Network Working Group M. Tuexen
Internet-Draft Muenster Univ. of Appl. Sciences
Intended status: Standards Track R. Stewart
Expires: September 12, 2012 Adara Networks
March 11, 2012
UDP Encapsulation of SCTP Packets
draft-ietf-tsvwg-sctp-udp-encaps-03.txt
Abstract
This document describes a simple method of encapsulating SCTP Packets
into UDP packets and its limitations. This allows the usage of SCTP
in networks with legacy NAT not supporting SCTP. It can also be used
to implement SCTP on hosts without directly accessing the IP-layer,
for example implementing it as part of the application without
requiring special privileges.
Status of this Memo
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This Internet-Draft will expire on September 12, 2012.
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Copyright (c) 2012 IETF Trust and the persons identified as the
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3.1. Portable SCTP Implementations . . . . . . . . . . . . . . 3
3.2. Legacy NAT Traversal . . . . . . . . . . . . . . . . . . . 4
4. SCTP over UDP . . . . . . . . . . . . . . . . . . . . . . . . 4
4.1. Architectural Considerations . . . . . . . . . . . . . . . 4
4.2. Packet Format . . . . . . . . . . . . . . . . . . . . . . 4
4.3. Encapsulation Procedure . . . . . . . . . . . . . . . . . 6
4.4. Decapsulation Procedure . . . . . . . . . . . . . . . . . 6
4.5. ICMP Considerations . . . . . . . . . . . . . . . . . . . 6
4.6. Path MTU Considerations . . . . . . . . . . . . . . . . . 7
4.7. Handling of Embedded IP-addresses . . . . . . . . . . . . 7
4.8. ECN Considerations . . . . . . . . . . . . . . . . . . . . 7
5. Socket API Considerations . . . . . . . . . . . . . . . . . . 7
5.1. Get or Set the Remote UDP Encapsulation Port Number
(SCTP_REMOTE_UDP_ENCAPS_PORT) . . . . . . . . . . . . . . 7
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
7. Security Considerations . . . . . . . . . . . . . . . . . . . 8
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 8
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8
9.1. Normative References . . . . . . . . . . . . . . . . . . . 8
9.2. Informative References . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10
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1. Introduction
This document describes a simple method of encapsulating SCTP packets
into UDP packets. SCTP as defined in [RFC4960] runs directly over
IPv4 or IPv6. There are two main reasons for encapsulating SCTP
packets:
o Allow SCTP traffic to pass legacy NATs, which do not provide
native SCTP support as specified in [I-D.ietf-behave-sctpnat] and
[I-D.ietf-tsvwg-natsupp].
o Allow SCTP to be implemented on hosts which do not provide direct
access to the IP-layer. In particular, applications can use their
own SCTP implementation if the operating system does not provide
one.
2. Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. Use Cases
This section discusses two important use cases for encapsulating SCTP
into UDP.
3.1. Portable SCTP Implementations
Some operating systems support SCTP natively. For other operating
systems implementations are available, but require special privileges
to install and/or use them. In some cases no kernel implementation
might be available at all. When proving an SCTP implementation as
part of a user process, most operating systems require special
privileges to access the IP layer directly.
Using UDP encapsulation makes it possible to provide an SCTP
implementation as part of a user process which does not require any
special privileges.
A crucial point for implementing SCTP in user-land is controlling the
source address of outgoing packets. This is not an issue when using
all available addresses. However, this is not the case when also
using the address management required for NAT traversal described in
Section 4.7.
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3.2. Legacy NAT Traversal
Using UDP encapsulation allows SCTP communication when traversing
legacy NATs (i.e those NATs not supporting SCTP as described in
[I-D.ietf-behave-sctpnat] and [I-D.ietf-tsvwg-natsupp]). It is
important to realize that for single homed associations it is only
necessary that no IP addresses are listed in the INIT and INIT-ACK
chunks. To use multiple addresses, the dynamic address
reconfiguration extension described in [RFC5061] MUST be used with
wildcard addresses in combination with [RFC4895].
