python-tcp-ip-stack 2.7.1
TCP/IP stack written in Python.
Requires Python: >=3.10

# PyTCP ver. 2.7

PyTCP is a fully functional TCP/IP stack written in Python. It supports TCP stream-based transport with reliable packet delivery based on a sliding window mechanism and basic congestion control. It also supports IPv6/ICMPv6 protocols with SLAAC address configuration. It operates as a user space program attached to the Linux TAP interface. Today, the stack can send and receive traffic over the Internet using IPv4 and IPv6 default gateways for routing. 

Version 2.7, unlike its predecessors, contains the PyTCP stack code as a library so that it can be easily imported and used in external code. This should make the user experience smoother and eventually provide the full ability to replace the standard Linux stack calls (e.g., socket library) with the PyTcp calls in any 3rd party application.

This program is a work in progress, and it often changes due to new features being implemented, changes being made to implement features, bug fixes, etc. Therefore, if the current version is not working as expected, try to clone it again the next day or email me the problem. Any input is appreciated. Also, remember that some stack features may be implemented only partially (as needed for stack operation). They may be implemented in a sub-optimal fashion or not 100% RFC-compliant way (due to lack of time), or they may contain bug(s) that I still need to fix.

Please feel free to check my two other related projects:
 - [RusTCP](https://github.com/ccie18643/RusTCP) - Attempt to rewrite some of PyTCP funcionality in Rust and use it to create IPv6/SRv6 lab router.
 - [SeaTCP](https://github.com/ccie18643/SeaTCP) - Attempt to create low latency stack using C and Assembly languages.


#### Already implemented:

 - Stack - *fast packet parser using 'zero copy' approach*
 - Stack - *fast packet assembler using 'zero copy' approach*
 - Stack - *MAC address manipulation library - compatible with buffer protocol (Memoryview)*
 - Stack - *IPv4 address manipulation library - compatible with buffer protocol (Memoryview) (not dependent on Python standard library)*
 - Stack - *IPv6 address manipulation library - compatible with buffer protocol (Memoryview) (not dependent on Python standard library)*
 - Code - *Unit testing for some of libraries and modules (based on Facebook's Testslide library)*
 - Ethernet protocol - *support of Ethernet II standard frame*
 - Ethernet protocol - *unicast, IPv4 multicast, IPv6 multicast and broadcast addressing supported*
 - ARP protocol - *replies, queries, ARP cache mechanism*
 - ARP protocol - *ARP Probe/Announcement IP conflict detection (ACD) mechanism*
 - IPv4 protocol - *default routing, stack can talk to hosts over Internet using IPv4 protocol*
 - IPv4 protocol - *automatic address configuration using DHCP protocol*
 - IPv4 protocol - *inbound packet defragmentation, robust mechanism able to handle out of order and overlapping data fragments*
 - IPv4 protocol - *outbound packet fragmentation*
 - IPv4 protocol - *IPv4 options accepted but not supported*
 - IPv4 protocol -  *multiple stack's IPv4 addresses supported, each of them acts as it was assigned to separate VRF* 
 - ICMPv4 protocol - *echo request, echo reply, port unreachable*
 - IPv6 protocol - *default routing, stack can talk to hosts over Internet using IPv6 protocol*
 - IPv6 protocol - *automatic Link Local address configuration using EUI64 and Duplicate Address Detection*
 - IPv6 protocol - *automatic GUA address configuration using Router Advertisement / EUI64*
 - IPv6 protocol - *automatic assignment of Solicited Node Multicast addresses*
 - IPv6 protocol - *automatic assignment of IPv6 multicast MAC addresses*
 - IPv6 protocol - *inbound packet defragmentation, robust mechanism able to handle out of order and overlapping data fragments*
 - IPv6 protocol - *outbound packet fragmentation*
 - ICMPv6 protocol - *echo request, echo reply, port unreachable*
 - ICMPv6 protocol - *Neighbor Discovery, Duplicate Address Detection*
 - ICMPv6 protocol - *Neighbor Discovery cache mechanism*
 - ICMPv6 protocol - *Multicast Listener Discovery v2 (MLDv2) protocol implementation (only messages needed by stack)*
 - UDP protocol - *full support, stack is able to exchange data with other hosts using UDP protocol*
 - UDP sockets - *full support, stack's 'end user' API similar to Berkeley sockets*
 - UDP services - *UDP Echo, Discard, Daytime implemented for testing purposes*
 - TCP protocol - *full implementation of TCP Finite State Machine, at this point stack is able to exchange bulk data with other hosts over TCP protocol*
 - TCP protocol - *TCP option support for: MSS, WSCALE, SACKPERM, TIMESTAMP*
 - TCP protocol - *TCP sliding window mechanism with and data retransmission (fast retransmit and time based scenarios)*
 - TCP protocol - *TCP backoff mechanism / basic congestion control*
 - TCP protocol - *TCP SYN/FIN packet retransmission*
 - TCP sockets - *full support, stack's 'end user' API similar to Berkeley sockets*

