Internet DRAFT - draft-xie-sctp4megaco
Network Working Group Q. Xie
Category: Informational C. Sharp
Expires in six months March 2000
Use SCTP as MEGACO Transport
Status of this Memo
This document is an Internet-Draft and is in full conformance with all
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This document discusses the use of the Simple Control Transmission
Protocol (SCTP) , for carrying MEGACO  messages between an MGC
and an MG. SCTP is currently being developed by IETF SIGTRAN Working
Group. The framework architecture defined in RFC 2719  can be used
as a guideline for transporting MEGACO messages using SCTP.
MEGACO implementations targeting for high capacity and high
availability deployment can especially benefit from the stream
capability, redundant network support, congestion avoidance, and
strong security features provided by SCTP.
MEGACO protocol messages may be transmitted over the Simple Control
Transmission Protocol (SCTP) .
The MEGACO implementation may take advantage of the following services
provided by SCTP:
o Datagram-based transport
o Reliable delivery --- As a reliable transport protocol, SCTP
provides recovery mechanisms for transmission loss and duplicate
packet receipt. This simplifies the design of application level
repetition and timer control.
o Ordered and unordered reliable message delivery --- Settable on a
per-message basis by the application, SCTP allows high priority
transactions be sent through unordered delivery for possible
o Stream capability --- SCTP can provide up to 65536 unidirectional
streams in each direction of an MGC-MG association. SCTP transmits
messages and processes received messages in one stream independent
to the order or status of messages in any other streams. The
application may effectively avoid head-of-line blocking by
transmitting unrelated transactions on different streams.
o Protection against "SYN" attacks --- The encryption cookie
mechanism built into the SCTP provides protection against the
equivalent of TCP "SYN" attacks on a MG or MGC node.
o Network congestion management --- SCTP provides effective means
for detecting and handling network congestion.
o Redundant path management --- It may become strongly desirable for
a large MG to have fault resilient network-level connectivity
towards an MGC. SCTP supports multi-homed IP nodes for redundant
path deployment. SCTP provides reachability monitoring, fast switch-
over/fail-over, and potentially load balancing over redundant paths.
In a transaction-oriented protocol like MEGACO, there are still ways
for transaction requests or responses to be lost, e.g., caused by
entity/component failure. As such, it is recommended that
entities using SCTP transport implement application level timers for
2. Providing the At-Most-Once functionality
SCTP is designed to recover from transport losses or duplications, but
loss of a transaction request or its reply may nonetheless be noted in
real implementations. In the absence of a timely response, MEGACO may
repeat commands. Most MEGACO commands are not idempotent. The state
of the MG would become unpredictable if, for example, Add commands
were executed several times. To guard against such losses, it is
recommended that entities follow the procedures in Annex D.1.1 of
document , with the exception LONG-TIMER or the use of the
TransactionResponseAck parameter, which shall not be used.
3. Transaction identifiers and three way handshake
For the same reasons as discussed above, it is possible that
transaction replies may be lost even with a reliable delivery protocol
such as SCTP. It is recommended that entities use transaction
identifers following the procedures in Annex D.1.2.1 of document .
Three way handshake is not applicable when SCTP is used.
4. Computing retransmission timers
With reliable non-duplicate delivery guaranteed by SCTP, application
level timers are only used to guard against entity/component failure.
Therefore, only simple timer mechanisms are required. Exponential
back-off algorithms shall not be necessary. The first retransmission
of a request can occur after a short interval. If additional
retransmissions are required a longer time interval is recommended
between the retransmissions.
5. Provisional responses
The basic procedures in section 8.2.3 of document  apply.
6. Ordering of commands
SCTP provides both ordered and unordered reliable delivery, settable
on a per-transaction basis. Therefore, MEGACO can take advantage of
the ordered capability of SCTP. High priority transactions can get
expedited treatment by properly using unordered delivery. No special
procedures are therefore required.
7. Stream independence
SCTP can provide up to 65536 unidirectional streams in each direction
of an MGC-MG association. SCTP transmits messages and processes
received messages in one stream independent to the order or status of
messages in any other streams. MEGACO may avoid head-of-line blocking
by transmitting unrelated transactions on different
streams. Reliability is still provided. Ordering of messages is
available per-stream. It is recommended that transactions related to
one context are transported over the same stream.
8. Authors' Addresses
Qiaobing Xie Tel: +1-847-632-3028
Motorola, Inc. EMail: email@example.com
1501 W. Shure Drive, #2309
Arlington Heights, IL 60004
Chip Sharp Tel:
Cisco Systems Inc. EMail:firstname.lastname@example.org
7025 Kit Creek Road
Research Triangle Park, NC 27709
Randall R. Stewart Tel: +1-847-632-7438
Motorola, Inc. EMail: email@example.com
1501 W. Shure Drive, #2315
Arlington Heights, IL 60004
Ian Rytina Tel:
Ericsson Australia EMail:firstname.lastname@example.org
37/360 Elizabeth Street
Melbourne, Victoria 3000
 R.R. Stewart, Q. Xie, K. Morneault, C. Sharp, H.J. Schwarzbauer,
T. Taylor, I. Rytina, M. Kalla, L. Zhang, and V. Paxson, "Simple
Control Transmission Protocol," <draft-ietf-sigtran-sctp-07.txt>,
IETF SIGTRAN Working Group, March, 2000.
 F. Cuervo, B. Hill, N. Greene, C. Huitema, A. Rayhan, B. Rosen,
and J. Segers "Megaco Protocol,"
<draft-ietf-megaco-protocol-07.txt>, IETF MEGACO Working Group,
 L. Ong, I. Rytina, M. Garcia, H. Schwarzbauer, L. Coene, H. Lin,
I. Juhasz, M. Holdrege, and C. Sharp, "Framework Architecture for
Signaling Transport," RFC 2719, IETF, Oct. 1999.
This Internet Draft expires in 6 months from March 2000.