker in a neighboring autonomous
system, then the degree of preference shall be computed based on
preconfigured policy information. The exact nature of this policy
information and the computation involved is a local matter. The
local speaker shall then run the internal update process of 9.2.1 to
select and advertise the most preferable route.
9.1.2 Phase 2: Route Selection
The Phase 2 decision function shall be invoked on completion of Phase
1. The Phase 2 function is a separate process which completes when
it has no further work to do. The Phase 2 process shall consider all
routes that are present in the Adj-RIBs-In, including those received
from BGP speakers located in its own autonomous system and those
received from BGP speakers located in neighboring autonomous systems.
Rekhter & Li [Page 36]RFC 1771 BGP-4 March 1995
The Phase 2 decision function shall be blocked from running while the
Phase 3 decision function is in process. The Phase 2 function shall
lock all Adj-RIBs-In prior to commencing its function, and shall
unlock them on completion.
If the NEXT_HOP attribute of a BGP route depicts an address to which
the local BGP speaker doesn't have a route in its Loc-RIB, the BGP
route SHOULD be excluded from the Phase 2 decision function.
For each set of destinations for which a feasible route exists in the
Adj-RIBs-In, the local BGP speaker shall identify the route that has:
a) the highest degree of preference of any route to the same set
of destinations, or
b) is the only route to that destination, or
c) is selected as a result of the Phase 2 tie breaking rules
specified in 9.1.2.1.
The local speaker SHALL then install that route in the Loc-RIB,
replacing any route to the same destination that is currently being
held in the Loc-RIB. The local speaker MUST determine the immediate
next hop to the address depicted by the NEXT_HOP attribute of the
selected route by performing a lookup in the IGP and selecting one of
the possible paths in the IGP. This immediate next hop MUST be used
when installing the selected route in the Loc-RIB. If the route to
the address depicted by the NEXT_HOP attribute changes such that the
immediate next hop changes, route selection should be recalculated as
specified above.
Unfeasible routes shall be removed from the Loc-RIB, and
corresponding unfeasible routes shall then be removed from the Adj-
RIBs-In.
9.1.2.1 Breaking Ties (Phase 2)
In its Adj-RIBs-In a BGP speaker may have several routes to the same
destination that have the same degree of preference. The local
speaker can select only one of these routes for inclusion in the
associated Loc-RIB. The local speaker considers all equally
preferable routes, both those received from BGP speakers located in
neighboring autonomous systems, and those received from other BGP
speakers located in the local speaker's autonomous system.
The following tie-breaking procedure assumes that for each candidate
route all the BGP speakers within an autonomous system can ascertain
the cost of a path (interior distance) to the address depicted by the
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NEXT_HOP attribute of the route. Ties shall be broken according to
the following algorithm:
a) If the local system is configured to take into account
MULTI_EXIT_DISC, and the candidate routes differ in their
MULTI_EXIT_DISC attribute, select the route that has the lowest
value of the MULTI_EXIT_DISC attribute.
b) Otherwise, select the route that has the lowest cost (interior
distance) to the entity depicted by the NEXT_HOP attribute of the
route. If there are several routes with the same cost, then the
tie-breaking shall be broken as follows:
- if at least one of the candidate routes was advertised by the
BGP speaker in a neighboring autonomous system, select the
route that was advertised by the BGP speaker in a neighboring
autonomous system whose BGP Identifier has the lowest value
among all other BGP speakers in neighboring autonomous systems;
- otherwise, select the route that was advertised by the BGP
speaker whose BGP Identifier has the lowest value.
9.1.3 Phase 3: Route Dissemination
The Phase 3 decision function shall be invoked on completion of Phase
2, or when any of the following events occur:
a) when routes in a Loc-RIB to local destinations have changed
b) when locally generated routes learned by means outside of BGP
have changed
c) when a new BGP speaker - BGP speaker connection has been
established
The Phase 3 function is a separate process which completes when it
has no further work to do. The Phase 3 Routing Decision function
shall be blocked from running while the Phase 2 decision function is
in process.
All routes in the Loc-RIB shall be processed into a corresponding
entry in the associated Adj-RIBs-Out. Route aggregation and
information reduction techniques (see 9.2.4.1) may optionally be
applied.
