Network Working Group K. Lougheed

Network Working Group K. Lougheed
Request for Comments: 1163 cisco Systems
Obsoletes: RFC 1105 Y. Rekhter
T.J. Watson Research Center, IBM Corp
June 1990

A Border Gateway Protocol (BGP)

Status of this Memo

This RFC, together with its companion RFC-1164, "Application of the
Border Gateway Protocol in the Internet", define a Proposed Standard
for an inter-autonomous system routing protocol for the Internet.

This protocol, like any other at this initial stage, may undergo
modifications before reaching full Internet Standard status as a
result of deployment experience. Implementers are encouraged to
track the progress of this or any protocol as it moves through the
standardization process, and to report their own experience with the
protocol.

This protocol is being considered by the Interconnectivity Working
Group (IWG) of the Internet Engineering Task Force (IETF).
Information about the progress of BGP can be monitored and/or
reported on the IWG mailing list ([email protected]).

Please refer to the latest edition of the "IAB Official Protocol
Standards" RFC for current information on the state and status of
standard Internet protocols.

Distribution of this memo is unlimited.

Table of Contents

1. Acknowledgements...................................... 2
2. Introduction.......................................... 2
3. Summary of Operation.................................. 4
4. Message Formats....................................... 5
4.1 Message Header Format................................. 5
4.2 OPEN Message Format................................... 6
4.3 UPDATE Message Format................................. 8
4.4 KEEPALIVE Message Format.............................. 10
4.5 NOTIFICATION Message Format........................... 10
5. Path Attributes....................................... 12
6. BGP Error Handling.................................... 14
6.1 Message Header error handling......................... 14
6.2 OPEN message error handling........................... 15

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6.3 UPDATE message error handling......................... 16
6.4 NOTIFICATION message error handling................... 17
6.5 Hold Timer Expired error handling..................... 17
6.6 Finite State Machine error handling................... 18
6.7 Cease................................................. 18
7. BGP Version Negotiation............................... 18
8. BGP Finite State machine.............................. 18
9. UPDATE Message Handling............................... 22
10. Detection of Inter-AS Policy Contradictions........... 23
Appendix 1. BGP FSM State Transitions and Actions........ 25
Appendix 2. Comparison with RFC 1105..................... 28
Appendix 3. TCP options that may be used with BGP........ 28
References................................................ 29
Security Considerations................................... 29
Authors' Addresses........................................ 29

1. Acknowledgements

We would like to express our thanks to Guy Almes (Rice University),
Len Bosack (cisco Systems), Jeffrey C. Honig (Cornell Theory Center)
and all members of the Interconnectivity Working Group of the
Internet Engineering Task Force, chaired by Guy Almes, for their
contributions to this document.

We would also like to thank Bob Hinden, Director for Routing of the
Internet Engineering Steering Group, and the team of reviewers he
assembled to review earlier versions of this document. This team,
consisting of Deborah Estrin, Milo Medin, John Moy, Radia Perlman,
Martha Steenstrup, Mike St. Johns, and Paul Tsuchiya, acted with a
strong combination of toughness, professionalism, and courtesy.

2. Introduction

The Border Gateway Protocol (BGP) is an inter-Autonomous System
routing protocol. It is built on experience gained with EGP as
defined in RFC 904 [1] and EGP usage in the NSFNET Backbone as
described in RFC 1092 [2] and RFC 1093 [3].

The primary function of a BGP speaking system is to exchange network
reachability information with other BGP systems. This network
reachability information includes information on the full path of
Autonomous Systems (ASs) that traffic must transit to reach these
networks. This information is sufficient to construct a graph of AS
connectivity from which routing loops may be pruned and some policy
decisions at the AS level may be enforced.

To characterize the set of policy decisions that can be enforced
using BGP, one must focus on the rule that an AS advertize to its

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neighbor ASs only those routes that it itself uses. This rule
reflects the "hop-by-hop" routing paradigm generally used throughout
the current Internet. Note that some policies cannot be supported by
the "hop-by-hop" routing paradigm and thus require techniques such as
source routing to enforce. For example, BGP does not enable one AS
to send traffic to a neighbor AS intending that that traffic take a
different route from that taken by traffic originating in the
neighbor AS. On the other hand, BGP can support any policy
conforming to the "hop-by-hop" routing paradigm. Since the current
Internet uses only the "hop-by-hop" routing paradigm and since BGP
can support any policy that conforms to that paradigm, BGP is highly
applicable as an inter-AS routing protocol for the current Internet.

