EEC-484/584 Computer Networks Lecture 8 Wenbing Zhao

2 Spring Semester 2008EEC-484/584: Computer NetworksWenbing Zhao Outline Reminder: –3/3 (Monday): Lab #3 –3/5 (Wednesday): Quiz #2 CSMA protocols Manchester Encoding The Ethernet MAC Sublayer Protocol The Binary Exponential Backoff Algorithm Switched Ethernet ARP and DHCP

3 Spring Semester 2008EEC-484/584: Computer NetworksWenbing Zhao Carrier Sense Multiple Access When station has data to send, listens to channel to see if anyone else is transmitting If channel is idle, station transmits a frame Else station waits for it to become idle If collisions occurs, station waits random amount of time, tries again Also called 1-persistent CSMA –With probability 1 station will transmit if channel is idle

4 Spring Semester 2008EEC-484/584: Computer NetworksWenbing Zhao After a station starts sending, it takes a while before 2nd station receives 1st station ’ s signal –2nd station might start sending before it knows that another station has already been transmitting If two stations become ready while third station transmitting –Both wait until transmission ends and start transmitting, collision results Carrier Sense Multiple Access: Collision Still Possible

5 Spring Semester 2008EEC-484/584: Computer NetworksWenbing Zhao p-persistent CSMA: Reduce the Probability of Collision Sense continuously, but does not always send when channel is idle –Applicable for slotted channels When ready to send, station senses the channel –If channel idle, station transmits with probability p, defers to next slot with probability q = 1-p –Else (if channel is busy) station waits until next slot tries again –If next slot idle, station transmits with probability p, defers with probability q = 1-p –…–…

6 Spring Semester 2008EEC-484/584: Computer NetworksWenbing Zhao Non-Persistent CSMA Does not sense continuously, send if it senses the channel is idle Before sending, station senses the channel –If channel is idle, station begins sending –Else station does not continuously sense, waits random amount of time, tries again

7 Spring Semester 2008EEC-484/584: Computer NetworksWenbing Zhao Persistent and Nonpersistent CSMA Improves over ALOHA because they ensure no station to transmit when it senses channel is busy

8 Spring Semester 2008EEC-484/584: Computer NetworksWenbing Zhao Manchester Encoding Binary encoding –Hard to distinguish 0 bit (0-volt) from idle (0-volt) –Requires clocks of all stations synchronized Manchester encoding and differential Manchester encoding

9 Spring Semester 2008EEC-484/584: Computer NetworksWenbing Zhao Ethernet Frame Structure Preamble: for clock synchronization –First 7 bytes with pattern , last byte with pattern –The two consecutive 1 ’ s indicate the start of a frame How can the receiver tell the end of the frame? –No current on the wire >= 64 bytes Not considered as part of the header!

10 Spring Semester 2008EEC-484/584: Computer NetworksWenbing Zhao Ethernet Frame Structure Destination address: 6 bytes (48 bits) –Highest order bit: 0 individual, 1 multicast; all 1 ’ s broadcast –Frames received with non-matching destination address is discarded Type: type of network layer protocol Pad – used to produce valid frame >= 64 bytes Checksum – 32-bit cyclic redundancy check

11 Spring Semester 2008EEC-484/584: Computer NetworksWenbing Zhao Ethernet MAC Sublayer Protocol Uses 1-persistent CSMA/CD Binary exponential backoff Provides unreliable connectionless service

12 Spring Semester 2008EEC-484/584: Computer NetworksWenbing Zhao CSMA with Collision Detection If two stations start transmitting simultaneously, both detect collision and stop transmitting Minimum time to detect collision = time for signal to propagate Monitor collision while sending –Minimum time to detect collision => minimum frame length

13 Spring Semester 2008EEC-484/584: Computer NetworksWenbing Zhao Minimum Time to Detect Collision To ensure the sender can detect collision –All frames must take more than 2  to send so that transmission is still taking place when the noise burst gets back to the sender

14 Spring Semester 2008EEC-484/584: Computer NetworksWenbing Zhao Randomization and Binary Exponential Backoff Time divided into slots –Length of slot = 2  = worst-case round-trip propagation time –To accommodate longest path, slot time = 512 bit times = 51.2  sec (10Mbps Ethernet) Binary exponential backoff