For the benefit of future support of inter-AS multicast capabilities,
a BGP speaker that participates in inter-AS multicast routing shall
advertise a route it receives from one of its external peers and if
Rekhter & Li [Page 38]RFC 1771 BGP-4 March 1995
it installs it in its Loc-RIB, it shall advertise it back to the peer
from which the route was received. For a BGP speaker that does not
participate in inter-AS multicast routing such an advertisement is
optional. When doing such an advertisement, the NEXT_HOP attribute
should be set to the address of the peer. An implementation may also
optimize such an advertisement by truncating information in the
AS_PATH attribute to include only its own AS number and that of the
peer that advertised the route (such truncation requires the ORIGIN
attribute to be set to INCOMPLETE). In addition an implementation is
not required to pass optional or discretionary path attributes with
such an advertisement.
When the updating of the Adj-RIBs-Out and the Forwarding Information
Base (FIB) is complete, the local BGP speaker shall run the external
update process of 9.2.2.
9.1.4 Overlapping Routes
A BGP speaker may transmit routes with overlapping Network Layer
Reachability Information (NLRI) to another BGP speaker. NLRI overlap
occurs when a set of destinations are identified in non-matching
multiple routes. Since BGP encodes NLRI using IP prefixes, overlap
will always exhibit subset relationships. A route describing a
smaller set of destinations (a longer prefix) is said to be more
specific than a route describing a larger set of destinations (a
shorted prefix); similarly, a route describing a larger set of
destinations (a shorter prefix) is said to be less specific than a
route describing a smaller set of destinations (a longer prefix).
The precedence relationship effectively decomposes less specific
routes into two parts:
- a set of destinations described only by the less specific
route, and
- a set of destinations described by the overlap of the less
specific and the more specific routes
When overlapping routes are present in the same Adj-RIB-In, the more
specific route shall take precedence, in order from more specific to
least specific.
The set of destinations described by the overlap represents a portion
of the less specific route that is feasible, but is not currently in
use. If a more specific route is later withdrawn, the set of
destinations described by the overlap will still be reachable using
the less specific route.
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If a BGP speaker receives overlapping routes, the Decision Process
shall take into account the semantics of the overlapping routes. In
particular, if a BGP speaker accepts the less specific route while
rejecting the more specific route from the same peer, then the
destinations represented by the overlap may not forward along the ASs
listed in the AS_PATH attribute of that route. Therefore, a BGP
speaker has the following choices:
a) Install both the less and the more specific routes
b) Install the more specific route only
c) Install the non-overlapping part of the less specific
route only (that implies de-aggregation)
d) Aggregate the two routes and install the aggregated route
e) Install the less specific route only
f) Install neither route
If a BGP speaker chooses e), then it should add ATOMIC_AGGREGATE
attribute to the route. A route that carries ATOMIC_AGGREGATE
attribute can not be de-aggregated. That is, the NLRI of this route
can not be made more specific. Forwarding along such a route does
not guarantee that IP packets will actually traverse only ASs listed
in the AS_PATH attribute of the route. If a BGP speaker chooses a),
it must not advertise the more general route without the more
specific route.
9.2 Update-Send Process
The Update-Send process is responsible for advertising UPDATE
messages to all peers. For example, it distributes the routes chosen
by the Decision Process to other BGP speakers which may be located in
either the same autonomous system or a neighboring autonomous system.
rules for information exchange between BGP speakers located in
different autonomous systems are given in 9.2.2; rules for
information exchange between BGP speakers located in the same
autonomous system are given in 9.2.1.
Distribution of routing information between a set of BGP speakers,
all of which are located in the same autonomous system, is referred
to as internal distribution.
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9.2.1 Internal Updates
The Internal update process is concerned with the distribution of
routing information to BGP speakers located in the local speaker's
autonomous system.
When a BGP speaker receives an UPDATE message from another BGP
speaker located in its own autonomous system, the receiving BGP
speaker shall not re-distribute the routing information contained in
that UPDATE message to other BGP speakers located in its own
autonomous system.
When a BGP speaker receives a new route from a BGP speaker in a
neighboring autonomous system, it shall advertise that route to all
other BGP speakers in its autonomous system by means of an UPDATE
message if any of the following conditions occur:
1) the degree of preference assigned to the newly received route
by the local BGP speaker is higher than the degree of preference
that the local speaker has assigned to other routes that have been
received from BGP speakers in neighboring autonomous systems, or
2) there are no other routes that have been received from BGP
speakers in neighboring autonomous systems, or
3) the newly received route is selected as a result of breaking a
tie between several routes which have the highest degree of
preference, and the same destination (the tie-breaking procedure
is specified in 9.2.1.1).
When a BGP speaker receives an UPDATE message with a non-empty
WITHDRAWN ROUTES field, it shall remove from its Adj-RIB-In all
routes whose destinations was carried in this field (as IP prefixes).
