BGP-4
yakov@watson.ibm.com Mon, 13 April 1992 21:16 UTC
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Date: Mon, 13 Apr 1992 16:01:14 -0400
From: yakov@watson.ibm.com
To: iwg@rice.edu
Subject: BGP-4
Folks,
Here is the revised version of BGP-4 that attempts to incorporate
all the comments I received from the WG members. Unless I hear
objections by the end of this week, I plan to post it as an
Internet Draft.
Yakov.
--------------------------------cut here----------------------------------
Y. Rekhter
T.J. Watson Research Center, IBM Corp.
T.Li
cisco Systems
Editors
April 1992
A Border Gateway Protocol 4 (BGP-4)
Status of this Memo
This memo, together with its companion document, "Application of the
Border Gateway Protocol in the Internet", define an inter-autonomous
system routing protocol for the Internet. This RFC specifies an IAB
standards track protocol for the Internet community, and requests
discussion and suggestions for improvements. Please refer to the
current edition of the "IAB Official Protocol Standards" for the
standardization state and status of this protocol. Distribution of
this memo is unlimited.
1. Acknowledgements
This document was originally published as RFC 1267 in October 1991,
jointly authored by Kirk Lougheed (cisco Systems) and Yakov Rekhter
(IBM).
We would like to express our thanks to Guy Almes (Rice University),
Len Bosack (cisco Systems), and Jeffrey C. Honig (Cornell University)
for their contributions to the earlier version of this document.
We like to explicitly thank Bob Braden (ISI) for the review of the
earlier version of this document as well as his constructive and
valuable comments.
We would also like to thank Bob Hinden, Director for Routing of the
Internet Engineering Steering Group, and the team of reviewers he
assembled to review earlier versions 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 BGP
Working Group with Yakov Rekhter and Tony Li as editors. Certain
sections of the document borrowed heavily from IDRP [7], which is the
OSI counterpart of BGP. For this credit should be given to the
ANSI X3S3.3 group chaired by Lyman Chapin (BBN) and to Charles Kunzinger
(IBM Corp.) who is the IDRP editor within that group.
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 full path of
Autonomous Systems (ASs) that traffic must transit to reach these
networks. This information is sufficient to construct a graph of AS
connectivity from which routing loops may be pruned and some policy
decisions at the AS level may be enforced.
BGP-4 provides a new set of mechanisms for supporting classless
interdomain routing. These mechanisms include support for advertising
an IP prefix and eliminates the concept of network "class" within BGP.
BGP-4 also introduces mechanisms which allow aggregation of routes,
including aggregation of AS paths. These changes provide support for
the proposed supernetting scheme [8].
To characterize the set of policy decisions that can be enforced
using BGP, one must focus on the rule that an AS advertize to its
neighbor ASs only those routes that it itself uses. This rule
reflects the "hop-by-hop" routing paradigm generally used throughout
the current Internet. Note that some policies cannot be supported by
the "hop-by-hop" routing paradigm and thus require techniques such as
source routing to enforce. For example, BGP does not enable one AS
to send traffic to a neighbor AS intending that that traffic take a
different route from that taken by traffic originating in the
neighbor AS. On the other hand, BGP can support any policy
conforming to the "hop-by-hop" routing paradigm. Since the current
Internet uses only the "hop-by-hop" routing paradigm and since BGP
can support any policy that conforms to that paradigm, BGP is highly
applicable as an inter-AS routing protocol for the current Internet.
A more complete discussion of what policies can and cannot be
enforced with BGP is outside the scope of this document (but refer to
the companion document discussing BGP usage [5]).
BGP runs over a reliable transport protocol. This eliminates the
need to implement explicit update fragmentation, retransmission,
acknowledgement, and sequencing. Any authentication scheme used by
the transport protocol may be used in addition to BGP's own
authentication mechanisms. The error notification mechanism used in
BGP assumes that the transport protocol supports a "graceful" close,
i.e., that all outstanding data will be delivered before the
connection is closed.
BGP uses TCP [4] as its transport protocol. TCP meets BGP's
transport requirements and is present in virtually all commercial
routers and hosts. In the following descriptions the phrase
"transport protocol connection" can be understood to refer to a TCP
connection. BGP uses TCP port 179 for establishing its connections.
This memo uses the term `Autonomous System' (AS) throughout. The
classic definition of an Autonomous System is a set of routers under
a single technical administration, using an interior gateway protocol
and common metrics to route packets within the AS, and using an
exterior gateway protocol to route packets to other ASs. Since this
classic definition was developed, it has become common for a single
AS to use several interior gateway protocols and sometimes several
sets of metrics within an AS. The use of the term Autonomous System
here stresses the fact that, even when multiple IGPs and metrics are
used, the administration of an AS appears to other ASs to have a
single coherent interior routing plan and presents a consistent
picture of what networks are reachable through it. From the
standpoint of exterior routing, an AS can be viewed as monolithic:
reachability to networks directly connected to the AS must be
equivalent from all border gateways of the AS.
The planned use of BGP in the Internet environment, including such
issues as topology, the interaction between BGP and IGPs, and the
enforcement of routing policy rules is presented in a companion
document [5]. This document is the first of a series of documents
planned to explore various aspects of BGP application.
Please send comments to the BGP mailing list (iwg@rice.edu).
3. Summary of Operation
Two systems form a transport protocol connection between one another.
They exchange messages to open and confirm the connection parameters.
The initial data flow is the entire BGP routing table. Incremental
updates are sent as the routing tables change. BGP does not require
periodic refresh of the entire BGP routing table. Therefore, a BGP
speaker must retain the current version of the entire BGP routing
tables of all of its peers for the duration of the connection.
KeepAlive messages are sent periodically to ensure the liveness of
the connection. Notification messages are sent in response to errors
or special conditions. If a connection encounters an error
condition, a notification message is sent and the connection is
closed.
