BGP协议格式记录

[toc]

BGP协议主体格式

BGP协议主要由Header和BGP Message Types组成

Message Header Format

Header结构为Marker（16字节）、Length（2字节）、Type（1字节）组成
=> RFC4271 Sec 4.1

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     |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type存在以下状态：

1 - OPEN
2 - UPDATE
3 - NOTIFICATION
4 - KEEPALIVE
5 - ROUTE-REFRESH
6 - DYNAMIC CAPABILITY

P.S Type: 5 - ROUTE-REFRESH出自 RFC2918 Sec 3.
Type: 6 - DYNAMIC CAPABILITY出自 draft-ietf-idr-dynamic-cap-16 Sec 3.

OPEN Message Format

Open结构为Version（1字节）、My Autonomous System（2字节）、Hold Time（2字节）、BGP Identifier（4字节，BGP IP）、Optional Parameters Length（1字节）、Optional Parameters（变长）
=> RFC4271 Sec 4.2

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                        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Opt Parm Len  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                                                               |
|             Optional Parameters (variable)                    |
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
0                   1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...
|  Parm. Type   | Parm. Length  |  Parameter Value (variable)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...

Optional Parameters内部结构为1个或多个<Parameter Type, Parameter Length, Parameter Value>三元组（1字节，1字节，x字节）

This field contains a list of optional parameters, in which each parameter is encoded as a <Parameter Type, Parameter Length, Parameter Value> triplet.

Parameter Type可以为Reserved（Parameter Type 0）、Authentication (deprecated)（Parameter Type 1）、Capability Codes（Parameter Type 2）

最常用的还是Capability Codes，其内部格式为1个或多个<Capability Code, Capability Length, Capability Value>三元组
=> RFC3392 Sec 4.

+------------------------------+
| Capability Code (1 octet)    |
+------------------------------+
| Capability Length (1 octet)  |
+------------------------------+
| Capability Value (variable)  |
+------------------------------+

其中Capability-Code可为如下的值

BGP-Open-Capability-Codes.png
=> IANA-Capability-Codes

整个结构如下

graph LR
X(Optional Parameters)--> T1(Opt Param tuple)
T1(Opt Param tuple)--> A("Parameter Type(if Capability Codes)")
T1(Opt Param tuple)--> B(Parameter Length)
T1(Opt Param tuple)--> C(Parameter Value)
C(Parameter Value)--> CT1(Cap Code tuple)
CT1(Cap Code tuple)--> CTA(Capability Code)
CT1(Cap Code tuple)--> CTB(Capability Length)
CT1(Cap Code tuple)--> CTC(Capability Value)
C(Parameter Value)--> CT2(...)
X(Optional Parameters)--> T2(...)

(1) Multiprotocol Extensions for BGP-4

Capability Code字段为1
Capability Length字段为变长
Capability Value如下：Address Family Identifier（2字节）、Reserved（1字节，默认为0）、Subsequent Address Family Identifier（1字节）

0       7      15      23      31
+-------+-------+-------+-------+
|      AFI      | Res.  | SAFI  |
+-------+-------+-------+-------+

=> RFC2858 Sec 7.

(2) Route Refresh Capability for BGP-4

Capability Code字段为2
Capability Length字段为0
=> RFC2918 Sec 2.

(3) Outbound Route Filtering Capability

Capability Code字段为3
Capability Length字段为变长
Capability Value字段为一个或者多个如下结构

+--------------------------------------------------+
| Address Family Identifier (2 octets)             |
+--------------------------------------------------+
| Reserved (1 octet)                               |
+--------------------------------------------------+
| Subsequent Address Family Identifier (1 octet)   |
+--------------------------------------------------+
| Number of ORFs (1 octet)                         |
+--------------------------------------------------+
| ORF Type (1 octet)                               |
+--------------------------------------------------+
| Send/Receive (1 octet)                           |
+--------------------------------------------------+
| ...                                              |
+--------------------------------------------------+
| ORF Type (1 octet)                               |
+--------------------------------------------------+
| Send/Receive (1 octet)                           |
+--------------------------------------------------+

其中Send/Receive值只能为1、2、3
=> RFC5291 Sec 5.

(4) Multiple routes to a destination capability (deprecated)

Capability Code字段为4
但是被弃用
=> RFC8277 Sec 6.