The speaker shall take the following additional steps:
1) if the corresponding feasible route had not been previously
advertised, then no further action is necessary
2) if the corresponding feasible route had been previously
advertised, then:
i) if a new route is selected for advertisement that has the
same Network Layer Reachability Information as the unfeasible
routes, then the local BGP speaker shall advertise the
replacement route
ii) if a replacement route is not available for advertisement,
then the BGP speaker shall include the destinations of the
Rekhter & Li [Page 41]RFC 1771 BGP-4 March 1995
unfeasible route (in form of IP prefixes) in the WITHDRAWN
ROUTES field of an UPDATE message, and shall send this message
to each peer to whom it had previously advertised the
corresponding feasible route.
All feasible routes which are advertised shall be placed in the
appropriate Adj-RIBs-Out, and all unfeasible routes which are
advertised shall be removed from the Adj-RIBs-Out.
9.2.1.1 Breaking Ties (Internal Updates)
If a local BGP speaker has connections to several BGP speakers in
neighboring autonomous systems, there will be multiple Adj-RIBs-In
associated with these peers. These Adj-RIBs-In might contain several
equally preferable routes to the same destination, all of which were
advertised by BGP speakers located in neighboring autonomous systems.
The local BGP speaker shall select one of these routes according to
the following rules:
a) If the candidate route differ only in their NEXT_HOP and
MULTI_EXIT_DISC attributes, and the local system is configured to
take into account MULTI_EXIT_DISC attribute, select the routes
that has the lowest value of the MULTI_EXIT_DISC attribute.
b) If the local system can ascertain the cost of a path to the
entity depicted by the NEXT_HOP attribute of the candidate route,
select the route with the lowest cost.
c) In all other cases, select the route that was advertised by the
BGP speaker whose BGP Identifier has the lowest value.
9.2.2 External Updates
The external update process is concerned with the distribution of
routing information to BGP speakers located in neighboring autonomous
systems. As part of Phase 3 route selection process, the BGP speaker
has updated its Adj-RIBs-Out and its Forwarding Table. All newly
installed routes and all newly unfeasible routes for which there is
no replacement route shall be advertised to BGP speakers located in
neighboring autonomous systems by means of UPDATE message.
Any routes in the Loc-RIB marked as unfeasible shall be removed.
Changes to the reachable destinations within its own autonomous
system shall also be advertised in an UPDATE message.
Rekhter & Li [Page 42]RFC 1771 BGP-4 March 1995
9.2.3 Controlling Routing Traffic Overhead
The BGP protocol constrains the amount of routing traffic (that is,
UPDATE messages) in order to limit both the link bandwidth needed to
advertise UPDATE messages and the processing power needed by the
Decision Process to digest the information contained in the UPDATE
messages.
9.2.3.1 Frequency of Route Advertisement
The parameter MinRouteAdvertisementInterval determines the minimum
amount of time that must elapse between advertisement of routes to a
particular destination from a single BGP speaker. This rate limiting
procedure applies on a per-destination basis, although the value of
MinRouteAdvertisementInterval is set on a per BGP peer basis.
Two UPDATE messages sent from a single BGP speaker that advertise
feasible routes to some common set of destinations received from BGP
speakers in neighboring autonomous systems must be separated by at
least MinRouteAdvertisementInterval. Clearly, this can only be
achieved precisely by keeping a separate timer for each common set of
destinations. This would be unwarranted overhead. Any technique which
ensures that the interval between two UPDATE messages sent from a
single BGP speaker that advertise feasible routes to some common set
of destinations received from BGP speakers in neighboring autonomous
systems will be at least MinRouteAdvertisementInterval, and will also
ensure a constant upper bound on the interval is acceptable.
Since fast convergence is needed within an autonomous system, this
procedure does not apply for routes receives from other BGP speakers
in the same autonomous system. To avoid long-lived black holes, the
procedure does not apply to the explicit withdrawal of unfeasible
routes (that is, routes whose destinations (expressed as IP prefixes)
are listed in the WITHDRAWN ROUTES field of an UPDATE message).
This procedure does not limit the rate of route selection, but only
the rate of route advertisement. If new routes are selected multiple
times while awaiting the expiration of MinRouteAdvertisementInterval,
the last route selected shall be advertised at the end of
MinRouteAdvertisementInterval.
9.2.3.2 Frequency of Route Origination
The parameter MinASOriginationInterval determines the minimum amount
of time that must elapse between successive advertisements of UPDATE
messages that report changes within the advertising BGP speaker's own
autonomous systems.
