Stream Control Transmission Protocol (SCTP) Network Address Translation Support
draft-ietf-tsvwg-natsupp-15
The information below is for an old version of the document.
| Document | Type |
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|
|---|---|---|---|
| Authors | Randall R. Stewart , Michael Tüxen , Irene Ruengeler | ||
| Last updated | 2019-11-17 | ||
| Replaces | draft-stewart-natsupp-tsvwg, draft-ietf-behave-sctpnat | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
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| Send notices to | "Gorry Fairhurst" <gorry@erg.abdn.ac.uk> |
draft-ietf-tsvwg-natsupp-15
Network Working Group R. Stewart
Internet-Draft Netflix, Inc.
Intended status: Standards Track M. Tuexen
Expires: May 20, 2020 I. Ruengeler
Muenster Univ. of Appl. Sciences
November 17, 2019
Stream Control Transmission Protocol (SCTP) Network Address Translation
Support
draft-ietf-tsvwg-natsupp-15
Abstract
The Stream Control Transmission Protocol (SCTP) provides a reliable
communications channel between two end-hosts in many ways similar to
the Transmission Control Protocol (TCP). With the widespread
deployment of Network Address Translators (NAT), specialized code has
been added to NAT for TCP that allows multiple hosts to reside behind
a NAT and yet use only a single globally unique IPv4 address, even
when two hosts (behind a NAT) choose the same port numbers for their
connection. This additional code is sometimes classified as Network
Address and Port Translation (NAPT).
This document describes the protocol extensions required for the SCTP
endpoints and the mechanisms for NAT devices necessary to provide
similar features of NAPT in the single point and multi point
traversal scenario.
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 May 20, 2020.
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Copyright Notice
Copyright (c) 2019 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 . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . 6
4.1. SCTP NAT Traversal Scenarios . . . . . . . . . . . . . . 6
4.1.1. Single Point Traversal . . . . . . . . . . . . . . . 6
4.1.2. Multi Point Traversal . . . . . . . . . . . . . . . . 7
4.2. Limitations of Classical NAPT for SCTP . . . . . . . . . 8
4.3. The SCTP-Specific Variant of NAT . . . . . . . . . . . . 8
5. Data Formats . . . . . . . . . . . . . . . . . . . . . . . . 12
5.1. Modified Chunks . . . . . . . . . . . . . . . . . . . . . 12
5.1.1. Extended ABORT Chunk . . . . . . . . . . . . . . . . 12
5.1.2. Extended ERROR Chunk . . . . . . . . . . . . . . . . 13
5.2. New Error Causes . . . . . . . . . . . . . . . . . . . . 13
5.2.1. VTag and Port Number Collision Error Cause . . . . . 14
5.2.2. Missing State Error Cause . . . . . . . . . . . . . . 14
5.2.3. Port Number Collision Error Cause . . . . . . . . . . 15
5.3. New Parameters . . . . . . . . . . . . . . . . . . . . . 16
5.3.1. Disable Restart Parameter . . . . . . . . . . . . . . 16
5.3.2. VTags Parameter . . . . . . . . . . . . . . . . . . . 16
6. Procedures for SCTP Endpoints and NAT Devices . . . . . . . . 18
6.1. Association Setup Considerations for Endpoints . . . . . 18
6.2. Handling of Internal Port Number and Verification Tag
Collisions . . . . . . . . . . . . . . . . . . . . . . . 19
6.2.1. NAT Device Considerations . . . . . . . . . . . . . . 19
6.2.2. Endpoint Considerations . . . . . . . . . . . . . . . 19
6.3. Handling of Internal Port Number Collisions . . . . . . . 20
6.3.1. NAT Device Considerations . . . . . . . . . . . . . . 20
6.3.2. Endpoint Considerations . . . . . . . . . . . . . . . 21
6.4. Handling of Missing State . . . . . . . . . . . . . . . . 21
6.4.1. NAT Device Considerations . . . . . . . . . . . . . . 21
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6.4.2. Endpoint Considerations . . . . . . . . . . . . . . . 22
6.5. Handling of Fragmented SCTP Packets by NAT Devices . . . 23
6.6. Multi Point Traversal Considerations for Endpoints . . . 23
7. Various Examples of NAT Traversals . . . . . . . . . . . . . 23
7.1. Single-homed Client to Single-homed Server . . . . . . . 24
7.2. Single-homed Client to Multi-homed Server . . . . . . . . 26
7.3. Multihomed Client and Server . . . . . . . . . . . . . . 29
7.4. NAT Loses Its State . . . . . . . . . . . . . . . . . . . 33
7.5. Peer-to-Peer Communication . . . . . . . . . . . . . . . 35
8. Socket API Considerations . . . . . . . . . . . . . . . . . . 40
8.1. Get or Set the NAT Friendliness
(SCTP_NAT_FRIENDLY) . . . . . . . . . . . . . . . . . . . 41
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 41
9.1. New Chunk Flags for Two Existing Chunk Types . . . . . . 41
9.2. Three New Error Causes . . . . . . . . . . . . . . . . . 42
9.3. Two New Chunk Parameter Types . . . . . . . . . . . . . . 43
10. Security Considerations . . . . . . . . . . . . . . . . . . . 43
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 43
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 44
12.1. Normative References . . . . . . . . . . . . . . . . . . 44
12.2. Informative References . . . . . . . . . . . . . . . . . 44
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 45
1. Introduction
Stream Control Transmission Protocol [RFC4960] provides a reliable
communications channel between two end-hosts in many ways similar to
TCP [RFC0793]. With the widespread deployment of Network Address
Translators (NAT), specialized code has been added to NAT for TCP
that allows multiple hosts to reside behind a NAT using private
addresses (see [RFC6890]) and yet use only a single globally unique
IPv4 address, even when two hosts (behind a NAT) choose the same port
numbers for their connection. This additional code is sometimes
classified as Network Address and Port Translation (NAPT). Please
note that this document focuses on the case where the NAT maps a
single or multiple private addresses to a single public address and
vice versa. To date, specialized code for SCTP has not yet been
added to most NAT devices so that only a translation of IP addresses
is supported. The end result of this is that only one SCTP-capable
host can successfully operate behind such a NAT and this host can
only be single-homed. The only alternative for supporting legacy NAT
devices is to use UDP encapsulation as specified in [RFC6951].
This document specifies procedures allowing a NAT to support SCTP by
providing similar features to those provided by a NAPT for TCP and
other supported protocols. The document also specifies a set of data
formats for SCTP packets and a set of SCTP endpoint procedures to
support NAT traversal. An SCTP implementation supporting these
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procedures can assure that in both single-homed and multi-homed cases
a NAT will maintain the appropriate state without the NAT needing to
change port numbers.
It is possible and desirable to make these changes for a number of
reasons:
o It is desirable for SCTP internal end-hosts on multiple platforms
to be able to share a NAT's public IP address in the same way that
a TCP session can use a NAT.
o If a NAT does not need to change any data within an SCTP packet it
will reduce the processing burden of NAT'ing SCTP by NOT needing
to execute the CRC32c checksum required by SCTP.
o Not having to touch the IP payload makes the processing of ICMP
messages in NAT devices easier.
An SCTP-aware NAT will need to follow these procedures for generating
appropriate SCTP packet formats.
When considering this feature it is possible to have multiple levels
of support. At each level, the Internal Host, External Host and NAT
may or may not support the features described in this document. The
following table illustrates the results of the various combinations
of support and if communications can occur between two endpoints.
+---------------+------------+---------------+---------------+
| Internal Host | NAT Device | External Host | Communication |
+---------------+------------+---------------+---------------+
| Support | Support | Support | Yes |
| Support | Support | No Support | Limited |
| Support | No Support | Support | None |
| Support | No Support | No Support | None |
| No Support | Support | Support | Limited |
| No Support | Support | No Support | Limited |
| No Support | No Support | Support | None |
| No Support | No Support | No Support | None |
+---------------+------------+---------------+---------------+
Table 1: Communication possibilities
From the table it can be seen that when a NAT device does not support
the extension no communication can occur. This assumes that the NAT
device does not handle SCTP packets at all and all SCTP packets sent
externally from behind a NAT device are discarded by the NAT. In
some cases, where the NAT device supports the feature but one of the
two hosts does not support the feature, communication may occur but
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in a limited way. For example only one host may be able to have a
connection when a collision case occurs.
