ethernet and internet control protocols
DESCRIPTION
Ethernet and Internet Control Protocols. EE 122: Intro to Communication Networks Fall 2010 (MW 4-5:30 in 101 Barker) Scott Shenker TAs: Sameer Agarwal, Sara Alspaugh, Igor Ganichev, Prayag Narula http://inst.eecs.berkeley.edu/~ee122/ - PowerPoint PPT PresentationTRANSCRIPT
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Ethernet andInternet Control Protocols
EE 122: Intro to Communication Networks
Fall 2010 (MW 4-5:30 in 101 Barker)
Scott Shenker
TAs: Sameer Agarwal, Sara Alspaugh, Igor Ganichev, Prayag Narula
http://inst.eecs.berkeley.edu/~ee122/
Materials with thanks to Jennifer Rexford, Ion Stoica, Vern Paxsonand other colleagues at Princeton and UC Berkeley
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Questions to be answered today
• What must a host know before it can operate?– Local information– Remote information
• How do you avoid manual configuration?– Management: most important issue in networking today!
• How can host learn about local network?
• How can host learn about the rest of the Internet?
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Answers Involve….
• Bootstrapping an end host (local)– Learning its own configuration parameters (DHCP)– Learning the link-layer addresses of other nodes (ARP)
• Network control messages (global)– Internet Control Message Protocol (ICMP)– Exploiting ICMP for discovering Internet path properties
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Bootstrap and Control Protocols
• Very different mechanisms
• For very different environments
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Internet versus LAN
• Scale:– Huge vs Limited
• Management:– Ad Hoc vs Managed
• Delivery Model:– No broadcast vs broadcast
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As a result…..
• Local mechanisms: broadcast to find things– “Bootstrapping”
• Remote mechanisms: investigate path– How to use what routing has already found– “Network Control Messages”
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Preliminary Observations
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Broadcast at Link-Level
• Use broadcast address: ff:ff:ff:ff:ff:ff
• If have return MAC address, use that in response
• Unless want everyone to know result
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Broadcast at IP Level
• Can’t broadcast to all IP hosts
• But application might want to send “local” broadcast
• Uses IP broadcast address 225.225.225.225
• Link-layer then users link-layer broadcast
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Reaching a Host
• First look up IP address
• Need to know where local DNS server is– How does a host know this?
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Sending a Packet
• On same subnet:– Use MAC address of destination– How do a host know that?
• On some other subnet:– Use MAC address of first-hop router– How do a host know that?
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Bootstrapping a Host
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What Does a Host Need to Know?
• What IP address the host should use?
• What local DNS server to use?
• How to tell which destinations are local?
• How do we address them using local network?
• How to send packets to remote destinations?
host host DNS... host host DNS...
router router
1.2.3.0/23 5.6.7.0/24
1.2.3.7 1.2.3.156???
1.2.3.19
router
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Avoiding Manual Configuration
• Dynamic Host Configuration Protocol (DHCP)– End host learns how to send packets– Learn IP address, DNS servers, “gateway”, what’s local
• Address Resolution Protocol (ARP)– For local destinations, learn mapping between IP
address and MAC address
host host DNS... host host DNS...
router router
1.2.3.0/23255.255.254.0
5.6.7.0/24
1.2.3.7 1.2.3.1561.2.3.48
1.2.3.19
router
1A-2F-BB-76-09-AD
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Key Ideas in Both Protocols
• Broadcasting: when in doubt, shout!– Broadcast query to all hosts in the local-area-network– … when you don’t know how to identify the right one
• Caching: remember the past for a while– Store the information you learn to reduce overhead– Remember your own address & other host’s addresses
• 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 in the face of unpredictable change
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MAC Address vs. IP Address
• MAC addresses– Hard-coded in read-only memory when adaptor is built– Like a social security number– Flat name space of 48 bits (e.g., 00-0E-9B-6E-49-76)– Portable, and can stay the same as the host moves– Used to get packet between interfaces on same network
• IP addresses– Configured, or learned dynamically– Like a postal mailing address– Hierarchical name space of 32 bits (e.g., 12.178.66.9)– Not portable, and depends on where the host is attached– Used to get a packet to destination IP subnet
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Bootstrapping Problem
• Host doesn’t have an IP address yet– So, host doesn’t know what source address to use
• Host doesn’t know who to ask for an IP address– So, host doesn’t know what destination address to use
• Solution: shout to “discover” server that can help– Broadcast a server-discovery message (ff:ff:ff:ff:ff:ff)– Server(s) sends a reply offering an address
host host host...
