IP/ATM Networks with Straightforward Multicasting and QoS Assurance

Fujikawa Kenji <magician@kuis.kyoto-u.ac.jp>
Kyoto University

Ohta Masataka <mohta@necom830.hpcl.titech.ac.jp>
Tokyo Institute of Technology

Ikeda Katsuo <ikeda@kuis.kyoto-u.ac.jp>
Kyoto University


IP multicasting and broadband networks with QoS assurance will be the most critical issues in advanced applications in the IP world. We propose a method named PLASMA, which provides straightforward IP multicasting and autoconfiguration of an IP subnet in an environment where a network is constructed with point-to-point links such as ATM. IP/ATM networks based on PLASMA derive the best performance of ATM's cell switching fabric, supporting straightforward IP multicasting and QoS assurance. Experiments over the OLU network are also presented.

Keywords: IP, ATM, PLASMA, IP multicast, RSVP, CSR, QoS, point-to-point link, OLU network.


1. Introduction

IP multicasting is a technology that distributes messages and data to widely spread recipients on the Internet. It is utilized for broadcasting video and audio streams and is especially useful for applications like remote conferences. In the IPv6 environment, which will soon take the place of the current IPv4 environment, multicasting is more essential since many functions in IPv6 depend on multicasting. To broadcast video and audio streams requires far more bandwidth than ever. Thus, a network is needed that provides broad bandwidth and is capable of multicasting. Quality of Service (QoS) assurance is also desired since video and audio streams may not be delivered satisfactorily in practical use without it. Therefore, IP multicasting and broadband networks with QoS assurance will be the most critical issues in advanced applications.

Asynchronous Transfer Mode (ATM) networks are promising for broad bandwidth and QoS, and several IPs over ATM models that support IP multicasting are proposed and are being developed. However, they require some kinds of servers, so the LAN is not autoconfigurable like an Ethernet or FDDI LAN. In addition, the IP multicast mechanism of such a LAN is much more complicated than that of Ethernet or FDDI. The requirement of servers and the complication of IP multicasting impair the applicability and reliability of a LAN. Utilization of the UNI/NNI (User Network Interface/Network Network Interface) signaling[1] is the main cause of this problem.

We propose a method, named Point-to-point Link Assembly for Simple Multiple Access (PLASMA), which provides a simple and straightforward multicast mechanism in a subnet, such as an IP/ATM subnet. PLASMA also assures QoS of transportation using Resource ReSerVation Protocol (RSVP)[2] over IP/ATM networks. For this purpose, we add a feature to the RSVP protocol and introduce Cell Switching Routers (CSRs)[3, 4], which are routes with cell switching fabric, into IP/ATM LANs.

2. Current IP/ATM Models and Problems

The current IP/ATM models that support IP multicasting in subnets are further complicated. Utilization of the UNI/NNI signaling is the main cause of this problem. Originally, IP multicasting in a LAN is very simply defined in [5], where every sender need not know which hosts are receivers, or every receiver which hosts are senders. Ethernet- or FDDI-based subnets easily and simply support IP multicasting.

We briefly review two sorts of IP multicasting in LAN Emulation (LANE)[6] and in the Multicast Address Resolution Server (MARS) model[7], each of which provides an IP multicast mechanism over ATM.

LANE is a method that implements applications of the current LANs over ATM. A LAN based on LANE behaves as if it were an Ethernet or FDDI LAN. In LANE, a LAN is managed by a LAN Emulation Configuration Server (LECS) and a LAN Emulation Server (LES), and broad-/multicasting are provided by a Broadcast and Unknown Server (BUS). A host wanting to broadcast or multicast IP packets has to send IP packets to a BUS, and subsequently the BUS transmits the packets to all the hosts in the LAN over point-to-point VCs (Virtual Channels) or a point-to-multipoint VC.