2. Conventions
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. Terminology
This document uses the following terms, which are depicted in
Figure 1. Familiarity with the terminology used in [RFC4960] and
[RFC5061] is assumed.
Private-Address (Priv-Addr): The private address that is known to
the internal host.
Internal-Port (Int-Port): The port number that is in use by the host
holding the Private-Address.
Internal-VTag (Int-VTag): The SCTP Verification Tag (VTag) (see
Section 3.1 of [RFC4960]) that the internal host has chosen for
its communication. The VTag is a unique 32-bit tag that must
accompany any incoming SCTP packet for this association to the
Private-Address.
External-Address (Ext-Addr): The address that an internal host is
attempting to contact.
External-Port (Ext-Port): The port number of the peer process at the
External-Address.
External-VTag (Ext-VTag): The Verification Tag that the host holding
the External-Address has chosen for its communication. The VTag
is a unique 32-bit tag that must accompany any incoming SCTP
packet for this association to the External-Address.
Public-Address (Pub-Addr): The public address assigned to the NAT
device that it uses as a source address when sending packets
towards the External-Address.
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Internal Network | External Network
|
Private | Public External
+--------+ Address | Address /--\/--\ Address +--------+
| SCTP | +-----+ / \ | SCTP |
|endpoint|=========| NAT |=======| Internet |==========|endpoint|
| A | +-----+ \ / | B |
+--------+ Internal | \--/\--/ External+--------+
Internal Port | Port External
VTag | VTag
Figure 1: Basic network setup
4. Motivation
4.1. SCTP NAT Traversal Scenarios
This section defines the notion of single and multi point NAT
traversal.
4.1.1. Single Point Traversal
In this case, all packets in the SCTP association go through a single
NAT, as shown below:
Internal Network | External Network
|
+--------+ | /--\/--\ +--------+
| SCTP | +-----+ / \ | SCTP |
|endpoint|=========| NAT |========= | Internet | ========|endpoint|
| A | +-----+ \ / | B |
+--------+ | \--/\--/ +--------+
|
Single NAT scenario
A variation of this case is shown below, i.e., multiple NAT devices
in a single path:
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Internal | External : Internal | External
| : |
+--------+ | : | /--\/--\ +--------+
| SCTP | +-----+ : +-----+ / \ | SCTP |
|endpoint|==| NAT |=======:=======| NAT |==| Internet |==|endpoint|
| A | +-----+ : +-----+ \ / | B |
+--------+ | : | \--/\--/ +--------+
| : |
Serial NAT Devices scenario
Although one of the main benefits of SCTP multi-homing is redundant
paths, in the single point traversal scenario the NAT function
represents a single point of failure in the path of the SCTP multi-
homed association. However, the rest of the path may still benefit
from path diversity provided by SCTP multi-homing.
The two SCTP endpoints in this case can be either single-homed or
multi-homed. However, the important thing is that the NAT device (or
NAT devices) in this case sees all the packets of the SCTP
association.
4.1.2. Multi Point Traversal
This case involves multiple NAT devices and each NAT device only sees
some of the packets in the SCTP association. An example is shown
below:
Internal | External
+------+ /---\/---\
+--------+ /=======|NAT A |=========\ / \ +--------+
| SCTP | / +------+ \/ \ | SCTP |
|endpoint|/ ... | Internet |===|endpoint|
| A |\ \ / | B |
+--------+ \ +------+ / \ / +--------+
\=======|NAT B |=========/ \---\/---/
+------+
|
Parallel NAT devices scenario
This case does NOT apply to a single-homed SCTP association (i.e.,
BOTH endpoints in the association use only one IP address). The
advantage here is that the existence of multiple NAT traversal points
can preserve the path diversity of a multi-homed association for the
entire path. This in turn can improve the robustness of the
communication.
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4.2. Limitations of Classical NAPT for SCTP
Using classical NAPT may result in changing one of the SCTP port
numbers during the processing which requires the recomputation of the
transport layer checksum by the NAPT device. Whereas for UDP and TCP
this can be done very efficiently, for SCTP the checksum (CRC32c)
over the entire packet needs to be recomputed. See Appendix B of
[RFC4960] for details of the CRC32c computation. This would
considerably add to the NAT computational burden, however hardware
support may mitigate this in some implementations.
An SCTP endpoint may have multiple addresses but only has a single
port number. To make multipoint traversal work, all the NAT devices
involved must recognize the packets they see as belonging to the same
SCTP association and perform port number translation in a consistent
way. One possible way of doing this is to use a pre-defined table of
ports and addresses configured within each NAT. Other mechanisms
could make use of NAT to NAT communication. Such mechanisms have not
been deployabled on a wide scale base and thus are not a recommended
solution. Therefore the SCTP variant of NAT has been developed.
4.3. The SCTP-Specific Variant of NAT
In this section it is allowed that there are multiple SCTP capable
hosts behind a NAT that has one Public-Address. Furthermore this
section focuses on the single point traversal scenario.
The modification of SCTP packets sent to the public Internet is
simple: the source address of the packet has to be replaced with the
Public-Address. It may also be necessary to establish some state in
the NAT device to later handle incoming packets.
For the SCTP NAT processing the NAT device has to maintain a NAT
binding table of Internal-VTag, Internal-Port, External-VTag,
External-Port, Private-Address, and whether the restart procedure is
disabled or not. An entry in that NAT binding table is called a NAT
state control block. The function Create() obtains the just
mentioned parameters and returns a NAT-State control block.
For SCTP packets coming from the public Internet the destination
address of the packets has to be replaced with the Private-Address of
the host the packet has to be delivered to. The lookup of the
Private-Address is based on the External-VTag, External-Port,
Internal-VTag and the Internal-Port.
The entries in the NAT binding table need to fulfill some uniqueness
conditions. There must not be more than one entry NAT binding table
with the same pair of Internal-Port and External-Port. This rule can
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be relaxed, if all NAT binding table entries with the same Internal-
Port and External-Port have the support for the restart procedure
enabled. In this case there must be no more than one entry with the
same Internal-Port, External-Port and Ext-VTag and no more than one
NAT binding table entry with the same Internal-Port, External-Port
and Int-VTag.
The processing of outgoing SCTP packets containing an INIT chunk is
described in the following figure. The scenario shown is valid for
all message flows in this section.
/--\/--\
+--------+ +-----+ / \ +--------+
| Host A | <------> | NAT | <------> | Internet | <------> | Host B |
+--------+ +-----+ \ / +--------+
\--/\---/
INIT[Initiate-Tag]
Priv-Addr:Int-Port ------> Ext-Addr:Ext-Port
Ext-VTag=0
Create(Initiate-Tag, Int-Port, 0, Ext-Port, Priv-Addr,
RestartSupported)
Returns(NAT-State control block)
Translate To:
INIT[Initiate-Tag]
Pub-Addr:Int-Port ------> Ext-Addr:Ext-Port
Ext-VTag=0
Normally a NAT binding table entry will be created.
However, it is possible that there is already a NAT binding table
entry with the same External-Port, Internal-Port, and Internal-VTag
but different Private-Address. In this case the INIT MUST be dropped
by the NAT and an ABORT MUST be sent back to the SCTP host with the
M-Bit set and an appropriate error cause (see Section 5.1.1 for the
format). The source address of the packet containing the ABORT chunk
MUST be the destination address of the packet containing the INIT
chunk.
It is also possible that a connection to External-Address and
External-Port exists without an Internal-VTag conflict but there
exists a NAT binding table entry with the same port numbers but a
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different Private-Address. In such a case the INIT MUST be dropped
by the NAT and an ABORT SHOULD be sent back to the SCTP host with the
M-Bit set and an appropriate error cause (see Section 5.1.1 for the
format).
The processing of outgoing SCTP packets containing no INIT chunks is
described in the following figure.
