[Mip6-firewall] Latest version of the firewall Drafts
Suresh Krishnan
suresh.krishnan at ericsson.com
Thu Nov 15 18:51:46 EST 2007
Hi Folks,
I have enclosed the latest version of the firewall drafts. I believe
I have addressed all the comments I received. Please let me know if you
have any comments. I will submit the drafts this weekend if there are no
comments.
Cheers
Suresh
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Network Working Group S. Krishnan
Internet-Draft Ericsson
Intended status: Informational N. Steinleitner
Expires: May 18, 2008 University of Goettingen
Y. Qiu
Institute for Infocomm Research
November 15, 2007
Guidelines for firewall administrators regarding MIPv6 traffic
draft-krishnan-mip6-firewall-admin-01
Status of this Memo
By submitting this Internet-Draft, each author represents that any
applicable patent or other IPR claims of which he or she is aware
have been or will be disclosed, and any of which he or she becomes
aware will be disclosed, in accordance with Section 6 of BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on May 18, 2008.
Copyright Notice
Copyright (C) The IETF Trust (2007).
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Abstract
This document presents some recommendations for firewall
administrators to help them configure their firewalls in a way that
allows Mobile IPv6 signaling and data messages to pass through. This
document assumes that the firewalls in question include some kind of
stateful packet filtering capability.
Table of Contents
1. Requirements notation . . . . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Home Agent behind a firewall . . . . . . . . . . . . . . . . . 5
3.1. Signaling between the MN and the HA . . . . . . . . . . . 5
3.2. IKEv2 signaling between MN and HA for establishing SAs . . 5
3.3. Data traffic from and to MN passing through the HA . . . . 6
4. Correspondent Node behind a firewall . . . . . . . . . . . . . 7
4.1. Route optimization signaling between MN and CN through
HA . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
4.2. Route optimization signaling between MN and CN . . . . . . 7
4.3. Binding Update from MN to CN . . . . . . . . . . . . . . . 8
4.4. Route Optimization data traffic from MN . . . . . . . . . 8
4.5. Bi-directional tunnelled data traffic from the MN to
the CN through HA . . . . . . . . . . . . . . . . . . . . 8
5. Mobile Node behind a firewall . . . . . . . . . . . . . . . . 10
5.1. Signaling between MN and HA . . . . . . . . . . . . . . . 10
5.2. Signaling between MN and CN . . . . . . . . . . . . . . . 10
5.3. IKEv2 signaling between MN and HA for establishing SAs . . 11
6. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 12
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
8. Security Considerations . . . . . . . . . . . . . . . . . . . 14
9. Normative References . . . . . . . . . . . . . . . . . . . . . 15
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 16
Intellectual Property and Copyright Statements . . . . . . . . . . 17
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1. Requirements notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
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2. Introduction
Network elements such as firewalls are an integral aspect of a
majority of IP networks today, given the state of security in the
Internet, threats, and vulnerabilities to data networks. MIPv6
[RFC3775] defines mobility support for IPv6 nodes. Since firewalls
are not aware of MIPv6 protocol details, they will probably interfere
with the smooth operation of the protocol. The problems caused by
firewalls to Mobile IPv6 are documented in [RFC4487].
This document presents some recommendations for firewall
administrators to help them configure their firewalls in a way that
allows Mobile IPv6 signaling and data messags to pass through. This
document assumes that the firewalls in question include some kind of
stateful packet filtering capability. The static rules that need to
be configured are described in this document. The dynamic
capabilities needed for the firewalls to implement stateful filtering
of MIPv6 packers is described in a companion document [MIP6FWVENDOR].
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3. Home Agent behind a firewall
This section presents the recommendations for configuring a firewall
that protects a home agent. For each type of traffic that needs to
pass through this firewall, recommendations are presented on how to
identify that traffic. The following types of traffic are considered
o Signaling between the MN and the HA
o IKEv2 signaling between MN and HA for establishing SAs
o Data traffic from and to MN passing through the HA
3.1. Signaling between the MN and the HA
The signaling between the MN and HA is protected using IPSec ESP.
These messages are critical to the MIPv6 protocol and if these
messages are discarded, Mobile IPv6 as specified today will cease to
work. In order to permit these messages through, the firewall has to
detect the messages using the following patterns.
Destination Address: Address of HA
Next Header: 50 (ESP)
Mobility Header Type: 5 (BU)
Destination Address: Address of HA
Next Header: 50 (ESP)
Mobility Header Type: 1 (HoTI)
This pattern will allow the BU messages from MNs to HA to pass
through. It will also allow the HoTI messages (related to route
optimization) between the MN and the HA to pass through.
