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					Link layer, LANs: outline
   5.1 Introduction and         5.6 Link-layer switches
    services                     5.7 PPP
   5.2 Error detection and      5.8 Link virtualization:
    correction                    MPLS
   5.3Multiple access           5.9 A day in the life of a
    protocols                     web request
   5.4 Link-Layer
    Addressing
   5.5 Ethernet



                                                        Link Layer   5-1
MAC addresses and ARP
   32-bit IP address:
     network-layer address for interface
     used for layer 3 (network layer) forwarding

   MAC (or LAN or physical or Ethernet) address:
     function: used ‘locally” to get frame from one interface to
      another physically-connected interface (same network, 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 “number” represents 4 bits)

                                                             Link Layer   5-2
  MAC addresses and ARP
   each adapter on LAN has unique MAC address

                                1A-2F-BB-76-09-AD




                      LAN
                    (wired or                                adapter
                    wireless)
71-65-F7-2B-08-53
                                         58-23-D7-FA-20-B0



                                 0C-C4-11-6F-E3-98




                                                                   Link Layer   5-3
LAN addresses (more)
   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
     LAN card can be moved, but its MAC address is not
      changed
   Hierarchical IP address not portable
     IP address depends on IP subnet to which node is
      attached
                                                         Link Layer   5-4
   ARP: address resolution protocol
    Question: how to determine
    interface’s MAC address,
    knowing its IP address?                          ARP table: each IP node (host,
                                                     router) on LAN has table
                              137.196.7.78
                                                         IP/MAC address
                                                           mappings for some LAN
                             1A-2F-BB-76-09-AD
                                                           nodes:
  137.196.7.23
                                  137.196.7.14            < IP address; MAC address; TTL>
                                                         TTL (Time To Live):
                       LAN                                time after which address
71-65-F7-2B-08-53                                         mapping will be
                                                          forgotten (typically 20
                                 58-23-D7-FA-20-B0

                                                          min)
                             0C-C4-11-6F-E3-98
        137.196.7.88


                                                                              Link Layer   5-5
ARP: address resolution protocol

   ARP Packet




                                               No IP Header
       Ethernet Header
       Source MAC (Hardware) Address: …
       Destination MAC (Hardware) Address: …



                                                    Link Layer   5-6
ARP protocol: same LAN
   A wants to send datagram to B             A caches (saves) IP-to-
     A does not know B’s MAC                  MAC address pair in its
      address                                  ARP table until
        • B’s MAC address not in A’s ARP       information becomes old
          table.                               (times out)
   A broadcasts ARP query                      soft state: information that
    packet, containing B's IP                    times out (goes away)
    address                                      unless refreshed
     dest MAC address = FF-FF-FF-FF-
      FF-FF
                                              ARP is “plug-and-play”:
     all nodes on LAN receive ARP
      query                                     nodes create their ARP
                                                 tables without intervention
   B receives ARP packet, replies               from net administrator
    to A with its (B's) MAC
    address
     frame sent to A’s MAC address
      (unicast)
                                                                    Link Layer   5-7
 Addressing: routing to another LAN
walkthrough: send datagram from A to B via R
  focus on addressing – at IP (datagram) and MAC layer (frame)
  assume A knows B’s IP address
  assume A knows IP address of first hop router, R (how?)
  assume A knows R’s MAC address (how?)


  A                                                                        B
                                        R
  111.111.111.111
                                                           222.222.222.222
  74-29-9C-E8-FF-55
                                                           49-BD-D2-C7-56-2A
                                      222.222.222.220
                                    1A-23-F9-CD-06-9B


      111.111.111.112     111.111.111.110               222.222.222.221
      CC-49-DE-D0-AB-7D   E6-E9-00-17-BB-4B             88-B2-2F-54-1A-0F




                                                              Link Layer    5-8
Addressing: routing to another LAN
     A creates IP datagram with IP source A, destination B
     A creates link-layer frame with R's MAC address as destination MAC
      address
     the frame contains A-to-B IP datagram
          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
                                                                  222.222.222.222
74-29-9C-E8-FF-55
                                                                  49-BD-D2-C7-56-2A
                                             222.222.222.220
                                           1A-23-F9-CD-06-9B