The hosts executing the Border Gateway Protocol need not be routers.
A non-routing host could exchange routing information with routers
via EGP or even an interior routing protocol. That non-routing host
could then use BGP to exchange routing information with a border
router in another Autonomous System. The implications and
applications of this architecture are for further study.
If a particular AS has multiple BGP speakers and is providing transit
service for other ASs, then care must be taken to ensure a consistent
view of routing within the AS. A consistent view of the interior
routes of the AS is provided by the interior routing protocol. A
consistent view of the routes exterior to the AS can be provided by
having all BGP speakers within the AS maintain direct BGP connections
with each other. Using a common set of policies, the BGP speakers
arrive at an agreement as to which border routers will serve as
exit/entry points for particular networks outside the AS. This
information is communicated to the AS's internal routers, possibly
via the interior routing protocol. Care must be taken to ensure that
the interior routers have all been updated with transit information
before the BGP speakers announce to other ASs that transit service is
being provided.
Connections between BGP speakers of different ASs are referred to as
"external" links. BGP connections between BGP speakers within the
same AS are referred to as "internal" links.
3.1 Routes: Advertisement and Storage
For purposes of this protocol a route is defined as a unit of information
that pairs a destination with the attributes of a path to that destination:
- Routes are advertised between a pair of BGP speakers in UPDATE messages:
the destination is the systems whose IP addresses are reported in
the Network Layer Reachability Information (NLRI) field, and the
the path is the information reported in the path attributes fields
of the same UPDATE message.
- Routes are stored in the Routing Information Bases (RIBs): namely,
the Adj-RIBs-In, the Loc-RIB, and the Adj-RIBs-Out. Routes that will
be advertised to other BGP speakers must be present in the Adj-RIB-Out;
routes that will be used by the local BGP speaker must be present in
the Loc-RIB, and the next hop for each of these routes must be present
in the local BGP speaker's forwarding information base; and routes
that are received from other BGP speakers are present in the Adj-RIBs-In.
If a BGP speaker chooses to advertise the route, it may add to or modify
the path attributes of the route before advertising it to adjacent
BGP speaker.
BGP provides mechanisms by which a BGP speaker can inform its neighbor
that a previously advertised route is no longer available for use.
There are three methods by which a given BGP speaker can indicate that
a route has been withdrawn from service:
a) the IP prefix that expresses destinations for a previously advertised
route can be advertised in the WITHDRAWN ROUTES field in the UPDATE
message, thus marking the associated route as being no longer available
for use
b) a replacement route with the same Network Layer Reachability
Information can be advertised, or
c) the BGP speaker - BGP speaker connection can be closed, which
implicitly removes from service all routes which the pair
of speakers had advertised to each other.
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 neighbors. The
routing information stored in the Adj-RIBs-Out will be carried
in the local BGP speaker's UPDATE messages and advertised to its
neighbor BGP speakers.
In summary, the Adj-RIBs-In contain unprocessed routing information that
has been advertised to the local BGP speaker by its neighbors; 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 neighbor BGP speakers by means
of the local speaker's UPDATE messages.
Although the conceptual model distinguishes between Adj-RIBs-In,
Loc-RIB, and Adj-RIBs-Out, this neither implies nor requires
that an implementation must maintain three separate copies of
the routing information. The choice of implementation (for example,
3 copies of the information vs 1 copy with pointers) is not constrained
by the protocol.
4. Message Formats
This section describes message formats used by BGP.
Messages are sent over a reliable transport protocol connection. A
message is processed only after it is entirely received. The maximum
message size is 4096 octets. All implementations are required to
support this maximum message size. The smallest message that may be
sent consists of a BGP header without a data portion, or 19 octets.
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 Authentication Code used in the OPEN message of the connection
is zero, 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 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Auth. Code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Authentication Data |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Version:
This 1-octet unsigned integer indicates the protocol version
number of the message. The current BGP version number is 3.
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 maximum number of
seconds that may elapse between the receipt of successive
KEEPALIVE and/or UPDATE and/or NOTIFICATION messages.
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 the IP address of one of its interfaces.
The value of the BGP Identifier is determined on startup
and is the same for every local interface and every BGP peer.
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.
Only one authentication mechanism is specified as part of this
memo:
- its Authentication Code is zero,
- its Authentication Data must be empty (of zero length), and
- the Marker fields of all messages must be all ones.
The semantics of non-zero Authentication Codes lies outside the
scope of this memo.
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. If the value of Authentication
Code field is zero, the Authentication Data field must have zero
length. The semantics of the non-zero length Authentication Data
field is outside the scope of this memo.
Note that the length of the Authentication Data field can be
determined from the message Length field by the formula:
Message Length = 29 + Authentication Data Length
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 advertise a single feasible route to a
neighboring BGP speaker, 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 Route Count (2 octets) |
+-----------------------------------------------------+
| Withdrawn Routes (variable) |
+-----------------------------------------------------+
| Total Path Attribute Length (2 octets) |
+-----------------------------------------------------+
| Path Attributes (variable) |
+-----------------------------------------------------+
| Network Layer Reachability Information (variable) |
+-----------------------------------------------------+
Unfeasible Route Count:
This is a 2-octets long field that contains an integer whose
value is equal to the number of IP address prefixes that are included
in the subsequent WITHDRAWN ROUTES field. 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 (non-
negative integer) number of Network fields to be determined as
specified below.
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 values and their 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 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 1 octet
non-negative integer. The value of this attribute may be used
by a BGP speaker's decision process to discriminate between
multiple exit points to an adjacent 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 1
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.
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 Route Count * 4
where UPDATE message Length is the value encoded in the fixed-size
BGP header, Total Path Attribute Length and Unfeasible Route Count
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 Route Count
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 trailing
bits is irrelevant.
The minimum length of the UPDATE message is 33 octets (including
message header).
