RFC Excerpts

Contents:

IP Datagram Header
TCP Segment Header
ICMP Parameter Problem Message Header
Retrieving RFCs

" Troubleshooting TCP/IP" refers to specific TCP/IP headers that are documented here. This is not an exhaustive list of headers; only the headers used in the troubleshooting examples in " Troubleshooting TCP/IP" are covered:

Each header is presented using an excerpt from the RFC that defines the header. These are not exact quotes; the excerpts have been slightly edited to better fit this text. However, the importance of using primary sources for troubleshooting protocol problems is still emphasized. These headers are provided here to help you follow the examples in " Troubleshooting TCP/IP". For real troubleshooting, use the real RFCs. You can obtain your own copies of the RFCs by following the instructions at the end of this appendix.

IP Datagram Header

This description is taken from pages 11 to 15 of RFC 791, Internet Protocol. 

Internet Header Format 0 1 2 3 
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 
 |Version| IHL |Type of Service| Total Length | 
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 
 | Identification |Flags| Fragment Offset | 
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 
 | Time to Live | Protocol | Header Checksum | 
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 
 | Source Address | 
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 
 | Destination Address | 
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 
 | Options | Padding | 
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Version: 4 bits The Version field indicates the format of the internet header. 
 This document describes version 4. IHL: 4 bits Internet Header Length is the length of the internet header in 32 
 bit words. The minimum value for a correct header is 5. Type of Service: 8 bits The Type of Service indication the quality of service desired. 
 The meaning of the bits is explained below. Bits 0-2: Precedence. 
 Bit 3: 0 = Normal Delay, 1 = Low Delay. 
 Bits 4: 0 = Normal Throughput, 1 = High Throughput. 
 Bits 5: 0 = Normal Reliability 1 = High Reliability. 
 Bit 6-7: Reserved for Future Use. 0 1 2 3 4 5 6 7 
 +-----+-----+-----+-----+-----+-----+-----+-----+ 
 | | | | | | | 
 | PRECEDENCE | D | T | R | 0 | 0 | 
 | | | | | | | 
 +-----+-----+-----+-----+-----+-----+-----+-----+ Precedence 111 - Network Control 
 110 - Internetwork Control 
 101 - CRITIC/ECP 
 100 - Flash Override 
 011 - Flash 
 010 - Immediate 
 001 - Priority 
 000 - Routine Total Length: 16 bits Total Length is the length of the datagram, measured in octets 
 (bytes), including internet header and data. Identification: 16 bits An identifying value assigned by the sender to aid in assembling 
 the fragments of a datagram. Flags: 3 bits Various Control Flags. The Flag bits are explained below: 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. 0 1 2 
 +---+---+---+ 
 | | D | M | 
 | 0 | F | F | 
 +---+---+---+ Fragment Offset: 13 bits This field indicates where in the datagram this fragment belongs. 
 The fragment offset is measured in units of 8 octets (64 bits). 
 The first fragment has offset zero. Time to Live: 8 bits This field indicates the maximum time the datagram is allowed to 
 remain in the internet system. Protocol: 8 bits This field indicates the Transport Layer protocol that the data 
 portion of this datagram is passed to. The values for various 
 protocols are specified in the "Assigned Numbers" RFC. 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. The checksum 
 algorithm is: The checksum field is the 16 bit one's complement of the one's 
 complement sum of all 16 bit words in the header. For purposes 
 of computing the checksum, the value of the checksum field is 
 zero. Source Address: 32 bits The source IP address. See "Delivering the Data" for a 
 description of IP addresses. Destination Address: 32 bits The destination IP address. See "Delivering the Data" for a description of IP 
 addresses. Options: variable The options may or may not appear in datagrams, but they must be 
 implemented by all IP modules (host and gateways). No options
 were used in any of the datagrams examined in "
 Troubleshooting TCP/IP".