    111.111.111.112           111.111.111.110                  222.222.222.221
    CC-49-DE-D0-AB-7D         E6-E9-00-17-BB-4B                88-B2-2F-54-1A-0F


                                                                     Link Layer    5-9
Addressing: routing to another LAN
     frame sent from A to R
     frame received at R, datagram removed, passed up to IP

        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 src: 111.111.111.111
              IP dest: 222.222.222.222

 IP                                             IP
Eth                                            Eth
Phy                                            Phy

A                                                                             B
                                                R
111.111.111.111
                                                                  222.222.222.222
74-29-9C-E8-FF-55
                                                                  49-BD-D2-C7-56-2A
                                             222.222.222.220
                                           1A-23-F9-CD-06-9B


    111.111.111.112             111.111.111.110                222.222.222.221
    CC-49-DE-D0-AB-7D           E6-E9-00-17-BB-4B              88-B2-2F-54-1A-0F


                                                                     Link Layer 5-10
Addressing: routing to another LAN
    R forwards datagram with IP source A, destination B
    R creates link-layer frame with B's MAC address as destination MAC
     address
    the frame still contains A-to-B IP datagram
                                             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
                                      IP                                                       Eth
                                     Eth                                                       Phy
                                     Phy

A                                                                                             B
                                      R
111.111.111.111
                                                                                  222.222.222.222
74-29-9C-E8-FF-55
                                                                                  49-BD-D2-C7-56-2A
                                    222.222.222.220
                                  1A-23-F9-CD-06-9B


    111.111.111.112     111.111.111.110                                        222.222.222.221
    CC-49-DE-D0-AB-7D   E6-E9-00-17-BB-4B                                      88-B2-2F-54-1A-0F


                                                                                     Link Layer 5-11
Addressing: routing to another LAN
    R forwards datagram with IP source A, destination B
    R creates link-layer frame with B's MAC address as dest, frame
     contains A-to-B IP datagram

                                             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
                                      IP                                                       Eth
                                     Eth                                                       Phy
                                     Phy

A                                                                                             B
                                      R
111.111.111.111
                                                                                  222.222.222.222
74-29-9C-E8-FF-55
                                                                                  49-BD-D2-C7-56-2A
                                    222.222.222.220
                                  1A-23-F9-CD-06-9B


    111.111.111.112     111.111.111.110                                        222.222.222.221
    CC-49-DE-D0-AB-7D   E6-E9-00-17-BB-4B                                      88-B2-2F-54-1A-0F


                                                                                     Link Layer 5-12
Addressing: routing to another LAN
    R forwards datagram with IP source A, destination B
    R creates link-layer frame with B's MAC address as dest, frame
     contains A-to-B IP datagram
                                                      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
                                                                           222.222.222.222
74-29-9C-E8-FF-55
                                                                           49-BD-D2-C7-56-2A
                                    222.222.222.220
                                  1A-23-F9-CD-06-9B


    111.111.111.112     111.111.111.110                                222.222.222.221
    CC-49-DE-D0-AB-7D   E6-E9-00-17-BB-4B                              88-B2-2F-54-1A-0F


                                                                              Link Layer 5-13
Link layer, LANs: outline
   5.1 Introduction and         5.6 Link-layer switches
    services                     5.7 PPP
   5.2 Error detection and      5.8 Link virtualization:
    correction                    MPLS
   5.3Multiple access           5.9 A day in the life of a
    protocols                     web request
   5.4 Link-Layer
    Addressing
   5.5 Ethernet



                                                        Link Layer 5-14
Ethernet
“dominant” wired LAN technology:
 cheap $20 for NIC
 first widely used LAN technology
 simpler, cheaper than token LANs and ATM
 kept up with speed race: 10 Mbps – 10 Gbps




                          Metcalfe’s Ethernet sketch
                                                       Link Layer 5-15
Ethernet: physical topology
   bus: popular through mid 90s
     all nodes in same collision domain (can collide with each
      other)
   star: prevails today
     active switch in center
     each “spoke” runs a (separate) Ethernet protocol (nodes
      do not collide with each other)




