Network Router

Vocality equipment includes IPV4 routing software for forwarding data between 10/100base-T Ethernet ports and aggregate ports.  To utilize this facility, the equipment must be configured with information about the IP sub-networks that the Vocality network is interconnecting.  All configurations must include at least:

• IP address and subnet mask for the Ethernet port
• Configuration of how the IP Router connects to the Vocality network
• Configuration of static IP routes

The Ethernet is represented in the user interface with the port name “ENET”.   The Ethernet must be configured with the IP address of the Ethernet port on the local network, and the mask of the local subnet.  Any host stations (PCs), or routers on this local network, must be configured to use the IP address of this port as the next-hop gateway for all IP networks that the unit is providing interconnect services for.  The MTU (maximum transmission unit in bytes) is typically left at the default value of 1514 for the Ethernet port.

Virtual Ports

The Vocality subsystem is built on the concept of connecting peer tributary ports across the Vocality network via aggregate links. The IP Router is fully integrated with the other ports and services by this subsystem for connection across the Vocality network, which may use of a variety of technologies for the aggregate links, not just IP via Ethernet.

The concept of virtual ports is used to represent the Wide Area Networks (WANs) of the IP Router. IP connectivity is provided across the Vocality network as a set of point-to-point connections between units, linked by the subsystem.  The IP Router forwards traffic between the Ethernet ports of pairs of units via the virtual ports, which represent these point-to-point connections between the integrated IP Router in each unit.

In Vocality equipment, tributary ports are represented by a port number.  These typically provide information about the location of the tributary port – the 4 voice ports on a voice card are represented as n:s:c, where n is the node number, s is the slot number that the voice card is inserted in, and c is the channel number within the voice card.  A set of special virtual ports has been reserved to represent the IP Router WAN ports.  These follow the standard port terminology of n:s:c.  As there is not plug-in hardware module for the integrated IP Router, the slot number component (s) of the channel identifier is always 0.  Channels 10 to 31 have been allocated for IP Router operation.  A separate channel is required for each integrated IP Router that this unit peers with.  The network configuration requires that the peer channel is configured for each channel configured on the local router.  In the example below (Figure 3), Node1 is connected to both Nodes 2 and 3.  Channel 0:10 on Node 1 is peered with channel 0:10 on Node2.  Channel 0:11 on Node1 is peered with Channel 0:10 on Node 3.

Figure 3 : Example of virtual port use

The integrated IP Router acts as a half-router – a pair of IP Routers connected via the Vocality network provide the full router operation.  To preserve IP addresses, the point-to-point link between Vocality units does not require an IP subnet configuration.  When IP traffic is routed over the point-to-point connection, it can only be received by the peer unit, and therefore IP addressing is not necessary. 

Each virtual port requires DBA (Dynamic Bandwidth Allocation) and Destination parameters to be configured.  The DBA is the maximum bandwidth (in bits per second) of IP traffic that can be sent to the peer unit – the actual amount is determined by the subsystem in the Vocality network according to other offered loads.  The destination is the virtual port identifier on the remote peer. 

Maximum Transmission Units (MTUs)

The MTU for each configured IP network specifies the largest datagram that may be transmitted onto that network.  Routed packets that exceed the MTU for the onward network are fragmented using standard IP fragmentation mechanisms.  Packets are reassembled by the target station.  The default MTU for the Ethernet is 1514 – this allows transmission of the maximum sized Ethernet frame (the 4-byte CRC is not included in the MTU).  Configuration of the MTU for the channels across the Vocality network requires some care to avoid degradation of voice quality of voice channels that are multiplexed across the same aggregates as the IP traffic.  It is recommended that the IP MTU is set to avoid creation of packets that will take longer than 20ms on the slowest aggregate between the local unit and its peer.  The MTU should be set according to:

MTU = LS/40

where LS is the slowest link speed (in bits per second) on the aggregate link to the peer.  If the calculated MTU exceeds the MTU for the Ethernet, the figure should be limited to 1514. Note that fragmentation has an overhead in both the computation required in the unit, and the bandwidth required to send data (each fragment carries an IP header), therefore setting the MTU below the recommended value should be avoided.

Static Routes

Each Vocality IP Router must be configured with static routes to tell it how to reach IP networks other than the one to which it is locally attached.  An IP STATIC ROUTE TABLE menu screen is provided under the IP sub-menu to do this.  Each configured route consists of a description, a destination address, a mask for the destination address and a next-hop.  The description is an optional text field used to identify a route in the configuration. The next-hop is either the IP address of the next-hop gateway on the local Ethernet network, or the channel number for a unnumbered link to a peer Vocality unit across the IP network.  When a route lookup matches more than one configured route, the one with the longest mask is used to route the packet.  A default route (one to use if all other route lookups fail) can be configured by using a destination and mask value of 0.0.0.0.

Troubleshooting

A number of troubleshooting tools have been added. These tools allow the user to run the ping protocol (ICMP echo), view the current internal IP route table, view the statistics from the IP Router, view the statistics from the Ethernet device, and view the ARP (Address Resolution Protocol) table.  When the network is not performing as expected, these tools can be used to troubleshoot behaviour.

UDP Relay

Standard IPV4 router operation does not allow for the forwarding of subnet broadcasts.  However certain network applications rely on the relay of UDP packets sent to the subnet broadcast address.  For example Windows browsing service relies on Netbios datagram service packets (addressed to UDP port 138 and the IP subnet broadcast address) reaching all stations within the browsing domain.  To allow seamless operation of such applications across an IP network routed over a Vocality network, the ability to relay UDP subnet broadcasts has been provided, with an entry in the UDP relay table required for each service that must be relayed.  Some well-known service types are pre-configured for addition to this table – other services require the appropriate UDP port number to be configured.  When using UDP relay care must be taken to ensure that broadcast loops are not introduced into the network.  Vocality software ensures that the effects of such loops are minimized through TTL reduction, although network operation could still be adversely affected.

DHCP Server/Relay

Vocality provides three modes of operation for DHCP (Dynamic Host Configuration Protocol). 

The default mode is OFF – in this mode the unit does not take part in DHCP operation – it is assumed that all stations on the local network have a static IP configuration, or that a separate DHCP server is available on the local network, or an externally routed network. 

In SERVER mode, the Vocality equipment has an embedded DHCP server that will respond to received requests from clients. It can provide the following host configuration parameters in response to received DHCP requests:

• IP Address
• Lease Time
• WINS Server
• Domain
• DNS Server Address

The IP address range should be from the range of addresses on the subnet configured on the ENET port of the unit, excluding the address configured for the ENET port itself.  The DNS Server addresses sent to the DHCP client are entered at configuration time.

In RELAY mode, DHCP requests are relayed to a remote DHCP server through the unit – requests are relayed to the addresses configured in the primary and secondary server addresses in the DHCP server configuration.

Telnet Access

The IP stack allows the management screens to be accessed via the telnet protocol.  Telnet access is possible once IP has been configured.  To provide additional security to ensure that telnet access is only granted to the appropriate parties, an access table has been provided.  The access table must be configured to specify which station or group of stations are allowed access to IP host facilities on the unit.  Each access table entry comprises a description (simple text – not used in access decision), an IP address, an IP mask, and a service definition.  When an attempt is made to access the host service (e.g. a telnet connection is requested), the access table is checked to ensure that an entry matches the requesting host.