4.2 Multicast IP supports multicast to support one-to-many (radio, news, IP multicast was originally a many-to-many (any source MC or

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1 CS475 Networks Lecture 14 Chapter 4 Advanced Internetworking Assignments Reading for Lecture 15: Sections Homework 5, Wireshark Project 3 posted, due next Thursday; Programming Project 3 posted, due following Tuesday 4.2 Multicast IP supports multicast to support one-to-many (radio, news, updates) and many-to-many (teleconf, gaming) communication. IP multicast (MC) uses MC groups, where each group has its own IP MC address. A sending host sends a single copy of a packet to the MC address. The network routers the packet whenever it needs to be forwarded over more than one link Multicast Addresses IPv4 uses addresses through (Class D) for multicast. This address range uses a 4-bit prefix (1110) leaving to specify an MC group. Ethernet supports multicast (in addition to unicast and broadcast) IP multicast was originally a many-to-many (any source MC or ASM) model. A one-to-many MC model (source-specific MC or SSM) was developed in which a receiver specifies a MC group and a specific host. A host joins and leaves an MC group by using (IPv4) Internet Group Management Protocol ( ) or (IPv6) MC Listener Discovery (MLD) to communicate to the local router Multicast Routing While a unicast propagates along a path, a multicast propagates along a. Multicast routing is the process by which the multicast distribution tree is determined. but uses only 23 bits for an MC address. 32 (2 5 ) IP MC addresses map into each Ethernet MC address. After receiving an MC packet a host must examine the entire IP address to either accept or reject the MC packet Multicast Routing: DVMRP The Distance Vector Multicast Routing Protocol (DVMRP) is an extension of distance vector routing to support multicast. DVMRP is a protocol. In a flood protocol each router would copy and forward a multicast packet along all links except the one on which the packet arrived if and only if the packet arrived over the link that is on the shortest path to the source. 10/13/ of 5

2 Duplicate packets can still be sent to LANs connected to more than one router. DVMRP solves this by requiring one router on a LAN to be designated as the. Only the parent forwards packets to the LAN. We want to prune networks that contain no members of the MC group. This can be accomplished by having each member of the group periodically announce that it is a group member. Parent routers can prune networks that contain no members Multicast Routing: PIM-SM DVMRP does not scale well and Protocol Independent MC (PIM) was developed in response. There are dense and modes (PIM-DM and PIM-SM). PIM-SM is the dominant MC routing protocol and is the only one discussed here. In PIM-SM, routers join the MC tree by sending a Join message to a special router known as the rendezvous point (RP). Here R4 sends a join message to the RP. R2 will MC traffic only along the path from RP to R4. R5's Join will not propagate farther than R2. R2 will add the path to R5 to the MC tree. For MC group G, each router looks at the Join and creates a forwarding table entry for the shared tree, called a (*, G) entry. To send a message to the group a host sends a packet addressed to the MC group to a local router (DR) which encapsulates the message in a Register message which is tunneled to the RP. The RP removes the packet and sends it out to the shared tree. R1 is the DR for the host. The message is from R1 to RP at which point it is multicast along the distribution tree. A host sending a lot of data to the MC group can trigger construction of a source-specific tree rooted at the DR. A high data rate from a single source can trigger construction of a more optimal source specific tree that the shared tree. The DR in this case effectively replaces the RP. This tree can be significantly shorter than the original shared tree. A source specific tree that uses the shared tree is shown in (c). A shorter source tree that replaces the shared tree is shown in (d). 10/13/ of 5

