Internet DRAFT - draft-rosenau-request-v6-option
draft-rosenau-request-v6-option
Network Working Group M. Rosenau
Internet-Draft February 10, 2018
Intended status: Experimental
Expires: August 14, 2018
TCP option "request IPv6 connection"
draft-rosenau-request-v6-option-00
Abstract
This document describes an idea for a header "option" for the
Transmission Control Protocol (TCP).
The extension is used by dual-stack nodes to force the use of the
IPv6 protocol instead of the IPv4 protocol when exchanging data over
TCP.
Especially if one of the two nodes is a NAT64 or NAT46 router it
makes sense not to use IPv4 but IPv6 if both nodes are dual-stack
nodes.
This document also suggests to declare the usage of such a method
mandatory for all nodes that use IPv4 addresses that will be assigned
by RIRs in the future.
Doing so will make it impossible to (mis-) use IPv4 addresses
assigned by RIRs to ISPs for IPv4-only nodes that do not support
IPv6.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
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."
This Internet-Draft will expire on August 14, 2018.
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Copyright Notice
Copyright (c) 2018 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
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described in the Simplified BSD License.
1. Introduction
Because of the IPv4 address shortage the IPv6 protocol has been
developed. Unfortunately many nodes in the internet are still
IPv4-only and many ISPs use NATs to establish connections to servers.
Some ISPs use NAT46 to allow IPv4-only clients to connect to an
IPv6-only server. In many cases both nodes would be able to
establish an IPv6 connection instead of an IPv4 connection because
the use of the NAT can be avoided in this case.
This document describes a TCP [RFC0793] header "option" which allows
both nodes to negotiate the use of the IPv6 protocol instead of the
IPv4 protocol.
2. Terminology
2.1. Keywords in capital letters
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, RFC 2119 [RFC2119] and indicate requirement levels for compliant
implementations.
2.2. client
In the context of this document a "client" is a node which is
establishing a TCP connection to another node by sending the first
TCP packet.
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2.3. server
In the context of this document a "server" is a node which is waiting
for a client to establish a TCP connection with that node.
2.4. NAT64, NAT46
A NAT64 is a node which allows an IPv6-only client to connect to an
IPv4-only server by translating IPv4 to IPv6 packets and vice versa
or by acting as IPv6 server and IPv4 client the same time forwarding
the data from one TCP connection to the other one.
A NAT46 is a node allowing an IPv4-only client to connect to an
IPv6-only server.
Note that according to the definitions above NAT64 and NAT46 nodes
are both "clients" and "servers" the same time.
3. Theory of operation
3.1. Connection to an IPv4-only server
A connection to a server not supporting this extension looks the
following way:
client server
| |
| TCP/IPv4, SYN, V6REQ(00) |
+-------------------------------------->|
| |
| TCP/IPv4, SYN, ACK |
|<--------------------------------------+
| |
| ... |
Figure 1: Request not understood by server
The client adds an option described in this document to the "options"
field of the TCP header.
The server does not understand the option described in this document
and it will probably ignore it. The TCP connection will be
established "normally"; the option described in this document is not
used.
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3.2. IPv4 negotiated
If both the client and the server support this extension but both
nodes decide to use IPv4 for the connection the connection looks the
following way:
client server
| |
| TCP/IPv4, SYN, V6REQ(00) |
+-------------------------------------->|
| |
| TCP/IPv4, SYN, ACK, V6REQ(10) |
|<--------------------------------------+
| |
| ... |
Figure 2: IPv4 negotiated
The server will send the "normal" (SYN/ACK) packet back to the client
- as it is done for a normal TCP connection. It will add a certain
option to the "options" field of the TCP header indicating that the
option in the first packet is understood and supported by the server
but it is not used for the current connection.
3.3. IPv6 negotiated
If the client and the server decide to use IPv6 for the connection
the connection looks like this:
client server
| |
| TCP/IPv4, SYN, V6REQ(00) |
+-------------------------------------->|
| |
| TCP/IPv4, RST, V6REQ(11) |
|<--------------------------------------+
| |
| TCP/IPv6, SYN |
+-------------------------------------->|
| |
| ... |
Figure 3: IPv6 negotiated
The server sends back a TCP packet containing the IPv6 address of the
server and indicating that the IPv4 TCP connection is rejected.
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The client will re-send an initial TCP packet (SYN) to the server
using IPv6.