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 time
(as advertised in the OPEN message) to expire. A reasonable maximum
time between KEEPALIVE messages would be one third of the Hold Time
interval.
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 Authentication Code.
5 - Authentication Failure.
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.
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. Which well-known attributes are
mandatory or discretionary is noted in the table below.
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. It shall be recognized
upon receipt by all BGP speakers. It shall be included in each
UPDATE message that includes Network Layer Reachability Information.
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. It shall be presented
in every UPDATE message and shall be recognized upon receipt by
all BGP speakers. 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 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 an adjacent autonomous system, then the advertising
speaker shall update the AS_PATH attribute as follows:
1) if the last path segment of the AS_PATH is of type AS_SEQUENCE,
the local system shall append its own AS number as the last
element of the sequence
2) if the last path segment of the AS_PATH is of type AS_SET, the
local system shall append a new path segment of type AS_SEQUENCE
to the AS_PATH, including its own AS number in that segment.
The BGP speaker that modifies the AS_PATH attribute is free to
collapse the previously received AS_PATH information in accordance
with its local policies: for example, it may create an AS_SET from
a received AS_SEQUENCE.
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 adjacent 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 networks 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.
The NEXT_HOP path attribute has meaning only on external BGP
links. However, presence of the NEXT_HOP path attribute in the
UPDATE message received via an internal BGP link does not
constitute an error.
5.1.4 MULTI_EXIT_DISC
The MULTI_EXIT_DISC attribute may be used on external (inter-AS)
links to discriminate between multiple exit or entry points to the
same neighboring AS. The value of the MULTI_EXIT_DISC attribute
is a 1-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 neighbors. The lower 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).
A BGP speaker shall not include this attribute in UPDATE messages
that it sends to BGP speakers located in an adjacent autonomous
system. It 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.
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
Authentication Code in the BGP OPEN message and the actual
authentication mechanism (if the Authentication Code in the BGP OPEN
message is non-zero). 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 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 the Authentication Code of the OPEN message is not recognized,
then the Error Subcode is set to Unsupported Authentication Code. If
the Authentication Code is zero, then the Authentication Data must be
of zero length. Otherwise, the Error Subcode is set to
Authentication Failure.
If the Authentication Code is non-zero, 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 Total Attribute Length is too large (i.e., if
Total Attribute Length + 21 exceeds the message Length), or if the
(non-negative integer) Number of Network fields cannot be computed as
in Section 4.3, 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 or semantically
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.
The AS route specified by the AS_PATH attribute is checked for AS
loops. AS loop detection is done by scanning the full AS route (as
specified in the AS_PATH attribute) and checking that each AS occurs
at most once. If a loop is detected, then the Error Subcode is set
to AS Routing Loop. The Data field contains the incorrect attribute
(type, length and value).
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.
Each Network field in the UPDATE message is checked for syntactic
validity. If the Network field is syntactically incorrect, or
contains a subnet or a host address, 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 OpenSent state. If among them 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 OpenSent
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 OpenSent state).
Comparing BGP Identifiers is done by treating them as
(4-octet long) unsigned integers.
A connection collision with existing BGP connections that
are either in OpenConfirm or 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 BGP neighbor. 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.
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 BGP neighbor 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.
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 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 an arbitrary large value (see
above), is replaced with the value indicated in the OPEN
message. If the value of the Autonomous System field is the
same as our own, then the connection is "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 time 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 Holdtime timer.
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 Holdtime 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.
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.
9. UPDATE Message Handling
An UPDATE message may be received only in the Established state.
When an UPDATE message is received, each field is checked for
validity as specified in Section 6.3.
If an optional non-transitive attribute is unrecognized, it is
quietly ignored. If an optional transitive attribute is
unrecognized, the Partial bit (the third high-order bit) in the
attribute flags octet is set to 1, and the attribute is retained for
propagation to other BGP speakers.
If an optional attribute is recognized, and has a valid value, then,
depending on the type of the optional attribute, it is processed
locally, retained, and updated, if necessary, for possible
propagation to other BGP speakers.
If the UPDATE message contains a non-empty WITHDRAWN ROUTES field,
the previously advertised routes whose destinations (expressed as
IP prefixes) contained in this field shall be removed from the Adj-RIB-In.
This BGP speaker shall run its Decision Process since the previously
advertised route is not longer available for use.
If the UPDATE message contains a feasible route, it shall be placed
in the appropriate Adj-RIB-In, and the following additional actions
shall be taken:
i) If its Network Layer Reachability Information (NLRI) is identical
to the one of a route currently stored in the Adj-RIB-In, then the
new route shall replace the older route in the Adj-RIB-In, thus
implicitly withdrawing the older route from service. The BGP speaker
shall run its Decision Process since the older route is no longer
available for use.
ii) If the new route is an overlapping route that is more specific
(see 9.1.4) than an earlier route contained in the Adj-RIB-In, the
BGP speaker shall run its Decision Process since the more specific
route has implicitly made a portion of the less specific route unavailable
for use.
iii) If the new route has identical path attributes to an earlier route
contained in the Adj-RIB-In, and is more specific (see 9.1.4)
than the earlier route, no further actions are necessary.
iv) If the new route has NLRI that is not present in any of the routes
currently stored in the Adj-RIB-In, then the new route shall be placed
in the Adj-RIB-In. The BGP speaker shall run its Decision Process.
v) If the new route is an overlapping route that is less specific
(see 9.1.4) than an earlier route contained in the Adj-RIB-In,
the BGP speaker shall run its Decision Process on the set of destinations
described only by the less specific route.
9.1 Decision Process
The Decision Process selects routes for subsequent advertisement by
applying the policies in the local Policy Information Base (PIB)
to the routes stored in its Adj-RIB-In. The output of the Decision
Process is the set of routes that will be advertised to adjacent
BGP speakers; the selected routes will be stored in the local speaker's
Adj-RIB-Out.
