Make TCP more Robust to Long Connectivity Disruptions Alexander - - PowerPoint PPT Presentation

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Make TCP more Robust to Long Connectivity Disruptions Alexander - - PowerPoint PPT Presentation

Jul 19, 2023 241 likes •429 views

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Motivation Disruption Indication Disruption Reaction Evaluation Discussion

Make TCP more Robust to Long Connectivity Disruptions

Alexander Zimmermann and Arnd Hannemann

Department of Computer Science, Informatik 4 RWTH Aachen University, Ahornstr. 55, 52074 Aachen, Germany

75th IETF TCPM Meeting - Stockholm, Sweden July 26, 2009

Alexander Zimmermann and Arnd Hannemann RWTH Aachen University draft-zimmermann-tcp-lcd-01.txt

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Motivation Disruption Indication Disruption Reaction Evaluation Discussion

Changes from previous draft version

draft-zimmermann-tcp-lcd-00

◮ Miscellaneous editorial changes in Section 1, 2 and 3 ◮ Section 2: Updated motivation for the algorithm

⇒ Congestion versus Non-Congestion Events/Loss ⇒ In-line with RFC “Improving the Robustness of TCP to Non-Congestion Events” [RFC4653]

◮ Section 4.1: Add basic idea of the algorithm ◮ Section 4.2: Update algorithm (suggestions Tim Shepard)

◮ Special case of the first received ICMP destination unreachable

after an RTO could be removed

◮ “Backoff cnt” variable was introduced so it is no longer

possible to perform more reverts than backoff

◮ Section 4.3: Expanded according to the algorithm changes

Alexander Zimmermann and Arnd Hannemann RWTH Aachen University draft-zimmermann-tcp-lcd-01.txt

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Motivation Disruption Indication Disruption Reaction Evaluation Discussion

Problem of Long Connectivity Disruptions (1/2)

Observation

◮ Disruptions in e2e path connectivity which last longer than

  • ne RTO cause suboptimal TCP performance

Problem statement

◮ TCP interprets segment loss as a sign of congestion

⇒ Means to detect loss: DUPACKs and RTO

◮ RTO case: (repeated) backoff(s) of the retransmission timer ◮ Deferred detection of connection re-establishment since TCP

has to wait until next RTO before retransmit again

Alexander Zimmermann and Arnd Hannemann RWTH Aachen University draft-zimmermann-tcp-lcd-01.txt

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Motivation Disruption Indication Disruption Reaction Evaluation Discussion

Problem of Long Connectivity Disruptions (2/2)

100000 200000 300000 400000 500000 600000 0.000 10.000 20.000 30.000 40.000 50.000 60.000 70.000 Sequence Offset [b] Relative Time [s] Advertised Window Cumulative ACK Sent Segments Retransmitted Segment ICMP Destination Unreachable

SYN R R R R R R R R R R R RR R R R R R R R R R R R RR RR R R R R R RR R R R RR R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R RR R R R R

Alexander Zimmermann and Arnd Hannemann RWTH Aachen University draft-zimmermann-tcp-lcd-01.txt

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Motivation Disruption Indication Disruption Reaction Evaluation Discussion

Solution for Long Connectivity Disruptions

Disruption Indication

◮ Disambiguate true congestion loss from non-congestion loss

caused by long connectivity disruptions

◮ Exploit standard ICMP destination unreachable messages

during timeout-based loss recovery

Disruption Reaction

◮ Connectivity disruption loss: undoing one RTO backoff if an

ICMP unreachable message reports on a lost retransmission ⇒ Enables prompt detection when connectivity is restored

◮ Congestion loss: Retaining std. timeout-based loss recovery

⇒ Sender-only modification

Alexander Zimmermann and Arnd Hannemann RWTH Aachen University draft-zimmermann-tcp-lcd-01.txt

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Motivation Disruption Indication Disruption Reaction Evaluation Discussion

Connectivity Disruption Indication

Queue of the router experiencing the link outage is

◮ Deep enough: buffers all incoming packets

⇒ Cause only variation in delay ⇒ Eifel [RFC3522], F-RTO [RFC4138]

◮ Not deep enough: drops packets; discards according route

⇒ TCP sender is notified about the dropped packets via ICMP destination unreachable messages [RFC1812]

Idea

◮ Interpret ICMP unreachable messages of code

0 (net unreachable) or code 1 (host unreachable) as long connectivity disruption indication

