Chapter 4 Network Layer Computer Networking: A Top Down Approach Featuring the Internet, 2 nd edition. Jim Kurose, Keith Ross Addison-Wesley, July A note on the use of these ppt slides: We’re making these slides freely available to all (faculty, students, readers). They’re in powerpoint form so you can add, modify, and delete slides (including this one) and slide content to suit your needs. They obviously represent a lot of work on our part. In return for use, we only ask the following:  If you use these slides (e.g., in a class) in substantially unaltered form, that you mention their source (after all, we’d like people to use our book!)  If you post any slides in substantially unaltered form on a www site, that you note that they are adapted from (or perhaps identical to) our slides, and note our copyright of this material. Thanks and enjoy! JFK/KWR All material copyright J.F Kurose and K.W. Ross, All Rights Reserved Material modified by MLH, AK, AHR 3/03 *Last 3 slides from: Presentations/wirelesstech3.ppt

Chapter 4 roadmap 4.1 Introduction and Network Service Models 4.2 Routing Principles 4.3 Hierarchical Routing 4.4 The Internet (IP) Protocol 4.5 Routing in the Internet 4.6 What’s Inside a Router? 4.7 IPv6 4.8 Multicast Routing 4.9 Mobility

Multicast: one sender to many receivers Multicast: act of sending datagram to multiple receivers with single “transmit” operation –analogy: one teacher to many students Question: how to achieve multicast Multicast via unicast source sends N unicast datagrams, one addressed to each of N receivers multicast receiver (red) not a multicast receiver (red) routers forward unicast datagrams

Multicast: one sender to many receivers Multicast: act of sending datagram to multiple receivers with single “transmit” operation –analogy: one teacher to many students Question: how to achieve multicast Network multicast Router actively participate in multicast, making copies of packets as needed and forwarding towards multicast receivers Multicast routers (red) duplicate and forward multicast datagrams

Multicast: one sender to many receivers Multicast: act of sending datagram to multiple receivers with single “transmit” operation –analogy: one teacher to many students Question: how to achieve multicast Application-layer multicast end systems involved in multicast copy and forward unicast datagrams among themselves

Internet Multicast Service Model multicast group concept: use of indirection –hosts addresses IP datagram to multicast group –routers forward multicast datagrams to hosts that have “joined” that multicast group multicast group

Multicast groups  class D Internet addresses reserved for multicast:  host group semantics: oanyone can “join” (receive) multicast group oanyone can send to multicast group ono network-layer identification to hosts of members  needed: infrastructure to deliver mcast-addressed datagrams to all hosts that have joined that multicast group

Joining a mcast group: two-step process local: host informs local mcast router of desire to join group: IGMP (Internet Group Management Protocol) wide area: local router interacts with other routers to receive mcast datagram flow –many protocols (e.g., DVMRP, MOSPF, PIM) IGMP wide-area multicast routing

IGMP: Internet Group Management Protocol host: sends IGMP report when application joins mcast group –IP_ADD_MEMBERSHIP socket option –host need not explicitly “unjoin” group when leaving router: sends IGMP query at regular intervals –host belonging to a mcast group must reply to query query report

IGMP IGMP version 1 router: Host Membership Query msg broadcast on LAN to all hosts host: Host Membership Report msg to indicate group membership –randomized delay before responding –implicit leave via no reply to Query RFC 1112 IGMP v2: additions include group-specific Query Leave Group msg –last host replying to Query can send explicit Leave Group msg –router performs group- specific query to see if any hosts left in group –RFC 2236 IGMP v3: under development as Internet draft

Multicast Routing: Problem Statement Goal: find a tree (or trees) connecting routers having local mcast group members –tree: not all paths between routers used –source-based: different tree from each sender to rcvrs –shared-tree: same tree used by all group members Shared tree Source-based trees

Approaches for building mcast trees Approaches: source-based tree: one tree per source –shortest path trees –reverse path forwarding group-shared tree: group uses one tree –minimal spanning (Steiner) –center-based trees …we first look at basic approaches, then specific protocols adopting these approaches

