Chapter 5-2 Link Layer Computer Networking: Top Down pproach 6 th edition Jim Kurose, Keith Ross ddison-wesley March 2012 Link layer, LNs: outline 5.1 introduction, services 5.2 error detection, correction 5.3 multiple access protocols 5.4 LNs addressing, RP ernet switches VLNS 5.5 data center networking 5.6 a day in the life of a web request Link Layer 5-1 Link Layer 5-2 MC addresses 32-bit address: network-layer address for interface used for layer 3 (network layer) forwarding MC (or LN or physical) address: function: used locally to send from one interface to another physically-connected interface (same network, in addressing sense) 48 bit MC address (for most LNs) burned in NIC ROM, also sometimes software settable e.g.: 1-2F--76-09-D hexadecimal (base 16) notation (each number represents 4 bits) MC addresses (more) each adapter on LN has unique MC (or LN) address 71-65-F7-2-08-53 LN (wired or wireless) 1-2F--76-09-D 58-23-D7-F-20-0 0C-C4-11-6F-E3-98 adapter Link Layer 5-3 Link Layer 5-4
MC addresses (more) MC address allocation administered by IEEE manufacturer buys portion of MC address space (to assure uniqueness) Destination address unicast address multicast address broadcast = 111...111 ddresses local or global Global addresses first 24 bits assigned to vender (OUI) Cisco 00-00-0C 3COM 00-01-02 next 24 bits assigned by vender Link Layer 5-5 RP: address resolution protocol Question: how to determine interface s MC address, knowing its address? 137.196.7.23 71-65-F7-2-08-53 137.196.7.88 LN 137.196.7.78 1-2F--76-09-D 137.196.7.14 58-23-D7-F-20-0 0C-C4-11-6F-E3-98 RP table: each node (host, ) on LN has table /MC address mappings for some LN nodes: < address; MC address; TTL> TTL (Time To Live): time after which address mapping will be forgotten (typically 20 min) Link Layer 5-6 RP protocol: same LN wants to send datagram to s MC address not in s RP table. broadcasts RP query packet, containing 's address dest MC address = FF-FF- FF-FF-FF-FF all nodes on LN receive RP query receives RP query packet, replies to with its ('s) MC address sent to s MC address (unicast) caches (saves) -to- MC address pair in its RP table until information becomes old (times out) soft state: information that times out (goes away) unless refreshed RP is plug-and-play : nodes create their RP tables without intervention from net administrator Link Layer 5-7 Link Layer 5-8
ddressing: routing to another LN walkthrough: send datagram from to via R focus on addressing at (datagram) and MC layer () assume knows s address assume knows address of first hop, R (how?) assume knows R s MC address (how?) ddressing: routing to another LN creates datagram with source, destination creates link-layer with R's MC address as dest, contains -to- datagram MC src: 74-29-9C-E8-FF-55 MC dest: E6-E9-00-17--4 src: 111.111.111.111 dest: 222.222.222.222 111.111.111.111 74-29-9C-E8-FF-55 R 222.222.222.220 1-23-F9-CD-06-9 222.222.222.222 49-D-D2-C7-56-2 111.111.111.111 74-29-9C-E8-FF-55 R 222.222.222.220 1-23-F9-CD-06-9 222.222.222.222 49-D-D2-C7-56-2 111.111.111.112 CC-49-DE-D0--7D 111.111.111.110 E6-E9-00-17--4 222.222.222.221 88-2-2F-54-1-0F 111.111.111.112 CC-49-DE-D0--7D 111.111.111.110 E6-E9-00-17--4 222.222.222.221 88-2-2F-54-1-0F Link Layer 5-9 Link Layer 5-10 ddressing: routing to another LN sent from to R received at R, datagram removed, passed up to ddressing: routing to another LN R forwards datagram with source, destination R creates link-layer with 's MC address as dest, contains -to- datagram 111.111.111.111 74-29-9C-E8-FF-55 MC src: 74-29-9C-E8-FF-55 MC dest: E6-E9-00-17--4 src: 111.111.111.111 src: 111.111.111.111 dest: 222.222.222.222 dest: 222.222.222.222 R 222.222.222.220 1-23-F9-CD-06-9 222.222.222.222 49-D-D2-C7-56-2 111.111.111.111 74-29-9C-E8-FF-55 R 222.222.222.220 1-23-F9-CD-06-9 MC src: 1-23-F9-CD-06-9 MC dest: 49-D-D2-C7-56-2 src: 111.111.111.111 dest: 222.222.222.222 222.222.222.222 49-D-D2-C7-56-2 111.111.111.112 CC-49-DE-D0--7D 111.111.111.110 E6-E9-00-17--4 222.222.222.221 88-2-2F-54-1-0F 111.111.111.112 CC-49-DE-D0--7D 111.111.111.110 E6-E9-00-17--4 222.222.222.221 88-2-2F-54-1-0F Link Layer 5-11 Link Layer 5-12