Alexander Zimmermann and Arnd Hannemann RWTH Aachen University draft-zimmermann-tcp-lcd-01.txt

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Motivation Disruption Indication Disruption Reaction Evaluation Discussion

ICMP messages as Connectivity Disruptions Indication

Issues

◮ Do not ignore congestion indication from the network ◮ ICMP messages do not necessarily operate on the same

timescale as the packets eliciting them [RFC1812]

◮ ICMP messages are subject to rate limiting [RFC1812]

Useful

◮ ICMP unreachable messages contain the IP header of the

datagram eliciting the ICMP messages plus the first 64 bit

  • f the payload [RFC0791]

⇒ Allows to identify which segment of the respective connection triggered the ICMP unreachable message

Alexander Zimmermann and Arnd Hannemann RWTH Aachen University draft-zimmermann-tcp-lcd-01.txt

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Motivation Disruption Indication Disruption Reaction Evaluation Discussion

Connectivity Disruption Reaction

Goal

◮ Prompt detection of the end of the connectivity disruption ◮ Retaining appropriate behavior in case of congestion

Basic Idea

◮ Increase the TCP’s retransmission frequency by undoing one

RTO backoff if ICMP message reports on a presumably lost retransmission

◮ If either the (re-)transmission itself, or the corresponding

ICMP message is dropped the backoff is performed

Alexander Zimmermann and Arnd Hannemann RWTH Aachen University draft-zimmermann-tcp-lcd-01.txt

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Motivation Disruption Indication Disruption Reaction Evaluation Discussion

The Algorithm (1/2)

State: retransmission timer is expired

  • 1. Initialize backoff counter:

◮ Backoff cnt := 0

  • 2. Placeholder for standard TCP timeout-based loss recovery

◮ In particular RFC 2988 steps (5.4) – (5.6) go here

  • 3. If RTO was backed off in step 2, then:

◮ Backoff cnt := Backoff cnt + 1

  • 4. Wait either for

◮ RTO, then Goto 2 ◮ ACK, then Goto 9 ◮ ICMP unreachable, then Goto 5

  • 5. If Backoff cnt ≥ 0, i.e., if an undoing of the last RTO backoff

is allowed, then Goto 6, else Goto4

Alexander Zimmermann and Arnd Hannemann RWTH Aachen University draft-zimmermann-tcp-lcd-01.txt

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Motivation Disruption Indication Disruption Reaction Evaluation Discussion

The Algorithm (2/2)

  • 6. Extract TCP segment included in ICMP unreachable:

◮ SEG := Extract(ICMP DU)

  • 7. If SEG.SEQ == SND.UNA, i.e., ICMP DU reports on the oldest
  • utstanding segment, undo last RTO backoff:

◮ RTO := RTO/2 ◮ Backoff cnt := Backoff cnt − 1

  • 8. If the RTO expires due to undoing in step 7, then Goto 2,

else Goto 4

  • 9. Placeholder for standard TCP behavior when an ACK has

arrived; no further processing

Alexander Zimmermann and Arnd Hannemann RWTH Aachen University draft-zimmermann-tcp-lcd-01.txt

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Motivation Disruption Indication Disruption Reaction Evaluation Discussion

Methodology

Code

◮ Algorithm is implemented in Linux 2.6.28.7 ◮ Publicly available: http://www.umic-mesh.net/downloads

Setup

◮ Wireless mesh network with 51 nodes ◮ Routing protocol: OLSR [RFC3626]; standard parameter ◮ Path length: 2 to 4 hops ◮ Two parallel flows: standard and patched ◮ 60 s bulk TCP transfer; 500 measurements

Alexander Zimmermann and Arnd Hannemann RWTH Aachen University draft-zimmermann-tcp-lcd-01.txt

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Motivation Disruption Indication Disruption Reaction Evaluation Discussion

Evaluation (1/3)

100000 200000 300000 400000 500000 600000 0.000 10.000 20.000 30.000 40.000 50.000 60.000 70.000 Sequence Offset [b] Relative Time [s] Advertised Window Cumulative ACK Sent Segments Retransmitted Segment ICMP Destination Unreachable

SYN R R R R R R RR R R R RR R R R R R R R R R RR RR RR R R R RR RR R R R RR R R R R R R R R R R R R R R R R R RRRR R R R R R R R R R R R R R R R R RR R R R R

