Missing pieces + Putting the pieces together

Transcription

1 Missing pieces + Putting the pieces together EE 122, Fall 2013 Sylvia Ratnasamy Material thanks to Ion Stoica, Scott Shenker, Jennifer Rexford, Nick McKeown, and many other colleagues

2 Today Missing pieces DHCP ARP Pu6ng the pieces together What happens when I click on a link? More missing pieces

3 Naming ApplicaDon layer: URLs and domain names names resources - - hosts, content, program (recall: mixes the what and where of an object) Network layer: IP addresses host s network locadon Link layer: MAC addresses host idendfier Use all three for end- to- end communicadon!

4 Discovery A host is born knowing only its MAC address Must discover lots of informadon before it can communicate with a remote host B what is my IP address? what is B s IP address? (remote) what is B s MAC address? (if B is local) what is my first- hop router s address? (if B is not local)

5 ARP and DHCP Link layer discovery protocols Address ResoluDon Protocol, Dynamic Host ConfiguraDon Protocol confined to a single local- area network (LAN) rely on broadcast capability of a LAN Hosts Router

6 ARP and DHCP Link layer discovery protocols Serve two funcdons Discovery of local end- hosts for communicadon between hosts on the same LAN

7 ARP and DHCP Link layer discovery protocols Serve two funcdons Discovery of local end- hosts Bootstrap communicadon with remote hosts what s my IP address? who/where is my local DNS server? who/where is my first hop router?

8 DHCP Dynamic Host ConfiguraDon Protocol defined in RFC 2131 A host uses DHCP to discover its own IP address its netmask IP address(es) for its DNS name server(s) IP address(es) for its first- hop default router(s)

9 DHCP: operadon 1. One or more local DHCP servers maintain required informadon IP address pool, netmask, DNS servers, etc. applicadon that listens on UDP port 67

10 DHCP: operadon 1. One or more local DHCP servers maintain required informadon 2. Client broadcasts a DHCP discovery message L2 broadcast, to MAC address FF:FF:FF:FF:FF:FF

11 DHCP: operadon 1. One or more local DHCP servers maintain required informadon 2. Client broadcasts a DHCP discovery message 3. One or more DHCP servers responds with a DHCP offer message proposed IP address for client, lease Dme other parameters

12 DHCP: operadon 1. One or more local DHCP servers maintain required informadon 2. Client broadcasts a DHCP discovery message 3. One or more DHCP servers responds with a DHCP offer message 4. Client broadcasts a DHCP request message specifies which offer it wants echoes accepted parameters other DHCP servers learn they were not chosen

13 DHCP: operadon 1. One or more local DHCP servers maintain required informadon 2. Client broadcasts a DHCP discovery message 3. One or more DHCP servers responds with a DHCP offer message 4. Client broadcasts a DHCP request message 5. Selected DHCP server responds with an ACK (DHCP relay agents used when the DHCP server isn t on the same broadcast domain - - see text)

14 DHCP uses sob state Sob state: if not refreshed, state is forgocen hard state: allocadon is deliberately returned/withdrawn used to track address allocadon in DHCP ImplementaDon address allocadons are associated with a lease period server: sets a Dmer associated with the record of allocadon client: must request a refresh before lease period expires server: resets Dmer when a refresh arrives; sends ACK server: reclaims allocated address when Dmer expires Simple, yet robust under failure state always fixes itself in (small constant of) lease Dme

15 Sob state under failure a.b.c.d is XYZ s from (now, now+c.lease) DHCP Server a.b.c.d is mine from (now, now +lease) XYZ Router What happens when host XYZ fails? refreshes from XYZ stop server reclaims a.b.c.d aber O(lease period)

16 Router What happens when server fails? ACKs from server stop XYZ releases address aber O(lease period); send new request A new DHCP server can come up from a `cold start and we re back on track in ~lease Dme Sob state under failure a.b.c.d is XYZ s from (now, now+c.lease) DHCP Server a.b.c.d is mine from (now, now+lease) XYZ

17 Sob state under failure a.b.c.d is XYZ s from (now, now+c.lease) DHCP Server a.b.c.d is mine from (now, now+lease) XYZ Router What happens if the network fails? refreshes and ACKs don t get through XYZ release address; DHCP server reclaims it

18 Are we there yet? What I learnt from DHCP my IP: netmask: /24 ( ) DNS: router: DHCP Server Host Host Host Host DNS Server Router

