Module 13
Time Exceeded
An ICMPv4 Time Exceeded message is used by a router to indicate that a packet cannot be forwarded because the Time to Live (TTL) field of the packet was decremented to 0. If a router receives a packet and decrements the TTL field in the IPv4 packet to zero, it discards the packet and sends a Time Exceeded message to the source host. ICMPv6 also sends a Time Exceeded message if the router cannot forward an IPv6 packet because the packet has expired. Instead of the IPv4 TTL field, ICMPv6 uses the IPv6 Hop Limit field to determine if the packet has expired. Note: Time Exceeded messages are used by the traceroute tool. ???
Router Advertisement (RA) message
An IPv6-enabled router will send out an RA message in response to an RS message. RA messages are sent by IPv6-enabled routers every 200 seconds to provide addressing information to IPv6-enabled hosts. The RA message can include addressing information for the host such as the prefix, prefix length, DNS address, and domain name. A host using Stateless Address Autoconfiguration (SLAAC) will set its default gateway to the link-local address of the router that sent the RA. (what is link local address??)
solicited-Node multicast address
A Solicited-Node multicast address is an IPv6 multicast address used by the Neighbor Discovery Protocol to determine the link-layer address (MAC address) associated with a given IPv6 address. It is also used to check if an address is already being used by the local link or not, through a process called DAD (Duplicate Address Detection).
Router Solicitation (RS) message
A host sends a RS message to determine how to receive its IPv6 address information dynamically.
Neighbor Solicitation (NS) message Neighbor Advertisement (NA) message (for address resolution - IPv6 address to MAC)
Address resolution is used when a device on the LAN knows the IPv6 unicast address of a destination but does not know its Ethernet MAC address. To determine the MAC address for the destination, the device will send an NS message to the solicited node address. The message will include the known (targeted) IPv6 address. The device that has the targeted IPv6 address will respond with an NA message containing its Ethernet MAC address.
Internet Control Message Protocol (ICMP)
Although IP is only a best-effort protocol, the TCP/IP suite does provide for error messages and informational messages when communicating with another IP device. These messages are sent using the services of ICMP. The purpose of these messages is to provide feedback about issues related to the processing of IP packets under certain conditions, not to make IP reliable. ICMP messages are not required and are often not allowed within a network for security reasons. ICMP is available for both IPv4 and IPv6. ICMPv4 is the messaging protocol for IPv4. ICMPv6 provides these same services for IPv6 but includes additional functionality. In this course, the term ICMP will be used when referring to both ICMPv4 and ICMPv6. The types of ICMP messages, and the reasons why they are sent, are extensive. The ICMP messages common to both ICMPv4 and ICMPv6 and discussed in this module include: Host reachability Destination or Service Unreachable Time exceeded
Host Reachability, ICMP Echo Message, Ping
An ICMP Echo Message can be used to test the reachability of a host on an IP network. The local host sends an ICMP Echo Request to a host. If the host is available, the destination host responds with an Echo Reply. This use of the ICMP Echo messages is the basis of the ping utility.
link-local address
An IP address that is automatically assigned by an operating system to allow a node to communicate over its local subnet if a routable IP address is not available. ICANN has established the range of 169.254.0.0 through 169.254.254.255 as potential link-local IPv4 addresses. IPv6 link-local addresses begin with the prefix FE80::/10. The address that a computer running IPv6 gives itself after first booting. The first 64 bits of a link-local address are always FE80::/64. An address that is designed to be used for and is limited to communications on the local link
Local link
Any local area network (LAN) bounded by routers.
Pinging the remote host
Ping can also be used to test the ability of a local host to communicate across an internetwork. The local host can ping an operational IPv4 host of a remote network. The router uses its IP routing table to forward the packets. If this ping is successful, the operation of a large piece of the internetwork can be verified. A successful ping across the internetwork confirms communication on the local network, the operation of the router serving as the default gateway, and the operation of all other routers that might be in the path between the local network and the network of the remote host. Additionally, the functionality of the remote host can be verified. If the remote host could not communicate outside of its local network, it would not have responded. Note: Many network administrators limit or prohibit the entry of ICMP messages into the corporate network; therefore, the lack of a ping response could be due to security restrictions.
ping
Ping is an IPv4 and IPv6 testing utility that uses ICMP echo request and echo reply messages to test connectivity between hosts. To test connectivity to another host on a network, an echo request is sent to the host address using the ping command. If the host at the specified address receives the echo request, it responds with an echo reply. As each echo reply is received, ping provides feedback on the time between when the request was sent and when the reply was received. This can be a measure of network performance. Ping has a timeout value for the reply. If a reply is not received within the timeout, ping provides a message indicating that a response was not received. This may indicate that there is a problem, but could also indicate that security features blocking ping messages have been enabled on the network. It is common for the first ping to timeout if address resolution (ARP or ND) needs to be performed before sending the ICMP Echo Request. After all the requests are sent, the ping utility provides a summary that includes the success rate and average round-trip time to the destination.
link-layer address (MAC address)
A link-layer address is called a link address, called a physical address, and sometimes a MAC address. Since a link is controlled at the data-link layer, the addresses need to belong to the data-link layer.
