Computer Networking - Link Layer (PDF)

Computer Networking CCS-2201/CE-231: Introduction to Networks Dr. Ehab Abousaif PhD in Electrical and Computer Eng., University of Idaho, USA IEEE member, IMAPS member College of Computing and Information Technology, AASTMT Cell: 01114757888 Email: [email protected] Chapter 6 The Link Layer...

Computer Networking CCS-2201/CE-231: Introduction to Networks Dr. Ehab Abousaif PhD in Electrical and Computer Eng., University of Idaho, USA IEEE member, IMAPS member College of Computing and Information Technology, AASTMT Cell: 01114757888 Email: [email protected] Chapter 6 The Link Layer Computer Networking: A and LANs (Short Version) Top-Down Approach A note on the use of these PowerPoint slides: 8th edition  These slides are based on the original slides made by the authors of the book. Jim Kurose, Keith Ross  These slides are a modified version of the original slides and have the same copyrights for Pearson, 2020 the authors and for Dr. Ehab Abousaif who modify the slides and put it into this final form. Copyrights ©Dr. Ehab Abousaif – Link Layer: 4-1 Link layer, LANs: roadmap  introduction  error detection, correction  LANs addressing, ARP Ethernet switches VLANs Copyrights ©Dr. Ehab Abousaif – Link Layer: 6-2 Link layer: introduction terminology: mobile network  hosts and routers: nodes national or global ISP  communication channels that connect adjacent nodes along communication path: links wired wireless LANs  layer-2 packet: frame, encapsulates datagram datacenter network link layer has responsibility of transferring datagram from one node enterprise network to physically adjacent node over a link Copyrights ©Dr. Ehab Abousaif – Link Layer: 6-3 Link layer: context  datagram transferred by transportation analogy: different link protocols over  trip from Princeton to Lausanne different links: limo: Princeton to JFK e.g., WiFi on first link, Ethernet plane: JFK to Geneva on next link train: Geneva to Lausanne  each link protocol provides  tourist = datagram different services  transport segment = e.g., may or may not provide communication link reliable data transfer over link  transportation mode = link-layer protocol  travel agent = routing algorithm Copyrights ©Dr. Ehab Abousaif – Link Layer: 6-4 Link layer: services  framing, link access: … encapsulate datagram into frame, adding … header, trailer channel access if shared medium “MAC” addresses in frame headers identify source, destination (different from IP address!)  reliable delivery between adjacent nodes we already know how to do this! seldom used on low bit-error links wireless links: high error rates Q: why both link-level and end-end reliability? Copyrights ©Dr. Ehab Abousaif – Link Layer: 6-5 Link layer: services (more)  flow control: … pacing between adjacent sending and … receiving nodes  error detection: errors caused by signal attenuation, noise. receiver detects errors, signals retransmission, or drops frame  error correction: receiver identifies and corrects bit error(s) without retransmission  half-duplex and full-duplex: with half duplex, nodes at both ends of link can transmit, but not at same time Copyrights ©Dr. Ehab Abousaif – Link Layer: 6-6 Where is the link layer implemented?  in each-and-every host  link layer implemented in network interface card (NIC) or on a chip application transport cpu memory Ethernet, WiFi card or chip network link implements link, physical layer host bus (e.g., PCI)  attaches into host’s system link controller buses physical physical  combination of hardware, software, firmware network interface Copyrights ©Dr. Ehab Abousaif – Link Layer: 6-7 Interfaces communicating application application transport transport cpu memory memory CPU datagram network network link link linkh datagram controller controller datagram link link physical physical physical physical sending side: receiving side:  encapsulates datagram in frame  looks for errors, reliable