Lecture notes on Transport layer

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Transport Layer EECS 489 Computer Networks http://www.eecs.umich.edu/courses/eecs489/w07 Z. Morley Mao Wednesday Jan 24, 2007 1 Mao W07 Acknowledgement: Some slides taken from Kurose&Ross and Katz&Stoicaƒƒƒ Adminstrivia Homework 1 was due yesterday – 1/23 - You can use your late days PA1 has been posted - A simplified Web server - You need to find a project partner for this assignment - If you want to work in groups of three or by yourself, please email us to get permission. Reading assignment for this week - Chapter 3 of the book - You should have read Chapter 1 and 2. 2 Mao W07ƒƒ Discussion on email Why do we have so much spam? - How would you design the email system to prevent spam? How does anonymous email work? 3 Mao W07ƒƒƒƒƒ Discussion on DNS Is it easy to attack the DNS system? Why is DNS caching good? Why is DNS caching bad? DNS is “exploited” for server load balancing, how? - Local DNS servers are usually close to local clients If you were to design DNS differently today, how would you? - Any problems with the current DNS system? 4 Mao W07ƒƒ Discussion on P2P How would you design a P2P system that is scalable, decentralized, and guarantees the location of the files? One solution: DHT (Distributed Hash Table) - Guarantees that you can find the file - Mapping between the file and the node ID - Consistent hashing function assigns each node and key an m-bit identifier using SHA-1 base hash function. 5 Mao W07ƒƒƒƒ Chord protocol Consistent hashing function assigns each node and key an m-bit identifier using SHA-1 base hash function. Node’s IP address is hashed. Identifiers are ordered on a identifier circle m modulo 2 called a chord ring. succesor(k) = first node whose identifier is = identifier of k in identifier space. 6 Mao W07Chord protocol For m = 6, of identifiers is 64. The following Chord ring has 10 identifier nodes and stores 5 keys. node The successor of key 10 is node 14. key X 6 1 0 successor(1) = 1 1 7 identifier successor(6) = 0 successor(2) = 3 6 2 6 2 circle 5 3 4 2 7 Mao W07ƒƒƒ IP Addressing 32-bit number in dotted-quad notation (12.34.158.5) Divided into network & host portions (left and right) 8 12.34.158.0/24 is a 24-bit prefix with 2 addresses 12 34 158 5 00001100 00100010 10011110 00000101 Network (24 bits) Host (8 bits) 8 Mao W07ƒƒ Some History: Why Dotted-Quad Notation? In the olden days… - Class A: 0 • Very large /8 blocks (e.g., MIT has 18.0.0.0/8) - Class B: 10 • Large /16 blocks (e.g,. UM has 141.213.0.0/16) - Class C: 110 • Small /24 blocks (e.g., AT&T Labs has 192.20.225.0/24) - Class D: 1110 • Multicast groups - Class E: 11110 • Reserved for future use (sounds a bit scary…) And then, address space became scarce… 9 Mao W07Classless Inter-Domain Routing (CIDR) Use two 32-bit numbers to represent a network. Network number = IP address + Mask IP Address : 12.4.0.0 IP Mask: 255.254.0.0 Address 00001100 00000100 00000000 00000000 Mask 11111111 11111110 00000000 00000000 Network Prefix for hosts Usually written as 12.4.0.0/15 10 Mao W07ƒ CIDR = Hierarchy in Address Allocation Prefixes are key to Internet scalability - Address allocation by ARIN/RIPE/APNIC and by ISPs - Routing protocols and packet forwarding based on prefixes - Today, routing tables contain 150,000-200,000 prefixes 12.0.0.0/16 : 12.1.0.0/16 12.3.0.0/24 12.2.0.0/16 12.3.1.0/24 : : 12.3.0.0/16 : : : 12.0.0.0/8 12.3.254.0/24 : 12.253.0.0/19 12.253.32.0/19 12.253.64.0/19 : 