Network Working Group Y. Rekhter
INTERNET DRAFT Juniper Networks
T. Li
Procket Networks, Inc.
Editors
A Border Gateway Protocol 4 (BGP-4)
<draft-ietf-idr-bgp4-16.txt>
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as ``work in progress.''
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
1. Acknowledgments
This document was originally published as RFC 1267 in October 1991,
jointly authored by Kirk Lougheed and Yakov Rekhter.
We would like to express our thanks to Guy Almes, Len Bosack, and
Jeffrey C. Honig for their contributions to the earlier version of
this document.
We like to explicitly thank Bob Braden for the review of the earlier
version of this document as well as his constructive and valuable
comments.
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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 earlier 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.
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 and to Charles Kunzinger who was
the IDRP editor within that group. We would also like to thank Enke
Chen, Edward Crabbe, Mike Craren, Vincent Gillet, Eric Gray, Jeffrey
Haas, Dimitry Haskin, John Krawczyk, David LeRoy, Dan Massey, Dan
Pei, Mathew Richardson, John Scudder, John Stewart III, Dave Thaler,
Paul Traina, Russ White, Curtis Villamizar, and Alex Zinin for their
comments.
We would like to specially acknowledge numerous contributions by
Dennis Ferguson.
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
Inter-Domain Routing (CIDR) [8, 9]. 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.
To characterize the set of policy decisions that can be enforced
using BGP, one must focus on the rule that a BGP speaker advertises
to its peers (other BGP speakers which it communicates with) in
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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 (aka explicit 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 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" inter-AS 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,
acknowledgment, and sequencing. Any authentication scheme used by the
transport protocol (e.g., RFC2385 [10]) 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 determine how to route packets within the AS,
and using an exterior gateway protocol to determine how 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
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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.
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 portion of the BGP routing table that is
allowed by the export policy, called the Adj-Ribs-Out (see 3.2).
Incremental updates are sent as the routing tables change. BGP does
not require periodic refresh of the routing table. Therefore, a BGP
speaker must retain the current version of the routes advertised by
all of its peers for the duration of the connection. If the
implementation decides to not store the routes that have been
received from a peer, but have been filtered out according to
configured local policy, the BGP Route Refresh extension [12] may be
used to request the full set of routes from a peer without resetting
the BGP session when the local policy configuration changes.
KEEPALIVE messages may be 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.
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. Internal BGP and
external BGP are commonly abbreviated IBGP and EBGP.
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 IBGP
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connections with each other. Alternately the interior routing
protocol can pass BGP information among routers within an AS, taking
care not to lose BGP attributes that will be needed by EBGP speakers
if transit connectivity is being provided. For the purpose of
discussion, it is assumed that BGP information is passed within an AS
using IBGP. Care must be taken to ensure that the interior routers
have all been updated with transit information before the EBGP
speakers announce to other ASs that transit service is being
provided.
3.1 Routes: Advertisement and Storage
For the purpose of this protocol, a route is defined as a unit of
information that pairs a set of destinations with the attributes of a
path to those destinations. The set of destinations are the systems
whose IP addresses are reported in the Network Layer Reachability
Information (NLRI) field and the path is the information reported in
the path attributes field of the same UPDATE message.
Routes are advertised between BGP speakers in UPDATE messages.
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 resolvable via the local BGP speaker's Routing Table. 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 the destination 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 NLRI can be advertised, or
c) the BGP speaker - BGP speaker connection can be closed, which
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implicitly removes from service all routes which the pair of
speakers had advertised to each other.
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.
Routing information that the router uses to forward packets (or to
construct the forwarding table that is used for packet forwarding) is
maintained in the Routing Table. The Routing Table accumulates routes
to directly connected networks, static routes, routes learned from
the IGP protocols, and routes learned from BGP. Whether or not a
specific BGP route should be installed in the Routing Table, and
whether a BGP route should override a route to the same destination
installed by another source is a local policy decision, not specified
in this document. Besides actual packet forwarding, the Routing Table
is used for resolution of the next-hop addresses specified in BGP
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updates (see Section 9.1.2).
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.
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.
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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.
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 (variable) |
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| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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.