100000 200000 300000 400000 500000 600000 700000 800000 0.000 10.000 20.000 30.000 40.000 50.000 60.000 70.000 Sequence Offset [b] Relative Time [s] Advertised Window Cumulative ACK Sent Segments Retransmitted Segment ICMP Destination Unreachable

SYN RR R R R R R R RRR RR R R R R R R R R R R R R R R R R R R R R R R RR R R R R R RR RR RR R RR R R R R R RRR R R RR R R R R RRR R R R R R R R R R R R RR R R R RR R R R R R RRRRRRRRRRRRRRRRRRRRR R R RR RR RR R R R R RR R R R R R R R R R R R R R R R RR R R R R R R R R R R R R R R R R R

Alexander Zimmermann and Arnd Hannemann RWTH Aachen University draft-zimmermann-tcp-lcd-01.txt

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Motivation Disruption Indication Disruption Reaction Evaluation Discussion

Evaluation (2/3)

540000 550000 560000 570000 580000 590000 600000 610000 620000 630000 42.000 43.000 44.000 45.000 46.000 47.000 48.000 49.000 Sequence Offset [b] Relative Time [s] Advertised Window Cumulative ACK Sent Segments Retransmitted Segment ICMP Destination Unreachable

R R R R R R R R R R R R R R R R R R R R

Alexander Zimmermann and Arnd Hannemann RWTH Aachen University draft-zimmermann-tcp-lcd-01.txt

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Motivation Disruption Indication Disruption Reaction Evaluation Discussion

Evaluation (3/3)

50000 100000 150000 200000 250000 300000 350000 400000 450000 500000 0.000 10.000 20.000 30.000 40.000 50.000 60.000 70.000 Sequence Offset [b] Relative Time [s] Advertised Window Cumulative ACK Sent Segments Retransmitted Segment ICMP Destination Unreachable

SYN R R R RR R R R R R R R R R R R R R R R R R R R R R R RST_IN RST_IN RST_IN RST_IN RST_IN RST_IN RST_IN

200000 400000 600000 800000 1000000 1200000 1400000 1600000 1800000 2000000 0.000 10.000 20.000 30.000 40.000 50.000 60.000 70.000 Sequence Offset [b] Relative Time [s] Advertised Window Cumulative ACK Sent Segments Retransmitted Segment ICMP Destination Unreachable

SYN R R RRRRRRRRRRRRRRRRRR R R R R R R R R R R R R R R RR R R R R R R R R R R R R R R R R R R R RR R R R R R R R R R RRRRR R R RR R R RR R R RR RR R RR R R R R R R R R R RRRRRRRRRRRRRRRRRRRR R R R RRR R R R R R R R RR R R R R R R R R R R R R R R R R R R R R R R R R R R R RST_IN RST_IN RST_IN RST_IN RST_IN

Alexander Zimmermann and Arnd Hannemann RWTH Aachen University draft-zimmermann-tcp-lcd-01.txt

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Motivation Disruption Indication Disruption Reaction Evaluation Discussion

Features

We . . .

◮ React only on ICMP unreachable messages during

timeout-based loss recovery that reporting on SND.UNA

◮ Fall back to usual backoff in case there is congestion along

the path after connectivity is restored

◮ Modify only the sender ⇒ Easy to implement

We do not . . .

◮ Alter TCP’s behavior in case of

◮ slow-start, steady-state or fast recovery ◮ timeout-based loss recovery with CWND > 1 ◮ no receiving ICMP unreachable messages

◮ Probe for route repair faster than slowest TCP can send

Alexander Zimmermann and Arnd Hannemann RWTH Aachen University draft-zimmermann-tcp-lcd-01.txt

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Motivation Disruption Indication Disruption Reaction Evaluation Discussion

Special cases

Retransmission ambiguity problem

◮ No problem since the assumption that the ICMP message

provides evidence that one link loss was wrongly considered as congestion loss is still true

Wrapped sequence numbers

◮ Late ICMP unreachable message reporting on an old error

may coincidentally fit as input

◮ Possibility is minuscule, since ICMP message must contain the

exact sequence number of SND.UNA, while at the same TCP is in timeout-based loss recovery

Alexander Zimmermann and Arnd Hannemann RWTH Aachen University draft-zimmermann-tcp-lcd-01.txt

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Motivation Disruption Indication Disruption Reaction Evaluation Discussion

Next steps

Any interest from the WG?

Alexander Zimmermann and Arnd Hannemann RWTH Aachen University draft-zimmermann-tcp-lcd-01.txt