Networking Chapter 6: Link Layer and LANs
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Questions and Answers

What data is stored in a switch table entry?

  • MAC address, interface to reach host, and time stamp (correct)
  • Protocol type and device type
  • IP address and MAC address
  • MAC address and physical address

    • How does a switch learn the location of a sender?

  • By sending a broadcast frame to all connected devices
  • By using an external routing protocol to update its table
  • By keeping a log of all transmitted frames
  • By recording the incoming LAN segment upon receiving a frame (correct)

    • Which action is taken by the switch when it receives a frame?

  • It forwards the frame to all other devices in the network
  • It records the incoming link and the MAC address of the sending host (correct)
  • It only filters frames based on destination IP
  • It automatically deletes old entries in the switch table

    • What is the role of a switch's self-learning mechanism?

    <p>To learn which hosts can be reached through which interfaces</p> Signup and view all the answers

    What happens to the switch table when a new frame is received?

    <p>The relevant sender/location pair is added to the switch table</p> Signup and view all the answers

    What type of addresses does the switch table index use?

    <p>MAC destination addresses to filter frames</p> Signup and view all the answers

    What is the initial state of a switch's table when it first operates?

    <p>Initially empty until it learns through received frames</p> Signup and view all the answers

    What is the purpose of the time stamp in a switch table entry?

    <p>To determine the validity of the MAC address over time</p> Signup and view all the answers

    What type of links frequently experience high error rates?

    <p>Wireless links</p> Signup and view all the answers

    Which layer implements flow control services in a network?

    <p>Link layer</p> Signup and view all the answers

    What happens during error detection in link layer services?

    <p>The receiver detects errors and signals for retransmission or drops the frame.</p> Signup and view all the answers

    Which mechanism allows a receiver to fix bit errors without retransmission?

    <p>Error correction</p> Signup and view all the answers

    Which statement best describes half-duplex communication?

    <p>Data can flow in one direction only at a time.</p> Signup and view all the answers

    Where is the link layer typically implemented within a computer system?

    <p>In each and every host's network interface card (NIC)</p> Signup and view all the answers

    What is the primary function of the link layer when encapsulating datagrams?

    <p>To add error checking bits</p> Signup and view all the answers

    What role does the physical layer play in communication?

    <p>Handling the actual transmission of data over physical mediums</p> Signup and view all the answers

    Which technology is least likely to use full-duplex communication?

    <p>Half-duplex radio systems</p> Signup and view all the answers

    What is the primary purpose of flow control in the link layer?

    <p>To pace data transmission between sender and receiver</p> Signup and view all the answers

    What is the primary function of port-based VLANs?

    <p>To isolate traffic between defined ports.</p> Signup and view all the answers

    Which protocol is used to add VLAN ID information to frames carried over trunk ports?

    <p>802.1Q</p> Signup and view all the answers

    What happens to frames forwarded between VLANs that span multiple switches?

    <p>They must carry VLAN ID information.</p> Signup and view all the answers

    In a situation with dynamic VLAN membership, how can ports be assigned?

    <p>Dynamically among different VLANs.</p> Signup and view all the answers

    Which of the following is NOT a characteristic of VLANs?

    <p>They allow unfiltered traffic among all devices.</p> Signup and view all the answers

    What is the role of a trunk port in a VLAN system?

    <p>It carries frames for multiple VLANs across switches.</p> Signup and view all the answers

    What is the significance of the VLAN ID in the 802.1Q frame format?

    <p>It enables isolation and proper routing between VLANs.</p> Signup and view all the answers

    Which of these statements about traffic isolation in port-based VLANs is true?

    <p>Ports are restricted to communicate only within their assigned VLAN.</p> Signup and view all the answers

    What is the source MAC address used in the link-layer frame for communication between A and B?

    <p>1A-23-F9-CD-06-9B</p> Signup and view all the answers

    Which protocol stack layer does B transition the datagram to after receiving it?

    <p>Network layer</p> Signup and view all the answers

    What is the primary advantage of the switched Ethernet topology over the bus topology?

    <p>No collisions between packets</p> Signup and view all the answers

    What is the maximum speed of Ethernet technology mentioned in the content?

    <p>400 Gbps</p> Signup and view all the answers

    What type of network technology does Ethernet primarily represent?

