RFC1654 - A Border Gateway Protocol 4 (BGP-4)

Network Working Group Y. Rekhter

Request for Comments: 1654 T.J. Watson Research Center, IBM Corp.

Category: Standards Track T. Li

cisco Systems

Editors

July 1994

A Border Gateway Protocol 4 (BGP-4)

Status of this Memo

This document specifies an Internet standards track protocol for the

Internet community, and requests discussion and suggestions for

improvements. Please refer to the current edition of the "Internet

Official Protocol Standards" (STD 1) for the standardization state

and status of this protocol. Distribution of this memo is unlimited.

1. Acknowledgements

This document was originally published as RFC1267 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 the previous version (BGP-2) of this document.

This team, consisting of Deborah Estrin, Milo Medin, John Moy, Radia

Perlman, Martha Steenstrup, Mike St. Johns, and Paul Tsuchiya, acted

with a strong combination of toughness, professionalism, and

courtesy.

This updated version of the document is the product of the IETF 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 was the IDRP editor within that group. We would also

like to thank Mike Craren (Proteon, Inc.), Dimitry HaSKIN

(Wellfleet), John Krawczyk (Wellfleet), and Paul Traina (cisco) for

their insightful comments.

We would like to specially acknowledge numerous contributions by

Dennis Ferguson (ANS).

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 RFC904 [1] and EGP usage in the NSFNET Backbone as

described in RFC1092 [2] and RFC1093 [3].

The primary function of a BGP speaking system is to exchange network

reachability information with other BGP systems. This network

reachability information includes information on the list of

Autonomous Systems (ASs) that reachability information traverses.

This information is sufficient to construct a graph of AS

connectivity from which routing loops may be pruned and some policy

decisions at the AS level may be enforced.

BGP-4 provides a new set of mechanisms for supporting classless

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, 9].

To characterize the set of policy decisions that can be enforced

using BGP, one must focus on the rule that a BGP speaker advertise to

its peers (other BGP speakers which it communicates with) in

neighboring ASs only those routes that it itself uses. This rule

reflects the "hop-by-hop" routing paradigm generally used throughout

the current Internet. Note that some policies cannot be supported by

the "hop-by-hop" routing paradigm and thus require techniques such as

source routing to enforce. For example, BGP does not enable one AS

to send traffic to a neighboring AS intending that the traffic take a

different route from that taken by traffic originating in the

neighboring AS. On the other hand, BGP can support any policy

conforming to the "hop-by-hop" routing paradigm. Since the current

Internet uses only the "hop-by-hop" 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.

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@ans.net).

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. Similarly, a peer in a

different AS is referred to as an external peer, while a peer in the

same AS may be described as an internal peer.

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 a

peer.

BGP provides mechanisms by which a BGP speaker can inform its peer

that a previously advertised route is no longer available for use.

There are three methods by which a given BGP speaker can indicate

that a route has been withdrawn from service:

a) the IP prefix that expresses 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 peers. The

routing information stored in the Adj-RIBs-Out will be carried in

the local BGP speaker's UPDATE messages and advertised to its

peers.

In summary, the Adj-RIBs-In contain unprocessed routing information

that has been advertised to the local BGP speaker by its peers; the

Loc-RIB contains the routes that have been selected by the local BGP

speaker's Decision Process; and the Adj-RIBs-Out organize the routes

for advertisement to specific peers by means of the local speaker's

UPDATE messages.

Although the conceptual model distinguishes between Adj-RIBs-In,

Loc-RIB, and Adj-RIBs-Out, this neither implies nor requires that an

implementation must maintain three separate copies of the routing

information. The choice of implementation (for example, 3 copies of

the information vs 1 copy with pointers) is not constrained by the

protocol.

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 4.

My Autonomous System:

This 2-octet unsigned integer indicates the Autonomous System

number of the sender.

Hold Time:

This 2-octet unsigned integer indicates the number of seconds

that the sender proposes for the value of the Hold Timer. Upon

receipt of an OPEN message, a BGP speaker MUST calculate the

value of the Hold Timer by using the smaller of its configured

Hold Time and the Hold Time received in the OPEN message. The

Hold Time MUST be either zero or at least three seconds. An

implementation may reject connections on the basis of the Hold

Time. The calculated value indicates the maximum number of

seconds that may elapse between the receipt of successive

KEEPALIVE, and/or UPDATE messages by the sender.

BGP Identifier:

This 4-octet unsigned integer indicates the BGP Identifier of

the sender. A given BGP speaker sets the value of its BGP

Identifier to an IP address assigned to that BGP speaker. The

value of the BGP Identifier is determined on startup and is the

same for every local interface and every BGP peer.

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 to advertise a single feasible route to a

peer, or to withdraw multiple unfeasible routes from service (see

3.1). An UPDATE message may simultaneously advertise a feasible route

and withdraw multiple unfeasible routes from service. The UPDATE

message always includes the fixed-size BGP header, and can optionally

include the other fields as shown below:

+-----------------------------------------------------+

Unfeasible Routes Length (2 octets)

+-----------------------------------------------------+

Withdrawn Routes (variable)

+-----------------------------------------------------+

Total Path Attribute Length (2 octets)

+-----------------------------------------------------+

Path Attributes (variable)

+-----------------------------------------------------+

Network Layer Reachability Information (variable)

+-----------------------------------------------------+

Unfeasible Routes Length:

This 2-octets unsigned integer indicates the total length of

the Withdrawn Routes field in octets. Its value must allow the

length of the Network Layer Reachability Information field to

be determined as specified below.

A value of 0 indicates that no routes are being withdrawn from

service, and that the WITHDRAWN ROUTES field is not present in

this UPDATE message.

Withdrawn Routes:

This is a variable length field that contains a list of IP

address prefixes for the routes that are being withdrawn from

service. Each IP address prefix is encoded as a 2-tuple of the

form <length, prefix>, whose fields are described below:

+---------------------------+

Length (1 octet)

+---------------------------+

Prefix (variable)

+---------------------------+

The use and the meaning of these fields are as follows:

a) Length:

The Length field indicates the length in bits of the IP

address prefix. A length of zero indicates a prefix that

matches all IP addresses (with prefix, itself, of zero

octets).

b) Prefix:

The Prefix field contains IP address prefixes followed by

enough trailing bits to make the end of the field fall on an

octet boundary. Note that the value of trailing bits is

irrelevant.

Total Path Attribute Length:

This 2-octet unsigned integer indicates the total length of the

Path Attributes field in octets. Its value must allow the

length of the Network Layer Reachability field to be determined

as specified below.

A value of 0 indicates that no Network Layer Reachability

Information field is present in this UPDATE message.

Path Attributes:

A variable length sequence of path attributes is present in

every UPDATE. Each path attribute is a triple <attribute type,

attribute length, attribute value> of variable length.