The selection process is formalized by defining a function that
takes the attribute of a given route as an argument and returns
a non-negative integer denoting the degree of preference for the route.
The function that calculates the degree of preference for a given
route shall not use as its inputs any of the following:
the existence of other routes, the non-existence of other routes,
or the path attributes of other routes. Route selection then
consists of individual application of the degree of preference
function to each feasible route, followed by the choice of the one
with the highest degree of preference.
The Decision Process operates on routes contained in each Adj-RIB-In,
and is responsible for:
- selection of routes to be advertised to BGP speakers located
in the local speaker's autonomous system
- selection of routes to be advertised to BGP speakers located in
adjacent autonomous systems
- route aggregation and route information reduction
The Decision Process takes place in three distinct phases, each
triggered by a different event:
a) Phase 1 is responsible for calculating the degree of preference
for each route received from a BGP speaker located in an adjacent
autonomous system, and for advertising to the other BGP speakers
in the local autonomous system the routes that have the highest
degree of preference for each distinct destination.
b) Phase 2 is invoked on completion of phase 1. It is responsible
for choosing the best route out of all those available for each
distinct destination, and for installing each chosen route into
the appropriate Loc-RIB.
c) Phase 3 is invoked after the Loc-RIB has been modified. It is
responsible for disseminating routes in the Loc-RIB to each
adjacent BGP speaker located in an adjacent autonomous system,
according to the policies contained in the PIB. Route aggregation and
information reduction can optionally be performed within this phase.
9.1.1 Phase 1: Calculation of Degree of Preference
The Phase 1 decision function shall be invoked whenever the local
BGP speaker receives an UPDATE message from a neighboring BGP speaker
located in an adjacent autonomous system that advertises a new route,
a replacement route, or a withdrawn route.
The Phase 1 decision function is a separate process which completes
when it has no further work to do.
The Phase 1 decision function shall lock an Adj-RIB-In prior
to operating on any route contained within it, and shall unlock
it after operating on all new or unfeasible routes contained
within it.
For each newly received or replacement feasible route, the local
BGP speaker shall compute a degree of preference. Then, the local
speaker shall run the internal update process of 9.2.1 to select
and advertise the most preferable route.
9.1.2 Phase 2: Route Selection
The Phase 2 decision function shall be invoked on completion of Phase 1.
The Phase 2 function is a separate process which completes when it
has no further work to do. The Phase 2 process shall consider all
routes that are present in the Adj-RIBs-In, including those
received from BGP speakers located in its own autonomous system
and those received from BGP speakers located in adjacent autonomous
systems.
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.
For each set of destinations for which a feasible route exists in
the Adj-RIBs-In, the local BGP speaker shall identify the route that
has:
a) the highest degree of preference of any route to the same set
of destinations, or
b) is the only route to that destination, or
c) is selected as a result of the Phase 2 tie breaking rules
specified in 9.1.2.1.
The local speaker shall then install that route in the Loc-RIB,
replacing any route to the same destination that is currently being
held in the Loc-RIB.
Unfeasible routes shall be removed from the Loc-RIB, and corresponding
unfeasible routes shall then be removed from the Adj-RIBs-In.
9.1.2.1 Breaking Ties (Phase 2)
In its Adj-RIBs-In a BGP speaker may have several routes to the
same destination that have the same degree of preference. The local
speaker can select only one of these routes for inclusion in the
associated Loc-RIB. The local speaker considers all equally preferable
routes, both those received from BGP speakers located in adjacent
autonomous systems, and those received from other BGP speakers
located in the local speaker's autonomous system.
Ties shall be broken according to the following rules:
a) If the candidate routes have identical path attributes or differ
only in the NEXT_HOP attribute, select the route that was
advertised by the BGP speaker in an adjacent autonomous system
whose BGP Identifier has the lowest value. If none of the candidate
routes were received from a BGP speaker located in an adjacent
autonomous system, select the route that was advertised by the
BGP speaker in the local autonomous system whose BGP Identifier
has the lowest value.
b) If the candidate routes differ only in their NEXT_HOP and
MULTI_EXIT_DISC attributes, and the local system is configured to
take into account MULTI_EXIT_DISC, select the route that has the
lowest value of the MULTI_EXIT_DISC attribute.
If the local system is configured to ignore MULTI_EXIT_DISC, select
the route advertised by the BGP speaker in an adjacent autonomous
system whose BGP Identifier has the lowest value. If none of the
candidate routes were received from a BGP speaker located in an
adjacent autonomous system, select the route that was advertised
by the BGP speaker in the local autonomous system whose BGP
Identifier has the lowest value.
c) If the candidate route differ in any path attributes other than
NEXT_HOP and MULTI_EXIT_DISC, select the route that was advertised
by the BGP speaker whose BGP Identifier has the lowest value.
9.1.3 Phase 3: Route Dissemination
The Phase 3 decision function shall be invoked on completion of Phase
2, or when any of the following events occur:
a) when routes in a Loc-RIB to local destinations have changed
b) when locally generated routes learned by means outside of BGP
have changed
c) when a new BGP speaker - BGP speaker connection has been
established
The Phase 3 function is a separate process which completes when it
has no further work to do. The Phase 3 Routing Decision function shall
be blocked from running while the Phase 2 decision function is in
process.
All routes in the Loc-RIB shall be processed into a corresponding
entry in the associated Adj-RIBs-Out. Route aggregation and
information reduction techniques (see 9.2.4.1) may optionally
be applied.
When the updating of the Adj-RIBs-Out and the FIBs is complete,
the local BGP speaker shall run the external update process
of 9.2.2.
9.1.4 Overlapping Routes
A BGP speaker may transmit routes with overlapping Network Layer
Reachability Information (NLRI) to another BGP speaker. NLRI
overlap occurs when a set of destinations are identified in
non-matching multiple routes. Since BGP encodes NLRI using
IP prefixes, overlap will always exhibit subset relationships.
