WO2005081499A1

WO2005081499A1 - Voice-over-internet telecommunication networks

Info

Publication number: WO2005081499A1
Application number: PCT/EP2004/001762
Authority: WO
Inventors: Jari Vikberg; Tomas Nylander
Original assignee: Telefonaktiebolaget L.M. Ericsson (Publ)
Priority date: 2004-02-23
Filing date: 2004-02-23
Publication date: 2005-09-01

Abstract

When certain types of data, such as voice data, is sent over an IP-based network a problem can occur if this data passes a network address translator NAT, which is unable to identify and translate address information contained in the payload portion of a message. Voice-over-IP requires the initial establishment of a connection including the signalling of the address information for receipt of voice data. In accordance with the invention, a node (60) connected to a public network (50) is adapted to determine when address information received from a second node (10) in the payload portion of a voice-path-establishment message is different from the source address in the header of a voice packet received subsequently from the second node. A difference indicates that a NAT function is operating and the node (60) sends voice packets to the address identified in the voice packet header rather to that identified in the earlier message.

Description

Voice-over-Internet Telecommunication networks

Field of invention The invention relates to telecommunication systems wherein voice data is carried at least partially over an Internet Protocol network.

Background art Private telecommunication networks that access a public telecommunication network are commonly secured by providing a combined Network Address

Translator (NAT) and Firewall (FW) at the connection point to the public network. For example in home networks that are connected to a public network via a cable modem or xDSL, such as an ADSL modem, the modem device will commonly perform the NAT/FW function or a further node located adjacent the modem will take care of this function.

When voice-over-Internet-Protocol (VoIP) traffic is transmitted between a private and public network problems can arise as a result of the presence of a NAT. Specifically, the combined NAT/FW function usually allows outgoing traffic, that is traffic passing from the private to the public network, but disallows incoming traffic unless specific port forwarding rules have been configured. As a result, voice traffic may be passed successfully only in one direction.

The function of a Network Access Translator (NAT) is to map private network

(IP) addresses used in the home network to public network addresses used in the public network. The NAT - and Firewall, if integrated in the same device - is connected to both networks and knows both public and private network addresses. Private network addresses are not recognisable - and therefore not routable - from the public network. The NAT function performs the mapping between the two. Translation of an address may include translation of both an IP-address and a transport level port number. Network Address Translators that are capable of translating both IP-addresses and ports are sometimes referred to as NAPT (Network Address and Port Translator). However, in the present document all types of address translators will be referred to simply as

NATs.

This means that when a device connected to the private network sends a packet to the public network, the NAT will change the sending address in the packet header so that it looks as if the packet originated at the public address of the NAT. The receiving node is then able to respond towards the public address of the NAT.

However, a NAT is normally only able to translate a network address contained in the IP header. If network addresses are contained in the payload portion of the message, then the NAT will generally not be able to change them. Voice-over-IP is generally transmitted using Real Time Protocol (RTP) and User Datagram Protocol (UDP). When two nodes exchange messages signalling call establishment, the network addresses and UDP voice ports that are to be used for the voice payload traffic are contained in the payload portion of the messages. A node on the public network receiving such a message from a private network will naturally attempt to send voice traffic to the indicated address. However, this address will not have been mapped by the NAT and will consequently be a private address that is not recognised in the public network.

Solutions to this problem do exist when a suitable protocol is used to implement voice-over-IP. For example when Session Initiation Protocol (SIP) or ITU-T H.323 teleconference protocol is used the NAT can be adapted to support a protocol specific application level gateway (ALG), which is capable of changing network addresses included in the payload portion of protocol signalling messages as well as making the necessary mappings in the NAT. However, not all NAT devices support the required ALGs. Furthermore, even when a NAT device does support the ALG, any modification of the voice- over-IP protocol will require a corresponding adaptation of the NAT function.

It is also possible to rely on a network server and a specific client in the node of the private network. Two implementations are known for such an arrangement. These are the simple traversal of user datagram protocol (UDP) through network address translator, also commonly referred to as STUN and the traversal using relay NAT, which is referred to as TURN. Both these implementations require the installation of a special client software in the node of the private network. However, it is not always desirable to have a specific server for this need in the network. Also this solution requires that the nodes in the private network be updated to include the specific client.

Summary of the Invention

In the light of the above problems, it is thus an object of the present invention to enable voice-over-IP traffic to be handled easily when a NAT is connected between the communicating nodes, regardless of the support provided in the

NAT.

