Internet Architecture and Protocols: Link Layer

1. School of Computing Science Simon Fraser University CMPT 771/471: Internet Architecture and Protocols Link Layer Instructor: Dr. Mohamed Hefeeda

2. Review of Basic Networking Concepts • Internet structure • Protocol layering and encapsulation • Internet services and socket programming • Network Layer • Network types: Circuit switching, Packet switching • Addressing, Forwarding, Routing • Transport layer • Reliability, congestion and flow control • TCP, UDP • Link Layer • Multiple Access Protocols • Ethernet, MAC addressing

3. Some terminology: hosts and routers are nodes communication channels that connect adjacent nodes along communication path are links wired links wireless links LANs layer-2 packet is a frame,encapsulates datagram “link” Link Layer data-link layer has responsibility of transferring datagram from one node to adjacent node over a link

4. Datagram transferred by different link protocols over different links: e.g., Ethernet on first link, frame relay on intermediate links, 802.11 on last link Each link protocol provides different services e.g., may or may not provide rdt over link transportation analogy trip from Burnaby to Lausanne, Switzerland limo: Burnaby to YVR plane: YVR to Geneva train: Geneva to Lausanne tourist = datagram transport segment = communication link transportation mode = link layer protocol travel agent = routing algorithm Link layer: context

5. Link Layer Services • Framing, link access: • encapsulate datagram into frame, adding header, trailer • channel access if shared medium • “MAC” addresses used in frame headers to identify source, dest • Reliable delivery between adjacent nodes • we learned how to do this already (chapter 3)! • seldom used on low bit error link (e.g., fiber) • used in wireless links: high error rates • Q: why both link-level and end-end reliability? • LL: local correction (bet adjacent nodes)  faster • e-2-e: is still needed because not all LL protocols provide reliability

6. Link Layer Services (more) • Flow Control • pacing between adjacent sending and receiving nodes • 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 • Half-duplex and full-duplex • with half duplex, nodes at both ends of link can transmit, but not at same time

7. link layer implemented in “adaptor” (aka NIC) Ethernet card, PCMCI card, 802.11 card sending side: encapsulates datagram in a frame adds error checking bits, rdt, flow control, etc. receiving side looks for errors, rdt, flow control, etc extracts datagram, passes to rcving node adapter is semi-autonomous link & physical layers frame frame Adaptors Communicating datagram rcving node link layer protocol sending node adapter adapter

8. Multiple Access Links and Protocols Two types of “links”: • point-to-point • Single sender and single receiver • E.g., dial-up links  point-to-point protocol (PPP) • broadcast (shared wire or medium) • Multiple senders and multiple receivers • E.g., traditional Ethernet, 802.11 wireless LAN •  need Multiple Access protocol (MAC)

9. Multiple Access protocols • Two or more simultaneous transmissions on a shared channel  interference (collision) • Collision: node receives two or more signals at the same time Multiple Access (MAC) protocol • distributed algorithm that determines how nodes share channel, i.e., determine when node can transmit • communication about channel sharing must use channel itself! • no out-of-band channel for coordination

10. MAC Protocols: a taxonomy Three broad classes: • Channel Partitioning • Channel Partitioning by time, frequency or code • TDMA, FDMA, CDMA • 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 • E.g., Token bus and token ring

11. 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” • 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 • ALOHA • CSMA, CSMA/CD, CSMA/CA

12. CSMA (Carrier Sense Multiple Access) CSMA: listen before transmit: • If channel sensed idle: transmit entire frame • If channel sensed busy, defer transmission • Can collisions still occur? • Yes, because of propagation delay • two nodes may not hear each other’s transmission • During collision, entire packet transmission time is wasted detect collision and abort immediately (CSMA/CD)

13. Ethernet “dominant” wired LAN technology: • cheap $20 for 100Mbs! • 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

14. Ethernet Frame Structure 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

15. Ethernet Frame Structure (more) • Addresses: 6 bytes • if adapter receives frame with matching destination address, or with broadcast address (e.g., ARP packet), it passes data in frame to net-layer protocol • otherwise, adapter discards frame • 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

16. Unreliable, connectionless service • Connectionless: No handshaking between sending and receiving adapter. • Unreliable: receiving adapter doesn’t send acks or nacks to sending adapter • stream of datagrams passed to network layer can have gaps • gaps will be filled if app is using TCP • otherwise, app will see the gaps

17. 1. Adaptor receives datagram from net layer & creates frame 2. If adapter senses channel idle, it starts to transmit frame. If it senses channel busy, waits until channel idle and then transmits 3. If adapter transmits entire frame without detecting another transmission, the adapter is done with frame! 4. If adapter detects another transmission while transmitting, aborts and sends jam signal 5. After aborting, adapter enters exponential backoff: after the mth collision, adapter chooses K at random from {0,1,2,…,2m-1}. Adapter waits K·512 bit times and returns to Step 2 Ethernet CSMA/CD algorithm

18. Jam Signal: make sure all other transmitters are aware of collision; 48 bits Bit time: 0.1 microsec for 10 Mbps Ethernet ;for K=1023, wait time is about 50 msec 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 collisions, choose K from {0,1,2,3,4,…,1023} Ethernet’s CSMA/CD (more) See/interact with Java applet on AWL Web site: highly recommended !