3 4.2.2 Multicast Routing: MSDP PIM-SM is typically used only within a domain. The Multicast Source Discovery Protocol (MSDP) was developed to extend MC across domains that use PIM-SIM. Each domain has an RP with peer RPs in other domains. An RP sends Source Active messages periodically to its peers on behalf of the sources. A peer can join the MC group by sending a Join message to DR for the source. Construction of a source specific MC tree across domains using Multicast Routing: PIM-SSM PIM was originally designed as a many-to-many protocol. PIM source-specific multicast (PIM-SSM) was developed to support the demand for a protocol. PIM-SSM required changes only to IGMP rather than PIM. In Multicast Routing: BIDIR-PIM Bidirectional PIM (BIDIR-PIM) is an enhancement to PIM that is better suited for many-to-many multicasting within a domain when the senders and receivers may be the same (multiparty video-conference). PIM-SSM a source specific tree is constructed first, bypassing the construction of a shared tree. 4.3 Multiprotocol Label Switching (MPLS) MPLS tries to combine some of the properties of virtual circuits with those of a network. MPLS-enabled routers forward packets by examining short, fixed-length labels. MPLS is primarily used for (1) destination-based forwarding, (2) explicit routing, and (3) virtual private networks and tunnels MPLS: Destination Based Forwarding Router R2 has assigned and advertised labels for certain network prefixes. Arriving packets have labels attached by R1 (the label edge router or ). 10/13/ of 5

4 The IP longest match lookup algorithm has been replaced by exact match lookup. MPLS allows internal ATM switches to be used as IP routers. (This can be a cost effective way to carry IP traffic on an existing Conversion of an overlay network that uses ATM switches to a ATM network.) The switches are now called Label Switched peer network (no hardware changes are involved). Routers ( ). R1 now has one next hop instead of five, resulting in simpler Each MPLS label is associated with a forwarding equivalence tables. class (FEC). In this example, the FECs are network prefixes MPLS: Explicit Routing MPLS: Virtual Private Networks and Tunnels MPLS enabled routers also allow a network to use explicit routing (similar to source routing). R1 can use the Resource Reservation Protocol ( ) to specify the R4-R5 path to R7. R2 can specify the R6 path. R1 and R2 attach different labels to their packets. The FEC is now based on the source router instead of the MPLS enabled routers allow tunnels through a network that can carry data (ATM cells, Ethernet or Frame Relay frames). IP tunnels can be used similarly, but MPLS tunnels use a much shorter packet header. destination Tunneling of ATM cells through an MPLS tunnel. A label (DL) identifies the virtual circuit. The tunnel label (TL) is a standard MPLS label. An ISP can use a network of MPLS enabled routers to create networks for customers. A common infrastructure is used, but each customer appears to have their own private network. 10/13/ of 5

5 4.4 Routing among Mobile Hosts has been a key enabling technology that has made wireless hotspots feasible. It provides an IP address and the identities of a default router and DNS server for new devices on Other applications would break if we assigned a new IP address to a host when it moves to a new network. For example, Voice over IP telephone calls, when a device moves between hotspots or switches from wireless to 3G a network. This is adequate for a large class of applications. An example is shown at the right. The correspondent's packets need to be re-routed to the receiver's new network. The group developed a procedure that allows a host to keep its original IP address as it moves between networks. Applications then continue to work seamlessly Routing to Mobile Hosts Mobile IP requires special routers known as the and foreign agent respectively. The home agent intercepts packets intended for the mobile host and tunnels the packets to the foreign agent. The foreign agent delivers the packets to the host Routing to Mobile Hosts - Optimization Optimizing routes between correspondent node and mobile Routing to Mobile Hosts - Mobility in Ipv6 Mobility is designed into IPv6. Any IPv6-capable host can nodes is referred to as the triangle routing problem, since Mobile acquire an address whenever it is attached to a foreign network IP path takes two side of a triangle rather than the direct path. The solution is to tell the correspondent node the of the mobile node. Then the correspondent node can create its own tunnel directly to the mobile node. In the best case, if they are on the same network, and can act as a foreign agent. Packets to the care-of address can contain an extension header with the home address. This allows the mobile node to present the illusion that its IP address is fixed to the higher protocol layers. the packets are addressed directly. In-class Exercises: Start Homework 5 10/13/ of 5