GENWiki

Premier IT Outsourcing and Support Services within the UK

User Tools

Site Tools


rfc:rfc1164

Network Working Group J. Honig, Cornell Univ. Theory Center Request for Comments: 1164 D. Katz, Merit/NSFNET

                           M. Mathis, Pittsburgh Supercomputing Center
                     Y. Rekhter, T.J. Watson Research Center, IBM Corp
                                                   J. Yu, Merit/NSFNET
                                                             June 1990
     Application of the Border Gateway Protocol in the Internet

Status of this Memo

 This RFC, together with its companion RFC-1163, "A Border Gateway
 Protocol (BGP)", 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 (IWG@nri.reston.va.us).
 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. BGP Theory and Application.............................    3
 3.1 Topological Model.....................................    3
 3.2 BGP in the Internet...................................    4
 3.2.1 Topology Considerations.............................    4
 3.2.2 Global Nature of BGP................................    5
 3.2.3 BGP Neighbor Relationships..........................    5
 3.3 Policy Making with BGP................................    6
 4. Operational Issues.....................................    7
 4.1 Path Selection........................................    7
 4.2 Syntax and Semantics for BGP Configuration Files......    9
 5. The Interaction of BGP and an IGP......................   17

Interconnectivity Working Group [Page 1] RFC 1164 BGP - Application June 1990

 5.1 Overview..............................................   17
 5.2 Methods for Achieving Stable Interactions.............   17
 5.2.1 Propagation of BGP Information via the IGP..........   18
 5.2.2 Tagged Interior Gateway Protocol....................   18
 5.2.3 Encapsulation.......................................   19
 5.2.4 Other Cases.........................................   19
 6. Implementation Recommendations.........................   20
 6.1 Multiple Networks Per Message.........................   20
 6.2 Preventing Excessive Resource Utilization.............   20
 6.3 Processing Messages on a Stream Protocol..............   21
 6.4 Processing Update Messages............................   21
 7. Conclusion.............................................   22
 References................................................   22
 Security Considerations...................................   22
 Authors' Addresses........................................   22

1. Acknowledgements

 The authors would like to thank Guy Almes (Rice University), Kirk
 Lougheed (cisco Systems), Hans-Werner Braun (Merit/NSFNET), Sue Hares
 (Merit/NSFNET), and the Interconnectivity Working Group of the
 Internet Engineering Task Force (chaired by Guy Almes) for their
 contributions to this paper.

2. Introduction

 The Border Gateway Protocol (BGP), described in RFC 1163, is an
 interdomain routing protocol.  The network reachability information
 exchanged via BGP provides sufficient information to detect routing
 loops and enforce routing decisions based on performance preference
 and policy constraints as outlined in RFC 1104 [2].
 This memo uses the term "Autonomous System" 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.

Interconnectivity Working Group [Page 2] RFC 1164 BGP - Application June 1990

 This paper discusses the use of BGP in the Internet environment.
 Issues such as topology, the interaction between BGP and IGPs, and
 the enforcement of policy rules with BGP will be presented.
 All of the discussions in this paper are based on the assumption that
 the Internet is a collection of arbitrarily connected Autonomous
 Systems.  The AS is assumed to be administered by a single
 administrative entity, at least for the purposes of representation of
 routing information to systems outside of the AS.

3. BGP Theory and Application

3.1 Topological Model

 We will be concerned throughout this paper with a general graph whose
 nodes are ASs and whose edges are connections between pairs of ASs.
 The notion of AS is discussed above in Section 2.  When we say that a
 connection exists between two ASs, we mean both of two things:
 physical connection: there is a shared network between the two ASs,
    and on this shared network each AS has at least one border gateway
    belonging to that AS.  Thus the border gateway of each AS can
    forward packets to the border gateway of the other AS without
    resort to Inter-AS or Intra-AS routing.
 BGP connection: there is a BGP session between BGP speakers on each
    of the ASs, and this session communicates to each connected AS
    those routes through the physically connected border gateways of
    the other AS that can be used for specific networks.  Throughout
    this document we place an additional restriction on the BGP
    speakers that form the BGP connection: they must themselves share
    the same network that their border gateways share.  Thus, a BGP
    session between the adjacent ASs requires no support from either
    Inter-AS or Intra-AS routing.  Cases that do not conform to this
    restriction fall outside the scope of this document.
 Thus, at each connection, each AS has one or more BGP speakers and
 one or more border gateways, and these BGP speakers and border
 gateways are all located on a shared network.  Only the AS's border
 gateways on the connection's shared network may be used by that AS's
 BGP speakers on that shared network in NEXT_HOP attributes in Update
 messages.  Paths announced by a BGP speaker of one AS on a given
 connection are taken to be feasible for each of the border gateways
 of the other AS on the same connection.  In all BGP usage, we intend
 that the flow of packets from one AS to the other correspond to
 advertised AS paths.
 Much of the traffic carried within an AS either originates or

Interconnectivity Working Group [Page 3] RFC 1164 BGP - Application June 1990

 terminates at that AS (i.e., either the source IP address or the
 destination IP address of the IP packet identifies a host on a
 network directly connected to that AS).  Traffic that fits this
 description is called "local traffic".  Traffic that does not fit
 this description is called "transit traffic".  A major goal of BGP
 usage is to control the flow of transit traffic.
 Based on how a particular AS deals with transit traffic, the AS may
 now be placed into one of the following categories:
 stub AS: an AS that has only a single connection to another AS.
    Naturally, a stub AS only carries local traffic.
 multihomed AS: an AS that has more than one connection to other ASs,
    but refuses to carry transit traffic.
 transit AS: an AS that has more than one connection to other ASs and
    is designed (under certain policy restrictions) to carry both
    transit and local traffic.
 Since a full AS path provides an efficient and straightforward way of
 suppressing routing loops and eliminates the "count-to-infinity"
 problem associated with some distance vector algorithms, BGP imposes
 no topological restrictions on the interconnection of ASs.