                                        switch
                                    star
        bus: coaxial cable
                                                          Link Layer 5-16
Ethernet frame structure
sending adapter encapsulates IP datagram (or other
  network layer protocol packet) in Ethernet frame
                                  type
                 dest.   source            data
      preamble address address           (payload)   CRC



preamble:
 7 bytes with pattern 10101010 … 10101011
 used to synchronize receiver, sender clock rates




                                                           Link Layer 5-17
Ethernet frame structure (more)
   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)
   CRC: cyclic redundancy check at receiver
     error detected: frame is dropped
                                    type
                   dest.   source            data
        preamble address address           (payload)   CRC



                                                             Link Layer 5-18
Ethernet: unreliable, connectionless
   connectionless: no handshaking between sending and
    receiving NICs
   unreliable: receiving NIC doesn’t send acks or nacks
    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 wth exponential backoff




                                                   Link Layer 5-19
Ethernet CSMA/CD algorithm
1. NIC receives datagram from network layer, creates
   frame

2-1. If NIC senses channel idle, starts frame transmission.

2-2. If NIC senses channel busy, waits until channel idle,
  then transmits.

3. If NIC transmits entire frame without detecting another
   transmission, NIC is done with frame !



                                                       Link Layer 5-20
Ethernet CSMA/CD algorithm
4. If NIC detects another transmission while transmitting,
   aborts and sends jam signal

5. After aborting, NIC enters exponential backoff:
    after mth collision, NIC chooses K at random from
     {0,1,2, …, 2m-1}. (m = min{n,10})
    NIC waits K·512 bit times, returns to Step 2
    longer backoff interval with more collisions




                                                         Link Layer 5-21
Ethernet CSMA/CD algorithm (more)
Jam Signal: make sure all other      Exponential Backoff:
   transmitters are aware of          Goal: adapt retransmission
   collision; 48 bits                   attempts to estimated current
                                        load
Bit time: .1 microsec for 10 Mbps          heavy load: random wait will
    Ethernet ;                              be longer
    for K=1023, wait time is about    first collision: choose K from
    50 msec                             {0,1}; delay is K· 512 bit
                                        transmission times
                                      after second collision: choose K
                                        from {0,1,2,3}…
 See/interact with Java               after ten collisions, choose K from
 applet on AWL Web site:                {0,1,2,3,4,…,1023}
 highly recommended !




                                                               5: DataLink Layer 5-22
CSMA/CD efficiency
   dprop = max propagation delay between 2 nodes in LAN
   dtrans = time to transmit max-size frame

                                             1
                  efficiency =
                                    1 + 5d prop /d trans
   efficiency goes to 1
      as dprop goes to 0
      as dtrans goes to infinity

   better performance than ALOHA: and simple, cheap,
    decentralized!

                                                           Link Layer 5-23
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,
        10G bps
       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             fiber physical layer
                           pair) physical layer
                                                                    Link Layer 5-24
Link layer, LANs: outline
   5.1 Introduction and         5.6 Link-layer switches,
    services                      LANs
   5.2 Error detection and      5.7 PPP
    correction                   5.8 Link virtualization:
   5.3 Multiple access           MPLS
    protocols                    5.9 A day in the life of a
   5.4 Link-layer                web request
    Addressing
   5.5 Ethernet



                                                        Link Layer 5-25
Ethernet switch
   link-layer device
      store and 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 are unaware of presence of switches
   plug-and-play, self-learning
      switches do not need to be configured



                                                Link Layer 5-26
Switch: allow multiple simultaneous transmissions

   hosts have dedicated, direct                            A
    connection to switch
                                                                               B
   switches buffer packets             C’

   Ethernet protocol used on each                6    1        2
    incoming link, but no collisions;
    full duplex                                    5        4       3
      each link is its own collision
                                                                           C
        domain                          B’

   switching: A-to-A’ and B-to-B’
    can transmit simultaneously,                       A’
    without collisions                       switch with six interfaces
                                                  (1,2,3,4,5,6)



                                                                    Link Layer 5-27
Switch forwarding table

Q: how does switch know A’                                    A
reachable via interface 4, B’                                                    B
reachable via interface 5?                C’

 A: each switch has a switch                       6    1        2
  table, each entry:
                                                     5        4       3
    (MAC address of host, interface to
     reach host, time stamp)              B’                                 C

    looks like a routing table!
                                                         A’
Q: how are entries created,                    switch with six interfaces
maintained in switch table?                         (1,2,3,4,5,6)
    something like a routing protocol?

