Stream Control Transmission Protocol (SCTP) Readdressing

Retransmission Triggerdraft-micchie-tsvwg-fastmsctp-01

Michio HondaKeio University

micchie@sfc.wide.ad.jp

SCTP Signaling on Migration between Different Access Networks

• Dynamic Address Reconfiguration (ADDIP) [RFC5061] support mobility between different access network– ASCONF Chunk(s) signal a peer endpoint of addition and

deletion of addresses in an SCTP Association• End-to-end mobility approach

Connectivity Disruption on Migration to Another Wireless Network

• Passing an area of bad wireless coverage

• Overhead to switch the wireless network to connect• Layer 2 configuration

• Scanning available base stations• Establishing link layer connectivity

• Layer 3 configuration• DHCP, Stateless Address

Autoconfiguration, DHCPv6

Transmission Delay Caused by Retransmission Time Outs (RTOs)

• During connectivity disruption on a mobile node, SCTP endpoints on the mobile node and the correspondent node fail to transmit packets

• After connectivity revival, the lost packets are not retransmitted until the next RTO expires• The RTO value is doubled on

each RTO expiration between 1 and 60 seconds [RFC 2988]

New Retransmission Trigger based on Readdressing Events

• In order to avoid extra transmission delay, immediate retransmission after the connectivity revival is required– Exchanging ASCONF and ASCONF-ACK Chunks indicates

connectivity revival on the SCTP Association – On the other hand, ASCONFs are transmitted not only due to

migration, but also due to other events• E.g., )Configuring new IP addresses on another wireless interface

without deletion of existing IP addresses

– Exchanging ASCONF and ASCONF-ACK due to migration should be distinguished from other events

New Retransmission Trigger based on Readdressing Events (2)

• New Retransmission Trigger– When following readdressing events occur, the endpoint MAY

retransmit unacknowledged data without waiting for the next RTO expiration• Change of all local addresses

– It occurs on the mobile node-side endpoint– Data transmitted from the previous addresses are never

acknowledged by the peer» Previous addresses are already deleted by the ASCONF

• Change of all destination addresses– It occurs on the correspondent node-side endpoint– Data transmitted to the previous destinations are never delivered

» Previous destinations are already deleted by the ASCONF

New Retransmission Trigger based on Readdressing Events (3)

• A readdressing event means change of the communication path– Mobile node-side endpoint

• SCTP congestion control is based on a destination• When at least 1 RTO expires during connectivity disruption

– RTO has been doubled and cwnd has been decreased to 1• When no RTO expires during connectivity disruption

– RTO has not been doubled, but cwnd is increased – Correspondent node-side endpoint

• RTO and cwnd are naturally reset due to change of the destination

• Path parameters, such as cwnd and RTO should be reset and retransmission should be started with slow-start– RTO SHOULD be set to RTO.Initial on the retransmission– cwnd SHOULD be set to <= 2*MTU on the retransmission

Examples of Experimental Results

• 3.8 seconds disconnected period occurs before change of the IP address

• Large improvement case• RTO expires long after

connectivity revival

• 6.8 seconds disconnected period occurs before change of the IP address

• Small improvement case• RTO expires soon after

connectivity revival

Conclusion• A new retransmission trigger on SCTP– Migration to another access network often involves

connectivity disruption• Passing an area of bad wireless coverage• Layer 2/3 configuration

– During the connectivity disruption, SCTP endpoints fail to transmit data

– After connectivity revival, SCTP endpoints do not retransmit lost packets until the next RTO expiration

– The new retransmission trigger retransmits the packets based on readdressing event caused by migration• Connectivity revival is indicated by exchanging ASCONF Chunks

– Change of all local addresses or all destination addresses indicates migration