ICMP. Internet Control Message Protocol. The Context. IP-Related Protocol: ICMP. TCP/IP Protocol Stack. Transport. Layer. Network.

Transcription

1 Iteret Cotrol Message Protocol ICMP 1 The Cotext IP-Related Protocol: ICMP TCP UDP Trasport Layer ICMP IP IGMP Network Layer ARP Network Access Lik Layer Media TCP/IP Protocol Stack 2 1

2 Iteret Cotrol Message Protocol (ICMP) The Iteret Cotrol Message Protocol (ICMP) is used by routers ad hosts to sed etwork cotrol iformatio to each other From a layerig poit of view, ICMP is a separate protocol that sits above IP ad uses IP to trasport messages I practice, ICMP is a itegral part of IP ad all IP modules must support the ICMP protocol Trasport TCP/UDP ICMP IP 3 Iteret Cotrol Message Protocol ICMP messages are ecapsulated i IP datagrams ICMP Header ICMP Data Frame Header IP Header IP Data Frame Data ICMP frames are idetified by IP Protocol field value 1 Used by IP to sed error ad cotrol messages Uses IP to sed its messages 4 2

3 ICMP Message Format 8-bit type 8-bit code 16-bit checksum Cotets depeds o type ICMP message: cotet cotais the first 8 bytes of IP datagram causig error, plus other thigs 5 Types of ICMP Messages Iformatio messages Seder seds a query to aother machie (either host or router) ad expects a aswer. For example, a host might wat to kow if a router is alive Error idicatio messages The IP software o a host or router has ecoutered a problem processig a IP datagram. For example, it may be uable to route the datagram to its destiatio. 6 3

4 ICMP Types of Messages 7 ICMP Error Reportig 10 4

5 ICMP Error Messages ICMP always reports error messages to the origial source. Importat Poits No ICMP error message will be geerated i respose to a datagram carryig a ICMP error message No ICMP error message will be geerated for a fragmeted datagram that is ot the first fragmet No ICMP error message will be geerated for a datagram havig a multicast address No ICMP error message will be geerated for a datagram havig a special address such as or ICMP Data for Error Messages 12 5

6 ICMP Error Messages (1) Destiatio Ureachable (type 3) Whe a gateway (router) caot route a datagram (e.g., it does't have a appropriate route i its local table, or it eeds to fragmet ad the DF bit is set), it discards the message ad returs a ICMP "destiatio ureachable" message to the sedig host. Source Quech (type 4) Whe a gateway becomes cogested ad rus out of buffer space, it may discard a datagram ad retur a source quech message. Source quech messages are used to request that the seder reduce the rate at which it is sedig datagrams 13 ICMP Error Messages (2) Time Exceeded (type 11) As a datagram is processed, routers decremet its time-tolive (TTL) field. If the TTL value reaches 0, the gateway discards the datagram ad seds a time exceeded message (code 0) to the seder. Code 1 is used by a destiatio host to show that ot all fragmets have arrived withi a set time. 14 6

7 Recall: Traceroute Traceroute records the route that packets take A clever use of ICMP ad the TTL field Whe a router receives a packet, it decremets TTL If TTL=0, sed ICMP Time exceeded back to seder To determie a route Sed a packet with TTL = 1 (hop) The first router discards the packet ad seds ICMP Time Exceeded ; whe ICMP Time Exceeded is received, record the seder s (router s) address Icremet TTL Repeat util the destiatio host is received or a error occurs 15 Traceroute (cotd.) A R1 R2 R3 B 16 7

8 ICMP Error Messages (3) Parameter Problem (type 12) Whe a host or gateway ecouters a problem parsig a IP datagram, it returs a parameter problem message to the datagram's seder Redirectio (type 5) Set from a router to a local host o the same etwork Iforms the source of a better route to the destiatio A host usually starts with a small routig table that is gradually augmeted ad updated. Redirectio helps it. 17 ICMP Redirect Example (1) Host A has oe default router, which is R2 A LAN1 R1 R2 CS Net LAN2 18 8

9 ICMP Redirect Example (2) Whe A wats to sed a message to the Campus Net, it seds it to the default router (R2) To: csgate.csc.vill.edu A LAN1 R2 forwards the message to R1 R1 R2 CS Net LAN2 19 ICMP Redirect Example (3) ICMP: redirect to R1 A R2 also seds a ICMP redirect message to A, tellig it to use R1 for coectios to csgate.vill.edu R1 LAN1 R2 CS Net LAN2 20 9

10 ICMP Redirect Redirectio Message Format 21 ICMP Redirect Example (4) A seds subsequet packets directly to R1 To: csgate.csc.vill.edu A Note: some hosts deliberately igore ICMP Redirect messages as a precautio agaist etwork attacks. R1 CS Net LAN1 R2 LAN

11 ICMP Queries 23 ICMP Query Messages Used to diagose etwork problems I this type of ICMP message, a ode seds a message that is aswered i a specific format by the destiatio ode. Echo Request / Reply (types 8 / 0) If machie A seds a ICMP echo request message to machie B, machie B is required to respod with a ICMP echo reply I UNIX, the program pig allows a user to check whether a machie is reachable ad fuctioig 24 11

