RFC 1771
A Border Gateway Protocol 4 (BGP-4)
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. 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 |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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
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.
Authentication Data:
The form and meaning of this field is a variable-
length field depend on the Authentication Code.
The minimum length of the OPEN message is 29 octets (including
message header).
4.3 UPDATE Message Format UPDATE messages are used to transfer routing information between BGP peers. The information in the UPDATE packet can be used to construct a graph describing the relationships of the various Autonomous Systems. By applying rules to be discussed, routing information loops and some other anomalies may be detected and removed from inter-AS routing. An UPDATE message is used to advertise a single feasible route to a peer, or to withdraw multiple unfeasible routes from service (see 3.1). An UPDATE message may simultaneously advertise a feasible route and withdraw multiple unfeasible routes from service. The UPDATE message always includes the fixed-size BGP header, and can optionally include the other fields as shown below: +-----------------------------------------------------+ | Unfeasible Routes Length (2 octets) | +-----------------------------------------------------+ | Withdrawn Routes (variable) | +-----------------------------------------------------+ | Total Path Attribute Length (2 octets) | +-----------------------------------------------------+ | Path Attributes (variable) | +-----------------------------------------------------+ | Network Layer Reachability Information (variable) | +-----------------------------------------------------+ Unfeasible Routes Length: This 2-octets unsigned integer indicates the total length of the Withdrawn Routes field in octets. Its value must allow the length of the Network Layer Reachability Information field to be determined as specified below. A value of 0 indicates that no routes are being withdrawn from service, and that the WITHDRAWN ROUTES field is not present in this UPDATE message. Withdrawn Routes: This is a variable length field that contains a list of IP address prefixes for the routes that are being withdrawn from service. Each IP address prefix is encoded as a 2-tuple of the form <length, prefix>, whose fields are described below:
+---------------------------+
| Length (1 octet) |
+---------------------------+
| Prefix (variable) |
+---------------------------+
The use and the meaning of these fields are as follows:
a) Length:
The Length field indicates the length in bits of the IP
address prefix. A length of zero indicates a prefix that
matches all IP addresses (with prefix, itself, of zero
octets).
b) Prefix:
The Prefix field contains IP address prefixes followed by
enough trailing bits to make the end of the field fall on an
octet boundary. Note that the value of trailing bits is
irrelevant.
Total Path Attribute Length:
This 2-octet unsigned integer indicates the total length of the
Path Attributes field in octets. Its value must allow the
length of the Network Layer Reachability field to be determined
as specified below.
A value of 0 indicates that no Network Layer Reachability
Information field is present in this UPDATE message.
Path Attributes:
A variable length sequence of path attributes is present in
every UPDATE. Each path attribute is a triple <attribute type,
attribute length, attribute value> of variable length.
Attribute Type is a two-octet field that consists of the
Attribute Flags octet followed by the Attribute Type Code
octet.
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Attr. Flags |Attr. Type Code|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The high-order bit (bit 0) of the Attribute Flags octet is the
Optional bit. It defines whether the attribute is optional (if
set to 1) or well-known (if set to 0).
The second high-order bit (bit 1) of the Attribute Flags octet
is the Transitive bit. It defines whether an optional
attribute is transitive (if set to 1) or non-transitive (if set
to 0). For well-known attributes, the Transitive bit must be
set to 1. (See Section 5 for a discussion of transitive
attributes.)
The third high-order bit (bit 2) of the Attribute Flags octet
is the Partial bit. It defines whether the information
contained in the optional transitive attribute is partial (if
set to 1) or complete (if set to 0). For well-known attributes
and for optional non-transitive attributes the Partial bit must
be set to 0.
The fourth high-order bit (bit 3) of the Attribute Flags octet
is the Extended Length bit. It defines whether the Attribute
Length is one octet (if set to 0) or two octets (if set to 1).
Extended Length may be used only if the length of the attribute
value is greater than 255 octets.
The lower-order four bits of the Attribute Flags octet are .
unused. They must be zero (and must be ignored when received).
The Attribute Type Code octet contains the Attribute Type Code.
Currently defined Attribute Type Codes are discussed in Section
5.
If the Extended Length bit of the Attribute Flags octet is set
to 0, the third octet of the Path Attribute contains the length
of the attribute data in octets.
If the Extended Length bit of the Attribute Flags octet is set
to 1, then the third and the fourth octets of the path
attribute contain the length of the attribute data in octets.
