RFC 4271：BGP-4：互联网路由协议标准与功能概述

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"RFC 4271《边界网关协议版本4 (BGP-4)》" 《边界网关协议版本4 (BGP-4)》是一个互联网标准跟踪协议，由Network Working Group发布，其主要作者包括Y. Rekhter、T. Li等。这份文档在2006年1月发布，取代了之前的RFC 1771，旨在为互联网社区提供一个可靠的自治系统间路由协议（Inter-Autonomous System routing protocol）。 BGP-4的核心功能是网络可达性信息的交换，这些信息涵盖了可达性所经过的自治系统（ASes）列表。通过这种信息，BGP系统能够构建一个AS连接图，从而检测并避免路由环路。此外，BGP在AS级别上支持一定程度的策略控制，例如通过配置路由选择策略和联盟（peering）关系来管理流量。协议的设计目标是保证路由的稳定性和效率，确保数据包能够在互联网的不同部分之间正确、有效地传输。BGP-4采用TCP作为其传输层协议，以保证数据传输的可靠性和顺序。它采用了路径矢量路由选择算法，其中每个路由条目包含了一段指向目的地的路径，每个AS在其路径中都有一个标识符，这使得路由器能够识别和选择最佳路径。 RFC 4271对BGP的路由更新机制、公认属性（well-known attributes）、路由反射器（route reflector）和路由聚合（route summarization）进行了详细规定。此外，还包括了路径属性的分类和验证机制，如团体属性（community attribute）和下一跳属性（next-hop attribute），这些属性在路由选择和路由策略中扮演着关键角色。该文档强调了协议的标准化状态，并鼓励社群参与讨论和提出改进建议。由于BGP-4的重要性和广泛应用，其广泛的分发意味着所有网络服务提供商和设备制造商都应参考此标准以确保他们的网络服务与全球互联网保持兼容和高效。《BGP-4规范》是现代互联网路由架构的基础之一，对于理解网络间路由策略的制定和实施，以及保持互联网的稳定和可扩展性具有重要意义。随着互联网的发展和技术的进步，BGP-4虽然已经发布多年，但仍持续受到维护和更新，以适应不断变化的网络环境。

RFC 4271

BGP-4 January 2006

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

a) Length:

The Length field indicates the length in bits of the IP

address prefix. A length of zero indicates a prefix that

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

octets).

b) Prefix:

The Prefix field contains an IP address prefix, followed by

the minimum number of trailing bits needed to make the end

of the field fall on an octet boundary. Note that the value

of trailing bits is irrelevant.

Total Path Attribute Length:

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

Path Attributes field in octets. Its value allows the length

of the Network Layer Reachability field to be determined as

specified below.

A value of 0 indicates that neither the Network Layer

Reachability Information field nor the Path Attribute field is

present in this UPDATE message.

Path Attributes:

A variable-length sequence of path attributes is present in

every UPDATE message, except for an UPDATE message that carries

only the withdrawn routes. Each path attribute is a triple

<attribute type, attribute length, attribute value> of variable

length.

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

Attribute Flags octet, followed by the Attribute Type Code

octet.

0 1

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

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

| Attr. Flags |Attr. Type Code|

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

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

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

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

Rekhter, et al. Standards Track [Page 16]

RFC 4271

BGP-4 January 2006

The remaining octets of the Path Attribute represent the

attribute value and are interpreted according to the Attribute

Flags and the Attribute Type Code. The supported Attribute

Type Codes, and their attribute values and uses are as follows:

a) ORIGIN (Type Code 1):

ORIGIN is a well-known mandatory attribute that defines the

origin of the path information. The data octet can assume

the following values:

Value Meaning

0 IGP - Network Layer Reachability Information

is interior to the originating AS

1 EGP - Network Layer Reachability Information

learned via the EGP protocol [

RFC904]

2 INCOMPLETE - Network Layer Reachability

Information learned by some other means

Usage of this attribute is defined in 5.1.1.

b) AS_PATH (Type Code 2):

AS_PATH is a well-known mandatory attribute that is composed

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

represented by a triple <path segment type, path segment

length, path segment value>.

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

following values defined:

Value Segment Type

1 AS_SET: unordered set of ASes a route in the

UPDATE message has traversed

2 AS_SEQUENCE: ordered set of ASes a route in

the UPDATE message has traversed

The path segment length is a 1-octet length field,

containing the number of ASes (not the number of octets) in

the path segment value field.

The path segment value field contains one or more AS

numbers, each encoded as a 2-octet length field.

Rekhter, et al. Standards Track [Page 18]

RFC 4271

BGP-4 January 2006

Usage of this attribute is defined in 5.1.2.

c) NEXT_HOP (Type Code 3):

This is a well-known mandatory attribute that defines the

(unicast) IP address of the router that SHOULD be used as

the next hop to the destinations listed in the Network Layer

Reachability Information field of the UPDATE message.

Usage of this attribute is defined in 5.1.3.

d) MULTI_EXIT_DISC (Type Code 4):

This is an optional non-transitive attribute that is a

four-octet unsigned integer. The value of this attribute

MAY be used by a BGP speaker’s Decision Process to

discriminate among multiple entry points to a neighboring

autonomous system.

Usage of this attribute is defined in 5.1.4.

e) LOCAL_PREF (Type Code 5):

LOCAL_PREF is a well-known attribute that is a four-octet

unsigned integer. A BGP speaker uses it to inform its other

internal peers of the advertising speaker’s degree of

preference for an advertised route.

Usage of this attribute is defined in 5.1.5.

f) ATOMIC_AGGREGATE (Type Code 6)

ATOMIC_AGGREGATE is a well-known discretionary attribute of

length 0.

Usage of this attribute is defined in 5.1.6.

g) AGGREGATOR (Type Code 7)

AGGREGATOR is an optional transitive attribute of length 6.

The attribute contains the last AS number that formed the

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

address of the BGP speaker that formed the aggregate route

(encoded as 4 octets). This SHOULD be the same address as

the one used for the BGP Identifier of the speaker.

Usage of this attribute is defined in 5.1.7.

Rekhter, et al. Standards Track [Page 19]

RFC 4271

BGP-4 January 2006

Network Layer Reachability Information:

This variable length field contains a list of IP address

prefixes. The length, in octets, of the Network Layer

Reachability Information is not encoded explicitly, but can be

calculated as:

UPDATE message Length - 23 - Total Path Attributes Length

- Withdrawn Routes Length

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

size BGP header, Total Path Attribute Length, and Withdrawn

Routes Length are the values encoded in the variable part of

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

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

Withdrawn Routes Length field.

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

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

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

| Length (1 octet) |

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

| Prefix (variable) |

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

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

a) Length:

The Length field indicates the length in bits of the IP

address prefix. A length of zero indicates a prefix that

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

octets).

b) Prefix:

The Prefix field contains an IP address prefix, followed by

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

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

irrelevant.

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

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

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

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

0).

Rekhter, et al. Standards Track [Page 20]

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