(5) Extended Next Hop Encoding

Capability Code字段为5
Capability Length字段为变长

+-----------------------------------------------------+
| NLRI AFI - 1 (2 octets)                             |
+-----------------------------------------------------+
| NLRI SAFI - 1 (2 octets)                            |
+-----------------------------------------------------+
| Nexthop AFI - 1 (2 octets)                          |
+-----------------------------------------------------+
| .....                                               |
+-----------------------------------------------------+
| NLRI AFI - N (2 octets)                             |
+-----------------------------------------------------+
| NLRI SAFI - N (2 octets)                            |
+-----------------------------------------------------+
| Nexthop AFI - N (2 octets)                          |
+-----------------------------------------------------+

=> RFC8950 Sec 4.

(6) BGP Extended Message

Capability Code字段为6
Capability Length字段为0
=> RFC8654 Sec 3.

(7) BGPsec Capability

Capability Code字段为7
Capability Length字段为3
Capability Value字段如下

  0   1   2   3      4      5   6   7
+---------------------------------------+
| Version          | Dir |  Unassigned  |
+---------------------------------------+
|                                       |
+------           AFI              -----+
|                                       |
+---------------------------------------+

=> RFC8205 Sec 2.1.

(8) Multiple Labels Capability

Capability Code字段为8
Capability Length字段为变长
Capability Value字段为一个或多个<AFI, SAFI, Count>三元组

 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|              AFI              |    SAFI       |    Count      ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~              AFI              |    SAFI       |    Count      |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

=> RFC8277 Sec 2.1.

(9) BGP Role

Capability Code字段为9
Capability Length字段为1
Capability Value字段如下

Value	Role name (for the local AS)
0	Provider
1	RS
2	RS-Client
3	Customer
4	Peer (i.e., Lateral Peer)
5-255	Unassigned

=> RFC9234 Sec 4.1.

(64) Graceful Restart Capability

Capability Code字段为64
Capability Length字段为变长
Capability Value字段为ReStart Flags、Restart Time in seconds和一个或多个<AFI, SAFI, Flags for Address Family>三元组

+--------------------------------------------------+
| Restart Flags (4 bits)                           |
+--------------------------------------------------+
| Restart Time in seconds (12 bits)                |
+--------------------------------------------------+
| Address Family Identifier (16 bits)              |
+--------------------------------------------------+
| Subsequent Address Family Identifier (8 bits)    |
+--------------------------------------------------+
| Flags for Address Family (8 bits)                |
+--------------------------------------------------+
| ...                                              |
+--------------------------------------------------+
| Address Family Identifier (16 bits)              |
+--------------------------------------------------+
| Subsequent Address Family Identifier (8 bits)    |
+--------------------------------------------------+
| Flags for Address Family (8 bits)                |
+--------------------------------------------------+


Restart Flags:
 0 1 2 3
+-+-+-+-+
|R|Resv.|
+-+-+-+-+

Flags for Address Family:
 0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|F|   Reserved  |
+-+-+-+-+-+-+-+-+

=> RFC4724 Sec 3.

(65) Support for 4-octet AS number capability

Capability Code字段为65
Capability Length字段为4
Capability Value字段为4字节的AS number
=> RFC6793 Sec 3.

(67) Support for Dynamic Capability (capability specific)

Capability Code字段为67
Capability Length字段为变长
Capability Value字段是一串capability codes，每个一字节

The Capability Value field consists of a list of capability codes (one-octet for each) that specify the capabilities that MAY be revised dynamically by the remote speaker.

=> draft-ietf-idr-dynamic-cap-16 Sec 2.

(68) Multisession BGP Capability

Capability Code字段为68
Capability Length字段为变长
Capability Value字段是一个Flags（G+Reserved）和0个或多个capability codes用于区别不同的组

 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|G|  Reserved   |  Session Id   ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

=> draft-ietf-idr-bgp-multisession-07 Sec 4.

(69) ADD-PATH Capability

Capability Code字段为69
Capability Length字段为变长
Capability Value字段为一个或多个<AFI, SAFI, Send/Receive>三元组

+------------------------------------------------+
| Address Family Identifier (2 octets)           |
+------------------------------------------------+
| Subsequent Address Family Identifier (1 octet) |
+------------------------------------------------+
| Send/Receive (1 octet)                         |
+------------------------------------------------+

=> RFC7911 Sec 4.