3.2 BGP in the Internet

3.2.1 Topology Considerations

 The overall Internet topology may be viewed as an arbitrary
 interconnection of transit, multihomed, and stub ASs.  In order to
 minimize the impact on the current Internet infrastructure, stub and
 multihomed ASs need not use BGP.  These ASs may run other protocols
 (e.g., EGP) to exchange reachability information with transit ASs.
 Transit ASs then tag this information as having been learned via EGP
 or some other method.  The fact that BGP need not run on stub or
 multihomed ASs has no negative impact on the overall quality of
 inter-AS routing for traffic not local to the stub or multihomed ASs
 in question.
 Of course, BGP may be used for stub and multihomed ASs as well,
 providing advantage in bandwidth and performance over some of the
 currently used protocols (such as EGP). In addition, this would
 result in less need for the use of defaults and in better choices of
 Inter-AS routes for mulitihomed ASs.

Interconnectivity Working Group [Page 4] RFC 1164 BGP - Application June 1990

3.2.2 Global Nature of BGP

 At a global level, BGP is used to distribute routing information
 among multiple Autonomous Systems.  The information flows can be
 represented as follows:
                        +--------+       +--------+
                  BGP   |  BGP   |  BGP  |  BGP   |  BGP
                --------+        +-------+        +-------
                        |  IGP   |       |  IGP   |
                        +--------+       +--------+
                       {___AS A___}     {___AS B___}
 This diagram points out that, while BGP alone carries information
 between ASs, a combination of BGP and an IGP carries information
 across an AS.  Ensuring consistency of routing information between
 BGP and an IGP within an AS is a significant issue and is discussed
 at length later in this paper.

3.2.3 BGP Neighbor Relationships

 As discussed in the introduction, the Internet is viewed as a set of
 arbitrarily connected Autonomous Systems (ASs).  BGP gateways in each
 AS communicate with each other to exchange network reachability
 information based on a set of policies established within each AS.
 Computers that communicate directly with each other via BGP are known
 as BGP neighbors.  BGP neighbors can be located within the same AS or
 in different ASs.  For the sake of discussion, BGP communications
 with neighbors in different ASs will be referred to as External BGP,
 and with neighbors in the same AS as Internal BGP.
 External BGP In the case of External BGP, the BGP neighbors must
    belong to different ASs, but share a common network.  This common
    network should be used to carry the BGP messages between them.
    The use of BGP across an intervening AS invalidates the AS path
    information.  An Autonomous System number must be used with BGP to
    specify which Autonomous System the BGP speaker belongs to.
 Internal BGP There can be as many BGP gateways as deemed necessary
    within an AS.  Usually, if an AS has multiple connections to other
    ASs, multiple BGP gateways are needed.  All BGP gateways
    representing the same AS must give a consistent image of the AS to
    the outside.  This requires that the BGP gateways have consistent
    routing information among them.  These gateways can communicate
    with each other via BGP or by other means.  The policy constraints
    applied to all BGP gateways within an AS must be consistent.

Interconnectivity Working Group [Page 5] RFC 1164 BGP - Application June 1990

3.3 Policy Making with BGP

 BGP provides the capability of enforcing some policies based on
 various preferences and constraints.  Policies are determined by the
 AS administration and are provided to BGP in the form of
 configuration information.  These policies are enforced within a BGP
 speaker by affecting the selection of paths from multiple
 alternatives, and by controlling the redistribution of routing
 information.  Policies are not directly encoded in the protocol.
 Non-technical constraints are related to political, security, or
 economic considerations.  For example, if an AS is unwilling to carry
 traffic to another AS, it can enforce a policy prohibiting this.  The
 following examples of non-technical constraints can be enforced with
 the use of BGP:
    1. A multihomed AS can refuse to act as a transit AS for other
       ASs.  (It does so by not advertising routes to networks other
       than those directly connected to it.)
    2. A multihomed AS can become a transit AS by allowing a certain
       set of ASs to use it as such.  (It does so by advertising
       routes to networks to this set of ASs.)
    3. An AS can favor or disfavor the use of certain ASs for carrying
       transit traffic from itself to networks advertised with
       competing AS paths.
 A number of performance-related criteria can be controlled with the
 use of BGP:
    1. An AS can minimize the number of transit ASs.  (Shorter AS
       paths can be preferred over longer ones.)
    2. The quality of transit ASs.  If an AS determines, using BGP,
       that two or more AS paths can be used to reach a given
       destination, that AS can use a variety of means to decide which
       of the candidate AS paths it will use.  The quality of an AS
       can be measured by such things as diameter, link speed,
       capacity, tendency to become congested, and quality of
       operation.  Information about these qualities might be
       determined by means other than BGP.
    3. Preference of internal routes over external routes.
 Non-technical policy will typically override performance issues.
 For consistency, combinations of policies and route selection

Interconnectivity Working Group [Page 6] RFC 1164 BGP - Application June 1990

 procedures that might result in equal cost paths must be resolved in
 a deterministic fashion.
 Fundamental to BGP usage is the rule that an AS advertizes to its
 neighboring ASs only those routes that it uses.  This rule reflects
 the "hop-by-hop" routing paradigm generally used by the current
 Internet.  Note that some policies that cannot be supported by the
 "hop-by-hop" routing paradigm and which require such techniques 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.

