Network Working Group D. Sprague
Request for Comments: 3094 R. Benedyk
Category: Informational D. Brendes
J. Keller
Tekelec
April 2001
Tekelec"s Transport Adapter Layer Interface
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2001). All Rights Reserved.
IESG Note:
Readers should note that this memo presents a vendor"s alternative to
standards track technology being developed by the IETF SIGTRAN
Working Group. The technology presented in this memo has not been
reviewed by the IETF for its technical soundness or completeness.
Potential users of this type of technology are urged to examine the
SIGTRAN work before deciding to use the technology described here.
Abstract
This document proposes the interfaces of a Signaling Gateway, which
provides interworking between the Switched Circuit Network (SCN) and
an IP network. Since the Gateway is the central point of signaling
information, not only does it provide transportation of signaling
from one network to another, but it can also provide additional
functions sUCh as protocol translation, security screening, routing
information, and seamless Access to Intelligent Network (IN) services
on both networks.
The Transport Adapter Layer Interface (TALI) is the proposed
interface, which provides TCAP (Transaction Capability Application
Part), ISUP (ISDN User Part), and MTP (Mail Transport Protocol)
messaging over TCP/IP. In addition, TALI provides SCCP (Signalling
Connection Control Part) Management (SCMG), MTP Primitives, dynamic
registration of circuits, and routing of call control messages based
on circuit location.
Table of Contents
1. Introduction 4
2. Overview of the TALI Protocol 6
2.1 Traditional PSTN SS7 Networks 6
2.2 Converged SS7 Networks 8
2.3 TALI Protocol Stack Overview 10
2.3.1 An Alternate TALI Protocol Stack using the SAAL Layer 13
2.3.2 An Alternate TALI Protocol Stack using SCTP 15
2.4 Inputs to the TALI Version 1.0 State Machine 15
3. TALI Version 1.0 17
3.1 Overview of the TALI Message Structure 17
3.1.1 Types of TALI Fields 19
3.2 Detailed TALI Message Structure 20
3.2.1 TALI Peer to Peer Messages 20
3.2.1.1 Test Message (test) 20
3.2.1.2 Allow Message (allo) 21
3.2.1.3 Prohibit Message (proh) 21
3.2.1.4 Prohibit Acknowledgement Message (proa) 21
3.2.1.5 Monitor Message (moni) 22
3.2.1.6 Monitor Acknowledge Message (mona) 22
3.2.2 Service Messages 23
3.2.2.1 SCCP Service Message (sccp) 23
3.2.2.1.1 SCCP Encapsulation using TALI 25
3.2.2.2 ISUP Service Message (isot) 27
3.2.2.2.1 ISUP Encapsulation using TALI 27
3.2.2.3 MTP3 Service Message (mtp3) 28
3.2.2.3.1 MTP3 Encapsulation using TALI 29
3.2.2.4 SAAL Service Message (saal) 30
3.2.2.4.1 MTP3 and SAAL Peer to Peer Encapsulation using TALI 31
3.3 TALI Timers 34
3.3.1 T1 Timer 34
3.3.2 T2 Timer 34
3.3.3 T3 Timer 34
3.3.4 T4 Timer 34
3.3.5 Recommended Defaults and Ranges for the TALI Timers 35
3.4 TALI User Events 35
3.4.1 Management Open Socket Event 35
3.4.2 Management Close Socket Event 36
3.4.3 Management Allow Traffic Event 36
3.4.4 Management Prohibit Traffic Event 36
3.5 Other Implementation Dependent TALI Events 37
3.6 TALI States 37
3.7 TALI Version 1.0 State Machine 38
3.7.1 State Machine Concepts 38
3.7.1.1 General Protocol Rules 38
3.7.1.2 Graceful Shutdown of a Socket 39
3.7.1.3 TALI Protocol Violations 39
3.7.2 The State Machine 40
3.8 TALI 1.0 Implementation Notes 42
3.8.1 Failure on a TCP/IP Socket 42
3.8.2 Congestion on a TCP/IP Socket 43
3.9 TALI 1.0 Limitations 43
4. TALI Version 2.0 43
4.1 Overview of TALI Version 2.0 Features 45
4.2 TALI Version Identification 47
4.3 Backwards Compatibility 50
4.3.1 Generating Protocol Violations based on Received Messages 53
4.4 Overview of the TALI Message Structure 55
4.4.1 Types of TALI Fields 55
4.5 Detailed TALI Message Structures for New 2.0 Opcodes 58
4.5.1 Management Message (mgmt) 60
4.5.1.1 Routing Key Registration Primitive (rkrp) 61
4.5.1.1.1 RKRP Data Structures 65
4.5.1.1.1.1 Common Fields in all RKRP Messages 65
4.5.1.1.1.2 CIC Based Routing Key Operations 67
4.5.1.1.1.3 SCCP Routing Key Operations 71
4.5.1.1.1.4 DPC-SI, DPC and SI based Routing Key Operations 74
4.5.1.1.1.5 Default Routing Key Operations 76
4.5.1.1.1.6 Support for Multiple RKRP Registration Operations 78
4.5.1.1.1.6.1 Multiple Registrations Support 78
4.5.1.1.1.6.2 Multiple RKRP Operations in a Single Message 80
4.5.1.2 MTP3 Primitive (mtpp) 82
4.5.1.3 Socket Option Registration Primitive (sorp) 87
4.5.2 Extended Service Message (xsrv) 91
4.5.3 Special Message (spcl) 92
4.5.3.1 Special Messages Not Supported (smns) 93
4.5.3.2 Query Message (qury) 93
4.5.3.3 Reply Message (rply) 94
4.5.3.4 Unsolicited Information Message (USIM) 95
4.6 TALI Timers 95
4.7 TALI User Events 95
4.8 TALI States 96
4.9 TALI Version 2.0 State Machine 96
4.9.1 State Machine Concepts 96
4.9.1.1 General Protocol Rules 96
4.9.1.2 Graceful Shutdown of a Socket 97
4.9.1.3 TALI Protocol Violations 97
4.9.2 The State Machine 97
4.10 TALI 2.0 Specification Limitations 101
5. Success/Failure Codes 101
6. Security Considerations 102
7. References 102
8. Acknowledgments 103
9. Authors" Addresses 104
10. Full Copyright Statement 105
1. Introduction
This document is organized into the following 6 sections:
- Introduction to the document
- Overview of the TALI Protocol
- TALI Version 1.0
- TALI Version 2.0
- Success/Failure Codes
- Security Considerations
The following terms are used throughout this document.
Circuit Identification Code (CIC):
A field identifying the circuit being setup or released. Depending
on SI and MSU Type, this field can be 12, 14 or 32 bits.
