Enhancing TCP Performance: Understanding Maximum Window Size
Explore the concept of increasing the maximum window size of TCP to improve performance. Delve into discussions on the current limitations, proposals for enhancement, and the importance of understanding TCP sequencing. Discover insights on why the maximum window size must be less than 2^30 and whether distinguishing between old and new data segments is necessary. Learn about potential solutions and signaling approaches for implementing these enhancements.
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Presentation Transcript
Increasing Maximum Window Size of TCP Yoshifumi Nishida Hirochika Asai
Background Current maximum window size 2^30 - 2^14 (1,073,725,440) Bytes Defined by RFC7323 Can we increase this? TCP s seqnum space is 2^32 Nearly 4 times bigger than the maximum window size Why we cannot use 2^31 2^15 bytes? Use shiftcount =15 for window scale option
Descriptions in RFC7323 TCP determines if a data segment is "old" or "new" by testing whether its sequence number is within 2^31 bytes of the left edge of the window, and if it is not, discarding the data as "old". Left Edge 2^31 bytes old old new
Why max window must be < 2^30 ? In worst case, receiver may receive left edge + 2 * window size seqnum from sender S seqnum for sender s left edge W window size Sender window Receiver Window S+2W+1 S S+W+1 S should be considered as old from S+2W+1 2W +1 should be less than 2^31 So, W must be < 2^30 Looks reasonable! But, is this really true?
Do we need to know old or new? Our answer: Probably not. Because important point is whether seqnum S is inside of window or not! If maxwin is 2^30 2^14 S is new from left ledge S is new from right ledge S Window If maxwin is 2^31 2^15 S is new from right ledge S is old from left ledge S S Window We cannot tell S is old or new. But, we can tell S is outside of the window!
Proposal Increase max shift count is window scale option Use 15 as max shift count New maximum window size will be 2^31-2^15 bytes
Signaling Possible approach Use new TCP option Notify peer using new max shift count No signaling. Just use shiftcount=15 in WS option RFC7323 allows to receive shiftcount=15 Parse as shiftcount=14 As long as a node can advertise new maximum window size (or close value), there is almost no harm Conventional peer will parse it as current max window size (or close value)
Thank You! Please read the draft for more detailed info