Attribute Type is a two-octet field that consists of the

Attribute Flags octet followed by the Attribute Type Code

octet.

0 1

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Attr. Flags Attr. Type Code

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The high-order bit (bit 0) of the Attribute Flags octet is the

Optional bit. It defines whether the attribute is optional (if

set to 1) or well-known (if set to 0).

The second high-order bit (bit 1) of the Attribute Flags octet

is the Transitive bit. It defines whether an optional

attribute is transitive (if set to 1) or non-transitive (if set

to 0). For well-known attributes, the Transitive bit must be

set to 1. (See Section 5 for a discussion of transitive

attributes.)

The third high-order bit (bit 2) of the Attribute Flags octet

is the Partial bit. It defines whether the information

contained in the optional transitive attribute is partial (if

set to 1) or complete (if set to 0). For well-known attributes

and for optional non-transitive attributes the Partial bit must

be set to 0.

The fourth high-order bit (bit 3) of the Attribute Flags octet

is the Extended Length bit. It defines whether the Attribute

Length is one octet (if set to 0) or two octets (if set to 1).

Extended Length may be used only if the length of the attribute

value is greater than 255 octets.

The lower-order four bits of the Attribute Flags octet are .

unused. They must be zero (and must be ignored when received).

The Attribute Type Code octet contains the Attribute Type Code.

Currently defined Attribute Type Codes are discussed in Section

5.

If the Extended Length bit of the Attribute Flags octet is set

to 0, the third octet of the Path Attribute contains the length

of the attribute data in octets.

If the Extended Length bit of the Attribute Flags octet is set

to 1, then the third and the fourth octets of the path

attribute contain the length of the attribute data in octets.

The remaining octets of the Path Attribute represent the

attribute value and are interpreted according to the Attribute

Flags and the Attribute Type Code. The supported Attribute Type

Codes, their attribute values and uses are the following:

a) ORIGIN (Type Code 1):

ORIGIN is a well-known mandatory attribute that defines the

origin of the path information. The data octet can assume

the following values:

Value Meaning

0 IGP - Network Layer Reachability Information

is interior to the originating AS

1 EGP - Network Layer Reachability Information

learned via EGP

2 INCOMPLETE - Network Layer Reachability

Information learned by some other means

Its usage is defined in 5.1.1

b) AS_PATH (Type Code 2):

AS_PATH is a well-known mandatory attribute that is composed

of a sequence of AS path segments. Each AS path segment is

represented by a triple <path segment type, path segment

length, path segment value>.

The path segment type is a 1-octet long field with the

following values defined:

Value Segment Type

1 AS_SET: unordered set of ASs a route in the

UPDATE message has traversed

2 AS_SEQUENCE: ordered set of ASs a route in

the UPDATE message has traversed

The path segment length is a 1-octet long field containing

the number of ASs in the path segment value field.

The path segment value field contains one or more AS

numbers, each encoded as a 2-octets long field.

Usage of this attribute is defined in 5.1.2.

c) NEXT_HOP (Type Code 3):

This is a well-known mandatory attribute that defines the IP

address of the border router that should be used as the next

hop to the destinations listed in the Network Layer

Reachability field of the UPDATE message.

Usage of this attribute is defined in 5.1.3.

d) MULTI_EXIT_DISC (Type Code 4):

This is an optional non-transitive attribute that is a four

octet non-negative integer. The value of this attribute may

be used by a BGP speaker's decision process to discriminate

among multiple exit points to a neighboring autonomous

system.

Its usage is defined in 5.1.4.

e) LOCAL_PREF (Type Code 5):

LOCAL_PREF is a well-known discretionary attribute that is a

four octet non-negative integer. It is used by a BGP speaker

to inform other BGP speakers in its own autonomous system of

the originating speaker's degree of preference for an

advertised route. Usage of this attribute is described in

5.1.5.

f) ATOMIC_AGGREGATE (Type Code 6)

ATOMIC_AGGREGATE is a well-known discretionary attribute of

length 0. It is used by a BGP speaker to inform other BGP

speakers that the local system selected a less specific

route without selecting a more specific route which is

included in it. Usage of this attribute is described in

5.1.6.

g) AGGREGATOR (Type Code 7)

AGGREGATOR is an optional transitive attribute of length 6.

The attribute contains the last AS number that formed the

aggregate route (encoded as 2 octets), followed by the IP

address of the BGP speaker that formed the aggregate route

(encoded as 4 octets). Usage of this attribute is described

in 5.1.7

Network Layer Reachability Information:

This variable length field contains a list of IP address

prefixes. The length in octets of the Network Layer

Reachability Information is not encoded explicitly, but can be

calculated as:

UPDATE message Length - 23 - Total Path Attributes Length -

Unfeasible Routes Length

where UPDATE message Length is the value encoded in the fixed-

size BGP header, Total Path Attribute Length and Unfeasible

Routes Length are the values encoded in the variable part of

the UPDATE message, and 23 is a combined length of the fixed-

size BGP header, the Total Path Attribute Length field and the

Unfeasible Routes Length field.

Reachability information is encoded as one or more 2-tuples of

the form <length, prefix>, whose fields are described below:

+---------------------------+

Length (1 octet)

+---------------------------+

Prefix (variable)

+---------------------------+

The use and the meaning of these fields are as follows:

a) Length:

The Length field indicates the length in bits of the IP

address prefix. A length of zero indicates a prefix that

matches all IP addresses (with prefix, itself, of zero

octets).

b) Prefix:

The Prefix field contains IP address prefixes followed by

enough trailing bits to make the end of the field fall on an

octet boundary. Note that the value of the trailing bits is

irrelevant.

The minimum length of the UPDATE message is 23 octets -- 19 octets

for the fixed header + 2 octets for the Unfeasible Routes Length + 2

octets for the Total Path Attribute Length (the value of Unfeasible

Routes Length is 0 and the value of Total Path Attribute Length is

0).

An UPDATE message can advertise at most one route, which may be

described by several path attributes. All path attributes contained

in a given UPDATE messages apply to the destinations carried in the

Network Layer Reachability Information field of the UPDATE message.

An UPDATE message can list multiple routes to be withdrawn from

service. Each such route is identified by its destination (expressed

as an IP prefix), which unambiguously identifies the route in the

context of the BGP speaker - BGP speaker connection to which it has

been previously been advertised.

An UPDATE message may advertise only routes to be withdrawn from

service, in which case it will not include path attributes or Network

Layer Reachability Information. Conversely, it may advertise only a

feasible route, in which case the WITHDRAWN ROUTES field need not be

present.

