CSCI 6433 Internet Protocols Class 7

1. CSCI 6433Internet ProtocolsClass 7 David C. Roberts

2. Topics • Internet Multicasting • Mobile IP • VPN

3. Broadcasting • Broadcasting—most common form of multipoint delivery • One copy of a packet to each destination • Bus networks broadcast just one packet • Switched networks must forward the packet • Broadcast addresses recognized in hardware • Computing resources consumed on every host

4. Multicasting • Multicast—single packet delivered to a group of machines • Range of multicast addresses is reserved for specific hardware • Hosts set network adapters to recognize some particular multicast address • Multicast addressed packets must be forwarded to all parts of the network

5. Functions • Addressing • Group management • Routing

6. Addressing • Addresses are reserved for multicast • All class D addresses are reserved

7. Group Management • IGMP, Internet Group Management Protocol, is used to manage groups

8. 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

9. IP Multicast • Group address—each group has a unique class D address, permanent or temporary • Number of groups—addresses for up to 228 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

10. The Concept • Multicast addresses • Local autonomy in address assignment • 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

11. IP Multicast Addresses 1110—multicast address Bits 4 through 31 identify a specific multicast group Address range: 224.0.0.0 through 239.255.255.255 Up to 228 simultaneous multicast groups

12. Permanent IP 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 multicast address • Multicast routers listen for 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 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.

17. 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

19. Group Membership State

20. IGMP Message Format

21. Special Properties of Multicast Routing • Multicast routes can change simply because an application decides to leave or join a group • Multicast forwarding requires a router to examine more than the destination address • 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 Dot, X are two different multicast groups

23. 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 Consider the case where a multicast datagram headed for host B is sent by host A

25. Multicast Trees Number of copies received depends on the source

26. 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 • 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

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

32. 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 • Label edge router pushes label, pops at other edge of MPLS network • Label switch router routes based on the label only

33. 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

34. 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 • Allows hierarchical routing of MPLS packets • 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

35. Multi-Protocol Label Switching Use of labels to direct forwarding

36. Routers Can Change Labels MPLS allows definition of a path of switches, without requiring the same label along the entire path

37. MPLS Encapsulation

38. 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

39. 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

40. Mobile IP • IP was not designed with hand-held or book-sized mobile computers in mind • Mobile IP has its limitations in today’s world, where IP address is tied to network address which is geographic

41. Mobile IP Allows portable computers to move from one network to another. Hosts move from one network to another, not in the original design of IP! Either • Host address must change, or • Routers must send a host-specific route across the entire Internet

42. General Characteristics of Mobile IP • Transparency—mobility transparent to applications, transport layer protocols, routers not involved in the change. • Interoperability with IPv4—mobile host can interoperate with stationery hosts using IP • Scalability—scales to large internets • Security—authentication for all messages • Macro mobility—focuses on long-duration moves, rather than roving as in a cellular phone system

43. Overview of Mobile IP • Host can have primary and secondary address • Primary is obtained at “home” location, permanent and fixed • Secondary obtained after a move. Sent to agent (router) at home. • Agent intercepts datagrams, encapsulates in IP datagrams, sends to secondary address. • Mobile host deregisters when returning home, notifies agent of new address after another move

44. Mobile Addressing • Home address—conventional IP address • Temporary address is called care-of address • Two forms of care-of address: • Co-located: mobile host does forwarding • Foreign: foreign agent (router) on network being visited assigns care-of address, handles forwarding

45. Foreign Agent Discovery ICMP router discovery mechanism used to discover a foreign agent.

46. Agent Registration

47. Summary • Mobile IP allows a computer to move from one network to another without changing its IP address • Mobile either obtains a co-located care-of address or discovers a foreign mobility agent and requests a care-of address. • Once registered, mobile can communicate with an arbitrary computer on the Internet. • Datagrams from mobile go directly to destination • Return datagrams go through mobile’s home agent

48. Private Networks VPN • Allow external connections • Keep internal datagrams private • Can build a private, internet not connected to the public Internet • Can build private network, but connect each site to the Internet also (hybrid network)

49. Virtual Private Network

50. VPN Addressing