ECS-TV System Details

Part 2 - LIVE Services

Technology behind the LIVE

ECS-TV takes advantage of many cutting edge technologies to provide high quality of service without sacraficing network bandwidth. Television like (non interactive) services are provided via Multicast IPv6 which works in a very similar way to digital terestrial television. Interactive programs and lectures are provided direct to the client (unicast) where content is viewed in a browser with other embedded information provided which is applicable to the stream.

Network Transmission Methods

Unicast: Provides a way of sending data directly from one computer to another, this is a one to one connection. Anycast: A method of sending data from a single source to the "nearest" or "best" destination as viewed by the routing topology. Multicast: One source to multiple clinets, here the traffic is not removed from a network when one client reads the data.

Multicast

Multicast data is transmitted once from a central point (broadcast node) and can be picked up by any number of client nodes simultaniously. Special reserved addresses are used which represent sepcific multicast addresses or groups and these are handled differently by routers on the network to ensure full coverage of a transmission. Multicast provides a TV like broadcast service for the internet where a central antenna sends out a high power signal which can be picked up by thousands of individual users. The internet is a far more distributed than the star topology of the classic terestrial television system hense more data is required to manage the data and decide where and when the data is not required.

Addressing Schemes - IPv6 Only

IPv4 multicast addressing is available however the features provided are no where near advanced as those provided by IPv6 and as a result services become difficult to manage. IPv6 provides a 128 bit addressesing which as can be seen in later sections is essential for use in embedded RP addressing.

IPv6 Addressing Basics

The 128 bit address: XXXX:XXXX:XXXX:XXXX:XXXX:XXXX:XXXX:XXXX

Often encased in '[]' to tell programs this is an IPv6 address
IPv6 addressing now contains HEX charecters supporting the full range of 0-F.
Blank spacers are automatically assumed to be the number '0'

Addressing Southampton University
       The University of Southampton has the IPv6 prefix: 2001:630:d0::/48 (or 2001:0630:00d0::/48).
       This essentially gives the University enough addressing space for a few trillion systems (1.2 x 10^24) to be more precise!
       Advantages of this many addresses:

No need for Network Address Translation (NAT) as system is not able to have it's own address (IPv4 space is running out)
More space for expansion into currently reserved address spaces.

Multicast IPv6

      Prefix: FFXX::/102
      First X flag: Represents the type of address '0' is reserved.
      Second X flag: The scope of the address (figure 2.2).

Scope ID Value

Multicast Address Scope

Reserved

Node-Local Scope

Link-Local Scope

Site-Local Scope

Organisation-Local Scope

Global Scope

Reserved

Figure 2.2 - IPv6 Multicast addressing scopes

ECS-TV Multicast - First Stages

ECS-TV makes use of Organisation-Local (8) addressing thus enabling coverage accross the entire university. After much deliberation it was decided that the feed prefix would be reserved for use in the project. Thus the addresses bacame:

ff18::feed:X

ff18:0000:0000:0000:0000:0000:feed:X

Figure 2.3 - Basic IPv6 multicast addressing

The X in figure 2.3 represents the channel number which is taken from Freeview TV.

ECS-TV Multicast - Netowrk Upgrade

A major ECS network upgrade in the summer of 2005 led to a complete replacement of all the main IPv6 routers on the network. Gigabit switches were all installed throughout the ECS Zepler building (building 59) enabling higher data rates arround the network. All switches and routers were provided by Cisco as top of the range enabling all latest IPv6 protocols to be supported including MLDv2 snooping and PIM-SM for use in conjunction with Embedded-RP addressing schemes.

IPv6 - Embedded-RP Addressing

Multicast addresses use a special reserved address space which routers recognise and handle appropriatly. However without some form of management over who can use which addresees there is nothing to stop 2 or more people tranmitting on the same address and thus interfearing with each other at the client side. As an attempt to solve this solutuion source specific addressing was introduced where the client would specify the source as well as the multicast group it wanted to join (Figure 2.4).

Multicast_Group@Source_Address

Figure 2.4 - Source Specific Addressing

Here we now need to know the source machine of the multicast group, however in most circumstances this is subject to change depending on network topology and renumbering. Also we have to have routers which can support this address@ format.
As a solution to this problem IPv6 introduced Rendezvous Point (RP) addressing where a multicast group has a central point which knows both the source and the clients and can connect the 2 together. The central points address is embedded within the multicast address and routers are able to decode this to get both the group address the client wishes to join and the address of the node (central pont) which handles that multicast group. Figure 2.5 shows the processes in connecting to a multicast group which contains an Embedded-RP multicast address.

1) Server sends data to Embedded-RP Multicast address, thus the Rendezvous Point knows about the multicast group.

2) Client requests a connection to the multicast group which is decoded and sent to the RP.

3) The RP connects the client to the multicast group and sends a few multicast packets directly to the client.

Figure 2.5 - Embedded-RP Connections

Example Multicast-RP Addressing
ECS-TV uses the main core IPv6 router in the School of Electronics and Computer Science as the Rendezvous Point. This Cisco 7200 router is the head router and connects ECS to the rest of the world via GEANT to the 6BONE. The following make up an Embedded-RP address:

2001:630:d0:f000::1 - The RP's (Ford.6core) address.
feed:X - ECS-TV address ending.
ff78 - An Embedded-RP address prefix with organisation-local scope.
Note:140 - Contains the 1 from the RP's address with the 40 denoting the length of the RP's address.

Figure 2.6 shows an example Embedded-RP address as used in ECS-TV, in this case BBC NEWS 24 will be recieved.

news24.zepler.tv (BBC News 24) - ff78:140:2001:630:d0:f000:feed:40

Figure 2.6 - Embedded-RP address of BBC News 24

PIM Sparse Mode & MLDv2
Once connected to your multicast group via an Embedded-RP address the first few packets come directly from the router the client machine in a Unicast type link. Once the clients and all the routers in the chain have recieved these first few multicast packets they can then see the source address of the multicast group which is contained within the packets header. The routers can then attempt to find a quicker connection to the source and connect the client directly. If this link cannot be obtained the RP can continue sending the data to the client thus acting as a relay, else prune messages will be sent back to the RP and the client will continue to decode the packets now being sent directly from the source.
The new Cisco Gigabit 3750G switches are also MLDv2 snooping aware and rather than the multicast packets being sent to the entire network subnet the switch manages the data and only sends the multicast packets to the machine which has requested connection to that multicast group. This prevents the side effects of unnecessary network flooding which can have adverse affects on older machine still running on 10Mbit network interface cards.

MORE INFORMATION: For further deatils on the operations outlined above please refer to Tim Chown's set of notes on IPv6 Multicast.

Part 2 - LIVE Services