19. CSMA/CD efficiency • Tprop = max prop between 2 nodes in LAN • ttrans = time to transmit max-size frame • Efficiency goes to 1 as tprop goes to 0 • Goes to 1 as ttrans goes to infinity • Much better than ALOHA, but still decentralized, simple, and cheap

20. application transport network link physical fiber physical layer copper (twister pair) physical layer 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 and frame format 100BASE-T2 100BASE-FX 100BASE-TX 100BASE-BX 100BASE-SX 100BASE-T4

21. Manchester encoding • used in 10BaseT • each bit has a transition • allows clocks in sending and receiving nodes to synchronize to each other • no need for a centralized, global clock among nodes! • Hey, this is physical-layer stuff!

22. twisted pair hub Hubs … physical-layer (“dumb”) repeaters: • bits coming in one link go out all other links at same rate • all nodes connected to hub can collide with one another • no frame buffering • no CSMA/CD at hub: host NICs detect collisions 5: DataLink Layer

23. Switch • link-layer device: smarter than hubs, take active role • store, forward Ethernet frames • examine incoming frame’s MAC address, selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment, uses CSMA/CD to access segment • transparent • hosts are unaware of presence of switches • plug-and-play, self-learning • switches do not need to be configured 5: DataLink Layer

24. Switch: multiple simultaneous transmissions A • 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’ simultaneously, without collisions • not possible with dumb hub C’ B 1 2 3 6 4 5 C B’ A’ switch with six interfaces (1,2,3,4,5,6) 5: DataLink Layer

25. Switch Table A • Q: how does switch know that A’ reachable via interface 4, B’ reachable via interface 5? • A: each switch has a switch table, each entry: • (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? C’ B 1 2 3 6 4 5 C B’ A’ switch with six interfaces (1,2,3,4,5,6) 5: DataLink Layer

26. Source: A Dest: A’ MAC addr interface TTL 60 1 A A A’ Switch: self-learning A • switchlearns which hosts can be reached through which interfaces • when frame received, switch “learns” location of sender: incoming LAN segment • records sender/location pair in switch table C’ B 1 2 3 6 4 5 C B’ A’ Switch table (initially empty) 5: DataLink Layer

27. Switch: frame filtering/forwarding When frame received: 1. record link associated with sending host 2. index switch table using MAC dest address 3. if entry found for destinationthen { if dest on segment from which frame arrivedthen drop the frame else forward the frame on interface indicated } else flood forward on all but the interface on which the frame arrived 5: DataLink Layer

28. Source: A Dest: A’ A’ A MAC addr interface TTL 60 60 1 4 A A’ A A’ A A’ A A’ A A’ A A’ A A’ Self-learning, forwarding: example A • frame destination unknown: C’ B 1 2 3 flood 6 4 5 • destination A location known: C selective send B’ A’ Switch table (initially empty) 5: DataLink Layer

29. S4 S3 S2 F I D H G E Interconnecting switches • switches can be connected together S1 A C B • 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!) 5: DataLink Layer

30. Institutional network mail server to external network web server router IP subnet 5: DataLink Layer

31. Switches vs. Routers • both store-and-forward devices • routers: network layer devices (examine network layer headers) • switches are link layer devices • routers maintain routing tables, implement routing algorithms • switches maintain switch tables, implement filtering, learning algorithms 5: DataLink Layer

32. MAC Addresses • 32-bit IP address: • network-layer address • used to get datagram to destination IP subnet • MAC (LAN, physical, or Ethernet) address: • used to get frame from one interface to another physically-connected interface (same network) • 48 bit MAC address (for most LANs) burned in the adapter ROM

33. 1A-2F-BB-76-09-AD LAN (wired or wireless) 71-65-F7-2B-08-53 58-23-D7-FA-20-B0 0C-C4-11-6F-E3-98 MAC Address Each adapter on LAN has unique LAN address Broadcast address = FF-FF-FF-FF-FF-FF = adapter

34. MAC Address (more) • MAC address allocation administered by IEEE • manufacturer buys portion of MAC address space (to assure uniqueness) • Analogy: (a) MAC address: like Social Insurance Number (b) IP address: like postal address • MAC flat address  portability • can move LAN card from one LAN to another • IP hierarchical address  NOT portable • depends on IP subnet to which node is attached

35. MAC and IP addresses • Why do we have TWO addresses (IP,MAC)? Do we have to have MAC addresses? • Yes, we must have both • To allow different network-layer protocols over same card (e.g., IP, Novell IPX, DECnet) • Enable flexibility, mobility of cards • Efficiency: imagine that nodes have only IP addresses  ALL packets sent over LAN will be forwarded by NIC to the IP layer  too many useless interrupts