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
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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.
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.
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Authentication Data:
Authentication Data is a variable length field that is
interpreted according to the value of the
Authentication Code field.
The minimum length of the OPEN message is 29 octets (including
message header).
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 feasible routes sharing common
path attribute 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 also includes the other fields as shown below (note, some
of the shown fields may not be present in every UPDATE message):
+-----------------------------------------------------+
| Withdrawn Routes Length (2 octets) |
+-----------------------------------------------------+
| Withdrawn Routes (variable) |
+-----------------------------------------------------+
| Total Path Attribute Length (2 octets) |
+-----------------------------------------------------+
| Path Attributes (variable) |
+-----------------------------------------------------+
| Network Layer Reachability Information (variable) |
+-----------------------------------------------------+
Withdrawn Routes Length:
This 2-octets unsigned integer indicates the total length of
the Withdrawn Routes field in octets. Its value must allow the
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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:
+---------------------------+
| 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 an IP address prefix 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.
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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|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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).
The lower-order four bits of the Attribute Flags octet are
unused. They must be zero when sent 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.
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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:
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 the EGP protocol
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>.
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
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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 Information 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 entry 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 attribute that is a four octet
non-negative integer. A BGP speaker uses it to inform other
internal peers of the advertising speaker's degree of
preference for an advertised route. Usage of this attribute
is described in 5.1.5.
f) ATOMIC_AGGREGATE (Type Code 6)
ATOMIC_AGGREGATE is a well-known discretionary attribute of
length 0. 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
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address of the BGP speaker that formed the aggregate route
(encoded as 4 octets). This should be the same address as
the one used for the BGP Identifier of the speaker. 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 -
Withdrawn Routes Length
where UPDATE message Length is the value encoded in the fixed-
size BGP header, Total Path Attribute Length and Withdrawn
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
Withdrawn 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) |
+---------------------------+
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.
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The minimum length of the UPDATE message is 23 octets -- 19 octets
for the fixed header + 2 octets for the Withdrawn Routes Length + 2
octets for the Total Path Attribute Length (the value of Withdrawn
Routes Length is 0 and the value of Total Path Attribute Length is
0).
An UPDATE message can advertise at most one set of path attributes,
but multiple destinations, provided that the destinations share these
attributes. All path attributes contained in a given UPDATE message
apply to all destinations carried in the NLRI 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 advertised.
An UPDATE message might 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.
An UPDATE message should not include the same address prefix in the
WITHDRAWN ROUTES and Network Layer Reachability Information fields,
however a BGP speaker MUST be able to process UPDATE messages in this
form. A BGP speaker should treat an UPDATE message of this form as if
the WITHDRAWN ROUTES doesn't contain the address prefix.
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 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.
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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 (variable) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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. If
no appropriate Error Subcode is defined, then a zero
(Unspecific) value is used for the Error Subcode field.
Message Header Error subcodes:
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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
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).
5. Path Attributes
This section discusses the path attributes of the UPDATE message.
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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 that contains NLRI. 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 BGP speaker 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 BGP speakers in the path.
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.
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The same attribute cannot appear more than once within the Path
Attributes field of a particular UPDATE message.
The mandatory category refers to an attribute which must be present
in both IBGP and EBGP exchanges if NLRI are contained in the UPDATE
message. Attributes classified as optional for the purpose of the
protocol extension mechanism may be purely discretionary, or
discretionary, required, or disallowed in certain contexts.
attribute EBGP IBGP
ORIGIN mandatory mandatory
AS_PATH mandatory mandatory
NEXT_HOP mandatory mandatory
MULTI_EXIT_DISC discretionary discretionary
LOCAL_PREF disallowed required
ATOMIC_AGGREGATE see section 5.1.6 and 9.1.4
AGGREGATOR discretionary discretionary
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
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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 an internal
peer, the advertising speaker shall not modify the AS_PATH
attribute associated with the route.
b) When a given BGP speaker advertises the route to an external
peer, 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). If the act of prepending will cause an overflow in
the AS_PATH segment, i.e. more than 255 elements, it shall be
legal to prepend a new segment of type AS_SEQUENCE and prepend
its own AS number to this new segment.
