Address and Switching in the Link Layer

Transcription

1 Address and Switching in the Link Layer Brad Karp (slides contributed by Kyle Jamieson, Scott Shenker, and adapted from Kurose and Ross) UCL Computer Science CS 05/GZ01 18 th November 014 1

2 The link layer: Func0onality IP datagram Link- layer protocol Sending host frame frame Receiving host Enables the exchange of messages (frames) between end hosts Func7onality: 1. Framing: Determine start and end of bits and frames. Error control: Detect and/or correct errors. Reliable delivery: Deliver frames exactly once 4. Medium access control: Control hosts access to a shared medium, if applicable (medium access control)

3 Today We finish the func7onality of the link layer, and 7e it in to IP 1. Addressing. Repeaters, hubs, and switches. Bootstrapping a host

4 Comparing addressing schemes Network layer address (IP address) Func7on: move datagram to des7na7on network - bit address, doned quad nota7on a.b.c.d where each component is an eight- bit unsigned integer Hierarchical address space Link layer address (MAC address, Ethernet address): Func7on: move frame from one point to another point on the same network Unique 48- bit address (in most LANs) Burned in NIC ROM, also some7mes soyware senable Usually a flat address space 4

5 Ethernet addresses 48- bit source and des7na7on addresses Receiver s link layer passes frame up to network- level protocol: If des7na7on address matches the adaptor s Or the des7na7on address is the broadcast address (ff:ff:ff:ff:ff:ff) Or the card is in a mode of opera7on that receives all frames (promiscuous mode) Addresses are globally unique Assigned by NIC vendors (top three bytes specify vendor) 5

6 Today We finish the func7onality of the link layer, and 7e it in to IP 1. Addressing. Repeaters, hubs, and switches Comparison Self- learning switches The Spanning Tree Protocol. Bootstrapping a host 6

7 Message, segment, datagram, and frame host host HTTP HTTP TCP TCP IP IP IP IP Ethernet interface Ethernet interface SONET interface SONET interface Ethernet interface Ethernet interface 7

8 Message, segment, datagram, and frame HTTP host HTTP message host HTTP TCP TCP IP IP IP IP Ethernet interface Ethernet interface SONET interface SONET interface Ethernet interface Ethernet interface 8

9 Message, segment, datagram, and frame HTTP host HTTP message host HTTP TCP TCP segment TCP IP IP IP IP Ethernet interface Ethernet interface SONET interface SONET interface Ethernet interface Ethernet interface 9

10 Message, segment, datagram, and frame HTTP host HTTP message host HTTP TCP TCP segment TCP IP IP datagram IP IP datagram IP IP datagram IP Ethernet interface Ethernet interface SONET interface SONET interface Ethernet interface Ethernet interface 10

11 Message, segment, datagram, and frame HTTP host HTTP message host HTTP TCP TCP segment TCP IP IP datagram IP IP datagram IP IP datagram IP Ethernet interface Ethernet interface SONET interface SONET interface Ethernet interface Ethernet interface Ethernet frame SONET frame Ethernet frame 11

12 Different devices switch on different informa0on Routers: forward IP datagrams based on network- layer addresses in the IP header H H H H data Network Link Physical Router IP datagram H H data Switches (Bridges): forward link- layer frames based on link- layer addresses in the link- layer header H H H H Repeaters/Hubs: rebroadcast all bits in the physical- layer frame data Link Physical Switch Link layer frame H H H data Hub Physical- layer frame H H H H data Physical H H H H data 1

13 Physical Layer: Repeaters Distance limita7on in local- area networks Electrical signal becomes weaker as it travels Imposes a limit on the length of a LAN In addi7on to limit imposed by collision detec7on Repeaters join LANs together Analog electronic device Con7nuously monitors electrical signals on each LAN Transmits an amplified copy Repeater 1

14 Physical Layer: Hubs Joins mul7ple input lines electrically Do not necessarily amplify the signal Very similar to repeaters Also operate at the physical layer hub hub hub hub 14

15 Limita0ons of repeaters and hubs One large place where packets collide (collision domain), since every bit is sent everywhere So, aggregate throughput is limited e.g., three departments each get 10 Mbps independently and then if connect via a hub must share 10 Mbps Cannot support mul7ple LAN technologies Repeaters/hubs do not buffer or interpret frames So, can t interconnect between different rates or formats e.g., no mixing 100 Mbit/s Ethernet and Gigabit Ethernet Limita7ons on maximum nodes and distances Does not circumvent limita7ons of the shared medium e.g., s7ll cannot go beyond 500 m in commercial Ethernet 15

