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INTERNET-DRAFT L. Coene(Editor)
Internet Engineering Task Force Siemens
Issued: November 2001
Expires: May 2002
Stream Control Transmission Protocol Applicability Statement
<draft-ietf-sigtran-sctp-applicability-08.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]. It also contrasts SCTP with
the two dominant transport protocols, UDP & TCP, and gives some
guidelines for when best to use SCTP and when not best to use SCTP.
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Table of contents
Stream Control Transmission Protocol Applicability statement
................................................................ ii
Chapter 1: Introduction ........................................ 2
Chapter 1.1: Terminology ....................................... 3
Chapter 1.2: Contributors ...................................... 3
Chapter 2: Transport protocols ................................. 3
Chapter 2.1: TCP service model ................................. 3
Chapter 2.2: SCTP service model ................................ 4
Chapter 2.3: UDP service model ................................. 5
Chapter 3: SCTP Multihoming issues ............................. 5
Chapter 4: SCTP Network Address Translators (NAT) issues
[RFC2663] ...................................................... 6
Chapter 5: Security considerations ............................. 7
Chapter 5.1: Security issues with TCP .......................... 7
Chapter 5.2: Security issues with SCTP ......................... 8
Chapter 5.3: Security issues with both TCP and SCTP ............ 9
Chapter 6: References and related work ......................... 9
Chapter 7: Acknowledgments ..................................... 11
Chapter 8: Author's address .................................... 11
Appendix A: Major functions provided by SCTP ................... 13
1 Introduction
SCTP is a reliable transport protocol [RFC2960], which along with
TCP [RFC793], RTP [RFC1889] and UDP [RFC768], provides
transport-layer services for upper layer protocols and services.
UDP, RTP, TCP and SCTP are currently the IETF standards-track
transport-layer protocols. Each protocol has a domain of
applicability and services it provides, albeit with some overlaps.
By clarifying the situations where the functionality of these
protocols are applicable, this document can guide implementers and
protocol designers in selecting which protocol to use.
Special attention is given to services SCTP provides which would
make a decision to use SCTP the right one.
Major functions provided by SCTP can be found in Appendix A.
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1.1 Terminology
The following terms are commonly identified in this 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
adaptation layer, a session layer protocol, or the user application
directly.
Multihoming: Assigning more than one IP network interface to a
single endpoint.
1.2 Contributors
The following people contributed to the document: L. Coene(Editor),
M. Tuexen, G. Verwimp, J. Loughney, R.R. Stewart, Qiaobing Xie,
M. Holdrege, M.C. Belinchon, A. Jungmaier, and L. Ong.
2 Transport protocols
2.1 TCP service model
TCP is a connection-oriented (a.k.a. session-oriented) transport
protocol. This means that it requires both the establishment of a
connection prior to exchange of application data and a connection
tear-down to release system resource after the completion of data
transfer.
TCP is currently the most widely used connection-oriented transport
protocol for the Internet.
TCP provides the upper layer with the following transport services:
- data reliability;
- data sequence preservation; and
- flow and congestion control.
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2.2 SCTP service model
SCTP is also connection-oriented and provides all the transport
services that TCP provides. Many Internet applications therefore
should find that either TCP or SCTP will meet their transport
requirements. Note, for applications conscious about processing
cost, there might be a difference in processing cost associated with
running SCTP with only a single ordered stream and one address pair
in comparison to running TCP.
However, SCTP has some additional capabilities that TCP lacks and
This can make SCTP a better choice for some applications and
environments:
- multi-streams support:
SCTP supports the delivery of multiple independent user message
streams within a single SCTP association. This capability, when
properly used, can alleviate the so-called head-of-line-blocking
problem caused by the strict sequence delivery constraint imposed to
the user data by TCP.
This can be particularly useful for applications that need to
exchange multiple, logically separate message streams between two
endpoints.
- multi-homing support:
SCTP provides transparent support for communications between two
endpoints of which one or both is multi-homed.
SCTP provides monitoring of the reachability of the addresses on the
remote endpoint and in the case of failure can transparently
failover from the primary address to an alternate address, without
upper layer intervention.
This capability can be used to build redundant paths between two
SCTP endpoints and can be particularly useful for applications that
seek transport-level fault tolerance.
Achieving path redundancy between two SCTP endpoints normally
requires that the two endpoints being equipped with multiple
interfaces assigned with multiple addresses and that routing is
configured appropriately (see Section 3).