    <p>Wired LAN technology</p> Signup and view all the answers

    Which device is primarily used in switched Ethernet topologies to manage data traffic?

    <p>Switch</p> Signup and view all the answers

    What does the MAC address of node A indicate?

    <p>Unique identifier for node A</p> Signup and view all the answers

    What is the function of Address Resolution Protocol (ARP) in the context of LANs?

    <p>To map IP addresses to MAC addresses</p> Signup and view all the answers

    In Ethernet, the frame structure is used to encapsulate which type of packet?

    <p>IP datagram</p> Signup and view all the answers

    What key feature distinguishes switched Ethernet from earlier network topologies?

    <p>Individual collision domains for each network node</p> Signup and view all the answers

    What is the primary function of a load balancer in a datacenter network?

    <p>To direct workload within the data center</p> Signup and view all the answers

    Which network element connects outside the datacenter?

    <p>Border routers</p> Signup and view all the answers

    What does the term 'multipath' refer to in datacenter networks?

    <p>The use of multiple routing paths to increase throughput</p> Signup and view all the answers

    Which technology allows remote DMA over Ethernet in datacenter networks?

    <p>RoCE</p> Signup and view all the answers

    What is a key challenge faced by datacenter networks?

    <p>Managing/balancing load across multiple applications</p> Signup and view all the answers

    Which layer uses ECN for congestion control in datacenter networks?

    <p>Transport layer</p> Signup and view all the answers

    What advantage does a rich interconnection among switches provide in a datacenter?

    <p>Increases available bandwidth and redundancy</p> Signup and view all the answers

    What is the function of Orion in Google's datacenter network?

    <p>A SDN control plane for routing and traffic management</p> Signup and view all the answers

    What does SDN stand for in the context of datacenter networks?

    <p>Software-Defined Networking</p> Signup and view all the answers

    How many server blades are typically housed in a server rack?

    <p>20-40</p> Signup and view all the answers

    What is the purpose of the preamble in an Ethernet frame?

    <p>To synchronize the clock rates of sender and receiver.</p> Signup and view all the answers

    What happens to a frame if the MAC address does not match the destination address?

    <p>The frame is discarded by the adapter.</p> Signup and view all the answers

    Which of the following correctly describes Ethernet's reliability?

    <p>Ethernet is unreliable and connectionless.</p> Signup and view all the answers

    Which layer of the networking model does Ethernet operate at?

    <p>Link layer</p> Signup and view all the answers

    What is the function of the CRC in an Ethernet frame?

    <p>To detect errors during frame transmission.</p> Signup and view all the answers

    What type of protocol does Ethernet utilize for multiple access?

    <p>CSMA/CD</p> Signup and view all the answers

    How does a switch maintain its forwarding table?

    <p>By observing incoming frames and their MAC addresses.</p> Signup and view all the answers

    Which of the following Ethernet standards indicates a transmission speed of 1 Gbps?

    <p>1000BASE-T</p> Signup and view all the answers

    What is a key feature of switches in a local area network?

    <p>They do not need any configuration to operate.</p> Signup and view all the answers

    In an Ethernet switch network, what allows for multiple transmissions simultaneously?

    <p>Each host has a direct connection to the switch.</p> Signup and view all the answers

    Which statement about Ethernet's MAC protocol is correct?

    <p>It employs binary backoff to manage collisions.</p> Signup and view all the answers

    Which of the following statements about the Ethernet frame structure is incorrect?

    <p>The payload can only contain IP packets.</p> Signup and view all the answers

    What characteristic of Ethernet frames makes them suitable for connectionless communication?

    <p>They do not maintain state information between transmissions.</p> Signup and view all the answers

    Which of the following is a property of a switch in an Ethernet network?