The remaining octets of the Path Attribute represent the
attribute value and are interpreted according to the Attribute
Flags and the Attribute Type Code. The supported Attribute Type
Codes, their attribute values and uses are the following:
a) ORIGIN (Type Code 1):
ORIGIN is a well-known mandatory attribute that defines the
origin of the path information. The data octet can assume
the following values:
Value Meaning
0 IGP - Network Layer Reachability Information
is interior to the originating AS
1 EGP - Network Layer Reachability Information
learned via EGP
2 INCOMPLETE - Network Layer Reachability
Information learned by some other means
Its usage is defined in 5.1.1
b) AS_PATH (Type Code 2):
AS_PATH is a well-known mandatory attribute that is composed
of a sequence of AS path segments. Each AS path segment is
represented by a triple <path segment type, path segment
length, path segment value>.
The path segment type is a 1-octet long field with the
following values defined:
Value Segment Type
1 AS_SET: unordered set of ASs a route in the
UPDATE message has traversed
2 AS_SEQUENCE: ordered set of ASs a route in
the UPDATE message has traversed
The path segment length is a 1-octet long field containing
the number of ASs in the path segment value field.
The path segment value field contains one or more AS
numbers, each encoded as a 2-octets long field.
Usage of this attribute is defined in 5.1.2.
c) NEXT_HOP (Type Code 3):
This is a well-known mandatory attribute that defines the IP
address of the border router that should be used as the next
hop to the destinations listed in the Network Layer
Reachability field of the UPDATE message.
Usage of this attribute is defined in 5.1.3.
d) MULTI_EXIT_DISC (Type Code 4):
This is an optional non-transitive attribute that is a four
octet non-negative integer. The value of this attribute may
be used by a BGP speaker's decision process to discriminate
among multiple exit points to a neighboring autonomous
system.
Its usage is defined in 5.1.4.
e) LOCAL_PREF (Type Code 5):
LOCAL_PREF is a well-known discretionary attribute that is a
four octet non-negative integer. It is used by a BGP speaker
to inform other BGP speakers in its own autonomous system of
the originating speaker's degree of preference for an
advertised route. Usage of this attribute is described in
5.1.5.
f) ATOMIC_AGGREGATE (Type Code 6)
ATOMIC_AGGREGATE is a well-known discretionary attribute of
length 0. It is used by a BGP speaker to inform other BGP
speakers that the local system selected a less specific
route without selecting a more specific route which is
included in it. Usage of this attribute is described in
5.1.6.
g) AGGREGATOR (Type Code 7)
AGGREGATOR is an optional transitive attribute of length 6.
The attribute contains the last AS number that formed the
aggregate route (encoded as 2 octets), followed by the IP
address of the BGP speaker that formed the aggregate route
(encoded as 4 octets). Usage of this attribute is described
in 5.1.7
Network Layer Reachability Information:
This variable length field contains a list of IP address
prefixes. The length in octets of the Network Layer
Reachability Information is not encoded explicitly, but can be
calculated as:
UPDATE message Length - 23 - Total Path Attributes Length -
Unfeasible Routes Length
where UPDATE message Length is the value encoded in the fixed-
size BGP header, Total Path Attribute Length and Unfeasible
Routes Length are the values encoded in the variable part of
the UPDATE message, and 23 is a combined length of the fixed-
size BGP header, the Total Path Attribute Length field and the
Unfeasible Routes Length field.
Reachability information is encoded as one or more 2-tuples of
the form <length, prefix>, whose fields are described below:
+---------------------------+
| Length (1 octet) |
+---------------------------+
| Prefix (variable) |
+---------------------------+
The use and the meaning of these fields are as follows:
a) Length:
The Length field indicates the length in bits of the IP
address prefix. A length of zero indicates a prefix that
matches all IP addresses (with prefix, itself, of zero
octets).
b) Prefix:
The Prefix field contains IP address prefixes followed by
enough trailing bits to make the end of the field fall on an
octet boundary. Note that the value of the trailing bits is
irrelevant.
The minimum length of the UPDATE message is 23 octets -- 19 octets
for the fixed header + 2 octets for the Unfeasible Routes Length + 2
octets for the Total Path Attribute Length (the value of Unfeasible
Routes Length is 0 and the value of Total Path Attribute Length is
0).
An UPDATE message can advertise at most one route, which may be
described by several path attributes. All path attributes contained
in a given UPDATE messages apply to the destinations carried in the
Network Layer Reachability Information field of the UPDATE message.
An UPDATE message can list multiple routes to be withdrawn from
service. Each such route is identified by its destination (expressed
as an IP prefix), which unambiguously identifies the route in the
context of the BGP speaker - BGP speaker connection to which it has
been previously been advertised.