16 Link Layer: Switches Switches also connect two or more LANs at the link layer Extracts des7na7on address from the frame Looks up the des7na7on in a table Forwards the frame to the appropriate LAN segment Or point- to- point link, for higher- speed Ethernet Each port is its own collision domain (if not a link to one host) Switch collision domain hub Extended LAN 16

17 Switches and concurrent communica0on Host A can talk to C, while B talks to D B A switch C If host has (dedicated) point- to- point link to switch: Full duplex: each connec7on can send in both direc7ons Completely avoids collisions! No need for carrier sense, collision detec7on, and so on! Change in medium access control, but same framing D 17

18 Switches: Advantages over hubs and repeaters Only forwards frames as needed Filters frames to avoid unnecessary load on segments Sends frames only to segments that need to see them Extends the geographic span of the network Separate collision domains allow longer distances Improves privacy by limi7ng scope of frames Hosts can snoop the traffic traversing their segment but not all the rest of the traffic Applies CSMA/CD in segment (not whole net) Smaller collision domain Joins segments using different technologies 18

19 Disadvantages over hubs and repeaters Higher cost More complicated devices that cost more money Delay in forwarding frames Bridge/switch must receive and parse the frame and perform a look- up to decide where to forward Introduces store- and- forward delay Can ameliorate using cut- through switching Start forwarding ayer only header received Need to learn where to forward frames Bridge/switch needs to construct a forwarding table Ideally, without interven7on from network administrators Solu7on: Self- learning algorithm 19

20 Mo0va0on for self learning Benefit if switch forwards frame only on segment(s) that need it Allows concurrent use of other links Switch forwarding table Maps des0na0on link- layer address to outgoing interface Goal: construct the switch table automa7cally B A switch C D 0

21 Self learning algorithm: Building the table When a frame (e.g., from A to B) arrives at the switch: Inspect the source link- layer address Associate that address with the incoming switch port Store the mapping in the switch table Use ;me- to- live field to eventually forget the mapping an amount of 7me later equal to its value This is an example of sox state A A " B data 1 switch 4 B Switch forwarding table: Address Port Time- to- live A 1 minutes C D Switch just learned how to reach A. 1

22 Self learning algorithm: Handling misses When frame arrives with unfamiliar des7na7on (e.g., B) Forward the frame out all ports except for the one on which the frame arrived This is called flooding Hopefully, this case won t happen very oyen When e.g. B replies, switch will learn that node, too Switch forwarding table: B Address Port Time- to- live A 1 minutes A " B data 1 A switch 4 C D

23 Self- learning algorithm When switch receives a frame: index into the forwarding table using link- layer des0na0on address if entry found for des7na7on { if dest on segment from which frame arrived then drop frame else forward frame on interface indicated } else flood the frame Forward on all ports except the port on which the frame arrived

24 Self- learning algorithm When switch receives a frame: index into the forwarding table using link- layer des0na0on address if entry found for des7na7on { if dest on segment from which frame arrived then drop frame else forward frame on interface indicated } else flood the frame Forward on all ports except the port on which the frame arrived Problems? 4

25 Flooding can lead to loops Switches some7mes need to flood frames: Upon receiving a frame with an unfamiliar des7na7on Upon receiving a frame sent to the broadcast address Flooding can lead to forwarding loops e.g., if the network contains a cycle of switches Either accidentally, or by design for higher reliability This is catastrophic, for two reasons: 1. Unlike IP, layer has no way of preven7ng frame looping. Ethernet duplicates frames, leading to an exponen7al increase, quickly crashing the extended LAN (this is called a broadcast storm) 5

26 Flooding can lead to loops Switches some7mes need to flood frames: Upon receiving a frame with an unfamiliar des7na7on Upon receiving a frame sent to the broadcast address Flooding can lead to forwarding loops e.g., if the network contains a cycle of switches Either accidentally, or by design for higher reliability How can we revise the bridge learning This is catastrophic, algorithm for two to reasons: avoid broadcast storms? 1. Unlike IP, layer has no way of preven7ng frame looping. Ethernet duplicates frames, leading to an exponen7al increase, quickly crashing the extended LAN (this is called a broadcast storm) 6

