EEC484584 Computer Networks Lecture 14 Wenbing Zhao wenbingieee
EEC-484/584 Computer Networks Lecture 14 Wenbing Zhao wenbing@ieee. org
2 Outline • Reminder – Lab#5: Wednesday (attendance optional) • Switched Ethernet • ARP and DHCP 2/26/2021 EEC-484/584: Computer Networks Wenbing Zhao
Switched Ethernet 3 • Switch – contains a high-speed backplane and room for typically 4 to 32 plug-in line cards, each containing 1 -8 connectors – Possibly each card forms its own collision domain, or – Full-duplex operation if each input port is buffered 2/26/2021 EEC-484/584: Computer Networks Wenbing Zhao
4 Ethernet/MAC Addresses • Ethernet (or MAC or LAN or physical) address: – Function: get frame from one interface to another physically-connected interface (same network) – 48 bit MAC address • Burned in NIC ROM, also sometimes software settable 2/26/2021 EEC-484/584: Computer Networks 5 -4
5 MAC Addresses Each adapter on Ethernet has unique MAC address 1 A-2 F-BB-76 -09 -AD LAN (wired or wireless) 71 -65 -F 7 -2 B-08 -53 Broadcast address = FF-FF-FF-FF = adapter 58 -23 -D 7 -FA-20 -B 0 0 C-C 4 -11 -6 F-E 3 -98 2/26/2021 EEC-484/584: Computer Networks 5 -5
6 MAC Addresses • MAC address allocation administered by IEEE • Manufacturer buys portion of MAC address space (to assure uniqueness) • 32 -bit IP address: – network-layer address – used to get datagram to destination IP subnet • MAC flat address ➜ portability – Can move LAN card from one LAN to another • IP hierarchical address NOT portable – Address depends on IP subnet to which node is attached 2/26/2021 EEC-484/584: Computer Networks 5 -6
7 ARP: Address Resolution Protocol Question: how to determine MAC address of B knowing B’s IP address? 137. 196. 7. 78 1 A-2 F-BB-76 -09 -AD 137. 196. 7. 23 • Each IP node (host, router) on LAN has ARP table • ARP table: IP/MAC address mappings for some LAN nodes 137. 196. 7. 14 < IP address; MAC address; TTL> – LAN 71 -65 -F 7 -2 B-08 -53 58 -23 -D 7 -FA-20 -B 0 TTL (Time To Live): time after which address mapping will be forgotten (typically 20 min) 0 C-C 4 -11 -6 F-E 3 -98 137. 196. 7. 88 2/26/2021 EEC-484/584: Computer Networks 5 -7
8 ARP Protocol: Same LAN • A wants to send datagram to B, and B’s MAC address not in A’s ARP table. • A broadcasts ARP query packet, containing B's IP address – Dest MAC address = FFFF-FF-FF – All machines on LAN receive ARP query • B receives ARP packet, replies to A with its (B's) MAC address • A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) – Soft state: information that times out (goes away) unless refreshed • ARP is “plug-and-play”: – Nodes create their ARP tables without intervention from net administrator – Frame sent to A’s MAC address (unicast) 2/26/2021 EEC-484/584: Computer Networks 5 -8
9 Addressing: Routing to Another LAN Walkthrough: send datagram from A to B via R Assume A knows B’s IP address 88 -B 2 -2 F-54 -1 A-0 F 74 -29 -9 C-E 8 -FF-55 A 111 E 6 -E 9 -00 -17 -BB-4 B 1 A-23 -F 9 -CD-06 -9 B 222. 220 111. 112 R 222. 221 222 B 49 -BD-D 2 -C 7 -56 -2 A CC-49 -DE-D 0 -AB-7 D • Two ARP tables in router R, one for each IP network 2/26/2021 EEC-484/584: Computer Networks 5 -9
• A creates IP datagram with source A, destination B • A uses ARP to get R’s MAC address for 111. 110 • A creates link-layer frame with R's MAC address as dest, frame contains A-to-B IP datagram This is a really important • A’s NIC sends frame example – make sure you understand! • R’s NIC receives frame • R removes IP datagram from Ethernet frame, sees its destined to B • R uses ARP to get B’s MAC address • R creates frame containing A-to-B IP datagram sends to B 88 -B 2 -2 F-54 -1 A-0 F 74 -29 -9 C-E 8 -FF-55 A E 6 -E 9 -00 -17 -BB-4 B 111 1 A-23 -F 9 -CD-06 -9 B 222. 220 111. 112 R 222. 221 222 B 49 -BD-D 2 -C 7 -56 -2 A CC-49 -DE-D 0 -AB-7 D 2/26/2021 EEC-484/584: Computer Networks 5 -10 10
11 ARP – Exercise • Node 1 wants to send a packet to node 4, what will be returned by ARP? • Node 1 wants to send a packet to node 2, what will be returned by ARP? 2/26/2021 EEC-484/584: Computer Networks Wenbing Zhao
RARP – Reverse Address Resolution Protocol 32 -bit Internet address ARP RARP 48 -bit Ethernet address • RARP - Allows a newly-booted disklessworkstation (e. g. , X terminal) to broadcast its Ethernet address and ask for its IP address – RARP server responds to a RARP request with the assigned IP address 2/26/2021 EEC-484/584: Computer Networks Wenbing Zhao 12
13 Limitations of RARP • RARP uses a link-layer broadcast, RARP requests are not forwarded by routers, therefore, an RARP server must be present on every network • The only thing returned by the RARP server is the IP address 2/26/2021 EEC-484/584: Computer Networks Wenbing Zhao
14 BOOTP – Bootstrap Protocol • BOOTP – uses UDP – A client broadcasts to 255 – The source IP address is set to 0. 0 if client does not know its own IP address yet – Port number: 67 for server, 68 for client • BOOTP drawbacks – Requires manual configuration of tables mapping IP address to Ethernet address at the BOOTP server • Replaced by DHCP 2/26/2021 EEC-484/584: Computer Networks Wenbing Zhao
