Application Note

Load Balancing on Dialogic HMP Software using OpenSER-Based SIP Call Balancer

Application Note

Load Balancing on Dialogic HMP Software using OpenSER-Based SIP Call Balancer

Executive Summary
This application note explains how to balance SIP calls using OpenSER the Open Source SIP Server. OpenSER provides many features, but this application note focuses on use of OpenSER as a SIP call loadbalancer. The ability to balance SIP calls is required for high-density systems and High Availability (HA) systems. This application note does not discuss how to achieve high-density and HA systems.

Load Balancing on Dialogic HMP Software using OpenSER-Based SIP Call Balancer

Application Note

Table of Contents
Introduction............................................................................................................ 2 Environment ........................................................................................................... 2 Call Flow ................................................................................................................ 2 OpenSER and Dispatcher Module Usage ................................................................ 3 OpenSER ........................................................................................................ 3 Dispatcher Module .......................................................................................... 4 Call Distribution Statistics........................................................................................ 4 Conguration Files .................................................................................................. 5 Considerations........................................................................................................ 5 Downloads.............................................................................................................. 6 References ............................................................................................................. 6 For More Information.............................................................................................. 6

Application Note

Load Balancing on Dialogic HMP Software using OpenSER-Based SIP Call Balancer

Introduction
When using multiple IP-based media servers in an inbound or outbound call center environment, resources can be optimized by load balancing across multiple media server platforms. Load balancing allows multiple VoIP devices to operate in parallel, and provides fault tolerance in that if one server goes down, the load balancer can redistribute trafc over the remaining servers.

Environment
The hardware and software used in the test environment for the method described herein consisted of various software packages running on four servers as shown in Figure 1: SIP call balancer Utilizes OpenSER on Linux Bulk call generator Utilizes Dialogic Host Media Processing Software Release 3.0 for Windows Inbound SIP call receiver Utilizes Dialogic HMP Software 3.0 Inbound SIP call receiver Utilizes Dialogic HMP Software Release 3.1LIN

Figure 1. Network Topology

Call Flow
The basic SIP call ow is discussed here. The bulk call generator initiates calls using gc_basic_call demo. The INVITE is sent to OpenSER, which picks one of the two Inbound SIP Call Termination servers. OpenSER then forwards the INVITE to one of those servers. Figure 2 shows some sample call ows that were made by capturing call trafc on OpenSER using Tethereal and analyzing the captured trafc using Wireshark. Note: Tethereal is a console-based tool, which unlike Wireshark, does not require a GUI environment. Thus, Tethereal is used to capture the packets into a le and then the trafc is analyzed using Wireshark (formerly known as Ethereal).
2

Load Balancing on Dialogic HMP Software using OpenSER-Based SIP Call Balancer

Application Note

Bulker
146.152.123.202

OpenSER
146.152.123.203

Inbound 1
146.152.123.201

INVITE SDP (g711A)

(5060) (5060)

100 Giving a try

(5060) (5060) (5060)

INVITE SDP (g711A)

(5060)

180 Ringing
(5060) (5060) (5060)

180 Ringing
(5060) (5060)

200 OK SDP (g711A)

(5060) (5060)

200 OK SDP (g711A)

(5060)

ACK
(5060) (5060) (5060) (5060)

ACK ACK
(5060) (5060) (5060)

BYE
(5060) (5060)

BYE
(5060)

200 OK
(5060) (5060) (5060)

200 OK
(5060)

Figure 2. Sample Call Flows using Tethereal and Wireshark

OpenSER and Dispatcher Module Usage

OpenSER OpenSER has built-in functionality. If features need to be added to OpenSER, they can be added via modules, which are actually shared libraries. For this test, the Dispatcher module was used (see the References section for downloading it). Modules are loaded via OpenSER conguration, and the OpenSER conguration le that was used is available for downloading (see the For More Information section). Lines of interest from that conguration include:

1. 2. 3. 4. 5. 6. 7. 8. 9.

loadmodule "dispatcher.so" modparam("dispatcher", "list_file", "/usr/local/etc/openser/dispatcher.list") modparam("dispatcher", "force_dst", 1) if (loose_route()) { t_relay(); }; ds_select_dst("1", "0"); record_route(); t_relay();

Application Note

Load Balancing on Dialogic HMP Software using OpenSER-Based SIP Call Balancer

Line 1: Loads the Dispatcher module for use by OpenSER Line 2: Location of conguration le used by Dispatcher module Line 3: Forces overwriting of the destination address when that is already set Lines 4 and 5: Messages within a dialog should take the path determined by record-routing Line 7: Selects a destination address using Dispatcher modules hash over called method Line 8: Packets that reach this line of the conguration le will get OpenSER added to the via list so that subsequent messages in the dialog go through OpenSER Line 9: Uses stateful forwarding of messages The lines pertaining to the Dispatcher module are described in the next section, Dispatcher Module. Dispatcher Module The Dispatcher module implements a dispatcher for destination addresses. It computes hashes over parts of the request and selects an address from a destination set. The selected address is used then as outbound proxy. The module can be used as a stateless load balancer, having no guarantee of fair distribution. [Mierla] The Dispatcher module is loaded during the OpenSER startup based on conguration, which is similar to that previously discussed in the OpenSER section. The Dispatcher module uses a simple conguration le. The tested conguration le contents are as follows:

hash over callid hash over from uri hash over to uri hash over request-uri round-robin (next destination) For the particular setup in this application note, it was decided that the hash over callid was the most suitable method. The round-robin method requires more conguration, since OpenSER does not track call state. The uri-based hash methods may be suitable for some network topologies where the source and destination addresses are random. It is likely that to uri and requesturi hash methods will not be suitable, however, since those hosts are not random that is, the service provider has a nite set of those hosts and they do not change often. Callids are globally unique identiers; thus, hash over callid is a good candidate for selection of destination IP address for distributing calls. Using hash over callid" over a large amount of calls should converge on even distribution, and the test results, discussed in the Call Distribution Statistics section, appear to show that being the case. Line 7 of the OpenSER conguration le indicates that destination selection is based on hash over callid by using a 0 parameter.