/--\/--\
+--------+ +-----+ / \ +--------+
| Host A | <------> | NAT | <------> | Internet | <------> | Host B |
+--------+ +-----+ \ / +--------+
\--/\---/
Priv-Addr:Int-Port ------> Ext-Addr:Ext-Port
Ext-VTag
Translate To:
Pub-Addr:Int-Port ------> Ext-Addr:Ext-Port
Ext-VTag
The processing of incoming SCTP packets containing an INIT ACK chunk
is described in the following figure. The Lookup() function getting
as input the Internal-VTag, Internal-Port, External-VTag, and
External-Port, returns the corresponding entry of the NAT binding
table and updates the External-VTag by substituting it with the value
of the Initiate-Tag of the INIT ACK chunk. The wildcard character
signifies that the parameter's value is not considered in the
Lookup() function or changed in the Update() function, respectively.
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/--\/--\
+--------+ +-----+ / \ +--------+
| Host A | <------> | NAT | <------> | Internet | <------> | Host B |
+--------+ +-----+ \ / +--------+
\--/\---/
INIT ACK[Initiate-Tag]
Pub-Addr:Int-Port <---- Ext-Addr:Ext-Port
Int-VTag
Lookup(Int-VTag, Int-Port, *, Ext-Port)
Update(*, *, Initiate-Tag, *)
Returns(NAT-State control block containing Priv-Addr)
INIT ACK[Initiate-Tag]
Priv-Addr:Int-Port <------ Ext-Addr:Ext-Port
Int-VTag
In the case Lookup fails, the SCTP packet is dropped. If it
succeeds, the Update routine inserts the External-VTag (the Initiate-
Tag of the INIT ACK chunk) in the NAT state control block.
The processing of incoming SCTP packets containing an ABORT or
SHUTDOWN COMPLETE chunk with the T-Bit set is described in the
following figure.
/--\/--\
+--------+ +-----+ / \ +--------+
| Host A | <------> | NAT | <------> | Internet | <------> | Host B |
+--------+ +-----+ \ / +--------+
\--/\---/
Pub-Addr:Int-Port <------ Ext-Addr:Ext-Port
Ext-VTag
Lookup(*, Int-Port, Ext-VTag, Ext-Port)
Returns(NAT-State control block containing Priv-Addr)
Priv-Addr:Int-Port <------ Ext-Addr:Ext-Port
Ext-VTag
For an incoming packet containing an INIT chunk a table lookup is
made only based on the addresses and port numbers. If an entry with
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an External-VTag of zero is found, it is considered a match and the
External-VTag is updated. This allows the handling of INIT collision
through NAT.
The processing of other incoming SCTP packets is described in the
following figure.
/--\/--\
+--------+ +-----+ / \ +--------+
| Host A | <------> | NAT | <------> | Internet | <------> | Host B |
+--------+ +-----+ \ / +--------+
\--/\---/
Pub-Addr:Int-Port <------ Ext-Addr:Ext-Port
Int-VTag
Lookup(Int-VTag, Int-Port, *, Ext-Port)
Returns(NAT-State control block containing Private-Address)
Priv-Addr:Int-Port <------ Ext-Addr:Ext-Port
Int-VTag
5. Data Formats
This section defines the formats used to support NAT traversal.
Section 5.1 and Section 5.2 describe chunks and error causes sent by
NAT devices and received by SCTP endpoints. Section 5.3 describes
parameters sent by SCTP endpoints and used by NAT devices and SCTP
endpoints.
5.1. Modified Chunks
This section presents existing chunks defined in [RFC4960] that are
modified by this document.
5.1.1. Extended ABORT Chunk
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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 = 6 | Reserved |M|T| Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ \
/ zero or more Error Causes /
\ \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The ABORT chunk is extended to add the new 'M-bit'. The M-bit
indicates to the receiver of the ABORT chunk that the chunk was not
generated by the peer SCTP endpoint, but instead by a middle box.
[NOTE to RFC-Editor:
Assignment of M-bit to be confirmed by IANA.
]
5.1.2. Extended ERROR Chunk
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 = 9 | Reserved |M|T| Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ \
/ zero or more Error Causes /
\ \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The ERROR chunk defined in [RFC4960] is extended to add the new
'M-bit'. The M-bit indicates to the receiver of the ERROR chunk that
the chunk was not generated by the peer SCTP endpoint, but instead by
a middle box.
[NOTE to RFC-Editor:
Assignment of M-bit to be confirmed by IANA.
]
5.2. New Error Causes
This section defines the new error causes added by this document.
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5.2.1. VTag and Port Number Collision Error Cause
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Cause Code = 0x00B0 | Cause Length = Variable |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ Chunk /
/ \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Cause Code: 2 bytes (unsigned integer)
This field holds the IANA defined cause code for the 'VTag and
Port Number Collision' Error Cause. IANA is requested to assign
the value 0x00B0 for this cause code.
Cause Length: 2 bytes (unsigned integer)
This field holds the length in bytes of the error cause. The
value MUST be the length of the Cause-Specific Information plus 4.
Chunk: variable length
The Cause-Specific Information is filled with the chunk that
caused this error. This can be an INIT, INIT ACK, or ASCONF
chunk. Note that if the entire chunk will not fit in the ERROR
chunk or ABORT chunk being sent then the bytes that do not fit are
truncated.
[NOTE to RFC-Editor:
Assignment of cause code to be confirmed by IANA.
]
5.2.2. Missing State Error Cause
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Cause Code = 0x00B1 | Cause Length = Variable |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ Incoming Packet /
/ \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Cause Code: 2 bytes (unsigned integer)
This field holds the IANA defined cause code for the 'Missing
State' Error Cause. IANA is requested to assign the value 0x00B1
for this cause code.
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Cause Length: 2 bytes (unsigned integer)
This field holds the length in bytes of the error cause. The
value MUST be the length of the Cause-Specific Information plus 4.
Incoming Packet: variable length
The Cause-Specific Information is filled with the IPv4 or IPv6
packet that caused this error. The IPv4 or IPv6 header MUST be
included. Note that if the packet will not fit in the ERROR chunk
or ABORT chunk being sent then the bytes that do not fit are
truncated.
[NOTE to RFC-Editor:
Assignment of cause code to be confirmed by IANA.
]
5.2.3. Port Number Collision Error Cause
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Cause Code = 0x00B2 | Cause Length = Variable |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ Chunk /
/ \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Cause Code: 2 bytes (unsigned integer)
This field holds the IANA defined cause code for the 'Port Number
Collision' Error Cause. IANA is requested to assign the value
0x00B2 for this cause code.
Cause Length: 2 bytes (unsigned integer)
This field holds the length in bytes of the error cause. The
value MUST be the length of the Cause-Specific Information plus 4.
Chunk: variable length
The Cause-Specific Information is filled with the chunk that
caused this error. This can be an INIT, INIT ACK, or ASCONF
chunk. Note that if the entire chunk will not fit in the ERROR
chunk or ABORT chunk being sent then the bytes that do not fit are
truncated.
[NOTE to RFC-Editor:
Assignment of cause code to be confirmed by IANA.
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]
5.3. New Parameters
This section defines new parameters and their valid appearance
defined by this document.
5.3.1. Disable Restart Parameter
This parameter is used to indicate that the restart procedure is
requested to be disabled. Both endpoints of an association MUST
include this parameter in the INIT chunk and INIT ACK chunk when
establishing an association and MUST include it in the ASCONF chunk
when adding an address to successfully disable the restart procedure.
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 = 0xC007 | Length = 4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Parameter Type: 2 bytes (unsigned integer)
This field holds the IANA defined parameter type for the Disable
Restart Parameter. IANA is requested to assign the value 0xC007
for this parameter type.
Parameter Length: 2 bytes (unsigned integer)
This field holds the length in bytes of the parameter. The value
MUST be 4.
[NOTE to RFC-Editor:
Assignment of parameter type to be confirmed by IANA.
]
This parameter MAY appear in INIT, INIT ACK and ASCONF chunks and
MUST NOT appear in any other chunk.