DHCP server
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Response from the DHCP Server
• DHCP “offer” message from the server– Configuration parameters (proposed IP address, mask,
gateway router, DNS server, ...)– Lease time (duration the information remains valid)
• Multiple servers may respond– Multiple servers on the same broadcast network– Each may respond with an offer
• Accepting one of the offers– Client sends a DHCP “request” echoing the parameters– The DHCP server responds with an “ACK” to confirm– … and the other servers see they were not chosen
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Dynamic Host Configuration Protocol
arrivingclient
DHCP server203.1.2.5
DHCP discover(broadcast)
DHCP offer
DHCP request
DHCP ACK
(broadcast)
Why all the broadcasts?
(broadcast)
(broadcast)
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Soft State: Refresh or Forget
• Why is a lease time necessary?– Client can release the IP address (DHCP RELEASE)
o E.g., “ipconfig /release” at the DOS prompto E.g., clean shutdown of the computer
– But, host might not release the addresso E.g., the host crashes (blue screen of death!)o E.g., buggy client software
– And you don’t want the address to be allocated forever
• Performance trade-offs– Short lease time: returns inactive addresses quickly– Long lease time: avoids overhead of frequent renewals &
lessens frequency of lease being denied
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So, Now the Host Knows Things
• IP address
• Mask
• Gateway router
• DNS server
• And can send packets to other IP addresses
• But: how to use the local network to do this?
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Figuring Out Where To Send Locally• Two cases:
– Destination is on the local networko So need to address it directly
– Destination is not local (“remote”)o Need to figure out the first “hop” on the local network
• Determining if it’s local: use the netmask– E.g., mask destination IP address w/ 255.255.254.0– Is it the same value as when we mask our own address?
o Yes = localo No = remote
host host DNS... host host DNS...
router router
1.2.3.0/23255.255.254.0
5.6.7.0/24
1.2.3.7 1.2.3.1561.2.3.48
1.2.3.19
router
1A-2F-BB-76-09-AD
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Where To Send Locally, con’t
• If it’s remote, look up first hop in (very small) local routing table– E.g., by default, route via 1.2.3.19– Now do the local case but for 1.2.3.19 rather than
ultimate destination IP address
• For the local case, need to determine the destination’s MAC address
host host DNS... host host DNS...
router router
1.2.3.0/23255.255.254.0
5.6.7.0/24
1.2.3.7 1.2.3.1561.2.3.48
1.2.3.19
router
1A-2F-BB-76-09-AD
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Sending Packets Over a Link
• Adaptors only understand MAC addresses– Translate the destination IP address to MAC address– Encapsulate the IP packet inside a link-level frame
host host DNS...1.2.3.156
router
1.2.3.53
1.2.3.53
1.2.3.156
IP packet
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5 Minute Break
Questions Before We Proceed?
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Address Resolution Protocol
• Every node maintains an ARP table– <IP address, MAC address> pair
• Consult the table when sending a packet– Map destination IP address to destination MAC address– Encapsulate and transmit the data packet
• But: what if IP address not in the table?– Sender broadcasts: “Who has IP address 1.2.3.156?”– Receiver responds: “MAC address 58-23-D7-FA-20-B0”– Sender caches result in its ARP table
• Link-layer protocol (RFC 826)– Because necessary to bootstrap IP connectivity
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Example: A Sending a Packet to B
How does host A send an IP packet to host B?