An UPDATE message may advertise only routes to be withdrawn from
service, in which case it will not include path attributes or Network
Layer Reachability Information. Conversely, it may advertise only a
feasible route, in which case the WITHDRAWN ROUTES field need not be
present.
4.4 KEEPALIVE Message Format
BGP does not use any transport protocol-based keep-alive mechanism to
determine if peers are reachable. Instead, KEEPALIVE messages are
exchanged between peers often enough as not to cause the Hold Timer
to expire. A reasonable maximum time between KEEPALIVE messages
would be one third of the Hold Time interval. KEEPALIVE messages
MUST NOT be sent more frequently than one per second. An
implementation MAY adjust the rate at which it sends KEEPALIVE
messages as a function of the Hold Time interval. If the negotiated Hold Time interval is zero, then periodic KEEPALIVE messages MUST NOT be sent. KEEPALIVE message consists of only message header and has a length of 19 octets. 4.5 NOTIFICATION Message Format A NOTIFICATION message is sent when an error condition is detected. The BGP connection is closed immediately after sending it. In addition to the fixed-size BGP header, the NOTIFICATION message contains the following fields: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Error code | Error subcode | Data | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Error Code: This 1-octet unsigned integer indicates the type of NOTIFICATION. The following Error Codes have been defined: Error Code Symbolic Name Reference 1 Message Header Error Section 6.1 2 OPEN Message Error Section 6.2 3 UPDATE Message Error Section 6.3 4 Hold Timer Expired Section 6.5 5 Finite State Machine Error Section 6.6 6 Cease Section 6.7 Error subcode: This 1-octet unsigned integer provides more specific information about the nature of the reported error. Each Error Code may have one or more Error Subcodes associated with it.
If no appropriate Error Subcode is defined, then a zero
(Unspecific) value is used for the Error Subcode field.
Message Header Error subcodes:
1 - Connection Not Synchronized.
2 - Bad Message Length.
3 - Bad Message Type.
OPEN Message Error subcodes:
1 - Unsupported Version Number.
2 - Bad Peer AS.
3 - Bad BGP Identifier. '
4 - Unsupported Optional Parameter.
5 - Authentication Failure.
6 - Unacceptable Hold Time.
UPDATE Message Error subcodes:
1 - Malformed Attribute List.
2 - Unrecognized Well-known Attribute.
3 - Missing Well-known Attribute.
4 - Attribute Flags Error.
5 - Attribute Length Error.
6 - Invalid ORIGIN Attribute
7 - AS Routing Loop.
8 - Invalid NEXT_HOP Attribute.
9 - Optional Attribute Error.
10 - Invalid Network Field.
11 - Malformed AS_PATH.
Data:
This variable-length field is used to diagnose the reason for
the NOTIFICATION. The contents of the Data field depend upon
the Error Code and Error Subcode. See Section 6 below for more
details.
Note that the length of the Data field can be determined from
the message Length field by the formula:
Message Length = 21 + Data Length
The minimum length of the NOTIFICATION message is 21 octets
(including message header).
5. Path Attributes This section discusses the path attributes of the UPDATE message. Path attributes fall into four separate categories: 1. Well-known mandatory. 2. Well-known discretionary. 3. Optional transitive. 4. Optional non-transitive. Well-known attributes must be recognized by all BGP implementations. Some of these attributes are mandatory and must be included in every UPDATE message. Others are discretionary and may or may not be sent in a particular UPDATE message. All well-known attributes must be passed along (after proper updating, if necessary) to other BGP peers. In addition to well-known attributes, each path may contain one or more optional attributes. It is not required or expected that all BGP implementations support all optional attributes. The handling of an unrecognized optional attribute is determined by the setting of the Transitive bit in the attribute flags octet. Paths with unrecognized transitive optional attributes should be accepted. If a path with unrecognized transitive optional attribute is accepted and passed along to other BGP peers, then the unrecognized transitive optional attribute of that path must be passed along with the path to other BGP peers with the Partial bit in the Attribute Flags octet set to 1. If a path with recognized transitive optional attribute is accepted and passed along to other BGP peers and the Partial bit in the Attribute Flags octet is set to 1 by some previous AS, it is not set back to 0 by the current AS. Unrecognized non-transitive optional attributes must be quietly ignored and not passed along to other BGP peers. New transitive optional attributes may be attached to the path by the originator or by any other AS in the path. If they are not attached by the originator, the Partial bit in the Attribute Flags octet is set to 1. The rules for attaching new non-transitive optional attributes will depend on the nature of the specific attribute. The documentation of each new non-transitive optional attribute will be expected to include such rules. (The description of the MULTI_EXIT_DISC attribute gives an example.) All optional attributes (both transitive and non-transitive) may be updated (if appropriate) by ASs in the path.