A more complete discussion of what policies can and cannot be
enforced with BGP is outside the scope of this document (but refer to
the companion document discussing BGP usage [5]).

BGP runs over a reliable transport protocol. This eliminates the
need to implement explicit update fragmentation, retransmission,
acknowledgement, and sequencing. Any authentication scheme used by
the transport protocol may be used in addition to BGP's own
authentication mechanisms. The error notification mechanism used in
BGP assumes that the transport protocol supports a "graceful" close,
i.e., that all outstanding data will be delivered before the
connection is closed.

BGP uses TCP [4] as its transport protocol. TCP meets BGP's
transport requirements and is present in virtually all commercial
routers and hosts. In the following descriptions the phrase
"transport protocol connection" can be understood to refer to a TCP
connection. BGP uses TCP port 179 for establishing its connections.

This memo uses the term `Autonomous System' (AS) throughout. The
classic definition of an Autonomous System is a set of routers under
a single technical administration, using an interior gateway protocol
and common metrics to route packets within the AS, and using an
exterior gateway protocol to route packets to other ASs. Since this
classic definition was developed, it has become common for a single
AS to use several interior gateway protocols and sometimes several
sets of metrics within an AS. The use of the term Autonomous System
here stresses the fact that, even when multiple IGPs and metrics are
used, the administration of an AS appears to other ASs to have a
single coherent interior routing plan and presents a consistent
picture of what networks are reachable through it. From the
standpoint of exterior routing, an AS can be viewed as monolithic:
reachability to networks directly connected to the AS must be
equivalent from all border gateways of the AS.

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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.

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.

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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

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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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Auth. Code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Authentication Data |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Version:

This 1-octet unsigned integer indicates the protocol version
number of the message. The current BGP version number is 2.

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My Autonomous System:

This 2-octet unsigned integer indicates the Autonomous System
number of the sender.

Hold Time:

This 2-octet unsigned integer indicates the maximum number of
seconds that may elapse between the receipt of successive
KEEPALIVE and/or UPDATE and/or NOTIFICATION messages.

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 = 25 + Authentication Data Length

The minimum length of the OPEN message is 25 octets (including
message header).

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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.

In addition to the fixed-size BGP header, the UPDATE message contains
the following fields (note that all fields may have arbitrary
alignment):

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Total Path Attributes Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
/ Path Attributes /
/ /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Network 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ /
/ /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Network n |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Total Path Attribute Length:

This 2-octet unsigned integer indicates the total length of the
Path Attributes field in octets. Its value must allow the (non-
negative integer) number of Network fields to be determined as
specified below.

Path Attributes:

A variable length sequence of path attributes is present in every
UPDATE. Each path attribute is a triple <attribute type,
attribute length, attribute value> of variable length.

Attribute Type is a two-octet field that consists of the Attribute
Flags octet followed by the Attribute Type Code octet.

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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 should 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 meaning and handling of Path Attributes is discussed in

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Section 5.

Network:

Each 4-octet Internet network number indicates one network whose
Inter-Autonomous System routing is described by the Path
Attributes. Subnets and host addresses are specifically not
allowed. The total number of Network fields in the UPDATE message
can be determined by the formula:

Message Length = 19 + Total Path Attribute Length + 4 * #Nets

The message Length field of the message header and the Path
Attributes Length field of the UPDATE message must be such that
the formula results in a non-negative integer number of Network
fields.

The minimum length of the UPDATE message is 37 octets (including
message header).

4.4 KEEPALIVE Message Format

BGP does not use any transport protocol-based keep-alive mechanism to
determine if peers are reachable. Instead, KEEPALIVE messages are
exchanged between peers often enough as not to cause the hold time
(as advertised in the OPEN message) to expire. A reasonable maximum
time between KEEPALIVE messages would be one third of the Hold Time
interval.

KEEPALIVE message consists of only message header and has a length of
19 octets.

4.5 NOTIFICATION Message Format

A NOTIFICATION message is sent when an error condition is detected.
The BGP connection is closed immediately after sending it.

In addition to the fixed-size BGP header, the NOTIFICATION message
contains the following fields:

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Error code | Error subcode | Data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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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 - Unsupported Authentication Code.
4 - Authentication Failure.