#### To do:

 - [ ] ICMPv6 - *MLDv2 support is quite a mess now, need to finish it*
 - [ ] Testing - *need to refactor packet flow tests (tests/packet_flow_*.py) to use the same format and dir as FPA tests based on test_frames*
 - [ ] Testing - *Create FPA unit tests for MLDv2 Report (len, str, assemble)*
 - [ ] Ip4 - *Reimplement packet defragmentation to store whole packets in flow db instead of making copies of IP header and data*
 - [ ] Stack - *Implement RAW socket support - to be used by 'user space' ping client*
 - [ ] Code - *Unit testing for libraries and modules (based on Facebook's Testslide library)*
 - [ ] Code - *Rewrite DHCPv4 protocol support to use standard fpa/fpp approach instead of legacy*
 - [ ] Stack - *get back to implementing stack debugging console so certain information about stack components can be displayed on demand by sending commands. eg 'show icmpv6 nd cache', 'show ipv6 route', etc... it should also let run interactive commands like ping or stack's udp/tcp echo clients*
 - [ ] QUIC protocol - *research and plan for the implementation, this depends on ability to create lab environment for it*
 - [ ] IPv6 protocol - *redesign the RA PI option handling and ND prefix auto configuration to properly use A nad L flags, some research also needed on case when different than /64 prefix is being advertised*
 - [ ] IPv6 protocol - *implement optional headers*
 - [ ] IPv6 protocol - *validate and possibly re-implements certain IPv6 mechanisms/processes according to RFC rules*
 - [ ] IPv6 protocol - *research and possibly implement support for certain optional IPv6 headers*
 - [ ] IPv6 protocol - *investigate Hop-by-Hop Options header and its relation to MLD2 Report message, implement if needed for MLD2 to work properly*
 - [ ] ICMPv6 protocol - *validate and possibly re-implements certain IPv6 mechanisms/processes according to RFC rules*
 - [ ] ICMPv6 protocol - *implement ND Redirect message*
 - [ ] ICMPv6 protocol - *Multicast Listener Discovery v2 (MLDv2) full implementation <-- it may be required by stack to respond to MLD queries*
 - [ ] TCP protocol - *ongoing effort of improving code and bug fixing while simulating more advanced traffic scenarios*
 - [ ] TCP protocol - *proper handling on RST packets in various states, need to do research on this*
 - [ ] TCP protocol - *need to rework the CLOSE syscall mechanism so FIN flag can be set on last data packet instead of being carried in separate one*
 - [ ] TCP protocol - *ACK packet retransmission in case we got FIN retransmission in TIME_WAIT state <-- need to investigate this*
 - [ ] TCP protocol - *implement proper response to packets containing old SEQ and/or ACK numbersi <-- need to investigate this*
 - [ ] ICMP protocols - *need to come up with some sort of "icmp socket" mechanism so ping client can bind to particular ICMP echo-reply stream*
 - [ ] IPv6/IPv4 protocols - *proper routing mechanism, route tables, etc...*
 - [ ] IPv6/IPv4 protocols - *ability of stack to act as a router*
 - [ ] ARP cache - *implement proper FSM*
 - [ ] ICMPv6 ND cache - *implement proper FSM*
 - [x] Logging - *Replace loguru with homegrown logger to improve performance and flexibility*
 - [x] Stack - *Packet flow counters to help gathering packet statistics and to let packet flow tracing for unit testing*
 - [x] Stack - *Implement feedback mechanism for TX path so packet sending failures can be communicated to sockets*
 - [x] IPv6 protocol - *ability to route traffic to external destinations via default gateway*
 - [x] TCP protocol - *ensure that event communication from TCP session to socket works properly (eg. connection reset by peer)*
 - [x] IPv4 protocol - *improvements in IP defragmentation mechanism are needed, out of order fragment handling, purging of orphaned fragments*
 - [x] UDP protocol - *need UDP echo client and mechanism to report receiving ICMP Port Unreachable message to UDP socket*
 - [x] UDP sockets - *overhaul is needed to make 'end user' interface match Berkeley sockets more closely so 3rd party aps can use it without porting*
 - [x] TCP sockets - *overhaul is needed to make 'end user' interface match Berkeley sockets more closely so 3rd party aps can use it without porting*