    <p>It uses a forwarding table to direct traffic.</p> Signup and view all the answers

    Study Notes

    • The PowerPoint slides are freely available for faculty, students, and readers.
    • To use these slides, users are asked to credit source.
    • For revision history, refer to the page's slide notes.
    • Understand the principles behind link layer services, error detection, correction, and sharing a broadcast channel (multiple access).
    • Understand link layer addressing, local area networks (LANs) including Ethernet, VLANs, and data center networks.
    • Understand instantiation and implementation of various link layer technologies.
    • Introduction
    • Error detection, correction
    • Multiple access protocols
    • LANs, including addressing, ARP, Ethernet, switches, VLANs.
    • Link virtualization: MPLS
    • Data center networking
    • a day in the life of a web request
    • Terminology: hosts, routers (nodes)
    • Communication channels that connect adjacent nodes are known as communication paths/links.
    • Wired and wireless LANs are part of communication channels
    • Layer-2 packet: frame that encapsulates a datagram
    • The link layer is responsible for transferring datagrams between physically adjacent nodes over a link.
    • Datagrams are transferred by different link protocols over different links (e.g., WiFi on first, Ethernet on next link).
    • Each link protocol provides different services (e.g., may or may not provide reliable data transfer over a link).

    Transportation analogy

    • Analogy uses a trip from Princeton to Lausanne: limo (Princeton to JFK), plane (JFK to Geneva), train (Geneva to Lausanne).
    • Tourist = datagram
    • Transport segment = communication link
    • Transportation mode = link-layer protocol
    • Travel agent = routing algorithm
    • Framing, link access
    • Encapsulating a datagram into a frame and adding a header and a trailer.
    • Channel access if required for a shared medium
    • MAC addresses in frame headers identify both the source and destination.
    • Reliable delivery between adjacent nodes, often done without retransmission on links with low error rates.
    • Wireless links have high error rates, thus it is useful to consider how to deal with reliability issues at both the link level and the application level.
    • Flow control manages pacing between adjacent sender and receiver nodes.
    • Error detection identifies errors caused by signal attenuation, noise, or other signal issues.
    • Error correction identifies and corrects bit errors without retransmission.
    • Half-duplex and full-duplex: nodes at both ends of a link can transmit but preferably not at the same time (half-duplex).
    • Link layer is implemented on chip(s) or in Network Interface Cards (NICs).
    • It includes physical layer and link layer implementations.
    • Attaches to host's system buses
    • Combination of hardware, software, and firmware.

    Interfaces communicating

    • Sending side: encapsulates datagram in frame, adds error checking bits, reliable data transfer, and flow control.
    • Receiving side: looks for errors, does reliable data transfer, flow control, and extracts datagrams passed to upper layer protocols.
    • Introduction
    • Error detection, correction
    • Multiple access protocols (LANs)
    • Addressing, ARP
    • Ethernet
    • Switches
    • VLANs
    • Link virtualization: MPLS
    • Data center networking
    • a day in the life of a web request

    Error detection

    • EDC: error detection and correction bits (e.g., redundancy).
    • D: data protected by error checking, may include header fields.
    • Error detection not 100% reliable. Protocol may miss some errors, but larger EDC fields yield better detection and correction.

    Parity checking

    • Single bit parity detects single bit errors.
    • Even/odd parity: sets parity bit for an even or odd number of 1's.
    • At receiver: compute parity of received bits, compare with received parity bit; if different from expected value, an error is detected

    Internet checksum

    • Goal: detect errors (e.g., flipped bits) in transmitted segment
    • Sender treats contents of UDP segments (including UDP header fields and IP addresses) as sequence of 16-bit integers and computes checksum. It puts checksum value into UDP checksum field.
    • Receiver computes checksum of received segment and checks if computed checksum equals checksum field value.

    Cyclic Redundancy Check (CRC)

    • More powerful error-detection coding.
    • D: data bits (given).
    • G: bit pattern (generator), of r+1 bits.
    • Sender computes CRC bits (R) such that (D,R) is exactly divisible by G (mod 2).
    • Receiver knows G, divides (D,R) by G. If non-zero remainder, error is detected!
    • Widely used in Ethernet, 802.11 WiFi.
    • Introduction
    • Error detection, correction
    • Multiple access protocols (LANs)
    • Addressing, ARP
    • Ethernet
    • Switches
    • VLANs
    • Link virtualization: MPLS
    • Data center networking
    • A day in the life of a web request
    • Two types of links:
    • Point-to-point: link between Ethernet switch and host; PPP for dial-up access
    • Broadcast: shared wire (e.g., old-school Ethernet, HFC in cable-based access network, 802.11 Wireless LAN, 4G/5G, satellite

    Multiple access protocols

    • Single shared broadcast channel
    • Two or more simultaneous transmissions by nodes
    • Collision if node receives two or more signals at same time
    • Distributed algorithm that determines how nodes share channel
    • Communication about channel sharing must use channel itself!
    • No out-of-band channel for coordination

    An ideal multiple access protocol

    • Given a multiple access channel (MAC) of rate R bps
    • Desiderata:
    • When one node wants to transmit, can send at rate R.
    • When M nodes want to transmit, each can send at average rate R/M.
    • Fully decentralized (no special node to coordinate transmissions, no synchronization of clocks/slots).

