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Signalling Transport Working Group L. Coene, Siemens
Request for Comments: J. Loughney, Nokia
I. Rytina, Ericsson
L. Ong, Nortel Networks
Simple Control Transmission Protocol(SCTP) applicability statement
draft-ietf-sigtran-sctp-applicability-00.txt
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Abstract
This document describes the applicability of the Simple Control
Transmission Protocol for general usage. A few general application
are descibed such as the transport of signalling information(SS7,
DSS1/2...) over IP infrastructure. The use and specification of
adaptation layers in conjuction with SCTP is described.
1 Introduction
This document covers subject terminology and makes a overview of
the solutions for transporting information over Internet Protocol
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infrastructure. The transport medium used is the Simple Control
Transmission Protocol(SCTP). However some of the issues may also
relate to the transport of information via TCP.
SCTP provides the following services to its users:
- acknowledged error-free non-duplicated transfer of user data
- application-level segmentation to conform to discovered MTU
size
- sequenced delivery of user datagrams within multiple streams,
with an option for order-of-arrival delivery of individual
datagrams
- optional multiplexing of user datagrams into SCTP datagrams,
subject to MTU size restrictions
- enhanced reliability through support of multi-homing at either
or both ends of the association.
- Explicit indication in the message of the application protocol
SCTP is carrying.
1.1 Terminology
The following functions are commonly identified in related work:
Portnumber: Indicates on the tranport level which application
needs to be reached in the layer above.
Transport Address: An IP address and a portnumber forms a tran-
sport address which identifies a SCTP association.
Protocol Identifier: Indicates the upper layer protocol that is
using SCTP for the tranport of its data.
Chunk: a unit of information within an SCTP datagram, consist-
ing of a chunk header and chunk-specific content. Each chunk can
contain user or data information about the particular SCTP
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association.
2 Simple Control Transmission Protocol -- SCTP
2.1 Introduction
The Simple Control Transmission Protocol(SCTP) provides a high reli-
alable, redundant transport between 2 endpoints. The interface
between SCTP and its applications is handled via adaptation layers
which provide a intermediation layer so that the upper layer proto-
cols of a certain protocol stack architecture do not have to change
their interface towards the transport medium and internal functional-
ity when they start using SCTP instead of a other transport protocol
The following function are provided by SCTP:
- Initialization of transport association
- Synchronization of association state
- Synchronization of sequence numbering
- Reliable Data Transfer
- Forward and backward sequence numbering
- Timers for transmission and acknowledgement
- Notification of out-of-sequence - Retransmission of
lost messages
- Support of multiple control streams
- Separate sequence control and delivery of each
stream
- Congestion control
- Window flow control
- Congestion avoidance based on on TCP methods, e.g.
using
retransmission backoff, window reduction, etc.
- Detection of session failure by active means, e.g. heart-
beat
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- Termination of association
SCTP does support a number of functions that are not provided by
current TCP:
- no head-of-line blocking, i.e. multiple streams
- multilink failover for added reliability
- keep-alive function for active rapid failure detection
- message verses byte sequence numbering
- tighter timer control (than standard TCP implementations)
- greater fan out (than standard TCP implementations)
By defining the approriate User adaptation module, a reliable
transport mechanism can be provided:
- relialable transmission of packets with end-to-end congestion
control provided using methods similar to TCP
- choice between sequenced and unsequenced, relialable msg
delivery
- keep-alive msg
Within a association between the 2 endpoint, 1 or more stream(s) may
be avialable. These streams are visible to the adaptation layers but
are invisible to any layer above the adaptation layer.
2.2 Issues affecting deployement of SCTP
2.2.1 SCTP Multihoming
Redundant communication between 2 SCTP endpoints is achieved by using
multihoming where the endpoint is able to send/receive over more than
one IP address.
Under the assumption that every IP address will have a different path
towards the remote endpoint, (this is the responsability of the rout-
ing protocols(3.2.4) 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).
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Multihoming provides redundant communication in SCTP by allowing com-
munication between two endpoints to continue in the event of failure
along a path between the endpoints.