Changeover/Changeback (co/cb):
SS7 MTP3 procedure related to link failure and re-establishment.
Far End (FE):
The remote endpoint of a socket connection.
Far End Allowed (FEA):
The FE is ready to use the socket for service PDUs.
Far End Prohibited (FEP):
The FE is not ready to use the socket for service PDUs.
Intelligent Network (IN):
A network that allows functionality to be distributed flexibly at a
variety of nodes on and off the network and allows the architecture
to be modified to control the services.
Management ATM Adaptation Layer (MAAL):
This layer is a component of SAAL. This layer maps requests and
indications between the System Management for the SG and the other
SAAL layers. MAAL includes interfaces to/from SSCOP, SSCF, and
system management. More information can be found in T1.652.
Media Gateway (MG):
A MG terminates SCN media streams, packetizes the media data, if it
is not already packetized, and delivers packetized traffic to the
packet network. It performs these functions in reverse order for
media streams flowing from the packet network to the SCN.
Media Gateway Controller (MGC):
An MGC handles the registration and management of resources at the
MG. The MGC may have the ability to authorize resource usage based
on local policy. For signaling transport purposes, the MGC serves as
a possible termination and origination point for SCN application
protocols, such as SS7 ISDN User Part and Q.931/DSS1.
MTP3 Framing (MTP3F):
TALI does not require full MTP3 procedures support but rather uses
the MTP3 framing structure (ie: SIO, Routing Label, etc)
Near End (NE):
The local endpoint of a socket connection.
Near End Allowed (NEA):
The NE is ready to use the socket for service PDUs.
Near End Prohibited (NEP):
The NE is not ready to use the socket for service PDUs.
Q.BICC ISUP:
An ISUP+ variant that uses 32 bit CIC codes instead of 14/12 bit CIC
codes. ISUP+, or Q.BICC ISUP, is based on the Q.765.BICC
specification currently being developed in ITU Study Group 11.
Signaling ATM Adaptation Layer (SAAL):
This layer is the equivalent of MTP-2 for ATM High Speed Links
carrying SS7 Traffic as described in GR-2878-CORE [8]. SAAL includes
SSCF, SSCOP and MAAL.
Signaling Gateway (SG):
An SG is a signaling agent that receives/sends SCN native signaling
at the edge of the IP network. The SG function may relay, translate
or terminate SS7 signaling in an SS7-Internet Gateway. The SG
function may also be co-resident with the MGC/MG functions to process
SCN signaling associated with line or trunk terminations controlled
by the MG (e.g., signaling backhaul).
Service Specific Coordination Function (SSCF):
This layer is a component of SAAL. This layer maps the services
provided by the lower layers of the SAAL to the needs of a specific
higher layer user. In the case of the STP, the higher layer user is
the MTP-3 protocol, and the SSCF required is that as defined by
T1.645: SSCF for Support of Signaling at the Network Node Interface
(SSCF at the NNI). More information can be found in T1.645. SSCF
provides the interface between SSCOP and MTP3 and includes the
following functions:
- Local Retrieve of messages to support link changeover procedures
- Flow control with four levels of congestion
Switched Circuit Network (SCN):
The term SCN is used to refer to a network that carries traffic
within channelized bearers of pre-defined sizes. Examples include
Public Switched Telephone Networks (PSTNs) and Public Land Mobile
Networks (PLMNs). Examples of signaling protocols used in SCN
include Q.931, SS7 MTP Level 3 and SS7 Application/User parts.
Service Specific Connection Oriented Protocol (SSCOP):
This layer is a component of SAAL. This layer provides reliable
point to point data transfer with sequence integrity and error
recovery by selective retransmission. Protocol layer interfaces are
described in T1.637. ASPects of the protocol include flow control,
connection control, error reporting to layer management, connection
maintenance in the prolonged absence of data transfer, local data
retrieval by the user of the SSCOP, error detection of protocol
control information and status reporting. SSCOP provides the link
layer functions that are:
- In-Sequence Delivery
- Flow Control
- Error Detection/Correction
- Keep Alive
- Local Data Retrieval
- Connection Control
- Protocol Error Detection and Recovery
Signaling Transfer Point (STP):
Packet switches that provide CCS message routing and transport. They
are stored programmed switches that use information contained in the
message in conjunction with information stored in memory to route the
message to the appropriate destination signaling point.
2. Overview of the TALI Protocol
2.1 Traditional PSTN SS7 Networks
The traditional PSTN SS7 network consists of 3 types of devices
connected via dedicated SS7 signaling links.
The 3 primary device types for PSTN networks are:
* SSP: Signaling Service Point. These nodes act as endpoints in
the SS7 network, originating SS7 messages as users attempt to
place phone calls. These nodes contain interfaces into the SS7
data network and the SS7 voice network.
* STP: Signaling Transfer Point. These nodes act primarily as
switches, switching SS7 traffic from node to node throughout the
network until it reaches another endpoint. An important feature
of each STP is to provide SS7 network management functionality
that allows messages to be delivered even when links and devices
fail. STPs also sometimes provide database type services, such as
Global Title Translations and Local Number Portability.
* SCP: Signaling Control Point. These nodes act as databases.
These nodes contain stored data that is used to turn SS7 Queries
into SS7 Replies.
There are 3 primary types of dedicated SS7 signaling links:
* 56Kbps SS7 (DS0, V35, OCU) links. These links implement the MTP-1
and MTP-2 protocols as defined in [1].
* DS1 High Speed Links. These links use the SAAL protocol to
provide an alternative to 56Kbps SS7 links that is based on newer,
faster technology. These links implement the SS7 protocol as
defined in [8].
* E1 Links.
Figure 1 provides an overview of the traditional PSTN network. In
this network, any of the links can be implemented via either 56
Kbps, DS1, or E1 links.
^
/
/SCP
/-----
/
/
/
/
/--- +---+ +---+ /---
SSP -----STP----STP----- SSP
---/ /+-+-+ /+-+-+ / ---/
/ / /
/ / /
/--- / +-+-+/ +-+-+ / /---
SSP /----STP----STP/---- SSP
---/ +---+ +---+ ---/
/
/
/
^ /
/ /
/SCP
/-----
Figure 1: The Traditional PSTN Network
2.2 Converged SS7 Networks
In the converged SS7 network, SS7 devices will reside on both the
traditional PSTN network (with dedicated 56 Kbps and DS1 links) and
on the IP network (with Ethernet links based on IP protocol). The
services of SSPs, STPs, and SCPs can be provided by new types of
devices that reside on IP networks. The IP network is not intended
to completely replace the PSTN, rather devices on the 2 types of
networks must be able to communicate with one another and convert
from 1 lower layer protocol to the other.