3.2. IKEv2 signaling between MN and HA for establishing SAs
The MN and HA exchange IKEv2 signaling in order to establish the
security associations. The security associations so established will
later be used for securing the mobility signaling messages. Hence
these messages need to be permitted to pass through the firewalls.
The following pattern will detect these messages.
Destination Address: Address of HA
Transport Protocol: UDP
Destination UDP Port: 500
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3.3. Data traffic from and to MN passing through the HA
If a CN tries to initiate traffic to an MN, a stateful firewall would
prevent these connection requests to pass through as there is no
established state on the firewall. Since MNs do not usually provide
services, this is not usually a problem. But if this is necessary to
do, the pattern to look for is
Destination Address: MN HoA
Allowing this traffic might allow any kind of traffic, including
malicious traffic, to pass through unfiltered to the MN. This would
expose the MN to any type of possibly malicious traffic, resulting in
a denial of service or exploitation of known security
vulnerabilities. This practice is NOT RECOMMENDED.
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4. Correspondent Node behind a firewall
This section presents the recommendations for configuring a firewall
if a node behind it should be able to act as Mobile IPv6 CN. For
each type of traffic that needs to pass through this firewall,
recommendations are presented on how to identify that traffic. The
following types of traffic are considered
o Route optimization signaling between MN and CN through HA
o Route optimization signaling between MN and CN
o Binding Update from MN to CN
o Route Optimization data traffic from MN
o Bi-directional tunnelled data traffic from the MN to the CN
through HA
4.1. Route optimization signaling between MN and CN through HA
Parts of the initial route optimization signaling has to pass through
the HA, namely the HoTI and the HoT messages. Without assistance,
the HoTI message from the HA to the CN is not able to traverse the
firewall. The following pattern will allow these messages to
traverse.
Destination Address: CN Address
Mobility Header Type: 1
This pinhole allows the HoTI message from the HA to the CN to
traverse the firewall. The HoT message from the CN to the MN through
the HA can traverse the firewall without any assistance. Hence no
pinhole is required.
4.2. Route optimization signaling between MN and CN
Route Optimization allows direct communication of data packets
between the MN and a CN without tunnelling it back through the HA.
To get route optimization work, the MN has to send a CoTI message
directly to the CN, which response with a CoT message. However, a
stateful firewall would prevent the CoTI message to pass through as
there is no established state on the firewall. The following pinhole
will allow the CoTI message to traverse.
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Destination Address: CN Address
Mobility Header Type: 2
The CoT message from the CN to the MN can traverse the firewall
without any assistance. Hence no pinhole is required.
4.3. Binding Update from MN to CN
After successfully performing the return routability procedure, the
MN sends the BU to the CN and expects the BA. Since this BU does not
match any previous installed pinhole rules, an additional pinhole
with the following format is required.
Destination Address: CN Address
Mobility Header Type: 5
This allows the BU to traverse the firewall and the BA can pass the
firewall without any assistance. Therefore, the Binding Update
sequence can be performed successfully.
4.4. Route Optimization data traffic from MN
Also the Route Optimization data traffic from MN directly to the CN
can not traverse the firewall without assistance. The stateful
firewall rules specified in [MIP6FWVENDOR] will open a pinhole for
this traffic.
4.5. Bi-directional tunnelled data traffic from the MN to the CN
through HA
If a MN tries to initiate traffic to a CN through the HA using bi-
directional tunnelling, a stateful firewall would prevent these
connection requests to pass through as there is no established state
on the firewall. This is usually a problem as CNs often provide
services. A solution is to static configure the firewall to let this
traffic pass through. However, this is only an acceptable option if
it is not necessary to open an all-embracing pinhole, e.g. if the
destination ports are well-known. In this case, the pinhole has to
look like
Destination Address: CN Address
Destination Port: Application Ports
If the ports are unknown, it is necessary to install a pinhole with
only the Destination Address as pattern. Allowing this traffic might
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allow any kind of traffic, including malicious traffic, to traverse
to the CN. Allowing this traffic might allow any kind of traffic,
including malicious traffic, to pass through unfiltered to the CN.
This would expose the CN to any type of possibly malicious traffic,
resulting in a denial of service or exploitation of known security
vulnerabilities. This practice is NOT RECOMMENDED.