27 The spanning tree protocol (STP) Early 1980s: Digital Equipment Corpora7on, a key Ethernet vendor, wanted to leverage the benefits of loops while avoiding broadcast storms Radia Perlman s idea: Switches agree on a loop- free and connected spanning tree Spanning tree: a sub- graph that touches all ver7ces but contains no cycles Graph with cycles Spanning tree has no cycles Once the spanning tree is formed: Switches use the switch learning algorithm to forward data frames over the tree links only 7

28 Spanning Tree Protocol (STP): Overview Users connect Ethernet switches and shared- medium Ethernet LANs together Arbitrarily, possibly crea7ng forwarding loops 4 Need a distributed algorithm so that: 1. Switches cooperate to build the spanning tree. Switches adapt automa7cally when failures occur 1 8

29 STP: Key ingredients of the algorithm Switches elect one root switch from which to build the tree Switch iden7fier = link- layer address on one port Switches block some ports from sending or receiving frames of Ethernet type IP (or other L data) To form tree, switches exchange configura;on messages (R, d, X): From switch X Proposing switch R (which is d hops away) as the root Configura7on messages are never blocked B Blocked ports B 1 4 Root switch 9

30 STP: Key ingredients of the algorithm Switches elect one root switch from which to build the tree Switch iden7fier = link- layer address on one port 4 Switches block some ports from sending or receiving frames of Ethernet type Let s IP (or begin other with L data) a simplified version of the full STP distributed algorithm To form tree, switches exchange configura;on messages (R, d, X): From switch X Proposing switch R (which is d hops away) as the root Configura7on messages are never blocked B Blocked ports 1 B Root switch 0

31 Simplified STP: State at each switch Each switch X keeps the following state: 1. Its view of who the root is Ini7ally, itself: X X Root id: X 1

32 Simplified STP: Startup and calcula0ng the root Note: Ini7ally, each switch X periodically sends (X, 0, X) from all its ports Root ID rule: Root ID r at switch X is the minimum of X and root IDs received at all ports Root id: 4 Root id: 4 Root id: 1

33 Simplified STP: Startup and calcula0ng the root Note: Ini7ally, each switch X periodically sends (X, 0, X) from all its ports Root ID rule: Root ID r at switch X is the minimum of X and root IDs received at all ports Switch sends (, 0, ); switch sets its root id to, switch 1 ignores Root id: # (, 0, ) 4 Root id: 4 Root id: 1

34 Simplified STP: Startup and calcula0ng the root Note: Ini7ally, each switch X periodically sends (X, 0, X) from all its ports Root ID rule: Root ID r at switch X is the minimum of X and root IDs received at all ports Switch 1 sends (1, 0, 1); switches and set their root ids to 1 4 Root id: 4 # (1, 0, 1) 1 4

35 Simplified STP: Startup and calcula0ng the root Note: Ini7ally, each switch X periodically sends (X, 0, X) from all its ports Root ID rule: Root ID r at switch X is the minimum of X and root IDs received at all ports Switch sends (, 0, ); switch 4 sets its root id to, others ignore 4 Root id: 1 5

36 STP: Startup and calcula0ng the root Note: Ini7ally, each switch X periodically sends (X, 0, X) from all its ports Root ID rule: Root ID r at switch X is the minimum of X and root IDs received at all ports Switch 4 sends (4, 0, 4); switch ignores 4 Root id: 1 6

37 STP: Startup and calcula0ng the root Note: Ini7ally, each switch X periodically sends (X, 0, X) from all its ports Root ID rule: Root ID r at switch X is the minimum of X and root IDs received at all ports 4 Root id: Switch Not yet 4 sends agreeing (4, 0, 4); on switch the iden0ty of the root: Root let s id: now 1 see ignores how switches propagate informa0on through the network 1 7

38 Simplified STP: State at each switch Each switch X keeps the following state: 1. Its view of who the root is Ini7ally, itself: X. Its configura0on message to send Ini7ally, announcing itself as root with zero distance to root: (X, 0, X) X Root id: X Msg: (X, 0, X) 8

39 Simplified STP: Calcula0ng the message Switch X finds its distance from the root (d): 1. If X thinks it is the root, d # 0. Otherwise, d # the minimum distance from messages received matching X s root id (call it r), plus one Configura0on message rule: Switch X sets its configura7on message to (r, d, X). If configura7on message changes, sends updated message immediately Root id: Msg: (, 0, ) 4 Root id: 4 Msg: (4, 0, 4) Root id: Msg: (, 0, ) 1 Msg: (1, 0, 1) 9