Signaling Gateways are new devices that may also function as an STP
in the converged network. SGs provide interfaces to:
* devices on the SCN (traditional SSPs, STPs, and SCPs)
* other SGs
* new devices on the IP network
SGs also continue to perform STP functions such as SS7 network
management and some database services (such as GTT and LNP).
New devices on the IP network include:
* Media Gateway Controllers. In addition to other functions, these
devices control Media Gateways and perform call processing.
* Media Gateways. In addition to other functions, these devices
control voice circuits that are used to carry telephone calls.
MGs + MGCs combine to provide the functionality of traditional
SSPs.
* IP based SCPs. The database services that are related to SS7 can
be moved onto devices on the IP network.
Figure 2 provides an overview of the converged SS7 network.
----- +----+
/ / ------------- SG
/ ---- SCN +----+ +----+
/SCP /------ SG
------ ----- +----+


-----
/ /
IP ----/
/--- / /SCP
SSP ----- ------
---/ /
/
/--- /
SSP +---+ +---+
---/ +----+ MGC MGC
MG +---+ +---+
+----+ /
/
-----
/
+----+ IP
MG ----------- /
+----+ -----
Figure 2: The Converged SS7 Network
In theory, the TALI protocol can be used between 2 nodes to carry SS7
traffic across TCP/IP. Some of the areas that TALI could be used
include:
- For SG to SG communication across IP
- For SG to MGC communication across IP
- For SG to IP based SCP communication across IP
- For communication between multiple IP based SCPs
- For communication between multiple MGCs
- For communication between MGCs and MGs
- For other IP devices such as DNS, Policy Servers, etc.
In reality, the communication between MGCs, or between MGC and MG is
probably better suited to using other protocols. With respect to the
Signaling Gateway implementation, the TALI protocol is used to carry
SS7 traffic:
- For SG to SG communication
- For SG to MGC communication
- For SG to IP based SCP communication
2.3 TALI Protocol Stack Overview
The Transport Adapter Layer Interface is the proposed interface that
provides SCCP, ISUP, and MTP messaging encapsulation within a TCP/IP
packet between two switching elements. In addition, TALI provides
SCCP Management (SCMG), MTP Primitives, dynamic registration of
circuits, and routing of call control messages based on circuit
location.
The major purpose of the TALI protocol is to provide a bridge between
the SS7 Signaling Network and applications that reside within an IP
network. Figure 3 provides a simple illustration that highlights the
protocol stacks used for transport of SS7 MSUs on both the SS7 side
and the IP side of the SG.
SS7 traffic SS7 traffic
via 56Kbps links via TALI
+-----------+ +----+ +--------+
Traditional SG IP
SS7 Devices<------> <--------> Devices
+-----------+ +----+ +--------+
SS7 SS7, TALI, TCP/IP
protocol stack protocol stack
+---------------+ +---------------+
SS7 application SS7 application
layer layer
+-------+-------+ +-------+-------+
TCAP ISUP TCAP ISUP
+-------+ +-------+
SCCP SCCP
+-------+-------+ +-------+-------+
MTP3 MTP3
+---------------+ +---------------+
MTP2 TALI
+---------------+ +---------------+
MTP1 TCP
(& phy. +---------------+
layer) IP
+---------------+ +---------------+
MAC
(& phy.
layer)
+---------------+
Figure 3: TALI Protocol to carry SS7 over TCP/IP
From Figure 3, several observations can be made:
* The TALI layer is used when transferring SS7 over IP.
* When SS7 traffic is carried over a IP network, the MTP2 and MTP1
layers of a traditional 56 Kbps link are replaced by the TALI,
TCP, IP, and MAC layers
* The TALI layer sits on top of the TCP layer.
* The TALI layer sits below the various SS7 layers (MTP3, SCCP/TCAP,
ISUP, and applications). The data from these SS7 layers is
carried as the data portion of TALI service data packets.
Some of the facts concerning the TALI protocol which are important to
understanding how TALI works that are not evident from Figure 3
include the following:
* Each TALI connection is provided over a single TCP socket.
* The standard Berkeley sockets interface to the TCP is used by
the TALI layer to provide connection oriented service from
endpoint to peer endpoint.
* TCP sockets are based on a Client/Server architecture; one end
of the TALI connection must be defined as the "server side",
the other end is a "client".
* The client/server roles are important only in bringing up the
TCP connection between the 2 endpoint, once the connection is
established both ends use the same Berkeley sockets calls
(send, recv) to transfer data.
* The TCP socket must be connected before the 2 TALI endpoints
can begin communicating.
* TALI provides user control over each TALI connection that is
defined. This control:
* Allows the user to control when each TALI connection will be
made
* Allows the user to control when each TALI connection is allowed
to carry SS7 traffic
* Allows the user to control the graceful shutdown of each socket
* TALI provides Peer to Peer messages. These messages originate
from the TALI layer of one endpoint of the connection and are
terminated at the TALI layer of the other endpoint. Peer to Peer
messages are used:
* To provide test and watchdog maintenance messages
* To control the ability of each socket to carry SS7 service
messages
* TALI provides Service messages. These messages originate from the
layer above the TALI layer of one endpoint of the connection and
are transferred to and terminated at the layer above the TALI
layer of the other endpoint.
* The service messages provide several different ways to
encapsulate the SS7 messages (SCCP/TCAP, ISUP, and other MTP3
layer data) across the TCP/IP connection.
* As we will see later, different Service opcodes are used to
communicate across the TALI socket exactly how each SS7 message
has been encapsulated.
* A set of TALI timers is defined. These timers are used to
correctly implement the TALI state machine.
2.3.1 An Alternate TALI Protocol Stack using the SAAL Layer
This section presents a different, slightly more complex, TALI
protocol stack that can be used in place of the protocol stack in the
previous section.
Figure 3 in the previous section provided a simple illustration that
highlighted the basic TALI protocol stack that can be used to
transport SS7 MSUs between 56 Kbps links on the SS7 side of an SG and
the IP devices.
Figure 4 below illustrates an alternate TALI protocol stack that
includes the SAAL layer as part of the data transferred across the
TCP/IP connection.