    MAC protocols: taxonomy

    • Channel partitioning
    • Divide channel into smaller pieces (e.g., time slots, frequency, code) and allocate piece to node for exclusive use.
    • Random access
    • Channel not divided; allow collisions; "recover" from collisions; "taking turns"
    • Nodes take turns, but nodes with more to send can take longer turns.

    Channel partitioning MAC protocols: TDMA

    • Time division multiple access (TDMA)
    • Channel access in "rounds."
    • Each station gets fixed-length slot (length = packet transmission time) in each round.
    • Unused slots are idle.

    Channel partitioning MAC protocols: FDMA

    • Frequency division multiple access (FDMA)
    • Channel spectrum divided into frequency bands.
    • Each station is assigned a fixed frequency band.
    • Unused transmission time in frequency bands are idle.

    Random access protocols

    • When node has packet to send, transmit at full channel data rate R; no a priori coordination among nodes when two or more nodes transmit: collision.
    • Random access protocol specifies how to detect and recover from collisions.

    Slotted ALOHA

    • Assumptions: all frames same size, time divided into equal size slots (time to transmit 1 frame), nodes start to transmit only at slot beginning, and nodes are synchronized.
    • If 2 or more nodes transmit in slot, all nodes detect collision.
    • Operation: when node obtains fresh frame, transmits in next slot. No collision: node can send new frame on next slot. Collision: node retransmits frame in each subsequent slot with probability p until success.

    Slotted ALOHA

    • Pros: single active node can continuously transmit at full channel rate, highly decentralized, simple.
    • Cons: collisions, wasting slots, idle slots, nodes may be able to detect collision in less than time to transmit packet, clock synchronization required.
    • Efficiency: long-run fraction of successful slots (many nodes, all with many frames to send). Suppose: N nodes, probability p = 1/(2e) ~ .37. At best, channel used for useful transmissions 37% of the time.

    Pure ALOHA

    • Unslotted Aloha: simpler, no synchronization
    • When frame first arrives, transmit immediately.
    • Collision probability increases with no synchronization; frame sent at to collides with other frames sent in [to-1,to+1]
    • Pure Aloha efficiency: 18%.

    CSMA (carrier sense multiple access)

    • Simple CSMA: listen before transmit; if channel idle, transmit entire frame; if channel busy, defer transmission.

    CSMA/CD (CSMA with collision detection)

    • Collisions detected within short time.
    • Colliding transmissions aborted. Reduction in channel wastage.
    • Collision detection easy in wired, difficult with wireless; human analogy: the polite conversationalist.

    CSMA: collisions

    • Collisions can still occur with carrier sensing due to propagation delays.
    • Propagation delay causes to nodes may not hear each other's just-started transmission.
    • Collision: entire packet transmission time wasted; distance and propagation delay in determining collision probability.

    CSMA/CD

    • CSMA/CD reduces amount of time wasted in collisions; transmission aborted on collision detection.

    Ethernet CSMA/CD algorithm

    • Ethernet receives datagram from network layer, creates a frame.
    • If Ethernet senses channel is idle, start frame transmission. If busy, wait until channel idle, then transmit.
    • If entire frame transmitted without collision—done.
    • If another transmission detected while sending, abort, and send jam signal.
    • After aborting, enter binary (exponential) backoff. After mth collision, chooses K from {0,1,2,…2m-1}, Ethernet waits K-512 bit times, returns to step 2. More collisions=longer backoff interval.

    CSMA/CD efficiency

    • T = max prop delay between 2 nodes in LAN, ttrans= time to transmit max size frame.
    • Efficiency = 1 / (1 + 5tprop/ttrans), as ttrans goes to 0, efficiency goes to 1; as tprop goes to infinity, efficiency goes to 0.

    "Taking turns" MAC protocols

    • Channel partitioning MAC protocols: efficient at high load, inefficient at low load. Delay in channel access, 1/N bandwidth allocated even if only 1 active node.
    • Random access MAC protocols efficient at low load; single node can fully utilize channel. High load means significant collision overhead
    • "Taking turns" protocols: look for best of both worlds.

