Communication across the ether

Transcription

1 Cmmunicatin acrss the ether Ethernet CS242 Cmputer Netwrks Department f Cmputer Science Wellesley Cllege Lcal Area Netwrks (LANs) Multiple access prtcls are used fr satellite and wireless channels. They are als used in upstream channel fr cable access t the Internet and in LANs*. Recall that a LAN is a cmputer netwrk cncentrated in a gegraphical area, e.g., Wellesley Cllege. *Less s nw. Wellesley Cllege, riginally n a single multiple access channel, is nw a switched LAN. Ethernet 19-2 Surprise Hsts and ruters have link-layer addresses in additin t netwrk-layer addresses. Why in the wrld wuld they want bth? Link-layer addresses A LAN nde s link-layer address (variusly physical, Ethernet, r MAC* address) prperly belngs its adapter. Generally these are 48 bits lng. Althugh MAC addresses were designed t be permanent, (riginally burnt int the adapter s ROM) it is nw pssible t set an adapter's MAC via sftware. Expressed in dashed hexadecimal ntatin *Media Access Cntrl Ethernet 19-3 Ethernet

2 N tw are ever alike Flat tax? Hw is this pssible? MAC address space is managed by IEEE. When a cmpany want t manufacture adapters, it purchases a chunk f 2 24 addresses fr a nminal fee. The first 24 bits f a MAC address are fixed by IEEE, and cmpany gets t create unique cmbinatins f the last 24 bits. An adapter's MAC address has a flat structure, as ppsed t IP's hierarchical structure. A slaptp* has the same MAC address, n matter where s ever it ges*; meanwhile its IP address is buncing all ver the place**. *Kinda like yur scial security number. Ethernet 19-5 **Kinda like yur pstal address (at least when yu are yung). Ethernet 19-6 An adaptr sends a frame int the LAN MAC Bradcast address If it is a bradcast LAN*, it is received and prcessed by all ther adapters n the LAN. On the receiving end, If a destinatin address matches the nde s MAC address, it extracts the netwrk-layer datagram and passes it up the prtcl stack. If the destinatin address desn t match, the nde discards the frame. The hst is never disturbed. Smetimes the sending adaptr wants all ther adapters n the LAN t receive and prcess the frame. The sending adapter inserts a MAC bradcast address int the destinatin field f the frame.* *Such as r the ld fashin Ethernet LANs. Ethernet 19-7 *Fr an Ethernet and tken passing LAN, the bradcast address is FF-FF-FF-FF-FF-FF. Ethernet

3 Address reslutin prtcl (ARP) Every hst and ruter n a LAN has an ARP mdule that perfrms directry assistance between netwrk-layer and link-layer addresses. An ARP table, kept in RAM, helps ut. IP Address LAN Address TTL B2-2F-54-1A-0F 13:45: C-66-AB B1 13:52:00 APR is similar t DNS, except DNS reslves hst names fr hsts anywhere in the Internet. ARP reslves IP addresses nly fr ndes n the same subnet. Suppse nde wants t send a datagram t nde Ethernet 19-9 Ethernet Okay, s nw suppse nde wants t send a datagram t nde IP Address LAN Address TTL B2-2F-54-1A-0F 13:45: C-66-AB B1 13:52:00 Ethernet ARP prtcl 1. Nde LAN bradcasts an ARP query packet asking fr s physical address. 2. All ndes n the LAN examine the query and the queried party, recgnizing its IP address, sends a standard frame t the querying party cntaining the desired infrmatin. 3. Nde enters the infrmatin int its ARP table and cntinues as befre. Nte that APR is plug-and-play; a nde s ARP table is cnstructed autmatically withut an administratr s help. Ethernet