SS7 traffic SS7 traffic
via DS1 links via TALI
+-----------+ +----+ +--------+
Traditional SG IP
SS7 Devices<------> <--------> Devices
+-----------+ +----+ +--------+
SS7 DS1 SS7, TALI, TCP/IP
protocol stack protocol stack
+-----------------+ +-----------------+
SS7 application SS7 application
layer layer
+--------+--------+ +--------+--------+
TCAP ISUP TCAP ISUP
+--------+ +--------+
SCCP SCCP
+--------+--------+ +--------+--------+
MTP3 MTP3
+-----------------+ +-----------------+
SAAL SAAL
(SSCF,MAAL,SSCOP) (SSCF,MAAL,SSCOP)
+-----------------+ +-----------------+
AAL5 TALI
+-----------------+ +-----------------+
ATM TCP
(& phy. +-----------------+
layer) IP
+-----------------+ +-----------------+
MAC
(& phy.
layer)
+-----------------+
Figure 4: An Alternate TALI Protocol Stack with SAAL
The following bullets provide a discussion regarding the differences
between these 2 protocol stacks, the reasons for having 2 protocol
stacks, and the advantages of each:
* When the TALI protocol stack is implemented without the SAAL
layer, as in Figure 3, the SEQUENCE NUMBER of the SS7 MSU is NOT
part of the data transferred across the TCP/IP connection. In 56
Kbps SS7 links, the MTP2 header contains an 8 bit sequence number
for each MSU. The sequence number is used to preserve message
sequencing and to support complex SS7 procedures involving MSU
retrieval during link changeover and changeback. As indicated in
Figure 3, the MTP2 header is NOT part of the data transferred
across the TCP/IP connection. The TALI protocol stack without
SAAL still guarantees correct sequencing of SS7 data (this
sequencing is provided by sequence numbers in the TCP layer),
however that protocol stack can not support SS7 changeover and
changeback procedures.
* When the TALI protocol stack is implemented with the SAAL layer,
as in Figure 4, the SEQUENCE NUMBER of the SS7 MSU IS part of the
data transferred across TCP/IP. In SS7 DS1 links, the SSCOP
trailer contains a 24 bit sequence number for each MSU. This 24
bit sequence number serves the same purposes as the 8 bit SS7
sequence number. As indicated in Figure 4, the SSCOP trailer IS
part of the data transferred across the TCP/IP connection. The
protocol stack in Figure 4 can support SS7 changeover and
changeback procedures.
* Implementing the TALI protocol with SAAL therefore provides
support for SS7 co/cb and data retrieval and can help to minimize
MSU loss as SS7 links are deactivated. However, implementing SAAL
is not a trivial matter. The SAAL layer consists of 3 sublayers
(SSCF, SSCOP, and MAAL), one of which (SSCOP) is quite involved.
It is envisioned that most SS7 to TCP/IP applications will NOT
choose to implement SAAL.
2.3.2 An Alternate TALI Protocol Stack using SCTP
The TALI protocol is dependent on a reliable transport layer below
it. At the initial design of TALI, TCP was the only reliable, proven
transport layer. Simple Control Transport Protocol (SCTP) is
currently being designed as a transport later specifically for
signalling. Once SCTP is a proven and accepted transport protocol,
SCTP can then be used in place of TCP as shown in Figures 3 and 4.
2.4 Inputs to the TALI Version 1.0 State Machine
Figure 5 illustrates the inputs that affect the TALI State Machine.
Inputs to the state machine include:
* Management events (ie: requests from the human user of the TALI
connection) to control the operation of a particular TALI session.
* TALI messages received from the Peer. These messages include peer
to peer messages as well as service data messages.
* Events from the User of the TALI layer. The user is the layer
above TALI in the protocol stack, either the SS7 or SAAL layer.
* Implementation Dependent Events. Each implementation must provide
inputs into the TALI state machine such as:
* Socket Events
* TALI protocol violations. The TALI state machine must detect
protocol violations and act accordingly.
* Timer events.
+====+ +============+
+---------+ +-------------+
User Service Mgmt. Open MANAGEMENT
Part<--> Message Mgmt. Close <-->
Mgmt. Proh.
+---------+ Mgmt. Allow +============+
+====+ ^ +-------------+
^

v v
+========================================================+
TALI State Machine
+========================================================+
^ ^ ^ ^


v
+---------+ +-----------------+ +-----------+ +------------+
Received Connection est. Protocol T1 EXPired
"test" Connection lost Violation T2 Expired
"allo" T3 Expired
"proh" +-----------------+ +-----------+ T4 Expired
"proa" ^ ^ +------------+
"moni" ^
"mona"
or
Service
Message +========================================+
+---------+ IMPLEMENTATION
^ DEPENDENT
+========================================+

v
+============+
PEER

+============+
Figure 5: Overview of Inputs to the TALI 1.0 State Machine
3. TALI Version 1.0
This chapter provides the states, messages, message exchange rules
and state machine that must be implemented to provide a TALI version
1.0 protocol layer.
3.1 Overview of the TALI Message Structure
Table 2 provides a summary of the messages and message structure used
in TALI version 1.0.
+------------------------------------------------------------------+
OCTET DESCRIPTION SIZE VALUE TYPE
+------------------------------------------------------------------+
0..3 SYNC 4 Octets 4 byte
ASCII
+------------------------------------------------------------------+
TALI "TALI"
+------------------------------------------------------------------+
4..7 OPCODE 4 Octets 4 byte
ASCII
+------------------------------------------------------------------+
Test Service "test"
Allow Service "allo"
Prohibit Service "proh"
Prohibit Service Ack "proa"
Monitor Socket "moni"
Monitor Socket Ack "mona"
SCCP Service "sccp"
ISUP Service over TALI "isot"
MTP3 Service over TALI "mtp3"
Service over SAAL "saal"
+------------------------------------------------------------------+
8..9 LENGTH 2 Octets integer
(least significant
byte first) non-0
if Service or
Socket monitor message
+------------------------------------------------------------------+
10..X DATA PAYLOAD variable variable
+------------------------------------------------------------------+
Table 2: Message Structure for TALI 1.0
Table 3 indicates the valid values of the LENGTH field for each
version 1.0 opcode. The LENGTH field is always an indication of the
# of bytes contained in the DATA PAYLOAD portion of a general TALI
message.
+------------------------------------------------------------------+
OPCODE VALID LENGTH VALUES COMMENTS
+------------------------------------------------------------------+
test 0 bytes
+------------------------------------------------------------------+
allo 0 bytes
+------------------------------------------------------------------+
proh 0 bytes
+------------------------------------------------------------------+
proa 0 bytes
+------------------------------------------------------------------+
moni 0-200 bytes A maximum length is provided so
that the maximum ethernet frame
size is not exceeded.