40 Simplified STP: Calcula0ng the message Switch X finds its distance from the root (d): 1. If X thinks it is the root, d # 0. Otherwise, d # the minimum distance from messages received matching X s root id (call it r), plus one Configura0on message rule: Switch X sets its configura7on message to (r, d, X). If configura7on message changes, sends updated message immediately Switch 1 sends (1, 0, 1), switches and update their root ids and msgs Msg: (1, 1, ) 1 Msg: (1, 0, 1) 4 Root id: Msg: (4, 0, 4) Msg: (1, 1, ) 40

41 Simplified STP: Calcula0ng the message Switch X finds its distance from the root (d): 1. If X thinks it is the root, d # 0. Otherwise, d # the minimum distance from messages received matching X s root id (call it r), plus one Configura0on message rule: Switch X sets its configura7on message to (r, d, X). If configura7on message changes, sends updated message immediately Switch sends (1, 1, ), switch 4 updates its root id and message Msg: (1, 1, ) 1 Msg: (1, 0, 1) 4 Msg: (1,, 4) Msg: (1, 1, ) 41

42 Simplified STP: Calcula0ng the message Switch X finds its distance from the root (d): 1. If X thinks it is the root, d # 0. Otherwise, d # the minimum distance from messages received matching X s root id (call it r), plus one 4 Msg: (1,, 4) Configura0on message rule: Now Switch all X switches sets its configura7on agree the root iden0fier. Root But id: 1 how do they message decide to (r, which d, X) ports to block to form the Msg: spanning (1, 1, ) tree? Msg: (1, 1, ) 1 Msg: (1, 0, 1) 4

43 STP: Port status All switches connected to a Ethernet LAN (or the two at the ends of a cable) agree on a single designated port 4 Msg: (1,, 4) Designated port: The port on the shortest path from the LAN or cable to the root is the designated port (D) D Msg: (1, 1, ) D The designated port forwards frames from the LAN to the root Only designated ports send configura7on messages D 1 Msg: (1, 0, 1) Msg: (1, 1, ) D 4

44 STP: Port status Root port: Each non- root switch notes which of its ports is on the shortest path to the root; this port is the root port (R) R D D R Msg: (1, 1, ) 1 Msg: (1, 0, 1) 4 Msg: (1,, 4) Msg: (1, 1, ) D D R 44

45 STP: Port status Blocked port: If neither designated nor root, a port is a blocked port (B), not forwarding data traffic. R 4 Msg: (1,, 4) R D D Msg: (1, 1, ) B 1 Msg: (1, 0, 1) B Msg: (1, 1, ) D D R 45

46 STP: State at each switch Each switch X keeps the following state: 1. Its view of who the root is Ini7ally, itself: X. Its configura0on message to send Ini7ally, announcing itself as root with zero distance to root: (X, 0, X) X Root id: X Msg: (X, 0, X) D: (X, 0, X). For each of X s ports: Whether designated (D), root (R), or blocking (B) data traffic Ini7ally, designated (D) Best configura7on message heard on that port Ini7ally, its own configura7on message (X, 0, X) 46

47 STP: Designated port rule At a switch, for each port p: Consider all configura7on messages received on port p and the configura7on message the switch would send If switch receives a bener configura7on message on a port p, don t send configura0on messages on port p Else, p is designated: send configura0on message on p Rule for comparing configura7on messages: (R 1, d 1, X 1 ) beger than (R, d, X ) if R 1 < R or (R 1 = R and d 1 < d ) or (R 1 = R and d 1 = d and X 1 < X ) 47

48 STP: Complete example All switches begin thinking they are root with all ports in the designated state D: (4,0,4) 4 Root id: 4 Msg: (4,0,4) D: (,0,) D: (,0,) Root id: Msg: (,0,) D: (,0,) D: (,0,) D: (,0,) Root id: Msg: (,0,) D: (,0,) D: (1,0,1) 1 Msg: (1,0,1) D: (1,0,1) 48

