The Link Layer and LNs: Ethernet and Swiches EECS3214 2018-03-21 Link layer, LNs: outline 6.1 introduction, services 6.2 error detection, correction 6.3 multiple access protocols 6.4 LNs addressing, RP Ethernet switches VLNS 6.5 link virtualization: MPLS 6.6 data center networking 6.7 a day in the life of a web request Link Layer and LNs 6-2 1
MC addresses and RP 32-bit IP address: network-layer address for interface used for layer 3 (network layer) forwarding MC (or LN or physical or Ethernet) address: function: used locally to get frame from one interface to another physically-connected interface (same network, in IPaddressing 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 numeral represents 4 bits) Link Layer and LNs 6-3 LN addresses and RP each adapter on LN has unique LN address 1-2F--76-09-D 71-65-F7-2-08-53 LN (wired or wireless) 58-23-D7-F-20-0 adapter 0C-C4-11-6F-E3-98 Link Layer and LNs 6-4 2
LN addresses (more) MC address allocation administered by IEEE manufacturer buys portion of MC address space (to assure uniqueness) analogy: MC address: like Social Security Number IP address: like postal address MC flat address portability can move LN card from one LN to another IP hierarchical address not portable address depends on IP subnet to which node is attached Link Layer and LNs 6-5 RP: address resolution protocol Question: how to determine interface s MC address, knowing its IP 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 IP node (host, router) on LN has table IP/MC address mappings for some LN nodes: < IP address; MC address; TTL> TTL (Time To Live): time after which address mapping will be forgotten (typically 20 min) Link Layer and LNs 6-6 3
RP protocol: same LN wants to send datagram to s MC address not in s RP table. broadcasts RP query packet, containing 's IP address destination MC address = FF-FF-FF-FF-FF-FF all nodes on LN receive RP query receives RP packet, replies to with its ('s) MC address frame sent to s MC address (unicast) caches (saves) IP-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 and LNs 6-7 ddressing: routing to another LN walkthrough: send datagram from to via R focus on addressing at IP (datagram) and MC layer (frame) assume knows s IP address assume knows IP address of first hop router, R (how?) assume knows R s MC address (how?) 111.111.111.111 74-29-9C-E8-FF-55 111.111.111.112 CC-49-DE-D0--7D R 111.111.111.110 E6-E9-00-17--4 222.222.222.220 1-23-F9-CD-06-9 222.222.222.222 49-D-D2-C7-56-2 222.222.222.221 88-2-2F-54-1-0F Link Layer and LNs 6-8 4
ddressing: routing to another LN creates IP datagram with IP source, destination creates link-layer frame with R's MC address as destination address, frame contains -to- IP datagram IP Eth Phy MC src: 74-29-9C-E8-FF-55 MC dest: E6-E9-00-17--4 IP src: 111.111.111.111 IP dest: 222.222.222.222 111.111.111.111 74-29-9C-E8-FF-55 111.111.111.112 CC-49-DE-D0--7D R 111.111.111.110 E6-E9-00-17--4 222.222.222.220 1-23-F9-CD-06-9 222.222.222.222 49-D-D2-C7-56-2 222.222.222.221 88-2-2F-54-1-0F Link Layer and LNs 6-9 ddressing: routing to another LN frame sent from to R frame received at R, datagram removed, passed up to IP MC src: 74-29-9C-E8-FF-55 MC dest: E6-E9-00-17--4 IP src: 111.111.111.111 IP src: 111.111.111.111 IP dest: 222.222.222.222 IP dest: 222.222.222.222 IP Eth Phy 111.111.111.111 74-29-9C-E8-FF-55 111.111.111.112 CC-49-DE-D0--7D IP Eth Phy R 111.111.111.110 E6-E9-00-17--4 222.222.222.220 1-23-F9-CD-06-9 222.222.222.222 49-D-D2-C7-56-2 222.222.222.221 88-2-2F-54-1-0F Link Layer and LNs 6-10 5
ddressing: routing to another LN R forwards datagram with IP source, destination R creates link-layer frame with 's MC address as destination address, frame contains -to- IP datagram 111.111.111.111 74-29-9C-E8-FF-55 IP Eth Phy 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 IP src: 111.111.111.111 IP dest: 222.222.222.222 IP Eth Phy 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 Link Layer and LNs 6-11 ddressing: routing to another LN R forwards datagram with IP source, destination R creates link-layer frame with 's MC address as destination address, frame contains -to- IP datagram 111.111.111.111 74-29-9C-E8-FF-55 IP Eth Phy 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 IP src: 111.111.111.111 IP dest: 222.222.222.222 IP Eth Phy 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 Link Layer and LNs 6-12 6
ddressing: routing to another LN R forwards datagram with IP source, destination R creates link-layer frame with 's MC address as dest, frame contains -to- IP datagram MC src: 1-23-F9-CD-06-9 MC dest: 49-D-D2-C7-56-2 IP src: 111.111.111.111 IP dest: 222.222.222.222 IP Eth Phy 111.111.111.111 74-29-9C-E8-FF-55 111.111.111.112 CC-49-DE-D0--7D R 111.111.111.110 E6-E9-00-17--4 222.222.222.220 1-23-F9-CD-06-9 222.222.222.222 49-D-D2-C7-56-2 222.222.222.221 88-2-2F-54-1-0F * Check out the online interactive exercises for more examples: http://gaia.cs.umass.edu/kurose_ross/interactive/ Link Layer and LNs 6-13 Link layer, LNs: outline 6.1 introduction, services 6.2 error detection, correction 6.3 multiple access protocols 6.4 LNs addressing, RP Ethernet switches VLNS 6.5 link virtualization: MPLS 6.6 data center networking 6.7 a day in the life of a web request Link Layer and LNs 6-14 7