+------------------------------------------------------------------+
mona 0-200 bytes Mona reply length and content must
match the original moni (with the
exception of the opcode)
+------------------------------------------------------------------+
sccp 12-265 bytes These are the valid sizes for the
SCCP-ONLY portions of SCCP UDT
MSUs
+------------------------------------------------------------------+
isot 8-273 bytes The length is the number of octets
in the MTP3 and higher layer(s) of
the SS7 MSU. This length includes
the SIO byte, the MTP3 routing
label, the CIC code, and the
ISUP Message Type field, and any
other bytes that may exist as part
of the SIF (Service Information
Field)
+------------------------------------------------------------------+
mtp3 5-280 bytes The length is the number of octets
in the MTP3 and higher layer(s) of
the SS7 MSU. This length includes
the SIO byte and the MTP3 routing
labeld, and any other bytes that
may exist as part of the SIF
(Service Information Field)
+------------------------------------------------------------------+
saal 11-280 bytes The length is the number of octets
in the MTP3 and higher layer(s) of
the SS7 MSU. This length includes
the SIO byte and all bytes in the
SIF (Service Information Field)
field. The MTP3 routing label is
part of the SIF field. Seven (7)
octets of SSCOP trailer is added
to the message. The SSCOP trailer
bytes are also included in the
length.
+------------------------------------------------------------------+
Table 3: Valid Length Fields for Each Opcode in TALI 1.0
3.1.1 Types of TALI Fields
Several field types are used in the general TALI message structure.
+------------------------------------------------------------------+
Field Type Implementation Notes for that Type
+------------------------------------------------------------------+
4 byte * 4 byte ASCII text strings are used to define the
ASCII text sync code and the opcode of the basic TALI message.
* These fields are case sensitive, the coding for
each sync and opcode literal needs to match the
case specified in Table 2.
* The standard ASCII conversion table is used to
transform each character into a byte.
* The order of the ASCII characters is important.
The first character in the string must be the
first character transmitted across the wire.
* For example, if the string being encoded is "abCD",
the order of the bytes as they are transferred
over the wire must be:
1st byte: 0x61 ("a") 3rd byte: 0x43 ("C")
2nd byte: 0x62 ("b") 4th byte: 0x44 ("D")
* The software for each implementation should be
written in a manner that accounts for the required
byte order of transmission (ie: the Big Endian/
Little Endian characteristics of the processor
need to be dealt with in the software.
+------------------------------------------------------------------+
Integer * A 1, 2 or 4 byte field to be treated as an integer
value. Integer fields should be transmitted Least
Significant Byte first across the wire.
* The software for each implementation should be
written in a manner that accounts for the required
byte order of transmission (ie: the Big Endian/
Little Endian characteristics of the processor
need to be dealt with in the software.
+------------------------------------------------------------------+
Variable * The definition of the message structure for this
field is governed by other specifications.
* For example, when transferring MTP3 service data
via a "mtp3" opcode, the DATA PAYLOAD begins with
the SIO byte of the MTP3 routing label. The
structure for the entire DATA PAYLOAD is governed
by the MTP3 message structure defined in [1].
+------------------------------------------------------------------+
X byte * ASCII text fields of sizes other than 4 bytes
ASCII text should be supported according to the same rules
presented for the 4 byte ASCII text fields. For
instance, an 8 byte string such as "ab01cd23" could
be used, where the "a" would be the first byte of
the field transmitted out the wire.
+------------------------------------------------------------------+
Table 4: Implementation Notes for each Type of TALI field
3.2 Detailed TALI Message Structure
3.2.1 TALI Peer to Peer Messages
The following subsections provide more information regarding the TALI
Peer to Peer messages that are implemented in version 1.0. The TALI
peer to peer messages originate at the TALI layer of 1 end of the
socket connection (the near end) and are terminated at the TALI layer
of the far end of the connection.
3.2.1.1 Test Message (test)
The "test" message is used by a TALI implementation to query the
remote end of the TALI connection with respect to the willingness of
the remote end to carry SS7 service data. This message asks the
other end: are you ready to carry service data? This message is sent
periodically by each TALI implementation based on a T1 timer
interval. Upon receiving "test", a TALI implementation must reply
with either "proh" or "allo" to indicate the nodes willingness to
carry SS7 service data over that TALI connection.
+------------------------------------------------------------------+
Octets Field Name Description
+------------------------------------------------------------------+
0..3 SYNC "TALI"
+------------------------------------------------------------------+
4..7 OPCODE "test"
+------------------------------------------------------------------+
8..9 LENGTH Length = 0
+------------------------------------------------------------------+
3.2.1.2 Allow Message (allo)
The "allo" message is sent in reply to a "test" query, or in response
to some internal implementation event, to indicate that a TALI
implementation IS willing to carry SS7 service data over the TALI
session. This message informs the far end that SS7 traffic can be
transmitted on the socket. "allo" is one of the 2 possible replies
to a "test" message. Before SS7 traffic can be carried over a
socket, both ends of the connection need to send "allo" messages.
+------------------------------------------------------------------+
Octets Field Name Description
+------------------------------------------------------------------+
0..3 SYNC "TALI"
+------------------------------------------------------------------+
4..7 OPCODE "allo"
+------------------------------------------------------------------+
8..9 LENGTH Length = 0
+------------------------------------------------------------------+
3.2.1.3 Prohibit Message (proh)
The "proh" message is sent in reply to a "test" query, or in response
to some internal implementation event, to indicate that a TALI
implementation is NOT willing to carry SS7 service data over the TALI
session. This message informs the far end that SS7 traffic can not
be transmitted on the socket. "proh" is one of the 2 possible
replies to a "test" message. As long as 1 end of the connection
remains in the "prohibited" state, SS7 traffic can not be carried
over the socket.
+------------------------------------------------------------------+
Octets Field Name Description
+------------------------------------------------------------------+
0..3 SYNC "TALI"
+------------------------------------------------------------------+
4..7 OPCODE "proh"
+------------------------------------------------------------------+
8..9 LENGTH Length = 0
+------------------------------------------------------------------+
3.2.1.4 Prohibit Acknowledgement Message (proa)
The "proa" message is sent by a TALI implementation each time a
"proh" is received from the far end. This message is sent to
indicate to the far end that his "prohibit" message was received
correctly and will be acted on accordingly.
+------------------------------------------------------------------+
Octets Field Name Description
+------------------------------------------------------------------+
0..3 SYNC "TALI"
+------------------------------------------------------------------+
4..7 OPCODE "proa"
+------------------------------------------------------------------+
8..9 LENGTH Length = 0
+------------------------------------------------------------------+
3.2.1.5 Monitor Message (moni)
The "moni" message provides a generic ECHO capability that can be
used by each TALI implementation as that implementation sees fit. A
TALI version 1.0 implementation does not have to originate a "moni"
message to be compliant with the 1.0 specification. The primary
intent of this message is to provide a way for the TALI layer to test
the round-trip message transfer time on a socket. A "mona" message
must be sent in reply to each received "moni" message. The DATA
portion of a "moni" message is vendor implementation dependent. The
DATA portion of each "mona" reply must exactly match the DATA portion
of the "moni" that is replied to. Regardless of whether an
implementation chooses to send "moni" or not, "mona" must be sent in
response to each "moni" in order to remain compliant with the TALI
protocol.