49 STP: Complete example All switches begin thinking they are root with all ports in the designated state D: (4,0,4) 4 Root id: 4 Msg: (4,0,4) Switch 1 sends (1,0,1), switches and update their root ids, ports, and msgs Switch breaks 7e between the two copies of (1,0,1) locally by numbering its ports Each switch s port remembers the best configura7on message seen so far R: (1,0,1) # (1, 0, 1) D: (1,0,1) D: (,0,) Msg: (1,1,) B: (1,0,1) 1 Msg: (1,0,1) D: (,0,) D: (,0,) Msg: (1,1,) 1 R: (1,0,1) D: (1,0,1) (1, 0, 1) " 49

50 STP: Complete example R: (1,0,1) (1,1,) " R: (1,1,) D: (,0,) Msg: (1,1,) B: (1,0,1) 1 4 Msg: (1,,4) D: (,0,) D: (1,1,) Msg: (1,1,) 1 R: (1,0,1) D: (1,0,1) D: (1,0,1) 1 Msg: (1,0,1) 50

51 STP: Complete example Switch sends (1,1,) from its designated ports, switch 4 updates its root id and message Switch, port remains designated because Switch s message (1,1,) is bener than (1,1,) Switch 1, port 1 remains designated because Switch 1 s message (1,0,1) is bener than (1,1,) R: (1,0,1) (1,1,) " R: (1,1,) D: (,0,) Msg: (1,1,) B: (1,0,1) 1 4 Msg: (1,,4) D: (,0,) D: (1,1,) Msg: (1,1,) 1 R: (1,0,1) D: (1,0,1) D: (1,0,1) 1 Msg: (1,0,1) 51

52 STP: Complete example Switch sends (1,1,) from port only Switch blocks its port since (1,1,) is bener than its message (1,1,) 1 R: (1,0,1) R: (1,1,) D: (,0,) Msg: (1,1,) B: (1,0,1) 4 Msg: (1,,4) B: (1,1,) D: (1,1,) Msg: (1,1,) 1 R: (1,0,1) D: (1,0,1) 1 Msg: (1,0,1) D: (1,0,1) 5

53 STP: Dynamics When do switches send configura<on messages? If you think you re the root, send periodically with parameter hello <me (two seconds recommended in 80.1d) Other switches send on all designated ports upon receiving root s message How does the algorithm adapt to topology changes? State table contains age field, which is updated con7nuously Aging rule: If age reaches a threshold max age (0 sec in 80.1d), discard that table entry and recalculate using all rules What happens if max age is too big? Too small? Recalculate when receive bener or newer configura7on message on port p (resul7ng in a table entry being overwrinen) 5

54 STP: Handling failures Suppose the Ethernet LAN fails R: (1,1,) 4 Msg: (1,,4) 1 R: (1,0,1) D: (,0,) Msg: (1,1,) B: (1,0,1) B: (1,1,) D: (1,1,) Msg: (1,1,) 1 R: (1,0,1) D: (1,0,1) 1 Msg: (1,0,1) D: (1,0,1) 54

55 STP: Handling failures Suppose the Ethernet LAN fails Switch : Stops hearing the root s messages through port 1, so it becomes designated Port becomes root Updates its own message 1 D: (1,,) R: (1,1,) D: (,0,) Msg: (1,,) B: (1,0,1) 4 Msg: (1,,4) R: (1,1,) D: (1,1,) Msg: (1,1,) 1 R: (1,0,1) D: (1,0,1) 1 Msg: (1,0,1) D: (1,0,1) 55

56 STP: Handling failures Suppose the Ethernet LAN fails Switch 4: Updates message heard on root port Updates its own message Switch : Stops hearing the root s messages through port, so it becomes designated 1 D: (1,,) R: (1,,) D: (,0,) Msg: (1,,) D: (1,1,) 4 Msg: (1,,4) R: (1,1,) D: (1,1,) Msg: (1,1,) 1 R: (1,0,1) D: (1,0,1) 1 Msg: (1,0,1) D: (1,0,1) 56

57 STP: Handling topology change Suppose we fix the LAN. Now we have created (temporary) forwarding loops R: (1,,) 4 Msg: (1,,4) This also happens when switches are powered- up 1 D: (1,,) D: (,0,) Msg: (1,,) D: (1,1,) R: (1,1,) D: (1,1,) Msg: (1,1,) 1 R: (1,0,1) D: (1,0,1) 1 Msg: (1,0,1) D: (1,0,1) 57