ddressing: routing to another LN R forwards datagram with source, destination R creates link-layer with 's MC address as dest, contains -to- datagram 111.111.111.111 74-29-9C-E8-FF-55 R 222.222.222.220 1-23-F9-CD-06-9 MC src: 1-23-F9-CD-06-9 MC dest: 49-D-D2-C7-56-2 src: 111.111.111.111 dest: 222.222.222.222 222.222.222.222 49-D-D2-C7-56-2 ddressing: routing to another LN R forwards datagram with source, destination R creates link-layer with 's MC address as dest, contains -to- datagram 111.111.111.111 74-29-9C-E8-FF-55 R 222.222.222.220 1-23-F9-CD-06-9 MC src: 1-23-F9-CD-06-9 MC dest: 49-D-D2-C7-56-2 src: 111.111.111.111 dest: 222.222.222.222 222.222.222.222 49-D-D2-C7-56-2 111.111.111.112 CC-49-DE-D0--7D 111.111.111.110 E6-E9-00-17--4 222.222.222.221 88-2-2F-54-1-0F 111.111.111.112 CC-49-DE-D0--7D 111.111.111.110 E6-E9-00-17--4 222.222.222.221 88-2-2F-54-1-0F Link Layer 5-13 Link Layer 5-14 Link layer, LNs: outline 5.1 introduction, services 5.2 error detection, correction 5.3 multiple access protocols 5.4 LNs addressing, RP ernet switches VLNS 5.5 data center networking 5.6 a day in the life of a web request ernet dominant wired LN technology: cheap $20 for NIC first widely used LN technology simpler, cheaper than token LNs and TM kept up with speed race: 10 Mbps 10 Gbps Metcalfe s ernet sketch Link Layer 5-15 Link Layer 5-16
ernet history IEEE 802.3 structure (1) 1970 LOHnet radio network deployed in Hawaiian islands 1973 Metcalfe and oggs invent ernet, random access in wired net 1979 DIX ernet II Standard 1985 IEEE 802.3 LN Standard (10 Mbps) 1995 Fast ernet (100 Mbps) 1998 Gigabit ernet 2002 10 Gigabit ernet ernet is the dominant LN standard 802.3 MC Frame 7 1 6 6 2 4 Preamble SFD Destination Source Length Information Pad FCS address address Synch Start of delimiter 64-1518 bytes preamble helps receivers synchronize their clocks to transmitter clock preamble: 7 bytes of 10101010 start of delimiter: 10101011 Link Layer 5-17 Link Layer 5-18 IEEE 802.3 structure (2) 7 1 6 6 2 4 Preamble SFD Destination Source Length Information Pad FCS address address Synch Start 802.3 MC Frame 64-1518 bytes addresses: 6 byte source, destination MC addresses if adapter receives with matching destination address, or with broadcast address (e.g. RP packet), it passes data in to network layer protocol otherwise, adapter discards IEEE 802.3 structure (3) 7 1 6 6 2 4 Preamble SFD Destination Source Length Information Pad FCS address address Synch Start 802.3 MC Frame 64-1518 bytes length: # bytes in information field max 1518 bytes, excluding preamble & SFD max information 1500 bytes: 05DC pad: ensures min of 64 bytes FCS: CCITT-32 CRC, covers addresses, length, information, pad fields NIC discards s with improper lengths or failed CRC Link Layer 5-19 Link Layer 5-20
ernet structure 7 1 6 6 2 4 Preamble SFD Destination Source Type Information FCS address address Synch Start ernet 64-1518 bytes DIX: Digital, Intel, Xerox joint ernet specification type field (2 bytes): to identify protocol of PDU in information field, e.g., RP 0800: 0806: RP 8137: Netware X 8189: NetIOS ernet: unreliable, connectionless connectionless: no handshaking between sending and receiving NICs unreliable: receiving NIC does not send acks or nacks to sending NIC data in dropped s recovered only if initial sender uses higher layer rdt (e.g., TCP), otherwise dropped data lost ernet s MC protocol: unslotted CSM/CD wth binary backoff Link Layer 5-21 Link Layer 5-22 ernet: physical topology bus: popular through mid 90s all nodes in same collision domain (can collide with each other) star: prevails today active switch in center each spoke runs a (separate) ernet protocol (nodes do not collide with each other) 802.3 ernet standards: link & physical layers many different ernet standards common MC protocol and format different speeds: 2 Mbps, 10 Mbps, 100 Mbps, 1Gbps, 10G bps different physical layer media: twisted pair, cable, fiber bus: coaxial cable star switch Link Layer 5-23 application transport network link physical 100SE-TX 100SE-T4 copper (twisted pair) physical layer MC protocol and format 100SE-T2 100SE-SX 100SE-FX 100SE-X fiber physical layer Link Layer 5-24