4. Operational Issues

4.1 Path Selection

 One of the major tasks of a BGP speaker for a given AS at a given
 connection is to evaluate different paths to a destination network
 from its border gateways at that connection, select the best one, and
 then advertise it to all of its BGP neighbors at that same connection
 (subject to policy constraints).  The key issue is how different
 paths are evaluated and compared.
 In traditional distance vector protocols (e.g., RIP) there is only
 one metric (e.g., hop count) associated with a path.  As such,
 comparison of different paths is reduced to simply comparing two
 numbers.  A complication in Inter-AS routing arises from the lack of
 a universally agreed-upon metric among ASs that can be used to
 evaluate external paths.  Rather, each AS may have its own set of
 criteria for path evaluation.
 A BGP speaker within an Autonomous System builds a routing database
 consisting of the set of all feasible paths and the list of networks
 reachable through each path.  In an efficient implementation, it will
 be important to store and process these paths and bundle the networks
 reachable through them.  For purposes of precise discussion, however,
 it's useful to consider the set of feasible paths for a given
 destination network.  In most cases, we would expect to find only one
 feasible path in the set.  This will often, however, not be the case.
 All feasible paths must be maintained, and their maintenance speeds
 adaptation to the loss of the primary path, but only the primary path
 at any given time will ever be advertised.

Interconnectivity Working Group [Page 7] RFC 1164 BGP - Application June 1990

 The path selection process can be formalized by defining a partial
 order over the set of all possible paths to a given destination
 network.  One way to define this partial order is to define a
 function that maps each full AS path to a non-negative integer that
 denotes the path's degree of preference.  Path selection is then
 reduced to applying this function to all feasible paths and choosing
 the one with the highest degree of preference.
 In actual BGP implementations, criteria for assigning degree of
 preferences to a path can be specified in a configuration file.
 The process of assigning a degree of preference to a path can be
 based on several sources of information:
    1. Information explicitly present in the full AS path.
    2. A combination of information that can be derived from the full
       AS path and information outside the scope of BGP.
 The criteria used to assign a degree of preference to a path can be
 classified as primitive or compound.  Possible primitive criteria
 include:
  1. AS count. Paths with a smaller AS count are generally better.
  1. Presence or absence of a certain AS or ASs in the path. By

means of information outside the scope of BGP, an AS may know

       some performance characteristics (e.g., bandwidth, MTU, intra-
       AS diameter) of certain ASs and may try to avoid or prefer
       them.
  1. Path origin. A path whose endpoint is internal to the last AS

on the path (BGP is used over the entire path) is generally

       better than one for which part of the path was learned via EGP
       or some other means.
  1. AS path subsets. An AS path that is a subset of a longer AS

path to the same destination should be preferred over the

       longer path.  Any problem in the shorter path (such as an
       outage) will also be a problem in the longer path.
  1. Link dynamics. Stable paths should be preferred over unstable

ones. Note that this criterion must be used in a very careful

       way to avoid causing unnecessary route fluctuation.  Generally,
       any criteria that depend on dynamic information might cause
       routing instability and should be treated very carefully.
  1. Policy consideration. BGP supports policy based routing based

Interconnectivity Working Group [Page 8] RFC 1164 BGP - Application June 1990

       on the policy based distribution of routing information defined
       in RFC 1104 [2].  A BGP gateway may be aware of some policy
       constraints (both within and outside of its own AS) and do
       appropriate path selection.  Paths that do not comply with
       policy requirements are not considered further.
 Metrics based on compound criteria can be computed as a weighted
 combination of the degree of preferences of primitive criteria.  The
 use of compound criteria should be done with extreme caution since it
 involves comparing potentially uncomparable quantities.

4.2 Syntax and Semantics for BGP Configuration Files

 A major task in using BGP is thus to assign a degree of preference to
 each available AS-path.  This degree of preference will generally be
 a function of the number of ASs in the path, properties of the
 specific ASs in the path, the origin of the route, and properties of
 the specific border router to be used in the first hop.  In this
 section we consider how a network administrator might articulate this
 function by means of a configuration file.  In the future, we can
 imagine using tools based on network management protocols such as
 SNMP for this task, but the protocols do not currently support this
 ability.
 In addition to controlling the selection of the best path to a given
 network, the network administrator must control the advertisement of
 this best path to neighboring ASs.  Therefore, path selection and
 path distribution emerge as the two key aspects of policy expression
 in BGP usage.
 Since different aspects of one AS's policy interact, and since the
 policies of different ASs interact, it is important to facilitate the
 analysis of such interactions by means of high-quality and consistent
 tools.
 There is also a need for tools to translate the expression of the
 network administrator's policy to some technical mechanism within a
 BGP speaker to implement that policy.
 These factors suggest that there should be a globally consistent way
 of describing policies in the configuration file.  The syntax and
 semantics of these policies should be capable of expressing the path
 selection phase within the local AS as well as the path
 redistribution phase to other ASs.
 Because it may be desirable to coordinate routing policy at an
 external level, it may prove worthwhile to create a language to
 describe this information in a globally consistent way.  Policies

