Network Layer

Address Mapping, Error Reporting, and

Multicasting

Address Mapping

• The delivery of a packet to a host or a router requires two levels of addressing: logical and physical.

• We need to be able to map a logical address to its corresponding physical address and vice versa.

• This can be done by using either static or dynamic mapping.– Static mapping: creating of a table that associates a logical

address with a physical address.

– Dynamic mapping: uses a protocol.

Address Resolution Protocol (ARP)• Anytime a host or a router has an IP datagram to send to another host

or router, it has the logical (IP) address of the receiver.

• An IP datagram must be encapsulated in a frame to be able to pass through the physical network.

• This means that the sender needs the physical address of the receiver.

• Address Resolution Protocol (ARP)– The host or the router sends a Address Resolution Protocol (ARP) query

request, containing the physical and IP address of the sender and IP address of the destination.

– The query is broadcast over the network.

– The intended recipient recognizes its IP address and sends back the ARP response containing its physical address by unicasting.

Mapping Logical to Physical Address: ARP

• Cache Memory: ARP reply can be cached (kept in cache memory for a while) to improve efficiency.

ARP packetThe type of the ARP

message being sent.

ARP request (1) &

ARP reply (2).

Type of the network on which the ARP

is running. E.g., Ethernet: 1, ATM: 16

The type of layer three addresses used

in the message. For IPv4 addresses, this

value is 0800 hex.

The length of the physical address in

bytes. E.g., for Ethernet the value is 6The length of the logical address in

bytes. E.g., for IPv4 the value is 4

Encapsulation of ARP packet

0806 for ARP (on an Ethernet)

Physical address of the sender

Physical broadcast address as

the destination

• Preamble: 56 bits of alternating

1s and 0s

• SFD: start frame delimiter flag

10101011

ARP Request and Reply (Example)

• A host with IP address 130.23.43.20 and physical address B2:34:55:10:22:10 has a packet to send to another host with IP address 130.23.43.25 and physical address A4:6E:F4:59:83:AB. The two hosts are on the same Ethernet network. Show the ARP request and reply packets encapsulated in Ethernet frames.

Reverse Address Resolution Protocol (RARP)

• Finds the logical address for a machine that knows only its physical address.

• A RARP request is created and broadcast on the local network. Another machine that knows all the IP addresses with respond with an RARP reply.

• The requesting machine must run an RARP client and responding machine must run an RARP server.

• Problem with RARP: broadcasting is done at the data link layer, and it does not pass the boundaries of a network. This means that if an administrator has several networks or subnets, it needs to assign a RARP server for each network or subnet.

• This is the reason that RARP is almost obsolete.

• Two protocols, BOOTP and DHCP, are replacing RARP.

Bootstrap Protocol (BOOTP)

• BOOTP is an application layer, client/server protocol designed to provide physical to logical address mapping.

• The administrator can put the client and the server on the same network or on different networks.

• BOOTP messages are encapsulated in a UDP packet, which is again encapsulated • BOOTP messages are encapsulated in a UDP packet, which is again encapsulated in an IP packet.

• The client uses all 0s as the source address and all 1s as the destination address.

• A broadcast IP datagram cannot pass through any router. Since a BOOTP request is a broadcast message and it cannot pass through any router. For this we use a relay agent. The relay agent knows the unicast address of a BOOTP server. When it receives a BOOTP request it encapsulates the message in a unicast datagram and send the request to the BOOTP server.

BOOTP client and server on the same and

different networks

• BOOTP is not a dynamic configuration protocol.

• If a client requests its IP address, the BOOTP server consults a table that matches the physical address of a client with its IP address.

• This binding is predetermined.

Dynamic Host Configuration Protocol (DHCP)

• Provides static and dynamic address allocation.

• Static address allocation:– Statically binds physical addresses to IP addresses.

– In this capacity DHCP acts as BOOTP.

– It is backward compatible with BOOTP, which means that BOOTP client can request a static address from DHCP server.

• Dynamic Address Allocation:• Dynamic Address Allocation:– DHCP also maintains a pool of available IP addresses.

– When a DHCP clients request for a temporary address the DHCP server goes to the pool of unused IP addresses and assigns an IP address for a negotiable period of time.

– After the time period expires, the client must either stop using the IP address or renew the lease.

– The server can either agree or disagree with the renewal.

Internet Control Message Protocol

(ICMP)• The IP protocol has no error-reporting or error-correcting mechanism.

• The IP protocol also lacks mechanism for host and management queries.

