1Internet Control ProtocolsEE 122: Intro to Communication NetworksFall 2009Prof. Vern [email protected] with thanks to Jennifer Rexford, Ion Stoica,and colleagues at Princeton and UC Berkeley2Goals of Today’s Lecture• Bootstrapping an end host– Learning its own configuration parameters (DHCP)– Learning the link-layer addresses of other nodes (ARP)• Network control messages– Error/status reporting– Monitoring– Internet Control Message Protocol (ICMP)– Exploiting ICMP for discovering Internet path propertieso Largest packet that can be sent w/o fragmenting (PMTU)o Route taking by packets through the network (traceroute)3How To Bootstrap an End Host?• What IP address the host should use?• What DNS server(s) to use?• How to tell which destinations are local?• How do we address them using local network?• How to send packets to remote destinations?hosthost DNS...hosthost DNS...router router1.2.3.0/235.6.7.0/241.2.3.7 1.2.3.156???1.2.3.19router4Avoiding 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 IPaddress and MAC addresshosthost DNS...hosthost DNS...router router1.2.3.0/23255.255.254.05.6.7.0/241.2.3.7 1.2.3.1561.2.3.481.2.3.19router1A-2F- BB-76-09-AD5Key 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 change6MAC 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 subnet7Bootstrapping 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 addresshosthost host...DHCP server8Response 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 chosen9Dynamic Host Configuration ProtocolarrivingclientDHCP server203.1.2.5DHCP discover(broadcast)DHCP offerDHCP requestDHCP ACK(broadcast)10Soft 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 softwareo E.g., you shut your laptop off and put it in your backpack– 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 denied11So, 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?12Figuring 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 = remotehosthost DNS...hosthost DNS...router router1.2.3.0/23255.255.254.05.6.7.0/241.2.3.7 1.2.3.1561.2.3.481.2.3.19router1A-2F- BB-76-09-AD13Where To Send Locally, con’t• If it’s remote, look up first hop in (very small) localrouting table– E.g., by default, route via 1.2.3.19– Now do to the local case but for 1.2.3.19 rather thanultimate destination IP address• For the local case, need to determine thedestination’s MAC addresshosthost DNS...hosthost DNS...router router1.2.3.0/23255.255.254.05.6.7.0/241.2.3.7 1.2.3.1561.2.3.481.2.3.19router1A-2F- BB-76-09-AD14Sending 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 framehosthost DNS...1.2.3.156router1.2.3.531.2.3.531.2.3.156IP packet15Address 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 connectivity16Example: A Sending a Packet to BHow does host A send an IP packet to host B?ARB1. A sends packet to R.2. R sends packet to B.17Host A Decides to Send Through RARB• 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
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