Ethernet dominant wired LN technology: first widely used LN technology simpler, cheap kept up with speed race: 10 Mbps 10 Gbps Metcalfe s Ethernet sketch Link Layer and LNs 6-15 Ethernet: physical topology bus: popular through mid 90s all nodes in same collision domain (can collide with each other) use CSM/CD with binary exponential backoff star: prevails today active switch in center each spoke runs a (separate) Ethernet protocol (nodes do not collide with each other) bus: coaxial cable star switch Link Layer and LNs 6-16 8
Ethernet frame structure sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame preamble dest. address source address type data (payload) CRC preamble: 7 bytes with pattern 10101010 followed by one byte with pattern 10101011 used to synchronize receiver, sender clock rates Link Layer and LNs 6-17 Ethernet frame structure (more) addresses: 6 byte source, destination MC addresses if adapter receives frame with matching destination address, or with broadcast address (e.g. RP packet), it passes data in frame to network layer protocol otherwise, adapter discards frame type: indicates higher layer protocol (mostly IP but others possible, e.g., Novell IPX, ppletalk) CRC: cyclic redundancy check at receiver error detected: frame is dropped preamble dest. address source address type data (payload) CRC Link Layer and LNs 6-18 9
Ethernet: unreliable, connectionless connectionless: no handshaking between sending and receiving NICs unreliable: receiving NIC doesn't send CK or NCK to sending NIC data in dropped frames recovered only if initial sender uses higher layer reliable data transfer (e.g., TCP), otherwise dropped data lost Ethernet s MC protocol: unslotted CSM/CD with binary exponential backoff Link Layer and LNs 6-19 802.3 Ethernet standards: link and physical layers many different Ethernet standards common MC protocol and frame format different speeds: 2 Mbps, 10 Mbps, 100 Mbps, 1Gbps, 10 Gbps, 40 Gbps different physical layer media: optical fiber, cable application transport network link physical 100SE-TX 100SE-T4 MC protocol and frame format 100SE-T2 100SE-SX 100SE-FX 100SE-X copper (twister pair) physical layer fiber physical layer Link Layer and LNs 6-20 10
Link layer, LNs: outline 6.1 introduction, services 6.2 error detection, correction 6.3 multiple access protocols 6.4 LNs addressing, RP Ethernet switches VLNS 6.5 link virtualization: MPLS 6.6 data center networking 6.7 a day in the life of a web request Link Layer and LNs 6-21 Ethernet switch link-layer device: takes an active role store, forward Ethernet frames examine incoming frame s MC address, selectively forward frame to one-or-more outgoing links when frame 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 and LNs 6-22 11
Switch: multiple simultaneous transmissions hosts have dedicated, direct connection to switch switches buffer packets Ethernet protocol used on each incoming link, but no collisions; full duplex 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) Link Layer and LNs 6-23 Switch forwarding table 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 and LNs 6-24 12
Switch: self-learning Source: Dest: switch learns which hosts can be reached through which interfaces when frame received, switch learns location of sender: incoming LN segment records sender/location pair in switch table C 6 1 2 5 4 3 C MC addr interface TS 1 9:32 Switch table (initially empty) Link Layer and LNs 6-25 Switch: frame filtering/forwarding when frame received at switch: 1. record incoming link, MC address of source if entry for source does not exist 2. index switch table using MC destination address 3. if entry found for destination then { if destination on LN segment from which frame arrived then drop frame /* frame filtering */ else forward frame on interface indicated by entry } else flood /* forward on all interfaces except arriving interface */ Link Layer and LNs 6-26 13
Self-learning, forwarding: example Source: Dest: frame destination,, location unknown: flood destination location known: selectively send on just one link C 6 1 2 5 4 3 C MC addr interface TS 1 9:32 4 9:33 switch table (initially empty) Link Layer and LNs 6-27 Frame filtering Data Link Layer 5-28 14
Interconnecting switches self-learning switches can be connected together: S 4 S 1 C S 2 D E F G S 3 H I Q: sending from to G - how does S 1 know to forward frame destined to G via S 4 and S 3? : self learning! (works exactly the same as in single-switch case!) Link Layer and LNs 6-29 Self-learning multi-switch example Suppose C sends frame to I, I responds to C S 4 S 1 C S 2 D E F G S 3 H I Q: show switch tables and packet forwarding in S 1, S 2, S 3, S 4 Link Layer and LNs 6-30 15
Institutional network to external network router mail server web server IP subnet Link Layer and LNs 6-31 Switches vs. routers both are store-and-forward: routers: network-layer devices (examine networklayer headers) switches: link-layer devices (examine link-layer headers) datagram frame application transport network link physical switch link physical frame both have forwarding tables: routers: compute tables using routing algorithms, IP addresses switches: learn forwarding table using flooding, learning, MC addresses application transport network link physical network link physical datagram frame Link Layer and LNs 6-32 16
Link layer, LNs: outline 6.1 introduction, services 6.2 error detection, correction 6.3 multiple access protocols 6.4 LNs addressing, RP Ethernet switches VLNS 6.5 link virtualization: MPLS 6.6 data center networking 6.7 a day in the life of a web request Link Layer and LNs 6-33 17