A
RB
1. A sends packet to R.2. R sends packet to B.
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Host A Decides to Send Through R
A
RB
• Host A constructs an IP packet to send to B– Source 111.111.111.111, destination 222.222.222.222
• Host A has a gateway router R– Used to reach destinations outside of 111.111.111.0/24– Address 111.111.111.110 for R learned via DHCP
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Host A Sends Packet Through R
• Host A learns the MAC address of R’s interface– ARP request: broadcast request for 111.111.111.110– ARP response: R responds with E6-E9-00-17-BB-4B
• Host A encapsulates the packet and sends to R
A
RB
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R Decides how to Forward Packet• Router R’s adaptor receives the packet
– R extracts the IP packet from the Ethernet frame– R sees the IP packet is destined to 222.222.222.222
• Router R consults its forwarding table– Packet matches 222.222.222.0/24 via other adaptor
A
RB
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R Sends Packet to B
• Router R’s learns the MAC address of host B– ARP request: broadcast request for 222.222.222.222– ARP response: B responds with 49-BD-D2-C7-56-2A
• Router R encapsulates the packet and sends to B
A
RB
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Security Analysis of ARP
• Impersonation– 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
• Or: Man-in-the-middle attack– Imposter updates frames w/ correct MAC address and
forwards whatever it receives to the legit destination …o …. but gets to inspect (& maybe alter) it first
• Does the attacker have to “win” a race?– Maybe not, if sender blindly believes ARP responses
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Network Control Messages(and how to use them for discovery)
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Error/Status Reporting
• Examples of errors a router may see– Router doesn’t know where to forward a packet– Packet’s time-to-live (hop count) field expires– Packet is too big for link-layer router needs to use
• Router doesn’t really need to respond– Best effort means never having to say you’re sorry– So, IP could conceivably just silently drop packets
• But: silent failures are really hard to diagnose– IP includes basic feedback about network problems– Internet Control Message Protocol (ICMP / RFC 792)
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Internet Control Message Protocol
• ICMP runs on top of IP– Same level as TCP and UDP– Though viewed as an integral part of IP (not transport)
• Diagnostics– Triggered when an IP packet encounters a problem
o E.g., Time Exceeded or Destination Unreachable– ICMP packet sent back to the source IP address
o Includes the error information (e.g., type and code)o … and IP header plus 8+ byte excerpt from original packet
– Source host receives the ICMP packeto Inspects excerpt (e.g., protocol and ports) to identify socket
– Exception: ICMP not sent if problem packet is ICMPo And just for fragment 0 of a group of fragments
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Types of Control Messages
• Need Fragmentation– IP packet too large for link layer, DF set
• TTL Expired– Decremented at each hop; generated if 0
• Unreachable– Subtypes: network / host / port
o (who generates Port Unreachable?)
• Source Quench– Old-style signal asking sender to slow down
• Redirect– Tells source to use a different local router
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Discovering Network Path Properties
• ICMP provides way for routers to talk to end hosts
• Can be used in clever ways to probe the network to discover things about its internals:–PMTU Discovery:
o What is largest packet that can be sent completely through the network w/o needing fragmentation?• Most efficient size to use• (Plus fragmentation can amplify loss)
–Traceroute:o What is the series of routers that a packet traverses
as it travels through the network?
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Path MTU Discovery
• MTU = Maximum Transmission Unit– Largest IP packet that a link supports
• Path MTU (PMTU) = minimum end-to-end MTU– Sender must keep datagrams no larger to avoid
fragmentation
• How does the sender know the PMTU is?
• Strategy (RFC 1191):– Try a desired value– Set DF to prevent fragmentation– Upon receiving Need Fragmentation ICMP …
o … oops, that didn’t work, try a smaller value
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Issues with Path MTU Discovery
• What set of values should the sender try?– Usual strategy: work through “likely suspects”– E.g., 4352 (FDDI), 1500 (Ethernet),
1480 (IP-in-IP over Ethernet), 296 (some modems)
• What if the PMTU changes? (how could it?)– Sender will immediately see reductions in PMTU (how?)– Sender can periodically try larger values
• What if Needs Fragmentation ICMP is lost?– Retransmission will elicit another one
• How can The Whole Thing Fail?– “PMTU Black Holes”: routers that don’t send the ICMP
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Discovering Routing via Time Exceeded
host DNS... host host DNS...