SCTP will always send its traffic to a certain transport address(=
destination address + portnumber combination) for as long as the
transmission is uninterupted(=primary). The other transport
addresses(secondary paths) will act as a backup in case the primary
path goes out of service. The changeover between primary are backup
will occur without packetloss and is completely transparent to the
application.
The portnumber is the same for all transport addresses of that
specific association.
Applications using directly SCTP may choose to control the multihom-
ing service themselves. The applications has then to supply the
specifc IP address to SCTP for each datagram. This might be done for
reasons of loadsharing and loadbalancing across the different paths.
This might not be advisable as the througput 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.
Applications using adaptation layers to run over SCTP do not have
that kind of control. The adaptation layers will have to take care of
this.
By sending a keepalive message on all the multiple paths that are not
used for active transmission of messages accross 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 keepalive msg should be engineerable
and the possible loss of keepalive msg could be used for the monitor-
ing and measurements of the concerned paths.
2.2.2 Fast retransmit of chunks
The retransmission of a msg is basically governed by the retransmis-
sion timer. So if no acknowledgement is received after a certain
time, then the msg is retransmitted. However there is a faster way
for retransmitting which is not dependant on that timer.
Every second msg that a node received will be acknowledge to the
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remote peer. If gaps occur in the acknowledge msg at the remote side,
then the remote side will wait 3 further gap
reports(acknowledgements) before it retransmit the msg. This
retransmission will happen far sooner than with a timer. Especially
if the traffic volume increases in SCTP, those retransmissions of the
chunks would happen faster and faster(and hopefully, they would also
be faster acknowledged)
See also the paragraph on congestion control and avoidance.
2.2.3 Use of SCTP in Network Adress Translator(NAT) networks
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:
A) Indicate that only one address can be used by including no tran-
sport addresses in the INIT message. This will make the endpoint that
receives this Initiation message to consider the sender as only hav-
ing that one address. This method can be used for a dynamic 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.
B) Indicate all of its networks in the Initiation by specifying all
the actual IP addresses and ports that the NAT will substitute for
the endpoint. This method requires that the endpoint behind the NAT
must have pre-knowledge of all the IP addresses and ports that the
NAT will assign.
This requires the adaptation of NAT boxes to go searching in SCTP
outgoing INIT and incoming INIT_ACK for the addresses and replace
them with the NAT internal address in addition to replace the
addresses in the IP header.
C) Use RSIP[] where the connection is tunnelled from host till the
NAT border and the host layers above IP networklayer have no
knowledge of the NAT internal addresses.
2.2.4 MTU path discovery
SCTP discovers the minimal length of the msg that can be transported
through the network to the final destination without having to frag-
ment the msg in IP network layer. This avoids using IP fragmenting
which if a segemented of a fragmented msg is discarded, only that
segment will be transmitted by SCTP (contrasted with segementing in
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IP where the whole unsegmented msg will have to be retransmitted and
after a longer time) -> fast retransmit of SCTP. See [07].
2.2.5 Use of multiple streams
The application can choose on which stream he can send it data. Some
application level protocols may standardize some stream number usage
convention, which, for instance, allows to send jpeg and gif portions
of a page through certain stream while text through others, so as to
avoid large graphics from blocking text content.
Each stream within a association should be looked upon as a link
between two points. If multiple streams are used then the application
is dealing with mulitple links towards the destination. Some applica-
tion require the use of sequence delivery, which would require for
them to select a certain link to send their message on.
2.2.6 Congestion control & avoidance
Congestion control and/or avoidance is of primordial importance in
any connectionless network. Congestion is the result of approaching
or exceeding the processing capacity of the link, network , applica-
tion and/or transport layers. If the processing capacity is exceeded,
then the congestion can be avoided(example taking a other non-
congested path towards the destination) or controlled(example :
reducing the rate of messages to that destination).
The reaction of SCTP to congestion is detailed in the next para-
graphs.
Congestion can be controlled and/or avoided on different levels:
- Transport: congestion control/avoidance within SCTP, TCP(fig
2.1.2)
- Network : Congestion control/avoidance present in the network
layers( example: SCCP, MTP ...)