A route describing a smaller set of destinations (a longer prefix)
is said to be more specific than a route describing a larger
set of destinations (a shorted prefix); similarly, a route
describing a larger set of destinations (a shorter prefix)
is said to be less specific than a route describing a smaller
set of destinations ( a longer prefix).
When overlapping routes are present in the same Adj-RIB-In, the
more specific route shall take precedence, in order from
more specific to least specific.
The 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
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
shall not alter the semantics of the overlapping routes. In particular,
a BGP speaker shall not accept the less specific route while rejecting
the more specific route, because the destinations represented by
the overlap will not forward along that route. Therefore, a BGP
speaker has the following choices:
a) Install both the less and the more specific routes
b) Install the more specific route only
c) Install the non-overlapping part of the less specific route only
d) Aggregate the two routes and install the aggregated route
e) Install neither route
9.2 Update-Send Process
The Update-Send process is responsible for advertising UPDATE messages
to adjacent BGP speakers. 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 an adjacent autonomous system.
Rules for information exchange between BGP speakers located in different
autonomous systems are given in 9.2.2; rules for information exchange
between BGP speakers located in the same autonomous system are given
in 9.2.1.
Distribution of routing information between a set of BGP speakers,
all of which are located in the same autonomous system, is
referred to as internal distribution.
9.2.1 Internal Updates
The Internal update process is concerned with the distribution of routing
information to BGP speakers located in the local speaker's autonomous
system.
When a BGP speaker receives an UPDATE message from another BGP speaker
located in its own autonomous system, the receiving BGP speaker shall
not re-distribute the routing information contained in that UPDATE
message to other BGP speakers located in its own autonomous system.
When a BGP speaker receives a new route from a BGP speaker in an
adjacent autonomous system, it shall advertise that route to
all other BGP speakers in its autonomous system by means of an UPDATE
message if any of the following conditions occur:
1) the degree of preference assigned to the newly received route by
the local BGP speaker is higher than the degree of preference that
the local speaker has assigned to other routes that have been
received from BGP speakers in adjacent autonomous systems, or
2) there are no other routes that have been received from BGP speakers
in adjacent autonomous systems, or
3) the newly received route is selected as a result of breaking a tie
between several routes which have the highest degree of preference,
and the same destination.
When a BGP speaker receives an UPDATE message with a non-empty
WITHDRAWN ROUTES field, it shall remove from its Adj-RIB-In all routes
whose destinations was carried in this field (as IP prefixes).
The speaker shall take the following additional steps:
1) if the corresponding feasible route had not been previously advertised,
then no further action is necessary
2) if the corresponding feasible route had been previously advertised,
then:
i) if a new route is selected for advertisement that has the same
Network Layer Reachability Information as the unfeasible routes,
then the local BGP speaker shall advertise the replacement route
ii) if a replacement route is not available for advertisement, then
the BGP speaker shall include the destinations of the unfeasible
route (in form of IP prefixes) in the WITHDRAWN ROUTES field of
an UPDATE message, and shall send this message to each neighbor
BGP speaker to whom it had previously advertised the corresponding
feasible route.
All feasible routes which are advertised shall be placed in the
appropriate Adj-RIBs-Out, and all unfeasible routes which are advertised
shall be removed from the Adj-RIBs-Out.
9.2.1.1 Breaking Ties (Internal Updates)
If a local BGP speaker has connections to several BGP speakers in
adjacent autonomous systems, there will be multiple Adj-RIBs-In
associated with these neighbors. These Adj-RIBs-In might contain
several equally preferable routes to the same destination, all of
which were advertised by BGP speakers located in adjacent autonomous
systems. The local BGP speaker shall select one of these routes
according to the following rules:
a) If the candidate route differ only in their NEXT_HOP and MULTI_EXIT_DISC
attributes, and the local system is configured to take into account
MULTI_EXIT_DISC attribute, select the routes that has the lowest value
of the MULTI_EXIT_DISC attribute.
b) In all other cases, select the route that was advertised by the BGP
speaker whose BGP Identifier has the lowest value.
9.2.2 External Updates
The external update process is concerned with the distribution
of routing information to BGP speakers located in adjacent autonomous
systems. As part of Phase 3 route selection process, the BGP speaker
has updated its Adj-RIBs-Out and its Forwarding Table. All newly
installed routes and all newly unfeasible routes for which there
is no replacement route shall be advertised to BGP speakers located
in adjacent autonomous systems by means of UPDATE message.
Any routes in the Loc-RIB marked as unfeasible shall be removed. Changes
to the reachable destinations within its own autonomous system
shall also be advertised in an UPDATE message.
9.2.3 Controlling Routing Traffic Overhead
The BGP protocol constrains the amount of routing traffic (that is,
UPDATE messages) in order to limit both the link bandwidth needed
to advertise UPDATE messages and the processing power needed by
the Decision Process to digest the information contained in the
UPDATE messages.
9.2.3.1 Frequency of Route Advertisement
The parameter MinRouteAdvertisementInterval determines the minimum
amount of time that must elapse between advertisement of routes to a
particular destination from a single BGP speaker. This rate limiting
procedure applies on a per-destination basis, although the value
of MinRouteAdvertisementInterval is set on a per BGP peer basis.
Two UPDATE messages sent from a single BGP speaker that advertise
feasible routes to some common set of destinations received
from BGP speakers in adjacent autonomous systems must be separated
by at least MinRouteAdvertisementInterval.
Since fast convergence is needed within an autonomous system, this
procedure does not apply for routes receives from other BGP speakers
in the same autonomous system. To avoid long-lived black holes, the
procedure does not apply to the explicit withdrawal of unfeasible routes
(that is, routes whose destinations (expressed as IP prefixes) are
listed in the WITHDRAWN ROUTES field of an UPDATE message).