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 a
path segment of type AS_SEQUENCE in the AS_PATH attribute of all
UPDATE messages sent to an external peer. (In this case, the AS
number of the originating speaker's autonomous system will be the
only entry the path segment, and this path segment will be the
only segment in the AS_PATH attribute).
b) the originating speaker shall include an empty AS_PATH
attribute in all UPDATE messages sent to internal peers. (An
empty AS_PATH attribute is one whose length field contains the
value zero).
Whenever the modification of the AS_PATH attribute calls for
including or prepending the AS number of the local system, the local
system may include/prepend more than one instance of its own AS
number in the AS_PATH attribute. This is controlled via local
configuration.
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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. The NEXT_HOP attribute is calculated as
follows.
1) When sending a message to an internal peer, the BGP speaker
should not modify the NEXT_HOP attribute, unless it has been
explicitly configured to announce its own IP address as the
NEXT_HOP.
2) When sending a message to an external peer X, and the peer is
one IP hop away from the speaker:
- If the route being announced was learned from an internal
peer or is locally originated, the BGP speaker can use for the
NEXT_HOP attribute an interface address of the internal peer
router (or the internal router) through which the announced
network is reachable for the speaker, provided that peer X
shares a common subnet with this address. This is a form of
"third party" NEXT_HOP attribute.
- If the route being announced was learned from an external
peer, the speaker can use in the NEXT_HOP attribute an IP
address of any adjacent router (known from the received
NEXT_HOP attribute) that the speaker itself uses for local
route calculation, provided that peer X shares a common subnet
with this address. This is a second form of "third party"
NEXT_HOP attribute.
- If the external peer to which the route is being advertised
shares a common subnet with one of the announcing router's own
interfaces, the router may use the IP address associated with
such an interface in the NEXT_HOP attribute. This is known as a
"first party" NEXT_HOP attribute.
- By default (if none of the above conditions apply), the BGP
speaker should use in the NEXT_HOP attribute the IP address of
the interface that the speaker uses to establish the BGP
session to peer X.
3) When sending a message to an external peer X, and the peer is
multiple IP hops away from the speaker (aka "multihop EBGP"):
- The speaker may be configured to propagate the NEXT_HOP
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attribute. In this case when advertising a route that the
speaker learned from one of its peers, the NEXT_HOP attribute
of the advertised route is exactly the same as the NEXT_HOP
attribute of the learned route (the speaker just doesn't modify
the NEXT_HOP attribute).
- By default, the BGP speaker should use in the NEXT_HOP
attribute the IP address of the interface that the speaker uses
to establish the BGP session to peer X.
Normally the NEXT_HOP attribute is chosen such that the shortest
available path will be taken. A BGP speaker must be able to support
disabling advertisement of third party NEXT_HOP attributes to handle
imperfectly bridged media.
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.
The NEXT_HOP attribute is used by the BGP speaker to determine the
actual outbound interface and immediate next-hop address that should
be used to forward transit packets to the associated destinations.
The immediate next-hop address is determined by performing a
recursive route lookup operation for the IP address in the NEXT_HOP
attribute using the contents of the Routing Table (see Section
9.1.2.2). The resolving route will always specify the outbound
interface. If the resolving route specifies the next-hop address,
this address should be used as the immediate address for packet
forwarding. If the address in the NEXT_HOP attribute is directly
resolved through a route to an attached subnet (such a route will not
specify the next-hop address), the outbound interface should be taken
from the resolving route and the address in the NEXT_HOP attribute
should be used as the immediate next-hop address.
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 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 received from a neighboring AS MUST
NOT be propagated to other neighboring ASs.
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A BGP speaker MUST IMPLEMENT a mechanism based on local configuration
which allows the MULTI_EXIT_DISC attribute to be removed from a
route. This MAY be done prior to determining the degree of preference
of the route and performing route selection (decision process phases
1 and 2).
An implementation MAY also (based on local configuration) alter the
value of the MULTI_EXIT_DISC attribute received over an external
link. If it does so, it shall do so prior to determining the degree
of preference of the route and performing route selection (decision
process phases 1 and 2).