5.3.2. VTags Parameter
This parameter is used to help a NAT recover from state loss.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Parameter Type = 0xC008 | Parameter Length = 16 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ASCONF-Request Correlation ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Internal Verification Tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| External Verification Tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Parameter Type: 2 bytes (unsigned integer)
This field holds the IANA defined parameter type for the VTags
Parameter. IANA is requested to assign the value 0xC008 for this
parameter type.
Parameter Length: 2 bytes (unsigned integer)
This field holds the length in bytes of the parameter. The value
MUST be 16.
ASCONF-Request Correlation ID: 4 bytes (unsigned integer)
This is an opaque integer assigned by the sender to identify each
request parameter. The receiver of the ASCONF Chunk will copy
this 32-bit value into the ASCONF Response Correlation ID field of
the ASCONF ACK response parameter. The sender of the ASCONF can
use this same value in the ASCONF ACK to find which request the
response is for. Note that the receiver MUST NOT change this
32-bit value.
Internal Verification Tag: 4 bytes (unsigned integer)
The Verification Tag that the internal host has chosen for its
communication. The Verification Tag is a unique 32-bit tag that
must accompany any incoming SCTP packet for this association to
the Private-Address.
External Verification Tag: 4 bytes (unsigned integer) The
Verification Tag that the host holding the External-Address has
chosen for its communication. The VTag is a unique 32-bit tag
that must accompany any incoming SCTP packet for this association
to the External-Address.
[NOTE to RFC-Editor:
Assignment of parameter type to be confirmed by IANA.
]
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This parameter MAY appear in ASCONF chunks and MUST NOT appear in any
other chunk.
6. Procedures for SCTP Endpoints and NAT Devices
When an SCTP endpoint is behind an SCTP-aware NAT a number of
problems may arise as it tries to communicate with its peer:
o IP addresses can not not be included in the SCTP packet. This is
discussed in Section 6.1.
o More than one host behind a NAT device could select the same VTag
and source port when talking to the same peer server. This
creates a situation where the NAT will not be able to tell the two
associations apart. This situation is discussed in Section 6.2.
o When an SCTP endpoint is a server communicating with multiple
peers and the peers are behind the same NAT, then the two
endpoints cannot be distinguished by the server. This case is
discussed in Section 6.3.
o A restart of a NAT during a conversation could cause a loss of its
state. This problem and its solution is discussed in Section 6.4.
o NAT devices need to deal with SCTP packets being fragmented at the
IP layer. This is discussed in Section 6.5.
o An SCTP endpoint may be behind two NAT devices providing
redundancy. The method to set up this scenario is discussed in
Section 6.6.
Each of these mechanisms requires additional chunks and parameters,
defined in this document, and possibly modified handling procedures
from those specified in [RFC4960].
6.1. Association Setup Considerations for Endpoints
The association setup procedure defined in [RFC4960] allows multi-
homed SCTP endpoints to exchange its IP-addresses by using IPv4 or
IPv6 address parameters in the INIT and INIT ACK chunks. However,
this doesn't work when NAT devices are present.
Every association setup from a host behind a NAT MUST NOT use
multiple private addresses. There MUST NOT be any IPv4 Address
parameter, IPv6 Address parameter, or Supported Address Types
parameter in the INIT chunk. The INIT ACK chunk MUST NOT contain any
IPv4 Address parameter or IPv6 Address parameter using non-global
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addresses. The INIT chunk and the INIT ACK chunk MUST NOT contain
any Host Name parameters.
If the association should finally be multi-homed, the procedure in
Section 6.6 MUST be used.
The INIT and INIT ACK chunk SHOULD contain the Disable Restart
parameter defined in Section 5.3.1.
6.2. Handling of Internal Port Number and Verification Tag Collisions
Consider the case where two hosts in the Private-Address space want
to set up an SCTP association with the same service provided by some
hosts in the Internet. This means that the External-Port is the
same. If they both choose the same Internal-Port and Internal-VTag,
the NAT device cannot distinguish between incoming packets anymore.
But this is very unlikely. The Internal-VTags are chosen at random
and if the Internal-Ports are also chosen from the ephemeral port
range at random this gives a 46-bit random number that has to match.
A NAPT device can control the 16-bit Natted Port and therefore avoid
collisions deterministically.
The same can happen with the External-VTag when an INIT ACK chunk or
an ASCONF chunk is processed by the NAT.
6.2.1. NAT Device Considerations
If the NAT device detects a collision of internal port numbers and
verification tags, it MUST send an ABORT chunk with the M-bit set if
the collision is triggered by an INIT or INIT ACK chunk. If such a
collision is triggered by an ASCONF chunk, it MUST send an ERROR
chunk with the M-bit. The M-bit is a new bit defined by this
document to express to SCTP that the source of this packet is a
"middle" box, not the peer SCTP endpoint (see Section 5.1.1). If a
packet containing an INIT ACK chunk triggers the collision, the
corresponding packet containing the ABORT chunk MUST contain the same
source and destination address and port numbers as the packet
containing the INIT ACK chunk. In the other two cases, the source
and destination address and port numbers MUST be swapped.
The sender of the ERROR or ABORT chunk MUST include the error cause
with cause code 'VTag and Port Number Collision' (see Section 5.2.1).
6.2.2. Endpoint Considerations
The sender of the packet containing the INIT chunk or the receiver of
the INIT ACK chunk, upon reception of an ABORT chunk with M-bit set
and the appropriate error cause code for colliding NAT binding table
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state is included, SHOULD reinitiate the association setup procedure
after choosing a new initiate tag, if the association is in COOKIE-
WAIT state. In any other state, the SCTP endpoint MUST NOT respond.
The sender of the ASCONF chunk, upon reception of an ERROR chunk with
M-bit set, MUST stop adding the path to the association.
6.3. Handling of Internal Port Number Collisions
When two SCTP hosts are behind an SCTP-aware NAT it is possible that
two SCTP hosts in the Private-Address space will want to set up an
SCTP association with the same server running on the same host in the
Internet. If the two hosts choose the same internal port, this is
considered an internal port number collision.
For the NAT, appropriate tracking may be performed by assuring that
the VTags are unique between the two hosts.
6.3.1. NAT Device Considerations
The NAT, when processing the INIT ACK, should note in its NAT binding
table that the association supports the disable restart extension.
This note is used when establishing future associations (i.e. when
processing an INIT from an internal host) to decide if the connection
should be allowed. The NAT device does the following when processing
an INIT:
o If the INIT is originating from an internal port to an external
port for which the NAT device has no matching NAT binding table
entry, it MUST allow the INIT creating an NAT binding table entry.
o If the INIT matches an existing NAT binding table entry, it MUST
validate that the disable restart feature is supported and, if it
does, allow the INIT to be forwarded.
o If the disable restart feature is not supported, the NAT device
MUST send an ABORT with the M-bit set.
The 'Port Number Collision' error cause (see Section 5.2.3) MUST be
included in the ABORT chunk sent in response to the INIT chunk.
If the collision is triggered by an ASCONF chunk, a packet containing
an ERROR chunk with the 'Port Number Collision' error cause MUST be
sent in response to the ASCONF chunk.
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6.3.2. Endpoint Considerations
For the external SCTP server on the Internet this means that the
External-Port and the External-Address are the same. If they both
have chosen the same Internal-Port the server cannot distinguish
between both associations based on the address and port numbers. For
the server it looks like the association is being restarted. To
overcome this limitation the client sends a Disable Restart parameter
in the INIT chunk.
When the server receives this parameter it does the following:
o It MUST include a Disable Restart parameter in the INIT ACK to
inform the client that it will support the feature.
o It MUST disable the restart procedures defined in [RFC4960] for
this association.
Servers that support this feature will need to be capable of
maintaining multiple connections to what appears to be the same peer
(behind the NAT) differentiated only by the VTags.
6.4. Handling of Missing State
6.4.1. NAT Device Considerations
If the NAT device receives a packet from the internal network for
which the lookup procedure does not find an entry in the NAT binding
table, a packet containing an ERROR chunk is sent back with the M-bit
set. The source address of the packet containing the ERROR chunk
MUST be the destination address of the incoming SCTP packet. The
verification tag is reflected and the T-bit is set. Such a packet
containing an ERROR chunk SHOULD NOT be sent if the received packet
contains an ABORT, SHUTDOWN COMPLETE or INIT ACK chunk. An ERROR
chunk MUST NOT be sent if the received packet contains an ERROR chunk
with the M-bit set. In any case, the packet SHOULD NOT be forwarded
to the external address.
