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CSE 331: Introduction to Networks and Security
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  1. CSE 331: Introduction to Networks and Security Slide Set 4 Fall 2000 Instructor: Carl A. Gunter

  2. Hop by Hop Vs. End to End • Link Layer Protocols • Ethernet (802.3) • Token Rings (802.5, FDDI) • Wireless Ethernet (802.11) • ATM • End to End Protocols • UDP • TCP • RPC • Sockets

  3. Ethernet (802.3) • Originated mid-1970’s at Xerox PARC, roots in Aloha packet radio. • Carrier Sense, Multiple Access with Collision Detect (CSMA/CD). • 10Mbps available in 1978, now 100Mbps (Fast Ethernet) and 1000Mbps (Gigabit Ethernet). • First ran on coax cables, now typically on twisted pair wires.

  4. Frame Format 1500 bytes 64 48 48 16 32 Packet Body Dest Address Src Address Packet Body Type Preamble CRC For Demux!

  5. Addresses • 48 bits, uniquely assigned to interface cards. • Written with hexadecimal; colons separate each pair of nibbles, leading zeros are omitted. • Example: 8:0:2b:e4:b1:2 • Kinds of addressing/reception: unicast, broadcast (all ones), multicast (first bit is one but not all bits), promiscuous.

  6. Media Access Protocol • Send frame immediately if line is idle. • If the line is busy, send frame as soon as it becomes idle. • Two hosts may begin transmitting at the same time. If a transmission is underway and a collision is detected, emit a 32 bit jamming sequence. • If you were unable to transmit, wait before trying again. Backoff formula: if n attempts have occurred, choose a number k between 0 and 2n-1; wait k* 51.2s before trying again.

  7. Worst Case Scenario A B A begins to omit a frame at time t

  8. Worst Case Scenario A B The beginning of A’s frame almost reaches b at t + d

  9. Worst Case Scenario A B B begins transmission just before A’s frame arrives, while B still detects an idle link, but a collision immediately occurs.

  10. Worst Case Scenario A B At approximately time t + 2*d, node A detects the collision. Ethernet requirements ensure that A is still transmitting at this time so it can detect the failure and treat its frame as dropped.

  11. Ethernet Pragmatics • Limited to about 2500m length to achieve 51.2s round trip delay (recall this number from the backoff rules). • Works best with significant over-provisioning: fewer than 200 hosts, and utilization of less than a third. • Cheap and easy to maintain. • Very widely deployed on LANs.

  12. Token Rings (802.5, FDDI) • IBM Token Ring earliest. 802.5 is IEEE standard based on this. They support 4Mbps or 16Mbps over twisted pair for about 250 nodes. • FDDI recent fast technology for optical fiber. It supports 100Mbps for as much as 200km of fiber and 500 nodes (with at most 2km between nodes).

  13. Architecture • Nodes are organized in a ring. • They pass a token around the ring. • The node holding the token can use the media to transmit. • To avoid breaking the ring, hosts use a relay that is open when the host is available, but closed (so the host is bypassed) otherwise.

  14. 802.5 Packet Format Demux! 8 8 8 48 48 Start Delimiter Access Control Frame Control Destination Address Source Address Variable Length Body 32 8 8 Checksum End Delimiter Frame Status

  15. Ring Structure

  16. Media Access Protocol • Nodes forward messages from other nodes. • A node can transmit only when it gets the token, which circulates around the ring. • When a node transmits, it removes its transmission from the network by not relaying it. • Token Holding Time (THT) is usually about 10ms. • Token Rotation Time (TRT) is determined by the latency of the ring and the number of nodes on it.

  17. Design Tradeoffs • A longer THT gives better link utilization, but increases the potential delay from the TRT. • Release strategy. • Early release: release the token immediately after transmission. • Delayed release: release the token only after confirming that it made it around the ring.