Interconnectivity Working Group [Page 9] RFC 1164 BGP - Application June 1990

 expressed in such a language could conceivably be used by some high-
 level tools to analyze the interaction among the routing policies of
 different Autonomous Systems.
 The following defines one possible syntax and semantics for
 describing AS path policies from the point of view of the local AS.
 Alternative syntaxes with equivalent richness of functionality are
 not precluded.  Other mechanisms may be needed to provide a fully
 functional configuration language.
 A complete AS path, supplied by BGP, provides the most important
 mechanism for policy enforcement.  Assigning a degree of preference
 to a particular AS path can be modelled as a matching between this
 path and one or more predefined AS path patterns.  Each predefined AS
 path pattern has a degree of preference that will be assigned to any
 AS path that matches it.
 Since patterns are naturally expressed by regular expressions, one
 can use regular expressions over the alphabet of AS numbers to define
 AS path patterns and, therefore, to formulate policies.
 Since certain constructs occur frequently in regular expressions, the
 following notational shorthand (operators) is defined:
    .  matches any AS number.  To improve readability, "." can be
       replaced by "any" so long as this does not introduce ambiguity.
  • a regular expression followed by * means zero or more

repetitions

    +  a regular expression followed by + means one or more
       repetitions
    ?  a regular expression followed by ? means zero or one repetition
    |  alternation
    () parentheses group subexpressions--an operator, such as * or
       works on a single element or on a regular expression enclosed
       in parentheses
    {m,n}  a regular expression followed by {m,n} (where m and n are
       both non-negative integers and m <= n) means at least m and at
       most n repetitions.
    {m}  a regular expression followed by {m} (where m is a positive
       integer) means exactly m repetitions.

Interconnectivity Working Group [Page 10] RFC 1164 BGP - Application June 1990

    {m,} a regular expression followed by {m,} (where m is a positive
       integer) means m or more repetitions.

Any regular expression is generated by these rules.

The Policy Based Routing Language can then be defined as follows:

    <Policy-Based-Routing> ::= { <policy-statement> }
    Semantics: each policy statement might cause a given possible BGP
    advertisement (possibility) to be installed into the routing table
    as the route to a given (set of) networks.  Thus, an empty
    Policy-Based-Routing means that no possibilities will be accepted.
    <policy-statement> ::=
                <policy-expression> '=' <dop-expression> ';'
    Semantics: if a given possibility matches the policy-expression,
    then that possibility will be accepted with a degree of preference
    denoted by the integer value dop-expression.
    <policy-expression> ::=
            <policy-term> |
            <policy-term> <policy-operator> <policy-term>
    <policy-term> ::=
            <network-list> <AS-path> <origin> <distribution-list> |
            '(' <policy-expression> ')' |
            NOT <policy-expression> |
            <>
    <policy-operator> ::= OR | AND
    Semantics: the intersection of the network list of a possibility
    and the network-list must be non-empty; the AS-path of the
    possibility must match the AS-path as a sequence; the origin of
    the possibility must be a member of the origin set; if these
    conditions are met, the route denoted by the possibility is
    accepted as a possible route to those networks of the intersection
    of the possibility network list and the network-list.
    <AS-path> ::= "regular expression over AS numbers"
    Semantics: the AS-path of the possibility must be generated by the
    regular expression <AS-path>.

Interconnectivity Working Group [Page 11] RFC 1164 BGP - Application June 1990

    <network-list> ::= '<' { network network-list } '>' |
                       '<' ANY '>'
    Semantics: A non-empty sequence enumerates the network numbers of
    the network-list; ANY denotes the set of all network numbers.
    <origin> ::= IGP | EGP | INCOMPLETE | ANY
    Semantics: origin enumerates the sequence of acceptable origins;
    ANY denotes the set of all origins.
    <distribution-list> ::= '<' { AS } '>' |
                            '<' ANY '>'
    Semantics: if a given possibility as accepted and installed into
    the routing table, then distribution-list is the set of
    (neighboring) autonomous systems to whose border routers we will
    distribute the BGP-derived routes.
    <dop-expression> ::= <dop-term> |
                         <dop-term> '+' <dop-term> |
                         <dop-term> '-' <dop-term> |
                         <dop-term> '*' <dop-term> |
                         <dop-term> '/' <dop-term> |
                         REJECT
    <dop-term> ::= <integer> |
                   <function> |
                   '(' <dop-expression> ')'
    Semantics: if a possibility matches with degree of preference
    REJECT, then that possibility will not be used.  Otherwise, the
    integer value of the degree of preference indicates the degree of
    preference of the possibility, with higher values preferred over
    lower ones.
 White spaces can be used between symbols to improve readability.
 "<>" denotes the empty sequence.
 There are two built-in functions, PathLength() and PathWeight().
 PathLength() takes the AS path as an argument and returns the number
 of ASs in that path.  PathWeight() takes the AS path and an AS weight
 table as arguments and returns the sum of weights of the ASs in the
 AS path as defined by the AS weight table.  In order to preserve
 determinism, the AS weight table must always have a default weight
 which will be assigned to any AS which is not in that table.
 The AS path, as used above, is constructed from right to left which