When sending the ERROR chunk, the error cause 'Missing State' (see
Section 5.2.2) MUST be included and the M-bit of the ERROR chunk MUST
be set (see Section 5.1.2).
If the NAT device receives a packet for which it has no NAT binding
table entry and the packet contains an ASCONF chunk with the VTags
parameter, the NAT device MUST update its NAT binding table according
to the verification tags in the VTags parameter and the optional
Disable Restart parameter.
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6.4.2. Endpoint Considerations
Upon reception of this ERROR chunk by an SCTP endpoint the receiver
takes the following actions:
o It SHOULD validate that the verification tag is reflected by
looking at the VTag that would have been included in the outgoing
packet. If the validation fails, discard the incoming ERROR
chunk.
o It SHOULD validate that the peer of the SCTP association supports
the dynamic address extension. If the validation fails, discard
the incoming ERROR chunk.
o It SHOULD generate a new ASCONF chunk containing the VTags
parameter (see Section 5.3.2) and the Disable Restart parameter
(see Section 5.3.1) if the association is using the disable
restart feature. By processing this packet the NAT device can
recover the appropriate state. The procedures for generating an
ASCONF chunk can be found in [RFC5061].
The peer SCTP endpoint receiving such an ASCONF chunk SHOULD either
add the address and respond with an acknowledgment, if the address is
new to the association (following all procedures defined in
[RFC5061]). Or, if the address is already part of the association,
the SCTP endpoint MUST NOT respond with an error, but instead SHOULD
respond with an ASCONF ACK chunk acknowledging the address and take
no action (since the address is already in the association).
Note that it is possible that upon receiving an ASCONF chunk
containing the VTags parameter the NAT will realize that it has an
'Internal Port Number and Verification Tag collision'. In such a
case the NAT MUST send an ERROR chunk with the error cause code set
to 'VTag and Port Number Collision' (see Section 5.2.1).
If an SCTP endpoint receives an ERROR with 'Internal Port Number and
Verification Tag collision' as the error cause and the packet in the
Error Chunk contains an ASCONF with the VTags parameter, careful
examination of the association is required. The endpoint does the
following:
o It MUST validate that the verification tag is reflected by looking
at the VTag that would have been included in the outgoing packet.
If the validation fails, it MUST discard the packet.
o It MUST validate that the peer of the SCTP association supports
the dynamic address extension. If the peer does not support it,
the NAT Device MUST discard the incoming ERROR chunk.
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o If the association is attempting to add an address (i.e. following
the procedures in Section 6.6) then the endpoint MUST NOT consider
the address part of the association and SHOULD make no further
attempt to add the address (i.e. cancel any ASCONF timers and
remove any record of the path), since the NAT devie has a VTag
collision and the association cannot easily create a new VTag (as
it would if the error occurred when sending an INIT).
o If the endpoint has no other path, i.e. the procedure was executed
due to missing a state in the NAT device, then the endpoint MUST
abort the association. This would occur only if the local NAT
device restarted and accepted a new association before attempting
to repair the missing state (Note that this is no different than
what happens to all TCP connections when a NAT device looses its
state).
6.5. Handling of Fragmented SCTP Packets by NAT Devices
A NAT device MUST support IP reassembly of received fragmented SCTP
packets. The fragments may arrive in any order.
When an SCTP packet has to be fragmented by the NAT device and the IP
header forbids fragmentation a corresponding ICMP packet SHOULD be
sent.
6.6. Multi Point Traversal Considerations for Endpoints
If a multi-homed SCTP endpoint behind a NAT connects to a peer, it
MUST first set up the association single-homed with only one address
causing the first NAT to populate its state. Then it SHOULD add each
IP address using ASCONF chunks sent via their respective NAT devices.
The address to add is the wildcard address and the lookup address
SHOULD also contain the VTags parameter and optionally the Disable
Restart parameter.
7. Various Examples of NAT Traversals
Please note that this section is informational only.
The addresses being used in the following examples are IPv4 addresses
for private-use networks and for documentation as specified in
[RFC6890]. However, the method described here is not limited to this
NAT44 case.
The NAT binding table entries shown in the following examples do not
include the flag indicating whether the restart procedure is
supported or not. This flag is not relevant for these examples.
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7.1. Single-homed Client to Single-homed Server
The internal client starts the association with the external server
via a four-way-handshake. Host A starts by sending an INIT chunk.
/--\/--\
+--------+ +-----+ / \ +--------+
| Host A | <------> | NAT | <------> | Internet | <------> | Host B |
+--------+ +-----+ \ / +--------+
\--/\---/
+---------+--------+----------+--------+-----------+
NAT | Int | Int | Ext | Ext | Priv |
| VTag | Port | VTag | Port | Addr |
+---------+--------+----------+--------+-----------+
INIT[Initiate-Tag = 1234]
10.0.0.1:1 ------> 203.0.113.1:2
Ext-VTtag = 0
A NAT binding tabled entry is created, the source address is
substituted and the packet is sent on:
NAT creates entry:
+---------+--------+----------+--------+-----------+
NAT | Int | Int | Ext | Ext | Priv |
| VTag | Port | VTag | Port | Addr |
+---------+--------+----------+--------+-----------+
| 1234 | 1 | 0 | 2 | 10.0.0.1 |
+---------+--------+----------+--------+-----------+
INIT[Initiate-Tag = 1234]
192.0.2.1:1 ------------------------> 203.0.113.1:2
Ext-VTtag = 0
Host B receives the INIT and sends an INIT ACK with the NAT's
external address as destination address.
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/--\/--\
+--------+ +-----+ / \ +--------+
| Host A | <------> | NAT | <------> | Internet | <------> | Host B |
+--------+ +-----+ \ / +--------+
\--/\---/
INIT ACK[Initiate-Tag = 5678]
192.0.2.1:1 <----------------------- 203.0.113.1:2
Int-VTag = 1234
NAT updates entry:
+---------+--------+----------+--------+-----------+
NAT | Int | Int | Ext | Ext | Priv |
| VTag | Port | VTag | Port | Addr |
+---------+--------+----------+--------+-----------+
| 1234 | 1 | 5678 | 2 | 10.0.0.1 |
+---------+--------+----------+--------+-----------+
INIT ACK[Initiate-Tag = 5678]
10.0.0.1:1 <------ 203.0.113.1:2
Int-VTag = 1234
The handshake finishes with a COOKIE ECHO acknowledged by a COOKIE
ACK.
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/--\/--\
+--------+ +-----+ / \ +--------+
| Host A | <------> | NAT | <------> | Internet | <------> | Host B |
+--------+ +-----+ \ / +--------+
\--/\---/
COOKIE ECHO
10.0.0.1:1 ------> 203.0.113.1:2
Ext-VTag = 5678
COOKIE ECHO
192.0.2.1:1 -----------------------> 203.0.113.1:2
Ext-VTag = 5678
COOKIE ACK
192.0.2.1:1 <----------------------- 203.0.113.1:2
Int-VTag = 1234
COOKIE ACK
10.0.0.1:1 <------ 203.0.113.1:2
Int-VTag = 1234
7.2. Single-homed Client to Multi-homed Server
The internal client is single-homed whereas the external server is
multi-homed. The client (Host A) sends an INIT like in the single-
homed case.