3.4. NAT scenario
There is even a situation where using this option in an IPv6 packet
makes sense: When using with NAT64. Such a connection looks like
this:
client NAT server
| | |
| TCP/IPv6, SYN, V6REQ(00) | |
+----------------------------------->| TCP/IP(v4 or v6), SYN, V6REQ(00) |
| +------------------------------------>|
| | |
| | TCP/IP, RST, V6REQ(11) |
| TCP/IPv6, RST, V6REQ(11) |<------------------------------------+
|<-----------------------------------+ |
| | |
| TCP/IPv6, SYN |
+------------------------------------------------------------------------->|
| |
| ... |
Figure 4: Using with NATs
The NAT64 sends back a TCP packet containing the IPv6 address of the
server and indicating that the TCP connection to the NAT's address is
rejected.
The client will re-send an initial TCP packet (SYN) directly to the
server using IPv6 not using the NAT64.
4. Option formats
4.1. Request packet
In the initial TCP packet (the packet sent by the client to the
server that has the SYN field set and the ACK field clear) the
following TCP header option will be added by the client:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TYPE=V6REQ | LENGTH=3 |0 0| CODE |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: Request option format
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The first octet of the option is the code "V6REQ" to be assigned by
the IANA
The second octet of the option is the length of the option (as
described in RFC 793 [RFC0793]). All codes described in this
document use a length of 3 octets.
The high two bits of the third octet are zero and the low six bits
are the code described below.
A TCP implementation MUST ignore this option (with the two upper bits
in the third octet being zero) when it is found in any other packet
but the initial TCP packet (SYN) sent from the client to the server.
If a TCP implementation does not understand the "CODE" (or the
"LENGTH" field does not match the "CODE" field) it MUST ignore the
option but it SHOULD add a "request not understood" option to the
packet sent back to the client.
4.2. Informational response
If client and server decide to use IPv4 for the TCP connection the
server answers as if the option was not present (SYN, ACK) but it
adds the following option to the "options" field of the TCP header:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+--//---+
| TYPE=V6REQ | LENGTH |1 0| CODE | DATA |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+--//---+
Figure 6: Informational response option format
The highest bit of the third octet is one and the bit below is set to
zero. The low six bits are the code described below.
A TCP implementation MUST ignore this option (with the two upper bits
in the third octet being one and zero) when it is found in any other
packet but the TCP packet (SYN, ACK) sent as response to the first
packet.
DATA is additional data depending on the "CODE" field.
If a TCP implementation does not understand the "CODE" (or the
"LENGTH" field does not match the "CODE" field) it MUST ignore the
option.
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4.3. Force restart response
If client and server decide to use IPv6 for the TCP connection the
server denies the IPv4-based connection and the client shall re-
establish a connection via IPv6.
The server sends a packet with a special form that any TCP
implementation that supports this option MUST understand:
The "source port" field contains the "destination port" of the
initial packet and vice versa (just like in the case of a "normal"
TCP response).
The "acknowledgement number" field contains the value of the
"sequence number" field in the initial packet and vice versa. (Note
that none of the two numbers is incremented.)
The "RST" bit MUST be set and "ACK" and "SYN" MUST be zero. The
other control bits SHALL be zero.
The "options" field in the TCP header contains the following option:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+--//---+
| TYPE=V6REQ | LENGTH |1 1| CODE | DATA |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+--//---+
Figure 7: Force restart response option format
The highest two bits of the third octet are one. The low six bits
are the code described below.
A TCP implementation MUST ignore this option (with the two upper bits
in the third octet being one) when it is found in any other packet
but the TCP packet sent as response to the first packet.
No TCP connection has been established between client and server when
the server answered with this packet.
Typically the "V6REQ" option will be the only option in the TCP
header in this case because more options make no sense in this case.
4.4. Reserved combination
The combination "zero-one" for the highest two bits in the third
octet is reserved for future use:
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+--//---+
| TYPE=V6REQ | LENGTH |0 1| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+--//---+
Figure 8: Reserved option format
A TCP implementation MUST ignore this option (with the two upper bits
in the third octet being zero and one) when it is found in any TCP
packet.
