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BGP Message Generation and Transport, and General Message Format
(Page 2 of 3)
BGP General Message Format
The use of TCP also has an interesting
impact on the way BGP messages are structured. One thing that stands
out when you look at the BGP message format (as we will see shortly)
is that a BGP message can have an odd number of bytes. Most routing
protocols are sized in units of 4 or 8 bytes, but since TCP sends data
as a stream of octets there is no need for BGP messages to break on
a 32-bit or 64-bit boundary. The other impact is the need of a special
Marker field to help ensure that BGP messages can be differentiated
from each other in the TCP stream (more about this in a moment).
Like most messaging protocols, BGP
uses a common message format for each of its four message types. Each
BGP message is conceptually divided into a header and a body (called
the data portion in the BGP standard). The header has three fields
and is fixed in length at 19 bytes. The body is variable in length,
and in fact is omitted entirely in Keepalive messages, since
it is not needed for them.
The general format for all BGP message
types is shown in Table 136
and Figure 192.
Table 136: BGP General Message Format
Field
Name
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Size (bytes)
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Description
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Marker
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16
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Marker: This large
field at the start of each BGP message is used for synchronization and
authentication.
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Length
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2
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Length:
The total length of the message in bytes, including the fields of the
header. The minimum value of this field is 19 for a Keepalive
message; it may be as high as 4,096.
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Type
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1
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Message
Body / Data Portion
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Variable
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Message
Body / Data Portion: Contains the specific fields used to implement
each message type, for Open, Update and Notification
messages.
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