A MARS server is a multicast extension of an ATM-ARP server. In the MARS model, each sender arranges one VC against one multicast address, that is, a set of point-to-multipoint VCs from different senders is dedicated to one multicast address (There is an approach that employs a MultiCastServer (MCS)[8], the discussion on which is almost the same as that on LANE.) A MARS server holds the information on which hosts join which IP multicast address, and is responsible for notifying hosts sending data to a multicast address of receivers' ATM addresses. In addition, a MARS server has to re-send the information to the senders every time the information changes. In order to keep a set of point-to-multipoint VCs toward all the receivers, every receiver must notify the MARS server of the multicast address it joins. Every sender must keep the information of all the receivers and must send SETUP and ADD PARTY signaling messages.

Thus, multicasting in both LANE and MARS requires some kinds of servers. Such superfluous elements are not favorable with respect to LAN's applicability and reliability. Autoconfiguration of a LAN is almost impossible, and configuration of a LAN becomes more complicated. In addition, initial setting up of servers is required, and many types of bottlenecks emerge. For instance, introducing a server like BUS prevents utilizing ATM's cell switching fabric. In such an IP/ATM subnet, QoS can hardly be assured because a VC cannot be assigned to each data flow path.

3. PLASMA Mechanisms

We propose a method named Point-to-point Link Assembly for Simple Multiple Access (PLASMA), which provides a simple and straightforward multicast mechanism in a small network comprising nodes and point-to-point links. An IP/ATM subnet is classified into this type of network. The term "small network" refers to a network that has a small number of hosts (less than about 300), regardless of whether it is a LAN, MAN, or WAN.

3.1. Features of PLASMA

The purpose of PLASMA is to provide a simple and straightforward multiple access mechanism in nonmultiple-access networks such as ISDN, Frame Relay, and ATM. In PLASMA, every sender does not need to know which hosts are receivers, nor does every receiver need to know which hosts are senders. One PLASMA domain constructs just one multicast-capable IP subnet.

PLASMA utilizes layer 2 (L2) label switching architecture, which detects the destination(s) of an L2 data frame from its L2 label and forwards the L2 frame to the destination(s), alternating the value of the L2 label. In ATM, an L2 frame and its L2 label correspond to a cell and the VPI/VCI value of the cell, respectively. In addition, ATM switches provide hardware-level L2 label switching architecture. For other point-to-point link networks, we are proposing a method for placing L2 labels in Point-to-Point Protocol (PPP)[9] frames and designing software-level L2 label switching architecture.

Data flow paths are created as a result of L2 label switching at the nodes en route. Each node employs the PLASMA Protocol (PLASMAP), which advertises L2 label switching information in a single IP subnet. Here a "node" is defined as an entity that sends and receives PLASMAP messages and sets up L2 label switching fabric. A PLASMA node does not have to be assigned its own identifier for processing PLASMAP if it is not an end in terms of data transportation on layer 2. Therefore, PLASMA enables autoconfiguration of IP subnets, that is, all users have to do is connect PLASMA nodes. For example, in the case of IP/ATM networks based on PLASMA, ATM switches are PLASMA nodes just as ATM hosts are. However, they are not required to have their own identifiers.

In a PLASMA network, where PLASMAP messages are exchanged, nodes can be connected in any topological manner. Of course, a PLASMA network is allowed to contain some loops, thus improving network flexibility.

3.2. PLASMAP Messages

PLASMAP uses the following three types of messages:

Table 1 shows the key fields of the PLASMAP messages.

Table 1. Key fields of JOIN, NOTIFY, and ACCEPT messages
Message Key fields
JOIN Join addresses
NOTIFY Source address, Flow ID, Hop count, Destination address, Flow spec
ACCEPT Source address, Flow ID, L2 label

Nodes create a data flow path, in other words, begin to receive and/or to send the data when they are sending NOTIFY messages related to the data and receive related ACCEPT messages. If nodes are pure receivers, then they are not required to receive ACCEPT messages. Each node that is not one of the ends makes use of L2 label switching fabric for forwarding the data.

Nodes send PLASMAP messages periodically. They expire a data flow path after a defined period of time for which they are not receiving related PLASMAP messages. Therefore, data flow paths in PLASMA are "soft-state."

