RFC 4271: A Border Gateway Protocol 4 (BGP-4)
- Y. Rekhter, Ed.,
- T. Li, Ed.,
- S. Hares, Ed.
Draft Standard
Network Working Group Y. Rekhter, Ed. Request for Comments: 4271 T. Li, Ed. Obsoletes: 1771 S. Hares, Ed. Category: Standards Track January 2006 A Border Gateway Protocol 4 (BGP-4) Status of This Memo This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited. Copyright Notice Copyright (C) The Internet Society (2006). Abstract This document discusses the Border Gateway Protocol (BGP), which is an inter-Autonomous System routing protocol. The primary function of a BGP speaking system is to exchange network reachability information with other BGP systems. This network reachability information includes information on the list of Autonomous Systems (ASes) that reachability information traverses. This information is sufficient for constructing a graph of AS connectivity for this reachability from which routing loops may be pruned, and, at the AS level, some policy decisions may be enforced. BGP-4 provides a set of mechanisms for supporting Classless Inter- Domain Routing (CIDR). These mechanisms include support for advertising a set of destinations as an IP prefix, and eliminating the concept of network "class" within BGP. BGP-4 also introduces mechanisms that allow aggregation of routes, including aggregation of AS paths. This document obsoletes RFC 1771. Rekhter, et al. Standards Track [Page 1]
RFC 4271 BGP-4 January 2006 Table of Contents 1. Introduction ....................................................4 1.1. Definition of Commonly Used Terms ..........................4 1.2. Specification of Requirements ..............................6 2. Acknowledgements ................................................6 3. Summary of Operation ............................................7 3.1. Routes: Advertisement and Storage ..........................9 3.2. Routing Information Base ..................................10 4. Message Formats ................................................11 4.1. Message Header Format .....................................12 4.2. OPEN Message Format .......................................13 4.3. UPDATE Message Format .....................................14 4.4. KEEPALIVE Message Format ..................................21 4.5. NOTIFICATION Message Format ...............................21 5. Path Attributes ................................................23 5.1. Path Attribute Usage ......................................25 5.1.1. ORIGIN .............................................25 5.1.2. AS_PATH ............................................25 5.1.3. NEXT_HOP ...........................................26 5.1.4. MULTI_EXIT_DISC ....................................28 5.1.5. LOCAL_PREF .........................................29 5.1.6. ATOMIC_AGGREGATE ...................................29 5.1.7. AGGREGATOR .........................................30 6. BGP Error Handling. ............................................30 6.1. Message Header Error Handling .............................31 6.2. OPEN Message Error Handling ...............................31 6.3. UPDATE Message Error Handling .............................32 6.4. NOTIFICATION Message Error Handling .......................34 6.5. Hold Timer Expired Error Handling .........................34 6.6. Finite State Machine Error Handling .......................35 6.7. Cease .....................................................35 6.8. BGP Connection Collision Detection ........................35 7. BGP Version Negotiation ........................................36 8. BGP Finite State Machine (FSM) .................................37 8.1. Events for the BGP FSM ....................................38 8.1.1. Optional Events Linked to Optional Session Attributes .........................................38 8.1.2. Administrative Events ..............................42 8.1.3. Timer Events .......................................46 8.1.4. TCP Connection-Based Events ........................47 8.1.5. BGP Message-Based Events ...........................49 8.2. Description of FSM ........................................51 8.2.1. FSM Definition .....................................51 8.2.1.1. Terms "active" and "passive" ..............52 8.2.1.2. FSM and Collision Detection ...............52 8.2.1.3. FSM and Optional Session Attributes .......52 8.2.1.4. FSM Event Numbers .........................53 Rekhter, et al. Standards Track [Page 2]