12.253.0.0/16 12.253.96.0/19 12.254.0.0/16 12.253.128.0/19 12.253.160.0/19 11 Mao W07 12.253.192.0/19ƒƒ Figuring Out Who Owns an Address Address registries - Public record of address allocations - ISPs should update when giving addresses to customers - However, records are notoriously out-of-date Ways to query - UNIX: “whois –h whois.arin.net 128.112.136.35” - http://www.arin.net/whois/ - http://www.geektools.com/whois.php -… 12 Mao W07Example Output for 141.213.4.5 (galileo.eecs.umich.edu) OrgName: University of Michigan OrgID: UNIVER-118 Address: IT Communications Services Address: 4251 Plymouth Road City: Ann Arbor StateProv: MI PostalCode: 48105-2785 Country: US NetRange: 141.213.0.0 - 141.213.255.255 CIDR: 141.213.0.0/16 NetName: UMNET3 NetHandle: NET-141-213-0-0-1 Parent: NET-141-0-0-0-0 NetType: Direct Assignment NameServer: SRVR8.ENGIN.UMICH.EDU NameServer: SRVR7.ENGIN.UMICH.EDU NameServer: DNS2.ITD.UMICH.EDU 13 Mao W07There is more… Comment: Abuse contact for 141.213.128.0/17 is abuseumich.edu. Comment: For DMCA info see http://www.umich.edu/itua/copyright/ RegDate: 1990-08-02 Updated: 2003-03-27 AbuseHandle: CEAC-ARIN AbuseName: College of Engineering Abuse Contact AbusePhone: +1-734-936-2486 AbuseEmail: abuseengin.umich.edu TechHandle: PMK5-ARIN TechName: Killey, Paul M. TechPhone: +1-734-763-4910 TechEmail: paulengin.umich.edu OrgTechHandle: UA11-ORG-ARIN OrgTechName: UMnet Administration OrgTechPhone: +1-734-647-4200 OrgTechEmail: umnet-adminumich.edu 14 Mao W07ƒƒ Longest Prefix Match Forwarding Forwarding tables in IP routers - Maps each IP prefix to next-hop link(s) Destination-based forwarding - Packet has a destination address - Router identifies longest-matching prefix - Cute algorithmic problem: very fast lookups forwarding table 4.0.0.0/8 destination 4.83.128.0/17 12.34.158.5 outgoing link 12.0.0.0/8 Serial0/0.1 12.34.158.0/24 126.255.103.0/24 15 Mao W07ƒƒƒƒ How are packets forwarded? Routers have forwarding tables - Map prefix to outgoing link(s) Entries can be statically configured - E.g., “map 12.34.158.0/24 to Serial0/0.1” But, this doesn’t adapt - To failures - To new equipment - To the need to balance load -… That is where routing protocols come in… more on this in the next lectures 16 Mao W07ƒƒƒƒ Discussions IP address space scarcity - What can we do about it? Increased IP address fragmentation Does an IP address identify the actual user? How does one achieve mobility while maintaining the same IP address? 17 Mao W07ƒƒƒ I/O Models Five different I/O models - Blocking I/O - Nonblocking I/O - I/O multiplexing (select and poll) Synchronous • Threads with blocking I/O - Signal driven I/O (SIGIO) - Asynchronous I/O (aio_ functions) • Not widely supported Blocking vs. nonblocking (system call) - Whether the system call blocks until data is ready Synchronous vs. asynchronous (I/O operation) - Whether the I/O operation causes requesting process to be blocked 18 Mao W07Blocking I/O Model Kernel Application System call No datagram ready recvfrom Wait for data Process datagram ready blocks copy datagram Copy data from Kernel to Return OK user copy complete process datagram 19 Mao W07Nonblocking I/O Model Kernel Application System call No datagram ready recvfrom EAGAIN Wait for System call recvfrom data EAGAIN System call recvfrom Process datagram ready repeatedly copy datagram Copy calls data recvfrom from (polling) Kernel to Return OK user copy complete process datagram 20 Mao W07

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