5. Option codes
5.1. List of option codes
The following option codes are defined:
Type* Code Length Code
00 1 3 Strong IPv6 request
00 2 3 Weak IPv6 request
10 3 3 Supported for this port
10 4 3 Not supported for this port
10 5 4 Request not supported
11 6 21 Address to be used
(*) 00 = valid in request
10 = valid in an informational response
11 = valid in a force restart response
Figure 9: List of option codes
5.2. Strong IPv6 request
A client sends this option in the initial TCP packet whenever it
desires to use IPv6 for the connection as far as it is possible.
If the server understands this option but there is no TCP/IPv6 port
which is equivalent to the TCP/IPv4 port the client wants to connect
to it MUST answer with "not supported for this port".
If the server understands this option AND there is a TCP/IPv6 port
which is equivalent to the TCP/IPv4 port the client wants to connect
to the server MUST answer with "address to be used".
5.3. Weak IPv6 request
This option is similar to the "strong IPv6 request" however it is
used if the client does not necessarily prefer IPv6 over IPv4.
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If the server understands the option and there is a TCP/IPv6
equivalent for the TCP/IPv4 port but the server also does not prefer
using IPv6 over IPv4 the server MUST answer with "supported for this
port".
The client may later send a "strong IPv6 request" to get information
about the TCP/IPv6 port that can be used instead of the TCP/IPv4
port.
In all other cases the server MUST react on this option the same way
as it reacts on a "strong IPv6 request". Especially in the case that
the server wants to communicate with the client over IPv6 instead of
IPv4 it will answer with "address to be used".
5.4. Supported for this port
A server sends this option in its first packet (in the answer to the
first packet of the TCP connection) as a response to a "weak IPv6
request".
Using this option it indicates that it would support a "strong IPv6
request" for this TCP/IPv4 port.
5.5. Not supported for this port
A server sends this option in its first packet as a response to a
"strong" or "weak IPv6 request".
Using this option it indicates that the it is not possible to
establish the desired TCP connection via IPv6.
5.6. Request not supported
A server sends this option in its first packet as a response to any
"request" option that was not understood by the server.
This is the case when the initial packet from the client contained a
"V6REQ" option whose topmost two bits of the third octet were zero
but the server did not understand the meaning of that option. (For
example if the "CODE" field did not have a value understood by the
server.)
The server indicates that it ignored the option it did not
understand.
Note that the "request not supported" option is only of informational
kind; the client SHOULD NOT treat this as error message.
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If the server did not understand the value of the "CODE" field the
"request not supported" option has the following form:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TYPE=V6REQ | LENGTH=4 |1 0| CODE=5 |L|0| ORG. CODE |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 10: Request not supported option format
"ORG. CODE" is the value of the "CODE" field in the request not
understood by the server.
"L" is zero if the code is not understood by the server at all. "L"
is one if the server understands the code but the length of the
option is not correct (currently: 3).
Note: If there will be "requests" (type="00") in the future being
longer than three octets a "response" indicating "invalid arguments"
should be defined. "Request not supported" should not be used in
this case.
5.7. Address to be used
A server sends this option in a "force restart response"
(Section 4.3) described above.
Doing so the server instructs the client to use IPv6 instead of IPv4
to connect to the server.
The option has the following form:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TYPE=V6REQ | LENGTH=21 |1 1| CODE=6 | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
| IPv6 ADDRESS |
| +-+-+-+-+-+-+-+-+
| | TCP ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... PORT |
+-+-+-+-+-+-+-+-+
Figure 11: Address to be used option format
The "IPv6 ADDRESS" and "TCP PORT" fields are the IPv6 address and the
TCP/IPv6 port number of a TCP port which is equivalent to the port
the client wanted to connect to using IPv4.
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Note that especially in the case of NAT46 the IPv6 address may be
different for different TCP ports. The TCP port number to be used
with IPv6 may also differ from the TCP port number to be used with
IPv4 - especially in a NAT46 scenario.
The TCP/IPv6 port returned MUST be absolutely equivalent to the TCP/
IPv4 port the client wanted to connect to. This means that when
establishing a connection from an IPv6 client to the server there
must be no difference in functionality when directly connecting to
the TCP/IPv6 port returned by the server compared to connecting to
the TCP/IPv4 port via a NAT64.
6. Using the option with IPv6
As described above the option may also used in TCP/IPv6 packets.
This typically only makes sense if the server is a NAT.
If the server understands this option, it is a NAT and the client can
connect to the actual server (behind the NAT) directly the NAT will
respond to a "strong IPv6 request" with an "address to be used"
answer.