  18. FIDDI Dual Ring

  19. FIDDI Recovery

  20. Wireless Ethernet (802.11) • Wireless links over three possible physical layers. • Frequency hopping over 79 one MHz bands. • Direct sequence with 11 bit chipping. • Diffused infrared for use in buildings. • First two at 2.4GHz.

  21. Hidden Nodes A B C If A and C both transmit to B, there is a collision at B, but, since A and C are out of range, they cannot sense the collision.

  22. Exposed Nodes A B C D Node B is transmitting to A. A communication from C will collide with this at B, but if C is out of range from A, it can transmit to D without colliding at A.

  23. Media Access Protocol • Both problems are addressed with Multiple Access with Collision Avoidance (MACA). • Sender and receiver send control frames before transmitting data. • Sender transmits Request to Send (RTS). • Receiver replies with Clear To Send (CTS). • Any node that sees the CTS knows it is too close to the receiver to transmit. • Any node that sees the RTS but not the CTS is free to transmit to nodes other than the sender.

  24. Distribution System X Y B A Access Points

  25. Distribution System X Y B A Changing Access Points A

  26. Frame Format • 16 Control (CTS or RTS?) (DS?) • 16 Duration • 48 Addr1 • 48 Addr2 • 48 Addr3 • SeqCtrl • 48 Addr4 • 0-18,496 (variable) Payload • 32 CRC 3 2 1 4

  27. Ad Hoc Networks • Routing for a wireless internetwork without the aid of a central base station. • Connections are low-bandwidth, lossy, and highly transient. • Unique routing assumptions: • Most routes are seldom used. • Bandwidth must be protected.

  28. Illustration Part 1 of 2 Movement Routing

  29. Illustration Part 2 of 2 New Routing

  30. AODV Protocol • If a node S needs a route to a destination D and does not have one, it floods a route-request (RREQ) packet through the network. • Each recipient R of this RREQ keeps a return pointer. • R broadcasts the request to its neighbors if it is not D and does not have a route to D. • If R is D, or has a route to D, it responds with a route-reply (RREP) packet using the return pointers for S. Perkins and Royer 99

  31. Asynchronous Transfer Mode • ATM is connection-oriented • ATM is packet-switched • Packets (aka “cells”) are fixed length • 53 = 5 bytes header + 48 bytes payload • Small in size (max Ethernet 1500 bytes) • Many decisions driven by HW requirements • Simplicity (know length) • Parallelism (lots of little clocked activities)

  32. Virtual Circuits Have a friend go ahead of you. At every road they reserve a lane just for you. At every intersection they post a big sign that says for a given lane which way to turn and what new lane to take. LANE#1 TURN RIGHT USE LANE#2 LANE#1 LANE#2

  33. Switching Tables

  34. Circuit-Switching Tradeoffs • - Delay for call setup • -/+ Statefullness • - loss of state on failures • + fast lookups (small Ids) • + QoS associations

  35. ATM Cell Format (UNI) • Three-Letter Acronyms (TLAs): • GFC - Generic Flow Control • UNI - User/Network Interface • VPI - Virtual Path Identifier • VCI - Virtual Circuit Identifier • CLP - Cell Loss Priority • HEC - Header Error Check GFC VPI VCI Type CLP HEC (CRC-8) Payload Bits: 4 8 16 3 1 8 384 (48 bytes)

  36. Segmentation-and-Reassembly • Convert between variable-sized packet abstraction and fixed-size cells • Packet->cell: Segmentation • Cells->Packet: Reassembly • Done by ATM Adaptation Layer (AAL) • AAL 3/4: • Convergence Sublayer Protocol Data Unit (PDU) format for encapsulating variable length data • Extra 32 bits per cell of overhead

  37. AAL5: Better for computers • Less overhead: • AAL 3/4 cell format: • AAL 5 cell format: • A bit in the ATM header is used to determine start and end for AAL5. 40 2 4 10 352 6 10 ATM Header Type SEQ MID Payload Length CRC-10 40 384 ATM Header Payload