5.1.5 LOCAL_PREF
LOCAL_PREF is a well-known attribute that SHALL be included in all
UPDATE messages that a given BGP speaker sends to the other internal
peers. A BGP speaker SHALL calculate the degree of preference for
each external route based on the locally configured policy, and
include the degree of preference when advertising a route to its
internal peers. The higher degree of preference MUST 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 MUST NOT include this attribute in UPDATE messages that
it sends to external peers, except for the case of BGP Confederations
[13]. If it is contained in an UPDATE message that is received from
an external peer, then this attribute MUST be ignored by the
receiving speaker, except for the case of BGP Confederations [13].
5.1.6 ATOMIC_AGGREGATE
ATOMIC_AGGREGATE is a well-known discretionary attribute.
When a router aggregates several routes for the purpose of
advertisement to a particular peer, and the AS_PATH of the aggregated
route excludes at least some of the AS numbers present in the AS_PATH
of the routes that are aggregated, the aggregated route, when
advertised to the peer, MUST include the ATOMIC_AGGREGATE attribute.
A BGP speaker that receives a route with the ATOMIC_AGGREGATE
attribute MUST NOT remove the attribute from the route when
propagating it to other speakers.
A BGP speaker that receives a route with the ATOMIC_AGGREGATE
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attribute MUST 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 not be the path specified in the AS_PATH
attribute of the route.
5.1.7 AGGREGATOR
AGGREGATOR is an optional transitive attribute which may be included
in updates which are formed by aggregation (see Section 9.2.2.2). A
BGP speaker which performs route aggregation may add the AGGREGATOR
attribute which shall contain its own AS number and IP address. The
IP address should be the same as the BGP Identifier of the speaker.
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, the associated Adj-RIB-In has
been cleared, and that all resources for that BGP connection have
been deallocated. Entries in the Loc-RIB 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
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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). The
Marker field should be all ones if the OPEN message carried no
authentication information. 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.
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-octets 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), or if the smallest locally supported version
number is greater than the version the remote BGP peer bid, then the
smallest locally supported version number.
If the Autonomous System field of the OPEN message is unacceptable,
then the Error Subcode is set to Bad Peer AS. The determination of
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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.
If one of the Optional Parameters in the OPEN message is recognized,
but is malformed, then the Error Subcode is set to 0 (Unspecific).
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 Withdrawn Routes Length or Total Attribute Length
is too large (i.e., if Withdrawn 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,
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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
links, and only when the sender and the receiving speaker are one IP
hop away from each other. To be semantically correct, the IP address
in the NEXT_HOP must not be the IP address of the receiving speaker,
and the NEXT_HOP IP address must either be the sender's IP address
(used to establish the BGP session), or the interface associated with
the NEXT_HOP IP address must share a common subnet with the receiving
BGP speaker. If the NEXT_HOP attribute is semantically incorrect, the
error should be logged, and 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.
The information carried by the AS_PATH attribute is checked for AS
loops. AS loop detection is done by scanning the full AS path (as
specified in the AS_PATH attribute), and checking that the autonomous
system number of the local system does not appear in the AS path. If
the autonomous system number appears in the AS path the route may be
stored in the Adj-RIB-In, but unless the router is configured to
accept routes with its own autonomous system in the AS path, the
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route shall not be passed to the BGP Decision Process. Operations of
a router that is configured to accept routes with its own autonomous
system number in the AS path are outside the scope of this document.
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.
If a prefix in the NLRI field is semantically incorrect (e.g., an
unexpected multicast IP address), an error should be logged locally,
and the prefix should be ignored.
An UPDATE message that contains correct path attributes, but no NLRI,
shall be treated as a valid UPDATE message.
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.
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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.
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.
A BGP speaker may support the ability to impose an (locally
configured) upper bound on the number of address prefixes the speaker
is willing to accept from a neighbor. When the upper bound is
reached, the speaker (under control of local configuration) may
either (a) discard new address prefixes from the neighbor, or (b)
terminate the BGP peering with the neighbor. If the BGP speaker
decides to terminate its peering with a neighbor because the number
of address prefixes received from the neighbor exceeds the locally
configured upper bound, then the speaker must send to the neighbor a
NOTIFICATION message with the Error Code Cease.