58 STP: Pre- forwarding port state Suppose any of the following apply to a port: 1. Transi7on from B à D. Any newly- connected port (detect Ethernet carrier). Any port on a freshly- powered switch The port then enters the pre- forwarding (PF) state, where: It sends configura7on messages and transi7ons to blocked and root states as if designated 1 PF: (1,,) R: (1,,) D: (,0,) Msg: (1,,) PF: (1,1,) 4 Msg: (1,,4) R: (1,1,) D: (1,1,) Msg: (1,1,) 1 R: (1,0,1) But it does not forward data frames, so can t create loops PF: (1,0,1) 1 Msg: (1,0,1) D: (1,0,1) 58

59 STP: Pre- forwarding port state Switches returns to old state R: (1,,) 4 Msg: (1,,4) 1 R: (1,0,1) D: (,0,) Msg: (1,1,) PF: (1,1,) R: (1,1,) D: (1,1,) Msg: (1,1,) 1 R: (1,0,1) PF: (1,0,1) 1 Msg: (1,0,1) D: (1,0,1) 59

60 STP: Pre- forwarding port state Switch returns to old state Switch returns to old state R: (1,,) 4 Msg: (1,,4) 1 R: (1,0,1) D: (,0,) Msg: (1,1,) B: (1,0,1) R: (1,1,) D: (1,1,) Msg: (1,1,) 1 R: (1,0,1) PF: (1,0,1) 1 Msg: (1,0,1) D: (1,0,1) 60

61 STP: Pre- forwarding port state Switch returns to old state Switch returns to old state Switch 4 returns to old state R: (1,1,) 4 Msg: (1,,4) Now switch 1, port 1 remains in the pre- forwarding state 1 R: (1,0,1) D: (,0,) Msg: (1,1,) B: (1,0,1) R: (1,1,) D: (1,1,) Msg: (1,1,) 1 R: (1,0,1) 1 1 PF: (1,0,1) D: (1,0,1) Msg: (1,0,1) 61

62 STP: Leaving the pre- forwarding state If s7ll in PF state ayer some number of seconds (forwarding delay parameter) then the port becomes designated (D) How long should forwarding delay be? Long enough for the en7re spanning tree to re- form, i.e.: Twice the maximum transit 7me across the extended LAN 0 seconds in 80.1d 1 R: (1,0,1) R: (1,1,) D: (,0,) Msg: (1,1,) B: (1,0,1) 4 Msg: (1,,4) B: (1,1,) D: (1,1,) Msg: (1,1,) 1 R: (1,0,1) 1 1 D: (1,0,1) D: (1,0,1) Msg: (1,0,1) 6

63 The evolu0on of Ethernet From the coaxial cable shared medium to switches Even more capacity, with simultaneous conversa7ons From Mbit/s experimental Ethernet to 100 Gbit/s recent standards From electrical signaling to op7cal Changed everything except the frame format Lesson: The right interface can accommodate many changes Implementa7on is hidden behind interface 6

64 Today We finish the func7onality of the link layer, and 7e it in to IP 1. Addressing. Repeaters, hubs, and switches. Bootstrapping a host Protocols for bootstrapping: DHCP, ARP Communica7ng over the same, different networks 64

65 What does a host need to know???? host host... DNS host host... DNS 1...0/ /

66 What does a host need to know? What IP address should the host use? What local DNS server to use? How to tell which des7na7ons are local? How to address them using the local network? How to send packets to remote des7na7ons???? host host... DNS host host... DNS 1...0/ /

67 Avoiding manual configura0on host host... DNS 1A- F- BB AD host host... DNS 1...0/ /4 67

68 Avoiding manual configura0on Dynamic Host Configura7on Protocol (DHCP) End host learns how to send packets Learn IP address, DNS servers, gateway, what s local Address Resolu7on Protocol (ARP) For local des7na7ons, learn the mapping between IP address and MAC address host host... DNS 1A- F- BB AD host host... DNS 1...0/ /4 68

69 Avoiding manual configura0on Dynamic Host Configura7on Protocol (DHCP) End host learns how to send packets Learn IP address, DNS servers, gateway, what s local Address Resolu7on Protocol (ARP) For local des7na7ons, learn the mapping between IP address and MAC address host host... DNS 1A- F- BB AD host host... DNS 1...0/ /4 69