Interconnectivity Working Group [Page 12] RFC 1164 BGP - Application June 1990

 is consistent with BGP), so that the most recent AS in the path
 occupies the leftmost position.
 Each network (and its associated complete AS path) received from
 other BGP neighbors is matched against local Routing Policies.
 If either no match occurs or the degree of preference associated with
 the matched policy is REJECT, then the received information is
 rejected.  Otherwise, a degree of preference associated with the
 matched policy is assigned to that path.  Notice that the process
 terminates on the first successful match.  Therefore, policy-terms
 should be ordered from more specific to more general.
 The semantics of a matched policy is as follows:  If a network in
 <network-list> that was originally introduced into BGP from <origin>
 is received via <AS-path>, that network should be redistributed to
 all ASs in <distribution-list>.
 The following examples (some taken from RFC 1102 [3]) illustrate how
 Policy Terms can be written.
 In the following topology, H elements are hosts, G elements are
 Policy Gateways running BGP, and numbered elements are ASs.
      H1 --- 1 -G12...G21 - 2 -- G23...G32 -- 3 ----- H2
             |                                |
             |                                |
             |                                |
             |- G14...G41 - 4 -- G43...G34 ---|- G35...G53 - 5
                            |                                |
                            |                                |
                            |                               H4
                            H3
 In this picture, there are four hosts, ten gateways, and five
 Autonomous Systems.  Gateways G12 and G14 belong to AS 1.  Gateways
 G21 and G23 belong to AS 2.  Gateways G41 and G43 belongs to AS 4.
 Gateways G32, G34, and G35 belong to AS 3.  Gateway G53 belongs to AS
 5.  Dashed lines denote intra-AS connections.  Dotted lines denote
 inter-AS connections.
 First, consider AS 2.  It has no hosts attached, and models a transit
 service, such as the NSFNET backbone network.  It may have a very
 simple policy: it will carry any traffic between any two ASs, without
 further constraint.  If AS 1 and AS 3 are neighboring domains, then
 its policy term could be written as:
    AS 2: < ANY > < (1 | 3) .* > < IGP > < 1 3 > = 10

Interconnectivity Working Group [Page 13] RFC 1164 BGP - Application June 1990

 The first component in this policy, the network list
    < ANY >
 says that any network is subject to this policy.  The second
 component, the AS path
    < (1 | 3) .* >
 says that routing information that came from either AS 1 or AS 3
 matches this policy, including routes from ASs that lie beyond AS 1
 and AS 3.  The third component, the origin
    < IGP >
 says that this route must be interior with respect to the originating
 AS, implying that routes imported via EGP or some other mechanism
 would not match this policy.  The fourth component, the distribution
 list
    < 1  3 >
 says that this route may be redistributed to both AS 1 and AS 3.
 Finally, the degree of preference assigned to any route which matches
 this policy is set to 10.
 To improve readability, the above policy can be rewritten as:
    AS 2: < ANY > < (1 | 3) ANY* > < IGP > < 1  3 > = 10
 Next, consider AS 3.  It is willing to provide transit service to AS
 4 and AS 5, presumably due to multilateral agreements.  AS 3 should
 set its policy as follows:
    AS 3: < ANY > < (4 | 5) > < IGP > < 2  4  5 > = 10
    AS 3: < ANY > < 2  .* > < ANY > < 4  5 > = 10
    AS 3: < ANY > < 3 > < ANY > < 2  4  5 > = 10
 This would allow AS 3 to distribute internal routes received from ASs
 4 and 5 to ASs 2, 4, and 5, and all backbone routes through AS 2
 would be distributed to ASs 4 and 5.  AS 3 would advertise its own
 networks to ASs 2, 4, and 5.  Hosts in AS 4 and AS 5 would be able to
 reach each other, as well as hosts in ASs 1 and 3 and anything beyond
 them.  AS 3 allows any origin in routes from AS 2.  This implies that
 AS 3 trusts AS 2 to impose policy on routes imported by means other
 than BGP.  Note that although the policy statement would appear to
 allow AS 3 to send ASs 4 and 5 their own routes, the BGP protocol
 would detect this as a routing loop and prevent it.