• The Internet Control Message Protocol (ICMP) has been designed to • The Internet Control Message Protocol (ICMP) has been designed to compensate for the above two deficiencies. It is a companion to the IP protocol.

• ICMP messages are divided into two broad categories: error-reporting messages and query messages.

ICMP – Message Format

• An ICMP message has an 8-byte header and a variable size data section. First 4 bytes are common for all ICMP messages.

• The code field specifies the reason for the particular message type.

• ICMP always reports error messages to the original source – because the only information available in the datagram about the route is the source and destination IP addresses.

ICMP – Error Reporting

• All error messages contain a data section that include the IP header of the original datagram plus the first 8 bytes of data in that datagram. – The header of the original datagram provides information to the source

about the datagram itself.

– The 8 byte of the data are included because it provides information about the port numbers (UDP & TCP) and the sequence number (TCP).

• ICMP forms an error packet, which is encapsulated in an IP datagram.

ICMP – Error Reporting

• Important points about ICMP error messages:– No ICMP error message will be generated in response to a

datagram carrying an ICMP error message.

– No ICMP error message will be generated for a fragmented datagram that is not the first fragment.

– No ICMP error message will be generated for a datagram having – No ICMP error message will be generated for a datagram having a multicast address.

– No ICMP error message will be generated for a datagram having a special address such as 127.0.0.0 or 0.0.0.0.

ICMP – Error Reporting

• When a router cannot route a datagram or a host cannot deliver a datagram.

• IP protocol lacks flow control. So, the source host never knows if the router or the destination

datagram or a host cannot deliver a datagram.

• The datagram is discarded and the router or host sends a destination-unreachable message.

• Codes – 1 Host Unreachable

– 2 Protocol Unreachable

– 3 Port Unreachable

– 4 Fragmentation Needed and Don't Fragment was Set

– 6 Destination Network Unknown

– 7 Destination Host Unknown

the source host never knows if the router or the destination host has been overwhelmed with datagrams.

• In such cases, a router or destination discards the datagram and sends a source-quench message to the source.

• The source-quench message in ICMP was designed to add a flow control to the IP.

• Codes – 0 No code

ICMP – Error Reporting

• The time-exceeded messages are generated in two cases:

• Reports ambiguity in the header part of a datagram. generated in two cases:

– If the TTL value of a datagram is 0. the datagram is discarded and a time-exceeded messages is sent to the original source.

– Not all fragments that make up a message arrive at the destination host within a certain time limit.

• Codes – 0 Time to Live exceeded in Transit

– 1 Fragment Reassembly Time Exceeded

part of a datagram.

• If a router or the destination host discovers an ambiguity or missing value in any field of the datagram, it discards the datagram and sends a parameter-problem message back to the source

• Codes• 0 invalid IP header

ICMP – Error Reporting

• A host may send a datagram, destined for another network, to a wrong router. wrong router.

• In this case, the route that receives the datagram will forward it to the correct router. Also to update the routing table of the host, it sends a redirection-message to the host.

ICMP – Query

• ICMP query message is encapsulated in an IP packet, which in turn is encapsulated in a data link layer frame.

• No bytes of the original IP are included in the message.

ICMP – Query

• Designed for diagnostic purposes.

• It determines if two nodes can

• Used to determine the round-trip time needed for an IP datagram between two nodes.

• It determines if two nodes can communicate with each other at the IP level.

• The ping command generates a series of echo-request and echo-reply messages.

• Codes– 0 No code

time needed for an IP datagram between two nodes.

• It can also be used to synchronize the clocks in two machines.

• Codes– 0 No code

• Used to obtain the mask of an IP address.

• Codes– 0 No code

ICMP – Query

• To send data a host needs to know the address of routers connected • To send data a host needs to know the address of routers connected to its own network. Also, if the router is alive and functioning.

• For this, a host can broadcast a route-solicitation message. The routers receiving the message can send their routing information using route-advertisement messages.

• A router can also periodically send route-advertisement messages even if no host has solicited.

Debugging Tools - Ping

• Ping is used to find if a host is alive and responding. The source host sends ICMP echo-request messages (type:8, code 0); the destination, if alive, responds with echo-reply messages.

• The ping program sets the identifier field in the echo-request • The ping program sets the identifier field in the echo-request and echo-reply message and starts the sequence number from 0; this is incremented by 1 for each new messages.