70 Avoiding manual configura0on Dynamic Host Configura7on Protocol (DHCP) End host learns how to send packets Learn IP address, DNS servers, gateway, what s local Address Resolu7on Protocol (ARP) For local des7na7ons, learn the mapping between IP address and MAC address host host... DNS 1A- F- BB AD host host... DNS 1...0/ /4 70

71 Avoiding manual configura0on Dynamic Host Configura7on Protocol (DHCP) End host learns how to send packets Learn IP address, DNS servers, gateway, what s local Address Resolu7on Protocol (ARP) For local des7na7ons, learn the mapping between IP address and MAC address host host... DNS 1A- F- BB AD host host... DNS 1...0/ /4 71

72 Avoiding manual configura0on Dynamic Host Configura7on Protocol (DHCP) End host learns how to send packets Learn IP address, DNS servers, gateway, what s local Address Resolu7on Protocol (ARP) For local des7na7ons, learn the mapping between IP address and MAC address host host... DNS 1A- F- BB AD host host... DNS 1...0/ /4 7

73 Avoiding manual configura0on Dynamic Host Configura7on Protocol (DHCP) End host learns how to send packets Learn IP address, DNS servers, gateway, what s local Address Resolu7on Protocol (ARP) For local des7na7ons, learn the mapping between IP address and MAC address host host... DNS 1A- F- BB AD host host... DNS 1...0/ /4 7

74 Key ideas in both protocols Broadcas0ng: when in doubt, shout! Broadcast query to all hosts in the local- area- network when you don t know how to iden7fy the right one Caching: remember the past for a while Store the informa7on you learn to reduce overhead Remember your own address and other host s addresses SoX state: eventually forget the past Associate a ;me- to- live field with the informa7on On expiry either refresh or discard the informa7on This is key for robustness in the face of unpredictable change 74

75 Bootstrapping problem Host doesn t have an IP address yet So, host doesn t know what source address to use Host doesn t know whom to ask for an IP address So, host doesn t know what des0na0on address to use host host 75

76 DHCP discovery, from the client DHCP Solu0on: shout to discover a server that can help Client broadcasts a DHCP discover message (to the broadcast IP address, ) Two possibili7es: 1. Server on same subnet sends a reply offering an address. Or: a DHCP relay agent (configured only with DHCP server s IP address) unicasts to a DHCP server on another network DHCP server replies unicast to relay agent; agent forwards replies to the new host s network host host DHCP server DHCP server DHCP relay 76

77 Response from the DHCP server The server responds with a DHCP offer message Contains configura7on parameters (including proposed IP address, mask, gateway, DNS server) Contains lease ;me (dura7on the informa7on remains valid) Mul7ple servers may respond Mul7ple servers on the same subnetwork Each may respond with an offer Accep7ng one of the offers Client sends a DHCP request echoing the parameters The DHCP server responds with a DHCP ACK to confirm The other servers see they were not chosen They can then safely offer those same parameters to other clients 77

78 Dynamic Host Configura0on Protocol Arriving client DHCP discover (broadcast) DHCP offer (broadcast) DHCP ACK (broadcast) DHCP request (broadcast) DHCP server Why all the broadcasts? Discover broadcast: client doesn t know DHCP server s iden7ty Offer, ACK broadcast: client doesn t have an IP yet Request broadcast: so other servers can see 78

79 SoX state: Refresh or forget Why is a lease 7me necessary? Client can release the IP address (DHCP release) e.g., clean shutdown of the computer But, host might not release the address e.g., the host crashes e.g., buggy client soyware And you don t want the address to be allocated forever Performance trade- offs Short lease 7me: returns inac7ve addresses quickly Long lease 7me: avoids overhead of frequent renewals & lessens frequency of lease being denied 79

80 So, now the host knows things! IP address! Mask! Gateway! DNS server And can send packets to other IP addresses But: how to use the local network to do this? 80

81 Figuring out where to send locally Two cases: 1. Des7na7on is on the local network: need to address it directly. Des7na7on is not local (remote): need to figure out the first hop on the local network Determining if it s local: use the netmask e.g., bitwise- AND the des7na7on IP address with Is it the same value as when we do the same with own IP address? Yes " des7na7on IP is local; no " des7na7on IP is remote 81

82 Figuring out where to send locally Two cases: 1. Des7na7on is on the local network: need to address it directly. Des7na7on is not local (remote): need to figure out the first hop on the local network Determining if it s local: use the netmask e.g., bitwise- AND the des7na7on IP address with Is it the same value as when we do the same with own IP address? Yes " des7na7on IP is local; no " des7na7on IP is remote host host... DNS 1A- F- BB AD host host... DNS 1...0/ /4 8