19 Sending Packets Over Link- Layer IP packet Host Host Host Host DNS 90- E2- A B D7- FA- 20- B0 Router Link layer only understands MAC addresses Translate the desdnadon IP address to MAC address Encapsulate the IP packet inside a link- level frame

20 ARP: Address ResoluDon Protocol Every host maintains an ARP table list of (IP address à MAC address) pairs Consult the table when sending a packet Map desdnadon IP address to desdnadon MAC address Encapsulate the (IP) data packet with MAC header; transmit But: what if IP address not in the table? Sender broadcasts: Who has IP address ? Receiver responds: MAC address D7- FA- 20- B0 Sender caches result in its ARP table

21 What if the desdnadon is remote? Look up the MAC address of the first hop router uses ARP to find MAC address for first- hop router rather than uldmate desdnadon IP address How does the red host know the desdnadon is not local? Uses netmask (discovered via DHCP) How does the red host know about ? Also DHCP /24 ( ) host host... DNS /24 host host... host router router router

22 Security Analysis of ARP ImpersonaDon Any node that hears request can answer and can say whatever they want Actual legit receiver never sees a problem Because even though later packets carry its IP address, its NIC doesn t capture them since not its MAC address

23 Steps in Sending a Packet" What do hosts need to know? And how do they find out?

24 Steps in reaching a Host" First look up destination s IP address Need to know where local DNS server is DHCP Also needs to know its own IP address DHCP

25 Sending a Packet" On same subnet: Use MAC address of destination. ARP On some other subnet: Use MAC address of first-hop router. DHCP + ARP And how can a host tell whether destination is on same or other subnet? Use the netmask DHCP

26 Example: A Sending a Packet to B" A R B How does host A send an IP packet to host B?

27 Example: A Sending a Packet to B" A R B 1. A sends packet to R. 2. R sends packet to B."

28 Host A Decides to Send Through R" Host A constructs an IP packet to send to B Source , destination Host A has a gateway router R Used to reach destinations outside of /24 Address for R learned via DHCP A 28 R B

29 Host A Sends Packet Through R" Host A learns the MAC address of R s interface ARP request: broadcast request for ARP response: R responds with E6-E BB-4B Host A encapsulates the packet and sends to R A 29 R B

30 Two points: RouDng table points to this port DesDnaDon address is within mask of port s address (i.e., local) R Decides how to Forward Packet" Router R s adapter receives the packet R extracts the IP packet from the Ethernet frame R sees the IP packet is destined to Router R consults its forwarding table Packet matches /24 via other adapter (port) A 30 R B

31 R Sends Packet to B" Router R s learns the MAC address of host B ARP request: broadcast request for ARP response: B responds with 49-BD-D2-C7-56-2A Router R encapsulates the packet and sends to B A 31 R B

32 Key Ideas in Both ARP and DHCP" Broadcasting: used for initial bootstrap Caching: remember the past for a while Store the information you learn to reduce overhead Remember your own address & other host s addresses Key optimization for performance Soft state: eventually forget the past Associate a time-to-live field with the information and either refresh or discard the information Key for robustness

33 Discovery mechanisms We ve seen two broad approaches Broadcast (ARP, DHCP) flooding doesn t scale no centralized point of failure zero configuradon Directory service (DNS) no flooding root of the directory is vulnerable (caching is key) needs configuradon to bootstrap (local, root servers, etc.) Can we get the best of both? Internet- scale yet zero config?

34 Are we there yet? Yes!

35 Pu6ng the pieces together Walk through the steps required to download from your laptop yourdns yourdhcp Google s datacenter You R router UCB Count the number of protocols that come into play! Dorm Assume: `cold start - - nothing cached anywhere Assume: yourdns on a different subnet from yourdhcp Ignore intra- and interdomain roudng protocols

36 Step 1: Self discovery You use DHCP to discover bootstrap parameters your IP addr (u.u.u.u) your DNS server s IP (u.dns.ip.addr) R s IP address (r.r.r.r).. Exchange between you and yourdhcp yourdhcp You R router Ethernet IP UDP DHCP Protocol count = 4 Dorm

37 Next You are ready to contact à need an IP address for à need to ask google s DNS server à need to ask my DNS server to ask google s DNS à I know my DNS server s IP addr is u.dns.ip.addr à create a packet to send source: u.u.u..u dst: u.dns.ip.addr Ethernet IP UDP DNS desdnadon MAC?