Link layer, LNs: outline 5.1 introduction, services 5.2 error detection, correction 5.3 multiple access protocols 5.4 LNs addressing, RP ernet switches VLNS 5.5 data center networking 5.6 a day in the life of a web request ernet switch link-layer device: takes an active role store, forward ernet s examine incoming s MC address, selectively forward to one-or-more outgoing links when is to be forwarded on segment, uses CSM/CD to access segment transparent hosts are unaware of presence of switches plug-and-play, self-learning switches do not need to be configured Link Layer 5-25 Link Layer 5-26 Switch: multiple simultaneous transmissions Switch forwarding table hosts have dedicated, direct connection to switch switches buffer packets ernet protocol used on each incoming link, but no collisions; each link is its own collision domain switching: -to- and -to- can transmit simultaneously, without collisions C 6 5 1 2 C 4 3 switch with six interfaces (1,2,3,4,5,6) Q: how does switch know reachable via interface 4, reachable via interface 5? : each switch has a switch table, each entry: (MC address of host, interface to reach host, time stamp) looks like a routing table! Q: how are entries created, maintained in switch table? something like a routing protocol? C 6 5 1 2 C 4 3 switch with six interfaces (1,2,3,4,5,6) Link Layer 5-27 Link Layer 5-28
Switch: self-learning Source: Dest: Switch: filtering/forwarding switch learns which hosts can be reached through which interfaces when received, switch learns location of sender: incoming LN segment records sender/location pair in switch table C MC addr interface TTL 6 5 1 2 C 1 60 4 3 Switch table (initially empty) when received at switch: 1. record incoming link, MC address of sending host 2. index switch table using MC destination address 3. if entry found for destination then { if destination on segment from which arrived then drop else forward on interface indicated by entry } else flood /* forward on all interfaces except arriving interface */ Link Layer 5-29 Link Layer 5-30 Self-learning, forwarding: example Source: Dest: Interconnecting switches destination,, locaton unknown: flood destination location known: selectively send on just one link C MC addr interface TTL 6 1 2 5 4 C 1 60 4 60 3 switch table (initially empty) switches can be connected together S 1 C S 2 D Q: sending from to G - how does S 1 know to forward destined to G via S 4 and S 3? : self learning! (works exactly the same as in single-switch case!) E S 4 F G S 3 H I Link Layer 5-31 Link Layer 5-32
Institutional network to external network 1 Gbps 100 Mbps 10 Gbps switch 10 Gbps 1 Gbps 1 Gbps 100 Mbps mail server web server subnet 100 Mbps Switches vs. s both are store-and-forward: s: network-layer devices (examine networklayer headers) switches: link-layer devices (examine link-layer headers) both have forwarding tables: s: compute tables using routing algorithms, addresses switches: learn forwarding table using flooding, learning, MC addresses datagram application transport network link physical switch application transport network link physical link physical network link physical datagram Link Layer 5-33 Link Layer 5-34 VLNs (Virtual LNs): motivation Computer Science Electrical Engineering Computer Engineering consider: single broadcast domain: all layer-2 broadcast traffic must cross entire LN security/privacy, efficiency issues for traffic isolation, replace the center switch with a CS user moves office to EE, but wants connect to CS switch? VLNs Virtual Local rea Network switch(es) supporting VLN capabilities can be configured to define multiple virtual LNS over single physical LN infrastructure. port-based VLN: switch ports grouped (by switch management software) so that single physical switch 1 2 Electrical Engineering (VLN ports 1-8) 7 8 9 10 15 16 Computer Science (VLN ports 9-15) operates as multiple virtual switches 1 2 Electrical Engineering (VLN ports 1-8) 7 9 8 10 15 16 Computer Science (VLN ports 9-15) Link Layer 5-35 Link Layer 5-36