4.4 KEEPALIVE Message Format

BGP does not use any transport protocol-based keep-alive mechanism to

determine if peers are reachable. Instead, KEEPALIVE messages are

exchanged between peers often enough as not to cause the Hold Timer

to expire. A reasonable maximum time between KEEPALIVE messages

would be one third of the Hold Time interval. KEEPALIVE messages

MUST NOT be sent more frequently than one per second. An

implementation MAY adjust the rate at which it sends KEEPALIVE

messages as a function of the Hold Time interval.

If the negotiated Hold Time interval is zero, then periodic KEEPALIVE

messages MUST NOT be sent.

KEEPALIVE message consists of only message header and has a length of

19 octets.

4.5 NOTIFICATION Message Format

A NOTIFICATION message is sent when an error condition is detected.

The BGP connection is closed immediately after sending it.

In addition to the fixed-size BGP header, the NOTIFICATION message

contains the following fields:

0 1 2 3

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Error code Error subcode Data

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Error Code:

This 1-octet unsigned integer indicates the type of

NOTIFICATION. The following Error Codes have been defined:

Error Code Symbolic Name Reference

1 Message Header Error Section 6.1

2 OPEN Message Error Section 6.2

3 UPDATE Message Error Section 6.3

4 Hold Timer Expired Section 6.5

5 Finite State Machine Error Section 6.6

6 Cease Section 6.7

Error subcode:

This 1-octet unsigned integer provides more specific

information about the nature of the reported error. Each Error

Code may have one or more Error Subcodes associated with it.

If no appropriate Error Subcode is defined, then a zero

(Unspecific) value is used for the Error Subcode field.

Message Header Error subcodes:

1 - Connection Not Synchronized.

2 - Bad Message Length.

3 - Bad Message Type.

OPEN Message Error subcodes:

1 - Unsupported Version Number.

2 - Bad Peer AS.

3 - Bad BGP Identifier.

4 - Unsupported Authentication Code.

5 - Authentication Failure.

6 - Unacceptable Hold Time.

UPDATE Message Error subcodes:

1 - Malformed Attribute List.

2 - Unrecognized Well-known Attribute.

3 - Missing Well-known Attribute.

4 - Attribute Flags Error.

5 - Attribute Length Error.

6 - Invalid ORIGIN Attribute

7 - AS Routing Loop.

8 - Invalid NEXT_HOP Attribute.

9 - Optional Attribute Error.

10 - Invalid Network Field.

11 - Malformed AS_PATH.

Data:

This variable-length field is used to diagnose the reason for

the NOTIFICATION. The contents of the Data field depend upon

the Error Code and Error Subcode. See Section 6 below for more

details.

Note that the length of the Data field can be determined from

the message Length field by the formula:

Message Length = 21 + Data Length

The minimum length of the NOTIFICATION message is 21 octets

(including message header).

5. Path Attributes

This section discusses the path attributes of the UPDATE message.

Path attributes fall into four separate categories:

1. Well-known mandatory.

2. Well-known discretionary.

3. Optional transitive.

4. Optional non-transitive.

Well-known attributes must be recognized by all BGP implementations.

Some of these attributes are mandatory and must be included in every

UPDATE message. Others are discretionary and may or may not be sent

in a particular UPDATE message.

All well-known attributes must be passed along (after proper

updating, if necessary) to other BGP peers.

In addition to well-known attributes, each path may contain one or

more optional attributes. It is not required or expected that all

BGP implementations support all optional attributes. The handling of

an unrecognized optional attribute is determined by the setting of

the Transitive bit in the attribute flags octet. Paths with

unrecognized transitive optional attributes should be accepted. If a

path with unrecognized transitive optional attribute is accepted and

passed along to other BGP peers, then the unrecognized transitive

optional attribute of that path must be passed along with the path to

other BGP peers with the Partial bit in the Attribute Flags octet set

to 1. If a path with recognized transitive optional attribute is

accepted and passed along to other BGP peers and the Partial bit in

the Attribute Flags octet is set to 1 by some previous AS, it is not

set back to 0 by the current AS. Unrecognized non-transitive optional

attributes must be quietly ignored and not passed along to other BGP

peers.

New transitive optional attributes may be attached to the path by the

originator or by any other AS in the path. If they are not attached

by the originator, the Partial bit in the Attribute Flags octet is

set to 1. The rules for attaching new non-transitive optional

attributes will depend on the nature of the specific attribute. The

documentation of each new non-transitive optional attribute will be

expected to include such rules. (The description of the

MULTI_EXIT_DISC attribute gives an example.) All optional attributes

(both transitive and non-transitive) may be updated (if appropriate)

by ASs in the path.

The sender of an UPDATE message should order path attributes within

the UPDATE message in ascending order of attribute type. The

receiver of an UPDATE message must be prepared to handle path

attributes within the UPDATE message that are out of order.

The same attribute cannot appear more than once within the Path

Attributes field of a particular UPDATE message.

5.1 Path Attribute Usage

The usage of each BGP path attributes is described in the following

clauses.

5.1.1 ORIGIN

ORIGIN is a well-known mandatory attribute. The ORIGIN attribute

shall be generated by the autonomous system that originates the

associated routing information. It shall be included in the UPDATE

messages of all BGP speakers that choose to propagate this

information to other BGP speakers.

5.1.2 AS_PATH

AS_PATH is a well-known mandatory attribute. This attribute

identifies the autonomous systems through which routing information

carried in this UPDATE message has passed. The components of this

list can be AS_SETs or AS_SEQUENCEs.

When a BGP speaker propagates a route which it has learned from

another BGP speaker's UPDATE message, it shall modify the route's

AS_PATH attribute based on the location of the BGP speaker to which

the route will be sent:

a) When a given BGP speaker advertises the route to another BGP

speaker located in its own autonomous system, the advertising

speaker shall not modify the AS_PATH attribute associated with the

route.

b) When a given BGP speaker advertises the route to a BGP speaker

located in a neighboring autonomous system, then the advertising

speaker shall update the AS_PATH attribute as follows:

1) if the first path segment of the AS_PATH is of type

AS_SEQUENCE, the local system shall prepend its own AS number

as the last element of the sequence (put it in the leftmost

position).

2) if the first path segment of the AS_PATH is of type AS_SET,

the local system shall prepend a new path segment of type

AS_SEQUENCE to the AS_PATH, including its own AS number in that

segment.

When a BGP speaker originates a route then:

a) the originating speaker shall include its own AS number in

the AS_PATH attribute of all UPDATE messages sent to BGP

speakers located in neighboring autonomous systems. (In this

case, the AS number of the originating speaker's autonomous

system will be the only entry in the AS_PATH attribute).

b) the originating speaker shall include an empty AS_PATH

attribute in all UPDATE messages sent to BGP speakers located

in its own autonomous system. (An empty AS_PATH attribute is

one whose length field contains the value zero).

