Transcript CSE331-10
CSE331: Introduction to Networks and Security Lecture 10 Fall 2002 Announcements • HW 2 Graded – Mean 82 – Pick up after class CSE331 Fall 2002 2 Recap • Domain Name System • IPv4 Routing – RIP – Routing Information Protocol – OSPF – Open Shortest Path First CSE331 Fall 2002 3 Today • Interdomain Routing • IPv6 • UDP & TCP CSE331 Fall 2002 4 Interdomain Routing • EGP - Exterior Gateway Protocol – Early; constrained Internet to tree and hence shares some problems of bridging (scalability, path exploitation, etc. ….. ) Internet tree structure circa 1990. CSE331 Fall 2002 5 Today’s Multibackbone Internet Peering point Large Corporation Consumer ISP Backbone Service Backbone ProviderService Backbone ProviderService Provider Consumer ISP Large Corporation Consumer ISP Small Company CSE331 Fall 2002 6 BGP - Border Gateway Protocol • Arbitrary topology • Interconnected of Administrative Domains – – – – – local traffic: originates or terminates in A.D. transit traffic: passes through stub: one A.D.-A.D. link multihomed: multiple A.D. links, no transit transit: connections to other A.D.s; carry transit CSE331 Fall 2002 7 BGP Continued • AD Connectivity – A border gateway is an IP router that forwards packets between A.D.s – A.D. has one “BGP speaker” • Border Gateway Protocol – Advertises reachability, not costs – Advertises complete paths between A.D.s • Needed for policy decisions like “Use provider B” • Helps detect loops – Not LSP or DV. – Router state ~# A.D.s, hence large CSE331 Fall 2002 8 Next Generation IP (IPv6 or IPng) • • • • • Lots of addresses (128 bits) Real-time Security Autoconfiguration Mobile hosts and other enhanced routing functions CSE331 Fall 2002 9 Packet Format 0 31 Ver. Prio FlowLabel PayloadLEN NextHeader HopLimit Source Address (128 bits) Destination Address (128 bits) Next Header/Data CSE331 Fall 2002 10 Protocol Stack Revisited Application Presentation Session Transport UDP and TCP/IP Network Data Link So far… Physical CSE331 Fall 2002 11 Application vs. Network Application Needs Network Char. Reliable, Ordered, Single-Copy Message Delivery Drops , Duplicates and Reorders Messages Arbitrarily large messages Finite message size Allows Flow Control by Receiver Arbitrary Delay Supports multiple applications per-host … CSE331 Fall 2002 12 User Datagram Protocol (UDP) 0 16 31 SrcPort DestPort Length Checksum IP Packet Data • Simplest transport-layer protocol • Just exposes IP packet functionality to application level • Ports identify sending/receiving process – Demultiplexing information – (port, host) pair identifies a network process CSE331 Fall 2002 13 UDP End-to-End Model • Multiplexing/Demultiplexing with Port number Application Application UDP Sender (Multiplexer) Application Application UDP Receiver (Demultiplexer) CSE331 Fall 2002 14 Using Ports • Client contacts Server at a well-known port – – – – DNS: port 53 POP3: port 110 Unix talk : port 517 In unix, ports are listed in /etc/services • Sometimes Client and Server agree on a different port for subsequent communication • Ports are an abstraction – Implemented differently on different OS’s – Typically a message queue CSE331 Fall 2002 15 Transmission Control Protocol (TCP) • Most widely used protocol for reliable byte streams – – – – Reliable, in-order delivery of a stream of bytes Full duplex: pair of streams, one in each direction Flow and congestion control mechanisms Like UDP, supports ports • Built on top of IP (hence TCP/IP) CSE331 Fall 2002 16 TCP End-to-End Model • Buffering corrects errors but may introduce delays Application Application Application TCP Sender: Send Buffers Application TCP Receiver Receive Buffers segment segment CSE331 Fall 2002 segment segment 17 Packet Format • Flags – – – – – – SYN FIN RESET PUSH URG ACK • Fields 0 15 SrcPort 31 DstPort SequenceNum Acknowledgment HL 0 Flags Advert.Wind. Checksum UrgPtr Options (variable) DATA CSE331 Fall 2002 18 Three-Way Handshake CSE331 Fall 2002 19 TCP State Transitions CSE331 Fall 2002 20 TCP Receiver • Maintains a buffer from which application reads • Advertises < buffer size as the window for sliding window • Responds with Acknowledge and AdvertisedWindow on each send; updates byte counts when data O.K. • Application blocked until read() O.K. CSE331 Fall 2002 21 TCP Sender • Maintains a buffer; sending application is blocked until room in the buffer for its write • Holds data until acknowledged by receiver as successfully received • Implement window expansion and contraction; note difference between flow and congestion control CSE331 Fall 2002 22 Flow vs. Congestion Control • Flow control protects the recipient from being overwhelmed. • Congestion control protects the network from being overwhelmed. CSE331 Fall 2002 23 TCP Congestion Control • Additive Increase / Multiplicative Decrease • Slow Start • Fast Retransmit and Fast Recovery CSE331 Fall 2002 24 Increase and Decrease • A value CongestionWindow is used to control the number of unacknowledged transmissions. • This value is increased linearly until timeouts for ACKs are missed. • When timeouts occur, CongestionWindow is decreased by half to reduce the pressure on the network quickly. • The strategy is called “additive increase / multiplicative decrease”. CSE331 Fall 2002 25 Additive Increase CSE331 Fall 2002 26 KB TCP Sawtooth Pattern Time CSE331 Fall 2002 27 Slow Start • Sending the entire window immediately could cause a traffic jam in the network. • Begin “slowly” by setting the congestion window to one packet. • When acknowledgements arrive, double the congestion window. • Continue until ACKs do not arrive or flow control dominates. CSE331 Fall 2002 28 Slow Start CSE331 Fall 2002 29