Options or payload? Costin Raiciu UCL IETF 78, Maastricht

Outline We need explicit data ACKs How do we encode Data ACK? – Data ACKs in the payload create problems – Data ACKs must be encoded in options How do we encode DSN mapping?

We need explicit data ACKs 10,1 Data ACK Window Receive Window [, ] [1, ] [1,2] ACK 10,WND 1 20,2 ACK 20, WND ,3 Drop Packet loss = congestion

We need explicit data ACKs 10,1 Data ACK Window Receive Window [, ] [1, ] [2, ] ACK 10,WND 1 20,2 ACK 20, WND , APP READ 20,3

Data ACKs must be explicit Otherwise flow control does not correctly work This may matter more often than we think – Stacks normally use as little buffering as possible How do we encode data ACKs?

Problems with Data ACKs in the payload Data ACKs are congestion controlled Head of line blocking of Data ACKs due to packet loss affect forward throughput – HoL blocking during Fast Retransmit, Timeouts Data ACKs are flow controlled – Potential deadlocks, can in principle be avoided

Experiment OpenVPN plaintext tunneling – TCP over TCP – TCP over UDP Baseline: – TCP over TCP tunneling: 93Mbps – TCP over UDP tunneling: 94Mbps 100Mbps,<1ms OpenVPN Client OpenVPN Server Router

Experiment: ACK path congestion TCP transfers OpenVPN Transfer Throughput(Mb/s) Number of Reverse Path TCP connections

Fixing Head of Line blocking Possible fix: use escape sequences instead of TLV – Overhead to escape payload Need to scan all payload in kernel No hardware acceleration (for now) – Need to linearly search payload when HoL – Performance will be poor Better fix: use options

Problems with Data ACKs in payload Head of Line Blocking Congestion Control Flow Control PayloadPoor performance FIX: do not cc signaling – but what if data and ACKs are mixed? Possible deadlock: FIX: save last 1kb of receive buffer for signaling OptionsNo problem

What about Data Sequence Mapping? In payload – Can get away with simpler solutions for finding payload length changes due to middleboxes – No space restrictions In options – We are using options anyways, we might as well keep the payload unmodified – Space is tighter, but can fit multipath without removing existing options – Future TCP extensions require extending the options space

Conclusions Options are the architecturally cleaner approach Can get through middleboxes – Otherwise protocol detects and quickly falls back to TCP Data ACKs must be transmitted out of band – Options are the only feasible alternative Data Sequence Mapping may be sent in band – Slight advantage for payload – Which one do people prefer?

Backup Slides

Experiment: bidirectional transfer TCP transfers OpenVPN Transfer Throughput(Mb/s) Number of TCP connections