- preservation of message boundaries:
SCTP preserves application messages boundaries. This is useful when
the application data is not a continuous byte stream but comes in
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logical chunks that the receiver handles separately.
In contrast, TCP offers a reliable data stream that has no
indication of what an application may consider logical chunks of the
data.
- unordered reliable message delivery:
SCTP supports the transportation of user messages that have no
application-specified order, yet need guaranteed reliable delivery.
Applications that need to send un-ordered reliable messages or
prefer to using their own message sequencing and ordering mechanisms
may find this SCTP capability useful.
2.3 UDP Service model
UDP is connectionless. This means that applications that use UDP do
not need to perform connection establishment or tear-down.
As transport services to its upper layer, UDP provides only:
- best-effort data delivery, and
- preservation of message boundaries.
Applications that do not require a reliable transfer of more than a
packet's worth of data will find UDP adequate. Some
transaction-based applications fall into this category.
3 SCTP Multihoming Issues
SCTP provides transport-layer support for multihoming. Multihoming
has the potential of providing additional robustness against network
failures. In some applications, this may be extremely important, for
example in signaling transport of PSTN signaling messages [RFC2719].
It should be noted that SCTP multihoming support only deals with
communication between two endpoints of which one or both is assigned
with multiple IP addresses on possibly multiple network interfaces.
It does NOT deal with communication ends that contain multiple
endpoints (i.e., clustered endpoints) that can switch over to an
alternate endpoint in case of failure of the original endpoint.
Generally, for truly fault resilient communication between two
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end-points, the multihoming feature needs more than one IP network
interface for each endpoint. The number of paths used is the minimum
of network interfaces used by any of the endpoints. 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. When
the endpoint chooses a source address, it should always select the
source address of the packet to correspond to the IP address of the
Network interface where the packet will be emitted subject to the
binding address constraint. The binding address constraint is put
simply that the endpoint must never choose a source address that is
not part of the association i.e. the peer endpoint must recognize
any source address used as being part of the association.
The availability of the association will benefit greatly from having
multiple addresses bound to the association endpoint when the
endpoint is on a multi-homed host.
4 SCTP Network Address Translators (NAT) issues [RFC2663]
When two endpoints are to setup an SCTP association and one (or
both) of them is behind a NAT (i.e., it does not have any publicly
available network addresses), the endpoint(s) behind the NAT should
consider 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 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 1.
+-------+ +---------+ *~~~~~~~~~~* +------+
|Host A | | NAT | * Cloud * |Host B|
| 10.2 +--|10.1|2.1 |----|--------------|---------+ 1.2 |
| | | | | * * | |
+-------+ +---------+ *~~~~~~~~~~* +------+
Fig 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
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the IP addresses in the INIT and INIT ACK chunks. See Figure 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 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.
5 Security considerations
In this section, some relevant security issues found in the
deployment of the connection-oriented transport protocols will be
discussed.
5.1 Security issues with TCP
Some TCP implementations have been known to be vulnerable to blind
denial of service attacks, i.e. attacks that had been executed by an
attacker that could not see most of the traffic to or from the
target host.
The attacker would send a large number of connection establishment
requests (TCP-SYN packets) to the attacked target, possibly from
faked IP source addresses. The attacked host would reply by sending
SYN-ACK packets and entering SYN-received state, thereby allocating
space for a TCB. At some point the SYN-queue would fill up,
(i.e. the number of connections waiting to be established would
rise to a limit) and the host under attack would have to start
turning down new connection establishment requests.
TCP implementations with SYN-cookies algorithm [SYN-COOK] reduce the
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risk of such blind denial of service attacks. TCP implementations
can switch to using this algorithm in times when their SYN-queues
are filled up while still fully conforming to the TCP specification
[RFC793]. However, use of options such as window scale [RFC1323],
is not possible, then. With the SYN-cookie mechanism, a TCB is only
created when the client sends back a valid ACK packet to the server,
and the 3-way handshake has thus been successfully completed.
Blind connection forgery is another potential threat to TCP. By
guessing valid sequence numbers, an attacker would be able to forge
a connection. However, with a secure hashsum algorithm, for some of
the current SYN-cookie implementations the likelihood of achieving
this attack is on the order of magnitude of 1 in 2^24, i.e. the
attacker would have to send 2^24 packets before obtaining one forged
connection when SYN-cookies are used.
