Internet Protocol

Source quench

Flow control message, request to decrease the traffic rate of data messages sent to an internet destination. A gateway (router) has to discard datagrams if it doesn't have the buffer space needed to queue the datagrams for output to the next network on the route to the destination network. If a gateway discards a datagram, it may send a source quench message to the source host. May also send if dgs arrive too fast to be processed. SQ is a request to the host to cut back the rate at which it is sending traffic to the internet destination. On receipt of a SQ, source host should reduce rate to destination until it no longer receives source quench messages from the gateway. It again gradually increases rate at which it sends traffic until it again receives SQ messages. parameter unused.

Destination unreachable

Generated by router to inform source host that it doesn't know how to reach the destination. Occurs if IP header set incorrectly. Parameters unused.

Time exceeded

Generated when TTL field becomes zero, sent to the source of the datagram. May also be sent by the destination if it fails to reassemble a fragmented datagram within its time limit. parameter unused.

Source IP address (32)

IP address of source sending the datagram

Destination IP address (32)

IP address of the final destination of the datagram

Version (4)

IP version number, generally 4, now 6 being deployed. Checked first so that correct version of IP software is used to process the datagram. Datagram rejected if version is different from 4.

Internet Header Length (4)

Length of IP header in 32 bit words. Excluding options and padding, min is 20 bytes, so IHL will be 5. Maximum 60 bytes including options and padding.

Timestamp/ Timestamp reply

Mechanism for finding delay characteristics of the internet if source routing is used. Timestamp message has an identifier (16) and sequence number (16), along with the originate timestamp (32). Timestamp reply has additionally receive timestamp (32) and transmit timestamp (32).

Internet control message protocol

Protocol for transferring messages about problems in communication environments from routers and others hosts to a host, *sent in response to a datagram*. They are sent in several situations - when a datagram can't reach its destination, when a gateway does not have the buffering capacity to forward a datagram, and when the gateway can direct the host to send traffic on a shorter route.

What is IP

Rightly the heart of the internet. The protocol that allows various networks to communicate by defining the data formats for transferring data b/w various networks, the addressing and routing mechanisms by hosts and routers, the rules for discarding packets, generating error messages. The IP service is unreliable connectionless packet service. Unreliable because there is no guarantee that packets will be delivered - may be lost due to congestion, may not arrive in sequence, may arrive with variable delays. In fact, IP does not guarantee a desired QoS. Connectionless since each packet is handled independently. It is implemented as software that must run on every end system and every router in any internet using the TCP/IP protocol suite.

Redirect

Router sends to source to advice about a better route to a particular destination. Router checks its routing table to obtain address for next router on the road to the destination and finds that the next one is on the same network as the source, a redirect message is sent to host recommending sending data to the next router directly. Router forwards the dg to the next router anyway. Address of R2 in parameter.

Fragment offset (13)

Specifies offset of fragment in the datagram, starting from 0. Destination can now assemble fragments in order depending on the offset.

Protocol (8)

Specifies which higher layer protocol is encapsulated in the data area of the IP datagram. Common protocols and decimal values are: 1. internet control message protocol (ICMP) 6. Transport Control Protocol (TCP) 17. User Datagram Protocol (UDP)

Class A IP address

0, network (7 bits), host (24 bits) used when site contains a small #networks (126) and each network has many host (node) range from 1.0.0.0 to 126.255.255.255, all allocated

Types of ICMP messages (8)

1. destination unreachable (3) 2. time exceeded (11) 3. source quench 4. redirect (5) 5. timestamp (13) 6. timestamp reply (14) 7. echo (8) 8. echo reply (0)

Class B IP address

10, network (14 bits), host (16 bits) used for medium #networks and each network has b/w 256 and 65536 hosts range from 128.0.0.0 to 191.255.255.255, all 2^(14) network addresses allocated

Class C IP address

110, network (21 bits), host (8 bits) used for large #networks, and each network has <256 hosts range 192.0.0.0 to 223.255.255.255, nearly all 2^(21) addresses allocated

Class D IP address

1110, multicast used when multicasting is reqd, i.e. when a datagram has to be sent to multiple hosts simultaneously range 224.0.0.0 to 239.255.255.255

Class E IP address

11110, future use reserved for future use

ICMP header

64 bit, with type (8) (specifies type of ICMP message), code (8) (used to specify parameters of message), checksum (16) (checksum of the entire ICMP message), parameters (32) (optional, used to specify lengthier parameters)

Flags (3)

Bit 0: reserved, must be 0 Bit 1: DF (do not fragment) 0 = may fragment, 1 = do not fragment Bit 2: MF (more fragments) 0 = last fragment, 1 = more fragments

Type of Service (8)

These bits specify how the datagram has to be handled by systems. Bits 0-2 set the precedence of the datagram i.e. how much importance to attach to its processing. Bit 4 is D, setting it requests low delay. Bit 5 is T, setting it requests high throughput/ data rate. Bit 6 is R, setting it requests high reliability. However, these are only requests - there is no guaranty.

Time to Live (8)

To prevent the datagram circulating in network endlessly and cause congestion, the max number of hops the packet can travel is specified. Default hop count is 64, much more than actual number of hops required. Every router that forwards the packet reduces this field value by 1. If any router finds that this is 0, packet is discarded.

Rules for IP addressing scheme

To uniquely identify every end system on the network, address must contain both an ID for the network and an ID for the host. Each node is identified by a 32 bit address, and can be of Classes A to E. Written in dotted decimal notation for easy readability. If host address bits are all 0, address is the network address. If host address bits are all 1, address is the broadcast address for all hosts on network.

Length (16)

Total length of the datagram *in bytes* including header and data. Max size is 2^(16) - 1. Length of data field is calculated by subtracting IHL from length. An IP datagram is larger than can be accommodated by a LAN - can only handle 1526 bytes in a frame. In such cases, the datagram has to be fragmented with most of the header copied, and sent over the network. Min datagram size that every host and router must handle is 576 bytes. Fragmentation done at routers, reassembly at the destination

Identification (16)

Unique ID assigned by sender to identify the datagram. Along with SA and DA, uniquely identifies the datagram, and these 3 fields are copied into all fragments to help in reassembly.

Header Checksum (16)

Used for error checking of header. Every router must recalculate it since TTL and offset fields changed. At each hop, CRC checked - if there is mismatch, discarded.

Echo/ Echo reply

Used to test reachability of a system. Source sends to destination, gets back echo reply. Request can have an optional data field returned in the reply, which, if returned properly, shows entire route is OK. Parameters are identifier (16) and sequence number (16)

Option

Variable length, contains data for network testing and debugging. Options for operations such as recording the route of a datagram (if set, each router adds its IP address in the options field and forwards it), and source routing (if set, IP addresses of all hopes mentioned in options field, and datagram only takes that route)

Data field

Variable length, length specified in length - IHL fields. Carries user data from the next layer up, integer multiple of 8 bits, max length of header + data is 2^(16) - 1

Padding

Variable length, used to make the header an exact multiple of 32 bits

IPv4 header fields (14)

Version (4), IHL (4), ToS (8), length (16), ID (16), flags (3), offset (13), TTL (8), protocol (8), checksum (8), Source address (32), destination address (32), options (variable), pad (variable). Minimum legnth of header = 20 bytes, max length 60 bytes