data  adds error checking bits, reliable data transfer, flow control, etc. transfer, flow control, etc.  extracts datagram, passes to upper layer at receiving side Copyrights ©Dr. Ehab Abousaif – Link Layer: 6-8 Link layer, LANs: roadmap  introduction  error detection, correction  LANs addressing, ARP Ethernet switches VLANs Copyrights ©Dr. Ehab Abousaif – Link Layer: 6-9 Error detection EDC: error detection and correction bits (e.g., redundancy) D: data protected by error checking, may include header fields datagram datagram Error detection not 100% otherwise reliable! all  protocol may miss bits in D’ N OK ? detected error some errors, but rarely d data bits  larger EDC field yields D EDC D’ EDC’ better detection and correction bit-error prone link Copyrights ©Dr. Ehab Abousaif – Link Layer: 6-10 Parity checking single bit parity: two-dimensional bit parity:  detect single bit errors  detect and correct single bit errors row parity 0111000110101011 1 d1,1... d1,j d1,j+1 d data bits d2,1... d2,j d2,j+1 parity............ bit di,1... di,j di,j+1 column parity di+1,1... Even parity: set parity di+1,j di+1,j+1 bit so there is an even number of 1’s no errors: 1 0 1 0 1 1 detected 10101 1 11110 0 and 10110 0 parity error correctable 01110 1 single-bit 01110 1 10101 0 error: 10101 0 parity error Copyrights ©Dr. Ehab Abousaif – Link Layer: 6-11 Internet checksum (review) Goal: detect errors (i.e., flipped bits) in transmitted segment sender: receiver:  treat contents of UDP  compute checksum of received segment (including UDP header segment fields and IP addresses) as sequence of 16-bit integers  check if computed checksum equals  checksum: addition (one’s checksum field value: complement sum) of segment not equal - error detected content equal - no error detected. But maybe  checksum value put into errors nonetheless? More later …. UDP checksum field Copyrights ©Dr. Ehab Abousaif – Link Layer: 6-12 Cyclic Redundancy Check (CRC)  more powerful error-detection coding  D: data bits (given, think of these as a binary number)  G: bit pattern (generator), of r+1 bits (given) r CRC bits d data bits D R bit pattern = D *2r XOR R formula for bit pattern goal: choose r CRC bits, R, such that exactly divisible by G (mod 2) receiver knows G, divides by G. If non-zero remainder: error detected! can detect all burst errors less than r+1 bits widely used in practice (Ethernet, 802.11 WiFi) Copyrights ©Dr. Ehab Abousaif – Link Layer: 6-13 Cyclic Redundancy Check (CRC): example We want: G 1 0 1 0 1 1 D.2r XOR R = nG 1 0 0 1 1 0 1 1 1 0 0 0 0 or equivalently: 1 0 0 1 D.2r = nG XOR R 1 0 1 D * 2r 0 0 0 or equivalently: 1 0 1 0 1 0 0 1 if we divide D.2r by G, want 1 1 0 remainder R to satisfy: 0 0 0 D.2r 1 1 0 0 R = remainder [ ] 1 0 0 1 G 1 0 1 0 1 0 0 1 0 1 1 R Copyrights ©Dr. Ehab Abousaif – Link Layer: 6-14 Link layer, LANs: roadmap  introduction  error detection, correction  LANs addressing, ARP Ethernet switches VLANs Copyrights ©Dr. Ehab Abousaif – Link Layer: 6-15 MAC addresses  32-bit IP address: network-layer address for interface used for layer 3 (network layer) forwarding e.g.: 128.119.40.136  MAC (or LAN or physical or Ethernet) address: function: used “locally” to get frame from one interface to another physically-connected interface (same subnet, in IP-addressing sense) 48-bit MAC address (for most LANs) burned in NIC ROM, also sometimes software settable e.g.: 1A-2F-BB-76-09-AD hexadecimal (base 16) notation (each “numeral” represents 4 bits) Copyrights ©Dr. Ehab Abousaif – Link Layer: 6-16 MAC addresses each interface on LAN  has unique 48-bit MAC address  has a locally unique 32-bit IP address (as we’ve seen) 137.196.7.78 1A-2F-BB-76-09-AD LAN (wired or wireless) 137.196.7/24 71-65-F7-2B-08-53 58-23-D7-FA-20-B0 137.196.7.23 137.196.7.14 0C-C4-11-6F-E3-98 137.196.7.88 Copyrights ©Dr. Ehab Abousaif – Link Layer: 6-17 MAC addresses  MAC address allocation administered by IEEE  