IPv4 Options. Lecture 3: IPv4 cont d, ICMP, and UDP. IP Options Encoding. Categories of IP Options. IP options types. IP Options: Record Route

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1 Internetworking Lecture 3: IPv4 cont d, ICMP, and Literature: Forouzan, TCP/IP Protocol Suite: Ch 8-9, 11 IPv4 Options IPv4 options are intended for network testing or debugging Options are variable size and comes after the fixed header. Contiguous no separators Not required fields, but all IP implementations must include processing of options In practice many implementations do not! Max 40 bytes - very limited use Max header length is 60 bytes (fixed part is 20 bytes) IP Options Encoding Categories of IP Options Two styles Single byte (only code) Multiple byte Option Code: 1 byte Copy (to fragments) (1 bit) Class (2 bits) 0 (00): Datagram or network control 2 (10): Debugging and measurement Number (5 bits) Option Length (len): 1 byte, defines total length of option (including code and len fields) Data: option specific code len data copy class option number IP options types IP Options: Record Route Each router records its address Number Option End of option No option Loose source route Timestamp Record route Strict source route The destination processes the trace E.g. sends the result back to the sender Pointer is next available slot Source creates an empty list Every router adds its address. Increments pointer Limited to nine hops IP header size limit code len pointer First IPv4 addr Second IPv4 addr... Second IPv4 addr First IPv4 addr 1

2 IP Options: Record Route Example IP Options: Source Route The sender dictates a path through the network Strict Source Routing The path is exactly as specified. Loose Source Routing Allows multiple hops between successive addresses. The routers records their addresses Just like record route code len pointer First IPv4 addr Second IPv4 addr... Second IPv4 addr First IPv4 addr Note that pointer is an index, starting with code at index 1 IP Options: Source Route Example IP Options: Timestamp Similar to record route, but also adds a timestamp Source creates an empty list Every router adds its address and a timestamp. An overflow field (O-flow) specifies how many routers could not specify a timestamp. A flags field specifies the visited router responsibilities E.g., add only timestamp or add timestamp + outgoing IP address Code: 68 Length (total) Pointer O-flow Flags Data IP Options: Timestamp cont d ICMP Internet Control Message Protocol - RFC 792 2

3 ICMP ICMP is a signalling protocol for IPv4. Report IP problems back to sender Control and Management Considered a part of IP, but uses IP for transfers. Query ICMPs Control purposes Examples: Echo, Router advertisement, Timestamp, etc. Error ICMPs Sent when an error in IP detected Includes the first 8 bytes of the data field of the original datagram which caused the error. Not sent for: icmp errors, broadcasts, fragments, etc. Examples: Dest unreachable, Redirect, etc. Type Error-reporting Message Destination unreachable Source quench Time exceeded Parameter problem Redirection ICMP Messages ICMP messages Type 8/0 13/14 17/18 10/9 Query Message Echo request/reply Timestamp request/reply Address mask request/reply Router solicitation/advertisement General Format of ICMP Messages ICMP Header ICMP error messages returns original IP datagram Original IP header (+ options) and 8 bytes of payload Example: ICMP Destination Unreachable ( packet) ICMP Message Ethernet header IP header ICMP header IP header of datagram that generated the error header Type: specifies type of message Code: specifies reason for the particular message type ICMP Header varies depending on type Example: ICMP Destination Unreachable (type 3) type (3) code (0-15) checksum Unused (all 0s) Common for all types IP header (including options) + first 8 bytes of original IP data ICMP Error Reporting One of the main responsibilities of ICMP Recall that IP is an unreliable protocol, and errors may occur ICMP does not correct errors Left to higher level protocols Error messages are always sent back to the original source Because the only information available in the datagram about the route is the source and destination IP addresses ICMP uses the source address of the IP packet to send the error message back to the source (originator) ICMP Error Restrictions An ICMP Error is not returned in response to: A datagram carrying another ICMP Error A datagram destined to IP broadcast or multicast address A datagram sent as link-layer broadcast (e.g., Ethernet) An IP fragment other than the first A datagram whose source address does not define a single host (e.g., ) Reason is the risk of creating: Loops Packet explosions (broadcast storms) 3

4 ICMP Error Reporting Messages ICMP Destination Unreachable Different types (Code 0-15): Code 0 network unreachable Returned by routers Code 1 host unreachable Returned by routers Code 3 port unreachable Returned by hosts when /TCP port does not exist... (Code 0 15 are defined) ICMP Source Quench Error Attempt to add a kind of flow control to IP! ICMP source quench may be generated if the system receives data faster than it can process it New Router Requirements RFC: routers should not generate source quench errors Consumes network bandwidth An ineffective and unfair fix for congestion Thus, for reliability you have to do end-to-end (transport level) flow control, error checking, and use acknowledgements TCP ICMP Time Exceeded This type (11) of error message is sent in 2 cases Code 0: when TTL is zero after decrementation, the router discards the datagram and sends an ICMP Time Exceed back to the source Code 1: when all fragments of a datagram do not arrive at the destination host within a certain time limit Timer is started at reception of first fragment Tool Using ICMP: Traceroute Traceroute traces a path to a destination by exploring every IP hop on the way Note: only receiving interfaces are traced, not sendig. Traceroute algorithm uses two steps: 1. Set small TTL fields and receive ICMP time exceeded incrementally 2. When final host reached, use unlikely port and get ICMP port unreachable back Alternative: use an IP datagram with record route option But this is not always implemented Limited number of hops can be traced due to maximum size of IP options (Record route records IP addresses of outgoing interfaces) ICMP Parameter Problem Code 0: Main IP header field problem Pointer points to byte with problem Code 1: Problem in IP option field Pointer not used IP routers and hosts do sanity checks on IP header 4