- Link layer: flow control
SCTP conforms to the model of end-to-end congestion control(Fig
2.2.6.2) while ISUP and SCCP model themselves on a link and network
based congestion control/overload mechanism(Fig 2.2.6.3).
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| |
| Application and/or transport layer |
+---------------------------------------------------+
| A
| |
| +-------------------------------------+ |
---->| |----
| Network layer |
---->| |----
| +-------------------------------------+ |
| |
| V
+---------------------------------------------------+
| |
| Link layer |
Fig 2.2.6.1 General Congestion model
| |
|transport layer| Congestion control present based on
| SCTP | windows
+---------------+
| A
V |
+---------------+
| |
| Network layer | No congestion control present
| IP(v4/v6) | in the IP layer
+---------------+
| A
V |
+---------------+
| Ethernet | No congestion control present
| Link layer | in the ethernnet link layer
Fig 2.2.6.2 End-to-End congestion control
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| |
|Application layer| Congestion control present for
| TC + MAP,IN... | specific applications
+-----------------+ - MAP: No congestion control
| A - IN: Call gapping
V |
+-----------------+
| |
| Network layer | Congestion control present in the
| SCCP & MTP | in MTP and SCCP based on link and
+-----------------+ destination status
| A
V |
+-----------------+
| MTP lvl 2 | Congestion control present
| Link layer | in the link layer
Fig 2.2.6.3 Distributed congestion control
By default, SCTP associations do not have a fixed capacity assigned
to them unless other QOS mechanisms are employed.Thus congestion
within SCTP association can and will be affected by all traffic using
the same links including other SCTP, TCP, RTP, UDP... traffic going
through the same links of the path followed by the SCTP association.
2.2.6.1 Application of Congestion control in SCTP - 3-SACK rule
The Selective Acknowledgement(SACK) is one of the cornerstones of
SCTP. It selective Acknowledges datagrams that have been successfully
received by the remote node. It serves 2 purposes:
- it indicates till a certain datagram that all previous
datagrams have been received(without any holes in the
sequence) and
- it indicates the datagrams sequence ranges which have
been received(and so does indicate the holes/gaps between
them). It provides us with a form of gap/hole report on
messages that have been lost or delayed. A hole can consist
of one or more messages.
The SACK is always generated and send back to to the sender either
- after every second message received(delayed ack).
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- after at most 200ms after receiving the last msg.
The reason for the holes may be diverse:
- simple message loss
- different round trip times of messages being transmitted
on different interfaces
At the sender end, whenever the sender notices a hole in a SACK, it
should wait for 3 further SACKs(identifying the same hole) before
taking action. This is 3 strikes besides the first one, so that means
4. Thus after 4 SACK, the datagrams belonging to the hole should be
retransmitted(and only those).
The 3 SACKs rule might be relaxed in certain network provided certain
condition are met:
- private IP network
- if the operator felt confident enough of his own closed
network.
The SACK rule might be configurable in such a networks. This would
mean that in case of message drops, retransmission would be "immedi-
ate".
2.2.6.2 Congestion Control
The number of messages in flight is determined by the Congestion
window(Cwnd). Every time a msg is SACK, a new msg might be send to
the remote side(up till the Cwnd), even if gaps exists which might
ultimatly lead to retransmissions.
The value of the Cwnd is dependant on the slow start and/or conges-
tion avoidance/control.
2.2.6.3 Use of Explicit Congestion notification(ECN)
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Explicit Congestion control is a experimental method for communicat-
ing congestion back to the end node.
2.2.6.4 Duplicated messages
SACK can get lost. The receiving node would then received duppli-
cated packets. A reason for such a behaviour is unbalance between the
2 traffic direction, use of different up and down path.
2.2.7 Use of the protocol identifier in SCTP
Indicates the sort of adaptation layers that is using the associa-
tions. The protocol identifier is avialable to the application and is
included in each chunk. 0 is the unknown protocol. This protocol id
can be used by firewalls for filtering out certain protocols. If
firewalls drops certain protocol id then then association will fail
in the end because the TSN will be lost. If the chunk(without its
user data) is simulated with the TSN in it, then the user data will
be dropped, but the association is preserved.