(70) Enhanced Route Refresh Capability

Capability Code字段为70
Capability Length字段为0
=> RFC7313 Sec 3.1.

(71) Long-Lived Graceful Restart (LLGR) Capability

Capability Code字段为71
Capability Length字段为变长
Capability Value字段为0个或多个<AFI, SAFI, Flags, Long-lived Stale Time>四元组

+--------------------------------------------------+
| Address Family Identifier (16 bits)              |
+--------------------------------------------------+
| Subsequent Address Family Identifier (8 bits)    |
+--------------------------------------------------+
| Flags for Address Family (8 bits)                |
+--------------------------------------------------+
| Long-lived Stale Time (24 bits)                  |
+--------------------------------------------------+
| ...                                              |
+--------------------------------------------------+
| Address Family Identifier (16 bits)              |
+--------------------------------------------------+
| Subsequent Address Family Identifier (8 bits)    |
+--------------------------------------------------+
| Flags for Address Family (8 bits)                |
+--------------------------------------------------+
| Long-lived Stale Time (24 bits)                  |
+--------------------------------------------------+

=> RFC-ietf-idr-long-lived-gr-06 Sec 3.1

(72) Routing Policy Distribution

Capability Code字段为72
Capability Length字段为变长
Capability Value字段为1个或多个<AFI, SAFI, Send/Receive>三元组

+--------------------------------------------------+
|  Address Family Identifier (2 octets)            |
+--------------------------------------------------+
|  Subsequent Address Family Identifier (1 octet)  |
+--------------------------------------------------+
|  Send/Receive (1 octet)                          |
+--------------------------------------------------+

=> draft-ietf-idr-rpd-04 Sec 4.3

(128) Prestandard Route Refresh (deprecated)

同 (2) Route Refresh Capability for BGP-4

(130) Prestandard Outbound Route Filtering (deprecated)

同 (3) Outbound Route Filtering Capability

UPDATE Message Format

UPDATE Message主要用来构建描述各种自治域的关系图，用于向peer通告共享公共路径属性的可行路由或者撤回多个不可行路由

update结构为Withdrawn Routes Length（2字节）、Withdrawn Routes（变长）、Total Path Attribute Length（2字节）、Path Attributes（变长，其长度值为前一个字段）、Network Layer Reachability Information（变长，其长度值为Header长度减去Total Path Attribute Length）

+-----------------------------------------------------+
|   Withdrawn Routes Length (2 octets)                |
+-----------------------------------------------------+
|   Withdrawn Routes (variable)                       |
+-----------------------------------------------------+
|   Total Path Attribute Length (2 octets)            |
+-----------------------------------------------------+
|   Path Attributes (variable)                        |
+-----------------------------------------------------+
|   Network Layer Reachability Information (variable) |
+-----------------------------------------------------+

Withdrawn Routes中是一些IP address prefix，每个IP address prefix都是<length, prefix>的二元组

+---------------------------+
|   Length (1 octet)        |
+---------------------------+
|   Prefix (variable)       |
+---------------------------+

每个Path Attributes都是一组<attribute type, attribute length, attribute value>三元组

Attribute Type是二个字节的字段。Attr. Flags、Attr. Type Code均一个字节

0                   1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|  Attr. Flags  |Attr. Type Code|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

=> RFC4271 Sec 4.3

Attr. Flags每个bits的0/1均有不同的含义，例如第四个bit（bit 3）表示是否是一个Extended长度的比特位，如果为0，则Attribute Length是1个字节，如果为1，则是2个字节

Attr. Type Code值如下

BGP-Update-Path-Attribute

整个结构如下

graph LR
X(Path Attributes)--> T1(tuple)
T1(tuple)--> A(attribute type)
A-->A1(Attr. Flags)
A-->A2(Attr. Type Code)
T1(tuple)--> B(attribute length)
T1(tuple)--> C(attribute value)
X(Path Attributes)--> T2(...)