Call Distribution Statistics

The tables below show call statistics from a test run using the setup explained in the OpenSER and Dispatcher Module Usage section. The Dispatcher module in the tested setup uses SIP callid as input to a hash function to select the destination SIP call termination server. The HMP PC2 was running gc_basic_call_demo with two inbound SIP channels and HMP PC3 was running with ve inbound SIP channels.
Call Count Outbound Calls Channel 1 HMP PC1 7519 7457 7441 7390 29807 29807 Inbound Calls HMP PC2 7237 7262 14499 29807 HMP PC3 3068 3067 3064 3054 3055 15308

# HMP Media Servers 1 sip:146.152.123.201:5060 1 sip:146.152.123.204:5060

The 1 is a set ID. One or more hosts are in a set. This example has two hosts. Line 7 of the OpenSER conguration le indicates that set 1 the rst parameter of ds_select_dst() is used for selecting destination hosts. The Dispatcher module can use the following methods to determine the destination IP address selection:
4

2 3 4 5 Total/server Total

Load Balancing on Dialogic HMP Software using OpenSER-Based SIP Call Balancer

Application Note

Calls by Percentage Outbound Calls Channel 1 2 3 4 5 Total/server HMP PC1 25.23 25.02 24.96 24.79 100.00 Inbound Calls HMP PC2 49.91 50.09 100.00 HMP PC3 20.04 20.04 20.02 19.95 19.96 100.00

If the hash-based method is used in the Dispatcher module, it is likely that the number of calls being sent to a User Agent Server (UAS) will be greater than the number of channels available in that UAS. Therefore, some sort of call accounting would need to be done to make OpenSERs routing more intelligent. An example of a SIP dialog with call-overow is shown in Figure 3. One possibility is to have OpenSER serially fork calls to UASs. For this to occur, the rst call can be sent to a hashbased UAS. When OpenSER receives the 486 Busy Here from that UAS, it forks calls to other UASs. Another possibility is to have OpenSER not use the Dispatcher module, but instead use a database. The

Configuration Files
Two conguration les discussed in the OpenSER and Dispatcher Module Usage section are available as downloads with this application note (see the For More Information section). They are called openser.cfg and dispatcher.list.

Considerations
The configuration tested uses more inbound channels than outbound channels. There are seven inbound channels and four outbound channels. As the number of outbound channels approaches the number of inbound channels, some challenges can be considered.

database would store the call states of all calls and provide a destination address based on UAS availability. These methods can increase the complexity of the OpenSER setup and are outside the scope of this document, although they are being considered for a future application note.

Bulker
146.152.123.202

OpenSER
146.152.123.203

Inbound 2
146.152.123.204

INVITE SDP (g711A)

(5060) (5060)

100 Giving a try

(5060) (5060) (5060)

INVITE SDP (g711A)

(5060)

486 Busy Here

(5060) (5060)

ACK
(5060) (5060) (5060)

486 Busy Here

(5060)

ACK
(5060) (5060)

Figure 3. Example of a SIP Dialog with Call-Overow

5

Application Note

Load Balancing on Dialogic HMP Software using OpenSER-Based SIP Call Balancer

Downloads
Both OpenSER and Dispatch module are available on the web (see the For More Information section). The version of OpenSER used is 1.2.1-notls:
# openser -V version: openser 1.2.1-notls (i386/linux) flags: STATS: Off, USE_IPV6, USE_TCP, DISABLE_NAGLE, USE_MCAST, SHM_MEM, SHM_MMAP, PKG_MALLOC, F_MALLOC, FAST_LOCK-ADAPTIVE_WAIT ADAPTIVE_WAIT_LOOPS=1024, MAX_RECV_BUFFER_SIZE 262144, MAX_LISTEN 16, MAX_URI_SIZE 1024, BUF_SIZE 65535 poll method support: poll, epoll_lt, epoll_et, sigio_rt, select. svnrevision: unknown @(#) $Id: main.c 1827 2007-03-12 15:22:53Z bogdan_iancu $ main.c compiled on 17:15:59 May 24 2007 with gcc 3.2.3

References
[Mierla] Daniel-Constantin Mierla, Editor, DISPATCHER Module, 2005 Voice-System.RO is available at http://www.openser.org/docs/modules/1.1.x/dispatcher.html

For More Information

A Zip le containing the conguration les can be downloaded at http://www.dialogic.com/goto/?10854 OpenSER is available at http://openser.org/pub/openser/ Dispatcher module is available at http://www.openser.org/docs/modules/1.1.x/dispatcher.html Dialogic Host Media Processing Software http://www.dialogic.com/products/ip_enabled/hmp_software.htm

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