38 Step 2: Ge6ng out the door You use ARP to discover the MAC address of R Exchange between you and R yourdhcp dst MAC? Ethernet ARP You R router Protocol count = 5 Dorm

39 Step 3: Send a DNS request Exchange between you and yourdns Now ready to send that packet yourdns Ethernet IP UDP DNS R s MAC source: u.u.u..u dst: u.dns.ip.addr You R router UCB Protocol count = 6

40 Step 4: yourdns does its thing yourdns resolves You yourdns root name server top- level name server s IP address is g.g.g.g google s name server Protocol count = 6

41 Step 5: Ge6ng the content (at last) You R UCB Google s datacenter Exchange between you and google s server at g.g.g.g R s MAC Protocol count = 8 Ethernet IP TCP HTTP source: u.u.u..u dst: g.g.g.g

42 Recap: Name discovery/resoludon MAC addresses? my own: hardcoded others: ARP (given IP address) IP addresses? my own: DHCP others: DNS (given domain name) how do I bootstrap DNS communicadon? (DHCP) Domain names? search engines

43 Switched Ethernet"

44 Why Switched Ethernet?" B" A" C" switch D" Enables concurrent communication Host A can talk to C, while B talks to D No collisions à no need for CSMA, CD No constraints on link lengths, etc.

45 Switched Ethernet " Constraints (for backward compatibility) same framing (MAC address structure, headers) maintain plug-n-play aspect (Old) Ethernet achieved plug-n-play by leveraging a broadcast medium how do we do this in a switched topology? Natural first thought: flood (to mimic broadcast)

46 Problem: Flooding can lead to loops Example: A wants to broadcast a message - A sends packet to 1-1 Floods to 2 and 4-2 Floods to B and 3-4 Floods to D and 3-3 Floods packet from 2 to C and 4-3 Floods packet from 4 to C and 2-4 Floods packet from 3 to D and 1-2 Floods packet from 3 to B and 1-1 Floods packet from 2 to A and 4-1 Floods packet from 4 to B and 2 -. B A C D - A broadcast storm if the network contains a cycle of switches

47 Easiest Way to Avoid Loops Use a topology where loops are impossible! Take arbitrary topology Build spanning tree (algorithm covered later) Sub-graph that covers all vertices but contains no cycles Ignore all links not on the spanning tree Only one path to desdnadons on spanning trees So don t have to worry about loops!

48 Consider graph

49 49 A Spanning Tree

50 50 Another Spanning Tree

51 Flooding on a Spanning Tree Switches flood using the following rule: (Ignoring all ports not on spanning tree!) OriginaDng switch sends flood packet out all ports When a flood packet arrives on one incoming port, send it out all other ports

52 Flooding on Spanning Tree

53 Flooding on Spanning Tree

54 But isn t flooding wasteful? Yes, but we can use it to bootstrap more efficient forwarding Idea: watch the packets going by, and learn from that There is a single path between any two nodes If node A sees a packet from node B come in on a pardcular port, it knows what port to use to reach B!

55 Nodes can learn roudng tables Switch can learn how to reach nodes by remembering where flooding packets came from! If flood packet from Node A entered switch from port 4, then switch uses port 4 to reach Node A 55

56 Learning from Flood Packets Node B Node A can be reached through this port Node A can be reached through this port Node A 56 Once a node has sent a flood message, all other switches know how to reach it.

57 Node B Responds Node B can be reached through this port Node B Node A 57 When a node responds, some of the switches learn where it is

58 General Approach Flood first packet to node you are trying to reach All switches learn where you are When desdnadon responds, some switches learn where it is Only some switches, because packet to you follows direct path, and is not flooded

59 Self-Learning Switch" When a packet arrives Inspect source MAC address, associate with incoming port Store mapping in the switch table Use time-to-live field to eventually forget mapping Packet tells switch how to reach A." B" A" C" D"

60 Self Learning: Handling Misses" When packet arrives with unfamiliar destination Forward packet out all other ports Response will teach switch about that destination B" A" C" D"

61 Summary of Learning Approach Avoids loop by restricdng to spanning tree This makes flooding possible Flooding allows packet to reach desdnadon And in the process switches learn how to reach source of flood No route computadon