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INTERNET-DRAFT L. Coene
Internet Engineering Task Force M. Tuexen
Issued: April 2001 G. Verwimp
Expires: October 2001 Siemens
J. Loughney
Nokia
R.R. Stewart
Cisco
Qiaobing Xie
Motorola
M. Holdrege
ipVerse
M.C. Belinchon
Ericsson
A. Jungmayer
University of Essen
L. Ong
Ciena
Stream Control Transmission Protocol Applicability Statement
<draft-ietf-sigtran-sctp-applicability-06.txt>
Status of this Memo
This document is an Internet-Draft and is in full conformance with
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Abstract
This document describes the applicability of the Stream Control
Transmission Protocol (SCTP)[RFC2960] for general usage in the
Internet. This document describes the key features of SCTP and how
they are used for general purpose data transport.
Coene, et al. Informational [Page 1]
Table of Contents
Stream Control Transmission Protocol Applicability statement
................................................................ ii
Chapter 1: Introduction ........................................ 2
Chapter 1.1: Terminology ....................................... 2
Chapter 1.2: Protocol Overview ................................. 3
Chapter 2: Applicability of SCTP ............................... 3
Chapter 2.1: Features provided by SCTP ......................... 3
Chapter 2.2: Applications that would benefit from using SCTP
................................................................ 4
Chapter 2.3: Applications that may receive little benefit ...... 4
Chapter 3: Issues affecting deployment of SCTP ................. 5
Chapter 3.1 : SCTP multihoming and interaction with routing .... 5
Chapter 3.2 : Use of SCTP in Network Address Translators
(NAT) Networks ................................................. 9
Chapter 4: Security considerations ............................. 10
Chapter 5: References and related work ......................... 10
Chapter 6: Acknowledgments ..................................... 11
Chapter 7: Author's address .................................... 11
1 Introduction
1.1 Terminology
The following terms are commonly identified in related work:
Association: SCTP connection between two endpoints.
Transport address: A combination of IP address and SCTP port
number.
Upper layer: The user of the SCTP protocol, which may be an adap-
tation layer, a session layer protocol, or the user application
directly.
TLA: Top level aggregation, part of a aggregatable unicast address
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1.2 Protocol Overview
The Stream Control Transmission Protocol (SCTP) provides a reliable
transport between two endpoints.
The following functions are provided by SCTP:
- Reliable Data Transfer
- Multiple streams to help avoid head-of-line blocking
- Ordered and unordered data delivery on a per-stream basis
- Bundling and fragmentation of user data
- TCP friendly Congestion and flow control
- Support continuous monitoring of reachability
- Graceful termination of association
- Support of multi-homing for added reliability
- Some protection against blind denial-of-service attacks
- Some protection against blind masquerade attacks
2 Applicability of SCTP
When making a choice between reliable transport protocols, namely UDP,
TCP and SCTP, various factors will enter in to deciding which one to
choose. Certain applications will find an extreme advantage to using
SCTP, while others may find little advantage.
2.1 Features provided by SCTP
SCTP provides acknowledged, error-free, non-duplicated transfer of user
data, with framing to preserve user protocol data unit boundaries, and
transport-level data fragmentation to conform to discovered path MTU
size. It provides sequenced delivery within streams, with the option
for designating messages for order-of-arrival delivery instead.
SCTP also provides mechanisms for network-level fault tolerance through
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the use of multi-homing at either or both ends of an SCTP association.
SCTP automatically detects failure to reach a destination address and
compensates through the use of available alternate addresses.
2.2 Applications that would benefit from using SCTP.
Applications using SCTP should have sufficient traffic levels to justify
the overhead and benefit from SCTP association establishment and conges-
tion and flow control procedures.
Applications that require framing of reliable data streams can get that
feature from SCTP.
Applications which require ordered transport of messages, but transfer
multiple independent message sequences that are unrelated (sometimes
called transactions) will benefit from the partial ordering provided by
SCTP streams.
Applications that need to transfer messages that hold no particular
sequence or relationship to one another or can be correlated and
sequenced at the application level can benefit from the unordered
delivery service and transport-level fragmentation provided by SCTP.
Application which depend on fast retransmit of data will have a reduced
dependence on timeouts(thus easing the load on the Operating System).
Applications requiring network layer redundancy can use SCTP's multi-
homing feature to support this, provided that the host supports multiple
addresses and routing is configured appropriately (see Section 3).
Transport of PSTN signaling protocols such as Signaling System No. 7
over IP networks is an example application fitting these requirements.
