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Versions: 00                                                            
Internet Draft                                           Cengiz Alaettinoglu
Expires  May 17, 1996                                                USC/ISI
draft-ietf-rps-aspath-00.txt                                      Jessica Yu
                                                            Merit Networking
                                                                  July, 1995


          Autonomous System Path Expression Extension to Ripe-181



Status of this Memo

This document extends Ripe-181 to enable the specification of AS path
expressions in the aut-num objects.

This document is an Internet Draft, and can be found as
draft-ietf-rps-aspath-00.txt in any standard internet drafts repository.
Internet Drafts are working documents of the Internet Engineering Task Force
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1  Introduction

Ripe-181 [Bates et al., 1994] is a language to specify routing policy
constraints for inter-domain routing in the Internet.  Ripe-181 allows the
specification of both import policy constraints (by specifying as-in and
interas-in attributes) and export policy constraints (by specifying as-out
and interas-out attributes).  There are limitations in the types of
preference and access control policies that can be described by RIPE-181 and
the limitations became evident when several enterprises tried to use
RIPE-181 to describe their routing policies.

One such limitation is the lack of ability to specify autonomous-system path
expressions, henceforth referred to as AS path expressions, in the policy
attributes (i.e. as-in, interas-in, as-out and interas-out attributes).
This document extends Ripe-181 to enable the specification of AS path
expressions in the aut-num objects.  It is assumed that the reader is

Internet Draft      AS Path Expression Extension to Ripe-181      July, 1995

familiar with Ripe-181.


2  Extended Attributes

We define four new policy attributes for the aut-num object, extended-as-in,
extended-as-out, extended-interas-in, and extended-interas-out to augment
the as-in, as-out, interas-in, and interas-out attributes respectively.  The
short names for these attributes are as follows:  xa for extended-as-in, xb
for extended-as-out, xc for extended-interas-in, and xd for
extended-interas-out.

The original attributes have the following forms [Bates et al., 1994]:

   as-in:       from <aut-num> <cost>
                accept <routing policy expression>
   as-out:      to <aut-num>
                announce <routing policy expression>
   interas-in:  from <aut-num> <local-rid> <neighbor-rid> <preference>
                accept <routing policy expression>
   interas-out: to <aut-num> <local-rid> <neighbor-rid> [<metric>]
                announce <routing policy expression>.

Please refer to RIPE-181 [Bates et al., 1994] for the details of their
syntax and semantics.

The extended attributes have the same syntax and semantics as the
corresponding original attributes except that the <routing policy
expression> is extended.  That is, we have the following new syntax for the
extended attributes:

   extended-as-in:       from <aut-num> <cost>
                         accept <extended routing policy expression>
   extended-as-out:      to <aut-num>
                         announce <extended routing policy expression>
   extended-interas-in:  from <aut-num> <local-rid> <neighbor-rid> <preference>
                         accept <extended routing policy expression>
   extended-interas-out: to <aut-num> <local-rid> <neighbor-rid> [<metric>]
                         announce <extended routing policy expression>.

<extended routing policy expression> is a logical combination of AS
numbers, AS macros, community names, route lists, predefined keywords
(currently only the keyword ANY is defined), and AS path expressions.  The
syntax and semantics of <extended routing policy expression> are identical
to the syntax and semantics of <routing policy expression> except that it
can contain AS path expressions.

An AS path expression has the following syntax:

     `<' <AS path regular expression> `>'.


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Internet Draft      AS Path Expression Extension to Ripe-181      July, 1995

The syntax and semantics of AS path regular expressions are given in
Appendix A.  An AS path expression represents the set of routes which
traverses a sequence of autonomous systems matched by the AS path regular
expression(1).  Note that the AS path regular expression only needs to match
a part of the complete AS path traversed.  For example, AS path expression
<AS1> matches the AS path ``1'' as well as the AS path ``2 1 3''.

The following are examples of extended attributes:

   extended-as-in: from AS1 5 accept <^AS1 . * AS2$> AND NOT <AS3>
   extended-as-out: to AS690 announce (<^AS1> OR <^AS2>) AND COMM_NSF_AUP.

The first example means ``accept the set of routes from AS1 with an AS_PATH
to AS2 that do not transit AS3 with preference 5.  The second example means
announce the set of routes learned from AS1 and AS2 which are in
COMM_NSF_AUP to AS690.

The same Ripe-181 rules that apply to original attributes also apply to the
extended attributes.  For example, in RIPE-181, one can only specify an
interas-in/out attribute if a corresponding as-in/out attributed is
specified for that peer AS. Hence, one can only specify an
extended-interas-in/out attribute if a corresponding extended-as-in/out
attribute is specified for that peer AS. Extended attributes are paired only
with corresponding extended attributes.


