1 CSCI 6433 Internet Protocols Class 7 Dave Roberts

Topics Internet Multicasting MPLS 2

IP Datagram Messaging Unicast: datagram from one source device to one recipient device Broadcast: datagram from one source to all recipients on a network Multicast: datagram from one source to a collection of recipient devices 3

Functions Needed Multicast addressing ◦ Class D of IPv4 addresses Multicast group management ◦ IGMP—Internet Group Management Protocol Multicast datagram routing ◦ Multicast routing protocols are employed 4

Group Management IGMP, Internet Group Management Protocol, is used to manage groups 5

Multicast addressing ARP is designed for unicast only ARP for multicast would be very inefficient Direct mapping to data link layer multicast groups is used 23 of the 28 bits in the multicast group address in Ethernet are used 6

Routing Routing is complicated! Must create multiple copies of the datagram for multicasting Routers need special algorithms to handle these multiple copies with efficiency Routers must be able to handle datagrams to a multicast group even if the source is not a group member 7

IP Multicast Group address—each group has a unique class D address, permanent or temporary Number of groups—addresses for up to 2 28 simultaneous multicast groups Dynamic group membership—host can join, leave a group any time Hardware—can use hardware multicast Internetwork forwarding—multicast routers needed to forward multicast between networks Transmission—any host can send to any group 8

Building Blocks Multicast addressing scheme Effective notification and delivery mechanisms Internetwork forwarding facility 9

The Concept Multicast addresses ◦ One IP multicast address per group ◦ Addresses useful worldwide Notification and delivery mechanism ◦ Inform routers about groups ◦ Transfer multicast packets to hosts Internetwork forwarding ◦ Efficient routing along shortest paths ◦ Don’t route to networks with no group members 10

IP Multicast Addresses 11

Permanent IPv4 Multicast Addresses 12

Permanent IPv6 Multicast Addresses 13

Multicast Semantics Multicast address only a destination address No ICMP error messages generated about multicast datagrams TTL field in an multicast datagram is honored Question: what happens when I ping a multicast address? Why? 14

Multicast Routing Not all routers are multicast routers Host does not know which are multicast Host sends packet using local network multicast address Multicast routers listen for all multicast datagrams, route them appropriately Host does not have to address multicast datagram to a multicast router Multicast capability is provided by routers, not hosts 15

Multicast Scope Scope of a multicast group—range of group members ◦ All on same network: scope is restricted to one network ◦ All in same organization: scope limited to one organization Methods of controlling scope: ◦ TTL controls the range ◦ Administrative scoping: routers forbidden to forward to the restricted space 16

Host Participation 17 Possible levels of participation of a host in multicasting: For an application to multicast, host must have an API for application to declare intent to join or leave a multicast group. Host must track application participation in groups, remember that it leaves a group when all applications leave.

IGMP—Internet Group Management Protocol IGMP carries group membership information Thought of as a part of IP, like ICMP IGMP is required for all machines that receive IP multicast Phase 1: host joins a group, sends IGMP message to multicast address declaring membership Phase 2: multicast routers poll hosts to determine whether any are still members of each group, stops advertising to other routers after no response to several polls 18

IGMP Implementation All communications between hosts and multicast routers use IGMP Every 125 seconds, multicast routers poll all groups for membership information, not just one Multicast routers on a single network choose just one to do all polling Hosts don’t all respond to a query at the same time Each host listens for responses from other hosts in the group, suppresses unnecessary response traffic Question: Why are hosts listening? What would they not send? 19

Group Membership State 20

IGMP Message Format 21

Special Properties of Multicast Routing 1. Multicast routes can change simply because an application decides to leave or join a group 2. Multicast forwarding requires a router to examine more than the destination address 3. A multicast datagram may originate on a host that is not part of the group, and may be routed across networks with no group members attached 22

Multicast Forwarding and Routing 23 Dot, X are two different multicast groups

Multicast Routing Paradigms RPF—reverse path forwarding ◦ Router looks up interface that leads to source address (interface I) ◦ Forwards datagram over all interfaces other than I ◦ If datagram arrived other than through I, discard it ◦ RPF sends datagrams to networks with no members TRPF—truncated reverse path forwarding ◦ Uses list of multicast groups reachable through each interface ◦ First, RPF is applied ◦ Then, interfaces that do not lead to members of the group are skipped Multicast routing is based on the datagram’s source and destination addresses 24