5.1.3 NEXT_HOP

The NEXT_HOP path attribute defines the IP address of the border

router that should be used as the next hop to the 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.

A BGP speaker must never advertise an address of a peer to that peer

as a NEXT_HOP, for a route that the speaker is originating. A BGP

speaker must never install a route with itself as the next hop.

When a BGP speaker advertises the route to a BGP speaker located in

its own autonomous system, the advertising speaker shall not modify

the NEXT_HOP attribute associated with the route. When a BGP speaker

receives the route via an internal link, it may forward packets to

the NEXT_HOP address if the address contained in the attribute is on

a common subnet with the local and remote BGP speakers.

5.1.4 MULTI_EXIT_DISC

The MULTI_EXIT_DISC attribute may be used on external (inter-AS)

links to discriminate among multiple exit or entry points to the same

neighboring AS. The value of the MULTI_EXIT_DISC attribute is a four

octet unsigned number which is called a metric. All other factors

being equal, the exit or entry point with lower metric should be

preferred. If received over external links, the MULTI_EXIT_DISC

attribute may be propagated over internal links to other BGP speakers

within the same AS. The MULTI_EXIT_DISC attribute is never

propagated to other BGP speakers in neighboring AS's.

5.1.5 LOCAL_PREF

LOCAL_PREF is a well-known discretionary attribute that shall be

included in all UPDATE messages that a given BGP speaker sends to the

other BGP speakers located in its own autonomous system. A BGP

speaker shall calculate the degree of preference for each external

route and include the degree of preference when advertising a route

to its internal peers. The higher degree of preference should be

preferred. A BGP speaker shall use the degree of preference learned

via LOCAL_PREF in its decision process (see section 9.1.1).

A BGP speaker shall not include this attribute in UPDATE messages

that it sends to BGP speakers located in a neighboring autonomous

system. If it is contained in an UPDATE message that is received from

a BGP speaker which is not located in the same autonomous system as

the receiving speaker, then this attribute shall be ignored by the

receiving speaker.

5.1.6 ATOMIC_AGGREGATE

ATOMIC_AGGREGATE is a well-known discretionary attribute. If a BGP

speaker, when presented with a set of overlapping routes from one of

its peers (see 9.1.4), selects the less specific route without

selecting the more specific one, then the local system shall attach

the ATOMIC_AGGREGATE attribute to the route when propagating it to

other BGP speakers (if that attribute is not already present in the

received less specific route). A BGP speaker that receives a route

with the ATOMIC_AGGREGATE attribute shall not remove the attribute

from the route when propagating it to other speakers. A BGP speaker

that receives a route with the ATOMIC_AGGREGATE attribute shall not

make any NLRI of that route more specific (as defined in 9.1.4) when

advertising this route to other BGP speakers. A BGP speaker that

receives a route with the ATOMIC_AGGREGATE attribute needs to be

cognizant of the fact that the actual path to destinations, as

specified in the NLRI of the route, while having the loop-free

property, may traverse ASs that are not listed in the AS_PATH

attribute.

5.1.7 AGGREGATOR

AGGREGATOR is an optional transitive attribute which may be included

in updates which are formed by aggregation (see Section 9.2.4.2). A

BGP speaker which performs route aggregation may add the AGGREGATOR

attribute which shall contain its own AS number and IP address.

6. BGP Error Handling.

This section describes actions to be taken when errors are detected

while processing BGP messages.

When any of the conditions described here are detected, a

NOTIFICATION message with the indicated Error Code, Error Subcode,

and Data fields is sent, and the BGP connection is closed. If no

Error Subcode is specified, then a zero must be used.

The phrase "the BGP connection is closed" means that the transport

protocol connection has been closed and that all resources for that

BGP connection have been deallocated. Routing table entries

associated with the remote peer are marked as invalid. The fact that

the routes have become invalid is passed to other BGP peers before

the routes are deleted from the system.

Unless specified explicitly, the Data field of the NOTIFICATION

message that is sent to indicate an error is empty.

6.1 Message Header error handling.

All errors detected while processing the Message Header are indicated

by sending the NOTIFICATION message with Error Code Message Header

Error. The Error Subcode elaborates on the specific nature of the

error.

The expected value of the Marker field of the message header is all

ones if the message type is OPEN. The expected value of the Marker

field for all other types of BGP messages determined based on the

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 Hold Time field of the OPEN message is unacceptable, then the

Error Subcode MUST be set to Unacceptable Hold Time. An

implementation MUST reject Hold Time values of one or two seconds.

An implementation MAY reject any proposed Hold Time. An

implementation which accepts a Hold Time MUST use the negotiated

value for the Hold Time.

If the BGP Identifier field of the OPEN message is syntactically

incorrect, then the Error Subcode is set to Bad BGP Identifier.

Syntactic correctness means that the BGP Identifier field represents

a valid IP host address.

If 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 Unfeasible Routes Length or Total Attribute

Length is too large (i.e., if Unfeasible Routes Length + Total

Attribute Length + 23 exceeds the message Length), then the Error

Subcode is set to Malformed Attribute List.

If any recognized attribute has Attribute Flags that conflict with

the Attribute Type Code, then the Error Subcode is set to Attribute

Flags Error. The Data field contains the erroneous attribute (type,

length and value).

If any recognized attribute has Attribute Length that conflicts with

the expected length (based on the attribute type code), then the

Error Subcode is set to Attribute Length Error. The Data field

contains the erroneous attribute (type, length and value).

If any of the mandatory well-known attributes are not present, then

the Error Subcode is set to Missing Well-known Attribute. The Data

field contains the Attribute Type Code of the missing well-known

attribute.

If any of the mandatory well-known attributes are not recognized,

then the Error Subcode is set to Unrecognized Well-known Attribute.

The Data field contains the unrecognized attribute (type, length and

value).

If the ORIGIN attribute has an undefined value, then the Error

Subcode is set to Invalid Origin Attribute. The Data field contains

the unrecognized attribute (type, length and value).

If the NEXT_HOP attribute field is syntactically incorrect, then the

Error Subcode is set to Invalid NEXT_HOP Attribute. The Data field

contains the incorrect attribute (type, length and value). Syntactic

correctness means that the NEXT_HOP attribute represents a valid IP

host address. Semantic correctness applies only to the external BGP

links. It means that the interface associated with the IP address, as

specified in the NEXT_HOP attribute, shares a common subnet with the

receiving BGP speaker and is not the IP address of the receiving BGP

speaker. If the NEXT_HOP attribute is semantically incorrect, the

error should be logged, and the the route should be ignored. In this

case, no NOTIFICATION message should be sent.

The AS_PATH attribute is checked for syntactic correctness. If the

path is syntactically incorrect, then the Error Subcode is set to

Malformed AS_PATH.

