Bug 195617 - TCP may send small segments limited to initial window size
Summary: TCP may send small segments limited to initial window size
Status: NEW
Alias: None
Product: Networking
Classification: Unclassified
Component: IPV4 (show other bugs)
Hardware: All Linux
: P1 normal
Assignee: Stephen Hemminger
URL:
Keywords:
Depends on:
Blocks:
 
Reported: 2017-04-30 00:50 UTC by Ambroz Bizjak
Modified: 2017-04-30 00:50 UTC (History)
0 users

See Also:
Kernel Version: 4.9.24
Subsystem:
Regression: No
Bisected commit-id:

Attachments
packet trace (1.54 KB, application/octet-stream)
2017-04-30 00:50 UTC, Ambroz Bizjak 		Details
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Description Ambroz Bizjak 2017-04-30 00:50:22 UTC 
Created attachment 256141 [details]
packet trace

I am seeing what I believe is faulty behavior when a Linux client connects to a TCP/IPv4 server which advertises a very small initial window. After the client sends a small segment limited to this small window, the server ACKs the data and extends the window considerably, but the client proceeds to send very small segments.

Here is a summary of the packets:
- Client: SYN
- Server: SYN,ACK,Win=1
- Client: ACK
- Client: ACK,Data=1B
- Server: ACK,Win=49984
- Client: ACK,Data=47B
- Client: ACK,Data=48B
- Client: ACK,Data=48B
- Client: ACK,Data=48B
...

This example was with initially advertised window (WIN) equal to 1, but generally it appears that Linux will limit outgoing segments to no more than max(48,WIN).

I have tested the server with a Windows 10 client which does not have this problem, the client starts sending fully sized segments when the window is extended.
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