2.3 Applications that may receive little benefit.
Applications requiring strict ordering of all data sent between communi-
cating endpoints would not benefit from the multi-stream capability pro-
vided by SCTP. In addition, applications oriented towards byte stream
transfer would not benefit from SCTP framing.
An example application that would derives no benefit from framing and
multi-stream capabilities of SCTP is file transfer.
Applications which do not require network-level redundancy or with
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physical limitations, e.g., only one network interface card is present,
would derive no benefit from SCTP's multi-homing feature.
Applications which generate small amounts of unrelated transactions
towards a destination do not gain a great benefit from using TCP
friendly congestion control and may experience a conservative
retransmission policy.
3 Issues affecting deployment of SCTP
3.1 SCTP multihoming and interaction with routing
For fault resilient communication between two SCTP endpoints, the mul-
tihoming feature needs more than one IP network interface for each end-
point. The number of paths used is the minimum of network interfaces
used by any of the endpoints. It is recommended to bind the association
to all the IP source addresses of the endpoint. Note that in IPv6, each
network interface will have more than one IP address.
Under the assumption that every IP address will have a different,
seperate paths towards the remote endpoint, (this is the responsibility
of the routing protocols or of manual configuration) , if the transport
to one of the IP address (= 1 particular path) fails then the traffic
can migrate to the other remaining IP address (= other paths) within the
SCTP association.
+------------+ *~~~~~~~~~* +------------+
| Endpoint A | * Cloud * | Endpoint B |
| 1.2 +---------+ 1.1<--->3.1 +----------+ 3.2 |
| | | | | |
| 2.2 +---------+ 2.1<--->4.1 +----------+ 4.2 |
| | * * | |
+------------+ *~~~~~~~~~* +------------+
Figure 3.1.1: Two hosts with redundant networks.
Consider figure 3.1.1, if the host routing tables look as follows the
endpoint will achieve maximum use of the multi-homing feature:
Endpoint A Endpoint B
Destination Gateway Destination Gateway
------------------------ -------------------------
3.0 1.1 1.0 3.1
4.0 2.1 2.0 4.1
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Now if you consider figure 3.1.1, if the host routing table looks as
follows, the association is subject to a single point of failure in that
if any interface breaks, the whole association will break(See figure
3.1.2).
Host A Host B
Destination Gateway Destination Gateway
------------------------ -------------------------
3.0 1.1 1.0 4.1
4.0 2.1 2.0 3.1
Example: link 4.2-4.1 fails
Primary path: link 1.2-1.1 - link 3.1-3.2
Second Path : Link 2.2-2.1 - link 4.1-4.2
Endpoint A
+-------+--------+------+
|S= 1.2 | D= 3.2 | DATA | ------->----- Arrives at Endpoint B
+-------+--------+------+
Endpoint B answers with SACK
+-------+--------+------+
|S= 4.2 | D= 1.2 | SACK | Gets lost, because send out on the failed
+-------+--------+------+ 4.1-4.2 link
After X time, retransmit on the other path by endpoint A
Endpoint A
+-------+--------+------+
|S= 2.2 | D= 4.2 | DATA | Is send out on link 2.2-2.1, but gets lost,
+-------+--------+------+ as msg has to pass via failed 4.1-4.2 link
The same scenario will play out for failures on the other links
Note : S = Source address
D = Destination address
Figure 3.1.2: Single point of failure case in redundant network
due to routing table in host B
When an endpoint selects its source address, careful consideration must
be taken. If the same source address is always used, then it is possible
that the endpoint will be subject to the same single point of failure
illustrated above. If possible the endpoint should always select the
source address of the packet to correspond to the IP address of the Net-
work interface where the packet will be emitted.
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+------------+ *~~~~~~~~~* +------------+
| Endpoint A | * Cloud * | Endpoint B |
| 1.2 +---------+ 1.1<--+ | | |
| | | |->3.1|----------+ 3.2 |
| 2.2 +---------+ 2.1<--+ | | |
| | * * | |
+------------+ *~~~~~~~~~* +------------+
Figure 3.1.3: Two hosts with asymmetric networks.
In Figure 3.1.3 consider the following host routing table:
Endpoint A Endpoint B
Destination Gateway Destination Gateway
------------------------ -------------------------
3.0 1.1 1.0 3.1
2.0 3.1
In this case the fault tolerance becomes limited by two seperate issues.