This procedure does not limit the rate of route selection, but only
the rate of route advertisement. If new routes are selected multiple
times while awaiting the expiration of MinRouteAdvertisementInterval,
the last route selected shall be advertised at the end of
MinRouteAdvertisementInterval.
9.2.3.2 Frequency of Route Origination
The parameter MinASOriginationInterval determines the minimum amount of
time that must elapse between successive advertisements of UPDATE
messages that report changes within the advertising BGP speaker's
own autonomous systems.
9.2.3.3 Jitter
To minimize the likelihood that the distribution of BGP messages
by a given BGP speaker will contain peaks, jitter should be applied
to the timers associated with MinRouteSelectionInterval 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.
9.2.4 Efficient Organization of Routing Information
Having selected the routing information which it will advertise,
a BGP speaker may avail itself of several methods to organize
this information in an efficient manner.
9.2.4.1 Information Reduction
Information reduction may imply a reduction in granularity of policy
control - after information is collapsed, the same policies will apply to
all destinations and paths in the equivalence class.
The Decision Process may optionally reduce the amount of information
that it will place in the Adj-RIBs-Out by any of the following methods:
a) Network Layer Reachability Information (NLRI):
Destination IP addresses can be represented as IP address prefixes.
In case where there is a correspondence between the address structure
and the systems under control of an autonomous system administrator,
it will be possible to reduce the size of the NLRI carried in the
UPDATE messages.
b) AS_PATHs:
AS path information can be represented as ordered AS_SEQUENCEs or
unordered AS_SETs. AS_SETs are used in the route aggregation
algorithm described in 9.2.4.2. They reduce the size of the AS_PATH
information by listing each AS number only once, regardless of how
many times it may have appeared in multiple AS_PATHs that were
aggregated.
An AS_SET implies that the destinations listed in the NLRI
can be reached through paths that traverse at least some of
the constituent autonomous systems. AS_SETs provide sufficient
information to avoid routing information looping; however their
use may prune potentially feasible paths, since such paths
are no longer listed individually as in the form of AS_SEQUENCEs.
In practice this is not likely to be a problem, since once
an IP packet arrives at the edge of a group of autonomous systems,
the BGP speaker at that point is likely to have more detailed
path information and can distinguish individual paths to destinations.
9.2.4.2 Aggregating Routing Information
Aggregation is the process of combining the characteristics of several
different routes in such a way that a single route can be advertised.
Aggregation can occur as part of the decision process to reduce the
amount of routing information that will be placed in the Adj-RIBs-Out.
Aggregation reduces the amount of information that a BGP speaker
must store and exchange with other BGP speakers. Routes can be
aggregated by applying the following procedure separately to path
attributes of like type and to the Network Layer Reachability Information.
Routes that have the following attributes shall not be aggregated
unless the corresponding attributes of each route are identical:
MULTI_EXIT_DISC, NEXT_HOP.
Path attributes that have different type codes can not be aggregated
together. Path of the same type code may be aggregated, according to
the following rules:
ORIGIN attribute: If at least one route among routes that are aggregated
has ORIGIN with the value INCOMPLETE, then the aggregated route
must have the ORIGIN attribute with the value INCOMPLETE. Otherwise,
if at least one route among routes that are aggregated has ORIGIN
with the value EGP, then the aggregated route must have the origin
attribute with the value EGP. In all other case the value of the
ORIGIN attribute of the aggregated route is INTERNAL.
AS_PATH attribute: If routes to be aggregated have identical AS_PATH
attributes, then the aggregated route has the same AS_PATH attribute
as each individual route.
If routes to be aggregated have different AS_PATH attributes, then the
rules for determining the AS_PATH attribute of the aggregated route are
specified by 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:
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 places 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.
9.3.6 Route Selection Criteria
Generally speaking, the rules for comparing routes among several
alternatives are outside the scope of this document. There are two
exceptions:
- If the local AS appears in the AS path of the new route being
considered, then that new route cannot be viewed as better than
any other route. If such a route were ever used, a routing loop
would result.
- In order to achieve successful distributed operation, only routes
with a likelihood of stability can be chosen. Thus, an AS must
avoid using unstable routes, and it must not make rapid
spontaneous changes to its choice of route. Quantifying the terms
"unstable" and "rapid" in the previous sentence will require
experience, but the principle is clear.
10. Detection of Inter-AS Policy Contradictions
Since BGP requires no central authority for coordinating routing
policies among ASs, and since routing policies are not exchanged via
the protocol itself, it is possible for a group of ASs to have a set
of routing policies that cannot simultaneously be satisfied. This
may cause an indefinite oscillation of the routes in this group of
ASs.
To help detect such a situation, all BGP speakers must observe the
following rule. If a route to a destination that is currently used
by the local system is determined to be unreachable (e.g., as a
result of receiving an UPDATE message for this route with the
UNREACHABLE attribute), then, before switching to another route, this
local system must advertize this route as unreachable to all the BGP
neighbors to which it previously advertized this route.
This rule will allow other ASs to distinguish between two different
situations:
- The local system has chosen to use a new route because the old
route become unreachable.
- The local system has chosen to use a new route because it
preferred it over the old route. The old route is still
viable.
In the former case, an UPDATE message with the UNREACHABLE attribute
will be received for the old route. In the latter case it will not.
In some cases, this may allow a BGP speaker to detect the fact that
its policies, taken together with the policies of some other AS,
cannot simultaneously be satisfied. For example, consider the
following situation involving AS A and its neighbor AS B. B
advertises a route with a path of the form <B,...>, where A is not
present in the path. A then decides to use this path, and advertises
<A,B,...> to all its neighbors. B later advertises <B,...,A,...>
back to A, without ever declaring its previous path <B,...> to be
unreachable. Evidently, A prefers routes via B and B prefers routes
via A. The combined policies of A and B, taken together, cannot be
satisfied. Such an event should be noticed, logged locally, and
brought to the attention of AS A's administration. The means to do
this, however, lies outside the scope of this document. Also outside
the document is a more complete procedure for detecting such
contradictions of policy.
