Subject: issue 32.2

From: Yakov Rekhter <[email-masked]>
Date: Thu, 26 Sep 2002 12:13:56 -0700

32.2) BGP Desitnation-based Forwarding Paradigm
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  Status: No Consensus
  Change: Potentially
  Summary: Proposal to clarify BGP's support for the destination-based
    forwarding paradigm.  There seems to be consensus that some sort of 
    updated text is good, but there is not yet consensus on which text
    that should be.
  
  Discussion:
  
  In response to these proposals, Yakov proposed that the real problem is that
  it is not clear that BGP is build to support the destination-based
  forwarding paradigm.  To fix this, it was propsed that:
  
  <OLD>
  
     To characterize the set of policy decisions that can be enforced
     using BGP, one must focus on the rule that a BGP speaker advertises
     to its peers (other BGP speakers which it communicates with) in
     neighboring ASs only those routes that it itself uses. This rule
     reflects the "hop-by-hop" routing paradigm generally used
     throughout the current Internet. Note that some policies cannot
     be supported by the "hop-by-hop" routing paradigm and thus
     require techniques such as source routing (aka explicit routing)
     to enforce. For example, BGP does not enable one AS to send
     traffic to a neighboring AS intending that the traffic take a
     different route from that taken by traffic originating in the
     neighboring 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" inter-AS 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.
  
  <NEW>
  
     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 can not) 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 enforce. Such
     policies can not be enforced using BGP either. For example, BGP
     does not enable one AS to send traffic to a neighboring AS
     intending that the traffic take a different route from that
     taken by traffic originating in the neighboring AS.	On the
     other hand, BGP can support any policy conforming to the
     destination-based forwarding paradigm.
  
  Curtis thinks the newer text here is more clear.
  
  In reponse to the new text, Christian Martin propsed a slightly different
  new text:
  
     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 can not) be enforces
     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 enforce. Such
     policies can not be enforced using BGP either. For example, BGP
     does not enable one AS to send traffic to a neighboring AS
     based on prefixes originating from the local AS.  On the
     other hand, BGP can support any policy conforming to the
     destination-based forwarding paradigm.
  
  To which Yakov replied:
  
      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 can not) be enforces
      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 enforce. Such
      policies can not be enforced using BGP either. For example,
      BGP does not enable one AS to send traffic through a neighboring
      AS to some destination (which is outside of the neighboring
      AS, but is reachable through the neighboring AS) intending that
      the traffic take a different route from that taken by the traffic
      to the same destination that originating in the neighboring AS.
      On the other hand, BGP can support any policy conforming to
      the destination-based forwarding paradigm.
  
  And Chris reponsed:
  
      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 can not) be enforces
      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 enforce. Such
      policies can not be enforced using BGP either. For example,
      BGP does not enable one AS to send traffic through a neighboring
      AS to some destination beyond the neighboring AS intending that
      the traffic take a different route from that taken by traffic
      to the same destination which originates in the neighboring AS.
      In other words, the BGP policy of a local AS cannot affect the
      downstream (aka, away from the local AS) forwarding policy of a
      remote AS.	On the other hand, BGP can support any policy conforming
      to the destination-based forwarding paradigm.
  
  Tom Petch prefered Yakov's second formulation, with these changes:
  
       policies can not be enforced using BGP either. For example,
       BGP does not enable one AS to send 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.
  
  Yakov agreed to Tom's suggested changes.

I would suggest we change the status of this item to Consensus
and indicate that the following paragraph

     To characterize the set of policy decisions that can be enforced
     using BGP, one must focus on the rule that a BGP speaker advertises
     to its peers (other BGP speakers which it communicates with) in
     neighboring ASs only those routes that it itself uses. This rule
     reflects the "hop-by-hop" routing paradigm generally used 
     throughout the current Internet. Note that some policies cannot
     be supported by the "hop-by-hop" routing paradigm and thus
     require techniques such as source routing (aka explicit routing)
     to enforce. For example, BGP does not enable one AS to send
     traffic to a neighboring AS intending that the traffic take a
     different route from that taken by traffic originating in the
     neighboring 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" inter-AS 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.

will be replaced in -18 with the following text:

   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 can not) 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 can not be enforced using BGP either. For example,
   BGP does not enable one AS to send 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.

Yakov.