CSE 331: Introduction to Networks and Security Slide Set 4 Fall 2000 Instructor: Carl A. Gunter
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
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.
Frame Format 1500 bytes 64 48 48 16 32 Packet Body Dest Address Src Address Packet Body Type Preamble CRC For Demux!
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.
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* 51.2s before trying again.
Worst Case Scenario A B A begins to omit a frame at time t
Worst Case Scenario A B The beginning of A’s frame almost reaches b at t + d
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.
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.
Ethernet Pragmatics • Limited to about 2500m length to achieve 51.2s 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.
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).
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.
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
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.
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.
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.
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.
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.
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.
Distribution System X Y B A Access Points
Distribution System X Y B A Changing Access Points A
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
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.
Illustration Part 1 of 2 Movement Routing
Illustration Part 2 of 2 New Routing
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
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)
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
Circuit-Switching Tradeoffs • - Delay for call setup • -/+ Statefullness • - loss of state on failures • + fast lookups (small Ids) • + QoS associations
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)
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
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