3  Interaction Between Extended and Original Attributes

In specifying policies for a given peer AS, if an AS only uses the original
attributes, or only uses the extended attributes, the semantics are clearly
defined in the previous section.  However, the semantics are not so clear
when an AS uses both the original and extended attributes for a given peer
AS. We allow only one kind of mixing of the original and the extended
attributes as depicted by the following two rules:  For a given peer AS, if
an AS uses either an extended-as-in or extended-interas-in attribute, then
for every extended-as-in policy it specifies, it also has to specify an
as-in policy; and for every extended-interas-in policy it specifies, it also
has to specify an interas-in policy.  For a given peer AS, if an AS uses
either an extended-as-out or extended-interas-out attribute, then for every
extended-as-out policy it specifies, it also has to specify an as-out
policy; and for every extended-interas-out policy it specifies, it also has
to specify an interas-out policy.  Note that the *-in attributes and *-out
attributes are treated separately.  When mixing, care should be taken that
the policies specified using the original and the matching extended
attributes are as equivalent as possible and certainly not contradictory.
_______________________________
(1)  A router can check this using the AS_PATH attribute in the Border
Gateway Protocol [Rekhter and Li, 1994] or RD_PATH attribute in the
Inter-Domain Routing Protocol [Rekhter, 1993] (other protocols have similar
mechanisms).


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Internet Draft      AS Path Expression Extension to Ripe-181      July, 1995

When a domain mixes the extended and the original attributes for a peer AS,
the tools that do not support the extended attributes use only the original
attributes.  The tools that support the extended attributes only consider
the original as-in/interas-in attributes if there is no
extended-as-in/extended-interas-in attributes, for a peer AS. Otherwise,
they only consider the extended-as-in/extended-interas-in attributes.  The
tools that support the extended attributes only consider the original
as-out/interas-out attributes if there is no
extended-as-out/extended-interas-out attributes, for a peer AS. Otherwise,
they only consider the extended-as-out/extended-interas-out attributes.  We
expect that all tools will be upgraded eventually to support the extended
attributes, at which time, original attributes can be replaced by the
extended attributes.

Here are some valid examples of mixing:

Eg. 1. (No mixing, original attributes only)
aut-num: AS1
as-in: from AS2 1 accept ANY
as-out: to AS2 announce ANY

Eg. 2. (No mixing, extended attributes only)
aut-num: AS1
extended-as-in: from AS2 1 accept ANY
extended-as-out: to AS2 announce ANY

Eg. 3. (Perfect mixing)
aut-num: AS1
as-in: from AS2 1 accept ANY
as-out: to AS2 announce ANY
extended-as-in: from AS2 1 accept ANY
extended-as-out: to AS2 announce ANY

And the following example is not valid because an extended-as-out attribute
is used without using an as-out attribute.

Eg. 4. (Invalid mixing)
aut-num: AS1
as-in: from AS2 1 accept ANY
extended-as-in: from AS2 1 accept ANY
extended-as-out: to AS2 announce ANY AND NOT <AS1 . * AS5 AS6 .* AS10$>

Note that the following example, in which as-out line does not reflect the
exact export policy of AS1, is valid.

Eg. 5. (Valid mixing)
aut-num: AS1
as-in: from AS2 1 accept ANY
as-out: to AS2 announce NOT AS10
extended-as-in: from AS2 1 accept NOT <AS5>
extended-as-out: to AS2 announce NOT <AS1 . * AS5 AS6 .* AS10>

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Internet Draft      AS Path Expression Extension to Ripe-181      July, 1995

We allow this so that the existing tools can function and produce
approximate results (using the approximate policy constraints as illustrated
in the example) at the discretion of the AS.

Eg. 6. (Valid mixing, as-out/extended-as-out only)
aut-num: AS1
as-in: from AS2 1 accept AS2
as-out: to AS2 announce AS1
extended-as-out: to AS2 announce <^[AS3 AS4 AS5]> OR AS1

In this example, as-out and extended-as-out attributes are mixed, and as-in
attribute is not mixed with an extended attribute.


4  Suggestions

We reccomend administrators who register extended attributes to also
register original attributes when possible as explained in Section 3.

We also caution that specifying AS path policy expressions which are too
topology specific can have serious consequences (see [Bates et al., 1994]).
This is because Internet's topology is dynamic and the changes in the
topology can easily affect the set of routes matched by these expressions
drastically.

The following three <extended routing policy expression> represent the same
set of routes, however the last one can be translated into a finte state
automota much more efficiently than the second one, and the second one can
be translated much more efficiently than the first one:

  o <AS1$> OR < AS2$> OR < AS3$>

  o <(AS1 | AS2 | AS3)$>

  o <[AS1 AS2 AS3]$>

We recommend the use of the last form whenever possible.