Each node is required to process NOTIFY, ACCEPT, and JOIN-ALL messages, and is recommended to process JOIN messages. If a node that cannot process JOIN messages receives JOIN messages, then it simply discards them.

3.3. Managing Join States by JOIN Messages

Each node keeps a "join state," which holds join addresses, for itself and for every point-to-point link interface. A join state at an interface is created and managed in accordance with the JOIN messages received from the interface.

A node that cannot send JOIN messages is required to send at least a JOIN-ALL message to its peers. From a different point of view, this implies that a node can send a JOIN-ALL message to any peer at any time instead of sending a JOIN message.

The join addresses placed in a JOIN message that is to be transmitted from one interface are determined from the join states of the node and at the other interfaces. That is, the join addresses are the merged ones of the node and at the other interfaces. If a join state at another interface holds all addresses (this means that this interface is receiving a JOIN-ALL message), a JOIN-ALL message is sent from the interface.

                                  (Join D)



                         D        (Join D)    (Join A,B)


             *           *           BC

             |           |           |

             |           *           |        (Join B,C)

             +--------*[N3]          +---------*[N8]


                         |        (Join A)


                 N1, N2, N3...N8 : Node

                 A, B, C and D   : Address

                 *               : All addresses

                 (Join A,B)      : N7 joins A and B


                 [N5]AB--        : N5 has a join state of A and B

                                   at this interface

Figure 1: Managing join states.

Figure 1 shows sample join states in a PLASMA network. In this network, for instance, Node N5 joins Address D, and is receiving a JOIN message of all addresses (*), a JOIN message of Addresses A and B, and a JOIN message of Addresses B and C from Nodes N2, N7, and N8, respectively. As a result, Node N5 has the join states shown in the figure, each of which corresponds to one of the receiving JOIN messages, and is sending a JOIN message of Addresses A, B, C, and D to Node N2 and a JOIN message of all addresses (*) to Nodes N7 and N8.

There is a case in which some of the join states of all addresses (*) become join states of Addresses A, B, C, and D. Assuming that the link between Node N1 and N3 breaks and resumes after some period of time, such a case will occur. Even in this case, PLASMAP works correctly and make the state converge to the one illustrated in the figure in time. PLASMAP very simply supports this function as follows: When a node receives the same NOTIFY messages from different interfaces, it makes join states at those interfaces to hold all addresses (*). This function avoids loops of redundant JOIN messages.

3.4. Creating Data Flow Paths by NOTIFY and ACCEPT Messages

When a node wants to send data to a certain uni-/multicast address, it sends a NOTIFY message to all the peers. Then the NOTIFY message is flooded within the subnet. Note that flooded messages are not data but signaling messages, thus the load on flooding is not so large. A NOTIFY message is transmitted along the following procedures:
  1. When a node receives a NOTIFY message, or when it is an originator of the message, it goes to the next stage.
  2. The node checks whether the same NOTIFY message has come from another interface recently. If the same NOTIFY message that has a smaller or equal hop count has already come, then the node discards the new NOTIFY message.
  3. The node sends the NOTIFY message, incrementing the hop count, to a peer beyond each interface that has a join state of all addresses or the destination address. If the node is not the originator, it must not send the NOTIFY message to the interface from which the message has come.

A node sends an ACCEPT message to the peer that sends a related NOTIFY message to it if it joins the destination address placed in the NOTIFY message or receives a related ACCEPT message from a downstream node.

                                  (Join D)


             (discarded) |

                ----->   D -----> (Join D)    (Join A,B)


             *           * ^         BC ----->

             |           | |         |

             | <-----    * |         |  ----->(Join B,C)

             +--------*[N3]          +---------*[N8]


                         | <----- (Join A)


                                 (Notify B)

                    (a) Sending a NOTIFY message

                                  (Join D)



                         D <----- (Join D)    (Join A,B)


             *           * |         BC <-----

             |           | |         |

             |           * V         |  <-----(Join B,C)

             +--------*[N3]          +---------*[N8]


                         | -----> (Join A)


                                 (Notify B)

                    (b) Sending ACCEPT messages

Figure 2: Creating data flow path by NOTIFY and ACCEPT messages.