### Examples:

#### Couple ping packets and two monkeys delivered via TCP over IPv6 protocol

![Sample PyTCP log output](https://github.com/ccie18643/PyTCP/blob/master/doc/images/malpi_00.png)
![Sample PyTCP log output](https://github.com/ccie18643/PyTCP/blob/master/doc/images/malpi_01.png)
![Sample PyTCP log output](https://github.com/ccie18643/PyTCP/blob/master/doc/images/malpi_02.png)
![Sample PyTCP log output](https://github.com/ccie18643/PyTCP/blob/master/doc/images/malpi_03.png)
![Sample PyTCP log output](https://github.com/ccie18643/PyTCP/blob/master/doc/images/malpi_04.png)
![Sample PyTCP log output](https://github.com/ccie18643/PyTCP/blob/master/doc/images/malpi_05.png)
![Sample PyTCP log output](https://github.com/ccie18643/PyTCP/blob/master/doc/images/malpi_06.png)
![Sample PyTCP log output](https://github.com/ccie18643/PyTCP/blob/master/doc/images/malpi_07.png)
![Sample PyTCP log output](https://github.com/ccie18643/PyTCP/blob/master/doc/images/malpi_08.png)
![Sample PyTCP log output](https://github.com/ccie18643/PyTCP/blob/master/doc/images/malpi_09.png)
![Sample PyTCP log output](https://github.com/ccie18643/PyTCP/blob/master/doc/images/malpi_10.png)


#### IPv6 Neighbor Discovery / Duplicate Address Detection / Address Auto Configuration
 - stack tries to auto configure it's link local address, it generates it as EUI64 address, as part od DAD process it joins appropriate solicited node multicast group and sends neighbor solicitation for the address it generated
 - stack doesn't receive any Neighbor Advertisement for the address it generated so assigns it to its interface
 - stack tries to assign preconfigured static address, as part of DAD process it joins appropriate solicited node multicast group and sends neighbor solicitation for the static address
 - another host that has the same address already assigned replies with Neighbor Advertisement message, this tells the stack that the address its trying to assign has been already
y assigned by another host so stack cannot us it
 - stack sends out Router Solicitation message to check if there are any global prefixes it should use
 - router responds with Router Advertisement containing additional prefix
 - stack tries to assign address generated based on received prefix and EUI64 host portion, as part of DAD process it joins appropriate solicited node multicast group and sends neighbor solicitation for the static address
 - stack doesn't receive any Neighbor Advertisement for the address it generated so assigns it to its interface
 - after all addresses are assigned stacks sends out one more Multicast Listener report listing all of the multicast addresses it wants to listen to

![Sample PyTCP log output](https://github.com/ccie18643/PyTCP/blob/master/doc/images/ipv6_nd_dad_01.png)
![Sample PyTCP log output](https://github.com/ccie18643/PyTCP/blob/master/doc/images/ipv6_nd_dad_02.png)
![Sample PyTCP log output](https://github.com/ccie18643/PyTCP/blob/master/doc/images/ipv6_nd_dad_03.png)
![Sample PyTCP log output](https://github.com/ccie18643/PyTCP/blob/master/doc/images/ipv6_nd_dad_04.png)
![Sample PyTCP log output](https://github.com/ccie18643/PyTCP/blob/master/doc/images/ipv6_nd_dad_05.png)