This and other objects are achieved in a node and a method of handling voice traffic transmitted between first and second nodes over a fixed broadband network as defined in the accompanying claims.

Specifically the invention provides a node that is connected to a fixed broadband network, is assigned a public routable address on the fixed broadband network and is adapted to receive voice data originating from other nodes connected to the fixed broadband network. The node is specifically adapted to receive a message over an established connection on the fixed broadband network from a second node and to identify in a payload portion of this message a desired destination address for voice packets, to receive a voice data packet from the second node and to identify a source address in a header of the packet, to compare the source address with the desired destination address, and if the source address differs from the desired destination address to configure a return destination address for voice packets transmitted to the second node as the source address.

By comparing the desired destination address contained in the payload portion of the voice packet with a source address indicated in a header of the same packet or earlier messages the node can determine whether a Network Address Translator is functioning over the connection. But over and above this, the node is able to ensure that voice data is correctly routed to the desired node regardless of whether address translation has occurred or not. The operation of the node is independent of whether the communicating sending node has a public address on the fixed broadband network or connected to a private network with access to the public network. The claimed solution is thus independent of changes in the NAT functions. Also any private network can communicate effectively with this node without the need to modify the NAT function, for example as a consequence of a change in the voice-over-IP protocol.

Preferably the fixed broadband network is an IP based network such that the public routable address is an Internet Protocol address.

It is advantageous if the node is further adapted to allocate an address for receiving voice packets from the second node and to transmit voice packets configured with the source address as destination address from this allocated address. In this way any restrictive gateway function of the NAT can be bypassed.

In a particularly preferred embodiment, the node is an access controller that is adapted to control access to a mobile cellular core network by a second node, wherein the second node is adapted to communicate with a mobile station and is connected to a private network connected to the fixed broadband network.

In accordance with a further aspect of the invention a method is proposed for handling voice traffic transmitted between first and second nodes over a fixed broadband network. The method includes the following steps by the first node: establishing a connection with the second node, receiving a message from the second node over the established connection, identifying in a payload portion of the message a desired destination address for voice packets, receiving a voice data packet from the second node and identifying a first source address in a header of the packet, comparing the first source address with the desired destination address, and if said first source address differs from said desired destination address configuring a return destination address for voice packets transmitted to the second node as said first source address.

BRIEF DESCRIPTION OF THE DRAWINGS

Further objects and advantages of the present invention will become apparent from the following description of the preferred embodiments that are given by way of example with reference to the accompanying drawings. In the figures:

Fig. 1 schematically illustrates a private telecommunications network connected to a public network,

Fig. 2 schematically depicts a network connection between a node of a public network and a node of a private network connected to the public network,

Fig. 3 schematically illustrates the signalling sequence for establishing a voice connection between the two nodes shown in Fig. 1 in accordance with the prior art, and

Fig. 4 schematically illustrates the signalling sequence for establishing a voice connection between the two nodes shown in Fig. 1 in accordance with the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

In the following description the present invention will be described with reference to an access network to a mobile telecommunications network. However, it will be understood by those skilled in the art that the invention can be applied to any devices communicating voice-over-IP between two networks when the connection between the network is protected by a NAT function.

Referring to Fig. 1, an access network is shown that comprises a low-power transceiver, called a home base station HBS 10, that communicates with mobile stations over an unlicensed-radio interface, such as DECT, WLAN or

Bluetooth. The home base station HBS 10 is connected to a small private network 20, that may also include a PC 30 or other devices not illustrated here.

The private network has its own IP-addresses, hereinafter referred to as private addresses. Accordingly, all devices connected to this network are identified by different private addresses. The private network is connected via a suitable modem 40 to a fixed broadband IP-based network 50. The modem 40 has network address translator (NAT) functionality and also performs Firewall

(FW) functions. The fixed broadband IP-based network 50 is a public network with its own IP-addresses, hereinafter referred to as public addresses. A home base station controller HBSC 60 is connected to this fixed broadband network 50 and has a public address on the network. The home base station controller HBSC 60 is connected to a mobile core network NSS 70. The home base station controller HBSC 60 serves to control access of voice and data traffic from a mobile station to the core network NSS 70 via the home base station HBS 10. The NAT modem 40 is also present on the public network and consequently will have its own public IP-address.