• Ping can calculate the round-trip time (RTT) –– it inserts the sending time in the data section of the message,

when the message arrives, it subtract the arrival time from the departure time to get RTT.

Ping

• 56 byte ICMP data + 8 bytes ICMP header + 20 bytes IP header = 84 bytes

• 56 byte ICMP data + 8 bytes ICMP header = 64 bytes in the ICMP packet

Internet Group Management Protocol

(IGMP)

• IGMP is a protocol that manages group membership.

• The IGMP protocol gives the multicast routers information about the membership status of hosts (routers) connected to the network.

• IGMP is a group management protocol. It helps a multicast router create and update a list of loyal members related to each router interface.

IGMP Messages

• IGMP has three types of messages: the query, the membership report, and the leave report.

• There are two types of query messages, general and special.

IGMP message format

• 1/10 sec

• The value is non-zero

for query messages

and zero for other two

• Covers the entire IGMP message

IGMP Operation• A multicast router connected to a

network has a list of multicast addresses of the group with at least one loyal member in the network.

• For each group there is one router with duty of distributing multicast packets for that group.multicast packets for that group.

• Lists of groups for each router are mutually exclusive (i.e. only one multicast router is responsible for each group). A router can also be a member of a group.

• Routers R1 and R2 can be distributors for one or several groups from the list of router R, but for other networks, not for the network above.

Membership report

• Host or a router can join a group.

• A host maintains a list of processes that have membership in a group.

• When a process wants to join a new group, it sends its request to the host.

• The host then adds the name of the process and the name of the requested group to its list, and sends the membership report to the router.

• The report is send twice (in case the first report get lost or damaged)

Leave Report• When a host sees that no process is interested in a specific group G, it sends a

leave report.

• If router receives a leave report it won’t purge the list if there are still other hosts interested in that group.

• For that purpose the router sends a special query message with a specified response time for the group in question to see if there is anyone interested in that group.

• If there is no response a membership report, it purges the list.

General Query Message• Membership report and leave report are not enough to maintain the

membership information.

• Example: a host that is a member of a group can shut down and the multicast router would never receive the leave report.

• Therefore the multicast router monitors the hosts and routers in LAN by periodically sending (by default every 125 sec) general query message.

• Hosts/routers respond by membership report if there is still interest in groups.

• No group specified

Delayed response

• In order to keep the traffic low the response to general query message must be done by only one host for a given group. This is achieved with delayed response:

• When a host receives general query message it delays the response: it sets a timer for each group to a different random value between 0 and 10 seconds, then broadcasts the response(s) according to the timers. 10 seconds, then broadcasts the response(s) according to the timers.

• If the host receives a response from another host, whose timer for that group has expired earlier, the host cancels the corresponding timer and doesn’t send the duplicate response for the group.

• Only one router on the LAN is designated for sending the query messages – the query router. This further reduces the traffic.

Delayed response - Example

• Imagine there are three hosts in a network, as shown in Figure. A query message was received at time 0; the random delay time (in tenths of seconds) for each group is shown next to the group address. Show the sequence of report messages.

• Solution

Four messages are sent

instead of 7 messages

Encapsulation of IGMP Messages

• IGMP messages are encapsulated in IP datagrams

Encapsulation of IGMP Messages

• The IP packet that carries an IGMP packet has a value of 1 in its TTL field.

Mapping class D to Ethernet physical

address

An Ethernet multicast physical address is in the range

01:00:5E:00:00:00 – 01:00:5E:7F:FF:FF

Mapping Class D to Ethernet Physical

Address – (Example-1)

• Change the multicast IP address 230.43.14.7 to an Ethernet multicast physical address.

• We can do this in two steps:– We write the rightmost 23 bits of the IP address in hexadecimal. – We write the rightmost 23 bits of the IP address in hexadecimal.

This can be done by changing the rightmost 3 bytes to hexadecimal and then subtracting 8 from the leftmost digit if it is greater than or equal to 8. In our example, the result is 2B:0E:07.

– We add the result of part a to the starting Ethernet multicast address, which is 01:00:5E:00:00:00. The result is

Mapping Class D to Ethernet Physical

Address – (Example-2)

• Change the multicast IP address 238.212.24.9 to an Ethernet multicast address.

• We can do this in two steps:– The rightmost 3 bytes in hexadecimal is D4:18:09. We need – The rightmost 3 bytes in hexadecimal is D4:18:09. We need

to subtract 8 from the leftmost digit, resulting in 54:18:09

– We add the result of part a to the Ethernet multicast starting address. The result is: