IST480. Protocol headers.

Protocol headers.


1	Source port number (UDP service ID for sender of this packet)
2	Destination port number (UDP service ID for receiver of this packet)
3	UDP packet length (byte length includes UDP header and data)
4	UDP checksum (UDP header and data and portions of IP header included)

source port: port number of the sender
destination port: port number of the receiver
packet length: length in bytes of UDP header and data
checksum (of header and data): standard IP checksum algorithm applied to UDP header and data


1	Source port number (TCP service ID for sender of this packet)
2	Destination port number (TCP service ID for receiver of this packet)
3	TCP sequence number
4	(byte count of data sent by source, starting at random value)
5	Acknowledgment number
6	(this is the sequence number of the last data byte received from destination)
7	TCP header length (offset data, in 32-bit words)	Reserved (6 bits in length)	TCP option bit flags
8	Current TCP window size (amount of data sender is willing to accept in bytes)
9	Checksum
10	Urgent data pointer (byte offset into data to last urgent data byte)
11	Options and padding (length varies, to calculate: (TCP header length - 5) * 4 bytes )

source port: port number of the sender
destination port: port number of the receiver
sequence number: octet sequence number (starts with clock-driven random value)
acknowledgment number: sequence number of last octet received (and implicitly acknowledges all prior octets)
TCP header length: number of 32-bit words in TCP header (5 unless TCP options are present)

TCP bit fags:

URG

ACK

PSH

RST

SYN

FIN

Bit #	Acronym	Description
13	RST	reset (abort connection)
14	SYN	synchronize (establish connection)
15	FIN	finish (close connection)

TCP window size: amount of buffer space available for data on sender
Checksum: standard IP checksum of TCP header and data
Urgent pointer: valid urgent pointer bit flag set in TCP header; there are two different and incompatible ways to interpret it:
- RFC 1129 style: points to last urgent byte
- RFC 793 style: points to byte after last urgent byte
Options: options are in 32-bit words, formatted as follows:
- byte 1: option
  value=0: end of options
  value=1: padding byte
- byte 2: size
- byte 3 and beyond: data


1	Type	Code
2	Checksum
3	Pointer	*Reserved*
4	*Reserved*
5	Data: Internet header + 64 bits of original datagram header
...	(the IP packet that generated this ICMP message)

Type and Code:

Type	Description	Code	Description
0	echo reply	0	unused; data is returned from echo request
3	Destination unreachable	0	Network unreachable
		1	Host unreachable
		2	Protocol unreachable
		3	Port unreachable
		4	Flag needed but IP df set
		5	Source route failed
4	Source quench	0	unused
5	Redirect	0	Network redirect
		1	Host redirect
		2	Type of service net redirect
		3	Type of service host redirect
8	Echo request	0	unused
9	Router solicitation	0	unused
10	Router advertisement	0	unused
11	Time exceeded	0	IP time-to-live exceeded
11	Time exceeded	1	flag reassembly timeout
12	Parameter problem	0	Pointer field in ICMP header indicates error
13	Request time	0	unused
14	Reply time	0	unused
15	Information request	0	unused
16	Information reply	0	unused
17	Address mask request	0	unused
18	Address mask replyt	0	unused


1	IP version (current version:4)		IP Header length (in 32-bit words)	Type of service
2	Total packet length (in bytes from the beginning of IP header)			IP packet ID number (packet serial number)
3	Fragment bit flags	IP fragment offset (location of this packet fragment in original packet, in 8-byte units)
4	Time to live (maximum router hops remaining)			Transport protocol (ICMP:1, IGMP:2, TCP:6, UDP:17)
5	Header checksum (Note: Only the header is checksummed)
6	Source IP address
7	(4-byte IP address for host from which the packet originated)
8	Destination IP address
9	(4-byte IP address of the host where the packet is going)
10	IP options (Length of this part is usually 0 but may vary)

IP version: current version is 4
IP header length: length of the packet plus all the data it contains (which usually includes transport header like TCP) measured in 32-bit words.

Type of service: three bit fields to determine precedence, and one of each to indicate delay, throughput, and reliability:

precedence			delay	through put	relia bility	reserved
0	0	0

IP packet ID: the serial number of an IP packet (same for all fragments)
Fragment bit
source port: port number of the sender
destination port: port number of the receiver flag (3 bits):
- bit 0: reserved
- bit 1: value 0, may fragment value 1, do not fragment
- bit 2: value 0, last fragment value 1, more fragments
IP fragment offset: location of this fragment in original packet in 8-byte units
Time to live: number of router "hopes" possible

Transport protocol:

0	IP	# internet protocol, pseudo protocol number
1	ICMP	# internet control message protocol
2	IGMP	# internet group management protocol
6	TCP	# transmission control protocol
17	UDP	# user datagram protocol

Header checksum: one's complement sum of 16-bit words in the header (does not include data). For computing the checksum, the checksum field is zeroed.
Destination and source IP: 32-bit Internet addresses in network byte order.

IP options (length varies):

bits 3-8:

value=0: end of list		value=1: NOP
value=2: security		value=3: loose resource routing
value=4: internet timestamp		value=7: record route
value=8: stream ID		value=9: strict source routing