For multi-homed SCTP association the address management as described
in Section 4.7 MUST be performed.
4. SCTP over UDP
4.1. Architectural Considerations
An SCTP implementation supporting UDP encapsulation MUST store a
remote UDP encapsulation port number per destination address for each
SCTP association.
Each SCTP stack uses a single local UDP encapsulation port number as
the destination port for all its incoming SCTP packets. The IANA
assigned value of 9989 MAY be used as this port number. If there is
only a single SCTP implementation on a host (for example, a kernel
implementation being part of the operating system), using a single
UDP encapsulation port number per host can be advantageous (e.g.,
this reduces the number of mappings in firewalls and NATs, among
other things). However, this is not possible if the SCTP stack is
implemented as part of an application.
4.2. Packet Format
To encapsulate an SCTP packet, a UDP header as defined in [RFC0768]
is inserted between the IP header as defined in [RFC0791] and the
SCTP common header as defined in [RFC4960].
Figure 1 shows the packet format of an encapsulated SCTP packet when
IPv4 is used.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv4 Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| UDP Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SCTP Common Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SCTP Chunk #1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SCTP Chunk #n |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: An SCTP/UDP/IPv4 packet
The packet format for an encapsulated SCTP packet when using IPv6 as
defined in [RFC2460] is shown in Figure 2. Please note the the
number m of IPv6 extension headers can be 0.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv6 Base Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv6 Extension Header #1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv6 Extension Header #m |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| UDP Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SCTP Common Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SCTP Chunk #1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SCTP Chunk #n |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: An SCTP/UDP/IPv6 packet
The UDP checksum MUST NOT be zero.
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4.3. Encapsulation Procedure
When inserting the UDP header, the source port is the local UDP
encapsulation port number of the SCTP stack, the destination port is
the remote UDP encapsulation port number stored for the destination
address the packet is sent to (see Section 4.1).
The length of the UDP packet is the length of the SCTP packet plus
the size of the UDP header.
The UDP checksum and the SCTP checksum MUST be computed.
4.4. Decapsulation Procedure
When an encapsulated packet is received, the UDP header is removed.
Then a lookup is performed to find the association the received SCTP
packet belongs to. The UDP source port is stored as the
encapsulation port for the destination address the SCTP packet is
received from (see Section 4.1).
Please note that when a non-encapsulated SCTP packet is received, the
encapsulation of outgoing packets belonging to the same association
and the corresponding destination address is disabled.
4.5. ICMP Considerations
When receiving ICMP or ICMPv6 response packets, there might not be
enough bytes in the payload to identify the SCTP association which
the SCTP packet triggering the ICMP or ICMPv6 packet belongs to. If
a received ICMP or ICMPv6 packet can not be related to a specific
SCTP association, it MUST be discarded silently. This means in
particular that the SCTP stack MUST NOT rely on receiving ICMP or
ICMPv6 messages. There MAY be implementation constraints not
allowing to process received ICMP or ICMPv6 messages at all.
If received ICMP or ICMPv6 messages are processed, the following
mapping SHOULD apply:
1. ICMP messages with type 'Destination Unreachable' and code 'Port
Unreachable' SHOULD be treated as ICMP messages with type
'Protocol Unreachable' and code 'Destination Port unreachable.
See [RFC0792] for more details.
2. ICMPv6 messages with type 'Destination Unreachable' and code
'Port unreachable' SHOULD be treated as ICMPv6 messages with type
'Parameter Problem' and code 'Unrecognized Next Header type
encountered'. See [RFC4443] for more details.
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4.6. Path MTU Considerations
If an SCTP endpoint starts to encapsulate the packets of a path, it
MUST decrease the path MTU of that path by the size of the UDP
header. If it stops encapsulating them, the path MTU SHOULD be
increased by the size of the UDP header.
When performing path MTU discovery as described in [RFC4820] and
[RFC4821] it MUST be taken into account that one cannot rely on the
feedback provided by ICMP or ICMPv6 due to the limitation laid out in
Section 4.5.