Traceroute - Test the Path
Ping is used to test connectivity between two hosts but does not provide information about the details of devices between the hosts. Traceroute (tracert) is a utility that generates a list of hops that were successfully reached along the path. This list can provide important verification and troubleshooting information. If the data reaches the destination, then the trace lists the interface of every router in the path between the hosts. If the data fails at some hop along the way, the address of the last router that responded to the trace can provide an indication of where the problem or security restrictions are found.
Type of connectivity tests performed with ping
Pinging the local loopback Pinging the default gateway Pinging the remote host
Pinging the local loopback
Pinging the local loopback address causes a device to ping itself. Ping can be used to test the internal configuration of IPv4 or IPv6 on the local host. To perform this test, ping the local loopback address of 127.0.0.1 for IPv4 (::1 for IPv6). A response from 127.0.0.1 for IPv4, or ::1 for IPv6, indicates that IP is properly installed on the host. This response comes from the network layer (layer 3). This response is not, however, an indication that the addresses, masks, or gateways are properly configured. Nor does it indicate anything about the status of the lower layer of the network stack. This simply tests IP down through the network layer of IP. An error message indicates that TCP/IP is not operational on the host. ????? aks baba to explain loopback!!!
Traceroute (its a command)
The first sequence of messages sent from traceroute will have a TTL field value of 1. This causes the TTL to time out the IPv4 packet at the first router. This router then responds with an ICMPv4 Time Exceeded message. Traceroute now has the address of the first hop. Traceroute then progressively increments the TTL field (2, 3, 4...) for each sequence of messages. This provides the trace with the address of each hop as the packets time out further down the path. The TTL field continues to be increased until the destination is reached, or it is incremented to a predefined maximum. After the final destination is reached, the host responds with either an ICMP Port Unreachable message or an ICMP Echo Reply message instead of the ICMP Time Exceeded message.
ICMPv6 Messages
The informational and error messages found in ICMPv6 are very similar to the control and error messages implemented by ICMPv4. However, ICMPv6 has new features and improved functionality not found in ICMPv4. ICMPv6 messages are encapsulated in IPv6. ICMPv6 includes four new protocols as part of the Neighbor Discovery Protocol (ND or NDP). Messaging between an IPv6 router and an IPv6 device, including dynamic address allocation are as follows: Router Solicitation (RS) message Router Advertisement (RA) message Messaging between IPv6 devices, including duplicate address detection and address resolution are as follows: Neighbor Solicitation (NS) message Neighbor Advertisement (NA) message Note: ICMPv6 ND also includes the redirect message, which has a similar function to the redirect message used in ICMPv4.
TTL
Time to live (TTL) refers to the amount of time or "hops" that a packet is set to exist inside a network before being discarded by a router. TTL is also used in other contexts including CDN caching and DNS caching.
IPv4 TTL and IPv6 Hop Limit
Traceroute makes use of a function of the TTL field in IPv4 and the Hop Limit field in IPv6 in the Layer 3 headers, along with the ICMP Time Exceeded message.
Round Trip Time (RTT)
Using traceroute provides round-trip time for each hop along the path and indicates if a hop fails to respond. The round-trip time is the time a packet takes to reach the remote host and for the response from the host to return. An asterisk (*) is used to indicate a lost or unreplied packet. This information can be used to locate a problematic router in the path or may indicate that the router is configured not to reply. If the display shows high response times or data losses from a particular hop, this is an indication that the resources of the router or its connections may be stressed.
Neighbor Solicitation (NS) message Neighbor Advertisement (NA) message Duplicate Address Detection (DAD)
When a device is assigned a global IPv6 unicast or link-local unicast address, it may perform duplicate address detection (DAD) to ensure that the IPv6 address is unique. To check the uniqueness of an address, the device will send an NS message with its own IPv6 address as the targeted IPv6 address, as shown in the figure. If another device on the network has this address, it will respond with an NA message. This NA message will notify the sending device that the address is in use. If a corresponding NA message is not returned within a certain amount of time, the unicast address is unique and acceptable for use. This message is sent out as a broadcast.??? Note: DAD is not required, but RFC 4861 recommends that DAD is performed on unicast addresses.
ICMP Destination Unreachable message
When a host or gateway receives a packet that it cannot deliver, it can use an ICMP Destination Unreachable message to notify the source that the destination or service is unreachable. The message will include a code that indicates why the packet could not be delivered. Some of the Destination Unreachable codes for ICMPv4 are as follows: 0 - Net unreachable 1 - Host unreachable 2 - Protocol unreachable 3 - Port unreachable Some of the Destination Unreachable codes for ICMPv6 are as follows: 0 - No route to destination 1 - Communication with the destination is administratively prohibited (e.g., firewall) 2 - Beyond scope of the source address 3 - Address unreachable 4 - Port unreachable
Pinging the default gateway
You can also use ping to test the ability of a host to communicate on the local network. This is generally done by pinging the IP address of the default gateway of the host. A successful ping to the default gateway indicates that the host and the router interface serving as the default gateway are both operational on the local network. For this test, the default gateway address is most often used because the router is normally always operational. If the default gateway address does not respond, a ping can be sent to the IP address of another host on the local network that is known to be operational. If either the default gateway or another host responds, then the local host can successfully communicate over the local network. If the default gateway does not respond but another host does, this could indicate a problem with the router interface serving as the default gateway. One possibility is that the wrong default gateway address has been configured on the host. Another possibility is that the router interface may be fully operational but have security applied to it that prevents it from processing or responding to ping requests.