#### TCP Fast Retransmit in action after lost TX packet
 - outgoing packet is 'lost' as result of simulated packet loss mechanism
 - peer notices the inconsistence in packet SEQ numbers and sends out 'fast retransmit request'
 - stack eceives the request and retransmits lost packet

![Sample PyTCP log output](https://github.com/ccie18643/PyTCP/blob/master/doc/images/tcp_tx_fst_ret_01.png)
![Sample PyTCP log output](https://github.com/ccie18643/PyTCP/blob/master/doc/images/tcp_tx_fst_ret_02.png)
![Sample PyTCP log output](https://github.com/ccie18643/PyTCP/blob/master/doc/images/tcp_tx_fst_ret_03.png)
![Sample PyTCP log output](https://github.com/ccie18643/PyTCP/blob/master/doc/images/tcp_tx_fst_ret_04.png)


#### Out of order queue in action during RX packet loss event
 - incoming packet is 'lost' as reult of simulated packet loss mechanism
 - stack notices that there is an inconsistence in inbound packet's SEQ number and sends out 'fast retransmit' request
 - before peer receives the request it already sends multiple packets with higher SEQ than what stack is expecting, stack queues all those packets
 - peer retransmits lost packet
 - stack receives lost packet, pulls all the packets stored in ooo queue so far and processes them
 - stacks sends out ACK packet to acknowledge latest of the packets pulled from queue

![Sample PyTCP log output](https://github.com/ccie18643/PyTCP/blob/master/docs/images/tcp_ooo_ret_01.png)
![Sample PyTCP log output](https://github.com/ccie18643/PyTCP/blob/master/docs/images/tcp_ooo_ret_02.png)
![Sample PyTCP log output](https://github.com/ccie18643/PyTCP/blob/master/docs/images/tcp_ooo_ret_03.png)
![Sample PyTCP log output](https://github.com/ccie18643/PyTCP/blob/master/docs/images/tcp_ooo_ret_04.png)
![Sample PyTCP log output](https://github.com/ccie18643/PyTCP/blob/master/docs/images/tcp_ooo_ret_05.png)
![Sample PyTCP log output](https://github.com/ccie18643/PyTCP/blob/master/docs/images/tcp_ooo_ret_06.png)


#### TCP Finite State Machine - stack is running TCP Echo service
 - peer opens connection
 - peer sends data
 - stack echoes the data back
 - peer closes connection

![Sample PyTCP log output](https://github.com/ccie18643/PyTCP/blob/master/docs/images/tcp_fsm_srv_01.png)
![Sample PyTCP log output](https://github.com/ccie18643/PyTCP/blob/master/docs/images/tcp_fsm_srv_02.png)
![Sample PyTCP log output](https://github.com/ccie18643/PyTCP/blob/master/docs/images/tcp_fsm_srv_03.png)
![Sample PyTCP log output](https://github.com/ccie18643/PyTCP/blob/master/docs/images/tcp_fsm_srv_04.png)


#### TCP Finite State Machine - stack is running TCP Echo client
 - stack opens connection
 - stack sends data
 - peer echoes the data back
 - stack closes connection

![Sample PyTCP log output](https://github.com/ccie18643/PyTCP/blob/master/docs/images/tcp_fsm_clt_01.png)
![Sample PyTCP log output](https://github.com/ccie18643/PyTCP/blob/master/docs/images/tcp_fsm_clt_02.png)
![Sample PyTCP log output](https://github.com/ccie18643/PyTCP/blob/master/docs/images/tcp_fsm_clt_03.png)
![Sample PyTCP log output](https://github.com/ccie18643/PyTCP/blob/master/docs/images/tcp_fsm_clt_04.png)


#### Pre-parse packet sanity check in action
 - first screenshot shows stack with sanity check turned off, malformed ICMPv6 packet is able to crash it
 - second screenshot shows stack with sanity check turned on, malformed ICMPv6 packet is being discarded before being passed to ICMPv6 protocol parser
 - third screenshot shows the malformed packet, number of MA records field has been set to 777 despite packet contains only one record