+------------------------------------------------------------------+
Octets Field Name Description
+------------------------------------------------------------------+
0..3 SYNC "TALI"
+------------------------------------------------------------------+
4..7 OPCODE "moni"
+------------------------------------------------------------------+
8..9 LENGTH Length
+------------------------------------------------------------------+
10..X DATA PAYLOAD Vendor Dependent
+------------------------------------------------------------------+
3.2.1.6 Monitor Acknowledge Message (mona)
As mentioned above, the "mona" must be sent in reply to each received
"moni". The contents of the "mona" DATA area must match the DATA
area of the received "moni" message.
+------------------------------------------------------------------+
Octets Field Name Description
+------------------------------------------------------------------+
0..3 SYNC "TALI"
+------------------------------------------------------------------+
4..7 OPCODE "mona"
+------------------------------------------------------------------+
8..9 LENGTH Length
+------------------------------------------------------------------+
10..X DATA PAYLOAD Vendor Dependent
+------------------------------------------------------------------+
3.2.2 Service Messages
The following subsections provide more information regarding the TALI
Service messages that are implemented in version 1.0. TALI Service
messages are used to carry SS7 MSUs across the IP network. The
information in this section includes details with respect to how to
encapsulate SS7 MSUs into TCP/IP frames using each of the TALI
service opcodes. The TALI service messages originate at the layer
above TALI, are transported across the IP network via a TALI service
message, and are delivered to the layer above TALI at the far end of
the TALI connection.
3.2.2.1 SCCP Service Message (sccp)
The "sccp" opcode is used to deliver SS7 MSUs with a Service
Indicator of 3 (SCCP) over a TALI connection. This opcode is only
used on TALI protocol stacks that are implemented without SAAL. The
MTP3 layer of the SS7 MSU is NOT part of the data transferred across
TCP/IP for this opcode; the data portion of the TALI "sccp" message
begins with the first byte of the SCCP data area in the SS7 MSU
(after the MTP3 routing label). The first byte in the SCCP data area
is an SCCP message type field.
Several restrictions on the SCCP messages that this TALI opcode can
carry exist. These restrictions are as follows:
* SCCP messages contain an SCCP message type field. The SCCP
messages that are supported by TALI 1.0 implementations are
limited to Class 0 and Class 1 SCCP messages with a message type
field of either:
* UDT
* UDTS
* XUDT
* XUDTS
* SCCP messages must contain a Point Code in the "calling party"
area in order to be transferred across the TCP/IP connection as a
"sccp" message. An implementation may choose to modify the
original SCCP MSU to add an appropriate calling party point code
before transmission across TALI if desired.
* SCCP messages must contain a Point Code in the "called party" area
in order to be transferred across the TCP/IP connection as a
"sccp" message. An implementation may choose to modify the
original SCCP MSU to add an appropriate called party point code
before transmission across TALI if desired.
* The encoding of the SS7 SCCP MSUs, as they are transmitted across
TALI via "sccp", should remain compliant with the ANSI
specifications (T1.112 and T1.114) that apply to the SCCP and TCAP
portions of the message respectively.
NOTE 1: SCCP Subsystem Management for the IP based SCP"s is supported
via this "sccp" opcode. SS7 SCCP Management messages are controlled
by an SCMG SS7 process. SCMG sends the management messages via SCCP
UNITDATA (UDT) messages. Therefore, the SCMG messages can be sent
across the TALI connection.
NOTE 2: "sccp" TALI messages will not include the MTP3 header and
therefore will not retain the original DPC/OPC of the SS7 MSU. Each
TALI implementation needs to consider if/how to provide this DPC/OPC
information in the SCCP portion of the message. For example the DPC
can be replicated to the point code in the SCCP Called Party Address
area and the OPC can be replicated to the point code in the SCCP
Calling Party Address area.
+------------------------------------------------------------------+
Octets Field Name Description
+------------------------------------------------------------------+
0..3 SYNC "TALI"
+------------------------------------------------------------------+
4..7 OPCODE "sccp"
+------------------------------------------------------------------+
8..9 LENGTH Length
+------------------------------------------------------------------+
10..X SCCP Data SCCP data starting at the first byte after
the Layer 3 Routing Label (data does not
include the SIO or Routing Label)
+------------------------------------------------------------------+
3.2.2.1.1 SCCP Encapsulation using TALI
When an SCCP MSU arrives at an SG from a 56 Kbps or DS1 link and is
routed within the SG for transmission to an IP device, the SG
performs the following processing on the SS7 MSU:
* discards the MTP Layer 2 information, CRC and flags
* places the DPC from MTP Layer 3 into the Called Party Address
field of the SCCP layer; the Calling Party Address field is
created if it does not exist and then filled
* places the OPC from MTP Layer 3 into the Calling Party Address
field of the SCCP layer if there is no Calling Party Point Code
* places the modified SCCP and unchanged TCAP data in the service
payload area of the TALI packet
* The SYNC field is set
* The OPCODE is set to "sccp"
* The LENGTH is set to the number of octets in the SERVICE field
Once the fully formed "sccp" TALI packet is created, it is handed to
the TCP socket layer and transmitted. The transmission process will
add TCP, IP and MAC header information.
Since the routing information from MTP Layer 3 is placed in the SCCP
part of the outgoing message, no routing information needs to be
saved by the SG.
SS7 MSU
Layer 3 Layer 2

+----+---+-----+-----+-------+---+--+---+---+---+---+----+
FlagFCSTCAP SCCP RoutingSIOLIFIBFSNBIBBSNFlag
LayerLayer Label
+----+---+-----+-----+-------+---+--+---+---+---+---+----+



TALI +-----------+---+------+----+
Packet Service LENOpcodeSYNC
+-----------+---+------+----+



+---------------------------+------+------+------+
IP TALI Packet TCP IP MAC
Packet HeaderHeaderHeader
+---------------------------+------+------+------+
Figure 6: Encapsulation of SCCP MSUs using the TALI "sccp" opcode
When an "sccp" TALI packet is received on by an SG from an IP device,
the SG performs the following processing on the "sccp" packet:
* validates the TALI header
* Allocates space for a new SS7 message
* Regenerates the SIO with the Sub-Service Field set to National
Network, priority of zero (0), Service Indicator set to SCCP
* extracts the SCCP/TCAP data from the SERVICE area and places it in
the new SS7 message
* sets the DPC to the SCCP Called Party Point Code
* sets the OPC to the SCCP Calling Party Point Code
* randomly generates the SLS
Once the "sccp" packet is transformed back into a normal SS7 MSU, the
MSU is routed within the SG according to the normal SS7 routing
procedures.