15 Spring Semester 2008EEC-484/584: Computer NetworksWenbing Zhao Randomization and Binary Exponential Backoff After 1 st collision, station picks 0 or 1 at random, waits that number of slots and tries again After 2 nd collision, station picks 0,1,2,3 at random, waits that number of slots and tries again …. After i-th collision, station picks 0,1, …,2 i -1 at random, … If 10 <= i < 16, station picks 0,1, …, at random If i=16, controller reports failure to computer

16 Spring Semester 2008EEC-484/584: Computer NetworksWenbing Zhao Ethernet Performance Binary exponential backoff results in –Low delay when few stations collide –Reasonable delay for collision resolution when many stations collide When other factors are fixed, channel efficiency decreases when –Network bandwidth increases –Cable length increases –Number of stations increases –Frame length decreases

17 Spring Semester 2008EEC-484/584: Computer NetworksWenbing Zhao Ethernet Performance Efficiency of Ethernet at 10 Mbps with 512-bit slot times

18 Spring Semester 2008EEC-484/584: Computer NetworksWenbing Zhao Switched Ethernet 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

19 Spring Semester 2008EEC-484/584: Computer NetworksWenbing Zhao ARP – Address Resolution Protocol How do IP addresses get mapped onto data link layer addresses, such as Ethernet?

20 Spring Semester 2008EEC-484/584: Computer NetworksWenbing Zhao ARP Optimization ARP result is cached (step 5 in figure) When A wants to communicate with B, A includes its IP-to-Ethernet mapping in the ARP packet so that B knows the mapping right away (step 3 in figure) Have every machine broadcast its mapping when it boots, so that everyone else knows the mapping To accommodate changes, entries in the ARP cache time out after a few minutes

21 Spring Semester 2008EEC-484/584: Computer NetworksWenbing Zhao ARP: How to Handle Remote Traffic Proxy ARP – A router is configured to answer ARP requests on one of its networks for a host on another network

22 Spring Semester 2008EEC-484/584: Computer NetworksWenbing Zhao ARP – Exercise Node 1 wants to send a packet to node 3, what will be returned by ARP? Node 1 wants to send a packet to node 2, what will be returned by ARP?

23 Spring Semester 2008EEC-484/584: Computer NetworksWenbing Zhao RARP – Reverse Address Resolution Protocol RARP - Allows a newly-booted diskless- workstation (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 32-bit Internet address 48-bit Ethernet address ARP RARP

24 Spring Semester 2008EEC-484/584: Computer NetworksWenbing Zhao 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

25 Spring Semester 2008EEC-484/584: Computer NetworksWenbing Zhao BOOTP – Bootstrap Protocol BOOTP – uses UDP –A client broadcasts to –The source IP address is set to 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

26 Spring Semester 2008EEC-484/584: Computer NetworksWenbing Zhao 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) –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

27 Spring Semester 2008EEC-484/584: Computer NetworksWenbing Zhao DHCP Client-Server Scenario A B E DHCP server arriving DHCP client needs address in this network

28 Spring Semester 2008EEC-484/584: Computer NetworksWenbing Zhao DHCP Client-Server Scenario DHCP server: arriving client time DHCP discover src : , 68 dest.: ,67 yiaddr: transaction ID: 654 DHCP offer src: , 67 dest: , 68 yiaddr: transaction ID: 654 Lifetime: 3600 secs DHCP request src: , 68 dest:: , 67 yiaddr: transaction ID: 655 Lifetime: 3600 secs DHCP ACK src: , 67 dest: , 68 yiaddr: transaction ID: 655 Lifetime: 3600 secs

29 Spring Semester 2008EEC-484/584: Computer NetworksWenbing Zhao 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

30 Spring Semester 2008EEC-484/584: Computer NetworksWenbing Zhao 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

31 Spring Semester 2008EEC-484/584: Computer NetworksWenbing Zhao Exercise An IP packet to be transmitted by Ethernet is 60 bytes long. Is padding needed in the Ethernet frame, and if so, how many bytes?

32 Spring Semester 2008EEC-484/584: Computer NetworksWenbing Zhao Exercise Consider building a CSMA/CD network running at 1 Gbps over a 1-km cable. The signal speed in the cable is 200,000 km/sec. What is the minimum frame size?