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@Yuan Xue A special acknowledge goes to J.F Kurose and K.W. Ross Some of the slides used in this lecture are adapted from their.

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Presentation on theme: "@Yuan Xue A special acknowledge goes to J.F Kurose and K.W. Ross Some of the slides used in this lecture are adapted from their."— Presentation transcript:

1 @Yuan Xue (yuan.xue@vanderbilt.edu) A special acknowledge goes to J.F Kurose and K.W. Ross Some of the slides used in this lecture are adapted from their original slides that accompany the book “Computer Networking, A Top-Down Approach” All material copyright 1996-2009 J.F Kurose and K.W. Ross, All Rights Reserved CS 283Computer Networks Spring 2013 Instructor: Yuan Xue

2 @Yuan Xue (yuan.xue@vanderbilt.edu) Chapter 4: Network Layer Outline Overview and Service Model 4. 1 Introduction 4.2 Virtual circuit and datagram networks Principle 4.5 Routing algorithms  Link state  Distance Vector Practice (in the context of Internet) 4.4 IP: Internet Protocol  IP packet format/Fragmentation  IP address/DHCP/NAT Routing Protocol  4.5.3 Hierarchical Routing / Addressing  4.6 Routing protocols in Internet 4.6.1 Intra-AS Routing: RIP/OSPF 4.6.3 Inter-AS Routing: BGP Forwarding and Implementation 4.3 What’s inside a router Broadcast and multicast routing ICMP Special IP addresses IPv6

3 @Yuan Xue (yuan.xue@vanderbilt.edu) Broadcast and Multicast Broadcast Deliver packets from source to all other nodes in the network Multicast Deliver packets from source to only a subset of network nodes

4 @Yuan Xue (yuan.xue@vanderbilt.edu) R1 R2 R3R4 source duplication R1 R2 R3R4 in-network duplication duplicate creation/transmission duplicate Broadcast Routing Deliver packets from source to all other nodes source duplication is inefficient:  source duplication how does source determine recipient addresses?

5 @Yuan Xue (yuan.xue@vanderbilt.edu) In-network duplication flooding: when node receives broadcast packet, sends copy to all neighbors problems: cycles & broadcast storm controlled flooding: node only broqdcqsts pkt if it hasn’t broadcst same packet before Sequence-number-controlled flooding: node keeps track of packet ids [src_node, sequence_num] already broadacsted. If the packet was received and forwarded before, it will get dropped. Reverse path forwarding (RPF): node only forwards a packet if it arrived on the shortest path between the node and the source spanning tree broadcast No redundant packets received by any node

6 @Yuan Xue (yuan.xue@vanderbilt.edu) Reverse Path Forwarding: example R1 R2 R3 R4 R5 R6 R7 datagram will be forwarded LEGEND S: source datagram will not be forwarded beyond receiving router Different from sequence-number-controlled flooding: In sequence-number-controlled flooding, the packet forwarding does not follow a specific path. It rather depends on the timing of p. In this example, if the packet from R5 is received at R6 earlier than the packet from R2, R6 will forward the packet to R2. Similar to sequence-number-controlled flooding For each packet, each node in the network will only perform one forwarding operation to all its neighbors (other than the one from which the packet is received.).

7 @Yuan Xue (yuan.xue@vanderbilt.edu) A B G D E c F A B G D E c F (a) Broadcast initiated at A (b) Broadcast initiated at D Spanning Tree Construct a spanning tree Spanning tree: a tree that contains every node in the graph Nodes forward copies only along spanning tree

8 @Yuan Xue (yuan.xue@vanderbilt.edu) IP multicast Multicast at network layer is called IP multicast It has two components Group management  IGMP Multicast routing  DVMRP, PIM, MSDP IP multicast vs. Application layer multicast

9 @Yuan Xue (yuan.xue@vanderbilt.edu) Multicast Address and Group Management Deliver packets from source to only a subset of network nodes  how to address them? List of IP addresses of all receivers  too much overhead in the packet header  The sender also needs to maintain this list (node join/leave) Internet solution  Multicast address  a class D IP address  The group of receivers associated with a class D address is referred to as a multicast group How to manage this group? Internet Group Management Protocol  A host informs its attached router to join a specific multicast group  Three message types: membership_query, membership_report, leave_group  Using soft state

10 @Yuan Xue (yuan.xue@vanderbilt.edu) Multicast Routing: Problem Statement Goal: find a tree (or trees) connecting routers having local mcast group members tree: not all paths between routers are used source-based: different tree from each sender to rcvrs shared-tree: same tree used by all group members Shared tree Source-based trees

11 @Yuan Xue (yuan.xue@vanderbilt.edu) Approaches for building mcast trees Approaches: source-based tree: one tree per source shortest path trees reverse path forwarding Pruning group-shared tree: group uses one tree minimal spanning (all nodes on the tree are part of the multicast group) or Steiner (there are nodes on the tree which are not part of the multicast group) tree center-based trees we first look at basic approaches, then specific protocols adopting these approaches

12 @Yuan Xue (yuan.xue@vanderbilt.edu) 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 R7 2 1 6 3 4 5 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

13 @Yuan Xue (yuan.xue@vanderbilt.edu) 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:

14 @Yuan Xue (yuan.xue@vanderbilt.edu) 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 beyond receiving router

15 @Yuan Xue (yuan.xue@vanderbilt.edu) 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

16 @Yuan Xue (yuan.xue@vanderbilt.edu) 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

17 @Yuan Xue (yuan.xue@vanderbilt.edu) 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

18 @Yuan Xue (yuan.xue@vanderbilt.edu) 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 2 1 3 1