Interconnectivity Working Group [Page 14] RFC 1164 BGP - Application June 1990

 Now consider AS 1.  AS 1 wishes to use the backbone service provided
 by AS 2, and is willing to carry transit traffic for AS 4.  The
 policy statements for AS 1 might read:
    AS 1: < ANY > < 4 > < IGP > < 2 > = 150
    AS 1: < ANY > < 2  .* > < ANY > < 4 > = 150
    AS 1: < ANY > < 1 > < ANY > < 2  4 > = 150
 AS 1 will redistribute all routes learned from the AS 2 backbone to
 AS 4, and vice versa, and distribute routes to its own networks to
 both AS 2 and AS 4.  The degree of preference assigned to any route
 which matches this policy is set to 150.
 AS 5 is a more interesting case.  AS 5 wishes to use the backbone
 service, but is not directly connected to AS 2.  Its policy
 statements could be as follows:
    AS 5: < ANY > < 3  4 > < IGP > < > = 10
    AS 5: < ANY > < 3  2  .* > < . > < > = 10
    AS 5: < ANY > < 5 > < . > < 3 > = 10
 This policy imports routes through AS 2 and AS 3 into AS 5, and
 allows AS 5 and AS 4 to communicate through AS 3.  Since AS 5 does
 not redistribute any routes other than its own, it is a stub AS.
 Note that AS 5 does not trust AS 3 to advertise only routes through
 AS 2, and thus applies its own filter to ensure that it only uses the
 backbone.  This lack of trust makes it necessary to add the second
 policy term.
 AS 4 is a good example of a multihomed AS.  AS 4 wishes to use AS 3
 as is primary path to the backbone, with AS 1 as a backup.
 Furthermore, AS 4 does not wish to provide any transit service
 between ASs 1 and 3.  Its policy statement could read:
    AS 4: < ANY > < 3  .* > < ANY > < > = 10
    AS 4: < ANY > < 1  .* > < ANY > < > = 20
    AS 4: < ANY > < 4 > < ANY > < 1  3 > = 10
 Paths to any network through AS 3 are preferred, but AS 1 will be
 used as a backup if necessary.  Note that since AS 4 trusts AS 3 to
 provide it with reasonable routes, it is not necessary to explicitly
 import routes from AS 5.  Since the redistribution terms are null
 except for networks within AS 4, AS 4 will never carry any transit
 traffic.
 Given the topology and policies described above, it becomes apparent
 that two paths of equal preference would be available from AS 2 to
 any of the networks in AS 4.  Since ties are not allowed, an

Interconnectivity Working Group [Page 15] RFC 1164 BGP - Application June 1990

 arbitrary tie-breaking mechanism would come into play (as described
 above), which might result in less than optimal routes to some
 networks.  An alternative mechanism that would provide optimal routes
 while still allowing fallback paths would be to provide network-by-
 network policies in specific cases, and explicit tie-breaking
 policies for the remaining networks.  For example, the policies for
 AS 2 could be rewritten as follows:
    AS 2: < 35 > < 1  .* > < IGP > < 3 > = 10
    AS 2: < 35 > < 3  .* > < IGP > < 1 > = 20
    AS 2: < ANY > < 1  .* > < IGP > < 3 > = 20
    AS 2: < ANY > < 3  .* > < IGP > < 1 > = 10
 Paths to network 35 through AS 1 would be preferred, with AS 3 as a
 fallback; paths to all other networks through AS 3 would be preferred
 over those through AS 1.  Such optimizations may become arbitrarily
 complex.
 There may be other, simpler ways to assign a degree of preference to
 an AS path.
 The simplest way to assign a degree of preference to a particular
 path is to use the number of ASs in the AS path as the degree of
 preference.  This approach reflects the heuristic that shorter paths
 are usually better than longer ones.  This policy can be implemented
 by using the PathLength() built-in function in the following policy
 statement:
    < ANY > < .* > < ANY > < ANY > = PathLength(ASpath)
 This policy assigns to any network with an arbitrary AS path a degree
 of preference equal to the number of ASs in the AS path; it then
 redistributes this information to all other BGP speakers. As an
 example, an AS path which traverses three different Autonomous
 Systems will be assigned the degree of preference 3.
 Another approach is to assign a certain degree of preference to each
 individual AS, and then determine the degree of preference of a
 particular AS path as the sum of the degree of preferences of the ASs
 in that path.  Note that this approach does not require the
 assignment of a specific degree of preference to every AS in the
 Internet.  For ASs with an unknown degree of preference, a default
 can be used.  This policy can be implemented by using the
 PathWeight() built-in function in the following policy statement:
    < ANY > < .* > < ANY > < ANY >
                         = PathWeight(ASpath, ASWeightTable)

Interconnectivity Working Group [Page 16] RFC 1164 BGP - Application June 1990

 As an example, if Autonomous Systems 145 and 55 have 10 and 15 as
 their weights in the ASWeightTable, and if the default degree of
 preference in the ASWeightTable is 50, then an AS path that traverses
 Autonomous Systems 145, 164, and 55 will be assigned degree of
 preference 75.
 The above examples demonstrate some of the simple policies that can
 be implemented with BGP.  In general, very sophisticated policies
 based on partial or complete AS path discrimination can be written
 and enforced.  It should be emphasized that movement toward more
 sophisticated policies will require parallel effort in creating more
 sophisticated tools for policy interaction analysis.

5. The Interaction of BGP and an IGP

5.1 Overview

 By definition, all transit ASs must be able to carry traffic external
 to that AS (neither the source nor destination host belongs to the
 AS).  This requires a certain degree of interaction and coordination
 between the Interior Gateway Protocol (IGP) used by that particular
 AS and BGP.  In general, traffic exterior to a given AS is going to
 pass through both interior gateways (gateways that support IGP only)
 and border gateways (gateways that support both IGP and BGP).  All
 interior gateways receive information about external routes from one
 or more of the border gateways of the AS via the IGP.
 Depending on the mechanism used to propagate BGP information within a
 given AS, special care must be taken to ensure consistency between
 BGP and the IGP, since changes in state are likely to propagate at
 different rates across the AS.  There may be a time window between
 the moment when some border gateway (A) receives new BGP routing
 information which was originated from another border gateway (B)
 within the same AS, and the moment the IGP within this AS is capable
 of routing transit traffic to that border gateway (B).  During that
 time window, either incorrect routing or "black holes" can occur.
 In order to minimize such routing problems, border gateway (A) should
 not advertise a route to some exterior network X to all of its BGP
 neighbors in other ASs until all of the interior gateways within the
 AS are ready to route traffic destined to X via the correct exit
 border gateway (B).  In other words, interior routing should converge
 on the proper exit gateway before advertising routes via that exit
 gateway to other ASs.