3.2.2.2 ISUP Service Message (isot)
The "isot" opcode is used to deliver SS7 MSUs with a Service
Indicator of 5 (ISUP) over a TALI connection. This opcode is only
used on TALI protocol stacks that are implemented without SAAL. The
MTP3 layer of the SS7 MSU IS part of the data transferred across
TCP/IP for this opcode; the data portion of the TALI "isot" message
begins with the SIO byte of the MTP3 header in the SS7 MSU.
+------------------------------------------------------------------+
Octets Field Name Description
+------------------------------------------------------------------+
0..3 SYNC "TALI"
+------------------------------------------------------------------+
4..7 OPCODE "isot"
+------------------------------------------------------------------+
8..9 LENGTH Length
+------------------------------------------------------------------+
10..X ISUP Data Raw ISUP data starting at the Layer 3 SIO
field.
+------------------------------------------------------------------+
3.2.2.2.1 ISUP Encapsulation using TALI
When an ISUP MSU arrives at an SG from a 56 Kbps or DS1 link and is
routed within the SG to a IP device, the SG performs the following
processing on the SS7 MSU:
* discards the MTP Layer 2 information, CRC and flags
* places MTP Layer 3 into the SERVICE payload area of the TALI
packet
* The SYNC field is set
* The OPCODE is set to "isot"
* The LENGTH is set to the number of octets in the SERVICE field
Once the fully formed "isot" TALI packet is created, it is handed to
the TCP socket layer and transmitted. The transmission process will
add TCP, IP and MAC header information.
Since the routing information is placed in the TALI Packet, no
routing information needs to be saved by the SG.
SS7 MSU
Layer 3 Layer 2

+----+---+----+----+---+-------+---+--+---+---+---+---+----+
FlagFCSISUPMsg.CICRoutingSIOLIFIBFSNBIBBSNFlag
PartType Label
+----+---+----+----+---+-------+---+--+---+---+---+---+----+
/
/

TALI +-----------------------+---+------+----+
Packet Service LENOpcodeSYNC
+-----------------------+---+------+----+
/
---------
/
+----------------------------+------+------+------+
IP TALI Packet TCP IP MAC
Packet HeaderHeaderHeader
+----------------------------+------+------+------+
Figure 7: Encapsulation of ISUP MSUs using the TALI "isot" opcode
When an "isot" TALI packet is received on an SG from an IP device,
the SG performs the following processing on the "isot" packet:
* validates the TALI header
* Allocates space for a new SS7 message
* extracts the MTP Layer 3 data from the SERVICE area and places it
in the new SS7 message
Once the "isot" packet is transformed back into a normal SS7 MSU, the
MSU is routed within the SG according to the normal SS7 routing
procedures.
3.2.2.3 MTP3 Service Message (mtp3)
The "mtp3" opcode is used to deliver SS7 MSUs with a Service
Indicator of 0-2, 4, 6-15 (non-SCCP, non-ISUP) over a TALI
connection. This opcode is only used on TALI protocol stacks that
are implemented without SAAL. The MTP3 layer of the SS7 MSU IS part
of the data transferred across TCP/IP for this opcode; the data
portion of the TALI "mtp3" message begins with the SIO byte of the
MTP3 header in the SS7 MSU.
+------------------------------------------------------------------+
Octets Field Name Description
+------------------------------------------------------------------+
0..3 SYNC "TALI"
+------------------------------------------------------------------+
4..7 OPCODE "mtp3"
+------------------------------------------------------------------+
8..9 LENGTH Length
+------------------------------------------------------------------+
10..X Layer 3 MSU Raw MSU data starting at the Layer 3 SIO
Data field.
+------------------------------------------------------------------+
3.2.2.3.1 MTP3 Encapsulation using TALI
When an SS7 MSU with SI=0-2,4,6-15 arrives at an SG from a 56 Kbps or
DS1 link and is routed within the SG to an IP device, the SG performs
the following processing on the SS7 MSU:
* discards the MTP Layer 2 information, CRC and flags
* places MTP Layer 3 into the SERVICE payload area of TALI packet
* The SYNC field is set
* The OPCODE is set to "mtp3"
* The LENGTH is set to the number of octets in the SERVICE field
Once the fully formed "mtp3" TALI packet is created, it is handed to
the TCP socket layer and transmitted. The transmission process will
add TCP, IP and MAC header information.
SS7 MSU
Layer 3 Layer 2

+----+---+-----------+-------+---+--+---+---+---+---+----+
FlagFCSOther LayerRoutingSIOLIFIBFSNBIBBSNFlag
3 Data Label
+----+---+-----------+-------+---+--+---+---+---+---+----+
/
------
/
TALI +----------------+---+------+----+
Packet Service LENOpcodeSYNC
+----------------+---+------+----+
/
--
/
+----------------------------+------+------+------+
IP TALI Packet TCP IP MAC
Packet HeaderHeaderHeader
+----------------------------+------+------+------+
Figure 8: Encapsulation of SS7 MSUs with SI!=3,5,13 using "mtp3"
When an "mtp3" TALI packet is received by an SG from an IP device,
the SG performs the following processing on the "mtp3" packet:
* validates the TALI header
* Allocates space for a new SS7 message
* extracts the MTP Layer 3 data from the SERVICE area and places it
in the new SS7 message
Once the "mtp3" packet is transformed back into a normal SS7 MSU, the
MSU is routed within the SG according to the normal SS7 routing
procedures.
3.2.2.4 SAAL Service Message (saal)
The "saal" opcode is used to deliver SS7 MSUs with any Service
Indicator over a TALI connection. This opcode is only used on TALI
protocol stacks that are implemented with SAAL. The "saal" opcode is
also used to transmit SAAL peer to peer packets (SSCF peer to peer
packets and SSCOP peer to peer packets other than SS7 service data)
over a TALI connection.
When used to transfer SS7 MSUs, the MTP3 layer of the SS7 MSU IS part
of the data transferred across TCP/IP for this opcode; the data
portion of the TALI "saal" message begins with the SIO byte of the
MTP3 header in the SS7 MSU and ends with the last byte of the SSCOP
trailer.
When used to transfer SSCF/SSCOP peer to peer messages the data
portion of the TALI "saal" message includes the entire SSCOP PDU.
+------------------------------------------------------------------+
Octets Field Name Description
+------------------------------------------------------------------+
0..3 SYNC "TALI"
+------------------------------------------------------------------+
4..7 OPCODE "saal"
+------------------------------------------------------------------+
8..9 LENGTH Length
+------------------------------------------------------------------+
10..X Layer 3 Raw MSU data starting at the Layer 3 SIO
Data field.
+------------------------------------------------------------------+
(X+1) SSCOP Zero (0) to three (3) octets of padding
..Y Trailer plus 4 octets for the trailer data. The
total length of the Layer 3 Data and the
SSCOP trailer must be a multiple of 4.
