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Keywords: SADR, source address-dependent routing, source address, multihoming, multihoming with multiple addresses, multiple addresses, source address selection, multiple routes, multipath, disjoint routes, route diversity

Internet Engineering Task Force (IETF) M. Boutier
Request for Comments: 9079 J. Chroboczek
Category: Standards Track IRIF, University of Paris
ISSN: 2070-1721 August 2021
Source-Specific Routing in the Babel Routing Protocol
Abstract
Source-specific routing, also known as Source Address Dependent
Routing (SADR), is an extension to traditional next-hop routing where
packets are forwarded according to both their destination address and
their source address. This document describes an extension for
source-specific routing to the Babel routing protocol.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9079.
Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction and Background
1.1. Application to Multihoming
1.2. Other Applications
1.3. Specificity of Prefix Pairs
2. Specification of Requirements
3. Data Structures
3.1. The Source Table
3.2. The Route Table
3.3. The Table of Pending Seqno Requests
4. Data Forwarding
5. Protocol Operation
5.1. Protocol Messages
5.2. Wildcard Messages
6. Compatibility with the Base Protocol
6.1. Starvation and Blackholes
7. Protocol Encoding
7.1. Source Prefix Sub-TLV
7.2. Source-Specific Update
7.3. Source-Specific Route Request
7.4. Source-Specific Seqno Request
8. IANA Considerations
9. Security Considerations
10. References
10.1. Normative References
10.2. Informative References
Acknowledgments
Authors' Addresses
1. Introduction and Background
The Babel routing protocol [RFC8966] is a distance vector routing
protocol for next-hop routing. In next-hop routing, each node
maintains a forwarding table that maps destination prefixes to next
hops. The forwarding decision is a per-packet operation that depends
on the destination address of the packets and on the entries of the
forwarding table. When a packet is about to be routed, its
destination address is compared to the prefixes of the routing table:
the entry with the most specific prefix containing the destination
address of the packet is chosen, and the packet is forwarded to the
associated next hop. Next-hop routing is a simple, well-understood
paradigm that works satisfactorily in a large number of cases.
The use of next-hop routing limits the flexibility of the routing
system in two ways. First, since the routing decision is local to
each router, a router A can only select a route ABC...Z if its
neighbouring router B has selected the route BC...Z. Second, the
only criterion used by a router to choose a route is the destination
address: two packets with the same destination follow the same route.
Yet, there are other data in the IP header that could conceivably be
used to guide the routing decision -- the Type of Service (ToS) octet
and, of course, the source address.
Source-specific routing [SS-ROUTING], or Source Address Dependent
Routing (SADR), is a modest extension to next-hop routing where the
forwarding decision depends not only on the destination address but
also on the source address of the packet being routed, which makes it
possible for two packets with the same destination but different
source addresses to be routed following different paths.
This document describes a source-specific routing extension for the
Babel routing protocol [RFC8966]. This involves minor changes to the
data structures, which must include a source prefix in addition to
the destination prefix already present, and some changes to the
Update, Route Request, and Seqno Request TLVs, which are extended
with a source prefix. The source prefix is encoded using a mandatory
sub-TLV ([RFC8966], Section 4.4).
1.1. Application to Multihoming
Multihoming is the practice of connecting a single network to two or
more transit networks. The main application of source-specific
routing is a form of multihoming known as "multihoming with multiple
addresses".
Classical multihoming consists of assigning a provider-independent
range of addresses to the multihomed network and announcing it to all
transit providers. While classical multihoming works well for large
networks, the cost of obtaining a provider-independent address range
and announcing it globally in the Internet is prohibitive for small
networks. Unfortunately, it is not possible to implement classical
multihoming with ordinary provider-dependent addresses: in a network
connected to two providers A and B, a packet with a source address
allocated by A needs to be routed through the edge router connected
to A. If it is routed through the edge router connected to B, it
will most likely be filtered (dropped), in accordance with [BCP84].
In multihoming with multiple addresses, every host in the multihomed
network is assigned multiple addresses, one for each transit
provider. Additional mechanisms are needed in order (i) to choose,
for each packet, a source address that is associated with a provider
that is currently up, and (ii) to route each packet towards the
router connected to the provider associated with its source address.
One might argue that multihoming with multiple addresses splits the
difficult problem of multihoming into two simpler sub-problems.
The issue of choosing a suitable source address is a decision local
to the sending host and is an area of active research. The simplest
solution is to use a traditional transport-layer protocol, such as
TCP, and to probe all available source addresses at connection time,
analogously to what is already done with destination addresses,
either sequentially [RFC6724] or in parallel [RFC8305]. Since the
transport-layer protocol is not aware of the multiple available
addresses, flows are interrupted when the selected provider goes down
(from the point of view of the user, all TCP connections are dropped
when the network environment changes). A better user experience can
be provided by making all of the potential source and destination
addresses available to higher-layer protocols, either at the
transport layer [RFC8684] [RFC4960] or at the application layer
[RFC8445].
Source-specific routing solves the problem of routing a packet to the
edge router indicated by its source address. Every edge router
announces into the routing domain a default route specific to the
prefix associated with the provider it is connected to. This route
is propagated all the way to the routers on the access link, which
are therefore able to route every packet to the correct router.
Hosts simply send packets to their default router -- no host changes
are necessary at the network layer.
1.2. Other Applications
In addition to multihoming with multiple addresses, we are aware of
two applications of source-specific routing. Tunnels and VPNs are
packet encapsulation techniques that are commonly used in the
Internet to establish a network-layer topology that is different from
the physical topology. In some deployments, the default route points
at the tunnel; this causes the network stack to attempt to send
encapsulated packets through the tunnel, which causes it to break.
Various solutions to this problem are possible, the most common of
which is to point a host route at the tunnel endpoint.
When source-specific routing is available, it becomes possible to
announce through the tunnel a default route that is specific to the
prefix served by the tunnel. Since the encapsulated packets have a
source address that is not within that prefix, they are not routed
through the tunnel.
The third application of source-specific routing is controlled
anycast. Anycast is a technique in which a single destination
address is used to represent multiple network endpoints, collectively
called an "anycast group". A packet destined to the anycast group is
routed to an arbitrary member of the group, typically the one that is
nearest according to the routing protocol.
In many applications of anycast, such as DNS root servers, the
nondeterminism of anycast is acceptable; some applications, however,
require finer control. For example, in some Content Distribution
Networks (CDNs), every endpoint is expected to handle a well-defined
subset of the client population. With source-specific routing, it is
possible for each member of the anycast group to announce a route
specific to its client population, a technique that is both simpler
and more robust than manually tweaking the routing protocol's metric
("prepending" in BGP).
1.3. Specificity of Prefix Pairs
In ordinary next-hop routing, when multiple routing table entries
match the destination of a packet, the "longest prefix rule" mandates
that the most specific entry applies. The reason why this rule makes
sense is that the set of prefixes has the following "tree property":
For any prefixes P and P', either P and P' are disjoint, or one is
more specific than the other.
It would be a natural proposition to order pairs of prefixes
pointwise: to define that (D,S) is more specific than (D',S') when D
is more specific than D and S is more specific than S'.
Unfortunately, the set of pairs of prefixes with the pointwise
ordering doesn't satisfy the tree property. Indeed, consider the
following two pairs:
(2001:db8:0:1::/64, ::/0) and (::/0, 2001:db8:0:2::/64)
These two pairs are not disjoint (a packet with destination
2001:db8:0:1::1 and source 2001:db8:0:2::1 is matched by both), but
neither is more specific than the other. The effect is that there is
no natural, unambiguous way to interpret a routing table such as the
following:
destination source next-hop
2001:db8:0:1::/64 ::/0 A
::/0 2001:db8:0:2::/64 B
A finer ordering of pairs of prefixes is required in order to avoid
all ambiguities. There are two natural choices: destination-first
ordering, where (D,S) is more specific than (D',S') when
* D is strictly more specific than D', or
* D = D', and S is more specific than S'
and, symmetrically, source-first ordering, in which sources are
compared first and destinations second.