5  Acknowledgments

We thank Elise Gerich, Curtis Villamizar, Daniel Karrenberg, Dale Johnson,
Laurent Joncheray and various members of the RA team for various discussions
and their suggestions.


References

[Bates et al., 1994] T. Bates, E. Gerich, L. Joncheray, J-M. Jouanigot,
   D. Karrenberg, M. Terpstra, and J. Yu. Representation of IP Routing
   Policies in a Routing Registry. Technical Report ripe-181, RIPE, RIPE

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Internet Draft      AS Path Expression Extension to Ripe-181      July, 1995

   NCC, Amsterdam, Netherlands, October 1994.
[Rekhter and Li, 1994] Y. Rekhter and T. Li. A Border Gateway Protocol 4
   (BGP-4). Request for Comment RFC-1654, Network Information Center, July
   1994.

[Rekhter, 1993] Y. Rekhter. Inter-Domain Routing Protocol (IDRP). Journal
   of Internetworking Research and Experience, 4:61--80, 1993.



A  AS Path Regular Expressions

AS Path Regular Expressions are POSIX compliant regular expressions over the
alphabet of AS numbers.  The regular expression constructs are informally
summarized below:

ASN      where ASN is an AS number in Ripe-181 notation (e.g. AS1, AS690).
             ASN matches the AS path that is of length 1 and contains the
             corresponding AS number (e.g. AS path regular expression AS1
             matches the AS path ``1'').

ASMacro  where ASMacro is an AS macro name (see [Bates et al., 1994]).
             ASMacro matches any AS path that is matched by one of the AS's
             in the expansion of ASMacro.

.        matches any AS path matched by any AS number.
[...]    is an AS number set.  It matches any AS path matched by any AS
             number listed between the brackets.  If a `-' is used between
             two AS numbers in this set, all AS numbers between the two AS
             numbers are included in the set.  If an AS macro ASMacro is
             used in this set, all AS numbers in the expansion of ASMacro
             are included in the set.

[^...]   is a complemented AS number set.  It matches any AS path which is
             not matched by the AS numbers in the set.
^        Matches the empty string at the beginning of an AS path.

$        Matches the empty string at the end of an AS path.


We next list the regular expression operators in the decreasing order of
evaluation.  These operators are left associative, i.e. performed left to
right.

Unary postfix operators * + ? For a regular expression A, A* matches zero
             or more occurrence of A; A+ matches one or more occurrence of
             A; A?  matches zero or one occurrence of A.

Binary catanation operator This is an implicit operator and exists between
             two regular expressions A and B when no other explicit
             operator is specified.  The resulting expression AB matches

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Internet Draft      AS Path Expression Extension to Ripe-181      July, 1995

             an AS path if A matches some amount of the beginning of that
             AS path and B matches the rest of the AS path.
Binary alternative (or) operator | For a regular expressions A and B, A |
             B matches any AS path that is matched by A or B.


Paranthesis can be used to override the default order of evaluation.  Space,
tab, and underscore characters can be used to increase readibility and they
are ignored.

The BNF that parses the AS path regular expressions is as follows:

aspathre: aspathre_term
  | aspathre '|' aspathre_term

aspathre_term: aspathre_closure
  | aspathre_term aspathre_closure

aspathre_closure: aspathre_factor
  | aspathre_closure '*'
  | aspathre_closure '?'
  | aspathre_closure '+'

aspathre_factor: aspathre_no
  | '^'
  | '$'
  | '(' aspathre ')'

aspathre_no: ASNUM_TOK
  | ASMACRO_TOK
  | '.'
  | '[' aspathre_range ']'
  | '[' '^' aspathre_range ']'

aspathre_range: aspathre_subrange
  | aspathre_range aspathre_subrange

aspathre_subrange: ASNUM_TOK
  | ASMACRO_TOK
  | '.'
  | ASNUM_TOK '-' ASNUM_TOK

where ASNUM_TOK is a string whose first two characters are ``AS'' and the
rest of the characters form a valid positive integer and ASMACRO_TOK is a
string whose first three characters are ``AS-'' and the rest of the
characters are uppercase letters from the English alphabet.






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Internet Draft      AS Path Expression Extension to Ripe-181      July, 1995

Author's Present Addresses

Cengiz Alaettinoglu
Information Sciences Institute
University of Southern California
Marina del Rey, CA 90292
e-mail:  cengiz@isi.edu

Jessica Yu
Merit Networking
University of Michigan
Ann Arbor, MI 48105
e-mail:  jyy@merit.edu







































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