Part I: Introduction

Part I: Introduction

Part 5: Data Link Layer CSE 3461/5461 Reading: Chapter 5, Kurose and Ross ( 6th ed.); Chapter 6, Kurose and Ross (7th ed.) 1 Part 5: Data Link Layer Our goals: Understand principles behind data link layer services: Error detection, correction Sharing a broadcast channel: multiple access Link layer addressing Reliable data transfer, flow control: done!

Overview: Instantiation and implementation of various link layer technologies Link layer services Error detection, correction Multiple access protocols and LANs Link layer addressing, ARP Specific link layer technologies: Ethernet Hubs, bridges, switches MPLS Datacenter networking 2

Outline Overview: Link Layer Services Error Detection and Correction Multiple Access Control (MAC) Sublayer Ethernet Switches and Switch Self-Learning Multiprotocol Label Switching (MPLS) Datacenter Networking Synthesis: Requesting a Web Page 3 Link Layer: Setting the Context (1) 4

Link Layer: Setting the Context (2) Two physically connected devices: Host-router, router-router, host-host, router-switch, etc. Unit of data: frame 5 Link Layer Services (1) Framing, link access: Encapsulate datagram into frame, adding header, trailer Implement channel access if shared medium, Physical addresses used in frame headers to identify source, dest Different from IP address! Reliable delivery between two physically connected devices: We learned how to do this already (chapter 3)! Seldom used on low bit error link (fiber, some twisted pair) Wireless links: high error rates Q: why both link-level and end-end reliability? 6

Link Layer Services (2) Flow Control: Pacing between sender and receivers Error Detection: Errors caused by signal attenuation, noise. Receiver detects presence of errors: Signals sender for retransmission or drops frame Error Correction: Receiver identifies and corrects bit error(s) without resorting to retransmission 7 Link Layer: Implementation Implemented in adapter e.g., PCMCIA card, Ethernet card Typically includes: RAM, DSP chips, host bus interface, and link interface 8 Outline

Overview: Link Layer Services Error Detection and Correction Multiple Access Control (MAC) Sublayer Ethernet and LAN Technologies Switches and Switch Self-Learning Multiprotocol Label Switching (MPLS) Datacenter Networking Synthesis: Requesting a Web Page 9 Error Detection EDC = Error Detection and Correction bits (redundancy) D = Data protected by error checking, may include header fields Error detection not 100% reliable!

Protocol may miss some errors, but rarely Larger EDC field yields better detection and correction 10 Parity Checking Single Bit Parity: Detect single bit errors Two Dimensional Bit Parity: Detect and correct single bit errors 0 0 11 Internet Checksum Goal: detect errors (e.g., flipped bits) in transmitted segment (note: used at transport layer only) Sender:

Treat segment contents as sequence of 16-bit integers Checksum: addition (1s complement sum) of segment contents Sender puts checksum value into UDP checksum field Receiver: Compute checksum of received segment Check if computed checksum equals checksum field value: NO - error detected YES - no error detected. But maybe errors nonetheless? More later .

12 Checksum: Cyclic Redundancy Check View data bits, D, as a binary number Choose r + 1 bit pattern (generator), G Goal: choose r CRC bits, R, such that D, R exactly divisible by G (modulo 2) Receiver knows G, divides D, R by G. If non-zero remainder: error detected! Can detect all burst errors less than r + 1 bits Widely used in practice (ATM, HDCL) 13 CRC Example Want: D . 2r XOR R = nG Equivalently: D . 2r = nG XOR R Equivalently: If we divide D . 2r by G,

want reminder R 14 Outline Overview: Link Layer Services Error Detection and Correction Multiple Access Control (MAC) Sublayer Ethernet and LAN Technologies Switches and Switch Self-Learning Multiprotocol Label Switching (MPLS) Datacenter Networking Synthesis: Requesting a Web Page 15

Multiple Access Links & Protocols Three types of links: Point-to-point (single wire, e.g. PPP, SLIP) Broadcast (shared wire or medium; e.g., Ethernet, Wi-Fi, etc.) Switched (e.g., switched Ethernet) 16 Multiple Access (MAC) Protocols Single shared communication channel Two or more simultaneous transmissions by nodes: interference only one node can send successfully at a time Multiple access protocol: Distributed algorithm that determines how stations share channel, i.e., determine when station can transmit Communication about channel sharing must use channel itself! What to look for in multiple access protocols: Synchronous or asynchronous Information needed about other stations Robustness (e.g., to channel errors) performance