Expedient as it would be to leave the choice to the implementation,
this is not possible: all routers in a routing domain must use the
same ordering lest persistent routing loops occur. Indeed, consider
the following topology:
A --- B --- C --- D
Suppose that A announces a route for (::/0, 2001:db8:0:2::/64), while
D announces a route for (2001:db8:0:1::/64, ::/0). Suppose further
that B uses destination-first ordering while C uses source-first
ordering. Then a packet that matches both routes, say, with
destination 2001:db8:0:1::1 and source 2001:db8:0:2::1, would be sent
by B towards D and by C towards A and would therefore loop
indefinitely between B and C.
This document mandates (Section 4) that all routers use destination-
first ordering, which is generally believed to be more useful than
source-first ordering. Consider the following topology, where A is
an edge router connected to the Internet and B is an internal router
connected to an access network N:
(::/0, S) (D, ::/0)
Internet --- A --- B --- N
A announces a source-specific default route with source S (::/0, S),
while B announces a nonspecific route to prefix D. Consider what
happens to a packet with a destination in D and a source in S. With
destination-first ordering, the packet is routed towards the network
N, which is the only way it can possibly reach its destination. With
source-first ordering, on the other hand, the packet is sent towards
the Internet, with no hope of ever reaching its destination in N.
2. Specification of Requirements
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. Data Structures
A number of the conceptual data structures described in Section 3.2
of [RFC8966] contain a destination prefix. This specification
extends these data structures with a source prefix. Data from the
original protocol, which do not specify a source prefix, are stored
with a zero-length source prefix, which matches the exact same set of
packets as the original, non-source-specific data.
3.1. The Source Table
Every Babel node maintains a source table, as described in [RFC8966],
Section 3.2.5. A source-specific Babel node extends this table with
the following field:
* The source prefix (sprefix, splen) specifying the source address
of packets to which this entry applies.
The source table is now indexed by 5-tuples of the form (prefix,
plen, sprefix, splen, router-id).
Note that the route entry contains a source (see Sections 2 and 3.2.5
of [RFC8966]) that itself contains both destination and source
prefixes. These are two different concepts and must not be confused.
3.2. The Route Table
Every Babel node maintains a route table, as described in [RFC8966],
Section 3.2.6. Each route table entry contains, among other data, a
source, which this specification extends with a source prefix as
described above. The route table is now indexed by 5-tuples of the
form (prefix, plen, sprefix, splen, neighbour), where the first four
components are obtained from the source.
3.3. The Table of Pending Seqno Requests
Every Babel node maintains a table of pending seqno requests, as
described in [RFC8966], Section 3.2.7. A source-specific Babel node
extends this table with the following entry:
* The source prefix (sprefix, splen) being requested.
The table of pending seqno requests is now indexed by 5-tuples of the
form (prefix, plen, sprefix, splen, router-id).
4. Data Forwarding
As noted in Section 1.3, source-specific tables can, in general, be
ambiguous, and all routers in a routing domain must use the same
algorithm for choosing applicable routes. An implementation of the
extension described in this document MUST choose routing table
entries by using destination-first ordering, where routing table
entry R1 is preferred to routing table entry R2 when either R1's
destination prefix is more specific than R2's or the destination
prefixes are equal and R1's source prefix is more specific than R2's.
In practice, this means that a source-specific Babel implementation
must take care that any lower layer that performs packet forwarding
obey these semantics. More precisely:
* if the lower layers implement destination-first ordering, then the
Babel implementation SHOULD use them directly;
* if the lower layers can hold source-specific routes but not with
the right semantics, then the Babel implementation MUST either
silently ignore any source-specific routes or disambiguate the
routing table by using a suitable disambiguation algorithm (see
Section V.B of [SS-ROUTING] for such an algorithm);
* if the lower layers cannot hold source-specific routes, then a
Babel implementation MUST silently ignore any source-specific
routes.
