Read the four option bytes
02The UTO option occupies four octets in the TCP header’s option area. Its final two octets contain a granularity bit and a 15-bit unsigned suggestion.
The value field is zero in 1C 04 00 00 and 1C 04 80 00. Both are ignored, not adopted as a zero-second timeout.
Key idea: Kind 28, Length 4; G chooses seconds or minutes, and zero is reserved.
Terms
- Octet
- Eight bits, conventionally displayed as two hexadecimal digits.
- Granularity
- The unit used for the timeout number: seconds or minutes.
- Reserved value
- A value current senders must not use and receivers must ignore.
Rules & examples
Kind 28, Length 4; G chooses seconds or minutes, and zero is reserved.
- Kind MUST be 28 and Length MUST be 4. The last 16 bits are G in the high bit and a 15-bit unsigned User Timeout field.
- G = 0 means seconds; G = 1 means minutes. The example 180 seconds encodes as 1C 04 00 B4. Three minutes encodes as 1C 04 80 03; both suggest a 180-second interval.
- The 15-bit field ranges from 1 through 32767 for currently sendable values. Its maximum is 32767 seconds or 32767 minutes, depending on G. This is a wire range, not permission to accept every suggested timeout.
- A zero User Timeout field is reserved with either G value. Implementations MUST NOT send it and MUST ignore it on reception (§3.4). Neither 1C 04 00 00 nor 1C 04 80 00 means an immediate abort.
- TCP option space is limited to 40 bytes. Other options can consume the space needed for this four-octet option (§3). The decoder is bounded to these four bytes, not a full TCP packet parser.
Widths in the field drawing follow the 8/8/1/15-bit split; printed hexadecimal rows are real option bytes. TCP header/checksum and other options are outside this fixture.
Sources
New to this? Start hereA quick picture + helpful background
Suggest how long to keep waiting
Imagine two people holding a shared call open during a brief outage. Telling the other person your waiting limit can help, but they still choose their own limit. This analogy is not a promise that the network or either endpoint will stay available.
Helpful first, if these ideas are unfamiliar
These pictures are analogies to help you learn. The linked RFC explains the actual rules.
Related RFCs and changes
“Updates” changes part of an earlier RFC. “Obsoletes” replaces it. This list shows selected related documents.