RFC 4271 BGP-4 January 2006 8.2.1.5. FSM Actions that are Implementation Dependent .................................53 8.2.2. Finite State Machine ...............................53 9. UPDATE Message Handling ........................................75 9.1. Decision Process ..........................................76 9.1.1. Phase 1: Calculation of Degree of Preference .......77 9.1.2. Phase 2: Route Selection ...........................77 9.1.2.1. Route Resolvability Condition .............79 9.1.2.2. Breaking Ties (Phase 2) ...................80 9.1.3. Phase 3: Route Dissemination .......................82 9.1.4. Overlapping Routes .................................83 9.2. Update-Send Process .......................................84 9.2.1. Controlling Routing Traffic Overhead ...............85 9.2.1.1. Frequency of Route Advertisement ..........85 9.2.1.2. Frequency of Route Origination ............85 9.2.2. Efficient Organization of Routing Information ......86 9.2.2.1. Information Reduction .....................86 9.2.2.2. Aggregating Routing Information ...........87 9.3. Route Selection Criteria ..................................89 9.4. Originating BGP routes ....................................89 10. BGP Timers ....................................................90 Appendix A. Comparison with RFC 1771 .............................92 Appendix B. Comparison with RFC 1267 .............................93 Appendix C. Comparison with RFC 1163 .............................93 Appendix D. Comparison with RFC 1105 .............................94 Appendix E. TCP Options that May Be Used with BGP ................94 Appendix F. Implementation Recommendations .......................95 Appendix F.1. Multiple Networks Per Message .........95 Appendix F.2. Reducing Route Flapping ...............96 Appendix F.3. Path Attribute Ordering ...............96 Appendix F.4. AS_SET Sorting ........................96 Appendix F.5. Control Over Version Negotiation ......96 Appendix F.6. Complex AS_PATH Aggregation ...........96 Security Considerations ...........................................97 IANA Considerations ...............................................99 Normative References .............................................101 Informative References ...........................................101 Rekhter, et al. Standards Track [Page 3]
RFC 4271 BGP-4 January 2006 1. Introduction The Border Gateway Protocol (BGP) is an inter-Autonomous System routing protocol. The primary function of a BGP speaking system is to exchange network reachability information with other BGP systems. This network reachability information includes information on the list of Autonomous Systems (ASes) that reachability information traverses. This information is sufficient for constructing a graph of AS connectivity for this reachability, from which routing loops may be pruned and, at the AS level, some policy decisions may be enforced. BGP-4 provides a set of mechanisms for supporting Classless Inter- Domain Routing (CIDR) [RFC1518, RFC1519]. These mechanisms include support for advertising a set of destinations as an IP prefix and eliminating the concept of network "class" within BGP. BGP-4 also introduces mechanisms that allow aggregation of routes, including aggregation of AS paths. Routing information exchanged via BGP supports only the destination- based forwarding paradigm, which assumes that a router forwards a packet based solely on the destination address carried in the IP header of the packet. This, in turn, reflects the set of policy decisions that can (and cannot) be enforced using BGP. BGP can support only those policies conforming to the destination-based forwarding paradigm. 1.1. Definition of Commonly Used Terms This section provides definitions for terms that have a specific meaning to the BGP protocol and that are used throughout the text. Adj-RIB-In The Adj-RIBs-In contains unprocessed routing information that has been advertised to the local BGP speaker by its peers. Adj-RIB-Out The Adj-RIBs-Out contains the routes for advertisement to specific peers by means of the local speaker's UPDATE messages. Autonomous System (AS) The classic definition of an Autonomous System is a set of routers under a single technical administration, using an interior gateway protocol (IGP) and common metrics to determine how to route packets within the AS, and using an inter-AS routing protocol to determine how to route packets to other ASes. Since this classic definition was developed, it has become common for a single AS to Rekhter, et al. Standards Track [Page 4]
