Internet Protocols

Address Resolution IP Addresses are not recognized by hardware. If we know the IP address of a host, how do we find out the hardware address ? Address Resolution. The process of finding the hardware address of a host given the IP address is called Address Resolution.

Address Resolution Protocol (ARP) The Address Resolution Protocol is used by a sending host when it knows the IP address of the destination but needs the Ethernet address. ARP is a broadcast protocol - every host on the network receives the request. Each host checks the request against it’s IP address - the right one responds.

Address Resolution Protocol (ARP) ARP does not need to be done every time an IP datagram is sent - hosts remember the hardware addresses of each other. Part of the ARP protocol specifies that the receiving host should also remember the IP and hardware addresses of the sending host.

ARP

Map IP MAC Local ARP IP: Ethernet: IP: Ethernet: IP: = ??? I heard that broadcast. The message is for me. Here is my Ethernet address. I need the Ethernet address of

Reverse ARP Map MAC IP Ethernet: IP: Ethernet: IP: Ethernet: IP = ??? What is my IP address? I heard that broadcast. Your IP address is

Internet Control Message Protocol ICMP is a protocol used for exchanging control messages. ICMP uses IP to deliver messages. ICMP messages are usually generated and processed by the IP software, not the user process.

ICMP Features Used by IP to send error and control messages Uses IP to send its messages Does not report errors on ICMP messages. ICMP message are not required on datagram checksum errors. ICMP reports error only on the first fragment ICMP HeaderICMP Data IP HeaderIP Data Datalink HeaderDatalink Data

ICMP Message Format IP Header Type of Message Error Code Checksum Parameters, if any Information 8b 16b Var

Sample ICMP Messages Source Quench: Please slow down! I just dropped one of your datagrams. Time Exceeded: Time to live field in one of your packets became zero.” or “Reassembly timer expired at the destination. Fragmentation Required: Datagram was longer than and “No Fragment bit” was set.

Sample ICMP Messages (Continued) Address Mask Request/Reply: What is the subnet mask on this net? Replied by “Address mask agent” Redirect: Send to router X instead of me. Time Stamp Request/Reply: used to find current time. ICMP error messages normally include the IP header of the datagram that generated the error, plus at least 8 bytes following the IP header => ICMP message sizes = 70 bytes

ICMP: Message Types Summary TypeMessage 0Echo reply 3Destination unreachable 4Source quench 5 Redirect 8Echo request 11Time exceeded 12Parameter unintelligible 13Time-stamp request 14Time-stamp reply 15 Information request 16Information reply 17Address mask request 18Address mask reply

Ping Ping: Used to test  destination reachability,  compute round trip time  count the # of hops to destination  may provide record route option. Sample output: Reply from : 48 bytes in 47 msec. TTL: 253

Traceroute Traceroute: Exploit TTL and ICMP  Send the packet with time-to-live = 1 (hop)  The first router discards the packet and sends an ICMP “time-to-live exceeded message”  Send the packet with time-to-live = 2 (hops) etc…  Does not use optional features like record route

Path MTU Discovery Send a large IP datagram with “Don’t fragment” bit set.  Failure to fragment at a link will result in ICMP message.

Summary ICMP is the control sibling of IP ICMP is used by IP and uses IP as network layer protocol ICMP is used for ping, traceroute, and path MTU discovery.