The protocol identifier is administreted by IANA.
2.2.8 Use of QOS methods
SCTP is a end-to-end protocol which cannot guarantee the quality-of-
service along the complete path(s) taken by the messages of that par-
ticular association. If more guarantees are required for improving
the relialability of the transport, some form of QOS mechanism may be
needed.
The possible schemes are as follows.
2.2.8.1 Overprovisioning
Overprovisioning of the links so that the total traffic running over
over the link never excedes the link capacity. In practice, this may
be difficult to ensure reliably.
2.2.8.2 Private Internets
Use of a private network solely for transport purposes. Private net-
works may allow better control and monitoring of resources available.
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2.2.8.3 Differentiated services
By providing a certain codepoint in the Type-of-service field (TOS),
certain Differential services can be selected. [09,10]
Setting the code point for transport requires some thought. It is
dependant on the kind of differentiate service selected. Also the use
of traffic is important: example signalling info should have a higher
priorty than the user data traffic for which the signalling is
responsible(and that relation does not always exist).
2.2.8.4 Integrated services
By use of integrated services [08], resources are reserved for sig-
naling transport.
If resources are unavailable for to initiate a new signaling tran-
sport, that request will be denied. In practice, RSVP does not scale
well and this solution may prove to be unfeasable.
An example is Multi Protocol Label Switching.
2.2.9 SCTP Checksum
SCTP uses the Adler-32 checksum algorithm. This algorithm will per-
form better than a 16 bit(CRC or not) checksum or even a 32 bit CRC
checsum.
The msg can also be protected by IPSEC. In that case, the checksum
migth be turned off(field set to 0).
2.2.10 Tunneling of SCTP association over UDP
The basic operation of SCTP is to run directly on top of IP. However,
due to restrictions placed on implementers by Operating Systems, not
all implementations may be able to run over IP directly. Therefore an
alternative is given which might circonvent some or all of the res-
trictions.
The STCP messages are transported over UDP instead. The following
issues must be observed:
- the portnumber in the UDP header should be the portnumber
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Draft SCTP applicability statement March 2000
assigned to SCTP. The portnumber in the SCTP common header
should be the one assigned to the user adaptation layer or to
the application of SCTP. This means that portnumbers previously
used in UDP and/or TCP can be reused for the same application
using SCTP. SCTP DOES NOT change the semantics of the portnumber
just because the protocol identifier is added to the SCTP mes-
sage. - the checksum field might be used as a additional guard
against errors(particular errors in the UDP header). However,
the SCTP checksum employed is far better at catching errors, but
does not take the UDP header into account.
2.2.11 How to define and Use adaptation layers
Many different applications may use SCTP for different purposes. They
go from Filetransfer over HTTP transport till signalling information
transport.
Some applications might want to have a unchanged interface with its
lower layer(in this case SCTP) while for other applications, this
does not pose a problem. A architecture has been devised to let the
application chosse whether they want to run over SCTP directly(just a
many applications run over TCP) or let application run on top of a
adaptation layer over SCTP.
The basic architecture is as in Figure 2.11.1 :
User/Application level Protocols
| | |
+------------------------------------+
| User Adaptation modules |
+------------------------------------+
|
+------------------------------------+
|Simple Control Transmission protocol|
+------------------------------------+
|
+------------------------------------+
| Standard IP Transport |
+------------------------------------+
|
Network Layer (IP)
Figure 2.11.1: Transport Components
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The three components of the transport protocol are :
(1) Adaptation modules that support specific primitives, e.g.
management indications, required by a particular user/ applica-
tion protocol. The use of a adaptation protocol is optional. It
is only used in case in which the application protocol does not
want to change its interface with the underlaying layer.
(2) the Simple Control Transmission Protocol itself that supports a
common set of reliable transport functions.
(3) a standard IP transport/network protocol provided by the
operating system. In some network scenarios, it has been recog-
nised that TCP can provide limited (but sufficient) reliable
transport functionality for some applications.