![Sample PyTCP log output](https://github.com/ccie18643/PyTCP/blob/master/docs/images/pre_sanity_chk_01.png)
![Sample PyTCP log output](https://github.com/ccie18643/PyTCP/blob/master/docs/images/pre_sanity_chk_02.png)
![Sample PyTCP log output](https://github.com/ccie18643/PyTCP/blob/master/docs/images/pre_sanity_chk_03.png)


#### ARP Probe/Announcement mechanism
 - stack is using ARP Probes to find any possible conflicts for every IP address that has been configured
 - one of IP addresses (192.168.9.102) is already taken so stack gets notified about it and skips it
 - rest of IP addresses are free so stack claims them by sending ARP Announcement for each of them

![Sample PyTCP log output](https://github.com/ccie18643/PyTCP/blob/master/docs/images/ip_arp_probe_01.png)
![Sample PyTCP log output](https://github.com/ccie18643/PyTCP/blob/master/docs/images/ip_arp_probe_02.png)
![Sample PyTCP log output](https://github.com/ccie18643/PyTCP/blob/master/docs/images/ip_arp_probe_03.png)
![Sample PyTCP log output](https://github.com/ccie18643/PyTCP/blob/master/docs/images/ip_arp_probe_04.png)


#### ARP resolution and handling ping packets
 - host 192.168.9.20 tries to ping the stack, to be able to do it it first sends ARP Request packet to find out stack's MAC address
 - stack responds by sending ARP Reply packet (stack doesn't need to send out its own request since it already made note of the host's MAC from host's request)
 - hosts sends ping packets, stack responds to them

![Sample PyTCP log output](https://github.com/ccie18643/PyTCP/blob/master/docs/images/arp_ping_01.png)
![Sample PyTCP log output](https://github.com/ccie18643/PyTCP/blob/master/docs/images/arp_ping_02.png)


#### IP fragmentation
 - host sends 4Kb UDP datagram using three fragmented IP packet (three fragments) 
 - stack receives packets and assembles them into single piece, then passes it (via UDP protocol handler and UDP Socket) to UDO Echo service
 - UDP Echo service picks data up and puts it back into UDP Socket
 - UDP datagram is being passed to IP protocol handler which creates IP packet and after checking that it exceedes link MTU fragments it into three separate IP packets
 - IP packets are being encapsulated in Ethernet frames and put on TX ring

![Sample PyTCP log output](https://github.com/ccie18643/PyTCP/blob/master/docs/images/ip_udp_frag_01.png)
![Sample PyTCP log output](https://github.com/ccie18643/PyTCP/blob/master/docs/images/ip_udp_frag_02.png)
![Sample PyTCP log output](https://github.com/ccie18643/PyTCP/blob/master/docs/images/ip_udp_frag_03.png)


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of sections 4 and 5, provided that you also convey the
machine-readable Corresponding Source under the terms of this License,
in one of these ways:

    a) Convey the object code in, or embodied in, a physical product
    (including a physical distribution medium), accompanied by the
    Corresponding Source fixed on a durable physical medium
    customarily used for software interchange.

    b) Convey the object code in, or embodied in, a physical product
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    written offer, valid for at least three years and valid for as
    long as you offer spare parts or customer support for that product
    model, to give anyone who possesses the object code either (1) a
    copy of the Corresponding Source for all the software in the
    product that is covered by this License, on a durable physical
    medium customarily used for software interchange, for a price no
    more than your reasonable cost of physically performing this
    conveying of source, or (2) access to copy the
    Corresponding Source from a network server at no charge.

    c) Convey individual copies of the object code with a copy of the
    written offer to provide the Corresponding Source.  This
    alternative is allowed only occasionally and noncommercially, and
    only if you received the object code with such an offer, in accord
    with subsection 6b.

    d) Convey the object code by offering access from a designated
    place (gratis or for a charge), and offer equivalent access to the
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    e) Convey the object code using peer-to-peer transmission, provided
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    Source of the work are being offered to the general public at no
    charge under subsection 6d.

  A separable portion of the object code, whose source code is excluded
from the Corresponding Source as a System Library, need not be
included in conveying the object code work.