5. Protocol Operation
This extension does not fundamentally change the operation of the
Babel protocol, and we therefore only describe differences between
the original protocol and the extended protocol.
In the original protocol, three TLVs carry a destination prefix:
Update, Route Request, and Seqno Request TLVs. This specification
extends these messages so that they may carry a Source Prefix sub-
TLV, as described in Section 7. The sub-TLV is marked as mandatory
so that an unextended implementation will silently ignore the whole
enclosing TLV. A node obeying this specification MUST NOT send a TLV
with a zero-length source prefix; instead, it sends a TLV with no
Source Prefix sub-TLV. Conversely, an extended implementation MUST
interpret an unextended TLV as carrying a source prefix of zero
length. Taken together, these properties ensure interoperability
between the original and extended protocols (see Section 6).
5.1. Protocol Messages
This extension allows three TLVs of the original Babel protocol to
carry a source prefix: Update TLVs, Route Request TLVs, and Seqno
Request TLVs.
In order to advertise a route with a non-zero length source prefix, a
node sends a source-specific update, i.e., an update with a Source
Prefix sub-TLV. When a node receives a source-specific update
(prefix, source prefix, router-id, seqno, metric) from a neighbour
neigh, it behaves as described in [RFC8966], Section 3.5.3, except
that the entry under consideration is indexed by (prefix, plen,
sprefix, splen, neigh) rather than just (prefix, plen, neigh).
Similarly, when a node needs to send a request of either kind that
applies to a route with a non-zero length source prefix, it sends a
source-specific request, i.e., a request with a Source Prefix sub-
TLV. When a node receives a source-specific request, it behaves as
described in Section 3.8 of [RFC8966], except that the request
applies to the route table entry carrying the source prefix indicated
by the Source Prefix sub-TLV.
5.2. Wildcard Messages
In the original protocol, the address encoding (AE) value 0 is used
for wildcard messages: messages that apply to all routes of any
address family and with any destination prefix. Wildcard messages
are allowed in two places in the protocol: wildcard retractions are
used to retract all of the routes previously advertised by a node on
a given interface, and wildcard route requests are used to request a
full dump of the route table from a given node. Wildcard messages
are intended to apply to all routes, including routes decorated with
additional data and AE values to be defined by future extensions;
hence, this specification extends wildcard operations to apply to all
routes, whatever the value of the source prefix.
More precisely, a node receiving an update with the AE field set to 0
and the Metric field set to infinity (a wildcard retraction) MUST
apply the route acquisition procedure described in Section 3.5.3 of
[RFC8966] to all of the routes that it has learned from the sending
node, whatever the value of the source prefix. A node MUST NOT send
a wildcard retraction with an attached source prefix, and a node that
receives a wildcard retraction with a source prefix MUST ignore the
retraction.
Similarly, a node that receives a route request with the AE field set
to 0 (a wildcard route request) SHOULD send a full routing table
dump, including routes with a non-zero length source prefix. A node
MUST NOT send a wildcard request that carries a source prefix, and a
node receiving a wildcard request with a source prefix MUST ignore
the request.
6. Compatibility with the Base Protocol
The protocol extension defined in this document is, to a great
extent, interoperable with the base protocol defined in [RFC8966]
(and all previously standardised extensions). More precisely, if
non-source-specific routers and source-specific routers are mixed in
a single routing domain, Babel's loop-avoidance properties are
preserved, and, in particular, no persistent routing loops will
occur.
However, this extension is encoded using mandatory sub-TLVs,
introduced in [RFC8966], and therefore is not compatible with the
older version of the Babel routing protocol [RFC6126], which does not
support mandatory sub-TLVs. Consequently, this extension MUST NOT be
used in a routing domain in which some routers implement [RFC6126];
otherwise, persistent routing loops may occur.