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RFC 1771 BGP-4 March 1995
9.2.3.3 Jitter
To minimize the likelihood that the distribution of BGP messages by a
given BGP speaker will contain peaks, jitter should be applied to the
timers associated with MinASOriginationInterval, Keepalive, and
MinRouteAdvertisementInterval. A given BGP speaker shall apply the
same jitter to each of these quantities regardless of the
destinations to which the updates are being sent; that is, jitter
will not be applied on a "per peer" basis.
The amount of jitter to be introduced shall be determined by
multiplying the base value of the appropriate timer by a random
factor which is uniformly distributed in the range from 0.75 to 1.0.
9.2.4 Efficient Organization of Routing Information
Having selected the routing information which it will advertise, a
BGP speaker may avail itself of several methods to organize this
information in an efficient manner.
9.2.4.1 Information Reduction
Information reduction may imply a reduction in granularity of policy
control - after information is collapsed, the same policies will
apply to all destinations and paths in the equivalence class.
The Decision Process may optionally reduce the amount of information
that it will place in the Adj-RIBs-Out by any of the following
methods:
a) Network Layer Reachability Information (NLRI):
Destination IP addresses can be represented as IP address
prefixes. In cases where there is a correspondence between the
address structure and the systems under control of an autonomous
system administrator, it will be possible to reduce the size of
the NLRI carried in the UPDATE messages.
b) AS_PATHs:
AS path information can be represented as ordered AS_SEQUENCEs or
unordered AS_SETs. AS_SETs are used in the route aggregation
algorithm described in 9.2.4.2. They reduce the size of the
AS_PATH information by listing each AS number only once,
regardless of how many times it may have appeared in multiple
AS_PATHs that were aggregated.
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An AS_SET implies that the destinations listed in the NLRI can be
reached through paths that traverse at least some of the
constituent autonomous systems. AS_SETs provide sufficient
information to avoid routing information looping; however their
use may prune potentially feasible paths, since such paths are no
longer listed individually as in the form of AS_SEQUENCEs. In
practice this is not likely to be a problem, since once an IP
packet arrives at the edge of a group of autonomous systems, the
BGP speaker at that point is likely to have more detailed path
information and can distinguish individual paths to destinations.
9.2.4.2 Aggregating Routing Information
Aggregation is the process of combining the characteristics of
several different routes in such a way that a single route can be
advertised. Aggregation can occur as part of the decision process
to reduce the amount of routing information that will be placed in
the Adj-RIBs-Out.
Aggregation reduces the amount of information that a BGP speaker must
store and exchange with other BGP speakers. Routes can be aggregated
by applying the following procedure separately to path attributes of
like type and to the Network Layer Reachability Information.
Routes that have the following attributes shall not be aggregated
unless the corresponding attributes of each route are identical:
MULTI_EXIT_DISC, NEXT_HOP.
Path attributes that have different type codes can not be aggregated
together. Path of the same type code may be aggregated, according to
the following rules:
ORIGIN attribute: If at least one route among routes that are
aggregated has ORIGIN with the value INCOMPLETE, then the
aggregated route must have the ORIGIN attribute with the value
INCOMPLETE. Otherwise, if at least one route among routes that are
aggregated has ORIGIN with the value EGP, then the aggregated
route must have the origin attribute with the value EGP. In all
other case the value of the ORIGIN attribute of the aggregated
route is INTERNAL.
AS_PATH attribute: If routes to be aggregated have identical
AS_PATH attributes, then the aggregated route has the same AS_PATH
attribute as each individual route.
For the purpose of aggregating AS_PATH attributes we model each AS
within the AS_PATH attribute as a tuple <type, value>, where
"type" identifies a type of the path segment the AS belongs to
Rekhter & Li [Page 45]RFC 1771 BGP-4 March 1995
(e.g. AS_SEQUENCE, AS_SET), and "value" is the AS number. If the
routes to be aggregated have different AS_PATH attributes, then
the aggregated AS_PATH attribute shall satisfy all of the
following conditions:
- all tuples of the type AS_SEQUENCE in the aggregated AS_PATH
shall appear in all of the AS_PATH in the initial set of routes
to be aggregated.
- all tuples of the type AS_SET in the aggregated AS_PATH shall
appear in at least one of the AS_PATH in the initial set (they
may appear as either AS_SET or AS_SEQUENCE types).