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+--------+
/--\/--\ /-|Router 1| \
+------+ +-----+ / \ / +--------+ \ +------+
| Host | <-----> | NAT | <-> | Internet | == =| Host |
| A | +-----+ \ / \ +--------+ / | B |
+------+ \--/\--/ \-|Router 2|-/ +------+
+--------+
+---------+--------+----------+--------+-----------+
NAT | Int | Int | Ext | Ext | Priv |
| VTag | Port | VTag | Port | Addr |
+---------+--------+----------+--------+-----------+
INIT[Initiate-Tag = 1234]
10.0.0.1:1 ---> 203.0.113.1:2
Ext-VTag = 0
NAT creates entry:
+---------+--------+----------+--------+-----------+
NAT | Int | Int | Ext | Ext | Priv |
| VTag | Port | VTag | Port | Addr |
+---------+--------+----------+--------+-----------+
| 1234 | 1 | 0 | 2 | 10.0.0.1 |
+---------+--------+----------+--------+-----------+
INIT[Initiate-Tag = 1234]
192.0.2.1:1 --------------------------> 203.0.113.1:2
Ext-VTag = 0
The server (Host B) includes its two addresses in the INIT ACK chunk.
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+--------+
/--\/--\ /-|Router 1| \
+------+ +-----+ / \ / +--------+ \ +------+
| Host | <-----> | NAT | <-> | Internet | == =| Host |
| A | +-----+ \ / \ +--------+ / | B |
+------+ \--/\--/ \-|Router 2|-/ +------+
+--------+
INIT ACK[Initiate-tag = 5678, IP-Addr = 203.0.113.129]
192.0.2.1:1 <-------------------------- 203.0.113.1:2
Int-VTag = 1234
NAT does not need to change the NAT binding table for the second
address:
+---------+--------+----------+--------+-----------+
NAT | Int | Int | Ext | Ext | Priv |
| VTag | Port | VTag | Port | Addr |
+---------+--------+----------+--------+-----------+
| 1234 | 1 | 5678 | 2 | 10.0.0.1 |
+---------+--------+----------+--------+-----------+
INIT ACK[Initiate-Tag = 5678]
10.0.0.1:1 <--- 203.0.113.1:2
Int-VTag = 1234
The handshake finishes with a COOKIE ECHO acknowledged by a COOKIE
ACK.
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+--------+
/--\/--\ /-|Router 1| \
+------+ +-----+ / \ / +--------+ \ +------+
| Host | <-----> | NAT | <-> | Internet | == =| Host |
| A | +-----+ \ / \ +--------+ / | B |
+------+ \--/\--/ \-|Router 2|-/ +------+
+--------+
COOKIE ECHO
10.0.0.1:1 ---> 203.0.113.1:2
ExtVTag = 5678
COOKIE ECHO
192.0.2.1:1 --------------------------> 203.0.113.1:2
Ext-VTag = 5678
COOKIE ACK
192.0.2.1:1 <-------------------------- 203.0.113.1:2
Int-VTag = 1234
COOKIE ACK
10.0.0.1:1 <--- 203.0.113.1:2
Int-VTag = 1234
7.3. Multihomed Client and Server
The client (Host A) sends an INIT to the server (Host B), but does
not include the second address.
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+-------+
/--| NAT 1 |--\ /--\/--\
+------+ / +-------+ \ / \ +--------+
| Host |=== ====| Internet |====| Host B |
| A | \ +-------+ / \ / +--------+
+------+ \--| NAT 2 |--/ \--/\--/
+-------+
+---------+--------+----------+--------+-----------+
NAT 1 | Int | Int | Ext | Ext | Priv |
| VTag | Port | VTag | Port | Addr |
+---------+--------+----------+--------+-----------+
INIT[Initiate-Tag = 1234]
10.0.0.1:1 --------> 203.0.113.1:2
Ext-VTag = 0
NAT 1 creates entry:
+---------+--------+----------+--------+-----------+
NAT 1 | Int | Int | Ext | Ext | Priv |
| VTag | Port | VTag | Port | Addr |
+---------+--------+----------+--------+-----------+
| 1234 | 1 | 0 | 2 | 10.0.0.1 |
+---------+--------+----------+--------+-----------+
INIT[Initiate-Tag = 1234]
192.0.2.1:1 ---------------------> 203.0.113.1:2
ExtVTag = 0
Host B includes its second address in the INIT ACK.
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+-------+
/--------| NAT 1 |--------\ /--\/--\
+------+ / +-------+ \ / \ +--------+
| Host |=== ====| Internet |===| Host B |
| A | \ +-------+ / \ / +--------+
+------+ \--------| NAT 2 |--------/ \--/\--/
+-------+
INIT ACK[Initiate-Tag = 5678, IP-Addr = 203.0.113.129]
192.0.2.1:1 <----------------------- 203.0.113.1:2
Int-VTag = 1234
NAT 1 does not need to update the NAT binding table for the second
address:
+---------+--------+----------+--------+-----------+
NAT 1 | Int | Int | Ext | Ext | Priv |
| VTag | Port | VTag | Port | Addr |
+---------+--------+----------+--------+-----------+
| 1234 | 1 | 5678 | 2 | 10.0.0.1 |
+---------+--------+----------+--------+-----------+
INIT ACK[Initiate-Tag = 5678]
10.0.0.1:1 <-------- 203.0.113.1:2
Int-VTag = 1234
The handshake finishes with a COOKIE ECHO acknowledged by a COOKIE
ACK.
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+-------+
/--------| NAT 1 |--------\ /--\/--\
+------+ / +-------+ \ / \ +--------+
| Host |=== ====| Internet |===| Host B |
| A | \ +-------+ / \ / +--------+
+------+ \--------| NAT 2 |--------/ \--/\--/
+-------+
COOKIE ECHO
10.0.0.1:1 --------> 203.0.113.1:2
Ext-VTag = 5678
COOKIE ECHO
192.0.2.1:1 ------------------> 203.0.113.1:2
Ext-VTag = 5678
COOKIE ACK
192.0.2.1:1 <------------------ 203.0.113.1:2
Int-VTag = 1234
COOKIE ACK
10.0.0.1:1 <------- 203.0.113.1:2
Int-VTag = 1234
Host A announces its second address in an ASCONF chunk. The address
parameter contains an undefined address (0) to indicate that the
source address should be added. The lookup address parameter within
the ASCONF chunk will also contain the pair of VTags (external and
internal) so that the NAT may populate its NAT binding table entry
completely with this single packet.
+-------+
/--------| NAT 1 |--------\ /--\/--\
+------+ / +-------+ \ / \ +--------+
| Host |=== ====| Internet |===| Host B |
| A | \ +-------+ / \ / +--------+
+------+ \--------| NAT 2 |--------/ \--/\--/
+-------+
ASCONF [ADD-IP=0.0.0.0, INT-VTag=1234, Ext-VTag = 5678]
10.1.0.1:1 --------> 203.0.113.129:2
Ext-VTag = 5678
NAT 2 creates a complete entry:
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+---------+--------+----------+--------+-----------+
NAT 2 | Int | Int | Ext | Ext | Priv |
| VTag | Port | VTag | Port | Addr |
+---------+--------+----------+--------+-----------+
| 1234 | 1 | 5678 | 2 | 10.1.0.1 |
+---------+--------+----------+--------+-----------+
ASCONF [ADD-IP, Int-VTag=1234, Ext-VTag = 5678]
192.0.2.129:1 ---------------------> 203.0.113.129:2
Ext-VTag = 5678
ASCONF ACK
192.0.2.129:1 <--------------------- 203.0.113.129:2
Int-VTag = 1234
ASCONF ACK
10.1.0.1:1 <----- 203.0.113.129:2
Int-VTag = 1234
7.4. NAT Loses Its State
Association is already established between Host A and Host B, when
the NAT loses its state and obtains a new public address. Host A
sends a DATA chunk to Host B.
/--\/--\
+--------+ +-----+ / \ +--------+
| Host A | <----------> | NAT | <----> | Internet | <----> | Host B |
+--------+ +-----+ \ / +--------+
\--/\--/
+---------+--------+----------+--------+-----------+
NAT | Int | Int | Ext | Ext | Priv |
| VTag | Port | VTag | Port | Addr |
+---------+--------+----------+--------+-----------+
DATA
10.0.0.1:1 ----------> 203.0.113.1:2
Ext-VTag = 5678
The NAT device cannot find an entry in the NAT binding table for the
association. It sends ERROR an message with the M-Bit set and the
cause "NAT state missing".