19 @Yuan Xue (yuan.xue@vanderbilt.edu) Internet Multicasting Routing DVMRP: distance vector multicast routing protocol, RFC1075 Source-based tree with reverse path forwarding and pruning PIM: Protocol-Independent Multicast routing protocol, RFC3973 Dense mode: flood-and-prune reverse path forwarding techniques (similar to DVMRP) Sparse mode: rendezvous point MSDP: Multicast Source Discovery Protocol Reality: IP multicast has yet to take off in a big way Application-level multicast In UK, BBC Multicast

20 @Yuan Xue (yuan.xue@vanderbilt.edu) 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 part of the group: send upstream prune msgs

21 @Yuan Xue (yuan.xue@vanderbilt.edu) 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

22 @Yuan Xue (yuan.xue@vanderbilt.edu) 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

23 @Yuan Xue (yuan.xue@vanderbilt.edu) 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

24 @Yuan Xue (yuan.xue@vanderbilt.edu) 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

25 @Yuan Xue (yuan.xue@vanderbilt.edu) 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

26 @Yuan Xue (yuan.xue@vanderbilt.edu) 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

27 @Yuan Xue (yuan.xue@vanderbilt.edu) ICMP: Internet Control Message Protocol used by hosts & routers to communicate network-level information error reporting: unreachable host, network, port, protocol echo request/reply (used by ping) network-layer “above” IP: ICMP msgs carried in IP datagrams ICMP message: type, code plus first 8 bytes of IP datagram causing error Type Code description 0 0 echo reply (ping) 3 0 dest. network unreachable 3 1 dest host unreachable 3 2 dest protocol unreachable 3 3 dest port unreachable 3 6 dest network unknown 3 7 dest host unknown 4 0 source quench (congestion control - not used) 8 0 echo request (ping) 9 0 route advertisement 10 0 router discovery 11 0 TTL expired 12 0 bad IP header

28 @Yuan Xue (yuan.xue@vanderbilt.edu) Traceroute and ICMP Source sends series of UDP segments to dest first has TTL =1 second has TTL=2, etc. When nth datagram arrives to nth router: router discards datagram and sends to source an ICMP message (type 11, code 0) ICMP message includes name of router & IP address when ICMP message arrives, source calculates RTT traceroute does this 3 times Stopping criterion UDP segment eventually arrives at destination host destination returns ICMP “port unreachable” packet (type 3, code 3) when source gets this ICMP, stops. Refer to http://en.wikipedia.org/wiki/Traceroute

29 @Yuan Xue (yuan.xue@vanderbilt.edu) Ping http://en.wikipedia.org/wiki/Ping Ping operates by sending Internet Control Message Protocol (ICMP) echo request packets to the target host and waiting for an ICMP response.Internet Control Message Protocolpackets In the process it measures the time from transmission to reception (round-trip time) and records any packet loss.packet loss The results of the test are printed in form of a statistical summary of the response packets received, including the minimum, maximum, and the mean round-trip times, and sometimes the standard deviation of the mean.meanstandard deviation

30 @Yuan Xue (yuan.xue@vanderbilt.edu) Special IP addresses Local broadcast address 255.255.255.255  all nodes within the local area network For more information: http://en.wikipedia.org/wiki/Broadcast_address http://en.wikipedia.org/wiki/Broadcast_address The node itself 127.0.0.1  Local host (loopback network interface) For more information http://en.wikipedia.org/wiki/Localhost

31 @Yuan Xue (yuan.xue@vanderbilt.edu) IPv6 Initial motivation: 32-bit address space soon to be completely allocated. Additional motivation: header format helps speed processing/forwarding header changes to facilitate QoS IPv6 datagram format: fixed-length 40 byte header no fragmentation allowed IPv6 addresses consist of eight groups of four hexadecimal digits separated by colons,hexadecimal for example 2001:0db8:85a3:0042:1000:8a2e:0370:7334.

32 @Yuan Xue (yuan.xue@vanderbilt.edu) IPv6 Header (Cont) Priority: identify priority among datagrams in flow Flow Label: identify datagrams in same “flow.” (concept of“flow” not well defined). Next header: identify upper layer protocol for data data destination address (128 bits) source address (128 bits) payload len next hdr hop limit flow label pri ver 32 bits

33 @Yuan Xue (yuan.xue@vanderbilt.edu) Other Changes from IPv4 Checksum: removed entirely to reduce processing time at each hop Options: allowed, but outside of header, indicated by “Next Header” field ICMPv6: new version of ICMP additional message types, e.g. “Packet Too Big” multicast group management functions

34 @Yuan Xue (yuan.xue@vanderbilt.edu) Transition From IPv4 To IPv6 Not all routers can be upgraded simultaneous no “flag days” How will the network operate with mixed IPv4 and IPv6 routers? Dual-stack : side-by-side implementation of IPv4 and IPv6- both protocols run on the same network infrastructure. Tunneling: IPv6 carried as payload in IPv4 datagram among IPv4 routers

35 @Yuan Xue (yuan.xue@vanderbilt.edu) Tunneling A B E F IPv6 tunnel Logical view: Physical view: A B E F IPv6 IPv4

36 @Yuan Xue (yuan.xue@vanderbilt.edu) Tunneling A B E F IPv6 tunnel Logical view: Physical view: A B E F IPv6 C D IPv4 Flow: X Src: A Dest: F data Flow: X Src: A Dest: F data Flow: X Src: A Dest: F data Src:B Dest: E Flow: X Src: A Dest: F data Src:B Dest: E A-to-B: IPv6 E-to-F: IPv6 B-to-C: IPv6 inside IPv4 B-to-C: IPv6 inside IPv4

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