    "Taking turns" MAC protocols: polling

    • Centralized controller "invites" other nodes to transmit in turn.
    • Typically used with "dumb" devices.
    • Concerns: polling overhead, latency, single point of failure (master).
    • Bluetooth uses polling.

    "Taking turns" MAC protocols: token passing

    • Control token message explicitly passed from one node to the next, sequentially.
    • Transmit while holding token.
    • Concerns: token overhead, latency, single point of failure (token).

    Cable access network: FDM, TDM and random access

    • Multiple downstream (broadcast) FDM channels (up to 1.6 Gbps/channel).
    • Single CMTS transmits into channels.
    • Multiple upstream channels (up to 1 Gbps/channel).
    • Multiple access: all users contend (random access) for certain upstream channel time slots.

    Cable access network

    • DOCSIS: Data over Cable Service Interface Specification
    • FDM over upstream, downstream frequency channels.
    • TDM upstream: some slots assigned, others have contention.
    • Downstream MAP frame: assigns upstream slots.
    • Request for upstream slots (and data) transmitted random access (binary backoff) in selected slots.

    Summary of MAC protocols

    • Channel partitioning (by time, frequency, or code).
    • Random access (dynamic): ALOHA, S-ALOHA, CSMA, CSMA/CD, CSMA/CA
    • Carrier sensing: easy in some technologies, hard in others (wireless); CSMA/CD is used in Ethernet, and CSMA/CA used in 802.11
    • Taking turns: polling from central site, token passing, Bluetooth, FDDI, token ring.
    • Introduction
    • Error detection, correction
    • Multiple access protocols (LANs)
    • Addressing, ARP
    • Ethernet
    • Switches
    • VLANs
    • Link virtualization: MPLS
    • Data center networking
    • A day in the life of a web request

    MAC addresses

    • 32-bit IP address used for network-layer addresses and forwarding.
    • MAC (or LAN or physical or Ethernet) address functions locally to move frames from one interface to another physically connected interface (same subnet).
    • 48-bit MAC address burned in NIC ROM, sometimes software settable. Example: 1A-2F-BB-76-09-AD (hexadecimal notation)

    MAC addresses

    • Each interface on LAN has a unique 48-bit MAC address and a locally unique 32-bit IP address. MAC address allocation administered by IEEE; manufacturer buys a portion of MAC address space to ensure uniqueness. Analogy: MAC address like Social Security Number; IP address like postal address; MAC flat address is portable compared to IP address (which depends on IP subnet to which node is attached).

    ARP: address resolution protocol

    • ARP table: each IP node (host, router) on LAN has IP/MAC address mappings for some LAN nodes. TTL (Time To Live): time after which an address mapping will be forgotten (typically 20 minutes).

    ARP protocol in action

    • A wants to send datagram to B. If B's MAC address not in A's ARP table, A uses ARP to find it. A broadcasts ARP query containing B's IP address; all nodes receive ARP query, and B replies with ARP response, giving its MAC address. A receives the reply, adding B's entry into its local ARP table.

    Routing to another subnet: addressing

    • Walkthrough: sending datagram from A to B via R. Assume A knows B's IP address, the IP address of first hop router (R), and R's MAC address.

    Routing to another subnet: addressing

    • A creates an IP datagram with IP source A, destination B.
    • A creates a link-layer frame containing the A-to-B IP datagram.
    • R's MAC address is the frame's destination.

    Routing to another subnet: addressing

    • R determines outgoing interface, passes datagram with IP source A, destination B.
    • R creates link-layer frame containing A-to-B IP datagram.
    • R transmits link-layer frame.

    Routing to another subnet: addressing

    • B receives frame, extracts IP datagram, destination B.
    • B passes datagram up protocol stack to IP.
    • Introduction
    • Error detection, correction
    • Multiple access protocols (LANs)
    • Addressing, ARP
    • Ethernet
    • Switches
    • VLANs
    • Link virtualization: MPLS
    • Data center networking
    • A day in the life of a web request

    Ethernet

    • "Dominant" wired LAN technology; first widely used LAN tech; simpler, cheap; single chip, multiple speeds (e.g., Broadcom BCM5761); kept up with speed race: 10 Mbps—400 Gbps.