  A "User Product" is either (1) a "consumer product", which means any
tangible personal property which is normally used for personal, family,
or household purposes, or (2) anything designed or sold for incorporation
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is a consumer product regardless of whether the product has substantial
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  "Installation Information" for a User Product means any methods,
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and execute modified versions of a covered work in that User Product from
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code is in no case prevented or interfered with solely because
modification has been made.

  If you convey an object code work under this section in, or with, or
specifically for use in, a User Product, and the conveying occurs as
part of a transaction in which the right of possession and use of the
User Product is transferred to the recipient in perpetuity or for a
fixed term (regardless of how the transaction is characterized), the
Corresponding Source conveyed under this section must be accompanied
by the Installation Information.  But this requirement does not apply
if neither you nor any third party retains the ability to install
modified object code on the User Product (for example, the work has
been installed in ROM).

  The requirement to provide Installation Information does not include a
requirement to continue to provide support service, warranty, or updates
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the User Product in which it has been modified or installed.  Access to a
network may be denied when the modification itself materially and
adversely affects the operation of the network or violates the rules and
protocols for communication across the network.

  Corresponding Source conveyed, and Installation Information provided,
in accord with this section must be in a format that is publicly
documented (and with an implementation available to the public in
source code form), and must require no special password or key for
unpacking, reading or copying.

  7. Additional Terms.

  "Additional permissions" are terms that supplement the terms of this
License by making exceptions from one or more of its conditions.
Additional permissions that are applicable to the entire Program shall
be treated as though they were included in this License, to the extent
that they are valid under applicable law.  If additional permissions
apply only to part of the Program, that part may be used separately
under those permissions, but the entire Program remains governed by
this License without regard to the additional permissions.

  When you convey a copy of a covered work, you may at your option
remove any additional permissions from that copy, or from any part of
it.  (Additional permissions may be written to require their own
removal in certain cases when you modify the work.)  You may place
additional permissions on material, added by you to a covered work,
for which you have or can give appropriate copyright permission.

  Notwithstanding any other provision of this License, for material you
add to a covered work, you may (if authorized by the copyright holders of
that material) supplement the terms of this License with terms:

    a) Disclaiming warranty or limiting liability differently from the
    terms of sections 15 and 16 of this License; or

    b) Requiring preservation of specified reasonable legal notices or
    author attributions in that material or in the Appropriate Legal
    Notices displayed by works containing it; or

    c) Prohibiting misrepresentation of the origin of that material, or
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    d) Limiting the use for publicity purposes of names of licensors or
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    e) Declining to grant rights under trademark law for use of some
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    f) Requiring indemnification of licensors and authors of that
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  All other non-permissive additional terms are considered "further
restrictions" within the meaning of section 10.  If the Program as you
received it, or any part of it, contains a notice stating that it is
governed by this License along with a term that is a further
restriction, you may remove that term.  If a license document contains
a further restriction but permits relicensing or conveying under this
License, you may add to a covered work material governed by the terms
of that license document, provided that the further restriction does
not survive such relicensing or conveying.

  If you add terms to a covered work in accord with this section, you
must place, in the relevant source files, a statement of the
additional terms that apply to those files, or a notice indicating
where to find the applicable terms.

  Additional terms, permissive or non-permissive, may be stated in the
form of a separately written license, or stated as exceptions;
the above requirements apply either way.

  8. Termination.

  You may not propagate or modify a covered work except as expressly
provided under this License.  Any attempt otherwise to propagate or
modify it is void, and will automatically terminate your rights under
this License (including any patent licenses granted under the third
paragraph of section 11).

  However, if you cease all violation of this License, then your
license from a particular copyright holder is reinstated (a)
provisionally, unless and until the copyright holder explicitly and
finally terminates your license, and (b) permanently, if the copyright
holder fails to notify you of the violation by some reasonable means
prior to 60 days after the cessation.

  Moreover, your license from a particular copyright holder is
reinstated permanently if the copyright holder notifies you of the
violation by some reasonable means, this is the first time you have
received notice of violation of this License (for any work) from that
copyright holder, and you cure the violation prior to 30 days after
your receipt of the notice.

  Termination of your rights under this section does not terminate the
licenses of parties who have received copies or rights from you under
this License.  If your rights have been terminated and not permanently
reinstated, you do not qualify to receive new licenses for the same
material under section 10.