UPDATE Message Error subcodes:

1 - Malformed Attribute List.
2 - Unrecognized Well-known Attribute.
3 - Missing Well-known Attribute.
4 - Attribute Flags Error.
5 - Attribute Length Error.
6 - Invalid ORIGIN Attribute
7 - AS Routing Loop.
8 - Invalid NEXT_HOP Attribute.
9 - Optional Attribute Error.
10 - Invalid Network Field.

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Data:

This variable-length field is used to diagnose the reason for the
NOTIFICATION. The contents of the Data field depend upon the
Error Code and Error Subcode. See Section 6 below for more
details.

Note that the length of the Data field can be determined from the
message Length field by the formula:

Message Length = 21 + Data Length

The minimum length of the NOTIFICATION message is 21 octets
(including message header).

5. Path Attributes

This section discusses the path attributes of the UPDATE message.

Path attributes fall into four separate categories:

1. Well-known mandatory.
2. Well-known discretionary.
3. Optional transitive.
4. Optional non-transitive.

Well-known attributes must be recognized by all BGP implementations.
Some of these attributes are mandatory and must be included in every
UPDATE message. Others are discretionary and may or may not be sent
in a particular UPDATE message. Which well-known attributes are
mandatory or discretionary is noted in the table below.

All well-known attributes must be passed along (after proper
updating, if necessary) to other BGP peers.

In addition to well-known attributes, each path may contain one or
more optional attributes. It is not required or expected that all
BGP implementations support all optional attributes. The handling of
an unrecognized optional attribute is determined by the setting of
the Transitive bit in the attribute flags octet. Unrecognized
transitive optional attributes should be accepted and passed along to
other BGP peers. If a path with unrecognized transitive optional
attribute is accepted and passed along to other BGP peers, the
Partial bit in the Attribute Flags octet is 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 should

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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 INTER-AS
METRIC attribute gives an example.) All optional attributes (both
transitive and non-transitive) may be updated (if appropriate) by ASs
in the path.

The order of attributes within the Path Attributes field of a
particular UPDATE message is irrelevant.

The same attribute cannot appear more than once within the Path
Attributes field of a particular UPDATE message.

Following table specifies attribute type code, attribute length, and
attribute category for path attributes defined in this document:

Attribute Name Type Code Length Attribute category
ORIGIN 1 1 well-known, mandatory
AS_PATH 2 variable well-known, mandatory
NEXT_HOP 3 4 well-known, mandatory
UNREACHABLE 4 0 well-known, discretionary
INTER-AS METRIC 5 2 optional, non-transitive

ORIGIN:

The ORIGIN path attribute defines the origin of the path
information. The data octet can assume the following values:

Value Meaning
0 IGP - network(s) are interior to the originating AS
1 EGP - network(s) learned via EGP
2 INCOMPLETE - network(s) learned by some other means

AS_PATH:

The AS_PATH attribute enumerates the ASs that must be traversed to
reach the networks listed in the UPDATE message. Since an AS
identifier is 2 octets, the length of an AS_PATH attribute is
twice the number of ASs in the path. Rules for constructing an
AS_PATH attribute are discussed in Section 9.

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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. This border router must belong to the same
AS as the BGP peer that advertises it.

UNREACHABLE:

The UNREACHABLE attribute is used to notify a BGP peer that some
of the previously advertised routes have become unreachable.

INTER-AS METRIC:

The INTER-AS METRIC attribute may be used on external (inter-AS)
links to discriminate between multiple exit or entry points to the
same neighboring AS. The value of the INTER-AS METRIC attribute
is a 2-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 INTER-
AS METRIC attribute may be propagated over internal links to other
BGP speaker within the same AS. The INTER-AS METRIC attribute is
never propagated to other BGP speakers in neighboring AS's.

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 should 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

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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 Authentication Code of the OPEN message is not recognized,
then the Error Subcode is set to Unsupported Authentication Code.

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If the Authentication Code is zero, then the Authentication Data must
be of zero length. Otherwise, the Error Subcode is set to
Authentication Failure.

If the Authentication Code is non-zero, then the corresponding
authentication procedure is invoked. If the authentication procedure
(based on Authentication Code and Authentication Data) fails, then
the Error Subcode is set to Authentication Failure.