While the above rules provide a mechanism to detect a set of routing
policies that cannot be satisfied simultaneously, the protocol itself
does not provide any mechanism for suppressing the route oscillation
that may result from these unsatisfiable policies. The reason for
doing this is that routing policies are viewed as external to the
protocol and as determined by the local AS administrator.
Appendix 1. BGP FSM State Transitions and Actions.
This Appendix discusses the transitions between states in the BGP FSM
in response to BGP events. The following is the list of these states
and events.
BGP States:
1 - Idle
2 - Connect
3 - Active
4 - OpenSent
5 - OpenConfirm
6 - Established
BGP Events:
1 - BGP Start
2 - BGP Stop
3 - BGP Transport connection open
4 - BGP Transport connection closed
5 - BGP Transport connection open failed
6 - BGP Transport fatal error
7 - ConnectRetry timer expired
8 - Holdtime timer expired
9 - KeepAlive timer expired
10 - Receive OPEN message
11 - Receive KEEPALIVE message
12 - Receive UPDATE messages
13 - Receive NOTIFICATION message
The following table describes the state transitions of the BGP FSM
and the actions triggered by these transitions.
Event Actions Message Sent Next State
--------------------------------------------------------------------
Idle (1)
1 Initialize resources none 2
Start ConnectRetry timer
Initiate a transport connection
others none none 1
Connect(2)
1 none none 2
3 Complete initialization OPEN 4
Clear ConnectRetry timer
5 Restart ConnectRetry timer none 3
7 Restart ConnectRetry timer none 2
Initiate a transport connection
others Release resources none 1
Active (3)
1 none none 3
3 Complete initialization OPEN 4
Clear ConnectRetry timer
5 Close connection 3
Restart ConnectRetry timer
7 Restart ConnectRetry timer none 2
Initiate a transport connection
others Release resources none 1
OpenSent(4)
1 none none 4
4 Close transport connection none 3
Restart ConnectRetry timer
6 Release resources none 1
10 Process OPEN is OK KEEPALIVE 5
Process OPEN failed NOTIFICATION 1
others Close transport connection NOTIFICATION 1
Release resources
OpenConfirm (5)
1 none none 5
4 Release resources none 1
6 Release resources none 1
9 Restart KeepAlive timer KEEPALIVE 5
11 Complete initialization none 6
Restart Holdtime timer
13 Close transport connection 1
Release resources
others Close transport connection NOTIFICATION 1
Release resources
Established (6)
1 none none 6
4 Release resources none 1
6 Release resources none 1
9 Restart KeepAlive timer KEEPALIVE 6
11 Restart Holdtime timer KEEPALIVE 6
12 Process UPDATE is OK UPDATE 6
Process UPDATE failed NOTIFICATION 1
13 Close transport connection 1
Release resources
others Close transport connection NOTIFICATION 1
Release resources
---------------------------------------------------------------------
The following is a condensed version of the above state transition
table.
Events| Idle | Active | Connect | OpenSent | OpenConfirm | Estab
| (1) | (2) | (3) | (4) | (5) | (6)
|--------------------------------------------------------------
1 | 2 | 2 | 3 | 4 | 5 | 6
| | | | | |
2 | 1 | 1 | 1 | 1 | 1 | 1
| | | | | |
3 | 1 | 4 | 4 | 1 | 1 | 1
| | | | | |
4 | 1 | 1 | 1 | 3 | 1 | 1
| | | | | |
5 | 1 | 3 | 3 | 1 | 1 | 1
| | | | | |
6 | 1 | 1 | 1 | 1 | 1 | 1
| | | | | |
7 | 1 | 2 | 2 | 1 | 1 | 1
| | | | | |
8 | 1 | 1 | 1 | 1 | 1 | 1
| | | | | |
9 | 1 | 1 | 1 | 1 | 5 | 6
| | | | | |
10 | 1 | 1 | 1 | 1 or 5 | 1 | 1
| | | | | |
11 | 1 | 1 | 1 | 1 | 6 | 6
| | | | | |
12 | 1 | 1 | 1 | 1 | 1 | 1 or 6
| | | | | |
13 | 1 | 1 | 1 | 1 | 1 | 1
| | | | | |
---------------------------------------------------------------
Appendix 2. Comparison with RFC1167
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 new attribute,
LOC_PREF, that facilitates route selection procedures.
Appendix 3. Comparison with RFC 1163
All of the changes listed in Appendix 2, plus the following.
To detect and recover from BGP connection collision, a new field (BGP
Identifier) has been added to the OPEN message. New text (Section
6.8) has been added to specify the procedure for detecting and
recovering from collision.
The new document no longer restricts the border router that is passed
in the NEXT_HOP path attribute to be part of the same Autonomous
System as the BGP Speaker.
New document optimizes and simplifies the exchange of the information
about previously reachable routes.
Appendix 4. Comparison with RFC 1105
All of the changes listed in Appendices 2 and 3, plus the following.
Minor changes to the RFC1105 Finite State Machine were necessary to
accommodate the TCP user interface provided by 4.3 BSD.
The notion of Up/Down/Horizontal relations present in RFC1105 has
been removed from the protocol.
The changes in the message format from RFC1105 are as follows:
1. The Hold Time field has been removed from the BGP header and
added to the OPEN message.
2. The version field has been removed from the BGP header and
added to the OPEN message.
3. The Link Type field has been removed from the OPEN message.
4. The OPEN CONFIRM message has been eliminated and replaced
with implicit confirmation provided by the KEEPALIVE message.
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, and
BGP, as specified in this document is referred to as BGP-3.
Appendix 5. TCP options that may be used with BGP
If a local system TCP user interface supports TCP PUSH function, then
each BGP message should be transmitted with PUSH flag set. Setting
PUSH flag forces BGP messages to be transmitted promptly to the
receiver.