Shared-Tree: Steiner Tree Steiner Tree: minimum cost tree connecting all routers with attached group members problem is NP-complete excellent heuristics exists not used in practice: –computational complexity –information about entire network needed –monolithic: rerun whenever a router needs to join/leave

Center-based trees single delivery tree shared by all one router identified as “center” of tree to join: –edge router sends unicast join-msg addressed to center router –join-msg “processed” by intermediate routers and forwarded towards center –join-msg either hits existing tree branch for this center, or arrives at center –path taken by join-msg becomes new branch of tree for this router

Center-based trees: an example Suppose R6 chosen as center: R1 R2 R3 R4 R5 R6 R7 router with attached group member router with no attached group member path order in which join messages generated LEGEND

Multicast Routing: Problem Statement Goal: find a tree (or trees) connecting routers having local mcast group members –tree: not all paths between routers used –source-based: different tree from each sender to rcvrs –shared-tree: same tree used by all group members Shared tree Source-based trees

Approaches for building mcast trees Approaches: source-based tree: one tree per source –shortest path trees –reverse path forwarding group-shared tree: group uses one tree –minimal spanning (Steiner) –center-based trees …we first look at basic approaches, then specific protocols adopting these approaches

Shortest Path Tree mcast forwarding tree: tree of shortest path routes from source to all receivers –Dijkstra’s algorithm R1 R2 R3 R4 R5 R6 R i router with attached group member router with no attached group member link used for forwarding, i indicates order link added by algorithm LEGEND S: source

Reverse Path Forwarding if (mcast datagram received on incoming link on shortest path back to center) then flood datagram onto all outgoing links else ignore datagram  rely on router’s knowledge of unicast shortest path from it to sender  each router has simple forwarding behavior:

Reverse Path Forwarding: example result is a source-specific reverse SPT –may be a bad choice with asymmetric links R1 R2 R3 R4 R5 R6 R7 router with attached group member router with no attached group member datagram will be forwarded LEGEND S: source datagram will not be forwarded

Reverse Path Forwarding: pruning forwarding tree contains subtrees with no mcast group members –no need to forward datagrams down subtree –“prune” msgs sent upstream by router with no downstream group members R1 R2 R3 R4 R5 R6 R7 router with attached group member router with no attached group member prune message LEGEND S: source links with multicast forwarding P P P

Internet Multicasting Routing: DVMRP DVMRP: distance vector multicast routing protocol, RFC1075 flood and prune: reverse path forwarding, source- based tree –RPF tree based on DVMRP’s own routing tables constructed by communicating DVMRP routers –no assumptions about underlying unicast –initial datagram to mcast group flooded everywhere via RPF –routers not wanting group: send upstream prune msgs

DVMRP: continued… soft state: DVMRP router periodically (1 min.) “forgets” branches are pruned: –mcast data again flows down unpruned branch –downstream router: reprune or else continue to receive data routers can quickly regraft to tree –following IGMP join at leaf odds and ends –commonly implemented in commercial routers –Mbone routing done using DVMRP

Tunneling Q: How to connect “islands” of multicast routers in a “sea” of unicast routers?  mcast datagram encapsulated inside “normal” (non-multicast- addressed) datagram  normal IP datagram sent thru “tunnel” via regular IP unicast to receiving mcast router  receiving mcast router unencapsulates to get mcast datagram physical topology logical topology

PIM: Protocol Independent Multicast not dependent on any specific underlying unicast routing algorithm (works with all) two different multicast distribution scenarios : Dense:  group members densely packed, in “close” proximity.  bandwidth more plentiful Sparse:  # networks with group members small wrt # interconnected networks  group members “widely dispersed”  bandwidth not plentiful

Consequences of Sparse-Dense Dichotomy: Dense group membership by routers assumed until routers explicitly prune data-driven construction on mcast tree (e.g., RPF) bandwidth and non- group-router processing profligate Sparse : no membership until routers explicitly join receiver- driven construction of mcast tree (e.g., center-based) bandwidth and non-group- router processing conservative