- for any tuple X of the type AS_SEQUENCE in the aggregated
AS_PATH which precedes tuple Y in the aggregated AS_PATH, X
precedes Y in each AS_PATH in the initial set which contains Y,
regardless of the type of Y.
- No tuple with the same value shall appear more than once in
the aggregated AS_PATH, regardless of the tuple's type.
An implementation may choose any algorithm which conforms to these
rules. At a minimum a conformant implementation shall be able to
perform the following algorithm that meets all of the above
conditions:
- determine the longest leading sequence of tuples (as defined
above) common to all the AS_PATH attributes of the routes to be
aggregated. Make this sequence the leading sequence of the
aggregated AS_PATH attribute.
- set the type of the rest of the tuples from the AS_PATH
attributes of the routes to be aggregated to AS_SET, and append
them to the aggregated AS_PATH attribute.
- if the aggregated AS_PATH has more than one tuple with the
same value (regardless of tuple's type), eliminate all, but one
such tuple by deleting tuples of the type AS_SET from the
aggregated AS_PATH attribute.
Appendix 6, section 6.8 presents another algorithm that satisfies
the conditions and allows for more complex policy configurations.
ATOMIC_AGGREGATE: If at least one of the routes to be aggregated
has ATOMIC_AGGREGATE path attribute, then the aggregated route
shall have this attribute as well.
Rekhter & Li [Page 46]RFC 1771 BGP-4 March 1995
AGGREGATOR: All AGGREGATOR attributes of all routes to be
aggregated should be ignored.
9.3 Route Selection Criteria
Generally speaking, additional rules for comparing routes among
several alternatives are outside the scope of this document. There
are two exceptions:
- If the local AS appears in the AS path of the new route being
considered, then that new route cannot be viewed as better than
any other route. If such a route were ever used, a routing loop
would result.
- In order to achieve successful distributed operation, only
routes with a likelihood of stability can be chosen. Thus, an AS
must avoid using unstable routes, and it must not make rapid
spontaneous changes to its choice of route. Quantifying the terms
"unstable" and "rapid" in the previous sentence will require
experience, but the principle is clear.
9.4 Originating BGP routes
A BGP speaker may originate BGP routes by injecting routing
information acquired by some other means (e.g. via an IGP) into BGP.
A BGP speaker that originates BGP routes shall assign the degree of
preference to these routes by passing them through the Decision
Process (see Section 9.1). These routes may also be distributed to
other BGP speakers within the local AS as part of the Internal update
process (see Section 9.2.1). The decision whether to distribute non-
BGP acquired routes within an AS via BGP or not depends on the
environment within the AS (e.g. type of IGP) and should be controlled
via configuration.
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Appendix 1. BGP FSM State Transitions and Actions.
This Appendix discusses the transitions between states in the BGP FSM
in response to BGP events. The following is the list of these states
and events when the negotiated Hold Time value is non-zero.
BGP States:
1 - Idle
2 - Connect
3 - Active
4 - OpenSent
5 - OpenConfirm
6 - Established
BGP Events:
1 - BGP Start
2 - BGP Stop
3 - BGP Transport connection open
4 - BGP Transport connection closed
5 - BGP Transport connection open failed
6 - BGP Transport fatal error
7 - ConnectRetry timer expired
8 - Hold Timer expired
9 - KeepAlive timer expired
10 - Receive OPEN message
11 - Receive KEEPALIVE message
12 - Receive UPDATE messages
13 - Receive NOTIFICATION message
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The following table describes the state transitions of the BGP FSM
and the actions triggered by these transitions.
Event Actions Message Sent Next State
--------------------------------------------------------------------
Idle (1)
1 Initialize resources none 2
Start ConnectRetry timer
Initiate a transport connection
others none none 1
Connect(2)
1 none none 2
3 Complete initialization OPEN 4
Clear ConnectRetry timer
5 Restart ConnectRetry timer none 3
7 Restart ConnectRetry timer none 2
Initiate a transport connection
others Release resources none 1
Active (3)
1 none none 3
3 Complete initialization OPEN 4
Clear ConnectRetry timer
5 Close connection 3
Restart ConnectRetry timer
7 Restart ConnectRetry timer none 2
Initiate a transport connection
others Release resources none 1
OpenSent(4)
1 none none 4
4 Close transport connection none 3
Restart ConnectRetry timer
6 Release resources none 1
10 Process OPEN is OK KEEPALIVE 5
Process OPEN failed NOTIFICATION 1
others Close transport connection NOTIFICATION 1
Release resources
Rekhter & Li [Page 49]RFC 1771 BGP-4 March 1995
OpenConfirm (5)
1 none none 5
4 Release resources none 1
6 Release resources none 1
9 Restart KeepAlive timer KEEPALIVE 5
11 Complete initialization none 6
Restart Hold Timer
13 Close transport connection 1
Release resources
others Close transport connection NOTIFICATION 1
Release resources
Established (6)
1 none none 6
4 Release resources none 1
6 Release resources none 1
9 Restart KeepAlive timer KEEPALIVE 6
11 Restart Hold Timer KEEPALIVE 6
12 Process UPDATE is OK UPDATE 6
Process UPDATE failed NOTIFICATION 1
13 Close transport connection 1
Release resources
others Close transport connection NOTIFICATION 1
Release resources
---------------------------------------------------------------------
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The following is a condensed version of the above state transition
table.
