Network Layer (4): ICMP

Transcription

1 1 Network Layer (4): ICMP Required reading: Kurose 4.4.3, CSE 4213, Fall 2006 Instructor: N. Vlajic 2 1. Introduction 2. Network Service Models 3. Architecture 4. Network Layer Protocols in the Internet 4.1 IPv4 4.2 IP Addressing and Subnetting 4.3 ICMP 4.4 IPv6 5. Routing Algorithms 6. Routing in the Internet 1

2 IP Protocol Deficiencies 3 IP Deficiencies lack of error control lack of error control (i.e. error-reporting and error-correcting) and network assistance mechanisms what if a router must discard a datagram because it cannot find a router to the final host, or because datagram s TTL = 0?! what if the final host must discard a number of fragments because it has not received all fragments by a certain time limit?! what if a host needs to determine if another host/router is alive?! malfunctioning router source destination ICMP 4 ICMP Internet Control Message Protocol companion to IP, intended to compensate for IP deficiencies ICMP is network layer protocol above IP its messages are not directly passed to data-link layer; instead they are first encapsulated inside IP datagrams IP header s protocol field set to 1 indicates that IP data carries an ICMP message The ultimate destination of an ICMP message is not an application program or user on the destination machine, but the Internet Protocol software of that machine! 2

3 Types of ICMP Messages 5 ICMP Messages are divided into two broad categories: Error-reporting reporting report problems that a router or destination host may encounter when processing one specific IP packet Query help a host or a network manager get specific info from a router or another host occur in pairs request/reply Category Type Message Category Type Message 3 Destination Unreachable 8 / 10 Echo Request / Reply Error-reporting Messages Source Quench Time Exceeded Parameter Problem Redirection Query Messages 13 / / / 9 Timestamp Request / Reply Address Mask Request / Reply Solicitation or Advertisement ICMP Message Format 6 ICMP Message Format 8 byte header + variable size data section first 4 bytes of header are the same for all message types, last 4 differ type field in header defines type of message code field in header specified reason for particular message type checksum in header in calculated over entire message data in error messages carries information for finding original packet that had error data in query messages carries extra information based on type of query 3

4 ICMP Error Reporting 7 Error Reporting ICMP does not correct errors, it simply reports them error correction is left to higher-level protocols error messages are always sent back to original source only information available in datagram about route is source and destination IP address data section in all error messages contains IP header of original datagram + 8 bytes of data in that datagram am in case of UDP and TCP protocol first 8 bytes provide info about port and sequence number this info is needed so that source can inform UDP and TCP about error ICMP Error Reporting (cont.) 8 Error Reporting Messages (1) Destination Unreachable when router cannot route or host cannot deliver datagram, datagram is discarded and destination unreachable message is sent back to source host Code 2: (destination host) protocol is unreachable Code 3: (destination host) port is unreachable application program is not running presently Code 7: destination host is unknown Code 9: communication with destination network is administratively prohibited 4

5 ICMP Error Reporting (cont.) 9 (2) Source Quench when router / host discards datagram due to congestion it sends source-quench message to sender of datagram in order to (a) inform source that datagram has been discarded why optional?! (b) warn source that there is congestion somewhere in the path source should slow down (unreliable flow control!) Optional. Optional. (3) Time Exceeded (a) whenever router decrements TTL to 0 in a datagram, it discards the datagram and sends time-exceeded message (Code=0) (b) when final destination does not receive all fragments in certain time interval it discards received fragments and sends timeexceeded message to original source (Code=1) Optional. ICMP Error Reporting (cont.) 10 (4) Parameter Problem if router / destination host discovers ambiguous or missing value in any field of datagram it discards datagram and sends parameter problem message back to source Pointer to the octet that caused the problem. (5) Redirection host usually starts with small routing table that is gradually updated - one of tools to accomplish this is redirection message (a) when host comes up its routing table has limited number of entries (b) for this reason, host may send datagram to wrong router (c) router that receives datagram will forward it to correct router (d) to update host s routing table, router sends redirection message to host NOTE: although the the redirection messages is is considered an an error reporting messages, it it has different form from other error messages it it does not not discard the the datagram; it it sends it it to to appropriate router! 5

6 ICMP Error Reporting (cont.) 11 Code 0: redirect for network error redirect all future datagrams sent to this network Code 1: redirect for host error redirect all future datagrams sent only to this specific device Code 2: redirect for TOS and network error Code 0 AND future datagrams with this ToS value Code 3: redirect for TOS and host error Code 1 and future datagrams with this ToS value source destination ICMP Query 12 Query Messages (1) Echo Request and Reply used for diagnostic purposes two messages in combination determine whether two systems (host and routers) can communicate with each other node to be tested is sent an echo request ; optional data field contains a message that must be repeated exactly by responding node in echo-reply message echo request and echo reply are used by ping when checking if another host is reachable 6

7 ICMP Query (cont.) 13 Example [ capturing ping s ICMP packets using Ethereal ] STEPS: 1) Start a new live capture in Ethereal. 2) Print on command line: ping ) Stop Ethereal capture. ICMP Query (cont.) 14 4) Filter ICMP. 7

8 ICMP Query (cont.) 15 When calculating RTT, does Ping considers the same path in both directions?! ICMP Query (cont.) 16 (2) Timestamp Request and Reply used to determine RTT needed for datagram to travel between two hosts or routers original timestamp is filled by source at timestamp request departure time receive timestamp is filled by destination at time timestamp request was received transmit timestamp is filled by destination at time timestamp reply departs roundtrip calculations are correct even if two clocks are not synchronized each clock contributes 2 times to calculation Sending time = value of receive timestamp -value of original timestamp Receiving time = time the packet returned - value of transmit timestamp RTT = sending time + receiving time Optional. Optional. 8