Figure 2(a) shows a sample network, where Nodes N7 and N8 join Address B and N6 is sending a NOTIFY message so that it can send data to Address B. The NOTIFY message is finally delivered to Nodes N7 and N8 according to the above-mentioned procedures. Discarding the NOTIFY message from N1 to N2 avoids creating a loop of the NOTIFY message. Consequently, the ACCEPT messages are delivered as shown in Figure 2(b), and the data flow path is created along the reverse path of the ACCEPT messages.

3.5. Connecting PLASMA networks via Routers

PLASMA routers, which are PLASMA nodes as well, reside on IP subnet boundaries as the conventional routers. Each router is set up to manage sets of point-to-point links, each of which belongs to a specified subnet, so that it can prevent forwarding PLASMAP messages from one subnet to another. Thus, PLASMAP is terminated at routers and is valid only in a single subnet. This also implies that routers usually forward data from one subnet to another in the conventional packet forwarding, not in L2 label switching.

3.6. PLASMA Application to IP/ATM Networks

The PLASMA mechanisms can be easily applied to IP/ATM networks. Nodes of PLASMA, L2 labels, and data flow paths correspond to ATM hosts and switches, VPI/VCI values, and VCs, respectively. All the ATM hosts and switches in an IP/ATM subnet exchange PLASMAP messages with each other. The advantage of utilizing ATM is that ATM cell switching fabric, which corresponds to L2 label switching, transmits data at a high speed with low delay and low jitter.

In IP/ATM networks based on PLASMA, IP unicasting and multicasting are simply implemented by using IPv4 (or IPv6) uni-/multicast addresses as PLASMA addresses. Servers like MARS, LECS, LES, and/or BUS are not required. Therefore, PLASMA enables straightforward IP multicasting in IP/ATM networks without any kinds of servers. In addition, autoconfiguration of an IP/ATM LAN is provided, since ATM switches do not need to have their own identifiers.

4. QoS Assurance in PLASMA

PLASMA is suitable for assuring QoS of data flows since it can distinguish data flows just by their L2 labels. In this section, we describe how to utilize RSVP in PLASMA and CSRs over IP/ATM networks based on PLASMA.

4.1. Introducing RSVP for QoS Specified Data Flows

We incorporate RSVP as a data flow setup protocol for PLASMA networks to assure QoS. In addition, PLASMA supports a straightforward multicast mechanism and RSVP is considered to manage IP multicasting, so they cooperate with each other and accomplish QoS-assured IP multicasting together.

In PLASMA with RSVP, QoS-specified transportation is implemented by utilizing an independent data flow path for each service, while non-QoS-specified transportation (i.e., best-effort transportation), is supported with a shared data flow path. Thus, PLASMA assign an independent data flow path to each RSVP flow. The nodes on the way arrange a queue for the data flow, distinguish the data by the L2 label, and queue it in the dedicated queue.

4.2. Extension of LIH Field for QoS-assured Flows across Subnets

As mentioned above, routers do not usually forward data in L2 label switching. A PLASMA router is required to detect the correspondence between an ingress QoS-specified data flow path in one subnet and an egress one in another for the purpose of forwarding the data from the ingress to the egress in L2 label switching. PLASMAP cannot make this detection by itself, Thus, we propose a method of extending the usage of the LIH field in the RSVP messages.

Each RSVP sender transmits an RSVP PATH message, which is transferred via a non-QoS specified data flow path, placing the flow ID of a PLASMA data flow path in the LIH field. A router detects that the ingress RSVP flow corresponds to an ingress data flow path by the LIH field placed in the PATH message. Consequently, the router detects the correspondence between the ingress and egress data flow paths, since it already knows the correspondence among the ingress PATH message, the egress PATH message, and the egress data flow path.

