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4.3. UPDATE Message Format

4.3. UPDATE Message Format​

UPDATE messages are used to transfer routing information between BGP peers. The information in the UPDATE message can be used to construct a graph that describes 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 feasible routes that share common path attributes 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 also includes the other fields, as shown below (note, some of the shown fields may not be present in every UPDATE message):

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      |   Withdrawn Routes Length (2 octets)                |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      |   Withdrawn Routes (variable)                       |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

Withdrawn Routes Length:

This 2-octets unsigned integer indicates the total length of the Withdrawn Routes field in octets. Its value allows 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A value of 0 indicates that no routes are being withdrawn from service, and that the WITHDRAWN ROUTES field is not present in this UPDATE message.

Withdrawn Routes:

This is a variable-length field that contains a list of IP address prefixes for the routes that are being withdrawn from service.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 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).

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).

The lower-order four bits of the Attribute Flags octet are unused. They MUST be zero when sent 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, the third and 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, 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 0 (IGP) - Network Layer Reachability Information is interior to the originating AS
  • Value 1 (EGP) - Network Layer Reachability Information learned via the EGP protocol [RFC904]
  • Value 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 1 (AS_SET): unordered set of ASes a route in the UPDATE message has traversed
  • Value 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.

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.

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).

An UPDATE message can advertise, at most, one set of path attributes, but multiple destinations, provided that the destinations share these attributes. All path attributes contained in a given UPDATE message apply to all destinations carried in the NLRI field of the UPDATE message.

An UPDATE message can list multiple routes that are 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 advertised.

An UPDATE message might advertise only routes that are to be withdrawn from service, in which case the message 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.

An UPDATE message SHOULD NOT include the same address prefix in the WITHDRAWN ROUTES and Network Layer Reachability Information fields. However, a BGP speaker MUST be able to process UPDATE messages in this form. A BGP speaker SHOULD treat an UPDATE message of this form as though the WITHDRAWN ROUTES do not contain the address prefix.

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