Events| Idle | Connect | Active | OpenSent | OpenConfirm | Estab
| (1) | (2) | (3) | (4) | (5) | (6)
|--------------------------------------------------------------
1 | 2 | 2 | 3 | 4 | 5 | 6
| | | | | |
2 | 1 | 1 | 1 | 1 | 1 | 1
| | | | | |
3 | 1 | 4 | 4 | 1 | 1 | 1
| | | | | |
4 | 1 | 1 | 1 | 3 | 1 | 1
| | | | | |
5 | 1 | 3 | 3 | 1 | 1 | 1
| | | | | |
6 | 1 | 1 | 1 | 1 | 1 | 1
| | | | | |
7 | 1 | 2 | 2 | 1 | 1 | 1
| | | | | |
8 | 1 | 1 | 1 | 1 | 1 | 1
| | | | | |
9 | 1 | 1 | 1 | 1 | 5 | 6
| | | | | |
10 | 1 | 1 | 1 | 1 or 5 | 1 | 1
| | | | | |
11 | 1 | 1 | 1 | 1 | 6 | 6
| | | | | |
12 | 1 | 1 | 1 | 1 | 1 | 1 or 6
| | | | | |
13 | 1 | 1 | 1 | 1 | 1 | 1
| | | | | |
---------------------------------------------------------------
Appendix 2. Comparison with RFC1267
BGP-4 is capable of operating in an environment where a set of
reachable destinations may be expressed via a single IP prefix. The
concept of network classes, or subnetting is foreign to BGP-4. To
accommodate these capabilities BGP-4 changes semantics and encoding
associated with the AS_PATH attribute. New text has been added to
define semantics associated with IP prefixes. These abilities allow
BGP-4 to support the proposed supernetting scheme [9].
To simplify configuration this version introduces a new attribute,
LOCAL_PREF, that facilitates route selection procedures.
Rekhter & Li [Page 51]RFC 1771 BGP-4 March 1995
The INTER_AS_METRIC attribute has been renamed to be MULTI_EXIT_DISC.
A new attribute, ATOMIC_AGGREGATE, has been introduced to insure that
certain aggregates are not de-aggregated. Another new attribute,
AGGREGATOR, can be added to aggregate routes in order to advertise
which AS and which BGP speaker within that AS caused the aggregation.
To insure that Hold Timers are symmetric, the Hold Time is now
negotiated on a per-connection basis. Hold Times of zero are now
supported.
Appendix 3. Comparison with RFC 1163
All of the changes listed in Appendix 2, plus the following.
To detect and recover from BGP connection collision, a new field (BGP
Identifier) has been added to the OPEN message. New text (Section
6.8) has been added to specify the procedure for detecting and
recovering from collision.
The new document no longer restricts the border router that is passed
in the NEXT_HOP path attribute to be part of the same Autonomous
System as the BGP Speaker.
New document optimizes and simplifies the exchange of the information
about previously reachable routes.
Appendix 4. Comparison with RFC 1105
All of the changes listed in Appendices 2 and 3, plus the following.
Minor changes to the RFC1105 Finite State Machine were necessary to
accommodate the TCP user interface provided by 4.3 BSD.
The notion of Up/Down/Horizontal relations present in RFC1105 has
been removed from the protocol.
The changes in the message format from RFC1105 are as follows:
1. The Hold Time field has been removed from the BGP header and
added to the OPEN message.
2. The version field has been removed from the BGP header and
added to the OPEN message.
3. The Link Type field has been removed from the OPEN message.
4. The OPEN CONFIRM message has been eliminated and replaced with
implicit confirmation provided by the KEEPALIVE message.
