Ch 21. Other IP-related Protocols

21.1 Address Mapping

• IP packets use logical address, and need to be encapsulated in a frame at data link layer, which requires physical address

• Address Resolution Protocol (ARP)– ARP request

– broadcast– ARP reply

– unicast– Information is cached

at the sender

ARP Packet Format

Length of the physical address

Length of the logical address

Example1. The sender A and the receiver B are hosts (local network). IP is

mapped to a physical address.2. The sender A is a host and the receiver B is a router. (packet is sent to

another network)3. The sender A and receiver B are routers (sent to another network). 4. The sender A is a router and the receiver B is a host.

Proxy ARP

• When a local network consists of multiple subnetworks

Other Address Mapping

• Reverse Address Resolution Protocol (RARP)– Find a logical address corresponding to a physical address

• Bootstrap Protocol (BOOTP)– Application level protocol to provide mapping from physical address

to logical address– Set source IP address to all 0’s, and set destination IP address to all 1’s

• Dynamic Host Configuration Protocol (DHCP)– Provides static and dynamic address allocation that can be manual or

automatic– A host may move from one network to another– DHCP provides temporary IP addresses

21.2 ICMP• IP provides unreliable and connectionless datagram delivery. It lacks of error

control and assistance mechanisms.• What happens if

– a router must discard a datagram because it cannot fined a router;– or time to live field has a zero value– The final destination host must discard all fragments of a datagram because it is not

received all fragments, with a predetermined time limit.• Internet Control Message Protocol

– Error-reporting (about a router or a host) or query message

Error Reporting

• Encapsulation of error reporting message– Copy (IP headers + 8 bytes data) of the received pkt

Query Message

• Encapsulation of query message

ICMP Application

• “ping”– To find if a host is alive– ICMP echo-request and echo-reply message

ICMP Application

• “traceroute” (in Unix; “tracert” in Windows)– To find the route of a packet– ICMP time-exceeded message

Nowadays, many hosts and routers do not respond (or a gateway blocks the packets) due to security reason

21.3 IGMP

• Multicasting: one-to-many communication• Internet Group Management Protocol (IGMP)

– One of protocols necessary for multicasting– Manage group membership

• Operations– Join a group– Leave a group– Monitor membership – Server periodically sends a query to

ensure membership. Hosts use a delayed response to reduce traffic

Encapsulation

• Encapsulation into IP datagram and frame

• IP packet with an ICMP message has a value of “1” in its TTL field IGMP works locally

• IP destinationaddress

• Encapsulation at data link layer– Multicast support (most LANs) – map multicast IP

address to physical address (many-to-one)– No multicast support (most WANs) – use tunneling

• Application “netstat”

Homework

• Exercise– 17– 19– 22– 23– 25