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5. Mobile Node behind a firewall
This section presents the recommendations for configuring a firewall
that protects the network a mobile node visiting. For each type of
traffic that needs to pass through this firewall, recommendations are
presented on how to identify that traffic. The following types of
traffic are considered
o Signaling between MN and HA
o Route Optimization Signaling between MN and CN
o IKEv2 signaling between MN and HA for establishing SAs
5.1. Signaling between MN and HA
As described in Section 3.1, the signaling between the MN and HA is
protected using IPSec ESP. Currently, a lot of firewalls are
configured to block the incoming ESP packets. Moreover, from the
view of the firewall, both source and destination addresses of these
messages from/to mobile node are variable. Fortunately, for a
stateful firewall, if the initial traffic is allowed through the
firewall, then the return traffic is also allowed. A mobile node is
always the initiator for the BU. Since MN's CoA is not able to be
known in advance, the firewall can use following patterns to permit
these messages through.
Source Address: Visited subnet prefix
Destination Address: Address of HA
Next Header: 50 (ESP)
Mobility Header Type: 1 (HoTI)
Source Address: Visited subnet prefix
Destination Address: Address of HA
Next Header: 50 (ESP)
Mobility Header Type: 5 (BU)
This pattern will allow the initial packets (e.g. BU from MNs to HA,
HoTI, etc.) to pass through the firewall. Then the return packets
(BA from HA to MN, HoT) is also able to pass through accordingly.
5.2. Signaling between MN and CN
Route Optimization allows direct communication of data packets
between the MN and a CN without tunneling it back through the HA. It
includes 3 pairs of messages: HoTI/HoT, CoTI/CoT and BU/BA. The
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first pair can pass through the firewall using the pattern described
in section 5.1. Here we discuss CoTI/CoT and BU/BA messages.
Following pattern permits these messages through the firewall.
Source Address: Visited subnet prefix
Mobility Header Type: 2 (CoTI)
Source Address: Visited subnet prefix
Mobility Header Type: 5 (BU)
This pattern allows the initial messages (CoTI and BU) from the MN to
the CN pass through the firewall. The return messages (CoT and BA)
from the CN to the MN can also passes through the firewall
accordingly.
5.3. IKEv2 signaling between MN and HA for establishing SAs
The MN and HA exchange IKEv2 signaling in order to establish the
security associations. The security associations so established will
later be used for securing the mobility signaling messages. Due to
variable source/destination IP addresses and MN always as initiator,
the following pattern will let the negotiation pass.
Source Address: Visited subnet prefix
Transport Protocol: UDP
Destination UDP Port: 500
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6. Contributors
This document is one of the deliverables of the MIPv6 firewall
design. The following members of the team were involved in the
creation of this document.
Hannes Tschofenig Hannes.Tschofenig at gmx.net
Gabor Bajko Gabor.Bajko at nokia.com
Suresh Krishnan suresh.krishnan at ericsson.com
Hesham Soliman solimanhs at gmail.com
Yaron Sheffer yaronf at checkpoint.com
Qiu Ying qiuying at i2r.a-star.edu.sg
Niklas Steinleitner steinleitner at cs.uni-goettingen.de
Vijay Devarapalli vijay.devarapalli at AzaireNet.com
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7. IANA Considerations
This document does not require any IANA action.
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8. Security Considerations
This document specifies recommendations for firewall administrators
to allow Mobile IPv6 traffic to pass through unhindered. Since some
of this traffic is encrypted it is not possible for firewalls to
discern whether it is safe or not. This document recommends a
liberal setting so that all legitimate traffic can pass. This means
that some malicious traffic may be permitted by these rules. These
rules may allow the initiation of Denial of Service attacks against
Mobile IPv6 capable nodes (the MNs, CNs and the HAs). Especially the
rules specified in Section 3.3 and Section 4.5 are broadly defined
and hence possess the most potential for abuse. Hence, if these
rules are implemented, the firewalls SHOULD be configured to rate-
limit such traffic on a per-destination basis. This would allow the
firewall to mitigate possible denial of service attacks on the
endpoints. Please note that such measures would not mitigate other
potential security issues.
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9. Normative References
[MIP6FWVENDOR]
Krishnan, S., "Guidelines for firewall vendors regarding
MIPv6 traffic", draft-krishnan-mip6-firewall-vendor-01
(work in progress), November 2007.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3775] Johnson, D., Perkins, C., and J. Arkko, "Mobility Support
in IPv6", RFC 3775, June 2004.
[RFC4487] Le, F., Faccin, S., Patil, B., and H. Tschofenig, "Mobile
IPv6 and Firewalls: Problem Statement", RFC 4487,
May 2006.