9 ICMP Query (cont.) 17 (3) Address-Mask Request and Reply host may know its full IP address, but it may not know which part defines its hostid and which netid all 0-s in request, actual mask in reply to obtain its mask host sends addressmask request message to a router on LAN, or it broadcasts such message Optional. (4.1) Solicitation host that wants to send data to host on another network needs to know address of routers connected to its own network to obtain these addresses host broadcast or multicast router solicitation message Optional. ICMP Query (cont.) 18 (4.2) Advertisement routers that receive router solicitation message broadcast their routing information using routing advertisement message router can also periodically send such advertisement messages even if no host has solicited NOTE: when router sends out advertisement it announces not only its own but also presence of all routers on network of which it is aware Transmission Reception Prohibited. Optional. Optional. # of 32-bit words of info per each router value is normally 2 (router address + preference level) # of seconds that entries can be considered valid Preferability of router relative to other routers used to select default router for which preference level = 0 9

10 ICMP Message Processing 19 Input module handles all received ICMP messages. It is invoked when an ICMP packet gets delivered to it from IP layer. 1) If the received packet is a request or solicitation, the module creates a reply or an advertisement and sends it out. 2) If the received packet is a redirection message the module updates the routing table. 3) If the received packet is an error message, the module informs respective protocol Output module is responsible for creating request, solicitation, or error messages requested by higher layers or the IP protocol. The module receives a demand from IP, UDP, TCP, or an application program. ICMP and Window s Tracert 20 TRACERT determines the route taken to a destination by sending ICMP echo packets in IP packets of varying Time-To-Live (TTL) values to the destination. Each router along the path is required to decrement the TTL on a packet by 1 before forwarding it, so the TTL is effectively a hop count. When the TTL on a packet reaches 0, the router should send an ICMP Time Exceeded message back to the source computer. TRACERT determines the route by sending the first echo packet with a TTL of 1 and incrementing the TTL by 1 on each subsequent transmission until the target responds or the maximum TTL is reached. The route is determined by examining the ICMP Time Exceeded messages sent back by intermediate routers. Note that some routers silently drop packets with expired TTLs and are invisible to TRACERT. IP TTL=1 ICMP echo to destination 3 probes IP 3 probes ICMP Time Exced. 10

11 ICMP and Window s Tracert (cont.) 21 Example [ capturing tracert s ICMP packets using Ethereal ] STEPS: 1) Start a new live capture in Ethereal. 2) Print on command line: tracert (IP = ) 3) Stop Ethereal capture. ICMP and Window s Tracert (cont.) 22 4) Filter ICMP. 11

12 ICMP and Window s Tracert (cont.) 23 5) Expand Internet Protocol Field at the last ICMP Echo Request packets. Verify for yourself that the value of TTL in this packet corresponds to the number of hops seen on the command line Introduction 2. Network Service Models 3. Architecture 4. Network Layer Protocols in the Internet 4.1 IPv4 4.2 IP Addressing and Subnetting 4.3 ICMP 4.4 IPv6 5. Routing Algorithms 6. Routing in the Internet Why has the adoption of IPv6 slowed down??? 1) NAT 2) DHCP 12

13 IPv6 25 IPv6 also known as IPng provides several advantages over IPv4 larger address space: IPv6 address is 128 bits long huge increase in address space better header format: number of IPv4 fields have been dropped or made optional resulting 40-byte fixed length header allows for faster processing IPv6 options are placed in separate optional headers that are located between IPv6 header and transport-layer header 4 bytes 4 bytes 16 bytes 16 bytes support for resource allocation: in IPv6 type-of-service field has been removed, but new mechanism flow labelling has been added to enable special handling of real-time audio and video IPv6 (cont.) 26 Transition from IPv4 to IPv6 it will take time before every system on the Internet moves moves from IPv4 to IPv6 possible transition strategies: Header Translation Tunneling Header Translation IPv6-capable stations should also run IPv4 until all (Dual Stack) of the Internet uses IPv6 DNS would help nodes determine whether other nodes are IPv6-capable by returning addresses in IPv6 format if either the receiver or any of the routers along the path is only IPv4 capable, an IPv4 datagram must be used problem: IPv6 nodes may be forced to exchange only IPv4 packets if there is at least one IPv6-incapable node in between them 13

14 IPv6 (cont.) 27 Tunneling place entire IPv6 packet in payload field of IPv4 should be used when two computers using IPv6 want to communicate with each other, but packet must pass through a region that uses IPv4 when IPv6 packet reaches boundary of IPv4 network, last IPv6 router encapsulates it in IPv4 packet - IPv4 packet is then sent to IPv6 node on other side, i.e. receiving side of tunnel IPv6 node on receiving side of tunnel recognizes that received IPv4 packet contains an IPv6 packet, extracts packet, and continues with its routing Exercise The purpose of echo request and echo reply is to. (a) report errors (b) check nod-to-node communication (c) check packet lifetime (d) find IP addresses 2. When all fragments of a message have not been received within the designated amount of time, a error message is sent? (a) source-quench (b) time-exceeded (c) parameter-problem (d) timestamp-request 3. A can learn about network by sending out a routersolicitation packet? (a) router, routers (b) router, hosts (c) host, hosts (d) host routers 14