© Jörg Liebeherr (modified by M. Veeraraghavan) 1 ICMP The PING Tool Traceroute program IGMP

© Jörg Liebeherr (modified by M. Veeraraghavan) 2 The IP (Internet Protocol) relies on several other protocols to perform necessary control and routing functions. Orientation

© Jörg Liebeherr (modified by M. Veeraraghavan) 3 ICMP The Internet Control Message Protocol (ICMP) is the protocol used for error and control messages in the Internet. ICMP provides an error reporting mechanism of routers to the sources. All ICMP packets are encapsulated as IP datagrams. The packet format is simple:

© Jörg Liebeherr (modified by M. Veeraraghavan) 4 Types of ICMP Packets Many ICMP packet types exist, each with its own format. A Selection: Type Field:Message Type: 0Echo Reply 3Destination Unreachable 4Source Quench 5Redirect (Change Route) 8Echo Request 11Time Exceeded 12Parameter Problem in Datagram 13Timestamp Request 17Address Mask Request

© Jörg Liebeherr (modified by M. Veeraraghavan) 5 ICMP Message Types ICMP messages are either query messages or error messages. ICMP query messages: Echo request / Echo reply Router advertisement / Router solicitation Timestamp request / Timestamp reply Address mask request / Address mask reply ICMP error messages: Host unreachable Source quench Time exceeded Parameter problem

© Jörg Liebeherr (modified by M. Veeraraghavan) 6 Each ICMP error message contains the header and at least the first 8 bytes of the IP datagram payload that triggered the error message. Problem: How to prevent that too many ICMP messages are sent ? (e.g., an ICMP packet could trigger an ICMP packet, which triggers …). ICMP Error Messages ICMP error messages are not sent......for multiple fragments of the same IP datagrams … in response to an error message … in response to a broadcast packet … etc.

© Jörg Liebeherr (modified by M. Veeraraghavan) 7 Example of a Query: ICMP Timestamp A system (host or router) asks another system for the current time. Time is measured in milliseconds after midnight UTC (Coordinated Universal Time). Sender sends a request, receiver responds with reply. Sender Receiver Timestamp Request Timestamp Reply

© Jörg Liebeherr (modified by M. Veeraraghavan) 8 Example of an Error Message: Port Unreachable There are 16 different ICMP error messages (‘codes’) of type “Destination Unreachable”(Type = 3) Code: Message Type: 0Network unreachable 1Host unreachable 2 Protocol unreachable 3 Port unreachable 4 Fragmentation needed but bit not set 5 Source route failed 6Destination network unknown 7 Destination node unknown 8Source host isolated Code: Message Type: 9Destination network administratively prohibited 10 Destination host administratively prohibited 11Network unreachable for TOS 12Host unreachable for TOS 13 Communication administra- tively prohibited by filtering 14 host precedence violation 15 precedence cutoff in effect

© Jörg Liebeherr (modified by M. Veeraraghavan) 9 ICMP Port Unreachable RFC 792: If, in the destination host, the IP module cannot deliver the datagram because the indicated protocol module or process port is not active, the destination host may send a port unreachable message to the source host. Scenario: Client Request a service at a port No Server No process is waiting at Port 1234 Port Unreachable

© Jörg Liebeherr (modified by M. Veeraraghavan) 10 ICMP Port Unreachable Format of the Port Unreachable Message Code = 3 for Port Unreachable

© Jörg Liebeherr (modified by M. Veeraraghavan) 11 The PING program PING (=Packet InterNet Gopher) is a program that utilizes the ICMP echo request and echo reply messages. PING is used to verify if a certain host is up and running. It is used extensively for fault isolation in IP networks. PING can be used with a wide variety of options, e.g, : -R Record route. Includes the RECORD_ROUTE option in the ECHO_REQUEST packet and displays the route buffer on returned packets. -s packetsize Specifies the number of data bytes to be sent (Default is 56) (In newer implementations, -s is used to continuously generate queries)

© Jörg Liebeherr (modified by M. Veeraraghavan) 12 Echo Request and Reply PING’s are handled directly by the kernel. Each Ping is translated into an ICMP Echo Request. The Ping’ed host responds with an ICMP Echo Reply. AIDA ICMP ECHO REQUEST MNG ICMP ECHO REPLY

© Jörg Liebeherr (modified by M. Veeraraghavan) 13 Format of Echo Request and Reply Identifier is set to process Id of querying process. Sequence number is incremented for each new echo request.

