1. TCP over Constrained-Node Networks draft-gomez-lwig-tcp-constrained- node-networks-01
Carles Gomez
Universitat Politècnica de Catalunya (UPC)/Fundació i2cat
Jon Crowcroft
University of Cambridge
IETF 97 – Seoul, November 2016

2. Status draft-gomez-core-tcp-constrained- node-networks-00
Presented in Berlin (LWIG and TCPM WGs)
draft-gomez-lwig-tcp-constrained- node-networks-00
Not modified
draft-gomez-lwig-tcp-constrained- node-networks-01
Several updates

3. Motivation
Several application layer protocols being used for the Internet of Things (IoT)
Constrained Application Protocol (CoAP)
Originally over UDP
CoAP over TCP in progress
To overcome middlebox problems
HTTP/2 and HTTP/1.1
XMPP
MQTT
TCP is being / will be used in many IoT scenarios
Offer simple measures for suitable TCP implementation/operation over CNNs
TCP

4. Updates in -01 (I/X) RFC 2119 language removed 1. Introduction
Purpose is to document how TCP can be used in CNNs (i.e. not proposing a new TCP variant)
1. Introduction
XMPP added to the list of protocols used in CNNs that use TCP
2. Characteristics of CNNs relevant for TCP
Often, low transmission rates (typically < 1 Mbit/s)

5. Updates in -01 (II/X) 3. Scenario Asymmetric Resource availability
Constrained devices: data sent > data received
Majority of constrained devices will be sensors

6. Updates in -01 (III/X) 4.1. TCP connection initiation
Typically initiated by the constrained device
Better support sleep periods in some RDC techniques
4.2. Maximum Segment Size
If a link layer offers MTU > 1280 bytes, still set MSS so that IPv6 datagram size ≤ 1280 bytes
Avoid issues with Internet links with not as high MTU

7. Updates in -01 (IV/X) 4.3. Window Size 4.4. RTO
Recommends single-MSS window
Receive and transmit
If device can afford it, 5-MSS window may be useful
Allow Fast Retransmit and Fast Recovery
4.4. RTO
draft-ietf-core-cocoa outperforms RFC 6298 RTO-based improvements (Peak Hopper, Linux RTO)
TBD: how to proceed?
Using CoCoA for TCP conflicts with RFC 6298 and Karn algorithm...

8. Updates in -01 (V/X) 4.5. TCP connection lifetime
Long TCP connection lifetime
Minimize overhead
However, firewalls may delete filter state records early
TCP keep-alives not useful to solve the issue
Short TCP connection lifetime
TCP Fast Open (TFO) - RFC 7413
Data carried in SYN or SYN-ACK segments
Cookie must be obtained, included in SYNs and refreshed
More efficient than frequently opening new TCP connections (with traditional 3WHS)
As long as cookie refresh rate well below new connection rate

9. Updates in -01 (VI/X) 4.7. TCP options
For single-MSS receive/transmit window
Not supporting, and ignoring if received
Window Scale
TCP timestamps
SACK, SACK-Permitted
For less constrained devices
SACK avoids unnecessary retries, and reduces latency, bandwidth and energy consumption

10. Updates in -01 (VII/X) 4.8. Delayed Acknowledgments
Device advertising single-MSS receive window needs to avoid supporting delayed ACKs
Avoid contributing up to 500 ms extra delay to the RTT
Not recommended in CNNs
Since traffic mostly of transactional type
Transaction size often < MSS
Could be useful to reduce # of ACKs in bulk transfers
Infrequent, compared to rest of transactions
5. Security considerations
If TFO used, security considerations of RFC 7413 apply

11. Updates in -01 (VIII/X)
Annex: TCP implementations for constrained devices
uIP
TCP/IP stack for 8- and 16-bit CPUs
Code size ~5 kB (checksumming, IP, ICMP and TCP)
Global, single-packet sized buffer for incoming packets
No buffer for outgoing data
MSS not modified during a connection
Stop-and-wait
Avoids sliding window operations (32-bit arithmetic, expensive on 8-bit CPUs)

12. Updates in -01 (IX/X)
Annex: TCP implementations for constrained devices
lwIP
TCP/IP stack for 8- and 16-bit CPUs
Code size: ~14 kB to ~22 kB
Memory management, checksumming, network interfaces, IP, ICMP and TCP
TCP code size: ~9 kB to ~14 kB
Buffering of incoming and outgoing data
Applications decoupled from the network stack
Transmission window greater than a single segment
Slow start, congestion avoidance, fast retransmit, fast recovery
SACK and Window Scale not implemented

13. Updates in -01 (X/X)
Annex: TCP implementations for constrained devices
RIOT
TBD

14. WG adoption ?
IETF 97 – Seoul, November 2016