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/--\/--\
+--------+ +-----+ / \ +--------+
| Host A | <----------> | NAT | <----> | Internet | <----> | Host B |
+--------+ +-----+ \ / +--------+
\--/\--/
ERROR [M-Bit, NAT state missing]
10.0.0.1:1 <---------- 203.0.113.1:2
Ext-VTag = 5678
On reception of the ERROR message, Host A sends an ASCONF chunk
indicating that the former information has to be deleted and the
source address of the actual packet added.
/--\/--\
+--------+ +-----+ / \ +--------+
| Host A | <----------> | NAT | <----> | Internet | <----> | Host B |
+--------+ +-----+ \ / +--------+
\--/\--/
ASCONF [ADD-IP, DELETE-IP, Int-VTag=1234, Ext-VTag = 5678]
10.0.0.1:1 ----------> 203.0.113.129:2
Ext-VTag = 5678
+---------+--------+----------+--------+-----------+
NAT | Int | Int | Ext | Ext | Priv |
| VTag | Port | VTag | Port | Addr |
+---------+--------+----------+--------+-----------+
| 1234 | 1 | 5678 | 2 | 10.0.0.1 |
+---------+--------+----------+--------+-----------+
ASCONF [ADD-IP, DELETE-IP, Int-VTag=1234, Ext-VTag = 5678]
192.0.2.2:1 -------------------> 203.0.113.129:2
Ext-VTag = 5678
Host B adds the new source address to this association and deletes
all other addresses from this association.
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/--\/--\
+--------+ +-----+ / \ +--------+
| Host A | <----------> | NAT | <----> | Internet | <----> | Host B |
+--------+ +-----+ \ / +--------+
\--/\--/
ASCONF ACK
192.0.2.2:1 <------------------- 203.0.113.129:2
Int-VTag = 1234
ASCONF ACK
10.1.0.1:1 <---------- 203.0.113.129:2
Int-VTag = 1234
DATA
10.0.0.1:1 ----------> 203.0.113.1:2
Ext-VTag = 5678
DATA
192.0.2.2:1 -------------------> 203.0.113.129:2
Ext-VTag = 5678
7.5. Peer-to-Peer Communication
If two hosts are behind NAT devices and want to communicate with each
other, they have to get knowledge of the peer's public address. This
can be achieved with a so-called rendezvous server. Afterwards the
destination addresses are public, and the association is set up with
the help of the INIT collision. The NAT devices create their entries
according to their internal peer's point of view. Therefore, NAT A's
Internal-VTag and Internal-Port are NAT B's External-VTag and
External-Port, respectively. The naming (internal/external) of the
verification tag in the packet flow is done from the sending host's
point of view.
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Internal | External External | Internal
| |
| /--\/---\ |
+--------+ +-------+ / \ +-------+ +--------+
| Host A |<--->| NAT A |<-->| Internet |<-->| NAT B |<--->| Host B |
+--------+ +-------+ \ / +-------+ +--------+
| \--/\---/ |
NAT Binding Tables
+---------+--------+----------+--------+-----------+
NAT A | Int | Int | Ext | Ext | Priv |
| VTag | Port | VTag | Port | Addr |
+---------+--------+----------+--------+-----------+
+---------+--------+----------+--------+-----------+
NAT B | Int | Int | Ext | Ext | Priv |
| v-tag | port | v-tag | port | Addr |
+---------+--------+----------+--------+-----------+
INIT[Initiate-Tag = 1234]
10.0.0.1:1 --> 203.0.113.1:2
Ext-VTag = 0
NAT A creates entry:
+---------+--------+----------+--------+-----------+
NAT A | Int | Int | Ext | Ext | Priv |
| VTag | Port | VTag | Port | Addr |
+---------+--------+----------+--------+-----------+
| 1234 | 1 | 0 | 2 | 10.0.0.1 |
+---------+--------+----------+--------+-----------+
INIT[Initiate-Tag = 1234]
192.0.2.1:1 ----------------> 203.0.113.1:2
Ext-VTag = 0
NAT B processes INIT, but cannot find an entry. The SCTP packet is
silently discarded and leaves the NAT binding table of NAT B
unchanged.
+---------+--------+----------+--------+-----------+
NAT B | Int | Int | Ext | Ext | Priv |
| VTag | Port | VTag | Port | Addr |
+---------+--------+----------+--------+-----------+
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Now Host B sends INIT, which is processed by NAT B. Its parameters
are used to create an entry.
Internal | External External | Internal
| |
| /--\/---\ |
+--------+ +-------+ / \ +-------+ +--------+
| Host A |<--->| NAT A |<-->| Internet |<-->| NAT B |<--->| Host B |
+--------+ +-------+ \ / +-------+ +--------+
| \--/\---/ |
INIT[Initiate-Tag = 5678]
192.0.2.1:1 <-- 10.1.0.1:2
Ext-VTag = 0
+---------+--------+-----------+----------+--------+
NAT B | Int | Int | Priv | Ext | Ext |
| VTag | Port | Addr | VTag | Port |
+---------+--------+-----------+----------+--------+
| 5678 | 2 | 10.1.0.1 | 0 | 1 |
+---------+--------+-----------+----------+--------+
INIT[Initiate-Tag = 5678]
192.0.2.1:1 <--------------- 203.0.113.1:2
Ext-VTag = 0
NAT A processes INIT. As the outgoing INIT of Host A has already
created an entry, the entry is found and updated:
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Internal | External External | Internal
| |
| /--\/---\ |
+--------+ +-------+ / \ +-------+ +--------+
| Host A |<--->| NAT A |<-->| Internet |<-->| NAT B |<--->| Host B |
+--------+ +-------+ \ / +-------+ +--------+
| \--/\---/ |
VTag != Int-VTag, but Ext-VTag == 0, find entry.
+---------+--------+----------+--------+-----------+
NAT A | Int | Int | Ext | Ext | Priv |
| VTag | Port | VTag | Port | Addr |
+---------+--------+----------+--------+-----------+
| 1234 | 1 | 5678 | 2 | 10.0.0.1 |
+---------+--------+----------+--------+-----------+
INIT[Initiate-tag = 5678]
10.0.0.1:1 <-- 203.0.113.1:2
Ext-VTag = 0
Host A sends INIT ACK, which can pass through NAT B:
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Internal | External External | Internal
| |
| /--\/---\ |
+--------+ +-------+ / \ +-------+ +--------+
| Host A |<--->| NAT A |<-->| Internet |<-->| NAT B |<--->| Host B |
+--------+ +-------+ \ / +-------+ +--------+
| \--/\---/ |
INIT ACK[Initiate-Tag = 1234]
10.0.0.1:1 --> 203.0.113.1:2
Ext-VTag = 5678
INIT ACK[Initiate-Tag = 1234]
192.0.2.1:1 ----------------> 203.0.113.1:2
Ext-VTag = 5678
NAT B updates entry:
+---------+--------+----------+--------+-----------+
NAT B | Int | Int | Ext | Ext | Priv |
| VTag | Port | VTag | Port | Addr |
+---------+--------+----------+--------+-----------+
| 5678 | 2 | 1234 | 1 | 10.1.0.1 |
+---------+--------+----------+--------+-----------+
INIT ACK[Initiate-Tag = 1234]
192.0.2.1:1 --> 10.1.0.1:2
Ext-VTag = 5678
The lookup for COOKIE ECHO and COOKIE ACK is successful.
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Internal | External External | Internal
| |
| /--\/---\ |
+--------+ +-------+ / \ +-------+ +--------+
| Host A |<--->| NAT A |<-->| Internet |<-->| NAT B |<--->| Host B |
+--------+ +-------+ \ / +-------+ +--------+
| \--/\---/ |
COOKIE ECHO
192.0.2.1:1 <-- 10.1.0.1:2
Ext-VTag = 1234
COOKIE ECHO
192.0.2.1:1 <------------- 203.0.113.1:2
Ext-VTag = 1234
COOKIE ECHO
10.0.0.1:1 <-- 203.0.113.1:2
Ext-VTag = 1234
COOKIE ACK
10.0.0.1:1 --> 203.0.113.1:2
Ext-VTag = 5678
COOKIE ACK
192.0.2.1:1 ----------------> 203.0.113.1:2
Ext-VTag = 5678
COOKIE ACK
192.0.2.1:1 --> 10.1.0.1:2
Ext-VTag = 5678
8. Socket API Considerations
This section describes how the socket API defined in [RFC6458] is
extended to provide a way for the application to control NAT
friendliness.