If the path between 3.1 and 3.2 breaks in both directions any associa-
tion will break between endpoint A and endpoint B. The second failure
will occur for the whole the association as well due to a breakage
between 1.2 and 1.1 in both directions, since no alternative route
exists to 3.2 and all traffic is being routed through one interface.
Now one of these issues can be remedied by the following modification
even when only one interface exists on endpoint B.
+------------+ *~~~~~~~~~~* +------------+
| Endpoint A | * Cloud * | Endpoint B |
| 1.2 +---------+ 1.1<---+ | | |
| | | +->3.1+----------+ 3.2 & 4.2 |
| 2.2 +---------+ 2.1<---+ | | |
| | * * | |
+------------+ *~~~~~~~~~~* +------------+
Figure 3.1.4: Two hosts with asymmetric networks, but symmetric
addresses.
In Figure 3.1.4 consider the following host routing table:
Endpoint A Endpoint B
Destination Gateway Destination Gateway
------------------------ -------------------------
3.0 1.1 1.0 3.1
4.0 2.1 2.0 3.1
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Now with the duplicate IP addresses assigned to the same interface and
the above routing tables, even if the interface between 1.1 and 1.2
breaks, an association will still survive this failure.
As a practical matter, it is recommended that IP addresses in a mul-
tihomed endpoint be assigned IP endpoints from different TLA's to ensure
against network failure.
In IP implementations the outgoing interface of multihomed hosts is
often determined by the destination IP address. The mapping is done by a
lookup in a routing table maintained by the operating system. Therefore
the outgoing interface is not determined by SCTP. Using such implemen-
tations, it should be noted that a multihomed host cannot make use of
the multiple local IP addresses if the peer is singlehomed. The mul-
tihomed host has only one path and will normally use only one of its
interfaces to send the SCTP datagrams to the peer. If this physical path
fails, the IP routing table in the multihome host has to be changed.
This problem is out of scope for SCTP.
SCTP will always send its traffic to a certain transport address (= des-
tination address + port number combination) for as long as the transmis-
sion is uninterrupted (= primary). The other transport addresses (secon-
dary paths) will act as a backup in case the primary path goes out of
service. The changeover between primary and backup will occur without
packet loss and is completely transparent to the application. The secon-
dary path can also be used for retransmissions(per section 6.4 of
[RFC2960]).
The port number is the same for all transport addresses of that specific
association.
Applications directly using SCTP may choose to control the multihoming
service themselves. The applications have then to supply the specific IP
address to SCTP for each outbound user message. This might be done for
reasons of load-sharing and load-balancing across the different paths.
This might not be advisable as the throughput of any of the paths is not
known in advance and constantly changes due to the actions of other
associations and transport protocols along that particular path, would
require very tight feedback of each of the paths to the loadsharing
functions of the user.
By sending a keep alive message on all the multiple paths that are not
used for active transmission of messages across the association, it is
possible for SCTP to detect whether one or more paths have failed. SCTP
will not use these failed paths when a changeover is required.
The transmission rate of sending keep alive message should be modifiable
and the possible loss of keep alive message could be used for the
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monitoring and measurements of the concerned paths.
3.2 Use of SCTP in Network Address Translators (NAT) Networks [RFC2663]
When a NAT is present between two endpoints, the endpoint that is behind
the NAT, i.e., one that does not have a publicly available network
address, shall take one of the following options:
(1) When single homed sessions are to be used, no transport addresses
should be sent in the INIT or INIT ACK chunk(Refer to section 3.3
of RFC2960 for chunk definitions). This will force the endpoint
that receives this initiation message to use the source address in
the IP header as the only destination address for this association.
This method can be used for a NAT, but any multi-homing configura-
tion at the endpoint that is behind the NAT will not be visible to
its peer, and thus not be taken advantage of. See figure 3.2.1.
+-------+ +---------+ *~~~~~~~~~~* +------+
|Host A | | NAT | * Cloud * |Host B|
| 10.2 +--|10.1|2.1 |----|--------------|---------+ 1.2 |
| | | | | * * | |
+-------+ +---------+ *~~~~~~~~~~* +------+
Fig 3.2.1: SCTP through NAT without multihoming
For multihoming the NAT must have a public IP address for each
represented internal IP address. The host can preconfigure IP
address that the NAT can substitute. Or the NAT can have internal
Application Layer Gateway (ALG) which will intelligently translate
the IP addresses in the INIT and INIT ACK chunks. See Figure 3.2.2.
If Network Address Port Translation is used with a multihomed SCTP
endpoint, then any port translation must be applied on a per-
association basis such that an SCTP endpoint continues to receive
the same port number for all messages within a given association.