17 MAC Protocols: A Taxonomy Three broad classes: Channel Partitioning TDMA: time division multiple access FDMA: frequency division multiple access CDMA (Code Division Multiple Access) Read (6.2.1) Random Access Allow collisions Recover from collisions Taking turns Tightly coordinate shared access to avoid collisions Goal: Efficient, fair, simple, decentralized 18 Channel Partitioning MAC Protocols: TDMA TDMA: Time Division Multiple Access Access to channel in rounds Each station gets fixed length slot (length = pkt trans time) in each round

Unused slots go idle Example: 6-station LAN, 1,3,4 have pkt, slots 2,5,6 idle 6-slot frame 6-slot frame 1 3 4 1 3 4 19 Channel Partitioning MAC Protocols: FDMA FDMA: Frequency Division Multiple Access

Channel spectrum divided into frequency bands Each station assigned fixed frequency band Unused transmission time in frequency bands go idle Example: 6-station LAN, 1,3,4 have pkt, frequency bands 2,5,6 idle FDM cable frequency bands time 20 Random Access Protocols When node has packet to send Transmit at full channel data rate R. No a priori coordination among nodes

Two or more transmitting nodes collision, collision, Random access MAC protocol specifies: How to detect collisions How to recover from collisions (e.g., via delayed retransmissions) Examples of random access MAC protocols: Slotted ALOHA and ALOHA CSMA and CSMA/CD 21 Slotted ALOHA (1) Assumptions: Operation: When node obtains fresh All frames same size frame, transmits in next slot Time divided into equal If no collision: node size slots (time to transmit 1 can send new frame in frame)

next slot Nodes start to transmit only If collision: node slot beginning retransmits frame in Nodes are synchronized each subsequent slot If 2 or more nodes transmit with probability p until in slot, all nodes detect success collision 22 Slotted ALOHA node 1 1 1 node 2 2

2 node 3 3 C 1 1 2 3 E C S E Pros:

Single active node can continuously transmit at full rate of channel Highly decentralized: only slots in nodes need to be in sync Simple C 3 E S S Cons: Collisions, wasting slots Idle slots Nodes may be able to detect collision in less than time to transmit

packet Clock synchronization 23 Pure (unslotted) ALOHA Unslotted Aloha: simpler, no synchronization When frame first arrives: transmit immediately Collision probability increases: frame sent at t0 collides with other frames sent in [t01, t0+1] 24 Pure/Slotted ALOHA efficiency Efficiency: long-run fraction of successful slots (many nodes, all with many frames to send) Protocol Efficiency Slotted ALOHA 1/e Pure ALOHA

1/2e The derivation is given in the textbook [Sect. 5.3 (5th, 6th ed.); Sect. 6.3 (7th ed.)]; it is a homework problem. Hint: 25 CSMA: Carrier Sense Multiple Access CSMA: listen before transmit: If channel sensed idle: transmit entire pkt If channel sensed busy, defer transmission Persistent CSMA: retry immediately with probability p when channel becomes idle (may cause instability) Non-persistent CSMA: retry after random interval Human analogy: dont interrupt others! 26 CSMA Collisions Spatial layout of nodes along Ethernet

Collisions can still occur: Propagation delay means two nodes may not year hear each others transmission Collision: entire packet transmission time wasted Note: role of distance and propagation delay in determining collision probability 27 CSMA/CD (Collision Detection) (1) CSMA/CD: carrier sensing, deferral as in CSMA Collisions detected within short time Colliding transmissions aborted, reducing channel wastage Persistent or non-persistent retransmission Collision detection: Easy in wired LANs: measure signal strengths, compare transmitted, received signals

Difficult in wireless LANs: receiver shut off while transmitting Human analogy: the polite conversationalist 28 CSMA/CD (2) 29 Taking Turns MAC Protocols (1) Channel partitioning MAC protocols: Share channel efficiently 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: collision overhead Taking turns protocols Look for best of both worlds! 30

Taking Turns MAC Protocols (2) Polling: Master node invites slave nodes to transmit in turn Request to Send, Clear to Send msgs Concerns: Polling overhead Latency Single point of failure (master) Token passing: Control token passed from one node to next sequentially. Token message Concerns: Token overhead