6.8 Connection collision detection.
If a pair of BGP speakers try simultaneously to establish a BGP
connection to each other, then two parallel connections between this
pair of speakers might well be formed. If the source IP address used
by one of these connections is the same as the destination IP address
used by the other, and the destination IP address used by the first
connection is the same as the source IP address used by the other, we
refer to this situation as connection collision. Clearly in the
presence of connection collision, 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.
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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, and this
connection collides with the connection over which the OPEN message
is received 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.
Unless allowed via configuration, a connection collision with an
existing BGP connection that is in Established state causes
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
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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.
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
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. An implementation MAY
impose a configurable upper bound on that time. Once the upper
bound is reached, the speaker shall no longer automatically
generate the Start event for the peer.
Any other event received in the Idle state is ignored.
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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.
The Start event is ignored in the Connect 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 listening for
and accepting 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
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 the 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 allows BGP connections with unconfigured
peers, then when the local system detects that a remote peer is
trying to establish a BGP connection to it, and the IP address
of the remote peer is not a configured one, the local system
creates a temporary peer entry, completes initialization, sends
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an OPEN message to its peer, sets its Hold Timer to a large
value, and changes its state to OpenSent.
If the local system does not allow BGP connections with
unconfigured peers, then the local system rejects connections
from IP addresses that are not configured peers, and remains in
the Active state.
The 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 affect 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.
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
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Error Code Cease and changes its state to Idle.
The 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.
The 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.
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:
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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.
The 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
routes derived from that connection.
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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 no longer available for use.
If the UPDATE message contains a feasible route, the Adj-RIB-In will
be updated with this route as follows: if the NLRI of the new route
is identical to the one of the 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.
Otherwise, if the Adj-RIB-In has no route with NLRI identical to the
new route, the new route shall be placed in the Adj-RIB-In.
Once the BGP speaker updates the Adj-RIB-In, the speaker shall run
its Decision Process.
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-RIBs-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 either (a)
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a non-negative integer denoting the degree of preference for the
route, or (b) a value denoting that this route is ineligible to be
installed in LocRib and will be excluded from the next phase of route
selection.
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 the Adj-RIB-In,
and is responsible for:
- selection of routes to be used locally by the speaker
- selection of routes to be advertised to other BGP peers
- 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 peer.
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 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,
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 from a peer an UPDATE message that advertises a new
route, a replacement route, or withdrawn routes.
The Phase 1 decision function is a separate process which completes
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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 as follows:
If the route is learned from an internal peer, either the value of
the LOCAL_PREF attribute shall be taken as the degree of
preference, or the local system may compute the degree of
preference of the route based on preconfigured policy information.
Note that the latter (computing the degree of preference based on
preconfigured policy information) may result in formation of
persistent routing loops.
If the route is learned from an external peer, then the local BGP
speaker computes the degree of preference based on preconfigured
policy information. If the return value indicates that the route
is ineligible, the route may not serve as an input to the next
phase of route selection; otherwise the return value is used as
the LOCAL_PREF value in any IBGP readvertisement.
The exact nature of this policy information and the computation
involved is a local matter.
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 eligible in the Adj-RIBs-In.
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 that is
not resolvable, or it would become unresolvable if the route was
installed in the routing table the BGP route should be excluded from
the Phase 2 decision function.
It is critical that routers within an AS do not make conflicting
decisions regarding route selection that would cause forwarding loops
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to occur.
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 9.1.2.2.
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. If the new BGP route is installed in the Routing
Table (as a result of the local policy decision), care must be taken
to ensure that invalid BGP routes to the same destination are removed
from the Routing Table. Whether or not the new route replaces an
already existing non-BGP route in the routing table depends on the
policy configured on the BGP speaker.
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 best matching route lookup in the Routing Table and
selecting one of the possible paths (if multiple best paths to the
same prefix are available). If the route to the address depicted by
the NEXT_HOP attribute changes such that the immediate next hop or
the IGP cost to the NEXT_HOP (if the NEXT_HOP is resolved through an
IGP route) changes, route selection should be recalculated as
specified above.