If the server understands this option but it is not a NAT or the
client cannot connect to the actual server directly it will respond
with "not supported for this port".
A NAT may behave differently for different ports - e.g. if one port
is forwarded to a server which can be accessed directly and another
port is forwarded to a server which cannot be accessed directly.
The same is true when a "weak IPv6 request" is received; in this case
the NAT may decide if it answers with a "supported for this port"
option or with "address to be used".
7. Suggestion for IPv4 addresses assigned by the RIRs
IPv4 addresses are very rare.
Unfortunately there still seem to be ISPs who receive IPv4 addresses
from the RIRs using these addresses for IPv4-only nodes.
To force these ISPs to use the IPv4 addresses they receive from the
RIRs for dual-stack nodes and translation mechanisms only the author
of the document has the following suggestion:
- RIRs are only allowed to assign IPv4 addresses if the receiver
guarantees that the receiver will observe some rules for ALL IPv4
addresses used by the receiver.
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- "ALL IPv4 addresses" means:
- An organisation already having IPv4 addresses may only
receive new IPv4 addresses from a RIR if it already observes
the rules for all IPv4 addresses in use by this organisation.
As soon as it receives the new IPv4 addresses observing the
rules will be mandatory for the "old" IPv4 addresses!
- In a situation where an organisation using IPv4 addresses
being subject to these rules and one organisation using IPv4
addresses not being subject to these rules merge (e.g. in a
company takeover) the merged organisation MUST decide if ALL
IPv4 used by the organisation will be subject to these rules or
if the IPv4 addresses being subject to these rules before the
merge are returned to the RIR.
- The RIR should revoke the IPv4 addresses when the rules are
violated.
Possible rules could be (note that most of these rules are already
mandatory for "new implementations" according to RFC 6540 [RFC6540]):
- All functionality a node supports via IPv4 MUST also be
supported via IPv6 - regardless if the functionality uses TCP, UDP
or other protocols.
- This also applies to the DNS entry: There MUST be an "AAAA"
record for each host name having an "A" record using such an
address.
- The only exception is the use of a secondary host name for
forcing an IPv4 connection like "www.ipv4.example.com" (force
IPv4) instead of "www.example.com" (use the default protocol); the
operator of the host MUST ensure that the secondary host name is
only used in case of problems with the IPv6 connection.
Especially it is not allowed to publish statements like: "The
address of our home page is www.ipv4.example.com".
- On the other hand such a node MAY support functionality which is
only accessible via IPv6 but not via IPv4.
- Nodes using these IPv4 addresses MUST prefer IPv6 over IPv4
(Example: If a node wants to establish a connection to a host
having both an "AAAA" and an "A" DNS record these nodes MUST first
try to use the "AAAA" record; only if this fails they MAY use the
"A" entry.)
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- All clients using these IPv4 addresses MUST use the "strong IPv6
request" option in outgoing TCP connections over IPv4. This will
force the clients to use the direct IPv6 connection in the case of
a NAT46. It will also guarantee that it is verifiable if the
client using this IPv4 address supports IPv6.
- All servers using these IPv4 addresses MUST respond to a
"strong" and/or "weak IPv6 request" using "address to be used"
(and refuse a TCP connection via IPv4). This will reduce the load
of NAT64 servers. It will also guarantee that it is verifiable if
the server using this IPv4 address supports IPv6.
8. References
8.1. Normative References
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, DOI 10.17487/RFC0793, September 1981,
<https://www.rfc-editor.org/info/rfc793>.
[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>.
8.2. Informational References
[RFC0792] Postel, J., "Internet Control Message Protocol", STD 5,
RFC 792, DOI 10.17487/RFC0792, September 1981,
<https://www.rfc-editor.org/info/rfc792>.
[RFC6146] Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful
NAT64: Network Address and Protocol Translation from IPv6
Clients to IPv4 Servers", RFC 6146, DOI 10.17487/RFC6146,
April 2011, <https://www.rfc-editor.org/info/rfc6146>.
[RFC6540] George, W., Donley, C., Liljenstolpe, C., and L. Howard,
"IPv6 Support Required for All IP-Capable Nodes", BCP 177,
RFC 6540, DOI 10.17487/RFC6540, April 2012,
<https://www.rfc-editor.org/info/rfc6540>.
Author's Address
Martin D. J. Rosenau
Email: martin@rosenau-ka.de
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