Latency Single point of failure (token) 31 Summary of MAC Protocols What do you do with a shared medium? Channel partitioning via time, frequency, or code Time Division, Code Division, Frequency Division Random partitioning (dynamic), ALOHA, S-ALOHA, CSMA, CSMA/CD Carrier sensing: easy in some technologies (wire), hard in others (wireless) CSMA/CD used in Ethernet Taking Turns Polling from a central cite, token passing 32 Outline

Overview: Link Layer Services Error Detection and Correction Multiple Access Control (MAC) Sublayer Ethernet and LAN Technologies Switches and Switch Self-Learning Multiprotocol Label Switching (MPLS) Datacenter Networking Synthesis: Requesting a Web Page 33 LAN Technologies Data link layer so far: Services, error detection/correction, multiple access Next: LAN technologies Addressing Ethernet Hubs, bridges, switches

802.11 PPP ATM 34 LAN Addresses and ARP 32-bit IP address: Network-layer address Used to get datagram to destination network (recall IP network definition) LAN (or MAC or physical) address: Used to get datagram from one interface to another physicallyconnected interface (same network) 48 bit MAC address (for most LANs) burned in the adapter ROM 35 LAN Addressing (1) Each adapter on LAN has unique LAN address 36

LAN Addressing (2) MAC address allocation administered by IEEE Manufacturer buys portion of MAC address space (to assure uniqueness) Analogy: (a) MAC address: like Social Security Number (b) IP address: like postal address MAC flat address collision, portability Can move LAN card from one LAN to another IP hierarchical address NOT portable Depends on network to which one attaches 37 Recall Earlier Routing Discussion Starting at A, given IP datagram addressed to B: Look up net. address of B, find B on same net. as A Link layer sends datagram to

B inside link-layer frame Frame source, dest address Datagram source, dest address As IP addr Bs MAC As MAC addr addr Bs IP addr IP payload datagram frame 38 ARP: Address Resolution Protocol (1) Question: How to determine

MAC address of B given Bs IP address? Each IP node (Host, Router) on LAN has ARP module, table ARP Table: IP/MAC address mappings for some LAN nodes IP Address MAC Address TTL TTL (Time To Live): time after which address mapping will be forgotten (typically 20

min) 39 ARP (2) A knows Bs IP address, wants to learn Bs physical address A broadcasts ARP query pkt containing Bs IP address All machines on LAN receive ARP query B receives ARP packet, replies to A with its (Bs) physical layer address A caches (saves) IP-to-physical address pairs until information becomes old (times out) Soft state: information that times out (goes away) unless refreshed 40 Routing to Another LAN (1) Walkthrough: routing from A to B via R A

R B In routing table at source Host, find router 111.111.111.110 In ARP table at source, find MAC address E6-E9-00-17-BB-4B, etc 41 Routing to Another LAN (2) A creates IP packet with source A, destination B A uses ARP to get Rs physical layer address for 111.111.111.110 A creates Ethernet frame with Rs physical address as dest, Ethernet frame contains A-to-B IP datagram As data link layer sends Ethernet frame Rs data link layer receives Ethernet frame

R removes IP datagram from Ethernet frame, sees its destined to B R uses ARP to get Bs physical layer address R creates frame containing A-to-B IP datagram, sends it to B A R B 42 Ethernet Dominant LAN technology (aka IEEE 802.3): Cheap: $35 for 1 Gbps (Raspberry Pi 4) First wildly used LAN technology Simpler, cheaper than token LANs and ATM Kept up with speed race: 10/100 Mbps; 1/10/40/100 Gbps Robert Metcalfes Ethernet sketch 43 Ethernet Frame Structure (1)

Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame Preamble: 7 bytes with pattern 10101010 followed by one byte with pattern 10101011 Used to synchronize receiver, sender clock rates 44 Ethernet Frame Structure (2) Addresses: 6 bytes, frame is received by all adapters on a LAN and dropped if address does not match Type: indicates the higher layer protocol, mostly IP but others may be supported such as Novell IPX and AppleTalk) CRC: checked at receiver, if error is detected, the frame is simply dropped 45 Ethernets CSMA/CD (1) 46

Ethernets CSMA/CD (2) Jam Signal: make sure all other transmitters are aware of collision; 48 bits Exponential Backoff: Goal: adapt retransmission attempts to estimated current load Heavy load: random wait will be longer First collision: choose k from {0, 1}; delay is k 512 bit transmission times After second collision: choose k from {0, 1, 2, 3} After ten or more collisions, choose k from {0, 1, , 1023} In general: After k-th collision, choose k from {0, , 2k 1}, where 1 k 10; stations give up after 10 collisions. 47 Ethernet Technologies: 10Base2 10: 10 Mbps; 2: under 200 meters max cable length Thin coaxial cable in a bus topology Repeaters used to connect up to multiple segments Repeater repeats bits it hears on one interface to its other interfaces: physical layer device only! 48