5.2 Security issues with SCTP
SCTP has been designed with the experiences made with TCP in
mind. To make it hard for blind attackers (i.e. attackers that are
not man-in-the-middle) to inject forged SCTP datagrams into existing
associations, each side of an SCTP association uses a 32 bit value
called "Verification Tag" to ensure that a datagram really belongs
to the existing association. So in addition to a combination of
source and destination transport addresses that belong to an
established association, a valid SCTP datagram must also have the
correct tag to be accepted by the recipient.
Unlike in TCP, usage of cookie in association establishment is made
mandatory in SCTP. For the server, a new association is fully
established after three messages (containing INIT, INIT-ACK,
COOKIE-ECHO chunks) have been exchanged. The cookie is a variable
length parameter that contains all relevant data to initialize the
TCB on the server side, plus a HMAC used to secure it. This HMAC
(MD5 as per [RFC1321] or SHA-1 [SHA1]) is computed over the cookie
and a secret, server-owned key.
As specifically prescribed for SCTP implementations [RFC2960],
additional resources for new associations may only be reserved in
case a valid COOKIE-ECHO chunk is received by a client, and the
computed HMAC for this new cookie matches that contained in the
cookie.
With SCTP the chances of an attacker being able to blindly forge a
connection are even lower than in the case of TCP using SYN-cookies,
since the attacker would have to guess a correct value for the HMAC
contained in the cookie, i.e. lower than 1 in 2^128 which for all
practical purposes is negligible.
It should be noted that SCTP only tries to increase the availability
of a network. SCTP does not contain any protocol mechanisms which
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are directly related to user message authentication, integrity and
confidentiality functions. For such features, it depends on the
IPsec protocols and architecture and/or on security features of the
application protocols.
Transport Layer security(TLS)[RFC2246] using SCTP must always use
in-order streams.
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.
5.3 Security Issues with both TCP and SCTP
It is important to note that neither TCP nor SCTP protect itself
from man-in-the-middle attacks where an established session might be
hijacked (assuming the attacker can see the traffic from and inject
its own packets to either endpoints).
Also, for preventing blind connection/session setup forgery, both
TCP implementations supporting SYN-cookies and SCTP implementations
rely on a server-known, secret key to protect the HMAC data. It must
be ensured that this key is created subject to the recommendations
mentioned in [RFC1750].
Although SCTP has been designed carefully as to avoid some of the
problems that have appeared with TCP, it has as of yet not been
widely deployed. It is therefore possible that new security issues
will be identified that will have to be addressed in further
revisions of [RFC2960].
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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[RFC2694] Srisuresh, P., Tsirtsis, G., Akkiraju, P. and Heffernan,
A., "DNS extensions to Network Address Translators (DNS_ALG)",
RFC2694, September 1999
[RFC768] Postel, J. (ed.), "User Datagram Protocol", STD 6, RFC 768,
August 1980.
[RFC793] Postel, J. (ed.), "Transmission Control Protocol", STD 7,
RFC 793, September 1981.
[RFC2719] Ong, L., Rytina, I., Garcia, M., Schwarzbauer, H., Coene,
L., Lin, H., Juhasz, I., Holdrege, M., and C. Sharp, "Architectural
Framework for Signaling Transport", RFC 2719, October 1999.
[RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
MIT Laboratory for Computer Science, April 1992.
[RFC1323] Jacobson, V., Braden, R, and D. Borman, "TCP Extensions
for High Performance", RFC 1323, LBL, USC/Information Sciences
Institute, Cray Research, May 1992.
[RFC1750] Eastlake, D., Crocker, S., and J. Schiller, "Randomness
Recommendations for Security", RFC 1750, December 1994.
[SHA1] NIST FIPS PUB 180-1, "Secure Hash Standard," National
Institute of Standards and Technology, U.S. Department of Commerce,
April 1995.
[SYNCOOK] Dan J. Bernstein, SYN cookies, 1997, see also
<http://cr.yp.to/syncookies.html>
[RFC2246] Dierks, T. and Allen, C.,"The TLS Protocol Version 1.0",
RFC 2246, January 1999.
[RFC1889] Schulzrinne, H., Casner, S., Frederick, R., and Jacobson,
V., "RTP: A Transport Protocol for Real-Time Applications", RFC
1889, January 1996.
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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 ,
R.J. Atkinson and many others for their invaluable comments.
7 Author's Address
The following authors have contributed to this document.
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
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
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Retama 7, 5th floor
Madrid, 28045
Spain
Andreas Jungmaier Phone: +49 201 1837636
University of Essen Email: [email protected]
Networking Technology Group at the IEM
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: May 31, 2002
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HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
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Appendix A: Major functions 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
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