6.3 UPDATE message error handling.

All errors detected while processing the UPDATE message are indicated
by sending the NOTIFICATION message with Error Code UPDATE Message
Error. The error subcode elaborates on the specific nature of the
error.

Error checking of an UPDATE message begins by examining the path
attributes. If the Total Attribute Length is too large (i.e., if
Total Attribute Length + 21 exceeds the message Length), or if the
(non-negative integer) Number of Network fields cannot be computed as
in Section 4.3, then the Error Subcode is set to Malformed Attribute
List.

If any recognized attribute has Attribute Flags that conflict with
the Attribute Type Code, then the Error Subcode is set to Attribute
Flags Error. The Data field contains the erroneous attribute (type,
length and value).

If any recognized attribute has Attribute Length that conflicts with
the expected length (based on the attribute type code), then the
Error Subcode is set to Attribute Length Error. The Data field
contains the erroneous attribute (type, length and value).

If any of the mandatory well-known attributes are not present, then
the Error Subcode is set to Missing Well-known Attribute. The Data
field contains the Attribute Type Code of the missing well-known
attribute.

If any of the mandatory well-known attributes are not recognized,
then the Error Subcode is set to Unrecognized Well-known Attribute.
The Data field contains the unrecognized attribute (type, length and
value).

If the ORIGIN attribute has an undefined value, then the Error
Subcode is set to Invalid Origin Attribute. The Data field contains
the unrecognized attribute (type, length and value).

If the NEXT_HOP attribute field is syntactically incorrect, then the

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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.

The AS route specified by the AS_PATH attribute is checked for AS
loops. AS loop detection is done by scanning the full AS route (as
specified in the AS_PATH attribute) and checking that each AS occurs
at most once. If a loop is detected, then the Error Subcode is set
to AS Routing Loop. The Data field contains the incorrect attribute
(type, length and value).

If an optional attribute is recognized, then the value of this
attribute is checked. If an error is detected, the attribute is
discarded, and the Error Subcode is set to Optional Attribute Error.
The Data field contains the attribute (type, length and value).

If any attribute appears more than once in the UPDATE message, then
the Error Subcode is set to Malformed Attribute List.

Each Network field in the UPDATE message is checked for syntactic
validity. If the Network field is syntactically incorrect, or
contains a subnet or a host address, then the Error Subcode is set to
Invalid Network Field.

6.4 NOTIFICATION message error handling.

If a peer sends a NOTIFICATION message, and there is an error in that
message, there is unfortunately no means of reporting this error via
a subsequent NOTIFICATION message. Any such error, such as an
unrecognized Error Code or Error Subcode, should be noticed, logged
locally, and brought to the attention of the administration of the
peer. The means to do this, however, lies outside the scope of this
document.

6.5 Hold Timer Expired error handling.

If a system does not receive successive KEEPALIVE and/or UPDATE
and/or NOTIFICATION messages within the period specified in the Hold
Time field of the OPEN message, then the NOTIFICATION message with
Hold Timer Expired Error Code should be sent and the BGP connection
closed.

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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 should not be used when a fatal error
indicated by this section does exist.

7. BGP Version Negotiation.

BGP speakers may negotiate the version of the protocol by making
multiple attempts to open a BGP connection, starting with the highest
version number each supports. If an open attempt fails with an Error
Code OPEN Message Error, and an Error Subcode Unsupported Version
Number, then the BGP speaker has available the version number it
tried, the version number its peer tried, the version number passed
by its peer in the NOTIFICATION message, and the version numbers that
it supports. If the two peers do support one or more common
versions, then this will allow them to rapidly determine the highest
common version. In order to support BGP version negotiation, future
versions of BGP must retain the format of the OPEN and NOTIFICATION
messages.

8. BGP Finite State machine.

This section specifies BGP operation in terms of a Finite State
Machine (FSM). Following is a brief summary and overview of BGP
operations by state as determined by this FSM. A condensed version
of the BGP FSM is found in Appendix 1.

Initially BGP is in the Idle state.

Idle state:

In this state BGP refuses all incoming BGP connections. No
resources are allocated to the BGP neighbor. In response to
the Start event (initiated by either system or operator) the
local system initializes all BGP resources, starts the
ConnectRetry timer, initiates a transport connection to other
BGP peer, while listening for connection that may be initiated
by the remote BGP peer, and changes its state to Connect.