The sender of an UPDATE message should order path attributes within the UPDATE message in ascending order of attribute type. The receiver of an UPDATE message must be prepared to handle path attributes within the UPDATE message that are out of order. The same attribute cannot appear more than once within the Path Attributes field of a particular UPDATE message. 5.1 Path Attribute Usage The usage of each BGP path attributes is described in the following clauses. 5.1.1 ORIGIN ORIGIN is a well-known mandatory attribute. The ORIGIN attribute shall be generated by the autonomous system that originates the associated routing information. It shall be included in the UPDATE messages of all BGP speakers that choose to propagate this information to other BGP speakers. 5.1.2 AS_PATH AS_PATH is a well-known mandatory attribute. This attribute identifies the autonomous systems through which routing information carried in this UPDATE message has passed. The components of this list can be AS_SETs or AS_SEQUENCEs. When a BGP speaker propagates a route which it has learned from another BGP speaker's UPDATE message, it shall modify the route's AS_PATH attribute based on the location of the BGP speaker to which the route will be sent: a) When a given BGP speaker advertises the route to another BGP speaker located in its own autonomous system, the advertising speaker shall not modify the AS_PATH attribute associated with the route. b) When a given BGP speaker advertises the route to a BGP speaker located in a neighboring autonomous system, then the advertising speaker shall update the AS_PATH attribute as follows: 1) if the first path segment of the AS_PATH is of type AS_SEQUENCE, the local system shall prepend its own AS number as the last element of the sequence (put it in the leftmost position).
2) if the first path segment of the AS_PATH is of type AS_SET,
the local system shall prepend a new path segment of type
AS_SEQUENCE to the AS_PATH, including its own AS number in that
segment.
When a BGP speaker originates a route then:
a) the originating speaker shall include its own AS number in
the AS_PATH attribute of all UPDATE messages sent to BGP
speakers located in neighboring autonomous systems. (In this
case, the AS number of the originating speaker's autonomous
system will be the only entry in the AS_PATH attribute).
b) the originating speaker shall include an empty AS_PATH
attribute in all UPDATE messages sent to BGP speakers located
in its own autonomous system. (An empty AS_PATH attribute is
one whose length field contains the value zero).
5.1.3 NEXT_HOP
The NEXT_HOP path attribute defines the IP address of the border
router that should be used as the next hop to the destinations listed
in the UPDATE message. If a border router belongs to the same AS as
its peer, then the peer is an internal border router. Otherwise, it
is an external border router. A BGP speaker can advertise any
internal border router as the next hop provided that the interface
associated with the IP address of this border router (as specified in
the NEXT_HOP path attribute) shares a common subnet with both the
local and remote BGP speakers. A BGP speaker can advertise any
external border router as the next hop, provided that the IP address
of this border router was learned from one of the BGP speaker's
peers, and the interface associated with the IP address of this
border router (as specified in the NEXT_HOP path attribute) shares a
common subnet with the local and remote BGP speakers. A BGP speaker
needs to be able to support disabling advertisement of external
border routers.
A BGP speaker must never advertise an address of a peer to that peer
as a NEXT_HOP, for a route that the speaker is originating. A BGP
speaker must never install a route with itself as the next hop.
When a BGP speaker advertises the route to a BGP speaker located in
its own autonomous system, the advertising speaker shall not modify
the NEXT_HOP attribute associated with the route. When a BGP speaker
receives the route via an internal link, it may forward packets to
the NEXT_HOP address if the address contained in the attribute is on
a common subnet with the local and remote BGP speakers.