                                                                      Link Layer 5-28
Switch: self-learning                                                 Source: A
                                                                      Dest: A’

                                                       A       A A’
   switch learns which hosts
    can be reached through                                                B
    which interfaces                C’

      when frame received,                 6     1        2
       switch “learns”
       location of sender:                  5          4       3
       incoming LAN segment
      records sender/location    B’                                  C
       pair in switch table
                                                  A’

                       MAC addr interface   TTL
                           A         1      60           Switch table
                                                       (initially empty)


                                                                        Link Layer 5-29
Self-learning, forwarding: example                                    Source: A
                                                                      Dest: A’

                                                       A       A A’
   frame destination, A’,
    locaton unknown: flood          C’                                    B


   destination A location                  6     1        2

    known: selectively send                 A A’
                                            5    4             3
    on just one link              B’                                  C
                                                  A’ A

                                                  A’

                       MAC addr interface   TTL
                          A          1      60           switch table
                          A’         4      60         (initially empty)


                                                                        Link Layer 5-30
Switch: frame filtering/forwarding
when frame received at switch:

  1. record incoming link(=leaning), MAC address of sending
     host
  2. search switch table using MAC destination address
  3. if entry found for destination
       then {
         if destination on segment from which frame arrived
            then drop frame (=filtering)
            else forward frame on interface indicated by entry
       }                                                   Selectively send
       else flood /* forward on all interfaces except arriving interface */


                                                                Link Layer 5-31
Interconnecting switches
   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!)
                                                    Link Layer 5-32
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




                                                            Link Layer 5-33
Self-learning multi-switch example
Suppose C sends frame to I, I responds to C

                                                S4
                                    1
                                               2
                    S1 2                                    2   S3
        A                        S2                                    1
                     1                         F
                                     D                                     I
            B              C
                                                        G          H
                                         E
   A:
S1                             S4                               S3
MAC addr interface TTL         MAC addr interface TTL           MAC addr interface TTL


    C           1   60           C         1       60             C         2     60
    I           2   60           I         2       60             I         1     60



                                                                                  Link Layer 5-34
    Institutional Layer 2 network
                                                        mail server
                                to external
                                network
                                              router            web server



                                                                  IP subnet




   Pros:
     Self-leaning switches are easy to maintain the network (switch
      is plug-&-play device).
     Throughput will increase (why? Layer 2 processing is fast).
   Cons:
     Broadcast domain is very large (think about 255.255.255.255)
     Large L2 network can be overwhelmed by ARP broadcast.
     Complex switch network does not provide efficient routing.
                                                                Link Layer 5-35
Switches vs. Routers
                                                application
                                                 transport
both are store-and-forward:          datagram     network
routers: network-layer devices      frame          link
(examine network-layer headers)                   physical          link   frame
                                                                  physical
switches: link-layer devices
(examine link-layer headers)                          switch

both have forwarding tables:                                   network    datagram

routers: compute tables using                                   link      frame
                                                               physical
routing algorithms, IP addresses
   Entry:                                   application
   <destination NET ID, interface>            transport
switches: learn forwarding table              network
using flooding, learning, MAC                    link
addresses                                      physical

   Entry:
   <source MAC Addr, interface>
                                                                    Link Layer 5-36
Link layer, LANs: outline
   5.1 Introduction and         5.6 Link-layer switches
    services                     5.7 PPP (simple summary)
   5.2 Error detection and      5.8 Link virtualization:
    correction                    MPLS
   5.3Multiple access           5.9 A day in the life of a
    protocols                     web request
   5.4 Link-Layer
    Addressing
   5.5 Ethernet