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The exact value of the ConnectRetry timer is a local matter,
but should be sufficiently large to allow TCP initialization.

Any other event received in the Idle state is ignored.

Connect state:

In this state BGP is waiting for the transport protocol
connection to be completed.

If the transport protocol connection succeeds, the local system
clears the ConnectRetry timer, completes initialization, sends
an OPEN message to its peer, and changes its state to OpenSent.

If the transport protocol connect fails (e.g., retransmission
timeout), the local system restarts the ConnectRetry timer,
continues to listen for a connection that may be initiated by
the remote BGP peer, and changes its state to Active state.

In response to the ConnectRetry timer expired event, the local
system restarts the ConnectRetry timer, initiates a transport
connection to other BGP peer, continues to listen for a
connection that may be initiated by the remote BGP peer, and
stays in the Connect state.

Start event is ignored in the Active state.

In response to any other event (initiated by either system or
operator), the local system releases all BGP resources
associated with this connection and changes its state to Idle.

Active state:

In this state BGP is trying to acquire a BGP neighbor by
initiating a transport protocol connection.

If the transport protocol connection succeeds, the local system
clears the ConnectRetry timer, completes initialization, sends
an OPEN message to its peer, sets its hold timer to a large
value, and changes its state to OpenSent.

In response to the ConnectRetry timer expired event, the local
system restarts the ConnectRetry timer, initiates a transport
connection to other BGP peer, continues to listen for a
connection that may be be initiated by the remote BGP peer, and
changes its state to Connect.

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

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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), the local
system sends a NOTIFICATION message and changes its state to
Idle.

If there are no errors in the OPEN message, BGP sends a
KEEPALIVE message and sets a KeepAlive timer. The hold timer,
which was originally set to an arbitrary large value (see
above), is replaced with the value indicated in the OPEN
message. If the value of the Autonomous System field is the
same as our own , then the connection is "internal" connection;
otherwise, it is "external". (This will effect UPDATE
processing as described below.) Finally, the state is changed
to OpenConfirm.

If a disconnect notification is received from the underlying
transport protocol, the local system closes the BGP connection,
restarts the ConnectRetry timer, while continue listening for
connection that may be initiated by the remote BGP peer, and
goes into the Active state.

If the hold time expires, the local system sends NOTIFICATION
message with error code Hold Timer Expired and changes its
state to Idle.

In response to the Stop event (initiated by either system or
operator) the local system sends NOTIFICATION message with
Error Code Cease and changes its state to Idle.

Start event is ignored in the OpenSent state.

In response to any other event the local system sends

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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

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restarts its Holdtime timer.

If the local system receives a NOTIFICATION message, it changes
its state to Idle.

If the local system receives an UPDATE message and the UPDATE
message error handling procedure (see Section 6.3) detects an
error, the local system sends a NOTIFICATION message and
changes its state to Idle.

If a disconnect notification is received from the underlying
transport protocol, the local system changes its state to
Idle.

If the Holdtime timer expires, the local system sends a
NOTIFICATION message with Error Code Hold Timer Expired and
changes its state to Idle.

If the KeepAlive timer expires, the local system sends a
KEEPALIVE message and restarts its KeepAlive timer.

Each time the local system sends a KEEPALIVE or UPDATE message,
it restarts its KeepAlive timer.

In response to the Stop event (initiated by either system or
operator), the local system sends a NOTIFICATION message with
Error Code Cease and changes its state to Idle.

Start event is ignored in the Established state.

In response to any other event, the local system sends
NOTIFICATION message with Error Code Finite State Machine Error
and changes its state to Idle.

Whenever BGP changes its state from Established to Idle, it
closes the BGP (and transport-level) connection, releases all
resources associated with that connection, and deletes all
routes derived from that connection.

9. UPDATE Message Handling

An UPDATE message may be received only in the Established state.
When an UPDATE message is received, each field is checked for
validity as specified in Section 6.3.

If an optional non-transitive attribute is unrecognized, it is
quietly ignored. If an optional transitive attribute is
unrecognized, the Partial bit (the third high-order bit) in the

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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 network and the path attributes associated with a route to
that network are correct, then the route is compared with other
routes to the same network. If the new route is better than the
current one, then it is propagated via an UPDATE message to adjacent
BGP speakers as follows:

- If a route in the UPDATE was received over an internal link, it is
not propagated over any other internal link. This restriction is
due to the fact that all BGP speakers within a single AS form a
completely connected graph (see above).