RFC 4271 BGP-4 January 2006 use several IGPs and, sometimes, several sets of metrics within an AS. The use of the term Autonomous System stresses the fact that, even when multiple IGPs and metrics are used, the administration of an AS appears to other ASes to have a single coherent interior routing plan, and presents a consistent picture of the destinations that are reachable through it. BGP Identifier A 4-octet unsigned integer that indicates the BGP Identifier of the sender of BGP messages. A given BGP speaker sets the value of its BGP Identifier to an IP address assigned to that BGP speaker. The value of the BGP Identifier is determined upon startup and is the same for every local interface and BGP peer. BGP speaker A router that implements BGP. EBGP External BGP (BGP connection between external peers). External peer Peer that is in a different Autonomous System than the local system. Feasible route An advertised route that is available for use by the recipient. IBGP Internal BGP (BGP connection between internal peers). Internal peer Peer that is in the same Autonomous System as the local system. IGP Interior Gateway Protocol - a routing protocol used to exchange routing information among routers within a single Autonomous System. Loc-RIB The Loc-RIB contains the routes that have been selected by the local BGP speaker's Decision Process. NLRI Network Layer Reachability Information. Route A unit of information that pairs a set of destinations with the attributes of a path to those destinations. The set of Rekhter, et al. Standards Track [Page 5]
RFC 4271 BGP-4 January 2006 destinations are systems whose IP addresses are contained in one IP address prefix carried in the Network Layer Reachability Information (NLRI) field of an UPDATE message. The path is the information reported in the path attributes field of the same UPDATE message. RIB Routing Information Base. Unfeasible route A previously advertised feasible route that is no longer available for use. 1.2. Specification of Requirements The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119]. 2. Acknowledgements This document was originally published as [RFC1267] in October 1991, jointly authored by Kirk Lougheed and Yakov Rekhter. We would like to express our thanks to Guy Almes, Len Bosack, and Jeffrey C. Honig for their contributions to the earlier version (BGP-1) of this document. We would like to specially acknowledge numerous contributions by Dennis Ferguson to the earlier version of this document. We would like to explicitly thank Bob Braden for the review of the earlier version (BGP-2) of this document, and for his constructive and valuable comments. We would also like to thank Bob Hinden, Director for Routing of the Internet Engineering Steering Group, and the team of reviewers he assembled to review the earlier version (BGP-2) of this document. This team, consisting of Deborah Estrin, Milo Medin, John Moy, Radia Perlman, Martha Steenstrup, Mike St. Johns, and Paul Tsuchiya, acted with a strong combination of toughness, professionalism, and courtesy. Certain sections of the document borrowed heavily from IDRP [IS10747], which is the OSI counterpart of BGP. For this, credit should be given to the ANSI X3S3.3 group chaired by Lyman Chapin and to Charles Kunzinger, who was the IDRP editor within that group. Rekhter, et al. Standards Track [Page 6]
RFC 4271 BGP-4 January 2006 We would also like to thank Benjamin Abarbanel, Enke Chen, Edward Crabbe, Mike Craren, Vincent Gillet, Eric Gray, Jeffrey Haas, Dimitry Haskin, Stephen Kent, John Krawczyk, David LeRoy, Dan Massey, Jonathan Natale, Dan Pei, Mathew Richardson, John Scudder, John Stewart III, Dave Thaler, Paul Traina, Russ White, Curtis Villamizar, and Alex Zinin for their comments. We would like to specially acknowledge Andrew Lange for his help in preparing the final version of this document. Finally, we would like to thank all the members of the IDR Working Group for their ideas and the support they have given to this document. 3. Summary of Operation The Border Gateway Protocol (BGP) is an inter-Autonomous System routing protocol. It is built on experience gained with EGP (as defined in [RFC904]) and EGP usage in the NSFNET Backbone (as described in [RFC1092] and [RFC1093]). For more BGP-related information, see [RFC1772], [RFC1930], [RFC1997], and [RFC2858]. The primary function of a BGP speaking system is to exchange network reachability information with other BGP systems. This network reachability information includes information on the list of Autonomous Systems (ASes) that reachability information traverses. This information is sufficient for constructing a graph of AS connectivity, from which routing loops may be pruned, and, at the AS level, some policy decisions may be enforced. In the context of this document, we assume that a BGP speaker advertises to its peers only those routes that it uses itself (in this context, a BGP speaker is said to "use" a BGP route if it is the most preferred BGP route and is used in forwarding). All other cases are outside the scope of this document. In the context of this document, the term "IP address" refers to an IP Version 4 address [RFC791]. Routing information exchanged via BGP supports only the destination- based forwarding paradigm, which assumes that a router forwards a packet based solely on the destination address carried in the IP header of the packet. This, in turn, reflects the set of policy decisions that can (and cannot) be enforced using BGP. Note that some policies cannot be supported by the destination-based forwarding paradigm, and thus require techniques such as source routing (aka explicit routing) to be enforced. Such policies cannot be enforced using BGP either. For example, BGP does not enable one AS to send Rekhter, et al. Standards Track [Page 7]
