One-line summary: Soft-state in BS's (the snoop agent) are used to perform local retransmissions across a wireless link to avoid end-to-end congestion control and retransmissions; IP-multicast is used to allow secondary BS's to cache packets destined to a MH in case that MH hands off to one of the secondary BS's.

Overview/Main Points

• Motivation: The usual TCP problems over wireless links are given as motivation for this work. Also, interruption during handoff further exascerbates TCP performance.
• Snoop agent:
  • FH->MH:For data transfer from a fixed host (FH) to a mobile host (MH), the base-station routing code is modified by adding a network-level snoop module. Snoop keeps a cache of unacknowledged FH->MH packets. When packet loss is detected (duplicate acknowledgement or local timeout), snoop retransmits the lost packet, hiding the FH from duplicate acknowledgements, thereby preventing congestion control from kicking in. If a new, in-sequence packet arrives from the FH, it is cached and forwarded to the MH. If an old packet rearrives, it is a sender retransmission, and is forwarded to the mH, and the snoop retransmit counter reset. If a new, out-of-sequence packet arrives, it is marked as having experienced congestion loss, and is forwarded.
    A new ACK from the MH allows snoop to clean its cache, update its RTT estimate, and relay the ACK to the sender. A spurious ack is ignored. A duplicate ack for a packet not in the snoop cache or marked as retransmitted by the sender is forwarded to the, since the FH TCP stack maintains state based on this ACK. A duplicate ack that snoop doesn't expect indicates loss on the wireless link - the lost packet is retransmitted with high priority. A duplicate ack that snoop does expect (based on the highest expected received sequence number) is discarded; such expected dupacks occur after snoop has retransmitted a packet but before the MH sequence numbers reflect the retransmitted packet.
  • MH->FH: Modification to the TCP state at the MH was done to process TCP negative acknowledgements (NACKs), which are generated at the BS for packets lost within a transmission window. The MH uses these NACKs to selectively retransmit lost packets.
• Routing for Handoff: A mobile-IP like strategy is used. The major difference is that each MH is assigned a home address and a temporary IP multicast address, instead of a home address and a foreign address. Packets are forwarded from the HA to the MH's primary BS, which forwards them to the MH. Secondary BSs that are identified as potential handoff targets also are asked join the multicast group, and buffer packets for the MH in case the MH hand-offs to its cell. Data "in flight" to the MH during a handoff is thus delivered directly fromthe new primary's buffered packets instead of causing data loss or forwarding delays. The new primary BS determines the first packet to forward based on the first ack recieved from the MH. The BS also maintains a finite-sized FIFO queue of buffered packets instead of keeping all of them; the size of the queue is determined by the expected handoff delay.
• Interaction: The new primary BS's snoop agent may not have cached all packets, since not all packets may have been buffered due to losses. Since snoop does not change TCP semantics, it is resistant to such gaps.
• Performance: Performance is strictly improved using these schemes, both in terms of handoff delay and throughput.

Relevance

Flaws

• Invariably, some overhead is incurred by a scheme such as snoop, although it does perform without extra packet copying.
• State and resources must be maintained by secondary BS's for the handoff routing to work.
• IP multicast must be used for the handoff routing to work - how much more expensive than plain multicast is IP multicast?
• The TCP stack on the MH had to be modified to get the fully snoop optimization.

  ---

  Back to index