5.2 Methods for Achieving Stable Interactions

 The following discussion outlines several techniques capable of

Interconnectivity Working Group [Page 17] RFC 1164 BGP - Application June 1990

 achieving stable interactions between BGP and the IGP within an
 Autonomous System.

5.2.1 Propagation of BGP Information via the IGP

 While BGP can provide its own mechanism for carrying BGP information
 within an AS, one can also use an IGP to transport this information,
 as long as the IGP supports complete flooding of routing information
 (providing the mechanism to distribute the BGP information) and one-
 pass convergence (making the mechanism effectively atomic).  If an
 IGP is used to carry BGP information, then the period of
 desynchronization described earlier does not occur at all, since BGP
 information propagates within the AS synchronously with the IGP, and
 the IGP converges more or less simultaneously with the arrival of the
 new routing information.  Note that the IGP only carries BGP
 information and should not interpret or process this information.

5.2.2 Tagged Interior Gateway Protocol

 Certain IGPs can tag routes exterior to an AS with the identity of
 their exit points while propagating them within the AS.  Each border
 gateway should use identical tags for announcing exterior routing
 information (received via BGP) both into the IGP and into Internal
 BGP when propagating this information to other border gateways within
 the same AS.  Tags generated by a border gateway must uniquely
 identify that particular border gateway--different border gateways
 must use different tags.
 All Border Gateways within a single AS must observe the following two
 rules:
 1. Information received via Internal BGP by a border gateway A
    declaring a network to be unreachable must immediately be
    propagated to all of the External BGP neighbors of A.
 2. Information received via Internal BGP by a border gateway A about
    a reachable network X cannot be propagated to any of the External
    BGP neighbors of A unless/until A has an IGP route to X and both
    the IGP and the BGP routing information have identical tags.
 These rules guarantee that no routing information is announced
 externally unless the IGP is capable of correctly supporting it.  It
 also avoids some causes of "black holes".
 One possible method for tagging BGP and IGP routes within an AS is to
 use the IP address of the exit border gateway announcing the exterior
 route into the AS.  In this case the "gateway" field in the BGP
 UPDATE message is used as the tag.

Interconnectivity Working Group [Page 18] RFC 1164 BGP - Application June 1990

5.2.3 Encapsulation

 Encapsulation provides the simplest (in terms of the interaction
 between the IGP and BGP) mechanism for carrying transit traffic
 across the AS.  In this approach, transit traffic is encapsulated
 within an IP datagram addressed to the exit gateway.  The only
 requirement imposed on the IGP by this approach is that it should be
 capable of supporting routing between border gateways within the same
 AS.
 The address of the exit gateway A for some exterior network X is
 specified in the "gateway" field of the BGP UPDATE message received
 from gateway A via Internal BGP by all other border gateways within
 the same AS.  In order to route traffic to network X, each border
 gateway within the AS encapsulates it in datagrams addressed to
 gateway A.  Gateway A then performs decapsulation and forwards the
 original packet to the proper gateway in another AS.
 Since encapsulation does not rely on the IGP to carry exterior
 routing information, no synchronization between BGP and the IGP is
 required.
 Some means of identifying datagrams containing encapsulated IP, such
 as an IP protocol type code, must be defined if this method is to be
 used.
 Note, that if a packet to be encapsulated has length that is very
 close to the MTU, that packet would be fragmented at the gateway that
 performs encapsulation.

5.2.4 Other Cases

 There may be ASs with IGPs which can neither carry BGP information
 nor tag exterior routes (e.g., RIP).  In addition, encapsulation may
 be either infeasible or undesirable.  In such situations, the
 following two rules must be observed:
 1. Information received via Internal BGP by a border gateway A
    declaring a network to be unreachable must immediately be
    propagated to all of the External BGP neighbors of A.
 2. Information received via Internal BGP by a border gateway A about
    a reachable network X cannot be propagated to any of the External
    BGP neighbors of A unless A has an IGP route to X and sufficient
    time (holddown) has passed for the IGP routes to have converged.
 The above rules present necessary (but not sufficient) conditions for
 propagating BGP routing information to other ASs.  In contrast to

Interconnectivity Working Group [Page 19] RFC 1164 BGP - Application June 1990

 tagged IGPs, these rules cannot ensure that interior routes to the
 proper exit gateways are in place before propagating the routes to
 other ASs.
 If the convergence time of an IGP is less than some small value X,
 then the time window during which the IGP and BGP are unsynchronized
 is less than X as well, and the whole issue can be ignored at the
 cost of transient periods (of less than length X) of routing
 instability.  A reasonable value for X is a matter for further study,
 but X should probably be less than one second.
 If the convergence time of an IGP cannot be ignored, a different
 approach is needed.  Mechanisms and techniques which might be
 appropriate in this situation are subjects for further study.