Turning now to Fig. 2 a simplified block diagram of the home base station 10, NAT modem 40 and home base station controller 60 is shown with the address allocation of these elements. The home base station HBS 10 being connected only to the private network 20 has a single private IP-address. In the example of Fig. 2 this is given as 10.0.0.1. The NAT modem also has a private IP- address for communication on the private network 20. This is shown as 10.0.0.100. The NAT modem also has a public IP-address for communication over the public network 50. This public address is 130.100.118.66. The home base station controller HBSC 60 has one public IP-address, which is 130.100.1 18.69 in the illustrated example. The public addresses allocated to the NAT and home base station controller HBSC are for example only. While they may imply that these elements are located very close to one another on a network, in reality the public network could be extremely large part of, for example, the internet and these elements could even be located in different countries. It is important only that both element are connected to the same public IP-based network.

Fig. 3 illustrates how messages are exchanged between the home base station

HBS 10 and the home base station controller HBSC 60 via the NAT modem

40 using these addresses in accordance with the prior art, assuming that the NAT function is unable to alter network addresses contained in the payload portion of a message. At event 1 in Fig. 3 the home base station HBS 10 sets up a signalling connection towards the home base station HBSC 60 using Transmission Control Protocol (TCP), and some initial signalling is performed. The NAT function of the modem 40 maps the addresses (IP- address and TCP-port) in the network level (IP) header and these addresses are shown as source and destination addresses in Fig. 3. The NAT function also stores mappings between the changed addresses so that is can perform the same mapping in the other direction. These mappings also open the firewall FW, so that traffic towards the private network 20 is possible. The call establishment then comes to a point when the home base station controller

HBSC 60 decides to set up the voice payload part of the call. The home base station controller HBSC 60 selects one local (public) IP-address and UDP-port for the voice payload traffic. In the illustrated example this address is 130.100.118.69 with UDP-port 2300. At event 2 this IP-address and UDP- port is communicated to the home base station HBS 10 by the home base station controller HBSC 60 in a Payload Establishment protocol message on the established signalling connection. The address information forms the body or payload portion of the message. When the home base station HBS 10 receives this message it also allocates a local IP-address and UDP-port (e.g. 10.0.0.1, 5300). In event 3 a payload establishment acknowledgement message is created carrying this address information as payload and is sent to the home base station controller HBSC 60 on the established signalling connection.

At event 4 the home base station HBS 10 starts to send voice traffic to the IP- address and UDP-port received. As the traffic passes through the NAT and

FW function of the modem an address mapping is performed and the source address information (IP-address and UDP-port) are changed to one that is recognised and routable on the public network 50. The packets are then routed by the network 50 to the home base station controller HBSC 60. At event 5, the home base station controller 60 attempts to send voice traffic also. However, the address information received from the home base station in the payload establishment message is a private network address (10.0.0.1, 5300). The NAT function was not able to detect and translate this address as it was buried in the payload portion of the message. The home base station controller HBSC 60 thus transmits packets with voice payload to the indicated address. This address is not recognised by the public network 50 and the packets are consequently lost. As a result voice traffic is transmitted only in one direction.

In accordance with the present invention, this problem is alleviated regardless of the type of NAT function present at the interface between the two networks by configuring the home base station controller HBSC 60 to perform a particular function. This is illustrated in Fig. 4.

Fig. 4 shows the same sequence of signalling as that illustrated in Fig. 3. Events 1 to 4 are thus identical to those in the earlier figure and will not be described in further detail. However, between receipt of the destination address information by the home base station controller HBSC 60 over the established TCP signalling connection at event 3 and the transmission of UDP voice packets, the home base station controller HBSC 60 performs an address translation of its own.

Specifically, when the home base station controller HBSC 60 receives the payload establishment acknowledgement message at event 3 it stores the address information contained in the payload part of this message (i.e. 10.0.0.1, 5300). Then when the first UDP voice packets arrive from the home base station HBS 10 the home base station controller HBSC 60 compares the source address information contained in the headers of these packets, i.e. the IP network address and UDP-port (130.100.118.66, 1235), with those already stored. If these are different, this means that some address translation has occurred that cannot access address information carried in the payload portion of a message and the address information in the voice packet headers is the public address of the NAT function. The home base station controller HBSC 60 is thus able to ascertain whether a NAT is present on the connection. The home base station controller HBSC 60 then replaces the stored voice address information (10.0.0.1, 5300) with the NAT address information (130.100.118.66, 1235) and uses this as the destination address for the voice packets sent to the home base station HBS 10 sent at event 5. These packets are thus routed successfully to the NAT modem by the public network, mapped to the private IP-address and UDP port of the home base station HBS 10 by the NAT function and subsequently routed successfully to the home base station HBS 10 by the private network 20.