83 Figuring out where to send locally Two cases: 1. Des7na7on is on the local network: need to address it directly. Des7na7on is not local (remote): need to figure out the first hop on the local network Determining if it s local: use the netmask e.g., bitwise- AND the des7na7on IP address with Is it the same value as when we do the same with own IP address? Yes " des7na7on IP is local; no " des7na7on IP is remote host host... DNS 1A- F- BB AD host host... DNS 1...0/ /4 8

84 Figuring out where to send locally () If it s remote, look up the first hop in a (very small) local rou7ng table e.g., by default, route via Now do the local case but for rather than ul7mate des7na7on IP address host host... DNS 1A- F- BB AD host host... DNS 1...0/ /4 For the local case, need to determine the des7na7on s link- layer address How does a host translate the next hop IP address to a link- layer address? 84

85 Address Resolu0on Protocol (ARP) Every node maintains an ARP table (IP address, link- layer address) pairs Consult the table when sending a packet Map des7na7on IP address to des7na7on MAC address Encapsulate and transmit the data packet But: what if IP address not in the table? Sender broadcasts: Who has IP address ? Receiver responds (unicast, to the source of the broadcast): link- layer address D7- FA- 0- B0 Sender caches result in its ARP table Sender may include its own <IP, link- layer> address mapping in request, so that receiver can reply back to the sender 85

86 Example: Pukng it all together How does host A send a datagram to host B? 1. A sends packet to R. R sends packet to B A host 74:9:9c:e8:ff: netmask 0xfffff000 49:bd:d:C7:56:a B host Network /0 e6:e9:00:17:bb:4b R 1a::f9:cd:06:9b Network /0 86

87 Host A decides to send through R Host A constructs an IP packet to send to B IP source , IP des7na7on Host A has a gateway R Used to reach any des7na7on outside of /0 Address for R learned via DHCP A host 74:9:9c:e8:ff: netmask 0xfffff000 49:bd:d:C7:56:a B host Network /0 e6:e9:00:17:bb:4b R 1a::f9:cd:06:9b Network /0 87

88 Host A sends packet through R Host A learns the MAC address of R s interface ARP request: broadcast request for ARP response: R responds with e6:e9:00:17:bb:4b Host A encapsulates the packet in a link- layer header and sends to R A host 74:9:9c:e8:ff: netmask 0xfffff000 49:bd:d:C7:56:a B host Network /0 To: R A " B data e6:e9:00:17:bb:4b R 1a::f9:cd:06:9b Network /0 88

89 R decides how to forward datagram Router R s ley interface receives the packet R extracts the IP packet from the Ethernet frame R sees the IP packet is des7ned to Router R consults its forwarding table Packet matches /0 via right interface A host 74:9:9c:e8:ff: netmask 0xfffff000 49:bd:d:C7:56:a B host Network /0 e6:e9:00:17:bb:4b A " B data R 1a::f9:cd:06:9b Network /0 89

90 R sends datagram to B Router R s right interface learns the link- layer address of host B ARP request: broadcast request for ARP response: B responds with 49:bd:d:C7:56:a Router R encapsulates the packet and sends to B A host 74:9:9c:e8:ff: netmask 0xfffff000 49:bd:d:C7:56:a B host Network /0 e6:e9:00:17:bb:4b R To: B A " B data 1a::f9:cd:06:9b Network /0 90

91 Security analysis of ARP Impersona0on Any node that hears an ARP request can answer and can say whatever they want Actual legit receiver never sees a problem Because even though later packets carry its IP address, its NIC doesn t capture them since not its link- layer address Man- in- the- middle alack Imposter updates frames with correct link- layer address and forwards whatever it receives to the legit des7na7on but gets to inspect (and maybe alter) it first Does the anacker have to win a race? Maybe not, if sender blindly believes ARP responses 91

92 Drawbacks of extended LANs Switched LANs afford greater scalability, but extended LANs do not isolate traffic Three resul7ng drawbacks: 1. Security: Allows eavesdropping across LANs, just by puvng an interface in promiscuous mode. Load: Some LANs are more heavily- used than others, may be desirable to separate them at 7mes.. Broadcast scalability: Broadcast frames traverse the en7re extended LAN; this reduces overall performance 9