4 Neither fish nr ful Sending t a nde ff the LAN ARP packets are encapsulated with a linklayer frame and thus lie architecturally abve the link layer. Suppse wants t send a message t Shuld it address the message t physical address 49-BD-D2-C7-56-2A? Hwever, ARP packets have fields cntaining linklayer addresses and thus might be cnsidered a link-layer prtcl. Ethernet Ethernet Well it culd Instead, hst the message wuldn t get there. The prblem is that 49-BD-D2-C7-56-2A isn t listening n s LAN. 1) Lks up in its frwarding table. 3) The apprpriate MAC address fr wuld either be lked up in the s ARP table r queried using ARP prtcl. Ethernet ) Said table indicates that t reach hst the datagram must be sent first t ) The interface s adapter encapsulates the datagram and the bits are shipped addressed t MAC address E6-E BB-4B. Ethernet

5 Nw it s the ruter s turn 1) Ruter lks up in its frwarding table and discvers its LAN is hanging ff f interface ) The ruter passes the datagram t the prper interface. Ethernet 3) Interface passes datagram t its adapter, which encapsulates the datagram in a new frame addressed t the destinatin s MAC address 49-BDD2-C7-56-2A.* Since the mid 1990s Ethernet has dminated the LAN market. Partially because, First t be widely deplyed. Generally cheaper and simpler than its cmpetitrs. Always managed t maintain cmparable data rates with emerging technlgies. *Discvered using ARP. Ethernet Original bus tplgy fr Ethernet LANs* Metcalfe and Bggs's riginal design used a caxial bus t intercnnect the ndes. A tplgy that lingered n int the mid-nineties. *Bradcast LAN - all transmitted frames travel t and are prcessed by all adapters cnnected t the bus. Ethernet Ethernet By the late 1990s the bus LANs had been replaced with Ethernet installins using a hubbased star tplgy. A hub is a physical-layer device that acts n individual bits rather than frames. When a bit arrives frm ne interface, the hub recreates the bit, bsts its energy, and transmits the bit nt all ther interfaces. Ethernet

6 Hubs als prvide netwrk management features If an adapter is misbehaving (e.g., hgging the netwrk), the hub can detect this and discnnect the ffending ne. Mst hubs als gather infrmatin and reprt the infrmatin back t a hst that cnnects directly t the hub. In the early 2000s the hub at the center f the star tplgy was replaced with a switch. A switch is a cllisin-less, stre-and-frward packet switching device which perates at the physical and link layers.* Ethernet *Yu guessed it, mre later. Ethernet Ethernet frame structure Rise and shine Seven bytes f and ne byte f LAN physical addresses 6 bytes each Our friend frm previus lecture 4 bytes The first seven bytes f the preamble* serves as a wake up call t the receiving adapters and t synchrnize their clcks t that f the sender. Why Sync? Tw bytes used fr multiplexed netwrk -layer prtcls; wh d I pass the data up t? Carries IP datagram; has MTU f 1500 bytes and minimum f 46 (if less, it is stuffed) *Seven bytes f (fllwed by ne byte f ). Ethernet Ethernet

7 Demultiplexing using the type field. What culd pssible g wrng? Hsts can use multiple netwrk-layer prtcls.* When Ethernet frame arrives at adapter B, B needs t knw t which netwrk-layer prtcl it shuld pass its cntents. This field is analgus t the prtcl field in the netwrk-layer datagram and the prt number in the transprt-layer segment. Causes f errrs include attenuatin in signal strength and ambient electrmagnetic energy that leaks int the Ethernet cables and interface cards. CRC calculated n everything except preamble. *Each prtcl has its wn type number (e..g., a frame cntenting an ARP packet has type field hex 0806). Ethernet Ethernet Unreliable cnnectinless service Ethernet prvides cnnectinless service; n handshaking, the sending adapter simply encapsulates the datagram and drps it int the LAN. Similarly, the service is unreliable. Receiver checks fr errrs, but sends n acknwledgements ne way r the ther.* Baseband transmissin and Manchester encding* *Very much like my family at Christmas. Ethernet *Manchester encding prperly belngs t the physical layer, but used extensively in Ethernet. Ethernet