Consequences of TRPF 25 Consider the case where a multicast datagram headed for host B is sent by host A

Multicast Trees 26 Number of copies received depends on the source

Multicast Routing TRPF doesn’t forward to network if it has no members of the group Thus router must know about group membership Membership information must be communicated across the Internet Multicast design a tradeoff of routing traffic overhead and inefficient data transmission 27

Reverse Path Multicasting Basis: ◦ First priority given to reach all group members rather than avoid retransmission ◦ Presume that multicast routers have a routing table that is correct ◦ Routing should improve performance where possible Use RPF to send across all networks Use RPM to identify routers that don’t reach members Cease forwarding to routers that don’t reach members RPM is called broadcast and prune strategy 28

Where It’s Used Multicasting is widely used within organizations for video and audio distribution Use on the Internet has begun with Internet radio and television 29

Summary IP multicasting an abstraction of hardware multicasting IP multicasting uses class D addresses Hosts communicate group membership to multicast routers using IGMP IGMP introduces periodic message from a multicast router and a reply for each group Several protocols have been designed for multicast routing 30

MULTI-PROTOCOL LABEL SWITCHING (MPLS) 31

Disadvantages of Table Routing Takes log 2 n probes to find an address in forwarding table of n entries Array index can be performed in one step Switching methods use a label from a packet as in index to a table that gives action to take 32

Multi-Protocol Label Switching Data packets are assigned labels Packet-forwarding decisions made based on the label, no need to examine the packet Can create end-to-end circuits across any transport medium with any protocol Can carry many types of packets Traffic management is improved Loss of visibility to IT departments Path labels identify virtual links between distant nodes, not necessarily endpoints 33

MPLS Routers Routers that carry MPLS traffic are called label switch routers (LSRs) Entry and exit point routers are called label edge routers (LERs), which push an MPLS label onto a packet and pop it off an outgoing packet 34

Label Switch Router Routers inside an MPLS network perform routing based only on the label When an LSR receives a packet, it uses the label included in the packet header as an index to determine the next hop on the label-switched path and a corresponding label for the packet from a lookup table. The old label is removed and replaced with the new label before the packet is routed 35

Label Distribution Protocol LDP used to distribute labels between LERs and LSRs LSRs exchange reachability and label information to build a complete diagram of the MPLS network Label switch paths are used to create virtual paths through MPLS networks LER determines forwarding class of a packet, pushes label into MPLS header LSR looks at topmost label, does a label swap, push or pop to label stack 36

MPLS in the Stack 37 MPLS is called a “layer 2.5” protocol

Understanding MPLS Instead of focusing on destinations, focus on packet flows Instead of thinking about forwarding tables that are relatively static, think about a system that can change forwarding tables quickly Switching technologies like MPLS use the flow abstraction and create forwarding for flows rather than for destinations 38

How It Works Each packet is prefixed by an MPLS header, called a label stack with some number of these: ◦ 20-bit label value ◦ 3-bit traffic class field for QoS ◦ 1-bit bottom of stack flag ◦ 8-bit TTL field Label edge router pushes label, pops at other edge of MPLS network Label switch router routes based on the label only 39

Use of the Label Stack Label swap ◦ Label is swapped with a new label ◦ Packet is forwarded along path of new label Label push ◦ New label pushed on top of existing label ◦ Encapsulates packet in another layer of MPLS ◦ Commonly used for MPLS VPNs Label pop ◦ Label is removed from packet ◦ May reveal an inner label ◦ If label is last one, packet leaves MPLS tunnel At egress router, only original payload remains, so that router must be able to process it 40

Multi-Protocol Label Switching 41 Use of labels to direct forwarding

MPLS Encapsulation 42

Traffic Engineering Can use MPLS to define full mesh between two ISPs, can monitor traffic to other ISPs Can use MPLS to use fastest routes for most critical traffic 43

Summary Indexing can be used instead of table lookup to determine routing, saving router time Paths can be defined and engineered MPLS prepends a header onto each message LSRs along the route use labels to forward the datagram without lookups MPLS was defined to save router time, is less important today because of faster routers MPLS is mostly used today to provide VPNs 44