10BaseT and 100BaseT (1) 10/100 Mbps rate; latter called Fast Ethernet T stands for Twisted Pair Hub to which nodes are connected by twisted pair, thus star topology CSMA/CD implemented at hub 49 10BaseT and 100BaseT (2) Max distance from node to Hub is 100 meters Hub can disconnect jabbering adapter Hub can gather monitoring information, statistics for display to LAN administrators 50 Hubs (1) Physical Layer devices: essentially repeaters operating at bit levels: repeat received bits on one interface to all other interfaces Hubs can be arranged in a hierarchy (or multi-tier design), with backbone hub at its top

51 Hubs (2) Each connected LAN referred to as LAN segment Hubs do not isolate collision domains: node may collide with any node residing at any segment in LAN Hub Advantages: Simple, inexpensive device Multi-tier provides graceful degradation: portions of the LAN continue to operate if one hub malfunctions Extends maximum distance between node pairs (100 m per hub) 52 Hub Limitations Single collision domain results in no increase in max throughput Multi-tier throughput same as single segment throughput Individual LAN restrictions pose limits on number of nodes in same collision domain and on total allowed geographical coverage Cannot connect different Ethernet types

(e.g., 10BaseT and 100baseT) 53 Token Passing: IEEE 802.5 Standard (1) 4 Mbps Max token holding time: 10 ms, limiting frame length SD, ED mark start, end of packet AC: access control byte: Token bit: value 0 means token can be seized, value 1 means data follows FC Priority bits: priority of packet Reservation bits: station can write these bits to prevent stations with lower priority packet from seizing token after token becomes free 54 Token Passing: IEEE 802.5 Standard (2) FC: frame control used for monitoring and maintenance Source, destination address: 48 bit physical address, as in Ethernet Data: packet from network layer

Checksum: CRC FS: frame status: set by destination, read by sender Set to indicate destination up, frame copied OK from ring DLC-level ACKing 55 Outline Overview: Link Layer Services Error Detection and Correction Multiple Access Control (MAC) Sublayer Ethernet and LAN Technologies Switches and Switch Self-Learning

Multiprotocol Label Switching (MPLS) Datacenter Networking 56 Ethernet Switch Link-layer device: takes an active role Store, forward Ethernet frames Examine incoming frames 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 are unaware of presence of switches Plug-and-play, self-learning Switches do not need to be configured 57 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: A-to-A and B-to-B can transmit simultaneously, without collisions A B C 6 1 2 4 5 B 3

C A Switch with six interfaces (1,2,3,4,5,6) 58 Switch Forwarding Table Q: how does switch know A reachable via interface 4, B reachable via interface 5? A A: each switch has a switch table, each entry: (MAC address of host, interface to reach host, time stamp) Looks like a routing table! B

C 6 1 2 4 5 B 3 C A Q: How are entries created, maintained in switch table? Switch with six interfaces (1,2,3,4,5,6) Something like a routing protocol?

59 Switch: Self-Learning Switch learns which hosts can be reached through which interfaces A Source: A Dest: A A A B C 6 When frame received, switch 5 learns location of B

sender: incoming LAN segment Records MAC addr Interface sender/location pair 1 A in switch table 1 2 4 3 C A TTL 60 Switch table (initially empty) 60

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 /* forward on all interfaces except arriving interface */ 61 Self-Learning, Forwarding: Example Frame dest. A, location unknown: flood Destination A location known: selectively send on just one link

A Source: A Dest: A A A B C 1 6 2 A A 4 5 3 B

C A A A MAC addr Interface A A 1 4 TTL 60 60 Switch table (initially empty) 62 Interconnecting Switches Switches can be connected together S4 S1

S3 S2 A B C F D E I G H Q: Sending from A to G how does S1 know to forward frame destined to F via S4 and S3? A: Self-learning! (works exactly the same as in single-switch case!) 63

Self-Learning Multi-Switch Example Suppose C sends frame to I, I responds to C S4 S1 S3 S2 A B C F D E I G

H Q: Show switch tables and packet forwarding in S1, S2, S3, S4. 64 Institutional Network Mail server To external network Router Web server IP subnet 65 Switches vs. Routers Both are store-and-forward: Routers: network-layer devices (examine networklayer headers)