+------------------------------------------------------------------+
or
+------------------------------------------------------------------+
Octets Field Name Description
+------------------------------------------------------------------+
0..3 SYNC "TALI"
+------------------------------------------------------------------+
4..7 OPCODE "saal"
+------------------------------------------------------------------+
8..9 LENGTH Length
+------------------------------------------------------------------+
10..X SAAL Peer Raw SSCF/SSCOP peer to peer packets are
to Peer also transferred over the TALI connection
message using this "saal" opcode.
+------------------------------------------------------------------+
3.2.2.4.1 MTP3 and SAAL Peer to Peer Encapsulation using TALI
When an SS7 MSU (with any SI) arrives at an SG from a 56 Kbps or DS1
link and is routed within the SG for transmission to an IP device,
the SG performs the following processing on the SS7 MSU:
* discards the MTP Layer 2 information, CRC and flags
* the MSU is passed from an MTP3 processing software layer to the
SSCF and SSCOP layers (the SAAL layers). These layers convert the
SS7 MSU into an SSCOP PDU. Part of this conversion includes
adding an SSCOP trailer.
* the SSCOP PDU (whether it is a peer to peer SAAL message or SS7
MSU in an SSCOP PDU) is copied into the SERVICE payload area of
the TALI packet
* The SYNC field is set
* The OPCODE is set to "saal"
* The LENGTH is set to the number of octets in the SERVICE field
Once the fully formed "saal" TALI packet is created, it is handed to
the TCP socket layer and transmitted. The transmission process will
add TCP, IP and MAC header information.
Since the routing information is placed in the TALI Packet, no
routing information needs to be saved by the SG.
SS7 MSU
Layer 3 Layer 2

+----+---+-----------+-------+---+--+---+---+---+---+----+
FlagFCSOther LayerRoutingSIOLIFIBFSNBIBBSNFlag
3 Data Label
+----+---+-----------+-------+---+--+---+---+---+---+----+



+-------+-----------------------+
SSCOP Service
Trailer
+-------+-----------------------+

+-------+-----------------------+---+------+----+
Service with SSCOP Trailer LENOpcodeSYNC
+-------+-----------------------+---+------+----+
/
-----------------
/
+----------------------------+------+------+------+
TALI Packet TCP IP MAC
HeaderHeaderHeader
+----------------------------+------+------+------+
Figure 9: Encapsulation of SAAL PDUs using the TALI "saal" opcode
When an "saal" TALI packet is received at the SG from an IP device,
the SG performs the following processing on the "saal" packet:
* validates the TALI header
* Allocates space for a new SSCOP PDU message
* extracts the SSCOP PDU data from the SERVICE area and places it in
the new SSCOP PDU message
Once the "saal" packet is transformed back into a normal DS1 SSCOP
PDU, the SSCOP PDU is passed to the SAAL layer for receive
processing. If the SSCOP PDU is a peer to peer pdu, it is processed
completely in the appropriate SAAL layer. If the SSCOP PDU is an SS7
MSU, the MSU is transformed back to a normal SS7 MSU and is routed
within the SG according to the normal SS7 routing procedures.
3.3 TALI Timers
Version 1.0 of the TALI specification defined 4 TALI timers that are
used as part of the TALI state machine. These timers are generically
named "T1" through "T4". Brief descriptions of each timer are
provided in the following subsections. Timer expiration events for
each of the T1-T4 timers appear as inputs to the TALI state machine.
For exact processing of each timer (when to start/stop, how to
process timer expirations), refer to the TALI state machine.
Both ends of the TALI connection have there own T1-T4 timers. The
T1-T4 timer values can be set on each end of the connection
independent of the settings on the far end. For each timer, a
default value and range is recommended in the following sections.
3.3.1 T1 Timer
The T1 timer represents the time interval between the origination of
a "test" message at each TALI implementation. Each time T1 expires,
the TALI implementation should send a "test".
3.3.2 T2 Timer
The T2 timer represents the amount of time that the Peer has to
return an "allo" or a "proh" in response to a "test". If the far end
fails to reply with "allo" or "proh" before T2 expires, the sender of
the "test" treats the T2 expiration as a protocol violation. Note
that T2 must be < T1 in order for these timers to work as designed.
3.3.3 T3 Timer
The T3 timer controls how long the near end should continue to
process Service Data that is received from the far end after a
Management Prohibit Traffic Event has occurred (at the near end).
This timer is used when a transition from NEA-FEA (both ends allowed
to send service data) to NEP-FEA (only far end willing to send
service data) occurs. On that transition, it is reasonable to expect
that the far end needs some amount of time to adjust its TALI state
machine and divert service data traffic away from this socket. The
T3 timer controls the amount of time the far end has to divert
traffic.
3.3.4 T4 Timer
The T4 timer represents the time interval between the origination of
a "moni" message at each TALI implementation. Each time T4 expires,
the TALI implementation should send a "moni".
3.3.5 Recommended Defaults and Ranges for the TALI Timers
The following table provides the recommended default and configurable
range for each TALI timer.
+------------------------------------------------------------------+
Name Min Max Default Description
+------------------------------------------------------------------+
T1 100ms 60sec 4 sec Send test PDU timer
+------------------------------------------------------------------+
T2 100ms 60sec 3 sec Response timer for an allo or proh
response to test message.
+------------------------------------------------------------------+
T3 100ms 60sec 5 sec Timer controls how long to process
rcvd serv data after an NE
transition from NEA to NEP. System
is waiting for a proa response to
the first proh send when NE
transitions from NEA to NEP.
+------------------------------------------------------------------+
T4 100ms 60sec 10 sec Send moni PDU timer
+------------------------------------------------------------------+
Table 5: Timers
NOTE: The value of T1 must be at least one (1) millisecond greater
than T2. This is to prevent the system from a lockup in the T1
expired condition. If T1 is equal or less than T2, it will expire
and restart T2 and not enforce responses to the test message.
Enforcement of minimum and maximum timer values is implementation
dependent.
3.4 TALI User Events
Each TALI implementation must provide several user event controls
over the behavior of the TALI state machine for each TALI connection.
The user interface to provide these capabilities is implementation
specific.
3.4.1 Management Open Socket Event
The "mgmt open socket" event, together with the "mgmt close socket"
event, allows the user to control when each defined TALI connection
will form a TCP socket connection. When "open socket" for a
particular TALI connection occurs, the TALI connection should begin
trying to form a TCP socket connection to the peer.
The steps that are taken to connect are dependent on the
client/server role of that end of the TALI connection. The exact
steps to perform these tasks are implementation dependent and may
differ based on the TCP stack being used.
In general, TALI clients form socket connections by using the BSD