  9. Acceptance Not Required for Having Copies.

  You are not required to accept this License in order to receive or
run a copy of the Program.  Ancillary propagation of a covered work
occurring solely as a consequence of using peer-to-peer transmission
to receive a copy likewise does not require acceptance.  However,
nothing other than this License grants you permission to propagate or
modify any covered work.  These actions infringe copyright if you do
not accept this License.  Therefore, by modifying or propagating a
covered work, you indicate your acceptance of this License to do so.

  10. Automatic Licensing of Downstream Recipients.

  Each time you convey a covered work, the recipient automatically
receives a license from the original licensors, to run, modify and
propagate that work, subject to this License.  You are not responsible
for enforcing compliance by third parties with this License.

  An "entity transaction" is a transaction transferring control of an
organization, or substantially all assets of one, or subdividing an
organization, or merging organizations.  If propagation of a covered
work results from an entity transaction, each party to that
transaction who receives a copy of the work also receives whatever
licenses to the work the party's predecessor in interest had or could
give under the previous paragraph, plus a right to possession of the
Corresponding Source of the work from the predecessor in interest, if
the predecessor has it or can get it with reasonable efforts.

  You may not impose any further restrictions on the exercise of the
rights granted or affirmed under this License.  For example, you may
not impose a license fee, royalty, or other charge for exercise of
rights granted under this License, and you may not initiate litigation
(including a cross-claim or counterclaim in a lawsuit) alleging that
any patent claim is infringed by making, using, selling, offering for
sale, or importing the Program or any portion of it.

  11. Patents.

  A "contributor" is a copyright holder who authorizes use under this
License of the Program or a work on which the Program is based.  The
work thus licensed is called the contributor's "contributor version".

  A contributor's "essential patent claims" are all patent claims
owned or controlled by the contributor, whether already acquired or
hereafter acquired, that would be infringed by some manner, permitted
by this License, of making, using, or selling its contributor version,
but do not include claims that would be infringed only as a
consequence of further modification of the contributor version.  For
purposes of this definition, "control" includes the right to grant
patent sublicenses in a manner consistent with the requirements of
this License.

  Each contributor grants you a non-exclusive, worldwide, royalty-free
patent license under the contributor's essential patent claims, to
make, use, sell, offer for sale, import and otherwise run, modify and
propagate the contents of its contributor version.

  In the following three paragraphs, a "patent license" is any express
agreement or commitment, however denominated, not to enforce a patent
(such as an express permission to practice a patent or covenant not to
sue for patent infringement).  To "grant" such a patent license to a
party means to make such an agreement or commitment not to enforce a
patent against the party.

  If you convey a covered work, knowingly relying on a patent license,
and the Corresponding Source of the work is not available for anyone
to copy, free of charge and under the terms of this License, through a
publicly available network server or other readily accessible means,
then you must either (1) cause the Corresponding Source to be so
available, or (2) arrange to deprive yourself of the benefit of the
patent license for this particular work, or (3) arrange, in a manner
consistent with the requirements of this License, to extend the patent
license to downstream recipients.  "Knowingly relying" means you have
actual knowledge that, but for the patent license, your conveying the
covered work in a country, or your recipient's use of the covered work
in a country, would infringe one or more identifiable patents in that
country that you have reason to believe are valid.

  If, pursuant to or in connection with a single transaction or
arrangement, you convey, or propagate by procuring conveyance of, a
covered work, and grant a patent license to some of the parties
receiving the covered work authorizing them to use, propagate, modify
or convey a specific copy of the covered work, then the patent license
you grant is automatically extended to all recipients of the covered
work and works based on it.

  A patent license is "discriminatory" if it does not include within
the scope of its coverage, prohibits the exercise of, or is
conditioned on the non-exercise of one or more of the rights that are
specifically granted under this License.  You may not convey a covered
work if you are a party to an arrangement with a third party that is
in the business of distributing software, under which you make payment
to the third party based on the extent of your activity of conveying
the work, and under which the third party grants, to any of the
parties who would receive the covered work from you, a discriminatory
patent license (a) in connection with copies of the covered work
conveyed by you (or copies made from those copies), or (b) primarily
for and in connection with specific products or compilations that
contain the covered work, unless you entered into that arrangement,
or that patent license was granted, prior to 28 March 2007.

