IP Header
Version identifies the IP version to which the packet belongs. This four-bit field is set to binary 0100 to indicate version 4 (IPv4) or binary 0110 to indicate version 6(IPv6).
Header Length is a four-bit field that tells the length of the IP header in 32-bit words. This field is included because the Options field(described later in this section) can vary in size. The minimum length of the IP header is 20 octets, and the options might increase this size up to a maximum of 60 octets the maximum length in 32-bit words that can be described by this field.
Type of Service(TOS) is an eight-bit field that can be used for specifying special handling of the packet. This field actually can be broken down into two subfields: Precedence and TOS.
Total Length is a 16-bit field specifying the total length of the packet, including the header, in
octets. By subtracting the header length, a receiver might determine the size of the packet's data
payload. Because the largest decimal number that can be described with 16 bits is 65,535, the
maximum possible size of an IP packet is 65,535 octets.
Identifier is a 16-bit field used in conjunction with the Flags and Fragment Offset fields forfragmentation of a packet. Packets must be fragmented into smaller packets if the original length exceeds the Maximum Transmission Unit (MTU) of a data link through which they pass. For example, consider a 5000-byte packet traveling through a network. It encounters a data link with a 1500 byte MTU. That is, the frame can contain a maximum packet size of 1500 bytes. The router that places the packet onto this data link must first fragment the packet into chunks of no
more than 1500 octets each. The router then marks each fragment with the same number in the Identifier field so that a receiving device can identify the fragments that go together. A fragmented packet is not reassembled at the other end of the data link; the packet stays fragmented until it reaches its final destination.
Flags is a three-bit field in which the first bit is unused. The second is the Don't Fragment (DF) bit. When the DF bit is set to one, a router cannot fragment the packet. If the packet cannot be forwarded without fragmenting, the router drops the packet and sends an error message to the source. This function enables the testing of MTUs in a network. The DF bit can be set using the Extended Ping utility in IOS. The third bit is the More Fragments (MF) bit. When a router fragments a packet, it sets the MF bit to one in all but the last fragment so that the receiver knows to keep expecting fragments until it encounters a fragment with MF = 0.
Fragment Offset is a 13-bit field that specifies the offset, in units of eight octets, from the beginning of the header to the beginning of the fragment.[6] Because fragments might not always arrive in sequence, the Fragment Offset field allows the pieces to be reassembled in the correct order.
Note that if a single fragment is lost during a transmission, the entire packet must be resent and refragmented at the same point in the network. Therefore, error-prone data links could cause a disproportionate delay. And if a fragment is lost because of congestion, the retransmission of the entire series of fragments might increase the congestion.
Time to Live(TTL) is an eight-bit field that will be set with a certain number when the packet is first generated.
Protocol is an eight-bit field that gives the "address," or protocol number, of the host-to-host or transport layer protocol for which the information in the packet is destined.
Header Checksum is the error detection field for the IP header. The checksum is not calculated forthe encapsulated data; UDP, TCP, and ICMP have their own checksums for doing this.
Source and Destination Addresses are the 32-bit IP addresses of the originator of the packet and the destination of the packet.
Options is a variable-length field and, as the name says, is optional. Space is added to the packet header to contain either source-generated information or for other routers to enter information;the options are used primarily for testing. The most frequently used options are
-Loose source routing, in which a series of IP addresses for router interfaces is listed. The packet must pass through each of these addresses, although multiple hops might be taken between the addresses.
-Strict source routing, where again a series of router addresses is listed. Unlike loose source routing, the packet must follow the route exactly. If the next hop is not the next address on the list, an error occurs.
-Record route provides room for each router to enter the address of its outgoing interface as the packet transits so that a record is kept of all routers the packet encounters. Record route provides a function similar to trace except that the outgoing interfaces, both on the path to the destination and on the return path, are recorded.
-Timestamp is an option similar to record route except each router also enters a timestamp: the packet not only keeps track of where it has been but also records when it was there.
All these options might be invoked by using the Extended Ping on Cisco routers.
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TCP Header
Source and Destination Port are 16-bit fields that specify the source and destination applications for the encapsulated data. A port number for an application, when coupled with the IP address of the host the application resides on, is called a socket. A socket uniquely identifies every application in a network.
Sequence Number is a 32-bit number that identifies where the encapsulated data fits within a data stream from the sender. For example, if the sequence number of a segment is 1343 and the segment contains 512 octets of data, the next segment should have a sequence number of 1343 + 512 + 1 = 1856.
Acknowledgment Number is a 32-bit field that identifies the sequence number the source next expects to receive from the destination. If a host receives an acknowledgment number that does not match the next sequence number it intends to send (or has sent), it knows that packets have been lost.
Header Length, sometimes called Data Offset, is a four-bit field indicating the length of the header in 32-bit words. This field is necessary to identify the beginning of the data because the length of the Options field is variable.
The Reserved field is four bits, which are always set to zero.
Flags are eight 1-bit flags that are used for data flow and connection control. The flags, from left to right, are Congestion Window Reduced (CWR), ECN-Echo (ECE), Urgent (URG), Acknowledgment (ACK), Push (PSH), Reset (RST), Synchronize (SYN), and Final (FIN).
Window Size is a 16-bit field used for flow control.
Checksum is 16 bits, covering both the header and the encapsulated data, allowing error detection.
Urgent Pointer is used only when the URG flag is set. The 16-bit number is added to the Sequence Number to indicate the end of the urgent data.
Options, as the name implies, specifies options required by the sender's TCP process. The most commonly used option is Maximum Segment Size, which informs the receiver of the largest segment the sender is willing to accept. The remainder of the field is padded with zeros to ensure that the header length is a multiple of 32 octets.
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UDP Header
A UDP packet is much smaller header than TCP. The Source and Destination Port fields are the same as they are in the TCP header; the UDP length indicates the length of the entire segment in octets. The checksum covers the entire segment, but unlike TCP, the checksum here is optional; when no checksum is used, the field is set to all zeros.
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