Switches: link-layer devices (examine link-layer headers) Both have forwarding tables: Routers: compute tables using routing algorithms, IP addresses Switches: learn forwarding table using flooding, learning, MAC addresses datagram frame application transport network link physical link physical

frame switch network link physical datagram frame application transport network link physical 66 VLANs: Motivation Consider: Computer Science

Electrical Engineering Computer Engineering CS user moves office to EE, but wants connect to CS switch? Single broadcast domain: All layer-2 broadcast traffic (ARP, DHCP, unknown location of destination MAC address) must cross entire LAN Security/privacy, efficiency issues 67 VLANs Virtual Local Area Network Switch(es) supporting

VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure. Port-Based VLAN: switch ports grouped (by switch management software) so that single physical switch 1 7 9 15 2 8 10

16 Electrical Engineering (VLAN ports 1-8) Computer Science (VLAN ports 9-15) operates as multiple virtual switches 1 7 9 15 2 8

10 16 Electrical Engineering (VLAN ports 1-8) Computer Science (VLAN ports 9-16) 68 Port-Based VLANs Router 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

Electrical Engineering (VLAN ports 1-8) Computer Science (VLAN ports 9-15) Forwarding between VLANs: done via routing (just as with separate switches) In practice, vendors sell combined switches plus routers 69 VLANs Spanning Multiple Switches 1 7 9 15 1

3 5 7 2 8 10 16 2 4 6 8

Electrical Engineering (VLAN ports 1-8) Computer Science (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 cant be vanilla 802.1 frames (must carry VLAN ID info) 802.1q protocol adds/removed additional header fields for frames forwarded between trunk ports 70 802.1Q VLAN Frame Format Note: Frame fields are not to scale. 71

Interconnecting LANs Q: Why not just one big LAN? Limited amount of supportable traffic: on single LAN, all stations must share bandwidth Limited length: 802.3 specifies maximum cable length Large collision domain (can collide with many stations) Limited number of stations: 802.5 have token passing delays at each station 72 Outline Overview: Link Layer Services Error Detection and Correction Multiple Access Control (MAC) Sublayer

Ethernet and LAN Technologies Switches and Switch Self-Learning Multiprotocol Label Switching (MPLS) Datacenter Networking Synthesis: Requesting a Web Page 73 Multiprotocol Label Switching (MPLS) Initial goal: high-speed IP forwarding using fixed length label (instead of IP address) Fast lookup using fixed length identifier (rather than shortest prefix matching) Borrowing ideas from Virtual Circuit (VC) approach But IP datagrams still keep their IP addresses! PPP or Ethernet header MPLS header Label 20 IP header

Remainder of link-layer frame Exp S TTL 3 1 5 74 MPLS-Capable Routers A.k.a. label-switched router Forward packets to outgoing interface based only on label value (dont inspect IP address) MPLS forwarding table distinct from IP forwarding tables Flexibility: MPLS forwarding decisions can differ from those of IP Use destination and source addresses to route flows to same destination differently (traffic engineering) Re-route flows quickly if link fails: pre-computed backup paths (useful for VoIP)

75 MPLS vs. IP Paths (1) R6 D R4 R3 R5 A R2 IP routing: Path to destination determined by destination address alone IP router 76

MPLS vs. IP Paths (2) Entry router (R4) can use different MPLS routes to A based, e.g., on source address R6 D R4 R3 R5 A R2 IP routing: path to destination determined by destination address alone IP-only router

MPLS routing: path to destination can be based on source and dest. address MPLS and IP router Fast reroute: precompute backup routes in case of link failure 77 MPLS Signaling Modify OSPF, IS-IS link-state flooding protocols to carry info used by MPLS routing, e.g., link bandwidth, amount of reserved link bandwidth Entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers RSVP-TE R6 D

R4 R5 Modified link state flooding A 78 MPLS Forwarding Tables In Label Out Out Label Dest Interface 10 12 8 A D