4.1. RSVP over IP/ATM Networks Based on PLASMA

ATM supports four sorts of bit rate services, CBR, VBR, ABR, and UBR. VCs of CBR and UBR (or ABR if available) are established for RSVP data flows and non-RSVP data flows over IP/ATM networks based on PLASMA as follows:

Cell switching routers (CSRs) are proposed in [3, 4], which are routers that can forward data in cell switching as well as in packet forwarding. Since this function equals that of our PLASMA routers in IP/ATM networks, we introduce CSRs in IP/ATM networks based on PLASMA, and add the LIH extension to CSR's features.

5. Experiments over OLU Network

The On-Line University (OLU) network is a Japanese nationwide experimental ATM network offered by NTT (Nippon Telegraph and Telephone) for cooperative multimedia communication experiments carried out by a group of more than 20 universities and companies. It is managed by the JAIN (Japan Academic Inter-university Network) consortium under the OLU project. The OLU network offers Virtual Path (VP) exchange and high bandwidth of 155Mbps. We have been adapting PLASMA to the OLU network.

                     Waseda      Osaka    Nara Inst. of

                      Univ.       Univ.    Sci. & Tech.

   Fujitsu [R]---------[R]---------[R]---------[R]-----------[R] Kobe

            |                                                 |  Univ.

            |                                                 |

  Univ. of [R]                            +------------------[R] Kyoto

     Tokyo  |                             |                   |  Univ.

            |                             |                   |

  Univ. of [R]                            |                  [R] Kyushu


Elec.-Comm. |                             |                   |  of Tech.

            |                             |                   |

       NTT [R]-------[R]-------[R]-------[R]-------[R]-------[R] Hiroshima

                     NEC   Tokyo Inst.  Nagoya    Tohoku      |  Univ.

                             of Tech.    Univ.     Univ.      |

                                                             [R] Hirshima

                                                                 City Univ.

Figure 3: OLU network.

The topology of the OLU network is basically a ring. Each node is connected to two adjacent nodes. Some nodes have extra connections to nonadjacent nodes in the ring, thus creating cut-through paths. Figure 3 shows only 15 nodes that run an IP router.

We have been developing three types of PLASMA nodes for IP/ATM, an ATM host, an ATM switch, and a CSR, which all employ PLASMAP for establishing VCs. Either a PLASMA ATM switch or a PLASMA CSR consists of an ATM switch and an ATM host that controls the switch. Each PLASMA CSR runs routing daemons, a gated and a mrouted, on it.

On some nodes, PLASMA ATM hosts, switches, and CSRs are introduced. The ATM hosts can make use of IP multicasting as well as IP unicasting without any kinds of servers, so all of the current IP services are available in the OLU network. In addition, PLASMA allows autoconfiguration of LANs, since ATM hosts do not have to be preconfigured.

In the OLU network, hosts can possess an independent VC per service using RSVP, regardless of whether intra-subnet or inter-subnet. The OLU network also shows how to set up VCs automatically in a VP exchange environment. We are also making an experiment in which end-to-end MPEG2 video streams of more than 6 Mbps are transmitted via RSVP flows through several CSRs. This will provide QoS-assured video conference systems of higher quality.

6. Conclusions

We argued about the importance of broad bandwidth, QoS assurance, and multicast capability for advanced applications in the Internet. Though an ATM network is promising for this purpose, the current IP over ATM models supporting IP multicasting have some severe problems.

We proposed PLASMA, which provides straightforward IP multicasting and autoconfiguration of an IP subnet in an environment where a network is constructed with point-to-point links. PLASMA can be easily applied to IP/ATM networks, derives the best performance of ATM's cell switching fabric, and also assure QoS using RSVP. Particularly for enabling QoS assurance across subnet boundaries, we proposed an RSVP LIH extension method. Finally, ongoing experiments over the OLU network were presented. IP/ATM networks based on PLASMA are suitable for future advanced applications, supporting IP multicasting and QoS assurance.


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