CSE 331: Introduction to Networks and Security Fall 2001 Instructor: Carl A. Gunter Slide Set 5

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

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

Frame Format Preamble Dest Address Src Address Type CRC Packet Body Packet Body  1500 bytes For Demux!

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

Media Access Protocol l Send frame immediately if line is idle. l If the line is busy, send frame as soon as it becomes idle. l 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. l If you were unable to transmit, wait before trying again. Backoff formula: if n attempts have occurred, choose a number k between 0 and 2 n -1; wait k* 51.2  s before trying again.

Worst Case Scenario AB A begins to omit a frame at time t

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

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

Worst Case Scenario AB 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.

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

Token Rings (802.5, FDDI) l IBM Token Ring earliest is IEEE standard based on this. They support 4Mbps or 16Mbps over twisted pair for about 250 nodes. l 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).

Architecture l Nodes are organized in a ring. l They pass a token around the ring. l The node holding the token can use the media to transmit. l 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.

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

Ring Structure

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

Design Tradeoffs l A longer THT gives better link utilization, but increases the potential delay from the TRT. l 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.

FIDDI Dual Ring

FIDDI Recovery

Wireless Ethernet (802.11) l 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. l First two at 2.4GHz.

Hidden Nodes ABC 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.

Exposed Nodes ABC 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. D

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

Access Points Distribution System A B XY

Changing Access Points Distribution System A B XY A

Frame Format l 16 Control (CTS or RTS?) (DS?) l 16 Duration l 48 Addr1 l 48 Addr2 l 48 Addr3 l SeqCtrl l 48 Addr4 l 0-18,496 (variable) Payload l 32 CRC

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

Illustration Part 1 of 2 Routing Movement

Illustration Part 2 of 2 New Routing

AODV Protocol l 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. l Each recipient R of this RREQ keeps a return pointer. l R broadcasts the request to its neighbors if it is not D and does not have a route to D. l 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

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

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 LANE#2 LANE#1 TURN RIGHT USE LANE#2

Switching Tables

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

ATM Cell Format (UNI) l 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: (48 bytes)

ATM Segmentation and Reassembly l With a 48 byte payload, it isn’t practical to fragment IP packets into ATM cells and reassemble them at the destination. l ATM provides a Assembly Adaptation Layer (AAL) that takes a Protocol Data Unit (PDU) from a higher level protocol and sends it across an ATM network as a family of cells. l AAL5 uses type bits in the cells to delimit cells holding each PDU.

Assembly Adaptation Layer ATM AAL PDU ATM AAL PDU ATM Cells