Port-based VLN traffic isolation: s to/from ports 1-8 can only reach ports 1-8 can also define VLN based on MC addresses of endpoints, rather than switch port dynamic membership: ports can be dynamically assigned among VLNs forwarding between VLNS: done via routing (just as with separate switches) in practice vendors sell combined switches plus s 1 2 Electrical Engineering (VLN ports 1-8) 7 8 9 10 15 16 Computer Science (VLN ports 9-15) Link layer, LNs: outline 5.1 introduction, services 5.2 error detection, correction 5.3 multiple access protocols 5.4 LNs addressing, RP ernet switches VLNS 5.5 data center networking 5.6 a day in the life of a web request Link Layer 5-37 Link Layer 5-38 Data center networks Data center networks 10 s to 100 s of thousands of hosts, often closely coupled, in close proximity: e-business (e.g. mazon) content-servers (e.g., YouTube, kamai, pple, Microsoft) search engines, data mining (e.g., Google) challenges: multiple applications, each serving massive numbers of clients managing/balancing load, avoiding processing, networking, data bottlenecks Inside a 40-ft Microsoft container, Chicago data center Link Layer 5-39 Load balancer Internet 1 2 3 4 5 6 7 8 C ccess load balancer: application-layer routing receives external client requests directs workload within data center returns results to external client (hiding data center internals from client) order Load balancer Tier 1 switches Tier 2 switches TOR switches (Top of Rack) Server racks Link Layer 5-40
Link layer, LNs: outline Synthesis: a day in the life of a web request 5.1 introduction, services 5.2 error detection, correction 5.3 multiple access protocols 5.4 LNs addressing, RP ernet switches VLNS 5.5 data center networking 5.6 a day in the life of a web request journey down protocol stack complete! application, transport, network, link putting-it-all-together: synthesis! goal: identify, review, understand protocols (at all layers) involved in seemingly simple scenario: requesting www page scenario: student attaches laptop to campus network, requests/receives www.google.com Link Layer 5-41 Link Layer 5-42 day in the life: scenario day in the life connecting to the Internet browser Comcast network 68.80.0.0/13 server connecting laptop needs to get its own address, addr of first-hop, addr of server: use school network 68.80.2.0/24 web page web server 64.233.169.105 Google s network 64.233.160.0/19 (runs ) request encapsulated in, encapsulated in, encapsulated in 802.3 ernet ernet broadcast (dest: FFFFFFFFFFFF) on LN, received at running server ernet demuxed to demuxed, demuxed to Link Layer 5-43 Link Layer 5-44
day in the life connecting to the Internet day in the life RP (before, before ) (runs ) Client now has address, knows name & addr of server, address of its first-hop server formulates CK containing client s address, address of first-hop for client, name & address of server encapsulation at server, forwarded (switch learning) through LN, demultiplexing at client client receives CK reply RP query RP RP reply RP (runs ) before sending request, need address of www.google.com: query created, encapsulated in, encapsulated in, encapsulated in. To send to, need MC address of interface: RP RP query broadcast, received by, which replies with RP reply giving MC address of interface client now knows MC address of first hop, so can now send containing query Link Layer 5-45 Link Layer 5-46 day in the life using (runs ) datagram containing query forwarded via LN switch from client to 1 st hop Comcast network 68.80.0.0/13 server datagram forwarded from campus network into comcast network, routed (tables created by R, OSPF, IS-IS and/or GP routing protocols) to server demux ed to server server replies to client with address of www.google.com Link Layer 5-47 day in the life TCP connection carrying SYNCK SYNCK SYNCK SYNCK SYNCK SYNCK TCP TCP web server 64.233.169.105 (runs ) to send request, client first opens TCP socket to web server TCP SYN segment (step 1 in 3- way handshake) inter-domain routed to web server web server responds with TCP SYNCK (step 2 in 3-way handshake) client responds with TCP CK (step 3 in 3-way handshake) TCP connection established! Link Layer 5-48
day in the life request/reply TCP web page finally (!!!) displayed TCP web server 64.233.169.105 (runs ) request sent into TCP socket datagram containing request routed to www.google.com web server responds with reply (containing web page) datagram containing reply routed back to client Link Layer 5-49