5.1.4 MULTI_EXIT_DISC The MULTI_EXIT_DISC attribute may be used on external (inter-AS) links to discriminate among multiple exit or entry points to the same neighboring AS. The value of the MULTI_EXIT_DISC attribute is a four octet unsigned number which is called a metric. All other factors being equal, the exit or entry point with lower metric should be preferred. If received over external links, the MULTI_EXIT_DISC attribute may be propagated over internal links to other BGP speakers within the same AS. The MULTI_EXIT_DISC attribute is never propagated to other BGP speakers in neighboring AS's. 5.1.5 LOCAL_PREF LOCAL_PREF is a well-known discretionary attribute that shall be included in all UPDATE messages that a given BGP speaker sends to the other BGP speakers located in its own autonomous system. A BGP speaker shall calculate the degree of preference for each external route and include the degree of preference when advertising a route to its internal peers. The higher degree of preference should be preferred. A BGP speaker shall use the degree of preference learned via LOCAL_PREF in its decision process (see section 9.1.1). A BGP speaker shall not include this attribute in UPDATE messages that it sends to BGP speakers located in a neighboring autonomous system. If it is contained in an UPDATE message that is received from a BGP speaker which is not located in the same autonomous system as the receiving speaker, then this attribute shall be ignored by the receiving speaker. 5.1.6 ATOMIC_AGGREGATE ATOMIC_AGGREGATE is a well-known discretionary attribute. If a BGP speaker, when presented with a set of overlapping routes from one of its peers (see 9.1.4), selects the less specific route without selecting the more specific one, then the local system shall attach the ATOMIC_AGGREGATE attribute to the route when propagating it to other BGP speakers (if that attribute is not already present in the received less specific route). A BGP speaker that receives a route with the ATOMIC_AGGREGATE attribute shall not remove the attribute from the route when propagating it to other speakers. A BGP speaker that receives a route with the ATOMIC_AGGREGATE attribute shall not make any NLRI of that route more specific (as defined in 9.1.4) when advertising this route to other BGP speakers. A BGP speaker that receives a route with the ATOMIC_AGGREGATE attribute needs to be cognizant of the fact that the actual path to destinations, as specified in the NLRI of the route, while having the loop-free property, may traverse ASs that are not listed in the AS_PATH
attribute. 5.1.7 AGGREGATOR AGGREGATOR is an optional transitive attribute which may be included in updates which are formed by aggregation (see Section 9.2.4.2). A BGP speaker which performs route aggregation may add the AGGREGATOR attribute which shall contain its own AS number and IP address. 6. BGP Error Handling. This section describes actions to be taken when errors are detected while processing BGP messages. When any of the conditions described here are detected, a NOTIFICATION message with the indicated Error Code, Error Subcode, and Data fields is sent, and the BGP connection is closed. If no Error Subcode is specified, then a zero must be used. The phrase "the BGP connection is closed" means that the transport protocol connection has been closed and that all resources for that BGP connection have been deallocated. Routing table entries associated with the remote peer are marked as invalid. The fact that the routes have become invalid is passed to other BGP peers before the routes are deleted from the system. Unless specified explicitly, the Data field of the NOTIFICATION message that is sent to indicate an error is empty. 6.1 Message Header error handling. All errors detected while processing the Message Header are indicated by sending the NOTIFICATION message with Error Code Message Header Error. The Error Subcode elaborates on the specific nature of the error. The expected value of the Marker field of the message header is all ones if the message type is OPEN. The expected value of the Marker field for all other types of BGP messages determined based on the presence of the Authentication Information Optional Parameter in the BGP OPEN message and the actual authentication mechanism (if the Authentication Information in the BGP OPEN message is present). If the Marker field of the message header is not the expected one, then a synchronization error has occurred and the Error Subcode is set to Connection Not Synchronized.
If the Length field of the message header is less than 19 or greater than 4096, or if the Length field of an OPEN message is less than the minimum length of the OPEN message, or if the Length field of an UPDATE message is less than the minimum length of the UPDATE message, or if the Length field of a KEEPALIVE message is not equal to 19, or if the Length field of a NOTIFICATION message is less than the minimum length of the NOTIFICATION message, then the Error Subcode is set to Bad Message Length. The Data field contains the erroneous Length field. If the Type field of the message header is not recognized, then the Error Subcode is set to Bad Message Type. The Data field contains the erroneous Type field. 6.2 OPEN message error handling. All errors detected while processing the OPEN message are indicated by sending the NOTIFICATION message with Error Code OPEN Message Error. The Error Subcode elaborates on the specific nature of the error. If the version number contained in the Version field of the received OPEN message is not supported, then the Error Subcode is set to Unsupported Version Number. The Data field is a 2-octet unsigned integer, which indicates the largest locally supported version number less than the version the remote BGP peer bid (as indicated in the received OPEN message). If the Autonomous System field of the OPEN message is unacceptable, then the Error Subcode is set to Bad Peer AS. The determination of acceptable Autonomous System numbers is outside the scope of this protocol. If the Hold Time field of the OPEN message is unacceptable, then the Error Subcode MUST be set to Unacceptable Hold Time. An implementation MUST reject Hold Time values of one or two seconds. An implementation MAY reject any proposed Hold Time. An implementation which accepts a Hold Time MUST use the negotiated value for the Hold Time. If the BGP Identifier field of the OPEN message is syntactically incorrect, then the Error Subcode is set to Bad BGP Identifier. Syntactic correctness means that the BGP Identifier field represents a valid IP host address. If one of the Optional Parameters in the OPEN message is not recognized, then the Error Subcode is set to Unsupported Optional Parameters.
