Using the Flow Label Field in IPv6 (RFC1809)
Original Publication Date: 1995-Jun-01
Included in the Prior Art Database: 2019-Feb-12
Internet Society Requests For Comment (RFCs)
The purpose of this memo is to distill various opinions and suggestions of the End-to-End Research Group regarding the handling of Flow Labels into a set of suggestions for IPv6. This memo provides information for the Internet community. This memo does not specify an Internet standard of any kind.
Network Working Group C. Partridge Request for Comments: 1809 BBN Systems and Technologies Category: Informational June 1995
Using the Flow Label Field in IPv6
Status of this Memo
This memo provides information for the Internet community. This memo does not specify an Internet standard of any kind. Distribution of this memo is unlimited.
The purpose of this memo is to distill various opinions and suggestions of the End-to-End Research Group regarding the handling of Flow Labels into a set of suggestions for IPv6. This memo is for information purposes only and is not one of the IPv6 specifications. Distribution of this memo is unlimited.
This memo originated as the report of a discussion at an End-to-End Research Group meeting in November 1994. At that meeting the group discussed several issues regarding how to manage flow identifiers in IPv6. A report of the meeting was then circulated to the IPv6 community. Feedback from that community resulted in changes to this memo and in changes to the IPv6 specification to fix some minor problems the End-to-End Group had raised.
While many of the ideas in this memo have found their way into the IPv6 specification, the explanation of why various design decisions were made have not. This memo is intended to provide some additional context for interested parties.
Brief Description of the Flow Label
The current draft of the IPv6 specification states that every IPv6 header contains a 24-bit Flow Label. (Originally the specification called for a 28-bit Flow ID field, which included the flow label and a 4-bit priority field. The priority field is now distinct, for reasons discussed at the end of this memo).
Partridge Informational [Page 1]
RFC 1809 June 1995
The Flow Label is a pseudo-random number between 1 and FFFFFF (hex) that is unique when combined with the source address. The zero Flow Label is reserved to say that no Flow Label is being used. The specification requires that a source must not reuse a Flow Label value until all state information for the previous use of the Flow Label has been flushed from all routers in the internet.
The specification further requires that all datagrams with the same (non-zero) Flow Label must have the same Destination Address, Hop- by-Hop Options header, Routing Header and Source Address contents. The notion is that by simply looking up the Flow Label in a table, the router can decide how to route and forward the datagram without examining the rest of the header.
Flow Label Issues
The IPv6 specification originally left open a number of questions, of which these three were among the most important:
1. What should a router do if a datagram with a (non-zero) Flow Label arrives and the router has no state for that Flow Label?
2. How does an internet flush old Flow Labels?
3. Which datagrams should carry (non-zero) Flow Labels?
This memo summarizes the End-to-End Group’s attempts to answer these questions.
What Does a Router Do With Flow Labels for Which...