A R6 0 0 1 In Label 0 R4 R5 Out Out Label Dest Interface 10 6

A 1 12 9 D 0 0 1 R3 D 1 0

0 R2 In Label 8 Out Out Label Dest Interface 6 A 0 In Label 6

A OutR1 Out Label Dest Interface - A 0 79 Outline Overview: Link Layer Services

Error Detection and Correction Multiple Access Control (MAC) Sublayer Ethernet and LAN Technologies Switches and Switch Self-Learning Multiprotocol Label Switching (MPLS) Datacenter Networking Synthesis: Requesting a Web Page 80 Datacenter Networks (1) 10,000s100,000s of thousands of hosts, often closely coupled, in close proximity: E-business (e.g. Amazon) Content servers (e.g., YouTube, Akamai, Apple, 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 81 Datacenter Networks (2) Load balancer: application-layer routing Internet Receives external client requests Directs workload within data center Returns results to external client (hiding datacenter internals from client) Border router Load balancer Access router

Tier-1 switches B A Load balancer C Tier-2 switches TOR switches Server racks 1 2 3 4

5 6 7 8 82 Datacenter Networks (3) Rich interconnection among switches, racks: Increased throughput between racks (multiple routing paths possible) Increased reliability via redundancy Tier-1 switches Tier-2 switches TOR switches Server racks

1 2 3 4 5 6 7 8 83 Outline

Overview: Link Layer Services Error Detection and Correction Multiple Access Control (MAC) Sublayer Ethernet and LAN Technologies Switches and Switch Self-Learning Multiprotocol Label Switching (MPLS) Datacenter Networking Synthesis: Requesting a Web Page 84 Synthesis: 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

85 A Day in the Life: Scenario DNS server browser Comcast network 68.80.0.0/13 School network 68.80.2.0/24 web page Web server 64.233.169.105 Googles network 64.233.160.0/19 86 A Day in the Life Connecting to the Internet (1)

DHCP UDP IP Eth Phy DHCP DHCP DHCP DHCP Connecting laptop needs to get its own IP address, addr of firsthop router, addr of DNS server: use DHCP DHCP DHCP DHCP DHCP DHCP DHCP UDP

IP Eth Phy Router (Runs DHCP) DHCP request encapsulated in UDP, encapsulated in IP, encapsulated in 802.3 Ethernet Ethernet frame broadcast (dest: FFFFFFFFFFFF) on LAN, received at router running DHCP server Ethernet demuxed to IP

demuxed, UDP demuxed to DHCP 87 A Day in the Life Connecting to the Internet (2) DHCP UDP IP Eth Phy DHCP DHCP DHCP DHCP DHCP DHCP DHCP DHCP DHCP DHCP UDP

IP Eth Phy Router (runs DHCP) DHCP server formulates DHCP ACK containing clients IP address, IP address of first-hop router for client, name & IP address of DNS server Encapsulation at DHCP server, frame forwarded (switch learning) through LAN, demultiplexing at client DHCP client receives DHCP ACK reply Client now has IP address, knows name & addr of DNS

server, IP address of its first-hop router 88 A Day in the Life ARP (Before DNS, HTTP) DNS DNS DNS ARP query DNS UDP IP ARP Eth Phy ARP ARP reply Eth Phy Router (runs DHCP)

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 now send frame containing DNS query 89 A Day in the Life Using DNS

DNS DNS DNS DNS DNS DNS DNS UDP IP Eth Phy DNS DNS DNS DNS server Comcast network 68.80.0.0/13

Router (runs DHCP) DNS UDP IP Eth Phy IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router IP datagram forwarded from campus network into Comcast network, routed (tables created by RIP, OSPF, IS-IS and/or BGP routing protocols) to DNS server

Demuxed to DNS server DNS server replies to client with IP address of www.google.com 90 A Day in the LifeTCP Connection Carrying HTTP HTTP HTTP TCP IP Eth Phy SYNACK SYN SYNACK SYN

SYNACK SYN SYNACK SYN SYNACK SYN SYNACK SYN TCP IP Eth Phy Web server 64.233.169.105 Router (runs DHCP)

To send HTTP 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 SYNACK (step 2 in 3-way handshake) TCP connection established! 91 A Day in the Life HTTP Request/Reply HTTP HTTP HTTP

TCP IP Eth Phy HTTP HTTP HTTP HTTP HTTP HTTP HTTP HTTP HTTP HTTP HTTP TCP IP Eth Phy Web server

64.233.169.105 Router (runs DHCP) Web page finally (!!!) displayed 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 92 Part 5: Summary Principles behind data link layer services: Error detection, correction Sharing a broadcast channel: multiple access Link layer addressing, ARP Various link layer technologies

Ethernet hubs, bridges, switches IEEE 802.11 LANs PPP ATM X.25, Frame Relay MPLS Datacenter Networking Journey down the protocol stack now OVER! 93

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