IP Protocol Details

1. IP Protocol Details Yen-Cheng Chen ycchen@ncnu.edu.tw

2. IP Header RFC 791

3. IP Version, IHL • Version: 4 bits • Indicates the format of the internet header. • IHL: 4 bits • The length of the internet header in 32 bit words, and thus points to the beginning of the data. • Note that the minimum value for a correct header is 5.

4. Precedence D T R Type of Service • Provides an indication of the abstract parameters of the quality of service desired. • Bits 0-2: Precedence. • Bit 3: 0 = Normal Delay, 1 = Low Delay. • Bits 4: 0 = Normal Throughput, 1 = High Throughput. • Bits 5: 0 = Normal Relibility, 1 = High Relibility. • Bit 6-7: Reserved for Future Use. 0 1 2 3 4 5 6 7

5. RFC 1349

6. Total Length, Identification • Total Length: 16 bits • Total Length is the length of the datagram, measured in octets, including internet header and data. • Identification: 16 bits • An identifying value assigned by the sender to aid in assembling the fragments of a datagram.

7. 0 DF MF Flags, Fragment Offset • Flags: 3 bits • Various Control Flags. • Bit 0: reserved, must be zero • Bit 1: (DF) • 0 = May Fragment, 1 = Don't Fragment. • Bit 2: (MF) • 0 = Last Fragment, 1 = More Fragments. • Fragment Offset: 13bits • This field indicates where in the datagram this fragment belongs. The fragment offset is measured in units of 8 octets (64 bits).

8. Time To Live (TTL) • Time to Live: 8 bits • Indicates the maximum time the datagram is allowed to remain in the internet system. • If this field contains the value zero, then the datagram must be destroyed. • This field is modified in internet header processing. • Measured in units of seconds, but since every module that processes a datagram must decrease the TTL by at least one even if it process the datagram in less than a second. • To bound the maximum datagram lifetime.

9. Traceroute RFC 1393 • To provide a trace of the path the packet took to reach the destination. • Operates by first sending out a packet with a Time To Live (TTL) of 1. The first hop then sends back an ICMP error message indicating that the packet could not be forwarded because the TTL expired. • The packet is then resent with a TTL of 2, and the second hop returns the TTL expired. This process continues until the destination is reached. • Record the source of each ICMP TTL exceeded message http://www.visualroute.com/

10. Protocol • Protocol: 8 bits http://www.iana.org/assignments/protocol-numbers http://www.iana.org/assignments/protocol-numbers

11. Header Checksum • Header Checksum: 16 bits • A checksum on the header only. • Since some header fields change (e.g., time to live), this is recomputed and verified at each point that the internet header is processed.

12. 0 1 2 3 4 5 6 7 Copy Class Option Number Options • There may be zero or more options. • Each option can be • a 8-bit Option-Type • Option-Type + Option-Length + Option-Data • Option-Type

13. Option-Type: Copy • Copy: 1 bit • The copied flag indicates that this option is copied into all fragments on fragmentation. • 0 = not copied • 1 = copied 0 1 2 3 4 5 6 7 Copy Class Option Number

14. Option-Type: Class • 0 (00) = control • 1 (01) = reserved for future use • 2 (10) = debugging and measurement • 3 (11) = reserved for future use 0 1 2 3 4 5 6 7 Copy Class Option Number

15. Option-Type: Option Number • Option Number: 5 bits • 0 = Endof Option list • 1 = No Option • 2 = Security • 3 = Loose Source Routing • 4 = Timestamp • 7 = Record Route • 8 = Stream ID • 9 = Strict Source Routing 0 1 2 3 4 5 6 7 Copy Class Option Number

16. Source Routing • The sender of a packet can specify the route that a packet should take through the network. • Loose Source (Record) Route (LSRR) • Sender gives one or more hops that the packet must go through. • Strict Source Routing • Sender specifies the exactroute the packet must take.