2.2.12 Security considerations
The following aspects of security are :
Authentication:
Information is sent/received from a known and/or trusted
partner.
Intergrity:
Information may not be modified while in transit. The integrity
of a msg in a public network is not guaranteed.
Confidentiality:
Confidentiality of the user data must be ensured. User data can
not be examined by unauthorized users.
Availability:
The communicating endpoint must remain in service in all
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Draft SCTP applicability statement March 2000
circonstances. Some services have very high availability
requirements: example , all SS7 nodes have to remain active for
the 99.999% of the time.
SCTP only tries to increase the availability of a network. SCTP does
not contain any protocol elements in its messages which are directly
related to Authentication, Intergrity and Confidentiality functions.
It depends for such a features on the IPSEC protocols and architec-
ture.
The only function which has some bearing on security of SCTP is the
integrity of message in SCTP, which is guarded by a Checksum. This
checksum is manadatory if IPSEC is NOT used. If IPSEC is used then
the SCTP checksum becomes optional. THe use of IPSEC in the SCTP
association must in this case be END-TO-END. The use of IPSEC on a
part of a path of a SCTP association does NOT relieve SCTP from using
the checksum(as this ain't end-to-end transport)
The general rule is that IPSEC should be turned on unconditionaly.
The description of the internet security architecture and the use of
it is described in [06].
3 Recommendations
4 References and related work
[01] Stewart, R. R., Xie, Q., Morneault, K., Sharp, C. , ,
Schwarzbauer, H. J., Taylor, T., Rytina, I., Kalla, M., Zhang,
L. and Paxson, V."Simple Control Transmission Protocol",
RFCxxxx, March 2000.
[02] SG11, ITU-T Recommendation Q.1400, " architecture framework for
the development of signaling and OA&M protocols using OSI con-
cepts ",1993
[03] Huitema, C., "Routing in the Internet", Prentice-Hall, 1995.
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Draft SCTP applicability statement March 2000
[03] Hinden, R. and Deering, S., "IP Version 6 Addressing Architec-
ture", RFC 2373, July 1998.
[04] Hinden, R. and Deering, S., "Internet Protocol, Version 6 (IPv6)
Specification", RFC 2460, December 1998.
[05] Clark, D.D., "Names, addresses, ports and routes", RFC 0814,
July 1982.
[06] Kent, S., and Atkinson, R., "Security Architecture for the
Internet Protocol", RFC 2401, November 1998.
[07] McCann, J., Deering, S., and Mogul, J., "Path MTU Discovery for
IP version 6", RFC 1981, August 1996.
[08] Mankin, A. Ed., Baker, F., , Braden, B., Bradner, S.,, O`Dell,
M., Romanow, A., Weinrib, A. and Zhang, L., "Resource ReSerVa-
tion Protocol (RSVP) -- Version 1 Applicability Statement Some
Guidelines on Deployment" , RFC 2208, September 1997.
[09] Heinanen, J., Baker, F., Weiss, W. and Wroclawski, J., "Assured
Forwarding PHB Group", RFC2597, June 1999
[10] Jacobson, V., Nichols, K. and Poduri, K., "An Expedited Forward-
ing PHB", RFC2598, June 1999
5. Acknowledgments
The authors wish to thank Renee Revis and many others for their
invaluable comments.
6 Author's Address
Lode Coene
Siemens Atea
Atealaan 34
B-2200 Herentals
Belgium
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Draft SCTP applicability statement March 2000
Phone: +32-14-252081
EMail: lode.coene@siemens.atea.be
John Loughney
Nokia
Research centre
Itamerenkatu 11-13
FIN-00180 Helsinki
Finland
Phone: +358-9-43761
EMail: john.loughney@nokia.com
Ian Rytina
Ericsson Australia
37/360 Elizabeth Street
Melbourne, Victoria 3000
Australia
Phone : -
EMail:ian.rytina@ericsson.com
Lyndon Ong
Nortel Networks
4401 Great America Parkway
Santa Clara, CA 95054
USA
Phone: -
EMail: long@nortelnetworks.com
Expires: November 2000
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