© Jörg Liebeherr (modified by M. Veeraraghavan) 14 Running Ping aida: ping mng.poly.edu PING mng.poly.edu ( ): 56 data bytes 64 bytes from : icmp_seq=0 ttl=128 time=0.718 ms 64 bytes from : icmp_seq=1 ttl=128 time=3.408 ms 64 bytes from : icmp_seq=2 ttl=128 time=3.171 ms 64 bytes from : icmp_seq=3 ttl=128 time=0.701 ms 64 bytes from : icmp_seq=4 ttl=128 time=0.693 ms 64 bytes from : icmp_seq=5 ttl=128 time=1.528 ms 64 bytes from : icmp_seq=6 ttl=128 time=0.689 ms 64 bytes from : icmp_seq=7 ttl=128 time=3.077 ms ^C --- mng.poly.edu ping statistics packets transmitted, 8 packets received, 0% packet loss round-trip min/avg/max = 0.689/1.748/3.408 ms

© Jörg Liebeherr (modified by M. Veeraraghavan) 15 Running Ping to a different machine Aida: ping PING fileserv1.cologne.de ( ): 56 data bytes 64 bytes from : icmp_seq=0 ttl=240 time= ms 64 bytes from : icmp_seq=1 ttl=240 time= ms 64 bytes from : icmp_seq=2 ttl=240 time= ms 64 bytes from : icmp_seq=3 ttl=240 time= ms 64 bytes from : icmp_seq=4 ttl=240 time= ms 64 bytes from : icmp_seq=5 ttl=240 time= ms ^C --- fileserv1.cologne.de ping statistics packets transmitted, 6 packets received, 14% packet loss round-trip min/avg/max = / /

© Jörg Liebeherr (modified by M. Veeraraghavan) 16 Running Ping on a different machine duke% ping mng mng.poly.edu is alive

© Jörg Liebeherr (modified by M. Veeraraghavan) 17 Traceroute program Uses ICMP and TTL rather than the IP Record Route option Why not use RR option? –RR option not always implemented in routers –RR is a one-way option - need to get a return message –Room allocated in options field not sufficient

© Jörg Liebeherr (modified by M. Veeraraghavan) 18 Traceroute operation

© Jörg Liebeherr (modified by M. Veeraraghavan) 19 LAN Output Svr4% traceroute slip traceroute to slip ( ), 30 hops max, 40 byte packets 1 bsdi ( ) 20ms 10ms 10ms 2 slip ( ) 120ms 120ms 120ms At each TTL value, three datagrams are sent –Times correspond to the round-trip times for each datagram

© Jörg Liebeherr (modified by M. Veeraraghavan) 20 IP Source routing option Sender specifies the route –Strict source routing: sender specifies exact path; if the next hop in the source route isn’t a directly connected network, an ICMP “source route failed” error is returned traceroute -G netb -G gateway -G gabby westgate –Loose source routing: sender specifies a list of IP addresses that the datagram must traverse, but in addition,it can traverse other routers between any two addresses in the list traceroute -g netb -g gabby westgate

© Jörg Liebeherr (modified by M. Veeraraghavan) 21 Format of source route option in IP header Code: 0x83 for loose source routing; 0x89 for strict source routing Ptr: points to the next router address in the list

© Jörg Liebeherr (modified by M. Veeraraghavan) 22 Fields in the source route option Len: length of the option in bytes. It can be a maximum of 39 bytes (36 for the 9 IP addresses and 3 bytes for code, len, ptr) Ptr: 1-based index into the 39-byte option of where to store (or use) the next address. In the record route option, it indicates where to store; in source route option, it indicates the next address to use. Min. value is 4; it changes to 8, 12, 16, upto 36. When it is 40, it means the option is full.

© Jörg Liebeherr (modified by M. Veeraraghavan) 23 Example of IP source routing Host S sends a source routed datagram to destination D Note: Destination IP address changed on each hop RFC 791 does not say anything about the source address in the IP header changing. So the source address stays the same even with source routing. In hop-by-hop routing, both source and destination addresses stay unchanged.

