installed by another source, is a local policy decision, and is not
specified in this document. In addition to actual packet forwarding,
the Routing Table is used for resolution of the next-hop addresses
specified in BGP updates (see Section 5.1.3).
4. Message Formats
This section describes message formats used by BGP.
BGP messages are sent over TCP connections. 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 (19 octets).
All multi-octet fields are in network byte order.
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 is included for compatibility; it MUST be
set to all ones.
Length:
This 2-octet unsigned integer indicates the total length of the
message, including the header in octets. Thus, it allows one
to locate the (Marker field of the) next message in the TCP
stream. 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. "padding" of extra data after
the message is not allowed. Therefore, 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. This document defines the following type codes:
1 - OPEN
2 - UPDATE
3 - NOTIFICATION
4 - KEEPALIVE
[RFC2918] defines one more type code.
4.2. OPEN Message Format
After a TCP 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.
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) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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
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 from 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 that is assigned to that BGP
speaker. The value of the BGP Identifier is determined upon
startup and is the same for every local interface and 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 contains a list of optional parameters, in which
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.
[RFC3392] defines the Capabilities Optional Parameter.
The minimum length of the OPEN message is 29 octets (including the
message header).
4.3. UPDATE Message Format
UPDATE messages are used to transfer routing information between BGP
peers. The information in the UPDATE message can be used to
construct a graph that describes 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 that share
common path attributes 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 allows 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 an IP address prefix, followed by
the minimum number of trailing bits needed 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 allows the length
of the Network Layer Reachability field to be determined as
specified below.
A value of 0 indicates that neither the Network Layer
Reachability Information field nor the Path Attribute field is
present in this UPDATE message.
Path Attributes:
A variable-length sequence of path attributes is present in
every UPDATE message, except for an UPDATE message that carries
only the withdrawn routes. 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.
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, the third and 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, and their attribute values and uses are as follows:
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 [RFC904]
2 INCOMPLETE - Network Layer Reachability
Information learned by some other means
Usage of this attribute 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 length field with the
following values defined:
Value Segment Type
1 AS_SET: unordered set of ASes a route in the
UPDATE message has traversed
2 AS_SEQUENCE: ordered set of ASes a route in
the UPDATE message has traversed
The path segment length is a 1-octet length field,
containing the number of ASes (not the number of octets) in
the path segment value field.
The path segment value field contains one or more AS
numbers, each encoded as a 2-octet length 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
(unicast) IP address of the 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 unsigned 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.
Usage of this attribute is defined in 5.1.4.
e) LOCAL_PREF (Type Code 5):
LOCAL_PREF is a well-known attribute that is a four-octet
unsigned integer. A BGP speaker uses it to inform its other
internal peers of the advertising speaker’s degree of
preference for an advertised route.
Usage of this attribute is defined 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 defined 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). This SHOULD be the same address as
the one used for the BGP Identifier of the speaker.
Usage of this attribute is defined 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 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 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 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