6. Implementation Recommendations

6.1 Multiple Networks Per Message

 The BGP protocol allows for multiple networks with the same AS path
 and next-hop gateway to be specified in one message.  Making use of
 this capability is highly recommended.  With one network per message
 there is a substantial increase in overhead in the receiver.  Not
 only does the system overhead increase due to the reception of
 multiple messages, but the overhead of scanning the routing table for
 flash updates to BGP peers and other routing protocols (and sending
 the associated messages) is incurred multiple times as well.  One
 method of building messages containing many networks per AS path and
 gateway from a routing table that is not organized per AS path is to
 build many messages as the routing table is scanned.  As each network
 is processed, a message for the associated AS path and gateway is
 allocated, if it does not exist, and the new network is added to it.
 If such a message exists, the new network is just appended to it.  If
 the message lacks the space to hold the new network, it is
 transmitted, a new message is allocated, and the new network is
 inserted into the new message.  When the entire routing table has
 been scanned, all allocated messages are sent and their resources
 released.  Maximum compression is achieved when all networks share a
 gateway and common path attributes, making it possible to send many
 networks in one 4096-byte message.

6.2 Preventing Excessive Resource Utilization

 When peering with a BGP implementation that does not compress
 multiple networks into one message, it may be necessary to take steps
 to reduce the overhead from the flood of data received when a peer is
 acquired or a significant network topology change occurs.  One method
 of doing this is to rate limit flash updates.  This will eliminate

Interconnectivity Working Group [Page 20] RFC 1164 BGP - Application June 1990

 the redundant scanning of the routing table to provide flash updates
 for BGP peers and other routing protocols.  A disadvantage of this
 approach is that it increases the propagation latency of routing
 information.  By choosing a minimum flash update interval that is not
 much greater than the time it takes to process the multiple messages,
 this latency should be minimized.

6.3 Processing Messages on a Stream Protocol

 Due to the stream nature of TCP, all the data for received messages
 does not necessarily arrive at the same time, due to the nature of
 TCP.  This can make it difficult to process the data as messages,
 especially on systems such as BSD Unix where it is not possible to
 determine how much data has been received but not yet processed.  One
 method that can be used in this situation is to first try to read
 just the message header.  For the KeepAlive message type, this is a
 complete message; for other message types, the header should first be
 verified, in particular the total length.  If all checks are
 successful, the specified length, minus the size of the message
 header is the amount of data left to read.  An implementation that
 would "hang" the routing information process while trying to read
 from a peer could set up a message buffer (1024 bytes) per peer and
 fill it with data as available until a complete message has been
 received.

6.4 Processing Update Messages

 In BGP, all Update messages are incremental.  Once a particular
 network is listed in an Update message as being reachable through an
 AS path and gateway, that piece of information is expected to be
 retained indefinitely.  In order for a route to a network to be
 removed, it must be explicitly listed in an Update message as being
 unreachable or with new routing information to replace the old.  Note
 that a BGP peer will only advertise one route to a given network, so
 any announcement of that network by a particular peer replaces any
 previous information about that network received from the same peer.
 This approach has the obvious advantage of low overhead; if all
 routes are stable, only KeepAlive messages will be sent.  There is no
 periodic flood of route information.
 However, this means that a consistent view of routing information
 between BGP peers is only possible over the course of a single
 transport connection, since there is no mechanism for a complete
 update.  This requirement is accommodated by specifying that BGP
 peers must transition to the Idle state upon the failure of a
 transport connection.

Interconnectivity Working Group [Page 21] RFC 1164 BGP - Application June 1990

7. Conclusion

 The BGP protocol provides a high degree of control and flexibility
 for doing interdomain routing while enforcing policy and performance
 constraints and avoiding routing loops.  It is hoped that the
 guidelines presented here will provide a starting point for more
 sophisticated and manageable routing in the Internet as it grows.

References

 [1]  Lougheed, K. and Y. Rekhter, "A Border Gateway Protocol", RFC
      1163, cisco Systems and IBM Watson Research Center, June 1990.
 [2]  Braun, H-W., "Models of Policy Based Routing", RFC 1104,
      Merit/NSFNET, June 1989.
 [3]  Clark, D., "Policy Routing in Internet Protocols", RFC 1102,
      M.I.T., May 1989.

Security Considerations

 Security issues are not discussed in this memo.

Authors' Addresses

 Jeffrey C. Honig
 Theory Center
 265 Olin Hall
 Cornell University
 Ithaca, NY  14853-5201
 Phone:  (607) 255-8686
 Email:  JCH@TCGOULD.TN.CORNELL.EDU
 Dave Katz
 Merit/NSFNET
 1075 Beal Ave.
 Ann Arbor, MI  48109
 Phone:  (313) 763-4898
 Email:  DKATZ@MERIT.EDU

Interconnectivity Working Group [Page 22] RFC 1164 BGP - Application June 1990

 Matt Mathis
 Pittsburgh Supercomputing Center
 4400 Fifth Ave.
 Pittsburgh, PA  15213
 Phone:  (412) 268-3319
 Email:  MATHIS@FARADAY.ECE.CMU.EDU
 Yakov Rekhter
 T.J. Watson Research Center
 IBM Corporation
 P.O. Box 218
 Yorktown Heights, NY  10598
 Phone:  (914) 945-3896
 Email:  YAKOV@IBM.COM
 Jie Yun (Jessica) Yu
 Merit/NSFNET
 1075 Beal Ave.
 Ann Arbor, MI  48109
 Phone:  (313) 936-3000
 Email:  JYY@MERIT.EDU

Interconnectivity Working Group [Page 23]

/data/webs/external/dokuwiki/data/pages/rfc/rfc1164.txt · Last modified: 1990/06/21 00:13 by 127.0.0.1

Donate Powered by PHP Valid HTML5 Valid CSS Driven by DokuWiki