Notice that even though BGP routes do not have to be installed in the
Routing Table with the immediate next hop(s), implementations must
take care that before any packets are forwarded along a BGP route,
its associated NEXT_HOP address is resolved to the immediate
(directly connected) next-hop address and this address (or multiple
addresses) is finally used for actual packet forwarding.
Unresolvable routes SHALL be removed from the Loc-RIB and the routing
table. However, corresponding unresolvable routes SHOULD be kept in
the Adj-RIBs-In.
9.1.2.1 Route Resolvability Condition
As indicated in Section 9.1.2, BGP routers should exclude
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unresolvable routes from the Phase 2 decision. This ensures that only
valid routes are installed in Loc-RIB and the Routing Table.
The route resolvability condition is defined as follows.
1. A route Rte1, referencing only the intermediate network
address, is considered resolvable if the Routing Table contains at
least one resolvable route Rte2 that matches Rte1's intermediate
network address and is not recursively resolved (directly or
indirectly) through Rte1. If multiple matching routes are
available, only the longest matching route should be considered.
2. Routes referencing interfaces (with or without intermediate
addresses) are considered resolvable if the state of the
referenced interface is up and IP processing is enabled on this
interface.
BGP routes do not refer to interfaces, but can be resolved through
the routes in the Routing Table that can be of both types. IGP routes
and routes to directly connected networks are expected to specify the
outbound interface.
Note that a BGP route is considered unresolvable not only in
situations where the router's Routing Table contains no route
matching the BGP route's NEXT_HOP. Mutually recursive routes (routes
resolving each other or themselves), also fail the resolvability
check.
It is also important that implementations do not consider feasible
routes that would become unresolvable if they were installed in the
Routing Table even if their NEXT_HOPs are resolvable using the
current contents of the Routing Table (an example of such routes
would be mutually recursive routes). This check ensures that a BGP
speaker does not install in the Routing Table routes that will be
removed and not used by the speaker. Therefore, in addition to local
Routing Table stability, this check also improves behavior of the
protocol in the network.
Whenever a BGP speaker identifies a route that fails the
resolvability check because of mutual recursion, an error message
should be logged.
9.1.2.2 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
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speaker can select only one of these routes for inclusion in the
associated Loc-RIB. The local speaker considers all routes with the
same degrees of preference, both those received from internal peers,
and those received from external peers.
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
NEXT_HOP attribute of the route, and follow the same route selection
algorithm.
The tie-breaking algorithm begins by considering all equally
preferable routes to the same destination, and then selects routes to
be removed from consideration. The algorithm terminates as soon as
only one route remains in consideration. The criteria must be
applied in the order specified.
Several of the criteria are described using pseudo-code. Note that
the pseudo-code shown was chosen for clarity, not efficiency. It is
not intended to specify any particular implementation. BGP
implementations MAY use any algorithm which produces the same results
as those described here.
a) Remove from consideration all routes which are not tied for
having the smallest number of AS numbers present in their AS_PATH
attributes. Note, that when counting this number, an AS_SET counts
as 1, no matter how many ASs are in the set, and that, if the
implementation supports [13], then AS numbers present in segments
of type AS_CONFED_SEQUENCE or AS_CONFED_SET are not included in
the count of AS numbers present in the AS_PATH.
b) Remove from consideration all routes which are not tied for
having the lowest Origin number in their Origin attribute.
c) Remove from consideration routes with less-preferred
MULTI_EXIT_DISC attributes. MULTI_EXIT_DISC is only comparable
between routes learned from the same neighboring AS. Routes which
do not have the MULTI_EXIT_DISC attribute are considered to have
the lowest possible MULTI_EXIT_DISC value.
This is also described in the following procedure:
for m = all routes still under consideration
for n = all routes still under consideration
if (neighborAS(m) == neighborAS(n)) and (MED(n) < MED(m))
remove route m from consideration
In the pseudo-code above, MED(n) is a function which returns the
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value of route n's MULTI_EXIT_DISC attribute. If route n has no
MULTI_EXIT_DISC attribute, the function returns the lowest
possible MULTI_EXIT_DISC value, i.e. 0.