36. ARP: determines MAC address of node given its IP address ARP: Address Resolution Protocol • Each IP node (Host, Router) on LAN has ARP 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) 237.196.7.78 1A-2F-BB-76-09-AD 237.196.7.23 237.196.7.14 LAN 71-65-F7-2B-08-53 58-23-D7-FA-20-B0 0C-C4-11-6F-E3-98 237.196.7.88

37. A wants to send datagram to B, and B’s MAC address not in A’s ARP table. A broadcasts ARP query packet, containing B's IP address Dest MAC address = FF-FF-FF-FF-FF-FF all machines on LAN receive ARP query B receives ARP packet, replies to A with its (B's) MAC address frame sent to A’s MAC address (unicast) A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state: information that times out (goes away) unless refreshed ARP is “plug-and-play”: nodes create their ARP tables without intervention from net administrator ARP protocol: Same LAN (network)

38. Routing to another LAN walkthrough: send datagram from A to B via R assume A knows B’s IP address • Two ARP tables in router R, one for each IP network (LAN) A R B

39. Routing to another LAN (cont’d) • Detailed steps: • A creates datagram with source A, destination B • A uses ARP to get R’s MAC address for 111.111.111.110 • A creates link-layer frame with R's MAC address as dest, frame contains A-to-B IP datagram • A’s adapter sends frame • R’s adapter receives frame • R removes IP datagram from Ethernet frame, sees it is destined to B • R uses ARP to get B’s MAC address • R creates frame containing A-to-B IP datagram sends to B

40. VLANs: motivation What happens if: • CS user moves office to EE, but wants connect to CS switch? • single broadcast domain: • all layer-2 broadcast traffic (ARP, DHCP) crosses entire LAN (security/privacy, efficiency issues) • each lowest level switch has only few ports in use What’s wrong with this picture? Computer Science Computer Engineering Electrical Engineering 5: DataLink Layer

41. 7 1 2 8 15 9 10 16 VLANs Port-based VLAN: switch ports grouped (by switch management software) so that single physical switch …… 15 7 9 1 Virtual Local Area Network 2 8 10 16 … … Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure. Computer Science (VLAN ports 9-15) Electrical Engineering (VLAN ports 1-8) … operates as multiple virtual switches … … Computer Science (VLAN ports 9-16) Electrical Engineering (VLAN ports 1-8) 5: DataLink Layer

42. forwarding between VLANS: done via routing (just as with separate switches) • in practice vendors sell combined switches plus routers Port-based VLAN 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 15 7 9 1 2 8 10 16 • dynamic membership: ports can be dynamically assigned among VLANs … … Computer Science (VLAN ports 9-15) Electrical Engineering (VLAN ports 1-8) 5: DataLink Layer

43. 1 16 VLANS spanning multiple switches • trunk port: carries frames between VLANS defined over multiple physical switches • frames forwarded within VLAN between switches can’t be vanilla 802.1 frames (must carry VLAN ID info) • 802.1Q protocol adds/removes additional header fields for frames forwarded between trunk ports 15 7 9 7 1 3 5 2 8 10 4 6 2 8 … … Computer Science (VLAN ports 9-15) Ports 2,3,5 belong to EE VLAN Ports 4,6,7,8 belong to CS VLAN Electrical Engineering (VLAN ports 1-8) 5: DataLink Layer

44. Type 802.1Q VLAN frame format 802.1 frame 802.1Q frame 2-byte Tag Protocol Identifier (value: 81-00) Recomputed CRC Tag Control Information (12 bit VLAN ID field, 3 bit priority field like IP TOS) 5: DataLink Layer

45. Virtualization of networks Virtualization of resources: powerful abstraction in systems engineering: • computing examples: virtual memory, virtual devices • Virtual machines: e.g., java • IBM VM os from 1960’s/70’s • Layer abstraction • don’t sweat the details of the lower layer, only deal with lower layers abstractly 5: DataLink Layer

46. The Internet: virtualizing networks 1974: multiple unconnected nets • ARPAnet • data-over-cable networks • packet satellite network (Aloha) • packet radio network … differing in: • addressing conventions • packet formats • error recovery • routing satellite net ARPAnet "A Protocol for Packet Network Intercommunication", V. Cerf, R. Kahn, IEEE Transactions on Communications, May, 1974, pp. 637-648. 5: DataLink Layer

47. Internetwork layer (IP): • addressing: internetwork appears as single, uniform entity, despite underlying local network heterogeneity • network of networks The Internet: virtualizing networks Gateway: • “embed internetwork packets in local packet format or extract them” • route (at internetwork level) to next gateway gateway satellite net ARPAnet 5: DataLink Layer

48. Cerf & Kahn’s Internetwork Architecture What is virtualized? • two layers of addressing: internetwork and local network • new layer (IP) makes everything homogeneous at internetwork layer • underlying local network technology • cable • satellite • 56K telephone modem • today: ATM, MPLS … “invisible” at internetwork layer. Looks like a link layer technology to IP! 5: DataLink Layer

49. 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 5: DataLink Layer

50. browser A day in the life: scenario DNS server Comcast network 68.80.0.0/13 school network 68.80.2.0/24 web page web server Google’s network 64.233.160.0/19 64.233.169.105 5: DataLink Layer