+-------+ +----------+ *~~~~~~~~~~* +------+
|Host A | | NAT | * Cloud * |Host B|
| 10.2 +---+ 10.1|5.2 +-----+ 1.1<+->3.1--+---------+ 1.2 |
| 11.2 +---+ 11.1|6.2 | | +->4.2--+---------+ 2.2 |
| | | | * * | |
+-------+ +----------+ *~~~~~~~~~* +------+
Fig 3.2.2: SCTP through NAT with multihoming
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(2) Another alternative is to use the hostname feature and DNS to
resolve the addresses. The hostname is included in the INIT of the
association or in the INIT ACK. The hostname must be resolved by
DNS before the association is completely set up. There are special
issues regarding NAT and DNS, refer to RFC2694 for details.
4 Security considerations
SCTP only tries to increase the availability of a network. SCTP does not
contain any protocol mechanisms which are directly related to user mes-
sage authentication, integrity and confidentiality functions. For such
features, it depends on the IPSEC protocols and architecture and/or on
security features of its user protocols.
Mechanisms for reducing the risk of blind denial-of-service attacks and
masquerade attacks are built into SCTP protocol. See RFC2960, section 11
for detailed information.
Currently the IPSEC working group is investigating the support of mul-
tihoming by IPSEC protocols. At the present time to use IPSEC, one must
use 2 * N * M security associations if one endpoint uses N addresses and
the other M addresses.
5 References and related work
[RFC2960] Stewart, R. R., Xie, Q., Morneault, K., Sharp, C. , ,
Schwarzbauer, H. J., Taylor, T., Rytina, I., Kalla, M., Zhang, L.
and Paxson, V."Stream Control Transmission Protocol", RFC2960,
October 2000.
[RFC2401] Kent, S., and Atkinson, R., "Security Architecture for the
Internet Protocol", RFC 2401, November 1998.
[RFC2663] Srisuresh, P. and Holdrege, M., "IP Network Address Translator
(NAT) Terminology and Considerations", RFC2663, August 1999
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Draft SCTP applicability statement April 2001
[RFC2694] Srisuresh, P., Tsirtsis, G., Akkiraju, P. and Heffernan, A.,
"DNS extensions to Network Address Translators (DNS_ALG)", RFC2694,
September 1999
6 Acknowledgments
The authors wish to thank Renee Revis, I. Rytina, H.J. Schwarzbauer,
J.P. Martin-Flatin, T. Taylor, G. Sidebottom, K. Morneault, T. George,
M. Stillman, N. Makinae, S. Bradner, A. Mankin, G. Camarillo, H.
Schulzrinne, R. Kantola, J. Rosenberg and many others for their invalu-
able comments.
7 Author's Address
Lode Coene Phone: +32-14-252081
Siemens Atea EMail: [email protected]
Atealaan 34
B-2200 Herentals
Belgium
John Loughney Phone: +358-9-43761
Nokia Research Center EMail: [email protected]
Itamerenkatu 11-13
FIN-00180 Helsinki
Finland
Michel Tuexen Phone: +49-89-722-47210
Siemens AG EMail: [email protected]
Hofmannstr. 51
81359 Munich
Germany
Randall R. Stewart Phone: +1-815-477-2127
24 Burning Bush Trail. EMail: [email protected]
Crystal Lake, IL 60012
USA
Qiaobing Xie Phone: +1-847-632-3028
Motorola, Inc. EMail: [email protected]
1501 W. Shure Drive
Arlington Heights, IL 60004
USA
Coene, et al. Informational [Page 11]
Draft SCTP applicability statement April 2001
Matt Holdrege Phone: -
ipVerse Email: [email protected]
223 Ximeno Avenue
Long Beach, CA 90803-1616
USA
Maria-Carmen Belinchon Phone: +34-91-339-3535
Ericsson Espana S. A. EMail: [email protected]
Network Communication Services
Retama 7, 5th floor
Madrid, 28045
Spain
Andreas Jungmayer Phone: +49-201-1837636
University of Essen EMail: [email protected]
Institute for experimental Mathematics
Ellernstrasse 29
D-45326 Essen
Germany
Gery Verwimp Phone: +32-14-253424
Siemens Atea EMail: [email protected]
Atealaan 34
B-2200 Herentals
Belgium
Lyndon Ong Phone: -
EMail: [email protected]
USA
Expires: October 31, 2001
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Coene, et al. Informational [Page 12]
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