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INTERNET-DRAFT L. Coene
Internet Engineering Task Force M. Tuexen
Issued: January 2001 G. Verwimp
Expires: July 2001 Siemens
J. Loughney
Nokia
R.R. Stewart
Cisco
Qiaobing Xie
Motorola
M. Holdrege
Ipverse
M.C. Belinchon
Ericsson
A. Jungmayer
University of Essen
Stream Control Transmission Protocol Applicability Statement
<draft-ietf-sigtran-sctp-applicability-04.txt>
Status of this Memo
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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 ................................. 2
Chapter 2: Applicability of the Stream Control Transmission
Protocol -- SCTP ............................................... 3
Chapter 3: Issues affecting deployment of SCTP ................. 4
Chapter 3.1 : SCTP multihoming and interaction with routing .... 4
Chapter 3.2 : Use of SCTP in Network Address Translators
(NAT) Networks ................................................. 8
Chapter 4: Security considerations ............................. 9
Chapter 5: References and related work ......................... 9
Chapter 6: Acknowledgments ..................................... 10
Chapter 7: Author's address .................................... 10
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.
1.2 Protocol Overview
The Stream Control Transmission Protocol (SCTP) provides a reliable
transport between two endpoints.
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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
- Congestion and flow control
- Support continuous monitoring of reachability
- Graceful termination of association
- Support of multi-homing for added reliability
- Protection against blind denial-of-service attacks
- Protection against blind masquerade attacks
2 Applicability of the Stream Control Transmission Protocol -- SCTP
This section describes where you could use SCTP for transporting appli-
cation data.
- SCTP can be used as a general-purpose transport protocol for
message-oriented applications. Message boundaries are preserved
during data transport and so no message delineation is needed. The
user data can be delivered by the order of transmission within a
stream or the order of arrival.
- For data streams that lack boundaries or markers, the application
must force an arbitrary boundary for the data it sends to SCTP. The
choice of the boundary should result in a reasonable frame size
based on the network MTU.
- SCTP can be used to provide redundancy and fault tolerance at the
transport layer and below. Applications needing this level of fault
tolerance can make use of SCTP's multi-homing support.
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- SCTP can be used as the transport protocol for applications where
head-of-line blocking is a concern. Such an application should use
multiple streams to provide independent ordering of user messages.
- SCTP can be used as the transport protocol for applications where
the average size of the user messages is small. For such applica-
tions, the bundling feature of SCTP will combine multiple small
messages for efficient bandwidth utilization.
- SCTP can be used as the transport protocol for applications where
the average size of the user messages is large. For such applica-
tions, SCTP will transparently fragment large user messages to con-
form to the appropriate packet size.
- SCTP can be used as the transport protocol for applications where
multiple message streams need to be multiplexed over a single asso-
ciation. For such applications, SCTP will transparently combine
messages from multiple streams at the transmission and demultiplex
upon reception.
- SCTP can be used as the transport protocol for applications where
monitoring and detection of loss of session is required. For such
applications, the SCTP path/session failure detection mechanisms,
especially the heartbeat, will actively monitor the connectivity of
the session.
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 address for each endpoint. The
number of paths used is the minimum of IP addresses used by any of the
endpoints. It is recommended to bind the association to all the IP
source addresses of the endpoint.
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
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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
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
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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
+------------+ *~~~~~~~~~* +------------+
| 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.
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+------------+ *~~~~~~~~~~* +------------+
| 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
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 TLV's to ensure
against network failure.
In IP implementations the outgoing interface of multihomed hosts is
ofter 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.
Something which is out of scope of 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 port number is the same for all transport addresses of that specific
association.
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Draft SCTP applicability statement January 2001
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 moni-
toring 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 consider the sender as
only having that one address. This method can be used for a NAT,
but any multi-homing configuration 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
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Draft SCTP applicability statement January 2001
Application Layer Gateway (ALG) which will intelligently translate
the IP addresses in the INIT and INIT ACK chunks. See Figure 3.2.2.
+-------+ +----------+ *~~~~~~~~~~* +------+
|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
(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.
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Draft SCTP applicability statement January 2001
[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
[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, L. Ong, H.J.
Schwarzbauer, J.P. Martin-Flatin, T. Taylor, G. Sidebottom, K. Mor-
neault, T. George, M. Stillman and many others for their invaluable com-
ments.
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
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Draft SCTP applicability statement January 2001
Qiaobing Xie Phone: +1-847-632-3028
Motorola, Inc. EMail: [email protected]
1501 W. Shure Drive
Arlington Heights, IL 60004
USA
Matt Holdrege Phone: +1-408-830-3239
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
Expires: July 31, 2001
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