4.7. Handling of Embedded IP-addresses
When using UDP encapsulation for legacy NAT traversal, IP addresses
that might require translation MUST NOT be put into any SCTP packet.
This means that a multi homed SCTP association is setup initially as
a singled homed one and the protocol extension [RFC5061] in
combination with [RFC4895] is used to add the other addresses. Only
wildcard addresses are put into the SCTP packet.
When addresses are changed during the lifetime of an association
[RFC5061] MUST be used with wildcard addresses only.
4.8. ECN Considerations
During encapsulation and decapsulation the ECN bits MUST NOT be
changed.
5. Socket API Considerations
This section describes how the socket API defined in [RFC6458] is
extended to provide a way for the application to control the UDP
encapsulation.
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.
5.1. Get or Set the Remote UDP Encapsulation Port Number
(SCTP_REMOTE_UDP_ENCAPS_PORT)
This socket option can be used to set and retrieve the UDP
encapsulation port number. This allows an endpoint to encapsulate
initial packets.
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struct sctp_udpencaps {
sctp_assoc_t sue_assoc_id;
struct sockaddr_storage sue_address;
uint16_t sue_port;
};
sue_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 sue_assoc_id.
sue_address: This specifies which address is of interest. If a
wildcard address is provided it applies only to future paths.
sue_port: The UDP port number in network byte order used as the
destination port number for UDP encapsulation. Providing a value
of 0 disables UDP encapsulation.
6. IANA Considerations
This document does not require any actions from IANA.
7. Security Considerations
Encapsulating SCTP into UDP does not add any additional security
considerations to the ones given in [RFC4960] and [RFC5061].
8. Acknowledgments
The authors wish to thank Irene Ruengeler and Dan Wing for their
invaluable comments.
9. References
9.1. Normative References
[RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
August 1980.
[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791,
September 1981.
[RFC0792] Postel, J., "Internet Control Message Protocol", STD 5,
RFC 792, September 1981.
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[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
[RFC4443] Conta, A., Deering, S., and M. Gupta, "Internet Control
Message Protocol (ICMPv6) for the Internet Protocol
Version 6 (IPv6) Specification", RFC 4443, March 2006.
[RFC4820] Tuexen, M., Stewart, R., and P. Lei, "Padding Chunk and
Parameter for the Stream Control Transmission Protocol
(SCTP)", RFC 4820, March 2007.
[RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU
Discovery", RFC 4821, March 2007.
[RFC4895] Tuexen, M., Stewart, R., Lei, P., and E. Rescorla,
"Authenticated Chunks for the Stream Control Transmission
Protocol (SCTP)", RFC 4895, August 2007.
[RFC4960] Stewart, R., "Stream Control Transmission Protocol",
RFC 4960, September 2007.
[RFC5061] Stewart, R., Xie, Q., Tuexen, M., Maruyama, S., and M.
Kozuka, "Stream Control Transmission Protocol (SCTP)
Dynamic Address Reconfiguration", RFC 5061,
September 2007.
9.2. Informative References
[RFC6458] Stewart, R., Tuexen, M., Poon, K., Lei, P., and V.
Yasevich, "Sockets API Extensions for the Stream Control
Transmission Protocol (SCTP)", RFC 6458, December 2011.
[I-D.ietf-behave-sctpnat]
Stewart, R., Tuexen, M., and I. Ruengeler, "Stream Control
Transmission Protocol (SCTP) Network Address Translation",
draft-ietf-behave-sctpnat-05 (work in progress),
June 2011.
[I-D.ietf-tsvwg-natsupp]
Stewart, R., Tuexen, M., and I. Ruengeler, "Stream Control
Transmission Protocol (SCTP) Network Address Translation
Support", draft-ietf-tsvwg-natsupp-01 (work in progress),
June 2011.
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Authors' Addresses
Michael Tuexen
Muenster University of Applied Sciences
Stegerwaldstrasse 39
48565 Steinfurt
DE
Email: tuexen@fh-muenster.de
Randall R. Stewart
Adara Networks
Chapin, SC 29036
US
Email: randall@lakerest.net
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