If an optional attribute is recognized, then the value of this

attribute is checked. If an error is detected, the attribute is

discarded, and the Error Subcode is set to Optional Attribute Error.

The Data field contains the attribute (type, length and value).

If any attribute appears more than once in the UPDATE message, then

the Error Subcode is set to Malformed Attribute List.

The NLRI field in the UPDATE message is checked for syntactic

validity. If the field is syntactically incorrect, then the Error

Subcode is set to Invalid Network Field.

6.4 NOTIFICATION message error handling.

If a peer sends a NOTIFICATION message, and there is an error in that

message, there is unfortunately no means of reporting this error via

a subsequent NOTIFICATION message. Any such error, such as an

unrecognized Error Code or Error Subcode, should be noticed, logged

locally, and brought to the attention of the administration of the

peer. The means to do this, however, lies outside the scope of this

document.

6.5 Hold Timer Expired error handling.

If a system does not receive successive KEEPALIVE and/or UPDATE

and/or NOTIFICATION messages within the period specified in the Hold

Time field of the OPEN message, then the NOTIFICATION message with

Hold Timer Expired Error Code must be sent and the BGP connection

closed.

6.6 Finite State Machine error handling.

Any error detected by the BGP Finite State Machine (e.g., receipt of

an unexpected event) is indicated by sending the NOTIFICATION message

with Error Code Finite State Machine Error.

6.7 Cease.

In absence of any fatal errors (that are indicated in this section),

a BGP peer may choose at any given time to close its BGP connection

by sending the NOTIFICATION message with Error Code Cease. However,

the Cease NOTIFICATION message must not be used when a fatal error

indicated by this section does exist.

6.8 Connection collision detection.

If a pair of BGP speakers try simultaneously to establish a TCP

connection to each other, then two parallel connections between this

pair of speakers might well be formed. We refer to this situation as

connection collision. Clearly, one of these connections must be

closed.

Based on the value of the BGP Identifier a convention is established

for detecting which BGP connection is to be preserved when a

collision does occur. The convention is to compare the BGP

Identifiers of the peers involved in the collision and to retain only

the connection initiated by the BGP speaker with the higher-valued

BGP Identifier.

Upon receipt of an OPEN message, the local system must examine all of

its connections that are in the OpenConfirm state. A BGP speaker may

also examine connections in an OpenSent state if it knows the BGP

Identifier of the peer by means outside of the protocol. If among

these connections there is a connection to a remote BGP speaker whose

BGP Identifier equals the one in the OPEN message, then the local

system performs the following collision resolution procedure:

1. The BGP Identifier of the local system is compared to the BGP

Identifier of the remote system (as specified in the OPEN

message).

2. If the value of the local BGP Identifier is less than the

remote one, the local system closes BGP connection that already

exists (the one that is already in the OpenConfirm state), and

accepts BGP connection initiated by the remote system.

3. Otherwise, the local system closes newly created BGP connection

(the one associated with the newly received OPEN message), and

continues to use the existing one (the one that is already in the

OpenConfirm state).

Comparing BGP Identifiers is done by treating them as (4-octet

long) unsigned integers.

A connection collision with an existing BGP connection that is in

Established states causes unconditional closing of the newly

created connection. Note that a connection collision cannot be

detected with connections that are in Idle, or Connect, or Active

states.

Closing the BGP connection (that results from the collision

resolution procedure) is accomplished by sending the NOTIFICATION

message with the Error Code Cease.

7. BGP Version Negotiation.

BGP speakers may negotiate the version of the protocol by making

multiple attempts to open a BGP connection, starting with the highest

version number each supports. If an open attempt fails with an Error

Code OPEN Message Error, and an Error Subcode Unsupported Version

Number, then the BGP speaker has available the version number it

tried, the version number its peer tried, the version number passed

by its peer in the NOTIFICATION message, and the version numbers that

it supports. If the two peers do support one or more common

versions, then this will allow them to rapidly determine the highest

common version. In order to support BGP version negotiation, future

versions of BGP must retain the format of the OPEN and NOTIFICATION

messages.

8. BGP Finite State machine.

This section specifies BGP operation in terms of a Finite State

Machine (FSM). Following is a brief summary and overview of BGP

operations by state as determined by this FSM. A condensed version

of the BGP FSM is found in Appendix 1.

Initially BGP is in the Idle state.

Idle state:

In this state BGP refuses all incoming BGP connections. No

resources are allocated to the peer. In response to the Start

event (initiated by either system or operator) the local system

initializes all BGP resources, starts the ConnectRetry timer,

initiates a transport connection to other BGP peer, while

listening for connection that may be initiated by the remote

BGP peer, and changes its state to Connect. The exact value of

the ConnectRetry timer is a local matter, but should be

sufficiently large to allow TCP initialization.

If a BGP speaker detects an error, it shuts down the connection

and changes its state to Idle. Getting out of the Idle state

requires generation of the Start event. If such an event is

generated automatically, then persistent BGP errors may result

in persistent flapping of the speaker. To avoid such a

condition it is recommended that Start events should not be

generated immediately for a peer that was previously

transitioned to Idle due to an error. For a peer that was

previously transitioned to Idle due to an error, the time

between consecutive generation of Start events, if such events

are generated automatically, shall exponentially increase. The

value of the initial timer shall be 60 seconds. The time shall

be doubled for each consecutive retry.

Any other event received in the Idle state is ignored.

Connect state:

In this state BGP is waiting for the transport protocol

connection to be completed.

If the transport protocol connection succeeds, the local system

clears the ConnectRetry timer, completes initialization, sends

an OPEN message to its peer, and changes its state to OpenSent.

If the transport protocol connect fails (e.g., retransmission

timeout), the local system restarts the ConnectRetry timer,

continues to listen for a connection that may be initiated by

the remote BGP peer, and changes its state to Active state.

In response to the ConnectRetry timer expired event, the local

system restarts the ConnectRetry timer, initiates a transport

connection to other BGP peer, continues to listen for a

connection that may be initiated by the remote BGP peer, and

stays in the Connect state.

Start event is ignored in the Active state.

In response to any other event (initiated by either system or

operator), the local system releases all BGP resources

associated with this connection and changes its state to Idle.

Active state:

In this state BGP is trying to acquire a peer by initiating a

transport protocol connection.

If the transport protocol connection succeeds, the local system

clears the ConnectRetry timer, completes initialization, sends

an OPEN message to its peer, sets its Hold Timer to a large

value, and changes its state to OpenSent. A Hold Timer value

of 4 minutes is suggested.

In response to the ConnectRetry timer expired event, the local

system restarts the ConnectRetry timer, initiates a transport

connection to other BGP peer, continues to listen for a

connection that may be initiated by the remote BGP peer, and

changes its state to Connect.