                                                      Link Layer 5-37
Point to Point Data Link Control
   one sender, one receiver, one link: easier than broadcast
    link:
       no Media Access Control
       no need for explicit MAC addressing
       e.g., dialup link, ISDN line

   popular point-to-point DLC protocols:
      PPP: point-to-point protocol
      HDLC: high level data link control




                                                     5: DataLink Layer 5-38
PPP Design Requirements [RFC 1557]
   packet framing: encapsulation of network-layer datagram
    in data link frame
       carry network layer data of any network layer
        protocol (not just IP)
   ability to demultiplex upwards
   bit transparency: must carry any bit pattern in the data
    field
   error detection (no correction)
   connection liveness: detect, signal link failure to network
    layer
   network layer address negotiation: endpoints can
    learn/configure each other’s network address
   simple


                                                        5: DataLink Layer 5-39
PPP non-requirements

   no error correction/recovery
   no flow control
   no order control
     out of order delivery is allowed
   no need to support multipoint links


      Error recovery, flow control, data re-ordering
              all delegated to higher layers!




                                               5: DataLink Layer 5-40
PPP Data Frame
   flag: delimiter (framing)
   address: always 11111111
     does nothing
   control: always 00000011
     does nothing; in the future possible control fields
   protocol: upper layer protocol to which frame delivered
    (eg, PPP-LCP, IP, IPCP, etc)




                                                            5: DataLink Layer 5-41
PPP Data Frame
   info: upper layer data being carried
     Max: 1500 bytes
   check: cyclic redundancy check for error detection




                                                    5: DataLink Layer 5-42
 PPP Data Control Protocol
Before exchanging network-layer data, data link peers must
 LCP (Link Control Protocol)
     configure PPP link (max. frame length, authentication)

   IP Control Protocol (IPCP)
      learn/configure network layer information
     for IP: configure/learn IP address




                                                               5: DataLink Layer 5-43
Link layer, LANs: outline
   5.1 Introduction and         5.6 Link-layer switches
    services                     5.7 PPP
   5.2 Error detection and      5.8 Link virtualization:
    correction                    MPLS (생략)
   5.3Multiple access           5.9 A day in the life of a
    protocols                     web request
   5.4 Link-Layer
    Addressing
   5.5 Ethernet



                                                        Link Layer 5-44
Link layer, LANs: outline
   5.1 Introduction and         5.6 Link-layer switches
    services                     5.7 PPP
   5.2 Error detection and      5.8 Link virtualization:
    correction                    MPLS
   5.3Multiple access           5.9 A day in the life of a
    protocols                     web request
   5.4 Link-Layer                 2학기 동안의 컴퓨터
    Addressing                       네트워크 교과목 종합
   5.5 Ethernet                     리뷰




                                                        Link Layer 5-45
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




                                                        Link Layer 5-46
 A day in the life: scenario

browser                                             DNS server
                                 Comcast network
                                 68.80.0.0/13




      school network
      68.80.2.0/24

            web page




                web server       Google’s network
                64.233.169.105   64.233.160.0/19




                                                    Link Layer 5-47
A day in the life… connecting to the Internet
 DHCP      DHCP                        connecting laptop needs to
 DHCP      UDP
             IP
                                        get its own IP address, addr
                                        of first-hop router, addr of
 DHCP

 DHCP       Eth
            Phy                         DNS server: use DHCP
        DHCP

                                       DHCP request encapsulated
                                        in UDP, encapsulated in IP,
               DHCP
    DHCP
    DHCP       UDP
                                        encapsulated in 802.3
    DHCP         IP                     Ethernet
    DHCP        Eth   router
                Phy   (runs DHCP)      Ethernet frame broadcast
                                        (dest: FFFFFFFFFFFF) on LAN,
                                        received at router running
                                        DHCP server
                                       Ethernet demuxed to IP
                                        demuxed, UDP demuxed to
                                        DHCP
                                                           Link Layer 5-48
A day in the life… connecting to the Internet
 DHCP      DHCP                                DHCP server formulates
 DHCP      UDP                                  DHCP ACK containing
 DHCP        IP                                 client’s IP address, IP
 DHCP       Eth                                 address of first-hop router
            Phy                                 for client, name & IP
                                                address of DNS server
                                               encapsulation at DHCP
    DHCP       DHCP                             server, frame forwarded
    DHCP       UDP                              (switch learning) through
    DHCP         IP                             LAN, demultiplexing at
    DHCP        Eth     router                  client
                        (runs DHCP)
        DHCP
                Phy                            DHCP client receives
                                                DHCP ACK reply