  Nothing in this License shall be construed as excluding or limiting
any implied license or other defenses to infringement that may
otherwise be available to you under applicable patent law.

  12. No Surrender of Others' Freedom.

  If conditions are imposed on you (whether by court order, agreement or
otherwise) that contradict the conditions of this License, they do not
excuse you from the conditions of this License.  If you cannot convey a
covered work so as to satisfy simultaneously your obligations under this
License and any other pertinent obligations, then as a consequence you may
not convey it at all.  For example, if you agree to terms that obligate you
to collect a royalty for further conveying from those to whom you convey
the Program, the only way you could satisfy both those terms and this
License would be to refrain entirely from conveying the Program.

  13. Use with the GNU Affero General Public License.

  Notwithstanding any other provision of this License, you have
permission to link or combine any covered work with a work licensed
under version 3 of the GNU Affero General Public License into a single
combined work, and to convey the resulting work.  The terms of this
License will continue to apply to the part which is the covered work,
but the special requirements of the GNU Affero General Public License,
section 13, concerning interaction through a network will apply to the
combination as such.

  14. Revised Versions of this License.

  The Free Software Foundation may publish revised and/or new versions of
the GNU General Public License from time to time.  Such new versions will
be similar in spirit to the present version, but may differ in detail to
address new problems or concerns.

  Each version is given a distinguishing version number.  If the
Program specifies that a certain numbered version of the GNU General
Public License "or any later version" applies to it, you have the
option of following the terms and conditions either of that numbered
version or of any later version published by the Free Software
Foundation.  If the Program does not specify a version number of the
GNU General Public License, you may choose any version ever published
by the Free Software Foundation.

  If the Program specifies that a proxy can decide which future
versions of the GNU General Public License can be used, that proxy's
public statement of acceptance of a version permanently authorizes you
to choose that version for the Program.

  Later license versions may give you additional or different
permissions.  However, no additional obligations are imposed on any
author or copyright holder as a result of your choosing to follow a
later version.

  15. Disclaimer of Warranty.

  THERE IS NO WARRANTY FOR THE PROGRAM, TO THE EXTENT PERMITTED BY
APPLICABLE LAW.  EXCEPT WHEN OTHERWISE STATED IN WRITING THE COPYRIGHT
HOLDERS AND/OR OTHER PARTIES PROVIDE THE PROGRAM "AS IS" WITHOUT WARRANTY
OF ANY KIND, EITHER EXPRESSED OR IMPLIED, INCLUDING, BUT NOT LIMITED TO,
THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
PURPOSE.  THE ENTIRE RISK AS TO THE QUALITY AND PERFORMANCE OF THE PROGRAM
IS WITH YOU.  SHOULD THE PROGRAM PROVE DEFECTIVE, YOU ASSUME THE COST OF
ALL NECESSARY SERVICING, REPAIR OR CORRECTION.

  16. Limitation of Liability.

  IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING
WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MODIFIES AND/OR CONVEYS
THE PROGRAM AS PERMITTED ABOVE, BE LIABLE TO YOU FOR DAMAGES, INCLUDING ANY
GENERAL, SPECIAL, INCIDENTAL OR CONSEQUENTIAL DAMAGES ARISING OUT OF THE
USE OR INABILITY TO USE THE PROGRAM (INCLUDING BUT NOT LIMITED TO LOSS OF
DATA OR DATA BEING RENDERED INACCURATE OR LOSSES SUSTAINED BY YOU OR THIRD
PARTIES OR A FAILURE OF THE PROGRAM TO OPERATE WITH ANY OTHER PROGRAMS),
EVEN IF SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF THE POSSIBILITY OF
SUCH DAMAGES.

  17. Interpretation of Sections 15 and 16.

  If the disclaimer of warranty and limitation of liability provided
above cannot be given local legal effect according to their terms,
reviewing courts shall apply local law that most closely approximates
an absolute waiver of all civil liability in connection with the
Program, unless a warranty or assumption of liability accompanies a
copy of the Program in return for a fee.

                     END OF TERMS AND CONDITIONS