(1) ORIGIN

Type Code为1
Attribute Len为1
Attribute Value可以为IGP(0)，EGP(1)，INCOMPLETE(2)

=> RFC4271 Sec 4.3

(2) AS_PATH

Type Code为2
Attribute Len为变长
Attribute Value为一组AS path segments，每一个AS path segments表示为<path segment type（1字节）, path segment length（1字节）, path segment value（变长）>三元组，

path segment type可以为AS_SET(1)，AS_SEQUENCE(2)
path segment length为ASes的个数（不是path segment value的字节长度）
path segment value为一个或多个AS编号，每个编号2个字节

=> RFC4271 Sec 4.3

整个结构如下

graph LR
X(Attribute Value)--> T1(AS path segments)
T1(AS path segments)--> A(path segment type)
T1(AS path segments)--> B(path segment length)
T1(AS path segments)--> C(path segment value)
C(path segment value)--> C1(AS number)
C(path segment value)--> C2(...)
X(Path Attributes)--> T2(...)

(3) NEXT_HOP

Type Code为3
Attribute Len为4
Attribute Value为IP address

=> RFC4271 Sec 4.3

(4) MULTI_EXIT_DISC

Type Code为4
Attribute Len为4
Attribute Value为4个字节的无符号整数，用于区别邻居自治域的入口点

=> RFC4271 Sec 4.3

(5) LOCAL_PREF

Type Code为5
Attribute Len为4
Attribute Value为4个字节的无符号整数，用于通知内部对等节点，广告路由的偏好程度（不是很理解）

=> RFC4271 Sec 4.3

(6) ATOMIC_AGGREGATE

Type Code为6
Attribute Len为0

=> RFC4271 Sec 4.3

(7) AGGREGATOR

Type Code为7
Attribute Len为6（某些实现下为8，即AS number(4字节)）
Attribute Value为the last AS number(2字节)，the IP address(4字节)

(8) COMMUNITIES

Type Code为8
Attribute Len为变长
Attribute Value为一组4字节的community

=> RFC1997

(9) ORIGINATOR_ID

Type Code为9
Attribute Len为4
Attribute Value为本地自治域的BGP Identifier

ORIGINATOR_ID和CLUSTER_LIST一起是防止路由环路的

=> RFC4456 Sec 8.

(10) CLUSTER_LIST

Type Code为10
Attribute Len为变长
Attribute Value为一组4字节的CLUSTER_ID

=> RFC4456 Sec 8.

(14) MP_REACH_NLRI

Type Code为14
Attribute Len为变长
Attribute Value为AFI(2字节)、SAFI(1字节)、Length of Next Hop Network Address(1字节)、Network Address of Next Hop(变长)、Reserved(1字节)、NLRI(变长)

Network Address of Next Hop和NLRI均由<AFI, SAFI>标识

+---------------------------------------------------------+
| Address Family Identifier (2 octets)                    |
+---------------------------------------------------------+
| Subsequent Address Family Identifier (1 octet)          |
+---------------------------------------------------------+
| Length of Next Hop Network Address (1 octet)            |
+---------------------------------------------------------+
| Network Address of Next Hop (variable)                  |
+---------------------------------------------------------+
| Reserved (1 octet)                                      |
+---------------------------------------------------------+
| Network Layer Reachability Information (variable)       |
+---------------------------------------------------------+

=> RFC4760 Sec 3.

(15) MP_UNREACH_NLRI

Type Code为15
Attribute Len为变长
Attribute Value为AFI(2字节)、SAFI(1字节)、Withdrawn Routes(变长)

+---------------------------------------------------------+
| Address Family Identifier (2 octets)                    |
+---------------------------------------------------------+
| Subsequent Address Family Identifier (1 octet)          |
+---------------------------------------------------------+
| Withdrawn Routes (variable)                             |
+---------------------------------------------------------+

Withdrawn Routes由<AFI, SAFI>标识

=> RFC4760 Sec 4.

(16) EXTENDED COMMUNITIES

Type Code为16
Attribute Len为变长
Attribute Value为一系列extended communities，每个Extended Community均是8个字节的如下结构

如果是Regular type，则是1个字节，如果是Extended type，则是2个字节

- Type Field  : 1 or 2 octets
- Value Field : Remaining octets

 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|  Type high    |  Type low(*)  |                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+          Value                |
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

=> RFC4360 Sec 2.

(17) AS4_PATH

Type Code为17
Attribute Len为4
Attribute Value为4字节的AS4_PATH

=> RFC6793 Sec 9.

(18) AS4_AGGREGATOR

Type Code为18
Attribute Len为8
Attribute Value为the last AS number(4字节)，the IP address(4字节)

=> RFC6793 Sec 9.

protocol
BGP

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