12 Pig Uses ICMP Echo request/reply to test destiatio reachability compute roud trip time cout the # of hops to destiatio Source seds ICMP echo request message to the destiatio address echo request packet cotais timestamp also Destiatio replies with a ICMP echo reply message cotaiig the data i the origial request message Source ca calculate RTT of packets If o echo reply comes back, destiatio ureachable 25 Pig (cotd.) A R1 R2 R3 B Sample output: Reply from : 48 bytes i 47 msec. TTL:

13 ICMP Echo Request / Reply Message The idetifier ad sequece umber may be used by the echo seder to aid i matchig the replies with the echo requests. 27 ICMP Timestamp Request / Reply Message Timestamp-request ad timestamp-reply messages ca be used to calculate the roud-trip time betwee a source ad a destiatio machie

14 ICMP Mask Request / Reply Message Timestamp-request ad timestamp-reply messages ca be used to calculate the roud-trip time betwee a source ad a destiatio machie. 29 ICMP Router Solicitatio / Advertisemet 30 14

15 New ICMP Versio: ICMPv6 31 Cotext for ICMPv6 ICMPv6 follows the same strategy ad purposes of versio 4. It is oly slightly more complex tha ICMPv

16 Iteret Cotrol Message Protocol ICMPv6 ICMPv6 is more complex tha ICMPv4: some protocols that were idepedet i versio 4 are ow part of ICMPv6 ew messages have bee added to ICMPv6 to make it more useful Itroduces some simplificatios by elimiatig obsolete types of messages o loger i use 33 ICMPv6 Header Type (8 bits) High order bit = 0 (0 127) idicates error message High-order bit = 1 ( ) idicates iformatio message. Code ( 8 bits) depeds o the message type Checksum (16 bits) Used to detect errors i ICMP ad part of IPv

17 ICMPv6 Messages Trasported withi a IPv6 packet i which extesio headers ca also be preset. Idetified by a value of 58 i the Next Header field of the IPv6 header or of the precedig Header. 35 ICMPv6 Error-Reportig Messages Destiatio-Ureachable Packet-Too-Big Time-Exceeded Parameter-Problem Type 1 Type 2 Type 3 Type

18 ICMPv6 Error Reportig Messages Type Message Name Summary Descriptio of Message Type 1 Destiatio Ureachable Idicates that a datagram could ot be delivered to its destiatio. Code value provides more iformatio o the ature of the error. 2 Packet Too Big Set whe a datagram caot be forwarded because it is too big for the MTU of the ext hop i the route. This message is eeded i IPv6 ad ot IPv4 because i IPv4, routers ca fragmet oversized messages, while i IPv6 they caot. 3 Time Exceeded Set whe a datagram has bee discarded prior to delivery due to the Hop Limit field reduced to zero. 4 Parameter Problem Idicates a miscellaeous problem (specified by the Code value) i deliverig a datagram. 37 Error Reportig i ICMP (v4 vs. v6) 38 18

19 ICMPv6 Iformatioal Messages Echo-Request Echo-Reply Router-Solicitatio Router-Advertisemet Neighbor-Solicitatio Neighbor-Advertisemet Redirect 39 ICMPv6 Echo Request / Reply Echo-Request: Set to test coectivity to aother device Echo-Reply: Set i respose to Echo request 40 19

20 Neighbor Discovery Messages Used by odes (hosts or routers) o the same lik Router-Solicitatio Message Router-Advertisemet Message Neighbor-Solicitatio Message Neighbor-Advertisemet Message 41 ICMPv6 Neighbor Discovery Messages Type Message Name Summary Descriptio of Message Type 133 Router Solicitatio 134 Router Advertisemet 135 Neighbor Solicitatio 136 Neighbor Advertisemet Prompts a router to sed a Router Advertisemet Set by routers to tell hosts o the local etwork that the router exists ad describe its capabilities Set by a device to request the MAC address of aother local device ad provide its ow Provides iformatio about a host to the local etwork 42 20

21 Iformatio Messages i ICMP (v4 vs. v6) 43 Path MTU Discovery for ICMPv6 (PMTUD) 21

22 PMTUDv6 Overview To eable hosts to discover the mi. MTU o a path to a particular destiatio. Fragmetatio i IPv6 is ot performed by itermediary routers. The source ode may fragmet packets by itself oly whe the path MTU is smaller tha the packets to deliver PMTUD for IPv6 uses ICMPv6 error message Type 2 Packet Too Big 45 IPv4 vs. IPV6 MTU Icreased Default MTU I IPv4 miimum required MTU = 576 bytes. I IPv6 miimum required MTU = 1280 bytes. Improves efficiecy by icreasig the ratio of maximum payload to header legth, ad reduces the frequecy of fragmetatio Elimiatio of E Route Fragmetatio I IPv4, datagrams may be fragmeted by either the source device, or by routers durig delivery. I IPv6, oly the source ode ca fragmet; routers do ot. The source must therefore fragmet to the size of the smallest MTU o the route before trasmissio

23 How Do Hosts Kow What Size to Use? Two choices: 1. Use Default MTU Use the default MTU of 1280, which all physical etworks must be able to hadle. Good choice especially for short commuicatios or for sedig small amouts of data. 2. Use Path MTU Discovery feature A ode seds messages over a route to determie what the overall miimum MTU for the path is 47 Path MTU Discovery 48 23