PIM- Dense Mode flood-and-prune RPF, similar to DVMRP but  underlying unicast protocol provides RPF info for incoming datagram  less complicated (less efficient) downstream flood than DVMRP reduces reliance on underlying routing algorithm  has protocol mechanism for router to detect it is a leaf-node router

PIM - Sparse Mode center-based approach router sends join msg to rendezvous point (RP) –intermediate routers update state and forward join after joining via RP, router can switch to source- specific tree –increased performance: less concentration, shorter paths R1 R2 R3 R4 R5 R6 R7 join all data multicast from rendezvous point rendezvous point

PIM - Sparse Mode sender(s): unicast data to RP, which distributes down RP-rooted tree RP can extend mcast tree upstream to source RP can send stop msg if no attached receivers –“no one is listening!” R1 R2 R3 R4 R5 R6 R7 join all data multicast from rendezvous point rendezvous point

Chapter 4 roadmap 4.1 Introduction and Network Service Models 4.2 Routing Principles 4.3 Hierarchical Routing 4.4 The Internet (IP) Protocol 4.5 Routing in the Internet 4.6 What’s Inside a Router? 4.7 IPv6 4.8 Multicast Routing 4.9 Mobility But first! Merrie’s Spring Break!Merrie’s Spring Break!

What is mobility? spectrum of mobility, from the network perspective: no mobility high mobility mobile user, using same access point mobile user, passing through multiple access point while maintaining ongoing connections ( like cell phone) mobile user, connecting/ disconnecting from network using DHCP.

Mobility: Vocabulary home network: permanent “home” of mobile (e.g., /24) Permanent address: address in home network, can always be used to reach mobile e.g., wide area network Correspondent; IM, , web browsing McConnell Network home agent: entity that will perform mobility functions on behalf of mobile, when mobile is remote; not necessarily a router. Can be a server/host on network.

Mobility: More Vocabulary Care-of-address: address in visited network. (e.g., 79, ) wide area network visited network: network in which mobile currently resides (e.g., /24) Permanent address: remains constant ( e.g., ) foreign agent: entity in visited network that performs mobility functions on behalf of mobile. correspondent: wants to communicate with mobile McConnell Network Autobahn Network

How do you contact a mobile friend: search all phone books? call her parents? expect her to let you know where he/she is? I wonder where Alice moved to? Consider a friend frequently changing addresses, how do you find her? I hope I hope Bob doesn’t find me! Question: How do you think a correspondent contacts a mobile user?

Mobility: approaches Let routing handle it: routers advertise permanent address of mobile-nodes-in-residence via usual routing table exchange. –routing tables indicate where each mobile located –no changes to end-systems –Routers, potentially, would have to maintain forwarding table entries for millions of mobile nodes Let end-systems handle it: –indirect routing: communication from correspondent to mobile goes through home agent, then forwarded to remote –direct routing: correspondent gets foreign address of mobile, sends directly to mobile

Mobility: approaches Let routing handle it: routers advertise permanent address of mobile-nodes-in-residence via usual routing table exchange. –routing tables indicate where each mobile located –no changes to end-systems –Routers, potentially, would have to maintain forwarding table entries for millions of mobile nodes let end-systems handle it: –indirect routing: communication from correspondent to mobile goes through home agent, then forwarded to remote –direct routing: correspondent gets foreign address of mobile, sends directly to mobile not scalable to millions of mobiles

Mobility: registration End result: Foreign agent knows about mobile Home agent knows location of mobile wide area network McConnell Network Autobahn Network 1 Merrie contacts foreign agent on entering Autobahn network 2 Autobahn agent contacts McConnell agent: “This Merrie is a resident in my network”