manufacturer buys portion of MAC address space (to assure uniqueness)  analogy: MAC address: like Social Security Number IP address: like postal address  MAC flat address: portability can move interface from one LAN to another recall IP address not portable: depends on IP subnet to which node is attached Copyrights ©Dr. Ehab Abousaif – Link Layer: 6-18 ARP: address resolution protocol Question: how to determine interface’s MAC address, knowing its IP address? ARP table: each IP node (host, ARP router) on LAN has table 137.196.7.78 ARP 1A-2F-BB-76-09-AD IP/MAC address mappings for ARP some LAN nodes: LAN < IP address; MAC address; TTL> 71-65-F7-2B-08-53 137.196.7.23 58-23-D7-FA-20-B0 137.196.7.14 TTL (Time To Live): time after ARP 0C-C4-11-6F-E3-98 which address mapping will be 137.196.7.88 forgotten (typically 20 min) Copyrights ©Dr. Ehab Abousaif – Link Layer: 6-19 ARP protocol in action example: A wants to send datagram to B B’s MAC address not in A’s ARP table, so A uses ARP to find B’s MAC address A broadcasts ARP query, containing B's IP addr Ethernet frame (sent to FF-FF-FF-FF-FF-FF) 1 destination MAC address = FF-FF-FF-FF-FF-FF all nodes on LAN receive ARP query C Source MAC: 71-65-F7-2B-08-53 Source IP: 137.196.7.23 ARP table in A Target IP address: 137.196.7.14 … IP addr MAC addr TTL TTL A B 1 71-65-F7-2B-08-53 58-23-D7-FA-20-B0 137.196.7.23 137.196.7.14 D Copyrights ©Dr. Ehab Abousaif – Link Layer: 6-20 ARP protocol in action example: A wants to send datagram to B B’s MAC address not in A’s ARP table, so A uses ARP to find B’s MAC address ARP message into Ethernet frame (sent to 71-65-F7-2B-08-53) C Target IP address: 137.196.7.14 Target MAC address: ARP table in A 58-23-D7-FA-20-B0 … IP addr MAC addr TTL TTL A B 2 71-65-F7-2B-08-53 58-23-D7-FA-20-B0 137.196.7.23 137.196.7.14 2 B replies to A with ARP response, giving its MAC address D Copyrights ©Dr. Ehab Abousaif – Link Layer: 6-21 ARP protocol in action example: A wants to send datagram to B B’s MAC address not in A’s ARP table, so A uses ARP to find B’s MAC address C ARP table in A IP addr MAC addr TTL TTL 137.196. 58-23-D7-FA-20-B0 500 A B 7.14 71-65-F7-2B-08-53 58-23-D7-FA-20-B0 137.196.7.23 137.196.7.14 3 A receives B’s reply, adds B entry into its local ARP table D Copyrights ©Dr. Ehab Abousaif – Link Layer: 6-22 Routing to another subnet: addressing walkthrough: sending a datagram from A to B via R  focus on addressing – at IP (datagram) and MAC layer (frame) levels  assume that: A knows B’s IP address A knows IP address of first hop router, R (how?) A knows R’s MAC address (how?) A B R 111.111.111.111 74-29-9C-E8-FF-55 222.222.222.222 49-BD-D2-C7-56-2A 222.222.222.220 1A-23-F9-CD-06-9B 111.111.111.112 111.111.111.110 CC-49-DE-D0-AB-7D E6-E9-00-17-BB-4B 222.222.222.221 88-B2-2F-54-1A-0F Copyrights ©Dr. Ehab Abousaif – Link Layer: 6-23 Routing to another subnet: addressing  A creates IP datagram with IP source A, destination B  A creates link-layer frame containing A-to-B IP datagram R's MAC address is frame’s destination MAC src: 74-29-9C-E8-FF-55 MAC dest: E6-E9-00-17-BB-4B IP src: 111.111.111.111 IP dest: 222.222.222.222 IP Eth Phy A B R 111.111.111.111 74-29-9C-E8-FF-55 222.222.222.222 49-BD-D2-C7-56-2A 222.222.222.220 1A-23-F9-CD-06-9B 111.111.111.112 111.111.111.110 CC-49-DE-D0-AB-7D E6-E9-00-17-BB-4B 222.222.222.221 88-B2-2F-54-1A-0F Copyrights ©Dr. Ehab Abousaif – Link Layer: 6-24 Routing to another subnet: addressing  frame sent from A to R  frame received at R, datagram removed, passed up to IP MAC src: 74-29-9C-E8-FF-55 IP src: 111.111.111.111 MAC dest: E6-E9-00-17-BB-4B IP dest: 222.222.222.222 IP src: 111.111.111.111 IP dest: 222.222.222.222 IP IP Eth Eth Phy Phy A B R 111.111.111.111 74-29-9C-E8-FF-55 222.222.222.222 49-BD-D2-C7-56-2A 222.222.222.220 1A-23-F9-CD-06-9B 111.111.111.112 111.111.111.110 CC-49-DE-D0-AB-7D E6-E9-00-17-BB-4B 222.222.222.221 88-B2-2F-54-1A-0F Copyrights ©Dr. Ehab Abousaif – Link Layer: 6-25 Routing to another subnet: addressing  R determines outgoing interface, passes datagram with IP source A, destination B to link layer  R creates link-layer frame containing A-to-B IP datagram. Frame destination address: B's MAC address MAC src: 1A-23-F9-CD-06-9B MAC dest: 49-BD-D2-C7-56-2A IP src: 111.111.111.111 IP dest: 222.222.222.222 IP Eth Phy A B R 111.111.111.111 74-29-9C-E8-FF-55 222.222.222.222 49-BD-D2-C7-56-2A 222.222.222.220 1A-23-F9-CD-06-9B 111.111.111.112 111.111.111.110 CC-49-DE-D0-AB-7D E6-E9-00-17-BB-4B 222.222.222.221 88-B2-2F-54-1A-0F Copyrights ©Dr. Ehab Abousaif – Link Layer: 6-26 Routing to another subnet: addressing  R determines outgoing interface, passes datagram with IP source A, destination B to link layer  R creates link-layer frame containing A-to-B IP datagram. Frame destination address: B's MAC address MAC src: 1A-23-F9-CD-06-9B  transmits link-layer frame MAC dest: 49-BD-D2-C7-56-2A IP src: 111.111.111.111 IP dest: 222.222.222.222 IP IP Eth Eth Phy Phy A B R 111.111.111.111 74-29-9C-E8-FF-55 222.222.222.222 49-BD-D2-C7-56-2A 222.222.222.220 1A-23-F9-CD-06-9B 111.111.111.112 111.111.111.110 CC-49-DE-D0-AB-7D E6-E9-00-17-BB-4B 222.222.222.221 88-B2-2F-54-1A-0F Copyrights ©Dr. Ehab Abousaif – Link Layer: 6-27 Routing to another subnet: addressing  B receives frame, extracts IP datagram destination B  B passes datagram up protocol stack to IP IP src: 111.111.111.111 IP dest: 222.222.222.222 IP IP Eth Eth Phy Phy A B R 111.111.111.111 74-29-9C-E8-FF-55 222.222.222.222 49-BD-D2-C7-56-2A 222.222.222.220 1A-23-F9-CD-06-9B 111.111.111.112 111.111.111.110 CC-49-DE-D0-AB-7D E6-E9-00-17-BB-4B 222.222.222.221 88-B2-2F-54-1A-0F Copyrights ©Dr. Ehab Abousaif – Link Layer: 6-28 Link layer, LANs: roadmap  introduction  error detection, correction  multiple access protocols  LANs addressing, ARP Ethernet switches VLANs  a day in the life of a web  link virtualization: MPLS request  data center networking Copyrights ©Dr. Ehab Abousaif – Link Layer: 6-29 Ethernet “dominant” wired LAN technology:  first widely used LAN technology  simpler, cheap  kept up with speed race: 10 Mbps – 400 Gbps  single chip, multiple speeds (e.g., Broadcom BCM5761) Metcalfe’s Ethernet sketch Copyrights ©Dr. Ehab Abousaif – Link Layer: 6-30 Ethernet: physical topology  bus: popular through mid 90s all nodes in same collision domain (can collide with each other)  switched: prevails today active link-layer 2 switch in center each “spoke” runs a (separate) Ethernet protocol (nodes do not collide with each other) bus: coaxial cable switched Copyrights ©Dr. Ehab Abousaif – Link Layer: 6-31 Ethernet frame structure sending interface encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame type dest. source data (payload) CRC preamble address address preamble:  used to synchronize receiver, sender clock rates  7 bytes of 10101010 followed by one byte of 10101011 Copyrights ©Dr. Ehab Abousaif – Link Layer: 6-32 Ethernet frame structure (more) type dest. source data (payload) CRC preamble address address  addresses: 6 byte source, destination MAC addresses if adapter receives frame with matching destination address, or with broadcast address (e.g., ARP 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, AppleTalk used to demultiplex up at receiver  CRC: cyclic redundancy check at receiver error detected: frame is dropped Copyrights ©Dr. Ehab Abousaif – Link Layer: 6-33 Ethernet: unreliable, connectionless connectionless: no handshaking between sending and receiving NICs unreliable: receiving NIC doesn’t send ACKs or NAKs to sending NIC data in dropped frames recovered only if initial sender uses higher layer rdt (e.g., TCP), otherwise dropped data lost Ethernet’s MAC protocol: unslotted CSMA/CD with binary backoff Copyrights ©Dr. Ehab Abousaif – Link Layer: 6-34 802.3 Ethernet standards: link & physical layers  many different Ethernet standards common MAC protocol and