RFC 4271 BGP-4 January 2006 traffic to a neighboring AS for forwarding to some destination (reachable through but) beyond that neighboring AS, intending that the traffic take a different route to that taken by the traffic originating in the neighboring AS (for that same destination). On the other hand, BGP can support any policy conforming to the destination-based forwarding paradigm. BGP-4 provides a new set of mechanisms for supporting Classless Inter-Domain Routing (CIDR) [RFC1518, RFC1519]. These mechanisms include support for advertising a set of destinations as an IP prefix and eliminating the concept of a network "class" within BGP. BGP-4 also introduces mechanisms that allow aggregation of routes, including aggregation of AS paths. This document 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 (IGP) and common metrics to determine how to route packets within the AS, and using an inter-AS routing protocol to determine how to route packets to other ASes. Since this classic definition was developed, it has become common for a single AS to use several IGPs and, sometimes, several sets of metrics within an AS. The use of the term Autonomous System stresses the fact that, even when multiple IGPs and metrics are used, the administration of an AS appears to other ASes to have a single coherent interior routing plan and presents a consistent picture of the destinations that are reachable through it. BGP uses TCP [RFC793] as its transport protocol. This eliminates the need to implement explicit update fragmentation, retransmission, acknowledgement, and sequencing. BGP listens on TCP port 179. The error notification mechanism used in BGP assumes that TCP supports a "graceful" close (i.e., that all outstanding data will be delivered before the connection is closed). A TCP connection is formed between two systems. They exchange messages to open and confirm the connection parameters. The initial data flow is the portion of the BGP routing table that is allowed by the export policy, called the Adj-Ribs-Out (see 3.2). Incremental updates are sent as the routing tables change. BGP does not require a periodic refresh of the routing table. To allow local policy changes to have the correct effect without resetting any BGP connections, a BGP speaker SHOULD either (a) retain the current version of the routes advertised to it by all of its peers for the duration of the connection, or (b) make use of the Route Refresh extension [RFC2918]. Rekhter, et al. Standards Track [Page 8]
RFC 4271 BGP-4 January 2006 KEEPALIVE messages may be sent periodically to ensure that the connection is live. 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. A peer in a different AS is referred to as an external peer, while a peer in the same AS is referred to as an internal peer. Internal BGP and external BGP are commonly abbreviated as IBGP and EBGP. If a particular AS has multiple BGP speakers and is providing transit service for other ASes, 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 IGP used within the AS. For the purpose of this document, it is assumed that a consistent view of the routes exterior to the AS is provided by having all BGP speakers within the AS maintain IBGP with each other. This document specifies the base behavior of the BGP protocol. This behavior can be, and is, modified by extension specifications. When the protocol is extended, the new behavior is fully documented in the extension specifications. 3.1. Routes: Advertisement and Storage For the purpose of this protocol, a route is defined as a unit of information that pairs a set of destinations with the attributes of a path to those destinations. The set of destinations are systems whose IP addresses are contained in one IP address prefix that is carried in the Network Layer Reachability Information (NLRI) field of an UPDATE message, and the path is the information reported in the path attributes field of the same UPDATE message. Routes are advertised between BGP speakers in UPDATE messages. Multiple routes that have the same path attributes can be advertised in a single UPDATE message by including multiple prefixes in the NLRI field of the UPDATE message. Routes are stored in the Routing Information Bases (RIBs): namely, the Adj-RIBs-In, the Loc-RIB, and the Adj-RIBs-Out, as described in Section 3.2. If a BGP speaker chooses to advertise a previously received route, it MAY add to, or modify, the path attributes of the route before advertising it to a peer. Rekhter, et al. Standards Track [Page 9]