6.1. Starvation and Blackholes
In general, the discarding of source-specific routes by non-source-
specific routers will cause route starvation. Intuitively, unless
there are enough non-source-specific routes in the network, non-
source-specific routers will suffer starvation and discard packets
for destinations that are only announced by source-specific routers.
In the common case where all source-specific routes are originated at
one of a small set of edge routers, a simple yet sufficient condition
for avoiding starvation is to build a connected source-specific
backbone that includes all of the edge routers and announce a non-
source-specific default route towards the backbone.
7. Protocol Encoding
This extension defines a new sub-TLV used to carry a source prefix:
the Source Prefix sub-TLV. It can be used within an Update, Route
Request, or Seqno Request TLV to match a source-specific entry of the
route table in conjunction with the destination prefix natively
carried by these TLVs.
Since a source-specific routing entry is characterised by a single
destination prefix and a single source prefix, a source-specific
message contains exactly one Source Prefix sub-TLV. A node MUST NOT
send more than one Source Prefix sub-TLV in a TLV, and a node
receiving more than one Source Prefix sub-TLV in a single TLV MUST
ignore the TLV. It MAY ignore the whole packet.
7.1. Source Prefix Sub-TLV
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 128 | Length | Source Plen | Source Prefix...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
Fields:
Type Set to 128 to indicate a Source Prefix sub-TLV.
Length The length of the body, in octets, exclusive of the
Type and Length fields.
Source Plen The length of the advertised source prefix, in bits.
This MUST NOT be 0.
Source Prefix The source prefix being advertised. This field's size
is (Source Plen)/8 octets rounded upwards.
The length of the body TLV is normally of size 1+(Source Plen)/8
rounded upwards. If the Length field indicates a length smaller than
that, then the sub-TLV is corrupt, and the whole enclosing TLV must
be ignored; if the Length field indicates a length that is larger,
then the extra octets contained in the sub-TLV MUST be silently
ignored.
The contents of the Source Prefix sub-TLV are interpreted according
to the AE of the enclosing TLV. If a TLV with AE equal to 0 contains
a Source Prefix sub-TLV, then the whole enclosing TLV MUST be
ignored. If a TLV contains multiple Source Prefix sub-TLVs, then the
whole TLV MUST be ignored.
Note that this sub-TLV is a mandatory sub-TLV. Therefore, as
described in Section 4.4 of [RFC8966], the whole TLV MUST be ignored
if that sub-TLV is not understood (or malformed).
7.2. Source-Specific Update
The source-specific update is an Update TLV with a Source Prefix sub-
TLV. It advertises or retracts source-specific routes in the same
manner as routes with non-source-specific updates (see [RFC8966]). A
wildcard retraction (update with AE equal to 0) MUST NOT carry a
Source Prefix sub-TLV.
Babel uses a stateful compression scheme to reduce the size taken by
destination prefixes in Update TLVs (see Section 4.5 of [RFC8966]).
The source prefix defined by this extension is not compressed. On
the other hand, compression is allowed for the destination prefixes
carried by source-specific updates. As described in Section 4.5 of
[RFC8966], unextended implementations will correctly update their
parser state while otherwise ignoring the whole TLV.
7.3. Source-Specific Route Request
A source-specific route request is a Route Request TLV with a Source
Prefix sub-TLV. It prompts the receiver to send an update for a
given pair of destination and source prefixes, as described in
Section 3.8.1.1 of [RFC8966]. A wildcard request (route request with
AE equal to 0) MUST NOT carry a Source Prefix sub-TLV; if a wildcard
request with a Source Prefix sub-TLV is received, then the request
MUST be ignored.
7.4. Source-Specific Seqno Request
A source-specific seqno request is a Seqno Request TLV with a Source
Prefix sub-TLV. It requests that the receiving node perform the
procedure described in Section 3.8.1.2 of [RFC8966] but applied to a
pair consisting of a destination and source prefix.