© Jörg Liebeherr (modified by M. Veeraraghavan) 24 How source routing works Sending host gets source list from the application. It removes the first entry and makes it the destination address of the IP header (R1). It leaves the ptr at 4, moves entries 1 position to the left; hence #R2, R3, D. The # sign indicates where the ptr is pointing. It adds the destination address to the end of the source route list in the source route option field. Each router checks if it is the destination; –if not, it just forwards the datagram. This can happen if loose source routing is used; only then the source routing module can get the packet.

© Jörg Liebeherr (modified by M. Veeraraghavan) 25 How source routing works (Contd.) –If it is the destination, and ptr is not greater than length, next address in the source list becomes the destination address of the IP packet. –IP address of outgoing interface becomes source address just used and ptr is incremented. Hence {R1, #R3, D}. –Using above approach the source list received at the far end can be used to supply a reverse route in its reply.

© Jörg Liebeherr (modified by M. Veeraraghavan) 26 IP Multicasting Multicasting is one-to-many or many-to-many communications. IP supports multicasting via the help of additional routing protocols.

© Jörg Liebeherr (modified by M. Veeraraghavan) 27 The IP Protocol Stack IP Multicasting only supports UDP at the higher layers (there is no multicast TCP !).

© Jörg Liebeherr (modified by M. Veeraraghavan) 28 IP Multicasting There are three essential components of the IP multicast service: IP Multicast Addressing IP Group Management Multicast Routing We will discuss addressing and group management

© Jörg Liebeherr (modified by M. Veeraraghavan) 29 Semantics of IP Multicast There are many different ways to implement multicast communications. IP multicast works as follows: Multicast groups are identified by a class D address. Hosts ( more precisely: interfaces) can join and leave a multicast group dynamically Every IP datagram sent to a multicast group is transmitted to all members of the group

© Jörg Liebeherr (modified by M. Veeraraghavan) 30 Multicast Addressing All Class D addresses are multicast addresses:

© Jörg Liebeherr (modified by M. Veeraraghavan) 31 Types of Multicast addresses Special and reserved Class D addresses, e.g,

© Jörg Liebeherr (modified by M. Veeraraghavan) 32 Multicast Address Translation The leftmost byte (first byte) of any ethernet address must be 01 to specify a multicast address Ethernet addresses corresponding to IP multicasting are in the range of 01:00:5e:00:00:00 to 01:00:5e:7f:ff:ff Why map an IP multicast address to an ethernet multicast address? –To avoid sending one ethernet frame per host –Hosts in multicast group will enable their ethernet device drivers to receive these multicast frames - called “joining the multicast group ”

© Jörg Liebeherr (modified by M. Veeraraghavan) 33 Multicast Address Mapping

© Jörg Liebeherr (modified by M. Veeraraghavan) 34 Not unique mapping Mapping procedure is not unique –32 different multicast IP (Class D) addresses will map to the same multicast ethernet address –Example: (hex: e ) and (hex: e0:00:40:20) both map to the ethernet address 01:00:5e:00:40:20 Ethernet device driver or IP module may need to perform filtering since the interface card may receive multicast frames in which the host is really not interested Interface card Device Driver IP

© Jörg Liebeherr (modified by M. Veeraraghavan) 35 IGMP The Internet Group Management Protocol (IGMP) is a simple protocol for the support of IP multicast. IGMP is defined in RFC IGMP is used by multicast routers to keep track of membership in a multicast group. Support for: – Joining a multicast group – Query membership –Send membership reports

© Jörg Liebeherr (modified by M. Veeraraghavan) 36 IGMP Packet Format IGMP messages are only 8 bytes long Type: 1 = query sent by router, 2 = report sent by host

© Jörg Liebeherr (modified by M. Veeraraghavan) 37 IGMP Protocol

© Jörg Liebeherr (modified by M. Veeraraghavan) 38 A host sends an IGMP report when it joins a multicast group (Note: multiple processes on a host can join. A report is sent only for the first process). No report is sent when a process leaves a group. A multicast router regularly multicasts an IGMP query to all hosts (group address is set to zero). A host responds to an IGMP query with an IGMP report. Multicast router keeps a table of which of its interfaces have one or more hosts in a multicast group. When the router receives a multicast datagram to forward, it forwards the datagram only out the interfaces that still have hosts with processes belonging to that group. IGMP Protocol

© Jörg Liebeherr (modified by M. Veeraraghavan) 39 IGMP Protocol means all systems on this subnet Router is asking each host to identify each group on that interface