RFC 4271 BGP-4 January 2006 BGP provides mechanisms by which a BGP speaker can inform its peers that a previously advertised route is no longer available for use. There are three methods by which a given BGP speaker can indicate that a route has been withdrawn from service: a) the IP prefix that expresses the destination for a previously advertised route can be advertised in the WITHDRAWN ROUTES field in the UPDATE message, thus marking the associated route as being no longer available for use, b) a replacement route with the same NLRI can be advertised, or c) the BGP speaker connection can be closed, which implicitly removes all routes the pair of speakers had advertised to each other from service. Changing the attribute(s) of a route is accomplished by advertising a replacement route. The replacement route carries new (changed) attributes and has the same address prefix as the original route. 3.2. Routing Information Base The Routing Information Base (RIB) within a BGP speaker consists of three distinct parts: a) Adj-RIBs-In: The Adj-RIBs-In stores routing information learned from inbound UPDATE messages that were received from other BGP speakers. Their contents represent routes that are available as input to the Decision Process. b) Loc-RIB: The Loc-RIB contains the local routing information the BGP speaker selected by applying its local policies to the routing information contained in its Adj-RIBs-In. These are the routes that will be used by the local BGP speaker. The next hop for each of these routes MUST be resolvable via the local BGP speaker's Routing Table. c) Adj-RIBs-Out: The Adj-RIBs-Out stores information the local BGP speaker selected for advertisement to its peers. The routing information stored in the Adj-RIBs-Out will be carried in the local BGP speaker's UPDATE messages and advertised to its peers. In summary, the Adj-RIBs-In contains unprocessed routing information that has been advertised to the local BGP speaker by its peers; the Loc-RIB contains the routes that have been selected by the local BGP Rekhter, et al. Standards Track [Page 10]
RFC 4271 BGP-4 January 2006 speaker's Decision Process; and the Adj-RIBs-Out organizes the routes for advertisement to specific peers (by means of the local speaker's UPDATE messages). Although the conceptual model distinguishes between Adj-RIBs-In, Loc-RIB, and Adj-RIBs-Out, this neither implies nor requires that an implementation must maintain three separate copies of the routing information. The choice of implementation (for example, 3 copies of the information vs 1 copy with pointers) is not constrained by the protocol. Routing information that the BGP speaker uses to forward packets (or to construct the forwarding table used for packet forwarding) is maintained in the Routing Table. The Routing Table accumulates routes to directly connected networks, static routes, routes learned from the IGP protocols, and routes learned from BGP. Whether a specific BGP route should be installed in the Routing Table, and whether a BGP route should override a route to the same destination installed by another source, is a local policy decision, and is not specified in this document. In addition to actual packet forwarding, the Routing Table is used for resolution of the next-hop addresses specified in BGP updates (see Section 5.1.3). 4. Message Formats This section describes message formats used by BGP. BGP messages are sent over TCP connections. 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 (19 octets). All multi-octet fields are in network byte order. Rekhter, et al. Standards Track [Page 11]
RFC 4271 BGP-4 January 2006 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 is included for compatibility; it MUST be set to all ones. Length: This 2-octet unsigned integer indicates the total length of the message, including the header in octets. Thus, it allows one to locate the (Marker field of the) next message in the TCP stream. The value of the Length field MUST always be at least 19 and no greater than 4096, and MAY be further constrained, depending on the message type. "padding" of extra data after the message is not allowed. Therefore, 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. This document defines the following type codes: 1 - OPEN 2 - UPDATE 3 - NOTIFICATION 4 - KEEPALIVE [RFC2918] defines one more type code. Rekhter, et al. Standards Track [Page 12]