frame format different speeds: 2 Mbps, 10 Mbps, 100 Mbps, 1Gbps, 10 Gbps, 40 Gbps different physical layer media: fiber, cable MAC protocol application and frame format transport network 100BASE-TX 100BASE-T2 100BASE-FX link 100BASE-T4 100BASE-SX 100BASE-BX physical copper (twister pair) physical layer fiber physical layer Copyrights ©Dr. Ehab Abousaif – Link Layer: 6-35 Link layer, LANs: roadmap  introduction  error detection, correction  multiple access protocols  LANs addressing, ARP Ethernet switches VLANs  a day in the life of a web  link virtualization: MPLS request  data center networking Copyrights ©Dr. Ehab Abousaif – Link Layer: 6-36 Ethernet switch  Switch is a link-layer device: takes an active role store, forward Ethernet frames examine incoming frame’s MAC address, selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment, uses CSMA/CD to access segment  transparent: hosts unaware of presence of switches  plug-and-play, self-learning switches do not need to be configured Copyrights ©Dr. Ehab Abousaif – Link Layer: 6-37 Switch: multiple simultaneous transmissions  hosts have dedicated, direct connection to switch A  switches buffer packets C’ B  Ethernet protocol used on each 1 2 incoming link, so: 6 3 no collisions; full duplex 5 4 each link is its own collision domain B’ C A’  switching: A-to-A’ and B-to-B’ can transmit simultaneously, without collisions switch with six interfaces (1,2,3,4,5,6) Copyrights ©Dr. Ehab Abousaif – Link Layer: 6-38 Switch: multiple simultaneous transmissions  hosts have dedicated, direct connection to switch A  switches buffer packets C’ B  Ethernet protocol used on each 1 2 incoming link, so: 6 3 no collisions; full duplex 5 4 each link is its own collision domain B’ C A’  switching: A-to-A’ and B-to-B’ can transmit simultaneously, without collisions switch with six interfaces (1,2,3,4,5,6) but A-to-A’ and C to A’ can not happen simultaneously Copyrights ©Dr. Ehab Abousaif – Link Layer: 6-39 Switch forwarding table Q: how does switch know A’ reachable via interface 4, B’ reachable via interface 5? A C’ B A: each switch has a switch table, each entry: 1 2 6  (MAC address of host, interface to reach 3 5 4 host, time stamp)  looks like a routing table! B’ C A’ Q: how are entries created, maintained in switch table?  something like a routing protocol? Copyrights ©Dr. Ehab Abousaif – Link Layer: 6-40 Switch: self-learning Source: A  switch learns which hosts Dest: A’ A A’ can be reached through A which interfaces C’ B when frame received, switch 1 2 “learns” location of sender: 6 3 incoming LAN segment 5 4 records sender/location pair B’ C A’ in switch table MAC addr interface TTL Switch table (initially empty) A 1 60 Copyrights ©Dr. Ehab Abousaif – Link Layer: 6-41 Switch: frame filtering/forwarding when frame received at switch: 1. record incoming link, MAC address of sending host 2. index switch table using MAC destination address 3. if entry found for destination then { if destination on segment from which frame arrived then drop frame else forward frame on interface indicated by entry } else flood Copyrights ©Dr. Ehab Abousaif – Link Layer: 6-42 Self-learning, forwarding: example Source: A Dest: A’  frame destination, A’, A A’ location unknown: flood A C’ B  destination A location 1 known: selectively send 6A A’ 2 on just one link 5 4 3 B’ C A’ A A’ MAC addr interface TTL A 1 60 switch table A’ 4 60 (initially empty) Copyrights ©Dr. Ehab Abousaif – Link Layer: 6-43 Interconnecting switches self-learning switches can be connected together: S4 S1 S3 A S2 F D I B C G H E Q: sending from A to G - how does S1 know to forward frame destined to G via S4 and S3?  