Similarly, neighborAS(n) is a function which returns the neighbor
AS from which the route was received.
d) If at least one of the candidate routes was received from an
external peer in a neighboring autonomous system, remove from
consideration all routes which were received from internal peers.
e) Remove from consideration any routes with less-preferred
interior cost. The interior cost of a route is determined by
calculating the metric to the next hop for the route using the
Routing Table. If the next hop for a route is reachable, but no
cost can be determined, then this step should be skipped
(equivalently, consider all routes to have equal costs).
This is also described in the following procedure.
for m = all routes still under consideration
for n = all routes in still under consideration
if (cost(n) is better than cost(m))
remove m from consideration
In the pseudo-code above, cost(n) is a function which returns the
cost of the path (interior distance) to the address given in the
NEXT_HOP attribute of the route.
f) Remove from consideration all routes other than 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 the 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
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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 Adj-RIBs-Out
according to configured policy. This policy may exclude a route in
the Loc-RIB from being installed in a particular Adj-RIB-Out. A
route shall not be installed in the Adj-Rib-Out unless the
destination and NEXT_HOP described by this route may be forwarded
appropriately by the Routing Table. If a route in Loc-RIB is excluded
from a particular Adj-RIB-Out the previously advertised route in that
Adj-RIB-Out must be withdrawn from service by means of an UPDATE
message (see 9.2).
Route aggregation and information reduction techniques (see 9.2.2.1)
may optionally be applied.
When the updating of the Adj-RIBs-Out and the Routing Table is
complete, the local BGP speaker shall run the Update-Send process of
9.2.
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
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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.
If a BGP speaker receives overlapping routes, the Decision Process
MUST consider both routes based on the configured acceptance policy.
If both a less and a more specific route are accepted, then the
Decision Process MUST either install both the less and the more
specific routes or it MUST aggregate the two routes and install the
aggregated route, provided that both routes have the same value of
the NEXT_HOP attribute.
If a BGP speaker chooses to aggregate, then it MUST 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.
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.
When a BGP speaker receives an UPDATE message from an internal peer,
the receiving BGP speaker shall not re-distribute the routing
information contained in that UPDATE message to other internal peers,
unless the speaker acts as a BGP Route Reflector [11].
As part of Phase 3 of the route selection process, the BGP speaker
has updated its Adj-RIBs-Out. All newly installed routes and all
newly unfeasible routes for which there is no replacement route shall
be advertised to its peers by means of an UPDATE message.
A BGP speaker should not advertise a given feasible BGP route from
its Adj-RIB-Out if it would produce an UPDATE message containing the
same BGP route as was previously advertised.
Any routes in the Loc-RIB marked as unfeasible shall be removed.
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Changes to the reachable destinations within its own autonomous
system shall also be advertised in an UPDATE message.
9.2.1 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.
9.2.1.1 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
external peers 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 external peers 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 received from other internal
peers. 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.
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9.2.1.2 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.
9.2.1.3 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.
9.2.2 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.
9.2.2.1 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
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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 9.2.2.2. 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.
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.
9.2.2.2 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.
If the aggregation occurs as part of the update process, routes with
different NEXT_HOP values can be aggregated when announced through an
external BGP session.
Path attributes that have different type codes can not be aggregated
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together. Path attributes 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 IGP.
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
(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 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 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 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 of type AS_SET with the same value shall appear more
than once in the aggregated AS_PATH.
- Multiple tuples of type AS_SEQUENCE with the same value may
appear in the aggregated AS_PATH only when adjacent to another
tuple of the same type and value.
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:
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- 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.
AGGREGATOR: All AGGREGATOR attributes of all routes to be
aggregated should be ignored. The BGP speaker performing the route
aggregation may attach a new AGGREGATOR attribute (see Section
5.1.7).
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 (provided that the speaker is configured to accept
such routes). If such a route were ever used, a routing loop could
result (see Section 6.3).
- 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.
Care must be taken to ensure that BGP speakers in the same AS do
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not make inconsistent decisions.
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 update process
(see Section 9.2). 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.
Appendix 1. Comparison with RFC1771
There are numerous editorial changes (too many to list here).