If the OPEN message carries Authentication Information (as an Optional Parameter), then the corresponding authentication procedure is invoked. If the authentication procedure (based on Authentication Code and Authentication Data) fails, then the Error Subcode is set to Authentication Failure. 6.3 UPDATE message error handling. All errors detected while processing the UPDATE message are indicated by sending the NOTIFICATION message with Error Code UPDATE Message Error. The error subcode elaborates on the specific nature of the error. Error checking of an UPDATE message begins by examining the path attributes. If the Unfeasible Routes Length or Total Attribute Length is too large (i.e., if Unfeasible Routes Length + Total Attribute Length + 23 exceeds the message Length), then the Error Subcode is set to Malformed Attribute List. If any recognized attribute has Attribute Flags that conflict with the Attribute Type Code, then the Error Subcode is set to Attribute Flags Error. The Data field contains the erroneous attribute (type, length and value). If any recognized attribute has Attribute Length that conflicts with the expected length (based on the attribute type code), then the Error Subcode is set to Attribute Length Error. The Data field contains the erroneous attribute (type, length and value). If any of the mandatory well-known attributes are not present, then the Error Subcode is set to Missing Well-known Attribute. The Data field contains the Attribute Type Code of the missing well-known attribute. If any of the mandatory well-known attributes are not recognized, then the Error Subcode is set to Unrecognized Well-known Attribute. The Data field contains the unrecognized attribute (type, length and value). If the ORIGIN attribute has an undefined value, then the Error Subcode is set to Invalid Origin Attribute. The Data field contains the unrecognized attribute (type, length and value). If the NEXT_HOP attribute field is syntactically incorrect, then the Error Subcode is set to Invalid NEXT_HOP Attribute. The Data field contains the incorrect attribute (type, length and value). Syntactic correctness means that the NEXT_HOP attribute represents a valid IP host address. Semantic correctness applies only to the external BGP
links. It means that the interface associated with the IP address, as specified in the NEXT_HOP attribute, shares a common subnet with the receiving BGP speaker and is not the IP address of the receiving BGP speaker. If the NEXT_HOP attribute is semantically incorrect, the error should be logged, and the the route should be ignored. In this case, no NOTIFICATION message should be sent. The AS_PATH attribute is checked for syntactic correctness. If the path is syntactically incorrect, then the Error Subcode is set to Malformed AS_PATH. If an optional attribute is recognized, then the value of this attribute is checked. If an error is detected, the attribute is discarded, and the Error Subcode is set to Optional Attribute Error. The Data field contains the attribute (type, length and value). If any attribute appears more than once in the UPDATE message, then the Error Subcode is set to Malformed Attribute List. The NLRI field in the UPDATE message is checked for syntactic validity. If the field is syntactically incorrect, then the Error Subcode is set to Invalid Network Field. 6.4 NOTIFICATION message error handling. If a peer sends a NOTIFICATION message, and there is an error in that message, there is unfortunately no means of reporting this error via a subsequent NOTIFICATION message. Any such error, such as an unrecognized Error Code or Error Subcode, should be noticed, logged locally, and brought to the attention of the administration of the peer. The means to do this, however, lies outside the scope of this document. 6.5 Hold Timer Expired error handling. If a system does not receive successive KEEPALIVE and/or UPDATE and/or NOTIFICATION messages within the period specified in the Hold Time field of the OPEN message, then the NOTIFICATION message with Hold Timer Expired Error Code must be sent and the BGP connection closed. 6.6 Finite State Machine error handling. Any error detected by the BGP Finite State Machine (e.g., receipt of an unexpected event) is indicated by sending the NOTIFICATION message with Error Code Finite State Machine Error.