               Client now has IP address, knows name & addr of DNS
                        server, IP address of its first-hop router

                                                                     Link Layer 5-49
A day in the life… ARP (before DNS, before HTTP)
    DNS       DNS                                before sending HTTP request, need
    DNS       UDP                                 IP address of www.google.com:
    DNS
            ARP
                IP                                DNS
ARP query     Eth
              Phy                                DNS query created, encapsulated in
                                                  UDP, encapsulated in IP,
                                                  encapsulated in Eth. To send frame
                          ARP
                                                  to router, need MAC address of
              ARP reply     Eth
                            Phy                   router interface: ARP
                                router           ARP query broadcast, received by
                                (runs DHCP)       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
                                                                          Link Layer 5-50
A day in the life… using DNS                         DNS
                                              DNS    UDP          DNS server
                                              DNS     IP
 DNS      DNS                                 DNS    Eth
 DNS      UDP                                 DNS    Phy
 DNS       IP
 DNS      Eth
          Phy
        DNS
                                              Comcast network
                                              68.80.0.0/13




                      router
                                          IP datagram forwarded from
                      (runs DHCP)          campus network into comcast
      IP datagram containing DNS          network, routed (tables created
       query forwarded via LAN             by RIP, OSPF, IS-IS and/or BGP
       switch from client to 1st hop       routing protocols) to DNS server
       router                             demux’ed to DNS server
                                          DNS server replies to client
                                           with IP address of
                                           www.google.com        Link Layer   5-51
A day in the life…TCP connection carrying HTTP
HTTP
           HTTP
SYNACK
  SYN      TCP
SYNACK
 SYN         IP
SYNACK
  SYN       Eth
            Phy



                                           to send HTTP request,
                                            client first opens TCP socket
                                            to web server
                          router           TCP SYN segment (step 1 in 3-
                          (runs DHCP)
  SYNACK
    SYN        TCP                          way handshake) inter-domain
  SYNACK
    SYN         IP                          routed to web server
  SYNACK
    SYN        Eth
               Phy                         web server responds with TCP
                                            SYNACK (step 2 in 3-way
         web server                         handshake)
         64.233.169.105                    TCP connection established!

                                                                Link Layer 5-52
A day in the life… HTTP request/reply
HTTP
 HTTP          HTTP                    web page finally (!!!) displayed
 HTTP
 HTTP          TCP
 HTTP
 HTTP            IP
 HTTP
 HTTP           Eth
                Phy




                                                           HTTP request sent into TCP
                                                            socket
                          router                           IP datagram containing HTTP
                 HTTP
        HTTP
        HTTP     TCP
                          (runs DHCP)                       request routed to
        HTTP       IP                                       www.google.com
        HTTP      Eth                                      web server responds with
                  Phy                                       HTTP reply (containing web
                                                            page)
         web server
         64.233.169.105
                                                           IP datagram containing HTTP
                                                            reply routed back to client
                                                                               Link Layer 5-53
Chapter 5: Summary
   principles behind data link layer services:
     error detection, correction
     sharing a broadcast channel: multiple access
     link layer addressing
   instantiation and implementation of various link
    layer technologies
     Ethernet
     switched LANS
   synthesis: a day in the life of a web request




                                                     Link Layer 5-54
Chapter 5: let’s take a breath
   journey down protocol stack complete (except
    PHY)
   solid understanding of networking principles,
    practice
   ….. could stop here …. but lots of interesting
    topics!
       wireless
       multimedia
       security
       network management



                                                     Link Layer 5-55

				
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