Rekhter & Li [Page 52]RFC 1771 BGP-4 March 1995
5. The format of the UPDATE message has been changed
significantly. New fields were added to the UPDATE message to
support multiple path attributes.
6. The Marker field has been expanded and its role broadened to
support authentication.
Note that quite often BGP, as specified in RFC 1105, is referred
to as BGP-1, BGP, as specified in RFC 1163, is referred to as
BGP-2, BGP, as specified in RFC1267 is referred to as BGP-3, and
BGP, as specified in this document is referred to as BGP-4.
Appendix 5. TCP options that may be used with BGP
If a local system TCP user interface supports TCP PUSH function, then
each BGP message should be transmitted with PUSH flag set. Setting
PUSH flag forces BGP messages to be transmitted promptly to the
receiver.
If a local system TCP user interface supports setting precedence for
TCP connection, then the BGP transport connection should be opened
with precedence set to Internetwork Control (110) value (see also
[6]).
Appendix 6. Implementation Recommendations
This section presents some implementation recommendations.
6.1 Multiple Networks Per Message
The BGP protocol allows for multiple address prefixes with the same
AS path and next-hop gateway to be specified in one message. Making
use of this capability is highly recommended. With one address prefix
per message there is a substantial increase in overhead in the
receiver. Not only does the system overhead increase due to the
reception of multiple messages, but the overhead of scanning the
routing table for updates to BGP peers and other routing protocols
(and sending the associated messages) is incurred multiple times as
well. One method of building messages containing many address
prefixes per AS path and gateway from a routing table that is not
organized per AS path is to build many messages as the routing table
is scanned. As each address prefix is processed, a message for the
associated AS path and gateway is allocated, if it does not exist,
and the new address prefix is added to it. If such a message exists,
the new address prefix is just appended to it. If the message lacks
the space to hold the new address prefix, it is transmitted, a new
message is allocated, and the new address prefix is inserted into the
new message. When the entire routing table has been scanned, all
Rekhter & Li [Page 53]RFC 1771 BGP-4 March 1995
allocated messages are sent and their resources released. Maximum
compression is achieved when all the destinations covered by the
address prefixes share a gateway and common path attributes, making
it possible to send many address prefixes in one 4096-byte message.
When peering with a BGP implementation that does not compress
multiple address prefixes into one message, it may be necessary to
take steps to reduce the overhead from the flood of data received
when a peer is acquired or a significant network topology change
occurs. One method of doing this is to limit the rate of updates.
This will eliminate the redundant scanning of the routing table to
provide flash updates for BGP peers and other routing protocols. A
disadvantage of this approach is that it increases the propagation
latency of routing information. By choosing a minimum flash update
interval that is not much greater than the time it takes to process
the multiple messages this latency should be minimized. A better
method would be to read all received messages before sending updates.
6.2 Processing Messages on a Stream Protocol
BGP uses TCP as a transport mechanism. Due to the stream nature of
TCP, all the data for received messages does not necessarily arrive
at the same time. This can make it difficult to process the data as
messages, especially on systems such as BSD Unix where it is not
possible to determine how much data has been received but not yet
processed.
One method that can be used in this situation is to first try to read
just the message header. For the KEEPALIVE message type, this is a
complete message; for other message types, the header should first be
verified, in particular the total length. If all checks are
successful, the specified length, minus the size of the message
header is the amount of data left to read. An implementation that
would "hang" the routing information process while trying to read
from a peer could set up a message buffer (4096 bytes) per peer and
fill it with data as available until a complete message has been
received.
6.3 Reducing route flapping
To avoid excessive route flapping a BGP speaker which needs to
withdraw a destination and send an update about a more specific or
less specific route shall combine them into the same UPDATE message.
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RFC 1771 BGP-4 March 1995
6.4 BGP Timers
BGP employs five timers: ConnectRetry, Hold Time, KeepAlive,
MinASOriginationInterval, and MinRouteAdvertisementInterval The
suggested value for the ConnectRetry timer is 120 seconds. The
suggested value for the Hold Time is 90 seconds. The suggested value
for the KeepAlive timer is 30 seconds. The suggested value for the
MinASOriginationInterval is 15 seconds. The suggested value for the
MinRouteAdvertisementInterval is 30 seconds.
An implementation of BGP MUST allow these timers to be configurable.
6.5 Path attribute ordering
Implementations which combine update messages as described above in
6.1 may prefer to see all path attributes presented in a known order.
This permits them to quickly identify sets of attributes from
different update messages which are semantically identical. To
facilitate this, it is a useful optimization to order the path
attributes according to type code. This optimization is entirely
optional.