    Ethernet: physical topology

    • Bus: popular through mid-90s; all nodes in same collision domain.
    • Switched: prevails today; active link-layer 2 switch in center; each "spoke" runs a (separate) Ethernet protocol; nodes do not collide with each other.

    Ethernet frame structure

    • Sending interface encapsulates IP datagram in Ethernet frame. Includes preamble, destination address, source address, type, data (payload), and CRC.
    • Preamble: used to synchronize receiver/sender clock rates; 7 bytes of 10101010 followed by 1 byte of 10101011.

    Ethernet frame structure (more)

    • Addresses: 6-byte source/destination MAC addresses. If adapter receives frame with matching destination or broadcast address (e.g., ARP), passes data to network layer protocol, otherwise discards frame.
    • Type: indicates higher layer protocol (mostly IP, but others possible).
    • CRC: cyclic redundancy check at receiver; error detected: frame is dropped.

    Ethernet: unreliable, connectionless

    • Connectionless: no handshaking between sending/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 is lost. Ethernet's MAC protocol: unslotted CSMA/CD with binary backoff.

    802.3 Ethernet standards: link & physical layers

    • Many different Ethernet standards; common MAC protocol and frame format; different speeds (2 Mbps, 100 Mbps, 1 Gbps, 10 Gbps, 40 Gbps, 80 Gbps); different physical layer media (fiber, cable).
    • Introduction
    • Error detection, correction
    • Multiple access protocols (LANs)
    • Addressing, ARP
    • Ethernet
    • Switches
    • VLANs
    • Link virtualization: MPLS
    • Data center networking
    • A day in the life of a web request

    Ethernet switch

    • Switch is a link-layer device; takes an active role.
    • Store, forward Ethernet (or other type of) frames; examine incoming frame's MAC address, selectively forward frame to one-or-more outgoing links when frame is to be forwarded on the 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.

    Switch: multiple simultaneous transmissions

    • Hosts have dedicated, direct connection to switch; switches buffer packets; Ethernet protocol used on each incoming link, so no collisions; each link is its own collision domain, and switching allows A-to-A’ and B-to-B’ simultaneous transmissions without collisions.

    Switch forwarding table

    • Q: How does switch know A' reachable via interface 4, B' reachable via interface 5?
    • A: Each switch has a switch table, each entry is (MAC 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.

    Switch: self-learning

    • Switch learns which hosts can be reached through which interfaces when frame received.
    • Switch learns locations of sender (incoming LAN segment); records sender/location pair in switch table.

    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, if destination is on segment from which frame arrived, then drop frame, else forward frame on interface indicated by entry.
      4. Else, flood (forward) frame on all interfaces except arriving interface.

    Self-learning, forwarding: example

    • Frame destination (A'), location unknown: flood.
    • Destination A location known: selectively send on just one link.

    Interconnecting switches

    • Self-learning switches can be connected together.
    • Q: Sending from A to G - how does S3 know to forward frame destined to G via S1 and S2? A: self learning.

    Self-learning multi-switch example

    • Suppose C sends frame to I, I responds to C.
    • Q: Show switch tables and packet forwarding in S1, S2, S3, and S4.

    UMass Campus Network - Detail

    • UMass network details (4 firewalls, 10 routers, ~2000+ network switches, ~6000 wireless access points, ~30000 active wired network jacks, ~55000 active end-user wireless devices, all built, operated, and maintained by ~15 people).

    Datacenter networks: network elements

    • Border routers (connections outside datacenter).
    • Tier-1 switches (connecting to ~16 T-2s below).
    • Tier-2 switches (connecting to ~16 TORs below).
    • Top of Rack (TOR) switch (one per rack, 100G-400G Ethernet to blades).
    • Server racks (20-40 server blades: hosts).

    Datacenter networks: multipath

    • Rich interconnection among switches/racks increases throughput between racks (multiple routing paths possible) and reliability via redundancy. -Highlighted disjoint paths between racks 1 and 11 showing two different possible routes.

    Datacenter networks: application-layer routing

    • Load balancer: receives external client requests; directs workload within data center; returns results to external client (data center internals hidden from client).