15 Dynamic Host Configuration Protocol • Allow host to dynamically obtain its IP address from network server when it joins network – IP address assignment is lease-based (to cope with client failure, also enables reuse of addresses) – Can renew its lease on address in use • DHCP overview (UDP is used for communication) – – 2/26/2021 Host broadcasts “DHCP discover” msg DHCP server responds with “DHCP offer” msg Host requests IP address: “DHCP request” msg DHCP server sends address: “DHCP ack” msg EEC-484/584: Computer Networks Wenbing Zhao
16 DHCP Replay • A DHCP relay agent can be configured on each LAN • The agent stores the IP address of the DHCP server and forward the request to the server 2/26/2021 EEC-484/584: Computer Networks Wenbing Zhao
17 DHCP with Replay Agent • To find its IP address, a newly-booted machine broadcasts a DHCP Discover packet • The DHCP relay agent on its LAN receives all DHCP broadcasts • On receiving a DHCP Discover packet, the agent sends the packet as a unicast packet to the DHCP server, possibly on a distant network 2/26/2021 EEC-484/584: Computer Networks Wenbing Zhao
18 Link Layer Devices • Hubs • Switches 2/26/2021 EEC-484/584: Computer Networks Wenbing Zhao
19 Hubs … physical-layer (“dumb”) repeaters: – – Bits coming in one link go out all other links at same rate All nodes connected to hub can collide with one another No frame buffering No CSMA/CD at hub: host NICs detect collisions twisted pair hub 2/26/2021 EEC-484/584: Computer Networks 5 -19
20 Switch • Link-layer device: smarter than hubs, take active role – Store, forward Ethernet frames – Examine incoming frame’s MAC address, selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment, uses CSMA/CD to access segment • Transparent – Hosts are unaware of presence of switches • Plug-and-play, self-learning – Switches do not need to be configured 2/26/2021 EEC-484/584: Computer Networks 5 -20
21 Switch: Allows Multiple Simultaneous Transmissions A • Hosts have dedicated, direct connection to switch • Switches buffer packets • Ethernet protocol used on each incoming link, but no collisions; full duplex C’ – Each link is its own collision domain • Switching: a-to-a’ and b-to-b’ simultaneously, without collisions B 6 1 2 5 3 4 C B’ A’ switch with six interfaces (1, 2, 3, 4, 5, 6) – Not possible with dumb hub 2/26/2021 EEC-484/584: Computer Networks 5 -21
22 Switch Table A • Q: how does switch know that A’ reachable via interface 4, B’ reachable via interface 5? • A: each switch has a switch table, each entry: C’ B 6 1 2 5 4 – (MAC address of host, interface to reach host, time stamp) • Looks like a routing table! • Q: how are entries created, maintained in switch table? – Something like a routing protocol? 2/26/2021 EEC-484/584: Computer Networks 3 C B’ A’ switch with six interfaces (1, 2, 3, 4, 5, 6) 5 -22
23 Switch: Self-Learning Source: A Dest: A’ A A A’ • Switch learns which hosts can be reached through which interfaces C’ B – When frame received, switch “learns” location of sender: incoming LAN segment – Records sender/location pair in switch table 6 1 2 5 4 3 C B’ A’ Switch table (initially empty) 2/26/2021 EEC-484/584: Computer Networks 5 -23
24 Switch: Frame Filtering/Forwarding When frame received: 1. record link associated with sending host 2. index switch table using MAC dest address 3. if entry found for destination then { if dest on segment from which frame arrived then drop the frame else forward the frame on interface indicated } else flood forward on all but the interface on which the frame arrived 2/26/2021 EEC-484/584: Computer Networks 5 -24
25 Self-Learning, Forwarding: Example Source: A Dest: A’ A A A’ C’ B • Frame destination unknown: flood • Destination A location known: selective send A 6 A’ 1 2 4 5 C A’ A B’ 3 A’ Switch table (initially empty) 2/26/2021 EEC-484/584: Computer Networks 5 -25
26 Interconnecting Switches • Switches can be connected together S 4 S 1 S 3 S 2 A B C F D E I G H r Q: sending from A to G - how does S 1 know to forward frame destined to F via S 4 and S 3? r A: self learning! (works exactly the same as in singleswitch case!) 2/26/2021 EEC-484/584: Computer Networks 5 -26
27 Self-Learning Multi-Switch Example Suppose C sends frame to I, I responds to C S 4 1 S 1 2 S 2 A B C S 3 F D E I G H r Q: show switch tables and packet forwarding in S 1, S 2, S 3, S 4 2/26/2021 EEC-484/584: Computer Networks 5 -27
28 Institutional Network 2/26/2021 EEC-484/584: Computer Networks 5 -28
29 Switches vs. Routers • Both store-and-forward devices – Routers: network layer devices (examine network layer headers) – Switches are link layer devices • Routers maintain routing tables, implement routing algorithms • Switches maintain switch tables, implement filtering, learning algorithms 2/26/2021 EEC-484/584: Computer Networks 5 -29
- Slides: 29