The following list the technical changes:
Changes to reflect the usages of such features as TCP MD5 [10],
BGP Route Reflectors [11], BGP Confederations [13], and BGP Route
Refresh [12].
Clarification on the use of the BGP Identifier in the AGGREGATOR
attribute.
Procedures for imposing an upper bound on the number of prefixes
that a BGP speaker would accept from a peer.
The ability of a BGP speaker to include more than one instance of
its own AS in the AS_PATH attribute for the purpose of inter-AS
traffic engineering.
Clarifications on the various types of NEXT_HOPs.
Clarifications to the use of the ATOMIC_AGGREGATE attribute.
The relationship between the immediate next hop, and the next hop
as specified in the NEXT_HOP path attribute.
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Clarifications on the tie-breaking procedures.
Appendix 2. Comparison with RFC1267
All the changes listed in Appendix 1, plus the following.
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.
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 Appendices 1 and 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.
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Appendix 4. Comparison with RFC 1105
All of the changes listed in Appendices 1, 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.
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
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with precedence set to Internetwork Control (110) value (see also
[6]).
A local system may protect its BGP sessions by using the TCP MD5
Signature Option [10].
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
path attributes 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
a path attribute set from a routing table that is not organized on a
per path attribute set basis is to build many messages as the routing
table is scanned. As each address prefix is processed, a message for
the associated set of path attributes 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
allocated messages are sent and their resources released. Maximum
compression is achieved when all the destinations covered by the
address prefixes share a common set of 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
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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 SHOULD combine them into the same UPDATE message.
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 1/3 of the Hold Time. The suggested value
for the MinASOriginationInterval is 15 seconds. The suggested value
for the MinRouteAdvertisementInterval is 30 seconds.
An implementation of BGP MUST allow the Hold Time timer to be
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configurable, and MAY allow the other 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.
The algorithm to aggregate two AS_PATH attributes works as
follows:
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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.
Security Considerations
BGP supports the ability to authenticate BGP messages by using BGP
authentication. The authentication could be done on a per peer basis.
In addition, BGP supports the ability to authenticate its data stream
by using [10]. This authentication could be done on a per peer basis.
Finally, BGP could also use IPSec to authenticate its data stream.
Among the mechanisms mentioned in this paragraph, [10] is the most
widely deployed.
References
[1] Mills, D., "Exterior Gateway Protocol Formal Specification",
RFC904, April 1984.
[2] Rekhter, Y., "EGP and Policy Based Routing in the New NSFNET
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Backbone", RFC1092, February 1989.
[3] Braun, H-W., "The NSFNET Routing Architecture", RFC1093, February
1989.
[4] Postel, J., "Transmission Control Protocol - DARPA Internet
Program Protocol Specification", RFC793, September 1981.
[5] Rekhter, Y., and P. Gross, "Application of the Border Gateway
Protocol in the Internet", RFC1772, March 1995.
[6] Postel, J., "Internet Protocol - DARPA Internet Program Protocol
Specification", RFC791, 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 Varadhan, K., ""Classless Inter-
Domain Routing (CIDR): an Address Assignment and Aggregation
Strategy", RFC1519, September 1993.
[9] Rekhter, Y., Li, T., "An Architecture for IP Address Allocation
with CIDR", RFC 1518, September 1993.
[10] Heffernan, A., "Protection of BGP Sessions via the TCP MD5
Signature Option", RFC2385, August 1998.
[11] Bates, T., Chandra, R., Chen, E., "BGP Route Reflection - An
Alternative to Full Mesh IBGP", RFC2796, April 2000.
[12] Chen, E., "Route Refresh Capability for BGP-4", RFC2918,
September 2000.
[13] Traina, P, McPherson, D., Scudder, J., "Autonomous System
Confederations for BGP", RFC3065, February 2001.
Editors' Addresses
Yakov Rekhter
Juniper Networks
1194 N. Mathilda Avenue
Sunnyvale, CA 94089
email: yakov@juniper.net
Tony Li
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Procket Networks
1100 Cadillac Ct.
Milpitas, CA 95035
Email: tli@procket.com
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