If the local system detects that a remote peer is trying to

establish BGP connection to it, and the IP address of the

remote peer is not an expected one, the local system restarts

the ConnectRetry timer, rejects the attempted connection,

continues to listen for a connection that may be initiated by

the remote BGP peer, and stays in the Active state.

Start event is ignored in the Active state.

In response to any other event (initiated by either system or

operator), the local system releases all BGP resources

associated with this connection and changes its state to Idle.

OpenSent state:

In this state BGP waits for an OPEN message from its peer.

When an OPEN message is received, all fields are checked for

correctness. If the BGP message header checking or OPEN

message checking detects an error (see Section 6.2), or a

connection collision (see Section 6.8) the local system sends a

NOTIFICATION message and changes its state to Idle.

If there are no errors in the OPEN message, BGP sends a

KEEPALIVE message and sets a KeepAlive timer. The Hold Timer,

which was originally set to a large value (see above), is

replaced with the negotiated Hold Time value (see section 4.2).

If the negotiated Hold Time value is zero, then the Hold Time

timer and KeepAlive timers are not started. If the value of

the Autonomous System field is the same as the local Autonomous

System number, then the connection is an "internal" connection;

otherwise, it is "external". (This will effect UPDATE

processing as described below.) Finally, the state is changed

to OpenConfirm.

If a disconnect notification is received from the underlying

transport protocol, the local system closes the BGP connection,

restarts the ConnectRetry timer, while continue listening for

connection that may be initiated by the remote BGP peer, and

goes into the Active state.

If the Hold Timer expires, the local system sends NOTIFICATION

message with error code Hold Timer Expired and changes its

state to Idle.

In response to the Stop event (initiated by either system or

operator) the local system sends NOTIFICATION message with

Error Code Cease and changes its state to Idle.

Start event is ignored in the OpenSent state.

In response to any other event the local system sends

NOTIFICATION message with Error Code Finite State Machine Error

and changes its state to Idle.

Whenever BGP changes its state from OpenSent to Idle, it closes

the BGP (and transport-level) connection and releases all

resources associated with that connection.

OpenConfirm state:

In this state BGP waits for a KEEPALIVE or NOTIFICATION

message.

If the local system receives a KEEPALIVE message, it changes

its state to Established.

If the Hold Timer expires before a KEEPALIVE message is

received, the local system sends NOTIFICATION message with

error code Hold Timer Expired and changes its state to Idle.

If the local system receives a NOTIFICATION message, it changes

its state to Idle.

If the KeepAlive timer expires, the local system sends a

KEEPALIVE message and restarts its KeepAlive timer.

If a disconnect notification is received from the underlying

transport protocol, the local system changes its state to Idle.

In response to the Stop event (initiated by either system or

operator) the local system sends NOTIFICATION message with

Error Code Cease and changes its state to Idle.

Start event is ignored in the OpenConfirm state.

In response to any other event the local system sends

NOTIFICATION message with Error Code Finite State Machine Error

and changes its state to Idle.

Whenever BGP changes its state from OpenConfirm to Idle, it

closes the BGP (and transport-level) connection and releases

all resources associated with that connection.

Established state:

In the Established state BGP can exchange UPDATE, NOTIFICATION,

and KEEPALIVE messages with its peer.

If the local system receives an UPDATE or KEEPALIVE message, it

restarts its Hold Timer, if the negotiated Hold Time value is

non-zero.

If the local system receives a NOTIFICATION message, it changes

its state to Idle.

If the local system receives an UPDATE message and the UPDATE

message error handling procedure (see Section 6.3) detects an

error, the local system sends a NOTIFICATION message and

changes its state to Idle.

If a disconnect notification is received from the underlying

transport protocol, the local system changes its state to Idle.

If the Hold Timer expires, the local system sends a

NOTIFICATION message with Error Code Hold Timer Expired and

changes its state to Idle.

If the KeepAlive timer expires, the local system sends a

KEEPALIVE message and restarts its KeepAlive timer.

Each time the local system sends a KEEPALIVE or UPDATE message,

it restarts its KeepAlive timer, unless the negotiated Hold

Time value is zero.

In response to the Stop event (initiated by either system or

operator), the local system sends a NOTIFICATION message with

Error Code Cease and changes its state to Idle.

Start event is ignored in the Established state.

In response to any other event, the local system sends

NOTIFICATION message with Error Code Finite State Machine Error

and changes its state to Idle.

Whenever BGP changes its state from Established to Idle, it

closes the BGP (and transport-level) connection, releases all

resources associated with that connection, and deletes all

routes derived from that connection.

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 included (see

9.1.4) in 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 all peers;

the selected routes will be stored in the local speaker's Adj-RIB-

Out.

The selection process is formalized by defining a function that takes

the attribute of a given route as an argument and returns 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

neighboring 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 a

neighboring 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 peer

located in a neighboring 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 peer located in a

neighboring 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 determine a degree of preference. If the route is

learned from a BGP speaker in the local autonomous system, either the

value of the LOCAL_PREF attribute shall be taken as the degree of

preference, or the local system shall compute the degree of

preference of the route based on preconfigured policy information. If

the route is learned from a BGP speaker in a neighboring autonomous

system, then the degree of preference shall be computed based on

preconfigured policy information. The exact nature of this policy

information and the computation involved is a local matter. The

local speaker shall then 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 neighboring 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.

If the NEXT_HOP attribute of a BGP route depicts an address to which

the local BGP speaker doesn't have a route in its Loc-RIB, the BGP

route SHOULD be excluded from the Phase 2 decision function.

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. The local speaker MUST determine the immediate

next hop to the address depicted by the NEXT_HOP attribute of the

selected route by performing a lookup in the IGP and selecting one of

the possible paths in the IGP. This immediate next hop MUST be used

when installing the selected route in the Loc-RIB. If the route to

the address depicted by the NEXT_HOP attribute changes such that the

immediate next hop changes, route selection should be recalculated as

specified above.

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

neighboring autonomous systems, and those received from other BGP

speakers located in the local speaker's autonomous system.

The following tie-breaking procedure assumes that for each candidate

route all the BGP speakers within an autonomous system can ascertain

the cost of a path (interior distance) to the address depicted by the

NEXT_HOP attribute of the route. Ties shall be broken according to

the following algorithm:

a) If the local system is configured to take into account

MULTI_EXIT_DISC, and the candidate routes differ in their

MULTI_EXIT_DISC attribute, select the route that has the

lowest value of the MULTI_EXIT_DISC attribute.

b) Otherwise, select the route that has the lowest cost

(interior distance) to the entity depicted by the NEXT_HOP

attribute of the route. If there are several routes with the

same cost, then the tie-breaking shall be broken as follows:

- if at least one of the candidate routes was advertised by

the BGP speaker in a neighboring autonomous system, select

the route that was advertised by the BGP speaker in a

neighboring autonomous system whose BGP Identifier has the

lowest value among all other BGP speakers in neighboring

autonomous systems;

- otherwise, 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.