Mobility via Indirect Routing wide area network McConnell Network Autobahn Network correspondent addresses packets using McConnell (home) address of mobile McConnell agent intercepts packets, forwards to foreign agent, using the COA Autobahn agent receives packets, forwards to Merrie Merrie replies directly to correspondent

Forwarding datagrams to remote mobile McConnell Permanent address: Merrie’s Care-of address: dest: packet sent by correspondent dest: dest: packet sent by home agent to foreign agent: a packet within a packet dest: foreign-agent-to-mobile packet

Leaving the Autobahn Autobahn Network Bye-bye Autobahn agent! On to France! Upon leaving the Autobahn, Merrie will deregister with the Autobahn agent. The foreign agent does not necessarily deregister Merrie’s Care of Address. When Merrie applies for a new COA, the old one is dropped as the Autobahn agent no longer receives encapsulations from McConnell network. Essentially, a time out process. As long as time between Autobahn and French Network is short, very few datagrams are lost; therefore a continuous connection. Those datagrams lost will be resent via transport protocols. French Network

Indirect Routing: comments Mobile uses two addresses: –permanent address: used by correspondent (hence mobile location is transparent to correspondent) –care-of-address: used by home agent to forward datagrams to mobile Routing is based on tunneling Question: When is this inefficient? Triangle Routing Problem: –inefficient when correspondent and mobile are in same network Other problems: Delay Burdens networks

Mobility via Direct Routing wide area network McConnell Network Autobahn Network correspondent requests, receives Merrie’s COA correspondent forwards to Autobahn agent Autobahn agent receives packets, forwards to Merrie Merrie replies directly to correspondent 3

Mobility via Direct Routing: comments overcome triangle routing problem non-transparent to correspondent: correspondent must get care-of-address from home agent What happens if mobile changes networks? Need protocol to notify correspondent of changing COA Chaining: Autobahn forwards to France, forwards to England, forwards... Etc What about security? This approach not considered secure by the IETF (Internet Engineering Task Force)

Mobile IP RFC has many features we’ve seen: –home agents, foreign agents, foreign-agent registration, care-of-addresses, encapsulation (packet-within-a- packet) three components to standard: –agent discovery –registration with home agent –indirect routing of datagrams

Mobile IP: agent discovery agent advertisement: foreign/home agents advertise service by broadcasting ICMP messages (typefield = 9) R bit: registration required H,F bits: home and/or foreign agent

Functions of Agent Advertisement Allow for the detection of mobility agents Let the mobile node (MN) know whether the agent is a host or foreign agent List one or more available care-of addresses Inform the MN about special features provided by FA –Example: Alternative encapsulation techniques MN compares the network portion of the agent’s IP address with the network portion of its home address. If the network portion do not match, then the MN is on a foreign network.

Mobile IP: registration example

Mobile IP: Registration The registration process involves 4 steps: –The MN requests the fowarding service by sending a registration request to the foreign agent that the mobile node wants to use. –The FA relays this request to the mobile node’s home agent. –The HA either accepts or denies the request and sends a registration reply to the FA. –The FA relays this reply to the MN.

Mobile IP: Registration Registration fields include: –Lifetime: The number of seconds before the registration is considered expired. A value of 0 is a request for deregistration. –Home address: The home IP address of the mobile node. –Home agent: The IP address of the mobile node’s home agent. –Care of Address: The home agent should forward IP datagrams that it receives with MN’s home address to this destination address. –Identification: Generated by MN; used for matching registration requests to registration replies (for security). Should be unique for each registration request.

Mobile IP: Registration The registration reply message includes the following fields: –Home address: The home IP address of the mobile node. –Home agent: The IP address of the MN’s home agent. –Lifetime: Number of seconds before registration expires –Identification: Used for matching registration requests to registration replies.

Ad-hoc Networks RFC No routing infrastructure: two users establish a network between themselves (think peer-to-peer) Problems: –Unreliable links can be jammed(Denial of Service attacks)/someone can listen in –What if a node signs off (to save battery power)? –How to authenticate? –How to control access? –Need way to multihop

The Future?