If a local system TCP user interface supports setting precedence for
TCP connection, then the BGP transport connection should be opened
with precedence set to Internetwork Control (110) value (see also
[6]).
Appendix 6. Implementation Recommendations
This section presents some implementation recommendations.
6.1 Multiple Networks Per Message
The BGP protocol allows for multiple networks with the same AS path
and next-hop gateway to be specified in one message. Making use of
this capability is highly recommended. With one network 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 flash
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 networks per AS path and gateway
from a routing table that is not organized per AS path is to build
many messages as the routing table is scanned. As each network is
processed, a message for the associated AS path and gateway is
allocated, if it does not exist, and the new network is added to it.
If such a message exists, the new network is just appended to it. If
the message lacks the space to hold the new network, it is
transmitted, a new message is allocated, and the new network 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 networks share a
gateway and common path attributes, making it possible to send many
networks in one 4096-byte message.
When peering with a BGP implementation that does not compress
multiple networks 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 flash updates. This will
eliminate the redundant scanning of the routing table to provide
flash updates for BGP peers and other routing protocols. A
disadvantage of this approach is that it increases the propagation
latency of routing information. By choosing a minimum flash update
interval that is not much greater than the time it takes to process
the multiple messages this latency should be minimized. A better
method would be to read all received messages before sending updates.
6.2 Processing Messages on a Stream Protocol
BGP uses TCP as a transport mechanism. Due to the stream nature of
TCP, all the data for received messages does not necessarily arrive
at the same time. This can make it difficult to process the data as
messages, especially on systems such as BSD Unix where it is not
possible to determine how much data has been received but not yet
processed.
One method that can be used in this situation is to first try to read
just the message header. For the KEEPALIVE message type, this is a
complete message; for other message types, the header should first be
verified, in particular the total length. If all checks are
successful, the specified length, minus the size of the message
header is the amount of data left to read. An implementation that
would "hang" the routing information process while trying to read
from a peer could set up a message buffer (4096 bytes) per peer and
fill it with data as available until a complete message has been
received.
6.3 Reducing route flapping
To avoid excessive route flapping a BGP speaker which needs to
withdraw a destination and send an update about a more specific
or less specific route shall combine them into the same UPDATE
message.
6.4 BGP Timers
BGP employs five timers: ConnectRetry, Holdtime, KeepAlive,
MinRouteOriginationInterval, and MinRouteAdvertisementInterval
Suggested value for the ConnectRetry timer is 120 seconds.
Suggested value for the Holdtime timer is 90 seconds.
Suggested value for the KeepAlive timer is 30 seconds.
Suggested value for the MinRouteOriginationInterval is 15 minutes.
Suggested value for the MinRouteAdvertisementInterval is 30 seconds.
An implementation of BGP shall allow any of these timers to be
configurable.
References
[1] Mills, D., "Exterior Gateway Protocol Formal Specification", RFC
904, BBN, April 1984.
[2] Rekhter, Y., "EGP and Policy Based Routing in the New NSFNET
Backbone", RFC 1092, T.J. Watson Research Center, February 1989.
[3] Braun, H-W., "The NSFNET Routing Architecture", RFC 1093,
MERIT/NSFNET Project, February 1989.
[4] Postel, J., "Transmission Control Protocol - DARPA Internet
Program Protocol Specification", RFC 793, DARPA, September 1981.
[5] Rekhter, Y., and P. Gross, "Application of the Border Gateway
Protocol in the Internet", RFC 1268, T.J. Watson Research Center,
IBM Corp., ANS, October 1991.
[6] Postel, J., "Internet Protocol - DARPA Internet Program Protocol
Specification", RFC 791, DARPA, September 1981.
[7] "Information Processing Systems - Telecommunications and Information
Exchange between Systems - Protocol for Exchange of Inter-domain
Routeing Information among Intermediate Systems to Support Forwarding
of ISO 8473 PDUs", ISO/IEC JTC 1/SC 6 N7196, March 1992.
[8] Fuller, V., Li, T., Yu, J., and Varadhan, K., "Supernetting: an
Address Assignment and Aggregation Strategy", to appear as an
Internet Draft, April 9, 1992.
Security Considerations
Security issues are not discussed in this memo.
Editors' Addresses
Yakov Rekhter
T.J. Watson Research Center IBM Corporation
P.O. Box 218
Yorktown Heights, NY 10598
Phone: (914) 945-3896
email: yakov@watson.ibm.com
Tony Li
cisco Systems, Inc.
1525 O'Brien Drive
Menlo Park, CA 94025
email: tli@cisco.com
- Re: BGP-4 Dennis Ferguson
- Re: BGP-4 Dimitry Haskin
- BGP-4 yakov
- BGP-4 yakov
- BGP-4 yakov
- Re: BGP-4 Tim Symchych
- BGP-4 yakov
- BGP-4 yakov
- Re: BGP-4 Dennis Ferguson
- BGP-4 yakov
- BGP-4 Tony Li
- BGP-4 Tony Li
- Re: BGP-4 Dimitry Haskin
- BGP-4 Tony Li
- Re: BGP-4 Jessica Yu
- Re: BGP-4 Steve Heimlich
- Re: BGP-4 Scott Brim
- Re: BGP-4 Dimitry Haskin
- Re: BGP-4 Dimitry Haskin
- BGP-4 Tony Li
- BGP-4 Tony Li
- Re: BGP-4 Jessica Yu
- Re: BGP-4 Dimitry Haskin
- BGP-4 Tony Li
- Re: BGP-4 Dimitry Haskin
- BGP-4 Tony Li
- BGP-4 Tony Li
- Re: BGP-4 Dimitry Haskin
- Re: BGP-4 Dimitry Haskin
- BGP-4 yakov
- Re: BGP-4 Dimitry Haskin
- BGP-4 yakov