    Datacenter networks: protocol innovations

    • Link layer: ROCE (remote DMA over Converged Ethernet).
    • Transport layer: ECN (explicit congestion notification) in transport-layer congestion control (DCTCP, DCQCN); experimentation with hop-by-hop (backpressure) congestion control.
    • Routing, management: SDN widely used within/among organizations' datacenters, place related services/data as close as possible (e.g., in same rack or nearby rack) to minimize tier-2/tier-1 communication.

    ORION: Google's new SDN control plane

    • Routing (intradomain, iBGP), traffic engineering (implemented in applications on top of Orion core), edge-edge flow-based controls (e.g., CoFlow scheduling) to meet contract SLAs, and management (pub-sub distributed microservices in Orion core; OpenFlow for switch signaling/monitoring).
    • Note: no routing protocols, congestion control (partially) also managed by SDN rather than by protocol; are protocols dying?
    • Introduction
    • Error detection, correction
    • Multiple access protocols (LANs)
    • Addressing, ARP
    • Ethernet
    • Switches
    • VLANs
    • Link virtualization: MPLS
    • Data center networking
    • A day in the life of a web request

    Synthesis: a day in the life of a web request

    • Our journey down the protocol stack is complete (except PHY).
    • Solid understanding of networking principles, practice!.
    • Could stop here... but may be other topics of interest like wireless or security.

    A day in the life: scenario

    • Arriving mobile client attaches to network, requests a web page, gets a simple sound effect.

    A day in the life: connecting to the Internet

    • Connecting laptop needs to get its own IP address, address of first-hop router, and address of DNS server. Use DHCP.
    • DHCP request encapsulated in UDP, encapsulated in IP, encapsulated in 802.3 Ethernet.
    • Ethernet frame broadcast, received by router running DHCP server.
    • Ethernet de-muxed to IP, de-muxed to UDP, and de-muxed to DHCP.

    A day in the life: connecting to the Internet

    • DHCP server formulates DHCP ACK containing client's IP address, IP address of first-hop router, and name and IP address of DNS server.
    • Encapsulation at DHCP server, frame forwarded (switch learning), through LAN; demultiplexing at client.
    • Client receives DHCP ACK reply. Client now has IP address, knows name and address of DNS server and IP address of its first-hop router.

    A day in the life... ARP (before DNS, before HTTP)

    • Before sending HTTP request, need IP address of www.google.com: DNS.
    • DNS query created, encapsulated in UDP, encapsulated in IP, encapsulated in Eth. To send frame to router, need MAC address of router interface: ARP.
    • ARP query broadcast, received by router, which replies with ARP reply giving MAC address of router interface.
    • Client now knows MAC address of first hop router, so can send a frame containing a DNS query.

    A day in the life... using DNS

    • IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router.
    • IP datagram forwarded from campus network to Comcast network, routed (tables created by RIP, OSPF, IS-IS, and/or BGP routing protocols) to DNS server.
    • Demuxed to DNS; DNS reply to client with IP address of www.google.com.

    A day in the life... TCP connection carrying HTTP

    • To send HTTP request, client first opens TCP socket to web server.
    • TCP SYN segment (Step 1 in TCP 3-way handshake) inter-domain routed to web server.
    • Web server responds with TCP SYNACK (Step 2 in TCP 3-way handshake).
    • TCP connection established.

    A day in the life... HTTP request/reply

    • HTTP request sent into TCP socket; IP datagram containing HTTP request routed to www.google.com; web server responds with HTTP reply (containing web page); IP datagram containing HTTP Reply routed back to client.

    Chapter 6: Summary

    • Principles behind data link layer services (error detection, correction, sharing a broadcast channel: multiple access; link layer addressing).
    • Instantiation/implementation of various link layer technologies (Ethernet, switched LANs, VLANs, virtualized networks as a link layer: MPLS)
    • Synthesis: a day in the life of a web request.

    Chapter 6: Let's take a breath

    • Journey down protocol stack complete (except PHY).
    • Solid understanding of networking principles and practice.
    • Could stop here... but more interesting topics exist (e.g., wireless or security).

    Additional Chapter 6 slides

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    Description

    This quiz covers key concepts from Chapter 6, focusing on the link layer services, error detection, and local area networks (LANs). You'll explore Ethernet, VLANs, and data center networking, while understanding multiple access protocols and link layer addressing. Ideal for students and educators looking to enhance their understanding of networking principles.

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