6.7 Cease. In absence of any fatal errors (that are indicated in this section), a BGP peer may choose at any given time to close its BGP connection by sending the NOTIFICATION message with Error Code Cease. However, the Cease NOTIFICATION message must not be used when a fatal error indicated by this section does exist. 6.8 Connection collision detection. If a pair of BGP speakers try simultaneously to establish a TCP connection to each other, then two parallel connections between this pair of speakers might well be formed. We refer to this situation as connection collision. Clearly, one of these connections must be closed. Based on the value of the BGP Identifier a convention is established for detecting which BGP connection is to be preserved when a collision does occur. The convention is to compare the BGP Identifiers of the peers involved in the collision and to retain only the connection initiated by the BGP speaker with the higher-valued BGP Identifier. Upon receipt of an OPEN message, the local system must examine all of its connections that are in the OpenConfirm state. A BGP speaker may also examine connections in an OpenSent state if it knows the BGP Identifier of the peer by means outside of the protocol. If among these connections there is a connection to a remote BGP speaker whose BGP Identifier equals the one in the OPEN message, then the local system performs the following collision resolution procedure: 1. The BGP Identifier of the local system is compared to the BGP Identifier of the remote system (as specified in the OPEN message). 2. If the value of the local BGP Identifier is less than the remote one, the local system closes BGP connection that already exists (the one that is already in the OpenConfirm state), and accepts BGP connection initiated by the remote system. 3. Otherwise, the local system closes newly created BGP connection (the one associated with the newly received OPEN message), and continues to use the existing one (the one that is already in the OpenConfirm state). Comparing BGP Identifiers is done by treating them as (4-octet long) unsigned integers.
A connection collision with an existing BGP connection that is in
Established states causes unconditional closing of the newly
created connection. Note that a connection collision cannot be
detected with connections that are in Idle, or Connect, or Active
states.
Closing the BGP connection (that results from the collision
resolution procedure) is accomplished by sending the NOTIFICATION
message with the Error Code Cease.
7. BGP Version Negotiation.
BGP speakers may negotiate the version of the protocol by making
multiple attempts to open a BGP connection, starting with the highest
version number each supports. If an open attempt fails with an Error
Code OPEN Message Error, and an Error Subcode Unsupported Version
Number, then the BGP speaker has available the version number it
tried, the version number its peer tried, the version number passed
by its peer in the NOTIFICATION message, and the version numbers that
it supports. If the two peers do support one or more common
versions, then this will allow them to rapidly determine the highest
common version. In order to support BGP version negotiation, future
versions of BGP must retain the format of the OPEN and NOTIFICATION
messages.
8. BGP Finite State machine.
This section specifies BGP operation in terms of a Finite State
Machine (FSM). Following is a brief summary and overview of BGP
operations by state as determined by this FSM. A condensed version
of the BGP FSM is found in Appendix 1.
Initially BGP is in the Idle state.
Idle state:
In this state BGP refuses all incoming BGP connections. No
resources are allocated to the peer. In response to the Start
event (initiated by either system or operator) the local system
initializes all BGP resources, starts the ConnectRetry timer,
initiates a transport connection to other BGP peer, while
listening for connection that may be initiated by the remote
BGP peer, and changes its state to Connect. The exact value of
the ConnectRetry timer is a local matter, but should be
sufficiently large to allow TCP initialization.
If a BGP speaker detects an error, it shuts down the connection
and changes its state to Idle. Getting out of the Idle state
requires generation of the Start event. If such an event is
generated automatically, then persistent BGP errors may result
in persistent flapping of the speaker. To avoid such a
condition it is recommended that Start events should not be
generated immediately for a peer that was previously
transitioned to Idle due to an error. For a peer that was
previously transitioned to Idle due to an error, the time
between consecutive generation of Start events, if such events
are generated automatically, shall exponentially increase. The
value of the initial timer shall be 60 seconds. The time shall
be doubled for each consecutive retry.
Any other event received in the Idle state is ignored.
Connect state:
In this state BGP is waiting for the transport protocol
connection to be completed.
If the transport protocol connection succeeds, the local system
clears the ConnectRetry timer, completes initialization, sends
an OPEN message to its peer, and changes its state to OpenSent.
If the transport protocol connect fails (e.g., retransmission
timeout), the local system restarts the ConnectRetry timer,
continues to listen for a connection that may be initiated by
the remote BGP peer, and changes its state to Active state.
In response to the ConnectRetry timer expired event, the local
system restarts the ConnectRetry timer, initiates a transport
connection to other BGP peer, continues to listen for a
connection that may be initiated by the remote BGP peer, and
stays in the Connect state.
Start event is ignored in the Active state.
In response to any other event (initiated by either system or
operator), the local system releases all BGP resources
associated with this connection and changes its state to Idle.
Active state:
In this state BGP is trying to acquire a peer by initiating a
transport protocol connection.
If the transport protocol connection succeeds, the local system
clears the ConnectRetry timer, completes initialization, sends
an OPEN message to its peer, sets its Hold Timer to a large
value, and changes its state to OpenSent. A Hold Timer value
of 4 minutes is suggested.
In response to the ConnectRetry timer expired event, the local
system restarts the ConnectRetry timer, initiates a transport
connection to other BGP peer, continues to listen for a
connection that may be initiated by the remote BGP peer, and
changes its state to Connect.