RFC 4271 BGP-4 January 2006 4.2. OPEN Message Format After a TCP 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. In addition to the fixed-size BGP header, the OPEN message contains the following fields: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+ | Version | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | My Autonomous System | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Hold Time | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | BGP Identifier | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Opt Parm Len | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | Optional Parameters (variable) | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Version: This 1-octet unsigned integer indicates the protocol version number of the message. The current BGP version number is 4. My Autonomous System: This 2-octet unsigned integer indicates the Autonomous System number of the sender. Hold Time: This 2-octet unsigned integer indicates the number of seconds the sender proposes for the value of the Hold Timer. Upon receipt of an OPEN message, a BGP speaker MUST calculate the value of the Hold Timer by using the smaller of its configured Hold Time and the Hold Time received in the OPEN message. The Hold Time MUST be either zero or at least three seconds. An implementation MAY reject connections on the basis of the Hold Rekhter, et al. Standards Track [Page 13]
RFC 4271 BGP-4 January 2006 Time. The calculated value indicates the maximum number of seconds that may elapse between the receipt of successive KEEPALIVE and/or UPDATE messages from the sender. BGP Identifier: This 4-octet unsigned integer indicates the BGP Identifier of the sender. A given BGP speaker sets the value of its BGP Identifier to an IP address that is assigned to that BGP speaker. The value of the BGP Identifier is determined upon startup and is the same for every local interface and BGP peer. Optional Parameters Length: This 1-octet unsigned integer indicates the total length of the Optional Parameters field in octets. If the value of this field is zero, no Optional Parameters are present. Optional Parameters: This field contains a list of optional parameters, in which each parameter is encoded as a <Parameter Type, Parameter Length, Parameter Value> triplet. 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) +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-... Parameter Type is a one octet field that unambiguously identifies individual parameters. Parameter Length is a one octet field that contains the length of the Parameter Value field in octets. Parameter Value is a variable length field that is interpreted according to the value of the Parameter Type field. [RFC3392] defines the Capabilities Optional Parameter. The minimum length of the OPEN message is 29 octets (including the message header). 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 Rekhter, et al. Standards Track [Page 14]
RFC 4271 BGP-4 January 2006 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) | +---------------------------+ Rekhter, et al. Standards Track [Page 15]
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 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. Rekhter, et al. Standards Track [Page 17]
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]
RFC 4271 BGP-4 January 2006 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. 4.4. KEEPALIVE Message Format BGP does not use any TCP-based, keep-alive mechanism to determine if peers are reachable. Instead, KEEPALIVE messages are exchanged between peers often enough not to cause the Hold Timer to expire. A reasonable maximum time between KEEPALIVE messages would be one third of the Hold Time interval. KEEPALIVE messages MUST NOT be sent more frequently than one per second. An implementation MAY adjust the rate at which it sends KEEPALIVE messages as a function of the Hold Time interval. If the negotiated Hold Time interval is zero, then periodic KEEPALIVE messages MUST NOT be sent. A KEEPALIVE message consists of only the 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 it is sent. Rekhter, et al. Standards Track [Page 21]
RFC 4271 BGP-4 January 2006 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 (variable) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 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. Rekhter, et al. Standards Track [Page 22]
RFC 4271 BGP-4 January 2006
OPEN Message Error subcodes:
1 - Unsupported Version Number.
2 - Bad Peer AS.
3 - Bad BGP Identifier.
4 - Unsupported Optional Parameter.
5 - [Deprecated - see