Please note that this section is informational only.
A socket API implementation based on [RFC6458] is extended by
supporting one new read/write socket option.
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8.1. Get or Set the NAT Friendliness (SCTP_NAT_FRIENDLY)
This socket option uses the option_level IPPROTO_SCTP and the
option_name SCTP_NAT_FRIENDLY. It can be used to enable/disable the
NAT friendliness for future associations and retrieve the value for
future and specific ones.
struct sctp_assoc_value {
sctp_assoc_t assoc_id;
uint32_t assoc_value;
};
assoc_id: This parameter is ignored for one-to-one style sockets.
For one-to-many style sockets the application may fill in an
association identifier or SCTP_FUTURE_ASSOC for this query. It is
an error to use SCTP_{CURRENT|ALL}_ASSOC in assoc_id.
assoc_value: A non-zero value indicates a NAT-friendly mode.
9. IANA Considerations
[NOTE to RFC-Editor:
"RFCXXXX" is to be replaced by the RFC number you assign this
document.
]
[NOTE to RFC-Editor:
The requested values for the chunk type and the chunk parameter
types are tentative and to be confirmed by IANA.
]
This document (RFCXXXX) is the reference for all registrations
described in this section. The requested changes are described
below.
9.1. New Chunk Flags for Two Existing Chunk Types
As defined in [RFC6096] two chunk flags have to be assigned by IANA
for the ERROR chunk. The requested value for the T bit is 0x01 and
for the M bit is 0x02.
This requires an update of the "ERROR Chunk Flags" registry for SCTP:
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ERROR Chunk Flags
+------------------+-----------------+-----------+
| Chunk Flag Value | Chunk Flag Name | Reference |
+------------------+-----------------+-----------+
| 0x01 | T bit | [RFCXXXX] |
| 0x02 | M bit | [RFCXXXX] |
| 0x04 | Unassigned | |
| 0x08 | Unassigned | |
| 0x10 | Unassigned | |
| 0x20 | Unassigned | |
| 0x40 | Unassigned | |
| 0x80 | Unassigned | |
+------------------+-----------------+-----------+
As defined in [RFC6096] one chunk flag has to be assigned by IANA for
the ABORT chunk. The requested value of the M bit is 0x02.
This requires an update of the "ABORT Chunk Flags" registry for SCTP:
ABORT Chunk Flags
+------------------+-----------------+-----------+
| Chunk Flag Value | Chunk Flag Name | Reference |
+------------------+-----------------+-----------+
| 0x01 | T bit | [RFC4960] |
| 0x02 | M bit | [RFCXXXX] |
| 0x04 | Unassigned | |
| 0x08 | Unassigned | |
| 0x10 | Unassigned | |
| 0x20 | Unassigned | |
| 0x40 | Unassigned | |
| 0x80 | Unassigned | |
+------------------+-----------------+-----------+
9.2. Three New Error Causes
Three error causes have to be assigned by IANA. It is requested to
use the values given below.
This requires three additional lines in the "Error Cause Codes"
registry for SCTP:
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Error Cause Codes
+-------+--------------------------------+-----------+
| Value | Cause Code | Reference |
+-------+--------------------------------+-----------+
| 176 | VTag and Port Number Collision | [RFCXXXX] |
| 177 | Missing State | [RFCXXXX] |
| 178 | Port Number Collision | [RFCXXXX] |
+-------+--------------------------------+-----------+
9.3. Two New Chunk Parameter Types
Two chunk parameter types have to be assigned by IANA. It is
requested to use the values given below. IANA should assign these
values from the pool of parameters with the upper two bits set to
'11'.
This requires two additional lines in the "Chunk Parameter Types"
registry for SCTP:
Chunk Parameter Types
+----------+--------------------------+-----------+
| ID Value | Chunk Parameter Type | Reference |
+----------+--------------------------+-----------+
| 49159 | Disable Restart (0xC007) | [RFCXXXX] |
| 49160 | VTags (0xC008) | [RFCXXXX] |
+----------+--------------------------+-----------+
10. Security Considerations
State maintenance within a NAT is always a subject of possible Denial
Of Service attacks. This document recommends that at a minimum a NAT
runs a timer on any SCTP state so that old association state can be
cleaned up.
For SCTP endpoints, this document does not add any additional
security considerations to the ones given in [RFC4960], [RFC4895],
and [RFC5061]. In particular, SCTP is protected by the verification
tags and the usage of [RFC4895] against off-path attackers.
11. Acknowledgments
The authors wish to thank Gorry Fairhurst, Bryan Ford, David Hayes,
Alfred Hines, Karen E. E. Nielsen, Henning Peters, Maksim Proshin,
Timo Voelker, Dan Wing, and Qiaobing Xie for their invaluable
comments.
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In addition, the authors wish to thank David Hayes, Jason But, and
Grenville Armitage, the authors of [DOI_10.1145_1496091.1496095], for
their suggestions.
12. References
12.1. Normative References
[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>.
[RFC4895] Tuexen, M., Stewart, R., Lei, P., and E. Rescorla,
"Authenticated Chunks for the Stream Control Transmission
Protocol (SCTP)", RFC 4895, DOI 10.17487/RFC4895, August
2007, <https://www.rfc-editor.org/info/rfc4895>.
[RFC4960] Stewart, R., Ed., "Stream Control Transmission Protocol",
RFC 4960, DOI 10.17487/RFC4960, September 2007,
<https://www.rfc-editor.org/info/rfc4960>.
[RFC5061] Stewart, R., Xie, Q., Tuexen, M., Maruyama, S., and M.
Kozuka, "Stream Control Transmission Protocol (SCTP)
Dynamic Address Reconfiguration", RFC 5061,
DOI 10.17487/RFC5061, September 2007,
<https://www.rfc-editor.org/info/rfc5061>.
[RFC6096] Tuexen, M. and R. Stewart, "Stream Control Transmission
Protocol (SCTP) Chunk Flags Registration", RFC 6096,
DOI 10.17487/RFC6096, January 2011,
<https://www.rfc-editor.org/info/rfc6096>.
[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>.
12.2. Informative References
[DOI_10.1145_1496091.1496095]
Hayes, D., But, J., and G. Armitage, "Issues with network
address translation for SCTP", ACM SIGCOMM Computer
Communication Review Vol. 39, pp. 23,
DOI 10.1145/1496091.1496095, December 2008.
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, DOI 10.17487/RFC0793, September 1981,
<https://www.rfc-editor.org/info/rfc793>.
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[RFC6458] Stewart, R., Tuexen, M., Poon, K., Lei, P., and V.
Yasevich, "Sockets API Extensions for the Stream Control
Transmission Protocol (SCTP)", RFC 6458,
DOI 10.17487/RFC6458, December 2011,
<https://www.rfc-editor.org/info/rfc6458>.
[RFC6890] Cotton, M., Vegoda, L., Bonica, R., Ed., and B. Haberman,
"Special-Purpose IP Address Registries", BCP 153,
RFC 6890, DOI 10.17487/RFC6890, April 2013,
<https://www.rfc-editor.org/info/rfc6890>.
[RFC6951] Tuexen, M. and R. Stewart, "UDP Encapsulation of Stream
Control Transmission Protocol (SCTP) Packets for End-Host
to End-Host Communication", RFC 6951,
DOI 10.17487/RFC6951, May 2013,
<https://www.rfc-editor.org/info/rfc6951>.
Authors' Addresses
Randall R. Stewart
Netflix, Inc.
Chapin, SC 29036
US
Email: randall@lakerest.net
Michael Tuexen
Muenster University of Applied Sciences
Stegerwaldstrasse 39
48565 Steinfurt
DE
Email: tuexen@fh-muenster.de
Irene Ruengeler
Muenster University of Applied Sciences
Stegerwaldstrasse 39
48565 Steinfurt
DE
Email: i.ruengeler@fh-muenster.de
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