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Authors' Addresses
Suresh Krishnan
Ericsson
8400 Decarie Blvd.
Town of Mount Royal, QC
Canada
Phone: +1 514 345 7900 x42871
Email: suresh.krishnan at ericsson.com
Niklas Steinleitner
University of Goettingen
Lotzestr. 16-18
Goettingen
Germany
Email: steinleitner at cs.uni-goettingen.de
Ying Qiu
Institute for Infocomm Research
21 Heng Mui Keng Terrace
Singapore
Phone: +65-6874-6742
Email: qiuying at i2r.a-star.edu.sg
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Network Working Group S. Krishnan
Internet-Draft Ericsson
Intended status: Informational Y. Sheffer
Expires: May 18, 2008 Checkpoint
N. Steinleitner
University of Goettingen
November 15, 2007
Guidelines for firewall vendors regarding MIPv6 traffic
draft-krishnan-mip6-firewall-vendor-01
Status of this Memo
By submitting this Internet-Draft, each author represents that any
applicable patent or other IPR claims of which he or she is aware
have been or will be disclosed, and any of which he or she becomes
aware will be disclosed, in accordance with Section 6 of BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on May 18, 2008.
Copyright Notice
Copyright (C) The IETF Trust (2007).
Krishnan, et al. Expires May 18, 2008 [Page 1]
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Abstract
This document presents some recommendations for firewall vendors to
help them implement their firewalls in a way that allows Mobile IPv6
signaling and data messages to pass through. This document describes
how to implement stateful packet filtering capability for MIPv6.
Table of Contents
1. Requirements notation . . . . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. MIPv6 Firewall Primitives . . . . . . . . . . . . . . . . . . 5
3.1. Requirements . . . . . . . . . . . . . . . . . . . . . . . 5
3.2. Detecting and parsing the Mobility Header . . . . . . . . 5
3.3. Parsing Mobility Options . . . . . . . . . . . . . . . . . 5
4. Allowing signaling response packets . . . . . . . . . . . . . 6
5. Allowing data packets based on signaling . . . . . . . . . . . 7
6. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 8
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
8. Security Considerations . . . . . . . . . . . . . . . . . . . 10
9. Normative References . . . . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12
Intellectual Property and Copyright Statements . . . . . . . . . . 13
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1. Requirements notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
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2. Introduction
Network elements such as firewalls are an integral aspect of a
majority of IP networks today, given the state of security in the
Internet, threats, and vulnerabilities to data networks. MIPv6
[RFC3775] defines mobility support for IPv6 nodes. Since firewalls
are not aware of MIPv6 protocol details, they will probably interfere
with the smooth operation of the protocol. The problems caused by
firewalls to Mobile IPv6 are documented in [RFC4487].
This document presents some recommendations for firewall vendors to
help them implement their firewalls in a way that allows Mobile IPv6
signaling and data messags to pass through. This document describes
how to implement stateful packet filtering capability for MIPv6.
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3. MIPv6 Firewall Primitives
3.1. Requirements
This document assumes that the firewalls are capable of deep packet
inspection at least until the mobility header. It also assumes that
the firewalls are capable of creating filters based on arbitrary
fields based on the contents of a signaling packet.
3.2. Detecting and parsing the Mobility Header
The Mobility Header is the basic primitive in all MIPv6 signaling
messages. Thus the firewalls need to be able to recognize the
presence of the mobility header and be able to parse the contents of
the Mobility Header. The MH is described in section 6.1 of [RFC3775]
and the format of the same is scribed in section 6.1.1 of [RFC3775].
Firewalls need to be able to at least understand the contents of the
MH Type field that describes the type of signaling message carried.
3.3. Parsing Mobility Options
The Mobility Header can carry additional information in the form of
mobility options as described in section 6.2 of [RFC3775]. Some of
these mobility options need to be understood for proper creation of
state on the firewalls. Hence firewalls must be able to parse the
mobility options defined in [RFC3775].
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4. Allowing signaling response packets
The MIPv6 signalling messages are usually performed as a request-
response pair. The request message is usually allowed by setting up
a static firewall rule to allow the traffic to pass through. The
response message on the other hand can be dynamically allowed if the
firewall can automatically setup a filter for the response packets
when the request packet passes through. This is not trivial, but
fortunately is straightforward. There are 3 message pairs that are
of importance to MIPv6 signaling. They are the BU/BA, HoTI/HoT and
CoTI/CoT pairs. When the first message in the pair traverses the
firewall in one direction, the firewall must setup a filter rule to
allow the second message through in the other direction.
Consider a packet that matches a static rule configured on a firewall
Destination Address: Address of HA
Next Header: 50 (ESP)
Mobility Header Type: 5 (BU)
This rule allows a binding update message from a MN to pass through
to the HA. Once a packet that matches this rule passes through the
firewall, the firewall must setup a dynamic filter for the return
packet
Source Address: Destination Address from Packet
Destination Address: Source Address from Packet
Next Header: 50 (ESP)
Mobility Header Type: 6 (BA)
This rule ensures that the return BA packet will pass through
unhindered. The rules can be generalized as summarized in the table
below.