6.6 AS_SET sorting
Another useful optimization that can be done to simplify this
situation is to sort the AS numbers found in an AS_SET. This
optimization is entirely optional.
6.7 Control over version negotiation
Since BGP-4 is capable of carrying aggregated routes which cannot be
properly represented in BGP-3, an implementation which supports BGP-4
and another BGP version should provide the capability to only speak
BGP-4 on a per-peer basis.
6.8 Complex AS_PATH aggregation
An implementation which chooses to provide a path aggregation
algorithm which retains significant amounts of path information may
wish to use the following procedure:
For the purpose of aggregating AS_PATH attributes of two routes,
we model each AS as a tuple <type, value>, where "type" identifies
a type of the path segment the AS belongs to (e.g. AS_SEQUENCE,
AS_SET), and "value" is the AS number. Two ASs are said to be the
same if their corresponding <type, value> tuples are the same.
Rekhter & Li [Page 55]RFC 1771 BGP-4 March 1995
The algorithm to aggregate two AS_PATH attributes works as
follows:
a) Identify the same ASs (as defined above) within each AS_PATH
attribute that are in the same relative order within both
AS_PATH attributes. Two ASs, X and Y, are said to be in the
same order if either:
- X precedes Y in both AS_PATH attributes, or - Y precedes X
in both AS_PATH attributes.
b) The aggregated AS_PATH attribute consists of ASs identified
in (a) in exactly the same order as they appear in the AS_PATH
attributes to be aggregated. If two consecutive ASs identified
in (a) do not immediately follow each other in both of the
AS_PATH attributes to be aggregated, then the intervening ASs
(ASs that are between the two consecutive ASs that are the
same) in both attributes are combined into an AS_SET path
segment that consists of the intervening ASs from both AS_PATH
attributes; this segment is then placed in between the two
consecutive ASs identified in (a) of the aggregated attribute.
If two consecutive ASs identified in (a) immediately follow
each other in one attribute, but do not follow in another, then
the intervening ASs of the latter are combined into an AS_SET
path segment; this segment is then placed in between the two
consecutive ASs identified in (a) of the aggregated attribute.
If as a result of the above procedure a given AS number appears
more than once within the aggregated AS_PATH attribute, all, but
the last instance (rightmost occurrence) of that AS number should
be removed from the aggregated AS_PATH attribute.
References
[1] Mills, D., "Exterior Gateway Protocol Formal Specification", RFC
904, BBN, April 1984.
[2] Rekhter, Y., "EGP and Policy Based Routing in the New NSFNET
Backbone", RFC 1092, T.J. Watson Research Center, February 1989.
[3] Braun, H-W., "The NSFNET Routing Architecture", RFC 1093,
MERIT/NSFNET Project, February 1989.
[4] Postel, J., "Transmission Control Protocol - DARPA Internet
Program Protocol Specification", STD 7, RFC 793, DARPA, September
1981.
Rekhter & Li [Page 56]RFC 1771 BGP-4 March 1995
[5] Rekhter, Y., and P. Gross, "Application of the Border Gateway
Protocol in the Internet", RFC 1772, T.J. Watson Research Center,
IBM Corp., MCI, March 1995.
[6] Postel, J., "Internet Protocol - DARPA Internet Program Protocol
Specification", STD 5, RFC 791, DARPA, September 1981.
[7] "Information Processing Systems - Telecommunications and
Information Exchange between Systems - Protocol for Exchange of
Inter-domain Routeing Information among Intermediate Systems to
Support Forwarding of ISO 8473 PDUs", ISO/IEC IS10747, 1993
[8] Fuller, V., Li, T., Yu, J., and K. Varadhan, "Classless Inter-
Domain Routing (CIDR): an Address Assignment and Aggregation
Strategy", RFC 1519, BARRNet, cisco, MERIT, OARnet, September
1993
[9] Rekhter, Y., Li, T., "An Architecture for IP Address Allocation
with CIDR", RFC 1518, T.J. Watson Research Center, cisco,
September 1993
Security Considerations
Security issues are not discussed in this document.
Editors' Addresses
Yakov Rekhter
T.J. Watson Research Center IBM Corporation
P.O. Box 704, Office H3-D40
Yorktown Heights, NY 10598
Phone: +1 914 784 7361
EMail: yakov@watson.ibm.com
Tony Li
cisco Systems, Inc.
170 W. Tasman Dr.
San Jose, CA 95134
EMail: tli@cisco.com
Rekhter & Li [Page 57]
