This document discusses TCP and its turbulent relationship with mobile networks. It provides an overview of TCP including that it is connection-oriented, reliable, uses cumulative ACKs, delivers packets in-order, is full-duplex, and uses a sliding window. The document then discusses issues TCP has over mobile networks, including that the congestion window remains small due to disconnections and high bit error rates. It proposes collecting data from mobile operators anonymously to send to the IETF to develop new, more mobile-friendly protocols.

1. @thisNatasha
TCP and
Mobile Networks
Turbulent Relationship
Natasha Rooney
@thisNatasha
GSMA Web & Internet Technologist
IETF SLIM Chair

2. @thisNatasha Website https://www.ietf.org/
StandardsandIETF

3. @thisNatasha
IETF TCP RFCs
“The original specification for TCP is [RFC0793] , although
some errors in that RFC are corrected in the Host
Requirements RFC, [1122RFC]. Since then, specifications for
TCP have been revised and extended to include clarified and
improved congestion control behavior [RFC5681] [RFC3782]
[RFC3517] [RFC3390] [RFC3168], retransmission timeouts
[RFC6298] [RFC5682] [RFC4015], operation with NATs
[RFC5382], acknowledgment behavior [RFC2883], security
[RFC6056] [RFC5927] [RFC5926], connection management
[RFC5482], and urgent mechanism implementation guidelines
[RFC6093]. There have also been a rich variety of experimental
modifications covering retransmission behaviors [RFC5827]
[RFC3708], congestion detection and control [RFC5690]
[RFC5562] [RFC4782] [RFC3649] [RFC2861], and other
features…”
StandardsandIETF

4. @thisNatasha
A Protocol Stack
TCPIntro

5. @thisNatasha
Protocol Stack: IP Datagram
TCPIntro

6. @thisNatasha
TCP: Qualities
- Connection Orientated
- Reliable
- Cumulative ACKs
TCPIntro
- In-order Delivery
- Full Duplex
- Sliding window

7. @thisNatasha
- Connection Orientated
- Reliable
- Cumulative ACKs
- In-order Delivery
- Full Duplex
- Sliding window
TCP: Qualities
TCPIntro

8. @thisNatasha
- Connection Orientated
- Reliable
- Cumulative ACKs
- In-order Delivery
- Full Duplex
- Sliding window
TCP: Qualities
TCPIntro

9. @thisNatasha
- Connection Orientated
- Reliable
- Cumulative ACKs
TCP: Qualities
TCPIntro
- In-order Delivery
- Full Duplex
- Sliding window

10. @thisNatasha
Protocol Stack: TCP Header
TCPIntro
Source Port
(16 bits)
Destination Port
(16 bits)
Sequence Number
(32 bits)
Acknowledgement Number
(32 bits)
Header
Length
(4 bits)
Resv
(4 bits)
C
W
R
E
C
E
U
R
G
A
C
K
P
S
H
R
S
T
S
Y
N
F
I
N
Window SIze
(16 bits)
TCP Checksum
(16 bits)
Urgent Pointer
(16 bits)
Options
(Variable)

11. @thisNatasha
Why Does TCP Suck
Over Mobile?
TCPRetransmitandCongestion

12. @thisNatasha
Missing a TCP Segment?
Need to retransmit!
Detect retransmission needs using ACKS:
1. Time based
2. Fast Retransmit
TCPRetransmitandCongestion

13. @thisNatasha
Missing a TCP Segment?
1. Time based
- After timeout will retransmit
- Retransmission Timeout (RTO)
based on RTT
- Binary Exponential Backoff: RTO
doubles for each new retransmit
- Major event.
TCPRetransmitandCongestion
timeout

14. @thisNatasha
Missing a TCP Segment?
2. Fast Retransmit
- Duplicate ACKs
- Usually 3
- RTT and RTO then increases
- SACK helps.
TCPRetransmitandCongestion

15. @thisNatasha
What if the Network is Congested?
TCPRetransmitandCongestion
Slow Start Congestion Avoidance

16. @thisNatasha
TCP Sawtooth
TCPRetransmitandCongestion

17. @thisNatasha
Issues on Mobile
TCPonMobile
Congestion Window (cwnd) remains small as…
- Disconnections means cwnd never reaches
optimum
- The effect of a High Bit Error Rate (BER)

18. @thisNatasha
How do we fix it…?
TCPonMobile
Current suggestions are bad and broken...
- TCP Optimisers interrupt traffic
- Create two TCP connections
- Optimise for only certain scenarios
- Rely on unencrypted traffic
- yeah…
Mobile Operators should not &
cannot solve this alone.

19. @thisNatasha
Heh. So, how do we fix it…?
SolveitatIETF!
Back to IETF!
- Evolve the core protocols
- Created Stack Evolution
Programme
- Mostly issues with Transport
- Need data to make protocol
improvements.
But operators don’t want to share data with IETF...

20. @thisNatasha
Collect Data at GSMA: horrid but convenient IPR rules.
Anonymise it: ain’t great but at least you’re getting
something.
Send to IETF: happy protocol makers!
Work together: to make a new set of mobile friendly
protocols.
Long Term Goal: better protocols, smoother
and faster internet experience on mobile, no
need for TCP optimisers in the network!
Collecting Data at GSMA
SolveitatIETF!

21. @thisNatasha
ありがとう！
Natasha Rooney
@thisNatasha
Especially: Patrick McManus (Mozilla), Martin Thomson (Mozilla)
and Mark Nottingham (Akamai) for being unasked mentors!
Phew!It’sover.Where’sthepizza?

22. @thisNatasha
Unused Slides Past this Point!

23. @thisNatasha

24. @thisNatasha
HOPs IETF Group
(soon to be MAP)