- If the UPDATE message is propagated over an external link, then the
local AS number is prepended to the AS_PATH attribute, and the
NEXT_HOP attribute is updated with an IP address of the router that
should be used as a next hop to the network. If the UPDATE message
is propagated over an internal link, then the AS_PATH attribute is
passed unmodified and the NEXT_HOP attribute is replaced with the
sender's own IP address.

Generally speaking, the rules for comparing routes among several
alternatives are outside the scope of this document. There are two
exceptions:

- If the local AS appears in the AS path of the new route being
considered, then that new route cannot be viewed as better than any
other route. If such a route were ever used, a routing loop would
result.

- In order to achieve successful distributed operation, only routes
with a likelihood of stability can be chosen. Thus, an AS must
avoid using unstable routes, and it must not make rapid spontaneous
changes to its choice of route. Quantifying the terms "unstable"
and "rapid" in the previous sentence will require experience, but
the principle is clear.

10. Detection of Inter-AS Policy Contradictions

Since BGP requires no central authority for coordinating routing
policies among ASs, and since routing policies are not exchanged via
the protocol itself, it is possible for a group of ASs to have a set

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of routing policies that cannot simultaneously be satisfied. This
may cause an indefinite oscillation of the routes in this group of
ASs.

To help detect such a situation, all BGP speakers must observe the
following rule. If a route to a destination that is currently used
by the local system is determined to be unreachable (e.g., as a
result of receiving an UPDATE message for this route with the
UNREACHABLE attribute), then, before switching to another route, this
local system must advertize this route as unreachable to all the BGP
neighbors to which it previously advertized this route.

This rule will allow other ASs to distinguish between two different
situations:

- The local system has chosen to use a new route because the old
route become unreachable.

- The local system has chosen to use a new route because it preferred
it over the old route. The old route is still viable.

In the former case, an UPDATE message with the UNREACHABLE attribute
will be received for the old route. In the latter case it will not.

In some cases, this may allow a BGP speaker to detect the fact that
its policies, taken together with the policies of some other AS,
cannot simultaneously be satisfied. For example, consider the
following situation involving AS A and its neighbor AS B. B
advertises a route with a path of the form <B,...>, where A is not
present in the path. A then decides to use this path, and advertises
<A,B,...> to all its neighbors. B later advertises <B,...,A,...>
back to A, without ever declaring its previous path <B,...> to be
unreachable. Evidently, A prefers routes via B and B prefers routes
via A. The combined policies of A and B, taken together, cannot be
satisfied. Such an event should be noticed, logged locally, and
brought to the attention of AS A's administration. The means to do
this, however, lies outside the scope of this document. Also outside
the document is a more complete procedure for detecting such
contradictions of policy.

While the above rules provide a mechanism to detect a set of routing
policies that cannot be satisfied simultaneously, the protocol itself
does not provide any mechanism for suppressing the route oscillation
that may result from these unsatisfiable policies. The reason for
doing this is that routing policies are viewed as external to the
protocol and as determined by the local AS administrator.

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Appendix 1. BGP FSM State Transitions and Actions.

This Appendix discusses the transitions between states in the BGP FSM
in response to BGP events. The following is the list of these states
and events.

BGP States:

1 - Idle
2 - Connect
3 - Active
4 - OpenSent
5 - OpenConfirm
6 - Established

BGP Events:

1 - BGP Start
2 - BGP Stop
3 - BGP Transport connection open
4 - BGP Transport connection closed
5 - BGP Transport connection open failed
6 - BGP Transport fatal error
7 - ConnectRetry timer expired
8 - Holdtime timer expired
9 - KeepAlive timer expired
10 - Receive OPEN message
11 - Receive KEEPALIVE message
12 - Receive UPDATE messages
13 - Receive NOTIFICATION message

The following table describes the state transitions of the BGP FSM
and the actions triggered by these transitions.