A: self learning! (works exactly the same as in single-switch case!) Copyrights ©Dr. Ehab Abousaif – Link Layer: 6-44 Self-learning multi-switch example Suppose C sends frame to I, I responds to C S4 S1 S3 A S2 F D I B C G H E Q: show switch tables and packet forwarding in S1, S2, S3, S4 Copyrights ©Dr. Ehab Abousaif – Link Layer: 6-45 Small institutional network mail server to external network router web server IP subnet Copyrights ©Dr. Ehab Abousaif – Link Layer: 6-46 Switches vs. routers application transport both are store-and-forward: datagram frame network link  routers: network-layer devices (examine physical link frame network-layer headers) physical  switches: link-layer devices (examine switch link-layer headers) network datagram link both have forwarding tables: physical frame  routers: compute tables using routing application algorithms, IP addresses transport  switches: learn forwarding table using network flooding, learning, MAC addresses link physical Copyrights ©Dr. Ehab Abousaif – Link Layer: 6-47 Link layer, LANs: roadmap  introduction  error detection, correction  multiple access protocols  LANs addressing, ARP Ethernet switches VLANs  a day in the life of a web  link virtualization: MPLS request  data center networking Copyrights ©Dr. Ehab Abousaif – Link Layer: 6-48 Virtual LANs (VLANs): motivation Q: what happens as LAN sizes scale, users change point of attachment? single broadcast domain:  scaling: all layer-2 broadcast traffic (ARP, DHCP, unknown MAC) must cross entire LAN Computer  efficiency, security, privacy issues Science EE Copyrights ©Dr. Ehab Abousaif – Link Layer: 6-49 Virtual LANs (VLANs): motivation Q: what happens as LAN sizes scale, users change point of attachment? single broadcast domain:  scaling: all layer-2 broadcast traffic (ARP, DHCP, unknown MAC) must cross entire LAN Computer  efficiency, security, privacy, efficiency Science EE issues administrative issues:  CS user moves office to EE - physically attached to EE switch, but wants to remain logically attached to CS switch Copyrights ©Dr. Ehab Abousaif – Link Layer: 6-50 Port-based VLANs port-based VLAN: switch ports grouped (by switch management software) so that single physical switch …… Virtual Local Area Network (VLAN) 7 9 15 1 switch(es) supporting 2 8 10 16 … … VLAN capabilities can be configured to define EE (VLAN ports 1-8) CS (VLAN ports 9-15) multiple virtual LANS … operates as multiple virtual switches over single physical LAN infrastructure. 1 7 9 15 2 8 10 16 … … EE (VLAN ports 1-8) CS (VLAN ports 9-15) Copyrights ©Dr. Ehab Abousaif – Link Layer: 6-51 Port-based VLANs  traffic isolation: frames to/from ports 1-8 can only reach ports 1-8 can also define VLAN based on MAC addresses of endpoints, rather than switch port  dynamic membership: ports can be dynamically assigned among VLANs 1 7 9 15 2 8 10 16  forwarding between VLANS: done via … … routing (just as with separate switches) EE (VLAN ports 1-8) CS (VLAN ports 9-15) in practice vendors sell combined switches plus routers Copyrights ©Dr. Ehab Abousaif – Link Layer: 6-52 VLANS spanning multiple switches 1 7 9 15 1 3 5 7 2 8 10 16 2 4 6 8 … … … EE (VLAN ports 1-8) CS (VLAN ports 9-15) Ports 2,3,5 belong to EE VLAN Ports 4,6,7,8 belong to CS VLAN trunk port: carries frames between VLANS defined over multiple physical switches  frames forwarded within VLAN between switches can’t be vanilla 802.1 frames (must carry VLAN ID info)  802.1q protocol adds/removed additional header fields for frames forwarded between trunk ports Copyrights ©Dr. Ehab Abousaif – Link Layer: 6-53 802.1Q VLAN frame format type dest. source data (payload) CRC preamble address address 802.1 Ethernet frame type dest. source CRC preamble address address data (payload) 802.1Q frame 2-byte Tag Protocol Identifier Recomputed (value: 81-00) CRC Tag Control Information (12 bit VLAN ID field, 3 bit priority field like IP TOS) Copyrights ©Dr. Ehab Abousaif – Link Layer: 6-54 Chapter 6: Summary  principles behind data link layer services: error detection, correction link layer addressing  instantiation, implementation of various link layer technologies Ethernet switched LANS, VLANs Copyrights ©Dr. Ehab Abousaif – Link Layer: 6-55