5 ICMP Redirect concept ICMP Redirect is sent by a router (R1) to the sender of an IP datagram (host) when the datagram should have been sent to a different router (R2) ICMP Redirect message format Code 0: Redirection for network specific route Code 1: Redirection for host specific route... (3) ICMP Redirect (1) IP datagram (4) Subsequent IP datagrams R1 (2) IP datagram R 2 ICMP Query Messages Echo Request and Reply Can you think of a widely used program that uses ICMP Echo request/reply? Ping uses ICMP Echo Request/Reply Timestamp Request and Reply Ping tests host reachability. Uses ICMP echo request/response, Almost all IP implementations support Ping server. Sends an ICMP echo request to a node Server replies with ICMP echo response With IP record route (RR) option, the route of the ping datagram can be traced ICMP Echo Request ICMP Echo Respond... Can be used to calculate round-trip time Even if clocks are not synchronized Can be used to synchronize clocks if one-way duration is known 5

6 Address-Mask Request and Reply Router Solicitation/Advertisement Can be used by diskless clients to find out the address mask Recall that RARP gives only the IP address Not used very much Other protocols (DHCP, BOOTP) are used for autoconfiguration (later lecture) On booting, hosts send ~3 ICMP router solicitation messages (~3 seconds apart) to find a default router. Dynamic discovery of the default router. Routers periodically broadcast or multicast advertisements of their existence and desire to provide routing service Advertisements typically every seconds Advertisements have a stated lifetime (typically 30 minutes) Seldom implemented Mobile IP discovery mechanisms IPv6 ICMP Summary IP and ICMP Summary Destination Unreachable Network/Host/Protocol/Port/... Time Exceeded TTL expired Used in the traceroute tool Parameter problem IP header error Source Quench Requests source to decrease its data rate. Redirect Tell source to send its messages to a better address Echo Request/Echo reply For testing (e.g., ping program sends an Echo request) Timestamp Request/Reply Clock synchronization RTT Address Mask Request/Reply Diskless systems Router Solicitation and Advertisment Hosts query routers Routers advertise presence and routes IPv4 is engineered to solve problems encountered at the network level Each field in the IPv4 header addresses a networking issue Logical addressing Different L2 characteristics (MTUs) QoS Bit errors Multiplexing The control and error mechanism of IP is provided by ICMP IPv4 is a very successful protocol, but there are many flaws and unused features IPv6 has cleaned up the IP layer considerably Transport Layer Responsible for end-to-end delivery of entire messages Transport Layer Service-point addressing (Protocol Port or Port Number) Address the specific running process on a computer Segmentation and Reassembly Divide message into transmittable segments and reassemble message at receiver Connection Control For connection-oriented transport protocols End-to-end Flow Control (in contrast to link level flow control) End-to-end Error Control (in contrast to link level error control) 6

7 TCP/IP Transport Layer Protocols 2 transport layer protocols in the TCP/IP stack User Datagram Protocol Connectionless unreliable service TCP Transmission Control Protocol Connection-oriented reliable stream service Telnet FTP... DNS ICMP TCP IP Underlying link technology ARP Protocol Ports Ultimate source/destination of/for a transport level message is a protocol port A process sends/listens to a protocol port (identified with an integer) Most operating systems provide synchronous access to ports A process gets blocked if it attempts to extract data from a port prior to arrival of data In general, ports are buffered Data arriving before a process is ready to accept is placed in a (finite) queue To communicate with a port, sender needs to know both the IP address of the machine and the protocol port number within the machine The combination of an IP address and a port number is called a socket Each message must carry destination port and source port Port Numbers in Three Groups Range Purpose Well-known ports Registered ports Dynamic ports Servers are normally known by their well-known port number (e.g., 80 for HTTP). Assigned and controlled by IANA Dynamic ports are ephemeral and can be used by any process (normally used by client processes) User Datagram Protocol - RFC 768 Message Format User Datagram Protocol Datagram-oriented transport layer protocol Provides connectionless unreliable service Provides optional end-to-end checksum covering header and data Provides no feedback to control data rate An datagram is silently discarded if checksum errors messages can be lost, duplicated, or arrive out of order Application programs using must deal with reliability problems DNS, DHCP, SNMP, NFS, VoIP, etc. use An advantage of is that it is a base to build your own protocols on 8 byte header + possible data bit source port number 16 bit destination port number 16 bit length 16 bit checksum data (if any) length field is redundant, since the IP software can pass this info to IP datagram datagram IP header header data 20 bytes 8 bytes 0..(2 16 1) ( ) = bytes 7

8 Checksum and Pseudo-header checksum covers application data, header,a pseudoheader, and pad byte (if needed) Purpose with pseudo-header: double-check that packet arrived to correct destination check that IP delivered the packet to the correct protocol (/TCP) Pseudoheader and pad byte not transmitted, only used for computation 32-bit src IP addr Pseudoheader 32-bit dst IP addr All 0s 8-bit protocol 16-bit total length 16-bit src port number 16-bit dst port number header 16-bit total length 16-bit checksum Maximum Datagram Size Theoretical limit: 65,535 bytes - due to (IP s) 16-bit total length field with 20 bytes of IP header + 8 bytes of header 65,507 bytes of user data Two limitations: sockets API limits size of send and receive buffer; generally 8 kbytes, but you can call a routine to change this TCP/IP implementation - Stevens found various limits to the sizes - even with loopback interface Hosts are required to handle at least 576 byte IP datagrams lots of protocols limit themselves to 512 bytes or less of data to avoid fragmentation DNS, TFTP, BOOTP, and SNMP Data, padded to multiple of 2 bytes (16 bits) Summary Transport Layer Basics end-to-end delivery of messages a fairly simple connectionless protocol 8