Event Actions Message Sent Next State
--------------------------------------------------------------------
Idle (1)
1 Initialize resources none 2
Start ConnectRetry timer
Initiate a transport connection
others none none 1

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Connect(2)
1 none none 2
3 Complete initialization OPEN 4
Clear ConnectRetry timer
5 Restart ConnectRetry timer none 3
7 Restart ConnectRetry timer none 2
Initiate a transport connection
others Release resources none 1

Active (3)
1 none none 3
3 Complete initialization OPEN 4
Clear ConnectRetry timer
5 Close connection 3
Restart ConnectRetry timer
7 Restart ConnectRetry timer none 2
Initiate a transport connection
others Release resources none 1

OpenSent(4)
1 none none 4
4 Close transport connection none 3
Restart ConnectRetry timer
6 Release resources none 1
10 Process OPEN is OK KEEPALIVE 5
Process OPEN failed NOTIFICATION 1
others Close transport connection NOTIFICATION 1
Release resources

OpenConfirm (5)
1 none none 5
4 Release resources none 1
6 Release resources none 1
9 Restart KeepAlive timer KEEPALIVE 5
11 Complete initialization none 6
Restart Holdtime timer
13 Close transport connection 1
Release resources
others Close transport connection NOTIFICATION 1
Release resources

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Established (6)
1 none none 6
4 Release resources none 1
6 Release resources none 1
9 Restart KeepAlive timer KEEPALIVE 6
11 Restart Holdtime timer KEEPALIVE 6
12 Process UPDATE is OK UPDATE 6
Process UPDATE failed NOTIFICATION 1
13 Close transport connection 1
Release resources
others Close transport connection NOTIFICATION 1
Release resources
---------------------------------------------------------------------

The following is a condensed version of the above state transition
table.

Events| Idle | Active | Connect | OpenSent | OpenConfirm | Estab
| (1) | (2) | (3) | (4) | (5) | (6)
|--------------------------------------------------------------
1 | 2 | 2 | 3 | 4 | 5 | 6
| | | | | |
2 | 1 | 1 | 1 | 1 | 1 | 1
| | | | | |
3 | 1 | 4 | 4 | 1 | 1 | 1
| | | | | |
4 | 1 | 1 | 1 | 3 | 1 | 1
| | | | | |
5 | 1 | 3 | 3 | 1 | 1 | 1
| | | | | |
6 | 1 | 1 | 1 | 1 | 1 | 1
| | | | | |
7 | 1 | 2 | 2 | 1 | 1 | 1
| | | | | |
8 | 1 | 1 | 1 | 1 | 1 | 1
| | | | | |
9 | 1 | 1 | 1 | 1 | 5 | 6
| | | | | |
10 | 1 | 1 | 1 | 1 or 5 | 1 | 1
| | | | | |
11 | 1 | 1 | 1 | 1 | 6 | 6
| | | | | |
12 | 1 | 1 | 1 | 1 | 1 | 1 or 6
| | | | | |
13 | 1 | 1 | 1 | 1 | 1 | 1
| | | | | |
---------------------------------------------------------------

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Appendix 2. Comparison with RFC 1105

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.

Appendix 3. 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]).

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References

[1] Mills, D., "Exterior Gateway Protocol Formal Specification", RFC
904, BBN, April 1984.

[2] Rekhter, Y., "EGP and Policy Based Routing in the New NSFNET
Backbone", RFC 1092, T.J. Watson Research Center, February 1989.

[3] Braun, H-W., "The NSFNET Routing Architecture", RFC 1093,
MERIT/NSFNET Project, February 1989.

[4] Postel, J., "Transmission Control Protocol - DARPA Internet
Program Protocol Specification", RFC 793, DARPA, September 1981.

[5] Honig, J., Katz, D., Mathis, M., Rekhter, Y., and J. Yu,
"Application of the Border Gateway Protocol in the Internet",
RFC 1164, Cornell University Theory Center, Merit/NSFNET,
Pittsburgh Supercomputing Center, IBM, Merit/NSFNET, June 1990.

[6] Postel, J., "Internet Protocol - DARPA Internet Program Protocol
Specification", RFC 791, DARPA, September 1981.

Security Considerations

Security issues are not discussed in this memo.

Authors' Addresses

Kirk Lougheed
cisco Systems, Inc.
1525 O'Brien Drive
Menlo Park, CA 94025

Phone: (415) 326-1941

Email: [email protected]

Yakov Rekhter
T.J. Watson Research Center IBM Corporation
P.O. Box 218
Yorktown Heights, NY 10598

Phone: (914) 945-3896

Email: [email protected]

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