8 CSMA/CD Ethernet CSMA/CD algrithm 1. An adapter may begin t transmit any time; n slts. 2. An adapter never transmits when it senses that sme ther adapter is transmitting; uses carrier detect. 3. A transmitting adapter abrts transmissin as sn as it detects anther transmissin; uses cllisin detect. 4. Befre attempting retransmissin, an adapter waits a randm time. 1. Adaptr gets datagram frm and creates frame. 2. If adapter senses channel idle, it starts t transmit frame. If it senses channel busy, waits until channel idle and then transmits. 3. If adapter transmits entire frame withut detecting anther transmissin, the adapter is dne with frame. 4. If adapter detects anther transmissin while transmitting, abrts and sends as 48-bit jam signal. 5. After abrting, adapter enters expnential backff: after the m th cllisin, adapter chses a K at randm frm {0, 1, 2,, 2 m -1}. Adapter waits K*512 bit times and returns t Step 2. Ethernet Ethernet Why a jam signal? Expnential backff Suppse A begins t transmit a frame, and just befre A s signal reaches B, B begins t transmit. In this case, B will have transmitted nly a few bits when it abrts its transmissin. These bits will prpagate t A, but they may nt cnstitute enugh energy fr A t detect the cllisin. The idea f the backff is the first time a cllisin is detected, adapter ges 50/50 the next frame. If anther cllisin, the adapter is mre cnservative and chses with prbability 1/4 frm {0,1,2,3}, and s frth. Ethernet impses limits n the distance between any tw ndes t ensure that nce an adaptr starts t transmit, it can finish withut a new cllisin. Ethernet Ethernet

9 Ethernet efficiency Many flavrs The efficiency f Ethernet is defined t be the lng-run fractin f time during which frames are begin transmitted n the channel withut cllisins when there are a large number f active ndes. Ethernet efficiency is apprximated by the expressin t prp / t trans We've been talking abut Ethernet as if it were ne prtcl standard when fact it is many. Many f these have been standardized by the IEEE CSMA/CS (Ethernet) wrking grup. where t prp = max prp between 2 ndes in LAN t trans = time t transmit max-size frame Ethernet Ethernet BASE-2 and 10BASE-5 Ethernet Generatins pass Legacy technlgy frm the 90s, 10Base-2 used thin caxial cable t cnnect ndes in a bus tplgy. The 10 stands fr 10 Mbps, BASE refers t baseband Ethernet and the 2 (5) stands fr 200 meters (500 meters), the length limit f a caxial cable. Tday's Ethernet is very different frm the riginal caxial bus technlgy. Ndes are cnnected t a switch via pint-t-pint segments made f twisted-pair cpper wires r fiber-ptic cable. Lnger runs culd be btained by using repeaters, physical-layer devices t bast signals. Ethernet Ethernet

10 100BASE-T, 100-BASE-FX, 100BASE-SX, 100BASE-BX Gigabit and 10 Gbps Ethernet In the mid-nineties, Ethernet was standardized at 100Mbps. Gigabit Ethernet (IEEE 802.3z) is backwardly cmpatible with 10BASE- T/100BASE-T technlgies and uses the same frame frmat. The riginal MAC prtcl and frame frmat were preserved, 100 Mbps Ethernet is limited t a 100 meter distance ver twisted pair, and several kilmeters ver fiber. *The T stands fr twisted pair, the FX, SX, and BX fr fiber. Allws fr pint-t-pint links (switches) as well as shared bradcast channels (hubs). Uses CSMA/CD fr shared bradcast channels. Max distances restricted fr acceptable efficiency. Ethernet Ethernet This week's puzzler In the lden days f buses and hubs, Ethernet was a bradcast link with frame cllisins when ndes transmitted at the same time. Tday, Ethernet is a switch-based star tplgy, using stre-andfrward packet switching. Is there really a need fr an Ethernet MAC prtcl? Ethernet