12.  Buffering and congestion control

      One of the major factors in the performance of a protocol is the buffering policy used. Lack of a proper buffering policy can force packets to be dropped, cause falsified windowing information to be emitted by protocols, fragment host memory, degrade the overall host performance, etc. Due to problems such as these, most systems allocate a fixed pool of memory to the networking system and impose a policy optimized for ``normal'' network operation.

      The networking system developed for UNIX is little different in this respect. At boot time a fixed amount of memory is allocated by the networking system. At later times more system memory may be requested as the need arises, but at no time is memory ever returned to the system. It is possible to garbage collect memory from the network, but difficult. In order to perform this garbage collection properly, some portion of the network will have to be ``turned off'' as data structures are updated. The interval over which this occurs must kept small compared to the average inter-packet arrival time, or too much traffic may be lost, impacting other hosts on the network, as well as increasing load on the interconnecting mediums. In our environment we have not experienced a need for such compaction, and thus have left the problem unresolved.

      The mbuf structure was introduced in chapter 5. In this section a brief description will be given of the allocation mechanisms, and policies used by the protocols in performing connection level buffering.

12.1.  Memory management

      The basic memory allocation routines manage a private page map, the size of which determines the maximum amount of memory that may be allocated by the network. A small amount of memory is allocated at boot time to initialize the mbuf and mbuf page cluster free lists. When the free lists are exhausted, more memory is requested from the system memory allocator if space remains in the map. If memory cannot be allocated, callers may block awaiting free memory, or the failure may be reflected to the caller immediately. The allocator will not block awaiting free map entries, however, as exhaustion of the page map usually indicates that buffers have been lost due to a ``leak.'' The private page table is used by the network buffer management routines in remapping pages to be logically contiguous as the need arises. In addition, an array of reference counts parallels the page table and is used when multiple references to a page are present.

      Mbufs are 128 byte structures, 8 fitting in a 1Kbyte page of memory. When data is placed in mbufs, it is copied or remapped into logically contiguous pages of memory from the network page pool if possible. Data smaller than half of the size of a page is copied into one or more 112 byte mbuf data areas.

12.2.  Protocol buffering policies

      Protocols reserve fixed amounts of buffering for send and receive queues at socket creation time. These amounts define the high and low water marks used by the socket routines in deciding when to block and unblock a process. The reservation of space does not currently result in any action by the memory management routines.

      Protocols which provide connection level flow control do this based on the amount of space in the associated socket queues. That is, send windows are calculated based on the amount of free space in the socket's receive queue, while receive windows are adjusted based on the amount of data awaiting transmission in the send queue. Care has been taken to avoid the ``silly window syndrome'' described in [Clark82] at both the sending and receiving ends.

12.3.  Queue limiting

      Incoming packets from the network are always received unless memory allocation fails. However, each Level 1 protocol input queue has an upper bound on the queue's length, and any packets exceeding that bound are discarded. It is possible for a host to be overwhelmed by excessive network traffic (for instance a host acting as a gateway from a high bandwidth network to a low bandwidth network). As a ``defensive'' mechanism the queue limits may be adjusted to throttle network traffic load on a host. Consider a host willing to devote some percentage of its machine to handling network traffic. If the cost of handling an incoming packet can be calculated so that an acceptable ``packet handling rate'' can be determined, then input queue lengths may be dynamically adjusted based on a host's network load and the number of packets awaiting processing. Obviously, discarding packets is not a satisfactory solution to a problem such as this (simply dropping packets is likely to increase the load on a network); the queue lengths were incorporated mainly as a safeguard mechanism.

12.4.  Packet forwarding

      When packets can not be forwarded because of memory limitations, the system attempts to generate a ``source quench'' message. In addition, any other problems encountered during packet forwarding are also reflected back to the sender in the form of ICMP packets. This helps hosts avoid unneeded retransmissions.

      Broadcast packets are never forwarded due to possible dire consequences. In an early stage of network development, broadcast packets were forwarded and a ``routing loop'' resulted in network saturation and every host on the network crashing.