If the local system detects that a remote peer is trying to
establish BGP connection to it, and the IP address of the
remote peer is not an expected one, the local system restarts
the ConnectRetry timer, rejects the attempted connection,
continues to listen for a connection that may be initiated by
the remote BGP peer, and stays in the Active state.
Start event is ignored in the Active state.
In response to any other event (initiated by either system or
operator), the local system releases all BGP resources
associated with this connection and changes its state to Idle.
OpenSent state:
In this state BGP waits for an OPEN message from its peer.
When an OPEN message is received, all fields are checked for
correctness. If the BGP message header checking or OPEN
message checking detects an error (see Section 6.2), or a
connection collision (see Section 6.8) the local system sends a
NOTIFICATION message and changes its state to Idle.
If there are no errors in the OPEN message, BGP sends a
KEEPALIVE message and sets a KeepAlive timer. The Hold Timer,
which was originally set to a large value (see above), is
replaced with the negotiated Hold Time value (see section 4.2).
If the negotiated Hold Time value is zero, then the Hold Time
timer and KeepAlive timers are not started. If the value of
the Autonomous System field is the same as the local Autonomous
System number, then the connection is an "internal" connection;
otherwise, it is "external". (This will effect UPDATE
processing as described below.) Finally, the state is changed
to OpenConfirm.
If a disconnect notification is received from the underlying
transport protocol, the local system closes the BGP connection,
restarts the ConnectRetry timer, while continue listening for
connection that may be initiated by the remote BGP peer, and
goes into the Active state.
If the Hold Timer expires, the local system sends NOTIFICATION
message with error code Hold Timer Expired and changes its
state to Idle.
In response to the Stop event (initiated by either system or
operator) the local system sends NOTIFICATION message with
Error Code Cease and changes its state to Idle.
Start event is ignored in the OpenSent state.
In response to any other event the local system sends
NOTIFICATION message with Error Code Finite State Machine Error
and changes its state to Idle.
Whenever BGP changes its state from OpenSent to Idle, it closes
the BGP (and transport-level) connection and releases all
resources associated with that connection.
OpenConfirm state:
In this state BGP waits for a KEEPALIVE or NOTIFICATION
message.
If the local system receives a KEEPALIVE message, it changes
its state to Established.
If the Hold Timer expires before a KEEPALIVE message is
received, the local system sends NOTIFICATION message with
error code Hold Timer Expired and changes its state to Idle.
If the local system receives a NOTIFICATION message, it changes
its state to Idle.
If the KeepAlive timer expires, the local system sends a
KEEPALIVE message and restarts its KeepAlive timer.
If a disconnect notification is received from the underlying
transport protocol, the local system changes its state to Idle.
In response to the Stop event (initiated by either system or
operator) the local system sends NOTIFICATION message with
Error Code Cease and changes its state to Idle.
Start event is ignored in the OpenConfirm state.
In response to any other event the local system sends
NOTIFICATION message with Error Code Finite State Machine Error
and changes its state to Idle.
Whenever BGP changes its state from OpenConfirm to Idle, it
closes the BGP (and transport-level) connection and releases
all resources associated with that connection.
Established state:
In the Established state BGP can exchange UPDATE, NOTIFICATION,
and KEEPALIVE messages with its peer.
If the local system receives an UPDATE or KEEPALIVE message, it
restarts its Hold Timer, if the negotiated Hold Time value is
non-zero.
If the local system receives a NOTIFICATION message, it changes
its state to Idle.
If the local system receives an UPDATE message and the UPDATE
message error handling procedure (see Section 6.3) detects an
error, the local system sends a NOTIFICATION message and
changes its state to Idle.
If a disconnect notification is received from the underlying
transport protocol, the local system changes its state to Idle.
If the Hold Timer expires, the local system sends a
NOTIFICATION message with Error Code Hold Timer Expired and
changes its state to Idle.
If the KeepAlive timer expires, the local system sends a
KEEPALIVE message and restarts its KeepAlive timer.
Each time the local system sends a KEEPALIVE or UPDATE message,
it restarts its KeepAlive timer, unless the negotiated Hold
Time value is zero.
In response to the Stop event (initiated by either system or
operator), the local system sends a NOTIFICATION message with
Error Code Cease and changes its state to Idle.
Start event is ignored in the Established state.
In response to any other event, the local system sends
NOTIFICATION message with Error Code Finite State Machine Error
and changes its state to Idle.
Whenever BGP changes its state from Established to Idle, it
closes the BGP (and transport-level) connection, releases all
resources associated with that connection, and deletes all
routes derived from that connection.
(page 34 continued on part 3)