For the benefit of future support of inter-AS multicast capabilities,

a BGP speaker that participates in inter-AS multicast routing shall

advertise a route it receives from one of its external peers and if

it installs it in its Loc-RIB, it shall advertise it back to the peer

from which the route was received. For a BGP speaker that does not

participate in inter-AS multicast routing such an advertisement is

optional. When doing such an advertisement, the NEXT_HOP attribute

should be set to the address of the peer. An implementation may also

optimize such an advertisement by truncating information in the

AS_PATH attribute to include only its own AS number and that of the

peer that advertised the route (such truncation requires the ORIGIN

attribute to be set to INCOMPLETE). In addition an implementation is

not required to pass optional or discretionary path attributes with

such an advertisement.

When the updating of the Adj-RIBs-Out and the Forwarding Information

Base (FIB) 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).

The precedence relationship effectively decomposes less specific

routes into two parts:

- a set of destinations described only by the less specific

route, and

- a set of destinations described by the overlap of the less

specific and the more specific routes

When overlapping routes are present in the same Adj-RIB-In, the more

specific route shall take precedence, in order from more specific to

least specific.

The set of destinations described by the overlap represents a portion

of the less specific route that is feasible, but is not currently in

use. If a more specific route is later withdrawn, the set of

destinations described by the overlap will still be reachable using

the less specific route.

If a BGP speaker receives overlapping routes, the Decision Process

shall take into account the semantics of the overlapping routes. In

particular, if a BGP speaker accepts the less specific route while

rejecting the more specific route from the same peer, then the

destinations represented by the overlap may not forward along the ASs

listed in the AS_PATH attribute of 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 (that implies de-aggregation)

d) Aggregate the two routes and install the aggregated route

e) Install the less specific route only

f) Install neither route

If a BGP speaker chooses e), then it should add ATOMIC_AGGREGATE

attribute to the route. A route that carries ATOMIC_AGGREGATE

attribute can not be de-aggregated. That is, the NLRI of this route

can not be made more specific. Forwarding along such a route does

not guarantee that IP packets will actually traverse only ASs listed

in the AS_PATH attribute of the route. If a BGP speaker chooses a),

it must not advertise the more general route without the more

specific route.

9.2 Update-Send Process

The Update-Send process is responsible for advertising UPDATE

messages to all peers. For example, it distributes the routes chosen

by the Decision Process to other BGP speakers which may be located in

either the same autonomous system or a neighboring autonomous system.

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 a

neighboring 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 neighboring autonomous systems, or

2) there are no other routes that have been received from BGP

speakers in neighboring 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 (the tie-breaking procedure

is specified in 9.2.1.1).

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 peer 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

neighboring autonomous systems, there will be multiple Adj-RIBs-In

associated with these peers. These Adj-RIBs-In might contain several

equally preferable routes to the same destination, all of which were

advertised by BGP speakers located in neighboring 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) If the local system can ascertain the cost of a path to the

entity depicted by the NEXT_HOP attribute of the candidate route,

select the route with the lowest cost.

c) 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 neighboring 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

neighboring 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 neighboring autonomous systems must be separated by at

least MinRouteAdvertisementInterval. Clearly, this can only be

achieved precisely by keeping a separate timer for each common set of

destinations. This would be unwarranted overhead. Any technique which

ensures that the interval between two UPDATE messages sent from a

single BGP speaker that advertise feasible routes to some common set

of destinations received from BGP speakers in neighboring autonomous

systems will be at least MinRouteAdvertisementInterval, and will also

ensure a constant upper bound on the interval is acceptable.

Since fast convergence is needed within an autonomous system, this

procedure does not apply for routes 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 MinASOriginationInterval, Keepalive, and

MinRouteAdvertisementInterval. A given BGP speaker shall apply the

same jitter to each of these quantities regardless of the

destinations to which the updates are being sent; that is, jitter

will not be applied on a "per peer" basis.

The amount of jitter to be introduced shall be determined by

multiplying the base value of the appropriate timer by a random

factor which is uniformly distributed in the range from 0.75 to 1.0.

9.2.4 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 cases 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.

For the purpose of aggregating AS_PATH attributes we model each AS

within the AS_PATH attribute as a tuple <type, value>, where

"type" identifies a type of the path segment the AS belongs to

(e.g., AS_SEQUENCE, AS_SET), and "value" is the AS number. If the

routes to be aggregated have different AS_PATH attributes, then

the aggregated AS_PATH attribute shall satisfy all of the

following conditions:

- all tuples of the type AS_SEQUENCE in the aggregated AS_PATH

shall appear in all of the AS_PATH in the initial set of routes

to be aggregated.

- all tuples of the type AS_SET in the aggregated AS_PATH shall

appear in at least one of the AS_PATH in the initial set (they

may appear as either AS_SET or AS_SEQUENCE types).

- for any tuple X of the type AS_SEQUENCE in the aggregated

AS_PATH which precedes tuple Y in the aggregated AS_PATH, X

precedes Y in each AS_PATH in the initial set which contains Y,

regardless of the type of Y.

- No tuple with the same value shall appear more than once in

the aggregated AS_PATH, regardless of the tuple's type.

An implementation may choose any algorithm which conforms to these

rules. At a minimum a conformant implementation shall be able to

perform the following algorithm that meets all of the above

conditions:

- determine the longest leading sequence of tuples (as defined

above) common to all the AS_PATH attributes of the routes to be

aggregated. Make this sequence the leading sequence of the

aggregated AS_PATH attribute.

- set the type of the rest of the tuples from the AS_PATH

attributes of the routes to be aggregated to AS_SET, and append

them to the aggregated AS_PATH attribute.

- if the aggregated AS_PATH has more than one tuple with the

same value (regardless of tuple's type), eliminate all, but one

such tuple by deleting tuples of the type AS_SET from the

aggregated AS_PATH attribute.

Appendix 6, section 6.8 presents another algorithm that satisfies

the conditions and allows for more complex policy configurations.

ATOMIC_AGGREGATE: If at least one of the routes to be aggregated

has ATOMIC_AGGREGATE path attribute, then the aggregated route

shall have this attribute as well.

AGGREGATOR: All AGGREGATOR attributes of all routes to be

aggregated should be ignored.

9.3 Route Selection Criteria

Generally speaking, additional rules for comparing routes among

several alternatives are outside the scope of this document. There

are two exceptions:

- If the local AS appears in the AS path of the new route being

considered, then that new route cannot be viewed as better than

any other route. 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.