+---------------------------------+---------------------------------+
| Passing packet MH Type | Setup return filter with MH |
| | Type |
+---------------------------------+---------------------------------+
| Mobility Header Type:1(HoTI) | Mobility Header Type:3(HoT) |
| | |
| Mobility Header Type:2(CoTI) | Mobility Header Type:4(CoT) |
| | |
| Mobility Header Type:5(BU) | Mobility Header Type:6(BA) |
+---------------------------------+---------------------------------+
Table 1: Glossary
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5. Allowing data packets based on signaling
Once the MIPv6 signaling completes, the data traffic can begin to
flow. The traffic filters for the data traffic can be inferred from
the contents of the signaling messages that setup the session. This
section describes how firewalls can intelligently setup filters for
data traffic based on signaling traffic.The following example
describes how to setup a filter for allowing incoming route optimized
messages from a CN to an MN after the MN sent a BU message to a CN.
When the BU message from MN to CN (MH Type 5) traverses through the
firewall the firewall extracts the home address (HoA) from the Home
Address Option (section 6.3 of [RFC3775]) of the packet.
The firewall adds the following rule in order to let the return
traffic pass.
Destination Address: Source Address of the packet (MN CoA)
Source Address: Destination Address of packet (CN)
Routing Header Type 2 Address: HoA
This pattern allows all route optimized traffic coming from the CN to
the MN to pass through.
Additionally, the firewall adds a second rule in order to let the
data traffic from the MN to the CN pass through.
Source Address: Source Address of the packet (MN CoA)
Destination Address: Destination Address of packet (CN)
Next Header: IPv6 Destination Options Header(60)
Home Address Dest. Option: MN HoA
This pattern allows all route optimized traffic coming from the MN to
the CN to pass through.
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6. Contributors
This document is one of the deliverables of the MIPv6 firewall
design. The following members of the team were involved in the
creation of this document.
Hannes Tschofenig Hannes.Tschofenig at gmx.net
Gabor Bajko Gabor.Bajko at nokia.com
Suresh Krishnan suresh.krishnan at ericsson.com
Hesham Soliman solimanhs at gmail.com
Yaron Sheffer yaronf at checkpoint.com
Qiu Ying qiuying at i2r.a-star.edu.sg
Niklas Steinleitner steinleitner at cs.uni-goettingen.de
Vijay Devarapalli vijay.devarapalli at AzaireNet.com
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7. IANA Considerations
This document does not require any IANA action.
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8. Security Considerations
This document specifies recommendations for firewall vendors to allow
Mobile IPv6 traffic to pass through unhindered. This document
recommends a liberal setting of firewall rules so that all legitimate
traffic may be allowed to pass. This means that some malicious
traffic may be permitted by these rules. These rules may allow the
initiation of Denial of Service attacks against Mobile IPv6 capable
nodes (the MNs, CNs and the HAs).
One of the main goals of any firewall is to prevent unsolicited
traffic from entering the network. The proposed solution allows such
traffic into the network, albeit with a number of restrictions.
In a typical enterprise environment, an administrator cannot
distinguish Mobile IPv6 capable nodes from other nodes. In such a
situation any node in the protected network may end up receiving
unsolicited packets from outside the firewall. The risk in this case
is that such packets could trigger unknown vulnerabilities in any of
these nodes, causing denial-of-service or worse attacks. This issue
is compounded in a mobile service provider environment by the risks
specific to such environments like endpoint battery exhaustion and
spectrum misuse.
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9. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3775] Johnson, D., Perkins, C., and J. Arkko, "Mobility Support
in IPv6", RFC 3775, June 2004.
[RFC4487] Le, F., Faccin, S., Patil, B., and H. Tschofenig, "Mobile
IPv6 and Firewalls: Problem Statement", RFC 4487,
May 2006.
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Authors' Addresses
Suresh Krishnan
Ericsson
8400 Decarie Blvd.
Town of Mount Royal, QC
Canada
Phone: +1 514 345 7900 x42871
Email: suresh.krishnan at ericsson.com
Yaron Sheffer
Checkpoint
5 Hasolelim St.
Tel Aviv 67897
Israel
Email: yaronf at checkpoint.com
Niklas Steinleitner
University of Goettingen
Lotzestr. 16-18
Goettingen
Germany
Email: steinleitner at cs.uni-goettingen.de
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Full Copyright Statement
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