Transcript ppt
CMPT 471 Networking II ICMP © Janice Regan, 2012 1 Errors in Packet Switching Networks Possible causes of errors include Hardware failure Network congestion Inability to fragment (DF set) Routing loops Unavailable host (disconnected or failed) Queue overrun on routers IP offers best effort delivery, it needs a mechanism to inform the source of packets dropped because of errors (except transmission errors). In the remainder of this lecture errors will mean errors not cause by transmission impairments © Janice Regan, 2012 2 Dealing with errors IP datagram delivery (network level) has a header checksum to detect transmission errors in the IP header TCP has a checksum which covers the TCP header, pseudo header and data Higher level protocols (for example TCP) also handle more types of errors Higher level protocols may deal with any errors not caught at lower levels. It may be more efficient to catch errors in the IP layer than in the TCP layer Within IP need an error reporting mechanism to report such errors, one such mechanism is the ICMP protocol. © Janice Regan, 2012 3 ICMP Internet Control Message Protocol is an integral part of IP ICMP messages are carried as the data portion of an IP datagram Communicates between the IP software on different hosts Used (primarily by routers) to report errors back to the original source. Used to enquire about the state and configuration of routers and hosts Reports errors and in some cases suggests corrective action © Janice Regan, 2012 4 ICMP encapsulation © Janice Regan, 2012 5 ICMP message format There are several types of ICMP messages designed to report different types of errors Each ICMP message has its own format, but all start with the same three fields A type field (1 octet) indicating which type of ICMP message follows A 1 octet code following the type that further defines the message (see text for list) For example type specifies destination unreachable, code specifies router or host The 3rd common field is a 2 octet checksum. The checksum is calculated the same way as the ARP checksum, including the entire ICPM PDU © Janice Regan, 2012 6 ICMPv4 Message Types © Janice Regan, 2012 Comer 2000: 7 IPv4 Destination Unreachable Codes 0 Net Unreachable Host Unreachable Protocol Unreachable Port Unreachable Fragmentation Needed and Don't Fragment was Set Source Route Failed Destination Network Unknown Destination Host Unknown Source Host Isolated Communication with Destination Network is Administratively Prohibited 1 Communication with Destination Host is Administratively Prohibited Destination Network Unreachable for Type of Service Destination Host Unreachable for Type of Service © Janice Regan, 2012 8 ICMP Echo Request/Reply Comer 2000: fig 9.2 Echo request is sent by the ping command to test for reachability Echo reply is sent in response to a received echo reply to confirm reachability Type: request 8, reply 0, Code 0 : no additional qualifying codes Identifier and sequence number are optional, they can be used to match replies with requests The optional data in a echo request must be returned in the resulting echo reply Linux ping has a record route and a timestamp option © Janice Regan, 2012 9 ICMP destination unreachable Comer 2000: fig 9.3 Sent when a router or host cannot deliver a datagram due to an identified failure (not all failures are identified) Can be disabled, not all hosts or routers will send ICMP messages The codes indicate what destination could not be reached and why (see table in text) The header and datagram information is provided to identify the packet needing retransmission (port numbers and sequence number for TCP UDP) © Janice Regan, 2012 10 ICMP Source Quench Message Comer 2000: fig 9.4 Used to help control congestion When a packet must be dropped due to congestion a source quench packet may be sent When the source receives a source quench message it may reduce the rate at which it transmits to the network 1 quench message per round trip travel time should cause change © Janice Regan, 2012 11 ICMP redirect message Comer 2000: fig 9.6 Temporary of permanent changes to network routing on one host may cause other routes on other hosts to become incorrect When a router detects a host on a directly connected network using non optimal routing it can send an ICMP redirect message This mechanism allows dynamic calculation of routes by routers (not hosts) to interact with host routing tables Allowing redirct messages to prompt routing changes can be a security issue and redirect messages are often disabled to protect from the security risk © Janice Regan, 2012 12 ICMP time exceeded message Comer 2000: fig 9.7 Possible causes for timeout To much time in transit (TTL reaches 0). Code 0 Missing fragments in reassembly of fragmented packets. Code 1 Similar format to source quench message © Janice Regan, 2012 13 ICMP parameter problem Comer 2000: fig 9.8 When a router finds problems not handled by previous messages that are severe enough to require the packet to be discarded Can deal with problems such as incorrect parameters for IP options © Janice Regan, 2012 14 ICMP timestamp request/reply Comer 2000: fig 9.9 Used for synchronization and estimation of travel times Originate timestamp added just before transmission Receive timestamp added immediately after receipt Transmit timestamp added immediately before the reply is transmitted This means that transit time (not queuing time) is measured In practice, because clocks on different systems may show different times, travel time and synchronization is complicated © Janice Regan, 2012 15 ICMP address mask request/reply Comer 2000: fig 9.10 To determine the subnetwork address mask for the local subnetwork a host may send an address mask reply to a directly connected router When the router replies the address mask field will contain the subnet mask Identifier and sequence number are used to associate the request with the reply © Janice Regan, 2012 16 ICMP Router discovery Comer 2000: fig 9.11 Used to dynamically determine the address of directly connected routers An alternative to BOOTP and DHCP (both based on manually configured databases) This is a router advertisement packet that is sent periodically (default every 10 minutes, with lifetime of 30 minutes) If an advertisment is not received by a host during the lifetime, the host will discard the routing entry for that router (until it hears again) © Janice Regan, 2012 17 ICMP Router Solicitation Comer 2000: fig 9.12 A host that has just booted will not wish to wait 10 minutes to discover the routers on the network A host can send a solicitation requesting an additional immediate router discovery message © Janice Regan, 2012 18 ICMPv6 (RFC 2463) Retains functionality of IPv4 Adds functionality of ARP and IGMP Adds new functionality, autoconfiguration and unreachability detection Includes neighbor discovery (RFC 2461) a sub protocol that replaces ARP and router discovery in ICMP. Can determine when a neighbor has become unreachable Includes Multicast Listener Discoverer (replacing IGMP) © Janice Regan, 2012 19