router routerrouter
host
1.2.3.7
8.9.10.11
5.6.7.156
• Host sends an IP packet– Each router decrements the time-to-live field
• If TTL reaches 0– Router sends Time Exceeded ICMP back to the source– Message identifies router sending it
o Since ICMP is sent using IP, it’s just the IP source address
Time exceeded
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Traceroute: Exploiting Time Exceeded
• Time-To-Live field in IP packet header– Source sends a packet with TTL ranging from 1 to n– Each router along the path decrements the TTL– “TTL exceeded” sent when TTL reaches 0
• Traceroute tool exploits this TTL behavior
source destination
TTL=1
Time exceeded
TTL=2
Send packets with TTL=1, 2, … and record source of Time Exceeded message
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traceroute to www.whitehouse.gov (204.102.114.49), 30 hops max, 40 byte packets
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traceroute to www.whitehouse.gov (204.102.114.49), 30 hops max, 40 byte packets 1 cory115-1-gw.EECS.Berkeley.EDU (128.32.48.1) 0.829 ms 0.660 ms 0.565 ms
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traceroute to www.whitehouse.gov (204.102.114.49), 30 hops max, 40 byte packets 1 cory115-1-gw.EECS.Berkeley.EDU (128.32.48.1) 0.829 ms 0.660 ms 0.565 ms 2 cory-cr-1-1-soda-cr-1-2.EECS.Berkeley.EDU (169.229.59.233) 0.953 ms 0.857 ms 0.727 ms
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traceroute to www.whitehouse.gov (204.102.114.49), 30 hops max, 40 byte packets 1 cory115-1-gw.EECS.Berkeley.EDU (128.32.48.1) 0.829 ms 0.660 ms 0.565 ms 2 cory-cr-1-1-soda-cr-1-2.EECS.Berkeley.EDU (169.229.59.233) 0.953 ms 0.857 ms 0.727 ms 3 soda-cr-1-1-soda-br-6-2.EECS.Berkeley.EDU (169.229.59.225) 1.461 ms 1.260 ms 1.137 ms 4 g3-8.inr-202-reccev.Berkeley.EDU (128.32.255.169) 1.402 ms 1.298 ms * 5 ge-1-3-0.inr-002-reccev.Berkeley.EDU (128.32.0.38) 1.428 ms 1.889 ms 1.378 ms 6 oak-dc2--ucb-ge.cenic.net (137.164.23.29) 1.731 ms 1.643 ms 1.680 ms 7 dc-oak-dc1--oak-dc2-p2p-2.cenic.net (137.164.22.194) 3.045 ms 1.640 ms 1.630 ms 8 * * * 9 dc-lax-dc1--sac-dc1-pos.cenic.net (137.164.22.126) 13.104 ms 13.163 ms 12.988 ms10 137.164.22.21 (137.164.22.21) 13.328 ms 42.981 ms 13.548 ms11 dc-tus-dc1--lax-dc2-pos.cenic.net (137.164.22.43) 18.775 ms 17.469 ms 21.652 ms12 a204-102-114-49.deploy.akamaitechnologies.com (204.102.114.49) 18.137 ms 14.905 ms 19.730 ms
Lost Reply
Router doesn’t send ICMPs
Final HopNo PTR record for address
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Ping: Echo and Reply• ICMP includes simple “echo” functionality
– Sending node sends an ICMP Echo Request message– Receiving node sends an ICMP Echo Reply
• Ping tool– Tests connectivity with a remote host– … by sending regularly spaced Echo Request– … and measuring delay until receiving replies
• ICMP includes other forms of probing– See /usr/include/netinet/ip_icmp.h on a Unix system– However, very often disabled … :-(
• Probing hosts– Try (say) traceroute www.cs.duke.edu
and ping www.cs.duke.edu
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Security Implications of ICMP?
• Attacker can cause host to accept an ICMP if the excerpt looks correct (assuming the host checks)– Must guess recent IP packet header & 8B of payload– All that really matters is source/destination addresses
and ports
• Threat:– Denial-of-Service (DoS)
o Unreachable, Redirect
– Impaired performanceo Need Fragmentation, Source Quench
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Summary
• Important control functions– Bootstrapping– Error/status reporting and monitoring
• Internet control protocols– Dynamic Host Configuration Protocol (DHCP)– Address Resolution Protocol (ARP)– Internet Control Message Protocol (ICMP)
• Next lecture: Shortest-Path Routing– K&R 4.5, 4.6.1, 4.6.2