As illustrated in Fig. 4, the home base station controller HBSC 60 uses the same source UDP port - and also the same source IP-address - for voice packets as the home base station HBS 10 is sending towards. In other words, if the home base station sends voice packets towards address 130.100.1158.69 (2300), this address is the source address for packets transmitted from the home base station controller HBSC 60 to the home base station HBS. This is necessary in those cases when the NAT and firewall (FW) function performs a restrictive mapping the permits only traffic towards the private network from the same public IP-address and UDP port that the private network device is sending to. However, many NAT functions are not so restrictive when mapping, so this limitation is not always necessary.

Preferably, the number of times the home base station controller HBSC 60 is permitted to change the destination IP-address and UDP-port during a call is restricted, ideally to only once. This minimises the risk of a hostile node sabotaging voice payload traffic by sending UDP-traffic towards the home base station controller HBSC 60.

If the NAT uses the same public IP-address for signalling messages as for transmitting voice packets, the home base station controller HBSC 60 can extract the IP-address given as source address in the headers of signalling message, i.e. at event 3 and subsequently use this IP-address as a filter when setting the destination address for voice payload traffic. In other words the IP- address for voice packets can be set as soon as it receives the payload establishment message at event 3. The UDP port can then be set or changed when the first UDP-packets are received from that IP-address.

By providing this function in the home base station controller HBSC 60 there is no need to provide protocol-specific Application Level Gateway support in the NAT function. The home base station controller HBSC 60 will be able to ensure the correct routing of voice packets regardless of the NAT functions present. This renders the network independent of changes in the voice-over-IP signalling protocol, which would otherwise require a corresponding update of the NAT function. There is also no need for a specific network server. Moreover the private network is unaffected by this solution, and hence requires no particular modification or update development.

Claims

Claims: 1. A node (60) in a telecommunications network, said node being connected to a fixed broadband network (50) and being assigned a public routable address on said fixed broadband network, said node being adapted to receive voice data originating from other nodes connected to said fixed broadband network, characterised in that said node is adapted to receive a message over an established connection on said fixed broadband network from a second node (10) and to identify in a payload portion of said message a desired destination address for voice packets, to receive a voice data packet from said second node and identify a source address in a header of said packet, to compare said source address with said desired destination address, and if said source address differs from said desired destination address to configure a return destination address for voice packets transmitted to said second node (10) as said source address.

2. A node as claimed in claim 1, characterised in that said public routable address on said broadband network is an Internet Protocol address.

3. A node as claimed in claim 1 or 2, characterised in that said source address, desired destination address and return destination address for voice packets each contain an Internet Protocol network address and a User Datagram Protocol Port identifier.

4. A node as claimed in any previous claim, characterised in that said node (60) is adapted to allocate an address for receiving voice packets from said second node (10) and to transmit voice packets configured with said source address as destination address from said allocated address.

5. A node as claimed in any previous claim, characterised in that it is an access controller (60) adapted to control access to a mobile cellular core network (70) from a second node (10) that is adapted to communicate with a mobile station and is connected to a private network (20) connected to said fixed broadband network.

6. A method of handling voice traffic transmitted between first and second nodes over a fixed broadband network, the method including the steps of said first node, establishing a connection with said second node, receiving a message from said second node over said established connection, identifying in a payload portion of said message a desired destination address for voice packets, receiving a voice data packet from said second node and identifying a first source address in a header of said packet, comparing said first source address with said desired destination address, and if said first source address differs from said desired destination address configuring a return destination address for voice packets transmitted to said second node as said first source address.

7. A method as claimed in claim 6, characterised by identifying both an IP network address and UDP-port in said desired destination address and source address.

8. A method as claimed in claim 6 or 7, characterised by the first node allocating an address for receiving voice packets from said second node and transmitting voice packets configured with said source address as destination address from said allocated address.

9. A method as claimed in any one of claims 6 to 8, characterised by said first node receiving a voice data packet and identifying a second source address different from said first source address in a header of said packet, maintaining a return destination address for voice packets transmitted to said second node as said first source address.