8. IANA Considerations
IANA has allocated sub-TLV number 128 for the Source Prefix sub-TLV
in the "Babel Sub-TLV Types" registry.
9. Security Considerations
The extension defined in this document adds a new sub-TLV to three
sub-TLVs already present in the original Babel protocol and does not
change the security properties of the protocol itself. However, the
additional flexibility provided by source-specific routing might
invalidate the assumptions made by some network administrators, which
could conceivably lead to security issues.
For example, a network administrator might be tempted to abuse route
filtering (Appendix C of [RFC8966]) as a security mechanism. Unless
the filtering rules are designed to take source-specific routing into
account, they might be bypassed by a source-specific route, which
might cause traffic to reach a portion of a network that was thought
to be protected. A network administrator might also assume that no
route is more specific than a host route and use a host route in
order to direct traffic for a given destination through a security
device (e.g., a firewall); source-specific routing invalidates this
assumption, and, in some topologies, announcing a source-specific
route might conceivably be used to bypass the security device.
10. References
10.1. Normative References
[BCP84] Baker, F. and P. Savola, "Ingress Filtering for Multihomed
Networks", BCP 84, RFC 3704, March 2004.
Sriram, K., Montgomery, D., and J. Haas, "Enhanced
Feasible-Path Unicast Reverse Path Forwarding", BCP 84,
RFC 8704, February 2020.
<https://www.rfc-editor.org/info/bcp84>
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8966] Chroboczek, J. and D. Schinazi, "The Babel Routing
Protocol", RFC 8966, DOI 10.17487/RFC8966, January 2021,
<https://www.rfc-editor.org/info/rfc8966>.
10.2. Informative References
[RFC4960] Stewart, R., Ed., "Stream Control Transmission Protocol",
RFC 4960, DOI 10.17487/RFC4960, September 2007,
<https://www.rfc-editor.org/info/rfc4960>.
[RFC6126] Chroboczek, J., "The Babel Routing Protocol", RFC 6126,
DOI 10.17487/RFC6126, April 2011,
<https://www.rfc-editor.org/info/rfc6126>.
[RFC6724] Thaler, D., Ed., Draves, R., Matsumoto, A., and T. Chown,
"Default Address Selection for Internet Protocol Version 6
(IPv6)", RFC 6724, DOI 10.17487/RFC6724, September 2012,
<https://www.rfc-editor.org/info/rfc6724>.
[RFC8305] Schinazi, D. and T. Pauly, "Happy Eyeballs Version 2:
Better Connectivity Using Concurrency", RFC 8305,
DOI 10.17487/RFC8305, December 2017,
<https://www.rfc-editor.org/info/rfc8305>.
[RFC8445] Keranen, A., Holmberg, C., and J. Rosenberg, "Interactive
Connectivity Establishment (ICE): A Protocol for Network
Address Translator (NAT) Traversal", RFC 8445,
DOI 10.17487/RFC8445, July 2018,
<https://www.rfc-editor.org/info/rfc8445>.
[RFC8684] Ford, A., Raiciu, C., Handley, M., Bonaventure, O., and C.
Paasch, "TCP Extensions for Multipath Operation with
Multiple Addresses", RFC 8684, DOI 10.17487/RFC8684, March
2020, <https://www.rfc-editor.org/info/rfc8684>.
[SS-ROUTING]
Boutier, M. and J. Chroboczek, "Source-Specific Routing",
IFIP Networking Conference,
DOI 10.1109/IFIPNetworking.2015.7145305, May 2015,
<http://arxiv.org/pdf/1403.0445>.
Acknowledgments
The authors are indebted to Donald Eastlake, Joel Halpern, and Toke
Hoiland-Jorgensen for their help with this document.
Authors' Addresses
Matthieu Boutier
IRIF, University of Paris
Case 7014
75205 Paris Cedex 13
France
Email: boutier@irif.fr
Juliusz Chroboczek
IRIF, University of Paris
Case 7014
75205 Paris Cedex 13
France
Email: jch@irif.fr