9.4 Originating BGP routes

A BGP speaker may originate BGP routes by injecting routing

information acquired by some other means (e.g., via an IGP) into BGP.

A BGP speaker that originates BGP routes shall assign the degree of

preference to these routes by passing them through the Decision

Process (see Section 9.1). These routes may also be distributed to

other BGP speakers within the local AS as part of the Internal update

process (see Section 9.2.1). The decision whether to distribute non-

BGP acquired routes within an AS via BGP or not depends on the

environment within the AS (e.g., type of IGP) and should be

controlled via configuration.

Appendix 1. 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 when the negotiated Hold Time value is non-zero.

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 - Hold 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 Hold 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 Hold 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 Connect Active 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 RFC1267

BGP-4 is capable of operating in an environment where a set of

reachable destinations may be expressed via a single IP prefix. The

concept of network classes, or subnetting is foreign to BGP-4. To

accommodate these capabilities BGP-4 changes semantics and encoding

associated with the AS_PATH attribute. New text has been added to

define semantics associated with IP prefixes. These abilities allow

BGP-4 to support the proposed supernetting scheme [9].

To simplify configuration this version introduces a new attribute,

LOCAL_PREF, that facilitates route selection procedures.

The INTER_AS_METRIC attribute has been renamed to be MULTI_EXIT_DISC.

A new attribute, ATOMIC_AGGREGATE, has been introduced to insure that

certain aggregates are not de-aggregated. Another new attribute,

AGGREGATOR, can be added to aggregate routes in order to advertise

which AS and which BGP speaker within that AS caused the aggregation.

To insure that Hold Timers are symmetric, the Hold Time is now

negotiated on a per-connection basis. Hold Times of zero are now

supported.

Appendix 3. Comparison with RFC1163

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 RFC1105

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 RFC1105, is referred

to as BGP-1, BGP, as specified in RFC1163, is referred to as

BGP-2, BGP, as specified in RFC1267 is referred to as BGP-3, and

BGP, as specified in this document is referred to as BGP-4.

Appendix 5. TCP options that may be used with BGP

If a local system TCP user interface supports TCP PUSH function, then

each BGP message should be transmitted with PUSH flag set. Setting

PUSH flag forces BGP messages to be transmitted promptly to the

receiver.

If a local system TCP user interface supports setting precedence for

TCP connection, then the BGP transport connection should be opened

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 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 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, Hold Time, KeepAlive,

MinASOriginationInterval, and MinRouteAdvertisementInterval The

suggested value for the ConnectRetry timer is 120 seconds. The

suggested value for the Hold Time is 90 seconds. The suggested value

for the KeepAlive timer is 30 seconds. The suggested value for the

MinASOriginationInterval is 15 seconds. The suggested value for the

MinRouteAdvertisementInterval is 30 seconds.

An implementation of BGP MUST allow these timers to be configurable.

6.5 Path attribute ordering

Implementations which combine update messages as described above in

6.1 may prefer to see all path attributes presented in a known order.

This permits them to quickly identify sets of attributes from

different update messages which are semantically identical. To

facilitate this, it is a useful optimization to order the path

attributes according to type code. This optimization is entirely

optional.

6.6 AS_SET sorting

Another useful optimization that can be done to simplify this

situation is to sort the AS numbers found in an AS_SET. This

optimization is entirely optional.

6.7 Control over version negotiation

Since BGP-4 is capable of carrying aggregated routes which cannot be

properly represented in BGP-3, an implementation which supports BGP-4

and another BGP version should provide the capability to only speak

BGP-4 on a per-peer basis.

6.8 Complex AS_PATH aggregation

An implementation which chooses to provide a path aggregation

algorithm which retains significant amounts of path information may

wish to use the following procedure:

For the purpose of aggregating AS_PATH attributes of two routes,

we model each AS as a tuple <type, value>, where "type" identifies

a type of the path segment the AS belongs to (e.g., AS_SEQUENCE,

AS_SET), and "value" is the AS number. Two ASs are said to be the

same if their corresponding <type, value> tuples are the same.

The algorithm to aggregate two AS_PATH attributes works as

follows:

a) Identify the same ASs (as defined above) within each AS_PATH

attribute that are in the same relative order within both

AS_PATH attributes. Two ASs, X and Y, are said to be in the

same order if either:

- X precedes Y in both AS_PATH attributes, or - Y precedes X

in both AS_PATH attributes.

b) The aggregated AS_PATH attribute consists of ASs identified

in (a) in exactly the same order as they appear in the AS_PATH

attributes to be aggregated. If two consecutive ASs identified

in (a) do not immediately follow each other in both of the

AS_PATH attributes to be aggregated, then the intervening ASs

(ASs that are between the two consecutive ASs that are the

same) in both attributes are combined into an AS_SET path

segment that consists of the intervening ASs from both AS_PATH

attributes; this segment is then placed in between the two

consecutive ASs identified in (a) of the aggregated attribute.

If two consecutive ASs identified in (a) immediately follow

each other in one attribute, but do not follow in another, then

the intervening ASs of the latter are combined into an AS_SET

path segment; this segment is then placed in between the two

consecutive ASs identified in (a) of the aggregated attribute.

If as a result of the above procedure a given AS number appears

more than once within the aggregated AS_PATH attribute, all, but

the last instance (rightmost occurrence) of that AS number should

be removed from the aggregated AS_PATH attribute.

References

[1] Mills, D., "Exterior Gateway Protocol Formal Specification", STD

18, RFC904, BBN, April 1984.

[2] Rekhter, Y., "EGP and Policy Based Routing in the New NSFNET

Backbone", RFC1092, T.J. Watson Research Center, February 1989.

[3] Braun, H-W., "The NSFNET Routing Architecture", RFC1093,

MERIT/NSFNET Project, February 1989.

[4] Postel, J., "Transmission Control Protocol - DARPA Internet

Program Protocol Specification", RFC793, DARPA, September 1981.

[5] Rekhter, Y., and P. Gross, "Application of the Border Gateway

Protocol in the Internet", T.J. Watson Research Center, IBM

Corp., ANS, RFC1655, T.J. Watson Research Center, MCI, July

1994.

[6] Postel, J., "Internet Protocol - DARPA Internet Program Protocol

Specification", STD 5, RFC791, 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 IS10747, 1993

[8] Fuller, V., Li, T., Yu, J., and K. Varadhan, "Classless Inter-

Domain Routing (CIDR): an Address Assignment and Aggregation

Strategy", RFC1519, BARRNet, cisco, MERIT, OARnet, September

1993.

[9] Rekhter, Y., and T. Li, "An Architecture for IP Address

Allocation with CIDR", RFC1518, T.J. Watson Research Center,

cisco, September 1993.

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

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